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Spin polarized tunneling and spin injection in Fe-GaAs hybrid structures [Elektronische Ressource] / vorgelegt von Peifeng Chen

151 pages
Spin polarized tunneling and spin injection inFe-GaAs hybrid structuresDISSERTATIONzur Erlangung des Doktorgrades der Naturwissenschaften(Dr. rer. nat.)der Naturwissenschaftlichen Fakulta¨t II - Physikder Universit¨at Regensburgvorgelegt vonPeifeng Chenaus Shanghai, ChinaRegensburg 2006Die Arbeit wurde angeleitet von: Prof. Dr. Werner WegscheiderDatum des Promotionskolloquiums: 19.09.2006Pru¨fungsausschuss: Vorsitzender: Prof. Dr. Jaroslav FabianErstgutachter: Prof. Dr. Werner WegscheiderZweitgutachter: Prof. Dr. Dieter WeissWeiterer Pru¨fer: Prof. Dr. Christian BackContentsAbstract 11 Introduction 31.1 A brief introduction to spintronics . . . . . . . . . . . . . . . 41.2 This thesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Fundamental concepts of spin polarized tunneling 112.1 Ferromagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2 Schottky Barrier . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.1 Ideal metal-semiconductor contact . . . . . . . . . . . 132.2.2 Image force and surface states. . . . . . . . . . . . . . 142.3 Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3.1 One dimensional rectangular barrier . . . . . . . . . . 152.3.2 WKB approximation . . . . . . . . . . . . . . . . . . . 162.3.3 Current density calculation . . . . . . . . . . . . . . . 172.3.4 Simmons Model . . . . . . . . . . . . . . . . . . . . . 172.3.5 Determining the Schottky barrier height . . . .
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Spin polarized tunneling and spin injection in
Fe-GaAs hybrid structures
DISSERTATION
zur Erlangung des Doktorgrades der Naturwissenschaften
(Dr. rer. nat.)
der Naturwissenschaftlichen Fakulta¨t II - Physik
der Universit¨at Regensburg
vorgelegt von
Peifeng Chen
aus Shanghai, China
Regensburg 2006Die Arbeit wurde angeleitet von: Prof. Dr. Werner Wegscheider
Datum des Promotionskolloquiums: 19.09.2006
Pru¨fungsausschuss: Vorsitzender: Prof. Dr. Jaroslav Fabian
Erstgutachter: Prof. Dr. Werner Wegscheider
Zweitgutachter: Prof. Dr. Dieter Weiss
Weiterer Pru¨fer: Prof. Dr. Christian BackContents
Abstract 1
1 Introduction 3
1.1 A brief introduction to spintronics . . . . . . . . . . . . . . . 4
1.2 This thesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Fundamental concepts of spin polarized tunneling 11
2.1 Ferromagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Schottky Barrier . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Ideal metal-semiconductor contact . . . . . . . . . . . 13
2.2.2 Image force and surface states. . . . . . . . . . . . . . 14
2.3 Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.1 One dimensional rectangular barrier . . . . . . . . . . 15
2.3.2 WKB approximation . . . . . . . . . . . . . . . . . . . 16
2.3.3 Current density calculation . . . . . . . . . . . . . . . 17
2.3.4 Simmons Model . . . . . . . . . . . . . . . . . . . . . 17
2.3.5 Determining the Schottky barrier height . . . . . . . . 19
2.4 Spin-polarized tunneling . . . . . . . . . . . . . . . . . . . . . 21
2.4.1 Magnetic tunneling Junction . . . . . . . . . . . . . . 22
2.4.2 Julli`ere model . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.3 Spin polarization measurement . . . . . . . . . . . . . 25
3 Device fabrication and test technology 29
3.1 Semiconductor wafer . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Process flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Critical processes in fabrication . . . . . . . . . . . . . . . . . 31
3.3.1 Photolithography . . . . . . . . . . . . . . . . . . . . . 31
iii CONTENTS
3.3.2 Wet chemical etching . . . . . . . . . . . . . . . . . . 33
3.3.3 Metal deposition . . . . . . . . . . . . . . . . . . . . . 37
3.3.4 Lift-off process . . . . . . . . . . . . . . . . . . . . . . 40
3.3.5 Epoxy bonding . . . . . . . . . . . . . . . . . . . . . . 41
3.4 Measurement set-up . . . . . . . . . . . . . . . . . . . . . . . 41
4 Temperature dependence of the TMR effect 45
4.1 Hints for low TMR effect . . . . . . . . . . . . . . . . . . . . 45
4.2 Sample preparation and test results. . . . . . . . . . . . . . . 48
4.3 Temperature dependence model of the TMR effect . . . . . . 50
4.4 Experimental data analysis . . . . . . . . . . . . . . . . . . . 52
4.4.1 Determination of the Fe/GaAs Schottky barrier height 53
4.4.2 Determination of P(T) and G . . . . . . . . . . . . 55eff
4.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5 Surface pretreatment in Fe/GaAs/Fe junctions 61
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . 63
5.3 Tunneling through sulphur-passivated GaAs barriers . . . . . 63
5.3.1 Barrier height . . . . . . . . . . . . . . . . . . . . . . . 63
5.3.2 The TMR effect . . . . . . . . . . . . . . . . . . . . . 65
5.3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 65
6 Theories of spin injection 69
6.1 Introduction to the spin injection model . . . . . . . . . . . . 69
6.2 The F/N junction . . . . . . . . . . . . . . . . . . . . . . . . 71
6.3 The F/N/F junction . . . . . . . . . . . . . . . . . . . . . . . 73
6.4 The magnetic p-n junction . . . . . . . . . . . . . . . . . . . . 75
6.4.1 Magnetic semiconductors . . . . . . . . . . . . . . . . 75
6.4.2 Spin injection through the depletion layer . . . . . . . 76
7 Interface resistivity of the Fe/GaAs Schottky barrier 79
7.1 Current transport mechanisms in the Schottky barrier . . . . 79
7.1.1 Thermionic emission model . . . . . . . . . . . . . . . 80
7.1.2 Thermionic-field emission and field emission . . . . . . 80
7.2 Depletion layer and Fermi level of Fe/GaAs Schottky barrier 82
7.2.1 Depletion layer width . . . . . . . . . . . . . . . . . . 82CONTENTS iii
7.2.2 Position of the Fermi level . . . . . . . . . . . . . . . . 83
7.2.3 Numerical evaluation . . . . . . . . . . . . . . . . . . . 84
7.3 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . 84
7.4 Measurement results and discussion . . . . . . . . . . . . . . 86
8 Spin injection experiments 93
8.1 Spin injection in a magnetic p-n junction diode . . . . . . . . 93
8.1.1 Device design . . . . . . . . . . . . . . . . . . . . . . . 93
8.1.2 Layer sequence of the semiconductor wafer . . . . . . 94
8.1.3 Sample fabrication . . . . . . . . . . . . . . . . . . . . 95
8.1.4 Magnetic properties of GaMnAs . . . . . . . . . . . . 95
8.1.5 Spin injection results and discussion . . . . . . . . . . 96
8.2 Spin injection in a F/SC/F structure . . . . . . . . . . . . . . 102
8.2.1 Sample design and structure . . . . . . . . . . . . . . 102
8.2.2 Spin injection results and discussion . . . . . . . . . . 103
Summary 107
Appendix 111
Publications 121
Bibliography 123List of Figures
2.1 Band structure of the magnetic transition metals . . . . . . . 12
2.2 Electron energy band diagram of a Schottky barrier . . . . . 13
2.3 The rectangular tunnel barrier . . . . . . . . . . . . . . . . . 15
2.4 Schematic diagram of tunnel barrier in Simmon’s model . . . 18
2.5 Intuitive understanding of the g(V) curve . . . . . . . . . . . 20
2.6 Simulation of the logarithmic derivative g(V) . . . . . . . . . 21
2.7 Magnetoresistance of Fe/Al O /Fe junction . . . . . . . . . . 222 3
2.8 Electron tunneling in a F/I/F tunnel junction . . . . . . . . . 24
2.9 Tunneling in F/I/S junctions . . . . . . . . . . . . . . . . . . 26
3.1 Epitaxialstructureofthewaferfortunnelingmagneticjunctions 30
3.2 Process flow of MTJ fabrication . . . . . . . . . . . . . . . . . 32
3.3 Two-step etching of GaAs and AlGaAs before second contact
deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 Selectivity of GaAs and AlGaAs etching . . . . . . . . . . . . 36
3.5 Schematic of the DC sputtering system . . . . . . . . . . . . 38
3.6 Schematic description of the lift-off process . . . . . . . . . . 40
3.7 Schematic diagram of the tunneling magnetic junction with
four contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1 TMR at 4.2K as a function of GaAs barrier thickness . . . . 46
4.2 Tunneling magnetoresistance comparison of MTJs . . . . . . 47
4.3 I-V characteristicsofaFe/GaAs/Fe/Comagnetictunneljunc-
tion at different temperatures . . . . . . . . . . . . . . . . . . 49
4.4 MagnetoresistanceofaFe/GaAs/Fe/Comagnetictunneljunc-
tion at different temperatures . . . . . . . . . . . . . . . . . . 51
ivLIST OF FIGURES v
4.5 J(V) curves and Simmons fitting results of the tunneling
junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.6 I-V curve measured using four point technique on the tunnel
junction of Fe/GaAs/Fe/Co . . . . . . . . . . . . . . . . . . . 55
4.7 Maximum and minimum conductance of a MTJ versus tem-
perature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.8 Linear fitting to obtain spin wave parameter α and parame-
′ters S and γ . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.9 Theoretical fitting of temperature dependence of the magne-
toresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.1 Interdiffusion and effective magnetization of Fe deposited on
the S-passivated GaAs . . . . . . . . . . . . . . . . . . . . . . 62
5.2 TemperaturedependenceofI-V andg(V)curvesofMTJwith
S-passivated GaAs barrier . . . . . . . . . . . . . . . . . . . . 64
5.3 Hysteresis loops and magnetoresistance of Fe/GaAs/Fe junc-
tions with and without passivation . . . . . . . . . . . . . . . 66
5.4 Comparison of the voltage dependence of TMR traces with
and without S-passivated GaAs barrier . . . . . . . . . . . . . 67
6.1 Spatial variation of the electrochemical potential in a F/N
junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.2 MagnetoresistanceversusinterfaceresistanceofaF/N/Fjunc-
tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.3 Spin injection through the space-charge region of a magnetic
p-n junction. . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
7.1 Schematic current transport mechanisms of Schottky barrier . 81
7.2 Band diagram of Schottky barriers for Fe/GaAs junctions of
different doping densities. . . . . . . . . . . . . . . . . . . . . 85
7.3 Schematic diagram of the interface resistivity measurements
of Fe/GaAs Schottky barrier . . . . . . . . . . . . . . . . . . 86
7.4 I-V characteristics of the Fe/GaAs Schottky barriers with
different doping densities. . . . . . . . . . . . . . . . . . . . . 87
7.5 Temperature dependence of the forward I-V characteristics
and E of the Fe/GaAs Schottky barrier . . . . . . . . . . . . 880vi LIST OF FIGURES
7.6 TemperaturedependenceoftheI-V characteristicsofaheav-
ily doped Fe/GaAs Schottky barrier . . . . . . . . . . . . . . 89
7.7 Schematic Fert’s conditions for spin injection in a F/N/F
junction with Fe/GaAs Schottky barriers . . . . . . . . . . . 90
8.1 Band diagram of a magnetic p-n junction diode . . . . . . . . 94
8.2 Epitaxialstructureofthewaferforthemagneticp-njunction
diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.3 Magneticp-njunctiondiodegeometryforfour-pointmeasure-
ments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
8.4 Hysteresis loop and T of GaMnAs . . . . . . . . . . . . . . . 97c
8.5 LogarithmicplotoftheforwardI-V characteristicofthemag-
netic p-n junction diode . . . . . . . . . . . . . . . . . . . . . 98
8.6 Hysteresis loop and magnetoresistance of magnetic p-n junc-
tion diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
8.7 Magnetoresistanceratioversustemperatureofamagneticp-n
junction diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.8 Magnetoresistance ratio versus voltage of magneticp-n junc-
tion diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.9 Geometry and band diagram of a Fe/GaAs/Fe spin injection
device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8.10 Magnetoresistanceasafunctionoftheexternalmagneticfield
in the plane of the Fe/GaAs/Fe structure . . . . . . . . . . . 104List of Tables
2.1 Polarization of the ferromagnets . . . . . . . . . . . . . . . . 26
3.1 List of lithography parameters . . . . . . . . . . . . . . . . . 33
3.2 List of chemical solutions for wet etching. . . . . . . . . . . . 34
4.1 Schottky barrier height and thickness extracted from Sim-
mons fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2 List of parameters to characterize the temperature depen-
dence of spin-polarized tunneling and junction quality . . . . 59
7.1 List of Fermi level and depletion layer width of Fe/GaAs
Schottky barrier . . . . . . . . . . . . . . . . . . . . . . . . . 84
8.1 List of spin injection measurements on Fe/GaAs/Fe structures103
viiList of abbreviations and
symbols
Abbreviations
2DEG Two Dimensional Electron Gas
AES Auger Electron Spectroscopy
AP Anti-parallel
BCS Bardeen Cooper Schrieffer
CMOS Complementary Metal-Oxide Semiconductor
COM Common Ground
DC Direct Current
DMS Diluted Magnetic Semiconductor
DOS Density of States
EBASE Epoxy Bond and Stop Etch-technique
FE Field Emission
F/I/F Ferromagnet/Insulator/Ferromagnet
F/I/S Ferromagnet/Insulator/Superconductor
F/N/F Ferromagnet/Non-magnetic metal/Ferromagnet
F/SC/F Ferromagnet/Semiconductor/Ferromagnet
GMR Giant Magnetoresistance
MBE Molecular Beam Epitaxy
MRAM Magnetic Random Access Memory
MTJ Magnetic Tunneling Junction
P Parallel
PMMA Polymethyl Methacrylate
QW Quantum Well
SI Semi-insulating
viii

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