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Citation: Qiao, Y.; Zhao, J.; Sun, H.;
Song, Z.; Xue, Y.; Li, J.; Song, S. Pt
Modified Sb2Te3Alloy Ensuring
High−Performance Phase Change
Memory. Nanomaterials 2022,12, 1996.
https://doi.org/10.3390/
nano12121996
Academic Editor: Jeremy Sloan
Received: 9 April 2022
Accepted: 13 May 2022
Published: 10 June 2022
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nanomaterials
Article
Pt Modified Sb2Te3Alloy Ensuring High−Performance Phase
Change Memory
Yang Qiao 1, Jin Zhao 2,3, Haodong Sun 1, Zhitang Song 2, Yuan Xue 2 ,*, Jiao Li 1 ,4 ,* and Sannian Song 2 ,*
1The Microelectronic Research & Development Center, Shanghai University, Shanghai 200444, China;
yangqiao@shu.edu.cn (Y.Q.); shd_1013484830@shu.edu.cn (H.S.)
2State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and
Information, Chinese Academy of Sciences, Shanghai 200050, China; zhaojin@mail.sim.ac.cn (J.Z.);
ztsong@mail.sim.ac.cn (Z.S.)
3University of Chinese Academy of Sciences, Beijing 100049, China
4Department of Mechatronics Engineering and Automation, Shanghai University, Shanghai 200444, China
*Correspondence: xueyuan@mail.sim.ac.cn (Y.X.); lijiaoshu@shu.edu.cn (J.L.);
songsannian@mail.sim.ac.cn (S.S.)
Abstract:
Phase change memory (PCM), due to the advantages in capacity and endurance, has the
opportunity to become the next generation of general
−
purpose memory. However, operation speed
and data retention are still bottlenecks for PCM development. The most direct way to solve this prob-
lem is to find a material with high speed and good thermal stability. In this paper, platinum doping is
proposed to improve performance. The 10-year data retention temperature of the doped material is
up to 104
◦
C; the device achieves an operation speed of 6 ns and more than
3×105operation cycles
.
An excellent performance was derived from the reduced grain size (10 nm) and the smaller density
change rate (4.76%), which are less than those of Ge
2
Sb
2
Te
5
(GST) and Sb
2
Te
3
. Hence, platinum
doping is an effective approach to improve the performance of PCM and provide both good thermal
stability and high operation speed.
Keywords: phase change memory; phase change material; high speed; thermal stability
1. Introduction
In the past decades, rapid advances in artificial intelligence [
1
,
2
], supercomputing [
3
],
and big data [
4
] have required ever
−
faster data exchange. While traditional hard disk
drives and solid
−
state drives struggle to meet demand, new types of memory have taken
the challenge. Phase change memory (PCM) is considered a promising non
−
volatile
memory technology due to its advantages of high speed, high density, high scalability,
low operating voltage, and high endurance [
2
,
5
–
8
]. As the storage medium of PCM,
phase change material can achieve reversible phase transitions between crystalline and
amorphous states under the action of electrical pulses. The memory relies on the resistance
difference between the crystalline and amorphous states of phase change materials to store
“0” and “1” [
9
–
12
]. The common phase change material Ge
2
Sb
2
Te
5
(GST) is currently the
most successful commercialized material. However, poor 10
−
year data retention (~85
◦
C),
slow operating speed (~20 ns), and a density change rate of 6.5% limit its wider application
in electrical devices [
13
,
14
]. Therefore, looking for a phase change material with high
amorphous thermal stability and fast speed is the key to improving the performance of
PCM [2,15,16].
The PCM device based on Sb
2
Te
3
shows fast operation speed. However, the low
crystallization temperature (<100
◦
C) makes the amorphous state unstable, which means
that Sb
2
Te
3
is not suitable for PCM application. Doping is a good way to improve thermal
stability and speed. Some researchers have obtained high
−
performance phase change
materials by doping Sb
2
Te
3
, such as Sc
0.2
Sb
2
Te
3
. It achieved an ultra
−
fast operation
Nanomaterials 2022,12, 1996. https://doi.org/10.3390/nano12121996 https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2022,12, 1996 2 of 8
speed of 700 ps and the data retention of ~87
◦
C [
5
], which satisfies the requirements of
subnanosecond high
−
speed cache memory. However, in some applications filed [
13
,
14
],
higher data retention is required. Since the thermal stability of materials is related to
data retention, we need to find a phase change material with high thermal stability. The
traditional precious metals materials (Au, Ag, Pt) have excellent chemical stability and are
conducive to engineering applications. Our selection principle is that the element with high
electronegativity is used as the doped element, so as to form a stable chemical bond with
the elements of the parent material to ensure no phase separation during the operation
of the device. Silver was, therefore, rejected as a candidate material. At the same time,
considering the cost of gold and platinum, platinum is finally selected as the dopant.
In this work, we have performed electrical tests based on Pt
−
Sb
2
Te
3
devices and mi-
croscopic characterization of films. The PCM devices based on Pt
0.14
Sb
2
Te
3
(PST) show fast
operation speed, high data retention, and good endurance. Meanwhile, the corresponding
microstructure of PST explains the origin of its high performance.
2. Materials and Methods
2.1. Film Preparation and Testing
The Sb
2
Te
3
, Pt
0.1
Sb
2
Te
3
, Pt
0.14
Sb
2
Te
3
(PST), and Pt
0.22
Sb
2
Te
3
films are deposited by
sputtering of Pt and Sb
2
Te
3
targets. The compositions of these films were measured by
energy
−
dispersive spectroscopy (EDS). Films with a thickness of 200 nm were deposited
on SiO
2
/Si (100) substrates for resistance
−
temperature (R
−
T) and X
−
ray diffraction (XRD)
tests. In situ R
−
T measurement was conducted by a homemade vacuum heating table,
and the heating rate was 20
◦
C /min. The film was heated in a vacuum chamber with
a heating rate of 60
◦
C/min, and the isothermal change in resistance with increasing
temperature was recorded to estimate the 10
−
year data retention. The X
−
ray reflectivity
(XRR) experiment (Bruker D8 Discover) was used to test the density change of films
before and after crystallization. X
−
ray photoelectron spectroscopy (XPS) experiment was
used to evaluate the bonding situation. Then the film (about 20 nm) was deposited on
the ultra
−
thin carbon film, and its microstructure was studied by Transmission Electron
Microscope (TEM). TEM is manufactured by Hitachi Limited in Tokyo, Japan.
2.2. Device Fabrication
T
−
shaped PCM devices were prepared by 0.13
µ
m complementary metal
−
oxide
semiconductor technology. The diameter of the tungsten bottom electrode is about
60 nm
.
The 70 nm
−
thick phase change material and 20 nm
−
thick TiN as adhesion layer were
deposited through the sputtering method over a 60 nm diameter of tungsten heating
electrode. The device is measured by the Keithley 2400 C source meter and Tektronix
AWG5002B pulse generator. The Keithley 2400 C source meter and Tektronix AWG5002B
pulse generator are manufactured in the Beaverton, OR, United States by Tektronix.
3. Results
3.1. Improved Device Performance
The films with different Pt compositions were performed by resistance
−
temperature
(R
−
T) tests, as shown in Figure 1b. The R
−
T curves show that doping Pt into Sb
2
Te
3
can
enhance the crystallization temperature of the material, and the crystallization temperature
increases with more Pt. The amorphous resistance of the material first increases and then
decreases with the content of Pt. This is due to the low crystallization temperature of
as
−
deposited Sb
2
Te
3
film and partly crystallization, which will be confirmed by subse-
quent XRD experiments. Dopant atoms can increase scattering probability, so the effect of
scattering is enhanced as the doping concentration increases and results in an increase in
resistivity. However, when the doping concentration is too high, the metallicity of the mate-
rial increases and the resistivity decreases. The crystallization temperature can be measured
via Raman or XRD measurements and is simply approximated by the curve of resistivity.
In this paper, we chose to use the R
−
T curve to calculate the crystallization temperature. In
Nanomaterials 2022,12, 1996 3 of 8
the R
−
T diagram, the crystallization temperatures of Pt
0.1
Sb
2
Te
3
, Pt
0.14
Sb
2
Te
3
(PST), and
Pt
0.22
Sb
2
Te
3
are 137
◦
C, 199
◦
C, and 236
◦
C, respectively, which indicates that the thermal
stability of the Sb
2
Te
3
alloy is improved after Pt doping. The resistance of the PST drops by
more than an order of magnitude, which is enough to distinguish the ON/OFF states used
in the PCM storage devices. Therefore, we believe that the performance of the PST film
is greatly improved. Figure 1c shows the resistance time (R
−
T) curve. The 10
−
year data
retention can be estimated by the Arrhenius equation:
t=τex p(Ea/KBT)(1)
Nanomaterials 2022, 12, x FOR PEER REVIEW 3 of 8
measured via Raman or XRD measurements and is simply approximated by the curve of
resistivity. In this paper, we chose to use the R−T curve to calculate the crystallization
temperature. In the R−T diagram, the crystallization temperatures of Pt
0.1
Sb
2
Te
3
,
Pt
0.14
Sb
2
Te
3
(PST), and Pt
0.22
Sb
2
Te
3
are 137 °C, 199 °C, and 236 °C, respectively, which indi-
cates that the thermal stability of the Sb
2
Te
3
alloy is improved after Pt doping. The re-
sistance of the PST drops by more than an order of magnitude, which is enough to distin-
guish the ON/OFF states used in the PCM storage devices. Therefore, we believe that the
performance of the PST film is greatly improved. Figure 1c shows the resistance time
(R−T) curve. The 10−year data retention can be estimated by the Arrhenius equation:
𝑡=𝜏𝑒𝑥𝑝
(𝐸
𝐾
⁄𝑇)
(1)
The 10−year data retention for GST and PST are expected to be 85 °C and 104 °C,
respectively, with corresponding activation energies (E
a
) of 2.57 eV and 1.86 eV. The acti-
vation energy errors are 0.05 eV and 0.40 eV, respectively. We find that 10−year data re-
tention of PST films is higher than that of most phase change memories, such as GST (~85
°C) and SST (~87 °C) [5].
Accordingly, based on standard 0.13 μm complementary metal−oxide semiconductor
(CMOS) technology, T−shape PCM devices based on PST were fabricated, as shown in
Figure 1a. Then, the electrical properties of the device are characterized. RESET, SET, and
READ functions can be realized by using different pulse waveforms. Figure 1d shows the
SET−RESET windows using the resistance−voltage (R−V) curves. The high/low resistance
ratio (R
RESET
/R
SET
) is about two orders of magnitude, which can meet the requirement of
the ON/OFF ratio used in PCM. When the voltage pulse width of 6 ns, the SET/RESET
voltage of the PST device requires 1.2V/3.8V. However, GST requires 4.6 V/5.5 V with a
10 ns operation speed [5]. A pre−program voltage applied by pre−operation to GST ena-
bles a SET speed of 500 ps in a restricted device structure [17]. This competitive recording
speed is already comparable to DRAM and SRAM (1−10 ns) [18]. As shown in Figure 1e,
the endurance period is revealed after we alternately apply two appropriate SET and RE-
SET voltage pulses. Figure 1e shows that the reversible phase transition characteristic is
up to 5 × 10
5
switching cycles with a resistance ratio of two orders of magnitude. The
switching cycles and resistance ratio of PST are better than Sb
2
Te
3
[19]. The endurance
performance is higher than GST [20] using the T−shaped device structure. All above, com-
pared with GST, faster operation speed and better endurance of PST have proved Pt dop-
ing Sb
2
Te
3
with suitable composition is a promising novel phase−change material.
Figure 1. Device performance. (a) The schematic diagram of the T−shaped phase change memory
(PCM) device. Schematic diagram of three pulse voltages of RESET, SET, and READ of PCM. (b)
Figure 1.
Device performance. (
a
) The schematic diagram of the T
−
shaped phase change memory
(PCM) device. Schematic diagram of three pulse voltages of RESET, SET, and READ of PCM. (
b
) The
temperature dependence of the resistance of Sb
2
Te
3
, Pt
0.1
Sb
2
Te
3
, Pt
0.14
Sb
2
Te
3
(PST), and Pt
0.22
Sb
2
Te
3
films at the same heating rate of 20
◦
C/min. (
c
) At the heating rate of 60
◦
C/min, the extrapolated
fitting line based on the Arrhenius formula shows the 10
−
year data retention temperature and
crystallization activation energy. (
d
) Resistance
−
voltage characteristics of PST based T
−
shaped
PCM device. The SET
−
RESET programming windows are obtained under different pulse widths.
(e) Endurance characteristic of PST based PCM T−shaped devices.
The 10
−
year data retention for GST and PST are expected to be 85
◦
C and 104
◦
C,
respectively, with corresponding activation energies (E
a
)of 2.57 eV and 1.86 eV. The activa-
tion energy errors are 0.05 eV and 0.40 eV, respectively. We find that 10
−
year data retention
of PST films is higher than that of most phase change memories, such as GST (~85
◦
C) and
SST (~87 ◦C) [5].
Accordingly, based on standard 0.13
µ
m complementary metal
−
oxide semiconductor
(CMOS) technology, T
−
shape PCM devices based on PST were fabricated, as shown in
Figure 1a. Then, the electrical properties of the device are characterized. RESET, SET, and
READ functions can be realized by using different pulse waveforms. Figure 1d shows the
SET
−
RESET windows using the resistance
−
voltage (R
−
V) curves. The high/low resistance
ratio (R
RESET
/R
SET
) is about two orders of magnitude, which can meet the requirement of
the ON/OFF ratio used in PCM. When the voltage pulse width of 6 ns, the SET/RESET
voltage of the PST device requires 1.2 V/3.8 V. However, GST requires
4.6 V/5.5 V
with
a
10 ns
operation speed [
5
]. A pre
−
program voltage applied by pre
−
operation to GST
enables a SET speed of 500 ps in a restricted device structure [
17
]. This competitive
recording speed is already comparable to DRAM and SRAM (1
−
10 ns) [
18
]. As shown
in Figure 1e, the endurance period is revealed after we alternately apply two appropriate
SET and RESET voltage pulses. Figure 1e shows that the reversible phase transition
Nanomaterials 2022,12, 1996 4 of 8
characteristic is up to
5×105
switching cycles with a resistance ratio of two orders of
magnitude. The switching cycles and resistance ratio of PST are better than Sb
2
Te
3
[
19
]. The
endurance performance is higher than GST [
20
] using the T
−
shaped device structure. All
above, compared with GST, faster operation speed and better endurance of PST have proved
Pt doping Sb2Te3with suitable composition is a promising novel phase−change material.
3.2. Characterization of Thin Film Structure
The XRD method was employed to characterize the lattice structure of PST film.
Figure 2a,b
shows the XRD results of PST and Sb
2
Te
3
films at different annealing temper-
atures. The diffraction peak of Sb
2
Te
3
appears in the deposited state, indicating that the
deposited Sb
2
Te
3
has crystallized. At this time, there is no diffraction peak of PST, so the PST
has not crystallized. At 200
◦
C, the FCC phase appeared in the PST, which indicated that
Pt inhibited the formation of the FCC phase and increased the crystallization temperature.
When the annealing temperature is 260 ◦C, both PST and Sb2Te3have only the diffraction
peaks of the hexagonal phase. Compared with pure Sb
2
Te
3
film, the diffraction peaks of
PST film become wider, the intensity of the peak becomes lower, and some diffraction
peaks disappear. In addition, a difference in the full width at half maximum (FWHM) of
the diffraction peak is observed on the XRD curves. According to the Scherrer formula:
β=Kλ/L(cosθ)(2)
Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 8
The temperature dependence of the resistance of Sb2Te3, Pt0.1Sb2Te3, Pt0.14Sb2Te3 (PST), and
Pt0.22Sb2Te3 films at the same heating rate of 20 °C/min. (c) At the heating rate of 60 °C/min, the
extrapolated fitting line based on the Arrhenius formula shows the 10−year data retention temper-
ature and crystallization activation energy. (d) Resistance−voltage characteristics of PST based
T−shaped PCM device. The SET−RESET programming windows are obtained under different pulse
widths. (e) Endurance characteristic of PST based PCM T−shaped devices.
3.2. Characterization of Thin Film Structure
The XRD method was employed to characterize the lattice structure of PST film. Fig-
ure 2a,b shows the XRD results of PST and Sb2Te3 films at different annealing tempera-
tures. The diffraction peak of Sb2Te3 appears in the deposited state, indicating that the
deposited Sb2Te3 has crystallized. At this time, there is no diffraction peak of PST, so the
PST has not crystallized. At 200 °C, the FCC phase appeared in the PST, which indicated
that Pt inhibited the formation of the FCC phase and increased the crystallization temper-
ature. When the annealing temperature is 260 °C, both PST and Sb2Te3 have only the dif-
fraction peaks of the hexagonal phase. Compared with pure Sb2Te3 film, the diffraction
peaks of PST film become wider, the intensity of the peak becomes lower, and some dif-
fraction peaks disappear. In addition, a difference in the full width at half maximum
(FWHM) of the diffraction peak is observed on the XRD curves. According to the Scherrer
formula:
𝛽=𝐾𝜆 𝐿(𝑐𝑜𝑠𝜃)
⁄ (2)
K in the equation is the Scherrer constant (K = 0.89), β is the grain size, L is the full
width at half maximum (FWHM) of the diffraction peak of the sample, θ is the diffraction
angle, and λ is the X−ray wavelength (0.154056 nm). The FWHM of PST was significantly
higher than that of Sb2Te3, indicating that the incorporation of Pt inhibited the crystalliza-
tion growth process, and grain refinement was obvious. Reducing grain size is ideal for
programming areas [21].
Figure 2. XRD results of the Sb2Te3 and PST. (a,b) XRD curves of PST and Sb2Te3 films were annealed
at 150 °C, 200 °C, and 260 °C for 5 min in an N2 atmosphere.
To study the crystalline phase and grain size more intuitively, high−resolution trans-
mission electron microscopy (HRTEM) and the associated selected area electron diffrac-
tion (SAED) patterns for Sb2Te3 film and PST films are presented in Figure 3. In total, 2
samples were annealed at the temperature of 260 °C for 5 min. The annealed films are both
in a polycrystalline state. Comparing Figure 3a,b, it can clearly be seen that the grain size
decreases significantly. In Figure 3c, doping Pt reduces the grain size of Sb2Te3 from 50
nm to about 5~10 nm, which confirms that the half−height width of PST is much larger
Figure 2.
XRD results of the Sb
2
Te
3
and PST. (
a
,
b
) XRD curves of PST and Sb
2
Te
3
films were annealed
at 150 ◦C, 200 ◦C, and 260 ◦C for 5 min in an N2atmosphere.
Kin the equation is the Scherrer constant (K= 0.89),
β
is the grain size, Lis the full
width at half maximum (FWHM) of the diffraction peak of the sample,
θ
is the diffraction
angle, and
λ
is the X
−
ray wavelength (0.154056 nm). The FWHM of PST was significantly
higher than that of Sb2Te3, indicating that the incorporation of Pt inhibited the crystalliza-
tion growth process, and grain refinement was obvious. Reducing grain size is ideal for
programming areas [21].
To study the crystalline phase and grain size more intuitively, high
−
resolution trans-
mission electron microscopy (HRTEM) and the associated selected area electron diffraction
(SAED) patterns for Sb
2
Te
3
film and PST films are presented in Figure 3. In total, 2 samples
were annealed at the temperature of 260
◦
C for 5 min. The annealed films are both in
a polycrystalline state. Comparing Figure 3a,b, it can clearly be seen that the grain size
decreases significantly. In Figure 3c, doping Pt reduces the grain size of Sb
2
Te
3
from 50 nm
to about 5~10 nm, which confirms that the half
−
height width of PST is much larger than
that of Sb
2
Te
3
. Meanwhile, according to Figure 3a,b, small crystal grains of the PST film
Nanomaterials 2022,12, 1996 5 of 8
can be also inferred from the continuous diffraction rings [
22
]. Smaller grain size increases
the surface volume ratio, thus generating more grain boundaries [
23
]. As the number of
grain boundaries increases, the crystal diffusion and slippage can be reduced. Hence, the
residual stress in the bulk of films can be degraded [
24
,
25
]. Moreover, the increased grain
boundaries provide a phonon and electron scattering center, and the decreased thermal
and electrical conductivity will improve the energy efficiency of the Joule heating [
26
].
According to the HRTEM image in Figure 3d,e, the crystal structure is in the hexagonal
phase after the calculation of inter
−
planar distance. They all belong to the (1010) and (105)
families, which indicates the crystalline state of the PST film is composed of the hexagonal
phase. The result of SAED in Figure 3b matches the HRTEM perfectly. In other words, Pt
doping affects the crystallization behavior of the Sb
2
Te
3
film without forming any new
phase or structure.
Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 8
than that of Sb
2
Te
3
. Meanwhile, according to Figure 3a,b, small crystal grains of the PST
film can be also inferred from the continuous diffraction rings [22]. Smaller grain size in-
creases the surface volume ratio, thus generating more grain boundaries [23]. As the num-
ber of grain boundaries increases, the crystal diffusion and slippage can be reduced.
Hence, the residual stress in the bulk of films can be degraded [24,25]. Moreover, the in-
creased grain boundaries provide a phonon and electron scattering center, and the de-
creased thermal and electrical conductivity will improve the energy efficiency of the Joule
heating [26]. According to the HRTEM image in Figure 3d, e, the crystal structure is in the
hexagonal phase after the calculation of inter−planar distance. They all belong to the (1010)
and (105) families, which indicates the crystalline state of the PST film is composed of the
hexagonal phase. The result of SAED in Figure 3b matches the HRTEM perfectly. In other
words, Pt doping affects the crystallization behavior of the Sb
2
Te
3
film without forming
any new phase or structure.
Figure 3. (a) TEM image of Sb
2
Te
3
film after annealed at 260 °C. (b) TEM image of PST film after
annealed at 260 °C. (c–e) HRTEM images of PST film after annealed at 260 °C.
Crystallization usually leads to an increase in film density and a reduction in film
thickness. The information on the density change upon crystallization is of paramount
importance in phase change media technology since it is related to the stresses induced in
the system during the write/erase cycle. The change of density before and after the phase
transition of the sample was measured by XRR. Figure 4 separately depicts the XRR curves
of PST films in amorphous and crystalline states. Based on the peak position shift, a linear
fit calculation is performed, as shown in Figure 4b. During the transition from the amor-
phous to the crystalline state, the thickness change rate of PST film is only 4.7%, while the
thickness change rate of Sb
2
Te
3
and GST films are 7.5% [27] and 6.5%, respectively. This
enhancement is responsible for the improved cyclability.
Figure 3.
(
a
) TEM image of Sb
2
Te
3
film after annealed at 260
◦
C. (
b
) TEM image of PST film after
annealed at 260 ◦C. (c–e) HRTEM images of PST film after annealed at 260 ◦C.
Crystallization usually leads to an increase in film density and a reduction in film
thickness. The information on the density change upon crystallization is of paramount
importance in phase change media technology since it is related to the stresses induced
in the system during the write/erase cycle. The change of density before and after the
phase transition of the sample was measured by XRR. Figure 4separately depicts the XRR
curves of PST films in amorphous and crystalline states. Based on the peak position shift, a
linear fit calculation is performed, as shown in Figure 4b. During the transition from the
amorphous to the crystalline state, the thickness change rate of PST film is only 4.7%, while
the thickness change rate of Sb
2
Te
3
and GST films are 7.5% [
27
] and 6.5%, respectively. This
enhancement is responsible for the improved cyclability.
Nanomaterials 2022,12, 1996 6 of 8
Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 8
Figure 4. The density−change rate before and after PST crystallization (a) XRR curves of amorphous
and crystalline PST films. (b) Bragg fitting curves of amorphous and crystalline films.
3.3. Evidence of Pt Occupying Positions
Experiments have proved that when element B is replaced by element C and bonded
with element A, if the electronegativity of element C is greater than that of element B, the
binding energy of element A increases [28]. In Figure 5a,b, the binding state of Sb2Te3 and
PST is revealed by XPS. When the Pt atom enters Sb2Te3, if the Pt atom replaces the Sb
atom and combines with the Te atom, since the electronegativity of Pt (2.2) is higher than
that of Sb (2.05) and Te (2.12), the binding energy of Te will shift towards the high binding
energy, which is consistent with the phenomenon in the experiment in Figure 5. Com-
bined with the XRD result that shows there is no new phase, this confirms that Pt replaces
the position of Sb.
Figure 5. XPS spectra of Sb2Te3 and PST films annealed at 260 °C (a) Sb 3d and (b) Te 3d.
4. Conclusions
In this work, we systematically studied the performance of PST. The PCM devices
based on PST can achieve higher speed and data retention than GST devices. According
to XPS and TEM analyses, the microstructure feature of Pt−modification Sb2Te3 film is ex-
plained clearly. The reduced grain size and formation of Pt−Te bonds are the main reasons
for the improved properties. Subsequently, a boost in device endurance gave the credit to
the reduced density change rate. The improvement of these properties is conducive to the
Figure 4.
The density
−
change rate before and after PST crystallization (
a
) XRR curves of amorphous
and crystalline PST films. (b) Bragg fitting curves of amorphous and crystalline films.
3.3. Evidence of Pt Occupying Positions
Experiments have proved that when element B is replaced by element C and bonded
with element A, if the electronegativity of element C is greater than that of element B, the
binding energy of element A increases [
28
]. In Figure 5a,b, the binding state of Sb
2
Te
3
and
PST is revealed by XPS. When the Pt atom enters Sb
2
Te
3
, if the Pt atom replaces the Sb
atom and combines with the Te atom, since the electronegativity of Pt (2.2) is higher than
that of Sb (2.05) and Te (2.12), the binding energy of Te will shift towards the high binding
energy, which is consistent with the phenomenon in the experiment in Figure 5. Combined
with the XRD result that shows there is no new phase, this confirms that Pt replaces the
position of Sb.
Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 8
Figure 4. The density−change rate before and after PST crystallization (a) XRR curves of amorphous
and crystalline PST films. (b) Bragg fitting curves of amorphous and crystalline films.
3.3. Evidence of Pt Occupying Positions
Experiments have proved that when element B is replaced by element C and bonded
with element A, if the electronegativity of element C is greater than that of element B, the
binding energy of element A increases [28]. In Figure 5a,b, the binding state of Sb2Te3 and
PST is revealed by XPS. When the Pt atom enters Sb2Te3, if the Pt atom replaces the Sb
atom and combines with the Te atom, since the electronegativity of Pt (2.2) is higher than
that of Sb (2.05) and Te (2.12), the binding energy of Te will shift towards the high binding
energy, which is consistent with the phenomenon in the experiment in Figure 5. Com-
bined with the XRD result that shows there is no new phase, this confirms that Pt replaces
the position of Sb.
Figure 5. XPS spectra of Sb2Te3 and PST films annealed at 260 °C (a) Sb 3d and (b) Te 3d.
4. Conclusions
In this work, we systematically studied the performance of PST. The PCM devices
based on PST can achieve higher speed and data retention than GST devices. According
to XPS and TEM analyses, the microstructure feature of Pt−modification Sb2Te3 film is ex-
plained clearly. The reduced grain size and formation of Pt−Te bonds are the main reasons
for the improved properties. Subsequently, a boost in device endurance gave the credit to
the reduced density change rate. The improvement of these properties is conducive to the
Figure 5. XPS spectra of Sb2Te3and PST films annealed at 260 ◦C (a) Sb 3d and (b) Te 3d.
4. Conclusions
In this work, we systematically studied the performance of PST. The PCM devices
based on PST can achieve higher speed and data retention than GST devices. According
to XPS and TEM analyses, the microstructure feature of Pt
−
modification Sb
2
Te
3
film is
explained clearly. The reduced grain size and formation of Pt
−
Te bonds are the main
reasons for the improved properties. Subsequently, a boost in device endurance gave
the credit to the reduced density change rate. The improvement of these properties is
Nanomaterials 2022,12, 1996 7 of 8
conducive to the commercial application of the material. Such experimental results show
that PST has broad application prospects in complex environments.
Author Contributions:
Conceptualization, S.S., Z.S., and Y.X.; methodology, Y.Q., Y.X. and J.Z.;
formal analysis, investigation, data curation, and writing—original draft preparation, Y.Q.; writing—
review and editing, Y.X., S.S., J.Z., H.S. and J.L.; visualization, Y.X.; supervision, Y.X., S.S. and J.L.;
project administration, S.S. and Z.S.; funding acquisition, S.S. and Z.S. All authors have read and
agreed to the published version of the manuscript.
Funding:
This research was funded by the National Key Research and Development Program
of China (2017YFA0206101), Strategic Priority Research Program of the Chinese Academy of Sci-
ences (XDB44010200), National Natural Science Foundation of China (91964204, 61874129, 61874178,
61775008, 61904046), Science and Technology Council of Shanghai (20501120300, 19JC1416800,
19YF1456100), Shanghai Sailing Program (19YF1456100), fund of the State Key Laboratory of Ad-
vanced Technologies for Comprehensive Utilization of Platinum Metals, Genetic Engineering of
Precious Metal Materials in Yunnan Province (I)–Construction and Application of Precious Metal
Materials Professional Database (I) (202002AB080001–1).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
The authors acknowledge the financial support from the State Key Laboratory
of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information, Chinese
Academy of Sciences, and the Microelectronic Research & Development Center, Shanghai University.
Conflicts of Interest: The authors declare no conflict of interest.
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