Hydrogen release in SiO 2 : Source sites and release mechanisms R.M. Van Ginhoven a, * , H.P. Hjalmarson a , A.H. Edwards b , B.R. Tuttle c a Sandia National Labs, MS 1110, P.O. Box 5800, Albuquerque, NM 87185, United States b Air Force Research Labs, Albuquerque, NM 87117, United States c Penn State Erie, Erie, PA 16563, United States Available online 13 June 2006 Abstract We investigate molecular scale mechanisms for radiation-induced release of hydrogen from precursor sites using density functional theory applied to a fully periodic model of SiO 2 . We focus on proton release from H-decorated oxygen vacancies in the bulk oxide. After hole-capture at the vacancy, a proton can hop to an energetically favorable bound state at a neighboring oxygen atom. In a-quartz, this release mechanism has an activation energy of about 1.2 eV. In amorphous silica, this hop has a range of low barriers, from 0.1 to 0.5 eV. Furthermore, another proton release mechanism involves cracking of H 2 molecules by a reaction with an isolated, positively charged Si- dangling bond. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.82.d; 61.43.Fs; 61.82.Ms Keywords: Silicon dioxide; DFT; Radiation effects; Hydrogen; Amorphous silica 1. Introduction The performance of Si/SiO 2 -based transistors is known to be affected by ionizing radiation through the production of interface traps and the buildup of trapped charge in the oxide [1]. The creation of interface traps has been shown to be associated with the transport of hydrogen to the inter- face region, which leads to depassivation of H-terminated Si-dangling bonds at the silicon surface [2,3]. The resulting defect states, one of which is the well known P b center [3], act as recombination centers for electrons and holes and degrade the performance of the transistor. The silicon dioxide portion of Si/SiO 2 transistors con- tains hydrogen from initial manufacturing processes [4]. This hydrogen may be present in several forms, primarily as O–H and Si–H groups, passivated Si–H sites at the Si interface and H 2 and H 2 O interstitial molecules. Ionizing radiation stimulates the release of hydrogen from source sites in the oxide. This hydrogen is then able to migrate to the interface, where it can react to form interface traps. Pre- vious work [5] has generally assumed that the irradiation process produces primarily protons, however, experimental evidence shows the presence of neutral atomic hydrogen [6]. Measurement of interface trap buildup under negative gate bias suggests the presence of a neutral species as well as pro- tons [7]. It has been shown that neutral hydrogen in SiO 2 spontaneously ionizes to form protons as it approaches the interface with Si [8]. Therefore, the hydrogen arriving at the interface from the oxide layer may originate from either proton (S A H) or neutral hydrogen (S B H) sources. In this work, we describe density functional theory (DFT) calculations for the source sites of hydrogen. We also consider release mechanisms. We identify the oxygen vacancy or oxygen-deficient network site as a likely source site in the oxide, both for direct release of protons and cracking of molecular hydrogen. 2. Model The hydrogen may come from several types of sites in the oxide. For example, hole trapping may lead to the 0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.123 * Corresponding author. Tel.: +1 505 284 8829; fax: +1 505 845 7442. E-mail address: rmvangi@sandia.gov (R.M. Van Ginhoven). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 250 (2006) 274–278 NIM B Beam Interactions with Materials & Atoms