Method of programming variable resistance element, method of initializing variable resistance element, and nonvolatile storage device

A nonvolatile storage device comprising:

• a first electrode;
• a second electrode;
• a variable resistance element which is provided between said first electrode and said second electrode, and includes a transition metal oxide having resistance values that increase and decrease according to electric pulses applied between said first and said second electrodes;
• and
• a drive unit,
• wherein the transition metal oxide comprises a first transition metal oxide layer and a second transition metal oxide layer which are stacked, the second transition metal oxide layer having a degree of oxygen deficiency smaller than a degree of oxygen deficiency of the first transition metal oxide layer,
• said drive unit is configured to perform:
• a writing step by applying a writing voltage pulse having a first polarity between said first electrode and said second electrode so as to change a resistance state of the transition metal oxide from high to low;
• and
• an erasing step by applying an erasing voltage pulse having a second polarity between said first electrode and said second electrode so as to change the resistance state of the transition metal oxide from low to high, the second polarity being different from the first polarity,
• |Vw1|>|Vw2| is satisfied, where Vw1 represents a voltage value of the writing voltage pulse for first to N-th writing steps, and Vw2 represents a voltage value of the writing voltage pulse for (N+1)-th and subsequent writing steps, where N is equal to or more than 1,
• te1>te2 is satisfied, where te1 represents a pulse width of the erasing voltage pulse for first to M-th erasing steps, and te2 represents a pulse width of the erasing voltage pulse for (M+1)-th and subsequent erasing steps, where M is equal to or more than 1, and
• the (N+1)-th writing step follows the M-th erasing step.

A method of initializing a variable resistance element which includes a transition metal oxide having resistance values that increase and decrease according to applied electric pulses,

• wherein the transition metal oxide comprises a first transition metal oxide layer and a second transition metal oxide layer which are stacked, the second transition metal oxide layer having a degree of oxygen deficiency smaller than a degree of oxygen deficiency of the first transition metal oxide layer,
• in the case where data is written to and erased from the variable resistance element, by repeating in sequence:
• a writing step by applying, to the transition metal oxide, a writing voltage pulse having a first polarity and a voltage value Vw2 so as to change a resistance state of the transition metal oxide from high to low;
• and an erasing step by applying, to the transition metal oxide, an erasing voltage pulse having a second polarity and a pulse width te2 so as to change the resistance state of the transition metal oxide from low to high, the second polarity being different from the first polarity,
• said method comprising:
• performing at least one initial writing step by applying, to the transition metal oxide, a voltage pulse having the first polarity and a voltage value Vw1 satisfying |Vw1|>|Vw2| so as to change a resistance state of the transition metal oxide from high to low;
• and
• subsequently performing at least one initial erasing step by applying, to the transition metal oxide, an erasing voltage pulse having a second polarity and a pulse width te1 satisfying te1>te2 so as to change the resistance state of the transition metal oxide from low to high,
• wherein a first one of the repeated writing steps follows a last one of the at least one initial erasing step.