The Crystal Quality and Surface Wettability of Various Tellurium-Based Chalcopyrite Layers Grown by the Closed Space Sublimation

AgGaTe₂, AgAlTe₂, CuGaTe₂, and Ag(Ga,Al)Te₂ layers were deposited by the close spaced sublimation method. The surface morphology and crystal quality of these Te-based chalcopyrite layers were systematically evaluated. Controlling the stoichiometry of these layers grown by the closed space sublimation was very difficult because Te preferentially detached from the source materials during the sublimation process and then Te vapor leaked out from the reactor. To solve this problem, the gap between the lid and reactor boat was minimized, and the vapor was encapsulated. As a result, the crystal quality of the AgGaTe₂ and AgAlTe₂ was improved. However, the controlling stoichiometry of CuGaTe₂ remained difficult even after the Te vapor leakage was minimized. This behavior was attributed to the large vapor pressure difference between Cu and Te. The surface morphology of the grown AgGaTe₂ and CuGaTe₂ layers exhibited scattered grain structure, while that of AgAlTe₂ possessed a continuous film structure. These different surface structures contributed to differing wettability between the chalcopyrite materials and substrates. It was found that AgAlTe₂ exhibited a high wettability against a sapphire substrate, which promoted continuous film formation.     

Effects of Si-doping on structural and electrical characteristics of polar,semi-polar,and non-polar AlGaN epi-layers

The effects of Si-doping on the structural and electrical properties of the wurtzite AlGaN epi-layers grown on polar, semi-polar, and non-polar sapphire substrates by metal organic chemical vapor deposition (MOCVD) were studied with X-ray diffraction, scanning electron microscopy, and hall effect measurement. The characterization results showed that both the surface morphology and the crystal quality of the polar AlGaN samples grown on a-plane sapphire substrates was improved with increasing Si concentration due to the Si-induced increase in dislocation movement. It was also found that the folds on the surfaces of the semi-polar and non-polar AlGaN samples grown on m- and r-plane sapphire substrates, respectively were significantly reduced in consequence of the growth suppression along c direction by Si-doping. Moreover, owing to the enhanced crystal quality, an increase in both the mobility and the carrier density for the polar AlGaN samples grown on a-plane sapphire substrates was achieved as the Si-doping level was increased. In addition, a relatively high electron concentration was obtained from the undoped semi-polar AlGaN samples grown on m-plane sapphire substrate, which is helpful to fabricate high quality semi-polar AlGaN-based ultra-violet light emitting diodes (UV-LEDs).     

Growth of AgGaTe2 Layers by a Closed-Space Sublimation Method

AgGaTe₂ layers were deposited on Si substrates by the closed-space sublimation method. Multiple samples were deposited with various source temperatures and holding times, and constant temperature differential. Variation of the source temperature was used primarily to improve the stoichiometry of the film. Deposited films were evaluated by the θ–2θ method of x-ray diffraction (XRD) and transmission electron microscopy. These results confirmed that the deposited films were stoichiometric (after optimizing the above parameters). From XRD, it was also clear that films deposited on Si (111) have strong preference for (112) orientation.     

(211)-Oriented Domain Formation During Growth of ZnTe on m-Plane Sapphire by MBE

ZnTe epilayers have been grown on 2°-tilted m-plane $\left( {10\overline{1} 0} \right)$ sapphire substrates by molecular beam epitaxy. Pole figure imaging was used to study the domain distribution within the layer, and the pole figures of 111, 220, 004, and 422 ZnTe and $30\overline{3} 0$ sapphire were measured. Computer simulation was used to analyze the symmetry of the diffraction patterns seen in the pole figure images. Stereographic projections were also compared with the pole figures of 422 and 211 ZnTe, confirming that single-domain (211)-oriented ZnTe epilayers had been grown on the 2°-tilted m-plane sapphire substrates. Although differences in crystal structure and lattice mismatch were severe in these heterostructures, precise control of the substrate surface’s lattice arrangement would result in the formation of high-quality epitaxial layers.     

Comparative Study on MOCVD Growth of a-Plane GaN Films on r-Plane Sapphire Substrates Using GaN, AlGaN, and AlN Buffer Layers

In this work, we have comparatively investigated the effects of the GaN, AlGaN, and AlN low-temperature buffer layers (BL) on the crystal quality of a-plane GaN thin films grown on r-plane sapphire substrates. Scanning electron microscopy images of the a-plane GaN epilayers show that using an AlGaN BL can significantly reduce the density of surface pits. The full-width at half-maximum values of the x-ray rocking curve (XRC) are 0.19°, 0.36°, and 0.48° for the films grown using Al_0.15Ga_0.85N, GaN, and AlN BLs, respectively, indicating that an AlGaN BL can effectively reduce the mosaicity of the films. Room-temperature photoluminescence shows that the AlGaN BL results in lower impurity incorporation in the subsequent a-plane GaN films, as compared with the case of GaN and AlN BLs. The higher crystal quality of a-plane GaN films produced by the Al_0.15Ga_0.85N BL could be due to improvement of BL quality by reducing the lattice mismatch between the BL and r-sapphire substrates, while still keeping the lattice mismatch between the BL and epitaxial a-plane GaN films relatively small.     

The influence of thickness and ammonia flow rate on the properties of AlN layers

Undoped AlN layers have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition in order to study the effects of ammonia (NH₃) flow rate and layer thickness on the structural quality and surface morphology of AlN layers by high-resolution X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Lower NH₃ flow rate improves crystallinity of the symmetric (0 0 0 2) plane in AlN layers. Ammonia flow rate is also correlated with surface quality; pit-free and smooth AlN surfaces have been obtained at a flow rate of 70 standard cm³ per minute. Thicker AlN films improve the crystallinity of the asymmetric (1 0 1̄ 2) plane.     

Tuning the Phase and Microstructural Properties of TiO<Subscript>2</Subscript> Films Through Pulsed Laser Deposition and Exploring Their Role as Buffer Layers for Conductive Films

Titanium oxide (TiO₂) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO₂ were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO₂, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO₂ films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO₂; however, when employed as a buffer layer, anatase TiO₂ deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO₂ phases and reveals another interesting application for TiO₂ as a buffer layer for transparent conducting oxides.     

Silver gallium telluride (AgGaTe2) single crystal: Synthesis,accelerated crucible rotation-Bridgman growth and characterization

A silver gallium telluride single crystal of diameter 12 mm and length 80 mm was successfully grown by the vertical Bridgman method using accelerated crucible rotation technique. To confirm the unit cell parameters of the grown silver gallium telluride (AgGaTe₂) crystal, single crystal X-ray diffraction studies were carried out. AgGaTe₂ has been studied using differential scanning calorimetry and is found to have supercooling of 13 °C. AgGaTe₂ shows Fourier transform infrared transmission in the spectral range of 6000–500 cm⁻¹. The composition of Ag, Ga and Te in the grown crystal was measured with energy dispersive X-ray analysis spectroscopy. The optical band gap for AgGaTe₂ is 1.8 eV. In the positron lifetime measurements, the average lifetime 267 ps corresponds to vacancy clusters in AgGaTe₂ crystal. Hall measurements confirm p-type nature of AgGaTe₂.     

Compositional modulation in In(x)Ga(1-x)N: TEM and X-ray studies

Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) have been used to study compositional modulation in In(x)Ga(1-x) N layers grown with compositions close to miscibility gap. The samples (0.34 < x < 0.8) were deposited by molecular beam epitaxy using either a 200 nm thick AlN or GaN buffer layer grown on a sapphire substrate. Periodic compositional modulation leads to extra electron diffraction spots and satellite reflections in XRD in the theta-2theta coupled geometry. The ordering period delta measured along c-axis was about delta = 45 A for x = 0.5 and delta = 66 A for x = 0.78 for samples grown on AlN buffer layer. TEM and XRD determinations of delta were in good agreement. Compositional modulation was not observed for the sample with x = 0.34 grown on a GaN buffer layer. Larger values of delta were observed for layers with higher In content and for those having larger mismatch with the underlying AlN buffer layer. The possibility that the roughness of the AlN growth surface promotes strong In segregation on particular crystallographic planes leading to compositional modulation is considered.     

Surface and Interface Properties of Metal-Organic Chemical Vapor Deposition Grown a -Plane Mg x Zn1– x O (0?≤ x ≤?0.3) Films

The a-plane Mg _x Zn_1−x O (0 ≤ x ≤ 0.3) films were grown on r-plane () sapphire substrates using metal-organic chemical vapor deposition (MOCVD). Growth was done at temperatures from 450°C to 500°C, with a typical growth rate of ∼500 nm/h. Field emission scanning electron microscopy (FESEM) images show that the films are smooth and dense. X-ray diffraction (XRD) scans confirm good crystallinity of the films. The interface of Mg _x Zn_1−x O films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy (HRTEM). The Mg _x Zn_1−x O surfaces were characterized using scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence (PL) measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements.     

A Comparison of ZnO Nanowires and Nanorods Grown Using MOCVD and Hydrothermal Processes

A comparison of ZnO nanowires (NWs) and nanorods (NRs) grown using metalorganic chemical vapor deposition (MOCVD) and hydrothermal synthesis, respectively, on p-Si (100), GaN/sapphire, and SiO₂ substrates is reported. Scanning electron microscopy (SEM) images reveal that ZnO NWs grown using MOCVD had diameters varying from 20 nm to 150 nm and approximate lengths ranging from 0.7 μm to 2 μm. The NWs exhibited clean termination/tips in the absence of any secondary nucleation. The NRs grown using the hydrothermal method had diameters varying between 200 nm and 350 nm with approximate lengths between 0.7 μm and 1 μm. However, the NRs grown on p-Si overlapped with each other and showed secondary nucleation. x-Ray diffraction (XRD) of (0002)-oriented ZnO NWs grown on GaN using MOCVD demonstrated a full-width at half-maximum (FWHM) of 0.0498 (θ) compared with 0.052 (θ) for ZnO NRs grown on similar substrates using hydrothermal synthesis, showing better crystal quality. Similar crystal quality was observed for NWs grown on p-Si and SiO₂ substrates. Photoluminescence (PL) of the NWs grown on p-Si and SiO₂ showed a single absorption peak attributed to exciton–exciton recombination. ZnO NWs grown on GaN/sapphire had defects associated with oxygen interstitials and oxygen vacancies.     

Epitaxial growth of (11\bar 20) ZnO on (01\bar 12) Al2O3 by metalorganic chemical vapor deposition2) Al2O3 by metalorganic chemical vapor deposition

There has been increased interest in high quality ZnO films for use in a diverse range of applications such as in high frequency surface acoustic wave filters, buffer layers for GaN growth, transparent and conductive electrodes, and solid state lasers. In the present paper, ZnO films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350–450°C. X-ray diffraction and electron microscopy results indicate that the ZnO films are epitaxially grown on ( $01\bar 12$ ) Al₂O₃ surface with the ( $11\bar 20$ ) plane parallel to the surface. Cross-sectional high resolution-transmission electron microscopy imaging of the as-grown film shows that the interface is semi-coherent and atomically sharp, with misfit dislocations relieving the misfit strain between ZnO and sapphire. In order to check the thermal stability of the as-grown ZnO films, annealing in an O₂+N₂ ambience at 850°C for 30 min was performed. The annealed films showed improved crystallinity. At the same time, limited reaction between ZnO and sapphire occurred, resulting in the formation of a 15–20 nm thick spinel layer at the interface.     

ZnO growth toward optical devices by MOVPE using N2O

Wide bandgap semiconductor zinc oxide (ZnO) layers were grown by metalorganic vapor phase epitaxy (MOVPE) using nitrous oxide (N₂O). Strong ultraviolet (UV) photoluminescence emissions with 1000 times less deep ones at room temperature were observed from ZnO layers grown on sapphire. Alow temperature (500 C)-grown buffer layer of ZnO was effective to enhance the initial nucleation process and to achieve high quality ZnO layers on it at higher growth temperatures (600–700 C). ZnO layers grown on III–V semiconductor substrates showed dominant UV luminescence in spite of low temperature growth. These results imply the abilities of high quality ZnO growth by MOVPE.     

Internal microstrain and distribution of composition and cathodoluminescence over lapped AlxGa1−x N epilayers on sapphire

Structural properties and spatial inhomogeneity of MOCVD-grown Al_xGa_1?x N layers on (0001) sapphire substrates were studied. A nonuniform distribution of Al across the epilayer was observed in layers grown at constant flux rates of precursors. The model of compositionally graded layer formation is proposed on the basis of cathodoluminescence and X-ray data. It is established that homogeneous samples can be obtained by increasing the flux rate of trimethylaluminum at the initial stage of epilayer growth compared with that in all further stages. Lowering the growth rate reduces strain in epitaxial Al_xGa_1?x N layers. The influence of strain on the luminescence properties of the layers is discussed.     

Growth of GaN on Buffer Layers with Different Polarities by Hydride Vapor-Phase Epitaxy

This paper reports the properties of GaN grown by the hydride vapor-phase epitaxy (HVPE) technique on buffer layers with different polarities. The N-, mixed-, and Ga-polarity buffer layers were grown by molecular-beam epitaxy (MBE) on sapphire (0001) substrates; then, thicker GaN epilayers were grown on these by HVPE. The surface morphology, structural, and optical properties of these HVPE-GaN epilayers were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), scanning electron microscopy, and photoluminescence (PL) spectroscopy. The results indicate that the crystallinity of these HVPE-GaN epilayers depends on the polarity of the buffer layer.     

Epitaxial growth of nonpolar GaN films on r-plane sapphire substrates by pulsed laser deposition

Single-crystalline nonpolar GaN epitaxial films have been successfully grown on r-plane sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) with an in-plane epitaxial relationship of GaN[1-100]//Al₂O₃[11-20]. The properties of the ~500 nm-thick nonpolar GaN epitaxial films grown at temperatures ranging from 450 to 880 °C are studied in detail. It is revealed that the surface morphology, the crystalline quality, and the interfacial property of as-grown ~500 nm-thick nonpolar GaN epitaxial films are firstly improved and then decreased with the growth temperature changing from 450 to 880 °C. It shows an optimized result at the growth temperature of 850 °C, and the ~500 nm-thick nonpolar GaN epitaxial films grown at 850 °C show very smooth surface with a root-mean-square surface roughness of 5.5 nm and the best crystalline quality with the full-width at half-maximum values of X-ray rocking curves for GaN(11-20) and GaN(10-11) of 0.8° and 0.9°, respectively. Additionally, there is a 1.7 nm-thick interfacial layer existing between GaN epitaxial films and r-plane sapphire substrates. This work offers an effective approach for achieving single-crystalline nonpolar GaN epitaxial films for the fabrication of nonpolar GaN-based devices.     

Modeling growth rate of HfO2 thin films grown by metal-organic molecular beam epitaxy

HfO₂ dielectric layers were grown on the p-type Si (100) substrate by metal-organic molecular beam epitaxy (MOMBE). Hafnium-tetra-butoxide, Hf(O·t-C₄H₉)₄ was used as a Hf precursor and Argon gas was used as a carrier gas. The thickness of the HfO₂ film and intermediate SiO₂ layer were measured by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The properties of the HfO₂ layers were evaluated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high frequency (HF) capacitance-voltage (C-V) measurement, and current-voltage (I-V) measurement. C-V and I-V measurements have shown that HfO₂ layer grown by MOMBE has a high dielectric constant (k) of 20-22 and a low-level of leakage current density. The growth rate is affected by various process variables such as substrate temperature, bubbler temperature, Ar and O₂ gas flows and growth time. Since the ratio of O₂ and Ar gas flows are closely correlated, the effect of variations in O₂/Ar flow ratio on growth rate is also investigated using statistical modeling methodology.     

Growth of high-quality, uniform c-axis-oriented zinc oxide nano-wires on a-plane sapphire substrate with zinc oxide templates

High-quality, vertically aligned zinc oxide (ZnO) nano-wires were grown by the vapour-transport method on (1 1 2¯ 0) (a-plane) sapphire substrate covered by a uniform ZnO nano-crystalline seed layer which was deposited in a preceding growth step via simple chemical vapour deposition. A thin layer of close-packed nano-seeds with an average size of 12 nm was formed rapidly on the substrate by sublimation and thermal decomposition of zinc acetate dihydrate as the precursor at moderate temperatures and pressures. Subsequently, growth of ZnO nano-wires was performed by a carbo-thermal vapour-transport method yielding nano-wires with high reproducibility and homogeneity. The as-grown, c-axis-oriented nano-wires exhibit excellent luminescence properties and perfect alignment with respect to the substrate surface.     

Carbothermal reduction growth of ZnO nanostructures on sapphire—comparisons between graphite and activated charcoal powders

Zinc oxide (ZnO) nanostructures were grown by the vapour phase transport (VPT) method on a-plane sapphire substrates via carbothermal reduction of ZnO powders with various carbon powders. Specifically, graphite powder and activated charcoal powder (of larger total surface area but similar mesh size) were used. ZnO nanostructures can be grown at lower temperatures (∼800 °C) using activated charcoal than those required using graphite powder. Furthermore, the morphologies of ZnO nanostructures obtained using activated charcoal were different to those obtained using graphite. At higher temperatures (∼950 °C), where well-aligned nanorods were obtained using graphite powder, no nanostructures were found using activated charcoal. In contrast to previous results on Si substrates we find that the effects on ZnO nanostructure growth on a-sapphire cannot be explained solely in terms of increased Zn vapour pressure due to the enhancement of the carbothermal reduction reaction rate by the high surface area activated charcoal.     

Effect of substrate material on the rate of growth and the optical parameters of a-C:H layers

a-C:H layers were grown by dc magnetron reactive sputtering of a graphite target in Ar + H₂ plasma. Ellipsometric measurements were carried out and analyzed at a wavelength of 6328Å for three sets of a-C:H layers with different thicknesses (different sputtering times) on silicon, fused quartz, and glass-ceramic substrates. It was shown that the substrate material had a substantial effect upon a-C:H growth: a-C:H layers on Si substrates were uniform up to ～ 7000Å thickness; for thin layers (<1000 Å) the growth rate was greater on quartz than on Si; the refractive index values of a-C:H were slightly different on quartz and Si substrates (1.60–1.65 and 1.65–1.72, respectively); a-C:H layers on glass-ceramic substrates were not uniform and had variable refractive index.