Hafnium silicate as control oxide in non-volatile memories

SONOS-type MIS capacitors with hafnium silicate as a control oxide are characterized and compared to devices featuring a conventional SONOS gate stack. Write operation is comparable for both gate stack types. Erase operation for the devices with hafnium silicate is improved since the parasitic injection of electrons from the gate is suppressed due to the low electric field in the high-k material. This reduction in leakage current through the gate enhances oxide stability. However, measurements indicate that charge retention for the gate stack with hafnium silicate is degraded for high charge densities. Band bending of the control oxide under high electric fields increases the tunneling probability for trapped charges. Additionally, initial flatband voltage decay is observed due to charge trapping in the hafnium silicate layer. Reducing the thickness of the hafnium silicate layer is possible, maintaining favorable erase properties while minimizing the charge decay rate during retention.