Electronic and vibrational Raman studies of the superconducting proximity effect in Nb:InAs heterojunctions
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Electronic Raman scattering from coupled phonon-plasmon model and vibrational scattering from LO phonon modes confined to that near-surface region of InAs have been used to probe the change electron density, n, associated with transition through T. of a thin Nb film electrode. In contrast to the vast majority of other III-V surfaces, InAs displays an electron charge accumulation region (CAR). At the doping densities used in these experiments the width of the CAR is of the order 50A. Confining electrons to such a small region of space gives rise to quantum size effects, and the result in the Raman spectrum is large-q contributions to the scattering mechanism for LO phonons. Because the LO phonon is effectively screened in the bulk of the InAs the relative magnitudes of the phonon and coupled phonon-plasmon bands can be used to monitor relative changes in the n in the near-surface region. Experiments with Nb:InAs heterojunctions make use of direct excitation through the Nb overlayer and demonstrate that cooling the superconductor through its transition temperature, can have a dramatic effect on n near the interface.Keywords:
Overlayer
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InAs quantum wells (QWs) have been used as weak links in many recent studies of novel superconductor-normal metal–superconductor junctions. The degree of coupling between the superconducting electrodes depends sensitively on both the superconductor/InAs interface and the QW material in the weak link, factors that are difficult to separate in dc transport studies. Here we used midinfrared spectroscopy to investigate the superconductor/semiconductor contact region. The remnant intersubband absorption we observe in Nb-clad InAs shows that the superconductor/InAs interface produced some confinement of electrons in the InAs. This confinement is, however, consistent with phase coherent transport in the InAs. We find no evidence for charge transfer from the superconductor to the InAs on cooling below the critical temperature of Nb.
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The scattering effects (specifically LO-phonon scattering) in a 45 Å AlAs/80 Å GaAs/33 Å AlAs asymmetric double barrier resonant tunneling (ADBRT) structure with a short period GaAS/Al0.3Ga0.7As superlattice incorporated on one side of the double barrier have been studied and characterized. Enhanced levels of current conduction were produced in the ADBRT due to the superlattice miniband electron transport under forward bias. And the effect of the said superlattice on the phonon scattering phenomena exhibited by the entire device was subsequently examined. Magnetic field fan diagrams at 4.2 K under reverse bias showed a new feature at an energy of 22 meV that could be explained on the basis of previously unreported GaAs LO-phonon scattering processes from the first excited emitter level. Finally, quenching of phonon-assisted tunneling in reverse bias on decreasing the period of the superlattice was also observed.
Phonon scattering
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The mobility and carrier density of two-dimensional hole systems formed at the interface of GaAs-Ga1−xAlxAs heterojunctions have been measured in the temperature range 1.9–100 K. The mobility increased monotonically with decreasing temperature, and in one sample reached 2.35×105 cm2 V−1 s−1, the highest value reported for holes. Optical phonon scattering (for T>40 K) and acoustic phonon scattering (for 15 K≤T≤40 K) are the mechanisms limiting the mobility down to low temperature, where Coulomb scattering dominates (for T<15 K). An observed linear increase of the inverse mobility with temperature cannot be explained quantitatively with a theory that was able to account for a similar behavior found in two-dimensional electrons.
Electron Mobility
Atmospheric temperature range
Phonon scattering
Limiting
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We have investigated the interaction between carriers and polar phonons by using Raman scattering spectroscopy in highly conductive Al-doped ZnO films grown by metalorganic chemical vapor deposition. Different from the longitudinal optical phonon-plasmon coupled modes (LOPPCM) observed in nondegenerate ZnO, an A1(LO)-like mode appears at the low frequency side of the uncoupled A1(LO) mode, and it monotonically shifts to higher frequencies and approaches to the uncoupled A1(LO) mode as Al composition increases. Based on line shape calculations, the A1(LO)-like mode is assigned to the large wave-vector LOPPCM arising from nonconserving scattering dominated by the Al impurity-induced Fröhlich mechanism. Benefiting from the nonmonotonic Al composition dependence of the electron density, it is revealed that the LOPPCM depends mainly on the doping level but not the carrier concentration.
Degenerate semiconductor
Wave vector
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Interactions between tunneling electrons and a variety of phonons have been previously reported in MIS tunnel junctions including zone boundary phonons, the k = 0 longitudinal optical phonon, and the B local-mode phonon. We report here the observation of interactions between the tunneling electron and the local-mode phonon associated with N in n-type SiC and P, O, and perhaps C and C–O in n-type Si. The data in SiC suggest that the interaction with the tunneling electrons arise from N substituted for Si atoms. The identification of the local modes in Si is based on agreement with infrared absorption measurements, by observation of at least two local-mode peaks for each impurity, and a correlation between the strength of the interaction and the concentration of P and O impurities present in various crystals.
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We present a Raman study of the vibrational modes in InAs/InP (001) quantum wires. The energy of the observed phonon modes evidences the confinement properties of the wires, their strain anisotropy and the effect of atomic intermixing. Resonance effects in confined and interface phonons are discussed for excitation in the vicinity of the E1 critical point. The observed vibrations and their variation with sample characteristics are in agreement with the conclusions of previous structural and optical characterization performed in the same samples.
Characterization
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Tunnel effect
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Atmospheric temperature range
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The energy distribution curves (EDCs) of photoelectrons emitted from the GaN(0001) negative electron affinity (NEA) surfaces are investigated along with GaAs(100) NEA surfaces. These experiments are performed at room temperature using 3.82eV laser excitation for GaN and 1.96eV laser excitation for GaAs. We find the main contribution to the total emitted current is the electrons that have lost an average energy of 310meV and 140meV, respectively, in the bandbending region (BBR). We propose that the origin of the energy loss as the electrons exit the solid is due to intervalley phonon scattering in the BBR (scattering of Γ electrons into the L–M valleys for GaN and L valley for GaAs). EDC studies on semiconductor NEA surfaces enable us to investigate the semiconductor electron transport property in the high-field region, which is established internally by the bandbending voltage at the surface.
Photoelectric effect
Electron scattering
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