Nanoscale physics

Concentration changes in the energy levels of 2D excitons in the Cd0.8Mg0.2Te / Cd1-хMnхTe structures

A. Komarov1a, P. Bukivskyj1, Yu. Gnatenko1, T.J. Wojtowicz2, Z. Zhuchenko3, G.G. Tarasov3, G. Vertsymakha4 

1 Institute of Physics of NASU, 03028 Kyiv, Pr. Nauky 46, Ukraine
2 Institute of Physics PAS, 02-668 Warsaw, Al. Lotników 32/46, Poland
3 Institute of Semiconductor Physics of NASU, 03028 Kyiv, Pr. Nauky 41, Ukraine
4 Institute for Nuclear Research of NASU, 03680 Kyiv, Pr. Nauky 47, Ukraine 
a) E-mail of corresponding author: a.v.komarov@i.ua

 	In this study the low-temperature (T=2K) reflection spectra of quasi-two-dimensional excitons (2D-excitons) measured in 4 samples (305A, 305B, 305C, 305D) of Cd0.8Mg0.2Te /Cd1-хMnхTe heterostructures with xA=0, xB=0.014, xC=0.025, and xD=0.05 are presented. Each of the samples grown by MBE technique consists of four Cd0.8Mg0.2Te /Cd1-хMnхTe /Cd0.8Mg0.2Te quantum wells (QWs) with different widths di of the Cd1-хMnхTe layer: di=19, 32, 58, and 300 Å for i=1,2,3,4 respectively. The layers of mixed semiconductor compound Cd0.8Mg0.2Te formed the barriers in the structures. The Cd0.8Mg0.2Te cap layer of 50 nm thickness was grown over each sample, whereas three inner barrier layers were of 30 nm thickness. The wide buffer layer was of 500 nm. All the samples were grown under the same growth conditions. In this case, the parameters of the width of the four quantum wells and all the barrier layers were characterized by the same values in each of the four studied samples. 
The concentration dependences of the energy levels of 2D‑excitons localized in the QWs with di values varying in the range from 1.9 to 30 nm were obtained. The dependence found in the narrowest QW with d1<aB, turned out to be different in form and quantitatively from the dependences obtained for 2D‑excitons localized in wider QWs. Here, aB is the Bohr radius of 2D‑excitons, which is estimated as aB ≈5 nm for QW with d4 = 30 nm and aB≈10 nm for QW with d1= 1.9 nm. The concentration dependences recorded at the beginning of the xj interval from xA=0 to xB=0.014 for 2D‑excitons localized in the wide QWs with d2=32, d3=58, and d4=300 Å are well described by linear functions similar to those observed in bulk Cd1-хMnхTe single crystals. In the rest of the range under study, from xB=0.014 to xC=0.025 and up to xD=0.05, the nonlinear rise section was recorded, the larger in magnitude, the larger the xJ value. When analyzing the results of this study, we suggest, that the mechanisms that complicate the shape of the concentration dependences obtained for 2D‑excitons localized in the QWs with thickness changing from 1.9 to 30 nm, are associated with thermal diffusion processes occurring at high temperature (300-500° C) and long-term (3-6 hours) MBE growth of the studied structures. We conclude 1) that the thermal diffusion of Mg ions into the i-th QW leads to a decrease in the average value of its di*- width and 2) the diffusion rate of Mg ions increases with an increase in the xj concentration of Mn ions in the i-th layer of Cd1-хMnхTe forming the i-th QW. Recently, an additional experimental study has been carried out, the results of which confirm the correctness of the assumption that the thermal diffusion of Mg and Mn ions changes the QW parameters precisely during the time interval (Δt=3-6 hours), when the structures grow using the MBE technology. This conclusion is important for the technology of producing the thin QW in dilute magnetic heterostructures.