Metal selenide films

Rastogi AC, Balakiishnan KS, Garg A (1993) A new electrochemical selenization technique for preparation of metal-selenide semiconductor thin films. J Electrochem Soc 140 2373-2375... 

In contrast, the analogous diethyl-diselenocarbamates (dsc) have been shown to be poor sources for the deposition of ZnSe or CdSe films. Under similar reaction conditions (10 -10 Torr, 370-420° C) the diethyl dsc precursors give films of the metal selenide heavily contaminated with selenium [108]. However the mixed alkyl dsc complex Eq. 4 have been used successfully to deposit thin films of CdSe or ZnSe [109,110]. 

Mechanisms of electrochemical and photoelectrochemical deposition of metal selenide clusters (Me = Pb, Cd, Zn, Bi, In) onto the surface as well as into the selenium films have been studied. These clusters are formed as a result of underpotential and overpotential deposition of the metals onto Se. Photoinduced underpotential deposition of Bi onto Se was used to cover selenium colloidal particles with BiiSes clusters. The PbSe and Bi2Sc3 clusters modify the Se surface and form electronic surface states in the Se bandgap, thus promoting electron exchange processes between the valence band and redox species in solution and the rise of the subbandgap photocurrent. 

RAFT polymerization lends itself to the synthesis of polymers with thiol end groups. Several groups have utilized the property of thiols and dilhioesLers to bind heavy metals such as gold or cadmium in preparing brushes based on gold film or nanoparticles1 8 761 763 and cadmium selenide nanoparticles.763 76 1... 

A quantitative analysis of the kinetics of CdSe deposition from selenosulfate, Cd(II)-EDTA baths in terms of a mechanism involving nucleation and electrode kinetics has been given by Kutzmutz et al. [65], Note also that selenosulfate-containing baths have been used for the anodic selenization of vacuum-deposited metal films in order to synthesize CdSe and other binary selenide semiconductor thin films such as CuSe and InSe [66],... 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

While most studies on CD have been on snlphides and selenides, considerable work has also been carried ont on oxide films. The fdms are most often formed by reaction of hydroxide ions with a metal salt. While it might be expected that the prodnct is a hydroxide rather than an oxide, in many reported cases oxides are directly formed. This is probably dne to two factors Many of the metal ions nsed (e.g., Pb, Sn, Tl,) do not readily, if at all, form simple hydroxides others (Ag, Cu, Mn) are very readily oxidized even in aqueous solutions. Ni(II) hydroxide is fairly readily dehydrated, particnlarly in the presence of the persulphate ion used to deposit the oxides in some cases. Zn(OH)2, Cd(OH)2 and In(OH)3 are reasonably stable Zn(OH)2 can be easily dehydrated while the other two require annealing to form the oxide. 

There is one example of a CD process (for deposition of tin sulphides) in which elemental sulphur dissolved in a nonaqueous solvent is used as a source for S. Since this appears to be the only example in the literature for this type of film deposition, it will be discussed in Chapter 6 together with the relevant study on tin sulphides. However, there is no reason to believe that this process may not be applicable to other materials. Metal sulphides (and selenides) are known to form, as precipitates, by reacting certain metal salts with dissolved elemental chalcogen, although visible film formation seems to be limited, up to now, to this one example. 

The rate-limiting step in CD for the first two mechanisms is almost always formation of the chalcogenide ion. This reaction should be slow otherwise fast, homogeneous precipitation of the metal chalcogenide will occur with little fihn formation. (Even rapid precipitation can lead to a film however, this film will be extremely thin and in most cases not visible.) Almost all the literature on CD is limited to sulfides (mostly), selenides, and oxides (including hydrated oxides and hydroxides). Anion-forming reactions are described in this section. 

Considering that homogeneous precipitation of metal chalcogenides (mainly sulphides) by reaction between metal ions and dissolved chalcogen is well established, the main difference between this deposition and similar reactions seems to be that the products adhere to a substrate to give a visible fdm (in this case) rather than only precipitate. Whether this is connected with the redissolu-tion/redeposition process that occurs with the Sn-S system or has some other explanation is important. If the former, it may be limited to only those systems that behave similarly. Otherwise it is not unreasonable to expect that other metal sulphides and selenides (possibly also tellurides, although tellurium tends to be much less soluble, if at all, in such solvents) may be deposited as films in this manner. 

In 1970 Jamison and Cosgrove reported an interesting study of the relationship between the crystal structure and the lubrication performance of the sulphides and selenides of several of the transition metals in Groups 4, 5, 6 and 7 of the Periodic Table (see Table 14.2). They showed experimentally that satisfactory film formation and low friction were only obtained within certain closely-defined limits of crystal structure. It is not proposed here to attempt to present full details of the rather complex crystallographic considerations, but only to give a simplified version of the essential aspects of their findings. 

The approach to HTSC electrosynthesis described in this section is entirely analogous to that used for the production of semiconductive films such as CujcInj,S(Se)j [205,206]. The thermal treatment of electrodeposited metallic precursors carried out in an H2S(Se) atmosphere in some cases proved more successful than the cathodic codeposition of sulfur or selenium with the metals [207]. There is also an analogy with other combined methods of selenide preparation the sputtering of copper or indium followed by cathodic deposition of selenium [208], and the chemical reaction of selenium with metals. In this case, as in the deposition of incomplete HTSC precursors [189], the use of the deposition mode is much easier. 

Works on chemical deposition of tellurides are very limited, despite in principle the expectation that deposition should be similar to the sulfide or selenide systems. As indicated by Bode [3], telluride precusors (such as tellurourea) are very unstable which renders the deposition very difficult to achieve. Deposition has been reported with using a different process [94, 95]. Telluride precursor is introduced as dissolved Te02 (TeO in basic solutions) in the solution containing complexed metallic ions. Upon the addition of hydrazine, the reduction to Te (-II) can be slowly achieved, leading to the formation of the metallic telluride as for PbTe [95]. More recently CdTe films have been prepared this way [94]. This process can be considered as an extension of the electroless process used for the deposition of metals [2]. It is probable that other routes similar to the selenosulfite route for selenides are possible for tellurides too, but have not yet been investigated in depth. 

Chemical bath deposition is a technique in which thin semiconductor films are deposited on substrates immersed in dilute solutions containing metal ions and a source of hydroxide, telluride, sulfide, selenide, etc., ions. One of the first chemically deposited semiconductors, reported in 1869, was a PbS thin film [26]. During the ensuing 140 years, CBD has been used to deposit films of metal sulfides, selenides, and oxides, and various other compounds. While it is a well-known technique in a few specific areas (notably photoconductive lead salt detectors, photoelectrodes, and, more recently, thin-film solar cells), it is by and large an under-appreciated technique. 

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