K. Zellama

Crystallization in amorphous germanium

Crystallization has been studied in amorphous germanium layers produced by evaporation. The crystalline fraction is deduced from conductivity measurements. Depending upon the conditions of evaporation homogeneous or heterogeneous nucleation is observed and the crystallization is induced at the surface or in the bulk. The variations with temperature of the growth rate of crystallization and of the nucleation rate are obtained from the kinetics of the crystallization measured at various temperatures. The results allow to provide orders of magnitude for the thermodynamical parameters which characterize the crystallization.

Structure and crystal growth of atmospheric and low-pressure chemical-vapor-deposited silicon films

Structure and crystal growth of undoped, as‐deposited, and annealed silicon films prepared by chemical vapor deposition (CVD) and low‐pressure chemical vapor deposition (LPCVD) of silane have been studied with use of x‐ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). The grain size and a complete texture analysis are performed on CVD films grown at atmospheric pressure and temperature range 600≤Td≤805 °C, LPCVD films grown in the pressure range 0.1≤Pd≤2 Torr and temperature range 500≤Td≤650 °C and annealed amorphous CVD and LPCVD films near Ta=600 °C. We obtain systematically amorphous, strong 〈220〉 polycrystalline, and inhomogeneous partially crystallized films 〈111〉 or 〈311〉 oriented depending on the deposition conditions. The presence of a given texture is explained by a model which takes into account the specific free surface energies of the starting equilibrium forms and the extinction of some crystalline planes by {111} slow growing facets. The appearance of the 〈220〉 tex...

Exodiffusion of hydrogen in amorphous silicon

The exodiffusion of hydrogen in a-Si prepared by silane decomposition has been previously studied using H(B11,α)αα nuclear reaction and conductivity measurements which show that it takes place in two stages, one centered at 723 K, the other at 853 K. The first stage is due to the diffusion of “weakly” bond H. The second stage which we study quantitatively here by EPR, has an activation energy of 3,6 eV corresponding to the liberation of a bond hydrogen. The fact that the EPR signal is related to the second stage governed by a process whose activation energy is equal to the SiH bond energy is a direct evidence that the EPR signal is associated with dangling bonds which were saturated by hydrogen.

Experimental study of disorder and defects in undoped a-Si:H as a function of annealing and hydrogen evolution

Abstract Information on the incorporation of hydrogen and its effects on the disorder and the defects in undoped PECVD a-Si:H films deposited under different conditions is gained from systematicannealing studies up to 500–600°C. The results reveal a better bonded hydrogen stability in he samples deposited at high rate, related to their particular microstructure.

A new probe of the density of gap states and dangling bonds in a-Si: crystallization studies

Abstract We have used the temperature dependence of the crystallization growth velocity to determine the relative separation of the charged dangling bond states in a-Si. The D+ and D− states are found to be separated by approximately 0.35eV, and are roughly centered about midgap. Experimental evidence indicates that the crystal growth rates are accurately described by an Arrhenius equation and that doping enhances the growth velocity by increasing the concentration of charged dangling bonds and by lowering the activation energy. The lowering of the activation energy is described by a shifting of the Fermi level from near midgap to the D− (D+) states. The change in the exponential prefactor upon doping is used to determine that less than 1% of the dangling bonds in intrinsic a-Si are charged.

Compton profiles of amorphous and hydrogenated amorphous silicon

Abstract High resolution Compton profiles were measured by hydrogenated, dehydrogenated amorphous silicon powder and finally on the recrystallized sample, using synchrotron radiation. The observed structures seen in difference profiles are discussed, using two different rigid-band approaches for hydrogen insertion process. In particular, the analysis of results can support the hypothesis of very short HH bonds in the a -Si : H host. The long range order in the recrystallized sample was clearly evidenced by the present high resolution results. Nevertheless, these experimental results require a more accurate model than the rigid band assumption for the understanding of electronic distorsion due to hydrogen.

Transient Bulk Induced Nucleation in Amorphous Group IV Semiconductors

We present in this seminar an interpretation of our experimental results on bulk induced transient nucleation and growth in amorphous germanium and silicon with the theory of transient nucleation[ 1] . The phenomena has been observed [ 2,3] in a temperature range of 660–678 K in germanium and of 833–873 K in silicon. Before the steady state regime, a transient period of duration τ occurs. We determine an expression for τ and show that it is thermally activated with an activation energy 1.8 eV for germanium and 3.66 eV for silicon, very close to the respective growth rate activation energies. From this, we obtain the number of atoms of the nucleus of critical size for germanium and silicon. These values are in agreement with those previously obtained from the study of the steady state regime [ 2,3].