Physics and chemistry of nitrogen dioxide (NO₂) adsorption on gallium nitride (GaN) surface and its interaction with the yellow-luminescence-associated surface state

Surface states have been a longstanding and sometimes underestimated problem in gallium nitride (GaN) based devices. The instability caused by surface-charge-trapping in GaN-based transistors is practically the same problem faced by the inventors of the silicon (Si) field effect transistors more than half a century ago. Although in Si this problem was eventually solved by oxygen and hydrogen-based passivation, in GaN, such breakthrough has yet to be made. Apparently, some of this surface charge originates in molecules adsorbed on its surface. Here, it is shown that the charge density associated with the GaN yellow band desorbs upon mild heat treatment in vacuum and re-adsorbs on exposure to the air. Selective exposure of GaN to nitrogen dioxide (NO₂) reproduces this surface charge to its original distribution, as does exposure to air. Residual gas analysis of the gases desorbed during heat treatment shows a large concentration of nitric oxide (NO). These observations suggest that selective adsorption of NO₂ is responsible for the surface charge that deleteriously affects the electrical properties of GaN. The physics and chemistry of this NO₂ adsorption, reported here, may open a new path in the search for passivation to improve GaN device reliability.