Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

Mechanism of Electrical Conduction in Polycrystalline InGaAs Films
T. Okuzako*, Y. Torii , and Y. Kajikawa
Interdisciplinary Graduate School of Science and Engineering, Shimane University,
1060 Nishikawatu, Matsue 690-8504, Japan
E-mail : s099411@matsu.shimane-u.ac.jp
Keywords: InGaAs, molecular-beam deposition, Hall-effect measurement

Abstract. Polycrystalline In1-xGaxAs films having Ga compositions of x = 0-1 and the thicknessof about 1 µm were grown on glass substrates at 240 and 350 °C by molecular-beam deposition.
Hall-effect measurements at 100-390 K revealed that the electron density of the samples having
Ga compositions of x ≤ 0.30 was almost independent of the measurement temperature, whilethat of the samples having x ≥ 0.36 decreased with decreasing temperature. The electron
mobility of the samples having Ga compositions of x ≤ 0.21 was almost independent of themeasurement temperature, while that of the samples having x ≥ 0.30 decreased with decreasing
temperature. The temperature dependence of electron density is explained assuming theexistence of three donor levels at the most. The temperature dependence of electron mobility is
explained in terms of thermionic electron emission over the grain-boundary barriers.

Introduction
Polycrystalline Si-TFTs are used for driving organic electroluminescence (EL) elements.
However, in order to manufacture polycrystalline Si-TFTs, it is necessary to recrystallize
amorphous Si by high-temperature process at 600 °C or higher[1]. Therefore, polycrystalline
Si-TFTs can be grown only on thermally resistant substrates. This leads a problem that one
cannot make use of the advantage of organic EL elements which can be manufactured on plastic
films.
Besides Si polycrystal, polycrystals of oxide semiconductors such as ZnO[2] are investigatedas materials for TFTs. There is, however, a problem of low mobility though polycrystalline ZnO
can be grown at lower temperatures than polycrystalline Si.
On the other hand, the polycrystals of III-V compound semiconductors have been scarcely
investigated as materials for TFTs. In our previous study, however, polycrystalline InAs was
deposited on a plastic film at 230 °C, and it showed an electron mobility higher than 400
cm2/Vs[3]. Moreover, when we deposited InGaAs polycrystal on glass by molecular-beamdeposition at 240 °C, the mobilities of the samples with low Ga compositions were almost equal
to those of the polycrystalline InAs[4]. Since InGaAs has larger band-gap energies than InAs,
polycrystalline InGaAs is expected as the most suitable material for flexible TFTs. However,
the influence of substrate temperature on the mobility and the conduction mechanism inpolycrystalline InGaAs are not clear. In this study, we clarify the influence of substratetemperature on the electrical properties, and study the mechanism of electrical conduction in thepolycrystalline InGaAs.

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Experiments
Polycrystalline In1-xGaxAs films with various Ga compositions (x = 0, 0.21, 0.30, 0.36,
0.44, 0.57, 0.64, 1) were deposited on non-alkali glass (Corning7059) substrates at substratetemperatures of Tg = 240 and 350 °C by molecular-beam deposition. The beam equivalentpressures of Ga and In at each deposition were adjusted so that the growth rate becomes about
1µm/h. The films of 1µm in thickness were deposited so that their Ga compositions can bemeasured in terms of electron probe microanalysis (EPMA). The temperature dependences ofelectron density and electron mobility were examined by Hall-effect measurements using thevan der Pauw method in the temperature range of 100-390 K. The surface was observed by
AFM, and the mean radius r of crystal grains was estimated by assuming the following
expression:
A = N G πr 2 ,(1)

where A = 2µm×2µm is the scanning area of an AFM image and NG is the number of crystal
grains observed in the scanning area of A.

Experimental results
Fig.1 shows the Ga composition dependence ofthe mean grain radius. As can be seen in Fig.1, themean radius at 350 °C is larger than that at 240 °C
by about 20 nm at each Ga composition. The mean
radius decreases slightly as the Ga composition
increases.
Figs.2(a) and 2(b) respectively show theArrhenius plots of the electron density and theelectron mobility of the polycrystalline InGaAsgrown at 240 °C. As can be seen in Figs.2(a) and2(b), both the electron density and the electron
mobility showed little change within the

Mean grain radius r [nm]

200

100

0
0

(a)

the mean grain radius.

1017

x=0.44x=0.361016
1015
x=0.641014

0x=0.575
10
103/T [K-1]

200

103

15

100
x=0.21(b)2

x=0x=0.21x=0.301018

1Temperature T [K]100Electron mobility μ[cm /Vs]

-3

Electron density n [cm ]

10

200

0.2 0.4 0.6 0.8
Ga composition x

Fig.1 Ga composition dependence ofTemperature T [K]19

Tg=350℃
Tg=240℃

x=0102

x=0.30x=0.64101

x=0.36x=0.57x=0.44

100

0

5
10
3
-1
10 /T [K ]15

Fig.2 Temperature dependence of (a) electron density and (b) electron mobility ofpolycrystalline InGaAs grown at 240°C.

100228051-2

measurement temperature range of 300-100 K for the samples having lower Ga compositions
than 0.21. On the other hand, for higher Ga compositions than 0.30, both the electron density
and the electron mobility decreased with decreasing measurement temperature. Thosetemperature dependences were enhanced as the Ga composition increases, and were diminishedwith decreasing measurement temperature. The mobility of the sample having a Ga compositionof 0.57 can not be measured at lower temperatures than 170 K because of its high resistivity.
The similar temperature dependences of the electron density and the electronic mobility wereobserved also for the samples grown at 350 °C.

Discussion
Temperature dependences of the Fermi leveland electron density. The temperature dependenceof the Fermi level in each sample was calculated
from the measurement results of the electron
density using the relation of(2)

where η=(EF－EC) / kBT, EC is the energy of the
conduction band minimum(CBM), NC(T) is theeffective density of states of the conduction band,and the Fermi-Dirac integral F1/2(η) is defined as

F1 / 2 (η ) =

∞

π

x

1/ 2

dx .
2 ∫1+ e η(3)( x− )

0.3

200

100

0.2
EF-EC [eV]n = N C (T ) F1 / 2 (η ) ,Temperature T [K]x=0
x=0.210.1

x=0.30x=0.44x=0.360x=0.57-0.1
x=0.64-0.2
0Tg=240℃

5

10
3

15

-1

10 /T [K ]Fig.3 Temperature dependence of the
Fermi level with respect to the CBM.

0

Fig.3 shows the temperature dependence of EF-EC in the samples grown at 240 °C. As can beseen in Fig.3, in the samples having lower Ga compositions than 0.44, electrons are degenerated
while the spatially averaged values of EC lie higher than EF in the samples having higher Ga
compositions than 0.57.
As can be seen in Fig.2(a), the slope of the Arrhenius plot of the electron density decreased
with decreasing temperature in the samples having lower Ga compositions than 0.36. Thissuggests that more than two donor levels exist in these samples. When assuming three donor
levels, the temperature dependence of the electron density is given by the following relation of
the neutrality condition among the electric charges [5]:
3

3

i =1

i =1

n = ∑ N Di − ∑ N Di f D (E Di ) ,(4)

where NDi’s are the densities of the donor levels while EDi’s are the energies of the donor levels.
fD(E) is defined as−1

 1
 E − E F  .
f D ( E ) = 1 + exp k B T 
 2

(5)

The values of NDi’s and EDi’s were obtained through fitting the measurement results of the100228051-3

temperature-dependent electron density using Eq.(4). The curves in Figs. 2(a) and 3 show thefitting results using Eq.(4). The densities and ionization energies of the donor levels obtained by
the fitting are shown in TableⅠfor the samples grown at 240 °C. The negative values of theionization energies εD1 in TableⅠindicate that the dominant donor level lies in the conduction
band.
Table I. The densities and ionization energies of donor levelsin the samples grown at 240 °C.
x

ND1[1018cm-3]

εD1
[eV]

ND2
[1018cm-3]

εD2
[eV]

ND3
[1018cm-3]

εD3
[eV]

0

20

-0.17

0

0

0.21

8

-0.11

0

0

0.30

5

-0.055

0

0.36

17

0.13

1.4

0.037

0.057

-0.05

0.44

20

0.12

1.0

0.021

0.14

-0.06

0

0.57

900

0.37

0

0.15

0.155

0.64

1400

0.52

0

0.002

0.13

Fig.4(a) shows the Ga composition dependences of densities NDi of the donor levels. As canbe seen in Fig.4, the density of the dominant donor level in the samples grown at 350 °C is
almost independent of Ga composition, while that in the samples having Ga compositions of
0.36-0.64 grown at 240 °C was very high. Fig.4(b) shows the Ga composition dependences ofenergies EDi of the donor levels with respect to the CBM together with EF at room temperature.
As can be seen in Fig.4(b), ED1 as well as EF is lowered as the Ga composition increases.
In Fig.5, we plot the energy level ED1-EV of the dominant donor level with respect to thevalence band maximum(VBM) as a function of the Ga composition, and compare it with theenergy of the EL2 level in InGaAs estimated by Khirouni et al. [6]. The band gap energy Eg =EC-EV = 0.36+0.505x+0.555x2 (eV) at room temperature is also plotted in Fig.5. As can be seenin Fig.5, the energy level of the dominant donor almost coincides with that of the EL2.
Therefore, the dominant donor level may be identical to the EL2 level.

Donor density NDi [cm-3]

1021

(a)

0.2

(b)

E-EC [eV]

1020
1019
18

10

ND1(Tg=350℃)
ND2(Tg=350℃)
ND3(Tg=350℃)
ND1(Tg=240℃)
ND2(Tg=240℃)
ND3(Tg=240℃)

1017
1016

0

0.2

0
-0.2
-0.4

0.4

0.6

Ga composition x-0.6
0EF(Tg =350℃)
ED1 (Tg =350℃)
ED2 (Tg =350℃)
ED3 (Tg =350℃)
EF(Tg =240℃)
ED1 (Tg =240℃)
ED2 (Tg =240℃)
ED3 (Tg =240℃)

0.2

0.4

0.6

Ga composition x

Fig.4 Dependence of (a) the densities and (b) the energies of donor levels on
Ga composition.

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Temperature T [K]
200
100

1.5
EC

104

μT 1/2 [cm2 K 1/2/Vs]

E-EV [eV]

ED1 (Tg =350℃)
ED1 (Tg =240℃)

1

x=0x=0.21

103

EL2

0.5

x=0.30x=0.64

102

0
0

0.2

0.4

0.6

0.8

1

Ga composition x

Fig.5 Ga composition dependence ofthe energy level ED1 of the dominant donor
and the CBM with respect to the VBM.

101

x=0.36

Tg=240℃ x=0.57

0x=0.44

5
10
103/T [K-1]

15

Fig.6 Arrhenius plots ofμT 1/2.

Temperature dependence of electron mobility. The electric conduction in polycrystalline
films is dominated in many cases by thermionic emission over potential barriers at grainboundaries. Then, the mobility can be expressed by the following expression with the meanvalue qφ and the standard deviation qu of the barrier height when assuming the distribution
in potential barrier height [7]:
1

 qφ

2
1
q 2u 2  exp −µ = Lq+ k T 2k 2 T 2  ,B 2πm * k B T 
B



(6)

where L is the grain size.
The logarithmic plots of μT 1/2 against the reciprocal of T are shown in Fig.6. The meanvalue and the standard deviation of the barrier height as well as the grain size L were obtained
through fitting by Eq. (6). In Fig.6, the curves express the fitted results by Eq. (6). The Ga
composition dependence of the mean value of the barrier height is shown in Fig.7. In Fig.7, thecircles and the error bars express the mean values ( qφ ) and the standard deviations ( ± qu ) of
the barrier height, respectively. As can be seen in Fig.7, the barrier height is less than 20meV in
the samples having lower Ga compositions than 0.21 while the barrier height increases withincreasing Ga composition in the samples having higher Ga compositions than 0.30.
Fig.8 shows the Ga composition dependence of the estimated grain sizes L by the fitting. As
can be seen in Fig.8, the estimated grain sizes L from Eq.(6) are comparable to the estimatedmean grain radius r from AFM. This suggests the validity of applying the grain-boundary
limiting model to the electron mobility.

100228051-5

200
Tg=350℃
Tg=240℃

Tg =350℃
Tg =240℃

Grain size L [nm]Barrier height qφ [meV]

150

100

100

50

0
0

0
0

0.2
0.4
0.6
Ga composition x

Fig.7 Ga composition dependenceof barrier height.

0.2 0.4 0.6 0.8
Ga composition x

1

Fig.8 Ga composition dependenceof grain size.

Summary
Polycrystalline In1-xGaxAs films were grown on glass substrates at 240 and 350 °C by
molecular-beam deposition. Temperature dependences of the electron density and the electron
mobility of the polycrystalline In1-xGaxAs films were investigated in order to study themechanism of electric conduction.
The temperature dependence of the electron density can be explained by assuming threedonor levels at the most. The density of the dominant donor level in the samples grown at
350 °C is almost independent of Ga composition. The density of the dominant donor level in thesamples having Ga compositions of 0.36-0.64 grown at 240 °C was very high. The energy ofthe dominant donor level as well as the Fermi level decreased with increasing Ga composition.
The energy level of the dominant donor coincides with that of EL2.
Being regardless of the substrate temperature, the temperature dependence of the electron
mobility can be explained by thermionic emission over the barriers at the grain boundaries withthe distribution of the barrier height taken into account. The barrier height is less than 20meV in
the samples having lower Ga compositions than 0.21 while the barrier height increases withincreasing Ga composition in the samples having higher Ga compositions than 0.30. Thus, thehigh electron mobilities in polycrystalline InGaAs having low Ga compositions are owing to
the low barriers. As a result of the comparably high mobility and the larger band gap than InAs,
InGaAs with a Ga composition around 0.2 is expected to be most suitable for high-speed TFTs.

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