Properties of thin film, amorphous cadmium sulfide, cadmium selenide, and cadmium telluride electrodeposited on platinum substrates

Amorphous CdS, CdSe, and CdTe thin films were electrochemically fabricated on Pt by cycling deposition potentials in aqueous NH{dollar}sb4{dollar}OH, NaSO{dollar}sb3,{dollar} CdCl{dollar}sb2,{dollar} and elemental chalcogen solutions. Proper solution preparation produced viable growth solutions. Thus, procedures were developed to ensure solution reliability. Chalcogen-OH{dollar}sp-{dollar} complexes aided in chalcogen dissolution. Cd(II)-NH{dollar}sb3{dollar} complexes pushed Cd(II) reduction potentials more negative. N{dollar}sb2{dollar}-flushed solutions prevented cadmium-tellurium-oxide precipitates from forming, and solution pH {dollar}>{dollar} 10 prevented CdSO{dollar}sb3{dollar} precipitates from forming. a-CdS and a-CdSe were produced from spontaneous precipitation of Cd{dollar}sp{lcub}+2{rcub}{dollar} and S{dollar}sp{lcub}-2{rcub}{dollar}/Se{dollar}sp{lcub}-2{rcub}{dollar} ions and a-CdTe from spontaneous bonding of adjacent Cd{dollar}sp0{dollar} and Te{dollar}sp0{dollar} atoms formed on the Pt substrate. Deposition rates of 0.22 nm/s (a-CdS), 0.107 nm/s (a-CdSe), and 0.09 nm/s (a-CdTe) were determined from photocurrent vs. film thickness experiments. Films were identified through Auger, XPS, and Raman spectroscopy. Raman spectra demonstrated {dollar}k=0{dollar} LO mode for a-CdSe; fundamental and first overtone for a-CdS; LO/LA modes and sharp peaks for a-CdTe. SEM photos revealed a homogeneous morphology consistent with noncrystalline materials, but proof of amorphism was provided by missing X-ray diffraction signals.; Electrical and optical properties were examined through impedance, photoconduction, and photoluminescence experiments. All three as-deposited amorphous films are n-type semiconductors with donor densities around 10{dollar}sp{lcub}18{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}.{dollar} Evidence of an additional deep donor state in the a-CdS films and a complicated distribution of charged states in the band gap for a-CdSe and a-CdTe was also obtained. The following, mobility-to-mobility-edge and band-to-band energies were determined: 2.55 eV, 1.73 eV (a-CdS); 1.82 eV, 0.84 eV (a-CdSe); and 1.67 eV, 0.75 eV (a-CdTe). Film properties are consistent with known ideas about amorphous materials, i.e., continuous distribution of localized states is present, optical transitions are strongest when delocalized state is involved, and localized states form highest and lowest energy states of an electron band. However, the intrinsic n-type character of these materials contradicts the commonly-held belief that amorphous semiconductors cannot be doped.