Atomistic Simulations of the Silicon Surface Structure at the Interface of Silver Thick Film Contacts on n-Type Silicon
- PDF / 279,508 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 21 Downloads / 216 Views
Atomistic Simulations of the Silicon Surface Structure at the Interface of Silver Thick Film Contacts on n-Type Silicon Stefan Kontermann1, Alexander Ruf2 and Ralf Preu3 1 Fraunhofer Heinrich Hertz Institute, Am Stollen 19, 38640 Goslar, Germany 2 Fraunhofer Institute for Physical Measurement Techniques, Heidenhofstr. 8, 79110 Freiburg, Germany 3 Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany ABSTRACT Nanoscale silver crystals at the interface of silver thick film contacts on n-type silicon carry the current across the contact and therefore control the contact resistance, which is a main performance limiting parameter for semiconductor devices. The silver crystals are located in pits at the silicon surface. The shape of the pits is different on Si-(111) and Si-(100). During contact formation, these pits form before the silver crystals. Hence they determine the crystal size and shape. Consequently, the pits with the crystals influence the contact resistance. We investigate these pits experimentally by scanning electron microscopy. We are the first to simulate the mechanism of pit formation at a contact interface by considering a model that is based on the removal probability of silicon surface atoms. This model leads to good agreement between experimental and simulated data. It enables the prediction of pit formation for arbitrary process parameters like temperature and duration for silver thick film contact formation on silicon. INTRODUCTION Metal-semiconductor contacts have been studied for many decades [1,2]. They found extensive application on integrated circuits, silicon solar cells, and other semiconductor devices. The most common contact types are evaporated and thick film contacts. The specific contact resistance of an evaporated metal contact on silicon is two orders of magnitude smaller compared to a thick film contact [3]. However, nearly 90% of today’s manufactured silicon solar cells are metallized with silver thick film contacts [4], because this technology allows for high throughput rates, requires only a limited number of process steps, and is inexpensive compared to other metallization techniques [5]. Reducing the specific contact resistance of silver thick film contacts is significantly important to further increase solar cell and other semiconductor device power output, what can be achieved by a better knowledge of the formation and function of these contacts. Within the last few years, several studies improved the understanding of silver thick film contacts [6-9]. The mechanism of the contact formation process was proposed by Schubert in [10]. Schubert describes the chemical reactions of the substances that are present at the contact interface: silicon from the substrate, silver and lead oxide from the metallization paste. Lead oxide is added to the metallization paste, because lead acts as a catalyst during the silicon silver interaction and, in the case of silicon solar cells, assists to corrosively open the silicon nitride antireflection coating, which i
Data Loading...
