Peculiarities of the current-voltage and capacitance-voltage characteristics of plasma etched GaN and their relevance to n-GaN Schottky photodetectors

Abstract

The impact of reactive ion etching (RIE) induced damage on the optoelectronic properties of GaN epitaxial layers and the photoresponse of Schottky detectors is investigated. Plasma induced surface damage in epitaxial layers is noticed which leads to a significant reduction of the intensity of the photoluminescence signal and also the photoresponse of detector devices post dry etching process. Electrical characterization of Au/Ni/GaN Schottky diodes along with secondary ion mass spectroscopy results indicate that the ion bombardment induced damage is mostly confined close to the surface of the GaN layer. It is found that the current-voltage characteristics of Schottky contacts on pristine n-GaN layers can be understood by considering a model based on the thermionic emission of carriers across the junction. However, the same is not possible in the case of plasma etched samples where the involvement of the thermionic field emission of carriers is essential. It is proposed that the RIE process leads to the generation of nitrogen vacancies in strongly localized domains near the surface. Such vacancies act as shallow donors shifting the Fermi level into the conduction band, thus enabling the tunnelling of carriers across the junction. However, this is not evident in capacitance-voltage characteristics since the damage is much prior to the depletion edge and is confined to extremely small domains. A method for the recovery of dry etch induced damage through O2 plasma treatment is demonstrated which is found to be very effective in improving the post-etch surface morphology and also the optoelectronic properties of etched GaN epitaxial layers. The spectral response of the Schottky photodetector is seen to degrade by 90% due to the plasma etching process. However, the same can be recovered along with an enhancement of the deep ultraviolet response of the detector after O2 plasma treatment of etched layers within the RIE chamber. The understanding developed here is crucial for the optimization of the RIE process and is found to be very helpful in recovery of damage caused by the dry etching process.