Noriyuki Hasuike

Epitaxial growth and characteristics of N-doped anatase TiO2 films grown using a free-radical nitrogen oxide source

This paper reports on a systematic investigation of nitrogen (N) doping in single-crystalline TiO2 films possessing an anatase phase. TiO2 films heavily doped with N were grown with high crystallinity by employing a pulsed laser ablation technique using a close lattice-matched LaAlO3 (100) substrate and NO as the source gases for N doping. N15 and O18 isotope tracing by secondary-ion-mass spectroscopy confirmed that N and O atoms were simultaneously incorporated into the films as a consequence of the gas phase reactions between Ti-related chemical species ablated by the laser and NO free radicals. The films yielded an absorption band at around 3.0 eV, as expected from the yellowish color, with band tailing becoming stronger with increasing N concentration. It is suggested that N-related complex defects in TiO2 acted as carrier compensation centers. Micro-Raman spectroscopy with a spatial resolution of 1μm2 revealed host-lattice defects and generation of secondary phases at the microscopic scale by N dopin...

Critical thickness and lattice relaxation of Mg-rich strained Mg0.37Zn0.63O (0001) layers towards multi-quantum-wells

The heteroepitaxy of Mg-rich Mg0.37Zn0.63O layers on ZnO (0001) substrates was carried out using laser molecular-beam epitaxy. Mg0.37Zn0.63O layers changed from a two-dimensional (2D) to three-dimensional growth mode at a layer thickness (tc) between 38 and 100 nm through lattice-strain relaxation. For tc>100nm, hexagonal nanodots with a density in the order of 109cm−2 formed naturally by the Stranski-Krastanov mode. The individual nanodots possessed pyramidal hillocks with lateral sizes raging from 100 to 200 nm, and phase separation to Mg-rich and Mg-poor regions in the Mg0.37Zn0.63O alloys was found from the results of atomic force microscopy and microphotoluminescence spectroscopy. A suitable layer thickness of Mg0.37Zn0.63O concerning quantum barriers was speculated as being 38 nm from a theoretical calculation based on the Matthew and Blakeslee model [J. Cryst. Growth 27, 118 (1974)]. For tc≦38nm, the top surface of the Mg0.37Zn0.63O layer was very flat due to the curtailment of 2D growth. This cont...

Correlation between structural and luminescent properties of Eu3+-doped ZnO epitaxial layers

We have studied the epitaxial growth and photoluminescent (PL) properties of Eu3+-doped ZnO layers in correlation with structural analyses. Incorporation of Eu3+ ions into a ZnO host resulted in deterioration in crystal quality and lattice distortion. The spectral structure of Eu3+ emission revealed that Eu3+ ions were located at site symmetries lower than the original C3v of the Zn2+ ion sites in a hexagonal wurtzite structure of ZnO, which allowed for efficient red luminescence based on the electric dipole transitions. Moreover, we have found an anticorrelation of PL intensity between the excitonic and Eu3+ emissions, related to energy transfer from the ZnO host to the Eu3+ ions. The excitonic structure at the band edge was obscured upon doping with Eu3+ ions due to the formation of a band tail around 3.2 eV. This correlated with a reduction in excitonic emissions, leading to enhancement of Eu3+ emission. Strong PL emission from Eu3+ ions was only observed at low temperatures, and was remarkably suppres...

Characterization of ZnO nanoparticles by resonant Raman scattering and cathodoluminescence spectroscopies

We have measured the Raman and cathodoluminescence (CL) spectra of ZnO nanoparticles with different crystallite sizes. The Raman spectra show that the frequency of the E2 (high) phonon in these nanoparticles, which is initially high, decreases with the crystallite size, while its linewidth increases. This result is explained by the size effect. The CL spectra show that the relative intensity ratio of the CL peak at around 500–600nm to the band-edge CL peak at approximately 360nm drastically increases as the crystallite size decreases. We use the surface recombination model to explain this result. According to this model, the relative intensity ratio increases because of an increase in the concentration of deep levels on the surface of the crystallites and/or an increase in the free-carrier concentration.

Systematic investigation on structure and excitonic-related transitions: An evidence for Zn1−xCoxO alloy film as a wide gap semiconductor

Systematic investigations were performed concerning the crystal structure and excitonic-related transitions of Zn1−xCoxO layers. The substitution of Co atoms for Zn sites was initially identified from the intra-emissions originating from the 3d levels of the Co2+ ions. Microresonant Raman scattering (μ‐RRS) revealed that the local crystal structure of the host was disordered even at Co concentrations of a few percent, as determined by monitoring the A1(LO) mode and also supported by the ω-rocking curves obtained from x-ray diffraction. A critical point (Γ-edge) derived from magnetic circular dichroism systematically shifted to higher energy with increasing x, representing clear evidence for a widening of the band gap, and that the photoluminescent peaks correlated closely with absorption peaks related to excitonic transitions. The obvious excitonic structures gradually became diffuse with the appearance of photoexcited charge-transfer levels of Co2+ ions when x exceeded 2%. This allowed for observations o...

Few-layer epitaxial graphene grown on vicinal 6H–SiC studied by deep ultraviolet Raman spectroscopy

Few layer epitaxial graphenes (1.8–3.0 layers) grown on vicinal 6H–SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

Synthesis and Characterization of Mn-Doped BiFeO_3 Nanoparticles

BiFeO3 is a multiferroic material showing antiferromagnetic ordering and ferroelectric behavior simultaneously. Here, Mn-doped BiFeO3 nanoparticles were synthesized up to 10% of Mn composition by a sol-gel process. The samples showed high crystallinity with no secondary phase up to 2% of Mn doping. A phonon peak at 1250 cm−1 in undoped BiFeO3 showed anomalous intensity enhancement in the magnetically ordered phase below TN = 643 K due to a spin-phonon coupling. This behavior was less pronounced in the Mn-doped samples, suggesting a suppression of magnetic ordering between Fe spins by Mn doping.

Stranski-Krastanov growth in Mg0.37Zn0.63O∕ZnO heteroepitaxy: Self-organized nanodots and local composition separation

Heteroepitaxy of strained Mg0.37Zn0.63O layers on ZnO (0001) substrates introduces a transition from two-dimensional to Stranski-Krastanov growth accompanying lattice relaxation at a critical thickness, as indicated by the nanodots formation on the growing planar layer. Due to differences in the atomic size of Mg and Zn, the strained layer was compressively distorted at the heterointerface and alloy compositions fluctuated locally. Elastic strain induced by the lattice misfit acted as a driving force for the self-organization of surface nanodots. Moreover, a strained field generated by alloy fluctuation caused local composition separation with a correlation between surface roughening and a lateral Mg segregation.

Noncontact In-Line Monitoring of Ge Content and Thickness Variations of Epitaxial Si1-xGex Layers on Si(100) Using Polychromator-Based Multiwavelength Micro-Raman Spectroscopy

Ge content and thickness variations of epitaxial Si1-xGex layers on Si(100) were nondestructively measured using a long-focal-length, polychromator-based, multiwavelength micro-Raman spectroscopy system, which is designed for noncontact, in-line process and material property monitoring. The Ge content and thickness variations measured using the Raman system showed good agreement with results from both high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS) analysis. The nondestructive depth profiling capability of the multiwavelength Raman spectroscopy was verified on Si1-xGex layers on Si(100) intentionally profiled with a Ge content gradient.

Polymorphism in the ferromagnetic GaCrN-diluted magnetic semiconductor: Luminescence and structural investigations

Luminescence properties of the transition-metal chromium-doped gallium nitride (GaCrN) epitaxial layers on sapphire substrates have been extensively studied by varying the doping level, excitation intensity, and temperature of study. Photoluminescence spectra of the doped films were found to be dominated by an excitonic peak around 3.25–3.29 eV in accordance with the doping level, due to the transition of neutral-donor bound excitions of GaCrN cubic modification. It is attributed that Cr, which is an acceptor in GaN matrix, acts as a surfactant and modifies the growth kinetics to induce phase modification from wurtzite to zinc blende of GaN. Moreover, Cr doping induced a blue band centered around 2.9 eV, which is a donor-acceptor pair, and also two bound excitonic lines at 3.36 and 3.31 eV at some locations of the sample: the expected origin of these is due to the transitions of excitons bound at the stacking faults which are formed due to the co-existence of hexagon and cubic modifications of GaN. The Ra...