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Results of gallium nitride crystallization on native seeds by Hydride Vapor Phase Epitaxy method are described. The seeds are high quality ammonothermal GaN crystals. Properties of unintentionally doped HVPE-GaN are briefly presented. A review on doping with donors and acceptors is prepared. Intentional incorporation of silicon or germanium is proposed in order to grow highly conductive HVPE-GaN. Carbon, iron, or manganese was introduced to increase the resistivity of crystallized material. GaN samples with different dopants are described in terms of their structural, optical, and electrical properties. [ABSTRACT FROM AUTHOR]

GaN/AlxGa1−xN quantum wells were grown by molecular beam epitaxy on high quality bulk (0001) GaN substrates. The quantum well thickness was set in the 6–8 nm range to favor the photoluminescence emission of indirect excitons. Indeed, such excitons are known to be spatially indirect due to the presence of the internal electric field which spatially separates the electron and hole wave functions. The growth conditions were optimized in view of minimizing the photoluminescence peak broadening. In particular, the impact of growth temperature (up to 900 °C) on the surface morphology, structural, and photoluminescence properties was studied. The diffusion of indirect excitons on the scale of tens of micrometers was measured with a micro-photoluminescence setup equipped with a spatially resolved detection. A dedicated model and its analysis allow us to extract from these measurements the exciton diffusion constant and to conclude on the optimum growth conditions for the GaN/AlxGa1−xN quantum well structures suited for studies of quantum collective effects in indirect exciton liquids. [ABSTRACT FROM AUTHOR]

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing. [ABSTRACT FROM AUTHOR]

Abstract: The growth and physical properties of GaN crystallized in a multi feed-seed (MFS) configuration by High Nitrogen Pressure Solution (HNPS) growth method are presented in detail. The conversion of free standing HVPE-GaN crystals to free standing HNPS-GaN is the basis of the MFS configuration. The influence of the experimental conditions (i.e. growth temperature, temperature gradient, etc.), the c-plane bowing of the initial substrate, the electrical properties of HNPS-GaN, and the rate and mode of growth from solution are analyzed. We show that the HNPS-GaN crystals have better structural quality than their HVPE-GaN seeds. The defect density decreases with increasing growth temperature, reaching 5×105 cm−2 for crystals grown at 1420°C or higher. In contrast, the free carrier concentration in HNPS-GaN increases with increasing growth temperature, reaching 7×1019 cm−3 for samples crystallized at 1440°C. Thus the possibility to obtain good quality plasmonic GaN substrates for laser diodes can be realized. [Copyright &y& Elsevier]

Abstract: Recent results of the bulk GaN crystallization by the high-pressure solution (HPS) growth method are presented. The new experimental configurations for seeded growth on HVPE free-standing GaN crystals are proposed. The growth rate, morphology and basic physical properties of the pressure-grown materials are reported. The finite element calculation is used for modeling the convective transport in the solution. The convectional flow velocity vectors in liquid gallium are determined from experimentally measured temperatures and by solving the set of Navier–Stokes equations. [Copyright &y& Elsevier]

Abstract: Seeded growth of GaN on 1in sapphire/GaN templates by high-pressure solution method is presented. Five crystallization runs at the same growth conditions (temperatures, supercooling, nitrogen pressure, time) but different in geometry are described in details. The finite element calculation is used for modeling the convective transport in the liquid gallium. The stream lines, convectional flow velocity vectors and isotherm lines in liquid metal are determined based on experimentally measured temperatures in the crucible wall. The influence of the seed and the baffle for convection in liquid metal is analyzed in details. [Copyright &y& Elsevier]

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T = 4 K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A − + h ν → A 0 + e C B −) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A 0 + h ν → A − + h V B +) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with C N being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral C N . [ABSTRACT FROM AUTHOR]

Magnetization measurements of the strain-free bulk GaN:Cr single crystals of wurtzite structure are reported. Strong magnetic anisotropy at low temperatures (2-10 K) was observed. The data were analyzed assuming Cr2+(d4) configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static Jahn-Teller distortion, and the spin-orbit interaction provide a good description of the experimental magnetization data. [ABSTRACT FROM AUTHOR]

GaN samples were implanted with Be and annealed in different conditions in order to activate the shallow BeGa acceptor. Low-temperature photoluminescence spectra were studied to find BeGa-related defects in the implanted samples. A yellow band with a maximum at about 2.2 eV (the YLBe band) was observed in nearly all samples protected with an AlN cap during the annealing and in samples annealed under ultrahigh N2 pressure. A green band with a maximum at 2.35 eV (the GL2 band), attributed to the nitrogen vacancy, was the dominant defect-related luminescence band in GaN samples annealed without a protective AlN layer. The ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV attributed to the shallow BeGa acceptor with the ionization energy of 0.113 eV appeared in implanted samples only after annealing at high temperatures and ultrahigh N2 pressure. This is the first observation of the UVLBe band in Be-implanted GaN, indicating successful activation of the BeGa acceptor. [ABSTRACT FROM AUTHOR]

Ammonothermal GaN samples with the concentration of free electrons of 1018 and 1019 cm−3 were annealed in a wide range of temperatures (Tann = 300–1400 °C) under atmospheric N2 pressure and under ultra-high N2 pressure conditions to avoid the GaN decomposition. Photoluminescence (PL) studies reveal the YL2 band with a maximum at 2.3 eV before annealing and two new PL bands after annealing at Tann > 600 °C: the OL3 band with a maximum at 2.1 eV and the RL4 band with a maximum at 1.6–1.7 eV. The ammonothermal GaN samples have high concentrations of complexes containing gallium vacancy (VGa), hydrogen, and oxygen. The first-principles calculations suggest that the VGa-3Hi complex is the origin of the YL2 band, while the VGa-3ON complex is responsible for the RL4 band. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from GaN substrates fabricated by the ammonothermal growth method was studied in a wide range of temperatures and excitation intensities, both with steady-state and time-resolved PL techniques. Three defect-related PL bands were detected: the ultraviolet luminescence band with the zero-phonon line at 3.27 eV, the Zn-related BL1 band with a maximum at 2.9 eV, and the yellow luminescence band (labeled YL2) with a maximum at 2.3 eV. The YL2 band belongs to an unknown defect and is different from the CN-related YL1 band. Its maximum blueshifts by 0.06 eV with increasing excitation intensity and redshifts by more than 0.1 eV with a time delay after a laser pulse. The YL2 band is preliminarily attributed to a defect complex containing the gallium vacancy. [ABSTRACT FROM AUTHOR]

Perspectives about growth of bulk gallium nitride crystals, fabricating high structural quality gallium nitride wafers and the market demand for them are presented. Three basic crystal growth technologies, halide vapor phase epitaxy, sodium flux, and ammonothermal, are described. Their advantages and disadvantages, recent development, and possibilities are discussed. The main difficulty with crystallization of thick GaN is determined. Some new solutions for bulk growth are proposed. It is shown that only crystallization on high structural quality native seeds will ensure proper progress. New ideas for fabricating gallium nitride crystals and wafers with a better control of their structural properties and point defect concentration are proposed. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) spectroscopy was used to study the point defects in 2 × 1017–1019 cm−3 C-doped GaN substrates grown by hydride vapor phase epitaxy. The intensity of an isotropic signal with g = 1.987 ± 0.001 increased monotonically with the carbon concentration, indicating that the EPR signal represents a carbon-related defect. In each sample, the signal intensity increased under illumination with photon energy greater than 2.75 eV, and the photo-induced signal decreased with subsequent illumination at 0.95 eV. A second signal, well-documented to be a shallow donor, appeared along with the g = 1.987 signal in the most lightly doped samples. The appearance of the donor confirms that the photo-induced increase is caused by excitation of an electron to the conduction band and implies that a defect level for the carbon-related center is about 1 eV above the valence band edge, consistent with temperature-dependent Hall measurements. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate a type of point defect present in 1019 cm−3 carbon-doped GaN substrates grown by hydride vapor phase epitaxy. A broad, isotropic resonance at g ∼ 1.987 was observed at 3.5 K, and the EPR intensity increased with illumination at energies greater than 2.75 eV and decreased with photon energies greater than 0.95 eV. The latter is consistent with a deep level of 0.95 eV above the valence band maximum and implies that the associated defect likely participates in donor compensation. The ionization energy for this defect is close to the predicted value for the (−/0) transition level of CN and transition levels associated with Ga vacancies such as VGa and VGa-ON-2H. [ABSTRACT FROM AUTHOR]

A point defect in acceptor-doped bulk free standing GaN grown by the high nitrogen pressure solution method (HNPS) was investigated using electron paramagnetic resonance (EPR) spectroscopy. A nearly isotropic spectrum with g-value of 1.984 is observed after illumination with 2.8 eV light and is stable after removal of the light. Comparison with earlier work on p-type GaN films and SIMS data on the HNPS substrates suggests that the defect is related to an acceptor, heavily compensated during growth by oxygen. Time-dependent photo-EPR data are well modeled as a single defect-to-band transition where the defect level is 0.67 eV above the valence band edge. Although Mg- and Be-doped samples were studied, only those samples containing Be produced the EPR signal. This observation, coupled with the similarity of the defect level measured to that predicted from the theory of others, suggests that the defect is related to a Be impurity. [ABSTRACT FROM AUTHOR]

A photo-induced electron paramagnetic resonance (EPR) attributed to beryllium-related acceptors was identified in GaN:Be substrates grown by the high nitrogen pressure solution technique. The acceptors, initially compensated by shallow O-related donors, were observed after illumination with photon energy greater than 2.7 eV. To adequately fit the time-dependent photo-EPR data over time periods up to 90 min, a two-defect model was developed based on three charge transfer processes: (1) photo-excitation of electrons from compensated acceptors, (2) electron capture by the positively charged donors and neutral acceptors directly from the conduction band, and (3) electron transfer from the donors to acceptors. The analysis of the spectral dependence of the optical cross section leads to the Be-related acceptor level lying 0.7 eV above the valence band maximum, consistent with the role of the acceptor as a compensating center as well as the 2.2 eV luminescence that others observed from these and other GaN:Be samples. [ABSTRACT FROM AUTHOR]

• Crystallization of GaN:Si,C was carried out to examine the role of C in n-type GaN crystals. • Intentional incorporation of high concentrations of Si and C is possible. • Raman spectroscopy shows low-frequency modes in co-doped crystals, suggesting SiC precipitates. • YL peak's position and intensity vary with dopant specifics, hinting at Si-C complexes or V Ga. In this work co-doping with silicon and carbon in gallium nitride grown by halide vapor phase epitaxy was performed. Native seeds of high structural quality with bowing radii of (0 0 0 1) crystallographic planes higher than 15 m and threading dislocation density on the order of 5 × 104 cm−3 were used. The crystallized material was examined in terms of structural, optical, and electrical properties. For this purpose different characterization methods: X-ray diffraction, Raman spectroscopy, low-temperature photoluminescence, and Hall measurements, were applied. The physical properties of samples doped only with silicon and co-doped with carbon were analyzed and compared. However, during the crystallization process, due to high amount of Si and C, clusters of silicon carbide were unintentionally formed. The formation of these SiC clusters had an impact on the properties of the GaN crystals that were being studied. The research provides an initial insight into the complex studies of co-doping gallium nitride crystals. The results of this paper give us groundwork for more in-depth exploration of this topic. [ABSTRACT FROM AUTHOR]

Manganese diffusion in ion implanted gallium nitride crystals was investigated. The ions were implanted into a sample composed of a GaN layer grown using halide vapor phase epitaxy on an ammonothermal gallium nitride substrate. The sample was polished into an atomically flat state prior to the implantation. Manganese ions were implanted into the surface of a (0001) plane (also known as the c-plane) using the beam energy of 230 keV and ion fluence of 5E15 cm−2. The beam was tilted relative to the sample's normal to prevent channeling. After the implantation, ultra-high pressure annealing method was used to both, remove the post-implantation damage and induce the diffusion of implanted Mn ions. Manganese profiles were analyzed along the c-direction using secondary ion mass spectrometry after 5 and 30 h of annealing in temperatures ranging from 1473 K to 1753 K. Two diffusion mechanisms were observed. The diffusion parameters, activation energy and temperature-independent diffusion coefficients, are presented for each mechanism. Additionally, the structural quality of the samples is assessed after 5 and 30-hour annealing in the aforementioned temperature range through the means of X-ray diffractometry. • Mn-implanted HVPE-GaN was annealed in ultra-high-pressure conditions. • Two Mn diffusion mechanisms were identified & described. • Models for the aforementioned diffusion mechanisms are proposed. • X-ray diffractometry presents a high degree of recovery of the crystal lattice. [ABSTRACT FROM AUTHOR]

Electrical properties of carbon-doped halide vapor phase epitaxy-GaN are presented and discussed. Crystals of the highest structural quality and with different carbon concentrations are investigated. Resistivity and Hall measurements as a function of temperature are analyzed in detail. It is found that the concentration of free holes systematically decreases with the increase of carbon concentration. Such behavior results from the fact that the compensation of the carbon acceptor level increases with the carbon concentration. It is accepted that carbon is amphoteric impurity in GaN, creating an acceptor as well as a donor state, which leads to self-compensation. The analysis of existing electron paramagnetic resonance results is extremely important. It enabled us to determine the compensation ratio as a function of carbon concentration. A combination of electron paramagnetic resonance, secondary ion mass spectrometry, and Hall data allowed us to conclude that the acceptor level (CN) exhibits rather significant temperature shift equal to 0.35 meV/K. [ABSTRACT FROM AUTHOR]

We report on optically pumped lasing at 500 nm on InGaN laser structures grown by plasma assisted molecular beam epitaxy. The InGaN laser structures were grown under group III-rich conditions on bulk (0001) GaN substrates. The influence of the nitrogen flux and growth temperature on the indium content of InGaN layers was studied. We demonstrate that at elevated growth temperatures, where appreciable dissociation rate for In-N bonds is observed, the indium content of InGaN layers increases with increasing nitrogen flux. We show that growth of InGaN at higher temperatures improves optical quality of InGaN quantum wells, which is crucial for green emitters. The influence of piezoelectric fields on the lasing wavelength is also discussed. In particular, the controversial issue of partial versus complete screening of built-in electric field at lasing conditions is examined, supporting the former case. [ABSTRACT FROM AUTHOR]

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 °C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values ρ and the ρ(T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, E[sub A]approx. 0.15 eV), and the second one much more deep, E[sub 2]approx. 0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ<10[sup 4] Ω cm at ambient temperature) in samples with dominant E[sub A] states and very high resistivity (ρ>10[sup 6] Ω cm at ambient temperature) in samples with dominant E[sub 2] states. For the first type of samples, annealing at T[sub ann]<500 °C leads to a decrease of their resistivity and is associated with an increase of the effective concentration of the shallow Mg acceptors. Annealing of both types of samples at temperatures between 600 and 750 °C leads to an increase of the deep state concentration. The presence of hydrogen ambient during annealing of the low-resistivity samples strongly influences their properties. The increase of the sample resistivity and an appearance of a local vibrational mode of hydrogen at 3125 cm-1 were observed. These effects can be removed by annealing in hydrogen-free ambient. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract We report on electrical properties of vertical n-GaN high voltage Schottky diodes (SBDs) grown by two different techniques: Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) on highly-conductive n-type Ammono-GaN substrate. The thermionic emission (TE) current model has been applied for diodes parameters analysis. The fabricated SBDs exhibited a breakdown voltage of 670 V and 220 V, barrier height of 1.05 eV and 0.92 eV, ideality factor of 1.65 and 1.42 and series resistance of 440 Ω and 12 Ω for HVPE and MOCVD samples, respectively. Finally, we demonstrate preliminary point defects analysis for both types of the samples. [ABSTRACT FROM AUTHOR]

Abstract A comprehensive study of the bulk ~400 µm thick GaN samples, grown by HVPE technology on AT-GaN seeds, and containing different carbon doping is reported. Spectroscopy of point centres has been implemented by combining several contactless/optical techniques: time resolved photoluminescence (TR-PL), microwave probed photoconductivity (MW-PC) transient and pulsed-photo-ionization spectroscopy (PPIS) techniques, together with steady-state PL (SS-PL) and transmission spectral measurements. The dynamics of the non-radiative and radiative recombination dependent on the excess carrier density and incorporated carbon concentration has been revealed. The cross-section of the photon-electron interaction and broadening factors due to electron-phonon coupling have been evaluated for various centres. Profiling of the excess carrier decay transients by scanning a wafer-edge boundary has been employed for estimation of surface recombination velocity. The asymmetry of surface recombination in heavily carbon doped samples has been corroborated by SS-PL intensity changes when comparing spectra recorded by exciting the opposite wafer surfaces using UV light. It has been inferred that the surface and bulk non-radiative recombination with 70 ns carrier lifetime is efficient only within initial stages of MW-PC and TR-PL transients. Several traps, tentatively attributed to C N O N , C N , C I , V Ga point defects, have been deduced from pulsed photo-ionization spectroscopy. The C N - defect appeared to be the most efficient in redistribution of the radiative recombination. For these centres, the parameters such as the photon-electron interaction cross-section, broadening factor due to electron-phonon coupling, and concentration have been evaluated for GaN samples, carbon doped with different levels. The C N V Ga , C I C Ga, C Ga V N complexes have also been implied by considering TR-PL and SS-PL spectra. The conversion from absorption to emission spectra has been revealed and explained based on van Roosbroeck-Shockley approach. The long-tail photoluminescence decay with durations up to tens of ms has been explained by nearly resonant photo-ionization and photoluminescence transitions in sequence of self-sustaining processes when local excitation travels by hopping over the re-absorption lengths. [ABSTRACT FROM AUTHOR]

Abstract AlGaN/GaN-superlattice based diode structures under study were grown by metal organic chemical vapour deposition, then implanted with two different Eu ion beam fluences, and further submitted to high temperature and high pressure thermal annealing treatments. The effects of Eu implantation and annealing on the diode structure emission characteristics were investigated at 14 K and room temperature. Eu3+ luminescence was observed in both Eu-implanted and annealed samples, being the most intense emission assigned to the 5D 0 → 7F 2 transition in the red spectral region (~621 nm), in line with the main observed behaviour of Eu3+ ions implanted in GaN layers. The combined excitation/emission spectra revealed that the measured intraionic emission corresponds to an overlap of two principal emitting centres which can be excited through above and below GaN bandgap. In addition, a considerable reduction of the non-radiative channels was observed for the diode structure implanted with the highest Eu fluence, inducing a higher thermal stability of the intra-4 f 6 lines intensity. In contrast to GaN layers, the perceived Eu3+ red emission observed at room temperature was found to be strongly sensitive to the GaN above bandgap excitation (out of the resonance conditions) suggesting that additional energy transfer between the AlGaN/GaN superlattice excitons and the Eu3+ ions occurs, therefore widening the excitation pathways for the red luminescence. Highlights • Eu implantation/annealing effects on AlGaN/GaN-superlattice diode optical properties. • Implantation/annealing conditions favour the multiple Eu3+ centre optical activation. • Higher thermal stability of the intra-4 f 6 lines for the highest Eu fluence diode. • Faster decay obtained for the yellow luminescence versus red Eu3+ luminescence. • The 325 nm excitation is the most efficient for providing the perceived red emission. [ABSTRACT FROM AUTHOR]

Highlights • Thick freestanding HVPE-GaN with very low dislocation density. • Biaxial stress free substrate for homoepitaxy growth. • Probing uniform areal and volumetric distribution of pervasive impurities. • Identification of intrinsic pervasive donor and acceptor impurities. • High free carrier mobility. Abstract We investigated the structural, optical, and electrical properties of thick freestanding HVPE GaN films deposited on and then removed from substrates consisting of thick HVPE-GaN deposited on ammonothemal-substrates. High-resolution XR diffraction of both Ga- and N-polar faces and XR topography studies confirmed high crystalline quality. Raman scattering measurements carried on both polar faces of these samples confirm that they are free of biaxial stress, and also confirm that the Si concentration increases with growth, as verified by SIMS analysis. This variation in the incorporation of the Si shallow impurity was also observed in the photoluminescence of these samples. Electrical transport measurements are consistent with good electrical properties. The results strongly suggest that this type of substrate could be conveniently tailored for the fabrication of high performance devices. [ABSTRACT FROM AUTHOR]

We report on vertical n-GaN high voltage Schottky diodes grown by metal organic chemical vapour deposition and hydride vapour phase epitaxy on conductive ammono-GaN substrate. The thermionic emission current model has been applied for diodes analysis and parameters extraction. Finally, we demonstrate that breakdown voltage as high as 670 V for such structures can be achieved. [ABSTRACT FROM AUTHOR]

The paper describes a Transmission Electron Microscopy study on the structural quality of GaN samples grown by Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal (Am) GaN seeds. The only difference between these samples is the crystallographic direction of misorientation of the Am GaN seeds for the further growth. The studied samples were 1° misoriented in two perpendicular crystallographic directions: the [1 1 0 0] and the [1 1 2 0]. Cross-section samples were studied and showed that growth of homo-epitaxial GaN on the substrate tilted toward the [1 1 0 0] direction appeared to lead to lower defect density than those tilted toward the [1 1 2 0] direction. This was associated with slow growth on {1 0 1 1} planes that are oriented “edge-on” for the growth along [1 1 2 0] direction leading to the formation of facets while the [1 1 0 0] direction is devoid of such facets. The competition between slow and fast growth rates on particular planes would lead to the change of the crystal shape such as triangle or trapezoidal one. Our present study also found that cracks in the samples tilted toward the [1 1 2 0] direction are formed and they are the origin for half loops. Moreover, dislocation propagation from these cracks is observed. Such cracks were not observed for the samples tilted toward the [1 1 0 0] directions, probably due to more uniform growth. There were no dislocations at the sample surface. However, some parasitic growth occurred on the wafer edges for both samples. Cross-section microscopy found high dislocation density in these areas with and a majority of them are edge dislocations. [ABSTRACT FROM AUTHOR]

Abstract Recent progress in ammonothermal technology of bulk GaN growth in basic environment is presented and discussed in this paper. This method enables growth of two-inch in diameter crystals of outstanding structural properties, with radius of curvature above tens of meters and low threading dislocation density of the order of 5 × 104 cm−2. Crystals with different types of conductivity, n-type with free electron concentration up to 1019 cm−3, p-type with free hole concentration of 1016 cm−3, and semi-insulating with resistivity exceeding 1011 Ω cm, can be obtained. Ammonothermal GaN of various electrical properties is described in terms of point defects present in the material. Potential applications of high-quality GaN substrates are also briefly shown. [ABSTRACT FROM AUTHOR]

Presents a study that investigated homoepitaxial deposition of a GaN overlayer onto a bulk single-crystal GaN substrate using molecular beam epitaxy. Comparison of structural quality of deposited GaN overlayer with heteroepitaxially grown layers; Photoluminescence of bulk GaN and the deposited homoepitaxial GaN layer; Benefits for the development of benchmark values for the optoelectronic properties of GaN thin films.

Room-temperature photoluminescence (λ exc = 320 nm) of Eu-implanted samples as seen by an observer. The emitted spectrum depends on both the Eu fluence and the annealing temperature. Both of the aforementioned parameters change the relative concentrations of Eu-X (X = C N , V N , O N , V Ga , Si Ga , Mg Ga and O N -Mg Ga) type luminescent defects which results in unique light emitting characteristics. [Display omitted] • Eu-implanted ammonothermal GaN was annealed in ultra-high-pressure conditions. • Diffusion of Eu was negligible after 26 h-long annealing at 1753 K. • Photoluminescence attributable to Eu-X defect complexes was observed. • Surface roughness drastically increased when Eu solubility limit was surpassed. Europium doping of gallium nitride using a novel ultra-high pressure annealing method was investigated. Ammonothermal gallium nitride substrates (n-type) were used as europium ion implantation targets using beam energy of 490 keV and ion fluences ranging from 1 × 1015 to 1 × 1016 Eu/cm2. The implanted samples were annealed at temperatures between 1473 K and 1753 K in high nitrogen pressure. Europium concentration profiles were analyzed along the [0001] crystallographic direction. The morphology and structural quality of implanted and annealed samples were examined. The "out-diffusion" phenomenon was noted. The presence of optically active Eu Ga -X defect complexes was suggested based on the analysis of photoluminescence spectra measured at low temperature (20 K). Limitations of diffusion-based europium doping of ammonothermal gallium nitride are discussed. [ABSTRACT FROM AUTHOR]

Contactless electroreflectance (CER) has been applied to study optical transitions between the ground and excited states in polar and a-plane nonpolar 2 nm wide GaN/Al0.12Ga0.88N quantum well (QW) structures. In addition to the fundamental transition, CER features related to optical transitions between excited states were clearly observed for the polar QW structure whereas such features were not observed for the nonpolar QW structure. This experimental result clearly shows that the polarization-related electric field leads to a quantum confinement of some extra states in the polar QW system. Such states are not confined in the nonpolar QW and, therefore, optical transitions between them are not detected, i.e., only the fundamental transition is observed in CER spectrum of the nonpolar QW. [ABSTRACT FROM AUTHOR]

We have used positron annihilation spectroscopy to study the thermal behavior of different native vacancy defects typical of freestanding GaN grown by hydride vapor phase epitaxy under high pressure annealing at different annealing temperatures. The results show that the VGa-ON pairs dissociate and the Ga vacancies anneal out from the bulk of the material at temperatures 1500–1700 K. A binding energy of Eb=1.6(4) eV can be determined for the pair. Thermal formation of Ga vacancies is observed at the annealing temperatures above 1700 K, indicating that Ga vacancies are created thermally at the high growth temperature, but their ability to form complexes such as VGa-ON determines the fraction of vacancy defects surviving the cooling down. The formation energy of the isolated Ga vacancy is experimentally determined. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Gallium nitride crystals were grown by hydride vapor phase epitaxy using solid iron as a source of dopants. Three crystal growth processes were performed at constant HCl flow over the solid iron and with different gallium chloride flows. High structural quality ammonothermal GaN was used as seed material. No yellow luminescence and only weak near band edge luminescence were visible in all grown crystals. A sharp peak was observed at 1.298 eV. This was shown before as an intrinsic transition of Fe impurity in GaN. The grown crystals were highly resistive at room temperature. High-temperature Hall effect measurements revealed n-type conductivity with activation energy equal to 1.8 eV. Secondary ion mass spectrometry indicated the presence of manganese in all three samples. The concentration of manganese was always higher than concentration of iron in the doped GaN. [ABSTRACT FROM AUTHOR]

Results of growth of high structural quality gallium nitride single crystals doped with silicon are described in this paper. Dichlorosilane was used as precursor of silicon in the hydride vapor phase epitaxy method. Crystallization runs with different flows of dichlorosilane were performed and compared. One-inch free-standing HVPE-GaN crystals of high structural quality and high purity, previously grown on ammonothermal GaN substrates, were used as seeds. Structural, electrical, and optical properties of HVPE-GaN doped with silicon are presented and discussed in detail. A laser diode built on the homoepitaxially grown GaN is demonstrated. [ABSTRACT FROM AUTHOR]

Investigation of source of stress in HVPE-GaN grown in c-direction on native seeds is reported. The study was based on experimental results. Ammonothermally grown GaN seed was used for growth of a thick HVPE-GaN layer in the c-direction. Deterioration of quality of the HVPE-GaN layer appeared. Numerical simulations were used for determining and examining radial, tangential, and axial stress distributions in the grown GaN crystal and the seed. The maximum equivalent stress (von Mises stress) was calculated and analyzed. It was shown that growth in non-polar and semi-polar directions, which appears on the edges of the crystal during crystallization process, can generate a very large tensile stress in GaN growing in the c-direction. This stress is much larger than the one generated due to a lattice mismatch between the ammonothermal seed and the deposited HVPE layer. [ABSTRACT FROM AUTHOR]

Advanced Substrates consist of a thin GaN layer bonded to a carrier wafer. The layer is separated from starting material by Smart Cut™ technology. GaN on sapphire Advanced Substrates were successfully used as seeds for HVPE-GaN growth. Unintentionally doped and silicon-doped thick GaN layers were crystallized. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates. Free-standing HVPE-GaN with high free carrier concentration was obtained. A laser diode was built on the n-type doped HVPE-GaN grown on an Advanced Substrate. [ABSTRACT FROM AUTHOR]

High crystallinity thick films with low free carrier concentration (≤1×10 15 /cm 3 ) and low compensation are required for many GaN-based electronic device applications. It has been demonstrated that low pressure chemical vapor and molecular beam epitaxy techniques can reproducibility deposit homoepitaxial films with low residual impurity concentrations. However, their typical slow growth rates prevent their utilization for thick film growth. Presently, hydride vapor phase epitaxy is the sole method that can deposit films with residual impurity concentrations ≤5×10 16 /cm 3 at hundreds of microns per hour growth rate. It is crucial to verify if this method can reproducibly deliver thick free-standing GaN films of high crystalline quality with exceptionally low and uniform free carrier concentration. X-ray diffraction, Raman scattering, and low temperature photoluminescence experiments were carried out on a number of samples prepared by dicing a free-standing wafer into several pieces perpendicular and parallel to the major growth directions; namely, c -plane {0001}, a -plane {11−20}, and m -plane {1−100}. SIMS depth profiles were employed to identify and quantify the concentration of the pervasive impurities. Spatial maps of a Raman line sensitive to free-carrier concentration were measured to determine the spatial distribution of the net impurity concentration. The reduced concentration of un-compensated shallow donors was also verified by low temperature electron paramagnetic resonance. [ABSTRACT FROM AUTHOR]

Study on the sources of stress in HVPE-GaN layer crystallized on 1-in. ammonothermally grown GaN seed is presented in this paper. Characterization by means of X-ray diffraction and transmission electron microscopy is performed. HVPE-GaN samples of high quality and those with visible quality deterioration are investigated on c-plane and m-plane cross-sections. Special attention is paid to HVPE material growing in semi-polar and non-polar directions on the edges of the seed and the growing layer. It is shown that this material generates significant stress leading to a structural deterioration of HVPE-GaN growing in the c -direction. [ABSTRACT FROM AUTHOR]

We studied the incorporation of Mg-related impurity centers in GaN crystals depending on the direction of the crystallization front. Two series of GaN crystals – (i) undoped and (ii) Mg-doped – were grown by High Nitrogen Pressure Solution (HNPS) method under otherwise identical conditions. Each series contained four samples with ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) orientations. The low-temperature photoluminescence (PL) spectroscopy was used for characterization of the obtained crystals. The observed differences in the PL spectra of GaN:Mg crystals suggested that Mg incorporation in GaN grown by HNPS method depends considerably on the orientation of crystallization front. The concentration of Mg impurity incorporated into the GaN crystals subsequently increases for the following sequence of planes: ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) , ( 20 2 ¯ 1 ¯ ) and ( 20 2 ¯ 1 ) . For ( 10 1 ¯ 0 ) , ( 11 2 ¯ 0 ) and ( 20 2 ¯ 1 ¯ ) planes the blue band is related only to O N – Mg Ga donor-acceptor pair (DAP) transitions, while for ( 20 2 ¯ 1 ) plane the incorporation of Mg-H complexes occurs additionally to the formation O N – Mg Ga DAP. [ABSTRACT FROM AUTHOR]

Results of gallium nitride crystallization by ammonothermal method are presented. GaN crystals grown earlier by a HVPE method on an ammonothermal GaN substrate and an MOCVD-GaN/sapphire template were used as seeds. Structural and optical properties of the obtained materials are studied and compared. Large radius of curvature (>100 m) and low dislocation density (7×10 4 cm −2 ) can be reproduced in the ammonothermal method using an HVPE-GaN seed grown before on ammonothermal GaN. This proves that the use of HVPE-GaN grown on ammonothermal seeds allows to reproduce high crystallinity in a subsequent ammonothermal growth. It also demonstrates that a much more effective multiplication process of high quality GaN can be launched using a combination of the ammonothermal and HVPE methods. [ABSTRACT FROM AUTHOR]

One of the main advantages of the hydride vapor phase epitaxy (HVPE) method for crystallizing bulk gallium nitride is the crystallization of GaN with a relatively high growth rate (>100 μm/h). In this paper various growth rates in the c -direction during crystallization of GaN by HVPE on ammonothermally grown GaN crystals are determined and examined. The influence of the highest (380 μm/h) and the lowest (40 μm/h) growth rate on the structural quality and purity of the HVPE-GaN crystals is analyzed. The optimal macroscopically stable growth rate (without cracks and pits) and the way of achieving it are presented and discussed. [ABSTRACT FROM AUTHOR]

The results of the Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal GaN seeds (Am-GaN) with various misorientations: 0.3, 0.5 and 1 degree to the [1 0 −1 0] and [1 1 −2 0] directions are presented. Growth rate and structural quality of the HVPE-GaN layers are analyzed. Morphology of the crystal growing surface at the beginning of the crystallization process (after one and two hours of growth) and at the end of it (after eight hours) is presented as a function of the Am-GaN seed misorientation. Based on these results a rough growth model is proposed and discussed in detail. [ABSTRACT FROM AUTHOR]

Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices. [ABSTRACT FROM AUTHOR]

In this work homoepitaxial HVPE-GaN growth on non-polar and semi-polar GaN seeds was described. Two crystallization processes, in the same experimental conditions but using different carrier gases: N 2 and H 2 , were performed. An influence of growth directions and growth conditions on the growth rate and properties (morphology, structural quality and oxygen and silicon contaminations) of obtained crystals were investigated and discussed. It was shown that the growth rate strongly depends on the growth direction and the carrier gas. It was demonstrated that for the semi-polar [20–21] direction it was possible to obtain high quality and highly conductive (without intentional doping) gallium nitride layers. [ABSTRACT FROM AUTHOR]

Two series of N- and Ga-face GaN Van Hoof structures were grown by plasma-assisted molecular beam epitaxy to study the surface potential barrier by contactless electroreflectance (CER). A clear CER resonance followed by strong Franz-Keldysh oscillation of period varying with the thickness of undoped GaN layer was observed for these structures. This period was much shorter for N-polar structures that means smaller surface potential barrier in these structures than in Ga-polar structures. From the analysis of built-in electric field it was determined that the Fermi-level is located 0.27 ± 0.05 and 0.60 ± 0.05 eV below the conduction band for N- and Ga-face GaN surface, respectively. [ABSTRACT FROM AUTHOR]

Single crystals of Mg-doped GaN grown by high nitrogen pressure solution method in different crystallographic directions ([0001], [...], and [...]) were investigated in order to determine thermal stability of their electrical and optical properties. Obtained dependences of resistivity, the Hall coefficient and energy shift of Mg-related photoluminescence peak on annealing temperature allow to suggest that incorporation of Mg in GaN is significantly influenced by the direction of the crystallization front. [ABSTRACT FROM AUTHOR]

Recent advances in fabrication and commercialization of high-brightness blue and green light-emitting devices based on gallium nitride have renewed intense research of its basic properties. Since information on electron transport processes in GaN is scarce, their systematic studies are highly desirable. The electron inelastic mean free path (IMFP) is a crucial parameter for quantitative interpretation of surface electron spectra. The energy dependence of IMFP for bulk GaN crystals with different surface concentrations of their constituents was obtained from elastic peak electron spectroscopy (EPES) with use of the Ni standard in the energy range 200–2000 eV. The measured IMFPs were compared with the values predicted by the TPP-2M and G-1 formulae. A reasonable agreement was found between the measured IMFPs in bulk GaN with an ideal stoichiometric surface composition and the corresponding calculated IMFPs. Compared with the bulk IMFPs, experimental IMFPs valid for the GaN sample with a thin surface layer enriched in ∼70 at.% Ga are only slightly smaller by 5–10%, depending on the electron energy. [Copyright &y& Elsevier]

In this paper, an overview will be given of the growth of thick GaN layers by hydride vapor-phase epitaxy. Two different kinds of substrates were used, that is MOCVD-grown GaN templates on sapphire and GaN single crystals. The layers grown on sapphire-based substrates suffer from the problem of cracking and pit formation. Although the morphology is not mirror-like, the optical and electrical quality of the material is excellent as demonstrated by photoluminescence and Hall–Van der Pauw measurements. The layers grown on Ga-polar GaN single crystals have almost perfect morphologies with only a very low density of pits. For the N-polar substrates the morphology is very rough, exhibiting the same features as are observed for the N-face MOCVD-grown GaN layers, both on sapphire and on N-face GaN single crystals. [Copyright &y& Elsevier]

Gallium vacancy complexes have been identified in n-type bulk GaN by applying positron annihilation spectroscopy. Their formation is suppressed when the material becomes resistive by Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. In Be-doped GaN vacancies are observed even in resistive material. The positron lifetimes show that their open volume is larger than expected for the N vacancy. A possible identification is a VN–BeGa complex, where the atoms neighboring the N vacancy are strongly relaxed outwards, thus increasing the open volume. [Copyright &y& Elsevier]