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GaN layer
Superlattices and Microstructures 40 (2006) 452–457 www.elsevier.com/locate/superlattices
Optical studies on a coherent InGaN/GaN layer M.R. Correia a,∗ , S. Pereira a,b , E. Alves c , B. Arnaudov d a Universidade de Aveiro, Departmento de F´ısica, Aveiro, Portugal b CICECO, Aveiro, Portugal c Instituto Tecnol´ogico e Nuclear, Departmento de F´ısica, Sacav´em, Portugal d Faculty of Physics, Sofia University, Sofia 1164, Bulgaria
Received 6 September 2006; accepted 7 September 2006 Available online 27 October 2006
Abstract Photoluminescence (PL), photoluminescence excitation (PLE) and selective excitation (SE-PL) studies were performed in an attempt to identify the origin of the emission bands in a pseudomorphic In0.05 Ga0.95 N/GaN film. Besides the InGaN near-band-edge PL emission centred at 3.25 eV an additional blue band centred at 2.74 eV was observed. The lower energy PL peak is characterized by an energy separation between absorption and emission – the Stokes’ shift – (∼500 meV) much larger than expected. A systematic PLE and selective excitation analysis has shown that the PL peak at 2.74 eV is related to an absorption band observed below the InGaN band gap. We propose the blue emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap. c 2006 Elsevier Ltd. All rights reserved.
Keywords: InGaN alloys; Photoluminescence; Defect levels
1. Introduction The optical recombination processes have been extensively studied in the InGaN/GaN system. In general the luminescence mechanisms of InGaN are discussed in view of the excitation localization due to the potential minima originating from spatial fluctuations of alloy composition, the so-called In rich quantum dots (QDs) or potential domains [1,2]. However, it has been demonstrated that strain effects might also be considered to interpret the optical properties of InGaN samples grown above the critical thickness [3]. Blue, green and yellow luminescence bands assigned to deep defects in the InGaN have been reported [4–7]. In particular, a slow blue emission (2.8 eV) within the range of milliseconds was ∗ Corresponding author. Tel.: +351 234 370943; fax: +351 234 424965.
E-mail address: [email protected] (M.R. Correia). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.004
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Fig. 1. Scanning electron microscopy image of the InGaN grown on a GaN buffer layer onto a sapphire substrate.
investigated [5]. Emissions within the same spectral region also occur in GaN [8–13]. A broad blue band emission located at 2.7–2.9 eV, observed in undoped [8,10,12] and doped GaN [9,11, 13] samples has been studied and different recombination models have been proposed. In this work we focus our attention on the luminescence bands observed within the yellow–violet spectral region in a InGaN single layer of high crystalline quality. A detailed analysis based on the luminescence excitation (PLE) and selective excitation of luminescence (SE-PL) measurements was done in an attempt to separate the InGaN film and GaN buffer contributions. 2. Experimental procedure The sample studied is a nominally undoped wurtzite InGaN (∼100 nm)/GaN (0001) (2 µm) layer, grown by metalorganic vapour phase epitaxy in a commercial reactor onto the sapphire substrate. The sample surface morphology was inspected by scanning electron microscopy. Rutherford backscattering spectrometry (RBS) was used to evaluate the composition homogeneity over depth. This analysis was performed using a 1 mm collimated beam of 2.0 MeV 4 He+ . The backscattered particles were detected at 160◦ and close to 180◦ , with respect to the beam direction, using silicon surface barrier detectors with resolution of 12 keV and 16 keV, respectively. The luminescence spectra were recorded at 14 K and room temperature, with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Jobin Yvon-Spex) coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter. The excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. 3. Results and discussion Fig. 1 display the surface morphology of the sample, obtained by scanning electron microscopy. The picture shows a smooth surface, with visible atomic terraces and a distribution
454
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
Fig. 2. Random (2◦ off axis), aligned h0001i and simulated RBS spectra.
of hexagonal pits. This is a typical surface of a pseudomorphic InGaN layer grown on GaN (0001). A pit density of ∼2 × 109 cm2 was determined. Fig. 2 shows the h0001i aligned and random (2◦ off axis) RBS spectra. Vertical arrows indicate the scattering energies of In and Ga elements. The horizontal arrows indicate the depth location in the sample. The crystalline disorder can be quantified by the parameter χmin , defined as the ratio between backscattering yields from the aligned and random spectra after the near surface region. The measured value of χmin along the h0001i direction is 3.2% which is comparable with the values obtained for high quality crystals. Observing the trend on the shape of random RBS spectra within the energy window corresponding to the InGaN, we can infer that InGaN film has a homogeneous composition over depth, because it follows the increase of the backscattering yield with ∝ 1/E 2 as the beam penetrates deeper in the film. For a detailed quantitative analysis, simulation of RBS spectra was performed with RUMP code [14]. A model consisting on a 151 ± 10 nm thick InGaN single layer with x = 0.05 ± 0.01 provides a good fit to experiment, as shown in Fig. 2. Fig. 3 shows the PL spectra at 14 K of the InGaN sample when excited at 325 nm. An emission at 3.48 eV, attributed to the bound exciton of GaN [15] is clearly visible. At lower energies, three bands are observed: a strong peak located at about 3.25 eV (violet), and two weaker peaks at 2.76 eV (blue) and 2.13 eV (yellow), respectively. The full widths at half-maximum (FWHM) are about 100 meV for violet emission and larger than 300 meV for blue and green bands. Focusing our attention in the violet and blue PL peaks we now compare our results with that which would be expected if each of these peaks originated in the full coherent and relaxed regions, respectively [3]. According to Ref. [3], an energy separation of ∼86 meV between the higher energy component and lower energy PL peak is expected. In this work we obtain ∼500 meV for the difference between violet and blue PL peaks; thus we concluded that the two emissions cannot be interpreted as reference for a strain relaxation process. As we are inspecting the transparency region for both materials, the spectral features observed can be originated either from GaN buffer layer or InGaN film. Therefore, PLE measurements
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
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Fig. 3. Low temperature photoluminescence spectra of the InGaN grown on a GaN (0001) buffer layer onto a sapphire substrate.
Fig. 4. Photoluminescence excitation spectra of the InGaN/GaN grown onto a sapphire substrate, monitored at (a) 3.26, (b) 3.10, (c) 2.76, (d) 2.58 and (e) 2.21 eV. In the inset the PLE spectrum monitored at the blue PL peak is plotted in logarithmic scale to put in evidence the absorption band located below the InGaN band gap.
were performed to identify the maximum of absorption of each PL peak. Fig. 4 shows the PLE spectra at 15 K monitored from 3.26 to 2.21 eV. When the detection energy is at 3.26 eV, the PLE spectrum is dominated by the InGaN near band edge. The clear observation of the exciton
456
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
Fig. 5. Photoluminescence spectra under selective excitation energies.
peak at 3.33 eV, as shown in Fig. 4, indicates the alloy disordering effect in the free exciton sates is rather small. The GaN band edge is also detected, however its relative PLE intensity is weaker than that at the InGaN band gap. This indicates the main PL peak is fundamentally populated trough the band edge of InGaN. Moreover, this emission has a Stokes’ shift of ∼80 meV, which is within the range of the values expected for a band to band transition in InGaN for this PL energy [2]. Therefore we assign the violet PL peak to a band to band transition in the InGaN. When the monochromator of emission is positioned at 3.10 eV the intensity of the PLE spectrum, at the GaN band edge, increases relative to the intensity of the InGaN absorption. Therefore, the luminescence observed in this region is an overlap of the GaN and InGaN related emissions which contribute to the asymmetry of the main PL peak in the lower energy side. An additional broad absorption band arises below the InGaN band gap as the PLE spectrum is taken at the maximum of the blue band (2.76 eV) (see selected dashed area at Fig. 4). Simultaneously the relative intensity at the GaN band edge decreases. Setting the detection energy at 2.58 eV, the blue related absorption remains. Nevertheless as the detection energy is set at 2.2 eV (yellow PL peak) only the InGaN and the GaN band gap are observed. The characteristics of this spectrum also indicate the emission at 2.2 eV is essentially populated from the GaN band edge, and corresponds to the “yellow band”. Moreover the absence of the absorption band, below the InGaN band gap, indicates that it is associated to the blue PL peak and occurs within a spectral width lower than 560 meV. In order to explore the nature of the blue emission additional SE-PL measurements were carried out. Fig. 5 shows the PL spectra for the blue luminescence taken by varying the energy of excitation from 3.54 to 3.18 eV. The experimental results indicate this PL peak is excited for energies well below the GaN band gap. A well defined broad band centred at 2.74 eV and with FWHM of ∼350 meV was observed exciting at 3.35 eV. This emission is also excited at 3.25 eV, which coincides with the violet PL peak (see Fig. 4). This is the reason why we have observed the InGaN band edge when the PLE spectrum was monitored at the blue PL peak. Nevertheless, this result is not a clear indication that the blue component is originated from the InGaN film, since both GaN and InGaN are transparent at 3.25 eV. At the excitation energy of 3.18 eV the relative intensity of blue PL peak decreases. These results suggest the width of the absorption
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
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band is compared to the blue PL peak FWHM. If the maximum of absorption is assumed at 3.25 eV, the difference between the absorption and emission energies is close to 500 meV, which is larger than the values expected for the Stokes’ shift for this PL energy in InGaN [2]. This indicates that the blue band cannot be interpreted as reference to an exciton localization on InGaN band tail. Comparing the PLE and SE-PL experimental results we propose the blue PL peak is likely related to defect levels. However further optical studies with depth resolution such as cathodoluminescence or by using a sequential etching process are needed in order to identify if the defect levels are related to the InGaN layer or the GaN buffer. 4. Conclusions Optical properties in a representative pseudomorphic In0.05 Ga0.95 N/GaN film are investigated. The PL peak at 3.25 eV revealed the typical behaviour of a band to band transition in InGaN. However, a systematic PLE and selective excitation analysis has shown that the additional PL peak at 2.74 eV is related to the absorption band observed below the InGaN band gap. This PL peak is characterized by a Stokes’ shift (∼500 meV) much larger than the expected for this energy in the InGaN. The results of this investigation indicate the blue band emission is neither due to the recombination of localized excitons at In-rich domains or as evidence of strain relaxation. We propose that this emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap. This investigation demonstrates the difficulty of unambiguously ascribing the origin of luminescence bands observed in InGaN/GaN heterostructures by common optical characterization. Acknowledgments M.R. Correia would like to thank Professor Teresa Monteiro from the University of Aveiro for useful comments and discussions. The authors acknowledge Rute A. S´a Ferreira from the University of Aveiro for assistance in the optical measurements. The financial support by FCT (POCTI/FIS/57550/2004) is gratefully acknowledged. References [1] S. Chichibu, T. Azuhata, T. Sota, S. Narukava, Appl. Phys. Lett. 70 (1997) 2822. [2] K.P. O’Donnell, R.W. Martin, P.G. Middleton, Phys. Rev. Lett. 82 (1999) 237. [3] S. Pereira, M.R. Correia, E. Pereira, C. Trager-Cowan, K.P. O’Donnell, E. Alves, N. Franco, A.D. Sequeira, Appl. Phys. Lett. 81 (2002) 1207. [4] Ch. Manz, M. Kunzer, H. Obloh, A. Ramakrishnan, U. Kaufmann, Appl. Phys. Lett. 74 (1999) 3993. [5] R. Seitz, C. Gaspar, M. Correia, T. Monteiro, E. Pereira, M. Heuken, O. Schoen, J. Lumin. 87 (2000) 1202. [6] M.R. Correia, S. Pereira, E. Pereira, R.A. S´a Ferreira, J. Frandon, E. Alves, C. Liu, A. Morel, B. Gil, Superlattice. Microstruct. 36 (2004) 625. [7] B. Han, M.P. Ulmer, B.W. Wessels, Physica B 340 (2006) 474. [8] M.A. Reshchikov, F. Shahedipour, R.Y. Korotkov, B.W. Wessels, M.P. Ulmer, Appl. Phys. Lett. 87 (2000) 3551. [9] H.C. Yang, T.Y. Lin, Y.F. Chen, Phys. Rev. B 62 (2000) 12593. [10] M.A. Reshchikov, P. Visconti, H. Morkoc¸, Appl. Phys. Lett. 78 (2001) 177. [11] S.O. Kucheyev, M. Toth, M.R. Phillips, J.S. Williams, C. Jagadish, G. Li, J. Appl. Phys. 91 (2002) 5867. [12] C. D´ıaz-Guerra, J. Piqueras, A. Cavallini, Appl. Phys. Lett. 82 (2003) 2050. [13] R. Armitage, W. Hong, Q. Yang, H. Feick, J. Gebauer, E.R. Weber, S. Hautakangas, K. Saarinen, Appl. Phys. Lett. 82 (2003) 3457. [14] L.R. Doolitle, Nucl. Instrum. Methods Phys. Res. B9 (1985) 344. [15] W. Shan, R.J. Hauenstein, A.J. Fisher, J.J. Song, W.G. Perry, M.D. Bremser, R.F. Davis, B. Goldenberg, Phys. Rev. B 54 (1996) 13460.
Optical studies on a coherent InGaN/GaN layer M.R. Correia a,∗ , S. Pereira a,b , E. Alves c , B. Arnaudov d a Universidade de Aveiro, Departmento de F´ısica, Aveiro, Portugal b CICECO, Aveiro, Portugal c Instituto Tecnol´ogico e Nuclear, Departmento de F´ısica, Sacav´em, Portugal d Faculty of Physics, Sofia University, Sofia 1164, Bulgaria
Received 6 September 2006; accepted 7 September 2006 Available online 27 October 2006
Abstract Photoluminescence (PL), photoluminescence excitation (PLE) and selective excitation (SE-PL) studies were performed in an attempt to identify the origin of the emission bands in a pseudomorphic In0.05 Ga0.95 N/GaN film. Besides the InGaN near-band-edge PL emission centred at 3.25 eV an additional blue band centred at 2.74 eV was observed. The lower energy PL peak is characterized by an energy separation between absorption and emission – the Stokes’ shift – (∼500 meV) much larger than expected. A systematic PLE and selective excitation analysis has shown that the PL peak at 2.74 eV is related to an absorption band observed below the InGaN band gap. We propose the blue emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap. c 2006 Elsevier Ltd. All rights reserved.
Keywords: InGaN alloys; Photoluminescence; Defect levels
1. Introduction The optical recombination processes have been extensively studied in the InGaN/GaN system. In general the luminescence mechanisms of InGaN are discussed in view of the excitation localization due to the potential minima originating from spatial fluctuations of alloy composition, the so-called In rich quantum dots (QDs) or potential domains [1,2]. However, it has been demonstrated that strain effects might also be considered to interpret the optical properties of InGaN samples grown above the critical thickness [3]. Blue, green and yellow luminescence bands assigned to deep defects in the InGaN have been reported [4–7]. In particular, a slow blue emission (2.8 eV) within the range of milliseconds was ∗ Corresponding author. Tel.: +351 234 370943; fax: +351 234 424965.
E-mail address: [email protected] (M.R. Correia). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.004
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
453
Fig. 1. Scanning electron microscopy image of the InGaN grown on a GaN buffer layer onto a sapphire substrate.
investigated [5]. Emissions within the same spectral region also occur in GaN [8–13]. A broad blue band emission located at 2.7–2.9 eV, observed in undoped [8,10,12] and doped GaN [9,11, 13] samples has been studied and different recombination models have been proposed. In this work we focus our attention on the luminescence bands observed within the yellow–violet spectral region in a InGaN single layer of high crystalline quality. A detailed analysis based on the luminescence excitation (PLE) and selective excitation of luminescence (SE-PL) measurements was done in an attempt to separate the InGaN film and GaN buffer contributions. 2. Experimental procedure The sample studied is a nominally undoped wurtzite InGaN (∼100 nm)/GaN (0001) (2 µm) layer, grown by metalorganic vapour phase epitaxy in a commercial reactor onto the sapphire substrate. The sample surface morphology was inspected by scanning electron microscopy. Rutherford backscattering spectrometry (RBS) was used to evaluate the composition homogeneity over depth. This analysis was performed using a 1 mm collimated beam of 2.0 MeV 4 He+ . The backscattered particles were detected at 160◦ and close to 180◦ , with respect to the beam direction, using silicon surface barrier detectors with resolution of 12 keV and 16 keV, respectively. The luminescence spectra were recorded at 14 K and room temperature, with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Jobin Yvon-Spex) coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter. The excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. 3. Results and discussion Fig. 1 display the surface morphology of the sample, obtained by scanning electron microscopy. The picture shows a smooth surface, with visible atomic terraces and a distribution
454
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
Fig. 2. Random (2◦ off axis), aligned h0001i and simulated RBS spectra.
of hexagonal pits. This is a typical surface of a pseudomorphic InGaN layer grown on GaN (0001). A pit density of ∼2 × 109 cm2 was determined. Fig. 2 shows the h0001i aligned and random (2◦ off axis) RBS spectra. Vertical arrows indicate the scattering energies of In and Ga elements. The horizontal arrows indicate the depth location in the sample. The crystalline disorder can be quantified by the parameter χmin , defined as the ratio between backscattering yields from the aligned and random spectra after the near surface region. The measured value of χmin along the h0001i direction is 3.2% which is comparable with the values obtained for high quality crystals. Observing the trend on the shape of random RBS spectra within the energy window corresponding to the InGaN, we can infer that InGaN film has a homogeneous composition over depth, because it follows the increase of the backscattering yield with ∝ 1/E 2 as the beam penetrates deeper in the film. For a detailed quantitative analysis, simulation of RBS spectra was performed with RUMP code [14]. A model consisting on a 151 ± 10 nm thick InGaN single layer with x = 0.05 ± 0.01 provides a good fit to experiment, as shown in Fig. 2. Fig. 3 shows the PL spectra at 14 K of the InGaN sample when excited at 325 nm. An emission at 3.48 eV, attributed to the bound exciton of GaN [15] is clearly visible. At lower energies, three bands are observed: a strong peak located at about 3.25 eV (violet), and two weaker peaks at 2.76 eV (blue) and 2.13 eV (yellow), respectively. The full widths at half-maximum (FWHM) are about 100 meV for violet emission and larger than 300 meV for blue and green bands. Focusing our attention in the violet and blue PL peaks we now compare our results with that which would be expected if each of these peaks originated in the full coherent and relaxed regions, respectively [3]. According to Ref. [3], an energy separation of ∼86 meV between the higher energy component and lower energy PL peak is expected. In this work we obtain ∼500 meV for the difference between violet and blue PL peaks; thus we concluded that the two emissions cannot be interpreted as reference for a strain relaxation process. As we are inspecting the transparency region for both materials, the spectral features observed can be originated either from GaN buffer layer or InGaN film. Therefore, PLE measurements
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455
Fig. 3. Low temperature photoluminescence spectra of the InGaN grown on a GaN (0001) buffer layer onto a sapphire substrate.
Fig. 4. Photoluminescence excitation spectra of the InGaN/GaN grown onto a sapphire substrate, monitored at (a) 3.26, (b) 3.10, (c) 2.76, (d) 2.58 and (e) 2.21 eV. In the inset the PLE spectrum monitored at the blue PL peak is plotted in logarithmic scale to put in evidence the absorption band located below the InGaN band gap.
were performed to identify the maximum of absorption of each PL peak. Fig. 4 shows the PLE spectra at 15 K monitored from 3.26 to 2.21 eV. When the detection energy is at 3.26 eV, the PLE spectrum is dominated by the InGaN near band edge. The clear observation of the exciton
456
M.R. Correia et al. / Superlattices and Microstructures 40 (2006) 452–457
Fig. 5. Photoluminescence spectra under selective excitation energies.
peak at 3.33 eV, as shown in Fig. 4, indicates the alloy disordering effect in the free exciton sates is rather small. The GaN band edge is also detected, however its relative PLE intensity is weaker than that at the InGaN band gap. This indicates the main PL peak is fundamentally populated trough the band edge of InGaN. Moreover, this emission has a Stokes’ shift of ∼80 meV, which is within the range of the values expected for a band to band transition in InGaN for this PL energy [2]. Therefore we assign the violet PL peak to a band to band transition in the InGaN. When the monochromator of emission is positioned at 3.10 eV the intensity of the PLE spectrum, at the GaN band edge, increases relative to the intensity of the InGaN absorption. Therefore, the luminescence observed in this region is an overlap of the GaN and InGaN related emissions which contribute to the asymmetry of the main PL peak in the lower energy side. An additional broad absorption band arises below the InGaN band gap as the PLE spectrum is taken at the maximum of the blue band (2.76 eV) (see selected dashed area at Fig. 4). Simultaneously the relative intensity at the GaN band edge decreases. Setting the detection energy at 2.58 eV, the blue related absorption remains. Nevertheless as the detection energy is set at 2.2 eV (yellow PL peak) only the InGaN and the GaN band gap are observed. The characteristics of this spectrum also indicate the emission at 2.2 eV is essentially populated from the GaN band edge, and corresponds to the “yellow band”. Moreover the absence of the absorption band, below the InGaN band gap, indicates that it is associated to the blue PL peak and occurs within a spectral width lower than 560 meV. In order to explore the nature of the blue emission additional SE-PL measurements were carried out. Fig. 5 shows the PL spectra for the blue luminescence taken by varying the energy of excitation from 3.54 to 3.18 eV. The experimental results indicate this PL peak is excited for energies well below the GaN band gap. A well defined broad band centred at 2.74 eV and with FWHM of ∼350 meV was observed exciting at 3.35 eV. This emission is also excited at 3.25 eV, which coincides with the violet PL peak (see Fig. 4). This is the reason why we have observed the InGaN band edge when the PLE spectrum was monitored at the blue PL peak. Nevertheless, this result is not a clear indication that the blue component is originated from the InGaN film, since both GaN and InGaN are transparent at 3.25 eV. At the excitation energy of 3.18 eV the relative intensity of blue PL peak decreases. These results suggest the width of the absorption
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band is compared to the blue PL peak FWHM. If the maximum of absorption is assumed at 3.25 eV, the difference between the absorption and emission energies is close to 500 meV, which is larger than the values expected for the Stokes’ shift for this PL energy in InGaN [2]. This indicates that the blue band cannot be interpreted as reference to an exciton localization on InGaN band tail. Comparing the PLE and SE-PL experimental results we propose the blue PL peak is likely related to defect levels. However further optical studies with depth resolution such as cathodoluminescence or by using a sequential etching process are needed in order to identify if the defect levels are related to the InGaN layer or the GaN buffer. 4. Conclusions Optical properties in a representative pseudomorphic In0.05 Ga0.95 N/GaN film are investigated. The PL peak at 3.25 eV revealed the typical behaviour of a band to band transition in InGaN. However, a systematic PLE and selective excitation analysis has shown that the additional PL peak at 2.74 eV is related to the absorption band observed below the InGaN band gap. This PL peak is characterized by a Stokes’ shift (∼500 meV) much larger than the expected for this energy in the InGaN. The results of this investigation indicate the blue band emission is neither due to the recombination of localized excitons at In-rich domains or as evidence of strain relaxation. We propose that this emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap. This investigation demonstrates the difficulty of unambiguously ascribing the origin of luminescence bands observed in InGaN/GaN heterostructures by common optical characterization. Acknowledgments M.R. Correia would like to thank Professor Teresa Monteiro from the University of Aveiro for useful comments and discussions. The authors acknowledge Rute A. S´a Ferreira from the University of Aveiro for assistance in the optical measurements. The financial support by FCT (POCTI/FIS/57550/2004) is gratefully acknowledged. References [1] S. Chichibu, T. Azuhata, T. Sota, S. Narukava, Appl. Phys. Lett. 70 (1997) 2822. [2] K.P. O’Donnell, R.W. Martin, P.G. Middleton, Phys. Rev. Lett. 82 (1999) 237. [3] S. Pereira, M.R. Correia, E. Pereira, C. Trager-Cowan, K.P. O’Donnell, E. Alves, N. Franco, A.D. Sequeira, Appl. Phys. Lett. 81 (2002) 1207. [4] Ch. Manz, M. Kunzer, H. Obloh, A. Ramakrishnan, U. Kaufmann, Appl. Phys. Lett. 74 (1999) 3993. [5] R. Seitz, C. Gaspar, M. Correia, T. Monteiro, E. Pereira, M. Heuken, O. Schoen, J. Lumin. 87 (2000) 1202. [6] M.R. Correia, S. Pereira, E. Pereira, R.A. S´a Ferreira, J. Frandon, E. Alves, C. Liu, A. Morel, B. Gil, Superlattice. Microstruct. 36 (2004) 625. [7] B. Han, M.P. Ulmer, B.W. Wessels, Physica B 340 (2006) 474. [8] M.A. Reshchikov, F. Shahedipour, R.Y. Korotkov, B.W. Wessels, M.P. Ulmer, Appl. Phys. Lett. 87 (2000) 3551. [9] H.C. Yang, T.Y. Lin, Y.F. Chen, Phys. Rev. B 62 (2000) 12593. [10] M.A. Reshchikov, P. Visconti, H. Morkoc¸, Appl. Phys. Lett. 78 (2001) 177. [11] S.O. Kucheyev, M. Toth, M.R. Phillips, J.S. Williams, C. Jagadish, G. Li, J. Appl. Phys. 91 (2002) 5867. [12] C. D´ıaz-Guerra, J. Piqueras, A. Cavallini, Appl. Phys. Lett. 82 (2003) 2050. [13] R. Armitage, W. Hong, Q. Yang, H. Feick, J. Gebauer, E.R. Weber, S. Hautakangas, K. Saarinen, Appl. Phys. Lett. 82 (2003) 3457. [14] L.R. Doolitle, Nucl. Instrum. Methods Phys. Res. B9 (1985) 344. [15] W. Shan, R.J. Hauenstein, A.J. Fisher, J.J. Song, W.G. Perry, M.D. Bremser, R.F. Davis, B. Goldenberg, Phys. Rev. B 54 (1996) 13460.