Differences in nucleation and properties of InN islands formed using two different deposition procedures

The initial stages of metalorganic chemical vapor deposition of InN have been investigated using two different growth procedures: growth of InN over a GaN buffer layer in one continuous run and growth of InN on a pre-deposited GaN template. While the growth conditions and material quality of the GaN underlying layers are nominally the same, characterization by AFM, X-ray diffraction and PL spectroscopy reveals significantly different material properties of InN islands formed using the two procedures and suggests a different path of evolution during the initial stages of growth. In particular, InN islands grown on a pre-deposited GaN template seem to nucleate directly on the GaN template and are 5 times larger in volume and 2 times lower in surface density as compared with InN growth in one continuous run with the GaN underlying layer. Our studies suggest that the Ga incorporation into the InN during the growth on a GaN buffer layer in one continuous run plays a significant role in altering InN growth mechanisms and material properties.     

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.     

Electron-carrier generation by edge dislocations in InN films: First-principles study

Electronic structures of edge dislocations in InN films are studied using the first-principles calculation. We found that dangling-bond states of In atoms localized in the dislocation core are located above the conduction-band bottom and thus supplies the electron carriers to the conduction band of bulk InN, in agreement with the experimental suggestion by Wang et al. [Appl. Phys. Lett. 90 (2007) 151901]. Moreover, it is shown that the Fermi energy in the conduction band has the tendency to be pinned at the energy positions of N-related dangling-bond states.     

Synthesis and optical properties of purified translucent,orthorhombic boron nitride films

Large-area (>1 cm²) freestanding translucent orthorhombic boron nitride (oBN) films have been synthesized by magnetron sputtering at a low radio-frequency power of 120 W. The structural characterizations were performed by means of X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It is demonstrated that oBN is a direct band gap semiconductor (E_g∼3.43 eV). Excited by ultraviolet laser (wavelength at 325 nm), the oBN films emit strong white light, which can be seen by the naked eyes in the dark. In the photoluminescence spectrum, besides the ultraviolet near-band-edge radiative recombination emission, there are three visible emission bands (centered at 400, 538, and 700 nm) arising from the defect-related deep-level centers of oBN, which are mixed to form the white light emission. The hardness and elastic modulus of oBN films are 11.5 and 94 GPa, respectively, examined by nanoindentation measurements.     

Elucidation of factors obstructing quality improvement of MOVPE-grown InN

A systematic study of the crystallographic and electrical/optical properties of MOVPE-grown InN was performed, and the factors that restrict the quality of MOVPE InN were elucidated. The quality of grown InN is highly dependant on the thermal decomposition of NH₃ as a nitrogen source. At a lower growth temperature (~550 °C) a shortage of active nitrogen, due to a lower decomposition rate of NH₃, causes the formation of N vacancies in the grown InN. With increasing growth temperature, a more stoichiometric crystal is grown and the electrical/optical properties improve. At temperatures above 600 °C, however, deterioration occurs at the N-face of In-polar InN near the substrate interface. This deterioration results in the formation of a porous layer during high temperature (~650 °C) growth. There are a few evidences that show that the hydrogen produced by NH₃ decomposition causes this degradation. Thus, improving the quality of MOVPE-grown InN by changing the growth temperature can be difficult. However, a short growth time at a high growth rate and a relatively high temperature is one effective way to solve this dilemma, and one can achieve carrier concentrations as low as 4×10¹⁸ cm⁻³ by growth at 650 °C for 30 min.     

InGaN growth with various InN mole fractions on m-plane ZnO substrate by metalorganic vapor phase epitaxy

We have demonstrated In_xGa_1−xN epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate.     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Method for HVPE growth of thick crack-free GaN layers

Twenty-five micrometer thick GaN was grown with hydride vapor phase epitaxy (HVPE) on metal-organic chemical vapor deposition (MOCVD) grown templates on sapphire substrates with the gallium treatment step (GTS) technique with varying buffer layer thickness. The samples are studied with atomic force microscopy (AFM), etching and scanning electron microscopy (SEM), photo-luminescence (PL), X-ray diffraction (XRD) and optical microscopy. The results show that the thickness of the buffer layer is not important for the layer quality once the growth in MOCVD starts to make the transition from 3D growth to 2D growth and HVPE continues in the same growth mode. We show that the MOCVD templates with GTS technique make excellent templates for HVPE growth, allowing growth of GaN without cracks in either sapphire or GaN.     

Growth of good quality InGaN multiple quantum wells by MOCVD

We report the optical properties of the blue emitting LED material that belongs to the III–V nitrides family. Quantum wells of InGaN were grown by metal organic chemical vapor deposition. Sapphire was used as the substrate material. Continuous wave and time resolved luminescence experiments were conducted in the temperature range of liquid helium to room temperature. Very intense blue luminescence was observed upto the room temperature. From the very broad linewidth of the emission it was concluded that the compositional fluctuations were dominant. At very low temperatures the lifetimes were found to be somewhat longer than the free exciton case which indicated the localization of excitons at potential fluctuations in the InGaN lattice.     

In2O3 nanorod arrays grown at grain-boundary triple junctions of Cu–Sn alloy substrate

In this article, an alternative method for site-specific growth of In₂O₃ nanorod arrays, which relies on the vapor–liquid–solid growth mechanism, is demonstrated using Cu–Sn (5 at% Sn) alloy as substrate. By annealing Cu–Sn alloy slightly below the solidus line, grain-boundary triple junctions can be wetted preferentially. As a result, the catalyzing Cu droplets will be present at the sites of grain-boundary triple junctions, which will control the growth of In₂O₃ nanorods at defined locations. This growth technique provides a cost-effective and simple approach to fabricate ordered nanorod arrays with the sites controlled, which may benefit nanorod device applications.     

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.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

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.     

Characterization of polycrystalline GaN grown on silica glass substrates

The structural, optical, and electrical properties of GaN films grown on silica glass substrate by metalorganic chemical vapor deposition were studied. X-ray diffraction showed that the films were grown in hexagonal structure with a predominant (0 0 0 2) peak. A broad and strong band-edge emission and very weak yellow luminescence in photoluminescence (PL) spectra were observed. And the temperature dependence of the PL spectra was extensively studied. The thermal quenching activation energy was found to be very close to the donor activation energy determined from the temperature dependence of the carrier concentration. Longitudinal optical phonons were found to be responsible for the PL broadening above 100 K.     

Electrical properties and deep levels spectra of bulk SiC crystals grown by hybrid physical–chemical vapor transport method

Concentrations of nitrogen shallow donors, boron shallow acceptors, charge carriers, and electron traps were measured as a function of position along the growth axis in a series of undoped 6H–SiC boules grown by sublimation method with and without addition of hydrogen to the growth atmosphere. Elemental analysis by secondary ion mass spectrometry and measurements of electrical properties indicate that the addition of hydrogen suppresses nitrogen incorporation and formation of all electron traps. Concentration of boron is not affected by hydrogen presence. The addition of hydrogen to the growth ambient improves the uniformity of nitrogen incorporation and deep trap distribution along the growth axis. The results are interpreted as due to increased carbon transport and corresponding shift of crystal stoichiometry toward carbon-rich side of the SiC existence range.     

Growth and characterization of epitaxial layers on aluminum nitride substrates prepared from bulk, single crystals

A comparative study of epitaxy of AlN, GaN and their alloys, grown on c-axis and off-axis substrates of single-crystal aluminum nitride has been carried out. Growth on off-axis (>30°) substrates appears to result in rough surfaces and the absence of two-dimensional electron gas (2DEG). However, smooth morphologies were demonstrated for both homoepitaxial and heteroepitaxial growth on on-axis (<2°) substrates. On one of these oriented substrates a 2DEG, with a mobility of 1000 cm²/V s and a sheet density of 8.5×10¹² cm⁻² at room temperature, was also demonstrated for the first time.     

On the isothermal closed space sublimation growth of CdSe using a mixed source for selenium

The use of a selenium–tellurium (SeTe) mixed source in the isothermal close space sublimation growth of CdSe epilayers is considered. The epitaxial growth was performed in flowing helium by sequential exposures of the substrate to vapors of the mixed SeTe source and elemental cadmium at temperatures within 350–410 °C. In spite of the mixed source (proposed to decrease the partial pressure of Se), tellurium incorporation was small and CdSe_xTe_(1−x) (x∼0.98) epilayers were obtained. X-ray diffraction reciprocal space mapping shows the existence of hexagonal inclusions mainly on the (1 1 1) facets of the cubic phase. Material deposition on areas of the graphite crucible exposed to the sources, contamination of the Cd source and large growth rates suggest the existence of a selenium transport process via graphite. This transport might be the result of the combination of selenium deposition on graphite with a subsequent activated desorption of selenium under cadmium exposure. It affects Cd source purity and growth kinetic bringing on a modification of the usual atomic layer deposition regimen; however, a reproducible growth rate of the epilayers was obtained.     

Growth of height-controlled InGaN quantum dots on GaN

InGaN height-controlled quantum dots (HCQDs) were grown by alternately depositing In_0.4Ga_0.6N QD and In_0.1Ga_0.9N spacer layers on a seed In_0.4Ga_0.6N QD layer. Structural and optical studies showed that the height of the InGaN QDs was controlled by the deposition cycle of In_0.4Ga_0.6N/In_0.1Ga_0.9N layers. Photoluminescence studies showed that the In_0.4Ga_0.6N HCQDs provided deep potential wells and the piezoelectric field-induced quantum-confined Stark effect was negligibly small. These phenomena are attributed to variation in quantum confinement energy in the electronically coupled InGaN HCQDs providing deep potential wells.     

Rate-determining process in chemical vapor deposition of SiC on off-axis -SiC

Homoepitaxial growth on off-axis α-SiC at reduced pressures in a horizontal cold-wall chemical vapor deposition (CVD) system operating at has been investigated. The growth rate was found inversely proportional to the square root of total pressure or the partial pressure of H₂, a carrier gas. A model to explain the experimental results is proposed, where the rate-determining process in CVD is competition between Si species and hydrogen atoms for C (carbon) dangling bonds at SiC step edges.