THERMAL STABILITY INVESTIGATIONS OF PECVD Al 2 O 3 FILMS DISCUSSING A POSSIBILITY OF IMPROVING SURFACE PASSIVATION BY RE-HYDROGENATION AFTER HIGH TEMPERATURE PROCESSES Bishal Kafle 1 , Saskia Kuehnhold 1 , Wolfhard Beyer 2 , Stefan Lindekugel 1 , Pierre Saint-Cast 1 , Marc Hofmann 1 , Jochen Rentsch 1 1 Fraunhofer-Institute for Solar Energy Systems (ISE) 2 IEK-5, Forschungszentrum Juelich GmbH 1 Heidenhofstr. 2, 79110 Freiburg, Germany 2 Wilhelm-Johnen-Str., 52428 Juelich, Germany ABSTRACT: This work investigates the changing passivation behavior of Al 2 O 3 after different thermal treatments based on carrier lifetime (τ eff ), interface defect density (D it ) and fixed charge density (Q tot ) measurements. A concept of diffusing H species into the dehydrogenated Al 2 O 3 films, termed as re-hydrogenation, has also been investigated for the PECVD deposited Al 2 O 3 samples. Use of a-SiN x :H as a capping layer as in Al 2 O 3 /a-SiN x :H stack provides a better thermal stability for a thin PECVD Al 2 O 3 layer. A comparison between single Al 2 O 3 layer and Al 2 O 3 / a-SiN x :H passivation stack after high temperature processes has been also performed in this work. Keywords: Passivation, PECVD, defect density, electrical properties, re-hydrogenation. 1 INTRODUCTION Al 2 O 3 as a surface passivation layer for lowly and highly doped p-type c-Si is extensively researched for inclusion in the solar cell production sequence. In as- deposited state, Al 2 O 3 provides nearly no surface passivation. An increase of minority charge carrier lifetime by 2-3 orders of magnitudes usually occurs after subsequent annealing, due to the activation of the passivation mechanism. This mechanism is partly due to an inherent high density of negative charges (Q tot ) in the layer (field-effect) and partly due to a lowered interface defect density (D it ) after annealing [1-3]. Though the passivation improves during the high temperature processes, the passivation level has been found to significantly degrade after a certain temperature and time period (>800 °C) [4]. This has been mostly attributed to the loss of H species from Al 2 O 3 layer, thus leaving unsaturated dangling bonds acting as active recombination centers [1]. The current work investigates the changes in properties of Al 2 O 3 layer after thermal processing at different temperatures. Effusion measurements for PECVD Al 2 O 3 are used to investigate the role of H species in defining passivation quality of the layer at high temperatures. With an increasing temperature, gradual effusion of H species from the Al 2 O 3 layer occurs leaving behind a large number of interface defect states. Changes in effective lifetime (τ eff ), interface defect density (D it ) and fixed charges (Q tot ) are used to explain the changing electrical properties of the layer with temperature. In this work, a new concept of re-hydrogenation is investigated in order to check the possibility of diffusing H species into the Al 2 O 3 /Si interface and revive the deteriorated passivation quality of the de-hydrogenated samples. This investigation provides deep insight about the effect on Al 2 O 3 /Si layer system due to a varying amount of H species in the Al 2 O 3 layer. In the last section, the thermal stability of PECVD Al 2 O 3 /a-SiN x :H stack has been investigated based upon carrier lifetime measurements performed after annealing processes performed at different temperatures. 2 THERMAL INVESTIGATION OF PECVD Al 2 O 3 2.1 Experimental Shiny etched surface float zone (FZ), low resistivity (1 Ωcm) p-type crystalline silicon (c-Si) wafers with a thickness of 250 μm and (100) crystal orientation were used for the investigations. Before the PECVD Al 2 O 3 deposition process, the wafers were cleaned in a hot HNO 3 bath followed by a short HF dip. The final HF dip is applied to remove both the native oxide and the grown oxide (formed as a result of the HNO 3 bath) on top of the Si substrate. An inline industrial PECVD machine (Roth & Rau SiNA) was used to deposit Al 2 O 3 films as well as a-SiN x :H films on the c-Si samples. 2.2 The role of H species in passivation mechanism Hydrogen species are believed to play an important role in defect passivation at the Al 2 O 3 /Si interface. Effusion experiments in cooperation with FZ Juelich were carried out to monitor the H species effusing out of the PECVD Al 2 O 3 sample with an increasing temperature. For that, the sample was heated up inside a high vacuum quartz tube up to high temperatures with a defined heating rate of 20 K/min [6]. The effused gases from the sample were analyzed by a quadrupole mass spectrometer (QMA) to detect different mass-to-charge (m/z) ratios. For QMA used in H effusion measurements, the electrons generated by a heated filament are accelerated by applying a voltage of 50-100 V to cause ionization, and under such conditions, z is typically 1 [5]. Effusion signals for H species were detected in the form of mass-to-charge ratio (m/z) of 18, 17 and 2. Neglecting the presence of deuterium, these m/z values also feature in the cracking pattern of water molecule [6]. Overall effusion intensity of H 2 species (m/z=2) detected was measured by the effusion set up. Comparing the effusion signal of H species for PECVD Al 2 O 3 to that of ALD Al 2 O 3 layers, only small differences have been detected [7]. This also suggests that the thermal effusion mechanism of H species from the Al 2 O 3 layer is virtually independent of these two deposition techniques.