Zingway Pei

Carrier Transport Mechanism in a Nanoparticle-Incorporated Organic Bistable Memory Device

In this letter, the conduction mechanism in nanoparticle-contained polymer memory was investigated experimentally and theoretically. The current-voltage characteristics showed that the device switches from an initial low-conductivity state to a high-conductivity state upon application of an external electric field at room temperature. The current transition exhibited a very narrow voltage range that causes an abrupt increase of current. A trap-filled space-charge-limited current model was proposed and supported by the experimental data to explain the transport mechanism in organic memory.

White electroluminescence from hydrogenated amorphous-SiNx thin films

White electroluminescence (EL) was observed from hydrogenated amorphous-SiNx-based light-emitting device. Silicon nitride thin films were deposited on the indium-tin-oxide (ITO)-coated glass substrate by plasma enhanced chemical vapor deposition method with a mixture of Ar-diluted 5% SiH4 and pure N2 gases, in the ratio 2 to 1. Measured x value of the film is 0.56, and the corresponding photoluminescence of a-SiN0.56:H thin film exhibited a red-infrared spectrum, centered at 630 nm. The layer structure of the EL device is ITO/a-SiN0.56:H (80 nm)/Al, with light emitting from the ITO layer, recognizable by the naked eye in the dark, under the 14 V forward bias conditions. White EL spectra from ∼400 to 750 nm, with a central peak at 560 nm, were observed in the hydrogenated amorphous silicon nitride EL device. A carrier transport mechanism was suggested, and the EL was attributed to the recombination of carriers through the luminescent states.

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si self-assembled quantum dots

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si quantum-dot light-emitting diodes is reported. The devices were fabricated in a mesa-type structure, with a silicon oxide layer on the top for surface/sidewall passivation. Different passivation processes were employed. We found that the integrated electroluminescence intensities were relatively less sensitive to temperature, persisting at nearly the same intensity up to RT. The fabricated device shows an internal quantum efficiency of about 0.015% at RT. The improved emission property is attributed to the reduced nonradiative recombination centers due to the surface passivation and thermal treatment.

A high efficient 820 nm MOS Ge quantum dot photodetector

A Ge quantum dot photodetector has been demonstrated using a metal-oxide-semiconductor (MOS) tunneling structure. The oxide film was grown by liquid phase deposition (LPD) at 50/spl deg/C. The photodetector with five-period Ge quantum dot has responsivity of 130, 0.16, and 0.08 mA/W at wavelengths of 820 nm, 1300 nm, and 1550 nm, respectively. The device with 20-period Ge quantum dot shows responsivity of 600 mA/W at the wavelength of 850 nm. The room temperature dark current density is as low as 0.06 mA/cm/sup 2/. The high performance of the photodetectors at 820 nm makes it feasible to integrate electrooptical devices into Si chips for short-range optical communication.

Ultra high-density silicon nanowires for extremely low reflection in visible regime

We fabricated large-area, vertically aligned silicon nanowire (SiNW) arrays on Si substrates employing catalytic etching on a polystyrene nanosphere template. The density of SiNWs was as high as 1010/cm2, and the bottom radii of SiNWs ranged between 30 and 60 nm. The reflection from the SiNW layer was approximately 0.1% over the spectral range of 300–800 nm for SiNWs longer than 750 nm. Effective medium theory was applied to explain this extremely low reflection, and it was confirmed that the incident light scatters randomly inside cone-like SiNWs, which lengthens the actual traveling path of light.

A New Nonvolatile Bistable Polymer-Nanoparticle Memory Device

In this letter, we demonstrate a new organic bistable nonvolatile memory device that is adopting polymer-chain-stabilized gold (Au) nanoparticles in a host polymer as a memory active layer. In this letter, the Au nanoparticles are well dispersed in the host polymer so as to enhance stability of memory devices. Current-voltage characteristics show that the device switches from an initial low-conductivity state to a high-conductivity state upon applying an external electric field at room temperature. This memory can be switched ON and OFF for over 150 times without an apparent performance degradation. In addition, the memory state can retain for over 36 000 s in air. This memory device is thus considered to be a suitable candidate for flexible electronics applications.

Numerical Simulation on the Photovoltaic Behavior of an Amorphous-Silicon Nanowire-Array Solar Cell

In this letter, we propose an amorphous-silicon (a-Si) solar cell with a nanowire-array structure. The proposed structure has photon absorption and carrier transport that are perpendicular to each other, which could overcome the efficiency limit of an a-Si solar cell. This nanowire structure has an n-type a-Si nanowire array in which the i-layer and the p-layer a-Si are sequentially grown along the surface of the nanowire. Under illumination, light is absorbed along the axial direction of the nanowire, and carrier transport is along the radial direction. Numerical simulations show that the photocurrent of the a-Si solar cell with a 4000-nm-long nanowire is nearly 40% more than that of a planar a-Si solar cell. A conversion efficiency of 11.6% was obtained, which is around 32% enhancement.

A high-performance SiGe-Si multiple-quantum-well heterojunction phototransistor

A novel phototransistor is fabricated by placing Si/sub 0.5/Ge/sub 0.5//Si multiple quantum wells (MQWs) between the base and the collector of Si-SiGe heterojunction bipolar transistors (HPT). The SiGe-Si MQWs are used as a light absorption layer. The cutoff frequency (f/sub T/) and maximum oscillation frequency (f/sub MAX/) of the phototransistor are found to be 25 GHz, which is suitable for gigabit integrated circuit. The responsivity of 1.3 A/W (external quantum efficiency = 194%) and the pulsewidth of 184 ps at a wavelength of 850 nm are observed. The excellent electrical and optical performance of the Si-SiGe MQW phototransistor makes it attractive for future Si-based optoelectronic integrated circuit applications.

Artificial electrical dipole in polymer multilayers for nonvolatile thin film transistor memory

In this letter, an organic nonvolatile thin film transistor (TFT) memory on a plastic substrate is reported. The cross-linked poly-4-vinyl phenol (PVP) is used as a polymer dielectric layer in the form of a triple layer structure to achieve the memory function. Two interfaces between the PVP triple layers are the main trapping centers for electrons and holes, respectively, which are verified by the capacitance-voltage analysis. The electric dipole is established by the separated electrons and holes in the two interfaces of the PVP triple layer structure and results in an 11V memory window for the TFT nonvolatile memory.

Effects of spacer thickness on optical properties of stacked Ge/Si quantum dots grown by chemical vapor deposition

Photoluminescence investigations on stacked Ge/Si dots with different spacer thicknesses are presented. According to the emission energy shift in the Ge dots, we found that a thinner spacer layer will lead to remarkable Ge–Si intermixing during the stacking of the Ge/Si dots. Such material intermixing not only shallows the dot potential depth, but also softens the sharpness of the dot/spacer interface. In addition, the temperature of photoluminescence quenching also varies with the spacer thickness. Finally, we point out some important factors that are relevant to the room-temperature luminescence efficiency of stacked Ge/Si quantum dots.