Christophe Longeaud

Anisotropy in the transport of microcrystalline silicon

We have measured the transport properties of several microcrystalline silicon films prepared by hot-wire chemical vapour deposition (HWCVD). The photocurrent measurements indicate an anisotropy in the optoelectronic properties of the material. We have investigated the defect structure in the samples by modulated and transient photocurrent, constant photocurrent method, and capacitance spectroscopy. The results indicate spatial inhomogeneities in the distribution of defects. A likely distribution of defects is proposed which can consistently explain the results from the different measurements.

Nanostructures and defects in silicon–hydrogen alloys prepared by argon dilution

Abstract Nanostructural heterogeneity of silicon–hydrogen (Si:H) alloy films deposited in a conventional radio frequency plasma-enhanced chemical vapour deposition unit from silane argon mixture has been studied by small-angle X-ray scattering (SAXS). The densities of defect states of Si:H films have been estimated by dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS) and the modulated photocurrent (MPC) method. From the structural and defect studies we identify two regions of Ar dilution, where the structure of the films are distinctly different. Up to 90% Ar dilution, the nanostructural as well as the large-scale (>30 nm) structural heterogeneities in the amorphous Si:H (a-Si:H) network decrease. A lowering of the bulk defect density has also been observed in this Ar dilution region. Increasing Ar dilution to greater than 90% of the mixture, the a-Si:H films show some dense regions embedded in the amorphous matrix. The high- and low-density amorphous structures within the films can explain the experimental results obtained from SAXS, DBP and PDS. A negligible conduction band tail, as observed from MPC result, suggests the formation of high degree of crystallinity in the film deposited with 99% Ar dilution and higher rf power density (80 mW/cm 2 ).

Electronic properties of silicon thin films prepared by hot-wire chemical vapour deposition

Abstract A transition from amorphous to microcrystalline silicon occurs in hot-wire chemical vapour deposition silicon films with increasing dilution of silane with hydrogen. This transition is detected for a dilution ratio R = [SiH 4 ]/[H 2 ] between 10% and 9%, where [SiH 4 ] and [H 2 ] are the silane and hydrogen flow rates, by Raman and optical absorption spectra, and by dark conductivities which are several orders of magnitude larger in microcrystalline as compared to amorphous films. In the microcrystalline films we observe a simultaneous increase of both majority and minority carrier mobility-lifetime products with increasing hydrogen dilution, which is consistent with the measured decrease in sub-gap absorption and defect density deduced from transient photocurrent measurements. This simultaneous increase is in contrast with the general trend observed in amorphous films, where these two quantities vary in opposite ways, and are associated with an improvement of the transport properties of the material. The microcrystalline samples did not show light-induced degradation after prolonged illumination.

The influence of a temperature dependent bandgap on the energy scale of modulated photocurrent experiments

Amorphous semiconductors and chalcogenide glasses exhibit a high density of localized states in their bandgap as a consequence of structural defects or due to a lack of long range order. These defect states have a strong influence on the electronic transport properties. Thus, many theories attribute the “resistance drift” or the “threshold switching” effects, both observed in amorphous phase change alloys, to defects within the bandgap. The energetic distribution of states within the bandgap can be probed via modulated photocurrent (MPC) experiments that enable a spectroscopy of the relative density of these defect states by varying the modulation frequency at various temperatures T. It is also a common feature that the bandgap decreases with temperature. Nevertheless, the consequences of a shrinking bandgap with increasing temperature have been neglected in the classical analysis of MPC experiments. In this paper, we propose to add correction terms to the classical MPC energy scaling to take the temperat...

On the density of states of germanium telluride

Germanium telluride (GeTe) is one of the most studied phase change materials. Surprisingly, only little is known about the density of states (DOS) in its band gap. In this paper, the DOS of amorphous GeTe films is investigated both experimentally and theoretically. We propose a model for this DOS as well as estimates of some of the transport parameters of this material. Thin films of amorphous GeTe have been deposited by sputtering. Their dark and photoconductivity have been measured as a function of temperature. By means of the modulated photocurrent technique their DOS was probed, while their absorption was investigated by photothermal deflection spectroscopy at room temperature. Numerical calculations were employed to reproduce the experimental results with a proper set of transport parameters and choice of DOS. These data constitute a good basis for further study on the influence of the DOS on the aging of the sample resistance (“resistance drift”).

Properties of Si:H thin films deposited by rf-PECVD of silane–argon mixtures with variation of the plasma condition

Abstract Structural evolution of hydrogenated silicon (Si:H) films deposited from silane–argon mixtures varying the rf power density in the conventional plasma enhanced chemical vapour deposition (PECVD) process were studied by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) absorption and Raman spectroscopy. It has been observed that the evolution is analogous to that seen with hydrogen dilution and the transition from amorphous (a-Si:H) to microcrystalline (μc-Si:H) phase is faciliated by high argon dilution. The results have been explained on the basis of bombardment of the growth surface by excited Ar atoms ( Ar ∗ ) from the plasma.

Light induced changes in the amorphous—crystalline silicon heterointerface

The photostability of the amorphous—crystalline silicon heterointerface is investigated. It is revealed that the metastability of hydrogenated amorphous silicon (a-Si:H) causes significant light induced changes in the heterointerface. Unlike bulk a-Si:H, the photostability of the heterointerface is not controlled by the microstructural properties of a-Si:H but rather by the initial heterointerface properties. Interfaces that initially have low interface defect density show the greatest degradation while those that initially have high interface defect density actually show light-induced improvement. It is shown that the degree of light induced change in the interface defect density is linearly proportional to the natural logarithm of the initial interface defect density. Further, it is revealed that the kinetics of light-induced change in the heterointerface defect density can be faster or slower than light-induced changes in bulk a-Si:H films depending on the initial properties of the heterointerface. Lig...

Evolution with light-soaking of polymorphous material prepared at 423 K

We have studied the density of states and the structural properties of a new class of material. called hydrogenated polymorphous silicon, prepared by decomposition of a mixture of silane and hydrogen in a radio frequency powered plasma enhanced chemical vapour deposition system. The evolution of the density of states and of the optoelectronic properties of these films upon light-soaking and annealing cycles is different from that observed on standard hydrogenated amorphous silicon. The presence of platelet-like microstructures. revealed by an infrared absorption band centred at 2030 cm(-1). may explain this evolution, (C) 2002 Elsevier Science B.V. All rights reserved.

Influence of light-soaking and annealing on the microstructure of a-Si:H deposited at 423 K

Evolution upon light-soaking and annealing of the transport parameters and of the density of states of hydrogenated amorphous silicon prepared under standard conditions (pure silane, low pressure, low power) at 423 K by radio-frequency powered plasma enhanced chemical vapour deposition has been investigated by many techniques. It is shown that both light-soaking and annealing at a higher temperature than the deposition temperature induce structural modifications mainly revealed by an enlargement of the conduction band tail. The structural modifications induced by each process are probably not of the same nature and we conclude that annealing is not always the reverse process of the light degradation.

Link between crystal quality and electrical properties of metalorganic vapour phase epitaxy InxGa1−xN thin films

We report on the crystal quality of metalorganic vapour phase epitaxy-grown InGaN with indium content ranging from 0% to 20%. Absorbance measurements are fit to a model including band tails and a defect represented as a Brendel oscillator (R. Brendel, Appl. Phys. A 50, 587, 1990). Band tail absorbance, corresponding to contorted bonds, increases with increased In content. Above 10% of In, the presence of another defect, the concentration of which increases with In content, has been correlated with x-ray diffraction and Raman. We suggest that this defect corresponds to nitrogen vacancies, in agreement with a reported model for GaN.