The authors have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage.

Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads

A spin-polarized vertical cavity surface emitting laser, with InAs∕GaAs self-organized quantum dots as the active gain media, has been fabricated and characterized. Electron spin injection is achieved via a MnAs∕GaAs Schottky tunnel contact. The laser is operated at 200K and, at this temperature, the degree of circular polarization in the output is 8% and the maximum threshold current reduction is 14%. These effects are not observed in identical control devices with nonmagnetic contacts.

Electrically injected InAs∕GaAs quantum dot spin laser operating at 200K

We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission.

Spin relaxation dynamics in highly uniform InAs quantum dots

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = ±1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = ±1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = ±2 contributes as the size of the nano...

Mechanism and origin of exciton spin relaxation in CdSe nanorods.

The poor radiative efficiency in quantum-box luminescence is tentatively explained as an intrinsic effect rather than the usually invoked effect of etch damages. From the recently calculated decreased relaxation rate in zero-dimensional (OD) systems under 100-200-nm lateral quantization, we propose that electrons captured from the barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds. Due to the mutual orthogonality of quantum states in a box, no luminescence, or much less than in 2D or 3D, can be obtained from these stored electrons with reasonable assumptions for the hole population. Magnetic-confinement experiments in quantum-well lasers support our conclusion. A realistic model at low temperature describes more quantitatively the observed strong decay of the radiative efficiency in quantum boxes and pseudowires with decreased lateral dimensions.

Intrinsic mechanism for the poor luminescence properties of quantum-box systems

The intersubband carrier relaxation dynamics in InAs quantum dots of high-uniformity has been investigated using time-resolved photoluminescence measurement. The clearly separated photoluminescence spectra of the ground state and the second state were observed. The rate equation analysis gives the intersubband relaxation time from the second state to the ground state of 360 ps at 10 K. The slow intersubband relaxation can be attributed to the phonon relaxation bottleneck. The intersubband relaxation time at room temperature is evaluated to be 320 ps by extrapolation of 10–200 K data.

Direct observation of phonon relaxation bottleneck in InAs quantum dots of high‐uniformity

We have investigated carrier spin dynamics in self-assembled InAs/GaAs high-uniform quantum dots. The high-uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of the inhomogeneous broadening. Spin Pauli blocking due to which the spin polarization in the second state is greater than that in the ground state has been clearly observed by time-resolved spin-dependent photoluminescence measurements. The spin relaxation times in the ground state and the second state were measured to be 1 ns and 0.8 ns, respectively.

Observation of spin Pauli blocking in InAs high-uniform quantum dots

We have investigated carrier spin dynamics in highly uniform self‐assembled InAs quantum dots The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the first excited state separately, without the disturbance of inhomogeneous broadening The spin relaxation times in the ground state and the first excited state were measured to be 10 ns and 06 ns, respectively Our measurements reveal the absence of the carrier density dependence of the spin relaxation time The measured spin relaxation time decreases rapidly from 11 ns at 10 K to 200 ps at 130 K This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission

M. Murayama

Papers

Spin relaxation dynamics in highly uniform InAs quantum dots

Direct observation of phonon relaxation bottleneck in InAs quantum dots of high‐uniformity

Observation of spin Pauli blocking in InAs high-uniform quantum dots

Spin Relaxation Dynamics in Highly Uniform InAs Quantum Dots