Si3N4-Nanolayers for Metal-Insulator-Silicon Solar Cells
V. Zakhvalinskii1, E. Piliuk1, I. Goncharov1, V. Rodriges1, A.Simashkevich2, D.Sherban2, L.Bruc2, N.Curmei2, M.Rusu3,4
1Belgorod National Research University, 85, Pobedy St., 308015 Belgorod, Russia.
2Institute of Applied Physics, 5 Academiei str., Chisinau MD- 2028Republic of Moldova
3Moldova State University, 60 A. Mateevici str., MD-2009 Chisinau, Republic of Moldova
4Institut Heterogene Materialsysteme, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
Corresponding author e-mail:
ABSTRACT: Si3N4-nanolayers were prepared by non-reactive magnetron sputtering in an Ar atmosphere. A previously synthesized silicon nitride was used as a solid-state target. The deposition was carried out on a cold substrate of p-Si (100) with a resistivity of 2 Ohm×cm. The film thicknesses were up to 20nm with the height of the structural units of about 1-2 nm. Vibrations at frequencies of 288, 307 and 347 cm-1 in the spectrum of Raman scattering are detected, which correspond to the cubic modification of silicon nitride, the shape of the spectrum is characteristic for the nanocrystalline state. A heterostructure was obtained by the deposition of a Si3N4 nanolayer on the surface of pSi wafer and a p-Si/n- Si3N4 nanolayer photovoltaic cell was fabricated. The barrier height at the Si/Si3N4 interface is 0.9-1.0 eV. The investigation of the electrical and photoelectric properties of such cells shows that a MIS/IL solar cell is formed. The spectral dependence of the Si/Si3N4 solar cells photo sensitivity entirely corresponds to the respective characteristic of the Si solar cell. Load I-V characteristics of the elaborated photovoltaic devices demonstrate conversion efficiencies of 7.41%.
Keywords: Magnetron Sputtering, Characterisation, Solar Cell, Nanolayers, Si3N4, Silicon.
1 INTRODUCTION
Crystalline Si is still the mostly used material for the fabrication of solar cells (SCs). However, the efficiency of silicon SCs has almost reached the theoretical limit. Therefore, the efforts of the scientific community are focused on the elaboration of new types of low-cost photovoltaic (PV) devices. The cost reduction is achieved by simplifying the fabrication technology and reducing the material consumption by using thinner Si wafers. In addition, different nanolayers, e.g. ITO, SiC, Si3N4 are used for the preparation of Si based SC heterojunctions. Such devices are usually based on metal-insulator-Silicon (MIS) surface barrier structures with an inversion layer (IL) located in silicon near the heterojunction interface [1, 2].
As Si3N4 is one of the key materials in microelectronics. Silicon carbide thin films become also of particular interest for SC manufacturing. For the first time, plasma-enhanced chemical vapor deposited silicon nitride was introduced into PV for MIS/IL SC fabrication [3]. Further investigations showed that very low surface recombination velocities can be achieved using silicon carbide films in SC fabrication [4-6], at the same time using these films as AR coating [7]. Silicon nitride films are mainly prepared by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) [8-11], or electron cyclotron resonance [12]. Even though CVD is widely used for obtaining those films, their main disadvantages are the incorporation of H2 in the films and high substrate temperatures [13], which deteriorates the properties of Si3N4 [14]. The deposition of Si3N4 by reactive magnetron sputtering at a low substrate temperature can provide films with extremely low hydrogen content [15, 16]. The high-frequency non-reactive magnetron sputtering (HFNRMS) [17, 18] is very promising, because is a non-toxic and a low material consumption deposition method. This method ensures an accurate control of the film thickness and composition. Hence, the aim of this communication is the demonstration of a possibility to fabricate MIS/IL SCs by a simple and low-cost HFNRMS technology using silicon nitride nanolayers.
2 PREPARATION AND CHARACTERIZATION OF SI3N4 NANOLAYERS
Thin films of Si3N4 were obtained by the HFNRMS method using the Ukrrospribor VN-2000 setup. A previously synthesized silicon nitride was used as a solid-state target. The deposition was carried out on a cold substrate of p-Si (100) with a resistivity of 2 Ohm×cm. The layer of silicon oxide was removed from the Si substrate by chemical etching in HF before the Si3N4 film deposition. Silicon carbide nanolayers with the thickness up to 20 nm were obtained. The electron diffraction investigations were carried out on thin foils of Si3N4 nanofilms in a high resolution (2 Å) transmission electron microscope (TEM) JEOL JEM-2100. This study demonstrates that Si3N4 films, deposited on the Si surface by HFNRMS, are a mixture of microcrystalline and amorphous states. Diffusion rings around the central reflex are the evidence that the film material is predominantly of amorphous character, while not-well defined concentric rings denote the presence of the second phase of a nano –or microcrystalline state. Measurements of the thickness and surface morphology of silicon nitride films were performed using a scanning probe microscope (SPM) in the mode atomic force microscopy (AFM) (NTEGRA Aura, NT-MDT) in a controlled atmosphere of a partial vacuum (See Figs. 1-3 and Table I).
Figure 1: Left panel: AFM image of a Si3N4 thin film on Si (100). Right panel: Si3N4 layer profile at the film edge
Fig.1 provides the estimation of the step height at the edge of Si3N4 film, the defined film thickness being 20 nm. The 3D image of the silicon carbide nanofilm morphology is shown in Fig.2.
Figure 2: 3D image of surface morphology of a Si3N4 thin film obtained by AFM
2D image of the film in Fig. 2 is shown in Fig. 3(a). Fig. 3(b) illustrates the processed image from Fig. 3(b).
a
b
Figure 3: The morphology of Si3N4 nanofilm surface.
2D AFM images: (a) as-recorded; (b) processed
The results of the AFM studies on the analysis of the sizes of the objects on the film surface are consistent with the results of transmission electron microscopy (in terms of observations of large clusters, e.g. of the order of tens of nanometers, including texture objects of about 1 nm.) Mathematical processing of the AFM images was performed using the software package «Image Analysis P9» (NT-MDT). The respective results of determined particle sizes are presented in Table I below.
Table I: Determined particle sizes of Si3N4 nanolayers
The results of the image processing / Average / Standard deviationArea (nm2) / 1925.1 / 1204.7
Average Size (nm) / 41.3 / 14.9
Length (nm) / 58.0 / 17.8
Mean Width (nm) / 29.7 / 12.5
Aspect Ratio / 2.1 / 0.7
Volume (nm3) / 3451.2 / 3152.9
Z Range (nm) / 2.7 / 1.4
Max Z (nm) / 4.3 / 1.4
Min Z (nm) / 1.5 / 0.4
Mean Z (nm) / 3.0 / 0.7
Local Mean Z (nm) / 1.5 / 0.7
Perimeter (nm) / 171.6 / 64.2
Diameter (nm) / 46.6 / 16.8
The investigation by TEM of the cross-section of the Si3N4/Si (100) structure shows small objects with the dimension of the order of 1 nm, which build a texture in the form of parallel lines perpendicular to the Si3N4/Si interface. Behind this texture, clusters with dimensions up to 10 nm have been observed.
The compositions of deposited films were characterized by Raman Spectroscopy (RS) techniques using co-focal nanometric resolution Omega Scope AIST-NT Raman microscope with Ar+ laser excitation at 532 nm.
The position of the maximum in the spectrum of Raman scattering (Fig. 4) corresponds to the compound Si3N4, and the shape of the spectrum is characteristic for the nanocrystalline state. Besides the lines assigned to Si atoms oscillations - 528 cm-1 and its harmonics in the vicinity of 950 cm-1, vibrations at frequencies of 288, 307 and 347 cm-1 are detected, which according to Ref. 19 correspond to the cubic modification of silicon nitride.
Figure 4: Raman spectrum of Si3N4 nano films
3 PREPARATION AND CHARACTERIZATION OF p-Si/Si3N4 NANOFILM SCs
Photovoltaic cells consisting of a substrate of p-type (100) oriented Si crystal covered by an amorphous and nanocrystalline mixture of Si3N4 layer were prepared. The Si substrate was specially treated with chemical etchant before the silicon nitride layer deposition. Best results were achieved with Si3N4 layers with the thickness up to 20 nm. An Ag grid was deposited onto Si3N4 thin film to form the front electrode, while a continuous Cu layer was deposited on the opposite side of the Si wafer to form the rear electrode.
The dark I-V characteristic of the elaborated p-Si/n-Si3N4 SCs is presented in Fig.5.
Figure 5: Dark I-V characteristic of a p-Si/n-Si3N4 SC
The devices were studied by performing dark I–V measurements and investigating spectral dependences of the SCs photo sensitivity as well as by measuring illuminated I-V load characteristics under AM1.5 standard conditions (1000 W/m2, 25°C) with an ST-1000 solar simulator. The barrier height at the Si/Si3N4 interface estimated from dark I-V measurements in the temperature range 300 – 450 K varied between 0.9 eV and 1.0 eV. These values are much higher than the half of the Si band gap. Therefore we conclude that a MIS/IL type SC is obtained and that the entire space charge region, where the light absorption takes place and charge carriers are generated and separated, is located in Si. This fact is in addition confirmed by the spectral dependence of the Si/Si3N4 photo sensitivity (see Fig. 6), which entirely corresponds to the respective characteristic of Si SCs.
Figure 6: Spectral dependence of the pSi/Si3N4 solar cell photo sensitivity
In Fig. 7 an I-V load characteristic of a Si/Si3N4 SC is presented.
Figure 7: Load I-V characteristic of a p-Si/n- Si3N4 SC
From this I-V curve the solar cell PV parameters were determined: the short-circuit current density value is 23.2 mA/cm2, the open circuit voltage is 0.538 V, the fill factor is 59.6% and the efficiency is 7.41%.
4 CONCLUSIONS
· Silicon nitride thin films consisting of a mixture of amorphous and microcrystalline phases were prepared by HFNRMS. Depositions were carried out on a cold substrate of p-Si (100) with a resistivity of 2 Ohm×cm. The film thicknesses were up to 20nm with the height of the structural units of about 1-2 nm.
· A heterostructure was obtained by HFNRMS of a Si3N4 nanolayer on the surface of p-type Si wafer. Based on this heterostructure, SCs were fabricated.
· The investigation of the electric and photoelectric properties of the p-Si/n- Si3N4 nanolayer SCs shows that a MIS/IL SC is formed. The barrier height at the Si/Si3N4 interface is 0.9-1.0 eV.
· The spectral dependence of the Si/Si3N4 SC photo sensitivity entirely corresponds to the respective characteristic of the Si solar cell. Load I-V characteristics of the elaborated SCs demonstrate conversion efficiencies of 7.41%.
Acknowledgements Research was accomplished with the support of Ministry of Education and Science of the Russian Federation: reference number 2014/420-367. Thank you for contribution prof. Kuzmenko A.P. from Regional center nanotehology Southwest State University for carrying out an experiment on Raman spectroscopy.
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