(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]