Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties

Abstract

We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn-Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105).

Self-organizing structures in immiscible crystals

“…It is interesting to note that our simulations predict that even at such low temperatures as T = 230 • C the layers undergo morphological transitions. These observations are in agreement with the results of the experiment on the multilayer PbTe/CdTe structures 10 .…”

“…Assuming the lattice constants a of 6.46 Å for PbTe and 6.48 Å for CdTe, we need approximately 12 atomic layers of PbTe and 32 atomic layers of CdTe to simulate the real growth conditions. It corresponds to the thicknesses of the layers used in the experiment 10 . We have performed Monte Carlo simulations based on the microscopic model described in this section.…”

“…By using the MC model described in the previous section we simulated the morphological transformations observed experimentally in PbTe/CdTe material system 10 . Below, we study specific situation of the reorganization process during growth leading to the appearance of zero-, one-, two-or three-dimensional patterns.…”

“…The case of morphological transformations in PbTe/CdTe multilayer heterostuctures was studied experimentally in Ref. 10. It was shown that depending on the growth conditions structures of different dimensionality were formed.…”

“…When the temperature is increased the layers transform into zero-dimensional dots that then merge creating one-dimensional columns. At sufficiently high growth temperature PbTe and CdTe deposits separate completely forming thick, quasi bulk objects 10 . Self-organization by spinodal decomposition in solids is interesting not only from the point of view of the practical applications but also from the point of view of theoretical study of processes leading to morphological reorganization of deposited structures.…”