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Fundamental understanding of quantum dots in III-V heterostructures

contact person: Li He (lh5n@virginia.edu)

UT-Austin/U. Virginia/Harvard

    Outline: III-P based quantum dot structures are developed as possible light emitters in the yellow and green spectral regions. At University of Virginia, we focus in the structural characterization by TEM techniques.

TEM sample preparation was as follows. For cross-section samples, two sections (3x2mm² each) of the same wafer were bonded with epitaxial films attached such that the [110] directions of each sample were perpendicular to each other.  The samples were then thinned to electron transparency by a combination of mechanical polishing and argon ion milling.  Planview samples were thinned by dimple-grinding and ion milling of the substrate side.

Fig1 shows one plan-view image of Sample A (InP QD/InAlP/GaAs) along [001] zone axis. Quantum dots are densely packed (1.6x10⁸ mm² ).

Figure1 1 Plan-view –220 dark field TEM image of Sample A.

Representative cross section images of Sample A are shown in Fig. 2 and Fig. 3.  The quantum dots are dislocation free.  Furthermore, we measured the angles between sides and bases in various samples. It was found that two angles 56.2^o and 35.2^o on the average prevailed on the –110 side and 110 side respectively, as shown in Fig 2 and Fig 3. We proposed one shape model that is consistent with this data as shown in Fig 4.  In this model, semi-pyramidal growth occurs for which {111}-type and {112} type facets develop.  Presumably, these facets develop relatively later in the growth process, as smaller dots shown in the images do not yet exhibit faceted sides.

Figure4 4 Schematic graph of Model 1

Quantum dots precursor dose appears to be the decisive factor that controls dot size. When the dose was 7.5 ML, the dots were generally 50 nm wide in base and were several nms high. For the case of 11.25ML, the dots were around 70 nm wide in base and about 10 nm high. The base width was then stable at over 70nm while the height grew to 20 nm or so for the case of 15ML.  It was also seen that the quantum dots embedded with capping layer were smaller in height than the dots without capping, which may be ascribed to the atom diffusion from dots into the capping layer.

In conclusion, we have applied conventional TEM to acquire structure information on III-V quantum dots.  Specifically, we are able to correlate quantum dot distribution measured by TEM with the distribution measured by AFM.  From cross-section imaging we were able to model the specific facets that developed during growth.  Finally, the size of the quantum dots was shown to be dependent on precursor dose.  Future experiments using analytical techniques such as EDXS and EELS are planned in order to elucidate precise compositional variations within quantum dots.