Electronic and optical properties of boron doped hydrogenated amorphous silicon thin films

Solar Energy Materials 10 (1984) 335-347 North-Holland, Amsterdam

335

E L E C T R O N I C A N D OPTICAL P R O P E R T I E S OF B O R O N D O P E D HYDROGENATED AMORPHOUS SILICON THIN FILMS Swati RAY, P. C H A U D H U R I , A.K. B A T A B Y A L and A.K. B A R U A Indian Association for the Cultivation of Science, Calcutta - 700032, India

Received in revised form 8 March 1984 The electronic and optical properties of p-type a-Si : H films prepared by the rf glow discharge decomposition of a mixture of silane and diborane gases have been studied. The films have been prepared under different conditions which include variation of volume ratio of B2H 6 and Sill 4 and rf power. The properties of compensated a-Si : H films prepared with very small boron doping have also been studied. The effects of mixing Ar with Sill 4 + B2H6 mixture have been investigated. The properties actually studied include (1) dark conductivity, (2) steady state photoconductivity, (3) spectral response, (4) optical absorption and band gap. Attempts have been made to analyse the data to yield information about the transport mechanism, and the existence of hole and electron traps.

1. Introduction In recent years rf glow discharge produced a-Si : H films have proved to be a very promising material for the fabrication of solar cells. This has led to considerable a m o u n t of work on the characterization of this material [1]. As p h o p h o r u s and b o r o n d o p e d a-Si : H films are used, respectively, as the n- and p-layers for the solar cells their detailed characterization is also essential. C o m p a r e d to the n-type a-Si : H films less work has been done on p-type films [2-5]. The p-type films prepared by the usual method have some properties which stand in the way of improving the efficiency of p - i - n structure a-Si solar cells. It is therefore essential to study the properties of these films which will lead to a better understanding of the material. In this paper we report the results of our studies on the electronic and optical properties of p-type a-Si : H films prepared by rf glow discharge decomposition of a mixture of silane and diborane gases. The B 2 H 6 / S i H 4 volume ratio and rf power have been varied. The effect of using A r as a diluent has also been studied. The concentration of B2H 6 has been lowered to the extent that compensated a - S i : H films have been produced. The actual properties studied include: (1) dark conductivity, (2) steady state photoconductivity, (3) optical absorption from which b a n d gaps have been obtained and (4) spectral response. Attempts have been made to obtain information about the transport mechanism in these films from the dark conductivity data. F r o m the photoconductivity and spectral response data the possibility of existence of hole and electron traps has been analysed. 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

336

S. Ray et al. / Properties of boron doped a - S i : H thin films

2. Experimental

The apparatus for the preparation of a - S i : H films by the rf glow discharge decomposition of silane gas has been described in a previous paper [6]. The p-type a - S i : H films were prepared on Coming 7059 glass substrate from a mixture of diborane and silane gases. The films were prepared at different volume ratios of diborane and silane (B2H6/SiH4) the range of which varies from 1 × 10 6 to 5 × 10 2. The gases used were of electronic grade supplied by M / s Air Products (UK). The substrate temperature was kept at 300 ° C and the rf power to the coil was varied from 2 to 30 W. The thickness of the film was measured by a stylus type instrument (Planer Products, U K ) and the thicknesses were in the range 3000-4000 A. The rate of deposition varied from 17 to 56 .A/min. The dark and photoconductivity of the films were measured using gap cell geometry under a vacuum of --- 10 -5 Torr and a Keithley 610 C Electrometer was used for the measurements. Before measurements the films had been annealed at 150°C for one hour under vacuum in darkness in order to eliminate the effect of light and adsorbed gases. Electric field applied to the Al-electrodes was 700 V / c m and photoconductivity was studied under white light of intensity 20 m W / c m 2. The intensity dependence of the photoconductivity was measured by using radiation of wavelength 5600 A from a monochromator (Jobin Yvon, France) and neutral density filters. For studying the spectral response the intensities at different wavelengths were measured by a calibrated silicon photodiode (EG and G, USA). The absorption coefficients and band gaps were studied with a Cary 17D double beam spectrophotometer.

3. Results and discussions

3.1. Dark conductivity The room temperature (300 K) dark conductivity of a-Si : H films prepared at an rf power of 10 W have been plotted in fig. 1 as function of the volume ratio of B2H 6 and Sill 4. The dark conductivity o D of the undoped a-Si : H film as obtained by us is 2.09 × 10-9 ~-~-i cm-1 (not shown in the figure). It is seen from the figure that with very small boron doping (up to B2H6/SiH 4 ~< 1 × 10 -5) the o D value decreases. This is due to the fact that the undoped a - S i : H film is n-type and a small amount of boron doping produces compensated a-Si : H. Room temperature o D decreases up to 3.63 × 10 -12 f l - i cm-1 as the B2H6/SiH 4 ratio increases to 1 × 10 -5. With further increase in doping concentration o D increases up to 2.75 x 10 -3 ~2-~cm -1 at a B2H6/SiH 4 ratio of 5 x 10 -2. The nature of the log o D vs. B2H6/SiH 4 curve is similar to that obtained by Jan et al. [4]. Jan et al. have obtained best compensation with B2H6/SiH 4 ratio = 1 × 10 -6 whereas w e have obtained it with a value - 1 × 1 0 -s. In fig. 2, log o D values are plotted against 1/T. The activation energy AE obtained from these plots are given in table 1. For the undoped a-Si : H film AE is

S.

Ray et al. / Properties of boron doped a- Si: H thin films

337

I0 -2 ,

10 - 3

10 -~

10-~ tE

10-6

o

7~ 10-';

%

to10-~

1(y~° 10-11 l(y t2 10-1

t 10-2

t 10-~

I 10-4

I 10-5

I 10-6

B2H6/$iH4 Fig. 1. Variation of room temperature dark conductivity (OD) and photoconductivity (oph) of gas flow ratio of B2H 6 to that of Sill 4.

as

a function

0.81 eV. With initial diborane doping AE increases up to 0.84 eV which is due to the compensation effect of boron doping resulting in a lowering of the Fermi energy E F towards the center of the gap. With the further increase in the diborane concentration, hole conduction sets in and the specimen is p-type. As has been reported by other workers [2,4] AE decreases with increasing boron concentration. For lightly doped films (B2H6/SiH4~< 1 × 10 -3) the o D vs. 1/T plots show single slopes throughout the temperature range of our measurements apart from kinks at high temperature, the origin of which has been discussed in a previous paper [6]. The activation energies are in the range 0.84 to 0.50 eV which indicate that the electrical transport occurs only through extended states. At higher doping concentrations (curves 6, 7 and 8 of fig. 2) the log o D vs. 1/T plots cannot be fitted by a single activation energy. The data may be interpreted in terms of a two channel model. At comparatively high boron doping an acceptor band is likely to be introduced below the Fermi level and near the valence band [4]. Two parallel transport paths now have to be considered viz: (1) extended state conduction at valence band and (2) hopping conduction in the acceptor band at a mean energy E A of the acceptor band. The

338

S. Ray et al. / Properties of boron doped a- Si." H thin films 100

162

10-4

6 .--.

'8 v~ 16'

16 8

1(~t¢

10-1",

zo

%

41o !~( K-1)

51o

6,o

Fig. 2. Temperature dependence of the dark conductivity for boron doped samples of a-Si : H. Curves 1, 2, 3, 4, 5, 6, 7 and 8 correspond to I~ H 6 / S i H 4 ratio 0.0, 1.0× 10 -6, 1 × 10 -5, 1 x 10 -4, 1 × 10 -3, 3 × 10-3~ 1 x 10 .2 and 2.5 x 10 -2, respectively.

Table 1 Activation energies and band gaps of intrinsic and boron doped a-Si : H films B2 H 6

Activation energy

Sill 4

A E 1 (eV)

0.0 1 ×10-6 1 ×10 -5 1 x 1 0 -4 1 ×10 .3 3 x 10 .3 1 ×10 -2 2.5 x 10 -2

0,81 0.83 0.84 0.73 0.50 0.45 0.37 0.28

Band gap A E2 (eV)

(eV)

0.43 0.34 0.20

1,75 1,75 1.75 1,75 1,74 1,61 1.44 1,39

S. Ray et al. / Properties of boron doped a-Si: H thin films

339

process (1) may be represented as, o, = o0,exp[ - a E t / k r l

(1)

with AE 1 = ( E v - E v ) 0 and process (2) as, °2 = °02exp[ - A E 2 / k T I

(2)

with AE 2 = ( E v - EA) 0 + AW, where AW is the energy for hopping [7]. The values of AE 1 and AE 2 are shown in table 1. The extended state conduction predominates above 250 K and at lower temperatures the other transport path predominates. It is seen that the deposition rate increases with the increase in diborane concentration. The higher rate of deposition obtained by using a mixture of diborane and silane has been reported earlier by Street et al. [8]. We have studied the effect of variation of rf power (supplied to the coil) on the dark conductivity of p-type a-Si : H prepared with ~ H 6 / S i H 4 ratios of 1 x 10 -3 and 1 × 10 -2. The increase of rf power is likely to have two effects viz: (1) increase of bonded hydrogen concentration in a-Si: H films and (2) the increase of doping efficiency. The effects of (1) and (2) will have opposite influence on o o values i.e. increase of bonded hydrogen concentration will decrease or) and increase of doping efficiency will increase or). For the films prepared with BzH6/SiH 4 ratio of 1 x 10 -3 , Or) decreases as rf power increases from 2 to 10 W as effect (1) is predominant. The results together with the activation energies are given in table 2. This result is in agreement with that reported previously for n-type films [6]. However, as the power is increased further to 30 W, or) increases which is probably due to the predominant effect of increased doping. The deposition rate also increases from 17 to 28 ,~,/min as the rf power increases from 2 to 30 W. At a B2H6/SiH 4 ratio of 1 x 10 -2 the effect of increased doping due to the incresae of rf power is likely to be less and the increase in hydrogen concentration is likely to be predomi-

Table 2 D a r k a n d p h o t o c o n d u c t i v i t i e s , a c t i v a t i o n energies a n d b a n d g a p o f b o r o n d o p e d a-Si : H films d e p o s i t e d under various conditions B2 H 6 Sill4

Percentage of Ar in S i l l 4

RF power (W)

Ol~ (fl-1 cm-a)

o ~,h (fl-1 cm-i)

Activation energy (eV)

Band gap (eV)

1×10 -3

0.0

2 10 30

2.5×10 -6 6.9 x 10 - s 7.0 x 10 - 6

2.6×10 -6 1.6 × 10 - 6 4.9 × 10 - 6

0.52 0.57 0.54

1.68 1.73 1.64

1 × 10 - 2

0.0

2 10 30

5.6 × 10 - 4 3.6 X 1 0 - 4 1 . 4 × 10 - 4

very lOW

0.37 0.37 0.40

1.39 1.5 1.61

10 10 10 10

6.9×10 -8 4.9 X 10 - 6 2.8 × 1 0 - 6 3.4×10 -6

1.6×10 -6 5.1 × 10 - 6 5 × 10-7 5 ×10 -6

0.50 0.40 0.38 0.38

1.74 1.65 1.61 1.59

1>':10 - 3

0 10 50 90

340

S. Ray et al. / Properties of boron doped a- Si ,"H thin film.~

nant. This is reflected in the monotonic decrease of o D with increasing rf power as shown in table 2. In this table we have also shown the effect of mixing Ar with Sill 4 on the o o values of p-type a-Si : H films. It is seen that by mixing 10% argon with Sill4, the dark conductivity increases about two orders of magnitude, activation energy and optical band gap decreases. This may be due to increased efficiency of doping due to the presence of argon. However, no significant change of the above mentioned parameters occur by increasing the argon concentration in the mixture. The deposition rate increases from 28 to 56 , ~ / m i n as Ar concentration increases from 10 to 90%.

3.2. Photoconductivity The room temperature photoconductivity Oph under white light radiation is shown in fig. 1 for p-type a-Si : H films prepared with different concentrations of diborane. Oph value for the undoped a - S i : H film under a radiation of 20 m W / c m 2 is 2.6 × 10 - 4 ~ - 1 c m - 1 which drops to 3.2 x 10 -7 f~-~ cm - t by slight diborane doping. With further increase in diborane doping there is a tendency of increase of photoconductivity as shown in fig. 1. According to Anderson and Spear [2], in the intrinsic a-Si : H film there will be equal and opposite charge distributions at the equilibrium Fermi level arising from the overlapping tails of the two types of defect center. The densities of the positive and negative charges are indicated by Pr and n r, respectively. The undoped a-Si : H film is slightly n-type so that or,h is determined by the capture of electrons into the Pr distribution. With the incorporation of a slight amount of boron pr increases and n r decreases as the Fermi level is lowered. However, since the photocurrent is still carried by excess electrons, their lifetime reduces due to the increase of pf. This phenomenon is probably responsible for the decrease of or,h with slight diborane doping. However, with heavier diborane doping the photocurrent is carried by excess holes. Thus for p-type films due to the decrease of n f, photoconductivity increases slightly. In fig. 3, log or, h vs. 1/T curves are plotted for p-type sample (prepared with BEH6/SiH 4 = 1 × 10 -4) at four different light intensities. Similar types of curves were also obtained by Anderson and Spear [2] and Paul and Anderson [9]. The prominent features of the curves are a peak near room temperature, a minimum and onset of a high temperature maximum. With the rise in incident light intensity the peak near room temperature shifts towards higher temperatures and the minimum gets shallower. The minimum for this p-type sample may be due to the release of electrons from an electron trap leading to recombination. This is followed by an increase of oeh with temperature which may be due to release of holes from a hole trap that enhances photoconductivity. In fig. 4, log Ovh vs. 1/T plots for undoped and two lightly boron doped ( B 2 H 6 / S i H 4 = 1 × 10 - 5 and 1 × 10 -4) films at intensity of radiation ~ 6.5 × 1013 p h o t o n s / c m 2 s have been shown. For the undoped film at about 250 K there is a small hump which is followed by a small depression at higher temperature. This is further followed by another hump at still higher temperature. Since the undoped

S. Ray et aL / Properties of boron doped a- Si: H thin films

341

10- 7

T

Eu

14

Tcl..~ IO-8 13 I0-9

Iz

I0~o

10112

g 3

l 4

.-T

I 5

~-~CK-1 )

I 6

Fig. 3. Temperature dependence of photoconductivity (aph) for the film prepared at B2H6/SiH 4 = 1 × 10-'* with incident radiation of wavelength 5600 ~ at different intensities. Curves Ii, 12, 13, I4, correspond to intensities 3.0× 1011, 5.4× 1012, 6.5 × 1013, 7.7 × 1014 p h o t o n s / c m 2 s, respectively.

io-6

io-7

10- 9

10~£

lt'~

"~ 2.(

I

I

3.0

4.0

~,

I

I

5.0

6.0

7.0

•~ - CK~ )

Fig. 4. Temperature dependence of photoconductivity (oph) at radiation of wavelength 5600 ~= and intensity 6.5 × 1013 p h o t o n s / c m 2 s. Curves 1, 2, 3 correspond to B2H6/SiH 4 ratio 0.0, 1 x 10 -2, 1 x 10 -4, respectively.

342

S. RcO' et al. / Properties of boron doped a- Si ."H thin films

film is n-type the first quenching may be explained as due to the release of holes at that temperature from a hole trap leading to a recombination. Similar explanation has been given by Paul and Anderson [9]. The second hump may be due to the existence of another hole trap. The nature of curves for the two boron doped films is similar. The temperature and flux dependences of photoconductivity of a-Si : H films have also been investigated by Vanier et al. [10]. They obtained various new features for the films prepared under different deposition conditions and interpreted the data with a model of competing recombination centres. The photoconductivity depends on the intensity I according to the relation (3)

Oph = K1 ~.

In fig. 5, we have plotted log Oph against log I at different temperatures for a p-type a-Si: H film ( B z H 6 / S i H 4 = ] X 1 0 - 4 ) , In different intensity ranges the slopes are different from which the y values can be determined. The values of 3' obtained under radiation of wavelength 5600 A and flux of 6.5 × 1013 p h o t o n s / c m 2 s are given in table 3. For the p-type film at 238 K, "r = 0.58 which indicates predominance of the bimolecular recombination process. At 345 K, the recombination process is monomolecular and again at higher temperatures the process tends to be bimolecular. Similar behaviour at slightly different temperatures has also been observed for undoped and compensated a - S i : H films and the corresponding -/ values have been given in table 3.

10-6

10 -~

T IO-E

3

vg

1o-9

10t°

t0"

I I 1012 1013 Intensity ( photons/cm2)

I 1014

1015

Fig. 5. Photoconductivity (oph) as a function of intensity for the film prepared at I ~ H 6 / S I H 4 = 1 × l0 -a at different temperatures of measurements. Curves 1, 2, 3 correspond to 238, 345 and 400 K, respectively.

S. Ray et aL / Properties of boron doped a - Si: H thin films

343

Table 3 Values of "t at different temperatures and at different values of B2H6/SiH 4 with radiation of intensity 6.5 × 1013 photons/cm2s B2H6 Sill4

T (K)

0.0

208 294 385

0.56 0.87 0.69

1 X10 5

233 313 385

0.69 1.02 0.62

1 X 10 -4

238 345 400

0.58 1.03 0.66

3.3. Optical band gap The optical absorption a(hv) were investigated for films deposited with different diborane concentrations under different conditions. The plots of (ah v)1/2 vs. photon energy hv are shown in fig. 6. From the extrapolations of the straight line portion of these plots to a = 0, optical band gaps were obtained which are given in tables 1 and 2. As shown in table 1 up to B2H6/SiH 4 = 1 × 10 -3, the optical band gap does not vary significantly and then ,with higher doping the band gap decreases steeply. Similar result has been obtained by" Okamoto et al. [11]. The values obtained by us are also very close to those of, Okamoto et al. The change in absorption spectra for different diborane doping can be s e e n from fig. 6. At high boron doping (B2H6/SiH 4 -- 1 × 102) a broad absorption band appears at lower hv region. This effect was also observed by Tsai and Yamasaki et al. [3,12]. For boron doped films Tsai [3] obtained optical band gaps varying between 1.6 and 1.85 eV. According to Yamasuki et al. [12], the gap states introduced by the incorporation of boron are located energetically close to a band edge which causes a decrease of slope of the (othv) 1/2 vs. hl) plot and thus contributing partially to band gap narrowing. The optical band gap does not change very much by mixing Ar in different proportions with Sill 4. ( e t h v ) 1/2 vs. hv plots obtained for films deposited with BzH6/SiH 4 = 1 × 10 -2 under different rf power are shown in fig. 7. The band gap increases with increase in power which corroborates with the result obtained by Matsuda et al. [5]. For low diborane doped films (B2H6/SiH 4 = 1 × 10 -3) the change of optical band gap with power is much less (table 2). As shown by Matsuda et al. [5], in our case also at higher power (30 W) the variation of optical gap with doping is much less (table 2) than the variation obtained under low power (2 W). Doping efficiency may be greater in case of high power deposited films than that under low power.

S. Ray et al. / Properties of boron doped a- Si: H thin films

344

~00(]

80C

E 60C t.)

.g: 400

200

O0

1.0

I

I

I

1.5

2.0

2.5

3.0

h~(ev) Fig. 6. Plots of (ah~') 1/2 against photon energy. Curve 1, 2 correspond to B2H6/SiH 4 = 1 × 10 -3 and 1 x 1 0 -2, respectively, with no argon. Curve 3: B2H6/SiH 4 =1 x l 0 -3 and 50% Ar in Sill4.

3.4. Spectral response The normalized photoresponse vs. hp plots for the films prepared with different doping concentrations have been investigated. The photocurrent ip is given by the expression [13] ip = eN0(1 - R)[1 - e x p ( - a d ) ] *llt'rEW,

(4)

where N0(1 - R) is the number of photons per second falling on to the specimen, corrected for surface reflection, ~ is the generation efficiency, • is the lifetime of carriers and/~ is the mobility, a denotes the absorption coefficient, d is the thickness of the film, E is the electric field and W is the length of the electrodes. Using eq. (4) and the photoresponse and absorption data T//~ for the films have been calculated and the results are shown in fig. 8. It is seen from fig. 8 that 7//~z increases sharply as hp increases from about 1.5 to 2 eV and then decreases very slowly as h v increases further. For the compensated a-Si: H film [B2H6/SiH4 = 1 × 10-5],//~ " (curve 2, fig. 9) is lower by three orders of magnitude than that of the undoped film (curve 1, fig. 8). As the boron doping

345

S. Ray et aL / Properties of boron doped a - Si." H thin films

800

60(3

E o

~

40C

e--

20C -

0 1.0

I

1.5

21.0

2.5

h,~(eV) Fig. 7. Plots of (ahp) ]/2 against photon energy for the film deposited with B 2 H 6 / S i H 4 = I ×10 -2 at different rf power. Curves 1, 2, 3 correspond to power 30, 10 and 2 W. t0 -<

1 I0 -s

10- 6

E

4~

10- 7

lo-8

10-9

16 I° 1.0

I

13

I

2.0 hxS(eV)

I

2.5

3.0

Fig. 8. The product of photoconductivitylifetime, mobility and quantum efficiency as a function of photon energy. Curves 1, 2, 3, 4 correspond to l~zH6/SiH4 ratio 0, 1×10 -3, 1×10 -4, 1x10 -3, respectively.

346

S. Ray et aL / Properties of boron doped a - Si .' H thin films

increases, r//.t~- increases (curves 3 and 4, fig. 8) although it is still about two orders of magnitude lower than that of the undoped film. This behaviour of ~p,r with boron concentration is in conformity with that reported recently by Haruki et al. [14] who studied photoconductivity under A M 1 radiation. They obtained a m i n i m u m value 1 0 - 7 c m 2 V 1 which gradually increased to = 10 6 c m 2 V 1 as the boron concentration was increased, The r//~- for u n d o p e d a-Si: H film was = 10 5 cm 2 V ~. The decrease of ~ - with initial b o r o n doping was explained as due to the decrease of ~n~'n for electrons with b o r o n doping. As seen in fig. 8 we have observed similar behaviour of T//~r with a change in b o r o n concentration. The increase in ~/~" with increase in diborane doping at higher boron concentrations may be explained as follows. Although/~p~'p for holes is much lower than ~nr n for the u n d o p e d film as b o r o n doping is increased the increase in p,p~-p predominates over the decrease in ~ n~-,, so that r//~- shows an overall increase.

3.5. The effects of adsorbates and Staebler- Wronski effect To assess the actual effect of adsorption the dark conductivity of p-type a-Si : H films prepared with diborane concentrations of 0.01, 0.1 and 1% was measured before and after annealing at 150 ° C in v a c u u m for one hour. The o o values of the films at r o o m temperature decreased by factors of 4, 30 and 40, respectively, after annealing. Since 02 is k n o w n to act as acceptors on silicon surface their removal by heating in v a c u u m p r o b a b l y decreases o o in p-type a-Si: H films. The increase in diborane concentration is likely to increase the concentration of voids in a-Si: H films so that oxygen absorption is more. This m a y be responsible for the increase in the effect of adsorbates as diborane concentration increases. To check whether our measurements of o D and oph had been affected by the Staebler-Wronski effect, p-type a-Si: H film prepared with 0.1% diborane had been subjected to a radiation of 20 m W / c m 2 for a period of one hour. The p h o t o c o n d u c tivity and darkqonductivity values did not show any change due to exposure to light. In our actual measurements of photoconductivity the samples were subjected to radiation of 20 m W / c m 2 for a few seconds only at a time so that the results were not influenced by Staebler-Wronski effect.

4. Conclusions The studies on the b o r o n doped a-Si : H film show that at low b o r o n concentrations, the conduction is in the extended states in the valence band. But at higher b o r o n doping probably an acceptor b a n d is created below the Fermi level near the valence b a n d and two parallel transport paths are effective for conduction. The b a n d gap of p-type a-Si : H remains almost constant up to a B 2 H 4 / S i H 4 volume ratio of 1 × 10 -3 and decreases sharply as the ratio increases further. For B 2 H 6 / S i H 4 ratio = 1 × 10 -2, the b a n d gap increases from 1.39 to 1.61 eV as the rf power increases from 2 to 30 W. The activation energy is more or less independent of the change of rf power.

S. Ray et al. / Properties of boron doped a- Si: H thin films

347

The nature of the temperature dependence of photoconductivity plots depends on the distributions of hole and electron traps in the material.

Acknowledgements The work has been done under a project funded by the Department of Non-Conventional Energy Sources, Govt. of India. The authors are grateful to Prof. M. Choudhury for the use of the Cary 17D Spectrophotometer.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

H. Fritzsche, Solar Energy Mater. 3 (1980) 447. D.A. Anderson and W.E. Spear, Phil. Mag. 36 (1977) 695. C.C. Tsai, Phys. Rev. B19 (1979) 2041. Z.S. Jan, R.H. Bube and J.C. Knights, J. Appl. Phys. 51 (1980) 3278. A. Matsuda, M. Matsumura, S. Yamasaki, H. Yamamoto, T. Imura, H. Okushi, S. lizima and K. Tanaka, Japan. J. Appi. Phys. 20 (1981) L183. S. Ray, P. Chaudhuri, A.K. Batabyal and A.K. Barua, Japan. J. Appl. Phys. 22 (1982) 23. H. Fritzsche, Amorphous and Liquid Semiconductors, ed. J. Tauc (Plenum Press, New York, 1974) p. 221. R.A. Street, D.K. Biegelsem and J.C. Knights, Phys. Rev. B24 (1981) 969. W. Paul and D.A. Anderson, Solar Energy Mater. 5 (1981) 229. P.E. Vanier, A.E. Delahoy and R.W. Griffith, J. Appl. Phys. 52 (1981) 5235. H. Okamoto, Y. Nitta, T. Yamaguchi and Y. Hamakawa, Solar Energy Mater. 2 (1980) 313. S. Yamasaki, A. Matsuda and K. Tanaka, Japan. J. Appl. Phys. 21 (1982) L789. P.J. Zanzucchi, C.R. Wronski and D.E. Carlson, J. Appl. Phys. 48 (1977) 5227. H. Haruki, H. Sakai, M. Kamiyama and Y. Uchida, Solar Energy Mater. 8 (1983) 441.