1. (In,Ga)As/(Al,Ga)As quantum wells on GaAs(110) R. Hey, M. Höricke, A. Trampert, U. Jahn, P. Santos Paul-Drude-Institut für Festkörperelektronik, Berlin Outline Motivation Experimental Results 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 transport Summary EuroMBE 2005, Grindelwald, Switzerland

2. 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

3. 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 / T G, BEP composition

4. 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:T s =440°C, BEP=50, v GaAs = 0.2 µm/h GaAs/AlGaAs:T s =480°C, BEP=70 InGaAs/GaAs:T s =440°C, BEP=70 InAs/GaAs/AlGaAsT s =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

5. 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, T G = 450°C and BEP-ratio=70 A x In = 0.1 d QW : 8 nm (110) B x In = 0.1 d QW : 20 nm (110) C x In = 0.2 d QW : 8 nm (110) D x In = 0.2 d QW : 8 nm (001) C1 as-grown C2 annealed ex-situ, 640°C/1h

6. Rapid Thermal Annealing Improvement PL efficiency PL linewidth narrowing Blue-shift diffusion broadened

7. Cathodoluminescence CL reveals relaxation: (001) vs (110) cf. a, c 8 nm In 0.2 Ga 0.8 As-(110)SQW relaxed, on (001) not thickness cf. b, c 8 nm In 0.2 Ga 0.8 As-(110)SQW relaxed, for 4 nm not post growth processing cf. d, e 8 nm In 0.15 Ga 0.85 As-(110)SQW relaxed after RTA Indium concentration cf. d, f grain-like CL emission distribution for higher x In x In = 0.2 (001) 8 nm d = 4 nm (110) d = 8 nm as grown RTA 900°C/10 s (110) x In = 0.15 d = 8 nm (110) 0.6 nm InAs 37 µm [-110] (a)(b)(c) (d)(e)(f)

8. 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, x In =0.2 This feature does not appear on: - single-heterostructure (surface SQW) with x In =0.2, - double-heterostructures with x In < 0.2, (same d QW ) AFM image of an (In 0.2 Ga 0.8 As)/GaAs-SQW T G = 420°C, BEP-ratio=70 dotted line - ML steps due to miscut dashed line - alignment of straight step segments

9. 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 (a) (b) 30 nm SF QW g 002 g 110 (c) 1  m [-110] Cross-sectional views of an 8 nm In 0.2 Ga 0.8 As 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). Cross-sectional view Plan-view

10. Spin transport by SAW: GaAs/Al 0.3 Ga 0.7 As-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 µm 2 Degree of circular polarization  of the PL from spin-polarized carriers generated at x = 0 and transported by SAW to the position x [-110] [001]

11. Summary GaAs/(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.

12. Dark Field Microscopy: InGaAs/AlGaAs 8 nm (In,Ga)As/(Al,Ga)As-SQW, x In =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]