Photoluminescence In GaN Research Articles

Effect of carrier drift-diffusion transport process on thermal quenching of photoluminescence in GaN

Thermal quenching of the defect-related photoluminescence (PL) has been widely used to determine the fundamental properties of point defects in GaN such as the activation energy (E t ) and capture cross section. However, in the present work, we have shown using drift-diffusion modeling and experimental analysis that the thermal quenching of defect-related PL from GaN does not only depend of the defect parameters but also is strongly governed by the carrier drift and diffusion in the depletion region. In particular, we have found that these processes significantly influence the slope of PL thermal quenching and temperature T 0 at which the quenching begins. As a result, E t obtained from the Arrhenius plot of the defect-related PL intensity in GaN provides the correct values of the defect activation energy only in the case of low surface state density ( cm−2eV−1) corresponding to the weak surface band bending. However, in the case of high D 0 giving rise to the significant surface band bending, the E t value should be cautiously interpreted because it may not correspond entirely to the real defect activation energy due to the drift-diffusion process. Finally, we have shown that our finding can explain in a coherent manner various anomalous behaviors of PL thermal quenching reported in the literature, such as tunable, abrupt or sample dependent thermal quenching. We believe that our findings can be very interesting for nitride growth technology community and optoelectronic device applications.

Optimization of the Epitaxial Growth of Undoped GaN Waveguides in GaN-Based Laser Diodes Evaluated by Photoluminescence

Non-intentionally doped c-plane GaN layers are generally employed as p-side waveguide layers in violet/blue-emitting laser diodes. The recombination and diffusion of charge carriers in the p-side GaN waveguide influence the injection efficiency of holes into the InGaN quantum wells of these devices. In this study, the non-radiative recombination and the diffusivity in the [000-1] direction for charge carriers in such GaN layers are investigated by the photoluminescence of buried InGaN quantum wells, in addition to the GaN photoluminescence. The vertical charge carrier diffusion length and the diffusion constant in GaN were determined by evaluating the intensity from InGaN quantum wells in different depths below a top GaN layer. Additionally, the intensity from the buried InGaN quantum wells was found to be more sensitive to variations in the non-radiative recombination rate in the GaN layer than the intensity from the GaN itself. The study enables conclusions to be drawn on how the growth of a p-side GaN waveguide layer has to be optimized: (1) The charge carrier diffusivity in the [000-1] direction at device operation temperature is limited by phonon scattering and can be only slightly improved by material quality. (2) The use of TMGa (trimethylgallium) instead of TEGa (triethylgallium) as a precursor for the growth of GaN lowers the background silicon doping level and is advantageous for a large hole diffusion length. (3) Small growth rates below 0.5 μm/h when using TMGa or below 0.12 μm/h when using TEGa enhance non-radiative recombination. (4) A V/III gas ratio of 2200 or more is needed for low non-radiative recombination rates in GaN.

Room-temperature infrared photoluminescence in GaN doped with various impurities

The steady-state infrared-photoluminescence spectra (IR-PL) emitted from about 400 μm thick, free-standing GaN wafers, grown by the ammono-thermal and hydride vapour-phase epitaxy GaN, and containing carbon, magnesium, manganese and iron doping have been examined. The room-temperature IR-PL spectra are correlated with pulsed-photo-ionization spectra using van Roosbroeck-Schockley approach for spectrum conversion. It has been revealed that iron and carbon dopants appear as the most efficient impurities for the room temperature of infra-red emission from GaN grown using different technologies.

Photoluminescence of planar and 3D InGaN/GaN LED structures excited with femtosecond laser pulses close to the damage threshold

We study the photoluminescence emission from planar and 3D InGaN/GaN LED structures, excited using a femtosecond laser with fluences close to sample’s damage threshold. For a typical laser system consisting of a titanium-sapphire regenerative amplifier, which is pumping an optical parametric amplifier delivering output pulses of a few tens of MW pulse power with ∼100 fs pulse duration, 1 kHz repetition rate and a wavelength of 325 nm, we determine the damage threshold of the InGaN/GaN LEDs to be about 0.05 J/cm2. We find that the relative intensity of the GaN photoluminescence (PL) and InGaN PL changes significantly close to the damage threshold. The changes are irreversible once the damage threshold is exceeded. As the damage threshold is approached, the InGaN luminescence band blue-shifts by several tens of meV, which is attributed to band filling effects. The PL decay time reduces substantially, by about 30%, when the excitation energy density is increased by approximately two orders of magnitude. The results are comparable for 2D and 3D LED structures, where in the latter case m-plane QWs exhibit different recombination dynamics because of the absence of the quantum confined Stark effect.

Hopping conductivity and dielectric relaxation in Schottky barriers on GaN

A study of the current and capacitance dependences on the forward voltage in Au/n-GaN Schottky diodes, the sub-band optical absorption spectra, and the defect photoluminescence in n-GaN bulk crystals and thin layers is reported. It is shown that defect-assisted tunneling is the dominant transport mechanism for forward-biased Schottky contacts on n-GaN. The dependences of the current and capacitance on forward bias reflect the energy spectrum of defects in the band gap of n-GaN: the rise in the density of deep states responsible for yellow photoluminescence in GaN with increasing energy and the steep exponential tail of states with an Urbach energy of E U = 50 meV near the conduction-band edge. A decrease in the frequency of electron hops near the Au/n-GaN interface results in a wide distribution of local dielectric relaxation times and in a dramatic transformation of the electric-field distribution in the space-charge region under forward biases.

Photo-induced Doping in GaN Epilayers with Graphene Quantum Dots.

We demonstrate a new doping scheme where photo-induced carriers from graphene quantum dots (GQDs) can be injected into GaN and greatly enhance photoluminescence (PL) in GaN epilayers. An 8.3-fold enhancement of PL in GaN is observed after the doping. On the basis of time-resolved PL studies, the PL enhancement is attributed to the carrier transfer from GQDs to GaN. Such a carrier transfer process is caused by the work function difference between GQDs and GaN, which is verified by Kelvin probe measurements. We have also observed that photocurrent in GaN can be enhanced by 23-fold due to photo-induced doping with GQDs. The improved optical and transport properties from photo-induced doping are promising for applications in GaN-based optoelectronic devices.

Formation of scandium nitride (ScN) layer on gallium arsenide (GaAs) substrate using a combined technique of e-beam evaporator and ammonia annealing treatment

A demonstration on a new technique of growing ScN using electron beam (e-beam) evaporator, coupled with successive ammonia (NH3) annealing treatment is presented in this paper. The annealing temperature was varied at 750, 800, 850, 900 and 980°C in order to obtain the best ScN layer. It was found that as the annealing temperature increased, the surface morphology of the ScN layer changed and ScN grains formed abundantly on the surface. The best surface of ScN layer was found in the 900°C annealed sample. However, the roughness of the ScN increased with temperature. The photoluminescence (PL) peak of the near-to-band-edge (NBE) of ScN was observable in all samples and its intensity was the highest in the 900°C annealed sample. Note that when the annealing treatment was conducted at 980°C, the GaN PL peak is observable. Raman peaks of TO(X) of ScN were much evident at the annealing temperature above 900°C. The formation of ScN bonds was confirmed by X-ray spectroscopy (XPS) measurement. In the end of this work, we propose that the formation of ScN using the above techniques was successful, with thermal annealing at the temperature of 900°C.

Properties of sub-band edge states in AlInN studied by time-resolved photoluminescence of a AlInN/GaN heterostructure

Time-resolved photoluminescence (PL) measurements of Al0.82In0.18N/GaN heterostructures revealed a large enhancement of low temperature PL from the GaN layer and a strong temperature dependence of this effect. Analysis of different phenomena that might affect the photoexcited carrier dynamics suggests that the enhanced GaN PL should be attributed to photoexcited hole transfer from the AlInN layer. The hole transport most probably takes place via dense sub-band edge valence band states related to nanoscale In-rich clusters. Scanning near-field optical microscopy data support this assignment.

In situ measurement of GaN film photoluminescence under plasma etching

This work focuses on an improved method that enables us to measure in situ GaN film photoluminescence (PL) under Ar or N2 plasma etching. Although the background signal is large and increases with bias voltage, the GaN PL can be obtained after subtraction of this background from the total luminescence. Moreover, after plasma etching, the intensities of the near-band-edge and the yellow luminescence decrease significantly. It is suggested that this behaviour is strongly related to the heavy plasma-induced damage, which includes non-radiative defects and deep-level defects, such as Ga vacancies and/or C impurities.

Carrier dynamics and localization in AlInN/GaN heterostructures

AbstractPhotoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructure have been examined by time‐resolved photoluminescence (PL), transient photoreflectance and scanning near‐field optical spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited carrier transfer from AlInN to GaN via sub‐band edge states. Near‐field PL scans and photoreflectance data show that the diameter of the localization sites and the distance between them are well below 100 nm. Majority of these states is assigned to In clusters, in which the valence band has a higher energy than the valence band in a uniform AlInN alloy. It is likely that the carrier transport through the sub‐band edge states proceeds via high conductivity channels involving extended defects. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Higher than 90% internal quantum efficiency of photoluminescence in GaN:Si,Zn

AbstractPhotoluminescence (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)

Photoexcited carrier dynamics in AlInN/GaN heterostructures

Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface.

Determination of the absolute internal quantum efficiency of photoluminescence in GaN co-doped with Si and Zn

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.

The Impact of Bulk Defects, Surface States, and Excitons on Yellow and Ultraviolet Photoluminescence in GaN

The quantitative analysis of the in uence of deep bulk levels, surface states and excitons on yellow, green and ultraviolet photoluminescence from n-type GaN was performed. The theoretical calculations of recombination rates in the bulk and at n-GaN surface versus UV-excitation intensity were done numerically using nite element method basing on drift-di usion model assuming point deep levels, continuous energetic distribution of surface states, as well as excitons. The obtained results of the photoluminescence intensity were compared with experimental data (measured within the range from 10 to 10 photon cm−2 s−1) for n-GaN samples with various surface passivating layers (Al2O3, SiO2).

Extremely high absolute internal quantum efficiency of photoluminescence in co-doped GaN:Zn,Si

We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phase epitaxy and a detailed study of photoluminescence in this material. We observe an exceptionally high absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of 0.93±0.04 has been obtained from several approaches based on rate equations.

Optical properties, luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

Abstract We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ∼0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu 3+ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu 3+ ion transition ( 5 D 0 → 7 F 2 ) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu 3+ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu 3+ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu 3+ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu 3+ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10 12 to 5 × 10 13 cm −2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.

Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure

This paper reports on a comparative study of the spatial distributions of the electrical, optical, and structural properties in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN template layer is shown to decrease in the regions of the wafer where the heterostructure sheet resistance increases and the GaN photoluminescence band-edge energy peak shifts to a high wavelength. This phenomenon is found to be attributed to the local compressive strain surrounding edge dislocation, which will generate a local piezoelectric polarization field in the GaN layer in the opposite direction to the piezoelectric polarization field in the AlGaN layer and thus help to increase the two-dimensional electron gas concentration.

Influence of a Strained AlGaN Overlayer on the GaN Matrix near AlGaN/GaN Heterointerfaces

Unusual splitting of the GaN peaks in the X-ray diffraction (XRD) and the photoluminescence (PL) spectra has been observed in strained AlGaN/GaN heterostructures. As the Al mole fraction increases, a single GaN peak splits into three peaks in both the XRD and the PL spectra, and the GaN PL peak due to the donor-bound exciton moves to higher energy in parallel with a pair of extrinsic peaks. The splittings of the peak and the blue shifts may possibly be associated with different crystalline domains in the GaN matrix layer, those domain being separated by the strong tensile stress in the AlGaN overlayer. This implies that the crystallinity of the GaN matrix near the AlGaN/GaN heterointerface can be influenced by strained the AlGaN overlayer.

Time- and frequency-domain measurements of carrier lifetimes in GaN epilayers

Abstract Results on time-resolved study of GaN photoluminescence (PL) in a power density range from 0.5 mW/cm2 under CW excitation by ultraviolet light emitting diode (UV LED) to 1 GW/cm2 under pulsed excitation by YAG:Nd laser in the temperature range from 8 to 300 K are presented. Measurements of PL response in the frequency domain by using amplitude-modulated emission of a UV LED as well as time-resolved PL measurements using a streak camera and light-induced transient grating technique have been used in the study. Yellow luminescence (YL) intensity increases with increasing temperature up to ∼120 K and faster components in YL decay switch to slower components with increasing temperature under UV LED excitation. At low carrier densities, the trapping decreases the carrier lifetime below 250 ps, while the carrier lifetime in the same GaN sample under excitation ensuring saturation of the traps equals 2 ns.

Enhanced second- and third-harmonic generation and induced photoluminescence in a two-dimensional GaN photonic crystal

We observed visible second-harmonic and ultraviolet third-harmonic fields generated in reflection from the surface of a two-dimensional triangular GaN/sapphire photonic crystal. When the pump radiation resonates with a photonic mode, enhancement factors as high as 250 and 3500 occurred for the second- and third-harmonic signals, respectively, as compared to the unpatterned GaN slab. The very large increase of third-harmonic field, with a photon energy exceeding that of the electronic band gap, was used as an efficient mechanism to induce GaN photoluminescence.