Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “EPITAXY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

High Carrier Lifetimes in Epitaxial Germanium-Tin/Al(In)As Heterostructures with Variable Tin Compositions

Group IV-based germanium-tin (Ge1-ySny) compositional materials have recently shown great promise for infrared detection, light emission and ultra-low power transistors. High carrier lifetimes are desirable for enhancing the detection limit and efficiency of photodetectors, low threshold current density in lasers, and low tunneling barrier height by lowering defects and dislocations at the heterointerface of a source and a channel. Here, carrier lifetimes in epitaxial germanium (Ge) and variable tin (Sn) compositional Ge1-ySny materials were experimentally determined on GaAs substrates using the contactless microwave photoconductive decay (u-PCD) technique at an excitation wavelength of 1500 nm. Sharp (2 x 2) reflection high energy electron diffraction patterns and low surface roughness were observed from the surface of the Ge0.97Sn0.03 epilayer. X-ray rocking curves from Ge0.97Sn0.03 and Ge0.94Sn0.06 layers demonstrated the pseudomorphic and lattice-matched growth on AlAs and In0.12Al0.88As buffers, respectively, further substantiated by reciprocal space maps and abrupt heterointerfaces evident from the presence of Pendellosung oscillations. High effective carrier lifetimes of 150 ns to 450 ns were measured for Ge1-ySny epilayers as a function of Sn composition, surface roughness, growth temperature, and layer thickness. The observed increase in the carrier lifetime with an increasing Ge layer thickness and a reducing surface roughness, by incorporating Sn, were explained. The enhancement of the carrier lifetime with an increasing Sn concentration was achieved by controlling the defects with lattice-matched Ge0.94Sn0.06/In0.12Al0.88As heterointerfaces or the pseudomorphic growth of Ge0.94Sn0.06 on GaAs. Therefore, our monolithic integration of variable Sn alloy compositional Ge1-ySny materials with high carrier lifetimes opens avenues to realize electronic and optoelectronic devices.

carrier lifetimes↗
DOE OSTI2010

Hydrogen refinement during solid phase epitaxy of buried amorphous silicon layers

The effect of hydrogen on the kinetics of solid phase epitaxy (SPE) have been studied in buried amorphous Si layers. The crystallization rate of the front amorphous/crystalline (a/c) interface is monitored with time resolved reflectivity. Secondary ion mass spectrometry (SIMS) is used to examine H implanted profiles at selected stages of the anneals. The H retardation of the SPE rate is determined up to a H concentration of 2.3x10{sup 20} cm{sup -3} where the SPE rate decreases by 80%. Numerical simulations are performed to model the H diffusion, the moving a/c interfaces and the refinement of the H profile at these interfaces. Despite the high H concentration involved, a simple Fickian diffusion model results in good agreement with the SIMS data. The segregation coefficient is estimated to be 0.07 at 575 deg. C. A significant fraction of the H escapes from the a-Si layer during SPE especially once the two a/c interfaces meet which is signified by the lack of H-related voids after a subsequent high temperature anneal.

36 MATERIALS SCIENCE↗
DOE OSTI2010

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy.

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {l_brace}1{bar 1}01{r_brace} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to {approx}20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650-780 C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Microwave properties of epitaxial (111)-oriented Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3} thin films on Al{sub 2}O{sub 3}(0001) up to 40 GHz

Perovskite Ba{sub 0.6}Sr{sub 0.4}TiO{sub 3} (BST) thin films have been grown on Al{sub 2}O{sub 3}(0001) substrates without/with inserting an ultrathin TiO{sub x} seeding layer by rf magnetron sputtering. X-ray diffraction and pole figure studies reveal that the film with the TiO{sub x} layer (12-A-thick) is highly oriented along the (111) direction and exhibits a good in-plane relationship of BST(111)||Al{sub 2}O{sub 3}(0001). The high frequency dielectric measurements demonstrate that the complex permittivity ({epsilon}={epsilon}{sup '}-j{epsilon}{sup ''}) is well described by a Curie-von Scheidler dispersion with an exponent of 0.40. The resulting epitaxial BST films show high permittivity ({approx}428) and tunability ({approx}41%, at 300 kV/cm and 40 GHz) and their microwave properties (1-40 GHz) potentially could be made suitable for tunable devices.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Formation of epitaxial metastable NiGe{sub 2} thin film on Ge(100) by pulsed excimer laser anneal

Epitaxial nickel digermanide (NiGe{sub 2}), a metastable phase, was formed by laser annealing Ni on (100) germanium-on-silicon substrates. The NiGe{sub 2} formation was investigated using transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectroscopy, and first-principles calculations. The formation mechanism of NiGe{sub 2} is discussed and is attributed to both the reduced interfacial energy at the NiGe{sub 2}/Ge(100) interface and the kinetic aspects of the laser annealing reaction associated with phase transformation and film agglomeration.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T{sub c}) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Clear correspondence between magnetoresistance and magnetization of epitaxially grown ordered FeRh thin films

Magnetoresistance and magnetization of the CsCl-type ordered FeRh epitaxial thin films grown on MgO(001) substrates are investigated as a function of temperature and film thickness. All the films show a clear first-order magnetic phase transition from the antiferromagnetic state to the ferromagnetic state at around 380 K. A large negative variation in the field-dependent magnetoresistance of the FeRh thin films, which is accompanied by the field-induced magnetic phase transition, is found to be well scaled with the magnetization squared M{sup 2}. The results indicate that the magnetoresistance primarily arises from spin-dependent scattering through the s-d exchange interactions between conduction electrons and the localized magnetic moments.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Distribution of bismuth atoms in epitaxial GaAsBi

The distribution of Bi atoms in epitaxial GaAs{sub (1-x)}Bi{sub x} is analyzed through aberration-corrected Z-contrast images. The relation between the atomic number and the intensity of the images allows quantifying the distribution of Bi atoms in this material. A bidimensional map of Bi atoms is extracted showing areas where nanoclustering is possible and evidencing the location of Bi at As-substitutional positions in the lattice. The distribution of Bi atoms differs from a random spatial pattern of Bi atoms in the material.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings

It was demonstrated that Fe-N martensite ({alpha}') films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe{sub 16}N{sub 2} films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe{sub 8}N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe{sub 8}N phase measured by VSM was evaluated in the range of 2.0-2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in {alpha}''-Fe{sub 16}N{sub 2} phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].

36 MATERIALS SCIENCE↗
DOE OSTI2011

Neutron doping effects in epitaxially laterally overgrown n-GaN

Thermal neutron irradiation and annealing effects were studied for undoped n-GaN prepared by epitaxial lateral overgrowth (ELOG). Electron beam induced current (EBIC) imaging and profiling prior to irradiation showed that the residual donor doping in our ELOG samples was about three times higher in the high-dislocation-density ELOG wing than in the low-dislocation-density ELOG window regions. Irradiation with thermal neutrons and subsequent annealing led to greatly improved doping uniformity, as evidenced by EBIC imaging. The neutron transmutation doping avoids the anisotropy of donor incorporation efficiency for different planes during ELOG and provides a uniform doping environment. Capacitance-voltage profiling on such samples showed the presence of electrically active centers with concentration close to the concentration of Ge donors produced by Ga interaction with thermal neutrons.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial growth of Heusler alloy Co{sub 2}MnSi/MgO heterostructures on Ge(001) substrates

We prepared Heusler alloy Co{sub 2}MnSi/MgO heterostructures on single-crystal Ge(001) substrates through magnetron sputtering for Co{sub 2}MnSi and electron beam evaporation for MgO as a promising candidate for future generation spin-based functional devices. Structural investigations showed that the Co{sub 2}MnSi/MgO heterostructure was grown epitaxially on a Ge(001) substrate with extremely smooth and abrupt interfaces and showed the L2{sub 1} structure for the Co{sub 2}MnSi film. Furthermore, a sufficiently high saturation magnetization ({mu}{sub s}) value of 5.1 {mu}{sub B}/f.u. at 10 K, which is close to the theoretically predicted {mu}{sub s} of 5.0 {mu}{sub B}/f.u. for half-metallic Co{sub 2}MnSi, was obtained for prepared Co{sub 2}MnSi films.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Transport Properties Of Fe{sub 3-x}Ti{sub x}O{sub 4}(x = 0.0 and 0.0206) Epitaxial Thin Films

Fe{sub 3-x}Ti{sub x}O{sub 4}(x = 0 and 0.0206) epitaxial thin films with thickness of 150 nm were prepared by pulsed-laser deposition technique on SrTiO{sub 3}(100), {alpha}-Al{sub 2}O{sub 3}(0001), Si (111) and Float Glass substrates. X-ray diffraction (XRD) pattern infers that parent and Ti doped magnetites are grown in single phase with (111) orientation. The Verwey transition temperature for Fe{sub 3}O{sub 4} thin films are 121 K (Float Glass), 123 K (SrTiO{sub 3}), 123.5 K ({alpha}-Al{sub 2}O{sub 3}) and 128 K (Si). It is shown that the Ti doping at B site results in the disappearance of Verwey transition in {l_brace}Ti{sub x}{sup 4+}Fe{sub 1-x}{sup 3+}Fe{sup 2+}{r_brace}O{sub 4}{sup 2-}.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Huge Anisotropic Magnetoresistance In Epitaxial Sm{sub 0.53}Sr{sub 0.47}MnO{sub 3} Thin Films

In the present work we have studied the variation of LFMR as a function of the angle ({theta}) between the applied magnetic field and the plane of the film. DC magnetron sputtering was used for film preparation on LSAT single crystal substrates from Sm{sub 0.55}Sr{sub 0.45}MnO{sub 3} targets synthesized by the solid state reaction route. X-ray diffraction confirmed the epitaxial nature of these films. These films have simultaneous ferromagnetic and metal insulator transition at {approx}91 K. These films exhibit enhanced low field colossal magnetoresistance {approx}99% at 3.6 kOe at 80 K. Huge anisotropy is observed in the MR, which decreases from {approx}80% to 6% as {theta} increases from 0 to 90 deg. The evolution of the resistance as a function of time and the magnetic field at a constant temperature (T{approx_equal}77 K) has also been studied.

36 MATERIALS SCIENCE↗
DOE OSTI2011

High quality InAlN single layers lattice-matched to GaN grown by molecular beam epitaxy

We report on properties of high quality {approx}60 nm thick InAlN layers nearly in-plane lattice-matched to GaN, grown on c-plane GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Excellent crystalline quality and low surface roughness are confirmed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. High annular dark field observations reveal a periodic in-plane indium content variation (8 nm period), whereas optical measurements evidence certain residual absorption below the band-gap. The indium fluctuation is estimated to be {+-} 1.2% around the nominal 17% indium content via plasmon energy oscillations assessed by electron energy loss spectroscopy with sub-nanometric spatial resolution.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Strain relaxation in GaN/Al{sub x}Ga{sub 1-x}N superlattices grown by plasma-assisted molecular-beam epitaxy

We have investigated the misfit relaxation process in GaN/Al{sub x}Ga{sub 1-x}N (x = 0.1, 0.3, 0.44) superlattices (SL) deposited by plasma-assisted molecular beam epitaxy. The SLs under consideration were designed to achieve intersubband absorption in the mid-infrared spectral range. We have considered the case of growth on GaN (tensile stress) and on AlGaN (compressive stress) buffer layers, both deposited on GaN-on-sapphire templates. Using GaN buffer layers, the SL remains almost pseudomorphic for x = 0.1, 0.3, with edge-type threading dislocation densities below 9 x 10{sup 8} cm{sup -2} to 2 x 10{sup 9} cm{sup -2}. Increasing the Al mole fraction to 0.44, we observe an enhancement of misfit relaxation resulting in dislocation densities above 10{sup 10} cm{sup -2}. In the case of growth on AlGaN, strain relaxation is systematically stronger, with the corresponding increase in the dislocation density. In addition to the average relaxation trend of the SL, in situ measurements indicate a periodic fluctuation of the in-plane lattice parameter, which is explained by the different elastic response of the GaN and AlGaN surfaces to the Ga excess at the growth front. The results are compared with GaN/AlN SLs designed for near-infrared intersubband absorption.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Epitaxial TiN(001) wetting layer for growth of thin single-crystal Cu(001)

Single-crystal Cu(001) layers, 4-1400 nm thick, were deposited on MgO(001) with and without a 2.5-nm-thick TiN(001) buffer layer. X-ray diffraction and reflection indicate that the TiN(001) surface suppresses Cu-dewetting, yielding a 4 x lower defect density and a 9 x smaller surface roughness than if grown on MgO(001) at 25 deg. C. In situ and low temperature electron transport measurements indicate that ultra-thin (4 nm) Cu(001) remains continuous and exhibits partial specular scattering at the Cu-vacuum boundary with a Fuchs-Sondheimer specularity parameter p = 0.6 {+-} 0.2, suggesting that the use of epitaxial wetting layers is a promising approach to create low-resistivity single-crystal Cu nanoelectronic interconnects.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optical detection of the defects associated with the magnetic properties observed in GaN:Gd layers grown by reactive molecular beam epitaxy

The effect of annealing on the magnetic and the optoelectronic properties of Gd-doped GaN layers, which are grown by reactive molecular beam epitaxy technique, has been investigated using photoluminescence, photoconductivity, and vibrating sample magnetometry techniques. The study reveals that the multiple types of defects are formed in GaN layers during the Gd incorporation. However, the density of only one defect type, which results in a strong luminescence feature at 3.05 eV in the photoluminescence spectra recorded at low temperatures in GaN:Gd layers, has been found to decrease significantly upon annealing and at the same time, the saturation magnetization is also observed to reduce, suggesting that these defects are responsible for the magnetic properties observed in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Point contact Andreev spectroscopy of epitaxial Co{sub 2}FeSi Heusler alloys on GaAs (001)

The predicted half-metallicity of Co{sub 2}FeSi in combination with its high Curie temperature of above 980 K makes this Heusler alloy interesting for spinelectronics. Thin Co{sub 2}FeSi films are grown by molecular-beam epitaxy on GaAs (001) with a close lattice match. We present a study of point-contact measurements on different films, varying in thickness between 18 nm and 48 nm and in substrate temperature during deposition between 100 deg. C and 300 deg. C. Transport spin polarizations at the Fermi level are determined from differential conductance curves obtained by point-contact Andreev-reflection spectroscopy. A maximum transport spin polarization of about 60% is measured for a 18 nm thin Co{sub 2}FeSi film grown at 200 deg. C.

36 MATERIALS SCIENCE↗