Zhong-Hui Fang

Resistive switching mechanism in silicon highly rich SiOx (x < 0.75) films based on silicon dangling bonds percolation model

The unipolar resistive switches are investigated in silicon highly rich SiOx (x   1.8) based devices, our Pt/SiO0.73/Pt devices operate at lower voltage regime (<2.0 V) and exhibit much lower resistance (∼30 Ω). The reset voltage (∼0.7 V) is lower than set voltage (∼1.7 V) and the performance is reduced in the vacuum environment. We propose a Si-DBs percolation model to explain the above characteristics. The experimental evidences for supporting our model are presented and discussed.

The x dependent two kinds of resistive switching behaviors in SiOx films with different x component

We discover the transition phenomenon of two kinds of different resistive switching behaviors in SiOx based Pt/SiOx/Pt devices with different x component. When x   0.95, the operations do not need a current compliance and Vreset is higher than Vset. We use the silicon dangling bonds (Si-DBs) percolation model to explain the x dependent transition phenomenon. The microstructural transitions of tetrahedral Si-O configurations and related Si-DBs in as-deposited SiOx films with different x and the hopping conductance of low-resistance-states support our model.

Higher than 60% internal quantum efficiency of photoluminescence from amorphous silicon oxynitride thin films at wavelength of 470 nm

We reported the study on the photoluminescence internal quantum efficiency (PL IQE) and external quantum efficiency (PL EQE) from the amorphous silicon oxynitride (a-SiNO) films, which were fabricated by plasma-enhanced chemical vapor deposition followed by in situ plasma oxidation. We employed the direct measurement of absolute quantum efficiency within a calibrated integration sphere to obtain the PL EQE. Then, we calculated the PL IQE by combing the measured EQE and optical parameters of light extraction factor, reflectivity, and transmittance of the a-SiNO thin films. We also derived the PL QE through investigating the characteristic of the temperature dependent PL. These results show that the PL IQE as high as 60% has been achieved at peak wavelength of about 470 nm, which is much higher than that of Si nanocrystal embedded thin films.

Fabricating a silicon nanowire by using the proximity effect in electron beam lithography for investigation of the Coulomb blockade effect.

We present an approach to fabricate a silicon nanowire relying on the proximity effect in electron beam lithography with a low acceleration voltage system by designing the exposure patterns with a rhombus sandwiched between two symmetric wedges. The reproducibility is investigated by changing the number of rhombuses. A device with a silicon nanowire is constructed on a highly doped silicon-on-insulator wafer to measure the electronic transport characteristics. Significant nonlinear behavior of current-voltage curves is observed at up to 150 K. The dependence of current on the drain voltage and back-gate voltage shows Coulomb blockade oscillations at 5.4 K, revealing a Coulomb island naturally formed in the nanowire. The mechanism of formation of the Coulomb island is discussed.

Different charging behaviors between electrons and holes in Si nanocrystals embedded in SiNx matrix by the influence of near-interface oxide traps*

Si-rich silicon nitride films are prepared by plasma-enhanced chemical vapor deposition method, followed by thermal annealing to form the Si nanocrystals (Si-NCs) embedded in SiNx floating gate MOS structures. The capacitance–voltage (C–V), current–voltage (I–V), and admittance–voltage (G–V) measurements are used to investigate the charging characteristics. It is found that the maximum flat band voltage shift (ΔVFB) due to full charged holes (~ 6.2 V) is much larger than that due to full charged electrons (~ 1 V). The charging displacement current peaks of electrons and holes can be also observed by the I–V measurements, respectively. From the G–V measurements we find that the hole injection is influenced by the oxide hole traps which are located near the SiO2/Si-substrate interface. Combining the results of C–V and G–V measurements, we find that the hole charging of the Si-NCs occurs via a two-step tunneling mechanism. The evolution of G–V peak originated from oxide traps exhibits the process of hole injection into these defects and transferring to the Si-NCs.

Fabrication of silicon highly-rich SiO x (x<0.75) and its novel resistive switching behaviors

The unipolar resistive switching characteristics are investigated in silicon highly-rich SiO_x (x<;0.75) films. Pt/SiO_0.732/Pt structure exhibits a good switching behavior with low operation voltage, long retention time, uniformly distributed resistance value and large memory widows etc. As-deposited SiO0732 films contain a large concentration (1×10¹⁹cm³) of dangling bonds and are rich in unsymmetric and polar O₃=Si-Si and SiO₂=Si-Si metastable configurations. We propose a model of silicon dangling bonds percolation path to explain the switching behavior.

Improvement of retention and endurance characteristics of Si nanocrystal nonvolatile memory device

The nitrided nc-Si floating gate nonvolatile memory device with an ultrathin tunnel (3.5 nm) oxide layer was fabricated. A memory window of 1.3 V was obtained under P/E voltages of ±7 V for 1 ms, and it kept still about 1.1 V after ten years from the extrapolated data of retention characteristics. These results can be attributed to the nitrogen passivation of the traps at the surface of nc-Si. In addition, no detectable variation of the memory window has been displayed after 107 P/E cycles, showing superior endurance characteristics.

Multipeak negative differential resistance effects in nanocrystalline Si stacking MOS structures

The nanocrystalline Si quantum dot (nc-Si QD) stacking MOS structure is fabricated by plasma-enhanced chemical vapor depositon (PECVD). Negative differential resistance (NDR) characteristics are investigated in the nc-Si quantum dot floating gate MOS structure. Clear multi NDR peaks for electrons and holes are observed in the I-V curves and calculations indicate that these NDR characteristics should be associated to Coulomb blockade effect and quantum confinement effect of the nc-Si quantum dots.

The development of nanometer solid state storage memory

Based on the single electron tunneling and storage in an uniform nc-Si single layer within a-SiNx (or a-SiOx) / nc-Si/a-SiNx (or a-SiOx) structures, the nc-Si floating gate nonvolatile memories (NVM) have been fabricated. The performance of the program and erase speed as well as the retention time have been improved by using the nitrogen plasma treatments. In order to increase the storage density, the double level charge storage has been realized in the stacked nc-Si layer based MIS structures. Considering the scaling limit of charge storage in nc-Si NVM, the Si high-rich SiOx based electrically switchable resistance random access memory (RRAM) has been investigated. The mechanism of electrically switchable resistance in Pt/SiOx/Pt structure has been also discussed.

Performance improvement of nc-Si nonvolatile memory by novel design of tunnel and control layer

The nc-Si nonvolatile memory devices with high performance have been fabricated by using general CMOS techniques. High resolution transmission electronic microscope (HRTEM) shows that the average size of nc-Si is 8 nm and its density is 3×1011/cm2. The performance of programming/ erasing and retention time is mainly depending on the quality and thickness of tunnel layer and control layer. The results show the relation between the performance and novel design of tunnel and control layers.