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(In,Ga)As/(Al,Ga)As quantum wells on GaAs(110)
R. Hey, M. Höricke, A. Trampert, U. Jahn, P. SantosPaul-Drude-Institut für Festkörperelektronik, Berlin
OutlineMotivationExperimentalResults of optical and structural characterization Materials: GaAs/(Al,Ga)As
(In,Ga)As/GaAs/(Al,Ga)As, (In,Ga)As/GaAs, InAs/GaAs Techniques: PL, CL, AFM, TEM Example for spin transportSummary
EuroMBE 2005, Grindelwald, Switzerland
For the manipulation and transport of spins in semiconductor hetero-junctions long spin lifetimes are beneficial.
By choosing appropriate crystal orientations this requirement can be accomplished as demonstrated by
Y. Ohno et al. [Phys. Rev. Lett. 83 (1999) 4196].
(110)-oriented GaAs/(Al,Ga)As-QW: spin relaxation time exceeds that of its (001) counterpart by one order of magnitude.
The use of InGaAs- instead of GaAs-QWs enhances the capability of spin manipulation by external magnetic fields and may allow for RT application.
Motivation
Challenges: PL Linewidth
Spin transport of carriers requires high structural perfection for low spin scattering
PL reflects structural perfection and homogeneity
Broadening of PL emission depending on:
substrate orientation / TG, BEP
composition
MBE growth of InGaAs/AlGaAs on GaAs(110) substrates is carried out at lower temperatures and higher III-V BEP ratios as compared to GaAs(001)
• Substrate smoothing: 10 nm GaAs — migration enhanced epitaxy mode
• Subsequent layers: MBE mode with growth interruptions and annealing
steps similarly as described by
M. Yoshita et al. [Appl. Phys. Lett. 81 49 (2002)]; this work: 600°C/1 min
• Best results with respect to morphology, PL line width & efficiency:
GaAs: Ts=440°C , BEP=50, vGaAs = 0.2 µm/h
GaAs/AlGaAs: Ts=480°C , BEP=70
InGaAs/GaAs: Ts=440°C , BEP=70
InAs/GaAs/AlGaAs Ts=425°C , BEP=50
• Annealing: post growth 640°C /1 h in an As atmosphere or
RTA up to 900°C/ 30 s, proximity capping
Sample preparation
Photoluminescence: InGaAs/GaAs-SQW
The PL-FWHM values: - larger for (110)-oriented SQWs compared to (001)-oriented ones grown under identical conditions (C1: 19.3 meV, D: 4.1 meV) - increase with SQW-thickness (A: 8.2 meV, B: 13.2 meV) - decrease with annealing and blue-shifting (C1: 19.3 meV, C2: 12.8 meV)
5 K PL of (In,Ga)As/GaAs-SQWs, TG= 450°C and BEP-ratio=70
A xIn= 0.1 dQW: 8 nm (110)
B xIn= 0.1 dQW: 20 nm (110)
C xIn= 0.2 dQW: 8 nm (110)
D xIn= 0.2 dQW: 8 nm (001)
C1 as-grown
C2 annealed ex-situ, 640°C/1h
Rapid Thermal Annealing
• Improvement PL efficiency• PL linewidth narrowing• Blue-shift
diffusionbroadened
Cathodoluminescence
CL reveals relaxation:
(001) vs (110) cf. a, c
8 nm In0.2Ga0.8As-(110)SQW
relaxed, on (001) not
thickness cf. b, c
8 nm In0.2 Ga0.8 As-(110)SQW
relaxed, for 4 nm not
post growth processing cf. d, e
8 nm In0.15 Ga0.85 As-(110)SQW
relaxed after RTA
Indium concentration cf. d, f
grain-like CL emission distribution
for higher xIn
xIn = 0.2(001) 8 nm d = 4 nm (110) d = 8 nm
as grown RTA 900°C/10 s
(110) xIn= 0.15 d = 8 nm(110) 0.6 nm InAs
37 µm
[-110]
(a) (b) (c)
(d) (e) (f)
Atomic Force Microscopy: InGaAs/GaAs
Straight step segments aligned parallel [-110], irrespective of the main misorientation step direction.
This feature is perpendicular to the spin transport direction [001].
This feature appears on:
- double-heterostructures, xIn=0.2
This feature does not appear on:
- single-heterostructure (surface
SQW) with xIn=0.2,
- double-heterostructures with
xIn< 0.2, (same dQW)
AFM image of an (In0.2Ga0.8As)/GaAs-SQW
TG= 420°C, BEP-ratio=70
dotted line - ML steps due to miscut
dashed line - alignment of straight step segments
Transmission Electron Microscopy
The non-equilibrium growth conditions causes deviation from stoichiometry.
The non-equilibrium point defect concentration, in combination with strain and/or
temperature cycles may initiate point defect condensation leading to stacking
fault formation as well as clustering and climbing of dislocations.
QW c
QW
(a)
(b)
30 nm
30 nm
SF QW
QW
g002
g110
(c)
1 m
[-110]
Cross-sectional views of an 8 nm In0.2Ga0.8As SQW sandwiched between
GaAs/(Al,Ga)As barriers: stacking faults (SF) in the QW region (a) and dislocation bundles propagating to the surface (b). Plan-view: dislocations || [-110] and dislocation bundles (c).
Cro
ss-s
ect
ion
al v
iew
Pla
n-v
iew
Spin transport by SAW: GaAs/Al0.3Ga0.7As-QW on GaAs(110)
• Surface/interface of a GaAs/(Al,Ga)As(110) QW
- composed of single and multiple monolayer steps
• Spin polarization of SAW-tranported carriers along [001] is detected up to 20 µm.
AFM image 10 µm2
Degree of circular polarization of the PL from spin-polarized carriers generated atx = 0 and transported by SAW to the position x
[-110]
[001]
SummaryGaAs/(Al,Ga)As-QW• GaAs/(Al,Ga)As-QWs with smooth interfaces are grown on
GaAs(110).
• Spins of photogenerated carriers which are transported by surface acoustic waves in a GaAs/(Al,Ga)As-QW are detected up to a distance of 20 µm.
(In,Ga)As/(Al,Ga)As-QWs• Structural degradation and relaxation in (In,Ga)As/GaAs-QWs
start at lower total net strain on GaAs(110) than on GaAs(001).
• Annealing improves the PL linewidth and efficiency.
• For an efficient transport of spins extended defects in (In,Ga)As and compositional fluctuations have to be minimized.
• For large spin transport distances the range of Indium compositions and SQW thicknesses are limited.
Dark Field Microscopy: InGaAs/AlGaAs
8 nm (In,Ga)As/(Al,Ga)As-SQW, xIn=0.2
- Lines and dots are aligned along [-110]
The lines correspond to the alignment of straight step egde segments in the AFM image as an early stage of development.
- Lines are shallow V-shaped depressions made of vicinal planes.
- In areas of a large line & dot density a pronounced PL emission at about 1040 nm is observed pointing to „quantum“ dot formation even for a single QW.
Optical dark field image (top) and 5K PL spectrum (bottom)
50 µm[-110]