Effect of the RF Power of PECVD on the Crystalline Fractions of Microcrystalline Silicon ( µ c-Si:H) Films and Their Structural, Optical, and Electronic Properties

: In this work, we report on the deposition of microcrystalline silicon ( µ c-Si:H) ﬁlms produced from silane (SiH 4 ), hydrogen (H 2 ), and argon (Ar) mixtures using the plasma-enhanced chemical vapor deposition (PECVD) technique at 200 ◦ C. Particularly, we studied the effect of RF power on the crystalline fraction (X C ) of the deposited ﬁlms, and we have correlated the X C with their optical, electrical, and structural characteristics. Different types of characterization were performed in the µ c-Si:H ﬁlm series. We used several techniques, such as Raman scattering spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), ﬁeld emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM), among others. Our results show that RF power had a strong effect on the X C of the ﬁlms, and there is an optimal value for producing ﬁlms with the largest X C .


Introduction
Hydrogenated amorphous silicon (a-Si:H) is a mature material that has been extensively studied because of its compatibility with silicon complementary metal-oxidesemiconductor (CMOS) technology, its low substrate temperature deposition (≤200 • C) via the plasma-enhanced chemical vapor deposition (PECVD) technique, the possibility of doping it (n-or p-type), and the fact that it can be deposited on large substrate areas (>1 m 2 ), such as metal foils, glasses, and flexible substrates [1][2][3].In photovoltaics and microelectronics, it has been extensively used for the development of large-area thin-film solar cells [4] and thin-film transistors (TFTs) [5,6].
However, a-Si:H has several drawbacks because of the short-range order of the atoms that make it up; this order is mainly related to the length of covalent bonds and the bond angle, which is only maintained between the nearest neighbor atoms [7].As a result of this, a-Si:H has poor transport properties, low electron mobility (µ e ), a large density of dangling bonds (defects), and degradation from light exposure (the Staebler-Wronski effect) [8,9].
Polycrystalline silicon (poly-Si) has a long-range order since it is composed of crystalline grains of sizes as large as 1 µm [10].This results in better transport properties, such as larger µ e and better stability; however, it requires deposition temperatures of about 600 • C using the low-pressure chemical vapor deposition (LPCVD) technique.
Microcrystalline silicon (µc-Si:H) is a semiconductor material also produced by PECVD at low temperatures (≤200 • C) that exhibits different properties from those of a-Si:H and poly-Si.µc-Si:H films grow in crystalline columns of different orientations, separated by boundaries composed of amorphous silicon.The crystals in the columns have sizes in the order of hundreds of nanometers, and the crystalline fraction (X C ) of the films can be tailored by the deposition conditions.Compared with a-Si:H, µc-Si:H has higher room temperature conductivity, higher µ e , is more stable against radiation, and the bandgap (E g ) and absorption coefficient (α) are different [11,12].When compared with poly-Si, µc-Si:H has lower crystal sizes, a lower X C , and lower µ e .Nevertheless, its lower deposition temperature allows for its integration on glass and flexible substrates, which is the main advantage over poly-Si.
For higher-performance devices, such as tandem solar cells (a-Si:H/µc-Si:H), it is necessary to have both large absorption in the ultraviolet/visible part of the electromagnetic spectrum, provided by an a-Si:H solar cell, and large absorption in the infrared part of the spectrum, provided by a µc-Si:H solar cell [13,14].The latter is achieved with a large X C in the µc-Si:H film.For TFTs, high µ e in the active layer is required to create faster devices, which is also achieved with a large X C in the µc-Si:H film [15].
Therefore, there is interest in controlling and optimizing the X C of µc-Si:H films using PECVD deposition conditions.In this work, we studied how RF power affects a film's X C using silane (SiH 4 ), hydrogen (H 2 ), and argon (Ar) mixtures.RF power is an important parameter to control since it affects the X C of films; in addition, large RF values produce excessive powder in plasma [16], which can result in damage to the vacuum pumps of the deposition system.Therefore, producing µc-Si:H films with large X C values with relatively low RF power is an issue.
Several techniques were employed to characterize the structural, optical, and electrical properties of the films.We used Fourier transform infrared (FTIR) spectroscopy, Raman scattering spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM).Moreover, several parameters were measured and calculated, such as the photoconductivity (σ ph ), activation energy (E A ), absorption coefficient (α), and bandgap (E g ).
Our results show that RF power had a strong effect on the X C of the films, where the use of an optimized value results in films with a large X C , but this also affects the electrical and optical parameters.From these results, we were able to select optimized films for applications in both thin-film transistors (TFTs) and thin-film solar cells where µc-Si:H films with a large X C are required.

Materials and Methods
A series of µc-Si:H films were deposited using a capacitively coupled PECVD reactor (Cluster Tool, MVSystems LLC, Arvada, CO, USA) at a radio frequency (RF) of 13.56 MHz with a chamber pressure of 1500 mTorr at a substrate temperature of 200 • C and a deposition time of 30 min.The gas mixture was composed of SiH 4 (10% in H 2 ), H 2 , and Ar, and the RF power levels were 20, 25, 30, 35, 40, and 45 W. Table 1 shows the deposition conditions of the films, including the RF power density.For the deposition of microcrystalline semiconductors, a high H 2 dilution, a high chamber pressure, and optimized RF power are required; therefore, the latter is the parameter that was varied in the series.The films were characterized structurally, optically, and electrically, and we employed several techniques for the characterization.Various substrates with a 1-square-inch area were used for the deposition of each film since they are necessary for different types of characterization.The substrates used were 1.1 mm thick corning glass 2934 (Corning, New York, NY, USA), 0.7 mm thick corning glass 1737 (Precision Glass & Optics, PG&O, Santa Ana, CA, USA), a 300 µm thick p-type <100> crystalline silicon wafer (Addison Engineering, San Jose, CA, USA), and 1.1 mm thick corning glass 2934 (Corning, New York, NY, USA) with 0.2 µm thick aluminum (Al) electrodes.For the fabrication of the electrodes, an Al metal layer with a thickness of 0.2 µm was evaporated via electron beam evaporation (e-beam evaporator, Oerlikon Balzers, Balzers Liechtenstein), and a photolithographic step was performed using a mask with patterns of 7 × 2 mm 2 , separated by 2 mm, using a positive photoresist (AZ 1512 HS, MicroChemicals GmbH, Ulm, Germany).A mask aligner was used for UV exposure (Model 800, OAI, San Jose, CA, USA), and wet etching was employed to define the electrodes.Finally, the photoresist was removed with acetone in an ultrasonic bath for ten minutes.
Note that the films were deposited in the four substrates at the same time (for each RF power); therefore, a standard deviation of the films' characteristics cannot be calculated at this time, and future work will be necessary.
The films deposited on corning glass 2947 were used for the following characterization.The films' thicknesses were measured with a profilometer (Dektak XT, Bruker, MA, USA).For the analysis of the films' roughness, we used an AFM (EasyScan, Nanosurf, Liestal, Switzerland).To determine the X C of the films, Raman spectroscopy was performed using a Raman spectrometer (LabRam HR 800, Horiba Jobin-Yvon, CA, USA) with a 632.8 nm laser.
The surface structural analysis of the selected film deposited on corning glass 2947 was performed using a high-resolution FE-SEM (JSM-7800F Prime, Jeol, Tokyo, Japan) with a Schottky field emission gun and an accelerating voltage range in the range of 10 V-30 kV.The transversal structural analysis of the same film was performed using a TEM (Talos F200X, FEI, OR, USA) with a 200 kV field emission gun (FEG) emitter.
FTIR spectroscopy characterization was performed on the films deposited on p-type <100> silicon wafers using an infrared spectrometer (Vector 22, Bruker, Massachusetts, USA).We also used a clean silicon wafer without native silicon oxide, for reference.
The absorption spectra on films deposited on corning glass 1737 substrates were calculated through transmittance measurements using a spectrophotometer (Lambda 3 Series, Perkin-Elmer, MA, USA) in a wavelength range of 400-900 nm, and the Tauc method was used for the E g determination [17].The selection of corning glass 1737 was due to its large optical transmission (up to 90%) in a range of 400-2500 nm.
The films deposited on corning glass 2947 substrates with aluminum electrodes were used for electrical characterization.The σ ph of the films was obtained using a solar simulator (Oriel Sol 2A, Newport, CA, USA) with AM 1.5 standard radiation, providing a power intensity of 100 mW/cm 2 .The films' temperature dependence of conductivity (σ(T)) was measured in a vacuum at a pressure of 50 mTorr in a cryostat (Janis Research, MA, USA) with a substrate temperature range of 300-400 • K, and the E A of the films was determined.

Results
As mentioned previously, the µc-Si:H films were characterized structurally, optically, and electrically; therefore, this section is divided into different subsections containing the different types of analysis.

Thickness Measurements
A photolithographic step was performed on the µc-Si:H film series deposited on corning glass 2947 substrates using a mask aligner for UV exposure (Model 800, OAI, California, USA).To protect part of the films, we used a positive photoresist (AZ 1512 HS, from MicroChemicals GmbH, Ulm, Germany).The uncovered part was etched with carbon tetrafluoride (CF 4 ) plasma using a Reactive Ion Etching (RIE) system (micro RIE, Technics).After removing the photoresist with acetone in an ultrasonic bath for ten minutes, the films' thicknesses were measured using the profilometer described in the previous section.Table 2 shows the thicknesses of the different films deposited for 30 min, and the deposition rate (V d ) was calculated.We can see that the V d is in a range of 0.80 to 1.09 Å/s.The AFM characterization was performed on the µc-Si:H films deposited on corning glass 2947 substrates in an area of 4 × 4 µm 2 .Figure 1 shows 3D images of the µc-Si:H films deposited using different RF powers.From these measurements, we extracted the average roughness (S a ) and the root-mean-square (RMS) roughness values, which are included in Table 2.

Raman Spectroscopy
As mentioned previously, Raman spectroscopy was used for the c-Si:H films' characterization; Figure 2a shows the Raman spectra of the films deposited with different RF power levels, where we can observe a pronounced peak at 520 cm −1 in the spectra.The XC of the films was calculated using the following equation: We employed the Scanning Probe Image Processor (SPIP) software as described in [18] to calculate the average roughness (S a ), which is defined by Equation (1): where M is the data number in the X point, N is the data number in the Y point of the rectangular image, and µ is the mean height defined by Equation (2): It is clear that there was an increase in film roughness upon increasing the RF power to deposit the films.It has been reported that larger roughness values in µc-Si:H films are associated with large crystal sizes [11].However, in our characterization, we did not observe a relationship between surface roughness and the X C .A possible explanation is that, with Raman spectroscopy, we are able to calculate the crystalline fraction (Xc), but we do not gain information on crystal sizes.For determining the sizes of crystals, other techniques can be used, such as X-ray diffraction and the Scherrer equation [19,20].

Raman Spectroscopy
As mentioned previously, Raman spectroscopy was used for the µc-Si:H films' characterization; Figure 2a shows the Raman spectra of the films deposited with different RF power levels, where we can observe a pronounced peak at 520 cm −1 in the spectra.The X C of the films was calculated using the following equation: where I C , I B , and I A are the integrated intensities of peaks associated with the crystalline phase at 520 cm −1 , an intermediate phase (correlated with small crystal sizes) at 500-514 cm −1 , and the amorphous phase at 480 cm −1 , respectively [21]. Figure 2b shows the spectrum of a µc-Si:H film with deconvolution in three Gaussian peaks located at 520 cm −1 , 508 cm −1 , and 480 cm −1 .We determined the X C of the µc-Si:H films using Equation (3).Table 2 contains the X C (%) values calculated, where we observed that the X C was in a range of 64-75%, and the film deposited with an RF of 25 W (86.5 mW/cm 2 ) had the largest X C .Si:H with a low XC, while on the other hand, high RF power can promote the H2 etching process, also affecting the XC., where the deconvolution of the spectrum in three Gaussian peaks at 520 cm -1 , 508 cm -1 , and 480 cm -1 is shown.

Field Emission Scanning Electron Microscopy
We selected the film with the largest XC (µc-Si:H-2) for scanning electron microscopy analysis (FE-SEM).Figure 3a shows the surface of the c-Si:H film obtained using FE-SEM with a magnification of 50,000× and an accelerating voltage of 3 kV for the electron beam; we can see that the film surface is composed of grains.Figure 3b shows an amplified image with a magnification of 200,000× and an accelerating voltage of 3 kV, and we can see that the silicon grains are approximately 30-80 nm.As mentioned in Section 2, for the deposition of µc-Si:H, three conditions are required, which are the high dilution of SiH 4 in H 2 , higher chamber pressure, and higher RF power levels than those used for the deposition of a-Si:H.During the deposition of the µc-Si:H films, a H 2 etching process can occur [22,23].From our results, we can see that the etching process is sensitive to the RF power; on the one hand, low RF power produces µc-Si:H with a low X C , while on the other hand, high RF power can promote the H 2 etching process, also affecting the X C .

Field Emission Scanning Electron Microscopy
We selected the film with the largest X C (µc-Si:H-2) for scanning electron microscopy analysis (FE-SEM).Figure 3a shows the surface of the µc-Si:H film obtained using FE-SEM with a magnification of 50,000× and an accelerating voltage of 3 kV for the electron beam; we can see that the film surface is composed of grains.Figure 3b shows an amplified image with a magnification of 200,000× and an accelerating voltage of 3 kV, and we can see that the silicon grains are approximately 30-80 nm., where the deconvolution of the spectrum in three Gaussian peaks at 520 cm -1 , 508 cm -1 , and 480 cm -1 is shown.

Field Emission Scanning Electron Microscopy
We selected the film with the largest XC (µc-Si:H-2) for scanning electron microscopy analysis (FE-SEM).Figure 3a shows the surface of the c-Si:H film obtained using FE-SEM with a magnification of 50,000× and an accelerating voltage of 3 kV for the electron beam; we can see that the film surface is composed of grains.Figure 3b shows an amplified image with a magnification of 200,000× and an accelerating voltage of 3 kV, and we can see that the silicon grains are approximately 30-80 nm.One important aspect is that the film was not prepared for observation; it was analyzed as it was deposited.The quality of the image is an indication that the film is electrically conductive, and this is of importance for the development of TFTs.In Section 3.3, the electrical characterization of the films is reported.One important aspect is that the film was not prepared for observation; it was analyzed as it was deposited.The quality of the image is an indication that the film is electrically conductive, and this is of importance for the development of TFTs.In Section 3.3, the electrical characterization of the films is reported.

Transmission Electron Microscopy
Even though grains can be observed on the surface of the µc-Si:H film shown in Figure 3, the bulk of the microcrystalline semiconductors was composed of crystalline columns [11,24].Therefore, in order to analyze the transversal structure of the µc-Si:H film, we used TEM characterization, as described in the previous section.Figure 4a shows a darkfield transversal view of the film (µc-Si:H-2) in an area of 250 × 250 nm 2 , obtained with an accelerating voltage of 200 keV.In the image, it can be seen that the film effectively grows in crystalline columns, and these columns are of different orientations.Figure 4b shows an amplified transversal view of an area of 50 × 50 nm 2 , obtained with an accelerating voltage of 200 keV.In this image, we can see, with more detail, different orientations of silicon atoms, and moreover, practically all the area is microcrystalline.
we used TEM characterization, as described in the previous section.Figure 4a shows a dark-field transversal view of the film (µc-Si:H-2) in an area of 250 × 250 nm 2 , obtained with an accelerating voltage of 200 keV.In the image, it can be seen that the film effectively grows in crystalline columns, and these columns are of different orientations.Figure 4b shows an amplified transversal view of an area of 50 × 50 nm 2 , obtained with an accelerating voltage of 200 keV.In this image, we can see, with more detail, different orientations of silicon atoms, and moreover, practically all the area is microcrystalline.

Fourier Transform Infrared Spectroscopy
FTIR spectroscopy was used to characterize the films, as described in Section 2. Figure 5 shows the absorbance spectra of the µc-Si:H film series in a range of 570-2150 cm −1 , where several bands are observed, corresponding to different vibration modes.
We observed a band located close to 640 cm −1 , which is associated with SiH bonds in bending mode [25].At approximately 890 cm −1 is a band associated with SiH2 bonds in bending mode [26].Moreover, two bands associated with Si-O bonds at 750 cm −1 and 1100 cm −1 are observed [26].Finally, the band at 2000-2200 cm −1 is attributed to SiH, SiH2, and SiH3 stretching modes [27].
The total H2-bonded content (CH) of the films was calculated by integrating the area under the Si-H peak at 640 cm −1 and using Equation (4) [28]: where ∫ α(ω)/ω dω is the area of the deconvolution of the peak at 640 cm −1 ; α is the absorption coefficient at frequency (ω); Aω is the oscillator strength in wagging mode at 640 cm −1 (A(640) = 1.6 × 10 19 cm −2 ) [28]; and NSi = 5 × 10 22 cm −3 , the atomic density of pure silicon.Table 3 includes the calculated H2-bonded content for the µc-Si:H films, which are in a range of 7.0 to 13.1%.

Fourier Transform Infrared Spectroscopy
FTIR spectroscopy was used to characterize the films, as described in Section 2. Figure 5 shows the absorbance spectra of the µc-Si:H film series in a range of 570-2150 cm −1 , where several bands are observed, corresponding to different vibration modes.The absorption (α) spectra of the µc-Si:H films deposited on corning glass 1737 sub- We observed a band located close to 640 cm −1 , which is associated with SiH bonds in bending mode [25].At approximately 890 cm −1 is a band associated with SiH 2 bonds in bending mode [26].Moreover, two bands associated with Si-O bonds at 750 cm −1 and 1100 cm −1 are observed [26].Finally, the band at 2000-2200 cm −1 is attributed to SiH, SiH 2 , and SiH 3 stretching modes [27].
The total H 2 -bonded content (C H ) of the films was calculated by integrating the area under the Si-H peak at 640 cm −1 and using Equation (4) [28]: where α(ω)/ω dω is the area of the deconvolution of the peak at 640 cm −1 ; α is the absorption coefficient at frequency (ω); A ω is the oscillator strength in wagging mode at 640 cm −1 (A (640) = 1.6 × 10 19 cm −2 ) [28]; and N Si = 5 × 10 22 cm −3 , the atomic density of pure silicon.Table 3 includes the calculated H 2 -bonded content for the µc-Si:H films, which are in a range of 7.0 to 13.1%.

Absorption Coefficient and Determination of the Bandgap
The absorption (α) spectra of the µc-Si:H films deposited on corning glass 1737 substrates were calculated via transmittance measurements using a spectrometer (Perkin-Elmer, Lambda Series) in a wavelength range of 400-900 nm and the Pointwise Unconstrained Minimization Approach [29,30].Figure 6 shows the α spectra of the µc-Si:H film series in a range of 2 to 3.5 eV.The films with lower Xcs have more absorption in the UV region of the spectra (2.5-3.5 eV), which is in agreement with our previous work, where higher α in the UV region was observed in films with a lower fraction of large crystalline grains [31].The bandgap (Eg) can be calculated using the Tauc plot method.Figure 7a shows the Tauc plots of the different µc-Si:H thin films; α is related to the Eg of the material via the expression of Tauc, that is, Equation ( 5): where h is Planck's constant, v is the frequency of the photon, and B is a constant.From the plot, (αhv) 1/2 vs. (hv); the extrapolation of (αhv) 1/2 to α =0 (when α > 10 3 cm -1 ) provides the Eg.This procedure is accurate for direct bandgap materials such as a-Si:H, but for indirect bandgap materials, such as µc-Si:H, it could provide erroneous results.To obtain more accurate results, we followed the process reported in [17], where a baseline in the sub-bandgap region of the Tauc plot was used, as shown in Figure 7b for the film marked as µc-Si:H-2.The extrapolation of the curve with the baseline provides the Eg value.The Eg values are in a range of 2.1 to 2.48 eV and are shown in Table 2.The above is related to the fact that a-Si:H has a larger absorption coefficient in the UV region of the electromagnetic spectrum than µc-Si:H.On the other hand, µc-Si:H has a larger absorption coefficient in the infrared region than a-Si:H [13,14]; these properties are related to their structural arrangements.The µc-Si:H films with lower X C s have a larger amorphous fraction, and therefore, they have larger absorption in the UV region.
The bandgap (E g ) can be calculated using the Tauc plot method.Figure 7a shows the Tauc plots of the different µc-Si:H thin films; α is related to the E g of the material via the expression of Tauc, that is, Equation ( 5): where h is Planck's constant, v is the frequency of the photon, and B is a constant.From the plot, (αhv) 1/2 vs. (hv); the extrapolation of (αhv) 1/2 to α =0 (when α > 10 3 cm −1 ) provides the E g .This procedure is accurate for direct bandgap materials such as a-Si:H, but for indirect bandgap materials, such as µc-Si:H, it could provide erroneous results.To obtain more accurate results, we followed the process reported in [17], where a baseline in the sub-bandgap region of the Tauc plot was used, as shown in Figure 7b for the film marked as µc-Si:H-2.The extrapolation of the curve with the baseline provides the E g value.The E g values are in a range of 2.1 to 2.48 eV and are shown in Table 2.The bandgap (Eg) can be calculated using the Tauc plot method.Figure 7a shows the Tauc plots of the different µc-Si:H thin films; α is related to the Eg of the material via the expression of Tauc, that is, Equation ( 5): where h is Planck's constant, v is the frequency of the photon, and B is a constant.From the plot, (αhv) 1/2 vs. (hv); the extrapolation of (αhv) 1/2 to α =0 (when α > 10 3 cm -1 ) provides the Eg.This procedure is accurate for direct bandgap materials such as a-Si:H, but for indirect bandgap materials, such as µc-Si:H, it could provide erroneous results.To obtain more accurate results, we followed the process reported in [17], where a baseline in the sub-bandgap region of the Tauc plot was used, as shown in Figure 7b for the film marked as µc-Si:H-2.The extrapolation of the curve with the baseline provides the Eg value.The Eg values are in a range of 2.1 to 2.48 eV and are shown in Table 2.
(a) (b)  To determine the room temperature conductivity (σ RT ) and photoconductivity (σ ph ) of the films, current-voltage (I-V) measurements were performed in the dark and under illumination, respectively, on the samples deposited on corning glass 2947 with aluminum contacts.Measurements under light were made using the ORIEL SOL 2 solar simulator, which provides light with a standard AM 1.5 spectrum and a power density of 100 mW/cm 2 .In both measurements, an electrometer (Keithley6517A) was used, configured as a voltage source and current meter.
Figure 8a-f show the current-voltage (I-V) characteristics of the films in the dark and under AM 1.5 radiation, applying a voltage ranging from 0 to 18 V.With the current values and the geometrical parameters of the films, the σ RT and σ ph were calculated.Table 3 shows the σ RT and σ ph values and the photo-response (σ ph /σ RT ) of the deposited µc-Si:H films.The film marked as µc-Si:H-3 has the largest σ RT value, while the film labeled as µc-Si:H-4 has the largest σ ph /σ RT value.
A µc-Si:H film is structurally composed of a crystalline fraction, an amorphous fraction, and voids [31,32].If the film has a large X C , its σ RT value is large as well; on the other hand, if the film has a low X C (and a large amorphous fraction), its σ RT value is lower.However, we did not see a correlation between the X C and the σ RT .A possible reason is the undesirable oxygen doping of the films during deposition, which can increase the σ RT .In fact, the film with the lowest E A (as shown below) had the largest σ RT , which makes sense since doping reduces the E A .
Furthermore, oxygen doping can be the reason for larger σ RT values in these µc-Si:H films than those reported in the literature [33].Moreover, in µc-Si:H, the σ ph is affected by the X C of the film and the SiH 4 concentration during deposition.It has been reported that

Temperature Dependence of Conductivity and Activation Energy
Temperature dependence of conductivity (σ(T)) measurements were performed on the films deposited on corning glass 2947 with aluminum contacts.The samples were collocated inside a cryostat at a pressure of 60 mTorr.Voltage was applied in a range of −20 to 20 V on the samples, and the current was measured using an electrometer (Keithley6517A) in a range of temperature of 27 to 127 °C.Finally, the dark conductivity (σD) was calculated using the films' geometrical parameters.Figure 9a shows the Log(σD) vs. 1/kT, where k is the Boltzmann constant, and T is the temperature.

Temperature Dependence of Conductivity and Activation Energy
Temperature dependence of conductivity (σ(T)) measurements were performed on the films deposited on corning glass 2947 with aluminum contacts.The samples were collocated inside a cryostat at a pressure of 60 mTorr.Voltage was applied in a range of −20 to 20 V on the samples, and the current was measured using an electrometer (Keithley6517A) in a range of temperature of 27 to 127 • C. Finally, the dark conductivity (σ D ) was calculated using the films' geometrical parameters.Figure 9a shows the Log(σ D ) vs. 1/kT, where k is the Boltzmann constant, and T is the temperature.The σD, T, and EA are related by an Arrhenius-type equation ( 6): where k is the Boltzmann constant, and σ0 is a pre-exponential factor; the previous equation can be expressed as Equation ( 7): Ln (σD) = Ln (σ0) -EA/kT (7) Equation ( 7) has the form of the equation depicting a straight line (y = A+ Bx).Experimentally, the activation energy is obtained from the slope of the graph of ln (σD) vs. 1/kT, as shown in Figure 9b, corresponding to the film labeled as µc-Si:H-2.A linear fit of the experimental data points was performed, and the slope was extracted.
Table 3 shows the EA values of the µc-Si:H film series, where we can see that the EA values are low, indicating that the films were unintentionally doped, possibly with oxygen, which was incorporated into the film during deposition.This result agrees with the FTIR characterization, where pronounced peaks can be observed in the 1000-1200 cm −1 region, which is related to Si-O bonds.Moreover, as the dopant is oxygen, the µc-Si:H films are n-type.

Discussion
From the previous analysis of the µc-Si:H film series, we saw that, effectively, there is an optimal RF power level that can obtain a large XC in films deposited by PECVD.An RF power of 25 W (86.5 mW/cm 2 ), coupled with a large H2/SiH4 ratio, resulted in an XC of 75%.Larger or lower RF power levels resulted in films with lower XCs.
From the FE-SEM characterization, we observed that the film surface seems to be composed of grains of sizes ranging from 30 to 80 nm; however, from the TEM analysis, we observed that the films grew in columns (Figure 4a) with a thin initial incubation layer, which is of an amorphous nature.A larger magnification in the analysis of the columns (Figure 4b) gave us information on the microcrystalline nature of the film, and several crystalline orientations are observed.
From the optical characterization, we observed that the total H2 content (CH) of the µc-Si:H films was in a range of 7.0-13.1%,which agrees with the results reported in [28, The σD, T, and EA are related by an Arrhenius-type Equation ( 6): where k is the Boltzmann constant, and σ 0 is a pre-exponential factor; the previous equation can be expressed as Equation (7): Equation ( 7) has the form of the equation depicting a straight line (y = A+ Bx).Experimentally, the activation energy is obtained from the slope of the graph of ln (σ D ) vs. 1/kT, as shown in Figure 9b, corresponding to the film labeled as µc-Si:H-2.A linear fit of the experimental data points was performed, and the slope was extracted.
Table 3 shows the E A values of the µc-Si:H film series, where we can see that the E A values are low, indicating that the films were unintentionally doped, possibly with oxygen, which was incorporated into the film during deposition.This result agrees with the FTIR characterization, where pronounced peaks can be observed in the 1000-1200 cm−1 region, which is related to Si-O bonds.Moreover, as the dopant is oxygen, the µc-Si:H films are n-type.

Discussion
From the previous analysis of the µc-Si:H film series, we saw that, effectively, there is an optimal RF power level that can obtain a large X C in films deposited by PECVD.An RF power of 25 W (86.5 mW/cm2), coupled with a large H 2 /SiH 4 ratio, resulted in an X C of 75%.Larger or lower RF power levels resulted in films with lower X C s.
From the FE-SEM characterization, we observed that the film surface seems to be composed of grains of sizes ranging from 30 to 80 nm; however, from the TEM analysis, we observed that the films grew in columns (Figure 4a) with a thin initial incubation layer, which is of an amorphous nature.A larger magnification in the analysis of the columns (Figure 4b) gave us information on the microcrystalline nature of the film, and several crystalline orientations are observed.
From the optical characterization, we observed that the total H 2 content (C H ) of the µc-Si:H films was in a range of 7.0-13.1%,which agrees with the results reported in [28,34].Furthermore, we observed that the E g values of the µc-Si:H film series were in a range of 2.1-2.48eV; these values are larger than those reported in [35] and are also larger than those values of a-Si:H, which are in a range of 1.6-1.88eV [36].The film with the largest X C had an E g of 2.2 eV, and we did not observe any trend related to the deposition RF power.
The electrical characterization gave us information on several parameters of the µc-Si:H films.The σ RT was in a range of 8.3 × 10 −5 -1.2 × 10 −3 (Ωcm) −1 , while the film with the largest X C had a σ RT of 6.4 × 10 −4 (Ωcm) −1 .This value is larger than those reported in the literature [24], and it is possibly related to unintentional doping with O 2 during the deposition of the films.The above is corroborated by a peak at 1000-1200 cm −1 in the films' absorbance spectra obtained via FTIR spectroscopy, which was related to Si-O bonds.Furthermore, the small E A values were in a range of 0.18 to 0.28 eV, indicating that the films are not intrinsic.
On the other hand, the σ ph values were in a range of 1.4 × 10 −4 -1.8 × 10 −3 (Ωcm) −1 , and the σ ph /σ RT ratio was in a range of 1.5-6.4.The film with the lowest X C had the largest σ ph /σ RT ratio (of 6.4), which agrees with the fact that the more amorphous a film is, the more photoconductive it is.
It is important to produce µc-Si:H films with large X C values since this is related to both larger µ e and larger absorption in the infrared region of the electromagnetic spectrum [13][14][15][16].These properties are important for the development of faster TFTs and tandem (a-Si:H/µc-Si:H) solar cells with high conversion efficiency, respectively.Moreover, it is important to control the deposition RF power since large values produce excessive powder in the deposition chamber and can damage the vacuum pumps of the PECVD deposition system.In this work, we demonstrated that a high RF power is not necessary to obtain µc-Si:H films with a large X C , and consequently, the powder formed in the deposition chamber was reduced significantly, preserving the integrity of the vacuum pumps.

Conclusions
In this work, we studied µc-Si:H films deposited using the PECVD technique at 200 • C with a SiH 4 , H 2 , and Ar mixture.Particularly, we focused our study on the effect of the deposition RF power and the X C of µc-Si:H films, and these were correlated with their structural, optical, and electrical properties.We found an optimal RF power value of 25 W (an RF power density of 86.5 mW/cm 2 ) for producing films with high X C values (75%), coupled with a high H 2 /SiH 4 ratio (of 50) and a high chamber pressure (of 1500 mTorr).Even though the films were deposited without any unintentional doping, they were highly conductive, and furthermore, they had low E A values, indicating their non-intrinsic nature.Using electron microscopy, we observed that, effectively, the films grew in crystalline columns, and these columns were of different orientations.We can stress that producing µc-Si:H films with high X C s will result in high-performance devices, such as TFTs and a-Si:H/µc-Si:H tandem solar cells, where high µ e and high absorption in the infrared spectrum are necessary, respectively.Finally, we produced µc-Si:H films with large X C values using relatively low RF power densities, significantly reducing powder formation in the plasma and preserving the integrity of the vacuum pumps of the PECVD system.

Figure 2 .
Figure 2. (a) Raman spectra of the series of µc-Si:H films deposited with different RF power levels;(b) Raman spectra of the film deposited with an RF power of 25 W (86.5 mW/cm 2 ), where the deconvolution of the spectrum in three Gaussian peaks at 520 cm -1 , 508 cm -1 , and 480 cm -1 is shown.

Figure 2 .
Figure 2. (a) Raman spectra of the series of µc-Si:H films deposited with different RF power levels; (b) Raman spectra of the film deposited with an RF power of 25 W (86.5 mW/cm 2 ), where the deconvolution of the spectrum in three Gaussian peaks at 520 cm −1 , 508 cm −1 , and 480 cm −1 is shown.

Figure 2 .
Figure 2. (a) Raman spectra of the series of µc-Si:H films deposited with different RF power levels;(b) Raman spectra of the film deposited with an RF power of 25 W (86.5 mW/cm 2 ), where the deconvolution of the spectrum in three Gaussian peaks at 520 cm -1 , 508 cm -1 , and 480 cm -1 is shown.

Figure 5 .
Figure 5. FTIR absorbance spectra of the series of µc-Si:H films deposited at different RF power levels.

Figure 5 .
Figure 5. FTIR absorbance spectra of the series of µc-Si:H films deposited at different RF power levels.

Figure 6 .
Figure 6.Absorption coefficient of the series of µc-Si:H films deposited at different RF power levels.

Figure 6 .
Figure 6.Absorption coefficient of the series of µc-Si:H films deposited at different RF power levels.

Figure 6 .
Figure 6.Absorption coefficient of the series of µc-Si:H films deposited at different RF power levels.

Figure 7 .
Figure 7. (a) Tauc plots of the series of µc-Si:H films deposited with different RF powers; (b) extraction of Eg for the film marked as µc-Si:H-2.Figure 7. (a) Tauc plots of the series of µc-Si:H films deposited with different RF powers; (b) extraction of E g for the film marked as µc-Si:H-2.

Figure 9 .
Figure 9. (a) Temperature dependence of conductivity (σ(T)) of the series of µc-Si:H films deposited with different RF power levels; (b) extraction of EA for the film marked as µc-Si:H-2, which has a larger XC.

Figure 9 .
Figure 9. (a) Temperature dependence of conductivity (σ(T)) of the series of µc-Si:H films deposited with different RF power levels; (b) extraction of E A for the film marked as µc-Si:H-2, which has a larger X C .

Table 1 .
Deposition parameters of the series of µc-Si:H films.The deposition time was 30 min, and the temperature was 200 • C.

Table 2 .
Properties of the series of µc-Si:H films obtained from structural characterization.

Table 3 .
Properties of the series of µc-Si:H films obtained using optical and electrical characterization; XC is included to correlate it with the other film parameters.

Table 3 .
Properties of the series of µc-Si:H films obtained using optical and electrical characterization; X C is included to correlate it with the other film parameters.