W. Walukiewicz

We present evidence for a strong interaction between the conduction band and a narrow resonant band formed by nitrogen states in $\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{In}{}_{x}\mathrm{N}{}_{y}\mathrm{As}{}_{1\ensuremath{-}y}$ alloys. The interaction leads to a splitting of the conduction band into two subbands and a reduction of the fundamental band gap. An anticrossing of the extended states of the conduction band of the $\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{In}{}_{x}\mathrm{As}$ matrix and the localized nitrogen resonant states is used to model the interaction. Optical transitions associated with the energy minima of the two subbands and the characteristic anticrossing behavior of the transitions under applied hydrostatic pressure have been unambiguously observed using photomodulation spectroscopy. The experimental results are in excellent quantitative agreement with the model.

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 In1−xGaxN films (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 In1−xGaxN alloys is well fit with a bowing parameter of ∼1.4 eV. The direct band gap of the In1−xGaxN 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 In1−xGaxN 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 In1−xGaxN alloys.

We report a strong correlation between the location of Mn sites in ferromagnetic ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ measured by combined channeling Rutherford backscattering and by particle-induced x-ray emission experiments and its Curie temperature. The concentrations of free holes determined by electrochemical capacitance-voltage profiling and of uncompensated ${\mathrm{Mn}}^{2+}$ spins determined from superconducting quantum-interference device magnetization measurements are found to depend on the concentration of unstable defects involving highly mobile Mn interstitials. This leads to large variations in ${T}_{C}$ of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ when it is annealed at different temperatures in a narrow temperature range. The fact that annealing under various conditions has failed to produce Curie temperatures above \ensuremath{\sim}110 K is attributed to the existence of an upper limit on the free hole concentration in low-temperature-grown ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}.$