J. D. McNamara

Determination of the electron-capture coefficients and the concentration of free electrons in GaN from time-resolved photoluminescence

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN.

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.

Two yellow luminescence bands in undoped GaN

Two yellow luminescence bands related to different defects have been revealed in undoped GaN grown by hydride vapor phase epitaxy (HVPE). One of them, labeled YL1, has the zero-phonon line (ZPL) at 2.57 eV and the band maximum at 2.20 eV at low temperature. This luminescence band is the ubiquitous yellow band observed in GaN grown by metalorganic chemical vapor deposition, either undoped (but containing carbon with high concentration) or doped with Si. Another yellow band, labeled YL3, has the ZPL at 2.36 eV and the band maximum at 2.09 eV. Previously, the ZPL and fine structure of this band were erroneously attributed to the red luminescence band. Both the YL1 and YL3 bands show phonon-related fine structure at the high-energy side, which is caused by strong electron-phonon coupling involving the LO and pseudo-local phonon modes. The shapes of the bands are described with a one-dimensional configuration coordinate model, and the Huang-Rhys factors are found. Possible origins of the defect-related luminescence bands are discussed.

Surface photovoltage in heavily doped GaN:Si,Zn

In n-type GaN, an upward band bending of about 1 eV is caused by negative charge at the surface. UV light reduces the band bending by creating a surface photovoltage (SPV), which can be measured by a Kelvin probe. Previously, we reported a fast SPV signal of about 0.6 eV in undoped and moderately doped GaN. In this work, we have studied degenerate GaN co-doped with Zn and Si, with a Si concentration of about 1019 cm−3 and a Zn concentration of 6×1017 cm−3. At room temperature, a fast component of about 0.6 eV was observed. However, after preheating the sample at 600 K for one hour and subsequently cooling the sample to 300 K (all steps performed in vacuum), the fast component disappeared. Instead, a very slow (minutes) and logarithmic in time rise of the SPV was observed with UV illumination. The total change in SPV was about 0.4 eV. This slow SPV transient can be reversibly converted into the “normal” fast (subsecond) rise by letting air or dry oxygen in at room temperature. Possible explanations of the ...

Surface photovoltage studies of p-type AlGaN layers after reactive-ion etching

The surface photovoltage (SPV) technique was used to study the surface and electrical properties of Mg-doped, p-type AlxGa1−xN (0.06 < x < 0.17) layers. SPV measurements reveal significant deviation from previous SPV studies on p-GaN:Mg thin films and from the predictions of a thermionic model for the SPV behavior. In particular, the SPV of the p-AlGaN:Mg layers exhibited slower-than-expected transients under ultraviolet illumination and delayed restoration to the initial dark value. The slow transients and delayed restorations can be attributed to a defective surface region which interferes with normal thermionic processes. The top 45 nm of the p-AlGaN:Mg layer was etched using a reactive-ion etch which caused the SPV behavior to be substantially different. From this study, it can be concluded that a defective, near-surface region is inhibiting the change in positive surface charge by allowing tunneling or hopping conductivity of holes from the bulk to the surface, or by the trapping of electrons traveli...