Local electric field measurement in GaN diodes by exciton Franz–Keldysh photocurrent spectroscopy

Abstract

The eXciton Franz-Keldysh (XFK) effect is observed in GaN p-n junction diodes via the spectral variation of photocurrent responsivity data that redshift and broaden with increasing reverse bias. Photocurrent spectra are quantitatively fit over a broad photon energy range to an XFK model using only a single fit parameter that determines the lineshape, the local bias ( ), uniquely determining the local electric field maximum and depletion widths. As expected, the spectrally determined values of vary linearly with the applied bias ( ) and reveal a large reduction in the local electric field due to electrostatic non-uniformity. The built-in bias ( ) is estimated by extrapolating at , which compared with independent C-V measurements indicates an overall ±0.31 V accuracy of . This demonstrates sub-bandgap photocurrent spectroscopy as a local probe of electric field in wide bandgap diodes that can be used to map out regions of device breakdown (hot spots) for improving electrostatic design of high voltage devices.GaN is a III-V compound semiconductor with a wide bandgap (Eg~3.4 eV) and a high breakdown electric field ( > 3.3 MV/cm), making it useful for power devices and optoelectronics. 1 5Heterostructures based on GaN result in high responsivity Metal-Heterojunction-Metal (MHM) ultraviolet photodetectors. 6,7 However, the stability and lifetime at high voltages for GaN based power electronics still remains a challenge to the device industry. 7 12 Vertical devices such as p-n junctions have electric field peaks near the device edge, which are managed using junction termination structures such as field plates and guard rings. 13 16 Lateral devices such as AlGaN/GaN HEMTs also show field variation across the gate-drain region. The built-in electric field in such heterostructures can be measured using contactless electroreflectance, 17,18 however it is not suitable at high fields when the reflectance oscillations decay. It is a challenge to measure the actual field distribution in such devices, and device engineers usually rely on device simulations to estimate the electric field profiles.Furthermore models used to estimate electric field profiles may ignore inhomogeneity, and thus may not