| There are many areas of potential utilization of the alloy AlxGa 1−xN, not the least of which are ultraviolet detector applications. These include flame and heat sensors, missile plume detection, and secure-from-earth inter-satellite communications. The AlxGa1−xN bandgap can be tailored from 4.3eV (xAl = 0) to 6.2eV (xAl = 1), corresponding to a band-edge wavelength range of 365nm to 200nm, to suit each unique application. Solar radiation below approximately 300nm wavelength is absorbed by ozone in the atmosphere. Thus, for applications in the presence of a large solar radiation background, a solar-blind detector that exhibits no spectral response for wavelengths at or above 300nm is extremely desirable. For solar-blind detectors a composition of greater than 30% Al is needed.; The first phase of this work involved the optimization of metalorganic chemical vapor deposition epitaxial growth of AlxGa1−x N (xAl ≥ 0.3) on GaN templates. It was found that defect formation in the AlxGa1−xN is exacerbated as x Al is raised. This could be partially counteracted by maximizing the mobility of Al surface species during growth, and by reducing threading dislocation density in the GaN template.; The second phase of the work examined the effect of defects on GaN-based p-i-n photodiodes. The presence of threading dislocations (especially mixed-character dislocations) was found to increase the leakage current, broaden the spectral response, and lengthen the response decay time. Defects in the i-layer, most likely point defects, were another source of leakage current. The sidewalls did not contribute to leakage current.; The final phase of the project was the development of bulk AlGaN growth on SiC substrates for use in solar-blind UV detectors. A nucleation layer or ramped growth rate was required to avoid cracking due to tensile strain. Si-doping of the n-type buffer introduced further tension in the film. The threading dislocation density in the AlGaN grown on SiC was very high—2–5 × 1010 cm−2—much higher than for GaN. Because the threading dislocations were primarily edge-type, reasonably low diode leakage currents could still be obtained. An insulating buffer was required to suppress photocurrent contributions from the SiC substrate. Solar-blind p-GaN/i-AlGaN/n-AlGaN photodiodes with a peak responsivity of 0.08 A/W were fabricated. |
