GeTe Thin Films Research Articles

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.

Influence of substrate dimensionality on the growth mode of epitaxial 3D-bonded GeTe thin films: From 3D to 2D growth

The pseudo-binary line of Sb2Te3-GeTe contains alloys featuring different crystalline characteristics from two-dimensionally (2D-) bonded Sb2Te3 to three-dimensionally (3D-) bonded GeTe. Here, the growth scenario of 3D-bonded GeTe is investigated by depositing epitaxial GeTe thin films on Si(111) and Sb2Te3-buffered Si(111) substrates using pulsed laser deposition (PLD). GeTe thin films were grown in trigonal structure within a temperature window for epitaxial growth of 210–270 °C on unbuffered Si(111) substrates. An unconventional growth onset was characterized by the formation of a thin amorphous GeTe layer. Nonetheless, the as-grown film is found to be crystalline. Furthermore, by employing a 2D-bonded Sb2Te3 thin film as a seeding layer on Si(111), a 2D growth of GeTe is harnessed. The epitaxial window can substantially be extended especially towards lower temperatures down to 145 °C. Additionally, the surface quality is significantly improved. The inspection of the local structure of the epitaxial films reveals the presence of a superposition of twinned domains, which is assumed to be an intrinsic feature of such thin films. This work might open a way for an improvement of an epitaxy of a 3D-bonded material on a highly-mismatched substrate (e.g. Si (111)) by employing a 2D-bonded seeding layer (e.g. Sb2Te3).

Self-screening induced abnormal stability of ferroelectric phase in GeTe ultrathin films

A stable ferroelectric phase or Curie temperature (Tc) above room temperature is essential for ferroelectric devices. However, the fast Tc drop of ferroelectric thin films has become a vital factor inhibiting the scaling down of ferroelectric devices. Many efforts have been made to slow down the Tc drop in conventional insulating ferroelectric thin films, but they are costly and complex. A semiconducting ferroelectric, GeTe, is proposed in this letter, which shows an abnormal stability of the ferroelectric phase as the film thickness decreases to as thin as 3 nm. The ferroelectric Curie temperatures of GeTe thin films with different thicknesses have been studied using variable-temperature Raman measurements. The self-screening process of free carriers provided by intrinsic Ge vacancies in GeTe films plays an important role in cancelling the depolarization field according to first principles calculations and results in an abnormal stability of ferroelectricity in ultrathin GeTe films. Self-screening by free carriers in GeTe unravels that the ferroelectric phase could remain stable in low dimension semiconducting ferroelectrics. These results provide a straightforward solution for high density ferroelectric devices and show promising application of semiconducting ferroelectrics in large current ferroelectric diodes.

In situ monitoring of stress change in GeTe thin films during thermal annealing and crystallization

Abstract Stress changes in GeTe thin films on silicon have been studied in situ as a function of temperature by optical curvature measurements. Crystallization of the initially amorphous layers is evidenced by a steep tensile stress buildup. The crystallization temperature is shown to be thickness-dependent for the thinner films. Various annealing conditions, such as cooling/re-heating steps and isothermal stages, allow exploring the thermo-mechanical behavior of the films. A non-thermoelastic temperature-dependent behavior is observed in the amorphous phase before crystallization.

Ferroelastic-strain-induced multiple nonvolatile resistance states in GeTe/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructures

We prepared 300-nm GeTe thin films on (111)-oriented and piezoelectrically active 0.71 Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-0.29 PT) single-crystal substrates by the pulsed laser deposition and investigated the effects of in situ electric-field-controllable non-180° ferroelastic domain switching of the PMN-0.29 PT on the electronic properties of the GeTe films. The in-plane strain of the PMN-0.29 PT could be modulated continuously and reversibly by electric fields in a nonvolatile manner and could be effectively transferred to the GeTe films. Based on this, we realized reversible and nonvolatile resistance switching and obtained multilevel stable nonvolatile resistance states with good stability and endurance at T = 300 K by applying appropriate asymmetrical bipolar electric fields to the PMN-0.29 PT(111) substrates along the thickness direction. Such heterostructures may be used for multilevel data storage that allows each unit to store multiple bits of information and thus improve the memory density. Our investigation would be beneficial for the fabrication of nonvolatile memory devices using PMN-xPT-based heterostructures.

Tailoring the epitaxy of Sb2Te3 and GeTe thin films using surface passivation

Depending on the substrate surface termination the epitaxy of chalcogenide thin films can be drastically altered. While GeTe grows with many randomly oriented domains on H-terminated Si(111), the in-plane alignment is significantly improved on Sb-terminated Si(111).

Atomic-scale study of the amorphous-to-crystalline phase transition mechanism in GeTe thin films

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.

Realization of Multilevel States in Phase‐Change Thin Films by Fast Laser Pulse Irradiation

Multilevel storage techniques are promising for increasing storage density and for reducing energy consumption in the application of phase‐change materials based memory devices. However, accurately controlling the phase transitions as well as understanding the underlying switching mechanisms are still under investigation. In this study, nonvolatile optical multilevel switching in single‐layered GeTe phase‐change films prepared by laser ablation is demonstrated to be feasible and accurately controllable at a time scale of nanoseconds. For this purpose, an ns UV laser pump‐probe setup is adapted for the investigations. It is found that each laser pulse excitation (duration: 20 ns, wavelength: 248 nm) with a fluence of 26 mJ cm−2 induced a partial crystallization whereas complete crystallization is achieved by superposition of single pulses. In the reverse process, a single pulse excitation at a fluence of 112 mJ cm−2 leads to reamorphization of GeTe thin films. Regarding the switching times, complete crystallization and reamorphization are determined to be accomplished after ≈300 ns and ≈3 ns, respectively. Furthermore, it is shown that the local microstructure phase transformation processes are correlated to the observed reversible switching of the optical reflectivity, which are important for understanding the underlying mechanisms of the phase‐change‐materials‐based multilevel phase‐change memory.

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.

Nanopatterning of GeTe phase change films via heated-probe lithography.

The crystallization of amorphous germanium telluride (GeTe) thin films is controlled with nanoscale resolution using the heat from a thermal AFM probe. The dramatic differences between the amorphous and crystalline GeTe phases yield embedded nanoscale features with strong topographic, electronic, and optical contrast. The flexibility of scanning probe lithography enables the width and depth of the features, as well as the extent of their crystallization, to be controlled by varying probe temperature and write speed. Together, these technologies suggest a new approach to nanoelectronic and opto-electronic device fabrication.

Nanosecond Switching in Superlattice-Like GeTe/Sb Thin Film for High Speed and Low Power Phase Change Memory Application

Superlattice-like GeTe/Sb thin films were investigated for their potential application in phase change memory. Compared with monolayer GeTe thin film, GeTe/Sb thin film has lower crystallization temperature (208°C), crystallization activation energy (2.38 eV) and thermal conductivity (0.15 W/(m·K)). In GeTe/Sb thin film, GeTe and Sb phases exist independently. The superlattice-like structure is observed even after annealing at 250°C for 10 min. The shorter phase change time is achieved for GeTe/Sb thin film (4.3 ns). The GeTe/Sb-based phase change memory devices show an operation speed of as fast as 9 ns and a low RESET operation power of 4.09 × 10−13 J.

The Effect of InP Doping on Phase Transformation in GeTe Thin Films

The Effect of InP Doping on Phase Transformation in GeTe Thin Films

Atomic Layer Deposition of GeTe Films Using Ge{N[Si(CH3)3]2}2, {(CH3)3Si}2Te, and Methanol

In this report, a new method to deposit GeTe thin film by atomic layer deposition (ALD) is described. Ge(N(Si(CH3)3)2)2, in which Ge is in +2 oxidation state, and ((CH3)3Si)2Te were used as Ge and Te precursors, respectively. GeTe films were deposited at a low wafer temperature of 70–120 °C. To improve low reactivity of Ge(N(Si(CH3)3)2)2 toward ALD-type reaction with ((CH3)3Si)2Te, methanol vapor was coinjected with the Ge precursor and the Te precursor to form a reactive Ge intermediate having methoxy ligands and a Te intermediate having hydrogen ligands in the gas phase. This chemistry-specific ALD process deposited films various compositional GeTe alloys including the desired composition of Ge:Te = 1:1. Detailed mechanism study on the probable chemical reactions and film composition analysis revealed that the film growth could proceed via the formations of GeTe2, GeTe, and Ge2Te phases depending on the relative Ge-precursor concentration during the ALD process. The films showed feasibility for phase-ch...

Thermal conductivity of carbon doped GeTe thin films in amorphous and crystalline state measured by modulated photo thermal radiometry

The thermal conductivity and thermal boundary resistance of GeTe and carbon doped GeTe thin films, designed for phase change memory (PCM) applications, were investigated by modulated photo thermal radiometry. It was found that C doping has no significant effect on the thermal conductivity of these chalcogenides in amorphous state. The thermal boundary resistance between the amorphous films and SiO2 substrate is also not affected by C doping. The films were then crystallized by an annealing at 450°C as confirmed by optical reflectivity analysis. The thermal conductivity of non-doped GeTe significantly increases after crystallization annealing. But, surprisingly the thermal conductivity of the crystallized C doped GeTe was found to be similar from that of the amorphous state and independent of C concentration. As for the amorphous phase, C doping does not affect the thermal boundary resistance between the crystalline GeTe films and SiO2 substrate. This behaviour is discussed thanks to XRD and FTIR analysis. In particular, XRD shows a decrease of crystalline grain size in crystalline films as C concentration is increased. FTIR analysis of the film before and after crystallization evidenced that this evolution could be attributed to the disappearing of Ge-C bonds and migration of C atoms out of the GeTe phase upon crystallization, limiting then the growth of GeTe crystallites in C-doped films.

Tunable split-ring resonators using germanium telluride

We demonstrate terahertz (THz) split-ring resonator (SRR) designs with incorporated germanium telluride (GeTe) thin films. GeTe is a chalcogenide that undergoes a nonvolatile phase change from the amorphous to crystalline state at approximately 200 °C, depending on the film thickness and stoichiometry. The phase change also causes a drop in the material's resistivity by six orders of magnitude. In this study, two GeTe-incorporated SRR designs were investigated. The first was an SRR made entirely out of GeTe and the second was a gold SRR structure with a GeTe film incorporated into the gap region of the split ring. These devices were characterized using THz time-domain spectroscopy and were heated in-situ to determine the change in the design operation with varying temperatures.

Impact of interfaces on scenario of crystallization of phase change materials

Chalcogenide phase change materials (PCMs), such as Ge-Sb-Te alloys, have outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, more recently, in Phase Change Random Access Memories. The latter are the most promising candidate to replace the current FLASH memories. The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In the present work, a new picture of the prototypical GeTe and Ge2Sb2Te5 thin film crystallization is provided from optical reflectivity, resistivity and Scanning Transmission Electron Microscopy studies of 100 nm thick films. The crystallization temperature Tx significantly varies as a function of surface oxidation. Whereas Ge2Sb2Te5 has been, and is still, the subject of many publications, the present results reveal that its crystallization mechanism was not fully understood. Heterogeneous crystallization of Ge2Sb2Te5 at the upper film surface, reported so far, results from a surface alteration. Adequately protected Ge2Sb2Te5 and GeTe crystallize through nucleation inside the amorphous phase. Preventing surface alteration due to oxidation as demonstrated in the present study, through suitable capping layer deposition methods, yields a spectacular increase in Tx above the values that have been reported in the literature. The present study demonstrates that interface engineering allows one to select the crystallization mechanism, and hence control the stability of the amorphous phase in PCMs.

Structural change with the resistance drift phenomenon in amorphous GeTe phase change materials’ thin films

Ageing of the amorphous phase of chalcogenide phase change materials is characterized by a large increase of their resistivity with time. This phenomenon, known as resistance drift and commonly attributed to structural relaxation, the nature of which remains unknown, has until now hindered the development of ultra-high multilevel storage devices. The origin of the resistance drift of amorphous GeTe thin films is studied here by resistivity measurements and grazing incidence x-ray absorption spectroscopy (GIXAS). The local order around Ge atoms is investigated at the Ge K-edge on a-GeTe samples previously set at different resistance drift levels by thermal annealing. In all samples, Ge–Ge and Ge–Te bonds coexist. This study demonstrates that the drift phenomenon is concomitant with structural changes linked to Ge–Ge homopolar bonds.

In-situ characterization of the optical and electronic properties in GeTe and GaSb thin films

GeTe and GaSb thin films obtained by pulsed laser deposition were investigated by spectroscopic ellipsometry at controlled temperatures. The GeTe films were fully amorphous, while the GaSb films were partially crystalized in the as-deposited state. The Tauc-Lorentz model was employed to fit the experimental data. From the temperature study of the optical constants, it was observed the crystallization in the 150–160 °C range of GeTe amorphous films and between 230 and 240 °C of GaSb amorphous phase. A second transition in the resonance energy and the broadening parameter of the Lorentz oscillator was observed due to the crystallization of Sb after 250 °C. The temperatures of 85 °C and 130 °C are noticed as the start of the relaxation of the amorphous GeTe phase and as-deposited GaSb. The peaks of the imaginary part of the dielectric function red shifted after the phase change, while the variation with temperature of the crystalline phase follows the Varshni law. The electron-phonon coupling constants are 2.88 and 1.64 for c-GeTe and c-GaSb, respectively. An optical contrast up to 60% was obtained for GeTe films and a maximum value of 7.5% is revealed in the case GaSb, which is altered by the partial crystallinity of the as-deposited films.

Improved terahertz modulation using germanium telluride (GeTe) chalcogenide thin films

We demonstrate improved terahertz (THz) modulation using thermally crystallized germanium telluride (GeTe) thin films. GeTe is a chalcogenide material that exhibits a nonvolatile, amorphous to crystalline phase change at approximately 200 °C, as well as six orders of magnitude decreased electrical resistivity. In this study, amorphous GeTe thin films were sputtered on sapphire substrates and then tested using THz time-domain spectroscopy (THz-TDS). The test samples, heated in-situ while collecting THz-TDS measurements, exhibited a gradual absorbance increase, an abrupt nonvolatile reduction at the transition temperature, followed by another gradual increase in absorbance. The transition temperature was verified by conducting similar thermal tests while monitoring electrical resistivity. THz transmittance modulation data were investigated between 10 and 110 cm−1 (0.3–3.3 THz). A peak modulation of approximately 99% was achieved at 2.3 THz with a 100 nm GeTe film on a sapphire substrate. After isolating the sapphire and the crystalline GeTe (c-GeTe) absorbance contributions, the results showed THz modulations ranging from 88.5% to 91.5% that were attributed solely to the single layer of transitioned c-GeTe. These results strongly motivate using GeTe or other chalcogenide thin films in THz modulators, filters, and metamaterial applications.

Crystallization kinetics of GeTe phase-change thin films grown by pulsed laser deposition

Pulsed laser deposition was employed to the growth of GeTe thin films on Silicon substrates. X-ray diffraction measurements reveal that the critical crystallization temperature lies between 220 and 240 °C. Differential scanning calorimetry was used to investigate the crystallization kinetics of the as-deposited films, determining the activation energy to be 3.14 eV. Optical reflectivity and in situ resistance measurements exhibited a high reflectivity contrast of ~21% and 3–4 orders of magnitude drop in resistivity of the films upon crystallization. The results show that pulsed laser deposited GeTe films can be a promising candidate for phase-change applications.