GaN Single Crystals Research Articles

Basic 암모노써멀 방법에 의한 벌크 GaN 단결정의 성장 및 특성

Basic 암모노써멀 방법에 의한 벌크 GaN 단결정의 성장 및 특성

FTIR exhaust gas analysis of GaN pseudo-halide vapor phase growth

FTIR exhaust gas analysis is used to establish chemical reactions paths in chemical vapor growth of GaN single crystals. The growth process utilizes the classical Degussa process on a lab scale to synthesize HCN as a transport agent for Ga. With GaCN as Ga and C precursor, the process is very similar to halide vapor phase epitaxy (HVPE), where GaCl is the Ga precursor and in both methods, the source of reactive nitrogen is NH3. Spectra are presented for the pure HCN synthesis and its combination with vapor growth, allowing the effective optimization of growth parameters, especially of gas flows.

Electronic and optical characteristics of an m-plane GaN single crystal grown by hydride vapor phase epitaxy on a GaN seed synthesized by the ammonothermal method using an acidic mineralizer

Fundamental electronic and optical properties of a low-resistivity m-plane GaN single crystal, which was grown by hydride vapor phase epitaxy on a bulk GaN seed crystal synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer, were investigated. The threading dislocation and basal-plane staking-fault densities of the crystal were around 104 cm−2 and less than 100 cm−1, respectively. Oxygen doping achieved a high electron concentration of 4 × 1018 cm−3 at room temperature. Accordingly, a photoluminescence (PL) band originating from the recombination of hot carriers was observed at low temperatures, even under weak excitation conditions. The simultaneous realization of low-level incorporation of Ga vacancies (VGa) less than 1016 cm−3 was confirmed by using the positron annihilation technique. Consistent with our long-standing claim that VGa complexes are the major nonradiative recombination centers in GaN, the fast-component PL lifetime of the near-band-edge emission at room temperature longer than 2 ns was achieved.

Mechanism for enhanced single-crystal GaN growth in the C-assisted Na-flux method

First-principles molecular dynamics simulations are used to examine the effect of C addition in Na-flux growth of GaN. The mechanism for suppression of polycrystalline growth and the enhancement of single-crystal growth was identified by systematically calculating activation free energies for the formation and dissociation of C–N bonds. The energy barrier for C–N dissociation in a Ga–Na melt is ≥3 eV; thus, dissociation is inhibited and the growth of polycrystals is suppressed. However, at kink sites at a Na/GaN interface with excess Ga atoms, the barrier is only ∼1.0 eV, allowing C–N dissociation and growth of GaN single crystals.

High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm2 is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm2 is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

Single-crystal GaN/AlN layers on CVD diamond

Original approach to fabricating a GaN/AlN/nanocrystalline diamond structure has been suggested and implemented. The stages of deposition of a structure of this kind include the following: (a) growth of nanocrystalline CVD-diamond on single-crystal AlN (preliminarily grown on a silicon substrate), (b) etch removal of the silicon substrate, and (c) growth of single-crystal GaN on the surface of single-crystal AlN. Single-crystal gallium nitride with a width of the X-ray rocking curve for the (0002) reflection of 0.35° was obtained on a nanocrystalline-diamond substrate.

Complete orientational access for semipolar GaN devices on sapphire

Semipolar () GaN light‐emitting diodes and () GaN films are demonstrated on sapphire substrates. The processes developed here embody all the required steps in order to produce planar semipolar devices on sapphire substrates with complete control of the crystallographic orientation. Theoretical and experimentally observed aspects of semipolar materials and devices on bulk substrates are reviewed, including device performance and orientation‐dependent material properties. This summary motivates the use of particular semipolar orientations having Miller indices (hkil), with l < 0, such as () and (). These orientations have not, as of yet, been shown to be obtained using sapphire substrates. With the appropriate nitridation and buffer layer growth condition on stripe‐patterned sapphire, high quality single crystal () GaN films have been achieved. Demonstration of surface planarization, device layer regrowth, and LED operation, performed here on semipolar () GaN, detail the processes to enable planar semipolar‐based devices on sapphire with these unique orientations. With these advances, the required foundation has been put into place to allow access to the potentially highest performance semipolar planes while using economically feasible, large area sapphire substrates.Processes to produce () LED devices (top left) on GaN films grown on full 2″ sapphire substrates (bottom left) are described in this paper. The achievement of () GaN paves the way toward achieving full orientational access on sapphire substrates (right).

Effect of Fe-doping on nonlinear optical responses and carrier trapping dynamics in GaN single crystals

We presented a quantitative study on the Fe-doping concentration dependence of optical nonlinearities and ultrafast carrier dynamics in Fe-doped GaN (GaN:Fe) single crystals using picosecond Z-scan and femtosecond pump-probe with phase object techniques under two-photon excitation. In contrast to the two-photon absorption that was found to be independent on the Fe-doping, the nonlinear refraction decreased with the Fe concentration due to the fast carrier trapping effect of Fe3+/Fe2+ deep acceptors, which simultaneously acted as an efficient non-radiative recombination channels for excess carriers. Remarkably, compared to that of Si-doped GaN bulk crystal, the free-carrier refraction effect in GaN:Fe crystals was found to be enhanced considerably since Fe-doping and the effective carrier lifetime (∼10 ps) could be tuned over three orders of magnitude at high Fe-doping level of 1 × 1019 cm−3.

Liquid-target reactive magnetron sputter epitaxy of High quality GaN(0001̄) nanorods on Si(111)

Direct current magnetron sputter epitaxy with a liquid Ga sputtering target has been used to grow single-crystal GaN(0001̄) nanorods directly on Si(111) substrates at different working pressures ranging from 5 to 20mTorr of pure N2. The as-grown GaN nanorods exhibit very good crystal quality from bottom to top without stacking faults, as determined by transmission electron microscopy. The crystal quality is found to increase with increasing working pressure. X-ray diffraction results show that the rods are highly (0001)-oriented. The nanorods exhibit an N-polarity, as determined by convergent beam electron diffraction methods. Sharp and well-resolved 4K photoluminescence peaks at ~3.474eV with a FWHM ranging from 1.7meV to 35meV are attributed to the intrinsic GaN band edge emission and corroborate the superior structural properties of the material. Texture measurements reveal that the rods have random in-plane orientation when grown on Si(111) with native oxide, while they have an in-plane epitaxial relationship of GaN[112̅0]//Si[11̄0] when grown on substrates without surface oxide.

Analysis of channel confined selective area growth in evolutionary growth of GaN on SiO2

Here, we analyze the chemical vapor deposition of semiconductor crystals by selective area growth in a non-planar geometry. Specifically, the growth process in laterally and vertically confined masks forming single-crystal GaN on SiO2 by metal-organic chemical vapor deposition is considered in detail. A textured AlN seed is used to initiate growth of oriented GaN selectively through the mask, allowing the reduction of degrees of freedom by the evolutionary grain selection process. As shown by measurements of growth rates within the mask, the sub micron length scale of the channel opening is comparable to the mean free path of precursors in the gas phase, resulting in transport characteristics that can be described by an intermediate flow regime between continuum and free-molecular. Mass transport is modeled through kinetic theory to explain the growth rate enhancements of more than a factor of two by changes in reactor pressure. The growth conditions that enable the modification of nucleation density within the channel are then discussed, and are measured by electron-back scatter diffraction of the nucleated grains on the AlN seed. Finally, the selectivity behavior using the low fill factor masks needed in these configurations has been optimized by control of precursor flow rates and the H2 enhanced etching of the polycrystalline GaN nuclei.

Determination of GaN solubility in supercritical ammonia with NH4F and NH4Cl mineralizer by in situ x-ray imaging of crystal dissolution

Quantitative data on the solubility of GaN in supercritical ammonia using NH4F as mineralizer are reported. The solubility is determined by in situ x-ray imaging of the dissolution of GaN single crystals. First, solubility values obtained by this method with NH4Cl as mineralizer are presented and discussed with respect to existing literature data. Monitoring the dissolution process in situ reveals the time when the solubility limit is reached. Thus, it allows to distinguish the saturation of the solution from dissolution based on mass transport and deposition. This is a key advantage of solubility measurements by in situ x-ray imaging compared to gravimetric methods. Our results indicate that the solubility limit is reached much faster than usually assumed in gravimetric solubility studies and the solubility of GaN in ammonothermal media is significantly lower than reported so far.

Modeling grown-in dislocation multiplication on prismatic slip planes for GaN single crystals

To dynamically model the grown-in dislocation multiplication on prismatic slip planes for GaN single crystal growth, the Alexander–Haasen (AH) model, which was originally used to model the plastic deformation of silicon crystals, is extended to GaN single crystals. By fitting the model to the experimental data, we found that it can accurately describe the plastic deformation of GaN caused by prismatic slip. A set of unified parameters for the AH model at different temperatures can be found. This model provides a possible method to minimize grown-in dislocations caused due to prismatic slip by optimizing growing and cooling conditions during GaN single crystal growth.

Electrochemical Deposition of Antimony on Single-Crystal Gallium Nitride(0001) Electrode: Nucleation and Growth Mechanism

The electrodeposition of antimony (Sb) on single-crystal n-type GaN(0001) from hydrochlorate solution has been investigated by scanning electron microscopy, X-ray diffraction, energy dispersive X-ray analysis and electrochemical techniques including cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and Tafel measurement. It was found that the Sb deposition on GaN(0001) commenced at a potential of −190 mV vs. Ag/AgCl without underpotential deposition process. On the basis of Tafel plot, an exchange current density of ∼8.05 × 10−5 mA cm−2 was calculated. In addition, the current transient measurements revealed that Sb deposition process followed the progressive nucleation in 5 mM SbCl3 + 0.5 M NaCl (pH = 1).

Preparation and Characterization of Mg-Doped GaN Nanowires by Au-Catalyzed Magnetron Sputtering Deposition

Mg-doped GaN nanowires have been successfully synthesized on Si (111) substrates by magnetron sputtering deposition through ammoniating Ga2O3/Au thin films at 900 °C for 15 min. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and photoluminescence (PL) spectrum were carried out to characterize microstructure, morphology, and optical property of GaN sample. The results demonstrate that the nanowires are single-crystal Mg-doped GaN with hexagonal wurtzite structure and high crystalline quality, which have the size of 40 nm in diameter and several tens of microns in length and good emission property. The growth procedure mainly follows the VLS mechanism, and Au plays an important role as catalyst, and more defect energy is formed due to metallic Au and thus promote the growth of GaN nanowires.

Design and fabrication of single-crystal GaN nano-bridge on homogeneous substrate for nanoindentation

This study reports a simple method to design and fabricate a freestanding GaN nano-bridge over a homogeneous short column as supporting leg. Test samples were fabricated from MOCVD-grown single-crystal GaN films over sapphire substrate using a FIB milling to leave freestanding short spans. We also investigated the nanoindentation characteristics and the corresponding nanoscopic mechanism of the GaN nano-bridge and its short column with a conical indenter inside transmission electron microscopy. The stress–strain mechanical properties and Young’s modulus have also been examined and calculated as 108 GPa ± 4.8 % by the strain energy method. The significant slope switch of the L–D curve corresponds to the transition from the single-point bending indentation to the surface stretching indentation and has been interpreted with the evolution of TEM images. This freestanding fabrication and test have key advantages to characterize nanoscale behavior of one-dimensional bridge structure and greater ease of sample preparation over other micro-fabrication techniques.

화학적 습식 에칭을 통한 AlN와 GaN의 결함 및 표면 특성 분석

화학적 습식 에칭을 통해 AlN와 GaN의 결함 및 표면 특성을 분석했다. 화학적 습식 에칭은 단결정의 결함을 선택적으로 에칭하기 때문에 결정의 품질을 평가하는 좋은 방법으로 주목 받고 있다. AlN와 GaN의 단결정은 NaOH/KOH 용융액을 이용하여 에칭을 했으며, 에칭 후 표면 특성을 알아보기 위해 주사전자현미경(SEM)과 원자힘 현미경(AFM)을 촬영했다. 에치 핏의 깊이를 측정하여 표면에 따른 에칭 속도를 계산했다. 그 결과 AlN와 GaN 표면에는 두 개의 다른 형태에 에치 핏이 형성 되었다. (0001)면의 metal-face(Al, Ga)는 육각 추를 뒤집어 놓은 형태를 갖는 반면 N-face는 육각형 형태의 소구 모양(hillock structure)을 하고 있었다. 에칭 속도는 N-face가 metal-face(Al, Ga)보다 각 각 약 109배(AlN)와 5배 정도 빨랐다. 에칭이 진행되는 동안 에치 핏은 일정한 크기로 증가하다 서로 이웃한 에치 핏들과 합쳐지는 것으로 보여졌다. 또한 AlN와 GaN의 에칭 공정을 화학적 메커니즘을 통해 알아 보았는데, 수산화 이온(<TEX>$OH^-$</TEX>)과 질소의 dangling bond에 영향을 받아 metal-face(Al, Ga)와 N-face가 선택적으로 에칭되는 것으로 추론되었다. We investigated defects and surface polarity in AlN and GaN by using wet chemical etching. Therefore, the effectiveness and reliability of estimating the single crystals by defect selective etching in NaOH/KOH eutectic alloy have been successfully demonstrated. High-quality AlN and GaN single crystals were etched in molten NaOH/KOH eutectic alloy. The etching characteristics and surface morphologies were carried out by scanning electron microscope (SEM) and atomic force microscope (AFM). The etch rates of AlN and GaN surface were calculated by etching depth as a function of etching time. As a result, two-types of etch pits with different sizes were revealed on AlN and GaN surface, respectively. Etching produced hexagonal pits on the metal-face (Al, Ga) (0001) plane, while hexagonal hillocks formed on the N-face. On etching rate calibration, it was found that N-face had approximately 109 and 15 times higher etch rate than the metal-face of AlN and GaN, respectively. The size of etch pits increased with an increase of the etching time and they tend to merge together with a neighbouring etch pits. Also, the chemical mechanism of each etching process was discussed. It was found that hydroxide ion (<TEX>$OH^-$</TEX>) and the dangling bond of nitrogen play an important role in the selective etching of the metal-face and N-face.

Optical nonlinearities and carrier dynamics in Fe doped GaN single crystal

Optical nonlinearities and transient dynamics of Fe doped GaN (GaN:Fe) were studied by Z-scan and pump-probe with phase object techniques under picosecond and nanosecond at 532 nm. From the pump-probe results, an additional decay pathway subsequent to two photon excitation was observed due to the carrier trapping of Fe3+/Fe2+ deep acceptors. The trapping state in the band gap results in a pronounced modulation to nonlinear responses of GaN:Fe compared to that of undoped GaN. With an effective three-level model as well as carrier trapping effect we described the photo-physical dynamics in GaN:Fe unambiguously.

HVPE로 성장된 GaN의 용융 KOH/NaOH 습식화학에칭

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea*UNIMO Photron, Seoul 137-063 Korea(Received June 10, 2014)(Revised July 7, 2014)(Accepted July 18, 2014)Abstract The hydride vapor phase epitaxy (HVPE) grown GaN samples to precisely measure the surface characteristicswas applied to a molten KOH/NaOH wet chemical etching. The etching rate by molten KOH/NaOH wet chemical etchingmethod was slower than that by conventional etching methods, such as phosphoric and sulfuric acid etching, which may bedue to the formation of insoluble coating layer. Therefore, the molten KOH/NaOH wet chemical etching is a betterefficient method for the evaluation of etch pits density. The grown GaN single crystals were characterized by using X-raydiffraction (XRD) and X-ray rocking curve (XRC). The etching characteristics and surface morphologies were studied byscanning electron microscopy (SEM). From etching results, the optimum etching condition that the etch pits were wellindependently separated in space and clearly showed their shape, was 410

Gigahertz acoustic wave velocity measurement in GaN single crystals considering acousto-electric effect.

The resistivity-frequency characteristics of longitudinal wave velocities propagating parallel to the c-axis in a GaN single crystal were theoretically estimated by considering the piezoelectric acousto-electric effect. The temperature and frequency dependences of longitudinal and shear wave velocities in conductive and semiconductive GaN single-crystal samples were experimentally investigated by Brillouin scattering. The temperature dependence of longitudinal and shear wave velocities had a linear tendency in the conductive sample, whereas in the semiconductive sample, those had a similar tendency to the predicted velocity changes resulting from the piezoelectric stiffening effect. However, the temperature dependence of shear wave velocity, which does not possess piezoelectric coupling, had a tendency similar to that of the longitudinal wave in the semiconductive sample, unexpectedly. The frequency dependence of longitudinal wave velocities in the semiconductive sample had a tendency similar to the predicted velocity changes resulting from the piezoelectric stiffening effect.

Dependence on pressure of the refractive indices of wurtzite ZnO, GaN, and AlN

We have measured both the ordinary and extraordinary refractive index of $m$-plane cuts of wurtzite ZnO, GaN, and AlN single crystals at room temperature and as a function of hydrostatic pressure up to 8 GPa. For that purpose we have developed an alternative optical interference method, called bisected-beam method, which leads, in general, to high contrast interference fringes. Its main feature, however, is to be particularly suitable for high pressure experiments with the diamond anvil cell, when the refractive index of the sample is low and similar to that of diamond and/or the pressure transmitting medium, as is the case here. For all three wide-gap materials we observe a monotonous decrease of the ordinary and extraordinary refractive indices with increasing pressure, being most pronounced for GaN, less marked for ZnO, and the smallest for AlN. The frequency dependence of the refractive indices was extrapolated to zero energy using a critical-point-plus-Lorentz-oscillator model of the ordinary and extraordinary dielectric function. In this way, we determined the variation with pressure of the electronic part (no-phonon contribution) of the static dielectric constant ${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}$. Its volume derivative, $r=d\phantom{\rule{0.16em}{0ex}}ln{\ensuremath{\varepsilon}}_{\ensuremath{\infty}}/d\phantom{\rule{0.16em}{0ex}}lnV$, serves as single scaling coefficient for comparison with experimental and/or theoretical results for other semiconductors, regarding the pressure effects on the dielectric properties. We have obtained an ordinary/extraordinary average value $\overline{r}$ of 0.49(15) for ZnO, 1.22(9) for GaN, and 0.32(4) for AlN. With the values for the ordinary and extraordinary case being within experimental uncertainty, there is thus no apparent change in dielectric anisotropy under pressure for these wurtzite semiconductors. Results are discussed in terms of the pressure-dependent electronic band structure of the materials.