Phase-Change Behavior of Stoichiometric Ge[sub 2]Sb[sub 2]Te[sub 5] in Phase-Change Random Access Memory

Abstract

We observed the atomic structures for each reset and set state in a phase-change random access memory fabricated using stoichiometric crystalline Ge 2 Sb 2 Te 5 . The reset state clearly showed a mixture of dome-shaped amorphous and crystal structure surrounding amorphous, but the set state showed abnormally grown large grains due to recrystallization of the amorphous structure. The crystal structure of the recrystallized grain was face-centered cubic. The element analysis indicated that the atomic composition changes to nonstoichiometric phase in the active regions of the reset and the set state, which is Sb-rich and Te-deficient compared to the pristine stoichiometric composition. Analysis showed that thermal interdiffusion of Sb and Te caused nonstoichiometric nature of the material to reach the energetically stable state in the active region.Great efforts to develop next generation memories with fewer technical barriers, longer lifetime, and better performance in comparison with existing memories, such as flash, and dynamic-static random access memories, have been attempted. Recently, there has been a great deal of interest in phase-change random access memory ͑PRAM͒ based on stoichiometric crystalline Ge 2 Sb 2 Te 5 ͓referred to as GST ͑2/2/5͔͒ films. PRAM takes advantage of electrically bistable status of resistance difference between its amorphous ͑reset state with high resistance͒ and crystalline phase ͑set state with low resistance͒. 1,2 Amorphous resistance is at least 10 2 times higher than crystalline resistance. Hence, PRAM is considered to be one of the promising memories owing to its nonvolatility, good scalability, large sensing signal, fast reading and moderately fast writing time, and long data endurance.In the operation, PRAM uses a high current pulse to make the amorphous state, and a low current pulse to get the crystalline state. The current is applied to heat up a contact region between GST and a bottom electrode ͑role of resistive heater͒ using Joule heating. During the high current pulse, the GST loses the polycrystalline ordering as temperature exceeds its melting point. After that, the GST is quenched and stays in a structurally disordered state. The low current pulse and its sufficient duration is used to maintain temperature in the crystallization range for both nucleation and crystallization of the GST. The amorphous and the recrystallized GST right near the contact region is the active region. The typical setreset characteristics of the device in this study are shown in Fig. 1, which is fully integrated in complementary metal-oxidesemiconductor ͑CMOS͒ technology.Because of high current driving operation mechanism, PRAM has suffered from device failure such as delamination. Recently, Ryu et al. reported another important issue regarding compositional alteration of GST͑2/2/5͒ during operation. 3 In their report GST͑2/2/5͒ phase is not stable and has changed to GST͑15/47/38͒. However, they have has not mentioned the detailed phenomena of the GST/TiN contact region, especially element t...

Temperature dependence on the contact size of GeSbTe films for phase change memories

“…2(b) is a conformal bi-layer GST structure with a HCP phase in the top GST layer and a FCC phase in the bottom GST layer [9,10]. The FCC and HCP are two different phases [9] with different electric and thermal conductivities for the GST materials. The second structure is used to simulate the curves of resistance versus applied currents (R-I curves).…”

RESET failure analysis of phase change memory based on Ge₂Sb₂Te₅

“…As the potential candidate, PCM based on GST-225 must have good SET-RESET performance and high reliability. As the PCM density increases, PCM experiences harsher operating conditions, such as high current density and temperature, which can cause failure and reliability issues induced by compositional change and void formation [4,5]. While phase transition processes are well understood, issues on performance and reliability are getting more and more attention in recent years, which makes the failure analysis of PCM becomes more and more important.…”

Study on the phase change material Cr-doped Sb 3 Te 1 for application in phase change memory

“…The characteristics of phase change materials have deep influence on the performance of PCM, thus many phase change materials are studied. Ge 2 Sb 2 Te 5 (GST) is widely utilized in PCM because of good crystallization speed and thermal stability, but low crystallization temperature (T c ) and inferior data retention make it not meet the demand of high thermal stable and high speed PCM [6][7][8][9][10].…”