Heteroepitaxial Growth Research Articles

Highly Stable and Efficient Light-Emitting Diodes Based on Orthorhombic γ-CsPbI3 Nanocrystals

Orthorhombic γ-CsPbI3 possesses the highest structural stability among the optically active (light-emissive) CsPbI3 perovskites. Here, we make use of a seed-assisted heteroepitaxial growth to fabricate seed/core/shell CaIx/γ-CsPbI3/CaI2 nanocrystals. Ultrasmall CaIx nanoparticles serve as seeds to template the Pb-centered octahedral arrangement which enables the formation of the γ-CsPbI3 phase and at the same time inhibit lattice strain by blocking the force transfer that otherwise leads to an octahedral twist and so improve the structural stability of the resulting nanocrystals. An outer shell composed from the same material, CaI2, isolates the formed γ-CsPbI3 nanocrystals from the environment, which also significantly improves their stability under ambient conditions. Optical and electrical studies indicate that the seed/core/shell CaIx/γ-CsPbI3/CaI2 structure possesses a shallower set of trap states as compared to cubic α-CsPbI3 nanocrystals. Light-emitting diodes utilizing these γ-CsPbI3 nanocrystals show a record high external quantum efficiency of 25.3%, high brightness of over 13600 cd/m2, and an operational lifetime of ∼14 h before reaching 50% of their initial luminance. These devices can repeatedly be illuminated over 650 times at ∼500 cd/m2 with no decline of brightness, which indicates their great commercial potential.

Growth of α-In2O3 films with different concentrations of In2O3 powder used as source precursor by mist chemical vapor deposition

The heteroepitaxial growth of In2O3 on a (0001) α-Al2O3 substrate was carried out using the mist chemical vapor deposition method. The concentrations of In2O3 powder in the source precursor solution varied between 0.025 and 0.250 mol l−1. An increase in the growth rate was observed with the increasing In2O3 powder concentration, and the highest growth rate of 4.2 μm h−1 was obtained at a powder concentration of 0.150 mol l−1. An improvement in the electrical properties was eventually observed, i.e. the lowest carrier concentration of 3.1 × 1017 cm−3 and the highest Hall mobility of 241 cm2 V−1 s−1 at room temperature were obtained at a powder concentration of 0.150 mol l−1. The temperature-dependent Hall effect measurement for the sample showed pronounced carrier freeze-out in a temperature range of 300–30 K, indicating that the α-In2O3 film had near-non-degenerate conductivity.

Realizing a Superconducting Square-Lattice Bismuth Monolayer

Interplay of crystal symmetry, strong spin-orbit coupling (SOC), and many-body interactions in low-dimensional materials provides a fertile ground for the discovery of unconventional electronic and magnetic properties and versatile functionalities. Two-dimensional (2D) allotropes of group 15 elements are appealing due to their structures and controllability over symmetries and topology under strong SOC. Here, we report the heteroepitaxial growth of a proximity-induced superconducting 2D square-lattice bismuth monolayer on superconducting Pb films. The square lattice of monolayer bismuth films in a C4 symmetry together with a stripey moiré structure is clearly resolved by our scanning tunneling microscopy, and its atomic structure is revealed by density functional theory (DFT) calculations. A Rashba-type spin-split Dirac band is predicted by DFT calculations to exist at the Fermi level and becomes superconducting through the proximity effect from the Pb substrate. We suggest the possibility of a topological superconducting state in this system with magnetic dopants/field. This work introduces an intriguing material platform with 2D Dirac bands, strong SOC, topological superconductivity, and the moiré superstructure.

Study on the Structure of GaN films deposited on MoS2/Sapphire via Plasma-Assisted Molecular Beam Epitaxy

The gallium nitride (GaN) films were grown on molybdenum disulfide (MoS2) layers via plasma-assisted molecular beam epitaxy (PA-MBE). The heterostructures of the GaN film were studied using reflection high-energy electron diffraction (RHEED) and HR-XRD. The heterostructures of GaN/MoS2/sapphire were revealed through cross-sectional transmission electron microscopy (TEM). The surface texture of the GaN films was analyzed using FE-SEM. Single-crystal heterostructure GaN films can be obtained on 2D MoS2/c-sapphire. The RHEED demonstrated spot patterns with high intensity showing the single crystal structure constructed in the GaN films. The GaN films on the surface exhibited a hexagonal structure. TEM images taken perpendicular to the surface revealed that, even after 60 minutes of epitaxial growth, the thickness of the GaN films remained consistent at approximately 4 nm. However, the 2D MoS2 layer was not observable in the images due to harm incurred during heteroepitaxial growth. Based on the surface structure, it was found that GaN films were successfully grown on the MoS2 layers using the PA-MBE system.

Epitaxial Growth of Lattice‐Matched BaWO4/CH3NH3PbX3(X = Cl, Br, I) Hetero‐Micro/Nanostructure with Suppressed Halide Ion Migration

AbstractIntrinsic instability of lead halide perovskites has always been a key obstacle to manifest their excellent optoelectronic applications. Here, a solvent‐free thermally driven heteroepitaxial growth strategy is proposed to assemble BaWO4/MAPbX3(MA = CH3NH3+; X = Cl−, Br−, or I−) micro‐nanostructure. Tetragonal BaWO4matrix restrains the halogen ion migration in epitaxial MAPbX3thanks to their perfect lattice matching degrees, also imparting the integration of exceptional durability to light, heat, polarity solvents, and capabilities to fine‐tune the luminescent colors. Furthermore, MABr‐triggered surface nucleation of MAPbBr3and its removal by H2O appear with switchable and reversible color changes, demonstrating great potential in advanced anticounterfeiting. The general approach in engineering heteroepitaxy of MAPbX3heterostructure opens up a new idea toward improved stability by the lattice matching principle.

Heteroepitaxial growth of CaGe2 films on high-resistivity Si(111) substrates and its application for germanane synthesizing

Realizing a germanane (GeH) film on insulating substrates is strongly required to understand its intrinsic electric properties in discussing the potential applications for the next-generation field-effect transistor channels. This study investigates the heteroepitaxial growth of CaGe2—the precursor of the GeH—films on a high resistive substrate of FZ-Si(111) using a molecular beam epitaxy with the deposition temperature (Tdepo) as a variable. X-ray diffraction and Raman spectroscopy analyses indicated that a metastable polytype of 2H was dominated in the epitaxial CaGe2 films; the Tdepo must be less than 700 °C to prevent Si diffusion from the substrates. We also synthesize GeH films by immersing some CaGe2 films in HCl acid solutions at a low temperature of −40 °C. Although the obtained grain size of GeH was still as small as several hundred nanometers, we believe that the present study is the first step in obtaining the GeH continuous films directly on insulators necessary for electrical characterizations.

Directional Self-Assembly of Facet-Aligned Organic Hierarchical Super-Heterostructures for Spatially Resolved Photonic Barcodes.

Organic multicolor heterostructures with spatially resolved luminescent colors and identifiable patterns have exhibited considerable potential for achieving micro-/nanoscale photonic barcodes. Nevertheless, such types of barcodes reported thus far are exclusively based on a single heterostructure with limited coding elements. Here, a directional self-assembly strategy is proposed to achieve high-coding-capacity spatially resolved photonic barcodes through rationally constructing organic hierarchical super-heterostructures, where numerous subheterostructure blocks with flat hexagonal facets are precisely oriented with their specific facets via a reconfigurable capillary force. The building blocks were prepared through a one-pot sequential heteroepitaxial growth, which enables the effective modulation of the structural and color characteristics in coding structures. Significantly, a directional facet-to-facet attraction between particles via facet registration leads to the formation of well-defined 1D super-heterostructures, which contain multiple coding elements, thus providing a good platform for constructing the high-coding-capacity photonic barcodes. The results may be useful in fabricating organic hierarchical hybrid super-heterostructures for security labels and optical data recording.

Heteroepitaxy of 2D CuCr2 Te4 with Robust Room-temperature Ferromagnetism.

Magnetic materials in 2D have attracted widespread attention for their intriguing magnetic properties. 2D magnetic heterostructures can provide unprecedented opportunities for exploring fundamental physics and novel spintronic devices. Here, the heteroepitaxial growth of ferromagnetic CuCr2 Te4 nanosheets is reported on Cr2 Te3 and mica by chemical vapor deposition. Magneto-optical Kerr effect measurements reveal the thickness-dependent ferromagnetism of CuCr2 Te4 nanosheets on mica, where a decrease of Curie temperature (TC ) from 320 to 260K and an enhancement of perpendicular magnetic anisotropy with reducing thickness are observed. Moreover, lattice-matched heteroepitaxial ultrathin CuCr2 Te4 on Cr2 Te3 exhibits an enhanced robust ferromagnetism with TC up to 340K due to the interfacial charge transfer. Stripe-type magnetic domains and single magnetic domain are discovered in this heterostructure with different thicknesses. The work provides a way to construct robust room-temperature 2D magnetic heterostructures for functional spintronic devices.

Lattice-matched heteroepitaxial preparation of InSb/CdTe on Si (111) substrate by magnetron sputtering

InSb films were deposited on Si (111) substrate using the CdTe buffer layers by magnetron sputtering. The effects of CdTe buffer layers on the crystal structure, morphological optical and electrical properties of InSb films were studied. The study found that the CdTe buffer layer can improve the grain size and reduce the stress of InSb films. Compared with direct growth on Si substrate, the use of the CdTe buffer layer can effectively control the appearance of dislocations at the interface and thus improve the epitaxial quality of InSb films. Furthermore, the surface roughness and carrier mobility of the InSb film depends on the CdTe buffer layer thickness, which is mainly caused by the difference in surface morphology of the buffer layer with different thicknesses. InSb/CdTe structure shows the advantages of lattice-matched heteroepitaxial growth, indicating that CdTe is an effective buffer layer to improve the performance of InSb. The insertion of a buffer layer provides a promising method for the preparation of III-V/II-VI hybrid system on a Si substrate, which can be used for high-performance infrared detectors.

Effect of dislocations on the performance of GaSb-based diode lasers grown on silicon

Silicon photonics is a promising technology for the fabrication of dense photonic chips, thanks to the very mature silicon industry. The direct epitaxial growth of III–V lasers on silicon is one of the main challenges for the realization of compact and robust mid-infrared sensors based on photonic integrated circuits. The crystal defects arising from this heteroepitaxial growth affect the laser performance and, therefore, need to be mitigated but also studied to better understand their impact on the laser operation. Here, we studied the effect of threading dislocations on laser performance by comparing the series of GaSb-based diode lasers grown on native GaSb and Si substrates with different numbers of quantum wells (nQW) in their active zones. As expected, the laser threshold currents are higher in the case of diode lasers on Si, and they rapidly vary with nQW. Still, the lowest threshold current densities are achieved with nQW = 1 for both substrates. With the help of a theoretical gain model, we attribute these results to the fact that dislocations create non-radiative recombination but do not introduce additional optical losses. This work allows a better understanding of the origin of performance degradation and the decision to be made regarding the heterostructure design.

ZnO nanofiber skeleton induced robust zeolitic imidazolate framework membranes for gas separation

Rationally designing compact metal-organic framework (MOF) membrane is highly desired but challenging. Herein, we proposed a ZnO nanofiber skeleton induced zeolitic imidazolate framework (ZIF) membrane inspired by the reinforced concrete structure. In this process, the ZnO nanofiber skeleton was employed as active anchor sites to assist the heteroepitaxial growth of continuous membranes, like the reinforcing steel in structure. The formed ZIFs particles were tightly embedded in the skeleton like the concrete. With this approach, highly compact Co-based ZIF-9 membrane and Zn-based ZIF-8 membrane were successfully achieved and exhibit effective H2 separation performance. For ZIF-9 membrane, the H2 permeance and the ideal selectivity of H2/CO2 are 2.19 × 10−7 mol m−2 s−1 Pa−1 and 15.3, respectively. For ZIF-8 membrane, the H2 permeance and the ideal selectivity of H2/CH4 are 2.26 × 10−6 mol m−2 s−1 Pa−1 and 9.7, respectively. More importantly, benefiting from the novel structure, the membrane showed a highly robust architecture under different pressures, good durability against rubbing, and separation stability of 100 h. This strategy provides a new approach toward other compact and robust MOF membrane.

Optical, electronic, and structural properties of ScAlMgO4

Magnesium aluminate scandium oxide (ScAlMgO4) is a promising lattice-matched substrate material for GaN- and ZnO-based optoelectronic devices. Yet, despite its clear advantages over substrates commonly used in heteroepitaxial growth, several fundamental properties of ScAlMgO4 remain unsettled. Here, we provide a comprehensive picture of its optical, electronic and structural properties by studying ScAlMgO4 single crystals grown by the Czochralski method. We use variable angle spectroscopic ellipsometry to determine complex in-plane and out-of-plane refractive indices in the range from 193 to 1690 nm. An oscillator-based model provides a phenomenological description of the ellipsometric spectra with excellent agreement over the entire range of wavelengths. For convenience, we supply the reader also with Cauchy formulas describing the real part of the anisotropic refractive index for wavelengths above 400 nm. Ab initio many-body perturbation theory modeling provides information about the electronic structure of ScAlMgO4, and successfully validated experimentally obtained refractive index values. Simulations also show exciton binding energy as large as a few hundred of meV, indicating ScAlMgO4 as a promising material for implementation in low-threshold, deep-UV lasing devices operating at room temperature. X-ray diffraction measurements confirm lattice constants of ScAlMgO4 previously reported, but in addition, reveal that dominant crystallographic planes (001) are mutually inclined by about 0.009{\deg}. In view of our work, ScAlMgO4 is a highly transparent, low refractive index, birefringent material similar to a sapphire, but with a much more favorable lattice constant and simpler processing.

Defect-Induced Efficient Heteroepitaxial Growth of Single-Wall Carbon Nanotubes @ Hexagonal Boron Nitride Films.

Carbon nanotube-based derivatives have attracted considerable research interest due to their unique structure and fascinating physicochemical properties. However, the controlled growth mechanism of these derivatives remains unclear, and the synthesis efficiency is low. Herein, we proposed a defect-induced strategy for the efficient heteroepitaxial growth of single-wall carbon nanotubes (SWCNTs)@hexagonal boron nitride (h-BN) films. Air plasma treatment was first performed to generate defects on the wall of SWCNTs. Then, atmospheric pressure chemical vapor deposition was conducted to grow h-BN on the surface of SWCNTs. Controlled experiments combined with first-principles calculations revealed that the induced defects on the wall of SWCNTs function as nucleation sites for the efficient heteroepitaxial growth of h-BN.

Step flow growth of β-Ga2O3 films on off-axis 4H-SiC substrates by LPCVD

In this work, β-Ga2O3 films were prepared on 4H-SiC (0001) substrates with 4° off-axis by low-pressure chemical vapor deposition (LPCVD). The crystalline structures, chemical compositions, optical properties, and surface/interface of the as-prepared films and heterostructures were all comprehensively investigated. X-ray diffraction and room temperature Raman spectrometry demonstrated β-Ga2O3 films are pure phase with highly preferential growth orientation along the (-201) plane. The Ga/O ratio estimated by X-ray photoelectron spectroscopy was 0.7, indicating the existence of Ga-O bonding. In addition, β-Ga2O3 films showed high transmittance at wavelengths over 300 nm, corresponding to an optical bandgap of about 4.88 eV. Remarkably, the growth of β-Ga2O3 films on 4H-SiC substrates with 4° off-axis followed the step-flow growth mode, reported for the first time. The relevant growth mechanism was discussed. These results demonstrated the feasible formation of heteroepitaxial growth of β-Ga2O3 films with high crystal quality and superior electrical properties on 4H-SiC substrates with off-axis angle by LPCVD, promising for applications in Ga2O3 based high power and optoelectronic devices.

HBN-Layer-Promoted Heteroepitaxy in Reactively Sputter-Deposited MoSx≈2(0001)/Al2O3(0001) Thin Films: Implications for Nanoelectronics

We show that van der Waals (vdW)-bonded hexagonal boron nitride (hBN) promotes heteroepitaxial growth of semiconducting MoSx(0001) (x = 2.0 ± 0.1) thin films on Al2O3(0001) substrates. hBN layers are grown on Al2O3(0001) via pyrolytic cracking of borazine (∼6 × 104 L) at 1373 K and the MoSx layers are deposited in an ultrahigh-vacuum system via reactive direct-current magnetron sputtering of Mo in an Ar/H2S gas mixture at 1073 K on bare and hBN-covered Al2O3(0001). Using in situ low-energy electron diffraction and Auger electron spectroscopy along with ex situ X-ray diffraction, X-ray photoelectron and Raman spectroscopies, and transmission electron microscopy, we determine the as-deposited MoSx layer composition and crystallinity. We obtain highly 0001-oriented, ∼20-nm-thick, 2H-structured MoSx multilayers with better crystalline quality on hBN/Al2O3(0001) than on Al2O3(0001). We suggest that hBN buffer layer enhances surface diffusion of depositing species, compared to bare Al2O3(0001), leading to an observed improvement in the crystallinity of MoSx layers. We expect that our results are likely to have broad implications in nanoelectronic device fabrication.

Epitaxial growth of β-Ga2O3 thin films on SrTiO3 (1 1 1) and (1 0 0) substrates by chemical vapor deposition

Due to good lattice matching and bipolar doping behavior, wide bandgap SrTiO3 (STO) is a promising substrate for the heteroepitaxial growth of β-Ga2O3 to construct heterojunctions for (opto-)electronic device applications. However, the investigation of the growth of β-Ga2O3 film on the STO (111) substrate is still lacking. In this work, for the first time, the β-Ga2O3 (2¯ 01) thin films were epitaxially grown on the supercell matched STO (111) substrates by low pressure chemical vapor deposition. The effects of the source and growth temperatures on the surface roughness and crystalline quality of the β-Ga2O3 films were systemically investigated. By controlling the supply of Ga source, the heteroepitaxial β-Ga2O3 film grown under the optimized conditions exhibits a narrow X-ray diffraction rocking curve of 0.75° and a low root-mean-square roughness of 1.10 nm. Furthermore, the epitaxial growth of β-Ga2O3 films on the STO (100) substrate was also investigated. The heteroepitaxy of β-Ga2O3 films on STO lays the foundation for future device applications of β-Ga2O3-based heterojunctions.

Investigation of OP-GaP Grown on OP-GaAs Templates Using Nondestructive Reciprocal Space Mapping

Orientation-patterned gallium phosphide (OP-GaP) has been grown heteroepitaxially on OP gallium arsenide (GaAs) templates using hydride vapor phase epitaxy (HVPE). The effect of OP-GaAs template fabrication methods of epitaxial-inversion and wafer bonding on the heteroepitaxial OP-GaP growth has been investigated. OP-GaP layers with a growth rate of up to 35 µm/h and excellent domain fidelity were obtained. The growth rate and the domain fidelity have been revealed/studied by scanning electron microscope (SEM). In addition, we demonstrate that the crystalline quality of the individual domains, namely, the substrate-oriented domains (ODs) and the inverted domains (IDs), can be investigated by high-resolution x-ray diffraction reciprocal space mapping (HRXRDRSM), which can also indicate the domain fidelity. Attempts to increase the growth rate and improve the domain fidelity by increasing the III and V group precursors resulted in either an increase in the growth rate in the OP-GaP layers grown on epitaxial inversion OP-GaAs template at the expense of the domain crystalline quality and fidelity or an improvement in the crystalline quality of the domains at the expense of the growth rate in the OP-GaP layers grown on wafer-bonded OP-GaAs templates. In the case of OP-GaP grown on OP-GaAs templates prepared by epitaxial inversion, the crystalline quality of the ODs is better than that of the IDs, but it shows that the quality of the inverted layer in the template influences the quality and fidelity of the grown domains. To the authors’ knowledge, exploitation of HRXRDRSM studies on OP-GaP to establish the crystalline quality of its individual domains (ODs and IDs) is the first of its kind. OP-ZnSe grown on OP-GaAs templates has also been included in this study to further emphasize the potential of this method. We propose from this study that once the growth rate is optimized from SEM studies, HRXRDRSM analysis alone can be used to assess the structural quality and to infer the domain fidelity of the OP structures.

Hierarchical copper-based metal-organic frameworks nanosheet assemblies for electrochemical ascorbic acid sensing

Noninvasive human health monitoring requires the development of efficient electrochemical sensors for the quantitative analysis of infinitesimal biomolecules. In this work, we reported a novel hierarchical nanosheet assemblies (HSA) of copper-based metal-organic frameworks (MOFs) as an electrochemical sensor for ascorbic acid (AA) detection. Copper 1,4-benzenedicarboxylate (CuBDC) HSA was constructed by three steps of in situ growth on stone paper, including hydrolysis, anion exchange, and heteroepitaxy growth. The monodispersed two-dimensional MOFs nanosheet units were aligned in an orderly manner and arranged into three-dimensional hierarchical assemblies. The CuBDC HSA-based AA sensor displayed a high sensitivity of 396.8 μA mM-1 cm-2 and a low detection limit of 0.1 μM. Excellent selectivity, stability and reproducibility were also obtained. Benefiting from the advantages of ultrathin nanosheets and nature-inspired hierarchy, this unique architecture facilitated reactant dispersion and maximized the accessible active sites and charge-transport capability and thus had superior catalytic ability for the electro-oxidation of ascorbic acid compared to bulk MOFs. Moreover, the CuBDC HSA sensor performed AA level detection in juice samples with acceptable accuracy and verified the feasibility for sweat AA sensing. This novel MOFs architecture holds great potential as an electrochemical sensor to detect AA for noninvasive human health monitoring in the future.

Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films

Willemite-type Zn2GeO4 is a promising ultrawide bandgap semiconductor material. To date, experimental results on growth and physical properties of epitaxial thin films of willemite-type Zn2GeO4 are not available. Here, we report the heteroepitaxial growth of (00.1)-oriented Zn2GeO4 thin films on c-plane sapphire substrates using pulsed laser deposition. The in-plane orientation relationships are [11.0] Zn2GeO4//[11.0] Al2O3 and [11¯.0] Zn2GeO4//[11¯.0] Al2O3. A 450 nm thick epitaxial film with a surface roughness of 2.5 nm deposited under 0.1 mbar oxygen partial pressure exhibits a full width at half maximum (FWHM) of rocking curve of (00.6) reflex of 0.35°. The direct bandgap is evaluated to be 4.9 ± 0.1 eV. The valence band maximum is determined to be 3.7 ± 0.1 eV below the Fermi level. Together with the density-functional theory band structure calculation, it is suggested that the O 2p orbital and Zn 3d orbital dominantly contribute to the valence band of Zn2GeO4. The steady-state photoluminescence (PL) spectra of the films under 266 nm excitation at room temperature exhibit a broad defect-related emission band centered at 2.62 eV with a FWHM of 0.55 eV. The origin of this native defect-related PL is suggested to correlate with Zn interstitials. This work advances the fundamental study on willemite-type Zn2GeO4 epitaxial thin films for potential device application.

Fractal Branched Microwires of Organic Semiconductor with Controlled Branching and Low-Threshold Amplified Spontaneous Emission

Fractals are quite normal in nature. However, fractal self-assembly of organic semiconductors remains challenging. Herein, we develop a facile solution assembly route to access organic microwires (MWs) comprising an oligo(p-phenylenevinylene) derivative (OPV-A) with and without branching. Instead of kinetically controlled β-OPV-A microrods (MRs), thermodynamically favored α-OPV-A gives fractal branching MW patterns. As-prepared 9,10-dicyanoanthracene (DCA) alloyed assemblies function as seeds to allow for the heteroepitaxial growth of branching α-OPV-A MWs via either coassembly or two-step seeded growth. Consequently, fractal MWs with single- and multisite growth were both achieved, accompanied by tailorable branching densities and hierarchies. Thermodynamic control and a well-matched epitaxial relationship should be crucial to the formation of fractal MW patterns. Importantly, the aligned α-OPV-A MW array functions as a multichannel optical gain medium and exhibits low-threshold amplified spontaneous emission (ASE). The present work deepens the research into fractal self-assembly of functional organic semiconductors.