Growth and Characterization of Zn 1-x Mn x Se 1-y S y Epilayers and Related Heterostructures
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Introduction The epitaxial growth of heterostructures derived from the wide bandgap 1I-VI magnetic semiconductor (MS) alloys has received substantial attention for several years primarily because of the fundamentally interesting magneto-optical phenomena (e.g. spin superlattice formation and a giant quantum confined Faraday effect) that result from the exchange interaction between electrons in extended band states and in localized 3d states [1-4]. A parallel outcome of the growth of such heterostructures is that they extend the possibilities of lattice constants and bandgaps available within the context of optoelectronic applications of II-VI heterostructures at short visible wavelengths [5]. For instance, at a superficial level, the (Zn,Mn)(S,Se) alloy system [6-9] offers an interesting alternative to the better investigated (Zn,Mg)(S,Se) buffer layers [5] currently used to confine the optically active region of these devices. Like Mg, the addition of Mn into a IIVI semiconductor such as ZnSe opens up the bandgap and increases the lattice constant. However, the presence of Mn also introduces additional effects such as an absorption band in the vicinity of 2.3 eV as well as a renormalization of the bandgap by spin fluctuations. While such phenomena raise interesting fundamental questions, their influence on purported device applications of MS needs to be carefully considered. This paper discusses our research into the (Zn,Mn)(S,Se) material system and related heterostructures. We first discuss our efforts to lattice-match (Zn,Mn)(S,Se) to GaAs substrates in an effort to produce buffer layers of higher structural integrity than than the
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Mat. Res. Soc. Symp. Proc. Vol. 452 © 1997 Materials Research Society
ZnSe buffer layers typically employed for MS quantum structures. The successful achievement of this goal is important for the fabricaton of high quality MS heterostructures for sophisticated magneto-optical studies. Next, we discuss the growth and optical characterization of larger regions of the quaternary phase diagram and explore the potential of this material to act as a confining layer for Il-VI quantum wells (QWs). Sample growth is carried out on (100) GaAs substrates in a cryoshrouded MBE chamber using Zn, Mn, Se, and ZnS sources of 6N, 5N, 5N and 5N purity, respectively, evaporated from standard effusion cells. In situ reflective high energy electron diffraction (RIEED) at 12 keV monitors the surface during growth. Background pressures in the chamber during growth are typically -10-1 torr. Growth rates of the various materials grown are measured using RHEED intensity oscillations and sample compositions of calibration epilayers are determined by electron probe microscopy. Structural characterization is carried out using double-crystal x-ray diffraction measurements. The steady state photoluminescence (PL) of these samples is measured using lock-in techniques in a 0.4 m spectrometer equipped with a cooled PMT over the temperature range 4K - 300K in a continuous flow optical cryostat using the 325 nm
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