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Title: Nanometric characterisation of III-nitride semiconductors
Author: Tan, Lay-Theng
ISNI:       0000 0004 2673 7473
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2009
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An investigation on the optical, composition and surface properties of the III-nitride ternary alloys InxGa₁-xN and Al₁-xInxN is presented. The structures studied are single quantum wells (SQWs) and epilayers. The effects of various GaN cap thicknesses, well widths, Si doping concentrations in the barriers, InN compositions, epilayer thicknesses and substrates are examined. The techniques used are photoluminescence (PL) and PL excitation (PLE) spectroscopy for investigating the optical properties, wavelength dispersive x-ray (WDX) and Rutherford backscattering spectrometry (RBS) for measuring InN composition. RBS is also used to study the crystalline quality of the samples; and secondary electron microscopy (SEM) and atomic force microscopy (AFM) for examining the surface quality. InxGa₁-xN/GaN SQWs with a variety of GaN cap thicknesses and different barrier doping levels were studied using PL spectroscopy. It was demonstrated that the InxGa₁-xN/GaN SQWs with thin caps give reduced PL intensity and larger PL linewidths, indicating degradation due to the increasing effects of the surface. As the doping level in the GaN barrier layers increases the luminescence intensity decreases, the linewidth increases and the SQW transition energy red-shifts. The latter is attributed to band-gap renormalisation. The properties of GaN/A₁-xInxN SQWs were investigated for varying quantum well width and for different substrates. The GaN/A₁-xInxN SQW luminescence peak energy decreases as the well-width increases, mainly due to the intense spontaneous polarisation fields, and its dependence on excitation power reveals the effects of carrier screening. Al₁-xInxN epilayers were also characterised and here the PL peak emission energies decrease with increasing InN composition. The energy bandgap bowing parameter is found to be linearly decreasing with increasing InN composition in the limited composition range studied. Cracks are observed in highly tensile strained epilayers with x < 0.10. For variations in layer thickness the surface quality and crystal quality worsens with increasing thickness. Finally, the samples on FS-GaN substrates have 3 to 4 atomic % more InN than those grown on sapphire. Surface quality degradation is observed in samples grown on non-polished sapphire substrates and AlyGa₁-yN templates with high Al content.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral