Yingchun Fu

Photocurrents of 14 μm quantum-well infrared photodetectors

We study the factors that determine photogenerated carriers and response wavelengths of photocurrents of long wavelength (∼14 μm) quantum well (QW) infrared photodetectors (QWIPs). The material structures of QWIPs are first characterized by the photoluminescence measurements (PL). By calculating the density of photogenerated carriers in the continuum above the energy barriers using the PL calibrated QWIP structures, we have demonstrated that due to the sample quality, the photocarriers can be either in miniband states (Bloch states in the multiple quantum wells), or they transport from one quantum well to the next in the form of running waves. By including possible scattering processes at the QWIP working temperature to link the theoretically calculated photocarrier density with the experimentally measured photocurrent, it is shown that the width of the photocurrent peaks of 14 μm GaAs/AlGaAs QWIPs under investigation is determined by the optical phonon emissions of photocarriers. We have further calculat...

A self-aligned process for phase-change material nanowire confined within metal electrode nanogap

A self-aligned fabrication process is presented by which phase-change material nanowire (NW) perfectly confined within metal electrode nanogap based on electron-beam lithography and inductively coupled plasma etching process. Lateral phase-change random access memory device fabrication is demonstrated by this process with Ge2Sb2Te5 NW confined within 39 nm tungsten electrode nanogap and the electrical characterizations are illustrated. It is found that the threshold current is only 2 μA and the dc power consumption is remarkably low. The process is simple, flexible and achieves localization filling. In addition, the process can be easily transferred to other types of phase-change and nanoelectronics materials.

Two-photon-absorption-induced nonlinear photoresponse in GaAs∕AlGaAs quantum-well infrared photodetectors

Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.

Understanding of Thermal Engineering for Vertical Nanowire Phase-Change Random Access Memory Partially Wrapped by Low-Conductivity Layer

A phase-change random access memory (PCRAM) geometric model is proposed to improve the thermal efficiency. A low-κ layer with very low thermal conductivity is inserted into the insulation layer. Then, the effective programming area and the programming current are greatly reduced. In addition, the selection conditions for the low-κ layer are carefully studied to help us find a suitable material that can be used as a low-κ layer. It is believed that this simple geometric model is a useful tool for increasing the thermal efficiency of PCRAM devices and for selecting the appropriate conditions for a low-κ layer allowing low-current operation.

A Self-Aligned Process to Fabricate a Metal Electrode-Quantum Dot/Nanowire-Metal Electrode Structure with 100% Yield

Using lateral phase change random access memory (PCRAM) for demonstration, we report a self-aligned process to fabricate a metal electrode-quantum dot(QD)/nanowire(NW)-metal electrode structure. Due to the good confinement and coupling between the Ge2Sb2Te5 (GST) QD and the tungsten electrodes, the device shows a threshold current and voltage as small as 2.50 μA and 1.08 V, respectively. Our process is highlighted with good controllability and repeatability with 100% yield, making it a promising fabrication process for nanoelectronics.