Mechanisms for Metastability in Hydrogenated Amorphous Silicon
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MECHANISMS FOR METASTABILITY IN HYDROGENATED AMORPHOUS SILICON R. BISWAS*, Y.-P. LI** AND B.C. PAN** *Department of Physics and Astronomy, Microelectronics Research Center and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011 **Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China ABSTRACT We propose metastabilities in amorphous silicon fall into two classes. One class is the local changes of structure affecting a macroscopic fraction of sites. The other class is the metastable generation of dangling bonds with mid-gap states. The local metastability is explained by a new metastable state formed when H is flipped to the backside of the Si-H bond at monohydride sites. The dipole moment of this H-flip defect is larger and increases the infrared absorption. This H-flip defect accounts for large structural changes observed on light soaking including larger absorption and volume dilation. We propose a new model for the generation of metastable dangling bonds. The new ‘silicon network rebonding model’ involves breaking of weak silicon bonds and formation of isolated dangling bonds, through rebonding of the silicon network. Hydrogen motion is not involved in metastable defect formation. Defect formation proceeds by breaking weak silicon bonds and formation of dangling bond-floating bond pairs. The floating bonds migrate through the network and annihilate, producing isolated dangling bonds. This new model provides a new platform for understanding the atomistic origins of lightinduced degradation. INTRODUCTION Metastability is among the most central issues facing amorphous silicon materials and devices. The understanding of metastability will benefit solar cell technology and have a fundamental impact on the physics of amorphous materials. The Staebler-Wronski (SW) effect [1] or light-induced degradation of hydrogenated amorphous silicon (a-Si:H), continues to be a leading area of study. We focus in this paper on understanding the atomistic origins of metastabilities of a-Si:H. We propose a division of metastable behavior into two natural classes. One is the StaeblerWronski degradation where light-soaking generates midgap electronic states at densities of 1016– 1017 cm-3. It is well established [2] that these defects are neutral silicon dangling bonds (D0) that are essentially indistinguishable from native (as-grown) dangling bonds. Several experiments have established that these dangling bonds are separated by more than 40 Å from each other [3] and are at least 4 Å separated from hydrogen sites [4]. These metastable dangling bond defects occur at an exceedingly low density of 1 atom in 105-106 atoms. The second aspect of metastability is that several experiments have detected additional larger changes to the structure that occur at much higher densities, perhaps affecting
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~ 1% of all the sites in the material[5]. These changes cannot be explained by the much lower density of dangling bond defects. Infrared absorption [6] finds a remarkable incre
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