The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.

High-quality wurtzite-structured In-rich ${\mathrm{In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{N}$ films $\text{(0⩽x⩽0.5)}$ have been grown on sapphire substrates by molecular beam epitaxy. Their optical properties were characterized by optical absorption and photoluminescence spectroscopy. The investigation reveals that the narrow fundamental band gap for InN is near 0.8 eV and that the band gap increases with increasing Ga content. Combined with previously reported results on the Ga-rich side, the band gap versus composition plot for ${\mathrm{In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{N}$ alloys is well fit with a bowing parameter of ∼1.4 eV. The direct band gap of the ${\mathrm{In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{N}$ system covers a very broad spectral region ranging from near-infrared to near-ultraviolet.

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of ${\mathrm{In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{N}$ alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edges relative to the average dangling bond defect energy represented by the Fermi level stabilization energy in ${\mathrm{In}}_{\text{1−x}}{\mathrm{Ga}}_x\mathrm{N}$ alloys.

We present strong experimental evidence suggesting that we have synthesized the β-${\mathrm{C}}_3$ ${\mathrm{N}}_4$ phase. This is a material predicted by theory to have bulk moduli comparable to diamond. Thin films containing small crystals were deposited on Si and Ge wafers by rf diode sputtering of a pure graphite target with pure ${\mathrm{N}}_2$. The crystal structure of these phases was investigated using x-ray and electron-beam diffractometry as well as transmission electron microscopy. Our results indicate that the β-${\mathrm{C}}_3$ ${\mathrm{N}}_4$ particles with typical dimensions of ∼0.5 μm are embedded in a 1-μm-thick layer of a C-N polymer. These particles are believed to crystallize preferentially on Si (100) wafers.

Infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy. Measurements of the plasma edge frequencies were used to determine electron effective masses. The results show a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN. We have also found a large Burstein-Moss shift of the fundamental band gap. The observed effects are quantitatively described by the $\mathbf{k}\cdot \mathbf{p}$ interaction within the two-band Kane model of narrow-gap semiconductors.

We show that the band gap bowing trends observed in III-V alloys containing dilute concentrations of Sb or Bi can be explained within the framework of the valence-band anticrossing model. Hybridization of the extended $p$-like states comprising the valence band of the host semiconductor with the close-lying localized $p$-like states of Sb or Bi leads to a nonlinear shift of the valence-band edge and a reduction of the band gap. The two alloys $\mathrm{Ga}{\mathrm{Sb}}_x{\mathrm{As}}_{1-x}$ and $\mathrm{Ga}{\mathrm{Bi}}_x{\mathrm{As}}_{1-x}$ are explored in detail, and the results are extrapolated to additional systems.