Growth of semiconductor nanostructures by MBE for the study of electron and nuclear spin enhancement and other physical phenomena.

摘要

Molecular beam epitaxy (MBE) is an extremely versatile thin film technique, which can produce single-crystal layers with atomic dimensional controls and thus permit the preparation of novel structures and devices tailored to meet specific needs. Spin relaxation time ts is one of the key features in spin-related phenomena and thus of great importance for spintronics. In this work, we prepare high quality samples, mainly of CdTe epilayers, by MBE, characterize their spin relaxation dynamics, and discuss the results theoretically.;First, with the goal of understanding the mechanisms of electron relaxation dynamics and nuclear spin enhancement, we focus on the growth and characterization of CdTe epilayers. By changing the shutter sequences and inserting ZnSe buffer layer, we have reproducibly grown (111) and (100) CdTe epilayers of high crystalline qualities by MBE, despite the large lattice mismatch between CdTe and GaAs substrate. Then we investigate ts for the (111) and (100) CdTe epilayers. It is found that for the (111) CdTe, spin relaxation rate t-1s is significantly enhanced and shows no temperature dependence through 130K to 300K, while t-1s for the (100) CdTe is strongly affected by the temperature. It is also found that t-1s is dependent on material quality for both (111) and (100) CdTe. We theoretically discuss the effect of strain and defect on spin relaxation time of CdTe. It is the first experimental observation of the effect of strain on t-1s in a II-VI semiconductor material.;Second, the growth and characterization of ZnTe/ZnSe related type II quantum structures, or quantum dots (QDs), are also presented in this work. The PL of Zn-Se-Te related type II quantum structures show blue shifts with higher intensities of exciting laser, an indication of type II QDs. Besides being an attractive method to p-type dope wide bandgap materials, the resulting material may be a promising structure for spin enhancement properties.;Third, we present the study of the enhancement of nuclear spin polarization through pumping laser. We find strong enhancement both in bulk CdTe as well as in CdTe epilayers, independent of the helicity of the laser, which is on the contrary to the prior reports by others. Compared with GaAs crystal, we ascribe this independence to the surface spin-dependent recombination. GaAs/AlAs and GaAs/GaAlAs multiple coupled double quantum wells (QWs), and CdTe/CdMgTe QW have also been grown and explored. The measurements show good quality of the material and are consistent with the designed structures.;Last, we summary the work and propose the future directions.;Samples are in-situ monitored by reflection high energy electron diffraction (RHEED). Post growth characterization techniques, such as time resolved Kerr rotation (TRKR), X-ray diffraction (XRD), photoluminescence (PL), and optical pumping nuclear magnetic resonance (OPNMR), are introduced and applied to the samples.