Low temperature SiNx:H films deposited by inductively coupled plasma for solar cell applications
Highlights
► Low-frequency inductively coupled SiH4 + N2 + H2 plasma is innovatively employed to deposit silicon nitride thin film. ► The deposition temperature is very low (100 °C) and the deposition rate is competitive with that of PECVD. ► The surface recombination velocity in n-type Si (2–3 Ω cm) is reduced to 36 cm/s without post-deposition annealing. ► The chemical compositions and refractive index of SiN can be modulated by altering the gas flow rate ratio of N2/SiH4.
Introduction
The reduction of surface light loss and minimization of photocarrier recombination are two essential issues accompanying with Si-based solar cells with high energy conversion efficiency. Indeed, thermally grownn SiO2 was used to fullfill these two requirements in despite of the extremlly high temperature (∼1000 °C) process [1]. The most successful alternative to the thermally grown SiO2 is hydrogenated silicon nitride (SiNx:H) desposited at a low temperature (∼300 °C) process. Indeed, plasma-enhanced-chemical-vapour-deposition (PECVD) grown SiNx:H has been widely used to serve as the passivation and antireflecction layer in industrial massive mono- and multi-crystalline Si solar cell production. The appropriate refractive index of about 2 enables SiNx:H to be a good antireflection candidate for silicon surface. Meanwhile, the inner hydrorgen atom from hydrogen-containing plasma, and the positive fixed charge mainly from the so-called K centers are responsible for the chemical and field passivation of Si, respectively [2]. There are many reports on SiNx:H grown by varying PECVD methods including the direct-plasma [3], remote-plasma [4], and high frequency [5] as well as low frequency plasma [6]. However, available literatures about inductively coupled plasma growth of SiNx:H are very few [7], [8], not to mention the related applications in solar cells. In most of the previous works, the source gas of nitrogen is NH3 due to the low ionization energy for NH3 and high incorporation of hydrogen in the deposited films. When NH3 is substituted by inexpensive nitrogen gas in PECVD, it was found that the hydrogen content incorporated in the film significantly decreases [3]. The resulting low hydrogen content in SiNx:H is undesirable to the silicion surface passivation.
In our earlier works, we reported the development of a low frequency (460 kHz) inductively coupled plasma (LFICP) source [9] and its applications in nano-technology fabrication [10], [11] and also, the deposition of intrinsic and doped silicon thin films [12], [13]. It was acknowledged that this LFICP source features various inherent advantages such as a high density (1019/m3), axial/radial uniformity plasma, and independent control of electron number density and the energy of ions impinging on the growing surface. Furthermore, the plasma generation zone can be separated from the deposition zone through the optimization of gas inlet scheme. Two gas dispersal rings are located at different zones in the vacuum chamber: the top one near the quartz window is used to import the diluent gas like H2; the bottom one 10 cm above the substrate stage feeds reactant gases like SiH4 and N2 into the reaction chamber.
In the present work, we report the deposition of SiNx:H thin films by meas of LFICP from the precursor gas of hydrogen diluted mixture of SiH4 and N2 at a very low temperature of 100 °C. The chemical composition of the films is modulated by varying the gas flow rate ratio R = N2/SiH4. It is found that as-deposited SiNx:H films exhibit an excellent passivation effect on p- and n-type silicon due to the high density bonded hydrogen (∼1022/cm3) and/or positive fixed charges (∼1012/cm2). The reflectivity of Si surface with a antireflection layer of SiNx:H is reduced by about 7%. Combination of the antireflection and passivation effects efficiently imroves the photovoltaic performance of the crystalline silicon solar cell.
Section snippets
Experimental details
SiNx:H thin films were deposited on mirror polished Si substrate for Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements. The used substrate for passivation experiments is p- and n-type Czochralski silicon with resistivity of 2–3 Ω cm, thickness of 200 μm, and (1 0 0) orientation. The Si substrates were cleaned using standard RCA method followed by 5% HF acid dip and transported into the deposition chamber through a load-lock chamber. The substrate
Experimental results
Fig. 1 shows a typical surface SEM image (a) and the corresponding cross-sectional SEM image (b) of a SiNx:H thin film with gas ratio of R = 0.24. There is no observable voids and clusters on the surface as shown in Fig. 1(a). The surface, coverd by grains in the size level of several nanometers, is smooth and uniform. In Fig. 1(b), the interface between the film and Si substrate is smooth and clear. We cannot find the conventional columnar growth observed in Si film deposited under similar
Discussions
As mentioned above, N2 can be efficiently dissociated in the LFICP reactor and resulting N can be incorporated into the deposited films with tunable chemical compositions. Lucovsky et al. [22] described four-step growth process in a remote ICP mode: (i) the RF excitation of reactant gases containing N atom; (ii) the transport of the excited species and electrons; (iii) the mixing and gas phase reaction of the excited nitrogen species with neutral silane out of the plasma region; and (iv) a CVD
Conclusions
In summary, amorphous SiNx:H thin films with different chemical compositions were fabricated by using the LFICP method with the precursor gas of hrdrogen diluted silane and N2 mixture. The bond density, refrative index, deposition rate and chemical composition depend on the gas ratio of N2/SiH4. The reflectivity on the alkaline-textured Si surface is reduced by 7% after the deposition of 80 nm SiN81:H layer with refractive index about 2. As-deposited SiNx:H has excellent passivation effect on p-
Acknowledgments
This work was supported by the National Research Foundation and the Agency for Science, Technology and Research for Singapore, and 8th Singapore-China Joint Grant. H.P. Zhou and D.Y. Wei acknowledge the research scholarship provided by the Nanyang Technological University.
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