Theory of Hydrogen Interactions with Amorphous Silicon
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molecules. First-principles computations have significantly contributed to our understanding of hydrogen-related phenomena. In this paper we will focus on recent work closely connected to the issues outlined above. The computational results have all been obtained using a state-of-the-art first-principles approach based on density-functional theory, ab inihio pseudopotentials, and a supercell geometry. In the first part of the paper, we will focus on hydrogen molecules. We will review experimental observations of interstitial H 2 molecules in crystalline and amorphous semiconductors, and describe the theoretical framework for understanding the physics of incorporation of a strongly bound molecule in a semiconducting environment. The second part of the paper will deal with hydrogen motion, as occurs in diffusion and in light-induced defect generation. We first discuss an exchange process between trapped and interstitial H that plays a significant role in diffusion. We have determined a low-energy pathway for exchange which involves an intermediate, metastable -=SiH 2 complex with both H atoms strongly bound to the Si atom. The energy barrier for the exchange is less than 0.2 eV, consistent with observations of hydrogen- deuterium exchange in a-Si:H(D) films. We also discuss potential implications of the =SiH2 complex for metastability and defect generation. On the issue of stability of Si-H bonds, finally, we discuss the dissociation path and the connection to vibrational properties. We then show how these insights into the microscopic mechanisms immediately explain the enhanced stability of Si-D bonds. 275 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
METHODS We have performed comprehensive and systematic calculations for hydrogen interactions with silicon using a state-of-the-art first-principles approach based on density-functional theory in the local-density approximation [1]. We employ a plane-wave basis set and a supercell geometry, with ab initio pseudopotentials for the semiconductor host atoms [2, 3]. Relaxation of host atoms was always included, and 32-atom supercells were typically used. This approach has produced reliable results for bulk properties of many materials, as well as properties of surfaces, interfaces, impurities, and defects. More details about the application of the method to the study of hydrogen can be found in Refs. [4], [5], and [6]. We estimate the uncertainty on the energies quoted here to be ±0.1 eV. HYDROGEN MOLECULES It has been known for some time that H 2 molecules are one of the more stable forms of hydrogen in many semiconductors. This knowledge was based on computational studies (see, e.g., Ref. [4, 5, 7]) as well as on interpretation of experimental data. Direct observation of 112 molecules proved very challenging, however, because of sensitivity problems in techniques such as NMR (nuclear magnetic resonance) and vibrational spectroscopy. Recently, however, great progress has been made in this area. A thorough understanding of the incorporation of H2 i
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