Depletion mode optoelectronic modulation spectroscopy

Abstract

The depletion mode of opto-electronic modulation spectroscopy (OEMS), spectroscopically senses the electrical response to wavelength modulated monochromatic illumination as trap states in the depletion region of a semiconductor device are cyclically excited. The method is demonstrated using metal–Si3N4–GaAs field-effect transistors in which the charge in the gate depletion region is detected through its effect on the channel current. The optical transition energies of trap levels were revealed and the charge exchange mechanisms identified unambiguously by examining the phase of the detected signal with reference to the phase of the photon energy variation. In-phase responses originate from electron trap states while responses of opposite sign derive from hole traps. Many of the states corresponded closely in energy with ones previously reported in vapor-phase-epitaxy GaAs materials. Charge exchange with energetically discrete and continuously distributed traps is theoretically described when they are excited by photons having periodic energy modulation. The results indicate that the magnitude and phase of the OEMS response spectra are determined by the relative thermal and optical emission rates of trapped charges. Deep level transient spectroscopy (DLTS) measurements made on the same samples could not reveal the responses of bulk levels that the OEMS technique showed were present; this was because the DLTS spectra were dominated by the interface continuum response. This illustrates clearly an important advantage of the technique. The method is equally applicable to other devices in which a space-charge region regulates the channel current.