Light-emitting diodes (LEDs) fabricated from gallium nitride (GaN) have led to the realization of high-efficiency white solid-state lighting. Currently, GaN white LEDs exhibit luminous efficacy ...greater than 150lmW−1, and external quantum efficiencies higher than 60%. This has enabled LEDs to compete with traditional lighting technologies, such as incandescent and compact fluorescent (CFL) lighting. Further improvements in materials quality and cost reduction are necessary for widespread adoption of LEDs for lighting. A review of the unique polarization anisotropy in GaN is included for the different crystal orientations. The emphasis on nonpolar and semipolar LEDs highlights high-power violet and blue emitters, and we consider the effects of indium incorporation and well width. Semipolar GaN materials have enabled the development of high-efficiency LEDs in the blue region and recent achievements of green laser diodes at 520nm.
We study vertical GaN p-n and Schottky power diodes with different buffer layer thicknesses grown on free-standing GaN substrates, using metalorganic chemical vapor deposition. High breakdown voltage ...of > 1 kV and low specific on-resistance of 3 mΩ·cm 2 are achieved on GaN p-n diode with 1 μm buffer layer and 9 μm drift layer without passivation or field plate. Detailed device analysis on GaN Schottky diodes indicates that buffer layer has significant impacts on the electrical properties of drift layer and thus device performances of GaN p-n diodes. A thicker buffer layer will significantly enhance the breakdown voltages of these devices, which is possibly due to the improved material quality of drift layers with reduced defect densities. Higher doping concentration in drift layer with thicker buffer layer will, however, lower breakdown voltage. More discussions reveal improving the material quality of drift layer plays a more dominant role in achieving high breakdown GaN-on-GaN p-n and Schottky diodes with increasing buffer layer thickness.
III-nitride semiconductors have drawn considerable attentions for its wide success in photonic devices including light-emitting diodes (LEDs), laser diodes, solar cells, and photodetectors. In ...addition to discrete devices, recent studies on III-nitride waveguides have now opened up new opportunities in integrated photonics circuits for biochemical sensing, beam steering, nonlinear optics, and quantum photonics applications in the ultraviolet and visible spectral region. For example, due to their wide bandgap, low material dispersion, and active integration capability, III-nitride materials are particularly attractive for integrated nonlinear optics applications, including second harmonic generation, comb generation, and parametric down conversion.
We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (β-Ga
O
), where both (010) β-Ga
O
and (2¯01) β-Ga
O
were examined for two-photon absorption ...coefficient, Kerr nonlinear refractive index, and their polarization dependence. The wavelength dependence of two-photo absorption coefficient and Kerr nonlinear refractive index were also estimated by a widely used analytical model. β-Ga
O
exhibits a two photon absorption (TPA) coefficient of 1.2 cm/GW for (010) β-Ga
O
and 0.6 cm/GW for (2¯01) β-Ga
O
. The Kerr nonlinear refractive index is -2.1 × 10
cm
/W for (010) β-Ga
O
and -2.9 × 10
cm
/W for (2¯01) β-Ga
O
. In addition, β-Ga
O
shows stronger in-plane nonlinear optical anisotropy on (2¯01) plane than on (010) plane. Compared with GaN, TPA coefficient of β-Ga
O
is 20 times smaller, and the Kerr nonlinear refractive index of β-Ga
O
is also found to be 4-5 times smaller. These results indicate that β-Ga
O
have the potential for ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in UV and visible wavelength spectral range.
Abstract
High-surface-area α-Al
2
O
3
nanoparticles are used in high-strength ceramics and stable catalyst supports. The production of α-Al
2
O
3
by phase transformation from γ-Al
2
O
3
is hampered ...by a high activation energy barrier, which usually requires extended high-temperature annealing (~1500 K, > 10 h) and suffers from aggregation. Here, we report the synthesis of dehydrated α-Al
2
O
3
nanoparticles (phase purity ~100%, particle size ~23 nm, surface area ~65 m
2
g
−1
) by a pulsed direct current Joule heating of γ-Al
2
O
3
. The phase transformation is completed at a reduced bulk temperature and duration (~573 K, < 1 s) via an intermediate δʹ-Al
2
O
3
phase. Numerical simulations reveal the resistive hotspot-induced local heating in the pulsed current process enables the rapid transformation. Theoretical calculations show the topotactic transition (from γ- to δʹ- to α-Al
2
O
3
) is driven by their surface energy differences. The α-Al
2
O
3
nanoparticles are sintered to nanograined ceramics with hardness superior to commercial alumina and approaching that of sapphire.
We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under ...low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.
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