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Steady-state and time-resolved luminescence in InGaN layers
Journal of Luminescence 87}89 (2000) 1202}1205
Steady-state and time-resolved luminescence in InGaN layers R. Seitz , C. Gaspar , M. Correia , T. Monteiro *, E. Pereira , M. Heuken, O. Schoen Departamento de Fn& sica, Universidade de Aveiro, 3810 Aveiro, Portugal Aixtron, Kackertstr. 15-17, D-52072 Aachen, Germany
Abstract InGaN layers are used as active layers of high brightness in nitride-based LEDs and lasers. Despite the progress in device development many of the fundamental optical properties are not completely understood. InGaN samples with di!erent In content are studied by steady-state and time-resolved photoluminescence. The low-temperature photoluminescence spectra show a near band edge emission that shifts to lower energies with increasing In content, excitation wavelength and delay times. The emission is broader than typical excitonic emission from binary material. Temperature ependent measurements indicate that the near band edge emission is an overlap of various emission bands with di!erent quenching behaviour. 2000 Elsevier Science B.V. All rights reserved. Keywords: InGaN ternary alloys; Photoluminescence; Time-resolved spectroscopy
1. Introduction
2. Experiment
The band gap of InGaN can be tuned from visible red to ultraviolet by changing the alloy composition. The material is used as an active layer of high brightness in multiple quantum well LEDs and lasers [1]. However, the fundamental recombination mechanisms involved in the emission bands are still a matter of controversy. The near band edge emission is usually identi"ed as localised exciton recombination caused by energy #uctuations in the band edge induced by alloy disorder [2}11]. In this work we study the near band edge emission of single InGaN layers by steady-state luminescence (PL), time-resolved spectroscopy (TRPL) and photoluminescence excitation (PLE). From the results a model for the recombination processes is discussed.
All the InGaN samples were grown by metal-organic chemical deposition on (0 0 0 1) oriented Al O substra tes. Three samples with thickness of 0.2 lm and average In mole fraction (estimated by XRD) of 10% (sample A), 13% (sample B) and 26% (sample C) are analysed. Typically, PL mapping shows a good compositional uniformity with standard deviations of 1.11 nm for each sample. For PL measurements we use the 325 nm line of a He}Cd laser or a cw Xe lamp coupled to a monochromator (also used for the PLE measurements). Neutral density "lters varied excitation intensity. TRPL measurements were carried out with a pulsed Xe lamp as an excitation source and a boxcar system for detection.
3. Results
* Corresponding author. Tel.: #351-34-370-824; fax: 351-34424965. E-mail address: tita@"s.ua.pt (T. Monteiro)
The PL peak position of the InGaN samples with di!erent In content shifts to lower energies with increasing In concentration as expected. In this paper we present the results of the detailed studies of sample. Fig. 1 shows PL and PLE spectra taken at 10 K. PLE spectra are
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 1 1 - 6
R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
1203
Fig. 1. PL and PLE spectra taken at 10 K of sample B.
monitored at several energies of the emission band. When excited by the He}Cd laser the near band edge emission clearly shows a shoulder on the high-energy side at 3.01 eV and a maximum at lower energies at 2.83 eV. The full-width at half-maximum (FWHM) of the overall emission band is 304 meV, suggesting a large alloy potential #uctuation due to compositional inhomogeneity. From PLE spectra taken at di!erent emission energies large Stokes shift are observed, in agreement with previous reports [3,7,8]. PL emission at 10 K under di!erent excitation energies is shown in Fig. 2. A shift towards lower energies of the PL peak position accompanied with a narrowing of the FWHM of the emission band is clearly observed when the excitation energy decreases. For "xed excitation energy (He}Cd laser line) and under di!erent excitation intensities no shift of the PL band has been found. The luminescence intensity depends superlinearily on the excitation intensity (I &I@ ., b"1.23). A comparison of TRPL and PL spectra shows that the band is an overlap of fast and slow (decay times&ms) emissions (Fig. 3). To our knowledge this is the "rst time that slow emission in the ms range is reported for InGaN samples. The recombination lifetime depends on the emission energy, increasing for lower energies.
Fig. 2. PL spectra of sample B taken at 10 K for di!erent excitation energies. Inset: FWHM dependence on excitation energy.
4. Discussion The PL peak position of InGaN layers clearly depends on In mole fraction and shifts to lower energies with increasing In content as expected from the reduction in the energy gap. The overall FWHM of the near band
Fig. 3. PL and TRPL (between 0.08 and 0.18 ms) under di!erent excitation energies taken at 10 K. Inset: PL and TRPL energy peak position dependence on excitation energy.
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R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
edge emission varies from 50 meV [2] to hundreds of meV [6}11]. In our sample B the FWHM of the emission is nearly constant between 10 and 50 K. From the PL spectra of Fig. 1, obtained by excitation at 3.815 eV, a shoulder on the higher-energy side at 3.01 eV and a maximum at lower energies peaked at 2.83 eV can be seen. When PLE is monitored at the higher-energy side of the emission we "nd a steeper slope in the spectrum than for lower energies. This is interpreted as due to In #uctuations in the sample. The lower-energy side of the emission band is due to a superposition of emissions originating in regions of di!erent In content. While from both PL mapping and Micro-Raman spectra no signi"cant alloy #uctuations are observed, the PLE data clearly shows an inhomogeneous In distribution in the sample, indicating that heterogeneities at a lower scale than lm exist. This assumption is evidenced by the PL spectra obtained under decreasing excitation energies (Fig. 2), where a shift to lower energy of the band maxima and a narrowing of the FWHM of the emission band is clearly observed (inset Fig. 2). The overall shape of the PL emission is similar to the one observed when the sample is excited by the He}Cd laser. A shoulder always accompanies the band maxima on the higher-energy side of the bands. These results indicate, that for lower excitation energy only the regions of high In content contribute to the luminescence. A PL peak position dependent on the excitation photon energy has been found on disordered solid solutions and a similar result on a In Ga N sample was previously reported [6]. TRPL studies in the ls}ms region taken with di!erent excitation energies clearly show a slow emission band besides a fast luminescence. The slow emission is narrower and occurs at the low-energy side (Fig. 3). The analysis for longer delays is complicated due to the increase in the yellow band emission. Thus, we used a narrow time window for the TRPL spectrum (0.08}0.18 ms). This explains why there is no overlap in the low-energy side of the PL and TRPL spectra. The decay of the lower-energy band is dependent on the emission energy, increasing with decreasing emission energy. These results are typical of donor}acceptor pair (DAP) recombinations. If only mass e!ective like electronic states are involved in the recombination processes [9] we can assume that besides intrinsic excitonic emissions that give rise to fast decays on the high-energy side, DAP recombinations can be present in inhomogeneous InGaN alloys on the low-energy side. PL emission from DAP recombinations are known to shift to higher energies with increasing excitation intensity due to saturation e!ects. Our measurements clearly indicate that no shift of the PL maximum occurs. In the overall near band edge emission we have an overlap of excitonic and DAP emissions that occur at di!erent energies depending on the band gap #uctuations and on the distance of the donor and acceptor pair. Therefore, we can understand the data as an
Fig. 4. PL spectra taken at di!erent temperatures using a He}Cd laser as excitation source.
averaging of the described processes. As we know that an emission at a given energy is the superposition of slow and fast components originating in regions of di!erent In content, it is likely that compensating e!ects occur suppressing a signi"cant shift in the band maximum with excitation intensity. The superlinear dependence of the emission intensity on the excitation intensity was observed previously but with a higher value of the exponent [2]. This may be due to wider #uctuations of In concentration in our samples that reduce the exciton-exciton interactions. The overall PL emission intensity decreases by three orders of magnitude in the temperature range from 10 K to RT when excited by the He}Cd laser. This is in agreement with a similar dependence previously reported by Shan et al. [9]. The overall band shape and position remains practically unchanged from 10 to 40 K. However, the band shape is strongly a!ected for temperatures above 50 K (Fig. 4), indicating that several recombination processes with di!erent quenching behaviours are involved.
5. Conclusions PL, PLE, TRPL behaviour has been studied in InGaN layers. From the data we are able to assume that the PL emission is an overlap of several emission bands originating in regions of di!erent In content. PLE clearly indicates that di!erent In mole fractions are present in the sample. It is therefore a powerful technique to assess In #uctuations in the layers. From TRPL measurements slow emissions in the ms range were observed for the "rst time in InGaN samples. TRPL spectra as well as decay
R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
times are consistent with DAP recombination. However, besides the slow component fast emissions are observed on the higher-energy side mainly due to excitonic transitions as indicated by the superlinear dependence of the emission intensity on the excitation intensity. Acknowledgements One of the authors (R.S.) thanks to FCT for maintenance grant (Praxis XXI/BD/16284/98). The samples were provided in cooperation within the Brite-Euram Project Rainbow contract no. BRPR-CT96-0340. References [1] S. Nakamura, M. Senoh, N. Naruhito, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [2] M. Smith, G.D. Chen, J.Y. Lin, H.X. Jiang, M.A. Khan, Q. Chen, Appl. Phys. Lett. 69 (1996) 2837.
1205
[3] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 70 (1997) 2822. [4] Y. Narukawa, Y. Kawakami, S. Fujita, S. Fujita, S. Nakamura, Phys. Rev. B 55 (1997) R1938. [5] Y. Narukawa, K. Sawada, Y. Kawakami, S. Fujita, S. Fujita, S. Nakamura, J. Cryst. Growth 189&190 (1998) 606. [6] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, F. Nakamura, M. Ikeda, Phys. Rev. B 57 (1998) R2041. [7] Y. Yamada, T. Taguchi, F. Sasaki, S. Kobayashi, T. Tani, J. Cryst. Growth 189&190 (1998) 611. [8] H. Kanie, N. Tsukamoto, H. Koami, T. Kawano, T. Totsuka, J. Cryst. Growth 189&190 (1998) 52. [9] W. Shan, W. Walukiewicz, E.E. Haller, B.D. Little, J.J. Song, M.D. McCluskey, N.M. Johnson, Z.C. Feng, M. Schurman, R.A. Stall, J. Appl. Phys. 84 (1998) 4452. [10] Y.H. Cho, B.D. Little, G.H. Gainer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G2.4. [11] T.Y. Lin, J.C. Fan, Y.F. Chen, Semicond. Sci. Technol. 14 (1999) 406.
Steady-state and time-resolved luminescence in InGaN layers R. Seitz , C. Gaspar , M. Correia , T. Monteiro *, E. Pereira , M. Heuken, O. Schoen Departamento de Fn& sica, Universidade de Aveiro, 3810 Aveiro, Portugal Aixtron, Kackertstr. 15-17, D-52072 Aachen, Germany
Abstract InGaN layers are used as active layers of high brightness in nitride-based LEDs and lasers. Despite the progress in device development many of the fundamental optical properties are not completely understood. InGaN samples with di!erent In content are studied by steady-state and time-resolved photoluminescence. The low-temperature photoluminescence spectra show a near band edge emission that shifts to lower energies with increasing In content, excitation wavelength and delay times. The emission is broader than typical excitonic emission from binary material. Temperature ependent measurements indicate that the near band edge emission is an overlap of various emission bands with di!erent quenching behaviour. 2000 Elsevier Science B.V. All rights reserved. Keywords: InGaN ternary alloys; Photoluminescence; Time-resolved spectroscopy
1. Introduction
2. Experiment
The band gap of InGaN can be tuned from visible red to ultraviolet by changing the alloy composition. The material is used as an active layer of high brightness in multiple quantum well LEDs and lasers [1]. However, the fundamental recombination mechanisms involved in the emission bands are still a matter of controversy. The near band edge emission is usually identi"ed as localised exciton recombination caused by energy #uctuations in the band edge induced by alloy disorder [2}11]. In this work we study the near band edge emission of single InGaN layers by steady-state luminescence (PL), time-resolved spectroscopy (TRPL) and photoluminescence excitation (PLE). From the results a model for the recombination processes is discussed.
All the InGaN samples were grown by metal-organic chemical deposition on (0 0 0 1) oriented Al O substra tes. Three samples with thickness of 0.2 lm and average In mole fraction (estimated by XRD) of 10% (sample A), 13% (sample B) and 26% (sample C) are analysed. Typically, PL mapping shows a good compositional uniformity with standard deviations of 1.11 nm for each sample. For PL measurements we use the 325 nm line of a He}Cd laser or a cw Xe lamp coupled to a monochromator (also used for the PLE measurements). Neutral density "lters varied excitation intensity. TRPL measurements were carried out with a pulsed Xe lamp as an excitation source and a boxcar system for detection.
3. Results
* Corresponding author. Tel.: #351-34-370-824; fax: 351-34424965. E-mail address: tita@"s.ua.pt (T. Monteiro)
The PL peak position of the InGaN samples with di!erent In content shifts to lower energies with increasing In concentration as expected. In this paper we present the results of the detailed studies of sample. Fig. 1 shows PL and PLE spectra taken at 10 K. PLE spectra are
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 1 1 - 6
R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
1203
Fig. 1. PL and PLE spectra taken at 10 K of sample B.
monitored at several energies of the emission band. When excited by the He}Cd laser the near band edge emission clearly shows a shoulder on the high-energy side at 3.01 eV and a maximum at lower energies at 2.83 eV. The full-width at half-maximum (FWHM) of the overall emission band is 304 meV, suggesting a large alloy potential #uctuation due to compositional inhomogeneity. From PLE spectra taken at di!erent emission energies large Stokes shift are observed, in agreement with previous reports [3,7,8]. PL emission at 10 K under di!erent excitation energies is shown in Fig. 2. A shift towards lower energies of the PL peak position accompanied with a narrowing of the FWHM of the emission band is clearly observed when the excitation energy decreases. For "xed excitation energy (He}Cd laser line) and under di!erent excitation intensities no shift of the PL band has been found. The luminescence intensity depends superlinearily on the excitation intensity (I &I@ ., b"1.23). A comparison of TRPL and PL spectra shows that the band is an overlap of fast and slow (decay times&ms) emissions (Fig. 3). To our knowledge this is the "rst time that slow emission in the ms range is reported for InGaN samples. The recombination lifetime depends on the emission energy, increasing for lower energies.
Fig. 2. PL spectra of sample B taken at 10 K for di!erent excitation energies. Inset: FWHM dependence on excitation energy.
4. Discussion The PL peak position of InGaN layers clearly depends on In mole fraction and shifts to lower energies with increasing In content as expected from the reduction in the energy gap. The overall FWHM of the near band
Fig. 3. PL and TRPL (between 0.08 and 0.18 ms) under di!erent excitation energies taken at 10 K. Inset: PL and TRPL energy peak position dependence on excitation energy.
1204
R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
edge emission varies from 50 meV [2] to hundreds of meV [6}11]. In our sample B the FWHM of the emission is nearly constant between 10 and 50 K. From the PL spectra of Fig. 1, obtained by excitation at 3.815 eV, a shoulder on the higher-energy side at 3.01 eV and a maximum at lower energies peaked at 2.83 eV can be seen. When PLE is monitored at the higher-energy side of the emission we "nd a steeper slope in the spectrum than for lower energies. This is interpreted as due to In #uctuations in the sample. The lower-energy side of the emission band is due to a superposition of emissions originating in regions of di!erent In content. While from both PL mapping and Micro-Raman spectra no signi"cant alloy #uctuations are observed, the PLE data clearly shows an inhomogeneous In distribution in the sample, indicating that heterogeneities at a lower scale than lm exist. This assumption is evidenced by the PL spectra obtained under decreasing excitation energies (Fig. 2), where a shift to lower energy of the band maxima and a narrowing of the FWHM of the emission band is clearly observed (inset Fig. 2). The overall shape of the PL emission is similar to the one observed when the sample is excited by the He}Cd laser. A shoulder always accompanies the band maxima on the higher-energy side of the bands. These results indicate, that for lower excitation energy only the regions of high In content contribute to the luminescence. A PL peak position dependent on the excitation photon energy has been found on disordered solid solutions and a similar result on a In Ga N sample was previously reported [6]. TRPL studies in the ls}ms region taken with di!erent excitation energies clearly show a slow emission band besides a fast luminescence. The slow emission is narrower and occurs at the low-energy side (Fig. 3). The analysis for longer delays is complicated due to the increase in the yellow band emission. Thus, we used a narrow time window for the TRPL spectrum (0.08}0.18 ms). This explains why there is no overlap in the low-energy side of the PL and TRPL spectra. The decay of the lower-energy band is dependent on the emission energy, increasing with decreasing emission energy. These results are typical of donor}acceptor pair (DAP) recombinations. If only mass e!ective like electronic states are involved in the recombination processes [9] we can assume that besides intrinsic excitonic emissions that give rise to fast decays on the high-energy side, DAP recombinations can be present in inhomogeneous InGaN alloys on the low-energy side. PL emission from DAP recombinations are known to shift to higher energies with increasing excitation intensity due to saturation e!ects. Our measurements clearly indicate that no shift of the PL maximum occurs. In the overall near band edge emission we have an overlap of excitonic and DAP emissions that occur at di!erent energies depending on the band gap #uctuations and on the distance of the donor and acceptor pair. Therefore, we can understand the data as an
Fig. 4. PL spectra taken at di!erent temperatures using a He}Cd laser as excitation source.
averaging of the described processes. As we know that an emission at a given energy is the superposition of slow and fast components originating in regions of di!erent In content, it is likely that compensating e!ects occur suppressing a signi"cant shift in the band maximum with excitation intensity. The superlinear dependence of the emission intensity on the excitation intensity was observed previously but with a higher value of the exponent [2]. This may be due to wider #uctuations of In concentration in our samples that reduce the exciton-exciton interactions. The overall PL emission intensity decreases by three orders of magnitude in the temperature range from 10 K to RT when excited by the He}Cd laser. This is in agreement with a similar dependence previously reported by Shan et al. [9]. The overall band shape and position remains practically unchanged from 10 to 40 K. However, the band shape is strongly a!ected for temperatures above 50 K (Fig. 4), indicating that several recombination processes with di!erent quenching behaviours are involved.
5. Conclusions PL, PLE, TRPL behaviour has been studied in InGaN layers. From the data we are able to assume that the PL emission is an overlap of several emission bands originating in regions of di!erent In content. PLE clearly indicates that di!erent In mole fractions are present in the sample. It is therefore a powerful technique to assess In #uctuations in the layers. From TRPL measurements slow emissions in the ms range were observed for the "rst time in InGaN samples. TRPL spectra as well as decay
R. Seitz et al. / Journal of Luminescence 87 }89 (2000) 1202}1205
times are consistent with DAP recombination. However, besides the slow component fast emissions are observed on the higher-energy side mainly due to excitonic transitions as indicated by the superlinear dependence of the emission intensity on the excitation intensity. Acknowledgements One of the authors (R.S.) thanks to FCT for maintenance grant (Praxis XXI/BD/16284/98). The samples were provided in cooperation within the Brite-Euram Project Rainbow contract no. BRPR-CT96-0340. References [1] S. Nakamura, M. Senoh, N. Naruhito, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [2] M. Smith, G.D. Chen, J.Y. Lin, H.X. Jiang, M.A. Khan, Q. Chen, Appl. Phys. Lett. 69 (1996) 2837.
1205
[3] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 70 (1997) 2822. [4] Y. Narukawa, Y. Kawakami, S. Fujita, S. Fujita, S. Nakamura, Phys. Rev. B 55 (1997) R1938. [5] Y. Narukawa, K. Sawada, Y. Kawakami, S. Fujita, S. Fujita, S. Nakamura, J. Cryst. Growth 189&190 (1998) 606. [6] A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, F. Nakamura, M. Ikeda, Phys. Rev. B 57 (1998) R2041. [7] Y. Yamada, T. Taguchi, F. Sasaki, S. Kobayashi, T. Tani, J. Cryst. Growth 189&190 (1998) 611. [8] H. Kanie, N. Tsukamoto, H. Koami, T. Kawano, T. Totsuka, J. Cryst. Growth 189&190 (1998) 52. [9] W. Shan, W. Walukiewicz, E.E. Haller, B.D. Little, J.J. Song, M.D. McCluskey, N.M. Johnson, Z.C. Feng, M. Schurman, R.A. Stall, J. Appl. Phys. 84 (1998) 4452. [10] Y.H. Cho, B.D. Little, G.H. Gainer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G2.4. [11] T.Y. Lin, J.C. Fan, Y.F. Chen, Semicond. Sci. Technol. 14 (1999) 406.