Thesis

Investigation of aluminium nitride phosphide alloys by X-ray microanalysis

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17504
Person Identifier (Local)
  • 202387186
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • The present thesis investigates the impact of growth parameters on the elemental composition of AlPN or AlNP samples designed for high-electron-mobility transistors (HEMTs). The study involved a series of AlNP thin films synthesised via metal organic vapour phase epitaxy MOVPE method on a sapphire substrate using AlN layers at growth temperatures between 680° and 1080°C. The samples were investigated using an electron probe micro analyser (EPMA) to determine the best experimental conditions for determining composition using wavelength dispersive X-ray analysis. Three different carbon coatings were applied namely thin, double thin and thick. The findings demonstrated that the different types of carbon coating had a significant impact on the measured total weight percentage (wt%), (e.g. 110% for thin, 105% for double thin and near 100% for thick), while variations in beam current had minimal influence. This underscores the necessity for careful selection of coatings and beam current in quantitative compositional analysis. This study highlighted the pivotal role of P integration in the thin films. Wavelength dispersive X- ray spectroscopy (WDX) measurements revealed that at growth temperature of 1080°C, a robust stoichiometric relationship was found between AlN, with less than 1.3% of P incorporation, for 250 sccm and 10 sccm of tertiary butyl phosphine (tBP) respectively. The incorporated P substitutes the N sublattice at this high growth temperature. However, at a reduced growth temperature of 680°C, the P incorporation level was found to exceed (8.11 ± 0.16) %, which resulted in the formation stoichiometry, whilst the aluminium (Al) remains 50% and nitrogen (N) content was reduced, ensuring AlN0.84P0.16. One sample at this temperature shows distributed stoichiometry. This defect was attributed to significant lattice strain, which may induce by the larger atomic size and the higher incorporation level of P atoms. Furthermore, at an intermediate temperature of 880°C, the nitrogen (N) content (At%) remains stable around (50 ± 1) % and the incorporated phosphorus (P) replaces the aluminium (Al) sublattice. So, the study revealed an inverse correlation between P(At%) and growth temperature, at higher growth temperature of 1080oC less than 1.3 % of P while at lower growth temperature of 680oC above 8% of incorporated P was measured by WDX. Moreover, a comparative analysis shows very good agreement for P (At%) in WDX measurements and EDX measurements. Additionally, a small amount O (At%) was found in WDX study, but it has no significant impact on total wt%. In comparative study, EDX investigations revealed a marginally elevated O (At%) in the range between 2% to 13% than WDX measured values that were between 1% to 8%. WDX study indicates that the composition of AlNP alloys is highly dependent on growth conditions. However, reliable results can be obtained by careful control of analytical variables, particularly carbon coating thickness. The WDX outcomes suggest that phosphorus incorporation at a growth temperature of 1080°C can effectively modify the alloy's composition and enhance its material properties. In such cases, AlNP alloys could play a crucial role in enhancing material quality, reducing defect density, and enabling higher two-dimensional electron gas (2DEG) charge density for next generation HEMT.
Advisor / supervisor
  • Martin, Robert
Resource Type
DOI

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