Oxidic Field Effect Structures with Memory

Abstract

Two types of switching memory devices have been made by integrating a ferroelectric layer in a device. A Schottky diode consisting of a ferroelectric semiconductor and a high work function metal was found to show a bistable I-V behaviour. Experimentally an On/Off current ratio of two orders of magnitude was found in a structure consisting of a semiconducting PbTiO3 layer with a gold top contact, grown on a La0.5Sr0.5CoO3 bottom contact layer. The ferroelectric polarization parallel or antiparallel to the internal field of the diode gives rise to a change in the Schottky barrier height and depletion width. This will, depending on the polarization direction of the ferroelectric layer, enhance or diminish the tunnel probability of charge carriers through the Schottky barrier and thereby increase or decrease the current through the device. In the Ferroelectric Field Effect Transistor an insulating ferroelectric gate oxide was brought in close contact to a gated n-type Sb-doped SnO2 semiconducting channel layer. A maximum On/Off Source-Drain current (IsD) ratio at zero gate voltage of 5.7 was found. Gate pulse measurements showed retention of IsD after polarisation reversal of the ferroelectric gate oxide in both polarization directions. After more than 104 switching cycles the device shows a gradual decrease in ISD both in the On and Off state.