Effect of etching current density on photoluminescence propertiesof porous silicon

Due to the excellent properties such as efficient visible room-temperature photoluminescence (PL) and extremely large surface-to-volume ratio, porous silicon (PS) is material of interest in many applications such as photodetectors [1]. In this research, effect of etching current density (J) on optical properties of photo-electrochemically synthesized PS samples was characterized in order to achieve a porous morphology with maximum PL properties. PS samples were synthesized byphoto-assisted electrochemical etching (PECE) of (100) oriented phosphorus doped crystalline silicon (c-Si) substrates with specific resistance of 0.05. and thickness of 680. A thin film of silver (Ag) with the thickness of ~150nm was deposited on the back side of c-Si substrates in order to facilitate anodization process. The metallization was taken place in a vacuum chamber of a sputtering system and an ultra-pure Ag plate was used as a target. This process decreased thesurface resistivity to 0.001 after 15 min of annealing in tube furnace at a temperature of 430 under nitrogen gas flow [2]. The chemical solution used here was containing a mixture of hydrofluoric acid (HF 38-40%) and ethanol (C2H5OH 96%) in a volumetric ratio of 1:4. Currentdensities as critical experimental parameters were fixed at 10, 20, 40, 60 and 80 mA. cm−2 for sample a, b, c and d respectively using DC source meter (Keitlly 2400, USA) throughout the etching time (20 min). A platinum mesh was used as a counter electrode (cathode) in PECE celland it was located approximately 10mm directly above the surface of c-Si sample (anode). To create the required holes for occurrence of dissolution reaction on the surface layer of working electrode (anode) and PS formation, front side of the c-Si was irradiated by means of one 100 W tungsten lamp at a distance of 20cm above it during the etching process. Fig. 1 shows SEM micrograph of synthesized porous substrate. SEM view reveals that while the applied J increases from 20mA. cm−2to 80mA. cm−2 , pore diameter of the PS samples increases and total volume of Si nanocrystallites on the surface decreases. Also, the decrease in pore density Np(the number of pores per unit area) was observed on PS surfaces by increasing J for sample c and d. The logical reason for the evolution in the morphological characteristics of the pores of these PS samples is influence of the electric field on carrier’s movement and hence on dissolution [3]. The effect of J on the band gap structure (Eg) and emission properties of the PS samples was identify using PL spectra. In this experiment, in order to record S-band transition, excitation wavelength of PL was 405 nm. Fig. 2 shows the PL spectra of PS samples. A remarkable increase in intensities of the emitted photon from porous samples was observed with initial increase in J from 10mA. cm−2 to20mA. cm−2. Then a slight decrease in PL intensity was shown by increasing J from 20 bank. cm−2to 80 mA. cm−2 . Maximum intensity was detected in sample b with J = 20 mA.cm-2, at a wavelength of ~658.2 nm that can be due to the total amount of nanocrystallites on the surface of this sample compared to sample a. Applying higher current densities (more than 20 mA.cm-2) will increase dissolution of silicon material on the surface (as shown in Fig. 1) and thus will reduce PL intensities in samples c and d compare to sample b. The larger macro-cavity pores reduce the amount of nanocrystallite and result in light trapping. Therefore, the specific surface area of PS will reduce by increasing macro-cavities which lead to decrease in PL peak in samples c and d . Consequently, our PL analysis reveals that applied J has a significant effect on structural properties such as size of pores and crystalline properties of PS. Therefore, etching current density can beassumed as an outstanding parameter to optimized electronic structure of PS (e.g. band gap) and it has an important role in its light emitting properties. In the current report, it was shown that the porosity of PS is not the only parameter which affects the optical properties. The morphology of pores acts an important role to determine PS properties.

Keywords:

Porous silicon , Photo-electrochemical etching , Etching current density , Photoluminescence