  Vertical structures of SiO$_{2}$ sandwiched between a top tungsten electrode
and conducting non-metal substrate were fabricated by dry and wet etching
methods. Both structures exhibit similar voltage-controlled memory behaviors,
in which short voltage pulses (1 $\mu$s) can switch the devices between high-
and low-impedance states. Through the comparison of current-voltage
characteristics in structures made by different methods, filamentary conduction
at the etched oxide edges is most consistent with the results, providing
insights into similar behaviors in metal/SiO/metal systems. High ON/OFF ratios
of over 10$^{4}$ were demonstrated.
