Ulf Ekenberg

Superefficient electric-field–induced spin-orbit splitting in strained p-type quantum wells

We investigate theoretically the efficiency of the Rashba effect, i.e. the spin-orbit splitting resulting from an electric field. In contrast to previous studies, where the carriers have usually been taken to be electrons, we focus on holes and are able to demonstrate remarkable improvements of the effect by several orders of magnitude. We also show that the frequently-neglected lattice-mismatch between GaAs and AlGaAs can be used to further enhance the efficiency of the wave vector splitting mechanism. The Rashba effect is the fundamental mechanism behind the Datta-Das spin transistor and we find that for a small electric field of 2 kV/cm the spin precession length becomes only 36 nm.

Effective mass and band nonparabolicity in remote doped Si/Si0.8Ge0.2 quantum wells

The effective masses in remote doped Si/Si0.8Ge0.2/Si quantum wells having sheet densities, Ns in the range 2×1011–1.1×1012 cm−2 have been determined from the temperature dependencies of the Shubnikov–de Haas oscillations. The values obtained increase with magnetic field and Ns. This behavior is taken as evidence for the nonparabolicity of the valence band and accounts for the discrepancies in previously reported masses. Self‐consistent band structure calculations for a triangular confinement of the carriers have also been carried out and provide confirmation of the increase in mass with Ns. Theory and experiment give extrapolated Γ point effective masses of 0.21 and 0.20 of the free‐electron mass, respectively.

Efficient switching of Rashba spin splitting in wide modulation-doped quantum wells

The authors demonstrate that the size of the electric-field-induced Rashba spin splitting in an 80 nm wide modulation-doped InGaSb quantum well can depend strongly on the spatial variation of the e ...

Photoluminescence and photoresponse from InSb/InAs-based quantum dot structures

InSb-based quantum dots grown by metal-organic vapor-phase epitaxy (MOVPE) on InAs substrates are studied for use as the active material in interband photon detectors. Long-wavelength infrared (LWIR) photoluminescence is demonstrated with peak emission at 8.5 µm and photoresponse, interpreted to originate from type-II interband transitions in a p-i-n photodiode, was measured up to 6 µm, both at 80 K. The possibilities and benefits of operation in the LWIR range (8-12 µm) are discussed and the results suggest that InSb-based quantum dot structures can be suitable candidates for photon detection in the LWIR regime.

A high-speed intersubband modulator based on quantum interference in double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 /spl mu/m is predicted for a device length of 18 /spl mu/m and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect.

Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells

We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on k and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.

Terahertz magneto-optical spectroscopy of a two-dimensional hole gas

Terahertz magneto-optical spectroscopy on a two-dimensional hole gas reveal a nonlinear dependence on the applied magnetic field. This is due to its complex non-parabolic valence band structure, as verified by multiband Landau level theoretical calculations.

Efficient infrared electroabsorption with 1 V applied voltage swing using intersubband transitions

We have demonstrated efficient intersubband electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor-phase epitaxy. An absorption modulation of 6 dB (Delta alpha= ...

Efficient electroabsorption for mid-infrared wavelengths using intersubband transitions

We have demonstrated efficient intersubband (IS) electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). An absorption modulation of 2300 cm-1 at λ = 5.7 μm due to Stark shift of the IS resonance was achieved with a low applied voltage swing of ± 0.5 V in a multipass waveguide structure. Two useful wavelength ranges of λ≈5.4–5.8 μm and 6.3–6.6 μm were obtained by considering the two flanks of the IS resonance. Based on the experimental results it is estimated that an electroabsorption modulator with a low peak-to-peak voltage of VPP = 0.9 V can yield a modulation speed of f3dB = 120 GHz with the present material by using a strongly confining surface plasmon waveguide of 30 μm length.

Superiority of p-type spin transistors

The spintronic device that has probably stimulated the most research interest is the Datta-Das spin transistor. The mechanism behind it, called the Rashba effect, is that an applied voltage gives r ...