Molecular beam epitaxy of semiconductor heterostructures for spintroics

by Ku, Keh-Chiang.

In this dissertation, we use molecular beam epitaxy to engineer a variety of materials of relevance to the emerging research field known as semiconductor spintronics. The broad aim of this research is to establish a fundamental framework that exploits electronic spin states in semiconductors for the manipulation, transfer, detection and storage of information. We explore two complementary approaches towards the implementation of semiconductor spintronics: 1. We use materials known as diluted magnetic semiconductors wherein itinerant charge carriers are exchange coupled with magnetic ions incorporated into the semiconductor lattice. This results in a spin polarization of both the carriers as well as the magnetic moments themselves. 2. Alternatively, we use optical techniques to introduce spin polarization into the Fermi sea in a conventional (non-magnetic) semiconductor, and then subsequently monitor the coherent dynamic evolution of this spin polarization. This enables the study of electron spin coherence in semiconductors for applications where the quantum mechanical phase of a wave function is important. We begin this dissertation by discussing the underlying basis for diluted magnetic semiconductors. We then follow this with a discussion of experimental studies of the crystal growth and physical properties of a “canonical” case: Ga1?xMnxAs. We first review how the structural, electrical and magnetic properties of as-grown Ga1?xMnxAs epilayers are substantially altered by post-growth annealing. We then highlight an important advance achieved during this dissertation, namely the identification of growth and annealing parameters that result in samples with consistently reproducible Curie temperatures up to 150 K. We also discuss experiments that show how the the proximity of a free surface influences the maximum attainable Curie temperature in a given sample architecture. In particular, we show that a GaAs capping layer as thin as a few monolayers can significantly suppress the enhancement of the Curie temperature associated with annealing. We next consider heterostructures that integrate Ga1?xMnxAs with other materials, including the fabrication of Ga1?xMnxAs on ZnSe(001) using a recrystallized GaAs temiii plate. It is found that n-doping of ZnSe using Cl does not affect the ferromagnetism of Ga1?xMnxAs, paving a pathway to potential applications with Ga1?xMnxAs/ZnSe heterostructures. We then demonstrate efficient spin-polarization tunneling between a ferromagnetic metal and a ferromagnetic semiconductor using epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (Ga1?xMnxAs) separated by a non-magnetic semiconductor (AlAs). In this system, a large tunneling magnetoresistance up to 30% at low temperatures is observed. Analysis of current-voltage characteristics allow us to understand the nature of the tunnel barrier. The next experiment demonstrates the exchange coupling of Ga1?xMnxAs with an epitaxially overgrown antiferromagnet (Mn). A clear shift in the magnetization hysteresis loop, as well as an enhancement of the coercivity, is observed when the heterostructure is cooled in the presence of an applied magnetic field. Both coercivity and the exchange field decrease monotonically with increasing temperature and vanish at the TC of the ferromagnetic Ga1?xMnxAs layer. Finally, we turn our attention to conventional non-magnetic semiconductor heterostructures in which spin polarization is introduced via optical pumping. The effect of different crystallographic orientations on spin relaxation processes in modulation-doped ZnSe quantum wells is examined. We describe the fabrication of modulation-doped ZnSe(110) quantum wells using a low-temperature-grown GaAs template. Time-resolved Faraday and Kerr spectroscopy probes the transverse spin relaxation times in these new heterostructures, revealing little orientation dependence in spin lifetimes. We finish this dissertation with a briefly discussion of the future direction of coherent control of electron g-factor using magnetic ZnSe parabolic quantum wells. iv