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Mutual quenching of Er3+ photoluminescence under two laser excitation in GaN:Er
Superlattices and Microstructures 36 (2004) 755–761 www.elsevier.com/locate/superlattices
Mutual quenching of Er3+ photoluminescence under two laser excitation in GaN:Er M. Wojdaka, A. Brauda,∗, J.L. Doualana, R. Moncorgéa, B. Pipeleersb, A. Vantommeb, O. Briotc a CIRIL-ENSICAEN, 6 bd du Maréchal Juin, 14050 Caen Cedex 04, France b Instituut voor Kern-en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D,
B-3001 Leuven, Belgium c UMR 5650 CNRS-Université Montpellier II, Bt 21, CC074, Place Eugne Bataillon,
34095 Montpellier Cedex 05, France Available online 27 October 2004
Abstract Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He–Cd (λ = 325 nm) laser and below-band-gap excitation by a tunable Ti–Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er3+ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lockin amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er3+ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er3+ emission. © 2004 Elsevier Ltd. All rights reserved. PACS: 78.55.Cr; 76.30.Kg; 95.85.J
∗ Corresponding address: CIRILL-ISMRA, 6 Boulevard Maréchal Juin, 14050 Caen Cedex, France. Tel.: +33 231 45 25 60; fax: +33 231 45 25 57. E-mail address: [email protected] (A. Braud).
0749-6036/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2004.09.032
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1. Introduction The research on semiconductors doped with rare earth (RE) ions is driven by the need for optimization and development of optoelectronic devices [1]. A particular interest in erbium is raised by the fact that its characteristic emission meets technologically important spectral ranges. The choice of the host material is also crucial, and since it was found that wide band gap materials exhibit a low thermal quenching of Er3+ emission [2,3], GaN:Er is intensively investigated. RE-related photoluminescence (PL) in semiconductors can be observed with aboveband-gap excitation. This occurs by the generation of free carriers, which are subsequently trapped on defects coupled to RE ions, finally recombine and transfer their energy to the RE ions [4]. When the incident photon energy is too low to either induce a band-to-band transition or create free excitons, the excitation of the RE can nevertheless occur. This below-band-gap excitation can be explained by the release of free carriers from acceptor levels or by transitions from the valence band to RE-coupled defects [4]. When above- or below-band-gap excitation is used in GaN:Er, the Er3+ emission exhibits a different spectral shape. Moreover, when the below-band-gap excitation wavelength is varied, the PL spectrum changes, indicating that Er3+ ions in different local lattice sites are excited. Kim et al. have identified primarily four [5], and later nine different Er3+ centers [6,7], which reveal different photoluminescence spectra. It was found that some of these centers participate in above- and below-band-gap excitation, while some are involved in only one of the excitation paths. 2. Experimental Undoped GaN layers were grown by MOCVD on a sapphire substrate, and implanted with 80 keV erbium ions at room temperature with a dose of 2.5 × 1014 at./cm2 . During implantation, the GaN0001 axis was aligned with the ion beam, providing channeled implantation. Afterwards, it was annealed at 950 ◦ C for 30 min in N2 atmosphere. More details about the samples’ preparation and their properties can be found in Refs. [8,9]. The PL measurements were performed with the set-up depicted in Fig. 1. The sample was placed in a closed cycle APD cryostat and cooled down to T = 7 K. Above-band-gap excitation was provided by a He–Cd laser (λ = 325 nm) and below-band-gap illumination by a tunable Ti–Sa laser set to λ = 764 nm. To assure a good overlap of both laser beams a small metallic diaphragm with a 2 mm diameter hole was fixed on the sample. The PL signal was dispersed by a single grating 0.6 m spectrometer and detected by a Hammamatsu InGaAs photodiode. A lock-in amplifier was used to filter the PL signal related to the modulated laser. 3. Results Fig. 2 presents the PL spectra corresponding to the Er3+ 4 I 13/2 → 4 I 15/2 transition obtained with above-band-gap (λ = 325 nm) (a), and below-band-gap (λ = 764 nm) (b) excitations. They exhibit a very different spectral shape. It should be noted that both
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Fig. 1. Set-up for two-laser experiment.
Fig. 2. Low temperature photoluminescence spectra recorded with different excitation wavelengths.
excitation wavelengths are not resonant with any Er3+ transition. Therefore, the excitation can only occur via the semiconductor host, either by a band to band transition, or due to an absorption by deep defects inside the band gap (λ = 764 nm). When the He–Cd and Ti–Sa lasers are used simultaneously and only the He–Cd laser is modulated (as shown in Fig. 1), we observe that the PL signal related to the He–Cd excitation is increasingly quenched, as the Ti–Sa photon flux is increased. This is presented in Fig. 3. Moreover, as can be seen, the shape of the recorded spectra drastically changes. These spectra are found to be a linear combination of the spectra shown in Fig. 2, which were recorded with each of the lasers separately. It is surprising that the PL spectrum related to the Ti–Sa laser plays a role in the recorded spectra, because this laser is not modulated and the related PL signal should be filtered out by the lock-in amplifier. The explanation for this is that the photoluminescence related to the Ti–Sa laser is also
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Fig. 3. Photoluminescence spectra recorded with He–Cd excitation and additional illumination of Ti–Sa laser (λ = 764 nm).
quenched by the illumination of the He–Cd laser. Therefore, there occurs a decrease of this PL signal which is coincident with the He–Cd laser modulation frequency (as depicted in the inset in Fig. 3). If the PL intensity of spectra (a) and (b) from Fig. 2 is referred to as IHe–Cd and ITi–Sa , respectively, then the PL signal Itwo-lasers detected by the lock-in amplifier under simultaneous pumping by both lasers and shown in Fig. 3 can be accurately reconstructed as Itwo-lasers = P1 × IHe–Cd + (1 − P2 ) × ITi–Sa .
(1)
By a multiple regression procedure we can resolve the spectra obtained under the twolaser excitation into their spectral components and determine the values of the coefficients P1 and P2 introduced in the above equation. It should be noted that in Eq. (1) the term (1 − P2 ) represents the quenched part of the PL signal related to the Ti–Sa excitation. However, for further analysis it is more relevant to consider the remaining part of the PL signal which is represented by the coefficient P2 . When the photon flux of the Ti–Sa laser is increased, the composition of the recorded spectra changes and the corresponding values of the coefficients P1 and P2 can be obtained. This dependence is presented in Fig. 4. The coefficient P1 decreases, indicating that the quenching of the He–Cd related PL signal is stronger when a higher Ti–Sa photon flux is used. It should be noted that P1 should be equal to 1 when the Ti–Sa photon flux is zero (as indicated by the fitted line). On the other hand, the dependence of the coefficient P2 shows that the Ti–Sa related PL is quenched by the He–Cd illumination to about 60% of its original value.
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759
Fig. 4. Dependence of coefficients P1 and P2 on the photon flux of the Ti–Sa laser.
4. Discussion These results show that we observe a clear mutual quenching of the PL signal related to one laser by the action of the other laser. As mentioned earlier, the excitation path associated with each laser excitation involves defects which act as mediators for the RE excitation. The observed mutual quenching could be explained by the saturation of one type of defects, which would be common to the two excitation paths. With this assumption the excited Er3+ ions should be the same ions excited by both lasers. But this explanation is not consistent with the difference in PL spectral shapes we observe when each laser is used separately. Indeed, the fact that we observe different spectra indicates that the Er3+ local site symmetry is different and therefore that the excited Er3+ centers are different in both cases. Therefore, this mutual quenching can only be explained by assuming that each type of excitation involves a specific type of Er3+ center coupled to a specific type of defect. In this simple model, we assume that there are specific defects, which are populated by one laser, and photo-ionized by the other laser. This leads to a quenching of the corresponding PL intensity. To describe this process we will use the following rate equation: N∗ dN ∗ pi ex ∗ = σ325 − σ764 nm Φ2 N ∗ . Φ (N − N ) − (2) 1 nm dt τ In this equation, N corresponds to the total concentration of traps capable of transferring excitation to Er3+ ions and N ∗ to the concentration of traps that actually captured electrons and are ready to transfer their excitation to nearby erbium ions. Eq. (2) describes the experiment where the He–Cd (λ = 325 nm) laser is the excitation source and the number ex N ∗ corresponds to traps populated by this laser. The parameter σ325 nm is the effective
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excitation cross-section associated with the He–Cd laser, and Φ1 is its photon flux. The last term corresponds to the photo-ionization of the traps by the Ti–Sa laser. The parameter pi σ764 nm is the photo-ionization cross-section by the Ti–Sa laser of the electron traps populated by the He–Cd laser and Φ2 is the photon flux of the Ti–Sa laser. To simplify the approach to this problem we assume that each populated defect will lead to the excitation of one Er3+ ion. Therefore, this model can be used to describe the quenching of the PL signal related to one laser by the other laser source and more specifically the evolution of the P1 and P2 coefficients representing the part of the PL signal, which is not quenched. For this purpose, solving Eq. (2) gives the dependence of N ∗ on both photon fluxes. It is to be noted that the P1 and P2 coefficients give the ratio of the quenched PL intensity (Φ2 = 0) to the non-quenched PL intensity (Φ2 = 0). Therefore the P1 coefficient is then given by −1 pi σ764 nm τ N∗ P1 = ∗ = 1+ . (3) Φ2 ex NΦ2 =0 1 + σ325 nm Φ1 τ This formula can be used to model the dependence of the coefficient P1 on the Ti–Sa photon flux Φ2 , represented by the square points in Fig. 4. The He–Cd photon flux is fixed at Φ1 = 5.2 × 1016 s−1 cm−2 . This simple model is in fact in good agreement with the observed data, as shown in Fig. 4. From this fitting procedure it is possible to find a relationship between the effective excitation cross-section and the photo-ionization crosssection as given by pi
σ764 nm τ = 1.08 × 10−20 s cm−2 . ex 1 + σ325 Φ τ nm 1
(4)
ex 4 Besides, separate experiments gave values for τ and σ325 nm . The erbium I13/2 emitting level exhibits a lifetime τ of about 1.2 ms under above-band-gap excitation and the effective excitation cross-section associated with the He–Cd laser was estimated to be ex −15 cm2 by measuring the dependence of the PL intensity on the He–Cd σ325 nm ≈ 1.2×10 ex photon flux without the Ti–Sa laser (Φ2 = 0). When we substitute these parameters σ325 nm pi and τ by their values in Eq. (4), we get σ764 nm ≈ 9.7 × 10−18 cm2 , which corresponds to the photo-ionization cross-section of the He–Cd related defects by the Ti–Sa light. The other set of points (triangles in Fig. 4), representing the dependence of the P2 coefficient on Φ2 , can also be modeled by Eq. (3). The only difference is that the role of the fluxes is exchanged. The Ti–Sa laser now acts as an excitation source, and the He–Cd laser is used to photo-ionize the traps populated by the Ti–Sa laser. Nevertheless, the photon flux notations are kept the same, i.e. Φ1 corresponds to the He–Cd flux and Φ2 to the Ti–Sa flux. The P2 coefficient is then given by −1 pi σ325 nm τ N∗ Φ1 P2 = ∗ = 1+ . (5) ex NΦ1 =0 1 + σ764 nm Φ2 τ
Fig. 4 shows that the modeling of the P2 coefficient evolution is also in good agreement with the experimental results. During this specific fitting procedure we pi ex used two fitting parameters, σ325 nm and σ764 nm . We obtained the following values:
M. Wojdak et al. / Superlattices and Microstructures 36 (2004) 755–761
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σ325 nm ≈ 1.2 × 10−14 cm2 for the photo-ionization cross-section of the traps populated by ex −18 cm2 the Ti–Sa excitation, and ionized by the He–Cd illumination, and σ764 nm ≈ 5 ×10 for the effective excitation cross-section by the Ti–Sa laser of Er3+ related traps. This two-laser experiment enables us to determine various excitation and photoionization cross-sections and by changing the excitation wavelengths should give us information about the defects which mediate the excitation from the conduction band to the rare-earth ions. pi
5. Summary We report here the observation of a large mutual quenching of the Er3+ photoluminescence by combining two laser excitations in GaN:Er. The He–Cd related Er3+ PL emission is quenched by a Ti–Sa laser and vice versa. The dependence of this mutual quenching on the Ti–Sa photon flux is presented. The experimental data are well explained by a simple model assuming the existence of two types of defects specific to each laser excitation. One laser quenches the PL intensity induced by the other laser by photo-ionizing the traps mediating the excitation towards the Er3+ ions. Using this model, pi we have determined the photo-ionization cross-section σ325 nm ≈ 1.2 × 10−14 cm2 of the deep defect populated by the Ti–Sa laser (λ = 764 nm) and photo-ionized by the He–Cd laser (λ = 325 nm). On the other hand, the photo-ionization cross-section by the Ti–Sa laser (λ = 764 nm) of traps populated by the He–Cd laser (λ = 325 nm) is pi determined: σ764 nm ≈ 9.7 × 10−18 cm2 . The effective excitation cross-sections associated ex −18 cm2 for the Ti–Sa laser and with each laser are also determined: σ764 nm ≈ 5 × 10 ex −15 cm2 for the He–Cd laser. σ325 nm ≈ 1.2 × 10 Acknowledgements This work was supported by the EU contract no. HPRN-CT-2001-00297 (RENIBEL) and by the Fund for Scientific Research, Flanders (FWO). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
A. Steckl, R. Birkhahn, Appl. Phys. Lett. 73 (1998) 1700. P.N. Favenec, H. L’Haridon, M. Salvi, D. Moutonnet, Y. Le Guillou, Electron. Lett. 25 (1989) 718–719. A.J. Neuhalfen, B.W. Wessels, Appl. Phys. Lett. 60 (1992) 2657. K. Takahei, A. Taguchi, H. Nakagome, K. Uwai, P.S. Whitney, J. Appl. Phys. 66 (1989) 4941. S. Kim, S.J. Rhee, D.A. Turnbull, X. Li, J.J. Coleman, S.G. Bishop, P.B. Klein, Appl. Phys. Lett. 71 (1997) 2662. S. Kim, S.J. Rhee, X. Li, J.J. Coleman, S.G. Bishop, Appl. Phys. Lett. 76 (2000) 2403. S. Kim, S.J. Rhee, J.O. White, A.M. Mitofsky, X. Li, G.C. Papen, J.J. Coleman, S.G. Bishop, Mater. Sci. Eng. B 81 (2001) 136. B. Pipeleers, S.M. Hogg, A. Vantomme, NIM B 206 (2003) 95–98. A. Braud, J.L. Doualan, R. Moncorge, B. Pipeleers, A. Vantomme, Mater. Sci. Eng. B 105 (2003) 101–105.
Mutual quenching of Er3+ photoluminescence under two laser excitation in GaN:Er M. Wojdaka, A. Brauda,∗, J.L. Doualana, R. Moncorgéa, B. Pipeleersb, A. Vantommeb, O. Briotc a CIRIL-ENSICAEN, 6 bd du Maréchal Juin, 14050 Caen Cedex 04, France b Instituut voor Kern-en Stralingsfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D,
B-3001 Leuven, Belgium c UMR 5650 CNRS-Université Montpellier II, Bt 21, CC074, Place Eugne Bataillon,
34095 Montpellier Cedex 05, France Available online 27 October 2004
Abstract Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He–Cd (λ = 325 nm) laser and below-band-gap excitation by a tunable Ti–Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er3+ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lockin amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er3+ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er3+ emission. © 2004 Elsevier Ltd. All rights reserved. PACS: 78.55.Cr; 76.30.Kg; 95.85.J
∗ Corresponding address: CIRILL-ISMRA, 6 Boulevard Maréchal Juin, 14050 Caen Cedex, France. Tel.: +33 231 45 25 60; fax: +33 231 45 25 57. E-mail address: [email protected] (A. Braud).
0749-6036/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2004.09.032
756
M. Wojdak et al. / Superlattices and Microstructures 36 (2004) 755–761
1. Introduction The research on semiconductors doped with rare earth (RE) ions is driven by the need for optimization and development of optoelectronic devices [1]. A particular interest in erbium is raised by the fact that its characteristic emission meets technologically important spectral ranges. The choice of the host material is also crucial, and since it was found that wide band gap materials exhibit a low thermal quenching of Er3+ emission [2,3], GaN:Er is intensively investigated. RE-related photoluminescence (PL) in semiconductors can be observed with aboveband-gap excitation. This occurs by the generation of free carriers, which are subsequently trapped on defects coupled to RE ions, finally recombine and transfer their energy to the RE ions [4]. When the incident photon energy is too low to either induce a band-to-band transition or create free excitons, the excitation of the RE can nevertheless occur. This below-band-gap excitation can be explained by the release of free carriers from acceptor levels or by transitions from the valence band to RE-coupled defects [4]. When above- or below-band-gap excitation is used in GaN:Er, the Er3+ emission exhibits a different spectral shape. Moreover, when the below-band-gap excitation wavelength is varied, the PL spectrum changes, indicating that Er3+ ions in different local lattice sites are excited. Kim et al. have identified primarily four [5], and later nine different Er3+ centers [6,7], which reveal different photoluminescence spectra. It was found that some of these centers participate in above- and below-band-gap excitation, while some are involved in only one of the excitation paths. 2. Experimental Undoped GaN layers were grown by MOCVD on a sapphire substrate, and implanted with 80 keV erbium ions at room temperature with a dose of 2.5 × 1014 at./cm2 . During implantation, the GaN0001 axis was aligned with the ion beam, providing channeled implantation. Afterwards, it was annealed at 950 ◦ C for 30 min in N2 atmosphere. More details about the samples’ preparation and their properties can be found in Refs. [8,9]. The PL measurements were performed with the set-up depicted in Fig. 1. The sample was placed in a closed cycle APD cryostat and cooled down to T = 7 K. Above-band-gap excitation was provided by a He–Cd laser (λ = 325 nm) and below-band-gap illumination by a tunable Ti–Sa laser set to λ = 764 nm. To assure a good overlap of both laser beams a small metallic diaphragm with a 2 mm diameter hole was fixed on the sample. The PL signal was dispersed by a single grating 0.6 m spectrometer and detected by a Hammamatsu InGaAs photodiode. A lock-in amplifier was used to filter the PL signal related to the modulated laser. 3. Results Fig. 2 presents the PL spectra corresponding to the Er3+ 4 I 13/2 → 4 I 15/2 transition obtained with above-band-gap (λ = 325 nm) (a), and below-band-gap (λ = 764 nm) (b) excitations. They exhibit a very different spectral shape. It should be noted that both
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Fig. 1. Set-up for two-laser experiment.
Fig. 2. Low temperature photoluminescence spectra recorded with different excitation wavelengths.
excitation wavelengths are not resonant with any Er3+ transition. Therefore, the excitation can only occur via the semiconductor host, either by a band to band transition, or due to an absorption by deep defects inside the band gap (λ = 764 nm). When the He–Cd and Ti–Sa lasers are used simultaneously and only the He–Cd laser is modulated (as shown in Fig. 1), we observe that the PL signal related to the He–Cd excitation is increasingly quenched, as the Ti–Sa photon flux is increased. This is presented in Fig. 3. Moreover, as can be seen, the shape of the recorded spectra drastically changes. These spectra are found to be a linear combination of the spectra shown in Fig. 2, which were recorded with each of the lasers separately. It is surprising that the PL spectrum related to the Ti–Sa laser plays a role in the recorded spectra, because this laser is not modulated and the related PL signal should be filtered out by the lock-in amplifier. The explanation for this is that the photoluminescence related to the Ti–Sa laser is also
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Fig. 3. Photoluminescence spectra recorded with He–Cd excitation and additional illumination of Ti–Sa laser (λ = 764 nm).
quenched by the illumination of the He–Cd laser. Therefore, there occurs a decrease of this PL signal which is coincident with the He–Cd laser modulation frequency (as depicted in the inset in Fig. 3). If the PL intensity of spectra (a) and (b) from Fig. 2 is referred to as IHe–Cd and ITi–Sa , respectively, then the PL signal Itwo-lasers detected by the lock-in amplifier under simultaneous pumping by both lasers and shown in Fig. 3 can be accurately reconstructed as Itwo-lasers = P1 × IHe–Cd + (1 − P2 ) × ITi–Sa .
(1)
By a multiple regression procedure we can resolve the spectra obtained under the twolaser excitation into their spectral components and determine the values of the coefficients P1 and P2 introduced in the above equation. It should be noted that in Eq. (1) the term (1 − P2 ) represents the quenched part of the PL signal related to the Ti–Sa excitation. However, for further analysis it is more relevant to consider the remaining part of the PL signal which is represented by the coefficient P2 . When the photon flux of the Ti–Sa laser is increased, the composition of the recorded spectra changes and the corresponding values of the coefficients P1 and P2 can be obtained. This dependence is presented in Fig. 4. The coefficient P1 decreases, indicating that the quenching of the He–Cd related PL signal is stronger when a higher Ti–Sa photon flux is used. It should be noted that P1 should be equal to 1 when the Ti–Sa photon flux is zero (as indicated by the fitted line). On the other hand, the dependence of the coefficient P2 shows that the Ti–Sa related PL is quenched by the He–Cd illumination to about 60% of its original value.
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Fig. 4. Dependence of coefficients P1 and P2 on the photon flux of the Ti–Sa laser.
4. Discussion These results show that we observe a clear mutual quenching of the PL signal related to one laser by the action of the other laser. As mentioned earlier, the excitation path associated with each laser excitation involves defects which act as mediators for the RE excitation. The observed mutual quenching could be explained by the saturation of one type of defects, which would be common to the two excitation paths. With this assumption the excited Er3+ ions should be the same ions excited by both lasers. But this explanation is not consistent with the difference in PL spectral shapes we observe when each laser is used separately. Indeed, the fact that we observe different spectra indicates that the Er3+ local site symmetry is different and therefore that the excited Er3+ centers are different in both cases. Therefore, this mutual quenching can only be explained by assuming that each type of excitation involves a specific type of Er3+ center coupled to a specific type of defect. In this simple model, we assume that there are specific defects, which are populated by one laser, and photo-ionized by the other laser. This leads to a quenching of the corresponding PL intensity. To describe this process we will use the following rate equation: N∗ dN ∗ pi ex ∗ = σ325 − σ764 nm Φ2 N ∗ . Φ (N − N ) − (2) 1 nm dt τ In this equation, N corresponds to the total concentration of traps capable of transferring excitation to Er3+ ions and N ∗ to the concentration of traps that actually captured electrons and are ready to transfer their excitation to nearby erbium ions. Eq. (2) describes the experiment where the He–Cd (λ = 325 nm) laser is the excitation source and the number ex N ∗ corresponds to traps populated by this laser. The parameter σ325 nm is the effective
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excitation cross-section associated with the He–Cd laser, and Φ1 is its photon flux. The last term corresponds to the photo-ionization of the traps by the Ti–Sa laser. The parameter pi σ764 nm is the photo-ionization cross-section by the Ti–Sa laser of the electron traps populated by the He–Cd laser and Φ2 is the photon flux of the Ti–Sa laser. To simplify the approach to this problem we assume that each populated defect will lead to the excitation of one Er3+ ion. Therefore, this model can be used to describe the quenching of the PL signal related to one laser by the other laser source and more specifically the evolution of the P1 and P2 coefficients representing the part of the PL signal, which is not quenched. For this purpose, solving Eq. (2) gives the dependence of N ∗ on both photon fluxes. It is to be noted that the P1 and P2 coefficients give the ratio of the quenched PL intensity (Φ2 = 0) to the non-quenched PL intensity (Φ2 = 0). Therefore the P1 coefficient is then given by −1 pi σ764 nm τ N∗ P1 = ∗ = 1+ . (3) Φ2 ex NΦ2 =0 1 + σ325 nm Φ1 τ This formula can be used to model the dependence of the coefficient P1 on the Ti–Sa photon flux Φ2 , represented by the square points in Fig. 4. The He–Cd photon flux is fixed at Φ1 = 5.2 × 1016 s−1 cm−2 . This simple model is in fact in good agreement with the observed data, as shown in Fig. 4. From this fitting procedure it is possible to find a relationship between the effective excitation cross-section and the photo-ionization crosssection as given by pi
σ764 nm τ = 1.08 × 10−20 s cm−2 . ex 1 + σ325 Φ τ nm 1
(4)
ex 4 Besides, separate experiments gave values for τ and σ325 nm . The erbium I13/2 emitting level exhibits a lifetime τ of about 1.2 ms under above-band-gap excitation and the effective excitation cross-section associated with the He–Cd laser was estimated to be ex −15 cm2 by measuring the dependence of the PL intensity on the He–Cd σ325 nm ≈ 1.2×10 ex photon flux without the Ti–Sa laser (Φ2 = 0). When we substitute these parameters σ325 nm pi and τ by their values in Eq. (4), we get σ764 nm ≈ 9.7 × 10−18 cm2 , which corresponds to the photo-ionization cross-section of the He–Cd related defects by the Ti–Sa light. The other set of points (triangles in Fig. 4), representing the dependence of the P2 coefficient on Φ2 , can also be modeled by Eq. (3). The only difference is that the role of the fluxes is exchanged. The Ti–Sa laser now acts as an excitation source, and the He–Cd laser is used to photo-ionize the traps populated by the Ti–Sa laser. Nevertheless, the photon flux notations are kept the same, i.e. Φ1 corresponds to the He–Cd flux and Φ2 to the Ti–Sa flux. The P2 coefficient is then given by −1 pi σ325 nm τ N∗ Φ1 P2 = ∗ = 1+ . (5) ex NΦ1 =0 1 + σ764 nm Φ2 τ
Fig. 4 shows that the modeling of the P2 coefficient evolution is also in good agreement with the experimental results. During this specific fitting procedure we pi ex used two fitting parameters, σ325 nm and σ764 nm . We obtained the following values:
M. Wojdak et al. / Superlattices and Microstructures 36 (2004) 755–761
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σ325 nm ≈ 1.2 × 10−14 cm2 for the photo-ionization cross-section of the traps populated by ex −18 cm2 the Ti–Sa excitation, and ionized by the He–Cd illumination, and σ764 nm ≈ 5 ×10 for the effective excitation cross-section by the Ti–Sa laser of Er3+ related traps. This two-laser experiment enables us to determine various excitation and photoionization cross-sections and by changing the excitation wavelengths should give us information about the defects which mediate the excitation from the conduction band to the rare-earth ions. pi
5. Summary We report here the observation of a large mutual quenching of the Er3+ photoluminescence by combining two laser excitations in GaN:Er. The He–Cd related Er3+ PL emission is quenched by a Ti–Sa laser and vice versa. The dependence of this mutual quenching on the Ti–Sa photon flux is presented. The experimental data are well explained by a simple model assuming the existence of two types of defects specific to each laser excitation. One laser quenches the PL intensity induced by the other laser by photo-ionizing the traps mediating the excitation towards the Er3+ ions. Using this model, pi we have determined the photo-ionization cross-section σ325 nm ≈ 1.2 × 10−14 cm2 of the deep defect populated by the Ti–Sa laser (λ = 764 nm) and photo-ionized by the He–Cd laser (λ = 325 nm). On the other hand, the photo-ionization cross-section by the Ti–Sa laser (λ = 764 nm) of traps populated by the He–Cd laser (λ = 325 nm) is pi determined: σ764 nm ≈ 9.7 × 10−18 cm2 . The effective excitation cross-sections associated ex −18 cm2 for the Ti–Sa laser and with each laser are also determined: σ764 nm ≈ 5 × 10 ex −15 cm2 for the He–Cd laser. σ325 nm ≈ 1.2 × 10 Acknowledgements This work was supported by the EU contract no. HPRN-CT-2001-00297 (RENIBEL) and by the Fund for Scientific Research, Flanders (FWO). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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