Photoluminescent (PL) porous layers were formed on p-type silicon by a metal-assisted chemical etching method using H2O2 as an oxidizing agent. Silver particles were deposited on the (100) Si surface prior to immersion in a solution of HF and H2O2. The morphology of the porous silicon (PS) layer formed by this method was investigated by atomic force microscopy (AFM). Depending on the metal-assisted chemical etching conditions, the macro- or microporous structures could be formed. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. It was found the shift of PL peak to a green region with increasing of deposition time can be attributed to the change in porous morphology. Finally, the PL spectra of samples formed by high concentrated solution of AgNO3 showed two narrow peaks of emission at 520 and 550 nm. These peaks can be attributed to formation of AgF and AgF2 on a silicon surface.
The indirect band-gap structure of silicon strongly restricts its application in the area of optoelectronics. The indirect transition that requires the participation of phonons leads to the low radiative recombination efficiency and the poor luminescence property. Many efforts have been carried out to solve this physical inability. One of the possible candidate systems is silicon nanostructures such as porous silicon (PS) [
PL from porous silicon is observable at wavelengths ranging from ultraviolet to the infrared. PS prepared using the electrochemical method shows strong visible emission, which is considered to originate from the quantum wires in the samples. The color of luminescence light could be well controlled by the variation of the size of the SiNCs. Various mechanisms, such as quantum confinement effect, surface state, and defect-center luminescence, were proposed to uncover the luminescence origins of PS and SiNCs. It is now generally agreed that various mechanisms are responsible for the different PL bands [
Recently, a new method, termed metal-assisted chemical etching, has been developed, which is relatively simple compared to the electrochemical method. The method does not need an external bias and enables a formation of uniform PS layers more rapidly than the conventional stain etching method. Thin metallic films or particles (Au, Pt, Al, Pd, etc.) are generally deposited directly on a silicon surface prior to immersion in an etchant composed of HF and an oxidizing agent [
In present paper, we report on the formation of visible-light-emitting layers on porous p-type silicon using H2O2 as an oxidizing agent and silver (Ag) as deposited metal. We discuss the etching time dependence on the morphology of PS layers and the PL peak intensity. The changes in the PL spectrum are attributed to the structural modifications of PS layers caused by etching procedure. Besides, we have observed two narrow PL peaks, which can be associated with formation of new phase or material.
Metal-assisted chemical etching (MaCE) is fundamentally a wet etch technique. MaCE was first used as an electroless etching technique to produce porous Si and porous III–V compound semiconductor by Li et al. in 2000 and 2002, respectively [
The metal-assisted chemical etching processes were applied to p-type Si wafers Czochralski-grown (100) 100 Ohm
Structural properties of porous silicon prepared by metal-assisted chemical etching have been investigated by Atomic Force Microscope (AFM) NT-206. AFM studies were done at atmospheric conditions. Using AFM, we could characterize the shape and sizes of isolated particles, their distribution depending on the chemicals conditions.
The luminescence was stimulated by UV laser LCS-DTL-374QT with excitation wavelength
During the experiment, we obtained samples with different surface morphology. Figure
AFM image of
In the case of low deposition times, the Ag particles were deposited nonuniformly. Distribution of pores on the surface and form are not orderly, as evident in the AFM image shown in Figure
AFM image of
The next step was to figure out how the increasing of oxidant concentration affects surface morphology. The increase of oxidant concentration (H2O2/H2O/HF = 25/80/40) leads to a change in the surface structure of silicon from microporous to highly porous structure. We have observed highly porous structure with the dimensions of the pores having an approximate size of 50–200 nm depending on the deposition time (Figure
AFM image of
For samples series no. 2, that had higher concentration of AgNO3 in immersion solution (10−3 M), we have obtained more remarkable experimental results. The color of the silicon surface after etching has been almost black (1–15 minutes of immersion time) or light brown (15–30 minutes of immersion time) depending on the immersion time. First, we supposed that it is similar to “black” silicon havinga needle-shaped surface structure where needles are made ofsingle-crystal siliconand have a height above 10 microns and diameter less than 1 micron [
AFM image of
The macropores grow parallel and perpendicular to the surface, a result similar to that generally observed on p-type highly resistive silicon electrochemically etched [
It was obvious that the transition of morphology from microporous to macroporous structure would modify the optical properties of the silicon surface. It was indeed observed in luminescence properties of silicon porous structure.
Figure
Photoluminescence spectra as a function of the deposition time taken from Ag-assisted chemically etched silicon. Etching was performed in solution—H2O2/H2O/HF = 10/80/40 within 10 min. Ag particles were deposited before etching in solution 0.23 M HF and 5 × 10−5 M AgNO3.
Figure
Photoluminescence spectra as a function of the deposition time taken from Ag-assisted chemically etched silicon. Etching was performed in solution—H2O2/H2O/HF = 25/80/40 within 6 min. Ag particles were deposited before etching in solution 0.23 M HF and 5 × 10−5 M AgNO3.
Besides, it can be seen the complex shape of the PL curve. By Origin pro 8.5 software, the PL spectrum was split onto separate peaks (Figure
Photoluminescence spectrum of highly porous silicon. Etching was performed in solution—H2O2/H2O/HF = 25/80/40 within 6 min. Deposition time—6 min in solution 0.23 M HF and 5 × 10−5 M AgNO3.
The next stage of the experiment was to investigate the photoluminescence spectra for samples series no. 2 (
Photoluminescence spectra as a function of the deposition time taken from Ag-assisted chemically etched silicon. Etching was performed in solution—H2O2/H2O/HF = 15/80/40 within 6 min. Ag particles were deposited before etching in solution 0.23 M HF and 10−3 M AgNO3.
The red peak in Figure
This result is very interesting, since it is difficult to form light-emitting porous layers on highly resistive silicon by the electrochemical method. Porous silicon layers exhibit strong red luminescence with PL peaks ranged within 630–680 nm, which are similar to those found for medium doped anodically etched silicon [
Photoluminescence and surface morphologies of nanostructured porous silicon prepared by metal-assisted chemical etching using H2O2 as an oxidizing agent have been studied. Depending on the metal-assisted chemical etching conditions, the macro- or microporous structures could be formed. Red band emissions were observed at 680–700 nm for silicon microporous structure. For highly porous silicon structures, we have observed PL peaks at 620, 610, and 560 nm. It was shown that the PL intensity increases with increasing etching time, which can be attributed to the increase of the silicon nanostructure density in agreement with the quantum confinement model prediction. It was found the shift of red PL peak to a green region with increasing of deposition time that can be attributed to the changing in porous morphology. The PL spectra of samples prepared with high concentrated solution of AgNO3 present two narrow peaks centered at 520 and 550 nm. These peaks can be attributed to formation of AgF and AgF2 on a silicon surface.