Structure and bonding of amorphous silicon alloys

Silicon is used extensively for making semiconductor devices such as solar cells. Many of its useful properties depend on its structure and bonding, which determine the density of states (DOS) in the valence band. This work has used lineshape analysis of spectra from valence band X-ray photoelectron spectroscopy (XPS) and Auger L1-L2,3V and L2,3-VV lines to study differences in the DOS of silicon in various forms and after various treatments. These included • crystalline silicon (c-Si) • argon-, neon- and xenon-ion bombarded c-Si to produce amorphous silicon (a-Si) • as deposited hydrogenated amorphous silicon (a-Si:H) produced by plasma • enhanced CVD (Chemical Vapour Deposition) • hydrogenated amorphous silicon (a-Si:H) after heating. The spectra were numerically treated to remove instrument and XPS lineshape broadening, Coster-Kronig effects and backgrounds. They were then normalised and decoupled into their component ss-like, sp-like and pp-like peaks. The sensitivity of lineshape analysis to the degree of disordering was determined by comparing the decoupled spectra of c-Si bombarded with argon ions of different energies. The bombarded spectra showed how the disorder decreased as the ion energy increased due to the increasing depth of the implanted ions and hence concentration of defects. Analysis of the XPS valence band (VB) spectra of argon-, neon- and xenon-ion bombarded c-Si showed that these noble gases have core level peaks close to, or coinciding with, the valence band of the silicon. Argon and xenon have significant core peaks that overlap the valence band and distort it. Neon’s core peak lies just below the valence band and can be compensated for. Hence, it was possible, using neon ion bombardment, to produce an XPS VB spectrum of a-Si, which was compared to those of c-Si and a-Si:H. There were significant lineshape differences between the three, showing that differences in structure are visible and the presence of hydrogen can be indirectly detected through XPS Valence Band lineshape analysis. The XPS VB, Auger L1-L2,3V and L2,3-VV lines were compared for c-Si, a-Si and a-Si:H spectra. All three spectra reflect either directly or indirectly the DOS but do show differences. The Auger L1-L2,3V lineshape is much more sensitive to dangling bonds at the top of the valence band than the XPS VB. The L2,3-VV, despite its relatively strong signal strength, is the hardest to analyse, being a self-convolution of the p-like DOS and having interference from plasmons. It also shows the least change between different samples. The XPS spectrum is relatively easy to collect, but reflects the bulk region far more than the surface. This analysis demonstrated that there are observable differences in the lineshapes of the DOS for c-Si, a-Si and a-Si:H. The a-Si had the widest component peaks when decoupled, which indicates a strained and disordered lattice. The pp-like area was narrower in the a-Si:H samples. The disordered samples showed strong features in the middle of the valence band, and these could be used to give a measure of the disorder. If the differences between the techniques were due only to differences in the matrix elements, it would be expected that the differences between the L1-L2,3V Auger and XPS VB spectra would be consistent, but this is not the case, indicating that the different techniques respond differently to the presence of disorder, dangling bonds and hydrogen. The techniques developed in this thesis provide the basis for further work on studies of outgassing and photodegradation in a-Si:H. These could provide insights into the bonding and degradation mechanisms in a-Si:H