Oblique Ar⁺ Sputtered SiC Thin Films: Structural, Optical, and Electrical Properties

Abstract

State-of-the-art experimental facility 200 kV ion accelerator, with energy range of 30–200 kV has been running successfully at Ion Beam Center, KUK. The versatility of this facility lies in providing single charge state and large area irradiation in a single step. In this regard, present work investigates the structural, optical, and electrical behavior of as-deposited and argon-sputtered SiC thin films at varying fluences keeping ion incidence at 500. Raman measurements reveal that the opening of sp2 sites on a-C results in increased sp3 content in the surface layers. Both FWHM of G peak and I(D)/I(G) ratio decrease with increasing argon fluence. UV-Vis-NIR spectroscopy reveals an increase in the optical absorption and a shifting of absorption edge toward longer wavelengths. I-V characteristics reveal ohmic behavior of all the samples in the voltage range of −5 to +5 V. The conductivity of all the samples is found to decrease with increase in argon ion fluence. The observed structural transformations are attributed to the different degree of sputtering yield of silicon and carbon at different argon ion fluences.

1. Introduction

Silicon carbide (SiC), a compound composed of silicon and carbon, has emerged as a remarkable material with a wide range of applications, revolutionizing various industries and research fields. Known for its exceptional mechanical strength and high thermal conductivity to its excellent electrical properties and wide bandgap, SiC has captured the attention of scientists and offers a myriad of possibilities for technological advancements [1, 2, 3, 4, 5, 6, 7, 8, 9].

Ion implantation stands as a pioneering technique at the forefront of materials engineering. This technique has opened new avenues for tailoring and enhancing the structural, optical, and electrical characteristics of materials. Interestingly, oblique ion implantation, a specialized technique within ion beam modification, has emerged as a powerful method for tailoring material properties with unprecedented precision. Unlike traditional perpendicular ion implantation, oblique implantation involves the controlled bombardment of a target material at an angle, introducing ions into the surface with an oblique trajectory. This unique approach opens a realm of possibilities, allowing engineers and researchers to engineer intricate structures, manipulate surface morphology, and optimize material characteristics in ways that were once considered challenging or even unattainable [8, 9, 10, 11, 12, 13, 14, 15, 16, 17].

Oblique ion implantation offers a distinct set of advantages, enabling the creation of anisotropic material modifications, surface patterning, and enhanced doping profiles. By varying the implantation angle, energy, and ion species, it becomes possible to engineer intricate 2D and 3D surface features effectively on a micro and nanoscale level. These tailored structures find applications across diverse fields, from microelectronics and optoelectronics to biomaterials and sensor technologies. Thus, by carefully controlling the ion implantation parameters, it is possible to engineer SiC with tailored structural and surface properties to meet the requirements of specific applications [9, 10, 11, 12, 13, 14, 15].

Thus, the present work investigates the structural, optical, and electrical properties of oblique argon-sputtered silicon carbide surfaces. As a novelty, we study the surface characteristics of unexplored SiC/Si(111) surfaces at oblique argon ion implantation with the aim of quantifying the dependence of structural, optical, and electrical behavior on the ion fluence. The oblique ion beam erosion, causing the removal of the surface target atoms, is the pivotal mechanism in this low-energy region. The investigations of optical and electrical response in combination with the structural modifications taking place in the silicon carbide thin film surfaces have been studied.

2. Experimental details

2.1 Sample preparation

SiC thin films were grown onto Si(111) substrate by radio-frequency (RF) sputtering technique, detailed in our earlier publication [18]. Then, these as-deposited thin film samples were mounted on a substrate holder with the possibility that angle of ion beam incidence could be fixed to 50⁰ with respect to the surface normal. For ion beam sputtering experiments, these samples were irradiated with 80 keV Ar⁺ ion beam to fluences of 5 × 10¹⁷ Ar⁺cm⁻² and 7 × 10¹⁷ Ar⁺cm⁻² at incident angle of 50⁰ with respect to the surface normal using 200 kV ion accelerator facility available at Ion Beam Center, Kurukshetra University, Kurukshetra. The base pressure in the target chamber was 1.2 × 10⁻⁶ Torr. The ion current density during irradiation was kept constant at 2.8 μA cm⁻² [19].

2.2 Instrumentation

The structural behavior has been recorded by Raman spectroscopy. Raman spectra of the argon-sputtered films were acquired by micro-Raman spectrometer with a laser excitation wavelength at 532 nm. The optical behavior has been studied by UV–VIS spectrophotometer.

The diffuse reflectance measurements have been performed on Si(111), SiC, and argon-sputtered SiC samples using a Shimadzu UV-VIS-NIR spectrophotometer (UV-3600 Plus), equipped with integrating sphere assembly ISR-603 in the wavelength range of 200–800 nm with a resolution of 0.1 nm. All the reflectance spectra were recorded keeping air as the reference.

Current–voltage (I-V) characteristics and conductivity measurements have been performed by employing Keithley 4200A-SCS parameter analyzer by applying voltage of −5 V to +5 V and measuring the resulting current.

3. Results and discussion

3.1 Structural investigations of RF sputtered SiC over Si(111)

In order to investigate the structural changes after 80 keV argon ion sputtering in SiC/Si(111) surfaces, we have performed Raman scattering in backscattering geometry at room temperature.

Figure 1 displays the room temperature (RT) first-order Raman spectra acquired from a bare Si(111) substrate, the as-deposited SiC film and 80 keV argon ion sputtering at various argon ion fluences, respectively.

Figure 1.

Raman spectra of (a) bare Si(111) substrate, (b) as grown SiC on Si(111), 80 keV argon-sputtered SiC surfaces for fluence of (c) 5 × 10¹⁷ Ar⁺ cm⁻² and (d) 7 × 10¹⁷ Ar⁺ cm⁻². TA-transverse acoustical, TO-transverse optical, LA-longitudinal acoustical, and LO-longitudinal optical.

Figure 1(a) reveals that Raman spectra of Si(111) consists of one sharp peak at 520 cm⁻¹ corresponding to first-order transverse optical phonon (1TO) vibrations of crystalline silicon [1, 2, 3, 4]. In addition, the presence of second-order transverse optical mode (2TO) at 929–1027 cm⁻¹ has been identified. This validates the crystalline nature of the surface layers in Si(111).

Figure 1(b) exhibits that Raman spectra of SiC RF sputtered over Si(111) display the presence of longitudinal and transverse phonon modes of Si-C bonds in SiC at wavenumber of 972 and 796 cm⁻¹, respectively. Generally, the 3C-SiC single crystal has these two strong phonon modes corresponding to longitudinal optical (LO) and transverse optical (TO) vibrations, whose Raman shifts are 972 and 796 cm⁻¹, respectively [1, 2, 3, 4]. The presence of these two phonon modes but with reduced intensity and board linewidth in Raman spectra in Figure 1(b) confirms that the SiC layers, analyzed in this work, consist mainly of cubic poly-type structure [1, 2, 3, 4, 5]. Due to the broad distribution of these LO and TO phonon modes, we can assume that the lattice of 3C-SiC is amorphous in nature.

Additionally, Raman spectrum (Figure 1(b)) shows prominent Si peak at 520 cm⁻¹ as observed in Si sample. The peak is detected due to the extended depth of the 532 nm laser light in 3C-SiC. In principle, the penetration depth of a given laser in material depends on the incident wavelength used in Raman measurements. The efficient penetration depth of Ar⁺ ion laser having wavelength of 532 nm in 3C-SiC is larger than 83 μm, and its penetration depth in Si is about 0.935 μm [1, 2, 3, 4, 5, 6, 7, 8]. Hence, our as-deposited 3C-SiC films of thickness 0.28 μm appear transparent. Thus, the penetrating light is absorbed within about 0.935 μm of the Si substrate, not in the 0.28 μm thick 3C-SiC, whose phonon intensity is much smaller than that of silicon.

Interestingly, selection rule for the (111) surface of a zinc blende crystal allows both LO and TO phonon mode. In our case, the broadband seen in the range 900–1010 cm⁻¹, which we have devoted to LO vibrations of Si-C bonds in SiC is also associated with Si second-order, that is, 2TO Raman scattering [4, 5, 6, 7, 8, 9, 10, 11, 12]. As we have already discussed the penetration depth of the 532 nm laser is more than the thicknesses of 3C-SiC films, so, here, LO phonon peak of SiC layer peak is overlapped with the second order Raman spectrum of silicon as evidenced from Figure 1(b). Interestingly, one can observe small humps on both sides of the LO band. These shoulders are attributed to 2TO peak spreading in the wavenumber region of 920–1050 cm⁻¹. This, in turn, complicates the use of LO peak position for drawing the conclusion on SiC structure.

Figure 1(c and d) reveals that sputtered SiC surfaces have undergone a substantial change indicative of structural rearrangement. The argon beam interaction has produced severe structural modifications in the surface layers of irradiated specimens. These spectra present three main broad peaks in the 100–550, 550–1100, and 1100–1700 cm⁻¹ spectral region corresponding to Si-Si, Si-C, and C-C bonding, respectively.

Similar to as-deposited sample, Raman spectrum in Figure 1(c and d) also display sharp 1TO peak at 520 cm⁻¹ characteristic of predominant crystalline structure of the Si substrate. This is due to the large penetration depth of the 532 nm light in 3C-SiC.

Additionally, Raman spectra display the presence of longitudinal phonon mode of Si-C bonds in SiC at wavenumber of 972 cm⁻¹, which is being overlapped with Si-Si bonding 2TO peak.

To qualitatively asses the structure of 3C-SiC, we have further studied in details the contribution from C-C bonds in as-deposited and 80 keV argon-sputtered SiC thin films. For this, we have deconvoluted the region from 1100 to 1700 cm⁻¹ of the Raman spectrum of Figure 1(b,c and d). The deconvoluted Raman spectra of SiC thin films within 1100–1700 cm⁻¹ are shown in Figure 2.

Figure 2.

Raman spectra of (a) as grown SiC on Si(111), 80 keV argon-sputtered SiC surfaces for fluence of (b) 5 × 10¹⁷ Ar⁺ cm⁻² and (c) 7 × 10¹⁷ Ar⁺ cm⁻² in the wavenumber region 1100–1700 cm⁻¹.

Figure 2 depicts that after Gaussian fitting of the Raman spectra, splitting into two peaks has been seen. One typical peak of amorphous carbon (a-C) centered at 1582 cm⁻¹ characteristic of G peak has been observed [13]. The G peak is the result of hybrid C-C bond diffusion of symmetrical stretching vibrating graphite sp² carbon [6, 7, 8]. Interestingly, one a-C peak at 1463 cm⁻¹ also appears. The presence of this peak is attributed to the amorphous carbon content in the sample. Further, this peak presents the incomplete crystallization of carbon clusters [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Zhang et al. [15] reported the presence of these amorphous carbon bands and ascribed to the mixed sp² and sp³ C-C bonds in amorphous SiC.

Figure 2(b and c) depicts that after Gaussian fitting of the Raman spectra, splitting into three peaks has been seen for SiC films following argon ion bombardment at oblique incidence of 50⁰ for fluences of 5 × 10¹⁷ Ar⁺cm⁻², 7 × 10¹⁷ Ar⁺cm⁻². Two typical peaks of amorphous carbon (a-C) centered at 1373 and 1570 cm⁻¹ characteristic of G peak and D peak have been observed, respectively [19]. Interestingly, one a-C peak at 1469 cm⁻¹ also appears. This symmetric peak can be ascribed to the presence of very small and highly disordered carbon clusters. The Raman parameters, that is, peak positions of G and D peak, FWHM of G peak and D peak and their integrated intensities have been determined from deconvolution procedure. These values have been listed in Table 1.

Specimen SiC/Si(111) Ar^+ cm^−2	Center			FWHM			Intensity			I(D)/I(G)
	D Peak	a-C Peak	G Peak	D Peak	a-C Peak	G Peak	D Peak	a-C Peak	G Peak
0	—	1466	1587	—	155	69	—	2100	352	—
5 × 10^17	1373	1469	1570	207	99	121	647	518	326	1.98
7 × 10^17	1353	1470	1559	145	122	99	321	851	360	0.89

Table 1.

Raman deconvolution parameters for varying argon ion fluences.

From Table 1, it can be seen that the position of G peak lies in the spectral region of 1559–1587 cm⁻¹, which is evidence for mixture of sp² and sp³ hybridization. In general, decreasing intensity ratio can be interpreted in terms of corresponding increase in sp³ content.

Table 1 depicts that G peak lies at 1587 cm⁻¹ for as-deposited SiC/Si(111) surfaces. This position of G peak, that is, P(G) moves progressively toward lower wavenumber for SiC surfaces following argon beam exposure at 5 × 10¹⁷ Ar⁺ cm⁻². This downshift is attributed to the lowering of C=C vibration modes by the heavier Si atoms in the surface layers. An additional cause of downshifting may be due to higher electronegativity of carbon atoms directly linked to silicon atoms. This direct linkage subsequently reduced the strength of C=C bonds and hence, absorption with lower frequencies. Along with this, linewidth broadening of G peak is observed with respect to the argon fluence. This clearly reveals the formation of more disordered amorphous structure at this stage of sputtering. Interestingly, intensity ratio of D peak to G peak is 1.98 revealing increase in sp³ content of C-C bonding in the surface layers. Thus, it can be concluded that initial amorphous SiC surfaces are transforming to highly disorder amorphous structure subjected to argon ion irradiation at lowest ion fluence of 5 × 10¹⁷ Ar⁺cm⁻².

With increase in ion fluence from 5 × 10¹⁷ to 7 × 10¹⁷ Ar⁺ cm⁻², further shift in G peak position toward lower wavenumber has been seen. In contrast to earlier case, we observed less dispersion in the G peak. Moreover, intensity ratio of D peak to G peak decreases drastically. Further, the fraction of amorphous Si increase at this stage of sputtering. Increasing Si content opens up the sp² sites, thereby creating more sp³ content on the surface.

Hence, Raman measurements reveal that the opening of sp² sites on a-C results in increased sp³ content in the surface layers. Both FWHM of G peak and I(D)/I(G) ratio decrease with increasing argon fluence.

3.2 Optical investigations of RF sputtered SiC over Si(111)

The optical behavior of RF sputtered SiC thin film and argon-irradiated SiC samples have been investigated by means of UV–Vis diffuse reflectance spectroscopy. Figure 3 presents the diffuse reflectance spectra of as-deposited and argon-sputtered SiC thin films at different fluences.

Figure 3.

Diffuse reflectance spectra of as-deposited and argon-sputtered SiC thin films.

It is clear from Figure 3 that diffuse reflectance decreases uniformly with increase in wavelength and shows prominent peak near 370 nm indicating the presence of fundamental absorption band in this region for as-deposited thin film. Multiple fringes can be clearly seen for SiC thin film sputtered with argon ion fluence of 5 × 10¹⁷ Ar⁺ cm⁻². Surprisingly, for samples sputtered with argon ion fluence of 7 × 10¹⁷ Ar⁺ cm⁻², only one broad hump near 280 nm is observed. These diffuse reflectance spectra are converted into corresponding absorption spectra using the Kubelka-Munk theory.

Figure 4 presents the variation of Kubelka-Munk function as a function of energy for different samples.

Figure 4.

Kubelka-Munk function F(R) against energy for SiC thin film as-deposited and irradiated with different argon ion fluences.

Firstly, the square of Kubelka-Munk function was calculated. Thereafter, this quantity is plotted with incident energy. This variation of F(R) with energy as plotted in Figure 4 is now extrapolated onto the x-axis (energy values). This extrapolation gives the measure of the values of the optical energy gap. The values of optical energy gap for SiC thin film as-deposited and irradiated with varying argon ion fluences are summarized in Table 2.

S. no.	Sample	Optical energy gap (eV)
1.	As-deposited SiC	1.44; 2.49; 3.32
2.	5 × 10^17 Ar^+ cm^−2	1.44; 2.58; 3.60
3.	7 × 10^17 Ar^+ cm^−2	1.18; 1.37

Table 2.

Values of optical energy gap.

Table 2 reveals that optical energy gap varies nonlinearly with argon ion fluence. An increase in the optical absorption and a shifting of absorption edge toward longer wavelengths has been observed. The observed behavior is attributed to the different degree of sputtering yield at different argon ion fluences and non-stochiometric sputtering of silicon and carbon.

3.3 Electrical investigations of RF sputtered SiC over Si(111)

Figure 5 illustrates the current–voltage (I-V) characteristics of bare Si(111), as-deposited SiC thin films and argon-sputtered SiC thin films.

Figure 5.

Current–voltage (I-V) characteristics of (a) bare Si(111) substrate, (b) as grown SiC on Si(111), 80 keV argon-sputtered SiC surface for fluence of (c) 5 × 10¹⁷ Ar⁺ cm⁻² and (d) 7 × 10¹⁷ Ar⁺ cm⁻².

The current passing through the films exhibited a noticeable reduction transitioning from (a) Si(111) to (b) SiC thin film and then with increase in argon ion fluence (c) and (d). This decrease in current exhibited a linear relationship graphically represented in Figure 5. The linear correlation between the applied voltage and the resulting current indicates that the as-deposited and argon-sputtered SiC thin films exhibit ohmic behavior within the designated voltage range of −5 to 5 V.

Moreover, the resistivity of the thin films can be determined by the equation [20, 21]:

$$\rho =R\frac{A}{l}$$E1

In this equation, $\rho$ represents the resistivity, R stands for the resistance, A denotes the surface area of the SiC thin films, and l represents the distance between the two measurement probes.

Interestingly, using these values of resistivity, conductivity can be calculated as

$$\sigma =\frac{1}{\rho }$$E2

Here, $\sigma$ signifies the conductivity of the thin films and $\rho$ corresponds to the resistivity of the SiC films. Figure 6 represents the variation of conductivity as a function of argon ion fluence.

Figure 6.

Variation of conductivity of SiC thin films with Ar⁺ ion fluence.

Interestingly, the conductivity of thin SiC thin films was found to decrease with increasing argon ion fluence. The increase in resistivity and decrease in conductivity of SiC thin films with increasing ion fluence can be attributed to the introduction of defects and disruptions in the crystal lattice. The ion beam bombardment leads to the creation of vacancies, interstitials, and lattice distortions in the SiC thin films. These defects act as scattering centers for charge carriers (electrons or holes), hindering their mobility and thereby increasing resistivity. The mobility of charge carriers is reduced due to an increase in scattering leads to a decrease in the conductivity of SiC thin films. In the case of the Si(111) wafer, the observed highest conductivity could be due to its single-crystalline structure and minimal defect density compared to the irradiated SiC thin films [22].

The increase in the optical band gap value with ion fluence suggests that the material’s electronic structure is being modified. The ion bombardment can induce changes in the electronic states of the material, including the creation of localized energy levels within the band gap. These levels can trap charge carriers, reducing their mobility and contributing to a decrease in conductivity.

4. Conclusions

In summary, the present work investigates the structural, optical, and electrical behavior of as-deposited and argon-sputtered SiC thin films at varying fluences keeping ion incidence at 50⁰. Raman studies reveal increased sp³ content in the surface layers. Both FWHM of G peak and I(D)/I(G) ratio decrease monotonically with increasing argon fluence. Further, an increase in the optical absorption and a shifting of absorption edge toward longer wavelengths has been observed. The I-V Characteristics reveal that the as-deposited and argon-sputtered SiC thin films exhibit ohmic behavior within the designated voltage range of −5 to 5 V. The observed structural behavior is attributed to the different degree of sputtering yield of silicon and carbon at different argon ion fluences. Hence, we propose that these structurally altered SiC thin films find more practical applications in advanced optoelectronics, photovoltaics, media recording, and storage devices.

Acknowledgments

This research is primarily supported by the Department of Science and Technology (DST), New Delhi by funding major research project for utilizing 200 kV ion accelerator and related characterization facilities. The authors are highly thankful to the Ministry of Human Resource and Development (MHRD) for RUSA 2.0 grants for the Center for Advanced Material Research (CAMR) at Kurukshetra University. The authors are grateful to Dr. Dinakar Kanjilal, Raja Ramana Fellow, and Dr. Sundeep Chopra, Inter University Accelerator Center (IUAC), New Delhi for fruitful discussions.