M. Holub

Spin-polarized light-emitting diodes and lasers

Spin-polarized light sources are a new class of devices in which the radiative recombination of spin-polarized carriers results in luminescence exhibiting a net circular polarization. The operation principles and design of spin-polarized light sources are discussed. A comprehensive review of experimental work on spin-polarized light-emitting diodes and surface-emitting lasers is provided, concluding with a discussion of future prospects.

Mn-doped InAs self-organized diluted magnetic quantum-dot layers with Curie temperatures above 300 K

The magnetic and structural properties of InAs:Mn self-organized diluted magnetic quantum dots grown by low-temperature (∼270°C), solid-source molecular-beam epitaxy using a very low InAs growth rate (<0.1ML∕s) are investigated. A Curie temperature (TC) of ∼350K is measured in a sample grown with a Mn∕In flux ratio of 0.15. Electron energy-loss spectroscopy confirms that most of the Mn remains within the InAs quantum dots. We propose as a possible explanation for this high TC the effects of magnetic and structural disorder introduced by a random incorporation and inhomogeneous distribution of Mn atoms amongst the InAs quantum dots.

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

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.

Epitaxially grown MnAs/GaAs lateral spin valves

The authors report magnetoresistance of lateral spin valves fabricated from an epitaxially grown MnAs∕GaAs heterostructure and utilizing a Schottky tunnel barrier for efficient spin injection. A coercive field difference between the two ferromagnetic MnAs contacts is obtained by a difference in aspect ratio. Peak magnetoresistances of 3.6% at 10K and 1.1% at 125K are measured for a 0.5μm channel length spin valve. The authors observe an exponential decay of the peak magnetoresistance with increasing channel length, which is indicative of diffusive spin transport. The magnetoresistance increases with increasing bias and with decreasing temperature. Control experiments have been carried out to confirm the spin-valve effect.

Electrically injected spin-polarized vertical-cavity surface-emitting lasers

We report the design, fabrication, and characterization of an electrically injected, spin-polarized, vertical-cavity surface-emitting laser. We have demonstrated spin injection from the ferromagnetic semiconductor (Ga,Mn)As into In0.2Ga0.8As∕GaAs quantum wells, spin transport across a distance of ∼0.25μm for temperatures ranging from 80to105K, and spin detection through optical polarization measurements with coherent light emission. Controlled switching between right- and left-elliptically polarized modes is achieved with a maximum degree of circular polarization of 4.6% measured at 80K.

Amplification of spin-current polarization

A ferromagnet/semiconductor based electrically controlled spin-current amplifier using a dual-drain nonlocal lateral spin valve is demonstrated. The spin polarization injected by the source into the channel is amplified at the second drain contact. An amplified current spin polarization of 100% is measured. The controlled variation of amplifier gain with bias is also demonstrated. The observations are explained in the framework of the spin drift-diffusion model.

Influence of Mn dopants on InAs/GaAs quantum dot electronic states

We have investigated the influence of Mn dopants on the electronic states in the vicinity of InAs/GaAs quantum dots(QDs) and the surrounding GaAs matrix. A comparison of cross-sectional scanning tunneling microscopy, scanning tunneling spectroscopy, and tight binding calculations of the local density of states reveals that the Mn dopants primarily influence the electronic states at the QD edges and the surrounding GaAs matrix. These results suggest that the Mn dopants reside at the QD edge, consistent with the predictions of a thermodynamic model for the nanoscale-size dependence of dopant incorporation in nanostructures.

Two-dimensional spin diffusion in multiterminal lateral spin valves

The effects of two-dimensional spin diffusion on spin extraction in lateral semiconductor spin valves have been investigated experimentally and theoretically. A ferromagnetic collector terminal of variable size is placed between the ferromagnetic electron spin injector and detector of a conventional lateral spin valve for spin extraction. It is observed that transverse spin diffusion beneath the collector terminal plays an important role along with the conventional longitudinal spin diffusion in describing the overall transport of spin carriers. Two-dimensional spin diffusion reduces the perturbation of the channel electrochemical potentials and improves spin extraction.

Response to “Comment on ‘Electrically injected spin-polarized vertical-cavity surface-emitting laser’ ” [Appl. Phys. Lett.88, 056101 (2006)]

We thank Hägele and Oestreich for a critical reading of our recent letter reporting on an electrically injected spinpolarized vertical-cavity surface-emitting laser spinVCSEL . In their comment, Hägele and Oestreich’s assert that electrical injection of spin-polarized holes in our structure is unlikely since the hole transit time from the spin aligner to the laser active region exceeds spin relaxation times reported in the literature and instead attribute the slight circular polarization to spin-dependent reabsorption. Clearly, considerable care must be taken in verifying spin injection from optical polarization measurements and in designing meaningful control experiments aimed at separating real from spurious contributions. Our control experiment which compares the optical polarization between VCSELs containing magnetic and nonmagnetic layers does not eliminate a potential contribution from magnetic circular dichrosim MCD , i.e., preferential absorption of one circularly-polarized component occurring in Ga,Mn As. To investigate the severity of MCD in our previous experiment, we prepared a spin-VCSEL heterostructure for magnetophotoluminescence measurements using a portion of the original, unprocessed wafer. The sample was subjected to annealing and heat treatments mimicking those experienced during device fabrication. A dielectric ZnSe/MgF2 dielectric Bragg reflector DBR stack was deposited by electron-beam evaporation to complete the spinVCSEL heterostructure such that any light generated from radiative recombination in the quantum wells QWs would undergo numerous internal reflections before escaping. Our sample was mounted in a magneto-optical cryostat and illuminated by linearly polarized laser excitation, generating unpolarized carriers in the QW active region. The optical polarization of the photoluminescence PL was studied using a measurement geometry comparable to our previous electroluminescence EL experiment, collecting light along the surface normal and parallel to the direction of the applied magnetic field. Since this light travels an identical path in both experiments, the PL will exhibit all polarization effects which result from magneto-optical effects and not direct spin injection from the Ga,Mn As spin aligner. Figure 1 shows the degree of circular polarization for the PL and EL taken at T=80 K. It is apparent that MCD constitutes a sizable portion of the measured response as suggested by Hägele and Oestreich. However, the contribution is incomplete and a small 1% degree of circular polarization remains to be

Magnetoresistance of fully epitaxial MnAs∕GaAs lateral spin valves

The authors report the growth, fabrication, and characterization of lateral MnAs∕GaAs spin valves where Schottky tunnel barriers enable all-electrical spin injection and detection. Through a difference in geometric aspect ratio for the MnAs contacts, parallel and antiparallel alignment between the contact magnetization is obtained by varying the external magnetic field. Temperature-dependent conductivity measurements indicate that tunneling is the dominant transport mechanism for the MnAs∕GaAs Schottky diode polarizer and analyzer contacts. Peak magnetoresistances of 3.6% at 10K and 1.1% at 125K are observed for a 0.5μm channel length spin valve.