Some theoretical considerations relating to switching and remanence in ferroelectric/photoconductor memory devices

An energy band model is proposed for the metal-ferroelectric-photoconductor combination of a ferroelectric/photo-conductor storage device. We consider a photoconductor with its space-charge region confined near to the surface contacting the ferroelectric; it is taken to be nondegenerate, trapless, and having uniform constant concentrations of fully ionized shallow donors and acceptors. The ferroelectric is assumed to be a perfect insulator. The metal-photo-conductor work function function difference is taken to be zero.Poisson's Equation is solved for the photoconductor at the ferroelectric/photoconductor interface. Calculations indicate that under illumination the voltage drop across the photoconductor is of the order of 0.53 V when a drive pulse of about 7 V is applied to the device. The center of charge is of the order of 10-8 cm away from the interface even though the space-charge layer is distributed from 0.4 to 0.8μ from the interface. This suggests that the presence of the space charge in the photoconductor under illumination should not hinder switching of the polarization in the ferroelectric.After switching is completed, the applied voltage is removed. Under these conditions, due to the space charge in the photoconductor which results from the compensation charge of the ferroelectric at the interface, there will be a voltage drop in the photoconductor. This voltage drop also appears across the ferroelectric. It is shown that, when the polarization in the ferroelectric points toward the ferroelectric/photoconductor interface, the voltage across the photoconductor and the ferroelectric is 0.54 V under illumination and 0.82 V under no illumination. In the case when the polarization points away from the interface, the voltage drop across the photoconductor and the ferroelectric is -0.54 V under illumination and -1.35 V under no illumination.The voltage drop across the ferroelectric when the photoconductor is not illuminated, resulting from the space-charge layers, produces a field in the ferroelectric in direction and magnitude so as to depolarize the ferroelectric. This mechanism of depolarization becomes increasingly significant as the thickness of the ferroelectric is reduced to very thin films.