Epitaxial Layers Research Articles

K/Ka‐Band–GaN–High‐Electron‐Mobility Transistors Technology with 700 mS mm−1 Extrinsic Transconductance

In this study, the impacts of fabrication technology and epitaxial layer design on the transconductance (gm) of radio frequency AlGaN/GaN high‐electron‐mobility transistors (HEMTs) are examined. Optimization of the SiNx passivation and AlGaN barrier design of 150 nm gate HEMTs enhances the extrinsic (at Vds = 10 V) and intrinsic (at Vds = 15 V) transconductance from ≈0.47/0.65 to ≈0.62/1.1 S mm−1. Notably, an extrinsic gm of 0.70 S mm−1 at Vds = 5 V is achieved, setting a new benchmark for the extrinsic transconductance of AlGaN/GaN HEMTs designed for the K/Ka frequency range with a breakdown voltage exceeding 100 V.

Simple Smoothing of the Bottom Silicon Surface Using Wet Chemical Etching Methods for Epitaxial III‐V/Silicon Tandem Manufacturing

The implementation of diverse technologies has recently facilitated the production of cost‐effective and highly efficient solar cells. High‐efficiency solar cells with III‐V compounds tandem crystalline silicon cells have achieved photovoltaic efficiency of higher than 39%. Etching silicon wafers plays a vital role in the deposition of epitaxial layers, neutralizing dangling bonds, and surface passivation for tandem solar cells. The wafers are polished using a solution of HF–HNO3–CH3COOH (HNA) and 20% KOH to smoothen the wafer surface. When HNA wet etching is performed for 3.5 min and the 20% KOH etching lasts for 6 min, the microroughness of the wafer is 1.9 nm with a measurement area of 10 × 10 μm2 and 0.816 nm within an area of 1 × 1 μm2. Compared with the as‐cut wafer, the reflectance increases from 31.7% to 34.7%, and the effective minority carrier lifetime, with 30 nm Al2O3 passivation after 450 °C activated, increases from 1.4 to 1.8 ms in a carrier density of 1.0 × 1015 cm−3.

An improved 4H-SiC trench MOS barrier Schottky diode with current spreading layer and low resistance layer

This paper investigates an improved trench MOS barrier Schottky (TMBS) structure with extra double epitaxial layers (DE-TMBS) based on the conventional TMBS (C-TMBS) featuring a high-doped N-type current spreading layer (CSL) and a high-doped low resistance layer (LRL). Compared to the C-TMBS, the CSL in the improved structure is grown on the N-type drift region, and the LRL is extended on the CSL. According to the numerical simulations and analytical models, the specific on-resistance (Ron,sp) of DE-TMBS can be significantly reduced compared to the conventional one. This is primarily due to the high doping concentration in CSL, which effectively lowers both the JFET resistance and spreading resistance. Additionally, the increased doping concentration in LRL reduces the channel resistance and JFET resistance. Moreover, the doping concentration and thickness of CSL and LRL are optimized to maximize the figure of merit (FOM), that Ron,sp is reduced by 40.9 % and the FOM (BV2/Ron,sp) is improved by 67.9 % compared to the C-TMBS structure.

Structures of the First Epitaxial Layer Created in Colloidal Heteroepitaxy.

Brownian dynamics simulations have been performed to investigate the structural dependence of the first epitaxial layer in colloidal heteroepitaxy. When the epitaxial particles were larger than the substrate particles and the interactions were dominated by the depletion force, a hexagonal structure formed on a closely packed hexagonal substrate. The orientation of this hexagonal structure varied with the size ratio of the epitaxial to substrate particles to make the interaction between the substrate and epitaxial particles strong. When the sizes of the substrate and epitaxial particles were similar, long-period structures formed instead of hexagonal structures to strengthen the interaction between the substrate and epitaxial layer at the expense of the interaction between particles in the first epitaxial layer.

Enabling 2D Electron Gas with High Room-Temperature Electron Mobility Exceeding 100cm2 Vs-1 at a Perovskite Oxide Interface.

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a 2D electron gas (2DEG) with possible applications in, e.g., high-electron-mobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature (RT) 2DEG performance due to notoriously low electron mobility. In this work, the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO3 and LaInO3 is demonstrated with enhanced RT electron mobility values up to 119 cm2 Vs-1-the highest RT value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, this approach opens up another degree of freedom in optimization and in situ control of the interface between two epitaxial oxide layers away from the substrate interface. Thus this approach is expected to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materialsplatforms.

Out‐Diffusion and Uphill‐Diffusion of Mg in Czochralski‐Grown (100) β‐Ga2O3 Under High‐Temperature Annealing and Its Influence on Lateral MOSFET Devices

AbstractIn this work, the out‐diffusion and uphill‐diffusion of Mg inside (100) β‐Ga2O3 epilayers and substrates are reported. The Mg accumulates toward the (100) surface upon annealing under an oxidizing environment, whereas the concentration profile changes with annealing temperatures and durations. Furthermore, the out‐diffusion of Mg from the substrate into the epilayer is observed at temperatures above 800 °C, which continues during the film growth. The substitutional‐interstitial‐diffusion (SID) mechanism is suggested to be the driving mechanism for the former, and the latter is related to the diffusion of mobile Mg interstitials. The accumulation profile of Mg can be used to identify the interface between the epilayer and the substrate. Furthermore, significant differences in device performance are observed for power transistors fabricated on annealed and non‐annealed epitaxial β‐Ga2O3 wafers. Increased breakdown voltages of annealed samples are attributed to the Mg diffusion into the first few nanometers of the epitaxial layer close to the interface to the semi‐insulating substrate, leading to compensation of residual dopants (donors) in that region.

Active interface characteristics of heterogeneously integrated GaAsSb/Si photodiodes

There is increased interest in the heterogeneous integration of various compound semiconductors with Si for a variety of electronic and photonic applications. This paper focuses on integrating GaAsSb (with absorption in the C-band at 1550 nm) with silicon to fabricate photodiodes, leveraging epitaxial layer transfer (ELT) methods. Two ELT techniques—nanomembrane transfer printing and macro-transfer printing—are compared for transferring GaAsSb films from InP substrates to Si, forming PIN diodes. Characterization through atomic force microscopy and transmission electron microscopy exhibits a high-quality, defect-free interface. Current–voltage (IV) measurements and capacitance–voltage analysis validate the quality and functionality of the heterostructures. Photocurrent measurements at room temperature and 200 K demonstrate the device's photo-response at 1.55 μm, highlighting the presence of an active interface.

Van der Waals Epitaxy of High-Quality Transition Metal Dichalcogenides on Single-Crystal Hexagonal Boron Nitride.

Van der Waals (vdW) heterostructures comprising of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN) are promising building blocks for novel 2D devices. The vdW epitaxy provides a straightforward integration method for fabricating high-quality TMDs/h-BN vertical heterostructures. In this work, the vdW epitaxy of high-quality single-crystal HfSe2 on epitaxial h-BN/sapphire substrates by chemical vapor deposition is demonstrated. The epitaxial HfSe2 layers exhibit a uniform and atomically sharp interface with the underlying h-BN template, and the epitaxial relationship between HfSe2 and h-BN/sapphire is determined to HfSe2 (0001)[1 10]//h-BN (0001)[1 00]//sapphire (0001)[1 00]. Impressively, the full width at half maximum of the rocking curve for the epitaxial HfSe2 layer on single-crystal h-BN is as narrow as 9.6 arcmin, indicating an extremely high degree of out-plane orientation and high crystallinity. Benefitting from the high crystalline quality of HfSe2 epilayers and the weak interfacial scattering of HfSe2/h-BN, the photodetector fabricated from the vdW epitaxial HfSe2 on single-crystal h-BN shows the best performance with an on/off ratio of 1×104 and a responsivity up to 43mAW-1. Furthermore, the vdW epitaxy of other TMDs such as HfS2, ZrS2, and ZrSe2 is also experimentally demonstrated on single-crystal h-BN, suggesting the broad applicability of the h-BN template for the vdW epitaxy.

Pressure tuning of HgCdTe epitaxial layers—the role of the highly disordered buffer layer

Pressure tuning of HgCdTe epitaxial layers—the role of the highly disordered buffer layer

The mechanism of degradation and failure in NiO/ β -Ga2O3 heterojunction diodes induced by the high-energy ion irradiation

This work investigated the single-event effects (SEE) of NiO/β-Ga2O3 heterojunction diodes (HJDs) irradiated by 1.86 GeV tantalum ions with linear energy transfer over 80 MeV cm2/mg. The HJDs exhibited radiation responses with the early single-event leakage current (SELC) degradation until the fatal single-event burnout (SEB) failure, which was far below their breakdown voltages. Meanwhile, the surface morphology revealed the SELC damage expressed as burnout of topside NiO and metal stacks, while the SEB damage was observed as a burned hole in the β-Ga2O3 epitaxial layer. According to technology computer aided design simulations, the thicker p-type region in HJDs could further alleviate the electric field crowding effect exacerbated by the heavy-ion strike because of the extension of charge distribution in the p-type region. The SEB threshold was raised to 250 V by thickening the NiO layer to 300 nm. As for the SELC degradation process along with the burnout of topside stacks in HJDs, we supposed the probable reason was the intolerance of NiO to the high electric field under the SEE. This paper analyzed the SEE mechanism in β-Ga2O3 diodes and paved the way for heavy-ion irradiation hardening.

Passivation mechanisms of oxygen-vacancy-induced hole traps by Mg acceptor atoms at GaN/SiO2 interface

Experiments in the past suggest that hole traps at GaN/SiO2 interfaces are reduced with heavily Mg-doped GaN epitaxial layers, but its physical origin is unclear. In this study, we use first-principles calculations to investigate interactions between substitutional Mg atoms at Ga sites (MgGa) and O vacancies (VO) in the nanometer-scale Ga-oxide (GaOx) interfacial layers, which are inevitably formed at GaN/SiO2 interfaces. We clarify the physical reason why Mg passivates hole traps. Our calculations show that MgGa and VO become stable by forming 2MgGa–VO complex in GaOx. In addition, while VO form deep hole traps in the bandgap of GaN, Mg attachment to VO makes the hole-trap level shift upward substantially and thus 2MgGa–VO induces no hole traps. These results indicate that hole traps originated from VO are passivated by the formation of a complex of MgGa atoms and VO.

Hexagonal Enhanced Porous GaN with Delayed Integrated Pulse Electrochemical (iPEC) Etching

This present study investigates the effect of time delay (Td) on the formation of porous GaN (P-GaN) using integrated pulse electrochemical (iPEC) etching. Porous GaN (P-GaN) was formed by etching an N-type GaN wafer with a 4% KOH electrolyte for 60 minutes under an ultraviolet (UV) lamp at a current density of 80 mA/cm2. A Td of 120 minutes was applied before electrochemically etching the P-GaN sample. The top view image of the field emission scanning electron microscopy (FESEM) revealed a significant difference when a Td was applied. A dense and uniform hexagonal P-GaN was obtained from the Td iPEC sample, while the non-Td sample exhibited a multi-layered hexagonal porous structure with unfinished pore-etched areas. Higher porosity and deeper pores were observed in the Td sample. Intense high-resolution X-ray diffraction (HR-XRD) peak intensity was observed in the Td iPEC sample with a lower full width half maximum (FWHM), indicating that the sample had better crystallinity. The Raman spectra of the sample anodized with a Td exhibited higher Raman peak intensity and a slight shift to a higher frequency concerning as-grown GaN, indicating better crystallinity and a tensile stress relaxation of 0.24 GPa. Post etching, a blue shift of the photoluminescence (PL) peak, from 364 nm (as-grown GaN) to 363 nm (P-GaN), was observed, and a small PL peak started to form around 385 nm compared to the as-grown GaN due to the relaxation of the tensile stress, which modified the bandgap. The PL peak intensity of the Td sample was higher than the non-Td sample, indicating that the porosity and uniformity allowed more light interaction with the material, resulting in more efficient photon absorption and emission. The results indicated that potentially efficient optoelectronics devices can be fabricated on a P-GaN using a combination of electroless and electrochemical etching of the GaN epitaxial layer.

Ultrahigh Power Factor of Sputtered Nanocrystalline N-Type Bi2Te3 Thin Film via Vacancy Defect Modulation and Ti Additives.

Magnetron-sputtered thermoelectric thin films have the potential for reproducibility and scalability. However, lattice mismatch during sputtering can lead to increased defects in the epitaxial layer, which poses a significant challenge to improving their thermoelectric performance. In this work, nanocrystalline n-type Bi2Te3 thin films with an average grain size of ≈110nm are prepared using high-temperature sputtering and post-annealing. Herein, it is demonstrated that high-temperature treatment exacerbates Te evaporation, creating Te vacancies and electron-like effects. Annealing improves crystallinity, increases grain size, and reduces defects, which significantly increases carrier mobility. Furthermore, the pre-deposited Ti additives are ionized at high temperatures and partially diffused into Bi2Te3, resulting in a Ti doping effect that increases the carrier concentration. Overall, the 1µm thick n-type Bi2Te3 thin film exhibits a room temperature resistivity as low as 3.56 × 10-6 Ω∙m. Notably, a 5µm thick Bi2Te3 thin film achieves a record power factor of 6.66mWmK-2 at room temperature, which is the highest value reported to date for n-type Bi2Te3 thin films using magnetron sputtering. This work demonstrates the potential for large-scale of high-quality Bi2Te3-based thin films and devices for room-temperature TE applications.

Epilayer thickness effect on the electrical and breakdown characteristics of vertical β-Ga2O3 Schottky barrier diode

The current–voltage and breakdown characteristics of Au/Ni/β-Ga2O3 Schottky barrier diodes were investigated as a function of β-Ga2O3 epilayer thickness in the range of 6–19 μm. The X-ray rocking curves indicated that the full-width half-maximum is reduced with increasing epilayer thickness, implying an improvement in the crystallinity of β-Ga2O3 epilayer. The barrier heights of the field-plated β-Ga2O3 Schottky barrier diode with epitaxial layer thickness of 6, 12, and 19 μm were obtained as 1.01, 1.03, and 1.04 eV, with the ideality factor values being 1.21, 1.19, and 1.46, respectively. The higher ideality factors could be associated with the existence of inhomogeneity at the metal–semiconductor interface. The series resistance of the Schottky diode obtained increased with increasing epilayer thickness. The Schottky diode fabricated on 19 μm thick epitaxial layer exhibited a higher breakdown voltage of 490 V. The increase in epilayer thickness led to the widening of depletion region, resulting in lower electric field over a larger distance. This could be a main cause of the enhancement of the breakdown voltage characteristics of β-Ga2O3 Schottky barrier diode.

Unveiling the oxidation stability of 2D gallium-intercalated monolayer epitaxial graphene through correlative microscopy

The oxidation- and air-stability of 2D gallium-intercalated monolayer epitaxial graphene was determined using correlative microscopy. Site-specific studies including AFM, scanning electron microscope, cross section STEM-HAADF, and EELS revealed that the oxygen signal detected by XPS and AES analyses originated from oxidized surface carbon contaminants without the presence of oxygen at the 2D gallium layers. In addition, the air-stability of the 2D gallium was correlated with the presence of intact epitaxial graphene. The absence of graphene leads to oxidation of the 2D gallium in air, consequently losing the crystallinity of the epitaxial gallium layer. This study invokes the importance of correlative microscopy to better understand defects in 2D metals that have been recently recognized through the confinement heteroepitaxy. In addition, this study highlights the advantage of using high spatial resolution STEM techniques in comparison with XPS that has relatively lower resolution.

Reviving Sodium Tunnel Oxide Cathodes Based on Structural Modulation and Sodium Compensation Strategy Toward Practical Sodium-Ion Cylindrical Battery.

As a typical tunnel oxide, Na0.44MnO2 features excellent electrochemical performance and outstanding structural stability, making it a promising cathode for sodium-ion batteries (SIBs). However, it suffers from undesirable challenges such as surface residual alkali, multiple voltage plateaus, and low initial charge specific capacity. Herein, an internal and external synergistic modulation strategy is adopted by replacing part of the Mn with Ti to optimize the bulk phase and construct a Ti-containing epitaxial stabilization layer, resulting in reduced surface residual alkali, excellent Na+ transport kinetics and improved water/air stability. Specifically, the Na0.44Mn0.85Ti0.15O2 using water-soluble carboxymethyl cellulose as a binder can realize a capacity retention rate of 94.30% after 1,000 cycles at 2C, and excellent stability is further verified in kilogram large-up applications. In addition, taking advantage of the rich Na content in Prussian blue analog (PBA), PBA-Na0.44Mn1-xTixO2 composites are designed to compensate for the insufficient Na in the tunnel oxide and are matched with hard carbon to achieve the preparation of coin full cell and 18650 cylindrical battery with satisfactory electrochemical performance. This work enables the application of tunnel oxides cathode for SIBs in 18650 cylindrical batteries for the first time and promotes the commercialization of SIBs.

Mercury-probe measurement of electron mobility in β-Ga2O3 using junction moderated dielectric relaxation

We demonstrate the junction-moderated dielectric relaxation method to measure the in-plane electron mobility in β-Ga2O3 epitaxial layers. Unlike the Hall technique and channel mobility measurement in field-effect transistors, this method does not require the deposition of permanent metal contacts. Rather, it measures the bias voltage and frequency dependence of the equivalent capacitance of the Mercury/β-Ga2O3/Mercury structure consisting of a Schottky contact, a quasi-neutral thin film semiconductor, and an Ohmic contact connected in series. The intrinsic dielectric relaxation of the bulk β-Ga2O3 semiconductor typically occurs at ∼1012 s−1, but when moderated by the Mercury/β-Ga2O3 Schottky junction, it manifests itself as an inflection in the capacitance-frequency characteristics at a much lower frequency of ∼106 s−1 within the range of most capacitance measuring instruments. Using carrier density and layer thickness determined from capacitance-voltage measurement, we extract the electron mobility of β-Ga2O3 from the junction-moderated dielectric relaxation frequency.

Methods for Studying the Electrical Characteristics of the Epitaxial Layers of n/p-InxGa1 – xAs Solid Solutions for Large-Area Device Structures

Methods for Studying the Electrical Characteristics of the Epitaxial Layers of n/p-InxGa1 – xAs Solid Solutions for Large-Area Device Structures

A Novel Isolation Approach for GaN-Based Power Integrated Devices.

This paper introduces a novel technology for the monolithic integration of GaN-based vertical and lateral devices. This approach is groundbreaking as it facilitates the drive of high-power GaN vertical switching devices through lateral GaN HEMTs with minimal losses and enhanced stability. A significant challenge in this technology is ensuring electrical isolation between the two types of devices. We propose a new isolation method designed to prevent any degradation of the lateral transistor's performance. Specifically, high voltage applied to the drain of the vertical GaN power FinFET can adversely affect the lateral GaN HEMT's performance, leading to a shift in the threshold voltage and potentially compromising device stability and driver performance. To address this issue, we introduce a highly doped n+ GaN layer positioned between the epitaxial layers of the two devices. This approach is validated using the TCAD-Sentaurus simulator, demonstrating that the n+ GaN layer effectively blocks the vertical electric field and prevents any depletion or enhancement of the 2D electron gas (2DEG) in the lateral GaN HEMT. To our knowledge, this represents the first publication of such an innovative isolation strategy between vertical and lateral GaN devices.

Comprehensive study on epitaxial growth of GeSn(111) layers with high Sn content on Si(111) featuring Ge buffer layer

The study comprehensively discussed the crystalline properties of GeSn(111) epitaxial layers on Si(111) using Ge double buffer layers toward the formation of the direct transition GeSn(111) with a Sn content around 10%. We found that the introduction of Ge double buffer layers is rather effective to improve the crystallinity of GeSn(111) than the single buffer case. However, thicker Ge buffer layer degrades the crystallinity of GeSn(111) because the strain relaxation of GeSn layer is likely to occur. In this study, through optimizing the thicknesses of Ge buffer layers formed at low and high temperatures (LT-Ge and HT-Ge, respectively), it was clarified that 100 and 500 nm for LT- and HT-Ge thicknesses, respectively, are suited to realize a high quality GeSn(111). The grown GeSn(111) epitaxial layer includes a Sn content of ~8–11% with a compressive strain of ~1–2%, which is contained within the region of direct transition GeSn(111).