| AVS 57th International Symposium & Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM+SS-ThM |
| Session: | Nitride Surfaces and Interfaces |
| Presenter: | A.A. Baski, Virginia Commonwealth |
| Authors: | A.A. Baski, Virginia Commonwealth M. Foussekis, Virginia Commonwealth M.A. Reshchikov, Virginia Commonwealth |
| Correspondent: | Click to Email |
Devices based on wide-bandgap GaN are successfully being used today, but a better understanding of surface effects such as band bending could further improve their performance. Many results have been reported on the upward surface band bending for n-type GaN, but fewer exist on the downward band bending for p-type GaN. Surface photovoltage (SPV) measurements using a Kelvin probe can directly measure the change in surface potential during UV illumination, and thereby indirectly measure the resulting decrease in band bending. We have studied steady-state and transient SPV for band-to-band (365 nm) illumination on a variety of p-type (Mg-doped) and n-type (Si-doped ) GaN samples grown by hydride vapor phase epitaxy and metal organic chemical vapor deposition.
For n-type and p-type GaN samples, short (5 s) UV exposures generate an SPV magnitude of about 0.5 eV in both air and vacuum environments. The sign of the SPV signal is positive for n-type GaN and negative for p-type GaN, corresponding to a positive or negative change in surface potential, respectively. This fast component of the SPV corresponds to the accumulation of photo-generated holes (n-type) or electrons (p-type) at the semiconductor/oxide interface and leads to a corresponding decrease in band bending. As expected, this "internal" mechanism does not appear to depend on the sample environment. After ceasing illumination, decay of the SPV in dark and under all environments is slow with a logarithmic time dependence.
It is during longer UV exposures that the SPV behavior becomes noticeably different for n- vs. p-type samples. For n-type GaN, a long UV exposure (1 h) typically causes the SPV to decrease to 0.35 eV in air, but to increase to 0.6 eV in vacuum. This SPV behavior is consistent with the photo-induced adsorption of negatively charged oxygen species in air and their desorption in vacuum. In contrast, long UV exposure for some p-type samples has caused the SPV to reach a surprisingly large value of -1.1 eV in air, but to not change significantly in vacuum. The more negative SPV in air again indicates the adsorption of negative surface species, but the rather large change over extended illumination is not expected, particularly given that a comparable opposite change does not occur in vacuum (as for n-type). Therefore, the charging of the surface layer on p-type GaN and how it affects band bending is still under investigation. We have developed a phenomenological model that is able to distinguish contributions from internal (fast) and external (slow) mechanisms in SPV transients, and have demonstrated that the native oxide layer can play a significant role.