IBN-1 Quantum and Spin Electronics: Research

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Spin Electronics

In the field of spin electronics, or spintronics, the quantum mechanical property of the electron spin is used for switching purpose instead of its charge. Potential advantages of this concept are lower power consuption as well as faster switching. A typical example of a spinelectronic device is the spintransistor proposed by Datta und Das [Applied Physics Letters 1990]. Here, ferromagnetic (FM) source and drain contacts are used as spin injector and analyzer. The spin orientation is controlled by the Rashba effect (see below) by means of a gate electrode.

Rashba Effect

The Rashba effect originates from the macroscopic electric field in a semiconductor quantum well. In the figure shown below a typical conduction band profile of a semiconductor quantum well is depicted. Due to the band offsets at the interface of two different materials the electrons are confined in a quantum well. A two-dimensional electron gas (2DEG) is formed. If the potential well is asymmetric, the electrons are moving in an effective electric field E. In the reference system of the electron this electrical field transforms into a magnetic field B. Depending of the spin orientation and the corresponding magnetic moment an energy lowering or an energy increase occurs, respectively. For applications it is essential, that the strength of the Rashba effect and thus the spin splitting can be controlled by means of a gate electrode.

InGaAs/InP Heterostructures

The Rashba effect is found to be very pronouced in two-dimensional electron gases with a low band gap channel layer. In this respect a 2DEG in an InGaAs/InP heterostructures is an almost ideal system. As can be seen in the figure below, the band profile (red curve) is asymmetric so that the electrons are propagating in an effective electric field. In order to enhance the electron mobility, the concept of modulation doping is used. Here, the dopant atoms are separated by an undoped InP spacer layer from the two-dimensional channel in order to suppress scattering processes.

Shubnikov-de Haas Oscillations

The strength of the Rashba spin-orbit coupling can be extracted from magnetotransport measurements. Shubnikov-de Haas oscillations are observed if the longitudinal resistance is measured as a function of a magnetic field. The oscillation frequency in 1/B is directly related to the electron concentration in the 2DEG. As mentioned above, the presence of the Rashba effect results in a spin splitting and thus to an effective separation in two different two-dimensional systems with different electron concentrations. The superposition of these slightly different oscillation frequencies results in a beating pattern of the Shubnikov-de Haas oscillations, as shown below. By analyzing the beating pattern the strength of the Rashba effect can be determined.

Quantum Wires

For novel concepts of spintronic devices often one-dimensional channels (quantum wires) are used, in order to enhance their efficiency. The figure below an InGaAs/InP quantum wire with a gate electrode is shown. The gate electrode is used to control the Rashba effect.

AlGaN/GaN Heterostructures

GaN-based heterostructures are a very promising material system for future spinelectronic devices. For GaN doped with 3d transistion metals, e.g. Fe or Cr, Curie temperatures higher than room temperatures are theoretically predicted. Similar to InGaAs/InP heterostructures two-dimensional electron gases can be realized as well in AlGaN/GaN layer systems. Spin effects, like spin-orbit coupling are currently under study.


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