Z. H. Mai

The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition

ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600u2009°C. The films were then annealed at 600u2009°C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.

Dry laser cleaning of particles from solid substrates: Experiments and theory

The experimental analysis of dry laser cleaning efficiency is done for certified spherical particle (SiO2, 5.0, 2.5, 1.0, and 0.5 μm) from different substrates (Si, Ge, and NiP). The influence of different options (laser wavelength, incident angle, substrate properties, i.e., type of material, surface roughness, etc.) on the cleaning efficiency is presented in addition to commonly analyzed options (cleaning efficiency versus laser fluence and particle size). Found laser cleaning efficiency demonstrates a great sensitivity to some of these options (e.g., laser wavelength, angle of incidence, etc.). Partially these effects can be explained within the frame of the microelectronics engineering (MIE) theory of scattering. Other effects (e.g., influence of roughness) can be explained along the more complex line, related to examination of the problem “particle on the surface” beyond the MIE theory. The theory of dry laser cleaning, based on one-dimensional thermal expansion of the substrate, demonstrates a great...

Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip

Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip in air has been investigated. A 532 nm Nd:YAG pulsed laser with a pulse duration of 7 ns was used. A 2×2 oxide dot array with dot sizes between 20 and 30 nm and an oxide single line with a width less than 30 nm have been created using an electrochemical-etched tungsten tip under laser irradiation. The modified regions were characterized by atomic force microscope. The apparent depth of oxide layer as a function of laser intensity has been studied. The advantages and drawbacks of using a continuous wave laser and a pulsed laser will be discussed.

Nanostructure fabrication using pulsed lasers in combination with a scanning tunneling microscope: Mechanism investigation

Nanostructure fabrication using lasers in combination with a scanning tunneling microscope has been reported in the past several years. Different mechanisms have been discussed for the formation of these nanostructures. However, they are controversial. In this study, we investigated the mechanism of nanostructure fabrication on both gold films and hydrogen-passivated Ge surfaces. Current-distance curves for a gold film and for an H-passivated Ge surface under an electrochemically etched tungsten tip were measured to determine the tip-sample distance. An analytical model was proposed to explain different mechanisms for nanostructure fabrication on gold films and on H-passivated Ge surfaces. Thermal expansion of the tip under laser irradiation was calculated. With comparison between the tip-sample distance and the thermal expansion of the tip, we can determine whether the mechanism is based on optical enhancement or on thermal mechanical indentation.

In-Depth Electrical Analysis to Reveal the Failure Mechanisms With Nanoprobing

This paper highlights the use of a localized probing technique, nanoprobing, to reveal some of the subtle defects affecting the yield of integrated circuits in the nanometer generation nodes. The tool is equipped with the capability to isolate and characterize the exact failing transistors of the malfunctioned devices. As a result, the identification process of the failure mechanisms, and hence the root cause, can be accelerated. The electrical characterization at the transistor level also offers an appropriate guide to the required physical analysis that has to be carried out in order to ldquovisualizerdquo the defects. Based on the in-depth diagnosis of the defective site, the three case studies covered in this paper demonstrate the importance of this advanced failure-analysis methodology. For the analysis, static random-access-memory test-chips were used. With that, marginal failures or degradations relating to the ultrathin gate oxides, variations in the resistance of the implanted layers in the substrate, and abnormal passive-voltage-contrast signature were determined.

Electrochromic writing and erasing on tungsten oxide films in air by scanning tunneling microscopy

The scanning tunneling microscope (STM) was used for both spot coloring and bleaching on α-WO3−x thin films. By wetting the STM tip with 1 M KOH solutions or de-ionized water before the tip approached, and keeping the humidity higher than 40%, the electrochemical reaction occurred at the tip–surface gap while the electrochromic reactions took place at the film surface. With a constant negative sample bias and grounded tip, a brown color circle or a light blue disk was formed on the surface with an enlarged diameter. The circle and disk formations were attributed to the production of alkali and hydrogen tungsten bronzes. By applying a positive sample bias with a grounded tip, the color of the hydrogen tungsten bronze could be bleached. The cathodic reduction of W6+ forms a color center at the W5+ site, while the anodic reaction removes an electron from the color center W5+ state. Raman spectroscopy indicated that for the colored state, the frequency for the W–O bond stretching mode increased, while for the...

Nano-modification on hydrogen-passivated Si surfaces by a laser-assisted scanning tunneling microscope operating in air

Abstract A novel method for nanofabrication, i.e., laser-induced nano-modification on hydrogen (H)-passivated Si surfaces under a tip of a scanning tunneling microscope (STM) is proposed. The theory and mechanism are discussed. Enhanced laser irradiation under a STM tip induces a local temperature rise on the nanometer scale, which causes thermal desorption of hydrogen atoms on an H-passivated Si surface. Oxidation occurs after desorption of hydrogen atoms. A vertical polarized Nd:YAG pulsed laser with a duration of 7 ns was used in our investigation. STM tips were electrochemically (EC) etched from a tungsten wire. A 3×2 dot array and a single line were created. The sizes of the dots are from 20 to 30 nm, and the width of the line is less than 30 nm. The dependence of the apparent depth on the laser intensity shows that there is a threshold of the intensity and a saturation value of the apparent depth is reached at high intensity.

Optimization of AC Electrochemical Etching for Fabricating Tungsten Nanotips with Controlled Tip Profile

AC electrochemical etching in diluted potassium hydroxide (KOH) solution was optimized to fabricate tungsten (W) nanotips with a controllable sharpness and aspect ratio using an additional lift-up step. The final tip profile was dependent on the extent of interaction between the KOH solution and the side of the W surface, and effective bubble shielding effects near the apex region during the lift-up. Lateral etching rate along the W material was affected by parameters such as electrolyte-cathode positioning, etching voltage, and electrode size that influenced the flow or replenishment rate of OH- ions to the W surface submerged in the solution and at the meniscus region. With the lift-up step, the dense layer of bubbles that formed during etching could provide a good shield in minimizing the etch-back effects on the tip apex. Combining the above investigated effects, sharp nanotips with the required aspect ratio could be achieved with the enhanced lateral etching and the protective shield of bubbles.

Carbon nitride thin film synthesized on iron buffer layers

Carbon nitride thin films were deposited on iron buffer layers by pulsed laser deposition assisted with ion implantation. Two types of samples (A) and (B) were prepared with and without iron layers. Several techniques were used to study the properties of the samples. Scanning tunneling microscopy (STM) was used to observe the surface structures of the samples. The difference in their surface morphologies was studied. The STM measurements also provided the relation between tunneling current and bias voltage to study the local density of states of the sample surface by calculating (dI/dV)/(I/V). Three band edges were observed from the calculated curve. Measurements by Raman and Fourier transform infrared (FTIR) spectra were carried out to study the electronic properties of the samples. The Raman spectra showed the presence of triply bonded carbon nitride bonds (C≡N) in sample (A), while only single bonds were observed in sample (B) by FTIR spectra. The mechanical properties were studied by nanoindentation. ...

Mechanism of laser-induced nanomodification on hydrogen-passivated Si(100) surfaces underneath the tip of a scanning tunneling microscope

Laser-induced nanomodification on hydrogen (H)-passivated Si(100) surfaces has been carried out underneath the tip of a scanning tunneling microscope (STM) in ambient air. The created features were characterized using STM, atomic-force microscopy (AFM), and Auger electron spectroscopy (AES). The features appeared as depressed regions in STM images, while they appeared as protruded regions in AFM images. Oxygen was detected in a modified 2×2u200aμm2 square region by AES, while no oxygen was detected elsewhere on the same sample surface. The experiment results and mechanism are discussed. Nano-oxide patterns, such as lines and dots, have been created. Dependence of oxide apparent depth on laser intensity, laser pulse numbers, tunneling current during modification, and laser incidence angle has been investigated.