Characterizations of HgTe nanocrystals induced by controlled precipitation in PbTe-4HgTe semiconductor alloys
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Characterizations of HgTe nanocrystals induced by controlled precipitation in PbTe-4HgTe semiconductor alloys Manjong Lee and Choong-Un Kim Materials Science and Engineering Program The University of Texas at Arlington, P.O. Box 19031, Arlington, TX 76019, USA ABSTRACT This paper reports the observation of HgTe precipitate formation and its subsequent evolution in a PbTe semiconductor matrix containing 4 mol. % HgTe (PbTe-4HgTe) produced by a controlled precipitation process. The controlled precipitation process is being developed as a new way to produce a large number of quality semiconductor nanocrystals more simply than the conventional methods. Characterization of the HgTe precipitates resulting from application of controlled precipitation to the PbTe-4HgTe alloy system provides evidence that this method is effective in generating nanocrystals. Transmission electron microscopy of the processed alloys reveals that HgTe precipitates are extremely small in size, maintain a coherent interface, and experience a unique shape evolution with aging. INTRODUCTION Nanocrystals have various unique properties that make them desirable for future applications and are considered to be a new class of material. For this reason, an extensive amount of research has been conducted over the past decade to understand the fundamentals governing nanocrystal properties as well as to develop effective fabrication methods [1-3]. While those efforts have yielded numerous advances, realizing nanocrystals with superior quality and in sufficient quantity is still a challenging task. Especially challenging is the production of semiconductor nanocrystals within another semiconductor phase because of the stringent microstructural requirements, such as an epitaxial interface, not to mention the smallness in size. While the conventional methods rely heavily on the photolithographic patterning of thin films, more recent methods, known as "self-assembly", use phase instability triggered by strain energy in epitaxially grown semiconductor films. The applicability of these methods is, however, limited to a few specialized materials because of their complexity and the rigorous nature of the processing. In a recent investigation, we introduced a new method [4] that may be effective in producing semiconductor nanocrystals in another semiconductor matrix. The principle of this method, controlled precipitation, is analogous to the aging-hardening process [5] routinely practiced in metallic alloys. In metallic alloys, controlled precipitation is used to produce well-dispersed high-density nanocrystals as a way to control the mechanical strength of structural alloys. It utilizes a sensitive interplay between phase instability and diffusion kinetics. Alloys containing elements with limited solubility experience phase instability when they are subjected to temperatures where equilibrium solubility is lower than the content of the alloying element. The phase instability results in the continuous formation of precipitates until the equilibrium concentration is re
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