GexSbyTez (GST)

Ge_xSb_yTe_z (GST)
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In general, crystalline GST (Ge_xSb_yTe_z) 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 Ge_xSb_yTe_z (GST) compositions.

Composition	HRTEM image	Crystal structure	Electrical resistivity	Lattice constant (nm)	Atomic density (atom·Å^-3)	Crystall-ization tempera-ture (°C)	Melting point (°C)	Note	Reference
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]
Ag-Sb₂Te								Crystalline growth velocity 5.99 m/s.	[15]
AgInSbTe						200	537
Amorphous Ag₄In₃Sb₆₇Te₂₆					0.0309				[20]
Crystalline In₃Sb₆₇Te₂₆					0.0324				[20]
GaSb						195	589
GeCu₂Te₃						250	500		[24]
Amorphous GeTe	Medium-range ordering with fringe spacing of 0.317 nm:	Asymmetrical tetrahedral structure:			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°.
GeTe				Rhombohedral; a = 0.5996; α = 88.18°	~0.0358			<400 °C has a trigonal (R-3m) structure (distorted NaCl type structure)	[1, 2, 17]
GeTe				a = 0.60; α = 90°	~0.0358			>400 °C has NaCl (B1) type structure	[1, 2, 17]
GeTe				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]
Sb₂Te₂				a = 0.426; c = 2.39				Space group P-3m1
Amorphous Sb₂Te₃					0.0289				[13]
Sb₂Te₃				a = 1.0426; α = 23°31'		90-100	570- 621	Trigonal R-3m (tetradymite) structure; poor stability	[3]
Sb₂Te₃				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
Sb₇₀Te₃₀							544.5
W-Sb_xTe_y						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 Ge_xSb_yTe_z								Electromigration: Ge and Sb atoms migrate to the cathode, while Te atoms migrate to the anode:	[40, 42 - 44]
Amorphous 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				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				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]
Ag_x(Ge₂Sb₂Te₅)_100-x (x = 0-3 at.%)									[50]
Bi-Ge₂Sb₂Te₅									[52]
C-Ge₂Sb₂Te₅						279 (for C5%); 314 (for C8%); 342 (for C15%)			[27]
Co-Ge₂Sb₂Te₅									[52]
Cr-Ge₂Sb₂Te₅									[52]
Ge-rich Ge_xSb_yTe_z									[39, 41]
N-Ge₂Sb₂Te₅						250 (for 15 at.% N) ---------			[28, 29]
(Si+N)-Ge₂Sb₂Te₅								Crystallization inhibition of GST by SiN_x; 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]
N-Ge₅Sb₇₅Te₂₀								Crystallization time:	[51]
Fe-Ge₂Sb₂Te₅				a = 0.4205; c = 1.732				UV-visible reflectance spectra:	[38]
In-Ge₂Sb₂Te₅									[52]
O-Ge₂Sb₂Te₅									[45]
Pb-Ge₂Sb₂Te₅						Increases with higher Pb at%: 124 - 138			[52, 53]
Sb-Ge₂Sb₂Te₅								Conductive Sb filaments with electrical current are formed due to excess Sb atoms.	[58]
Sn-Ge₂Sb₂Te₅									[49]
Ti-Ge₂Sb₂Te₅									[52]
Ge_xSb₂Te_3+x								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:			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				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]
GeSb₄Te₇			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				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]
Ge₁₅Sb₈₅									[13]
(O+N)-Ge₂₁Sb₂₆Te₅₃									[57]
InSb						168	490
InSbTe						200	500-600
In₃SbTe₂					0.0351				[13, 21]
In₁₀Ge_xSb_52-xSn₂₃Te₁₅						192.6 (for x = 2); 201.5 (for x = 5); 208.6 (for x = 7); 213.1 (for x = 9).			[25]
InSe						200 & 650	890
Ga₃Sb₈Te						227	567.5		[23]
Si_3.9Sb_45.6Te_50.5						180	550		[24]
Si-Ga₂TeSb₇						260–361 (increase with increasing Si-composition)			[26]
Te₈₁Ge₁₅Sb₂S₂							380
TeGa₂Sb₁₄						232	584	Activation energy of crystallization 3.66 eV;	[22]

Note that for Ge_xSb_yTe_z, a higher vacancy concentration results in a higher crystallization speed. [31] The elemental concentrations of Ge_xSb_yTe_z 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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