Electro-Optical Properties of 2

Optical and electro-optical properties of WSe(,2) single crystals have been investigated in order to obtain a better understanding of its band structure. The crystals were grown by the iodine vapor transport method using a "two-step" technique. The optical properties were studied by means of electric field modulation technique. The effects of dc bias superimposed on the ac electric field was investigated. Photoelectric measurements were also carried out including studies of photoconductivity and current-voltage characteristic curves. Four independent excitonic series designated A,B,A',B' were observed located at 1.80, 2.49, 2.38 and 2.76 eV respectively. The value of R(,e) = 1.110 eV is an average value calculated from the R(,e) values deduced for the A' and B' excitonic series. This method avoids direct calculation of R(,e) from only the two observed levels of the A exciton, since the first level deviates from the hydrogenic sequence. The optical band gap was found to be 1.80 eV. Nine band-to-band transitions were observed. These were designated d,C,D,E,F,G,H,(alpha),(beta). Of these, E,F,G,H have not been previously reported in modulation of reflectance studies. The effects of dc bias superimposed on the ac electric field were studied and analyzed in terms of surface states. The photoconductivity measurements confirmed the existence and position of the A and B excitons. The band structure model proposed by Kasowski was extended to WSe(,2).