GaN Epilayers Research Articles

Sub-bandgap excited photoluminescence probing of deep defect complexes in GaN doped by Si, Ge and C impurities

Abstract With the sub-bandgap optical excitation, thermal dynamics of holes among multiple levels in n-type GaN epilayers with different dopants of Si, Ge and C are investigated via measuring and modelling variable-temperature yellow luminescence (YL) band of the samples. In sharp contrast to the case of above-bandgap optical excitation, the variable-temperature YL band of all the studied GaN samples including unintentionally-doped sample exhibit unusual negative thermal quenching (NTQ) behavior, suggesting a possible physical mechanism, namely thermally induced migration of holes from shallower levels to the luminescent deep level. By considering the possible presence of multiple hole levels in the doped GaN samples, a phenomenological model is developed for the thermal transfer of holes among the multi-levels and the interpretation of the observed NTQ phenomenon of the YL band. Different activation energies of 347.9, 520.8 and 348.5 meV are obtained for the Ge-doped, high C-containing, and Si-doped GaN samples, respectively. The results reveal the existence of multiple hole defect levels in the n-type GaN. Possible microstructural origins causing these different hole levels are further argued. The study may shed some light on the nature of various defect complexes in the technologically important GaN epilayers. Combined microstructural and optical investigations need to be further done for elucidating various optically- and electrically-active defect complexes in GaN.

Graphene-assisted remote epitaxy wrinkle-free GaN films on flexible mica

When GaN films were directly deposited onto fluorophlogopite mica substrates, spontaneous wrinkling was frequently observed in GaN films. Because of the non-uniform interface adhesion between GaN and mica, two types of delamination phenomena have been observed by scanning electron microscopy. The maximum thickness of wrinkle-free GaN films on mica substrates was ∼200 nm. Interface pinning effect occurred due to the dangling bonds caused by the damage to mica during the deposition of GaN films. To avoid the formation of dangling bonds, the surface of mica was shielded by a graphene interlayer which was transferred via a wet method. Wrinkles were eradicated after the formation of sliding contact between GaN films and fully graphene-covered mica substrates. Despite the presence of the graphene lattice, the growth of GaN epilayer remained governed by the potential field of mica, indicating that remote epitaxy occurred on a 2D mica support substrate.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

Low‐Pressure, Modified Halide Vapor‐Phase Epitaxy for Chemically Pure GaN Epilayers

The halide vapor‐phase epitaxy (HVPE) technique has been extensively used for the production of freestanding GaN crystals for GaN wafer manufacturing, but has had renewed interest in recent years as a technique for producing high‐quality GaN epilayers at high growth rates and without the issues of carbon incorporation which complicate the growth of GaN using metal–organic vapor phase epitaxy. Herein, thick epilayers grown by HVPE using a low‐pressure modified quartz reactor with controlled H2 injection are presented which exhibit surface root‐mean‐square roughnesses <0.5 nm with low background doping concentrations <2E15 cm−3 and room‐temperature mobilities >1200 cm2 V−1 s−1. Such films are considered to be strong contenders for the production of lightly doped GaN drift layers for vertical power electronics devices.

Controlled nano-roughening of the GaN surface by post-growth thermal annealing

GaN-based semiconductor structures have been widely used in photonic, electronic, and optoelectronic devices. However, due to the lack of lattice-matched substrates for GaN, several schemes related to material growth and device structure, including surface roughening, have been proposed to improve the efficiency of GaN-based devices. In our work, we developed an experimental approach to achieving a controllable roughening of the GaN epilayer surface by thermal post-annealing. The degree of surface roughness, i.e., nanometric modification, is controlled by the number of annealing cycles. The samples were annealed at 1200 °C under N2 ambient. Under this condition, the first stage of GaN decomposition occurs. This annealing process led to the formation of Ga-rich GaN nanoparticles at the film surface with no significant change in the dislocation density. Furthermore, this treatment greatly decreases the compressive stress degree, resulting in a considerable improvement in the optical properties of the GaN samples.

Fully-vertical GaN-on-SiC Schottky barrier diode with ultrathin AlGaN buffer layer

In this letter, vertically conductive GaN epilayer on SiC substrate was achieved without the typically used conductive buffer layer. Here, in order to reduce the impacts of band offset of different layers on vertical conductivity and improve the vertical carrier transportation, an ultrathin AlGaN buffer layer was employed to replace the thick conductive buffer layer. Fully-vertical Schottky barrier diode (SBD) based on this vertically conductive epi-stack demonstrated a much lower specific on-resistance of 0.84 mΩ·cm2 with superior thermal stability. Moreover, the SBD also exhibited an on/off ratio of ∼5 × 109 and a nearly unity ideality factor of 1.08. This approach lays the foundation for the heterogeneous integration of GaN/SiC based devices.

Impact of swift heavy ion irradiation on as-grown gallium nitride epilayers by MOCVD technique

The as-grown GaN by Metalorganic Chemical Vapor Deposition was utilized to study the irradiation effects on GaN epilayers. The stopping range of ions in the matter is studied to choose the energy range for irradiation. The grown epilayers were irradiated with light and heavy ions of O7+ and Ag12+ by varying ion fluence. The X-ray Diffraction confirmed the wurtzite crystal structure for pristine and all the irradiated GaN epilayers. Atomic force microscopy showed high surface roughness for heavy ion irradiation when compared with light ion. Micro-Raman analysis was carried out through the Lorentz line fitting and calculated the carrier concentration and phonon lifetime. In light and heavy ion irradiation, the carrier concentration enhances with defect formations. X-ray Photoelectron Spectroscopy analysis showed that light ion irradiation caused oxygen bonding, while heavy ion irradiation led to the Ga–Ga bond shift, revealing film degradation.

Study on the Influence of KOH Wet Treatment on Red μLEDs

InGaN-based red micro-light-emitting diodes (µLEDs) of different sizes were prepared in this work. The red GaN epilayers were grown on 4-inch sapphire substrates through metal-organic chemical vapor deposition (MOCVD). Etching, sidewall treatment, and p- and n-contact deposition were involved in the fabrication process. Initially, the etching process would cause undesirable damage to the GaN sidewalls, which leads to an increase in leakage current. Hence, we employed KOH wet treatment to rectify the defects on the sidewalls and conducted a comparative and systematic analysis of electrical as well as optical properties. We observed that the µLEDs with a size of 5 µm exhibited a substantial leakage current, which was effectively mitigated by the application of KOH wet treatment. In terms of optical performance, the arrays with KOH demonstrated improved light output power (LOP). Additionally, while photoelectric performance exhibited a decline with increased current density, the devices treated with KOH consistently outperformed their counterparts in terms of optoelectronic efficiency. It is noteworthy that the optimized devices displayed enhanced photoelectric characteristics without significantly altering their original peak wavelength and FWHM. Our findings point to the elimination of surface non-radiative recombination by KOH wet treatment, thereby enhancing the performance of small-sized red µLEDs, which has significant potential in realizing full-color micro-displays in near-eye projection applications.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Passivation of hole traps in SiO2/GaN metal-oxide-semiconductor devices by high-density magnesium doping

A major challenge in GaN-based metal-oxide-semiconductor (MOS) devices is significant hole trapping near the oxide/GaN interface. In this study, we show that the density and energy level of the hole traps depends crucially on the concentration of magnesium (Mg) dopants in GaN layers. Although the surface potential of a conventional SiO2/p-GaN MOS device is severely pinned by hole trapping, hole accumulation and very low interface state densities below 1011 cm−2 eV−1 are demonstrated for MOS capacitors on heavily Mg-doped GaN epilayers regardless of the degree of dopant activation. These findings indicate the decisive role of Mg atoms in defect passivation.

Trap characterization of high-growth-rate laser-assisted MOCVD GaN

A detailed study comparing defect incorporation between laser-assisted metal-organic chemical vapor deposition (MOCVD)-grown GaN and conventional low- and high-growth-rate MOCVD GaN was conducted. Using deep-level transient and optical spectroscopy, traps throughout the bandgap were characterized where traps were found at EC-0.25 eV, EC-0.57 eV, EC-0.72 eV, EC-0.9 eV, EC-1.35 eV, EC-2.6 eV, and EC-3.28 eV in all three samples. This indicates no new traps were observed in the laser-assisted MOCVD GaN sample. Overall, the trap concentrations in the laser-assisted MOCVD sample were ∼2× higher than the optimal low-growth-rate sample, but this is primarily due to the increase in gallium vacancy EC-2.6 eV and carbon-related EC-3.28 eV trap concentrations. The EC-0.9 eV trap concentration was ∼2× higher in the laser-assisted sample, so proton irradiation experiments were conducted to identify the physical source of this level. The results indicated this was a native point defect likely related to gallium interstitials. Overall, this study shows that the laser-assisted MOCVD growth method is promising for future thick, high-quality GaN epilayers after further growth optimizations.

Wafer-scale heteroepitaxy GaN film free of high-density dislocation region with hexagonal 3D serpentine mask

GaN-based III-Nitride compound semiconductors are fundamental materials for high-performance optoelectronic and electronic devices. Low-defect-density substrate has been a major bottleneck in achieving high internal quantum efficiency and high breakdown voltage in those devices. However, a large-scale and uniform low dislocation density GaN film is still hard to obtain on foreign substrates, which is the mainstream economical method in scientific research and industrial production. Here, we present a 3D serpentine mask method of growing high-quality GaN film on foreign substrates without high dislocation density (HDD) areas by designing both sectional and planar shapes of stacked masks. The serpentine channel in the cross-section mechanically blocks the extended dislocations and the hexagonal planar pattern reduces the dimension of coalescence geometry. A wafer-scale GaN epilayer, free of HDD zone, with the overall threading dislocation density of 1.7 × 107 cm−2 estimated by plan-view cathode luminescence, is achieved in the metal-organic chemical vapor deposition system. Then, an array of light-emitting diodes based on this substrate with low forward voltages and high light output powers are fabricated, which introduces a practical method to provide high-quality GaN films for high-performance optoelectronic devices.

Quantitative analysis of carbon impurity concentrations in GaN epilayers by cathodoluminescence

In this work, a technique for quantifying carbon doping concentrations in GaN:C/AlGaN buffer structures using cathodoluminescence (CL) is presented. The method stems from the knowledge that the blue and yellow luminescence intensity in CL spectra of GaN varies with the carbon doping concentration. By calculating the blue and yellow luminescence peak intensities normalised to the peak GaN near-band-edge intensity for GaN layers of known carbon concentrations, calibration curves that show the change in normalised blue and yellow luminescence intensity with carbon concentration in the 1016 − 1019 cm−3 range were derived at both room temperature and 10 K. The utility of such calibration curves was then examined by testing against an unknown sample containing multiple carbon-doped GaN layers. The results obtained from CL using the normalised blue luminescence calibration curves are in close agreement with those from secondary-ion mass spectroscopy (SIMS). However,the method fails when applying calibration curves obtained from the normalised yellow luminescence likely due to the influence of native VGa defects acting in this luminescence region. Although this work shows that indeed CL can be used as a quantitative tool to measure carbon doping concentrations in GaN:C, it is noted that the intrinsic broadening effects innate to CL can make it difficult to differentiate between the intensity variations in thin ( < 500 nm) multilayered GaN:C structures such as the ones studied in this work.

Modeling of temperature-dependent photoluminescence of GaN epilayer by artificial neural network

Modeling of temperature-dependent photoluminescence of GaN epilayer by artificial neural network

Effect of Carrier Gas on Silicon Doped GaN Epilayer Characteristics

Effect of Carrier Gas on Silicon Doped GaN Epilayer Characteristics

Influence of Pulse Atomic-Layer Epitaxy (PALE) AlN Buffer Layer on Quality of MOCVD Grown GaN on Si(111) Substrate.

A crack-free GaN-on-Si with a high capability to grow a thick GaN layer is important for the performance of optoelectronic devices. Regrettably, a large lattice and thermal mismatch degraded the quality of GaN-on-Si. Therefore, we introduce the pulse atomic-layer epitaxy (PALE) AlN buffer layer to overcome the surface crack issues and improve the quality of GaN epilayers. The effect of PALE AlN buffer layer with 0, 35, 70, and 140 cycle numbers towards the quality of grown GaN on Si(111) substrate is studied. All samples have been observed using AFM, FESEM, XRD and Raman spectroscopy. AFM imaging revealed a fully-coalesced and clear step flow GaN layer with the lowest RMS roughness of 0.606 nm. FESEM surface morphology shows a crack occurs on a sample without AlN PALE (0 pulse cycle) while crack-free GaN is obtained for samples with AlN PALE layer (35, 70 and 140 cycles). It was observed by XRD that the AlN PALE pulse cycle number greatly affects the structural properties of the top GaN layer where the lowest x-ray rocking curve for (002) and (102) achieved at 70 cycles, indicating the reduction of threading dislocations density in the growth structure. Further, the transition of the GaN E2 (high) from the Raman spectroscopy revealed a tensile strain in the GaN epilayers reduces with the increasing number of pulse cycles.

Improvement of Mg-Doped GaN with Shutter-Controlled Process in Plasma-Assisted Molecular Beam Epitaxy

Mg-doped GaN was grown by plasma-assisted molecular beam epitaxy (PAMBE) on a Fe-doped GaN template substrate by employing a shutter-controlled process. The transition from n-type to p-type conductivity of Mg-doped GaN in relation to the N/Ga flux ratio was studied. The highest p-type carrier concentration in this series was 3.12 × 1018 cm−3 under the most N-rich condition. By modulating the shutters of different effusion cells for the shutter-controlled process, a wide growth window for p-type GaN was obtained. It was found that the presence of Mg flux effectively prevents the formation of structural defects in GaN epi-layers, resulting in the improvement of crystal quality and carrier mobility.

Enhancement of gallium nitride on silicon (111) using pulse atomic-layer epitaxy (PALE) AlN with composition-graded AlGaN buffer

The ability to configure the optimal buffer layer for GaN growth depends on the knowledge of relaxation processes that occurs during the cooling step while countering the tensile stresses due to the contrast of thermal expansion coefficient between GaN and Si(111) substrate. Here, we inaugurate the pulse atomic-layer epitaxy (PALE) AlN layer to reinforce the buffer layer to achieve a thick GaN epilayer which is crucial for high performance power devices. The characteristics of grown GaN on Si substrate based on PALE AlN thickness of 0 ~ 100 nm are investigated along with microstructural evolution between AlN NL and composition-graded AlGaN buffer layer. PALE AlN layer deposited with an optimum thickness of 50 nm and above was observed to exhibit a highly uniform coalesced GaN epilayer surface with root-mean square (RMS) roughness of 0.512 nm. The thickness of the PALE AlN layer substantially affected the crystallinity of the top GaN epilayer where the lowest value for symmetric (0 0 0 2) and asymmetric (1 0 -1 2) x-ray rocking curve analysis were achieved, indicating the reduction of threading dislocation density in the growth structure. Transition of the E2 (high) peak from the Raman spectrum shows that the strain compression in GaN epilayer is directly proportional to the thickness of the PALE AlN layer.

Micro-Raman analysis of HVPE grown etched GaN epilayer with porous formation

The GaN epilayer grown by hydride vapor phase epitaxy was wet etched by phosphoric acid as the etchant. X-ray diffraction confirms that the GaN has a wurtzite structure. Scanning electron microscopy shows various sizes of hexagonal pits for different times of etchant reactions. Atomic force microscopy shows increase in surface roughness with different etchant rate. The photoluminescence gives a 3.4 eV luminescence for the pristine GaN epilayer. In the etched films, the deep-level defect belonging to yellow and green luminescence was found. The deconvoluted Ga 3d peaks of etched samples show Ga-rich epilayers. Micro-Raman spectroscopy is a non-destructive method for measuring carrier concentration, phonon lifetime and strain using A 1 (LO) spectra of Raman vibration mode was utilized via the Lorentz fitting method. The carrier concentration increases while the phonon lifetime decreases with etching rate. Overall, in the 9 min reaction, the epilayer was etched heavily with a perfect hexagonal etch pit structure.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.