Poole-Frenkel conductivity prior to electroforming in evaporated Au-SiOx-Au sandwich structures

It is well known that evaporated metal-silicon oxide-metal sandwich structu res exhibit a field-assisted lowering of the potential barrier between dono r-like centres and the conduction band edge, known as the Poole-Frenkel eff ect. This behaviour is indicated by a Linear dependence of logJ on V-1/2, w here J is the current density and V is the applied voltage. Less well known is that when a suitable electrode metal is used, and after the application of a few volts under moderate vacuum conditions, the structure undergoes a process known as 'electroforming', after which characteristic features suc h as voltage-controlled differential negative resistance (VCNR) and electro n emission are exhibited. The work described in the present investigation i s concerned with the d.c. properties during voltage cycling prior to electr oforming. In agreement with previous work Poole-Frenkel conductivity was ob served during the first voltage cycle, but the value of the Poole-Frenkel f ield-lowering coefficient beta generally increased with the number of volta ge cycles before the onset of electroforming, frequently attaining a value of up to twice the theoretical value. No significant variation in the value of beta was observed in samples which did not subsequently electroform. En hanced beta values have previously been identified with the existence of a non-uniform electric field region within the insulator, and it was therefor e concluded that a high-held region is established during the voltage cycli ng, which is necessary before electroforming can take place. (C) 1999 Elsev ier Science S.A. All rights reserved.