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GeSbTe ( germanium-antimony-tellurium or GST) is a phase-change material from the group of chalcogenide glasses used in rewritable and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing of up to 35 megabit/s to be written and direct overwrite capability up to 106 cycles. It is suitable for land-groove recording formats. It is often used in DVD-RW. New phase-change memories are possible using n-doped GeSbTe semiconductor. The melting point of the alloy is about 600 °C (900 K) and the crystallization temperature is between 100 and 150 °C.
During writing, the material is erased, initialized into its crystalline state, with low-intensity laser irradiation. The material heats up to its crystallization temperature, but not its melting point, and crystallizes. The information is written at the crystalline phase, by heating spots of it with short (<10 ns), high-intensity laser pulses; the material melts locally and is quickly cooled, remaining in the amorphous phase. As the amorphous phase has lower reflectivity than the crystalline phase, data can be recorded as dark spots on the crystalline background. Recently, novel liquid organogermanium precursors, such as isobutylgermane (IBGe) and tetrakis(dimethylamino)germane (TDMAGe) were developed and used in conjunction with the metalorganics of antimony and tellurium, such as tris-dimethylamino antimony (TDMASb) and di-isopropyl telluride (DIPTe) respectively, to grow GeSbTe and other chalcogenide films of very high purity by metalorganic chemical vapor deposition (MOCVD). Dimethylamino germanium trichloride (DMAGeC) is also reported as the chloride containing and superior dimethylaminogermanium precursor for Ge deposition by MOCVD.
The early devices were slow, power consuming and broke down easily due to the large currents. Therefore, it did not succeed as SRAM and flash memory took over. In the 1980s though, the discovery of germanium-antimony-tellurium (GeSbTe) meant that phase-change memory now needed less time and power to function. This resulted in the success of the rewriteable optical disk and created renewed interest in the phase-change memory. The advances in lithography also meant that previously excessive programming current has now become much smaller as the volume of GeSbTe that changes phase is reduced.
Phase-change memory has many near ideal memory qualities such as non-volatility, fast switching speed, high endurance of more than 1013 read –write cycles, non-destructive read, direct overwriting and long data retention time of more than 10 years. The one advantage that distinguishes it from other next generation non-volatile memory like magnetic random access memory (MRAM) is the unique scaling advantage of having better performance with smaller sizes. The limit to which phase-change memory can be scaled is hence limited by lithography at least until 45 nm. Thus, it offers the biggest potential of achieving ultra-high memory density cells that can be commercialized.
Though phase-change memory offers much promise, there are still certain technical problems that need to be solved before it can reach ultra-high density and commercialized. The most important challenge for phase-change memory is to reduce the programming current to the level that is compatible with the minimum MOSFET transistor drive current for high-density integration. Currently, the programming current in phase-change memory is substantially high. This high current limits the memory density of the phase-change memory cells as the current supplied by the transistor is not sufficient due to their high current requirement. Hence, the unique scaling advantage of phase-change memory cannot be fully utilized.
The typical phase-change memory device design is shown. It has layers including the top electrode, GST, the GeSbTe layer, BEC, the bottom electrode and the dielectric layers. The programmable volume is the GeSbTe volume that is in contact with the bottom electrode. This is the part that can be scaled down with lithography. The thermal time constant of the device is also important. The thermal time constant must be fast enough for GeSbTe to cool rapidly into the amorphous state during RESET but slow enough to allow crystallization to occur during SET state. The thermal time constant depends on the design and material the cell is built. To read, a low current pulse is applied to the device. A small current ensures the material does not heat up. Information stored is read out by measuring the resistance of the device.
Using the most possible crystalline and amorphous local structures for GeSbTe, the fact that density of crystalline GeSbTe is less than 10% larger than amorphous GeSbTe, and the fact that free energies of both amorphous and crystalline GeSbTe have to be around the same magnitude, it was hypothesized from density functional theory simulations that the most stable amorphous state was the spinel structure, where Ge occupies tetrahedral positions and Sb and Te occupy octahedral positions, as the ground state energy was the lowest of all the possible configurations. By means of Car-Parrinello molecular dynamics simulations this conjecture have been theoretically confirmed.
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