Bulk GaN Single Crystals Research Articles

Vertical Schottky Contacts to Bulk GaN Single Crystals and Current Transport Mechanisms: A Review

Vertical Schottky Contacts to Bulk GaN Single Crystals and Current Transport Mechanisms: A Review

Local piezoelectric properties in Na-flux GaN bulk single crystals

The local piezoelectricity of a Na-flux GaN crystal grown on a multipoint-seed-GaN template is investigated using piezoresponse force microscopy. The piezoresponse is critically dependent on two types of growth regions that are dominantly formed in the Na-flux GaN crystal: the c-growth sector (cGS), which is grown on top of the point-seed GaN surface with a growth front of (0001) planes, and the facet-growth sector (FGS), which is grown on the side of cGS with {101¯1} facets. Quantitative analyses reveal the GaN surface displacements at cGS that result from the piezoresponses increase with the applied AC voltage: the measured values well reflect the piezoelectric constant of d33 in GaN. The piezoresponses at the FGS and the boundary between the cGS and FGS are less sensitive than that at the cGS. A combination of cathodoluminescence and multiphoton excitation photoluminescence techniques clarifies that a local reduction of the piezoresponse observed in cGS is attributed to microscale FGSs that exist randomly in cGS. The dependence of the piezoresponse on the growth regions is quantitatively discussed from three possible viewpoints that potentially affect the polarization properties of GaN: residual strain, local crystallographic tilting, and inherent carrier distribution. As a result, a carrier screening effect is the most probable candidate to induce reduction of the piezoresponse in the FGSs of GaN crystals.

Computational fluid dynamics simulation study of the gas flow balance in a vertical HVPE reactor with a showerhead for low cost bulk GaN crystal growth

In order to make low cost bulk GaN single crystal with a high growth rate, we have built a vertical HVPE reactor with a showerhead configuration. The flow from the showerhead of the reactor is independently controlled by inner/outer two sets of gas supply lines. This special showerhead design makes the flow model different from the conventional showerhead reactors. We have employed a finite element-based simulator to study the fluid dynamic and crystal growth of this inner/outer flow independently controlled showerhead. Two simulation experiments were performed. One experiment has demonstrated that both growth rate and Ga yield can be improved without compromising the uniformity by controlling the different input parameters of inner and outer flow on the showerhead, as compared with the conventional showerhead. The other experiment has demonstrated that the showerhead can be further optimized by changing the inner/outer area ratio and input parameters.

Correlation between current leakage and structural properties of threading dislocations in GaN bulk single crystals grown using a Na-flux method

This work investigated the electrical and microstructural properties of threading dislocations (TDs) in Na-flux-grown GaN bulk single crystals composed entirely of a c-plane growth sector (cGS). Several microscale Schottky-like contacts fabricated on TD-related etch pits and flat (0001) surfaces were analyzed via a combination of conductive atomic force microscopy, multi-photon excitation photoluminescence and transmission electron microscopy. Significant current leakage was observed in microscale facet growth sectors (μ-FGSs), having high local oxygen impurity concentrations, that had formed in the cGS. Higher leakage currents were found to be associated with bundled a type and a + c type TDs, whereas lower leakage currents were produced by single a type TDs. The leakage current evidently increased as the size of etch pits formed on the μ-FGSs increased. These observations establish a close correlation between current leakage and TD-induced strain fields that affect the distribution of point defects such as oxygen impurities around TDs.

Quantitative analysis of lattice plane microstructure in the growth direction of a modified Na-flux GaN crystal using nanobeam X-ray diffraction

We quantitatively evaluated lattice plane microstructure, which includes lattice plane tilt, spacing, twist, and their fluctuations, in a modified Na-flux GaN bulk single crystal using the synchrotron-based nanobeam X-ray diffraction method. The GaN crystal was fabricated by two-step growth; the first layer had coalescence boundaries as a consequence of faceted growth from the multipoint-seed GaN template, and the second layer grew on the first without faceted growth. Position-dependent ω-2θ-φ mapping analysis revealed in-plane distribution of local lattice plane microstructure along with dislocation morphology around the coalescence boundary and the growth-stage boundary (GSB). Faceted growth from the multipoint seed template led to concentration of a-type dislocations at the coalescence boundary. These dislocations would glide widely on basal planes above the GSB, and then homogeneously propagate toward the surface. As a result, the modified Na-flux GaN crystal had a homogeneous lattice plane microstructure with little bunching of threading dislocations.

Local current leakage at threading dislocations in GaN bulk single crystals grown by a modified Na-flux method

We have investigated the local electrical and structural properties of threading dislocations (TDs) in Na-flux-grown GaN bulk single crystals that consist entirely of c-plane growth sectors. Conductive atomic force microscopy analysis of Schottky-like contacts locally fabricated on dislocation-related etch pits showed a variety of leakage current under reverse bias conditions. Microstructural analysis around TDs revealed a correlation between the current leakage and the structure of the TDs and their surroundings. Microscale faceted growth sectors with a higher oxygen impurity concentration locally formed in the vicinity of TDs had a larger influence on the current leakage than the structure of TDs.

Leakage current analysis for dislocations in Na-flux GaN bulk single crystals by conductive atomic force microscopy

The mechanisms associated with electrical conduction through individual threading dislocations (TDs) in a Na-flux GaN crystal grown with a multipoint-seed-GaN technique were investigated by conductive atomic force microscopy (C-AFM). To focus on individual TDs, dislocation-related etch pits (DREPs) were formed on the Na-flux GaN surface by wet chemical etching, after which microscopic Pt electrodes were locally fabricated on the DREPs to form conformal contacts to the Na-flux GaN crystal, using electron beam assisted deposition. The C-AFM data clearly demonstrate that the leakage current flows through the individual TD sites. It is also evident that the leakage current and the electrical conduction mechanism vary significantly based on the area within the Na-flux GaN crystal where the TDs are formed. These regions include the c-growth sector (cGS) in which the GaN grows in the [0001] direction on top of the point-seed with a c-plane growth front, the facet-growth sector (FGS) in which the GaN grows with {101¯1} facets on the side of the cGS, the boundary region between the cGS and FGS (BR), and the coalescence boundary region between FGSs (CBR). The local current-voltage (I–V) characteristics of the specimen demonstrate space charge limited current conduction and conduction related to band-like trap states associated with TDs in the FGS, BR, and CBR. A detailed analysis of the I–V data indicates that the electrical conduction through TDs in the cGS may proceed via the Poole-Frenkel emission mechanism.

Modification of the yellow luminescence in gamma-ray irradiated GaN bulk single crystal

We report the variation of the yellow luminescence (YL) in GaN bulk single crystals by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17 and 1.33 MeV from a cobalt-60 source. Gamma-ray dose is 160 kGy. The resistivity varies from 30 Ωcm for an un-irradiated sample to 104 Ωcm for gamma-ray irradiated one. The nitrogen displacement in gamma-ray irradiated samples is observed by Rutherford backscattering channeling experiments using proton beam, suggesting the existence of the deep energy level relating to interstitial nitrogen atoms. The YL from the un-irradiated GaN with a peak at 557 nm (2.22 eV) is observed at around 440 nm to 800 nm, whereas that of the gamma-ray irradiated GaN shows a peak at 532 nm (2.33 eV) although the YL spectrum is almost overlapped with un-irradiated ones. Compton electrons emitted by the gamma-ray irradiation induce the shallow donor located at about 50 meV bellow the conduction band. This energy level is close to that of nitrogen vacancy. The modification of YL is attributed to a transition from the shallow donor induced by the gamma-ray irradiation to the native gallium vacancy.

高圧溶液成長法によるバルクGaN単結晶の成長( バルク成長分科会特集 : 結晶成長の科学と技術)

高圧溶液成長法によるバルクGaN単結晶の成長( バルク成長分科会特集 : 結晶成長の科学と技術)

Halogen-free vapor phase epitaxy for high-rate growth of GaN bulk crystals

Here, we propose a halogen-free vapor phase epitaxy (HF-VPE) technique to grow bulk GaN single crystals. This technique employs the simplest reaction for GaN synthesis (reaction of Ga vapor with NH3) and can potentially achieve a high growth rate, a prolonged growth duration, a high crystal quality, and a low cost. The analyses of thick HF-VPE-GaN layers grown under optimized growth conditions revealed that high-quality crystals, both in terms of dislocation density and impurity concentration, are obtained at high growth rates of over 100 µm/h.

Basic 암모노써멀 방법에 의한 벌크 GaN 단결정의 성장 및 특성

Basic 암모노써멀 방법에 의한 벌크 GaN 단결정의 성장 및 특성

Convection patterns and temperature fields of ammonothermal GaN bulk crystal growth process

The natural convection heat transfer in an ammonothermal process for growing GaN bulk single crystals has been examined numerically. We consider only one crystal to simplify the calculation and discuss the relationship between convection patterns and temperature fields. Two types of convection patterns are observed owing to the difference in the crystal radius. When the convection pattern is transformed, the crystal surface temperature decreases as the crystal radius increases.

Numerical Simulation of Heat and Fluid Flow in Ammonothermal GaN Bulk Crystal Growth Process

A numerical simulation of an ammonothermal process for growing GaN bulk single crystals has been performed. The autoclave and growing crystal are assumed to be axisymmetric and the raw material is assumed to be a porous medium. Heat transfer by natural convection is discussed in terms of the open-space ratio of the baffle and the height of the crystals. Simulation results show that a larger open-space ratio, at least 30%, is better for the crystal growth from the viewpoints of average surface temperature and flow direction.

Hydrophobic Growth of GaN Material: Current Status and Future Potential

Due in part to the high breakdown voltage, gallium nitride is among a select few semiconductors investigated for next generation high power electronic devices. However, GaN based devices are currently performing far below their theoretical potential. Manufacturers of GaN HEMTs and FETs must de-rate the voltage of the device to maintain a high reliability. Conventional epitaxial growth of GaN thin films by HVPE, MOCVD, and MBE have been performed on nonnative substrates of Si, SiC, or sapphire, which results in defect formation and ultimately lowers operating voltages. To reduce the defects, several growth techniques have been utilized by researchers to produce bulk GaN single crystals. However, the fabrication of bulk GaN crystals of substantial size and suitable quality has proved quite difficult. This paper will review the current state of the art for the Ammonothermal, Flux growth, and HVPE techniques currently being pursued to produce bulk GaN.

Observation of symmetrically decay of A1(longitudinal optical) mode in free-standing GaN bulk single crystal from Li3N flux method

We report Raman analysis of A1(LO) (longitudinal optical) and E2(high) phonon lifetimes in a bulk GaN single crystal and their temperature dependence from 77 K to 770 K. Both the A1(LO) and E2(high) phonons in GaN were observed decaying primarily into two phonons of equal energy [Klemens model, P. G. Klemens, Phys. Rev. 148, 845 (1966)]. This is a rare example of a high-quality free-standing GaN bulk single crystal displaying abnormal A1(LO) phonon decay. These results will have significant impact on designing and understanding of GaN-based high-speed, high-power electric devices.

Centimeter-Sized Bulk GaN Single Crystals Grown by the Na-Flux Method with a Necking Technique

Centimeter-sized bulk GaN single crystals with large dislocation-free areas were fabricated by the Na-flux method with a necking technique. This necking, which is the key technique in Czochralski growth of dislocation-free Si ingots, was realized using a newly developed GaN point seed. Structural properties of grown crystals were investigated using panchromatic cathodoluminescence (CL) measurements and X-ray diffraction. Prism-shape and well-faceted bulk GaN crystals with dimensions as large as 0.85 cm (width) and 1 cm (length) were grown by this technique. The GaN single crystal grown for 400 h have full-width at half-maximum values for c// and c⊥ as narrow as 42.8 and 32.5 arcsec, respectively, indicating an extremely high quality. Panchromatic CL images of (0001) GaN wafers sliced from grown crystals revealed that large areas of the wafers were dislocation free. We concluded that the necking technique in Na flux GaN growth may be a major breakthrough for fabricating large dislocation-free GaN ingots.

HVPE법으로 성장시킨 GaN substrate 제작과 특성 평가

Bulk GaN single crystal with 1.5 mm thickness was successfully grown by hydride vapor phase epitaxy (HVPE) technique. Free-standing GaN substrates of mm size were fabricate after lift-off of sapphire substrate and their optical properties were characterized properties for device applications. X-ray diffraction patterns showed (002) and (004) peak, and the FWHM of the X-ray rocking curve (XRC) measurement in (002) was 98 arcsec. A sharp photoluminescence spectrum at 363 nm was observed and defect spectrum at visible range was not detected. The hexagonal-shaped etch-pits are formed on the GaN surface in at 5 minutes. The defect density calculated from observed etch-pits on surface was around . This indicates that the fabricated GaN substrates can be used for applications in the field of optodevice, and high power electronics.

Numerical simulation of GaN single-crystal growth process in ammonothermal autoclave – Effects of baffle shape

The numerical simulation of an ammonothermal process for growing GaN bulk single crystals has been performed by using the SC/Tetra computational fluid dynamics software. The autoclave is assumed to be axisymmetric. Heat transfer by natural convection is discussed in the case of flat and funnel-shaped baffles. Simulation results show that the optimum baffle angle is approximately 20°. This result is identical to that obtained in our previous study on the hydrothermal ZnO crystal growth process.

Epitaxial electrodeposition of ZnO thin film on GaN(0001) bulk single crystal

Ga terminated surface of heavily doped conductive GaN(0001) bulk single crystal was used as a rotating disk electrode (RDE) to electrodeposit ZnO thin film employing reduction of O2. Although the native surface was rather inactive for the reduction of O2, it was activated by dipping in HCl and further by prolonged electrolysis to reduce O2 in a Zn2+ free solution. Koutecky–Levich analysis revealed important kinetic constants, such as the standard charge transfer rate constant (k0) of 2.4 × 10–14 cm s–1 and the transfer coefficient (α) of 0.11 at 70 °C for the reduction of O2 at the most activated GaN(0001). Electrodeposition of ZnO from the bath containing ZnCl2 lead to an epitaxial growth of ZnO in a ZnO[100] ‖ GaN[100] alignment as confirmed from the XRD ω scan with θ adjusted to ZnO(102). The higher level of epitaxy was achieved for the more active surfaces of GaN as estimated from narrowing of the full width at half maximum (fwhm) of the peaks in the XRD ω -scan. Such films were also fully covering the surface of GaN as found in the SEM observation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultra-long-lived coherent acoustic phonons in GaN single crystals

We report our experimental and theoretical studies on generation and time-resolved detection of coherent acoustic phonons (CAPs) in bulk GaN single crystals, using the femtosecond optical pump-probe spectroscopy. A train of 100-fs-wide, ultraviolet laser pump pulses with the photon energy far above of the GaN bandgap induced electronic stress at the crystal surface and triggered propagation of CAP oscillations. Subsequently, time-delayed, low-intensity, 100-fs-wide, ultraviolet/infrared (one/two-colour setup) probe pulses monitored CAP propagation either near the surface or deeply inside the GaN crystal. The amplitude of CAP oscillations was on the order of 10-5-10-6 of the probe-beam transient differential reflectivity signal, and was only dependent on the pump-beam absorption coefficient. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to 8000 m/s-the speed of sound in GaN. In our two-colour arrangement, we could detect CAP oscillations as deeply as ~0.1 mm inside the crystal (the pump-probe delay ~13.2 ns), since the CAP signal attenuation was only limited by the absorption of the infrared probe light in GaN. The latter indicated that the intrinsic CAP lifetime in our crystals was actually ultra long and was estimated to be of the order of at least 100 ns.