Epitaxial Relationship Research Articles

ZnO Nanowires Grown on Al2O3-ZnAl2O4 Nanostructure Using Solid-Vapor Mechanism

We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given.

In-plane anisotropy in van der Waals epitaxial MoS2 on MoO2(010)

Transition metal dichalcogenide based 2D/2D or 2D/3D van der Waals heterostructures exhibit superior properties for high-performance electronics, tunneling transistors, and catalysts. Here, we report on the fabrication of high quality 2D/3D MoS2/MoO2 heterostructures with an atomic clean interface by one-step chemical vapor deposition. Optical microscopy, atomic force microscopy, Raman spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy measurements reveal the high-quality of as-grown samples and the specific epitaxial relationship between MoS2 and MoO2: MoS2[1¯1¯20]//MoO2[001] and MoS2[1¯100]//MoO2[201]. Photoluminescence and Kelvin probe force microscopy measurements combined with density functional theory calculations confirm the interfacial charge transfer from MoS2 to the underlying MoO2. Furthermore, MoO2 induced in-plane anisotropy in MoS2 was revealed using angle-resolved polarized Raman and photoluminescence spectroscopy with anisotropic ratios of 1.27 (Raman) and 1.29 (photoluminescence), respectively, which is most possibly attributed to anisotropic interfacial charge interactions. Our findings provide an excellent platform for the investigation on interfacial effects. Moreover, the in-plane anisotropy in MoS2 induced by MoO2 has expanded the application of isotropic MoS2 in the polarization-dependent fields.

Epitaxial growth and characterization of (001) [NiFe/M]20 (M = Cu, CuPt and Pt) superlattices

We present optimization of [(15Å) Ni80Fe20/(5Å) M]20 single crystal multilayers on (001) MgO substrates, with M being Cu, Cu50Pt50 and Pt. These superlattices were characterized by high resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at substrate temperature of 100 °C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at ∼10° off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021).

Tuning the Liquid-Vapour Interface of VLS Epitaxy for Creating Novel Semiconductor Nanostructures.

Controlling the morphology and composition of semiconductor nano- and micro-structures is crucial for fundamental studies and applications. Here, Si-Ge semiconductor nanostructures were fabricated using photolithographically defined micro-crucibles on Si substrates. Interestingly, the nanostructure morphology and composition of these structures are strongly dependent on the size of the liquid-vapour interface (i.e., the opening of the micro-crucible) in the CVD deposition step of Ge. In particular, Ge crystallites nucleate in micro-crucibles with larger opening sizes (3.74-4.73 μm2), while no such crystallites are found in micro-crucibles with smaller openings of 1.15 μm2. This interface area tuning also results in the formation of unique semiconductor nanostructures: lateral nano-trees (for smaller openings) and nano-rods (for larger openings). Further TEM imaging reveals that these nanostructures have an epitaxial relationship with the underlying Si substrate. This geometrical dependence on the micro-scale vapour-liquid-solid (VLS) nucleation and growth is explained within a dedicated model, where the incubation time for the VLS Ge nucleation is inversely proportional to the opening size. The geometric effect on the VLS nucleation can be used for the fine tuning of the morphology and composition of different lateral nano- and micro-structures by simply changing the area of the liquid-vapour interface.

Epitaxial CsPbBr3 /CdS Janus Nanocrystal Heterostructures for Efficient Charge Separation.

Epitaxial heterostructures of colloidal lead halide perovskite nanocrystals (NCs) with other semiconductors, especially the technologically important metal chalcogenides, can offer an unprecedented level of control in wavefunction design and exciton/charge carrier engineering. These NC heterostructures are ideal material platforms for efficient optoelectronics and other applications. Existing methods, however, can only yield heterostructures with random connections and distributions of the two components. The lack of epitaxial relation and uniform geometry hinders the structure-function correlation and impedes the electronic coupling at the heterointerface. This work reports the synthesis of uniform, epitaxially grown CsPbBr3 /CdS Janus NC heterostructures with ultrafast charge separation across the electronically coupled interface. Each Janus NC contains a CdS domain that grows exclusively on a single {220} facet of CsPbBr3 NCs. Varying reaction parameters allows for precise control in the sizes of each domain and readily modulates the optical properties of Janus NCs. Transient absorption measurements and modeling results reveal a type II band alignment, where photoexcited electrons rapidly transfer (within ≈9picoseconds) from CsPbBr3 to CdS. The promoted charge separation and extraction in epitaxial Janus NCs leadsto photoconductors with drastically improved (approximately three orders of magnitude) responsivity and detectivity, whichispromising for ultrasensitive photodetection.

Competitive actions of MnSi in the epitaxial growth of Mn5Si3 thin films on Si(111)

Some magnetically ordered phases of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ crystal are proving to be prototypes for the study of the new fundamental spin physics related to the spontaneous breaking of the time-reversal symmetry despite a zero net magnetization. Here, we report on a route to grow epitaxial ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ thin films on Si(111). To this end, we use Mn and Si codeposition in a molecular beam epitaxy system and carefully tune the deposition rates, the growth temperature, and the annealing temperature. We assessed the silicide phase-formation and morphology using reflection high-energy electron diffraction, x-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy. Layers containing only ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ could be stabilized under very restrictive conditions, by tuning the Mn/Si flux ratio to match the compound stoichiometry and adjusting the substrate temperature during growth to 443 K. HRTEM imaging revealed the existence of an interfacial amorphous layer of few nanometers thickness. Annealing the heterostructure up to 573 K led to the formation of MnSi at the vicinity of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$/Si(111) interface, which significantly reduced the nucleation barrier of ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$. High-quality crystalline ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ thin films were then formed with the following epitaxial relationships: ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$(0001)$[01\overline{1}0]//\mathrm{MnSi}(111)[\overline{2}11]//\mathrm{Si}(111)[1\overline{1}0]$. Our experiments showed that the formation of MnSi is enhanced at a growth temperature above 473 K or for a longer annealing step, while the crystalline quality of the ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ overlayer is correspondingly degraded leading to textured thin films. The growth pathways and structural properties of the manganese silicides can be rationalized in terms of reactions maximizing the free-energy lowering rate. Moreover, we found that the magnetic and the magnetotransport properties can be used as an efficient tool to track both ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ crystallinity and proportion in the deposited layers.

Tailoring the epitaxial growth of oriented Te nanoribbon arrays

As an elemental semiconductor, tellurium (Te) has been famous for its high hole-mobility, excellent ambient stability and topological states. Here, we realize the controllable synthesis of horizontal Te nanoribbon arrays (TRAs) with an angular interval of 60°on mica substrates by physical vapor deposition strategy. The growth of Te nanoribbons (TRs) is driven by two factors, where the intrinsic quasi-one-dimensional spiral chain structure promotes the elongation of their length; the epitaxy relationship between [110] direction of Te and [110] direction of mica facilitates the oriented growth and the expansion of their width. The bending of TRs which have not been reported is induced by grain boundary. Field-effect transistors based on TRs demonstrate high mobility and on/off ratio corresponding to 397cm2V-1s-1 and 1.5×105, respectively. These phenomena supply an opportunity to deep insight into the vapor-transport synthesis of low-dimensional Te and explore its underlying application in monolithic integration.

Modulation of Remote Epitaxial Heterointerface by Graphene-Assisted Attenuative Charge Transfer

Remote epitaxy (RE), substrate polarity can "penetrate" two-dimensional materials (2DMs) and act on the epi-layer, showing a prospective universal growth strategy. However, essentially, the role that 2DMs plays in RE has not been deeply investigated so far. Here, the RE of single-crystal films on the weakest polarity/iconicity substrate is realized to reveal its essence physical properties. Graphene facilitates attenuative charge transfer (ACT) from a substrate to epi-layer to construct remote interactions. Interfacial atoms are assembled into "incommensurate" epitaxial relationships through graphene to reduce misfit dislocations in the epi-layer. Moreover, graphene reduces the atomic migration barrier, leading to a tendency toward a "layer-by-layer" growth mode. Such film growth mode is different with the conventional epitaxy (CE), and it is beneficial for the fast growth of epi-layers and the reduction of dislocations at coalescence boundaries. The insightful revelation of the role of graphene reveals the interface physics of RE and provides a more valuable guide to using 2DMs to expand three-dimensional materials (3DMs) for application in devices.

Interplay between boron doping and epitaxial relationships in VO2 films grown by laser ablation

We investigated the effect of boron doping on the functional and structural properties of VO2 thin films. Temperature-dependent measurements were performed on pure and boron-doped (0.5 and 1.3 at.%) VO2 films grown on Al2O3(0001) by Reactive Pulsed Laser Deposition. The transition temperature is initially reduced by the insertion of boron; decreasing from ca. 346 K in the case of the pure sample to 335 K for a 0.5 at.% boron amount. Further increase of the boron concentration leads to a slight decrease of the transition temperature (−2 K), a significantly broader hysteresis loop and a reduced resistivity contrast (less than 3 orders of magnitude). Raman spectroscopy and high-resolution X-ray diffraction were performed to elucidate the nature of the involved phases and their structure. A correlation was found between increasing the boron concentration and the formation of two peculiar in-plane epitaxial relationships, the VO2 films evolving from one to the other. We also evidenced the presence of the transient M2 phase in the highest doped sample.

Direct growth of GaN film on ScAlMgO4 substrate by radio-frequency plasma-excited molecular beam epitaxy

ScAlMgO4 (SAM) has attracted attention as a substrate for nitride semiconductor crystal growth owing to its small a-axis lattice mismatch with GaN and InGaN. In this study, we investigated GaN growth on a SAM substrate via radio-frequency plasma-excited molecular beam epitaxy. By optimizing the growth conditions, GaN with the following epitaxial orientation relations (0001)GaN//(0001)SAM and [11–20]GaN//[11–20]SAM was successfully grown directly on the SAM substrate. The atomically flat and abrupt interface of GaN directly grown on the SAM substrate was observed via high-resolution transmission electron microscopy, and uniform GaN growth on a two-inch SAM substrate was also demonstrated.

Structural, magnetic, and transport properties of Co2CrAl epitaxial thin film

We report the physical properties of Co2CrAl Heusler alloy epitaxial thin films grown on a single-crystalline MgO(001) substrate using a pulsed laser deposition technique. The x-ray diffraction pattern in the θ-2θ mode showed the film growth in a single phase B2-type ordered cubic structure with the presence of (002) and (004) peaks, and the film oriented along the MgO(001) direction. The ϕ scan along the (220) plane confirms the fourfold symmetry, and the epitaxial growth relation is found to be Co2CrAl(001)[100]||MgO(001)[110]. The thickness of about 12 nm is extracted through the analysis of x-ray reflectivity data. The isothermal magnetization (M–H) curves confirm the ferromagnetic (FM) nature of the thin film having significant hysteresis at 5 and 300 K. From the in-plane M–H curves, the saturation magnetization values are determined to be 2.1 μB/f.u. at 5 K and 1.6 μB/f.u. at 300 K, which suggest the soft FM behavior in the film having the coercive field ≈522 Oe at 5 K. The thermomagnetization measurements at 500 Oe magnetic field show the bifurcation between field-cooled and zero-field-cooled curves below about 100 K. The normalized field-cooled magnetization curve follows the T2 dependency, and the analysis reveals the Curie temperature around 335±11 K. Moreover, the low-temperature resistivity indicates semiconducting behavior with the temperature, and we find a negative temperature coefficient of resistivity (5.2×10−4/K).

Large-area epitaxial growth of 2D ZrS2(1−x)Se2x semiconductor alloys with fully tunable compositions and bandgaps for optoelectronics

Bandgap engineering of transition metal dichalcogenides (TMDs) is significant for broadening their applications in electronics and optoelectronics devices. Herein, we report the new epitaxial growth of large-area uniform ZrS2(1−x)Se2x alloy films with fully tunable composition on sapphire substrates via a facile single-step chemical vapor deposition method. The ZrS2(1−x)Se2x alloys exhibit good single crystallinity and epitaxial quality, as well as uniform elemental distribution, and the epitaxial relationship with the substrate is determined to be ZrS2(1−x)Se2x (0001)[10-10]∥sapphire (0001)[11-20]. The bandgap of ZrS2(1−x)Se2x alloy exhibits a pronounced bowing behavior with continuously tunable bandgaps from 1.86 to 1.15 eV, depending on the Se composition. The ZrS2(1−x)Se2x-based photodetectors demonstrate a sensitive photoresponse to visible light with a fast response time of ∼100 µs, and their performances are significantly improved as the Se composition decreases. This work provides an efficient way to synthesize ZrS2(1−x)Se2x alloys with fully tunable bandgaps, providing great flexibility in designing TMD-based optoelectronic devices.

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.

Anisotropic material depletion in epitaxial polymer crystallization.

The physical properties of a semicrystalline polymer thin film are intimately related to the morphology of its crystalline domains. While the mechanisms underlying crystallization of flat-on oriented polymer crystals are well known, similar mechanisms remain elusive for edge-on oriented thin films due to the propensity of substantially thin films to adopt flat-on orientations. Here, we employ an epitaxial polymer-substrate relationship to enforce edge-on crystallization in thin films. Using matrix-assisted pulsed laser evaporation (MAPLE), we deposit films in which crystal nucleation is spatially separated from subsequent epitaxial crystallization. These experiments, together with phase-field simulations, demonstrate a highly anisotropic and localized material depletion during edge-on crystallization. These results provide deeper insight into the physics of polymer crystallization under confinement and introduce a processing motif in the crystallization of ultrathin structured films.

Epitaxial van der Waals contacts of 2D TaSe2-WSe2 metal-semiconductor heterostructures.

The electronic contact between two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors and metal electrodes is a formidable challenge due to the undesired Schottky barrier, which severely limits the electrical performance of TMD devices and impedes the exploration of their unconventional physical properties and potential electronic applications. In this study, we report a two-step chemical vapor deposition (CVD) growth of 2D TaSe2-WSe2 metal-semiconductor heterostructures. Raman mapping confirms the precise spatial modulation of the as-grown 2D TaSe2-WSe2 heterostructures. Transmission electron microscopy (TEM) characterization reveals that this two-step method provides a high-quality and clean interface of the 2D TaSe2-WSe2 heterostructures. Meanwhile, the upper 1T-TaSe2 is formed heteroepitaxially on/around the pre-synthesized 2H-WSe2 monolayers, exhibiting an epitaxial relationship of (20-20)TaSe2//(20-20)WSe2 and [0001]TaSe2//[0001]WSe2. Furthermore, characterization studies using a Kelvin probe force microscope (KPFM) and electrical transport measurements present compelling evidence that the 2D metal-semiconductor heterostructures under investigation can improve the performance of electrical devices. These results bear substantial significance in augmenting the properties of field-effect transistors (FETs), leading to notable improvements in FET mobility and on/off ratio. Our study not only broadens the horizons of direct growth of high-quality 2D metal-semiconductor heterostructures but also sheds light on potential applications in future high-performance integrated circuits.

Reducing the β-Ga2O3 Epitaxy Temperature to 240 °C via Atomic Layer Plasma Processing

In this work, we report on low-temperature as-grown single-phase epitaxial beta-gallium oxide (β-Ga2O3) films on c-plane sapphire at the substrate temperature of 240 °C using plasma-enhanced atomic layer deposition (PEALD). The films were deposited using triethylgallium (TEG) and Ar/O2 plasma as the metal precursor and oxygen co-reactant, respectively. Growth experiments were performed with an rf-power level of 50 W. Additionally, each unit ALD cycle was followed by in situ Ar-plasma annealing treatment, which consisted of Ar-plasma exposure for 20 s at 250 W rf power. X-ray diffraction (XRD) of the sample on sapphire revealed epitaxial Ga2O3 film signature with the monoclinic β-phase. On the other hand, GIXRD of the samples grown on Si and glass displayed polycrystalline β-Ga2O3 films. High-resolution transmission electron microscopy (HR-TEM) confirmed the epitaxial relationship of the Ga2O3 layers grown on sapphire substrates. X-ray photoelectron spectroscopy (XPS) shed light on the chemical bonding states of pure β-Ga2O3 layers outlining the Ga–O bonding states. STEM-EDX measurements confirmed the pristine nature of the β-Ga2O3 film with carbon atomic composition falling below the tool’s detection levels.

MoS2 -Mediated Epitaxial Plating of Zn Metal Anodes.

Metal-based anodes (Li, Zn, etc.) are regarded as promising solutions for next-generation advanced batteries due to their high theoretical specific capacities. However, most of these metal anodes suffer from dendrite growth, which severely restricts their practical applications. Recently, epitaxial anode metal deposition by choosing a suitable substrate has received tremendous attention as an effective strategy to suppress dendrites. However, the epitaxial relationship between plated metal and the substrate has been a subject of debate. Herein, large-area, mono-orientated 2D material (MoS2 ) is used, for the first time, to electrodeposit truly epitaxial Zn anodes. The continuous (without edges) mono-orientated MoS2 films are shown to be an effective strategy for suppressing metal dendrites. In addition, the epitaxial nature of the electrodeposited Zn anode is proven by pole figure analysis, which provides the first demonstration of truly epitaxial Zn anode growth over large area as metal anode protection strategy through epitaxy.

Epitaxy of tungsten on polycrystalline molybdenum using chemical vapor transport deposition technology

Polycrystalline tungsten (W) coatings were deposited on molybdenum (Mo) polycrystalline substrates via chemical vapor transport deposition (CVTD). The structural characterization of the samples indicates an epitaxial relationship between W and Mo; however, there are voids at the W–Mo interface, and the number of voids increases as the temperature decreases. Further, the deposition rate of W coatings decreases with decreasing temperature and pressure. The grains of the coatings deposited at 1420–1590 K have diverse orientations when the pressure is 250 Pa; however, the [100]-oriented grains are the majority. When the temperature reaches 1590 K, penetration twins form inside the coatings as the pressure decreases from 250 to 50 Pa. Further, the coating grains have no obvious preferred orientation with decreasing pressure. Compositional penetration was discovered at the W–Mo interface, and the penetration depth of Mo into W decreased with decreasing temperature. This study analyzes the competitive growth of W grains and the formation of penetration twins according to influence of the environment on the growth process. The voids and compositional penetration at the W–Mo interface are caused by the diffusion at the defects; thus, the distribution of the diffusion sites can demonstrate the perfection of the epitaxial W–Mo coherent interface. This study contributes to the structural regulation of epitaxial W on Mo.

Influence of the temperature and substrate modification on the formation of continuous GaSb film on Si(111) by solid phase epitaxy

The solid-state formation of gallium antimonide on Si(111) from a stoichiometric mixture of GaSb in the temperature range of 300 °C–500 °C and thicknesses of 12–40 nm was studied under ultrahigh vacuum conditions. The influence of the preformed GaSb seed islands on the morphology, composition, and structure was studied. It has been found that at 300 °C a strained continuous polycrystalline film is formed, which rupture at 350 °C. It has been shown that a continuous single-crystal GaSb film grows at 400 °C–500 °C if the sample is annealed at a weak antimony flow. This is also facilitated by the preliminary formation of a high density of nanosize GaSb seed islands. As a result, a continuous relaxed film with epitaxial relations GaSb(111)∣∣Si(111) and GaSb[1–10]∣∣Si[1–10] was obtained from a GaSb mixture 40 nm thick at 500 °C. We demonstrate a possibility of direct formation of GaSb on Si(111) without buffer layers of other chemical elements.

Epitaxial Growth of AgxCu1−xI on Al2O3(0001)

Herein, the epitaxial growth of alloy layers and isolated small crystals on using close distance sublimation (CDS) technique is presented. Single‐phase thin films are fabricated up to an Ag content of , while at higher Ag contents the β‐phase is also observed. The epitaxial relationships between the deposited layers and the substrate as well as the structure type are discussed for different alloy compositions. In addition, a further approach is presented for deposition of polycrystalline single‐phase thin films for Ag contents up to based on the solid‐state reaction of AgI layers on substrate with CuI. Furthermore, for differently prepared samples it is shown that the near‐band‐edge emission at 2 K is dominated by excitonic recombination, whereas the spectral position of the emission profile can be tuned by the alloy composition.