Zhenjie Tang

Dependence of memory characteristics on the (ZrO2)x(SiO2)1−x elemental composition for charge trap flash memory applications

Charge trap flash memory capacitors incorporating various (ZrO2)x(SiO2)1−x films (x = 1.0, 0.92, 0.79, 0.63, 0.46, 0.28, 0.17 and 0.08) as the charge trapping layer were fabricated, and the dependence of the memory window, data retention and program/erase speed on the mole fraction value x were investigated. It was observed that changing the elemental composition affects the dielectric constant and equivalent oxide thickness of (ZrO2)x(SiO2)1−x films, and the memory capacitor (x = 0.63) exhibits a memory window as large as 10.7 V for a ±10 V sweeping voltage range, and a low extrapolated charge loss of 9.0% over a period of 10 years. A faster program/erase speed can be obtained for memory capacitors when x = 0.79 and 0.63. The results should be attributed to the different (ZrO2)x(SiO2)1−x microstructures, defect state densities and energy band alignments resulting from the change of compositions. In order to achieve the trade-off between the memory window, data retention, and program/erase speed, the optimal x values of the (ZrO2)x(SiO2)1−x trapping layer are in the range of 0.63 to 0.79 for charge trap flash memory applications. The present study provides useful insights for the composition selection for a complex oxide-based charge trap flash memory.

Efficient photovoltaic devices based on p-ZnSe/n-CdS core–shell heterojunctions with high open-circuit voltage

For solar cells, high open circuit voltage is necessary for their high output voltage. Usually, two methods are used to increase the open circuit voltage: using suitable and matched large band-gap materials and reduction of saturation current. Herein, large band-gap p-type ZnSe nanowires and n-type CdS films are used to construct core–shell p–n heterojunctions with high open circuit voltage. Furthermore, a 4 nm Si3N4 layer is inserted between the ZnSe core and the CdS shell in order to passivate the interface defects and to reduce recombination and the saturation current. Additionally, a fast annealing process is employed to reduce the series resistance for improved performance. For a typical device, an optimum high open circuit voltage of ∼1.3 V and an energy conversion efficiency of ∼5.27% were achieved for its photovoltaic operation. Moreover, tens of devices were fabricated to demonstrate their stable and consistent performance.

Investigation of a broadband and polarization-insensitive optical absorber based on closed-ring resonator

A broadband and polarization-insensitive optical metamaterial absorber (MA) based on the refractory metal chromium (Cr) closed-ring resonator is theoretically investigated. The semiconducting silicon dioxide (SiO2) thin film is introduced as the space layer in this sandwiched structure. Utilizing the symmetrical geometry of the proposed MA structure, polarization insensitivity of the broadband absorption is gained. The simulation results show that the absorber with Cr closed-ring array obtains an average absorption of 99.25% from 400nm to 900nm, covering the total visible wavelength range. This compact design may have potential applications in solar energy harvesting, thermal imaging, and emissivity control.

Design of a multiband terahertz perfect absorber

A thin-flexible multiband terahertz metamaterial absorber (MA) has been investigated. Each unit cell of the MA consists of a simple metal structure, which includes the top metal resonator ring and the bottom metallic ground plane, separated by a thin-flexible dielectric spacer. Finite-difference time domain simulation indicates that this MA can achieve over 99% absorption at frequencies of 1.50 THz, 3.33 THz, and 5.40 THz by properly assembling the sandwiched structure. However, because of its asymmetric structure, the MA is polarization-sensitive and can tune the absorptivity of the second absorption peak by changing the incident polarization angle. The effect of the error of the structural parameters on the absorption efficiency is also carefully analyzed in detail to guide the fabrication. Moreover, the proposed MA exhibits high refractive-index sensing sensitivity, which has potential applications in multi-wavelength sensing in the terahertz region.

Surface-dominated negative photoresponse of phosphorus-doped ZnSe nanowires and their detecting performance

Abstract Phosphorus-doped ZnSe nanowires were synthesized by using phosphorus (P) as dopant via thermal evaporation method. Their doping effect and obvious p-type conduction were confirmed through a top-gate MISFET based on individual P-doped ZnSeNW. Negative photoconductivity (the photocurrent was lower than the dark current) was found in p-type ZnSeNWs due to their surface effect. Two terminal devices based on p-type ZnSeNWs, as gas sensors, were also constructed and performed effective detecting behaviors to oxidizing gas. Furthermore, a complementary photodetector containing both n-type ZnSeNWs and p-type ZnSeNWs was first manufactured to eliminate the effects of crosstalk or false signal exist in conventional photodetectors. These results are expected to be used to develop new-type nano-devices and expand their applications.

Construction of coaxial ZnSe/ZnO p–n junctions and their photovoltaic applications

Coaxial ZnSe/ZnO nanostructures were fabricated by coating a ZnO thin film on the surface of presynthesized p-type ZnSe 1D nanostructures by a sputtering method. Owing to the n-type behavior of ZnO resulting from intrinsic defects, coaxial ZnSe/ZnO p–n junctions were realized and showed a pronounced rectifying behavior. Photovoltaic devices based on the coaxial ZnSe/ZnO p–n junction showed a power conversion efficiency of 1.24% and a large open-circuit voltage of 0.87 V under UV light. The large bandgaps of ZnSe and ZnO and the high quality of the ZnSe/ZnO interface were considered to be related to the high performance of the devices.

Theoretical investigation of a five-band terahertz absorber based on an asymmetric split-ring resonator

A simple five-band terahertz metamaterial perfect absorber, composed of an asymmetric double-gap square split ring and a metallic ground plate spaced by a thin polyimide dielectric layer, is proposed and theoretically investigated. The results show that it can perform absorption peaks at five resonant frequencies whose peaks average 98.85%. The perfect absorption is mainly attributed to the combined effect of LC, dipole, and surface response of the structure. Compared to previously reported multiband absorbers, our design only has a single and compact structure, which can drastically simplify the design and fabrication process. Furthermore, the resonance absorption properties of the absorber can be tuned by changing the geometric parameters of the structure. Such a simple and effective design holds great promise for potential applications in spectroscopic imaging, biological sensing, and detecting of drugs and explosives.

Charge storage phenomenon derived from composition redistribution for single Hf0.8Si0.2Ox film after high-temperature treatment

Simple metal-Hf0.8Si0.2Ox-silicon capacitors have been fabricated. It is observed that the capacitor after high-temperature rapid thermal annealing treatment exhibits a significant charge storage phenomenon, with large hysteresis windows of 3.93 V in a ±8 V gate sweeping voltage range, faster operating speed and good data retention characteristics. The occurrence of charge memory should be attributed to the high-temperature treatment, which gives rise to the HfO2 crystallization and elemental composition redistribution in the Hf0.8Si0.2Ox film, forming a typical metal-oxide-high-κ-oxide-silicon memory structure. Therefore, the high-temperature treatment that induced the internal structure transformation is an appealing approach, and provides a guide for future charge-trap memory design.