Atomic Layer Deposition of Al2O3 and SiO2 for Passivation of Si Surfaces

Minimizing the recombination losses of charge carriers at Si and other semiconductor surfaces is an important topic for many electronic and photonic devices. It is a particularly important issue in the field of crystalline Si photovoltaics as recombination losses cannot be tolerated for efficient solar cells. Surface recombination can be reduced by passivating the surface by dielectric films that lower the (impact of the) density of surface defect states. Thermally grown SiO2 films have traditionally provided the best surface passivation on a variety of Si surfaces with a wide range of doping levels. However the high temperatures and the long oxidation times required have prevented their widespread use in industrial solar cells. State-of-the-art passivation of such solar cells, in particular the n-type emitter at the front-side of p-type cells, is currently provided by hydrogenated amorphous silicon nitride (a-SiNx:H), a thin film material generally deposited by plasma-enhanced CVD (PECVD). Advances in solar cell technology require however the introduction of new solar cell passivation schemes. The development and study of surface passivation films is therefore currently a very active field of research. In this contribution recent results will be presented which demonstrate that excellent surface passivation (on par or superior to thermal SiO2) can be obtained by Al2O3 prepared by atomic layer deposition (ALD). These films can be ultrathin when employing plasma ALD (down to 5 nm) with Al(CH3)3 and O2 plasma or thermal ALD (down to 10 nm) with Al(CH3)3 and H2O. The key distinguishing property of Al2O3 is the fact that the films can contain a very high density of negative fixed charges (~5×10 cm ) at the interfacial region (with typically 1-2 nm interfacial SiOx) between the Al2O3 and the Si [1]. These fixed charges lead to a surface space region lowering the electron density at the surface causing “field-effect” passivation. Together with a significant reduction of the defect state density (<10 eV cm, “chemical” passivation) Al2O3 leads to ultralow recombination velocities on n-, pand p-type Si surfaces [2]. The merits of ALD are the fact that it yields extremely uniform films, its precise thickness control, and the possibility of processing at low substrate temperatures (<400 oC) [3]. Another benefit of ALD that can be of importance for solar cells is the fact that it leads to very conformal films on demanding 3-D topologies, including very rough and non-planar surfaces for light trapping and wrap-through technologies, respectively. To extend the applicability of ALD for crystalline Si photovoltaics, also for applications that not benefit from the field-effect passivation by negative fixed charges, an ALD process has been developed for SiO2 employing SiH2(N(C2H5)2)2 (Air Liquide SAM.24) and O2 plasma. The ALD process with this easy-to-apply precursor (high vapor pressure, Fig. 1) leads to high quality SiO2 films over a wide temperature range (Fig. 2). The synthesis and passivation quality of this ALD SiO2 will be addressed (Fig. 3) and possible functionalities in surface passivation schemes will be discussed. Figure 1: Relationship between pressure and temperature of TMA [Al(CH3)3] and SAM.24 [SiH2(N(C2H5)2)2] precursors.