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In Situ Surface Studies of III-V Semiconductor Compounds

Bastiman, Faebian (2010) In Situ Surface Studies of III-V Semiconductor Compounds. PhD thesis, University of Sheffield.

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Abstract

Since its advent in the early 1980s, Scanning Tunnelling Microscopy (STM) has been used to advance the knowledge of semiconductor grow processes. Hybridisation of STM with other analytical methods and the Molecular Beam Epitaxy (MBE) growth technique allowed a flexible and diverse approach to growth front exploration. The first hybrid, limited the applicability of STM to in vacuo operation whereby the sample is rapidly cooled or “quenched” in an attempt to preserve the growing surface, before imaging can commence. This technique suffers dually from the unknown effects of the quenching procedure and the limiting ability to only capture frozen-in-time images of the surface. The ultimate evolution of STM would be to allow concurrent or in situ MBE and STM operation. The ability to perform concurrent MBE and STM requires three basic criteria: accurate and stable control of the sample temperature, reliable and maintainable STM tunnelling tip procedures and controlled, sustained emission from the MBE effusion cells within the STM chamber. Samples are slivers 8 x 1 mm2 to 12 x 4 mm2 of wafer mounted for either direct current heating or radiative pyrolytic boron nitride heating within the STM chamber. No direct temperature monitoring method is available and thus a myriad of techniques were employed to map the current-temperature response for samples including Reflection High Energy Electron Diffraction (RHEED), thermocouples and thermography, yielding a reliable heating profile. Tunnelling tip fabrication involves manufacturing an atomically sharp tip via a two-step electrochemical etching and annealing procedure. An extensive and exhaustive investigation sought to produce a quantitative method for tip identification and etching parameterisation based on the available variables of differential sensitivity, etching voltage, immersion depth and etchant concentration. An optimised tip type transfer diagram of tip fabrication resulted, after which, an anneal algorithm was formulated resulting in clean, sharp tips without the side effect of apex distortion and melting. Quality of the initial growth layer depends strongly on the clean-up conditions. As a prequel to growth, sample preparation methods are investigated via STM analysis to determine the best preparation conditions in order to achieve high quality MBE growth in the STM chamber. The final stage involves MBE source operation during STM. Initial investigation focused on flux alteration of surface reconstructions and allowed the effects of As4 on the STM stage to be investigated. This is the first documented case where an e-beam As4 source has been successfully operated within an STM system, during imaging. The inclusion of group III elements in the evaporation flux proves unequivocally that III-V Molecular Beam Scanning Tunnelling Microscopy (MBSTM) is a realisable investigatory technique. Simultaneous deposition of In and As whilst imaging allowed dynamic observation of the InAs/GaAs wetting layer evolution on GaAs(001)-(2 × 4). The experiment followed initial heteroepitaxial growth through wetting layer evolution to the onset of 3D growth.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Electronic and Electrical Engineering (Sheffield)
Depositing User: Dr Faebian Bastiman
Date Deposited: 27 Sep 2010 08:37
Last Modified: 08 Aug 2013 08:45
URI: http://etheses.whiterose.ac.uk/id/eprint/1023

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