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In order to clarify the strain effect on the GaN-based lasers and to give the important guideline on their device design, the subband structure and the optical gains of strained wurtzite GaN/AlGaN quantum wells are theoretically investigated on the basis of k⋅p theory. First-principles band calculations are used for deriving the unknown physical parameters. It is found that neither compressive nor tensile biaxial strains in the c plane are so effective on the reduction of the threshold carrier density as conventional zinc-blende lasers and that the uniaxial strain in the c plane is very useful for reducing it. The relation between the uniaxial strain’s direction and the optical polarization is also clarified. As a result, we suggest that the uniaxial strain in the c plane is one of the preferable approaches for the efficient improvement of the GaN-based lasers performance.

Subband structures and optical gains of both unstrained and biaxial strained wurtzite GaN/AlGaN quantum well (QW) laser diodes (LDs) are theoretically investigated by the 8×8 k\cdotpp theory, with the assistance of the first-principles calculations in the derivation of the required parameters such as deformation potentials. The strong electron affinity and the small spin-orbit coupling of a nitrogen yield much heavier effective masses even in the QWs. It plays an essential role in causing a higher threshold current density for any well length than GaAs/AlGaAs QW LDs. Considering a biaxial strain induced by the lattice mismatch, the optical gain property qualitatively improves for any well length. However, the effect on the reduction of the threshold current density is quantitatively not so effective as GaAs/AlGaAs QW LDs.