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II.Nanocrystal Quantum Dots  >> 3. Integrated Hybrid Nanostructures: Overgrowth on InAs NCQDs


Integrated Hybrid Nanostructures: Overgrowth on InAs NCQDs


This work is supported by: DURINT-01 AFOSR Grant No. F49620-01-1-0474


Our approach to epitaxical integration of NCQDs (schematically shown in fig. 3 of the introduction) follows a four step procedure:
    1. Preparation of a fresh atomically smooth GaAs(001) surface via MBE growth.
    2. Deposition of NCQDs on the substrate from the toluene solution in a controlled atmosphere glove box.
    3. Surface cleaning to remove the organic moieties coming from the NCQD solution and also the TOP ligand molecules from the NCQD surface.
    4. High quality III-V epitaxical overgrowth.
As a prototypical systemthe we have investigated epitaxical integration of InAs NCQDs into GaAs matrix. Due to the reduced thermal stability of the InAs NCQDs compared to bulk InAs, a consequence of "Kelvin" effect, steps 3 and 4 have to carried out with the lowest possible thermal budget consistent with good quality overgrowth. To minimize the thermal budget during step 3 we carry out the cleaning with the assistance of H-radical flux. And for step 4 we employ Migration Enhanced Epitaxy (MEE) a special MBE based technique which allows high quality epitaxy at low temperatures. Cleaning under optimized conditions leads to substantial size-reduction of NCQDs, from ~ 4nm starting size to ~ 1nm. Figure 1 shows AFM measurement of the NCQDs after optimized cleaning.
Figure 1 (a) AFM image of the NCQDs on GaAs(001) after H-assisted cleaning. (b) Height profile across the line shown in panel (a).

To determine the quality of the epitaxical overgrowth, the MEE growth is done simultaneously on three types of substrates loaded on the same holder:
    (1) Untreated sample - this sample is exposed only to the glove-box and serves as the reference,
    (2) Toluene exposed sample, and
    (3) InAs NCQD deposited sample.
The overgrowth consisted of deposition of 1ML InAs followed by 170 ML of GaAs. The 1 ML InAs layer serves as an marker to determine the optical quality.

HRTEM and PL studies were performed to determine the structural and optical nature of the overgrowth. The cross-sectional HRTEM image shown in fig. 2 evidences the good structural quality. Large regions of the sample were examined (compared to the mean spacing between the NCQDs) and no extended defects were observed. However, the limitations (spatial resolution and lack of chemical information) of employed TEM precludes observation of the embedded ~ 1nm InAs NCQDs.

Figure 2 Cross-sectional HRTEM image of overgrowth on InAs NCQDs.


The results from optical studies (see in figure 3) show that the 1ML InAs PL of the toluene-exposed and NCQD-deposited samples is comparable to that of reference, a result consistent with the good structural quality. Though we do not see a clear optical signature from the NCQDs, the NCQD deposited sample shows a shoulder on the low-energy side of the 1 ML InAs PL peak. The location of this shoulder is qualitatively consistent with that expected from sub-nm InAs NCQDs. Following these encouraging results (for details see reference 1), we are proceeding with the obvious next step of using InAs NCQDs with a larger starting size than the 4nm NCQDs used in these studies. We have recently succeeded in synthesizing 8nm InAs NCQDs with large yield by using punctuated growth, a concept that was developed and demonstrated in our lab in the context of synthesizing the InAs self-assembled quantum dots (SAQDs) with independent control over their density and size (reference 2).

Figure 3 Photoluminescence behavior of the overgrowth


    1. "Integrated semiconductor nanocrystal and epitaxical nanostructure systems: structural and optical behavior", Anupam Madhukar, Siyuan Lu, Atul Konkar, Yi Zhang, Max Ho, Steven M. Hughes, and A. Paul Alivisatos, Nano Lett. 5, 492 (2005).
    2. "Punctuated island growth: An approach to examination and control of quantum dot density, size, and shape evolution", I. Mukhametzhanov, Z. Wei, R. Heitz, and A. Madhukar, Appl. Phys. Lett. 75, 85 (1999)


II.1 Study of Energy Transfer
II.2 TEM Study

DURINT Project
    - Nanocrystal/Eptaxial 2D integration
    - Surface Modification (Chemical & Biochemical)
    - Nanoscale Simultaneous Morphological & Optical Imaging

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