Epitaxial Structures Research Articles

Molecular beam epitaxy and band structures of type-II antiferromagnetic semiconductor EuTe thin films

The rare-earth Eu-based compounds with a unique half-filled 4f orbital have attracted an amount of research interest recently. Here, we synthesized EuTe(001) single-crystal thin films on SrTiO3(001) substrate via molecular beam epitaxy (MBE). The scanning tunneling microscopy and x-ray diffraction results indicate that the grown EuTe thin films orientated as EuTe[100]//SrTiO3[110] in plane. In the angle-resolved photoemission spectroscopic (ARPES) measurements, the grown EuTe films show a semiconductive band structure with the valence band maximum lying on the center point of the Brillouin zone. The bandgap size of EuTe was further identified by the optical transmission spectra as 2.2 eV. The antiferromagnetic transition temperature of the grown EuTe film is 10.5 K measured by a superconductive quantum interference device (SQUID). Our results provide important information on the fundamental electronic structures for the further research and applications of the Eu-based compounds.

Wafer-scale vertical injection III-nitride deep-ultraviolet light emitters

A ground-breaking roadmap of III-nitride solid-state deep-ultraviolet light emitters is demonstrated to realize the wafer-scale fabrication of devices in vertical injection configuration, from 2 to 4 inches. The epitaxial device structure is stacked on a GaN template instead of conventionally adopted AlN, where the primary concern of the tensile strain for Al-rich AlGaN on GaN is addressed via an innovative decoupling strategy, making the device structure decoupled from the underlying GaN template. Moreover, the strategy provides a protection cushion against the stress mutation during the removal of substrates. As such, large-sized wafers can be obtained without surface cracks, even after the removal of the sapphire substrates by laser lift-off. Wafer-scale fabrication of 280 nm vertical injection deep-ultraviolet light-emitting diodes is eventually demonstrated, where a light output power of 65.2 mW is achieved at a current of 200 mA, largely thanks to the significant improvement of light extraction. This work will definitely speed up the application of III-nitride solid-state deep-ultraviolet light emitters featuring high performance and scalability.

GaN-on-sapphire JTE-anode lateral field-effect rectifier for improved breakdown voltage (>2.5 kV) and dynamic RON

In high-power switching applications such as electric grids, transportation, and industrial electronics, power devices are supposed to have kilo-voltage (kV) level blocking capability. In this work, 1200-V gallium nitride (GaN) lateral field-effect rectifiers (LFERs) are demonstrated. The GaN-on-sapphire epitaxial structure is adopted to prevent vertical breakdown. To address electric field crowding, a p-GaN/AlGaN/GaN junction termination extension (JTE) is embedded in the anode region of the LFER. Comparing to the conventional LFER (Conv-LFER) fabricated on the same wafer, the JTE-anode LFER (JTE-LFER) achieves an improved breakdown voltage (>2.5 kV) and a lower dynamic ON-resistance (RON). The proposed p-GaN/AlGaN/GaN JTE offers a semiconductor-based solution (contrasted to the dielectric-based solution, i.e., field plate) to mitigate the high electric field, which is highly desirable for wide bandgap semiconductor power devices as it enhances the dielectric reliability.

Photoluminescence and Raman spectroscopy of Ce3+ doped Y3Al5O12 single crystalline films grown onto Y3Al5O12 and Lu3Al5O12 substrates

Raman spectroscopy, high spectral resolution luminescence, and X-ray diffraction techniques were employed to study two Ce3+ doped Y3Al5O12 single crystalline films grown by liquid phase epitaxy method onto Y3Al5O12 and Lu3Al5O12 single crystal substrates. Optical spectra were obtained with a micrometer step along the cross-sections of epitaxial structures, allowing excellent differentiation of the film, the transition layer, and the substrate of each sample. X-ray measurements demonstrate the mismatch between the lattice constants of the Y3Al5O12:Ce3+ film and its Lu3Al5O12 substrate, an effect related to different compositions. Consequently, the film grown onto Lu3Al5O12 exhibits higher residual stresses than its counterpart grown onto Y3Al5O12. This was confirmed by a mutual comparison of the Raman bands positions of the films. The luminescence spectra of both samples consist mainly of cerium 5d-4f emission, the intensity of which allows for additional study of epitaxial cross section and estimation of the size of transition layer.

Improved DC and RF Characteristics of GaN-Based Double-Channel HEMTs by Ultra-Thin AlN Back Barrier Layer.

In order to improve the off-state and breakdown characteristics of double-channel GaN HEMTs, an ultra-thin barrier layer was chosen as the second barrier layer. The strongly polarized and ultra-thin AlN sub-barrier and the InAlN sub-barrier are great candidates. In this article, the two epitaxial structures, AlGaN/GaN/AlN/GaN (sub-AlN) HEMTs and AlGaN/GaN/InAlN/GaN (sub-InAlN) HEMTs, were compared to select a more suitable sub-barrier layer. Through TEM images of the InAlN barrier layer, the segregation of In components can be seen, which decreases the mobility of the second channel. Thus, the sub-AlN HEMTs have a higher output current density and transconductance than those of the sub-InAlN HEMTs. Because the high-quality AlN barrier layer shields the gate leakage current, a 294 V breakdown voltage was achieved by the sub-AlN HEMTs, which is higher than the 121 V of the sub-InAlN HEMTs. The current gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of the sub-AlN HEMTs are higher than that of the sub-InAlN HEMTs from low to high bias voltage. The power-added efficiency (PAE) and output power density (Pout) of the sub-AlN HEMTs are 57% and 11.3 W/mm at 3.6 GHz and 50 V of drain voltage (Vd), respectively. For the sub-InAlN HEMTs, the PAE and Pout are 41.4% and 8.69 W/mm, because of the worse drain lag ratio. Thus, the Pout of the sub-AlN HEMTs is higher than that of the sub-InAlN HEMTs.

Internal quantum efficiency improvement of InGaN-based red LED by using double V-pits layers as strain relief layer

Micro/mini light emitting diodes (LEDs) based on AlInGaN material system have vast potential in display applications. Nevertheless, the low internal quantum efficiency (IQE) of InGaN-based red LED limits its development and application. In the epitaxial structure of our designed red LED, double V-pits layers were used as strain relief layers to reduce compressive strain and improve the IQE of the active layer. First, InGaN/GaN superlattices (SLs) were grown below the active layer to form low-density large V-pits layer. Subsequently, multi-period green and red composite quantum wells were adopted as the active layer. A high-density small V-pits layer was introduced into the active region to release the compressive strain by adjusting the growth parameters of green multiple quantum wells (MQWs). The V-shaped pits divide the continuous large-area of active layer into mutually isolated small pieces, which prevents the transmission of strain and converts the long-range strain into separated local strain. The peak IQEs of LED A2 with single V-pits layer and LED B4 with double V-pits layers were measured to be 10.5% at 613 nm and 21.5% at 612.1 nm, respectively. The IQE is greatly improved by 204.7%. The research results indicate that the double V-pits layers structure can alleviate the compressive strain of InGaN QWs more effectively, reduce the influence of piezoelectric polarization field, and improve the IQE.

Atomic‐Scale Structural Dynamics at a‐Si:H/c‐Si Heterointerface During Low‐Temperature Thermal Annealing

AbstractThe integrity of the hydrogenated amorphous silicon/crystalline silicon (a‐Si:H/c‐Si) interface is essential for the enhanced performance of a‐Si:H/c‐Si heterojunction‐based devices. However, during annealing processes aimed at passivating silicon dangling bonds, unexpected Si epitaxy and nanotwin formation tend to emerge, even under low‐temperature conditions. Therefore, understanding the influence of such annealing on the a‐Si:H/c‐Si interfacial structure is therefore pivotal for device optimization. In this study, the atomic‐scale structural transformation of the a‐Si:H/c‐Si heterointerface subjected to low‐temperature annealing is delved into. The dynamic evolution of this interface is captured by employing in situ spherical aberration (CS)‐corrected transmission electron microscopy (TEM), molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. The TEM observations indicated that Si epitaxy initiated before Si nanotwin formation, and these nanotwins are inclined to revert to epitaxial structures upon sustained annealing. Through MD and DFT insights, the thermodynamic and kinetic intricacies driving the concerted tri‐layer atomic shift characterizing the Si nanotwin‐to‐epitaxy transition are decoded. The findings shed light on the thermal behavior of a‐Si:H/c‐Si interfaces, offering new perspectives on the thermal management in silicon heterojunction devices.

Epitaxial Growth of Ga2O3: A Review.

Beta-phase gallium oxide (β-Ga2O3) is a cutting-edge ultrawide bandgap (UWBG) semiconductor, featuring a bandgap energy of around 4.8 eV and a highly critical electric field strength of about 8 MV/cm. These properties make it highly suitable for next-generation power electronics and deep ultraviolet optoelectronics. Key advantages of β-Ga2O3 include the availability of large-size single-crystal bulk native substrates produced from melt and the precise control of n-type doping during both bulk growth and thin-film epitaxy. A comprehensive understanding of the fundamental growth processes, control parameters, and underlying mechanisms is essential to enable scalable manufacturing of high-performance epitaxial structures. This review highlights recent advancements in the epitaxial growth of β-Ga2O3 through various techniques, including Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), Mist Chemical Vapor Deposition (Mist CVD), Pulsed Laser Deposition (PLD), and Low-Pressure Chemical Vapor Deposition (LPCVD). This review concentrates on the progress of Ga2O3 growth in achieving high growth rates, low defect densities, excellent crystalline quality, and high carrier mobilities through different approaches. It aims to advance the development of device-grade epitaxial Ga2O3 thin films and serves as a crucial resource for researchers and engineers focused on UWBG semiconductors and the future of power electronics.

Antiferromagnetic coupling in ferrimagnetic Mn4N-based bilayer structures

Mn4N/(Mn,Cu)4N epitaxial bilayer structures with (Mn,Cu)4N compositions below and above the magnetization compensation composition were prepared on SrTiO3(001) substrates by molecular beam epitaxy. The thickness of the (Mn,Cu)4N layer was fixed at approximately 20 nm, while that of the Mn4N layer was changed from 7.5 to 19.6 nm. Cross-sectional elemental mapping proved that the diffusion of Cu from the (Mn,Cu)4N layer to the Mn4N layer was negligible. The magnetization curves showed that the magnetic moments of Mn4N and (Mn,Cu)4N were antiferromagnetically coupled, independent of the Mn4N film thickness, indicating a synthetic ferrimagnetic structure. The dependence of magnetic order on Mn4N film thickness was confirmed by surface-sensitive measurements using polar magneto-optical Kerr effect and x-ray magnetic circular dichroism. This is due to the change in the layer with dominant magnetization and the strength of the antiferromagnetic coupling. The temperature dependence of the anomalous Hall effect showed that the antiferromagnetic coupling was retained in the Mn4N(7.5 nm)/(Mn,Cu)4N(22.4 nm) structure over a wide temperature range of 10–350 K.

Three-Layered Composite Scintillator Based on the Epitaxial Structures of YAG and LuAG Garnets Doped with Ce3+ and Sc3+ Impurities.

In this study, we propose novel three-layer composite scintillators designed for the simultaneous detection of different ionizing radiation components. These scintillators are based on epitaxial structures of LuAG and YAG garnets, doped with Ce3+ and Sc3+ ions. Samples of these composite scintillators, containing YAG:Ce and LuAG:Ce single crystalline films with different thicknesses and LuAG:Sc single crystal substrates, were grown using the liquid phase epitaxy method from melt solutions based on PbO-B2O3 fluxes. The scintillation properties of the proposed composites, YAG:Ce film/LuAG:Sc film/LuAG:Ce crystal and YAG:Ce film/LuAG:Ce film/LuAG:Sc crystal, were investigated under excitation by radiation with α-particles from a 239Pu source, β-particles from 90Sr sources and γ-rays from a 137Cs source. Considering the properties of the mentioned composite scintillators, special attention was paid to the ability of simultaneous separation of the different components of mixed ionizing radiation containing the mentioned particles and quanta using scintillation decay kinetics. The differences in scintillation decay curves under α- and β-particle and γ-ray excitations were characterized using figure of merit (FOM) values at various scintillation decay intensity levels (1/e, 0.1, 0.05, 0.01).

Conductive single-phase SrMoO3 epitaxial films synthesized in pure Ar ambience via plasma-assisted radio frequency sputtering

ABSTRACT The cubic perovskite SrMoO3 with a paramagnetic ground state and remarkably low room-temperature resistivity has been considered as a suitable candidate for the new-era oxide-based technology. However, the difficulty of preparing single-phase SrMoO3 thin films by hydrogen-free sputtering has hindered their practical use, especially due to the formation of thermodynamically favorable SrMoO4 impurity. In this work, we developed a radio frequency sputtering technology enabling the reduction reaction and achieved conductive epitaxial SrMoO3 films with pure phase from a SrMoO4 target in a hydrogen-free, pure argon environment. We demonstrated the significance of controlling the target-to-substrate distance (TSD) on the synthesis of SrMoO3; the film resistivity drastically changes from 1.46 × 105 μΩ·cm to 250 μΩ·cm by adjusting the TSD. Cross-sectional microstructural analyses demonstrated that films with the lowest resistivity, deposited for TSD = 2.5 cm, possess a single-phase SrMoO3 with an epitaxial perovskite structure. The formation mechanism of the conductive single-phase SrMoO3 films can be attributed to the plasma-assisted growth process by tuning the TSD. Temperature-dependent resistivity and Hall effect studies revealed metal-like conducting properties for low-resistive SrMoO3 films, while the high-resistive ones displayed semiconductor-like behavior. Our approach makes hydrogen-free, reliable and cost-efficient scalable deposition of SrMoO3 films possible, which may open up promising prospects for a wide range of future applications of oxide materials.

Ab Initio Molecular Dynamics Insight to Structural Phase Transition and Thermal Decomposition of InN.

Extensive ab initio density functional theory molecular dynamics calculations were used to evaluate stability conditions for relevant phases of InN. In particular, the p-T conditions of the thermal decomposition of InN and pressure-induced wurtzite-rocksalt solid-solid phase transition were established. The comparison of the simulation results with the available experimental data allowed for a critical evaluation of the capabilities and limitations of the proposed simulation method. It is shown that ab initio molecular dynamics can be used as an efficient tool for simulations of phase transformations of InN, including solid-solid structural transition and thermal decomposition with formation of N2 molecules. It is of high interest, because InN is an important component of epitaxial quantum structures, but it has not been obtained as a bulk single crystal. This makes it difficult to determine its basic physical properties to develop new applications.

Performance improvement of 1.3 μm InAlGaAs MQW modulators grown by MOVPE using C-doped InAl(Ga)As cladding layers

We propose a novel epitaxial layer structure to prevent Zn from diffusing into a multiple quantum well (MQW) layer of an electro-absorption modulator (EAM). Zn is practically employed as a p-type dopant in an InP material system, but there is a technical issue of unintentional Zn diffusion. In this work, we introduce a novel C-doped p-type layer between a Zn-doped p-cladding layer of InP and MQW to both reduce Zn diffusion and accurately define a p-i-n junction interface. The Zn concentrations in the MQW for each device are compared by measuring secondary ion mass spectrometry (SIMS). To clarify how the reduction of diffused Zn affects the EAM characteristics, we fabricate the EAMs with each epitaxial structure and compare their extinction ratio characteristics. The evaluation of extinction ratio per EAM capacitance successfully demonstrates that employing the C-doped layer effectively improves the EAM characteristics.

Vertical InGaN Light-Emitting Diode with Hybrid Distributed Bragg Reflectors.

A vertical-type InGaN light-emitting diode with a resonant cavity was demonstrated with a 9 μm aperture size and a short cavity formed by hybrid distributed Bragg reflectors (DBRs). The approach involved designing epitaxial structures and utilizing an electrochemical etching process to convert heavily doped n-type gallium nitride (n+-GaN) layers into porous GaN layers as a porous-GaN DBR structure. Thirteen pairs of the conductive porous-GaN:Si/GaN:Si DBR structure provided a vertical current path in a vertical-type light-emitting diodes (LED) structure. The LED epitaxial layers were separated from sapphire for membrane-type LED structures through a laser lift-off process. During the free-standing membrane fabrication process, the dielectric DBR deposited on ITO/p-GaN:Mg layers was inverted from top to bottom, thereby establishing the concept of higher reflectivity for the bottom DBR compared to the porous-GaN DBR. The physical cavity length was reduced from about 2.3 μm for the LED membrane to 0.74 μm for the membrane-type LED with the embedded porous-GaN DBR structure. The divergent angles and line width of EL emission light were reduced from 124°/31.7 nm to 44°/3.3 nm due to the resonant cavity effect. The membrane-type LED structures with hybrid DBRs consisted of small divergent angles, narrow line width, and vertical current injection properties that have potential for directional emission light sources and vertical-cavity surface-emitting diode laser applications.

Solar-blind photonic integrated chips for real-time on-chip communication

The monolithically integrated self-driven photoelectric detector (PD) with the light-emitting diode (LED) epitaxial structure completely relies on the built-in electric field in the multi-quantum wells region to separate the photogenerated carriers. Here, we propose a novel superlattices–electron barrier layer structure to expand the potential field region and enhance the detection capability of the integrated PD. The PD exhibits a record-breaking photo-to-dark current ratio of 5.14 × 107, responsivity of 110.3 A/W, and specific detectivity of 2.2 × 1013 Jones at 0 V bias, respectively. A clear open-eyed diagram of the monolithically integrated chip, including the PD, LED, and waveguide, is realized under a high-speed communication rate of 150 Mbps. The obtained transient response (rise/decay) time of 2.16/2.28 ns also illustrates the outstanding transient response capability of the integrated chip. The on-chip optical communication system is built to achieve the practical video signals transmission application, which is a formidable contender for the core module of future large-scale photonic integrated circuits.

Research on the effect of remelting on the epitaxial growth process of single crystal superalloy

In this paper, the remelting process is applied to Laser Directed Energy Deposition (L-DED) to repair single crystal superalloys. The main formation modes of stray grains in single crystal epitaxial growth were analyzed, and the inhibitory effect of remelting process on stray grains in single crystal epitaxial growth using L-DED was studied. Comparative experiments between remelting and non-remelting processes have been conducted. Three process parameters in the remelting process: power, z-axis lift and cladding-remelting interval time were studied. The impact of different remelting process parameters on single crystal epitaxial growth and the formation of stray grains has been studied. Remelting has been shown to effectively prevent the formation of stray grains during the solidification of the melt pool, which are otherwise caused by the presence of unmelted powder during the powder feeding phase of the L-DED process. A cladding layer without stray grains on both sides is obtained. After remelting, the epitaxial growth structure can achieve larger cellular and columnar dendritic zones and smaller primary dendrite arms. A favorable epitaxial growth structure was obtained in this paper, under the optimized remelting process window.

Wavelength Tuning in Resonant Cavity Interband Cascade Light Emitting Diodes (RCICLEDs) via Post Growth Cavity Length Adjustment.

We demonstrate substrate-emitting resonant cavity interband cascade light emitting diodes (RCICLEDs) based on a single distributed Bragg reflector (DBR). These devices operate in continuous wave mode at room temperature. Compared to standard ICLEDs without a cavity, we achieved an 89% reduction in the emission spectrum width, as indicated by the Full Width Half Maximum (FWHM) of 70 nm. Furthermore, we observed far-field narrowing and improved thermal stability. A single DBR configuration allows the cavity length to be adjusted by adding refractive index-matched material to the top of the epitaxial structure after epitaxial growth. This modification effectively shifts the cavity response towards longer wavelengths. We fabricated emitters comprising two cavities of different lengths, resulting in the emission of two distinct spectral lines that can be independently controlled. This dual-color capability enables one of the emission lines to serve as a built-in reference channel, making these LEDs highly suitable for cost-effective gas-sensing applications.

Investigation of GaAs-Based Laser Diode Adopting an Al Composition Gradient Double Waveguide Structure and its Photoelectric Properties

The double waveguide structure of a 1060 nm laser diode with different Al composition linear gradient was designed for achieving high output power. In contrast to the single waveguide layer with Al-free composition gradient structure, the double waveguide layer with a reverse Al composition gradient from n side to p side showed excellent optoelectronic properties. We found that the reverse Al composition gradient double waveguide layer could decrease injection potential barrier for electrons and holes, as well as has a high block leakage potential barrier, which can be help to increase carrier transport and optical confined factor. Meanwhile, it can improve sheet carrier concentration in order to decrease non-radiative recombination. When the injection current is 6 A, the maximal output power and peak wall-plug conversion efficiency are 6.12 W and 81.1%, respectively. The influencing mechanism of these photoelectric parameters on power and wall-plug conversion efficiency was discussed. The novel waveguide structure will be favorable for designing epitaxial structure and providing a theoretical basis for high-power laser diode.

Optimization of Epitaxial Structures on GaN-on-Si(111) HEMTs with Step-Graded AlGaN Buffer Layer and AlGaN Back Barrier

Recently, crack-free GaN-on-Si growth technology has become increasingly important due to the high demand for power semiconductor devices with high performances. In this paper, we have experimentally optimized the buffer structures such as the AlN nucleation layer and step-graded AlGaN layer for AlGaN/GaN HEMTs on Si (111) substrate by varying growth conditions and thickness, which is very crucial for achieving crack-free GaN-on-Si epitaxial growth. Moreover, an AlGaN back barrier was inserted to reduce the buffer trapping effects, resulting in the enhancement of carrier confinement and suppression of current dispersion. Firstly, the AlN nucleation layer was optimized with a thickness of 285 nm, providing the smoothest surface confirmed by SEM image. On the AlN nucleation layer, four step-graded AlGaN layers were sequentially grown by increasing the Al composition from undermost layer to uppermost layer, meaning that the undermost one was close to AlN, and the uppermost was close to GaN, to reduce the stress and strain in the epitaxial layer gradually. It was also verified that the thicker step-graded AlGaN buffer layer is suitable for better crystalline quality and surface morphology and lower buffer leakage current, as expected. On these optimized buffer structures, the AlGaN back barrier was introduced, and the effects of the back barrier were clearly observed in the device characteristics of the AlGaN/GaN HEMTs on Si (111) substrate such as the transfer characteristics, output characteristics and pulsed I-V characteristics.

60‐4: Minimal Efficiency Degradation and Elevated Radiometric Power Density of Ultraviolet‐A Micro‐LED with Homoepitaxial Structure

As display technology continues to advance, UVA micro‐LEDs are becoming increasingly important in a variety of display and beyond‐display applications. In this study, we investigate the optoelectronic characteristics of UVA micro‐LEDs based on a homogeneously epitaxial structure on freestanding gallium nitride (GaN) substrates. The device exhibits a central wavelength of 390 nm, positioned at the overlap of UVA and visible light spectra. It demonstrates an impressively low ideality factor of 1.49 at 2.86 V and a series resistance of 9.88 O beyond 3 V. Optically, our device shows virtually no wavelength shift across a wide current density range of 0.1‐1000 A/cm2, indicating exceptional optical stability and color accuracy. The emitted light is typical purple, reaching an expansive color gamut of 115.3% of Rec.2020. Furthermore, the GaN‐on‐GaN structure, with its superior crystal structure and heat dissipation properties, results in a very low droop ratio in EQE. This ensures sustained highpower output at high current densities, showcasing great potential in applications such as 3D printing, maskless photolithography, and fluorescence tagging.