R-cut Sapphire Substrates Research Articles

Substrate-dependent carrier mobility in polycrystalline Ge thin films

We present the effect of substrate choice on the carrier mobility of polycrystalline Ge thin films on SiO2/Si, C-cut sapphire, and R-cut sapphire substrates. The Hall mobility of the polycrystalline Ge thin films exhibited a strong correlation with the substrates, with mobility increasing sequentially for Ge thin films on SiO2/Si, C-cut sapphire, and R-cut sapphire substrates. Hall Effect measurements further suggested minimal influence of grain boundary scattering. Electron backscatter diffraction analysis revealed similar grain sizes of ∼200 nm across different substrates, but distinct grain orientation distributions, particularly with pronounced (110) grains on R-cut sapphire substrates. This work sheds light on the importance of substrate selection in optimizing the electrical performance of polycrystalline Ge thin films and highlights avenues for further investigation in understanding the underlying mechanisms governing substrate-dependent carrier mobility.

Deep Ultraviolet Optical Anisotropy of β-Gallium Oxide Thin Films.

β-Crystalline phase gallium oxide (β-Ga2O3) is an ultrawide bandgap material with prospective applications in electronics and deep ultraviolet (DUV) optoelectronics and optics. The monoclinic crystal structure of β-Ga2O3 results in optical anisotropy to incident light with different polarization states. This attribute can lead to different optical applications in the DUV. In this article, we investigated the optical properties of β-Ga2O3 thin films grown by pulsed laser deposition technique on sapphire substrates with different crystallographic orientations. Marked in-plane polarization anisotropy, determined by reflectance and Raman spectroscopy, was observed for β-Ga2O3 films deposited on an r-cut sapphire substrate. In contrast, isotropic optical properties were observed in β-Ga2O3 films deposited on a c-cut sapphire substrate.

Impact of the crystallographic variants of VO2 thin films on c- and r-cut sapphire on structural phase transition and radiofrequency properties

Vanadium dioxide exhibits a metal to insulator transition close to room temperature, making it very interesting in particular for radio frequency (RF) device applications. Here, we compare the structural and RF properties of VO2 thin films grown by magnetron sputtering on c-cut and r-cut sapphire substrates. The epitaxial growth of VO2 on c-cut sapphire gives rise to several crystallographic variants for the insulating M1 phase. Moreover, during the structural transition, simultaneous presence of both metallic and insulating phases is evidenced by x-ray diffraction over a large temperature range. Films grown on r-cut sapphire exhibit only two variants and present a very narrow temperature range of their structural transition. Interestingly, such structural differences of the films grown on c- and r-cut sapphire substrates have very little influence on their dc resistivity, while the transmission of the RF signal through the metallic phase was found much lower on c-cut than on r-cut sapphire. This supports the fact that r-cut sapphire is preferable for VO2-based RF device fabrication.

Ion irradiation induced modification in the electrical properties and the electronic structures of vanadium dioxide thin films

Ion irradiation modifies the physical properties of oxide materials. The present study focuses on the effect of 150 keV Fe and Ni ion irradiations in VO2 thin films. These films were deposited by pulsed laser on an r-cut sapphire substrate (1 1‾ 02). The range of ion fluences was from 1 × 1014 to 1 × 1016 ions/cm2 and these samples were characterized by using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Temperature-dependent resistivity measurements (R-T), and Hall effect measurement. While the presence of monoclinic phases is confirmed by GIXRD and Raman measurements, the ion irradiation on VO2 modifies the metal to insulator transition temperature (MIT). The R-T measurement with supporting analysis of carrier concentration authenticates a decrease in transition temperature and an increase in conductivity with an increment of ion-fluences. Synchrotron-based X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) revealed that Vanadium ions are in mixed oxidation states of 4+ and 5+ after ion irradiation. Further, the Photo-Electron Spectroscopy (PES) measurement indicates the metallization of VO2 after ion irradiation consistent with R-T measurements.

Growth and scintillation properties of Ce 3+:LuAG–Al2O3 chemically deposited eutectics

Directionally solidified eutectics (DSEs) in Al2O3-based systems have been studied as high-temperature structural ceramics, also recently attracting attention as a phosphor screen for high-resolution radiation imaging. However, it is difficult to obtain uniform and thinly processed DSE in large areas. Here, we report a chemically deposited eutectic (CDE) of Lu3Al5O12 (LuAG)–Al2O3 using a laser-assisted chemical vapor deposition method. The Ce3+-doped LuAG lamella were epitaxially grew with an α-Al2O3 matrix on an r-cut sapphire substrate. The Ce3+:LuAG–α-Al2O3 CDE film exhibited green emissions due to the 5d–4f transitions of the Ce3+ center under UV light and α-/X-ray irradiations. The α-ray dose in the Ce3+:LuAG phase was estimated with Monte Carlo simulations to study a scintillation light yield of the CDE film. The X-ray radiograph of the semiconductor storage device was successfully obtained using a 15-μm-thick Ce3+:LuAG–α-Al2O3 CDE film as an X-ray phosphor screen.

Normal-state transport in superconducting NbN films on r-cut sapphire

High-quality thin NbN films are very crucial for realizing quantum devices. Here, we investigated electrical transport and noise properties of a series of thin NbN films of various thicknesses grown on r-cut sapphire substrate using a DC magnetron sputtering technique. The films exhibit non-uniform thickness dependences for superconducting transition temperature (Te ) and normal-state resistivity. Morphological characterization of NbN samples of various thicknesses reveals uniform structure in thin films and granular structure in thick films. By measuring transport and noise properties in a normal state, we observe that the granular structure of NbN films does not have a strong effect on resistivity and does not cause an additional source of current noise.

Evolution of the Ferromagnetism of MnxSi1 –x (x ≈ 0.5) Films Produced by Laser Synthesis on c- and r-Cut Sapphire Substrates as the Laser Energy Density on the Target Changes

MnxSi1 – x (x ≈ 0.5) thin films have been produced on c- and r-cut sapphire substrates by the droplet-free pulsed laser deposition method at various laser energy densities E on the target. Their magnetic, electric, and X-ray diffraction properties as a function of E and substrate orientation have been investigated. It has been established that the high-temperature ferromagnetic phase in the films at E ≥ 6 J cm–2 is more pronounced than that at E ≈ 4–5 J cm–2, when the low-temperature ferromagnetic phase dominates and there is no influence of the sapphire substrate orientation. The attained Curie temperature TC was 330 K at E ≈ 7.4 J cm–2 for the MnxSi1 – x films produced on c- and r-cut sapphire substrates. The magnetization of the MnxSi1 – x films produced at E ≥ 6 J cm–2 on c-cut sapphire is higher than that on r-cut sapphire and, conversely, lower when E ≤ 5.5 J cm–2. A change in the ratio of the diffuse signal amplitudes in the X-ray spectra for the films grown on different substrates is also observed under these conditions. Such a correlated behavior with magnetization is explained by the existence of e-MnSi nanocrystallites of optimal sizes, which, on the one hand, are responsible for the emergence of a diffuse signal in the X-ray spectra and, on the other hand, determine the high-temperature ferromagnetism of the films. The concentration of such nanocrystallites and the distribution of defects in the films are controlled by the type of substrate and the energy density on the target.

Topology of conductive clusters in sputtered high-quality VO2 thin films on the brink of percolation threshold during insulator-to-metal and metal-to-insulator transitions

Electrical conductivity (σ) and optical transmittance of high quality VO2 thin films deposited by DC reactive magnetron sputtering on r-cut sapphire substrates (at 650 °C) have been measured simultaneously as a function of temperature by heating and cooling scans through the phase transition region. The partial concentration of the metallic phase (Xm) has been calculated from the optical transmittance, and the σ(Xm) dependence has been analyzed through an insulator-to-metal transition (IMT) during heating and through a metal-to-insulator transition (MIT) during cooling. The results have shown to be consistent with the Efros–Shklovskii percolation theory, predicting the formation of two-dimensional infinite conductive cluster (ICC) during IMT and the preservation of three-dimensional ICC during MIT. The critical concentrations (Xc) corresponding to the appearance of ICC at IMT and the disappearance of ICC at MIT were found to be very different, 0.57 and 0.06, respectively. A mathematical model explaining very small Xc at MIT was developed. The dissimilarity of the ICC topology during IMT and MIT is connected with the appearance and disappearance of local mechanical stresses imminent in VO2 phase transitions.

Synthesis, structure and optical properties of high-quality VO2 thin films grown on silicon, quartz and sapphire substrates by high temperature magnetron sputtering: Properties through the transition temperature

Vanadium dioxide (VO2) thin films were deposited on silicon (100), quartz, and r-cut sapphire substrates by DC magnetron sputtering at 650 °C. The thin films were characterized by Raman spectroscopy and imaging, scanning electron microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS), and Transmission Electron Microscopy (TEM). The optical transmittance and hence the insulator-to-metal transition (IMT) and metal-to-insulator transition (MIT) were characterized as a function of temperature. The effects of substrate material on VO2 thin film’s growth, microstructure, stoichiometry and optical properties were investigated. The results show that the as-deposited VO2 thin films on Si, quartz, and sapphire all show a near-zero IR transmission in switched metallic state; especially VO2 thin film grown on sapphire shows superior IMT characteristics compared with previously published works on thin VO2 films, and very close to its bulk single crystal form, which is probably due to the significantly reduced imperfections in the film and the interface, and especially the highly preferred growth with similar paralleled grain orientation of VO2 on sapphire arising from both the high deposition temperature and epitaxial growth. The spectral transmittance of the films on quartz and sapphire substrates was analyzed to extract the optical constants n and k as a function of wavelength. No significant difference between optical properties could be observed for films deposited on quartz and sapphire substrates indicating that the differences in microstructure of these films play a minor role in their optical properties. n and k spectra are, in general, in agreement with previously reported works. The absorption coefficient versus photon energy characteristics indicate that VO2 in the insulating phase possesses a direct bandgap at 2.6 eV and an indirect bandgap of 0.52 eV. Raman imaging was used to map the phase mixture of various regions of the film at micron scale (1.2 μm × 1.2 μm) during heating and cooling scans through the IMT and MIT. The dependence of the integrated Raman intensities at 220 cm−1 and 600 cm−1 on the temperature shows an excellent correlation with the optical transmittance through the IMT and MIT, both during heating and cooling scans. It is shown that the transformation occurs by the growth of one phase at the expense of the other and, in the IMT/MIT region, both phases coexist.

BaTiO3/LaSrMnO3 Heterostructure Grown on Sapphire for Ferroelectric Tunneling Junctions

BaTiO3/LaSrMnO3 (BTO/LSMO) structures with a ferroelectric layer thickness of 10 nm have been formed on r-cut sapphire substrate by means of high-frequency magnetron sputtering. Investigations of the sample structure showed the presence of a crystalline phase, while measurements of electrical properties revealed piezoelectric response of the BTO films. The study of local current–voltage characteristics showed dependence of the sample resistance on the prehistory of voltage application as determined by ferroelectric hysteresis of the BTO layer.

VO2 microrods synthesized from V2O5 thin films

Self-assembled single crystal VO2 microrods (MRs) with a length up to 600 µm have been synthesized on Si and sapphire substrates by the annealing of V2O5 thin films. The nucleation and growth of VO2 MRs from V2O5 thin films were investigated. The morphology and microstructure evolutions of the intermediate phases during the reduction process were characterized by SEM, HRTEM, XPEEM, and Raman spectroscopy. The results have shown that the conversion of V2O5 thin films to VO2 MRs is dominated by a melting-nucleation-growth mechanism. The growth of VO2 MRs can be controlled by V2O5 melting process and a consistent feeding of V2O5 liquids to the growing VO2 MR would lead to ultra-long VO2 MRs. Further, epitaxial growth of VO2 MRs can be induced by using an r-cut sapphire substrate.

The Growth of (00l)-Oriented CeO<sub>2</sub> Buffer Layers on R-Cut Sapphire Substrates for the Epitaxy of Tl-2212 Superconducting Films

Tl-2212 superconducting films were fabricated on r-cut sapphire substrates buffered with (00l)-oriented CeO2films. The buffer layers were deposited by the cerium dioxide sputtering target and the RF magnetron sputtering method. The epitaxial growth of CeO2films on r-cut sapphire substrates was obtained over a wide range of sputtering parameters, such as temperature, pressure, power and Ar/O2ratio. The Tl-2212 films grown on these buffer layers subsequently were purely c-axis orientation. The critical transition temperature of the best film was 105.6 K, the critical current density was 2.8 MA/cm2(77 K, 0 T) and the surface resistance was 435 μΩ (10 GHz, 77 K).

K x Na1−xNbO3 perovskite thin films grown by pulsed laser deposition on R-plane sapphire for tunable microwave devices

K x Na 1-x NbO 3 thin films with x = 0.5 and x = 0.7 were deposited by pulsed laser deposition onto R-cut sapphire substrates to be suitable for microwave applications. The 500-800 nm-thick films present a preferential (100) orientation. The ω-scans show a weak mosaicity (full-width-at-half-maximum equal to 0.36° and 0.60° for x = 0.5 and x = 0.7, respectively). In addition to this texture, the in-plane ordering evidenced by X-ray diffraction φ-scan for the (100) orientation is in agreement with an epitaxial-like growth in spite of the high lattice mismatch between K x Na 1-x NbO 3 and sapphire. The dielectric characteristics and the frequency tunability at microwave frequencies were obtained from coplanar waveguide devices (transmission lines and stub resonators). For the K 0.5 Na 0.5 NbO 3 and K 0.7 Na 0.3 NbO 3 compositions, high dielectric permittivity e r values of 360 and 1 250, and loss tangent tanδ values of 0.36 and 0.43 without biasing were retrieved from the transmission lines measurements at 10 GHz, respectively. Frequency tunabilities of 15 % and 12 % have been assessed under 80 kV/cm biasing from stub resonator measurements for the K 0.5 Na 0.5 NbO 3 and K 0.7 Na 0.3 NbO 3 compositions, respectively. K 0.5 Na 0.5 NbO 3 composition is therefore a promising solution for miniaturized tunable devices at microwave frequencies.

Anisotropic behaviour of transmission through thin superconducting NbN film in parallel magnetic field

Transmission of terahertz waves through a thin layer of the superconductor NbN deposited on an anisotropic R-cut sapphire substrate is studied as a function of temperature in a magnetic field oriented parallel with the sample. A significant difference is found between transmitted intensities of beams linearly polarised parallel with and perpendicular to the direction of applied magnetic field.

Stereo-epitaxial growth of single-crystal Ni nanowires and nanoplates from aligned seed crystals.

Epitaxially grown anisotropic Ni nanostructures are promising building blocks for the development of miniaturized and stereo-integrated data storage kits because they can store multiple magnetic domain walls (DWs). Here, we report stereo-epitaxially grown single-crystalline Ni nanowires (NWs) and nanoplates, and their magnetic properties. Vertical and inclined Ni NWs were grown at the center and edge regions of c-cut sapphire substrates, respectively. Vertical Ni nanoplates were grown on r-cut sapphire substrates. The morphology and growth direction of Ni nanostructures can be steered by seed crystals. Cubic Ni seeds grow into vertical Ni NWs, tetrahedral Ni seeds grow into inclined Ni NWs, and triangular Ni seeds grow into vertical Ni nanoplates. The shapes of the Ni seeds are determined by the interfacial energy between the bottom plane of the seeds and the substrates. The as-synthesized Ni NWs and nanoplates have blocking temperature values greater than 300 K at 500 Oe, verifying that these Ni nanostructures can form large magnetic DWs with high magnetic anisotropy properties. We anticipate that epitaxially grown Ni NWs and nanoplates will be used in various types of 3-dimensional magnetic devices.

(001) and (111) Single-Oriented Highly Epitaxial CeO2 Thin Films on r-Cut Sapphire Substrates

We have studied the growth of CeO2 thin films by molecular beam epitaxy on r-cut sapphire substrates. The oxidation state of the substrate surface controls the growth direction of CeO2. Oxygen pre-annealed substrates favor (001) growth, while oxygen vacancies lead to a mixed (001) and (111) orientation. Combining pre- and post-annealing, it is possible to achieve single- oriented CeO2 in both growth directions. Furthermore, post-annealing results in a dramatic increase of crystallinity with a rocking curve width of the (002) reflection as small as 0.004°. We provide a consistent growth model involving oxygen vacancies at the substrate to thin film interface.

Electrically Conductive Pt-Zr-B and Pt-Si Thin Films for Use in High Temperature Harsh Environments

ABSTRACTStable, electrically conductive, thin film materials are key components for high temperature sensors operating in harsh environments. In this work, nanocomposite Pt-Zr-B and Pt-Si thin film materials were grown to a nominal thickness of 200 nm on both r-cut sapphire (α-Al2O3) substrates using e-beam evaporation, and their structure, morphology, and chemical composition was characterized following thermal treatments in an air laboratory furnace up to 1300°C. In the Pt-Zr-B system, oxidation of a nanolaminate architecture consisting of ZrB2 and pure Pt layers leads to boron oxide evaporation and the formation of Pt grains decorated by tetragonal-ZrO2 nanocrystallites at high temperature. Electrical conductivity measurements with a 4-point probe show that this nanocomposite film structure can maintain a film conductivity > 1x106 S/m up to 1300°C, depending on the Pt/ZrB2 layer thickness ratio. In the Pt-Si system, film compositions were varied to yield either nanocrystalline Pt3Si, Pt2Si, or PtSi phases depending on the Pt-Si ratio, or an amorphous phase at high Si content. Above 1000°C in air, Pt-oxide and Si-oxide phases form and coexist with the Pt-Si phases, and some Pt-Si film conductivities remain as high as 1x106 S/m after annealing at 1000°C for 6 hours. It was found that a 100 nm thick amorphous alumina capping layer grown by atomic layer deposition (ALD) aids in limiting film oxidation, but film stress leads to regions of delamination.

Lateral epitaxial growth of faceted SnO2nanowires with self-alignment

Self-aligned, lateral SnO2 nanowires (NWs) were grown on r-cut (012) sapphire substrate by thermal evaporation via a Au-catalyzed vapor–liquid–solid (VSL) mechanism. The lateral SnO2 NWs were hetero-epitaxially grown on r-cut sapphire and the growth direction was [01] of SnO2, which was confirmed by an X-ray pole figure and high-resolution transmission electron microscopy (HRTEM). The SnO2 NWs exhibited sawtooth-like morphology in the length direction, and rectangular shape with round corners in the perpendicular direction. The top surface of the SnO2 NW was (101) and the side surfaces were (020). Consequently, the lateral SnO2 NWs grew in the direction of a large lattice mismatch with the formation of surface facets and coherent interface in the perpendicular direction. In addition, the effects of gas supply direction and the size of Au catalysts on the growth direction and growth mode of the SnO2 NWs were examined.

Properties of amorphous SiAlON thin films grown by RF magnetron co-sputtering

SiAlON thin films were deposited by RF magnetron co-sputtering of Al and Si targets in Ar/O2/N2 mixtures to a thickness of ~200nm onto both bare and Pt-coated r-cut sapphire substrates. Films deposited at 200°C are amorphous as determined from X-ray diffraction (XRD) and have <1nm RMS roughness as measured by X-ray reflectivity (XRR). In situ X-ray photoelectron spectroscopy (XPS) measurements from films grown over a wide range of SixAlyOzN100−x−y−z stoichiometries indicate that a homogenous amorphous phase is formed over all compositional regions of the quaternary thin film phase diagram. After annealing at 1000°C for 10h in vacuum, the film stoichiometries remained nearly unchanged and the films retained an amorphous structure, as verified by XRD. The films lost nitrogen during air exposure at 1000°C, leading to the formation of an amorphous aluminum silicate layer at the surface. No crystalline SiAlON phases, which have been reported for bulk SiAlON materials, were observed in films even after heating at 1500°C for 10days. Pin-on-disk measurements showed that SiAlON films have negligible wear up to 80gram loads, while significant wear occurs on the sapphire pin in sliding contact, indicating that the SiAlON films have excellent wear resistance.

Magnetic and structural investigations on as-grown and oxygen annealed PLD SnO2 : Co thin films

Detailed structural and magnetic characterization has been performed on dilute magnetic oxide SnO2 : Co thin films deposited on R-cut sapphire substrates via pulsed laser deposition from a doped target of 5 at.% nominal Co concentration. These studies demonstrate that the films are highly crystalline, composed of single phase SnO2 : Co grains, with no formation of Co nanoclusters in the oxide matrix, and are ferromagnetic at room temperature. It was found that ferromagnetism and electrical conduction are strongly dependent on the growth rate, film thickness and post deposition annealing protocols and suggest that the magnetic properties are related to the number of defects, and thus the crystallinity of the films. For thicknesses above 70 nm, films are electrically conducting and ferromagnetic, with a relatively constant MS of about 20 emu cm−3 at room temperature. Moreover, the moment per unit area varies linearly with the film thickness, showing that the ferromagnetism of the studied materials is a volume property. Samples annealed in conditions similar to their growth display a dramatic increase of MS in the first 5 min of annealing. Temperature dependent magnetization measurements reveal that approximately one third of the cobalt couples ferromagnetically with the remaining two thirds existing in the form of paramagnetic isolated cobalt spins. Experimental evidence supports the conclusion that films are formed of single crystal grains of SnO2 doped with Co, held together by defect-rich grain boundaries.