Jinhan Zhang

Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.

Structural and optical properties of BaxSr1−xTiO3 thin films on indium tin oxide/quartz substrates prepared by radio-frequency magnetron sputtering

BaxSr1−xTiO3 (x=0.6 and 0.8) thin films have been prepared on indium-doped tin oxide (ITO) coated quartz substrates using radio-frequency magnetron sputtering. Their structural properties and surface morphologies were examined by x-ray diffraction and atomic force microscopy, respectively. The BaxSr1−xTiO3 (BST) thin films with x=0.6 and 0.8 annealed at 650°C for 20min exhibit good surface morphology and well-crystallized perovskite structure. High quality BST ferroelectric thin films were further investigated by electrical measurements, showing the remnant polarization (Pr) of 6.75μC∕cm2 and the coercive field (Ec) of 43.2kV∕cm. Optical transmittance measurement indicated that the Ba concentration has an effect on the band gap energy (Eg) structure of the BaxSr1−xTiO3 thin films. The Eg decreases linearly with the increase of the Ba content. The refractive index (n) and extinction coefficient (k) of the BST films with x=0.6 and 0.8 were obtained by fitting the spectroscopic ellipsometric data using a par...

Bondlength alternation of nanoparticles Fe2O3 coated with organic surfactants probed by EXAFS

Abstract The nanoparticles γ-Fe2O3 coated with surfactants, DBS, ST and CTAB, repsectively, were synthesized by microemulsion method. The coated ferric oxides, which show enhanced nonlinear optical properties compared with their bulk counterpart, were probed by EXAFS in solution forms. The x-ray absorption spectra for Fe K edge were measured at room temperature by help of synchrotron radiation light source. Extended x-ray absorption fine structure analysis shows that the coating layers of organic molecules interact with surface atoms of nanoparticles, the Fe-O bond length was extended.

Normally OFF GaN-on-Si MIS-HEMTs Fabricated With LPCVD-SiN x Passivation and High-Temperature Gate Recess

Low-current-collapse normally OFF GaN-on-Si MIS high-electron-mobility transistors (MIS-HEMTs) are fabricated with low-pressure chemical-vapor-deposited SiN_x (LPCVD-SiN_x) passivation and high-temperature low-damage gate-recess technique. The high-thermal-stability LPCVD-SiN_x enables a passivation-prior-to-ohmic process strategy and effectively suppresses deep states at the passivation/HEMT interface. The fabricated MIS-HEMTs feature a high V_TH of +0.85 V at the drain current of 1 μA/mm and a remarkable ON/OFF current ratio of 10¹⁰ while reduced dynamic ON-resistance as compared to plasma-enhanced chemical-vapor-deposited SiO₂ passivation. High field-effect channel mobility of 180 cm²/V·s is achieved, leading to a high maximum drain current density of 663 mA/mm.

Structure-related optical properties of (Pb,La)(Zr,Ti)O3 thin films on indium tin oxide∕quartz substrates

To be suitable for integrated optical devices, (Pb,La)(Zr,Ti)O3 (PLZT) ferroelectric thin films require high crystalline quality, low surface roughness, high optical index, and high transparency. In this paper, PLZT thin films have been grown in situ on indium tin oxide (ITO) coated quartz substrates by rf magnetron sputtering. X-ray diffraction, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate the structural properties of these PLZT films. The results show that the ferroelectric films exhibit satisfying crystallization with the highly (110)-oriented growth from 550°C, and the surface roughness value (∼3.1nm) in studied films is within the optimum range so that a low optical loss can be obtained. High quality PLZT ferroelectric thin films were further investigated by electrical measurements, showing that the remnant polarization Pr and coercive field Ec are approximately 11.3μC∕cm2 and 56.2kV∕cm, respectively. Spectroscopic ellipsometry (SE) was employed to ch...

High RF Performance Enhancement-Mode Al 2 O 3 /AlGaN/GaN MIS-HEMTs Fabricated With High-Temperature Gate-Recess Technique

In this letter, we report high-performance enhancement-mode (E-mode) Al2O3/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) fabricated with high-temperature low-damage gate recess technique. The high-temperature gate recess is implemented by increasing the substrate temperature to 180 °C to enhance the desorption of chlorine-based etching residues during the dry etching of AlGaN barrier. High-crystal-quality Al2O3 gate dielectric was grown by atomic-layer deposition using O3 as the oxygen source to suppress hydrogen-induced weak bonds. The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of 1.6 V, a pulsed drive current of 1.13 A/mm, and very low OFF-state standby power of

High-temperature low-damage gate recess technique and ozone-assisted ALD-grown Al 2 O 3 gate dielectric for high-performance normally-off GaN MIS-HEMTs

6.8 \times 10-8 W/mm at VGS = 0 V and VDS = 30 V. At 4 GHz and in pulse-mode operation, the output power density and power-added efficiency were measured to be 5.76 W/mm and 57%, both of which are the highest for GaN-based E-mode MIS-HEMTs reported to date.

Effects of interface oxidation on the transport behavior of the two-dimensional-electron-gas in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN passivation

A high-temperature (180 °C) gate recess technique featuring low damage and in-situ self-clean capability, in combination with O₃-assisted atomic-layer-deposition (ALD) of Al₂O₃ gate dielectric, is developed for fabrication of high performance normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs), which exhibit a threshold voltage of +1.6 V, a pulsed drive current of 1.1 A/mm, and low dynamic ON-resistance under hard-switching operation. Chlorine-based dry-etching residues (e.g. AlCl₃ and GaCl₃) are significantly reduced by increasing the wafer temperature during the gate recess to their characteristic desorption temperature, while defective bonds like Al-O-H and positive fixed charges in ALD-Al₂O₃ are significantly suppressed by substitution of H₂O with O₃ precursor.

Large third-order optical nonlinearity in (Pb, La)(Zr,Ti)O3 ferroelectric thin film

The effects of interface oxidation on the transport behavior of the 2-D electron gas (2DEG) in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN (PEALD-AlN) passivation were investigated using temperature-dependent Hall-effect and X-ray photoelectron spectroscopy (XPS) characterizations. AlGaN/GaN heterostructure with a 4-nm-thick PEALD-AlN passivation exhibits good 2DEG transport behavior and stability at moderately high temperature (e.g., 275 °C). However, serious oxidation of the AlN/GaN (cap layer) interface occurs as the sample is heated up to 400 °C in low-pressure atmosphere, as verified by an increased Ga-O bond in Ga 3d core-level spectra. The oxidation leads to a significant reduction of 2.47 × 1012 cm−2 in the 2DEG density in the channel. A modified AlN passivation structure with Al2O3/AlN (10/4 nm) stack is shown to be able to effectively suppress the oxidation of the AlN/GaN interface, demonstrating an enhanced 2DEG density and high-temperature stability even when the s...

Mechanism of Ti/Al/Ti/W Au-free ohmic contacts to AlGaN/GaN heterostructures via pre-ohmic recess etching and low temperature annealing

Ferroelectric (Pb0.92La0.08)(Zr0.65Ti0.35)O3 (PLZT) film with good surface morphology and perovskite structure was grown in situ on quartz substrate by radio-frequency magnetron sputtering at 650°C. The fundamental optical constants (the band gap energy, linear refractive index, and absorption coefficient) of the film are obtained by optical transmittance measurement. The third-order nonlinear optical properties of the films were investigated by the Z-scan technique. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. A large nonlinear refractive index as high as 1.21×10−6esu is displayed in the film. All results show that PLZT ferroelectric thin film has potential applications in nonlinear optical devices.