Different bias-voltage dependences of photocurrent in Pt/InGaN/GaN and Pt/GaN Schottky photodetectors on sapphire

Two structures under back illumination showed opposite bias polarity dependence in the photocurrent of Schottky barrier contacts, where a combination of Pt/Au metal films was formed on unintentionally doped n-type layers. The contact with a 2-mu m-thick GaN layer exhibited a higher photocurrent for reverse biasing as expected, whereas the same contact with an additional 20-nm-thick InGaN layer on GaN exhibited a much higher current for forward biasing. This current was maintained down to a small reverse bias voltage, which indicates that the thin InGaN layer with an In content of 15% behaves like a p-type semiconductor. The result can be understood by the internal electric field in the InGaN layer as well as the fact that 400 nm light illuminated from the back side is absorbed in the thin layer just under the contact metal.