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

Characterization of SiOx/Si Interface Properties by Photo Induced Carrier
Microwave Absorption Method
M. Hasumi, J. Takenezawa, Y. Kanda, T. Nagao and T. Sameshima
Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo
184-8588, Japan
E-mail : mhasumi@cc.tuat.ac.jp
Keywords: free carrier absorption, photo induced carrier, effective lifetime, interface properties,
H2O vapor treatment

Abstract. We investigated photo induced carrier density and effective minority carrier lifetimeby a 9.35 GHz microwave free carrier absorption caused by 532 nm light induced photo carriers
for crystalline silicon samples coated with vacuum evaporated SiOx layers and spin-coatedpolysilazane (-(SiH2NH)-) layers. In the case of 55 nm thick SiOx film evaporated on n-typesilicon substrate, the photo induced carrier density and the effective minority carrier lifetime
were increased from 1.5 x 1012 to 1.6 x 1013 cm-2 and from 31 to 350 μs, respectively by 1.2 x
106 Pa H2O vapor heat treatment at 260oC for 1 h under the light illumination at 20.8 mW/cm2
because of passivation of SiOx/Si interfaces. High pressure H2O vapor treatment was more
effective in decrease the density of carrier recombination sites for hole minority carriers. Fromthe investigation of polysirazane coated samples, we confirmed that our microwave
measurement system could detect small photo carrier density on the order of 1010 cm-2 andsmall effective minority carrier lifetime on the order of 10-6 s.
Introduction
Defect passivation has an important role for improvement in device characteristics
especially for silicon solar cells and thin film transistors. Low temperature fabrication process
for the passivation films is required especially for the devices on glass or plastic substrates.
Therefore, non destructive and non contact measurement method of interface electricalproperties is attractive as monitoring samples during the device fabrication. Analysis of photo
induced carrier behavior is also important for developing photovoltaic devices. Photo induced
minority carrier lifetime is one of the most important characteristics. Measurements of
microwave photo conductive decay (μ-PCD) [1] and quasi steady state photoconductance [2]have been widely used for the measurement of the photo induced minority carrier lifetime. We
recently have proposed that the microwave free carrier absorption effect is attractive for theinvestigation of photo induced minority carrier properties [3, 4].
In this paper, we report a precise investigation of photo induced carrier properties in bothn-type and p-type crystalline silicon samples coated with thermally grown SiO2 layers as wellas SiOx layers fabricated by low temperature processes. Photo induced carrier density and its
spatial distribution depend on the bulk and interface properties. Defects induce carrier
recombination and reduce the density of photo induced carriers. We demonstrate quantitativeevaluation of carrier recombination properties using photo induced carrier microwave
absorption measurement. We also discuss changes in the photo induced carrier density and the100228114-1

effective minority carrier lifetime induced by surface passivation by high pressure H2O vapor
heat treatment [5].
Experimental
In order to investigate SiOx/Si interface properties on photo induced carriers, both n-type
and p-type single crystalline silicon wafers were prepared. Carrier concentration of n-typesilicon wafer was 2.8 x 1014 cm-3 and that of p-type wafer was 1.4 x 1015 cm-3. The thicknessesof both types of wafers were 525 μm. The both surfaces of silicon wafer were coated with 100
nm thick thermally grown SiO2 layers at first. The silicon wafers coated with as-thermallygrown SiO2 layers were used as reference.
SiOx films were deposited at room temperature on the top surface of silicon wafers by thevacuum evaporation of powdered SiO after removing the thermally grown SiO2 layer by
buffered HF solution at the top surface and still keeping the thermally grown SiO2 layer at therear surface. Heat treatment in 1.2 x 106 Pa H2O vapor at 260oC for 1h was also applied to thesamples [5].
Polysilazane (-(SiH2NH)-) films were coated on the top surface of silicon wafers after
removing the top surface of thermally grown SiO2 layer. Thermally grown SiO2 layer at the rearsurface was still keeping. Heat treatments in 1.2 x 106 Pa H2O vapor at 260oC for 1h and 6h
were also applied to the samples.
Optical reflectivity spectra in ultraviolet and visible range were measured and analyzed forinvestigating film thickness and optical properties of films. Thickness of SiOx and polysilazane
films were evaluated 55 nm and 180 nm, respectively by the spectral fitting of opticalreflectivity.
Figure 1 shows a schematic of photo induced carrier microwave absorption measurement
system. The 9.35 GHz microwave was emitted by a field effect transistor type oscillator andthat was introduced using a waveguide tube. There was a 1 mm gap for measurement of sample
wafers. A thin light scattering plate was inserted into the gap facing to the top surface ofsamples as shown in Fig. 1. A laser light at 532 nm was introduced using optical fibers to thelight scattering plate, which gave uniform illumination of the green light to the samples. The
intensity of the laser light was modulated at a frequency of 1 Hz (sufficiently slower than thephoto carrier generation and recombination) and controlled from 0 to 20.8 mW/cm2 at thesurface. The intensity of microwave transmitted through the sample was measured by Agilent
U2000 power sensor. The transmissivity of sample T was obtained by the ratio of intensities of
microwave with the sample Is to without sample Iair, as T = Is / Iair. A finite element numericalcalculation including Fresnel optical interference effect induced by in-depth change in
1 Hz Modulation

Optical fiber

YAG-Laser
532 nmLight scattering plate
Waveguide

Filter
Detector

Source9.35 GHz Microwave

SampleAgilent U2000
Power Sensor

Fig. 1 Schematic diagram of photo induced carrier microwave absorption measurement
system.

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refractive index owing to photo induced free carrier diffusion was constructed to estimate thedensity of free carriers from the experimental transmissivity [6, 7].
Results and Discussion
Figure 2 shows the microwave transmissivity as a function of the intensity of 532 nm lightillumination for n-type samples (a) and p-type samples (b). Solid circles and squares show thetransmissivity for the samples with the top surface coated with as-deposited SiOx layers, andsamples subsequently treated with treated with 1.2 x 106 Pa H2O vapor at 260oC for 1h,
respectively. Open circles show the transmissivity for the sample with both surfaces coated withthermally grown SiO2 layers as reference. Transmissivity without light illumination wereranging from 24.0 to 24.5 % for the n-type samples and from 13.8 to 15.2 % for the p-typesamples, respectively. The difference of transmissivity resulted in the carrier concentration
ranging from 2.8 x 1014 to 2.9 x 1014 cm-3 for the n-type samples and from 1.4 x 1015 to 1.5 x1015 cm-3 for the p-type samples, respectively.
The transmissivity monotonously decreased as the light intensity increased from 0 to 20.8
mW/cm2 for every sample, because of free carrier absorption by photo induced carriers. The
n-type silicon sample with SiOx layer on the top surface annealed with high pressure H2O vapor
for 1h showed the highest decrease in transmissivity from 24.5 to 9.6 % as the light intensity
increased from 0 to 20.8 mW/cm2, while the n-type silicon sample with as-deposited SiOx layeron the top surface showed the decrease in transmissivity from 24.2 to 21.9 %, as shown in Fig.
2(a). On the other hand, the p-type silicon sample with SiOx layer on the top surface annealedwith high pressure H2O vapor for 1h showed decrease in transmissivity from 15.2 to 12.8 % as
the light intensity increased from 0 to 20.8 mW/cm2, while the p-type silicon sample with
as-deposited SiOx layer on the top surface showed the lowest decrease in transmissivity from
14.7 to 14.4 %, as shown in Fig. 2(b).
The density of photo induced minority carriers per unit area was obtained by analysis of theexperimental transmissivity using the free carrier absorption theory. Figure 3 shows thedensities of photo induced minority carriers per unit area as a function of the intensity of 532
nm light illumination for n-type samples (a) and p-type samples (b). Photo induced minority

Fig. 2 Transmissivity as a function of the intensity of 532 nm light illumination for n-type (a)and p-type (b) crystalline silicon samples coated with thermally grown SiO2 (open circles) andas-deposited SiOx layers on the top surface (solid circles) and annealed with high-pressure
H2O vapor for 1h (solid squares).

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carriers increased as the intensity of light illumination increased for every sample. Both n-type
and p-type samples with as-deposited SiOx layer on the top surface show the low photo induced
carrier density. The n-type silicon sample with SiOx layer on the top surface annealed with highpressure H2O vapor for 1h had the highest density of photo induced carriers of 1.6 x 1013 cm-2
at 20.8 mW/cm2. It was larger than the density of photo induced carriers for the sample withboth surfaces coated with thermally grown SiO2 (1.2 x 1013 cm-2 at 20.8 mW/cm2). This result
indicates that the extensive defect passivation was achieved for the SiOx deposited sample by
the high pressure H2O vapor heat treatment. The p-type silicon samples with as-deposited SiOx
layer on the top surface also shows increase in photo induced carrier density by the highpressure H2O vapor annealing for 1h. It was also higher that for sample coated with thermallygrown SiO2 layers. However the photo induced carrier density increased to 3.7 x 1012 cm-2 atmost at 20.8 mW/cm2, which was lower than that for n-type samples. The origin of thedifference of photo induced carrier density between n-type samples and p-type samples is
supposed to be the difference of effective minority carrier lifetime.

Fig. 3 Density of photo induced minority carriers per unit area as a function of the intensity of
532 nm light illumination for n-type samples (a) and p-type samples (b).
Figure 4 shows the effective minority carrier lifetime τeff as a function of the intensity of
532 nm light illumination for n-type samples (a) and p-type samples (b). The lowest τeff at
around 6.0 μs was obtained for the p-type silicon with as-deposited SiOx layer on the top
surface. It indicated the as-deposited SiOx/Si interface had substantial defect states that reduced
τeff. τeff was increased by high pressure H2O vapor heat treatment because of the passivation of
SiOx/Si interfaces. The samples H2O vapor heat treated had the highest τeff ranging from 350 to660 μs for the n-type silicon and ranging from 73 to 87 μs for the p-type silicon under the lightintensity ranging from 4.1 to 20.8 mW/cm2. These τeff were higher than that of samples bothside coated with thermally grown SiO2 and showed the profitability of high pressure H2O vapor
heat treatment.
In the case of as-deposited SiOx samples, τeff was small because of high surface
recombination velocity S due to high density of defects states at the top surface. In the case ofvery long minority bulk carrier lifetime and very small recombination velocity at the rearsurface, τeff is approximately given as τeff = d / S, where d is the substrate thickness and S is thesurface recombination velocity at the front surface [8]. The ratio of τeff on p-type and n-typesamples directly gives the ratio of S, as τeff-e/τeff-h = Sh / Se, where h and e of subscript expressedhole and electron (minority carrier), respectively. Experimental Sh / Se ranged from 0.17 to 0.27,
which depended on the light intensity for as-deposited SiOx samples. This means thatrecombination of electron minority carriers seriously occurred. On the other hand, it ranged

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from 0.11 to 0.24 after the high pressure H2O vapor heat treatment. The results of Fig. 4 clearlyshow that high pressure H2O vapor heat treatment effectively decreased the density of carrier
recombination sites and increased the effective minority carrier lifetime for both n-type andp-type samples. Moreover, slight decrease in the ratio of the minority carrier surface
recombination velocity given above indicate that high pressure H2O vapor heat treatment ismore effective in decrease the density of carrier recombination sites for hole minority carriers.

Fig. 4 Effective minority carrier lifetime analyzed from the results of Fig. 2 as a function of theintensity of 532 nm light illumination for n-type samples (a) and p-type samples (b).
Figure 5 shows (a) transmissivity, (b) density of photo induced minority carriers per unit
area and (c) effective minority carrier lifetime as a function of the intensity of 532 nm lightillumination for n-type silicon samples coated with polysilazane (-(SiH2NH)-) on the top
surface after removing the thermally grown SiO2 layer. Decreases in transmissivity as the lightintensity increased from 0 to 20.8 mW/cm2 for the samples of as-coated and H2O vapor heat
treated for 1 and 6 h were 0.2, 0.6 and 1.0%, respectively. The density of photo induced
minority carriers per unit area obtained by analysis of fig. 5(a) increased as the intensity of lightillumination increased for every sample, as shown in fig. 5(b). The highest density of photo
induced carriers of 6.0 x 1011 cm-2 and effective minority carrier lifetime of 17 μs at 20.8
mW/cm2 was obtained for the sample H2O vapor heat treated for 6 h. These results indicatedFig.5 Transmissivity (a), density of photo
induced minority carriers per unit area (b)and effective minority carrier lifetime (c) as
a function of the intensity of 532 nm lightillumination for the n-type silicon samples
coated with polysilazane and heat treatedwith high-pressure H2O vapor for 1 and 6 h.

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that the interface between polysilazane and silicon was passivated by high pressure H2O vapor
heat treatment, though the improvement was smaller than the SiOx coated samples. We
confirmed that our microwave measurement system could detect small photo carrier density on
the order of 1010 cm-2 and small effective minority carrier lifetime on the order of 10-6 s fromthe analysis of polysilazane coated samples. This method has high sensitivity compared withthe conventional μ-PCD. It is because our method measures the quasi static density of photo
induced carrier not the decay of carrier density.
Summary
We investigated photo induced carrier recombination properties for crystalline siliconcoated thermally grown SiO2 layers, vacuum evaporated SiOx layers and spin-coatedpolysilazane layers by the transmissivity measurements of 9.35 GHz microwave underillumination of 532 nm green light to the top surface. Photo induced carrier density andeffective minority carrier lifetime were analyzed by free carrier photo absorption and carrier
diffusion theories.
In the case of 55 nm thick SiOx film evaporated on n-type silicon substrate, the photo
induced carrier density and the effective minority carrier lifetime under the light illumination at
20.8 mW/cm2 were increased from 1.5 x 1012 cm-2 to 1.6 x 1013 cm-2 and from 31 μs to 350 μs,
respectively by 1.2 x 106 Pa H2O vapor heat treatment at 260oC for 1h. The minority carrier
lifetime after the H2O vapor heat treatment exceeded that of sample both side coated withthermally grown SiO2. This result showed the profitability of high pressure H2O vapor heat
treatment to reduce the carrier recombination defects.
From the result of polysirazane film coated on silicon substrate, we confirmed that our
microwave measurement system could detect small photo carrier density on the order of 1010
cm-2 and small effective minority carrier lifetime on the order of 10-6 s.
Acknowledgments
The authors thank Ms. M. Kimura and Mr. M. Shimokawa for their support. This work waspartially supported by New Energy and Industrial Technology Development Organization(NEDO) as part of the Innovative Solar Cells R&D Program P07026.
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