Tomoyuki Tanikawa

Tunnel magnetoresistance in MgO-barrier magnetic tunnel junctions with bcc-CoFe(B) and fcc-CoFe free layers

We have investigated the spin tunneling transport in magnetic tunnel junctions (MTJs) with Co40Fe40B20, Co50Fe50, and Co90Fe10 free layers which were deposited on the lower electrode consisting of the same Co40Fe40B20 reference layer/MgO barrier. The tunnel magnetoresistance (TMR) ratio depends critically on the choice of the free layer; the TMR ratios up to 355% were obtained for the MTJs with Co40Fe40B20 free layers and up to 277% with Co50Fe50 free layers, both of which have highly (001)-oriented bcc structures. No high TMR ratio was observed for the MTJs with Co90Fe10 free layer having a polycrystalline fcc structure.

Reduction of Efficiency Droop in Semipolar (1101) InGaN/GaN Light Emitting Diodes Grown on Patterned Silicon Substrates

The semi-polar InGaN-based LEDs exhibits low efficiency droop because the reduction of the polarization field not only made the band diagram smoother but also restricted electron overflow to the p-GaN layer as shown in simulations.

Partial strain relaxation by stacking fault generation in InGaN multiple quantum wells grown on (11¯01) semipolar GaN

We have investigated the structural properties and relaxation phenomenon of InGaN multiple quantum wells (QWs) on (11¯01) semipolar GaN templates grown on patterned (001) silicon substrates by selective area growth technique. Our studies by transmission electron microscopy and x-ray diffraction reciprocal space mapping reveal that QWs emitting light at 540 nm experience significant strain relaxation along the in-plane [11¯02¯] direction by the generation of an array of basal stacking faults (BSF). The generation of BSFs in 540 nm QWs could be an important factor limiting its luminescence efficiency.

Red to blue wavelength emission of N-polar

N-polar (−c-plane) InGaN light-emitting diodes with emission wavelengths ranging from blue to green to red were fabricated on a c-plane sapphire substrate by metalorganic vapor phase epitaxy. The optimization of growth conditions for −c-plane InGaN/GaN multiple quantum wells was performed. As a result, the extension of the emission wavelength from 444 to 633 nm under a constant current of 20 mA was achieved by changing the growth temperature of quantum wells from 880 to 790 °C.

(000\bar{1})

We demonstrated lasing action and investigated the optical properties of multiquantum-well (MQW) stripe crystals on patterned (001) Si substrates. Longitudinal and higher order transverse modes were observed from a ridge waveguide structure. These results strongly suggest the possibility of fabricating InGaN MQW laser diodes on (001) Si.

InGaN light-emitting diodes grown by metalorganic vapor phase epitaxy

Erbium doped InGaN alloys (InGaN:Er) were grown on Si (001) substrates using metal organic chemical vapor deposition. The growth of epitaxial films was accomplished by depositing InGaN:Er on GaN templates deposited on 7.3° off-oriented Si (001) substrates which were prepared by etching and subsequent selective area growth. X-ray diffraction measurements confirmed the formation of wurtzite InGaN (11¯01) epilayers, which exhibit strong photoluminescence emission at 1.54 μm. The observed emission intensity at 1.54 μm was comparable to that from similar alloys grown on GaN/AlN/Al2O3 templates. These results indicate the high potential for on-chip integration of erbium based photonic devices with complementary metal oxide semiconductor technology.

Optically pumped lasing properties of

With respect to N-polar GaN grown on a sapphire substrate, the effects of Mg doping on the surface morphology, and the optical, and electrical properties are precisely investigated. By doping Mg, hillocks observed on the surface of GaN can be suppressed, while step bunching becomes severe. The atomic terrace width is extended with increasing Mg/Ga precursor ratio. Mg doping can promote the surface migration of Ga adatoms on a GaN surface during growth. In the case of heavily Mg-doped GaN, atomic steps become wavy. From photoluminescence spectra, the dominant transition was found to change from near-band-edge transition to donor–acceptor-pair transition. Hall-effect measurement shows p-type conduction at room temperature for a sample grown with the Mg/Ga precursor ratio of 4.5 × 10−3. The activation energy is 143 meV, which is comparable to that of Mg in the conventional Ga-polar (0001) GaN.

(1\bar{1}01)

Indium was introduced as a surfactant during metalorganic vapor phase epitaxial growth of nitrogen-polar GaN on c-plane sapphire tilted from 0.2 to 1.0° toward m-axis. For each sample with different tilted angle, the surfactant effect was confirmed from a scanning electron microscope and an atomic force microscope observations. In particular, for the tilted angle of 0.2°, all the hillocks, which usually exist on the nitrogen-polar GaN films on sapphire substrates with low tilted angles, disappeared. The microscopic terrace width was enlarged, and the surface roughness became small irrespective of the tilted angle, however, for the samples with higher tilted angles, the occurrence of the giant step bunching was rather enhanced. Therefore, it was confirmed that indium introduced during the nitrogen-polar GaN growth surely plays a role as a surfactant.

InGaN/GaN stripe multiquantum wells with ridge cavity structure on patterned (001) Si substrates

GaN films were successfully grown on Si(111) substrates using a reactive-sputter-deposited AlN intermediate layer (sp-AlN) at 350 °C. Uniform GaN growth was obtained with the pre sputtering of a thin Al film before the deposition of sp-AlN. Atomic force microscopy (AFM) measurements revealed that the density of pits originating from the ends of mixed/screw-type dislocations is about 109 cm-2, which is much lower than that of GaN grown on a metalorganic vapor phase epitaxy (MOVPE) deposited AlN intermediate layer. The decrease in the pit density might have originated from the small number of initial nuclei of GaN.

Photonic properties of erbium doped InGaN alloys grown on Si "001… substrates

InGaN/GaN multiple quantum wells (MQWs) on semipolar (101) GaN microstripes on a Si substrate were fabricated and their optical properties were investigated. From cathodoluminescence (CL) analysis, strong CL emission was obtained in an MQW emitting at 433 nm. However, dark lines appeared in MQWs emitting at longer wavelengths. These dark lines are attributed to lattice relaxation and the generation of misfit dislocations and stacking faults in an MQW, resulting in nonradiative centers. The internal quantum efficiency (IQE) was estimated from excitation-power-dependent photoluminescence analysis. The (101) InGaN/GaN MQW had a high IQE owing to the high crystalline quality of the underlying GaN and the reduced piezoelectric field. The IQE at a carrier concentration of 1×1018 cm-3 in a sample emitting at 490 nm was as high as 90%. The efficiency decreased in a sample with a higher indium content in the MQW due to the generation of misfit dislocations and stacking faults.