Optical absorption of SiH0.16 films near the optical GAP

Journal of Non-Crystalline Sollds 35 ~North-Holland Publishing Company

&

36 (1980) 463-468

OPTICAL ABSORPTIONOF StIEO.16 FILMS NEAR THE OPTICAL GAP G. D. Cody, S. Abeles, C. R. Wronskl and B. Brooks Exxon Corporate Research Linden, New Jersey 07036 W. A. Lanford Yale University New Haven, Connecticut 06520 We have made detat|ed measurements of the opttcal transmission and reflection tn the photon energy range of 0.5 to 5.0 eV on $tH0.16 ftlms deposited by discharge decomposition of StH4 on quartz substrates held at 240°C. Above 2.0 eV, the spectral dependence of c2 is in excellent agreement wtth a model derived for parabolic bands In a disordered material. The range of coherence ts about 20A very close to other materials (a-St, a:Ge, As2Se3). The extrapolated value of the band gap (Eg) ts 1.8 eV. Below 1.8 eV we have uttltzed photoconductlvttyand optical absorption and have observed an exponential dependence tn to 1.5 eY wtth an activation energy of 0.06 eV. From 1.5 eV to 1.2 eV absorption occurs to a photoconducttng state wtth a maxtmum at 1.3 eV. INTRODUCTION Amorphous stllcon hydride (a-StHx) ts ~ new semtconducttng material. I t ts the f i r s t thin film matertal that exhtbtts a f u l l range of semtconducttng properties (optical ~ap, extended state mobility, doping s e n s i t i v i t y ) independent of substrata.m~urrent interest in the performance of thts material as a thin ftlm photovoltatc J enhances the Importance o f characterizing as f u l l y as possible the semtconducttng properties of thts Interesting new material. In thts paper we focus on the opttcal properties of ftlms prepared by RF capacitive glow discharge decomposition on anodtc substrates held at temperatures above 250eC (RFGD ftlms). The optical properties of the ftlm, ~the complex index ~ (~ = n + t K) and di= e l e c t r i c constant ~ ('~ = Cl + I~2 = nZ) were measured by transmission and reflection (with respect ~o c-St). At very low energies, we uttltzed the film photoconducttvtty normalized to the transmission measurements.i[t a photon energy of 1.75 eV. Hydrogen concentrations were determined by the Nj ° resonance reaction4 as well as by calibrated measurements of the integrated absorption in the St-H stretch vibration at 2000 cm-1. The major interest of the paper is the spectral dependence of the optical absorption constant ~(~ = 4~ EG; E ;~ EG; and E < EG, where E ts the e,ergy of the tnctdent l t g h ) and EG is t~he opttcal gap defl~ed by the "Tauc" linear extrapolation of (¢2EZ)l/2 or almost equivalently, (~E} / , to zero. A basic assumption of the present work is that ftlms, whose thickness (D) ranges from 0.03 to 5U, are i d e n t i c a l , although surface regions (substrate or free) may play a dominant role tn certain regtons of E, 0 space.

463

46h

GoD. Cod9 et al. / Optical Absorption of SiHo~I6 Films

FILM PREPARATION The StHx films were prepared by glow discharge decomposition of stlane (StH4) tn a capacitive reactor operated at 13.66 Mhz. The discharge electrodes were parallel plates, 6 or 8 inches tn diameter, spaced 2 inches apart. The substrates were placed on the lower electrode (anode) which was grounded e l e c t r t c a l l y and heated by a resistance heater. The temperature of the substrates was determined by c a l i b r a t i o n wtth a thermocouple embedded tn a dummy substrate. Undiluted CVD stlane was injected into the reactor through a mass flow c o n t r o l l e r . The system was pumped by a turbomolecular pump and pressure in the reactor was maintained at 30 mtorr by an adjustable t h r o t t l e valve. A mass spectrometer and capacitance manometer were used to measure the parttal pressure of StH4 rich plasma. Low RF power and high SIH4 i n l e t rate resulted in a SIH4 rich plasma; low flow rate and high RF power resulted in a StH4 depleted plasma. For opttcal measurements, ftlms were deposited on quartz substrates; films for IR measurements were made on o p t i c a l l y poltshed c-St substrates. OPTICAL ANALYSIS In the absence of multtple r e f l e c t i o n s (E > EG) we analyzed the transmission and free surface r e f l e c t i o n c o e f f i c i e n t by TO = (C 16

nsl 12A)/(! ] + 12lns + ki2); Ro

- ]12/i

+ 112.

(l)

In the above expressions, ns is the real index of the substrate; C is a c o e f f i c ient to account for the m u l t i p l e incoherent r e f l e c t i o n s tn the substrate (for quartz, C = 0 . 9 7 ) ; ~l is the complex index of a-SiN x and A = exp (-~D) where D is the @]lm thickness. R e f l e c t t v t t y was measured in non-polarized l i g h t at an incidence angle of less than 20 ° ~The c-St sp~ctmen was calibrated at NBS. Iteration o f To and Ro determined n and hence c. In the low energy regime, we chose to average adjacent fringes to obtain the incoherent form for the transmission, : = (Tmax Tmtn)1/2 = To/[l-(r2]r23A)23,

(2)

where r21 = (n - ] ) / ( n + 1) and r23 = (n - ns)/(n + n ). In thts regime, we neglect the imaginary part of ?( in r21 and r23. We o~taln a measurement of n by the magnitude of the transmission minima. These values joined smoothly to those determined by Ro for E • Eg. The thickness of our films was determined both by contact measurements of a "step',' or by analysls of the fringes. The two approaches agreed to about +_ 5%. Both the transmission and photoconduct|vlty measure an average attenuation and cannot, by themselves, distinguish l,homogeneous absorption from homogeneous. To separate surface effects requires films of the same properties, but different thlcRnesses. For E • 1.75 eV, we believe volume absorption dominates. For 1.60 < E < 1.75, there Is some evidence for surface absorption in our films. The "absorption" coefficient in this case is denoted by <~D>ID. EXPERIMENTAL RESULTS E • EG In Fig. 1 we e x h i b i t ~ and n for anodlc films o f normal co,nposttton StHo.16 prepared at substrate temperatures of 240oC. We ascribe the scatter in ~ to thickness uncertanty. Fig. 2 displays the data in a form suggested by the Tauc mode] for optical absorption in a disordered material, with a free electron density of states in both the occupied and unoccupied levels5:

465

G.D, Cody et alo / Optical Absorption of SiHOol6 Films

(¢2E2) 1/2 = (E-EG) (e2,/2~2) 1/2 (V Dal) 1/2

(3)

In the above, EG ts the optical gap, V ~s the volume of the specimen and Da2 is the square of the gradient matrtx element for the opttcal transition at energy E. U t i l i z i n g ~2 = 2n~c the above expression ts equivalent to (~E)I/2 ~ (E-EG) I f n(E) does not vary appreciably in energy. Following Tauc5 and Davis andMott6: VDa2 = (Vo/V)(VDx 2) - VoDx2 where Dx ts the matrix element equivalent to Da for the direct transition In the crystal. Vo ts the "coherence volume" In the disordered solid, t . e . that volume over which sufficient order exists to define momentum. From r i g . 2 and a value for Dx for c-St (E • 3.2~V) we obtain for E • 2,2 Vo (23A) 3 and EG = 1.84 eV. For 1.75 < E < 2 . 2 , Vo~ (16A)3 and EG = 1.72 eV: The solid curves in Fig. 1 correspond to Eq. (3) and these parameters. The striking ltnear variation for (clEl) 1/2 over a wide range of E(1.5 eV) Is also exhibited for a-GeT and a-St8, but with about one half the coherence volume. For a-St, EG = 1.5 eV, the Vo for a-StHo.16 corresponds to about 600 atoms. I t is Interesting to note a recent calculation of Chtng and Ltn ~ on a 54 atom continuous random network which exhibits good agreement wtth Eq. (3) above 2.4 eV for EG ~ 1.5 eV and Vo ~, 1/2 (23A)3. The extended agreement wtth Eq. (3) appears to be a property of "ideal" amorphous materials both tn theory and experiment. EG is a "one-parameter" characteristic of the opttcal properties of the ftlm for E > EG and ts denoted the optical gap. In Ftg. 3 we exhibit thts quantity as a function of measurement temperature for one ftlm (a-StH 0 24). In Fig. 3 we also show the temperature dependence of the indi,ect gap of c'St scaled by the ratio of optical gaps at 30OAK.lu The Eo3 data of Freeman and Paul for sputtered ftlms appears to vary s l t g h t l y more linearly wtth temperature than the glow discharge matertal but has a stmtlar temperature derivative at 30OK.mm We conttnue to focus on EG and tn Fig. 4 exhibit Its dependence on the temperature of the ftlm during growth. For comparison we exhibit a curve gtven by the RCA group12 as well as EG for an anodlc ftlm prepared by Tsat and Frttzche.2 The agreement ts satisfactory. In Ftg. 5 we show EG as a function of hydrogen concentration where .the comparison ts again to the work of Tsal and FrttzcheZ as well as Janai et. al.13 (silane pyrolysis). Where we have not measured the hydrogen directly we have utilized the calibration plot shown tn Ftg. 5 between the opttcal density at the St-H stretch and the measured hydrogen concentration. In Ftg. 5 the notation S (small), L (large), and VL (very large) refers to the "dthydrtde"- ~mponent in the stretch absorption l i n e . EXPERINENTAL RESULTS E ~ EG In order to conttnue the optical measurements to regtons well below Er., we have made photoconducttvtty measurements. We u t t l t z e the following relatt6n for weakly adsorbed l i g h t : F<~D>~ I1/y where ! Is the photocurrent, F ts the tnctdent flux and 7 ts a measure of recombination kinetics, y has the value uf 0,7 independent of E in thts range. The photoconducttvlty data was scaled to the optical absorption at 1.8 eV. Ftg. 6 shows the results. There are several noteworthy features of, Ftg. 6. One is the exponential drop tn opttcal absorption, over two orders of magnitude in 0.25 eV, which has a mtnlmum In the v t c t n t t y of a photon energy of 1.5 eV. This is the f i r s t experimental evtdence for an exponential tat l tn a-SiHx .Z The.,excitation energy (~ 0.06 eV) ts surprisingly close to that observed for other amorphous materials. 5

466

G.D. Cody et al. / Optical Absorption of SiHo.16 Films

The other noteworthy point ts the maximum at 1.3 eV which may be ascl~tbed to structure in the gap states wlthtn 0.5 eV of the valence band edge. SUHHARY We have shown that the opttcal properties of a-StH 0 16 exh|btt reproducible and almost idealized behavior. The absorption edge ts a sharp exponential between 1.5 eV and 1.8 eV and t s p a r a b o l l c from 1.8 eV to 4.1 eV. Below 1.5 eV there is evtdence for an absorption to a photoconducttng state down to at least 1.2 eV, with a maximum at 1.3 eV. The optical gap defined by Tauc ts 1.84 wtth a coherence volume o~ 600 atoms. The energy dependence of (~E)I/2 can be associated wtth a reduction of the coherence volume by a factor of two between 1.84 and 1.72 eV for STH0.16. The opttcal gap ts a convenient and well-defined parameter for measurements at different temperatures or to characterize ftlms prepared under d i f f e r e n t conditions. The temperature dependence of E~ can be scaled to the temperature ~ependence of the indirect gap of c-St ~tnd, by just the ratto of EG to Eind ('I .63). ~CKNOWLEDGHENTS We want to acknowledge the interest and encouragement of F. R. Gamble. We have benefited from discussions wtth T. Ttedje, J. P. de Neufvtlle, A. Ruppert, T. D. Moustakas, and D. Morel. We are p a r t i c u l a r l y indebted to H. Stasiewskt. We are tndebted to Robert Sherrter, who made significant experimental contributions early in the program.

467

GoD~ Cody et al. / Optical Absorption of SiHOol6 Films

I

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100

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EG = 1.84

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Fig. 3 EG(T) compared with the indirect gap of c-Si scaled by a factor of 1.63. (~E)1/2 as a fu'nctton of E is shown at 90K and 297K.

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Fig. 4 EG as a function of substrate temperature, T F, for anodic RFGD films.

468

G.Do Cody et al. / Optical Absorption of SiHo,16 Films

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EG and integrated absorption at 2000 cm-I (A s : Si-H stretch} as a function of at % H. The notation S, L and VL refers to "dihydride" content.

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Fig. 6 Comparison of optical absorp tion inferred from photoconductivity (PC) and derived from optical transmission in fringe region (average absorptance) and in non-fringe region (Fig. 1 ).

REFERENCES

I. R. C. Chlttick, J. Non-crystl, Solids 3255 (Ig70). 2. C. C. Tsai and H. Frltzsche, Solar Enermgy Materials 1 2 9 {1979). 3. C. R. Wronski, D. E. Carlson and R. E. Daniel, Appl .--Phys. Letters 2_.99602 (1976). 4. J. F. Ziegler et. a l . , Nucl. Inst. Meth. 149, 19 (1978). 5. J Tauc in Opt. Prop. o f Solids, ed. by F.-~beles, North Holland 1972, pp. 279-313. 6 E . A . Davis and N. F. Mott, Phil. Mag. 22 903 (1970). 7. g. Paul, G. Connell and R. Temkin, Adv.~n Phys. 22 529 (1973). 8. D. T. Pierce and W. E. Spicer, Phys. Rev. B5 30 17 {1972). 9. 14. Y. Ching and C. C. Lin, Phys. Rev. B "197F. 10. G. C. Macfarlane et. al. Phys. Rev. 111 1245 (1958). 11. Eva C. Freeman and William Paul, to be published (1979). 12. P. J. Zanzucchi, at. a l . J. Appl. Phys. 48 5227 (1978). 13. M. Janai, et. al. Solar Energy Materials-~_ 11 (1979).