M. L. Theye

Relations between the deposition conditions, the microstructure and the defects in PECVD hydrogenated amorphous carbon films; influence on the electronic density of states

The relationships between the deposition conditions, the growth mechanisms, the microstructure and the electronic density of states of hydrogenated amorphous carbon (a-C:H) films prepared by PECVD from hydrocarbons are not yet fully understood. We therefore performed a systematic study using several complementary techniques to determine the changes in the microstructure and in the optical properties of a-C:H samples deposited from a dual micro-wave/radio-frequency discharge in methane as a function of the negative r.f. bias voltage applied to the substrate −Vb. The results reveal the existence of two successive film growth regimes when varying −Vb from −30 to −600 V, i.e. when increasing the ion bombardment during deposition, which lead to different types of sp2 C atoms clustering and H incorporation, but to the same density of paramagnetic defects. Models of the a-C:H microstructure are proposed in each case, and their influence on the electronic states density is analysed in detail. It is shown that the H content and the proportion of sp2 C atoms play a minor role on the electronic properties, as compared to the nature, the size, the number and probably the distortions of the π-bonded clusters.

Optical investigations of the microstructure of hydrogenated amorphous carbon films

The electronic and vibrational properties of hydrogenated amorphous carbon (a-C:H) films deposited by a dual electron cyclotron resonance-radio frequency (ECR-RF) plasma technique from methane at different negative RF substrate bias voltages have been investigated by several complementary optical methods covering a wide spectral range (0.06 to 30 eV). The results as a whole are interpreted in terms of local C/C and C/H bonding configurations and their effects on the electronic states introduced by the π-bonded clusters.

Influence of disorder on the electronic distribution of InP by X-ray and photoelectron spectroscopies

Abstract Soft X-ray spectroscopy (SXS) and X-ray induced photoelectron spectroscopy (XPS) measurements have been performed on amorphous InP samples prepared by flash evaporation, comparatively to crystalline InP. SXS results reveal striking differences in the 3p states of the valence band distribution of a-InP as compared to c-InP. The presence of a partial chemical disorder in a-InP is deduced from XPS measurements.

Hydrogen incorporation in device-quality a-Si:H deposited at low temperature

Abstract The nature of H bonding and its influence on the optoelectronic properties of device-quality glow-discharge a-Si:H deposited at 50°C using high dilution of SiH 4 in H 2 are investigated by H evolution and annealing experiments. The results give clear evidence for H incorporation modes very different from those in standard low temperature a-Si:H.

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.

Investigations on the influence of the dihedral angle distribution on the atomic radial distribution function in amorphous Ge and Si

Abstract The relations between the dihedral angle distribution function and the atomic radial distribution in tetrahedrally coordinated amorphous semiconductors Ge and Si are investigated theoretically by considering the contribution of the 3-bond connected atoms in different models. The results are discussed and compared with experimental diffraction data.

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 Hue5f8H 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.

X-Ray Spectra of Amorphous and Crystalline GaP

The modification of the electronic density of states of tetra-hedrally bonded semiconductors on going from the crystalline to the amorphous form, have been related to the different elements of disorder: topological disorder (in particular odd-membered rings of bonds) and quantitative disorder (fluctuations in bond-length and bond-angle, and distribution of dihedral angles). In the case of compound, additional effects may arise from atoms of the same kind, ‘like-atom’ (maverik) bonds, which result from the chemical disorder or are introduced by the network topology. Extensive theoretical studies performed on GaAs (taken as the prototype of III-V compounds) have shown that the effects of like-atom bonds on the electronic density of states may depend appreciably on their clustering configuration2.