DC-free Method to Evaluate Nanoscale Equivalent Oxide Thickness: Dark-Mode Scanning Capacitance Microscopy.

This study developed a DC-free technique that used dark-mode scanning capacitance microscopy (DM-SCM) with a small-area contact electrode to evaluate and image equivalent oxide thicknesses (EOTs). In contrast to the conventional capacitance-voltage (C-V) method, which requires a large-area contact electrode and DC voltage sweeping to provide reliable C-V curves from which the EOT can be determined, the proposed method enabled the evaluation of the EOT to a few nanometers for thermal and high-k oxides. The signal intensity equation defining the voltage modulation efficiency in scanning capacitance microscopy (SCM) indicates that thermal oxide films on silicon can serve as calibration references for the establishment of a linear relationship between the SCM signal ratio and the EOT ratio; the EOT is then determined from this relationship. Experimental results for thermal oxide films demonstrated that the EOT obtained using the DM-SCM approach closely matched the value obtained using the typical C-V method for frequencies ranging from 90 kHz to 1 MHz. The percentage differences in EOT values between the C-V and SCM measurements were smaller than 0.5%. For high-k oxide films, DM-SCM with a DC-free operation may mitigate the effect of DC voltages on evaluations of EOTs. In addition, image operations were performed to obtain EOT images showing the EOT variation induced by DC-stress-induced charge trapping. Compared with the typical C-V method, the proposed DM-SCM approach not only provides a DC-free approach for EOT evaluation, but also offers a valuable opportunity to visualize the EOT distribution before and after the application of DC stress.

Voltage modulation efficiency in scanning capacitance microscopy.

This paper reports a novel investigation of the voltage modulation efficiency (VME) in scanning capacitance microscopy (SCM). A signal intensity model was used to define the VME, which is dependent on the impedance components in an SCM setup. In SCM, the VME was found to play a key mediating role in the close relationship between the signal intensity and the modulation voltage, providing an indicator for the surface treatment and the back-contact process of an SCM specimen. We observed that, for silicon-based specimens, ultraviolet-assisted oxidation and microwave annealing improved the specimen surface and the back-contact, respectively, which increased the VME. It was also found that a high modulation voltage and a large back-contact area may induce a significant stray capacitance around the conductive tip and, hence, lower the VME. The VME degradation not only decreased the SCM signal intensity but also reduced the image contrast in the regions with high carrier concentrations. In addition, our experimental results further revealed that the signal intensity model also provided a promising opportunity to establish a precise and quantitative method for measuring the thickness of insulating layers.

Facile Preparation of a Platinum Silicide Nanoparticle-Modified Tip Apex for Scanning Kelvin Probe Microscopy.

In this study, we propose an ultra-facile approach to prepare a platinum silicide nanoparticle-modified tip apex (PSM tip) used for scanning Kelvin probe microscopy (SKPM). We combined a localized fluoride-assisted galvanic replacement reaction (LFAGRR) and atmospheric microwave annealing (AMA) to deposit a single platinum silicide nanoparticle with a diameter of 32 nm on the apex of a bare silicon tip of atomic force microscopy (AFM). The total process was completed in an ambient environment in less than 3 min. The improved potential resolution in the SKPM measurement was verified. Moreover, the resolution of the topography is comparable to that of a bare silicon tip. In addition, the negative charges found on the PSM tips suggest the possibility of exploring the use of current PSM tips to sense electric fields more precisely. The ultra-fast and cost-effective preparation of the PSM tips provides a new direction for the preparation of functional tips for scanning probe microscopy.

A facile approach to prepare silicon-based Pt-Ag tubular dendritic nano-forests (tDNFs) for solar-light-enhanced methanol oxidation reaction.

In this paper, a facile two-step Galvanic replacement reaction (GRR) is proposed to prepare Pt-Ag tubular dendritic nano-forests (tDNFs) in ambient condition for enhancing methanol oxidation reaction (MOR) under solar illumination. In the first GRR, a homogeneous layer of silver dendritic nano-forests (DNFs) with 10 µm in thickness was grown on Si wafer in 5 min in silver nitride (AgNO3) and buffer oxide etchant (BOE) solution. In the second GRR, we utilized chloroplatinic acid (H2PtCl6) as the precursor for platinum (Pt) deposition to further transform the prepared Ag DNFs into Pt-Ag tDNFs. The catalytic performance and solar response of the Pt-Ag tDNFs toward methanol electro-oxidation are also studied by cyclic voltammetry (CV) and chronoamperometry (CA). The methanol oxidation current was boosted by 6.4% under solar illumination on the Pt-Ag tDNFs due to the induced localized surface plasmon resonance (LSPR) on the dendritic structure. Current results provide a cost-effective and facile approach to prepare solar-driven metallic electrodes potentially applicable to photo-electro-chemical fuel cells.

The manufacturing of a metallic nano-cluster at a tip apex for field-sensitive microscopy applications.

Using a conductive atomic force microscopic setup, a metallic nano-cluster at a tip apex was successfully manufactured by an electrochemical redox process from an anodic aluminum oxide template. The diameter of the metallic nano-clusters ranged from 15 nm to 200 nm. The diameters of the nano-clusters could be well-controlled by adjusting the pore size of the templates. The formation of a variety of metallic nano-clusters at the tip apex was accomplished by preparing the electrolyte solution from different metallic salts. The formation mechanism for the nano-cluster is outlined and discussed. Moreover, we were able to enhance the performance of the nano-cluster tips for field-sensitive scanning probe microscopy, including electrostatic force microscopy and scanning Kelvin probe microscopy by laser annealing. Our experimental results indicated that for applications in field-sensitive scanning probe microscopy the stray field effect was significantly suppressed by the nano-cluster tip and hence the spatial resolution was improved.

Analysis of InAsN quantum dots by transmission electron microscopy and photoluminescence.

Quantum dots (QDs) have great potential in optical fiber communication applications were widely recognized. The structure of molecular beam epitaxy (MBE) grew InAsN QDs were investigated by transmission electron microscopy (TEM) and measured their optical properties by photoluminescence (PL). TEM images show that the InAsN QDs are irregular or oval shaped. Some of the InAsN QDs are observed to have defects, such as dislocations at or near the surface in contrast to InAs QDs, which appear to be defect free. PL results for InAsN QDs showed a red-shifted emission peak. In addition, the InAsN emission peak is broader than InAs QDs, which supports the TEM observation that the size distribution of the InAsN QDs is more random than InAs QDs. The results show that the addition of nitrogen to InAs QDs leads to a decrease in the average size of the QDs, bring changes in the QD's shape, compositional distribution, and optical properties.