Cody disorder: Absorption-edge relationships in hydrogenated amorphous silicon (original) (raw)
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Disorder and the Optical-Absorption Edge of Hydrogenated Amorphous Silicon
Physical Review Letters, 1981
Lie i n v e s t i g a t e the e f f e c t of thermal and s t r u c t u r a l disorder on the e l e c t r o n i c s t r u c t u r e of hydrogenated amorphous s i l i c o n , by measuring t h e shape o f the o p t i c a l absorption edge as a f u n c t i o n o f temperature and hydrogen content. The data i s c o n s i s t e n t w i t h the idea t h a t the thermal and s t r u c t u r a l disorder are a d d i t i v e , and suggests t h a t disorder i s t h e fundamental determining f a c t o r o f the o p t i c a l bandgap. he have measured the o p t i c a l absorption edge as a f u n c t i o n o f temperature on high density1 a-SiHx f i l m s . k! e f i n d t h a t our data i s c o n s i s t e n t w i t h t h e i n t e r y et a t i o n t h a t both the w i d t h o f the exponential edge2 4 the o p t i c a l band pap are c o n t r o l l e d by the amount o f disorder, s t r u c t u r a l and thermal, i n the network, and t h a t hydrogen e f f e c t s the band gap i n d i r e c t 1 , through i t s e f f e c t on d i s o r d e r . This r e l a t i o n s h i p between the o p t i c a l gap and t h i sharpness o f the absorption edge suggests t h a t there i s a fundamental t r a d e o f f i n a-Sit!, s o l a r c e l l s between o p t i c a l absorption and electron-hole p a i r e x t r a c t i o n e f f i c i e n c y . C e t a i l s o f f i l m preparation and the o p t i c a l measurements have been given previously.3 I n order t o compare the e f f e c t s o f s t r u c t u r a l an3 thermal disorder on the absorption edge we have a l s o induced s t r u c t u r a l disorder i n t e n t i o n a l l y i n the f i l m s by i n t r o d u c i n g dangling bonds through thermal e v o l u t i o n o f hydrogen. I n Fig. 1 are shown o p t i c a l measurements as a f u n c t i o n o f photon energy, E, a t T = 12.7K, 151K and 293K on an "as prepared" f i l m o f composition SiHg.13. 1, Je a l s o show data obtained a t T = 293K on a s i m i l a r f i l m , from which hydrogen was evolved i n a step-wise manner through isochronal heating i n a vacuum a t 25C i n t e r v a l s from 400-600C (TH) f o r 30 minutes a t a time. W e note t h a t the absorption edge broadens and s h i f t s t o lower energy w i t h e i t h e r increasing thermal disorder o r w i t h s t r u c t u r a l disorder due t o the isochronal h e a t i n The exponential dependence o f a on E, f o r 2 x 102 cm-1 < a < 5 x 103 cm-7; (See Fig. 1) has been shovin i n a previous paper t o extend f o r about t h r e e and h a l f orders o f magnitude i n a 2 . The departure from an exponential shown i n Fig. 1 a t low a i s due t o the i n s e n s i t i v i t y o f the o p t i c a l transmission measurement technique. From Fig. 1 we draw the conclusion t h a t u(E,T) can be expressed by the Urbach form4,5 where Eo(T,X) i s the w i d t h o f the exponential t a i l , X i s an as y e t t o be defined ?arameter d e s c r i b i n g s t r u c t u r a l disorder, and a, = (1.3 + 0.4) x 106 cm-l and El = 2.17 * 3.02 eV, as determined b a l e a s t square f i t o f Eq. (1) t o the data r f o r 2 x 102 cm-1 < a < 5 x 103 cm-. To deduce the temperature dependence o f the o p t i c a l energy gap Eg, we f i t t e d a(E,T) outside the exponential region ( a > 104 cm-I), by the expression [~( E , T ) E ] ' /~ = C(E-EG(T))
Disorder and Optical Absorption in Amorphous Silicon
Aps March Meeting Abstracts, 1996
The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.
Optical properties of hydrogenated amorphous silicon
Journal of Applied Physics, 1986
A detailed study of the optical properties of sputtered hydrogenated amorphous silicon films with varying hydrogen concentration is presented here. The energy dependence of the absorption coefficient is looked into, in detail, from a point of view of understanding the well known Tauc rule and the alternate relations being proposed in recent years. Spectroscopic and band-structural models like Wemple-Didomenico and Penn are then utilized to analyze the optical parameters near the band-gap region of the wavelength spectra. Extensive comparisons of our results are made with those of sputtered a-Si:H films of other workers, glow discharge prepared a-Si:H, chemically vapor deposited and evaporated a-Si, and also crystalline silicon. The similarities in the variation of the optical properties of a-Si:H with increasing hydrogen concentration (or decreasing measurement temperature) to that of crystalline silicon with decreasing measurement temperature lead us to interesting conclusions. Thus, it seems that decreasing disorder (topological or thermal) in a-Si:H is equivalent to decreasing thermal disorder in c-Si, at least as far as the disorder-optical property relationships are concerned.
An attempt is made to highlight the importance of inhomogeneities in hydrogenated amorphous silicon (a-Si:H), in controlling its electronic properties. We note that hydrogen increases the gap in a-Si:H and that hydrogen is distributed inhomogeneously in it. This gives rise to long-range potential fluctuations, which are mostly uncorrelated and usually ignored. These and other such considerations have not only enabled us to gain new insights into the behaviour of a-Si:H in general, but have also allowed us to resolve several unsolved puzzles. Among these are questions like why undoped a-Si:H is n-type, why the creation of dangling bonds upon light soaking (LS) so inefficient, why a-Si:H degrades more upon LS when it is doped, why the reciprocity fails for light-induced degradation, why presence of nanocrystalline silicon improves stability and so on. We provide evidence to support some of our ideas and make suggestions for verifying the others.
Defect transition energies and the density of electronic states in hydrogenated amorphous silicon
Journal of Non-Crystalline Solids, 2002
Using photoluminescence excitation (PLE) spectroscopy, we report detailed measurements of the fundamental absorption threshold below the optical gap in hydrogenated amorphous silicon (a-Si:H). These measurements suggest that the density of neutral defects is much greater than the densities of charged defects in intrinsic a-Si:H. The positions and widths of the corresponding transition energies are determined and agree with two models proposed to describe the density of states in a-Si:H.
Disorder and optical absorption in amorphous silicon and amorphous germanium
Journal of Non-Crystalline Solids, 1997
The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.
2012 IEEE 38th Photovoltaic Specialists Conference (PVSC) PART 2, 2012
Temperature annealing is used as a tool to study the validity of network models for the nanostructure of hydrogenated amorphous silicon (a-Si:H) and its relation to defect states. The changes in the size of the dominant open volume deficiencies have been studied using Doppler broadening positron annihilation spectroscopy and Fourier transform infrared spectroscopy. It is shown that the dominant open volume deficiencies for as-deposited films are divacancies, which appear to agglomerate into larger open volume deficiencies up to 400 • C. Above this temperature, the largest open volume deficiencies are suggested to be released at the surface of the sample. Fourier transform photocurrent spectroscopy results indicate a dramatic increase in the density of various subgap defect state distributions during temperature annealing. In addition, at least four defect states have been identified. These findings cannot be directly explained by assuming solely dangling bonds as the dominant defects in a-Si:H. We discuss that a model based on an anisotropic disordered network with volume deficiencies does explain our findings better than the classical model based on a continuous random network with solely an isotropic distribution of coordination defects. The claim is made that next to dangling bonds not fully hydrogen-passivated vacancies are significantly contributing to the dominant defect states in a-Si:H.
Electronic and transport properties of hydrogenated amorphous silicon
Physical review. B, Condensed matter, 1985
We have extended previous coherent-potential-approximation calculations of the electronic and transport properties of hydrogenated amorphous silicon (a-Si), in order to examine the effects of fully dispersed hydrogen in a-Si. The present calculation replaces random vacancies in the Si matrix by single H atoms instead of the four-H-atom clusters previously considered. In addition, to eliminate dangling-bond states in the gap we have introduced an ad hoc reconstruction of the lattice around the vacancy by effectively saturating the dangling orbitals with other Si atoms. Our results reinforce previous claims that an understanding of various experiments in a-Si:H can be obtained from first-principles calculations which neglect topological disorder and the precise configuration of the hydrogen atoms. The present calculations lead to an improved agreement with the photoemission and optical absorption data.
Applied Surface Science, 1997
Simple methods have been developed to enable the X-ray excited silicon L 2.3VV, and the experimentally more difficult L,L,.,V, Auger spectra to be treated routinely using numerical debroadening and deconvolution to obtain an indication of the valence band transition densities of states for the surface. A method based on the simplex algorithm has then been applied to enable both the Si L,,aVV and Si L ,L2,sV spectra to be decomposed (decoupled) into their component (pp-, spand ss-like) peaks. Changes in these components have been compared before and after disordering and hydrogenation of the surface to quantitatively probe the effect of these treatments on the surface valence band densities of states. It is shown that both the L,,,VV and L ,La,aV lines give a simple semi-quantitative method for monitoring hydrogen incorporation in hydrogenated amorphous silicon. Examples of these methods applied to artificially and naturally hydrogenated amorphous silicon surfaces are presented to illustrate their usefulness for studying materials fabricated for use in solar cells.