Digital communications advance with simple CdS thin films

Amplitude modulator using erbium doped polycrystalline CdS thin films

Demands on digital communications are increasing at an exponential rate. The need for innovative advances in this area means research on optical and electrical properties of CdS thin films are of interest. It is already known that CdS thin films show promise in conversion of energy applications such as for photovoltaic devices and sensors but they may also be suited for other applications. These applications include the use of CdS films for building analog multipliers or amplitude modulators.

The principal purpose of every amplitude, frequency or phase modulator for analog or digital communication systems is to incorporate information to be transmitted (modulating signal) into a carrier signal.

Researchers J. A. Davila-Pintle, O. Portillo-Moreno and E. Molina Flores from Benem¨¦rita Universidad Aut¨®noma de Puebla used a chemical bath deposition process to grow polycrystalline thin films of CdS on glass substrates. During the growth process, relative volumes of nitrate of erbium penta-hydrate (Er(NO3)35H2O) were added in aqueous solution of CdS in order to obtain different levels of doping. The samples obtained by this method were electrically and optically characterized determining the dark conductivity, carrier density, and the photoconductivity in the 590 to 451 nm range respectively.

After optimising the composition for electrical and optical properties, the researchers built an amplitude modulator for digital communications. The system worked successfully demonstrating that it could be used for optoelectronic purposes. Although the data transfer rate was relatively slow (less than 1 kbit¨Msecond), the system developed by J. A. Davila-Pintle et al. had the advantage of being much more simple compared with purely electronic modulators which incorporate non linear devices like diodes, transistors and require electronic filters.