Direct observation of coherent spin transfer processes in an InGaAs/GaAs quantum well via two-color time-resolved Kerr rotation measurements

A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch-Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

The influence of 1 nm AlN interlayer on properties of the Al0.3Ga0.7N/AlN/GaN HEMT structure

The sheet carrier concentrations, conduction band profiles and amount of free carriers in the barriers have been determined by solving coupled Schrodinger and Poisson equation self-consistently for coherently grown Al0.3Ga0.7N/GaN and Al0.3Ga0.7N/AlN/GaN structures on thick GaN. The Al0.3Ga0.7N/GaN heterojunction structures with and without 1 nm AlN interlayer have been grown by MOCVD on sapphire substrate, the physical properties for these two structures have been investigated by various instruments such as Hall measurement and X-ray diffraction. By comparison of the theoretical and experimental results, we demonstrate that the sheet carrier concentration and the electrons mobility would be improved by the introduction of an AlN interlayer for Al0.3Ga0.7N/GaN structure. Mechanisms for the increasing of the sheet carrier concentration and the electrons mobility will be discussed in this paper. (C) 2007 Elsevier Ltd. All rights reserved.

Fabrication and excitation-power-density-dependent micro-photoluminescence of hexagonal nanopillars with a single InGaAs/GaAs quantum well

Hexagonal nanopillars with a single InGaAs/GaAs quantum well (QW) were fabricated on a GaAs (111) B substrate by selective-area metal-organic vapor phase epitaxy. The standard deviations in diameter and height of the nanopillars are about 2% and 5%, respectively. Zincblende structure and rotation twins were identified in both the GaAs and the InGaAs layers by electron diffraction. The excitation-power-density-dependent micro-photoluminescence (mu-PL) of the nanopillars was measured at 4.2, 50, 100 and 150 K. It was shown that, with increasing excitation power density, the mu-PL peak's positions shift to a higher energy, and their intensity and width increase, which were rationalized using a model that includes the effects of piezoelectricity, photon-screening and band-filling. It was also revealed that the rotation twins significantly reduce the diffusion length of the carriers in the nanopillars, compared to that in the regular semiconductors.

Low threshold current density 1.3 mu m metamorphic InGaAs/GaAs quantum well laser diodes

Very low threshold current density InGaAs/ GaAs quantum well laser diodes grown by molecular beam epitaxy on InGaAs metamorphic buffers are reported. The lasing wavelength of the ridge waveguide laser diode with cavity length of 1200 mm is centred at 1337.2 nm; the threshold current density is 205 A/cm(2) at room temperature under continuous-wave operation.

Influence of spacer layer thickness on the current-voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes

This paper investigates the dependence of current-voltage characteristics of AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes (RTDs) on spacer layer thickness. It finds that the peak and the valley current density J in the negative differential resistance (NDR) region depends strongly on the thickness of the spacer layer. The measured peak to valley current ratio of RTDs studied here is shown to improve while the current density through RTDs decreases with increasing spacer layer thickness below a critical value.

Optical properties of InGaAs/GaAs quantum chains

We have investigated the steady-state and transient optical properties of InGaAs/GaAs quantum chains and found that the photoluminescence (PL) decay time exhibits a strong photon energy dependence. It increases with the decrease of the emission energy. It is also found that the PL decay time increases with the excitation power. When the excitation power is large enough the PL decay time tends to be saturated. All these experimental results show that there is a strong carrier coupling along the chain direction in the quantum dot chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.

Hydrogen sensors based on AlGaN/AIN/GaN HEMT

Pt/AlGaN/AIN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8 mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8 mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor. (c) 2007 Elsevier Ltd. All rights reserved.

1.58 mu m InGaAs quantum well laser on GaAs

We demonstrate the 1.58 mu m emission at room temperature from a metamorphic In0.6Ga0.4As quantum well laser grown on GaAs by molecular beam epitaxy. The large lattice mismatch was accommodated through growth of a linearly graded buffer layer to create a high quality virtual In0.32Ga0.68As substrate. Careful growth optimization ensured good optical and structural qualities. For a 1250x50 mu m(2) broad area laser, a minimum threshold current density of 490 A/cm(2) was achieved under pulsed operation. This result indicates that metamorphic InGaAs quantum wells can be an alternative approach for 1.55 mu m GaAs-based lasers. (C) 2007 American Institute of Physics.

Strong Spin-Orbit Interactions in an InAlAs/InGaAs/InAlAs Two-Dimensional Electron Gas by Weak Antilocalization Analysis

Spin-orbit interactions in a two-dimensional electron gas were studied in an InAlAs/InGaAs/InAlAs quantum well. Since weak anti localization effects take place far beyond the diffusive regime, (i.e., the ratio of the characteristic magnetic field, at which the magnetoresistance correction maximum occurs, to the transport magnetic field is more than ten) the experimental data are examined by the Golub theory, which is applicable to both diffusive regime and ballistic regime. Satisfactory fitting lines to the experimental data have been achieved using the Golub theory. In the strong spin-orbit interaction two-dimensional electron gas system, the large spin splitting energy of 6.08 meV is observed mainly due to the high electron concentration in the quantum well. The temperature dependence of the phase-breaking rate is qualitatively in agreement with the theoretical predictions. (C) 2009 The Japan Society of Applied Physics

Fabry-Perot microcavity modes observed in the micro-photoluminescence spectra of the single nanowire with InGaAs/GaAs heterostructure

We report on the fabrication of the nanowires with InGaAs/GaAs heterostructures on the GaAs(111) B substrate using selective-area metal organic vapor phase epitaxy. Fabry-Perot microcavity modes were observed in the nanowires with perfect end facets dispersed onto the silicon substrate and not observed in the free-standing nanowires. We find that the calculated group refractive indices only considering the material dispersion do not agree with the experimentally determined values although this method was used by some researchers. The calculated group refractive indices considering both the material dispersion and the waveguide dispersion agree with the experimentally determined values well. We also find that Fabry-Perot microcavity modes are not observable in the nanowires with the width less than about 180 nm, which is mainly caused by their poor reflectivity at the end facets due to their weak confinement to the optical field. (C) 2009 Optical Society of America

High responsivity resonant-cavity-enhanced InGaAs/GaAs quantum-dot photodetector for wavelength of similar to 1 mu m at room temperature

The characteristics of a resonant cavity-enhanced InGaAs/GaAs quantum-dot n-i-n photodiode with only a bottom distributed Bragg reflector used as the cavity mirror, are reported. To suppress the dark current, an AlAs layer is inserted into the device structure as the blocking layer. It turns out that the structure still possesses the resonant coupling nature, and makes Rabi splitting discernible in the photoluminescence spectra. The measured responsivity spectrum of the photocurrent shows a peak at lambda = 1030 nm, and increases rapidly as the bias voltage increases. A peak responsivity of 0.75 A/W, or equivalently an external quantum efficiency of 90.3%, is obtained at V-bias = -1.4 V.

Long-Wavelength Emission InAs Quantum Dots Grown on InGaAs Metamorphic Buffers

In this work, InAs quantum dots (QDs) grown on a linear graded InGaAs metamorphic buffer layer by molecular beam epitaxy have been investigated. The growth of the metamorphic buffer layers was carefully optimized, yielding a smooth surface with a minimum root mean square of roughness of less than 0.98 nm as measured by atomic force microscopy (AFM). InAs QDs were then grown on the buffer layers, and their emission wavelength at room-temperature is 1.49 mu m as measured by photoluminescence (PL). The effects of post-growth rapid thermal annealing (RTA) on the optical properties of the InAs QDs were investigated. After the RTA, the PL peak of the QDs was blue-shifted and the full width at half maximum decreased.

Metamorphic InGaAs Quantum Well Laser Diodes at 1.5 mu on GaAs Grown by Molecular Beam Epitaxy

We report a 1.5-mu m InGaAs/GaAs quantum well laser diode grown by molecular beam epitaxy on InGaAs metamorphic buffers. At 150 K, for a 1500 x 10 mu m(2) ridge waveguide laser, the lasing wavelength is centred at 1.508 mu m and the threshold current density is 667 A/cm(2) under pulsed operation. The pulsed lasers can operate up to 286 K.

Temperature Insensitivity of Optical Properties of InAs/GaAs Quantum Dots due to a Pregrown InGaAs Quantum Well

Both the peak position and linewidth in the photoluminescence spectrum of the InAs/GaAs quantum dots usually vary in an anomalous way with increasing temperature. Such anomalous optical behaviour is eliminated by inserting an In0.2Ga0.8As quantum well below the quantum dot layer in molecular beam epitaxy. The insensitivity of the photoluminescence spectra to temperature is explained in terms of the effective carrier redistribution between quantum dots through the In0.2Ga0.8As quantum well.

Dependence of current-voltage characteristics of pseudomorphic AlAs/In0.53Ga0.47As/InAs resonant tunnelling diodes on quantum well widths

This paper studies the dependence of I - V characteristics on quantum well widths in AlAs/In0.53Ga0.47As and AlAs/In0.53Ga0.47As/InAs resonant tunnelling structures grown on InP substrates. It shows that the peak and the valley current density in the negative differential resistance region are closely related with quantum well width. The measured peak current density, valley current densities and peak-to-valley current ratio of resonant tunnelling diodes are continually decreasing with increasing well width.