GexSbyTez (GST)
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In general, crystalline GST (GexSbyTez) thin films show two types of structures: [5, 61]
         i) Metastable NaCl-cubic structure (space group Fm-3m).
In this structure, the 4A sites are occupied by Te atoms and the 4B sites are randomly occupied by Ge atoms, Sb atoms and 10-20% vacancies depending on the GST composition [5]. On the other hand, there is a relatively large displacement of Ge-atoms.
         ii) Stable hexagonal structure (space group p-3m1).

Note that, in formation of GST crystals, a stacking disorder parallel to the basal plane increases with increasing cooling rates.

In poly-crystalline GST, excess Ge atoms and Sb atoms do not fill up the vacancies, but rather segregate on the grain boundaries. In amorphous binary GeTe and quasibinary GST, the Ge-Te bonds are shorter and stronger than those in the corresponding crystals, and thus the amorphous phase is locally more ordered than the crystalline phase [62].

Table 3161. Structural properties of some GexSbyTez (GST) compositions.

Electron diffraction
HRTEM image
Crystal structure
Electrical resistivity
Lattice constant
Atomic density (atom·Å-3)
Crystall-ization tempera-ture (°C)
Melting point (°C)
Amorphous Sb
            0.0321       [13]
Crystalline Sb
  a = 0.43; c = 1.12       Space group R-3m. Interatomic distances: 0.296 nm and 0.430 nm. [13]
                  Crystalline growth velocity 5.99 m/s. [15]
              200 537    
Amorphous Ag4In3Sb67Te26
            0.0309       [20]
Crystalline In3Sb67Te26
            0.0324       [20]
              195 589    
              250 500   [24]
Amorphous GeTe
    Medium-range ordering with fringe spacing of 0.317 nm:
medium-range ordering
Asymmetrical tetrahedral structure:
Asymmetrical tetrahedral structure GeTe
    0.0334 182 725 Bond length: Ge-Te = 0.255±0.004 nm; Te-Te = 0.425±0.004 nm. Bond angle: Te-Ge-Te = 112°. Still various structure models for the local order. [16, 59]
Amorphous 4%C- GeTe
              290   Improved amorphous phase stability compared to GeTe [63]
Amorphous 10%C- GeTe
              340     [63]
Crystalline GeTe
                  Bond length: Ge-Te = 0.290±0.004 nm; Te-Te = 0.428±0.004 nm. Bond angle: Te-Ge-Te = 93°.  
          Rhombohedral; a = 0.5996; α = 88.18° ~0.0358     <400 °C has a trigonal (R-3m) structure (distorted NaCl type structure) [1, 2, 17]
          a = 0.60; α = 90° ~0.0358     >400 °C has NaCl (B1) type structure [1, 2, 17]
          a' = 0.417; c' = 1.071 ~0.0358     With an a, b, c stacking sequence of close packed planes along the c'-axis of Te–Ge–Te–Ge–Te–Ge– [17]
Amorphous Ge8Sb2Te11
            0.0309       [17]
Amorphous Sb2Te
            0.0309       [13]
Sb2Te (δ)
          a = 0.4272; c = 1.7633       Space group P-3m1; crystalline growth velocity 13.16 m/s. [15]
          a = 0.426; c = 2.39       Space group P-3m1  
Amorphous Sb2Te3
            0.0289       [13]
          a = 1.0426; α = 23°31'   90-100 570- 621 Trigonal R-3m (tetradymite) structure; poor stability [3]
  a' = 0.425; c' = 3.04 0.0313 90-100 621 Space group R-3m; with an a, b, c stacking along the c'-axis Te–Sb–Te–Te–Sb– [19]
Sb4Te3 (γ)
a = 0.426; c = 4.155 Space group P-3m1
W-SbxTey XRD 132 (for 0% W), 158 (for 0.7% W), 189 (for 2.9% W), and 233 (for 5.2% W) 536 (for 0% W), 537 (for 0.7% W), 539 (for 2.9% W), and 539 (for 5.2% W) [47]
Molten GexSbyTez
        FCC HCP amorphous fcc Ge2Sb2Te5       melting temperature of GST Electromigration: Ge and Sb atoms migrate to the cathode, while Te atoms migrate to the anode:
GST electromigration
[40, 42 - 44]
Amorphous Ge2Sb2Te5
Ge2Sb2Te5 XRD
Ge2Sb2Te5 XRD
      FCC HCP amorphous fcc Ge2Sb2Te5   0.0300    

Stable at room temperature for more than 10 years. Both α- and c-Ge2Sb2Te5 can only be etched by nitric acid (HNO3) aqueous solution, but cannot be etched by H2C2O4, HClO4, CH3COOH, H2SO4, H3PO4, HCl; electric conductivity 3 Ω-1-m-1; thermal conductivity 0.2 W/K-m; specific heat 1.25 x 106 J/K-m3; activation energy of crystallization: 3.636 eV; activation energy of structural transformation from fcc to hcp: 1.579 eV; Band gap: 0.7 - 1.0 eV.

[13, 18, 30, 42, 54]
Metastable fcc Ge2Sb2Te5
      FCC HCP amorphous fcc Ge2Sb2Te5 a = 0.60 0.0335 151-174 632 Metastable phase NaCl-type structure with the Te atoms on one fcc sublattice and with the Ge and Sb atoms and 20% of vacancies distributed randomly over the other fcc sublattice; Fm-3m; electric conductivity 2770 Ω-1-m-1; thermal conductivity 0.5 W/K-m; specific heat 1.25 x 106 J/K-m3. [5, 9, 17, 42, 45]
Stable hcp Ge2Sb2Te5
Ge2Sb2Te5     FCC HCP amorphous fcc Ge2Sb2Te5 a' = 0.425; c' = 1.72 ~ 1.827 0.0335 151-174 632 Trigonal cell with stable phase hexagonal structure; (P-3m1); Stacking sequence along the c'-axis of a four layer block of GeTe and one repeat unit of Sb2Te3: Te–Sb–Te–Ge–Te– Te–Ge–Te–Sb–; thermal conductivity 1.59 W•m-1•K-1, density 6300 kg•m-3, specific heat 1.25 x 106 J/K-m3, Young's modulus 58.7 GPa, thermal expansion coefficient 17.4 × 10-6 K-1, Poisson's ratio 0.3. [4, 6 - 9, 17 - 18, 34 - 37, 42]
Agx(Ge2Sb2Te5)100-x (x = 0-3 at.%)
Agx(Ge2Sb2Te5)100-x             Agx(Ge2Sb2Te5)100-x     [50]
    279 (for C5%); 314 (for C8%); 342 (for C15%)     [27]
Ge-rich GexSbyTez
Ge-rich GexSbyTez       Ge-rich GeSbTe     Ge-rich GexSbyTez     [39, 41]
            250 (for 15 at.% N)
    [28, 29]
(Si+N)-Ge2Sb2Te5       (Si+N)-Ge2Sb2Te5         Crystallization inhibition of GST by SiNx; average grain sizes of pure, Si-, and (Si+N)-GST after 400 °C annealing for 10 min are 20.4, 10.7, and 5.6 nm, respectively. [55, 56]
Ge5Sb75Te20                 Crystallization time:
Fe-Ge2Sb2Te5         a = 0.4205; c = 1.732       UV-visible reflectance spectra:
O-Ge2Sb2Te5                   [45]
Pb-Ge2Sb2Te5             Increases with higher Pb at%: 124 - 138     [52, 53]
                  Conductive Sb filaments with electrical current are formed due to excess Sb atoms. [58]
Sn-Ge2Sb2Te5                   [49]
                  An alternation in c'-axis of one repeat unit of Sb2Te3 and a block consisting of 2x layers GeTe [6]
Amorphous GeSb2Te4
      Asymmetrical tetrahedral structure:
Asymmetrical tetrahedral structure
    0.0304     Bond length: Ge(Sb)-Te = 0.278±0.004 nm; Te-Te = 0.410±0.004 nm (shorter than in the corrsponding crystalline phases). Bond angle: Te-Ge-Te = 95°. [17]
Metastable cubic GeSb2Te4
            0.0316     Bond length: Ge(Sb)-Te = 0.295±0.004 nm; Te-Te = 0.417±0.004 nm. Bond angle: Te-Ge-Te = 90°. [17]
Stable hexagonal  GeSb2Te4
GeSb2Te4 resistance
a' = 0.425; c' = 4.10 0331     The periodicity is 21:
-Te-Ge-Te-Sb-Te-vac-Te-Sb-Te-Ge-Te-Sb-Te-vac-Te-Sb-Te-Ge-Te-Sb-Te-vac-Te-Sb-. Very high static dielectric constant 98.
[6 - 8, 14, 15, 17]
O-GeSb2Te4                 For O < 10 at.%, Te, Sb and most of Ge are in metallic state, and free O is located at tetrahedral interstitial sites and acts as nucleation center; Higher O at.%, segregated Te is in metallic state; Higher T (573 K) promoted Te phase. [48]
        3.47 X 10-4 Ω•m;        

Thermal conductivity 0.49 W•m-1•K-1, specific heat 193.55 J•kg-1•K-1, density 5685 kg•m-3 Young's modulus 37.8 GPa, thermal expansion coefficient 17.913 × 10-6 K-1, Poisson's ratio 0.3.

[32 - 34]
  Ge3Sb2Te6 Ge3Sb2Te6 HRTEM     a' = 0.425; c' = 6.26         [6 - 8]
Ge15Sb70Te15                 Space group R-3m; [12]
Diamond-type Ge
          a = 0.565735 at 20 °C       Fd-3m [10, 11]
              (O+N)-Ge21Sb26Te53     [57]
              168 490    
              200 500-600    
    0.0351       [13, 21]
              192.6 (for x = 2); 201.5 (for x = 5); 208.6 (for x = 7); 213.1 (for x = 9).     [25]
              200 & 650 890    
              227 567.5   [23]
              180 550   [24]
Si-Ga2TeSb7             260–361 (increase with
increasing Si-composition)
            232 584 Activation energy of crystallization 3.66 eV; [22]

Note that for GexSbyTez, a higher vacancy concentration results in a higher crystallization speed. [31] The elemental concentrations of GexSbyTez are normally quantified by using EDS measurements with Ge K, Sb L, and Te L lines. [46]

Application of GST alloys for optical storage

Figure 3161a. Application of GST alloys for optical storage. [60]

Figure 3161b shows the dependence of crystallized fraction of amorphous Ge2Sb2Te5 (GST) on the annealing time and temperature.

Dependence of amorphous Ge2Sb2Te5 (GST) on the annealing time and temperature

Figure 3161b. Dependence of crystallized fraction of amorphous Ge2Sb2Te5 (GST) on the annealing time and temperature.




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