Chunli Zou

Atomically smooth gallium nitride surface prepared by chemical-mechanical polishing with different abrasives:

For chemical-mechanical polishing of epitaxial gallium nitride (GaN), a two-step experiment method with two kinds of abrasives, aluminum oxide (Al2O3) and colloidal silica (SiO2), was put forward. The average material removal rates of GaN by the slurry with Al2O3 and SiO2 abrasives were 594.79 and 66.88 nm/h, respectively. An atomically flat surface with roughness (Ra) of 0.056 nm was obtained after the second chemical-mechanical polishing process with SiO2-based slurry, which presented an atomic step-terrace structure. The material removal characteristics of GaN surfaces were investigated in detail. A model was proposed to describe the different behaviors of the two kinds of abrasive during chemical-mechanical polishing process.

Synthesis of dual-doped non-precious metal electrocatalysts and their electrocatalytic activity for oxygen reduction reaction

Abstract The pyrolyzed carbon supported ferrum polypyrrole (Fe-N/C) catalysts are synthesized with or without selected dopants, p -toluenesulfonic acid (TsOH), by a facile thermal annealing approach at desired temperature for optimizing their activity for the oxygen reduction reaction (ORR) in O 2 -saturated 0.1 mol/L KOH solution. The electrochemical techniques such as cyclic voltammetry (CV) and rotating disk electrode (RDE) are employed with the Koutecky-Levich theory to quantitatively obtain the ORR kinetic constants and the reaction mechanisms. It is found that catalysts doped with TsOH show significantly improved ORR activity relative to the TsOH-free one. The average electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. The heat-treatment is found to be a necessary step for catalyst activity improvement, and the catalyst pyrolyzed at 600 °C gives the best ORR activity. An onset potential and the potential at the current density of – 1.5 mA/cm 2 for TsOH-doped catalyst after pyrolysis are 30 mV and 170 mV, which are more positive than those without pyrolized. Furthermore, the catalyst doped with TsOH shows higher tolerance to methanol compared with commercial Pt/C catalyst in 0.1 mol/L KOH. To understand this TsOH doping and pyrolyzed effect, X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) are used to characterize these catalysts in terms of their structure and composition. XPS results indicate that the pyrrolic-N groups are the most active sites, a finding that is supported by the correspondence between changes in pyridinic-N content and ORR activity that occur with changing temperature. Sulfur species are also structurally bound to carbon in the forms of C–S n –C, an additional beneficial factor for the ORR.

Effect of Modified Silica Abrasive Particles on Nanosized Particle Deposition in Final Polishing of Silicon Wafers

The silane coupling agent γ-aminopropyl triethoxysilane (APTS) and polyethylene oxide (PEO) are proposed to modify the SiO2 abrasive particles for final polishing of silicon wafers. The effects of the modified silica abrasive particles on nanosized particle deposition, roughness, and removal rate of the silicon wafer are explored in detail. PEO is proved to be a potential modifying agent for controlling deposition of large particles (∼410 nm diameter), leading to low roughness (Ra = 0.097 nm), and APTS is found to be effective in controlling deposition of both large and small particles (∼410 and ∼200 nm diameter, respectively), resulting in lower roughness (Ra = 0.054 nm).

Investigation on defect control for final chemical mechanical polishing of aluminum alloy

In acid final chemical mechanical polishing (CMP) of aluminum alloy, the addition of a small amount of nitric acid (HNO3) is proved to be the effective measure for reducing the orange peel defect due to the protective film produced by HNO3 on aluminum alloy; however, it makes point defects worse. The effect of hydrolyzed polymaleic anhydride (HPMA) on point defects is investigated. HPMA adsorbs on the surface of aluminum alloy first, and the composite film formed by both HPMA and HNO3 is proved to be efficient for controlling both the orange peel and point defects. When the aluminum alloy is polished by the acid original slurry with 0.7 wt% HNO3 and 0.1 wt% HPMA, the roughness of the aluminum alloy surface is low to 0.678 nm.

An in situ study of chemical-mechanical polishing behaviours on sapphire (0001) via simulating the chemical product-removal process by AFM-tapping mode in both liquid and air environments

Chemical-mechanical polishing (CMP) has drawn significant attention as one of the most advanced techniques for achieving an atomic-level smooth surface. However, the mechanism of CMP is still unclear, and the in situ characterization of CMP behaviors at the nanoscale has been a challenge for decades. In this study, we, for the first time, report an in situ study of CMP behaviors on sapphire (0001) via simulating the chemical product-removal process by using atomic force microscopy (AFM) in tapping mode. Through a combination of intensive experimental measurements and detailed structural characterizations, it is shown that the AFM probe in tapping mode can act as a polishing abrasive to realize simultaneous imaging and chemical product removal on sapphire (0001), thus achieving successful in situ characterizations in both liquid and air environments. This work fills in gaps relating to fundamental CMP mechanisms, and provides a new perspective for the study of CMP behaviors on different materials.

The effects of ultra-smooth surface atomic step morphology on CMP performances of sapphire and SiC wafers

Whether sapphire or SiC wafer, clear and regular atomic step morphology could be observed all over the ultra-smooth wafer surface via atomic force microscopy (AFM) using our CMP technology. However, towards sapphire and SiC wafers, the variations of atomic step widths and step directions on the whole of wafer surface are different. The step widths and step directions on the different positions of sapphire wafer are uniform, while that on SiC wafer are distinct. Thus, the effects of atomic step width on CMP removal rate of sapphire and SiC wafers were studied. On the other side, the CMP removal model of super-hard hexagonal crystalline wafer to realize atomically ultra-smooth surface is proposed. The variations of atomic step morphology towards different defects on sapphire and SiC wafers surface are analyzed, and the formation mechanism of the defects is discussed.

Atomically smooth gallium nitride surfaces generated by chemical mechanical polishing with non-noble metal catalyst(Fe-N x /C) in acid solution

In this paper, a novel method for preparing atomically smooth gallium nitride (GaN) wafer surfaces which involves chemical mechanical polishing with a non-noble metal catalyst (Fe-Nx) in acidic slurry is presented. It was confirmed that non-noble metal catalyst based slurry could be used for gallium face of GaN. Atomic force microscope images of the processed surface indicate that an atomically flat surface with Ra=0.0518 nm was achieved after planarization and the processed surface has an atomic step-terrace structure. Besides, the rate of removal of the GaN surface was measured to be approximately 66.9 nm/h, more than triple times higher than that nothing was used as catalyst.