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The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100--300 \ensuremath{\mu}s lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of \ensuremath{\sim}10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.

In this work, we report the observation of abrupt and tunable thermal quenching of photoluminescence (PL) in high-resistivity, undoped GaN. The ultraviolet luminescence (UVL) band in these samples exhibited abrupt and tunable quenching, which is similar to behavior observed for p -type GaN:Mg samples. Such behavior has never been observed for undoped GaN and was very rarely reported for other semiconductors. We describe the effect of temperature and excitation intensity on PL in undoped GaN with a phenomenological model involving three defect species. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence (PL) from high-quality GaN codoped with silicon and zinc was investigated in detail. The internal quantum efficiency (IQE) of PL was determined from the analysis of the dependencies of the PL intensity on the excitation intensity and temperature, and the simulation of these dependencies with a phenomenological model based on rate equations. The model reproduces an important phenomenon: the quenching of a recombination channel with a high IQE causes a rise in efficiency of all the other PL bands. Quantitative analysis of this phenomenon allows one to determine reliably the absolute IQE of PL. The absolute IQE of the PL in GaN co-doped with Si and Zn exceeds 90%, with the largest contribution coming from the blue luminescence band associated with the ZnGa acceptor. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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)

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.

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

An unusual temperature dependence of the photoluminescence lifetime for the green luminescence (GL) band in GaN is explained. This GL is caused by an internal transition of electrons from an excited state to the ground state of the 0/+ transition level of the isolated ${\mathrm{C}}_{\mathrm{N}}$ defect. The excited state appears only after the ${\mathrm{C}}_{\mathrm{N}}$ defect captures two photogenerated holes. The electron capture by the excited state is nonradiative, yet the lifetime of such can be probed by the temperature variation of the GL lifetime, whose temperature dependence shows a classic case of electron capture by a giant trap.

We have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at 1.8 eV is often the dominant PL band in HVPE GaN. It is caused by an unknown defect with the concentration of up to ∼1017 cm-3. The concentrations of defects responsible for other defect-related PL bands rarely exceed 1015 cm-3.

A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy $({V}_{\mathrm{N}})$. We propose that transitions of electrons from the conduction band to the +/2+ transition level of the ${V}_{\mathrm{N}}$ defect are responsible for the GL2 band in high-resistivity undoped and Mg-doped 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...

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 ZnGa 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 ZnGa acceptors and free electrons were also estimated from the photoluminescence measurements.