Aluminum gallium nitride-based solar-blind ultraviolet photodetectors
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High performance AlxGa1-xN-based ultraviolet photodetectors were designed, fabricated, characterized, and modeled for use in commercial and military solar-blind sensing applications. Chronologically, the first device structure studied was a heterojunction AlxGa1-xN/GaN p-i-n photodiode. These devices achieved record low dark current densities and record high external quantum efficiencies of ~77% with a semi-transparent recessed window device structure. Selective-area regrowth of Al0.30Ga0.70N epitaxial layers on top of GaN template layers was used to reduce the tensile-strain-induced cracking and move toward solar-blind devices. The zero bias external quantum efficiency peak was shifted 50 nm toward solar-blind with ~ 20% at λ = 315 nm. Our group’s first back-illuminated solar-blind photodetectors were achieved with zero bias external quantum efficiencies of ~ 12% at λ = 278 nm and a large detectivity of D* = 5.3 ×1013 cm·Hz1/2·W-1. These devices had the same percentage aluminum in both the n and i-regions. A new device structure was used to investigate the advantage of using a “window” Al0.50Ga0.50N n-region to increase the external quantum efficiency. With an Al0.41Ga0.59N absorption region, solar-blind photodetectors were fabricated with high zero-bias external quantum efficiencies of 26% at λ = 279 nm. Although the external quantum efficiency of the solar-blind detector was improved, the detectivity decreased to D* = 5.30×1012 cm·Hz1/2·W-1 at λ = 279 nm. This was attributed to the large leakage current, which caused a significant decrease in the differential resistance. Finally, two improved solar-blind detectors were fabricated with an innovative Al0.60Ga0.40N n-region. We report a zero bias external quantum efficiency of ~ 42% at λ = 269 nm for an Al0.48Ga0.52N i-region device. By slightly increasing the aluminum percentage in the i-region, the zero bias external quantum efficiency was increased to ~ 53% at λ = 275 nm for an Al0.45Ga0.55N i-region device. The low leakage currents of these devices leads to large differential resistances, which when combined with the high external quantum efficiency at zero bias, gives solar-blind detectivities of D* = 1.9×1014 cm-Hz1/2-W-1 at λ = 269 nm and D* = 3.2×1014 cm-Hz1/2-W-1 at λ = 275 nm for the Al0.48Ga0.52N and Al0.45Ga0.55N i-region devices, respectively.