Bulk GaN Crystals Research Articles

Growth of GaN crystals under ammonothermal conditions

ABSTRACTGrowth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 μm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM.

Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature

ABSTRACTWe report the growth and characterization of bulk GaN single crystals by temperature-gradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm−2 have been achieved. Additional characterization of the GaN crystals is also presented.

Study of absorption spectra of Er3+ ions in GaN crystals

The f–f absorption spectra of Er3+ introduced by diffusion into bulk GaN crystals grown by vapour-phase epitaxy in a chloride system have been studied. The wavelength interval of 518–527 nm corresponding to the region of 4I15/2 → 2H11/2 transitions has been studied in detail at 293, 77, and 2 K. At T = 2 K the 4I15/2 → 2H11/2 spectrum contains six lines in accordance with theoretical predictions for Er3+ in a noncubic crystal field. The positions of levels in the 2H11/2 multiplet correspond to 2.360, 2.361, 2.365, 2.369, 2.379, and 2.386 eV. Both the number and narrow width of lines observed at T = 2 K indicate that in our samples Er3+ ions occupy predominantly the same regular position in GaN crystals. Most probably, Er3+ ions substitute Ga atoms in the crystal lattice.

High-temperature strength of III–V nitride crystals

The hardness of AlN and GaN bulk crystals was investigated in the temperaturerange 20–1400°C by the Vickers indentation technique. The hardnesses of GaNand AlN at room temperature are 10.8 and 17.7 GPa, respectively. Withincreasing temperature the hardness shows a gradual decrease and then a steepdecrease from around 1000°C, which indicates the beginning of macroscopicdislocation motion and plastic deformation. The yield strength of GaN bulkcrystals determined directly by means of compressive deformation is around100–200 MPa in the temperature range 900–1000°C, i.e. similar to that of α-SiCand much higher than those of Si, Ge, GaP, GaAs, InP and InAs. On the basisof the observed temperature dependence of yield stress, the activationenergy for dislocation motion at elevated temperatures in GaN is deduced.

Optical spectroscopy of bulk GaN crystals grown from a Na–Ga melt

Colorless transparent platelet and prismatic GaN crystals up to 3–4 mm, grown from a Na–Ga melt (0.6–0.7 mol fraction of Na) at temperatures of 700–800 °C in a modest (5 MPa) pressure of N2, are characterized using Raman scattering, room and low temperature photoluminescence, and reflectance. They exhibit sharp free and bound exciton luminescence features (down to 0.22 meV full width at half maximum), including multiple excited states. Residual Mg and Zn acceptors and a 33.6 meV donor (possibly ON) are identified. Raman spectra suggest free carrier concentrations down to the low to mid 1016 cm−3 range.

Anisotropy of atomic bonds formed by p-type dopants in bulk GaN crystals

The anisotropy of atomic bonds formed by acceptor dopants with nitrogen in bulk wurtzite GaN crystals was studied by means of linearly polarized synchrotron radiation used in measurements of X-ray-absorption spectra for the K-edgeof Mg and Zn dopants. These spectra correspond to i) a single acceptor N bond along the c-axis and ii) three bonds realized with N atoms occupying the ab-plane perpendicular to the c-axis. The Zn dopant formed resonant spectra similar to that characteristic for Ga cations. In the case of the Mg dopant, similarity to Ga cations was observed for triple bonds in the ab-plane, only. Practically no resonant structure for spectra detected along the c-axis was observed. The absorption spectra were compared with ab initio calculations using the full-potential linear muffin-tin-orbital method. These calculations were also used for determination of the bond length for Mg–N and Zn–N in wurtzite GaN crystals and show that introducing dopants causes an increase of the lengths of the bonds formed by both dopants. Extended X-ray-absorption fine-structure measurements performed for bulk GaN:Zn confirmed the prediction of the theory in the case of the Zn–N bond. Finally, it is suggested that the anisotropy in the length of the Mg–N bonds, related to their larger strength in the case of bonds in the ab-plane, can explain preferential formation of a superlattice consisting of Mg-rich layers arranged in ab-planes of several bulk GaN:Mg crystals observed by transmission electron microscopy. Within the sensitivity of the method used, no parasitic metallic clusters or oxide compounds formed by the considered acceptors in GaN crystals were found.

Blue-Laser Structures Grown on Bulk GaN Crystals

Room temperature (RT) pulsed operation of blue (402–427 nm) InGaN multi-quantum well (MQW) structure laser diodes (LDs) grown on bulk GaN crystals have been demonstrated. Substrate crystals were grown with the high nitrogen pressure and high temperature solution method. After substrate preparation the laser structures were grown by the low pressure MOVPE process. The LD structures are fully strained on the whole area of GaN crystals, without mismatch-related defects, being confirmed by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Stripe geometry cavity has a contact area of 10 × 500 μm2. The mirror facets were cleaved and coated with reflection dielectric layers.

Temperature Dependence of the Thermal Conductivity and Phonon Scattering Time of a Bulk GaN Crystal

We have measured photothermal signals of a bulk GaN crystal by the photothermal divergence (PTD) method in the temperature range from 110 K to 370 K. The thermal conductivity and the scattering time of phonons contributing to the thermal conductivity have been evaluated from the photothermal signals. A phonon-defect interaction has been expected to play a crucial role in determining the thermal properties of the sample employed here in the above temperature range.

The absorption spectra of gallium nitride crystals doped with erbium ions

We have measured the absorption spectra of Er3+ ions introduced by diffusion into bulk GaN crystals grown by vapor phase epitaxy in a chloride system. The wavelength interval of 518–527 nm corresponding to the region of the 4I15/2→2H11/2 transition was studied in detail at 293, 77, and 2 K. At 2 K, the spectrum of this transition displays six lines, which correspond to the theoretically possible number of sublevels of the 2H11/2 state of Er3+ ion occurring in a noncubic crystal field. The positions of levels in the 2H11/2 multiplet correspond to 2.360, 2.361, 2.365, 2.369, 2.379, and 2.386 eV. Both the number and narrow width of lines observed at 2 K indicate that Er3+ ions predominantly occupy the same regular position in GaN crystals. Most probably, erbium ions occupy gallium sites in the crystal lattice.

Influence of pressure control on the growth of bulk GaN single crystal using a Na flux

We investigated the influence of N 2 pressure control on the growth of bulk GaN crystals by using a Na flux. GaN single crystals with a maximum dimension of 3 mm could be obtained by this method, though smaller crystals of 0.5–2 mm in size have been grown without pressure control. This suggested significant effects of additional NH 3 gas on growth of GaN crystals in the Na flux method.

Observation of Dark Spots and Dark Lines of GaN Microcrystals Grown by Nitridation of Gallium Sulfide

High-spatial-resolution (50 nm) cathodoluminescence images in the ultra-violet region (365 nm) and in the yellow region (570 nm) were taken at room temperature for GaN bulk crystals under a scanning electron microscope equipped with a monochromator and a real-time RGB-separation detector. The crystals were grown by nitridation of Ga 2 S 3 and had a c-facet and side facets with Miller indices of [110x] or [112y]. Dark spots were observed only on the side facets while dark lines only on a c-facet. Dark lines stretch in the direction of (1100) or (11200) and end as a dark spot on side facets. Dark spot density was 10 9 cm -2 . The nonuniformity of yellow luminescence and its irrelevance to the existence of dark lines suggest an extrinsic defect for the origin of yellow luminescence, such as carbon impurity.

Observation of Dark Spots and Dark Lines of GaN Microcrystals Grown by Nitridation of Gallium Sulfide

High-spatial-resolution (50 nm) cathodoluminescence images in the ultra-violet region (365 nm) and in the yellow region (570 nm) were taken at room temperature for GaN bulk crystals under a scanning electron microscope equipped with a monochromator and a real-time RGB-separation detector. The crystals were grown by nitridation of Ga2S3 and had a c-facet and side facets with Miller indices of 10x or 11y. Dark spots were observed only on the side facets while dark lines only on a c-facet. Dark lines stretch in the direction of 〈100〉 or 〈1100〉 and end as a dark spot on side facets. Dark spot density was 109 cm—2. The nonuniformity of yellow luminescence and its irrelevance to the existence of dark lines suggest an extrinsic defect for the origin of yellow luminescence, such as carbon impurity.

Thermal stability of isolated and complexed Ga vacancies in GaN bulk crystals

We have applied positron annihilation spectroscopy to show that 2-MeV electron irradiation at 300 K creates primary Ga vacancies in GaN with an introduction rate of 1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The Ga vacancies recover in long-range migration processes at 500--600 K with an estimated migration energy of 1.5 (2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300--1500 K), we conclude that they are stabilized by forming complexes with oxygen impurities. The estimated binding energy of 2.2 (4) eV of such complexes is in good agreement with the results of theoretical calculations.

Characterization of 2.5-Inch Diameter Bulk GaN Grown from Melt-Solution

GaN bulk crystals up to 2.5-inch diameter were grown from melt-solution. Properties of these crystals were investigated using Auger electron spectroscopy, X-ray diffraction and photoluminescence.

Characterization of 2.5-Inch Diameter Bulk GaN Grown from Melt-Solution

GaN bulk crystals up to 2.5-inch diameter were grown from melt-solution. Properties of these crystals were investigated using Auger electron spectroscopy, X-ray diffraction and photoluminescence.

Optical and electrical properties of Be doped GaN bulk crystals

Highly resistive GaN : Be was obtained by means of synthesis of Ga+Be with atomic nitrogen under high nitrogen pressure. Activation energy of resistivity is about 1.5 eV. This material exhibits features very different from those observed in highly resistive bulk GaN : Mg. Up to 300 K strong yellow band dominates photoluminescence spectrum in resistive GaN : Be crystals. Positron annihilation studies point to the presence of gallium vacancies, V Ga. In highly resistive GaN:Mg neither yellow band with considerable intensity, nor detectable concentration of V Ga was found. We also discuss the puzzling findings in highly resistive bulk GaN : Be of morphological features typical for highly conducting bulk n-GaN material.

Picosecond Z-scan measurements on bulk GaN crystals

Bulk GaN crystals were characterized by using picosecond laser pulses at λ=0.527 μm and Z-scan techniques. The role of the free-carrier absorption was evaluated by a dynamical, pump-and-probe-type transmitivity measurement. The values of two-photon absorption coefficient (17–20 cm/GW) and refractive index changes at high optical irradiances due to bound (n2=−4×10−12 esu) and free (σr=−1.0×10−20 cm3) electrons in that material were determined.

Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing

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, EA≈0.15 eV), and the second one much more deep, E2≈0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ<104 Ω cm at ambient temperature) in samples with dominant EA states and very high resistivity (ρ>106 Ω cm at ambient temperature) in samples with dominant E2 states. For the first type of samples, annealing at Tann<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.

Light emitters fabricated on bulk GaN substrates. Challenges and achievements.

AbstractWe used high-pressure grown GaN single crystal substrates to fabricate dislocation free optoelectronic devices like light emitting diodes and laser diodes structures. The latter ones demonstrated laser action under optical pumping condition with the threshold of about 200 kW/cm2 at room temperature. In the present paper we would focus on the specific aspects of the homoepitaxial growth by MOVPE method including epi-ready substrate preparation and surface polarity choice. We believe that our results demonstrate clearly the feasibility of device fabrication based on high-pressure grown GaN bulk crystals.

Pressure-Controlled Solution Growth of Bulk GaN Crystals under High Pressure

We have developed the pressure-controlled solution growth (PC-SG) method and indicated the validity of this method for the growth of GaN single crystals. We have investigated the effect of the supersaturation of nitrogen atoms on the size of a GaN single crystal and its morphology. GaN single crystals with a surface area of about 120 mm2 and/or with good morphology were obtained at a rate of increase of nitrogen pressure less than 49 MPa/h. Structural and photoluminescence properties of GaN single crystals were examined and determined that these crystals with good morphology had good crystallinity. Furthermore, we have introduced a new high-pressure furnace and developed the multi-crucible system. The GaN single crystal with a surface area of 300 mm2 was obtained by natural nucleation. The GaN crystal with 47 mm diameter, whose normal axis was the (0001) plane, could be grown on a sapphire substrate.