Optical and electrical characterization

Photoluminescence (PL) measurements are important for characterizing the optical properties of materials. Photons of energy exceeding the band gap of materials, are used to excite electrons from the valence band to the conduction band. Photoluminescence occurs as the excited electrons return radiatively to the valence band, either in a direct fashion or an indirect fashion through an energy level in the gap. The emission spectrum shows a fingerprint of the energy of the transition, offering fruitful probes of semiconductor band structures and impurity defect levels. In our current PL system, an Ar ion laser is used as the excitation source, and a monochromator coupled with photo-detectors (photo-multiplier tube and Ge detector) and a lock-in amplifier allows the detection of luminescence in the spectral range between near ultraviolet and near infrared. A cryostat makes it possible to study temperature dependence of PL between 4-650 K. The PL system is expected to be upgraded in the future by adding necessary accessories for Raman scattering experiments as well as for polarized PL and high-pressure PL measurements, which would enhance our capability in understanding local symmetry/environment of defects in semiconductors.

Current-voltage (I-V) and capacitance-voltage (C-V) measurements provide information about carrier injection and conduction in materials. With samples mounted on the low-temperature stage, I-V and C-V measurements will be used to examine metal/semiconductor interfaces and carrier traps in materials. Combining I-V and C-V apparatus with the PL detection system, electroluminescence (EL) of various optoelectronic devices can be investigated.