Formation of nanostructured silicon surfaces by stain etching
© Ayat et al.; licensee Springer. 2014
Received: 30 April 2014
Accepted: 26 August 2014
Published: 11 September 2014
In this work, we report the fabrication of ordered silicon structures by chemical etching of silicon in vanadium oxide (V2O5)/hydrofluoric acid (HF) solution. The effects of the different etching parameters including the solution concentration, temperature, and the presence of metal catalyst film deposition (Pd) on the morphologies and reflective properties of the etched Si surfaces were studied. Scanning electron microscopy (SEM) was carried out to explore the morphologies of the etched surfaces with and without the presence of catalyst. In this case, the attack on the surfaces with a palladium deposit begins by creating uniform circular pores on silicon in which we distinguish the formation of pyramidal structures of silicon. Fourier transform infrared spectroscopy (FTIR) demonstrates that the surfaces are H-terminated. A UV-Vis-NIR spectrophotometer was used to study the reflectance of the structures obtained. A reflectance of 2.21% from the etched Si surfaces in the wavelength range of 400 to 1,000 nm was obtained after 120 min of etching while it is of 4.33% from the Pd/Si surfaces etched for 15 min.
In the current semiconductor industry, nanostructures of silicon represent the basic material for the conception of several devices in the field of nanoelectronics[1, 2], optoelectronics, energy conversion[4, 5], energy storage[6, 7], and also (bio)chemical sensors[8, 9]. Various methods have been developed to fabricate Si nanostructures such as reactive-ion etching (RIE), electrochemical etching, metal-assisted etching, or stain etching. This last one is an electroless method of forming porous silicon (PSi) in a mixture based on hydrofluoric acid (HF) and an oxidant. The nanostructuring of silicon by stain etching has attracted increasing attention in recent years for several reasons. One of these reasons is that it is an inexpensive method with the ability to control various parameters and can be accomplished in a simple chemical laboratory. The most widely used oxidant is nitric acid (HNO3) which has been investigated in numerous studies for the development of silicon nanostructures (nanopillars and nanowires) for their interesting fields of application. However, the use of HNO3 leads to bubble formation, inhomogeneous films, and irreproducible results.
Recently, Kolasinski and Barclay demonstrated that using vanadium oxide (V2O5) in the etching solution seems to be a good way to avoid bubble formation (no gas is generated). V2O5 dissolves in HF(aq) to form VO2+, which according to the half reaction with an appropriate acceptor level at E0 = +1.0 V, which is able to inject holes into the Si valence band. Several studies on the kinetics of the stain etching using V2O5 have been demonstrated, but few works on the morphological properties of the structures have been obtained.
In this work, we report the fabrication of ordered array pillar silicon and silicon macropores by a simple chemical etching of silicon in vanadium (V) oxide/hydrofluoric acid solution. Different etching parameters including the solution concentration, temperature of Si substrates, and thin metal catalyst film deposition (Pd) on the Si surface were studied. The etched surfaces were characterized by scanning electron microscopy and spectrophotometry.
Twenty-nanometer palladium (Pd) films are deposited by evaporation technique, on one side of p-type (1 to 10 Ωcm) single crystal (1 0 0) silicon wafers (both sides polished), with 250 to 300 μm thickness, cleaned using a dilute HF aqueous solution (1:10) for 30 s prior to deposition. Deposition of a significantly thinner Pd film can result in uniform etching of the entire silicon surface during chemical etching. The last step of the fabrication process consists of etching the Si and the Pd/Si samples in a mixture of HF (49%; Sigma-Aldrich, St. Louis, MO, USA) and V2O5 (98%; Sigma-Aldrich), for a period of 30, 90, and 120 min. Only one side is in contact with the etchant by using an adapted cell, and the edges are protected by an O ring.
After etching, the samples are thoroughly rinsed in DI water and dried with nitrogen stream. The etch rate was obtained by dividing the etch amount, i.e., Si weight before and after the etching process of Si by reaction time, exposed surface area, and Si density (2.33 × 103 kg/m3). Photoluminescence analyses were performed with a PerkinElmer LS 50B (PerkinElmer, Waltham, MA, USA) spectrometer.
The UV-Visible reflectance of the etched silicon was measured using a Cary 500 Varian spectrophotometer (Varian Medical Systems Inc., Palo Alto, CA, USA) in the range of 400 to 1,100 nm. The surface morphology and microstructure of the etched silicon surface were investigated using JEOL JSM 6360 LV (JEOL Ltd., Akishima-shi, Japan) scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and a FEI Inspect F-SEM (FEI, Hillsboro, OR, USA) equipped with FEG (field emission gun). The Fourier transform infrared spectrometry (FTIR) was performed with a Thermo Nicolet spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). IR spectra were recorded at a resolution of 4 cm-1 by averaging 32 scans.
The presence of Pd traces during the etching was performed using secondary ion mass spectroscopy (SIMS) 4E7.
Results and discussion
The widening of the pores could be due to different reasons: (1) Si without metal is etched as well due to the diffusion of injected holes from the etching front to the side of the pore wall[17, 18], and (2) Si without Pd metal coverage is etched slowly in the etchant.
Etching time effect on the rate was examined by measuring the etch depth as a function of etching time in the range of 0 to 360 min. The etch depth was also compared with that measured by SEM.
Figure 6b corresponds to the etching of Pd-coated Si in solution as previously mentioned. It shows that the etch depth first increases linearly with time, reaches a maximum at about 20 min, then decreases to attain a minimum etch rate value for 40 min, and finally increases to stabilize at an etching time of 70 min. This behavior is interesting since it confirms that the etching of Pd-coated Si samples in HF/V2O5 occurs in two steps. In the first step, the metal-assisted chemical etching is predominant (Pd catalyst effect), and in the second step, the stain etch is involved (Pd is consumed in the etching).
The decrease of PL intensity with increasing time is probably due to the reduction of nanostructure (nanocrystallites) density.
The results show that Si etching in HF/V2O5 solution induces the formation of pyramidal or pillar structures which evolve with etching time. The presence of metal catalyst (Pd) does not only accelerate the etch rate but also creates pyramidal structures within macropores whose diameter increases with etching time till the total consummation and disappearance of the Pd film. A low mean reflectance value of 2.21% in the wavelength range of 400 to 1,000 nm was observed for Si samples etched in V2O5/HF for 120 min, while a mean reflectance value of 4.33% was obtained by etching the Si/Pd samples in the same conditions for 15 min. Finally, the reflectance spectra of both Si and Si/Pd surfaces indicate that low mean reflectance is obtained which suggest a potential application in photovoltaic solar cells.
Part of this work has been performed at the NanoFacility Piemonte, INRIM, a laboratory supported by Compagnia di San Paolo in the framework of the EU Project NaS-ERA. The authors thank Sabrina Sam from CRTSE for her contribution to the revisions of the FTIR characterization and in designing the manuscript. The authors also acknowledge Lakhdar Guerbouze from the Centre de Recherche Nucléaire d'Alger for photoluminescence characterization. The authors thank Hamid Menari for reflectance characterization.
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