Stimulated emission and laser action have been observed near 3.45 eV in single-crystal needles of GaN. These observations support the earlier suggestion that GaN is a direct band-gap semiconductor with Eg∼3.50 eV at 2°K. Furthermore, the occurrence of very high gain (g∼105 cm−1) in the stimulated emission emphasizes the possible device potential of this material.

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Single ribbons of gallium phosphide have been synthesized which are very thin (down to ∼0.35 μ) and yet possess relatively large cross-sectional areas with uniform thickness so that high-resolution optical-absorption measurements can be extended well above the lowest energy direct interband electronic transition (Γ15→Γ1). The Γ15→Γ1 exciton energy gap is 2.873 eV below 25°K, and the internal binding energy of the direct exciton is about 5 meV. Direct transitions involving hole states in the split-off valence band can be clearly seen at low temperatures, and the spin-orbit splitting is 82±1 meV. Weak structure corresponding to indirect transitions at the Γ15→X3 energy gap is also observed at low temperatures, and the separation of the X3 and X1 conduction-band minima is 0.29±0.01 eV, in excellent agreement with the value obtained from interconduction-band absorption spectra. The temperature coefficient of the Γ15→Γ1 direct energy gap is − (5.2±0.1)×10−4 eV(°K)−1 near 300°K, more than twice the value for the Γ15→X1 indirect gap.

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Type II donor-acceptor pair spectra involving the shallow group IIA acceptor Mg and the group VI donors S and Te have been observed from GaP crystals grown from Ga solution in an open-tube furnace system. Magnesium can be an optically active inadvertent contaminant in high-purity GaP. Crystals containing ∼1017 cm−3 of these desired impurities, added intentionally, exhibited sharp structure due to electron-hole recombinations at discrete donor-acceptor pairs, with negligible contamination from unwanted spectra. The transition energies in these spectra can be closely fitted for shell numbers m≳20 (pair separation ≳17 Å) by an expression which includes only the coulomb monopole term for the final state interaction and a van der Waals term for the initial state interaction, using low-temperature static dielectric constant. This analysis yields the activation energy of the Mg acceptor in GaP, (EA)Mg=53.5±1 meV, substantially different from previous estimates derived from electrical transport and impurity Raman scattering measurements. Trends in the activation energies of the three Ga-site acceptors and three P-site acceptors known in GaP are discussed qualitatively.

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The optical absorption edge of GaP is influenced in two principal ways when a few percent of the phosphorus atoms are replaced by arsenic. Besides the uniform decrease of the indirect band gap, the mixed crystal exhibits an extra absorption component. It is shown that this component is due to the no-phonon creation of free excitons and free-electron-hole pairs. Momentum is conserved through scattering at the arsenic impurities. The cross section for this process apparently decreases with increasing kinetic energy of the free excitons. In addition, the matrix element for indirect transitions assisted by the emission of LA phonons is enhanced by the decrease in the Xi e -Ti c energy separation in the mixed crystal. The replacement of phosphorus by arsenic in GaP is a relatively minor perturbation. Unlike the case of nitrogen in GaP, there is no bound state. The near-threshold absorption due to the arsenic-induced creation of free particles is only ~0.1% of that due to nitrogen substituents.