J.M. Berger

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.

The effects of surface characteristics on the visible and uv optical properties of CVD amorphous silicon films

Silicon films were deposited by the pyrolytic decomposition of silane onto fused quartz substrates held at various temperatures in the range 550 to 800°C. The films are amorphous when prepared below a substrate temperature Ts of 650 to 700°C. Scanning electron micrographs indicated that surface roughness increased with increasing Ts. Optical properties were determined from near-normal specular reflectance and transmittance measurements in the energy range 0.5 to 14 eV. Diffuse reflectance measurements in the energy range 0.5 to 5.5 eV permitted the determination of the hemispherical reflectance. These measurements indicated that the CVD amorphous silicon films significantly scattered photons having energies above 2 eV. In addition, the uv measurements indicated the presence of silicon dioxide layers which was confirmed by specular reflectance measurements of samples annealed in air. The presence of scattering through surface roughness and the influence of oxide layers must be taken into account carefully when optical measurements above 2 eV are made the basis for a subsequent Kramers-Kronig analysis. Theoretical conclusions based on the results of such an analysis deserve confidence only if the surface roughness and oxide layers do not detectably affect the specularity of the optical measurements.

Temperature dependent studies of the optical properties of post-hydrogenated sputtered a-Si:H

Abstract In the present study, post-hydrogenation and annealing effects on the optical properties of amorphous silicon films deposited by rf sputtering are described. It is essentially seen that large changes in the hydrogen concentration on the various sites occur and the optical properties appear to be strongly dependent on the tissue. A decrease of the residual disorder with post-hydrogenation for hydrogen saturated or unsaturated samples is deduced from the determination of the structural part Fs of the Urbach tail.

Variations of optical properties between 95 and 723 K of boron doped a:SiH films prepared by glow discharge

Abstract The study of optical properties of boron doped a:SiH films shows that the boron atoms disturb the silicon matrix. Contrary to hydrogen, boron doping makes the optical gap decrease. When the measurement temperature T m becomes higher than the preparation temperature T s , irreversible variations of optical gap appear in a sense opposite to those of undoped samples. This study leads us to believe that boron doping creates an increase in the density of states in the forbidden band.

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 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.

Boron doping and post-hydrogenation effects on optical gaps and urbach tails of amorphous hydrogenated silicon films prepared by chemical vapour deposition

Abstract The effect of doping and post-hydrogenation on the optical absorption edge in amorphous hydrogenated silicon prepared by chemical vapour deposition was studied. The width of the exponential tail increases as a result of boron doping and decreases as a result of post-hydrogenation. These results are discussed in terms of a model of electric field broadening of the edge by electrically active dopant atoms. The reduction in the slope due to post-hydrogenation implies some H-B interaction which is analysed and compared with previous conductivity and electron spin resonance results.