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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.

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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

High-quality Bi{sub 2}Te{sub 3} thin films grown on mica substrates for potential optoelectronic applications

We report high-quality topological insulator Bi{sub 2}Te{sub 3} thin films grown on muscovite mica substrates by molecular beam epitaxy. The topographic and structural analysis revealed that the Bi{sub 2}Te{sub 3} thin films exhibited atomically smooth terraces over a large area and a high crystalline quality. Both weak antilocalization effect and quantum oscillations were observed in the magnetotransport of the relatively thin samples. A phase coherence length of 277 nm for a 6 nm thin film and a high surface mobility of 0.58 m{sup 2} V{sup -1} s{sup -1} for a 4 nm thin film were achieved. These results confirm that the thin films grown on mica are of high quality.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Polarization doping and the efficiency of III-nitride optoelectronic devices

The intrinsic polarization is generally considered a nuisance in III-nitride devices, but recent studies have shown that it can be used to enhance p- and n-type conductivity and even to replace impurity doping. We show by numerical simulations that polarization-doped light-emitting diode (LED) structures have a significant performance advantage over conventional impurity-doped LED structures. Our results indicate that polarization doping decreases electric fields inside the active region and potential barriers in the depletion region, as well as the magnitude of the quantum-confined Stark effect. The simulations also predict at least an order of magnitude increase in the current density corresponding to the maximum efficiency (i.e., smaller droop) as compared to impurity-doped structures. The obtained high doping concentrations could also enable, e.g., fabrication of III-N resonant tunneling diodes and improved ohmic contacts.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Studies on optoelectronic properties of DC reactive magnetron sputtered CdTe thin films

Cadmium telluride continues to be a leading candidate for the development of cost effective photovoltaics for terrestrial applications. In the present work two individual metallic targets of Cd and Te were used for the deposition of CdTe thin films on mica substrates from room temperature to 300 °C by DC reactive magnetron sputtering method. XRD patterns of CdTe thin films deposited on mica substrates exhibit peaks at 2θ = 27.7°, 46.1° and 54.6°, which corresponds to reflection on (1 1 1), (2 2 0) and (3 1 1) planes of CdTe cubic structure. The intensities of XRD patterns increases with the increase of substrate temperature upto 150 °C and then it decreases at higher substrate temperatures. The conductivity of CdTe thin films measured from four probe method increases with the increase of substrate temperature. The activation energies (ΔE) are found to be decrease with the increase of substrate temperature. The optical transmittance spectra of CdTe thin films deposited on mica have a clear interference pattern in the longer wavelength region. The films have good transparency (T > 85 %) exhibiting interference pattern in the spectral region between 1200 – 2500 nm. The optical band gap of CdTe thin films are found to be in the range of 1.48 – 1.57. The refractive index, n decreases with the increase of wavelength, λ. The value of n and k increases with the increase of substrate temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Optoelectronic and low temperature thermoelectric studies on nanostructured thin films of silver gallium selenide

Polycrystalline thin films of silver gallium selenide were deposited on ultrasonically cleaned soda lime glass substrates by multi-source vacuum co-evaporation technique. The structural analysis done by X-ray diffraction ascertained the formation of nano structured tetragonal chalcopyrite thin films. The compound formation was confirmed by X-ray photo-electron spectroscopy. Atomic force microscopic technique has been used for surface morphological analysis. Direct allowed band gap ∼1.78eV with high absorption coefficient ∼10{sup 6}/m was estimated from absorbance spectra. Low temperature thermoelectric effects has been investigated in the temperature range 80–330K which manifested an unusual increase in Seebeck coefficient with negligible phonon drag toward the very low and room temperature regime. The electrical resistivity of these n-type films was assessed to be ∼2.6Ωm and the films showed good photo response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optoelectronic simulation of GaAs solar cells with angularly selective filters

We discuss the influence of angularly selective filters on thin film gallium arsenide solar cells. For this reason, the detailed balance model was refined to fit our needs with respect to Auger recombination, reflection, transmission, and realistic absorption. For calculating real systems, an approach was made to include optical effects of angularly selective filters into electron-hole dynamic equations implemented in PC1D, a one dimensional solar cell calculation tool. With this approach, we find a relative V{sub oc} increase of 5% for an idealized 100 nm GaAs cell, including Auger recombination.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A direct measurement of the electronic structure of Si nanocrystals and its effect on optoelectronic properties

Since reports that silicon nanocrystals (Si-NCs) can exhibit direct transition emission, the silicon laser field is at a juncture where the importance of this discovery needs to be evaluated. Most theoretical models predicted a monotonic increase in the bandgap and experimental information currently available on the electronic structure at the Γ valley of these promising materials is circumstantial as it is obtained from emission measurements where competing non-radiative relaxation and recombination processes only provide an incomplete picture of the electronic structure of Si-NCs. Optical absorption, the most immediate probe of the electronic structure beyond the band-edges, showing the evolution of the Γ valley states with nanocrystal size has not been measured. Here, we show such measurements, performed with high dynamic range, allowing us to observe directly the effect of crystal size on the Γ valley splitting far above the band-edges. We show that the splitting is 100 s of meV more pronounced than predicted by pseudo potential calculations and Luttinger-Kohn model. We also show that ultrafast red-shifting emission can be observed in plasma enhanced chemical vapor deposition prepared Si-NCs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigations on optoelectronic transition mechanisms of silicon nanoporous pillar array by using surface photovoltage spectroscopy and photoluminescence spectroscopy

We report the electronic transition mechanisms for hydrothermally prepared silicon nanoporous pillar array (Si-NPA), investigated by surface photovoltage (SPV) spectroscopy and photoluminescence (PL) spectroscopy. By comparing the SPV spectra of single crystal silicon (sc-Si) with that of Si-NPA, the silicon nano-crystallites (nc-Si)/SiO{sub x} nanostructure in the Si-NPA could produce SPV in the wavelength range of 300–580 nm. And 580 nm (∼2.14 eV) was considered as the absorption edge of the nc-Si/SiO{sub x} nanostructure. After the sample was annealed and oxidized in air at different temperatures, both the SPV in the wavelength range of 300–580 nm and the PL emission band around 690 nm from the nc-Si/SiO{sub x} nanostructure weakened and disappeared as the annealing temperature increased from 100 to 500 °C. But both the red-infrared PL band (>710 nm) and the violet-blue PL band were enhanced by increasing the annealing temperature. After 2 years of natural oxidation in air, the SPV features for sc-Si disappeared completely, and the SPV characteristics of the nc-Si/SiO{sub x} nanostructure could be clearly observed. After analysis, the Si–O structure related localized states at the nc-Si/SiO{sub x} interface dominated the electronic transitions during the red PL emission and the SPV for the nc-Si/SiO{sub x} nanostructure in Si-NPA, the red–infrared PL was due to the Si=O structure related electronic transitions, and the violet-blue PL emission could attribute to the oxygen-related defect related recombination of the photo induced carriers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors

A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k{sup →}) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg{sub 1−x}Cd{sub x}Te, and In{sub 1−x}Ga{sub x}As{sub y}P{sub 1−y} lattice matched to InP, as example of III–V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Heteroepitaxial ZnO films on diamond: Optoelectronic properties and the role of interface polarity

We demonstrate the growth of heteroepitaxial ZnO films on (110) diamond substrates by molecular beam epitaxy and report on a major advance in structural quality, as confirmed by XRD and high-resolution TEM measurements. The growth direction is found to be along the polar c-axis with Zn-polarity, deduced from annular bright-field scanning transmission electron microscopy imaging. This is important information, as simulations of the electronic band structure reveal the ZnO polarity to dominate the electronic structure of the interface: the formation of a two-dimensional electron gas on the ZnO side or a two-dimensional hole gas on the diamond side are predicted for Zn- and O-polarity, respectively. In addition, photoluminescence and absorption studies exhibit good optical properties and reveal stimulated emission for optical excitation above a threshold of 30 kW/cm{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Studies on optoelectronic properties of DC reactive magnetron sputtered chromium doped CdO thin films

Cr doped CdO thin films were deposited on glass substrates by DC reactive magnetron sputtering method and subsequently annealed from 200 °C to 500 °C. X-ray diffraction analysis showed that the films exhibit (1 1 1) preferred orientation. The optical transmittance of the films increases from 64% to 88% with increasing annealing temperature. The optical band gap values were found to be decreased from 2.77 to 2.65 eV with the increase of annealing temperature. The decrease in optical band gap energy with increasing annealing temperature can be attributed to improvement in the crystallinity of the films and may also be due to quantum confinement effect. A minimum resistivity of 2.23 × 10{sup −4} Ω.cm and sheet resistance of 6.3 Ω/sq is obtained for Cr doped CdO film annealed at 500 °C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of optoelectronic properties of cubic perovskite LaGaO{sub 3}

The structural, electronic, bonding and optical properties of cubic perovskite LaGaO{sub 3} have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method in the density functional theory (DFT) as embodied in WIEN2k code. The modified Becke-Johnson (mBJ) potential is applied for the calculation of electronic and optical properties. The calculated lattice constant is in good agreement with the experimental result. The predicted band structure shows an indirect (M-X) band gap of 4.22 eV. The bonding in the material is of mixed covalent and ionic nature. Optical properties like dielectric function, refractive index, reflectivity, conductivity and absorption coefficient are presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A novel synthesis of tin oxide thin films by the sol-gel process for optoelectronic applications

A novel and simple chemical method based on sol-gel processing was proposed to deposit metastable orthorhombic tin oxide (SnOx) thin films on glass substrates at room temperature. The resultant samples are labeled according to the solvents used: ethanol (SnO-EtOH), isopropanol (SnO-IPA) and methanol (SnO-MeOH). The variations in the structural, morphological and optical properties of the thin films deposited using different solvents were characterized by X-ray diffraction, atomic force microscopy, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy and photoluminescence (PL) analysis. The XRD patterns confirm that all the films, irrespective of the solvents used for preparation, were polycrystalline in nature and contained a mixed phases of tin (II) oxide and tin (IV) oxide in a metastable orthorhombic crystal structure. FTIR spectra confirmed the presence of Sn=O and Sn-O in all of the samples. PL spectra showed a violet emission band centered at 380 nm (3.25 eV) for all of the solvents. The UV-vis spectra indicated a maximum absorption band shown at 332 nm and the highest average transmittance around 97% was observed for the SnO-IPA and SnO-MeOH thin film samples. The AFM results show variations in the grain size with solvent. The structural and optical properties of the SnO thin films indicate that this method of fabricating tin oxide is promising and that future work is warranted to analyze the electrical properties of the films in order to determine the viability of these films for various transparent conducting oxide applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Optoelectronic transport properties in amorphous/crystalline silicon solar cell heterojunctions measured by frequency-domain photocarrier radiometry: Multi-parameter measurement reliability and precision studies

A theoretical one-dimensional two-layer linear photocarrier radiometry (PCR) model including the presence of effective interface carrier traps was used to evaluate the transport parameters of p-type hydrogenated amorphous silicon (a-Si:H) and n-type crystalline silicon (c-Si) passivated by an intrinsic hydrogenated amorphous silicon (i-layer) nanolayer. Several crystalline Si heterojunction structures were examined to investigate the influence of the i-layer thickness and the doping concentration of the a-Si:H layer. The experimental data of a series of heterojunction structures with intrinsic thin layers were fitted to PCR theory to gain insight into the transport properties of these devices. The quantitative multi-parameter results were studied with regard to measurement reliability (uniqueness) and precision using two independent computational best-fit programs. The considerable influence on the transport properties of the entire structure of two key parameters that can limit the performance of amorphous thin film solar cells, namely, the doping concentration of the a-Si:H layer and the i-layer thickness was demonstrated. It was shown that PCR can be applied to the non-destructive characterization of a-Si:H/c-Si heterojunction solar cells yielding reliable measurements of the key parameters.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Optoelectronic properties and interband transition of La-doped BaSnO{sub 3} transparent conducting films determined by variable temperature spectral transmittance

Perovskite-structured Ba{sub 1−x}La{sub x}SnO{sub 3} (x = 0–0.10) films have been directly grown on (0001) sapphire substrates by a sol-gel method. Optical properties and bandgap energy of the films have been investigated by transmittance spectra from 10 K to 450 K. It indicates that these films exhibit a high transmission of more than 80% in the visible region. With increasing temperature, there is a significant bandgap shrinkage of about 0.5 eV for lightly La doping (x ≤ 0.04) films. For heavily La doping concentration (x ≥ 0.06), the bandgap remains nearly stable with the temperature and La composition. This is due to the fact that the lattice expansion caused by La doping is close to the saturation for the film doped with x = 0.06. Moreover, temperature dependent conductivity behavior shows a similar pattern, which suggests that the doping concentration of La-doped BaSnO{sub 3} (BLSO) films has a saturated state. The La introduction can modify the Sn 5s-O 2p antibonding state and the nonbonding O 2p orbital, which remarkably affect the electronic bandgap of the BLSO films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of solvent ratio on the optoelectronic properties of fluorine doped tin oxide thin films

Fluorine doped tin oxide (FTO) thin films were deposited on to the well cleaned microscopic glass substrates using nebulized-spray pyrolysis (n-SP) technique by varying the water to ethanol solvent proportion. The deposited thin films were characterized by X-ray diffraction (XRD), UV-Vis-NIR spectroscopy, field emission scanning electron microscopy and Hall measurements to study the structural, optical, surface morphological and electrical properties of the films, respectively. Results of the analyzes show that the films are polycrystalline, having tetragonal structure with the preferred orientation along (110) plane. The grain size varies between 7 to 20 nm. The optimized films exhibit the optical transparency of 85 % at the wavelength of 580 nm. The optical bandgap lies in the range of 3.94 to 4 eV. The optimized films, deposited with 40 % of ethanol proportion are having the mean resistivity 4.72×10{sup −3} Ω-cm, carrier concentration 1.79×10{sup 20} cm{sup 3} and the mobility 7 cm{sup 2}/Vs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optoelectronic properties of p-diamond/n-GaN nanowire heterojunctions

In this work, nanodiodes comprised of n-GaN nanowires on p-diamond substrates are investigated. The electric transport properties are discussed on the basis of simulations and determined experimentally for individual p-diamond/n-GaN nanodiodes by applying conductive atomic force microscopy. For low doping concentrations, a high rectification ratio is observed. The fabrication of a prototype nanoLED device on the basis of ensemble nanowire contacts is presented, showing simultaneous electroluminescence in the UV and the green spectral range which can be ascribed to hole injection into the n-GaN nanowires and electron injection into the p-diamond, respectively. In addition, the operation and heat distribution of the nanoLED device are visualized by active thermographic imaging.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Al{sub 4}SiC{sub 4} wurtzite crystal: Structural, optoelectronic, elastic, and piezoelectric properties

New experimental results supported by theoretical analyses are proposed for aluminum silicon carbide (Al{sub 4}SiC{sub 4}). A state of the art implementation of the density functional theory is used to analyze the experimental crystal structure, the Born charges, the elastic properties, and the piezoelectric properties. The Born charge tensor is correlated to the local bonding environment for each atom. The electronic band structure is computed including self-consistent many-body corrections. Al{sub 4}SiC{sub 4} material properties are compared to other wide band gap wurtzite materials. From a comparison between an ellipsometry study of the optical properties and theoretical results, we conclude that the Al{sub 4}SiC{sub 4} material has indirect and direct band gap energies of about 2.5 eV and 3.2 eV, respectively.

36 MATERIALS SCIENCE↗