GeTe Films Research Articles

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.

Tuning electronic properties of graphene heterostructures by amorphous-to-crystalline phase transitions

The remarkable ability of phase change materials (PCM) to switch between amorphous and crystalline states on a nanosecond time scale could provide new opportunities for graphene engineering. We have used density functional calculations to investigate the structures and electronic properties of heterostructures of thin amorphous and crystalline films of the PCM GeTe (16 \AA{} thick) and ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (20 \AA{}) between graphene layers. The interaction between graphene and PCM is very weak, charge transfer is negligible, and the structures of the chalcogenide films differ little from those of bulk phases. A crystalline GeTe (111) layer induces a band gap opening of 80 meV at the Dirac point. This effect is absent for the amorphous film, but the Fermi energy shifts down along the Dirac cone by $\ensuremath{-}60$ meV. ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ shows similar features, although inherent disorder in the crystalline rocksalt structure reduces the contrast in band structure from that in the amorphous structure. These features originate in charge polarization within the crystalline films, which show electromechanical response (piezoelectricity) upon compression, and show that the electronic properties of graphene structures can be tuned by inducing ultrafast structural transitions within the chalcogenide layers. Graphene can also be used to manipulate the structural state of the PCM layer and its electronic and optical properties.

CHF3/O2-Based Plasma Reactive Ion Etching of GeTe for Nonvolatile Phase Change Memory

The reactive-ion-etching (RIE) characteristics of phase change material GeTe in CHF3/O2 plasma for back end of nonvolatile phase change memory devices were investigated in this paper. The etch rate and surface root-mean-square (RMS) roughness of crystalline GeTe and amorphous GeTe films were studied with various etching parameters, such as gas mixture ratio, chamber gas pressure, and power. It is found that the etch rate first increases with the increasing concentration of O2 and get at peak value 68.5 nm/min at 12% O2/(CHF3+O2), then decreases from 12% to 20% of O2 fraction. The surface RMS roughness slightly increases with the increasing oxygen content. By changing the gas pressure, the etch rate of GeTe films increases approximately linearly with increasing gas pressure up to 50 mTorr. In addition, the etch rate increases approximately linearly with increasing power below 400 W. The XPS spectra shows that some non-volatile GeFx, GeOx, TeFx, and TeOx by-products were left on the etched film surface. From these results of etched TiN films and GeTe films, the result of RIE with one step recipe shows the formation of under-cut on the profile of etched film. However, the result of RIE with two steps recipe shows better vertical characteristic and few residues were left on the profile of etched film.

Sub 10 ns fast switching and resistance control in lateral GeTe-based phase-change memory

In this study, we investigated the fast switching and resistance control in a lateral GeTe-based phase-change memory (PCM). The resistivity of GeTe as a function of annealing temperature showed that it changed by more than 6 orders of magnitude in a very narrow temperature range. X-ray diffraction patterns of GeTe films indicated that GeTe had only one crystal structure, that is, face-centered cubic. It was demonstrated that the lateral device with a top conducting layer had a good performance. The operation characteristics of the GeTe-based lateral PCM device showed that it could be operated even when sub-10-ns voltage pulses were applied, making it much faster than a Ge2Sb2Te5-based device. The device resistance was successfully controlled by applying a staircase-like pulse, which enables the device to be used for fast multilevel storage.

Etching characteristics of phase change material GeTe in inductively coupled BCl3/Ar plasma for phase change memory

The dry etching characteristics of phase change material GeTe were investigated in inductively coupled BCl3/Ar plasma. By changing gas ratio, gas pressure, substrate bias power, and inductively coupled plasma (ICP) source power, respectively, various characteristics of GeTe films were investigated about surface roughness, etch rate and profiles. Etching damage was studied by analyzing the X-ray photoelectron spectroscopy results of etched blank GeTe films. We found that the etch rate increases with the increase of BCl3 content and substrate bias power. However, it first increases then decreases with the increase of gas pressure and ICP power. Surface becomes smooth with the increase of gas pressure, but the higher power and substrate bias power lead rougher surface. Little C contamination, oxidation and halogenated layer were remained on the surface during etching process, which can be removed easily by Ar+ sputtering. The stoichiometric ratio of GeTe is stable after being sputtered in tens of seconds on the etched surface, indicating the etching damage is low.

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.

Interplay of spin–orbit coupling and superconducting correlations in germanium telluride thin films

There is much current interest in combining superconductivity and spin–orbit coupling in order to induce the topological superconductor phase and associated Majorana-like quasiparticles which hold great promise towards fault-tolerant quantum computing. Experimentally these effects have been combined by the proximity-coupling of super-conducting leads and high spin–orbit materials such as InSb and InAs, or by controlled Cu-doping of topological insu-lators such as Bi2Se3. However, for practical purposes, a single-phase material which intrinsically displays both these effects is highly desirable. Here we demonstrate coexisting superconducting correlations and spin–orbit coupling in molecular-beam-epitaxy-grown thin films of GeTe. The former is evidenced by a precipitous low-temperature drop in the electrical resistivity which is quelled by a magnetic field, and the latter manifests as a weak antilocalisation (WAL) cusp in the magnetotransport. Our studies reveal several other intriguing features such as the presence of two-dimensional rather than bulk transport channels below 2 K, possible signatures of topological superconductivity, and unexpected hysteresis in the magnetotransport. Our work demonstrates GeTe to be a potential host of topological SC and Majorana-like excitations, and to be a versatile platform to develop quantum information device architectures. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron conductivity in GeTe and GeSe upon ion implantation of Bi

This paper presents results on ion implantation of bismuth in GeTe and GeSe films. The conductivity and the thermopower of amorphous chalcogenide films are investigated. Electron conductivity in the films is attained at the Bi implantation doses higher than (1.5–2) × 1016 cm−2. In conjunction with the structural modification in the films as revealed Raman spectroscopy, the results suggest the structural re-arrangement of the amorphous network occurs via weakening the bonds of a lower energy. The onset of electron conductivity is hindered by a stronger bond in an alloy. In GeTe, this is the Ge-Ge bond.

Impact of deposition conditions on the crystallization kinetics of amorphous GeTe films

The speed at which phase change memory devices can operate depends strongly on the crystallization kinetics of the amorphous phase. To better understand factors that affect the crystallization rate, we have investigated crystallization of GeTe films as a function of their deposition temperatures and deposition rates, using X-ray synchrotron radiation and Raman spectroscopy. As-deposited films were found to be fully amorphous under all conditions, even though films deposited at higher temperatures and lower rates experienced lower effective quench rates. Non-isothermal transformation curves show that the apparent crystallization temperature of GeTe films decreases with increasing deposition temperature and decreasing deposition rate. It was found that this correlates with a decrease in the activation energy for nucleation (calculated using Kissinger’s analysis), while the activation energy for crystal growth remained unaffected. From Raman spectroscopy measurements, it was found that increasing the deposition temperature or decreasing the deposition rate, and therefore the effective quench rate, reduces the number of homopolar Te–Te bonds and thereby reduces the barrier to crystal nucleation.

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.

Structural singularities in Ge(x)Te(100-x) films.

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

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.

Control of carbon content in amorphous GeTe films deposited by plasma enhanced chemical vapor deposition (PE-MOCVD) for phase-change random access memory applications

Amorphous and smooth GeTe thin films are deposited on 200 mm silicon substrates by plasma enhanced—metal organic chemical vapor deposition (PE–MOCVD) using the commercial organometallic precursors TDMAGe and DIPTe as Ge and Te precursors, respectively. X-ray photoelectron spectroscopy (XPS) measurements show a stoichiometric composition of the deposited GeTe films but with high carbon contamination. Using information collected by Optical Emission Spectroscopy (OES) and XPS, the origin of carbon contamination is determined and the dissociation mechanisms of Ge and Te precursors in H2 + Ar plasma are proposed. As a result, carbon level is properly controlled by varying operating parameters such as plasma radio frequency power, pressure and H2 rate. Finally, GeTe films with carbon level as low as 5 at. % are obtained.

Irradiation Induced Modifications of Amorphous Phase in GeTe Film

The modified amorphous GeTe formed by pulsed laser irradiation in as-grown GeTe has been analyzed in terms of variations of local bonding structure using extended x-ray absorption fine structure (EXAFS). The modified GeTe film has octahedral-like Ge-Te bonding structure that can be effectively induced by irradiation process. The EXAFS data clearly shows that the irradiation can lead to reduction of the average coordination number. Variations in the transition temperature for the irradiated film during crystallization can be described by the presence of octahedral-like local structure.

Electrical properties of Bi-implanted amorphous chalcogenide films

The impact of Bi implantation on the conductivity and the thermopower of GeTe, Ge–Sb–Te, and Ga–La–S films is investigated. The enhanced conductivity appears to be notably sensitive to a dose of an implant. Incorporation of Bi in amorphous chalcogenide films at doses up to 1×1015 cm−2 is seen not to change the majority carrier type and activation energy for the conduction process. Higher implantation doses may reverse the majority carrier type in the studied films. Electron conductivity was observed in GeTe films implanted with Bi at a dose of 2×1016 cm−2. These studies indicate that native coordination defects present in amorphous chalcogenide semiconductors can be deactivated by means of ion implantation. A substantial density of implantation-induced traps in the studied films and their interfaces with silicon is inferred from analysis of the space-charge-limited current and capacitance-voltage characteristics taken on Au/amorphous chalcogenide/Si structures.

Thickness dependence and magnetization behavior of Mn-doped GeTe phase change materials

In this paper, the thickness dependences of microstructural, magnetic and electrical properties of Mn-doped GeTe phase change magnetic thin films were studied. The X-ray diffraction patterns demonstrate that the as-deposited films with difference thickness are amorphous while the annealed films are polycrystalline. The crystallographic structure investigation shows that the dominant orientation is enhanced with increasing thickness. The lattice parameters of Mn-doped GeTe thin films as function of the thickness were calculated. The grain sizes increase with the films thickness while the full width at half maximum, plane distance and the lattice strain decrease. The temperature dependences of magnetizations of all the films show that the maximum magnetization values occur at the lowest temperature. The data analysis results of the field cooling magnetization showed that Mn-doped GeTe films have the ferromagnetic ordering at TC = 48, 97 and 110 K for film with thickness 60, 120 and 200 nm respectively. It was found that the resistivity decreases from 12.6 to 0.863 mΩ cm as the film thickness increases from 60 to 200 nm.

Local structure of epitaxial GeTe and Ge2Sb2Te5 films grown on InAs and Si substrates with (100) and (111) orientations: An x-ray absorption near-edge structure study

GeTe is an end-point of the GeTe-Sb2Te3 quasibinary alloys often referred to as phase-change memory materials. The polycrystalline nature of the crystalline films used in devices and the concomitant presence of grain boundaries complicate detailed structural studies of the local structure. Recent progress in the epitaxial growth of phase-change materials offers unique possibilities for precise structural investigations. In this work, we report on results of x-ray absorption near-edge structure (XANES) studies of GeTe and Ge2Sb2Te5 epitaxial films grown on Si and InAs substrates with (100) and (111) orientations. The results show a strong dependence of the local structure on the substrate material and especially orientation and are discussed in conjunction with polycrystalline samples and ab-initio XANES simulations.

Laser irradiation-induced modification of the amorphous phase in GeTe films: the role of intermediate Ge–Te bonding in the crystallization mechanism

Modified amorphous GeTe was analyzed in terms of variations in the local bonding structure. Inducstion of intermediated Ge–Te bonding in modified structure critically affects the phase change process.

In-situ crystallization of GeTe\GaSb phase change memory stacked films

Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 °C and 198 °C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 °C. Furthermore, at 210 °C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples.