|Journal of Renewable |
Physical Properties of SiC Nanostructure for Optoelectronics Applications
1Applied Science Department, University of Technology-Iraq, Baghdad, 10066, Iraq
2Laser and Optoelectronic Engineering Department, University of Technology-Iraq, Baghdad, 10066, Iraq
*Corresponding Authors: Evan T. Salim. Email: firstname.lastname@example.org; Makram A. Fakhri. Email: email@example.com
Received: 20 December 2020; Accepted: 18 January 2021
Abstract: A SiC nanofilms have been deposited and investigated on quartz and silicon substrates using pulsed laser deposition technique with the 300 pulses of Nd: YAG laser at two different laser wavelengths of 1064 nm and 532 nm. The structural, morphological, and optical properties of the deposited nanostructure SiC were prepared and characterized as a function of the wavelengths of the used laser. The structural result shows four different pecks at (111), (200), (220), and (311) planes related to Nano SiC. The transmission result presents that the optical energy gap value for the SiC nanostructure is depended on the wavelength of the used laser and it is found about the range (3.03 eV–3.23 eV). The investigations of the SEM and AFM show that the prepared SiC Nano-films having a grain size range (36.34–48.75) nm and roughness about 4.462 to 3.062 nm. SiC/Si hetero-junction devices show an enhanced performance at 532 nm.
Keywords: Pulsed Laser deposition; nanostructure; 4H-SiC; PLD; heterostructure; nanofilms
Silicon carbide (SiC) are amongst the materials with the widest optical energy gap abut (2.0 eV ≤ E ≤ 7.0 eV) [1−4]. This behavior allows the production of UV detectors to be used [5,6]. The SiC-integrated photodetectors are capable of producing high signal-to-noise ratios that work for solar-blinds [7−9].
SiC Nanostructures are attracted due to their perfect properties such as the high values of the thermal, the wide-optical bandgap semiconducting materials, the stability of the chemical, large mobility, high breakdown of the field strength, and high-temperature resistance [10−12].
The structural, the morphological, and the optical properties (the physical properties) of the nanostructures of the SiC are greatly influenced by the carbon and the silicon film contents, and therefore, the preparation methods, as well as Si and C sources, are of interest [13−15].
In recent years, it got good competition for various traditional materials for different multi-disciplinary implementation. It is premium properties such as (the electronic and the electrical) make it favorable materials are using for the next-generation optoelectronic, and photonic devices [16–18]. Recently, Kaur et al.  deposited the Nanostructure of β-SiC, which was conducted selective growth for β-SiC by laser irradiation excimer of the PMMA (polymethyl methacrylate) [20,21].
It has been widely used in different applications such the Nanoelectronics, optoelectronics, the LED (light-emitting diodes), and the photonic , cells of the photoelectrochemical , photocatalyst , different sensor , and the energy storage .
Several different synthesis methods, techniques, and gases were used in the preparation and deposition of the SiC or its nanostructures with the electrical, electronic, and other required physical properties at the relatively low values of the substrate temperatures such as CVD (chemical vapor deposition) [27−29], MBE (molecular beam epitaxy) [30,31]. Sol-gel and electrospinning [32−34], Pulsed laser ablation in liquids  PE-CVD (chemical vapor deposition enhanced by plasma) , PLD (pulsed laser deposition) , and the MBE (molecular beam epitaxy) .
The (PLD) Laser pulse deposition, is used the method on a large scale and allows the growth of super-conductors with high temperatures. This technique is run at a low temperature relatively and is capable of producing Nano thin films (silicon carbide is amorphous or polycrystalline) [39−42]. This method has several advantages, such as the activation of the types extracted from the user and the selected target. This will enhance the chemical nature of film surface growth, which reduces the requirements for the activation of the surfaces [43−48]. Moreover, there is a control for the thickness of the deposited films by controlling the growth of the kinetics, simply by manipulating the (frequency) or the repetition rate and the used laser power in addition to the deposition pressure [49−53].
In this work, a two laser wavelengths has been used to prepare SiC nanostructure on the quartz and Si substrates. The physical properties of thin films and SiC/Si device performance have been investigated.
2 Experimental Work
A high purity powder (99.99%) of the SiC from Aldrich was used in the (Pulsed Laser Deposition) PLD method and was prepared by pressing the powder of the SiC using a hydraulic press (15 Tonne) to get a pellet property diameter of 25 mm and a height of 5 mm. A Q-Switching- Nd: YAG laser RY 280, Chin, of 300 pulses of laser, 6 ns of the pulse duration, With an average frequency of three Hz/sec., and 1500 mJ energy of the used laser to deposit the material of the prepared targets on a substrate of the quartz at the different laser wavelength. PLD mechanism include the ablation of the target material employing short pulses of laser wavelength as energy source, such that the ablated particle deposited on the desired substrate in a vacuum environment. The powder material is compressed under high mechanical pressure to take a pellet shape.
The different laser wavelength of (1064 nm and 532 nm) was used to ablate the substrate during the deposition process under vacuum pressure of about 10-3 mbar. After finishing the process of deposition, the deposited Nanofilms were undergoing a process of annealing at 400°C for 4 h. A Double-beam from (Shimadzu UV-Vis 1800, Japan) spectrophotometer has tested the transmissions and the absorptions of the deposited films at the range of the wavelength (300–800) nm, and also the value of the energy bandgap (Eg) was calculated depending on the (transmission) T%, and the (absorption ) A%, the relation between A, T, and reflection are A + T + R = 1 since the optical beam has a vertical incidence on the sample there for the R assume to be zero, and the results related to the absorption and hence the transmission from the film.
X-ray diffraction system (X’Pert Pro MRD PW3040) with the Cu-Kα radiation where λ = 0.15418 nm was used to tests the properties of the structure for the SiC Nanofilms.
Scanning Probe Microscope of (SPM-9600), from the Japan (Shimadzu company), and the (JOEL JSM-6460LV, Oxford Instruments Analytical, Ltd., Tarvonsalmenkatu, Finland) device from Japan has been used to exam the final properties of the topography for the Nanofilms deposited, that are represented by the AFM, and SEM, respectively.
A single crystalline silicon substrate of 1–6 Ω cm electrical resistivity’s was used for device manufacturing; a figure print like aluminum mask has been deposited to maintained the device performance. To investigate I-V in dark and illumination using (DF LT30/2) Power Supply, (UNI-T-UT33C) and (TEKR. CDM 250) Digital Multi-meters. The C-V results were tested using RCL programmable Meter from (Fluke PM6306). Finally, the value of the spectral responsivity of the silicon carbide heterostructure was tested at the region of the spectral of (200 nm–900 nm) using an optical monochromator. All the previous tests were carried out in the temperature room.
3 Results and Discussion
Fig. 1 shows the transmission spectrums of the SiC Nano thin films, the deposition conditions seem to have a strong and direct effect on the optical properties of the films. In general, the value of transmission displayed a relative increase in their value from 81.9% to 96.2%, with the changes in laser wavelength from 1064 nm to 532 nm, respectively. The low value of the transmission in the region of UV, as a part of the physical properties of the silicon carbide, where its values are related to the value of the optical bandgap in the Uv-region. At the visible region, the optical transmission as a high value indicates the low values of the surface roughness and very good homogeneous films [12,54,55].
The absorption spectrums of the SiC Nano-films at different laser wavelengths have been presented in Fig. 2. The absorption (A) values were estimated based on the values of the transmission. It is observed that the values of the optical absorption were decreased by using the Second Harmonic Generation (SHG) Nd-YAG laser; this will be suitable for using such film in optoelectronic device fabrication where window effect will be dependent for the hetero-junction device. Then elevated absorption value near 300 nm is related to cut off wavelength of the SiC bandgap after which the material is transparent to all incident wavelengths.
The value of the energy bandgap (Eg) for the deposited SiC was evaluated by extrapolated the straight line of the curve between the photon energy (hν) and (αhν)². The intersection of this drawn straight line with the main curve on the x-axis corresponds to the value of the optical energy bandgap as presents in Fig. 3. The bandgap values of the deposited Nano SiC films are around 303 eV and 3.23 eV for both 1064 nm and 532 nm, respectively. The reduction in bandgap value at the SHG wavelength may be related to increases in the laser flounce which resulted in decreases in the particles size, therefore according to the Burstein Moss–effect, the energy gap-related inversely to the particle size, and also because the increase in the sizes of the grains leads to a high crystallization which means less stress in the film. That could result in an increase in the bandgap as a result of the reduction of the crystal defects inside the material [56,57].
Fig. 4 present the patterns of the x-ray diffractions (XRD) for the Nano SiC films deposited at different wavelengths of Laser. The SiC Nanofilms X-ray pattern is deposited at 1064 nm exhibits four peaks manifest at the angles of diffraction of 35.42° and 42.54°, 60.82° and 73.24° corresponds to the planes of (111), (200), (220), and (311), respectively. When decreased the wavelength of the used laser to 532 nm, the pattern of the x-ray (XRD) of the deposited SiC shows the presence of the same peaks fastened at angles of the diffraction of 35.42° and 42.54°, 60.82° and 73.24° corresponds to the planes of (111), (200), (220), and (311) respectively but it differs in the intensities. All the present peaks in both wavelengths used in the preparation are indexed to the structure type of hexagonal and it appears to 4H-SiC depending on the card (JCPDS #22-1317). Moreover, it clearly presents that the values of the intensity for the pattern of the XRD peaks were enhanced and increased with the increase in the wavelength of the used laser as a result of an increase in grain size and as a result of promotion of the quality of the crystals (high crystenality) [25,58].
The estimated values of the average size of the grains, the roughness, and the root mean square RMS values for the Nanofilms of the deposited SiC on the quartz substrates at different laser wavelengths based on the atomic force microscopy (AFM) results are presented in Fig. 5. The results show the regular density surface with a uniform distribution of the deposited grains throughout the surface. The presented results are indicating that the average diameter of the grains is inversely proportional to the used Laser wavelength. Where these values changed from (36.34–48.75) nm where the grains of 86 nm for the used laser wavelength are 532 nm and the grains of 98 nm for the used laser wavelength are 1065 nm. Thus, it can be shown that the increase in the used laser wavelength means an identical increase in the values of the optical bandgap of the prepared and deposited Nanofilms. In conjunction with the decreasing of the used laser wavelength, the values of the mean roughness and the RMS roughness are decreased. Where, the values of the average roughness are increased from (2.1 to 3.3) nm, while the RMS roughness increased from (3.062 to 4.462) nm. Controlling the RMS roughness and the average roughness is very important in optoelectronics devices using SiC Nanofilm. All estimated constants of the surface are tabulated below in Tab. 1.
Fig. 6a presents the images of the results of SEM for the Nanofilms of the SiC production at different laser wavelengths using the PLD method. The size and the morphology of the deposited SiC Nanofilms are clearly dependent strongly on the wavelength of the used laser. It is clear that of the presented images there is a mixture of Nano-sized spherical particles and Nano flakes that are horizontally grown. For the deposited Nanofilms of the SiC synthesized with 532 nm wavelength of the used laser it clear that the number of Nano spherical grains is more distributed from the flakes, and the average of these particle sizes was about 65 nm and the average size of the flakes were about 85 nm. The concentration of SiC nanofilms increased as the used laser wavelength is decreased to 532 nm. The images of the SEM are confirmed that the number of lumpy grains (Nano flakes) was increased at the high laser wavelength because of the laser wavelength increase due to the reduction of the incident beam energy which results in lowering the particle size. The EDX result shows in Fig. 6b insure the successful formation of the SiC nanostructure.
Fig. 7 presents the dark electrical (I-V) properties at the temperature of the room over a range of applied voltage (−3–+3 volts) of nano silicon carbide deposited/Si heterostructure deposited at the different pulsed laser wavelength. All the variants of the heterojunctions present good characteristics of ratification comparing with the wafer of the Silicone which indicates the fabrication of a device diode-like. The value of the current at the forward bias has been doubled very dramatically with a value of the biasing voltage, where the tested currents are very small at the value of the biasing voltage are small, these current named a recombination current, while at the higher biasing voltage >1 V, the value of the diffusion current was controlled. From the figure below it is clear that the synthesized heterojunction using the second harmonic generation gives a larger value of a forward current as a result of reducing the resistivity of the layer of the silicon carbide.
Fig. 7 also shows the I-V properties of the silicon wafer that did not exhibit any correction properties (roughly seen as an ohmic contact). Fig. 8 presents the electrical (I-V) characteristics under the illumination of halogen light of the intensity of 65 mW/cm2 for the deposited nano SiC/Si photodetectors deposited at the various pulsed laser wavelength. It is clear that the value of the reverse current has been increased after illumination as a result of the pairs of the electrons- holes generation at the region of the depletion and the length of the carrier diffusion. It is clear that the reduction in pulsed laser wavelength (using second harmonic generation) leads to increasing the illumination current. Where no saturation is observed in the value of the photo-current when illuminated, and this indicates the quality of the linear characteristics used for fabricating an optoelectronic device such as the photodetectors. The presented figure also shows the heterogeneity of the fabricated heterojunction at the wavelength variation of the pulsed laser used in the deposition, which is due to the increase in the amount of light absorption and as a result of the increased concentration of the silicon carbide nanoparticles, as well as the expansion of the depletion region width and the increase in the carrier spread length. Moreover, there are some factors affecting the optical current values of the photodetectors such as grain size and surface morphology as well as the surface chemistry of SiC nanoparticles also affect.
Fig. 9 presented the relationship of the squared reciprocal of the tested capacitance (C−2) with the voltage at the reverse bias for the deposited nano SiC heterostructure, where the presented linear relationship shows that the type of the deposited junction is an abrupt junction. The value of the potential Built-in has been determined by extrapolating the linear portion of the presented curve to the value of the 1/C2 equal to 0 points. The estimated Vbi values have been found lower than the estimated values of the voltage turn-on from I-V properties as a result of the high resistivity of silicon carbide nanostructures. The values of the Built-in potential were reduced with the decrease in the laser wavelength (using second harmonic generation)can be related to the difference of each concentration of the electrons for the nano silicon carbide structure and the width of the depletion region with the wavelength of the pulsed laser.
Fig. 10 presents the spectral responsivity for the silicon carbide nano heterostructure at the bias of 5 volts, it has clearly seen that two featured regions of the response, the first one peaked at ~375 nm as the edge of the absorption of the nano silicon carbide heterostructure, and the second one is peaked at ~715 nm as the edge of the absorption of the substrate of the nano silicon wafer. The value of the optical responsivity of the photodetector at 375 nm has been increased from 1.3 AW−1 to 1.62 AW−1 as the wavelength of the pulsed laser decreased (using second harmonic generation) from 1064 nm to 532 nm, these increasing as a result of an increase in the width of the depletion region and the length of the diffusion, increasing the efficiency of the carrier collection, increasing the absorption of the photons at the region of the depletion.
Fig. 11 presents the effect of the pulsed laser wavelength on the specific detectivity D* of the heterostructure for the nano silicon carbide. The values of the detectivity are dependent on the current noise and the values of the spectral responsivity which is presented previously as a function of the pulsed laser wavelength. while the decrease in the values of the leakage current causes an increase in the values of the detectivity of the fabricated photodetector, the values of the specific detectivity change with a value of the wavelengths exactly the same presented in the values of the spectral responsivity. Also, it has been found two presented peaks along the applied wavelength with the different pulsed laser wavelength. The high-value detectivity (177 × 1012 W−1 cm Hz0.5 at the wavelength of 376 nm and 321 × 1012 W−1 cm Hz0.5 at the wavelength of 715 nm) whereas, the fabricated photodetectors and their results presented here can be used to and sense detects the UV signals and the weak Near Infra-Read signals.
High performance SiC/Si heterojunction device could be successfully prepared using simple, low temperature, PLD technique at second harmonic ND-Yag laser. A mixture of Nano-sized spherical particles about 36 nm size and Nano flake that are horizontally grown about 48 nm sizes with hexagonal single-crystal could be obtained. The bandgap values of the deposited Nano SiC films are increases as laser wavelength increase and its values around from 2.94 eV to 3.14 eV, the shifts towered the blue region were shown for SiC nanofilms prepared at low laser wavelength. The device performance results confirmed the presence of two response peaks at 375 nm and 715 nm, respectively. The high response of the fabricated devices (photodetectors) in the region of the Ultra-Violet indicates that the technology used here is encouraging and promising for fabricating cost-effective and simple Ultra-Violet detectors.
Funding Statement: The authors received no specific funding for this study.
Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.
|This work is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.|