Norbert Bernhard

The optical bandgap of multilayers of a-Si:H and alloys: A comparison of experiment and thin-film optics calculations

The optical transmission of compositional multilayers of a-Si:H and alloys with Ge and C has been calculated using coherent multilayer thin-film optics. The optical bandgap in dependence on different electronic well and barrier widths was derived from the calculated spectra in the same way as from experimental ones. Si-SiGe and Si-SiC multilayers of about 1 μ m total thickness and consisting of up to 400 peroids were deposited by fast gas switching. Their periodicity was verified by X-ray scattering, and the optical gaps compared to those derived from calculated spectra. Observed shifts of the gap in calculations agree well with experimental values without taking into account a quantum size effect. Remaining differences may be explained by an interfacial transition layer between well and barrier of about 10 A and/or a possible contribution to the shift due to redistribution of hydrogen in its bonding configuration. X-ray and IR data support the view that the width of the transition region is clearly more than the inter-atomic spacing.

Irregularities in current-voltage characteristics of hydrogenated-amorphous-silicon-based barrier structures: Resonant tunnelling against hopping and filamentary conduction through the barriers

Abstract The question has been investigated whether resonant tunnelling is responsible for observed bump-like irregularities in current-voltage characteristics of double-barrier structures based on hydrogenated amorphous Si (a-Si:H) and its alloys with C or N. In this paper, general physical considerations, as well as elementary calculations taking into account special features of amorphous barrier structures, are compared with the results of a systematic experimental study of a-Si: H/a-Sil _ XCX : H heterostructures. Different series of samples, each consisting of a double and a single barrier, and a sample without any heterostructure barrier, have been investigated. A wide variety of experimental features from complete smoothness of the I—V curves, to bumps, noise and even accidental step-like switching behaviour, were recorded at different temperatures. From an overall inconsistency of our experimental data with the calculations, and in agreement with basic physical considerations, resonant tunnelling ...

Random Telegraphic Noise in a-Si1−χCχ:H Schottky Barriers

Abstract Random telegraphic noise (RTN) was observed in a-Si 1−χ C χ :H Schottky barriers which were investigated in the temperature range from 23 to 300 K. Switching times down to the order of 10 μs and resistance fluctuations up to 20 % were found. Also a voltage induced forming of the samples and a temperature dependent probability of appearance of RTN were observed. Physical mechanisms are discussed, elementary calculations performed, and energetic and local position of the traps involved are estimated.

Transient forward bias switching behaviour of amorphous pin diodes

Abstract The switching-on behaviour of amorphous hydrogenated silicon (a-Si:H) based pin diodes under the application of a forward bias step-like voltage pulse was investigated. The characteristic transients of the electric current were recorded in dependence on the pulse height of applied forward bias, its repetition rate, an additional illumination bias, and finally on the degradation state of the samples and also on the spatial distribution of defects created by light soaking. The effect of these parameters was compared for thick (between 2 and 5 μm) samples and those with thicknesses relevant for solar cell applications. In the first 50 ns a resemblance of the current transients with those from nin devices was found. A qualitative consistent explanation of the observed experimental phenomenology is given which will be supported by the determination of the temperature dependent activation of the speed of the delayed current rise.

Numerical Simulation and Experimental Investigation of the Time-Of-Flight Technique Applied to a-Si:H/a-SiGe:H-Heterojunctions

We report on a comparative study, employing the TOF-technique for the characterization of the properties of a-Si:H/a-SiGe:H-heterojunctions. Both the simulated and experimental electron current transients exhibit a pronounced difference depending on the direction of movement of the excess carriers (from the a-Si:H into the a-SiGe:H or vice versa). For the movement from the low bandgap a-SiGe:H into the a-Si:H we find a char¬acteristic increase in the transient current at room temperature, which, as the simulation can reveal, is attributed to the higher drift mobility in the a-Si:H. A drop in the current is observed when the direction for the transit is from a-Si:H to a-SiGe:H. The post-transit behaviour is dominated by the large amount of trapped carriers which remain on the SiGe side of the sample. The situation for a high barrier between a-Si:H and a-SiC:H and the influence of various parameters on the shape of the current transient are discussed.

The Influence of the Bonding Structure on Disorder and Band-Tails in a-B:H

A thorough investigation of plasma-CVD amorphous hydrogenated boron (a-B:H) has been conducted with the main emphasis on how in this amorphous semiconductor the variation of the coordination number in comparison to a-Si:H influences the structural disorder and density of states in the band tails. a-B:H was deposited from different concentrations of B 2 H 6 diluted in H 2 by both DC- and RF-plasma-CVD. The influence of the change of substrate temperature, pressure, flow and deposition power on the structural, optical and electronic properties of the material was examined. Raman-scattering and IR-absorption reveal the clear non-crystallinity of the deposited films. Almost all samples show some photoconductivity with a σphoto/σdark ratio from 10 -1 to 3×10 1 . Although a strong influence of some of the deposition parameters on bandgap and refractive index, hydrogen content, dark and photoconductivity was observed, the density of states in the band tails as measured by PDS was relatively high, showing always a rather flat Urbach slope of about 180 – 220 meV. An explanation for this unexpected almost uniform huge Urbach slope might be that even in the amorphous state the behaviour of boron is still dominated by its electron deficiency character which leads to a certain amount of three centre bonds (as seen in IR-absorption) and results in comparison to amorphous hydrogenated silicon in even stronger constraints of the amorphous network and obviously in more potential fluctuations.

Characterization of amorphous silicon pin solar cells by transient measurement techniques

In order to provide more specific access to solar cell limitations (and to faults during line production as well) the authors report on space-charge limited time-of-flight (SCL-TOF) studies in the voltage mode, and on forward bias transient switching experiments in 500 nm thick pin structures. Complementing I-V characteristics and quantum yield, both techniques are easy to use and give a clear and reproducible measure of the degradation state of a-Si:H based solar cells. An effective carrier lifetime can be deduced from the extraction time of SCL-TOF. Time and temperature dependent forward current transients show a distinct voltage threshold in the onset of double-injection recombination currents which is related to the built-in voltage of the devices. Enhanced bulk recombination due to light or current induced degradation reduces saturated forward currents and increases the time initially needed for building up space charge in the pin-diodes. Differences upon light-soaking by front or back side illumination have also been investigated.

Analysis of the Physical Origin of the blue shift of the Optical Bandgap of a-Si:H based Multilayers

The physical origin of the blue shift of the optical Taue bandgap of compositional multilayers of a-Si:H and its alloys with Ge, C or N is analysed. It is shown that for multilayers with bandgap differences up to 0.7 eV between well and barrier Material, the effect can be understood completely classical in terms of the effective medium approximation, when the mean composition of the multilayer films is not kept constant. For the variation of both the electronic well and barrier widths a shift of the Taue bandgap is theoretically expected. It is of the same order of magnitude as the effect which is experimentally observed. Interfacial transition layers can account for a blue shift also for higher bandgap differences, as usually the case in a-Si:H/a-SiN x :H Multilayers, and when the mean composition of the multilayers is kept constant.