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 Al0.4Ga0.6N (window layer), Al0.4Ga0.6N (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].