Deposition of microcrystalline silicon in electron-cyclotron resonance discharge (24 GHz) plasma from silicon tetrafluoride precursor

    Deposition of microcrystalline silicon in electron-cyclotron resonance discharge(24 GHz) plasma from silicon tetrafluoride precursor D.. Mansfeld, .V. Vodopyanov, S.V. Golubev, P.G. Sennikov, L.A. Mochalov, B.. Andreev, Yu N. Drozdov, .N. Drozdov, V.I. Shashkin, P. Bulkin, P. Roca i Cabarrocas PII: DOI: Reference:

S0040-6090(14)00383-6 doi: 10.1016/j.tsf.2014.03.091 TSF 33343

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

8 May 2013 27 March 2014 28 March 2014

Please cite this article as: D.. Mansfeld, .V. Vodopyanov, S.V. Golubev, P.G. Sennikov, L.A. Mochalov, B.. Andreev, Yu N. Drozdov, .N. Drozdov, V.I. Shashkin, P. Bulkin, P. Roca i Cabarrocas, Deposition of microcrystalline silicon in electron-cyclotron resonance discharge(24 GHz) plasma from silicon tetrafluoride precursor, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.03.091

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Deposition of microcrystalline silicon in electron-cyclotron resonance discharge(24 GHz) plasma from silicon tetrafluoride precursor

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D А Mansfeld1, А V Vodopyanov2, S V Golubev1, P G Sennikov1,3, L.A. Mochalov1,3, B А

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Andreev4, Yu N Drozdov4, М N Drozdov4, V I Shashkin4, P Bulkin5 and P Roca i Cabarrocas5

Institute of Applied Physics of RAS, Nizhny Novgorod, Russia

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Lobachevsky State University of Nizhny Novgorod, Russia

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Institute of Chemistry of High-Purity Substances of RAS, Nizhny Novgorod, Russia

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Institute for Physics of Microstructures of RAS, Nizhny Novgorod,Russia

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LPICM, CNRS Ecole Polytechnique, 91128 Palaiseau, France

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E-mail : [email protected]

Abstract. Deposition of Si films is done in a high power 24 GHz gyrotron-based electron-

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cyclotron resonance plasma enhanced chemical vapour deposition setup. The possibility of highrate (2.5 nm/s) deposition of thin silicon films from SiF4 + H2 plasmas in an electron-cyclotron resonance reactor with gyrotron microwave plasma source at the frequency of 24 GHz is demonstrated. The analysis with spectroscopic ellipsometry and Raman scattering shows that films have large crystalline fraction (75%) with an average size of silicon grains of 3 nm. The use of gyrotron with high microwave power opens up the possibility to make depositions from high density plasmas at pressures of tens pascals, which results in high deposition rates (tens nm/s).

1. Introduction

ACCEPTED MANUSCRIPT Microcrystalline silicon is a promising material for various applications, especially in the field of thin film solar cells and thin film transistors. The modern trend in thin film photovoltaics is the use of cells based on micro-(μc-Si:H) or nanocrystalline (nc-Si:H) silicon in

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combination with amorphous silicon (a-Si:H) in tandem (so-called “micromorph”) or triple

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junction solar cells (Kaneka Co., Sharp Co., Unisolar Co. [1]) to achieve high stable efficiencies.

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The top a-Si:H based solar cell absorbs the visible light and leaves the infrared part of the spectrum for the bottom μc-Si:H based solar cell. Record stabilized efficiencies in triple junction

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solar cells have reached 12.5 % at EPFL [2] and 13.4% has been achieved by LG [3]. Semiconductor industry is also investigating the potential of p-doped μc-Si:H, especially in thin-

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film transistor (TFT) production, in order to realize complementary metal–oxide–semiconductor circuitry with TFT transistors. Moreover, silicon nanocrystals can be used as photosensitizers in

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photophysics and photochemistry, biology, in medicine (photodynamic cancer therapy) etc.

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One of the most pressing challenges in reducing the production costs of solar cell is the

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deposition of high quality μc-Si:H films at high rate (up to several nm/s) for intrinsic layer absorber. Numerous efforts are aiming now to achieve such high deposition rates, most notably through utilization of high-density plasmas. In radiofrequency (RF) capacitively coupled plasma

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(CCP) discharges this typically achieved with increase of RF power, but that increases the sheath voltage and above certain value radiation damage from ions renders films highly defective. One of the approaches is to use electron cyclotron resonance (ECR) discharge for plasma enhanced chemical vapour deposition (PECVD) of silicon from gas precursors. The important feature of ECR discharge plasmas is the possibility to deposit layers at very high rate due to their high electron density resulting in high dissociation efficiency. The advantage of ECR discharges in comparison with inductively coupled plasmas RF discharges is in the absence of any dielectric materials in the discharge area, like the absence of quartz chamber walls, required with the use of external coils. The area of ECR discharge is determined by location of the zones of resonant ECR absorption of microwave radiation and can be arranged to be far away from the walls.

ACCEPTED MANUSCRIPT Besides, the ion energy in ECR plasmas is significantly lower than in CCP discharges, for example, and that minimizes sputtering of the walls of the reactor as well as radiation damage on the growing layer. The reactor is placed in the external magnetic field, which also restricts the

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ion motion in the direction towards the walls. So ECR discharges are ideally suited for high-rate

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in RF-CCP systems, remains very challenging problem.

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production of high-purity materials, in particular, silicon thin films, however scale-up, achieved

Deposition of silicon films in ECR plasmas is traditionally done with 2.45 GHz

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microwave plasma in 875 Gauss resonant magnetic field at low pressure (0.1 Pa range). First amorphous silicon films were deposited from SiH4/H2 mixtures by Shing et al. [4] in 1989. Then

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numerous experiments were aimed at the ECR deposition of microcrystalline silicon films from different silane mixtures [5, 6]. In these works the deposition rate of silicon films didn’t exceed 1

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nm/s in the pressure range 0.1-2 Pa. The deposition of microcrystalline silicon films in ECR

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discharges was investigated later [7-13]. It was also reported [14] that the deposition of μс-Si:H

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from SiH4 in matrix distributed ECR plasma reactor can achieve the rate of up to 2.8 nm/s. But solar cell manufacturing demands even higher deposition rates and since critical electron density scales with microwave frequency, we may search for microwave generators for plasma sources

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along that direction. One such higher frequency microwave generator is a gyrotron. Gyrotrons are used for many industrial and high technology heating applications. For example, gyrotrons are used in nuclear fusion research experiments to heat plasmas, and also in manufacturing industry as a rapid heating tool in processing of glass, composites, and ceramics, as well as for annealing processes. In our previous study [15] the technological gyrotron with frequency 24 GHz and power level up to 5 kW was applied for the decomposition of silicon tetrafluoride. That allowed us to perform the deposition of silicon at high microwave power and to demonstrate a density of absorbed power in the plasma as high as 100 W/cm3. The use of microwave radiation of gyrotron with such a high frequency promises to increase significantly the plasma density in a PECVD

ACCEPTED MANUSCRIPT reactor, and as the result, it is expected to increase the rate and efficiency of dissociation reactions. The approach for producing high-purity silicon with isotopic enrichment of

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isotope in plasma of ECR discharge, sustained by microwave radiation of gyrotron with the

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frequency of 24 GHz has been demonstrated recently [16]. Moreover thick microcrystalline

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diamond films were grown on silicon substrates with 60-90 mm in diameter in the PECVD

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reactor based on 10 kW gyrotron operating at a frequency of 30 GHz [17,18]. In this article we present the results of experiments on the growth of thin μс-Si:H layers

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from SiF4+H2 plasma in ECR reactor with gyrotron microwave plasma source at a frequency of 24 GHz. The silicon tetrafluoride was chosen as a precursor because it is neither combustible nor

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explosive (unlike silane), and in the case of low humidity is very stable. SiF4 has the highest mass fraction (27%) of silicon in comparison with other silicon halogenides. The material source

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base of silicon tetrafluoride is essentially unlimited because it is the waste product of mineral

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2. Experimental setup

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processing.

We performed our studies using a gas discharge sustained by the electromagnetic

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radiation of a gyrotron in the magnetic field under electron cyclotron resonance conditions. A sketch of the experimental deposition reactor is shown in figure 1. Continuous-wave radiation (TE11 mode) of a gyrotron with a frequency of 24 GHz

propagates through the plasma-coupling device into the centre of the discharge chamber. The construction of plasma-coupling device provides the transmission of more than 90% of microwave power, also protecting the gyrotron window from plasma fluxes and reflected radiation from plasma. The microwave power can be varied from 0.1 to 5 kW. The details of experimental PECVD system can be found in [15].

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Figure 1. Sketch of the experimental system. The discharge chamber consists of a cylindrical stainless tube (38 mm in diameter with

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a length of 60 cm) with the flattened bottom inside the deposition area. In the central section of the tube, two ports are situated: from one side a quartz window for optical diagnostics and from

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opposite side a port which is used for gas injection and pressure monitoring. The working gas mixture of hydrogen (H2) and silicon tetrafluoride (SiF4) is introduced into the discharge

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chamber through the injection port. The flow of the gases is controlled by mass-flow controllers

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with a range of 1 to 500 sccm. The pressure of working gas in the chamber is measured by a

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baratron. The gas is evacuated from the chamber through the pumping port, located at the end of the tube, either by scroll pump or by turbomolecular pump The operating pressure range is 5-500 Pa in case of using scroll pump, and 0.001-1 Pa for turbomolecular pump. The discharge

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chamber is partly placed in magnetic field, produced by magnetic coil of Bitter type with water cooling. The maximum magnetic field strength is about 1 T, reached with a current of 750 A. Deposition time was always kept at four minutes. After the gas breakdown the plasma is resonantly sustained at the fundamental gyrofrequency. The ECR absorption region occupies the position near the centre of the magnetic coil and corresponds to the magnetic field strength of 0.86 T. Silicon was deposited on glass substrates (Corning 1737 2.5x2.5 cm, 1 mm thick), placed on the flat bottom of vacuum chamber. The bottom side of the chamber was cooled externally with water at room temperature (~200С). The substrate was placed in the region, where the ECR conditions for 24 GHz are fulfilled (ECR-zone).

ACCEPTED MANUSCRIPT The Raman spectrum of the sample was acquired under excitation by blue light laser at a wavelength of 473 nm at room temperature using LabRam Raman spectrometer coupled with confocal microscope. The power of laser beam was controlled in a way not to affect (crystallize)

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the sample.

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Secondary ion mass spectrometry (SIMS) measurements were used to qualitatively

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determine certain impurities in the silicon films. The elemental composition was analysed using SIMS with a TOF-SIMS-5 instrument (IONTOF). For the in-depth analysis, we have used an ion

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gun of O2+ ions with an energy of 2 keV, a beam current of 500 nA, and a beam diameter of 100μm or Cs+ ions (2 keV, 300 nA, and 50μm). These beams were scanned in the 120×120μm

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raster pattern.

A spectroscopic ellipsometer (UVISEL) was to determine the optical properties of the

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films. The spectra were taken between 1.5 and 4.8 eV with step of 0.01 eV. Data were fitted in

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the frame of classical for microcrystalline silicon 3-layer model (seed layer, main layer and

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roughness) taking into account incoherent reflection from back side of Corning glass. Thickness of each layer and its composition were obtained with DeltaPsi 2 software.

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3. Experimental results and discussion Here we present the results of experiments in which silicon was deposited from SiF4 on

Corning glass substrate. In all experiments the power of microwave emission of gyrotron was 250 W during the deposition time of four minutes. The SiF4 and H2 flows were 12 sccm and 18 sccm respectively and the working pressure was 11 Pa. Evidence of the presence of nanocrystals in the deposited layers was obtained from the analysis of the Raman spectra. Figure 2 shows the spectrum of the deposited layer (circles). Raman spectrum was deconvoluted into three bands related to amorphous phase ( 480 cm-1), crystalline phase (520 cm-1) and an intermediate phase (500-514 cm-1) representing grain

ACCEPTED MANUSCRIPT boundaries or small size nanocrystals [19-21].The best fitting results by Gaussian functions are presented in Figure 2 for crystalline phase (triangles), amorphous phase (dashed line), intermediate phase (crosses) and their sum (solid line). The peak of intermediate phase is

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centered at  511 cm-1 and is shifted from the crystalline peak (519.5 cm-1) by   8.5cm1 , indicating the presence of silicon nanocrystals. The mean crystalline size d was obtained using

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formula: d  2  /  , where  is the peak shift and   2cm 1nm2 [22,23]. The mean crystalline size is about 3 nm. Given the fact that calculations for intermediate phase suggest 3

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nm crystallite size, while the largest peak corresponds to the signal, identical to monocrystalline silicon (very large grains), this shift of 8.5 cm-1 can also be due to signal from the grain

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boundaries.

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The crystalline volume fraction was calculated as Fc = A1+A2/(A1+A3+A3), where A1, A2 and

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A3 are the integrated intensities of crystalline, intermediate and amorphous phase respectively.

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For the calculated intensities of presented spectrum Fc is about 75%.

Figure 2. Raman spectrum of the sample(circles), contribution of crystalline phase (triangles), contribution of amorphous phase (dashed line), contribution of intermediate phase (crosses), resulting fit (solid line). Figure 3 gives an example of the real and imaginary parts of the pseudo-dielectric function of a µc-Si:H film. It clearly shows the characteristic features of a crystalline material – two shoulders at 3.4 and 4.25 eV for crystalline silicon. The experimental data were modelled

ACCEPTED MANUSCRIPT using Bruggeman Effective Medium Approximation in the full spectrum range, taking into account back-reflections from glass substrate, in order to determine the thickness and composition of the film. The standard model consists of three layers: i) an interface layer with

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the substrate, a bulk layer, and a surface roughness. Each layer is characterized by its thickness

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and composition: amorphous fraction, crystalline fraction (large and small grain material), and

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voids (in surface layer). In Figure 3 the experimental spectrum is superimposed with results of modelling in full-range from 1.5 to 4.8 eV. In the table 1 fit result for a film, ellipsometric

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spectra of which is shown in Fig. 3, is presented. The roughness of the film is found to be about 11 nm and its bulk thickness of about 520 nm with a crystalline fraction of the order of 70%,

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which is close to the value obtained from Raman spectra. One should take into account that for the thick films the discrepancy between ellipsometric and Raman diagnostic can occur because a

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void fraction may be present in certain portion of the film volume. In a Raman spectroscopy,

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voids fraction does not contribute to the signal and peaks are only represent crystalline and

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amorphous phases, omitting the void content. In this sense, for highly crystallized films, Raman will always be upper limit, and typically overestimation of crystalline fraction when compared to ellipsometry. It was shown in the recent article [24] that films with exactly same Raman

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spectrum can be enormously different in morphology and ellipsometry spectra may show that unambiguously. Moreover, modelling also reveals the presence of a 70 nm thick amorphous interface layer between the glass substrate and the bulk material. The lower crystalline fraction in the interface layer is supported by its higher impurity content as demonstrated by the SIMS measurements below.

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Figure 3. Real and imaginary parts of the pseudo-dielectric function of a µc-Si:H film.

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The experimental data (points) are quite well reproduced by the model (lines) in the full spectral

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range.

a-Si :H (Tauc-Lorenz)

38.2 %

29.2 %

30.5 %

40.3 %

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Incubation layer 69 nm

micro-Si small grains

25.8 %

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Bulk layer 520 nm

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Surface layer 11 nm 36 percent voids

micro-Si large grains

Table 1. Fit results for a film, ellipsometric spectra of which are shown in Fig. 3.

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Secondary ion mass-spectrometry (SIMS) measurements were used for qualitative and quantitative determination of the presence of impurities in the microcrystalline silicon films. Figure 4 shows a depth profile distribution of silicon and some impurities obtained using Cs+ ions.

ACCEPTED MANUSCRIPT Figure 4. Distribution of the elements in the silicon layer obtained by SIMS method (sputtering by Cs+ ions)

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The depth profile of impurities can be divided into two regions: one close to the

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substrate and a bulk region. The impurities density decreases as the layer thickness increases,

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and comes to the steady state on the distance of 0.3-0.4 μm from the film/substrate interface. The concentration of oxygen in the bulk region is about 1019 cm-3, hydrogen ~ 1020 cm-3, carbon ~

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2*1019 cm-3, fluorine ~ 1020 cm-3 .The film thickness can be estimated as 600-650 nm by known rate of sputtering of Cs+ gun, in good agreement with the optical modelling by ellipsometry. The

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deposition time was about 4 minutes, so the deposition rate can be estimated at 2.5 nm/s. The transition layer Si+O+F is produced on the initial stages of the deposition, as a

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result of the intense interaction of fluorine atoms with the glass surface, which leads to the

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incorporation of oxygen in the early stages of µc-Si:H deposition. Moreover, the low

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temperature of the substrate (the substrate was not intentionally heated) may also contribute to the relatively high incorporation of impurities in the early stages of deposition. As the thin silicon layer grows, the influence of substrate material decreases and the impurities densities

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come to the steady state.

4. Conclusions The possibility of high-rate (2.5 nm/s) low-temperature deposition of thin silicon films in ECR gyrotron discharge plasma was demonstrated. The analysis of the samples by means of spectroscopic ellipsometry and Raman scattering showed that all films have large crystalline fraction. The use of gyrotron with high microwave power opens up the opportunity to make deposition in high density plasma at pressures of tens pascals which should result in even higher deposition rates (tens nm/s).

ACCEPTED MANUSCRIPT Acknowledgments The work was supported by Russian Academy of Sciences (Program of Presidium No.24)

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and partly by Presidential RSS Council (Grant No. SP-23.2012.1)

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ACCEPTED MANUSCRIPT Figure 1. Sketch of the experimental system. Figure 2. Raman spectrum of the sample(circles), contribution of crystalline phase (triangles), contribution of amorphous phase (dashed line), contribution of intermediate phase (crosses),

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resulting fit (solid line).

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Figure 3. Real and imaginary parts of the pseudo-dielectric function of a µc-Si:H film. The experimental data (points) are quite well reproduced by the model (lines) in the full spectral

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range.

Figure 4. Distribution of the elements in the silicon layer obtained by SIMS method (sputtering

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by Cs+ ions)

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Table 1. Fit results for a film, ellipsometric spectra of which are shown in Fig. 3

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Figure 1. Sketch of the experimental system.

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Figure 2. Raman spectrum of the sample(circles), contribution of crystalline phase

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(triangles), contribution of amorphous phase (dashed line), contribution of intermediate phase (crosses), resulting fit (solid line).

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Figure 3. Real and imaginary parts of the pseudo-dielectric function of a µc-Si:H film.

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The experimental data (points) are quite well reproduced by the model (lines) in the full spectral

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range.

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Figure 4. Distribution of the elements in the silicon layer obtained by SIMS method (sputtering by Cs+ ions)

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micro-Si large grains

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25.8 %

Bulk layer 520 nm

29.2 %

30.5 %

Incubation layer 69 nm

100 %

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38.2 %

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Surface layer 11 nm 36 percent voids

micro-Si small grains

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a-Si :H (Tauc-Lorenz)

40.3 % 0

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Table 1. Fit results for a film, ellipsometric spectra of which are shown in Fig. 3

ACCEPTED MANUSCRIPT Highlights

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Deposition of silicon films from silicon tetrafluoride with high rate is demonstrated

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Silicon tetrafluoride is efficiently decomposed in plasma with gyrotron pumping

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Gyrotrons extend the deposition pressure range to tens of pascals