Ge-doped SbTe Research Articles

Surface-tensile-stress induced polishing-voids in cross-point phase-change-memory cells: corrosion mechanism and solution

In the fabrication of cross-point phase-change-memory-cells through atomic-layer-deposition of Ge-doped SbTe (Ge-ST) film followed by chemical-mechanical-polarization (CMP), a remarkable surface tensile stress was generated, originating from the surface stress induced by the pad down force and the surface structure tensile stress in a confined memory-cell structure. It was maximized at the position of the Si3N4-film spacer where the curvature becomes zero. The maximized surface tensile stress produced polishing induced voids via the generation mechanism of the stress corrosion cracking. The generation frequency and nanoscale size of the polishing induced voids rapidly increased at the maximum surface tensile stress during CMP. Then, they slightly decreased and saturated when the surface tensile stress reduced during further CMP. A design of a spacer using a SiO2-film spacer rather a Si3N4-film spacer could prevent the generation of the polishing induced voids, confirming that the generation mechanism of the polishing induced voids is associated with the stress corrosion cracking.

Surface-Tensile-Stress Induced Polishing-Voids Suppression via H2O2 Oxidizer Effect in Cross-Point Phase-Change-Memory-Cells

The chemical-mechanical-planarization (CMP) of the Ge-doped SbTe (Ge-ST) film deposited by atomic layer deposition (ALD) is essentially necessary for 3-dimensional (3D) cross-point phase-change-memory (PCM) array, producing indispensably the surface-tensile-stress inducing polishing-voids due to the corrosion of the Ge-ST film and structural tensile stress in the confined memory-cells with ∼20-nm-diameter. The oxidizer (i.e., H2O2) in a CMP slurry played an important role to suppress the generation of the polishing voids via strong chemical oxidation of Sb2O5 and TeO2 of the Ge-ST film surface to avoid a corrosion process during CMP. The suppression efficiency of the polishing voids greatly depended on the H2O2 concentration in a ALD Ge-ST-film CMP of the confined memory-cell array; i.e., the polishing voids could disappear completely greater than a specific H2O2 concentration (i.e., 4wt%).

Local structural environments of Ge doped in eutectic Sb-Te film before and after crystallization

Electrical phase change device using the Ge-doped eutectic Sb-Te (e.g., Ge1Sb8Te2) film is known to exhibit improved energy efficiency thanks to lowered threshold voltage as well as decreased power consumption for the reset operation, as compared with Ge2Sb2Te5 film. Ge K-edge EXAFS analysis is employed in this study in an effort to elucidate such merits of Ge1Sb8Te2 film in connection with its local atomic arrangements. It is then verified that a Ge atom is four-fold coordinated in its nearest-neighboring shell both in the as-deposited and in the annealed films. It needs to be highlighted that approximately two Sb atoms constitute the Ge tetrahedral units in its amorphous state; however, after being crystallized, heteropolar Ge-Sb bonds hardly exist in this Ge1Sb8Te2 film. It has been known that crystallization temperature and activation energy for crystallization of this Ge1Sb8Te2 composition are greater than those of Ge2Sb2Te5 composition. In addition, these two phase change materials exhibit distinctly different crystallization mechanisms, i.e., nucleation-dominant for Ge2Sb2Te5 film but growth-dominant for Ge1Sb8Te2 film. These discrepancies in the crystallization-related properties are delineated in terms of the local structural changes verified from the present EXAFS analysis.

Au-catalyzed synthesis and characterisation of phase change Ge-doped Sb–Te nanowires by MOCVD

The interest in the Ge doped Sb–Te chalcogenide alloy is mainly related to phase change memory applications. In view of phase change device scaling and reduction of programming energy, Sb–Te nanowires (NWs) become an attractive option. In this work, in order to investigate their potential transferability to industrial implementation, the self-assembly of Sb2Te3 NWs and Ge–Sb–Te NWs with Ge content in the range of 1–13% (Ge doping) was studied by coupling the advantages of MOCVD and the Vapour–Liquid–Solid (VLS) mechanism. The results show the structural and compositional gradual changes occurring from pure Sb2Te3 NWs to the previously reported, stoichiometric Ge1Sb2Te4 NWs [[12] M. Longo et al., Nano Lett., 12 (2012) 1509]. The typical diameter of the obtained NWs resulted to be 50nm, with lengths up to 3μm. The typology of Au catalyst nanoislands influenced both the NW morphology and the Ge incorporation during the VLS self-assembly; the Ge metalorganic precursor partial pressure affected the NW morphology and their structure. Finally, TEM observations revealed that defect-free, monocrystalline Sb2Te3 and Ge-doped Sb–Te phase change NWs could be obtained.

Fast and scalable memory characteristics of Ge‐doped SbTe phase change materials

AbstractPhase change memory (PCM) has opportunities of various applications on the premise of its high performance operations, which are still to develop with innovations such as change of a memory material. In respects of high‐speed and high‐scalability memory characteristics, δ‐phase Ge‐doped SbTe (GeST) materials stand as highly promising candidates. An overview of the material and device characteristics of these materials is presented primarily based on our recent experimental and computational studies and with a particular regard to their Sb‐to‐Te ratio (STR) dependence.TEM images of the δ‐phase GeST microstructures of varying STR and a highly scaled PCM cell with a δ‐phase GeST of high STR. magnified image

Modified write-and-verify scheme for improving the endurance of multi-level cell phase-change memory using Ge-doped SbTe

In this study, a modified write-and-verify (WAV) scheme is proposed for improving the programming/erasing (P/E) endurance of multi-level cell (MLC) phase-change memory (PCM) using Ge-doped SbTe (GeST). A dual reference data read method is developed to detect the level margin decay during P/E cycling, and a trigger condition is designed to trigger self-repair for the degraded cells before any P/E error for the modified WAV scheme. Experimental results suggest that the modified WAV scheme effectively extends the P/E endurance of PCM using GeST during 4-level P/E by at least 10 times. The modified WAV scheme is expected to improve the endurance of MLC–PCM of system applications.

Infrared spectroscopic ellipsometry of Ge-doped SbTe alloys

We grew Ge-SbTe films on glass substrates using RF sputtering deposition. We measured the dielectric function of the Ge-doped SbTe thin films in the infrared and visible spectral region. By applying linear extrapolation to the absorption coefficient, we obtained accurate band gap energy values, which decreased with increasing Te composition. Using the Drude model, we estimated their resistivity and mean scattering time. Compared to the Hall effect parameters of carrier concentration and mobility, we estimated the effective mass of holes for the Ge-SbTe thin films.

Controlled recrystallization for low-current RESET programming characteristics of phase-change memory with Ge-doped SbTe

An investigation was conducted to examine the high RESET-current (IRESET) problem of phase-change memory (PCM) using a fast growth-dominated Ge-doped SbTe (GeST). By examining material and device characteristics of GeST with varying Sb-to-Te ratio from 1.80 to 3.82, the growth rate of crystallization was found to play an important role in determining IRESET and SET speed of the device. Lower IRESET obtained with decreasing Sb-to-Te ratio was ascribed to lower growth rate leading to smaller degree of recrystallization during melt-quenching. With shrinkage of device dimensions, GeST-PCM of a lower Sb-to-Te ratio may become increasingly promising due to its lower IRESET and scaled SET speed.

Device Characteristics of a Ge-doped SbTe alloy for High-speed Phase-change Random Access Memory

We investigated the influence of phase-change materials on the device performance for low-power and high-speed phase-change random access memory (PCRAM). The Ge-doped SbTe (Ge-ST) alloy was used as a phase-change material and was compared with the conventional Ge2Sb2Te5 (GST225) alloy. An as-deposited Ge-ST film grown by using chemical vapor deposition had an amorphous phase which showed the superior gap-fill capability compared to as-deposited crystalline GST225. The crystallization time for the set operation was decreased to 80 ns, and the reset current reduced to 0.16 mA when Ge-ST was introduced in sub-40 nm a PCRAM. In addition, the Ge-ST alloy showed a good data retention property of 10 years guaranteed at 100 ◦C. These results will enable us to realize low-power and high-speed PCRAM devices beyond the sub-40 nm design rule.

Multi-level cell storage with a modulated current method for phase-change memory using Ge-doped SbTe

We demonstrate a high-speed multi-level cell (MLC) storage for the phase-change memory using a Ge-doped SbTe (GeST) for the first time with a conventional pore-type device structure and a conventional modulated-current writing method. The GeST was selected to have a low Sb-to-Te ratio of 1.8 (GeST L), rendering a diminished growth rate relative to the case of a high Sb-to-Te ratio typically of fast, growth-dominated crystallization. With a writing time of less than 100 ns, each of the 4 resistance levels separated from one another at least by the factor of 4 is shown to form reliably and stay with a low drift coefficient of ∼0.1. GeST L may be regarded as a promising material for high-speed MLC phase-change memory application.

Investigation of the Structural and Optical Properties of Ge-doped SbTe Films with Various Sb:Te Ratios

In this study, we investigated the structural and optical properties of Ge-doped SbTe (Ge–ST) thin films with three differing compositions: , , and , grown on Si substrate by radio-frequency sputtering method. The films were annealed at for crystallization and their crystal structures were examined by X-ray diffraction. Compared to the X-ray diffraction spectra of the undoped SbTe, the Ge–ST thin films had a hexagonal structure with large stacking periods. Using Raman spectroscopy, we investigated the shift of the phonon mode frequencies ( and ) of the films with varying Sb:Te ratios. We compared the dependence of the phonon frequencies of Ge–ST on the Sb content to those of the corresponding undoped SbTe. The composition dependence of the phonon frequency could be explained in terms of the linear extrapolation of Sb and crystals. By using spectroscopic ellipsometry, we measured the dielectric function of the thin films in the near-IR, visible, and ultraviolet spectral regions. The optical energy gaps and bandgaps of the amorphous and crystalline phases, respectively, were determined using linear extrapolation of the absorption coefficient. The optical gap energies of the amorphous Ge–ST films were determined to be about , whereas the indirect bandgap energies of the crystalline films shrank substantially to about .

Growth and Phase Separation Behavior in Ge-Doped Sb−Te Thin Films Deposited by Combined Plasma-Enhanced Chemical Vapor and Atomic Layer Depositions

The growth, phase separation, crystallization behavior, and electrical properties of various Ge-doped Sb−Te thin films were examined. The films were deposited by combined plasma-enhanced chemical vapor and atomic layer depositions at a wafer temperature of 150 °C using Ge(i-C4H9)4, Sb(i-C3H7)3, and Te(i-C3H7)2 as the precursors for Ge, Sb, and Te, respectively. The different compositions were obtained by adjusting the Ge and Sb precursor injection times. Segregated crystalline Sb islands were observed in amorphous Sb-rich Sb−Te thin films containing over 10 atom % of Ge. The crystallization kinetics of the Ge-doped SbTe alloys, which depends on the concentration ratio of Sb/Te and the bond energy between Ge, Sb, and Te, provide clues to help understand the phase separation behavior of the films at growth temperature. Ge17Sb58Te25 and Ge31Sb38Te31 showed the highest resistivity of 67 Ω cm at the as-deposited state and exhibited a crystallization temperature of ∼260 °C.

Improved stability of a phase change memory device using Ge-doped SbTe at varying ambient temperature

Ge-doped SbTe (Ge–ST) was compared with Ge2Sb2Te5 (GST) for its potential use in the phase-change memory with improved stability at varying ambient temperature (TA). Device characteristics such as RESET current, RESET resistance, and SET resistance of Ge–ST devices were found to vary significantly less with TA than those of GST devices. From measured carrier density, mobility, and optical band gaps, these findings are interpreted to derive from a metallic nature of the crystalline Ge–ST in contrast with a semiconducting nature of the crystalline GST as well as a relatively weaker covalent bonding in amorphous Ge–ST.

The effect of Ge addition on the RESET operation of a phase-change memory (PCM) device using Ge-doped SbTe

Using Ge-doped SbTe (Ge–ST) materials of a fixed Sb:Te ratio (∼4.4), we investigated the effects of Ge addition (0–13.1 at.%) with a particular regard to the possibility of achieving highly fast and reliable programming characteristics of a phase-change memory device. From material characterization, we found that a higher Ge content led to the enhanced stability of the amorphous phase state and retarded nucleation of crystallites but the growth of crystallites remained very fast regardless of Ge content. Consistent with these findings, examination of device characteristics revealed that, with increasing Ge content up to 13.1 at.%, we could make a RESET programming more reliable by slower melt-quenching while maintaining a very high SET speed. The reliable RESET programming is considered as due to decreased driving force for recrystallization resulting from increased stability of the amorphous phase state.

Demonstration of a Reliable High Speed Phase-Change Memory Using Ge-Doped SbTe

For potential use in phase-change memory (PCM) devices, a Ge-doped SbTe (Ge-ST) was investigated with respect to both material properties and PCM device characteristics. Compared with (GST), the used Ge-ST was characterized to have a lower melting temperature (814 K), a higher crystallization temperature (436 K), and a lower electrical resistivity in both amorphous and crystalline states. The Ge-ST devices have a much faster operation speed by about 1 order and consume less RESET power compared with the GST devices. Nevertheless, a higher RESET-programming current was required due to a lower dynamic resistance. Device reliability properties such as write/erase cycling endurance and data retention properties were also examined and found to be comparable to those of the GST devices, suggesting that the Ge-ST is a promising phase-change material for high speed PCMs.

Analysis and improvement of interfacial adhesion of growth-dominant Ge-doped SbTe phase change materials

A semiquantitative characterization of the interfacial adhesion of phase change materials is developed, which consists of determining critical adhesion temperature (TCA) via measuring the probability of adhesion failure with temperature using patterned films. By comparison of TCA values, Ge-doped SbTe (Ge-ST) is shown to have weaker adhesion than Ge2Sb2Te5 (GST), which results from its limited ability in relaxation of crystallization-induced stress. Nitrogen or oxygen doping in Ge-ST produces significant increase in TCA, close to that of GST. This improvement is due to smaller grain size of N-/O-doped Ge-ST, which facilitates the relaxation of the stress via grain boundary diffusion or sliding.

Crystallization and memory programming characteristics of Ge-doped SbTe materials of varying Sb : Te ratio

A phase change memory (PCM) utilizes resistivity changes accompanying fast transitions from an amorphous to a crystalline phase (SET) and vice versa (RESET). An investigation was made on the SET characteristics of PCM cells with Ge-doped SbTe (Ge–ST) materials of two different Sb : Te ratios (4.53 and 2.08). For the material of higher Sb : Te (4.53), a SET operation was completed within several tens of nanoseconds via nucleation-free crystallization whereas the material of lower Sb : Te (2.08) rendered a slower SET operation requiring several hundred nanoseconds for a nucleation-mediated crystallization. From measurements of nucleation and growth kinetics via laser-induced crystallization, the observed SET characteristics of the former case were found to derive from a growth time about 103 times shorter than the nucleation time and those of the latter from a much shorter nucleation time as well as a longer growth time than in the former case. The measured nucleation kinetics of the lower Sb : Te (2.08) material is unexpected from the existing data, which has led us to advance an interesting finding that there occurs a trend-reversing change in the nucleation kinetics of the Ge-ST materials around the eutectic composition (Sb : Te ∼2.6); nucleation is accelerated with the increase in the Sb : Te ratio above Sb : Te of 2.6, but with a decrease in the Sb : Te ratio below it.

Preparation of Ge-doped SbTe Material Thin Film for Phase Change Random Access Memory by Magnetron Sputtering on Small Hole Patterns

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Bias polarity dependence of a phase change memory with a Ge-doped SbTe: A method for multilevel programing

We investigated the bias polarity dependence of the characteristics of a phase change memory device and found that the device showed higher resistance both at programed and erased states, extended erasing time, and higher threshold voltage (Vth) under negative bias than those under the conventional positive bias. Taking advantage of this dependence, we were able to obtain four highly distinguishable resistance states in such a reproducible manner that may be utilized for a reliable multilevel storage. We explain this polarity dependence in terms of the difference in the density of trap states at the interfaces between the phase change material and electrodes.