Sapphire provides base for AlGaN solar-blind photodiodes

Researchers at the Microelectronics Research Center at the University of Texas (Austin, TX) have grown aluminum gallium arsenide (AlGaN) heteroepitaxial back-illuminated solar-blind PIN photodiodes on (0001) sapphire substrates. The AlGaN compounds work well for ultraviolet photodetectors--especially in airborne and space applications--because the materials do not require heavy and expensive filtering systems for long wavelengths. The main fabrication problem is cracking due to the substantial lattice mismatch between Al_0.4Ga_0.6N (window layer), Al_0.4Ga_0.6N (active layers), and GaN. There also can be p-type doping difficulties related to the large activation energy necessary for the ionization of magnesium acceptors with high-Al-content AlGaN alloys. To resolve these issues, the heterostructures were grown by low-pressure metal-organic chemical-vapor deposition. The scientists used a two-step growth procedure to optimize the optical and electrical characteristics of the various component materials. They eliminated cracking problems and were able to obtain activated p-type active layers. The back-illuminated devices exhibit very low dark-current densities and external quantum efficiencies up to 35% at the peak of the photoresponse, which occurs at a wavelength of 280 nm. Contact Russell Dupuis at[email protected].