C. Ance

Refractive index dependence on optical gap in amorphous silicon—Part I. Si prepared by glow discharge

Abstract We present here our studies concerning the variation of refractive index with the optical gap in amorphous silicon prepared by glow discharge decomposition of silane. The study has been carried out in the light of the models of Penn, Wemple-Didomenico, Ravindra et al ., Moss and Bahl-Bhagat. It is essentially seen that the model of Bahl-Bhagat is good enough to explain the relative shifts in the refractive indices in terms of the changes in the gaps on introduction of hydrogen into amorphous silicon. However, because of weaknesses associated with the fitting parameters, we propose here an alternate model which explains fairly well the dependence of the index of refraction on the optical gap. Furthermore, to explain the gradient of the refractive index vs optical gap plots, we see that a simple model like that of Moss would suffice. This is all the more interesting by virtue of the fact that the Moss formula is basically representative of the atomic picture. Of course, the constant depends on conditions during formation of the sample. Under some limiting conditions, the Bahl-Bhagat relation is shown to reduce to the linear form like that of Ravindra et al . We also attempt to analyse qualitatively the dependence of dispersion energy and the average excitation energy on temperature in the light of the Wemple-Didomenico model. The present study has been carried out for samples prepared by glow discharge at different substrate temperatures and with different hydrogen concentrations.

Boron doping effect on optical properties of a-Si films prepared by CVD: Post-hydrogenated and H-exodiffusion study

Abstract The effect of post-hydrogenation and H-exodiffusion on the optical parameters is studied in boron doped amorphous silicon prepared by CVD. The optical parameters, which depend on boron concentration, vary when hydrogen is incorporated. The variations observed are compatible with a decreasing of the spin density. When the measurement temperature increases, irreversible effects appear due to evolution of hydrogen. These results are interpreted by a simple model of electronic compensation of defects by doping and a variation of the density of electrically active boron atoms with hydrogen concentration.

Temperature dependent optical absorption in hydrogenated silicon from amorphous to crystalline state

The optical absorption of hydrogenated silicon has been determined at low and elevated temperatures from 4 to 973 K for several amorphous and crystalline states obtained by annealing. The variation of the optical gap is deduced using the Tauc model and then interpreted using the Varshni relation. In the energy range considered, the absorption coefficient increases exponentially with temperature. A characteristic thermodynamic function such as heat capacity C(T) has been calculated using parameters occurring in the Varshni relation. This is shown to illustrate the differences between the amorphous states obtained during annealing.

Temperature effects on the optical properties of amorphous hydrogenated silicon nitrogen alloys prepared by RF sputtering

Amorphous hydrogenated silicon nitrogen alloys with variable nitrogen concentration were prepared by the reactive RF sputtering method. The optical gap shows a drastic increase at an atomic N/Si ratio near unity. The variations of the optical gap and static refractive index were measured at different temperatures and after several heat treatments. Irreversible effects related to hydrogen evolution and changes in the microscopic structure were observed, the importance of which depends on the nitrogen concentration.

Urbach tail and optical gap of G D a-Si:H films: Temperature and doping effects

Abstract Optical gap Eg and Urbach tail parameter Eo of undoped and Phosphorus doped hydrogenated amorphous silicon films have been determined in the temperature range Tm (95 to 750 K). P doping effects on optical gap and Urbach tail parameter are negligible. These two optical properties are seen to depend on measurement temperature, hydrogen content, structural disorder and doping.

Refractive index dependence on optical gap in amorphous silicon—part II. Si prepared by chemical vapour deposition

In the present study, we essentially attempt to extrapolate our earlier studies(1) to amorphous silicon films prepared by chemical vapour deposition using a slightly different approach. This work assumes significance especially in view of a rather dangerous trend prevailing today to generalize model calculations without attributing any importance to the method of preparation of the films.(2–4) Here, we adopt a spectroscopic approach to analyse the various properties of the a-Si films. Some of these films are doped with boron and most of them have been subjected to post-hydrogenation. This study leads us to conclude that the Moss formula(5) is again a fairly good representative of the optical gap-refractive index variation. The scheme developed by Ravindra et al.(6) is shown to be valid for these CVD films. In the passing, we attempt to analyse the properties of films prepared at different temperatures and at different rates of deposition. The model of Revesz(7) is utilised to explain the variation in the density. Our study leads us to believe that for the CVD technique (a high temperature technique with temperatures close to that of crystallisation(8), the rate of deposition seems to be very important in determining the surface conditions of the films. Further, it is seen that on post-hydrogenation, the spectroscopic parameters as have been defined in the Wemple-Didomenico model(9) behave randomly. The study has been carried out on a large number of samples.

Optical gap of hydrogenated amorphous and polycrystalline silicon between 4 and 973 K

A detailed study of the evolution with temperature of the optical properties of hydrogenated silicon from the amorphous state (a-Si:H) to the polycrystalline state (poly-Si) is presented. Optical measurements were performed up to 3 eV for several temperatures in the range 4–973 K and these were used to determine in particular the optical gaps Eg and the slopes B using the well-known Tauc rule (αE)12 = B(E − Eg). The effects of temperature annealing are analysed from the variations in Eg and B before and after crystallization. The influences of structural and thermal disorders in a-Si:H and poly-Si on the optical gap are discussed.

Static refractive index of hydrogenated amorphous and polycrystalline silicon between 293 K and 973 K

Optical measurements were performed from 0.5 eV to 3 eV in the temperature range 293–973 K on hydrogenated silicon thin films. The thicknesses of the films were chosen to obtain an accurate optical gap Eg and static refractive index n0. For a given sample, opposing variations in Eg and n0 are observed with measurement temperature for both amorphous and polycrystalline states. The observed decrease in these two parameters on crystallization is attributed to an increase in voids and to a variation in structural disorder.