Polarization sensitive solar-blind detector based on a-plane AlGaN.

We report polarization-sensitive solar-blind metal-semiconductor-metal UV photodetectors based on (112̄0) a-plane AlGaN. The epilayer shows anisotropic optical properties confirmed by polarization-resolved transmission and photocurrent measurements, in good agreement with band structure calculations. Solar blind UV (SBUV) detectors, with no photosensitivity above 280nm wavelength, have wide range of applications like – missile plume detection, UV astronomy, chemical/biological battlefield reagent detection etc.1−3. The wide-bandgap, high-temperature compatible AlGaN material system has been the workhorse for such SBUV detectors with many reports on high performance devices based on [0001] c-plane AlGaN layers. The inherent anisotropic optical properties and reduced crystal plane symmetry of “non-polar ” (112̄0) a-plane AlGaN epilayers allows the fabrication of polarization sensitive (PS) detectors. Such PS detectors give additional advantages of selectivity and narrow band detection in a differential configuration consisting of two or four photo-detectors, without using filters 4,5. We present, to the best of our knowledge, the first report of a PS SBUV detector. About 0.5µm thick Al0.6Ga0.4N epilayers were grown on AlN buffer layers via metal organic vapour phase epitaxy (MOVPE) in a closed-coupled showerhead reactor using standard pre-cursors. The details of the growth procedure, method to estimate the solid phase Al content and strain in the layer can be found in Refs.[6,7]. Metal-Semiconductor-Metal (MSM) type de-vices with interdigitated finger geometry Schottky contacts (metallization–200A ̊ Ni/1000ÅAu) were fabricated using standard optical photolithography, electron-beam evaporation and lift-off techniques. The III-nitride semiconductors have three closely-spaced valence bands near the center of Figure 1: (a) Schematic diagram showing the three closely spaced valence band at k=0 of the III-nitrides. (b) Orientation of hexagonal unit cell for a-plane nitrides. The in-plane strains are yy and zz