On-device cryogenic quenching enables robust amorphous tellurium for threshold switching

Domain: physics
Published: 2026-01-13
Editorial outcome: minor_revision

Abstract
Amorphous chalcogenide alloys exhibiting crystallization-free Ovonic threshold switching behaviour have gained immense attention as selector materials. While the switching characteristics depend on the chalcogen species, understanding device-level elemental behaviour, particularly for tellurium (Te), remains challenging due to its low crystallization temperature and poor glass-forming ability. Here, we realize an electrothermally induced amorphous Te (a-Te) phase via on-device cryogenic quenching, which rapidly suppresses crystallization in the supercooled liquid at low ambient temperature. The order-to-disorder transition yields a ~0.81 V increase in threshold voltage and a ~10³ reduction in subthreshold current, attributed to enhanced deep-level trap formation. The a-Te phase exhibits reliable self-regulated oscillations, driven by deep traps, distinguishing it from conventional capacitance-driven effects. These findings support that the threshold switching in Te originates from defect-mediated transitions occurring before melting, rather than solely from thermal phase-change effects. Our results provide insights into chalcogenide switching mechanisms and pave the way for stoichiometry-tuned selector devices, nano-oscillators, and selector-only memory applications.

Introduction
Ovonic threshold switching (OTS), a volatile electronic switching phenomenon observed in amorphous chalcogenide alloys, has generated intense interest as a crucial enabler for high-speed, energy-efficient memory and logic systems when integrated as a selector element1. OTS behavior is marked by a rapid, substantial current increase, often spanning several orders of magnitude, at a specific threshold voltage (Vth) without inducing crystallization2,3, thus retaining the amorphous structure over 106–108 operation cycles. These characteristics position amorphous chalcogenides as ideal candidates for selector devices in high-density, parallel-processing memory arrays, where precise write and read operations are paramount4,5. Over the past decades, extensive material-level design approaches, including doping6–8 and heterogeneous structuring9–11, have been explored to enhance the selector performances. More recently, unconventional behaviors such as the prior pulse polarity effect12,13 and self-regulating oscillation14,15 have been observed, highlighting the significant impact of chalcogen species selection: tellurium (Te) vs. selenium (Se). Unlike Se-based systems, Te-containing alloys typically exhibit smaller bandgaps, lower glass transition temperatures, and a higher density of shallow trap states. More specifically, Te becomes stabilized in the amorphous matrix through enhanced covalency with other elements, thereby enabling unipolar switching and reliable oscillation. However, even though the primary component is attributed to a threshold characteristic, the intrinsic contribution of the amorphous chalcogen itself has not been directly investigated.

In parallel, elemental Te itself has been intensively investigated across a broad spectrum of fields, from fundamental materials science to practical device applications. Te forms a quasi-two-dimensional atomic configuration, consisting of one-dimensional helical chains held together by van der Waals (vdW) interactions16,17. Highly crystalline Te (c-Te) films are typically produced via traditional physical vapor deposition methods18–20 and, very recently, by atomic layer deposition (ALD)21. Owing to their intrinsic p-type semiconducting behavior21–24, Te has enabled the development of diverse complementary logic devices21,25,26. In addition, Te has emerged as a potential selector component, exhibiting a first-order crystal-liquid phase transition through local Joule heating20,27. The high Schottky barrier between Te and TiN enables threshold switching with low and high current operation in the crystalline solid and metallic-like liquid states. Despite growing interest, prior studies have predominantly focused on highly c-Te. In contrast, the solid-state amorphous counterpart of Te remains unexplored, largely due to its low crystallization temperature (Tcry) of −10–0°C18 and poor glass-forming ability. While several attempts have been made to stabilize amorphous Te (a-Te) and characterize its physical properties, these efforts have been limited to a closed cryogenic deposition system, not yet extended to functional electronic device integration28–31. Meanwhile, computational investigations of monatomic amorphous chalcogen matrix have suggested hypervalent bonding networks in a-Te, distinct from conventional covalent bonding in Se- or S-based amorphous matrices32–35, further motivating experimental verification of its electronic properties.

In this work, we demonstrate a robust approach for stabilizing an electronic device that integrates both the ordered and disordered solid states of Te by simultaneously controlling the internal and external thermal conditions. The nanoscale chalcogen-metal contact effectively induced Joule heating within the proposed via-hole device, enabling selective phase transitions from the liquid chalcogen. Specifically, the rapid electrothermal quenching resulted in a disordered configuration by suppressing crystallization kinetics, sustaining its atomic arrangement under a cryogenic environment. The degradation of crystallinity generated thermally stable deep-level traps within the Te layer, resulting in a higher Vth, lower conductance, and reliable oscillation behavior. Furthermore, the formation of a highly oriented single-domain structure, thermodynamically grown from amorphous nuclei, distinctly contrasts with the polycrystalline morphology induced by electrical pulses, thereby providing indirect evidence of amorphization within the device. Although sputtered Te devices have previously been reported to show volatile switching behavior20,27, we now clarify that this behavior is governed by defect-driven switching that occurs prior to the solid-liquid transition. Furthermore, on-device cryogenic stabilization of both c- and a-Te allows us to resolve distinct physical and electrical features, most notably a self-regulating oscillation in a-Te that has not been demonstrated before. Our findings clarify the underlying electrical properties of the disordered elemental Te and so highlight its significance in the trap-assisted threshold switching of OTS materials and their applications from a device perspective.

Results
Threshold switching of tellurium under Joule heating
Figure 1a illustrates our 2-terminal Te-based electrothermal device employing a nanoscale via-hole architecture. This device configuration, commonly used in phase-change memory systems, is particularly well-suited for generating and modulating localized Joule heating via electrical pulse waveforms36,37. Specifically, the melting and subsequent phase transition of Te predominantly occur above the via-hole region−referred to as the programming region−where the generated electrothermal heat is maximally concentrated. Figure 1b summarizes the key experimental findings regarding the phase configurations of monatomic Te explored in this work. To achieve a disordered matrix, which is crucial for overcoming Te’s inherently poor glass-forming ability, we adopted a rapid quenching strategy from the liquid phase. This approach effectively suppresses nucleation and inhibits nuclei growth in the supercooled liquid, thereby preventing crystallization. Consequently, reversible and programmable transitions between ordered and disordered phases are achieved from the liquid phase in the via-hole region, governed by precisely controlled electrothermal pulses. Both solid states of Te can be demonstrated inside a nanoscale device at ambient room temperature (RT) and in a cryogenic environment. However, holding the programmed Te at RT provides sufficient thermal energy to induce recrystallization, thereby reverting the amorphous phase to the crystalline state and promoting defect relaxation even within the crystalline phase. In contrast, both solid states, particularly a-Te, remain stable under cryogenic conditions, enabling reliable characterization of their physical and electrical properties without thermal interference.

Detailed fabrication procedures for the Te device are provided in the Methods section and Supplementary Fig. 1. Cross-sectional high-resolution TEM (HR-TEM) and corresponding energy-dispersive X-ray spectroscopy (EDS) confirm successful device fabrication with a via-hole diameter of 400 nm (Fig. 1c and Supplementary Fig. 2). Initial characterization by Raman spectroscopy (Fig. 1d) with isotropic angle-resolved properties (Supplementary Fig. 3) and X-ray diffraction (Supplementary Fig. 4) revealed that the as-deposited Te thin films exhibit a randomly oriented hexagonal polycrystalline structure on both metal and dielectric layers. As discussed, two distinct Te solid states can be stabilized under cryogenic conditions, and their electrical characterization below RT necessitates minimizing the Schottky barrier height between Te and the metal electrode. To achieve this, tungsten (W) was selected as the bottom electrode contact (BEC) material to mitigate excessive Schottky contact with Te20. Kelvin probe force microscopy (KPFM) was employed to determine the work function difference between W and Te (Fig. 1e and Supplementary Fig. 5), yielding a low Schottky barrier height of 0.20–0.25 eV for the p-type monatomic Te film (Fig. 1f). This optimized contact design ensures that electrical contrasts originate from the intrinsic chalcogen phases via the electrothermal effect within the nanoscale device.

The Te device arrays were initially evaluated at RT using a triangular pulse with a 3 V input voltage and 1 μs rise/fall times (Fig. 1g). Figure 1h displays the output current−input voltage (I–V) curves over multiple pulse cycles, clearly demonstrating volatile OTS characteristics. This behavior is attributed to intrinsic trap states associated with lone-pair electrons within Te’s covalent bonding networks, which is experimentally discussed in the subsequent sections38,39. To capture the sequential phase transitions, we present the dynamic transient responses to the initial pulse injection, showing the progression from the initial solid state to the liquid phase, and finally to the rearranged solid state (Fig. 1i). Initially, the device exhibited a low current until reaching a threshold voltage (Vth) of 1.90 V. Beyond this point, a sufficient electric field induced a high current flow. The low melting temperature of Te (~450 °C) facilitated its transformation into the liquid phase with only a few mA of current20, solidifying as the pulse fell. Subsequently, the device reverted to a low current state below a holding voltage (Vh) of 0.72 V. During multiple 1 μs melt-cooling processes, the Vth stabilized around ~1.1 V, while the subthreshold current gradually increased (see Supplementary Fig. 6 for details). These results confirm that the nanoscale via-hole device effectively generates localized Joule heating within the programming region, inducing phase rearrangement of the solidified Te.

Reversible order to disorder phase transition
Figure 2a illustrates the conceptual framework and experimental design for regulating the quenching rate within the programming region, i.e., the Te melting zone induced by Joule heating above the BEC, so as to achieve reversible transitions between ordered and disordered Te phases. This approach emulates melt-quenching processes wherein a fast quenching speed on the nanosecond timescale leads to amorphization, while slower cooling favors crystallization. The output current directly correlates with the electrothermal temperature achieved during Joule heating, and critically, the pulse fall time determines the time-dependent temperature gradient (dT/dt). All electrical measurements presented in Fig. 2 were conducted at RT.

Based on such quenching-rate-guided phase regulation, we obtained I–V curves for two distinct Te phases−a high resistance state (HRS) and a low resistance state (LRS)−across 50 devices (Fig. 2b). The subthreshold current was measured using a direct current (DC) bias (IDC) of 0.5 V. The HRS was programmed with a 5 V pulse (20 ns rise, 100 ns width, 20 ns fall), while the LRS was achieved using a 3 V pulse (1 μs rise, 100 ns width, 1 μs fall). The reading pulses in the dynamic transient curves exhibited distinct contrasts in Vth and subthreshold current after programming HRS and LRS, but with identical switching times of ~45 ns (Supplementary Fig. 7). Based on these results, we analyzed the probability distributions of Vth and IDC (Fig. 2c). Gaussian fit curves reveal distinct distributions with an average Vth contrast of ~0.81 V and an IDC difference spanning approximately three orders of magnitude. This IDC difference notably aligns with the reported resistivity ratio between a-Te and c-Te40. The device also demonstrated cyclic reliability exceeding 104 cycles, confirming its durable phase transition (Fig. 2d). Additionally, the identical phenomenon was observed in devices utilizing an ALD-grown Te layer, indicating that the behavior is indeed intrinsic to the material itself (Supplementary Fig. 8). We also confirmed the negligible polarity effects in the Te device during both programming and reading pulses (Supplementary Figs. 9, 10), ruling out conductive paths within the chalcogen matrix generated from the metal electrode. We note that polar segregation and internal or interfacial dipole formation are further prohibited in our devices because the monatomic Te is non-polar and structurally homogeneous41,42. Finally, we investigated the final solidified arrangement of the liquid monatomic chalcogen across a range of input voltages and fall times (Supplementary Fig. 11), demonstrating how the atomic configuration is solely dictated by the quenching rate.

Notably, the threshold switching (TS) was identified as an inherent phenomenon, independent of the Te phases. This key observation prompted a deeper investigation into whether the TS mechanism, particularly in the crystalline phase, is defect-driven rather than exclusively associated with a liquid-solid phase transition. To elucidate the origin of this phenomenon, we compared our Te device with a standard Ge2Sb2Te5 (GST)-based PCM device, utilizing the concept of an order-to-disorder transition. As shown in Supplementary Fig. 12, both GST and Te devices transition from the initial LRS to the HRS via melt-slow quenching. However, under identical pulse rise regimes in the dynamic transient curves, TS occurred exclusively in the Te device, whereas crystalline GST exhibited linear I–V behavior. Furthermore, the S-shaped negative differential resistance observed in the current-driven I–V curves of both a-Te and c-Te provides additional evidence of OTS behavior in monatomic Te (see Supplementary Fig. 13 for details). This stark contrast strongly suggests that the TS in c-Te arises from intrinsic defect states rather than electrothermal phase transitions.

Significantly, thermally unstable Te phases reorganize upon exposure to ambient thermal energy (Fig. 2e). The HRS initially sustained its current, but rapidly increased and saturated within ~102 s due to spontaneous crystallization. Conversely, the LRS current level gradually decreased, as clearly monitored in the linear scaling retention results. To elucidate these phenomena, Fig. 2f and Fig. 2g explore the time-dependent Vth drift of the LRS. The relaxation time (trelax) was defined as the interval between two identical triangular pulses, ranging from 100 ns to 105 s. The Vth remained constant within ~100 μs of trelax but soon began to perturb after a few seconds. After about 105 s (24 h), it reached full saturation, showing a voltage difference of ~0.8 V. Typically, Te-containing amorphous OTS alloys exhibit a drift phenomenon within 100 μs, primarily due to latent heat from Joule heating43. Therefore, the Vth drift observed in our Te device is best explained by structural relaxation and subsequent defect annihilation in the electrothermally reconfigured ordered phase (see Supplementary Fig. 14 for details)44. As the fully annihilated current level of LRS is identical to the recrystallized HRS (Fig. 2e), it suggests that the disordered state spontaneously crystallizes towards a thermodynamically stable ordered state of elemental Te, further evident in time-dependent relaxation in HRS as presented in Supplementary Fig. 15. The observed current decrement of LRS is directly attributed to the Vth drift, which ultimately saturates as it reaches its equilibrium state at ambient temperatures. Furthermore, the pulse-induced defect relaxation in a-Te yields defect dynamics comparable to those of conventional OTS materials when using pulse conditions tailored to match prior reports44,45 (see Supplementary Fig. 16 for details).

During the order-disorder transitions, we obtained the conductance values for three Te phases: disordered, liquid, and ordered (Fig. 2h). Details of the extracted calculations are provided in Supplementary Note 1. Notably, the conductance extracted for each Te phase was converted to an estimated resistivity under the assumed device geometry, and the resulting values are in good agreement with previously reported experimental data (see Supplementary Table 1). Starting from the initial liquid phase, solidification into distinct atomic arrangements leads to significant contrasts in their conductance. Specifically, a 20 ns fall time effectively transforms supercooled liquid Te into an amorphous solid state by suppressing the stable nucleation growth. Given that these outcomes correlate with the defect population within the Te phases, the inherent trap characteristics are further investigated under cryogenic conditions, where both thermally unstable solid states can be reliably preserved.

Phase dependent trap characteristics in tellurium
The specific trap characteristics were examined under cryogenic conditions (T < Tcry) (Fig. 3). All the electrical analyses were conducted at 80 K, unless otherwise noted as RT or as part of temperature-dependent measurements. The order-disorder phase transition was also clearly demonstrated at 80 K, showing a ~0.4 V positive shift in Vth compared to RT conditions (Supplementary Fig. 17). Figure 3a presents robust phase retentions for over 103 s in both the ordered and disordered phases. Notably, the LRS maintained its current level without any disturbance from delocalized defect annihilation, emphasizing the necessity of cryogenic stabilization for preserving electrothermally reconfigured monatomic chalcogen phases. Distinct trap properties were extracted using a Poole-Frenkel-based thermally assisted hopping (TAH) conduction model, described by the following equation:

I = 2qANT,totτ e−Ea/kbT sinh(qVΔz/2u kbT) (1)

where q is the elementary charge, A is the BEC area, NT,tot is the integral of the trap distribution (approximately 1/Δz3), Δz is trap-to-trap distance, Ea is the activation energy, V is the applied bias, kb is the Boltzmann constant, T is temperature, and u is the chalcogenide thickness46,47. Given that the current flow in Te is mainly hole-driven below 150 K48, the trap-limited conduction model considers holes as the primary mobile charge carriers. Figure 3b showcases the DC I–V curves for individual HRS and LRS in the subthreshold voltage region. The extracted HRS trap characteristics exhibited a ~0.93 nm reduction in Δz and a 0.06 eV larger Ea, compared to those of the LRS. Additionally, the DC measurements of a-Te confirmed OTS functionality, showing an off-current of ~60 nA at 1/2 Vth and an on/off ratio of ~103 under compliance-limited conditions, which increases to ~104 without compliance (Supplementary Fig. 18).

Furthermore, Ea was validated through temperature-dependent conductance (G) measurements of the HRS, using the following equation:

G = G0 exp(−Ea/kbT) (2)

where G0 is the initial conductance (Fig. 3c). The disordered Te exhibited p-type semiconducting behavior, consistent with previous reports49. The inset of Fig. 3c presents a logarithmic conductance plot versus inverse temperature, from which two distinct Ea values are derived. The boundary point between these two linear regions correlates with Vth changes observed under varying measurement temperatures (Supplementary Fig. 19). Notably, the extracted energy of ~0.15 eV is comparable to the Ea obtained from the TAH model. A minimal Ea of ~0.004 eV indicates a localized trap forming an Urbach tail near the valence band edge50. Figure 3d–f further explore trap properties as a function of the quenching rate at 80 K. At a fixed input voltage of 5 V, the Te device abruptly transitioned from the HRS to the LRS when the fall time reached 300 ns (Fig. 3d). Concurrently, the subthreshold current gradually increased, even as the Vth remained similar between the HRS and LRS regions (Fig. 3e). The gradually reduced quenching rate consistently led to the annihilation of deep-level trap density, as reflected by clear trends in Δz and Ea dictated by the regulated fall time (Fig. 3f). Consequently, the disordered phase possesses energetically more stable and denser deep-level traps than the ordered phase.

The presence of such energetically stable and dense trap populations in disordered Te is presumed to be a key factor enabling reliable oscillations. Investigations into the trap nature revealed robust self-oscillation performance in the monatomic chalcogen device (Fig. 4). Experimental details are described in Fig. 4a and Supplementary Note 2. While the oscillations are generally observed in Te-containing amorphous chalcogenide materials, explicit mechanistic evidence has often been lacking51. Here, reliable oscillations were clearly demonstrated in the HRS, driven by self-modulation of the applied bias in response to a 100-μs square pulse (Fig. 4b). The average oscillation period was ~7 μs, composed of a switching time of ~0.28 μs and a relaxation time of ~6.72 μs (Fig. 4c). In contrast, Fig. 4d shows a constant current without any oscillatory behavior in the LRS. The consistent absence of oscillation across various input voltages in the LRS indicates a fundamental lack of physical factors required to establish self-regulated properties in this state (Supplementary Fig. 20). Further analysis of the HRS oscillatory behavior was conducted under prolonged square pulses (>300 μs). Despite the current injection being mitigated by external resistance, a continuous oscillation current range of 50–100 μA gradually transformed the initial amorphous phase into the crystalline phase; accordingly, the oscillation characteristics then vanished (Fig. 4e).

The defect-related oscillation behavior in disordered Te was further investigated (Fig. 4f–h). To systematically modulate the defect density in the amorphous matrix, the fall time of the HRS programming pulse was varied from 50 to 250 ns, while other parameters remained constant (details provided in Fig. 3d). Figure 4f showcases the oscillation waveforms corresponding to the quenching rate-dependent disordered Te phases. The corresponding oscillation components were then analyzed under a 100-μs square pulse (Fig. 4g, h). Specifically, the current rise time remained constant at 40–50 ns regardless of fall time, matching the HRS switching time (Supplementary Fig. 7). Conversely, the current decay time gradually decreased as the minimum oscillation current increased (Fig. 4h), which is a direct result of the reduction in deep-level trap density. Collectively, these results strongly underscore that the deep-level traps, originating from localized defects in Te, are essential for robust oscillation behavior, while shallow-level traps inhibit self-regulating performance. A detailed electrical and atomic-level description of oscillation in the Te devices is provided in Supplementary Note 3 and Supplementary Figs. 21–23.

Theoretical comparison of three tellurium phases
To support the experimental outcomes, various Te phases were rigorously analyzed using ab initio molecular-dynamics (AIMD) simulations. Figure 5a displays the disordered atomic configuration of the a-Te, constructed with 180 Te atoms. For direct comparison, the charge density distributions of both amorphous and crystalline phases were visualized along the same crystallographic orientation corresponding to the (200) plane in the crystalline structure (Fig. 5b), clearly revealing distinct differences between the disordered and ordered configurations. The amorphous phase exhibited only short-range order (SRO) without any periodicity. In contrast, a long-range order (LRO), characterized by periodic alignment between adjacent helical Te chains, was clearly observed in the crystalline phase (see Supplementary Fig. 24 for details). To further understand the amorphous matrix, we examined the local bonding environment of disordered Te. The bond-angle distribution revealed two dominant populations: one at 90–100° and another exceeding 160° (Supplementary Fig. 25a). The latter is attributed to the presence of hypervalent networks involving lone-pair electrons. Furthermore, the bond length distribution indicated a mid-range order (MRO) at ~4.351 Å and a prominent SRO at ~2.940 Å (Supplementary Fig. 25b).

To investigate the band gap (Eg) and the mid-gap state (MGS), we calculated the total density of states (TDOS) and corresponding normalized inverse participation ratios (IPR). Higher IPR values indicate more localized electron states. In the disordered Te phase, the calculated Eg was ~0.7 eV, significantly larger than that of c-Te (Fig. 5c, d). No MGS was observed in the ordered Te, consistent with the defect-free nature of the crystalline supercell. In contrast, a distinct MGS appeared in the disordered phase at ~0.18 eV, close to the valence band maximum. This theoretical result matches the Ea of trap states in disordered Te within 30 meV, as experimentally extracted from the p-type hole-driven analytic conduction model (Fig. 3b, c). Additionally, the statistical analysis of multiple independently constructed a-Te models yielded mean (μ) and standard deviation (σ) values of (0.704 eV, 0.040 eV) for Eg and (0.156 eV, 0.045 eV) for Ea, in good agreement with Fig. 5c (Supplementary Fig. 26). The high reproducibility across these models indicates that the calculated electronic properties capture intrinsic features of disordered Te. Partial DOS (PDOS) for the 4s, 4p, and 3d orbitals of Te further elucidated the orbital contributions to the MGS (Supplementary Fig. 27). All 4p orbitals (e.g., 4px, 4py, and 4pz) significantly contributed to forming the MGS, while the 4s orbital accounted for less than 5%. This suggests that the dominant trap states dispersed in disordered Te originate from hypervalent networks involving lone-pair electrons, a feature absent in amorphous Se35. Exceptionally, the liquid Te phase displayed continuously dispersed delocalized orbitals across all band energies (Supplementary Fig. 28). The absence of a well-defined bandgap and the extensive orbital overlay further support the metallic nature of the liquid Te.

The conductivity differences among the ordered, disordered, and liquid Te phases were subsequently interpreted in terms of Eg and the maximally localized Wannier function (MLWF) spread. Figure 5e displays a stepwise increase in Eg, corresponding to the transition from liquid to ordered, and then to disordered Te. The 0.58 eV Eg of the ordered phase slightly differs in the band gap extracted via UV-Vis-NIR spectroscopy (~0.33 eV for 45-nm-thick Te), presumably due to the exchange-correlation functional employed (Supplementary Fig. 29). The MLWF spread was then analyzed in Fig. 5f. Specifically, the spread of each MLWF, Ωn, represents the measure of spatial extent (i.e., variance) from the center of the Wannier function, expressed as following equation:

Ωn(Å2) = ⟨wn|r2|wn⟩ − ⟨wn|r|wn⟩2 (3)

where wn is the Wannier function, and r is the position vector. From the disordered, to ordered, to liquid phases, the average spread (Ωavg) gradually increased, indicating reinforced electron delocalization35. Collectively, the consistent trends in Eg and MLWF spread explain the abrupt conductance contrast between the liquid and two solid states (a-Te and c-Te), as discussed in Fig. 2h.

Structural characterization of the amorphization pathway
To confirm the formation of electrothermally programmed amorphous monatomic Te, we performed a detailed TEM analysis. Since direct observation of disordered Te is challenging without specialized in-situ cryogenic TEM capabilities, we instead investigated the atomic reconfiguration via post-annealed grain growth from the presumed a-Te within the nanoscale device18. To prepare the specimen, the Te device was initially programmed to the HRS at 80 K, and then subjected to a post-annealing process at 273 K for 1 h. Figure 6a presents the prepared device, analyzed via the cross-sectional TEM. Three distinct Te regions were defined based on the influence of Joule heating: (i) the programming region above the metal electrode (Areas 1–4), (ii) the non-programming region above the dielectric layer (Area 6), and (iii) the intermediate region (Area 5) between (i) and (ii). Fast Fourier transform (FFT) was applied to areas 1–4 within a 400 nm2 scale along the fixed zone axis of [0001] (Fig. 6b). The results strikingly revealed identical hexagonal configurations within the region that had been previously amorphized. While the entire region did not transform into a fully single-crystalline domain, each grown grain, exceeding 100 nm in size, exhibited a consistent orientation towards an identical crystallographic order. Furthermore, we compared FFT analyses of three distinct ordered Te phases: (i) as-deposited Te at RT, (ii) electrothermally crystallized Te induced by multiple LRS pulse injections, and (iii) post-annealed Te grown from amorphous nuclei above the metal electrode (Supplementary Fig. 30). Although injecting a slow cooling pulse also tends to align the crystallographic orientation of Te, misoriented grains are still clearly observed in that case. The highly uniform orientation observed only in the post-annealed amorphous region provides compelling and distinctive evidence for the successful amorphization process of monatomic Te within our device.

The origin of the hexagonal FFT images is further elucidated in Fig. 6c. High-angle annular dark-field STEM (HAADF-STEM) analysis revealed large-scale trigonal helical chains with a lattice constant of ~4.348 Å. This extracted lattice constant matched the d-spacing of (10-10) planes (~3.788 Å), within the zone axis of [01-10] (Supplementary Fig. 31). In the intermediate region, a Moiré pattern was observed, indicating the coexistence of parallel and vertical helical chains along the same zone axis (Fig. 6d, Supplementary Fig. 32). This is attributed to the radial dispersion of hemispherical Joule heating generated during device programming. Interestingly, the lattice constant of this hexagonal motif was slightly smaller than that of bulk trigonal Te (~4.5 Å). It is speculated that over-coordinated (>2-folded) MRO Te clusters act as crystalline nuclei, given that their interatomic distances are similar to the recrystallized lattice constant34,52. Selected area electron diffraction (SAED) patterns with the aperture size of ~100 nm were subsequently analyzed in the programming and non-programming regions (Fig. 6e, f). Consistent with prior XRD results (Supplementary Fig. 4), small Te grains above the dielectric layer, lacking a preferred orientation, produced irregular diffraction spots. In contrast, the SAED patterns from the programming region unequivocally confirmed the presence of a highly oriented hexagonal phase. Consequently, our results provide robust evidence that rapid melt-quenching successfully induces a-Te formation within the nanoscale device, reliably covering the BEC contact region. We further note that visualizing a-Te via on-device cryogenic quenching combined with in-situ cryogenic TEM could enable direct observation of the amorphization process, which is potentially investigated in future studies.

Methods
Device fabrication
The bottom electrode contact (BEC) was initially defined on the 300-nm-thick SiO2/Si substrate. Firstly, W was deposited using a DC sputter. Conventional photolithography and dry etching via an inductively coupled plasma-reactive ion etching (ICP-RIE) were sequentially processed for patterning. The photoresist was cleaned with the solvent (e.g., acetone and IPA). After that, a 30 nm Si3N4 dielectric layer was deposited using a plasma-enhanced chemical vapor deposition technique. An nanoscale hole was then patterned via a combination of e-beam lithography and ICP-RIE. Next, an RF sputtering system was used to cover the patterned area completely with monatomic tellurium (Te, 45 nm) or phase-change material (GST, 180 nm) inside the via hole. The base pressure was under 5×10−7 torr to obtain high-quality, non-oxidized chalcogen film. After that, TiN (~10 nm) by reactive sputtering with Ti target and N2 gas was immediately encapsulated Te film to prevent oxidation. It is patterned by the lift-off process. Prior to the metallization, BEC opening is defined by the ICP-RIE process, as the etching region is defined by photolithography. Finally, TiN (~10 nm) and W (100 nm) were deposited for metallization of both bottom and top electrodes within the lift-off and solvent-cleaning process. Note that, TiN should be interposed between chalcogenide materials and W due to the adhesion issues.

Thin-film characterization
Surface topography and film thickness were measured by AFM (MFP-3D Origin, Oxford Instruments) in a tapping mode using the Si cantilever (AC160TS-R3). The work function difference between adjacent materials was obtained by KPFM surface potential map using a Ti/Ir (5/20 nm)-coated tip (ASYELEC-01-R2, Asylum Research) with a resonance frequency of 75 kHz and a force constant of 2.8 Nm−1. Phonon vibration modes were inspected by a confocal micro-Raman system (customized based on XperRam 200S, NANOBASE Inc.) using a 532 nm laser with a power of 0.3 mW. Crystallinities of the thin film were obtained by XRD (Bruker, D8 Discover system) using Cu Kα radiation (λ=1.5405 Å) in the 2θ of 20–60°. For GI-XRD, the incident angle for the X-ray beam source and sample stage is tilted by 0.3°. Optical bandgaps were investigated using a UV-Vis spectrometer (Cary 5000, Agilent) analyzed by the Tauc plot53.

Electrical measurements
Electrical measurements were performed using a Keithley 4200A-SCS semiconductor analyzer and pulse measure unit (4225-PMU). Minimum rise/fall time and pulse width of electrical pulses are 20 ns and 50 ns, respectively. To obtain accurate resistance values, we employed a DC reading voltage of 0.5 V utilizing an embedded source measure unit (SMU). Also, we used remote preamplifier/switch modules (4225-RPM) to connect both PMU and SMU for comfortable conversion of DC and AC pulse measurements. The cryogenic environment was constructed using liquid nitrogen (LN2) gas, cooling the vacuum chamber. To achieve self-regulating oscillations, the external resistance of ~50 kΩ was series connected between the BE probe and the RPM module.

Cross-sectional TEM characterization
A cross-sectional TEM specimen was prepared using a focused ion beam (FIB) (Helios 450HP FIB). The Te device under investigation was first coated by a Pt thin film (FEI sputter), and then the amorphous carbon layer was deposited on the target area to minimize ion beam damage during the FIB preparation. The cross-sectional high-resolution bright-field high-resolution TEM and high-angle annular dark-field (HAA-DF) TEM images of the Te device were acquired using a Cs-corrected scanning TEM (JEOL, JEM-ARM300F) operated at 160 kV.

Ab initio molecular-dynamics simulations
Ab initio molecular-dynamics (AIMD) simulations based on density-functional theory (DFT) were employed to construct and analyze crystalline, amorphous, and liquid Te models, as implemented in the Vienna Ab initio Simulation Package (VASP) code54. All models used the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional55 within the projector augmented-wave (PAW) method56. For band gap calculations, the hybrid functional of HSE06 was employed57. A plane-wave energy cutoff of 300 eV was used. The crystalline model consisted of 81 Te atoms in a supercell. The amorphous model was generated with 180 Te atoms via a melt-quenching process, cooling from 900 K to 80 K at a rate of 5 K/ps. The temperature was controlled by a Nosé thermostat algorithm, and the MD timestep was 3 fs. The simulation volume was kept constant during the AIMD simulations. Structural relaxation of the quenched amorphous models was carried out using the conjugate-gradient method at 0 K, until the force on any atom was less than 0.01 eV/Å. Maximally localized Wannier functions and their spread were computed by the wannier90 code58.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled “On-device cryogenic quenching enables robust amorphous tellurium by threshold switching” by Prof. Joonki Suh’s group reports a fascinating ovonic threshold switching (OTS) behavior in amorphous Te achieved via 80 K cryogenic quenching. This behavior differs from the recently reported electrical switching behavior of crystalline Te—termed the phase-change switch (PCS)—as described in Science (2021, 374, 1390) and Advanced Functional Materials (2025, 35, 2423940). The findings are highly compelling and exceptionally well presented. They offer groundbreaking insights into the selector device field, particularly given the current surge of interest in both OTS and PCS technologies.\n\nThe manuscript can be accepted in its current form after minor revisions and clarifications as outlined below:\n1.Figure 2 shows a W/TiN/Te/W device structure, which is asymmetric—unlike the symmetric structure used in Shen et al. (Science, 2021, 374, 1390). This asymmetry may result in differing electrical behaviors under opposite voltage biases. Please provide data demonstrating the bidirectional threshold-switching characteristics of the Te devices.\n2. The main manuscript lacks the DC performance characterization of the Te device, which is essential for evaluating selector functionality. Please include the DC I–V characteristics in the main text.\n3. Figure 2i presents the conductance of amorphous, crystalline, and liquid Te. It would strengthen the discussion to compare these values with previously reported experimental data, such as those in Phys. Rev., 1957, 107, 412.\n4. The extracted activation energy (Ea) of the high-resistance (amorphous) Te state is only 0.148 eV. This is even lower than the Ea of crystalline Te after removing the Schottky barrier by applied voltage (~0.25 eV, as shown in Fig. 3e of Science, 2021, 374, 1390). Typically, the Fermi level (Ef) in amorphous chalcogenides is pinned near the mid-gap, implying an Ea of approximately half the band gap. With a calculated band gap of 0.71 eV for amorphous Te in Figure 5c, the expected Ea should be around 0.35 eV. The reported value seems too low given the estimated Eg and the material’s low conductivity. Please verify whether the fitted Ea is accurate and reasonable.", "reviewer_2": "The authors present a comprehensive study (electrical/physical characterization, as well as theoretical simulations) of the OTS/phase-change Te device. The low-temperature (80K) OTS behavior of amorphous phase was compared to the normal conditions, when a-Te crystallizes. While Te-based materials were already known for OTS/phase-change properties, the noteworthy result of this work is the observation of the oscillation phenomenon in pure aTe, attributed to the amorphous state of the Te material and its deep hole traps in the gap.\n\nWhile the work presents most of the time sufficient evidence for the claims and conclusions, and as such, it represents an important evidence of the importance of the deep gap traps in the threshold switching phenomenon, however, it still lacks any attempts to explain the origin of the oscillations.\n\nFor detailed comments, questions, correction and suggestions for improving the manuscript please see the attached annotated manuscript pdf files.", "reviewer_3": "The authors investigated the disordered amorphous Te phase that can be stabilized by fast quenching, and suggested that the rapid quenching suppresses nucleation and crystallization. By changing the pulse falling edge time, different degrees of disorder can be controlled with distinct threshold voltage shift and subthreshold current reduction. These phenomenon are attributed to enhanced deep-level trap formation, which also lead to self-regulated oscillations, instead of by the conventional capacitance-driven oscillation effects. These observations are supported by theoretical modelling and physical characterizations such as TEM. This work provides new insight into the amorphous-crystalline transition in a-Te from its liquid phase, and how it can be utilized for enhancing OTS performance.\n\nHowever, there are several issues that need to be addressed.\n1) The single-element Te OTS device has been investigated in recent literature, for example, many of the observations in this manuscript have been reported in Refs.20, 24, 27, but are not discussed in detail (for example, the manuscript only briefly mentioned that 'Highly crystalline Te (c-Te) films are typically produced via traditional physical vapour deposition methods' and 'Te has emerged as a potential selector component, exhibiting a first-order crystal liquid phase transition through local Joule heating' in ref 20, Science, 2021, doi: 10.1126/science.abi6332). The authors should clearly state in the manuscript which parts of their results has been reported, and which parts are not.\n2) It seems that both ordered (crystalline) and disordered (amorphous) phases can be achieved at both room (Fig.2) and cryogenic (Fig.3) temperatures, where low temperatures enhance the amorphous phase. If this is correct, it is misleading to state that 'electrothermally induced amorphous Te (a-Te) phase stabilized through on-device cryogenic quenching' as in the abstract and manuscript.\n3) The impact of falling edge time and relaxation conditions on Vth and Ioff in various OTS devices have also been extensively investigated and explained by defects dynamics, for example, in DOI: 10.1109/VLSITechnology18217.2020.9265011 and 10.1109/IRPS48228.2024.10529450. The authors should also compare their defects characterization to these existing explanations.\n4) It is not convincing that the so-called deep-level traps can lead to the higher Vth and lower Ioff in amorphous state. In contrast, additional traps in OTS are generally considered leading to lower Vth and higher Ioff, as being explained and modelled in decades of study. A few recent examples include https://doi.org/10.1002/aelm.202300037 and https://doi.org/10.1002/advs.202408028. There are also electron traps and hole traps as measured in the 2nd paper above. The authors need to provide more convincing experimental results to support their claim, and explain the contradiction against other explanations.\n5) The TEM results are supportive but indirect, it will be more convincing if in-situ TEM results can be obtained at cryogenic temperatures during the fast quenching processing.\n6) The explanation of self-oscillation is also not convincing. Self-oscillation that is only observable at amorphous states but not at crystalline states can be also be explained by the conventional RC driven effects. Although there is no external capacitor in the set-up, the parasitic capacitance associated with the large device size and cables/connections do exist. A large Te resistance at amorphous state is needed to contribute to a sufficient RC value for oscillation, while the small Te resistance at the crystalline state leads to a too small RC constant to cause oscillations. The authors should provide more evidence to exclude the impact of parasitic capacitance."}, "author_response": "General Responses to Referees’ Comments\nWe sincerely thank the editor and all three referees for their thoughtful and constructive assessments of our manuscript. We are encouraged that the referees recognized the significance and novelty of our work, as reflected in their following supportive comments:\n“The findings are highly compelling and exceptionally well presented. They offer groundbreaking insights into the selector device field, particularly given the current surge of interest in both OTS and PCS technologies.” – Reviewer #1\n“The work presents most of the time sufficient evidence for the claims and conclusions, and as such, it represents important evidence of the importance of the deep gap traps in the threshold switching phenomenon.” – Reviewer #2\n“This work provides new insight into the amorphous-crystalline transition in a-Te from its liquid phase, and how it can be utilized for enhancing OTS performance.” – Reviewer #3\nWhile acknowledging the strength of our approach, the referees also raised important points requiring clarification, primarily concerning: (i) the sub-threshold conduction mechanism and extracted activation energy in a-Te (Reviewer #1, #2), (ii) the statistical robustness/validation of the DFT analysis (Reviewer #2), and (iii) the physical origin of the oscillations observed exclusively in the amorphous state (Reviewer #2, #3). In addition, following the referees’ suggestions, we have performed further electrical characterizations, clarified key physical phenomena, improved data presentation, and better positioned our findings within the context of existing literature.\nIn response, we have carefully addressed all comments and thoroughly revised our manuscript on the basis of new experimental results, expanded our theoretical analysis, and provided more detailed explanations in both the main text and Supplementary Information.\nBelow, we present point-by-point responses to each comment, followed by a summary of relevant changes (at the end of each individual response) and a reference list cited in the revision (at the end of this letter). We hope that you and the referees find the revised manuscript suitable for publication in Nature Communications. We are deeply grateful for the insightful feedback, which has significantly contributed to improving our work.\nSincerely,\nJoonki Suh (on behalf of all co-authors)\n\nPoint-by-Point Responses to Reviewer #1’s Comments\n[...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have properly revised their manuscript according to the reviewers' comment, I suggest the publication of the manuscript as it is.", "reviewer_2": "The authors addressed satisfactorily all the raised questions and I recommend the mansucript for publication.", "reviewer_3": "The authors have addressed my comments appropriately with further theoretical and experimental evidence, and have clarified these in the revised manuscripts. I would recommend to publish as it is."}, "author_response": "General Responses to Editor’s and Referees’ Comments\nWe sincerely appreciate the time and effort the editor and reviewers have dedicated to evaluating our revised manuscript. We are delighted to note that all three referees have expressed their support for publication. In preparation for the final submission, we have carefully addressed all items in the Author Checklist. Particularly, we have ensured that (i) the abstract & editor’s summary, (ii) author information, (iii) article structure, (iv) main text, (v) figures and tables, (vi) data and code availability, and (vii) references are fully compliant with the formatting standards of Nature Communications.\nA point-by-point response to the final comments is provided below. We hope that the manuscript is now ready for publication in Nature Communications and thank the editor once again for the guidance throughout this process.\nSincerely,\nJoonki Suh (on behalf of all co-authors)\n\nPoint-by-Point Responses to Reviewer #1’s Comments\nThe authors have properly revised their manuscript according to the reviewers’ comment, I suggest the publication of the manuscript as it is.\nWe sincerely thank the reviewer for the positive assessment of our revision and for endorsing the manuscript for publication. We are deeply appreciative of the time and careful attention provided throughout the review process, which has significantly improved the quality of our work.\n\nPoint-by-Point Responses to Reviewer #2’s Comments\nThe authors addressed satisfactorily all the raised questions and I recommend the manuscript for publication.\nWe are grateful to the reviewer for their encouraging comments and for confirming that all concerns have been satisfactorily addressed. We thank the reviewer for his/her support and for recommending this work for publication.\n\nPoint-by-Point Responses to Reviewer #3’s Comments\nThe authors have addressed my comments appropriately with further theoretical and experimental evidence, and have clarified these in the revised manuscripts. I would recommend to publish as it is.\nWe thank the reviewer for his/her constructive feedback throughout this process. We are pleased that additional theoretical and experimental evidence has strengthened the manuscript and met the reviewer’s expectations. We appreciate the recommendation for publication."}]

Decision letter