GeTe Phase Change Material Research Articles

Optical properties of Sn-substituted GeTe phase-change materials under high pressure

The emerging phase change materials (PCMs) have attracted much attention due to their unique properties and applications. However, the optical properties of PCM at high pressures have not been well explored. In this work, the microstructure and optical properties of GeTe and Sn-substituted GeTe at high pressures up to 30 GPa have been systematically investigated. We find that the substitution of small amounts of Sn for Ge combined with the introduction of high pressure can induce an optical contrast of 117% in crystalline GeTe, which is even higher than the highest optical contrast reported so far (90%) caused by amorphous GeTe to crystalline GeTe. The pressure-induced contrast does not originate from any phase transition as previously, but from the combined effect of Peierls distortion elimination and Ge vacancy formation, which leads to an increase in carrier concentration. The relationship between pressure/atomic composition, Peierls distortion/vacancy formation, and optical contrast can be well explained by a combination of experiments and theoretical calculations. Therefore, this study reveals the effect of high pressure on the optical properties of PCMs and provides new ideas for the design of high-optical-contrast PCMs.

The effect of Cr atoms: From non-stoichiometric Ge-Te to Cr2Ge2Te6

The traditional phase change material Ge-Te presents a large difference with Cr2Ge2Te6 (CrGT) in the phase change properties. Compared with Ge-Te, the two-dimensional van der Waals material CrGT presents a reversed resistance state, in which the crystalline phase is in a high-resistance state and the amorphous phase is in a low-resistance state. Nevertheless, there are some structural similarities between the two materials. CrGT may be regarded as the Cr doping of Ge-Te. It is necessary to investigate the evolution of the structure and phase change properties of Ge-Te to CrGT through Cr doping. In this study, phase change materials Ge40Te60 (GT) and CrGT were fabricated. Subsequently, a series of intermediate state films were fabricated by changing the Ge/Te/Cr cation-to-anion ratio. Surprisingly, the coexistence of the CrGT and GT phases was observed in all intermediate films. As a result, a gradual reversal in the resistance between the amorphous and crystalline states can be observed. Transmission electron microscopy experiments were also performed to characterize the grain growth and the phase separation. Unlike GT, CrGT island-like crystal grains usually have boundaries with regular shapes. The pure CrGT-based device exhibited an excellent device performance. This work may provide a useful guide for further application of the CrGT phase change material.

Crystallization of GeTe phase change thin films grown by pulsed electron-beam deposition

In this work, a technique for crystallization of Germanium Telluride (GeTe) thin films was experimentally studied. The GeTe phase change material was grown by a novel high vacuum physical deposition method called pulse electron-beam deposition (PED). To evaluate the quality of the PED growth method, crystallized GeTe thin films were investigated by contemplating the crystallite quality and the electrical conductivity performance. X-ray diffractometer (XRD) spectrum and resistivity were used to evaluate the crystal quality. Alternative material characterization methods, like field emission scanning electron microscope (FESEM) and energy-dispersive X-ray spectroscopy (EDX), were used to explore the crystallization parameters further. Our previous study observed that the as-grown GeTe thin film was amorphous. In order to convert the GeTe thin film into a crystalline formation, post treatment annealing was performed. To optimize the post annealing procedure, several annealing parameters were experimentally investigated. The thin film resistivity dropped by six orders of magnitude upon the crystallization process.

Transmission/reflection/absorption individually control multifunctional metasurfaces

We propose a metasurface based on hybrid phase change materials GeTe and vanadium dioxide (VO2), which can manipulate the transmission /reflection /absorption of terahertz waves independently. By changing the external temperature from 25°C to 160°C, the function of this structure can be dramatically changed. When VO2 is in a dielectric state (i.e. 25°C), the designed structure behaves as a transmission-mode terahertz vortex beams manipulator. When VO2 is in a dielectric state (i.e. 68°C), the proposed structure serves as a reflection-mode terahertz vortex beams controller. When GeTe is in crystalline state (i.e. 160°C), the designed structure becomes as a terahertz perfect absorber at a frequency of 1.98THz within the incident angle of 30°. The proposed structure provides a new method toward the use of multifunctional terahertz devices for their potential in applications including terahertz wireless communication and detection.

Experimental evidence for non-purely electric field-induced threshold switching and modified thermal-assisted model in GeTe phase change material

In this paper, we present a multiple layer device for investigating the impact of electric field on the conductance switching of GeTe phase change material excluding the contribution from Joule heat. The device includes a dielectric layer with excellent current-blocking which can result in a large electric field generated in the amorphous GeTe film and almost no current. With the generated electric field far beyond its threshold value for the conductance switching, our experimental data indicate that the conductance switching has not happened in the GeTe film. This indicates that the ovonic threshold switching (OTS) could not be induced by the purely electric field in amorphous chalcogenide film. Meanwhile, a modified thermal-assist model based on the Poole–Frenkel (PF) mechanism has been proposed to verify the thermal assistance is indispensable in the OTS process. And the modified model is well applied on the GeTe devices with different scales, which further supports the current experimental conclusion. This contributes to the further study of the OTS mechanism and application of the phase-change memory (PCM).

Loss Compensated PCM GeTe-Based Latching Wideband 3-bit Switched True-Time-Delay Phase Shifters for mmWave Phased Arrays

This article reports the first implementation of 3-bit millimeter-wave switched true-time-delay (TTD) phase shifters based on phase-change material (PCM) germanium telluride (GeTe). Two TTD phase shifters are presented. The first phase shifter is designed using four monolithically integrated PCM single-pole triple-throw (SP3T) switches to route the signal through delay lines. The insertion loss variation between various states is minimized by integrating two fixed PCM GeTe elements maintained in the crystalline state, along with the optimized width of the delay lines. The PCM switching cells are latching type, thus, consume no static dc power. The SP3T switches are connected back-to-back in two stages to provide a 3-bit phase shift with 20° precision. The second phase shifter is designed using two back-to-back connected PCM single-pole eight-throw (SP8T) switches. Both phase shifters are designed to operate over an 8 GHz wide frequency band with a center frequency of 30 GHz. The devices are fabricated in-house using an eight-layer microfabrication process. The proposed devices are highly miniaturized with an overall device area of 0.42mm 2 and 1.4mm 2 for the first and second phase shifter, respectively. The first phase shifter exhibit a measured average loss of 4.3 dB with a variation of ±0.3 dB and a return loss better than 20 dB, while the second phase shifter demonstrates low average measured loss of 3.8 dB with only ±0.2 dB loss variation and returns loss better than 17 dB at 30 GHz. Both phase shifters provide 180° linear phase shift with lower than 18 ps delay in the worst case.

A unified mid-gap defect model for amorphous GeTe phase change material

Understanding the nature of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials is crucial to guide the development of multi-level memories, selectors, and neuromorphic devices. Currently, however, the knowledge of the mid-gap defects is fragmented and the known structural and electrical characteristics of the defects cannot be unified in a single atomic model. In this work, a Ge–Ge chain defect model is built by artificial bond rearrangement in an amorphous GeTe network generated by atomic distortion, revealing a possible valence alternation mechanism in its formation. Atoms in the Ge–Ge chain are found to have a crystalline-like environment. The chain is found to have a negative-U property and to introduce mid-gap states. This model unifies the main known properties of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials.

GeTe photoresist films for both positive and negative heat-mode nanolithography

Binary chalcogenide phase change material GeTe was proposed and investigated as a novel high-resolution inorganic photoresist for laser heat-mode lithography. GeTe possesses both positive and negative photoresist characteristics based on alkaline or acid developing solutions, which can improve the flexibility in fabricating micro/nano devices. Different relief gratings with feature sizes in the range of nanometer to micron were fabricated, and the minimum linewidths can be reduced to 137 nm (positive tone) and 98 nm (negative tone) respectively. The selective wet etching mechanism was discussed with local oxygen concentration mapping of nanopatterns. Moreover, the patterns were successfully transferred onto Si substrate by plasma etching.

Harnessing machine learning potentials to understand the functional properties of phase-change materials

Abstract

Near-infrared modulation by means of GeTe/SOI-based metamaterial.

Today, nanophotonics still lacks components for modulation that can be easily implementable in existing silicon-on-insulator (SOI) technology. Chalcogenide phase change materials (PCMs) are promising candidates for tuning in the near infrared: at the nanoscale, thin layers can provide enough contrast to control the optical response of a nanostructure. Moreover, all-dielectric metamaterials allow for resonant behavior without having ohmic losses in the telecom range. Here, a novel hybridization of a SOI-based metamaterial with PCM GeTe is experimentally investigated. A metamaterial based on Si nanorods, covered by a thin layer of GeTe, is designed and fabricated. Switching GeTe from amorphous to crystalline leads to a rather high resonance-governed reflection contrast at 1.55μm. Additional confocal Raman imaging is done to differentiate the crystallized zones of the metamaterials' unit cell. The findings are in good agreement with numerical analysis and show good perspectives of all-dielectric tunable near-infrared nanophotonics.

Terahertz generation measurements of multilayered GeTe-Sb2Te3 phase change materials.

Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always p polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3 and Bi2Te3, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in p-type semiconducting iPCM materials.

Growth study of GeTe phase change material using pulsed electron-beam deposition

Thin film growth of chalcogenides composition is an exciting field, owing to the interesting optical and electrical properties of them. In this work, a novel, physical vapor deposition (PVD) method called pulse electron-beam deposition (PED) experimentally studied to investigate the requirements for ablation of phase change materials (PCM). To evaluate the qualifications of the PED method, thin films of Germanium Telluride (GeTe) are grown and investigated by contemplating the surface morphology and the material composition. High-quality GeTe thin films with thicknesses from 30 to 200 nm, were successfully grown on top of silicon wafers at room temperature to demonstrate the ability of PED. To optimize the growth procedure, several growth parameters were thoroughly investigated, including background pressure, pulse energy, and growth temperature. A series of material characterization methods were adopted to study the GeTe material quality after the growth. These methods include field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and thickness profilometer. It was found that higher material growth rate can be obtained in lower background pressure (∼ 2.6 mTorr), lower temperature (room temperature) but higher pulse energy (e.g., 15 kV). Besides, by increasing the target-to-substrate distance, the surface quality (e.g., smoothness) was improved substantially, but the growth rate decreased linearly. Finally, after the growth optimization, FESEM images revealed that the as-grown GeTe films were of high smoothness and uniformity. EDX analysis indicated that the compositions of the GeTe films were pressure dependent. Through the XRD spectrum, it is found that the as-grown GeTe films were amorphous. In order to convert them into crystalline formation, further post-treatment approaches (e.g., annealing) will be required.

Simultaneously high thermal stability and low power based on Cu-doped GeTe phase change material

The Cu-doped GeTe material was investigated systematically for its potential application in phase change memory. Compared with GeTe, Ge40Cu20Te40 material had higher crystallization temperature (258 °C) and activation energy for crystallization (3.78 eV). After Cu adding, the band gap energy increased and the crystallization was restrained. A smoother surface for Ge40Cu20Te40 material was achieved due to smaller grain size. The phase change memory device based on Ge40Cu20Te40 material could have electrical switching with the RESET voltage of 2.65 V and pulse width 5 ns. The good thermal stability and low power demonstrated the promising application for Ge-Cu-Te material in phase change memory.

Atomistic insights toward strengthening of GeTe phase change material by impurity doping and grain boundary engineering

The interplay between impurities (i.e., C and N) and the twin boundary (TB) in GeTe was systematically investigated by first principles calculations. The strong segregation propensity of C and N at TBs was demonstrated. Moreover, TBs were shown to restrain impurity diffusion, exerting a trapping effect on impurities. With the presence of impurities, the mechanical strength of TB was significantly enhanced. Such a strengthening effect arises from the strong covalent bonding between the impurity (C and N) and host atoms at TB. The present work provides atomic-scale understanding underlying impurity-induced TB strengthening and offers new insights based on the synergy between grain boundary engineering and impurity doping into designing more robust and stable phase-change material devices.

Dynamics of the phase-change material GeTe across the structural phase transition

Results of inelastic neutron scattering experiments and ab initio molecular dynamics simulations for GeTe–the parent compound of phase-change materials are reported. The inelastic neutron spectra of GeTe powder samples have been determined in the temperature range extending from 300 to 700 K. The phonon peaks undergo thermal shifts resulting from anharmonic effects being weaker for acoustic than optic modes. A small concentration of free charge carries arising from the presence of Ge-vacancies was found not to affect significantly the neutron weighted phonon densities of states of GeTe. The spectral pattern changes qualitatively across the structural phase transition, but the local structure of GeTe remains hardly affected, as confirmed by the analysis of temperature dependence of the pairdistribution function obtained from ab initio molecular dynamics investigations. The present theoretical studies support in a wide extent our experimental observations and also those provided by local probe methods.

Thermal Frequency Reconfigurable Electromagnetic Absorber Using Phase Change Material.

In this study, we propose a thermal frequency reconfigurable electromagnetic absorber using germanium telluride (GeTe) phase change material. Thermally-induced phase transition of GeTe from an amorphous high-resistive state to a crystalline low-resistive state by heating is used to change the resonant frequency of the absorber. For full-wave simulation, the electromagnetic properties of GeTe at 25 °C and 250 °C are characterized at 10 GHz under normal incidence for electromagnetic waves. The proposed absorber is designed based on the characterized electromagnetic parameters of GeTe. A circular unit cell is designed and GeTe is placed at a gap in the circle to maximize the switching range. The performance of the proposed electromagnetic absorber is numerically and experimentally demonstrated. Measurement results indicate that the absorption frequency changes from 10.23 GHz to 9.6 GHz when the GeTe film is altered from an amorphous state at room temperature to a crystalline state by heating the sample to 250 °C. The absorptivity in these states is determined to be 91% and 92%, respectively.

Investigating the Influence of Resonant Bonding on the Optical Properties of Phase Change Materials (GeTe)xSnSb2Se4

Thin film samples of Ge9SnSb2Te9Se4, Ge4.5SnSb2Te4.5Se4, Ge2.5SnSb2Te2.5Se4, and GeSnSb2TeSe4 were prepared via co-sputtering of GeTe and SnSb2Se4 and compared to the well-investigated phase change material GeTe. All samples were obtained in an amorphous state. Temperature-dependent in situ X-ray diffraction experiments reveal a crystallization temperature that increases with an increasing SnSb2Se4 content, leading to a higher stability of the amorphous phase. The electrical contrast between the amorphous and metastable crystalline state investigated via the van der Pauw method is as large as 4 orders of magnitude up to a 4.5:1 GeTe:SnSb2Se4 ratio. Increasing the SnSb2Se4 content leads to a decrease in the electrical contrast. Investigations of the samples by applying Fourier transform infrared spectroscopy and variable incident angle spectroscopic ellipsometry show that the optical properties of the amorphous phase are not affected by changes in stoichiometry. In striking contrast, the impact of SnSb2Se4...

In situ observation of the impact of surface oxidation on the crystallization mechanism of GeTe phase-change thin films by scanning transmission electron microscopy

The crystallization mechanisms of prototypical GeTe phase-change material thin films have been investigated by in situ scanning transmission electron microscopy annealing experiments. A novel sample preparation method has been developed to improve sample quality and stability during in situ annealing, enabling quantitative analysis and live recording of phase change events. Results show that for an uncapped 100 nm thick GeTe layer, exposure to air after fabrication leads to composition changes which promote heterogeneous nucleation at the oxidized surface. We also demonstrate that protecting the GeTe layer with a 10 nm SiN capping layer prevents nucleation at the surface and allows volume nucleation at a temperature 50 °C higher than the onset of crystallization in the oxidized sample. Our results have important implications regarding the integration of these materials in confined memory cells.

Impact of In doping on GeTe phase-change materials thin films obtained by means of an innovative plasma enhanced metalorganic chemical vapor deposition process

We investigated the deposition and the phase-change properties of In-doped GeTe thin films obtained by plasma enhanced metalorganic chemical vapor deposition and doped with indium using a solid delivery system. The sublimated indium precursor flow rate was calculated as a function of sublimation and deposition parameters. Indium related optical emission recorded by means of optical emission spectroscopy during deposition plasma allowed proposing the dissociation mechanisms of the [In(CH3)2N(CH3)2]2 solid precursor. In particular, using an Ar + H2 + NH3 deposition plasma, sublimated indium molecules are completely dissociated and do not induce by-product contamination by addition of nitrogen or carbon in the films. X-ray photoelectron spectroscopy evidences the formation of In-Te bonds in amorphous as-deposited In-doped GeTe films. The formation of an InTe phase after 400 °C annealing is also evidenced by means of X-ray diffraction analysis. The crystallization temperature Tx, deduced from monitoring of optical reflectivity of In-doped GeTe films with doping up to 11 at. % slightly varies as a function of the In dopant level with a decrease of Tx down to a minimum value for an In doping level of about 6–8 at. %. In this In doping range, the structure of crystallized In-GeTe films changes and is dominated by the presence of a crystalline In2Te3 phase. Finally, the Kissinger activation energy for crystallization Ea is showing to monotonically decrease as the indium content in the GeTe film is increased indicating a promising effect of In doping on crystallization speed in memory devices while keeping a good thermal stability for data retention.

Insight into the role of oxygen in the phase-change material GeTe

Oxygen in GeTe induces the formation of defect clusters in the crystallization process of the amorphous state, which slows down the crystallization speed and improves the data retention of phase change material GeTe.