Investigations of the pore formation in the lead selenide films using glacial acetic acid- and nitric acid-based electrolyte
© Zimin et al.; licensee Springer. 2012
Received: 25 April 2012
Accepted: 12 June 2012
Published: 22 June 2012
We report a novel synthesis of porous PbSe layers on Si substrates by anodic electrochemical treatment of PbSe/CaF2/Si(111) epitaxial structures in an electrolyte solution based on glacial acetic acid and nitric acid. Electron microscopy, X-ray diffractometry, and local chemical microanalysis investigation results for the porous layers are presented. Average size of the synthesized mesopores with approximately 1010 cm−2 surface density was determined to be 22 nm. The observed phenomenon of the active selenium redeposition on the mesopore walls during anodic treatment is discussed.
KeywordsPorous semiconductors Lead selenide Anodic electrochemical treatment Electrolyte Mesopores 81.05.Rm; 71.20.Nr; 81.65.Cf.
Currently, there is a new rapidly emerging direction of modern nanotechnology research in the fabrication of porous semiconductor compound materials. Such nanostructured materials can be applied in many novel and unique practical applications, such as optoelectronic devices based on the quantum size effects due to the small dimensions of the interpore material, or biomedical compounds based on storing nanoinclusions in the pore volume, and many more. Of course, porous Si  and III-V and II-VI compound semiconductors [2, 3] to date have been studied rather extensively. On the other hand, IV-VI materials, in particular lead chalcogenides PbX (X = Te, Se, S), while being extremely useful for thermoelectric and optoelectronic applications [4, 5], are not researched at porous form almost at all. Among the reasons for that are the lack of the large monocrystalline wafer samples fitted for anodic cell treatment and also the absence of an established electrolyte etchant solution. Recently, while being able to be the first to demonstrate the pore formation in lead chalcogenide layers and study their properties [6–9], we also found out that a potassium hydroxide (KOH)-based electrolyte, which was originally proposed by Norr [10, 11] and is most commonly used for the PbX electropolishing, is not optimal for a stable pore formation in PbSe, in contrast to PbTe . It became necessary to optimize anodic treatment conditions for the fabrication of a well-defined pore morphology in lead selenide layers, and such new and more fruitful approach is presented in this work.
Initial samples were high-quality epitaxial monocrystalline PbSe films with a thickness of 2.2 to 5.2 μm grown on CaF2/Si(111) wafer substrates by molecular beam epitaxy (MBE) in ETH, Zürich . The thickness of the calcium fluoride buffer layer was 2 to 4 nm. Anodic electrochemical treatment experiments were based on a technological approach that allows us to prepare porous lead chalcogenide layers on silicon wafer substrates using a vertical-type electrochemical cell . The key treatment feature was the electrolyte solution. For the discussed study, the first batch of the initial epitaxial PbSe films was anodized using a KOH-based electrolyte solution (Norr electrolyte)  containing 20 g of KOH, 45 ml of distilled water, 35 ml of glycerol, and 20 ml of ethanol. Current density was 2 to 8 mA·cm−2, and treatment duration was 10 to 20 min, performed at room temperature. These film samples were further divided into two groups depending on the morphology type of their initial surface: the first group had a typical flat surface for MBE-grown PbX layers on CaF2/Si(111) with nanoterraces and dislocation exit pits with approximately 107 cm−2 density, while the second group had a peculiar granular surface (for details, see ). The second batch of PbSe films (containing 3% Sn, with flat initial surface) was anodized using a solution of 10 ml of nitric acid (HNO3), 10 ml of glacial (undiluted) acetic acid (CH3COOH), and 40 ml of glycerol (first used by EH Tompkins and GL Johnson  for the electropolishing of lead selenide at high current densities), for which, by analogy with porous silicon, we reduced the current density (1 mA·cm−2) and processing temperature (20°C) with a duration of 10 min. Scanning electron microscopy (SEM) studies were performed on Supra-40 (Carl Zeiss, Inc., Oberkochen, Germany); chemical energy-dispersive X-ray spectroscopy (EDS) local microanalysis was carried out simultaneously with SEM using an INCA-Energy spectrometer (Oxford Instruments, Abingdon, UK).
Results and discussion
It should be noted that the deposition of chalcogen during electrochemical processings of PbX materials is a known effect, and Se layers were reported to be fabricated electrochemically [11–14]. However, we are the first to observe the effect of selenium redeposition for the electrochemical synthesis of porous PbX layers. Thus, in , where Se thin films with PbSe nanoclusters were fabricated electrochemically in nitric acid-based solutions via anodic dissolution of PbSe, the analysis of the corresponding processes showed the reaction of PbSe dissolution to Pb2+ ions and Se atoms, which presumably takes place for the porous layers as well. The Se layer on the pore walls can potentially be very beneficial for the practical applications of the porous PbSe layers due to its modified semiconductor properties and high sensitivity in the visible range, and amorphous coatings in particular can have their own specific applications. At the same time, the redeposited Se film can serve as a protective, stabilizing layer by analogy with Te coatings. For example, in , PbTe has been coated with a thin sublimed porous elemental tellurium layer, in order to prevent surface oxidation and carbon absorption. Obviously, a further experimental research is required to learn the properties of Se-coated mesoporous PbSe layers fabricated in our study, but there are all reasons to believe that this nanostructured material holds much promise for the fabrication of stabilized porous layer-based devices on silicon wafer substrates.
In this work, our results demonstrated the potential of the developed novel technique for the synthesis of porous PbSe by anodic electrochemical treatment of PbSe/СaF2/Si(111) epitaxial structures in an electrolyte containing glacial acetic acid and nitric acid, with the former wetting the surface and stabilizing the growth of the porous layer. SEM studies showed the presence of mesopores with a surface density of approximately 1010 cm−2 and an average size of 22 nm, while EDS mapping showed the enrichment of the pore walls with selenium, and the latter effect can be useful for the practical applications of PbSe porous layers in various devices, as well as for the fundamental study of the electrochemical processes during the porous PbX material fabrication.
SPZ is a professor at the Microelectronics Department, Yaroslavl State University. ESG is a principal engineer at the Microelectronics Department, Yaroslavl State University and a research associate at the Yaroslavl Branch of the Institute of Physics and Technology of Russian Academy of Sciences. VVN is a senior research fellow at the Yaroslavl Branch of the Institute of Physics and Technology of Russian Academy of Sciences. FOS is a student at the Microelectronics Department, Yaroslavl State University.
Energy-dispersive X-ray spectroscopy
Molecular beam epitaxy
Scanning electron microscopy.
The authors would like to thank E Yu Buchin (Yaroslavl Branch of the Institute of Physics and Technology RAS) and VM Vasin (Yaroslavl State University) for their valuable contributions to the anodic electrochemical treatment experiments, and H Zogg (ETH, Zürich) for the provided epitaxial PbSe samples. Electron microscopy and chemical microanalysis investigations were performed at the Center for Collective Use of Scientific Equipment ‘Diagnostics of Micro- and Nanostructures’ (Yaroslavl). This study was financially supported by the Russian Foundation for Basic Research (RFBR) (grants 12-02-90029-Bel_a and 12-02-90419-Ukr_a).
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