The thermodynamics and atomic-scale mechanisms of metalinduced crystallization (MIC) of amorphous semiconductors are described in detail. It is shown that the MIC effect for a wide range of metal/amorphous semiconductor systems is in general an interfacecontrolled phenomenon, which includes three major aspects: (i) metal-induced weakening of covalent bonds at the interface, (ii) fast atomic transportation along the interface, and (iii) interface 26 Thermodynamics and Atomic Mechanisms thermodynamics, which critically controls whether low-temperature crystal nucleation and crystal growth can occur or not. By quantitative calculation of the interface thermodynamics, recognizing aspects (i) and (ii), the (very) different MIC temperatures and behaviors observed for various metal/amorphous semiconductor systems can now be understood and predicted on a unified basis. The theoretical predictions have been confirmed in particular by in situ heating high-resolution transmission electron microscopy (TEM) and valence energy-filtered TEM experiments, which also revealed the atomic-scale mechanisms of the MIC process. The fundamental understanding reached may lead to pronounced technological progress in applications of the MIC process, in particular regarding the low-temperature manufacturing of high-efficiency solar cells and other electronic components on cheap and flexible substrates such as glasses and plastics.
IntroductionAmorphous semiconductors such as silicon and germanium can crystallize at a temperature much lower than their "bulk" crystallization temperatures when they are put in direct contact with a metal, such as Al [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], Au [18-22], Ag [23-25], Ni [26-33], Cu [34-36], and Pd [37,38]. This phenomenon, which was firstly observed more than 40 years ago for amorphous germanium [39], is now commonly referred to as metal-induced crystallization (MIC) [40][41][42][43]. In the past decade, the MIC process in various metal/amorphous semiconductor systems has been extensively investigated, which has largely been driven by its many (potential) applications, for example, in the low-temperature production of high-performance crystalline semiconductor-based solar cells, flatpanel displays, and high-density data storage devices (see Chapters 5, 6, and 7 of the book).As a result of numerous investigations from different research groups all over the world, the MIC characteristics in various metal/ amorphous semiconductor systems have now been disclosed in great detail, as reported in the literature (see, e.g., Refs. ). It has been found that in MIC the (reduced) crystallization temperatures as well as the crystallization behaviors of amorphous semiconductors are 27 strongly dependent on the type of the contacting metal (e.g., ).For compound (e.g., silicide)-forming metals such as Ni, Cu, and Pd, the MIC of amorphous semiconductors usually occurs at a relatively high temperature of about 500°C [26][27][28][29][30][31][32][33][34][35][36][37][38]. The MIC process in such sys...