Vicinal Substrates Research Articles

Magnetization switching by spin-orbit torque in crystalline (Ga,Mn)(As,P) film deposited on a vicinal GaAs substrate

Magnetization switching by spin-orbit torque in crystalline (Ga,Mn)(As,P) film deposited on a vicinal GaAs substrate

Geometrically Controlled Microscale Patterning and Epitaxial Lateral Overgrowth of Nitrogen-Polar GaN.

In this Letter, we report on a novel two-step epitaxial growth technique that enables a significant improvement of the crystal quality of nitrogen-polar GaN. The starting material is grown on 4° vicinal sapphire substrates by metal-organic vapor-phase epitaxy, with an initial high-temperature sapphire nitridation to control polarity. The material is then converted to a regular array of hexagonal pyramids by wet-etching in a KOH solution and subsequently regrown to coalesce the pyramids back into a smooth layer of improved crystal quality. The key points that enable this technique are the control of the array geometry, obtained by exploiting the anisotropic behavior of the wet-etch step, and the use of regrowth conditions that preserve the orientation of the pyramids' sidewalls. In contrast, growth conditions that cause an excessive expansion of the residual (0001̅) facets on the pyramids' tops cause the onset of a very rough surface morphology upon full coalescence. An X-ray diffraction study confirms the reduction of the threading dislocation density as the regrowth step develops. The analysis of the relative position of the 0002̅ GaN peak, with respect to the 0006 sapphire peak, reveals a macroscopic tilt of the pyramids, probably induced by the large off-axis substrate orientation. This tilt correlates very well with an anomalous broadening of the 0002̅ diffraction peaks at the beginning of the regrowth step.

Electrical Properties of N‐Polar GaN/AlGaN/AlN Grown via Metal‐Organic Vapor Phase Epitaxy

The metal‐organic vapor phase epitaxy (MOVPE) method faces several challenges when used for the growth of N‐polar GaN on foreign substrates, including the presence of a rough surface morphology characterized by step bunching or hexagonal hillocks. In this study, it is aimed to address these issues by establishing optimal growth conditions for the MOVPE method, enabling the growth of N‐polar GaN/Al0.9Ga0.1N/AlN heterostructures with a smooth surface morphology on a vicinal sapphire substrate. The formation of 2D electron gas (2DEG) in N‐polar GaN/AlGaN/AlN prepared using MOVPE is discussed. Additionally, in the study, the impact of growth conditions, such as temperature and V/III ratio, on the electrical properties of N‐polar GaN is investigated. In the results, it is revealed that growth at lower temperatures and a V/III ratio of 30 000 effectively suppresses 3D growth. Moreover, an increase in the V/III ratio correlates with a decrease in residual impurity concentrations (C and H); hence, electron mobility improves. Moreover, the N‐polar GaN/AlGaN/AlN field‐effect transistor, grown under optimized conditions, exhibits a higher maximum drain–source current (IDmax). In these results, possibilities are broadened for the high performance of N‐polar GaN channel high‐electron‐mobility transistors through MOVPE.

Large Area Epitaxial Lateral Overgrowth of Semipolar (11¯$1 \left(\right. \macr \left.\right)$01) GaN Stripes on Patterned Si Substrates Prepared using Maskless Lithography

Selective area growth and epitaxial lateral overgrowth (ELOG) of semipolar (101) GaN stripes are demonstrated on a trench patterned vicinal (001) Si substrate fabricated by a maskless photolithography‐based process. High precision alignment enables selective mask formation to one sidewall of the trench. Selective area growth of GaN stripes is conducted from the (111) plane sidewall of Si, and ELOG region reaches ≈13 μm. The ELOG GaN crystal is dislocation‐free at most areas. The semipolar GaN stripes with atomically flat surface morphology are uniformly obtained. Light‐emitting diode structures with InGaN/GaN multiple quantum wells are grown on the 13‐μm ELOG (101) GaN stripes and a single photoluminescence emission peaked at 485 nm is obtained, suggesting potential for the cost‐effective semipolar micro light‐emitting diode fabrication technologies.

Universal Dimensionality of Ferroelectric Domain Walls in Ultrathin Films

AbstractThe dimensionality of dynamic interfaces—domain walls (DWs) —is greatly influenced by symmetry and physical dimensions, irrespective of the microscopic details of the system. To address this fundamental question for the ferroelectric model system, the DW scaling criticality and dimensionality is investigated in ultrathin films of varied ferroelectric materials, compositions, and electrode–ferroelectric interfaces, grown on nominally flat and vicinal substrates. In spite of significant variations among ferroelectric systems, the observed prevalence of 1D DWs is consistent with a random bond disorder, elucidated through a scenario of 2D nucleation and growth‐driven DW creep. These findings highlight the thickness size‐dominated universal behavior of ferroelectric DWs, uncovering fascinating prospects for dimensionally engineered DW‐based nanoelectronics.

The effect of surface-step-terrace on energy storage density and working temperature of BaZr0.2Ti0.8O3 epitaxial films

Dielectric film capacitors have been widely used in various electronic devices due to the high energy storage density and high charge–discharge speed. The energy storage performances are highly determined by the strain status of the films. Herein, we improved the energy storage performances of epitaxial BaZr0.2Ti0.8O3 film through surface-step-terrace in the vicinal substrate. It is found that the terraces in the vicinal substrate create an additional strain parallel to the terraces and a compressive strain along the out-of-plane direction. The strains result in the increase in the energy storage density of the BZT film up to 66.9 J/cm3 with an energy storage efficiency of 73.2% at room temperature, which is 2.2 times as high as that of the films on regular substrates. Moreover, the strains also lead to the reduction of the hysteresis loss and increase in the up-limited working temperature from 125 to 225 °C with an energy storage density of 52.35 J/cm3 and an efficiency of 70.25%, exhibiting excellent thermal stability. Our work demonstrates that the strains generated by vicinal substrates can improve the energy storage performance greatly and provide a method to improve the energy storage performances of dielectric thin film capacitors.

Field-free magnetization switching with full scale in Pt/Tm3Fe5O12 bilayer on vicinal substrate

The spin–orbit torques within a Pt/Tm3Fe5O12 (TmIG) bilayer offer an expedient method for manipulating the magnetization of TmIG. However, the practical application of TmIG is hindered by the presence of an external field during switching. Here, we demonstrate field-free magnetization switching in Pt/TmIG bilayer on a vicinal substrate with minimal sacrifice to the perpendicular magnetic anisotropy (PMA) of TmIG. With the assistance of tilt PMA, reversible perpendicular magnetization switching is realized in the absence of an external field. Our results offer an alternative solution for achieving field-free perpendicular magnetization switching in a Pt/TmIG bilayer, thereby fostering the advancement of emerging SOT-based devices.

Low‐Temperature Photoluminescence Investigation of Carbon‐Doped AlGaAs Grown by MOVPE on Vicinal Substrates

AlGaAs is promising absorber material for multi‐junction solar cells (MJSC) for its tunable bandgap to match various bandgap combination designs. However, the path to achieving high‐quality AlGaAs for solar cell application remains to be determined. In this study, we grow and characterize carbon‐doped p‐Al0.375Ga0.625As on exact and vicinal GaAs(0 0 1) substrates, using metal‐organic vapor phase epitaxy (MOVPE). Low‐temperature photoluminescence (PL) measurement is performed in the range from 6 K to 300 K. The Arrhenius plot of PL intensities reveals an increasing trend for non‐radiative recombination in conjunction with increasing miscut angle. The presence of atomic ordering that inversely correlates to the miscut angle is proposed to explain the PL linewidth and peak energy dependency on substrate orientation. The conflicting tendency of non‐radiative recombination and ordering with miscut angle are further validated using solar cell devices. A slightly vicinal substrate is recommended for AlGaAs solar cells according to our observation.

Encapsulation of a bis-amino phenol precatalyst for the deoxydehydration of 1,2-hexanediol

Nanoreactors encapsulating homogeneous catalysts behave as heterogeneous catalysts at the macroscopic scale, enabling the homogeneous catalyst to be easily recovered. One way to synthesize such nanoreactors is by inserting the homogeneous catalyst (or synthesizing it in situ) in the porosity of an inorganic oxide material, followed by a silylation reaction to narrow the pore entrance, preventing the outwards diffusion of the jailed homogeneous catalyst during the reaction. This way, the often-superior activity of homogeneous catalysts is combined with the robustness and the easy recovery of heterogeneous ones. Hereunder, we report for the first time the implementation of such a strategy towards the synthesis of a deoxydehydration (DODH) catalyst, namely a bis-amino phenol Re-complex. 1,2-hexanediol was used as the DODH vicinal diol model substrate. The encapsulation of the homogeneous catalyst in SBA-1 was demonstrated. It proved active in the DODH test reaction although not sustainably; the deactivation mechanism was therefore elucidated.

Terahertz Probing of Anisotropic Conductivity and Morphology of CuMnAs Epitaxial Thin Films

AbstractAntiferromagnetic CuMnAs thin films have attracted attention since the discovery of the manipulation of their magnetic structure via electrical, optical, and terahertz pulses, enabling convenient approaches for switching between magnetoresistive states of the film for information storage. However, the magnetic structure and, thus, the efficiency of the manipulation can be affected by the film morphology and growth defects. In this study, the properties of CuMnAs thin films are investigated by probing the asymmetrical growth‐related uniaxial anisotropy of electric conductivity by contact‐free terahertz transmission spectroscopy. It is shown that the terahertz measurements conveniently detect the conductivity anisotropy that is consistent with conventional DC Hall‐bar measurements. Moreover, the terahertz technique allows for considerably finer determination of anisotropy axes, and it is less sensitive to the local film degradation. Thanks to the averaging over a large detection area, the THz probing also allows an analysis of strongly non‐uniform thin films. Using scanning electron and near‐field terahertz microscopies, the observed anisotropic conductivity of CuMnAs is related to the elongation and orientation of growth defects, which both originate in the anisotropic growth of the films. In addition, control over the morphology of defects is demonstrated by using vicinal substrates.

Width-dependent band gap of arm-chair graphene nanoribbons formed on vicinal SiC substrates by MBE

Formation and electronic states of graphene nanoribbons with arm-chair edges (AGNR) are studied on the SiC(0001) vicinal surfaces toward the [] direction. The surface step and terrace structures of both 4H and 6H-SiC substrates are used as the growth templates of one-dimensional arrays of AGNRs, which are prepared using the carbon molecular beam epitaxy followed by hydrogen intercalation. A band gap is observed above the π-band maximum by angle-resolved photoelectron spectroscopy (ARPES) for the both samples. The average widths of the AGNRs are 6 and 10 nm, and the estimated average band gaps are 0.40 and 0.28 eV for the 4H- and 6H- substrates, respectively. A simple and phenomenological inverse relation between the energy gap and AGNR width works in the analyses of the ARPES data.

Effect of a niobium-doped PZT interfacial layer thickness on the properties of epitaxial PMN-PT thin films

We are reporting on high quality epitaxial thin films of [Pb(Mg1/3Nb2/3)O3]0.67-(PbTiO3)0.33 [PMN-PT (67/33)]. These films were deposited on (001) oriented, vicinal SrTiO3 single crystal substrates, using 1 mol. % niobium-doped Pb(Zr0.52,Ti0.48)O3 (Nb-PZT) as an interfacial layer. The functional properties of the epitaxial PMN-PT (67/33) thin films were investigated as a function of the layer thickness of the Nb-PZT layer. The deposited hetero-structures are perovskite phase pure and fully (001)-oriented. The variation in Nb-PZT interfacial layer thickness results in an increasing trend change of the in-plane lattice parameter of that layer, which in turn causes a decrease in the c/a ratio of the PMN-PT film on top. The most noticeable effect related to this is a decrease in built-in-bias (imprint) voltage. Thus, the built-in bias can be tuned by changing the interfacial layer thickness. The ferroelectric capacitor properties are found to be most stable for the thinnest interfacial layers under a high number (108) of switching cycles.

Polarity Inversion of GaN via AlN Oxidation Interlayer Using Metal–Organic Vapor Phase Epitaxy

The polarity inversion of GaN films grown on vicinal sapphire substrates is demonstrated using the metal–organic vapor phase epitaxy (MOVPE) method via an AlN oxidation interlayer for application in wavelength conversion devices. By annealing the N‐polar AlN surface at 900 °C in air, the pit‐free Ga‐polar GaN layer can be regrown. Anisotropic etching in KOH and transmission electron microscopy confirms the formation of a GaN polarity‐inverted structure. X‐ray diffraction measurements reveal an increase in threading dislocations due to the nucleation island formation by low‐temperature GaN layer and strain relaxation at the regrown interface. The simple fabrication process of the GaN polarity‐inverted structure using an AlN oxidation interlayer will pave the way for applications in new devices that require polarity‐controlled and/or polarity‐inverted stacking structures.

Improvements in characteristics of N-polar Si-doped AlGaN epi-layer grown on mis-oriented c-plane sapphire substrate

The N-polar Si-doped n-type AlGaN epi-layer with an Al composition up to 54% was successfully grown on vicinal c-plane sapphire substrates by metal-organic chemical vapor deposition technology. High-resolution X-ray diffraction, atomic force microscopy, and Hall effect measurement were used to evaluate the influence of the use of vicinal sapphire substrate on the crystalline quality, the surface morphology, and the electrical property of the N-polar Si-doped AlGaN epi-layer, respectively. The characterization results reveal that the use of an optimized vicinal sapphire substrate plays a vital role in the improvements in the crystalline quality, the surface morphology, and the electrical property for the N-polar Si-doped n-type AlGaN epi-layers. In fact, a relatively smooth surface morphology with a root-mean-square value as small as 17.3 nm, an electron concentration of 1.9 × 1019 cm−3, and an electron mobility of 23.5 cm2 V−1 S−1 were achieved with the N-polar Si-doped n-type AlGaN epi-layer sample grown on a 1° mis-oriented c-plane sapphire substrate in this study.

Squeezed Abrikosov-Josephson Vortex in Atomic-Layer Pb Superconductors Formed on Vicinal Si(111) Substrates

Unlike bulk counterparts, two-dimensional (2D) superconductors are sensitive to disorder. Here, we investigated superconductivity of Pb atomic layers formed on vicinal substrates to reveal how surface steps with an interval shorter than the coherence length ξ affect it. Electrical transport showed reduced critical temperature and enhanced critical magnetic field. Scanning tunneling microscopy exhibited vortices elongated along the steps, that is, Abrikosov-Josephson vortices squeezed normal to the steps due to the reduced ξ. These results demonstrate that steps work as disorder and vicinal substrates provide a unique platform to manipulate the degree of disorder on 2D superconductors.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Dislocation reduction mechanism os GaN films on vicinal sapphire substrates

<sec>GaN materials are widely used in optoelectronic devices, high-power devices and high-frequency microwave devices because of their excellent characteristics, such as wide frequency band, high breakdown electric field, high thermal conductivity, and direct band gap. Owing to the large lattice mismatch and thermal mismatch brought by the heterogeneous epitaxy of GaN material, the GaN epitaxial layer will produce a great many dislocations in the growth process, resulting in the poor crystal quality of GaN material and the difficulty in further improving the device performance. Therefore, researchers have proposed the use of vicinal substrate to reduce the dislocation density of GaN material, but the dislocation annihilation mechanism in GaN film on vicinal substrate has not been sufficiently studied. Therefore, in this paper, GaN thin films are grown on vicinal sapphire substrates at different angles by using metal organic chemical vapor deposition technique. Atomic force microscope, high resolution X-ray diffractometer, photoluminescence testing, and transmission electron microscopy are used to analyze in detail the effects of vicinal substrates on GaN materials. The use of vicinal substrates can significantly reduce the dislocation density of GaN materials, but lead to degradation of their surface morphology morphologies. And the larger the substrate vicinal angle, the lower the dislocation density of the sample is. The dislocation density of the sample with a 5º bevel cut on the substrate is reduced by about one-third compared to that of the sample with a flat substrate. The special dislocation termination on the mitered substrate is observed by transmission electron microscopy, which is one of the main reasons for the reducing the dislocation density on the mitered substrate. The step merging on the vicinal sapphire substrate surface leads to both transverse growth and longitudinal growth of GaN in the growth process. The transverse growth region blocks the dislocations, resulting in an abrupt interruption of the dislocations during propagation, which in turn reduces the dislocation density.</sec><sec>Based on the above phenomena, a model of GaN growth on vicinal substrate is proposed to explain the reason why the quality of GaN crystal can be improved by vicinal substrate.</sec>

Field-free switching through bulk spin — orbit torque in L10-FePt films deposited on vicinal substrates

L10-FePt distinguishes itself for its ultrahigh perpendicular magnetic anisotropy (PMA), enabling thermally stabile memory cells to scale down to 3 nm. The recently discovered “bulk” spin-orbit torques in L10-FePt provide an efficient and scalable way to manipulate the L10-FePt magnetization. However, the existence of an external field during the switching limits its practical application, and therefore field-free switching of L10-FePt is highly demanded. In this manuscript, by growing the L10-FePt film on vicinal MgO (001) substrates, we realize the field-free switching of L10-FePt. This method is different from previously established strategies as it does not need to add other functional layers or create asymmetry in the film structure. The dependence on the vicinal angle, film thickness, and growth temperature demonstrates a wide operation window for the field-free switching of L10-FePt. We confirm the physical origin of the field-free switching is due to the tilted anisotropy of L10-FePt induced by the vicinal surface. We also quantitatively characterize the spin-orbit torques in the L10-FePt films. Our results extend beyond the established strategies to realize field-free switching, and potentially could be applied to mass production.

Exciton Fine Structure in InAs Quantum Dots with Cavity-Enhanced Emission at Telecommunication Wavelength and Grown on a GaAs (111) A Vicinal Substrate

The effcient generation of entangled photons at telecom wavelength is crucial for the success of many quantum communication protocols and the development of fiber-based quantum networks. Entangled light can be generated by solid state quantum emitters with naturally low fine structure splitting, such as highly symmetric InAs quantum dots (QDs) grown on (111)-oriented surfaces. Incorporating this kind of QDs into optical cavities is critical to achieve sufficient signal intensitiesfor applications, but has so far shown major complications. In this work we present droplet epitaxy of telecom-wavelength InAs QDs within an optical cavity on a vicinal (2{\deg} miscut) GaAs(111)A substrate. We show a remarkable enhancement of the photon extraction efficiency compared to previous reports together with a reduction of the density that facilitates the isolation of single spectral lines. Moreover, we characterise the exciton fine structure splitting and employ numerical simulations under the framework of the empirical pseudopotential and configuration interaction methods to study the impact of the miscut on the optical properties of the QDs. We demonstrate that the presence of miscut steps influences the polarisation of the excitonic states and introduces a preferential orientation in the $C_{3v}$ symmetry of the surface.

N-polar GaN Film Epitaxy on Sapphire Substrate without Intentional Nitridation.

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer’s growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates.