MICROSTRUCTURE AND LUMINESCENCE PROPERTIES OF GaxIn1-xP/GaAs HETEROSTRUCTURE

The Ga_xIn_1-xP/GaAs heterostructures with different x values have been investigated by photoluminescence (PL), energy dispersive spectrum (EDS), and transmission electron microscopy (TEM). Experimental results suggested that the composition of epitaxial layers evidently affected the microstructure of the heterostructures. When x value was close to 0.5, the defects in the epitaxial layer decreased, and the lattice match between film and sub-strate approached optimum, Meanwhile, the heteroatructure provided a high luminescence efficiency.     

Effect of preferred orientation on magnetoelectric properties of multiferroic La0.7Sr0.3MnO3/BaTiO3 heterostructure

The epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃ bilayer heterostructures were deposited on LaAlO₃ (001) and (110) substrates by pulsed laser deposition. The inherent ferromagnetic, ferroelectric properties and strong magnetoelectric (ME) effect at room temperature were approved, which correlated to the preferred orientation of the films. Both heterostructures showed similar frequency-dependent ME behavior in 0.1 kHz-100 kHz, the ME voltage coefficients were around 140 mV/cm Oe and 104.8 mV/cm Oe at 1 kHz for (001) and (110) oriented bilayers, respectively. This was at least one order of magnitude higher than previously reported results of the related heterostructures, which is mainly ascribed to the lower dielectric constant of BTO film.     

两步化学气相沉积法制备In2Se3/MoSe2范德华尔斯异质结

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research,but also for the application of next generation electronic and optoelectronic devices.Herein,we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In₂Se₃/MoSe₂ heterostructures.Transmission electron microscopy characterization reveals clearly that the In₂Se₃ has well-aligned lattice orientation with the substrate of monolayer MoSe₂.Due to the interaction between the In₂Se₃ and MoSe₂ layers,the heterostructure shows the quenching and red-shift of photoluminescence.Moreover,the current rectification behavior and photovoltaic effect can be observed from the heterostructure,which is attributed to the unique band structure alignment of the heterostructure,and is further confirmed by Kevin probe force microscopy measurement.The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.     

Defect formation in Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>/Ge(111) epitaxial heterostructures

Selective chemical etching and transmission electron microscopy are used to study the defect formation in Ge_1?xSi_x/Ge(111) epitaxial heterostructures at 0.01<x<0.35. As the Si content in the solid solution (SS) increases, the dislocation densities in the epitaxial layer, at the interface, and in the near-interface region in the substrate are found to vary nonmonotonically. The difference in the depth distribution of dislocations observed in the heterostructures in three different SS composition ranges is caused by the effect of the SS composition on the kinetics of misfit-stress relaxation, in particular, on the intensity of misfit-dislocation generation and multiplication. It is found that, in the heterostructures grown by hydride epitaxy at 600°C, misfit-dislocation multiplication through a modified Frank-Read mechanism occurs only in the range 0.03<x<0.20. The results obtained are explained in the context of the effect of silicon-rich microprecipitates, which form during the spinodal decomposition of the SS, on dislocation generation and motion in the epitaxial layer. A mechanism is proposed for misfit-dislocation generation by heterogeneous sources in the epitaxial layer; the mechanism is based on the generation of interstitial dislocation loops near microprecipitates.     

Photosensitive structures using Hg1−xCdxTe solid solutions grown by molecular beam epitaxy

Results of a study of the parameters of Hg_1−xCd_xTe epitaxial structures produced by molecular beam epitaxy are presented. Results are given for measurements of the recombination and spectral properties and the noise properties of photosensitive elements have been studied. A high detectivity was obtained over a broad wavelength range. The parameters of MOS photodetector elements based on isotype n-Hg_1−xCd_xTe heterostructures with compositions x=0.2 and x=0.3 were estimated and compared with experimental data on the charge carrier accumulation time in MOS structures in the nonequilibrium depletion regine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 44–49, August, 1998.     

Multiferroic behaviour of epitaxial NiFe2O4--BaTiO3 heterostructures

Multiferroic NiFe2O4 （NFO）-BaTiO3 （BTO） bilayered thin films are epitaxially grown on （001） Nb-doped SrTiO3 （STO） substrates by pulsed-laser deposition （PLD）. Different growth sequences of NFO and BTO on the substrate yield two kinds of epitaxial heterostructures with （001）-orientation, i.e. （001）-NFO/（001）-BTO/substrate and （001）- BTO/（001）-NFO/substrate. Microstructure studies from x-ray diffraction （XRD） and electron microscopies show differences between these two heterostructures, which result in different multiferroic behaviours. The heterostructured composite films exhibit good coexistence of both ferroelectric and ferromagnetic properties, in particular, obvious magnetoelectric （ME） effect on coupling response.     

Parallel and perpendicular transport in Si/CoSi2 and Si/CoSi2/Si heterostructures

It has recently become possible to grow epitaxial Si/CoSi₂/Si semiconductor-metal-semiconductor heterostructures of almost perfect crystalline quality. Electronic transport in the plane of the metal film (parallel transport) is investigated by the extensive studies of resistivity and superconducting properties of these films. The sharp influence of film thickness on both phenomena is presented and its physical origin is briefly discussed. The transfer of hot electrons emitted by the top Si_epi/PrmCoSi₂ diode to the Si_bulk/CoSi₂ through the metal film (perpendicular transport) is studied. Experimental data strongly favor the hypothesis of ballistic transport with a ballistic mean free path close to the one deduced from resistivity measurements.     

Band offsets of epitaxial LaAlO3/TiO2 interface determined by X-ray photoelectron spectroscopy

Room-temperature ferromagnetism in doped anatase TiO₂ has previously been observed, ferromagnetic semiconductor heterostructures based on anatase TiO₂ can thus provide a new opportunity to study spin-dependent transport phenomena at room temperature. An accurate determination of barrier heights or band offsets at the TiO₂-based heterojunctions is of great importance for the spintronics application with semiconductors. X-ray photoelectron spectroscopy with high-energy resolution was used to determine the band offsets of epitaxial LaAlO₃/TiO₂ heterojunction on SrTiO₃(001) substrate fabricated by pulsed laser deposition. Results showed an upward band bending of 0.48(0.03) eV when the film thickness of the overlayer LaAlO₃ above 4 unit cells. The valence band offset obtained is about 0.35(0.16) eV. Assuming bulk band gaps for the LaAlO₃ and TiO₂ epitaxial films, the associated conduction band offset is about 2.95(0.16) eV. These results show that LaAlO₃ can be an ideal tunneling barrier for TiO₂-based heterojunctions.     

Surface roughening by anisotropic adatom kinetics in epitaxial growth of La0.67Ca0.33MnO3

The growth mechanisms and surface morphology of colossal magnetoresistance (CMR) La_0.67Ca_0.33MnO₃ films deposited by rf magnetron sputtering on SrTiO₃(0 0 1) substrates are investigated. The films are epitaxial, coherently strained and ferromagnetic. It is found that at early growth stages, in nanometric films, a layer-by-layer mechanism dominates, which results in step and terrace surface morphology. Upon further growth, the flat surface becomes unstable when large two-dimensional (2D) islands form. The Erlich-Schwoebel step-edge energy barrier induces an anisotropic adatom kinetics that favors 2D nucleation on top of the islands as it reduces downhill adatom current. As a result, there is an evolution with growth to mound-like structures of increasing height. Critical thickness for mound formation and average mound separation can be tuned by substrate miscut angle and growth temperature. These results provide a detailed understanding of the roughening process in manganites and are relevant for the controlled fabrication of CMR films, in particular for its use in epitaxial heterostructures.     

Epitaxy of AlxGa1−xSb films from a molten metal.

Conclusions Crystallization from a solution of antimony in a gallium-aluminum melt has been used to produce epitaxial Al_xGa_1–xSb films with x=0 to 0.4; isothermal (T = 450, 500, 550°C) and isoconcentration (0; 1; 2.4 at. % Al) sections have been drawn for the liquidus surface on the composition plane. X-ray microspectral analysis has been used to examine the composition of the Al_xGa_1–xSb films in relation to aluminum content in the melt, as well as the distribution of Al and Ga over the epitaxial film. These films had a perfect structure. The results enable one to determine the solution composition needed to grow Al_xGa_1–xSb epitaxial films at 450–550°C on gallium antimonide substrates, and it is shown to be possible to make heterostructures in the Al_xGa_1–xSb-GaSb system.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 146–148, September.We are indebted to Zh. I. Alferov and V. I. Shveikin for attention and support in this work.     

Reactance of the n-Au/p-La0.67Ca0.33MnO3 film contact

The responses of the resistance and reactance of Au/(20 nm)La_0.67Ca_0.33MnO₃ film heterostructures to temperature variation and magnetic field (f = 100 kHz) are investigated. At T = 300 K, the capacitance per unit area of the interface between a gold contact and the Au/(20 nm)La_0.67Ca_0.33MnO₃ epitaxial film is found to be about 1 μF/cm². The maximum value of the negative magnetoreactance (≈60% at μ₀ H = 0.4 T) of the heterostructures is almost twice as high as the extremal value of the active magnetoresistance at T ≈ 235 K. The effective depth of magnetic field penetration into the manganite film on the side of the gold contact deposited on its surface is about 3 nm at room temperature.     

Effect of the chromium layer thickness on the morphology and optical properties of heterostructures Si(111)/(CrSi<Subscript>2</Subscript> nanocrystallites)/Si(111)

Growth and the optical properties of epitaxial heterostructures Si(111)/(CrSi₂ nanocrystallites)/Si(111) based on nanosized islands of chromium disilicide (CrSi₂) on Si(111) were studied using low-energy electron diffraction, atomic-force microscopy, and optical reflection and transmission spectroscopy. The heterostructures with thicknesses of 0.1, 0.3, 0.6, 1.0, and 1.5 nm were formed by reactive epitaxy at a temperature of 500°C followed by the epitaxial growth of silicon at 750°C. The specific features of changes in the density and sizes of CrSi₂ islands on the silicon surface were determined at T = 750°C as the chromium layer thickness was increased. It was established that, in the heterostructures with chromium layer thicknesses exceeding 0.6 nm, a small part of faceted Cr₂Si₂ nanocrystallites (NCs) emerge into near-surface region of the silicon, which is confirmed by the data from optical reflectance spectroscopy and an analysis of the spectral dependence of the absorption coefficient. A critical size of NCs is shown to exist above which their shift to the silicon surface is hampered. The decreased density of emerging NCs at chromium layer thicknesses of 1.0–1.5 nm is associated with the formation of coarser NCs within a silicon layer, which is confirmed by the data from differential reflection spectroscopy.     

Effect of SrTiO3 thickness on the capacitance–voltage characteristics of (La,Sr)CoO3/(Pb,La)(Zr,Ti)O3/SrTiO3/LaVO3 epitaxial heterostructures

We report the effect of SrTiO₃ thickness on the capacitance?Cvoltage (C?CV) characteristics of (La,Sr)CoO₃/(Pb,La)(Zr,Ti)O₃/SrTiO₃/LaVO₃ metal?Cferroelectric?Cinsulator?Csemiconductor (MFIS) epitaxial heterostructures. The C?CV measurement of the heterostructure exhibited the asymmetry of capacitance with respect to gate bias. Within the given thickness range (5?C30 nm), the amount of capacitance reduction at positive gate bias and the rapidness of capacitance reduction decreased with increasing SrTiO₃ thickness, which is consistent with the C?CV characteristics of conventional silicon-based MFIS capacitors. These results suggest that quantitative understanding on the electrical behavior of oxide heterostructures is possible with C?CV analysis, with potentially important implications on their device applications.     

X-ray diffraction studies of heterostructures based on solid solutions Al x Ga1 − x As y P1 − y : Si

The growth of MOCVD-hydride epitaxial heterostructures based on ternary solid solutions Al _x Ga_1?x As heavily doped with phosphorus and silicon has been studied using high-resolution X-ray diffraction and X-ray microanalysis. The prepared epitaxial films are five-component solid solutions (As _x Ga_1?x As _y P_{1 ? y} )_{1 ? z} Si _z .     

Effect of polar distortion on electronic structures of (001) LaGaO3/SrTiO3 interface

First-principles calculations of electronic structures of (001) epitaxial LaGaO₃/SrTiO₃ heterostructures were performed in the framework of density functional theory. The effects of atomic relaxation on electronic characteristics of both n-type (LaO)⁺/(TiO₂)⁰ and p-type (GaO₂)⁻/(SrO)⁰ interfaces are investigated. It is found that the n-type interface remains metallic, whereas the p-type interface becomes insulating after atomic relaxation. Polar distortion in the LaGaO₃ layers associated with the atomic relaxation strongly screens the intrinsic electric field induced by periodically stacking (LaO)⁺ and (GaO₂)⁻ charged atomic layers on SrTiO₃ with charge neutral (001) atomic layers. This relieves the trend to a polar catastrophe and reduces the carrier charge density on the interface.     

Thermophotovoltaic converters on indium arsenide-based compounds

Thermophotovoltaic converters based on multicomponent solid solutions of III–V compounds, specifically, InAsSbP/InAs heterostructures (E _g = 0.35–0.60 eV), that are intended for fabricating IR emitters heated to 1000–2000°C are studied. The use of such narrow-gap heterostructures makes it possible to advance the sensitivity of the elements into the long-wave range and utilize the thermal energy of low-temperature sources more efficiently. Fresh physical approaches to fabricating epitaxial quaternary InAs-based InAsSbP solid solutions with a low carrier concentration and heterostructures with sharp interfaces are presented. Quaternary InAsSbP solid solutions and other related heterostructures offer a number of advantages, such as the possibility of growing perfect structures lattice-matched with the substrate, stress-free interfaces, good electrical and photoelectrical properties (low dark currents and a high external quantum efficiency), and the possibility of flexibly controlling the energy gap by varying the composition of the solid solution. It is shown that InAsSbP films grown on an InAs substrate by liquid-phase epitaxy from supercooled liquid solution and liquid-phase electro-epitaxy with replenishment of liquid solution by growing layer components are uniform in composition and have a perfect crystal structure. Thermophotovolatic p-InAsSbP/n-InAs diode-type heterostructures obtained by the above methods are found to have saturation dark currents close to theoretically predicted values and a wide range of spectral sensitivity, which makes them candidates for thermophotovoltaic elements.     

Structural and local electrical properties of AlInN/AlN/GaN heterostructures

GaN layers and Al_1−xIn_xN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures.     

Synchrotron X-ray diffraction study of SrRuO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> nano-sized heterostructures grown by laser MBE

Structural investigation using X-ray synchrotron radiation has been performed on SrRuO₃/SrTiO₃/SrRuO₃ epitaxial heterostructures, with each constituent layer a few unit cell thick grown on (001) SrTiO₃ substrate. Detailed information on the evolution of the in-plane lattice structure has been obtained, in these heterostructures, by grazing incidence diffraction measurements. The samples have been grown by low-pressure pulsed laser deposition. Under our deposition conditions, SrRuO₃ layers grow with an elongated cell perpendicular to the substrate surface. The in-plane pseudocubic lattice parameters do not match the in-plane square SrTiO₃ structure even in the case of very thin SrRuO₃ layers. Such a distortion was found to decrease with increasing the thickness of the SrTiO₃ barrier layer.     

MnxSny(x=2,3,4; y=18,24,30)团簇几何结构的密度泛函研究

采用密度泛函方法,研究了Mn_xSn_y(x=2,3,4; y=18,24,30)团簇的几何结构. 发现Mn_xSn_6x+6(x=2,3,4)倾向于形成 Mn 原子内掺入D_3d Sn团簇单笼结构,即Mn₂Sn₁₈, Mn₃Sn₂₄ 和Mn₄Sn₃₀.而Mn_xSn_6x+12(x=2,3)则倾向于形成由两个小笼连接 而成的双笼结构,即MnSn₁₂-MnSn₁₂ 和MnSn₁₂-Mn₂Sn₁₈.因此,可望通过控制掺杂Mn 原子的数量来组装成不同结构的Mn_xSn_y一维纳米线.     

Growth and conductivity of calcium cuprate films

Crystal and transport properties of epitaxial CaCuO₂ (CCO) films are studied for samples grown on (110)NdGaO₃, (001)SrTiO₃, and (001)LaAlO₃ substrates using the laser ablation technique. The resistivity is found to be dependent on the crystal quality of the films. The conductivity type varies depending on the doping. For lightly doped films with a resistance higher than 0.1 Ω cm, 3D hopping conductivity is observed. For low-resistance CCO films doped with Sr, a power law temperature dependence is found, which is inconsistent with the hopping conductivity. The influence of substrate tilting on the subsequent growth of CaCuO₂ films is studied. YBCO/CCO heterostructures preserve high critical temperatures and small widths of superconducting transitions, which is of special importance for growing Josephson heterostructures for superconducting electronics.