GeSbTe
Encyclopedia
GeSbTe, Germanium-Antimony-Tellurium or GST is a phase change material from the group of chalcogenide glass
es, used in rewritable optical disc
s and phase-change memory
applications. Its recrystallization time is 20 nanoseconds, allowing bitrate
s of up to 35 Mbit
/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 rewritable DVDs
. 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 100-150 °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 (abbr: IBGe) and tetrakis(dimethylamino)germane (abbr: TDMAGe) were developed and used in conjunction with the metalorganics
of antimony
and tellurium, such as tris-dimethylamino antimony (TDMASb) and Di-isopropyl telluride (abbr: DIPTe) respectively, to grow GeSbTe and other chalcogenide
films of very high purity by metalorganic chemical vapor deposition (MOCVD). Dimethylamino germanium trichloride (abbr: DMAGeC) is also reported as the chloride containing and a superior dimethylaminogermanium precursor for Ge deposition by MOCVD.
, antimony
, and tellurium, with composition GeTe-Sb2Te3. In the GeSbTe system, there is a pseudo-line as shown where most of the alloys lie on. Moving down this pseudo-line, it can be seen that as we go from Sb2Te3 to GeTe, the melting point and glass transition temperature of the materials increase, crystallization speed decreases and data retention increases. Hence, in order to get high data transfer rate, we need to use material with fast crystallization speed such as Sb2Te3. This material is not stable because of its low activation energy. On the other hand, materials with good amorphous stability like GeTe has slow crystallization speed because of its high activation energy. In its stable state, crystalline GeSbTe has two possible configurations: hexagonal and a metastable face centered cubic (FCC) lattice. When it is rapidly crystallized however, it was found to have a distorted rocksalt structure. GeSbTe has a glass transition temperature of around 100 °C. GeSbTe also has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair
of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are also common in GeSbTe and due to these defects, an Urbach
tail in the band structure is formed in these compounds. GeSbTe is generally p type
and there are many electronic states in the band gap
accounting for acceptor and donor like traps. GeSbTe has two stable states, crystalline and amorphous. The phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe.
useful as a memory is the ability to effect a reversible phase change when heated or cooled, switching between stable amorphous and crystalline states. These alloys have high resistance in the amorphous state ‘0’ and are semimetals in the crystalline state ‘1’. In amorphous state, the atoms have short-range atomic order and low free electron density. The alloy also has high resistivity and activation energy. This distinguishes it from the crystalline state having low resistivity and activation energy, long-range atomic order and high free electron density. When used in phase change memory, use of a short, high amplitude electric pulse such that the material reaches melting point and rapidly quenched changes the material from crystalline phase to amorphous phase is widely termed as RESET current and use of a relatively longer, low amplitude electric pulse such that the material reaches only the crystallization point and given time to crystallize allowing phase change from amorphous to crystalline is known as SET current.
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 MOS 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.
. This can be seen from the current
-voltage
(IV) plot, where current is very low in the amorphous state at low voltage until threshold voltage is reached. Current increases rapidly after the voltage snapback
. The material is now in the amorphous "ON" state, where the material is still amorphous, but in a pseudo- crystalline electric state. In crystalline state, the IV characteristics is ohmic
. There had been debate on whether threshold switching was an electrical or thermal
process. There were suggestions that the exponential
increase in current at threshold voltage must have been due to generation of carriers that vary exponentially with voltage such as impact ionization
or tunneling
.
s are intact and only the weaker bonds are broken.
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.
. It offers higher linear density, but has lower overwrite cycles by 1-2 orders of magnitude. It is used in groove-only recording formats, often in rewritable CDs
.AgInSbTe
is known as a growth dominated material while GeSbTe is known as a nucleation dominated material. In GeSbTe, the nucleation process of crystallization is long with many small crystalline nuclei being formed before a short growth process where the numerous small crystals are joined together. In AgInSbTe
, there are only a few nuclei formed in the nucleation stage and these nuclei grow bigger in the longer growth stage such that they eventually form 1 crystalline crystal.
Chalcogenide glass
A chalcogenide glass is a glass containing one or more chalcogenide elements. These are Group 16 in the periodic table e.g. sulfur, selenium or tellurium. Such glasses are covalently bonded materials and may be classified as network solids. In effect, the entire glass matrix acts like an...
es, used in rewritable optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
s and phase-change memory
Phase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
applications. Its recrystallization time is 20 nanoseconds, allowing bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
s of up to 35 Mbit
Megabit
The megabit is a multiple of the unit bit for digital information or computer storage. The prefix mega is defined in the International System of Units as a multiplier of 106 , and therefore...
/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 rewritable DVDs
DVD-RW
A DVD-RW disc is a rewritable optical disc with equal storage capacity to a DVD-R, typically 4.7 GB. The format was developed by Pioneer in November 1999 and has been approved by the DVD Forum. The smaller Mini DVD-RW holds 1.46 GB, with a diameter of 8 cm.The primary advantage of DVD-RW over...
. New phase-change memories are possible using n-doped GeSbTe semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
. The melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
of the alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
is about 600 °C (900 K) and the crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
temperature is between 100-150 °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
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulses; the material melts locally and is quickly cooled, remaining in the amorphous phase. As the amorphous phase has lower reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
than the crystalline phase, data can be recorded as dark spots on the crystalline background. Recently, novel liquid organogermanium precursors, such as isobutylgermane (abbr: IBGe) and tetrakis(dimethylamino)germane (abbr: TDMAGe) were developed and used in conjunction with the metalorganics
Metalorganics
Metalorganic compounds are a class of chemical compounds that contain metals and organic ligands. Metalorganic compounds are used extensively in materials science in applications such as metalorganic vapour phase epitaxy or sol-gel processing using alkoxides...
of antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
and tellurium, such as tris-dimethylamino antimony (TDMASb) and Di-isopropyl telluride (abbr: DIPTe) respectively, to grow GeSbTe and other chalcogenide
Chalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...
films of very high purity by metalorganic chemical vapor deposition (MOCVD). Dimethylamino germanium trichloride (abbr: DMAGeC) is also reported as the chloride containing and a superior dimethylaminogermanium precursor for Ge deposition by MOCVD.
Material Properties
GeSbTe is a ternary compound of germaniumGermanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
, antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
, and tellurium, with composition GeTe-Sb2Te3. In the GeSbTe system, there is a pseudo-line as shown where most of the alloys lie on. Moving down this pseudo-line, it can be seen that as we go from Sb2Te3 to GeTe, the melting point and glass transition temperature of the materials increase, crystallization speed decreases and data retention increases. Hence, in order to get high data transfer rate, we need to use material with fast crystallization speed such as Sb2Te3. This material is not stable because of its low activation energy. On the other hand, materials with good amorphous stability like GeTe has slow crystallization speed because of its high activation energy. In its stable state, crystalline GeSbTe has two possible configurations: hexagonal and a metastable face centered cubic (FCC) lattice. When it is rapidly crystallized however, it was found to have a distorted rocksalt structure. GeSbTe has a glass transition temperature of around 100 °C. GeSbTe also has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are also common in GeSbTe and due to these defects, an Urbach
Urbach
-Places:*several places in Germany:**Urbach, Baden-Württemberg**Urbach, Rhineland-Palatinate**Urbach, Thuringia*the village of Urbach in Moselle, France, which is the burial site of J. F. Oberlin.*a valley in Oberhasli, Switzerland-Scientific Term:...
tail in the band structure is formed in these compounds. GeSbTe is generally p type
P-type semiconductor
A P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....
and there are many electronic states in the band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
accounting for acceptor and donor like traps. GeSbTe has two stable states, crystalline and amorphous. The phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe.
Applications in phase-change memory
The unique characteristic that makes phase-change memoryPhase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
useful as a memory is the ability to effect a reversible phase change when heated or cooled, switching between stable amorphous and crystalline states. These alloys have high resistance in the amorphous state ‘0’ and are semimetals in the crystalline state ‘1’. In amorphous state, the atoms have short-range atomic order and low free electron density. The alloy also has high resistivity and activation energy. This distinguishes it from the crystalline state having low resistivity and activation energy, long-range atomic order and high free electron density. When used in phase change memory, use of a short, high amplitude electric pulse such that the material reaches melting point and rapidly quenched changes the material from crystalline phase to amorphous phase is widely termed as RESET current and use of a relatively longer, low amplitude electric pulse such that the material reaches only the crystallization point and given time to crystallize allowing phase change from amorphous to crystalline is known as SET current.
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
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
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
Lithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...
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 MOS 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
Phase-change memory
Phase-change memory is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous...
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
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
and the dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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.
Threshold Switching
Threshold switching occurs when GeSbTe goes from a high resistive state to a conductive state at the threshold voltageThreshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
. This can be seen from the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
-voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
(IV) plot, where current is very low in the amorphous state at low voltage until threshold voltage is reached. Current increases rapidly after the voltage snapback
Snapback
Snapback is an event played in several incarnations of the television series Gladiators. In this ground based event, contenders are attached to bungee cords and must race across the floor in order to score points...
. The material is now in the amorphous "ON" state, where the material is still amorphous, but in a pseudo- crystalline electric state. In crystalline state, the IV characteristics is ohmic
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
. There had been debate on whether threshold switching was an electrical or thermal
Thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...
process. There were suggestions that the exponential
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
increase in current at threshold voltage must have been due to generation of carriers that vary exponentially with voltage such as impact ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
or tunneling
Tunneling
Tunneling may refer to:* Digging tunnels * Quantum tunneling, the quantum-mechanical effect where a particle crosses through a classically-forbidden potential energy barrier...
.
Nano-timescale phase change
Recently, much research has focused on the material analysis of the phase change material in an attempt to explain the high speed phase change of GeSbTe. Using EXAFS, it was found that the most matching model for crystalline GeSbTe is a distorted rocksalt lattice and for amorphous a tetrahedral structure. The small change in configuration from distorted rocksalt to tetrahedral suggests that nano-timescale phase change is possible as the major covalent bondCovalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s are intact and only the weaker bonds are broken.
Using the most possible crystalline and amorphous local structures for GeSbTe, the fact that density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of crystalline GeSbTe is less than 10% larger than amorphous GeSbTe, and the fact that free energies
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
of both amorphous and crystalline GeSbTe have to be around the same magnitude, it was hypothesized from density functional theory
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...
simulations that the most stable amorphous state was the spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
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
Molecular dynamics
Molecular dynamics is a computer simulation of physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a period of time, giving a view of the motion of the atoms...
simulations this conjecture have been theoretically confirmed.
Nucleation dominated versus growth dominated
Another similar material is AgInSbTeAgInSbTe
AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications...
. It offers higher linear density, but has lower overwrite cycles by 1-2 orders of magnitude. It is used in groove-only recording formats, often in rewritable CDs
CD-RW
A CD-RW is a rewritable optical disc. It was introduced in 1997, and was known as "CD-Writable" during development. It was preceded by the CD-MO, which was never commercially released....
.AgInSbTe
AgInSbTe
AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications...
is known as a growth dominated material while GeSbTe is known as a nucleation dominated material. In GeSbTe, the nucleation process of crystallization is long with many small crystalline nuclei being formed before a short growth process where the numerous small crystals are joined together. In AgInSbTe
AgInSbTe
AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs and phase-change memory applications...
, there are only a few nuclei formed in the nucleation stage and these nuclei grow bigger in the longer growth stage such that they eventually form 1 crystalline crystal.