Growth, Structure, and Optical Characterization of Diluted Magnetic Semiconductor Nanowires
Open Access
- Author:
- Cooley, Benjamin Joseph
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 26, 2010
- Committee Members:
- Nitin Samarth, Dissertation Advisor/Co-Advisor
Nitin Samarth, Committee Chair/Co-Chair
Vincent Henry Crespi, Committee Member
Kenneth O Hara, Committee Member
Jian Xu, Committee Member - Keywords:
- diluted magnetic semiconductor
nanowire
semiconductor nanowire
nanowire growth - Abstract:
- Nanowires combining the usually disparate areas of semiconductors and magnetism are of contemporary relevance within the context of semiconductor spintronics. This is a relatively new field of research that seeks to exploit electron spin within electronic and opto-electronic semiconductor devices. While much of the effort within semiconductor spintronics has been directed toward fundamental studies and applications of 3D, 2D and 0D systems, there has been little work to date on 1D systems. The distinctive change in the electronic density of states with changing dimensionality provides a strong motivation for developing and exploring semiconductor nanowires in which one might be able to probe and control spin-dependent phenomena within a 1D or quasi-1D environment. This thesis explores the crystal growth, structural properties and magneto-optical behavior of quasi-1D semiconductor nanowires in which we incorporate magnetism through two approaches: first, by synthesizing nanowires of the diluted magnetic semiconductor (Zn,Mn)Se, wherein the <i>d</i>-shell electrons of substitutional Mn ions interact with the band states of the ZnSe host lattice via <i>sp–d</i> exchange; second, by making hybrid core-shell nanostructures wherein a metallic ferromagnetic shell (MnAs) is epitaxially deposited on a semiconductor nanowire (GaAs). After an introductory overview of past work in the field and a description of the experimental techniques relevant to the thesis, we discuss our experimental results.<br> The first set of experiments focuses on ZnSe and (Zn,Mn)Se nanowires grown in a single stage. The nanowires were grown on Si and GaAs substrates with a thin layer of gold evaporated onto them, which were then annealed before growth so that the gold formed nanoscale droplets. The growth yields samples covered in random arrays of nanowires growing out an an angle to the substrate, with an undergrowth of crooked nanowires and other small structures. The long thin nanowires vary in diameter, down to ~ 10 nm, with ~ 15–20 nm being fairly common. Structural characterization done by transmission electron microscopy (TEM) shows that the long nanowires are largely single crystal, though often with many stacking faults, and tend to form in the wurtzite crystal structure with <i>c</i>-axis growth direction. Elemental analysis performed on individual nanowires using X-ray energy dispersive spectroscopy shows that Mn can be incorporated into Zn<sub>1−<i>x</i></sub>Mn<sub><i>x</i></sub>Se nanowires in high concentrations, up to <i>x</i> ~ 0.6. Magneto-photoluminescence (magneto-PL) measurements of as-grown samples reveal substantial Zeeman shifts in the near band edge luminescence, but only partial polarization, indicating the luminescence is originating in a nanostructured environment. Surprisingly, however, nearly all of the PL signal comes from the nanostructured undergrowth rather than the long nanowires, as shown from magneto-PL measurements of samples which were sonicated to remove the long nanowires.<br> The next set of experiments covers ZnSe and (Zn,Mn)Se nanowires grown using a two-stage process. The substrates are prepared in a similar manner as the in the previous samples. The first stage of growth is carried out at lower than normal substrate temperature, which results in highly tapered nanoneedles with narrow tips (~ 10 nm). The second stage of growth is performed under normal nanowire growth conditions, yielding narrow nanowires growing from the tips of the nanoneedles. These nanowires have far fewer defects than nanowires grown in a single stage process. Magneto-PL measurements carried out in a micro-PL system show many localized emitters and near band edge emission that shifts in a magnetic field. Sonicated samples gave similar results, indicating that the undergrowth is still responsible for much of the luminescence, even though there is much less undergrowth than in the single stage nanowires. Measurements of dispersed samples suggests that some of the luminescence may be coming from nanowires, but the sources of emission from dispersed samples could not be conclusively identified.<br> The last set of experiments involves core-shell nanowires created by growing epitaxial shells on GaAs nanowire cores. The GaAs cores are grown using a self-seeded technique, in which a GaAs substrate is coated with SiO<sub>2</sub> and then etched to open nanoscale pinholes in the SiO<sub>2</sub>. During growth, Ga atoms accumulate in the pinholes and form droplets that then serve as seed particles for VLS nanowire growth. To create the shells, the growth conditions are changed to epitaxial growth conditions. Shells of the ferromagnetic metal MnAs have been grown epitaxially on the side faces of the GaAs nanowires. The epitaxial relationship between the MnAs and GaAs depends on whether the GaAs is in the zincblende or wurtzite crystal phase. ZnSe and (Zn,Mn)Se shells have also been grown on the GaAs nanowires, providing a possible alternative method to create quasi-1D systems of (Zn,Mn)Se.