Single Crystal Substrates Research Articles

Pulsed Laser Deposition of Epitaxial SrTiO3/Sr3Al2O6 Templates as a Water-Soluble Sacrificial Layer for GaAs Growth and Lift-Off.

Despite the record-high efficiency of GaAs solar cells, their terrestrial application is limited due to both the particularly high costs related to the required single-crystal substrates and epitaxial growth. A water-soluble lift-off layer could reduce costs by avoiding the need for toxic and dangerous etchants, substrate repolishing, and expensive process steps. Sr3Al2O6 (SAO) is a water-soluble cubic oxide, and SrTiO3 (STO) is a perovskite oxide, where a SAO ≈ 4 × a STO ≈ (2√2)a GaAs. Here, the pulsed laser-deposited epitaxial growth of SrTiO3/Sr3Al2O6 templates on STO and Ge substrates for epitaxial GaAs growth was investigated, where SAO works as a sacrificial layer and STO protects the hygroscopic SAO during substrate transfer between deposition chambers. We identified that the SAO film quality is strongly dependent on the growth temperature and the O2 partial pressure, where either a high T or a high P(O2) improves the quality. XRD spectra of the films with optimized deposition parameters showed an epitaxial STO/SAO stack aligned to the STO (100) substrate, and TEM analysis revealed that the grown films were epitaxially crystalline throughout the thickness. The STO/SAO growth on Ge substrates at a high T with no intentional O2 flow resulted in some nonepitaxial grains and surface pits, likely due to partial Ge oxidation. GaAs was grown by metalorganic vapor-phase epitaxy (MOVPE) on STO/SAO/STO templates. Lift-off after dissolving the sacrificial SAO in water resulted in free-standing ⟨001⟩ preferentially oriented polycrystalline GaAs.

Controlled growth of YBCO film with tungsten oxide buffered substrate

This study explores the influence of tungsten oxide (WO3-x) nanostructures, decorated on LaAlO3 (LAO) single crystal substrate, on the fabrication and superconducting performance of YBa2Cu3Oy (YBCO) films. The WO3-x in square-dot and nanowire forms, significantly enhance the biaxial texture of YBCO films, resulting in high critical current density Jc (square-dot, 3.05 MA/cm2 @ 77 K, nanowire, 2.1 MA/cm2 @ 77 K). However, the presence of WO3-x nanodots leads to a random nucleation of YBCO grains along the a/b-axis, driving a porous surface and broadening the full width at half maximum (FWHM) in both phi and omega scans. This poor texture deteriorates the film’s Jc to 0.71 MA/cm2 @ 77 K. Therefore, optimizing the WO3-x interface is crucial for enhancing the epitaxial growth and critical current density of YBCO film. This study provides an effective approach to optimize the superconducting performance of YBCO film by tailoring nanostructure interface.

Three-Layered Composite Scintillator Based on the Epitaxial Structures of YAG and LuAG Garnets Doped with Ce3+ and Sc3+ Impurities.

In this study, we propose novel three-layer composite scintillators designed for the simultaneous detection of different ionizing radiation components. These scintillators are based on epitaxial structures of LuAG and YAG garnets, doped with Ce3+ and Sc3+ ions. Samples of these composite scintillators, containing YAG:Ce and LuAG:Ce single crystalline films with different thicknesses and LuAG:Sc single crystal substrates, were grown using the liquid phase epitaxy method from melt solutions based on PbO-B2O3 fluxes. The scintillation properties of the proposed composites, YAG:Ce film/LuAG:Sc film/LuAG:Ce crystal and YAG:Ce film/LuAG:Ce film/LuAG:Sc crystal, were investigated under excitation by radiation with α-particles from a 239Pu source, β-particles from 90Sr sources and γ-rays from a 137Cs source. Considering the properties of the mentioned composite scintillators, special attention was paid to the ability of simultaneous separation of the different components of mixed ionizing radiation containing the mentioned particles and quanta using scintillation decay kinetics. The differences in scintillation decay curves under α- and β-particle and γ-ray excitations were characterized using figure of merit (FOM) values at various scintillation decay intensity levels (1/e, 0.1, 0.05, 0.01).

(Invited) The Evolving SiC Market and Impact on CMP Supply Chain

Silicon carbide (SiC) is a wide bandgap semiconductor with physical and electrical properties that provide superior performance in high-voltage or high-power applications. Compared to the more established silicon devices, SiC provides better thermal conductivity, ~10x higher breakdown voltage, faster switching speeds and lower resistivity. Even with the performance advantages, the adoption rate of SiC is slowed by high cost and the inherent difficulty of processing this material. SiC is extremely hard (just below diamond on the hardness scale) and chemically inert which makes processing boules into wafers rather difficult. This presentation will contrast some of the basic differences between process methods developed for silicon and other semiconductor materials versus those required for SiC.Demand for high-quality single crystal SiC substrates continues to rise as the worldwide demand for electric vehicles, charging infrastructure, green energy production, and more efficient power devices in general requires more and more SiC devices. SiC wafer manufacturers are trying to ramp output as quickly as possible but technical difficulties of processing SiC are often a limiting factor. In spite of aggressive capital spending by many of the major players, the actual growth in substrate production has been constrained to around 15% CAGR even though demand would support a much faster ramp. As end use markets continue to evolve, ripple effects are also evident in the underlying supply chains, such as CMP pads and slurries.

Epitaxial Growth and Doping of N-Polar AlN by Plasma-Assisted Molecular Beam Epitaxy

In recent years, AlN has garnered significant attention in the research community due to its excellent electronic properties. AlN, which is an ultra-wide bandgap material with a direct band gap of 6.2 eV, has a high breakdown electric field (12 MV/cm) and high thermal conductivity. In addition, band engineering can be achieved by alloying AlN with Gallium (Ga), Indium (In), and Scandium (Sc), and two-dimensional electron gas (2DEG) is formed at the AlGaN/AlN interface due to strong piezoelectric and spontaneous polarization. Furthermore, AlN has high-temperature stability, which is suitable for application in harsh environments. Due to these unique properties, AlN has gained attention as a promising candidate for the next generation of high power electronic and optoelectronic applications.In this talk, we present the demonstration of controllable Si doping in N-polar AlN films grown on single crystal AlN substrates by plasma assisted molecular beam epitaxy (PAMBE). Through optimization of growth conditions, we obtained high quality N-polar AlN films at high temperatures. However, our studies revealed that Si incorporation dramatically decreases at such high growth temperature. To enable higher Si incorporation, a hybrid growth scheme was developed, using a combination of low-temperature and high-temperature growth condition and by using Ga as a surfactant at low growth temperature. By lowering the growth temperature of AlN to 750°C, we were able to incorporate Si with concentrations as high as 2x1020 cm-3 and demonstrated an electron concentration as high as 1.25x1019 cm-3 at room temperature.

(Invited) Preparation of N-Type and P-Type Doped Single Crystal Diamonds with Laser-Induced Doping and Microwave Plasma Chemical Vapor Deposition

In this study, n-type doped single crystal diamonds were successfully prepared under normal temperature and pressure conditions using laser-induced doping. 248nm pulsed laser beams with nanosecond duration were irradiated on the single crystal diamond substrate immersing in an 85% phosphoric acid solution and it introduced phosphorus doping to form an n-type doped thin layer. The resistivity of the doped region significantly decreased compared to that of the single crystal diamond. After depositing a titanium electrode, the resistivity of the doped film obtained by Van der Pauw Technique was determined to be 1.3×10-6 Ω·m. Raman spectroscopy confirmed the absence of carbon or graphite phases in the laser-induced doped diamond, indicating that the enhanced conductivity was due to phosphorus incorporation.Furthermore, p-type doped single crystal diamonds were successfully prepared by introducing boron during the growth process using Microwave Plasma Chemical Vapor Deposition（MPCVD）. It was demonstrated that the proposed technique can introduce impurities into single crystal diamonds to form doped conductive thin layers, which has potential applications in the field of microdevices and integrated circuits.

(Invited) Engineering Nanocomposite Substrates for Metal Deposition

Reversible metal plating is at the heart of aqueous based energy storage. Since virtually all metal anodes, e.g., Fe, and Zn, operate outside the stability window of water, it is thus essential to minimize their surface area by controlling the nucleation and growth during deposition and the pit formation during stripping.While epitaxial growth has shown promise as exemplified by the horizontal zinc deposition on graphene, an approximate heteroepitaxy, we will focus our effort in developing a nanocomposite substrate that promotes uniform nucleation sites and fast surface transport. The strategy is potentially highly scalable to address the need for large scale storage. Recently, we have shown that a nanocomposite substrate can induce single crystalline Li growth. By reacting FeF3 with Li, a nanocomposite of Fe and LiF is formed which features Fe nanoparticles of < 2 nm in size uniformly distributed in a matrix of LiF. Despite the global lithiophobility of the substrate, nano Fe domains provide energetically uniform nucleation sites for Li deposition while LiF enables rapid surface transport. Corporative merging of neighboring seeds lead to the formation of single crystalline Li. Unlike single crystal substrates, the nanocomposite features uniformly distributed, energetically equivalent sites for nucleation.We have adopted a similar strategy for Fe and Zn deposition. Preliminary results show that significantly improved macroscopic uniformity is achieved with the nanocomposite substrate. We will discuss materials requirements for operation in an aqueous environment and the general applicability of the approach for other materials.

Preparation of high purity phase (Bi, Pb)-2223 superconducting films by sol–gel method

In this work, a (Bi, Pb)2Sr2Ca2Cu3O10+δ ((Bi, Pb)-2223) solution with good film-forming capability and chemical stability was synthesized through using metal acetates as raw materials, acrylic acid as an additive, and anhydrous methanol as a solvent to adjust the sol formula. Subsequently, (Bi, Pb)-2223 thin films were fabricated on LaAlO3 (LAO) single-crystal substrates via dip-coating technology at different heat treatment temperatures, and the structures and properties of (Bi, Pb)-2223 films were characterized. It was found that when the sol stoichiometric ratio of Bi: Pb: Sr: Ca: Cu was controlled at 1.9: 0.35: 2: 2: 3 and the gel film was heat-treated at 860 °C and dwelled for 120 min in an mixed atmosphere of N2: O2 = 96: 4, the prepared thin film with a pure (Bi, Pb)-2223 superconducting phase exhibited excellent biaxial texture, a dense surface structure, and had a T c of 106 K and a ΔT c of 6 K. The microstructure analysis revealed a layered epitaxial growth of the film along the LAO substrate with a regular arrangement of atoms and a consistent interlayer spacing.

Effect of bias voltage on the structure and properties of CoCrFeNi high entropy alloy thin films prepared by magnetron sputtering

In this study, CoCrFeNi high entropy alloy thin films were prepared on 304 stainless steel and single crystal Si substrates using the direct-current magnetron sputtering process under different bias voltages. The effects of these bias voltages on the film micrographs, phase structure, surface roughness, corrosion resistance, and mechanical characteristics of thin films were studied by a scanning electron microscope, transmission electron microscope, X-ray diffractometer, atomic force microscope, electrochemical workstation and nanoindentation tester, respectively. The results indicated that all films exhibited equiaxed nano-scale grains and developed a columnar structure. Fe, Co, Cr, and Ni were uniformly dispersed in all films and all films displayed face centered cubic solid solution (FCC). At a bias voltage of 0 V, the CoCrFeNi HEAs film formed a FCC single phase. With increasing bias voltage, the crystallinity decreased and then increased, reaching the worst crystalline state at bias voltage of −200 V due to the fact that the structure of the film was composed of FCC nanocrystalline and amorphous biphasic structure. Meanwhile, the surfaces of the film prepared at bias voltage of −200 V exhibited the densest structures and the least roughness, with grain sizes reaching a minimum value of approximately 10 nm. In addition, columnar grains of the film prepared at a bias voltage of −200 V displayed the densest state and had the least size, thus this film displayed the best corrosion resistance of a minimum value 1.51 × 10−7 A/cm2, a maximum hardness of approximately 11.18 ± 0.2 GPa and a maximum elastic modulus of approximately 193 ± 2.8 GPa. The passivation films of CoCrFeNi HEAs films were mainly composed of Cr2O3.

Evaluation of the effect of oxygen addition on charge carrier transport properties in single-crystal diamond growth

The effects of oxygen addition during single-crystal diamond growth by microwave plasma CVD method on crystal surface morphology and charge carrier transport properties were evaluated. Diamond single-crystals were homoepitaxially grown on (001) surface of high-pressure and high-temperature (HPHT) synthesized type IIa single-crystal diamond substrates with a fixed methane concentration of 1 % and oxygen concentrations of 0–0.8 %. Inverted pyramid-shaped pits were generated as the oxygen concentration increased. The free exciton recombination emission intensity in the cathodoluminescence spectrum reached a maximum at oxygen concentration of 0.5 %. The samples with oxygen concentrations of 0.5 % and 0.8 % showed excellent charge collection efficiency and energy resolution. On the other hand, the mobility-lifetime (μτ) product was only (7.1 ± 1.7) × 10−5 cm2/V, which is approximately 1/4 of that of the sample with 0.2 % methane and no oxygen addition.

Importance of growth method and substrate-induced crystalline quality in Al/Gd0.2Ca0.8MnO3/Au memristor devices

We report on the impact of the growth method and substrate-induced crystalline quality on the performance of planar Al/Gd0.2Ca0.8MnO3/Au memristor devices. Structural, magnetic, and resistive properties were thoroughly examined for fundamental characterizations, with a particular emphasis on their correlation with the memristive properties of fabricated devices. Our findings suggest that memristor structures grown on single crystal SrTiO3 substrates using pulsed laser deposition (PLD) consistently exhibit superior crystalline quality compared to those fabricated using chemical solution deposition and on silicon-based substrates. Despite variations in growth method and substrate, all memristor structures display typical resistive switching (RS) behaviour, distinguishing between high-resistance and low-resistance states. However, endurance and retention measurements demonstrate that memristor structures produced via PLD on single crystalline SrTiO3 demonstrate the most favourable RS properties. To elucidate the mechanisms underlying the differences in RS behavior across substrates and deposition methods, we extensively discuss these issues in the context of structural distortion and conduction mechanisms.

Ferroelastic strain control of multiple nonvolatile resistance tuning in Cr:In2O3/PMN-PT(111) multiferroic heterostructures

Strain can significantly affect the electronic structure and functional properties of dilute magnetic semiconductors. As a wide bandgap transparent semiconductor, doped In2O3 has also received extensive attention for the modulation of physical properties by lattice strain due to its excellent functional properties. Here, we epitaxially grew the Cr:In2O3 thin film on the (111)-oriented 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) ferroelectric single-crystal substrate. By applying an electric field to PMN–PT, multiple reversible and nonvolatile resistance states can be achieved at room temperature. Utilizing in situ XRD, different strain states corresponding to different resistance states induced by the ferroelastic domain switching of the PMN–PT were characterized. Based on first-principles calculations, the influence of lattice strain on the resistivity of the Cr:In2O3 was discussed. These results offer a route for the design of multiple-valued nonvolatile memory devices and multiple functional magneto-electric devices based on dilute magnetic semiconductors.

Manganite Heterostructures: SrIrO&lt;sub&gt;3&lt;/sub&gt;/La&lt;sub&gt;0.7&lt;/sub&gt;Sr&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; and Pt/La&lt;sub&gt;0.7&lt;/sub&gt;Sr&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; for Generation and Registration of Spin Current

This paper presents the results of experimental studies of the cross section of the boundaries of the SrIrO3/La0.7Sr0.3MnO3 и Pt/La0.7Sr0.3MnO3, heterostructures, in which, upon excitation of ferromagnetic resonance in a La0.7Sr0.3MnO3 film, a spin current arises that flows through the boundary in structure. Epitaxial growth of thin films of strontium iridate SrIrO3 and manganite La0.7Sr0.3MnO3 on a (110) NdGaO3 single-crystal substrate was carried out using magnetron sputtering at high temperature in a mixture of argon and oxygen gases. The spin mixing conductance, which determines the amplitude of the spin current and generally has real Re g↑↓ and imaginary Im g↑↓ parts, was determined from the frequency dependence of the FMR spectrum of the LSMO film and heterostructures. It is shown that the Im g↑↓ quantity, can play an important role in determining the spin Hall angle (θSH) from the angular dependence of the spin magnetoresistance. For the SrIrO3/La0.7Sr0.3MnO3 heterostructures, θSH turned out to be significantly higher (almost an order of magnitude) than for the Pt/La0.7Sr0.3MnO3 heterostructure.

Acoustic-assisted deposition of ordered self-assembled silica opal layers on hydrophilic single crystal diamond substrates

We deposited self-assembled films of a few monolayers of silica spheres from the water-based colloid on vertically moving hydrophilic single crystal diamond substrates. The use of acoustic agitation of the water suspension with the SiO2 nanospheres significantly improved the long-range order of the films with opal structure. The study examines the evolution of the structure, specifically the size of single crystal domains, with the assistance of sound frequency (0–2500 Hz) and intensity (0–125 dB). Optimal (resonance) parameters are determined based on scanning electron microscopy and optical reflection spectroscopy of the films. The production of high-quality photonic crystals, including templates for diamond-based ordered composites and inverse opal structures, is a promising application of acoustic crystallization of opals on diamond substrates.

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals.

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Enhanced cation ordering, electron-spin-phonon interactions and Fano resonance in half-metallic Sr2FeMoO6 thin films

We report the presence of electron-spin-phonon interactions in half-metallic ferromagnetic Sr2FeMoO6 (SFMO) double perovskite thin films using temperature-dependent Raman spectroscopy. A series of SFMO thin films have been prepared on LaAlO3 (001) single-crystal substrate using the Pulsed Laser Deposition technique. These depositions have been made in different gas-conditions such as in vacuum, and under nitrogen and oxygen gas pressures. At room temperature, Raman spectra manifest a Fano feature which indicates the presence of electron-phonon coupling in the films. The electron-phonon coupling strength further changes with a change in deposition conditions. Magnetization results show that the SFMO film grown in vacuum has the highest saturation magnetization which suggests better cation ordering as compared to the other films. For enhanced understanding, Raman spectra were recorded at varied temperatures and the data were analyzed by theoretical model fittings. A parameter quantifying temperature-dependent anharmonic nature of phonons has been derived using Balkanski model fits. This parameter shows a drastic deviation in the vicinity of Curie temperatures, manifesting a spin-phonon coupling in SFMO films. We further show that the spin-phonon coupling strengthens with improved Fe–Mo ordering. Any experimental observation of spin-phonon coupling has not been reported for SFMO systems till date. The magnetization data corroborate well with these observations made by Raman measurements. Our results of Raman spectroscopy, magnetization and resistivity collectively suggest that the SFMO films exhibit electron-spin-phonon interactions, which are influenced by the cation ordering. We also devised out the method of relating the anharmonic nature of Raman modes with the degree of Fe–Mo ordering and spin-phonon coupling in double-perovskite materials.

Strained single crystal high entropy oxide manganite thin films

The ability to accommodate multiple principal cations within a single crystallographic structure makes high entropy oxides (HEOs) ideal systems for exploring new composition–property relationships. In this work, the high-entropy design strategy is extended to strained single-crystal HEO-manganite (HEO-Mn) thin films. Phase-pure orthorhombic films of (Gd0.2La0.2Nd0.2Sm0.2Sr0.2)MnO3 were deposited on three different single-crystal substrates: SrTiO3 (STO) (100), NdGaO3 (110), and LaAlO3 (LAO) (100), each inducing different degrees of epitaxial strain. Fully coherent growth of the thin films is observed in all cases, despite the high degree of lattice mismatch between HEO-Mn and LAO. Magnetometry measurements reveal distinct differences in the magnetic properties between epitaxially strained HEO-Mn thin films and their bulk crystalline HEO counterparts. In particular, the bulk polycrystalline HEO-Mn shows two magnetic transitions as opposed to a single one observed in epitaxial thin films. Moreover, the HEO-Mn film deposited on LAO exhibits a significant reduction in the Curie temperature, which is attributed to the strong variation of the in-plane lattice parameter along the thickness of the film and the resulting changes in the Mn–O–Mn bond geometry. Thus, this preliminary study demonstrates the potential of combining high entropy design with strain engineering to tailor the structure and functionality of perovskite manganites.

Over 600 V Lateral AlN-on-AlN Schottky Barrier Diodes with Ultra-Low Ideality Factor

This letter reports the demonstration of lateral AlN Schottky barrier diodes (SBDs) on single-crystal AlN substrates by metalorganic CVD (MOCVD) with an ultra-low ideality factor (η) of 1.65, a high Schottky barrier height of 1.94 eV, a breakdown voltage (BV) of 640 V, and a record high normalized BV by the anode-to-cathode distance. The device current was dominated by thermionic emission, while most previously reported AlN SBDs suffered from defect-induced current with higher η (>4). This work represents a significant step towards high-performance ultra-wide bandgap AlN-based high-voltage and high-power devices.

Pulsed laser deposition and structural analysis of Ba2Ti6O13 epitaxial thin film on the (210) surface of SrTiO3 single crystal

A single crystalline thin film of Ba2Ti6O13 was grown on the (210) surface of a SrTiO3 single-crystal substrate by pulsed laser deposition. Scanning transmission electron microscopy shows that the Ba2Ti6O13 thin film grew epitaxially with the a-plane, and the b-/c-axes of Ba2Ti6O13 parallel to the (210) plane and the [00 1¯ ]/[ 1¯ 20] directions of SrTiO3, respectively. The electrical conductance of the Ba2Ti6O13 film was approximately one order larger than that of the reduced SrTiO3−δ single crystal with oxygen deficiency of roughly δ = 0.05. Electronic structure calculations predict that Ba2Ti6O13 is an n-type degenerate semiconductor with spin-split states primarily of Ti 3d orbital.

Microstructure evolution during oxidation of protruded NiCoCrAlYTa-Al2O3 abrasive coating produced by vacuum infiltration sintering

In this paper, protruded NiCoCrAlYTa-Al2O3 abrasive coatings were prepared by vacuum infiltration sintering, and the effect of the filler metal content on the microstructure before and after oxidation is investigated. The results show that, the Al2O3 grits are bonded to the coating matrix by a layer of Y-Al-O compound, and various types of G-phase are presented depending on the locations. Adjacent to the G-phase, larger γ' phase and γ/γ' eutectics are presented due to the expel of Al during the formation of G-phase. With the increase of the filler metal content, G-phase precipitation and the Si penetration depth into the single crystal superalloy substrate increase. After 1000 °C/100 h oxidation, the Y-Al-O intermediate layer between Al2O3 grits and the coating matrix is little changed, with an additional thermally grown oxide (TGO) layer between the Y-Al-O layer and the coating matrix presented. The selective oxidation of Al and Cr facilitates the formation of G-phase layer underneath the oxide film, and the rapid diffusion of Ti and the selective oxidation of Al yields the formation of γ' depletion region adjacent to the G-phase and on coating surface without Al2O3 grits.