Search Results (508)

In this paper, various Ga₂O₃ films, including amorphous Ga₂O₃ films, β-Ga₂O₃, and α-Ga₂O₃ epitaxial films, have been nitridated and converted to single-crystalline GaN layers on the surface. Although the original Ga₂O₃ films are different, all the converted GaN layers exhibit the (002) preferred orientation and the porous morphologies. The ~200 µm GaN thick films have been grown on the nitridated Ga₂O₃ films using the halide vapor phase epitaxy (HVPE) method. Raman analysis indicates that all the HVPE-GaN films grown on nitridated Ga₂O₃ films are almost stress-free. An obvious GaN porous layer/Ga₂O₃ structure has been observed in the interface between GaN thick films and sapphire substrates. The porous GaN layers can be used as promising templates for the preparation of free-standing GaN substrates. Full article

Graphene’s exceptional carrier mobility and broadband absorption make it promising for ultrafast photodetection. However, its low optical absorption limits responsivity, while the absence of a bandgap results in high dark current, constraining the signal-to-noise ratio and efficiency. Although silicon (Si) photodetectors normally offer fabrication compatibility, their performance is severely hindered by interface trap states and optical shading. To overcome these limitations, we demonstrate an epitaxial graphene/n-Si heterojunction photodiode. This device utilizes graphene epitaxially grown on germanium integrated with a transferred Si thin film, eliminating polymer residues and interface defects common in transferred graphene. As a result, the fabricated photodetector achieves an ultralow dark current of 1.2 × 10⁻⁹ A, a high responsivity of 1430 A/W, and self-powered operation at room temperature. This work provides a strategy for high-sensitivity and low-power photodetection and demonstrates the practical integration potential of graphene/Si heterostructures for advanced optoelectronics. Full article

This work presents a comprehensive study of the structural, luminescent, and photoconversion properties of epitaxial composite phosphor converters based on single crystalline films of Ce³⁺-activated Ca_2−xY_1+xMg_1+xSc_1−xSi₃O₁₂:Ce (x = 0–0.25) (CYMSSG:Ce) garnet, grown using the liquid phase epitaxy (LPE) method on single-crystal Y₃Al₅O₁₂ (YAG) and YAG:Ce substrates. The main goal of this study is to elucidate the structure–composition–property relationships that influence the photoluminescence and photoconversion efficiency of these film–substrate composite converters, aiming to optimize their performance in high-power white light-emitting diode (WLED) applications. Systematic variation in the Y³⁺/Sc³⁺/Mg²⁺ cationic ratios within the garnet structure, combined with the controlled tuning of film thickness (ranging from 19 to 67 µm for CYMSSG:Ce/YAG and 10–22 µm for CYMSSG:Ce/YAG:Ce structures), enabled the precise modulation of their photoconversion properties. Prototypes of phosphor-converted WLEDs (pc-WLEDs) were developed based on these epitaxial structures to assess their performance and investigate how the content and thickness of SCFs affect the colorimetric properties of SCFs and composite converters. Clear trends were observed in the Ce³⁺ emission peak position, intensity, and color rendering, induced by the Y³⁺/Sc³⁺/Mg²⁺ cation substitution in the film converter, film thickness, and activator concentrations in the substrate and film. These results may be useful for the design of epitaxial phosphor converters with tunable emission spectra based on the epitaxially grown structures of garnet compounds. Full article

A super-polishing procedure was performed on the Al₂O₃ (0001) surface for epitaxial purposes. The roughness of the final polished surface was measured to be 0.16 nm using atomic force microscopy and X-ray reflectivity techniques. After heat treatment at 130 °C, results from low-energy electron diffraction and Auger energy spectroscopy indicated that the top surface was well ordered and clean, rendering it suitable for epitaxial growth. The successful growth of a GaN thin film on an Al₂O₃ (0001) substrate was confirmed by the hk-circle scan in XRD and the presence of a sharp peak in the rocking curve of the GaN (0002) Bragg peak. These findings indicate that the top surface of the substrate is conducive to epitaxial growth. Full article

A wide-bandgap AgInGaSe₂ (AIGS) thin film was fabricated using molecular beam epitaxy (MBE) via a three-stage method. The influence of Selenium (Se) pressure on the properties of AIGS films and solar cells was studied in detail. It was found that Se pressure played a very important role during the fabrication process, whereby Se pressure was varied from 0.8 × 10⁻³ Torr to 2.5 × 10⁻³ Torr in order to specify the effect of Se pressure. A two-stage mechanism during the production of AIGS solar cells was concluded according to the experimental results. With an increase in Se pressure, the grain size significantly increased due to the supply of the Ag–Se phase; the superficial roughness also increased. When Se pressure was increased to 2.1 × 10⁻³ Torr, the morphology of AIGS changed abruptly and clear grain boundaries were observed with a typical grain size of over 1.5 μm. AIGS films fabricated with a low Se pressure tended to show a higher bandgap due to the formation of anti-site defects such as In and Ga on Ag sites as a result of the insufficient Ag–Se phase. With an increase in Se pressure, the crystallinity of the AIGS film changed from the (220)-orientation to the (112)-orientation. When Se pressure was 2.1 × 10⁻³ Torr, the AIGS solar cell demonstrated its best performance of about 9.6% (Voc: 810.2 mV; Jsc: 16.7 mA/cm²; FF: 71.1%) with an area of 0.2 cm². Full article

Recently, with the development of automation technology in various fields, much research has been conducted on infrared photodetectors, which are the core technology of LiDAR sensors. However, most infrared photodetectors are expensive because they use compound semiconductors based on epitaxial processes, and they have low safety because they use the near-infrared (NIR) region that can damage the retina. Therefore, they are difficult to apply to automation technologies such as automobiles and factories where humans can be constantly exposed. In contrast, short-wavelength infrared photodetectors based on PbS QDs are actively being developed because they can absorb infrared rays in the eye-safe region by controlling the particle size of QDs and can be easily and inexpensively manufactured through a solution process. However, PbS QDs-based SWIR photodetectors have low chemical stability due to the electron/hole extraction layer processed by the solution process, making it difficult to manufacture them in the form of patterning and arrays. In this study, bulk NiO and ZnO were deposited by sputtering to achieve uniformity and patterning of thin films, and the performance of PbS QDs-based photodetectors was improved by optimizing the thickness and annealing conditions of the thin films. The fabricated photodetector achieved a high response characteristic of 114.3% through optimized band gap and improved transmittance characteristics. Full article

This study provides new insight into the mechanisms governing solid state dewetting (SSD) in SiGe alloys and underscores the potential of this bottom-up technique for fabricating self-organized defect-free nanostructures for CMOS-compatible photonic and nanoimprint applications. In particular, we investigate the SSD of Si_1−xGe_x thin films grown by molecular beam epitaxy on silicon-on-insulator (SOI) substrates, focusing on and clarifying the interplay of dewetting dynamics, strain elastic relaxation, and SiGe/SOI interdiffusion. Samples were annealed at 820 °C, and their morphological and compositional evolution was tracked using atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Raman spectroscopy, considering different annealing time steps. A sequential process typical of the SiGe alloy has been identified, involving void nucleation, short finger formation, and ruptures of the fingers to form nanoislands. XRD and Raman data reveal strain relaxation and significant Si-Ge interdiffusion over time, with the Ge content decreasing from 29% to 20% due to mixing with the underlying SOI layer. EDX mapping confirms a Ge concentration gradient within the islands, with higher Ge content near the top. Full article

BaBiO₃ has recently gained significant research attention as a parent material for an interesting family of alloyed compositions with multiple technological applications. In order to grow a variety of structures, a versatile deposition tool such as molecular beam epitaxy must be employed. In this work, the molecular beam epitaxy growth of BaBiO₃ on SrTiO₃(001) and MgO(001) substrates is studied. When grown by molecular beam epitaxy on SrTiO₃(001) or MgO(001) substrates, BaBiO₃ is known to have two competing orientations, namely (001) and (011). Characterization of the thin film is carried out by X-ray diffraction, X-ray reflectivity, atomic force microscopy, Rutherford backscattering, and transmission electron microscopy. Pathways to block the growth of BaBiO₃(011) and to grow only the technologically relevant BaBiO₃(001) are described for both substrates. An understanding of the enabling mechanism of the co-growth is established from an epitaxial point of view. This can be beneficially utilized for the growth of different compositions in the BaBiO₃ material family in a more controlled manner. Full article

Magnetic shape memory alloys have attracted considerable attention due to their multifunctional properties. Among these materials, Ni-Mn-Ga alloys are distinguished by their ability to achieve up to 10% strain when exposed to a magnetic field, a characteristic predominantly observed in single-crystal samples. Consequently, it is essential to develop nanomaterials with a crystal structure closely resembling that of a single crystal. In this study, an epitaxial Ni-Mn-Ga thin film was fabricated using Pulsed Laser Deposition on an Al₂O₃ $\left(11\overline{2}0\right)$ single-crystal substrate. The crystal structure was characterised through X-ray diffraction methodologies, such as symmetrical $2\theta -\omega$ scans, pole figures, and reciprocal space maps. The results indicated that the sample was mainly in a slightly distorted cubic austenite phase, and some incipient martensite phase also appeared. A detailed microstructural analysis, performed by transmission electron microscopy, confirmed that certain regions of the sample exhibited an incipient transformation to the martensite phase. Regions closer to the substrate retained the austenite phase, suggesting that the constraint imposed by the substrate inhibits the phase transition. These results indicate that it is possible to grow high crystalline quality thin films of Ni-Mn-Ga by Pulsed Laser Deposition. Full article

SnSe₂, as a prominent member of the post-transition metal dichalcogenides, exhibits many intriguing physical phenomena and excellent thermoelectric properties, calling for both fundamental study and potential application in two-dimensional (2D) devices. In this article, we realized the molecular beam epitaxial growth of SnSe₂ films on a $\sqrt{3}\times \sqrt{3}$-Sn reconstructed Si(111) surface. The analysis of reflection high-energy electron diffraction reveals the in-plane lattice orientation as SnSe₂[110]//$\sqrt{3}$-Sn [112]//Si [110]. In addition, the flat morphology of SnSe₂ film was identified by scanning tunneling microscopy (STM), implying the relatively strong adsorption effect of $\sqrt{3}$-Sn/Si(111) substrate to the SnSe₂ adsorbates. Subsequently, the interfacial charge transfer was observed by X-ray photoemission spectroscopy. Afterwards, the direct characterization of electronic structures was obtained via angle-resolved photoemission spectroscopy. In addition to proving the presence of interfacial charge transfer again, a new relatively flat in-gap band was found in monolayer and few-layer SnSe₂, which disappeared in multi-layer SnSe₂. The interface strain-induced partial structural phase transition of thin SnSe₂ films is presumed to be the reason. Our results provide important information on the characterization and effective modulation of electronic structures of SnSe₂ grown on $\sqrt{3}$-Sn/Si(111), paving the way for the further study and application of SnSe₂ in 2D electronic devices. Full article

Various types of strain, as well as chemical pressure induced by dopants, can effectively tailor the performance of perovskite thin films, including their optical, electrical or photoelectrochemical properties. The control of these functional properties through such engineering techniques is key to fulfilling the application-specific requirements of ferroelectric devices in various fields. Numerous models and experimental data have been published on this subject, especially on ferrite-based ferroelectric materials. Within this paper, the mechanisms of tuning ferroelectric intrinsic properties, such as polarization and ferroelectric domain configurations, through epitaxial strain and doping, as well as the role of these techniques in influencing functional properties such as dielectric and photoelectrochemical ones, are presented. This review examines the significant improvements in dielectric properties and photoelectrochemical efficiency achieved by the strategical control of key functionalities including dielectric losses, domain structures, charge separation and surface reactions in strained/doped ferroelectric thin films, highlighting the advancements and research progress made in this field in recent years. Full article

Terahertz (THz) metamaterials based on phase-change materials (PCMs) offer promising approaches to the dynamic modulation of electromagnetic responses. In this study, we design and experimentally demonstrate a tunable THz metamaterial composed of a symmetric split-ring resonator (SRR) pair, with the left halves covered by a 35 nm thick epitaxial vanadium dioxide (VO₂) film, enabling the simultaneous exploitation of both permittivity- and conductivity-induced modulation mechanisms. During the metal–insulator transition (MIT) of VO₂, cooperative changes in permittivity and conductivity lead to the excitation, redshift, and eventual disappearance of a quasi-bound state in the continuum (QBIC) resonance. Finite element simulations, using optical parameters of VO₂ film defined by the Drude–Smith model, predict the evolution of the transmission spectra well. These results indicate that the permittivity change originating from mesoscopic carrier confinement is a non-negligible factor in THz metamaterials hybridized with VO₂ film and also reveal the potential for developing reconfigurable THz metamaterials based on the dielectric modulation effects of VO₂ film. Full article

Ferroelectric thin films with a high dielectric tunability (η) have great potential in electrically tunable applications, including microwave tunable devices such as phase shifters, filters, delay lines, etc. Using a modified Landau–Devonshire type thermodynamic potential, we show that the dielectric tunability η of a (001) tetragonal ferroelectric film can be analytically solved. After a survey of materials, a large η value above 60% was predicted to be achievable in a (001)-oriented tetragonal Pb(Zr_0.52Ti_0.48)O₃ (PZT) film. Experimentally, (001)-oriented PZT thin films were prepared on LaNiO₃-coated (100) SrTiO₃ substrates by using pulsed laser deposition (PLD). These films exhibited good dielectric tunability (η ~ 67.6%) measured at a small electric field E of ~250 kV/cm (corresponding to 5 volts for a 200 nm thick film). It only dropped down to ~54.2% when E was further reduced to 125 kV/cm (2.5 volts for 200 nm film). The measured dielectric tunability η as functions of the applied electric field E and measuring frequency f are discussed for a 500 nm thick PZT film, with the former well described by the theoretical η(E) curves and the latter showing a weak frequency dependence. These observations validate our integrated approach rooted in a theoretical understanding. Full article

We report the growth and optical characterization of single-crystal BiFe_1−xMn_xO₃ thin films directly on SrTiO₃/Si(001) substrates using molecular beam epitaxy. X-ray diffraction confirmed epitaxial growth, film crystallinity, and sharp interface quality. Scanning electron microscopy and energy dispersive X-ray spectroscopy verified uniform film morphology and successful Mn incorporation. Spectroscopic ellipsometry revealed a systematic bandgap reduction with increasing Mn concentration, from 2.7 eV in BiFeO₃ to 2.58 eV in BiFe_0.74Mn_0.26O₃, consistent with previous reports on Mn-doped BiFeO₃. These findings highlight the potential of BiFe₁₋_xMn_xO₃ films for bandgap engineering, advancing their integration into silicon-compatible multifunctional optoelectronic and photovoltaic applications. Full article

Ultrathin NiO films, ranging from 1 to 16 monolayers (ML) in thickness, have been stabilized via reactive molecular beam epitaxy on the (001) surface of a metastable body-centered cubic (BCC) Ni film. Low-energy electron diffraction (LEED) confirms that NiO grows as a crystalline film, exposing the (001) surface. Auger electron spectroscopy (AES) reveals a slight oxygen excess compared to a perfectly stoichiometric NiO film. Scanning tunneling microscopy (STM) shows that at low coverages the film exhibits atomically flat terraces, while at higher coverage a “wedding cake” morphology emerges. Scanning tunneling spectroscopy (STS) reveals a thickness-dependent evolution of the electronic band gap, which increases from 0.8 eV at 3 ML to 3.5 eV at 16 ML. The center of the band gap is approximately 0.2 eV above the Fermi level, indicating that NiO is p-doped. Full article