Introduction

Following the pioneering investigation of Ovshinsky,1 chalcogenide alloys, which can quickly change from an amorphous to a crystalline state at the nanosecond (ns) scale, have attracted the attention of scientists for many years. The striking electrical contrast between disordered and ordered phases has also been confirmed. Thanks to the excellent electrical conversion properties, chalcogenide compounds have been applied for phase-change random access memory (PCRAM). To perform the SET operation, a moderate electric pulse is employed to crystallize the amorphous phase-change memory (PCM) layer. In the RESET process, the crystalline PCM material transforms to the amorphous state under an intensive pulse.2 Data transfer is largely dependent on the phase transformation of the PCM alloy. Understanding the structural transition behavior is urgently needed.

Ge2Sb2Te5 is a prototype PCM material. Many studies have been performed on the crystallization behavior of Ge2Sb2Te5. Sutou et al.3 reported a decrease in the Avrami exponent with increasing crystallinity of Ge2Sb2Te5, indicating obvious crystal growth with decreasing nucleation rate. This means that crystallization is a multi-stage process where the properties of the PCM material change as a result. The non-Arrhenius grain growth behavior of Ge2Sb2Te5 was confirmed, implying the high-fragility crystallization characteristic.4 High-fragility crystallization promotes the stability of the amorphous structure at low temperature and rapid crystallization at high temperature, which are critical properties for PCM applications.5 Therefore, the crystallization behavior of the PCM layer largely determines storage applications. The structural origin of crystallization characteristics is worth exploring. In the umbrella-flip model, sliding Ge atoms induce rapid conversion of Ge-Te bonds, supporting fast crystallization of Ge2Sb2Te5,6 whereas the strong Ge-Te bond in the tetrahedral environment could increase structural rigidity and reduce the tendency for crystallization.7 Akola and Jones8 ascribed the crystallization of Ge2Sb2Te5 to the rearrangement of ABAB (A for Sb/Ge atom, B for Te atom) rings, which implies that the local structural motifs play an important role in the crystallization process. Analysis of crystallization characteristics allows to improve the properties and applications of PCM materials.

Research is continuously improving crystallization characteristics and exploring advanced PCM systems. Sb2Te, a novel Sb-based PCM material, exhibits rapid phase transition. The conversion time is 20 ns for the Sb2Te-based PCM device, close to that of the DRAM (< 10 ns).9 Nevertheless, the data retention temperature (55°C/10 years) limits its application in more challenging scenarios, such as the Internet of Things, neuron computing, and automotive electronic systems, where a minimum retention temperature of 120°C/10 years is required.10 Faster crystallization and lower amorphous thermal stability imply a higher crystallization trend, limiting the application of Sb2Te for more stable memory. To improve its performance, extensive studies have been performed on doped Sb2Te.11,12,13,14,15,16,17,18,19,20,21,22,23 Nonetheless, only a few studies have been conducted on the crystallization characteristics and related physical mechanisms of Sb2Te itself.24 A careful examination of the evolution of the microstructure and properties provides direct insight into the crystallization process. Analyzing the crystallization kinetics is useful for understanding the crystallization behavior. Attention should be paid to the amorphous local bonding of Sb2Te. It is worth exploring the correlation between local structural features and crystallization transition, which is important in advancing the application of Sb2Te for more stable storage.

Motivated by this topic, we focused on the crystallization behavior of Sb2Te thin films to explore the application potential for stable storage. First of all, the microstructure and photo-electronic properties were monitored to directly understand the crystallization transition. Then, the crystallization kinetics was studied, where the local crystallization activation energy and crystal growth velocity were calculated to quantitatively describe the crystallization behavior. Next, we linked the crystallization behavior to the amorphous local structure. The chemical bonding and atomic diffusion of the amorphous Sb2Te thin film were explored experimentally and theoretically. Finally, we made suggestions to adjust the amorphous local bonding of Sb2Te thin film for more stable optoelectronic memory applications.

Experimental Details

Preparation of the Thin Films

Sb2Te thin films with 200 nm thickness were deposited on SiO2/Si (100) wafers by DC magneton sputtering using a single stoichiometric Sb2Te target. The background pressure was below 10–6 kPa. The argon pressure was fixed at 6.6 × 10–4 kPa with a flow rate of 33 sccm. Substrate rotation with a speed of 20 rpm was used to ensure homogeneous deposition. The as-deposited thin film was annealed at 300°C for 30 min under a vacuum atmosphere (< 10–6 kPa) to obtain the crystalline state.

Optoelectronic Properties Measurements

The temperature-dependent resistance of the as-deposited thin film was monitored in situ by a four-point probe system with a heating rate of 5°C min−1. The optical reflectivity of the as-deposited and annealed thin films was measured by UV–Vis spectroscopy (Shimadzu UV-2550) at visible wavelengths. The microstructure of the annealed thin film was characterized by electron backscatter diffraction (EBSD) (Oxford Instruments).

Kinetics Analysis

Differential scanning calorimetry (DSC) was employed to record the crystallization process under various heating rates (10°C min−1, 20°C min−1, 30°C min−1, and 40°C min−1) in a high-purity argon protection environment. Sb2Te thin films of 1000 nm thickness were measured to improve the signal-to-noise ratio of DSC traces. The obtained DSC curves were used to investigate the crystallization kinetics of Sb2Te thin film.

Structure Characterization

X-ray photoelectron spectroscopy (XPS) was performed to survey the chemical bonding states of the as-deposited thin film. Raman measurement was used to identify the vibration modes in the as-deposited thin film. Theoretical investigations were carried out by means of density functional theory (DFT) using CASTEP.25 Molecular dynamics (MD) simulation was employed to generate an amorphous Sb2Te model. The ultrasoft pseudopotentials were used to describe electron–ion interactions. The electronic exchange–correlation energies were described by the generalized gradient approximation based on the Perdew–Burke–Ernzerhof functional.26 The energy cutoff was chosen to be 230 eV. The k points of 2 × 2 × 1 was set. The NVT ensemble was used. The initial configuration is a Sb2Te hexagonal supercell with 144 atoms. To ensure reliable computation, the atomic density was set to experimental value (30.5 atoms nm−3).16 Steps of 3 femtoseconds (fs) were employed. The model was melted at 2000 K for 2000 steps to erase the orderly arrangement of atoms. The molten system was equilibrated at 1200 K for 5000 steps. The model was quenched to 300 K and equilibrated for 10,000 steps to reach an amorphous state.

Results and Discussion

Crystallization Process of Sb2Te Thin Film

Electrical Resistance Change

The change in thin film resistance with increasing temperature is monitored to understand the crystallization transition in real time. As shown in Fig. 1, a sudden drop in thin film resistance can be observed around 130°C, which is the signature of crystallization. Before crystallization, the thin film resistance reduces slowly with increasing temperature, which may be due to the thermally induced transport.27 After the phase transition, the thin film resistance is almost unchanged with rising temperature, which implies a stable crystal phase. It is evident that the resistance is reduced at least 102 orders of magnitude after crystallization for Sb2Te thin film, enabling data reading reliability.18 To discuss the crystallization temperature (Tc) of Sb2Te thin film, the differential of the R–T curve has been computed (Fig. 1). The Tc is determined as the temperature corresponding to the minimum of the differential curve.20 The Tc of Sb2Te thin film, estimated to be ~ 132.9°C, is clearly lower than that of Ge2Sb2Te5 (~ 155°C),15 showing a strong crystallization tendency for amorphous Sb2Te thin film.

Fig. 1
figure 1

Resistance–temperature curve of Sb2Te thin film at a heating rate of 5°C min−1 and the calculated differential curve (top of panel).

Full size image

Optical Reflectivity Transition

The optical transition is an important signal of crystallization. As shown in Fig. 2, the reflectivity of the as-deposited and crystalline Sb2Te thin films are 45% and 63%. The optical reflectivity contrast (C) has been computed by using the following equation:28

$$C=2\times \left|\frac{{R}_{\mathrm{a}}-{R}_{\mathrm{c}}}{{R}_{\mathrm{a}}+{R}_{\mathrm{c}}}\right|\times 100\%$$
(1)

where Ra and Rc are the reflectance before and after crystallization for PCM films. The calculated reflectivity contrast of Sb2Te thin film is about 32%. A 10% threshold of optical contrast is necessary to maintain the minimum signal-to-noise ratio.29 GeSb alloy is a prominent optical switching material that exhibits reflectivity contrast over 15%.29 Multilevel reflectivity tunning of Sb2Te3 alloy has been proposed.30 Nevertheless, little is known about the optical application of Sb2Te thin film. This work demonstrates the potential of Sb2Te thin film as a promising candidate for optical memory material. In the crystallization process, the resistance of Sb2Te thin film drops obviously, while the reflectivity enhances significantly. Crystallization behavior determines the microstructure features that affect the optoelectronic properties, which need to be investigated.

Fig. 2
figure 2

Reflectivity versus wavelength curve for as-deposited and crystalline Sb2Te thin films.

Full size image

Grain Growth

We have used EBSD to study the microstructure features of crystalline Sb2Te thin film. Figure 3a displays the band contrast image of Sb2Te thin film. The average grain size has been obtained, as shown in Fig. 3b. The average grain size is 0.329 ± 0.101 μm for Sb2Te thin film annealed at 300°C, larger than that of Ge2Sb2Te5 (~ 0.08 μm, annealed at 280°C),31 suggesting the obvious grain growth tendency of Sb2Te thin film. It is worth noting that the standard deviation of the average grain size is slightly large, implying grain inhomogeneity. We have calculated the grain size distribution, as shown in Fig. 3b. Grains with a diameter of 0.10–0.20 μm are predominantly present in the Sb2Te thin film. Several grains with diameters ranging from 0.25 μm to 0.35 μm can be observed, displaying a broad range of grain sizes. Faster grain growth with limited nucleation rate causes differences in grain size of various nucleated areas.32 Thus, inhomogeneous coarse grain may result from the strong crystallization tendency of Sb2Te thin film, where the crystal growth controls the crystallization. The grain coarsening is accompanied by a decrease in the proportion of grain boundaries. The grain boundary shows stronger carrier and photon scattering.33,34 Thus, the low ratio of grain boundaries is related to the increase of reflectivity and decrease of resistance. An in-depth understanding of the crystallization tendency and structure origin provides a reference for tuning microstructure and optoelectronic properties, which are discussed below.

Fig. 3
figure 3

(a) EBSD band contrast images of crystalline Sb2Te thin film. The black lines indicate the high angle grain boundaries. The red lines denote the low-angle grain boundaries. (b) Grain size distribution and average grain size of crystalline Sb2Te thin film (Color figure online).

Full size image

Crystallization Kinetics of Sb2Te Thin Film

Average Crystallization Activation Energy

We can obtain information on the crystallization features of Sb2Te thin film by recording the DSC trace. Figure 4a shows the DSC curves for Sb2Te thin films at various heating rates. The exothermic peak of the DSC curve indicates the crystallization of Sb2Te thin film, which corresponds to the SET operation of the device, in which current pulses from sub-nanosecond to hundreds of nanoseconds are applied to crystallize the amorphous phase-change layer. Although heating speed of the DSC measurement employed in this work differs from the actual pulse rate, the kinetic parameters derived from the DSC traces at different heating rates can still provide insight into the crystallization behavior quantitatively. Tc is given by the exothermic peak position. The obtained Tcs at 10°C min−1, 20°C min−1, 30°C min−1, and 40°C min−1 are 127.6°C, 132.7°C, 136.3°C, and 138.7°C, respectively. Tc values derived from DSC and R–T curves are agreement with each other. The crystallization activation energy (Ea) that is a crucial index to depict the difficulty of crystallization can be evaluated from the DSC curve by employing the Kissinger method. The Kissinger equation is as follows:35

$$\mathrm{ln}\left(\alpha /{T}_{c}^{2}\right)=C+\left({E}_{\mathrm{a}}/{K}_{\mathrm{B}}{T}_{\mathrm{c}}\right)$$
(2)

where α, C, and KB are heating rate, constant, and Boltzmann constant, respectively. The plot of ln (α/Tc2) versus 1/KBTc is shown in Fig. 4b. The average Ea, 1.82 eV, can be estimated from the slope of the linearly fitted line. The Kissinger relation can be simplified, which is as follows:36

Fig. 4
figure 4

(a) DSC curves for Sb2Te thin films at different heating rates. (b) Plot of ln (α/Tc2) versus 1/KBTc for average Ea calculation. (c) Plot of ln (α) versus 1/KBTc for average Ea calculation.

Full size image
$$\mathrm{ln}\left(\alpha \right)=C+\left({E}_{\mathrm{a}}/{K}_{\mathrm{B}}{T}_{\mathrm{c}}\right)$$
(3)

Figure 4c shows the fitting for ln (α) versus 1/KBTc. The obtained average Ea is 1.88 eV, nearly equal to the value derived from Kissinger formula. The Tc and average Ea are sensitive to the experimental condition,37 which is insufficient to understand the multi-step crystallization described by Tc and average Ea.

Local Crystallization Activation Energy

The local Ea that is also known as effective Ea corresponds to the amorphous structural evolution, which may be a more suitable parameter to qualify the thermal stability. To obtain the local Ea, we have calculated the crystallization fraction by first integrating the DSC traces. The crystallization fraction as a function of temperature is shown in Fig. 5a, where the typical S-shaped transition curves38 can be observed. We can derive the local Ea by using Ozawa's method, which is as follows:39

$$\mathrm{log}\left(\alpha \right)=C-0.4567\left({E}_{\mathrm{a}}/RT\right)$$
(4)

where R is the gas constant and T is the heating temperature. The curve of log (α) versus 1000/T is shown in Fig. 5b. The local Ea can be estimated from the slope of the linearly fitted lines. Figure 5c displays the local Ea as a function of crystallization fraction in the range of 0.1–0.9. The local Ea decreases continuously with increasing crystallization fraction, arising from an easier ordering of local bonding. The gradual saturation of nucleation and growth is accompanied by the decrease of local Ea.3 1.72 eV, the maximum value in Fig. 5c, is considered to be local Ea corresponding to the amorphous Sb2Te thin film. The local Ea of Sb2Te thin film is lower than that of Ge2Sb2Te5 (~ 2.2 eV).40 A stronger crystallization tendency of Sb2Te thin film has been qualified by a lower local Ea and coarser grains. It may be accompanied with a higher crystal growth velocity, which is discussed in the following study.

Fig. 5
figure 5

(a) Crystallization fraction as a function of heating temperature at various heating rates. (b) Curve of log (α) versus 1000/T for calculation of local Ea. (c) Local Ea as a function of crystallization fraction in the range of 0.1 to 0.9.

Full size image

Crystal Growth Velocity

Now, we turn to the discussion of fast crystal growth, which is one of the most important crystallization features of Sb2Te. The crystal growth velocity (v) can be calculated by the following formula:

$$v=\frac{6D(T)}{\lambda }\left[1-\mathrm{exp}\left(-\frac{\Delta G(T)}{{K}_{\mathrm{B}}T}\right)\right]$$
(5)

where D(T) is atomic diffusion coefficient, λ is average diffusion jumping distance (~ 0.1 nm),41 and ΔG is Gibbs energy difference between amorphous and crystalline phases. In the crystallization process, atoms attach to the crystal nucleus at a certain rate. As temperature and composition fluctuate, atoms may become part of the stable nucleus. The gradual ordering of atomic arrangement results in the expansion of the crystal domain. Wilson42 suggested that the rate and probability of atoms occupying lattice sites correspond to the atomic mobility and crystallization driving force. Accordingly, the coupling effect of atomic diffusion and driving force determines the crystal growth velocity, as shown in Eq. 5. Various models43,44,45,46 have been developed to describe the growth velocity, where the driving force items are basically consistent with each other, while the kinetic coefficients in the diffusion term have been modified, which may yield different results. The Wilson method has been used to calculate the growth velocity of Ag4In3Sb67Te26,5 which is utilized to describe the grain growth of Sb2Te to provide a direct comparison.

The vital parameters (ΔG and D) are necessary to deduce the crystal growth velocity. The Gibbs energy difference can be calculated by Thompson-Spaepen47 and Turnbull48 approximations, as shown below:

$$\Delta G=\Delta {H}_{\mathrm{m}}\frac{{T}_{\mathrm{m}}-T}{{T}_{\mathrm{m}}}\left(\frac{2T}{{T}_{\mathrm{m}}+T}\right)$$
(6)
$$\Delta G=\Delta {H}_{\mathrm{m}}\frac{\Delta T}{T}$$
(7)

where ΔHm, Tm, and ΔT are melting enthalpy, melting temperature, and temperature difference, respectively. The melting enthalpy can be replaced by the crystallization enthalpy,49 which derives from the DSC curve. Nevertheless, the heating rate may affect the exothermic signal of Sb2Te thin film, causing a narrower exothermic peak, thus a smaller crystallization enthalpy. To calculate the Gibbs energy difference between amorphous and crystalline states, we have performed the DFT-MD to obtain the amorphous and crystalline models of Sb2Te. Figure 6 displays the schematic diagram of the relaxed crystalline and amorphous structures. Figure 7 shows the obtained Gibbs energy difference between the relaxed amorphous and crystalline configurations. The calculated Gibbs energy differences of Sb50 and Ge2Sb2Te551 are also displayed. A smaller Gibbs energy difference implies a lower crystallization drive force, meaning the increased amorphous stability.51 Although the Gibbs energy is tightly connected with the rapid crystallization, the atomic diffusion also plays a significant role on the crystallization and should be considered.

Fig. 6
figure 6

Schematic diagram of the relaxed crystalline (a) and amorphous (b) structures. Sb and Te atoms are marked with dark yellow and light violet, respectively (Color figure online).

Full size image
Fig. 7
figure 7

Gibbs energy difference between amorphous and crystalline Sb2Te, Sb ,50 and Ge2Sb2Te551 models. Data of Sb obtained by ab initio molecular dynamics simulation from Xu et al.50 Data of Ge2Sb2Te5 computed by ab initio calculation from Cho et al.51

Full size image

The mean square displacement (MSD) has been employed to describe the atomic mobility. A relatively lower temperature range (300–350 K) is chosen to observe the atomic diffusion behavior in stable amorphous state. Figure 8 shows the MSD of Sb, Te, and total atoms in amorphous Sb2Te as a function of simulated time at different temperatures. All MSD curves are close to a straight line, which indicates that the atomic diffusion in the amorphous Sb2Te tends to thermal equilibrium. Similar trace of amorphous PCM has been reported.52 The motion of atoms in the steady state can be qualified by diffusion coefficient (D), which is calculated by the following equation:

$$ D = \frac{1}{6}\frac{\partial }{\partial t}\mathop {\lim }\limits_{t \to \infty } MSD. $$
(8)
Fig. 8
figure 8

MSD of Sb, Te, and total atoms in amorphous Sb2Te as a function of simulated time at (a) 300 K, (b) 325 K, and (c) 350 K. (d) Diffusion coefficient of Sb, Te, and total atoms. The inset is an enlarged view of the diffusion coefficient in the range of 300 K to 325 K.

Full size image

Figure 8d shows the calculated diffusion coefficients of Sb, Te, and total atoms at 300 K, 325 K, and 350 K. The diffusion coefficient of total atoms in amorphous Sb2Te is 1.19 × 10–11 m2 s−1 at 300 K, close to the case in amorphous Ge1.3Bi0.5Sb1.5Te3 (~ 2 × 10–11 m2 s−1).53 It is important to stress that Sb atoms are more diffusive than Te atoms, especially at higher temperatures. Sb atoms with strong mobility have also been observed in Ge1.3Bi0.5Sb1.5Te353 and Ge2Sb2Te5,8 which means that more Sb atoms promote the rapid phase transition, while Te atoms have an opposite effect.

The Gibbs energy difference and diffusion coefficient are used to calculate the crystal growth rate of Sb2Te, as given in Table I. The grain growth velocity of Sb2Te at 350 K (4.96 m s−1) is higher than that of Ag4In3Sb67Te265 at 455 K (2.63 m s−1) but close to that at 503 K (4.12 m s−1), which confirms the rapid crystal growth of Sb2Te. Current calculations may not accurately describe the non-Arrhenius behavior of the viscosity.5 Nevertheless, knowledge of deduced crystal growth velocity could provide insights into the fast crystallization of PCM materials. Our results demonstrate the rapid crystal growth of Sb2Te. Meanwhile, the visible inhibition effect of Ag and In on crystallization is revealed. Additionally, the grain growth is faster in Sb2Te and Ag4In3Sb67Te26 than that in Ge2Sb2Te5 (3 m s−1 at 650 K),4 indicating the rapid crystallization of Sb-based PCM materials. Inhomogeneous and coarse grain, low crystallization activation energy, and high crystal growth velocity imply the strong crystallization tendency of Sb2Te, whose structural origins should be explored. Furthermore, it is vital to survey the structural factors affecting the photoelectric conversion.

Table I Crystal growth velocity of Sb2Te, Ge2Sb2Te5,4 and Ag4In3Sb67Te26,5 data for Ge2Sb2Te5 derived by ultrafast-heating calorimetry from Orava et al.,4 data for Ag4In3Sb67Te26 computed by ab initio molecular dynamics simulation from Zhang et al.5
Full size table

Local Structural Characteristics of Amorphous Sb2Te Thin Film

Local Bonding Features of Amorphous Sb2Te Thin Film

We have employed XPS to explore the bonding features of as-deposited Sb2Te thin film. Figure 9a shows the XPS core level spectra of Sb 3d. The presence of Sb-Sb bonds in amorphous Sb2Te thin film can be detected, whose binding energy peaks are located at 528.44 eV (3d5/2) and 537.88 eV (3d3/2). The binding energy of Sb 3d5/2 is close to that of O 1s, which may not facilitate in-depth discussion of the bonding environment of Sb atoms. As reported in previous works,16,22,54 the bonding of Sb to Te atoms has been clearly observed from the core level spectra of Sb 4d and Te 4d. We have analyzed the detailed spectra of Sb 4d and Te 4d, and recognized the significant Sb-Te binding energy peaks, as displayed in Fig. 9b, d. The dominance of Sb-Sb and Sb-Te bonds may constitute the matrix of amorphous Sb2Te thin film. Te-Te homopolar bonds that should not be overlooked have been reported in amorphous PCM materials. As seen in Fig. 9c, the binding energy peaks of the Te 3d core level spectra appear at 572.55 eV (Te 3d5/2) and 582.94 eV (Te 3d3/2), confirming the formation of Te-Te bonds. As inferred from XPS measurements, the amorphous Sb2Te thin film consists of Sb-Sb, Sb-Te, and Te-Te bonds.

Fig. 9
figure 9

XPS core level spectra of Sb 3d (a), Sb 4d (b), Te 3d (c), and Te 4d (d) for as-deposited Sb2Te thin film.

Full size image

We have examined the phonon vibration modes in the as-deposited Sb2Te thin film by Raman spectroscopy. A broad flat Raman band ranging from 60 cm−1 to 220 cm−1 has been obtained, as shown in Fig. 10. This implies the disordered local environment of amorphous Sb2Te thin film. A significant Raman characteristic peak occurs at ~ 150 cm−1, corresponding to the contribution of Sb-Sb bonds (A1g).55 The vibrational modes at ~ 117 cm−1 (Eg) and ~ 169 cm−1 (A1g) have also been observed, which are attributed to Sb-Te bonds.56 The vibrating of Te-Te bonds (Eg) can be supported by the Raman peak at 92 cm−1.57 The information on the existence of Sb-O bonds has been displayed, and the slightly weaker vibrational mode is at ~ 197 cm−1.58 Raman studies show that Sb-Sb, Sb-Te, and Te-Te bonds are formed in amorphous Sb2Te thin film, consistent with the results of XPS.

Fig. 10
figure 10

Raman spectrum of as-deposited Sb2Te thin film.

Full size image

Local bonding in amorphous Sb2Te is analyzed theoretically by using structure factor (S (k)). Last 3000 configurations in simulated quenching process have been used. We have calculated the pair correlation function (g (r)), which is a spherically averaged distribution of interatomic vectors. The S (k) can be obtained by Fourier transforming the (g (r)). The calculation details can be viewed in Caravati et al.7 and Akola and Jones.8 Figure 11 shows the obtained S (k)s. The main peak of total S (k) locates at 0.20 nm−1 with the intensity of 1.60 (Fig. 11a). Oscillations up to ~ 0.65 nm−1 can be observed for total S (k). As displayed in Fig. 11b, c, the decomposed partial S (k)s of Sb-Sb and Sb-Te pairs exhibit the strong peaks at ~ 0.20 nm−1. The principal peak intensities of the decomposed partial S (k)s are 1.19 and 1.14 for Sb-Sb and Sb-Te pairs, respectively. The decomposed partial S (k) of Sb-Sb pair also contributes to the medium range order, including the second peak at 0.31 nm−1 and the third one at 0.43 nm−1. It indicates the predominant contribution of Sb-Sb bonds. Simultaneously, numerous Sb-Te bonds are present. The weak peak of the decomposed partial S (k) is around 0.20 nm−1 for Te-Te pair (Fig. 10d), implying the existence of few Te-Te bonds. It is accepted that Sb atoms prefer to bond with Sb and Te atoms. The remaining Te atoms form chemical bonds with Te atoms. Therefore, amorphous Sb2Te has large amount of homopolar bonds (considerable Sb-Sb bonds and several Te-Te bonds), as well as many heteropolar bonds (Sb-Te bonds). Although simulated quenching speed is different from the actual deposited rate, the formation of Sb-Sb, Sb-Te, and Te-Te bonds in amorphous Sb2Te can be supported by theoretical and experimental results.

Fig. 11
figure 11

Total (a) and decomposed partial structure factors of Sb-Sb pair (b), Sb-Te pair (c), and Te-Te pair (d) for amorphous Sb2Te.

Full size image

Structural Origins of Higher Crystallization Tendency

The interaction and diffusion of atoms are the decisive factors in the crystallization process. Interactions between atoms can be characterized by the strength of chemical bonds. Nevertheless, the chemical bond length in the amorphous structure is not fixed. It is difficult to quantitatively calculate the strength of chemical bonds in the amorphous phase. Table II lists the measured bond energy of Sb-Sb, Sb-Te, and Te-Te bonds59 in the crystalline state to provide an explanation. The information on Ge-Ge and Ge-Te bonds59 is also given, which are the essential chemical bonds in amorphous Ge2Sb2Te5. The bond energy of Sb-Sb and Sb-Te bonds is much smaller than that of Ge-Te bonds. This may be an important structural origin that amorphous Sb2Te has lower thermal stability than Ge2Sb2Te5. The atomic mobility can be further discussed by calculating the pre-exponential diffusion factor (D0) and diffusion activation energy (Ediffusion). The calculation equation is as follows:

$$ D\left( T \right) = D_{0} {\text{exp}}\left( { - \frac{{E_{{{\text{diffusion}}}} }}{{k_{B} T}}} \right). $$
(9)

D(300 K), D(325 K), and D(350 K) are used. The obtained D0 and Ediffusion of total, Sb, and Te atoms are given in Table III. The calculated D0 of Sb atoms (1.87 × 10–7 m2 s−1) is larger than that of Te atoms (1.53 × 10–7 m2 s−1), which means that Sb atoms move faster than Te atoms, in agreement with the above result. Strong diffusion ability of Sb atoms may be another essential reason for the rapid crystallization of Sb2Te.

Table II Measured bond energy of Sb-Sb, Sb-Te, Te-Te, Ge-Ge, and Ge-Te bonds in crystalline state from Luo59
Full size table
Table III Calculated pre-exponential factor of diffusion (D0) and activation energy of diffusion (Ediffusion) of total, Sb, and Te atoms for Sb2Te
Full size table

Since the atomic geometry determines the crystallization mechanism, attention should be paid to the local structural primitives. It has been suggested that C atomic chains with C-C double bonds are formed in amorphous C-Ge2Sb2Te5.60 On the contrary, disordered C clusters were found in crystalline C-Ge2Sb2Te5.60 The two distinct configurations of C atoms increase the difference of local geometry before and after crystallization. The breaking of bonds and the diffusion of atoms become necessary processes for crystallization, which requires more activation energy and time. Consequently, crystallization is inhibited. In contrast, stable octahedral units induced by Sc atoms have been found in amorphous and crystalline Sb2Te3,61 making the local bonding of two phases more similar. In the crystallization process, there are fewer broken chemical bonds and weaker atomic diffusion. Thus, the increase of local geometric similarity promotes rapid crystallization. As regards amorphous Sb2Te, the Sb-centered octahedral bonding has been confirmed,24 as occurs in the crystalline state. Therefore, we believe that octahedral unit is the main structural basis of high crystallization tendency, which makes the local geometry of the amorphous and crystalline phases more similar. Weaker chemical bonding and stronger atomic mobility further promote high-speed and low-activation-energy crystallization. Namely, the structural origins of strong crystallization tendency of Sb2Te are local octahedral bonding, weak chemical bonds, and fast atomic movement.

Discussion on the Structural Factors of Optoelectronic Contrast

The role of amorphous local bonding on optoelectronic properties is also worth discussing. As stated in Caravati et al.7 and Wełnic et al.,62 the ratio of octahedron to tetrahedron changes enough to produce strong photoelectric transition, which means that differences in local geometry provide an optoelectronic contrast. The computational results showed that the variation of reflectivity results from the reduction of optical matrix elements in the amorphous phase, which is linked to the misalignment of the octahedron.63 Sb2Te exhibits local octahedral configuration. The misaligned octahedra decrease the medium range order of the amorphous phase, which is considered to be an important cause of optoelectronic difference. Numerous Sb-Sb bonds have been observed in amorphous Sb2Te, which promotes the formation of Sb-centered nanoclusters.64 The nanoclusters centered on Sb atoms change the motion path of carriers and photons and affect the photoelectric contrast. Furthermore, as previously mentioned, strong crystallization tendency coarsens the grains, which also plays a critical role in the optoelectronic properties.

Suggestion for More Stable Optoelectronic Memory Applications

In the present work, we have demonstrated the structural origins of higher crystallization tendency for Sb2Te thin film. Local octahedral geometry with weak bonds and rapidly diffusing atoms is considered to be the major contributor. Moreover, we also discussed the structural factors of optoelectronic contrast for Sb2Te thin film. Octahedral misalignment is believed to be the main cause. In order to achieve stable optoelectronic storage applications, the first step is to reduce the proportion of octahedral bonding in amorphous Sb2Te to increase the structural contrast, which will improve the amorphous stability and support optoelectronic difference. It is also important to introduce the strong chemical bonds so that the structural rigidity can be enhanced and atomic diffusion can be dragged. The ameliorated crystallization tendency can also alter the microstructure characteristics and improve the optoelectronic properties.

Conclusions

We have investigated the crystallization behavior of Sb2Te thin film to explore the potential as a sable photo-electronic memory material. The in situ resistance measurement results show that the resistance is reduced larger than 102 orders of magnitude in the crystallization process. The UV–Vis spectrum shows the optical reflectivity is increased by more than 30% before and after crystallization. The striking contrast meets the demands of optoelectronic storage applications. The deduced average crystallization activation energy is ~ 1.82 eV. The calculated local crystallization activation energy that corresponds to the amorphous state is 1.72 eV. The derived crystal growth velocity is 4.96 m s−1 at 350 K. The crystallization activation energy and crystal growth speed support the strong crystallization tendency of Sb2Te thin film. The structural origins of photoelectric contrast and crystallization tendency have been discussed. The misaligned octahedrons promote the optoelectronic contrast. Moreover, the weak Sb-Sb, Sb-Te, and Te-Te bonds and fast-diffusing Sb atoms in octahedral geometry facilitate the low-activation-energy and high-speed crystallization. Therefore, it is believed that reducing the ratio of octahedral bonding and strengthening the chemical bonds could improve the crystallization properties of Sb2Te thin film for more stable optoelectronic storage applications.