Magnetotransport in Cleaved-Edge-Overgrown F e/GaAs-based and Rare-Earth-Doped GaN-based Heterostructures Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften in der Fakultat fur Physik und Astronomie der Ruhr-Universitat Bochum vorgelegt von Fang-Yuh Lo geboren in Taipei, Taiwan Lehrstuhl fur Angewandte Festkorperphysik 2007 Der erste Gutachter: Prof. Dr. Andreas D. Wieck Der zweite Gutachter: Prof. Dr. Daniel Hagele Date of Disputation: 05.07.2007 1 Table of contents Table of contents 1 List of abbreviations 3 List of symbols 5 1 Introduction 7 2 Theoretical background 11 2.1 Introduction to GaAs- and GaN-based heterostructures 11 2.2 Magnetism 14 2.2.1 Isolated atoms 14 2.2.2 Magnetism in materials 15 2.2.2.1 Exchange interactions 15 2.2.2.2 Molecular field theory and ab-initio calculations 16 2.2.2.3 Magnetic materials 18 2.3 Magnetotransport and electrical spin injection 20 2.3.1 Magnetoresistance in nonmagnetic materials 20 2.3.2 Magnetoresistance in ferromagnetic materials 22 2.3.3 Electrical spin injection 24 2.4 GaN-based diluted magnetic semiconductors 29 2.5 Focused ion beam 32 2.5.1 Focused ion beam system 33 2.5.1.1 Liquid metal ion source 33 2.5.1.2 Focused ion beam column 34 2.5.2 Focused ion beam milling 36 2.5.2.1 sputtering 36 2.5.2.2 Redeposition 37 2.5.2.3 Implantation and amorphization 38 2 3 Magnetotransport in cleaved-edge-overgrown Fe/GaAs-based heterostructures 39 3.1 Cleaved-edge overgrowth 39 3.1.1 Properties of the interface and the metal thin films 41 3.1.2 Properties of the metal-semiconductor contacts 42 3.2 Focused ion beam milling 44 3.3 Electrical properties and magnetoresistances of the spin valves 48 3.3.1 Electrical properties of the spin valves 48 3.3.2 Magnetoresistances of the spin valves 49 3.4 Longitudinal magnetoresistances of the electrodes 52 3.5 Summary and Discussion 54 4 Magnetic properties in rare-earth elements doped GaN and its heterostructures 56 4.1 Gd-doped zinc-blende GaN with focused ion beam 57 4.2 Eu-doped wurtzite GaN with focused ion beam 62 4.3 Magnetotransport in Gd-implanted GaN-based heterostructures 64 4.3.1 I-V characteristics after Gd implantation 65 4.3.2 Magnetotransport in Gd-implanted GaN-based HEMT structures 66 4.3.2.1 Hall effect 67 4.3.2.2 Magnetoresistances of van der Pauw structures 69 4.3.2.3 Magnetoresistances of transmission line structures 70 4.4 Summary 72 5 Summary and outlook 73 5.1 Magnetotransport in cleaved-edge-overgrown Fe/GaAs-based heterostructures 73 5.2 Magnetotransport in rare-earth-doped GaN-based heterostructures 74 References 77 Acknowledgement 84 Curriculum Vitae 86 List of publications 87 3 List of abbreviations 2DEG two-dimensional electron gas 3DEG three-dimensional electron gas AFM atomic force microscope AlGaAs ALGa^As AlGaN AlxGa1-xN AMR anisotropic magnetoresistance CBM conduction band minimum CEO cleaved-edge overgrowth CPP-GMR the current perpendicular to the plane giant magnetoresistance DOS density of states DMS diluted magnetic semiconductor F ferromagnet/ferromagnetic region FC field cooled FET field-effect transistor FIB focused ion beam GMR giant magnetoresistance HB Hall-bar HCl salt acid HEMT high electron mobility transistor InGaAs In xGa1-xAs hh heavy hole LEED low-energy electron diffraction lh light hole LMIS liquid metal ion source LDA local density approximation LSDA local spin density approximation MOCVD metal-organic chemical vapor deposition MOKE magneto-optical Kerr effect 4 MBE molecular beam epitaxy MR magnetoresistance MRAM magnetoresistive random access memory MTJ magnetic tunnel junction N nonmagnet/nonmagnetic region PL photoluminescence RE rare-earth (elements) RKKY interaction Ruderman-Kittel-Kasuya-Yoshida interaction RT room temperature SEM scanning electron microscope (microscopy) SFET spin-FET, spin field-effect transistor SIC-LSDA self-interaction corrected local spin density approximation so split-off hole SRIM stopping and range of ions in matter SQUID superconducting quantum interference device TE total energy TM transition metal TML transmission line TMR tunneling magnetoresistance TR temperature-dependent remanent UHV ultra-high vacuum VBM valence band maximum vdP van der Pauw (geometry or structure) WZ wurtzite XRD X-ray diffraction ZB zinc-blende ZFC zero-field cooled 5 List of symbols In this work, the symbols for vectors are written in bold face. a area B magnetic field, magnetic induction D diffusion constant D(E) density of states d thickness E Energy e electron charge F, F force g g-factor H magnetic field H Hamiltonian h Planck constant h reduced Planck constant I, I electric current I± light intensity for left (+) and right (-) circularly polarized light, respectively J, J total angular momentum j current density ke Boltzmann constant L, L total orbital angular momentum M, M magnetization m magnetic moment m mass N number of charge carriers n density nxD, nxD x-dimensional carrier density P spin polarization p momentum 6 q charge R resistance r position r resistance S, S total spin angular momentum T temperature Y voltage v, v velocity w width X a certain physical quantity Y sputtering yield Z atomic number Y gyromagnetic ratio € electric field e permitivity (dielectric constant) ^0 permeability in vacuum ^B Bohr magneton q electro-chemical potential p resistivity G conductivity G± left (+) and right (-) circularly polarized light, respectively T relaxation time 0 electrical potential X susceptibility to (angular) frequency 7 1 Introduction Spintronics is an exciting and up and coming field in both scientific researches and the applicati­ ons and aims ambitiously at combining the spin and the charge characteristics of a charge carrier to create novel devices or to provide the existing devices new functionalities. An operating spintronic device requires efficient injection of nonequilibrium spins into a device and manipulation of the in ­ jected spin polarization at given locations. Electrical spin injection from ferromagnetic metals into paramagnetic metals was first observed by M. Johnson and R. H. Silsbee [1]. Such spin injection was proved to be efficient, and it led to the discovery of famous effects, such as giant magnetoresistance (GMR) and tunneling magnetoresi­ stance (TMR). Spintronics exploiting GMR and/or TMR can be called metal-based spintronics or magnetoelectronics [2, 3]. Unfortunately, spin injection from ferromagnetic metals into semicon ­ ductors are not as highly efficient as from ferromagnetic metals into paramagnetic metals, and the modern trends in semiconductor spintronics are based on employing spin-orbit coupling to achieve efficient spin injection and the manipulation of injected spins [4]. Generally, spintronics is interdis ­ ciplinary and integrates spins (magnetizations) with modern micro-, nano-, and opto-electronics, and people working on it may meet some of the following fields of physics: magnetism, semicon ­ ductor physics, mesoscopic physics, optics, and superconductivity. Historically, the observations of the magnetoresistances (MRs) date back to the 19th century. Lord Kelvin, then William Thomson, was the first to measure the anisotropic magnetoresistance (AMR) in 1856 [5], and the Hall effect was discovered by E. C. Hall in 1879. The magnetic tunnel junction (MTJ), or TMR, was discovered by M. Julliere in 1975 [6], and GMR was observed inde ­ pendently by two different groups in 1988 [7, 8]. Dieny et al. fabricated a spin-valve structure based on GMR effect [9], and this was later applied in the industry to make magnetoresistive read/write heads and the new type of nonvolatile memory, the magnetoresistive randon access memory (MRAM). The read/write heads and the MRAMs are, to the author's knowledge, the only commer­ cially available spintronic devices. 8 Semiconductor spintronics attracted great interest since S. Datta and B. Das proposed the prototy­ pe of a spin field-effect transistor (Spin-FET or SFET) by using Fe for the source and drain contacts on an InAs-based field-effect transistor (FET) in 1990 [10]. In this Spin-FET the spins of the elec­ trons flowing from source to drain can be controlled by the gate voltage. The first GaAs-based dilu ­ ted magnetic semiconductor (DMS) was fabricated by Ohno et al. in 1996 by introducing Mn into GaAs [11], and this achievement opened up new possibilities to study spin-related properties and phenomena in semiconductors and to create new spintronic devices. The theory of electrical spin injection was first developed by A. G. Aronov and G. E. Pikus in 1976 [12]. Later on, it was expanded separately by, just to name a few, E. I. Rashba, A. Fert and H. Jaffres, and others. Due to the conductivity mismatch [13] and spin-related properties of the me­ tal-semiconductor contact and the spin relaxation in semiconductors, the electrical spin injection from ferromagnetic metals into semiconductors has until now only successfully observed via optical methods [14 - 17]. Table 1.1 Historical events of spin electronics Year Event Contributor Source First observation of the anisotropic magnetore­ 1856 Lord Kelvin [5] sistance, AMR 1879 Discovery of the Hall effect E. C. Hall 1921 Discovery of (atomic) spins O. Stern and W. Gerlach First observation of the tunneling magnetoresi­ 1975 M. Julliere [6] stance, TMR First theoretical work on electrical spin injecti ­ 1976 A. G. Aronov and G. E. Pikus [12] on 1985 First observation of electrical spin injection M. Johnson and R. H. Silsbee [1] M. N. Baibich et al. [7] 1988 Discovery of the giant magnetoresistance, GMR G. Binasch et al. [8] 1990 Proposal of the spin field-effect transistor S. Datta and B. Das [10] 1995 First hot-electron spin transistor D. J. Monsma et al. [18] 1996 First appearance of the name, spintronics S. A. Wolf [3]1 End of Commercial magnetoresistive read/write head 1990s First commercial magnetoresistive random ac­ 2005 Freescale Semiconductor [19] cess memory, MRAM 1 Cited from footnote Nr.2 on page 324.