Thermal Stability of Amorphous Multilayer Structures
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THERMAL STABILITY OF AMORPHOUS MULTILAYER STRUCTURES K. TANAKA*, I. HONMA**, H. TAMAOKI** AND H. KOMIYAMA** *Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki **University of Tokyo, Bunkyo-ku, Tokyo 113
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ABSTRACT Firstly, a general discussion on the crystallization of amorphous multilayer structures is presented from the viewpoints of thermodynamics, where both a nature of the heterointerface and a repeat distance of the layer are theoretically shown to be key factors for determining the crystallization temperature (Tc). Secondly, experimental observations are described of the effect of each layer thickness and the heterointerface on the structural stability of reactively-sputtered a-Ge:H(a-Ge)/a-GeNx multilayer films. It is demonstrated that Tc of a-Ge:H(a-Ge) increases with decreasing its layer thickness and/or increasing the thickness of a-GeNx layer, which is interpreted qualitatively within the framework of the macroscopic thermodynamics. Relevant phenomena observed by other groups as well as the present results are discussed in a unified manner. INTRODUCTION A variety of optical and electronic properties have been reported in amorphous multilayer structures (AMS) consisting of alternately-stacked two different ultra-thin amorphous layers; quantum size effects [1-31, changes in photoluminescence [1,4] and band-tail absorption [5] due to hetero-interface defects, and persistent photoconductivity [6,7], in a-Si:H/a-SiNx:H, a-Si:H/aSiCx:H and a-Si:H/a-Ge:H superlattices. However, structural stability of those amorphous multilayer structures (AMS) has not yet been well understood although it is obviously one of the important issues in the device applications of AMS[81. Several works have been reported regarding thermal instability of amorphous multilayer structures at elevated temperatures; interdiffusion in Si/Ge amorphous multilayer films (8], amorphous-to-crystalline transition of Ge in Pb/amorphous Ge multilayer films [9], and nickel silicide formation from Ni/amorphous Si multilayer films [10]. It has been recognized in those works that the phase transition occurs at lower temperatures according as the repeat distance of AMS decreases [91(10]. On the other hand, in our previous work, the crystallization temperature of Ge in a-Ge:H/a-GeN, multilayer films increased with a decrease in each layer thickness, suggesting that the thermal stability of a-Ge:H is enhanced in amorphous multilayer structures [11]. Similar observation has been independently reported by Gonzalez-Hernandez et al. in aGe/a-SiNx and a-Si/a-SiNx multilayer structures [12]. In this paper, firstly, we discuss the thermal stability of amorphous multilayer structures using a simple free-energy model, and roughly estimate the crystallization temperature as a function of the layer thickness and the interfacial energy between layers. Secondly, we present systematic data on the annealing experiments performed on a-Ge:H/a-GeNx as well as aGe/a-GeNx multilayer structures and discuss the crystallization of a-Ge in AMS as well as pre-crystalliz
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