CuS多孔硅异质结的光学特性

In the present work, monocrystalline porous silicon was fabricated using the photoelectrochemical etching technique and then copper sulfide (CuS) nanoparticles were  deposited on a porous silicon substrate at 250°C using spray pyrolysis method at different  concentrations of CuS (0.1, 0.3 and 0.5M). The physical properties of CuS/PSi/c-Si  heterojunction were studied. The results of photoluminescence (PL) exhibited that the peak  position of PL spectra was shifted to the longer wavelength or lower energy when the  concentration of CuS was increased. On the other side, the energy gap of CuS decreased  with increasing concentration. The values of energy gap are 2.7, 2.59, and 2.55 eV at 0.1,  0.3 and 0.5M, respectively. The properties of the CuS/PSi/n-Si structure were enhanced.

1. Introduction

In recent years, many of papers have focused on  nanostructured semiconductors, which have their  unique structural properties. Copper sulfide (CuS) is a p-type semiconductor with direct energy gap of 1.6- 2.7eV. The composition of CuxS exists many stable phases. The composite Cu2S, the copper-rich,  the diorlite, dignite, and anilite, ending with the  covellite (CuS) phase, which is rich in sulfur.  Copper sulfide thin films were deposited using many  methods such as chemical bath deposition, sol-gel,  chemical vapor deposition (CVD), chemical spray, and spin coating. The thermochemical spray  pyrolysis method is considered the most common  among the chemical methods for preparing thin films.  These methods are summarized by spraying a  solution of the material from which the film is to be  prepared onto hot bases at a specific temperature that  depends on the type of material used, as a  thermochemical reaction occurs between the atoms of  the material and the hot base, and as a result of this  reaction a thin membrane is formed. Porous  silicon (PSi) is a material with strong emissivity,  showing effective illumination over a wide spectral  range. Many research works have focused on porous silicon and is based on optoelectronic techniques such as light-emitting diodes, wave  vectors, and photovoltaic diodes. The mixture  between CuS thin film and PSi layer can provide an  excellent homogenous of lattice constant.

2. Experimental Work

Figure (1a) exhibits a schematic diagram of the  anodization etching method. In this work, there are  two orientations; (100) and (111); of n-type silicon  wafers are used with resistivity of 0.01-0.02 Ω.cm and the thickness of the silicon wafer is 505±12µm.  The Si wafers were cleaned before the etching to  remove any contamination on the surface.

Fig. (1) (a) Diagram of adonization process, (b) diagram of  chemical spray

The hydrofluoric acid (16 wt.%) contraction has  been diluted by using an absolute ethanoic solution  (99%) (C2H5OH) to minimize the hydrogen bubbles  during the etching. Fixed current density was  maintained at 15 mA/cm2 and etching time of 5, 10  and 15 min were used. Additionally, a tungsten lamp  of 100 mW/cm2 intensity was carried out. The set-up  consists of two electrodes, the first part is gold and is  connected to the acid solution (HF), the second part  is a stainless-steel put under the Si substrate. After  that, CuS/PSi heterojunction was achieved by using  spray pyrolysis process. We used aqueous solution of  copper chloride and sodium sulfide (Na2S) at various concentrations. We mixed first in distilled water (H2O) and then sprayed on the substrate in order to  obtain ions of copper and sulfide. The CuS nanoparticles were deposited on PSi substrate at  250°C using spray pyrolysis method at different  concentrations of CuS (0.1, 0.3 and 0.5M). The spray  rate was used at 25 mL/min. The distance between the  nozzle and the sample were fixed about 30 cm and a  k-type thermocouple was used as shown Fig. (1b) and  table (1).

3. Results and Discussion 

The PL spectra shown in figures (2) and (3) are for CuS/Psi heterojunctions prepared at etching time  of 15 min with fixed current density (15 mA/cm2 ),  concentration of HF (18%) and concentration of CuS (0.1M and 0.5M) with orientation (100) and (111).  Obviously, the strong emission peaks of CuS/PSi at  orientation (100) were observed at 577.5 nm, 535.3  nm and 546.4 nm (green region) corresponding to  energies of 2.19 eV, 2.31 eV and 2.27 eV of pure PSi and concentration 0.1 and 0.5M respectively (Fig. 2).

Fig. (2) PL spectra of CuS/PSi nanostructures fabricated at  concentrations of (a) 0.1M, (b) 0.3M, (c) 0.5M of orientation  (100)

In the same way, the PL emission peaks at  orientation (111) were illustrated at 566.5 nm, 577.2 nm and 605.6 nm (green region) corresponding to  energies of 2.15 eV, 2.14 eV and 2.04 eV of pure PSi  and concentration 0.1 and 0.5M, respectively (Fig. 3)  because of the carriers restricted (electron-hole) recombining in the PSi surface, and this is further  supported by measurements of the particle sizes'  statistical distribution. The peak position was  shifted to the long wavelength or lower energy with  increasing concentration of CuS due to the density of state of PL and the absorbance of the carriers due to  quantum confinement effects. It can be seen  the change in the peaks intensity due to the increasing  of the number of smaller nanocrystallite sizes  contributes to the PL since it generates more holes for  further reactions as in tables (2) and (3). The silicon  atoms near the crystallite's surface modify their bonds  and dimension to adapt variations in the immediate environment, forming the recombination centers. The  possibility of electron and hole recombination is a  less dimension structure as the energy gap decreased  with increasing concentration due to the confinement  of PSi practical in a smaller dimension.

Fig. 3

4. Conclusion

In summary, CuS nanostructures were successfully deposited on porous Si wafer using  chemical spray method. The peak position of  photoluminescence was shifted to the long  wavelength or lower energy when the concentration  of CuS was increased. The energy gap increases with  increasing concentration of CuS. Consequently, the  experimental circumstances can be used to control the  size and shape of the final structures. In brief, the  characteristics of CuS/PSi photodetectors are  effectively dependent on fabrication requirements.