Search

Epitaxial heterostructures based on oxide perovskites and III-V, II-VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics.sup.1-7. Halide perovskites--an emerging family of tunable semiconductors with desirable properties--are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers.sup.8-15. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration.sup.16,17. Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility, which leads to interdiffusion and large junction widths.sup.18-21, and owing to their poor chemical stability, which leads to decomposition of prior layers during the fabrication of subsequent layers. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance.sup.22-26. Here we report an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid [pi]-conjugated organic ligands. We demonstrate highly stable and tunable lateral epitaxial heterostructures, multiheterostructures and superlattices. Near-atomically sharp interfaces and epitaxial growth are revealed by low-dose aberration-corrected high-resolution transmission electron microscopy. Molecular dynamics simulations confirm the reduced heterostructure disorder and larger vacancy formation energies of the two-dimensional perovskites in the presence of conjugated ligands. These findings provide insights into the immobilization and stabilization of halide perovskite semiconductors and demonstrate a materials platform for complex and molecularly thin superlattices, devices and integrated circuits. An epitaxial growth strategy that improves the stability of two-dimensional halide perovskites by inhibiting ion diffusion in their heterostructures using rigid [pi]-conjugated ligands is demonstrated, and shows near-atomically sharp interfaces., Author(s): Enzheng Shi [sup.1] , Biao Yuan [sup.2] , Stephen B. Shiring [sup.1] , Yao Gao [sup.1] , Akriti [sup.1] , Yunfan Guo [sup.3] , Cong Su [sup.4] , Minliang [...]

The development of two-dimensional (2D) materials has opened up possibilities for their application in electronics, optoelectronics and photovoltaics, because they can provide devices with smaller size, higher speed and additional functionalities compared with conventional silicon-based devices.sup.1. The ability to grow large, high-quality single crystals for 2D components--that is, conductors, semiconductors and insulators--is essential for the industrial application of 2D devices.sup.2-4. Atom-layered hexagonal boron nitride (hBN), with its excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator.sup.5-12. However, the size of 2D hBN single crystals is typically limited to less than one millimetre.sup.13-18, mainly because of difficulties in the growth of such crystals; these include excessive nucleation, which precludes growth from a single nucleus to large single crystals, and the threefold symmetry of the hBN lattice, which leads to antiparallel domains and twin boundaries on most substrates.sup.19. Here we report the epitaxial growth of a 100-square-centimetre single-crystal hBN monolayer on a low-symmetry Cu (110) vicinal surface, obtained by annealing an industrial copper foil. Structural characterizations and theoretical calculations indicate that epitaxial growth was achieved by the coupling of Cu step edges with hBN zigzag edges, which breaks the equivalence of antiparallel hBN domains, enabling unidirectional domain alignment better than 99 per cent. The growth kinetics, unidirectional alignment and seamless stitching of the hBN domains are unambiguously demonstrated using centimetre- to atomic-scale characterization techniques. Our findings are expected to facilitate the wide application of 2D devices and lead to the epitaxial growth of broad non-centrosymmetric 2D materials, such as various transition-metal dichalcogenides.sup.20-23, to produce large single crystals. The epitaxial growth of large single-crystal hexagonal boron nitride monolayers on low-symmetry copper foils is demonstrated., Author(s): Li Wang [sup.1] [sup.2] , Xiaozhi Xu [sup.1] , Leining Zhang [sup.3] [sup.4] , Ruixi Qiao [sup.1] , Muhong Wu [sup.1] [sup.5] , Zhichang Wang [sup.6] , Shuai Zhang [...]

Heteroepitaxy--atomically aligned growth of a crystalline film atop a different crystalline substrate--is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes (1-5). Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots (6-10). The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantum dots, combined in the solution phase, produce epitaxially aligned 'dots-in-a-matrix' crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantum dot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantum dots, we engineer a new platform to advance solution-processed infrared optoelectronics., Heteroepitaxy has so far largely relied on vacuum methods such as molecular-beam epitaxy, atomic-layer epitaxy, and metallo-organic vapour-phase epitaxy (1,2). These methods have been used to test and refine theoretical [...]

Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the [Formula omitted] orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact . The intergrowth is dictated by the formation of equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile-rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile-rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian-ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier-Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian-ilmenite crystals was close to Ilm.sub.67Hem.sub.33, typical for Fe-Ti-rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian-ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO.sub.2., Author(s): Aleksander Recnik[sup.1] , Nadezda Stankovic[sup.1] , Nina Daneu[sup.1] Author Affiliations: (1) Department for Nanostructured Materials, Jozef Stefan Institute, Jamova Cesta 39, 1000, Ljubljana, Slovenia Introduction Various intergrowths of rutile [...]

An effective method is described for improving the linearity of GaN/AlGaN high-electron mobility transistors (HEMTs) by optimizing barrier (AlGaN Layer) thickness or implementing doped GaN cap or a combination of both. Third-order intermodulation distortion and adjacent channel power ratio measurements are performed and a significant improvement is observed in linearity for an optimized architecture.

Epitaxial manganese substituted M-type barium ferrite thin films are deposited by alternating target laser ablation deposition (ATLAD) of Ba[Fe.sub.2][O.sub.4], [Fe.sub.2][O.sub.3], and Mn[Fe.sub.2][O.sub.4] targets. The crystal structure and the epitaxy of the films are investigated by X-ray diffraction. Surface morphology is studied by atomic force microscopy. Magnetic properties of the films are characterized by vibrating sample magnetometry and magnetization as a function of temperature measurements. Ferromagnetic resonance (FMR) measurements are utilized to study the dynamic properties of the films. Possible mechanisms for main FMR linewidth broadening as a result of Mn substitution, such as increased conductivity and the presence of Jahn-Teller effect associated with octahedrally coordinated [Mn.sup.3+] cations, are briefly discussed. Extended absorption X-ray fine structure measurements are performed to determine the cation distribution in the hexagonal unit cell. The observed 15-20% increase in saturation magnetization at 4 K and 50 K increase in the Neel temperature in comparison to bulk reference values are attributed to differences in cation distribution as a result of atomic scale deposition by the ATLAD technique. Index Terms--Cation substitution, ferromagnetic resonance, hexagonal ferrite, laser ablation deposition.

Strontium ferrite (Sr[Fe.sub.12][O.sub.19]) thin films have been deposited on thermally oxidized silicon wafer with Au underlayer by using DC magnetron sputtering system. Crystallization of the strontium hexaferrite phase on Au underlayer is achieved for deposition temperature as low as 475[degrees]C. The intensity of (111) diffraction line for Au and that of (00l) diffraction line for strontium ferrite decreases with increasing substrate temperature of underlayer. The maximum of coercivity and remanent squareness ratio in perpendicular direction are 5.7 kOe and 0.86, respectively, at substrate temperature of underlayer of 200[degrees]C. Index Terms--Au, hexagonal ferrite, perpendicular recording, sputtering, strontium ferrite.

Sm-Co thin films were prepared on Cu(111) single-crystal underlayers by changing the Sm composition from 12 to 35 at.% and the effects of Co/Sm composition on the ordered phase formation were investigated. The epitaxial Sm[Co.sub.5] thin films were obtained for all samples. The Sm[Co.sub.5] epitaxial layer grown on Cu underlayer has two different types of domains whose orientations are rotated around film normal by 30 degrees to each other. Cu atoms substitute a part of the Co sites in the Sm[Co.sub.5] structure, and an ahoy compound of Sm[(Co,Cu).sub.5] phase was formed for all samples. The Sm[(Co,Cu).sub.5] phase was more Cu enriched with increasing the Sm composition. The Cu atom diffusion into Sm-Co layer plays an important role in assisting the formation of Sm[(Co,Cu).sub.5] ordered phase. Index Terms--Cu(111) single-crystal underlayer, epitaxial growth, ordered phase, Sm[Co.sub.5] thin film.

A systematic study of in-plane and out-of-plane exchange biases in FePt/FeMn multilayers epitaxially deposited onto MgO(100), MgO(110), MgO(111) substrates by ion beam sputter-deposition system was performed. It is found that the surface and volume anisotropy constants are largest when samples are growth in (100) direction. While the FePt thickness dependence of the blocking temperature show a similar behavior for samples grown with different crystalline orientations, the dependences of exchange bias field and the unidirectional anisotropy constant on FePt thickness behave differently for different orientations. The ratio of in-plane and out-of-plane exchange biases is not only strongly dependent on crystalline orientation but also on thickness of FePt layers suggesting that the spin structure at the interface in multilayer thin films may not be the same as that of the antiferromagnetic bulk. Index Terms--Blocking temperatures, multilayer thin films, out-of-plane exchange bias, surface anisotropy.

Magnetic properties as a function of film thickness (2-55 nm) for magnetite ([Fe.sub.3] [O.sub.4]) thin films grown on MgO (100) substrates show that the small thickness films ([less than or equal to] 5 nm) are ferromagnetic and possess magnetization values which are much greater than that of bulk [Fe.sub.3] [O.sub.4]. The inverse thickness dependence of the magnetization points to an interface related mechanism. Whereas, our detailed spin polarized density functional theoretical calculations suggests that there is only a marginal enhancement in the local spin moments of Fe-atom in the vicinity of the [Fe.sub.3] [O.sub.4]-MgO interface and it turns out that the noncompensation of spin moments between the two magnetic sublattices of spinel [Fe.sub.3] [O.sub.4 ]is a major factor contributing to the enhancement of magnetization. Index Terms--Epitaxial layers, interface phenomena, magnetite, surfaces.

Epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs) were grown on MgO(001) substrates by molecular beam epitaxy. The epitaxial structure has been characterized using in-situ reflection high energy diffraction, ex-situ X-ray diffraction, and high-resolution transmission electron microscopy. The magnetic and transport properties have been investigated using vibrating sample magnetometry and physical properties measurement system. The temperature dependence of the TMR ratio, whose value rises from 170% at room temperature to 318% at 10 K, is found to be nonlinear and is fitted using a model based on the effect of magnetic disorder of the electrodes on the spin polarized tunneling. Index Terms--Magnetic disorder, Magnetic Tunnel Junctions (MTJs), Molecular Beam Epitaxy (MBE).

The characteristics of InGaAsSb/AlGaAsSb phototransistors are discussed. Findings reveal the observation of uniform resistivity between 1.2- and 2.4-[mu]m region with a mean value of 1000 A/W.

A low-pressure metal organic vapor phase epitaxy method is used to investigate the growth of GaAs metal gate semiconductor field effect transistor (MESFET) structures on a Si substrate. The MESFET device has displayed good current-voltage characteristics.

A structure of two-epitaxial-layer that was proposed in order to create a step doping profile to reduce the compensation effect is discussed. It is seen that the double-epilayer structure leads to 14% higher device output saturation current when compared to the single epilayer.

[Al.sub.1-x][ln.sub.x]N (0001) thin films of the pseudobinary AlN-InN system are grown epitaxially onto (111)-oriented MgO wafers with seed layers of T[i.sub.1-y]Z[r.sub.y]N by dual direct current magnetron sputtering under ultrahigh vacuum conditions. The highest relative deviations are found at low InN mole fractions while the largest absolute deviation is found at x=0.63, indicating that Vegard's rule is not directly applicable to determine the compositions in the wurtzite [Al.sub.1-x][ln.sub.x]N system.

Epitaxial silver tantalate niobate, Ag([Ta.sub.0.5][Nb.sub.0.5])[O.sub.3] (ATN), thin films on (001)SrRu[O.sub.3]/(001)LaAl[O.sub.3] substrates are deposited by a chemical solution deposition method. The crystal structure and dielectric properties of (00l) epitaxial ATN films on (001)SrRu[O.sub.3]/(001)LaAl[O.sub.3] (SRO/LAO) substrates are examined.

A detailed investigation of embedded source/drain SiGe stressors (eSiGes) on a silicon-on-insulator substrate for pMOS performance enhancement is presented. It is found that the integration with undoped SiGe epitaxy suffers strain relaxation from a post epitaxy implantation and SiGe growth with in situ doping is able to retain high strain for carrier enhancement.

A high tangential signal sensitivity (TSS) zero-bias GaAs planar doped barrier (PDB) diode for microwave and millimeter-wave power detection applications is presented. The fabricated PDB diodes have shown 4 dB better TSS at 35 GHz than that of reported devices, considerably increasing the minimum detectable power and widening the dynamic range. The high TSS was obtained by optimizing the PDB layer structures, namely, the [sigma]-doped [p.sup.++] layer and the two intrinsic layers, and by employing ion bombardment to better define the device and reduce parasitic effects. The isolation properties of ion bombarded epitaxial layers on GaAs substrates were examined and optimized to have a sheet resistivity of [l0.sup.8] [ohm]/sq. The temperature dependence of the barrier height of the PDB diode has been investigated experimentally, showing positive temperature coefficient and, hence, better thermal stability. We have also defined the critical barrier height and derived its analytical expression, which gives the theoretically lowest possible barrier height of a PDB diode. Index Terms--Ion bombardment, planar doped barrier (PDB) diodes power detectors and sensors, tangential signal sensitivity (TSS).

An investigation on the effect of ambient atmosphere on metal-to-insulator transition (MIT) in strained vanadium dioxide (VO2) ultrathin films (7-8 nm) grown epitaxially on TiO2 (001) single crystal substrate is conducted by pulsed laser deposition. Results reveal that the reduction of the MIT temperature is found to correlate with shortening c-axis length when decreasing the substrate temperature, indicating the significant strain effects.

A study demonstrates plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN.

The strain-induced defects and their effects on structural and optical properties in ZnO epitaxial layers deposited on SiC substrates are demonstrated. It is learnt that the biaxial strain in ZnO/SiC heterostructure is the main driving force for generation of defects, together with impurities and imbalanced interfaces.

Sr-ferrite films were prepared by heat treating spinel ferrite films containing 2 at. % of Sr at 1123 K for 1 h under an oxygen pressure of 4x[10.sup.-3] Pa. Data measured with a vibrating sample magnetometer revealed that the film exhibited a magnetic uniaxial anisotropy axis perpendicular to the film plane and the coercivity was 2.5 kOe.

Low-temperature photoluminescence (PL) measurements of Ga[As.xub.1-x][N.sub.x] epitaxial layers grown on GaAs have revealed an anomalous second peak in solid-source molecular beam epitaxy. The lower energy peak (gamma) is due to the accumulation of N content near the GaAs/GaAsN interface and hence this abnormal annealing behavior has occurred in layers with nonuniform N concentration at the GaAsN/GaAs interface.

A high-temperature anneal-resistant process, called promoting emitter diffusion (PED), which enables high-speed SiGe HBTs to embed scaled CMOS, is optimized in SiGe BiCMOS technology. The PED process is based on enhanced phosphorus diffusion from poly-Si emitter electrodes at high temperature to fabricate thin base layers and shorten the base transit time, and is effective in reducing an extrinsic base resistance due to deep boron diffusion from poly-Si base electrodes.

The actual growth process of ZnO on Si catalyzed by Au is not of real vapor-liquid-solid (VLS) character in most cases. It is found that if the O2 partial pressure during the initial growth stage is too low, a thin layer of SiO2 and big liquid droplets of Zn-rich alloys are formed first on the substrate surface.

Three methods used for fabricating nanostructure from GaAs-AlGaAs molecular beam epitaxy (MBE) is reviewed. The result demonstrates that the quantum wire laser is significantly more stable in emission energy versus pump power than are corresponding two dimensional quantum well lasers.

The detailed characteristics of long-wave-length infrared InP-InGaAs quantum-well infrared photodetectors (QWIPs) and 640 x 512 focal plane array (FPA) grown by molecular beam epitaxy are reported. The results show the potential of InP-InGaAs QWIPs for thermal imaging applications requiring high responsivity and short integration times.

A study is carried out on the dynamic solid phase epitaxy of alpha-quartz during Ba(super +) ion implantation at 300-1170 K, and its relationship to cathodoluminescence emission are reported. The results show that samples first implanted with Ba ions and then postannealed in air or (super 18)O(sub 2) atmosphere up to 1320 K did not show full epitaxy of the amorphized layer.

Graphite, graphite-tantalum (Ta) and Ta growth environment to the sublimination epitaxy of aluminum nitride (AIN) is implemented, and morphological and cathodoluminescence emission properties of AIN crystals are studied. Three different types of crystallites form in the three different type of growth environment, which presumably manifests the relationship between crystallite-habit-type and impurities are presented.

GaAs were grown directly on Si substrates at low temperature (-250C) to enable monolithic integration of low-temperature-GaAs photoconductive switches with finished Si-CMOS circuits. In situ RHEED patterns showed a clean, reconstructed Si surface before growth and reconstructed GaAs film after postgrowth annealing.

An account of the microwave performance of AlGaN/GaN HEMT at large drain bias is presented. The device structures were grown by organometallic vapor phase epitaxy on SiC substrates with a channel sheet resistance less than 280 ohms/square.

Molecular beam epitaxy of germanium is used along with kinetic Monte Carlo simulations to study time-varying processing parameters and their effect on surface morphology. The study presented is an initial demonstration of a general approach that can be used to optimize properties in other materials and deposition processes.

The surface morphology of GaN epitaxial films grown by plasma-assisted molecular-beam epitaxy was investigated. The study shows that the surface morphology is sensitive to the N to Ga flux ratio (N/Ga) when grown at a high temperature of 788 degree Celsius.

Limited-thickness homoepitaxial growth on a Si(001) surface at low temperatures was investigated by using the classical molecular-dynamics method with the Stillinger-Weber potential. Tracking a few atoms on the substrate surface reveals that these transient anisotropic surface structures are created by the breaking and reconstruction of dimers due to the impingement of deposited atoms and it is concluded that breakdown of epitaxy occurs when such renewal of surface dimers ceases.

The formation, the chemical and electronic properties, and the thermal stability of the Cu/ZnS(001) interface are investigated by x-ray photoelectron spectroscopy, x-ray fluorescence spectroscopy, and low energy electron diffraction (LEED). The study concludes that the Cu film grows in an island mode at room temperature, whereas a partially ordered arrangement is adopted at ZnS(001) surface following Cu deposition at 320 degree Celsius.

The Fe/Cr/Fe trilayers are epitaxially grown on atomically flat GaAs(001). The superconducting quantum interference device measurements confirm the interpretation of the magneto-optical Kerr effect (MOKE) loops and demonstrate homogeneity and sharpness of the interfaces in the structures.

Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials.

Ion-implanted Sb is activated with higher concentrations than at thermal equilibrium during solid phase epitaxy, this Sb being deactivated at thermal equilibrium during the subsequent thermal process. The onset of the increase in resistance is shown as being related to the diffusion length of Sb.

The device properties of the BiCMOS technology using selective epitaxial growth (SG) and lateral solid phase epitaxy (LSPE) are described. The 3-D BiCMOS technology is useful for lower power, high-density and high-speed integrated circuits (ICs) applications.