Effect of gas exposure on GaN surface quantum wells

Abstract

Some HEMT transistors and a variety of gas, pH, and molecular sensors use thin GaN capping layers that behave as surface quantum wells (SuQWs), such as those formed by a thin GaN layer with vacuum or gas on one side and a higher bandgap (Al0.2Ga0.8N) layer on the other. Their performance is highly sensitive to surface treatments and gas exposure, and the extent to which surface adsorbates alter the crystal polarization and non-radiative surface recombination velocity may be monitored using photoluminescence (PL) spectroscopy. Here, we examine how room temperature PL intensity and emission energy from a GaN SuQW changes when excited by a pulsed laser and exposed to vacuum, inert gases, or hydrogen- or oxygen-containing gas environments. Ambient water vapor readily adsorbs onto SuQW surfaces but may be desorbed by exposure to the gases studied, causing the number of available surface states to increase and SuQW PL efficiency to suffer. However, exposure to hydrogen-containing forming gas leads to hydrogen adsorption that replaces the desorbed water, passivates the newly available surface states, and enhances PL efficiency. These behaviors are confirmed by the observed blue or red shifts in SuQW PL emission energy, which correlate with the electronegativity of the adsorbing/desorbing species. Changes in radiative efficiency evolve over time as gas pressure equilibrates with the surface adsorbates. This study uses practical, non-idealized SuQWs to assess real-world sensitivities to ambient exposure and provides a path toward practical mitigation strategies.