The structural and surface quality of the material employed is one of the most critical issues as regards the large scale application of electronic devices based on hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and a-SiGe:H. Atomic hydrogen migration occurs in the amorphous network. The high temperatures applied during growth of those materials, e. g. by chemical vapor deposition, or reached during device operation enhance the diffusion of H atoms, in particular of those liberated from their bonds to the host atoms as a consequence of annealing. Such enhanced diffusion favors the migration of H atoms towards nanovoids where they very likely form molecular H2 since the reaction 2MeH ? H2 +Me-Me is an exothermic one (Me indicates the host atom: Si or Ge). The accumulation on the wall of voids causes the evolution of hydrogen bubbles and then the formation of blisters. Some efforts have been made to understand the microscopic mechanisms determining the rupture of the MeH bonds and formation of H2 rich voids at the origin of the blisters in order to get rid of them. The objective of this work was to find a way to determine the threshold temperature below which surface blistering does not occur in hydrogenated a-SixGe1-x, 0 <= x <=1. This is achieved by a theoretical model that takes into account both the kinetics of the rupture of the MeH bonds and, in particular, the diffusion of the atomic H. The experimental results suggesting our theoretical approach have been obtained by Secondary Neutral Mass Spectrometry (SNMS), as regards the depth distribution of H (Fig. 1), and by surface light reflectivity measurements, as regards the assessment, as a function of temperature, of the time of the onset of blistering and its activation energy by Arrhenius plots (Fig. 2). The data supplied by the latter plots allow the validation of the theoretical model. The calculated critical temperature for blistering, in fact, is on the same order of magnitude as the experimentally observed one. The experimentally determined Vegard's law-like dependence [1] of the blistering activation energy on the Si concentration in the a-SixGe1-x alloys is interpreted by a simple formula and related 3D-like diagram. [1] M. Serényi, C. Frigeri, R. Schiller, J. Alloys Compd. 763, 471 (2018). Fig.1. SNMS depth profile of hydrogen in annealed a-Si and a-Ge layer after 40 minutes of annealing (a), and after 10 and 40 minutes in a-Si layer (b). Fig. 2. Graphical representation n of blistering onset vs. temperature in a polar coordinate system. The left-hand vertical plane belongs to the Si and the right one to the Ge Arrhenius plot.
Diffusion and reaction kinetics governing surface blistering in RF sputtered hydrogenated a-SixGe1-x (0<= x <=1) thin films
C Frigeri
2019
Abstract
The structural and surface quality of the material employed is one of the most critical issues as regards the large scale application of electronic devices based on hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and a-SiGe:H. Atomic hydrogen migration occurs in the amorphous network. The high temperatures applied during growth of those materials, e. g. by chemical vapor deposition, or reached during device operation enhance the diffusion of H atoms, in particular of those liberated from their bonds to the host atoms as a consequence of annealing. Such enhanced diffusion favors the migration of H atoms towards nanovoids where they very likely form molecular H2 since the reaction 2MeH ? H2 +Me-Me is an exothermic one (Me indicates the host atom: Si or Ge). The accumulation on the wall of voids causes the evolution of hydrogen bubbles and then the formation of blisters. Some efforts have been made to understand the microscopic mechanisms determining the rupture of the MeH bonds and formation of H2 rich voids at the origin of the blisters in order to get rid of them. The objective of this work was to find a way to determine the threshold temperature below which surface blistering does not occur in hydrogenated a-SixGe1-x, 0 <= x <=1. This is achieved by a theoretical model that takes into account both the kinetics of the rupture of the MeH bonds and, in particular, the diffusion of the atomic H. The experimental results suggesting our theoretical approach have been obtained by Secondary Neutral Mass Spectrometry (SNMS), as regards the depth distribution of H (Fig. 1), and by surface light reflectivity measurements, as regards the assessment, as a function of temperature, of the time of the onset of blistering and its activation energy by Arrhenius plots (Fig. 2). The data supplied by the latter plots allow the validation of the theoretical model. The calculated critical temperature for blistering, in fact, is on the same order of magnitude as the experimentally observed one. The experimentally determined Vegard's law-like dependence [1] of the blistering activation energy on the Si concentration in the a-SixGe1-x alloys is interpreted by a simple formula and related 3D-like diagram. [1] M. Serényi, C. Frigeri, R. Schiller, J. Alloys Compd. 763, 471 (2018). Fig.1. SNMS depth profile of hydrogen in annealed a-Si and a-Ge layer after 40 minutes of annealing (a), and after 10 and 40 minutes in a-Si layer (b). Fig. 2. Graphical representation n of blistering onset vs. temperature in a polar coordinate system. The left-hand vertical plane belongs to the Si and the right one to the Ge Arrhenius plot.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
