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TEM and XSTM Characterization of Embedded InAs Quantum Dots

Department of Materials Science & Engineering University of California at Berkeley. TEM and XSTM Characterization of Embedded InAs Quantum Dots. Matt Lowry Presented for NSEC203/EEC235 April 16, 2008. Department of Materials Science & Engineering University of California at Berkeley.

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TEM and XSTM Characterization of Embedded InAs Quantum Dots

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  1. Department of Materials Science & Engineering University of California at Berkeley TEM and XSTM Characterization of Embedded InAs Quantum Dots Matt Lowry Presented for NSEC203/EEC235 April 16, 2008

  2. Department of Materials Science & Engineering University of California at Berkeley Motivation • Optical & electrical properties depend on shape, size, composition, and symmetry • AFM vs. XSTM vs. TEM • Changes due to capping • In segregation • TEM averaging effects • XSTM cleavage planes • Optimize growth strategies for potential applications • Good microscopy is difficult

  3. Department of Materials Science & Engineering University of California at Berkeley Cross-Sectional Scanning Tunneling Microscopy (XSTM) • Raster tip • I ~ exp (-kz) • Image or topography • http://www.insp.upmc.fr/axe1/Dispositifs%20quantiques/AxeI2_more/PRINCIPLE/STSprin.HTM

  4. Department of Materials Science & Engineering University of California at Berkeley Shape and Size Determination • Full and truncated pyramidal quantum dots • Cleaved on (110) planes • Only model 2 was observed • D.M. Bruls, et al., Appl. Phys. Lett. 81, 1708 (2002).

  5. Department of Materials Science & Engineering University of California at Berkeley Composition Determination • Stress relaxes on free surface due to 7% lattice mismatch • InxGa1-xAs alloy with linear In gradient • 60%-100% In gradient from bottom to top • Lattice constant • Increase of 35pm across dot due to In content increase • D.M. Bruls, et al., Appl. Phys. Lett. 81, 1708 (2002).

  6. Department of Materials Science & Engineering University of California at Berkeley Multi-Directional Analysis of an Isolated Quantum Dot • Multi-directional analysis to detect asymmetries • Splitting of exciton states • InAs dot with GaAs cap • Focused Ion Beam used to isolate a quantum dot in a micropillar • 80nm X 80nm pillar with 5nm amorphous region T. Kita, et al., Appl. Phys. Lett. 90, 041911 (2007).

  7. Department of Materials Science & Engineering University of California at Berkeley TEM of an Isolated Quantum Dot • Along [110] : 21nm Along [-110] : 23nm • Anisotropic growth due to faceting • Preferential strain in [-110] • Asymmetry of diffraction patterns T. Kita, et al., Appl. Phys. Lett. 90, 041911 (2007). [110] Zone Axis [-110] Zone Axis

  8. Department of Materials Science & Engineering University of California at Berkeley Conclusions • Cross-sectional STM has allowed for the determination of shape, size, and composition of InAs quantum dots embedded in GaAs. • A combined FIB/TEM approach has allowed for the determination of asymmetric crystal strain in an isolated InAs quantum dot embedded in GaAs. • Both procedures are applicable to other systems • Better characterization will hopefully aid in fulfilling the promise of applications

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