Philosophical Magazine, 21 February 2004 Vol. 84, No. 6, 595–609 Hydrogen-plasma etching of hydrogenated amorphous silicon: a study by a combination of spectroscopic ellipsometry and trap-limited diffusion model F. Kai« lyz, A. Fontcuberta i Morralz, A. Hadjadjy} , P. Roca i Cabarrocasz and A. Beorchiay y Dynamique des Transferts aux Interfaces (Unite´ Mixte de Recherche associe´ e au CNRS 6107), Faculte´ des Sciences, Moulin de la Housse BP 1039, 51687 Reims Cedex 2, France z Laboratoire de Physique des Interfaces et des Couches Minces (Unite´ Mixte de Recherche associe´ e au CNRS 7647), Ecole Polytechnique, 91128 Palaiseau Cedex, France [Received 23 July 2003 and accepted in revised form 9 October 2003] Abstract The kinetics of etching hydrogenated amorphous silicon by a hydrogen plasma has been studied by in-situ spectroscopic ellipsometry measurements. The formation of a hydrogen-rich sublayer is clearly emphasized. Its thickness increases from 7 to 27 nm when the temperature during the hydrogen-plasma treatment is raised from 100 to 250  C. This effect is interpreted by solving the differential equation for trap-limited hydrogen diffusion through a mobile surface. By assigning the thickness of this sublayer to the mean diffusion distance of hydrogen we determined values of the effective diffusion coefficient of hydrogen higher than 10 14 cm 2 s 1 with an activation energy of 0.22 eV. The density of hydrogen traps is found to decrease from 7.3  10 18 to 4.5  10 17 cm 3 as the temperature of the hydrogen treatment increases from 100 to 250  C with an activation energy of 0.43 eV. This effect is interpreted by a thermal equilibrium involving hydrogen transitions between shallow states and hydrogen-trapping sites. } 1. Introduction Hydrogen plays a crucial and antagonist role in hydrogenated amorphous silicon (a-Si : H). Its ability to move into, out of and within a-Si : H leads to both beneficial and undesirable properties. On the one hand it bonds to weak and broken silicon bonds and thus reduces the defect density to a level compatible with doping and useful electronic devices. On the other hand it is linked to metastable defect for- mation that limits the applications of a-Si : H (Staebler and Wronski 1977, Dersch et al. 1980, Sladek et al. 1998). In addition, all models describing the growth of hydrogenated microcrystalline silicon (mc-Si : H) by plasma-enhanced chemical vapour deposition involve either short-range effects of hydrogen or long-range effects, or both of them (Matsuda 1983, Tsai et al. 1989, Layadi et al. 1995, Nakamura et al. 1995). A considerable number of studies have been devoted to Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online # 2004 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/14786430310001635440 } Author for correspondence: Email: aomar.hadjadj@univ-reims.fr.