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Title: Spin dynamics and spin control in narrow gap semiconductors
Author: Leontiadou, Marina A.
ISNI:       0000 0004 2741 676X
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2012
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We report a spin dynamic investigation with the use of the optical polarisation pump-probe technique in bulk and quantum well narrow gap semiconductors (NGSs) of the III-V and IV-VI families. By taking advantage of their zinc-blende (III-Vs) and rock-salt (IV-VIs) crystal symmetries, their direct energy band-gap, small effective mass, high electron effective g-value, strong relativistic effects, and with the use of the Surrey Ultrafast Laser system and the Free Electron Laser system (FELIX) for interband excitation process we prove that they are attractive for high speed electronic and new proposed spintronic concepts. The electron spin relaxation times have been measured in InSb and InAs epilayers in Faraday and Voigt configurations. A strong and opposite field dependence of the spin lifetime has been observed. We report the existence of a new spin relaxation process, which was theoretically predicted but not observed before. We demonstrate that for these NGSs, and specifically for the III-Vs, the electron spin lifetime can be modified by more than one order of magnitude simply by changing the direction of a moderate, externally applied magnetic field. We used circular two-photon absorption (CTPA) and investigated the “allowed-forbidden” and “allowed-allowed” transitions in n-InSb, the theory of which has not been investigated experimentally until now. The detailed CTPA spectrum presented here shows that the sign of photo-excited spin polarisation can in principle be controlled by the excitation wavelength in the two-photon absorption processes. We report the analysis of optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field, for degenerately n-doped InSb/InAlSb QWs, one asymmetric and one symmetric. For the asymmetric QW, by making use of directly determined experimental parameters, we have made a direct measurement of the zero field spin splitting without the influence of the large Zeeman effect. The extracted Rashba parameter is more than an order of magnitude larger than that measured earlier for GaAs QWs. Pump-pump photoconductivity experiments have been performed and we successfully measured the recombination time in InSb-based devices of different geometries, allowing new investigation methods of low dimensional structures and manipulation of samples of weak transmittance signals. We investigated the dependence of spin lifetime on controlled asymmetry in IV-VI Multi-QWs. We show for the first time experimentally that lead-chalcogenide semiconductor heterostructures can exhibit long spin lifetimes by virtue of their centro-symmetric crystal structure, and a tuning of the lifetime of over one order of magnitude after appropriate structure control corresponding to a large zero field spin splitting. The results imply that this system can be the material of choice for certain semiconductor spintronic applications requiring control of spins. We report a significant temperature dependence of the transverse electron g*-factor in symmetric IV-VI Multi-QWs. A second oscillation frequency has been observed, suggesting a possible method for distinguishing the actual excited states and the activation of an electron-hole mechanism. Although temperature tuning of lead salt laser emission wavelengths has been the method of choice in these systems for many years, we demonstrate that temperature can also be used to modulate g*, and hence the spin lifetime in lead salt QW spintronic devices.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available