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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Chemical Solution Derived Planarization Layers for Highly Aligned IBAD MgO Templates

The main goal of this research is to develop a chemical solution derived planarization layer to fabricate highly aligned IBAD-MgO templates for the development of high temperature superconductor (HTS) based coated conductors. The standard IBAD-MgO template needs an additional electrochemical polishing step of the mechanically polished 50- m-thick Hastelloy C-276 substrates to ensure a flat and smooth surface for subsequent growth of multi-layer buffer architectures, which include: sputtered 80-nm Al2O3; sputtered 7-nm Y2O3; IBAD 10-nm MgO; sputtered 30-nm homo-epi MgO; and sputtered 30-nm LaMnO3 (LMO) layers. We have successfully developed a solution planarization layer that removes the electrochemical polishing step and also acts as a barrier layer. Crack-free, smooth Al2O3 layers were prepared on mechanically polished Hastelloy substrates using a chemical solution process. A nearly 10-15-nm thick Al2O3 layer was formed with each coating and the coating was repeated several times to achieve the desired film thickness with intermediate heat-treatments after each coating. The Al2O3 planarization layer significantly reduced the surface roughness of the substrate. The average surface roughness value, Ra for a starting substrate was 9-10 nm. After 8 coatings of Al2O3 layer, the Ra was reduced to 2 nm. Highly aligned IBAD-MgO layers with out-of-plane and in-plane textures comparable to the standard IBAD-MgO layers were successfully deposited on top of the solution planarization Al2O3 layers with an Y2O3 nucleation layer using a reel-to-reel ion-beam sputtering system. Both homo-epi MgO and LMO layers were subsequently deposited on the IBAD-MgO layers using rf sputtering to complete the buffer stack required for the growth of HTS films. YBa2Cu3O7- (YBCO) films with a thickness of 0.8 m deposited on these IBAD-MgO templates by pulsed laser deposition showed a high self-field critical current density, Jc of 3.04 MA/cm2 at 77 K and 6.05 MA/cm2 at 65 K. These results demonstrate that a low-cost chemical-solution-based, high-throughput Al2O3 planarization layer can remove the electro-polishing step and replace sputtered Al2O3 layers for the production of high Jc YBCO-coated conductors.

Paranthaman, Mariappan Parans↗
DOE OSTI2012

High quality ultrathin Bi{sub 2}Se{sub 3} films on CaF{sub 2} and CaF{sub 2}/Si by molecular beam epitaxy with a radio frequency cracker cell

We report a method to fabricate high quality Bi{sub 2}Se{sub 3} thin films using molecular beam epitaxy with a radio frequency cracker cell as an atomic selenium source. With Se-to-Bi ratios close to exact stoichiometry, optimal layer-by-layer growth of high quality Bi{sub 2}Se{sub 3} thin films with smooth surfaces has been achieved on CaF{sub 2}(111) substrates and Si(111) substrates with a thin epitaxial CaF{sub 2} buffer layer (CaF{sub 2}/Si). Transport measurements show a characteristic weak-antilocalization magnetoresistance in all the films, with the emergence of a weak-localization contribution in the ultrathin film limit. Quantum oscillations, attributed to the topological surface states have been observed, including in films grown on CaF{sub 2}/Si.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhancement of the open circuit voltage of Cu 2 O/Ga 2 O 3 heterojunction solar cells through the mitigation of interfacial recombination

Thin film solar cells were fabricated using cuprous oxide (Cu 2 O) absorber layers grown by chemical vapor deposition (CVD) and gallium oxide (Ga 2 O 3 ) buffer layers grown by atomic layer deposition (ALD) on the cuprous oxide CVD films. The in-situ formation of heterojunction in the same deposition system without exposure to oxygen-rich ambient was found to be effective in mitigating the creation of detrimental cupric oxide (CuO) at the interface, resulting in a pristine photovoltaic junction capable of delivering an enhanced open-circuit voltage of 1.78 V. Numerical device simulations of a novel two-layer absorber architecture (CVD-Cu 2 O on ECD-Cu 2 O) showed promising possibilities (theoretical 13.2 % efficiency) for a solar cell combining in-situ junction formation with electrochemical deposition of the absorber layer.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Ge/GeSn heterostructures grown on Si (100) by molecular-beam epitaxy

The growth of GeSn layers by molecular-beam epitaxy on Si (100) wafers coated with a germanium buffer layer is investigated. The properties of the fabricated structures are controlled by reflection high-energy electron diffraction, atomic-force microscopy, X-ray diffractometry, Rutherford backscattering, and Raman scattering. It is shown that GeSn layers with thicknesses up to 0.5 μm and Sn molar fractions up to 0.073 manifest no sign of plastic relaxation upon epitaxy. The lattice constant of the GeSn layers within the growth plane is precisely the same as that of Ge. The effect of rapid thermal annealing on the conversion of metastable elastically strained GeSn layers into a plastically relaxed state is examined. Ge/GeSn quantum wells with Sn molar fraction up to 0.11 are obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Investigation of Nitrogen Absorption Rate and Nitride Growth on SRF Cavity Grade RRR Niobium as a Function of Furnace Temperature

The current state of the art processing of niobium superconducting radio frequency cavities with nitrogen diffusion is performed at 800 C in a furnace with a partial pressure of approximately ~20 mTorr of nitrogen. Multiple studies have shown the bulk of the nitrogen absorbed by the niobium forms a thick (1-3 microns) non-superconducting nitride layer which must be removed to produce optimal RF results. The depth profiling of interstitial nitrogen and surface nitrides has already been probed using SIMS measurements for this recipe. These measurements have been successfully modelled by extrapolating data from nitride growth studies performed at atmospheric pressure and temperatures above 1000 C.1 One open question is whether there is a diffusion-only zone at lower temperature in the ~20 mTorr range in which the niobium will absorb nitrogen but not create a non-superconducting nitride layer; or is the absorption of nitrogen only possible by first forming a nitride buffer layer which then frees up nitrogen for absorption which is what has been shown down to 600 C at atmospheric pressures on long time scales. A systematic study of absorption vs. temperature and correlated SIMS measurements is required to answer this question. We report on measurements of the parabolic rate constant vs. temperature from 400 C to 900 C of cavity grade niobium with metallurgically flat witness samples. Depth profiles of witness samples using SIMS are presented together with SEM imaging for nitride crystals and correlated with N absorption.

Reece, Charles↗
DOE OSTI2013

Growth of coincident site lattice matched semiconductor layers and devices on crystalline substrates

Methods of fabricating a semiconductor layer or device and said devices are disclosed. The methods include but are not limited to providing a substrate having a crystalline surface with a known lattice parameter (a). The method further includes growing a crystalline semiconductor layer on the crystalline substrate surface by coincident site lattice matched epitaxy, without any buffer layer between the crystalline semiconductor layer and the crystalline surface of the substrate. The crystalline semiconductor layer will be prepared to have a lattice parameter (a') that is related to the substrate lattice parameter (a). The lattice parameter (a') maybe related to the lattice parameter (a) by a scaling factor derived from a geometric relationship between the respective crystal lattices.

36 MATERIALS SCIENCE↗
NASA NTRS2007

GaAs Photovoltaics on Polycrystalline Ge Substrates

High efficiency III-V multijunction solar cells deposited on metal foil or even polymer substrates can provide tremendous advantages in mass and stowage, particularly for planetary missions. As a first step towards that goal, poly-crystalline p/i/n GaAs solar cells are under development on polycrystalline Ge substrates. Organo Metallic Vapor Phase Epitaxy (OMVPE) parameters for pre-growth bake, nucleation and deposition have been examined. Single junction p/i/n GaAs photovoltaic devices, incorporating InGaP front and back window layers, have been grown and processed. Device performance has shown a dependence upon the thickness of a GaAs buffer layer deposited between the Ge substrate and the active device structure. A thick (2 m) GaAs buffer provides for both increased average device performance as well as reduced sensitivity to variations in grain size and orientation. Illumination under IR light (lambda > 1 micron), the cells showed a Voc, demonstrating the presence of an unintended photoactive junction at the GaAs/Ge interface. The presence of this junction limited the efficiency to approx.13% (estimated with an anti-refection coating) due to the current mismatch and lack of tunnel junction interconnect.

Wilt, David M.↗
DOE OSTI2012

Properties of Pb(0.92)La(0.08)Zr(0.52)Ti(0.48)O(3) thin films grown on SrRuO(3) buffered nickel and silicon substrates by chemical solution deposition

Ferroelectric film-on-foil capacitors are suitable to replace discrete passive components in the quest to develop electronic devices that show superior performance and are smaller in size. The film-on-foil approach is the most practical method to fabricate such components. Films of Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} (PLZT) were deposited on SrRuO{sub 3} (SRO) buffer films over nickel and silicon substrates. High-quality polycrystalline SRO thin-film electrodes were first deposited by chemical solution deposition. A phase pure, dense, uniform microstructure with grain size <100 nm was obtained in films crystallized at 700 C. The room-temperature resistivity of the SRO films crystallized at 700 C was {approx}800-900 {mu}{Omega}-cm. The dielectric properties of sol-gel derived PLZT capacitors on SRO-buffered nickel were evaluated as a function of temperature, bias field, and frequency, and the results were compared to those of the same films on silicon substrates. The comparison demonstrated the integrity of the buffer layer and its compatibility with nickel substrates. Device-quality dielectric properties were measured on PLZT films deposited on SRO-buffered nickel foils and found to be superior to those for PLZT on SRO-buffered silicon and expensive platinized silicon. These results suggest that SRO films can act as an effective barrier layer on nickel substrates suitable for embedded capacitor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Studying Magnetic Diodes with a GaMnAs Layer Formed by Pulsed Laser Deposition

A new design for diode heterostructures with (Ga, Mn)As ferromagnetic layers is experimentally investigated. The diode structures are fabricated using a combination of metal-organic chemical vapor deposition (MOCVD) epitaxy and pulsed laser deposition and contain (Ga, Mn)As/n-InGaAs heterojunctions. The electrical properties of the diodes in a magnetic field applied perpendicular to the p–n junction are examined. The negative magnetoresistance observed at temperatures of <50 K (below the (Ga, Mn)As Curie temperature) is attributed to a decrease in the carrier scattering caused by ferromagnetic ordering and in the potential barrier at the (Ga, Mn)As/n-InGaAs interface. The observed negative magnetoresistance depends nonmonotonically on the forward bias with a maximum in the voltage region close to the p–n-junction potential barrier height. The maximum can be caused by the decisive contribution of the spin-dependent resistances of the (Ga, Mn)As layer and (Ga, Mn)As/n-InGaAs interface to the total resistance of the structure. It is found that the dependence of the magnetoresistance on the external magnetic field is hysteretic due to the influence of tensile stresses in the (Ga, Mn)As layer grown on top of a relaxed In{sub x}Ga{sub 1 –x}As buffer layer with an indium content of x ≈ 0.1.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Features of molecular-beam epitaxy and structural properties of AlInSb-based heterostructures

AlInSb layers are grown on highly lattice-mismatched GaAs (100) substrates by molecular-beam epitaxy (MBE) and studied in situ by reflection high-energy electron diffraction and ex situ by scanning and transmission electron microscopy (SEM and TEM). It is shown that one feature of AlInSb/GaAs heterostructure features is a high probability of forming microtwins; methods for decreasing their concentration are proposed. To initiate AlInSb growth on GaAs substrates under high lattice-mismatch ({approx}14.5%) conditions and to stimulate the transition to 2D growth, the GaAs layer surface was preliminarily exposed to an antimony flux followed by deposition of an intermediate AlSb buffer layer. The optimization of initial MBE growth stages of Sb-containing layers on the GaAs surface allows a decrease in the defect density in the GaAs/AlInSb heterostructures more than by two orders of magnitude, including a drastic decrease in the microtwin density. Optimal MBE growth conditions for Al{sub x}Al{sub 1-x}Sb are determined in a wide composition range (0 < x < 0.3). The TEM and SEM studies confirm the high structural quality of grown GaAs/AlInSb heterostructures. Hall-effect measurements showed the dependence of the carrier mobility and concentration on the aluminum content in AlInSb layers and allowed preliminary conclusions on scattering mechanisms.

36 MATERIALS SCIENCE↗
NASA NTRS1983

Production of large-area single-crystal wafers of cubic SiC for semiconductor devices

A reproducible process is described for growing a thick single-crystal layer of cubic SiC on a single-crystal Si wafer by chemical vapor deposition. A buffer layer, grown in situ, is used between the cubic SiC and the Si substrate to minimize the effect of lattice mismatch. Layers of up to 34 microns thick and several sq cm in area have been grown. Wafers are obtained by chemically removing the Si substrates from the grown layers. Excellent electron channeling patterns produced by these wafers indicate very good crystal quality. Preliminary electrical measurements have yielded electron mobilities up to 380 sq cm/Vs.

Nishino, S.↗
NASA NTRS1996

Method of fabricating multiwavelength infrared focal plane array detector

A multiwavelength local plane array infrared detector is included on a common substrate having formed on its top face a plurality of In.sub.x Ga.sub.1-x As (x.ltoreq.0.53) absorption layers, between each pair of which a plurality of InAs.sub.y P.sub.1-y (y.ltoreq.1) buffer layers are formed having substantially increasing lattice parameters, respectively, relative to said substrate, for preventing lattice mismatch dislocations from propagating through successive ones of the absorption layers of decreasing bandgap relative to said substrate, whereby a plurality of detectors for detecting different wavelengths of light for a given pixel are provided by removing material above given areas of successive ones of the absorption layers, which areas are doped to form a pn junction with the surrounding unexposed portions of associated absorption layers, respectively, with metal contacts being formed on a portion of each of the exposed areas, and on the bottom of the substrate for facilitating electrical connections thereto.

Forrest, Stephen R.↗
NASA NTRS1995

Multiwavelength infrared focal plane array detector

A multiwavelength focal plane array infrared detector is included on a common substrate having formed on its top face a plurality of In.sub.x Ga.sub.1-x As (x.ltoreq.0.53) absorption layers, between each pair of which a plurality of InAs.sub.y P.sub.1-y (y<1) buffer layers are formed having substantially increasing lattice parameters, respectively, relative to said substrate, for preventing lattice mismatch dislocations from propagating through successive ones of the absorption layers of decreasing bandgap relative to said substrate, whereby a plurality of detectors for detecting different wavelengths of light for a given pixel are provided by removing material above given areas of successive ones of the absorption layers, which areas are doped to form a pn junction with the surrounding unexposed portions of associated absorption layers, respectively, with metal contacts being formed on a portion of each of the exposed areas, and on the bottom of the substrate for facilitating electrical connections thereto.

Forrest, Stephen R.↗
DOE OSTI2014

Enhanced catalyst-free nucleation of GaN nanowires on amorphous Al{sub 2}O{sub 3} by plasma-assisted molecular beam epitaxy

We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al{sub 2}O{sub 3} buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al{sub 2}O{sub 3} buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Real structure of the ZnO epitaxial films on (0001) leucosapphire substrates coated by ultrathin gold layers

The real structure of ZnO films formed by magnetron sputtering on (0001) leucosapphire substrates coated by an ultrathin (less than 0.7 nm) Au buffer layer has been studied by high-resolution microscopy. It is shown that modification of the leucosapphire substrate surface by depositing ultrathin Au layers does not lead to the formation of Au clusters at the film–substrate interface but significantly improves the structural quality of ZnO epitaxial films. It is demonstrated that the simplicity and scalability of the technique used to modify the substrate surface in combination with a high (above 2 nm/s) film growth rate under magnetron sputtering make it possible to obtain high-quality (0001) ZnO epitaxial films with an area of 5–6 cm{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Emission Properties of Heavily Doped Epitaxial Indium-Nitride Layers

The results of studies of the spontaneous photoluminescence and stimulated emission spectra of epitaxial n-InN layers with a concentration of free electrons of ~10{sup 19} cm{sup –3} are reported. The layers are grown by molecular-beam epitaxy with the plasma activation of nitrogen on sapphire substrates with AlN and GaN buffer layers. It is found that, as the InN layers are grown under conditions with enrichment with nitrogen at a growth temperature elevated to 470°C close to the beginning of the decomposition of InN, the crystal quality of the layers is improved and the stimulated-emission threshold is lowered. As the conditions of growth change to conditions with enrichment with the metal, two emission bands separated by an energy of 100 meV are observed in the spontaneous-photoluminescence spectra of InN. For such layers, a substantial increase in the stimulated-emission threshold and, in some cases, the lack of a transition to stimulated emission are observed. In the study, an interpretation of the observed emission bands is given and some inferences as to their nature are made.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1979

Characterization and growth of epitaxial layers of Gs exhibiting high resistivity for ionic implantation

Either classical or low temperature epitaxial growth techniques can be used to control the deposition of buffer layers of GaAs on semiconducting substrates and to obtain the resistivity and purity desired. Techniques developed to study, as a function of thickness, the evolution of mobilities by photoHall, and the spectroscopy of shallow and deep centers by cathodoluminescence and current transients reveal one very pure layer of medium resistivity and high mobility, and another "dead layer" of elevated resistivity far from the surface. The highly resistive layer remains pure over several microns, which appears interesting for implantation.

Source record↗