Luminescence of quasi-2DEG in heterostructures based on PbS films
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Luminescence of quasi-2DEG in heterostructures based on PbS films G. Khlyap, State Pedagogical University, 24 Franko str., Drogobych, 82100, Ukraine. ABSTRACT The paper deals with the problem of luminescence due to non-equilibrium charge carriers transfer between the quasi-2D electron system localized in the space-charge region of the heterostructures based on lead sulfide films (up to 3 µm thickness) and zinc selenide substrates surrounded by the wide gap semiconductor region. The processes of electro- and photoluminescence are studied, the band diagram is proposed and the main parameters of the structure PbS/quasi-2DEG/ZnSe are calculated. INTRODUCTION Highest performance of modern often requires more sophisticated structures and non-destructive methods of their properties analysis. Epitaxial techniques offer important advantages in comparison with the bulk grown ones: lower temperature, shorter growth time and reduced precipitation problems enable the growth of large-area samples with good parameters. At present, MBE (molecular-beam technology) is the most mature method of device-quality layer fabrication. In particular, semiconductor devices based on A2B6-A4B6 heterostructures are of special importance due to the well-known “windoweffect” as well as a possibility to act as a system of quasi-2D or quasi-3D charge carriers, as it was shown earlier [1]. The paper presents results of photoluminescence studies performed at 77 K on the isotype n-n-heterojunctions PbS/ZnSe obtained by the MBE technology. EXPERIMENTAL DETAILS Preparation of the samples. The investigated isotype n-n-heterostructures ZnSe/PbS were grown by the MBE technology of lead sulfide (Eg = 0.41 eV) films with thickness up to 3 µm on the (110)oriented ZnSe (Eg = 2.72 eV) wafers under the substrate temperature Ts = 540 K (the vacuum level in the effusion cell was estimated to be about 10-9 Tor). The samples of 1.5x3.5 mm2 size characterized by the film surface homogeneity were selected for the examinations. Parameters of the contacting materials are listed in Table 1. Table 1. Parameters of the components of the investigated heterostructures Material ZnSe
PbS:Na
Parameter Eg = 2.72 eV a = 5.668 Å ε = 9.1ε0 ne = (7.8.1016 - 2.6.1017) cm-3 Eg = 0.41 eV a = 5.940 Å ε = 175ε0 ne = (2 - 8).1015 cm-3 H6.39.1
Results of electric-field measurements. The electrophysical studies described for the first time in [2].were shown that the heterostructures based on the materials mentioned in the present article are abrupt heterojunctions.
Fig. 1. Field dependencies of the examined heterostructure n-PbS/nZnSe. Functions 1, 3 are forward sections and functions 2, 4 are reverse ones at 300 and 77 K, respectively. The further electric-field measurements (Fig. 1) carried out on the as-grown specimens demonstrated that the processes of the charge carriers transfer are strongly effected by the space-charge region of the heterojunction formed by the surface electron states at the interface. The numerical analysis of the experimental I-F-measurements (where I stands for
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