ARTICLE Reversible and irreversible effects after oxygen exposure in thick (>1 m) silicon films deposited by VHF-PECVD on glass substrates investigated by dual beam photoconductivity 1 Gökhan Yilmaz, Hamza Cansever, H. Muzaffer Sagban, Mehmet Günes, Vladimir Smirnov, Friedhelm Finger, and Rudi Brüggemann Abstract: Metastability and instability effects due to oxygen exposure in thick intrinsic hydrogenated microcrystalline silicon films deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investi- gated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measure- ments obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with I C RS  0.40. These changes are partially irreversible for samples I C RS  0.80 and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with I C RS  0.40 as well as in pure amorphous silicon. PACS Nos.: 73.61.Jc, 73.63.Bd, 73.50.Pz, 72.80.Ng. Résumé : Nous utilisons la conductivité en obscurité, la photoconductivité stationnaire et des mesures d’absorption sous la bande interdite obtenues de la méthode de photoconductivité a ` deux faisceaux (DBP), afin d’étudier les effets stables et métastables de l’exposition a ` l’oxygène de films de silicium microcristallins hydrogénés épais déposés par plasma a ` haute fréquence augmenté d’un dépôt de vapeur chimique sur un substrat de verre lisse. Nous ne détectons aucun changement de la conductivité en obscurité ni de la photoconductivité, même après une longue exposition a ` l’air a ` température ambiante, aussi bien qu’après exposition a ` l’oxygène où les échantillons sont examinés dans une atmosphère d’oxygène. Cependant, étudier sous vide les échantillons exposés a ` l’oxygène donne une augmentation de la conductivité en obscurité, aussi bien que de la photoconductivité, en même temps qu’une diminution significative de l’absorption sous la bande interdite dans les échantillons avec I C RS  0.40. Ces changements sont partiellement réversibles pour les échantillons avec I C RS  0.80 et majoritairement réversibles pour les matériaux compacts avec une fraction amorphe importante. Nous ne détectons aucun changement méta- stable dans les échantillons de silicium microcristallin avec I C RS  0.40 ni dans le silicium complètement amorphe. [Traduit par la Rédaction] 1. Introduction Single junction and multijunction thin-film silicon solar cells are generally deposited on rough light-scattering substrates with absorber layers of undoped hydrogenated amorphous and micro- crystalline silicon (c-Si:H) layers with thickness about 1 m or higher [1, 2]. Long-term stability of such solar cell structures is mainly determined by the properties of absorber layers as well as interfaces and contacts. Amorphous silicon absorber layers strongly suffer from the metastability called the Staebler– Wronski effect [3]. Solar cells having c-Si:H absorber layers with significant crystalline volume fraction were demonstrated to be immune to the Staebler–Wronski effect [1]. However, such c-Si:H layers were reported to exhibit different types of instability and metastability phenomena under air or oxygen exposure [4]. The former is due to irreversible formation of Si–O bonds on the grain boundaries as identified by infrared spectroscopy. The latter is the reversible adsorption of oxygen, which strongly decreases electri- cal conductivity by several orders of magnitude and increase the electron spin resonance spin density [4]. Two decades later, it was shown that two different metastable changes in dark conductivity exist in microcrystalline silicon, where  Dark generally decreases in highly crystalline materials (called type I) and it increases in more compact materials (called type II) with significant amor- phous fraction as samples are exposed to air or oxygen [5]. Corre- sponding electron spin resonance investigation carried out using powder samples indicated irreversible changes in spin density in highly crystalline materials with I C RS  0.70 after oxygen treatment at 80 °C. However, no significant change in electron spin reso- nance spins was detected in compact samples with I C RS  0.50 [5, 6]. Investigation of metastability and instability effects using photo- conductivity methods like steady-state photoconductivity, steady- state photocarrier grating method, transient photoconductivity, Received 25 October 2013. Accepted 22 November 2013. G. Yilmaz, H. Cansever, H.M. Sagban, and M. Günes. Mugla Sitki Koçman University, Faculty of Sciences, Physics Department, Kötekli Yerleskesi, 48000 Mug ˘ la, Turkey. V. Smirnov and F. Finger. Forschungszentrum Jülich, IEK-5 Photovoltaik 52425 Jülich, Germany. R. Brüggemann. Institut für Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany. Corresponding author: Mehmet Günes (e-mail: mehmet.gunes@mu.edu.tr). 1 This paper was presented at the 25th International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS25). 778 Can. J. Phys. 92: 778–782 (2014) dx.doi.org/10.1139/cjp-2013-0638 Published at www.nrcresearchpress.com/cjp on 29 November 2013. Can. J. Phys. Downloaded from www.nrcresearchpress.com by 88.244.24.249 on 07/11/14 For personal use only.