Depth profile analysis of solar cells by Secondary Neutral Mass Spectrometry using conducting mesh R. Lovics a , A. Csik a, * , V. Takáts a , J. Hakl a , K. Vad a , G.A. Langer b a Institute of Nuclear Research of the Hungarian Academy of Sciences, H-4001 Debrecen, P. O. Box 51, Hungary b University of Debrecen, Department of Solid State Physics, H-4010 Debrecen, P.O. Box 2, Hungary abstract Depth profile analysis of solar cells was performed by Secondary Neutral Mass Spectrometry (SNMS), which is a suitable technique for quantitative analysis of the composition of layered structures. However, in the case of insulating samples or samples prepared on non-conductive substrates (e.g. microslide, oxidized silicon wafer) the charge accumulation on the sample surface due to ion beam bombardment can cause a serious problem by destroying the resolution of depth profile. The high frequency (HF) mode of electron-gas SNMS seems to be a good solution for this problem. Another method to prevent the charge accumulation on a sample surface can be a conducting mesh (e.g. copper, stainless steel) placed on the surface. Using one of the two methods mentioned above can help us to get rid of the charging effect, i.e. to neutralize the surface charge during measurements. But in the case of solar cell analysis these two methods should be applied simultaneously during depth profiling. The experimental results performed on p-i-n:Si (p-type/intrinsic/n-type) diodes have proved that SNMS measurement in HF operation mode combined with a mesh is very efficient in the determination of doping levels of phos- phorus and boron with good depth resolution, even in the case of 500e600 nm thick samples. 1. Introduction In the last few decades continuously growing energy consumption has provided solid support for the development of renewable energy technologies. Among these technologies the most promising one is solar energy conversion. The silicon thin film solar cell is a preferable choice for the large-scale production of low-cost solar modules for numerous reasons:(i) an abundance of cheap raw material; (ii) the non-toxic component of the tech- nology; (iii) shorter energy payback time; and (iv) low temperature technology [1]. In low-power applications, amorphous silicon (a-Si) based systems are typically used and the commercially available modules provide relatively low stable efficiency (5e7%). From the early 1990s onwards, a new material, the microcrystalline silicon (m-Si) system appeared in laboratory scale research. This material can be deposited by a technology similar to the amorphous one, but offers major advantages: (i) 1.1 eV bandgap (1.7 eV of the a-Si) allows the utilization of the near infrared spectral range of the incident solar light; and (ii) a lower degradation rate, i.e. higher stable efficiency [2]. A new and promising way for photovoltaic application of m-Si is the so-called “micromorph tandem” structure, which is a serial combination of an amorphous and a microcrys- talline cell [3,4]. The development and implementation of this technology into an industrial manufacturing line may result in the reduction of specific processing costs and increased efficiency. On the other hand, not only production, but also analysis of the produced layers is important in order to increase the efficiency of energy conversion. One of the most effective methods for characterizing the layered structure of a solar cell is the depth profile analysis. Secondary Neutral Mass Spectrometry (SNMS) has proved to be an excellent technique for the quantitative analysis of the composi- tion of layered structures, especially when high depth resolution and sensitivity is required [5,6]. In the HF operation mode, when a square-wave type high frequency voltage is applied to the sample instead of a constant dc voltage [7,8], the SNMS in most cases is a suitable technique for the analysis of electrically insu- lating samples. According to the phase of the driving potential, the ion bombardment process is periodically interrupted at low voltage parts of the square-wave. During interruption times the electrons of the plasma reach the specimen and this process makes possible the neutralization of the positive charge * Corresponding author. Fax: þ36 52 416181. E-mail address: csik@atomki.hu (A. Csik). doi:10.1016/j.vacuum.2011.07.031 Vacuum 86 (2012) 721e723