Ordered Macroporous Silicon-germanium Fabricated By Electrodeposition In Ionic Liquid

SixGe1-xis a very important semiconductor with high refractive index whichcould be adjusted by varying the content of Ge. Ordered macroporous SixGe1-xcouldbe obtained by filling the voids of the opaline photonic crystals, which makes itpossible to achieve complete band gap. However, there are two obstacles on thisresearch. First, special equipments are necessary to get SixGe1-xand the energyconsumption in normal methods is extremely high. Low filling ratio is the otherproblem faced when an ordered macroporous Si_xGe_1-xis prepared.In this paper, ionic liquids are used as solvent to prepare SixGe1-xfilm andordered macroporous Si_xGe_1-xthrough electrical deposition, the obtained materialwell retains the ordered macroporous structure of the templates. The fabricationconditions and the relationship between the infiltration depth （film thickness） andthe morphology are systematically studied. The preparation of the laminatedsemiconductor layer at room temperature is briefly discussed as well as thefabrication of two-dimensional ordered macroporous SixGe1-xat elevatedtemperature. The main contents of this paper are as follows:Ionic liquid [EMIm]Tf₂N and Py_1,4Tf₂N are used as solvent while siliconchloride and germanium chloride are used as solute. Cyclic voltammetry is used toevaluate the electrochemical behaviors of ionic liquids on ITO, copper and platinumsputter-coated silicon substrates. The reduction potentials and the control step of theelectrode reaction are studied on ITO substrate. The results present that theelectrochemical process is a non-mass transfer limited reaction and there are tworeduction peaks during the process. The peak centered at-1.3V is correlated with thereduction of Ge（IV） to Ge（II）, and a more negetive wave centered at-1.9V could becontributed to the co-deposition of silicon and germanium.XPS results reveal that silicon contained in product is totally oxidized whilegermanium is partially oxidized. In Raman analysis, the peak at280cm^-1corresponds to longitudinal optical phonon vibration of amorphous Ge-Ge. Thepartially oxidized film is heated under high vacuum with rather low hydrogen partialpressure. The heat-treated products are then analysed by XPS and Raman, the resultsillustrate that GeO₂could be partially reduced to GeO and Ge. However, the thermaltreatment has no impact on SiO₂.The optical properties of polystyrene template are studied to get theinformation on the spatial distribution of light incidents on the ordered structure.The deviation between the measured position of bandgap and the calculated one according to Bragg-Snell equation is6.7%. The observed surface morphology of3-D ordered macroporous products along the （111） direction is discussed in terms ofsubsequently increasing film thickness. Models are set up to help with fullunderstanding of the evolution of the morphology （the shapes of the pore mouthsand pore walls）. When the film thickness isFeasibility and constraints of the preparation of ordered macroporous SixGe1-xare investigated. The contact angles of the solvent on PS template and on ITOsubstrate are tested, confirming that the solvent has better wettability to PS templateand the product is deposited around the microspheres priorly. The effect ofdeposition potential is studied, the result reveals that the ordered structre of thetemplate is destroyed if a more negative potential is applied during the deposition.The potential of-1.9V is more suitable for the co-deposition of silicon andgermanium. The effect of packing modes of templates, the adhesion of the templateto substrate and the diameters of PS spheres are also studies. The results indicatethat the deposit could not be infiltrated into the interstitial space of templateassembled from PS spheres with diameters smaller than400nm, and the physicalproperties of SixGe1-xobtained if the adhesion is weak is similar to film product.Si/Ge laminated and Al/Ge laminated ordered macroporous materials areprepared for the first time. The adhesion of the first layer to substrate plays aimportant role in determining the morphology of the product. For the fabrication ofSi/Ge laminated ordered macroporous material, Ge layer is deposited on PS-coatedITO substrate first, and then Si layer is deposited on the top of the Ge layer.However, the deposition of Al/Ge laminated ordered macroporous layers is morecomplicated for the fast deposition rate of aluminum. Al/Ge laminated product canbe obtained by modifying the thickness of germanium deposit, the depositionpotential and the deposition time of aluminum. The growth mode of aluminum isVolmer-Weber, leaving an uneven film surface.Research on the electrodeposition of SixGe1-xfilm and ordered macroporous Si_xGe_1-x in ionic liquid at elevated temperature is carried out. XPS and Ramanresults reveal that the grain sizes of the product are increased as the temperatureincreases, so does the resistance of the particles to oxide. Atwo-dimensional orderedmacroporous film is prepared through electrodeposition at elevated temperature. Theformation of the2-D architecture could be due to the soften of polystyrene templateaccompanied with the evaporation of silicon chloride and germanium chloride.Moreover, the thickness of the product is limited to a radiu of the PS spheres.Product obtained by this method could used as gratings, generating monochromaticlight.