Single-crystal Gallium Nitride Research Articles

Terahertz time-domain ellipsometry with high precision for the evaluation of GaN crystals with carrier densities up to 1020\xa0cm\u22123

Gallium nitride (GaN) is one of the most technologically important semiconductors and a fundamental component in many optoelectronic and power devices. Low-resistivity GaN wafers are in demand and actively being developed to improve the performance of vertical GaN power devices necessary for high-voltage and high-frequency applications. For the development of GaN devices, nondestructive characterization of electrical properties particularly for carrier densities in the order of 1019 cm−3 or higher is highly favorable. In this study, we investigated GaN single crystals with different carrier densities of up to 1020 cm−3 using THz time-domain ellipsometry in reflection configuration. The p- and s-polarized THz waves reflected off the GaN samples are measured and then corrected based on the analysis of multiple waveforms measured with a rotating analyzer. We show that performing such analysis leads to a ten times higher precision than by merely measuring the polarization components. As a result, the carrier density and mobility parameters can be unambiguously determined even at high conductivities.

Single-crystal GaN growth and polarity control using an E-beam evaporated aluminum layer

We report on a method for controlling the polarity of gallium nitride (GaN) using an E-beam evaporated aluminum (Al) layer on a sapphire substrate. A high-temperature nitridation process was designed to enable the amorphous Al layer to serve as a nucleation layer for single-crystal Ga-polar GaN growth. The Al layer also acts as a mask that prevents N-polar GaN growth. As a result, Ga-polar and N-polar GaN can be grown on the Al layer and sapphire surface, respectively. This method is not only advantageous for the selective polarity control but also to simplify the fabrication process of lateral polarity structures.

Research on material removal mechanism and radial cracks during scribing single crystal gallium nitride

A diamond conical indenter with the cone angle of 60° and a tip arc radius of 5 μm was used to perform the gradual loading scribing experiments on gallium nitride (0001) crystal plane along the (Li et al., 2019; Wang et al., 2018; Yao et al., 2020; Fang and Zhang, 2013; Feng et al., 2009; Zhang et al., 2007; Jing et al., 2007; Yonenaga et al., 2018; Nowak et al., 1999; Shimada et al., 1998) [11-20] crystal orientation, and the material removal mechanism and machining properties of gallium nitride are studied. Single crystal gallium nitride will have the surface and sub-surface damage during machining, and the study of the stress field during scribing can effectively analyze and control the surface and sub-surface damage. Therefore, the stress field for scribing gallium nitride is acquired by superposing the boussinesq field, cerruti field and sliding blister field which have been led-in anisotropy parameters. Then on the basis of the distribution of the maximum tensile stress on the scribing surface, the nucleation and initial spread angle of radial cracks under our experimental conditions are analyzed, and for gallium nitride, the more severe the brittle spalling accompanying radial crack spread is, the greater the initial spread angle of radial cracks is.

Multiple epitaxial lateral overgrowth of GaN thin films using a patterned graphene mask by metal organic chemical vapor deposition

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

An analytical model for estimation of the stress field and cracks caused by scratching anisotropic single crystal gallium nitride

Single crystal gallium nitride will produce subsurface damage during processing, and studying the total stress field during scratching gallium nitride is advantageous for predicting and controlling subsurface damage. The total stress field during scratching gallium nitride (0001) crystal plane is obtained by superimposing the elastic and residual stress field. Thus the distribution of the stress field under the indenter can be acquired. It was found that the median crack nucleated at the elastoplastic interface of the front of the indenter, and the extension angle of the median crack is linearly related to half-angle of the conical indenter. Next the anisotropic model in this paper and the method simplified to isotropic materials are used for calculating the total stress field, and the consequences of the two are obviously different. The ratio between the stresses obtained by the two methods under different indenter half-angle has been obtained. Then by introducing the corresponding ratio into the traditional equation of the median crack depth and the lateral crack depth, the results considering the anisotropy can be obtained by the isotropic method.

Effect of abrasive size on nano abrasive machining for wurtzite GaN single crystal via molecular dynamics study

Molecular dynamics simulations were used to explore the effect of abrasive size on the nano abrasive machining for wurtzite gallium nitride (GaN) single crystal. The deformation behaviours, varied with the abrasive size, were systematically analysed. The simulation results show that increasing the abrasive size results in more severe plastic deformation and machining-induced hardening, which consequently cause an increase in atomic temperature, strain, stress, and cutting forces. The use of larger abrasive produces greater compression, thus boosts the elastic recovery and damages the subsurface of GaN single crystal much heavily, with the enhancement of the nucleation and emission of dislocation, and the local phase transition from B4 to B3 structure or even to fivefold hexagonal structure. The development of pile-up and amorphization is impressively inhibited by increasing abrasive size, suggesting that the cutting effect could be impaired using a larger abrasive under a given depth of cut.

Investigation on the interaction between a gallium nitride surface and H2O using a nanometer-scale GaN lamella structure

Gallium nitride (GaN) is exceedingly apposite for liquid-based sensor applications because of their high internal piezoelectric polarization, chemical and high temperature stability. In this work, the interaction between GaN and H2O has been investigated using a novel methodology. We report the fabrication of single crystal GaN lamella with thickness of few hundreds of nanometer using focused ion-beam milling technique, for sensing applications. Results signify that the device resistivity increases with time at room temperature during the GaN-H2O interaction. Such a change in electrical resistivity is explained based on the electron transfer and electrochemical reactions at the surface of GaN. Study of the surface chemistry transformation of the tested GaN lamella is conducted using high-angle annular dark-field scanning transmission electron microscopy coupled with electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDS) techniques. EDS and EELS results signify the presence of a region containing Ga and O at the interface of the H2O/GaN which is a result due to the adsorption of molecular H2O and its dissociation products implying the occurrence of GaN-water reaction.

On the qualitative characteristics of a two-dimensional mathematical model of diffusion of minority charge carriers generated by a low-energy electron beam in a homogeneous semiconductor material

The problem of two-dimensional diffusion of excitons generated by a pulsating sharply focused low-energy electron beam in a single-crystal gallium nitride was considered by mathematical modeling methods. For the describing the energy loss by electrons in a semiconductor target, a mathematical model is used, based on a separate description of the energy loss by electrons that have experienced small-angle scattering and are absorbed by the target and electrons that have been reflected at large angles and left the target. The main attention is paid to the solution of the differential equations of heat and mass transfer and to the problems of their correctness and stability.

Atomization and Changes in Chemical Composition by Laser Ablation in Liquid prior to Determination of Trace Elements in Gallium Nitride.

Trace elements contained in single-crystal gallium nitride (GaN), which is a refractory material, were determined by inductively coupled plasma mass spectrometry with laser ablation in liquid (LAL) sampling. Particle size analysis by dynamic light scattering confirmed that LAL atomized the single-crystal GaN, and scanning electron microscopy/energy-dispersive X-ray spectrometry revealed that LAL sampling converted the GaN to gallium oxide particles. The atomization and changes in the chemical composition allowed for easy digestion of the sampled particles with nitric acid and a hot plate instead of with hydrochloric or sulfuric acid, a microwave digestion system, and a high-pressure vessel. Trace element analysis revealed the presence of trace Mg, Ge, Y, and Cd in single-crystal GaN, and the detection limits for these elements were 0.1 - 2 mg kg-1. The uncertainty derived from the small amount of sample used, which is a disadvantage of LAL sampling, was reduced by calculating the amount of sample ablated by measuring the concentration of the major element, Ga. We expect that the trace-element profiles of other refractory materials will be able to be determined by using this analytical approach.

Increase in the piezoelectric response of scandium-doped gallium nitride thin films sputtered using a metal interlayer for piezo MEMS

Wurtzite gallium nitride (GaN) has a polarity along the c-axis and piezoelectric properties the same as aluminum nitride. Since it has a high mechanical quality factor and high output sensitivity, it is expected to perform well in piezo micro-electro-mechanical system devices. This paper demonstrates that Hf and Mo interlayers enable the preparation of highly (001)-oriented GaN films via conventional sputtering at a low temperature (400 °C). The piezoelectric coefficient d33 of the prepared undoped GaN films is equivalent to that of a single-crystal GaN. Furthermore, the results demonstrate that the piezoelectric response of GaN films increases significantly when they are doped with scandium (Sc). Although this enhancement was predicted theoretically, the piezoelectric response of Sc-doped GaN films prepared on Hf and Mo interlayers has shown great improvement. Moreover, bulk acoustic wave resonators constructed using Sc-doped GaN films show a piezoelectric coupling factor that is three times larger than that of a single-crystal GaN.

On the correctness of mathematical models of time-of-flight cathodoluminescence of direct-gap semiconductors

Two-dimensional and three-dimensional mathematical models of diffusion and cathodoluminescence of excitons in single-crystal gallium nitride excited by a pulsating sharply focused electron beam in a homogeneous semiconductor material are compared. The correctness of these models has been carried out, estimates have been obtained to evaluate the effect of errors in the initial data on the distribution of the diffusing excitons and the cathodoluminescence intensity.

Growth of 2D GaN Single Crystals on Liquid Metals.

Two-dimensional (2D) gallium nitride (GaN) has been highly anticipated because its quantum confinement effect enables desirable deep-ultraviolet emission, excitonic effect and electronic transport properties. However, the currently obtained 2D GaN can only exist as intercalated layers of atomically thin quantum wells or nanometer-scale islands, limiting further exploration of its intrinsic characteristics. Here, we report, for the first time, the growth of micrometer-sized 2D GaN single crystals on liquid metals via a surface-confined nitridation reaction and demonstrate that the 2D GaN shows uniformly incremental lattice, unique phonon modes, blue-shifted photoluminescence emission and improved internal quantum efficiency, providing direct evidence to the previous theoretical predictions. The as-grown 2D GaN exhibits an electronic mobility of 160 cm2·V-1·s-1. These findings pave the way to potential optoelectronic applications of 2D GaN single crystals.

Observation of Threading Dislocations in Ammonothermal Gallium Nitride Single Crystal Using Synchrotron X-ray Topography

Synchrotron monochromatic-beam x-ray topography observation has been performed on high-quality ammonothermal gallium nitride single crystal to evaluate threading dislocations (TD) in a nondestructive manner. Asymmetric diffractions with six equivalent g-vectors of 11–26, in addition to a symmetric diffraction with g = 0008, were applied to determine the Burgers vectors (b) of dislocations. It was found that pure edge-type TDs with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 \) did not exist in the sample. A dominant proportion of TDs were of mixed type with \( {\varvec b} = \left\langle {11 - 20} \right\rangle /3 + \left\langle {0001} \right\rangle \), i.e., so-called c + a dislocations. Pure 1c screw dislocations with \( {\varvec b} = \left\langle {0001} \right\rangle \) and TDs with c-component larger than 1c were also observed.

Optical properties of bulk gallium nitride single crystals grown by chloride–hydride vapor-phase epitaxy

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.

In situ inward epitaxial growth of bulk macroporous single crystals

The functionalities of porous materials could be significantly enhanced if the materials themselves were in single-crystal form, which, owing to structural coherence, would reduce electronic and optical scattering effects. However, growing macroporous single crystals remains a fundamental challenge, let alone manufacturing crystals large enough to be of practical use. Here we demonstrate a straightforward, inexpensive, versatile method for creating macroporous gallium nitride single crystals on a centimetre scale. The synthetic strategy is built upon a disruptive crystal growth mechanism that utilises direct nitridation of a parent LiGaO2 single crystal rendering an inward epitaxial growth process. Strikingly, the resulting single crystals exhibit electron mobility comparable to that for bulk crystals grown by the conventional sodium flux method. This approach not only affords control of both crystal and pore size through synthetic modification, but proves generic, thus opening up the possibility of designing macroporous crystals in a wealth of other materials.

Crystal growth of HVPE-GaN doped with germanium

Crystallization by hydride vapor phase epitaxy method of gallium nitride single crystals doped with germanium and properties of the obtained material are described in this paper. Growth was performed in hydrogen and nitrogen carrier gas. The results were studied and compared. Influence of different flows of germanium tetrachloride, precursor of germanium, on the grown crystals was investigated. Ammonothermal GaN substrates were used as seeds for crystallization. Structural, electrical, and optical properties of HVPE-GaN doped with germanium are presented and discussed in detail. They were compared to properties of HVPE-GaN doped with silicon and also grown on native seeds of high quality.

Transferable single-crystal GaN thin films grown on chemical vapor-deposited hexagonal BN sheets

Single-crystal gallium nitride (GaN) layers were directly grown on centimeter-scale hexagonal boron nitride (h-BN). Using chemical vapor deposition (CVD), centimeter-scale h-BN films were synthesized on a single-crystal Ni(111) and readily transferred onto amorphous fused silica supporting substrates that had no epitaxial relationship with GaN. For growing fully coalescent GaN layers on h-BN, the achievement of high-density crystal growths was a critical growth step because the sp2-bonded h-BN layers are known to be free of dangling bonds. Unlike GaN layers grown on a typical heterogeneous sapphire substrate, the morphological and microstructural results strongly suggest a high-density growth feature that is driven by the atomic cliffs inherent in the CVD-grown h-BN layers. More importantly, the GaN layers grown on CVD-grown h-BN exhibited a flat and continuous surface morphology with well-aligned crystal orientations both along the c-axis and in-plane, indicating the characteristics of GaN heteroepitaxy on h-BN.

Large spin accumulation and crystallographic dependence of spin transport in single crystal gallium nitride nanowires

Semiconductor spintronics is an alternative to conventional electronics that offers devices with high performance, low power and multiple functionality. Although a large number of devices with mesoscopic dimensions have been successfully demonstrated at low temperatures for decades, room-temperature operation still needs to go further. Here we study spin injection in single-crystal gallium nitride nanowires and report robust spin accumulation at room temperature with enhanced spin injection polarization of 9%. A large Overhauser coupling between the electron spin accumulation and the lattice nuclei is observed. Finally, our single-crystal gallium nitride samples have a trigonal cross-section defined by the (001), () and () planes. Using the Hanle effect, we show that the spin accumulation is significantly different for injection across the (001) and () (or ()) planes. This provides a technique for increasing room temperature spin injection in mesoscopic systems.

Near-infrared waveguide in gallium nitride single crystal produced by carbon ion implantation

We report on the fabrication of planar waveguides in gallium nitride by 5 MeV carbon ion implantation with different fluences at room temperature. The waveguides are characterized by prism coupling, Rutherford backscattering/channeling, and high-resolution X-ray diffraction analysis. A positive change in ordinary refractive index is confirmed in the waveguide region at a near-infrared waveband. The thermal stability of the ion-implanted GaN waveguides is investigated by annealing the samples at different temperatures.

One-step fabrication of porous GaN crystal membrane and its application in energy storage

Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.