Microcrystalline silicon thin film deposition from silicon tetrafluoride: Isolating role of ion energy using tailored voltage waveform plasmas

TL;DR: In this article, the impact of ion energy on material quality was investigated in PECVD of hydrogenated microcrystalline silicon (µc-Si:H) thin films from an SiF4/H2/Ar chemistry.

About: This article is published in Solar Energy Materials and Solar Cells.The article was published on 2019-02-01 and is currently open access. It has received 6 citations till now. The article focuses on the topics: Thin film & Plasma-enhanced chemical vapor deposition.

Summary

1. Introduction

• In contrast to hydrogenated amorphous silicon (a-Si:H), hydrogenated microcrystalline (or nanocrystalline) silicon (µc-Si:H) allows light absorption in the infrared due to its lower band-gap.
• To overcome this inherent limitation, the best known strategies are to use high process pressures [2,3], very high frequency (VHF) excitation [4,5], or “classical” dual-frequency excitation [6,7].
• Driven by a non-sinusoidal voltage waveform generated as a finite Fourier series of multiple consecutive harmonics with individually adjustable amplitudes and phases, an electrically asymmetric plasma response can be formed, regardless of the reactor geometry.
• The authors report on the structural and electronic properties of µc-Si:H films deposited using this gas mixture, while using TVW plasma excitation to control the ion energy during deposition.

2. Experimental details

• The experiments have been carried out in a CCP RF-PECVD reactor, which comprises a cylindrical plasma confinement box surrounding two 100 mm diameter parallel electrodes with their inter-electrode distance set to 20 mm.
• Figure 1 shows some examples for such voltage waveforms with n set to four, where Vmin and Vmax are the minimum and maximum excursions in the applied voltage.
• One can further increase φ above π up to 2π.
• The material deposition has been performed on Corning “Eagle” glass, with co-deposited films on undoped crystalline silicon (c-Si) wafers for further characterization.
• The temperature of the grounded electrode was set to 150 ºC, and that of the powered electrode was 80 ºC.

3.1 Control over ion bombardment energy

• As this study deals with the impact of ion energy on µc-Si:H film growth, it is necessary to make an estimation of the time-averaged plasma potential (Vpl), as this will be the maximum kinetic energy, i.e., the maximum IBE , that an ion may acquire before arriving on the growing surface (on the grounded electrode).
• Figure 2 shows the variation of VDC and IBEmax as a function of φ.
• Since the reactor is geometrically asymmetric due to the grounded plasma box, large negative values of VDC are usually observed, as shown in Figure 2-(a).

3.2 Structural properties

• To quantify the impact of ion energy, the films have been characterized by Raman spectroscopy, and the results are presented in Figure 3.
• This suggests that although similar crystalline volume fractions are obtained for all process conditions, high ion energy conditions can lead to the growth of smaller grained material [40].
• Regardless of the reason, a greater <εi>max is commonly considered to be associated with improved material properties [41].
• Similar results have also been observed in Ref. [44].
• The discrepancy between these two techniques could originate from the portion of small grains that are indistinguishable from the amorphous phase by Raman scattering.

3.3 Infrared absorption

• Figure 7 shows the FTIR absorption spectra of the co-deposited “0-π” films on c-Si wafers, focusing on the hydride stretching modes (SMs) [48,49].
• In order to make a qualitative evaluation of the material microstructure, a microstructure parameter R* can be derived from the FTIR spectra.
• The deduced R* values for these two films are included in Figure 8 for comparison.
• One should note that this would lead to a sacrifice in deposition rate compared to the case of high power injection.

3.4 Ion-related interactions

• In contrast to neutrals, ions within the plasma can have a considerable amount of kinetic energy due to the acceleration process in the plasma sheath.
• In Figure 9, the theoretical energy thresholds at which impinging ions begin to induce effective surface and bulk atomic displacement on a silicon substrate are plotted versus the ion mass [61].
• Bulk displacement can also occur if the ion energy is high enough (IBEmax > ~35 eV).
• As for the H3+-induced surface displacement, no obvious transition is observed due to this ion-surface interaction.
• Finally, the SiFx+ species present can be Si+, SiF+, SiF2+, and SiF3+, of which have surface displacement energy thresholds in the range of 12-15 eV, and so like Ar+-induced surface displacement, are present for all conditions.

4.1 Transport properties

• To examine the impact of ion energy on the films' electronic properties, SSPG measurements have been carried out.
• Notably, it fits quite well within the trend.
• Moreover, SSPG measurements allows for the determination of the ambipolar mobility-lifetime products (holes mobility-lifetime products, µτmin) through [63]: L 2 μτ (4) Figure 10-(b) presents the results of µτmin plotted with increasing IBEmax.
• The “out-of-plane” transport properties will be more crucial for practical solar cell devices.
• It should be noted that a slightly thinner absorber layer is used for IBEmax = ~45 eV (see Table 2), with little impact on JSC.

5. Conclusions

• The authors have applied the multi-frequency TVW excitation technique to the RF-PECVD of µc-Si:H films from an SiF4/H2/Ar plasma chemistry.
• Through structural and electronic analysis, the authors find that a variation of ion energy - or precisely speaking the IBEmax - during processing can be of vital importance in depositing high quality materials.
• This is attributed to the effect of bulk atomic displacements induced by silicon-related ions during processing.
• H solar cells with an absorber layer deposited with IBEmax in the range of ~45-55 eV have shown an optimum performance, mainly due to an improvement in VOC, also known as Single-junction µc-Si.
• This is of great value when developing materials for practical photovoltaic applications.

Figures

Figure 2. (a) Experimentally obtained VDC as a function of φ during µc-Si:H film deposition. (b) Variation of the estimated IBEmax and rd with varying φ. The gray dash line is a guide to eye for the comparison of rd.

Figure 8. Variation of microstructure parameter R* with varying IBEmax. Two reference films deposited using standard single-frequency (13.56 MHz) excitation source with low (8 W) and high (15 W) power are shown by purple diamonds.

Figure 3. (a) Raman spectra for µc-Si:H films deposited using different types of TVW (only the “0-π” films are shown). The gray line indicates the silicon Raman crystalline peak at ~520 cm-1. (b) Variation of FWHM520 with varying IBEmax, where the up/down triangles correspond to the “0-π”/“π-2π” films, respectively.

Figure 6. Correlation between fraction of small/large grain in bulk layer and FWHM520.

Figure 7. FTIR spectra in range from 1800 to 2300 cm-1 for µc-Si:H films deposited using different types of TVW (only the “0-π” films are shown).

Figure 10. Variation of (a) Ld and (b) µτmin at room temperature for two sets of µc-Si:H films with varying

Figure 9. Energy threshold for ion-induced surface (dashed line) and bulk (solid line) atomic displacement on a silicon substrate versus impinging ion mass, calculated by Brice et al. [61]. The H3+, Ar+ and SiFx+-induced displacements are highlighted (circles), and higher order silicon-related ions (SiyFx+, y > 1) are indicated by the shadow area in gray. The blue and red bands correspond to the upper limit of IBEmax to sustain silicon epitaxy (from Ref. [62]) and the experimental threshold values found in this work.

Figure 4. (a) Ellipsometry spectra in high energy portion for µc-Si:H films deposited using different TVW (only the “0-π” films are shown). (b) Variation of the <εi>max at high energy portion with varying IBEmax.

Figure 1. Voltage waveforms obtained with equation (1) for different values of φ (in the range of 0- π), with n set to four. Vmin and Vmax represent the minimum and maximum amplitude of the applied signal.

Table 1. Process conditions for two reference µc-Si:H films deposited using standard single-frequency (13.56 MHz) excitation source. PRF represents the RF power injection.

Figure 5. Variation of (a) surface roughness, (b) small and large grain volume fraction, i.e., XSE sg and XSE lg , and (c) void fraction XSE v in bulk layer with varying IBEmax.

Table 2. J-V parameters of µc-Si:H solar cells with intrinsic absorber layers deposited under different ion energy conditions using TVW The reference cell (IBEmax = ~55 eV) was deposited using standard single-frequency (13.56 MHz) excitation source with 15W power injection.

Figure 11. Light J-V curves of µc-Si:H solar cells with absorber layers deposited using TVW, and with reference sample deposited by standard single-frequency (13.56 MHz) excitation source shown in purple.

Frequently asked questions

Q1. How many harmonics were used to generate different shapes of TVW?

The phase between harmonics φ was varied in the range from 0-2π, generating different shapes of TVW, while a constant value of VPP of 250 V was used. 

Q2. What contributions have the authors mentioned in the paper "Microcrystalline silicon thin film deposition from silicon tetrafluoride: isolating role of ion energy using tailored voltage waveform plasmas" ?

In this work, the authors utilize a novel radio-frequency ( RF ) excitation technique – tailored voltage waveforms ( TVW ) – as a solution to achieve ion flux-energy decoupling through the electrical asymmetry effect. This makes it possible to independently study the impact of ion energy on material deposition. The authors study the impact of ion energy more precisely the maximum ion bombardment energy ( IBEmax ) before collisions on the PECVD of hydrogenated microcrystalline silicon ( μc-Si: H ) thin films from an SiF4/H2/Ar chemistry. 

Q3. What have the authors contributed in "Microcrystalline silicon thin film deposition from silicon tetrafluoride: isolating role of ion energy using tailored voltage waveform plasmas" ?

In this work, the authors utilize a novel radio-frequency ( RF ) excitation technique – tailored voltage waveforms ( TVW ) – as a solution to achieve ion flux-energy decoupling through the electrical asymmetry effect. This makes it possible to independently study the impact of ion energy on material deposition. The authors study the impact of ion energy more precisely the maximum ion bombardment energy ( IBEmax ) before collisions on the PECVD of hydrogenated microcrystalline silicon ( μc-Si: H ) thin films from an SiF4/H2/Ar chemistry. 

Q4. What ion energy threshold is the likely to be present in the plasma?

In the plasma process studied in this work, the ions most likely to be present in the plasma are Hx+ (x = 0-3), Ar+ and SiFx+ (x = 0-3). 

Q5. Why do thicker films lead to a better photoelectronic response?

Because of the material anisotropy along the growth direction, giving improved grain features (larger grain sizes), and less influence from the bottom amorphous incubation layer (or less crystallized layer in the early growth stage), thicker films can lead to a better photoelectronic response. 

Q6. What can be used to make a qualitative evaluation of the material microstructure?

In order to make a qualitative evaluation of the material microstructure, a microstructure parameter R* can be derived from the FTIR spectra. 

Q7. What is the way to fit an optical model to the experimental data?

To fit an optical model to the experimental data, a three-layer structure consisting of an interface layer, a bulk layer and a surface roughness layer was used. 

Q8. What is the extent to which the deposited material can be affected?

The extent to which the deposited material can be affected depends on a number of factors: the ions' kinetic energy, the mass ratio between ions and target atoms, as well as the ions' angle of incidence. 

Q9. What is the film properties for a solar cell?

The best film properties (crystalline grain features, material density and photoelectronic response) have been obtained with IBEmax around 45-55 eV. 

Q10. What are the parameters allowed to change in the fitting process?

The parameters allowed to change in the fitting process are the thickness and the volume fraction of each material in each layer. 

Q11. What is the i>max of a c-Si:H film?

the <εi>max of a μc-Si:H film depends on a number of factors: the crystalline volume fraction, crystallite size, density, as well as surface roughness. 

Q12. What is the common technique used to grow high-quality c-Si:?

The growth of high-quality µc-Si:H is typically performed using the conventional radio-frequency (13.56 MHz)plasma enhanced chemical vapor deposition (RF-PECVD) technique. 

Q13. How was the dark saturation density calculated?

In order to access the physical properties of the photodiodes, J-V measurements have also been performed in the dark, from which the dark saturation current density J0 for these solar cells has been extracted. 

Q14. What is the ion energy threshold for the Ar+-induced surface displacement?

The ion energy threshold for the Ar+-induced surface displacement is around 12 eV, which meansthat this process is equally present for all the plasma conditions studied in this work. 

Q15. What is the likely explanation for the ion-induced bulk atomic displacements?

The experimentally observedtransitions of material properties are most likely to be associated with the silicon-related ion-induced bulk atomic displacements. 

Q16. What is the process condition used for the deposition of the intrinsic absorber layer?

The process conditions are shown in Table 1: one of them (15W) is similar to the conditions used in Ref. [53] for the deposition of the intrinsic absorber layer for high efficiency µc-Si:H solar cell devices; for the other one, the authors attempted to decrease the RF power to achieve a lower ion energy during deposition. 

Q17. How much VOC is observed with increasing ion energy?

As presented in Table 2, with increasing ion energy, an increase in VOC is observed, with an optimum value of 0.509 V occurring at IBEmax = ~45 eV.