Steady-state and transient surface photovoltage ͑SPV͒ in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity.

Local conductivity and surface photovoltage variations due to magnesium segregation in p-type GaNThe effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UV illumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaN surface. In contrast, the upward band bending gradually decreases in vacuum under UV illumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration.

The optical properties of high-quality GaN co-doped with silicon and zinc are investigated by using temperature-dependent continuous-wave and time-resolved photoluminescence measurements. The blue luminescence band is related to the Zn Ga acceptor in GaN:Si,Zn, which exhibits an exceptionally high absolute internal quantum efficiency (IQE). An IQE above 90% was calculated for several samples having different concentrations of Zn. Accurate and reliable values of the IQE were obtained by using several approaches based on rate equations. The concentrations of the Zn Ga acceptors and free electrons were also estimated from the photoluminescence measurements.

Metal-organic molecular-beam epitaxy of GaN with trimethylgallium and ammonia: Experiment and modeling J. Appl. Phys. 98, 053518 (2005); 10.1063/1.2039276In situ spectroscopic ellipsometry study of GaN nucleation layer growth and annealing on sapphire in metalorganic vapor-phase epitaxy J. Appl. Phys. 98, 033522 (2005); 10. 1063/1.1999033 Growth model for GaN with comparison to structural, optical, and electrical properties Using a Kelvin probe, the authors have studied changes in surface contact potential during illumination, i.e., surface photovoltage ͑SPV͒, for n-and p-type GaN films grown by hydride vapor phase epitaxy. Short ultraviolet ͑UV͒ exposures ͑3 s͒ generate a positive SPV of about 0.5 eV for n-type and a negative SPV of about Ϫ0.6 eV for p-type GaN, which is consistent with the expected surface band bending for these two surface types. The fast component of the SPV is attributed to the accumulation of photogenerated holes ͑n-type͒ or electrons ͑p-type͒ at the surface, which results in a decrease in band bending. During long UV exposures ͑1 h͒, however, slower photoinduced processes can cause the SPV signal to change as a function of the ambient conditions. For both nand p-type GaN, UV illumination causes the adsorption of negatively charged oxygen species on the surface in air and their subsequent desorption in vacuum. It appears that this adsorbate effect is most prevalent for n-type GaN in vacuum and p-type GaN in air, with significant changes in the SPV of up to 0.3 eV ͑n-type͒ and 0.6 eV ͑p-type͒. One important difference between the GaN films is the sensitivity of p-type GaN to the illumination geometry. When the sample contacts are exposed to near-bandgap light, unexpected offsets of the SPV signal can result. If such offsets are appropriately taken into account, however, then the SPV behaviors for both n-and p-type GaN can be reasonably modeled by thermionic expressions.Foussekis, Baski, and Reshchikov: Comparison of surface photovoltage behavior for n-type versus p-type GaN 041205-6 041205-6

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The band bending in a Mg-doped, p-type GaN film grown by hydride vapor phase epitaxy was studied at various temperatures. At 295 K, the band bending in dark was calculated to be approximately À1.5 eV. However, when the sample was heated to 600 K for 1 h in dark before performing a measurement at 295 K, the calculated value of band bending in dark became about À2.0 eV. These results are explained by the fact that increasing the sample temperature exponentially increases the rate at which the band bending restores and allows for a more accurate value of band bending to be measured. V

The effects of polarity and surface treatment on the morphological, electrical, and optical behaviors in bulk GaN have been investigated. Kelvin probe, atomic force microscopy (AFM), and photoluminescence (PL) techniques were utilized to examine a set of freestanding, bulk GaN samples, which were grown by halide vapor phase epitaxy. The Ga- and N-polar surfaces were treated with either a mechanical polish (MP) or chemical mechanical polish (CMP), which influences the morphology, surface photovoltage (SPV), and PL behaviors. Topography studies indicate that the CMP-treated, Ga-polar surface is the smoothest of the sample set, whereas the MP-treated, N-polar surface has the highest root mean square roughness. Local current–voltage spectra obtained with conducting AFM reveal a higher forward-bias, turn-on voltage for the N-polar versus Ga-polar surfaces. Using a Kelvin probe, intensity-dependent SPV measurements are performed on samples with CMP-treated, Ga- and N-polar surfaces, and provide band bending values of 0.83 and 0.70 eV, respectively. The restoration of the SPV from CMP-treated surfaces behaves as predicted by a thermionic model, whereas restoration from MP-treated surfaces has a faster rate than expected. This result is possibly due to enhanced electron conduction via hopping between defect states to the surface. The quantum efficiency of the PL from the CMP- and MP-treated surfaces at room temperature is ∼1% and 1 × 10−5%, respectively, suggesting high quenching of the PL for MP-treated surfaces by near-surface defects. Therefore, AFM, PL, and SPV data indicate that the MP-treated surfaces have a significantly higher density of surface defects.

We have investigated the N‐ and Ga‐polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi‐ready after a chemical‐mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga‐ and N‐polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP‐treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP‐treated surfaces restored with a much faster‐than‐predicted rate. This result can be interpreted by the hopping of charge carriers in the highly‐defective near‐surface layer of the MP‐treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)