Free-standing GaN Research Articles

A bow-free freestanding GaN wafer.

For applications as high-brightness light-emitting-diodes, a bow-free freestanding gallium nitride (GaN) wafer 2 inch in diameter and ∼185 μm in thickness was fabricated by process-designing pit and mirror GaN layers grown via hydride-vapor-phase epitaxy, laser lift-off, N-face polishing of the pit GaN layer, and three-step polishing of the mirror GaN layer using 3.0 μm-, 0.5 μm-, and 50 nm-diameter diamond abrasives and by inductively-coupled-plasma reactive-ion etching. The considerably large concave shape of the GaN wafer could be decreased by controlling the removal amount of the Ga-face mirror layer during the first step of the polishing process, which approached a bow-free shape or changed with further polishing; this well correlated with the residual stress of the polished GaN wafer.

GaN-based LEDs using Homo-Epitaxial technology and the Progress and Challenges of HVPE method

GaN and other group III-nitrides are promising materials for optoelectronic devices applications. Currently, sapphire is the most widely used substrate material for GaN LEDs due largely to economic considerations. During the past decade or so, free-standing GaN substrates fabricated via HVPE have gained technological importance and circumventing the shortcomings of sapphire substrates in terms of thermoconductivity and lattice mismatch. This technology, however, does have its own set of challenges. This review summarizes the current state of freestanding GaN substrate technology, the challenges it is facing, and outlook.

Surface morphology smoothing of a 2 inch-diameter GaN homoepitaxial layer observed by X-ray diffraction topography.

We investigated the surface morphology changes in a 2 inch-diameter, c-plane, free-standing GaN wafer using X-ray diffraction topography in a grazing-incidence geometry. We observed a decrease in the peak intensity and increase in the full width at half maximum of the GaN 112̄4 Bragg peak after the deposition of a homoepitaxial layer on the same GaN wafer. However, the lattice plane bending angles did not change after homoepitaxial layer deposition. Distorted-wave Born approximation calculations near the total external reflection condition revealed a decrease in the X-ray incidence angle of the 112̄4 Bragg peak after the homoepitaxial layer deposition. The decrease in both X-ray penetration and incidence angle induced broader and weaker diffraction peaks from the surface instead of the bulk GaN.

Fabrication of a 1.5-inch freestanding GaN substrate by selective dissolution of sapphire using Li after the Na-flux growth

Abstract Reducing the thermal stress induced in GaN crystals is key for the fabrication of large-diameter GaN crystals. In the Na-flux method, thermal stress is eliminated by sapphire dissolution at growth temperature after growth. We succeeded in fabricating a freestanding GaN substrate with this technique. However, the flux with Li used as a solvent to dissolve the sapphire can also dissolve the GaN. In this study, we demonstrate that increasing the amount of nitrogen can suppress GaN dissolution in the Li-added flux, without interfering in the dissolution of sapphire. This means that the sapphire and GaN have different dissolution mechanisms. By utilizing the different mechanisms, we succeeded in selectively dissolving sapphire and fabricating a 1.5-inch freestanding GaN substrate. It is revealed by secondary ion mass spectrometry (SIMS) that Li is incorporated in the crystal. The distribution of Li indicated that it was included in the crystal during sapphire dissolution.

Optical properties of AlGaN/GaN epitaxial layers grown on different face GaN substrates

The AlGaN/GaN epitaxial layers were grown by metalorganic chemical vapor deposition on both Ga-face and N-face free-standing GaN substrates, which were fabricated by halide vapor phase epitaxy method. Time-resolved photoluminescence technique was applied to study the optical properties of the grown AlGaN/GaN heterostructures. A strong emission at ~3.41 eV has been observed in the Ga-polar AlGaN/GaN structure, which is confirmed to associate to the two-dimensional electron gas. The temporal behavior of the two-dimensional electron gas emission is studied, which correlates well with the recombination processes in an asymmetric triangular potential well formed by an AlGaN/GaN structure grown in [0 0 0 1] direction.

Nonequilibrium plasma aerotaxy of size controlled GaN nanocrystals

High quality gallium nitride (GaN) nanocrystals (NCs) are promising materials in a wide range of applications including optoelectronics, photonics and biomedical devices. Unlike II–VI semiconductors, the synthesis of free-standing GaN NCs is not well-established, and there is a need for a synthesis platform that can provide GaN NCs with tunable size and photonic properties. In this work, we present a flexible gas-phase synthesis method that can deliver crystalline, free-standing, pure GaN NCs with controlled size and narrow size distributions. The method, termed nonequilibrium plasma aerotaxy (NPA), employs an aerosol of Ga and gaseous N2 as the precursors. The term aerotaxy means growth on an unsupported surface, in this case promoted by a nonequilibrium plasma. The key to narrow size distributions is that the NPA mechanism is based upon surface growth, as opposed to coagulation mechanisms that result in broad size distributions. The NPA process converts the Ga aerosol into GaN NCs within 10–100 ms of residence time. The mechanism involves non-thermal vaporization of the source Ga aerosol, which is followed by nucleation and reaction with the excited N2 species in the plasma. Particles can be made to be either hollow or solid. Solid NCs were found to be photoluminescent. Large NCs emitted photons at a peak wavelength near the bulk band-gap transition. Tuning the size to be smaller than 7 nm average diameter led to a blue-shifted photoluminescence. Inline processing of these bare GaN NCs into porous films by supersonic impact deposition is demonstrated. Moving beyond the specific example of GaN, the NPA mechanism is general and can be extended to many other binary, ternary or doped semiconductors.

Growth of uniform MoS2 layers on free-standing GaN semiconductor for vertical heterojunction device application

The feasibility of van der Waals (VdW) heteroepitaxy of molybdenum disulphide (MoS2) layers on gallium nitride (GaN) semiconductor has attracted significant interest in heterojunction optoelectronic device applications. Here, we report on the growth of uniform MoS2 layers on free-standing GaN semiconductor for vertical heterojunction device application. A uniform MoS2 layer was directly grown on the n-type GaN wafer by sulphurization process of molybdenum oxide thin layer. Raman and scanning electron microscopy (SEM) analyses showed homogenous growth of the few-layers MoS2 forming a continuous film, considering the suitability of GaN semiconductor substrate. The fabricated MoS2/GaN vertical heterojunction showed excellent rectifying diode characteristics with a photovoltaic photoresponsivity under monochromatic light illumination. The X-ray photoelectron spectroscopy (XPS) studies showed the conduction and valence band offset values are around 0.44 and 2.3 eV with type II band alignment in the fabricated heterojunction device. This will facilitate effective movement of photoexcited electrons across the MoS2–GaN junction, while a large valence band offset will prevent movement of holes towards the GaN, resulting in low recombination loss to obtain a photovoltage in the heterojunction device. Our study revealed the formation of large-area homogenous MoS2 layers on GaN wafer for vertical heterojunction device application.

Atomic-scale quantitative analysis of implanted Mg in annealed GaN layers on free-standing GaN substrates

Achieving efficient p-type conduction in Mg-implanted GaN depends largely on postimplantation annealing conditions. Here, we study the effect of postimplantation annealing on the evolution of defects and their interactions with implanted Mg ions by using scanning transmission electron microscopy and atom probe tomography. We found that Mg clusters start to form by annealing the implanted sample above 1000 °C. In addition to the Mg clusters, stacking faults form at an annealing temperature of 1300 °C. The Mg concentrations of about 2–3 orders of magnitude higher than implanted Mg were segregated at the stacking faults. Nanobeam electron diffraction analysis revealed no distinct phase other than GaN formed at the Mg-enriched defects, suggesting that Mg is substituted for Ga in the GaN lattice at the edge of the stacking faults.

Two-Step Mesa Structure GaN p-n Diodes With Low ON-Resistance, High Breakdown Voltage, and Excellent Avalanche Capabilities

Vertical structure GaN power devices fabricated on freestanding GaN substrates have high potentials in ideally efficient energy conversion systems. This letter describes successful acquisition of high avalanche capabilities for high breakdown voltage GaN p-n junction diodes. We have already reported the high avalanche capability by applying a punch-through structure in p-GaN layer, however; the structure caused a drawback of increase in on-resistance and turn-on voltage. By applying a newly developed two-step mesa structure consisting of the first inner mesa with partially thinned p-GaN layer and the second outer mesa etched to n-GaN drift layer, the high avalanche capabilities with reversible current-voltage characteristics have been realized at high breakdown voltages of 4.7-4.8 kV without sacrificing the forward I-V characteristics. The two-step mesa structure transferred the position of the peak electric field in the p-n junction from the dry-etch damaged second outer mesa to the first mesa covered by the thinned p-GaN layer, which could lead the mild breakdown. The high avalanche capability was obtained with good reproducibility regardless of the anode electrode diameter. This structure can contribute to the construction of robust power systems.

Experimental Determination of Velocity-Field Characteristic of Holes in GaN

This study presents a photo-assisted method to measure the drift velocity of carriers in semiconductors, and successfully used to determine the drift velocity of holes in GaN. A p-i-n diode with a buried p-type layer was designed and fabricated on a free-standing GaN substrate. By reverse-biasing the p-i-n diode and illuminating the cathode layer using an ultraviolet light simultaneously, photo-generated holes were injected into the depletion region and accelerated by the electric field to reach the saturation velocity. The drift velocity ( $\text{v}_{\text {d}}$ )–electric field (E) characteristic can be obtained from the photocurrent induced by photo-generated holes. The measured hole drift velocity can be written as $v_{\text {d}}=\mu _{\text {LF}}E /[1+(\mu _{\text {LF}}E/v_{{\mathrm {sat}}})^{{\beta }}]^{{1}/\beta }$ , where $\mu _{\text {LF}}\,\,=17$ cm2/Vs is the low-field hole mobility, $v_{\text {sat}}= 6.63\times 10^{{6}}$ cm/s is the saturation velocity, $\beta =1.75$ is the fitting parameter. The method presented in this study is a unique way of determining the saturation drift velocity of holes in GaN.

2 W mm−1 power density of an AlGaN/GaN HEMT grown on free-standing GaN substrate at 40 GHz

In this letter, a record performance at 40 GHz obtained on an AlGaN/GaN high electron mobility transistor (HEMT) grown on hydride vapor phase epitaxy free-standing GaN substrate is reported. An output power density of 2 W mm−1 associated with 20.5% power added efficiency and a linear power gain (Gp) of 4.2 dB is demonstrated for 70 nm gate length device. The device exhibits a maximum DC drain current density of 950 mA mm−1 and a peak extrinsic transconductance (gm Max) of 300 mS mm−1 at VDS = 6 V. A 100 GHz maximum intrinsic cutoff frequency fT, and a maximum intrinsic oscillation frequency fMax of 125 GHz are obtained from S-parameters measurement. This performance is very promising for HEMTs grown on free-standing GaN substrate.

Fabrication of Pyramid Structure Substrate Utilized for Epitaxial Growth Free-Standing GaN

Metal–organic chemical vapor deposition (MOCVD)-grown GaN on sapphire substrate was etched by hot phosphoric acids. Pyramid structures were obtained in the N-polar face of the MOCVD–GaN. Details of the formation process and morphology of the structures were discussed. The crystallographic plane index of the pyramid facet was calculated dependent on the symmetry of the wurtzite crystal structure and the tilt angle. The substrates with pyramid structures were utilized in subsequent hydride vapor phase epitaxy (HVPE) growth of GaN. Free-standing crystals were obtained, while HVPE-grown GaN achieved a certain thickness. Raman spectroscopy was employed to obtain the stress conditions of the HVPE–GaN without and with sapphire substrate. The mechanism of the self-separation process was discussed. This facile wet etching method may provide a simple way to acquire free-standing GaN by HVPE growth.

Boosting the doping efficiency of Mg in p-GaN grown on the free-standing GaN substrates

The Mg doping efficiency is found to be drastically enhanced in the p-GaN films grown on the free-standing GaN substrates by metal organic chemical vapor deposition. The free hole concentrations are five and ten times higher in the lightly and heavily Mg-doped homoepitaxial p-GaN, respectively, than those in the p-GaN-on-GaN/sapphires grown and activated at the same conditions although the Mg incorporation concentrations {[Mg]} are much lower. The indication of the p-type conductivity in the photoluminescence spectra at room temperature in p-GaN-on-GaN substrates is found to be the dominant ultraviolet luminescence band located at around 3.26 eV. This behavior is different in the heteroepitaxial p-GaN, for which the fingerprint of the p-type conductivity is the emergence of blue luminescence bands at around 2.9 eV. The markedly enhanced activation efficiency is attributed to the suppression of self-compensation centers in the high-quality homoepitaxial films. The Mg-Ga-O disordered layer, which is typically observed on the surface of p-GaN-on-GaN/sapphires due to the Mg diffusion along edge-type dislocations, is also inhibited on the homoepitaxial p-GaN film, which is beneficial for the stable operation of the vertical-type metal-oxide-semiconductor field effect transistors.

Analysis of the back-barrier effect in AlGaN/GaN high electron mobility transistor on free-standing GaN substrates

The effect of AlGaN back-barrier on AlGaN/GaN high electron mobility transistors (HEMTs) using free-standing GaN wafer has been investigated in this work. With the introducing back-barrier structure, the leakage path underneath the buffer (native nitrogen-vacancies and GaOx compounds of the HVPE-grown free-standing GaN surface) layer can be suppressed by lift-up the conduction band. As compared with AlGaN/GaN HEMTs on SiC wafer, AlGaN/GaN HEMTs on free-standing GaN wafer show enhanced drain current (700 mA/mm), improved transconductance (143 mS/mm), less current collapse (12%), higher current gain cut-off frequency (13 GHz), and maximum stable gain cut-off frequency (24 GHz), which is attributed to the higher epi quality layer on free-standing GaN wafer.

Pendeo-epitaxy of GaN on SOI nano-pillars: Freestanding and relaxed GaN platelets on silicon with a reduced dislocation density

Nanopendeo-epitaxy of gallium nitride (GaN) is considered in this study as a way of producing freestanding GaN with reduced strain and threading dislocation density (TDD) for optoelectronic applications. The novelty of this work lies in the use of silicon on insulator (SOI) substrates patterned into nano-pillars down to the buried oxide (BOX). We actually want to benefit from the creeping properties of SiO2 at the growth temperature of GaN for strain relaxation and grain-boundary dislocations reduction. In this paper, we report on the fabrication of 40 × 40 µm2 and 300 × 300 µm2 freestanding GaN platelets, up to 10 µm-thick, spontaneously separated from the initial pillars. Structural and optical characterizations show that the platelets are crack-free and almost fully relaxed, with a TDD of ~4 × 108/cm2. We underline the different benefits of this approach, but most importantly, we believe that it will be the founding-brick for transferable GaN-based devices.

Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors

With the increasing demand for high integration and multi-color photodetection for both military and civilian applications, the research of multi-wavelength detectors has become a new research hotspot. However, current research has been mainly in visible dual- or multi-wavelength detectors, while integration of both visible light and ultraviolet (UV) dual-wavelength detectors has rarely been studied. In this work, large-scale and high-quality monolayer MoS2 was grown by the chemical vapor deposition method on transparent free-standing GaN substrate. Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors was demonstrated using common semiconductor fabrication technologies such as photolithography, argon plasma etching, and metal deposition. High performance of a 280 nm and 405 nm dual-wavelength photodetector was realized. The responsivity of the UV detector reached 172.12 A/W, while that of the visible detector reached 17.5 A/W. Meanwhile, both photodetectors achieved high photocurrent gain, high external quantum efficiency, high normalized detection rate, and low noise equivalent power. Our study extends the future application of dual-wavelength detectors for image sensing and optical communication.

V-shaped dislocations in a GaN epitaxial layer on GaN substrate

In this study, V-shaped dislocations in a GaN epitaxial layer on a free-standing GaN substrate were observed. Our investigation further revealed that the V-shaped dislocations were newly generated at the interface in the epilayer rather than propagated from the GaN substrate. V-shaped dislocations consist of two straight parts. The straight parts of the V-shaped dislocations were separated from each other in the m-direction and tilted toward the step-flow direction of the GaN epitaxial layer. The V-shaped dislocations are continuous single dislocations having a Burgers vector component of 1a and an intrinsic stacking fault between their straight parts.

Investigation of Al2O3/GaN interface properties by sub-bandgap photo-assisted capacitance-voltage technique

Interfaces in Al2O3/n-GaN capacitors fabricated on free-standing GaN substrates were investigated using sub-bandgap photo-assisted capacitance-voltage measurements. After post-metallization annealing (PMA) at 300 °C, the metal-oxide-semiconductor (MOS) devices exhibited excellent capacitance-voltage (C-V) characteristics without frequency dispersion under dark conditions and a quite low interface state density (Dit) of ∼7×1010 cm-2 eV-1 for energies less than ∼1.2 eV from the conduction band edge. Despite the outstanding characteristics under dark conditions, it was found that sub-bandgap irradiation also resulted in shifts of the C-V curves toward the negative bias direction, which indicates the existence of donor-type interface traps. As a result, the Al2O3/n-GaN Dit distribution does not show the generally reported U-shape: for energies less than ∼1.2 eV from the conduction band edge, a Dit of ∼7×1010 cm-2 eV-1 was observed, although Dit rapidly increased to ∼2-4×1012 cm-2 eV-1 near the valence band edge. These interface states near the valence band edge are positively charged due to trapped holes when the states are located above the Fermi level.

Vertical-Type Ni/GaN UV Photodetectors Fabricated on Free-Standing GaN Substrates

The authors report on a vertical-type visible-blind ultraviolet (UV) Schottky-type photodetector fabricated on a homoepitaxial GaN layer grown on free-standing GaN substrates with a semi-transparent Ni Schottky contact. Owing to the high-quality GaN drift layer with low-density threading dislocation and high electron mobility, the UV photodetector shows a high specific detectivity of more than 1012 Jones and a UV/visible discrimination ratio of ~1530 at −5 V. The photodetector also shows the excellent self-powered photo-response and a high signal-to-noise ratio of more than 104 at zero voltage. It is found that a relatively lower growth rate for the GaN epilayer is preferred to improve the performance of the Schottky-type photodetectors due to the better microstructure and surface properties.

Vertical GaN-on-GaN PIN diodes fabricated on free-standing GaN wafer using an ammonothermal method

We report vertical GaN-on-GaN PIN diodes with a record high figure-of-merit (VBR2/Ron) of 29.7 GW/cm2 on free-standing GaN wafer using a complementary metal-oxide-semiconductor (CMOS) compatible contact materials. Due to the low substrate resistivity, low contact resistance, and high quality of GaN drift layer, a low on-state resistance Ron of 0.31 mΩ cm2 is obtained. With integrating of the metal filed plate structure in the vertical device, the peak electrical field along the GaN mesa edge can be significantly reduced, thus leading to a high breakdown voltage VBR of 3.04 kV. The vertical GaN-on-GaN PIN diodes in this work show turn-on voltage Von of ∼3.4 V, on/off current ratio of ∼1.3 × 107, and ideal factor n of ∼2.2. According to the reverse switching measurement, the reverse recovery time Trr (reverse recovery charge Qrr) is 22.8 ns (4.8 nC) and 24.0 ns (5.4 nC), respectively, under a testing temperature of 300 K and 500 K.