Nonclassical Crystallization in Low-Temperature Deposition of

Article pubs.acs.org/crystal

Nonclassical Crystallization in Low-Temperature Deposition of Crystalline Silicon by Hot-Wire Chemical Vapor Deposition Seung-Wan Yoo,† Ju-Seop Hong,† Sung-Soo Lee,† Chan-Soo Kim,‡ Tae-Sung Kim,§ and Nong-Moon Hwang*,† †

Department of Materials Science & Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-744, Republic of Korea ‡ Marine Energy Convergence & Integration Laboratory, JGRC of Korea Institute of Energy Research, Gimnyeong-ri, Gujwa-eup, Jeju-si, Jeju-do 695-971, Republic of Korea § Department of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyunggi-do 440-746, Republic of Korea S Supporting Information *

ABSTRACT: The effect of process pressure on the deposition behavior of crystalline Si films during hot-wire chemical vapor deposition was approached by nonclassical crystallization, wherein crystals grow not by individual atoms, but by nanoparticles. The size distribution of charged nanoparticles generated in the gas phase was measured using a particle beam mass spectrometer, and the nanoparticles were observed by transmission electron microscopy (TEM) after being captured from the gas phase on a TEM grid membrane. This found that, as the pressure is increased from 0.3 to 2 Torr, it is not only the size and the number of captured nanoparticles that are reduced but also the rate of deposition. An increase in the distance at which nanoparticles were captured from the hot wires under 1.5 Torr also reduced the size and number of particles; however, this tendency decreased markedly at 0.3 Torr. These results imply that the Si-H system should have a retrograde solubility, whose tendency increases with increasing pressure. The pressure dependence of the deposition behavior can be explained by nonclassical crystallization. On the basis of this understanding, the microcrystalline Si could be deposited on a glass substrate at 370 °C with an amorphous incubation layer of ∼10 nm.

1. INTRODUCTION Si thin films are widely used in large-area electronics such as the thin-film transistors (TFTs) of active matrix displays and solar cells. Since both of these applications require a high mobility, there has been intensive research effort made into the deposition of crystalline Si at low temperature.1,2 Of the methods developed, hot-wire chemical vapor deposition (HWCVD) has attracted particular attention of late, but it is still not clearly understood why it allows for crystalline Si to be deposited at substrate temperatures below 500 °C.3 Indeed, the diffusion of Si below 500 °C is so low that the amorphous Si is expected.4 Several growth mechanisms have been proposed to explain the deposition of crystalline Si at low temperature, the most popular among which is related to the atomic hydrogen hypothesis.5−7 Generally, it was known that the hydrogen molecule is decomposed into atomic hydrogen by catalytic reaction with hot wires,8 and so it has been suggested that this atomic hydrogen preferentially etches away weak Si−Si bonds, resulting in the deposition of crystalline Si.9 A drastically different mechanism has been suggested by Hwang et al.,10−13 © 2014 American Chemical Society

wherein charged crystalline Si nanoparticles are formed in the high-temperature region near the hot wires and subsequently contribute to the deposition of crystalline Si at low substrate temperatures. The crystal growth by the building block of nanoparticles is a relatively new concept and called nonclassical crystallization, which has been extensively studied over the past decade.14−20 Nonclassical crystallization is in contrast with the classical crystallization where individual atoms or molecules are the building unit. Recently, the crystal growth by nanoparticles is validated by direct observation of crystal growth by nanoparticles in solution using a liquid-cell transmission electron microscopy (TEM) technique.21−23 According to a newly established concept of nonclassical crystallization, many crystals that were believed to grow by atomic, molecular, or ionic entities may in fact actually grow by nanoparticles. Cölfen and Antonietti24 made an extensive review on nonclassical crystallization and made a meaningful Received: June 13, 2014 Revised: September 24, 2014 Published: October 30, 2014 6239

dx.doi.org/10.1021/cg5008582 | Cryst. Growth Des. 2014, 14, 6239−6247

Crystal Growth & Design

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Figure 1. (a) Raman spectra of Si films with a film thickness of 400 nm deposited at a wire temperature of 1800 °C under the process pressures from 0.3 to 2 Torr with a flowing gas mixture of 4% SiH4−96% H2. (b) The typical deconvolution of the Raman spectrum of the Si film deposited at a pressure of 1.5 Torr. (SiH4). The HWCVD reactor has a diameter of 30 cm with the shower head of a diameter of 5 cm lying above the hot wire. The substrate is placed below the hot wire, and the exhaust outlet is placed below the substrate. The gas flow is downward to the exhaust outlet. With this geometry and gas flow, the loss of the flux to the chamber wall in the HWCVD reactor is not so significant as that in the tube reactor. Other details of the HWCVD reactor have been described elsewhere.54,57 The flow rates of the hydrogen diluted silane (30% SiH4−70% H2) and hydrogen were 18 sccm (standard cubic centimeter per minute) and 117 sccm, respectively; the fraction of H2 in the mixture gas was 0.96. In the HWCVD process, the substrate temperature can be different between the top and bottom because of radiation heating on the top from the hot wires. For this reason, we tried to measure the temperature on the top of the substrate by putting the junction point of bimetal rods of the thermocouple. Each of the bimetal rods was shielded by the dielectric alumina tube except the junction point. The error of the thermocouple in measuring the temperature was less than ∼3 °C. The substrate temperature on the top was maintained at 370 °C, and the distance between the substrate and hot wires was 4 cm. The deposition rate of the Si films was determined from crosssectional analysis of its microstructure using field emission scanning electron microscopy (FESEM, JSM-6330F, JEOL). The crystalline volume fraction of the deposited films was then also examined using a Raman spectrometer (Jobin Yvon, LabRam HR Raman spectrometer). In order to prevent thermal crystallization by an incident laser beam of Raman spectroscopy, the power of the Ar ion laser (514.532 nm) was kept below 0.05 mW with a spot diameter being 1 μm. To confirm whether crystallization by local laser heating should occur or not, a point of the sample deposited at 0.3 Torr in Figure 1 was analyzed by the Raman spectrometer with the power from 0.001 to 0.1 mW (see Figure S1, Supporting Information). The intensity and position of the peak centered at 517 cm−1 that correspond to the nanocrystalline phase were not changed. From this result, it is concluded that crystallization of the sample during the Raman measurement would not occur. Using a PBMS installed on the exhaust line of the HWCVD reactor, the size distribution and polarity of charged Si nanoparticles were measured in real time under the same conditions as the Si film deposition. The PBMS can measure the size distribution of nanoparticles (5−500 nm) sampled from low pressure conditions (>100 mTorr) in spite of the low concentration values of particles (>20/cm−3).56,58,59 During the PBMS measurements, the nanoparticles that were generated in the HWCVD reactor are focused by aerodynamic lenses60,61 into collimated particle beam, and the nanoparticles are classified by an electrostatic energy analyzer. Unlike normal PBMS analysis, where incoming nanoparticles are made to be charged until they became saturated by the electrons emitted from the electron gun, no electron gun was used in this study as the nanoparticles were instead spontaneously charged in the HWCVD reactor. According to the previous results on maximum charges and size of an airborne particle,62 charged nanoparticles smaller than ∼10 nm have a single charge. Considering a soft charging condition in the

remark that, reanalyzing the literature, this mechanism turns out to be a “rediscovery”, as it seems that many important original observations are meanwhile forgotten and hidden in the past literature, as they simply did not comply with the classical crystallization model. In fact, there has been great resistance to this concept of nonclassical crystallization. For example, more than 40 years ago, a concept similar to nonclassical crystallization was suggested by Glasner et al.25−28 during their study of the crystal growth of KBr and KCl in the presence of Pb2+ in aqueous solution. At the time, their suggestion was so revolutionary as to receive severe criticism29 and was subsequently dismissed by the crystal growth community. Sunagawa30,31 made a similar suggestion that the growth unit of synthetic diamond is not an atom but a much larger unit, which has not been taken seriously in the community of diamond synthesis. A similar concept has been more recently suggested in the plasma-enhanced chemical vapor deposition (CVD) process by Cabarrocas,32,33 Vladimirov and Ostrikov,34 and Nunomura et al.35 In this process, the incorporation of crystalline Si nanoparticles into the films produces a so-called polymorphous structure.30,31 Moreover, the building block of nanoparticles can be utilized to synthesize various nanostructures by the plasma-aided nanofabrication technique.34−38 Through an extensive study of the nonclassical crystallization of thin films and nanostructures in nonplasma processes such as thermal and hot-wire CVD (HWCVD), Hwang et al.11,39−52 demonstrated that electrically charged nanoparticles play a critical role in the evolution of dense films, whereas neutral nanoparticles produce porous aggregations.53 The generation of charged nanoparticles (CNPs), which was predicted to form in the gas phase during CVD, was experimentally confirmed not only in the Si CVD process 12,50,52,54 but also in many other CVD processes.43,48,51,55 In this work, we investigate the effect of process pressure on the generation of CNPs and the deposition of crystalline Si films during a HWCVD process. To this end, the size distribution of CNPs generated in the gas phase during deposition of Si films was evaluated through particle beam mass spectrometry (PBMS),56 and Si nanoparticles were captured on the membrane of a transmission electron microscopy (TEM) grid and observed by TEM.

2. EXPERIMENTAL SECTION Si films were deposited by HWCVD onto a glass substrate (Eagle 2000XG) under pressures ranging from 0.3 to 2 Torr for 20 min using a wire temperature of 1800 °C and hydrogen (H2) diluted silane 6240



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HWCVD process,53 the charge state of nanoparticles is assumed to be ±1. This assumption was verified in the previous report.57 The incoming nanoparticle flux was formed by the pressure difference between the HWCVD reactor and the PBMS, from which it was found that a reactor pressure above 1 Torr created sufficient nanoparticle flux for PBMS measurement. Si nanoparticles in the gas phase were captured for 10 s on a TEM grid membrane (SiO film, 300 mesh copper grids, TED PELLA, INC.) under the same conditions as the Si film deposition. A circular stainless-steel shutter (Ø>60mm, 3 mm in height) was installed 3 mm away from the TEM grid to control the capture time of gas-phase Si nuclei. To investigate the initial microstructure of the Si films, crosssectional TEM specimens of the Si films were also prepared using a focused ion beam (FIB, SMI3050SE, SII Nanotechnology).

the experiment. In the previous reports, most Si HWCVD processes were done at pressures less than 0.5 Torr. This is supported by the work of Pant et al.,67 in which it was reported that the deposition rate of Si films increases with pressure from 25 mTorr to 0.25 Torr, but decreases with increasing pressure from 0.25 to 1 Torr. It is generally expected that a higher process pressure would produce a higher deposition rate due to the increase in the equilibrium amount of precipitation; however, Figure 2 shows quite the opposite to be true, with a decrease in deposition rate when increasing the pressure between 0.3 and 2 Torr. This implies that there must be a decrease in the equilibrium amount of Si precipitated from the gas phase, which needs to be considered in relation to the PBMS data in Figure 3. 3.2. Charged Nanoparticles in the Gas Phase. Figure 3 shows the PBMS data for the size distribution and polarity of charged Si nanoparticles generated in the gas phase of the HWCVD reactor with a 4% SiH4−96% H2 gas mixture and a wire temperature of 1800 °C at 1.0, 1.5, and 2.0 Torr. The peak of this size distribution is at 9−12 nm at which the concentration values of both positively and negatively charged nanoparticles are 106−107 /cm3. It is noted that the extracting sampling of nanoparticles for mass spectroscopy can have artifacts such as particle loss by diffusion in the transport line. The PBMS measurements also have such a possibility. The aerodynamic lenses that deliver nanoparticles from the CVD reactor to PBMS by focusing nanoparticles can transport nearly 100% of nanoparticles between 0.02 and 0.3 μm.58 However, the transport efficiency decreased rapidly as the size of particles decreased below 20 nm. Therefore, it was studied to design aerodynamic lenses affordable to focus the small-size nanoparticles.68−70 Wang et al. designed the single lens with the particle size limit of 5 nm, which is a minimum particle size that can be focused by aerodynamic lens, using helium as carrier gas.68 According to the result, lighter carrier gas is preferred to focus smaller particles and multiple lenses can focus particles smaller than the particle size limit of single lens. Considering the experimental conditions of this study such as using hydrogen as carrier gas and the aerodynamic lenses designed with multiple lenses, it is supposed that the aerodynamic lenses of the PBMS instrument have the minimum measurable particle size smaller than that of nanoparticles, as shown in Figure 3, although the transport efficiency of the aerodynamic lenses is not calculated yet by numerical simulations. Moreover, in Figure 3, the peak diameter of both positively and negatively charged nanoparticles decreases with increasing pressure. It is reported that increasing input pressure for aerodynamic lenses can increase the minimum measurable particle size, leading to classification effect.68 However, considering the result that the size and the number of nanoparticles decreased with increasing pressure, the classification effect seemed to be not so pronounced. The generation of CNPs during HWCVD shown in Figure 3 is consistent with previous observations that the generation of gas-phase CNPs is typical of many CVD processes.47,48,50,51 In addition to the PBMS data in Figure 3, the size of the Si gas-phase nuclei was also determined by capturing them on the TEM grid membrane in the HWCVD reactor and observing them ex situ by TEM. Figure 4 shows the TEM images of Si nanoparticles captured for 10 s at different pressures under the same deposition conditions as in Figure 2. Numerous Si nanoparticles were observed on the membrane under all conditions, with their size and number decreasing with

3. RESULTS AND DISCUSSION 3.1. Pressure Dependence of the Deposition Behavior of Si Films. Figure 1 shows the Raman spectra of Si films deposited at a wire temperature of 1800 °C at various reactor pressures from 0.3 to 2 Torr. The film thickness was fixed at 400 nm by adjusting the deposition time. To estimate the crystalline volume fraction of the films, these Raman spectra were deconvoluted into a nanocrystalline peak (517 cm−1), and an amorphous peak (480 cm−1), and an intermediate peak (507 cm−1) following the scheme of the previous reports.63,64 For example, the typical deconvolution of the Raman spectrum of the Si film deposited at a pressure of 1.5 Torr is shown in Figure 1b. The crystalline volume fraction, Xc, was determined by the equation, Xc = (Inc + Im)/(Inc + Im + Ia). Inc, Im, and Ia are integrated intensities of the nanocrystalline, intermediate, and amorphous peaks, respectively. The calculated crystalline volume fractions were 0.37 at 0.3 Torr, 0.52 at 0.5 Torr, and 0.59 at 1, 1.5, and 2 Torr. The grazing incidence X-ray diffraction measurements were also carried out to support the Raman data in Figure 1 (see Figure S2, Supporting Information). This dependence of the crystalline fraction of deposited Si films on the reactor pressure is consistent with the previous report.63 Figure 2 shows the deposition rate of Si films over 20 min at the same wire temperature and pressure as those in Figure 1.

Figure 2. Deposition rate of Si films deposited for 20 min at a wire temperature of 1800 °C with various process pressures from 0.3 to 2 Torr with a flowing gas mixture of 4% SiH4−96% H2.

The deposition rate of Si films decreased with increasing pressure, giving values of 3, 2.3, 1.5, 1.3, and 1 nm/s at 0.3, 0.5, 1, 1.5, and 2 Torr, respectively. This dependence of deposition rate on pressure is different from previous reports, wherein the deposition rate was found to increase with process pressure during the deposition of Si films by HWCVD.63,65,66 This is due to the difference in the range of the process pressure used in 6241



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Figure 3. Size distribution of positively (a) and negatively (b) charged Si nanoparticles at a wire temperature of 1800 °C with various process pressures.

Figure 4. TEM images of Si nanoparticles captured for 10 s on the TEM grid membrane at the process pressures of (a) 0.3, (b) 0.5, (c) 1, (d) 1.5, and (e) 2 Torr, respectively, at the same condition as in Figures 1−3.

grid being as low as ∼370 °C. The crystalline lattices shown in Figure 5 have important implications. Since the diffusion of Si atoms would be so low at ∼370 °C, it is expected that not crystalline but amorphous Si should be deposited if the nanoparticles in Figure 4 are formed by the atomic diffusion on the surface of the TEM grid membrane. Considering this, it is far more likely that the crystalline Si nanoparticles are instead formed in the high-temperature region near the hot wires, and subsequently land on the TEM grid over a capture time of 10 s. If this capture time is extended beyond the time required to cover the entire surface with Si nanoparticles, then a Si film with a microcrystalline structure will be formed. It should be noted that the low-temperature deposition of crystalline Si films by HWCVD or PECVD has been a quite puzzling phenomenon because of the low diffusivity of Si. However, it can be explained by assuming that these crystalline Si films grow by nonclassical crystallization. That is, they are deposited by crystalline Si nanoparticles formed in the high-temperature region of the gas phase. 3.4. Si Nanocrystallites Embedded in an Amorphous Matrix. The deposition of Si nanocrystallites embedded in the amorphous matrix, which is shown in Figure 5 and also in Figure 9, is puzzling from the viewpoint of the mechanism of

increasing process pressure. Note that this is consistent with the PBMS data in Figure 3 and the dependence of pressure on the deposition rate demonstrated in Figure 2. The Si nanoparticles captured at a pressure of 0.3 Torr (Figure 4a) were the greatest in terms of both size and quantity within the pressure range examined, and this corresponds to the highest film growth rate in Figure 2. In contrast, the size and quantity of nanoparticles were the least at 2 Torr (Figure 4e), which corresponds to the lowest film growth rate in Figure 2. It should be noted that the peak diameters of the CNPs at 1.0, 1.5, and 2.0 Torr in Figure 3 are very similar to the most frequently observed nanoparticles captured at the corresponding pressure, as shown in Figure 4c− e, respectively. This dependence of size and quantity on the process pressure is in agreement with the dependence of the growth rate on the pressure demonstrated in Figure 2. It, therefore, appears that the decrease in deposition rate with pressure evident in Figure 2 comes from the decrease in the size and quantity of Si nanoparticles with pressure shown in Figures 3 and 4. 3.3. Crystallinity of Si Films and Nanoparticles. The high-resolution TEM images in Figure 5, which are magnified views of those in Figure 4, show that all of the Si nanoparticles have a crystalline lattice, despite the temperature of the TEM 6242



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Figure 5. Magnified TEM images of Si nanoparticles in Figure 4.

Figure 6. TEM images of Si nanoparticles captured for 10 s on the membrane of TEM grids, which were placed 2 cm (a, d), 3 cm (b, e), and 4 cm (c, f) away from the hot wires, at the process pressures of 0.3 Torr (a−c) and 1.5 Torr (d−f), at the wire temperature of 1800 °C.

classical crystal growth because of the low diffusivity of Si at low temperature below 500 °C. In order for nanometer-sized crystallites to be formed on the growing amorphous surface by atomic growth at low temperature, three-dimensional nucleation or secondary nucleation should take place. In the classical crystal growth theory, where the growth unit is an atom or molecule, the growth mechanism depends on the structure of the surface, which has a structural transition between rough and smooth interfaces that is dominant, respectively, in entropy and enthalpy. The rough interface, which is atomically disordered and has numerous kink sites, has no barrier for atomic attachment. In contrast, the smooth interface, which is atomically ordered, has an appreciable barrier for atomic attachment and needs ledge-generating sources such as screw

dislocation for atomic growth. Considering that the amorphous matrix has a rough interface, the three-dimensional or secondary nucleation cannot occur on the amorphous surface because the supersaturation cannot build up. This means that nanocrystallites cannot form on the rough interface of the amorphous structure by atomic growth at low temperature. It should be noted that the three-dimensional or secondary nucleation can take place only on the smooth interface.53 Even if crystalline nuclei are artificially placed on the amorphous surface, they cannot continue to grow but will be enclosed by the amorphous phase because the growth rate of the rough interface of the amorphous phase is much faster than that of the smooth interface of the crystalline phase. Considering all of these, the Si nanocrystallites embedded in the amorphous 6243



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Figure 7. High-resolution TEM images of Si nanoparticles captured for 10 s on the membrane of TEM grids, which were placed 2 cm (a) and 4 cm (b) away from the hot wires, at the process pressure of 1.5 Torr at the wire temperature of 1800 °C.

the hot wire under 0.3 and 1.5 Torr at a wire temperature of 1800 °C. In Figure 6a−c, almost the entire surface is covered with nanoparticles, forming a continuous film. However, in Figure 6d−f, where the reactor pressure was increased from 0.3 to 1.5 Torr, a continuous film is not formed and instead the nanoparticles become isolated when the distance is increased to more than 3 cm. The size and quantity of nanoparticles produced at 1.5 Torr also decreases with increasing distance (Figure 6d−f), with the decrease in quantity being much more pronounced than that at 0.3 Torr. It can be argued that the increasing amount of Si deposition with decreasing distance in Figure 6 came from the temperature increase because the substrate temperature increased from ∼370 to ∼510 °C with decreasing distance of the substrate from 4 to 2 cm away from the hot wire. In order to address this argument, let us examine the high-resolution TEM images of Figure 6d,f to check whether the particles are amorphous or crystalline. Both TEM images in Figure 7a,b, which are magnified views of Figure 6d,f, respectively, show that the particles are not amorphous, but crystalline. The size of nanocrystallites deposited at 1.5 Torr on the substrate placed 2 cm away from the hot wire (Figure 7a) is larger than that of the nanocrystallites deposited 4 cm away (Figure 7b). Considering that the diffusivity of Si is so low at ∼510 °C that atomic diffusion hardly occurs, the crystalline nanoparticles in Figure 7a cannot have formed by atomic diffusion but must have formed by nonclassical crystallization by the building block of the charged crystalline nanoparticles formed in the gas phase. Therefore, the increasing amount of Si deposition with decreasing distance in Figure 6 should result from the increasing amount of nanoparticles formed in the gas phase. It is reported that a considerable amount of nanometer-sized particles is lost to the chamber wall by Brownian diffusion.76 Furthermore, if an electric field exists near the surface of the wall, charged nanoparticles are shown to be lost to the wall much more than neutral ones. In this work, the electric potential gradient exists between the hot wire and the chamber wall. Because one end of the hot wire connected with the DC power supply has a potential of +18 V to maintain the wire temperature of 1800 °C, the negatively charged nanoparticles tend to be attracted to and the positively charged nanoparticles tend to be repelled from the hot wire. This aspect was investigated in detail by Park et al.13,77 In addition, Si is often found to deposit on the chamber wall, although the amount is much less than that deposited in the thermal CVD. One

matrix cannot be formed by atomic growth but must have been formed by the incorporation of the crystalline nanoparticles formed in the gas phase. Therefore, the Si nanocrystallites embedded in the amorphous matrix provide an indirect evidence for the deposition of nanoparticles formed in the gas phase. However, it is commonly observed that nanocrystallites are embedded in an amorphous matrix not only in the hot wire but also plasma enhanced CVD.71−73 Cheng et al.72 deposited SiC films that contain crystalline Si quantum dots embedded in an amorphous SiC matrix by increasing hydrogen dilution in inductively coupled plasma chemical vapor deposition (ICP-CVD). In that study, the key factor for the generation of Si crystallites embedded in the amorphous SiC matrix was hydrogen dilution. In nonthermal plasmas, the temperature of the growing surface of film can be higher than expected due to surface heating by the ion bombardment or recombination.74 In that case, the elevated surface temperature by surface heating will enhance the mobility of atoms and increase the deposition rate. However, the result of Cheng et al. showed that the deposition rate of films decreased remarkably at the condition in which Si crystallites were deposited. Considering this, the formation of the Si crystallites embedded in the amorphous SiC matrix is not easy to explain. In our opinion, considering that nanometer-sized crystallites embedded in an amorphous matrix cannot be formed at low temperature, the Si crystallites embedded in the amorphous SiC matrix should come from the gas phase after being generated in the plasma. It is also commonly observed that nanostructures of various dimensionalities such as onedimensional nanowires, two-dimensional sheets, and threedimensional nanocrystalline films as well as zero-dimensional structure such as nanocrystallites or quantum dots in the hot wire and plasma enhanced CVD process. In fabricating such nanostructures of various dimensionalities, neutral nanoparticles are unfavorable as a building block because neutral nanoparticles do not undergo self-assembly, whereas charged nanoparticles undergo self-assembly by electrostatic interaction and play an important role as the building block of the various nanostructures.36,53,75 3.5. Si Nanoparticles Captured at Different Distances from the Hot Wire. As the distance from the hot wires decreases, the temperature of the gas phase increases. Thus, in order to examine the size and quantity of gas-phase nuclei formed at different distances, nanoparticles were captured for 10 s on a TEM grid membrane placed 2, 3, and 4 cm away from 6244



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distance in the same manner as is shown in Figure 6d−f. This means that nanoparticles formed near the hot wires are etched as they approach the substrate; and under these conditions, additional nucleation would not occur in the low-temperature region. It should also be noted that, if Si nucleation occurred in the low temperature, then the nucleus would be an amorphous phase. The suppression of nucleation in the low-temperature region would, therefore, increase the percentage of crystalline nanoparticles that are formed in the high-temperature region near the hot wires. If the percentage of these crystalline nanoparticles is high, then it would improve the crystallinity of films deposited even at low temperature. Moreover, since small CNPs have a higher tendency of epitaxial deposition than large ones, the size of the crystalline grains in films would increase under the condition of the high retrograde solubility. 3.7. Deposition of Crystalline Si Films Using Retrograde Solubility. Given the observations in section 3.6, it is apparent that the retrograde solubility could be used to increase the crystallinity of Si films. For example, using the concept of gas-phase nucleation and the fact that the Si-Cl-H system has retrograde solubility, Chung et al.79 were able to remarkably increase the crystalline fraction during low-temperature HWCVD of Si. Lee et al. could deposit p-type Si films having a crystalline volume fraction as high as 0.74.80 Through HCl injection, the nucleation in the low-temperature region was suppressed and the highly crystalline films could be deposited by smaller crystalline nanoparticles. Chung et al.78 also successfully deposited a crystalline Si film directly on a glass substrate at 200 °C without an amorphous incubation layer. The evolution of the bulk crystalline phase by the deposition of crystalline nanoparticles indicates that HWCVD of Si films occurs by nonclassical crystallization, which had been proposed in the theory of charged nanoparticles.46,53 Nonclassical crystallization implies that the charged nanoparticles should have a liquid-like property. In relation to this aspect, Youn et al.81 suggested that the presence of charge in nanoparticles enhances atomic diffusion and makes the nanoparticles liquidlike, inducing the epitaxial recrystallization of charged nanoparticles upon landing on the growing surface. Considering that an increase in process pressure increases the tendency for retrograde solubility in the Si-H system, it is expected that the crystallinity of the Si films would also increase with increasing pressure. This expectation agrees well with the Raman results in Figure 1, which indeed show that the crystallinity of the Si films increases with increasing pressure. It is further expected that the formation of an amorphous incubation layer may be reduced in a film deposited on a glass substrate at 1.5 Torr under the conditions used in this. To confirm this, the cross section of the Si film deposited at 1.5 Torr was observed by TEM, as shown in Figure 9a. Note that images (b)−(d) in Figure 9 correspond to magnified images of the boxed area in Figure 9a. In Figure 9b, we can see that the thickness of the amorphous incubation layer is about 10 nm, which is 5 times less than the thickness of the amorphous incubation layer that is typically deposited under a pressure of 75 mTorr.82,83 Figure 9c,d shows the nanocrystalline phase that is deposited after this initial amorphous layer. Although this study clearly shows that charged nanoparticles are generated in the gas phase during HWCVD of Si, it has not yet been determined just how exactly they are formed or how they are charged. With regards to the charging mechanism, one possibility is that clusters in the form of embryos or nuclei are initially formed, and then undergo surface ionization on the hot

possible source of Si is the decomposition of the unreacted SiH4 precursor on the wall. This possibility is low because the chamber wall is water-cooled and its temperature is too low for SiH4 to decompose. Therefore, it is more probable that some charged Si nanoparticles formed near the hot wire should be lost to the chamber wall, which is electrically grounded. The result of Figure 6 showing that the growth rate is decreased with increasing distance might be attributed to increasing loss of nanoparticles with increasing distance. It should be noted that, although the flux of nanoparticles might decrease with increasing distance, but the size of nanoparticles should not decrease with increasing distance if the driving force is for deposition, due to growth by atomic attachment or collision between nanoparticles. However, Figure 6d−f clearly shows that the size of nanoparticles decreased with increasing distance, which indicates that the nanoparticles underwent etching. 3.6. Retrograde Solubility in the Si-H System. The TEM results obtained indicate that the larger nanoparticles produced at a distance of 2 cm (Figure 6d) underwent etching and became smaller at 4 cm (Figure 6f), which is equivalent to saying that the equilibrium amount of precipitation decreases with decreasing temperature. This again means that the Si-H system has a retrograde solubility under the experimental conditions used. Furthermore, since the decreasing tendency of the nanoparticle size is higher at 1.5 Torr than at 0.3 Torr, the tendency of the retrograde solubility appears to increase with pressure, at least within the range tested. This then means that the decrease in the number of particles with distance shown in Figure 6d−f may also be related to the retrograde solubility and etching effect with increasing distance away from the hot wires. Thermodynamic calculations of the Si-H system using Thermo-Calc,78 which were based on the Scientific Group Thermodata Europe (SGTE) substance database, do not show a retrograde solubility. Instead, these calculations show that the temperature dependence of the equilibrium amount of precipitation follows curve P1 in Figure 8. Considering that

Figure 8. Schematic of an equilibrium amount of Si precipitation in the gas phase depending on the process pressure and temperature.

Figures 2, 3, 4, and 6 all indicate that the Si-H system has a retrograde solubility, whose tendency increases with increasing pressure, we suggest that the temperature dependence of the equilibrium amount of Si precipitation instead follows the curves P2 and P3 in Figure 8. Since the tendency of the retrograde solubility increases with pressure, it is expected that the pressure increases in the order of P1, P2, and P3. If gas-phase nucleation occurs in a system with retrograde solubility, then the size of gas-phase nuclei should decrease with 6245



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Figure 1 was analyzed by the Raman spectrometer with the power from 0.001 to 0.1 mW. For supporting to understand the structure of Si films in Raman data in Figure 1, we provide the grazing incidence X-ray diffraction (GI-XRD) data of the same samples in Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-2-880-8922. Fax: +822-883-8197 (N.-M.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Brain Korea (BK21) Program, Republic of Korea; and the Global Frontier R&D Program (2013-073298) of the Center for Hybrid Interface Materials (HIM), founded by the Ministry of Science, ICT & Future Planning.

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Figure 9. TEM images of a Si film with a film thickness of 400 nm prepared at the process pressure of 1.5 Torr at the same condition as in Figure 2. High-resolution images of (b)−(d) are for the locations of the film designated in (a).

wires. If such clusters were to undergo negative surface ionization, they would become negatively charged, and similarly, if they undergo positive surface ionization, they would become positively charged. Another possibility is that electrons are thermally emitted from the hot wires and that these electrons then induce nucleation through a mechanism similar to ion-induced nucleation84 that would eventually result in negatively charged nanoparticles being formed. As to the possible origin of positive charges, Peineke et al.85,86 suggested that these can be generated by the positive surface ionization of impurity alkali atoms on the hot wires. Once such positive ions are generated, ion-induced nucleation would proceed to produce positively charged nanoparticles. The third possibility is that electrons thermally emitted from the hot wires or ion existing in the gas phase are attached to nanoparticles.87 In either instance, it is clear that further theoretical and experimental study is still needed to understand clearly the charging mechanism that occurs in the HWCVD process.

4. CONCLUSION The deposition rate of Si films has been shown to decrease with an increase in process pressure from 0.3 to 2 Torr during the HWCVD of Si. The deposition rate of Si films was closely related to the size and number of charged nanoparticles generated in the gas phase. This dependence of the film growth rate and the size of the nanoparticles on pressure can be explained by nonclassical crystallization based on the retrograde solubility of the Si-H system. The increased crystallinity of the films with increasing pressure, as revealed by Raman analysis and cross-sectional TEM observation, can also be explained by nonclassical crystallization based on the retrograde solubility, the tendency of which increases with increasing pressure.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

In order to confirm whether crystallization by local laser heating occurs or not, the sample deposited at 0.3 Torr in 6246



Article

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