Close Lattice Match Research Articles

High Efficiency and Long-Lived Green and Blue Light Emitting Diodes Based on ZnSSe:Te Active Layer Grown by Molecular Beam Epitaxy

Bright green and blue light emitting diodes have been developed based on ZnSSe:Te active layer, with high crystal quality and a close lattice-match to GaAs substrate. The green LEDs exhibit a fairly long lifetime (>2000 h) at room temperature, which is attributable to the crystal-hardening effect by Te-doping. The devices show slow-mode degradation due to the gradual enhancement of microscopic defects in p-ZnMgSSe cladding layer. No detectable changes in dark-spot and dark-line formation due to macroscopic defects are observed during device operation.

Growth study of epitaxial FexZn1−xF2 thin films

The FexZn1−xF2 alloy has been shown to be a model system for studying the magnetic phase diagram of dilute magnets. Whereas the growth of bulk single crystals with fixed Zn concentrations is difficult, the thin film growth is comparatively simpler and more flexible. To gain an understanding of the growth of FexZn1−xF2 films, a method was developed to grow smooth films at fixed concentrations. This was done by depositing a MgF2 buffer layer on MgF2(001) substrates and then depositing FeF2 and ZnF2 [001]-orientated epitaxial thin films at different temperatures. Surprisingly, the lattice spacing depends strongly on the growth temperature, for 44-nm-thick FeF2 films and 77-nm-thick ZnF2 films. This indicates a significant amount of stress, despite the close lattice match between the films and the MgF2 substrate. Thick alloy samples (approximately 500 nm thick) were grown by co-evaporation from the FeF2 and ZnF2 sources at the ideal temperature determined from the growth study, and their concentration was accurately determined using x-ray diffraction.

Growth and Chemical Properties of Layered, Binary Mixed Oxides: NO on MgO−NiO and CaO−NiO

The growth mode of layered, binary mixed oxides consisting of NiO, MgO, and CaO has been studied using low-energy electron diffraction (LEED), ion scattering spectroscopy (ISS), and NO temperature-programmed desorption (NO-TPD). The NiO/MgO layered system shows greater long-range order relative to NiO/CaO. The superior wetting found for NiO/MgO compared to NiO/CaO is likely due to the close lattice match of NiO/MgO, in contrast to NiO/CaO. The influence of MgO and CaO on NO adsorption on NiO has been studied using TPD and infrared reflection absorption spectroscopy (IRAS) and correlated with the growth mode of the corresponding layered oxide system. A long-range, substrate-mediated interaction has been found between NO and coadsorbed MgO (or CaO) on the NiO(100)/Mo(100) surface.

Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

The effect of mineral substrates on the crystallization of lysozyme

The effects of exogenous mineral substrates on the induction time for nucleation, and on the number, morphology and purity of protein crystals were investigated in a series of experiments using chicken egg-white lysozyme (CEWL) as a model protein. CEWL was crystallized using the vapor-diffusion technique in the absence of substrates (control) and in the presence of mineral substrates exhibiting various degrees of crystalline lattice match to CEWL (experiments). Results indicate that mineral substrates with a close lattice match to CEWL had a greater influence on the induction time for nucleation and crystal properties.

Tunneling characteristics of epitaxial YBa 2Cu 3O 7−x/Y 2O 3/YBa 2Cu 3O 7−x planar type junctions

We deposited YBa 2Cu 3O 7−x/Y 2O 3/YBa 2Cu 3O 7−x heterostructures by a dc/rf sputtering process at high oxygen pressures on (001) SrTiO 3 substrates. The deposition conditions of Y 2O 3 are completely compatible with those of YBa 2Cu 3O 7−x. The crystal structure of Y 2O 3 provides a close lattice match with YBa 2Cu 3O 7−x which results in high quality epitaxial trilayers with no foreign phases in the structure. Both bottom and top YBa 2Cu 3O 7−x layers are superconducting above 90 K. The thicknesses of the intermediate artificial Y 2O 3 layers range from 3 to 5 nm. Tunneling measurements on these planar junctions exhibited an anisotropic gap structure without zero bias anomalies. Gap structures at 9.6 K are clearly seen at about 20 and 5 mV bias voltages. These values are discussed considering the contributions from both ab-plane and quasiparticle tunneling processes.

On-axis sputter deposition of Sr/sub 2/AlTaO/sub 6/ dielectric thin films for multilayer Y-Ba-Cu-O circuits

Sr/sub 2/AITaO/sub 6/(SAT) is a desirable dielectric for use in YBCO multilayer circuits due to the close lattice match to YBa/sub 2/Cu/sub 3/O/sub 7/(YBCO), and low dielectric constant, SAT thin films have been deposited using on axis rf magnetron sputtering. Epitaxial growth of SAT has been measured using x-ray diffraction. The electrical isolation of SAT sandwiched between two YBCO layers, and the superconducting critical current of YBCO deposited over SAT crossover structures are measured.

Temperature dependent exchange anisotropy in single-crystal ferrite thin films (abstract)

Single-crystal bilayers of (magnetically soft) (Mn,Zn)Fe2O4 and (magnetically hard) CoFe2O4 can exhibit nearly ideal exchange anisotropy. This is directly related to the extremely high structural quality that can be achieved by epitaxial growth on appropriate substrates. For example, we use (110)-oriented MgAl2O4 substrates with 100 nm CoCr2O4 buffer layers. The paramagnetic buffer layer provides a close lattice match for the ferrites, and is annealed at about 1000 °C to provide excellent crystallinity. Subsequent growth of the ferrite bilayer is executed at 400 °C, which is high enough to permit growth of highly crystalline epitaxial films, yet low enough to avoid substantial intermixing at the interfaces. Measurements at elevated temperatures (290 K<T<1000 K) permit characterization of the exchange anisotropy in regimes of low and moderate crystal anisotropy in the soft layer, as well as directly revealing the blocking temperature and the portion of the overall magnetization contributed by each layer. The results provide additional evidence that the exchange coupling energy across the interface is comparable to that within each layer, and in that sense is nearly ideal. The detailed shapes of the M–H loops are found to be in semiquantitative agreement with micromagnetic theory.

Mapping of CdZnTe substrates and CdHgTe epitaxial layers by X-ray diffraction

A simple procedure is described for mapping of lattice parameter and X-ray rocking curve width across Cd 1− y Zn y Te (CZT) substrates and Cd x Hg 1− x Te (CMT) epitaxial layers. It is shown how screening of the crystallinity and composition of CZT substrates can be utilized to make subsequent X-ray characterizations of epitaxial layers more reliable. This also allows selection of substrates with a close lattice matching to a given CMT composition, to reduce or avoid layer relaxation. Substrates with laterally varying Zn content give a varying lattice mismatch on a single epitaxial layer. Correlation of the lattice parameter maps of substrate and layer shows the resulting variations in layer strain, and the onset of relaxation can be clearly identified. Mapping of rocking curve peak widths on good quality CZT substrates and closely lattice matched CMT layers shows that the peak width can be below 6 arcsec, limited essentially by the intrinsic peak width of the materials. Additional broadening of the layer peaks is introduced primarily by substrate defects and misfit dislocations, unless epitaxial growth conditions deviate significantly from their optimum.

Growth of NLO Chalcopyrite Materials by Omvpe

AbstractThis paper describes the application of organometallic vapor phase epitaxy to the growth of II-1V-V2 chalcopyrite materials that have high figures of merit for nonlinear optical (NLO) applications. ZnGeAs2, although not a particularly interesting NLO material, is used as a model for the growth of ZnGeP2, which is. Both compounds, as well as others, have been successfully grown by vapor phase on III-V substrates that provide a close lattice match. Doping studies using Group II and VI elements have been undertaken to control the p-type conductivity found in both compounds. Except for the possible case of indium, the results of these experiments are less than encouraging. Minority-carrier lifetimes of 150 ns have been measured in ZnGeAs2.The results of this work are used to make projections about the growth of CdGeAs2. CdGeAs2 is promising for the next generation of NLO mid-infrared materials if an absorption band that occurs at about 5 μm can be reduced. The growth projections suggest that this compound will be difficult to grow epitaxially and has no III-V substrate that provides a close lattice match. Mixing CdGeAs2 with other II-IV-V2 materials may offer solutions to the substrate problem. The defect properties of CdGeAs2 have not, to our knowledge, been studied.

MOVPE Growth of GaPAsN Quaternary Alloys Lattice-Matched to GaP

AbstractGaP1-x-yAsyNx. (x ∼,2.3%, 0< y <19%) quaternary alloy semiconductor films on GaP substrates have been successfully grown by metalorganic vapor phase epitaxy (MOVPE). With the fixed supplies of trimethylgallium(TMG), AsH3 and dimethylhydrazine(DMHy) during the growth, the As concentration in the solid increased with increasing AsH3 supply, while the N concentration was almost unaffected. It has been demonstrated that the crystalline quality of the alloy films is much improved with the close lattice-matching to the GaP substrate, giving a superior surface morphology without any cross-hatches, a much narrower x-ray diffraction linewidth, and a significantly higher photoluminescence(PL) intensity.

Oriented growth of SrBi2Ta2O9 ferroelectric thin films

We report on the ferroelectric properties of c-axis oriented ferroelectric SrBi2Ta2O9 thin films. Pt/SrBi2Ta2O 9/Pt capacitors were grown on single crystal MgO (and/or SrTiO3) substrates using pulsed laser ablation. These substrates provide the necessary template for (100) texture in platinum due to their close lattice matching. This in turn facilitates the c-axis orientation in the ferroelectric films. The degree of orientation in the layered structure ferroelectric film was systematically varied from highly c-axis oriented to random polycrystalline by varying the growth conditions of the bottom metal electrode. The polarization and coercive field values were found to decrease with an increasing degree of c-axis orientation; while the randomly oriented films exhibited a remnant polarization of 5 μC/cm 2, a coercive field of 70 kV/cm, and a dielectric constant of 320, the c-axis oriented films exhibited very low polarization (∼1 μC/cm2), coercivity (22 kV/cm), and dielectric constant (∼200) values.

Mg2Si buffer layers on Si(100) prepared by a simple evaporation method

The formation of Mg2Si(100), ao= 6.39A, on Si(100) substrates has been investigated. Mg was first evaporated onto Si(100) surfaces and Mg2Si (100) films were formed in a subsequent annealing process. The Mg2Si layers were characterized by x-ray diffraction and transmission electron microscopy analysis. Optical and scanning electron microscopy analysis show the surface morphology to be smooth. The films are stable under thermal cycling and exhibit low resistivity. Epitaxial films of Mg2Si on Si(100) could be an ideal substrate for mercury cadmium telluride and antimonide based III-V semiconductor for mid-infrared devices because of its close lattice matching (the lattice misfit factor is less than 1.5%).

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a wurtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O2 into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 1018 cm−3 with an electron mobility of 260 cm2/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.

Molecular beam epitaxy growth and characterization of InGaAlAs-collector heterojunction bipolar transistors with 140 GHz fmax and 20 V breakdown

The molecular beam epitaxy (MBE) growth, material characterization, and device performance of InGaAlAs-collector heterojunction bipolar transistors that dramatically increase device breakdown without compromising high-frequency performance is described. This performance is achieved by the use of large band gap InGaAlAs collectors with a pulse-doped layer at the beginning of the graded collector-to-base junction to suppress current-blocking effects. Double-crystal x-ray diffraction measurements indicate these complex structure can be grown using MBE with very close lattice matching to the underlying InP substrate. From photoreflectance spectra the collector composition, field present at the collector-to-base junction, and various transitions associated with the graded emitter can be measured. The use of an In0.53(Ga0.50Al0.50)0.47As collector was found to increase breakdown characteristics (BVcbo=20 V, VA=250 V) substantially compared to an In0.53Ga0.47As collector (BVcbo=11 V, VA=7 V). For the In0.53(Ga0.50Al0.50)0.47As collector current blocking effects were only found at very high current levels (∼100 kA/cm2) allowing this device to achieve very good rf performance (ft=58 GHz and fmax=140 GHz). For higher Al compositions in the collector the breakdown voltage was found to increase further but current-blocking effects became more severe and rf performance began to suffer.

Epitaxial growth and lattice match of cobalt and nickel disilicides/Si(111) gold and cobalt doping of NiSi 2

We report on an X-ray diffraction study of the epitaxial growth of thick films of cobalt and nickel disilicides on silicon. The lattice parameters both parallel and normal to the interface are measured at room temperature (RT) for various samples prepared by solid phase epitaxy or reactive deposition epitaxy with annealing at high temperature (600–800°C). The formation of a rigid disilicide/silicon interface with a dislocation network frozen below a critical temperature T D, estimated at 600°C for CoSi 2 and at 710°C for NiSi 2, is the likely growth mechanism of the thick disilicide films. CoSi 2 and NiSi 2 layers are indeed tetragonally strained at RT with a lateral mismatch equal to that of the relaxed material at T D, i.e. −0.87% and +0.17% respectively. The tetragonal distortion results from the different thermal expansion coefficients of silicon and silicides. Doping NiSi 2 with 10% Co, i.e. smaller atoms, leads to a close in-plane lattice match. Introducing 6% Au atoms increases the lattice parameter of NiSi 2, and also favours the relaxation of the film after annealing. With 12% Au-doping, quasi-total relaxation is observed but the layer is no longer uniform. Doping in all cases induces a decrease of the critical temperature T D of dislocation freezing.

Optical Properties of Mg-GaN, GaN/AlGaN SCH structures, and GaN on ZnO Substrates

ABSTRACTGaN films and GaN/AlGaN heterostructures have been gro wn by MBE. GaN films doped with varying levels of Mg indicate effective mass acceptor at low doping concentrations, as determined from strong photoluminescence emission at about 380 nm. As the Mg concentration is increased the photoluminescence emission line red shifts considerably, indicating the formation of Mg-related or induced complexes whose lifetimes are relatively short. GaN/AlGaN separate confinement heterostructures grown on sapphire show strong near ultraviolet stimulated emission at room temperature in a side-pumping configuration. The pumping threshold for stimulated emission at room temperature was found to be ∼90 kW/cm2. Initial GaN films grown on ZnO substrates show the A exciton in low temperature photoluminescence. ZnO is being considered for nitride growth because of its stacking order and close lattice match.

BeTe/ZnSe graded band gap ohmic contacts to p-ZnSe

BeTe is not a very well known wide-band semiconductor. Due to the close lattice match to GaAs and ZnSe and p-type as-grown character, BexZn1−xTexSe1−x graded band-gap layers appear an ideal candidate for ohmic contact to p-type ZnSe based semiconductors. These contacts allow for an implementation of epitaxial structures of II-VI compound diode lasers entirely lattice matched to the GaAs substrate. The numerical calculations predict contact resistivity of BexZn1−xTexSe1−x graded gap contacts lower than ρc=10−4 Ω cm2 at acceptor doping level 1×1018 cm−3, which corresponds to a voltage drop across the contact layer of less than 0.1 V during lasing operation.

Diamond Nucleation Studies on Refractory Metals and Nickel

ABSTRACTDiamond nucleation has been investigated on a variety of potential heteroepitaxial substrate materials. Previous work in this laboratory has demonstrated heteroepitaxial nucleation on both Si and SiC substrates via bias-enhanced nucleation (BEN). In this study the effects of BEN of diamond on refractory metal substrates is investigated in detail. Initial data suggest a strong correlation between the carbide forming nature of the substrate material and the rate of nucleation during biasing. Our second avenue of research involves low pressure diamond growth on nickel. This material is a promising material due to its close lattice match and recent evidence of epitaxy reported by other researchers. To form heteroepitaxial diamond on nickel a high temperature pretreatment routine has been established and is the basis for this area of research. The relative importance of hydrogen absorption during this routine is explored and correlated to the formation and degree of diamond epitaxy.

Transmission Electron Microscopy of AlxGa1-xN/SiC multilayer structures

The potential of wide-band-gap III-V nitrides as solar-blind ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wideband-gap III-V nitrides: the growth of ultra-short period GaN/AlN superlattices (digital alloys); and the incorporation of SiC layers into AlxGa1-xN structures. By varying the relative periods in a GaN/AIN superlattice, it should bepossible to tailor the band gap of the composite to lie between the elemental values of 365nm for GaN and 200nm for AIN. The group IV semiconductor, SiC, has a close lattice match (&lt; 3%) to AlxGa1-xN forgrowth on the basal plane. Demonstration of epitaxial growth for AlxGa1-xN/SiC multilayers could enable an extension of direct band-gap material towards the visible.The superlattice samples were grown by low-pressure metalorganic chemical-vapor deposition (MOCVD)using a unique switched atomic-layer-epitaxy (SALE) procedure (as first used by Dapkus et al. for GaAs). GaN was grown on the basal-plane sapphire substrates above a thin AlN layer, which has been foundto maximize the crystalline quality of the GaN film.