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ABSTRACT We have evaluated the optical gain of GaN/AlGaN quantum well structures with localized states, taking into account the Coulomb interaction. The localized states axe introduced in the well as quantum dot-like subband states. We have used the temperature Green's function formalism to treat the many-body effects and have found a new excitonic enhancement of the optical gain involved the localized states. This enhancement is stronger than the conventional Coulomb enhancement. It might play an important role to reduce the threshold carrier density. INTRODUCTION A short wave length is one of the essential requirements of laser diodes (LDs) for high density storage devices. Recently, continuous-wave operation of the InGaN multi-quantumwell LDs with a life time of 35 hr has been achieved [1,2]. However, there is a key issue, such as high threshold carrier density, to lead them to commercial production. So far, we derived the electron and hole effective mass parameters of the group-III nitrides, using the firstprinciples calculation [3] and evaluated the optical gain of the wurzite GaN/AlGaN quantum well lasers within a free carrier model [4,5]. As a result, the threshold current density of the GaN/AlGaN LDs was estimated very high compared to conventional GaAs/AlGaAs quantum well LDs. It is caused by the large density of states due to the strong electronegativity and the weak spin-orbit coupling of the N atom in group-III nitrides. To overcome the problem, we proposed the strain effects [6] and the incorporation [7] of the GaAs or GaP in the well layer to reduced the density of states. On the other hand, the strong Coulomb interaction for wide-gap semiconductors might be helpful to reduce the threshold carrier density using some mechanism attributed to excitons. In this paper, we investigate the role of the electron-hole Coulomb interaction in the optical gain spectra and try to find a possibility of the reduction of the threshold carrier density by them. The LD structure we study here is a quantum well with localized states. The localized states axe introduced as the quantum dot-like subband states. Since the radiative recombination attributed to the localized excitons by the alloy fluctuation were measured recently [8,9], it becomes important to study the relation between the localized states and the optical gain. Generally, since the wide-gap semiconductors have the strong Coulomb interaction, the optical gain is enhanced, compaxed to the one within a free carrier model. This is so-called 'excitonic enhancement' or 'Coulomb enhancement'. We have found another excitonic enhancement of the optical gain involved in the localized states. This enhancement is stronger than the conventional one and it might play an important role to reduce the threshold carrier density. 805 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society

OPTICAL GAIN SPECTRA Optical gain is the negatives absorption coefficient which is proportional to the conductivity in terms of the recombination current. Kubo formula leads to