Size-controlled quantum dots fabricated by precipitation of epitaxially grown, immiscible semiconductor heterosystems

Abstract

Epitaxial quantum dots with symmetric and highly faceted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. This novel self-organization scheme is based on the immiscibility of the involved semiconductor materials, which originates from the different bulk bonding configurations and the concomitant lattice-type mismatch. By varying the thickness of the initial 2D layers, the dot size can be controlled in a range between 5 and 25 nm, with areal densities as high as 3 × 10 11 cm −2 . Control of the quantum dot size allows for photoluminescence tuning over a spectral range between 2.2 and 3.7 µm. Multilayer quantum dot stacks with systematically varying sizes yield ultra-broadband emission, and thus a precondition for the development of superluminescence diodes operating in the near-and mid-infrared.

Epitaxial Zinc-Blende CdTe Antidots in Rock-Salt PbTe Semiconductor Thermoelectric Matrix

“…After the MBE growth had been carried out in accordance with the technological protocol described above, both samples were annealed under the same conditions, namely, at 350 °C for 1 h. This annealing procedure resulted in the disintegration of ultrathin, continuous CdTe or PbTe layers into CdTe or PbTe nanoprecipitates. This suggests that the formation of CdTe antidots in a PbTe matrix proceeds in the same way as the formation of PbTe dots in a CdTe matrix − and is driven by the minimization of the CdTe/PbTe interface energy and phase separation near thermodynamic equilibrium. , Therefore, one can expect a correlation between the initial thickness of the CdTe layers in the CdTe/PbTe multilayer and the size of the CdTe antidots, as was established for PbTe dots in CdTe matrix . To verify this idea, we prepared three samples, each containing 10 CdTe layers with thicknesses of 1, 2, and 4 nm, separated by PbTe layers with a thickness of 25 nm.…”

“…Moreover, we expect that, using this method (under better-adjusted technological conditions), it should be possible to obtain CdTe dots with diameters smaller than 4 nm. Note that the lower “theoretical” limit for CdTe layers with an initial thickness of just one monolayer (0.3 nm) is about 1–2 nm . Similarly to PbTe dots, − ,, the CdTe antidots very often exhibit the same highly symmetric rhombicuboctahedral shape, as shown in Figure .…”

“…Thus, using this semiconductor heterosystem, it is possible to obtain stressfree thermoelectric devices. Similarly to the case of PbTe quantum dots in a CdTe matrix, 23 the technological control of the size distribution of the CdTe antidots is possible by changing the thicknesses of the CdTe and PbTe layers in the initial CdTe/ PbTe multilayer. The electrical and thermal properties of a nanocomposite material can be optimized in this way.…”

“…25,26 Therefore, one can expect a correlation between the initial thickness of the CdTe layers in the CdTe/PbTe multilayer and the size of the CdTe antidots, as was established for PbTe dots in CdTe matrix. 23 To verify this idea, we prepared three samples, each containing 10 CdTe layers with thicknesses of 1, 2, and 4 nm, separated by PbTe layers with a thickness of 25 nm. Additionally, we prepared one sample containing four CdTe layers with different initial thicknesses of 0.3, 0.6, 1, and 2 nm.…”

“…We applied a two-stage technological method that involves the molecular beam epitaxy (MBE) growth of a properly designed CdTe/PbTe epitaxial multilayer, followed by a nanostructure-forming in situ high-vacuum annealing procedure. This method exploits the almost complete immiscibility of PbTe and CdTe below a temperature of about 300 °C, which is related to the qualitative differences in chemical bonding and crystal structure of these semiconductor materials. ,− Even though these compounds crystallize in different cubic lattices, they exhibit excellent matching of their lattice parameters: the zinc-blende lattice parameter of CdTe is a 0 = 0.648 nm, whereas the rock-salt lattice parameter of PbTe is a 0 = 0.646 nm. Thus, using this semiconductor heterosystem, it is possible to obtain stress-free thermoelectric devices.…”