Bulk GaN Crystals Research Articles

Homogenous indium distribution in InGaN/GaN laser active structure grown byLP-MOCVD on bulk GaN crystal revealed by transmission electron microscopy and x-raydiffraction

We present transmission electron microscopy (TEM) and x-ray quantitative studies of the indium distribution inInxGa1−xN/GaN multiple quantumwells (MQWs) with x = 0.1 and 0.18. The quantum wells were grown by low-pressure metalorganic chemical vapourdeposition (LP-MOCVD) on a bulk, dislocation-free, mono-crystalline GaN substrate. Byusing the quantitative TEM methodology the absolute indium concentration wasdetermined from the 0002 lattice fringe images by the strain measurement coupled withfinite element (FE) simulations of surface relaxation of the TEM sample. In the x-raydiffraction (XRD) investigation, a new simulation program was applied to monitor theindium content and lateral composition gradients. We found a very high qualityof the multiple quantum wells with lateral indium fluctuations no higher thanΔxL = 0.025. The individual wells have very similar indium concentration and widths over the wholemultiple quantum well (MQW) stack. We also show that the formation of ‘false clusters’ isnot a limiting factor in indium distribution measurements. We interpreted the ‘falseclusters’ as small In-rich islands formed on a sample surface during electron-beam exposure.

Characterization of Bulk GaN Crystals Grown From the Solution at Moderate Pressure and Temperature

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Magneto-optical studies of iron impurity in HVPE GaN

We report on optical studies of bulk GaN crystals doped with iron. High-quality freestanding GaN crystals with varying Fermi level position were grown using the hydride vapor phase epitaxy on bulk GaN substrates. Samples with the dominant neutral Fe 3+(3d 5) acceptor state showed a characteristic near-infrared luminescence band around 1.3 eV, consisting of several sharp lines due to the fine structure of the 4T 1(G)– 6A 1(S) internal transitions of isolated Fe 3+(d 5) ions. In a magnetic field, these lines split into several components. This allowed us to determine energy structure of the 4T 1 multiplet in the magnetic field. For samples containing a singly ionized Fe 2+(3d 6) acceptor, absorption due to well-resolved 5E– 5T 2 internal transitions of Fe 2+ was observed. Measurements performed at different temperatures ranging from 7 to 50 K and at magnetic fields up to 13 T enabled us to analyze some sublevels of the 5E ground and the 5T 2 excited state.

A GaN bulk crystal with improved structural quality grown by the ammonothermal method

The realization of high-performance optoelectronic devices, based on GaN and other nitride semiconductors, requires the existence of a high-quality substrate. Non-polar or semipolar substrates have recently been proven to provide superior optical devices to those on conventional c-plane substrates. Bulk GaN growth enables GaN substrates sliced along various favourable crystal orientations. Ammonothermal growth is an attractive method for bulk GaN growth owing to its potential to grow GaN ingots at low cost. Here we report on improvement in the structural quality of GaN grown by the ammonothermal method. The threading dislocation densities estimated by plan-view transmission electron microscopy observations were less than 1 x 10(6) cm(-2) for the Ga face and 1 x 10(7) cm(-2) for the N face. No dislocation generation at the interface was observed on the Ga face, although a few defects were generated at the interface on the N face. The improvement in the structural quality, together with the previous report on growth rate and scalability, demonstrates the commercial feasibility of the ammonothermal GaN growth.

Pattern formation and directional and spatial ordering of edge dislocations in bulk GaN: Microphotoluminescence spectra and continuum elastic calculations

We present a detailed microphotoluminescence study of the long-range strain fields surrounding threading dislocations as well as their interaction and pattern formation in GaN bulk crystals. The stress patterns are detected by tiny energy shifts of the near-band-edge spectral lines and show a dipolelike stress state around the dislocation core of edge- or mixed-type dislocations, with an angular orientation coinciding with high-symmetry crystal directions. We detect tens of micrometers long linear lineups of edge dislocations with sizable effects on the local strain states and photoluminescence peak positions. With continuum elastic strain simulations, we demonstrate that the observed patterns of threading dislocations are energetically favorable. We calculated a binding energy per threading dislocation length of $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\ensuremath{\mu}\mathrm{m}$ against gliding of an edge dislocation within a lineup of edge dislocations of parallel Burgers vector orientation.

Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy

GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×10 9 cm −2 and finally decreased to 2×10 8 cm −2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of ∼8×10 6 cm −2.

Elastic–plastic transition during nanoindentation in bulk GaN crystal

Mechanical properties of a defect-free bulk GaN single crystal has been studied by nanoindentation in the C (0001) surface. Our experiments provide consistent evaluations of Young's modulus ( E = 320 GPa) measured with both Berkovich and spherical indenters. Additionally, Berkovich hardness ( H = 17 GPa) and true hardness ( H t = 25 GPa) were determined. Pop-in events are confirmed to indicate the elastic–plastic transition of the material, and give also consistent yield (maximum) shear stress, τ max = 19 GPa, for both the indenters. To achieve these precise analyses, the effective curvature of the indenter was determined by the Hertz analysis of the contact between the indenter and a diamond crystal, in addition to the Oliver–Pharr method with a standard fused quartz.

Thermal and Electrical Properties of High-Quality Freestanding GaN Wafers with High Carrier Concentration

The thermal and electrical properties of high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy were investigated in the high-carrier-concentration region between 1.0×1018 and 1.24×1019 cm-3. The carrier concentration was almost identical to the Si doping concentration. An electrical resistivity as low as 2.5 mΩ cm was obtained at the carrier concentration n=1.24×1019 cm-3. The electron mobilities from Hall measurements were 441 and 200 cm2 V-1 s-1 for n=1.0×1018 and 1.24×1019 cm-3, respectively, which are significantly higher than those reported in the literature. The thermal conductivity measured by the laser flash method was consistently high in the measured carrier concentration range, i.e., about 2.0 W cm-1 K-1 for n=1.0×1018 cm-3 and 1.87 W cm-1 K-1 for n=1.24×1019 cm-3.

Photoluminescence and Electron Paramagnetic Resonance Studies of Bulk GaN Doped with Gadolinium

Theoretical predictions of possible room temperature ferromagnetism in GaN diluted with manganese strongly motivated studies of nitrides based diluted magnetic semiconductors (DMS) with transition metals or rare-earths. Over the last years, above room temperature ferromagnetism for MBE grown GaN layers weakly doped with Gd has been reported by different researchers [for example see ref.1]. The estimated average value of magnetic moment per Gd ion has been obtained as high as 500 times that of Gd atomic value. It has been argued that the observed enhancement of the average magnetic moment resulted from strong contribution of GaN host lattice to macroscopic magnetization of the whole material [1]. The origin of such behavior remains still puzzling and more studies of GaN:Gd are required to clarify this issue. In this paper we present photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments on strain free GaN bulk crystals of wurtzite structure doped with gadolinium. The samples were grown from the solution of nitrogen in liquid gallium under high (1.5GPa) pressure (HP) of N2 and at elevated temperatures of about 1500 0 C. Doping with Gd was obtained by adding this element to the solution. Both PL and EPR techniques are well known as a powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth in semiconductors, and they have not been exploited up to now for GaN:Gd system. In EPR experiment sharp absorption lines were detected, which could be attributed to Gd 3+ (4f 7 ) state. Identification of the lines was possible by comparison with other semiconductors doped with Gd. The observed Dysonian shape of the lines originated from presence of high concentration of free electrons in measured GaN. However, the estimated value of free electron concentration was definitively lower than in undoped HP GaN, what indicated purification role of Gd from unintentional donors (presumably oxygen). PL studies revealed intensive spectra in two energy range: close to GaN band gap (3.13.6 eV) and in the “red” range of 1.6-1.8 eV. Band-to band transitions, typical for GaN doped with free electron concentration at a high 10 18 cm -3 level, were clearly seen. They were followed by several sharp peaks in direction of lower energy. In the “red” range sharp structure with main line at 1.77eV energy was detected. The FWHM of this line was as small as 5meV. This structure was related to Gd 3+ (4f 7 ) state. Judging from EPR and PL results Gd dopant was present in the studied samples. However, no ferromagnetic behavior of the samples was observed by means of EPR. This creates some doubts if ferromagnetism observed by others was really related to GaGdN system.

Optical properties of InGaN/GaN quantum wells on sapphire and bulk GaN substrate

AbstractWe investigated the recombination dynamics in InGaN quantum well (QW) structures, grown by metal organic chemical vapour deposition on sapphire and bulk GaN crystals. We present the results of time‐integrated and time‐resolved photoluminescence (PL) spectroscopy performed on single QWs with widths of 4.5 nm and 9.5 nm. The recombination mechanisms were studied at 5 K and elevated temperatures up to room temperature. Radiative and nonradiative life‐times were determined. The luminescence properties are strongly influenced by the localization of excitons in a complex potential landscape with localization sites of varying depth. Redistribution of charges from shallow to deeper localization sites were observed. The band profile fluctuations influence the thermal stability of the luminescence. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallization of free standing bulk GaN by HVPE

AbstractGallium nitride was crystallized on 2 inch MOVPE GaN/sapphire substrates by Hydride Vapor Phase Epitaxy. A stable growth has been achieved in long duration (>10 h) processes at growth rates bigger than 100 μm/h. As a result, entirely transparent and colorless bulk crystals with thickness exceeding 2 mm were obtained. The cracks in the thick HVPE GaN layers deposited on the MOVPE GaN/sapphire substrates can appear especially during cooling of the system after crystallization. It is shown that the formation of cracks at cooling is dependent on the gradients in the layer thickness The relaxation of strains in the resulting crystal coupled to the substrate leads to the self separation of GaN from sapphire. (At present 30 x 30 x 2 mm free standing bulk GaN crystals are obtained). The GaN crystals are characterized by defect selective etching (DSE) and X‐ray diffraction. The density of threading dislocations (measured by DSE of (0001) surface) decreases with the thickness of the HVPE layer and becomes lower than 107 cm–2 in the layers thicker than app. 1 mm. The X‐ray rocking curves for (0002) reflection (slit 0.5 x 0.1 mm) are in the range of 80–95 arcsec. However, larger scans reveal bending of crystallographic {0001} planes. The behavior of these “deformed” free standing crystals used as substrates for HVPE re‐growth is also analyzed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport growth of GaN crystals by the ammonothermal technique using various nutrients

The nutrient transport direction for the growth of GaN crystals in ammonobasic solutions was investigated. Growth of GaN crystals via nutrient transport from the cold zone to the hot zone in alkaline solutions by the ammonothermal technique was confirmed using three kinds of nutrients: gallium metal, gallium-containing intermediates and polycrystalline GaN. Ga-containing intermediates are easily formed when using gallium metal or GaN as nutrient, and remain stable at temperatures below 400 °C; GaN is deposited in the hot zone of the autoclave only at high temperatures greater than 500 °C. Transport growth of GaN crystals from polycrystalline GaN nutrient in the cold zone to hydride vapor phase epitaxy (HVPE) seeds in the hot zone in alkali supercritical ammonia was also achieved. The study has clearly demonstrated that gallium-containing intermediates are the stable phases at lower temperatures, and that GaN bulk crystals (substrates) can be grown in the hot zone at high temperatures from ammonobasic solutions by the ammonothermal technique.

Degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals

We have investigated the aging processes in InGaN laser diodes fabricated by metal organic vapor phase epitaxy on low-dislocation-density, high-pressure-grown bulk gallium nitride crystals. The measured threshold current turned out to be a square root function of aging time, indicating the importance of diffusion for device degradation. The differential efficiency, in contrast, was roughly constant during these experiments. From these two observations we can conclude that the main reason for degradation is the diffusion-enhanced increase of nonradiative recombination within the active layer of the laser diode. Additionally, microscopic studies of the degraded structures did not reveal any new dislocations within the active area of the aged diodes, thus identifying point defects as a source of nonradiative processes.

GaN single crystals grown under moderate nitrogen pressure by a new flux: Ca 3N 2

Using a new flux, Ca 3N 2, bulk GaN crystals were grown from Ga melt at 900 °C under a nitrogen pressure of about 0.2 MPa. Optical observations indicated that the grown GaN crystals were transparent hexagonal prisms with length up to 1.5 mm. The morphology of these GaN crystals was characterized by scanning electron microscopy (SEM) and compared with that of GaN crystals grown by using Li and Na flux. Raman scattering examinations revealed that the GaN crystals grew along [0 0 0 1] direction. These results demonstrated that Ca 3N 2 was an effective new flux in the crystal growth of GaN besides the known fluxes of Li, Na and Na–Ca.

Towards identification of degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals

In this work we present a reliability study of low dislocation density InGaN laser diodes fabricated on high-pressure grown GaN monocrystaline substrates. The aging process was performed under pulse current conditions. Degradation of these devices manifests primarily in the increase of the threshold current. Interestingly, the differential efficiency of lasers remains stable at all times. The aging time dependence of the increase of the threshold current precisely follows a square root dependence. These observations suggest that the degradation results from the enhancement of the nonradiative recombination within the device active layers and is related to point defect diffusion.

Growth of thin AlInN∕GaInN quantum wells for applications to high-speed intersubband devices at telecommunication wavelengths

In this article, we report on growth of AlInN∕GaInN multi-quantum-wells (MQWs) with high Al content (93%) by rf-plasma-assisted molecular-beam epitaxy on (0001) GaN/sapphire templates and on bulk GaN crystals. A series of samples with a barrier thickness of 3nm and with different well thicknesses of 1.5–3nm was grown. The wells were doped with Si at a concentration of 5×1019cm−3. Structures grown on (0001) GaN-based substrates are crack-free, as demonstrated by Nomarski contrast and scanning electron microscopy measurements. X-ray diffraction mapping of a and c lattice parameters shows that AlInN∕GaInN MQWs are fully strained and have up to 7% indium in the barriers and up to 10% In in the quantum wells. These structures exhibit intersubband absorptions at room temperature at a wavelength in the range of 2.45–1.52μm. The AlInN∕GaInN strain-compensated MQW structures, having good quality, are very attractive for ultrahigh-bit-rate telecommunication applications at 1.55μm wavelengths. In addition, because of their low average refractive index, they could be used as thick cladding layers for optical waveguides.

Crystallization of GaN by HVPE on pressure grown seeds

Growth of GaN under pressure from solution in gallium results in almost dislocation free plate-like crystals but with size limited to approx. 1 cm (lateral) and 100 μm (thickness). On the other hand, deposition of GaN by HVPE on the pressure grown substrates allows stable crystallization with rates of a few hundreds μm/h. The crystals with a thickness exceeding 2 mm are grown in this way. However, in these thick GaN crystals grown directly on almost dislocation free substrates quite high number of dislocations appears if the crystal thickness exceeds certain critical value probably due to a small lattice mismatch between GaN grown by HVPE and the heavily doped pressure grown substrates Since the critical thickness for defect generation is of the order of 100 μm, almost dislocation free layers (density below 10 4 cm -2 ) thinner than 100 μm were grown. The pressure grown substrates were then removed by mechanical polishing, or conductivity sensitive electrochemical etching (for strongly n-type substrates). Then the HVPE low dislocation density GaN platelets were used as substrates for the growth of a few mm thick bulk GaN crystals.

Ammonothermal Synthesis of III-Nitride Crystals

Ammonothermal synthesis of nitrides is reviewed, with an emphasis on gallium and aluminum nitrides due to their important applications as direct wide band gap semiconductors. Since the crystallization process of nitrides involves the formation of some intermediate compounds during ammonothermal synthesis where a mineralizer is used, some ternary amides and ammoniates of aluminum and gallium with alkali metals or halides are also reviewed briefly. The ammonothermal crystallization of GaN and AlN bulk crystals, which is analogous to the hydrothermal growth of oxides, is introduced. Retrograde solubility, mineralizers, pressure−temperature−volume−concentration (PTVC) relations, phase relations, and transport growth of GaN in alkaline solutions are discussed in detail. Recent progress of GaN single-crystal growth by the ammonothermal technique is reported. We have grown GaN bulk single crystals up to 10 mm2 by 1-mm thick. Issues such as ammonia breakdown, impurity incorporation, and scale-up of the ammonother...

Photoluminescence properties of GaN with dislocations induced by plastic deformation

Fresh (a/3)[1120] dislocations on the (1100) prismatic plane were introduced into GaN bulk crystals by plastic deformation at 950–1000°C. In photoluminescence studies at 11 K, the near-band-edge (3.48 eV) luminescence intensity decreased remarkably in the deformed GaN, which was attributed to the introduction of high-density nonradiative recombination centers during plastic deformation. The yellow-band luminescence (2.22 eV) decreased due to plastic deformation, while several luminescence bands centered at 1.79, 1.92, and 2.4 eV developed. The dependence of PL features on deformation and annealing suggests that yellow luminescence is not related to the native structure of edge dislocations in GaN.

A study of the chemical reactions involved in Li–Ca–N system

The chemical reactions and phases involved in the potential flux system of Li–Ca–N for the growth of bulk GaN crystals have been investigated under varying conditions. It is found that no preferential nitrification of Li or Ca by N 2 in Li–Ca melts at 500 °C. Only the ternary compound LiCaN is identified in the Li–Ca–N system under the present experimental conditions. Static N 2 pressures are found to enhance the formation of LiCaN compared to an N 2 stream. LiCaN forms from two possible pathways: one is a modified metathesis chemical reaction represented by Li 3N+Ca→LiCaN+2Li, and the other is a combination chemical reaction represented by Li 3N+Ca 3N 2→3LiCaN. The formation of LiCaN by the metathesis reaction is thermodynamically favored over the other pathway. In addition, the formation of LiCaN might benefit from a slightly larger initial amount of Li 3N compared with Ca or Ca 3N 2.