QUANTITATIVE MODEL FOR PHOTOCURRENT SPECTROSCOPY OF QUANTUM-WELL DIODES INCLUDING TRANSIT-TIME AND BACKGROUND-DOPING EFFECTS

We present a simple model for photocurrent spectroscopy of quantum-wel l p-i-n diodes that provides a quantitatively accurate desciption of t he dependence of photocurrent on absorption coefficient and applied bi as. The model incorporates the transit-time effect described previousl y [R. P. Leavitt and J L. Bradshaw, Appl. Phys. Lett 59, 2433) (1991)] as a limiting case. It also includes the two major effects of residua l background doping in the intrinsic region of the diode: nonuniform e lectric fields, which affect the transport of carriers, and incomplete depletion at low electric fields, which reduces the amount of photocu rrent collected. We show that the background-doping effect alone can m imic the transit-time effects: reduction in the overall carrier collec tion efficiency, saturation of photocurrent spectral features, and the presence of minima in photocurrent where absorption spectra show maxi ma. We obtain a closed-form expression for the photocurrent in the gen eral case where both transit-time and background-doping effects are si gnificant. Excellent agreement is obtained between model calculations and experimental room-temperature photocurrent spectra for an 89-perio d 100-angstrom GaAs/100-angstrom Al0.3Ga0.7As multiple-quantum-well di ode, where the background doping density, the effective electron mobil ity, and the built-in potential are treated as adjustable parameters. The background doping density and the built-in potential obtained from the fit are in excellent agreement with independent measurements. We applied the model to predict the dependence of photocurrent on the int rinsic-region thickness of the diode. We also show a dramatic asymmetr y between photocurrent spectra measured with light incident from the f ront and from the back of the diode, and we discuss the impact of this asymmetry on the performance of self-electro-optic-effect devices. We also find good agreement between the model predictions and the photoc urrent results of Whitehead et al. [Appl. Phys. Lett. 52, 345 ( 1988)] . Further, our model qualitatively describes the dependence of the pho toluminescence intensity on electric fields in multiple-quantum-well p -i-n diodes.