EuTe/PbTe Superlattices: Mbe Growth and Optical Characterization
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EuTe/PbTe SUPERLATTICES: MBE GROWTH AND OPTICAL CHARACTERIZATION G.SPRINGHOLZ, SHU YUAN, G.BAUER, M.KRIECHBAUM* AND H.KRENN InstitutfUr Halbleiterphysik,JohannesKepler UniversitldtLinz, A - 4040 Linz, Austria * InstitutftirTheoretischePhysik, Karl Franzens UniversitiitGraz, A - 8010 Graz, Austria ABSTRACT
The heteroepitaxial growth of EuTe on PbTe (11) by molecular beam epitaxy (MBE) was investigated using in situ reflection high energy electron diffraction (RHEED). As a function of substrate temperature and Te2 flux rate, the resulting EuTe (11) surfaces exhibit several different surface reconstructions corresponding to Te-stabilized or Eu-stabilized surfaces. The Eustabilized surface shows a (243 x 243)R30° surface reconstruction. Because of the strain induced tendency for 3D islanding, only in a narrow window of MBE growth parameters perfect 2D layer-by-layer heteroepitaxial growth exists. Using such optimized MBE growth conditions, we have fabricated a series of PbTe/EuTe superlattices. In such superlattices the wide band gap EuTe layers act as barriers and the narrow band gap PbTe as quantum wells. The superlattices were investigated by high resolution x-ray diffraction, showing their high structural perfection. Modulated low temperature Fourier transform infrared reflection measurements were performed in order to determine the confined energy levels in the PbTe quantum wells. The measurements indicate that mini-subbands are formed in the PbTe quantum wells with a mini-band width of 22 meV in agreement with envelope function calculations. 1. INTRODUCTION
Advances in molecular beam epitaxial (MBE) growth techniques have recently led to the fabrication of lead salt heterostructure quantum well diode lasers [1]. In such structures, usually PbTe is used for the quantum wells and the ternary PblEuxTe with x < 0.05 for the barriers. Because of the rapid increase of the energy gap with Eu content [1], PblxEuxTe is an attractive material in this context. Recently, also PbTe/EuTe superlattices, have been used in such structures [2], as well as in PbTe-EuTe transistor structures [3]. Because of the large difference of the energy band gaps (PbTe: Eg = 190 meV and EuTe: Eg = 2.0 eV), very large confinement energies of more than 1 eV can be achived in EuTe/PbTe quantum well structures. In addition, the EuTe - PbTe system has attracted attention because of the interesting combination of an antiferromagnetic (EuTe) and a diamagnetic (PbTe) material [5,6], which offers the possibility to study the influence of low dimensionality on the magnetic behaviour in short period EuTe/PbTe superlattices. In terms of heteroepitaxy, both materials crystallize in the fcc rocksalt crystal structure and their lattice mismatch is only 2.1%. 2. EXPERIMENTAL
Our experiments were carried out in a Riber MBE growth chamber and custom-built preparation and load-lock chambers. For optimum PbTe MBE growth, it is of crucial importance
to achieve an as close as possible stoichiometric flux [7]. Therefore, we use a Te2 effusion cell in addition to
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