R-plane Al2O3 Research Articles

Phase coexistence induced surface roughness in V2O3/Ni magnetic heterostructures

We present an investigation of the microstructure changes in V2O3 as it goes through its inherent structural phase transition. Using V2O3 films with a well-defined crystal structure deposited by reactive magnetron sputtering on r-plane Al2O3 substrates, we study the phase coexistence region and its impact on the surface roughness of the films and the magnetic properties of overlying Ni magnetic layers in V2O3/Ni hybrid magnetic heterostructures. The simultaneous presence of two phases in V2O3 during its structural phase transition was identified with high resolution x-ray diffraction and led to an increase in surface roughness observed using x-ray reflectivity. The roughness reaches its maximum at the midpoint of the transition. In V2O3/Ni hybrid heterostructures, we find a concomitant increase in the coercivity of the magnetic layer correlated with the increased roughness of the V2O3 surface. The chemical homogeneity of the V2O3 is confirmed through transmission electron microscopy analysis. High-angle annular dark field imaging and electron energy loss spectroscopy reveal an atomically flat interface between Al2O3 and V2O3, as well as a sharp interface between V2O3 and Ni.

Unusual tilted growth and epitaxial relationship of NaCl B1-structured NiO and CrN on r-plane Al2O3

Epitaxial NiO and CrN thin films were deposited on a single-crystal Al2O3(11¯02) (r-plane sapphire) using magnetron sputtering. The two materials were intentionally deposited into two different deposition chamber designs and under different conditions (temperature, pressure, gases, and energy of sputtered particles). Despite the differences in the deposition condition and material system, both materials had the same feature with uncommon tilted epitaxial growth. Through an in-depth x-ray diffraction analysis of the NaCl (B1)-structured materials on r-plane sapphire, the full twin domain epitaxial relations were determined and can be described as (110)NaCl(B1)∥(44¯03)Al2O3 and [11¯2]NaCl(B1)∥[1¯1¯20]Al2O3. This relationship differs from the previously observed orientation of (100)NaCl(B1)∥(11¯02)Al2O3 and [100]NaCl(B1)∥[101¯0]Al2O3. These results are of general relevance for the growth of the extended NaCl (B1)-structured cubic material family onto a r-plane sapphire substrate where similar epitaxial growth can be expected.

MOCVD deposited double-sided CeO2 buffer layer for YBCO superconducting film grow on R-plane Al2O3 substrate

Double-sided CeO2/YBCO films were grown on the double-sided polishing R-plane Al2O3 substrate using MOCVD technology. The effects of deposition parameters of the MOCVD setup on the structural orientation and surface morphology of CeO2 buffer layers grown on sapphire substrates were investigated. A series of CeO2 buffer layers were prepared by varying the substrate temperature and oxygen flow rate. The results show that when the substrate temperature is lower than 750 °C and the oxygen flow rate is lower than 400 sccm, CeO2 films is grown in a mixed (00l) and (111) orientations, and when the substrate temperature is 750 °C-840 °C and the oxygen flow rate is higher than 400 sccm, the CeO2 films are grown with a single (001) orientation. When the deposition temperature was 800 °C and the oxygen flow rate was 800 sccm, the CeO2 buffer layer structure had the best orientation with double-sided ω-scan FWHM of 0.019° and 0.02°; the FWHM of double-sided Ф-scan were 1.8° and 1.7°; the RMS roughness on both sides measured by AFM were 1.28 nm. YBCO films grown on this buffer layer have good c-axis epitaxial orientation, and the test results of the pole figure show an in-plane rotation of 45° between YBCO and CeO2 lattices. Thus, the obtained YBCO film has a critical current density of 2 MA/cm2, indicating that the films can be prepared to meet the needs of microwave devices using R-plane Al2O3 substrates.

Concurrent Structural and Electronic Phase Transitions in V2O3 Thin Films with Sharp Resistivity ChangeSupported by the National Key R&D Program of China (Grant No. 2017YFA0403600), and Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences Large-Scale Scientific Facility (Grant No. U1532149).

The relationship between structural and electronic phase transitions in V2O3 thin films is of critical importance for understanding of the mechanism behind metal–insulator transition (MIT) and related technological applications. Despite being extensively studied, there are currently no clear consensus and picture of the relation between structural and electronic phase transitions so far. Using V2O3 thin films grown on r-plane Al2O3 substrates, which exhibit abrupt MIT and structural phase transition, we show that the electronic phase transition occurs concurrently with the structural phase transition as revealed by the electrical transport and Raman spectra measurements. Our result provides experimental evidence for clarifying this issue, which could form the basis of theoretical studies as well as technological applications in V2O3.

Substrate dependent epitaxy of superconducting niobium nitride thin films grown by pulsed laser deposition

Niobium nitride (NbN) has suitable mechanical properties for application as protective coatings in mechanical engineering, and also its superconductivity can be utilized in thin film devices for sensorics or in combination with ferromagnet in spintronics. Long-range superconducting proximity effect at the heterostructures with a weak ferromagnet can be used for generation of spin-polarized current. For any operational heterostructure application the high quality NbN thin films need to be prepared. In this work we have investigated impact of the substrates on the structure and preferential orientation of niobium nitride thin films fabricated by pulsed laser deposition at 600 °C on Si (001), MgO (001), C-plane and R-plane Al2O3 substrates. Growth parameters have been tuned in order to obtain single superconducting δ-NbN phase. Microstructure was analyzed by X-ray diffraction and transmission electron microscopy. Si substrate does not induce the film preferential orientation. Films on MgO are epitaxial (001) oriented. Films on R-Al2O3 show (135) orientation with twinned crystallites on the lower symmetry substrate surface. The (111) epitaxial growth with the largest crystallites and their smallest tilting was achieved on C-Al2O3 substrate leading to the best superconducting properties.

Plastic strain relaxation and alloy instability in epitaxial corundum-phase (Al,Ga)2O3 thin films on r-plane Al2O3

The growth of (AlxGa1−x)2O3 alloy thin films in the corundum phase on r-plane (01.2) Al2O3 substrates is investigated.

Epitaxial growth and strain relaxation of corundum-phase (Al,Ga)2O3 thin films from pulsed laser deposition at 1000 °C on r-plane Al2O3

We investigate the epitaxial growth of (Al,Ga)2O3 alloy thin films in the corundum phase on r-plane (01.2) Al2O3 substrates. We compare films grown by pulsed laser deposition at substrate temperatures of 750 °C and 1000 °C. The initial strongly anisotropic plastic strain relaxation through the a-plane prismatic glide system is directly evidenced by imaging the associated slip lines. We find enhanced plastic relaxation at the higher substrate temperature. Details of the stoichiometry transfer from the target to the film are discussed.

Stress-Induced In Situ Modification of Transition Temperature in VO2 Films Capped by Chalcogenide.

We attempted to modify the monoclinic–rutile structural phase transition temperature (Ttr) of a VO2 thin film in situ through stress caused by amorphous–crystalline phase change of a chalcogenide layer on it. VO2 films on C- or R-plane Al2O3 substrates were capped by Ge2Sb2Te5 (GST) films by means of rf magnetron sputtering. Ttr of the VO2 layer was evaluated through temperature-controlled measurements of optical reflection intensity and electrical resistance. Crystallization of the GST capping layer was accompanied by a significant drop in Ttr of the VO2 layer underneath, either with or without a SiNx diffusion barrier layer between the two. The shift of Ttr was by ~30 °C for a GST/VO2 bilayered sample with thicknesses of 200/30 nm, and was by ~6 °C for a GST/SiNx/VO2 trilayered sample of 200/10/6 nm. The lowering of Ttr was most probably caused by the volume reduction in GST during the amorphous–crystalline phase change. The stress-induced in in situ modification of Ttr in VO2 films could pave the way for the application of nonvolatile changes of optical properties in optoelectronic devices.

Inhomogeneity-mediated systematic reduction of the Schottky barrier in a Au/GaN nanorod film interface

Self-aligned GaN nanorods of various densities are grown on an r-plane Al2O3 substrate with Stranski–Krastanov or layer-plus-island growth conditions by using a plasma-assisted molecular beam epitaxy system. These conditions result in the formation of a GaN nanorod matrix on an epitaxial GaN thin film. The orientation of the nanorods was found to be at an inclination of ∼60° from the substrate. As expected, the GaN thin film grows along the [11–20] direction, but interestingly the nanorods have a preferential growth direction along the [0002] axis. The overall structure mimics the Gaussian distribution of Schottky barriers at the metal–semiconductor interface. The GaN nanorod/thin-film matrix systematically causes the well-known Au/GaN Schottky metal–semiconductor interface to display an Ohmic type of behavior. A systematic reduction of the Schottky barrier is observed with an increase in the GaN nanorod density (from 5 to 65 nanorods micron−2). The overall configuration provides a tunable Gaussian distribution of Schottky barriers with nanorod density, which could be extremely useful for replacing conventional multi-level electrode stacking techniques.

Enhanced electromechanical coupling in SAW resonators based on sputtered non-polar Al0.77Sc0.23N 112¯ thin films

Non-polar a-plane Al0.77Sc0.23N 112¯0 thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3(11¯02) substrates. Different substrate off-cut angles were compared, and the off-cut angle of 3° resulted in the best structural quality of the AlScN layer. Structural characterization by x-ray diffraction confirmed that single phase, wurtzite-type, a-plane AlScN 112¯0, surface acoustic wave resonators were fabricated with wavelengths λ = 2–10 μm (central frequency up to 1.7 GHz) with two orthogonal in-plane propagation directions. A strong dependence of electromechanical coupling on the in-plane orientation was observed. Compared to conventional c-plane AlScN based resonators, an increase of 185–1000% in the effective electromechanical coupling was achieved with only a fractional decrease of <10.5% in series resonance frequency.

Anisotropic strain relaxation through prismatic and basal slip in α-(Al, Ga)2O3 on R-plane Al2O3

Pseudomorphic and partially relaxed layers of corundum phase (Al, Ga)2O3 epilayers on (01.2)-oriented Al2O3 fabricated by pulsed laser deposition (PLD) are investigated. An exact analytical (continuum elasticity) strain theory for rhombohedral heterostructures as a function of the two substrate orientation angles fits the strain state of pseudomorphic and relaxed samples very well. From reciprocal space maps and a quantitative analysis of x-ray diffraction peaks and tilts using the strain theory, it is concluded that in the present samples grown below 800 °C, plastic strain relaxation above the critical thickness occurs first through slip on the prismatic a-plane glide system and subsequently via the basal c-plane system. We also present a general PLD stoichiometry transfer model simultaneously explaining the epilayer alloy composition and growth rate in the entire composition range.

Remote Epitaxy of ZnO Microrods on Graphene-Coated Substrates

Two-dimensional nanomaterials are emerging for use as seed substrates toward transferrable, transparent, or flexible device applications.[1,2] In the van der Waals heteroepitaxy, graphene acted as seed for epitaxial overlayers of III–V or II–VI nanostructures or thin films.[3-5] Because the hydrophobic surface of graphene, given from lack of dangling bonds, substantially hinders the high-density nucleation in heteroepitaxy,[3] the use of graphene substrates was precluded for practical use as a substrate in the semiconductor epitaxy. Such surface properties of graphene have been known to be changeable by contacting other substances underneath graphene layer, because of the wetting transparency of graphene. More recently, the chemical property transparency was applied to the epitaxy technique to obtain single crystalline thin film overlayer whose crystal structure and orientation can be copied from underlying substrate across graphene. This is the so-called remote epitaxy, which was firstly demonstrated by Kim et al.[6,7] Here, we report on remote homoepitaxy of ZnO microrods on graphene-coated ZnO substrates with different a- and c-planes. The remote epitaxy was carried out via a hydrothermal growth using a nutrient solution with zinc nitrate hexahydrate and hexamethylenetetramine at 95 °C. For preparation of a- and c-plane ZnO substrate, ZnO thin films were heteroepitaxially coated on r-plane Al2O3 and c-plane GaN, respectively. Then, monolayer graphene, which was synthesized via a chemical vapor deposition (CVD) on copper foil, was transferred onto these substrates using a poly(methyl-methacrylate)-supported etching-and-transfer technique. ZnO epitaxial overlayers grew to be horizontally and vertically aligned microrod arrays on a-plane and c-plane ZnO substrates, respectively, across graphene layers, which were also found to show a well-defined long-range in-plane orientation over the entire surface of graphene-coated ZnO substrate, despite the presence of CVD-grown graphene with typical domain size of several micrometers. Cross-sectional transmission electron microscopic and selected area electron diffraction analyses revealed the homoepitaxial relationship with a remote gap of graphene. We further studied the successful remote homoepitaxy of ZnO across bilayer- and trilayer graphene, on which lower density of horizontal- and vertical ZnO microrods were obtained in comparison with that on monolayer graphene-coated substrate. This result strongly suggests that ZnO substrates played a crucial role in determining the crystallographic orientation and lattice arrangement of ZnO microrod overlayer across graphene layer, but as increasing the thickness of graphene, the impact of substrate is lesser for nucleation and growth of ZnO overlayer in the remote homoepitaxy. Such finding that electric field given from the substrate can be penetrated across graphene was simulated via density functional theory (DFT) calculations. The DFT calculations also indicated that the field penetration strength decreases with the increase of graphene thickness. The ability of graphene, which can be released from the host substrate without covalent bonds, was exploited to transfer the microrod arrays via a thermal release tape exfoliation technique. After the exfoliation, the host substrate was reused for repeating the remote epitaxy over again. This study opens a way for producing well aligned, transferrable epitaxial semiconductor microrod arrays while regenerating the substrate for cost-saving device manufacturing process. [1] K. Chung, C. H. Lee and G. C. Yi, Science, 2010, 330, 655–657. [2] C. H. Lee, Y. J. Kim, Y. J. Hong, S. R. Jeon, S. Bae, B. H. Hong, G. C. Yi, Adv. Mater., 2011, 23, 4614–4619. [3] Y. J. Hong, T. Fukui, ACS Nano, 2011, 5, 7576–7584. [4] A. M. Munshi, D. L. Dheeraj, V. T. Fauske, D. C. Kim, A. T. J. van Helvoort, B. O. Fimland, H. Weman, Nano Lett., 2012, 12, 4570–4576. [5] Y. J. Hong, J. W. Yang, W. H. Lee, R. S. Ruoff, K. S. Kim, T. Fukui, Adv. Mater., 2013, 25, 6847–6853. [6] Y. Kim, S. S. Cruz, K. Lee et al., Nature, 2017, 544, 340–343. [7] W. Kong, H. S. Li, K. Qiao et al., Nat. Mater., 2018, 17, 999–1004.

Heteroepitaxial growth of α-, β-, γ- and κ-Ga2O3 phases by metalorganic vapor phase epitaxy

Improved MOVPE (metalorganic vapor phase epitaxy) growth conditions for the different Ga2O3 phases are reported. The influence of the substrate material, the growth conditions and the variation of precursors on the phase formation was investigated. Trimethylgallium (TMG) and N2O were used as common precursors for gallium and oxygen, respectively. In addition, water was supplied for the growth of κ-Ga2O3. The influence of additional triethylboron (TEB) and dimethylhydrazin (DMHy) on the formation of the α-phase is discussed. Growth conditions could be determined, where α-, β-, γ- and κ-Ga2O3 is epitaxially deposited. The κ-phase was epitaxially grown on c-, a- and r-plane Al2O3, (1 1 1) MgO, (1 1 1) MgAl2O4 and on a (0 0 0 1) Al2O3/(0 0 0 1) GaN heterostructure. Successful MOVPE growth of γ-Ga2O3 with defect spinel structure has been achieved on (0 0 0 1) Al2O3, (1 1 1) and (1 0 0) MgAl2O4, (1 0 0) MgO as well as on (0 0 0 1) Al2O3/(0 0 0 1) GaN. Boron addition stabilizes the MOVPE growth of the α-phase on c-, a- and r-plane Al2O3. The various Ga2O3 phases were identified by X-ray diffraction (XRD). The epitaxial relationships between the Ga2O3 phases and the different substrates were determined.Optical transmission spectra measured in the spectral range from 200 to 2000 nm show a well-defined absorption edge at about 4.9 eV for layers of β-, γ- and κ-Ga2O3 and 5.3 eV for layers of α-Ga2O3 grown on c-plane sapphire.

Effect of impurities on morphology, growth mode, and thermoelectric properties of (1 1 1) and (0 0 1) epitaxial-like ScN films

ScN is an emerging semiconductor with an indirect bandgap. It has attracted attention for its thermoelectric properties, use as seed layers, and for alloys for piezoelectric application. ScN and other transition metal nitride semiconductors used for their interesting electrical properties are sensitive to contaminants, such as oxygen or fluorine. In this present article, the influence of depositions conditions on the amount of oxygen contaminants incorporated in ScN films were investigated and their effects on the electrical properties (electrical resistivity and Seebeck coefficient) were studied. Epitaxial-like films of thickness 125 ± 5 nm to 155 ± 5 nm were deposited by DC-magnetron sputtering on c-plane Al2O3, MgO(1 1 1) and r-plane Al2O3 at substrate temperatures ranging from 700 °C to 950 °C. The amount of oxygen contaminants in the film, dissolved into ScN or as an oxide, was related to the adatom mobility during growth, which is affected by the deposition temperature and the presence of twin domain growth. The lowest values of electrical resistivity of 50 µΩ cm were obtained on ScN(1 1 1)/MgO(1 1 1) and on ScN(0 0 1)/r-plane Al2O3 grown at 950 °C with no twin domains and the lowest amount of oxygen contaminant. At the best, the films exhibited an electrical resistivity of 50 µΩ cm with Seebeck coefficient values maintained at −40 µV K−1, thus a power factor estimated at 3.2 × 10−3 W m−1 K−2 (at room temperature).

Properties of ZnO/ZnMgO nanostructures grown on r-plane Al2O3 substrates by molecular beam epitaxy

Growths of ZnMgO/ZnO/ZnMgO multiple quantum well (MQW) structures were performed on semi-polar r-plane sapphire substrates by molecular beam epitaxy. It has been shown that it is possible to grow self-organized ZnMgO nanocolumns with MQW structures at a temperature of 550 °C without employing a catalyst. The nanocolumns are symmetrically tilted at an angle of 62° to the substrate surface. Deposition of a low-temperature (450 °C) ZnO buffer layer prior to the growth of ZnO/ZnMgO MQWs results in good quality planar structures. The crystalline quality of the nanostructures was characterized by X-ray diffraction and scanning electron microscopy. Luminescence properties of ZnO/ZnMgO heterostructures and MQWs were studied. The luminescence was dominated by localized exciton emissions below 100K, while free exciton recombination was the most intense emission at temperatures above 100 K.

Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

MgB2 thin films were fabricated on r-plane Al2O3 (\({1} \overline{{1}} {0} {2})\) substrates. First, deposition of boron was performed by rf magnetron sputtering on Al2O3 substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe2O3 nanoparticles on the structural and magnetic properties of films, MgB2 films were coated with different concentrations of Fe2O3 nanoparticles by spin coating process. The magnetic field dependence of the critical current density J c was calculated from the M–H loops and magnetic field dependence of the pinning force density, f p(b), was investigated for the films containing different concentrations of Fe2O3 nanoparticles. The critical current densities, J c, in 3T magnetic field at 5 K were found to be around 2·7 × 104 A/cm2, 4·3 × 104 A/cm2, 1·3 × 105 A/cm2 and 5·2 × 104 A/cm2 for films with concentrations of 0, 25, 50 and 100% Fe2O3, respectively. It was found that the films coated with Fe2O3 nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe2O3 became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe2O3 nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB2 thin films were deposited on MgO (100) substrate and r-plane Al2O3\((1\bar{1}02)\) substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB2 films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density Jc was calculated from M–H loops and also the magnetic field dependence of Fp was compared for the different temperature ranges from 5 to 25 K. The critical current density Jc was found to be around 1.0×106 A/cm2 and 1.7×106 A/cm2 in zero field at 5 K for MgB2 films deposited on MgO and r-plane Al2O3 substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al2O3 in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB2 films deposited on MgO and r-plane Al2O3 substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.

Magnetic field influence on the transient photoresistivity of defect-induced magnetic ZnO films

Magnetic field dependent photoresistivity was measured at 280 K in ZnO ferromagnetic films grown on r-plane Al2O3 under a N2 atmosphere. A correlation between the negative magneto photoresistivity and the existence of defect-induced magnetic order was found. The effect of magnetic field on the transient photoresistivity is to slow down the recombination process enhancing the photocarriers density. The experimental results demonstrate the possibility of tuning photocarriers life time using magnetic field in diluted magnetic semiconductors.

Growth Orientation Dependent Hardness for Epitaxial Wurtzite InN Films

We report the hardness of wurtzite InN thin film grown with different crystalline orientations of heteroepitaxial s-plane (1101) and a-plane (1120) on r-plane (1102) Al2O3, and homoepitaxial film on c-plane (0001) Al2O3. Hardness values along with elastic properties are studied using nanoindentation technique. Maximum hardness is reported for c-plane (approximately 8 GPa), which is followed by s-plane (approximately 5.5 GPa) and then a-plane (approximately 4 GPa) oriented InN Films. Peierls force in the slip system of different crystalline orientations for wurtzite sample is calculated for explaining the systematic variations in hardness values of these films grown with different crystalline orientations.

Microstructure, optical, and electrical properties of p-type SnO thin films

SnO thin films were fabricated by electron beam evaporation on (100) Si and c- and r-plane Al2O3 substrates. The films grown at 25 °C are nanocrystalline, while the films grown at 600 °C are epitaxial on r-plane Al2O3 and (001) textured on Si and c-plane Al2O3. The SnO films have an optical band gap of 2.82–2.97 eV and p-type conductivity, according to Hall measurements, with resistivities of 0.5–110 Ω cm, hole concentrations of 1017–1019 cm−3, and Hall mobilities of 0.1–2.6 cm2/Vs. The p-type conductivity, which appears to correlate with VSn, can be enhanced via Y- and Sb-doping. Defect complexes of SbSn−2VSn are suggested to be the acceptors in Sb-(or Y-) doped SnO films.