Carrier-Density Dependence of Faraday Rotation and Spin Splitting in Cd1?xMnxTe

We employ spin-quantum-beat spectroscopy to investigate the carrier-density dependence of the spin-precession frequency and the magnitude of the Faraday rotation of Cd_1–x Mn _x Te samples at fixed magnetic field. We find an onset of saturation of the Faraday rotation at carrier densities as low as 4× 10¹⁶ cm^–3 and attribute it to electrons (not holes which dominate in other types of experiments). The spin splitting at fixed magnetic field remains density dependent down to 3 × 10¹⁵ cm^–3 (the lowest density accessible in our measurements) which suggests a direct influence of the carrier-density on the sp–d exchange not mediated by screening effects.     

Fabrication of magnetic artificial atoms

We have fabricated gated vertical quantum dots made from a II-VI semiconductor heterostructure containing a paramagnetic quantum well. The absence of a known Schottky gate metal compatible with ZnSe based material precludes the traditional method of using a self-aligning shadow evaporated gate. Instead, we make use of a multi-step electron beam lithography process to surround a pillar with an insulating dielectric and gate. This process allows for the processing of dots with diameters down to 250?nm. Preliminary transport data confirming the magnetic nature of the resulting artificial atom are presented.     

A Flexible and Energy-Efficient Coarse-Grained Reconfigurable Architecture for Mobile Systems

Two of the most important design issues for next generation handheld devices are wireless networking and the processing of multimedia content. Both applications rely heavily on computationally intensive digital signal processing algorithms. Programmable architectures that keep pace with the increasing performance requirements become more and more power hungry. This is problematic for a battery powered mobile device, since it has only a limited amount of energy available. Conversely, dedicated architectures are too inflexible to keep pace with changing standards and feature sets. A mobile device requires high-performance, flexibility and (energy-)efficiency. These contradicting requirements need to be balanced in the system architecture of a mobile device. In this paper a heterogeneous architecture of domain specific processing tiles is proposed. The focal point is the coarse-grained reconfigurable architecture of the Montium processing tile, which is designed to execute digital signal processing algorithms energy-efficiently.     

Competition Between Rashba Effect and Exchange Interaction in a Hg0.98Mn0.02Te Magnetic 2DEG

Magnetotransport measurements are used to study the peculiar properties of a modulation doped n-type Hg_0.98Mn_0.02Te magnetic 2DEG. The Rashba effect and giant Zeeman spin splitting are observed simultaneously and can be separated by temperature and gate voltage dependent Shubnikov-de Haas oscillations. The Rashba effect is found to be the dominant term up to 5 T.     

Spin-Polarized Transport in II–VI Magnetic Resonant-Tunneling Devices

This paper investigates electronic transport through II-VI semiconductor resonant-tunneling structures containing diluted magnetic impurities. Due to the exchange interaction between the conduction electrons and the impurities, there arises a giant Zeeman splitting in the presence of a moderately low magnetic field. As a consequence, when the quantum well is magnetically doped, the current-voltage characteristics show two peaks corresponding to transport for each spin channel. This behavior is experimentally observed and can be reproduced with a simple tunneling model. The model thus allows to analyze other configurations. First, the magnetic field was further increased, which leads to a spin polarization of the electronic current injected from the leads, thus giving rise to a relative change in the current amplitude. The authors demonstrate that the spin polarization in the emitter can be determined from such a change. Furthermore, in the case of an injector with magnetic impurities, the model shows a large increase in peak amplitude accompanied by a shift of the resonance to higher voltages with increasing fields. It was found that this effect arises from a combination of 3D incident distribution, giant Zeeman splitting, and broad resonance linewidth     

Spin Manipulation Using Magnetic II–VI Semiconductors

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.     

A process for the fabrication of large areas of high resolution,high aspect ratio silicon structures using a negative tone Novolak based e-beam resist

The non-chemically amplified negative resist ARN 7520 produced by ALLRESIST GmbH shows excellent suitability for fabricating stamps for nanoimprinting with a pitch resolution as high as 70 nm and dense periodical structures using e-beam lithography and dry etching. Due to its chemical formulation, the resist does not swell in the developer, has good sensitivity and contrast. The adhesion of such class of resists is provided by silane containing promoters used before the spin-coating. However, for the lower exposure doses, the bonding of the nanostructures to the surface is still insufficient. Instead of the promoter, we evaporate 3–5 nm Ti layer before the resist spin-coating. This strongly improves the resist adhesion in a wide range of exposure doses, suppresses the influence of the substrate conductivity on the electron beam lithography parameters and also improves the structure profile during dry etching. Reducing ion voltage from 400 to 200 V midway through dry etching also helps to keep the structure walls more vertical. Silicon stamps with lines and spacings of 70–100 nm periods and an area of 3 × 3 mm² have been successfully fabricated.     

Suppressing Twin Formation in Bi2Se3 Thin Films

The microstructure of Bi₂Se₃ topological‐insulator thin films grown by molecular beam epitaxy on InP(111)A and InP(111)B substrates that have different surface roughnesses has been studied in detail using X‐ray diffraction, X‐ray reflectivity, atomic force microscopy and probe‐corrected scanning transmission electron microscopy. The use of a rough Fe‐doped InP(111)B substrate results in complete suppression of twin formation in the Bi₂Se₃ thin films and a perfect interface between the films and their substrates. The only type of structural defect that persists in the twin‐free films is an antiphase domain boundary, which is associated with variations in substrate height. We also show that the substrate surface termination influences which family of twin domains dominates.