A-plane Sapphire Research Articles

Growth of AgGaTe2 and AgAlTe2 Layers for Novel Photovoltaic Materials

AgGaTe2 and AgAlTe2 layers were grown on a-plane sapphire substrates by closed-space sublimation. These compounds replace Cd in CdTe with group I and III elements, and are, hence, expected to be ideal novel candidate materials for solar cells. The grown layers were confirmed to be stoichiometric AgGaTe2 and AgAlTe2 by x-ray diffraction (XRD). The AgAlTe2 layers had strong preference for the (112) orientation. The XRD spectrum of the AgGaTe2 layer was different from that of the AgAlTe2 layer, and strong peaks were observed for (103) and (110) diffraction. The variation in orientations of the grown layers was analyzed in detail by use of XRD pole figures, which revealed that the AgGaTe2 layers had an epitaxial relationship with the a-plane sapphire substrates.

X-ray multiple diffraction of ZnO substrates and heteroepitaxial thin films

We investigate Umweganregung reflections (X-ray multiple reflections) for , (00.3), and (00.5) for a number of wurtzite structure ZnO samples, namely various bulk crystals (substrates), two ZnO thin films and a ZnO:Mg thin film, grown by pulsed laser deposition on a-plane sapphire. Using a low divergence setup, we achieve a high angular resolution of 0.1 or better. We derive an analytical formula for peak positions as a function of wavelength and the lattice parameters a and c. The three investigated ZnO substrates exhibit all expected (kinematically allowed) reflections for Cu Kα1, Kα2, and Kβ. A lineshape analysis of the multiple reflection peaks shows that the width of certain reflections is determined by the spectral linewidth of the exciting copper lines. The thin films exhibit only slight broadening of the peaks but disappearance of several reflections. The a lattice constant can be determined from the azimuthal peak positions. Umweganregung scan for () of a hydrothermally grown ZnO substrate, excited with Cu Kα1 radiation. The pattern is symmetric with respect to zero angle representing the (10.0) azimuth.

A DLTS study of a ZnO microwire, a thin film and bulk material

ABSTRACTWe have investigated the electrical properties of a ZnO microwire grown by carbo-thermal evaporation, a ZnO thin film grown by pulsed-laser deposition on an a-plane sapphire, and a hydrothermally grown Zn-face ZnO single crystal (Tokyo Denpa Co. Ltd.). The samples were investigated by means of current-voltage measurements, capacitance-voltage measurements, and deep-level transient spectroscopy.The defects T2 [1,2] and E3 [1,3,4] were identified in all three sample types. Additionally, in the single crystal and thin film samples E64 [5] and E4 [1] were detected. These findings support the common opinion that T2 is an intrinsic defect since it is found in all the samples investigated and thus its occurrence is not related to any growth technique.

Effects of N2O addition on the properties of ZnO thin films grown using high-temperature H2O generated by catalytic reaction

ABSTRACTThe effects of N2O gas addition on the properties of zinc oxide films grown on a-plane (11-20) sapphire (a-Al2O3) substrates were investigated, using a chemical vapor deposition method based on the reaction between dimethylzinc and high-energy H2O produced by a Pt-catalyzed H2-O2 reaction. By employing an optimal N2O gas pressure, both the film crystallinity and crystal orientation were improved. Subsequent to treatment with N2O, the electron mobility of films at room temperature increased from 207 to 234 cm2/Vs while the electron concentration decreased at low temperatures. In addition, the photoluminescence peak intensity of the nearband-edge emission was increased.

Examining the crystal growth that influences the electronic device output from vertical arrays of ZnO nanowires

ABSTRACTZnO nanowire (NW) arrays were examined with Transmission Electron Microscopy (TEM) in cross-section after preparation by Focused Ion Beam (FIB) milling. This technique revealed that ZnO nanowires grown using a Au catalyzed vapor technique typically have Au particles at the NW tips, and also randomly dispersed across the base crystal growth that joins adjacent NWs. It is shown the adjacent NWs and the combined base growth is one crystal structure which can be used as a back electrical contact making fabrication of vertical array devices possible. However, the base growth displays detrimental features such as embedded Au particles and lattice defects which can affect the electrical output through depletion regions and scattering centers. In an effort to overcome these problems we investigate a growth method that is nucleated through a minor alteration of the a-plane sapphire surface roughness via a weak chemical etch. Observations of various stages of the growth show the growth nucleates as separate nanoislands that grow in c-plane alignment with Sapphire (1-210), and as growth continues these islands meet and form a polycrystalline film. Further growth initiates nanowire growth and the formation of a single crystal base layer and NW structure that can cover several square millimeter’s. This allows high quality arrays that are relatively free from defects to be formed without any metals contamination and ready for further device processing.

低温外延生长平整ZnO薄膜

ZnO thin film that grows at low temperature benefits the sharp interface in heterostructure and enough doping level of acceptor in ZnO. In general, the smooth growth can be easily obtained at temperature higher than 600C, but difficult in the case of low growth temperature. In this work, by controlling the growth ambient and growth rate, a series of ZnO thin films with smooth surface were grown on a-plane sapphire substrates at 450C by the plasma-assisted molecular beam epitaxy (P-MBE). The growth was performed in oxygen-rich atmosphere. To tune the growth rate, the zinc flux was changed by varying the K-cell temperature of zinc source (Tk) while keeping oxygen flux constant. The growth rate of the samples is only 40~100 nm/h. Scanning electron microscopy (SEM) images indicate that there are lots of irregular grains on the thin film surface at high zinc flux, and most of the grains disappear gradually from the surface with the zinc flux decreasing. This smooth growth with low growth rate is conducive to control finely the layer thickness and smoothness in the multilayer structure. The root mean square (RMS) surface roughness is only 0.238 nm, measured by atomic force microscopy (AFM). The smooth surface benefits from the low growth rate and small sub-lattice mismatch between the c-plane-ZnO thin film and a-plane-sapphire substrate.

Highly textured fresnoite thin films synthesized in situ by pulsed laser deposition with CO2 laser direct heating

Fresnoite Ba2TiSi2O8 (BTS) thin films were grown and crystallized in situ using pulsed laser deposition (PLD) with CO2 laser direct heating of the a-plane sapphire (1 1 0) substrates up to 1250 °C. Starting with 775 °C growth temperature, (0 0 1)- and (1 1 0)-textured BTS and BaTiO3 phases, respectively, could be assigned in the films, and the typical fern-like BTS crystallization patterns appear. For higher process temperatures of 1100 to 1250 °C, atomically smooth, terraced surface of the films was found, accompanied by crystalline high-temperature phases of Ba–Ti–Si oxides. HAADF micrographs taken in both scanning transmission electron microscopy and energy-dispersive x-ray spectrometry mode show details of morphology and elemental distribution inside the films and at the interface. To balance the inherent Si deficiency of the BTS films, growth from glassy BTS × 2 SiO2 and BTS × 2.5 SiO2 targets was considered as well. The latter targets are ideal for PLD since the employed glasses possess 100% of the theoretical density and are homogeneous at the atomic scale.

TEM Observation and Ionic Conductivity Study of Li<sub>2</sub>SiO<sub>3</sub> Thin-Film on Sapphire Substrate

Thin-film samples of the lithium ionic conductor Li2SiO3(LSO) were deposited on an A-plane sapphire substrate via the pulsed laser deposition (PLD) method, and the irreversible temperature dependence of the ionic conductivity in the thin-film samples was studied. Via transmission electron microscopy (TEM) observations of annealed LSO thin-film, it was found that the as-prepared LSO thin-film was amorphous over the temperature rangeT490 K, and that nanocrystals existed in the annealed LSO thin-film in the temperature rangeT550 K. Further more, it was clarified the irreversible temperature dependence of the ionic conductivity is due to the generation of nanocrystals.

Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes

High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co2MnGe(13nm)/Al2O3(3nm)/Co(13nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates.

Properties of zinc oxide films grown on sapphire substrates using high-temperature H2O generated by a catalytic reaction on platinum nanoparticles

The authors investigated the characteristics of ZnO films grown on a-plane (11-20) sapphire substrates at 773–873 K using a reaction between dimethylzinc and high-temperature H2O generated by a catalytic reaction on Pt nanoparticles. The growth rate was 0.02–0.07 μm min−1. The largest electron mobility and the smallest residual carrier concentration for the ZnO films were 169 cm2 V−1 s−1 and 1.6 × 1017 cm−3, respectively. X-ray diffraction patterns for the ZnO films exhibited intense (0002) and (0004) peaks associated with ZnO (0001) planes. The minimum full width at half maximum of the ω-rocking curve for ZnO (0002) was less than 0.1°. In a ZnO film with a high electron mobility, no rotational domains were identified using a ZnO (10-10) ϕ scan. From secondary ion mass spectroscopy, a hydrogen concentration of 3 × 1018 cm−3 and a boron concentration of 2–5 × 1017 cm−3 were determined. These were identified as extrinsic donor impurities.

Effects of substrate temperature, substrate orientation, and energetic atomic collisions on the structure of GaN films grown by reactive sputtering

The combined effects of substrate temperature, substrate orientation, and energetic particle impingement on the structure of GaN films grown by reactive radio-frequency magnetron sputtering are investigated. Monte-Carlo based simulations are employed to analyze the energies of the species generated in the plasma and colliding with the growing surface. Polycrystalline films grown at temperatures ranging from 500 to 1000 °C clearly showed a dependence of orientation texture and surface morphology on substrate orientation (c- and a-plane sapphire) in which the (0001) GaN planes were parallel to the substrate surface. A large increase in interplanar spacing associated with the increase in both a- and c-parameters of the hexagonal lattice and a redshift of the optical bandgap were observed at substrate temperatures higher than 600 °C. The results showed that the tensile stresses produced during the film's growth in high-temperature deposition ranges were much larger than the expected compressive stresses caused by the difference in the thermal expansion coefficients of the film and substrate in the cool-down process after the film growth. The best films were deposited at 500 °C, 30 W and 600 °C, 45 W, which corresponds to conditions where the out diffusion from the film is low. Under these conditions the benefits of the temperature increase because of the decrease in defect density are greater than the problems caused by the strongly strained lattice that occurr at higher temperatures. The results are useful to the analysis of the growth conditions of GaN films by reactive sputtering.

Magnetization Dynamics in Co<SUB>2</SUB>MnGe/Al<SUB>2</SUB>O<SUB>3</SUB>/Co Tunnel Junctions Grown on Different Substrates

We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions (TMJ), deposited on various single crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are compared to the magnetic properties of Co and of Co$_{2}$MnGe single films lying on sapphire substrates. X-rays diffraction always shows a (110) orientation of the Co$_{2}$MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the TMJ grown on sapphire. Our vibrating sample magnetometry measurements reveal in-plane anisotropy only in samples grown on a sapphire substrate. Depending on the substrate, the ferromagnetic resonance spectra of the TMJs, studied by the microstrip technique, show one or two pseudo-uniform modes. In the case of MgO and of Si substrates only one mode is observed: it is described by magnetic parameters (g-factor, effective magnetization, in-plane magnetic anisotropy) derived in the frame of a simple expression of the magnetic energy density; these parameters are practically identical to those obtained for the Co single film. With a sapphire substrate two modes are present: one of them does not appreciably differ from the observed mode in the Co single film while the other one is similar to the mode appearing in the Co$_{2}$MnGe single film: their magnetic parameters can thus be determined independently, using a classical model for the energy density in the absence of interlayer exchange coupling.

Ultraviolet detection from graphitic-C/Zn1−xMgxO Schottky devices fabricated at moderate temperatures

Ultraviolet (UV) Schottky detector devices were fabricated on polycrystalline wurtzite Zn1−xMgxO films energetically deposited onto a-plane sapphire at room-temperature (RT) and 200 °C. The unintentionally doped, transparent, n-Zn1−xMgxO films exhibit low surface roughness (<5% of film thickness), moderate carrier concentration, and Hall mobility up to 15 cm2 V−1 s−1. The direct bandgaps of the RT and 200 °C films (x = 0.24 and x = 0.20) were 3.57 eV and 3.40 eV. Schottky diodes with graphitic anodes formed on these films exhibited barrier heights up to 0.88 eV and ideality factors as low as 1.97. Spectral response measurements demonstrated UV/visible photo-current ratios up to ∼104.

AlN Grown on a- and n-Plane Sapphire Substrates by Low-Pressure Hydride Vapor Phase Epitaxy

We have studied the properties of AlN layers grown by low-pressure hydride vapor phase epitaxy (HVPE) on n-plane (112̄3) sapphire substrates and compared them with those of AlN layers on a-plane (112̄0) sapphire substrates. c-Plane AlN was grown on a-plane sapphire. In the case of AlN growth on n-plane sapphire, the c-axis of AlN was tilted by about 1.2° relative to the n-axis of sapphire, unlike AlN growth on a-plane sapphire. For AlN grown on a-plane sapphire, the in-plane epitaxial relationship between AlN and sapphire changed with nitridation temperature in the initial-stage of growth, but it remained constant for AlN grown on n-plane sapphire.

Electrical properties of ZnO thin films grown on a-plane sapphire substrates using catalytically generated high-energy H2O

The electrical properties of zinc oxide (ZnO) epitaxial films grown by chemical vapor deposition (CVD) using high-energy H2O generated by H2–O2 reactions on Pt nanoparticles were evaluated. High-energy ZnO precursors formed by the reaction between dimethylzinc gas molecules and H2O molecules were supplied to the substrate surface. The ZnO epitaxial films were grown directly on a-plane sapphire (a-Al2O3) substrates at 773K without any buffer layer. The electron mobility (μH) at room temperature increased from 30 to 190 cm2V−1s−1 with increasing film thickness from 100nm to 2800nm. The μH increased significantly with decreasing temperature to approximately 100–150K, but it decreased at temperatures less than 100K for films thicker than 500nm. The μH of the ZnO film (189 cm2V−1s−1) at 290K increased to 660 cm2V−1s−1 at 100K. In contrast, μH hardly changed with temperature for films thinner than 500nm. According to a two-layer Hall-effect model, the μH and electron concentration of the upper layer were corrected based on the above results, assuming that the degenerate layer had a thickness of 100nm.

Accurate determination of optical bandgap and lattice parameters of Zn1–xMgxO epitaxial films (≤x≤0.3) grown by plasma-assisted molecular beam epitaxy on a-plane sapphire

Zn1–xMgxO epitaxial films with Mg concentrations 0≤x≤0.3 were grown by plasma-assisted molecular beam epitaxy on a-plane sapphire substrates. Precise determination of the Mg concentration x was performed by elastic recoil detection analysis. The bandgap energy was extracted from absorption measurements with high accuracy taking electron-hole interaction and exciton-phonon complexes into account. From these results a linear relationship between bandgap energy and Mg concentration is established for x≤0.3. Due to alloy disorder, the increase of the photoluminescence emission energy with Mg concentration is less pronounced. An analysis of the lattice parameters reveals that the epitaxial films grow biaxially strained on a-plane sapphire.

High-Temperature Heat-Treatment of c-, a-, r-, and m-Plane Sapphire Substrates in Mixed Gases of H2 and N2

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixed gas flow of H2 and N2 (H2/N2= 3/1) was investigated within the temperature range 980–1480 °C. AlN was formed on sapphire in the temperature range 1030–1430 °C for c-, a-, and m-plane sapphire, and 980–1430 °C for the r-plane sapphire. At 1480 °C, AlN was not formed, and only sapphire was decomposed by H2 with the ranking of m- > r- > a- > c-plane. The ranking was contrary to that of the amount of AlN formation at 1380 °C, which occurred by the reaction of gaseous Al generated by the sapphire decomposition and N2. This discrepancy was due to the shape of AlN formed on sapphire; whisker-like AlN does not protect c- and a-plane sapphire from decomposing, while layer-like AlN protects r- and m-plane sapphire from decomposing.

Cross-sectional TEM imaging of β-Ga2 O3 thin films formed on c -plane and a -plane sapphire substrates

We investigated in detail the microstructure of -oriented β-Ga2O3 thin films on sapphire substrates by transmission electron microscopy (TEM) and electron diffraction. Cross-sectional TEM images of the films on (001) c-plane and (110) a-plane sapphire substrates are composed of four types of regions with different atomic arrangements. The consideration based on the crystal orientation of β-Ga2O3 showed that the type of TEM images corresponds to the projection direction caused by the rotation of the β-Ga2O3 unit cell round the normal to the plane. Electron diffraction patterns corresponding to each region can also be explained in the same manner. These support the results of X-ray pole figure measurements in our previous report. Finally, the relationship among the TEM images, the electron diffraction patterns and the rotation angle of unit cell for -oriented β-Ga2O3 was classified.

Role of nitridation on polarity and growth of InN by metal–organic vapor phase epitaxy

We report on metal–organic vapor phase epitaxy (MOVPE) of (0001)InN layers simultaneously grown on a-plane (112¯0) and c-plane (0001) sapphire substrates. The substrates were nitridated at temperatures from 500°C to 1050°C prior to the growth of c-plane InN layers. Nitridation determined the polarity, the crystallinity, and the surface morphology of the InN layers. Nitridation temperatures above 800°C lead to N-polar InN layers, while nitridation temperatures from 700°C to 750°C produce mixed-polar InN layers, and nitridation temperatures from 500°C to 650°C produce In-polar InN layers. The roughness and crystallinity of the InN layers are correlated with the changes of polarity. The incorporation of nitrogen into the nitridation layers at different nitridation temperatures was measured. A strong N–Al bond signal after nitridation is correlated with N-polarity layers after overgrowth.

Epitaxial Growth of Cubic and Hexagonal InN Thin Films via Plasma-Assisted Atomic Layer Epitaxy

InN thin films possessing either a novel cubic or a hexagonal phase were grown by plasma-assisted atomic layer epitaxy on an a-plane sapphire, Si(111), and GaN/sapphire templates, simultaneously. Two ALE growth temperature windows were found between 175–185 °C and 220–260 °C, in which the growth process is self-limiting. In the lower temperature ALE window, InN on an a-plane sapphire crystallized in a face-centered cubic lattice with a NaCl type structure, which has never been previously reported. InN grown on other substrates formed the more common hexagonal phase. In the higher temperature ALE window, the InN films grown on all substrates were of hexagonal phase. The NaCl phase and the epitaxial nature of the InN thin films on the a-plane sapphire grown at 183 °C are confirmed independently by X-ray diffraction, transmission electron microscopy, and numerical simulations. These results are very promising and demonstrate the tremendous potential for the PA-ALE in the growth of crystalline III-N materials with novel phases unachievable by other deposition techniques.