Elucidating temperature-dependent local structure change and optical properties in GeTe phase-change material

The increasing demand to integrate artificial intelligence (AI), pattern recognition, and other cutting-edge data-intensive technologies into the processing and storage of data has been sparked by research and development efforts in non-volatile memory and neuro-inspired computing devices.¹ The performance of data processing and energy efficiency have already been hampered by the traditional von Neumann architecture, which has separate storage and processing units, garnering the attention of neuromorphic, electronic, and nanophotonics devices that can emulate the human brain to shift the paradigm. The new neuromorphic architecture receives significant attention due to its low power consumption, integrated storage and computing, ability to perform complex cognitive tasks, and excellent parallelism.² Phase-change memory (PCM) exhibits rapid erasing (50 ns) and writing speeds (10 ns), long cycle life (>10¹²), high accuracy, and exceptional durability, making PCM an ideal synaptic device.^2,3 The phase-change material serves as the active region of this emerging non-volatile storage technology with rapid and reversible transitions between a highly conductive crystalline state (SET) and a highly resistive amorphous state (RESET) through Joule heating, which are assigned to binary 1 and 0, respectively, showcasing a pronounced resistivity contrast between phases.^4,5 This contrast arises from variations in structural order, bond length, and carrier concentration between the two phases.^6–9 Ge₂Sb₂Te₅ (GST) is the most extensively researched alloy for phase-change materials for next-generation high-speed and non-volatile memory applications^3,5 due to the fast and stable reversible transition between the two states. However, GST exhibits relatively poor thermal stability, as indicated by its low crystallization temperature (T_c) of 160 °C.^10,11 This impacts the cell-to-cell variability and the drift of the electrical resistance with time in the amorphous phase.^7,9 Thus, to mitigate these issues, a search for new materials with enhanced electrical and thermal properties is crucial for achieving superior programming capabilities and high-performance devices. Germanium telluride (GeTe) is a binary chalcogenide material that serves as a promising alternative to germanium antimony telluride (GeSbTe) for PCM due to its high-temperature applications, higher crystallization temperature (T_c), larger resistance, and faster crystallization rate.^12–14 GeTe serves as an outstanding active memory layer in PCM devices, offering long endurance, better data retention, and a rapid switching speed of less than 1 ns.^11,13 Also, the significant difference in electrical conductivity between the disordered amorphous phase and ordered crystalline phase of GeTe makes it ideal for multi-level data storage and neuromorphic computing applications.^15–18 These attributes establish GeTe as a prominent contender among various phase-change materials for emerging non-volatile memory technology. However, a proper understanding of temperature-dependent local structures to optical property change is essential for the further development of GeTe material-based PCM devices.

GeTe thin films were examined to understand how their electrical resistance varies with temperature using temperature-dependent electrical resistance measurements. The significant property changes during the phase change from amorphous to crystalline are attributed to differences in the bonding systems between these two phases.¹⁹ In GeTe, the transition from amorphous to the crystalline phase involves the conversion of covalent bonds into metavalent bonds.^8,19,20 Covalent bonds typically share two electrons, while metavalent bonds allow neighboring atoms to exchange one electron.¹⁹ The x-ray photoelectron spectroscopy (XPS) technique offers insights into the changes in the chemical states of materials during phase transition. Additionally, investigations into optical properties were conducted to comprehend the temperature-dependent characteristics of GeTe thin films using a UV-Vis-NIR spectrophotometer. In this paper, we provide enhanced insight into the process of Urbach tail width^17,18,21,22 and disorder effects^23–25 by estimating the Tauc parameter and the Urbach energy. Thus, the degree of disorder is a key paradigm in the design of multiple resistance states. In light of this, temperature-dependent investigations that link local structures and optical characteristics in GeTe are essential from a technological standpoint.

This study investigates the microstructural evolution of thin amorphous GeTe films at elevated temperatures from room temperature to 250 °C. We analyze this evolution using temperature-dependent resistivity, x-ray diffraction, and XPS measurements. Furthermore, the investigation explores the impact of temperature on the optical bandgap, structural disorder, Urbach energy, and local structural changes in the material during the phase change from amorphous to crystalline, employing UV–Vis–NIR spectroscopy techniques. These studies aim to systematically explore the temperature-dependent local structural and optical property changes throughout the crystallization process in GeTe thin films.

Thin films of GeTe with a thickness of 100 nm were sputter-deposited onto SiO₂ substrates through radiofrequency (RF) magnetron sputtering. To begin, the glass substrates were sequentially cleaned with de-ionized water–acetone–propanol in an ultrasonic bath each for 15 min and then dried by N₂ (99.99%) gas flow. A single stoichiometric target of 99.99% pure Ge₅₀Te₅₀ was used in the deposition procedure. Pre-sputtering of the target took place for about 25 min in an argon (99.999%) atmosphere before deposition, aiming to minimize surface contaminations. The chamber maintained a base pressure of 1.1 × 10⁻⁶ mbar, generated by a turbo molecular pump backed by a rotary pump. In the growth process, operating pressure inside the chamber was kept at 3.0 × 10⁻³ mbar by regulating the flow of Ar (99.999%) with mass flow controllers. Subsequently, the films were subjected to annealing at different temperatures employing a tubular furnace with a heating rate of 3 °C°min⁻¹ in an argon environment. The continuous argon gas flow within the tubular furnace acts as a protective barrier and ensures that the sample remains unoxidized during annealing.

The structural characterization of the GeTe thin films was examined by employing x-ray diffraction (XRD) conducted by a PANalytical X'Pert3 Powder XRD system (Cu-Kα, λ = 1.54 Å) in the 2θ range of 10–80 °C. The incidence angle was set at 0.5°, and data were collected at room temperature subsequent to the annealing of studied samples for a fixed duration. The temperature-dependent resistivity measurements were conducted on as-deposited amorphous GeTe films from room temperature to 250 °C with an annealing rate of 5 °C°min⁻¹ in the Ar environment. The sheet resistance measurements were performed using a setup of in situ four-probe resistivity measurements employing the van-der-Pauw technique.^26,27 XPS measurements using Al Ka radiation were utilized to verify each element’s atomic percentage and bonding states in GeTe. Also, the thin film's absorption spectra were recorded employing a JASCO V-670 UV–VIS–NIR spectrophotometer across the spectral range of 800–2500 nm. To determine the absorption spectra of the thin film, a sample of SiO₂, matching the substrate thickness, served as the reference material. The optical absorption spectra facilitated the determination of the energy bandgap and several other optical parameters.

Figure 1(a) depicts the typical resistivity–temperature-dependent measurement of a 100 nm as-deposited amorphous thin GeTe film heated at a rate of 5 °C°min⁻¹ in an argon ambiance. Large resistivity of around ⁓10⁸ Ω°cm is seen in the as-deposited amorphous phase. As the temperature increases, the resistance gradually decreases, demonstrating a semiconducting behavior up to 184 °C. Beyond this point, the sheet resistance experiences a sharp decline until 190 °C, corresponding to the formation of an amorphous to crystalline phase, and the resistivity of around 10¹ Ω°cm is measured during the crystalline phase. The temperature-dependent resistivity measurement reveals a single transition from an amorphous to crystalline phase. The changes in resistivity are mainly due to the phase change, coupled with alterations in the electronic structure.^28–31 The heating rate is essential while examining the crystallization kinetics of phase-change thin films where an increase in T_c with an increasing heating rate is clearly evident upon phase transition.^28,29 Upon crystallization, a sudden and significant reduction in resistivity at a transition temperature of 187 °C confirming a single T_c, aligns well with prior literature values^10,32–34 employing differential scanning calorimetric (DSC) and reflectivity measurements.²⁶ An abrupt drop of resistance is observed at the annealing process, marking a sharp decline in semiconductor behavior at their respective T_c ascertained by the minimum derivative of the in situ resistivity–temperature curve. In Fig. 1(b), the first derivative of resistivity exhibits a local minimum at approximately 187 °C, confirming the single crystallization of GeTe materials.³⁵ Notably, the first derivative of the resistivity remains consistent up to 182 °C. Furthermore, the resistivity difference in the crystalline and amorphous phases of GeTe films spans more than six orders of magnitude. This significant difference validates a substantial range of achieving multiple conductance states.²⁷ To gain a thorough understanding of the structural changes, XRD was employed to conform to the amorphous/crystalline characteristics of as-deposited and annealed films, which will be discussed in further sections.

Figure 2 depicts the XRD findings of both as-deposited and annealed GeTe thin films at temperatures of 100, 150, 200, and 250 °C under the steady flow of Ar, examining the impact of post-annealing on crystallization. The absence of sharp peaks in the RT, 100 °C, and 150 °C thin films of the XRD confirms their amorphous nature. At elevated temperatures of around 200 °C, sharp peaks emerge, eliminating the humps associated with the amorphous state and suggests the partial crystallization of the film.³⁶ This phenomenon aligns with the sharp decline in sheet resistance observed at 187 °C, corresponding to the phase transition from an amorphous to crystalline state, as illustrated in Fig. 1. The process of crystallization is found to start at 200 °C and evolve crystalline peaks with a trigonal GeTe structure (ICDD PDF 00-047-1079, space group: R3 m),²¹ acknowledges the complete coalition of the crystalline environment, exhibited by the annealed film at 250 °C. To shed more light on these structural rearrangements during annealing, the XPS for both amorphous and crystalline GeTe are analyzed.

The chemical states, elemental composition, and particle size at the surface of as-deposited and annealed GeTe thin films were investigated by XPS with an energy of 1486.6 eV. The XPS spectra obtained from the as-deposited and annealed GeTe thin film in the Ge 3d and Te 3d regions are depicted in Fig. 3. The XPS data represent the occurrence of germanium (Ge), tellurium (Te), and oxygen (O) on the film’s surface. The Ge 3d of the as-deposited film comprises two component peaks at 30.58 and 32.86 eV, corresponding to Ge–Te (Ge²⁺) and Ge–O (Ge⁴⁺), respectively, as illustrated in Fig. 3(a). The appearance of the Ge–O bond is attributed to air exposure.¹⁶ The peaks of Te 3d_5/2 and Te 3d_3/2 at a binding energy of 573.03 and 583.42 eV, respectively, are shown in Fig. 3(b). These values align with the reported Ge–Te values.²⁵  Figures 3(c) and 3(d) depict the XPS spectra of annealed crystalline GeTe films. Figure 3(c) demonstrates the post-annealing influence of the surface oxygen on the Ge 3d peak.^18,25,35 The XPS spectra peaks at 29.98 and 33.38 eV correspond to Ge 3d and Ge–O, respectively, for the crystalline phase. Most surface-bound Ge forms bonds with O, a finding consistent with the literature on the oxidation behavior of GeTe.

Whether in its amorphous/crystalline phase, the GeTe thin film initially develops Ge–O upon exposure to air, whereas the formation of Te oxide takes place after a particular duration. The peaks at 573.08 and 583.49 eV represent the Te 3d_5/2 and Te 3d_3/2, respectively, and agree with the previous literature that ensures the formation of crystalline GeTe as depicted in Fig. 3(d).^22,25,37 The Te 3d (Te⁴⁺) peaks at 573.33 eV (3d_5/2) and 585.01 eV (3d_3/2) are related to Te–O. The binding energy of an element increases when it forms a bond with a more electronegative element.^25,38 As a result, the peak shift of Te 3d_5/2 and Te 3d_3/2 toward higher binding energy, by 0.5 and 0.7 eV, respectively, is thus observed during the phase change from amorphous to crystalline. The XPS analysis delves into the chemical binding states and bond reorganization influenced by the annealing temperature.^39,40 At the same time, the UV–Vis–NIR tool explores the material's electronic transitions, offering the development of bandgap through optical absorption spectra discussed in further sections.

Figures 5(a) and 5(b) illustrate the temperature-dependency of $E g$ and $B$ for the temperature range of room temperature to 250 °C. Upon heating, $E g$ declines from 0.92 eV (room temperature) to 0.85 eV (150 °C) in its as-deposited amorphous phase. Further increasing the temperature to 200 °C results in an $E g$ of 0.74 eV. This steady decline in $E g$ is attributed to the thermal excitation of carriers and an increase in medium-range order at lower and elevated temperatures, respectively. A red shift of $E g$ has been witnessed from 0.74 to 0.70 eV at 250 °C, revealing an increase in crystallization. Numerous studies reveal that the heat treatment causes nearly ideal amorphous substances to crystallize, whereas dangling bonds accumulate over the surface of the crystallites.^23,36 Dangling bonds play a crucial role in creating defects within highly crystalline solids. As the annealing temperature exceeds 200 °C, dangling bonds and defects increase, leading to a gradual rise in localized states within the material's band structure. Consequently, heat treatment widens the energy range of localized states, reducing the optical energy gap.

In Fig. 5(b), slope $B$ exhibits a similar trend to $E g$ concerning temperature. However, there is a crucial difference: $E g$ decreases while $B$ increases upon crystallization. The Tauc parameter $B$ is associated with the electronic states and measures the structural disorder in the atomic configuration of the material. A higher $B$ indicates a lower disorder.^43–45 Decrement in the $B$ slope from room temperature to 150 °C is due to reduced short-range order caused by the induced thermal atomic vibrations. Subsequently, there is a substantial increase in the $B$ (1077–1482 eV²  cm⁻²) slope of about 27% from 150 to 200 °C seen around the GeTe crystallization temperature, rendering the onset of structural ordering in the material due to the evolution of the rhombohedral phase transition., $B$⁠, and $E u$ of GeTe are estimated for different temperatures and are shown in Table I.

The first term $( E u ) T$ represents electron–phonon interactions, varies with thermal vibrations, and increases with elevated temperatures. The other two terms $( ( E u ) X , stat a n d ( E u ) X , dyn)$ are reflected by the mean-square shift of atoms caused by the structural deviation from an ideally ordered lattice. The temperature-independent $( E u ) X , stat$ reflects a lack of long-range order, whereas amorphous phase-change materials exhibit only short-range order in the atomic arrangement causing this term to likely increase with temperature.^9,23 However, the overall $E u$ decreases as temperature rises. This is due to significant changes in the parameter $( E u ) X , dyn$⁠, which is associated with medium-range order. As temperature rises and medium-range order improves, $( E u ) X , dyn$ decreases. Among the terms $( E u ) T$ and $( E u ) X , dyn$⁠, the latter is anticipated to be more dominant. As a consequence, the term $( E u ) T$ is primarily influenced by lattice thermal vibrations, tending to increase with rising temperature. Thus, there is a decrement of Urbach energy experienced upon elevated temperatures, indicating an enhancement in medium-range order,²³ as depicted in Fig. 6(a). The overall $E u$ decreases upon increasing temperature. The obtained value of $E u$ at the as-deposited amorphous state is 224.42 meV at room temperature while 139.49 meV at 250 °C. One possible explanation for the higher probability of localized transitions (direct transitions) compared to other optical transitions is the presence of a large density of trap states near the band edge, as confirmed by the elevated values of $E u$⁠. Large resistivity at the amorphous phase is reflected as the Fermi level pinning in the middle of the gap caused by the high concentration of traps near the band edge.⁴⁷ As a result, the estimation of $E u$ provides strong evidence for the evolution of crystalline GeTe, which is demonstrated at phase transition temperatures by an abrupt drop of $E g$ and an increase in $B$⁠. Table II illustrates a comparative performance outlined in pertinent studies of GeTe and its stoichiometries, considering the parameters such as resistivity, thickness, and optical properties, according to the previous literature.

We have investigated the evolution of local structure changes during the crystallization process of GeTe thin films with annealing temperature. The resistivity of the GeTe thin film exhibits exponential temperature dependency typical of semiconducting materials around 184 °C. It drops several orders in magnitude beyond this threshold temperature, demonstrating multibit conductance of phase-change memory for synaptic activity. The sudden and significant decrease in sheet resistance at the transition temperature of 187 °C aligns well with previous findings, confirming a single T_c. The physical and optical properties of the GeTe film's pre- and post-annealing were studied. XPS analyses demonstrated that the GeTe thin film undergoes the structural changes and bond rearrangement for both amorphous and crystalline phases with annealing temperature. The binding energy of Ge–Te and Ge–O are found at 30.58 and 32.86 eV, respectively, whereas a shift of 0.5 eV (Te 3d_5/2) and 0.36 eV (Te 3d_3/2) is seen in Te3d spectra to conform the crystallinity of the material through XPS measurement. Furthermore, UV–Vis–NIR spectroscopy examined the optical properties of the as-deposited and annealed GeTe thin films. Amorphous to crystalline transformations induced by heat treatments result in significant changes in the optical property of the film, with $E g$ values of 0.92 and 0.70 eV for the amorphous and crystalline phases, respectively. An enormous optical bandgap value in thin GeTe films is desirable for multiple states of conductance for various synaptic and data storage applications. An increment of 27% in the $B$ slope during the phase transition of the GeTe material reveals its more ordered nature. Also, a reduction in disorder is observed at elevated temperatures as $E u$ decreases from 199.87 meV (amorphous) to 141.27 meV (crystalline) owing to the more ordered atomic arrangement in the film, thereby reducing the optical bandgap alignment with the resistivity measurements and XRD analysis at different annealing temperatures. The excellent thermal and structural capabilities, along with the evolution of local structural changes of thin GeTe films, are highlighted by these temperature-dependent experimental results. Hence, GeTe emerges as a promising alternative phase-change material for next-generation computing and data storage applications.

The authors acknowledge the Department of Physics, NIT Silchar, for the XRD and UV-Vis-NIR spectroscopy facilities and the Advanced Material Research Centre (AMRC), IIT Mandi, for the XPS facility.

The authors have no conflicts to disclose.

Amiya Kumar Mishra: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Writing – original draft (lead); Writing – review & editing (equal). Shivendra Kumar Pandey: Conceptualization (equal); Funding acquisition (lead); Investigation (equal); Resources (lead); Supervision (lead); Validation (equal); Writing – original draft (supporting); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.