Wet sulfur passivation of GaSb(1 0 0) surface for optoelectronic applications

A comparative analysis of the properties of the non-passivated and S-passivated GaSb(1 0 0) surfaces has been performed through PL, AFM and RHEED characterization. The samples treated with a 1 M Na₂S aqueous solution demonstrate an increase in the 5 K PL intensity. According to AFM data, the annealing of the S-passivated GaSb(1 0 0) leads to the formation of the clean flat (1 0 0) surface. Moreover, after annealing the PL intensity of the S-passivated GaSb(1 0 0) surfaces decreases by 20%, whereas for the non-passivated samples it drops by more than a factor of 4. The method of wet sulfur passivation has shown great effectiveness in pre-epitaxial processing for LPE and MBE growth of the GaSb-related materials for optoelectronics.     

On the influence of the surface roughness onto the ultrathin SiO2/Si structure properties

The surface roughness of the semiconductor substrate substantially influences properties of the whole semiconductor/oxide structure. SiO₂/Si structures were prepared by using low temperature nitric acid oxidation of silicon (NAOS) method and then the whole structure was passivated by the cyanidization procedure. The influence of the surface morphology of the silicon substrate onto the electrical properties of ultrathin NAOS SiO₂ layer was investigated. Surface height function properties were studied by the AFM method and electrical properties were studied by the STM method. The complexity of analyzed surface structure was sensitive to the oxidation and passivation steps. For describing changes in the oxide layer structure, several fractal measures in an analysis of the STM images were used. This fractal geometry approach enables quantifying the fine spatial changes in the tunneling current spectra.     

Investigation of material removal mechanism of silicon wafer in the chemical mechanical polishing process using molecular dynamics simulation method

Chemical mechanical polishing (CMP) technology, being the mainstream technique of acquiring global planarization and nanometer level surface, has already become an attractive research item. In the case of CMP process, the indentation depth lies in the range of nanometer or sub-nanometer, huge hydrostatic pressure induced in the local deformation area which makes the material removal and surface generation process different from traditional manufacturing process. In order to investigate the physical essence of CMP technique, the authors carry out molecular dynamics (MD) analysis of chemical mechanical polishing of a silicon wafer. The simulation result shows that huge hydrostatic pressure is induced in the local area and leads to the silicon atom transform from the classical diamond structure (α silicon) to metal structure (β silicon). This important factor results in the ductile fracture of silicon and then in the acquisition of a super-smooth surface.     

Crystalline instability of Bi-2212 superconducting whiskers near room temperature

We report new evidences for the thermodynamic instability of whisker crystals in the Bi–Sr–Ca–Cu–O (BSCCO) system. Annealing treatments at 90°C have been performed on two sets of samples, which were monitored by means of X-Rays Diffraction (XRD) and Atomic Force Microscopy (AFM) measurements, respectively. Two main crystalline domains of Bi₂Sr₂CuCa₂O_8+x (Bi-2212) were identified in the samples by the XRD data, which underwent an evident crystalline segregation after about 60 hours. Very fast dynamics of the surface modifications was also described by the AFM monitoring. Two typologies of surface structures formed after about 3 annealing hours: continuous arrays of dome shaped bodies were observed along the edges of the whiskers, while in the central regions a dense texture of flat bodies was found. These modifications are described in terms of the formation of simple oxide clusters involving a degradation of the internal layers.     

Molecular dynamics study of groove fabrication process using AFM-based nanometric cutting technique

Three-dimensional molecular dynamics simulation of groove fabrication using atomic force microscopy (AFM)-based nanometric cutting technique is set up, fabrication processes of grooves with two types (line, and folder line) and five folder angles (0°, 30°, 45°, 60°, 90°) are simulated to investigate the effect of groove geometry on the fabrication process. The results show that the Normal force, Lateral force, and Resultant forces are almost symmetric with respect to the critical folder angle of 45°. The best surface quality of fabricated groove can be obtained at the folder angle of 45°. It reveals that the groove geometry has a significant effect on the groove fabrication process due to the material anisotropy on the atomic scale.     

Organic thin-film transistors of pentacene films fabricated from a supersonic molecular beam source

Top-contact organic thin-film transistors (OTFTs) of pentacene have been fabricated on bare SiO₂ and SiO₂ modified with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The pentacene films were deposited from a supersonic molecular beam source with kinetic energy of incident molecules ranging from 1.5 to 6.7 eV. The field-effect mobility of OTFTs was found to increase systematically with increasing kinetic energy of the molecular beam. The improvements are more important on HMDS- and OTS-treated surfaces than on bare SiO₂. Tapping mode atomic force microscopy images reveal that pentacene thin films deposited at high kinetic energy form with significantly larger grains—independent of surface treatment—than films deposited using low-energy beams.     

The effect of nitrogen incorporation on surface properties of silicon oxynitride films

In order to investigate the surface heterogeneity of silicon oxynitride films, we observed the nanoscale variation of the surface potential by Kelvin probe force microscopy (KFM), the molecular bonding characteristics by Fourier transform infrared spectrometry (FTIR), and the wetting behavior by contact angle measurement. Nitrogen incorporation into silicon oxynitride films influenced the decrease in the surface potential and the polar component of the surface free energy. We present the first correlation between the nanoscale measurement of the surface potential and the macroscopic measurement of the surface free energy in silicon oxynitride films grown by a standard plasma‐enhanced chemical vapor deposition (PECVD) technique. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterisation of the double perovskite Sr2TaMnO6

The presented compound, Sr₂TaMnO₆, has a weak, disordered magnetic structure. The metal oxide was prepared under high isostatic oxygen pressure. The doubling of the perovskite structure was proven with electron diffraction and powder neutron diffraction. Combining neutron- and X-ray diffraction data, the room-temperature structure was modelled with the Rietveld method. Both octahedral positions are partially occupied by Mn and Ta, but with different Mn/Ta ratios. AC- and DC-magnetic measurements indicate a magnetic transition at about 17 K and the AC-magnetic susceptibility, both real and imaginary part, is frequency dependent, suggesting that the material has a spin-glass feature. The magnetic spins freeze during a wide temperature range and a possible explanation is a competative situation between the double exchange (ferromagnetism) and the super-exchange (anti-ferromagnetism).     

Influence of annealing atmosphere on ZnO thin films grown by MOCVD

ZnO films were deposited on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Annealing treatments for as-deposited samples were performed in different atmosphere under various pressures in the same chamber just after growth. The effect of annealing atmosphere on the electrical, structural, and optical properties of the deposited films has been investigated by means of X-ray diffraction (XRD), atomic force microscope (AFM), Hall effect, and optical absorption measurements. The results indicated that the electrical and structural properties of the films were highly influenced by annealing atmosphere, which was more pronounced for the films annealed in oxygen ambient. The most significant improvements for structural and electrical properties were obtained for the film annealed in oxygen under the pressure of 60 Pa. Under the optimum annealing condition, the lowest resistivity of 0.28 Ω cm and the highest mobility of 19.6 cm² v⁻¹ s⁻¹ were obtained. Meanwhile, the absorbance spectra turned steeper and the optical band gap red shifted back to the single-crystal value.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.