A. Gheorghiu

Determination of low absorption coefficients from absorptance measurements on thin films

Abstract We discuss the determination of the absorption coefficient (α) from absorptance (A) measurements on thin films in the low absorption range, especially in the case of amorphous semiconductor films studied by photothermal spectroscopies. We propose approximate expressions between A and α, determine their range of validity and compare them to expressions used by other authors.

Absorption edge in amorphous Ge and GaAs: Comparative study of the role of disorder and defects

Abstract The optical absorption edge of evaporated a-Ge films and flash-evaporated a-GaAs films is accurately determined as a function of annealing, down to 0.5 eV, by combining spectrophotometric and photothermal deflection spectroscopy measurements. The results are compared and interpreted in terms of disorder and defects. A dangling bond contribution is clearly observed in annealed a-Ge. The defect states associated with dangling bonds and wrong bonds in stoichiometric and As-rich a-GaAs are discussed.

Influence of π-bonded clusters on the electronic properties of diamond-like carbon films

Abstract A comparative study of two series of a-C:H films prepared by different techniques and presenting strongly contrasting properties was carried out using several complementary techniques probing the electronic density of states and the incorporation of hydrogen. The results were analysed using a two-phase model consisting of π-bonded clusters embedded in an amorphous sp3 C:H matrix. It is shown that the two series differ essentially in the sizes of the clusters and in the degree of confinement of the π electrons within the clusters, and that hydrogen plays a significant role in the film microstructure and its modifications upon annealing.

Adhesion of a thin silicon oxide film on a polycarbonate substrate

Abstract The present paper analyzes the adhesion mechanisms of a silicon oxide film on a polycarbonate substrate, combining XPS, UV-visible ellipsometry and Fourier transform infrared phase-modulated ellipsometry (FTPME) measurements. In particular, we present the new FTPME technique: Due to its submonolayer sensitivity to vibrational properties, it suits well to physico-chemical studies, such as plasma-surface interactions. A preliminary plasma treatment of the polymer substrate before film growth is found to increase the sticking of the first monolayers of the film, and therefore its adhesion, as shown by conventional thermal adhesion tests. The treatment consists of three successive steps: argon plasma, (NH3, Ar) plasma and (SiH4, He, Ar) plasma. The first two steps induce a densification of the bulk and an activation of the surface (formation of CN bonds), while the silane plasma leads to the growth of a very thin silicon oxide layer. In particular, the increase in the film adhesion appears to be closely related to the presence of a SiO vibrational mode located at 1030 cm−1.

Local order and H-bonding in N-rich amorphous silicon nitride

Abstract The local order in Plasma-Enhanced Chemical Vapor Deposition (PECVD) N-rich a-SiN x :H films is shown to depend strongly on the H bonding configuration. The SiH bonding affects the SiN lattice vibration, giving a shoulder at 1020 cm −1 , and shifts the Si3p - N2p peak position in the valence band distribution towards lower binding energies.

Photoelectron spectroscopy study of amorphous GaAs and Ge

Abstract The results of a detailed study of the effects of disorder and defects on the core level and valence band distributions in amorphous GaAs by X-ray photoelectron spectroscopy, are discussed and compared to those of similar experiments on amorphous Ge. We analyse the differences between the compound and the element in terms of medium-rangedisorder and we study the annealing effects. In the GaAs case, we show evidence for states associated with AsAs wrong bonds, in agreement with theoretical predictions.

Studies of short-range order in amorphous Ge x Se100−x compounds by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy experiments were performed on as-deposited evaporated a-Ge x Se100-x compound films, with x ranging from 20 to 50 at.%, in order to investigate the changes in the core level and valence band electron distribution as a function of composition. The results are interpreted in terms of short-range ordering, taking the stoichiometric a-GeSe2 compound as a reference. The valence band spectra are compared to theoretical predictions and discussed in relation to optical data obtained on the same samples. It is shown that 4:2 coordination is maintained over the whole composition range, and that chemically ordered structural units are favoured in all cases.

XPS studies of disorder effects in flash-evaporated amorphous GaAs

Abstract XPS measurements are used to compare the As and Ga core level spectra and the valence band spectrum of flash-evaporated stoichiometric amorphous GaAs to those of crystalline GaAs. The core level data suggest that the amorphous compound is essentially chemically ordered. The most prominent modifications of the valence band due to disorder are a broadening of the upper p-bonding peak, with a parallel shift upwards of the upper edge of the band, and a filling up of the valley between this peak and the intermediate one. The ionicity gap is maintained and the s-like lower peak is little affected.

Studies of evaporated amorphous GeSe2 films as a function of annealing

The optical properties of evaporated amorphous GeSe 2 films have been determined before and after annealing by transmission and photothermal deflection spectroscopy measurements. The results are analyzed together with those of X-ray photoemission and X-ray diffraction experimemts. The annealing effects are related to changes in medium-range order. The low-energy optical absorption shows a defect state density varying with deposition conditions and annealing

The incorporation of carbon in a-Si:H enhanced by boron doping

In order to better understand B(CH3)3 doping effect on a-Si:H characteristics, the local Si bonding have been studied by XPS in B(CH3)3 or B2H6 doped a-Si:H and a-Si1-xCx:H respectively. The results have been correlated to ellipsometry measurements. Boron is found to enhance the incorporation of C in the films; B(CH3)3 largely changes the film growth and the optical properties of the material. These effects can not be attributed to individual B and C effects.