1 Dr. Max Mustermann Referat Kommunikation & Marketing Verwaltung Spin Dynamics in Single GaAs Nanowires F. Dirnberger, S. Furthmeier, M. Forsch, A. Bayer, J. Hubmann, B. Bauer, J. Zweck, E. Reiger, C. Schüller, T. Korn and D. Bougeard Institute for Experimental and Applied Physics, University of Regensburg, Germany
2 Motivation Spin-FET in 1D Majorana fermions SOI and spin dynamics
3 Motivation GaAs nanowires: hexagonal wurtzite phase can be stabilized GaAs nanowires: due to large surface-area-to-volume ratio Spin-FET in 1D Majorana fermions SOI and spin dynamics
4 Motivation GaAs nanowires: hexagonal wurtzite phase can be stabilized GaAs nanowires: due to large surface-area-to-volume ratio Zincblende: Spin-FET in 1D Majorana fermions SOI and spin dynamics
5 Motivation GaAs nanowires: hexagonal wurtzite phase can be stabilized GaAs nanowires: due to large surface-area-to-volume ratio Wurtzite: Zincblende: Spin-FET in 1D Majorana fermions SOI and spin dynamics
6 Motivation Optical Orientation with circularly polarized light: Δm j = ±1 Zincblende:
7 Motivation Optical Orientation with circularly polarized light: Δm j = ±1 Wurtzite: Zincblende:
8 Motivation Optical Orientation with circularly polarized light: Δm j = ±1 Wurtzite: Non-invasive optical approach: Spin relaxation dynamics Spin-orbit interaction }?
9 Outline Demonstration of optical spin injection in a single nanowire Spin dynamics in wurtzite GaAs nanowires Spin-orbit interaction and the role of the core-shell interface
10 Optical spin injection into single freestanding wurtzite GaAs nanowires 2 µm Stacking-fault-free, pure wurtzite crystal structure: 4 5 µm length, 80 120 nm diameter Furthmeier et al., APL 105, 222109 (2014)
11 Optical spin injection into single freestanding wurtzite GaAs nanowires 2 µm Stacking-fault-free, pure wurtzite crystal structure: 4 5 µm length, 80 120 nm diameter Furthmeier et al., APL 105, 222109 (2014) Deposition of AlGaAs passivation shell to disable the dominant nonradiative recombination at the bare GaAs surface
12 Optical spin injection into single freestanding wurtzite GaAs nanowires 2 µm Optical axis wurtzite c-axis No linear polarization effects Stacking-fault-free, pure wurtzite crystal structure: 4 5 µm length, 80 120 nm diameter Furthmeier et al., APL 105, 222109 (2014) Deposition of AlGaAs passivation shell to disable the dominant nonradiative recombination at the bare GaAs surface
13 Optical spin orientation in GaAs nanowires Excitation and detection with same (I + ) and opposite (I ) helicity P C = I + I I + + I
14 Optical spin orientation in GaAs nanowires Excitation and detection with same (I + ) and opposite (I ) helicity P C = I + I I + + I PL Intensity (arb. u.) T = 4.2 K B = 0 Carbon defects (substrate) Spin I + Spin I P C = I + I I + + I ~50% 1.48 1.50 1.52 1.54 Emission Energy (ev) Detected degrees of circular polarization up to 50%
15 Hanle Experiment B
C 16 Hanle Experiment T = 4.2 K B P S (arb. u.) -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T)
C 17 Hanle Experiment T = 4.2 K B P S (arb. u.) C -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) ω L : Larmor frequency τ* : spin decay time
18 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S (B ) ~ exp( t/τ s ) cos(ω L t ) B = 300 mt P S (arb. u.) B = 200 mt B = 0 0 1 2 3 4 5 Time (ns)
19 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S (B ) ~ exp( t/τ s ) cos(ω L t ) B = 300 mt P S (arb. u.) B = 200 mt B = 0 0 1 2 3 4 5 Time (ns)
20 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S (B ) ~ exp( t/τ s ) cos(ω L t ) B = 300 mt P S (arb. u.) B = 200 mt B = 0 0 1 2 3 4 5 Time (ns) Exponential decay yields the spin relaxation time τ s (B ) Larmor frequency ω L delivers the electron g-factor via ω L (B) = g e μ B ħ B
25 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S (B ) ~ exp( t/τ s ) cos(ω L t ) B = 300 mt 1.6 P S (arb. u.) B = 200 mt B = 0 Spin relaxation time S (ns) 1.4 1.2 1.0 0.8 0.6 0.4 0 1 2 3 4 5 Time (ns) 0 100 200 300 400 Magnetic field (mt)
26 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S (B ) ~ exp( t/τ s ) cos(ω L t ) B = 300 mt 1.6 P S (arb. u.) B = 200 mt B = 0 Spin relaxation time S (ns) 1.4 1.2 1.0 0.8 0.6 0.4 τ s drops with B 0 1 2 3 4 5 Time (ns) 0 100 200 300 400 Magnetic field (mt)
27 Spin dynamics in wurtzite GaAs nanowires counterintuitive compared to related bulk wurtzite semiconductors Bulk wurtzite GaN Nanowire wurtzite GaAs Spin relaxation time S (ns) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 τ S 0 1 3 τ S 0 Buß et al., APL 95, 192107 (2009) 0 100 200 300 400 Magnetic field (mt)
SOI and the role of the core-shell interface 28
SOI and the role of the core-shell interface 29
30 SOI and the role of the core-shell interface Large GaAs/AlGaAs core shell interface
31 SOI and the role of the core-shell interface Large GaAs/AlGaAs core shell interface Strong Rashba-like spin orbit fields due to natural interface asymmetry (NIA)
32 SOI and the role of the core-shell interface Large GaAs/AlGaAs core shell interface Strong Rashba-like spin orbit fields due to natural interface asymmetry (NIA)
SOI and the role of the core-shell interface 33
C 34 Summary First optical spin injection in a single nanowire Hanle Measurement P S (arb. u.) -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) B = 400 mt Spin dynamics in wurtzite GaAs nanowires Spin relaxation time τ s ~ 1.5 ns Magnetic field: τ s drops Model: Rashba fields due to large GaAs/AlGaAs core shell interface P S (arb. u.) B = 300 mt B = 200 mt B = 0 florian.dirnberger@ur.de 0 1 2 3 4 5 Time (ns)
C 35 Summary First optical spin injection in a single nanowire Hanle Measurement P S (arb. u.) -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) Spin dynamics in wurtzite GaAs nanowires Spin relaxation time τ s ~ 1.5 ns Magnetic field: τ s drops Model: Rashba fields due to large GaAs/AlGaAs core shell interface florian.dirnberger@ur.de
36 Outlook Nanowire Quantum Wire florian.dirnberger@ur.de
37 Outlook Nanowire Quantum Wire Looking for further evidence: Shrinking the nanowire diameter towards quantum sizes florian.dirnberger@ur.de
38 Many thanks to Stephan Furthmeier Moritz Forsch Andreas Bayer Joachim Hubmann Benedikt Bauer Josef Zweck Elisabeth Reiger Christian Schüller Tobias Korn Dominique Bougeard the DFG for financial support via SFB 689 You for your attention!
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40 Outline Optical spin orientation in wurtzite GaAs nanowires Proof for optical spin injection in a single wurtzite GaAs nanowire Spin dynamics in wurtzite GaAs nanowires Spin relaxation: Model
41 Purely wurtzite GaAs nanowires with MBE growth Typical GaAs nanowire sample Pure wurtzite structure Pure wurtzite wires: 4 8 µm length, 80 120 nm diameter Deposition of AlGaAs passivation shell to disable the dominant nonradiative recombination at the bare GaAs surface
42 Optical spin injection in wurtzite GaAs requires: Circularly polarized light Direction of polarization c-axis Lying nanowires: Optical selection rules lead to different absorption coefficients Depolarization of circular excitation and detection
43 Optical spin injection in wurtzite GaAs nanowires Circularly polarized light Direction of polarization wurtzite c-axis
44 Optical spin injection in wurtzite GaAs nanowires Circularly polarized light Direction of polarization wurtzite c-axis Probing freestanding nanowires Optical axis c-axis No linear polarization effects
45 Probing freestanding nanowires μ-photoluminescence in confocal configuration λ/4 c-axis ħω circ Excitation and detection with same ( ) and opposite ( ) helicity P C = I I I + I
46 Probing freestanding nanowires P C = I Co I Contra I Co + I Contra λ/4 c-axis
Sample preparation for investigation of single nanowires 47
48 Sample preparation for investigation of single nanowires Reduction of wire density by ultrasonic bath dip
49 Sample preparation for investigation of single nanowires Creating patterns on the samples for the identification of single wires
50 Investigation of single freestanding nanowires free exciton substrate 2D scans allow identification of single nanowires
51 Optical spin injection P C = I Co I Contra Excitation and detection with same (Co) and opposite (Contra) helicity I Co + I Contra cb ħω circ hh lh
52 Crystal structure of GaAs nanowires 3D and 2D: only cubic zinc-blende Nanowires: unique access to the hexagonal wurtzite phase Properties of wurtzite GaAs not known exactly: - Band gap? - Effective masses? exciton binding energy? - Conduction band symmetry? - Spin dynamics / Landé factor g?
Crystal Structure & Electronic Properties 53 Electronic Structure of heterocrystalline Nanowires Most of the nanowires contain zinc-blende and wurtzite Type II (staggered) band alignment -> excitons bound to the ZB-WZ interfaces E phot = E gap - E X - ΔE VB
Crystal Structure & Electronic Properties 54 Electronic Structure of heterocrystalline Nanowires Most of the nanowires contain zinc-blende and wurtzite Type II (staggered) band alignment -> excitons bound to the ZB-WZ interfaces E phot = E gap - E X - ΔE VB Short segments of ZB / WZ -> Quantum confinement E phot = E gap - E X - ΔE VB + E conf, electron + E conf, hole
Crystal Structure & Electronic Properties 55 Luminescence Spectra of mixed Crystal Nanowires Broad emission below the free exciton peak Lowest energy gives lower bound of valence band offset: E phot, min = 1.455 ev > E g - ΔE VB - E X -> ΔE VB > 60 mev (Calculations: 65-85 mev) No free exciton signal
56 Luminescence of purely wurtzite GaAs nanowires T = 4.2 K free exciton PL Intensity (arb. u.) substrate 1.48 1.50 1.52 1.54 Emission Energy (ev) (Nearly) defect-free wurtzite structure Sharp free exciton emission peak at ~ 1. 52 ev, slightly larger than in zinc-blende (E = 1.515 ev)
57 Luminescence of purely wurtzite GaAs nanowires PL Intensity (arb. u.) 10 0 10-1 10-2 R ~ 10 ns 0 10 20 30 40 Time (ns) Monoexponential decay Long exciton lifetimes up to 10 ns, significantly longer than the pure spin dephasing times T 2 ~ 1 ns
58 Optical spin injection in wurtzite GaAs nanowires T = 4.2 K P C = I I I + I ~50% PL Intensity (arb. u.) C-GaAs substrate Spin Spin 1.48 1.50 1.52 1.54 Emission Energy (ev) Detected degrees of circular polarization up to 50%
59 Hanle - Experiment B P S (arb. u.) -0.2 0.0 0.2 Magnetic Field (T)
60 Hanle - Experiment B P S (arb. u.) -0.2 0.0 0.2 Magnetic Field (T)
61 Hanle - Experiment B P S (arb. u.) -0.2 0.0 0.2 Magnetic Field (T)
62 Hanle - Experiment P S B ~ 1 1 + (ω L τ )² P S (arb. u.) FWHM ~ 1/(ω L τ ) ω L = g e μ B ħ B 1 τ = 1 + 1 τ R T 2-0.2 0.0 0.2 Magnetic Field (T)
63 Hanle - Experiment SPG (arb. units) * novalue * 210ps * 420ps * 520ps * 730ps * 610ps T = 50K T = 40K T = 30K T = 20K T = 10K -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) T = 4.5K
Hanle Experiment 64
65 Hanle Experiment T = 4.2 K P S (arb. u.) -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T)
66 Hanle Experiment T = 4.2 K P S (arb. u.) P S B ~ 1 1 + (ω L τ )² -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) ω L : Larmor frequency τ : spin decay time
67 Outline Optical spin orientation in wurtzite GaAs nanowires Proof for optical spin injection in a single wurtzite GaAs nanowire Spin dynamics in wurtzite GaAs nanowires g-factor spin decay time Spin relaxation: Model
68 Time-resolved photoluminescence B > 0 P S (arb. u.) ~ exp t τ cos ω L t 0 1 2 3 4 5 Time (ns)
69 Time-resolved photoluminescence B > 0 P S (arb. u.) 0 1 2 3 4 5 Time (ns)
70 Time-resolved photoluminescence B > 0 P S (arb. u.) 0.5 1.0 1.5 Time (ns) ħω circ
71 Time-resolved photoluminescence B > 0 P S (arb. u.) 0.5 1.0 1.5 Time (ns) ħω circ ħω circ
72 Time-resolved photoluminescence B > 0 P S (arb. u.) 0.5 1.0 1.5 Time (ns) ħω circ ħω circ ħω circ
73 Spin dynamics in wurtzite GaAs nanowires T = 4.2 K B = 300 mt P S (arb. u.) 0 1 2 3 4 5 Time (ns)
74 Spin dynamics in wurtzite GaAs nanowires T = 4.2 K B = 300 mt P S (arb. u.) ~ exp t τ cos ω L t 0 1 2 3 4 5 Time (ns)
75 Spin dynamics in wurtzite GaAs nanowires B = 400 mt P S B ~ exp t τ cos ω L t B = 300 mt P S (arb. u.) B = 200 mt B = 0 0 1 2 3 4 5 Time (ns) Exponential decay yields the spin decay time τ (B) Larmor frequency ω L delivers the electron g-factor via ω L (B) = g e μ B ħ B
76 Spin dynamics in wurtzite GaAs nanowires Larmor frequency (1/ns) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 100 200 300 400 Magnetic Field (mt)
77 Spin dynamics in wurtzite GaAs nanowires Larmor frequency (1/ns) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 100 200 300 400 Magnetic Field (mt) ω L = g e μ B ħ B g e ~ 0. 25 Absolute value of the g-factor in wurtzite GaAs nanowires is g e For comparison: g-factor in bulk zinc-blende GaAs is g e = 0.44 ~ 0. 25
78 Spin dynamics in wurtzite GaAs nanowires T = 4.2 K B = 400 mt P S (arb. u.) ~ exp t τ 0 1 2 3 4 5 Time (ns)
Spin dynamics in wurtzite GaAs nanowires 79
80 Outline Optical spin orientation in wurtzite GaAs nanowires Proof for optical spin injection in a single wurtzite GaAs nanowire Spin dynamics in wurtzite GaAs nanowires g-factor spin decay time Spin relaxation: Model bulk wurtzite semiconductors wurtzite GaAs nanowires
81 Spin relaxation in bulk wurtzite semiconductors Dominant spin relaxation mechanism: Dyakonov-Perel k-dependent spin-orbit fields cause precession of the electron spins and lead to ensemble dephasing
82 Spin relaxation in bulk wurtzite semiconductors Spin orbit interaction: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0
83 Spin relaxation in bulk wurtzite semiconductors Spin orbit interaction: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0 k 3 -dependent term due to bulk inversion asymmetry, which also describes the spin-splitting in bulk zinc-blende semiconductors
84 Spin relaxation in bulk wurtzite semiconductors Spin orbit interaction: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0 k-linear contribution due to an intrinsic wurtzite structure inversion asymmetry
85 Spin relaxation in bulk wurtzite semiconductors Spin orbit interaction: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0 k-linear contribution due to an intrinsic wurtzite structure inversion asymmetry τ z = 1 2 τ x = 1 2 τ y
86 Spin relaxation in bulk wurtzite semiconductors Spin relaxation due to Dyakonov-Perel scattering: B = 0 τ S 0 = τ z τ z = 1 2 τ x = 1 2 τ y
87 Spin relaxation in bulk wurtzite semiconductors Spin relaxation due to Dyakonov-Perel scattering: B 0 x Spin precession τ eff = 1 τz + 2 1 τ y 1 = 4 3 τ S 0 τ z = 1 2 τ x = 1 2 τ y
Buß et al., APL 95, 192107(2009) 88 Spin relaxation in bulk wurtzite semiconductors Spin relaxation due to Dyakonov-Perel scattering: Spin dynamics in bulk wurtzite GaN τ eff = 1 τz + 2 1 τ y 1 = 4 3 τ S 0
89 Spin relaxation in wurtzite GaAs nanowires Spin relaxation due to Dyakonov-Perel scattering: Bulk wurtzite GaN Nanowire wurtzite GaAs Buß et al., APL 95, 192107(2009) τ z = 1 2 τ x = 1 2 τ y
90 Spin relaxation in wurtzite GaAs nanowires Spin relaxation due to Dyakonov-Perel scattering: Bulk wurtzite GaN Nanowire wurtzite GaAs Buß et al., APL 95, 192107(2009) τ z = 1 2 τ x = 1 2 τ y τ z = 5τ x = 5τ y
91 Spin relaxation in wurtzite GaAs nanowires Spin relaxation due to Dyakonov-Perel scattering: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0 +
92 Spin relaxation in wurtzite GaAs nanowires Spin relaxation due to Dyakonov-Perel scattering: Ω k = 2 ħ γ e bk z 2 k 2 k y k x 0 + α e k y k x 0 + Suggestion: Rashba contribution due to structure inversion asymme at the large GaAs/AlGaAs core-shell interface
93 Spin relaxation in wurtzite GaAs nanowires Suggestion: Rashba fields due to GaAs/AlGaAs core-shell interface
94 Summary T = 4.2 K First optical spin injection in a single nanowire P S (arb. u.) Hanle Measurement -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T)
95 Summary T = 4.2 K First optical spin injection in a single nanowire P S (arb. u.) Hanle Measurement -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) Spin dynamics in wurtzite GaAs nanowires g-factor: g e ~ 0.23 Spin decay time τ ~ 1.3 ns Magnetic field: τ drops Rasha fields due to large GaAs/AlGaAs core-shell interface? P S (arb. u.) B = 400 mt B = 300 mt B = 200 mt B = 0 0 1 2 3 4 5 Time (ns)
96 Summary T = 4.2 K First optical spin injection in a single nanowire P S (arb. u.) Hanle Measurement -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) Spin dynamics in wurtzite GaAs nanowires g-factor: g e ~ 0.25 Spin decay time τ ~ 1.3 ns Magnetic field: τ drops Rasha fields due to large GaAs/AlGaAs core-shell interface?
97 Summary T = 4.2 K First optical spin injection in a single nanowire P S (arb. u.) Hanle Measurement -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) Spin dynamics in wurtzite GaAs nanowires g-factor: g e ~ 0.25 Spin decay time τ ~ 1.5 ns Magnetic field: τ drops Rasha fields due to large GaAs/AlGaAs core-shell interface?
98 Summary T = 4.2 K First optical spin injection in a single nanowire P S (arb. u.) Hanle Measurement -0.3-0.2-0.1 0.0 0.1 0.2 0.3 Magnetic Field (T) Spin dynamics in wurtzite GaAs nanowires g-factor: g e ~ 0.25 Spin decay time τ ~ 1.5 ns Magnetic field: τ drops Rashba fields due to large GaAs/AlGaAs core-shell interface?