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Tailoring refractive index of a-Si:H by TBP (C4H11P) doping
Journal of Non-Crystalline Solids 209 Ž1997. 188–192
Tailoring refractive index of a-Si:H by TBP ž C 4 H 11 P/ doping R.M. Mehra ) , Inderbir, Jasmina, P.C. Mathur Department of Electronic Science, UniÕersity of Delhi South Campus, New Delhi 110021, India Received 7 December 1995; revised 20 May 1996
Abstract The refractive index, n, and extinction coefficient, k, of n-type TBP Žtertiarybutylphosphine. doped a-Si:H films is reported. TBP is preferred over phosphine because of its lower toxicity and pyrophoricity. The films were deposited using glow discharge Žplasma lab m P. technique. The optical constants, n, k and absorption coefficient, a are determined as functions of wavelength, l and doping concentration. The refractive index is found to decrease as the TBPrSiH 4 ratio increases from 0.1% to 3%. This decrease is due to increase in the internal strain in amorphous network. Also the refractive index and extinction coefficient decrease with increase in wavelength in the range of 600 to 900 nm.
1. Introduction The ability to control the refractive index of a semiconductor is important in the design of many photonic devices w1x, particularly those that make use of optical waveguides w2x, integrated optics w3x and injection laser diodes w4,5x. Semiconductor materials are dispersive so that the refractive index is dependent on wavelength w6x. The knowledge of real and imaginary parts of complex refractive index, n s n y i k, as a function of wavelength and thickness, t, of a-Si:H is necessary to make effective use of these materials for optoelectronic devices w7–9x, particularly as an antireflective coating w10x. The variation of refractive index with doping and growth parameters also provides the means to tailor the refractive index to any desired value required for use in filters w11x.
)
Corresponding author. Fax: q91-11 688 6427; e-mail: [email protected].
In the present paper, the effect of TBP doping on the refractive index and absorption coefficient of n-type a-Si:H films is reported.
2. Experimental details Films were obtained using tertiarybutylphosphine ŽTBP. as a dopant in place of phosphine 1 w12x, the advantage of using TBP over phosphine being its lower toxicity, less pyrophoricity and safer handling w13x. A glow discharge system Žplasma lab m P. was used to deposit a-Si:H films. TBP vapor was mixed with silane ŽSiH 4 . to produce a n-type amorphous silicon. Depositions were on glass substrates ŽCorning 7059.. The samples were grown using helium as the carrier gas. TBP was introduced into the reactor by bubbling He gas through a cooled ampoule con-
1
Samples were prepared at the University of Utah, Salt Lake City, USA, during the visit of one of the co-authors.
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 3 3 - 9
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
189
taining TBP. The doping level of phosphorous was controlled by adjusting the flow rate of TBP, SiH 4 and the carrier gas. The silane flow rate was 20 sccm while the TBPrcarrier gas flow rate was 0.35–1.0 sccm. The RF power during deposition was 25 W, and the deposition rate was 25 to 35 nm per minute. The substrate temperature was kept at 2508C. The transmission spectra were taken with a spectrophotometer ŽShimadzu, UV-260. in the range 500 to 900 nm. The thickness of the films were measured by a Talystep ŽRank Taylor–Hobson. for the sake of comparison with the results deduced from the transmission spectra.
3. Determination of optical constants In order to calculate the refractive index and extinction coefficient, we used the method suggested by Swanepoel w14x. The refractive index and absorption coefficient are obtained by using only the transmission spectrum. The film layer is assumed to be homogeneous. In the case of inhomogeneities in thin films there is a considerable shrinking of the interference fringes of the optical transmission spectrum w15x. In the present case no such shrinkage is observed indicating uniformity of the films. According to Swanepoel’s method which is based on the approach of Manifacier et al. w16x of creating the envelopes of interference maxima and minima, the value of refractive index in the region of weak and medium absorption Ž a / 0. was calculated by the expression n s N q Ž N 2 y s2 .
1r2 1r2
Ž 1.
where Ns2 s
Tmax y Tmin Tmax Tmin
s2 q 1 q
2
with s the refractive index of the substrate. Tmax and Tmin are the transmission maximum and the corresponding minimum at a certain wavelength as shown in Fig. 1. The values of n, are calculated using Eq. Ž1. at the extremes of the interference fringes of spectrum only at a particular wavelength. The accuracy to which l can be measured is "1 nm. The
Fig. 1. Transmission spectrum for n-type a-Si:H film with TBPrSiH 4 ratio 2%.
maximum absolute accuracy of Tmax and Tmin is "0.001. If n1 and n 2 are refractive indices at two adjacent maxima Žor minima. at l1 and l 2 , using the basic equation for interference fringes 2 nt s m l ,
Ž 2.
where m is an integer for maxima and half integer for minima, the thickness is given by ts
l1 l2
Ž 3.
2 Ž l1 n 2 y l 2 n1 .
The mean value of t so calculated is used with n1 to determine the order number for the extremes from Eq. Ž2.. More accurate values of t Ž tacc . are obtained by taking integral and half-integral values of m. Using the accurate values of m and t, n Ž n acc . is again calculated. The absorption coefficient, a , can be determined from the relation w14x x s exp Ž ya t .
Ž 4.
where x is the absorbance given by 3
xs
Em y Em2 y Ž n 2 y 1 . Ž n 2 y s 4 . 3
Ž n y 1. Ž n y s 2 .
1r2
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
190
Table 1 Values of l, Tma x , Tmin , n and t for the spectrum of Fig. 1
l Žnm. Ž"1 nm.
Tma x Ž"0.001.
Tmin Ž"0.001.
n acc Žnm.
tacc
860.4 836 819.4 800 782.8 764
0.809 0.793 0.748 0.71 0.681 0.638
0.48 0.467 0.451 0.44 0.424 0.407
2.850Ž"0.005. 2.863Ž"0.005. 2.886Ž"0.005. 2.868Ž"0.006. 2.897Ž"0.006. 2.906Ž"0.007.
2565 2535 2554 2580 2568 2573
tacc s 2562 nm, s s14.6 nm. Fig. 2. Variation of refractive index with doping at two different wavelengths Ž l s 700 and 800 nm.. Error bars have been shown at points.
Table 2 Constants a and b of the Cauchy dispersion relation for different doping ratios TBPrSiH 4 ratio 0.1% 0.3% 2% 3%
where 2
Em s
8n s Tmax
q Ž n2 y 1. Ž n2 y s 2 . .
Extinction coefficient, k, can be determined from the relation w14x ks
al 4p
.
Ž 5.
4. Results Fig. 2 shows the variation of n with doping for two different wavelengths Ž700 and 800 nm.. The value of n decreases from 3.173 to 2.853 at 700 nm
Fig. 3. Variation of refractive index with wavelength as a function of doping. The solid lines represent the least square fit to the corresponding experimental data shown by different symbols. Error bars have been shown at points.
a Žnm2 .
b 5
1.078=10 1.34=10 5 1.511=10 5 1.339=10 5
2.967 2.812 2.645 2.565
and from 3.153 to 2.767 at 800 nm, as the TBPrSiH 4 ratio increases from 0.1% to 3%. n is calculated to an accuracy of 2%. The maximum possible error in n varies from "0.003 to "0.011. Fig. 3 shows the variation of n with wavelength Ž650–900 nm. for different doping ratios. It is observed that the refractive index is a slowly varying function of wavelength and decreases with increase in wavelength. The values of n acc and tacc for dopant concentration 2% are shown in Table 1. The values of m and t, determined graphically, were found to agree with the calculated values.
Fig. 4. Variation of absorption coefficient with wavelength as a function of doping. The solid lines represent the least square fit. Error bars have been shown at a point on each curve.
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
Fig. 5. Variation of absorption coefficient with doping at l s800 nm. Error bar has been shown at a point.
Refractive index was also fitted to a function for extrapolation to shorter wavelengths. The least square fit to Cauchy dispersion relationship w6x a ns 2 qb l gives a s 1.078 = 10 5 nm2 and b s 2.967 for TBPrSiH 4 concentration 0.1%. For other dopant ratios, the best fit values of a and b are listed in Table 2. The variation of a with wavelength for different doping concentrations is shown in Fig. 4. a is found to decrease with increase in wavelength. Fig. 5 shows the variation of a with doping concentration for l s 800 nm. a is calculated to an accuracy of 1%. The maximum possible error in a varies from "0.479 to "0.923 cmy1 for l s 800 nm. The thickness of the films measured by Talystep is close to that calculated from the transmission spectra. For instance, for the film with TBPrSiH 4 ratio 2% the measured value of thickness is 2450 nm, where as the calculated value is 2562 nm.
5. Discussion Srinivasan et al. w17x have reported increase of refractive index with doping in PH 3 doped a-Si:H films. As the PH 3rSiH 4 ratio increases from 5000 to 50,000 vppm the refractive index increases from 3.6 to 4.0 for wavelength 700 nm and from 4.03 to 4.35 for wavelength 600 nm. The experimental data on the refractive index of several PH 3 doped a-Si:H films strongly favour the existence of P3o centres
191
Žwhere P3o is the neutral threefold coordinated phosy Ž phorous atom. andror Tq where Tq 3 y P2 3 is the positively charged silicon dangling bond and P2y is the negatively charged twofold coordinated phosphorous atom. pairs as an efficient strain reliever. It has been shown by Dusane et al. w18x that in PH 3 doped a-Si:H, bond angle deviations increase sharply with increasing dopant concentrations. The increase in bond angle deviations cause internal strain in amorphous network. The increase in internal strain in the present case is the probable cause for decrease of refractive index with doping in TBP doped a-Si:H. The observed decrease of a at high TBP concentrations indicates a lower density of deep defects in these samples and is not due to an increase in the optical band gap which is found to be negligible.
6. Conclusion The refractive index of the n-type a-Si:H obtained using TBP as dopant is found to vary with doping concentration and can be tailored by changing the TBPrSiH 4 ratio. It has also been found that the absorption is extremely low for wavelength greater than 850 nm for all the samples. In GaAs, the formation of optical waveguides takes advantage of the refractive index dependence on Al concentration of Ga 1y x Al x As. It has been observed that epitaxially grown GaAsrAlGaAs used for waveguide purpose has refractive index 3.5 and attenuation 5 dBrcm at l s 850 nm w19x. It is thus suggested that the TBP doped n-type a-Si:H can be used as a thin transparent layer whose index of refraction is larger than the refractive indices of the two surrounding media, e.g. glass and air. It is observed that the refractive index for 0.1% dopant concentration is 3.152 at 800 nm and the absorption coefficient is the lowest Ž0.041 = 10 3 cmy1 . at this wavelength. Hence a planar as well as multilayer structure of n-type a-Si:H ŽTBP doped. can be fabricated for use for optical waveguide application.
Acknowledgements The authors are thankful to Professor P.C. Taylor, University of Utah, Salt Lake City, USA, for useful
192
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
discussions and providing facilities for the growth of a-Si:H films. Two of the authors, Inderbir and Jasmina, acknowledge the help of UGC and CSIR for providing financial assistance for the research work.
References w1x J. Tauc, Amorphous and Liquid Semiconductors ŽNew York, Plenum, 1974.. w2x H.C. Casey, D.D. Sell and M.B. Panish, Appl. Phys. Lett. 24 Ž1974. 63. w3x T.L. Koch and U. Koren, IEEE J. Quantum Electron. 27 Ž1991. 641. w4x R.N. Hall, G.E. Fenner, J.D. Kingsley, T.J. Soltys and R.O. Carlson, Phys. Rev. Lett. 9 Ž1962. 366. w5x M.I. Nathan, W.P. Dumke, G. Burns, F.H. Dill and G. LLasher, Appl. Phys. Lett. 1 Ž1962. 62. w6x T.S. Moss, Optical Properties of Semiconductors ŽLondon, Butterworths, 1959..
w7x Y. Tawada, K. Tsuge, M. Kondo, H. Okamoto and Y. Hamakawa, J. Appl. Phys. 53 Ž1982. 5273. w8x J.C.C. Fan and B.S. Palm, Sol. Cells 10 Ž1983. 81. w9x H. Munekata and H. Kukimoto, Appl. Phys. Lett. 42 Ž1983. 432. w10x J.C. Jans and J.W. Gemmink, Appl. Opt. 32 Ž1993. 84. w11x H.A. Macleod, Thin Film Optical Filters ŽAdam Hilger, London and Macmillan, New York, 1987.. w12x R.M. Mehra, Gurinder, Inderbir, Jasmina, Amit Pundir and P.C. Mathur, J. Non-Cryst. Solids 164–166 Ž1993. 517. w13x C.A. Larsen, C.H. Chen, M. Kitamura and G.B. String, fellow, Appl. Phys. Lett. 48 Ž1986. 1531. w14x R. Swanepoel, J. Phys. E16 Ž1983. 1214. w15x R. Swanepoel, J. Phys. E17 Ž1984. 896. w16x J.C. Manifacier, J. Gasiot and J.P. Fillard, J. Phys. E, Sci. Instr. 9 Ž1976. 1002. w17x G. Srinivasan and A.S. Nigavekar, Mater. Sci. Eng. B8 Ž1991. 23. w18x R.O. Dusane, S.R. Dusane and V.G. Bhide, J. Non-Cryst. Solids 137&138 Ž1991. 115. w19x L.D. Hutcheson, Integrated Optical Circuits and Components, Design and Applications ŽMarcel Dekker, 1987..
Tailoring refractive index of a-Si:H by TBP ž C 4 H 11 P/ doping R.M. Mehra ) , Inderbir, Jasmina, P.C. Mathur Department of Electronic Science, UniÕersity of Delhi South Campus, New Delhi 110021, India Received 7 December 1995; revised 20 May 1996
Abstract The refractive index, n, and extinction coefficient, k, of n-type TBP Žtertiarybutylphosphine. doped a-Si:H films is reported. TBP is preferred over phosphine because of its lower toxicity and pyrophoricity. The films were deposited using glow discharge Žplasma lab m P. technique. The optical constants, n, k and absorption coefficient, a are determined as functions of wavelength, l and doping concentration. The refractive index is found to decrease as the TBPrSiH 4 ratio increases from 0.1% to 3%. This decrease is due to increase in the internal strain in amorphous network. Also the refractive index and extinction coefficient decrease with increase in wavelength in the range of 600 to 900 nm.
1. Introduction The ability to control the refractive index of a semiconductor is important in the design of many photonic devices w1x, particularly those that make use of optical waveguides w2x, integrated optics w3x and injection laser diodes w4,5x. Semiconductor materials are dispersive so that the refractive index is dependent on wavelength w6x. The knowledge of real and imaginary parts of complex refractive index, n s n y i k, as a function of wavelength and thickness, t, of a-Si:H is necessary to make effective use of these materials for optoelectronic devices w7–9x, particularly as an antireflective coating w10x. The variation of refractive index with doping and growth parameters also provides the means to tailor the refractive index to any desired value required for use in filters w11x.
)
Corresponding author. Fax: q91-11 688 6427; e-mail: [email protected].
In the present paper, the effect of TBP doping on the refractive index and absorption coefficient of n-type a-Si:H films is reported.
2. Experimental details Films were obtained using tertiarybutylphosphine ŽTBP. as a dopant in place of phosphine 1 w12x, the advantage of using TBP over phosphine being its lower toxicity, less pyrophoricity and safer handling w13x. A glow discharge system Žplasma lab m P. was used to deposit a-Si:H films. TBP vapor was mixed with silane ŽSiH 4 . to produce a n-type amorphous silicon. Depositions were on glass substrates ŽCorning 7059.. The samples were grown using helium as the carrier gas. TBP was introduced into the reactor by bubbling He gas through a cooled ampoule con-
1
Samples were prepared at the University of Utah, Salt Lake City, USA, during the visit of one of the co-authors.
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 3 3 - 9
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
189
taining TBP. The doping level of phosphorous was controlled by adjusting the flow rate of TBP, SiH 4 and the carrier gas. The silane flow rate was 20 sccm while the TBPrcarrier gas flow rate was 0.35–1.0 sccm. The RF power during deposition was 25 W, and the deposition rate was 25 to 35 nm per minute. The substrate temperature was kept at 2508C. The transmission spectra were taken with a spectrophotometer ŽShimadzu, UV-260. in the range 500 to 900 nm. The thickness of the films were measured by a Talystep ŽRank Taylor–Hobson. for the sake of comparison with the results deduced from the transmission spectra.
3. Determination of optical constants In order to calculate the refractive index and extinction coefficient, we used the method suggested by Swanepoel w14x. The refractive index and absorption coefficient are obtained by using only the transmission spectrum. The film layer is assumed to be homogeneous. In the case of inhomogeneities in thin films there is a considerable shrinking of the interference fringes of the optical transmission spectrum w15x. In the present case no such shrinkage is observed indicating uniformity of the films. According to Swanepoel’s method which is based on the approach of Manifacier et al. w16x of creating the envelopes of interference maxima and minima, the value of refractive index in the region of weak and medium absorption Ž a / 0. was calculated by the expression n s N q Ž N 2 y s2 .
1r2 1r2
Ž 1.
where Ns2 s
Tmax y Tmin Tmax Tmin
s2 q 1 q
2
with s the refractive index of the substrate. Tmax and Tmin are the transmission maximum and the corresponding minimum at a certain wavelength as shown in Fig. 1. The values of n, are calculated using Eq. Ž1. at the extremes of the interference fringes of spectrum only at a particular wavelength. The accuracy to which l can be measured is "1 nm. The
Fig. 1. Transmission spectrum for n-type a-Si:H film with TBPrSiH 4 ratio 2%.
maximum absolute accuracy of Tmax and Tmin is "0.001. If n1 and n 2 are refractive indices at two adjacent maxima Žor minima. at l1 and l 2 , using the basic equation for interference fringes 2 nt s m l ,
Ž 2.
where m is an integer for maxima and half integer for minima, the thickness is given by ts
l1 l2
Ž 3.
2 Ž l1 n 2 y l 2 n1 .
The mean value of t so calculated is used with n1 to determine the order number for the extremes from Eq. Ž2.. More accurate values of t Ž tacc . are obtained by taking integral and half-integral values of m. Using the accurate values of m and t, n Ž n acc . is again calculated. The absorption coefficient, a , can be determined from the relation w14x x s exp Ž ya t .
Ž 4.
where x is the absorbance given by 3
xs
Em y Em2 y Ž n 2 y 1 . Ž n 2 y s 4 . 3
Ž n y 1. Ž n y s 2 .
1r2
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
190
Table 1 Values of l, Tma x , Tmin , n and t for the spectrum of Fig. 1
l Žnm. Ž"1 nm.
Tma x Ž"0.001.
Tmin Ž"0.001.
n acc Žnm.
tacc
860.4 836 819.4 800 782.8 764
0.809 0.793 0.748 0.71 0.681 0.638
0.48 0.467 0.451 0.44 0.424 0.407
2.850Ž"0.005. 2.863Ž"0.005. 2.886Ž"0.005. 2.868Ž"0.006. 2.897Ž"0.006. 2.906Ž"0.007.
2565 2535 2554 2580 2568 2573
tacc s 2562 nm, s s14.6 nm. Fig. 2. Variation of refractive index with doping at two different wavelengths Ž l s 700 and 800 nm.. Error bars have been shown at points.
Table 2 Constants a and b of the Cauchy dispersion relation for different doping ratios TBPrSiH 4 ratio 0.1% 0.3% 2% 3%
where 2
Em s
8n s Tmax
q Ž n2 y 1. Ž n2 y s 2 . .
Extinction coefficient, k, can be determined from the relation w14x ks
al 4p
.
Ž 5.
4. Results Fig. 2 shows the variation of n with doping for two different wavelengths Ž700 and 800 nm.. The value of n decreases from 3.173 to 2.853 at 700 nm
Fig. 3. Variation of refractive index with wavelength as a function of doping. The solid lines represent the least square fit to the corresponding experimental data shown by different symbols. Error bars have been shown at points.
a Žnm2 .
b 5
1.078=10 1.34=10 5 1.511=10 5 1.339=10 5
2.967 2.812 2.645 2.565
and from 3.153 to 2.767 at 800 nm, as the TBPrSiH 4 ratio increases from 0.1% to 3%. n is calculated to an accuracy of 2%. The maximum possible error in n varies from "0.003 to "0.011. Fig. 3 shows the variation of n with wavelength Ž650–900 nm. for different doping ratios. It is observed that the refractive index is a slowly varying function of wavelength and decreases with increase in wavelength. The values of n acc and tacc for dopant concentration 2% are shown in Table 1. The values of m and t, determined graphically, were found to agree with the calculated values.
Fig. 4. Variation of absorption coefficient with wavelength as a function of doping. The solid lines represent the least square fit. Error bars have been shown at a point on each curve.
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
Fig. 5. Variation of absorption coefficient with doping at l s800 nm. Error bar has been shown at a point.
Refractive index was also fitted to a function for extrapolation to shorter wavelengths. The least square fit to Cauchy dispersion relationship w6x a ns 2 qb l gives a s 1.078 = 10 5 nm2 and b s 2.967 for TBPrSiH 4 concentration 0.1%. For other dopant ratios, the best fit values of a and b are listed in Table 2. The variation of a with wavelength for different doping concentrations is shown in Fig. 4. a is found to decrease with increase in wavelength. Fig. 5 shows the variation of a with doping concentration for l s 800 nm. a is calculated to an accuracy of 1%. The maximum possible error in a varies from "0.479 to "0.923 cmy1 for l s 800 nm. The thickness of the films measured by Talystep is close to that calculated from the transmission spectra. For instance, for the film with TBPrSiH 4 ratio 2% the measured value of thickness is 2450 nm, where as the calculated value is 2562 nm.
5. Discussion Srinivasan et al. w17x have reported increase of refractive index with doping in PH 3 doped a-Si:H films. As the PH 3rSiH 4 ratio increases from 5000 to 50,000 vppm the refractive index increases from 3.6 to 4.0 for wavelength 700 nm and from 4.03 to 4.35 for wavelength 600 nm. The experimental data on the refractive index of several PH 3 doped a-Si:H films strongly favour the existence of P3o centres
191
Žwhere P3o is the neutral threefold coordinated phosy Ž phorous atom. andror Tq where Tq 3 y P2 3 is the positively charged silicon dangling bond and P2y is the negatively charged twofold coordinated phosphorous atom. pairs as an efficient strain reliever. It has been shown by Dusane et al. w18x that in PH 3 doped a-Si:H, bond angle deviations increase sharply with increasing dopant concentrations. The increase in bond angle deviations cause internal strain in amorphous network. The increase in internal strain in the present case is the probable cause for decrease of refractive index with doping in TBP doped a-Si:H. The observed decrease of a at high TBP concentrations indicates a lower density of deep defects in these samples and is not due to an increase in the optical band gap which is found to be negligible.
6. Conclusion The refractive index of the n-type a-Si:H obtained using TBP as dopant is found to vary with doping concentration and can be tailored by changing the TBPrSiH 4 ratio. It has also been found that the absorption is extremely low for wavelength greater than 850 nm for all the samples. In GaAs, the formation of optical waveguides takes advantage of the refractive index dependence on Al concentration of Ga 1y x Al x As. It has been observed that epitaxially grown GaAsrAlGaAs used for waveguide purpose has refractive index 3.5 and attenuation 5 dBrcm at l s 850 nm w19x. It is thus suggested that the TBP doped n-type a-Si:H can be used as a thin transparent layer whose index of refraction is larger than the refractive indices of the two surrounding media, e.g. glass and air. It is observed that the refractive index for 0.1% dopant concentration is 3.152 at 800 nm and the absorption coefficient is the lowest Ž0.041 = 10 3 cmy1 . at this wavelength. Hence a planar as well as multilayer structure of n-type a-Si:H ŽTBP doped. can be fabricated for use for optical waveguide application.
Acknowledgements The authors are thankful to Professor P.C. Taylor, University of Utah, Salt Lake City, USA, for useful
192
R.M. Mehra et al.r Journal of Non-Crystalline Solids 209 (1997) 188–192
discussions and providing facilities for the growth of a-Si:H films. Two of the authors, Inderbir and Jasmina, acknowledge the help of UGC and CSIR for providing financial assistance for the research work.
References w1x J. Tauc, Amorphous and Liquid Semiconductors ŽNew York, Plenum, 1974.. w2x H.C. Casey, D.D. Sell and M.B. Panish, Appl. Phys. Lett. 24 Ž1974. 63. w3x T.L. Koch and U. Koren, IEEE J. Quantum Electron. 27 Ž1991. 641. w4x R.N. Hall, G.E. Fenner, J.D. Kingsley, T.J. Soltys and R.O. Carlson, Phys. Rev. Lett. 9 Ž1962. 366. w5x M.I. Nathan, W.P. Dumke, G. Burns, F.H. Dill and G. LLasher, Appl. Phys. Lett. 1 Ž1962. 62. w6x T.S. Moss, Optical Properties of Semiconductors ŽLondon, Butterworths, 1959..
w7x Y. Tawada, K. Tsuge, M. Kondo, H. Okamoto and Y. Hamakawa, J. Appl. Phys. 53 Ž1982. 5273. w8x J.C.C. Fan and B.S. Palm, Sol. Cells 10 Ž1983. 81. w9x H. Munekata and H. Kukimoto, Appl. Phys. Lett. 42 Ž1983. 432. w10x J.C. Jans and J.W. Gemmink, Appl. Opt. 32 Ž1993. 84. w11x H.A. Macleod, Thin Film Optical Filters ŽAdam Hilger, London and Macmillan, New York, 1987.. w12x R.M. Mehra, Gurinder, Inderbir, Jasmina, Amit Pundir and P.C. Mathur, J. Non-Cryst. Solids 164–166 Ž1993. 517. w13x C.A. Larsen, C.H. Chen, M. Kitamura and G.B. String, fellow, Appl. Phys. Lett. 48 Ž1986. 1531. w14x R. Swanepoel, J. Phys. E16 Ž1983. 1214. w15x R. Swanepoel, J. Phys. E17 Ž1984. 896. w16x J.C. Manifacier, J. Gasiot and J.P. Fillard, J. Phys. E, Sci. Instr. 9 Ž1976. 1002. w17x G. Srinivasan and A.S. Nigavekar, Mater. Sci. Eng. B8 Ž1991. 23. w18x R.O. Dusane, S.R. Dusane and V.G. Bhide, J. Non-Cryst. Solids 137&138 Ž1991. 115. w19x L.D. Hutcheson, Integrated Optical Circuits and Components, Design and Applications ŽMarcel Dekker, 1987..