INVESTIGATION OF III-V FERROMAGNETIC SEMICONDUCTORS

Open Access
Author:
Sheu, Ben-Li
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 09, 2007
Committee Members:
  • Peter E Schiffer, Committee Chair
  • Nitin Samarth, Committee Member
  • Roy F Willis, Committee Member
  • Darrell G Schlom, Committee Member
Keywords:
  • ferromagnetism
  • magnetic semiconductors
  • spintronics
Abstract:
III-Mn-V ferromagnetic semiconductors (FMSC) are attractive and promising material candidates for semiconductor spin-electronics, in which the carrier spin and charge are exploited for potential applications. In this thesis, we presented collective studies of important materials in this class, and focused on understanding and improving the physical properties of (Ga,Mn)As, as well as the investigation of new compounds. We explored the physical parameters required to induce ferromagnetism in extremely diluted Ga(1-x)Mn(x)As (x <= 0.015), where previous studies have focused mostly on highly doped samples (x > 0.02). Our data confirmed that insulating and ferromagnetic behaviors can coexist in this low doping regime, as the samples span from paramagnetic insulating to ferromagnetic conducting at this low doping level. In contrast to previous understanding, we found that the hopping energy and localization of hole-carriers critically determine the onset of ferromagnetism in this insulating system, regardless of the carrier density or the Mn impurity concentration. We fabricated nanometer-scale wires to rejuvenate the effectiveness of thermal annealing on GaAs/Ga(1-x)Mn(x)As (x ~ 0.06)/GaAs heterostructures, where lateral sidewalls of the wires serve as free surfaces to assist the removal of Mn interstitial (MnI) defects, so the Curie temperature and electrical conductivity can be enhanced by annealing. We also studied the efficiency of annealing as a function of the wire width, and compared the out-diffusion of MnI defects between (Ga,Mn)As epilayers and these wires. We reported preliminary studies on a novel quaternary FMSC alloy (In,Al,Mn)As grown on indium phosphide, as well as a series of closely lattice-matched (In,Al,Mn)As and (In,Ga,Mn)As superlattices (SLs), wherein the SL period is fixed and the thickness ratio of the constituents systematically varied. We found that the Curie temperature, effective ferromagnetic moment and magnetic anisotropy of the SLs are strongly correlated to the thickness ratio of (In,Al,Mn)As and (In,Ga,Mn)As.