With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.

We report the realization and properties of a high-resolution solid-state self-emissive microdisplay based on III-nitride semiconductor micro-size light emitting diodes (µLEDs) capable of delivering video graphics images. The luminance level of III-nitride microdisplays is several orders of magnitude higher than those of liquid crystal and organic-LED displays. The pixel emission intensity was almost constant over an operational temperature range from 100 to −100 °C. The outstanding performance is a direct attribute of III-nitride semiconductors. An energy efficient active drive scheme is accomplished by hybrid integration between µLED arrays and Si CMOS (complementary metal–oxide–semiconductor) active matrix integrated circuits. These integrated devices could play important roles in emerging fields such as biophotonics and optogenetics, as well as ultra-portable products such as next generation pico-projectors.

Deep $\mathrm{UV}$ photoluminescence spectroscopy has been employed to study the optical properties of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ alloys $(0⩽x⩽1)$ . The emission intensity with polarization of $\mathbf{E}\perp \mathbf{c}$ and the degree of polarization were found to decrease with increasing $x$ . This is a consequence of the fact that the dominant band edge emission in $\mathrm{GaN}$ $\left(\mathrm{AlN}\right)$ is with polarization of $\mathbf{E}\perp \mathbf{c}\left(\mathbf{E}\parallel \mathbf{c}\right)$ . Our experimental results suggest that the decreased emission efficiency in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ alloys and related $\mathrm{UV}$ emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of $\mathbf{E}\perp \mathbf{c}$ decreases with $x$ . It is thus concluded that $\mathrm{UV}$ emitters with $\mathrm{AlGaN}$ alloys as active layers have very different properties from $\mathrm{InGaN}$ and other semiconductor emitters.

We report on the fabrication and photovoltaic characteristics of InGaN solar cells by exploiting InGaN/GaN multiple quantum wells (MQWs) with In contents exceeding 0.3, attempting to alleviate to a certain degree the phase separation issue and demonstrate solar cell operation at wavelengths longer than previous attainments $\left(>420\text{nm}\right)$ . The fabricated solar cells based on ${\text{In}}_{0.3}{\text{Ga}}_{0.7}\text{N}/\text{GaN}$ MQWs exhibit an open circuit voltage of about 2 V, fill factor of about 60%, and an external efficiency of 40% (10%) at 420 nm (450 nm).