Dai Yuehua

Metal dopants in HfO2-based RRAM: first principle study

Based on density-functional theory (DFT), the effects of metal dopants in HfO2-based RRAM are studied by the Vienna ab initio simulation package (VASP). Metal dopants are classified into two types (interstitial and substitutional) according to the formation energy when they exist in HfO2 cell. Several conductive channels are observed through the isosurface plots of the partial charge density for HfO2 doped with interstitial metals, while this phenomenon cannot be found in HfO2 doped with substitutional metals. The electron density of states (DOS) and the projected electron density of states (PDOS) are calculated and analyzed; it is found that the conduction filament in HfO2 is directly formed by the interstitial metals and further, that the substitutional metals cannot directly generate conduction filament. However, all the metal dopants contribute to the formation of the oxygen vacancy (VO) filament. The formation energy of the VO and the interaction between metal dopants and VO are calculated; it is revealed that the P-type substitutional metal dopants have a strong enhanced effect on the VO filament, the interstitial metal dopants have a minor assistant effect, while Hf-like and N-type substitutional metal dopants have the weakest assistant effect.

Effects of interaction between defects on the uniformity of doping HfO2-based RRAM： a first principle study

The physical mechanism of doping effects on switching uniformity and operation voltage in Al-doped HfO2 resistive random access memory (RRAM) devices is proposed from another perspective: defects interactions, based on first principle calculations. In doped HfO2, dopant is proved to have a localized effect on the formation of defects and the interactions between them. In addition, both effects cause oxygen vacancies (VO) to have a tendency to form clusters and these clusters are easy to form around the dopant. It is proved that this process can improve the performance of material through projected density of states (PDOS) analysis. For VO filament-type RRAM devices, these clusters are concluded to be helpful for the controllability of the switching process in which oxygen vacancy filaments form and break. Therefore, improved uniformity and operation voltage of Al-doped HfO2 RRAM devices is achieved.

The process and performance of double doping polysilicon gate MOSFET

In early days, our project team has analyzed the electric field, threshold voltage, capacitance, cut-off frequency and other characteristics of the double doping polysilicon gate MOSFET (DDPG-MOS), see references [1]. In this study, the process steps of DDPG-MOS are designed and simulated with software TSUPREM. Then the frequency and transient characteristic of the device are analyzed using software MEDICI. The results show that, the process of DDPG-MOS is completely compatible with CMOS, and its performances are improved significantly. Specially, DDPG-MOS has a wider frequency range and faster response speed, which has good application prospects in the RF field.

A 2-D semi-analytical model of double-gate tunnel field-effect transistor*

A 2-D semi-analytical model of double gate (DG) tunneling field-effect transistor (TFET) is proposed. By aid of introducing two rectangular sources located in the gate dielectric layer and the channel, the 2-D Poisson equation is solved by using a semi-analytical method combined with an eigenfunction expansion method. The expression of the surface potential is obtained, which is a special function for the infinite series expressions. The influence of the mobile charges on the potential profile is taken into account in the proposed model. On the basis of the potential profile, the shortest tunneling length and the average electrical field can be derived, and the drain current is then constructed by using Kanes model. In particular, the changes of the tunneling parameters Ak and Bk influenced by the drain—source voltage are also incorporated in the predicted model. The proposed model shows a good agreement with TCAD simulation results under different drain—source voltages, silicon film thicknesses, gate dielectric layer thicknesses, and gate dielectric layer constants. Therefore, it is useful to optimize the DG TFET and this provides a physical insight for circuit level design.

Optimal migration path of Ag in HfO2: A first-principles study*

First-principles calculations are used to investigate the migration path of Ag in the HfO2-based resistive random access memory (ReRAM). The formation energy calculation suggests that there are two different sites (site 1 and site 3) for the incorporation of Ag atoms into the HfO2 unit cell. Thermodynamic analysis shows that the motion of Ag atom in the HfO2 supercell appears to be anisotropic, which is due to the fact that the Ag atom at site 3 moves along the orientation, but the Ag atom at site 1 moves along the [001] orientation. The migration barriers of the Ag atoms hopping between neighboring unit cells are calculated along five different orientations. Difficulty in producing motion of the Ag atom varies with the migration barrier: this motion is minimized along orientation. Furthermore, The optimal circulation path for Ag migration within the HfO2 supercells is obtained, and is found to be approximately along the orientation. Therefore, it is proposed that the positive voltage should be applied along this orientation, the conduction filament may form more easily, which could improve the response time and reduce the power consumption in ReRAM applications.

A first-principle investigation of the oxygen defects in Si3N4-based charge trapping memories

Based on first principle calculations, a comprehensive study of substitutional oxygen defects in hexagonal silicon nitride (β-Si3N4) has been carried out. Firstly, it is found that substitutional oxygen is most likely to form clusters at three sites in Si3N4 due to the intense attractive interaction between oxygen defects. Then, by using three analytical tools (trap energy, modified Bader analysis and charge density difference), we discuss the trap abilities of the three clusters. The result shows that each kind of cluster at the three specific sites presents very different abilities to trap charge carriers (electrons or holes): two of the three clusters can trap both kinds of charge carriers, confirming their amphoteric property; While the last remaining one is only able to trap hole carriers. Moreover, our studies reveal that the three clusters differ from each other in terms of endurance during the program/erase progress. Taking full account of capturing properties for the three oxygen clusters, including trap ability and endurance, we deem holes rather than electrons to be optimal to act as operational charge carriers for the oxygen defects in Si3N4-based charge trapping memories.

Compact model of surface potential in strong inversion channel

In this work, the expression of surface potential corresponding to strong inversion state is presented which is distinct from the classical value 2VB due to quantum effect. Then threshold voltage is calculated and it is found that quantum effect affects seriously threshold voltage.