Masashi Kubota

Blue Light-Emitting Diode Based on ZnO

A near-band-edge bluish electroluminescence (EL) band centered at around 440 nm was observed from ZnO p–i–n homojunction diodes through a semi-transparent electrode deposited on the p-type ZnO top layer. The EL peak energy coincided with the photoluminescence peak energy of an equivalent p-type ZnO layer, indicating that the electron injection from the n-type layer to the p-type layer dominates the current, giving rise to the radiative recombination in the p-type layer. The imbalance in charge injection is considered to originate from the lower majority carrier concentration in the p-type layer, which is one or two orders of magnitude lower than that in the n-type one. The current-voltage characteristics showed the presence of series resistance of several hundreds ohms, corresponding to the current spread resistance within the bottom n-type ZnO. The employment of conducting ZnO substrates may solve the latter problem.

Nonpolar m-plane InGaN multiple quantum well laser diodes with a lasing wavelength of 499.8 nm

We demonstrated nonpolar m-plane InGaN multiple quantum well laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 499.8 nm, which is the longest reported for GaN-based LDs. A maximum optical output power of 15 mW was achieved, with the threshold current and the corresponding threshold current density (Jth) of 46 mA and 3.1 kA/cm2, respectively. The correlation between lasing wavelength shift and electrical input power (Pin) under cw operation was investigated using LDs of which reflectivity of front facet were varied from 70% to 97%. The lasing wavelength increased with increasing Pin with a slope of 4.56 and 4.34 nm/W for 70% and 97% mirror, respectively. The result suggested that the redshift due to self-heating is more predominant than the blueshift due to band filling above Jth even at near green region for nonpolar GaN-based LDs and reduction in Pin is indispensable to improve wavelength stabilization.

Pure Blue Laser Diodes Based on Nonpolar

Blue laser diodes (LDs) based on m-plane gallium nitride were demonstrated by using m-plane GaN substrates. The lasing wavelength and the threshold current under pulsed operation were 451.8 nm and 134 mA (22.3 kA/cm2), respectively. The device structures consisted of InGaN-based multi-quantum wells, InGaN guiding layers, and Al-containing cladding layers. The InGaN guiding layers play two roles; as appropriate optical waveguides for longer lasing wavelengths and for the prevention of macroscopic cracks parallel to the c-plane. The latter is an indispensable technology in order to fabricate nonpolar LDs for longer wavelengths beyond the blue region.

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We demonstrated nonpolar m-plane GaN-based blue-green laser diodes (LDs) under continuous-wave (cw) operation with a lasing wavelength of 481 nm. A maximum output power of more than 20 mW was achieved, for which the threshold current and the corresponding threshold current density (Jth) were 61 mA and 6.1 kA/cm2, respectively. The value of Jth and the electroluminescence peak wavelength shift until lasing did not change with lasing wavelength in the range from 459 to 481 nm, though the reflectivities of the cavity facets were fixed for each LD. In addition, the slope efficiency increased with increasing lasing wavelength, from 0.37 W/A at 459 nm to 0.49 W/A at 481 nm. This is the remarkable advantages of nonpolar GaN-based material compared to c-plane material for the realization of green LDs.

-Plane Gallium Nitride with InGaN Waveguiding Layers

Polarized photoluminescence (PL) spectra from nonpolar m-plane InGaN multiple quantum wells (MQWs) in blue laser diode wafers fabricated on m-plane GaN substrates were measured as a function of temperature. The polarization ratio (ρ) and the energy difference between the highest and the second highest valence bands estimated from the energy difference between PL peaks (ΔE) increased with increasing InN molar fraction x (or the estimated anisotropic compressive strain along the m-axis eyy) in the MQWs. The values of ρ at 300K and ΔE were 0.71 and 76meV for the case of 430nm PL peak (x=0.104, eyy=+0.75%) and 0.92 and 123meV for the case of 485nm PL peak (x=0.175, eyy=+1.26%). These results suggest that the preferred stripe direction is the c axis for nonpolar m-plane laser diodes in the region from violet to near green.

High-Efficiency Continuous-Wave Operation of Blue-Green Laser Diodes Based on Nonpolar m-Plane Gallium Nitride

The internal quantum efficiency (ηint) of the near-band-edge (NBE) excitonic photoluminescence (PL) in ZnO epilayers was significantly improved by eliminating point defects, as well as by the use of ZnO high-temperature-annealed self-buffer layer (HITAB) on a ScAlMgO4 substrate as epitaxial templates. Negatively charged Zn vacancy (VZn) concentration was greatly reduced by high-temperature growth, and slower postgrowth cooling (annealing) under minimum oxygen pressure further reduced the gross concentration of positively and negatively charged and neutral point defects, according to the suppression of nonequilibrium defect quenching. The nonradiative PL lifetime (τnr) at room temperature was increased by decreasing the gross concentration of point defects, as well as by decreasing the concentration of VZn. Accordingly, certain point defect complexes incorporated with VZn (VZn-X complexes) are assigned to the dominant nonradiative recombination centers. As a result of the elimination of point defects, a re...

Temperature dependence of polarized photoluminescence from nonpolar m-plane InGaN multiple quantum wells for blue laser diodes

The longitudinal spin Seebeck effect has been investigated for a uniaxial antiferromagnetic insulator Cr(2)O(3), characterized by a spin-flop transition under magnetic field along the c axis. We have found that a temperature gradient applied normal to the Cr(2)O(3)/Pt interface induces inverse spin Hall voltage of spin-current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr(2)O(3). The possible contribution of the anomalous Nernst effect is confirmed to be negligibly small. The above results establish that an antiferromagnetic spin wave can be an effective carrier of spin current.

Improvements in quantum efficiency of excitonic emissions in ZnO epilayers by the elimination of point defects

Magnetic skyrmion, a topologically stable spin-swirling object, can host emergent electromagnetism, as exemplified by the topological Hall effect and electric-current-driven skyrmion motion. To achieve efficient manipulation of nano-sized functional spin textures, it is imperative to exploit the resonant motion of skyrmions, analogously to the role of the ferromagnetic resonance in spintronics. The magnetic resonance of skyrmions has recently been detected with oscillating magnetic fields at 1-2 GHz, launching a search for new skyrmion functionality operating at microwave frequencies. Here we show a microwave magnetoelectric effect in resonant skyrmion dynamics. Through microwave transmittance spectroscopy on the skyrmion-hosting multiferroic crystal Cu₂OSeO₃ combined with theoretical simulations, we reveal nonreciprocal directional dichroism (NDD) at the resonant mode, that is, oppositely propagating microwaves exhibit different absorption. The microscopic mechanism of the present NDD is not associated with the conventional Faraday effect but with the skyrmion magnetoelectric resonance instead, suggesting a conceptually new microwave functionality.

Thermal Generation of Spin Current in an Antiferromagnet.

The continuous-wave (cw) operation of m-plane InGaN-based blue (460 nm) laser diodes (LDs) has been achieved. The threshold current and the corresponding threshold current density were 40 mA and 5.0 kA/cm2, respectively, with a 459 nm lasing wavelength under cw operation. The electroluminescence peak wavelength shift in pulsed mode was only 10 nm (58 meV), from spontaneous emission (at 0.3 mA) to stimulated emission (at 32 mA), which is extremely small when compared with that of c-plane blue LDs. This is first clear experimental demonstration of the advantage in fabricating nonpolar InGaN-based LDs beyond the blue region.

Microwave magnetoelectric effect via skyrmion resonance modes in a helimagnetic multiferroic

An accurate method of estimating polarized light emission was presented for nonpolar m-plane InGaN-based blue light emitting diodes, where the unpolarized component caused by unintentional light scattering was eliminated as noise. The polarization ratios of electroluminescence (EL) at 300 and 100K were 0.85 and 0.98, respectively. The energy difference between the highest and the second highest valence bands was estimated to be 129meV from the temperature dependence of the spectrally integrated EL intensities under the assumption of Fermi statistics. This value agreed with the one (=118meV) obtained directly from the difference of the EL peak energies between two polarized components, the electric fields perpendicular and parallel to the c axis.