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Title: The electrical characterisation of Si and Si/Si₁₋ₓGeₓ/Si structures grown by molecular beam epitaxy
Author: Brighten, James Cordeaux
ISNI:       0000 0001 3480 0784
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1993
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Si/Si1-xGex strained layer heterostructures offer great promise for applications in field-effect and bipolar devices; however, their usefulness depends on a knowledge of the purity and perfection of both the bulk and heterointerfacial regions. Very little work has been published on the characteristics of the Si/Si1-xGex heterointerface via electrical means, the practice to date being to fabricate devices and indirectly assess the interface via the quality of the device characteristics. The present work has focused on the direct electrical characterisation of Si and Si/Si1-xGex/Si layers grown by Molecular Beam Epitaxy (MBE), in order to investigate the band offset at the Si/Si1-xGex heterointerface and electrically active centres in both the bulk and heterointerfacial regions. Sputtered Ti Schottky barriers were found to provide suitable rectifying barriers for electrical measurements on the MBE layers. Current-Voltage (I-V) characteristics revealed excessive reverse leakage currents in early work carried out; this was attributed to metallic contamination located preferentially in the alloy region. The use of liners around the Si and Ge charges reduced the in-diffusion of metallics and dramatic improvements were found in the material quality as observed from the I-V characteristics. The large extracted barrier heights from C-V measurements reflected the stability and reproducibility of the Schottky contacts and provided rectifying barriers of sufficient quality to access the bulk epilayers via depletion-capacitance techniques. Capacitance-Voltage (C-V) profiling allowed heterojunction features in the apparent free carrier distribution to be observed, for the first time, in Si/Si1-xGex/Si layers for 0 < x < 0.2. Numerical integration of these profiles allowed valence band offsets to be extracted which were in good agreement with those predicted theoretically. The effects of traps on C-V measurements were observed in Si and Si/Si1-xGex/Si layers; considerable carrier compensation was apparent, occurring to a greater extent in the alloy layers. Use of C-V simulations allowed the reconstruction of apparent free carrier distributions, using the extracted valence band offset, interface charge density and trap distributions (the latter determined by Deep Level Transient Spectroscopy (DLTS)) as input parameters; simulations were also used to assess the validity of the experimentally determined heterojunction parameters in the presence of interfacial traps. DLTS studies were used to determine the distribution and nature of deep states in both Si and Si/Si1-xGex/Si structures. Well defined deep level peaks were observed in the bulk Si and Si1-xGex regions whilst distortion occurred to the spectra in the Si/Si1-xGex heterointerfacial region. This latter distortion is attributed to the significant band bending that occurs in this region. Apparent trap concentrations increased with Ge composition and also showed an increase in the Si1-xGex as compared to the Si regions. Broadening of the deep level spectra was investigated via simulation in terms of carrier emission from a band of energies in the band-gap and allowed a correction to the underestimated trap concentrations. The donor-like nature of the well defined features, determined from the logarithmic filling and field-independent behaviour and their dependence on growth temperature and anneal, suggested the origin of the electrical activity could lie with point defect/dislocation interactions. Structural analysis carried out on the structures investigated here indicated the presence of Fe and O; it is considered that both impurities, in point defect form, could interact with the threading and misfit dislocations present to produce the electrical activity observed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics