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Title: Silicon molecular beam epitaxy : doping and material aspects
Author: Pindoria, Govind
ISNI:       0000 0001 3490 804X
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1990
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Silicon Molecular Beam Epitaxy (Si-MBE) allows independent control over the dopant and matrix species, offering the possibility of engineering device structures with resolution down to the monolayer level. However, significant improvements in material quality and doping capability are essential before the potential of the growth technique for applications in VLSI technologies and in evaluating new device designs can be assessed. Three key areas have been identified in this study, where advances were considered feasible in the time scale of this project: particulate contamination, the modelling of the co-evaporation dopant incorporation process and an evaluation of a new dopant in Si-MBE, phosphorus. A fourth area, that of metallic contamination, was also investigated. Particulate contamination in Si-MBE epilayers is of increasing concern, as the possible applications of this layer growth technology to VLSI are assessed. A systematic study of several hundred epilayers correlated the particulate contamination in the epilayers with the high electron flux in the deposition region arising from the electron beam evaporator and with the unstable excess silicon deposits in the growth chamber. Reduction of silicon accumulation in the deposition zone by containment of the silicon flux significantly reduces particulate densities in the epilayers. Two types of particulate-related features have been identified. The first type thought to be due to microscopic particulates is decorated by crystallographic defects, whereas the second type, which is free of these defects, appears to be related to shadowing by larger particulates. A correlation in the densities of both types of particulate defects in epilayers grown under a variety of experimental conditions suggests a common source. The incorporation of dopants in Si-MBE has proven to be the most difficult aspect of this growth technology. In this study it is shown that the atomic size of a dopant relative to the matrix, is the key parameter which determines whether or not a dopant exhibits surface accumulation behaviour during molecular beam epitaxy. Specifically, surface accumulation only occurs If the dopant atoms are larger than those of the matrix atom substituted. In compound and alloy matrix systems, this size effect strongly influences the net site occupation of a dopant, a process previously believed to be dominated by site availability. The physical basis of this phenomenon is discussed with particular reference to theories of equilibrium surface segregation and the nature of the surface stress. Although p-type doping in Si-MBE with boron is now well established, allowing high doping concentrations and good dopant control, n-type doping using antimony has not matched these achievements, the problems becoming acute at low growth temperatures, (S 500°C). A possible alternative is the use of phosphorus instead of antimony. In this study the first phosphorus doped Si-MBE epilayers were grown, using a tin phosphide source. Bulk-like mobilities were demonstrated. The behaviour of phosphorus as a function of growth parameters and of 'potential enhanced doping' indicates a non-unity, almost growth temperature independent incorporation efficiency. Preliminary evidence indicates that phosphorus does not accumulate on the growing silicon surface, in line with the predictions of the empirical model. Metallic impurities can have a variety of influences on semiconducting devices, the majority of them detrimental. A preliminary investigation into the levels of metallic impurities in MBE grown silicon was carried out and a methodology developed aimed at reducing these levels.
Supervisor: Not available Sponsor: Alvey Directorate ; General Electric Company ; British Telecom
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics