High Epitaxial Quality Research Articles

Low-temperature growth of high-quality VO2 epitaxial film on c-plane sapphire by reactive magnetron sputtering

Low-temperature growth of high-quality VO2 epitaxial film on c-plane sapphire by reactive magnetron sputtering

Flexible epitaxial LiNbO3 thin film with ferroelectricity and nonlinear optical response

Flexible or wearable devices are of great interests in the modern technologies, and oxide thin films are critical components for such applications. In this work, epitaxial LiNbO3 (LNO) thin film integrated on flexible mica substrate has been achieved. Al-doped ZnO (AZO) layer has been first deposited as buffer layer to obtain the epitaxial LNO film, furthermore AZO serves as bottom electrode for electrical measurement. The as-deposited LNO film exhibits high epitaxial quality with c-direction growth. The transmittance measurement indicates the high transparency of the film, ～85 % in visible range and ～90 % in near infrared region up to 1500 nm. The second harmonic generation (SHG) measurement confirms the nonlinearity of the flexible LNO film. The LNO/AZO/mica film also presents apparent ferroelectric response. Overall, the high-quality LNO integrated on flexible mica could be potentially applied in wearable optical and electrical devices.

High-quality Ge epitaxial film based on dislocation trapping mechanism in patterned Si substrate

Threading dislocation density (TDD) in a Ge heteroepitaxial film on Si is reduced based on a dislocation trapping mechanism in a patterned substrate. An array of V-shaped grooves on the submicron scale is patterned in the [110] direction on a (001) Si substrate. Epitaxial growth of Ge with a thickness of 1 μm by CVD realizes a reasonable flat surface despite the non-flat starting surface. The TDD in Ge for a V-shaped groove pattern of 0.5 μm in width with an inter-groove distance of 0.3 μm is obtained as low as 4 × 107 cm–2, which is lower than about 6 × 107 cm–2 for the rectangular one with the same groove width and inter-groove distance and about 22 × 107 cm–2 for the unpatterned one. The reduction is attributed to the dislocation trapping at the groove regions, as observed by cross-sectional transmission electron microscopy.

High performance junctionless FDSOI SiGe channel p-FinFET with high ION/IOFF ratio and excellent SS

This paper presents a novel FDSOI FinFETs with SiGe channel/Si cap layer (35 nm/5 nm) formed on SOI wafers with 20 nm Si body. Electrical characterization result shows that JL FDSOI FinFETs with SiGe channel exhibited low subthreshold slope (SS) of 86 mV/decade and record high ON-OFF current ratio (ION/IOFF = 2 × 106) at VD = -0.5V, respectively. Device performance improvement was mainly attributed to high-epitaxial quality compressive strained SiGe channel and novel device structure. The strain in the channel of the transistors was estimated locally in TEM using Nano beam diffraction (NBD) technique. The FDSOI FinFET with SiGe channel, lays the foundation for the further development of high speed, high density, low cost, and 3D monolithic integration circuits.

Point defect–dislocation interactions in BEOL-compatible Ge-on-Si epitaxy

Reduced thermal budget is required for back-end-of-line (BEOL) integration of application specific functionality into the multilevel metal stack of a processor “substrate.” We report 400 °C BEOL-compatible Ge-on-Si growth (LT Ge) that is epitaxial and single crystalline with a defect density similar to high temperature growth and a small 0.05% tensile strain. Room temperature methanol–iodine passivation is employed pre-growth in lieu of the typical 800 °C oxide removal step. Undoped LT Ge exhibits p-type conductivity initially and n-type conductivity conversion upon annealing. Hall effect measurements following post growth heat treatment between 400 and 600 °C reveal an acceptor removal reaction that follows first-order kinetics with an activation energy of 1.7 ± 0.5 eV and a pre-exponential factor of 2.3×107 s−1 consistent with a point defect, diffusion limited process. We also observe that 90° sessile dislocations identified via transmission electron microscopy are annihilated in the same temperature regime, which is evidence for point defect-mediated climb. Ensuring high-quality epitaxy by characterizing defect reactions in a BEOL-compatible Ge-on-Si process flow is key to enabling vertical integration of optical interconnects.

Growth rate control and phase diagram of wafer-scale Ga2O3 films by MOCVD

The potential of gallium oxide (Ga2O3) as a promising semiconductor for advanced electronics and optoelectronics has been propelled by its wide bandgap, high breakdown voltage tolerance, and excellent chemical and thermal stability. To develop it into an industrial-level application, extensive research needs to be conducted to advance wafer-scale and high-quality epitaxy growth techniques. In this work, a close-coupled showerhead (CCS) metal-organic chemical vapor deposition (MOCVD) system was used to grow wafer-scale Ga2O3 films. Through a detailed examination of growth parameters including substrate temperature, oxygen flow rate, precursor proportion, and chamber pressure, as well as their effect on film crystallinity, surface morphology, and optical properties, a comprehensive growth phase diagram and control over the growth rate of 2-inch Ga2O3 thin films were obtained at a wafer-scale. These findings suggest that the CCS-MOCVD system presents a competitive avenue for future industrial high-speed fabrication of single-phase Ga2O3 films.

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Design and Differentiation of Quantum States at Subnanometer Scale in La2CuO4-Sr2CuO4-δ Superlattices.

We present a study on the properties of superlattices made of ultrathin Sr2CuO4-δ layers sandwiched between La2CuO4 layers beyond the antiferromagnetic insulating nature of the individual layers of choice. Using molecular beam epitaxy, we synthesized these superlattices and observed superconductivity and metallicity at the interfaces. We probed the hole distribution to determine the discernible quantum states and found that the high-quality epitaxy, combined with mapping the electronic fine structure by electron energy-loss spectroscopy, allowed for the differentiation of insulating, metallic, and superconducting layers at the atomic-column scale. Our results demonstrate the possibility of exploring specific electronic properties at the subnanometer scale and highlight the potential of utilizing metastable Sr2CuO4-δ slabs.

Tunable Magnetic and Optical Anisotropy in ZrO2‐Co Vertically Aligned Nanocomposites

AbstractMetamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO2‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm−3, strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.

Epitaxial Growth of Aurivillius Bi3Fe2Mn2Ox Supercell Thin Films on Silicon

Nanoelectronic devices integrated with functional complex oxides have drawn much attention in recent years. However, due to material and processing compatibility issues, integrating functional complex oxides with Si-based devices is challenging, and success has been limited. As an example, the Bi3Fe2Mn2Ox (BFMO) supercell system, a single-phase layered oxide made by compositing BiFeO3 and BiMnO3, has been studied for much of the past decade for its unusual layered Aurivillius structure and magnetic and ferroelectric properties. However, most of the BFMO thin film growth has been demonstrated on single-crystal oxide substrates such as SrTiO3 and LaAlO3. In this work, we demonstrate that the BFMO layered supercell phase can be integrated on Si with high epitaxial quality using a buffer stack of TiN/SrTiO3/CeO2. Further understanding of the strain-controlled growth of the BFMO supercell phase has allowed such Si integration. Microstructure, magnetic, ferroelectric, and optical properties of the BFMO films on Si have been characterized and compared with those of BFMO on SrTiO3 single-crystal substrates, demonstrating comparable epitaxial quality and physical properties. Integrating multiferroic BFMO oxides on Si demonstrates the potential of layered supercell oxides in practical device applications such as ferroelectric field effect transistors.

(Invited) Vertical GaN Diodes: Effect of Growth Parameters, Indium Surfactants, and Device Development for High-Power Electronics

GaN has achieved widespread interest in high-power and high-speed electronics due to its wide-bandgap (WBG), large breakdown field, and high mobility compared to Si and SiC. There is also continued interest in the growth and development of GaN vertical devices owing to their advantageous properties of high current capability from backside ohmic, better field management, scaling feasibility, and enhanced thermal transport, which is not feasible with the lateral device topology. To realize the predicted high breakdown voltage and low on-resistance from vertical devices, high-quality drift layers with low unintentional doping and low compensating defects are fundamentally required. Hence, our research aims to pursue a comprehensive investigation of growth and device development with vertical GaN structures, building from the optimization of thick GaN homoepitaxy with high growth rate but low impurities, improving surface morphology using Indium as a surfactant as well as realizing punch-through field-plate p-n diodes to maximize the average electric field in high-voltage operation.Toward the goal of high-quality GaN epitaxy growth, we performed systematic studies of growth rate effects on the drift layer doping using ammonia molecular beam epitaxy (NH3 MBE) and plasma-assisted molecular beam epitaxy (PA MBE) [1]. The NH3 MBE homoepitaxy GaN demonstrated an unintentional doping (UID) of ~1015 cm-3, significantly lower than that of the conventional metalorganic chemical vapor deposition (MOCVD) reports, which can be potentially contributed by clean growth environment of MBE.We further focused on improving surface morphology using Indium as a surfactant. Through a combination of varying V/III ratios, In flux and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved for fast growth rates (on the order of 1 µm/hr and beam equivalent pressures on the order of 5 × 10-7 Torr). However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant can also reduce silicon impurities as revealed from the Secondary ion mass spectrometry (SIMS) (Fig. 2).Subsequently, we developed vertical p-n GaN power devices using the NH3 MBE-grown high-quality epitaxy [2]. The vertical p-n GaN diode consisted (from bottom up): ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer, ~4 µm UID GaN drift layer for n-type region, ~0.4 µm p+GaN ([Mg]: 3×1019 cm-3) for p-type region, and ~10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact. Circular p-n diodes of 80-100 µm diameter were fabricated with Pd/Pt anode and a backside Ti/ Au ohmic contact. The field management was achieved by field-plate of 15 µm formed with Ti/Au metals on top of a field-plate dielectric of a Al2O3 (26 nm)/ Si3N4 (205 nm) stack (Fig. 3).The p-n GaN diodes exhibited an outstanding forward J-V behavior with a low specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and a minimum ideality factor of 1.36 (Fig. 4), which are among the best achieved in vertical homoepitaxy GaN p-n diodes [2-4]. The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). These reverse-voltage properties imply a punch-through characteristics with peak electric field (EC) >2.6 MV/cm, as revealed by Silvaco simulation (Fig. 5). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly downscaled (~4 um) drift layer compared to the MOCVD GaN p-n diodes of ≥ 8 um (Fig. 6). Work is ongoing to develop NHE MBE p-n diodes with indium surfactant-assisted drift layer growth which will be promising to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

The Influence of Graphene on the Electrodeposition of Metals

Electrodeposition of metals with controlled crystallography and grain structure is critical for applications including batteries, electronic circuit fabrication, and industrial coating processes. For metal-anode-based batteries in particular, it is important to understand how the electrode surface affects the crystallography of electrodeposited metals. Graphene and other 2D materials have recently been shown to exhibit strong effects on metal crystallography during electrodeposition, but it is unclear the extent to which graphene is affecting epitaxy, and further understanding of the 3D metal/2D material interface is crucial for advancing interfacial electrodeposition control. In this work, we study the influence of graphene and the underlying Cu substrate on the crystal structure and morphology of electrodeposited metals. Based on extensive electrochemical testing and electron backscatter diffraction (EBSD), we find that the orientation of the substrate underneath graphene (usually copper) exerts a controlling crystallographic influence on the electrodeposited metal overlayer, rather than the graphene layer itself. Further transmission electron microscopy (TEM) and Raman spectroscopy investigations are used to probe the nature of the interface and the graphene layer. These findings are important because they show that graphene can potentially be used as an interlayer to enable high-quality remote epitaxy between a variety of metals that normally exhibit inhibited epitaxy because of surface oxide layers.

Tailoring interface epitaxy and magnetism in La1−xSrxMnO3/SrTiO3 heterostructures via temperature-driven defect engineering

Defect engineering of La1−xSrxMnO3 (LSMO)—a strongly correlated oxide displaying half metallicity and ferromagnetism above room temperature—has been the focus of a long-standing quest aimed at the exploitation of this material as a functional building block for memory storage and spintronic applications. Here, we discuss the correlation between structural defects and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures as a function of growth temperature and post-deposition annealing. Upon increasing the growth temperature from 500 to 700 °C at a fixed oxygen partial pressure of 0.007 mbar, the sputter-deposited epitaxial LSMO films experience a progressive increase in Curie temperature Tc from 110 to 270 K and saturation magnetization Ms from 1.4 to 3.3 μB/u.c. owing to a reduction in oxygen deficiencies. Concurrently, however, growth temperatures above 600 °C trigger the formation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface as a possible aftermath of temperature-driven diffusion of impurities from the STO substrate. Notably, although the interfacial spurious islands cause an increase in sample surface roughness, the heterostructure still preserves high-quality epitaxy. In general, the best compromise in terms of both structural and magnetic properties, comprising high-quality epitaxy, atomically flat surface, and robust ferromagnetism above room temperature, is obtained for LSMO films grown at a relatively low temperature of about 500–540 °C followed by a post-deposition annealing treatment at 900 °C for 1 h in air. Our study compares effective routes based on temperature-controlled defect engineering to finely tailor the complex interplay between microstructure and magnetism in LSMO thin films.

Photoinduced evolution of lattice orthorhombicity and conceivably enhanced ferromagnetism in LaMnO3 membranes

Ultrashort laser pulses have been utilized to dynamically drive phase transitions in correlated quantum materials. Of particular interest is whether phases not achievable in thermal equilibrium can be induced in complex oxides with intricately coupled lattice, electron and spin degrees of freedom. Here, we tracked atomic motions in LaMnO3 following photoexcitation with MeV ultrafast electron diffraction (MeV-UED) technique. We found that the light excited state exhibits numerous signatures different from thermal equilibrium ones, including nearly conserved Bragg intensities, strongly suppressed La cation and oxygen anion displacements, and the long-range lattice orthorhombicity evolution. Furthermore, using first-principles calculations, we predict that the ferromagnetic ordering and conductivity are both enhanced upon laser excitation due to the reduction of the lattice orthorhombicity. This work benefits from recent advance in fabrication of membrane films with high epitaxial quality and in MeV-UED with large momentum space access and high temporal resolution.

Bismuth doping enhanced tunability of strain-controlled magnetic anisotropy in epitaxial Y3Fe5O12(111) films

Y3Fe5O12 (YIG) and BiY2Fe5O12 (Bi:YIG) films were epitaxially grown on a series of (111)-oriented garnet substrates using pulsed laser deposition. Structural and ferromagnetic resonance characterizations demonstrated the high epitaxial quality, extremely low magnetic loss and coherent strain state in these films. Using these epitaxial films as model systems, we systematically investigated the evolution of magnetic anisotropy (MA) with epitaxial strain and chemical doping. For both the YIG and Bi:YIG films, the compressive strain tends to align the magnetic moment in the film plane while the tensile strain can compete with the demagnetization effect and stabilize perpendicular MA. We found that the strain-induced lattice elongation/compression along the out-of-plane [111] axis is the key parameter that determines the MA. More importantly, the strain-induced tunability of MA can be enhanced significantly by Bi doping; meanwhile, the ultralow damping feature persists. We clarified that the cooperation between strain and chemical doping could realize an effective control of MA in garnet-type ferrites, which is essential for spintronic applications.

Epitaxial growth and room-temperature ferromagnetism of quasi-2D layered Cr4Te5 thin film

Large-scale growth of two-dimensional (2D) ferromagnetic thin films will provide an ideal platform for studying 2D magnetism and active spintronic devices. However, controllable growth of 2D ferromagnets over large areas faces tremendous challenges. Herein, we report a large-area growth of 2D ferromagnetic single-crystal thin films Cr4Te5 on Al2O3 (0001) substrates using pulsed laser deposition. X-ray diffraction patterns and atomic force microscopy detection confirm that all thin films have high quality epitaxy together with being atom-level smooth. Magnetic measurements show the persistence of ferromagnetic ordering state to above room temperature, with a Curie temperature ∼320 K, atomic magnetic moment ∼0.307/Cr, and the easy-magnetization axis in the film plane. Comparing with bulk Cr4Te5 single-crystal, the critical exponent β = 0.491 indicates that the magnetic interactions of the thin film obey the mean-field model rather than the 3D Heisenberg model. This work will open an avenue for growing large-scale 2D ferromagnets and developing room temperature 2D magnet-based nanodevices.

GaN Power p–n Diodes on Hydride Vapor Epitaxy GaN Substrates with Near‐Unity Ideality Factor and <0.5 mΩ cm2Specific On‐Resistance

Herein, vertical GaN p–n diodes grown by metal organic chemical vapor deposition on hydride vapor epitaxy bulk GaN substrates are reported. The devices exhibit an ideality factor of 1.15, a turn‐on voltage of 3.2 V at 100 A cm−2, over 1 kV breakdown voltage, and specific on‐state resistance of 0.47 mΩ cm2, leading to a Baliga's figure of merit of ≈2.2 GW cm−2. The low specific on‐resistance, low turn‐on voltage, and the near‐unity ideality factor demonstrated here are attributed to the high epitaxial material quality and optimized low anode contact resistance.

High-quality AlGaN epitaxy on lattice-engineerable AlN template for high-power UVC light-emitting diodes

AlGaN-based UVC light-emitting diodes (LED) were fabricated on high-quality AlN templates with an engineerable in-plane lattice constant. The controllability of the in-plane strain originated from the vacancy formation in Si-doped AlN (AlN:Si) and their interaction with edge dislocations. The strain state of the Si:AlN top interface could be well depicted by a dislocation-tilt model depending on the buffer strain state, threading dislocation density (TDD), and regrown Si:AlN thickness. The validity of the model was verified by cross-sectional TEM analysis. With a gradually widened lattice constant of regrown Si:AlN layer, strain-induced defects of subsequently grown n-AlGaN was suppressed. Therefore, growing a current spreading layer which possesses a moderate Al content (<65%), decent thickness (>1.5 µm), and a low TDD (<1.0 × 109 cm−2) simultaneously becomes possible. Additionally, the idea of an optimal edge TDD (ρe,opt) in the AlN buffer was revealed for growing high-quality n-AlGaN layers with a targeted thickness. After a deliberate strain-TDD engineering for Si:AlN and n-AlGaN, high-power UVC LEDs (λ = 275 nm, P > 200 mW) with a low forward voltage (Vf = 5.7 volt) were demonstrated at I = 1.35 A. The low forward voltage under high current injection density was attributed to the success in preparation of a low series resistance and high-quality n-AlGaN current spreading layer.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

Transfer-free graphene-guided high-quality epitaxy of AlN film for deep ultraviolet light-emitting diodes

We report on quasi-van der Waals epitaxy of high-quality AlN film guided by transfer-free graphene (Gr) with low wrinkle density (LWD-Gr) on sapphire. The surface wrinkle density of LWD-Gr is greatly reduced by 94% compared to conventional Gr with high wrinkle density (HWD-Gr). Also noteworthy is that AlN nucleation islands grown on LWD-Gr exhibit better crystalline orientation consistency and more feasibility to coalesce with each other, thus forming AlN film with lower stress and dislocation density. Finally, 285 nm deep ultraviolet (DUV) light-emitting diodes are fabricated on the high-quality AlN template with LWD-Gr, which shows stronger electroluminescence intensity than its counterpart without and with HWD-Gr. The insights granted by this research pave a new pathway for improving the performance of nitride-based DUV optoelectronic devices through Gr engineering.