Dislocation-Induced Spatial Ordering of InAs Quantum Dots

1. Department of Materials Science & Engineering University of California at Berkeley Dislocation-Induced Spatial Ordering of InAs Quantum Dots Matt Lowry Presented for NSEC203/EEC235 March 19, 2008

2. Department of Materials Science & Engineering University of California at Berkeley Motivation • Stranski-Krastanov growth yields random distribution of QD’s • Misfit dislocation networks allow for preferential nucleation of QD’s • Alignment is a natural consequence of growth • No separate lithography C.C. Thet, S. Panyakeow, and S. Kanjanachuchai, Microelectronic Eng. 84, 1562 (2007).

3. Department of Materials Science & Engineering University of California at Berkeley Misfit Dislocation Network (MDN) • Lattice strain is relaxed by misfit dislocations and threading dislocations • Slip steps followed by surface diffusion allow for the formation of undulations • Cross-hatch pattern • C.C. Thet, S. Panyakeow, and S. Kanjanachuchai, Microelectronic Eng. 84, 1562 (2007).

4. Department of Materials Science & Engineering University of California at Berkeley Cross-Hatch Pattern Approach • Graded layer of InxGa1-xAs grown on a GaAs substrate. • 0.013 ≤ x ≤ 0.18 • Before QD growth, samples annealed to allow for dislocation motion • Also grew QD’s on dislocation free GaAs for comparison. R. Leon et al., J. Appl. Phys. 91, 5826 (2002)

5. Department of Materials Science & Engineering University of California at Berkeley Cross-Hatch Pattern Results • QD’s nucleate in trenches, both walls and along bottom • Strained: QD density = 109 / cm2 Control: QD density = 1010 / cm2 • Depending on diffusion time, up to 97% of QD’s along trenches R. Leon et al., J. Appl. Phys. 91, 5826 (2002)

6. Department of Materials Science & Engineering University of California at Berkeley Photoluminescence Characterization of CHP Method • Ordered QD’s: 1.22 eV peak, 76 meV FWHM • Control QD’s: 1.19.eV peak, 128 meV FWHM • Narrow peak due to smaller size variations and reduced interactions • Ordered carrier lifetime: 170 ps • Control carrier lifetime: 460 ps • Both are too short, indicates the presence of defects in dots or at interfaces • Neither is optimized R. Leon et al., J. Appl. Phys. 91, 5826 (2002)

7. Department of Materials Science & Engineering University of California at Berkeley Smooth Surface Approach • QD’s aligned without formation of cross-hatched pattern • InxGa1-xAs: • x = 10-30% • d= 8-40 nm • Critical coverage = 1.68 monolayers, but only 1.65 monolayers along dislocation lines θ<θc θ > θc 7 nm 0 nm H. Welsch et al., J. Crystal Growth 301-302, 759 (2007).

8. Department of Materials Science & Engineering University of California at Berkeley Conclusions • Misfit dislocation networks generated from InxGa1-xAs on GaAs lattice strain relaxation have been used to order InAs QD’s along <110> directions. • Ordering can occur with or without the formation of a cross-hatch pattern. • Ordered peaks have a shortened carrier lifetime and a narrower PL peak. • Control over the misfit dislocation network? • Application: quantum computation architectures such as quantum cellular automata

9. Department of Materials Science & Engineering University of California at Berkeley References R. Leon, et al., J. Appl. Phys. 91, 5826 (2002). H. Welsch, T. Kipp, Ch. Heyn, and W. Hansen, J. Crys. Growth 301-302, 759 (2007). C.C. Thet, S. Panyakeow, and S. Kanjanachuchai, Microelectronic Eng. 84, 1562 (2007).