Silicon Carbide Thin Films Research Articles

Effect of silver doping on the optical properties of SiC thin films

In this work, pulsed laser deposition technique (PLD) with the Nd: YAG laser (1064 nm, 6 Hz, 500 mJ) had been used to prepare the pure and Ag-doped silicon carbide (SiC) films on the quartz substrates with various dopant ratios (1% wt, 3% wt, 5% wt, and 7% wt) of Ag. The optical absorbance measurements of SiC films have been studied by ultraviolet-visible spectrophotometry (UV–Vis). The optical parameters that calculated are absorbance, transmittance, reflectivity, absorption coefficient, optical energy gap, extinction coefficient, refractive index and complex dielectric constant. The results showed that the absorbance spectrum had been decreased as the doping ratio increasing. The optical energy gap was decreased from (2.62 to 2.02 eV) as the doping ratio increasing.

Investigation of Thin Film Properties of SiCN:H deposited by ECR PECVD with Acetylene and Ethane Hydrocarbon Sources

Silicon carbonitride (SiCN) thin films have drawn considerable interest among the ternary compounds due to the combination of unique properties such as high hardness, wide band gap, high photosensitivity in the ultraviolet (UV) region and low dielectric coefficient (k). In the last few decades various fabrication methods including reactive sputtering and plasma-enhanced chemical vapour deposition (PECVD) have been intensively studied to achieve SiCN thin films having attractive mechanical, tribological and optoelectronic features. Applications range from hard, wear-resistant coatings, low-k interconnects, UV photodetectors to gas separation membranes [1].The properties of thin films are not only influenced by the deposition method, which mainly determines the energy of bombarding ions, but also the choice of source gas [2]. In PECVD processes, silicon (Si), carbon (C) and nitrogen (N) can either be introduced separately as silane (SiH4), methane (CH4), and molecular nitrogen (N2) or ammonia (NH3), or alternatively using organic single precursors such as methylsilazanes [3]. In this work we deposited our thin films with the electron cyclotron resonance (ECR) PECVD method, which differs from other PECVD methods due to it is capability of generating a dense, highly ionized plasma (1011 ions/cm3) and ion impingement energies on the substrate as low as 20 eV [4]. We present the compositional and mechanical properties of hydrogenated SiCN (SiCN:H) thin films which were deposited with two different C precursors, acetylene (C2H2) and ethane (C2H6). The stoichiometry, density of the thin film, optical constants, and the bonding structure of SiCN:H thin films as a function of hydrocarbon carbon gas source have been explored. Due to the hydrogen-containing precursors used, the silicon carbonitride films deposited by CVD methods contain a significant amount of hydrogen (H). From Rutherford backscattering spectrometry (RBS), elastic recoil detection (ERD) analysis, quantitative elemental composition distributions including H were found for films deposited with both carbon sources. For further investigation of bonding structure of SiCN:H, Fourier Transform Infrared (FTIR) Spectroscopy and X-ray Photoelectron Spectroscopy (XPS) measurements were performed. Furthermore, we studied the hardness and Young’s modulus by nanoindentation, and variable angle spectroscopic ellipsometry (VASE) measurements were performed to extract optical constants. To interpret the measurements further, nearly stoichiometric silicon nitride and silicon carbide thin films were also prepared.[1] C.W. Chen, C.C. Huang, Y.Y. Lin, L.C. Chen, K.H. Chen, W.F. Su, Optical prop- erties and photoconductivity of amorphous silicon carbon nitride thin film and its application for UV detection, Diamond Relat. Mater. 14 (3-7) (2005) 1010–1013.[2] Schwarz-Selinger, T., Von Keudell, A., & Jacob, W. (1999). Plasma chemical vapor deposition of hydrocarbon films: The influence of hydrocarbon source gas on the film properties. Journal of Applied Physics, 86(7), 3988-3996.[3] V.I. Ivashchenko, A.O. Kozak, O.K. Porada, L.A. Ivashchenko, O.K. Sinelnichenko, O.S. Lytvyn, T.V. Tomila, V.J. Malakhov, Characterization of SiCN thin films: experimental and theoretical investigations, Thin Solid Films 569 (2014) 57–63.[4] M. G. Boudreau, "SiOxNy Waveguides Deposited by ECR-PECVD", M.Eng. thesis, McMaster University, 1993.

Multilayer SiC/C Thin Film Coating on Fiber Tip for Raman Probe Application

A novel technique in the fabrication of an optical filter that is made up of multilayer thin film is introduced in this study. Multiple layers of silicon carbide and carbon with high and low refractive index materials respectively will be deposited alternatively with a various thickness on a glass substrate by radio frequency magnetron sputtering of 99% purity of silicon carbide as the target. The variation of structure and optical properties of the films are studied by using X-ray reflectivity (XRR), Raman scattering spectroscopy, and UV–vis-NIR spectroscopy until it reached the desired results and meet the specification as Raman optical filter. The thin film is then placed on a multimode fiber tip as Raman edge filter. Raman fingerprint of specific alcohol (ethanol) obtained is tested using a fabricated filter with the standard spectrum. The preliminary result of SiC thin film fabrication for 30, 60 and 90 minutes by sputtering process showed rate of deposition obtained is 3.17nm/min. The results demonstrate that the optical properties of silicon carbide thin film can be monitored by modifying the condition of silicon carbide and carbon deposition.

Analysis of the physical and photoelectrochemical properties of c-Si(p)/a-SiC:H(p) photocathodes for solar water splitting

The photoelectrochemical (PEC) properties of sputtered aluminum doped hydrogenated amorphous silicon carbide thin films grown on p-type crystalline silicon substrates were investigated in 1 M solution under chopped light illumination. Optical and structural properties of the top absorber layer were systematically assessed after post-deposition isochronical annealing treatments. Samples exhibited a noticeable improvement of the opto-electronic properties after thermal treatments. In addition, an abrupt enhancement of the photocurrent was observed reaching a saturation value of 17 mA cm−2 at −1.75 V vs. Ag/AgCl (3.5 M KCl). In this research we propose that this enhancement effect is associated to a charge transfer kinetic mechanism influenced by surface states and the p-type substrate. The latter most likely due to the space charge region extending beyond the absorber layer reaching the substrate. Current density-potential and electrochemical impedance spectroscopy measurements in dark revealed a reduction of the native layer at cathodic potentials higher than −1 V vs. Ag/AgCl (3.5 M KCl), which contributes to the high charge transfer kinetic of the system. We believe that these results will contribute to understand the substrate influence in the PEC performance of top absorber layers in multilayer structures for solar water splitting.

Carbon ratio effect on the electrical and morphological properties of Silicon Carbide thin films

An experimental method for preparing a homogenous mixture of Silicon Carbide by a small quantity of concentrated sulfuric acid at room temperature was introduced. SiC film exhibits an amorphous state due to acidic treatment that etches the crystalline surface of the film. SiC films were characterized using FTIR, SEM and optical images. SiC films porosity has been observed by SEM images show the morphology of the final composite. Electrical conductivity of thin films have been evaluated by four point probe Hall measurements. The best ratio of 6Si:4C demonstrated the highest conductivity 0.55 μS.cm.

Boron-doped silicon carbide (SiC) thin film on silicon (Si): a novel electrode material for supercapacitor application

In this work, we report the synthesis of silicon carbide (SiC) thin film on silicon by modified chemical vapour deposition technique using boron-doped liquid polycarbosilane as a precursor. Subsequent microscopic and physical characterizations of this film show the presence of SiC nanocrystal along with boron in the SiC thin film. The electrochemical characterizations of the film shows a highest capacitance of 232F/g at 2.2A/g current density from Galvanostatic charging–discharging method with good cyclic stability upto 2000 cycles. The high capacitance value is attributed to the surface defect states and amorphous carbon which acts as the charge active sites. The result further infers that the (B)SiC/Si is a promising electrode material for high-performance energy storage application.

Abrasive mechanisms and interfacial mechanics of amorphous silicon carbide thin films in chemical-mechanical planarization

In this paper, the polishing process of a silicon carbide (SiC) substrate covered with a thin amorphous SiC (a-SiC) film is investigated by molecular dynamics simulation method. A diamond abrasive particle slides or rolls over the SiC workpiece surface with different depths and speeds. The results indicated that in the sliding motion, increasing the sliding depth leads to a higher force, stress, temperature, and a higher number of atoms removed. The best surface quality gains by sliding at 10 Å depth or at the depth that equal to the a-SiC layer thickness. At this polishing depth, the a-SiC plays a role as a padding layer that prevents the SiC substrate from subsurface damage. At a deeper depth, the crystalline SiC atoms at the interfacial area are transformed into the amorphous state. In sliding motion, the ploughing and cutting regimes dominate the atomic removal mechanism. Similar to the sliding motion, improving the rolling depth results in a higher force, stress, temperature, and the number of atoms removed. However, the rolling movement generates higher stress, deeper subsurface damage (SSD) zone, rougher surface and lower number of atoms removed. In rolling motion, the removal mechanism is mainly adhering to which the workpiece atoms are adhered by the abrasive surface. This report also investigates the effect of the abrasive size and the a-SiC thickness to the removal process. The bigger abrasive the higher force, stress, and the number of atoms removed. While increasing the a-SiC film thickness leads to a lower force and shallower SSD zone. Despite the differences in the abrasive size and the thickness, the sliding motion always performs a dramatically better ability to remove the workpiece atoms than the rolling one. A SiC model with the ripple structure is also constructed and analyzed. The bigger ripple mostly generates a higher force, temperature, strain, and a higher number of atoms removed. The high-temperature and high-value atomic strain areas mainly exist in the a-SiC film, at the upper side of the interface. The sliding motion again wipes out more atoms and creates a smoother groove than the rolling motion. Additionally, in the rolling motion, the large ripple can be lengthened and stuck to the substrate.

Enhanced Photoluminescence of Hydrogenated Amorphous Silicon Carbide Thin Films by Means of a Fast Thermal Annealing Process.

In this paper, the photoluminescence (PL) of hydrogenated amorphous silicon carbide (a-Si1−xCx:H) thin films obtained by Plasma Enhancement Chemical Vapor Deposition (PECVD) is reported. Strong PL is obtained after a fast annealing process for 60 s at temperatures of 200, 400, 600, and 800 °C. The thin films are characterized using Fourier Transform Infrared spectroscopy (FTIR), PL spectroscopy, and Energy-Dispersive X-ray Spectroscopy (EDS). According to the results of the structural characterization, it is deduced that a structural rearrangement of the amorphous matrix is carried out during the fast annealing process, which results in different degrees of oxidation on the a-Si1−xCx:H films. The PL peak position shifts towards higher energies as the temperature increases. The sample deposited with a silane/methane flux ratio of 37.5 at an Radio Frequency (RF) power of 6 W experiences an increase in PL intensity of more than nine times, with a displacement in the peak position from 2.5 eV to 2.87 eV, at 800 °C. From the PL analysis, we observe two emission bands: one centered in the near infrared and other in the visible range (with a blue peak). This study opens the possibility to use such thin films in the development of optoelectronics devices, with potential for application in solar cells.

Resistive switching behaviour of amorphous silicon carbide thin films fabricated by a single composite magnetron sputter deposition method

Amorphous silicon carbide (a-SiC) films of thickness 50–300 nm are deposited by a single composite target magnetron sputtering process. Metal–SiC–metal structures are fabricated to demonstrate resistive switching. The top metal electrode is Cu, Pt or Ag and the bottom electrode is fixed as Au. Reversible resistive switching from high to low resistance states is observed for SiC films at voltages between 1 and 5 V. The interface between metal electrode and a-SiC films plays a significant role in achieving optimal switching performance. Resistance OFF/ON ratios of $$10^{8}$$ , retention times $${>}10^{4}$$ s and endurance of 50 cycles are achieved in the best devices. Cross-sectional scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy are employed to understand the mechanism of device operation. Raman spectroscopy indicates the formation of nanocrystalline graphite in these devices after a few cycles of operation.

Tailoring of stoichiometry and band-tail emission in PLD a-SiC thin films by varying He deposition pressure

The present work focuses on the effect of He pressure (vacuum, 10−2–5 mbar) on structural and optical properties of amorphous silicon carbide (a-SiC) thin films, deposited by pulsed laser deposition (PLD) technique onto fused silica. A stoichiometric change occurs in PLD a-SiC film, with increasing He deposition pressure, transforming a near stoichiometric SiC material to C-rich SiC films till 1 mbar of He pressure followed by drastic decrease in carbon (C) content at 5 mbar. The optical band gap of the films estimated using Tauc plot from UV–Vis–NIR transmittance spectra showed initial increase from 2.09 to 2.61 eV with increasing He pressure from 10−6 to 1 mbar, then decrease to 2.11 eV at 5 mbar. A broad band photoluminescence (PL) peak featured in each of the a-SiC films at room temperature. The PL spectra exhibited blue shift in peak energy from 1.8 to 2.2 eV with increase in atomic percentage of C from 49.4 to 62.4%. The origin of the PL exhibited in these films is clearly due to band tail emissions from amorphous SiC structures which has been explained by the observed variation of PL intensity and corresponding FWHM with band gap as well as Urbach energy.

Deposition of boron-doped nanocrystalline silicon carbide thin films using H2-Ar mixed dilution for the application on thin film solar cells

Hydrogen-argon mixed dilution has been applied for the deposition of boron-doped nanocrystalline silicon carbide (nc-SiCx) thin films. The variations of structural, compositional, electrical and optical properties with the varying H2/Ar ratio are systemically investigated through various characterizations. It is shown that by using H2-Ar mixed dilution for deposition, B-doped nc-SiCx thin film possessing both wide optical band gap (∼2.22 eV) and high conductivity (∼1.9 S cm−1) can be obtained at the H2/Ar flow ratio of 360/140. In addition, the B-doped nc-SiCx thin films are fabricated as the window layers of a-Si thin film solar cells, and the highest conversion efficiency (8.13%) is obtained when applying the window layer with the largest optical band gap energy.

Spectrometry studies of Ag implanted silicon carbide thin films for application as a diffusion barrier against transition metals

A diffusion kinetics study of implanted silver (Ag) ions into SiC films is conducted. SiC thin films have been processed on Si (1 0 0) substrates by electron beam deposition from a commercial high purity hot-pressed SiC sheet. The as-deposited films were thereafter implanted with Ag+ ions. Realtime Rutherford backscattering spectrometry experiments showed that Ag+ is stable until 600 °C, which is the highest temperature that the heater in the system used could achieve. Conventional RBS studies on isochronally annealed films at higher temperatures in the range between 650 °C and 950 °C reveal that there is no significant change in the spectra up to a temperature of ∼ 750 °C. At 850 °C a change in the Ag implantation profile was observed, suggesting that Ag is mobile at that temperature. The corresponding depth profile at 850 °C suggests that there is significant Ag in- diffusion, however the overall integrated intensity data indicate that most of the Ag+ is escaping out of the films. Upon annealing for one hour at 950 °C, all the Ag ions have essentially escaped out of the films and/or spread throughout the layer below the detection limit of RBS. Isothermal annealing studies over varied duration times at 850 °C confirm Ag in-diffusion, but at the same time there is evidence of depletion of Ag near-surface regions as the annealing time increases; the overall out-diffusion of the implanted metal, although steady, is found to be slow in an isothermal process. FTIR investigations point to a diffusion model through pores and grain boundaries.

Confinement induced variation of composition ratio in amorphous silicon carbide thin films and effect in optical properties

Thickness dependency in the composition ratio of amorphous silicon carbide (SiC) thin films and its effect in optical properties are reported. SiC thin films (~ 20 nm to 450 nm range) with very low surface roughness (<1 nm) were grown using dual ion beam sputtered (DIBS) deposition technique. Thinnest SiC film (~ 20 nm) exhibited higher atomic concentration of silicon (Si) relative to that of carbon (C), with C:Si = 0.7, whereas carbon to silicon atomic concentration ratio (C:Si) was found to increase with increasing SiC film thickness, and reached 1:1 ratio for thickest SiC film (about 450 nm), using X-ray photoelectron spectroscopy (XPS) analysis. XPS was employed to investigate the chemical composition and bonding configuration of SiC. A thickness dependent distinct change in band gap as a consequence of variation in the composition ratio in SiC films, is reported, which indicates potential control on electrical and optical properties of the system.

Effect of DC negative bias on microstructure and surface morphology of amorphous silicon carbide films prepared by HWP-CVD

The effect of DC negative bias (− Vs) on microstructure and surface morphology of amorphous silicon carbide thin films prepared by helicon wave plasma chemical vapor deposition is reported. Microstructure and surface morphology were obtained by scanning electron microscope (SEM) and atomic force microscope (AFM). The results show that the increase of − Vs on the substrate make a more compact film and lower surface roughness, which can reach 0.56 nm. The XRD analysis reveals that the SiC thin films are of an amorphous structure. Percentages of carbon and silicon atoms (C/Si) were measured by energy dispersive spectrometer (EDS), and the C/Si ratio can reach 1.45. The structural properties of the films were studied by Raman spectroscopy techniques and Fourier transform infrared (FTIR). It is found that the films contain not only Si–C bonds but also Si–CHx bonds. Raman spectra results show that the proportion of disordered carbon in the films decreases with the increase of − Vs. The results of ultra-microhardness tester show that the hardness of the films increases with the increase of − Vs and the maximum mechanical hardness can reach 18.5 GPa at − Vs = − 60 V.

Ultra-sensitive UV sensors based on porous silicon carbide thin films on silicon substrate

In this report, high-performance ultraviolet (UV) detectors were designed based on porous silicon carbide (SiC) thin films on silicon (Si) substrate. The results can broaden the applications of porous SiC structures in sensing applications. Here, n-type Si (100) was used as a substrate for epitaxial growth of SiC thin films. In order to fabricate porous SiC thin films, a two-electrode, photo-assisted electrochemical etching process was carried out using an integrated current source. The improvement in optical characteristics of porous SiC/Si was reported by tuning the anodization current density. It was illustrated that current density is an effective parameter for controlling the morphology of porous samples. The optical properties of samples were studied using photoluminescence (PL) and optical reflectometry. The results showed that by applying an optimized value of etching current density, the optical reflectivity is decreased which is due to the elevated specific surface area of porous samples that captures the incident light and reduces the reflection. Enhancement in porosity of optimized porous SiC/Si sample was illustrated by its elevated PL intensity. The UV sensing capability of fabricated metal-semiconductor-metal (MSM) detectors based on porous SiC/Si samples with different etching current densities was investigated. The device based on the optimized porous sample showed enhanced sensitivity (54.51) to UV illumination due to the elevated photogenerated current. Moreover, the ultrafast sensing behavior of this device indicates its improved optoelectrical performance in UV detection.

Pulsed laser deposition of SiC thin films and influence of laser-assisted annealing

Silicon carbide (SiC) thin films were grown by pulsed laser deposition (PLD) on Si (1 0 0) substrates at a substrate temperature of 800 °C. Besides, laser annealing was performed on the post deposited intrinsic amorphous SiC films using Nd3+:YAG 355 nm laser in the Ar environment. The laser-annealed samples showed the crystalline characteristics. Crystalline characteristics of PLD grown, laser annealed samples were identified by X-ray diffraction (XRD), Raman analysis. Numerical analysis performed on SiC/Si interface to investigate the temperature distribution, to understand the mechanism of laser assisted annealing.

X-ray Absorption Spectroscopy of Silicon Carbide Thin Films Improved by Nitrogen for All-Silicon Solar Cells

Synchrotron-based experiments in combination with optical measurements were used to explore the potential of a photovoltaic material based on silicon carbonitride (SiCN) thin films, in particular for the use in space solar cells. The large bandgap, SiCN films were fabricated using electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) followed by low-temperature annealing processes. X-ray absorption near edge structure (XANES) with excitations at the carbon, nitrogen, and silicon K-edges verifies that the presence of nitrogen tends to disrupt Si–C networks. This results in the enhancement of light absorption and bandgap widening, which is desirable for front emitters in all-silicon tandem solar cells. The ternary structure of SiCN allows bandgap engineering and tuning of the light absorption and refractive index through careful design of the composition. XANES showed that the thermal annealing at a medium temperature (500 °C) using N2 ambient promoted the formation of Si–Si and C–N sp2 bonds before disappearing in higher annealing temperatures. In our opinion unlocking the potential of robust SiC mixed with nitrogen in SiCN matrix has appeal in radiation-resistant solar cells, where it can serve as the top emitter layer in all-silicon tandem solar cells and at the same time benefits the antireflection properties.

Application of polycrystalline silicon carbide thin films as the passivating contacts for silicon solar cells

In this paper, silicon rich polycrystalline silicon carbide (poly-SiCx) thin films were prepared for passivating contact of silicon solar cells. The effect of different carbon doping ratios R [methane flow (sccm)/silane flow (sccm)] and the annealing temperatures on the passivation quality was investigated. The lifetime test showed that, given a lower annealing temperature, the passivation quality of poly-SiCx thin films was very poor at R over 0.2, and it can be improved by elevating the annealing temperature. The effective lifetime of over 1.8 ms can be obtained with implied open-circuit voltages (iVoc) of 715 mV and saturated dark current (J0) of 18 fA/cm2. The microstructure and optical property of poly-SiCx films was studied by Raman spectra, X-ray photoelectron spectroscopy and UV-VIS spectrophotometer. The results showed that the incorporation of carbon lowered the crystalline fraction of poly-SiCx and a higher annealing temperature was needed to achieve high crystallinity. The optical band gap of poly-SiCx was widened with increase in R, and got up to 2.3eV when R was 0.4. A proof-of-concept top/rear TOPCon solar cells, featuring a N+-poly-SiCx (R = 0.2) front contact, was fabricated to demonstrate the potential of this SiCx passivation contact. The decrease in the parasitic absorption of light at the front side resulted in higher photogenerated current. And the conversion efficiency of 20.17% was achieved.

A Technique for Estimation of Residual Stress and Young’s Modulus of Compressively Stressed Thin Films Using Microfabricated Beams

Predictive design of MEMS devices and their performance evaluation require post-fabrication measurement of essential material properties, such as Young's modulus and residual stress, as these are usually affected by fabrication process conditions. To our knowledge, few techniques exist for the measurement of residual stress in micromechanical structures fabricated from compressively-stressed amorphous dielectric thin films. Here, we propose a method for estimating residual stress and Young's modulus from the post-buckling dynamic response of fixed-fixed microbeams and frequency response of cantilevers, respectively. Static deflection profiles and first natural frequencies of buckled beams are used in this analysis. Further, the proposed method uses the length of the beams as an unknown to account for the variation in the actual length of a fabricated beam due to the presence of undercut. The effective length is simultaneously determined from the analysis of the measured data using multi-parameter curve fitting. Use of post-buckling analysis makes this technique useful for thin films with high compressive stresses. Since the proposed method does not involve electrostatic actuation, it is applicable for dielectric thin films in addition to crystalline and conducting films. Using the proposed method, residual stress is estimated in amorphous silicon carbide thin films deposited by PECVD and reactive-sputtering. In addition, the modulus values of three materials (PECVD SiNx, PECVD SiC, and sputtered SiC) are estimated and compared with reported results. [2019-0090]

Investigation of the Young's Modulus and the Residual Stress of 4H-SiC Circular Membranes on 4H-SiC Substrates.

The stress state is a crucial parameter for the design of innovative microelectromechanical systems based on silicon carbide (SiC) material. Hence, mechanical properties of such structures highly depend on the fabrication process. Despite significant progresses in thin-film growth and fabrication process, monitoring the strain of the suspended SiC thin-films is still challenging. However, 3C-SiC membranes on silicon (Si) substrates have been demonstrated, but due to the low quality of the SiC/Si heteroepitaxy, high levels of residual strains were always observed. In order to achieve promising self-standing films with low residual stress, an alternative micromachining technique based on electrochemical etching of high quality homoepitaxy 4H-SiC layers was evaluated. This work is dedicated to the determination of their mechanical properties and more specifically, to the characterization of a 4H-SiC freestanding film with a circular shape. An inverse problem method was implemented, where experimental results obtained from bulge test are fitted with theoretical static load-deflection curves of the stressed membrane. To assess data validity, the dynamic behavior of the membrane was also investigated: Experimentally, by means of laser Doppler vibrometry (LDV) and theoretically, by means of finite element computations. The two methods provided very similar results since one obtained a Young’s modulus of 410 GPa and a residual stress value of 41 MPa from bulge test against 400 GPa and 30 MPa for the LDV analysis. The determined Young’s modulus is in good agreement with literature values. Moreover, residual stress values demonstrate that the fabrication of low-stressed SiC films is achievable thanks to the micromachining process developed.