Zhang Wanli

Sodium beta-alumina thin films as gate dielectrics for AlGaN/GaN metal?insulator?semiconductor high-electron-mobility transistors

Sodium beta-alumina (SBA) is deposited on AlGaN/GaN by using a co-deposition process with sodium and Al2O3 as the precursors. The X-ray diffraction (XRD) spectrum reveals that the deposited thin film is amorphous. The binding energy and composition of the deposited thin film, obtained from the X-ray photoelectron spectroscopy (XPS) measurement, are consistent with those of SBA. The dielectric constant of the SBA thin film is about 50. Each of the capacitance?voltage characteristics obtained at five different frequencies shows a high-quality interface between SBA and AlGaN. The interface trap density of metal?insulator?semiconductor high-electron-mobility transistor (MISHEMT) is measured to be (3.5~9.5)?1010 cm?2?eV?1 by the conductance method. The fixed charge density of SBA dielectric is on the order of 2.7?1012 cm?2. Compared with the AlGaN/GaN metal?semiconductor heterostructure high-electron-mobility transistor (MESHEMT), the AlGaN/GaN MISHEMT usually has a threshold voltage that shifts negatively. However, the threshold voltage of the AlGaN/GaN MISHEMT using SBA as the gate dielectric shifts positively from ?5.5 V to ?3.5 V. From XPS results, the surface valence-band maximum (VBM-EF) of AlGaN is found to decrease from 2.56 eV to 2.25 eV after the SBA thin film deposition. The possible reasons why the threshold voltage of AlGaN/GaN MISHEMT with the SBA gate dielectric shifts positively are the influence of SBA on surface valence-band maximum (VBM-EF), the reduction of interface traps and the effects of sodium ions, and/or the fixed charges in SBA on the two-dimensional electron gas (2DEG).

Low platinum loading PtNPs/graphene composite catalyst with high electrocatalytic activity for dye-sensitized solar cells

Platinum nanoparticles (PtNPs)/graphene composite materials are synthesized by a controlled chemical reduction of H2PtCl6 on graphene sheets. The electrocatalytic activity of a PtNPs/graphene composite counter electrode for a dye-sensitized solar cell (DSSC) is investigated. The results demonstrate that the PtNPs/graphene composite has high electrocatalytic activity for the dye-sensitized solar cell. The cell employing PtNPs (1.6 wt%)/graphene counter electrode reaches an conversion efficiency (?) of 3.89% upon the excitation of 100 mW/cm2 AM 1.5 white light, which is comparable to that of the cell with a Pt-film counter electrode (? = 3.76%). It suggests that one can use only 14% Pt content of the conventional Pt-film counter electrode to obtain a comparable conversion efficiency. It may be possible to obtain a high performance DSSC using the PtNPs/graphene composite with a very low Pt content as a counter electrode due to its simplicity, low cost, and large scalability.

Pseudo-Spin-Valve Trilayer Using Amorphous CoNbZr Layer: Giant Magnetoresistance, Domain Structures and Potentials for Spin-Electronic Devices

We propose a pseudo-spin-valve (PSV) trilayer using amorphous CoNbZr alloy for soft magnetic layers. The giant magnetoresistance (GMR), domain structures and their variation upon thermal annealing are investigated. The GMR effect is not only stable up to 300 degrees C but also enhanced due to the improvement of the interfaces between Cu and magnetic layers. With high annealing temperature, the magnetoresistance (MR) ratio decreases rapidly as a result of serious layer interdiffusion. Dense stripe domains, which disappear after annealing at 300 degrees C for 1 h, are observed in the sandwiched films. It is found that after patterning to elliptic stripe with aspect ratio of 6:1, the trilayers have a single domain and their MR ratio increases. The dynamic MR behaviour under an ac magnetic field indicates that the patterned stripes have good linear MR responses. Therefore, it is believed that the CoNbZr/Cu/Co PSV trilayer have strong potentials for spin-electronic devices including magnetic random access memory.

Rectifying the Current-Voltage Characteristics of a LiNbO3 Film/GaN Heterojunction

Epitaxial LiNbO3 (LNO) films are grown on n-type GaN semiconductor substrates, forming LNO/GaN p-n junctions. The current-voltage (I—V) and capacitance-voltage (C—V) characteristics of the junctions are studied. The I—V curve shows a clear rectifying property with a turn-on voltage of 2.4 V. For the forward voltages, the conduction mechanism transits from Schottky thermionic emission for low voltages to space-charge-limited current for large voltages. Reverse C—V characteristics exhibit a linear 1/C2 versus V plot, from which a built-in potential of 0.34 V is deduced. These results are explained using the energy-band structure of the LNO/GaN junction.

AlGaN/GaN MISHEMTs with Sodium-Beta-Alumina as the Gate Dielectrics

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) with sodium beta-alumina (SBA) thin films as the gate dielectrics are studied. AlGaN/GaN metal-semiconductor high-electron-mobility transistors (MESHEMTs) and MISHEMTs with Al2O3 thin-film gate dielectrics are also fabricated for comparative study. The MISHEMTs with SBA gate dielectrics show nearly four orders of magnitude lower gate leakage current and an approximately 60% increase in maximum transconductance, indicating that SBA can serve as an effective dielectric for AlGaN/GaN MISHEMTs. However, SBA gate dielectrics result in threshold voltage modulation of AlGaN/GaN MISHEMTs. Compared with those of AlGaN/GaN MESHEMTs, the threshold voltages of AlGaN/GaN MISHEMTs with Al2O3 gate dielectrics shift negatively from −5.5 V to −7.5 V. In contrast with the normally used gate dielectrics, the threshold voltages of MISHEMTs with SBA gate dielectrics shift positively from −5.5 V to −3.5 V Based on an x-ray photoelectron spectrum study and energy band spectrum calculation, the primary mechanism of the threshold voltage modulation is attributed to the decrease in the surface valence-band maximum and the increase in the barrier height by SBA gate dielectrics.

Composition-Controlled Low Field Magnetostriction of TbFe Amorphous Films

Amorphous TbFe films are fabricated by dc magnetron sputtering, and their magnetostrictions at low field are examined over a wide range of terbium content (from 32 at.% to 70 at.%). It is found that the terbium content plays an important role in the magnetic and magnetostrictive properties of TbFe films. TbFe film soft magnetic properties and low field magnetostriction can be efficiently improved by controlling the terbium at an optimum content. The magnetostriction at lower magnetic field is increased with the increase of terbium content up to 48.2 at.%. After reaching the maximum value, further increase of terbium content would result in a great decrease of the low field magnetostriction. By contrast, at higher magnetic field, the magnetostriction is decreased monotonically with the increase of the terbium content.

FABRICATION AND CONDUCTIVE CHARACTERISTIC OF HIGHLY ORIENTED LANIO3 THIN FILMS

ABSTRACT LaNiO3 thin films were fabricated on Si substrates by sol-gel method under different annealing temperatures. The XRD pattern showed that the films annealed at 700°C were single perovskite phase and highly (200) oriented. Dense and homogeneously distributed surface was observed using AFM, the average grain size and the root mean square roughness (RMS) of the sample are 83.67 nm and 5.06 nm, respectively. It was also revealed that the electrical property of the films mainly depended on the crystallite size of LNO, the resistivity and sheet resistance of the LNO thin films annealed at 700°C are 0.0037 Ω · cm and 76 Ω /□, respectively.

Effects of Rapid Recurrent Thermal Annealing on Giant Magnetoresistance NiFe/Ag Multilayers

NiFe/Ag multilayers were prepared by dc sputtering onto glass-ceramic substrates directly at room temperature. The samples were thermally processed by rapid recurrent thermal anneal (RRTA). We studied the effects of RRTA on giant magnetoresistance (GMR) NiFe/Ag multilayer by controlling the anneal temperature as well as the rapid anneal cycle. The samples after three RRTA cycles have a similar annealing temperature dependence of GMR responses to the ordinary annealed samples. With the increasing anneal cycle, the GMR response improved at first and then reached an unexpected high value of 9% before descent rapidly. Microstructure study shows that this effect is ascribed to the transformation of continuous NiFe layer into discontinuous one, and then into a granular like film in a step-by-step way.

Performance of an AlGaN/GaN MISHEMT with sodium beta-alumina for gate insulation and surface passivation

An AlGaN/GaN metal—insulator—semiconductor high-electron-mobility transistor (MISHEMT), with sodium beta-alumina (SBA) for both gate insulation and surface passivation, was investigated and compared with a conventional metal—semiconductor high-electron-mobility transistor (MESHEMT). The measured gate leakage current of the MISHEMT was reduced by approximately one order of magnitude as compared with that of the conventional MESHEMT. The saturation drain current of the AlGaN/GaN MISHEMT reached 830 mA/mm, which was about 43% higher than that of a conventional MESHEMT. The peak extrinsic transconductance of the MISHEMT was 103 mS/mm, which was similarly higher than that of the MESHEMT. The results suggested that the SBA thin film is an effective candidate gate dielectric for AlGaN/GaN MISHEMTs.

The Design and Realization of the Material Series Database System

Abstract Aiming at the special material equipment, a multi-layer database structure was studied. The data characteristics, the intrinsic relationship of each layer, and the multi-layer database structure related to the material trademark and its performance were defined. The logical organization and the physical structure of the material series database and the material performance database were designed, as well as the software system was developed based on the Oracle and the J2EE technology. The data exchange between the two databases of the material trademark and the performance can be seamlessly connected, which shows that the system can effectively manage and intuitionisticly display the material series framework and detail information with text, picture, forms and so on.