Amorphous Silicon Films and Nanocolumns Deposited on Sapphire and GaN by DC Sputtering

Sputtering is a deposition technique used to fabricate low‐cost silicon films on crystalline and amorphous substrates. Herein, the deposition of amorphous silicon films by DC sputtering on both sapphire and GaN/sapphire substrates is reported. Films are deposited using argon plasma with a pressure of 0.47 Pa at 30–60 W of DC power and different deposition temperatures from RT to 550 °C. The effect of different deposition conditions is investigated on structural quality, layer morphology, and optical properties of the layers. X‐ray diffraction measurements do not show any peak associated to crystalline silicon, while energy‐dispersive X‐ray demonstrates the presence of silicon in the layers. Silicon films deposited on sapphire show a compact morphology but the formation of silicon columns on GaN. On both substrates, the growth rate increases a factor of 3 with the applied DC power (50–150 nm h−1). Finally, the optical bandgap energy extracted from transmission measurements decreases from 2.40 to 2.10 eV with the DC power, due to the reduction of impurity incorporation. This work offers a low‐cost alternative for the deposition of amorphous compact silicon films and silicon nanocolumns at low temperature, for application in sensing, photonic, electronic, and photovoltaic devices.


Introduction
Silicon is the most used material in the semiconductor industry and has special interest in the photovoltaic one. Hydrogenated amorphous silicon (a-Si:H) has proved to be a potential semiconductor choice for large-area electronics thanks to its interesting doping, photoconductivity, and junction formation properties. [1] Amorphous silicon owns the same chemical properties as crystalline silicon, so the material deposition techniques and the device fabrication technology take advantage of this background knowledge about crystalline silicon processing and plasma deposition processes. Amorphous Si thin films, which are typically fabricated by plasma-enhanced chemical vapor deposition (PECVD), are usually applied in advanced electronics devices such as highvoltage thin film transistors, [2,3] microelectromechanical systems (MEMS), [4] solar cell devices, [5,6] and passivation buffer layers. [7][8][9][10][11] On the contrary, investigations on a-Si nanocolumns, nanopillars, or nanoholes are undergoing. Kim et al. [12] reported amorphous silicon on copper nanopillars electrodes; similarly, Lin et al. [13] reported lithium fluoride-coated amorphous silicon nanocolumns for lithium-ion batteries. However, Brieger et al. [14] presented the fabrication of ordered arrays of cylindrical nanoholes, and Zhao et al. [15] fabricated a-Si nanocolumns on Si(100) and Si(100) covered with polystyrene nanospheres. For the time being, a-Si is mainly deposited on quartz, glass, and Si(100) wafers, but no studies of a-Si deposited on GaN substrates were found in the literature. So, as far as we know, this is the first report of a-Si nanocolumns on GaN. The sputtering deposition technique (RF or DC) is widely used in the thin film industry due to its low cost. This technique relies on the bombardment of the target material by energetic ions generated by the ionization of a working gas (reactive or nonreactive) under an applied voltage. The sputtered atoms of the material will then deposit on a substrate placed on the opposite site. Unlike other deposition methods, sputtering does not involve any chemical interactions among species. Sputtering can be used to deposit good quality films at high (%500°C), moderate (%300°C), and low (%100°C) temperatures, and even at room temperature. This fact enables the compatibility of the sputtering deposition technique with different type of substrates, including plastic and flexible ones, making it a promising technology for future thin film and low-cost applications. Moreover, the deposition of silicon by sputtering offers several advantages over other techniques, such as the elimination of using toxic/hazardous gases, the reduction of the process temperature, the ability to control the amount of hydrogen incorporated, and the capacity to predope the silicon target. [16] All things considered, the potential of an alternative low-cost deposition technique for the fabrication of a-Si should be further investigated.
This work presents the study of the structural, morphological, and optical properties of silicon layers deposited by DC sputtering simultaneously on sapphire and GaN-on-sapphire substrates under different deposition conditions, namely, substrate temperature and applied power. The aim of this study is to find Sputtering is a deposition technique used to fabricate low-cost silicon films on crystalline and amorphous substrates. Herein, the deposition of amorphous silicon films by DC sputtering on both sapphire and GaN/sapphire substrates is reported. Films are deposited using argon plasma with a pressure of 0.47 Pa at 30-60 W of DC power and different deposition temperatures from RT to 550°C. The effect of different deposition conditions is investigated on structural quality, layer morphology, and optical properties of the layers. X-ray diffraction measurements do not show any peak associated to crystalline silicon, while energy-dispersive X-ray demonstrates the presence of silicon in the layers. Silicon films deposited on sapphire show a compact morphology but the formation of silicon columns on GaN. On both substrates, the growth rate increases a factor of 3 with the applied DC power (50-150 nm h À1 ). Finally, the optical bandgap energy extracted from transmission measurements decreases from 2.40 to 2.10 eV with the DC power, due to the reduction of impurity incorporation. This work offers a low-cost alternative for the deposition of amorphous compact silicon films and silicon nanocolumns at low temperature, for application in sensing, photonic, electronic, and photovoltaic devices.
the optimal deposition conditions of silicon by DC sputtering for further applications in hybrid III-nitride on silicon photovoltaic devices. [17]

Experimental Section
Silicon layers were deposited using a direct current (DC) magnetron sputtering system (AJA International, ATC ORION-3-HV) on different substrates, namely, 500 μm-thick (0001)-oriented sapphire and 4 μm-thick GaN-on-sapphire (GaN template) substrates. This system is equipped with 2 in. confocal magnetron cathodes of p-type boron-doped (5-20 mΩ cm) Si (99.999%). The background pressure of the system was in the order of 10 À7 mBar, and the distance between target and substrate was fixed at 10.5 cm. Substrates were chemically cleaned in organic solvents and outgassed inside the chamber for 30 min at 550°C. Before deposition, the surface of the targets and substrates was cleaned using a soft plasma etching with Ar (2 sccm at 20 W).
Silicon thin films were deposited in a pure Ar (99.9999%) atmosphere with an argon flow of 2 sccm and a pressure of 0.47 Pa. Two set of samples were developed (see Table 1): samples T20, T300, and T550 correspond to a deposition temperature of 20, 300, and 550°C, respectively, with a DC power applied to the Si target of 30 W (set A of samples). Samples P30, P40, P50, and P60 correspond to layers deposited at 550°C with different DC power applied to the Si target, namely, 30, 40, 50, and 60 W, respectively (set B of samples). In all cases, the deposition time was 2 h.
Structural properties were studied using high-resolution X-ray diffraction (HRXRD) measurements using a PANalytical X'Pert Pro MRD system. The thicknesses of the layers were estimated by X-ray reflection (XRR) measurements and verified with cross-sectional field-emission scanning electron microscopy (FESEM) images. Energy-dispersive X-ray (EDX) measurements were carried out to confirm the presence of silicon in the layers. Additionally, the optical bandgap energy of the films was obtained from room-temperature optical transmission measurements.

Structural Characterization
The structural quality of the silicon layers was studied by HRXRD measurements on sapphire and GaN template substrates. Figure 1 shows the 2θ/ω scans of the samples at different temperatures (set A of samples) and applied DC power (set B of samples), showing no peak associated with crystalline silicon. However, EDX measurements performed in these samples (see Figure 2 for an EDX spectra of a representative sample) confirm the presence of silicon pointing out the deposition of amorphous Si. Oxygen and aluminum peaks detected in EDX measurements are attributed to sapphire substrate. Figure 3 shows the XRR measurements of silicon on sapphire samples as a function of the growth temperature (set A) (a), and as a function of the applied DC power (set B) on sapphire (b) and on GaN template (c). The thickness of each layers is obtained from the maxima of the interference oscillations in the XRR measurements. Using the critical angle (θ c ) and the peaks of the Kiessig fringes (θ), a linear model with θ 2 as a function of m 2 (i.e., the peak number) can be adjusted to extract the thickness of the layer (d) following the equation However, the intensity of the fringes drops when increasing the applied power due to the increase of the film thickness. The estimated deposition rate for each case is summarized in Table 1, showing that the temperature does not play a significant role in the deposition rate between RT and 550°C.
On the other hand, silicon samples deposited on GaN templates do not show any interference oscillations due to columnar morphology of the layers, as shown in the FESEM images in Figure 5.
Additionally, the density of the material can be estimated using the equation ρ ¼ 2δπA r e ZN A λ 2 , where δ is the dispersion, which is calculated from the critical angle as δ ¼ θ 2 c 2 , A is the atomic mass, r e is the electron radius, Z is the atomic number, N A is the Avogadro's number, and λ is the wavelength of the X-ray (1.5406 Å). The values of density obtained by XRR for the samples are listed in Table 1. The average density of the films is %2.1 g cm À3 , slightly below the theoretical one of 2.285 g cm À3 for amorphous silicon. [18] The values obtained in this work fall in line with values reported by previous studies, such as the work of Tan et al., [19] stating values of 2.21 g cm À3 , Chittick et al. [20] with 1.9 g cm À3 and Moss et al. [21] with values ranging from 1.75 to 2.2 g cm À3 depending on the deposition conditions.

Morphological Characterization
The morphology of the layers was investigated by FESEM measurements. Figure 4 and 5 show the FESEM images of the silicon films grown on sapphire and GaN template substrates as a function of the growth temperature ( Figure 4) and depending on the applied DC power ( Figure 5). Silicon films deposited on sapphire own a compact morphology, regardless the deposition temperature or applied power, even at room temperature. However, silicon samples grown on GaN template show a columnar morphology. This change on growth behavior observed on GaN substrate can be attributed to a different surface nucleation at the start of the process and to differences in thermal expansion coefficients, which reduces the adatom mobility leading to thicker layers with columnar morphology.  Finally, Figure 6 summarizes the deposition rate of silicon by DC sputtering on the three substrates obtained by the XRR and FESEM. A similar trend for all substrates is obtained, with a higher deposition rate for samples deposited on GaN template. This can be explained by the different growth mechanism causing a noncompact morphology as seen in FESEM images.  www.advancedsciencenews.com www.pss-b.com

Optical Characterization
The optical properties of layers deposited on sapphire were evaluated by optical transmittance measurements at room temperature. The absorption coefficient of the layers (α) can be derived from transmission spectra (T ) following the relation α(E)·L = Àln(T ), being L the thickness of the layer, and without considering optical scattering and reflection losses. The optical bandgap was determined from the Tauc's plot as follows: (αE) 1/n ∝ E, where α is the absorption coefficient, E is the photon energy in eV, and n is a coefficient related to the nature of the transition. [22] Then, as shown in Figure 7, the optical bandgap energy of the samples was calculated from the extrapolation of the linear part of the obtained plot as a function of the energy, with n = ½, which corresponds to a direct bandgap semiconductor. The behavior of the optical bandgap energy can be related to the crystalline structure and the role of hydrogen or impurities. [23] This was observed by Hossain et al. [24] where sputtered films with no hydrogen had a bandgap energy of 1.4 eV and increases to 2.0 eV for 4.4 of at%. They also prepared films by PECVD with 11 at% of hydrogen with a bandgap energy lower than that of sputtered films with similar hydrogen content.  The inconsistency of the bandgap energy between the two techniques can be related to the way hydrogen (or other impurities) is incorporated in the amorphous matrix in both methods. In our amorphous silicon samples, the obtained bandgap energy, summarized in Table 1, remains the same with the increasing substrate temperature. On the contrary, the narrowing of the optical bandgap energy with the applied DC power can be explained by the lower impurity incorporation in the film due to the higher growth rate. Almost identical values of bandgap energy were obtained on silicon films grown on both sapphire and GaN template substrates regardless their different morphology observed above.

Conclusions
Amorphous silicon films were deposited via DC sputtering on sapphire and GaN template substrates under different deposition conditions. In the first set of samples (set A), silicon was deposited on sapphire at 20, 300, and 550°C and 30 W of DC power. In the second set of samples (set B), silicon films were deposited at 550°C and DC powers of 30, 40, 50, and 60 Won sapphire and GaN template substrates. Even that in all cases no peak associated with crystalline silicon was found in the XRD 2θ/ω scans, the presence of silicon in the films was verified by EDX measurements.
In the set A of samples, compact silicon films were obtained with no significant differences in structural, morphological, and optical properties between them. The growth rate increased from 69 to 75 nm h À1 when increasing the temperature from 20 to 550°C and the bandgap energy remains around 2.4 eV.
On the other hand, in the set of samples B, compact silicon layers were obtained when deposition on sapphire, whereas silicon columns were formed when using GaN templates substrates. The deposition rate increased from 55 to 150 nm h À1 , and from 90 to 180 nm h À1 for sapphire and GaN template substrates, respectively, within this DC power range. In this case, a reduction of the bandgap energy, from 2.40 to 2.10 eV, was observed with the DC power, being attributed to a decrease of the impurity incorporation due to the increase of the growth rate. No significant difference in optical properties was observed due to the distinct morphology of the layers between samples on grown on different substrates.
In this work, we demonstrate the ability to successfully produce amorphous and compact Si layers on sapphire and amorphous silicon nanocolumns on GaN via DC sputtering. This study sets the ground for future optical or electrical device www.advancedsciencenews.com www.pss-b.com applications such as diodes, transistors, sensors, optical waveguides, and buffer/passivation layer in silicon-based solar cells using this low-cost technique.