GeTe Thin Films Research Articles

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

Phase-change memory emerges as a top contender for non-volatile data storage applications. We report here a systematic change in local structure and crystallization kinetics of binary GeTe thin films using temperature-dependent resistivity measurements, which offers single-stage crystallization at around 187 °C, corroborated with x-ray diffraction. Furthermore, the change in chemical bonding upon crystallization is determined through x-ray photoelectron spectroscopy core level spectra, which reveals the existence of Ge and Te components that align with the GeTe crystal structure. Also, an investigation was carried out employing a UV–Vis–NIR spectrophotometer to explore the evolution of optical bandgaps (Eg), Tauc parameter (B) representing the local disorder, and Urbach energy (Eu) of the GeTe material, as it undergoes the transition from a disordered amorphous state to a crystalline state. As crystallization progresses, a consistent shift of Eg from 0.92 to 0.70 eV corresponds to as-deposited amorphous at room temperature and crystalline at 250 °C, respectively. In addition, the reduction in Eu (from 199.87 to 141.27 meV) and a sudden increase of B around crystallization temperature is observed upon increasing temperature, indicating direct observation of enhanced medium-range order and distortion in short-range order, respectively, in GeTe thin films, revealing improved structural and optical properties. These enhancements make the GeTe material ideal for data storage applications of phase-change memory for next-generation computing technology.

Thickness dependence and crystallization properties of amorphous GeTe thin films on silicon dioxide

Radio frequency magnetron sputtering was used to prepare the amorphous GeTe thin films on silicon dioxide and the thickness effects on the crystallization behavior were investigated. With the film thickness reducing, the crystallization temperature, crystallization activation energy, amorphous and crystalline resistance increase remarkably, indicating the great improvement in thermal stability and power consumption. Ozawa’s model was used to estimate the crystallization kinetics of GeTe thin films, it shows that nucleation and grain growth occur simultaneously, and grain growth dominates ultimately. XRD analysis demonstrated that the grain size can be reduced and the crystallization process of GeTe thin film can be inhibited with the film thickness decreasing. Furthermore, the thinner film has smaller resistance drift index and surface roughness, which are beneficial to improve the reliability of storage device. T-type phase change memory devices based on 25 nm GeTe thin film were fabricated by 0.13 μm CMOS technology, and the current–voltage and resistance-voltage characteristics demonstrate the excellent electrical performance, including the fast resistance switching between SET and RESET processes, low threshold current and voltage. All the results proved the strong dependency relationships between the crystallization properties and film thickness of GeTe thin film, which paves the way for developing high-density phase change memory in the fields of big data and artificial intelligence.

Domain Wall Migration-Mediated Ferroelectric Switching and Rashba Effect Tuning in GeTe Thin Films

Domain Wall Migration-Mediated Ferroelectric Switching and Rashba Effect Tuning in GeTe Thin Films

Propagon boundary scattering relaxed via crystalline host on multiphase germanium telluride

The movement of heat through amorphous solids on an atomic level remains an outstanding question. Recent studies suggest that the primary thermal carrier in amorphous materials, propagons, essentially behaves like phonons. In this work, we provide experimental evidence that shows the interaction between propagons and phonons by utilizing the phase change chalcogenide germanium telluride. A series of ultra-long time-delay time-domain thermoreflectance measurements are used to analyze the scattering of vibrational thermal carriers at the boundaries of amorphous GeTe thin films relative to scattering across a crystalline-amorphous bilayer. We find that amorphous long wavelength propagons that would otherwise scatter can instead be hosted by a crystalline underlayer and its phonon population. This experimental evidence directly demonstrates propagon–phonon interactions in a clear experimental manner.

INVESTIGATION THE IMPACT OF DIFFERENT ANNEALING TEMPERATURES ON STRUCTURAL AND OPTICAL PROPERTIES OF BI-DOPED GETE (BI: 5 %) THIN FILMS FABRICATED BY PVD TECHNIQUE)

The impact of different annealing temperatures on the Bi-doped GeTe thin films were investigated. The thin films have been prepared by using physical vapor deposition techniques (PVD). The Bismuth (Bi) is doped GeTe with ratio of 5%. Different characterizations techniques have been used to study the different properties of thin films with several annealing temperatures. It has been found that the film thickness decreases as the annealing temperature increases. The XRD patterns show that as-deposited and annealed Bi-doped GeTe films at 150°C,200°C, and 250°C were fully amorphous, while the film annealed at 100 °C was crystalline. FESEM image shows that the structure is amorphous with no grain appearing for the crystallite GeTe compound. Whereas the annealed thin films at 100°C are well-appeared crystallites of GeTe with an average size of (110.64 nm). The thin films are annealed at (150, 200, and 250)°C which reveals that the crystallite or grain is increased. An increase in the annealing temperature has been found to cause a significant shift in the absorption edge toward an extended wavelength and an overall reduction in transmittance. At a wavelength of 1100 nm, the transmittance dropped from 65.25% for as-deposited thin films to 32.57% for annealed thin films at 250 °C. Furthermore, when the annealing temperature rises from 100°C to 250°C, the optical band gap reduces from 0.95 eV to 0.42 eV.

Self-doping enhancing thermoelectric properties of GeTe thin films

The thermoelectric film has broad application potential in the self-power supply of miniature electrical equipment. In this work, GeTe thermoelectric films were prepared using physical vapor deposition combined with annealing processes. Benefitting from the high mobility enabled by the increased crystallinity and the optimized carrier concentration via Ge self-doping, the power factor of a GeTe thin film was significantly improved to 18 μW cm−1 K−2 (300 K), and the maximum one (28 μW cm−1 K−2) was achieved at 576 K. Furthermore, thermoelectric thin film devices assembled with high-performance GeTe films exhibited superior output performance at a temperature difference of 40 K. The maximum open circuit voltage reached 12.2 mV and the power density was 2.4 mW cm−2, indicating that GeTe thin films have broad application prospects in the field of self-power supply.

Optimization of electrical properties and radio frequency applications of GeTe thin film

GeTe belongs to a chalcogenide phase change material, which can dynamically achieve reversible switching between the crystalline state of low resistivity and the amorphous state of high resistivity by utilizing the thermally induced phase change characteristics. The GeTe is an important functional material in the fields of memristors and nonvolatile radio frequency (RF) switches. For RF switch applications, this paper focuses on optimizing the electrical performance of GeTe thin films prepared by magnetron sputtering. By comprehensively analyzing the effects of substrate materials, sputtering conditions, and annealing conditions on the resistivity of crystalline GeTe films, effective conditions for preparing low resistivity GeTe films are explored. Fig. (a) shows that compared with the GeTe film on a SiO<sub>2</sub> substrate, the film on an Al<sub>2</sub>O<sub>3</sub> substrate can obtain higher crystallinity and lower resistivity. For the deposition power and pressure shown in Fig. (b), the combination of medium power (50–80 W) and low pressure (2–3 mTorr) is beneficial for low crystalline resistivity of GeTe film. Additionally, Fig. (c) shows that higher annealing temperature (350–400 ℃) can realize lower film resistivity. Finally, the experimental results show that the lowest crystalline resistivity of the prepared GeTe thin film reaches 3.6×10<sup>–6</sup> Ω·m, and the resistance ratio is more than 10<sup>6</sup>. Based on rectangular chips of GeTe film, a parallel millimeter-wave switch with zero static power is also constructed. As shown in Fig. (d), the insertion loss is less than 2.4 dB, and the isolation is greater than 19 dB in a 1–40 GHz frequency band, demonstrating the potential application of GeTe thin films in the field of broadband high-performance discrete nonvolatile RF switches.

Thermomechanic behavior of epitaxial GeTe ferroelectric films

A key development toward new electronic devices integrating memory and processing capabilities could be based on the electric control of the spin texture of charge carriers in semiconductors. In that respect, GeTe has been recently recognized as a promising ferroelectric Rashba semiconductor, with giant spin splitting of the band structure, due to the inversion symmetry breaking arising from ferroelectric polarization. Here, we address the temperature dependence of the ferroelectric structure of GeTe thin films grown on Si(111). We demonstrate the hysteretic behavior of the ferroelectric domain density upon heating/cooling cycles by low energy electron microscopy. This behavior is associated with an abnormal evolution of the GeTe lattice parameter as shown by x-ray diffraction. We explain these thermomechanical phenomena by a large difference of thermal expansion coefficients between the film and the substrate and to the pinning of the GeTe/Si interface. The accumulated elastic energy by the GeTe thin film during sample cooling is released by the formation of a-nanodomains with in-plane ferroelectric polarization components.

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

High Thermoelectric Performance of Non-Stoichiometric and Oriented GeTe Thin Films.

The increasing demand for micro-thermoelectric coolers and generators promotes the research on thermoelectric (TE) thin films. As a promising medium-temperature TE material, GeTe has attracted wide attention recently. However, the thermoelectric performance of thin-film GeTe remains inferior. Herein, oriented GeTe films with excessive Ge are obtained by magnetron co-sputtering technique, which can not only reduce the carrier concentration but also increase the carrier mobility, maintaining the high electrical conductivity of GeTe. Furthermore, higher structural symmetry and grain boundary scattering enhance the Seebeck coefficient of oriented GeTe films. As a result, the power factor (PF) value can reach as high as 2848 µW m-1 K-2 at room temperature and increase to 5263 µW m-1 K-2 at 600 K. Furthermore, a TE device with the Ge-rich GeTe thin film is fabricated and the maximum output power density (power per unit area) reaches 0.3W cm-2 at ΔT = 250 K. This work demonstrates that the stoichiometry and orientation modulations are effective strategies to improve the thermoelectric performance of GeTe thin films.

Nano-Scanning Calorimetry Applied to Phase Change Processes in GeTe Thin Films

The parallel nano-scanning calorimeter (PnSC) is a device for calorimetric measurement of nanoscale materials. Compared with traditional scanning calorimetry of bulk materials, PnSC has a faster heating rate and higher sensitivity when testing small masses of samples. Here, nano-scanning calorimetry is used to investigate the phase transition behavior of germanium telluride (GeTe). Amorphous GeTe is deposited on the heating element of PnSC, and the sample is analyzed in the process of heating crystallization. At a heating rate of approximately 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{3}}$</tex-math> </inline-formula> K/S, the amorphous deposited GeTe changes to rhombic GeTe at 185 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C and to cubic GeTe as the temperature continues to rise to 412 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C. The phase change process of GeTe was also determined using conventional differential scanning calorimetry (DSC). The obtained phase change enthalpies and phase change temperatures were compared with those determined by nano-scanning calorimetry. The composition of the low and high-temperature phases was determined by X-ray diffraction and Raman spectroscopy.

Novel four-port RF phase change switches based on GeTe thin film

An indirect-heated phase-change switch (PCS) using germanium telluride (GeTe) has been fabricated using thermal actuation driven by thin film heater on the model. Switches require a low ON-state resistance and a high OFF-state resistance with OFF/ON resistance ratio of 105. The finite element analysis simulation is applied to simulate the temperature of individual node GeTe with different microwave heating pulses. Finally, in order to reduce the phase-change time and increase the switching speed of indirectly heated switching structures, a new four-port indirectly heated phase change switching structure is proposed. In this paper, the heat dissipation of the switch is increased by etching deep grooves on the back of the switch. This structure obviously reduces the phase change time compared to conventional indirectly heated phase change switches the time between ON-state and OFF-state is reduced by more than 19% and the total process is reduced by more than 47%. The GeTe PCSs with etched grooves not only significantly increases the switching speed, but also reduces the risk of recrystallization of the phase change material.

Boosting Thermoelectric Performance in Epitaxial GeTe Film/Si by Domain Engineering and Point Defect Control

This study demonstrates a simultaneous realization of ultralow thermal conductivity and high thermoelectric power factor in epitaxial GeTe thin films/Si substrates by a combination of the interface introduction by domain engineering and the suppression of Ge vacancy generation by point defect control. We formed epitaxial Te-poor GeTe thin films having low-angle grain boundaries with a misorientation angle close to 0° or twin interfaces with a misorientation angle close to 180°. The control of interfaces and point defects gave rise to ultralow lattice thermal conductivity of ∼0.7 ± 0.2 W m-1 K-1. This value was the same in the order of magnitude as the theoretical minimum lattice thermal conductivity of ∼0.5 W m-1 K-1 calculated by the Cahill-Pohl model. At the same time, the GeTe thin films exhibited a high thermoelectric power factor because of the suppression of Ge vacancy generation and a small contribution of grain boundary carrier scattering. The outstanding combined technique of domain engineering and point defect control can be a great approach for developing high-performance thermoelectric films.

Influence of molybdenum doping on the structural, electrical, and optical properties of germanium telluride thin films

This study investigates the influence of molybdenum (Mo) doping on the local atomic structure, morphology, and electrical properties of amorphous germanium telluride (GeTe) thin films. Structural and morphological studies show that Mo-doping inhibits the crystallization of amorphous GeTe thin films. Moreover, the crystallization temperature of the GeTe thin film increases from 197 to 317 °C with increasing Mo dopant concentration. The electrical properties of Mo-doped GeTe thin films are discussed in terms of the capacitance–voltage characteristics in temperatures range 27–357 °C. The optical bandgap values decrease from 0.67 to 0.55 eV when increasing Mo doping in GeTe thin film. Mo integration significantly impacts the Ge–Te bond lengths and structural symmetry, increasing the degree of disorder in amorphous GeTe thin films. This study demonstrates the effectiveness of Mo doping in enhancing the thermal stability of the amorphous phase of GeTe for its envisioned memory and brain-inspired computing applications.

Understanding crystallization in undoped and nitrogen doped GeTe thin films using substrate curvature measurements

Herein, we study the crystallization of undoped and nitrogen doped amorphous GeTe thin films (slightly rich in Ge) obtained by sputtering using substrate curvature measurements to understand the underlying mechanisms controlling stress evolution in the film throughout the phase transformation. At temperatures below crystallization temperature, amorphous films showed stress relaxation and the stress gradually became tensile with annealing time. The GeTe samples show a two-step crystallization wherein amorphous GeTe crystallized first (at the crystallization temperature Tx) followed by crystallization of excess Ge (Ge precipitation) at -Tx+50 °C. Upon GeTe crystallization, a sharp increase in the tensile stress is explained using a coalescence mechanism. This interpretation resolves the issue of the discrepancy between the measured stress buildup reported by several authors and the predicted stress jump from elastic accommodation of density change. The precipitation of excess Ge (from amorphous to crystalline) along grain boundaries in GeTe leads to compressive stress build-up. Nitrogen doping affects both the GeTe and Ge crystallization events leading to lesser tensile and compressive stress. The models for stress relaxation in the amorphous phase, stress build-up due to GeTe, and excess Ge crystallization are discussed.

Broadband and ultrafast terahertz modulation with GeTe thin films

Chalcogenide phase change materials (PCMs) are gaining much interest for optoelectronic applications because of their capability to perform nonvolatile phase transition between the amorphous and crystalline states under Joule heat generated by applied electrical bias or optical excitation. Utilizing the high dielectric tunability of PCMs, we experimentally demonstrated the terahertz (THz) modulation with germanium telluride (GeTe) thin films. With THz time-domain spectroscopy (THz-TDS), we showed that the transmission of the 250-nm GeTe film can be tuned from 95% (amorphous) to 35% (crystallized) in the range of 0.1–1.2 THz. The high optical contrast of the GeTe film was also confirmed by Fourier transform infrared spectrometer and scattering-type scanning near-field optical microscope at higher frequencies. Ultrafast time-resolved optical pump–THz probe experiments were performed with 400-nm laser pulses. The amorphous and crystalline phases of GeTe exhibit different volatile THz modulation capabilities on the picosecond time scale and their decay times are 1.39 ps and 0.73 ps, respectively.

Investigation of thermal stability and crystallization mechanism of Er0.03(GeTe)0.97 phase change material

Er-doped GeTe thin films with different concentrations were prepared by magnetron co-sputtering technique. The chemical compositions of Er0.03(GeTe)0.97 thin films were measured by energy dispersive spectroscopy. The difference between the phase transition behavior of Er0.03(GeTe)0.97 and pure GeTe films was investigated by in-situ electrical measurements. The crystallization temperature, crystallization resistance and optical band gap of GeTe thin films obviously increase with the doping of Er elements. X-ray diffraction and x-ray photoelectron spectroscopy show that proper Er doping can inhibit grain growth and reduce grain size. The surface morphology of Er0.03(GeTe)0.97 and pure GeTe films were observed by atomic force microscopy and it was found that the surface of the film becomes smoother after Er doping. The flatter surface of Er0.03(GeTe)0.97 material means better interfacial properties and reliability. All the outcomes indicate that the proper doping of Er element can effectively improve the comprehensive performance of GeTe thin films for high thermal stability applications.

Polar surface of ferroelectric nanodomains in GeTe thin films

Ferroelectrics have polar surfaces that can undergo large structural and stoichiometric modifications to be neutral. These changes can have major implications on the surface stability and physicochemical properties. We have studied the growth and structure of ferroelectric GeTe thin films on Si(111) by a combination of scanning tunneling microscopy, low-energy electron microscopy, and low-energy electron diffraction. We show that GeTe growth occurs with a single epitaxy and proceeds via a step-flow mode hindered by the advance of electrically neutral step edges exhibiting triangular notches. We demonstrate the presence of ferroelectric nanodomains with in-plane components of polarization and a complex restructuring of their polar surface. $2\ifmmode\times\else\texttimes\fi{}2$ surface reconstruction, missing row reconstruction and extended 2D modulations of the surface structure are demonstrated on these nanodomains. We show that these structures stabilize the surface termination of the low-symmetry polar nanodomains.

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.

Model of a multilayer coating for an artificial optical synapse

Optical characteristics of germanium telluride (GeTe) thin films in multilayer structures are calculated. A model of a multilayer optical coating with a four-level transmission coefficient is developed. Based on the calculated data, thickness values are determined at which the transmittance change is greatest for 1550-nm modulated optical radiation. Experimental samples coated with antireflection GeTe films are synthesized and their optical transmission characteristics are investigated. Combinations of parameters of the multilayer structures for the implementation of contrast transmittance levels are determined. The results of the study represent the implementation of a multilevel artificial optical synapse for neuromorphic processors.