Addressing the fundamental obstacles of p-type doping, substrates, and defect densities in group III-nitrides grown by molecular beam epitaxy: Towards a III-nitride on silicon solar cell
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P-type doping of III-nitrides is explored in order to better understand the shortcomings of existing p-type material and achieve higher hole concentrations. Using a modified form of molecular beam epitaxy (MBE) called metal-modulated epitaxy (MME), hole concentrations as high as 7.9x1019 cm-3 are achieved, nearly 20-40x the previous state of the art. Impurity band conduction, resulting from proper incorporation of Mg at concentrations >1020 cm-3 by careful selection and monitoring of growth conditions, is suggested as the source of these unique properties and more than 10x improvement in hole concentrations over previous state-of-the-art. Electrical properties and Mg incorporation were found to be sensitive to growth conditions and an optimal growth window is elucidated. Diode devices using these highly p-type layers were found to be properly rectifying with turn-on voltages near 3 V, exhibiting room-temperature and cryogenic luminescence, and unambiguously confirming the p-type nature of the films. High MBE growth rates in GaN are demonstrated using a modified high-conductance nitrogen plasma source. Growth rates from 1.6 μm/hour up to 8.4 μm/hour are achieved for pure nitrogen gas. A mixed argon/nitrogen plasma chemistry expands the range of achievable growth rates from ~1 μm/hour up to 9.8 μm/hour due to reduced ion content and more efficient generation of reactive atomic nitrogen species compared to a pure nitrogen plasma. Repeatable background electron concentrations of 1-2x1015 cm-3 are reported, and germanium is demonstrated as an alternative n-type dopant for high growth rate applications with electron concentrations from 2x1016 cm-3 up to 4x1019 cm-3. GaN growth on Si is explored using several unique nucleation methods in order to achieve a fully vertical device structure. Preliminary photovoltaic response is demonstrated in spite of the poor crystal quality of the GaN films. Further study of various diode configurations exhibit either conductive or rectifying junctions. Alternative nucleation techniques exploiting the Al-Si eutectic demonstrate improved crystal quality and surface roughness, providing a path forward for improved PV devices. Finally, an exotic substrate called alexandrite (BeAl2O4) is examined for GaN growth to attempt to exploit a unique relaxation mechanism and reduce dislocation densities. Several nucleation and buffer layers demonstrate some improvements in crystal quality, but on the whole crystal quality remains poor in all cases compared to existing GaN growth on sapphire. Nonetheless, a diode structure is grown in spite of the defective films, which demonstrates proper rectifying behavior with ~3 V turn-on.