Study the Effect of Irradiation Time and HF Concentration on Porosity of Porous Silicon and Study Some of the Electrical Properties of Its Based Device ()
1. Introduction
Porous silicon (PSi) can be considered as a silicon crystal having a network of voids in it .The nanosized voids in the silicon bulk result in a sponge—like structure of pores and channels surrounded with a skeleton of crystalline silicon nanowires [1].
The interest in porous silicon has increased greatly over the last decades, mainly due to its photoluminescence properties and the potential applications, which arise from these [2,3]. There are, however, many other possible uses including chemical sensors, micromachining, micro scale optical components, and filters and diffraction grating. Porous silicon is usually prepared by iodization in aqueous hydrogen fluoride (HF). It is also possible to employ a photochemical technique, which does not involve an externally applied bias [2]. According to G. Lerondel et al., porosity is defined as the fraction of void within the PSi layer and can be easily determined by weight measurements. This macroscopic characteristic can be measured by gravimetry, preferably on thick layers. The accessible porous silicon values typically included between 15% and 90%. It is significant to note that porosity is an average value and that for the same porosity; various material morphologies can be obtained [4].
2. Experimental Work
Commercially available n-type (111) oriented silicon wafers of (500 ± 15 µm) thickness with resistivity (r = 3.84 × 105 W∙cm) have been used as substrate. Before the photochemical etching process, the silicon wafer has been cut out into small pieces in dimensions of (0.5 × 0.5 cm). After the cleaning, the silicon wafers were immersed in hydrofluoric acid (HF) aqueous solution. The HF concentration was changed in the range (24 to 36 wt%). All samples were front-side illuminated using a Halogen lamps of power density (0.28 & 0.7 W/cm2). The sample has been mounted on a Teflon cell in such away that the current required for the etching process, could pass from bottom surface to the top of the polished surface through the electrolyte. The light source was vertically aligned and focused by quartz lens of (3.87 cm) focal length.
The porosity of PSi layers has been determined by using the gravimetric method according to the following equation [4]:
(1)
where g is the porosity, m1 and m2 are the weight of the silicon substrate before and after the etching process respectively, and m3 is the weight after removing the porous silicon layer.
The fabricated CdS/PSi junction was achieved by using the chemical spray pyrolysis technique.
3. Results and Discussion
Figure 1 shows the relationship between porosity and etching time of prepared PSi layer at different irradiation time (30 - 90 min), 24% HF concentration, and power density of 0.28 W/cm2. From this figure, we can see that the values of porosity are increasing with increasing of etching time. This result is ascribed to the increasing of the number and width of the pores with increasing of etching time. Our result agrees with outcomes of [1,5].
Also, Figure 2 represents the relationship between porosity and HF concentration (24 - 36 wt%), etching time (60 min), and power density of 0.7 W/cm2. We can observe from this figure that the porosity decreases with increasing HF concentration. By lowering HF the porosity of the substrate is raised until there is enough HF serving F– ion in the electrolyte. This is necessary to oxidize Si and provide charge carriers for the electrolysis. This result is comparable with result of [6].
Figure 3 shows the I-V characteristics for CdS/PSi heterojunction at dark and reverse bias at room temperature, at etching time (30 min), 24% HF concentration, and power density of (0.28 W/cm2). In general, the forward dark current is generated due to the flow of majority carriers and the applied voltage inject majority carriers which leads to decrease the value of built—in potential, and decrease the width of the depletion layer. Then majority and minority carrier concentration is higher than