Al2O3 Single Crystal Substrates Research Articles

Origin of extra diffraction spots for high crystalline alpha-Ga2O3

This work has investigated the microstructure characteristics of high-quality alpha-Ga2O3 thin film grown on the Al2O3 single crystal substrate membrane. Hetero-epitaxial alpha Ga2O3 crystals reveal the formation of a three-fold symmetry at the initial stage of the growth by the oxygen template provided by the Al2O3. Inversion domains are found, and they have a 180° inverted configuration from the surroundings. These IDs lead to extra diffraction spots when observed along [110] and [010].

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor.

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al2O3 substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of pz orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

Effects of Si doping on the ferromagnetic properties of delta doped GaMnN nanorods

Delta doping (δ-doping) of group-III nitride-based nanostructures such as nanorods (NRs) with transition metals such as manganese (Mn) can lead to one-dimensional (1D) diluted magnetic semiconductors (DMSs). In order to investigate the effects of free carrier doping on the structural, electrical, and magnetic properties of such delta-doped 1D structures, we have used nanosphere lithography to grow uniform arrays of vertically aligned NRs with fixed aspect ratios on single crystal Al2O3 substrates using plasma-assisted molecular beam epitaxy (PAMBE). The precise control of the elemental flux intensity and duration, facilitated by PAMBE, enables the growth of phase-pure nanostructures, resulting in spatial separation on the order of few nanometers, between the δ-Mn layer and the free carriers in the Si:GaN layer. Chemical quantification verifies the presence of Mn and Si, while Raman spectroscopy shows that Si doping enhances the local vibration mode associated with Mn bonded to N as well as the disorder-activated mode. The free carriers do not diminish the inherent magnetic ordering in these 1D structures, while magnetic measurements show a stability in the signal.

The role of lattice mismatch in heterogeneous nucleation of pure Al on Al2O3 single-crystal substrates with different termination planes

Al solidification on Al2O3 single-crystal substrates with (0001) and (11 $$ \overline{2} $$ 0) termination surfaces was performed to investigate whether the lattice mismatch can influence the heterogeneous nucleation when the same kind of substrate was employed. Nucleation undercooling was measured via differential scanning calorimeter to present the nucleation potency of Al on selected substrates. The measured results show that the lower undercooling was achieved in Al/Al2O3(11 $$ \overline{2} $$ 0), which means that the Al has a higher nucleation potency on Al2O3(11 $$ \overline{2} $$ 0) substrates. This is consistent with the calculated lattice misfit of Al/Al2O3(0001) and Al/Al2O3(11 $$ \overline{2} $$ 0) based on Bramfitt two-dimensional model because the lowest lattice misfit of Al/Al2O3(11 $$ \overline{2} $$ 0) is less than that of Al/Al2O3(0001). Moreover, the corresponding orientation relationships at the interface achieved by high-resolution transmission electron microscopy are almost the same with the predicted one from lattice misfit calculation. It is proved that the lattice misfit does affect the heterogeneous nucleation even the melted metals nucleated on the substrates made of the same kind of material.

A new method for Synthesis of Epitaxial Films of Silicon Carbide on Sapphire Substrates (α-Al2O3)

Abstract A fundamentally new method for synthesis of thick (200-400 nm) epitaxial films of silicon carbide on single-crystal Al2O3 substrates is proposed. The method develops the previously discovered method of topochemical substitution of atoms during the transformation of Si into SiC. In the proposed method, a layer of epitaxial Si of orientation (100) or (111) is being preliminarily deposited on the sapphire via CVD or other technique. Then, using a topochemical reaction between the CO gas and the deposited epitaxial silicon film, a part of Si atoms is being substituted by carbon atoms. As a result, the silicon film transforms into SiC film. As experimental studies have shown, in the process of this transformation a high-quality epitaxial SiC layer can be formed on the sapphire surface under certain synthesis conditions. A detailed investigation of grown SiC layers using the methods of electron diffraction, ellipsometry, Raman spectroscopy, X-ray diffractometry and scanning microscopy is conducted and an analysis of experimental data is given. Formation of both cubic 3C-SiC and hexagonal SiC polytypes during the transformation of Si into SiC was revealed by the XRD method. A detailed analysis of the current state of the problems of growth of epitaxial SiC layers on Al2O3 using various methods is provided. The method developed in this study opens up completely new perspectives not only for synthesis of semiconductors, but also for creation of new classes of composite, heat-resistant and other solid coatings.

Evolution of Phase Composition and Structure in Sapphire Implanted with 64Zn+ Ions and Heat-Treated in Oxygen

Single-crystal Al2O3 substrates are implanted with 64Zn+ ions using doses of 5 × 1016 cm–2 and an energy of 100 keV. The samples are annealed in oxygen with a stepwise increase in temperature from 400 to 1000°C. The changes on the surface and in the bulk of the sample are analyzed via scanning electron microscopy, energy-dispersive analysis, transmission electron microscopy, and Auger electron spectroscopy.

Rise and fall of ferromagnetism in O-irradiated Al2O3 single crystals

In dilute magnetic semiconductors studies, sapphire was usually used as non-magnetic substrate for films. We observed weak ferromagnetic component in Al2O3 single crystal substrate, and excluded the possibility of ferromagnetic contaminations carefully by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. The ferromagnetism rise and fall during the process of annealing-oxygen irradiation-annealing of the sapphire. The ferromagnetic changes are consistent with Al-vacancy related defects detected by positron annihilation spectroscopy. With first-principle calculations, we confirm that Al-vacancy can introduce magnetic moment for 3 μB in Al2O3 crystal and form stable VAl-VAl ferromagnetic coupling at room temperature.

Electronic and magnetic properties of (1 1 1)-oriented CoCr2O4 epitaxial thin film

We report on the fabrication of high quality (1 1 1)-oriented ferrimagnetic normal spinel CoCr2O4 epitaxial thin films on single crystal Al2O3 substrates. The structural, electronic, and magnetic properties were characterized by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, dc magnetization measurement, and element resolved resonant X-ray magnetic scattering. The comprehensive characterization reveals that no disorder in the cation distribution or multivalency issue is present in the samples. As a result, Kagomé and triangular layers are naturally formed via this specific growth approach. These findings offer a pathway to fabricate two dimensional Kagomé heterostructures with exotic quantum many-body phenomena by means of geometrical design.

Enhancing the Thermoelectric Properties of Ca3Co4O9 Thin Films by the Addition of a Nanoscale NbN x Second Phase

High-quality Ca3Co4O9 (CCO) thin films have been epitaxially grown on c-cut Al2O3 single crystal substrates using pulsed laser deposition (PLD). Different doses of Nb ions were injected into the films using an ion beam injection technique, and a nanoscale NbN x second phase was generated in the films after annealing in pure N2. The resistivity and Seebeck coefficient of the films were measured in the temperature range 175–375 K. The results demonstrated that the power factor of the films increases when injected with appropriate quantities of Nb. When the injected Nb concentration was 1.46 × 1020/cm3, the power factor of the film reached 0.17 mW/m K2 at room temperature, which is nearly twice as large as that for pure CCO film. A maximum value of 0.22 mW/m K2 was obtained at 375 K.

Effect of grain size on the electrical and magnetic properties of MgB2 thick films deposited on the Al2O3 single crystal substrates

MgB2 films having thicknesses of approximately 10 μm have been prepared on the Al2O3 single crystal substrates by using ultrasonic spray pyrolysis technique. The films fabricated were then analyzed by means of microstructural, structural, transport and magnetic properties. The influence of grain size, depending on the ex-situ and in situ heating, were investigated. The results obtained showed that the size of MgB2 grains has an effect on the transport and magnetic properties of the films. Films prepared with in situ heating have small particle size (<50 nm) and were found to be much better properties than the films prepared ex-situ heating, which have larger grains (~1 μm). The best Tc value and critical current density, Jcmag, were obtained to be ~36 K and 9.88 × 105 A cm−2 respectively for the films prepared with small grains. The results were showed that the smaller grain size and in situ heating cycles are essential points for quality thick film MgB2 fabrication.

Mechanical properties of deposited carbon thin films on sapphire substrates using atomic force microscopy (AFM)

In this study, the mechanical properties of nano-layered carbon thin films deposited on Al2O3 single crystal substrates with varying orientations in A (1120), M (1010), R (1102) and C (0001) planes were investigated using depth-sensing nanoindentation techniques. All of the nano-layered films were deposited by a thermionic vacuum arc (TVA). In this experiment, a high purity amorphous carbon rod was used. Single crystal Al2O3 plates were used as the substrate material. All of the substrates were a commercial product that is readily available. For the surface topography, the roughness and depth-sensing nano-hardness of the deposited films were analyzed using an Ambios Q-scope atomic force microscope. An F20 thin film measurement system was used for the determination of thickness and reflection properties of the deposited thin films. Indention depths were determined as 5nm, 10nm and 15nm. In these indentation depths, hardness values were calculated in the range of 8–25GPa.

Deposition of perpendicular c-axis oriented BaM thin films on (001) Al2O3 substrates by introducing an interfacial BaM buffer layer

M-type barium ferrite (BaM) thin films with two different structures (single layered and double layered) were deposited on (001) Al2O3 single crystal substrates using an RF magnetron sputtering system. The changes in the surface morphologies, crystallographic and magnetic properties of BaM thin films corresponding to the different structures and substrate temperatures (Ts) were investigated in detail. The results indicated that the interfacial BaM buffer layer plays an important role in improving the crystallographic and magnetic properties of BaM thin films. The single layered BaM thin films deposited at Ts=300°C and Ts=500°C are both randomly c-axis oriented. Whereas, by introducing a thin BaM layer which acts as an interfacial buffer layer, an excellent perpendicular c-axis orientation with an epitaxial structure for the double layered BaM thin film was obtained. The c-axis dispersion angle (Δθc) decreased to 0.49°, and the perpendicular squareness ratio increased to 0.85 for the double layered film. The mechanism for improving the perpendicular c-axis orientation with an interfacial BaM buffer layer was attributed to two reasons. One is a relative increase in the nucleation sites for perpendicularly oriented grains over the nucleation sites for in-plane and/or randomly oriented grains, and the other is the release of the stress that comes from the interface between BaM thin film and Al2O3 substrate.

Enhancing the thermoelectric properties of Ca3Co4O9 thin films by Nb ion injection

High quality Ca3Co4O9 thin films have been grown epitaxially on single crystal Al2O3 substrates with pulsed laser deposition. Nb was implanted into the Ca3Co4O9 films using an ion beam injection technique. The microstructure of the thin films has been investigated by XRD, SEM and AFM. The epitaxial thin films were grown with the c-axis perpendicular to the substrate surface. The effect of Nb doping by ion beam injection was verified using resistivity measurements at room temperature. Resistivity and the Seebeck coefficient were also measured in the temperature range 150–380K. The results indicate that the power factors of Ca3Co4O9 thin films increase when doped with Nb. When the concentration of doped Nb was 3.65×1019atoms/cm3, the power factor of the thin films reached 0.10mW/mK2 at room temperature, and it approached a maximum of 0.17mW/mK2 at 380K.

Fabrication of ZnF2 thin films and their vacuum ultraviolet transparency

ZnF2 thin films were deposited on SiO2-glass and Al2O3-single crystal substrates using electron-beam evaporation, and the morphology of the films was studied in relation to the kind of substrate and deposition conditions. While the films deposited on SiO2-glass, (112¯0)-Al2O3 substrates exhibited an almost non-oriented polycrystalline nature, the films deposited on (0001)-Al2O3 substrates were epitaxially grown (100)-oriented films. The full-width at half maximum of the rocking curve of (200)-diffraction for the (100)-oriented film deposited at 400°C was 150″. This value was comparatively small compared with previously reported (100)-oriented SnO2 and TiO2 films and showed the high orientation and crystallinity of the ZnF2 film. From the transmission spectra from near-infrared (NIR) to vacuum ultraviolet (VUV), the optical band gap of the ZnF2 was evaluated to be 6.7eV, and the films exhibited high transparency in the wide range from NIR to VUV. We discuss the potential of the film as a transparent conducting material in the VUV region.

Interfacial Thermal Conductance of Transfer‐Printed Metal Films

IO N Deterministic assembly of microdevices by transfer-printing is an advanced manufacturing technology that enables the heterogeneous integration of disparate materials on largearea substrates.[1–5] All active electronic devices generate heat as a byproduct of their operation and thermal management of transfer-printed assemblies must be a consideration whenever the heat flux is large. While the thermal conductivities of most materials used in microelectronics are well known, the thermal conductance of interfaces formed by transfer-printing is unknown. We report studies of the thermal conductance of interfaces formed by transfer-printing of Au and Au(Pd) alloy thin films, 100 μm × 100 μm in area and 100 nm thick, on amorphous SiO2 , hydrogen-terminated Si(001), and singlecrystal Al2O3 substrates. We find that the thermal conductance Gt of transfer-printed interfaces spans a relatively small range, 10 10 kW cm−2. The thermal conductance G of interfaces formed by physical vapor deposition of metal films on dielectric substrates has been studied extensively. G is the transport coefficient that relates the heat flux JQ to the temperature drop ΔT at an interface, JQ = GT . The observed values span a large range, from a low conductance of G ≈ 10 MWm−2K−1 for Bi deposited on hydrogen-terminated diamond[6] to a high conductance[7] of G ≈ 700 MWm−2K−1 for epitaxial TiN/MgO. Often,

Structure Analysis of Co Epitaxial Thin Films Grown on Al2O3 Single-Crystal Substrates

Co thin films were prepared on Al2O3 single-crystal substrates with (0001) and (11-20) orientations by ultra-high vacuum molecular beam epitaxy. Effects of substrate temperature and substrate orientation on the film structure were investigated. Co epitaxial thin films with close-packed planes parallel to the substrate surfaces are obtained on Al2O3(0001) and Al2O3(11-20) substrates at 50-500 °C and 300-500 °C, respectively. The films grown on Al2O3(0001) substrates at temperatures lower than 100 °C consist of fcc-Co(111) crystal. With increasing the substrate temperature, hcp-Co(0001) crystal begins to coexist with the fcc crystal and the volume ratio of hcp to fcc crystal increases. The films grown at temperatures higher than 300 °C consist primarily of hcp crystal coexisting with a small volume of fcc crystal. On the contrary, the Co thin films grown on Al2O3(11-20) substrates at temperatures higher than 300 °C consist of only the hcp crystal. The in-plane lattice strains of these Co epitaxial films are larger than the out-of-plane lattice strains due to the accommodation of lattice mismatches between the films and the substrates. The film strain decreases as the substrate temperature increases.

Fast lithium ion conducting solid state thin-film electrolytes based on lithium thio-germanate materials

In this study, lithium thio-germanate thin-film electrolytes for lithium rechargeable batteries have been successfully prepared by radio-frequency (RF) magnetron sputtering deposition in Ar gas atmospheres. The targets for RF sputtering were prepared by melting, milling and pressing the appropriate amounts of the starting materials in the nLi2S+GeS2, n=1–4, binary system. 2mm wide×18mm long Au electrodes with a parallel configuration of 2mm spacing were sputtered to ∼100nm thick at a sputtering rate at ∼5nmmin−1 on Al2O3 single crystal substrates. Thin-film electrolytes were grown on this electrode assembly at 50W power and 25mtorr in Ar gas pressure. The ionic conductivities of the thin-film electrolytes were measured from −25°C to 100°C with 25°C increments over the frequency range 0.1Hz–1MHz. The d.c. ionic conductivities determined from complex plane plots of the impedance of the Li2GeS3, Li4GeS4, Li6GeS5, and Li8GeS6 amorphous thin films at 25°C were found to be 1.1×10−4Scm−1, 7.5×10−4Scm−1, 1.7×10−3Scm−1, and 7.0×10−5Scm−1, respectively. As the Li2S content in the thin film increases, the ionic conductivities of the thin films increase from n=1 to n=3. However, for the n=4 Li8GeS6 thin film, the ionic conductivity decreased and the activation energy increased. The maximum in the conductivity for the n=3 film is among the highest ever reported for an amorphous solid state Li+ ion conductor.

Fabrication of Atomically Flat ScAlMgO4 Epitaxial Buffer Layer and Low-Temperature Growth of High-Mobility ZnO Films

Atomically flat single-crystalline thin films of ScAlMgO4 (SCAM) were fabricated on yttria-stabilized zirconia (YSZ) (111) substrates by reactive solid-phase epitaxy using an ablation ceramic target with an optimized chemical composition. Owing to the good lattice matching, the SCAM layers enhanced two-dimensional growth of ZnO epitaxial films, suppressed the formation of threading dislocations at step edges, and provided larger Hall mobilities than those of ZnO films on YSZ and Al2O3 single-crystal substrates especially at low deposition temperatures <700 °C. The ZnO films exhibited Hall voltage anomaly at temperatures <140 K, which is explained by a two-layer model with a degenerated layer.

Metal-semiconductor transition in epitaxial ZnO thin films

We report on the formation and transport properties of ZnO thin films which are grown by pulsed-electron beam deposition under a low residual oxygen pressure (10−5 mbar). ZnO films presenting metallic conductivity at room temperature, and a metal-semiconductor transition at low temperature, were epitaxially grown on Al2O3 single crystal substrates for growth temperatures in the 300–450 °C range. These results have been interpreted through the quantum corrections to conductivity in a disordered oxide conductor, implying first a high density of carriers leading to degenerate semiconductor ZnO films, and then a sufficient disorder in these films. These characteristics could be related to the nature of the ZnO films formed by pulsed-electron deposition: a high density of carriers related to an oxygen deficiency in the films and a high density of defects related to the high deposition rate of the pulsed-electron beam deposition method.

Photoinduced conductivity in tin dioxide thin films

The effects of ultraviolet light irradiation on the conducting properties of SnO2−x thin films grown epitaxially on TiO2 or Al2O3 single-crystal substrates are studied at room temperature. A large increase in conductivity by two to four orders of magnitude is observed with light irradiation in an inert atmosphere and remains after the light is removed. The high-conducting state reverts to the original low-conducting state by exposing it to oxygen gas. These reversible phenomena are ascribed to the desorption and adsorption of negatively charged oxygen species at the grain boundaries, which critically change the mobility of electron carriers already present inside grains by changing the potential barrier height at the grain boundary. The UV light irradiation provides us with an easy and useful route to achieve a high-conducting state even at low carrier density in transparent conducting oxides and also to draw an invisible conducting wire or a specific pattern on an insulating film.