Showing papers on "Epitaxy published in 2000"

Pulsed laser deposition of thin films

Abstract: The pulsed laser deposition is a new technique for the growth of thin films,which has been attended generally by people recently The physical principle, unique characteristics and the proceeding of the study were introduced briefly In addation, the result of the PtSi nanometer thin film based on silicon prepared by the pulsed laser deposition was describedPULS

Material Design of Half-Metallic Zinc-Blende CrAs and the Synthesis by Molecular-Beam Epitaxy

Abstract: A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Ga-Doped ZnO Films Grown on GaN Templates by Plasma-Assisted Molecular-Beam Epitaxy

Abstract: We have investigated the structural and optical properties of Ga-doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. The carrier concentration in Ga-doped ZnO films can be controlled from 1.33×1018/cm3 to 1.13×1020/cm3. Despite high Ga incorporation, the linewidth of (0002) ω-rocking curves of Ga-doped ZnO films still lies in the range from 5 to 15 arc min. Photoluminescence (PL) spectra of Ga-doped ZnO films show dominant near-bandedge emission with negligibly weak deep-level emission, independent of carrier concentration. The PL spectrum exhibits a new emission line at 3.358 eV, which corresponds to exciton emission bound to a Ga donor. To avoid degradation of the PL intensity, the maximum dopability of Ga in ZnO is determined to be around 2.6×1019/cm3.

Ultraviolet detectors based on epitaxial ZnO films grown by MOVCD

Abstract: High-quality zinc oxide (ZnO) films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range of 350°C to 600°C. In-situ nitrogen compensation doping was performed using NH3. Microstructural and optical properties of the films, as well as the N-doping effects, were studied. The metal-semiconductor-metal ultraviolet sensitive photodetectors were fabricated on N-doped epitaxial ZnO films. The detector showed fast photoresponse, with a rise time of 1 µs and a fall time of 1.5 µs. Low-frequency photoresponsivity, on the order of 400 A/W at 5 V bias, was obtained.

High ε gate dielectrics Gd2O3 and Y2O3 for silicon

Abstract: We report on growth and characterization of both epitaxial and amorphous films Gd2O3 of (e=14) and Y2O3(e=18) as the gate dielectrics for Si prepared by ultrahigh vacuum vapor deposition. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single-domain films in the Mn2O3 structure. Typical electrical leakage results are 10−3 A/cm2 at 1 V for single domain epitaxial Gd2O3 and Y2O3 films with an equivalent SiO2 thickness, teq of 15 A, and 10−6 A/cm2 at 1 V for smooth amorphous Y2O3 films (e=18) with a teq of only 10 A. For all the Gd2O3 films, the absence of SiO2 segregation at the interface is established from infrared absorption measurements.

Fabrication of GaAs Quantum Dots by Modified Droplet Epitaxy

Abstract: We propose a modified droplet epitaxy method for fabricating self-organized GaAs/AlGaAs quantum dots (QDs) with a high As flux irradiation and a low substrate temperature. By our novel method, GaAs QDs were successfully formed, retaining their pyramidal shape, original base size and density of droplets, and preventing layer-by-layer growth. Quantum size effects of the QDs were distinctly observed by photoluminescence measurements. It was confirmed that this new modified droplet epitaxy method is promising for fabricating a high-quality GaAs/AlGaAs QD system.

Highly electrically conductive indium–tin–oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition

Abstract: Highly electrically conductive indium–tin–oxide thin films were epitaxially grown on an extremely flat (100) surface of yttria-stabilized zirconia single-crystal substrates at a substrate temperature of 600 °C by a pulsed-laser deposition technique. A resistivity down to 7.7×10−5 Ω cm was reproducibly obtained, maintaining optical transmission exceeding 85% at wavelengths from 340 to 780 nm. The carrier densities of the films were enhanced up to 1.9×1021 cm−3, while the Hall mobility showed a slight, almost linear, decrease from 55 to 40 cm2 V−1 s−1 with increasing SnO2 concentration. The low resistivity is most likely the result of the good crystal quality of the films.

Improvement on epitaxial grown of InN by migration enhanced epitaxy

Abstract: Epitaxial growth of InN on (0001) sapphire with an AlN buffer layer was studied by migration-enhanced epitaxy, which is composed of an alternative supply of pure In atoms and N2 plasma. A series of samples were prepared with different substrate temperatures ranging from 360 to 590 °C. As-grown films were characterized by x-ray diffraction (XRD), reflective high-energy electron diffraction, atomic-force microscopy (AFM), and Hall measurements. Both XRD θ–2θ and ω scans show that the full width at half maximum of the (0002) peak nearly continuously decrease with increasing growth temperature, while InN grown at 590 °C shows the poorest surface morphology from AFM. It is suggested that three-dimensional characterization is necessary for an accurate evaluation of the quality of the InN epilayer. Hall mobility as high as 542 cm2/V s was achieved on film grown at ∼500 °C with an electron concentration of 3×1018 cm−3 at room temperature. These results argue against the common view that nitrogen vacancies are res...

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

Abstract: A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Low-Dislocation-Density GaN from a Single Growth on a Textured Substrate

Abstract: The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.

Piezoelectric coefficient of aluminum nitride and gallium nitride

Abstract: The piezoelectric coefficient d33 of aluminum nitride (AlN) and gallium nitride (GaN) thin films grown on silicon substrates by molecular beam epitaxy have been measured using a laser interferometer. X-ray diffraction reveals that the AlN and GaN films consist mainly of crystals with a hexagonal wurtzite structure. In order to grow epitaxial GaN films, an AlN film was first deposited on silicon as the buffer layer, so the d33 measurement for GaN was actually performed on GaN/AlN/Si multilayer systems. The relative permittivity and electrical resistivity of each constituent layer of the film and the potential drop across each layer were determined as a function of frequency. The potential drops were then used to calculate the piezoelectric coefficient d33 of GaN. After correcting for substrate clamping, d33 of AlN and GaN were found to be (5.1±0.1) and (3.1±0.1) pm V−1, respectively.

Room Temperature 1.6 µm Electroluminescence from a Si-Based Light Emitting Diode with β-FeSi2 Active Region

Abstract: Room temperature electroluminescence (EL) was obtained for the first time from a Si-based light emitting diode with semiconducting silicide (β-FeSi2) active region. The peak wavelength was 1.6 µm and it is from β-FeSi2 balls embedded in a Si p-n junction. An a-axis oriented β-FeSi2 layer was grown on n+-(001) Si by reaction deposition epitaxy (RDE), then p- and p+-Si layers were grown by molecular beam epitaxy (MBE). The β-FeSi2 aggregated into balls with about 100 nm diameter in this process. The EL intensity increased superlinearly with increase of the injected current density, and reasonable EL was obtained at current density above 10 A/cm2, though the photoluminescence (PL) was difficult to be detected at room temperature.

Uniaxial locked epitaxy of ZnO on the a face of sapphire

Abstract: High-quality, c-oriented ZnO epitaxial films have been grown on the a surface using molecular-beam epitaxy. The use of a-oriented sapphire eliminates rotational domains and related structural defects which have limited the use of ZnO in electronic applications. The ZnO epitaxial layers are uniquely oriented with the ZnO/sapphire orientational relationship [0001]‖[1120] and 〈1120〉‖[0001]. This unique orientation is a consequence of the anisotropy of the a-sapphire surface in conjunction with a strong correlation along a single direction leading to the term uniaxial locked epitaxy. High-resolution x-ray diffraction measurements show an increase in x-ray lateral coherence length from several tens of nanometers to >0.7 μm for growth of c-oriented ZnO on the a surface as opposed to the c surface of sapphire.

High-density ultrasmall epitaxial Ge islands on Si(111) surfaces with a SiO 2 coverage

Abstract: Epitaxial Ge islands less than 7 nm in base diameter and 2.5 nm in height, and with a number density of about $2\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ were created on Si(111) surfaces covered with 0.3-nm-thick ${\mathrm{SiO}}_{2}$ films. The low- and high-temperature limits for the epitaxy were found to be dependent on the Ge deposition flux. The experimental results suggest that the island nuclei and the conditions for epitaxial growth appear through a reaction between individual Ge adatoms and ${\mathrm{SiO}}_{2},$ and the mechanism of island formation corresponds to a growth model with the critical island size ${i}^{*}=0.$ The relationship between the rate of the nucleation reaction and the diffusion coefficient of Ge adatoms was used to estimate the island density.

High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy

Abstract: A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free.

Influence of epitaxial growth and substrate-induced defects on the breakdown of 4H–SiC Schottky diodes

Abstract: Morphological defects and elementary screw dislocations in 4H-SiC were studied by high voltage Ni Schottky diodes. Micropipes were found to severely limit the performance of 4H-SiC power devices, w ...

Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy

Abstract: Thick gallium nitride films 250–350 µm in thickness were grown on 2-inch-diameter (0001) sapphire wafers by hydride vapor phase epitaxy. The size of the free-standing GaN substrates without cracks separated from the sapphire substrates by laser processing was equal to that of the initial sapphire substrates. The origin of bowing and the broad photoluminescence (PL) spectra of GaN films was considered the difference in the residual strain between the front and bottom surfaces caused by threading dislocations.

Role of 90° domains in lead zirconate titanate thin films

Abstract: We report observations of the ferroelectric domain structure in epitaxial lead zirconate titanate (PbZr0.2Ti0.8O3) ferroelectric thin films using piezoresponse microscopy. By manipulating the film thickness, a uniform two-dimensional grid of 90° domains (a domains, i.e., c axis in the plane of the film) has been induced. Our studies show that the out of plane polarization direction in the film is preferentially oriented. 90° domains have been observed as regions of low piezoresponse, as compared to the fully c axis oriented regions. We have studied the influence of these 90° domains and the domain walls on the nucleation of polarization reversal. We observe that the nucleation occurs preferentially at 90° domain interfaces. Polarization reversal is seen to occur through the nucleation and subsequent growth of “semicircular/elliptical” reverse domains, which eventually consume the entire region as a function of reversal time.

Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate

Abstract: The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (−c) polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the {1010} directions to cover a room among the facet...

Monolithic Mesophase Silica with Large Ordering Domains

Abstract: Two-dimensional X-ray diffraction (XRD) analyses of transparent mesoscopically ordered silica/block copolymer composite monoliths reveal single-crystal-like patterns that correspond to well-ordered hexagonal domains that are greater than 1 cm × 1 mm × 1 mm in size. Analyses of the diffraction patterns reveal highly uniform hexagonal domains with narrow distributions of orientational order that are characterized by a φ-mosaic smaller than 2° in the plane of the hexagonal lattice and a χ-mosaic smaller than 11° along the cylindrical aggregate axes. XRD and TEM results also show that the composite monoliths have large well-ordered cubic mesophase domains in the meniscus regions of the samples. Boundaries between the hexagonal and cubic domains in the monoliths reflect gradual transitions between the two epitaxially related structures. Such large single-domain hexagonal and cubic mesostructures were furthermore preserved following removal of the organic species by calcination to produce mesoporous silica mono...

High-mobility AlGaN/GaN heterostructures grown by molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy

Abstract: We report on the growth and transport properties of high-mobility two-dimensional electron gases (2DEGs) confined at the AlGaN/GaN interface grown by plasma-assisted molecular-beam epitaxy on GaN templates prepared by hydride vapor phase epitaxy. We have grown samples over a broad range of electron densities ranging from ns=6.9×1011 to 1.1×1013 cm−2, and at T=4.2 K, observe a peak mobility of 53 300 cm2/V s at a density of 2.8×1012 cm−2. Magnetotransport studies on these samples display exceptionally clean signatures of the quantum Hall effect. Our investigation of the dependence of 2DEG mobility on carrier concentration suggests that the low-temperature mobility in our AlGaN/GaN heterostructures is currently limited by the interplay between charged dislocation scattering and interface roughness.

Single-crystal material on non-single-crystalline substrate

Abstract: A method for making a multilayered structure with a single crystal film (12) bonded to a polycrystalline substrate (10) has the steps of bonding a single crystal film to a polycrystalline substrate, and growing an epitaxial layer (14, 14') on said single crystal film bonded to said polycrystalline substrate.

Spectroscopic ellipsometry of epitaxial ZnO layer on sapphire substrate

Abstract: Optical properties of epitaxial ZnO layers have been studied in the spectral region from 1.5 to 5.4 eV using four-zone null spectroscopic ellipsometry. An existing model dielectric function based on excitonic structure near direct band gap has been improved by including a high-energy absorption term. Surface layer, corresponding to the surface roughness, was found to be essential to fit the spectroellipsometric data obtained. Two kinds of samples have been studied: ZnO layers prepared on (0001) and (1120)-oriented sapphire substrates. The surfaces of the first ones were found to be more rough.

Epitaxial diamond encapsulation of metal microprobes for high pressure experiments

Abstract: Diamond anvils with diamond encapsulated thin-film microcircuits have been fabricated for ultrahigh pressure electrical conductivity experiments. The diamond films were homoepitaxially deposited onto the diamond anvil substrates with microwave plasma chemical vapor deposition using a 2% methane in hydrogen gas mixture and a diamond substrate temperature of 1300 °C. The diamond embedded thin-film microprobes remain functional to megabar pressures. We have applied this technology to the study of the pressure-induced metallization of KI under pressures up to 1.8 Mbar. This technology has the potential of greatly advancing the pressure range of a number of existing high-pressure diagnostic techniques, and for expanding the capabilities of diamond anvil cells into new directions.

Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films

Abstract: The lattice deformation of dense strained La0.7Sr0.3MnO3 (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 A, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO3 (STO) and LaAlO3 (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s pl...

Origin of antimony segregation in GaInSb/InAs strained-layer superlattices

Abstract: We show how cross-sectional scanning tunneling microscopy may be used to reconstruct the Sb segregation profiles in GaInSb $/$InAs strained-layer superlattices. These profiles are accurately described by a one-dimensional model parametrizing the spatial evolution of an Sb seed at the InAs-on-GaInSb interface in terms of two-anion-layer exchange. We argue that the segregation seed, which decreases from $\frac{2}{3}$ to $\frac{1}{2}$ monolayer when growth conditions are made less anion rich, has its origin in the Sb-bilayer reconstruction maintained during GaInSb epitaxy.

EPITAXIAL GROWTH OF SiO2 ON Mo(112)

Abstract: A new preparation is reported which, for the first time, results in a thin, crystalline SiO2 film on a Mo(112) single crystal. The procedure consists of repeated cycles of silicon deposition and subsequent oxidation, followed by a final annealing procedure. AES and XPS have been used to control film stoichiometry. LEED pictures of high contrast show a hexagonal, crystalline SiO2 overlayer with a commensurate relationship to the Mo(112) substrate. The wetting of the substrate by the film has been investigated by LEED, XPS and TDS, revealing that the film covers the substrate completely.

Biexciton emission from high-quality ZnO films grown on epitaxial GaN by plasma-assisted molecular-beam epitaxy

Abstract: We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for...

Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy

Abstract: GaN:Mg layers grown by plasma-assisted molecular-beam epitaxy at 650 °C are investigated. Secondary-ion-mass-spectroscopy measurements reveal uniform Mg doping profiles with very sharp boundaries. The amount of incorporated Mg atoms changes approximately linearly with incident Mg flux. Hall measurements on p-type GaN:Mg layers show that about 1%–2% of all Mg atoms are ionized at room temperature. The hole mobility depends strongly on the hole concentration, varying from μp=24 cm2/V s for p=1.8×1017 cm−3 to μp=7.5 cm2/V s for p=1.4×1018 cm−3. GaN p–n diodes with molecular-beam-epitaxy-grown p regions are analyzed using current–voltage measurements.