The photoluminescence droop effect, i.e., the decrease in emission efficiency with increasing excitation intensity, is observed and studied in GaN epilayers with different carrier lifetimes. Spontaneous and stimulated emissions have been studied in the front-face and edge emission configurations. The onset of stimulated recombination occurs simultaneously with the droop onset in the front-face configuration and might be considered as an origin of the droop effect in GaN epilayers.
© 2012 OSA
The origin of the efficiency droop, i.e., of a decrease in the emission efficiency of the III-nitride based high-power light-emitting diodes (LEDs) at elevated driving currents, is not yet clear. The explanations suggested up to now might be divided into two groups: i) external mechanisms related to a decreased capture of the injected carriers in the quantum wells or their enhanced escape out of the wells [1–3]; and ii) internal mechanisms related to screening of the localization potential fluctuations , thermally assisted current transport along dislocations , junction heating , density-activated defect recombination , thermal activation of localized carriers , nonradiative Auger recombination [7–9], phase-space filling, and saturated radiative recombination rate [10–12]. Since the efficiency droop is observed not only at increasing current injection, but also under increasing photoexcitation, the internal mechanisms are at least partially responsible for the droop. In this paper, we study the droop under photoexcitation (thus, eliminating external droop mechanisms) in GaN epilayers (eliminating recombination features specific for quantum wells and eliminating carrier localization effects due to composition fluctuations). The droop effect has been observed in GaN epilayers , but not studied in detail. We paid a special attention to the possible influence of stimulated recombination on the droop. The stimulated recombination is shown to play a significant role in the decrease of emission intensity perpendicularly to the cavity axis in InGaN laser diodes . We investigated photoluminescence of GaN epilayers containing no optical cavities both in the conventional front-face configuration and in the edge emission configuration that is convenient for studying stimulated emission.
Eight GaN epitaxial layers with different carrier lifetimes ranging from 41 ps to 3630 ps were selected for the study and labeled according to the lifetimes as T41, T58, etc. The carrier lifetimes were measured using light-induced transient grating technique, as previously reported in Ref . All the samples under study were grown on c-plane sapphire using a combination of Metal-Organic Chemical Vapor Deposition (MOCVD) and Migration Enhanced MOCVD (MEMOCVD®) techniques. The difference in carrier lifetimes was caused by different growth conditions and layer thicknesses.
The luminescence spectra have been measured at room temperature in two configurations. In the front-face configuration, the laser beam was focused on the layer surface into a spot of ~350 µm in diameter. The luminescence collected from the spot contained mainly the contribution from spontaneous emission. The absorption (or carrier diffusion) depth of 0.1-1 µm was too small for the single-pass amplification of light propagating perpendicular to the sample even when the population inversion was high enough for a strong optical gain. The contribution of stimulated emission might be detected in this configuration only due to scattering of the light amplified in a single pass by propagating parallel to the sample surface along the spot of ~350 µm in diameter. The edge emission configuration was used to extract the stimulated emission. The excitation light was focused into a narrow stripe (~30 µm in width and 2 mm in length) on the sample edge, and the light propagating along the stripe was detected. In both configurations, the emission was dispersed using a double monochromator (Jobin Yvon HRD-1) and recorded by a UV-enhanced photomultiplier. The 4th harmonic (266 nm) of the Q-switched YAG:Nd laser radiation (pulse duration 4 ns) served as an excitation source.
Figure 1 shows typical spectra recorded in both configurations below and above the threshold for the stimulated optical transitions. A broad spontaneous emission band (peaked at ~3.42 eV with minor position shifts from a sample to a sample) dominated the spectrum recorded in the front-face configuration at low excitations. A narrower stimulated emission band at the low-energy side of the spontaneous band was observed at high excitations in both configurations but, as expected, was considerably more pronounced in the edge configuration. Figures 2 and 3 present the PL efficiency, i.e. the spectrally-integrated PL intensity divided by the excitation intensity, as a function of the excitation power density for the front-face and edge configurations, respectively. The efficiency at a fixed excitation power density was approximately proportional to the carrier lifetime. This proportionality was slightly distorted due to the different surface morphology of the samples resulting in different light collection during the PL measurements. The efficiency increased with increasing the excitation power density. This increase might be primarily explained by two concurrent effects: i) the saturation of nonradiative recombination centers; and ii) an increasing fraction of carriers recombining via bimolecular band-to-band recombination in respect to the linear, mostly nonradiative, recombination. As expected, the efficiency increase was more pronounced in the samples with longer carrier lifetimes corresponding to higher carrier densities at the same excitation intensities (see Fig. 2). The samples with higher carrier lifetimes reached the onset of the droop effect at lower excitation intensities. A similar tendency was observed for the threshold of stimulated optical transitions.
The threshold was studied in all the samples under study in edge configuration. Emission efficiency as a function of excitation power density is plotted in Fig. 3. The threshold corresponding to the onset of stronger increase of emission intensity in Fig. 3 (encircled points) is plotted as a function of carrier lifetimes in different samples in Fig. 4(a) . The stimulated emission band was also observed in front-face configuration. The points when the stimulated emission band becomes distinguishable are encircled in Fig. 2. The carrier lifetime dependence of the onset of stimulated emission band in front-face configuration is presented in Fig. 4(b) and shows the same trend as the threshold of stimulated emission depicted in Fig. 4(a).
Since the front-face configuration is not favorable to observe the stimulated emission, the stimulated recombination might become important in the dependences presented in Fig. 2 earlier than the encircled points, which represent an obvious emergence of the stimulated emission band. Despite large uncertainties in comparison of excitation power densities in front-face and edge configurations (mainly due to uncertainties in spot size determination and, especially, edge quality at the selected position), it is clear that the stimulated optical transitions became important in the carrier recombination earlier than it appeared as a separate emission peak in the front-face configuration. Thus, the threshold for stimulated emission was close to the onset of the efficiency droop. Consequently, the stimulated recombination, being a faster recombination channel than the spontaneous recombination, might be the mechanism limiting the increase of carrier density at elevated excitation intensities and result in the droop in efficiency of spontaneous emission in GaN epilayers observed in the front-face configuration. Above the threshold, the dependence of the recombination rate due to stimulated transitions on carrier density is considerably stronger than that of the spontaneous bimolecular recombination. Thus, the stimulated emission tends to stabilize the carrier density just above the stimulated emission threshold due to a negative feedback: an increase in carrier density increases the gain coefficient and results, in turn, in reduction of the carrier density due to the enhanced stimulated recombination rate. This stabilization of carrier density at increasing excitation intensity results in the efficiency droop for the “useful” emission detected in front-surface recombination. Meanwhile, the efficiency of the total (spontaneous and stimulated) emission spatially-integrated in all directions does not suffer any droop. Note that at very high excitation power densities, when the signal detected in front-surface configuration is dominated by contribution of stimulated emission, the emission efficiency in this configuration increases again (see Fig. 2). The increase in the total emission intensity is very obvious in edge configuration (see Fig. 3), where the share of the stimulated emission is considerably larger than that in front-face configuration. It is worth noting that we could not expect such an increase in efficiency of the total emission in case of considerable influence of nonradiative Auger recombination, which is often considered as the main origin of the droop effect.
In conclusion, the droop of photoluminescence efficiency in GaN epilayers observed in the front-face configuration coincided with the onset of stimulated optical transitions. The droop onset and threshold of stimulated emission occurred at higher excitation power densities in the samples with shorter carrier lifetimes. These results imply that the onset of stimulated recombination, stabilizing the carrier density at higher excitation power density, might be sufficient to explain the droop effect in GaN epilayers. Stimulated optical transitions might also be an important contributor to the droop in other III-nitride epilayers, heterostructures, and LEDs. In the latter case, stimulated recombination initiated by the light propagating parallel to the well plane might inhibit the increase of carrier density at increased injection rate (driving current) and cause the droop in the efficiency of LED emission extracted mainly in the direction perpendicular to the well plane.
The research at Vilnius University was supported by the Lithuanian Research Council (contract No MIP-070/2011). The work at RPI was supported primarily by the Engineering Research Centers Program (ERC) of the National Science Foundation under NSF Cooperative Agreement No. EEC-0812056 and in part by New York State under NYSTAR contract C090145.
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