Substrate dependence of self-assembled quantum dots Sarah Felix 3/19/08 EE C235 - Nanoscale Fabrication

Design and process control Self-assembled QDs: Stranski-Krastanow (SK) growth using (MBE) PROCESS VARIABLES Material system Temperature Growth rate Substrate Orientation Cleavage interfaces Capping Layer Growth Interruption Substrate Patterning STRUCTURAL CHARACTERISTICS Size Shape Density Spatial arrangement OPTICAL AND ELECTRICAL PROPERTIES Electronic levels Emission energy M. Henini, Nanoscale Res Lett (2006)

GaAs crystal structure (100) typically used Large, well-developed process window for good MBE growth Normal cleavage planes Why look at other orientations? Interband transitions Strain field and growth kinetics Charge and surface polarity

Higher-index planes Lower energy, “corrugated” surface Different surface steps and altered strain energy field influence adatom migration (511) - AFM (211) - RHEED R. Notzel, L. Daweritz, and K. Plook, Phys. Rev. B 46, 4736 (1992)

Relevant studies 1990’s R. Notzel et al. (1992,1994,1996) K. Nishi et al. (1996, 1997) C.M. Reaves et al. (1996) Henini et al. (1998) This talk focuses on a more recent “systematic” studies (2001, 2006) Application: Mapping process/material design to desired electronic and optical properties

Study description Substrates: (100), (n11) A/B; (n=1,2,3,4,5) Molecular beam epitaxy (MBE) Process techniques GaAs capping layer Growth interruption Substrate rotated Characterization Atomic force microscopy (AFM) Photoluminescence (PL) GaAs AlGaAs GaAs/AlAs superlattice In0.5Ga0.5As W. Jiang, H. Xu, B. Xu, W. Zhou, Q. Gong, D. Ding, J. Liang, and Z. Wang, J. Vac. Sci. Technol. B 19(1) 2001

AFM Results Different shapes Highest density on (311) B (100): dome (311) A: arrowhead (311) B: pyramid (111) A: triangle/pyramid Highest density on (311) B Low density on (111) B known to be difficult to grow on Highly non-uniform nucleation on (511) A (100) (511) A (311) A (311) B W. Jiang, H. Xu, B. Xu, W. Zhou, Q. Gong, D. Ding, J. Liang, and Z. Wang, J. Vac. Sci. Technol. B 19(1) 2001

PL Results Blueshift of PL peak, more so for B-type B-type show smaller full-width-half-maximum values, indicating better homogeneity (311) A: High integrated PL intensity correlates with high QD density from AFM Multi-modal PL intensities correlate with size distributions from AFM W. Jiang, H. Xu, B. Xu, W. Zhou, Q. Gong, D. Ding, J. Liang, and Z. Wang, J. Vac. Sci. Technol. B 19(1) 2001

Newer work: B-type substrates Similar processing and methodology Uniform, lateral ordering observed More detailed correlation study B-type shows stronger integral PL intensity and narrower FWHM than (100) B.L. Liang, W.M. Wang, Y.I. Mazur, V.V. Strelchuck, K. Holmes, J.H. Lee, G.J. Salamo, Nanotechnology 17 (2006)

Conclusions Optical and structural properties highly dependent on substrate orientation B-type high-index substrates better suited for QD growth Some possible flaws GaAs capping Conditions optimized for (100) orientation Many interrelated factors at play Indium segregation? Piezoelectric field at interface of wetting layer?  Physical mechanisms not fully understood

Questions?