Barrier breakdown mechanism in nano-scale perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Hua Lv,Diana C Leitao,Susana Cardoso,Johannes Müller,Juergen Langer,Zhiwei Hou,Jerzy Wrona,Paulo P Freitas,Thomas Kämpfe

doi:10.1063/1.5007656

Abstract

Recently, the perpendicular magnetic tunnel junctions (p-MTJs) arouse great interest because of its unique features in the application of spin-transfer-torque magnetoresistive random access memory (STT-MRAM), such as low switching current density, good thermal stability and high access speed. In this paper, we investigated current induced switching (CIS) in ultrathin MgO barrier p-MTJs with dimension down to 50 nm. We obtained a CIS perpendicular tunnel magnetoresistance (p-TMR) of 123.9% and 7.0 Ω·μm2 resistance area product (RA) with a critical switching density of 1.4×1010 A/m2 in a 300 nm diameter junction. We observe that the extrinsic breakdown mechanism dominates, since the resistance of our p-MTJs decreases gradually with the increasing current. From the statistical analysis of differently sized p-MTJs, we observe that the breakdown voltage (Vb) of 1.4 V is 2 times the switching voltage (Vs) of 0.7 V and the breakdown process exhibits two different breakdown states, unsteady and steady state. Using Simmons’ model, we find that the steady state is related with the barrier height of the MgO layer. Furthermore, our study suggests a more efficient method to evaluate the MTJ stability under high bias rather than measuring Vb. In conclusion, we developed well performant p-MTJs for the use in STT-MRAM and demonstrate the mechanism and control of breakdown in nano-scale ultrathin MgO barrier p-MTJs.

Highlights

The spin-transfer-torque magnetoresistive random access memory (STT-MRAM) is one of the most promising generation memories due to its unique features, such as non-volatility, high density, high-speed operation, high endurance and low power consumption.1 Hereby, STT-MRAM with perpendicular anisotropy has shown to have a better performance because of its lower switching current density, better thermal stability and higher access speed.2,3 the dielectric breakdown of MTJ becomes a severe reliability issue upon shrinking the cell sizes and very thin barrier thicknesses
Our study demonstrates in detail how the breakdown happens step by step, applies a comprehensive understanding of the breakdown mechanism in such devices and suggests a more efficient method to improve the device stability under high bias stress to enhance the endurance
J-V curves in different MTJs were fitted by Eq [3] and we find that V b,stable are close to the values of barrier height (φ/e), 055908-5 Lv et al FIG. 4. (a) V b, V s and V b,stable distributions on junction size, the dashed blue line indicates the average value of φ/e, where φ is the barrier height obtained from J-V curve fitting with Eq [3]. (b) and (c) are schemes of unsteady and steady state during breakdown, respectively, where EF is the Fermi level of the ferromagnetic electrode, J is the tunneling current density

Summary

Introduction

The spin-transfer-torque magnetoresistive random access memory (STT-MRAM) is one of the most promising generation memories due to its unique features, such as non-volatility, high density, high-speed operation, high endurance and low power consumption. Hereby, STT-MRAM with perpendicular anisotropy has shown to have a better performance because of its lower switching current density, better thermal stability and higher access speed. the dielectric breakdown of MTJ becomes a severe reliability issue upon shrinking the cell sizes and very thin barrier thicknesses. The spin-transfer-torque magnetoresistive random access memory (STT-MRAM) is one of the most promising generation memories due to its unique features, such as non-volatility, high density, high-speed operation, high endurance and low power consumption.. STT-MRAM with perpendicular anisotropy has shown to have a better performance because of its lower switching current density, better thermal stability and higher access speed.. There are two known types of breakdown mechanisms in MTJs, the intrinsic and the extrinsic mechanisms. The former is a physical deterioration process and leads to a sudden decrease of the device resistance, whereas the later is related to the pinhole growth inside the barrier and reveals a.

Methods

Results

Conclusion