T.T. Chen

NANOMACHINING OF (110)-ORIENTED SILICON BY SCANNING PROBE LITHOGRAPHY AND ANISOTROPIC WET ETCHING

We have demonstrated that silicon nanostructures with high aspect ratios, having ∼400 nm structural height and ∼55 nm lateral dimension, may be fabricated by scanning probe lithography and aqueous KOH orientation-dependent etching on the H-passivated (110) Si wafer. The high spatial resolution of fabricated features is achieved by using the atomic force microscope based nano-oxidation process in ambient. Due to the large (110)/(111) anisotropic ratio of etch rate and the large Si/SiO2 etch selectivity at a relatively low etching temperature and an optimal KOH concentration, high-aspect-ratio gratings with (111)-oriented structural sidewalls as well as hexagonal etch pit structures determined by the terminal etch geometry can be obtained.

Nanometer-scale conversion of Si3N4 to SiOx

It has been found that atomic force microscope (AFM) induced local oxidation is an effective way for converting thin (<5 nm) Si3N4 films to SiOx. The threshold voltage for the 4.2 nm film is as low as 5 V and the initial growth rate is on the order of 103 nm/s at 10 V. Micro-Auger analysis of the selectively oxidized region revealed the formation of SiOx. Due to the large chemical selectivity in various etchants and great thermal oxidation rate difference between Si3N4, SiO2, and Si, AFM patterning of Si3N4 films can be a promising method for fabricating nanoscale structures.

Heteroepitaxy of GaN on Si(111) realized with a coincident-interface AlN/β-Si3N4(0001) double-buffer structure

We present a stacked buffer mechanism for heteroepitaxial growth with large lattice mismatch. The stacked buffer consists of constituent layers, which can form coincident lattices at layer/layer and layer/substrate interfaces. For the case of GaN-on-Si(111) heteroepitaxy, we utilize the 1:2 and 5:2 coincident lattices formed at the β-Si3N4(0001)/Si(111) and AlN(0001)/β-Si3N4(0001) interfaces, respectively, to facilitate the double-buffer layer for GaN-on-Si heteroepitaxial growth. By using this buffer technique, we resolve the issue of autodoping resulting from Si outdiffusion when grown with a single AlN(0001) buffer. As a result, the epitaxial quality of GaN film is also significantly improved.

Local electric-field-induced oxidation of titanium nitride films

Nanometer-scale patterning of TiN films grown on SiO2/Si(001) has been demonstrated using the local electric-field-induced oxidation process with a conductive-probe atomic force microscope. The chemical composition of the modified TiN region was determined by micro-Auger electron spectroscopy and was found to consist of Ti, some trace amount of N, and O, suggesting the formation of titanium oxynitride in the near surface region. The dependence of the oxide height on the sample bias voltage with a fixed scanning speed shows a nonlinear trend in the high electric field regime, indicating that the growth kinetics might be significantly different from previous studies using other film materials.

Surface-enhanced Raman scattering of water adsorbed on silver electrodes

Abstract Temporal evolution of the Raman spectra of H2O, D2O and HDO during an oxidation-reduction cycle of a Ag electrode in aqueous 1 M KCl or KBr has been recorded with an optical multichannel analyzer. Surface enhanced Raman spectra of the adsorbed water are readily observable and are different from the Raman spectra of bulk water.

Irreversible loss of adatoms on Ag electrodes during potential cycling determined from surface enhanced Raman intensities

Abstract Surface enhanced Raman scattering of adsorbates (Ag0-Cl−, H2O and pyridine) on Ag electrodes in 1M KC1 and 1M KCl+0.05M pyridine electrolytes was monitored continuously with an optical multichannel analyzer system as the electrode potential was cycled over various ranges within nonfaradaic regions of the oxidation-reduction cycle. A systematic investigation was performed of the potential dependence of SERS of pyridine in 1M KX + 0.05M pyridine electrolytes, where X = F, Cl, Br and I. Since the surface coverage of the adsorbates is reversible with potential cycling within a potential range, it was possible to determine potential dependences of the irreversible loss in the SERS enhancement factor which occurs as the electrode potential is ramped toward the potential of zero charge (PZC). The results provide strong support for the role of adatoms on the electrode surface in the overall enhancement mechanism. There is evidence that the strongly bound adsorbates immobilize the adatoms at positive potentials but allow the adatoms to migrate and become lost at surface defects as the potential approaches the PZC where the adsorbates are less tightly bound.

Nano-oxidation of silicon nitride films with an atomic force microscope: Chemical mapping, kinetics, and applications

We demonstrate that local oxidation of silicon nitride films deposited on conductive substrates with a conductive-probe atomic force microscope (AFM) is a very promising approach for nanofabrication. Scanning Auger microscopy and spectroscopy are employed to verify the chemical changes after AFM-induced oxidation. Furthermore, the growth kinetics are found to have a logarithmic relationship of oxide height versus pulse duration [h∝ln(t/t0)]. In contrast to rather slow thermal oxidation process, AFM-induced oxidation on silicon nitride has an anomalously high initial oxidation rate (∼30 000 nm/s at 10 V) and a small onset time t0 (∼10 μs). As for the applications in ultrahigh-density recording, an oxide dot array (∼100 Gbit/in.2) produced by this process is demonstrated. The nitride film patterned by AFM can be utilized as an etching mask to fabricate “subtractive” silicon nanostructures, due to the large etching selectivity of Si3N4:SiO2:Si in various etchants. With this method, which is entirely compatible with the existing microelectronic processes, synthesis of ultrahigh packing density and ordered nanostructures could become readily achievable.We demonstrate that local oxidation of silicon nitride films deposited on conductive substrates with a conductive-probe atomic force microscope (AFM) is a very promising approach for nanofabrication. Scanning Auger microscopy and spectroscopy are employed to verify the chemical changes after AFM-induced oxidation. Furthermore, the growth kinetics are found to have a logarithmic relationship of oxide height versus pulse duration [h∝ln(t/t0)]. In contrast to rather slow thermal oxidation process, AFM-induced oxidation on silicon nitride has an anomalously high initial oxidation rate (∼30 000 nm/s at 10 V) and a small onset time t0 (∼10 μs). As for the applications in ultrahigh-density recording, an oxide dot array (∼100 Gbit/in.2) produced by this process is demonstrated. The nitride film patterned by AFM can be utilized as an etching mask to fabricate “subtractive” silicon nanostructures, due to the large etching selectivity of Si3N4:SiO2:Si in various etchants. With this method, which is entirely compatib...

High Resolution Electron Tunneling Spectroscopy

A system which is capable of resolving changes in a tunneling junction conductance σ of about one part in 105 as well as providing calibrated values of dσ/dV is described. Methods of simultaneous analog and digital recording on magnetic tape suitable for subsequent computer analysis are given. The flow chart for the computer programming is also presented along with a typical result.

Local electron field emission characteristics of pulsed laser deposited diamondlike carbon films

Local electron field emission properties of diamondlike carbon (DLC) films were measured by a probe method in 1 atmosphere ambient pressure, using the modified scanning tunneling microscopic (STM) system, and the diode method. In the STM probe method, the field emission was turn on at a low bias as +2.67 V. A large emission current of 40 nA was attained at +7 volt bias voltage and the emission current density was estimated as Jt=5 A/cm2. By contrast, the current–voltage (I–V) characteristics of the same DLC films measured by diode method in 10−6 Torr ambient pressure revealed that the electron field emission was produced for a 13.2 V/μm electric field, and a high emission current density of J=160 μA/cm2 was obtained for a E=20 V/μm electric field. The geometrical enhancement factor (α), evaluated from the Fowler–Nordheim plot was around α=30, which is larger than what would be expected from a planar surface. This can be interpreted as the evidence that electrons are emitted locally from spherical SP3 clus...

Surface enhanced second-harmonic generation from Ag electrodes and adsorbates during electrochemical cycling

Second-harmonic generation from the Ag electrode surface and its adsorbate (AgCl or Ag2SO4) is measured. Temporal correlation of the second-harmonic intensity with the electro-chemical cyclic voltammogram is made and is compared with the evolution of the surface enhanced Raman scattering from the adsorbate during an oxidation-reduction cycle. The evolution of the diffused second-harmonic intensity from the Ag electrode and adsorbate from mechanically and etchant roughened surfaces to fully developed electrochemically roughened surfaces is presented. Laser radiation induced surface morphology changes of the silver electrode surface is noted from scanning electron micrographs.