Kanako Shojiki

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.

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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.

InGaN light-emitting diodes grown by metalorganic vapor phase epitaxy

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.

Enhancement of surface migration by Mg doping in the metalorganic vapor phase epitaxy of N-polar

The metastable zincblende (ZB) phase in N-polar (0001¯) (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy is elucidated by the electron backscatter diffraction measurements. From the comparison between the −c-plane and Ga-polar (0001) (+c-plane), the −c-plane MQWs were found to be suffered from the severe ZB-phase inclusion, while ZB-inclusion is negligible in the +c-plane MQWs grown under the same growth conditions. The ZB-phase inclusion is a hurdle for fabricating the −c-plane light-emitting diodes because the islands with a triangular shape appeared on a surface in the ZB-phase domains. To improve the purity of stable wurtzite (WZ)-phase, the optimum conditions were investigated. The ZB-phase is dramatically eliminated with decreasing the V/III ratio and increasing the growth temperature. To obtain much-higher-quality MQWs, the thinner InGaN wells and the hydrogen introduction during GaN barriers growth were tried. Consequently, MQWs with almost pure WZ phase...

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Nitrogen-polar (N-polar) InGaN films were grown on a GaN template/c-plane sapphire substrate by metal–organic vapor phase epitaxy (MOVPE). The effects of c-plane sapphire substrate miscut angle on the indium (In) content and crystal properties of N-polar InGaN films were investigated. The In content increased with increasing miscut angle in the vicinal region of less than 1.1°. This tendency is different from that of group-III-polar InGaN growth because of the difference in the atomic arrangement on the terraces and at step edges between these two inverted polar surfaces. In the case of N-polar growth, a spontaneous two-dimensional nucleation on terraces is difficult and the intentional introduction of steps is effective compared with group-III-polar growth. Furthermore, by observing the surface morphologies of GaN templates in view of both macroscopic and microscopic scales, a clear relationship between the macroscopic surface structure of GaN template and the In content of InGaN was revealed.

GaN/sapphire

We report on the polarity control of GaN regrown on pulsed-laser-deposition-grown N-polar AlN on a metalorganic-vapor-phase-epitaxy-grown Ga-polar GaN template. The polarity of the regrown GaN, which was confirmed using aqueous KOH solutions, can be inverted from that of AlN by inserting a low-temperature GaN (LT-GaN) buffer layer. We hypothetically ascribe the Ga-polarity selection of GaN on the LT-GaN buffer layer to the mixed polarity of LT-GaN grains and higher growth rate of the Ga-polar grain, which covers up the N-polar grain during the initial stage of the high-temperature growth. The X-ray rocking curve analysis revealed that the edge-dislocation density in the N-polar regrown GaN is 5 to 8 times smaller than that in the Ga-polar regrown GaN. N-polar GaN grows directly on N-polar AlN at higher temperatures. Therefore, nucleus islands grow larger than those of LT-GaN and the area fraction of coalescence boundaries between islands, where edge dislocations emerge, becomes smaller.

Improvement of surface morphology of nitrogen-polar GaN by introducing indium surfactant during MOVPE growth

To improve the homogeneity of the N-polar (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy (MOVPE), the growth of GaN and MQW on two c-plane sapphire substrates with an off-cut angle of 0.8° toward the a-plane (sub-A) and the m-plane (sub-M) was performed. The effects of the off-cut direction on the structural properties and surface morphologies of −c-plane GaN films were elucidated. It was found that the step bunching and meandering of −c-plane GaN were significantly suppressed on sub-A. The spatial homogeneity of the −c-plane InGaN/GaN MQWs along the off-cut direction was observed in the submicrometer scale using microbeam X-ray diffraction. By inhibiting the step bunching of the GaN template using sub-A, the thickness homogeneity of the MQWs on sub-A has been significantly improved in comparison with that on sub-M.

Suppression of metastable-phase inclusion in N-polar (0001¯) InGaN/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy

N-polar (−c-plane) InGaN/GaN light-emitting diodes (LEDs) were grown by metalorganic vapor phase epitaxy, and their optoelectronic properties were evaluated by electroreflectance (ER) and electroluminescence (EL) measurements. In −c-plane LEDs, the emission energy was much lower than that in c-plane LEDs. By comparing EL and ER results, we found that the emission energy was also much lower than the transition energy. The transition energy is in good agreement with X-ray diffraction analysis results. These results indicate that −c-plane LEDs exhibit a larger Stokes-like shift than do c-plane LEDs. This Stokes-like shift is due to the strong potential fluctuation, which is possibly caused by the specific growth patterns of −c-plane III–nitrides. The dominant emission centers of the −c-plane LEDs were suggested to be the localized states of InGaN islands.

Effect of c-plane sapphire substrate miscut angle on indium content of MOVPE-grown N-polar InGaN

The accurate alloy composition of a nonpolar InGaN grown on m-plane GaN is estimated from X-ray reciprocal-space maps (RSMs) of (201) and (210) diffractions. In this estimation, the anisotropic residual strain in m-plane is carefully considered. In order to avoide the error which may be generated by the anisotropic strain and tilted domains in the film of InGaN, the lattice constants along m-, a-, and c-directions are determined using a pair of two RSMs normalized to the unit reciprocal vector along m-direction. The indium content of InGaN is derived from RSMs data using Poisson effect and Vegards law. Based on this method, the incorporation of indium into InGaN is investigated. This incorporation is found to be promoted with the increase in the substrate miscut angle and the growth rate. From the precise analysis of RSMs, some of the InGaN domains on m-plane GaN substrates are found to be tilted toward ±a-direction despite of the substrate miscut toward c-direction.

Polarity control of GaN grown on pulsed-laser-deposited AlN/GaN template by metalorganic vapor phase epitaxy

The internal electric fields in III-polar (0001), N-polar , and semipolar InGaN/GaN light-emitting diodes were investigated by electroreflectance (ER) spectroscopy. The ER spectra reflected the difference in the direction and strength of internal electric fields. Phase analyses of the ER signal revealed that only III-polar InGaN wells have the opposite direction of the internal electric field at zero bias voltage; this finding is in good agreement with the results of numerical analyses. Quantitative analyses of internal electric fields were conducted by the linewidth analyses of ER spectra. Our experimental results indicate that the absolute value of internal electric fields can be measured from ER spectra.