Electrical and optical properties of bismuth telluride/gallium nitride heterojunction diodes

Semiconducting bismuth telluride has a band gap energy of about 0.15 eV at room temperature and is a good material for middle wavelength IR (MWIR) detection [S.K. Mishra, S. Satpathy, O. Jepsen, J. Phys. Condens. Matter, p. 461]. We have grown bismuth telluride thin films on gallium nitride (on sapphire) by pulsed laser deposition at a substrate temperature of 300 °C. The structural characteristics and surface morphology of the films were studied by X-ray diffraction and scanning electron microscopy, respectively. The chemical composition of the as-deposited bismuth telluride thin films was determined by X-ray photoelectron spectroscopy and was found to be different from that of the bulk target, changing from stoichiometric Bi2Te3 to bismuth rich. A bismuth rich p-Bi2Te3/n-GaN/Al2O3 heterojunction was fabricated for photovoltaic detection of low energy photons. The wide band gap semiconducting n-GaN layer and the Al2O3 substrate act as a window for IR transmission. A sensitive IR photoresponse of the heterojunction is obtained by back-side illumination. The photocurrent measured is 0.18 μA according to the irradiation change in the current–voltage characteristics. The current–voltage characteristics allow us to evaluate the series resistance (Rs), zero-bias resistance (R0) and ideality factors (n) of the junctions. Our results have suggested that bismuth telluride is a potential candidate for photovoltaic mid-infrared detection.