an ultra - high density data storage and retrieval unit has a data layer for storing and / or retrieving data and a second layer , wherein the data layer and / or the second layer comprise an ordered - defect material . the data layer is a phase - change layer capable of changing between a first state and a second state . the second layer forms a diode with the data layer for detecting a data state of the data layer . a method is provided for forming an ultra - high density data storage and retrieval device comprising forming a data layer for storing and / or retrieving data , and forming a second layer beneath the data layer , wherein the data layer and / or the second layer is an ordered - defect material .