材料研究の重要性：とくに窒化物半導体について

早稲田大学創立125周年記念シンポジウム 半導体・ナノエレクトロニクス―技術立国日本のこれから― 2007年10月23日 早稲田大学国際会議場井深大記念ホー

Observation of transient band-gap renormalization in quantum wells.

Mesure directe de la variation de la renormalisation de la bande interdite en fonction de la densite en porteurs de charge ainsi que son evolution temporelle dans un puits quantique GaAs/Al x Ga 1−x As. Le comportement transitoire de la renormalisation de la bande interdite est gouverne par la duree de vie du porteur de charg

Spatial Coherence of a Polariton Condensate

We perform Young's double-slit experiment to study the spatial coherence properties of a two-dimensional dynamic condensate of semiconductor microcavity polaritons. The coherence length of the system is measured as a function of the pump rate, which confirms a spontaneous buildup of macroscopic coherence in the condensed phase. An independent measurement reveals that the position and momentum uncertainty product of the condensate is close to the Heisenberg limit. An experimental realization of such a minimum uncertainty wave packet of the polariton condensate opens a door to coherent matter-wave phenomena such as Josephson oscillation, superfluidity, and solitons in solid state condensate systems

Quantum Degenerate Exciton-Polaritons in Thermal Equilibrium

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice.Comment: Updated with the published versio

Direct observation of miniband-edge singularities in the optical spectra of GaAs/AlAs superlattices.

Band-edge optical transitions in GaAs/AlAs superlattices with different miniband widths are investigated by low-temperature-photocurrent, photoluminescence-excitation, and reflectance spectroscopy. Direct spectroscopic evidence is given for excitonic effects at saddle-type critical points which are related to ${\mathit{M}}_{1}$ Van Hove singularities at the upper miniband edges, for both heavy-hole and light-hole excitons. Excellent agreement is obtained between the observed transition energies and the miniband widths deduced from Kronig-Penney model calculations

Mesoscopic spin confinement during acoustically induced transport

Long coherence lifetimes of electron spins transported using moving potential dots are shown to result from the mesoscopic confinement of the spin vector. The confinement dimensions required for spin control are governed by the characteristic spin-orbit length of the electron spins, which must be larger than the dimensions of the dot potential. We show that the coherence lifetime of the electron spins is independent of the local carrier densities within each potential dot and that the precession frequency, which is determined by the Dresselhaus contribution to the spin-orbit coupling, can be modified by varying the sample dimensions resulting in predictable changes in the spin-orbit length and, consequently, in the spin coherence lifetime.Comment: 10 pages, 2 figure

On the mechanisms of spontaneous growth of III-nitride nanocolumns by plasma-assisted molecular beam epitaxy

A study of the GaN nanocolumns nucleation and growth by molecular beam epitaxy on Si(1 1 1) is presented. Ga droplets with different diameters (340–90 nm) were deposited on the substrate, prior to growth, to determine any effect on the nanocolumns size and distribution. Results indicate that there is no difference in nanocolumnar size and density whether Ga droplets are used or not, meaning that Ga droplets do not act as catalysts for the nanocolumns nucleation. In addition, Ga droplets were never observed on the nanocolumn tips upon growth termination. These findings rule out the vapor–liquid–solid mechanism. Instead, driven by a strong lattice mismatch nanocolumnar nucleation occurs spontaneously by Volmer–Weber growth mechanism, whereas nitrogen excess prevents the nucleation sites coalescence. Further nanocolumnar growth proceeds by direct Ga incorporation on the nanocolumns top and by Ga diffusion along the nanocolumns sidewalls up to their apex. Related to this diffusion mechanism, we found that Ga droplets, when used, may act as reservoirs to feed Ga atoms to the neighboring nanocolumns. Nanocolumns preserve a constant diameter if growth conditions are not modified because of a strong metal ad-atom diffusion length along their sidewalls. The effect of using AlN buffer layers on the nanocolumnar growth and morphology is also addressed