Optoelectronic properties of aluminum gallium nitride / gallium nitride superlattices
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Abstract
In this thesis, three primary findings are presented concernmg optolelectronic properties of AlGaN superlattices. First, we obtain the lowest lateral p-type resistivity and highest lateral p-type mobility to date in the AlGaN material system. Second, we obtain the first experimental results of multi-subband photoluminescence in p-type AlGaN superlattices. Last, we report the first direct measurement of perpendicular electrical transport ( electrical transport perpendicular to the superlattice planes) in AlGaN superlattices. Our research into resistivity and mobility of AlGaN superlattices stems from the fact that p-type AlGaN is highly resistive. To overcome the problem of highly resistive p-type AlGaN, we propose and demonstrate modulation doping in p-type AlGa superlattices. Our measurements yield a low-temperature lateral resistivity and mobility of 0.068 S1 • cm and 36 cm 2 /(V • s), respectively. This is the lowest resistivity and highest mobility recorded to date in p-type AlGaN and results from reduced ionized impurity scattering inherent in modulation doping. The optical properties of AlGaN superlattices are of great interest because they are often used in light-emitting diodes and laser diodes. Specifically, the absorption
edge in AlGaN superlattices is typically thought of as being severely red-shifted due to internal electric fields present in AlGaN-based materials. We obtain experimental photoluminescence results on large-period superlattices that indicate that the redshifting of the absorption edge is much less than previously thought due to the combined effects of band-filling and oscillator strength on energy. We develop a computer model based on the self-consistent solution of the Poisson and Schrodinger system of equations. Our model predicts a drastic decrease in spontaneous recombination lifetime with increased transition energy, which is consistent with our experimental data. The perpendicular resistivity of AlGaN superlattices is also of critical importance to the development of AlGaN-based devices. We therefore measured the perpendicular resistivity of an n-type AlGaN superlattice and compared it to bulk n-type GaN. The superlattice has a perpendicular resistivity of 1.2 D • cm while bulk n-type GaN is 0.18 D • cm. We develop a theoretical model based on sequential tunneling and enhanced free carrier concentration to explain our experimental findings. Our model shows that perpendicular resistivity is dominated by two factors; carrier concentration and tunneling probability.
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