A Study on Fast Visible-Light Photodetectors Employing Indium Gallium Nitride

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The objective is characterizing thin films of indium gallium nitride as a material for photodetectors through fabrication of devices with high sensitivities and high speed responses. The idea was based on the observation that conventional studies placed too much emphases on light-emitting devices of this material system.In the device fabrication, practically acceptable characteristics have been achieved in large area detectors of a metal-semiconductor-metal structure. The characteristics include a low dark current of less than 100 pA at 10V, a high responsivity over 0.1 A/W, and pulse responses on the order of 10 ns, with detecting area up to 1 mm square. The In_xGa_<1-x>N films with x=0.12 on GaN/sapphire were characterized as a photoactive material in the wavelength range of 400-500 nm by examining the differences between front vs. back incidence configurations, especially an effect of illuminating the region just beneath the electrode metal. The observation resulted in finding an anomalous bias dependence of thin InGaN on GaN layers, which is explained by a built-in field existing in a distorted crystal of coherently grown InGaN layer. The effect was successfully applied in a two-color detector that can selectively sense optical signals of 350 nm or 400 nm by simply changing the bias voltage of the Schottky barrier diode. In addition, designing the thicknesses of layers and the operating voltage enabled us to realize a UV-blind blue light detector with similar structure. In relation to the response speed, measurements on deep energy levels were conducted finding an electron trap in the GaN crystal. This finding, however, did not reached to the practical merit of eliminating slow components in photocurrent responses.This study is continued to characterize the material system and apply it to practical photodetectors with an emphasis on the use of built-in field in the thin InGaN layer.