ABSTRACT
The thermodynamics and atomic-scale mechanisms of metal-induced 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
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. 2.1 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 [1-17], 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) [40-43]. 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, flat-panel 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. [1-42]). It has been found that in MIC the (reduced) crystallization temperatures as well as the crystallization behaviors of amorphous semiconductors are
strongly dependent on the type of the contacting metal (e.g., Refs. [40-43]). 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 [26-38]. The MIC process in such systems is often associated with initial formation of one or more compound phases at the metal/semiconductor interface and subsequent motion of the compound phase(s) reaction front into the amorphous semiconductor phase, leaving behind a crystalline semiconductor phase in the wake of the compound phase reaction front [26-38]. For non-compound-forming metals such as Al, Ag, and Au, the MIC of amorphous semiconductors can occur at much lower temperatures (as low as 150°C for amorphous Si [a-Si] in contact with Al [5, 11] or Au [21]). The MIC process in such systems is often associated with transportation of semiconductor material into the metal phase, and a crystalline semiconductor phase is formed at the location of the original metal phase [3, 9]. In such cases, even very small variations in the detailed microstructure of the metal phase (e.g., single-crystalline or polycrystalline, fine-grained or coarsegrained, presence of defects and/or contaminants, film thickness) can result in very different crystallization temperatures and crystallization behaviors [2, 9]. Since most previous studies on MIC are largely application orientated, the fundamental, scientific aspects of the MIC process have often been discussed only briefly and qualitatively and sometimes have even been fully discarded in the literature. The microscopic, atomistic mechanisms of the MIC process have remained largely unclear. In the last years, the present authors have devoted considerable effort to attain a fundamental understanding of the thermodynamics and atomic mechanisms of the MIC process in various metal/amorphous semiconductor systems. On the one hand theoretical modeling of the (interface) thermodynamics was performed by employing the macroscopic atom approach [10, 11, 44]. On the other hand dedicated, in particular in situ heating high-resolution transmission electron microscopy (HRTEM) and valence energyfiltered transmission electron microscopy (VEFTEM), experiments
were performed to reveal the operating atomistic mechanisms [14, 15]. It has been found that MIC is in general an interface-controlled phenomenon [10, 11, 14, 40]. The crucial roles of interfaces (including, especially, grain boundaries [GBs] in the metal phase) in MIC involve (1) bond weakening of the semiconductor material at the interface with the metal, leading to enhanced mobility of semiconductor atoms at the interface; (2) the interfaces (including GBs in the metal) serving as fast, short-circuit diffusion paths for material transportation; and (3) very importantly, the interface energetics (thermodynamics), which is decisive for whether crystal nucleation and crystal growth will occur. As will be described in detail in this chapter, the MIC process as observed in a wide range of metal/amorphous semiconductor systems can be described on the above basis. The theoretical predictions have been confirmed by in situ heating X-ray diffraction (XRD), HRTEM, and VEFTEM experiments, where the HRTEM and VEFTEM methods also revealed the atomistic mechanisms of the MIC process. The thus achieved fundamental understanding of the MIC process makes possible highly sophisticated application(s) of MIC in various technologies. For example, direct application of the model for interface thermodynamics enables us to tailor the crystallization temperature of a-Si systematically from 700°C down to 150°C (and any temperature in between) by controlling the Al overlayer thickness [10]. Such fundamental understanding has led, for example, to the development of a novel process for the growth of crystalline Si (c-Si) nanowires at exceedingly low temperatures [14].
