Charge trapping-detrapping mechanism of barrier breakdown in MgO magnetic tunnel junctions

Abstract

The endurance of MgO-based magnetic tunnel junctions has been studied using a time-dependent dielectric breakdown method. Series of successive electrical pulses of constant duration, varying voltage and varying time interval between successive pulses (t) were applied until electrical breakdown of the tunnel barrier occurred. We show here that two electrical breakdown regimes exist depending on the time interval t between pulses compared to a characteristic escape time  0 of the order of 100ns. For t< 0 , the breakdown is caused by a high average charge trapped in the barrier. For t> 0 , the breakdown is ascribed to large temporal variations of the amount of trapped charges causing alternating stress in the barrier oxide. Between these two regimes, the tunnel junction reaches a very high endurance. PACS numbers:Spin-transfer torque magnetoresistive random access memory (STT-MRAM) is a promising memory technology because of its non-volatility, high speed operation, very large endurance, high density and compatibility with standard CMOS process 1 . As the magnetic tunnel junction (MTJ) size shrinks, the MTJ resistance must remain comparable to the resistance of the selection transistor in a one transistor-one MTJ (1T-1MTJ) design. Hence, a thinner tunnel barrier that does not compromise on reliability is required. Oxide barrier breakdown is one of the key integration and reliability issues for advanced semiconductor memory technology. Despite numerous studies on the tunnel magnetoresistance of MgO-based magnetic tunnel junctions (MTJ), the breakdown mechanism of ultrathin MgO-MTJ has not been thoroughly investigated 1-8 .However, a comprehensive understanding of MTJ reliability issues is essential for the success of STT-MRAM or of other devices based on hybrid CMOS/MTJ technology.In this work, we studied the lifetime of junctions using a time dependent dielectric breakdown (TDDB) 3,4 technique. Studies of time-dependent dielectric breakdown (TDDB) in magnetic tunnel junctions (MTJ) are usually carried out by applying a DC voltage while recording the time to breakdown 3,4 . However, the normal operation conditions of an MTJ in an MRAM device require applying a large number of read/write voltage pulses a few nanoseconds long.This work reports on the breakdown behavior of MgO-based tunnel barriers submitted to successive voltage pulses. We studied their endurance as a function of the time interval between pulses and pulses amplitude and polarity. An earlier study has shown that MgO dielectric breakdown measurements carried under DC voltage and under cumulative pulsed voltage yield equivalent results for pulsewidths longer than 100 ns 7 . In the present study, we therefore chose to work with shorter constant pulse-width of 30ns. A very peculiar behavior was observed characterized by a very large enhancement of the MTJ endurance around an intermediate characteristic time interval  0 between successive pulses of the order of 100ns.We ascribe this behavior to charging/discharging effects within the MgO barrier du...

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

“…5. To not alter or destroy our MTJs through defect-mediated electromigration 61 , we have found it necessary to limit measurements up to T ¼ 155 K at |V| ¼ 2 V, and up to T ¼ 250 K at |V| ¼ 1.3 eV. At T ¼ 40 K, the data set in Fig.…”

Magnetoresistive Random Access Memory

“…Electron trapping/detrapping mechanism in the tunnel barrier was shown to play a key role in the early breakdown of MTJs and a correlation between endurance and 1/f electrical noise of MTJs has been demonstrated [113], [114]. Figure 23.…”

Tunnel Junction with Perpendicular Magnetic Anisotropy: Status and Challenges

“…As the MTJ scales, thinner MgO tunnel barriers are required for lower RA to remain comparable with the transistor, which contradicts the improvement of TMR ratio in the material point of view. Besides, the aging and even breakdown of ultra-thin MgO tunnel barriers caused by writing current passing through the MTJ stack leads to retention and reliability issues of conventional STT-MRAM [54].…”