Twin-mediated crystal growth
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The structure and origin of twin defects have been studied over the past half-century. Recently, there has been renewed interest in investigating the mechanisms by which twin defects facilitate the growth of bulk and nanoscale systems. This article reviews our understanding and experimental advances to unravel the complex role that twin defects play during crystal growth. The following topics are addressed: growth promotion at single and multiple, parallel and antiparallel twin boundaries; the role of {100} and {111} solid–liquid interfaces during crystallization; the application of realtime imaging to the study of crystal growth in the presence of twin defects; and suggested future research needed to shed light on the driving forces for twin-related phenomena. By providing a broad survey of the existing literature on twin-assisted crystal growth, we anticipate that our review will aid researchers in deciphering various growth forms that arise in materials processing applications.
I. INTRODUCTION
The development of the transistor by Bardeen, Brattain, and Shockley1,2 in the late 1940s brought about a new era in the field of crystal growth. The burgeoning commercial interest of transistors, and later, integrated circuits, placed an unprecedented demand on the control of the perfection of semiconductor crystals.3,4 Thus, it is to our advantage to understand the fundamental processes underlying the formation of semiconductor crystals, and in particular, the complex role that defects play during crystallization. One important class of defect is the twin boundary. Following Friedel,5 a twin is defined as a “polycrystalline edifice, built up of two or more homogeneous portions of the same crystal species in juxtaposition and oriented with respect to each other according to well-defined laws” (see also Refs. 6 and 7). The term homogeneous implies that each portion (also known as a grain in the metallurgical literature) is a single crystal, and juxtaposition means that the constitutive grains touch at the twin boundary. Furthermore, twins may be categorized by their mechanism of formation as either deformation or growth twins; the latter form during out-of-equilibrium materials processing, whereas the former arise to accommodate strain during plastic deformation.6,8 Deformation twinning is not covered in this review. Instead, we refer the interested reader to the reviews in Refs. 8 and 9.
Contributing Editor: Sung-Yoon Chung a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2016.308
Here, we focus on growth twins in semiconductor crystals, examples of which are given in Fig. 1. It is well known10–18 that growth twins that intersect the crystal surfaces act as favorable attachment sites for the nucleation of new monoatomic layers of crystal. Twins are thought to be favorable attachment sites in comparison to facets where layers must overcome a large nucleation barrier for the formation of an isolated island, as first pre
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