Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang phổ và Ứng dụng. 8/2006, Cần Thơ, Việt Namdifferent behavior of midgap levels (detected at 0.6-0.9 eV), shallower deep levels(detected at 1.2-1.3 eV), and shallow defects and band-tail states (detected at 1.45-1.50 eV). From Fig. 5 we conclude:- Device operation time increases the midgap level concentration substantially;in the case of Fig.5 (e) by a factor of 3. Generally, there is no correlationbetween the midgap-level concentration and the strain at various locations inthe emitter.- For transitions via shallow defects and band-tail states, the situation appears tobe quite different; see Fig.5(c). The change in strain is caused here by thecoactions of two independent parameters: local position and device age.Remarkably, linear fits to data sets, the pristine and the aged devices result innearly identical fit parameters, see Fig.5(c). From such a uniform strain-defectrelationship, we conclude that the creationFigure 5. (a) shows µPL peak-positions extracted by fitting the luminescence spectra. Full circlesindicate data measured at a trace 10 µm away from the active region, open circles and the full linerefer to data taken at 30 and 50 µm, respectively. The µPC data given in (b) represent the spectralposition of the 1hh-1e transition in the QW along the emitter before and after aging. (c), (d) and (e)show local defects absorption versus local strain. The plot shows the spread of the defect-relatedµPC signal for the different channels within one device versus the strain determined from thespectral position of the 1hh-1e transition at the corresponding local position. Full symbolsrepresent data obtained at 0 hours, open symbols mark data measured in the same spectral rangeafter 1500 hours of operation. The analysis includes those local positions only, which are locatedunderneath the metallized stripe of the central emitter. The lines are linear fits to thecorresponding data.of shallow defects and band tail states is strongly correlated with the strain,regardless of its origin.One expects a linear defect relationship, such as the one in Fig.5 (c), if strainfields grow due to subsequent defect creation. Thus, we consider the additionalcreation of shallow defects and band-tail states as the driving force. An alternative296