Single Crystalline Al2O3 Research Articles

Single-crystalline metal-oxide dielectrics for top-gate 2D transistors.

Two-dimensional (2D) structures composed of atomically thin materials with high carrier mobility have been studied as candidates for future transistors1-4. However, owing to the unavailability of suitable high-quality dielectrics, 2D field-effect transistors (FETs) cannot attain the full theoretical potential and advantages despite their superior physical and electrical properties3,5,6. Here we demonstrate the fabrication of atomically thin single-crystalline Al2O3 (c-Al2O3) as a high-quality top-gate dielectric in 2D FETs. By using intercalative oxidation techniques, a stable, stoichiometric and atomically thin c-Al2O3 layer with a thickness of 1.25 nm is formed on the single-crystalline Al surface at room temperature. Owing to the favourable crystalline structure and well-defined interfaces, the gate leakage current, interface state density and dielectric strength of c-Al2O3 meet the International Roadmap for Devices and Systems requirements3,5,7. Through a one-step transfer process consisting of the source, drain, dielectric materials and gate, we achieve top-gate MoS2 FETs characterized by a steep subthreshold swing of 61 mV dec-1, high on/off current ratio of 108 and very small hysteresis of 10 mV. This technique and material demonstrate the possibility of producing high-quality single-crystalline oxides suitable for integration into fully scalable advanced 2D FETs, including negative capacitance transistors and spin transistors.

Adsorption of Ferritin at Nanofaceted Al2O3 Surfaces.

The influence of nanoscale surface topography on protein adsorption is highly important for numerous applications in medicine and technology. Herein, ferritin adsorption at flat and nanofaceted, single-crystalline Al2O3 surfaces is investigated using atomic force microscopy and X-ray photoelectron spectroscopy. The nanofaceted surfaces are generated by the thermal annealing of Al2O3 wafers at temperatures above 1000 °C, which leads to the formation of faceted saw-tooth-like surface topographies with periodicities of about 160 nm and amplitudes of about 15 nm. Ferritin adsorption at these nanofaceted surfaces is notably suppressed compared to the flat surface at a concentration of 10 mg/mL, which is attributed to lower adsorption affinities of the newly formed facets. Consequently, adsorption is restricted mostly to the pattern grooves, where the proteins can maximize their contact area with the surface. However, this effect depends on the protein concentration, with an inverse trend being observed at 30 mg/mL. Furthermore, different ferritin adsorption behavior is observed at topographically similar nanofacet patterns fabricated at different annealing temperatures and attributed to different step and kink densities. These results demonstrate that while protein adsorption at solid surfaces can be notably affected by nanofacet patterns, fine-tuning protein adsorption in this way requires the precise control of facet properties.

Need for complementary techniques for reliable characterization of MoS2-like layers

The observation of characteristic A1g and E2g1 peaks, at around 408 and 382 cm−1, respectively, in Raman spectroscopy is considered the evidence of 2H-structured MoS2, probably the most extensively studied transition-metal dichalcogenide. Here, using a combination of x-ray diffraction, x-ray photoelectron spectroscopy, and resonant Raman spectroscopy, we show that the detection of A1g and E2g1 modes in Raman spectra alone may not necessarily imply the presence of MoS2. A series of Mo–S films, ≈ 20-nm-thick, are grown on single-crystalline Al2O3(0001) substrates at 1073 K as a function of H2S partial pressure, pH2S (= 0, 0.01%, 0.1%, and 1% of total pressure) via ultra-high vacuum dc magnetron sputtering of a Mo target in 20 m Torr (2.67 Pa) Ar/H2S gas mixtures. In pure Ar discharges and with pH2S up to 0.1%, i.e., pH2S ≤ 2.67 × 10−3 Pa, we obtain body centered cubic (bcc), 110-textured films with lattice parameter a increasing from 0.3148 nm (in pure Ar) to 0.3151 nm (at pH2S = 2.67 × 10−4 Pa), and 0.3170 nm (at pH2S = 2.67 × 10−3 Pa), which we attribute to increased incorporation of S in the Mo lattice. With 1% H2S, i.e., pH2S = 2.67 × 10−2 Pa, we obtain 000l oriented 2H-structured MoS2.0±0.1 layers. Raman spectra of the thin films grown using 0.1% (and 1%) H2S show peaks at around 412 (408) and 380 cm−1 (382 cm−1), which could be interpreted as A1g and E2g1 Raman modes for 2H-MoS2. By comparing the Raman spectra of MoS2.0±0.1 and Mo:S thin films, we identify differences in A1g and E2g1 peak positions and intensities of defect-sensitive peaks relative to the A1g peaks that can help distinguish pure MoS2 from non-stoichiometric MoS2−x and multiphase Mo:S materials.

Borazine Promoted Growth of Highly Oriented Thin Films.

We report on a phenomenon, where thin films sputter-deposited on single-crystalline Al2O3(0001) substrates exposed to borazine─a precursor commonly used for the synthesis of hexagonal boron nitride layers─are more highly oriented than those grown on bare Al2O3(0001) under the same conditions. We observed this phenomenon in face-centered cubic Pd, body-centered cubic Mo, and trigonal Ta2C thin films grown on Al2O3(0001). Interestingly, intermittent exposure to borazine during the growth of Ta2C thin films on Ta2C yields better crystallinity than direct deposition of monolithic Ta2C. We attribute these rather unusual results to a combination of both enhanced adatom mobilities on, and epitaxial registry with, surfaces exposed to borazine during the deposition. We expect that our approach can potentially help improve the crystalline quality of thin films deposited on a variety of substrates.

Epitaxial growth of high quality Mn3Sn thin films by pulsed laser deposition

Noncollinear antiferromagnet Weyl semimetal Mn3Sn has recently attracted great research interest. Although large anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and magneto-optical effect have been observed in Mn3Sn, most studies are based on single crystals. So far, it is still challenging to grow high quality epitaxial Mn3Sn thin films with transport and optical properties comparable to their single crystal counterparts. Here, we report the structure and magneto-optical and transport properties of epitaxial Mn3Sn thin films fabricated by pulsed laser deposition (PLD). Highly oriented Mn3+xSn1−x (0001) and (112¯0) epitaxial films are growth on single crystalline Al2O3 and MgO substrates. Large anomalous Hall effect up to ΔρH=3.02 μΩ cm and longitudinal magneto-optical Kerr effect with |θK| = 38.1 mdeg at 633 nm wavelength are measured at 300 K, which are comparable to Mn3Sn single crystals. Our work demonstrates that high quality Mn3Sn epitaxial thin films can be fabricated by PLD, paving the way for future device applications.

RBS/Channeling characterization of Ru(0001) and thin epitaxial Ru/Al[formula omitted]O[formula omitted](0001) films

Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al2O3(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orientation, a well-defined epitaxial relation with the substrate and a high degree of crystal quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7 nm in thickness, for which relaxation of epitaxial strain is not complete, produce a similar degree of dechanneling. The surface of the films can be prepared in a clean and ordered state in order to allow further epitaxial growth on top.

Structure and magnetic properties of the Co2FeAl and Co2NiSi Heusler alloy films

The structural and magnetic properties (field dependences of the magnetization in magnetic fields of up to 6 kOe) of thin-film Co2FeAl, and Co2NiSi Heusler alloys grown by pulsed laser deposition on glass and a single-crystalline Al2O3 R-plane substrate at different growth temperatures (20, 280 and 420 °C) were studied. It was found that the stoichiometric composition of the films depends on the substrate temperature during growth and repeats the composition of the target for films grown at low temperatures. The films deposited on a single crystalline Al2O3 substrate have uniaxial magnetic anisotropy in the plane.

Spectrally Resolved Thermoluminescence of Anion‐Deficient Al2O3–BeO Ceramics for High‐Dose Dosimetry

Thermoluminescence (TL) curves of Al2O3–BeO ceramic samples synthesized during high‐temperature treatment of nanostructured Al2O3 compacts in vacuum with the presence of graphite in BeO crucibles are studied. Temperature ranges of certain TL peaks emission at the synthesis temperature varied from 1200 to 1400 °C are found. A comparative analysis of the TL peaks temperature position and the nature of traps in the obtained ceramics with single crystalline Al2O3, BeO, and BeAl2O4 analogs is carried out. Spectral composition of different TL peaks is investigated. It is found that oxygen vacancies and their aggregates play the main role in the TL emission centers formation in the samples under study. A complex structure of the dominating luminescence band at 4.0 eV in the dosimetric peak at 530 K of the ceramics synthesized at 1400 °C is found. This band is shown to be a superposition of three bands with the maxima at 3.11, 3.80, and 4.17 eV, resulted from F, F+ centers and presumably from Fcat‐type centers in aluminum oxide. The obtained results are essential to control and modify TL properties of Al2O3–BeO ceramics to create luminescence detectors of high‐dose radiation.

Tunable resonance frequency of wavelike FeNi nanofilm using a double-layer structure

Abstract A variety of triangular wavelike FeNi films have been obtained on the annealed single-crystalline Al2O3 substrate by magnetron sputtering and photolithography. Nanoscale patterns and microscale stripes in double-layer FeNi nanofilm have also been studied. Properties relating to the morphology, and static and dynamic magnetic properties of materials were measured by AFM, SEM, vibrating sample magnetometers, and vector network analyzers. The surface of the annealed sample was wavy, and its wavelength and peak were 80 nm and 15.4 nm, respectively. Lithography formed 15-μm-wide lines on the surface. Compared with nano-scale patterns, the micro-scale stripes, and overlay patterns, fr increased to 4.5 GHz after superposition and FeNi was used as a soft magnetic material. Hc should have been small but, after superimposing the pattern, increased significantly. Hc along the easy axis increased to 18 Oe and that along the hard axis to 50 Oe. This was related to the coupling effect of the multilayer film.

Analysis of acid-base interactions at Al2O3 (11-20) interfaces by means of single molecule force spectroscopy

Single molecule force spectroscopy (SMFS) was employed to investigate the interaction forces between aliphatic amino, hydroxyl and ether groups and aluminum oxide single crystal surfaces in an aqueous electrolyte at pH=6. The force studies were based on the variation of the terminal group of polyethylene glycol which was bound via a SAu bond to the gold coated AFM tip. X-ray Photoelectron Spectroscopy (XPS) was performed to characterize the surface chemistry of the substrate. Force distance curves were measured between the PEG-NH2, OH and OCH3 functionalized atomic force microscope (AFM) tip and the non-polar single crystalline Al2O3(11-20) surface. The experimental results exhibit non-equilibrium desorption events which hint at acid-base interactions of the electron donating hydroxyl and amino groups with Al-ions in the surface of the oxide. The observed desorption forces for the NH2, OH/Al2O3(11-20) were in the range of 100–200pN.

Structural transformations in highly oriented seven modulated martensite Ni–Mn–Ga thin films on an Al2O3 substrate

Abstract

Single-Layer Graphene Synthesis on a Al2O3(0001)/Cu(111) Template Using Chemical Vapor Deposition

The parameter space of graphene growth using chemical vapor deposition on an untwinned single-crystalline Al2O3(0001)/Cu(111) template is discussed. The influence of growth temperature, methane flow and carbon dose is examined to assess graphene quality and multilayer coverage. An optimized growth window is identified yielding single-layer graphene. Production temperatures above 850°C result in a graphene quality improvement, but also an increase in multilayer coverage. Adapting graphene dosages by minimizing the growth time is the key element to reduce the amount of multilayer domains without affecting the quality. After graphene transfer, the multilayer graphene areas show a large increase in I2D/IG peak ratio, which indicates a turbostratic stacking order of graphene multilayer domains. Two growth models are put forward, i.e. a carbon penetration mechanism and an adsorption-diffusion mechanism.

Dose-dependent optical properties and laser damage of helium-implanted sapphire

A series of single crystalline Al2O3 samples are implanted with He+ ions at different nominal fluences up to 1 × 1018 ions/cm2 at room temperature. The microstructure evolution and optical properties as well as laser-induced damage threshold are investigated. Optical microscopic images show that the density and amount of defects increase with increasing implantation fluence. In addition, atomic force microscopic images indicate that the surface morphologies have changed distinctly when the fluence reaches 5 × 1017 ions/cm2 and above. After helium implantation, broad purple and green–yellow absorption bands as well as an obvious photoluminescence band at 330 nm are observed, respectively. With the increase of implantation fluence, the intensities of absorption bands increase greatly, whereas the intensity of the photoluminescence band decreases and tends to saturation. The original strong infrared band shifts and broadens with increasing implantation fluence. The mechanism for the shift and broadening of the infrared band is discussed. After laser irradiation, it is found that the implantation fluence has great effect on the laser-induced damage threshold, which decreases significantly from 5.43 J/cm2 to 4.62, 3.71, 2.64, and 1.80 J/cm2 with increasing implantation fluence. A mechanism for the degradation of laser damage resistance is presented.

High-temperature ferromagnetism of Si1−xMnx (x≈0.52−0.55) alloys

The paper reports on the comprehensive study of properties of nonstoichiometric Si1−xMnx alloys slightly enriched in Mn (x≈0.51–0.55) as compared to the stoichiometric monosilicide MnSi. Mosaic type Si1−xMnx films 55–70nm in thickness were produced by the pulsed laser deposition (PLD) method onto the single crystalline Al2O3 substrates at 340°C. The Curie temperature TC in nonstoichiometric Si1−xMnx (x≈0.52–0.55) films exceeds room temperature, while in their stoichiometric counterpart, MnSi, the TC value does not exceed ≈30К. The consistent data on anomalous Hall effect and transverse Kerr effect prove the global character of ferromagnetic (FM) order caused by magnetic defect formation rather than the presence of FM clusters. Аt Mn content x≤0.55, the magnetization data testify to a good homogeneity in the distribution of magnetic defects without their segregation: variations of the saturation magnetization Ms do not exceed 6% in the temperature range T=10–100К and are well described by the Bloch law. It is also revealed that textured high-quality Si1−xMnx films with x≈0.52 and ТС~300К could be formed by PLD method in the “shadow” geometry (at lower energy of deposited atoms).

Surface amorphization in Al2O3 induced by swift heavy ion irradiation

Microstructure in single crystalline Al2O3 developed during irradiation by swift heavy ions has been investigated. The specimens were irradiated by Xe ions with energies from 70 to 160MeV at ambient temperature. The fluences were in the range from 1.0×1013 to 1.0×1015ions/cm2. After irradiations, X-ray diffractometry (XRD) measurements and cross sectional transmission electron microscope (TEM) observations were conducted. The XRD results indicate that in the initial stage of amorphization in single crystalline Al2O3, high-density Se causes the formation of new planes and disordering. The new distorted lattice planes formed in the early stage of irradiation around the fluence of 5.0×1013ions/cm2 for single crystalline Al2O3 irradiated with 160MeV-Xe ions. Energy dependence on structural modification was also examined in single crystalline Al2O3 irradiated by swift heavy ions. The XRD results indicate that the swift heavy ion irradiation causes the lattice expansion and the structural modification leading to amorphization progresses above the energy around 100MeV in this XRD study. The TEM observations demonstrated that amorphization was induced in surface region in single crystalline Al2O3 irradiated by swift heavy ions above the fluence expected from the results of XRD. Obvious boundary was observed in the cross sectional TEM images. The crystal structure of surface region above the boundary was identified to be amorphous and deeper region to be single crystal. The threshold fluence of amorphization was found to be around 1.0×1014ions/cm2 in the case over 80MeV swift heavy ion irradiation and the fluence did not depend on the crystal structures.

Narrow size distribution of Au nanocrystals formed in sapphire by utilizing Ar ion irradiation and thermal annealing

We describe a method to form narrow size distribution of Au nanoparticles in Al2O3 substrate. A layer of Au about 30nm thick was deposited on surface of single crystalline Al2O3, and then was brought into the matrix by Ar ion irradiation to form Au nanoparticles. The size distribution of the nanoparticles was subsequently modified by annealing treatment. Rutherford Backscattering Spectrometry, X-ray Diffraction and Transmission Electron Microscopy were used to study the structure and the size distribution of the nanoparticles. The results show that a narrow size distribution of nanoparticles formed in single crystalline alumina with an average diameter of 3.9nm and a standard deviation of 0.5nm.

Magnetic domain patterns in Co2MnGe Heusler nanostripes

We have prepared thin films of the ferromagnetic Heusler alloy Co2MnGe on a-plane Al2O3 substrates exhibiting growth-induced, superimposed fourfold and uniaxial magnetic anisotropies, the symmetry being determined by the single-crystalline Al2O3 substrate. The magnitude of the uniaxial anisotropy compared to the cubic anisotropy can be tuned over a wide range by the film thickness and the growth conditions. On submicrometer-wide stripes of Co2MnGe prepared by electron beam lithography we studied magnetic domain patterns by magnetic force microscopy. For stripes with a sufficiently large uniaxial anisotropy and with the easy axis oriented perpendicular to the stripe axis, we find perfectly regular domain patterns with the magnetization direction perpendicular to the stripe axis and alternating from domain to domain. The highly regular and controllable domain patterns in Co2MnGe nanostripes could be useful for magnetic storage devices and applications related to spin transfer torque.

Approaching Terahertz Graphene Transistors

Graphene is emerging as an attractive electronic material for future electronics due to its exceptionally high carrier mobility and single-atomic thickness. However, the conventional dielectric integration and device fabrication processes cannot be readily applied to graphene transistors because they can often introduce substantial defects into the monolayer of carbon lattices and severely degrade the device performance. Here we describe a new strategy to fabricate high performance graphene transistors through the hetero-integration of graphene with inorganic nanostructures. First, I will describe the fabrication of high mobility top-gated graphene transistors by first synthesizing free-standing high-k oxide nanostructures and then transferring them onto graphene as top-gate dielectrics. We show that single crystalline Al2O3 nanoribbons can be synthesized with excellent dielectric properties, and can be used as the top-gate dielectrics to enable graphene transistors with the highest carrier mobility exceeding 20,000 cm2/V•s. Furthermore, we describe a self-aligned approach using a metal-dielectric core-shell nanowire as the top-gate, with the source and drain electrodes defined through a self-alignment process and the channel length defined by the nanowire diameter. This fabrication approach preserves the high carrier mobility in graphene, and ensures nearly perfect alignment between source, drain, and gate electrodes. It therefore affords unprecedented transistor performance with the projected intrinsic cut-off frequency approaching terahertz regime.

Inducing supercurrents in thin films of ferromagneticCrO2

It is becoming gradually clear that it is possible to induce superconducting correlations inferromagnets by generating so-called odd-frequency triplet Cooper pairs, which have twoequal (rather than opposite) spins. Half-metallic ferromagnets (HMF) such asCrO2 are of special interest here since the presence of only a single spin band and nopossibility for spin flip scattering can lead to very long lengths over which asupercurrent is induced. Early experiments with superconducting (S) contacts onCrO2 films found supercurrents over distances of up to a micrometer. The generation of tripletcorrelations is assumed to involve inhomogeneous magnetization or disordered magnetic moments inthe ferromagnet or at the interface, and a major question still is how this is brought about in theS/CrO2 case. Here we report on experiments in whichCrO2 thin films are grown on two different substrates,TiO2 and sapphire (singlecrystalline Al2O3).Using amorphous Mo70Ge30 superconducting contacts, we find supercurrents can flow in films grown onsapphire, but we have not been able yet to induce supercurrents in films grown onTiO2. We discuss this in terms of the differences in film morphology and the magneticanisotropy for both film systems. We also show that the characteristic behavior of the cleanS/HMF interface is a jump at the superconducting transition temperature of the resistanceof the device to larger values.

High Temperature Capacitors Based on [0001] and [1120] Sapphire Dielectrics

The test results of the dielectric properties of [0001] (C-plane) and [1120] (A-plane) sapphire (single crystalline Al2O3) at high temperatures indicate that these materials have very stable dielectric constants and low dielectric losses (compared with polycrystalline alumina) at low frequencies in the temperature range from room temperature to 550°C. Therefore, sapphire materials have become likely candidate dielectric materials for high temperature capacitors. This paper reports prototype low-volume (∼100pF) capacitors based on sapphire dielectrics for high temperature and low frequency applications. Low-volume parallel-plate capacitors using C-plane and A-plane sapphire as dielectric material were fabricated by stacking metallized sapphire substrates. These prototype capacitors were characterized in the temperature range from room temperature to 550°C by measuring the capacitance and parallel resistance of these devices at 120Hz, 1kHz, 10kHz, 100kHz, and 1MHz. The capacitance and equivalent parallel resistance of these capacitors were all directly measured by an AC LCZ impedance meter in controlled temperature environments. These prototype devices demonstrate stable capacitances over a wide temperature range, and therefore, have the potential to be integrated with silicon carbide (SiC) devices to enable high temperature electronics. The needs of thin-film metallization and encapsulation for these sapphire substrates are also discussed.