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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25195–25200
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Photoluminescence efficiency droop and stimulated recombination in GaN epilayers

Jūras Mickevičius, Jonas Jurkevičius, Michael S. Shur, Jinwei Yang, Remis Gaska, and Gintautas Tamulaitis  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25195-25200 (2012)
http://dx.doi.org/10.1364/OE.20.025195


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Abstract

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

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 [15

15. J. Mickevičius, M. S. Shur, R. S. Q. Fareed, J. P. Zhang, R. Gaska, and G. Tamulaitis, “Time-resolved experimental study of carrier lifetime in GaN epilayers,” Appl. Phys. Lett. 87(24), 241918 (2005). [CrossRef]

]. 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
Fig. 1 PL spectra of GaN epitaxial layer sample T434 measured above (solid lines) and below (dashed line) the threshold of stimulated recombination in edge and front-face configurations (indicated). Spectra are vertically shifted for clarity.
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
Fig. 2 PL efficiency dependence on excitation power density of epitaxial GaN samples measured in front-face configuration. The carrier lifetimes in the samples are indicated. Points corresponding to emergence of stimulated emission are encircled.
and 3
Fig. 3 PL efficiency dependence on excitation power density of epitaxial GaN samples measured in edge emission configuration. The carrier lifetimes in the samples are indicated. Stimulated emission threshold points are encircled.
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)
Fig. 4 Stimulated emission threshold dependence on carrier lifetime in epitaxial GaN samples measured at edge (a) and front face (b) emission configurations.
. 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.

Acknowledgments

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.

References and links

1.

I. V. Rozhansky and D. A. Zakheim, “Analysis of dependence of electroluminescence efficiency of AlInGaN LED heterostructures on pumping,” Phys. Status Solidi C 3(6), 2160–2164 (2006). [CrossRef]

2.

M.-H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. Park, “Origin of efficiency droop in GaN-based light-emitting diodes,” Appl. Phys. Lett. 91(18), 183507 (2007). [CrossRef]

3.

M. F. Schubert, S. Chhajed, J. K. Kim, E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes,” Appl. Phys. Lett. 91(23), 231114 (2007). [CrossRef]

4.

B. Monemar and B. E. Sernelius, “Defect related issues in the “current roll-off” in InGaN based light emitting diodes,” Appl. Phys. Lett. 91(18), 181103 (2007). [CrossRef]

5.

A. A. Efremov, N. I. Bochkareva, R. I. Gorbunov, D. A. Larinovich, Y. T. Rebane, D. V. Tarkhin, and Y. G. Shreter, “Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs,” Semiconductors 40(5), 605–610 (2006). [CrossRef]

6.

J. Hader, J. V. Moloney, and S. W. Koch, “Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes,” Appl. Phys. Lett. 96(22), 221106 (2010). [CrossRef]

7.

Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, “Auger recombination in InGaN measured by photoluminescence,” Appl. Phys. Lett. 91(14), 141101 (2007). [CrossRef]

8.

K. T. Delaney, P. Rinke, and C. G. Van de Walle, “Auger recombination rates in nitrides from first principles,” Appl. Phys. Lett. 94(19), 191109 (2009). [CrossRef]

9.

H.-Y. Ryu, H.-S. Kim, and J.-I. Shim, “Rate equation analysis of efficiency droop in InGaN light-emitting diodes,” Appl. Phys. Lett. 95(8), 081114 (2009). [CrossRef]

10.

G. Bourdon, I. Robert, I. Sagnes, and I. Abram, “Spontaneous emission in highly excited semiconductors: saturation of the radiative recombination rate,” J. Appl. Phys. 92(11), 6595–6600 (2002). [CrossRef]

11.

J.-I. Shim, H. Kim, D.-S. Shin, and H.-Y. Yoo, “An explanation of efficiency droop in InGaN-based light emitting diodes: saturated radiative recombination rate at randomly distributed In-rich active areas,” J. Korean Phys. Soc. 58(3), 503–508 (2011). [CrossRef]

12.

J. Hader, J. V. Moloney, and S. W. Koch, “Suppression of carrier recombination in semiconductor lasers by phase-space filling,” Appl. Phys. Lett. 87(20), 201112 (2005). [CrossRef]

13.

A. David and N. F. Gardner, “Droop in III-nitrides: comparison of bulk and injection contributions,” Appl. Phys. Lett. 97(19), 193508 (2010). [CrossRef]

14.

S. Grzanka, P. Perlin, R. Czernecki, L. Marona, M. Bockowski, B. Lucznik, M. Leszczynski, and T. Suski, “Effect of efficiency “droop” in violet and blue InGaN laser diodes,” Appl. Phys. Lett. 95(7), 071108 (2009). [CrossRef]

15.

J. Mickevičius, M. S. Shur, R. S. Q. Fareed, J. P. Zhang, R. Gaska, and G. Tamulaitis, “Time-resolved experimental study of carrier lifetime in GaN epilayers,” Appl. Phys. Lett. 87(24), 241918 (2005). [CrossRef]

OCIS Codes
(160.4760) Materials : Optical properties
(160.6000) Materials : Semiconductor materials
(250.5230) Optoelectronics : Photoluminescence

ToC Category:
Optoelectronics

History
Original Manuscript: May 1, 2012
Revised Manuscript: September 18, 2012
Manuscript Accepted: September 21, 2012
Published: October 22, 2012

Citation
Jūras Mickevičius, Jonas Jurkevičius, Michael S. Shur, Jinwei Yang, Remis Gaska, and Gintautas Tamulaitis, "Photoluminescence efficiency droop and stimulated recombination in GaN epilayers," Opt. Express 20, 25195-25200 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25195


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References

  1. I. V. Rozhansky and D. A. Zakheim, “Analysis of dependence of electroluminescence efficiency of AlInGaN LED heterostructures on pumping,” Phys. Status Solidi C3(6), 2160–2164 (2006). [CrossRef]
  2. M.-H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F. Schubert, J. Piprek, and Y. Park, “Origin of efficiency droop in GaN-based light-emitting diodes,” Appl. Phys. Lett.91(18), 183507 (2007). [CrossRef]
  3. M. F. Schubert, S. Chhajed, J. K. Kim, E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes,” Appl. Phys. Lett.91(23), 231114 (2007). [CrossRef]
  4. B. Monemar and B. E. Sernelius, “Defect related issues in the “current roll-off” in InGaN based light emitting diodes,” Appl. Phys. Lett.91(18), 181103 (2007). [CrossRef]
  5. A. A. Efremov, N. I. Bochkareva, R. I. Gorbunov, D. A. Larinovich, Y. T. Rebane, D. V. Tarkhin, and Y. G. Shreter, “Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs,” Semiconductors40(5), 605–610 (2006). [CrossRef]
  6. J. Hader, J. V. Moloney, and S. W. Koch, “Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes,” Appl. Phys. Lett.96(22), 221106 (2010). [CrossRef]
  7. Y. C. Shen, G. O. Mueller, S. Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, “Auger recombination in InGaN measured by photoluminescence,” Appl. Phys. Lett.91(14), 141101 (2007). [CrossRef]
  8. K. T. Delaney, P. Rinke, and C. G. Van de Walle, “Auger recombination rates in nitrides from first principles,” Appl. Phys. Lett.94(19), 191109 (2009). [CrossRef]
  9. H.-Y. Ryu, H.-S. Kim, and J.-I. Shim, “Rate equation analysis of efficiency droop in InGaN light-emitting diodes,” Appl. Phys. Lett.95(8), 081114 (2009). [CrossRef]
  10. G. Bourdon, I. Robert, I. Sagnes, and I. Abram, “Spontaneous emission in highly excited semiconductors: saturation of the radiative recombination rate,” J. Appl. Phys.92(11), 6595–6600 (2002). [CrossRef]
  11. J.-I. Shim, H. Kim, D.-S. Shin, and H.-Y. Yoo, “An explanation of efficiency droop in InGaN-based light emitting diodes: saturated radiative recombination rate at randomly distributed In-rich active areas,” J. Korean Phys. Soc.58(3), 503–508 (2011). [CrossRef]
  12. J. Hader, J. V. Moloney, and S. W. Koch, “Suppression of carrier recombination in semiconductor lasers by phase-space filling,” Appl. Phys. Lett.87(20), 201112 (2005). [CrossRef]
  13. A. David and N. F. Gardner, “Droop in III-nitrides: comparison of bulk and injection contributions,” Appl. Phys. Lett.97(19), 193508 (2010). [CrossRef]
  14. S. Grzanka, P. Perlin, R. Czernecki, L. Marona, M. Bockowski, B. Lucznik, M. Leszczynski, and T. Suski, “Effect of efficiency “droop” in violet and blue InGaN laser diodes,” Appl. Phys. Lett.95(7), 071108 (2009). [CrossRef]
  15. J. Mickevičius, M. S. Shur, R. S. Q. Fareed, J. P. Zhang, R. Gaska, and G. Tamulaitis, “Time-resolved experimental study of carrier lifetime in GaN epilayers,” Appl. Phys. Lett.87(24), 241918 (2005). [CrossRef]

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