Comparative Studies between Porous Silicon and Porous P-Type Gallium Nitride Prepared Using Alternating Current Photo-Assisted Electrochemical Etching Technique

Porous n-type Si and porous p-type GaN nanostructures were fabricated using alternating current photo-assisted electrochemical (ACPEC) etching in 1:4 volume ratio of hydrofluoric acid (HF) and ethanol (C2H5OH) for a duration of 30 min. The proposed approach to this work was to study pore formation on the Si and p-GaN substrates in the aspects of morphological and structural changes. The morphological and structural properties of porous Si and porous p-type GaN samples have been studied using field emission scanning electron microscopy (FESEM) measurement, energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and high-resolution X-ray diffraction (HR-XRD) in comparison to the respective as-grown sample. FESEM analysis revealed that uniform pore size with triangular-like shape was formed in porous Si sample while circular-like shape pores were formed in the porous p-type GaN sample. AFM measurement revealed that the root-mean-square surface roughness of porous Si and porous p-type GaN was 6.15 nm and 5.90 nm, respectively. Detailed investigation will be presented in this work to show that ACPEC etching technique is a viable technique to produce porous nanostructures in different substrates.


Introduction
Porous semiconductors such as silicon (Si), gallium nitride (GaN), and gallium arsenide (GaAs) have been employed for optoelectronic devices and sensors due to the enhanced properties over semiconductors bulk materials [1]. The versatility of porous semiconductors could help to diminish the deformity density and strain relief induced by lattice mismatch as well as help to understand the fundamental properties of nanotechnology advancement structures [1,2]. On the other hand, porous semiconductors could help to increase the surface area to volume ratio on the surface of the substrate that can be produced via etching process [3][4][5].
Photo-electrochemical (PEC) etching could be promising technique to fabricate porous structures due to the preferences, for example, most practical, low structural damage, and cost-effective method when contrasted with other method for instance dry etching method lead to high density nanostructures with finite porous structures [1,6,7]. PEC etching technique involves important conditions including electrolyte solution, duration, voltage, and illumination, which would greatly influence the structure of  [8,9]. Therefore, PEC etching technique has capability of controlling the pore size by varying the etching parameters.
Studies related to the fabrication of porous Si using alternating current photo-electrochemical (ACPEC) etching technique was done by [10] using hydrogen fluoride (HF) and ethanol (C2H5OH) etch solution with a volume ratio 1:4 at a steady current density 10 mA/cm 2 . However, the studies proposed two-step ACPEC to revive the pore consistency and porosity of the porous Si samples. The studies successfully uncovered that the porous Si structure displayed uniform and high density of pores fabricated via the two-step etching technique. Another study about the fabrication of porous p-type GaN by [11] in a mixed sulphuric acid (H2SO4) and methanol (CH3OH) etch solution with a volume ratio 2:1 has been performed at a constant etch duration and varied applied current (40, 60, 80, and 100 mA) under ultraviolet (UV) illumination. The research was to investigate the influence of various current value on structural and optical properties of resulting porous structures. Formation of porous at applied current of 60 mA was observed as the best porous structures among others without etching away the surface of p-GaN layer.
In this work, the formation of porous n-type Si(111) and porous p-type GaN via ACPEC etching technique in the same electrolyte solution, applied current, and duration under UV illumination will be studied. Detailed exploration towards structural and physical properties of porous n-type Si(111) and p-type GaN samples have been further discussed. N-type Si(111) and p-type GaN were used as substrate and were cleaved into square pieces. Prior to the etching technique, the cleaved samples were cleaned by the standard Radio Corporation of America (RCA) and aqua regia method respectively. This step has been done properly to remove organic residues from the substrates. Porous structures were produced using ACPEC etching technique. The samples were fixed at the bottom of a Teflon cavity cell contacted with metal plate. The etching electrolyte consisting of hydrogen fluoride (HF) and ethanol (C2H5OH) mixed solutions (1:4 volume ratios) was filled into the cavity cell. The constant AC current value (30 mA) was supplied by using variance AC regulator. The fundamental scheme behind variance AC regulator is to control the amount of voltage and to obtain the ideal amount of AC current value in which the current reading was checked by utilizing clamp meter. One output terminal of the variance AC regulator was joined to Pt wire that was submerged in the Teflon cavity cell loaded up with etch solution, while the other one was connected to the metal plate. Porous n-type Si (111) and porous p-type GaN was fabricated by etching for 30 min under external incandescent light and UV light illumination, respectively. After ACPEC etching, the resulting porous n-type Si (111) and porous p-type GaN were rinsed with deionized (DI) water and dried using nitrogen gas.

Experimental
FESEM (Model:FEI Nova NanoSEM 450) was used to examine the surface morphology of investigated samples while Atomic Force Microscopy (AFM) (Model:Dimension EDGE, BRUKER) was used to determine the 3-dimensional surface topography and root-mean-square (RMS) roughness of investigated samples. From the FESEM images, ImageJ software was utilized to ascertain the total pore area and the porosity percentage of non-porous and porous samples. NanoScope Analysis software was used to analyse the surface roughness of the non-porous and porous samples within the scan area 5 x 5 µm 2 . In addition, high resolution X-ray diffraction (HR-XRD) (Panalytical X'pert PRO MRD PW3040) was applied for the investigation of structural characteristic. HR-XRD analysis of Si and ptype GaN samples were carried out at 40kV, 40 mA with step time and step size of 1.0 s and 0.05° respectively. The scanning range of HR-XRD measurement was carried out from 20° to 80° for Si and GaN samples. Figure 2 shows FESEM images obtained for surface morphology of the non-porous and porous samples. The non-porous Si sample was smooth, and no pits could be observed (Figure 2(a)). A similar characteristic was observed for the non-porous p-type GaN sample (Figure 2(c)). Formation of pores was observed on the n-type Si (111) surface (Figure 2(b)). The distribution of pores was uniform, and the triangular-shape could be observed. On the other hand, formation of pores on p-type GaN film was consistently disseminated on the surface of film. However, it was assumed that the discovery of a few patches on the porous p-type GaN surface resulted from an incomplete removal of etch residues after etching process [11].

Results and Discussion
Image processing technique was analyzed in order to examine the total pore area and porosity of non-porous and porous samples by utilizing ImageJ software. Table 1 shows the summary of total pore area and porosity of non-porous and porous n-type Si (111) and p-type GaN samples. Interestingly, the porosity percentage was determined as a proportion of a surface area involved by pores to the entire surface area of the film [12]. From the Table 1, the results indicated that porous samples produced a higher total pore area and porosity percentage compared to the non-porous samples. Figure 3 delineates 3-dimensional surface topography of non-porous and porous samples analysed using utilizing atomic force microscopy (AFM. NanoScope Analysis programming was utilized to measure the RMS surface roughness on non-porous and porous n-type Si (111) and p-type GaN samples. The summary of RMS surface roughness of non-porous and porous samples was illustrated in Table 2. Low RMS surface roughness with value of 1.32 nm and 1.56 nm were obtained for the non-porous ntype Si (111) and p-type GaN, respectively, indicating the absence of pores on the substrate. Aside from elucidating of non-porous structures, the AFM measurements demonstrated that the RMS value of the porous n-type Si (111) and p-type GaN were 6.15 nm and 5.89 nm, respectively, were increased in contrast to non-porous samples. Interestingly, these perceptions are supported by FESEM images (Figure 2), which demonstrated that the surface morphologies of the n-type Si (111) and p-type GaN were altered after ACPEC etching process.    HR-XRD analysis was performed to analyse crystalline properties of non-porous and porous samples. Figure 4 demonstrates the 2θ-scan of HR-XRD patterns for non-porous and porous samples. Diffraction peaks attributed to Si (International Centre of Diffraction Data (ICDD) file no. of 01-077-2111) (Figure 4(a)), aluminium oxide (Al2O3) (ICDD file no. of 01-073-1512) and hexagonal phases of GaN (ICDD file no. of 00-050-0792) oriented in (0002) and (0004) planes (Figure 4(b)) were revealed in all of the investigated samples. As observed, the peak intensity of the non-porous n-type Si (111) appeared at ~28.41°. However, the peak of porous n-type Si (111) sample appeared at ~28.12°, which was degraded in the peak intensity when compared with the non-porous n-type Si (111). A conceivable reason contributing to the peak shifting occurred due to the presence of tensile strain in the porous ntype Si(111) sample after the etching process [10,13]. On the other hand, the peak intensity of porous ntype Si (111) sample diminishes compared to non-porous n-type Si (111) due to formation of pores influenced by etching process [14]. For non-porous p-type GaN sample, GaN (0002) and (0004) peaks showed up at ~34.58° and ~72.86°, respectively. The GaN peak intensity was likewise observable in porous p-type GaN sample. In correlation, the peak intensity of porous p-type GaN, GaN (0002) Figure 4(b) and the decrease in peak intensity of porous p-type GaN compared to non-porous p-type GaN sample due to etching process. The peak at ~41.73° was ascribed to the sapphire substrate.  The Full-Width-Half-Maximum (FWHM) and crystallite size of investigated samples was tabulated in Table 3. The average crystallite size of non-porous and porous Si and GaN samples can be determined by utilizing the Debye-Scherrer equation (1): where D is the average crystallite size, K is the Scherrer constant (K =0.9), λ is the X-ray wavelength (λ=1.5406 Å), β is the FWHM in radians, and θ is the diffraction angle calculated in radians. The nonporous n-type Si(111) and p-type GaN have revealed higher value of the crystallite size ~177.97 nm and 56.37 nm, respectively. The reduction in average crystallite size of porous n-type Si(111) and p-type GaN were affirmed in the broadening of FWHM, which was conversely corresponding to the crystallite size as shown in equation (1).

Conclusions
Formation of porous n-type Si and p-type GaN substrates was accomplished via ACPEC etching technique at a constant current of 30 mA under external incandescent light and UV light illumination, respectively, for 30 min in a constant volume ratio of HF:C2H5OH (1:4) solutions. Comparative studies about changes in terms of surface morphology, structural properties and RMS surface roughness of the investigated samples have been discussed. The results showed that the total pore area and porosity of (a) (b)