Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3-1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective g factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

Hole Spin Relaxation in an Ultrathin InAs Monolayer

We investigate the spin relaxation time of holes in an ultrathin neutral InAs monolayer (1.5 ML) and compare with that of electrons, using polarization-dependent time-resolved photoluminescence (TRPL) experiments. With excitation energies above the GaAs gap, we observe a rather slow relaxation of holes (tau(1h) = 196 +/- 17 ps) that is in the magnitude similar to electrons (tau(1e) = 354 +/- 32 ps) in this ultrathin sample. The results are in good agreement with earlier theoretical prediction, and the phonon scattering due to spin-orbit coupling is realized to play a dominant role in the carrier spin kinetics.

Time-resolved photoluminescence of sub-monolayer InGaAs/GaAs quantum-dot-quantum-well heterostructures

Time-resolved photoluminescence (PL) of sub-monolayer (SML) InGaAs/GaAs quantum-dot-quantum-well heterostructures was measured at 5 K for the first time. The radiative lifetime of SML quantum dots (QDs) increases from 500 ps to 800 ps with the increase of the size of QDs, which is related to the small confinement energy of the excitons inside SML QDs and the exciton transfer from smaller QDs to larger ones through tunneling. The rise time of quantum-dot state PL signal strongly depends on the excitation power density. At low excitation power density, the rise time is about 35 ps, the mechanism of carrier capture is dominated by the emission of longitudinal-optical phonons. At high excitation power density, the rise time decreases as the excitation density increases, and Auger process plays an important role in the carrier capture. These results are very useful for understanding the working properties of sub-monolayer quantum-dot devices.

Spin relaxation dynamics in InAs monolayer and submonolayer

By using time-resolved photoluminescence and time-resolved Kerr rotation, we have studied the unique electron spin dynamics in InAs monolayer (ML) and submonolayer (SML), which were sandwiched in GaAs matrix. Under non-resonant excitation, the spin relaxation lifetimes of 3.4 ns and 0.48 ns were observed for 1/3 ML and I ML InAs samples, respectively. More interestingly, the spin lifetime of the 1/3 ML InAs decreased dramatically under resonant excitation, down to 70 ps, while the spin lifetime of the 1 ML sample did not vary much, changing only from 400 to 340 ps. These interesting results come from the different electron-hole interactions caused by different spatial electron-hole correlation, and they provide a direct evidence of the dominant spin relaxation process, i.e. the BAP mechanism. Furthermore, these new results may provide a valuable enlightenment in controlling the spin relaxation and in seeking new material systems for spintronics application.

Structure and photoluminescence study of InGaAs/GaAs quantum dots grown via cycled (InAs)(n)/(GaAs)(n) monolayer deposition

We report the morphology of an InGaAs nanostructure grown by molecular beam epitaxy via cycled (InAs)(n)/(GaAs)(n) monolayer deposition. Atomic force microscopy images clearly show that varying monolayer deposition per cycle has significant influence on the size, density and shape of the InGaAs nanostructure. Low-temperature photoluminescence spectra show the effect of n on the optical quality, and 1.35mum photoluminescence with a linewidth of only 19.2meV at room temperature has been achieved in the (InAs)(1)/(GaAs)(1) structure.

1.35 mu m photoluminescence from In0.5Ga0.5As/GaAs islands grown by molecular beam epitaxy via cycled (InAs)(1)/(GaAs)(1) monolayer deposition

1.35 mum photoluminescence (PL) with a narrow linewidth of only 19.2 meV at room temperature has been achieved in In0.5Ga0.5As islands structure grown on GaAs (1 0 0) substrate by solid-source molecular beam epitaxy. Atomic force microscopy (AFM) measurement reveals that the 16-ML-thick In0.5Ga0.5As islands show quite uniform InGaAs mounds morphology along the [ 1(1) over bar 0] direction with a periodicity of about 90 nm in the [1 1 0] direction. Compared with the In0.5Ga0.5As alloy quantum well (QW) of the same width, the In0.5Ga0.5As islands structure always shows a lower PL peak energy and narrower full-width at half-maximum (FWHM), also a stronger PL intensity at low excitation power and more efficient confinement of the carriers. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2000 Elsevier Science B.V. All rights reserved.

Optical anisotropy of InAs monolayer in (311)-oriented GaAs matrix

In-plane optical anisotropy which comes from the heavy hole and the light hole transitions in an InAs monolayer inserted in (311)-oriented GaAs matrix is observed by reflectance difference spectroscopy. The observed steplike density of states demonstrates that the InAs layer behaves like a two-dimensional quantum well rather than isolated quantum dots. The magnitude of the anisotropy is in good agreement with the intrinsic anisotropy of (311) orientation quantum wells, indicating that there is little structural or strain anisotropy of the InAs layer grown on (311)-oriented GaAs surface.

Optical characterization of InAs monolayer quantum structures grown on (311)A, (311)B, and (100)GaAs substrates

Low-temperature photoluminescence and excitation spectra from InAs monolayer quantum structures, grown on (311)A, (311)B, and (100) GaAs substrates, are investigated, The structures were grown simultaneously by conventional molecular-beam epitaxy (MBE), The experimental results show that the quality of InAs monolayer on (311)B GaAs substrate is obviously better in crystal quality than those on the two other oriented GaAs substrates. In addition, the transition peaks of the InAs layer grown on (311) GaAs substrates shift to higher energy with respect to that from the InAs layer grown on (100) GaAs substrate.

The modification of self-assembled monolayer on indium tin oxide as cathode in inverted bottom-emitting organic light-emitting diodes

Self-assembled monolayers (SAMs) of a series of p-substituted benzoyl chlorides were formed on indium tin oxide as the cathode for the fabrication of inverted bottom-emitting organic light-emitting diodes (IBOLEDs). The studies on the efficiency of electron injection and device performances showed that the direct tunneling of electron and the formation of dipole associated with the monolayer-forming molecule lead to significant enhancement in electron injection. Consequently, the device efficiency is greatly improved.

Weak epitaxy growth and phase behavior of planar phthalocyanines on p-sexiphenyl monolayer film

We systematically investigated the weak epitaxy growth (WEG) behavior of a series of planar phthalocyanine compounds (MPc), i.e., metal-free phthalocyanine (H2PC), nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPc), iron phthalocyanine (FePc); cobalt phthalocyanine (CoPc), grown on a p-sexiphenyl (p-6P) monolayer film by selected area electron diffraction (SAED) and atomic force microscopy (AFM). Two types of epitaxial relations, named as incommensurate epitaxy and commensurate epitaxy, were identified between phthalocyanine compounds and the substrate of the p-6P film.

Weak epitaxy growth of metal-free phthalocyanine on p-sexiphenyl monolayer and double-layer films

Weak epitaxy growth (WEG) can afford high-mobility thin films of disk-like organic semiconductor of which mobility is up to the level of the corresponding single crystals. We investigated the WEG behavior and mechanism of planar phthalocyanine in the model system of metal-free phthalocyanine (H2Pc) grown on p-sexiphenyl (p-6P) ultrathin films (monolayers and double layers). Highly oriented H2Pc films with molecules standing up exhibited two kinds of different in-plane orientations, i.e., three sets of in-plane orientations and only one set of in-plane orientation, on p-6P monolayer and double-layer films, respectively.

Weak Epitaxy Growth of Copper Hexadecafluorophthalocyanine (F16CuPc) on p-Sexiphenyl Monolayer Film

Weak epitaxy growth (WEG) behavior and mechanism of copper hexadecafluorophthalocyanine (F16CuPc) on p-sexiphenyl (p-6P) monolayer film were investigated by atomic force microscopy (AFM), selected area electron diffraction (SEAD), and wide-angle X-ray diffraction (WAXD). High-quality F16CuPc films with high order, large size, and molecular-level smoothness were obtained successfully by WEG method. It was identified that there exists incommensurate epitaxial relation between highly oriented F16CuPc and p-6P films. The geometrical channels of p-6P monolayer surface induce the nucleation and growth of F16CuPc molecules.