Abrupt Heterointerfaces Research Articles

Interfacial dynamics of GaP/Si(100) heterostructure grown by molecular beam epitaxy

The atomic structure and surface chemistry of GaP/Si(100) heterostructure with different pre-layers grown by molecular beam epitaxy are studied. It is found that GaP epilayer with Ga-riched pre-layers on Si(100) substrate has regular surface morphology and stoichiometric abrupt heterointerfaces from atomic force microscopes (AFMs) and spherical aberration-corrected transmission electron microscopes (ACTEMs). The interfacial dynamics of GaP/Si(100) heterostructure is investigated by X-ray photoelectron spectroscopy (XPS) equipped with an Ar gas cluster ion beam, indicating that Ga pre-layers can lower the interface formation energy and the bond that is formed is more stable. These results suggest that Ga-riched pre-layers are more conducive to the GaP nucleation as well as the epitaxial growth of GaP material on Si(100) substrate.

Growth of high-quality near-ultraviolet light-emitting diodes used InGaN/GaN multiple quantum wells

High-quality near-ultraviolet (UV) light-emitting diodes (LEDs) used InGaN/GaN multiple quantum wells (MQWs) have been grown on ScMgAlO4 substrates by plasma enhanced metalorganic chemical vapor deposition (PEMOCVD). The structural properties and optoelectronic properties of InGaN/GaN MQWs are characterized carefully. The full-width at half-maximums (FWHMs) for GaN(0002) and GaN(10–12) X-ray rocking curves are 180 and 190 arcsec, respectively, indicating high crystalline quality of GaN epitaxial films. The as-grown InGaN/GaN MQWs with sharp and abrupt hetero-interfaces have been also obtained and confirmed by high-resolution X-ray diffraction and high-resolution transmission electron microscopy measurements. Meanwhile, there is a photoluminescence peak at 385 nm with an FWHM of 23.2 nm at 300 K and the internal quantum efficiency (IQE) for these InGaN/GaN MQWs is measured to be as high as 79.8%, revealing the high optoelectronic properties. This work sheds light on the preparation of high-performance near-UV LEDs for the application of UV curing, lighting, etc.

Growth and characterization of amber light-emitting diodes with dual-wavelength InGaN/GaN multiple-quantum-well structures

We report the growth of a dual-wavelength green-amber light-emitting diode (LED) structure. Such dual-wavelength multiple quantum wells (MQWs) structures not only assisted the releasing of the strain, accordingly improved the crystal quality of MQW heterointerfaces, but also enhanced the incorporation of indium atoms amid growth of the high indium-content QWs. The high-resolution x-ray diffraction (HR-XRD) measurement showed multi-order satellite peaks, implying the abrupt heterointerfaces formed between MQWs in the LED sample. It shows a distinct dual wavelength emission characteristics in the temperature-dependent photoluminescence (TDPL). Furthermore, only a single amber peak around 591–609 nm was emitted with a small blue shift at a current injection from 5 to 100 mA in the electroluminescence (EL) characterization. The full-width at half-maximum (FWHM) of these peaks just shows a little fluctuation.

Quasi One-Dimensional Metal-Semiconductor Heterostructures.

The band offsets occurring at the abrupt heterointerfaces of suitable material combinations offer a powerful design tool for high performance or even new kinds of devices. Because of a large variety of applications for metal-semiconductor heterostructures and the promise of low-dimensional systems to present exceptional device characteristics, nanowire heterostructures gained particular interest over the past decade. However, compared to those achieved by mature two-dimensional processing techniques, quasi one-dimensional (1D) heterostructures often suffer from low interface and crystalline quality. For the GaAs-Au system, we demonstrate exemplarily a new approach to generate epitaxial and single crystalline metal-semiconductor nanowire heterostructures with atomically sharp interfaces using standard semiconductor processing techniques. Spatially resolved Raman measurements exclude any significant strain at the lattice mismatched metal-semiconductor heterojunction. On the basis of experimental results and simulation work, a novel self-assembled mechanism is demonstrated which yields one-step reconfiguration of a semiconductor-metal core-shell nanowire to a quasi 1D axially stacked heterostructure via flash lamp annealing. Transmission electron microscopy imaging and electrical characterization confirm the high interface quality resulting in the lowest Schottky barrier for the GaAs-Au system reported to date. Without limiting the generality, this novel approach will open up new opportunities in the syntheses of other metal-semiconductor nanowire heterostructures and thus facilitate the research of high-quality interfaces in metal-semiconductor nanocontacts.

Epitaxial Growth and Characterization of Nonpolar <i>A</i>-Plane AlGaN-Based Multiple Quantum Wells

A serious of non-polar a-plane AlGaN-based multiple quantum wells (MQWs) were successfully grown on the semi-polar r-plane sapphire substrate with metal organic chemical vapor deposition technology. Intense MQWs-related emission peaks at an emission wavelength covered from 277-294 nm were observed based on the photoluminescence measurement. It was found that the employment of the trimethyl-aluminum (TMAl) flow duty-ratio modulation method which was developed based on the two-way pulsed-flows growth technique played a crucial role to control the Al composition of the non-polar a-plane AlGaN epi-layers. The non-polar a-plane AlGaN-based MQWs were deposited with the new developed TMAl flow duty-ratio modulation technique. Evident-3th order X-ray diffraction (XRD) satellite peak was observed from the high resolution-XRD measurement, proving the successful growth of non-polar a-plane AlGaN-based MQWs with abrupt hetero-interfaces.

Heterogeneous integration of InAs/GaSb tunnel diode structure on silicon using 200 nm GaAsSb dislocation filtering buffer

An InAs/GaSb tunnel diode structure was heterogeneously integrated on silicon by solid source molecular beam epitaxy using a 200 nm strained GaAs1-ySby dislocation filtering buffer. X-ray analysis demonstrated near complete strain relaxation of the metamorphic buffer and a quasi-lattice-matched InAs/GaSb heterostructure, while high-resolution transmission electron microscopy revealed sharp, atomically abrupt heterointerfaces between the GaSb and InAs epilayers. In-plane magnetotransport analysis revealed Shubnikov-de Haas oscillations, indicating the presence of a dominant high mobility carrier, thereby testifying to the quality of the heterostructure and interfaces. Temperature-dependent current-voltage characteristics of fabricated InAs/GaSb tunnel diodes demonstrated Shockley-Read-Hall generation-recombination at low bias and band-to-band tunneling transport at high bias. The extracted conductance slope from the fabricated tunnel diodes increased with increasing temperature due to thermal emission (Ea ∼ 0.48 eV) and trap-assisted tunneling. Thus, this work illustrates the significance of defect control in the heterointegration of metamorphic InAs/GaSb tunnel diode heterostructures on silicon when using GaAs1-ySby dislocation filtering buffers.

N-face GaN/InN/GaN double heterostructures emitting near-infrared photoluminescence grown by metalorganic vapor phase epitaxy

N-face double heterostructures (DHs) composed of 30-nm-thick InN layers sandwiched with GaN buffer and cap layers were grown by metalorganic vapor phase epitaxy. A high InN growth temperature is necessary for obtaining good radiative properties in the temperature range from 595 to 675 °C. Using a lower growth temperature for the GaN cap layer (550 °C) than for the InN layer (675 °C) enables us to fabricate a high-quality N-face GaN/InN/GaN DH having abrupt heterointerfaces as well as strong band-edge photoluminescence at around 0.75 eV at the measurement temperature of 83 K.

Growth dynamics of SiGe nanowires by the vapour–liquid–solid method and its impact on SiGe/Si axial heterojunction abruptness

The vapour–liquid–solid (VLS) method is by far the most extended procedure for bottom-up nanowire growth. This method also allows for the manufacture of nanowire axial heterojunctions in a straightforward way. To do this, during the growth process, precursor gases are switched on/off to obtain the desired change in the nanowire composition. Using this technique, axially heterostructured nanowires can be grown, which are crucial for the fabrication of electronic and optoelectronic devices. SiGe/Si nanowires are compatible with complementary metal oxide semiconductor (CMOS) technology, which improves their versatility and the possibility of integration with current electronic technologies. Abrupt heterointerfaces are fundamental for the development and correct operation of electronic and optoelectronic devices. Unfortunately, the VLS growth of SiGe/Si heterojunctions does not provide abrupt transitions because of the high solubility of group IV semiconductors in Au, with the corresponding reservoir effect that precludes the growth of sharp interfaces. In this work, we studied the growth dynamics of SiGe/Si heterojunctions based on already developed models for VLS growth. A composition map of the Si–Ge–Au liquid alloy is proposed to better understand the impact of the growing conditions on the nanowire growth process and the heterojunction formation. The solution of our model provides heterojunction profiles that are in good agreement with the experimental measurements. Finally, an in-depth study of the composition map provides a practical approach to the drastic reduction of heterojunction abruptness by reducing the Si and Ge concentrations in the catalyst droplet. This converges with previous approaches, which use catalysts aiming to reduce the solubility of the atomic species. This analysis opens new paths to the reduction of heterojunction abruptness using Au catalysts, but the model can be naturally extended to other catalysts and semiconductors.

Integrating AlInN interlayers into InGaN/GaN multiple quantum wells for enhanced green emission

Significant enhancement in green emission by integrating a thin AlInN barrier layer, or interlayer (IL), in an InGaN/GaN multiple quantum well (MQW) is demonstrated. The MQWs investigated here contains 5 periods of an InGaN QW, a 1 nm thick AlInN IL, and a 10 nm thick GaN barrier grown by metalorganic chemical vapor deposition. To accommodate the optimum low-pressure (20 Torr) growth of the AlInN layer a growth flow sequence with changing pressure is devised. The AlInN IL MQWs are compared to InGaN/AlGaN/GaN MQWs (AlGaN IL MQWs) and conventional InGaN/GaN MQWs. The AlInN IL MQWs provide benefits that are similar to AlGaN ILs, by aiding in the formation of abrupt heterointerfaces as indicated by X-ray diffraction omega-2theta (ω-2θ) scans, and also efficiency improvements due to high temperature annealing schedules during barrier growth. Room temperature photoluminescence of the MQW with AlInN ILs shows similar performance to MQWs with AlGaN ILs, and ∼4–7 times larger radiative efficiency (pump intensity dependent) at green wavelengths than conventional InGaN/GaN MQWs. This study shows the InGaN-based MQWs with AlInN ILs are capable of achieving superior performance to conventional InGaN MQWs emitting at green wavelengths.

Nanoparticle Stability in Axial InAs-InP Nanowire Heterostructures with Atomically Sharp Interfaces.

The possibility to expand the range of material combinations in defect-free heterostructures is one of the main motivations for the great interest in semiconductor nanowires. However, most axial nanowire heterostructures suffer from interface compositional gradients and kink formation, as a consequence of nanoparticle-nanowire interactions during the metal-assisted growth. Understanding such interactions and how they affect the growth mode is fundamental to achieve a full control over the morphology and the properties of nanowire heterostructures for device applications. Here we demonstrate that the sole parameter affecting the growth mode (straight or kinked) of InP segments on InAs nanowire stems by the Au-assisted method is the nanoparticle composition. Indeed, straight InAs-InP nanowire heterostructures are obtained only when the In/Au ratio in the nanoparticles is low, typically smaller than 1.5. For higher In content, the InP segments tend to kink. Tailoring the In/Au ratio by the precursor fluxes at a fixed growth temperature enables us to obtain straight and radius-uniform InAs-InP nanowire heterostructures (single and double) with atomically sharp interfaces. We present a model that is capable of describing all the experimentally observed phenomena: straight growth versus kinking, the stationary nanoparticle compositions in pure InAs and InAs-InP nanowires, the crystal phase trends, and the interfacial abruptness. By taking into account different nanowire/nanoparticle interfacial configurations (forming wetting or nonwetting monolayers in vertical or tapered geometry), our generalized model provides the conditions of nanoparticle stability and abrupt heterointerfaces for a rich variety of growth scenarios. Therefore, our results provide a powerful tool for obtaining high quality InAs-InP nanowire heterostructures with well-controlled properties and can be extended to other material combinations based on the group V interchange.

Investigating FinFET Sidewall Passivation Using Epitaxial (100)Ge and (110)Ge Metal–Oxide–Semiconductor Devices on AlAs/GaAs

Device-quality crystallographically oriented epitaxial (100)Ge and (110)Ge were grown on GaAs substrates using a large bandgap AlAs buffer. Electrical characteristics of p-type metal–oxide–semiconductor (pMOS) capacitors, fabricated from the aforementioned material stacks, are presented for the first time. High-resolution cross-sectional transmission electron microscopy analysis demonstrated atomically abrupt heterointerfaces between Al2O3/Ge as well as Ge/AlAs for both (100) and (110) orientations. Various process conditions were implemented during MOS capacitor fabrication to study their impact on the Al2O3/Ge interface. The fabricated pMOS devices demonstrated excellent electrical characteristics with efficient modulation of the Fermi level from midgap to the conduction band edge, corresponding to a minimum ${D}_{\text {it}}$ value of ${1.2} \times {10}^{11}$ cm $^{-2}\cdot $ ev−1 on (100)Ge, indicative of a high-quality oxide/Ge heterointerface, and an effective electrical passivation of the Ge surface. Postdeposition annealing under O2 was found to be less effective at reducing oxide trap density ( ${N}_{{\text {OT}}})$ as compared to forming gas or O2 postmetallization anneals (PMA), indicating that metal-induced bandgap states at the gate metal/dielectric interface have a notable impact on Ge pMOS ${N}_{{\text {OT}}}$ . On the other hand, a tradeoff must be made between ${N}_{{\text {OT}}}$ and the equivalent oxide thickness when performing PMA under O2 or forming gas ambient.

Enhancement of the hole photocurrent in layers of Ge/Si quantum dots with abrupt heterointerfaces

The photoconductive gain, hole photocurrent spectra in the mid-infrared range, and band-to-band photoluminescence spectra in arrays of Ge/Si quantum dots with different elemental compositions of the heterointerface are measured. The diffusive mixing of the materials of the matrix and the dots is controlled by varying the temperature at which the Ge layers are overgrown with Si. It is found that the formation of abrupt heterointerfaces leads to the enhancement of the hole photocurrent and quenching of photoluminescence. The results are explained by an increase in the lifetime of nonequilibrium holes owing to the suppression of their capture into the bound states of quantum dots.

Synthesis and structural characterization of vertical ferromagnetic MnAs/semiconducting InAs heterojunction nanowires

The purpose of this study is to synthesize vertical ferromagnetic/semiconducting heterojunction nanowires by combing the catalyst-free selective-area growth of InAs nanowires and the endotaxial nanoclustering of MnAs and to structurally and magnetically characterize them. MnAs penetrates the InAs nanowires to form nanoclusters. The surface migration length of manganese adatoms on the nanowires, which is estimated to be 600 nm at 580 °C, is a key to the successful fabrication of vertical MnAs/InAs heterojunction nanowires with atomically abrupt heterointerfaces.

Magnetic Oxide Heterostructures

Complex transition metal oxides have played a central role in the study of magnetic materials, serving as model systems for explorations of fundamental exchange interactions and the relationships between structural, electronic, and magnetic responses. Enabled by advances in epitaxial synthesis techniques, abrupt heterointerfaces and superlattices have emerged as a powerful platform for engineering novel magnetic behavior in oxides. Following a brief introduction to the dominant exchange mechanisms in metal oxides, we review the general means by which interfacial magnetism can be tailored in ABO3 perovskites, including interfacial charge transfer, epitaxial strain and structural coupling, orbital polarizations and reconstructions, and tailoring exchange interactions via cation ordering. Recent examples are provided to illustrate how these strategies have been employed at isolated interfaces and in short-period superlattices. We conclude by briefly discussing underexplored and emerging research directions in the field.

Epitaxial growth of homogeneous single-crystalline AlN films on single-crystalline Cu (1 1 1) substrates

The homogeneous and crack free single-crystalline AlN thin films have been epitaxially grown on single-crystalline Cu (111) substrates with an in-plane alignment of AlN [11–20]//Cu [1–10] by pulsed laser deposition (PLD) technology with an integrated laser rastering program. The as-grown AlN films are studied by spectroscopic ellipsometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), polarized light microscopy, high-resolution X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM). The spectroscopic ellipsometry reveals the excellent thickness uniformity of as-grown AlN films on the Cu (111) substrates with a root-mean-square (RMS) thickness inhomogeneity less than 2.6%. AFM and FESEM measurements indicate that very smooth and flat surface AlN films are obtained with a surface RMS roughness of 2.3nm. The X-ray reflectivity image illustrates that there is a maximum of 1.2nm thick interfacial layer existing between the as-grown AlN and Cu (111) substrates and is confirmed by HRTEM measurement, and reciprocal space mapping shows that almost fully relaxed AlN films are achieved only with a compressive strain of 0.48% within ∼321nm thick films. This work demonstrates a possibility to obtain homogeneous and crack free single-crystalline AlN films on metallic substrates by PLD with optimized laser rastering program, and brings up a broad prospect for the application of acoustic filters that require abrupt hetero-interfaces between the AlN films and the metallic electrodes.

Growth of straight one-dimensional Ge/ZnSe heterojunctions with atomically sharp interfaces by catalytic residue controls

One-dimensional (1D) heteroepitaxy with an abrupt interface is essential to construct the 1D heterojunctions required for photonic and electronic devices. During catalytic 1D heteroepitaxial growth, however, the heterojunctions are generically kinked and composition-diffused across the interfaces. Here, we report a simple synthetic route for straight 1D heteroepitaxy with atomically sharp interfaces of group IV(Ge)/group II–VI(ZnSe) nanowires (NWs) during Au-catalytic growth. Specifically, it is discovered that eliminating residues in Au catalysts by Se vapour treatments lowers the energy barrier for the Ge NW axial heteroepitaxy on ZnSe NWs, and forms atomically abrupt heterointerfaces. We verified such 1D variation in the local electronic band structure of the grown Ge/ZnSe NW heterojunctions with spatially resolved photocurrent measurements.

Control and understanding of kink formation in InAs–InP heterostructure nanowires

Nanowire heterostructures are of special interest for band structure engineering due to an expanded range of defect-free material combinations. However, the higher degree of freedom in nanowire heterostructure growth comes at the expense of challenges related to nanowire–seed particle interactions, such as undesired composition, grading and kink formation. To better understand the mechanisms of kink formation in nanowires, we here present a detailed study of the dependence of heterostructure nanowire morphology on indium pressure, nanowire diameter, and nanowire density. We investigate InAs–InP–InAs heterostructure nanowires grown with chemical beam epitaxy, which is a material system that allows for very abrupt heterointerfaces. Our observations indicate that the critical parameter for kink formation is the availability of indium, and that the resulting morphology is also highly dependent on the length of the InP segment. It is shown that kinking is associated with the formation of an inclined facet at the interface between InP and InAs, which destabilizes the growth and leads to a change in growth direction. By careful tuning of the growth parameters, it is possible to entirely suppress the formation of this inclined facet and thereby kinking at the heterointerface. Our results also indicate the possibility of producing controllably kinked nanowires with a high yield.

Epitaxial growth of 2 inch diameter homogeneous AlN single-crystalline films by pulsed laser deposition

2 inch diameter homogeneous AlN films are epitaxially grown on sapphire substrates by pulsed laser deposition (PLD). By optimizing laser rastering and PLD growth conditions, the 2 inch diameter single-crystalline AlN films exhibit excellent thickness uniformity with root-mean-square (RMS) inhomogeneity less than 4.5% and very smooth surface with RMS roughness less than 1.53 nm. There is a maximum of 1.5 nm thick interfacial layer, if there is any, existing between the as-grown AlN and the pre-nitrided sapphire substrate, and the as-grown AlN films are almost fully relaxed only with a 0.26% in-plane compressive strain. The achievement of high-quality large-scale AlN films with uniform thickness and atomically abrupt interface is of great interest for the commercial development of AlN-based devices, particularly acoustic filters where abrupt heterointerfaces with substrates and flat surfaces for AlN films are highly desired.

EXAFS study of GaN/AlN multiple quantum wells grown by ammonia MBE

AbstractExtended X‐ray absorption fine structure above the Ga–K edge has been used to study the local structure of heterointerfaces in GaN/AlN multiple quantum wells (MQWs) grown by ammonia molecular beam epitaxy. The thickness of AlN and GaN layers in MQWs were evaluated using transmission electron microscopy, X‐ray diffraction and Raman spectroscopy. Two sets of MQWs structures with the number of period about of 20 and more than 100 were studied. The Ga‐Ga interatomic distance in MQWs is lower than that in bulk GaN due to elastic compression of GaN layers in MQWs. This effect is less pronounced in thick structures because of almost full relaxation of GaN layers. The Ga‐Al interatomic distance in thick MQWs is too long, about 0.1 Å, longer than typical values for AlGaN alloys. Then number of Ga cations in the second coordination shell of Ga atoms is lower than that predicted for abrupt heterointerfaces that evidences intermixing of heterointerfaces. The intermixing degree depends on the total thickness of MQWs (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Homogeneous epitaxial growth of AlN single-crystalline films on 2 inch-diameter Si (111) substrates by pulsed laser deposition

Homogeneous and crack-free AlN films have been epitaxially grown on 2 inch Si (111) substrates by pulsed laser deposition (PLD). By optimising the laser rastering and PLD growth conditions, the 2 inch-diameter single-crystalline AlN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity of less than 3.6% and a very smooth surface with a RMS roughness of 1.4 nm. There is a 1.5 nm-thick interfacial layer between the as-grown AlN films and Si substrates, which is confirmed by HRTEM and a GIXR simulation, and the as-grown AlN films are almost fully relaxed with only 0.3% in-plane tensile strain. The achievement of high-quality and crack-free AlN films with a uniform thickness and abrupt interface on 2 inch Si (111) substrates is of great interest for AlN-based devices, particularly acoustic filters, where abrupt heterointerfaces with the substrates and flat surfaces of AlN films are highly desired.