GaN Epilayers Research Articles

Ab initio calculations on the initial stages of GaN and ZnO growth on lattice-matched ScAlMgO4 (0001) substrates

The initial stages of GaN and ZnO epitaxial growth on lattice-matched ScAlMgO4 substrates have been investigated by ab initio calculation. The geometrical parameters and electronic structure of ScAlMgO4 bulk and (0001) surface have been investigated by density-functional first-principles study. The effects of different surface terminations have been examined through surface energy and relaxation calculations. The O–Mg–O termination is more favorable than other terminations by comparing the calculated surface energies. It should be accepted as the appropriate surface structure in subsequent calculation. The initial stages of GaN and ZnO epitaxial growths are discussed based on the adsorption and diffusion of the adatoms on reconstructed ScAlMgO4 (0001) surface. According to theoretical characterizations, N adatom on the surface is more stable than Ga. O adatom is more favorable than Zn. These observations lead to the formation of GaN and ZnO epilayer and explain experimentally-confirmed in-plane alignment mechanisms of GaN and ZnO on ScAlMgO4 substrates. Furthermore, the polarity of GaN and ZnO surfaces on ScAlMgO4 (0001) at the initial growth stage have been explored by ab initio calculation. Theoretical studies indicate that the predominant growths of Ga-polar GaN and Zn-polar ZnO are determined by the initial growth stage.

Distribution of dislocations in AlN crystals grown on evaporating SiC substrates

By the use of high-resolution X-ray diffractometry and synchrotron radiation topography, the distribution of dislocations in AlN crystals grown on evaporating SiC substrates is studied. The growth of AlN layer in one process with the evaporation of SiC substrate gave the possibility to prevent the relaxation of thermal stresses through cracking of AlN during cooling the structure. The continuous 0.2-1.5 mm thick plates were used as the model objects for the study of dislocation structure near the AlN/SiC interface. Analysis of the broadening of the Bragg reflection peaks, the shape of scattering maps in reciprocal space and topographs showed that dislocations formed the mosaic structure, different from the distribution of threading dislocations in GaN epilayers. A theoretical model for misfit strain relaxation in the growing AlN layer is suggested. The results enable to clarify the dislocation processes during sublimation growth of industrial quality AlN crystals.

A detailed study of AlN and GaN grown on silicon-on-porous silicon substrate

In this study, we investigate the growth of AlN and GaN epilayers on silicon-on-porous silicon (SOP) as a compliant substrate. The porous silicon layers is obtained by electrochemical etching of silicon wafers in HF based solutions. Different porosities (10–30%) and porous layer thicknesses (3–30 μm) have been investigated. In order to obtain a single orientation GaN, a Si cap layer is grown on porous silicon by chemical vapour deposition leading to a smooth epi-ready surface. Then, the subsequent growth of AlN by molecular beam epitaxy and GaN by metal-organic chemical vapour deposition is investigated by atomic force microscopy and X-ray diffraction. The epilayers grown on SOP show similar surface morphology and structural quality to state of the art GaN layers grown on bare silicon. In addition, strain in the GaN layers grown on SOP and Si has been investigated. Interestingly, the results indicate that a slight compliant effect is achieved on SOP substrate with the thicker porous layer.

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.

The effect of transparent conductive nanocrystalline oxide thin layer on performance of UV detectors fabricated on Fe-doped GaN

Two kinds of metal-semiconductor-metal (MSM) Schottky UV detectors with and without an insertion layer of transparent conductive nanocrystalline oxide were fabricated on Fe-doped semi-insulating GaN epilayers. It is found that the optical responsivity of the detectors can be significantly improved by two orders of magnitude by inserting a thin layer of Ga-doped nanocrystalline ZnO (GZO) between metal electrode and Fe-doped GaN. Such improvement is suggested to be associated with the valence-band discontinuities and efficient suppression recombination of traps induced by Fe impurities in GaN due to the insertion of a thin layer of GZO.

Optical and Electrical Study of Defects in GaN In Situ Doped with Eu3+ Ion Grown by OMVPE

A silicon/europium-codoped GaN epilayer (GaN:Si,Eu3+) and a control sample (GaN:Si) have been investigated using optical spectroscopy and various deep-level transient spectroscopy (DLTS) techniques. Three electron traps in GaN:Si,Eu3+, as well as one electron trap and one hole trap in GaN:Si, were observed in the temperature range from 35 K to 400 K by DLTS. High-resolution Laplace DLTS revealed multiple, closely spaced defect energy levels associated with trap B of GaN:Si,Eu3+ and trap D of GaN:Si, respectively. Minority-carrier transient spectroscopy (MCTS) measurements revealed a shallow hole trap in GaN:Si,Eu3+, whose presence was validated using the rare-earth-structured isovalent (RESI) hole trap model. The identified DLTS energy traps were compared with other energy traps reported in literature. Also, the energy transfer mechanism between the GaN host and Eu3+ ion was considered and discussed using the combination of electrical and optical studies reported here and in literature.

Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb co-doped GaN epilayers

The Erbium (Er) doped GaN is a promising gain medium for optical amplifiers and solid-state high energy lasers due to its high thermal conductivity, wide bandgap, mechanical hardness, and ability to emit in the highly useful 1.5 μm window. Finding the mechanisms to enhance the optical absorption efficiency at a resonant pump wavelength and emission efficiency at 1.5 μm is highly desirable. We report here the in-situ synthesis of the Er and Yb co-doped GaN epilayers (Er + Yb:GaN) by metal-organic chemical vapor deposition (MOCVD). It was observed that the 1.5 μm emission intensity of the Er doped GaN (Er:GaN) under 980 nm resonant pump can be boosted by a factor of 7 by co-doping the sample with Yb. The temperature dependent PL emission at 1.5 μm in the Er + Yb:GaN epilayers under an above bandgap excitation revealed a small thermal quenching of 12% from 10 to 300 K. From these results, it can be inferred that the process of energy transfer from Yb3+ to Er3+ ions is highly efficient, and non-radiative recombination channels are limited in the Er + Yb:GaN epilayers synthesized in-situ by MOCVD. Our results point to an effective way to improve the emission efficiency of the Er doped GaN for optical amplification and lasing applications.

A combined growth process for state-of-the-art GaN on silicon

This study aims to simplify the heteroepitaxy of GaN on Si while keeping state-of-the-art III-nitride materials. The originality of this work is to combine the advantages of both NH3-MBE and metal organic chemical vapor deposition (MOCVD) growth techniques. First, the structural quality of AlN is assessed by AFM, XRD measurements, and TEM. Terraces and atomic step edges are observed by AFM showing a good structural quality of the AlN epilayer. TEM investigation reveals a sharp and well-controlled AlN/Si using NH3-MBE. Then, a 2-μm thick smooth GaN layers are obtained by MOCVD on top of these AlN templates with the use of a SiNx treatment allowing a 3D growth mode able to induce an efficient dislocation filtering to be obtained. GaN structural properties, measured by XRD, AFM, and SEM, are discussed and compared to GaN-on sapphire (Al2O3). In addition, the stress in the GaN layers grown on these AlN templates with or without a SiNx treatment is assessed. GaN epilayers with a dislocation density in the range of few 108 dislocations per cm2 have been achieved using a single AlN buffer layer (no interlayer) and only 2 μm of GaN.

Anelasticity of GaN Epitaxial Layer in GaN LED

In this work, the anelasticity of the GaN layer in the GaN light-emitting-diode device was studied. The present results show that the forward-voltage of GaN LED increases with time, as the GaN light-emitting-diode was maintained at a constant temperature of 100 °C. We found that the increase of the forward-voltage with time attributes to the delay-response of the piezoelectric fields (internal electrical fields in GaN LED device). And, the delay-response of the internal electrical fields with time is caused by the anelasticity (time-dependent strain) of the GaN layer. Therefore, using the correlation of strain-piezoelectric-forward voltage, a plot of thermal strain of the GaN layer against time can be obtained by measuring the forward-voltage of the studied GaN LED against time. With the curves of the thermal strain of GaN epi-layers versus time, the anelasticity of the GaN compound can be studied. The key anelasticity parameter, characteristic relaxation time, of the GaN is defined to be 2623.76 min in this work.

Highly Mg-doped GaN dots and films grown by VLS transport at low temperature

In this study, a new approach toward localized p-type doping of GaN was explored by implementing vapor–liquid–solid (VLS) transport for both reducing the growth temperature and reaching very high Mg incorporation levels. The starting seeds are GaN(0001) epilayers grown on Si(111). The growth cycle includes three steps. At first, Ga is deposited by MOCVD onto the GaN surface, resulting in a network of Ga droplets with sub-micrometer diameters. Then, Mg is incorporated into the droplets, from the gas phase, using (MeCP)2Mg precursor. Finally, droplets are nitridated at 600–700 °C in flowing NH3. Performing one complete growth cycle leads to a network of well-separated dots and/or ring-shaped GaN features. It is demonstrated that the Mg content in the droplets drastically influences the GaN growth mode. Increasing Mg incorporation promotes growth at the liquid/solid interface versus growth at the triple line. To improve the seed coverage, several successive cycles have been performed leading to GaN films with thickness up to ∼250 nm. SIMS analyses demonstrate the very high incorporation of Mg in the VLS-grown material, with concentrations from 3 × 1019 to 8 × 1021 cm−3. Nevertheless, pronounced O incorporation (a few 1020 cm−3) is also found.

Synchrotron radiation X-ray photoelectron spectroscopy of Ti/Al ohmic contacts to n-type GaN: Key role of Al capping layers in interface scavenging reactions

Interface reactions between Ti-based electrodes and n-type GaN epilayers were investigated by synchrotron radiation X-ray photoelectron spectroscopy. Metallic Ga and thin TiN alloys were formed at the interface by subsequently depositing Al capping layers on ultrathin Ti layers even at room temperature. By comparing results from stacked Ti/Al and single Ti electrodes, the essential role of Al capping layers serving as an oxygen-scavenging element to produce reactive Ti underlayers was demonstrated. Further growth of the metallic interlayer during annealing was observed. A strategy for achieving low-resistance ohmic contacts to n-GaN with low-thermal-budget processing is discussed.

Modeling of the wafer bow in GaN-on-Si epiwafers employing GaN/AlN multilayer buffer structures

We built a calculation model for the wafer bow in GaN-on-Si epiwafers employing GaN/AlN multilayer (ML) buffer structures by extending Stoney’s equation. The calculated bow and the derived strain in the epilayers were almost consistent with experimental results. The calculation quantitatively revealed that the ML buffers introduced an in-plane compressive stress in the epitaxial structures. Also, relationships between the epiwafer bow and the stress in the respective layers became clear to a certain extent. For instance, when considering the case where periodic structures with 20 nm thick GaN/5 nm thick AlN pairs were grown as ML buffers on 4-in.-diameter and 525 μm thick Si (111) substrates at a growth temperature (Tg) of 1125 °C, the stress in the MLs was derived to be 2.18 GPa in the in-plane compressive direction in the GaN layers and 5.89 GPa in the in-plane tensile direction in the AlN layers at Tg. Although magnitude of the in-plane stress in the GaN layers is obviously smaller than that in the AlN layers, the restoring force generated in the GaN layers becomes larger than that in the AlN layers, because the force is in proportion to the layer thickness rather than to just the stress. As a consequence, the generated stress in the MLs was considered to produce enough force to suppress the epiwafer bow. The calculation also demonstrated that GaN epilayers on GaN/AlN MLs were strained in the in-plane compressive direction at Tg and almost strain-free at room temperature.

Heteroepitaxial growth and characterization of monocrystal anatase TiO2 films on epi-GaN (0001)/sapphire substrates

Monocrystal anatase titanium dioxide (a-TiO2) films have been deposited on epi-GaN (0001)/sapphire substrates by the metalorganic chemical vapor deposition (MOCVD) method with the substrate temperature varied from 550 to 700 °C. The structural analyses showed that the film deposited at 650 °C was pure a-TiO2 monocrystal with the best crystalline quality. The epitaxial relationship between the film and GaN epilayer was determined as a-TiO2 (001)‖GaN (0001) with a-TiO2 [110]‖GaN 〈 $$ \bar{1}100 $$ 〉. The elemental composition and proportion were studied by the X-ray photoelectron spectroscopy (XPS) method, which proved the deposited film approximated stoichiometric TiO2. The films showed excellent transparency of ~90 % in the visible range with an optical band gap of 3.33 eV obtained for the 650 °C-deposited sample.

Defect formation and its effect on electronic structure and magnetic properties of GaN:Mn films

GaN:Mn thin films were fabricated by implanting Mn ions into Mg-doped GaN epilayer. X-ray diffraction study reveals that all the samples are in single phase wurtzite crystalline structure. Raman spectra exhibit additional excitations attributed to the vibrational mode of defects caused by Mn ion implantation and the Mn related local vibrational mode in the vicinity of E2high. The X-ray photoelectron spectroscopy results indicate that the Mn ions were successfully incorporated into the GaN host lattice by substituting the Ga sites. The magnetization curve as a function of the magnetic field at 300K indicates that room ferromagnetisms exist in GaN:Mn thin films. The effect of vacancy defect on the magnetic properties of MnMg co-doped GaN has been studied by first principles calculations. Based upon experimental and theoretical results, it is believed that the alteration of ferromagnetism with annealing temperature is directly related to the variation of magnetic exchange interaction with microstructure in this material system.

Nanostructuring GaN thin film for enhanced light emission and extraction

AbstractWe demonstrate here that nanostructuring of GaN thin film significantly enhances the band‐edge emission, due to structural and geometrical effects. Films of increasing roughness are formed by kinetic control in a PA‐MBE system and their morphological, structural and optical properties are compared by complementary characterization probes. The nanowall configuration with largest pore size (215 nm) shows a two orders of magnitude enhancement of integrated PL intensity in comparison to a GaN epilayer. Finite difference time domain (FDTD) simulation is performed to explain the role of total internal reflection and scattering on light extraction. The extended defects terminate proximal to interface, leading to most regions of the nanowalls are defect free, enhancing light generation. The observation of broad HRXRD rocking curves is attributed to a mosaicity that originates due to mutual misorientation of the nanowalls. Thus, the low dislocation density in the nanowalls and their suitable geometry promotes high light emission and extraction, respectively, offering this nanostructure as a potential material for high brightness LED fabrication.

Effect of initial growth on the quality of GaN on patterned sapphire substrate with ex situ physical vapor deposition AlN seed layer

GaN epitaxy was explored on a cone-patterned sapphire substrate with an ex situ AlN seed layer prepared by physical vapor deposition (PVD). The effect of initial growth on the quality of the GaN epilayer was investigated using both ex situ PVD-AlN seed layers with various thicknesses and various deposition parameters such as temperature and reactor pressure in metal–organic vapor-phase epitaxy (MOVPE). It was found that the quality of GaN is insensitive to both the thickness of the ex situ PVD-AlN seed layer and the MOVPE growth conditions. A high-quality GaN film was realized, as indicated by room-temperature CL mapping (dark spot density of 1.6 × 108 cm−2), on a patterned sapphire substrate with a wide growth condition window by simply employing an ex situ PVD-AlN seed layer.

Fabrication of extremely thermal-stable GaN template on Mo substrate using double bonding and step annealing process**Project supported by the Guangdong Innovative Research Team Program (No. 2009010044), the China Postdoctoral Science Foundation (No. 2014M562233), the National Natural Science Foundation of Guangdong, China (No. 2015A030312011), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2014KF17).

A new layer transfer technique which comprised double bonding and a step annealing process was utilized to transfer the GaN epilayer from a sapphire substrate to a Mo substrate. Combined with the application of the thermal-stable bonding medium, the resulting two-inch-diameter GaN template showed extremely good stability under high temperature and low stress state. Moreover, no cracks and winkles were observed. The transferred GaN template was suitable for homogeneous epitaxial, thus could be used for the direct fabrication of vertical LED chips as well as power electron devices. It has been confirmed that the double bonding and step annealing technique together with the thermal-stable bonding layer could significantly improve the bonding strength and stress relief, finally enhancing the thermal stability of the transferred GaN template.

Wafer-scale epitaxial lift-off of optoelectronic grade GaN from a GaN substrate using a sacrificial ZnO interlayer

Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer

We demonstrate the high structural and optical properties of InxGa1−xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 107 cm−2) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1−xN epilayers can be achieved with high optical quality of InxGa1−xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design.

Structural and strain anisotropies of N-polar GaN epilayers on offcut sapphire substrates

The dependence of the structure and strain upon the crystal direction (anisotropies) of N-polar GaN films grown on offcut c-plane sapphire substrates was examined by varying the degree of substrate offcut and the V/III ratio. On offcut substrates, the broadening of GaN (0002¯) rocking curves was found to depend upon the x-ray incidence azimuth, whereby the maxima and minima of the broadening, respectively, occurred parallel and perpendicular to the offcut direction. In contrast, no such structural anisotropies were observed for samples grown on the on-axis substrates. Mosaic tilt was suggested as the determining factor for the structural anisotropy, which was further aided by the different island coalescence rates in the two orthogonal directions during the initial nucleation stage. In addition, the off-axis samples exhibited larger in-plane strains occurring in the direction perpendicular to the substrate offcut than in other directions, which was significantly different from the uniform in-plane strain found in the on-axis sample. The in-plane strain anisotropy was attributed to preferential strain relaxation along the substrate offcut direction via the formation of geometrical partial misfit dislocations along the orthogonal direction. Moreover, the structural (strain) anisotropy decreased (increased) as the substrate offcut increased, and both anisotropies were affected only marginally by the V/III ratio.