Defect annealing in a II–VI laser diode structure under intense optical excitation

Abstract

Defect annealing under intense pulsed optical excitation has been observed in a II-VI laser diode structure at room temperature. More than one order of magnitude increase in photoluminescence intensity has been obtained when the annealed area is probed at low excitation intensity. High-resolution confocal photoluminescence images of the annealed region do not show any sign of degradation. Together, these results suggest that an initial density of intrinsic point defects present within the active region can be removed by the optical annealing. Recombination-enhanced defect reactions in the vicinity of the point defects are responsible for this nonthermal annealing effect. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00502-6͔Macroscopic defects originating at the GaAs/ZnSe heterointerface such as stacking faults and threading dislocations have been recognized to be the main cause of the rapid degradation and, therefore, the very short operating lifetime of II-VI-based laser diodes.1-4 Considerable effort has been put into reducing the stacking fault density to less than the critical value of 10 4 cm Ϫ2 in order to obtain a laser diode with no extended defects in its stripe area ͑typically, 10 mϫ600 m͒. At present, a 100 h lifetime continuouswave laser diode operating at room temperature has been achieved with a stacking fault density lower than 3 ϫ10 3 cm Ϫ2. 5 For further improvement, slower degradation mechanisms, such as point defect diffusion and reaction, need better understanding.It has been shown recently that recombination-enhanced defect reaction ͑REDR͒ is the driving force of the degradation in relatively long-lived II-VI light-emitting diodes. Recombination-enhanced processes in semiconductors have been thoroughly studied over the years.7-9 REDR describes how the energy released by nonradiative carrier trapping at point defect sites can excite localized vibrational modes of the defect and of the surrounding atoms, before being dissipated through lattice phonons. This energy can be used to promote defect motion and reaction, inducing defect multiplication ͑degradation͒, or, alternatively, defect reduction ͑annealing͒.In this letter we report an improvement in photoluminescence ͑PL͒ efficiency in a II-VI laser diode structure using high intensity laser excitation. Instead of observing degradation, we find that the PL efficiency increases as a function of time through optical annealing of the II-VI material. PL maps of the annealed area were also performed using a confocal microscope 10 setup ͑0.5 m resolution͒ and show that no degradation occurred in the active region during the annealing process.The sample studied was a ZnCdSe/ZnSSe/ZnMgSSe single quantum well separate-confinement heterostructure ͑SCH͒ grown by molecular beam epitaxy ͑MBE͒. The SCH structure is similar to the laser structure reported before, 5 except that a part of the p-type layer has been removed by chemical etching to allow optical pumping to be performed. The sample was annealed at room temperature by optical excitation using...