Surface band bending of a-plane GaN studied by scanning Kelvin probe microscopy

We report the value of surface band bending for undoped, a-plane GaN layers grown on r-plane sapphire by metalorganic vapor phase epitaxy. The surface potential was measured directly by ambient scanning Kelvin probe microscopy. The upward surface band bending of GaN films grown in the [112¯0] direction was found to be 1.1±0.1V. Because polarization effects are not present on a-plane GaN, we attribute such band bending to the presence of charged surface states. We have modeled the surface band bending assuming a localized level of surface states in the band gap on the surface. It should be noted that the band bending observed for a-plane layers is comparable to that obtained on polar c-plane layers, and both a-plane and c-plane GaN films with similar surface treatments demonstrate comparable band bending behavior, indicating that charged surface states dominate band banding in both cases

I-V characteristics of Au∕Ni Schottky diodes on GaN with SiNx nanonetwork

Room temperature and temperature dependent current-voltage characteristics of Ni∕AuSchottky diodes fabricated on undoped GaN prepared with and without in situ SiNxnanonetwork by metal organic chemical vapor deposition have been studied. The features of the Schottky diodes depend strongly on the SiNx deposition conditions, namely, its thickness. Reduction in the point and line defect densities caused the Schottky barrier height to increase to1.13eV for 5min SiNx deposition time as compared to 0.78eV without SiNx nanonetwork. Similarly, the breakdown voltage also improved from 76V for the reference to 250V when SiNx nanonetwork was used. With optimized SiNx nanonetwork, full width at half maximum values of (0002) and (101¯2) x-ray rocking curves improved to 217 and 211arcsec, respectively, for a 5.5μm thick layer, as compared to 252 and 405arcsec for a reference sample of the same thickness, which are comparable to literature values. The photoluminescence linewidth also reduced to 2.5meV at 15K with free excitons A and B clearly resolvable

Study of SiNx and SiO2 passivation of GaN surfaces

The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence(PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the surface potential measured by SKPM increases as a function of distance from the Schottky contact much faster after SiNx passivation. We suggest that the passivation reduces the total density of surface states and therefore reduces surface recombination

Comparative study of the (0001) and (0001) surfaces of ZnO

The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯)surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯)surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯)surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient