Band-gap engineering of enhanced spin-orbit interactions in InAs/AlGaAs heterostructures for Datta-Das spin transistor

摘要

Much effort has been done on spin injection and MR measurements after the proposal of spin-FET by Datta & Das (Fig.1). The remaining issue of gate controlled spin rotation in the quantum well has not been focused together with heterostructure design in conjunction with increased spin-orbit interaction. We will show that increased spin-orbit interaction is possible by band-gap engineering in InAs/AlGaAs heterostructure grown on InP, thereby spin 2π rotation in less than 70nm in channel length becomes possible. Importance of conduction band discontinuities in a quantum well was pointed out as the source of the energy shift by Pfeffer and Zawadzki in InGaAs/InAlAs heterostructures grown on InP, as opposed to the naive interpretation that the spin-orbit interaction is proportional to the average electric field in the quantum well (QW). We have designed and fabricated psuedomorphic InGaAs channel structures with high indium content and verified enhanced spin-orbit interaction theoretically and experimentally. The device structure is designed in such a way as to bring tha peak of the wavefunction lies on top of the band discontinuity on front side between main QW and sub-channel QW. The enhancement of spin-orbit interaction was verified experimentally and confirmed theoretically by k·p calculation a la Zawadzki. Samplel has In{sub}0.9Ga{sub}0.1As channel of 50A thickness and sample2 does InAs channel of 40A thickness (Fig.2a, b). The mobility and carrier concentration of sample1 and sample2 are 76000cm{sup}2/Vs, 1.6×10{sup}12cm{sup}(-2) and 58000 cm{sup}2/Vs, 2.0×10{sup}12cm{sup}(-2), respectively at 3.8K. Shubnikov de Haas (SdH) oscillations were measured and Fourier transformation was carried out to estimate Rashba coefficient α which indicates the magnitude of the spin splitting as ΔE=2 α |k{sub}//| (Fig.3). Measured Rashba coefficient was 52×10{sup}(-12)eVm for sample1 and 50×10{sup}(-12)eVm for sample2. These values turned out to be extremely large compared with the value, 30×10{sup}(-12)eVm, usually reported for InAs-based HEMTs in conventional simple quantum well structure whose wavefunction peak lies in the center of the well (Fig.4) and our control sample of In{sub}0.81Ga{sub}0.19As HEMT Rashba coefficient of 21×10{sup}(-12)eVm by SdH analysis (Fig.2c). This result is promising to provide a channel as short as 65nm for spin FET to the length become much shorter than spin relaxation length along the channel.