Lattice-matched Layers Research Articles

Polycrystalline inclusion-free growth of thick, lattice-matched SiGeC with high substitutional Carbon concentrations up to 2%

Lattice-matched SiGeC with ~20% of Ge was grown at 550°C, 10 Torr on a 300 mm Si substrate in an industrial standard Reduced Pressure Chemical Vapor Deposition reactor using commercially available Si2H6/GeH4/SiH3CH3 precursors. Thicknesses exceeding 250 nm resulted in cluster formation in SiGeC with [C]Sub~2.0% and [C]Int~0.4%, with a surface density up to 6.13x106 µm-2. Room temperature photoluminescence measurements showed that the clusters were non-radiative defects and Transmission Electron Microscopy, using the ASTAR system, proved they were polycrystalline with random orientation. Cluster-free growth was achieved for 1 µm thick SiGeC with [C]Sub~1.0% and no interstitial carbon atoms, however, without lattice-matched growth on a Si substrate. Injecting HCl during a co-flow process resulted in a smooth surface with a RMS roughness of 0.23 nm and a reduction of [C]Intfrom ~0.7% down to ~0.1%, close to the detection limit of X-ray Photoelectron Spectroscopy at 0.1%. This led to a significant cluster surface density reduction down to 1.15x104 µm-2. The injection of HCl caused an increase in the Ge concentration from 19.7% up to 28.9% and a non-lattice-matched growth. A Cyclic Deposition Etch (CDE) process resulted in no [C]Int reduction, but significantly reduced the cluster surface density down to 4.60x104 µm-2. CDE-grown SiGeC was lattice-matched to the Si substrate and yielded a smooth surface with a RMS roughness of 0.22 nm. This enabled the growth of high crystalline quality, thick, lattice-matched layers on a 300 mm Si substrate with material properties beyond pure Si.

Formation of GaN mesas with reverse-tapered edge structures on a lattice-matched AlInN layer for a positive beveled edge termination

GaN mesas were fabricated by sequential dry and wet etching of a +c-oriented GaN layer onto a lattice-matched AlInN layer for future applications of positive beveled edge termination, which is desirable for preventing premature breakdown of power devices. The dry etching produced hexagonal AlInN/GaN mesas surrounded by m-plane sidewalls with six protrusions at the vertices. The subsequent hot phosphoric acid etching selectively etched the AlInN layer to expose and etch the chemically unstable −c surface of the GaN layer, which formed reverse-tapered { 101¯2¯ } facets. The protrusions were sacrificed during the wet etching to prevent undesirable positive tapering at the vertices.

High-efficient spin injection in GaN through a lattice-matched tunnel layer

Semiconductor spintronics has brought about revolutionary application prospects in future electronic devices. The tunnel junction plays a key role in achieving efficient spin injection in semiconductors. This work employed the GaN semiconductor as a room-temperature spin injection system, taking advantage of its weak spin–orbit coupling and spin scattering. By introducing a lattice-matched AlN barrier layer to improve the tunneling interface, advanced spin injection and transport were realized compared with traditional oxide barriers. The spin polarization was further improved by modulating the applied bias, and a bias-controlled tunneling enhancement mechanism was revealed. Consequently, we demonstrated a high record of spin polarization of 20.5%. This work paves a feasible route for achieving efficient spin injection and transport in GaN, which will further promote the development of room-temperature and high-performance spintronic devices.

High-performance FeSiAl soft magnetic composites achieved by confined solid-state reaction

Soft magnetic composites (SMCs) consisting of insulated metallic magnetic particles are critical materials in modern electronics. However, the simultaneous optimization of power loss and permeability for SMCs remains challenging due to the vulnerable insulation layer inside. Here, we succeed in fabricating FeSiAl SMC by confined solid-state reaction between TiO2 and FeSiAl matrix, which leads to the formation of homogeneous and lattice-matched Al2O3 layer and brings about effective electrical insulation of FeSiAl particles. TiO2 can progressively release O atoms and confine the solid-state reaction strictly within the interfacial region. Meanwhile, itself becomes ferromagnetic with deficient oxygen, resulting in alleviated magnetic dilution. Fast domain wall displacement and small hysteresis loss are directly observed by in-situ Lorenz transmission electron microscopy. The obtained FeSiAl SMC exhibits low power loss of 130 mW·cm−3 (50 mT, 100 kHz) and high effective permeability of 143, which is desired in energy-saving and high-efficiency devices. This work relates the interfacial behavior with magnetic properties and highlights a new strategy for fabricating high-performance SMCs.

Pulsed laser ejection of single-crystalline III-V solar cells from GaAs substrates

The best III-V solar cells start out as single-crystalline multilayers on GaAs substrates. Separating these multilayers from their growth substrate enables higher performance and wafer reuse, which are both critical for terrestrial III-V solar cell viability. Here, we remove rigidly bonded, 16 mm2 × 3.5 μm thick devices from a GaAs substrate using an unfocused Nd:YAG laser pulse. The pulse is absorbed by a low-band-gap, lattice-matched layer below the device, driving an ablation event that ejected the crystalline multilayer from the substrate. Minutes of selective wet-chemical etching and device finishing yield a 0.1 cm2 device with a 17.4% power conversion efficiency and open-circuit voltage of 1.07 V, using AM1.5 direct with no anti-reflection coating. We show that the performance is comparable to similar cells produced via conventional processes. We discuss unique process characteristics, such as the potential to separate wafer-sized solar cells per laser pulse.

Enhanced Performance of N-Polar AlGaN-Based Ultraviolet Light-Emitting Diodes With Lattice- Matched AlInGaN Insertion in n-AlGaN Layer

AlGaN-based ultraviolet-A light-emitting diodes (UVA LEDs) inevitably suffer from current crowding effects at high injection levels due to their lateral device structure, resulting in non-uniform light emission and device overheating. In N-polar UV LEDs, the problem is further exacerbated by increased hole injection efficiency, leading to current crowding and aggravated hole leakage, which limits the device performance. An n-AlGaN/AlInGaN/AlGaN structure is adopted in this study, through modulation of the Al and In compositions in the AlInGaN quaternary alloy, lattice matching and greater bandgap of AlInGaN to AlGaN template is designed. The numerical results prove that the n-AlGaN/AlInGaN/AlGaN structure can promote current spreading and thus mitigate hole leakage, resulting in the significantly enhanced performance of N-polar UVA LEDs. Furthermore, the use of lattice-matched AlInGaN layers in practical epitaxy is feasible, which can avoid the defect introduction resulting from the lattice mismatch.

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The ${\mathrm{Gd}}^{3+}$ ion in GdSb has a high spin $S=7/2$ and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular-beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band-structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) calculations. ARPES shows a hole-carrier rich, topologically trivial, semimetallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum-well states. DFT-predicted Fermi wave vectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole-carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

Vapor–liquid assisted chemical vapor deposition of Cu2X materials

Transition metal dichalcogenides (TMDs) are known for their layered structure and tunable functional properties. However, a unified understanding on other transition metal chalcogenides (i.e. M2X) is still lacking. Here, the relatively new class of copper-based chalcogenides Cu2X (X = Te, Se, S) is thoroughly reported. Cu2X are synthesized by an unusual vapor–liquid assisted growth on a Al2O3/Cu/W stack. Liquid copper plays a significant role in synthesizing these layered systems, and sapphire assists with lateral growth and exfoliation. Similar to traditional TMDs, thickness dependent phonon signatures are observed, and high-resolution atomic images reveal the single phase Cu2Te that prefers to grow in lattice-matched layers. Charge transport measurements indicate a metallic nature at room temperature with a transition to a semiconducting nature at low temperatures accompanied by a phase transition, in agreement with band structure calculations. These findings establish a fundamental understanding and thrust Cu2Te as a flexible candidate for wide applications from photovoltaics and sensors to nanoelectronics.

Droplet epitaxy of InAs/InP quantum dots via MOVPE by using an InGaAs interlayer

InAs quantum dots (QDs) are grown on an In0.53Ga0.47As interlayer and embedded in an InP(100) matrix. They are fabricated via droplet epitaxy (DE) in a metal organic vapor phase epitaxy (MOVPE) reactor. Formation of metallic indium droplets on the In0.53Ga0.47As lattice-matched layer and their crystallization into QDs is demonstrated for the first time in MOVPE. The presence of the In0.53Ga0.47As layer prevents the formation of an unintentional non-stoichiometric 2D layer underneath and around the QDs, via suppression of the As-P exchange. The In0.53Ga0.47As layer affects the surface diffusion leading to a modified droplet crystallization process, where unexpectedly the size of the resulting QDs is found to be inversely proportional to the indium supply. Bright single dot emission is detected via micro-photoluminescence at low temperature, ranging from 1440 to 1600 nm, covering the technologically relevant telecom C-band. Transmission electron microscopy investigations reveal buried quantum dots with truncated pyramid shape without defects or dislocations.

Passivation of InP solar cells using large area hexagonal-BN layers

Surface passivation is crucial for many high-performance solid-state devices, especially solar cells. It has been proposed that 2D hexagonal boron nitride (hBN) films can provide near-ideal passivation due to their wide bandgap, lack of dangling bonds, high dielectric constant, and easy transferability to a range of substrates without disturbing their bulk properties. However, so far, the passivation of hBN has been studied for small areas, mainly because of its small sizes. Here, we report the passivation characteristics of wafer-scale, few monolayers thick, hBN grown by metalorganic chemical vapor deposition. Using a recently reported ITO/i-InP/p+-InP solar cell structure, we show a significant improvement in solar cell performance utilizing a few monolayers of hBN as the passivation layer. Interface defect density (at the hBN/i-InP) calculated using C–V measurement was 2 × 1012 eV−1cm−2 and was found comparable to several previously reported passivation layers. Thus, hBN may, in the future, be a possible candidate to achieve high-quality passivation. hBN-based passivation layers can mainly be useful in cases where the growth of lattice-matched passivation layers is complicated, as in the case of thin-film vapor–liquid–solid and close-spaced vapor transport-based III–V semiconductor growth techniques.

Design and growth of short-period long wavelength infrared InAsSb/InAsSb superlattices on lattice engineered metamorphic buffers

There has been steady effort to advance Ga-free InAs1−xSbx/InAs1−ySby superlattice (SL) materials for long-wavelength infrared (LWIR) detector applications. Although Ga-free SLs grown pseudomorphically on GaSb wafers work well in mid-wavelength applications, most LWIR Ga-free SL designs suffer from relatively low optical absorption and poor hole transport. One of the main problems in using the Ga-free material system for LWIR applications is a tremendous constraint on band engineering imposed by matching the SL lattice parameter to the lattice constant of conventional GaSb substrates. In this work, we propose several lattice engineered short-period LWIR InAs1−xSbx/InAs1−ySby SL designs that can enhance optical absorption and vertical hole mobility at an 11 μm cutoff wavelength. We also performed test growths of the designed structures on lattice-matched homogeneous buffer layers on GaAs wafers.

Insulator-to-Metal Transition of Cr2O3 Thin Films via Isovalent Ru3+ Substitution

In transition-metal oxides, the vast combinations of crystal structures and metal ions provide a fertile ground for tailoring their electronic properties. Using a thin-film growth technique creates an additional interface-related degree of freedom in the artificial heterostructures. However, in corundum-type and crystallographically related ternary compounds, the scarcity of oxides applicable to a lattice-matched electrode layer often impedes the electrical characterization of these heterostructures. Here, we report the synthesis of a thin film of a new corundum-type oxide that exhibits metallic conduction. By substituting Ru into the antiferromagnetic insulator Cr2O3 via pulsed laser deposition, we fabricated thin films of (Cr1–xRux)2O3 not known in bulk. The structural, electrical, and magnetic characterizations showed that the antiferromagnetic insulator became a paramagnetic metal at Ru contents x greater than approximately 0.40 while preserving the host corundum structure. Spectroscopic analyses revealed the formation of Ru 4d-derived bands at the Fermi level and the presence of Ru3+, which is a valence state of Ru rarely observed in oxides. The stabilization of the unusual Ru3+ state is attributed to the charge neutrality constraint in the robust Cr2O3-based framework as well as the kinetics-driven vacuum deposition process.

Defects in Cd3As2 epilayers via molecular beam epitaxy and strategies for reducing them

Molecular beam epitaxy offers an exciting avenue for investigating the behavior of topological semimetal Cd3As2, by providing routes for doping, alloying, strain engineering, and heterostructure formation. To date, however, minimal exploration has been devoted to the impact of defects that are incorporated into epilayers due to contraints imposed by the substrate and narrow growth window. Here, we use a combination of lattice-matched ZnxCd1-xTe buffer layers, miscut substrates and broadband illumination to study how dislocations, twins and point defects influence the electron mobility of Cd3As2. A combination of defect suppression approaches produces Cd3As2 epilayers with electron mobilities upwards of 15,000 cm2/V-s at room temperature.

Growth of GaP1 − x − yAsyNx on Si substrates by chemical beam epitaxy

Chemical beam epitaxy has been used to grow layers of GaP1 − xNx, GaP1 − yAsy, and nearly lattice-matched layers GaP1 − x − yAsyNx on Si substrates. To address the issue of antiphase domain generation associated with the growth of polar semiconductors on Si, misoriented Si(001) substrates have been used combined with a carefully designed GaP buffer layer growth. The reflection high-energy electron diffraction pattern exhibits a (2 × 4) surface reconstruction after GaP buffer layer and GaP(As,N) graded layer growth, indicating the good surface quality and planarity of the grown layers. Sample composition was obtained by simultaneous acquisition of Rutherford backscattering spectrometry and nuclear reaction analysis, indicating a linear dependence of N and As mole fractions on the flux of their respective precursor. GaP1 − x − yAsyNx layers grown on Si substrates have a lattice mismatch not larger than ±0.005 for N contents in the range 0.02 < x < 0.05. High-resolution X-ray diffraction reciprocal space maps demonstrate a good crystalline quality. Intense photoluminescence spectra have been measured in all GaP1 − xNx and GaP1 − x − yAsyNx layers, as it is expected for direct bandgap materials. Two wide overlapped emission peaks are observed in all the spectra, most likely related to near bandgap recombination. The position of the higher energy peak for GaP1 − xNx and GaP1 − x − yAsyNx layers has been compared to bandgap energy calculations using the band anticrossing model, showing good agreement.

Large-area ZnO/MoS2 heterostructure grown by pulsed laser deposition

A simple and efficient method to grow continuous large-area ZnO/MoS2 heterostructures is demonstrated by pulsed laser deposition. The layer number of MoS2 films is controlled by the number of laser pulses. The MoS2 film acts as a closely lattice-matched buffer layer for the subsequent ZnO growth, which improves the structural and optical quality of the ZnO film. ZnO film and multilayer MoS2 forms a type-II heterostructure, and the band offsets between them are quantitatively determined.

Structural Properties of Lattice-Matched InGaPN on GaAs (001)

Structural properties of lattice-matched InGaPN on GaAs (001) have comprehensively investigated by high resolution X-ray diffraction (HRXRD), Raman spectroscopy, and atomic force microscopy (AFM). The InGaPN layers were grown by metal organics vapor phase epitaxy (MOVPE). To obtain the lattice-matched InGaPN on GaAs, flow rates of trimethylindium (TMIn), trimethylgallium (TMGa) were kept, respectively, at 14.7 and 8.6 /umol/min. On the other hand, the N content optimized by varying the flow rate of dimethyhydrazine (DMHy, N precursor) was controlled at 300 /umol/min. With a combination of HRXRD and Raman scattering measurements, the In and N contents are estimated to be 55.8 and 0.9 at%, respectively. The lattice-mismatch lower than 0.47%, which corresponds to the lattice-matching condition, was confirmed for all the layers. The rapid thermal annealing (RTA) process was performed to improvement the crystalline quality of InGaPN layers. The annealing temperature was fixed at 650∘C, which is an optimum growth temperature of a GaAs buffer layer. The annealing time was varied in a range of 30 to 180 s to verify a composition uniformity. With increasing the annealing time up to 120 s, the In and N contents were slightly increased. The AFM-root mean square (RMS) roughness of the InGaPN surface was observed to be reduced. For higher annealing times, the N content was dramatically reduced, whereas the In content was still remained. Moreover, the RMS roughness was observed to be increased. RTA at 650∘C for 120 s demonstrated a significant improvement of structural properties of the lattice-matched InGaPN layers on GaAs (001).

Sub-picosecond acoustic pulses at buried GaP/Si interfaces

We report on the optical generation and detection of ultrashort acoustic pulses that propagate in three-dimensional semiconductor crystals. Photoexcitation of lattice-matched GaP layers grown on Si(001) gives rise to a sharp spike in transient reflectivity due to the acoustic pulse generated at the GaP/Si interface and detected at the GaP surface and vice versa. The extremely short width of the reflectivity spike, 0.5 ps, would translate to a spatial extent of 3 nm or 10 atomic bilayers, which is comparable with the width of the intermixing layer at the GaP/Si interface. The reflectivity signals are also modified by quasi-periodic Brillouin oscillations arising from the propagation of the acoustic pulses in the GaP and Si crystals. The present results demonstrate the potential applications of the simple optical pump-probe scheme in the nondestructive evaluation of the buried semiconductor interface quality.

Use of a bilayer lattice-matched AlInGaN barrier for improving the channel carrier confinement of enhancement-mode AlInGaN/GaN hetero-structure field-effect transistors

A quaternary lattice-matched layer structure based on employing a bilayer barrier for improving the carrier confinement in the channel of enhancement-mode metal-face c-plane wurtzite AlInGaN/GaN hetero-structure field effect transistors (HFETs) is for the first time proposed. Using the commercial self-consistent Poisson-Schrödinger solver Nextnano, electronic properties of the proposed hetero-structure, including the sheet charge density and carrier confinement on the GaN side of the hetero-interface, are evaluated. Based on these evaluations, it is shown that while the proposed layer structure substantially improves the carrier confinement in the GaN channel layer, it also upholds the merits of employing a lattice-matched barrier towards achieving an enhancement-mode operation (i.e., in the absence of the piezoelectric effect). According to these simulations, in terms of maintaining the required positive threshold-voltage for the enhancement-mode operation, it is also shown that the proposed layer structure substantially outperforms the quaternary AlInGaN/GaN HFETs employing a thin AlN spacer layer.

Structure and cation distribution of (Mn0.5Zn0.5)Fe2O4 thin films on SrTiO3(001)

A comprehensive study on growth of ferrimagnetic manganese zinc ferrite (Mn0.5Zn0.5Fe2O4) films on single crystalline strontium titanate(001) (SrTiO3) substrates was carried out. Under the optimized conditions, a thin film with a layer thickness of 200 nm was deposited, and the structural properties were investigated. Contrary to data published in literature, no buffer layer was necessary to achieve epitaxial growth of a poorly lattice-matched layer. This was confirmed for Mn0.5Zn0.5Fe2O4(001) on SrTiO3(001) by x-ray diffraction and the adjoined phi scans, which also revealed a lattice compression of 1.2% of the manganese zinc ferrite film in the out-of-plane direction. Using x-ray photoelectron spectroscopy, the near surface stoichiometry of the film could be shown to agree with the intended one within the uncertainty of the method. X-ray absorption spectroscopy showed an electronic structure close to that published for bulk samples. Additional x-ray magnetic circular dichroism investigations were performed to answer detailed structural questions by a comparison of experimental data with the calculated ones. The calculations took into account ion sites (tetrahedral vs. octahedral coordination) as well as the charge of Fe ions (Fe2+ vs. Fe3+). Contrary to the expectation for a perfect normal spinel that only Fe3+ ions are present in octahedral sites, hints regarding the presence of additional Fe2+ in octahedral sites as well as Fe3+ ions in tetrahedral sites have been obtained. Altogether, the layer could be shown to be mostly in a normal spinel configuration.

High-efficiency InGaN/AlInGaN multiple quantum wells with lattice-matched AlInGaN superlattices barrier**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274003, 61422401, 51461135002, and 61334009), the Natural Science Foundation of Jiangsu Province, China (Grant Nos. BY2013077, BK20141320, BE2015111, and BK20161324), the Program for New Century Excellent Talents in University, China (Grant No. NCET-11-0229), and the Special Semiconductor Materials and

A new approach to fabricating high-quality AlInGaN film as a lattice-matched barrier layer in multiple quantum wells (MQWs) is presented. The high-quality AlInGaN film is realized by growing the AlGaN/InGaN short period superlattices through metalorganic chemical vapor deposition, and then being used as a barrier in the MQWs. The crystalline quality of the MQWs with the lattice-matched AlInGaN barrier and that of the conventional InGaN/GaN MQWs are characterized by x-ray diffraction and scanning electron microscopy. The photoluminescence (PL) properties of the InGaN/AlInGaN MQWs are investigated by varying the excitation power density and temperature through comparing with those of the InGaN/GaN MQWs. The integral PL intensity of InGaN/AlInGaN MQWs is over 3 times higher than that of InGaN/GaN MQWs at room temperature under the highest excitation power. Temperature-dependent PL further demonstrates that the internal quantum efficiency of InGaN/AlInGaN MQWs (76.1%) is much higher than that of InGaN/GaN MQWs (21%). The improved luminescence performance of InGaN/AlInGaN MQWs can be attributed to the distinct reduction of the barrier-well lattice mismatch and the strain-induced non-radiative recombination centers.