M. Foussekis

Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) 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.

Photoadsorption and photodesorption for GaN

The 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.

Comparison of surface photovoltage behavior for n-type versus p-type GaN

Using a Kelvin probe, the authors have studied changes in surface contact potential during illumination, i.e., surface photovoltage (SPV), for n- and p-type GaN films grown by hydride vapor phase epitaxy. Short ultraviolet (UV) exposures (3 s) generate a positive SPV of about 0.5 eV for n-type and a negative SPV of about −0.6 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-type) or electrons (p-type) at the surface, which results in a decrease in band bending. During long UV exposures (1 h), however, slower photoinduced processes can cause the SPV signal to change as a function of the ambient conditions. For both n- and 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-typ...

Effects of polarity and surface treatment on Ga- and N-polar bulk GaN

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 val...

Temperature dependent behavior of the SPV for n-type GaN

N-type GaN exhibits upward, near-surface band bending that can be decreased by generating a surface photovoltage (SPV). Fitting SPV measurements with a thermionic model based on the emission of charge carriers over the nearsurface barrier provides information about the band bending in dark. We have studied the temperature dependent SPV behavior from a Si-doped, n-type GaN sample grown by hydride vapor phase epitaxy in order to determine how the magnitude of band bending changes at higher temperatures. We have measured the effect of temperature and oxygen on the steady-state SPV behavior, where oxygen is photo-adsorbed on the surface under band-to-band illumination in an air/oxygen ambient more efficiently at higher temperatures. As predicted, the intensity-dependent SPV measurements performed at temperatures between 295 and 500 K exhibit a decrease in the maximum SPV with increasing temperature. When illumination ceases, the band bending then begins to restore to its dark value with a rate proportional to the sample temperature, which also fits a thermionic model.

Temperature-dependent Kelvin probe measurements of band bending in p-type GaN

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 −1.5 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 −2.0 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.

Low-temperature surface photovoltage in p-type GaN

The surface photovoltage (SPV) behavior for Mg-doped, p-type GaN was studied using a Kelvin probe at temperatures from 80 to 300 K. Under band-to-band UV illumination at room temperature, the measured SPV signal for p-type GaN becomes negative as electrons are swept to the surface. However at low temperatures, the SPV signal becomes positive under UV illumination, contrary to the SPV behavior of p-type GaN at room temperature. This positive SPV resembles the behavior of an n-type semiconductor. It is assumed that at low temperatures and under UV illumination, the concentration of photogenerated electrons exceeds the concentration of free holes, leading to n-type conductivity. The positive SPV signal is caused by the alignment of a quasi-Fermi level for electrons with the Fermi level of the probe, since the band bending and its change under illumination are negligible at 80 K. Interestingly, the characteristic temperature at which this transition from p- to n-type SPV behavior occurs is dependent on illumination intensity. This effect is explained with a simple phenomenological model.

Manipulation of Surface Charge on GaN

We have characterized the surface charge on a variety of GaN samples using two surface potential techniques, conventional Kelvin probe and scanning Kelvin probe microscope (SKPM). Kelvin probe was primarily used to measure the change in surface potential under UV illumination, otherwise known as the surface photovoltage (SPV). Due to band bending near the semiconductor surface of about 1 eV in dark conditions, the SPV signal for n-type GaN typically reaches 0.5 to 0.6 eV upon switching on UV light. This value can slowly decrease by up to 0.3 eV during UV illumination in air ambient for 2-3 hours. We report that samples with many hours of ambient UV exposure do not show this slow decrease during SPV measurements, consistent with the UV-induced growth of a thicker surface oxide that limits charge transfer. In addition to prolonged UV exposure, the surface contact potential was also manipulated by local charge injection. In this procedure, the surface is charged using a metallized atomic force microscope tip which is scanned in contact with the sample. Subsequent SKPM measurements indicate an increase or decrease in the surface contact potential for the charged region, depending on the applied voltage polarity. Measurements of the discharge behavior in dark for these regions show a logarithmic time behavior, similar to the decay behavior during our observations of SPV transients after switching off the light. As expected, illumination of the surface increases the discharge rate and restores the charged area to its original state.

Effect of UV exposure on the surface charge behavior for GaN

The surface charge behavior on n-type GaN was investigated as a function of UV exposure in ambient. It was first noticed that the surface photovoltage (SPV), or change in surface contact potential under UV illumination, could slowly decrease during illumination. The SPV for as-grown samples is ~0.6 eV and can drop by up to 0.3 eV over 1 h of UV exposure in air ambient. We believe that this slow decrease is due to the photo-induced chemisorption of oxygen species. Interestingly, samples exhibit a smaller drop in SPV during continuous UV illumination after many hours of accumulated UV exposure. This can be explained by the UV-induced growth of a thicker surface oxide layer which inhibits electron transfer from the bulk to the surface. The original SPV behavior can be restored by chemical etching to remove the additional surface oxide. We have also investigated the surface behavior by locally charging the surface before and after UV-induced oxide growth. Measurements of the surface contact potential using scanning Kelvin probe microscopy (SKPM) demonstrate that negatively charged regions initially spread laterally on the UV-exposed surface, but not on the clean surface. The UV-induced oxide therefore appears to be relatively conductive and enhance the lateral motion of injected electrons. The discharge of both the negatively and positively charged regions under dark conditions is similar for both the initial and UV-exposed surfaces and shows a logarithmic time dependence.

Effect of ambient on electrical and optical properties of GaN

Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and air ambient. SPV transients were measured with two set-ups: traditional Kelvin probe attached to an optical cryostat and atomic force microscope in contact potential mode. It is found that upward band bending in GaN decreases from its dark value of about 0.9 eV to about 0.3 eV under intense UV light and then gradually increases in air ambient (due to photoadsorption of oxygen) and decreases in vacuum (due to photodesorption of oxygen). Manifestations of such changes were observed as changes in PL intensity and in SPV (both increased in vacuum and decreased in air). The effects were sample-dependent.