Deposition and characterization of amorphous silicon with embedded nanocrystals and microcrystalline silicon for thin film solar cells

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Highlights

  • Nanostructured silicon thin films were deposited by PECVD.

  • Polymorphous and microcrystalline were obtained varying the pressure and power.

  • Structural and optoelectronics properties were studied.

  • The σdark changed by 5 order of magnitude under illumination, Vd was at 2.5 A/s.

  • The evidence of embedded nanocrystals into the amorphous matrix was investigated.

Abstract

Amorphous silicon thin films with embedded nanocrystals and microcrystalline silicon were deposited by the standard Radio Frequency (RF) Plasma Enhanced Chemical Vapor Deposition (PECVD) technique, from SiH4, H2, Ar gas mixture at substrate temperature of 200 °C. Two series of films were produced varying deposition parameters as chamber pressure and RF power density. The chemical bonding in the films was characterized by Fourier transform infrared spectroscopy, where it was observed a correlation between the hydrogen content and the morphological and electrical properties in the films. Electrical and optical parameters were extracted in both series of films, as room temperature conductivity (σRT), activation energy (Ea), and optical band gap (Eg). As well, structural analysis in the films was performed by Raman spectroscopy and Atomic Force Microscopy (AFM), which gives an indication of the films crystallinity. The photoconductivity changed in a range of 2 and 6 orders of magnitude from dark to AM 1.5 illumination conditions, which is of interest for thin film solar cells applications.

Introduction

Hydrogenated amorphous silicon (a-Si:H) is a mature material in the electronics manufacturing industry, where is used for the development of thin film solar cells, infrared sensors and thin film transistors (TFT), due to the very large absorption coefficient, its compatibility with the standard Si-CMOS technology and the low process temperatures used (<300 °C) when is deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). The above features, allows use different substrates like metal foils, plastics and glasses [1], [2], [3], [4]. However a-Si:H has some disadvantages as the large density of states in the band gap, poor transport properties, and poor stability against radiation.

Recently, it has been found that it is possible to form nanocrystals in the amorphous silicon matrix, improving its electrical properties, as electron mobility and stability against radiation, such material is usually referred as polymorphous silicon (pm-Si:H) or nanocrystalline silicon (nc-Si:H), depending on the density of nanocrystals [1], [2], [3], [5], [6], [7].

Basically pm-Si:H is composed of a silicon amorphous matrix with embedded nanocrystals (with diameter in the range of 2–5 nm). The presence of such nanocrystals results on a lower density of states in the band gap and much better transport properties than a-Si:H [3], [5], which makes pm-Si:H an excellent candidate for more stable thin film solar cells.

It has ben found that by modifying the deposition conditions as deposition pressure and RF power density is possible to produce hydrogenated microcrystalline silicon (μc-Si:H) films, in which the size of nano/micro crystals are in the range of hundreds of nanometers [5], [6], [7], [8], [9]. The growth of nc-Si:H/μc-Si:H thin films by PECVD requires high hydrogen (H2) dilution of SiH4 [8]; H2 dilution has been widely used for the growth of microcrystalline silicon films and the transition from amorphous to microcrystalline has been reported as a function of the ratio R = (H2/SiH4) [8], [9], [10], [11].

As well, argon (Ar) has been used for the growth of μc-Si:H thin films in order to improve the molecules dissociation and the films crystalline fraction [12]. The effect of the RF power density and the H2 and Ar dilution on the film structural properties has been studied [8], [10], [11], [12], and also the deposition of nanocrystalline silicon thin films at very high frequency VHF-PECVD using SiH4 with a Ar dilution has been reported [1], [7].

For the development of much more stable large area electronic devices, such as solar cells and thin films transistors, the quality and stability of the nanostructured silicon thin films deposited in standard PECVD at 13.56 MHz, is still a challenge that is currently on study [1], [2], [3], [4], [5], [6], [7].

In this work we present the deposition and characterization of pm-Si:H and μc-Si:H thin films using the standard RF PECVD technique, at a substrate temperature of 200 °C. We varied the pressure from 500 m Torr (for a-Si:H deposition) to relative high pressures of about 1.5 Torr for the production of pm-Si:H thin films. As well, we produced μc-Si:H thin films by varying the RF power density of the PECVD reactor. The correlation of the deposition parameters and the structural, electrical and morphological properties are studied, with special attention on the photoconductivity of the films under AM 1.5 illumination conditions, which is of interest for thin film solar cells applications.

Section snippets

Experimental

Two series of nanostructured thin films (pm-Si:H and μc-Si:H) were deposited by PECVD at radio frequency RF = 13.56 MHz, from SiH4 and H2 mixture for the pm-Si:H films, and from SiH4, H2 and Ar mixture for the μc-Si:H films. Table 1 shows the deposition conditions for the pm-Si:H and μc-Si:H thin films, those films were deposited for 30 min, at substrate temperature Ts = 200 °C.

UV–VIS transmission measurements were carried out in the range of 200–900 nm, with a Perkin Elmer Spectrophotometer (Model

Deposition rate

The deposition rate (Vd) of the pm-Si:H and μc-Si:H films was calculated from the average thickness and the deposition time, notice that all the films were deposited for 30 min. Fig. 1(a) shows the variation of deposition rate of the pm-Si:H films as a function of deposition pressure, maintaining constant the other process parameters (see Table 1). In Fig. 1(a), it is seen that deposition rate of the pm-Si:H films, increases from 0.5 Å/s to 2.4 Å/s as the deposition pressure increases. For the

Conclusions

We have deposited and characterized pm-Si:H and μc-Si:H thin films by PECVD at low substrate temperature 200 °C. We have found that the deposition pressure and the RF power density have an important influence on the structural, optical and electrical characteristics of the films.

From AFM characterization we found that large deposition pressure (larger than the used for a-Si:H) results in an increment on the Ra values of the films surface, which is associated with the formation of nanocrystals in

Acknowledgements

M. Moreno acknowledges CONACyT for the support granted through the Project # 117212, R. Ambrosio acknowledges the CONACyT sabbatical Project # 232926.

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