c-Al2O3 Substrates Research Articles

Epitaxial growth of high-quality Mg3Sb2 thin films on annealed c-plane Al2O3 substrates and their thermoelectric properties

High-quality epitaxial Mg3Sb2 thin films are promising thermoelectric materials to enable practical applications of compact and environmentally friendly thermoelectric conversion at RT. In this study, high-quality single-crystal Mg3Sb2 with high c-plane orientation was epitaxially grown directly on annealed c-Al2O3 substrates without passive layers. These thin films exhibited about three times higher thermoelectric power factor than any previously reported values due to high carrier mobility. The ultra-smooth surface of the annealed c-Al2O3 substrate facilitated the formation of high-quality Mg3Sb2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.

Piezoelectric response of InN nanostructures grown on c-Al2O3 and Si substrates: Role of oxygen incorporation in improved piezoresponse

Materials with high piezoelectric coefficients are crucial in developing high piezopotential nanogenerators. The present study reports the effect of oxygen incorporation in improving the piezoelectric coefficient of InN nanostructures. InN nanostructures are grown on sapphire and Si substrates using the chemical vapor deposition technique. The nanostructures grown on Si substrate have a marginally higher carrier density (∼ 2.6 × 1018 cm−3) compared to that (∼ 1.8 × 1018 cm−3) for nanostructures grown on a sapphire substrate. The piezoresponse of nanostructures is studied using piezoresponse force microscopy (PFM). InN nanostructures grown on Si substrates showed one-order higher piezoresponse than nanostructures grown on a sapphire substrate. The piezoelectric coefficients (d33) are estimated to be 12 and 28 pm/V for the nanostructures grown on sapphire and Si substrates, respectively. The high piezoresponse of nanostructures grown on Si substrate is attributed to the structural instability and chemical heterogeneity induced by O defects.

Initial Growth Behavior in Catalyst-Free-Grown Vertical ZnO Nanorods on c-Al2O3, as Observed Using Synchrotron Radiation X-ray Scattering

In this study, synchrotron radiation X-ray diffraction analysis is used to investigate the initial growth dynamics of metal–organic vapor-phase epitaxy-grown ZnO nanorods on c-Al2O3 substrates. A two-dimensional area detector with a grazing-incidence geometry enables projected reciprocal space mapping (RSM) using coordinate transformation of diffraction images. The projected RSM reveals multiple heteroepitaxial relationships between ZnO and Al2O3, including the dominant relationship of ZnO(101̅0)∥Al2O3(21̅1̅0). Evolution of internal strain is revealed by an additional study on diffraction images and θ–2θ scans. Surface topology from atomic force microscopy estimates the growth rate, supported by the Scherrer equation analysis of the θ–2θ diffraction.

Effect of oxygen pressure on the properties of n-type cuprous oxide films prepared by pulsed laser deposition

The quality of the n-type cuprous oxide film is the key to improving the conversion efficiency of the cuprous oxide homojunction solar thin film battery, and its preparation is very difficult. In this paper, pulsed laser deposition (PLD) was used to stably prepare n-type Cu2O films on (0001) sapphire (c-Al2O3) substrates, and the influence of the oxygen pressure in the chamber on the structure and electrical properties of the films was studied in detail. The measurments of X-ray diffractometer, scanning electron microscope, atomic force microscope, and Hall effect show that the Cu2O films exhibited n-type conductivity uder the oxygen pressure of 0.5, 1, 2, and 5 Pa, respectively. At the same time, the film under the oxygen pressure of 0.5 Pa produce the best crystal quality and electrical properties of an electron concentration of 2.753×1011cm-3and mobility of 19.76cm2/vs.

Preparation of high performance Ga2O3 based ultraviolet photodetector by CVD

Here, β-Ga2O3 films have been grown on c-Al2O3 and p-GaN substrates by chemical vapor deposition method to fabricate solar-blind ultraviolet photodetectors with MSM-type planar structure and pn-junction vertical structure respectively. The β-Ga2O3 films grown with domain growth mode exhibit high crystallinity and flat surface, so the fabricated MSM-type photodetector shows spectral peak responsivity of 1.83 A/W at 254 ​nm and photo-to-dark current ratio of 104 under 25 ​V bias with 20% oxygen fraction and 120 sccm flow rate. On this basis, the constructed vertical structured β-Ga2O3/p-GaN heterojunction photodetector shows peak responsivity of 1.99 A/W under 25 ​V for the junction effect between β-Ga2O3 and p-GaN. Meanwhile, spectral responsivity peak of 1.36 ​× ​10−4 A/W at 0V claims significant self-powered effect induced by build-in electric field between β-Ga2O3 and p-GaN. These results confirm that high performance β-Ga2O3 solar-blind ultraviolet photodetectors can be obtained by simple chemical vapor deposition method.

Laser synthesis of volatile memristors based on niobium oxide thin films

Amorphous NbOx (x ≤ 2.5) thin films with the presence of nanocrystallites, which are promising for use as an active region of volatile memristors were fabricated on c-Al2O3 substrates by the laser synthesis. The films were obtained from targets of niobium metallic during ablation of laser with λ = 248 nm or λ = 532 nm in oxygen pressure range from 5 to 50 mTorr. The effect of the oxygen partial pressure and the wavelength of ablating laser on the chemical, structural, electrical, and optical properties of the films was studied. The optimal conditions for the formation of Nb2O5 and NbO2 stable phases in the films were determined. It was found that the amorphous NbOx phase, which was a source of defects associated with oxygen deficiency, required for the creation of memristors, appeared only at low oxygen pressures in the chamber (∼ 5 ⋅ 10−3 − 10−1 Ohm ⋅ cm) during the film synthesis by the laser with both λ = 248 nm and λ = 532 nm. Volatile memristors of Nb/NbОх/Nb2O5/Pt/с-Аl2O3 (x ≤ 2.5) in cross-bar geometry were created demonstrating long-term stability of operation for 60 DC cycles with ΔRHRS/RHRS < 8% at an operating switching voltage of ∼ 0.5 V.

Growth of highly conducting MoS2-xNx thin films with enhanced 1T' phase by pulsed laser deposition and exploration of their nanogenerator application

SummaryHigh-quality growth of MoS2-xNx films is realized on single-crystal c-Al2O3 substrates by the pulsed laser deposition (PLD) in ammonia rendering highly stable and tunable 1Tʹ/2H biphasic constitution. Raman spectroscopy reveals systematic enhancement of 1Tʹ phase component due to the incorporation of covalently bonded N-doping in MoS2 lattice, inducing compressive strain. Interestingly, the film deposited at 300 mTorr NH3 shows ∼80% 1Tʹ phase. The transport measurements performed on MoS2-xNx films deposited at 300 mTorr NH3 display very low room temperature resistivity of 0.03 mΩ-cm which is 100 times enhanced over the undoped MoS2 grown under comparable conditions. A triboelectric nanogenerator (TENG) device containing biphasic MoS2-xNx film as an electron acceptor exhibits a clear enhancement in the output voltage as compared to the pristine MoS2. Device architecture, p-type N doping in MoS2 lattice, favorably increased work-function, multiphasic component of MoS2, and increased surface roughness synergistically contribute to superior TENG performance.

Electronic structure modification in Fe-substituted β-Ga2O3 from resonant photoemission and soft x-ray absorption spectroscopies

Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

Behaviours of the lattice-polarity inversion in AlN growth on c-Al2O3 (0001) substrates by ammonia-free high temperature metalorganic chemical vapor deposition

A BF-STEM image and inserted magnified HR-HAADF-STEM images showing the different lattice-polarity of AlN at each position.

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Thin films of MnxSi1-x (x ≈ 0.5) alloy were synthesized by pulsed laser deposition in a droplet-free mode on sapphire substrates with different crystallographic orientations of the cut plane (c-Al2O3 and r-Al2O3) at various laser energy densities E at the polycrystalline MnSi target. The X-ray structural, as well as static and resonance magnetic properties of the films were studied depending on E and the orientation of the substrate. The films deposited at high E &gt; 6 J/cm2 are found to demonstrate the presence of a ferromagnetic phase with anomalously high Curie temperature TC ~ 300 K which is not typical of MnSi single crystals (TC ≈ 30 K). In the case of the c-Al2O3 sub-strates, the magnetic moment of the films turns out to be somewhat higher than in the case of the r-Al2O3 substrates. Moreover, the films deposited on the c-Al2O3 substrates exhibit noticeably higher values of the effective surface anisotropy field 4πMeff measured by ferromagnetic resonance. The data obtained indicate a significant effect of the substrate structure on the formation and magnetic properties of the high-temperature ferromagnetic phase in the MnxSi1-x films.

Residual Strain in the AlN Layers Deposited by Reactive-Gas Pulsed Sputtering Deposition

High-quality AlN films were sputtered onto c-Al2O3 substrates by reactive-gas pulsed sputtering deposition. The plasma power was varied from 300 to 800 W, and AlN films were sputtered at room temperature. AlN layer with a Full-Width at Half-Maximum (FWHM) at 1120 arcsec X-ray diffraction was achieved at the plasma power of 600 W. However, the AlN films revealed considerable residual compressive strain. The cause of the residual strain was discussed from the various points of view including surface color, Al-content in the film, residual strain and XRD FWHM, as well as the metallic cluster density. All results can be explained in terms of increase of Al-clustering especially at high sputtering power condition.

Optimization of ruthenium as a buffer layer for non-collinear antiferromagnetic Mn3X films

Two thin film deposition routes were studied for the growth of high quality single crystalline Ru (0001) epitaxial films on c-Al2O3 substrates using radio frequency-magnetron sputtering. Such films are very important as buffer layers for the deposition of epitaxial non-collinear antiferromagnetic Mn3X films. The first route involved depositing Ru at 700 °C, leading to a smooth 30 nm thick film. Although, high resolution x-ray diffraction revealed twinned Ru film orientations, in situ post-annealing eliminated one orientation, leaving the film orientation aligned with the substrate, with no in-plane lattice rotation and a large lattice mismatch (13.6%). The second route involved the deposition of Ru at room temperature followed by in situ post-annealing at 700 °C. Transmission electron microscopy confirmed a very high quality of these films, free of crystal twinning, and a 30° in-plane lattice rotation relative to the substrate, resulting in a small in-plane lattice mismatch of –1.6%. X-ray reflectivity demonstrated smooth surfaces for films down to 7 nm thickness. 30 nm thick high quality single-crystalline Mn3Ga and Mn3Sn films were grown on top of the Ru buffer deposited using the second route as a first step to realize Mn3X films for antiferromagnetic spintronics applications.

Stress evolution during growth of AlN templates on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy

We describe stress evolution during plasma-assisted molecular beam epitaxy (PAMBE) of AlN nucleation and buffer layers on c-Al2O3 substrates at varying growth temperatures of 780 and 850°C. Moreover, different mechanisms of stress generation in the growing AlN films, related to the processes of grain coalescence and impact of Me-excess during PA MBE by migration enhanced epitaxy and metal-modulated epitaxy, are considered.

Stress evolution in AlN layers grown on c-Al2O3 by plasma-assisted molecular beam epitaxy at metal-rich conditions

Stress generation and relaxation in AlN nucleation and buffer layers grown on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy at low (LT, TS = 780°C) and high (HT, TS = 850°C) substrate temperatures were studied. Oscillation behavior of the wafer curvature (stress×thickness) was revealed during pulsed deposition of AlN in metal modulated epitaxy and migration enhanced epitaxy modes under the metal-rich conditions, which corresponded to Al bilayer formation and consumption. Minimal average stress ∼ +0.05 GPa in AlN buffer layers was observed at LT growth, which increased up to +0.85 GPa at HT growth. In addition, the LT AlN layers demonstrated lower densities of screw and edge threading dislocations equal to 7.8·108, 5.1·109 cm−2, respectively, while in the case of HT AlN layers those densities were significantly higher.

Structural Characterization of Atomic Layer Deposited Vanadium Dioxide

Vanadium dioxide (VO2) is a promising smart material particularly appealing for added functionality in both electronic and optical applications. We investigate the structure of VO2 films fabricated using atomic layer deposition (ALD) on c-Al2O3 substrates and the structure of the same VO2 films after an optimized anneal for 2 h at 585 °C in 10–5 Torr O2. Synchrotron-based grazing incidence X-ray diffraction (GIXRD) measurements revealed that the as-deposited ALD VO2 contains small crystalline inclusions of both V2O5 and VO. Ex situ annealing of the ALD VO2 film results in formation of mixed phase VO2 (predominantly M1 with indications of a small amount of M2), with a twinned monoclinic structure that is highly oriented and lattice matched to the c-Al2O3 substrate. Using in situ temperature-dependent measurements, the M1 and M2 components were shown to transition across different temperature ranges, which could account for the hysteretic behavior seen in these VO2 thin films. Additionally, these high-quali...

The influence of thermal processing on microstructure of sol–gel-derived SrSnO3 thin films

Polycrystalline SrSnO3 thin films were fabricated using the aqueous sol–gel process and deposited on the single crystal sapphire (R-Al2O3 and C-Al2O3) and Si substrates by spin coating technique. Two different processing of thermal treatment was explored to produce SrSnO3 thin films of different porosity. The XRD analysis showed that polycrystalline films with preferential growth of SrSnO3 (200) plane were obtained on C-Al2O3 substrates, while films deposited on R-Al2O3 demonstrated a random polycrystalline growth. FE-SEM analysis revealed that the higher porosity of SrSnO3 films can be achieved by introducing additional thermal treatment step during the deposition procedure. The UV–Vis reflectance spectroscopy was used to study the effect of porosity on optical properties of the films.

X-ray photoelectron spectroscopic studies of CeO2 thin films deposited on Ni-W (100), c-Al2O3 (0001) and Si (100) substrates

The oxidation states of CeO2 films, deposited on biaxially textured Ni-W(100), c-Al2O3(0001) and Si(100) substrates, were investigated by using X-ray photoelectron spectroscopy (XPS). The CeO2 films were deposited by RF magnetron sputtering under same deposition conditions. X-ray diffraction (XRD) studies of the films revealed that preferred (200) orientation of CeO2 was observed in case of Ni-W substrate while (111) orientation on Al2O3 and Si substrates. The O1s and Ce3d spectra reveal that the films mostly consist of Ce4+ oxidation states with ∼72.6%, 74.7% and 74.9% on Ni-W, Al2O3 and Si substrates, respectively. The less Ce4+ content in case of Ni-W substrate has been attributed to the migration of oxygen atoms from film to the metallic substrate. The decrease of O2p states in the valence band spectra also in support with the presence of more oxygen vacancies in CeO2/Ni-W compared to that in other substrates. Further, the position of valance band maxima is almost identical (∼2.6 eV) for all the deposited films indicating that all films are n-type in nature.

Observation of near-band-edge photoluminescence and UV photoresponse in near-stoichiometric Zn2SnO4 nanowires

Single-phase, near-stoichiometric zinc stannate (Zn2SnO4) nanowires were synthesized by a direct vapor transport process on c-Al2O3 substrates under optimized growth conditions. The optimal growth temperature for Zn2SnO4 nanowires is above 700 °C. Structural characterization indicates that the nanowires had the single crystal cubic spinel structure and diameters of ∼90 nm and they grew in the [1 direction. Low-temperature photoluminescence (PL) shows a strong emission peak at 3.7 eV, attributed to the near-band-edge-emission of Zn2SnO4 nanowires. Temperature-dependent PL results are consistent with the Varshni equation, and the band gap is redshifted by ∼170 meV as the temperature is increased from 11 to 300 K. The obtained direct gap of Zn2SnO4 nanowires is 3.546 eV at 300 K. A UV photodetector based on as-grown Zn2SnO4 nanowires was fabricated by a simple and cost-effective process. The Zn2SnO4 nanowires exhibited UV photoconductivity, and good selectivity and decent response to UV. The efficient fabrication method, high chemical and thermal stability of the Zn2SnO4 nanowire UV photodetector made it very suitable for use in harsh environments.

Semi-transparent all-oxide ultraviolet light-emitting diodes based on ZnO/NiO-core/shell nanowires.

Semi-transparent all-oxide light-emitting diodes based on ZnO/NiO-core/shell nanowire structures were prepared on double-polished c-Al2O3 substrates. The entire heterojunction diode showed an average transparency of ∼65% in the ultraviolet and visible regions. Under forward bias, the diode displayed an intense ultraviolet emission at ∼382 nm, and its electroluminescence performance was remarkable in terms of a low emission onset, acceptable operating stability, and the ability to optically excite emissive semiconductor nanoparticle chromophores.

Preparation and characterization of bottom ferromagnetic electrode for graphene based magnetic junction

Magnetic multilayer stacks incorporating several layers of graphene have been predicted to produce very high magnetoresistance and high conductivity, a combination of properties that would be useful in magnetic sensors and future spin-based data storage and processing technologies such as MRAM. To realize the theoretically modeled heterostructures and probe their properties, a clean, high-quality graphene-ferromagnet interface, such as one that results from CVD of graphene directly on ferromagnetic films, is required. However, past works using Ni and Co films for CVD of graphene employ the ferromagnetic film as a sacrificial layer to be dissolved after graphene growth and ignore changes to its morphology and magnetic properties. Here we investigated the effect of graphene CVD growth conditions on the properties of Co, Ni, Co90Fe10 and Ni80Fe20 ferromagnetic films. The magnetic films were grown by dc magnetron sputtering with different growth conditions onto c-Al2O3, Si/AlN and MgO substrates. The crystalline orientation, surface morphology/roughness and magnetic properties of the films were measured using X-ray diffraction, atomic force microscopy and vibrating sample magnetometry, respectively. Cobalt films grown at 500°C were found to be hcp and heteroepitaxial on c-Al2O3. CoFe, Ni, and NiFe films on c-Al2O3 were found to be fcc and to be (111) textured but with grains having in-plane rotation differing by 60°. The CoFe and NiFe films on c-Al2O3 retained their small coercivity and high remanence while the pure Co and Ni films exhibited much smaller remanence after graphene growth, making them unsuitable for magnetic memory technologies. Films on Si/AlN were found to have the same rotational domains as those on sapphire c-Al2O3. The NiFe films on (111) MgO were found to be mostly single domain.