Electrodeposition of semiconductors

As well as compound semiconductors elemental semiconductors can be obtained from ionic liquids. Si and Ge are widely used as wafer material for different electronic applications furthermore junctions of n- and p-doped Si are still interesting for photovoltaic applications. A controlled electrodeposition of both elements and their mixtures would surely also be interesting for nanotechnology as Ge quantum dots made under UHV conditions show interesting photoluminescence. 

The direct electrodeposition of GaAs from ionic liquids has been studied mainly by two groups. Wicelinski et al. [66] used an acidic chloroaluminate melt at 35-40 °C to co-deposit Ga and As. However, it was reported that Al underpotential deposition occurs on Ga. Carpenter et al. employed an ionic liquid that was based on GaQs to which AsQs was added [18,67]. Unfortunately, in these studies the quality of the deposits was not convincing and both pure arsenic and gallium could be found in the deposits. Nevertheless, these studies have to be regarded as the first steps in the electrodeposition of Ga-based semiconductors. Furthermore, thermal annealing could improve the quality of the deposits. 

In situ STM studies on Ge electrodeposition on gold from an ionic liquid were performed by the Endres group [69,70]. In the first studies they used dry [BMIM][PF6] 

ZnTe The electrodeposition of ZnTe was published quite recently [58]. The authors prepared a Hquid that contained ZnCl2 and [EMIM]Cl in a molar raho of 40 60. Propylene carbonate was used as a co-solvent, to provide melting points near room temperature, and 8-quinolinol was added to shift the reduction potential for Te to more negative values. Under certain potenhostahc condihons, stoichiometric deposition could be obtained. After thermal armeaHng, the band gap was determined by absorption spectroscopy to be 2.3 eV, in excellent agreement with ZnTe made by other methods. This study convincingly demonstrated that wide band gap semiconductors can be made from ionic liquids. 

Cermanium In situ STM studies on Ge electrodeposition on gold from an ionic Hquid have quite recently been started at our insHtute [59, 60]. In these studies we used dry [BMIM][PF ] as a solvent and dissolved Gel4 at esHmated concentrations of 0.1-1 mmoir the substrate being Au(lll). This ionic Hquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposihon. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in SecHon 6.2.2.3. 

Nanoscale Processes at the Electrode/lonic Liquid Interface 

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Schlesinger TE (2000) Electrodeposition of Semiconductors, in Schlesinger M, Paunovic M (eds) Modern Electroplating, 4th edn. Wiley, New York... 

Hodes G, Rubinstein I (2001) Electrodeposition of semiconductor quantum dot films. In Hodes G (ed) Electrochemistry of Nanostructures, Wiley-VCH, Weinheim... 

In this chapter we report on the electrodeposition of semiconductors in ionic liquids. It is shown that ionic liquids are, due to their extraordinary physicochemical properties, well suited as a solvent medium for the electrodeposition of elemental semiconductors (like Si and Ge), their mixtures (Si Gei %) and compound semiconductors (GaAs, AlSb, InSb, ZnTe, CdTe, CuInSe2, etc.). 

In this chapter we have summarized selected literature data on the electrodeposition of semiconductors in ionic liquids. It has been demonstrated that elemental silicon, germanium, and selenium can be elecrodeposited in ionic liquids. Furthermore, it is shown that compound semiconductors like InSb, AlSb, CdTe and others can be made, especially at elevated temperatures where kinetic barriers are easier to overcome, even allowing the exclusive electrodeposition of grey selenium. In this context ionic liquids are very promising for semiconductor electrodeposition. Both wide electrochemical and thermal windows allow processes which are impossible in aqueous or organic solvents. 

Klein JD, Herrick RD, Palmer D, Sailor MJ, Brumhk CJ, Martin CR (1993) Electrochemical fabrication of cadmium chalcogenide microdiode arrays. Chem Mater 5 902-904 Sima M, Enculescu I, Visan T (2004) The electrodeposition of semiconductor nanowires with thermoelectric properties using template method. Revista De Chimie 55 743-746... 

Lincot, D. (2005) Electrodeposition of semiconductors. Thin Solid Films, 487, 40 8. 

Although this chapter is limited to electrodeposition of semiconductors, it is only fair to mention, even if briefly, some examples of electrodeposition of metal nanostructures. This is important because the principles and techniques used in electrodepositing metals are essentially the same as those used for depositing semiconductors - the main difference is that almost all studies on electrodeposition of nanocrystalline semiconductors involve compound semiconductors, with the added comphcations this entails. Examples include pulsed electrodeposition of metal multilayers [1, 2], porous membrane-templated electrodeposition of gold nanotubes [3], and Ni nanowires [4]. 

Semiconducting nanowires, nanorods, nanodots, nanocones, nanopins, etc. are interesting due to their broad range of applications. Electrochemically, the most easily fabricated semiconductors are Il-Vl semiconductors, for example, CdS, CdSe. There are three approaches for electrodeposition of semiconductors. The first method [131] is deposition of metal in alumina nanopores, followed by etching of alumina surface by phosphoric/chromic acid to access metallic surface for sulphur or arsenic vapour to attain metal sulphide or arsenide nanostructures. The second method deals with electrolysis of sulphuric acid, causing the sulphide atoms to be deposited in pores. 

In the past several decades ionic liquids have attracted a fast-growing research interest as potential electrolytes for electrodeposition of semiconductor materials [16-19]. Processes that are impossible in common aqueous and organic solutions become viable if ILs are used. However, although the possibility of electrodeposition from ILs has been demonstrated for a variety of semiconductor materials, there are some fundamental issues that still have to be clarified. In this chapter attention to semiconductors deposition shall be attracted as such studies are still rare in ionic hquids. 

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