T. Inoshita

Formation of GaAs ridge quantum wire structures by molecular beam epitaxy on patterned substrates

A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.

Electronic Structure of Potassium-Graphite Intercalation Compound: C8K

The electronic band structure of the first-stage potassium-graphite intercalation compound C 8 K was calculated by a semi-empirical tight-binding scheme. The calculated Fermi surfaces can be classified into two distinct types. One is potassium-like and nearly isotropic; the other is carbon-like and of cylindrical shape. In addition, the cylindrical portions show nesting property, which is likely to induce charge-density-wave instability. The isotropic portions of the Fermi surfaces are responsible for the large reduction of anisotropy in conductivity of C 8 K relative to graphite. The calculated density of states has a peak around the Fermi level and is in good agreement with the observed density of states derived from the specific heat measurements.

Near-infrared sideband generation induced by intense far-infrared radiation in GaAs quantum wells

GaAs quantum wells are simultaneously illuminated with near-infrared (NIR) radiation at frequency ωnir and intense far-infrared (FIR) radiation from a free-electron laser at ωfir. Magnetic fields up to 9 T are applied. Strong and narrow sidebands are observed at ωsideband=ωnir±2ωfir. The intensity of the sidebands is enhanced when either ωsideband or ωnir is near the onset of NIR absorption in the quantum well, or when ωfir is near the free-electron cyclotron frequency. We attribute these sidebands to four-wave mixing of NIR and FIR photons whose energies differ by more than a factor of 100.

Structure of pentacene/tetracene superlattices deposited on glass substrate

Pentacene/tetracene organic superlattices were formed by deposition on a glass substrate, and their structure was investigated by x‐ray diffraction and photoluminescence. For samples in which the number (n) of constituent pentacene (or tetracene) molecular layers within a period is such that n≥3, the first‐order x‐ray diffraction peak was found to consist of several equally spaced subpeaks. Comparison of the spectra with calculated structure factors established that a well‐defined superlattice structure was achieved in these samples. In contrast, samples with n<3 showed no evidence of superlattice formation. Photoluminescence spectra show clear correlation with the x‐ray results.

Electron-phonon interaction and the so-called phonon bottleneck effect in semiconductor quantum dots

Abstract Whether the phonon bottleneck effect, or slowed relaxation of electrons through phonon emission, in quantum dots is detrimental to their application as optical devices has been a matter of debate. We present a theoretical analysis of the bottleneck effect, based on a second-order perturbation calculation of the relaxation time and a Greens function calculation of the electron density of states. Emphasis will be put on multiple-phonon and coherence effects. A brief review of the relevant experiments is given in the second part of the paper.

Long‐wavelength lattice dynamics of In1−xGaxAsyP1−y alloys

The first investigation of the lattice dynamics of In1−xGaxAsyP1−y quaternary semiconductor alloys lattice matched to GaAs has been made by Raman scattering. The spectra in the optical frequency range consist of two separate bands, each having a peak at the high‐frequency end. These two peaks are shown to be GaP‐like and GaAs‐like, respectively. The cell‐isodisplacement theory for the long‐wavelength lattice dynamics by Zinger, Ipatova, and Subashiev [Sov. Phys. Semicond. 10, 286 (1976)] is reexamined in detail. It is improved upon by consistently treating the renormalization of the intracell force constants and then applied to InGaAsP. This improvement gives remarkably better fitting of the calculated phonon frequencies to the existing experiments in the entire composition range 0≤x, y≤1 and enables the construction of a coherent model for the long‐wavelength lattice dynamics of InGaAsP. The results of the Raman measurements are discussed in the light of the calculation. The oscillator strengths are also...

Theory of phonon-limited resistivity and electron-phonon interaction in higher-stage graphite intercalation compounds

Abstract The first principle calculations of the basal-plane resistivity and of the electron-phonon coupling constant are presented for higher stage graphite intercalation compounds using a pseudopotential method. On account of the intra- and inter-pocket scatterings in two inequivalent cylindrical Fermi surfaces, the temperature dependence of total resistivity deviates from the ordinary T linear law and follows the T 2 law in the high temperature region, which is consistent with experimental results. It is shown that the present theory also explains other observed characteristics of resistivity such as the existence of a minimum in the resistivity vs. stage curve, and higher mobility compared with those of ordinary metals. Finally, the electron-phonon coupling constant is calculated as a first step towards understanding the mechanism of superconductivity in first stage compounds. It is shown that the electron-phonon coupling constant is fairly large even for second stage compounds and it is comparable with those of weak coupling superconducting metals.

MBE growth of GaAs nanometer-scale ridge quantum wire structures and their structural and optical characterizations

Abstract A novel method for fabrication of the quantum wire structures has been investigated by which a quantum wire has been successfully fabricated on top of a (111)B facet structure with a very sharp ridge. Electron microscope study has shown that GaAs wires with the effective lateral width of 16–18 nm and with the thickness of 6–9 nm are formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that ID quantum confinement of electrons is realized at the ridge top and its blue shift agrees with the quantum confined energy calculated for the observed wire structure.

Electronic structure of nanometer-scale quantum dots created by a conductive atomic force microscope tip in resonant tunneling structures

We report on a self-consistent electronic structure calculation for an AlAs/InGaAs/AlAs near-surface quantum well with a conductive atomic force microscope tip placed immediately above and biased at a positive voltage. The result indicates that the application of a modest voltage (∼V) to the tip creates zero-dimensional electron states (quantum dot) in the well with lateral confinement size <10 nm. These discrete states can be directly probed by resonant tunneling using the tip as an electrode. We also propose that p doping in the cap layer is very useful for the achievement of tighter lateral confinement.

Electronic structure and properties of alkali-graphite intercalation compounds

Abstract The band structure of the first-stage graphite intercalation compound C8K has been calculated non-empirically by introducing the pseudopotentials of constituent atoms and the Madelung-type potential due to the charge transfer. The amount of charge transfer from K to C layers has been determined self-consistently to be 60 percent. We have further calculated the resistivity due to electron-phonon scattering for higher-stage compounds C12nM. It is found that the mobility along the layers is at least one order of magnitude larger than that in copper at 300 K, because of small contribution from the scattering with acoustic phonons. It is also found that the electron-phonon coupling constant due to the graphite layers is much larger than that in a potassium metal and this is possibly an origin of superconductivity in C8K.