  Due to the lack of effective p-type doping in GaN and the adverse effects of
surface band-bending of GaN on electron transport, developing practical GaN
heterojunction bipolar transistors has been impossible. The recently
demonstrated approach of grafting n-type GaN with p-type semiconductors, like
Si and GaAs, by employing ultrathin (UO) Al$_2$O$_3$ at the interface of Si/GaN
and GaAs/GaN, has shown the feasibility to overcome the poor p-type doping
challenge of GaN by providing epitaxy-like interface quality. However, the
surface band-bending of GaN that could be influenced by the UO Al2O3 has been
unknown. In this work, the band-bending of c-plane, Ga-face GaN with UO Al2O3
deposition at the surface of GaN was studied using X-ray photoelectron
spectroscopy (XPS). The study shows that the UO Al2O3 can help in suppressing
the upward band-bending of the c-plane, Ga-face GaN with a monotonic reduction
trend of the upward band-bending energy from 0.48 eV down to 0.12 eV as the
number of UO Al2O3 deposition cycles is increased from 0 to 20 cycles. The
study further shows that the band-bending can be mostly recovered after
removing the Al2O3 layer, concurring that the change in the density of fixed
charge at the GaN surface caused by UO Al2O3 is the main reason for the surface
band-bending modulation. The potential implication of the surface band-bending
results of AlGaAs/GaAs/GaN npn heterojunction bipolar transistor (HBT) was
preliminarily studied via Silvaco(R) simulations.
