High-quality GaN Single Crystals Research Articles

Review— Toyoda Gosei's History of R&D on Blue LEDs with Professor Isamu Akasaki

Toyoda Gosei (TG) started joint R&D with Prof. Akasaki and Prof. Amano (collectively, the Akasaki Group) in 1986, immediately after the Akasaki Group succeeded in the growth of high-quality GaN single crystals for the first time in the world. Then, the Akasaki Group and TG concluded an entrustment R&D agreement with Japan Science and Technology Agency (JST) in 1987 to promote R&D through the investment of JST. Under these circumstances, the Akasaki Group achieved the growth of high-quality p-type GaN and n-type GaN, believed impossible at that time. These breakthrough technologies were of fundamental importance for developing blue light-emitting diodes (LEDs). Namely, (1) high-quality GaN single crystals, (2) p-type GaN, and (3) n-type GaN were found to be applicable to industrial products. Thanks to the Akasaki Group, TG started the mass production of blue LEDs in 1995. However, at that time, these LEDs were not particularly bright, and no one believed that they would be used in general lighting. In spite of this prediction, the brightness of blue LEDs was improved markedly by industry-based developments, leading to their widespread adoption in many applications.

Determination of absolute value of quantum efficiency of radiation in high quality GaN single crystals using an integrating sphere

Omnidirectional photoluminescence (ODPL) measurement using an integrating sphere was carried out to absolutely quantify the quantum efficiency of radiation (η) in high quality GaN single crystals. The total numbers of photons belonging to photoluminescence (PL photons) and photons belonging to an excitation source (excitation photons) were simultaneously counted in the measurement, and η was defined as a ratio of the number of PL photons to the number of absorbed excitation photons. The ODPL spectra near the band edge commonly showed a two-peak structure, which originates from the sharp absorption edge of GaN. A methodology for quantifying internal quantum efficiency (ηint) from such experimentally obtained η is derived. A record high ηint of typically 15% is obtained for a freestanding GaN crystal grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method using an acidic mineralizer, when the excitation photon energy and power density were 3.81 eV and 60 W/cm2, respectively.

Growth of GaN Single Crystals by Li3N Flux with Mn as Addition

Metal Mn was introduced into Li3N flux to grow GaN single crystals from Ga melt. Colorless, transparent GaN single crystals with an average size of 2−3 mm were obtained at 800 °C under N2 pressure of about 2 atm. The effects of Mn on the quality, size distribution, morphologies, and formation mechanism of GaN crystals were investigated. It was found that the proper addition of Mn favored GaN growth, impeded the homogeneous nucleation to some extent, and resulted in a more uniform size distribution and high quality of GaN crystals as compared with that of GaN grown using Li3N as flux only. The formation mechanism was enhanced by Mn addition. These results suggested a promising new route for the growth of large size and high quality GaN single crystals from a Li−Ga−N system in the future by optimizing the molar ratio of Mn to Li3N.

Defect-pit-assisted growth of GaN nanostructures: nanowires, nanorods and nanobelts

A new method using defect-pit-assisted growth technology to successfully synthesize the high-quality single crystalline GaN nanostructures by ammoniating Ga(2)O(3) films was proposed in this paper. During the ammoniating process, the amorphous middle buffer layer may unavoidably produce some defects and dislocations. Some defect pits come out, which have the lowest surface energy and can subsequently be used as a mask/template or act as potential nucleation sites to fabricate the GaN actinomorphic nanostructures. The as-prepared products are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicate that all the reflections of the samples can be indexed to the hexagonal GaN phase and the clear lattice fringes in HRTEM further confirm the growth of high-quality single-crystal GaN nanostructures. The SEM images show that the nanostructures have been realized under different experimental conditions exhibiting different shapes: nanowires, nanorods, and nanobelts. No particles or other nanostructures are found in the SEM study, demonstrating that the product possesses pure nanostructures. These nanostructures show a very good emission peak at 366 nm, which will have a good advantage for applications in laser devices using one-dimensional structures. Finally, the growth mechanism is also briefly discussed.

Catalytic synthesis of large-scale GaN nanorods

A novel rare earth metal seed was employed as the catalyst for the growth of GaN nanorods. Large-scale GaN nanorods were synthesized successfully through ammoniating Ga 2O 3/Tb films sputtered on Si(1 1 1) substrates. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the structure, morphology, and composition of the samples. The results demonstrate that the nanorods are high-quality single-crystal GaN with hexagonal wurtzite structure. The growth mechanism of GaN nanorods is also discussed.

Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

The fundamental material parameters associated with GaN, which are important for the design of devices such as light-emitting diodes and laser diodes, were investigated using large high-quality GaN single crystals fabricated through hydride vapor phase epitaxy using the void-assisted separation method. The thermal-expansion coefficients (298–573K) along the C[0001], A[112¯0], and M[101¯0] axes (αC, αA, and αM) were measured. Thermal expansion in each direction, approximately proportional to the temperature, was observed throughout the measured temperature range. Although the thermal-expansion coefficients in the high-temperature range, i.e., αC(573K)=7.2±0.02×10−6∕K, αA(573K)=5.7±0.2×10−6∕K, and αM(573K)=5.8±0.2×10−6∕K,were relatively close to the reported values, the thermal-expansion coefficients along the C axis in the low-temperature range, i.e., αC(298K)=5.3±0.02×10−6∕K, was significantly larger than the reported values. Thermal conductivities parallel and perpendicular to the C axis were almost the same, i.e., 2.0±0.1Wcm−1K−1. This value is close to the values reported on the low-dislocation density part of epitaxially laterally overgrown GaN. This could indicate that the defect density of our crystals is very low and that the defects are distributed uniformly throughout the volume. The wavelength dependence of the refractive index and the optical-absorption coefficient was also investigated.

High-nitrogen-pressure growth of GaN single crystals: doping and physicalproperties

Growth of GaN under high-pressure high-temperature conditions allows one to obtainlarge-size high-quality GaN single crystals. These crystals have highconcentration of free electrons, most likely due to a high concentration of Oimpurity replacing nitrogen in the N sublattice. The incorporation of oxygen impurityduring high-pressure growth of GaN single crystals was investigated using quantum mechanical density functional theorycalculations. It was shown that the adsorption of oxygen in liquid groupIII metals (Al, Ga and In) leads to dissociation of the O2 molecule. Thedissociation process proceeds without energy barrier.The transition of oxygen from the adsorbed position into the interior of the Al has beenalso investigated. The results of calculations indicate that the directtransition energy barrier is about 3 eV. This indicates that the dissolution ofoxygen into liquid group III metals proceeds via Brownian motion of O-containing clusters.This also explains the difference between the solid and liquidsurfaces: the solid surfaces undergo passivation by oxygen, whereas in the liquid metalthe oxygen is dissolved.The doping of Mg during growth leads to a change of the electric properties of GaNcrystals - they become highly resistive. Mg doping changes the morphology of theplate-like GaN crystals. The physical properties of GaN:Mg crystals will bereviewed and compared with undoped GaN crystals.Beryllium doping is considered as an alternative route to obtaining p-type GaN. Thedoping with Be during growth increases the resistivity of the Be-doped GaN.However,the optical properties of Be-doped GaN crystals are different. These propertieswill be compared with Mg-doped and undoped GaN crystals.

Growth of nitride crystals, BN, AlN and GaN by using a Na flux

Bulk crystals of BN, AlN and GaN were grown by means of Na flux. All these crystals were grown at a temperature of 800°C and a nitrogen pressure of 100 atm, relatively lower than that required by many flux and melt growth methods. High-quality GaN single crystals as large as 0.5 mm were obtained. Furthermore, the oriented GaN crystals were obtained by means of the seeded Na flux method with the addition of oriented AlN (0001) film in the growth ambient. The nucleation of bulk GaN was spatially confined on top of the AlN film and grown with the GaN [0001] axis parallel to the AlN [0001] axis. In addition, the h-BN polycrystals were confirmed by the h-BN (0002) peak of X-ray diffraction (XRD) at 2θ=26.700. A hexagonal grain with a size as large as 2 μm was observed by scanning electron microscopy (SEM). Likewise, AlN crystals were also obtained from Al wires.

Stimulated emission in GaN thin films in the temperature range of 300–700 K

We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the unique properties of SE in GaN thin films at high temperatures could potentially be utilized in optoelectronic devices.

Near-threshold gain mechanisms in GaN thin films in the temperature range of 20–700 K

We report the results of an experimental study on near-threshold gain mechanisms in optically pumped GaN epilayers in the temperature range of 20–700 K. High-quality single-crystal GaN films grown on 6H–SiC and (0001) sapphire were used in this study. We show that the dominant near-threshold gain mechanism is inelastic exciton–exciton scattering for temperatures below ∼150 K, characterized by band-filling phenomena and a low stimulated emission (SE) threshold. An analysis of both the temperature dependence of the SE threshold and the relative shift between stimulated and band-edge-related emission indicates electron–hole plasma is the dominant gain mechanism for temperatures exceeding 150 K. Based on our results, we discuss possibilities of reducing the room-temperature lasing threshold in laser diode structures with a GaN active medium.

Binding energy for the intrinsic excitons in wurtzite GaN.

We present the results of an experimental study on the binding energy for intrinsic free excitons in wurtzite GaN. High-quality single-crystal GaN films grown by metalorganic chemical vapor deposition were used in this study. Various excitonic transitions in GaN were studied using reflectance measurements. The observation of a series of spectral features associated with the transitions involving the ground and excited exciton states allows us to make a straightforward estimate of exciton binding energy using the hydrogenic model. Our results yield a binding energy ${\mathit{E}}_{\mathit{b}}$=0.021\ifmmode\pm\else\textpm\fi{}0.001 eV for the A and B excitons, and 0.023\ifmmode\pm\else\textpm\fi{}0.001 eV for the C exciton in wurtzite GaN within the framework of the effective mass approximation. \textcopyright{} 1996 The American Physical Society.

Preparation and Characterization of Wurtzitic Gan Single Crystals in Nano and Micro Scale

AbstractGallium nitride and its alloys are the most promising materials for short wave light emitters. If high quality GaN single crystals can be prepared, the GaN base light emitters should be fabricated directly on the lattice-matched GaN substrate.In this work, GaN crystals in nano and micro scale with definite faces have been prepared by dc arc discharge using gallium and N2+NH3 as starting materials. Transmission electron microscope, selected area diffraction, x-ray microanalysis of energy dispersive spectroscopy, and x-ray diffraction investigation of the as grown GaN crystals show that the well faceted crystals are single crystalline GaN in wurtzite structure having lattice constants a0=3.18Å and c0=5.18Å. The crystal size of stoichoimetric GaN in wurtzite structure depends on the partial pressure of nitrogen in the plasma. The maximum crystal size in this work is about several micrometers.

Influence of buffer layers on the deposition of high quality single crystal GaN over sapphire substrates

Recently several research groups, including ours, have reported on the deposition of extremely high quality single crystal GaN layers over sapphire substrates. One of the keys to obtaining the high quality was the use of a thin AlN or GaN buffer layer between the sapphire substrate and the grown film. In this communication, we discuss the crystallinity and the influence of the buffer layer in controlling the crystalline, optical, and the electrical properties of the GaN depositions. We also compare the use of GaN and AlN as the buffer layer material. Our results indicate that the buffer layer thickness and the total film thickness are the key factors controlling the electrical, optical, and crystalline properties of the GaN depositions over sapphire substrates.

Deposition and surface characterization of high quality single crystal GaN layers

Recently, using low pressure metalorganic chemical vapor deposition we grew extremely high quality single crystal GaN films over basal plane sapphire substrates. Optimization of the buffer layers and the total film thickness played a key role in determining the electrical, optical, crystalline and surface qualities. In this communication we present the surface characterization results for these high quality GaN layers. Reflection high energy electron diffraction, low energy electron diffraction (LEED), and electron energy loss spectroscopy data are presented for clean GaN surfaces. The cleaning procedure was developed using Auger electron spectroscopy analysis. These electron diffraction data indicate the grown surface to be excellent single crystal GaN with a wurtzite structure. To the best of our knowledge ours is the first reported LEED data for single crystal GaN.

Electrical properties and ion implantation of epitaxial GaN, grown by low pressure metalorganic chemical vapor deposition

High quality single crystal GaN films with extremely uniform thicknesses have been grown in low pressure metalorganic chemical vapor deposition system using the reaction of (C2H5)3Ga with NH3. Electrical and optical properties of the layers were measured. Schottky barriers were fabricated after compensating the background donor concentration (typically ND ∼1×1019 cm−3) with Be+ or N+ implants. These Schottky barriers were electrically and optically characterized.