Free-standing Bulk GaN Research Articles

Freestanding GaN substrate enabled by dual-stack multilayer graphene via hydride vapor phase epitaxy

The freestanding GaN substrate has a great potential to homo-epitaxy of optoelectronic and electronic devices with high reliability and performances. Here, we realized the growth of freestanding bulk GaN on dual-stack multilayer graphene as an insertion layer by the hydride vapor phase epitaxy (HVPE) method. Stacked multilayer graphene was fabricated on copper foil using chemical vapor deposition (CVD), following the overlap of top and bottom layers of graphene after removing the copper. Based on Raman spectra and X-ray photoelectron spectroscopy (XPS) experiments, it is found that the dual-stack structure of multilayer graphene is crucial for the self-separation of GaN substrate from sapphire. The exfoliation was attributed to the weak interaction force between the dual-stack multilayer graphene. This study can provide an efficient, practical, and mass-producible technique to fabricate freestanding GaN substrates, which exhibit immense application potential for the nitride-based optoelectronic and electronic devices.

Effects of growth temperature on electrical properties of GaN/AlN based resonant tunneling diodes with peak current density up to 1.01 MA/cm2

Identical GaN/AlN resonant tunneling diode structures were grown on free-standing bulk GaN at substrate temperatures of 760 °C, 810 °C, 860 °C, and 900 °C via plasma-assisted molecular beam epitaxy. Each sample displayed negative differential resistance (NDR) at room temperature. The figures-of-merit quantified were peak-to-valley current ratio (PVCR), yield of the device with room-temperature NDR, and peak current density (Jp). The figures-of-merit demonstrate an inverse relationship between PVCR/yield and Jp over this growth temperature series. X-ray diffraction and transmission electron microscopy were used to determine the growth rates, and layer thicknesses were used to explain the varying figures-of-merit. Due to the high yield of devices grown at 760 °C and 810 °C, the PVCR, peak voltage (Vp), and Jp were plotted vs device area, which demonstrated high uniformity and application tunability. Peak current densities of up to 1.01 MA/cm2 were observed for the sample grown at 900 °C.

Bulk GaN alpha-particle detector with large depletion region and improved energy resolution

An alpha-particle detector was fabricated using a freestanding n-type bulk GaN wafer with a Au/Ni/GaN sandwich Schottky structure. Current–voltage measurements at room temperature revealed a Schottky contact with a leakage current of 7.53±0.3nA at a reverse bias of 200V. The detector had a large depletion depth that can capture much of the energy from 5.486MeV alpha particles emitted from a 241Am source. The resolution of its alpha-particle energy spectrum was improved to 2.2±0.2% at 5.486MeV under a bias of 550V. This superior resolution was attributed to the shortening of the carrier transit time and the large energy deposition within the large depletion depth, i.e., 27µm at −550V, which all resulted in a more complete charge collection. A model developed using the ATLAS simulation framework from Silvaco Inc. was employed to study the charge collection process. The simulation results were found to agree closely with the experimental results. This detector will be beneficial for research at neutron scattering facilities, the International Thermonuclear Experimental Reactor, and the Large Hadron Collider, among other institutions, where the Si-based charged particle detectors could be quickly degraded in an intense radiation field.

A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates

We report on a comparative study of the low temperature emission and polarisation properties of InGaN/GaN quantum wells grown on nonpolar () a-plane and () m-plane free-standing bulk GaN substrates where the In content varied from 0.14 to 0.28 in the m-plane series and 0.08 to 0.21 for the a-plane series. The low temperature photoluminescence spectra from both sets of samples are broad with full width at half maximum height increasing from 81 to 330 meV as the In fraction increases. Photoluminescence excitation spectroscopy indicates that the recombination mainly involves strongly localised carriers. At 10 K the degree of linear polarisation of the a-plane samples is much smaller than of the m-plane counterparts and also varies across the spectrum. From polarisation-resolved photoluminescence excitation spectroscopy we measured the energy splitting between the lowest valence sub-bands to lie in the range of 23–54 meV for the a- and m-plane samples in which we could observe distinct exciton features. Thus the thermal occupation of a higher valence sub-band cannot be responsible for the reduction of the degree of linear polarisation at 10 K. Time-resolved spectroscopy indicates that in a-plane samples there is an extra emission component which is at least partly responsible for the reduction in the degree of linear polarisation.

Structural and optical studies of thick freestanding GaN films deposited by Hydride vapor phase epitaxy

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

Molecular beam epitaxy as a method for the growth of freestanding zinc-blende (cubic) GaN layers and substrates

The authors have studied the growth of bulk, freestanding zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). They have established that the best structural properties of freestanding zinc-blende GaN can be achieved with initiation under Ga-rich conditions but without Ga droplet formation. It is difficult to initiate the growth of zinc-blende GaN, but it is even more difficult to sustain the growth of the pure zinc-blende polytype in thick layers without any wurtzite inclusions. In order to grow high quality freestanding cubic GaN layers, it is necessary to maintain the same growth conditions for about 1week. The best quality zinc-blende phase GaN was achieved in the first 10μm of the GaN layers. The authors have produced zinc-blende GaN substrates from our thick bulk GaN layers and they used the side previously attached to the GaAs substrate as the episide of these zinc-blende GaN substrates. They have demonstrated the scalability of the process by growing zinc-blende GaN layers on 2 and 3in. diameter wafers. The growth of freestanding bulk GaN layers has allowed them to refine the value for the lattice parameter of zinc-blende GaN as 4.510±0.005Å. They have demonstrated that the PA-MBE process developed has allowed them to grow freestanding AlxGa1−xN wafers.

Dislocation Clustering and Luminescence Nonuniformity in Bulk GaN and Its Homoepitaxial Film

Cathodoluminescence (CL) spectroscopy and mapping techniques were used to study defect and impurity distributions in free-standing bulk GaN substrates prepared by hydride vapor-phase epitaxy. It was found that, in the bulk GaN substrates investigated, dislocation clusters appearing as dark cores in the CL map were surrounded by bright disk-like regions with higher luminescence efficiency than that of the outside areas. This large-area luminescence nonuniformity disappeared in homoepitaxial GaN grown on top of the GaN substrate. Schottky barrier diodes fabricated on the homo-epilayer exhibited low average reverse leakage current, while dislocation clusters duplicated from the original bulk GaN substrate still limited device yield.

Low-noise GaN ultraviolet p-i-n photodiodes on GaN substrates

We report low-noise GaN visible-blind homojunction p-i-n photodiodes. The devices are grown on a freestanding bulk GaN substrate and are fabricated using a “ledged” surface depletion technique to suppress the mesa sidewall leakage. For an 80-μm-diameter photodetector, the dark current density is lower than 40 pA/cm2. A room-temperature noise equivalent power of 4.27×10−17 W Hz−0.5 and a detectivity of 1.66×1014 cm Hz0.5 W−1 are achieved at a reverse bias of 20 V. The noise performance of the reverse-biased GaN p-i-n photodiodes are among the best values reported to date and demonstrate the potential of GaN photodiodes for low-noise high-speed UV detection.

Development of Nonpolar and Semipolar InGaN/GaN Visible Light-Emitting Diodes

AbstractThis article reviews the development of nonpolar and semipolar InGaN/GaN light-emitting diodes (LEDs), emphasizing structures on freestanding bulk GaN. A brief history of LED development on each orientation is provided, followed by a discussion of the most relevant and recent results. The context is related to several current LED issues, such as the realization of high-efficiency white solid-state lighting, potential solutions to the “green gap,” and applications for polarized emitters. The section on nonpolar LEDs highlights high-power violet and blue emitters and considers the effects of indium incorporation and substrate miscut. The section on semipolar GaN reviews the development of LEDs in the violet, blue, green, and yellow regions and highlights the potential of InGaN/GaN LEDs as an alternative technology to AlInGaP for yellow emitters. A brief review of polarization anisotropy also is included for each orientation. Finally, a two source white light system utilizing a nonpolar blue LED and a semipolar yellow LED is presented.

High-resistivity GaN homoepitaxial layer studied by Schottky diode structure

An undoped GaN homoepitaxial layer has been grown by metal-organic chemical vapour deposition on a conductive free-standing bulk GaN substrate. The dislocation density of the homoepitaxial layer characterised by cathodoluminescence mapping technique is about 6×106 cm−2. By fabricating a vertical Schottky diode structure, the resistivity of the homoepitaxial layer is estimated in the order of 109 Ωcm at room temperature. This study indicates that highly-resistive GaN can be produced by a homoepitaxy approach.

Low leakage Schottky rectifiers fabricated on homoepitaxial GaN

The development of a vertical GaN Schottky rectifier on freestanding bulk GaN substrate by employing homoepitaxial growth technique is reported. The n−∕n+ Schottky diode structure was grown on conductive GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the homoepitaxial layer determined by cathodoluminescence mapping technique is ∼6×106cm−2 with surface roughness less than 1nm. The fabricated Schottky rectifiers with dielectric overlap edge termination show a low reverse leakage current of <3×10−8A at −100V, which can be explained by the low dislocation density in the epilayer and high Schottky barrier height (1.37eV) of the Pt∕GaN metal-semiconductor contact. Due to the reduced series resistance, the bulk GaN Schottky rectifiers also exhibit a low turn-on voltage of ∼1.3V and an on-resistance of ∼5.3mΩcm2.

Al$_{x}$Ga$_{1-x}$N Ultraviolet Avalanche Photodiodes Grown on GaN Substrates

AlxGa1-xN (x=0.05) ultraviolet (UV) avalanche photodiodes grown on a GaN substrate are reported. The epitaxial structure was grown by metal-organic chemical vapor deposition on a free-standing bulk GaN substrate having low dislocation density. The growth conditions for AlxGa1-xN epitaxial layers on GaN substrates were optimized to achieve improved crystalline and structural quality. With UV illumination at lambda~250 nm, devices with mesa diameters of ~30 mum achieve stable maximum optical gains of ~50 at a reverse bias voltage of ~87 V.

Growth and Characterization of High-Performance GaN and AlxGa1−xN Ultraviolet Avalanche Photodiodes Grown on GaN Substrates

AbstractWide-bandgap III-nitride-based avalanche photodiodes (APDs) are important for photodetectors operating in UV spectral region. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of III-nitride based APDs. In this study, we investigate the growth and characterization of GaN and AlGaN-based APDs on free-standing bulk GaN substrates. Epitaxial layers of GaN and AlxGa1−xN p-i-n ultraviolet avalanche photodiodes were grown by metalorganic chemical vapor deposition (MOCVD). Improved crystalline and structural quality of epitaxial layers was achieved by employing optimum growth parameters on low-dislocation-density bulk substrates in order to minimize the defect density in epitaxially grown materials. GaN and AlGaN APDs were fabricated into 30μm- and 50μm-diameter circular mesas and the electrical and optoelectronic characteristics were measured. APD epitaxial structure and device design, material growth optimization, material characterizations, device fabrication, and device performance characteristics are reported.

Crystallization of free standing bulk GaN by HVPE

AbstractGallium nitride was crystallized on 2 inch MOVPE GaN/sapphire substrates by Hydride Vapor Phase Epitaxy. A stable growth has been achieved in long duration (>10 h) processes at growth rates bigger than 100 μm/h. As a result, entirely transparent and colorless bulk crystals with thickness exceeding 2 mm were obtained. The cracks in the thick HVPE GaN layers deposited on the MOVPE GaN/sapphire substrates can appear especially during cooling of the system after crystallization. It is shown that the formation of cracks at cooling is dependent on the gradients in the layer thickness The relaxation of strains in the resulting crystal coupled to the substrate leads to the self separation of GaN from sapphire. (At present 30 x 30 x 2 mm free standing bulk GaN crystals are obtained). The GaN crystals are characterized by defect selective etching (DSE) and X‐ray diffraction. The density of threading dislocations (measured by DSE of (0001) surface) decreases with the thickness of the HVPE layer and becomes lower than 107 cm–2 in the layers thicker than app. 1 mm. The X‐ray rocking curves for (0002) reflection (slit 0.5 x 0.1 mm) are in the range of 80–95 arcsec. However, larger scans reveal bending of crystallographic {0001} planes. The behavior of these “deformed” free standing crystals used as substrates for HVPE re‐growth is also analyzed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cathodoluminescence mapping and selective etching of defects in bulk GaN

Abstract Cathodoluminescence (CL) mapping and selective-etching techniques were used to characterize defects in free-standing bulk GaN substrates prepared by hydride vapor phase epitaxy. Melton KOH–NaOH (1:1) solution was found to be effective to produce etch pits on bulk GaN with density close to the anticipated dislocation density. Under preferred etching and measurement conditions, a strong correlation between CL mapping and selective-etching techniques for revealing defects in the bulk GaN was demonstrated. All structural defects causing etch pits are confirmed to be non-radiative recombination centers showing dark contrast in CL map, while CL mapping could give additional information about certain internal defects underneath the GaN top-surface. The clear correlation established in this comparative study provides strong support for the viability of using both techniques for evaluating bulk GaN.

Measurement of second order susceptibilities of GaN and AlGaN

Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1−xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064nm. The AlxGa1−xN samples, ranging in thickness from roughly 0.5to4.4μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x=0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ∼70μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ∼226μm thick removed from its growth substrate, and a crystal ∼160μm thick grown by high-pressure techniques. For the AlxGa1−xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ∼0 for x=1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=−2χ31(2), was seldom observed, and the samples with x=0.660 and x=0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.

Exciton-exciton interaction and heterobiexcitons in GaN

The formation of not only A biexcitons ${(XX}_{\mathrm{AA}})$ but also heterobiexcitons that consist of A and B excitons ${(XX}_{\mathrm{AB}})$ in a free-standing bulk GaN is identified by polarization-sensitive spectrally resolved FWM measurements. The FWM spectra and delay-time dependence show that the interaction between A and B exciton gives rise to the energy shifts of the spectra and the phase shifts of the quantum beating, which is considered as the effect of the unbound state of ${\mathrm{XX}}_{\mathrm{AB}}$ (i.e., ${\mathrm{XX}}_{\mathrm{AB}}^{*})$ and ${\mathrm{XX}}_{\mathrm{AB}}^{*}$ is found to play an important role in the FWM signals for all polarizations. The unbound A biexciton ${(XX}_{\mathrm{AA}}^{*})$ is also observed clearly in spectral and temporal domains.

Hall mobility and carrier concentration in free-standing high quality GaN templates grown by hydride vapor phase epitaxy

Measured and calculated (without any adjustable material parameter) electron Hall mobility and carrier concentration in the range of 26.5–273 K are reported for a high-mobility free-standing bulk GaN grown by hydride vapor phase epitaxy. The peak electron mobility of 7386 cm 2/V s at 48 K and a value of 1425 cm 2/V s at 273 K were measured. An iterative solution of the Boltzmann equation was applied to calculate the mobility using the materials parameters either measured on the sample under study or recent values that are just becoming available with only the acceptor concentration being variable. Using only one donor and one conducting layer system, the donor and acceptor concentrations of 1.76×10 16 and 2.4×10 15 cm −3, respectively, satisfy simultaneously the charge neutrality and electron mobility at all temperatures within the framework of the iterative method and measurements. The donor activation energy was determined to be 25.2 meV and is consistent with the value of about 30 meV for the hydrogenic ground state in a dilute semiconductor. The high electron mobility, low background impurity concentration, low compensation ratio, and negligible dislocation scattering demonstrate the high quality of the material studied.