Epitaxial Device Research Articles

Transmission electron microscopy of epitaxial semiconductor materials and devices

Abstract The transmission electron microscope (TEM) is a powerful imaging, diffraction and spectroscopy tool that has revolutionized the field of microscopy. It has contributed to numerous breakthroughs in various scientific disciplines. TEM-based techniques can offer atomic resolution as well as elemental analysis, which benefit the study of epitaxial semiconductors and their related optoelectronic devices on the atomic scale. The design and optimization of the device performance depend on three key factors: the control of strain at nanometer scale, control of the formation and propagation of defects as well as the control of local electronic properties. Manipulation and optimization are only possible if the key factors can be characterized precisely. Herein, the TEM techniques for strain analysis, defect characterization and bandgap evaluation are reviewed and discussed. Lately, with the development of in-situ TEM techniques, researchers have been able to observe dynamic processes and study the behaviour of materials and devices under realistic conditions (in gaseous atmosphere or in liquids, at elevated or cryogenic temperatures, under strain, bias or illumination) in real-time with extremely high spatial resolution. This review explores the impact and significance of in-situ TEM in the field of semiconductors.

Improvement in CPP-GMR read head sensor performance using [001]-oriented polycrystalline half-metallic Heusler alloy Co2FeGa0.5Ge0.5 and CoFe bilayer electrode

ABSTRACT A current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) device with a half-metallic electrode is one of the most promising candidates of next-generation read head for hard disk drive. In this study, we fabricate [001]-oriented polycrystalline CPP-GMR devices with the normal ferromagnet (NFM) CoFe/half-metallic ferromagnet (HMFM) Co2FeGa0.5Ge0.5 (CFGG) bilayer electrodes to enhance the magnetoresistance (MR) ratio by large interfacial spin-dependent scattering at the NFM/HMFM interface. The CoFe/CFGG bilayer electrode provides the additional large interfacial spin-dependent scattering and achieves high MR ratio of 22.7% with the CoFe(4.5 nm)/CFGG(2.5 nm) bilayer electrodes, which is almost three(two) times larger than the MR ratio with the single CoFe(CFGG) (7 nm) electrodes. The bias voltage dependent study revealed an additional advantage of increasing the output voltage |ΔV| by using the CoFe/CFGG bilayer due to the improvement of the endurance against spin-transfer torque under high bias current. A maximum output voltage Δ V max of 6.5 mV was obtained with the CoFe(5.5 nm)/CFGG(1.5 nm) electrodes, which is the highest ever reported in the CPP-GMR devices with a uniform metallic spacer including high-quality epitaxial devices.

Improve the response frequency of epitaxial LiNbO3 film SAW devices by substrate strain gradient

Improve the response frequency of epitaxial LiNbO3 film SAW devices by substrate strain gradient

Compact light couplers for lateral III-V membrane devices grown on SOI platforms.

Compact light couplers between III-V devices and Si waveguides are crucial for advancing the scalability of Si photonics. Here, we present a compact light coupling strategy for lateral III-V membrane lasers and PDs directly grown on SOI platforms. Benefiting from the coplanar configuration of epitaxial III-V membranes and Si device layer, we designed novel, to our knowledge, butt couplers to achieve both small footprint and high efficiency coupling. We employed sub-wavelength grating structures to gradually bridge the effective refractive index between the III-V membranes and Si waveguide and obtained a coupling loss of less than 0.5 dB across the entire telecom band in a length of less than 10 μm. Our work here offers a fresh perspective for future densely integrated Si photonics.

Emission current enhancement from quasi-freestanding epitaxial graphene microstructure electron emitters through surface layered silicon dioxide

Enhanced electron emission from oxide-encapsulated quasi-freestanding bilayer epitaxial graphene devices is reported, including one emission current of 9.4 µA and successful emission even with oxide thicknesses of up to 1.25 µm. The low operating temperature (215 °C), and applied electric fields under which the devices operate indicate electron emission is due to phonon-assisted electron emission, wherein forward-scattering hot phonons impart the necessary energy for the electrons to escape the graphene as an emission current. A suite of device structures and behaviors are cataloged, and various emission behaviors are demonstrated through encapsulating oxide layers. Emission current enhancement due to electron multiplication in the oxide layers is observed across multiple devices and oxide thicknesses.

High performance CsPbBr3 epitaxial film photodetector with ultralow dark current and record detectivity

At present, spin coating is commonly used for perovskite film detectors, which has large photocurrent in the dark state due to the poor control on film growth and low crystal quality. In this Letter, pulsed laser deposition has been introduced to grow high quality CsPbBr3 epitaxial films, and the effect of substrate temperature on the film quality was studied during the epitaxial process. Planar metal–semiconductor–metal photoconductive detectors based on such epitaxial CsPbBr3 thin films with dark current as low as 11 pA at a bias voltage of 2 V was achieved. Under the illumination of a 450 nm laser with a power density of 0.65 μW cm−2, the responsivity, external quantum efficiency, and detectivity of the devices reach 12.796 AW−1, 2996%, and 3.38 × 1014 Jones, respectively. The maximum on/off ratio can be 2.38 × 105 under high-intensity 450 nm laser irradiation of 148 mW cm−2. In contrast, the spin-coated CsPbBr3 film-based detector with the same device configuration exhibit dark current that is two orders of magnitude higher and an on/off ratio of three orders of magnitude smaller than those of the epitaxial film devices. Therefore, due to their high-quality, thickness-control, and easy-integration, such epitaxial perovskite thin films can be used as a platform for the study of more functionalities of halide perovskite semiconductors and related devices.

Nano-crystal domains in Co-based fcc(111) epitaxial magnetic junctions and their impact on tunnel magnetoresistance

Nano-crystal domain structures formed in a MgO barrier and their effects on tunnel magnetoresistance (TMR) in epitaxial fcc-Co90Fe10 (CoFe)(111)/MgO(111)/CoFe(111) magnetic tunnel junctions (MTJs) have been systematically studied using scanning transmission electron microscopy and first-principles calculations. These domains are widely distributed in the (111)-textured MgO layer, being different from conventional bcc-CoFe/MgO(001)-based MTJs. The (111)-texture is formed by extension of {111} planes through several adjacent MgO domains. Three types of orientation relationships (ORs) between CoFe and MgO are identified, including cube-on-cube type (Type-1), twin-like type (Type-2), and unexpected type (Type-3). Crystallographic analysis indicated that Type-2 OR is a variant of Type-1 OR, triggered by different stacking orders of MgO(111) planes, while Type-3 OR is formed by a 30° in-plane rotation of MgO lattice relative to Type-1 OR. Due to the large in-plane lattice mismatch (19.6%) between Co(111) and MgO(111) in Type-1 and Type-2 ORs, Type-3 OR (mismatch 3.4%) can be stabilized. First-principles calculations uncovered that the theoretical TMR ratio of the MgO(111) MTJ with Type-3 OR is ∼2 orders of magnitude smaller than that with Type-1 and Type-2 ORs. The small contribution of Type-3 OR to the transport reasonably interprets why the experimental TMR ratio (∼37%) is much lower than the theoretical value (∼2100%) in the Co/MgO/Co(111) MTJ. This study has revealed the nano-crystal domain formation unique to the fcc-CoFe/MgO(111) MTJs, indicating that controlling the nano-crystal domains (e.g., lattice optimization by atomic doping) can be a guiding principle to develop MgO(111)-based epitaxial MTJs and related heterostructure devices.

Effect of Sub-Surface Damage Layer Removal by Sublimation Etching of 4H-SiC Bulk Wafers on PL Imaging of Crystal Defect Visibility

Improving the visibility of defects in nitrogen-doped 4H-SiC (0001) bare wafers by photoluminescence imaging (PLI) is essential for improving the epitaxial growth process and device yields. This study proposes sub-surface damage (SSD) introduced during the mechanical process of SiC wafers as a new factor in reducing defect visibility in PL images. To verify the effect of SSD, we observed the surface of a SiC wafer, which was thermally etched at about 3 μm. As a result, dramatic defect visibility improvement was observed when the surface roughness was sufficiently flat (Ra < 0.3 nm) after thermal etching. Thus, the results suggest that defect visibility in PL images can be improved by controlling SSD and surface roughness. Using the background noise reduction effect of the SSD removal, not only PLI but also many other wafer surface inspections are expected to be improved.

High‐Brightness GaN‐Based Blue Light‐Emitting Diodes Using AlON Buffer Layer on 4 In.‐Patterned Sapphire Substrate: Low‐Dislocation‐Defect Epitaxy, Growth Mechanisms, and Higher Device Performance

AlN is a common buffer layer in GaN‐based light emitting diode (LED) devices. However, application of AlN on industry‐relevant patterned sapphire substrate (PSS) causes unwanted screw dislocations in the epitaxial layer during GaN nucleation. Industry experience suggests that AlON buffer layer improves the quality of GaN epitaxial layer, but the low‐dislocation‐defect epitaxy mechanisms remain unclear. Herein, high‐brightness GaN‐based blue LEDs with the sputtered AlON buffer layer on industry‐grade four‐inch PSS is demonstrated. The effect of oxygen addition to AlN is revealed by growing GaN epitaxial layers on AlON buffer layers with different oxygen contents and testing these GaN layers in LED devices. Compared with conventional AlN, our AlON buffer layer reduces the density of screw dislocations, thereby improving the quality of the GaN epitaxial layers and the performance of GaN‐based LEDs. These insights allow the explanation of the growth mechanisms of GaN on PSS with AlN and AlON buffer layers and can guide the development of next‐generation optoelectronic technologies.

Failure Physics and Reliability of GaN‐Based HEMTs for Microwave and Millimeter‐Wave Applications: A Review of Consolidated Data and Recent Results

Herein, the results are reviewed concerning reliability of high‐electron mobility transistors (HEMTs) based on GaN, which currently represent the technology of choice for high‐efficiency microwave and millimeter‐wave power amplifiers. Several failure mechanisms of these devices are extensively studied, including converse piezoelectric effects, formation of conductive percolation paths at the edge of gate toward the drain, surface oxidation of GaN, time‐dependent breakdown of GaN buffer, and of field‐plate dielectric. For GaN HEMTs with scaled gate length, the simultaneous control of short‐channel effects, deep‐level dispersion, and hot‐electron‐induced degradation requires a careful optimization of epitaxial material quality and device design.

Investigation into the Influence of Substrate Dislocations in 4H-SiC on the Subsequent Epitaxy and Resultant Device Performance

As the demand for SiC based devices continues to grow, there is an aggressive push to drive down sources of yield loss and device failures. One route to mitigate this issue is via connection of device yield loss to defects in the bare substrate material. Here, high throughput X-ray topography in conjunction with KOH etching and photoluminescence imaging were utilized to detect and classify various substrate dislocations. Subsequent epitaxial layers, grown on the substrates from various vendors, were analyzed using a confocal microscope to determine which substrate defects propagated into the epi. Data indicates that there is a good correlation between substrate BPDs nucleating into epitaxial stacking faults. This is supported by data from hundreds of different boules demonstrating that there is a relationship between the number of BPDs in the substrate and the number of partials and stacking faults detected in the resultant epitaxial layer. BPDs are not the only substrate dislocation nucleation source as the data also suggests other parallel sources are involved. Thousands of MOSFET and Diode devices were analyzed to understand the influence that substrate defects, propagated to the epitaxial layers, have on device leakage and yield loss. It was determined that MOSFET devices are more sensitive to the stacking faults that nucleate from the substrate dislocations compared to Diodes. While most of the stacking faults and other dislocations that occur in the epi layers do not independently result in device failures or leakage, an increase in the density of these defects within a die, can lead to increased leakage and decreased device performance. The inherent variability of these relationships requires the analysis of very large numbers of wafers to see the clear trends. This work uses a combination of X-ray and photoluminescence detection techniques to trace dislocations in substrates that nucleate into epitaxy layers and result in device leakage and failure over hundreds of wafers and thousands of devices. This correlation between device characteristics and crystal dislocations provides necessary information in determining which defects are most necessary to reduce or eliminate to improve device yield and performance.

Record‐High Responsivity and High Detectivity Broadband Photodetectors Based on Upconversion/Gold/Prussian‐Blue Nanocomposite

AbstractThe development of high‐quality and strongly Au conjugated upconversion nanoparticles (UCNPs) is time‐consuming and requires specific chemicals. Therefore, a one‐pot hydrothermal method is adopted for novel in situ preparation of UCNP@Au composites using a binary functional ethylenediaminetetraacetic salt, which is employed as a surfactant and reducing agent. The composites are electrostatically conjugated with metal‐coordinated Prussian blue (PB) to yield UCNP@Au+PB nanocomposites (NCs), which demonstrate 21‐fold upconversion emission quenching by fluorescence resonance energy transfer compared to UCNPs. Additionally, the PB, UCNP@Au, and NCs demonstrate a synergistically reduced trap (α ≈ 0.85) and enhance ultrasensitive broadband (432–980 nm) photodetection. The NCs‐based gate‐free epitaxial graphene device demonstrates excellent high photoresponsivity (5.9 × 105 A W–1), detectivity (2.17 × 1014 ), and normalized gain (2.06 × 10−4 m2 V–1) at 318 nW cm–2 (532 nm) and a bias voltage of 1 V. The Au plasmons enhance the one‐photon‐enabled visible absorption of Er3+ ions, and PB exhibits broad absorption and enhances the carrier density of the device, resulting in an ultrahigh photoresponse. The obtained device performance is the highest to date among their class of nanohybrids. Also, these NCs can readily detect polychromatic light and signals from daily‐use appliances, indicating their potential for applications in consumer electronics.

Spin-Sensitive Epitaxial In2Se3 Tunnel Barrier in In2Se3/Bi2Se3 Topological van der Waals Heterostructure.

Current-generated spin arising from spin-momentum locking in topological insulator (TI) surface states has been shown to switch the magnetization of an adjacent ferromagnet (FM) via spin-orbit torque (SOT) with a much higher efficiency than heavy metals. However, in such FM/TI heterostructures, most of the current is shunted through the FM metal due to its lower resistance, and recent calculations have also shown that topological surface states can be significantly impacted when interfaced with an FM metal such as Ni and Co. Hence, placing an insulating layer between the TI and FM will not only prevent current shunting, therefore minimizing overall power consumption, but may also help preserve the topological surface states at the interface. Here, we report the van der Waals epitaxial growth of β-phase In2Se3 on Bi2Se3 by molecular beam epitaxy and demonstrate its spin sensitivity by the electrical detection of current-generated spin in Bi2Se3 surface states using a Fe/In2Se3 detector contact. Our density functional calculations further confirm that the linear dispersion and spin texture of the Bi2Se3 surface states are indeed preserved at the In2Se3/Bi2Se3 interface. This demonstration of an epitaxial crystalline spin-sensitive barrier that can be grown directly on Bi2Se3, and verification that it preserves the topological surface state, is electrically insulating and spin-sensitive, is an important step toward minimizing overall power consumption in SOT switching in TI/FM heterostructures in fully epitaxial topological spintronic devices.

Deposition-last lithographically defined epitaxial complex oxide devices on Si(100)

The epitaxial growth of SrTiO3 on Si(100) substrates that have been lithographically patterned to realize deposition-last, lithographically defined oxide devices on Si is explored. In contrast to traditional deposition-last techniques which create a physical hard mask on top of the substrate prior to epitaxial growth, a pseudomask is instead created by texturing the Si substrate surface itself. The Si is textured through a combination of reactive ion etching and wet-etching using a tetramethylammonium hydroxide solution. Desorbing the native SiOx at high temperatures prior to epitaxial growth in ultrahigh vacuum presents no complications as the patterned substrate is comprised entirely of Si. The inverted profile in which the epitaxial oxide device layer is above the textured pseudomask circumvents shadowing during deposition associated with conventional hard masks, thereby opening a pathway for highly scaled devices to be created.

Desorption timescales on epitaxial graphene via Fermi level shifting and Reststrahlen monitoring

This work reports information on the transience of hole doping in epitaxial graphene devices when nitric acid is used as an adsorbent. Under vacuum conditions, desorption processes are monitored by electrical and spectroscopic means to extract the relevant timescales from the corresponding data. It is of vital importance to understand the reversible nature of hole doping because such device processing can be a suitable alternative to large-scale, metallic gating. Most measurements are performed post-exposure at room temperature, and, for some electrical transport measurements, at 1.5 K. Vacuum conditions are applied to many measurements to replicate the laboratory conditions under which devices using this doping method would be measured. The relevant timescales from transport measurements are compared with results from X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy measurements, with the latter performed at ambient conditions and accompanied by calculations of the spectra in the Reststrahlen band.

Fabrication and Characterization of Epitaxial Graphene Field Effect Transistor Devices Based on a Monolithic Bottom Gate

Current electronics technology increasingly demands higher integration, flexibility, higher efficiency, and performance aspects such as compatibility with higher temperature operation of the semiconductor devices, which may find limitations when silicon is used. The superior intrinsic properties of SiC, eventually combined with the ability of growing monolithically epitaxial, high quality graphene on a SiC wafer (1), makes it a reliable alternative for some electronic applications, such as field effect transistors (FET), radio frequency (RF) power amplifiers, integrated circuits (IC), or sensors. In this work, we describe the fabrication and preliminary electrical characterizations of epitaxial graphene (EG) on a SiC substrate FET devices based on an alternative back gate architecture. We propose a new approach in which the FET device is built on a 4◦ off-axis cut, N+ doped 4H-SiC substrate (the back gate) with, on top of, it a 1μm semi-insulating homoepitaxial layer of SiC compensated with vanadium (the dielectric layer). EG will be used as FET conduction channel. Using this V-compensated dielectric layer is aimed to minimize effects on the FET characteristics such as from defects in the SiC crystal, especially below the FET active areas, which would have occurred when using ion implantation to create a buried gate. The EG film was grown by the high temperature Si sublimation method under an Ar ambient. Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were applied in the structural characterization of epigraphene. The lack of D-peak in Raman spectra, together with SEM and AFM images, indicate that high quality monolayer to few layer epitaxial graphene fully covering the SiC surface is deposited. The electrical characteristics of the EG channel-devices and the functionality of the bottom gate were examined with 2-probe and 4-probe method. The electrical properties of the FET devices were also investigated with 3 terminal configuration.

Dynamics of transient hole doping in epitaxial graphene

This work reports the dynamics of transient hole doping in epitaxial graphene devices by using nitric acid as an adsorbent. The timescales associated with corresponding desorption processes are extracted from the data. The understanding of reversible hole doping without gating is of crucial importance to those fabricating devices with a particular functionality. Measurements of the electrical and optical properties of several devices postexposure were performed with transport temperatures between 300 and 1.5 K. Ambient conditions are applied to nontransport measurements to replicate the most likely laboratory conditions for handling devices using this doping method. The relevant timescales from transport measurements are compared with results from Raman spectroscopy measurements.

Recent Developments of Quantum Dot Materials for High Speed and Ultrafast Lasers.

Owing to their high integration and functionality, nanometer-scale optoelectronic devices based on III-V semiconductor materials are emerging as an enabling technology for fiber-optic communication applications. Semiconductor quantum dots (QDs) with the three-dimensional carrier confinement offer potential advantages to such optoelectronic devices in terms of high modulation bandwidth, low threshold current density, temperature insensitivity, reduced saturation fluence, and wavelength flexibility. In this paper, we review the development of the molecular beam epitaxial (MBE) growth methods, material properties, and device characteristics of semiconductor QDs. Two kinds of III-V QD-based lasers for optical communication are summarized: one is the active electrical pumped lasers, such as the Fabry–Perot lasers, the distributed feedback lasers, and the vertical cavity surface emitting lasers, and the other is the passive lasers and the instance of the semiconductor saturable absorber mirrors mode-locked lasers. By analyzing the pros and cons of the different QD lasers by their structures, mechanisms, and performance, the challenges that arise when using these devices for the applications of fiber-optic communication have been presented.

Epitaxial all-bcc-Co50Fe50/Cu/Co50Fe50 current-in-plane giant magnetoresistive spin-valves on Si(0 0 1) substrate

Following the high magnetoresistance (MR) ratio reported for fully epitaxial CoFe/bcc-Cu/CoFe current-in-plane giant magnetoresistance (CIP-GMR) grown on MgO(001) single crystalline substrate, we attempted to grow epitaxial CoFe/bcc-Cu/CoFe CIP-GMR devices on industrially viable Si(001) substrate through an epitaxial NiAl buffer layer. We found that the deposition of MgO insulation layer on the NiAl buffer layer is crucial to prevent current shunting in the NiAl layer which reduces MR ratio. The epitaxial CIP-GMR device grown on Si(001) exhibited sufficiently high MR ratio up to 15.9(±0.1)% at room temperature; however, it is somehow lower than that grown on the MgO(001) substrate, 20.6(±0.1)%. Detailed microstructural analysis revealed that fcc phase of Cu presented in the bcc-Cu spacer, which is considered to be the reason for the lower MR ratio of the device grown on Si(001) substrate.

A novel strategy for GaN-on-diamond device with a high thermal boundary conductance

To achieve high device performance and high reliability for the gallium nitride (GaN)-based high electron mobility transistors (HEMTs), efficient heat dissipation is important but remains challenging. Enormous efforts have been made to transfer a GaN device layer onto a diamond substrate with a high thermal conductivity by bonding. In this work, two GaN-diamond bonded composites are prepared via modified surface activated bonding (SAB) at room temperature with silicon interlayers of different thicknesses (15 nm and 22 nm). Before and after post annealing process at 800 °C, thermal boundary conductance (TBC) across the bonded interface including the interlayer and the stress of GaN layer are investigated by time-domain thermoreflectance and Raman spectroscopy, respectively. In the case of as-bonded samples, TBC of the 15 nm Si interlayer (32.4 MW/m2-K) was higher than that of the 22 nm (28.0 MW/m2-K); but after annealing, TBC of the 15 nm Si interlayer (71.3 MW/m2-K) became lower than that of the 22 nm (85.9 MW/m2-K), because the annealing is especially effective for thicker interlayer to improved interfacial TBC. The obtained stress was less than 230 MPa for both before and after the annealing, and this high thermal stability indicates that the room-temperature bonding can realize a GaN-on-diamond template suitable for further epitaxial growth or device process.