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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 and with atomically smooth surface have been demonstrated. [ABSTRACT FROM AUTHOR]

A crack-free aluminum nitride (AlN) layer of 9 ± 1 μm thickness was grown on a nanopatterned sapphire substrate (NPSS) with a sputter-deposited AlN buffer layer. The buffer layer was thermally annealed and AlN was regrown by hydride vapor-phase epitaxy (HVPE). The dependence of the crystallinity of HVPE-grown AlN layers on the growth temperature was investigated. It was found that undesired misaligned AlN growth can be prevented by choosing an appropriate growth temperature. The full widths at half maximum of the (0002)- and (10–12)-plane X-ray rocking curves were improved to as low as 102 and 219 arcsec, respectively, by applying an NPSS with a sputter-deposited annealed AlN film. Compared with substrates without nanopatterning, the NPSS was also effective for suppressing cracks owing to the formation of voids at the interface of the HVPE-grown AlN layer and NPSS template. [ABSTRACT FROM AUTHOR]

Strain states in face-to-face-annealed sputter-deposited c-plane AlN (FFA-sp-AlN) films on c- and a-plane sapphire substrates were evaluated. As a result, an existence of anisotropic strain in the c-plane FFA-sp-AlN film on a-plane sapphire was ascertained by X-ray diffraction reciprocal space maps. Consequently, the peak splitting of polarized Raman spectra was observed from the c-plane FFA-sp-AlN film on a-plane sapphire for the first time. Meanwhile, there was no peak splitting of polarized Raman spectra of the c-plane FFA-sp-AlN film on c-plane sapphire, which has anisotropic strain. In addition, local and anisotropic strain relaxations in patterned FFA-sp-AlN films on c-plane sapphires were experimentally proven by polarized Raman spectra and two dimensional Raman spectra maps. [ABSTRACT FROM AUTHOR]

Combination of sputter deposition and high-temperature annealing is a promising technique for preparing AlN templates with a low threading dislocation density (TDD) at a lower film thickness compared to those prepared by the conventional metalorganic vapor phase epitaxy. However, cracking of AlN films during annealing is a critical issue. In this study, we controlled the residual stress of the sputter-deposited AlN films by modifying the sputtering conditions. Consequently, the occurrence of cracking was effectively suppressed. By optimizing the fabricating conditions, a TDD of 2.07 × 108 cm−2 was achieved for the AlN template with a thickness of 480 nm. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate homogeneously distributed In0.3Ga0.7N/GaN quantum disks (QDs), with an average diameter below 10 nm and a high density of 2.1 × 1011 cm−2, embedded in 20 nm tall nanopillars. The scalable top-down fabrication process involves the use of self-assembled ferritin bio-templates as the etch mask, spin coated on top of a strained In0.3Ga0.7N/GaN single quantum well (SQW) structure, followed by a neutral beam etch (NBE) method. The small dimensions of the iron cores inside ferritin and nearly damage-free process enabled by the NBE jointly contribute to the observation of photoluminescence (PL) from strain-relaxed In0.3Ga0.7N/GaN QDs at 6 K. The large blueshift of the peak wavelength by over 70 nm manifests a strong reduction of the quantum-confined Stark effect (QCSE) within the QD structure, which also agrees well with the theoretical prediction using a 3D Schrödinger equation solver. The current results hence pave the way towards the realization of large-scale III-N quantum structures using the combination of bio-templates and NBE, which is vital for the development of next-generation lighting and communication devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effects of growth conditions such as Mg/Ga and V/III ratios on the properties of N-polar p-type GaN grown by metalorganic vapor phase epitaxy were studied. Photoluminescence spectra from Mg-doped GaN depended on Mg/Ga and V/III ratios. For the lightly doped samples, the band-to-acceptor emission was observed at 3.3 eV and its relative intensity decreased with increasing V/III ratio. For the heavily doped samples, the donor–acceptor pair emission was observed at 2.8 eV and its peak intensity monotonically decreased with V/III ratio. The hole concentration was maximum for the Mg/Ga ratio. This is the same tendency as in group-III polar (0001) growth. The V/III ratio also reduced the hole concentration. The higher V/III ratio reduced the concentration of residual donors such as oxygen by substituting nitrogen atoms. The surface became rougher with increasing V/III ratio and the hillock density increased. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]