Photoconductivity as a tool for gallium nitride investigation

Fundamental purpose for the semiconductor technology is the control of defects, unintentionally introduced during growth or induced by processes, which can dramatically affect the device performance. The synthesis of new semiconductors with promising applications gives strong impulse to technological progress, but often represents a challenge for the defect characterization techniques, which are required to improve their efficiency of detection. Due to their good performance in communication and optoelectronic applications, wide gap semiconductors are gaining ever increasing interest among the scientific community. Despite the high efficiency of devices, however, wide-gap materials contain many defects, object of intensive investigation. Due to the high band-gap width, many widely diffused techniques are inadequate for a full detection of defects in these materials. This fact renewed the interest in photoconductivity-based methods, among the first methods developed for semiconductor characterization, which is very efficient in the detection of defect-related transitions. Photoconductivity spectroscopy analyzes the optically-induced conductivity variations and is often associated to luminescence spectroscopy, due to the complementarity of the two techniques. In this review we will describe the physical mechanisms of photoconductivity investigation and its analysis for the characterization of gallium nitride (GaN), a significant testing material to verify the capabilities of the method. GaN is a wide band gap semiconductor (3.4 eV at room temperature) typically grown by heteroepitaxy and containing a high density of point-like and extended-like defects. Photoconductivity investigation revealed to be particularly efficient in GaN determining that: i) the dislocation typically have a strong electrical activity; ii) the main deep levels give rise to extrinsic transitions towards the bands; iii) the defects are inhomogeneously distributed along the growth direction; iv) the residual strain due to heteroepitaxial growth induces important band-gap modification; v) the persistent photoconductivity effect can be ascribed to multiple factors. These features often appear strictly correlated, as illustrated in the results here illustrated.