The influence of indium on the growth of GaN from solution under high pressure

The influence of significant fraction (10–50 mole%) indium in liquid gallium on GaN crystallization from a ternary Ga–In–N solution was analyzed. Crystallization experiments of GaN on GaN-sapphire templates from Ga–In solutions, at 1350–1450 °C, with prior to the growth seed wetting at 1500 °C, and 1.0 GPa N₂ pressure, without solid GaN source showed faster growth of GaN on the seed (by a factor of 1.5–2) than using pure gallium solvent. Nevertheless the new grown crystals were morphologically unstable. The instability was reduced by decrease of the wetting temperature down to 1100 °C or by omitting the wetting procedure entirely, which indicated that GaN dissolves much faster in Ga–In melt than in pure Ga and that the unstable growth was caused most likely by complete dissolution of GaN template before the growth. It was observed that the crystals grown on bulk GaN substrates did not show morphological instability observed for GaN-sapphire templates. The influence of indium on thermodynamic and thermal properties of the investigated system is discussed.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

MOVPE of AlN-free hexagonal GaN/cubic SiC/Si heterostructures for vertical devices

The heterostructures of GaN/SiC/Si were prepared without using AlN or AlGaN buffer layers (AlN buffers) in the metalorganic vapor phase epitaxy of GaN on SiC. GaN (0 0 0 1) with specular surface was obtained. The AlN buffers are usually used in the conventional growth of GaN on SiC due to the poor nucleation of GaN on SiC. Instead, the nucleation of GaN was controlled by varying the partial pressure of H₂ in the carrier gas, the mixture of H₂ and N₂, during the low-temperature (600 °C) growth of GaN (LT-GaN). After the LT-GaN, the high-temperature (1000 °C) growth of GaN was performed using pure H₂ as the carrier gas. The epitaxial film of cubic SiC (1 1 1) on a Si (1 1 1) substrate was used as the SiC template. Increasing the partial pressure of H₂ in the carrier gas decreased the coverage of SiC surface by LT-GaN. It is suggested that the hydrogen atoms adsorbed on the surface of SiC is preventing the nucleation of GaN.     

Hydrothermal growth of ZnO nanorods on a-plane GaN/sapphire template

ZnO nanorod arrays are grown on a-plane GaN template/r-plane sapphire substrates by hydrothermal technique. Aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were employed as growth precursors. Electron microscopy and X-ray diffraction measurements were carried out for morphology, phase and growth orientation analysis. Single crystalline nanorods were found to have off-normal growth and showed well-defined in-plane epitaxial relationship with the GaN template. The 〈0 0 0 1〉 axis of the ZnO nanorods were observed to be parallel to the 〈1 0 1¯ 0〉 of the a-plane GaN layer. Optical property of the as-grown ZnO nanorods was analyzed by room temperature photoluminescence measurements.     

Stacking faults blocking process in (1 1 −2 2) semipolar GaN growth on sapphire using asymmetric lateral epitaxy

We have succeeded in effectively stopping the propagation of basal stacking faults in (1 1 −2 2) semipolar GaN films on sapphire using an original epitaxial lateral overgrowth process. The growth conditions were chosen to enhance the growth rate along the [0 0 0 1] inclined direction. Thus, the crystal expands laterally until growth above the a-facet of the adjacent crystal seed, where the basal stacking faults emerge. The growth anisotropy was monitored using scanning electron microscopy. The faults filtering and improvement of crystalline quality were attested by transmission electron microscopy, X-ray diffraction and low temperature photoluminescence, which exhibits high intensity band-edge emission with low stacking faults related emission.     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Crystal growth and characterization of GaCrN nanorods on Si substrate

GaCrN nanorods were grown on GaN nanorods by RF-plasma-assisted molecular beam epitaxy. GaN nanorods were grown on Si (0 0 1) substrates with native SiO₂. Cr doping into GaCrN nanorods was conducted at substrate temperatures of 800 and 550 °C. Cross-sectional transmission electron microscopy images revealed that the diameter of GaCrN nanorod gradually increases with growth proceeding at 550 °C, while the growth at 800 °C does not change the nanorod diameter. Low-temperature growth enhances the growth perpendicular to the c-axis and decreases the growth along the c-axis. It was found that the solubility limit of Cr atoms in GaCrN is much higher for the low-temperature growth than for the high-temperature growth. It was also found that the highest saturation magnetization is obtained at some optimum Cr cell temperature.     

Flux growth and characterizations of NdPO4 single crystals

Neodymium phosphate single crystals, NdPO₄, have been grown by a flux growth method using Li₂CO₃-2MoO₃ as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N₂. The specific heat of NdPO₄ crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO₄ crystal were also measured at room temperature.     

Characterization of bulk GaN crystals grown from solution at near atmospheric pressure

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm³, or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm³) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c-axis, with approximate g-values, g_∥=1.951 and g_⊥=1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

Hydride-assisted growth of GaN nanowires on Au/Si(0 0 1) via the reaction of Ga with NH3 and H2

High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH₃ and N₂:H₂ at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH₃, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga₂O₃ NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O₂ under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H₂ dissociates on the Au nanoparticles and reacts with Ga giving Ga_xH_y thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH₃ near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga₂O₃ NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Growth of cubic InN films with high phase purity by pulsed laser deposition

Cubic InN films have been grown on MgO (1 0 0) substrates with cubic GaN buffer layers by pulsed laser deposition (PLD). It has been found that cubic InN (1 0 0) films grow on the GaN (1 0 0)/MgO (1 0 0) structure with an in-plane epitaxial relationship of [0 0 1]_InN∥[0 0 1]_GaN∥[0 0 1]MgO. The phase purity of a cubic InN film grown at 440 °C was as high as 99% that can probably be attributed to the enhanced surface migration of film precursors in case of PLD. These results indicate that PLD is a suitable technique for the growth of high-quality cubic InN films, and will makes it possible to fabricate optical and electron devices based on cubic InN films.     

Thermogravimetric analysis and microstructural observations on the formation of GaN from the reaction between Ga2O3 and NH3

Thermogravimetric analysis (TGA) and microstructural observations were carried to investigate the nitridation mechanism of β-Ga₂O₃ powder to GaN under an NH₃/Ar atmosphere. Non-isothermal TGA showed that nitridation of β-Ga₂O₃ starts at ∼650 °C, followed by decomposition of GaN at ∼1100 °C. Isothermal TGA showed that nitridation follows linear kinetics in the temperature range 800–1000 °C. At an early stage of nitridation, small GaN particles (∼5 nm) are deposited on the β-Ga₂O₃ crystal surface and they increase with time. We proposed a mechanism for the nitridation of Ga₂O₃ by NH₃ whereby nitridation of β-Ga₂O₃ proceeds via the intermediate vapor species Ga₂O(g).     

Optical properties of MOVPE-grown a-plane GaN and AlGaN

a-Plane GaN and AlGaN were grown on r-plane sapphire by low-pressure metal-organic vapor epitaxy (LP-MOVPE), and the effects of reactor pressure (from 40 to 500 Torr) and growth temperature (from 1020 to 1100 °C) on the crystalline quality and surface morphology of a-plane GaN were studied. The a-plane GaN grown under 40 Torr had a smooth-surface morphology but a poor crystalline quality; however, the a-plane GaN grown under 500 Torr had higher crystalline quality and optical properties, whose full-width at half-maximum of the X-ray rocking curve (XRC-FWHM) and intensity of yellow luminescence (YL) were smaller. Furthermore, the optical properties of a-plane GaN were investigated by photoluminescence (PL) in detail. We also studied the emission properties of a-plane Al_0.35Ga_0.65N grown at room temperature.     

Growth of GaN films on PLD-deposited TaC substrates

GaN films were grown by metal organic chemical vapor deposition on TaC substrates that were created by pulsed laser deposition of TaC onto (0 0 0 1) SiC substrates at ∼1000 °C. This was done to determine if good quality TaC films could be grown, and if good quality GaN films could be grown on this closely lattice matched to GaN, conductive material. This was done by depositing the TaC on on-axis and 3° or 8° off-axis (0 0 0 1) SiC at temperatures ranging from 950 to 1200 °C, and examining them using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The GaN films were grown on as-deposited TaC films, and films annealed at 1200, 1400, or 1600 °C, and examined using the same techniques. The TaC films were polycrystalline with a slight (1 1 1) texture, and the grains were ∼200 nm in diameter. Films grown on-axis were found to be of higher quality than those grown on off-axis substrates, but the latter could be improved to a comparable quality by annealing them at 1200–1600 °C for 30 min. TaC films deposited at temperatures above 1000 °C were found to react with the SiC. GaN films could be deposited onto the TaC when the surface was nitrided with NH₃ for 3 min at 1100 °C and the low temperature buffer layer was AlN. However, the GaN did not nucleate easily on the TaC film, and the crystallites did not have the desired (0 0 0 1) preferred orientation. They were ∼10 times larger than those typically seen in films grown on SiC or sapphire. Also the etch pit concentration in the GaN films grown on the TaC was more than 2 orders of magnitude less than it was for growth on the SiC.     

Effects of temperature and carrier gas flow amount on the formation of GaN nanorods by the HVPE method

We fabricated one-dimensional GaN nanorods on AlN/Si (1 1 1) substrates at various temperatures, and carrier gas flow amount, using the hydride vapor phase epitaxy (HVPE) method. An AlN buffer layer of 50 nm thickness was deposited by RF sputtering for 25 min. Stalagmite-like GaN nanorods formed at a growth temperature of 650 °C. The diameters and lengths of GaN nanorods increase with growth time, whereas the density of nanorods decreases. And we performed the experiments by changing the carrier gas flow amount at a growth temperature of 650 °C and HCl:NH₃ flow ratio of 1:40. GaN nanorods, with an average diameter of 50 nm, were obtained at a carrier gas flow amount of 1340 sccm. The shape, structures, and optical characteristics of the nanorods were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence.     

Synthesis of GaN crystal by the reaction of Ga with Li3N in NH3 atmosphere

A novel method to synthesize GaN crystals was studied by the reaction of Ga with Li₃N under NH₃ atmosphere. We have already reported the synthesis technique of GaN by the reaction of Ga₂O₃ with Li₃N. However, the size of GaN crystals obtained by this method was limited to be smaller than several micrometers because of the solid phase reaction. In order to increase the size of GaN crystals, the method using liquid Ga as gallium source was studied for solid–liquid phase reaction. We found that the GaN crystals with the size of more than 100 μm were synthesized at 750 °C for 24 h under NH₃ atmosphere. We propose the possible reaction mechanism as follows. Lithium amide (LiNH₂) is synthesized by the reaction of Li₃N with NH₃ gas and then the crystal growth of GaN occurs by the reaction of Ga with LiNH₂. We found that LiNH₂ is a useful nitrogen source for the GaN synthesis method.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

Sublimation crystal growth of yttrium nitride

The sublimation–recombination crystal growth of bulk yttrium nitride crystals is reported. The YN source material was prepared by reacting yttrium metal with nitrogen at 1200 °C and 800 Torr total pressure. Crystals were produced by subliming this YN from the source zone, and recondensing it from the vapor as crystals at a lower temperature (by 50 °C). Crystals were grown from 2000 to 2100 °C and with a nitrogen pressure from 125 to 960 Torr. The highest rate was 9.64×10⁻⁵ mol/h (9.92 mg/h). The YN sublimation rate activation energy was 467.1±21.7 kJ/mol. Individual crystals up to 200 μm in dimension were prepared. X-ray diffraction confirmed that the crystals were rock salt YN, with a lattice constant of 4.88 Å. The YN crystals were unstable in air; they spontaneously converted to yttria (Y₂O₃) in 2–4 h. A small fraction of cubic yttria was detected in the XRD of a sample exposed to air for a limited time, while non-cubic yttria was detected in the Raman spectra for a sample exposed to air for more than 1 h.