Preparation of Semiconducting Materials in the Laboratory, Part 2: Microscale Chemical Bath Deposition of Materials with Band Gap Energies in the UV, VIS, and IR

The technique of chemical bath deposition (CBD), based upon the controlled precipitation of an insoluble compound, is used in the laboratory for the student production of thin films of three inorganic semiconducting sulfides: ZnS, CdS and Bi2S3 which have their band gap energy values (Eg) in the UV, VIS and IR regions of the electromagnetic spectrum, respectively. Here, a saturated solution of an electrolyte in contact with its own solid phase originates a well defined numerical relationship (Ksp) among the concentrations (or activities) of the ions in solution. Precipitation occurs when this product is exceeded. However, this precipitation is usually massive and the solid formed does not grow in a defined, homogeneous pattern. In order to achieve a controlled precipitation one can often add a complexing agent (L) as to form a stable complex (MnLp) with the metallic ion (M) that controls its concentration. The band gap energies, Eg of these semiconducting films produced on glass or polymer slides are then estimated from their absorbance spectra at the corresponding wavelengths, lg (i.e. Eg = hc/lg). Typical student values (in eV) are: 3.7, 2.3 and 1.4 for ZnS, CdS and Bi2S3, respectively which compare well to the literature values of 3.4, 2.42 and 1.47 eV.