Deng Ning

A Forming-Free Bipolar Resistive Switching in HfOx-Based Memory with a Thin Ti Cap

The electroforming process of Ti/HfOx stacked RRAM devices is removed via the combination of low temperature atomic layer deposition and post metal annealing. The Pt/Ti/HfOx/Pt RRAM devices show a forming-free bipolar resistive switching behavior. By x-ray photoelectron emission spectroscopy analysis, it is found that there are many oxygen vacancies and nonlattice oxygen pre-existing in the HfOx layer that play a key role in removing the electroforming process. In addition, when the thickness ratio of the Ti and HfOx layer is 1, the uniformity of the switching parameters of Pt/Ti/HfOx/Pt devices is significantly improved. The OFF/ON window maintains about 100 at the read voltage of 0.1 V.

Ge quantum dot infrared photo-detector

In our experiments, vertical aligned superlattices of multiple self-assembled Ge island layers separated by Si spacer layers on Si(100) substrates have been grown by ultra high vacuum chemical vapor deposition system (UHV/CVD). The photoluminescence of material was excited with 500nm laser and measured at 10 K, the PL peaks representing Ge islands and Si wetting layers were observed at 825 and 1010 mev respectively. Based on this material, samples of quantum dot infrared photo-detectors (QDIP), with p-i-n junctions, were fabricated. The responsivity of the detectors was measured with various semiconductor light-emitting diodes and semiconductor lasers. At room temperature and at -3V applied bias, for samples with 245 /spl times/ 245 /spl mu/m/sup 2/ large window and dark current I/sub d//spl sim/10/sup -9/A, the maximum photocurrent responsivity of 0.52 A/W at 774 nm was found, and 0.043mA/W at 1.31 /spl mu/m. Higher responsivities can be obtained with a waveguide geometry and optimization of the whole structure, such as thickness of layers and type of doping.

Dependences of spin polarization on the control parameters in the spin-polarized injection through the magnetic p-n junction

Effective spin-polarized injection from magnetic semiconductor (MS) to nonmagnetic semiconductor (NMS) has been highlighted in recent years. In this paper we study theoretically the dependence of nonequilibrium spin polarization (NESP) in NMS during spin-polarized injection through the magnetic p-n junction. Based on the theory in semiconductor physics, a model is established and the boundary conditions are determined in the case of no external spin-polarized injection and low bias. The control parameters that may influence the NESP in NMS are indicated by calculating the distribution of spin polarization. They are the doping concentrations, the equilibrium spin polarization in MS and the bias. The effective spin-polarized injection can be realized more easily by optimizing the above parameters.

Simulation study of new 3-terminal devices for high speed STT-RAM

To improve the performance of spin transfer torque random access memory (STT-RAM), especially writing speed, we propose three modified 3-terminal STT-RAM cells. A magnetic dynamic process in the new structures was investigated through micro-magnetic simulation. The best switching speed of the new structures is 120% faster than that of the rectangular 3-terminal device. The optimized 3-terminal device offers high speed while maintaining the high reliability of the 3-terminal structure.

Investigation of Composition in Nano-Scaled Self-Assembled SiGe Islands

A modified model is proposed to explain the influence of Si concentration on shape transition of self-assembled SiGe islands on Si substrates. The experimental results show that the critical sizes for shape transition from pyramids to domes (44, 50 and 60 nm) increase with the increasing Si concentration (0.032, 0.09 and 0.17) from sample A to C. Based on the proposed model, the quantitative relation between the Si concentration and the critical size is established. Then the composition exactly in nano-scaled self-assembled SiGe islands is calculated from the measured critical size. The result shows that the Si concentration deduced from Raman spectrum is much larger than the actual values. It is demonstrated that the quantitative relation we obtained can be used to investigate the composition in nano-scaled SiGe islands.