Strained germanium for applications in spintronics

Abstract

Germanium (Ge) is another group-IV semiconductor material, which recently started attracting tremendous attention in spintronics following success of silicon (Si). The crystal inversion symmetry of Si and Ge precludes the spin relaxation of conduction electrons by the Dyakonov-Perel mechanism, resulting in a long spin relaxation time. Since the proposal of the spin FET in 1990 by Datta and Das, semiconductor materials have been studied for their spin-orbit (S-O) interactions, particularly those that can be modified by an applied electric field, such as the Rashba S-O interaction, in order to create devices that utilise spin modulation and control to perform logic operations. Since then new proposals have appeared. Nowadays they include spin transistors with several different operating principles, spin-based diodes, spin-based field programmable gate arrays, dynamic spin-logic circuits, spin-only logic, spin communication and others. In this review, the focus will be made on presenting recent progress in Ge spintronics including the key advances made. The absence of Dresselhaus S-O coupling in Ge enables a longer spin diffusion length when compared to III-V semiconductor materials. Evidence of a strong Rashba S-O interaction in strained Ge quantum wells has begun to emerge. Also, the first experimental demonstration of room-temperature spin transport in Ge has recently been reported.

Spin-coherent dynamics and carrier lifetime in strained Ge1−xSnx semiconductors on silicon

“…The outstanding challenge of overcoming fundamental limits of conventional device electronics has stimulated various proposals and extensive investigations of radical alternatives 1 . The prospect of utilizing quantum information and communication processing has placed group IV semiconductors at the leading edge of current research efforts [2][3][4] . Such materials are ubiquitous in the mainstream microelectronic industry and naturally exhibit favourable properties for the solid-state implementation of logic-gate operations built upon quantum states 5 .…”

Comparison of cross‐sectional transmission electron microscope studies of thin germanium epilayers grown on differently oriented silicon wafers

“…There is considerable interest in growing thin, epitaxially strained layers (epilayers) of pure Ge (or an alloy of SiGe) onto either buffered Si wafers (Yeo et al ., ) or onto prepared SiGe virtual substrates (Myronov et al ., ; Myronov et al ., ) for research on various quantum phenomena in electronics (Foronda et al ., ) and spintronics (Morrison & Myronov, ). The latter virtual substrates are usually comprised of a thick (a few μm) relaxed layer of SiGe grown in a compositionally stepped (Baribeau et al ., ) or compositionally graded (Fitzgerald et al ., ; Shah et al ., ) manner onto a Si wafer on top of which the compressively strained Ge quantum well (QW) epilayer is deposited.…”

Hole weak anti-localization in a strained-Ge surface quantum well

“…By fitting the weak anti-localization peak to a model including a dominant cubic spin-orbit coupling, we extract the characteristic transport time scales and a spin splitting energy of ∼1 meV. Finally, we observe a weak anti-localization peak also for magnetic fields parallel to the quantum well and attribute this finding to a combined effect of surface roughness, Zeeman splitting, and virtual occupation of higher-energy hole subbands.Hole spins in p-type SiGe-based heterostructures are promising candidates for quantum spintronic applications [1,2]. They are expected to display a relatively small in-plane effective mass [3,4], favoring lateral confinement, as well as long spin coherence times [5], stemming from a reduced hyperfine coupling (natural Ge is predominantly constituted of isotopes with zero nuclear spin and holes are less coupled to nuclear spins due to the p-wave symmetry of their Bloch states [6]).…”

“…Further details of materials growth and characterization are described elsewhere [17]. The same epitaxial growth technology resulted in the creation of strained Ge QW heterostructures with superior low-and room-temperature electronic properties [18][19][20] enabling the observation of various quantum phenomena including the fractional quantum Hall effect [21], mesoscopic effects due to spin-orbit interaction [2,[22][23][24][25] and terahertz quantum Hall effect [26].The studied devices have a Hall-bar geometry defined by a top-gate electrode operated in accumulation mode ( Fig. 1.b).…”

“…Hole spins in p-type SiGe-based heterostructures are promising candidates for quantum spintronic applications [1,2]. They are expected to display a relatively small in-plane effective mass [3,4], favoring lateral confinement, as well as long spin coherence times [5], stemming from a reduced hyperfine coupling (natural Ge is predominantly constituted of isotopes with zero nuclear spin and holes are less coupled to nuclear spins due to the p-wave symmetry of their Bloch states [6]).…”