Oxide Dots Research Articles

Microdroplet and Atomic Force Microscopy Probe Assisted Formation of Acidic Thin Layers for Silicon Nanostructuring

AbstractIn this work, an aqueous acidic thin‐layer‐based strategy for fabricating nanostructures on silicon by using atomic force microscopy (AFM) nanolithography is presented. The approach involves the formation of microscale droplets via dilute hydrofluoride (DHF) etching, the conversion of the droplets to acidic thin layers by AFM‐probe scanning, and subsequent lithographic operations using a biased probe in the aqueous layers. By varying the concentration of the acidic DHF layers, the thin layers can facilitate the creation of both positive and negative patterns, such as oxide dots and Si pores, through anodic oxidation and dissolution. In particular, the anodic oxidation in the acidic media is associated with the field‐enhanced nonequilibrium dissociation of the weak electrolyte. The Si pore structure formation is related to the field‐assisted dissolution of anodic oxides and the Si substrate. The acidic‐layer‐based technique allows switching between different lithographic modes by changing the acidity of the DHF layers, and is complementary to bulk solution‐based and local meniscus‐based approaches in AFM nanolithography. In principle, this method can also be extended to other materials that have similar reactions with DHF.

Local Oxide Growth Mechanisms on Nickel Films

The oxidation characteristics of nickel thin films were investigated by atomic force microscopy (AFM) anodization. The anodization parameters, such as anodized voltages, oxidation times, pulse voltage periods and how they affected the creation and growth of the oxide nanostructures were explored. The results showed that the height of the nickel oxide nanodots grew as a result of either the anodization time or the anodized voltage being increased. The oxide growth rate was dependent on the anodized voltage and on the resulting electric field strength. Furthermore, as the electric field strength was at an order of 2×109 Vm−1, the anodization rate decreased quickly and the oxide dots stopped growing.

Kinetics of Atomic Force Microscope-Based Scanned Probe Oxidation on an Octadecylated Silicon(111) Surface

Atomic force microscope (AFM)-based scanned probe oxidation (SPO) nanolithography has been carried out on an octadecyl-terminated Si(111) surface to create dot-array patterns under ambient conditions in contact mode. The kinetics investigations indicate that this SPO process involves three stages. Within the steadily growing stage, the height of oxide dots increases logarithmically with pulse duration and linearly with pulse voltage. The lateral size of oxide dots tends to vary in a similar way. Our experiments show that a direct-log kinetic model is more applicable than a power-of-time law model for the SPO process on an alkylated silicon in demonstrating the dependence of oxide thickness on voltage exposure time within a relatively wide range. In contrast with the SPO on the octodecysilated SiO2/silicon surface, this process can be realized by a lower voltage with a shorter exposure time, which will be of great benefit to the fabrication of integrated nanometer-sized electronic devices on silicon-based substrates. This study demonstrates that the alkylated silicon is a new promising substrate material for silicon-based nanolithography.

Gallium arsenide second‐window quantum dot VCSEL

We have processed 1.3 µm range InAs quantum dot oxide confined vertical cavity surface emitting lasers with top distributed Bragg reflectors contacts, all on GaAs substrate. Our devices exhibit acceptable parameters for 10 Gigabit Ethernet standards. 1275 nm lasing operation is achieved with low operation voltage and low threshold current. We study the threshold current variation with temperature in continuous wave and pulsed operation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Local oxidation characteristics on titanium nitride film by electrochemical nanolithography with carbon nanotube tip

The oxidation characteristics of TiN thin films by atomic force microscopy (AFM) electrochemical nanolithography with multiwalled carbon nanotube tip was investigated. The TiN films were produced on silicon substrate by atomic layer chemical vapor deposition (ALCVD). The electrochemical parameters, such as anodized voltages, oxidation times, writing speeds, pulse voltage periods and how they affected the creation and growth of the oxide nanostructures were explored. The results showed that the height of the TiN oxide dots grew as a result of either the anodization time or the anodized voltage being increased. The oxide growth rate was dependent on the anodized voltage and on the resulting electric field strength. Furthermore, as the electric field strength was at an order of 2 × 10 7 V/cm, the anodization rate decreased quickly and the oxide dots stopped growing. Auger electron spectroscopy (AES) measurements confirm the modified structures took the form of anodized TiN.

Analysis of the process of anodization with AFM

Nanofabrication is necessary for building a nanodevice, and nanometer scale oxide structures are primary elements for achieving quantum effect. Local oxidation induced by electric field with AFM is a promising method. In this paper, an experimental setup with process monitor is established for analyzing the fabrication technology. Some oxide dots and lines on Si surface were fabricated using the conductive atomic force microscope, while the electric current between the tip and the sample is monitored. The process of anodization is analyzed based on the variational current. The electric charge dependence on the electric current plays an important part in the formation of oxide structures, thus the cross section of oxide dots or lines can be controlled by adjusting the current between the tip and the sample in the process of anodization.

Mechanisms of p-GaAs(100) surface by atomic force microscope nano-oxidation

Nanopattering using atomic force microscope (AFM) has become an important area of research, for both fundamental research and future nanodevice applications. Local oxidation of p-GaAs(100) surface by using a negatively biased conductive AFM tip is a universal method for this purpose. The dependences of the height, aspect ratio and volume on applied anodization times and voltages during which the anodization voltage is applied were studied. We explore the kinetics and mechanisms of the anodization process and how factors such as the electric field strength and the relative humidity influence its growth rate and the contribution of ionic diffusion. The results revealed that the protruding oxide dot's height, aspect ratio and volume increase during longer anodization time and at larger anodization voltage as well as in higher relative humidity conditions. The high initial growth rate (∼300 nm s−1 for 10 V) decreases quickly with decreasing electric field strength and the oxide practically ceases to grow at an order of (2–3) × 107 V cm−1. Auger electron spectroscopy measurements confirm that the modified structures take the form of anodized p-GaAs(100). Also, the contribution of ionic diffusion increases by about 80% at a higher relative humidity. In addition, the nanohardness of the oxide structures was measured with the aid of an AFM-based nanoindentation technique.

Spatially resolved diagnosis of stress-induced breakdown in oxide dots by in situ conducting atomic force microscopy

We report an investigation on the stress-induced breakdown (BD) in ultrathin oxide grown by atomic force microscopy (AFM oxide). A conducting atomic force microscopy (c-AFM) technique was employed to stress the AFM oxide and examine its BD behavior. It was found that thermal annealing has a strong impact on the dielectric strength of AFM oxide. The stress-induced trap generation probability, Pt, could be reduced by ∼50% after annealing the oxide at elevated temperatures. Such a thermal effect is related to the local structural relaxation and trap state minimization in AFM oxide upon annealing. The spatially resolved current images allow a microscopic diagnosis of the distribution of BD sites: isolated single BD spots and laterally propagated BD areas were observed in an oxide dot. Soft and hard breakdown sites were also distinguished on the current images.

Oxidation characteristics of nanodot and nanobump on TiN thin films by atomic force microscopy

The electrochemical oxidation characteristics of TiN thin films by atomic force microscopy (AFM) was investigated. The TiN films were produced on silicon substrate by atomic layer chemical vapor deposition (ALCVD). The anodization parameters, such as the anodized voltages, the oxidation times, and how they affected the creation and growth of the oxide nanostructures were explored. The results showed that the height of the TiN oxide dots grew as a result of either the anodization time or the anodized voltage being increased. The oxide growth rate was dependent on the anodized voltage and the resulting electric field strength. Furthermore, the oxide growth rate decreased immediately when the electric field strength reached (2–3) × 10 7 V/cm rapidly decrease to a growth rate of 0.

Formation of microscopic coloured oxide dots on the titanium foil surface irradiated by a laser

Experiments are performed on the formation of coloured dots of a submillimetre size on the titanium foil surface irradiated by pulsed IR lasers. It is shown that the parameters of laser pulses can be selected so that the dots produced on the titanium foil surface will change their colour from yellow to blue with increasing exposure time. The threshold laser power densities required for the formation of contrast coloured dots on the titanium foil surface irradiated by laser pulses with low repetition rates are estimated as ~1.5 × 103 J cm-2 for a 1.73-μm Xe laser and ~2.3 × 103 J cm-2 for a 10.6-μm CO2 laser.

Faradaic current detection during anodic oxidation of the H-passivated p-Si(001) surfacewith controlled relative humidity

Faradaic current during anodic oxidation is measured over a relative humidityrange of 40–70% using an atomic force microscope with humidity control. Thelevel of detected current during the fabrication of oxide dots on H-passivatedSi(001) is in the picoampere (pA) level. Current flow began immediately (within afew milliseconds) after applying an oxidation voltage above a threshold valueand decreased with time according to oxide growth. The total charge resultingfrom the current flow was calculated by integrating the current–time curve andwas found to agree well with an estimation of expected current from the volumeof the fabricated oxide dots. Actual monitoring of the oxidation process by theFaradaic current is demonstrated during the fabrication of a two-dimensionallattice.

Formation and electric property measurement of nanosized patterns of tantalum oxide by current sensing atomic force microscope

Nanosized patterns of tantalum oxide were fabricated on a tantalum substrate by applying a potential pulse utilizing current sensing atomic force microscopy (CSAFM). The dimensions of the dots were strongly dependent on the bias applied, scan rate, and potential pulse duration. By controlling these variables, the minimum size nanodots with full width at half maximum of 35 nm was achieved. Immediately after pattern formation, the electrical properties of the Ta oxide nanodots were measured using CSAFM. The charge transport at the CSAFM tip and the nanosized Ta oxide dot can be described by Poole–Frenkel type conduction. The relative dielectric constant of the nanosized Ta2O5 dots was calculated to be 17.8–24.3, showing that the quality of the oxide was high. In addition, by controlling the substrate bias applied, pulse duration, and tip scan speed, nanosized Ta oxide lines with the desired dimensions were prepared.

Fabrication of highly aligned nano-hole/trench structures by atomic force microscopy tip-induced oxidation and atomic hydrogen cleaning

We fabricated highly aligned nano-hole and trench structures on GaAs (0 0 1) by a combination of atomic force microscope (AFM) tip-induced oxidation and atomic hydrogen etching. Highly aligned oxide dot and line structures at the nano-meter scale were patterned by using AFM tip-induced oxidation. Then, the oxide structures and surface oxides were removed by using atomic hydrogen. Finally, nano-hole and trench structures with atomically flat surface were successfully achieved. The smallest hole diameter obtained in the present work was 13.4 nm with the depth of 0.47 nm.

Field emission from zinc oxide nanopins

Nanostructural zinc oxide pins have been fabricated by vapor transport on copper-coated silicon wafer. The nanopins are composed of hexagonal wurtzite-phase zinc oxide with single crystal quality. The growth process includes two steps: (1) growth of a micron-sized zinc oxide dot on the substrate and (2) growth of a sharp tip from the zinc oxide dot. The field emission of the nanopins shows a low field emission threshold (1.92 V/μm at a current density of 0.1 μA/cm−2) and high current density with a field enhancement factor of 657. The emission current density and the electric field follow Fowler–Nordheim relationship. The good performance for field emission is attributed to the single-crystalline structure and the nanopin geometry.

Precisely ordered quantum dot array formed using AFM lithography for all‐optical electron spin quantum computers

For application of quantum dots (QDs) in quantum computers, a precise control of self-assembled QDs was achieved in position and size. On a GaAs surface, atomic force microscope lithography is performed to fabricate nano-scaled oxide dots. When the oxide dots are suitably detached from the substrate, holes of the same size are formed. By overgrowing InGaAs on such a hole-patterned surface, one single quantum dot, whose size depends on the hole, is formed on the site of each hole. An ordered array of differently sized QDs can be fabricated as the qubits of all-optical quantum computer using electron spins of coupled QDs.

Nanoscale oxide structures induced by dynamic electric field on Si with AFM

Nanoscale oxide structures are very important for the study of functional nanodevices. Local oxidation induced by electric field with scanning probe microscopy is a promising method. Some oxide lines and dots on Si surface were fabricated using conductive atomic force microscope in this paper. Nanoscale oxide lines induced by dc voltages exhibit the characteristic of single peak, but the hollow structures with higher aspect ratio are observed under the effect of square wave voltages. We present the hollow structures result from the finite diffuse speed and concentration of oxygen ion, and the higher aspect ratio results from the effect of dynamic electric field in the conductor–nonconductor–semiconductor junction formed in the course of fabrication.

Fabrication of nanometric single zinc and zinc oxide dots by the selective photodissociation of adsorption-phase diethylzinc using a nonresonant optical near field

We demonstrated a deposition of nanometer-scale Zn dots using the selective photodissociation of adsorption-phase diethylzinc with a nonresonant optical near field, where the photon energy is lower than that of the absorption edge of gas-phase diethylzinc. We achieved nanometric prenucleation by dissociating diethylzinc molecules adsorbed on a substrate. Subsequent deposition was performed by dissociating the adsorbed molecules on the prenucleated Zn. The topographic image of the deposited Zn dot had a full width at half maximum (FWHM) of 25 nm. Furthermore, the photoluminescence intensity distribution from a single ZnO dot fabricated using laser annealing had a FWHM of 85 nm.

Branched Crystalline Patterns Formed Around Poly(ethylene oxide) Dots in Humidity

Branched patterns reminiscent of diffusion-limited aggregation form around poly(ethylene oxide) dots on Au surfaces in humidity by crystallization of polymer chains in water-assisted diffusion, involving fractal-like structure and dense branching-like morphology (DBM). Higher humidity leads to a slower growth of crystal lamellae. Crystal growth promotes outward diffusion. Fractal-like lamellae are viewed flat-on. Due to the reduction of chain availability in the diffusion field, the fractal-like structure turns into DBM.

Scanning tunneling microscopy and in situ spectroscopy of ultra thin Ti films and nano sized TiO x dots induced by STM

Scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) of a titanium (Ti) amorphous film and titanium oxide dots were investigated at room temperature and in ambient air by scanning tunneling microscope (STM). A smooth amorphous Ti film (3 nm thick) with an area of 1 μm×1 μm was chosen and imaged by STM. Tunneling current versus bias spectra for the film are symmetric to the origin in the STS measurements. Ultra thin films were oxidized at different positions by applying different voltage pulses ranging from 3.5 to 5 V for a period of 5 s. The subsequent images of STM show that the higher the bias applied to the film, the larger the dot. The in situ STS measurements were used, for the first time, to study the contaminant-free TiO x dots just as induced by STM. The in situ corresponding tunneling current versus bias spectrum was preceded by formation of a dot and followed by imaging the dot. STS I– V curves exhibit that the formed TiO x dots are n-type semiconductors. A transformation from slight asymmetry to drastic asymmetry of STS curves and increase in band gap indicate that nano-TiO x dots exhibit metallic to semiconducting behaviors with increase in the voltage pulse applied to sample.

Improvement in Aspect Ratio of P-GaAs Oxide Fabricated by Atomic Force Microscope (AFM)-Based Nanolithography Using Pulsed Voltage

An atomic force microscope (AFM)-based surface nanooxidation method was used to fabricate nanoscale p-GaAs oxide. We were able to realize an increase in the aspect ratio by a factor of 2 for oxide dots using a voltage modulation technique. The aspect ratios of oxide dots reached the maximum at a frequency of about 1000 Hz. Moreover, from a duty ratio dependence of aspect ratios of oxide dots, it was considered that optimization of an anodizing time per cycle of a pulsed voltage was necessary. The oxide could be etched by water. By adjusting both oxidation and etching process conditions, a groove with a 40 nm width and 6 nm depth was successfully fabricated. From these results, it was clear that the aspect ratio of p-GaAs oxide could be improved using a pulsed voltage, and optimization of process conditions, particularly the frequency and duty ratio of a pulsed voltage, was necessary to obtain p-GaAs oxide with a high aspect ratio.