Investigation of thin-film· polycrystalline silicon forsolar cells by a new solid phase crystallization method

February 1994

Takao MA TSUY AMAoInvestigation of thin-film polycrystalline silicon forsolar cells by a new solid phase crystallization method

February, 1994

Takao MA TSUY AMAContents

1 Introduction··················· ·······.· .............. ·······································1
1.1 History of solar cells ................................ ··· ........ · .. ·· .. · .................... 1

1.2 Conventional fabrication method of thin-film
polycrystalline silicon for solar cells
1.3 Solid phase crystallization................................................... 2

method .......... · ............................................. 6

1.4 A new solid phase crystallization (SPC) method .... · .. · ...... · ...................... 8
References························ ··· ··· ............................................................ 12

2 Theoretical background· ····. ·.· ..... .................... ... .. ........... .............. 14
2.1 Introduction······.··· ......................................................................... 14

2.2 Solid phase crystallization .................................................................. 15
2.3 Transport of polycrystalline silicon films .......................... · .......... · .... · .... 18
2.4 Current-voltage characteristics of p-n junction ......................................... 21
2 . 5 Sum mar y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
References······················ ................................................ ... ................. 27

3

Development of thin-film polycrystalline siliconb y a new S PC met hod· . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28
3.1 Introduction···· ··· ··· ......................................................................... 28
3.2 Effect of impurities(dopants)· .. · .......................................................... 33

3.2.1 Experimental technique· ............................................................ 33
3.2.2 Phosphorus and Boron .. · ......................................................... 35
3.2.3 Partial doping ........................................................................ 55
3.3 Optimum starting materials for the SPC method .. · .... · ...... ··· .... · .......... · .... 66
3.3.1 Experimental technique ............................................................. 66
3.3.2 Evaluation of structural disorder in starting materials ...... · .. · .. · .......... 69
3.3.3 Correlation between grain size of polycrystalline silicon andstructural disorder of starting materials .. · .. ·· .......... · ....................... 72
3.3.4 Factors for enlarging grain size in polycrystalline silicon .... · .. · .......... 76
3.4 Investigation of substrates ................................................................. 83
3.4.1 Experimental technique .. · .. · ....................................................... 83
3.4.2 Effects of surface morphology ................................................... 86

J

~..

.

1. Introduction
1.1 History of solar cells
3.4.3 Growth kinetics on textured subst rates · .. ·· .............. · .. ··· ................ 93

In 1954, the first solar cell using crystalline silicon (c-Si) was developed by Chapin,

3.5 Solid phase crystallization using crystal silico n as a seed ......................... 97
3 . 5 . 1 Ex per i men t a I t e c h n i que· .. ... . . . .. . . . . . . . . . . . . . . . . . .. . . .. . . . . . .. . . . . . .. . . . . . . .. . .. .. 97

Fuller, and Pearson of Bell Telephone Laboratories, 1) and a conversion efficiency of 6%

3 . 5 . 2 E f f e c t 0 fin t ern a I see din g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 99

was soon obtained. In 1958, the U. S. Vanguard space satellite used c-Si solar cells to

3 . 6 Sum mar y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

power its radio. The cells worked so well that solar cells were soon recognized as an

References ......... ........... .... ......... ......................................................... 110
effective power source for space operations. As for terrestrial applications of solar cells,
numerous products have been over the last 30 years for commercial use. This is because

4

Development of solar cells
using thin-film polycrystalline silicon ...... · .. · .. · .. · .... · .. · .. · .............. · III

photovoltaic conversion of solar energY' into electricity provides an abundant, clean, andrelatively homogeneously distributed source of energy compared with oil resources.

4.1 Introduction .··.· · ····· ········ ······· ·· ··· ····· · ····· ......................................... 111

Solar cells themselves require no fuel, and therefore, there is no pollution from exhaust

4.2 Experimental technique ..... ··············· ······· ·················· ........................ . 112
4.3 A new junction fabrication technology .... · .......... · .......... · .............. · ...... .. 116

gases. In addition, as solar cells do not have moving parts, they are silent in operation,

4.4 Solar cells using thin-film polycrystalline silicon

and their lifetime is long.

................................................. ....... .. 136

c-Si and polycrystalline silicon (poly-Si) solar cells are the most popular cells at

4 .5 Summary·.·.··········································· ............................ ....... .. .. 147

present, but, their cost is so high that they cannot compete with conventional energy

fabricated by the SPC method

References ...... ............... . . .. .. ...... ..... .. . ....... ...... .... ................................ 149
sources such as thermal-power generation. The main reason for the expensiveness of c-Si
and poly-Si solar cells is their fabrication process. A high temperature is required in the

5. Summary.... ...... ..................................... ..................................... 150

Acknowledgement

processes of crystal growth and junction formation. The necessity of slicing andpolishing also causes loss of materials. In addition, because a semiconductor layer serves
as the substrate, thickness of c-Si and poly-Si used in solar cells is as thick as 300ll-rn,
which also increases the cost. Therefore, expensive c-Si and poly-Si solar cells have beenutilized in restricted applications, such as satellites, lighthouses, and so on.
Amorphous silicon (a-Si) films are also good candidates for a solar cell material. Since
Spear and LeComber succeeded in the valency control of hydrogenated a-Si deposited bya glow discharge method with silane (SiH 4 ) in 1975,2) a-Si films have been gatheringmuch attention as a low-cost solar cell material. Their fabrication temperature is low, theamount of material required is small due to their high optical absorption coefficient, andthey can utilized low-cost substrates. The conversion efficiency of a-Si solar cells has
increased gradually, and presently reaching 12 % for a single junction a-Si solar cell witha size of 10 cm x 10 cm, 3 ) 10.05 % for a tandem type a-Si/a-Si solar cell with a size of-1-

30 cm x 40 cm 4 ) and 13.7 % for a three-stack type a-Si/a-Si/amorphous silicon

Concerning junction formation, homojunctions and heterojunctions have been

gennanium (a-SiGe) solar cell with a size of 0.25 cm 2 .5) But a-Si films show a light-

investigated. The fabrication methods for homojunctions use the diffusion 15) of

induced degradation,6) which becomes a large problem for a-Si solar cells in outdoor

phosphorus (P) and boron (B), and the ion implantation 16) of Pion s (p+) and B ionsuse . There are many reports on the origin of this light-induced degradation? -10) but it(B-) into poly-Si. Heterojunctions are fabricated by depositing a-Si 17) or microcrystalline

has not yet been resolved.

silicon (~c-Si) 18) onto poly-Si.

Recently, thin -fi lm poly-Si has gathered considerable attention as a solar cell material

Conventional fabrication methods require a hi g h-temperature process of more thanwhich does not exhibit the light -i nduced degradation of a-Si and offers high1000 °C. So the materials that can be used as the substrate are limited . Cheap substrates,

photosensiti vity in the long wavelength region of more than 800 nm, where a-Si films

such as glass and stainless steel, can not be used because the melting points of thesehave no photosensitivity. There is also a report on a solar cell with a high theoreticalsubstrates are lower than 1000 °C. Therefore, conventional fabrication methods do notconversion efficiency of 29 %, in spite of a thickness of only 10 ~m.11) But it presents asuit the preparation of thin-film poly-Si for low-cost solar cells.

problem from the viewpoint of mechanical strength when slicing thick poly-Si and c-Si toa thickness of less than 100 ~m. So, it is necessary to prepare thin-film poly-Si or c-Si
on substrates in order to fabricate solar cells with high conversion efficiency and low
cost. To lower the cost of solar cells, substrates such as glass, stainless steel, and so on,
should be used.

1.2 Conventional fabrication method of thin-film polycrystalline silicon for solar celIs
A number of methods are used to fabricate thin-film poly-Si on substrates for solar
cells, such as chemical vapour deposition (CVD) 12, 13) and dipping. 14) In the CVD
method, gases such as chlorosilane (SiH n CI 4 _n (n:O, 1,2)) and monosilane (SiH 4 ) areused as a source gas and the film is deposited onto substrates by the thermal
decomposition of the source gas in a reaction chamber, as shown in Figure 1.2-1. The
substrates are set onto a susceptor heated by a R.F. coil and the typical substratetern perature is about 1100 °C. The deposition rate of the films is on the order of 1
~m/min . at this temperature . It is difficult to apply the films deposited on substrates togood active layers in solar cells, because of their very small grain. In general, the films
after deposition are treated by thermal annealing again to increase their grain size. Figure
1.2-2 shows the principal of the dipping method. In this method, a substrate such asgraphite is introduced into liquid silicon, and poly-Si films are fabricated by pulling up
the graphite at a speed of a few cm/min. The thickness of the poly-Si is 100 ~m.

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Exhaust
Quartz tube

R F
,

'I

. COl

oooo~ooooooo

Substrate(Graphite)Thin-film poly-Si
Pulling up£

I

Substrate

000000000000

H2 Ar

Susceptor

Siliconmelt

SiH2CI2SiHCI3SiH4Fig. 1.2-1 A conventional CVD apparatus for the preparation of thin-film poly-Si.

Fig. 1.2-2 The principal of dipping method.

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--- -

-~-~~~~--~-~"-~~-

1.3 Solid phase crystallization methcxi
One of the methods for fabricating thin-film poly-Si at the low temperature of about60(rC is the solid phase crystallization (SPC) method. Thin-film poly-Si prepared by theconventional SPC method has a thickness of less than 0.5 Ilm and has been applied tothin-film transistors (TFT).19) The conventional SPC method,20, 21) in general,consists of three processes. Figure 1.3-1 shows a diagram of the conventional SPC
method. In the first process, a-Si films or poly-Si films with small grains of about 0.2
Ilm were deposited on substrates by low pressure chemical vapour deposition (LPCVD).

(a) Film deposition by LPCVD method

In the second process, P+, B- or silicon ions (Si+) were implanted into these films toenhance crystal growth. Poly -Si films were amorphized by implanting Si+. In the third

B or P or Si ionsprocess, these films were transferred into poly-Si films by thermal annealing at thetemperature of about 600°C. The merits of the conventional SPC method are as
follows:.
(1) The process temperature is lower than that of CVD or the dipping method.

I

T(2) A low cost substrate can be used instead of silicon.

Film(3) A large grain of more than 0.11lm can be obtained.
But, there are a number of problems in the preparation of thin-film poly-Si for a solar cell~

Substratematerial, as follows:.
(1) It is difficult to prepare poly-Si films with a thickness of more than lllm, because(b) Ion implantation

the ion distribution in depth is not uniform.
(2) It is difficult to prepare large-area poly-Si films, because large-area
implantation is not so easy.
(3) The process is complicated.
There are no reports on solar cells using poly-Si films prepared by the conventional SPC
method. So, to resolve these problems and to apply poly-Si films prepared by the SPC
method to solar cells, a new SPC method must be developed, while maintaining themerits of the low-temperature process.

I~'(c) Thermal annealing

Fig. 1.3-1 Diagram of the conventional SPC method.

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the silicon.

1.4 A new solid phase crystallization (SPC) method
A new solid phase crystallization (SPC) method was developed for the preparation ofthin-film poly-Si, using a-Si films as a starting material. This SPC method is a way of
fabricating thin-film poly-Si by the thermal annealing of plasma-CVD a-Si films.
Phosphorus and boron atoms were doped in-situ into a-Si films. Impurities such as(3) A flat surface must be obtained easily for the substrates.
(4) The composition elements of the substrates must not have an unfavorable
influence on the growth of thin-film silicon.
(5) The electrical resistivity of the substrates must be low.

oxygen, nitrogen, and carbon have various unfavorable influences on the crystallization

Table 1.4-1 shows the materials which fulfill the required conditions of (1) - (5). A newof silicon. a-Si films with low impurity concentration are prepared by a super chamber

SPC method is a way of adapting to the preparation of thin-film poly-Si on substrates

18

other than silicon, because the quartz and W substrates used in the new SPC method

cm- 3 , 1x10 17 cm- 3 and 2x10 18 cm- 3 , respectively, in the a-Si films fabricated using thesatisfy these conditions. Quartz and W can also be changed to glass and W -coatedsuper chamber. This oxygen concentration is almost the same as that in c-Si made by thestainless-steel, respectively.

method. 22 ) The impurity concentrations of oxygen, nitrogen and carbon are 2x 10

Czochralski process. So, a-Si films prepared by the super chamber method were used as

The purpose of this work is to develop thin-film poly-Si as a solar cell material.

a starting material for SPC.

In Chapter 2, the preparation of thin-film poly-Si by the new SPC method and the

Figure 1.4-1 shows a diagram of the SPC method, which consists of two processes.

improvement of the thin-film poly-Si properties by nucleation control are reported. I

In the first process, P or B doped a-Si films were deposited by the plasma-CVD method

propose three innovative technologies for nucleation control: a partial doping method, a

on a substrate. Quartz and tungsten (W) were used as substrates. In the second process,textured substrate, and a seed made of crystalline phase. For each technology, the grain

P-doped a-Si films and B-doped a-Si films were transferred to n-type poly-Si and p-type

size and Hall mobility of thin-film poly-Si are investigated.

poly-Si, respectively, by thermal annealing in a vacuum or a nitrogen atmosphere at

Chapter 3 describes solar cells using a new junction formation technology and solar

various temperatures. The typical annealing temperature is 600 °C. This SPC method has

cells applying this new junction formation technology to thin-film poly-Si prepared by thethree main features:new SPC method. The junction fabrication method, in which the process temperature(1) Impurities (phosphorus or boron atoms) are doped in-situ into a-Si as a starting

was below 200°C, was developed for solar cells using a-Si and c-Si. Thin-film poly-Si

material, without the need for the ion implantation process.

prepared by the new SPC method was also analyzed in solar cells.
Finally, the conclusions of the present study are given in Chapter 4.

(2) The process is very simple.
(3) n-type poly-Si (or p-type poly-Si) with a large-area can be prepared.
So, it is recognized that the problems described in Section 1.3 can be resolved by a new
SPC method.
The following conditions are required for the preparation of thin-film poly-Si on
substrates instead of silicon:(1) There must be no chemical reaction between the substrate materials and the siliconat the process temperature, and the silicon atoms must adhere to the substrates.
(2) The coefficient of thermal expansion of the substrate materials must fit that of

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Table 1.4-1 The properties of various substrates

n-type(or p-type)poly-Si

Materials

Coefficient of thermal
expansion

Other properties

Substrate

SubstrateSi

2.6 (X 10 -SoC)

Melting point: 1417°C

Plasma CVD

Thermal annealing

Ta

6.6

Melting point: 3007°CMo

3.7 - 5.3

Melting point: 2608°C

W

6.0

Melting point: 3380°C

C(Graphite)

2.0 - 2.4

Pore ratio: 1 - 3%

Si02 (Quartz)

0.4

P{or B)-doped
a-Si

SPC

, , , , , 1\ \ , , , ,'""" "",,\, , , , I I I
I I I I I I I
I I I I I I I'"
I , I'I I I I I I I
I I I I I I I

I , , , , "

"",,'

\\1

V

~------------------------------~

Fig. 1.4-1 Diagram of the SPC method consisting of two processes.

AI203

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---------Heat-resistance

7.8

temperature: 1750 °c-11-18) Y. Matsumoto, H. Okamoto, H. Takakura, M. Onega and Y. Hamakawa: Tech.

References
1) D . M. Chapin, C. S. Fuller and G. L. Pe arson: J. App!. Phys., 25 (1954) p. 676.
2) W. E. Spear and P . G. LeComber: Solid State Comm., 17 (1975) p. 1193 .
3) Y. Hi shi kawa , M. Sasaki, S. Tsuge, S. Okamoto and S. Tsuda:"MA TERIALCONTROL FOR HIGH-EFFICIENCY AMORPHOUS SILICON SOLAR CELLS" ,Proc. of Mat. Res. Soc. in 1993 to be published.
4) Y. Ichikawa , S. Fujikake, H . Ohta, T. Sasaki and H . Sakai: Proc. of 22nd IEEE
Photovoltaic Specialists Conf., 1991, p. 1296.
5) J. Yang, R. Ross , T. Glatfel ter, R. Mohr, G . Hammond, C . Bernotaitis, E. Chen, J.
Burdick , M. Hopson and S. Guha: Proc. of 20th IEEE Photovoltaic Specialists Conf.,

Digest Int. PVSEC-5 , Kyoto, Japan (1990) p. 693.
19) S. Takenaka, M. Kunii, H. Oka and H. Kurihara: Jpn. 1. Appl. Phys. Vo1.29,
No.12, (1990) p. L2380 .
20) Y. Saito, 1. Mizushima and H. Kuwan o: J. Appl. Phys. 57 (6), (1985) p. 2010.
21) I. Mizushima , W . Tabuchi and H. Kuvv ano: lpn. l. Appl. Phys. Vol.27, No.12,(1988 ) p. 2310.
22) S. Tsuda , T. Takahama , M. Isomura, H. Tarui, Y. l\akashima, Y. Hishikawa,N. Nakamura, T. Matsuoka, H. Ni~hiwaki, S. Nakano, M. Ohnishi and
Y. Kuwano: lpn. l. App!. Ph ys. Vol. 26 No.1, (19 7) p. 33.

1988, p. 241.
6) D. L. Staebler and C. R. Wron ski: Appl. Phys. Lett. 31 (1977) p. 292 .
7) D . L. Staebler, R. Crandall and R . Williams: Appl. Phys. Lett. 39 (1981) p. 733.
8) H. Okushi, M. Itoh, T. Okuno, Y. Hosokawa, S. Yamasaki and K. Tanaka: Proc.
Int. Conf. Optical Effects in Amorphous Semiconductors, Snowbird, Utah (1984)
p. 251.

9) S. Tsuda, N. Nakamura, K. Watanabe, M. Nishikuni, M. Ohnishi, S. Nakano, Y.
Kishi, H. Shibuya and Y. Kuwano: Tech. Digest Int. PVSEC-1, Kobe, Japan (1984)
p.213 .
10) M. Stutzmann, W. B. Jackson and C. C. Tsai: Phys. Rev. B32 (1985) p. 23.
11 ) T. Tiedje et a!.: IEEE Trans. Vol. ED-31, NO .5 (1984) p. 711.

12) T. L. Chu : Applied Physic s Letters, Vol. 29, No . 10, (1976) p. 675.
13) T. L. Chu, Shirley S. Chu, C. L. Lin and R. Abderrassoul: J. Appl. Phys . 50 (2),(1979) p. 919 .
14) J. D. Heaps, R. B. Maciolek, 1. D. Zook and M. W . Scott: Proc. IEEE 12th
Photovoltaic Specialists Conference, (1977 ) p. 147.
15) R. N. Hall et al.: Phys . Rev., 80, (1950) p. 467.
16). W. Shockley: Proe . IEEE , 56 (1968) p. 295.
17) M. Tanaka , M . Taguc hi , T. 1atsuyama, T. Sawada, S. Tsuda, S. Nakano, H.
Hanafusa and Y. Kuwano: Jpn. J. App!. Phys., Vo1.31, No.l1 (1992) p. 3518.

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2.2 Solid phase crystallization

2 Theoretical background

Crystalline silicon growth in solid phase consists of two processes such as nucleation

2.1 Introduction
In order to investigate polycrystalline silicon (poly-Si), it is necessary to study kinetics

and growth. It is necessary to study the theory of nucleation and growth. According toclassical theory, microcrystallites (containing only a few atoms) will nucleate frequently.

of growth and carrier transport.
As for growth process, many phase transformations occur by nucleation and growth.

Because of a large surface-to-volume ratio, these tend to shrink. From thermodynamic

In classical theory, it is proposed that clusters of atoms in the configuration of theconsiderations, however, a few will become large enough that further growth istransformation product arise in the original material. Initially, the clusters are small andenergetically favorable . Growth in the microcrystalline regime occurs by the same basic

consequently unstable because of their high surface-to-volume ratio. Eventually,mechanism as growth in the macrocrystalline regime: The rate at which each atom at an

however, some clusters grow to a size beyond which they are stable, i.e., they become

amorphous-crystalline interface makes'a transition between the amorphous state and thenuclei . At constant temperature, a constant rate of production of nuclei will becrystalline state is a function of the energy levels of the two states. Growth isestablished. In section 2.2, transient time, nucleation rate, and growth rate, which haveenergetically favorable when the free energy of the system is lowered by the transition ofmuch influence on grain size of poly-Si, are discussed.

an interface atom from the amorphous to the crystalline state.

The electrical properties of poly-Si films have been interpreted in terms of two distinct

The free energy of formation of a cluster of size n consists of two components: themodels: the segregation theory, 1, 2) according to which impurity atoms tend to segregate

Gibbs-free-energy difference between crystalline and amorphous phases (proportional toat the grain boundary where they are electrically inactive, and the grain boundary trapping

volume, or n ) and the energy required to maintain the amorphous-crystalline interface

theory3, 4) assuming the presence of a large amount of trapping states at the grain(proportional to surface area or n2/ 3 ). The Gibbs-free-energy difference between theboundary able to capture, and therefore immobilize, free carriers. These charged states atamorphous and crystalline phases is denoted as g and is in units of e V/atom. It is taken to

the grain boundary create potential barriers, which oppose the passage of carriers from abe positive. That is, the energy of each atom in the cluster is lower than the energy of an

grain to the neighboring ones.

atom in the amorphous phase by g.

The basic limitation of the segregation model is that it does not explain the temperature

At small cluster sizes, the interfacial energy outweighs the energy difference due to thedependence of the film resistivity. On the other hand, a theory combining the grain-phase change-the net free energy of formation ~Gn is positive. The maximum freeboundary trapping model with a thermionic-emission mechanism through the barriers canenergy of formation, denoted ~Gn *, occurs at the critical cluster size n* and enters into

explain most of electrical properties of poly-Si film. So, in section 2.3, this theory is

the temperature dependence of the steady-state nucleation rate r n as mentioned the later.

described.

The forward reaction rate is the rate that a cluster grows from n to n+ 1 atoms, and the

The p-n junctions are of supreme importance both in the modern electronic
applications and in the understanding of other semiconductor devices. The p-n junction
theory serves as the foundation of the physics of semiconductor devices . The basic
theory of current-voltage characteristics of p-n junction was established by Shockley.5)
This theory was then extended by Sah et a1. 6 ) and by Moll. 7 ) So, section 2.4 mentions

reverse reaction rate is the rate that a cluster shrinks from n+ 1 to n atoms as following
equations:+k n -1En- 1 + El ~ En'(2.2-1)k n-

the theory which is basis of device physics.

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k n+

temperature dependence . By ass um ing th at the free energy of formation is a weak
(2.2-2)

En + E1 ~ E n +1,function of temperature, the form is'tt DC T Ik n * ,k n- +1(2.2-5)where En represents a cluster of n molecules and E 1 a single molecule. k n + is the rate ofwhere k n * is the forward (or reverse ) reactio n rate at the critical clu ster size . From

addition of molecules to a cluster En, and k n - is the rate of loss of molecules from En·

Eq.(2.2-4),(2.2-6)

These rate s are found by defining an unbiased atomic jump frequency u at theamorphous-crystalline interface. Furthermore, since u is probably closely related to thejump rate in the amorphous phase, it can be characterized by the activation energy of self-

Kashchiev 's theoretical results , 13) which are in good agreement with experiment,
indicate that 'to is proportional to'tt :'to DC Texp (EdlkT~ .

diffusion in amorphous silicon Ed:(2.2-7)(2.2-3)

U DC exp(-Ed/kT),

where k is Boltzmann's constant, T is absolute temperature. Since all reaction rates areproportional to u, Ed is the activation energies for transient time 'to' steady-state(2) Nucleation rate
The theoretical temperature dependence of the steady-state nucleation rate rn 14) is
r n oc u ·exp( -6G n */kT).

nucleation rate r n, and growth velocity v g.
The reaction rates are also proportional to the number of atoms at the cluster surface(2.2-8)Since the unbiased atomic jump frequency U is proportional to l/'tO' using Eq .(2.2-7),
one obtainesand may be written in the following form:
k n _ n+ 1 DC n 2 / 3 exp(-Ed/kT) x exp[(~Gn+ 1-~Gn)/2kT],
kn+1 _ n DC n 2/ 3exp(-Ed/kT) x exp[(~Gn-~Gn+l)/2kT].

(2.2-9)(2.2-4)

A complete analysis of the reaction rates [Eq.(2.2-4)] and 6G n (described above)(3) Growth rate

provides values for transient time , nucleation rate, and growth velocity within the scope

The growth rate can be found from the net forward reaction rate for a large cluster.

of classical theory. The respective activation energies, as shown below, are related toAddition of one atom to a large cluster will not significantly change the surface energy.

~Gn*' g, and Ed. The transient time is the effective time to reach a steady-state nucleation

The change in free energy, then , is -g. From Eq.(2.2-4), by assuming g»2kT,

rate from the initial conditions (no clusters). At steady state the number of clusters N n of

dn/dt=k n - n+ }-kn+ 1 - neach cluster size n is time independent, and the net forward reaction rate (Nnk n - n+1 -DC

n 2/ 3 exp(-Ed/kT) x [exp(g/2kT)-exp(-g/2kT)]

Nn+ 1kn+ 1 - n) is equal to the steady-state nucleation rate rn for all n. Growth velocity isoc

n2/3exp[-(Ed-g/2)/kT].

obtained from the net forward reaction rate for a large cluster.

(2.2- 10)

Because n is proportional to the radius cubed (r 3 ), Eq.(2.2-10) is easily solved for Vg(=dr/dt):
v g DC exp[ -(Ed-g/2)/kT] .

(1) Transient nucleation time

(2.2- 11 )A number of analyses of transient nucleation have been published.9- 13 ) In each case,a characteristic tim e 'tt associated with transient nucleation was found to have the same

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Grain boundary

2.3 Transport of polycrystalline silicon films
A polycrystalline material is composed of small crystallites joined together by grainboundaries. Inside of each crystallite, the atoms are arranged in a periodic manner so that~.!

it can be considered as a small single crystal. The grain boundary is a complex structure,
usually consisting of a few atomic layers of disordered atoms. It is important to consider
the effects of the grain boundary on the electrical properties for discussing the transportof polycrystalline semiconductors. Seto considered that carrier trapping at the grainboundary governed the electrical transport properties of polycrystalline films. IS) In a real(a) Crystal structure

polycrystalline material, the crystallites have a distribution of sizes and irregular shapes.

Q

In order to simplify the model, following assumptions were used:(1) Poly-Si is composed of identical crystallites having a grain size of L em.

(2) Only one type of impurity atom presents, and the impurity atoms are totally
ionized and uniformly distributed with a concentration of N em -3.

-----.--~--.--+o~~~---,-----~x

qN(3) The grain boundary is of negligible thickness compared to L and contains Nt
em -2 of traps located at energy Et with respect to the intrinsic Fermi level.

Using these assumptions, an abrupt depletion approximation is used to calculate the-U2 -I

U2(b) Charge distributionenergy band diagram in the crystallites. In Figure 2.3-1, all the mobile carriers in a regionof (L/2-1) em from the grain boundary are trapped by the trapping states, resulting in a
depletion region. Using the above approximation, Poisson's equation becomesd 2 V/dx 2 = qN/£,I < x < L/2,

(2.3-1)

where £ is the dielectric permittivity of poly-Si. Integrating Eq.(2.3-1) twice and applying

Y

Y

-E t

-E t

Ec

-

Efi

the boundary conditions that Vex) is continuous and dV /dx is zero at x=l gives
V(x)=(qN/2£)(x-l)2+ V vO,

I < x < L/2,

(2.3-2)where V vO is the potential of the valence band edge at the center of the crystallite. In thiscalculation, the intrinsic Fermi level is taken to be at zero energy and energy is positive
towards the valence band. There exist two possible conditions depending on the doping

YEv! E{yEfEv(C) Energy band structure

concentration: (a) LN<Nt, and (b) LN>Nr
Under LN<N t condition, the crystallite is completely depleted of carriers and the traps
are partially filled, so that 1=0 and Eq.(2.3-2) becomes
V(x)=(qN/2£)x 2 + V vO,

x ~/2.

-18-

Fig. 2.3-1 (a) Model for the crystal structure of poly-Si films. (b) The charge
distribution within the crystallite and at the grain boundary. (c) The
energy band structure for poly-Si crystallites.

(2.3-3)-19-

The potential barrier height, VB' is the difference between YeO) and V(L/2), i.e.,
V B=qL 2 N/ 8£(2.3-4)showing that VB increases linearly with N. Using Boltzmann statistics, the mobile carrier

where m * is the effective mass of the carrier. If Va is small, q Va «kT, Eq.(2.3-10) canbe expanded to give
lth=q2p a(2m *1tkT)-1I2·exp(-qV B/kT)V a'(2.3- 11)which is a linear current-voltage relationship. From Eq.(2.3-11), the conductivity of aconcentration, p(x), becomes(2.3-5)p(x)=N y exp{ -[qV(x)-Ef]/kT},

where Ny is the density of states and E f is the Fermi level. The average carrier
concentration, P a' is obtained by integrating Eq.(2.3-5) from -L/2 to L/2 and dividing by

poly-Si film with a grain size L cm is
cr=Lq2 p a(2m *1tkT)-1I2. exp (-qV /kT).

(2.3-12)B

Insening Eqs. (2.3-6) and (2.3-9) into Eq.(2.3-12), following relationships are obtained.

the grain size. The result is
p a=(n/Lq) (21t£kT/N) 1/2.exp[(~+Ef)/kT]·erf[(qL/2)(N/2£kT) 1/2], (2.3-6)(Joeexp[ -(Eg-Ef)/kTj,

if NL < Nt,(2.3-13)(Joe T -1/2exp[-Eg/kT],

if NL > N

(2.3-14)r

where ~ =qV Band ni =N y exp[ -EgI(2kT)] is the intrinsic carrier concentration of single2.4 Current-voltage characteristics of p-n junction

crystalline silicon (with band gap Eg) at temperature T.
If LN>Nt, only part of the crystallite is depleted of carriers and 1>0. The potential

The ideal current-voltage characteristics in dark are derived on the basis of thefollowing four assumptions:barrier height then becomes from Eq.(2.3-2)
2
V B=qN t /8£N.

(2.3-7)(1) The built-in potential and applied voltages are supported by a dipole layer with

The average carrier concentration, P a' is obtained by averaging over the crystallite . In theabrupt boundaries, and outside the boundaries the semiconductor is assumed to be

undepleted region, the carrier concentration, P b , is the same as that of a doped single-

neutral.
(2) Throughout the depletion layer, the Boltzmann relations are valid.

crystalline silicon,(2.3-8)for a nondegenerately doped sample. The carrier concentration in the depletion region is
given in Eq.(2.3-5). The average carrier concentration, P a' can be shown to be
P a=P b {(l-NiLN)+ 1/(qL)(21t£kT/N) 1/2.erf[qNJ2.(2£kTN)-1/2]}.

(2.3-9)There are two important contributions to current across the grain boundary: thermionic(3) The injected minority carrier densities are small compared with the majority-carrier

densities.
(4) The facts that no generation current exists in the depletion layer, and the electron
and hole currents are constant through the depletion layer.
At thermal equilibrium, the Boltzmann relation is given byemission and tunneling. Thermionic emission results from those carriers possessing highne=njexp[ (EF-Ei)/kT]=njexp[ q (\}J -¢ )/kT](2.4-1 a)enough energy to surmount the potential barrier at the grain boundary. On the other hand,

Ph=njexp[(EtEF)/kT]=njexp[q(¢-\V)/kT](2.4-1 b)

the tunneling current arises from carriers with energy less than the barrier height. The

where ne is electron density, Ph is hole density, ni is intrinsic carrier density, EF is Fermi

barrier height decreases to a small value for a doped poly-Si, therefore, the tunnelinglevel, Ej is intrinsic Fermi level, and \V and ¢ are the potentials corresponding to thecurrent should be neglected. The thermionic emission current density, J th , for an applied

intrinsic level and the Fermi level, respectively (or 'V=-E/q, ¢=:-EF/q). When a voltage isvoltage, Va' across a grain boundary isapplied, the minority carrier densities on both sides of a junction are changed, and the

Jth=qP a(kT/2m *1t) 1I2· exp(-qV B/kT)[exp(qV ikT)-I],(2.3-10)

Phne product is no longer given by n? The imrefs is defined as follows:
ne=njexp[q(\}J-¢n)/kT]-20-(2.4-2a)-21-

(2.4-11 )(2.4-2b)where ¢n and ¢p are the imrefs or quasi-Fermi levels for electrons and holes,

It can be shown that from the assumption (3) (e.g., Pn«nn~nnO in the n-typerespectively. From Eqs.(2.4-2a) and (2.4-2b), ¢n and ¢p are obtained to be

semiconductor) Eq.(2.4-9) reduces to¢n=o/-kT/q·ln(nJnj)(2.4-3a)¢JT-=\V+kT/q·ln (pt!n j)(2.4-3b)-(Pn-PnO)/'tp-ll-pE 'dPn/dX+Dp'd2pn/dX2=0

In the neutral region where there is no electric field, Eq.(2.4-12) further reduces tod 2pn/dX 2-(Pn-PnO)/'tpDp=O.

the Phrle product becomes(2.4-4)

Ph ne=n j2ex p [q (¢p-¢n )/kT]

For a forward bias, (¢p-¢n»O, and Phne>nj2: on the other hand, for a reversed bias,(2.4-13)The solution of Eq.(2.4-13) with the boundary condition Eq.(2.4-7) and Pn(x=oo)=PnO,
gives
Pn-PnO=PnO(eqV /kT-1 )e-(x-xn)!Lp(¢p-¢n)<O, and Phne<nj2.
Since the electron density ne varies in the junction from the n side to the P side bymany orders of magnitude, whereas the electron current I n is almost constant, it follows
that ¢n must also be almost constant over the depletion layer. The electrostatic pOltential(2.4-14)

where
Lp=(D p't p) 1/2.

(2.4-15)

And at x=xn
Jp=-qDp 'dpn/dX Xn =qDpPno/Lp( eq v /kT -1 ).

difference across the junction is given by(2.4-5)Vs=¢p-¢n.

Eqs.(2.4-4) and (2.4-5) can be combined to give the electron density at the boundary of(2.4-16)

Similarly following equation is obtained for the p side

I n=qD n'dnpfdx _xp=qDnnpO"Ln(eqV IkL 1).

(2.4-17)

The total current is given by the sum of Eqs.(2.4-16) and (2.4-17) :the depletion-layer region on the p side (x=-x p)(2.4-6)

np=n?/ppexp(qV JkT)=npOexp(qV sfkT)

where npO is the equilibrium electron density on the p side. Similarly,
Pn=PnOexp(qV JkT)

(2.4-12)(2.4-7)at x=xn for the n type boundary.Eqs.(2.4-6) and (2.4-7) serve as the most important

l=Jp+Jn=Js(eqV/kL 1),(2.4- 18)

ls=qDpPno/Lp+qD nnpO"Ln.

(2.4-19)

The rate of generation of electron-j=hole pairs can be obtained with the condition p<ni
and n<nj:
U=-[O"pO"n uthNJ[O"nexp{ (Et-Ej)/kT) +O"pexp{ (Ej-Et)/kT} ]]nj=-n/'teboundary equations for ideal current-voltage equation.

(2.4-20)

From the continuity equations, following equations are obtained for the steadY-Slate:where O"p is the hole capture cross section, O"n is the electron capture cross section, uth is-(nn-nnO)/'tn+ll-nE .(dnn/dx)+ll-nnn'(dE /dX)+Dn'(d 2n n/dx 2)=O, (2.4-8a)

the carrier thermal velocity, Nt is the trap density, E j is the intrinsic Fermi level, E t is the-(Pn-PnO)/'tp+ll-pE '(dPn/dX)+ll-pPn'(dE /dx)+Dp·(d2Pn/dx2)=O.

trap energy level, and 'te is the effective lifetime.

(2.4-8b)

We can eliminate the term in dE Idx from the above equations with the condition that

Using Eq.(2.4-20), the total forward current considering the generation-recombination
process can be obtained (for PnO»np(Y' and V>kT/q):(Pn-PnO)/'t p equals (nn-nnO)/'t n. This gives

JF=q(Dpf'tp)1/2(nj2jN o)exp(q V/kT)+(qW/2)O"UthNtnjexp(q V/2kT),-(Pn-Pno)/'tp+ll-aE '(Pn'dnn/dx-nn'dPn/dX)/(Pn-nn)+Da'(Pn'd2nn/dx2+nn'd2pn/dX2)/(Pn+nn)=0(2.4-9)(2.4-21)

where W is the depletion width. The experimental results in general can be represented by
the following empirical form

wherel~exp(qV/nkT)-22-(2.4-22)-23-

where the factor n=2 when the recom bination current dominates, as shown in Figure 2.41 curve (a), and n=l when the diffusion current dominates, as shown in Fig. 2.4-1 curve(b). When both currents are comparable, n has a value between 1 and 2.

10
10

8

/

7

10

6

10

5

10

4

V ,.. " ,..

/ ,.. ,.. ,..

);f)'

-

~

kr
/
10
-~ ~-"/
10-FORWARD

(a)

~

3

J

10

,..'

j,/

(b)

CJ)

J

,..

,.. ,..

-REVERSE_E- - .. --- ---- ...

2

' I

/I

1

it

'---IDEAL FORWARD

/

/

10° V

5

I

I

I

I

15

20

~ IDEAL REVERSE

10

25

30

qlv/ / kT
Fig. 2.4-1 Current-voltage characteristics of a practional Si deode(a) generation-recombination current region(b) diffusion current region

-24-

-25-

References

2.5 Summary
The crystallized theory of poly-Si from a-Si was described. The transient time,1) M. E. Cower and T. O. Sedgwick: J. Electrochem . Soc. 119, (1972) p.1565.

nucleation rate, and growth velocity, which were very important factor for the2) A. L. Fripp: J. Appl. Phys. 46, (1975) p.1240.

crystallization, were explained using the Gibbs-free-energy difference between the3) T. I. Kamins: J. App!. Phys. 42, (1971) p.4357 .

amorphous and crystalline phases g, the maximum free energy of fonnation ~Gn *, and

4) J. Y. W. Seto: J. Electrochem. Soc. 122, (1975) p.70l.

the activation energy of self-diffusion in amorphous silicon Ed· These three factors were

5) W. Shockley: "The Theory of p-n Junctions in Semiconductors and p-n Junction

also found to have the temperature dependence.
The electrical transport properties of poly-Si films were also mentioned. A theory

Transistor," Bell Syst. Tech. J., 28, (1949) p.435.
6) C. T. Sah, R. N. Noyce, and W. Shockley: "Carrier Generation and Recombination

combining the grain boundary trapping model with a thennionic-emission mechanism

in p-n Junction and p-n Junction Characteristics," Proc. IRE, 45, (1957) p.1228.

was introduced and explained the conductivity of n-type poly-Si films. In this theory, it

7) J. L. Moll: "The Evolution of the Theory of the Current-Voltage Characteristics of p-n

was considered that the electrical transport properties of poly-Si films are governed bycarrier trapping at the grain boundary.
Finally, the theory on the current-voltage characteristics of p-n junction as the basic
transport theory of device was described.

Junctions," Proc. IRE, 46, (1958) p.1076.
8) H. Wakeshama: 1. Chern. Phys. 22, (1954) p.1614.
9) F. C. Collins: Z. Elektrochem. 59, (1955) p.404.
10) J. Feder, K. C. Russell, J. Lothe, and G. M. Pound: Adv. Phys. 15, (1966) p.lll.
11) B. K. Chakraverty: Surf. Sci. 4, (1966) p.205 .
12) D. Kashchiev: Surf. Sci. 14, (1969) p.209.
13) F. F. Kelton, A. L. Greer, and C. V . Thompson: J. Chern. Phys. 79, (1983)
p.6261.
14) J. Y. W. Seto: J. Appl. Phys. 46, (1975) p.5247 .

-27-

3 Development of thin-film polycrystalline silicon by a new SPC method

Despite using different textured substrates from "graphoepitaxy", thin-film poly-Si with a

3.1 Introduction

large grain size were successfully grown in this study. Factors that facilitate the

For the development of solar cells with a much higher conversion efficiency, it isenlargement of grain size on textured substrates were also studied. Furthennore, the film

necessary to develop materials with high photosensitivity in the long-wavelength region

properties of poly-Si were improved by combining partial doping and textured substrates.

where amorphous silicon (a-Si) films have no photosensitivity as shown in Figure 3.1-l.

Another candidate of starting material is the a-Si film including a crystalline phase toThin-film poly-Si is a hopeful material for this purpose, and it was fabricated by a newact as seed of solid-phase crystallization. Solid-phase crystallization using c-Si as seed4 )SPC method.

has also been reported. In this paper, heat treatment of a-Si in contact with mesa-striped

It is important for enlargement of grain size in thin-film poly-Si to control the density(1 OO)-oriented Si seed crystal was performed, as shown in Figure 3.1-3. This methcx:l isof nuclei generated in the initial stage of the SPC process. In order to find a way tocalled by external seeding technique. In' external seeding technique, there are problems ascontrol nucleation, the effects of phosphorus and boron atoms as dopants for siliconfollowing.
(1) It is necessary to remove impurities such as oxygen (0), carbon (C), and nitrogen

during solid-phase crystallization were investigated. For nucleation control, the positionand density of nuclei generated in the initial stage of SPC are tried to be restrained. The(N) on both Si surfaces of the seed and a-Si.

development of a way for controlling the position of nuclei generation led to the SPC(2) Both Si surfaces of the seed and a-Si must be flat.

method called by partial doping method. This partial doping method is the basis on the(3) The surface of the a-Si must be brought into contact with the seed at a pressure ofabout 1 kg/cm 2 , for a gocx:l contact with the seed.

improvement of thin-film poly-Si properties and most of starting materials has the
structure consisting of a nucleus generation layer and a crystal growth layer, as shown in

So, it is difficult to prepare large-area thin-film poly-Si using the external seeding

Figure 3.1-2.

technique and to apply it to a textured surface. The reproducibility for the preparation of

To enlarge grain size, it is essential to find an a-Si structure well-suited to solid-phasethin-film poly-Si is poor. Thus, I have developed internal seeding technique using grain

crystallization. The relationship between the structure of a-Si films before the SPC

made of single-crystalline phase as a seed. This internal seeding technique is a way ofprocess and the average grain size of thin-film poly-Si after the SPC process was

fabricating thin-film poly-Si by the SPC method using a-Si films containing a single-

investigated. To elucidate the growth kinetics, the nucleation rate and velocity of solid-

crystalline phase. The merits of this internal seeding technique are follows.

phase crystallization, and the detailed structure of a-Si films were analyzed. Growth(1) The concentrations of impurities such as 0, C, and N are very low at the interface

kinetics for the enlargement of grain size were thereby explained.

between the seed and a-Si.

The surface morphology of substrates is also very important in solid-phase(2) The incubation time for solid-phase crystallization is decreased because there is no

crystallization. It was found that textured substrates had a great effect on the achievement

nucleation process.

of thin-film poly-Si with large grains in the SPC method. There are a number of reports(3) It is easy to fabricate large-area thin-film poly-Si.

on the crystalline growth of materials such as silicon (Si) 1), germanium (Ge)2), and

The internal seeding technique thus enables significant improvement in thin-film poly-Si

gallium arsenide (GaAs)3) using textured substrates. These investigations are commonly

properties.

known as "graphoepitaxy". The surface morphology of textured substrates in these
papers were precise, but that of textured substrates used in this study was at random.

-28-

-29-

.

~

....

"

undopedlayer~0.2r-------------------------------­cSolar spectrum:J

Crystal growth
layer

Q)

~ 0.1
o

c...
en
Q.)

Nucleus generation
layer

a-Si

doped layer

~

o
+-'
o
..c

700
900
Wavelength (nm)0...

Fig. 3.1-1

1100

Solar spectrum and photoresponse of a-Si solar cells.

-30-

Substrate

Fig. 3.l-2 The structure of starting material (a-Si) used in partial
doping method.

-31-

3.2 Effect of impurities (dopants)
This section de cribes the effects of dopant atoms (phosphoru, and boron) on solidphase crystallization as well as those of partial doping using phosphorus atoms.

Pressure

3.2.1 Experimental technique
In order to investigate the effects of dopant atoms (phosphorus or boron) on solidphase crystallization, P-doped a-Si, B-doped a-Si and undoped a-Si films were deposited
on a quartz substrate by plasma-CVD using a capacitively-coupled parallel electrode.
Phosphorus and boron atoms were doped in-situ by mixing PH 3 with SiH4 and by--Gla ss substrate

mixing B2H6 with SiH 4 , respectively, as source gases for the spe process. The
preparation conditions of P-doped a-Si, B-doped a-Si and undoped a-Si films are shown-~Si02in Table 3.2-1. The thickness of P-doped a-Si films was 0.2 - 3.0 J.l.m and 0.09 - 0.4 jlm
for B-doped a-Si films and 0.09 - 3.0 jlm for undoped a-Si films. PH 3 and B2H6 gases
were diluted by H2 for 1 % and for 1000 ppm, respectively. Thermal annealing was--a-Si../\...

J

performed for P-doped a-Si, B-doped a-Si and undoped a-Si films under the conditIons\.

shown in Table 3.2-2. After SPC of the films, their structure was evaluated by Raman-~ c-Si seed

spectroscopy and scanning electron microscopy (SEM). Raman measurements werecarried out with a JASeO R800T Raman spectrophotometer with a triple monochrometer

Hot plate

and a cooled photomultiplier. The exciting light is a 488 nm line from an Ar-ion laser.
Wavenumbers were calibrated by uSlIlg the natural emission lines of the laser. Hall
mobility was measured. In the Hali effect measurements, the intensity of the magnetic
field was 5000 Gauss, the electrode was a van der Pauw configuration, and thetemperature was 300 K.
To provide high-quality thlll -film poly-Si using the partial doping method, aFig. 3.1-3 A schematic illustration of the external seeding method for SPC.
Stripes are fanned on (1 OO)-oriented Si seed crystal.
The same chemical treatment is applied to the both Si
surfaces of seed and substrate.

quartz/undoped a-Sl (crystal-growth layer)/P-doped a-Si (nucleus-generation layer)
structure was fabricated, where, in the nucleus generation layer, the density of nuclei
generated during the spe process was designed to be minimized.

-33-

-32-

- - , /'.

.

.

3.2.2. Phosphoru s and boron

Table 3.2-1

Preparation conditions of P-doped a-Si, B-doped
a-Si and un doped films.

SiH 4 (seem)(1) Effect of phosphorus atoms on SPC

P-dopeda-Si

8-dopeda-Si

undoped
a-Si

1-30

5

30

In order to inves tigate the effect of phosphoru s atom s on solid-phase crystallization ,the SPC process was performed at a temperature of 600 DC, using P-doped and undoped
a-Si films as starting materials . Figure 3.2-1 shows the Raman spectra of the films after
SPC of undoped and P-doped a-Si fIlms, whose thickness is 1.0 Jlm . After the SPC

PH 3 1% H 2 (seem)

3-70-------._------~H6 0.1 %-------

1-50--------

H2 (seem)

process, the film of undoped a-Si showed the broad peak at 480 cm -1 but did not show
the peak at 520 cm -1. On the other hand, the film after SPC of P-doped a-Si has the peakat 520 cm -1. Figure 3.2-2 shows the Cross-sectional SEM photographs taken after the

Substrate temperature(OC) 200-500

36

Pressure (Pa)200-300

650

40

36

SPC process of undoped and P-doped a-Si films at an SPC temperature of 600 DC, where
the SPC time is 10 hours. The SPC of the undoped a-Si film does not occur and the crosssection after the SPC process is of amorphous phase. On the other hand, SPC of p_

R.F. power (mW/em2)

120

80

120

doped a-Si film does occur and the cross section after SPC is of polycrystalline phasewith a grain size of about l)lm. RHEED measurements also confirmed that the films after
SPC of P-doped a-Si films were polycrystalline phase. Thus, it was found that SPC
occurred easily at low temperature when phosphorus atoms were doped. Therefore, thephosphorus atom has the effect of promoting polycrystallinity with large grains at a

Table 3.2-2

SPC conditions of P-doped a-Si, B-doped a-Si and undoped a-Si films.

relatively low temperature of 600 DC.
The influences of phosphorus concentration and SPC temperature on n-type poly-Si

Annealing temperature (OC)500-850

Annealing time (hour)

3-10

Annealing atmospherevaeeum or N2

were investigated. Figure 3.2-3 shows cross-sectional SEM photographs of n-type polySi with phosphorus concentrations of 1.2 x 10 20 - 1.7 x 10 21 cm- 3 , where the SPC
conditions are chosen to be 700°C for 10 hours to accelerate the crys tallization. In Fig.
3.2-3, the grain size of n-type poly-Si drastically increases with the reduction inphosphorus concentration. This indicates that the reduction of phosphorus concentration
leads to the decrease of the density of nuclei generated during SPC. As a result,polycrystalline Si of large grain size grew after SPC. In n-type poly-Si with aphosphorus concentration of 1.2 x 10 20 cm -3, a grain size of about 2.0 )lm was
obtained. Figure 3.2-4 shows the ab sorption coefficient of the films

-34-

-35-

-....
CI)C::l

6.0cm- 1

..c

~

cO

~

.-t=

n-type poly-Si

CI)

C

....c
Q)

c

('Ij

E
cO
a:

undopedundoped a-Sin-type
I

I

O.Spm
550

450

500

4010

Wavenumber (cm- 1 )Fig. 3.2-1

Fig. 3.2-2

The cross-sectional SEM photographs taken after SPC processof undoped a-Si and P-doped a-Si films at the SPC temperature of 600 C.
0

Raman spectra of the films after SPC process of undo~d a-Si
and P-doped a-Si films at the SPC temperature of 600 C.

-37-

-36-

"

....

~

..

.

after SPC of a-Si films with phosphorus concentrations of 1.2 x 10 20 - 1.7 x 1021 cm- 3 ,IIa.5pfficompared with that of single-crystalline silicon (c-Si) .5) The absorption coefficient wasdetennined precisely using transmittance and reflection pectra of the films. 6 ) The
absorption coefficient increases with increasing the phosphorus concentration .
Considering that thin-film poly-Si consists of grain and grain boundaries, this increase in
the absorption coefficient is thought to be due to the increase in grain-boundary regions
because the absorption coefficient of the grains are the same as that of c-Si. Thus, it wasalso confirmed by a optical measurement that grain size was enlarged by reduction of thephosphorus concentration .

..

CD P:1.2X 10 20cm -3

Figure 3.2-5 show the cross-sectional SEM photographs of n-type poly-Si for two
SPC temperatures , 600 °C and 700(7aOOe,10H)
I

I

a.5pffi·e, where the phosphoru s concentration is 1.2x 1020

cm -3 . The n-type poly-S i fabricated at the temperature of 600 °e had the larger grains
than that prepared at the temperature of 700°C in the region near the substrate. This
indicates that reducing the SPC temperature contributes to the growth of polycrystalline
Si of large grain size as a result of the decrease in the density of nuclei generated during
SPC. It was recognized that the control of nucleus generation led to growth ofpolycrystalline Si of large grain size.
Figure 3.2-6 shows the Hall mobility of n-type poly -Si prepared by the SPC attemperatures of 600 ·C and 700 ·C. The Hall mobility of n-type poly-Si crystallized at
600 °C was larger than that crystal lIzed at 700 ·C. This supports that n-type poly-Si@ P:1.2X 10 21 cm -3

crystallized at lower temperatures has the larger grain size, as shown in Fig. 3.2-5.

0(700 e,10H)

Minority carrier characteristics are very important for solar cell materials. Thus, theminority carrier trap density was analyzed in term of a trapping model. 8) Accorcling to

Fig. 3.2-3

The cross-sectional SEM photographs of n-type
poJy-Si with phosphorus concentration of 1.2 X 1020 cm- 3-----1.7 X 1021 cm-3 , where the SPC conditions are 700°C, 10hours.

the trapping model, the following equation can be introduced:
oT 1/ 2 oc exp(-q2 N ?/8C:NkT),(3.2-1)

where Ntis the minority carrier trap oensity , N the phosphorus concentration , a the dark
conductivity of POly-Sl, q the electrOnIC charge, £ the dielectric constant of silicon, k theBoltzmann's constant, and T the absolute temperature. In n-type poly-Si prepared by the
SPC method, the dependence of dark conductivity on temperature was analyzed from theslope of the In( aT 1/2 )-v s-lrr relationsh ip . Figure 3.2-7 shows the minority carrier trap

-38-

-39-

I

I

O .S~m105 ~--------~--------~--------~
3
P: 1. 7x 102 1

cin-

/1 /

:::.r

.a v

./' : ~

P: 6.0xl020 cm-3:::.r ;:,;.-: / ' P: 2.6xl0 20 cm-3

1 04::::~..

....................................

/./~

/"

/':::

..

../.~'
"

z,' /

••
%
• v /"

n-typepoly-S i

Expansion

·

/' ~

I

Quartz

I

O.Spm

:

..

:

c-Si5)•
103 ~••·------~------~--------~
, .50
2.00
2.50
3.00

Photon energy (e V)
Fig. 3.2-4 The absorption coefficient. of the films after
the SPC process for the a-Si films with various
phosphorus concentrations.

P:1.2X 10 20cm -3(600°C,10H)

Fig. 3.2-5-40-

P:1.2X 10 20cm -3(700°C,10H)

The cross-sectional SEM photographs of n-type poly-Si for two tem~raturesof 600°C and 700°C, where the phosphorus concentration is 1.2X 10 °cm- 3 .
T he upper part shows the whole portion in cro s-section " and the lower partshows a portion near the substrate for the temperatures.

-41-

density Nt in n-type poly-Si prepared by the SPC method as a function of the phosphorus
concentration in poly-Si. The phosphorus concentration was measured by secondary ion
mass spectroscopy (SIMS). As shown in Fig. 3.2-7 , Nt decreased with decreasing
phosphorus concentration. Especially in the region of low phosphorus concentration, theNt decreased significantly with a reduction in SPC temperature. As shown in Figs. 3.2-3
and 3.2-5, poly-Si of large grain size grows due to a reduction in phosphorus
concentration and SPC temperature. Thus, when the phosphorus concentration and SPC

10 3

temperature are decreased, poly-Si with a small grain boundary region is prepared.
Therefore, the decreases in Nt coincide 'with the results of grain-size enlargement induced

7)

by reduction in phosphorus concentration and SPC tern perature.

c-Si (after Sze)-

The preparation of hi ghly conductive n-type poly-Si (n +poly-Si) was also attemp teden>-.. 10 2

C\I

§

E
()

-

0

>.

0

+J

~

~

~600 oeby the SPC method, using highly P-doped a-Si films. Figure 3.2-8 shows the darko 700 econductivity of n + poly-Si after SPC as a function of the doping ratio (PH 3/SiH 4 ). The

0

~

0

dark conductivity of n"T" poly-Si increases with an increase in the doping ratio and has a~maximum value of 1.5x 10 3 (Qcmf 1. This conductivity is equal to a sheet resistance of

.D
0

E
ro

10

about 10 nJD at the thickness of 0.7 j.lm, and this value is close to that of Sn02 which is

0

1

0

I

used in TeO (transparent conductive oxide) electrode of solar cells. Thus, n+ poly-Si
with high conductivity can be prepared by using the SPC method.

Thin-film poly-Si by SPC

Carrier concentration (cm-3 )Fig. 3.2-6 The comparison of Hall mobility in the n-type poly-Si
prepared at the SPC temperature of 600°C and 700°C .

-42-

-43-

-E

C\J

SPC Time: 10hours

()

13
;:
10
:~

en

c

Q)

-

"'0

T"I

a.
ro
......

E
()
- 103

~

~

SPC condition
BOOoe, 10H

a

Q)

10 21
10 20
Phosphorus concentration (cm -3)

Fig. 3.2-7

The minority carrier trap density Nt in n-type poly-Si preparedby the SPC method as a function of the phosphorus
concentration in poly-Si.

Fig. 3.2-8

-44-

""'-

---_.

--

~

--

The dark conductivity of n+ poly-Si after SPC as a functionof the doping ratio (PH 3 /SiH 4 )

-45-

-

(2) Effect of boron atoms on the SPC
In order to find the effects of boron atoms on solid phase crystallization, a B-dopedhydrogenated amorphous silicon (a-Si:H) film was annealed at temperatures of 650°C
and 700 °C. Figure 3.2-9 shows the Raman spectra of the films before and after SPC of
the B-doped a-Si:H films. Before SPC, a broad peak was observed at 480 cm -1. The
film before SPC was amorphous. After SPC, a peak was observed at around 512 cm-

I

and no peak was observed in the 470 to 480 cm- 1 region. Thus, it was confinned thatfilms after SPC of B-doped a-Si:H films had a crystalline phase. Figure 3.2-10 shows
cross-sectional SEM photographs taken after the SPC process of undoped a-Si:H film

C/)and B-doped a-Si:H film at an SPC temperature of 600°C, where the SPC time was 3+-'hours. SPC of the undoped a-Si:H film did not occur, and the cross section after the SPC
process was amorphous phase. On the other hand, SPC of B-doped a-Si:H film did

p-type }1 c-Si

occur, and the cross section after SPC was polycrystalline phase. Thus, it was found that~

SPC occurred easily at low temperatures when boron atoms were doped. Therefore,boron has the effect of promoting crystallization at a relatively low temperature of 6CX) °C.

+-'

C/)

C
Q)
+-'

The p-type microcrystalline silicon (Ilc-Si) films with high conductivity were
investigated to determine their suitability as the p-type layer of solar cells. Figure 3.2-11

C
C

ro
ro

shows the dark conductivity before and after SPC as a function of the doping ratioE(B 2H /SiH 4 ). The SPC conditions were 600 - 700 °C and 3 hours. The film thickness
6a:

As grown

was 0.09 11m. The dark conductivity of p-type microcrystalline silicon (Ilc-Si) increased
as the SPC temperature was increased for each doping ratio, and reaching a maximum
value at a doping ratio of 5x 10- 3 . Another prominent feature was an increase in dark

550

500

450

400

Wavenumber (cm- 1 )conductivity by 9 toll orders after SPC at a doping ratio of 5xIO- 3 , using the film withlow dark conductivity. A maximum value of about 2.0x 10 3 (Qcmf 1 was achieved, avalue never before reported. In order to elucidate the reasons why p-type Ilc-Si films
prepared by the SPC method have the highest conductivity at a doping ratio 5x 10- 3 , theFig. 3.2-9

Raman spectra of films before and after SPC of B-doped a-Si:H films ,where SPC temperatures are 650t and 700'C.

quality of crystallinity for films after SPC was analyzed using their absorption
coefficient. Figure 3.2-12 shows the dependence of the absorption coefficient of p-type
Ilc-Si on the doping ratio, where the SPC conditions were 650°C for 3 hours and 700 °C
for 3 hours. The absorption coefficient of p-type Ilc-Si at a doping ratio of 5xlO- 3 was

-46-

-47-

4

10
2

10
I

I

I

I

0.5pm

0.5pm

,

0

.,...
~

E

,
,,

,

,
, .,

.,

.,
., .,

, ,0, ...

10

u

-

a

'-"

-2

10

•

~

~

.>
".;:;

u

-4

10

•

:::J

-0

c

-6

0

10

0

-8

10
• as grown• 600°C, 3H... 650°C, 3H
o 700°C, 3H

p-typeundoped

-12

10

10-4

10-3

10-2

10- 1

B2 H6/SiH 4
Fig. 3.2-10

The cross-sectional SEM photographs after SPC of B-doped a-Si andundoped a-Si films. Boron atoms were introduced in-situ into a-Si film.
Fig. 3.2-11-48-

Dark conductivity of the films before and after SPC as afunction of doping ratio (B2H 6 /SiH 4), where SPC temperaturesare 600°C, 650°C, and 700t.

-49-the lowest, and its value was nearest to that of c-Si. 5 ) Given that the absorption

106~------------------------~

coefficient of n-type poly-Si increases as the phosphorus concentration increases, the ptype J..lc-Si at a doping ratio of 5xl0- 3 had the largest grain size of these samples.

Eu--...-

3
Therefore, the dark conductivity ofp-type Ilc-Si is greatest at a doping ratio of 5xl0- .

5

10

~

The Hall effect on the p-type Ilc-Si films prepared by the SPC method was measured

Q)

o

u

and the carrier concentration in the films was analyzed to find the cause of high darkcconductivity of more than 1.0x102 COcmr1. Figure 3.2-13 shows the carrier

4

o
+-'
c..
'o

concentration in the p-type Ilc-Si films as a function of boron concentration, where the10_.. -"_ .. -" C~Si5)en

SPC conditions are 700°C for 3 hours . The boron concentration was measured by-..- -.,..

_.. -.. - .. -..

_.. -" -'-B2 H6 /SiH 4
0
O.SX 10.2

-

.0

«

SIMS. In Fig. 3.2-13, the dashed line shows that the activation ratio of boron atoms is

1.0X 10-23

10 2

100 %. In the boron concentration of 4xl020 - 2x1021 cm- 3 , the activation ratio ofboron atoms was nearly 100 %, and a maximum carrier concentration of about

3.5

3

2.5

Photon energy (eV)

2xl021 cm- 3 was obtained. It was found that the high activation ratio of boron atoms at
the high boron concentration was the main reason for the high dark conductivity in the ptype Ilc-Si films prepared by the SPC method.

106~------------------------~

~
I

E
u
--...-

Figure 3.2-14 compares the absorption coefficients of p-type J..lc-Si films prepared by
the SPC method, and p-type Ilc-Si:H9) and a-SiC:H 1 0) films prepared by the...
........................

~

conventional plasma-CVD method. The absorption coefficient of p-type J..lc-Si preparedQ)o",,,,,,,

uby the SPC method was smaller than that for the films prepared by the conventional

mobility of p-type J..lc-Si films prepared by the SPC method and the conventional plasma-

c
o
+-'
c..
'-

CVD method. In carrier concentrations as high as two orders of magnitude, the mobility.0plasma-CVD method, in the 2.1 to 3.0 e V region. Figure 3.2-15 shows a comparison of--.'.. ,, .-

4

10

o

en

«

of p-type Ilc-Si prepared by the SPC method, which was 7 - 13 cm 2Ns, was almost

3

10 2

3.5

3

2.5

equal to that of the p-type Ilc-Si:H prepared by the plasma-CVD method. Thus, using the

Photon energy (eV)SPC method, p-type J..lc-Si films with a higher conductivity and lower absorption
coefficient can be prepared.

Fig. 3.2-12

The dependence of absorption coefficient of p-type f1 c-Si
prepared by the SPC method on the doping ratio (B2H6/SiHt ),where SPC conditions are 650°C for 3 hours and 700°C for 3 hours.

-50-

-51-

~

..

•

1

,.

-

~

...-....
('t)

22

I

10

E
u
"'-""

--~

c

~
~

Q)
C,.)

21

0
10
.ro
L..

+-'

c
Q)
u
20
c 10
0
u

5

",/

,
,,

Q)
.-

19

,

,
,,

,

,,

plasma-CVD
a-SiC·

9)

10

---

0

C,.)

c

~

~

0

10

4

~

c..

,
,,

SPC
)1. c-Si

~

0

en
..c

« 103

~

()

2.0

2.5

3.0

Photon energy (eV)

1d

9

if0

1

1if1

1if2

Concentration of B atom (cm- 3)

Fig. 3.2-13

plasma-CVD)1. c-Si--Q)

Activation ratio,:100 %

L..

co 10

10

'+'+-

+-'

L..
L..

106

E
C,.)

SPC condition
700°C, 3H

I

c

-r--

Fig. 3.2-14

Comparison of absorption coefficient between p-ty~ f1 c-Si . .
films prepared by the SPC method and p-type f1 c-Sl.H and a-SlC.H
films prepared by the conventional plasma-CVD method.

Carrier concentraion in the p-type f1 c-Si films as a function ofboron concentration in the p-type f1 c-Si films, where the.SPC
conditions are 700CC for 3 hours and the boron concentralon wasmeasured by SIMS.

-52-

-53-

3.2.3. Partial doping
It is necessary to control nucleus generation durin g the SPC process for the

improvement of n-type poly-Si film properties. The partial doping method was developed...--...

to suppress nucleus generation. This method offers a means of preparing n-type poly-Si

(\')

-

I

E

from starting materials separated nucleus generation and crystal growth layers . P-doped

u

'--'

c

a
-+-'

0

SPC

21 f -

10

doped a-Si films could be occur at lower temperatures compared with that for undoped a-

00
0
0

~

C\S

'+-'

c

Q)

u

c

Si films, as mentioned in 3.2.2. So, starting materials had a quartzJP-doped a-Si/undoped
a-Si structure. Figure 3.2-16 shows cross-sectional SEM photographs of n-type poly-Si

20

10 -

prepared by uniform doping and partial doping. "Uniform doping" means that dopant-

atoms were included throughout the a-Si film. The phosphorus concentration was

au
'Q)
''-

a-Si films were used as the nucleus generation layer, because the crystallization of P-

1.2xl020 cm -3; the thicknesses of P-doped a-Si and undoped a-Si in partial doping were

19

10

C\S
()
18

10

- plasma-CVD-

0.2 )..lm and 2.0 )..lm, respectively; and, the SPC conditions were 600 °C for 10 hours.

• •

~

I

I

I

0

For comparison, an undoped sample is also shown. No SPC occurs for undoped a-Si.
The cross section after the SPC process has an amorphous phase. In uniform doping or
partial doping, SPC occurs and n-type poly-Si after SPC had a crystalline phase. This
indicates that nucleation took place in the P-doped layer during the initial stage of SPC,I

I I I III

1

10

10

then nuclei grew into undoped a-Si in partial doping, since no nucleation occurred inundoped a-Si at a temperature of 600 °C. Thus, it was proven that the partial doping

Hall mobility (cm2 IV· s)method could control the position of nucleation. Another interesting feature was that thephosphorus concentration in the n-type poly-Si prepared by the partial doping method
was uniformly about 2xl0 18 cm- 3 in depth after SPC.
Figure 3.2-17 shows the Nt for n-type poly-Si prepared by partial doping and unifonn

Fig. 3.2-15

Comparison of Hall mobility between p-type fJ. c-Si films preparedby the SPC method and the conventional plasma-CVD method.

doping. The Nt in n-type poly-Si prepared by partial doping decreased more than that forfilm prepared by unifonn doping, dropping to a minimum value of 4.6xl0 11 cm- 2 .
To clarify the effects of partial doping method on crystallization of a-Si films, nuclei
density was analyzed in P-doped a-Si films (nucleus-generation layer) for various
deposition temperatures (Ts) and SPC temperatures. Figure 3.2- 18 shows the density of
nucleus grown in the P-doped a-Si:H layer as a function of Ts, for SPC temperatures of500, 550 and 600 DC. At each SPC temperature, the density of nucleus generation

-55-

-54-

..

, . ,

-

"

".

.

-

C\J
I

E

undopedundoped

P-doRed
20

Quartz* Partial doping~} uniform. doping

C/)

c

(])

3

1.2 X 10 cni
Quartzu~ 1013......

"'0

P-dORed
20 -3
1.2X10 ern

0..

eot.......
t.-

Quartz

(])

1012

t.t.-

eo
u
~
......
t.oC~

Phosphorus concentration (Cm-3)

Fig. 3.2-17

The comparison of the minority carrier trap density Nt in
n-type poly-Si prepared by partial doping and uniform doping.

O.Spm
Fig. 3.2-16

The cross-sectional SEM photographs of n-type poly-Si prepared by
uniform doping and partial doping and undoped a-Si after the SPC
process, where the phosphorus concentration is 1.2 X 1()20cna- 3 andthe SPC conditions are 600°C for 10 hours.

-56-

-57-

,

. ,

'

increased as Ts increased and reach a maximum value at a Ts in the range of
300 - 350°C. Finally, no SPC becomes to occur at 500 ·C. For a-Si:H films prepared at
the same temperature, the density of nucleus decreased with decreasing the SPC
temperature. So, in order to minimize the number of nuclei, it is need to decrease both the
Ts of P-doped a-Si:H and the SPC temperature. In this experiment, it was found that thex : No cr stallization

density of nucleus could be decreased to 2 - 3 /~m2 by choosing a deposition temperature

The SPC of undoped a-Si (Ts: 650 ·C) was examined at 500 ·C, using P-doped a--

Si:H (Ts: 200 ·C) as a nucleus-generation layer. The structure of the sample was::t.

of 200 ·C for a-Si:H and an SPC process temperature of 500 ·C.

100\~--SPC . 600

C\J
I

E

\

0

e

\
\

\
\

"""-'

\
\

quartz/undoped a-Si/P-doped a-Si. P-doped a-Si was deposited on undoped a-Si to

CD

\

\

()

prevent thennal damage during the deposition of undoped a-Si. Figure 3.2-19 shows

::J
C
0>
C

cross-sectional SEM photographs of the films after SPC at 500 ·C for the quartz/undoped~

a-Si (Ts: 650 ·C) and the quartz/undoped a-Si (Ts: 650 ·C) /p-doped a-Si:H

\
\
\
\
\
\

50

\

SPC : 550 e
0

0

l.

'-

(Ts: 200 ·C). No crystallization of the quartz/undoped a-Si (Ts: 650 ·C) occurred at

0>

...

~

0

500 ·C. The SPC of the quartz/undoped a-Si (Ts: 650 ·C) /P-doped a-Si:H (Ts: 200 DC),

...

~

{\

~

however, did occur at a very low temperature of 500 ·C. It was proven again that nuclei(/)

cwere generated in only the P-doped a-Si:H (Ts: 200 ·C) layer and grew to a size of

\
\
\
\

...

...

,
\
\
\
\
\

SPC : 500°C

CD

0
2 - 3 ~m in the undoped a-Si (Ts: 650 ·C), which acted as a crystal-growth layer, as

,

x

0

x

illustrated in Figure 3.2-20. This experiment also showed an interesting result in that

300
400
500
Deposition temperature of p-doed a-Si (OC)solid-phase crystallization occurred at a temperature lower than Ts. In order to investigate
the influence of the number of nuclei generated in the P-doped a-Si:H (Ts: 200 ·C), SPC
of quartz/undoped a-Si (Ts: 650 ·C)/P-doped a-Si:H (Ts: 200 ·C) was perfonned at threedifferent temperatures, 500, 550 and 600°C. Figure 3.2-21 shows cross-sectional SEM
photographs of the films after SPC at the three different temperatures for 10 hours. At all

Fig. 3.2-18

SPC temperatures, SPC of undoped a-Si, which was the crystal-growth layer, occurred.

The density of growing nuclei in P-doped a-Si:H layer as afunction of its Ts, for various SPC temperatures of Soooe,
SSOoe and 600 e.
0

It was also found that the grain size of poly-Si increased with decreasing the SPC
temperature between 500 and 600 ·C, because the density of nuclei generated in the Pdoped a-Si:H (Ts: 200 ·C) decreased with decreasing the SPC temperature as shown in
Fig. 3.2-18. Therefore, it was proven that the poly-Si thin films with large grains could
be grown by decreasing the density of nuclei generated during the initial stage of SPC,

-58-

-59-

,
~

.

-

- r-- P-doped a-Si

- r-- undoped

P-doped a-Si (Ts:200°C)

a-Si
undoped a-Si (Ts:6S(rC)undoped a-Si (Ts:6S0°C)-

Substrate(a) Before the SPC

QuartzQuartz~(b) Initial stage of
the SPC

P-doped a-Si

Sub.

SUb.

Sub.

Sub.

SPC condition: 500°C, 10H 10.5 fl ~undopeda-Si

SPC condition: 500°C, 10H 10.5 J.L ~Substrate(c) Middle stage of
the SPC

Fig. 3.2-20
Fig. 3.2-19(b) Final stage of
the SPC

The model for growth kineties in the SPC, using the a-Si
film prepared by the partial doping method.

The cross-sectional SEM photographs of the films after the SPC at
500°C of the quartz/undoped a-Si (Ts:650°C) and the quartz/undopeda-Si(Ts:650'C) P-doped a-Si:H(Ts:200'C).

-60-

-61-

and that SPC of a-Si films prepared by plasma-CVD could occur at a temperature lower

P-doped a-Si:H (Ts: 2(XYC)

than the deposition temperature, if the partial doping method of separating the nucleusundoped a-Si (Ts:650·C)

generation and crystal-growth layers was used.
Figure 3.2-22 shows the Hall mobility of n-type poly-Si thin-films prepared by the
SPC method from a-Si films using partial doping (open circle) and uniform doping

QuartzO.5pm(closed triangle) as a function of carrier concentration. In Fig. 3.2-22, the solid line
shows the mobility of c-Si.7) The poly-Si thin-films prepared by uniform doping had amobility of 4 - 70 cm 2Ns in the carrier concentration region of 2xl0 18 to 5xl020 cm-3.
The maximum mobility value was 70 c'm2Ns at a carrier concentration of lxl0 19 cm- 3 .
On the other hand, poly-Si thin-films prepared by partial doping have a mobility of 40 to
196 cm 2Ns in the region of lxl0 18 to 8xl0 18 cm - 3 . Thus , carrier concentration inpoly-Si thin films prepared by partial doping is reduced. The highest value obtained was
196 cm 2Ns at a carrier concentration of lxl0 18 cm - 3 . This value is as high as about
60 % of value of c-Si at the same carrier concentration. So, it was found that the poly-Si
thin-films prepared by partial doping have higher mobility at lower carrier concentrations,compared with those prepared by uniform doping. The increase of Hall mobility supports

SPC conditions: 500°C, 10hours

SPC conditions: 550°C, 10hours

SPC conditions: 600°C, 10hours

the enlargement of the grain size by using the partial doping method. The partial doping
method is therefore more effective in controlling nucleus generation during SPC.
Consequently, it promotes n-type thin-film poly-Si with large grains and a high Hall
mobility.

Fig. 3.2-21

The cross-sectional SEM photographs of the films after the
SPC at the temperature of 500°C, 550°C and 600°C for 10 hours.
The film before SPC is the quartz/undoped a-Si(Ts:650°C)!P-dopeda-Si:H(Ts:200°C).

The partial doping method was found to have remarkable advantage on the
improvement in mobility, increase of grain size, and decrease of SPC temperature. It wasalso proven that SPC of a-Si films could occur at temperatures lower than the deposition
temperature. The lowest SPC temperature of 500 ·C was achieved. In n-type thin-film
poly-Si prepared by SPC at 500°C from a-Si films using partial doping, a high mobilityof 196 cm 2Ns was obtained at a carrier concentration of lxl0 18 cm-3.
In summary, it was found that phosphorus and boron used as dopants for siliconfacilitated nucleation during the SPC process and consequently, decreased the criticaltemperature of solid-phase crystallization, compared with undoped films. The partial
doping method that the nucleation occurred only in a nucleus-generation layer (doped a-

-62-

-63-

Si) and then, nuclei were grown up to crystal growth layer (undoped a-Si) was
developed. As a result, thin-film poly-Si with large grains could be prepared, and thecritical temperature of solid-phase crystallization was decreased to 500 °c. It was also
confinned that n-type poly-Si prepared by the partial doping method had mobility higher
than that prepared by unifonn doping.

o SPC:500oe "-'550 oe
Partial doping~ SPC:600oe "-'BOOoe
Uniform dopingc-Si(after Sze) 7)~

Thin-film poly-Si by SPC

1C/

10

8

10

9

10°

10

1

Carrier concentration (cm- 3 )Fig. 3.2-22-64-

The Hall mobility of n-type poly-Si thin-films prepared by the SPC
method from a-Si films using the partial doping (open circle) anduniform doping (closed triangle) as a function of carrier concentration.

3.3 Optimum starting materials for the SPC method
In Section 3.2, it was shown that separating the nucleus-generation layer from thecrystal-growth layer was effective for the improvement of thin-film poly-Si properties. In
this section, the structural analysis of the undoped a-Si (crystal-growth layer) suited tosolid-phase crystallization, and growth kinetics for thin-film poly-Si with large grains aredescribed.

3.3.1 Experimental technique
The sample structure before SPC was quartzJP-doped a-Si (0.17 Ilm)/undoped a-Si
(3 Ilm), as shown in Figure 3.3-1. The role of the P-doped layer was to enhance

undoped a-Si
(3 pm)

nucleation and increase grain size, which is called by the partial doping method as
mentioned in Section 3.2.3. In this experiment, P-doped a-Si layers were the same; only
undoped a-Si layers were deposited at various conditions. Undoped a-Si films were
deposited by glow discharge of SiH4 or Si 2H 6 gas on flat quartz. a-Si films with various

P-doped a-Si
(0.17 pm)

structures were prepared by changing deposition conditions such as pressure, R. F.
power and so on. The deposition conditions are summarized in Table 3.3-1. The

Substrate

deposition temperature (Ts) is 550 ·C which is lower than 650 ·C mentioned in section
3.2, since Ts had to be decreased to achieve a low-temperature process. Sample
annealing was performed in a vacuum at 550 - 650°C for 10 - 600 min. In order toinvestigate the relationship between the structure of the a-Si and grain size of poly-Si, thestructure of a-Si was evaluated by Raman spectroscopy. Grain size and density were

Fig. 3.3-1 The sample structure before SPC process.

measured using SEM images after Secco's etching or from transmission electron
microscopy (TEM) images.

-66-

-67-

3.3.2 Evaluation of structural disorder in starting materials
The structural disorder of a-Si films before SPC was analyzed. Figure 3.3-2 shows
the typical Raman spectrum of the a-Si film. Two prominent peaks are observed at about
480 cm -1 and 150 cm -1. These peaks are attributed to the transverse optical (TO) bandand transverse acoustic (TA) band, respectively. It has been reported that the TA peakheight divided by the TO peak height is the sensitive probe of the structural disorder of aSi. 11 ) So, T ArrO was used as a parameter for structural disorder. The peak heights were
measured from the dashed baseline in Fig. 3.3-2. The statistical error of T A{fO is lessthan ± 0.02. The full width at half maximum (F.W.H.M) of the TO peak was defined as

2xr, using r as shown in Fig. 3.3-2. The angle deviation from bond angle 109.5° in the
Si-Si bond of c-Si, ~e, has a relationship with the F.W.H.M. of the TO peak asfollows: 12)

Table 3.3-1 The deposition conditions of undoped a-Si films.

(F.W.H.M.=2xr)=15 +6~e.

(3.3-1)Equation 3.3-1 was confirmed to be correct both theoretically and experimentally. Using

Substrate temperature ( °C )Gas flow
SiH4
Si2H6
rate (seem)

500 - 600
30 - 50
10 - 20

various conditions. The ~e correlated well with TArrO and its value was found to

R. F. power (mW/cm2)

100 - 300

increase as T ArrO increased. The distortion energy in a-Si film was proportional to thePressure (Pa)30 - 70

Eq. 3.3-1, the ~9 was calculated for a-Si films with various TArrO values. Figure 3.3-3
shows the correlation between the T ArrO and the ~e in a-Si films deposited undersquare of ~e.13) So, it was found that a-Si films with large TArrO values stored large
distortion energy.

-68-

-69-

.--..
Q)

~

0>

9

Q)

--

--v/

1J

~ 8.5

--/

'-

0>

Q)

~ 8
0>

..... ~

o

~

E 7.5

100

o

TO

~

+-'

'-

c::

.2

~
.s;

(/)

C

7

Q)

Q)

1J
65
Q)
•

+-'

C

c

0>

C

50

ctj

E

V
•

V_

-

v

l?..

~

6

<l

0.25 0.27 0.29 0.31 0.33 0.35 0.37 0.39
T AlTO of a-Si (arb. units)

ctj

a::

0

700 600 500 400 300 200 100
Raman shift (cm-1)

Fig. 3.3-3 The relationship between the TAtrO and the AO
in a-Si films deposited under various conditions.

Fig. 3.3-2 The typical Raman spectrum of the a-Si film prepared byplasma-CVD method.

-70-

-71-

3.3.3 Correlation between grain size of polycrystalline silicon and structural disorder ofstarting materials
Figures 3.3-4 (a) and (b) show SEM images of thin-film poly-Si films after Secco's
etching, the T A{fO of which are 0.28 and 0.37, respectively. The SPC conditions were
600 °C for 10 hours. The grain size after SPC clearly becomes large by using the a-Si
film with a larger T A[fO. The correlation between the grain size of poly-Si and thestructural disorder of a-Si films was investigated. Figures 3.3-5 (a) and (b) show the
relationship between average grain size vs. TArrO and vs. F.W.H.M. of the TO peak,
respectively. The average grain size was defined as the average of all grains visible in

0.5pillsurface and cross-sectional SEM images. The average grain size of poly-Si after SPC
was found to have a better correlation with the TArrO of a-Si films before SPC,
compared with those of F.W.H.M. Thus, the average grain size of n-type poly-Si(a) T AlTO of 0.28

increased with increasing TArrO value of the a-Si films.
Figure 3.3-6 shows the Hall mobility of n-type thin-film poly-Si prepared by the SPC
method as a function of TArrO. As T AlTO increased, the Hall mobility of n-type poly-Si
increased, because the average grain size was enlarged by the T ArrO. Therefore, a-Si
films with large TArrO values have effects on the improvement of structural andelectronic properties in the poly-Si after SPC.

I

I

0.5pm

(b) TAiTO of 0.37
Fig. 3.3-4 SEM images of thin-film poly-Si films after Secco's etching,where (a) is the T A{fO of 0.28 and (b) is the T A{fO of 0.37.

-72-

-73-

...--...

E
::t.

'-"'"

1.5

CD
N
(/)

c

ro
~

1.0

0)

400 ~------~--------~------~

CD
0)

ro

350

~

CD

>
<C

0.30
0.35
TAiTO (arb. units)(a) Average grain size vs. T AlTO
~

8

~

'-"
Q)

N
• '1"""""1
C/:J~• '1"""""1

ro
~

Ol)
Q)

Ol)

ro

~

(l)

;>

~

..0 250
~

~

e·
~

:
i

. __ ......... .j. ......
.

······················· ···············r··············· .. -........ ................... .
.

.~

~ 200

.. _-- -_ .........__... +..................................... ·············r······································_--,---.

.

::ro 150 ....0. . ····································t··················--·······--··--··--·····--··--··f--············--······--····..... -- ...... --.. .

~

-

-

•

-

.....

1.2 =
=
-

1 =
0.9 ::
0.8 :
0.7 :0.6 :
0.5 ~
50
--

...
•
III- III I_I·
54

58

62

~

0.250

0 .300

I

I

I

66

I

I

70

F. W. H. M. of TO peak (em-I)(b) Average grain size vs. F. W. H. M. of TO peakFig. 3.3-5 The relationship between the average grain size and T ArrO (a), andbetween that and the F. W. H. M. of TO peak (b).

-74-

0.350

0 .400Fig. 3.3-6 The Hall mobility of n-type thin-film poly-Si
prepared by SPC method, as a function ofTA{fO.

•
I

100~------~--------~------~

TA{fO of a-Si (arb. units)--r: __ ···... __ ...._-_ ._-_................ _-_ ........:; -----.................
E:) --.................
···········_··· __ ·· __·············_···_··__ ·_-t··S

1.5
1.4 ::
1.3 :
1. 1

300

~

-75 -

3.3.4 Factors for enlarging grain size in polycrystalline silicon
In 3.3.2, it was asserted that a-Si films with large TA(TO value stored large distortion
energy. In order to investigate factors for the enlargement of grain size in SPC using a-Si
films with large T ArrO values, the dependences of grain size, growth velocity andnucleation rate on distortion energy in a-Si films were examined. Figure 3.3-7 shows theSEM photographs of thin-film poly-Si after Secco's etching, where the distortion
energies are 56.2 and 77.4 (degree)2. SPC conditions are 600°C for 10 hours. The grain
size of thin-film poly-Si after SPC increases by using a-Si films with large distortion
energy. Figure 3.3-8 shows cross-sectional TEM photographs of films after 3 hours'Iannealing at 600°C. When the distortion energy of a-Si film is large, the grain density isI

O.5pillsmaller, indicating a low nucleation rate. The length of growth is also longer, indicating alarge growth velocity. The relationships between the distortion energy of a-Si films, andgrowth velocity and nucleation rate were investigated. Figure 3.3-9 shows growth(a) Distortion energy:56.2

velocity and nucleation rate as a function of the distortion energy. As the distortion
energy increased, the nucleation rate decreased, and growth velocity increased. So, an
increase in the distortion energy increases growth velocity and decreases the nucleatilonrate, which are main factors for enlarging grain size in the SPC process.
I speculated the reasons for the dependence of nucleation rate and growth velocity on

distortion energy. According to a statistical picture' of the nucleation and the growth 14),many clusters of a small size are always created, and some of them are annihilated during
SPC. The free-energy change, ~G n' on the fonnation of a crystalline cluster having n
atoms in a-Si is given by the following equation:~Gn = - n·g + sn ·f(3.3-2)

O.5pill

where g is the Gibbs free energy difference between the amorphous phase and lthe
crystalline phase per atom (positive value), sn is the surface area of a cluster of n atoms,
and f i the interfacial free energy per unit area . ~G n has a maximum value of ~G n* at(b) Distortion energy:77.4

cluster size of n* which is called the critical size. From Eq. 3.3-2, ~G n * can be
expressed analytically by assuming that both f and g are independent of cluster size n:

Fig. 3.3-7 The SEM photographs of thin-film poly-Si after Secco's etching,where the distortion energies are 56.2 and 77.4.

(3.3-3)

-77-

-76-

... ~ ..~~ ... ~-~

-

-

_. .

--~.

2

6

5

c

E
C\J

E

t-,---4

::l..

O.Spm

4 ~
C\J
,
I
I

I..

I .
I .
I

o

•I
I

I

-- -----r-- --'-----r-- --'- ---- r---- ' --- --r----' ---(a) Distortion energy:56.2

I
I

I
.

I

~

I

I
I

or-

3 ><

---

I

I

I
I

~

:
•

>

2

c

o
+-'

rn

Q)

1

u

:J

Z

o

30 35 40 45 50 55 60 65 70 75 80(~e )2: (angle deviation from 109.5 degree)2 (degree)2t-'---4

O.Spm

Fig. 3.3-9 The growth velocity and nucleation rate as afunction of the distortion energy: (6. 8 ) 2.

(b) Distortion energy:75.7
Fig. 3.3-8 The cro s-sectional TEM photographs of films after 3hours'
annealing at 600°C.

-78-

0

-79-

where ex is the surface area factor (S n=ex'n 2/3 ). ~Gn * represents the nuclear formationenergy, which is the energy barrier to create stable nuclei. Smaller clusters (n < n*) tend
to shrink, and larger clusters (n > n* ) or nuclei tend to grow because growth isincreased with increasing the T A(fO. Enlarged grain size results from increasing the
T A(fO because the large distortion energy stored in a-Si film increase the growth velocity
and decrease the nucleation rate.

energetically favorable. Therefore, the nucleation rate is determined from ~Gn *. The
growth velocity is determined only from g because the first term in Eq. 3.3-2 is dominant
when a cluster grows large enou'gh.
From more detailed consideration 15), the nucleation rate and the growth velocity cam
be expressed by the following equations:
Vg DC exp(-(-g/2+Ed)/kT)
n DC exp( - (~Gn *+Ed)/kT)(3.3-4)(3.3-5)where Ed is the activation energy of self diffusion in a-Si. Based on Eq. 3.3-4, theincrease in the growth velocity is thought to be caused by an increase in the Gibbs energy
difference, g. The Gibbs energy difference consists mainly of the energy stored in a-Si in
the form of bond-angle distortion 13). The increase in stored energy due to the bond-angle
distortion increases g and consequently, increases growth velocity Vg.
From Eq. 3.3-5, the decrease in nucleation rate is thought to be caused by the increasein ~G n *. The increase in stored energy due to bond-angle distortion increases f, thenincreases ~G n * because ~G n * has the relationship indicated in Eq. 3.3-3. So, thenucleation rate, n, decreases by the amount of increase in stored energy due to bondangle distortion.
In subsection 3.3.2, it was asserted that a-Si films with large T AlTO value stored
large distortion energy. So, the growth kinetics for achieving large grains by increasing
TA(fO was proposed, as shown in Figure 3.3-10. The increases in TA(fO create large
distortion energies stored in a-Si films. This large distortion energy has two effects onsolid-phase crystallization. One is the increase of growth velocity caused by an increasein the Gibbs energy difference, g. The other is a decrease in the nucleation rate caused byan increase in ~Gn *. The increase in growth velocity and decrease in nucleation rateenlarge the grain size after SPC process.
In summary, it has been found that the TA(fO of a-Si films before SPC correlated
well with the average grain size of poly-Si after SPC, and that the average grain size

-80-

-81-

3.4 Investigation of substrates
In this section, the effects of textured substrate and growth kinetics on such substrates

TAlTO : Large

are descri bed.

3.4.1 Experimental technique.
Distortion energy: Increase

Quartz substrates were textured by mechanical etching. The surface roughness (thedistance between top and bottom) was between 10 and 20 )..tm. Figure 3.4-1 shows an
SEM photograph of the textured quartz substrates. To examine the effect of such
substrates on n-type poly-Si prepared using the SPC method, P-doped a-Si films with athickness of 2 )..tm were deposited on textured and flat quartz by plasma-CVD. The
preparation conditions were shown in Table 3.4-1. The improvement of p-type )..tc-Si film

Interface energy between
amorphous and crystal*.
phases, Gn : Increases

Energy difference between
amorphous and crystal
phases, g : increases

properties was also investigated using the textured substrate. B-doped a-Si:H films were
deposited on textured and flat quartz. The deposition conditions for the p-type a-Si:H
films were shown in Table 3.4-2. After the SPC process, the films were analyzed using
TEM and Hall effect measurements. The fabrication of high-quality n-type thin-film polySi was tried by combining the effects of the partial doping method with those of using a~,~,

ISPC velocity · large II Nucleation rate · small I

textured substrate.
In order to discover the effects of textured substrates on the growth kinetics of poly-Si
thin films, the structural disorder was analyzed in a-Si films before the SPC process. The
time dependence of SPC was also investigated.

Enlargement of grain size
Fig. 3.3-10 The growth kinetics for large grain by increasing TAffO.

-82-

-83-

Table 3.4-1

The preparation conditions of n-type a-Si films
deposited on the textured quartz substrate

SiH4 (seem)40

PH3 0.1 %/H2 (seem)

10

Substrate temperature CC)

500

Pressure (Pa)27
2

R.F.power (mW/em )Table 3.4-2

60 J1. m

Fig. 3.4-1

The preparation conditions of p-type a-Si films
deposited on the textured quartz substrate

SiH4 (seem)2.5-"10

B2H6 0.1 %/H2 (seem)

4-.,50Substrate temperature CC)

200-.,350

Pressure (Pa)13.3

R.F.power (mW/em2)

100

An SEM photograph of the textured quartz substrates

-84-

120

-85-

3.4.2 Effects of surface morphology
It was discovered that textured substrates greatly improve mobility and grain size inpoly-Si thin films prepared by the SPC method. Figure 3.4-2 shows the mobility of ntype poly-Si thin-films prepared by the SPC method on textured and flat substrates, as afunction of carrier concentration. P-doped a-Si films deposited using unifonn doping
were used as a starting material. The SPC conditions were 550°C for 10 hours changing

103~----------------------~

the ramp rate from 10 to 40 °C/s. The poly-Si thin-films on textured and flat substrateso Textured substrate

were found to have almost same carrier concentration of lxl0 18 to lxl0 19 cm- 3 , and• Flat substrate

the carrier concentration increased with increasing the ramp rate. In this region, the

SPC condition:~

mobility of the poly-Si on the textured substrate was about seven times higher than thaton flat substrate. A maximum value of 217 cm 2 N s was obtained at a carrier
concentration of lxl0 18 cm- 3 . Figure 3.4-3 shows cross-sectional TEM photographs of

(/')

--E
u

'-'

~

..

n-type poly-Si thin-films with a thickness of 2 ~m prepared by the SPC method on+-'

textured and flat substrates. The SPC conditions were 550°C for 8 hours. The grain size..0of poly-Si was between 4 and 6 ~m on both textured substrates, and between 2 and 3 ~m

E
10 1
rn

on the flat substrates. These grain sizes were confirmed by measuring the films after

550°C, 10H> 10 2
..~

..

0

~ ..

.. ..

..

~\

I

Secco's etching using SEM. Furthermore, the grain boundary of poly-Si on textUlred\\

substrates could not be observed clearly. In the case of the flat substrates, however, agrain boundary was clearly visible. Consequently, it is apparent that poly-Si thin films on

100~~------~----~------~

textured substrates have a higher mobility, larger grain size and a more rigid grain

1018

boundary than those on flat substrates. The larger grain size of 4 to 6 ~m contributes on

Carrier concentration (cm-3 )the achievement of the seven-times-higher mobility.
In 3.2.3, it was mentioned that the partial doping method effectively improved
mobility in the lower carrier concentration, compared with unifonn doping. So, thepossibility of further improving the mobility of n-type poly-Si thin-films prepared by the1019

Fig. 3.4-2

The comparison of Hall mobility of n-type poly-Si
thin-films prepared by the SPC method on textured
and flat substrates, where SPC conditions are 550°C
for IOhours

SPC method was examined by combining the effects of partial doping with those of atextured substrates. Figure 3.4-4 shows the mobility of n-type poly-Si thin films
prepared by the SPC method as a function of carrier concentration, where n-type poly-Si
thin films were fabric~ted under various conditions: namely, partial doping on textured

substrates (star), uniform doping on textured substrates (open circle), partial

-86-

-87-

doping on flat substrates (closed circle) and uniform doping on flat substrates(open triangle) . The n-type poly-Si thin films fabricated by the SPC method from a-Si
films using partial doping on textured substrates (star) were found to have low carrier
concentrations, ranging from l.Sxl0 1S to 3.Sxl0 1S cm- 3 . In these poly-Si thin films, amaximum mobility of 623 cm 2 /Vs was achieved at a carrier concentration of
3.0xl0 1S cm- 3 , for the first time. This value was highest in the poly-Si thin-films on
glass substrates and was about SO % the value for c-Si at the same carrierSub.

concentration?) So, it was proven that high-quality poly-Si thin-films could be obtainedby applying the partial doping method to films on textured substrate.
The preparation of p-type ~c-Si films was also investigated using the textured quartz.
Figure 3.4-S shows cross-sectional TEM photographs of n-type poly-Si thin films with athickness of O.S ~m prepared by the SPC method on textured and flat substrates. The
SPC conditions were 6S0 °C for 7 hours. The grain size of the films on the textured(a) Textured substratequartz after SPC was larger than that on the flat quartz. For example, the maximum grain
size was about 3 - 4 ~m on the textured quartz, compared with 1 - 2 ~m on the flat
quartz. Figure 3.4-6 shows the Hall mobility of the films on both the textured and flat
quartz after SPC as a function of carrier concentration. The SPC temperatures were 6S0
and 700°C. The ramp rate was in the range of 1 - 40 °C/s. The Hall mobility of the films
on the textured quartz was three to seven times larger than that of the films on the flat
quartz, at a high carrier concentration of 2xl020 - 2xl021 cm- 3 . Most of the films on the

O.5~mtextured quartz prepared at 6S0 ·C had a greater mobility than that of the films on the flat
quartz at 700°C. The maximum mobility was 32.2 cm 2/Vs at a carrier concentration of
2xl0 20 cm -3 on the textured quartz. This value is 64 % that of c-Si with the same carrier
concentration. Therefore, it was also confirmed for p-type ~c-Si films that using a

Sub.

Sub.

textured quartz was an effective means for enlarging grain size and improving Hall
mobility.

(b) Flat substrate
Fig.3.4-3

The cross-sectional TEM photographs of n-type poly-Si thin films
prepared by the SPC method on textured and flat substrates

-88-

-89-

* Partial doping on textured substrateo Uniform doping on textured substratE~• Partial doping on flat substrate
6 Uniform doping on flat substrate
0.5 j.t m ~

Sub.

Sub.
(a) Textured substrate

Fig. 3.4-4

The Hall mobility of n-type poly-Si thin-films prepared by the
SPC method as a function of carrier concentration, where n-ltype
poly-Si thin-films were fabricated under the various conditions:
namely, partial doping on textured substrates, uniform doping on
textured substrates, partial doping on flat substrates and unif:orm
doping on flat substrates(b) Flat substrate
Fig. 3.4-5-90-

The cross-sectional TEM photographs of p-type pc-Si thin fil ms
prepared by the SPC method on textured and flat substrates-91-

3.4.3 Growth kinetics on textured substrates
In order to investigate the origin of the differences between the poly-Si on textured
and flat substrates, the structural disorder of a-Si films before SPC was analyzed and
SPC time dependence was studied. Figure 3.4-7 shows the Raman spectra for a-Si films
deposited under the same conditions on textured and flat quartz. The a-Si film on textured
quartz have a larger distortion energy, compared with that of a-Si film on the flat quartz.
An SPC temperature of 550°C was used. The SPC time was varied: periods of 3, 30, 60
and 480 min. were perfonned. Figure 3.4-8 shows cross-sectional TEM photographs of~

the films after SPC using the various SPC times. In poly-Si thin films on textured
substrates, nuclei generation had already begun 3 min. after starting the SPC process,

•••
.0

o

nuclei growth proceeded both perpendicularly and horizontally on the substrate withincreasing the SPC time. In the poly-Si thin films on flat substrate, however, no nuclei

•

r-

E
=ro 10°:rI~.t0. Texturedr- • Flat sub.
- • Flat sub.

30 min. then grew perpendicular to the substrate with increasing the SPC time. The
growth velocities estimated from Fig. 3.4-8 are 8.4 A/s and 3.6 A/s for textured and flat
quartz, respectively. This high growth velocity on textured quartz was caused by an

SUb,} 650 e

increase in the distortion energy stored in the a-Si films as a result of using the textured

0

substrate. Therefore, it was found that the films on textured substrate had shorter

700 e
0

incubation times and especially, had the higher growth velocities in the horizontal

10-1 L-_~L-_ _I .....iIL-&...'.J...I_ _ _ _L--I_~I---LI_ _iL-_..LI.....JIL...J.l---1.

10

had emerged at 3 min . from the start of the SPC process. Nuclei began to generate at

2o

10

21

Carrier concentration (cm- 3 )direction compared with those for films grown on flat substrate. This high growthvelocity is one factor accounting for the large grain size of 4 to 6 ~m which can beobtained on textured substrates.
I discovered that textured substrate had the effect on the growth of high-quality polySi by the SPC method. By using textured substrates, the grain size of poly-Si increased

Fig. 3.4-6

The comparison of Hall mobility of p-type f-L c-Si thin-films
on textured and flat substrates prepared by SPC method, where
SPC temperatues of 650°C and 700°C

by about two times, and mobility increased to about seven times that of poly-Si on flat
substrate. It was found that one factor accounting for enlargement of grain size on
textured substrate was the increase in growth velocity in SPC as a result of large
distortion energy stored in a-Si films on such substrates. Thus, a large grain size of 4 to
6 11m and high mobility of 217 cm 2Ns were obtained without using partial doping. In a

-92-

-93-

poly-Si thin-film combining partial doping and a textured substrate, a maximum mobilityof 623 cm 2Ns was achieved at a carrier concentration of 3.0xl0 15 cm- 3 .

TO

100

.....

~

Distortion energy:51.8

en

c

.....
c

0,)

Flat substrate

c

co
E
co 50

a:

\
Textured
substrate

Distortion energy:70.6

\
0
400

300

200

Raman shift (cm- 1)

Fig. 3.4-7 Raman spectra of a-Si films deposited on textured and flat
substrates under a same cndition by plasma-CVD.

-94-

-95-

100

p
0
LO
LO

y

I~

0

Q)
'-

V)

rn

0-

E

I~

Q)

+-'

0

a..

E

~

O)

0

n..

~

E

ex:>

0

"Ii:j-

I ::J

0)

(l)
.....
"'0
c::
ro

ex:>

1.0
' Cf)

+-'

~

(/)

0

V)

I~

(l)

E
::t

E

'Lj

0..

C/.)

'T"'"-

+-'

C

E

+-'

E

ro
"--

(f)

1..0
I ::J
1Cf)

..0
::J
(f)

0
<.D

=

internal seeding layer/undoped a-Si was also fabricated. SiH 4 , PH and H2 were used as
3

.....
ro

>

source gases. The fabrication conditions were shown in Table 3.5-1. The internal seeding(1)Ulayer was prepared using a high hydrogen dilution. Such a high hydrogen dilution is

C'CS

C/.)

.........
"--

~I

CfJI

"D

...01

"--

:l

0)

(/)

:::J
X
CD

+-J

E

+-"

E

~

::t rnL0'T"'"-ro:

LL..-....

::t ...0..-....

T

seeding layer was deposited on quartz substrates by the plasma-CVD. The quartz/the

0

.~

C

/ ::J
, (/)

I define the films containing crystal phase as an internal seeding layer. The internalV)'T"'"-.0

It was tried to fabricate crystal phase in P-doped a-Si as nuclei generation layer. Here,

U

0

0
0)

, ..0

3.5.1 Experimental technique

c::

::t

0
<.D

new structure of starting material.

~

0

E

I

in this section, I propose SPC using a substrate/film containing crystal phase/a-Si as a

.....

0...

..0
::J
Cf)

modifying the structure of the substrate/nuclei generation layer/crystal growth layer. So,

"'0

C
.-

+-'

E

Q)

=
~
.....
=
><

~

C

CO
t-

The improvement of thin-film poly-Si properties have been succeeding on the basis of

1:2
V)
oO

'T"'"-::J+-'

3.5 Solid phase crystallization using crystal silicon as seed

V)

(l)
.....
ro

0

0)

'T"'"-

::::J

en

E
::t

0
to
to

E
::t

-

0

'T"'"-

..

0..

(l)

always used to prepare ~c-Si at low deposition temperatures of 200°C. But in this study,

¢::
~

V)

-'+=

E

it is different from conventional experiment in that the deposition temperature is as highas 550°C. The films were evaluated before and after SPC process by SEM after Secco's

(l)

..c
.....
4-1

etching and TEM. The Hall effect was measured as an electrical property.

0-'" ..
V)

..c
p.,
~

0.0

0
.....

.

E

E

I~

I~

0

C

C
.-

0
C'0

I-§

0

..c:
p.,

::;E

~

C'0

~

' Cf)

c::

.-.....
0

u

(l)

r/'.J
I

E

E

[ C\J::t.

::t

I C\J

0

..
V)
V)

0

u

Q)

..c
~

0
00
I

~

..0
::J
Cf)

C

C

E

E

I-§

C0

C'0

~

.-on
~

Cf)

-96-

-97-

3.5.2 Effect of internal seeding
Figure 3.5-1 shows a SEM photograph of a film deposited under the conditions asshown in Table 3.5-1 . It was found that this film consisted of two phases, where one
was toothed in shape when viewed in cross section and the other was amorphous phase.
In order to confrrm the nature of these phases, the diffraction of the TEM was measured
for a portion of the toothed and lattice images of TEM was measured for the films . Figure
3.5-2 shows the diffraction pattern of the TEM for the toothed portion. Figure 3.5-3shows the lattice image for the film. The diffraction pattern shows a spot pattern whichindicates single-crystal phase. The distance between two planes in the crystal phasecalculated from this pattern is 1.90 A. This value is almost same as the value of 1.92 A

Table 3.5-1 The deposition conditions of films under highhydrogen dilution.

Substrate temperature (OC)Gas flowrate (seem)

500 - 600

SiH4

1-5

PH3

0.003 - 0.01
40 - 300

H2

R. F. power (mW/cm2)
Pressure (Pa)120
30 - 60

for c-Si. Fig. 3.5-3 shows silicon atoms arranged regularly and arranged at random. So,
it was found that this film had a crystalline phase structure existing in the amorphous
phase. The size of the grain with a single-crystalline phase was about 1000 A, estimatedfrom Figs. 3.5-1 and 3.5-3.
A quartz/internal seeding layer/undoped a-Si structure was fabricated. The thicknessesof the internal seeding layer and undoped a-Si layer were 0.2 /lm and 3 /lm, respectively.
Figure 3.5-4 shows cross-sectional SEM photograph of the film after the SPC process.
For comparison, that of a film after SPC of quartz/P-doped a-Si/undoped a-Si is also
shown. In the film using the internal seeding layer, the grain boundary is almost
perpendicular to the substrate, but it is random in the film without seeding. The grain sizeis larger than that of the film without seeding. So, it was confirmed that the internal seed
had the effect of enlarging the grain size and directional control of solid phase growth.
Figure 3.5-5 shows the Hall mobility for the films prepared by the SPC method,using a-Si films with internal seed as a function of carrier concentration. The quartz
substrates are flat and textured. In this SPC method using internal seed, the effect of
texture was also shown. The Hall mobility of n-type poly-Si prepared using internal seed
was found to be higher than that of poly-Si without internal seed, and a maximum value

2of 808 cm Ns was achieved at a carrier concentration of 1.3 x 10 16 cm- 3 . So, from theviewpoints of both structural and electrical properties, this internal seeding method isclearly effective in improving solid phase crystallization.

-98-

-99-

I described the innovative technologies for the improvement of thin-film poly-Si
properties prepared by SPC method, above. I show the comparison between thin-film
poly-Si prepared by SPC method with c-Si seeds and thin-film poly-Si prepared by theconventional SPC method. Figure 3.5-6 shows the resistivity of the n-type thin-film
poly-Si described in this study and that prepared by the conventional SPC method 16) as afunction of phosphorus concentrati0n, compared with c-Si 17). The resistivity of n-typethin-film poly-Si prepared by the SPC method is almost the same as that of c-Si when thephosphorus concentration exceeds 10 15 to 10 21 cm -3. The resistivity of films preparedby the conventional SPC method, on the other hand, shows values higher by six to sevenorders than that of c-Si at phosphorus concentrations of 10 17 - 1019cm-3. Figure 3.5-7
shows the resistivity of p-type ~c-Si prepared by the SPC method and conventional SPC
method 16), compared with c-Si 17). The p-type films prepared by the SPC method show
resistivity lower than that prepared by conventional SPC method, and their values areclose to those of c-Si. So, the n- and p-type thin-film Si prepared by the SPC method
offer good properties, and the SPC method proposed in this study is well-suited to the
preparation of thin-film Si materials at low temperatures.

0.1 pmFig. 3.5-1 The SEM photograph of a film deposited under thehigh hydrogen dilution.

-100-

-101-

50A

Fig. 3.5-2 The difraction pattern of TEM for a film depositedunder the high hydrogen dilution condition.

Fig. 3.5-3 The lattice image for the film deposited under high hydrogen dilution conditionby plasma-CVD.

••

Texture quartz/internal seed
Flat quartz/internal seed

1000

I

j--- ............. • ..... . . • . . . . . .

·1~.. . . . .. . .. . .... . ..... . .... . . . ..... .

oA. . . . •. . •• • •• • • . ..•••. .. ••. . •••.•..•..

:~o+................................. ~.

O.Spm .
(a) quartz/internal seeding layer/undoped a-SI

Texture quartz/ no seed
Flat quartz/ no seed

=

••

~
r OO

••

A ....

-

•

-

....

100~------~------~------~----~

10 15

10 16

10 17

10 18

Carrier eoneen tration (em -3)

10 19

Fig. 3.5-5 The Hall mobility of the films prepared by SPC method using a-Si
films with and without internal seed, as a function of carrier
concentration. The substrates are f textured quartz.

O.Spm(b) quartz/P-doped a-Silu ndoped a-Si
Fig. 3.5-4 The cross-sectional SEM photographs of the films afte r SPC process of(a) quartz/internal seeding layer/undoped a-S i and (b) quartz/P-doped a-Si/
undoped a-Si. SPC conditions are 600°C for lOhours.

o•Flat sub./uniform
Flat sub./partial
6

10

0

o
•

Tex . sub./uniform
Tex. sub./partial

Flat sub./seed
Tex. sub./seed

EE

..

o
•

SPC temperature : 700°C
SPC temperature: 650°C•

SPC temperature: 600°C.......T • • • • r~ ~~t'~>fjTT\~Conventional!

!

1 0 0 0 ········· ······T •••••••••·...T •••••••••••••••

!

]

_

................... __ .. .. __ ........................ .

J: L ~~~~~~b~~l~.
........................+.............................~........................... .

1

1

0.01

L

~~~

...............~ ...~ .. -...: . j~ .. ~... :....................~ ....'*.~

S ·17 )

:

:

'. :

Phosphorus concentration (cm -3)

Fig. 3.5-6 The resistivity of n-type thin-film poly-Si prepared by various methods
as a function of phosphorus concentration, compared with c-Si..

-106-

~

.....................C.~Si.l?~ ................ j.............-.......:... _ ...

0 :····~· ··· c" .. l ····f····················+····················l...................:. ~ ..:.: .~........~ ............ .

10 - 4 ~~~~liill~~~-LUil~~~~~~~~
1 01 5
1 01 7
1 01 91 02 1. . . . . . . . . . . .. . . . . . . . .

·t·····························....•...: : ..•...• :. .....•. ..... ~.> -k .........................•..........................

O. 0 1 ......................./
:

+-

:

:

! . . ,~. . .~ . . .,:. . . ?. . . . . . . ..

0'

1 0-4 ~~~~~~~~~~~~~~~~~~
10

17

19

1 021
Boron concentration (cm- 3)
10

Fig. 3.5-7 The resistivity of the p-type films prepared by various methods
as a function of boron concentration, compared with c-Si.

-107-

method also became apparent. In this thin-film poly-Si, a maximum mobility of

3.6 Summary
The n- and p-type thin-film Si materials were fabricated by the SPC method and film

808 cm 2Ns was achieved at a carrier concentration of 1.3x10 16 cm- 3 .

properties were improved by innovative technologies . The phosphorus and boron atoms

In conclusion, n- and p-type thin-film Si prepared by the SPC method are materials

as dopant for silicon have the effect of promoting the creation of crystalline phase and,offering good properties. The SPC method proposed in this study is well-suited to thedecreasing the critical temperature of solid-phase crystallization.

preparation of thin-film Si materials at low temperatures.

The partial doping method which consists of separating the nucleus-generation andcrystal-growth layers has been developed. By using this partial doping method, control
of nucleus generation during the initial stage of SPC was performed and it was proventhat the partial doping method was an effective technology for improving the properties ofthin-film poly-Si. The lowest SPC temperature of 500 DC was also achieved, using thepartial doping method.
a-Si films suited to solid-phase crystallization were investigated. It was found that theTArro of Raman spectra of a-Si films before SPC correlated well with the average grain
size of poly-Si after SPC, and that average grain size increased with increasing the
T ArrO. The factors for enlargement of grain size by increasing the T ArrO are that thestore of large distortion energy in a-Si film increases the growth velocity and decreases
the nucleation rate.
The improvement of film properties was realized using textured substrate with random
surface morphology, for the first time. The films on textured substrate have about seventimes higher mobility and grain size two times larger than that on flat substrate at same
carrier concentration. It was found that a factor for enlargement of grain size on textured
substrate was the increase in the growth velocity during SPC by storage of large
distortion energy in the a-Si films. Combining the effects of the partial doping methodwith those of a textured substrate was also investigated and a high mobility of

623 cm 2Ns was achieved.
The SPC using an internal seed was investigated. The grain with a single-crystalline
phase can be fabricated in a-Si films by plasma-CVD method. The thin-film poly-Si
prepared by the SPC method using an internal seed had grain boundary perpendicular tothe substrate. The possibility of controlling the direction of solid-phase growth using this

-108-

-109-

4 Development of solar cells using thin-film polycrystalline siliconReferences

4.1 Introduction

1) M. W. Geis, D. C. Flanders and Henry I. Smith : Appl. Phys. Lett. 35 (1), (1979)

In conventional c-Si and poly-Si solar cells, the p-n junctions are, in general,p.ll.

fabricated using popular methods such as the thermal diffusion 1) of P and B, and the2) M. W. Geis, B-Y. Tsaur and D. C. Flanders : App!. Phys. Leu. 41 (6), (1982) p.

implantation 2 ) of p+ and B- into c-Si and poly-Si. In these methods, dopant atoms,

526.
3) T. Kanata, H. Takakura and Y. Hamakawa: App!. Phys. Lett. 54 (8), (1989) p. 706.

which are an opposite type of dopant contained in the host Si, are introduced into the host4) A. Doi, M. Kumikawa, 1. Konishi and Y. Nakamizo: Appl. Phys. Lett. 59 (20),Si. For example, when the host Si was n-type, boron atoms were implanted. As a result,atoms of both boron and phosphorus exist in the host Si. So, these methods have the(1991) p. 2518.

following problems.

5) W. C. Dash and R. Newman : Phys. Rev . 99 , (1955) p. 1151.

(1) The process temperatures are higher than 800°C.

6) Y. Hishikawa, N. Nakamura, S. Tsuda, S. Nakano, Y. Kishi and Y. Kuwano: Jpn.

(2) The quality of the c-Si and poly-Si is deteriorated, because opposite types of

J. Appl. Phys. Vol. 30, No. 5, (1991) p. 1008.

dopants are contained in the same host semiconductor.

7) S. M. Sze and J. C. Irvin: Solid State Electron. 11, (1968) p. 599.

a-Si solar cells are composed of three kinds of thin layers, i. e. p-type, i-type, and n-8) John Y. W. Seto: 1. App1. Phys. 46, (1975) p. 5247.

type layers, and these are deposited in order on the substrate. So, the junction is9) K. Mori, M. Kitagawa, T. Hirao, S. Ishihara and M. Ohno: Jpn. J. Appl. Phys.,fabricated by depositing thin layers in a-Si solar cells. Using this technology, two kinds

Vol. 20, (1981) p. 2431.

of junction fabrications by the deposition of thin layers were investigated at low10) Y. Hamakawa: Proc. of 8th E. C. Photovoltaic Solar Energy Conf.,( 1988)

temperatures of less than 700°C to resolve problems (1) and (2) mentioned above. One

p. 1211.

was a homojunction consisting of p-type thin-film crystalline silicon and a c-Si wafer,11) T. Shimada, Y. Katayama, K. Nakagawa, H. Matsubara, M. Migitaka and E.

and the other was a heterojunction composed of a-Si and a c-Si wafer.

Maruyama: 1. Non-Cryst. Solids. 59&60, (1983) p: 783.

There are few reports on homojunction fabrication by the deposition of thin layers in12) D. Beeman: Phys. Rev. B. Vol.32, No.2, (1985) p. 874.
13) T. Saito, T. Karasawa and I. Ohdomari: J. Non-Cryst Solids. 50, (1982) p. 271.

c-Si and poly-Si solar cells. Various researchers, however, have been working on a-Si/c-14) K. F. Kelton, A. L. Greer and C. V. Thompson: J. Chern. Phys. 79, (1982)

Si heterojunction solar cells. Hamakawa et a1. developed a tandem solar cell containing
both a-Si and poly-Si. 3) Their bottom cell (poly-Si cell) has an n-type poly-Si wafer/p-

p.6261.
15) R. B. Iverson and R. Reif: J. Appl. Phys. 62, (1987) p. 1675.

type J..lc-Si heterojunction structure. A conversion efficiency of more than 20% has been16) I. Mizushima, W. Tabuchi and H. Kuwano: Jpn. J. App!. Phys. Vol. 27, No.12,

reported with the tandem cell. 4 ) Morikawa et a1. 5 ) studied the characteristics of J..lcSi/poly-Si wafer heterojunction solar cells . They found that Ilc-Si deposition after(1988) p. 2310.

fabricating a thin layer of silicon oxide (Si0 2) improved solar cell characteristics such as17) J. C. Irvin: Bell Syst. Tech. J., 41 , (1962) p. 387.

the open circuit voltage (V oc )' Mimura and Hatanaka6 ) and Matsuura et al.7) investigated
the junction properties of a-Si/c-Si wafer heterojunctions, but they made no reports on
their application in solar cells.

-111-

-110-

r

-

-

-

In spite of the above studies, the conversion efficiencies of heterojunction solar cells
are lower than those of conventional homojunction cells; the maximum efficiency ofmode" with a monochromatic light of 2 x 10 14 photons/(cm 2 .s), under illumination witha white light bias of AM-1.5, 100 mW/cm 2 .

conventional cells is about 24 %.8) So, the heterojunction using a-Si and c-Si wasinvestigated to improve the junction properties.
By applying new junction fabrication technologies to the thin-film poly-Si described in
Chapter 3, thin-film poly-Si solar cells were fabricated on a metal substrate, and attempts
were made to improve solar cell performance.
This Chapter mentions these new junction fabrication technologies and thedevelopment of thin-film poly-Si solar cells using those technologies.

4.2 Experimental technique
To fabricate a homojunction composed of p-type thin-film crystalline silicon and c-Si
wafer by the SPC method, B-doped a-Si:H films were deposited by the plasma-CVD
method on n-type c-Si (p: 1.35 - 2.25 ncm) and then, thermal annealing was performed
for a solid-phase crystallization of B-doped a-Si:H layer. Boron atoms were in-situ dopedby mixing B2H6 with SiH 4 as a source gas for the SPC process. The deposition
conditions are shown in Table 4.2-1. Thermal annealing conditions were 600 - 700 °C
and for 1 - 7 hours in a vacuum or nitrogen atmosphere.
For the fabrication of a heterojunction consisting of'thin a-Si films and c-Si wafer, Bdoped a-Si:H and undoped a-Si:H films were also deposited on n-type c-Si (p: 1.35 2.25 ncm) by the plasma-CVD method. Table 4.2-2 shows the preparation conditions
for these films.
Thin-film poly-Si solar cells were fabricated using the n-type poly-Si thin-films as anactive layer. The p-n junction was an a-Si/poly-Si heterojunction. a-Si films were
deposited by plasma-CVD. In the solar cells, an indium tin oxide (ITO) transparent
electrode was fabricated on a-Si/poly-Si by a sputtering method at a substrate temperatureof 150 °C.
The output characteristics of the solar cells were measured under AM-1.5, 100
mW/cm2. Collection efficiency spectra were also measured at a "constant-photon density

-112-

-113-

Table 4.2-2 The deposition conditions of a-Si:H films for the fabrication ofheterojunction by plasma-CVD.

Table 4.2-1 The deposition conditions of B-doped a-Si:H
for the fabrication of homojunction by the SPC
method.

Substrate temperature (OC)SiH4
Gas flow
B2H6
rate (seem)
H2
R. F. power (mW/em2)
Pressure (Pa)-114-

200 - 350
2.5 - 10p-typei-typen-type

120

120

180

SiH4

5

, 5

10

B2H6

0.03 - 0.3--------------

PH3--------------

0.2

H2

0-70

-------

0-100

Substrate temperature CC)

Gas flowrate (seem)

0.004 - 0.05
40 - 50

R. F. power (mW/em 2)

25 - 75

25 -7555

95.5

Pressure (Pa)15 - 275 -27

8 - 27

13.3

-115-

4.3 A new junction fabrication technology
This section describes new junction fabrication methods using film deposition.

A~ mentioned in Chapter 3, p-type ~c-Si films prepared by the SPC method have highconductivity of more than 1x 102 (Qcmf 1. A homojunction fabrication method wasdeveloped using these films. The junction fabrication method consists of two processes.
Figure 4.3-1 shows a diagram of this junction fabrication method. In the first process, Bdoped a-Si:H films were deposited on n-type c-Si by the plasma-CVD method. In thesecond process, B-doped a-Si:H/n-type c-Si was annealed at temperatures of 600 - 700°C, and consequently p-type SPC-Si/n-type c-Si was fabricated. This method offers
following features.
(1) The impurity atom (B) is in-situ doped into a-Si:H.

B-doped a-Si:H

SPC(2) This process is very simple.
(3) The distribution of impurity atoms is sharp at the interface of the p-n junction.

p-type SPC-Si

n-type c-Sin-type c-Si

Plasma CVD

Thermal annealing(4) The process temperature is low.
Figure 4.3-2 shows the structure of a solar cell with a homojunction prepared by the
SPC method. The thicknesses of the n-type c-Si, p-type SPC-Si and ITO were 400 ~m,
0.2 ~m and 700 A, respectively. The SPC conditions were 650°C for 7 hours.
Aluminum was also evaporated for the front and back electrodes. Figure 4.3-3 shows thenormalized collection efficiency (l1 co ll) of the p-type SPC-Si/n-type c-Si solar cell,compared with a conventional solar cell. The Tlcoll at 1000 nm showed a high value of
79.4 %. This ratio is almost equal to that of the conventional p-n junction solar cell, in; I , , , Ii ( \ \ , \'",1" "~"~I"~r r r , r r r

11 I I 1 1 'r I I I I I I
I I I I I I I

I I I I I I I
I I I I I , I

I I 1 , , I I

r r r r r r '

\'I

l,'

~---_Q(a) 8-doped a-Si:H deposition(b) Crystallization

which the p-n juryction was prepared by a high-temperature process, such as a thermal
diffusion or ion implantation method. The p-type SPC-Si/n-type c-Si solar cell shows ashort circuit current (Isc) of 34.3 mA/cm 2 .

Fig. 4.3-1 Diagram of fabricating homojunction solar cells, using SPC method.

In order to evaluate the junction properties, the depth profile of boron concentrationand the dark current-voltage characteristics were measured. Figure 4.3-4 shows the
distributions in the depth of boron concentration, compared with thermal diffusion 9 ) andion implantation methods. 10) In the thermal diffusion method, boron concentration
C(x, t) is expressed by the following two equations. For a "limited source" condition,with the total amount of impurities S, C(x, t) is given by the Gaussian function

-116-

-117-

~
o

Incident light

:

".g
Q.)

~

0 . 8 ~·········· ·····7··· ···f --li~+--ITOp-type SPC-Si-+---

n-type c-t)i

AI back electrode

'\~

+

::

Corvent~bnal i:\:···············~················(·····solarcell~······· .........(.. . .. L............L............. .

i

! ~ ,

;S ~ O. 4 · · · · · · T b~typ~SpC~sil~~~~~si\ i§~0 .2 ....................... ...... ,..............."..............

~?l~..f~ll... :................!. . . . . . . . ~.
!~O~~~~~~~~~~~~~~~~

300

500

700

900

11 00

Wavelength (nm)Fig. 4.3-3 Nonnalized collection efficiency of a p-type
SPC-Si/n-type c-Si solar cell, compared withthat of conventional c-Si solar cells.

Fig. 4.3-2 The structure of a solar cell with homojunctn prepared by SPC method.

-118-

:

. : : : : : :8 5U a.6"2:§

AI front electrode

:

-119-

C(x, t)=S· (nDtf 1/2.exp (-x 2/(4Dt)}(4.3-1)

where D is the diffusion coefficient of boron atoms, t diffusion time, and x distance fromthe surface. For the "constant surface concentration" condition with a surface
concentration C ' C(x, t) is given by the error function compliments
C(x, t)=C erfc[x/{2(Dt)1/2}]s(4.3-2)In Fig. 4.3-4, C(x, t) for the two conditions was calculated introducing each value of D
202
1.96 x 10- 14 cm 2/s (at 1000 °C), t 1800 s, S 4 .33 x 1015 cm- and C s 4.11 x 101021cr~~~~~~~~~~~~~~~~

cm- 3 to eqs. of (4.3-1) and (4.3-2). In ion implantation method, boron was implanted. doses were 1 x 10 14 cm.
-2 After Imp. 1antatlOn,.
into the c-Si at 150-keY energy. Impunty" N

1 0 2 0 ......................"'....~....... '......... Gaussian"sample was annealed in nitrogen at 900 °C for 150sec. Fig. 4.3-4 shows ion implantation
data after annealing. In the SPC method, the boron concentration fell from 4.11 x 10

Thermal diffusion
!

20

"V

Error

funct~bn..........: ....................
'

cm- 3 to 1.0 x 10 17 cm- 3 with a depth of about 1200 A. In the thermal diffusion method,
the depth of 3000 - 3500 A dropped to the same value. In the ion implantation method,
the depth fell from 3.23 x 10 18 cm- 3 to 1.0 x 10 17 cm- 3 at 3250 A. This indicated that

1 01 7the SPC method was suitable for fabricating the step junction.

SPC method :Figure 4.3-5 shows the dark current-voltage characteristics of a p-type SPC-Si/n-typec-Si solar cell. Based on eq.(2.4-18) in Section 2.4 of Chapter 2, the dark current-voltage
28characteristics was evaluated. The backward current density was about 4 x 10- A/cm ata bias voltage of 1 Y, and the rectification ratio was of more than 5 orders. This showedthat the p-n junction prepared by the SPC method had good junction properties, despite
the low-temperature process of 650°C.
From the viewpoint of solar cell performance, the textured substrate is effective for theincrease of Isc' On the other hand, it was mentioned in 3.4.2 of Chapter 3 that the p-type
SPC-Si prepared on a textured substrate by the SPC method had large grains, andshowed high mobility in the high carrier concentration range of 2 x 10 20 - 2 x 10

. . ... . ... .,:?l\><! ,21

cm -3. The p-type SPC-Si/n-type c-Si (textured) solar cells prepared by the SPC method
are expected to have higher conversion efficiency. Figure 4.3-6 shows the illuminated1-Y characteristics of the p-type SPC-Si/n-type c-Si (textured) solar cell, compared withthat of the p-type SPC-Si/n-type c-Si (flat) solar cell. In the p-type SPC-Si/n-type c-Si
(textured) solar cell, I sc ' fill factor (F. F.) and open circuit voltage (V oc) increase,: \1 0 16

a

~,_

:
,:

1 000 2000 3000 4000 5000 6000

Depth (A)Fig. 4.3-4 The depth profiles of boron concentration in p-njunctions prepared by various methods.

compared with th e p-type SPC-Si/n-type c- Si (flat) solar cell. In particular, Isc was aslarge as 42 mA/cm 2 . These improvements in solar cell performance are attributed toincreases in mobility in the p-type SPC-Si film s on a textured substrate. As a result, ahigh conversion efficiency of 14.2 % has been achieved with a size of 2 cm 2 .
a-Si/c-Si heterojunction solar cells were also investigated using the structure as shownin Figure 4.3-7. Figure 4.3-8 shows the output characteristics of p-type a-Si/n-type c-Si
heterojunction solar cells as a function of p-type a-Si film thickness. Voc does not depend~

0.1 ----~----~----~--~----~--~

on the film thickness in this region. Isc decreases with increasing the film thickness.

N

a
0.01
~

.....................~ ...................... ··t·· ..................... -~ ................ .

............ . .......l....................... ;... .....................+. . . .. ...........). . .... . . . . .... . ;. . . ...............
~ 0.001
Forward :'-/1 0- 4

1 0- 5........ __...........~ ..__ .................. 1. ...._............. -.-... i... ........... -.... --...~ ........................ J. ................ ....
.

........ +................. --.-.~ .. - ............. ..... .~ ........................ ~.... ............ .... .

1 0- 6
1 0- 7.

......................~ ........................ +....
.

~

~

.

Backward~~.. ...................

.; ........................ ......... ........ ........ .............. ....... .... ... ........... .. .... ....

Figure 4.3-9 shows the collection efficiency spectra of the p-type a-Si/n -type c-Si
heterojunction solar cells at p-type a-Si film thicknesses of 50 and 270 A. The collection
efficiency in the short wavelength region was improved by decreasing the film thickness .
In Fig. 4.3-8, the decreases of Isc by increases in the film thickness are attributed to the
reduction of the collection efficiency in the short wavelength region. This comes fromlight absorption in the p-type a-Si, and it was found that the p(a-Si) film used in thisexperiment had no photovoltaic ability.

1 0- 8From these results, it was determined that the p-type a-Si film thickness should be as-9100.0~0--0~.2-0--0-.~4-0--0~.6-0---0~.8-0---1.~O-0--1~.20

thin as possible from the viewpoint of optical characteristics, although there is a minimum

Voltage (V)thickness above which good junction properties are assured. A maximum conversion

Fig. 4.3-5 Dark current-voltage characteristics ofp-type SPC-Si/n-type c-Si solar cell.

efficiency of 12.3 % was achieved at a film thickness of under 100A. In theseheterojunction solar cells, however, Voc and F. F. were somewhat lower than those of
conventional c-Si homojunction solar cells .
In order to explain this inferiority, the junction properties of the p-type a-Si/n-typec-Si heterojunctions were investigated. The p-type a-Si/n-type c-Si heterojunction solar
cells were evaluated by dark I-V characteristics, and low- and high-frequency C- V
characteristics. The dark current-voltage characteristics was analyzed, based on eq.(2.418) in Section 2.4 of Chapter 2. As a result , it became clear that the dark I-V
characteristics exhibited
prominent features. Figure 4.3-10 shows the dark 1-V characteristics of a p-type a-Si/ntype c-Si heterojunction solar cell. The backward current density was about 10- 6 A/cm2
at a bias voltage of IV. This value for the backward current density is considerably

-122-

-123-

45~----------~T~e-x~tu-r-e-s~iz-e--:~2~.0~c-m~2~

Isc : 42.0mAlcm2
Voe : 0.525V
F.F. : 0.6424035..--..

E 30
()

~ 25
E

+-'

a5 20
L..
L..

:J

o 15

Flat size: 2.0cm 2
Isc : 34.3mAlcm2
Vac: 0.489V
F.F. : 0.577
r; : 9.7 %Incident light

10

5

0.2
0.3
0.4
Voltage (V)

0.5

0.6

Fig. 4.3-6 illuminated I-V characteristics of p-type SPC-Si/n-type c-Si solar cells
prepared by the SPC method, where n-type c-Si substrates were flatand textured.

-124-

n-typec-Sit/L-----..L...--__~~

AM-1.5 100mW/em2
0.1

p-typea-Si:H

TeO

metal

Fig. 4.3-7 Structure of a p-type a-Si/n-type c-Si heterojunction solar cell.

-125-

..--.

G 0.60u0> 0.55
..--.

C\l

E

u

~

E

---u
(/)

... I

•

A

35
30
25
20

100

- 80
~
0

p(50 A)
,

~

(,)

c
LL.
LL

0.7 ~

... ...

Q.)

~
0

---~

u
c

Q)

"u
~

w

/\

60

....

(,)

:&

-

:/ p(270 A)

\+=

'+-

Q.)

c

40

,
,

0

......
(,)

0.5
..--.

,

Q.)

20

,,

--'

0

0

14
12
10
8

,

,

,
,,

0

200

500

800

1100

Wavelength (nm)0100 200 300 400 500

Thickness of p-type a-Si (A)Fig. 4.3-9 Collection efficiency spectra of a-Si/c-Si heterojunction solar cells
whose p (a-Si) layer thicknesses were 50 and 270A.

Fig. 4.3-8 Output characteristics of an a-Si/c-Si heterojunction solar cell
as a function of p (a-Si) layer thickness.

-126-

-127-

larger than that of the c-Si homojunction, and seems to be caused by a large interface state
density that originates from the heterojunction, and a localized state density in the a-Si
layer:In order to improve the junction properties, a new heterojunction technology wasdeveloped by thin-film deposition of le ss than 2ooA . The decrease of the backward
current in the p-n heterojunction was tried by inserting thin undoped a-Si film between ptype a-Si and n-type c-Si , as shown in Figure 4.3-11. This structure was called HIT

p-layer:50A(Heterojunction with Intrinsic Thin-layer). Figure 4.3-12 shows a comparison between
the dark I-V characteristics for the HIT cell and a-Si/c-Si heterojunction solar cell. The
backward current density was reduced to about 10- 8 A/cm2 with the HIT structure.
Figure 4.3-13 shows the output characteristics of the HIT structure solar cells as afunction of the undoped a-Si layer thickness. In this figure, the data at a film thickness ofzero are those for the p-type a-Siln-type c-Si heterojunction solar cell. In the HIT
structure, both Voc and F. F. were improved. Voc was particularly enhanced by about 30-

Forward10 0

1:u 10-2
~

"-""

.....

~

mY, and an F. F. of more than 0.8 was obtained. Figure 4.3-14 shows the collection
efficiency spectra of a p-type a-Siln-type c-Si heterojunction solar cell and a HIT structuresolar cell, where the total thickness of the a-Si layers was a constant to be 140 A. The

en 10-4c

BackwardQ)-0.....c 10-6Q)collection efficiency of the HIT structure solar cell was improved in the short wavelength~~

region by inserting undoped a-Si (100 A) between p-type a-Si and n-type c-Si. By using

()

::::J

10-8

the HIT structure, a conversion efficiency of 14.8 % was obtained at the undoped a-Si
layer thickness of 40 A. It was found that the insertion of undoped a-Si contributed to theimprovement of device properties such as the decrease of backward current, theenhancement of Voc' and the increase of collection efficiency in the short wavelength

0.4

0.8

1.2

Voltage (V)region. These improvements indicate that undoped a-Si has an effect on the reduction ofinterface recombination between a-Si and c-Si.
To achieve a higher conversion efficiency, textured substrate and back surface field

Fig. 4.3-10 Dark I-V characteristics of an a-Si/c-Si heterojunction solar cell.

(BSF) technologies were applied to HIT structure solar cells. In the application of the
textured substrate, it is important to deposit undoped a-Si thin layer unifonnly and toobtain a clean surface before the fabrication of the a-Si film . It was found that a decreasein the deposition rate and the optimization of hydrogen dilution to silane gas were

-128-

-129-

.... -- .. ---Forwardpin
heterojunction..... - ..

n-typec-Si

p-typea-Si:H

metal

TCO
undoped a-Si:H

0.4---------

0.8

Voltage (V)Fig. 4.3-11 Structure of a HIT solar cell.

-130-

Fig. 4.3-12 Dark I-V characteristics of an a-Si/c-Si heterojunction
solar cell and a HIT solar cell.

-131-

1.2

..., ..

>0.60

-g
>

N"'

E
u
~

0.58

~

0.56 I - - - - - - - - - - - - - l

35
30

- 25
E

,/

....

~

0.8

--

0.7

u

u

CJ)

100

~
0

LL
LL

~

u
c

Q)

p(40 A )/i(1 00 A)

80

"" '"

60

.. .

~

Q)

-

~
0

~

u

c

Q)

'u
-.;:::
~

w

15
14
13
12
11

c
0

•

40

+-J

:

U

Q)

20

0

.../

p(140 A)

:

,,

:

0

0
200

°

100 200 300

500

800
Wavelength (nm)1100

Thickness of undoped a-Si (A)Fig. 4.3-13 Output characteristics of an a-Si/c-Si heterojunction solar celland HIT solar cells as a function of undoped a-Si layer
thickness.

Fig. 4.3-14 Collection efficiency spectra of an a-Si/c-Si heterojunction solar celland a HIT solar cell. The thicknesses of the a-Si layers are 140A.

-132-

-133-

effective for the uniform deposition of a-Si films . Clean surfaces for the textured c··Siwere obtained by using a hydrogen plasma treatment before the deposition of the a··Si
layers. As a result, Isc was improved by about 20 % in a solar cell using textured c-Si. In
addition to the textured substrate, BSF structures using n-type a-Si were applied. Finally,a high conversion efficiency of 18.7 % was achieved, as shown in Figure 4.3-15. This is
the highest value ever reported for solar cells in which the junction was fabricated at a
low temperature of less than 200 ·C.
The summaries in this Section are following. Homojunction and heterojunctionpformation by film deposition was investigated. A p-n homojunction was fabricated by then AI

SPC method, and the junction had a sharp distribution of boron concentration in thedepth profile, compared with thermal diffusion and ion implantation methods. The
backward current density of the dark 1- Y characteristics was also as low as 4x
10- 8 A/cm 2 . In a solar cell using this technology, a high Isc of 42 mA/cm 2 and a50

conversion efficiency of 14.2 % were obtained at a low process temperature of 650 ·C.
A p-type a-Si/n-type c-Si heterojunction was also fabricated by plasma-CYD method.

AM-1.5, 100 mW/errf
Cell size: 1 errf

40

The interface property of this heterojunction was not so good, as the backward current
density was as high as 10- 6 A/cm 2 . It was found that the interface recombination

30

between p-type a-Si and n-type c-Si was reduced by inserting an undoped a-Si thin layer,
and consequently device properties such as backward current density , V oc' and the20collection efficiency in the short wavelength region were improved. In a solar cell using
the HIT structure, a high conversion efficiency of 18.7 % was achieved at a very low

10

Voe (V)
0.638
Ise(mAlem2) 37.9
F.F.
0.775
17 (%)
18.7

process temperature of less than 200 ·C.
It was confirmed that junction fabrication by film deposition was effective for
improving junction properties in both homojunctions and heterojunctions. A reduction in
the process temperature was also achieved.

o ~------------~----------------~------------~~~
0.2
0.4
0.6
o
Voltage (V)Fig. 4.3-15 Structure and illuminated I-V characteristics of a HIT solar cell.

-134-

-135-

4.4 Solar cells using thin-film polycrysta1line silicon fabricated by the SPC method
In this Section, thin-film solar cells which combine the new junction formation

Incident lighttechnology and n-type poly-Si prepared by the SPC method are described.
A thin-film solar cell has been developed, using thin-film poly-Si prepared by the SPC..I.

method. The structure of the poly-Si solar cell was ITO/p-type a-Si:Hln-type poly-

7 ooA
9 ooA

Si/n +poly-Si/flat quartz. The thicknesses of the n+poly-Si layer, n-type poly-Si layer,
and p-type a-Si:H layer are 0.7, 3.0, and 0.09 ~m, respectively. Figure 4.4-1 shows

3.0 J.lm

collection efficiency and inner quantum efficiency spectra of a thin-film poly-Si solar cell.
The n-type poly-Si as an active layer was prepared by the partial doping method, and the..I.

SPC conditions were 600 ·C for 9 hours. In the long wavelength region of 800 - 1000
nm, a collection efficiency of about 20 % was observed. The inner quantum efficiency

0.7 J.l m

ITO
p-type a-Si:H

SPC
conditions

n-type thin-film
poly-Si~----------

n+poly-Si
Quartzcalculated from the collection efficiency and absorption coefficient of each film was about
80 % in the range of 800 - 1000 nm. It was confirmed that n-type poly-Si prepared by
SPC method was effective for preparing a material with high photosensitivity in the long..-....

wavelength region.
The properties of n-type thin-film poly-Si were improved by SPC for a-Si films

~
0

- 80
~

u

deposited on textured substrates by plasma-CVD, using the partial doping method. Thus,c:a poly-Si solar cell was fabricated on a textured metal substrate using this poly-Si. The'ustructure of the poly-Si solar cell was ITO/p-type a-Si:H/i-type a-Si:H/n-type polySi/textured metal substrate, as shown in Figure 4.4-2. The a-Si films used as the starting

100

0)

60

"t"t-

,,

,

/ " --- ,,Inner collection' ...... " ,efficiency

O)

c
0

40

~

material for the n-type poly-Si, which is very important as the active layer in the solarcell, were prepared by the partial doping method. The thickness of the n-type poly-Si
layer was 12 ~m, and the total thickness of the i-type a-Si:H layer and the p-type a-Si:H
layer was 100 A. Figure 4.4-3 shows the illuminated 1-V characteristics of a poly-Si solar

u

0)

20

0

0

0

300 400 500 600 700 800 900 1000
Wavelength (nm)cell with a size of 4 mm 2 . The SPC conditions of the n-type poly-Si layer as an active
layer were 600 °C for 60 hours. The Isc ' Voc ' and F. F. are 24.73 mNcm 2 , 0.548 V and
0.457, respectively. Consequently, a conversion efficiency of 6.2 % was obtained.
Figure 4.4-4 shows the collection efficiency of the poly-Si solar cell. The dashed line
shows the calculated collection efficiency assuming a c-Si thickness of 12 ~m forcomparison. In the long-wavelength region of more than 800 nm, the photovoltaic ability

Fig.4.4-1 Collection efficiency and inner quantum efficiency spectraof a thin-film poly-Si solar cell prepared by SPC method.
The cell structure was also shown.

Incident light

30
Metal
ITO (700A)
p-type a-Si:HI}1 ooA
i-type a-Si:H

Heterojunction{-+--n-type poly-S;i(12pm)

AM-1.S 100mW/cm 24mm2.--...
C\J

E

~ 20
E

Isc =24.73mA/cm2
Voc=0.548V
F.F.=0.4S7
1]: :.- 6.2%-+--- Metal substrate°o~--~--~--~---L--~~~

0.1

0.2
0.3
0.4
Voltage (V)

0.5

0.6

Fig. 4.4-3 Illum.inated I-V characteristics of a thin-film poly-Si solar cell
applyIng the partial doping method to textured substrate.
'
Fig. 4.4-2 Structure of a thin-film poly-Si solar cell prepared by the SPC method
on a textured metal substrate.

-138-

-139-

of the solar cell was observed and a high collection efficiency of 50 % was obtained at thelong-wavelength region of 900 nm in thin n-type poly-Si of 12 11m. This high value isattribnted to the large grain size of 4 to 6 J.lm. The collection efficiency in the region of
more than 900 nm is higher than the calculated value. This is attributed mainly to theincrease in light absorption by the effect of optical scattering at the interface between the
n-type poly-Si and textured metal substrate. Therefore, it became clear that n-type poly-Si

100

prepared by the SPC method on a textured substrate was high potential as a solar cell~ ___...--..

material with high photosensitivity in the long-wavelength region.
The n-type thin-film poly-Si layer in the solar cell was prepared by applying undoped
a-Si films with a large TA/TO to the crystal-growth layer. Figure 4.4-5 shows the2illuminated I-V characteristics of a thin-film poly-Si solar cell with a size of 1 cm . The
thickness of the n-type poly-Si layer was 10 11m . The Isc' V oc' and F. F. are 28.4~
0iJ..<....

~

l

U
C

I
,I

Q)

~

v

\,
s

poly-Si solar cell. For comparison, that of the a-Si solar cell is also shown. The poly-Si
solar cell exhibits high collection efficiency in the long wavelength region where the a-Si
solar cell has no photosensitivity, and a high collection efficiency of 51 % was obtained
at a wavelength of 900 nm.
As mentioned above, it was found that the solar cell performance increased with the

...

...

... ,

.

Q) 50

\

poly-Si thin-film
solar cell(12}J. m thick)---------------;--- -- -----------\.
<% was obtained. Figure 4.4-6 shows the collection efficiency spectrum of the thin-film... ...

i

~

U
~
~

mA/cm2, 0.51 V and 0.434, respectively. Consequently, a conversion efficiency of 6.3

-<

~,~--

'-'"

c-Si(12 }J. m thick)(Calculated value)

.

\

C

0

/~,

+-'

r·· ··················.. ·.............··...·.·.-.......

U

\

os,

La-Si solar cell i ~\~.,

Q)

--0

\
\

"\

0

O~~~~~~~~~+-__L-~~~400 500 600 700 800 90010001100

improvement of the n-type poly-Si film properties by the innovative technologies
described in Chapter 3.
The diffusion length of the minority carrier of the poly-Si thin films prepared by the
SPC method was estimated from short-circuit current measurements. 11 ) When ashallow-junction solar cell is illuminated with monochromatic radiation with a low

Fig. 4.4-4

Collect}on effi~iency spectra of thin-film poly-Si solar cell andth~ typIc~1 a-SI solar celL The calculated spectrum using c-Si
WIth a thIckness of 12Jlffi was also shown.

absorption coefficient, such as that in the wavelength range of 0.8 to 1.03 11m for silicon,
the measured current is essentially the photocurrent from the n-type poly-Si region andcan be written 12)
ISC = (qFOLa/a2L 2_1 )[aLexp(-at) -exp(-at)coth {(d-t)JL}+exp(-ad)cosech (d-t)JL}],(4.4-1)

where FO is the light intensity incident on the surface, L is the diffusion length, t is the

-140-

-141-

35~------------------------~..-....

AM-1.5 100mW/em 2
1em 2

30

1.0~-------------~~..-a---S-i-ce-I-1----~

~ 0.8

c

C\J

Q)

E 25

~ 0.6

u

~

Q)

~ 20

•

,
•,

.

~

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 't •••••••.•••••••
~

....

,

o 0.4

11
t
It

+-'

::; 15

c

~
10
'::J

5

u

Ise =
Vae = 0.51 V
F.F. = 0.434
1J ~ . ·6.3 0/0

28.4 mA/em 20~~----~--~~~~~-71
o
0.1
0.2
0.3 ' 0.4
0.5
0. 6
Voltage (V)

..

Q)

,

'll

o 0.2

o

900

500

700

900

1100

Wavelength (nm)Fig. 4.4-6 Collection efficiency spectrum of the thin-film poly-Si solar cell
prepared by SPC method.

Fig. 4.4-5 Illuminated I-V characteristics of a thin-film poly-Si solar cell
prepared by SPC method.

-142-

poly-Si cell
Conversion eft.
6.3 % ( 1 cm2)

~

c

E

()

-

-143-

depletion layer thickness, d is the specimen thickness and a is the optical absorption

at a wavelength of 900 nm, despite its 10 ~m thickness. The high value is attributed to acoefficient. When ~ 1 and ~2L, the term exp( -at)cosech {(d-t)lL) becomes negligible,

diffusion length of 11 ~m, which is longer than the thickness.

and c{)th(d-t)lL} approaches unity. Equation (4.4-1) can then be written
ISC = qF Lexp(-at)/(a- 1 +L).

o

(4.4-2)

If the incident light intensity on the n-type poly-Si, F=FOexp( -at), is adjusted to maintaina given ISC for all wavelengths, then eq. (4.4-2) is reduced to
F = K(a- 1 +L),

(4.4-3)where K is a constant depending only on cell parameters and the selected value of ISC'
The absorption coefficient of single crystalline silicon at various wavelengths was used to
calculate F. 13) Figure 4.4-7 shows the plot of intensity (F) versus a-I for the solar cellshown in Figs.4.4-1, 4.4-4, and 4.4-5, where each F was calculated using the ISC of
11.5,24.7, and 28.4 mA/cm 2 , and the thickness of 3, 12, and 10 ~m, respectively. The
diffusion length is obtained from the negative intercept on the a-I axis and the estimateddiffusion lengths are 2.5, 11, and 11 ~m for poly-Si (3 ~m) on flat, poly-Si (12 ~m) on
texture, and poly-Si (10 ~m) on texture, respectively. The diffusion lengths were foundto increase as the property of the n-type thin-film poly-Si was improved. In the solar cellwith thickness of 10 ~m, the diffusion length of 11 ~m, which was longer than the
thickness, was obtained. This long diffusion length contributes to the high collection
efficiency of 51 % at a wavelength of 900 nm.
Thus, it was also confirmed from the result of solar cell characteristics that SPC
method in used in this study was an effective fabrication method for solar cell materials.
The summaries in this Section are following. The thin-film poly-Si solar cells weredeveloped, using n-type poly-Si prepared by the SPC method as an active layer. I
succeeded in the fabrication of the poly-Si solar cells on the substrates except for siliconat the low temperature of 600 °c. It was proved that n-type poly-Si prepared by the SPC
method exhibited high photosensitivity in the long wavelength region where a-Si solar
cells have no photosensitivity. As the film properties improved, the solar cellcharacteristics were enhanced, and a conversion efficiency of 6.3 % was obtained in asolar cell with a size of 1 cm 2 . This solar cell showed a high collection efficiency of 51 %

-144-

-145-

4.5 Summary
Junction fonnation by film deposition was investigated for both homojunctions andheterojunctions. A p-n homojunction was fabricated by the SPC method, and the junction
had a sharp distribution of boron concentration in the depth, compared with thermal
diffusion and ion implantation methods. The backward current density of dark the I-V
characteristics was also as low as 4 x 10- 8 Ncm 2. In a solar cell using this technology, ahigh Isc of 42 mNcm 2 and conversion efficiency of 14.2 % were obtained at a low
process temperature of 650°C.
An a-Si/c-Si heterojunction was fabricated by plasma-CVD method. The interface

2.5

property of this heterojunction was not so good, as backward current density was as highas 10- 6 A/cm 2 . It was found that the interface recombination between p-type a-Si and n-

2

type c-Si was reduced by inserting an undoped a-Si thin layer, which had an effect on
decreasing the backward current density, on enhancing V oc' and on increasing collection

1.5

efficiency in the short wavelength region. The HIT structure solar cell was developed byinserting the undoped a-Si layer, which had an effect on improving the interface

1

properties between p-type a-Si and n-type c-Si. In the solar cell using the HIT structure, apoly-Si(3}lm)
on flat0.5high conversion efficiency of 18.7 % was achieved at a very low process temperature ofless than 200°C.

o-15 -10 -5oIt was confinned that junction fabrication by film deposition was effective for

5

10

15

(Absorption coefficient)-l (pm)

improving of junction properties in both homojunctions and heterojunctions. A reductionin the process temperatures was also achieved.
Using the new junction fabrication technology, thin-film solar cells applying n-type
poly-Si prepared by the SPC method to the active layer were developed. It was revealed

Fig. 4.4-7 Diffusion length estimated by the constant shortcircuit current method, for solar cells msde of various
n-type thin-film poly-Si prepared by SPC method.

that the solar cells could be fabricated on the substrates except for silicon at the lowtemperature of 600°C, by using the SPC method. It was proven that n-type poly-Si
prepared by the SPC method exhibited high photosensitivity in the long wavelength
region where a-Si solar cells have no photosensitivity. As the film properties improved,
the solar cell characteristics were improved, and a conversion efficiency of 6.3 % was
obtained in a solar cell with a size of 1 cm 2 . This solar cell showed a high collection

-146-

-147-

efficiency of 51 % at a wavelength of 900 nm, despite its 10 ~m thickness. The highvalue is attributed to a diffusion length of 11 ~m, which is longer than the thickness.

References
1) M. Tanenbaum and D. E. Thomas: Bell Syst. Tech. J., 35, (1956) p. 1.
2).W. Shockley: Proc. IEEE, 56 (1968) p. 295.
3) K. Okuda, H. Okamoto and Y. Hamakawa: Jpn. J. Appl. Phys., 22 (1983) p. L603.
4) W. Ma, T. Horiuchi, M. Yoshimi, K. Hattori, H. Okamoto and Y. Hamakawa: Proc.
6th Int. Photovoltaic Science and Engineering Conf., New Delhi, India (Oxford &IBH Publishing Co. Pvt. Ltd., New Delhi, 1992) p. 463.
5) H. Morikawa, T. Itagaki, K. Kawabata, T. Ishihara, K. Sato and H. Namizaki: Tech.
Dig. Int. Photovoltaic Science and Engineering Conf., Kyoto, Japan (5th International
PVSEC Organizing Committee, Kyoto, 1990) p. 215.
6) H. Mimura and Y. Hatanaka: Jpn. J. Appl. Phys., 26 (1987) p. 60.
7) H. Matsuura, T. Ok uno, H. Okushi and K. Tanaka: J. Appl. Phys., 55 (1984)
p. 1012.
8) H. Matsunami and T. Fuyuki: Proc. 6th Int. Photovoltaic Science and Engineering
Conf., New Delhi, India (Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1992)
p. 11.
9) H. S. Carslaw and 1. C. Jaeger: Conduction of Heat in Solids, Oxford University

Press, 2nd Ed. (1959).
10) M. Miyake and S. Aoyama: 1. Appl. Phys., 63 (1988) p. 1754.
11) E. D. Stokes and T. L. Chu: Appl. Phys. Lett., 30 (8) (1977) p. 425.
12) T. S. Moss, G. J. Barrell and B. Ellis, Semiconductor Opto-Electronics, (Wiley,
New York, 1976) p. 182.
13) W. C. Dash and R. Newman: Phys. Rev., 99 (1955) p. 1151.
14) S. M. Sze: "Physics of Semiconductor Devices (Wiley, New York, 1969)" p. 41.
15) A. K. Ghosh, C. Fishman, and T. Feng: J. Appl. Phys., 51(1) (1980) p. 446.
16) C. Feldman, N. A. Blum, H. K. Charles, Jr. and F. G. Satkiewicz; 1. Electron.
Mat.,7 (1978) p. 309.
17) 1. Manolu and T. I. Kamins: Solid-State Electron., 15 (1972) p. 1103.

-148-

-149-

and 2 times larger grain size than that on flat substrate. It was found that a factor for the

5. Summary
In this thesis, a systematic study was performed to develop thin-film solar cells withenlargement of grain size was the increase of the growth velocity in SPC by the largehigh photosensitivity in the long wavelength region where a-Si solar cells had no

distortion energy stored in a-Si films. By optimizations of the poly-Si films on textured

photosensitivity, through the approaches of the fabrication process and the junction

substrate, the films with high mobility of 623 cm 2Ns was obtained.
P-doped a-Si films containing the grains with single-crystalline phase were used as aformation technology.
Concerning the fabrication process, a new solid phase crystallization (SPC) method

nucleus-generation layer. By using this internal seeding technology, the films after thewas developed, using a-Si films as a starting material. As for junction formation process,

SPC process had grain boundary perpendicular to substrate and it was indicated to bea new technology for p-n junctions (homo- and heterojunction) was developed using film

able to control the direction of solid-phase growth. In this thin-film poly-Si, a maximum

deposition.

mobility of 808 cm 2Ns was achieved at a cru-rier concentration of 1.3xl0 16 cm- 3 .

In Chapter 2, the theories on the growth and electrical transport of poly-Si, and on theSo, it was proven through the innovative technologies that the decrease of nucleation

current-voltage characteristics of p-n junction were described. In the growth theory, theand the increase of growth velocity in the SPC process were very effective for the

kinetics of crystallization in solid phase was discussed and it was shown that transient

improvements of the thin-film poly-Si properties.

time, nucleation, and growth velocity had the temperature dependence. The relationship

In Chapter 4, a new junction formation technology and the thin-film poly-Si solar cells

between the conductivity and trap density was also introduced on the basis of theusing this technology were studied. The formations of homo- and heterojunctions by film

assumption which carrier trapping at grain boundary governed the electrical transport

deposition were investigated for the purpose of preventing the existence of opposite types

properties.

of dopants in a same semiconductor. In a p-n homojunction fabricated by the SPC

silicon, on solid-phase crystallization were investigated. These atoms were found to havemethod, the backward current density of dark I-V characteristics was as low as
4x 10- 8 A/cm 2 . In the solar cell using this junction, a high Isc of 42 mA/cm 2 andthe effects of promoting crystalline phase. A partial dop'ing method of separating nucleus

conversion efficiency of 14.2% were obtained at a low process temperature of 650°C.

generation-layer and crystal-growth layer was developed using this feature of dopant, andp-type a-Si/n-type c-Si heterojunctions were also fabricated by plasma-CVD. HIT

it was shown that control of nucleation was a much effective means for improvements ofstructure solar cells, which undoped a-Si was inserted at interface between p-type a-Si

thin-film poly-Si properties.

and n-type c-Si, were also developed. This type solar cells had a low backward current

In Chapter 3, the effects of phosphorus and boron atoms, which are dopant for

Based on the partial doping method, improvements of the film properties were

density of lx10- 8 mA/cm 2 , and a maximum conversion efficiency of 18.7 % wasattempted. As for crystal-growth layer, a-Si films suited to solid-phase crystallizationachieved at a very low process temperature of less than 200°C. So, it was confmned thatwere investigated. It was found that a-Si films with the large T ArrO value in Raman

the junction fabrication by film deposition was a considerably effective means for the

spectrum increased the average grain size of poly-Si after the SPC process. This

improvements of junction properties in homo- and heterojunctions.

enlargement of grain size attributed to the increase of growth velocity and the decrease of
nucleation rate by the large distortion energy stored in a-Si films.

Since the thin-film poly-Si with good properties and the innovative technology forjunction formation were developed, thin-film solar cells applying n-type poly-Si prepared

The film properties were further improved using textured substrate with at random

by the SPC method to an active layer were fabricated using the new heterojunction

surface morphology. The films on textured ubstrate had about 7 times higher mobility

fabrication technology. It was revealed that the solar cells could be fabricated on the

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Acknowledgement

substrates except for silicon at a low process temperature of 600 °C. and, that the solar
cells exhibited high photosensitivity in the long wavelength region where a-Si solar cells

The author wishes to express his sincere gratitude to Professor Masataka Hirose of

had no photosensitivity. As the poly-Si film properties improved, the solar cell

the Faculty of Engineering of Hiroshima University for his kind guidance and hearty

characteristics was increased, and the conversion efficiency of 6.3 % was obtained in theencouragement for completing this thesis.

solar cell with a size of 1 cm 2. This solar cell showed a high collection efficiency of 51 %The author is also grateful to Professor Yukio Osaka, Professor Masamichi

at a wavelength of 900 nm despite 10 ~m thickness. This high value is attributed to the

Yamanishi, Professor Yasuhiro Horiike and Professor Takafumi Yao of the Faculty ofdiffusion length of 11 ~m longer than the thickness. Thus, it was found that the thin-film

Engineering of Hiroshima University for their guidance and suggestions during thepoly-Si prepared by the SPC method was very suitable for solar cell materials.

preparation of this thesis.
This work was carried out at the Functional Materials Research Center of SANYOElectric Co., Ltd., and supported in part by the New Energy and Iudustrial Technology
Development Organization as a part of the Sunshine Project under the Ministry ofInternational Trade and Industry.
The author expresses special thanks to Dr. Seiji Ohara and Dr. Yukinori Kuwano,executive directors of the R&D Headquarters of SANYO Electric Co., Ltd. for theirimmeasurable and valuable support.
The kind support provided by Dr. Shoichi Nakano, general manager of the Functional
Materials Research Center, and Dr. Shinya Tsuda, manager of the Thin Film Laboratoryis also greatly appreciated.
The author gratefully thanks Mr ~ Tsuyoshi Takahama, Mr. Mikio Taguchi, Mr.
Toshiaki Baba, and Mr. Tohru Sawada for their helpful discussions.
And the author thanks Ms. Ritsuyo Ohnishi and Ms. Michiyo Nishida for preparing
some of the figures.

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