This paper discusses the performance of InGaN multi-quantum well (MQW) laser diodes and the effects of composition fluctuations on the optical gain characteristics. Pulsed threshold current densities as low as 4.8 kA/cm2 have been observed for ridge-waveguide laser diodes allowing room-temperature continuous-wave (cw) operation with threshold currents of 125 mA and operating voltages of 6.5 V. The effects of composition fluctuations in the InGaN alloy were investigated by comparing theoretical and experimental gain spectra obtained from true spontaneous emission measurements, cavity length studies and cw laser emission spectra. We find that the distributed losses in our structures are of the order of 45–50 cm−1. Using a simple logarithmic gain–current relationship, we were able to describe the experimental results with a characteristic gain g0=72 cm−1 and a transparency current density Jtrans=2.5 kA/cm2. Optical gain calculations have been performed for InGaN quantum-well structures, based on band structures generated with an effective-mass Hamiltonian and taking pseudomorphic strain into account. We find that a very modest amount of composition fluctuation (standard deviation in the In content <0.008), which produces inhomogeneous broadening, is consistent with the experimental observations. This is consistent with TEM structural studies, which show that there is negligible phase separation in InxGa1−xN MQW with indium content smaller than x=0.1.

We present theoretical and experimental results for the band gap of InxGa1−xN alloys, showing significantly larger bowing than has been commonly assumed. We highlight the importance of properly including strain in the experimental analysis. Using X-ray diffraction (XRD) and Rutherford backscattering spectrometry the layers in our study were determined to be pseudomorphically strained. The In content determined by XRD depends strongly on the assumptions made about the strain in the InGaN layers. Strain also affects the band structure and hence the transition energies measured by optical-transmission spectroscopy. An analysis of the experimental results produces a bowing parameter b≈3.8 eV at x=0.1. First-principles calculations, based on pseudopotential–density–functional theory, produce values of the bowing parameter in agreement with the experimental determination, and also indicate a strong dependence of the bowing parameter on composition.

Band gap measurements have been performed on strained InxGa1−xN epilayers with x⩽0.12. The experimental data indicate that the bowing of the band gap is much larger than commonly assumed. We have performed first-principles calculations for the band gap as a function of alloy composition and find that the bowing is strongly composition dependent. At x=0.125 the calculated bowing parameter is b=3.5 eV, in good agreement with the experimental values.