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Bi2Te3-based thermoelectric materials with large thermoelectric figure of merit, ZT, at elevated temperatures are advantageous in power generation by using the low-grade waste heat. Here, we show that incorporation of small proportion (0.3 vol. %) of nanophase Cu2Se into BiSbTe matrix causes an enhanced high-temperature thermopower due to elevated energy filtering of carriers and inhibition of minority transport besides enhanced phonon blocking from scattering at interfaces, which concurrently result in an ~20% increase in the power factor and an ~60% reduction in the lattice thermal conductivity at 488 K. As a result, ZT=1.6 is achieved at 488K in the composite system with 0.3 vol.% of Cu2Se. Significantly, its ZT is larger than unit in broad high-temperature range (e.g., ZT=1.3 at 400K and ZT=1.6 at 488 K), which makes this material to be attractive for applications in energy harvesting from the low-grade waste heat. [ABSTRACT FROM AUTHOR]

To enhance thermoelectric performance by utilizing topological properties of topological insulators has attracted increasing attention. Here, we show that as grain size decreases from microns to ~80nm in thickness, the electron mobility μ increases steeply from 12-15 cm² V-1 s-1 to ~600cm² V-1 s-1 , owing to the contribution of increased topologically protected conducting surfaces. Simultaneously, its lattice thermal conductivity is lowered by ~30%-50% due to enhanced phonon scattering from the increased grain boundaries. As a result, thermoelectric figure of merit, ZT, of all the fine-grained samples is improved. Specifically, a maximum value of ZT =~0.63 is achieved for Bi2Se3 at T = ~570 K. [ABSTRACT FROM AUTHOR]

In Pb and Te-free β-Zn4Sb3 based composites incorporated with nanophase Cu3SbSe4 (~200 nm), we concurrently realize ~30% increase in thermoelectric power factor (PF) through an energy filtering effect caused by carrier scattering at interface barriers, and around twofold reduction in lattice thermal conductivity due to interface scattering allowing the figure of merit (ZT) to reach 1.37 at 648K in the composite system with 5 vol.% of Cu3SbSe4. Present results demonstrate that simultaneous enhancement of PF and phonon blocking can be achieved via proper design of a material-system and its microstructures, resulting in large increase in ZT of a material-system. [ABSTRACT FROM AUTHOR]

We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive molecular-beam epitaxy on different single crystalline substrates, varying the substrate-induced biaxial strain from -2.3% to +3.2%. Magnetization and electrical transport measurements reveal that the dependence of the Curie temperature on biaxial strain is in very good agreement with the theoretical predictions of Millis et al. [J. Appl. Phys. 83, 1588 (1998)]. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) properties of the E0, E0+Δ0, and E+ bands in an x=0.62% GaAs1-xNx alloy were investigated in detail, including their peak position, linewidth, and line shape dependences on the excitation energy, excitation power, and temperature, using micro-PL. The hot electrons within the E+ band are found to exhibit highly unusual thermalization, which results in a large blueshift in its PL peak energy by >2kBT, suggesting peculiar density of states and carrier dynamics of the E+ band. [ABSTRACT FROM AUTHOR]

Resonant Raman scattering has been applied to a dilute GaAs1-xNx alloy with 0.1% N. The Raman lines of GaAs and GaN related modes, their combinations, and multiple order replicas of GaAs-like longitudinal-optical modes have been observed with a lower N composition than those studied previously. All these Raman features are found to be strongly enhanced with excitations in resonance with a broad photoluminescence band that is associated with the so-called E+ transition. This study provides additional insights into how the GaAs host conduction band states are perturbed and thus the electron-phonon interaction is affected by the N doping. [ABSTRACT FROM AUTHOR]

The lifetimes of a series of N-related photoluminescence lines (A2–A6) in GaAs1-xNx (x=0.1%) were studied under hydrostatic pressures at ∼30 K. The lifetimes of A5 and A6 were found to increase rapidly with increasing pressure: from 2.1 ns at 0 GPa to more than 20 ns at 0.92 GPa for A5 and from 3.2 ns at 0.63 GPa to 10.8 ns at 0.92 GPa for A6. The lifetime is found to be closely correlated with the binding energy of the N impurity states, which is shown either in the pressure dependence for a given emission line or in the lifetime variation from A2 to A6. [ABSTRACT FROM AUTHOR]

Formation of complex defects involving a Ga interstitial (Gai) in (Al,In)yGa1-yNxP1-x alloys and their effects on optical quality are studied by photoluminescence (PL) and optically detected magnetic resonance spectroscopies. Introduction of these defects is shown to be largely promoted by incorporation of N. In quaternary alloys, concentrations of the defects are found to critically depend on the group III atoms that replace Ga, i.e., it is largely enhanced by the presence of Al in alloys, but is only marginally affected by In incorporation. The effect is attributed to differences in surface adatom mobilities of the group III atoms involved and their bonding strength with N. The revealed Gai complexes are shown to act as efficient nonradiative recombination centers degrading the PL efficiency. The defects exhibit high thermal stability and can only be partially removed by postgrowth rapid thermal annealing. [ABSTRACT FROM AUTHOR]

We demonstrate that a high magnetic field can be used effectively not only to probe the nature of the photoluminescence (PL) in a semiconductor, but also to reveal emission peaks that are unobservable at zero field since the magnetic field can alter energy relaxation processes and the statistical distribution of the photocarriers. Our systematic magneto-PL study of GaAs[SUB1-x]N[SUBx] (0.1% &les; x < 2.5%) in fields up to 30 T indicates that the character of the low-temperature PL in this system changes drastically with varying nitrogen composition x and exhibits transitions with applying strong magnetic fields. For x <0.7%, the PL spectrum shows many discrete features whose energies remain nearly stationary up to the highest applied field. However, the magnetic confinement gives rise to a feature emerging on the higher energy side of the zero-field spectrum. This feature does show a diamagnetic shift, but it is much slower that that of the GaAs band-edge transition. For x > 1%, the PL spectrum evolves into a broad band, and its diamagnetic shift resembles the band-edge transition in a conventional semiconductor, and the rate of shift is comparable to that of GaAs. From the diamagnetic shift of the band, the reduced effective masses for different composition of nitrogen have been derived for this system using the standard theory for the magneto-exciton in a three dimensional semiconductor. [ABSTRACT FROM AUTHOR]

A set of GaAs[sub l-x]N[sub x] samples with small nitrogen composition (x< 1%) were investigated by continuous-wave photoluminescence (PL), pulse-wave excitation PL, and time-resolved PL. In the PL spectra, an extra transition located at the higher-energy side of the commonly reported N-related emissions was observed. By measuring the PL dependence on temperature and excitation power, the PL peak was identified as a transition of alloy band edge-related recombination in GaAsN. The PL dynamics further confirms its intrinsic nature as being associated with the band edge rather than N-related bound states. [ABSTRACT FROM AUTHOR]

The absorption edge of GaN[sub x]P[sub 1-x] alloys (0.01≤x≤0.03) is shown to exhibit a direct-band gap-like behavior. Thermal variation of the band gap energy E[sub g], however, is found to be the same or even smaller than that for the indirect band gap of GaP and depends on the N content. The effect is tentatively attributed to the following counteracting contributions to the band edge formation. An interaction with N-related localized states, especially significant in the vicinity of band crossover (e.g., x=0.013), causes a substantial slow down of the E[sub g] shift with temperature. On the contrary, an increasing contribution of Γ[sub c] states, which becomes predominant for the higher compositions, leads to the larger thermal variation in E[sub g]. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time resolved photoluminescence spectroscopy is employed to monitor the effect of N incorporation on the band structure of GaNP alloys. Abrupt shortening in radiative lifetime of near-band gap emissions, arising from excitonic radiative recombination within N-related centers, is found to occur at very low N compositions of around 0.5%, i.e., within the same range as the appearance of the direct-band gap-like transitions in the photomodulated transmission spectra of GaNP reported previously. The effect has been attributed to an enhancement in oscillator strength of optical transitions due to band crossover from indirect to direct-band gap of the alloy. [ABSTRACT FROM AUTHOR]

Based on the results of temperature-dependent photoluminescence (PL) and absorption measurements, the PL emission in GaNP epilayers and GaNP/GaP multiple quantum well structures with N composition up to 4% is shown to be dominated by optical transitions within deep states likely related to N clusters. With increasing N composition, these states are shown to become resonant with conduction band of the alloy and thus optically inactive, leading to the apparent redshift of the PL maximum position. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The formation of two nonradiative defects (i.e., an As[sub Ga]-related complex and an unknown deep-level defect with g=2.03) in GaN[sub x]As[sub 1-x] epilayers and GaAs/GaN[sub x]As[sub 1-x] multiple-quantum-well structures, grown by molecular beam epitaxy, is studied by the optically detected magnetic resonance technique. It is shown that contributions by these defects in competing carrier recombination strongly vary with the nitrogen composition. An increase in the growth temperature or postgrowth rapid thermal annealing significantly reduces the influence of the nonradiative defects studied, and is accompanied by a remarkable improvement in the optical properties of the structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Raman measurements in backscattering configuration are employed to characterize the effect of nitrogen on the structural properties of a GaN[sub x]P[sub 1-x] alloy with x≤3%. The following effects of N incorporation on the vibrational spectra of GaNP are observed. First, frequencies of GaP-like and GaN-like longitudinal optical phonons exhibit strong compositional dependence, due to a combined effect of alloying and biaxial strain. Second, a dramatic quenching of two-phonon Raman scattering and an emergence of zone-edge GaP-like vibrations are observed. These effects are tentatively attributed to a local distortion of the GaNP lattice and/or compositional disorder in the alloy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dilute InN[sub x]P[sub 1-x] alloy thin films were synthesized by nitrogen ion implantation into InP using doses corresponding to N mole fraction up to 0.048. In the films with the highest N contents, it was shown using modulated photoreflectance that the fundamental band gap energy was decreased by up to 180 meV. The band gap reduction is similar in magnitude to that observed in epitaxially grown III-N[sub x]V[sub 1-x] alloys. The InN[sub x]P[sub 1-x] layers were thermally stable up to an annealing temperature of 850 °C. Using the recently developed band anticrossing model which relates the band gap reduction to the N content, we estimate that the maximum mole fraction of N achieved in the InN[sub x]P[sub 1-x] alloys is larger than that reported previously for film grown by chemical vapor deposition and exceeds 0.01. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A systematic investigation of the effect of rapid thermal annealing (RTA) on optical properties of undoped GaNAs/GaAs structures is reported. Two effects are suggested to account for the observed dramatic improvement in the quality of the GaN[sub x]As[sub 1-x]/GaAs quantum structures after RTA: (i) improved composition uniformity of the GaN[sub x]As[sub 1-x] alloy, deduced from the photoluminescence (PL), PL excitation and time-resolved measurements; and (ii) significant reduction in the concentration of competing nonradiative defects, revealed by the optically detected magnetic resonance studies. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Red light-emitting diodes (LEDs) emitting at 670 nm and employing GaN[sub 0.011]P[sub 0.989] p-n homojunction grown on a (100) GaP substrate by gas-source molecular beam epitaxy with a rf plasma nitrogen source have been obtained. The integrated photoluminescence intensity of GaNP p-n homojunction LED is 5 times stronger than that of Ga[sub 0.51]In[sub 0.49]P bulk layer, but the peak width is much broader. Compared to conventional high-brightness AlGaInP red LEDs, our LED structure saves two process steps of etch removing of the GaAs absorbing substrate and wafer bonding to a GaP transparent substrate. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical properties of GaN[sub x]P[sub 1-x] alloys (0.007≤x≤0.031) grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in GaN[sub x]P[sub 1-x] at energy below the indirect Γ[sub V]-X[sub C] transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in GaN[sub x]P[sub 1-x] indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report that the incorporation of N in GaN[sub x]P[sub 1-x] alloys (x>=0.43%) leads to a direct band-gap behavior of GaNP. For N concentration lower than 0.43%, a series of sharp emission lines from the various N pair centers are observed for GaNP bulk layers. With increasing N concentration higher than 0.43%, a strong photoluminescence (PL) emission from GaNP bulk layers is observed at room temperature. While the PL peak redshifts with increasing N concentration to 3.1%, the PL intensity remains as intense. Absorption measurements show a direct band-gap behavior of GaNP alloys. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]