Substrate dependence of self-assembled quantum dots

Sarah Felix3/19/08

EE C235 - Nanoscale Fabrication

Design and process control

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)

Self-assembled QDs: Stranski-Krastanow (SK) growth using (MBE)

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

R. Notzel, L. Daweritz, and K. Plook, Phys. Rev. B 46, 4736 (1992)

(511) - AFM

(211) - RHEED

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

In0.5Ga0.5As

GaAs/AlAs superlattice

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

– (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)

(311) B (311) A

(511) A

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

B.L. Liang, W.M. Wang, Y.I. Mazur, V.V. Strelchuck, K. Holmes, J.H. Lee, G.J. Salamo, Nanotechnology 17 (2006)

• Similar processing and methodology• Uniform, lateral ordering observed• More detailed correlation study• B-type shows stronger integral PL

intensity and narrower FWHM than (100)

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?