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

2. 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)

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

9. 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)

10. 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

11. Questions?