  Trapping of charge at surface states is a longstanding problem in GaN that
hinders a full realization of its potential as a semiconductor for
microelectronics. At least part of this charge originates in molecules adsorbed
on the GaN surface. Multiple studies have addressed the adsorption of different
substances, but the role of adsorbents in the charge-trapping mechanism remains
unclear. Here, we show that the GaN surface selectively adsorbs nitrogen
dioxide (NO2) existing in the air in trace amounts. NO2 appears to charge the
yellow-luminescence-related surface state. Mild heat treatment in vacuum
removes this surface charge, only to be re-absorbed on re-exposure to air.
Selective exposure of vacuum-annealed GaN to NO2 reproduces a similar surface
charge distribution, as does the exposure to air. Residual gas analysis of the
gases desorbed during heat treatment in vacuum shows a large concentration of
nitric oxide (NO) released from the surface. These observations suggest that
NO2 is selectively adsorbed from the air, deleteriously affecting the
electrical properties of air-exposed GaN. The trapping of free electrons as
part of the NO2chemisorption process changes the surface charge density,
resulting in a change in the surface band bending. Uncontrollable by nature,
NO2 adsorption may significantly affect any GaN-based electronic device.
However, as shown here, a rather mild heat treatment in vacuum restores the
surface state occupancy of GaN to its intrinsic state. If attempted before
passivation, this heat treatment may provide a possible solution to
longstanding stability problems associated with surface charge trapping in
GaN-based devices.
