Surface spectroscopy and Auger lineshape analysis studies of amorphous silicon surfaces

Christopher Paul Lund

This work deals with the experimental investigation of the surface of one of the most interesting and important new semiconductor materials, hydrogenated amorphous silicon (a-Si:H). Ultra-high vacuum surface spectroscopy methods, especially Auger lineshape analysis and X-ray photoelectron spectroscopy (XPS), have been used with a view to studying the effect on the local densities of states at the surface of various preparation methods and subsequent treatments. X-ray excited Si L2,3VV and Si L1L2,3V Auger lines as well as XPS valence band (XPS VB) spectra have been measured for a number of silicon materials and surfaces prepared in different ways. These materials included crystalline silicon (c-Si), amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H). Preparation techniques used included atmospheric pressure chemical vapour deposition (APCVD), glow discharge (GD) or plasma enhanced CVD and radio frequency (RF) sputtering. Surface treatments included disordering of c-Si by argon ion bombardment, hydrogenation by hydrogen ion bombardment, annealing and rehydrogenation from the bulk. Methods have been developed and thoroughly tested to enable X-ray excited silicon Auger spectra to be treated routinely using numerical debroadening and deconvolution to obtain an indication of the valence band transition densities of states (VBTDOS). These show good agreement with previously published results for electron initiated Auger spectra and theoretical results. In particular a method has been developed for treating the experimentally difficult Si L1L2,3V Auger line. Techniques have been developed to remove the sloping background and Coster-Kronig broadening to enable an indication of the transition densities of states. The L1L2,3V derived VBTDOS approximates closely to the theoretical DOS and the experimental results obtained from UPS and XPS. It is shown that because of this the Si L1L2,3V line is a more effective method of monitoring changes in the surface VBTDOS of a-Si:H due to various treatments than the more commonly used but harder to interpret Si L2,3VV line. A method based on the simplex algorithm has been applied to enable the Si L2,3VV and Si L1L2,3V spectra to be decomposed (decoupled) into their component (p-, sp- and s/L2H like) peaks. Changes in the relative contributions of these components have been compared with changes induced by disordering and hydrogenation. It is shown that both the Si L2,3VV and Si L1L2,3V lines give a semiquantitative method for monitoring hydrogen incorporation and changes in the localised states near the valence band edge. Results are presented for varying amounts of disorder (or amorphousness) produced by argon ion bombardment of the surface. A number of results are ·presented· for artificial and naturally hydrogenated surfaces as well as for different deposition techniques. AES and XPS are shown to be very sensitive to changes in disorder (amorphousness) and hydrogen bonding in a-Si:H. The L1L2,3V Auger spectrum is found to be particularly sensitive. Both the AES and XPS VB spectra for a disordered c-Si sample give new information on the affect of disorder on the DOS. The L1L2,3V Auger line is also shown to be sensitive to varying degrees of disorder. Si L2,3VV and L1L2,3V spectra are successfully used to study the effect of several rehydrogenation methods on a-Si. These methods are shown to lead to different amounts of hydrogen in the surface as well as differences in the type of hydrogen bonding. A-Si:H prepared using different techniques is shown to have differences in the amount of order and hydrogen present in the films produced. The deposition technique is also seen to effect the type of hydrogen bonding present in the surface. A novel transfer vessel has been constructed to enable samples prepared in one system to be analysed in another UHV system without exposure to air and the subsequent contamination of the surface. Results are presented for a pure, 'as deposited' surface of a-Si:H prepared by GD. The 'as deposited' surface is shown to be significantly different to one that has been argon ion cleaned and then rehydrogenated. Also using the transfer method changes in the Si L1L2,3V, Si L2,3VV and XPS VB spectra were studied for an a-Si:H surface after heating above the first desorption threshold for hydrogen. This enables the effect of different Si-H bonding configurations on the VBDOS to be studied.

Surface spectroscopy and Auger lineshape analysis studies of amorphous silicon surfaces

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