Electrical and optical properties of amorphous silicon films deposited from fluorodisilanes

Solar Cells, 27 (1989) 391 - 401

391

ELECTRICAL AND OPTICAL PROPERTIES OF AMORPHOUS SILICON FILMS DEPOSITED FROM FLUORODISILANES J O H N J. D'ERRICO*, U D O

C. P E R N I S Z and K E N N E T H

G. S H A R P

ElectronicsResearch, D o w Corning Corporation, Midland, M 1 4 8 6 8 6 (U.S.A.)

Summary Hydrofluorinated amorphous silicon films have been deposited from pentafluorodisilane and 1,1,1-trifluorodisilane by chemical vapor deposition. The films and gaseous by-products of the depositions were analyzed by IR spectroscopy. The optical bandgap of the films ranged from 1.85 to 2.87 eV as determined from Tauc plots. The dark conductivity as a function of temperature was measured; room-temperature conductivities ranged from 10 -13 to 10 -l° S cm -1. The films demonstrated high photoconductivity under white light and a small Staebler--Wronski effect characterized by the t -'/3 law.

1. Introduction Hydrofluorinated amorphous silicon films (a-Si:F:H) have previously been deposited from percursor gas mixtures such as SiF2/H2 and SiF4/SiH4 [1, 2] or from SiFxH4-x (x = 1 - 3) [1, 3, 4]. This report describes the results obtained from the deposition of a-Si:F:H films from SiF3SiF2H and SiF3SiH3 under chemical vapor deposition (CVD) conditions. The disilanes used in this study were chosen for several reasons: (1) fluorine and hydrogen are contained in a single molecule, thus eliminating the need for gas mixtures, (2) the presence of an Si-Si bond confers higher reactivity to these, resulting in lower deposition temperatures, and (3) t h e y are m u c h safer to handle than silane or disilane. Characterization of the films included measurement of the dark conductivity as a function of temperature and the photoconductivity under both white light (simulated sunlight) and monochromatic light. The mobility-lifetime product for the photogenerated charge carriers was determined. The degradation of the material under light soaking conditions was also investigated. *Present address: Monsanto Chemical Company, Indian Orchard Plant, 730 Worcester Street, Springfield, MA 01151, U.S.A. 0379-6787/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

392 2. Experimental technique 2.1. Materials and manipulations The preparation of SiF3SiH 3 and SiF3SiF2H has been described elsewhere [5]. All chemical manipulations were conducted on a grease-free highvacuum line (background pressure less than 10 -4 Torr), or in a recirculating nitrogen atmosphere dry box. 2.2. Depositions All depositions were performed under static CVD conditions in 100 cm 3 Pyrex tubes which contained Coming 7059 glass slides and crystalline silicon (100) substrates. The substrates were cleaned with Decontam TM, rinsed with distilled water and dried in vacuo before use. The deposition conditions and film thicknesses (measured with a Tencor Alpha-Step Model 100 profilometer) are listed in Table 1. The tubes were maintained at a particular deposition temperature for 40 rain; reddish golden films resulted from the depositions from SiF3SiF2H and SiF3SiH 3. The coated substrates were stored in a glove box.

TABLE 1 Deposition conditions Film

Precursor

Pressure (kPa (Torr))

Deposition temperature (°C)

Film thickness (nm)

A B C D E

SiFsSiF2H SiF~SiF2H SiF3SiF2H SiF3SiHs SiF3SiH3

13.3 32.0 21.1 9.3 12.7

405 490 350 380 350

50 300 400 70 260

(100) (240) (158) (70) (95)

2.2.1. SiF3SiF2H An IR spectrum of the volatile by-products indicated the presence of approximately equimolar quantities of SiF3H and SiFa [6]. The following IR data refer to the film (cm-1): 2106(m), l 1 7 5 ( s h ) , 1082(s, br), 977(w), 927(m), 896(m), 875(m), 835(m), 633(s). 2.2.2. SiF3SiHs An IR of the gaseous by-products indicated the presence of SiFsH and Sill4, together with small amounts of SiF4 and SiF2H 2. The following IR data refer to the film (cm-1): 2095(m), l 1 6 9 ( s h ) , l l 1 8 ( s , br), 979(w), 926(m), 895(m), 828(s), 636(s, br). A deposition at a relatively high pressure of 25 kPa (188 Tort) resulted in the formation of a brown powder, apparently from vapor nucleation.

393

2. 3. Spectral and electrical characterization Infrared spectra of the gases (10 cm cell, KBr windows, 2 cm -1 resolution) and a-Si:F:H (Si(100) substrates, 8 cm -1 resolution) were recorded on a Nicolet 5SXB Fourier transform IR spectrometer. Optical absorption data of the films (on Coming 7059 glass substrates) were obtained with a Bausch & Lomb Spectronic 2000 spectrophotometer. The spectra were evaluated in a Tauc [7] plot to determine the optical bandgap of the amorphous films. The electrical characterization of the films consisted of measuring the dark and photoconductivity as a function of temperature (between 25 and 180 °C at an average heating and cooling rate of 10 °C min-1). Coplanar carbon electrodes (typically 0.8 cm long, separated by a 0.2 cm gap) were painted onto the surface of the film and used for the conductivity measurements. The measurements were performed under a dry nitrogen purge. F r o m the photoresponse under m o n o c h r o m a t i c light (440 nm, 0.6 mW cm -2) the mobility-lifetime product was calculated. Photoresponse data obtained for weakly absorbed light at 640 nm (at 3.6 mW cm -2) yielded mobility-lifetime products of the same order of magnitude after correcting for the dependence of the photocurrent on irradiance. In a light-soaking experiment, the photoconductivity under irradiation with white light (150 W tungsten-iodine bulb, 10 cm water filter, about 160 mW cm -2 irradiance) was measured periodically over a 5 h period.

3. Results and discussion

3.1. Film deposition Disilanes typically liberate silylenes under CVD conditions [8, 9]. The following reactions demonstrate the possible decomposition routes available to the disilanes: > SiF4 + SiFH

(1)

SiF3SiF2H SiF3H + SiF2 SiF3H + Sill2

SiF3SiH 3

(2) > SiFH3 + SiF2

Infrared spectra were obtained of the gases which resulted from the decomposition of the disilanes. For SiF3SiF2H, approximately equimolar quantities of SiF4 and SiF3H were observed (see eqn. (1)). For SiF3SiH3, a mixture of SiF3H, Sill4, SiF4, and SiF2H 2 was observed. The formation of SiF3H is described in eqn. (2). The other monosilanes can be accounted for by con-

394

sidering that SiFH 3 is very susceptible to redistribution reactions such as eqns. (3) or (4) [10]: 2SiFH3

> Sill4 + SiF2H 2

SiFH~ + SiF3H

(3)

~ Sill4 + SiF4

(4)

The formation of these monosilanes appears to support the reactions proposed in eqns. (1) and (2). However, SiFxH4_x (x = 1 - 4 ) can also be generated from reactions which occur at the substrate surface. The IR spectra of a-Si:F:H deposited from SiF3SiF2H and SiF3SiH 3 were very similar. In both cases, there was an Si-H stretch at ca. 2100 cm-1; absorptions in this region have been associated with Sill2 groups [11]. This assignment is supported by the presence of absorptions at 896 cm -1 which are attributed to the Sill2 scissors mode (Fig. 1). There were also Sill2 rocking modes observed in the 630 - 640 cm -1 range [11]. SiF 2 absorptions were observed in the 9 7 5 - 980, 9 2 5 - 930, and 8 2 5 - 835 c m - ' ranges for the two films; the last two absorptions are assigned to the asymmetric and symmetric absorptions, respectively, of isolated SiF2 groups within the films [11]. The higher-frequency absorptions may be due to terminal SiF2 groups [11 ]. There was no evidence for the presence of--SiF3 groups (1000 - 1015 cm -1) or SiF4 in the films. The incorporation of oxygen, as shown by broad siloxane stretches in the 1000 - 1200 cm -1 region, may have resulted from exposure of these materials to the atmosphere. Oxygen contamination has

o9 ==1 Ioo

t~oo

1ioo

1~oo

§oo

8oo

~oo

8oo

~oo

WRVENUMBER

Fig. 1. I R s p e c t r u m ( 1 3 0 0 - 4 0 0 c m - 1 ) o f a - S i : F : H f i l m d e p o s i t e d f r o m S i F 3 S i H 3 (sample E, T a b l e 1 ). 1100 - 1200 cm-l: Si--O--Si stretch 979,925,828 cm -I : SiF2 stretches 893 cm -1 : Sill2 scissors 638 cm -I : Sill2 rock

395

also been noted in a-Si:F prepared from SiF3SiF3 [12]. Films deposited at relatively low temperatures are most susceptible to atmospheric oxidation. 3.2. Optical characteristics The optical bandgap Eopt was determined from a Tauc [7] plot of the optical absorption data in the visible part of the spectrum (extrapolation to the abscissa of ((xhv) 1/2 vs. hv where e and hv are the absorption coefficient and the p h o t o n energy, respectively). Figure 2 shows data from three different films: sample C derived from SiF3SiF2H, and sample E derived from SiF3SiH 3. Both films were prepared at a substrate temperature of 350 °(3. The optical bandgap for these two films (Table 2) is substantially higher than the typical value of 1.7 eV observed for a-Si: F [ 11 ] deposited under plasmaenhanced chemical vapor deposition (PECVD) conditions. This is probably because of the presence of Si--O bonds in the films (observed by IR) which would significantly increase the optical bandgap [11 ]. The effect of substrate temperature on the optical bandgap is seen in Fig. 2; sample B was also prepared from SiF3SiF2H, but at a temperature of 490 °(3. This decrease in optical bandgap with increasing deposition temperature could be a result of decreased oxygen uptake u p o n exposure of the film to the atmosphere. Similar results have been observed with a-Si:F [13]. It is noted that the slope (the inverse of which is a measure of the disorder parameter of amorphous materials [14]) is nearly the same for the three films in the Tauc plot, ranging from 2.0 X 10 -3 to 2.3 X 10 -3 eV z/2 cm 1/2. This indicates that the width of the tail states is approximately the same for films prepared from the two different precursors, and at different substrate temperatures. Typical values for a-Si:F:H prepared by PECVD

1500 2,87 eV 2.15 eV

E

/

o o o 1.85 eV

c

B

1000

500

J

¢r

oo~o# ° P

oI . O

2'

g

t

/2 . 0 Photon E n e r g y

3.

4.0 hv

[eV]

Fig. 2. Optical bandgap determination from optical absorption data (Tauc plot [16]). Samples are indicated in the figure. Compare with data in Tables 1 and 2.

396 TABLE 2 Dark conductivity and photoconductivity and for 100 m W cm -2 irradiance)

Film Precursor

A B

SiF3SiF2H

C D E

SiF3SiH3

d a t a o f a - S i : F : H t h i n f i l m s ( e v a l u a t e d at 30 °C

Dark conductivity

Activation Photoenergy, conductivity,

Odk (S cm -1)

Ea (eV)

Oph (S cm -1)

2.4 x 10 -1° 0.74 6.3 x 10 -13 0.95

6.5 × 10 - s 3.5 x 1 0 -8

7.7 × 1 0 -13 0.90

2.3 x 1 0 -11

3.4 × 10 -12 0.94

4.7 × 10 -8 3.1 x 10 -8 1.8 x 10 -9

< 1 x 10 - i s

1.17

Exponent of Mobilityirradiation lifetime, dependence, ~ ~pT a 0.87 0.88 1.01 b

Optical bandgap,

(cm 2 V -1)

Eop t (eV)

1.2 x 10 `-9 2.8 x 1 0 -1°

1.85 2.87

0.94 1.0 c 0.96

5.9 x 10 -11 9.2 x 10 12 2.15

a F o r m o n o c h r o m a t i c light (440 n m ) at 0.6 mW cm -2. b A f t e r light soaking for 6 h with white light at 160 mW cm -2. CAfter light soaking for 3 min.

from difluorosilane, SiF2H2, are between 1.2 X 10 -3 and 1.6 X 10 -3 eV 1/2 cm I n [15].

3.3. Electrical conductivity The Arrhenius plot of the dark conductivity for all films yielded a straight line corresponding to activation energy Ea over the temperature range investigated. Due to the low dark conductivity of these materials, the measurement at r o o m temperature was usually obscured b y noise. Accordingly, the values reported at 30 °C in Table 2 were extrapolated from the least-squares fit to the high-temperature data where necessary. Figure 3 shows an Arrhenius plot of b o t h dark photoconductivity and p h o t o c o n d u c t i v i t y obtained b y switching the light source on for brief periods while cooling the sample down from 187 °C (sample B, from SiFaSiF2H, white light, 160 mW cm-2). The photogeneration rate for charge carriers is constant and much larger than the thermal generation rate. Therefore, the slope observed for the photoconductivity pertains to the activation energy for charge carrier transport (mobility); the value found is 0.146 eV. This activation energy is due to trapping in the shallow states below the conduction band edge [16] and describes the width of these states. Comparable values (between 0.17 and 0.19 eV) have been f o u n d for a-Si:H deposited from silane b y PECVD [17 ]. The photoresponse of the a-Si:F:H films in monochromatic light was measured for strongly absorbed light at 440 nm (2.82 eV). The values of the mobility-lifetime product, H/aT, of the photogenerated charge carriers calculated from these data are listed in Table 2. The internal quantum efficiency

397 -5

\

wht. light O. 14E] eV

\ \ o 0

sample B

\\ dark

0.95 eV -13 1.75

2.25

reciprocal

2.75

t e m p e r a t u r e IO00/T

3.25

It/K]

Fig. 3. Dark conductivity and photoconductivity (sample B) as a function of temperature (Arrhenius plot). White light irradiance 165 mW cm-2. Values of activation energies given in figure.

77 was n o t separately determined as it has been f o u n d to be generally close to unity [18]. The irradiance dependence of the p h o t o c o n d u c t i v i t y was determined for white light, to scale the p h o t o c o n d u c t i v i t y data to a reference irradiation of 100 mW cm -2. The p h o t o c o n d u c t i v i t y o b e y s a power law of the form o B ~ where B is the irradiance. The e x p o n e n t determined for white light holds approximately for monochromatic radiation as well; the range of slopes is from 0.79 (440 nm) to 0.81 (640 nm) to 0.88 (as determined for sample B). Table 2 gives values of the e x p o n e n t ~ for white light. The 7#7 product can then be scaled to different values o f irradiation according to ~//~T cc 1/B (1-~). Characteristic values of the mobility-lifetime product for a-Si:F:H prepared from difluorosilane b y PECVD [4] fall b e t w e e n 10 -9 and 10 -s cm 2 V -1 s-1 and correlate roughly with the dark conductivity. The low values observed here are ascribed to the oxygen f o u n d in the material. The presence of Si--O--Si would n o t only result in a large optical bandgap b u t also increase the density o f recombination centers in the a-Si:F:H material, thus reducing the charge carrier lifetime. The exponents found for the irradiance dependence of the photoconductivity fall at the high end of a range of values which is predicted [19] from the position of the Fermi level. It is interesting to note that the exp o n e n t initially f o u n d for sample D increased to 1.0 after irradiation for only 2.5 min (at 160 mW cm-2), with an associated decrease of the photoconductivity. This behavior is compatible with the photogeneration [20] of mid-gap defects, otherwise k n o w n as the Staebler-Wronski effect [21].

398

3.4. P h o t o d e g r a d a t i o n

U p o n exposure to white light at an irradiance level of 160 mW cm -2 (light soaking) the a-Si:F:H samples showed only a small amount of photodegradation. The degradation stabilized after a b o u t 100 min of light soaking at values between 0.75 and 0.20 of the initial conductivity. No decrease in the dark conductivity after the light-soaking period was observed. However, the exponent of the irradiance dependence of the photoconductivity for white light increased from 0.88 to 1.01 at the end of the light-soaking experiment. These results are similar to those reported for other a-Si:F:H films [2], and are in contrast to a loss of photoconductivity and dark conductivity of several orders of magnitude for typical a-Si:H [21]. The degradation of the p h o t o c o n d u c t i v i t y as a function of time is plotted in Figs. 4(a) and 4(b) for samples B (from SiF3SiF2H ) and E (from SiF3SiH3), respectively. Following the t -1/3 model that was suggested for the kinetics of the StaeblerWronski effect (long-time limit) in a-Si:H [20], the quantity 1/aph(0) 3 1/Oph(t) 3 is plotted vs. time t where 1/o(0) is the initial value of the photoconductivity. After an initial small b u t rapid decay of less than 3 min duraa) 0.6

j

.,,"

,

W

W

0.4~

/

g

0.2

÷

O.C 1000

60

I20

180

/

b} 600

g 4o0

~ 200 oi

/

300 #

/

800

8

240

// 60

/ 120

light soak time

180

t

240

300

[min]

Fig. 4. D e g r a d a t i o n o f photoconductivity under white light soaking conditions. The data are p l o t t e d as F ( o ) = 10S/Oph(0)] 3 - [10S/Oph(t)] 3 vs. t i m e t w h e r e 1 / o ( 0 ) is the initial value o f the photoconductivity in S c m -1. T i m e t is p l o t t e d in m i n . (a) S a m p l e B. •...... , Initial rapid d e c a y ; ++++, e x p o n e n t i a l d e c a y ; o o o o , t -1/3 d e c a y law; * * * * , saturation. ( b ) S a m p l e E. T h e b r o k e n line indicates interruption of light soaking ( d a r k ) f o r 16 h, d u r i n g w h i c h t i m e s o m e r e c o v e r y o f degradation occurred. Time beyond dark p e r i o d is s h i f t e d b y 15 rain.

399

tion (values plotted as dots), sample B showed exponential decay for another 15 min (plotted with + symbols), and then o b e y e d the t - 1 / 3 law. After a b o u t 90 min, the photocurrent reached a constant value that was maintained for several hours. In non-fluorinated a-Si, saturation of the defect density has been observed ( b y subgap optical absorption) after a b o u t 110 h of light soaking [22] or after a b o u t 2 h at 100 °C [23]. In the a-Si:F:H films saturation is observed within 2 h at r o o m temperature. There are two mechanisms which could explain the saturation in the photodegradation process. (1) An equilibrium b e t w e e n generation and annealing of defects is reached. This possibility is unlikely at r o o m temperature, where annealing is not observed. (2) The initial density of Si--Si bonds that are sufficiently weak to be broken b y recombination of photogenerated charge carriers is so low that t h e y are rapidly consumed. As the formation of metastable dangling bonds is self-limiting [20] to densities typically below 1017 cm -3, it may be concluded that the initial density of weak Si--Si bonds is well below this value. The weak Si--Si bonds could have been replaced b y bonds to fluorine or oxygen b e y o n d what is seen in a-Si:H, where t h e y have an intrinsic density [20] b e t w e e n 10 is and 1019 cm -3. The stabilizing effect of oxygen on a-Si:H (with respect to photodegradation) has been previously reported [24]. Sample E does not go through an initial exponential phase b u t shows the t - 1 / 3 law directly after the rapid initial drop of conductivity (not visible on the scale used in Fig. 4(b)). For this sample the light soaking was interrupted b y a 16 h dark period, as indicated b y the broken abscissa. The degradation resumed with the same slope and continued for another 60 min. (The subsequent decrease in degradation rate could indicate the onset of saturation.) The slopes that describe the photodegradation of the two samples B and E are 3.8 × 1019 and 7.8 × 1022 cm 3 V A -3 s-1 (the different rates reflect the different levels of the initial photoconductivity). From the observed p h o t o c o n d u c t i v i t y data the ratio of the (initial) density of recombination centers Nr and the Staebler-Wronski coefficient Csw (which describes the probability of formation of metastable defects per tail-to-tail recombination transition [20]) can be determined as 9.1 × 102s and 2.6 × 1024 cm -s, respectively. Assuming that the second photodegradation process discussed above applies, an upper limit for Csw in sample B where saturation was observed can be calculated: Csw = 1.1 × 10 -7. This value is nearly three orders of magnitude lower than typical values determined for a-Si:H, which indicates a lower probability of photoinduced defect formation in this material. 4. Conclusions a-Si:H films were deposited from SiF3SiF2H and SiF3SiH 3 under static CVD conditions; temperatures as low as 350 °C were suitable. The electrical

400 p r o p e r t i e s o f t h e s e films w e r e c o m p a r a b l e w i t h t h o s e d e p o s i t e d f r o m p r e c u r s o r gas m i x t u r e such as SiF2/H 2. I R analysis o f t h e films i n d i c a t e d t h e p r e s e n c e o f Sill2, S i F : , a n d S i - - O - - S i g r o u p s w i t h i n t h e film. T h e last p r o b a b l y arose f r o m e x p o s u r e o f t h e films t o t h e a t m o s p h e r e ; this b e h a v i o r s e e m s especially p r e v a l e n t f o r films d e p o s i t e d at relatively low t e m p e r a t u r e . T h e o p t i c a l b a n d g a p o f t h e films w a s d e t e r m i n e d f r o m T a u c p l o t s o f t h e o p t i c a l a b s o r p t i o n d a t a a n d r a n g e d f r o m 1.85 t o 2.87 eV; t h e s e relatively large values are also c o n s i s t e n t w i t h t h e p r e s e n c e o f o x y g e n . T h e A r r h e n i u s p l o t o f t h e d a r k c o n d u c t i v i t y w a s linear w i t h a c t i v a t i o n energies ranging f r o m 0 . 7 4 to 1.17 eV. T h e r a t i o o f p h o t o c o n d u c t i v i t y ( w h i t e light) to d a r k c o n d u c t i v i t y r a n g e d f r o m 102 to 10 s. T h e d e g r a d a t i o n o f t h e p h o t o c o n d u c t i v i t y was f o u n d to o b e y t h e t -1/3 law p r o p o s e d f o r t h e kinetics of the photogeneration of metastable defects (Staebler-Wronski e f f e c t ) in a m o r p h o u s s e m i c o n d u c t o r s . T h e s a t u r a t i o n o f d e g r a d a t i o n observed in o n e s a m p l e is indicative o f a low initial d e n s i t y o f w e a k Si--Si b o n d s in this a - S i : F : H m a t e r i a l .

Acknowledgments This w o r k w a s s u p p o r t e d in p a r t b y t h e D e p a r t m e n t o f E n e r g y t h r o u g h t h e Solar E n e r g y R e s e a r c h I n s t i t u t e u n d e r S u b c o n t r a c t Z L - 5 - 0 4 0 7 4 - 6 . H e l p f u l discussions w i t h L. T a r h a y a n d t e c h n i c a l s u p p o r t b y J. Seifferly are g r a t e f u l l y a c k n o w l e d g e d .

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