Vicinal Substrates Research Articles

Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.

Purely one-dimensional bands with a giant spin-orbit splitting: Pb nanoribbons on Si(553) surface

We report on a giant Rashba type splitting of metallic bands observed in one-dimensional structures prepared on a vicinal silicon substrate. A single layer of Pb on Si(553) orders this vicinal surface making perfectly regular distribution of monatomic steps. Although there is only one layer of Pb, the system reveals very strong metallic and purely one-dimensional character, which manifests itself in multiple surface state bands crossing the Fermi level in the direction parallel to the step edges and a small band gap in the perpendicular direction. As shown by spin-polarized photoemission and density functional theory calculations these surface state bands are spin-polarized and completely decoupled from the rest of the system. The experimentally observed spin splitting of 0.6 eV at room temperature is the largest found to now in the silicon-based metallic nanostructures, which makes the considered system a promising candidate for application in spintronic devices.

Characterization of antimonide based material grown by molecular epitaxy on vicinal silicon substrates via a low temperature AlSb nucleation layer

We report on the characterization of GaSb layers grown on silicon substrates using an AlSb nucleation layer. In particular, we investigate the influence of the AlSb layer thickness when this nucleation layer is grown at low temperature (400°C). X-ray diffraction techniques, atomic force microscopy and transmission electron microscopy were used to characterize the material properties. We demonstrate that there exists a correlation between the micro-twin density, the surface roughness and the broadening of the ω-scan GaSb peaks. Moreover, the AlSb thickness has a strong influence on the micro-twin density, and must be carefully optimized to improve the GaSb quality.

Повышение быстродействия прибора PHEMT на основе GaAs с помощью профилированного дельта-легирования оловом

The PHEMT epitaxial structures with a doping profile in the form of tin nanowires were fabricated using vicinal GaAs substrate disoriented by 0.3° in relation to accurate orientation (100). Investigation of electron transport properties of the structures with nanowires of tin atoms on vicinal GaAs substrates have been presented. It was determined that non-optimal growth temperature of cap layers leads to degradation of electrophysical parameters of the samples and prevent forming of tin one-dimensional channels. Anisotropy of the saturation current on current-voltage characteristics was obtained when current flow along and across nanowires. Field effect transistors with special topology for orthogonal current flow were fabricated and frequency characteristics were measured. The evident anisotropy of the frequency characteristics for PHEMT was shown. Gain coefficient MSG (Maximum Stable Gain) of the fabricated PHEMT for current flow along nanowires correlates to best-achieved values of MSG for GaAs PHEMT.

Compositionally graded InGaN layers grown on vicinal N-face GaN substrates by plasma-assisted molecular beam epitaxy

This work reports on compositionally graded (0001¯) N-polar InxGa1−xN layers. The InGaN grades with different final In compositions xf up to 0.25 were grown by plasma-assisted molecular beam epitaxy on vicinal GaN base layers with a miscut angle of 4° towards the m-direction. When increasing xf the surface morphology evolved from an interlacing finger structure, attributed to the Ehrlich-Schwöbel effect, towards fully strain-relaxed columnar features. Regardless of the crystal morphology and the strain state each graded sample exhibited a bright photoluminescence signal at room temperature spanning the whole visible range. Cross-sectional nanoscale cathodoluminescence evidenced a red-shift of the luminesced signal from 420 to 580nm along the grade and also showed strong lateral emission inhomogeneities.

Temperature dependence of magnetic anisotropies in ultrathin Fe film on vicinal Si(111)

The temperature dependence of magnetic anisotropy of ultrathin Fe film with different thickness epitaxially grown on vicinal Si(111) substrate has been quantitatively investigated using the anisotropic magnetoresistance(AMR) measurements. Due to the effect of the vicinal substrate, the magnetic anisotropy is the superposition of a four-fold, a two-fold and a weakly six-fold contribution. It is found that the temperature dependence of the first-order magnetocrystalline anisotropies coefficient follows power laws of the reduced magnetization m(T)(=M(T)/M(0)) being consistent with the Callen and Callen's theory. However the temperature dependence of uniaxial magnetic anisotropy (UMA) shows novel behavior that decreases roughly as a function of temperature with different power law for samples with different thickness. We also found that the six-fold magnetocrystalline anisotropy is almost invariable over a wide temperature range. Possible mechanisms leading to the different exponents are discussed.

Investigation of Low Off-Angled 4H-SiC Epitaxial Wafers for Power Device Applications

We investigated the potential of low off angled 4H-SiC epitaxial wafers for use in power device applications. To this end, we successfully grew epitaxial layers on 2 degree off-angled and vicinal 4H-SiC Si-face substrate with good surface morphology and low defects density. Schottky diodes, metal oxide semiconductor (MOS) capacitors, and trench metal oxide semiconductor field effective transistors (MOSFETs) were fabricated on these epitaxial layers, and we investigated their characteristics. The results were comparable, with respect to the Schottky diodes and trench MOSFETs and the reliability of MOS capacitors, to those for similar devices fabricated on 4 degree off-angled 4H-SiC Si-face epitaxial wafers. These results suggest that low off-angled 4H-SiC Si-face epitaxial wafers may be effectively utilized in power device applications.

Influence of the vicinal surface on the anisotropic dielectric properties of highly epitaxial Ba0.7Sr0.3TiO3 thin films.

Epitaxial thin films of Ba0.7Sr0.3TiO3 (BST) were grown on the designed vicinal single-crystal LaAlO3 (001) substrates to systematically investigate the evolution of microstructures and in-plane dielectric properties of the as-grown films under the strains induced by surface step terraces. Anisotropic dielectric properties were observed, which can be attributed to different tetragonalities induced by vicinal LaAlO3 substrates with miscut orientations along the [100] and [110] directions with different miscut angles of 1.0°, 2.75° and 5.0°. A terrace geometric model with both compressive and tensile strained domains in the BST film was established, which is in good agreement with the experimental results. Our experimental studies not only shed new light on the heteroepitaxial growth mechanism, but also provide a promising platform for the design and integration of high performance device applications.

Determination of magnetic anisotropies and miscut angles in epitaxial thin films on vicinal (111) substrate by the ferromagnetic resonance

A method for determining magnetic anisotropy parameters of a thin single-crystal film on vicinal (111) substrate as well as substrate miscut angles from angular dependence of ferromagnetic resonance field has been proposed. The method is based on the following: (i) a new approach for the solution of the system of nonlinear equations for equilibrium and resonance conditions; (ii) a new expression of the objective function for the fitting problem. The study of the iron silicide films grown on vicinal Si(111) substrates with different miscut angles confirmed the efficiency of the method. The proposed method can be easily generalized to determine parameters of single-crystal films grown on substrates with an arbitrary cut.

Structural and Optical Properties of Al0.30Ga0.70N/AlN Multiple Quantum Wells Grown on Vicinal 4H p-SiC Substrates by Molecular Beam Epitaxy

ABSTRACTAlGaN based multiple quantum wells (MQWs) were grown on 8° vicinal 4H p-SiC substrates by plasma-assisted molecular beam epitaxy. The MQWs were designed to emit near 300 nm using the wurtzite k.p model. The MQW periodicity and strain state were measured with X-ray diffraction. The optical properties were characterized with temperature dependent photoluminescence (PL). The internal quantum efficiency was estimated from the ratio of room temperature to 18K integrated PL intensity. Internal quantum efficiency up to 48% was achieved. These data are encouraging for future vertical and inverted ultraviolet light emitting diodes grown on p-SiC substrates.

Pulsed laser deposition of SrRuO3 thin-films: The role of the pulse repetition rate

SrRuO3 thin-films were deposited with different pulse repetition rates, fdep, epitaxially on vicinal SrTiO3 substrates by means of pulsed laser deposition. The measurement of several physical properties (e.g., composition by means of X-ray photoelectron spectroscopy, the out-of-plane lattice parameter, the electric conductivity, and the Curie temperature) consistently reveals that an increase in laser repetition rate results in an increase in ruthenium deficiency in the films. By the same token, it is shown that when using low repetition rates, approaching a nearly stoichiometric cation ratio in SrRuO3 becomes feasible. Based on these results, we propose a mechanism to explain the widely observed Ru deficiency of SrRuO3 thin-films. Our findings demand these theoretical considerations to be based on kinetic rather than widely employed thermodynamic arguments.

Stability and controllability of InGaAs/GaAsP wire-on-well (WoW) structure for multi-junction solar cells

Wire on Well (WoW) structure embedded in a matrix is naturally formed by growing InxGa1−xAs/GaAs1−yPy strained multiple quantum wells (MQW) on vicinal substrates and employing triethylgallium (TEGa) as a precursor in low-temperature MOVPE. The structure is useful for the subcell in current-matched mult-junction solar cells with lattice-matched materials because of its ability of band-gap tuning. In this research, high density and uniform In0.30Ga0.70As/GaAs0.6P0.4 WoW was obtained up to 200 stacks and its structure was analyzed by X-ray diffraction reciprocal space mapping, atomic force microscopy and scanning transmission electron microscopy. The structure of the wire can be controlled by changing the equivalent layer thicknesses of In0.30Ga0.70As and GaAs0.6P0.4. The photoluminescence peak from the WoW shifted according to the size of InGaAs wires and the intensity was dependent on the accumulation of lattice-mismatch stress.

Increase in the emission wavelength (over 1800 nm) of InAs quantum dots grown on InP substrates using a dot‐in‐well structure

A dot‐in‐well structure for quantum dot (QD) growth on InP (001) vicinal substrates was introduced to increase the emission wavelength. The emission wavelength of the QDs changed from 1600 nm in conventional structures to 1850 nm in the proposed dot‐in‐well structure, when measured at room temperature. The emission intensity of this dot‐in‐well structure was the same as that of the conventional structure, implying that the dot‐in‐well structure did not degrade crystal quality.

Self‐organized growth of cubic InN dot arrays on cubic GaN using MgO (001) vicinal substrates

Cubic zincblende InN (c‐InN) nanoscale dot arrays are grown on cubic GaN (c‐GaN) by RF‐molecular beam epitaxy using MgO (001) vicinal substrates oriented 3.5° toward [110]. The obtained dot arrays have longer ordering length compared with those grown using conventional on‐axis (just) substrates. The c‐GaN underlayer grown on the vicinal substrate exhibits a single‐domain crystalline structure, while that grown on the just substrate is a mixture of two orthogonal crystalline domains. The change in the c‐GaN domain structure leads to the enlarged domain size and lower density of domain boundaries on the c‐GaN (001) surface in [1–10]. The longer ordering length of the c‐InN dot on the vicinal substrates is reflected by the decrease in the c‐GaN domain boundary that disrupts the lateral ordering of the dot arrays.Atomic force microscope image of c‐InN dots grown on a single‐domain c‐GaN. The sample was fabricated using a MgO (001) vicinal substrate.

Erratum: “Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength”

Erratum: “Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength”

Surface step terrace tuned microstructures and dielectric properties of highly epitaxial CaCu3Ti4O12 thin films on vicinal LaAlO3 substrates.

Controllable interfacial strain can manipulate the physical properties of epitaxial films and help understand the physical nature of the correlation between the properties and the atomic microstructures. By using a proper design of vicinal single-crystal substrate, the interface strain in epitaxial thin films can be well controlled by adjusting the miscut angle via a surface-step-terrace matching growth mode. Here, we demonstrate that LaAlO3 (LAO) substrates with various miscut angles of 1.0°, 2.75°, and 5.0° were used to tune the dielectric properties of epitaxial CaCu3Ti4O12 (CCTO) thin films. A model of coexistent compressive and tensile strained domains is proposed to understand the epitaxial nature. Our findings on the self-tuning of the compressive and tensile strained domain ratio along the interface depending on the miscut angle and the stress relaxation mechanism under this growth mode will open a new avenue to achieve CCTO films with high dielectric constant and low dielectric loss, which is critical for the design and integration of advanced heterostructures for high performance capacitance device applications.

Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2[Formula: see text]off miscut angle toward the [110] direction (2[Formula: see text]F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Pässler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Growth of InAs quantum dots on vicinal GaAs substrates by molecular beam epitaxy

Previous work shows the benefits of using vicinal substrates but there is currently a gap in the experimental studies of the effects under different MBE growth conditions. To fully realize controllable growth while using a vicinal substrate, we systematically explore and discuss the mechanism behind the dependence of the optical characteristics of MBE grown InAs QD ensembles with different growth parameters on a vicinal substrate. In addition, the potential improvement in optical quality with a vicinal substrate over an on-axis is demonstrated and an investigation into applying a two-step growth procedure on a vicinal substrate is conducted. Photoluminescence of the grown QD ensembles shows that increasing V/III ratio increased wavelength and decreased FWHM. Decreasing substrate temperature increased wavelength and FWHM. Utilizing the two-step growth method increased both wavelength and FWHM with increased interruption time.

Growth of high quality N-polar n-GaN on vicinal C-face n-SiC substrates for vertical conducting devices

High quality N-polar n-type GaN films were deposited on vicinal C-face n-SiC substrates by metal organic chemical vapor deposition for vertical conducting devices. We employed the SiNx interlayer to N-polar GaN growth and studied the effect of SiNx interlayer on the properties of N-polar GaN. We found that the properties of N-polar films were greatly improved by using SiNx interlayer as characterized by X-ray diffraction, photoluminescence, and I-V measurements. N-polar n-type GaN films with SiNx interlayer exhibited a full width at half maximum of 422 arcsec for (0002) omega scan and 335 arcsec for (101¯2) omega scan, respectively. Integrated intensity ratio of near-band-edge emission and yellow-luminescence was increased by a factor of 4 by using SiNx interlayer. Besides, the resistance of the vertical structure with SiNx interlayer was reduced by 25% in comparison with that without SiNx interlayer.

(Invited) Tuning the Strain and Physical Properties of Highly Epitaxial Perovskite Thin Films on Vicinal Substrates

Controllable interfacial strain can manipulate the physical properties of epitaxial films and help understand the physical nature of the correlation between the properties and the atomic microstructures. In this presentation, we demonstrated that the interface strain in epitaxial thin films can be well controlled by adjusting the miscutting angle of a vicinal single-crystal substrate via a surface-step-terrace matching growth mode. Epitaxial perovskite or perovskite-like oxide thin films such as CaCu3Ti4O12 (CCTO) and (Ba,Sr)TiO3 were grown on vicinal (001) single-crystal LaAlO3 (LAO) substrates with various miscutting angles of 1.0°, 2.5°, and 5.0° by a polymer assisted deposition (PAD) technique. High-resolution X-ray diffraction were employed to study the epitaxial nature and interface strains induced by the substrate step terrace dimensions. It is found that the strain is determined by the mismatch between the lattice of the thin film and the substrate step terrace dimensions. A model is proposed that both compressive and tensile strained domains would form in the film to fit the size of each step terrace whereas the ratio of the two kinds of domains can be tuned by the miscutting angle. Based on this growth mode, a proper design of the substrate step terrace dimension can tune the properties of the thin films. For example, an optimized design of a CCTO thin film on a 5o-miscutting (001) LaAlO3 substrate can maintain a similar dielectric constant and reduce the dielectric loss by half an order of magnitude compared with the films grown on the regular (001) LaAlO3 substrates, briefly by releasing a large part of the interfacial stress through the complementary strained domains instead of dislocations. This work is supported by the National Basic Research Program of China (973 Program) under Grant No. 2015CB351905, the National Natural Science Foundation of China (Nos. 51172036, 11329402, and 51372034), and the Technology Innovative Research Team of Sichuan Province of China (No.2015TD0005).