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Showing papers on "Epitaxy published in 2017"


Journal ArticleDOI
19 Apr 2017-Nature
TL;DR: It is shown that the weak van der Waals potential of graphene cannot completely screen the stronger potential field of many substrates, which enables epitaxial growth to occur despite its presence, and is also applicable to InP and GaP.
Abstract: Epitaxy-the growth of a crystalline material on a substrate-is crucial for the semiconductor industry, but is often limited by the need for lattice matching between the two material systems. This strict requirement is relaxed for van der Waals epitaxy, in which epitaxy on layered or two-dimensional (2D) materials is mediated by weak van der Waals interactions, and which also allows facile layer release from 2D surfaces. It has been thought that 2D materials are the only seed layers for van der Waals epitaxy. However, the substrates below 2D materials may still interact with the layers grown during epitaxy (epilayers), as in the case of the so-called wetting transparency documented for graphene. Here we show that the weak van der Waals potential of graphene cannot completely screen the stronger potential field of many substrates, which enables epitaxial growth to occur despite its presence. We use density functional theory calculations to establish that adatoms will experience remote epitaxial registry with a substrate through a substrate-epilayer gap of up to nine angstroms; this gap can accommodate a monolayer of graphene. We confirm the predictions with homoepitaxial growth of GaAs(001) on GaAs(001) substrates through monolayer graphene, and show that the approach is also applicable to InP and GaP. The grown single-crystalline films are rapidly released from the graphene-coated substrate and perform as well as conventionally prepared films when incorporated in light-emitting devices. This technique enables any type of semiconductor film to be copied from underlying substrates through 2D materials, and then the resultant epilayer to be rapidly released and transferred to a substrate of interest. This process is particularly attractive in the context of non-silicon electronics and photonics, where the ability to re-use the graphene-coated substrates allows savings on the high cost of non-silicon substrates.

372 citations


Journal ArticleDOI
21 Nov 2017-ACS Nano
TL;DR: The wafer-scale epitaxial growth of highly oriented continuous and uniform monolayer MoS2 films on single-crystalline sapphire wafers by chemical vapor deposition (CVD) method would facilitate the scalable fabrication of various electronic, valleytronic, and optoelectronic devices for practical applications.
Abstract: Large scale epitaxial growth and transfer of monolayer MoS2 has attracted great attention in recent years. Here, we report the wafer-scale epitaxial growth of highly oriented continuous and uniform monolayer MoS2 films on single-crystalline sapphire wafers by chemical vapor deposition (CVD) method. The epitaxial film is of high quality and stitched by many 0°, 60° domains and 60°-domain boundaries. Moreover, such wafer-scale monolayer MoS2 films can be transferred and stacked by a simple stamp-transfer process, and the substrate is reusable for subsequent growth. Our progress would facilitate the scalable fabrication of various electronic, valleytronic, and optoelectronic devices for practical applications.

348 citations


Journal ArticleDOI
TL;DR: It is revealed that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1 × 1) facet of (101̅0) of the bulk crystal.
Abstract: Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1 × 1) facet of (1010) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching the near-infrared band for the monolayer Te. An explicit band bending at the edge between the monolayer Te and graphene substrate is visualized. With the thickness controlled in the atomic scale, Te films show potential applications of electronics and optoelectronics.

204 citations


Journal ArticleDOI
TL;DR: In this article, a hierarchical ITO/Fe2O3/FeTiO5/FeNiOOH multi-layer composites for photoelectrochemical water splitting is presented, and a series of systematic experiments designed to elucidate the mechanism underlying the interfacial coupling effect of the quaternary hematite composite.
Abstract: The optimization of multiple interfaces in hematite (α-Fe2O3) based composites for photoelectrochemical water splitting to facilitate charge transport in the bulk is of paramount importance to obtain enhanced solar-to-fuel efficiency. Herein, we report the fabrication of ITO/Fe2O3/Fe2TiO5/FeNiOOH multi-layer nanowires and a series of systematic experiments designed to elucidate the mechanism underlying the interfacial coupling effect of the quaternary hematite composite. The hierarchical ITO/Fe2O3/Fe2TiO5/FeNiOOH nanowires display photocurrents that are more than an order of magnitude greater than those of pristine Fe2O3 nanowires (from 0.205 mA cm−2 to 2.2 mA cm−2 at 1.23 V vs. RHE and 1 Sun), and higher than those of most of the recently reported state-of-the-art hematite composites. Structural, compositional and electrochemical investigations disclose that the surface states (SS) are finely regulated via the atomic addition of an Fe2TiO5 layer and FeNiOOH nanodots, while the upgrading of back contact conductivity and charge donor densities originate from the epitaxial relationship and enhanced Sn doping contributed from the ITO underlayer. We attribute the superior water oxidation performance to the interfacial coupling effect of the ITO underlayer (Sn doping and back contact conductivity promoter), the atomic level Fe2TiO5 coating (Ti doping, surface state density and energy level modulation) and the FeNiOOH nanodot electrocatalyst (regulating surface state energy level). Our work suggests an effective pathway for rational designing of highly active and cost-effective integrated photoanodes for photoelectrochemical water splitting.

174 citations


Journal ArticleDOI
TL;DR: In this article, the fabrication of ultraviolet photodetector on non-polar (11−20), nearly stress free, Gallium Nitride (GaN) film epitaxially grown on r-plane (1−102) sapphire substrate was reported.
Abstract: We report the fabrication of ultraviolet photodetector on non-polar (11–20), nearly stress free, Gallium Nitride (GaN) film epitaxially grown on r-plane (1–102) sapphire substrate. High crystalline film leads to the formation of two faceted triangular islands like structures on the surface. The fabricated GaN ultraviolet photodetector exhibited a high responsivity of 340 mA/W at 5 V bias at room temperature which is the best performance reported for a-GaN/r-sapphire films. A detectivity of 1.24 × 109 Jones and noise equivalent power of 2.4 × 10−11 WHz−1/2 were also attained. The rise time and decay time of 280 ms and 450 ms have been calculated, respectively, which were the fastest response times reported for non-polar GaN ultraviolet photodetector. Such high performance devices substantiate that non-polar GaN can serve as an excellent photoconductive material for ultraviolet photodetector based applications.

162 citations


Journal ArticleDOI
TL;DR: This study reports the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS2 film on hexagonal boron nitride (h-BN) by molecular beam epitaxy, a significant step in the scalable synthesis ofhighly crystalline MoS1 films on atomically flat surfaces and paves the way to large-scale applications.
Abstract: Atomically thin molybdenum disulfide (MoS2), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics, but the scalable synthesis of highly crystalline film remains challenging. Here we report the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS2 film on hexagonal boron nitride (h-BN) by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that MoS2 grown on h-BN primarily consists of two types of nucleation grains (0° aligned and 60° antialigned domains). By adopting a high growth temperature and ultralow precursor flux, the formation of 60° antialigned grains is largely suppressed. The resulting perfectly aligned grains merge seamlessly into a highly crystalline film. Large-scale monolayer MoS2 film can be grown on a 2 in. h-BN/sapphire wafer, for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of hi...

152 citations


Journal ArticleDOI
TL;DR: It is shown that a diamond single crystal with a diameter of ~90 mm and a weight of 155 carat can be grown from such a carbon film which initially consisted of 2 · 1013 individual grains.
Abstract: A detailed mechanism for heteroepitaxial diamond nucleation under ion bombardment in a microwave plasma enhanced chemical vapour deposition setup on the single crystal surface of iridium is presented. The novel mechanism of Ion Bombardment Induced Buried Lateral Growth (IBI-BLG) is based on the ion bombardment induced formation and lateral spread of epitaxial diamond within a ~1 nm thick carbon layer. Starting from one single primary nucleation event the buried epitaxial island can expand laterally over distances of several microns. During this epitaxial lateral growth typically thousands of isolated secondary nuclei are generated continuously. The unique process is so far only observed on iridium surfaces. It is shown that a diamond single crystal with a diameter of ~90 mm and a weight of 155 carat can be grown from such a carbon film which initially consisted of 2 · 1013 individual grains.

143 citations


Journal ArticleDOI
TL;DR: An experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy using density functional theory calculations reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene.
Abstract: Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [100] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.

140 citations


Journal ArticleDOI
TL;DR: In this paper, a La-doped BaSnO3 films were grown in an adsorption-controlled regime by molecular-beam epitaxy, where the excess volatile SnO desorbs from the film surface.
Abstract: Epitaxial La-doped BaSnO3 films were grown in an adsorption-controlled regime by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A film grown on a (001) DyScO3 substrate exhibited a mobility of 183 cm2 V−1 s−1 at room temperature and 400 cm2 V−1 s−1 at 10 K despite the high concentration (1.2 × 1011 cm−2) of threading dislocations present. In comparison to other reports, we observe a much lower concentration of (BaO)2 Ruddlesden-Popper crystallographic shear faults. This suggests that in addition to threading dislocations, other defects—possibly (BaO)2 crystallographic shear defects or point defects—significantly reduce the electron mobility.

138 citations


Journal ArticleDOI
TL;DR: In this paper, the growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O 3) substrates.
Abstract: The growth of high quality epitaxial beta-gallium oxide (β-Ga2O3) using a compound source by molecular beam epitaxy has been demonstrated on c-plane sapphire (Al2O3) substrates. The compound source provides oxidized gallium molecules in addition to oxygen when heated from an iridium crucible in a high temperature effusion cell enabling a lower heat of formation for the growth of Ga2O3, resulting in a more efficient growth process. This source also enabled the growth of crystalline β-Ga2O3 without the need for additional oxygen. The influence of the substrate temperatures on the crystal structure and quality, chemical bonding, surface morphology, and optical properties has been systematically evaluated by x-ray diffraction, scanning transmission electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, spectroscopic ellipsometry, and UV-vis spectroscopy. Under optimized growth conditions, all films exhibited pure 2¯01 oriented β-Ga2O3 thin films with six-fold rotational symmetry when grown on a sapphire substrate. The thin films demonstrated significant absorption in the deep-ultraviolet (UV) region with an optical bandgap around 5.0 eV and a refractive index of 1.9. A deep-UV photodetector fabricated on the high quality β-Ga2O3 thin film exhibits high resistance and small dark current (4.25 nA) with expected photoresponse for 254 nm UV light irradiation suggesting that the material grown using the compound source is a potential candidate for deep-ultraviolet photodetectors.

118 citations


Journal ArticleDOI
TL;DR: In this paper, a β-(Al x Ga 1− x )2O3:Si/Ga 2O3 modulation-doped structure was fabricated by direct β-Ga 1−x 2O 3 epitaxial growth on a (010)-β-Ga2O 3 substrate, which exhibited a confined sheet carrier density of 3 × 1012 cm−2.
Abstract: A β-(Al x Ga1− x )2O3:Si/Ga2O3 modulation-doped structure was fabricated by direct β-(Al x Ga1− x )2O3 epitaxial growth on a (010) β-Ga2O3 substrate. Si on the order of 1018 cm−3 from adsorbed contaminants on the substrate surface was doped into the β-(Al x Ga1− x )2O3 layer. The heterojunction interface exhibited a confined sheet carrier density of ~3 × 1012 cm−2, which is on the same order as that of AlGaAs/GaAs. The successful modulation doping for the β-(Al x Ga1− x )2O3/Ga2O3 heterostructure encourages the development of β-Ga2O3-based heterojunction field-effect transistors.

Journal ArticleDOI
TL;DR: A topological crystalline to amorphous phase transition predicted in 2D systems is observed in freestanding oxide films, likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.
Abstract: Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3 membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.

Journal ArticleDOI
14 Sep 2017
TL;DR: In this article, the first successful epitaxial growth of high quality PtSe2 films by molecular beam epitaxy was reported, which provides new opportunities for growing large size single crystalline films to investigate the physical properties and potential applications of PtSe 2.
Abstract: Atomically thin PtSe2 films have attracted extensive research interests for potential applications in high-speed electronics, spintronics and photodetectors. Obtaining high quality thin films with large size and controlled thickness is critical. Here we report the first successful epitaxial growth of high quality PtSe2 films by molecular beam epitaxy. Atomically thin films from 1 ML to 22 ML have been grown and characterized by low-energy electron diffraction, Raman spectroscopy and x-ray photoemission spectroscopy. Moreover, a systematic thickness dependent study of the electronic structure is revealed by angle-resolved photoemission spectroscopy (ARPES), and helical spin texture is revealed by spin-ARPES. Our work provides new opportunities for growing large size single crystalline films to investigate the physical properties and potential applications of PtSe2.

Journal ArticleDOI
17 Mar 2017
TL;DR: In this paper, the opto-electronic properties of hexagonal boron nitride grown by high temperature plasma-assisted molecular beam epitaxy were investigated, and it was shown that the epilayers on highly oriented pyrolytic graphite demonstrate superior performance in the deep ultraviolet (down to 210 nm) compared to those on sapphire.
Abstract: We investigate the opto-electronic properties of hexagonal boron nitride grown by high temperature plasma-assisted molecular beam epitaxy. We combine atomic force microscopy, spectroscopic ellipsometry, and photoluminescence spectroscopy in the deep ultraviolet to compare the quality of hexagonal boron nitride grown either on sapphire or highly oriented pyrolytic graphite. For both substrates, the emission spectra peak at 235 nm, indicating the high optical quality of hexagonal boron nitride grown by molecular beam epitaxy. The epilayers on highly oriented pyrolytic graphite demonstrate superior performance in the deep ultraviolet (down to 210 nm) compared to those on sapphire. These results reveal the potential of molecular beam epitaxy for the growth of hexagonal boron nitride on graphene, and more generally, for fabricating van der Waals heterostructures and devices by means of a scalable technology.

Journal ArticleDOI
Yi Zhuo1, Zimin Chen1, Wenbin Tu1, Xuejin Ma1, Yanli Pei1, Gang Wang1 
TL;DR: In this article, the epitaxial relationship between e-Ga 2 O 3 and c-plane sapphire is analyzed using X-ray diffraction and a low growth temperature and low VI/III ratio were beneficial for the formation of hexagonal-type eGa 2O 3 films.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate growth of ScxAl1-xN on GaN and SiC substrates using plasma-assisted molecular beam epitaxy with x'='0.14'-0.24'.
Abstract: ScxAl1-xN is a promising ultra-wide bandgap material with a variety of potential applications in electronic, optoelectronic, and acoustoelectric devices related to its large piezoelectric and spontaneous polarization coefficients. We demonstrate growth of ScxAl1-xN on GaN and SiC substrates using plasma-assisted molecular beam epitaxy with x = 0.14–0.24. For metal-rich growth conditions, mixed cubic and wurtzite phases formed, while excellent film quality was demonstrated under N-rich growth conditions at temperatures between 520 and 730 °C. An rms roughness as low as 0.7 nm and 0002 rocking curve full-width at half maximum as low as 265 arc sec were measured for a Sc0.16Al0.84 N film on GaN. To further demonstrate the quality of the ScAlN material, a high-electron-mobility transistor heterostructure with a Sc0.14Al0.86 N barrier, GaN/AlN interlayers, and a GaN buffer was grown on SiC, which showed the presence of a two-dimensional electron gas with a sheet charge density of 3.4 × 1013 cm−2 and a Hall mob...

Journal ArticleDOI
TL;DR: With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed and suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halideperovskites.
Abstract: High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Journal ArticleDOI
G. Wisz1, I. S. Virt1, P. Sagan1, Piotr Potera1, R. Yavorskyi1 
TL;DR: The structural, optical, and electrical properties of zinc oxide (ZnO) layers manufactured at different process conditions were investigated and it was shown that the type of films conductivity is metallic and it is limited by charge transfer across grain boundaries.
Abstract: The structural, optical, and electrical properties of zinc oxide (ZnO) layers manufactured at different process conditions were investigated. ZnO epitaxial layers were grown on silicon, glass, and ITO/glass substrates by pulsed laser deposition (PLD) technique. The influence of power beam, substrate temperature, and deposition time on films properties was analysed. Morphological features of the film surface were investigated by scanning electron microscopy. A structural study shown planar orientation of films at low temperatures of substrate, but the columnar type of growth originated in temperature enhances. Electrical properties were determined in the temperature range 300–500 K. It was shown that the type of films conductivity is metallic and it is limited by charge transfer across grain boundaries.

Journal ArticleDOI
TL;DR: A double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported, which produces a highly crystalline film that features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude.
Abstract: Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high-performance, low-cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus-guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device-to-device variability. Here, a double-step method for organic semiconductor layers combining a solution-processed templating layer and a lateral homo-epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus-guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high-performance, large-area electronics based on highly crystalline thin films of organic semiconductors.

Journal ArticleDOI
TL;DR: In this paper, the thermal stability of e-Ga 2 O 3 polymorph was studied by complementary methods, including X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM).

Journal ArticleDOI
07 Mar 2017-Small
TL;DR: In this paper, the aligned growth of millimeter-size single-crystal hexagonal boron nitride (h-BN) domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated.
Abstract: Atomically thin hexagonal boron nitride (h-BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene-based devices. For these applications, synthesis of high-quality and large-area h-BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter-size single-crystal h-BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single-crystal h-BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large-size h-BN domains results mainly from the reduced Ni-grain boundaries and the improved crystallinity of Ni film. Furthermore, the h-BN domains show well-aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large-scale high-quality h-BN layers for electronic applications.

Journal ArticleDOI
TL;DR: In this article, the hydride-vapor-phase epitaxy (HVPE) system was used to produce device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness.
Abstract: Thick (20–30 µm) layers of highly pure GaN with device-quality smooth as-grown surfaces were prepared on freestanding GaN substrates by using our advanced hydride-vapor-phase epitaxy (HVPE) system. Removal of quartz parts from the HVPE system markedly reduced concentrations of residual impurities to below the limits of detection by secondary-ion mass spectrometry. Appropriate gas-flow management in the HVPE system realized device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness. The undoped GaN layer showed insulating properties. By Si doping, the electron concentration could be controlled over a wide range, down to 2 × 1014 cm−3, with a maximum mobility of 1150 cm2V−1s−1. The concentration of residual deep levels in lightly Si-doped layers was in the 1014 cm−3 range or less throughout the entire 2-in. wafer surface. These achievements clearly demonstrate the potential of HVPE as a tool for epitaxial growth of power-device structures.

Journal ArticleDOI
TL;DR: A new member of the layered pseudo-1D material family-monoclinic gallium telluride (GaTe)-is synthesized by physical vapor transport on a variety of substrates and the resulting anisotropic behavior is clearly revealed.
Abstract: GaTe flakes were synthesized by the PVT technique in a tube furnace using GaTe and Ga2Te3 polycrystalline powders as the source. The PVT synthesis of GaTe flakes was carried out in a tube furnace with a 1'' quartz tube. GaTe and Ga2Te3 powders were mixed together as the source materials, and Ar (15 sccm)+H2 (5 sccm) was used as the carrier gas. GaAs, Si, and c-cut sapphire wafers were used as growth substrates. Prior to growth the GaAs substrates were cleaned by piranha solution followed by dilute hydrogen chloride (HCl) solution. The source powders were loaded in a quartz boat and sent to the center of the tube. The substrate was located 10 cm away downstream. The temperature was kept at 650°C for 5 min and then cooled down to room temperature. The Ar flow rate was set at 15 sccm and the growth pressure was 30 Torr for the whole process. The GaTe bulk crystals were synthesized by the modified Bridgman growth technique in a single zone furnace at temperatures ranging from 850 to 1020°C for three weeks. Room temperature PL and Raman measurements for the GaTe flakes were performed. Interestingly, nanomaterials grown on sapphire exhibit well-defined, narrow, and bright PL emission peaks originating from localized emission due to a select type of imperfection site states that appear at energies well below the fundamental emission line (optical band gap). Bright emission within the forbidden band is observed for the first time in GaTe and may be the starting point for further defect engineering for optoelectronics in PTMCs. Lastly, angle-resolved PL and Raman studies suggest that the synthesized monoclinic GaTe flakes are also highly anisotropic due to their unique crystal structure, which is the first demonstration of the anisotropy in vapor phase synthesized pseudo 1D GaTe.

Journal ArticleDOI
TL;DR: In this paper, c-plane sapphire substrates were annealed under different temperatures in a vacuum furnace prior to the molecular beam epitaxy (MBE) of β-Ga2O3 thin film, yielding a smoother surface and even a terrace-and-step-like morphology on the substrate, resulting in improved crystallinity of the epitaxial film.
Abstract: Recently, monoclinic Ga2O3 (β-Ga2O3) photodetectors (PDs) have been extensively studied for various commercial and military applications due to the merits of intrinsic solar rejection, high gain, and great compactness. In this work, c-plane sapphire substrates were annealed under different temperatures in a vacuum furnace prior to the molecular beam epitaxy (MBE) of β-Ga2O3 thin film, which yielded a smoother surface and even a terrace-and-step-like morphology on the substrate, resulting in improved crystallinity of the epitaxial film. Accordingly, both the dark and photo currents of β-Ga2O3 metal-semiconductor-metal (MSM) PDs were increased by the enhanced carrier mobility (μ) of the more crystalline film. However, the substrate-annealing temperature must be sufficiently high to offset the rise of the dark current and thus achieve a remarkable improvement in the photodetection properties. As a result, the PD fabricated on the 1050 °C-annealed substrate exhibited extremely high sensitivity, for example, high responsivity (R) of 54.9 A/W and large specific detectivity (D*) of 3.71 × 1014 Jones. Both parameters were increased by one order of magnitude because of the combined effects of the dramatic increase in μ and the effective reduction in defect-related recombination centers. Nevertheless, the latter also prolonged the recovery time of the PD. These findings suggest another way to develop β-Ga2O3 PD with extremely high sensitivity.

Journal ArticleDOI
TL;DR: The result opens a window for further extension of the ALD applications from amorphous thin films to the high-quality low-temperature atomic layer epitaxy, which can be exploited in a variety of fields and applications in the near future.
Abstract: Low-temperature epitaxial growth of AlN ultrathin films was realized by atomic layer deposition (ALD) together with the layer-by-layer, in-situ atomic layer annealing (ALA), instead of a high growth temperature which is needed in conventional epitaxial growth techniques. By applying the ALA with the Ar plasma treatment in each ALD cycle, the AlN thin film was converted dramatically from the amorphous phase to a single-crystalline epitaxial layer, at a low deposition temperature of 300 °C. The energy transferred from plasma not only provides the crystallization energy but also enhances the migration of adatoms and the removal of ligands, which significantly improve the crystallinity of the epitaxial layer. The X-ray diffraction reveals that the full width at half-maximum of the AlN (0002) rocking curve is only 144 arcsec in the AlN ultrathin epilayer with a thickness of only a few tens of nm. The high-resolution transmission electron microscopy also indicates the high-quality single-crystal hexagonal phase of the AlN epitaxial layer on the sapphire substrate. The result opens a window for further extension of the ALD applications from amorphous thin films to the high-quality low-temperature atomic layer epitaxy, which can be exploited in a variety of fields and applications in the near future.

Journal ArticleDOI
TL;DR: In this paper, the in situ high temperature ellipsometry measurements of ultrasmooth and epitaxial quality crystalline films, along with electron beam evaporated polycrystalline silver films at temperatures up to 700 °C, in the wavelength range of 330-2000 nm.
Abstract: Silver holds a unique place in plasmonics compared to other noble metals owing to its low losses in the visible and near-IR wavelength ranges. With a growing interest in local heating and high temperature applications of plasmonics, it is becoming critical to characterize the dielectric function of nanometer-scale thin silver films at higher temperatures, especially near the breakdown temperature, which depends on the film thickness and crystallinity. So far, such a comprehensive study has been missing. Here we report the in situ high temperature ellipsometry measurements of ultrasmooth and epitaxial quality crystalline silver films, along with electron beam evaporated polycrystalline silver films at temperatures up to 700 °C, in the wavelength range of 330–2000 nm. Our findings show that the dielectric function of all the films changes remarkably at elevated temperatures with larger relative changes observed in polycrystalline films. In addition, low-loss epitaxial films were found to be thermally more s...

Journal ArticleDOI
01 Jun 2017-Vacuum
TL;DR: A vacuum annealing was conducted on c-plane sapphire substrate in a plasma assisted MBE system prior to the epitaxial growth of β-Ga 2 O 3 film as discussed by the authors.

Journal ArticleDOI
TL;DR: High‐quality graphene/h‐BN in‐plane heterostructure with epitaxial relationship can be formed, which is supported by extensive characterizations and will have important impact on future research and applications based on this unique material platform.
Abstract: Graphene/hexagonal boron nitride (h-BN) monolayer in-plane heterostructure offers a novel material platform for both fundamental research and device applications. To obtain such a heterostructure in high quality via controllable synthetic approaches is still challenging. In this work, in-plane epitaxy of graphene/h-BN heterostructure is demonstrated on Cu-Ni substrates. The introduction of nickel to copper substrate not only enhances the capability of decomposing polyaminoborane residues but also promotes graphene growth via isothermal segregation. On the alloy surface partially covered by h-BN, graphene is found to nucleate at the corners of the as-formed h-BN grains, and the high growth rate for graphene minimizes the damage of graphene-growth process on h-BN lattice. As a result, high-quality graphene/h-BN in-plane heterostructure with epitaxial relationship can be formed, which is supported by extensive characterizations. Photodetector device applications are demonstrated based on the in-plane heterostructure. The success will have important impact on future research and applications based on this unique material platform.

Journal ArticleDOI
TL;DR: In this paper, an oxide passivating layer on the Cu surface before the growth of h-BN by chemical vapor deposition (CVD) can lead to increased domain sizes from 1 to 20 μm by reducing the nucleation density from 106 to 103 mm-2.
Abstract: We show how an oxide passivating layer on the Cu surface before the growth of h-BN by chemical vapor deposition (CVD) can lead to increased domain sizes from 1 to 20 μm by reducing the nucleation density from 106 to 103 mm–2. The h-BN domains within each Cu grain are well-oriented, indicating an epitaxial relationship between the h-BN crystals and the Cu growth substrates that leads to larger crystal domains within the film of ∼100 μm. Continuous films are grown and show a high degree of monolayer uniformity. This CVD approach removes the need for low pressures, electrochemical polishing, and expensive substrates for large-area continuous films of h-BN monolayers, which is beneficial for industrial applications that require scalable synthesis.

Journal ArticleDOI
TL;DR: In this article, depletion mode vertical Ga2O3 trench metal-oxide-semiconductor field effect transistors were developed by using n+ contact and n− drift layers, which showed good DC characteristics with a specific on-resistance of 3.7 mΩ cm2 and clear current modulation.
Abstract: We developed depletion-mode vertical Ga2O3 trench metal–oxide–semiconductor field-effect transistors by using n+ contact and n− drift layers. These epilayers were grown on an n+ (001) Ga2O3 single-crystal substrate by halide vapor phase epitaxy. Cu and HfO2 were used for the gate metal and dielectric film, respectively. The mesa width and gate length were approximately 2 and 1 µm, respectively. The devices showed good DC characteristics, with a specific on-resistance of 3.7 mΩ cm2 and clear current modulation. An on–off ratio of approximately 103 was obtained.