Yoshinobu Nakada

GaN heteroepitaxial growth on silicon nitride buffer layers formed on Si (111) surfaces by plasma-assisted molecular beam epitaxy

Wurtzite GaN films were grown on silicon nitride buffer layers formed on Si (111) substrates by radio frequency plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction, Auger electron spectroscopy, transmission electron microscopy, and photoluminescence results indicate that the single crystalline wurtzite GaN was grown on the buffer layers of amorphouslike silicon nitride formed on Si (111) substrates by taking the following relationship with the substrate: GaN [0001]//Si [111] and GaN (1120)//Si (110). Both faces of the silicon nitride buffer layer were found to be flat and sharp, the thickness of the buffer layer (1–1.5 nm) being constant across the interface. Efficient bound exciton emission was observed at 3.46 eV. The growth technique described was found to be simple but very powerful for growing high quality GaN films on Si substrates.

Scanning tunneling microscopy studies of formation of 8×5 reconstructed structure of Ga on the Si(001) surface

The behavior of Ga on the Si(001) 2×1 surface has been studied for Ga coverage of 0.8 monolayers and annealing temperatures in the range 623–863 K by means of scanning tunneling microscopy. Annealing at the lowest annealing temperature Ta leads to the formation of the Ga 2×2 phase, which is a result of self-arrangement of Ga dimers, as well as irregular Ga clusters. An increase in Ta to 673 K results in the formation of a partially ordered metastable Ga 5×2 phase, whereas further increase in Ta leads to the transformation of the 5×2 structure into 8×n (n=4,5) structure, the degree of order of which gradually increases with an increase in the annealing temperature. At Ta=833 K the surface is uniformly covered by the periodically aligned arrays of 8×5 units, this 8×5 structure is believed to consist of the ordered double-layered Ga clusters having 24–26 Ga atoms in the first layer and four Ga atoms in the second layer. The 8×5 units are out of registry with the underlying Si surface in one direction, this u...

Auger electron spectroscopy studies of nitridation of the GaAs(001) surface

Auger electron spectroscopy has been used to investigate the processes taking place during the initial stages of nitridation of the As-stabilized GaAs(001)-2×4 surface by active nitrogen species generated by a radio-frequency plasma source. The results of analysis of the spectral shape of core-level Auger electron signals from Ga, As, and N, as well as dependencies of the intensities of those signals on the duration of nitridation combined with reflection high-energy electron diffraction results show that nitridation occurs in two distinct steps: the first step (with duration of only a few minutes) being the formation of 1 ML of nitrogen (partially mixed with arsenic) on the surface, and the second stage being the formation of the disordered GaAsN phase, which may be the GaAsxN1−x surface phase. The subsequent thermal annealing for several minutes at 600 °C leads to the desorption of arsenic and the resulting crystallization of the GaAsN phase into a cubic GaN layer of about 20 A thickness.

Nitridation of GaAs(001) surface: Auger electron spectroscopy and reflection high-energy electron diffraction

Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED) have been used to investigate the processes taking place during the initial stages of nitridation of GaAs(001)–2×4 surface by active nitrogen species. The results of analysis of the spectral shapes and the spectral positions of the Auger electron signals from Ga, As, and N, as well as their dependencies on the nitrogen exposure combined with RHEED results show that the processes taking place during nitridation greatly differ depending on the nitridation temperature. On the one hand, at low temperatures (Ts⩽200 °C) nitridation is hindered by kinetic restrictions on the atomic migration, leading to island growth with formation of the disordered GaAsN phase in the subsurface region, which cannot be completely re-crystallized even after prolonged annealing. On the other hand, at high temperatures (Ts⩾500 °C) the process of nitridation takes place simultaneously with etching of the surface due to decomposition of the subs...

Scanning tunneling microscopy study of reconstruction of 0.8 monolayers Ga on an Si (001) surface

The surface structure of 0.8 monolayers (ML) Ga on an Si (001) 2×1 surface has been studied with scanning tunneling microscopy (STM). Periodical clusters, aligned in 〈110〉 directions on an Si (001) surface and reflecting the polarity of terraces on that surface, were observed. The clusters of Ga consist of the 8×5 phase. From the line profiles of the clusters in STM images, each cluster of the 8×5 phase has been found to consist of Ga double layers, the top layer of Ga being composed of four Ga atoms. Clusters in contiguous arrays are out of phase in the direction of the arrays. Pairs of two Ga atoms exist in both longitudinal sides at each cluster along the contiguous arrays. These pairs of two Ga atoms at both sides of the valleys between the contiguous arrays are located alternatively. From the above information on positions of Ga atoms, arrangements of Ga atoms of the clusters of the 8×5 phase are presumed. Two cases are considered for the arrangements of the clusters based on the arrangements of Si a...

Nitridation of GaAs (001) Surface Studied by Auger Electron Spectroscopy

Auger electron spectroscopy (AES) has been used to investigate the processes taking place during the initial stages of nitridation of GaAs(001)-2×4 surface by active nitrogen species. The results of analysis of the spectral positions of the Auger electron signals from Ga, As and N, as well as their dependencies on the nitrogen exposure show that the processes taking place during nitridation greatly differ depending on the nitridation temperature. At low temperatures (≤200°C) nitridation is hindered by kinetic restrictions on the atomic migration, leading to the island growth with the formation of the disordered GaAsN phase in the subsurface region. At high temperatures (≥500°C) the process of nitridation takes place simultaneously with the etching of the surface due to decomposition of the substrate, resulting in the rough interface between the GaN and GaAs phases. However, for intermediate nitridation temperatures (300°C–400°C) AES results indicate that one monolayer of N atoms may be formed on the substrate during the initial stage of nitridation. The post-nitridation annealing of the samples nitrided at the intermediate temperatures results in the formation of a crystalline GaN layer, the line shape of the AES signals from which is identical to that for a GaN reference sample.

Fast Nanoimprint Thermal Lithography Using a Heated High-Aspect Ratio Mold

The throughput of the thermal nanoimprint lithography is improved. The proposed process uses a high-aspect ratio mold. The mold is heated to temperatures above the glass transition temperature of the resist, and only the top part of the mold is used to press the resist. The mold press time, the resist cooling time, and the mold releasing time can be reduced. A line pattern having a width of 100 nm and a pitch of 400 nm, and a dot pattern having a diameter of 75 nm and a pitch of 200 nm were successfully transferred to the resist using SiC molds. The imprint time was 5 s.

STM observation of wurtzite GaN(0 0 0 1) surface grown by MBE on 6H-SiC substrates

Surface structure of molecular beam epitaxy (MBE)-grown wurtzite GaN epilayers on 6H-SiC substrates was investigated by scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). Many columnar crystals were found to grow on 6H-SiC substrates and surfaces of the columnar crystals were found to be covered with several nanometer scale particle-like structures.

Energy Shifts of Auger Transitions of Ga, As and N during Plasma-assisted Nitridation of GaAs (001) Surface

Auger electron spectroscopy (AES) is used to study the processes taking place during the initial stages of nitridation of GaAs(001)-2×4 surface by active nitrogen species generated by a radio-frequency (RF) plasma source. Two effects, i.e. a chemical shift and a shift of the Fermi level due to band bending, take place during nitridation, these effects causing shifts of the Auger transitions of Ga, As and N. The shifts are the most pronounced for nitridation temperatures in the range 300°C–400°C, when a uniform GaN layer is believed to be formed on the surface. The post-nitridation thermal annealing of the samples nitrided in the above temperature range leads to re-crystallization of the GaAsN phase, formed as a result of nitridation, into cubic GaN layer.

Nitridation of GaAs (001)-2x4 Surface Studied by Auger-Electron Spectroscopy

Auger electron spectroscopy (AES) was used to investigate the processes taking place during the initial stages of nitridation of GaAs (001) surface. The analysis of the AES results combined with that of RHEED show that the processes taking place during nitridation greatly differ depending on the nitridation temperature. At low temperatures (≤ 200°C) nitridation is hindered by kinetic restrictions on atomic migration, whereas at high temperatures (≥ 500°C) the process of nitridation takes place simultaneously with the etching of the surface. However, for intermediate temperatures (300°C ∼ 400°C) the results indicate that a complete monolayer of N atoms may be formed on the substrate during the initial stage of nitridation. The post-nitridation annealing of the samples nitrided at the intermediate temperatures results in the formation of a crystalline GaN layer, the line shape of the AES signals from which is identical to that for a GaN reference sample.