Disorder Control in Crystalline GeSb₂Te₄ Using High Pressure

Abstract

disordered as possible. However, usually such chemical disorder is discouraged by the concomitant enthalpy rise, caused by the formation of homopolar bonds. Disorder can induce localization of electronic wavefunctions (e.g., Anderson localization [ 1 ] and thus can be crucial for the electronic properties of materials. [2][3][4][5][6][7] The current applications of phasechange materials (PCMs) take advantage of the fast transformations and large property contrast between the amorphous and the crystalline states. [8][9][10][11] Such binary memory devices, however, may fail to meet the increasingly demanding requirements of data storage. A possible solution to this issue is to record data on each memory cell with multiple states of electrical resistivity, in addition to "on" and "off" switches. This may be achieved by tuning disorder in PCMs. Therefore, it is important to understand and control disorder in these materials. The development of a multistate memory device would signifi cantly increase the data density and could change the way electronic devices work. [ 12,13 ] To manipulate the disorder in data storage media, Siegrist et al. have modifi ed the atomic arrangement in crystalline PCMs such as Ge-Sb-Te (GST), a prototype of PCMs, by annealing. [ 2 ] At low annealing temperatures, these GST samples form metastable cubic rocksalt phases ( c -GST) with different levels of electrical resistivity. In this phase, one sublattice contains Te atoms, whereas Ge, Sb, and vacancies occupy the sites of the second sublattice in a random fashion. High annealing temperatures induce the ordering of the vacancies in c -GST, which gradually evolves into a metallic hexagonal phase ( h -GST) with all vacancies diffusing into layers. Ab initio simulations [ 3 ] show that such multiple resistive states are indeed due to the different degrees of vacancy ordering. In particular, strong disorder results in the localization of electron wave functions at the Fermi energy.In this article, we report that the compositional disorder in PCMs can also be tuned by pressure in lieu of the thermal treatment. Large-scale ab initio molecular dynamics (AIMD) simulations reveal that pressure can expedite the antisite hopping in the vacancy-ridden c -GST by lowering the migration barrier. Accumulation of these antisites leads to severe atomic distortions. The resulting strong misalignment of bonds may trigger the loss of the long-range order in the crystal [ 14 ] and contributes to the amorphization of c -GST under high pressure, as observed in experiments at 15 GPa. [ 15,16 ] Our simulations identify a new disorder-triggered mechanism of the amorphization Electronic phase-change memory devices take advantage of the different resistivity of two states, amorphous and crystalline, and the swift transitions between them in active phase-change materials (PCMs). In addition to these two distinct phases, multiple resistive states can be obtained by tuning the atomic disorder in the crystalline phase with heat treatment, because the disorder can lead to...

Crystal growth of Ge2Sb2Te5 at high temperatures

“…[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Upon fast crystallization of the glass, GST forms a metastable rocksalt phase, [34] in which Te atoms occupy one sublattice, while Ge, Sb, and vacancies occupy the second one in a random fashion. It has been shown experimentally [35][36][37] and theoretically [38][39][40] that the degree of disorder in rocksalt GST and similar compounds on the pseudobinary line can be tuned by thermal annealing or even pressure, so as to induce Anderson insulator-metal transitions induced by disorder.…”

A Review on Disorder-Driven Metal–Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials

“…Recently, small fractions of antisite defects, yielding some Te–Te and Ge(Sb)–Ge(Sb) homopolar bonds, were found in recrystallized Ge 2 Sb 2 Te 5 obtained by DFT-based molecular dynamics (DFMD) simulations [79,80,81,82,83,84]. A thorough understanding of the effects of antisite defects on the localization properties is anticipated, nevertheless, in a recent study on pressuring rocksalt Ge 1 Sb 2 Te 4 at elevated temperatures, pairwise antisites were found but were shown to be ineffective for the localization properties of the occupied electronic states at the Fermi level [85]. To conclude, the presence and random distribution of a high amount of atomic vacancies in crystalline GST would unavoidably result in the formation of “vacancy clusters”, giving rise to the localization of electrons in crystalline GST.…”

“…It was predicted that the barrier of interlayer vacancy diffusion in GST crystals is not very high ~0.8–1.0 eV, and several such vacancy diffusion events were found in very short time scale ~50 picoseconds at 500 °C [17,42]. This transition barrier can be further reduced by pressure [85]. It is highly desirable to thoroughly test the vacancy ordering speed within the crystalline states in nanoscale phase change devices.…”

“…In addition to thermal annealing, pressure [85,107,108,109,110,111], strain [112,113], chemical composition [14,16], ion bombardment [114], focused electron beam irradiation [92], electric field [115,116], voltage pulses [117,118,119], laser and photonic excitation [120,121], and interface template [93,122,123] are shown to be effective approaches in tuning disorder in GST and related compounds. It is also worth mentioning that the remaining disorder in ultrathin hexagonal GST films ~7–14 nm would result in a weak anti-localization phenomenon.…”

Effects of stoichiometry on the transport properties of crystalline phase-change materials

“…The strength of disorder is tuned by increasing the annealing temperature. Recently, it has been shown that disorder in crystalline GST can also be tuned by pressure12. A disorder-driven MIT has been reported in GeTe nanowires as well13.…”