Thin Sohd Fdms, 113 (1984) 261-270

ELECTRONICS AND OPTICS 261

THE EFFECT OF MIXING H Y D R O G E N WITH SILANE ON TH E ELECTRONIC AND OPTICAL PROPERTIES O F H Y D R O G E N A T E D A M O R P H O U S SILICON THIN FILMS

P. CHAUDHURI, SWATI RAY AND A. K. BARUA

Indtan Assoczation for the Cultwatwn of Science, Calcutta 700032 (Indta)

(Recewed September 20, 1983; accepted January 24, 1984)

The electronic and optical properties of a-Si:H films prepared by the r.f. glow discharge method from a mixture of Sill 4 and H 2 were studied. The effects of variation in the concentration of H 2 in the mixture were also investigated. The dark conductivity of a-Si:H films reaches a minimum at a low concentration of H2 and then increases with increasing concentration of H2. In addition, the photoconduc- tive gain shows a maximum at a low concentration of H 2. The band gap, however, remains virtually constant up to 25% H 2 and then increases significantly as the H 2 concentration is increased further. The activation energy increases slowly with H 2 concentration up to 25% and then decreases with a further increase in the H2 concentration. IR vibrational spectra show an increase in the amount of S i - -H bonding as the H2 concentration increases to 25% and then a decrease with a further increase in the H 2 concentration.

1. INTRODUCTION

It is now well established that the electronic and optical properties of hydrogenated a-Si (a-Si:H) films depend mainly on the hydrogen content in the films as hydrogen acts as a dangling bond terminator in such films. The properties of a-Si:H films are now being extensively studied because of their importance in the fabrication ofa-Si solar cells. Some of the groups engaged in the development ofa-Si solar cells are using pure Sill4 as the discharge material whereas other groups are using a mixture of Sill 4 and H 2 gases 1-3. It is therefore interesting to study the effect of mixing Si l l , with hydrogen on the electronic and optical properties of a-Si:H films.

The influence of the addition of H2 in different proportions to argon on the properties of a-Si: H films prepared by the d.c. method or the r.f. sputtering method has been studied by a number of workers 4-7. However, no results of studies on the changes in the properties of a-Si: H prepared from a mixture of Sill 4 and H 2 by the r.f. glow discharge method have been reported. In this paper we report the electronic and optical properties of a-Si:H films prepared by the r.f. glow discharge method from mixtures with different concentrations of H2 in Sill, .

The properties which have been studied are the temperature dependence of the

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262 P CHAUDHUR1, S. RAY, A. K BARUA

dark conductiwty and the photoconductlvity, the intensity dependence of the photoconductivity, the absorption coeffioent, the spectral response and the IR vibrational spectra.

2. EXPERIMENTAL DETAILS

a-Si: H films were deposited onto Corning 7059 glass substrates by d~ssooatmg pure SIH4 or an SIH4-H2 mixture in an r.f. glow discharge apparatus The r f. power supply (7.6 MHz) was inductively coupled to the system. Details of the apparatus were given in an earlier paper s. The flow rates of Sill4 and H 2 were separately controlled by two electronic mass flowmeters and thus the ratio of H2 to the S iH4-H 2 mixture was accurately determined. The substrate temperature was normally kept at 250 °C and the r.f. power supplied to the coil was 20 W The H2 concentration (by volume) in the H2-SiH4 mixture was varied from 0°o to 90~o. The thickness of the films was m the range 2000-3000/~ and the deposition rate was 20-30 /~mln-1 For IR transmission measurements films with a thickness of 12 000-13 000/~ were deposited onto single-crystal sihcon wafers

Dark conductivity and photoconductivity measurements were carried out in vacuum (about 10-5 Torr) with a gap cell geometry using vacuum-evaporated aluminium electrodes. The electric field applied to the electrodes was 1 kV c m - 1. To study the variation in photoconductivity w~th temperature and intensity of incident radiation a monochromator was used. Radiation with a wavelength (5600/~) corresponding to a photon energy of 2.2 eV was selected as the exciting radiation and the intensity was varied by neutral density filters. The photoconductlwty was also studied under white light of intensity 20 mW c m - 2. The absorption coeffioent, band gap, transmittance and reflectance of a-Si:H films were studied with a Cary 17D double-beam spectrophotometer. The IR transmission was measured with a Perk in-Elmer 783 IR spectrophotometer.

3. RESULTS AND DISCUSSION

The dark conductwlty (7 D as a function of temperature was studied for a number of films prepared with different concentrations of H 2. The plots of log aD versus 1 /T are shown in Fig. 1. The values of the activation energies AE for these films were obtained from the gradients of the curves and are given m Table I. For a-Si. H film prepared from pure Sill 4 (Fig. 1, curve 1) the activation energy at temperatures above 27°C is 0.96 eV and the plot is a straight line. This suggests that the conduction takes place by transport in the extended states. At lower temperatures the slope is less steep, suggesting a change in transport mechanism which may now involve hopping through the locahzed states near the conduction band. As the concentration of H 2 increases (25~o; Fig. 1, curve 2) the straight portion of the log aD versus 1 /T curve, which has a corresponding activation energy of 1.01 eV, extends to a lower temperature than does curve 1. The ab values for curve 2 are also lower than those for curve 1. These results indicate that with glow discharge using H2-S1H4 mixtures where the H2 concentration is low the passivation of dangling bonds is more effective than for the films prepared from pure Sill4. The increase in the activation energy also indicates the incorporation of a larger quantity of H2 m the

EFFECT OF H 2 ON a-Si:H THIN FILMS PREPARED FROM Sill,, 263

10-4

10-(

ld 8

Io -9

2 0 215 310 5 103/X (K -I )

Fig 1 The dark conductivity plotted as a function of 1/T for the samples deposited with various H 2 concentrattons m the H2-S1H 4 gas mixture: curve 1, 0% H2; curve 2, 25% H2, curve 3, 50% H2, curve 4, 75% H2, curve 5, 90% H2

T A B L E I ACTIVATION ENERGIES, BAND GAPS AND CONDUCTIVITY PREFACTORS OF a-S1.H FILMS DIFFERENT PERCENTAGES OF S 2 IN THE SIH4-H 2 GAS MIXTURE

PREPARED WITH

Concentratwn of H 2 Conductwlty prefactor Acttvatwn energy AEvh Band gap m StH,-H2 (%) (f l - 1 c m - 1) (eV) (eV) (eV)

0 2.25 x 106 0 96 0.18 1.76 9 2.25 x 106 0.96 0.17 1.75

25 1.75 x 107 1 01 0.13 1.74 40 1 80 x 106 0.99 0.16 1 80 50 1.26 x 106 096 0.17 1 82 75 3.36 x 104 0 78 0.19 1 87 90 7 70 x 102 0.66 0 17 1.91

a-Si:H films. With a further increase in the n 2 concentration in the discharge gas mixture (Fig. 1, curve 3), the conductivity starts to increase again and the activation energy decreases. This result probably indicates an increase in the number of defect states due to excess hydrogen. The conductivities of the films prepared with 75% H 2 and 90% H 2 (Fig. 1, curves 4 and 5) are considerably higher than those prepared with pure Sill 4 and the activation energies are 0.78 eV and 0.66 eV respectively, which are much lower than the value (0.96 eV) obtained for pure Sill4. This result together with those obtained for band gaps (reported later in the present paper) show that EF is shifted towards the conduction band when a comparatively large amount of H2 is present in the SiH4-H 2 mixture. The slopes (Fig. 1, curves 4 and 5) also change at much higher temperatures, indicating that the activated hopping mechanism has

264 P. C H A U D H U R I , S. RAY, A. K BARUA

started to take place. Thus it is seen that by increasing the H 2 concentration in Sill 4 beyond a certain limit defect states are created m a-Si: H films.

The values of the prefactors ao obtained from the intercept at 103/T = 0 have also been given in Table I. For a-Si:H films prepared without H E and with 9% HE,

ao = 2.25 x 10 6 [')- 1 cm - 1. According to Fritzsche 9 a 0 reaches this value when E F is near the gap centre. With 25% HE, ~r 0 increases to 1.75 x l07 ~ - 1 c m - 1 and, with a further increase in the H 2 concentration, a0 gradually decreases to 7.7 x 10 2

D -1 c m - L By varying the deposition conditions in the r f. glow &scharge decomposition method, Spear et al. ~° obtained 102f~-~cm -~ < ao < l 0 7 [,')-1 cm-1. Paul and Anderson 11 obtained a 0 in the range 103-104 f~-1 cm-1 for a-Sl: H films prepared by the r.f. sputtering method.

The variation in the dark conductivity at 346 K with H 2 concentrat]on is shown in Fig. 2. The photoconductive gain aph/ tr o for the films obtained under white light radiation of intensity 20 mW c m - 2 is also shown. From this figure it can also be seen that, with the addition of a slight amount of n 2 to Sill4, aD decreases and the photoconductive gain increases: this shows a decrease in the density of dangling bonds and recombination centres. With a further increase in the H E concentration o D increases and ffph/O'O decreases, showing an increase in the defect density. It has also been found that, if the a-SI:H film prepared with 90% H E IS exposed to the atmosphere for some time, the conductivity changes considerably. This type of change has been reported previously by Tanielian et al. 12 and by Anderson and PauP. The films prepared with high concentrations of H E may have pores which absorb water vapour from the atmosphere. If the film is kept under vacuum for 1 h the conductivity decreases by up to one order of magnitude. This effect is negligible for the films deposited at lower H 2 concentrations.

From the reflectance and transmittance data the absorption coefficients ~ for the a-SI:H films were calculated. (cthv) 1/2 is plotted in Fig. 3 as a function of the photon energy hv for films deposited with different H2 concentrations. From the extension of the straight portion of the curve, band gaps for the corresponding films

lo-6

Y

b ~ 1(5 ~

lO -¸

104

03

bO 2

I I _ I ] ~0 20 40 60 ~0

142 Concentration ( % )

Fig 2 D a r k conductzvl ty at, (curve l) and pho toconduc l l ve gain O'ph/a D (curve 2) at 346 K as funct ions of H 2 concen t ra t ion

EFFECT OF H 2 ON a - S i : H THIN FILMS PREPARED FROM Sil l , 2 6 5

80£

60(

-f/,'? '// ///~'/ II / 1 1

// I l l f i l l /

/ / , / I l l I I [ 15 20 25 30

h ~(eV)

Fig 3. (~hv) 1/2 vs. hv plots for a-Si:H films: curve 1, 0 ~ H2, curve 2, 25% H2, curve 3, 50% H2; curve 4, 75~ H2, curve 5, 90°/0 H2.

were obtained, the values of which are given in Table I. It can be seen that the band gap remains virtually constant up to 259/0 H 2 and then increases significantly as the H 2 concentration is increased further.

The IR vibrational spectra of a few a-Si:H films prepared from SiH4-H 2 mixtures with different H2 concentrations have been studied to obtain information about the total bonded hydrogen content and the local bonding configurations of hydrogen. The spectra obtained for a-Si: H films prepared from mixtures having 09/0, 25~, 509/o and 759/0 H 2 are shown in Figs. 4 and 5. The a-Si:H films have two main absorption peaks at 2000 cm-1 (Fig. 4) and 640 c m - t (Fig. 5) which correspond respectively to stretching and wagging Si---H bonds. A weak peak was observed at 2100 cm- x which corresponds to the bond-stretching mode of Sill2 x3. This shows that the bonded hydrogen in the a-Si: H films prepared by us is mainly present in the monohydride form.

The concentration of bonded hydrogen in the a-Si:H films has been estimated from the intensities of IR absorption due to the Si--H wagging mode using the expression x 3, ~,t

Nn = A f c~(o9) d~ (1) Jo, O9

where ~ is the absorption coefficient and co is the wavenumber. The value of A was taken as 1.6 x 10 x9 cm -2. The hydrogen contents of the films prepared with 09/0 H2, 259/o H2, 509/o H2 and 75~ H 2 were estimated to be 8.6 at.~, 18.6 at.~, 15.5at.~ and 10.2 a t .~ respectively. Thus the concentration of hydrogen bonded in the form of

266 P. C H A U D H U R I , S. R A Y , A. K B A R U A

C E

200C

100c /

1900

J% \

2000 210o 2200 ~(~-~)

400C

3000

b E 2000

100(

550

t. ,\

650 75O a)(cm -~)

Fig 4 Absorption coefficient ~t vs frequency o~ for stretching SI--H bonds m a-S1 H films curve 1, 0°o H 2, curve 2, 25°o H2, curve 3, 50°o H2, curve 4, 75°0 H 2 Fig 5 Absorption coefficient ct vs frequency ~o for wagging S1--H bonds m a-Sl H films curve 1, 0°,o H z , curve 2, 25°'o H 2 . curve 3, 50°0 H2; curve 4.75°o H 2

S i - - H increases up to a n H 2 concen t ra t ion of 25~o and then s tar ts decreas ing with a fur ther increase in the H 2 concent ra t ion . The band gap, however, increases m o n o t o n i c a l l y with the increase in the H 2 concen t ra t ion in the S i H 4 - H 2 mixture. These results m a y be in te rpre ted as being due to the ent ry of H e into the a-Si ne twork in different conf igura t ions at different H E concent ra t ions , which m a y give different electronic states in the pseudogap . This in te rp re ta t ion has also been given by F r e e m a n and Pau l i 5 for s imilar results ob ta ined by them with sput te red a - S i : H films.

The pho tocu r r en t s of the films depos i ted with different hydrogen concent ra - t ions were measured as a funct ion of wavelength using a m o n o c h r o m a t o r . The p h o t o c u r r e n t at a given wavelength can be wri t ten as 16

tp = eNo(1 -- g){ 1 -- exp( - - ~d)}~ll~zEW (2)

where N o ( 1 - R ) is the number of pho tons per second incident on the specimen, cor rec ted for surface reflection, r/ is the genera t ion efficiency of free carriers, # is the e lec t ron mobi l i ty , z is the e lec t ron lifetime, e is the e lect ron charge, ~ is the a b s o r p t i o n coefficient, d is the thickness of the film, E is the electric field s t rength and W is the length of the electrodes. Us ing this equat ion , values of t//az for different samples were ca lcu la ted and are p lo t ted in Fig. 6. F o r energies above the band gap t/ should be uni ty 16. If we assume tha t # is the same for all the samples then the e lec t ron lifetimes for the samples depos i ted with h igher hydrogen concen t ra t ions are less as shown in Fig. 6.

The var ia t ion in the pho toconduc t l v i t y with l IT is shown in Figs. 7 and 8 for different intensi t ies of Incident r ad ia t ion for a - S i : H films p repa red f rom pure S i l l4 and a mixture of SIH4 and H2 with 75~o H2. It can be seen that near r o o m t empera tu r e there is a small depress ion in the Crph versus l IT plots which IS more p r o m i n e n t at lower intensities. This depress ion results in a small peak or h u m p at a sl ightly lower tempera ture . The depress ion m a y be expla ined as being due to the existence of a hole t rap level which s tar ts d e p o p u l a t i n g at tha t t empera tu re and

EFFECT OF H 2 ON a - S i : H THIN FILMS PREPARED FROM S i l l , , 2 6 7

j - E o

90 x

5

I I I 1 5 2 0 2 5 30

h~)(eV) Fig. 6. The product of the photoconductiwty hfeume, mobihty and quantum effimency as a function of the photon energy, curve 1, 0% H2; curve 2, 9% I-t2; curve 3, 51P~ H2, curve 4, 75% H2; curve 5, 9ff',/o H2

Io-4

IO-S

lo-6

T

'7 10 -7

lo-e

IG'

I 4

1() I° I I I I 20 30 4 0 50 60

IOVT (K -1)

Fig. 7. Photoconductivity %h of the films deposlted with pure SIH, as a funcUon of I/T at various light intensiUes: curve I1, 1.60 x 101 t photons cm-2 s-1; curve 12, 5.45 x 1012 photons cm-2 s-1; curve 13, 6.65 x 10 t3 photons crn- a s - t ; curve I4, 8.29 x 1014 photons cm- 2 s - t

268 P. C H A U D H U R I , S R A Y , A K . B A R U A

10 -4

10 -5

10 -~

E

-7 ~1o

io-8

1o -~

16

c ~

r~

\ - . . I 3

I1

o {o ~o ~o ---&o 7o 103/T ( K ~I )

Fig 8 PhotoconductlvRy aph of the films deposited with 75°0 I-I 2 m the H2-SIH4 mixture as a function of lIT at various hght mtensltms curve I1, 2 92 x 1011 photons cm-2 s- I, curve I2, 4 92 x 1012 photons cm-as -1 curveI3,635×1013photonscm-2s l curveI4,750x1014photonscm-2s 1

which may enhance the recombinat ion process. There is also a prominent peak in the trph versus l i T plots at a higher temperature which has been reported previously by other workers 14. The existence of this peak may also be due to the presence of another hole trap level which is depopulated at a higher temperature. The existence of a depresston in the Oph versus 1 /T plot has also been reported and discussed by Paul and Anderson 1~

The intensity dependence of the pho toconduc twi ty can be written as

O'ph ~--- K I ~ (3)

where K is a constant and I is the incident light intensity. The value o fy determines the nature of the recombinat ion mechanism. Figure 9 shows the variation in Y with l i T for samples deposited with 0%, 9% and 75% H 2. At lower temperatures 7 = 0.5, which indicates a bimolecular recombinat ion process. 7 increases gradually with increasing T but, at some higher temperature, y = 0.5, indicating again the predominance of a bimolecular recombinat ion process. As the temperature is increased further, ? becomes equal to unity, showing a monomolecu la r recombina- tion process which indicates uniformly distributed recombinat ion centres.

In the low temperature region where y ~ 0.5, i.e. where a blmolecular recombinat ion process is predominant ,

O'ph o( exp k T (4)

where E c - E k is a measure of the extent of the localized tail states ~ 7. F r o m the slopes of the log (rph versus 1 /T curves we have calculated the values of Agph -- E c -- E A for

EFFECT OF H 2 ON a-Si: H TmN FILMS PREPARED FROM Sill,, 269

1C

O~

1C

~ o ~

lo

o5

o 2

l o~/r (K -1

Fig 9 Temperature dependence of the parameter 7 m trvh CC I v: curve 1, 0% H2, curve 2, 9% H 2; curve 3, 75% H 2

films deposited at different H 2 concentrations; these are given in Table I. It can be seen from Table I that, with a slight addition of H2, the width of the tail states decreases but, with a further increase in the H 2 concentration, this width increases again.

4. CONCLUSIONS

It can be seen that small but significant changes in the electronic and optical properties ofa-Si:H films prepared by r.f. glow discharge decomposition occur when H 2 is added to Sill+. The quality of the film improves with the addition of a small quantity of H 2 and it deteriorates with a further increase in the concentration of H2.

Although we have seen that the presence of only a small quantity of H 2 in SiH+-H 2 mixtures is beneficial for producing a-Si:H films suitable for solar cells, the best composition will depend on various deposition parameters including the geometry of the apparatus.

ACKNOWLEDGMENTS

The work has been carried out under a project funded by the Department of Non-conventional Energy Sources, Government of India. The authors are grateful to Dr. J. I. B. Wilson of Heriot-Watt University, Edinburgh, for helpful discussions.

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270 P. CHAUDHURI, S. RAY, A. K. BARUA

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(1980) 575 13 M H. Brodsky, M CardonaandJ J Cuomo, Phys Rev B, 16(1977) 3556 14 A Morxmoto, T Mlura, M K u m e d a a n d T Shlmlzu, Jpn J Appl Phys,21(1982)Ll19 15 E C FreemanandW Paul, Phys Rev B, 20(1979) 716 16 P J Zanzucchl, C R Wronskl and E E Carlson, J Appl Phys, 48 (1977) 5227 17 W E Spear and P G Le Comber, m J Mort and D M Pa, (eds), Photoconductlvtty and Related

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