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.     

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.     

Effect of NH3 flow rate on m-plane GaN growth on m-plane SiC by metalorganic chemical vapor deposition

This paper reports a study of the effect of NH₃ flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH₃ flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (1 0 1¯ 0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH₃ flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (1 0 1¯ 0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy.     

Ammonothermal epitaxy of wurtzite GaN using an NH4I mineralizer

Purely wurtzite phase needle crystals and epitaxial layers of GaN were grown by the ammonothermal method using an NH₄I mineralizer. The inclusion of zincblende phase GaN was effectively eliminated by increasing the growth temperature higher than 500 °C. Accordingly, an approximately 20-μm-thick GaN epitaxial layer was achieved on the Ga-polar face of a c-plane GaN seed wafer at 520 °C. Although the characteristic deep state emission band dominated the room temperature photoluminescence spectrum, the near-band-edge emission of GaN was observed for both the needle crystals and the epitaxial layers. These results encourage one to grow better quality GaN crystals at a high growth rate under high-temperature growth conditions.     

Bulk GaN crystals grown by HVPE

We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10⁶ cm⁻² near the N-face surface to 1.2×10⁶ cm⁻² near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.     

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.     

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.     

Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs

In this study, we report on the enhancement in the light extraction efficiency of GaN blue LEDs topped with ZnO nanorods. The ZnO nanorods were grown by a two-step hydrothermal synthesis with pre-coated ZnO nanoparticles under optimized condition to give the appropriate size and quality, giving an increase in the light output efficiency of 66%. This improvement is attributed to the optimal rod size and spacing with improved thermal dissipation as compared to light extraction from plain GaN surface. During the ZnO growth on the LEDs, 0.55 M of NH₃ was added and the ZnO sample was later annealed at 475 °C in N₂ ambient, to drive out interstitial oxygen atoms from the tetrahedral unstable site. As a result, a high ratio of UV to orange defect band emission was achieved. The two-step growth of ZnO nanorods on GaN LEDs was effective in generating array of ZnO nanorods which serve as reflector to enhance light extraction from LEDs.     

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.     

In-situ and ex-situ ZnO nanorod growth on ZnO homo-buffer layers

Vertically well-aligned ZnO nanorods were fabricated in-situ and ex-situ on ZnO homo-buffer layers using catalyst-free metal-organic chemical vapor deposition. Field-emission electron microscopy measurements demonstrated that the nanorods were well aligned and had a uniform diameter of 70–100 nm depending on the growth temperature, irrespective of growth conditions, in-situ and ex-situ. X-ray diffraction measurements demonstrated that the ZnO nanorods and the ZnO buffer layers had a wurtzite structure, and that the crystal quality of the nanorods grown on a smooth surface was better than that of the nanorods grown on a rough surface. Field-emission transmission electron microscopy measurements revealed the presence of a disordered layer at the interface of the nanorod and the buffer layer.     

Lu5Ir4Si10 whiskers: Morphology,crystal structure,superconducting and charge density wave transition studies

Highly perfect single crystal whiskers of Lu₅Ir₄Si₁₀ were successfully grown out of the melt. Details of the surface and morphology of the whiskers are presented. X-ray diffraction data confirmed that the whisker structure has the same tetragonal P4/mbm space group symmetry as bulk single crystals with lattice parameters a=12.484(1) and c=4.190(2) Å. By means of field emission scanning electron microscopy, the morphology of the whiskers has been studied. Using a 4-circle X-ray diffractometer we found that whiskers grow along the c-axis direction and all side faces are oriented along the [1 1 0] direction. The mosaicity has been measured and is found to be almost perfect: below 0.15° along the c-axis. According to our transport measurements performed along the c-axis, the whiskers present a sharp superconducting transition at T_c=4.1 K and show a charge density wave (CDW) transition at 77 K. From the hysteresis of the temperature dependance of the electrical resistivity study, the CDW transition is found to be of first order.     

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.     

Growth and characterization of c-plane AlGaN on γ-LiAlO2

This study demonstrates a pure c-plane AlGaN epilayer grown on a γ-LiAlO₂ (1 0 0) (LAO) substrate with an AlN nucleation layer grown at a relatively low temperature (LT-AlN) by metal-organic chemical vapor deposition (MOCVD). The AlGaN film forms polycrystalline film with m- and c-plane when the nucleation layer grows at a temperature ranging from 660 to 680 °C. However, a pure c-plane AlGaN film with an Al content of approximately 20% can be obtained by increasing the LT-AlN nucleation layer growth temperature to 700 °C. This is because the nuclei density of AlN increases as the growth temperature increases, and a higher nuclei density of AlN deposited on LAO substrate helps prevent the deposition of m-plane AlGaN. Therefore, high-quality and crack-free AlGaN films can be obtained with a (0 0 0 2) ω-rocking curve FWHM of 547 arcsec and surface roughness of 0.79 nm (root-mean-square) using a 700-°C-grown LT-AlN nucleation layer.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.     

Heteroepitaxial growth of InN layers on (1 1 1) silicon substrates

Indium nitride (InN) layers were grown on (1 1 1) silicon substrates by reactive magnetron sputtering using an indium target. Atomic force microscope, X-ray diffraction, and Raman spectroscopy analysis revealed that highly c-axis preferred wurtzite InN layers with very smooth surface can be obtained on (1 1 1) silicon substrates at a substrate temperature as low as 100 °C. The results indicate that the reactive sputtering is a promising growth technique for obtaining InN layers on silicon substrates at low substrate temperature with low cost and good compatibility with microelectronic silicon-based devices.     

Structural and optical property studies of CdSe crystalline nanorods synthesized by a solvothermal method

CdSe nanorods are synthesized via a simple solvothermal method at a moderate temperature of 180 °C. The influences of introducing hydrazine hydrate (N₂H₄·H₂O) as the reducing agent, and ammonia (NH₃·H₂O) as the complexing agent and also the reaction temperature, on the morphology and size of the obtained CdSe nanorods are investigated and reported. CdSe nanorods with a mean diameter and length of 25 and 82 nm, respectively, are synthesized and the problem of handling the stacking faults present in the long CdSe nanorods is analyzed. The use of increased quantity of hydrazine hydrate and also prolonged reaction time is found to reduce the stacking faults on the synthesized nanorods. The morphology, phase and the optical properties of CdSe nanoparticles are studied using powder X-ray diffraction, TEM and high-resolution transmission electron microscope (HRTEM), UV–visible absorption spectroscopy and photoluminescence (PL) spectroscopy. The low-resolution TEM images confirm the formation of CdSe nanorods, and also the agglomeration of nanoparticles and the presence of few spherical nanoparticles. The strong PL intensity from the CdSe nanorod at 702 nm confirms a blue shift of 14 nm, when compared with the bulk wurtzite CdSe.     

Theoretical approach to structural stability of GaN: How to grow cubic GaN

We investigated the growth conditions of cubic GaN (c-GaN) by ab initio-based approach which incorporates free energy of vapor phase. It is known that a c-GaN is a metastable phase and wurtzite GaN (h-GaN), which is a stable phase of GaN, is easily incorporated in the c-GaN crystal during growth. h-GaN is formed in the area grown on {1 1 1} faceted surface. In the present study, therefore, we studied the growth conditions of {1 1 1} facet formation in order to suppress h-GaN mixing. The results suggest that we can suppress the {1 1 1} facet formation, i.e., h-GaN mixing, by controlling the growth conditions such as temperature and gallium beam equivalent pressure.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

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.