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

Analysis of Microwave Absorption by Photo-Induced Carriers at PN
Junction Formed by Ion Implantation
T.Sameshima(1), M.Shimokawa(1), J.Takenezawa(1), T.Nagao (1), M.Hasumi(1), S.Yoshidomi (1),
N.Sano (2), T.Mizuno(3)(1)
Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo,
184-8588(2)
Hightec Systems Corporation, 3-9-15, Shin Yokohama, Kohoku-ku, Yokohama, Kanagawa,
222-0033(3)
Kanagawa University, 2946, Tsuchiya, Hiratsuka 259-1293
E-mail :tsamesim@cc.tuat.ac.jp
Keywords: free carrier absorption, carrier density, carrier mobility, PN junction, effective
lifetime, photo-induced carrier, recombination velocity

Abstract.
We report photo-induced carrier recombination properties for crystalline silicon in steps of N+Pjunction formation. Transmissivity of 9.35GHz microwave was measured and analyzed underillumination of 532 nm green light at 4~20 mW/cm2 to the top or rear surface. Phosphorus ionswith 2x1014 cm-2 were implanted to 525 μm thick p-type silicon coated with 100 nm thickthermally grown SiO2 layers. The implantation markedly increased the surface recombination
velocity S much higher than 2.6x104 cm/s, while S was 501 cm/s for initial sample. The
implanted surface regions were activated by 940 nm infrared semiconductor laser irradiation at
100 kW/cm2 for 2.5 ms. 1.3×106 Pa H2O vapor heat treatment at 260oC for 3 h was also
conducted for defect reduction. These processes markedly decreased S to 307 cm/s. When the
SiO2 layer at the top surface was removed, S increased to 1500 cm/s, which indicated an abilityof photo-induced carrier separation by N+P junction.

Introduction
Analysis of photo-induced carrier behavior is important for developing photovoltaic devices
such as solar cells. Measurements of microwave photoconductive decay [1] andquasi-steady-state-photo-conductance [2] have been widely used for measurement of the photo
induced minority carrier lifetime. We recently have developed a microwave free carrier
absorption measurement system for precisely non-destructive and non-contact investigation ofphoto-induced carrier properties [3]. Photo induced carriers are much sensitive to properties ofthe surface and bulk semiconductor. Defects induce carrier recombination and reduce thedensity of photo induced carriers.
In this paper, we report a precise analysis of photo-induced carrier properties in processes of
PN junction formation followed by infrared semiconductor laser annealing at the silicon surface
using the free carrier microwave absorption method. We discuss changes in the effective
minority carrier lifetime at steps of ion implantation, laser annealing [4] and high pressure H2Ovapor heat treatment [5]. The surface recombination velocity is also analyzed. We also report arole of reduction of carrier recombination velocity of PN junction.

100228115-1

Experimental
P type silicon substrates with resistivity of 8-9 Ωcm and a thickness of 525 μm wereprepared. The both surfaces were coated with 100 nm thick thermally grown SiO2 layers.
Phosphorus atoms with a concentration of 2×1014 cm-2 were implanted to the top surface at 73
keV. They had a peak concentration of 2.0×1019 cm-3 at SiO2/Si interfaces. Phosphorus atoms
with 1×1014 cm-2 were effectively incorporated into silicon with Gaussian distribution from thesurface to 140 nm deep. Samples were annealed by 940 nm contineous wave infrared
semiconductor laser irradiation [6]. Laser beam at a power of 25 W was introduced to samples
using an optical fiber and lens mounted on the X-Y mobile stage. The laser beam was moved at
6~10 cm/s in the X-direction. It was also moved in the Y-direction with a 25 μm step. Laserbeam was focused to a spot with a Gaussian-like intensity distribution with a diameter of 150
μm at the sample surface. The laser light intensity was about 100 kW/cm2 at the sample surface.
The laser dwell time was estimated as laser beam size devided by laser moving velocity. It
ranged from 1.5 to 2.5 ms. The optical reflectivity of the sample surface was 15.8 % at awavelength of 940 nm. The optical absorption coefficient of silicon was about 3×103 cm-1 at
940 nm. Laser light was incorporated and absorbed in silicon substrate. Optical reflectivity
spectra in ultraviolet and visible ranges were also measured and analyzed for investigatingrecrystallization properties by laser annealing [7].
Photo induced free carrier absorption was measured using 9.35 GHz microwave circuit [3].
The microwave interferometer was constructed by waveguide tubes. It had narrow gaps formeasurement of transmissivity for sample wafers. A thin light illumination plate was also
inserted facing the samples. A laser light at 532 nm was introduced using optical fibers to thelight illumination plate. The light intensity was controlled from 4 to 20 mW/cm2 at the surface.
Transmissivity at 9.35 GHz during light illumination to the top or rear surface was measured forsamples initial, as-ion-implanted, laser annealed and heat treatment at 260oC with 1.3×106 Pa
H2O vapor for 3 h [5]. A finite element numerical calculation program including Fresnel opticalinterference effect induced by in-depth change in refractive index owing to photo-induced free
carrier diffusion was constructed to estimate the density of fee carriers from the experimental
transmissivity.
Results and discussion
Figure 1 shows in-depth profiles of crystalline volume ratio analyzed from the opticalreflectivity spectra for the top surface of samples. The crystalline volume ratio in top 140 nm
region was decreased by phosphorus ion implantation. It was 0.27 just at the silicon surface, as
shown in Fig. 1. It means that phosphorus ion implantation caused serious crystalline damage.
The crystalline volume ratio in the most of
implanted region was increased to 1.0 by laser
irradiation for 1.5 ms. But it was 0.7 only 2 nm
deep surface region. Moreover, it wascompletely 1.0 for sample laser annealed for 2.5
ms. The recrystallization was achieved bysemiconductor laser irradiation. There was noroughness and the surface was kept flat.
Figure 2 shows changes in normalized
transimissivity as a function of light intensity
illuminated to the top surface (ion-implanted
surface) and rear surface for samples initial andas-implanted (a), laser annealed for 1.5 and 2.5
ms (b), and heated with 1.3x106 Pa H2O vapor
for 3 h (c). Transmissivity was normalized bythat measured in the dark field. Initial sample
showed marked decreases in normalized Fig. 1 In-depth profiles of crystallinetransimissivity to 0.87 for light illumination at volume ratio analyzed from the optical
20 mW/cm2 to the top and rear surfaces. Thismeans that the silicon surfaces were well reflectivity spectra.

100228115-2

passivaed by thermally grown SiO2 and ahigh density of photo-induced carriers wasexisted under illumination. On the other
hand, no decrease in transmissivity wasobserved in the case of light illumination to
the top surface for as-implanted sample.
Normalized transmittance slightly decreased
to 0.98 in the case of light illumination at 18
mW/cm2 to the rear surface, as shown in Fig.
2(a). This shows that phosphorus ion
implantation caused substantial defects
associated with serious carrier recombinationin silicon.
Laser annealing for 2.5 ms caused
decreases in normalized transmissivity to
0.97 and 0.95 in the case of lightillumination at 20 mW/cm2 to the top surface
and 18 mW/cm2 to the rear surface,respectively. Laser annealing reduced
photo-induced
carrierrecombination Fig.2 Changes in normalized transimissivity
probability compared with as-implanted as a function of light intensity illuminated tostates. However, decrease in normalized the top and rear surfaces for samples initialtransmissivity was smaller than that for
initial samples, although analysis of optical and as-implanted (a), laser annealed for6 1.5
reflectivity spectra resulted in complete and 2.5 ms (b), and heated with 1.3x10 Pa
recrystallization by laser annealing for 2.5 H2O vapor for 3 h (c).
ms, as shown in Fig. 1. The results of Fig. 2(b) indicate that there were still defect states
inducing carrier recombination after laser
annealing for 2.5 ms. Normalized transmissivity slightly decreased to 0.99 and 0.96 in the caseof light illumination at 20 mW/cm2 to the top surface and 18 mW/cm2 to the rear surface,respectively, for samples laser annealed for 1.5 ms. Small decrease in normalized
transmissivity by light illumination to the top surface compared with laser annealing for 2.5 ms
indicates that there was a high density of residual defect states remained.
1.3x106 Pa H2O vapor heat treatment at 260oC for 3 h resulted in marked decrease innormalized transmissivity to 0.77 in light illumination at 17 mW/cm2 to the top and rearsurfaces for sample laser annealed for 2.5 ms, as shown in Fig. 2(c). This degree of normalized
transmissivity decrease was higher than that for initial sample, as shown in Fig. 2(a). It
indicates that H2O vapor heat treatment decreased the density of defect states inducing carrier
recombination velocity. On the other hand, H2O vapor heat treatment at 260oC for 3 h
decreased the normalized transmissivity only to 0.98 and 0.95 for light illumination at 17
mW/cm2 to the top and rear surfaces, respectively, for samples laser annealed for 1.5 ms. This
indicates that there were residually serious defect states causing carrier recombination at thesurface region for samples laser annealed for 1.5 ms.
The photo induced carrier density was analyzed from change in transmissivity shown in Fig.
2 by our numerical program with the effective carrier lifetime τeff and surface recombination
velocities. Light illumination induces the photo carriers in silicon substrate. The carrier
diffusion model gives minority carrier in-depth distribution Nphoto(x) as [3,8],⎛⎛ D⎛⎛ 2d − x ⎞ ⎞⎞⎞x ⎞ ⎛ DN photo ( x ) = N 0 ⎜ ⎜+ S2 ⎟ exp ⎜ −+⎜− S2 ⎟ exp ⎜ −
⎟
⎟⎟
⎜
⎜
⎜⎝ τb
Dτ b ⎟⎠ ⎝ τ b
Dτ b ⎟⎠ ⎟⎠
⎠
⎠
⎝
⎝
⎝
N0 =

η (1 − r )G
⎛
⎛ D
⎞⎛ D
⎞ ⎛ D
⎞⎛ D
⎞
2d ⎞
+ S2 ⎟ ⎜+ S1 ⎟ − ⎜− S1 ⎟ ⎜− S2 ⎟ exp ⎜ −⎟⎜⎜Dτ b ⎟⎠⎝ τb⎠⎝ τb⎠ ⎝ τb⎠⎝ τb⎠⎝100228115-3(1),where G is the photon flux per unit area with light intensity of W per unit area given by
G=W/hv, where h is the plank constant and v is the light frequency, η is the quantum efficiency
for photo induced carrier generation, which was assumed as 1 in this investigation, r is the
optical reflectivity of the green laser light, D is the electron carrier diffusion coefficient of 36
cm2/s and τb is the minority bulk carrier lifetime, S1 is the surface recombination velocity at alight illumination side. S2 is the recombination velocity at the other dark side, d is the substrate
thickness. The total photo induced minority carrier density per unit area n is obtained by the
integration of Nphoto(x) from 0 to d as,
⎛
⎛
⎛
⎛ D
⎞
d ⎞⎞⎛ Dd ⎞⎞n = N 0 Dτ b ⎜ 1 − exp ⎜ −+ S2 + ⎜− S2 ⎟ exp ⎜ −
⎟⎜
⎟
⎟⎟
⎜
⎜
⎜
Dτ b ⎟⎠ ⎟⎠ ⎜⎝ τ b
Dτ b ⎟⎠ ⎟⎠
⎝ τb
⎠
⎝
⎝
⎝

.

(2),

The carrier effective lifetime is also defined from eqs. (1) and (2) as,
⎛
⎛
⎛ D
⎞
D⎛
d ⎞⎞⎛ D+ S2 + ⎜− S2 ⎟ exp ⎜ −⎜ 1 − exp ⎜⎜ −⎟⎜⎟⎜
τ b ⎜⎝
Dτ b ⎟⎠ ⎟⎠ ⎜⎝ τ b
⎝ τb
⎠
⎝
⎝
τ eff = τ b
⎛
⎛ D
⎞⎛ D
⎞ ⎛ D
⎞⎛ D
⎞
+ S2 ⎟ ⎜+ S1 ⎟ − ⎜− S1 ⎟ ⎜− S2 ⎟ exp ⎜ −
⎜
⎜
⎝ τb
⎠⎝ τb
⎠ ⎝ τb
⎠⎝ τb
⎠
⎝

d ⎞⎞
⎟⎟
Dτ b ⎟⎠ ⎟⎠2d ⎞⎟Dτ b ⎟⎠(3),τeff is equivalent to τb when S1 and S2 are zero.
τeff becomes low in the case of light illumination to the top surface if S1 is very large. Especiallyin the case of very long τb, τeff is approximately given as,
d
(4).
S1
Our experimental accuracy gave a lower limit of τeff as 2 μs. The maximum S1 was therefore
estimated to be 2.6×104 cm/s for a substrate thickness of 525 μm.
On the other hand, a substantial carrier density can remain even if there is a surface with avery high carrier recombination velocity when light is illuminated to the other surface with alow carrier recombination velocity because photo induced carriers alive during travel across thesilicon substrate until reaching the other surface. The lower limit of τeff with a very large S2(dark side) and very long minority carrier bulk lifetime is given as,τ eff ~

2

⎛
D⎛
d ⎞⎞
⎜ 1 − exp ⎜⎜ −⎟⎟ ⎟⎟
τ b ⎜⎝
D
τ
d2
b
⎝
⎠⎠
~
τ eff ~ τ b
⎛⎛ D
⎛
⎞ ⎛ D
⎞
2d ⎞ ⎞ 2 ( D + S1d )+ S1 ⎟ + ⎜− S1 ⎟ exp ⎜ −
⎜⎜
⎟⎟
⎜
⎜⎝ τb
Dτ b ⎟⎠ ⎟⎠⎠ ⎝ τb⎠⎝⎝(5).

When S1 was 501 cm/s, τeff has a lower limit of 23μs for a substrate thickness of 525 μm.
Figure 3 shows average values of the effective carrier lifetime τeff (a) and surface
recombination velocity when τb was assumed to be large enough of 0.01 s (b). τeff for initialsamples was 50 μs in the cases of light illumination to the both surfaces. τeff gave a surface
recombination velocity of 501 cm/s for the both surfaces under the assumption of a very longbulk lifetime of 0.01 s. Ion implantation caused no change in transmissivity for lightillumination to the top surface (ion implantation side). Therefore τeff was less than themeasurement limit of 2 μs. On the other hand, τeff was 8 μs in the case of at light illumination

100228115-4

to the rear surface, which was lower than the lower limit of 23μs for a substrate thickness of
525 μm, as discussed above. τeff of 8 μs suggests that τb was decreased to very low in the top
surface region, and the effective thickness with initial τb decreased to 245 μm according to eq.
(4), although phosphorus atoms were implanted only 100 nm deep. Laser irradiation for 1.5
and 2.5 ms increased τeff to 20 and 24 μs, respectively, in the case of light illumination to therear surface. This result indicates laser
irradiation for 2.5 ms restored τb because τeff
became larger than the lower limit τeff of 23
μs for a thickness of 525 mm. But 1.5 mslaser irradiation was not enough to restore τb.
τeff in the case of light illumination to the top
surface was still low of 4 and 13 μs for laser
irradiation for 1.5 and 2.5 ms, respectively.
The recombination velocity at the top surface
was estimated to be 9.3×103 and 2.5×103
cm/s, respectively. Laser irradiation did notcomplete defect annihilation and the surface
recombination velocity was still high.
Especially sample had residual amorphous
region at the top surface region for sample
laser annealed for 1.5 ms as shown in Fig. 1.
Amorphous states probably play a role ofcarrier recombination.
1.3×106 Pa H2O vapor heat treatment at
270oC for 3 h increased τeff to150 μs in the Fig.3 Average values of the effective carriercases of light illumination to the both lifetime τeff for different processes (a) andsurfaces for sample laser annealed for 2.5 ms surface recombination velocity when τb was
as shown in Fig. 3(a). That value of τeff was assumed to be large enough of 0.01 s (b).
higher than the initial value of 50 μs. H2Ovapor heat treatment markedly decreased thedensity of defect state. The surface
recombination velocity was estimated 307
cm/s if τb was very large of 0.01 s, as shownin Fig. 3(b). On the other hand, H2O vapor
heat treatment did not increase τeff for sample
laser annealed for 1.5 ms as shown in Fig.
3(a). It probably results from thatrecrystallization did not complete, and that
amorphous structure remained.
In
order
to
investigate
carrier+recombination properties of N P junction
formed at the top surface, the SiO2 layer at
the top surface was removed for sample laser
annealed for 2.5 ms. τeff changed to 21 and48 μs, respectively, in the cases of lightillumination to the top and rear surfaces. The
recombination velocity at the top N+P Fig. 4 Changes in surface recombination
junction surface was estimated to be 1500 velocity by removing top SiO2 layer when τb
cm/s using those τeff results and a was assumed to be large enough of 0.01 s.

100228115-5

recombination velocity of 307 cm/s at the rear surface, as shown in Fig. 4. N+P junction
separated hole and electron until compensating the internal voltage formed in the depletion
region. The carrier recombination velocity of PN junction is important value for estimate solar
cell characteristics. The charge separation determines the open circuit voltage. The present
results means that passivation of the top surface of the N+P junction is still important to keep
photo induced carrier in silicon substrate.
Summary

We investigated photo-induced carrier recombination properties for crystalline silicon insteps of N+P junction formation. Phosphorus ions with a concentration of 2×1014 cm-2 were
implanted to 8-9 Ωcm 525 μm thick p-type silicon coated with 100 nm thick thermally grown
SiO2 layers. Phosphorus atoms were incorporated into silicon substrates. The surface region
was partially amorphized from to surface to 140 nm deep region. The implanted surface regions
were annealed by 940 nm infrared semiconductor laser irradiation at 100 kW/cm2 for 1.5 and2.5 ms. Analysis of optical reflectivity spectra at the surface revealed that the surface region
was completely recrystallized by laser annealing for 2.5 ms, while there was slightly residual
amorphous region just at the surface 2 nm deep for laser irradiation for 1.5 ms. 1.3×106 Pa
H2O vapor heat treatment at 260oC for 3 h was also conducted for defect reduction.
Transmissivity of 9.35GHz microwave was measured under illumination of 532 nm green light
4~20 mW/cm2 to the top or rear surfaces. Transmissivity was analyzed by free carrier photoabsorption and carrier diffusion theories. The implantation markedly increased the surface
recombination velocity higher than 2.6 × 104 cm/s, while initial samples had a surface
recombination velocity of 501 cm/s. Moreover, ion implantation decreased the carrier bulk
lifetime from to surface to 245 μm deep region. Laser irradiation for 2.5 ms returned the carrierbulk lifetime to the initial value. However the carrier recombination velocity at the top surface
was still high of 2.5×103 S/cm. 1.3×106 Pa H2O vapor heat treatment at 260oC for 3 hmarkedly increased the minority carrier lifetime to 150 μs and decreased the carrier
recombination velocity to 307 cm/s for the both surfaces. When the SiO2 layer at the top surface
was removed, the carrier recombination velocity increased to 1500 cm/s, which indicated anability of photo-induced carrier separation by N+P junction.
Acknowledgments
This work was partially supported by NEDO foundation P07026.
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