Design of novel phase-change materials based on In3SbTe2 for low-power consumption and fast switching phase-change memory

Abstract

Ternary alloys, especially Te-based chalcogenides have been investigated for use in phase-change random access memory (PRAM). PRAM exhibits reversible switching characteristics between amorphous and crystalline phases with fast transition, long retention, and excellent endurance as a non-volatile memory device. Among the various phase-change materials, Ge2Sb2Te5 (GST) phase-change material has been popularly used. The GST-based phase-change materials have critical limitations, and its characteristics need to be improved. GST has low crystallization temperature (~150 ˚C) and low thermal stability (2.33 eV of activation energy). These low thermal properties result in critical drawbacks such as resistance drift and high power consumption. For this reason, a phase-change material with high thermal stability is required to replace GST. Accordingly, In3SbTe2 (IST) phase-change material has been suggested as a promising candidate, because it has a higher crystallization temperature (~300 ˚C) and higher activation energy (~5.29 eV) than those of GST. From the perspective of thermal stability, IST phase-change material is fairly attractive for PRAM devices. However, the high thermal stability with high crystallization temperature and high activation energy results in slow operation speed and high power consumption. To overcome this drawback, bismuth (Bi) was doped into the IST phase-change material. In Bi-doped IST, Sb atoms are partially substituted by 3.2–5.5 at.% of Bi atoms. As a result, the NaCl-type crystal structure of IST is slightly distorted. The distorted inter-planar angles were observed with fast Fourier transformation of the lattice images, and were found to be within the maximum range of inter-planar angles calculated by the density functional theory (DFT). The calculated value of distortion angle is fairly well matched with the experimental result. With an increase in the Bi content, the crystallization temperature becomes relatively lower than that of IST and the activation energy decreases from 5.29 to 2.61 eV. Consequently, phase-change random access memory (PRAM) fabricated with Bi-doped IST (Bi-IST) can operate with lower power consumption than pure IST PRAM. The set and reset speeds of PRAM cells fabricated with Bi-IST are 100 ns at 5.5 at.% of Bi, which are obviously faster than the switching speeds of PRAM cells fabricated with IST and GST. These experimental results reveal that the switching speed is strongly related with the thermal properties of the distorted lattice structure. From these results for the Bi-doped IST, it can be seen that the relationship between the atomic structures changed with lattice distortion and operation speed. The change in thermal properties (decreases in crystallization temperature, activation energy, and melting temperature) are definitely related to the improved operation speeds. However, the improved operation speeds are not directly proportional to the reduction in thermal parameters. Therefore, we tried to design novel phase-change materials with the density function theory (DFT), and proved the theoretical predictions with experimental results. Using a computational high-throughput screening method, 29 doping elements were investigated for improving the thermal and electrical characteristics of IST phase-change material. Among the 29 dopants, it was found that Y offers the largest distortion in the lattice structure of IST with negative doping formation energy when Y is substituted in the In site. The atomic lattice images clearly show that the In site is substituted by the Y atom and the distortion angles of the Y-doped IST (Y-IST) are well matched with the calculated results of the density function theory (DFT). The set/reset speed of the Y-IST phase-change memory is faster than that of the IST and GST devices; the high set/reset speed is strongly related to the fast and stable phase transition due to the larger lattice distortion. The power consumption of the Y-IST device is also less than a fourth of that of the GST device. Therefore, lattice distortion effect can be considered as the most remarkable factor and may lead to the development of novel phase-change materials.