Crystallization of Amorphous Germanium in a Silver Germanium Layered System
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CRYSTALLIZATION OF AMORPHOUS GERMANIUM LAYERED SYSTEM
GERMANIUM
IN
A SILVER-
Toyohiko J. Konno and Robert Sinclair Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
ABSTRACT We studied crystallization of amorphous germanium (a-Ge) induced in a Ag/a-Ge layered system, using in situ transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Amorphous Ge was found to crystallize at about 270'C with the heat of reaction of 10±lkJ/mol. In situ TEM revealed that Ag grains migrate into the a-Ge phase leaving the crystalline Ge (c-Ge) phase behind. From this observation, we propose a model whereby the Ag provides the fastest path for the Ge atoms to diffuse from a-Ge to c-Ge phases. INTRODUCTION The thermal stability of amorphous semiconductors, such as amorphous Si (a-Si) or aGe is an important issue in the application of these materials. For example, it is reported that aSi based solar cells, where a-Si was deposited on aluminum, crystallized at relatively low temperatures. Such behavior when the amorphous semiconductor phase is in contact with metals has been known for a long time and it is sometimes called Metal-Contact Induced Crystallization (MCIC). For example, a-Ge crystallizes at about 100°C when it is in contact such a contact, the crystallization temperature of a-Ge is normally with gold, while without reported to be 500-600 0 C.[1-31 The mechanism of this crystallization process has been the subject of some research. Following a report on the lowering of crystallization temperatures found in a variety of metalsemiconductor systems by Old et al.[4], Herd et al. made the first TEM study on MCIC using ebeam deposited amorphous semiconductor-metal trilayer fidms.[5] These investigators reported that crystallization temperature of a-Si and a-Ge by MCIC correlates with their respective eutectic temperatures. Herd et al. suggested, based on their findings using plan view TEM, that the crystallization temperature is determined by kinetics of mass transport at the interface of the metal and semiconductor phases or by interdiffusion between the two phases. In order to clarify the mechanism of the reaction, direct observation of the metalsemiconductor interface is necessary. Recent development of cross-section TEM makes it possible to achieve such experiments. Radnoczi et al., in a study of a-Si/Al/a-Si layers and Si 5All.5 alloy systems using ex-situ cross-section TEM, showed that Al provides nucleation sites for the crystalline Si (c-Si).[6] The first in situ cross-section TEM on MCIC was carried out by Hung et al., who used an Al/a-Si bilayer film.[7] The a-Si in the bilayer film was, however, made by ion-bombardment after the Al was deposited, leading to a somewhat ambiguous interface. Indeed Hou et al. showed, in the study of crystallization in Ge/Au bilayer system, that intermixing caused by ion-implantation led to a considerable change in the morphology of the annealed film.[8 In the study of MCIC in Al/a-Si multilayer films using in situ cross-se
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