UV–assisted pulsed–current photo–electrochemical etching of silicon carbide thin films on silicon substrates

In order to fabricate porous silicon carbide (PSC) thin films, photo-electrochemical etching of silicon carbide (SiC) thin films was carried out using a pulsed-current generator with specific pulse properties. Prior etching, thin films of SiC were coated on polished surface of silicon (Si) using radio frequency (RF) magnetron sputtering. For best quality of SiC thin films, the surface of Si substrate was chemically cleaned using RCA clean method. Silicon carbide was immediately deposited on freshly prepared Si substrates using an RF magnetron sputtering unit (Edwards A500, UK). The RF power was set to 200 W and the substrate temperature was fixed at 400 °C. Substrate heating can act like an in-situ annealing process that promotes the allocation of the sputtered SiC atoms. After 2 h of sputtering, the obtained thickness of the deposited SiC was 1 μm (deposition rate = 500 nm/h). The samples were then placed in a quartz carrier and annealed in a tube furnace. Thermal annealing was performed for 1 h at 1200 °C under a nitrogen flow. The samples were then allowed to cool to room temperature inside the furnace. For formation of PSC, etching is an essential technique. Recently, dry etching methods based on electron cyclotron and inductively coupled plasma are mostly applied. However, these methods needs expensive and sophisticated techniques and suffer from the risk of creating damage in the surface [1-4]. In many cases, wet-chemical etching is an attractive alternative [5]. There are two main methods for wet chemical etching of SiC substrates: metal-assisted electroless chemical etching and photo-assisted electrochemical etching. The former has been suffering from the lack of controllability of the pore size and distribution. But the latter technique which was pioneered by Shor et al. [6, 7] can be controlled by several parameters like current density and shape of current. Many studies on SiC wafers were reported, but less effort has been focused on SiC thin films on Si substrates [8-10]. For electrochemical etching of p-type SiC, by applying an anodic potential to the substrate, holes accumulate at the surface and cause oxidation and dissolution. This process takes place in dark [10]. 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 302 The UV assisted-electrochemical etching method can be assumed as an attractive technique for fabricating porous n-type SiC and producing optical waveguides because of the ease with which layers can be fabricated over a large area of the substrate, and the uniformity of porous layers which are created by this method. At the SiC/HF electrolyte interface, the chemical treatment of SiC leads to the formation of SiF62+ complex which is soluble in the solution [5]. The role of ultraviolet radiation for enhancing the etch rate is partially due to the relatively shallow absorption depth of UV light. This phenomenon allows more carriers to be photo-generated in the space charge layer. In the current research, a two-electrode setup was used for the electrochemical etching of SiC thin films. The substrate (SiC) functioned as the anode electrode, whereas an inert metal wire (Pt) was used as the cathode. A pulsed current with a period time (T) of 14 ms and a pause time (Toff) of 4 ms was supplied by an integrated SourceMeter instrument (Keithley 2400). Current densities of 10, 15, and 20 mA/cm2 were used to prepare the porous samples A, B, and C, respectively. Then, all samples were washed with deionized water and air-dried in the laboratory at room temperature. Here, the improvement in optical characteristics of PSC/Si by optimization of the current density in UV-assisted electrochemical etching of SiC thin films on Si substrates is reported. It has been illustrated that current density can be considered as an important parameter for controlling the etching rate and morphology of the porous samples. Thus, it can enhance the optical properties of electrochemically etched PSC layers. Scanning electron microscopy (SEM) images demonstrate that by this technique the porosity and uniformity of PSC thin films can be controlled. Hence, the photoluminescence properties of porous samples can be optimized. Therefore, PL of PSC has been related to the morphology of the surface which is controllable by etching parameters. The PL Peak intensities of PSC samples are shown to be enhanced and the optical properties are improved with increase in etching current density as compared to non-porous SiC thin film.