Amorphous Silicon Carbide Thin Films Research Articles

Influence of the substrate conductivity type on the electroluminescence properties of PP+IN and PIN+N diodes based on amorphous silicon carbide (a-Si1-xCx:H) / crystalline silicon (c-Si) heterostructures

The study of the conduction and light emission mechanisms of PIN diodes based on amorphous silicon-carbide (a-Si1-xCx:H) thin film and crystalline silicon (c-Si) substrates is reported in this work. Using the same low-temperature process (200 °C), PP+IN and PIN+N structures were fabricated on p-type and n-type silicon (Si) substrates, respectively. The plasma enhanced chemical vapor deposition (PECVD) technique was employed for the active layers deposition where two different CH4/SiH4 flow rates ratios of 30 sccm/6 sccm and 30 sccm/1 sccm were used. All structures show red-orange light emission in forward bias (FB). The structures with the active layer deposited with a lower SiH4 flow rate, display light emission with a higher intensity at a lower voltage bias.In addition, we related the current-voltage measurements with different conduction models and the dominant charge transport mechanisms in the PIN structures were proposed. The highest Si content structures were related to Fowler-Nordheim (F-N) tunneling, because of its active layer with lower barrier height. On the other hand, the lowest Si content structures were associated with the Poole-Frenkel (P-F) emission, due to a higher density of defects in localized tail states. The results showed the same conduction and emission mechanisms for the PP+IN and PIN+N diodes with the same active layer, regardless of the conduction type of substrate, proving that it is feasible to use the same process for developing light emission devices on p-type and n-type Si substrates.

Study of the annealing effect in optical properties for phosphorus-doped a-Si x C1− x :H films deposited by PECVD

The optical properties of phosphorus-doped hydrogenated amorphous silicon carbide (P-doped a-Si x C1−x :H) thin films are studied. Films were deposited by plasma-enhanced chemical vapor deposition at 110 kHz of frequency and pressure of 1.5 Torr, with silane (SiH4) and methane (CH4) as precursor gases. Hydrogen (H2) and phosphine (PH3) were used as diluent and dopant gases, respectively. The impact of the gases flow rate and thermal annealing on the optical properties is evaluated. A concordance is observed between the atomic content of Si, C, and P in films, determined by Energy Dispersive X-ray Spectroscopy, as gases flow rate increases. The composition of the films is established by identifying the vibrational bonding modes using Fourier transform infrared spectroscopy measurements. Si−H, Si−C, and C−H bonds were identified and their density was obtained. Characterization with UV–Vis spectroscopy shows that the optical band gap (E gopt), oscillation energy (Eo ), Urbach energy (EU ), and iso-absorption energy (E 04) increase with the doping level but decrease with the annealing treatment. However, values of dispersion energy (Ed ), dielectric constant (ϵ) and the refractive index (n) generally become smaller as the phosphorus and carbon content grows, but with a tendency to fall during the annealing. The refractive index by UV–Vis is compared with measurements made with ellipsometry, resulting in a similarity between both techniques. High values of E gopt obtained in P-doped a-Si x C1−x :H films can be related to better values of the temperature coefficient of resistance for flow sensor applications.

Opto-electrical properties of amorphous silicon carbide thin films adjustably prepared by magnetron sputtering at room temperature

The application of silicon carbide (SiC) thin films is extensive, and they can be prepared at room temperature using magnetron sputtering. In this study, a wide range of deposition pressures (0.5–5.5 Pa) was utilized for the sputtering of amorphous SiC thin films. With an increase in deposition pressure, not only does the oxygen content increase, but also the carbon phase changes. The oxygen ratio increases from approximately 6.8% to 20.0% as the deposition pressure rises from 0.5 Pa to 1.5 Pa, and it gradually increases further to around 25.0% as the deposition pressure rises to 5.5 Pa. When the deposition pressure is>3.0 Pa, the amorphous carbon phase transforms into diamond-like and graphite-like carbons. As a result, the optical bandgap varies from about 1.7 eV to 3.6 eV with the change in deposition pressure, while the conductivity reduces from 2.5 × 10-4 S/cm to 1.3 × 10-8 S/cm. This study provides insights into the significant impact of high deposition pressure on the material properties of sputtered SiC thin films. It demonstrates that the chemical bonding, composition, and opto-electrical properties of SiC thin films can be effectively tailored for application in optoelectronic devices.

In-situ growth mechanism and structural evolution of silicon quantum dots embedded in Si-rich silicon carbide matrix prepared by RF-PECVD

Amorphous silicon-rich silicon carbide (a-SixC1-x) thin films that contain Si quantum dots (QDs) were prepared using radio-frequency plasma-assisted chemical vapor deposition (RF-PECVD) from reactive silane and methane precursor gases at a substrate temperature of 300 °C. The effect of the silane-to-methane flow ratio on the structural properties of the Si QDs/a-SixC1-x nanocomposite films, is systematically studied by means of high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The obtained results confirm the in-situ formation of Si QDs within the amorphous SiC matrix. Moreover, it is found that the Si crystalline fraction increases from 60.7 to 78.5% with increasing the gas flow ratio. Photoluminescence analyses reveal broader emission spectra which are explained in terms of quantum confinement-related emission corresponding to different QD sizes. Based on the obtained results, the growth mechanisms of Si QDs during the deposition of the a-SixC1-x film and the amorphous-to-crystalline transition are discussed.

High thermo-optic tunability in PECVD silicon-rich amorphous silicon carbide.

In this work, the thermo-optic coefficient (TOC) of the silicon-rich amorphous silicon carbide (a-SiC) thin film deposited by plasma-enhanced chemical vapor deposition (PECVD) was characterized. We found that the TOC of the film increases as its silicon content increases. A more than threefold improvement in the TOC was measured, reaching a TOC as high as 1.88×10-4 ∘C-1, which is comparable to that of crystalline silicon. An efficient thermo-optic phase shifter has also been demonstrated by integrating the silicon-rich a-SiC micro-ring structure with a NiCr heater. Tunability of 0.117 nm/mW was demonstrated, and a corresponding tuning efficiency P π as low as 4.2 mW has been measured at an optical wavelength of 1550 nm. These findings make silicon-rich a-SiC a good candidate material for thermo-optic applications in photonic integrated circuits.

Development of Heterojunction c-Si/a-Si1-xCx:H PIN Light-Emitting Diodes.

In this work, we explored the feasibility of the fabrication of PIN light-emitting diodes (LEDs) consisting of heterojunctions of amorphous silicon-carbide (a-Si1-xCx:H) thin films and crystalline silicon wafers (c-Si). The objective is the future development of electro-photonic systems in the same c-Si wafer, containing transistors, sensors, LEDs and waveguides. Two different heterojunction LEDs were fabricated consisting of PIN and PIN+N structures, where a-Si1-xCx:H thin films were used as P-type and I-type layers, while an N-type c-Si substrate was used as an active part of the device. The amorphous layers were deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique at a substrate temperature of 200 °C. The PIN device presented electroluminescence (EL) only in the forward bias, while the PIN+N device presented in both the forward and reverse biases. The EL in reverse bias was possible due to the addition of an N+-type a-Si:H layer between the c-Si substrate and the I-type a-Si1-xCx:H active layer. Likewise, the EL intensity of the PIN+N structure was higher than that of the PIN device in forward bias, indicating that the addition of the N-type a-Si:H layer makes electrons flow more efficiently to the I layer. In addition, both devices presented red EL in the full area, which is observed with the naked eye.

Elaboration and Characterization of Amorphous Silicon Carbide Thin Films (a-SiC) by Sputerring Magnetron Technique for Photoelectrochemical CO2 Conversion

Elaboration and Characterization of Amorphous Silicon Carbide Thin Films (a-SiC) by Sputerring Magnetron Technique for Photoelectrochemical CO2 Conversion

Thermal and Mechanical Properties of Amorphous Silicon Carbide Thin Films Using the Femtosecond Pump-Probe Technique

Nanoscale amorphous silicon carbide (a-SiC) thin films are widely used in engineering applications. It is important to obtain accurate information about their material properties because they often differ from those of the bulk state depending on the fabrication technique and process parameters. In this study, the thermal and mechanical properties of a-SiC thin films were evaluated using the femtosecond pump-probe technique, which provides high spatial and temporal resolutions sufficient to measure films that have a thickness of less than 300 nm. a-SiC films were grown using a plasma-enhanced chemical vapor deposition system, and the surface characteristics were analyzed using ellipsometry, atomic force microscopy, and X-ray reflectometry. The results show that the out-of-the-plane thermal conductivity of the films is lower than that of bulk crystalline SiC by two orders of magnitude, but the lower limit is dictated by the minimum thermal conductivity. In addition, a decrease in the mass density resulted in a reduced Young’s modulus by 13.6–78.4% compared to the literature values, implying low Si-C bond density in the microstructures. The scale effect on both thermal conductivity and Young’s modulus was not significant.

Si diffusion induced adhesion and corrosion resistance in annealed RF sputtered SiC films on graphite substrate

Silicon carbide (SiC) is one of the promising candidates for graphite protection in different applications involving high temperatures and a highly corrosive environment. An ideal Silicon carbide coating should withstand a corrosive environment without compromising its adhesion. Herein, RF magnetron sputtered silicon-rich SiC thin films were deposited on a graphite substrate followed by annealing at 1000 °C, 1200 °C, and 1400 °C in an inert atmosphere. The impact of annealing temperature on microstructure, adhesion and chemical stability of SiC thin films was demonstrated. Different analytical techniques like Scanning electron microscopy (SEM), X-Ray Diffraction (XRD), Fourier's Transform Infrared (FTIR) spectroscopy and nano-indentation were used to study microstructural evaluation and mechanical characteristics. Moreover, the electrochemical analysis (Tafel and Electrochemical impedance spectroscopy) was performed in 3.5% NaCl solution. The microstructural analysis revealed that the amorphous SiC thin film turned into a crystalline and dense film upon annealing. Scanning electron micrographs showed that silicon-rich regions at SiC film surface started to disappear as Si diffuses into graphite matrix at elevated temperatures. Both these factors contributed to improvement in the adhesion of SiC coating with graphite substrate as annealing temperature increased. In addition, the nano-indentation hardness of 5.2 GPa was obtained for as grown sample, which decreased at 1000 °C, and then increased at 1200 °C and 1400 °C. Furthermore, the electrochemical analysis confirmed the enhancement in corrosion resistance upon annealing at a temperature of 1200 °C and 1400 °C due to Si diffusion and film compactness in these samples.

Amorphous silicon carbide thin films doped with P or B for the photoelectrochemical water splitting devices

Photoelectrochemical water splitting devices require semiconductor photoelectrode material fulfilling a number of primary requirements such as band gap, band edge alignment and corrosion resistance to electrolyte. Amorphous silicon carbide films, undoped and doped (P or B), were deposited on Si substrates by PECVD technology. The concentration of elements in the films was determined by RBS and ERD analytical method. Raman spectroscopy study of the SiC films were performed by using a Raman microscope and chemical compositions were analyzed by FTIR, before and after immersion of samples to aqueous pH 2.0 and pH 1.0 sulphuric acid electrolyte. Electrical properties of SiC films before and after immersion of samples to aqueous pH 2.0 and pH 1.0 sulphuric acid electrolyte were studied by measurement of the I–V characteristics on structure Al/SiC/Si/Al. Differences between Raman spectra, FTIR spectra and I–V characteristics before and after immersion to electrolyte are discussed.

Electrical and optical properties of amorphous silicon carbide thin films prepared by e-beam evaporation at room temperature

Amorphous silicon carbide (SiC) thin films can be applied as window layers for perovskite solar cells, where low processing temperature and ion-bombardment free is favorable. By taking advantages of the e-beam evaporation technique, amorphous SiC thin films were deposited at room temperature in this work. The dependency of e-beam current in material composition was investigated, as well as the conductivity and optical bandgap. It is shown that SiC films consist of Si-O component and its ratio can be tuned from 57.5% to 10.0% by controlling the e-beam current from 38 mA to 78 mA. The CC component tends to form at e-beam current more than 58 mA. By adjusting the Si-O and CC components, amorphous SiC thin films with optical bandgap of 1.7–2.3 eV and conductivity of 10−6–10−4 S/cm were obtained with e-beam current of 38–52 mA, which possesses a good potential for applications in solar cells.

Ripple patterns over oblique Ar+ sputtered SiC/Si(1 1 1) surfaces: Role of preferential sputtering

Controlled surface structuring of amorphous silicon carbide (a-SiC) thin films as grown on silicon substrates Si(111) by Radio Frequency (RF) sputtering and irradiation of obliquely incident 80 keV Ar+ ion beam has been investigated. Sub-wavelength ripple patterns with wave-vector parallel to the ion beam projection are found to evolve on argon sputtered surfaces. Studies reveal that the temporal parameters such as ripple wavelength and amplitude, ordering and homogeneity of these patterns vary non-linearly with argon ion fluence. The formation of such surface structures is attributed to the preferential sputtering of silicon in comparison to carbon. Our results reveal that ion irradiation can be exploited to fabricate controlled surface nanostructures by optimizing the irradiation parameters.

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.

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.

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.

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]