Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

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.     

Influence of deposition conditions on nanocrystalline InN layers synthesized on Si(1 1 1) and GaN templates by RF sputtering

We present a detailed investigation on the influence of deposition conditions on morphological, structural and optical properties of InN films deposited on Si(1 1 1) and GaN-on-sapphire templates by reactive radio-frequency (RF) sputtering. The deposition parameters under study are nitrogen content in the sputtering gas, substrate–target distance, substrate temperature and RF power. X-ray diffraction measurements confirm the (0 0 0 1) preferred growth orientation and the wurtzite crystallographic structure of the material. For optimized deposition conditions, InN on Si(1 1 1) substrates presents smooth surface with root-mean-square roughness ∼1 nm. Surface quality of the InN films can be further improved by deposition on GaN-on-sapphire templates, achieving root-mean-square roughness as low as ∼0.4 nm, comparable to that of the underlying substrate. The room-temperature absorption edge is located at 1.70 eV. Intense low-temperature photoluminescence peaking at 1.60 eV is observed.     

Isothermal crystallization kinetic of ZnO thin films

Zinc oxide (ZnO) thin films deposited by DC magnetron sputtering were annealed in nitrogen atmosphere at different temperatures ranging from 100 to 500 °C with a step of 100 °C; the annealing time was 6 h. In order to study the film’s crystallization kinetic, their structures were monitored by means of X-ray diffraction (XRD) analysis each hour. Variation in grain size, calculated from the XRD patterns, with annealing time and temperature, obeys the classical parabolic law of grain growth. Exponent n was found to be dependent on the annealing temperature; it ranged from 5.13 to 3.8 with increase in annealing temperature. From the obtained exponent n values we inferred that the grain growth mechanism is mainly governed by the atom jumping across the grain boundary. We have found that the grain growth is characterized by a low activation energy ranging from 22 to 24 kJ/mol.     

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.     

MOVPE growth of InN films using 1,1-dimethylhydrazine as a nitrogen precursor

InN films have been successfully grown on sapphire substrates by MOVPE using trimethylindium (TMIn) and 1,1-dimethylhydrazine (DMHy) with N₂ carrier. DMHy is an advantageous precursor of N as it decomposes efficiently at relatively low temperature (T₅₀=420 °C) compatible with the InN growth. The reactor is specially designed so as to avoid parasitic reaction between TMIn and DMHy occurring at room temperature. The growth feature was studied by varying growth temperature, V/III ratio, TMIn flow and reactor pressure. The InN films were obtained at 500–570 °C and 60–200 Torr with a V/III ratio optimized to 100–200. The In droplets are seen on the grown surfaces, indicating an excess supply of TMIn. It is demonstrated that the InN films grows on the sapphire substrate in a single domain with an epitaxial relationship, [1 01¯ 0]_InN//[1 1 2¯ 0]_sapphire.     

Growth of ZnO crystal by self-flux method using Zn solvent

Zinc oxide (ZnO) single crystal was grown, for the first time, by the self-flux method using a high-purity zinc solvent saturated with oxygen. ZnO crystals were successfully grown in the solvent by moving a growing crystal slowly, at about 17 mm/day along a temperature gradient (1.5 °C/mm) in a furnace, from 1050 to 500 °C. Hexagonal single crystals 1–3 mm in length and 0.5–1 mm in thickness were obtained. Energy-dispersive X-ray diffraction, secondary ion mass spectrometry, and photoluminescence confirmed that the purity and crystallinity of the ZnO crystals grown in this work were high.     

Optimizing the growth of CoSi2 film with oxide-mediated CoSi2 template by silicon cap layer

Applying both template and Si cap technology, we achieved the epitaxial growth of CoSi₂ directly on Si(1 0 0) substrate by rapid thermal annealing (RTA). The crystal quality of CoSi₂ film is found to be significantly dependent on the Si cap thickness. In our work, a good-quality CoSi₂ film with a minimum of χ_min~11.6% and 3.3 Ω/square was obtained as a 15 nm Co with a subsequent 15 nm Si cap layer is deposited on an oxide-mediated CoSi₂ template and followed by an anneal at 1050 °C under N₂ protection; whereas too thin or thick Si cap layer will deteriorate the crystalline quality of CoSi₂. These experimental results are discussed in combination with the simulation of Rutherford backscattering spectroscopy and X-ray reflectivity.     

Improvement in crystallinity and optical properties of ZnO epitaxial layers by thermal annealed ZnO buffer layers with oxygen plasma

ZnO epitaxial layers with treated low-temperature (LT) ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si (1 0 0) substrates. The LT-ZnO buffer layers were treated by thermal annealing in O₂ plasma with various radio frequency (RF) power ranging from 100 to 300 W before the ZnO epilayers growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), and room-temperature (RT) photoluminescence (PL) were carried out to investigate their structural and optical properties. The surface roughness measured by AFM was improved from 2.71 to 0.59 nm. The full-width at half-maximum (FWHM) of the rocking curve observed for ZnO (0 0 2) XRD and photoluminescence of the ZnO epilayers was decreased from 0.24° to 0.18° and from 232 to 133 meV, respectively. The intensity of the XRD rocking curve and the PL emission peak were increased. The XRD intensity ratio of the ZnO (0 0 2) to Si substrates and PL intensity ratio of the near-band edge emissions (NBEE) to the deep-level emissions (DLE) as a function of the RF power was increased from 0.166 to 0.467 and from 2.54 to 4.01, respectively. These results imply that the structural and optical properties of ZnO epilayers were improved by the treatment process.     

1H-1,2,3-triazole,a promising precursor for chemical vapor deposition of hydrogenated carbon nitride

Well-crystallized hydrogenated carbon nitride thin films have been prepared by microwave plasma enhanced chemical vapor deposition (MWPECVD). 1H-1,2,3-triazole+N₂ and Si (1 0 0) were used as precursor and substrate, respectively. Substrate temperature during the deposition was recorded to be 850 °C. The synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photo-electron spectroscopy (XPS) analyses. The plasma compositions were checked by optical emission spectroscopy (OES). XRD observation strongly suggests that the films contain polycrystalline carbon nitride with graphitic structure of (1 0 0), (0 0 2), (2 0 0) and (0 0 4). XPS peak quantification reveals that the atomic ratio of the materials C:N:O:Si is 32:41:18:9. X-ray photo-electron peak deconvolution shows that the most dominant peak of C (1s) and N (1s) narrow scans correspond to sp² hybrid structure of C₃N₄. These observations indicate that 1H-1,2,3-triazole favors the formation of hydrogenated carbon nitride with graphitic phase by CVD method and thus is in good agreement with XRD results. SEM of surface and OES of plasma also support the formation of polycrystalline carbon nitride films from 1H-1,2,3-triazole+N₂ by CVD.     

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.     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

A catalyst-free method to silicon nanowires at relative low temperature

Well-crystallized straight Si nanowires (SiNWs) were successfully prepared in large scale via a facile reaction between NaN₃ and Na₂SiF₆ at 600 °C without using any catalyst. Characterization by X-ray powder diffraction and transmission electron microscopy demonstrates that the as-obtained product is pure-phase cubic SiNWs with diameters of 40 nm or so, and lengths of several micrometers. And the SiNWs with spherical tips can be obtained at a temperature as low as 300 °C. Heating temperature and holding time have crucial influence on the synthesis and morphology of the SiNWs. An oxide-assisted growth mechanism is responsible for the formation of the SiNWs.     

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.     

The effect of thermal annealing on the structural and mechanical properties of a-C:H thin films prepared by the CFUBM magnetron sputtering method

a-C:H films were prepared by closed-field unbalanced magnetron (CFUBM) sputtering on silicon substrates using argon (Ar) and acetylene (C₂H₂) gases, and the effects of post-annealing temperature on structural and mechanical properties were investigated. Films were annealed at temperatures ranging from 300 °C to 700 °C in increments of 200 °C using rapid thermal annealing equipment in vacuum ambient. Variations in microstructure were examined using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Surface and mechanical properties were investigated by atomic force microscopy (AFM), nano-indentation, residual stress tester, and nano-scratch tester. We found that the mechanical properties of a-C:H films deteriorated with increased annealing temperature.     

Investigation on the crystal growth process of spherical Si single crystals by melting

Spherical Si single crystals with a diameter of approximately 1 mm were grown by melting for solar cell applications. The start sources were spherical Si multicrystals fabricated by a dropping method, which had various irregular shapes. Spherical Si multicrystals were melted into droplets and recrystallized on a quartz plate sample holder that was coated with Si₃N₄. It was found that a surface coating of SiO₂ layer on the start sources and oxygen atmosphere during melting and recrystallization were essential to achieve almost perfect spherical shape. Defect-free single crystalline spherical Si could be obtained at recrystallization temperature ranging from 1400 to 1330 °C, corresponding to an undercooling ranging from 14 to 84 °C, with a yield of nearly 100%. At recrystallization temperatures higher than 1380 °C, the recrystallized spherical Si crystals were almost perfect spheres, whereas small protuberances were formed when the recrystallization temperature was lower than 1360 °C. It was also found that that melting at a temperature close to the melting point of Si (at ~1414 °C), a slow cooling rate of ~1 °C/min before recrystallization and relatively fast cooling rate of ~20 °C/min after recrystallization were important for achieving high carrier lifetime. The average carrier lifetime was greatly improved from lower than 2.5 μs of start sources up to ~7.5 μs by melting at optimized conditions. The influences of residual oxygen on the carrier lifetime of recrystallized spherical Si are discussed based on the measurement results with Fourier transform infrared spectrometer.     

Flux growth and morphology analysis of Na3VO2B6O11 crystals

Colorless and transparent Na₃VO₂B₆O₁₁ (NVB) crystal has been grown by the top seeded solution growth method using NaVO₃ as the flux at cooling rates of 0.8–1.5 °C/day, in the temperature range 610–650 °C. A well-developed morphology of the crystals was observed and analyzed. The grown crystals were characterized by powder X-ray diffraction (PXRD), infrared spectroscopy and second harmonic generation (SHG) test.     

Growth and characterization of calcium lithium tantalum gallium garnet single crystal

A new disordered garnet single crystal, calcium lithium tantalum gallium garnet (CLTGG) was grown by the Czochralski method. The lattice parameter of the crystal has been determined to be a=12.508±0.001 Å by X-ray powder diffraction (XRPD). The melting point of CLTGG was measured to be 1548 °C. The crystal possesses a wide transmission range from the ultraviolet to infrared, which is advantageous for applications as a laser host material. The refractive indices were also measured by the V-prism method.     

Structural and optical properties of 6CaO·6SrO·7Al2O3 thin films derived by sol-gel dip coating process

The nanostructured 6CaO·6SrO·7Al₂O₃ (C6S6A7) thin films with cubic structure using calcium, strontium metals, aluminium isopropoxide and ethylene glycol monomethyl ether as stating materials has been fabricated via sol-gel route. Based on hydrolysis of Ca²⁺, Sr²⁺ and Al³⁺ in the sol-gel processing using ethylene glycol monomethyl ether as solvent have been employed as the precursor material. The films were coated on soda lime float glass by the dip coating technique and annealed at 450 °C in air atmosphere. The structure, morphology and composition of the films were investigated by Fourier transformed infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy indicating that the films were composed of C6S6A7 nanoparticles with cubic structure. The spectral transmittance of the films was measured in the wavelength range of 200-1100 nm using an UV-visible spectrometer. It has been found that the optical properties of the films significantly affected by precursor chemistry and annealing temperature due to the improvement of the crystallinity of the films with increasing annealing temperature and became stable when the annealing temperature is higher than 450 °C. The C6S6A7 films annealed at 450 °C had high transparency about 80% in wide visible range.