Nanostructured silicon films produced by PECVD

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Nanostructured silicon films produced by PECVD R. Martins, H. Águas, V. Silva, I. Ferreira, A. Cabrita and E. Fortunato CENIMAT, DCM- FCT/UNL and CEMOP/UNINOVA, Quinta da Torre, 2825-114 Monte de Caparica, Portugal ABSTRACT This paper presents the process conditions that lead to the production of nanostructured silicon films grown by plasma enhanced chemical vapour deposition close to the so-called gamma regime (powder formation), highly dense and with low density of bulk states. Thus, the powder management is one important issue to be addressed in this paper. As a general rule we observed that high quality films (low density of states and high µτ products) are obtained when films are grown under low ion bombardment at high hydrogen dilution and deposition pressure conditions, to allow the proper surface passivation and surface activation. INTRODUCTION Amorphous silicon films (a-Si:H) produced by plasma enhanced chemical vapour deposition (PECVD) have stimulated great interest in the optoelectronic industry. This fact led to numerous studies to understand the formation process by plasma in order to obtain films with optimised optoelectronic properties in applications such as solar cells, photocopiers, flat panel displays and thin film transistors (TFT) [1]. To improve the transport and stability properties of the films, plasma conditions have been widely explored. Frequently, the formation of powder-particles has been observed in the plasma gas phase and considered as a source of contamination [2]. Thus, since the early 90’s, many investigations have been done to understand the silicon powder formation and its effects on the plasma properties [3,4]. Recently, silicon films deposited under conditions close to those for powder-particle formation have revealed interesting new optoelectronic properties, in particular better transport and stability properties as compared to aSi:H [5-7]. This new class of silicon thin films has been fabricated using a wide range of plasma conditions. These films are nanostructured, some of them containing ordered silicon crystals with nanometer sizes (1-2 nm) [6]. Therefore, proper powder management has to be considered. Oneway to do so it is through the r.f. power (P), the main carrier gas flow (F), gas dilution, grid bias Vg (voltage applied to a grid enclosing the rf electrode) and gas temperature Tg. Recently it has been proposed to express the role of those parameters through the momentum transfer of the energy coupled to the discharge process via cascade collisions induced on the gas species, per range of pressure (p) used [P/(Fp)] [8]. This phenomenon is similar to what was observed in plasma processes of polymers by Bell and other authors [9,10]. Thus, at low P/(Fp) the cascade collision process favours the formation of weakly bonded polymeric chains [9] while for high P/(Fp), the growth mechanism changes from weakly bonded polymeric chains to tightly bonded units. As the process takes place close to the γ-regime (region where powders are formed), the plasma resistance (R) is clos