Blanka Magyari-Köpe

Achieving direct band gap in germanium through integration of Sn alloying and external strain

GeSn is predicted to exhibit an indirect to direct band gap transition at alloy Sn composition of 6.5% and biaxial strain effects are investigated in order to further optimize GeSn band structure for optoelectronics and high speed electronic devices. A theoretical model has been developed based on the nonlocal empirical pseudopotential method to determine the electronic band structure of germanium tin (GeSn) alloys. Modifications to the virtual crystal potential accounting for disorder induced potential fluctuations are incorporated to reproduce the large direct band gap bowing observed in GeSn alloys.

ON-OFF switching mechanism of resistive–random–access–memories based on the formation and disruption of oxygen vacancy conducting channels

We study the ON-OFF switching mechanism of oxide-based resistive–random–access–memories using theoretical calculations. Electron deficient vacancies (VO) up to 1+ charge states would stabilize a cohesive filament, while further electron removal will stabilize the disrupted VO configurations with 2+ charges. The VO cohesion-isolation transition upon carrier injection and removal is shown to be a strong driving force in the ON-OFF switching process. We also propose that bipolar or unipolar behavior is determined by how the carriers are injected into VO. The control of the carrier injection by the electrode material selection is essential for desired bipolar switching.

GeSn technology: Extending the Ge electronics roadmap

First principles study showed indicated band gap of Ge can be tuned by alloying with Sn and metastable GeSn alloys can be synthesized at or above room temperature. Subsequently, high quality GeSn layers were grown using low temperature MBE. PL indicated good crystal quality of GeSn material with a reduced direct bandgap. Challenges involved in CMOS processing on GeSn were addressed through effective surface cleaning and a low thermal budget process flow. To the best of our knowledge this work is the first demonstration of a high-κ pMOSFET using 3% GeSn as channel material showing 20% improvement in hole mobility compared to Ge. Alloying Ge with Sn has thus been shown as a performance booster for Ge based devices. Further improvements in material quality and incorporation of higher substitutional Sn, coupled with strain and bandgap engineering, significant performance gains can be achieved from this alloy system.

Nanoscale (∼10nm) 3D vertical ReRAM and NbO 2 threshold selector with TiN electrode

The scaling and 3-D integration issues of NbO₂ with threshold switching characteristics were investigated for ReRAM selector device. To avoid the process problems of Pt electrode, we tested ReRAM and selector devices with conventional electrodes (TiN and W). By adopting 10nm-thick TiN bottom electrode with low thermal conductivity, we could significantly reduce the threshold current for insulator-metal transition (I-M-T) due to the heat confinement effect. We have evaluated for the first time both 1S1R (NbO₂/TaO_x) and hybrid (NbO₂/Nb₂O₅) devices. We have confirmed the feasibility of high density vertical memory device by adopting NbO₂ I-M-T selector device.

Physical model of the impact of metal grain work function variability on emerging dual metal gate MOSFETs and its implication for SRAM reliability

A new model of work function variability (WFV) based on grain orientation differences of the polycrystalline metal gate is reported. Our model predicts that at the 22 nm technology node, among the three device variability sources: random dopant fluctuation (RDF), line edge roughness (LER) and WFV, WFV will cross over RDF and becomes the dominating factor. The SRAM circuit analysis shows that write/read failures are underestimated by 9 orders of magnitude by the area weighted averaged work function model.

A Combined Ab Initio and Experimental Study on the Nature of Conductive Filaments in

Through ab initio calculations, we propose that the conductive filaments in Pt/HfO2/Pt resistive random access memories are due to HfOx suboxides, possibly tetragonal, where x ≤ 1.5. The electroforming process is initiated by a continuous supply of oxygen Frenkel defect pairs through an electrochemical process. The accumulation of oxygen vacancies leads to metallic suboxide phases, which remain conductive even as ultranarrow 1-nm2 filaments embedded in an insulating HfO2 matrix. Our experiments further show that the filaments remain as major leakage paths even in the OFF-state. Moreover, thermal heating may increase the OFF-state resistance, implying that there are oxygen interstitials left in the oxide layer, which may recombine with the oxygen vacancies in the filaments at high temperature.

{\rm Pt}/{\rm Hf}{\rm O}_{2}/{\rm Pt}

Ge channel is one of the promising performance boosters for replacing Si channel in future complementary metal-oxide-semiconductor technology. The uniaxial stress technology can further enhance the performance of Ge MOSFETs. In this paper, the uniaxial stress effect on Ge NMOSFETs was experimentally and theoretically investigated. The gate dielectric in the Ge NMOSFETs was fabricated by using the novel radical oxidation technique. The high quality of the gate dielectric allowed high vertical field mobility measurements. By applying mechanical uniaxial stress on the fabricated Ge NMOSFETs, the mobility enhancement was experimentally obtained. The physical mechanism of mobility enhancement under such strain indicates that the device performance of Ge NMOSFETs in the ballistic transport regime can achieve as much as 48% drive current gain beyond the 15 nm technology node.

Resistive Random Access Memory

We report on the electronic roles in filamentary-type switching of binary oxide-based resistive random access memories using ab initio calculations. We show that charge injection and removal determine the thermodynamic stability of the vacancy filament and the diffusion in the memory devices; electron injection induces the vacancy cohesion that stabilizes the filament, whereas removal of these electrons favors the vacancy isolation that destabilizes the filament; electron removal makes the energy barrier of the vacancy diffusion processes small enough to be overcome by joule heating. The vacancy cohesion-isolation processes are induced by charge injection and removal that leads to occupation of the bonding-like electron states, which can be controlled by shifting the system Fermi level via an applied voltage during memory operation. The vacancy cohesion-isolation phase transition upon charge injection and removal is thus one of the main factors that govern resistive switching. Based on the physics, we propose three-layer stack structures for further improvement of the memory characteristics.

Uniaxial Stress Engineering for High-Performance Ge NMOSFETs

We theoretically study an oxygen vacancy (VO) diffusion in Al2O3-based resistive-random-access-memories (ReRAMs). We find that the activation energy of VO diffusion in Al2O3 strongly depends on the charge state of VO. In ReRAM, the charge state of VO can be easily changed by applying voltage and the lowest activation energy is observed at q = 2+. The operation voltage on Al2O3-based ReRAM is close to the activation energy at q = 2+, indicating that VO diffuses with doubly positive state. Moreover, the activation energy at q = 0 is close to that observed in bulk Al2O3, which explains the discrepancy between previous experimental and theoretical studies.

Vacancy Cohesion-Isolation Phase Transition Upon Charge Injection and Removal in Binary Oxide-Based RRAM Filamentary-Type Switching

We present a detailed theoretical analysis to motivate GeSn for CMOS logic. High quality GeSn films have been obtained on Ge-on-Si using a CVD process. A novel surface passivation scheme is presented to achieve record low trap densities at high-κ/GeSn interface. Using the novel surface passivation method, combined with a low thermal budget device fabrication process, n-channel MOSFETs on GeSn with channel Sn content as high as 8.5% have been demonstrated for the first time.