Preparation and Characterization of Crystalline Silicon by Electrochemical Liquid-Liquid-Solid Crystal Growth in Ionic Liquid.

The electrodeposition at low temperature for the direct growth of crystalline thin films without any templating agent in ionic liquid (IL) is a relatively new electrochemical synthetic strategy. This work studied the role of the deposition temperature, deposition time, and different working electrodes in the electrodeposition of crystalline Si thin films from the byproduct silicon tetrachloride in IL at low temperature. X-ray diffraction (XRD) revealed that the as-deposited Si films were crystalline at the temperature of 80 °C. Scanning electron microscopy (SEM) and Raman spectroscopy further indicated that the crystalline quality of the as-deposited silicon film was relatively the best when the electrodeposition time reached 1 h at the temperature of 100 °C; excessive electrodeposition would yield amorphous silicon on the surface of the as-deposit crystalline Si, which decreased the crystal quality of the Si film. The SEM and XRD, respectively, revealed that the crystal structure of Si yielded on e-InGa was significantly different from that produced on Ga and more impurities existed in the film. Research on the influence of these parameters on crystallinity and morphological characteristics of Si gives better control over the growth of crystalline Si thin films for specific applications.

Introduction

So far, semiconductor crystalline silicon is one of the most important materials for a wide range of applications including integrated circuits,[1,2] solar cells,[3,4] biosensors,[5,6] and other fields. The process of preparing polycrystalline silicon has a wide variety in industries, such as the improved Siemens method, silane, metallurgy method, fluidized bed method, carbothermal reduction method, hot-wire method,[7] and so forth. However, for every ton of polysilicon produced in industry, several tons or even dozens of tons of byproduct silicon tetrachloride will be produced. Thus, it is extremely attractive to develop and utilize the byproduct silicon tetrachloride produced by the preparation of crystalline silicon in the industry for the re-preparation of crystalline silicon for implementing sustainable and green development.[8−10] Electrodeposition has always been identified as a potential alternative route for preparing Si because of its simple equipment, low cost, and relatively clean operation.[11] In recent years, the electrodeposition of Si has been achieved using organic solvents,[12,13] high-temperature molten salts,[14,15] and ionic liquids (ILs)[16,17] as a solution system to dissolve silicon sources. However, excessively high (>700 °C) temperatures are required for electrodeposition to yield crystalline Si.[18,19] Conventional electrodeposition of Si at solid electrodes cannot direct yield crystalline silicon at low temperatures, which still requires subsequent annealing and purification processes to transform it into the crystalline state.[12] The incompatibility of low temperature and a pure crystalline product have thus seriously limited the development of Si electrodeposition.

In 2011, Maldonado et al. proposed a new electrochemical synthetic strategy called “electrochemistry-liquid–liquid–solid” (ec-LLS) crystal growth, which can directly yield crystalline covalent group IV and III–V semiconductor materials under or near ambient temperature conditions. They successfully electrodeposited crystalline germanium (Ge) using liquid metal mercury as a working electrode at ambient temperature.[20] The liquid metal mercury acts as the source of electrons for the heterogeneous reduction of oxidized semiconductor precursors dissolved in the electrolyte and the solvent for dissolving zero-valent germanium. The supersaturation of germanium in the liquid metal causes the final crystal nucleation and growth. They also successfully produced crystalline silicon using liquid metal gallium (Ga) and PC/TBAC + SiCl4 as the working electrode and electrolyte, respectively, in several experiments from room temperature to 100 °C.[21] This strategy for Si electrodeposition solves the long-standing problem that crystalline silicon cannot be directly deposited without any templating agent at low temperature. However, the system suffers from high volatility, which results in a large amount of evaporation even at low temperatures. Therefore, high pressure should be applied to the electrolytic cell in the whole process of the experiment to prevent the volatilization of SiCl4. Recently, Zhang et al. reported an efficient method to prepare crystalline silicon films from silicon tetrachloride on the surface of liquid gallium at low temperature with IL as an electrolyte. In their studies, they noticed that the two sides of the silicon film have different morphological structures, which were smooth amorphous silicon and nano-level polycrystalline silicon.[22] Besides, there are reports of using photolithography to etch microhole arrays on silicon wafers and filling of Ga liquid metal nanodroplets as discrete ultra-microelectrodes to obtain Si nanowires by electrodeposition.[23] Currently, researchers have made relatively great progress in the preparation of crystalline silicon at low temperature. However, it is still necessary to further study the crystallization and growth mechanism of preparing silicon thin films to obtain high-quality crystalline silicon thin films for subsequent applications.

In this study, we used the hydrophobic [N1114] [TFSI] IL and Ga liquid metal electrode as a platform for directly electrodepositing crystalline Si thin films from the dissolved SiCl4 precursor under benign conditions and yielded Si thin films with excellent crystal quality at a lower temperature through adjusting the temperature and deposition time and different substrates. Additionally, the impact of different parameters on the growth and surface morphology of the silicon film has been studied.

Experimental Section

Materials and Chemicals

[N1114]TFSI (≥99%) was purchased from Shanghai Chengjie Chemical Co., Ltd. Gallium (Ga, 99.99%) was purchased from Aladdin. SiCl4 (≥99.5%) was purchased from Aike Reagent. Indium gallium eutectic e-GaIn (99.99%) was purchased from Alfa-Aesar. Bismuth particles (99.99%) and indium particles (99.99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals used in this work were commercially purchased products of analytical grade.

Electrodeposition of Si

All experiments were performed in an argon-filled glove box using three-electrode single-cell electrodeposition. The purchased 200 g (approximately 140 ml) bottle of IL “butyltrimethylammonium bis(trifluoroformyl) imide ([N1114]TFSI)” was poured into a 250 mL clean beaker (enough space is left to prevent overflow when dry). The ([N1114]TFSI) was dried in a vacuum-drying oven for 3 h at 110 °C to remove the volatile impurities and water. Among the three electrodes, the working electrode Ga(l) and the indium gallium eutectic e-GaIn(l) were contained in a small circular quartz groove (area about 0.5 cm2, depth 0.5 cm) and platinum wire was used with a 0.5 mm-diameter polytetrafluoroethylene tube for electrical connection. A silver wire was used as an quasi-reference electrode (99.99%, 0.5 mm in diameter). A platinum plate (99.99%, 10 mm × 10 mm) was used as a counter electrode. The counter electrode was first polished with sandpaper; then, the platinum wire and platinum plate were placed in 10% dilute nitric acid several times to clean. All the electrodes were put together in ethanol and deionized water and ultrasonically washed for 10 min and finally blew dry for use. In addition, custom-made digging quartz plates were placed in the electrolytic cell to fix the position of the gallium electrode.

Measurements

Scanning electron microscopy (SEM) (Hitachi S-4800) was used to characterize the surface morphology of the silicon thin film at 5 kv, and the magnification was above 20,000 times. The Bruker D8 X-ray single-crystal diffractometer was used to analyze the crystal quality and orientation of the silicon film. The instrument was equipped with a Cu Kα source (λ = 1.5406 Å). The scanning range is 10–90°. Raman spectroscopy of the electrodeposited silicon film was carried out using the Renishaw INVIA confocal Raman microscope (633 nm wavelength, British).

Results and Discussion

Influence of Deposition Temperature on the Electrodeposited Silicon Film

Electrochemical Analysis

The crystallization temperature of Si films was explored through characterization and analysis of Si films deposited at 60–120 °C. Figure shows the representative voltammograms of the electrolytic solution with a SiCl4 concentration of 0.3 mol/L at different electrodeposition temperatures. As shown in Figure , there are obvious diffusion-controlled peaks for a Ga(l) working electrode scanned to negative potentials. Si4+ was directly reduced to the zero-valent state in one step, and no reduction reaction of other substances occurred since there was only one diffusion-controlled peak during scanning to negative potentials. The peak current of the reduction peak gradually increased with the increase in temperature due to the significant increase in conductivity of the IL. Besides, a small raised oxidation peak appeared near −1.0 V when scanning back to the positive potentials, which may be partial oxidation of the electrolyte or a small amount of oxidation of the silicon film.

Figure 1

CVs of 300 mM SiCl4 in IL with a scan rate of 50 mV s–1 at different temperatures.

Potentiostatic electrodeposition was used to start the electrodeposition of Si at different temperatures at a reduction potential of −1.8 V (vs Ag QRE). Figure shows the current–time curve of electrodepositing Si for 1 h. Although the peak current of each reduction peak in the cyclic voltammogram (CV) was relatively large, the current dropped sharply in the initial stage of electrodeposition and reached a stable state after dropping to a very small value (<5 mA/cm2). It can be found that the higher the experimental temperature, the longer it takes to reach the steady state from the four curves of a, b, c, and d. The curve of Si mass deposited at 100 °C with a typical representative has three obvious sections: the rapid decrease in current in the first 5 min was due to the reduction of SiCl4 that caused the ion concentration of the solution to decrease rapidly, the current decline rate slowed down, and crystal growth may be occurring in the next 15 min at the electrode–electrolyte interface, which further led to a relatively smaller current. The presence of small current after 20 min might be due to the low conductivity of the grown Si film and reduction of ion concentration.

Figure 2

Current–time curve for the potentiostatic electrodeposition process at −1.8 V (vs Ag QRE) at different temperatures. Deposition time is 1 h.

Surface Morphology of the Si Film

The surface morphology of the as-prepared Si mass after magnification to 2–3 million times was observed by SEM in Figure . The as-prepared Si mass deposited at 60 °C exhibits a porous honeycomb-shaped white substance with no obvious rules. However, on the Si mass deposited at 80 °C, small polygonal particles can be seen piled together, which are similar to polycrystalline silicon particles. Flat and densely arranged crystal grains exist on the mass deposited at 100 °C, which have obvious characteristics of polysilicon.[15] The morphology of as-prepared Si mass deposited at 120 °C is quite different, and it is observed that a large number of amorphous agglomerates are adsorbed on the surface of the Si film. X-ray photoelectron spectroscopy (XPS) of the electrodeposition of Si is shown in Figure S1. The presence of a weak Si 2p state signal is observed at a binding energy of 99.3 eV, indicating that the as-deposited product is Si0. A single major peak is observed at 103.0 eV, which is probably due to the presence of Si with oxygen species such as SiO. It is believed that oxygen contamination of the Si surface occurred when the electrodeposited Si was ex situ analyzed by XPS.[24,25]

Figure 3

SEM image of the as-deposited Si thin films at the electrodeposition temperature of (a) 60, (b) 80, (c) 100, and (d) 120 °C. Electrodeposition of Si at −1.8 V (vs Ag QRE) for 1 h.

Internal Structure of the Si Film

As shown in Figure , from the X-ray diffraction (XRD) patterns of Si electrodeposition at different temperatures, the silicon films yielded by the liquid Ga electrode have a strong noise signal, wide diffraction peaks, and low intensity, indicating poor crystallinity and low purity. In terms of the impact of different temperatures on electrodeposition, the Si mass deposited at a temperature of 60 °C hardly has any diffraction peak, indicating that there is no crystalline silicon in the sample. The XRD patterns of Si mass deposited at temperatures of 80 and 100 °C have obvious diffraction peaks at 2θ = 28.4, 47.2, and 56.0°, corresponding to the (111), (220), and (311) crystallographic planes of silicon, respectively. The as-deposited Si produces a sharp diffraction pattern, consistent with the expected diamond cubic crystal structure of crystalline Si. However, the intensity of the diffraction pattern for the two as-prepared Si masses is slightly different. Among them, the diffraction peak of the (111) crystallographic plane for the as-prepared Si mass deposited at 100 °C is stronger, and the Si grains are preferentially growing along the (111) crystallographic plane. Si grains have a preferential crystal orientation along the (220) crystallographic plane (80 °C). The diffraction peak intensity of the sample deposited at 120 °C is weak, and there are only diffraction peaks of (220) and (311) crystallographic planes, indicating that the content of crystalline Si is very small and lots of amorphous Si exist.

Figure 4

XRD pattern of deposited Si at −1.8 V (vs Ag QRE) at different temperatures. Deposition time is 1 h.

A sharp and strong peak can be seen near 520 cm–1, which is the lateral optical mode (TO) of the Si–Si bond in single-crystal silicon for the first-order Raman spectrum of standard single crystal silicon.[26,27] As the disorder increases, the dynamical selection rules are relaxed, leading to the creation of new phonon modes. The first change during this period is that the peak width near 520 cm–1 increases, and the phonon mode near 520 cm–1 is red-shifted.[28,29] The Raman shift of the peak position is near 480 cm–1. Then, two new vibration modes[30] are generated, which are the longitudinal optical mode (Raman displacement is about 410 cm–1) and transverse acoustic mode (Raman displacement is about 170 cm–1), both of which have relatively low intensity. As shown in Figure , the Raman spectra of the as-prepared Si film deposited at different temperatures are obtained after multiple tests in different regions for each sample. The Si film deposited at 60 °C has no Raman scattering peak corresponding to the XRD result, indicating that the surface of the film consists of mostly SiO, and a small amount of amorphous silicon may not be detected in the bottom layer. The Raman scattering peak of the sample deposited at 80 °C is located at 509 cm–1 with a peak half height [full width at half maximum (fwhm)] of 13 cm–1, indicating poor crystallinity. The Si sample deposited at 100 °C has a sharp Raman peak signal of crystalline silicon at 520 cm–1, exhibiting a high degree of crystallinity. Only a broad scattering peak was measured at a Raman shift of 480 cm–1 at a temperature of 120 °C, indicating that the deposited silicon is amorphous, which leads to the inference that a large amount of amorphous Si has been deposited on the surface of the as-prepared crystalline silicon.

Figure 5

Raman spectrogram of electrodeposited Si at −1.8 V (vs Ag QRE) for 1 h at different temperatures.

With the increase in temperature, the number of silicon atoms in the liquid metal increases, and the facile rearrangement of silicon atoms can be promoted to adopt the correct position in the developing crystal lattice.[31] The rate of the electrodeposition process of silicon would be accelerated as the temperature rises, which may make the transition from liquid electrode electrodeposition to solid electrode electrodeposition faster, resulting in more amorphous oxides of silicon being produced on the surface of the crystalline silicon, which leads to the decrease in crystallinity of the silicon film. Also, the high volatility of the silicon source SiCl4 and the instability of deposition in a liquid environment can make the crystallization process unstable as the temperature continues to rise, which may result in more crystal defects.

Influence of Deposition Time on the Silicon Film

At present, there has not been a clear and systematic explanation for the growth process of liquid electrode-electrodeposited crystalline Si films. It is found above that the silicon films exhibited a distinct amorphous character at the temperature of 120 °C. As the temperature increases, the growth rate of the silicon film will be accelerated, which will result in excessive deposition time, affecting the crystallinity of the silicon film. We thus set different deposition times to observe and explore the growth process of silicon. For samples deposited at different deposition times, dense and uniform diamond cubic crystal grains can be seen after magnifying 30,000 times under a scanning electron microscope. As shown in Figure , the polysilicon crystal grain size was close to 100 nm after 5 min of electrodeposition, and the new crystal nuclei to be formed were filled between the large crystal grains. Besides, a small amount of silver-white impurity gallium can be seen. The silicon crystal nuclei grew to a size of 200 nm after 20 min of electrodeposition, and they were densely stacked in planar layers. The small crystal grains which were interspersed between large crystal grains had a size of tens of nanometers. The grain size had been relatively uniform and stabilized at about 200 nm after being deposited for 1 h. However, tiny particles would gradually grow on the surface of large Si crystal grains after 2 h, and there were more irregular amorphous particles on the surface after 3 h. The boundary between crystal Si grains became blurred, and the size of large grains was reduced by the extrusion effect.

Figure 6

SEM images of Si films were obtained by electrodeposition at −1.8 V (vs Ag QRE) for (a) 5 min, (b2) 20 min, (c) 1 h, (d) 2 h, and (e) 3 h at the temperature of 100 °C and (b1) SEM images of Si films electrodeposited for 20 min at low magnification.

Furthermore, as observed from the small-magnification electron microscopy in Figure b1, the bottom of the grain is a smooth plane, and the nucleation and growth of crystalline Si are overall heterogeneous, which is an island-like growth mode.[13]

Although the SEM image can obtain some structural and morphological information of the obtained silicon film, XRD and Raman spectroscopy still need to be further used to test the effect of different electrodeposition times on the crystallinity of the crystalline Si film under the same conditions. The XRD pattern in Figure shows that when the electrodeposition temperature is 100 °C, the Si films obtained from several experiments with the deposition time from 5 min to 3 h all show strong diffraction peaks at (111), (220), and (311) crystallographic planes of silicon, indicating that all the samples are crystalline.

Figure 7

XRD patterns for different times of electrodeposition. Deposition temperature is 100 °C.

Figure a–f shows the independent Raman spectra of the samples at each electrodeposition time and the calculated Raman peak position and its half-width (Figure f). As shown in Figure f, the Raman scattering peak is between 505 and 520 cm–1 and the fwhm is between 5 and 18 cm–1, indicating that the silicon films obtained by electrodeposition from 5 min to 3 h at a temperature of 100 °C were all crystalline silicon but with different crystallinities. In the initial stage, with the increase in the electrodeposition time, the crystallinity increased and reached the best at 1 h. The Raman peak red-shifted and broadened 3 h later, indicating that the crystallinity of the silicon film was significantly reduced. This is because the gallium surface is completely covered with the crystalline silicon layer, which prevents the reduced element silicon from dissolving into the liquid gallium to continue the nucleation and growth of crystals. However, if the applied potential has not been terminated, the reduced elemental silicon will still be produced but cannot dissolve into the liquid gallium, so it will adhere to the surface of the crystalline Si disorderly and form a film in the form of amorphous silicon, that is, the entire deposition process is surprisingly transformed from ec-LLS crystal growth with liquid gallium as a working electrode to electrodeposition of the solid-state electrode on the as-deposited crystal Si thin film, where the former-yielded crystalline Si and the latter-yielded amorphous silicon are.[21,32−34]

Figure 8

Raman spectra of Si films with different electrodeposition times: (a) 5 min, (b) 20 min, (c) 1 h, (d) 2 h, and (e) 3 h. (f) Location of Raman scattering peaks and fwhm statistics in panel (a–e) diagram.

Influence of Different Substrates on Silicon Thin Films

Different liquid metals have different melting points and have a greater impact on the migration rate and solubility of silicon atoms in them. When liquid Ga is used as a substrate, its surface tension is too large and an oxide layer is easily formed on the surface, which interferes with electrochemical measurements and changes the physical and chemical properties of the gallium surface and the hydrodynamic behavior of the metal.[35] We try to use different metals or alloys as working electrodes. On one hand, it can explore the influence of different substrates on the growth of silicon films. On the other hand, since the melting point of metal gallium is about 29 °C, the as-deposited Si and substrate gallium cannot form an effective solid-state device at room temperature. Therefore, it is very important to find a low-temperature liquid metal with a melting point higher than room temperature and capable of depositing Si thin films, which can directly form semiconductor devices for specific requirement.

Figure a shows representative voltammograms of electrodeposited silicon with three different metal or eutectic liquid electrodes, respectively. One of the most obvious differences in the voltammograms is that the diffusion-controlled peaks of curves a, b, and c are quite different. The diffusion-controlled peak current of the eutectic indium gallium (e-GaIn) electrode electrodeposition is the largest, indicating that its conductivity is the strongest, while the eutectic indium–bismuth (e-BiIn) electrode is less conductive during electrodeposition. In addition, two diffusion-controlled peaks appear at −0.6 and −1.5 V during electrodeposition at the e-BiIn electrode, which are smaller than those at both the e-GaIn and Ga electrodes, respectively. Because of the good thermal and electrochemical stability of our chosen IL [N1114]TFSI, the reduction reactions in the electrolyte cannot occur in the presence of pure IL and SiCl4, except for the reduction of SiCl4. Therefore, the extra reduction peaks may be caused by the instability of the e-BiIn electrode. As shown in Figure b, Ga and e-BiIn were used as electrodes to perform cyclic voltammetric scanning in an electrolytic cell with pure IL as the electrolyte. It was found that only curve 1 has a diffusion-controlled peak around −0.5 V, indicating that this peak is caused by the breaking of the metal bond in the e-InBi.

Figure 9

(a) CVs of silicon electrodeposited on different substrates. (b) Comparison of CVs of e-BiIn and Ga electrodes for the electrodeposition of silicon in pure IL. The scanning rate is 50 mV s–1, and the deposition temperature is 100 °C.

Electrodeposition of Si was carried out at −2.3 V potential to yield silicon films with indium gallium eutectic as the working electrode. SEM was performed on different areas of the film several times and two different microtopography images were obtained (Figure ). The first type has obvious boundary particles, which is similar to the polysilicon structure obtained by electrodeposition in Ga, except that its shape is cone-shaped and the surface is relatively rounded, which is similar to the texture of the crystalline silicon in the solar cell after corrosion. The second kind is closer to the disorderly accumulation of tiny particles. The electrodeposition of the indium–bismuth eutectic electrode is different from the electrodeposition of the indium–gallium eutectic electrode (Figure S2). Two representative morphologies were also selected, which were gray particles intermixed with each other and uniform regular octahedral particles. Energy-dispersive X-ray spectroscopy (EDS) shows that the first type is mainly composed of Si (24 at. %) and O (56 at. %), and the second type is In (about 90 at. %), indicating that the thin film electrodeposited by the e-BiIn electrode contained a large amount of indium and serious oxidation. Combined with the voltammograms, the electrodeposition process began with the breaking of metallic bonds in eutectic, and then, a large amount of indium overflowed to the surface.

Figure 10

Two representative SEM images of electrodeposition of Si using e-InGa for 1 h at the temperature of 100 °C. The concentration of SiCl4 is 300 mM.

XRD shows that more liquid metals were mixed in the as-deposited film deposited by the eutectic electrode (Figure ). The XRD pattern of the film electrodeposited with the indium gallium eutectic electrode contains strong Ga diffraction peaks, including the (111) and (002) crystallographic planes at 2θ = 30.2 and 39.8°, respectively. In addition, two weaker silicon diffraction peaks were observed at 28.4 and 47.2°, namely, (111) and (220) crystallographic planes, respectively. Only the (220) crystal plane diffraction peaks of silicon exist in the XRD pattern of the sample deposited with indium–bismuth eutectic as the working electrode. The stronger peaks are at 33.0, 39.2, and 54.5°, corresponding to the diffraction peaks of (101), (110), and (112) crystal planes of metallic indium, respectively. The internal metal impurities may include mixed attachment and doping, which makes it difficult to purify silicon thin-film samples.

Figure 11

XRD of electrodeposited Si films using e-GaIn and e-BiIn electrodes.

In the process of electrodeposition of crystalline silicon using ec-LLS, crystalline silicon grows outward in the form of an island after reaching supersaturation on the superficial surface of liquid metal, which means that this is a process of inhomogeneous growth of crystalline thin films. Therefore, the growth speed and effect of crystal silicon at different locations will be different under the influence of various factors, which may lead to the difference in the morphology of the films prepared. Besides, the samples were doped to varying degrees due to the mixing of In and Ga impurities during the deposition process, which in part led to inhomogeneous growth of the crystalline silicon.

Conclusions

In conclusion, we used SiCl4 as a silicon source to prepare a crystalline silicon film using ec-LLS in a mild experimental environment (atmospheric pressure, T < 120 °C) and studied in detail by controlling different electrodeposition temperatures and times, so as to obtain the key deposition conditions of the highest quality polysilicon film.

The voltammograms at different temperatures have a consistent −1.8 V reduction potential. SEM images show that temperature has a great impact on the morphology of silicon thin films. Polysilicon particles were observed in the samples deposited at 80 and 100 °C, which were later proved to be crystalline silicon by XRD and Raman spectroscopy. It was found that the electrodeposition of Si on the Ga surface experienced a rapid reduction, dissolution, and nucleation in a short time by controlling the constant temperature and analyzing the SEM images of products with different deposition times. The size of a part of the crystal grains was stable after reaching about 200 nm, and then, other polysilicon particles gradually grew to form a dense silicon film and completely covered the exposed gallium surface. As the deposition continued, the elemental silicon continuously reduced on the surface and could not be dissolved into the liquid gallium to complete the “ec-LLS” process, resulting in the deposition of an amorphous silicon film on the surface of the crystalline silicon film under low temperature and normal pressure, which reduced the crystallinity of the entire silicon film. Polysilicon thin films with a uniform and dense conical shape are formed in the electrodeposition of e-InGa as the working electrode. As an attractive strategy for preparing crystal semiconductors at low temperatures, liquid electrode-electrodeposited Si still has many problems to be solved. Further work is needed to find and study the liquid electrode which is easier to deposit and apply to promote the development and popularization of this new electrodeposition strategy.