The Effect of Composition Dependent Lattice Strain on the Chemical Potential of Tellurium in Pb 1-x Sn x Se 1-y Te y Qua
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The Effect of Composition Dependent Lattice Strain on the Chemical Potential of Tellurium in Pbl.xSnxSelyTey Quaternary Alloys
Patrick J. McCann School of Electrical Engineering, Universityof Oklahoma, Norman, OK 73019
ABSTRACT
IV-VI semiconductor Pbj xsSnxsSej-ysTe.. quaternary alloys were grown on (I 11) BaF2 by liquid phase epitaxy (LPE). X-ray diffraction analysis shows that liquid (Pbl.xSnxl)o. 9g(Sel.yTey1)o.oI solutions produce alloys lattice-matched with the substrate whenyj = 60%, 58%, and 57%, and x1 = 20%, 40%, and 60%, respectively. These data suggest that the chemical potential of tellurium in the Pbl-xsSnxsSe-ysTeys solid solution decreases as tin and tellurium concentrations increase. It is argued that this reduction is due to decreasing strain energy driven segregation of tellurium from the solid to the liquid as the tin concentration increases. I. INTRODUCTION
A promising materials system for fabrication of far-infrared optoelectronic devices is the quaternary PblxsSnxsSe,_ysTeys alloy. This material can be used as the active region in either detectors or lasers designed to operate in the 6 to 30 micron spectral range. Applications for such devices include thermal imaging and high resolution molecular spectroscopy. The bandgaps and lattice parameters ofthis IV-VI semiconductor quaternary alloy are independently controllable, allowing growth of lattice-matched layered heterostructures. Substrates for lattice-matched layer growth include Pbo.80Sno.2oTe [1], PbSe [2,3], and KCI [4]. The growth method in all of these cases has been liquid phase epitaxy (LPE). Since LPE growth takes place under near-equilibrium conditions, it is possible to extract thermodynamic information regarding the liquid-to-solid phase transformation. This thermodynamic information can then be used to elucidate various properties of the solid solution alloy. In this paper, data from recently performed lattice-matching experiments involving LPE growth of PblxsSnxsSel-ysTeys alloys on (111) BaF2 substrates are used to show that the chemical potential of tellurium in this solid solution is dependent upon tin content. It is hypothesized that increasing the tin content decreases the lattice strain associated with tellurium incorporation thus reducing tellurium chemical potential. II. EXPERIMENTAL PROCEDURE
(Pb1..xSnxl)o.99(Se1.y 1Tey,)o.oi growth solutions were prepared using weighed portions of six
nines lead and tin and polycrystalline pieces of PbTe and PbSe. With a chalcogen content fixed at 1%, the liquidus temperatures for these solutions were in the range of 530 0 C [3]. Pbl.xsSnxsSe_yjTeys solid solution alloys were grown from these liquid solutions on freshly
cleaved (111 )-oriented BaF2 substrates using a standard sliding-substrate LPE graphite boat under a flowing hydrogen ambient. Growth was initiated two degrees below the liquidus temperature by positioning the BaF 2 substrate under the growth solution. Growth proceeded for ten minutes while the furnace cooled at two degrees per minute. Growth was terminated by pul
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