Excitation Path of Rare Earth Ions in Nitride
Transcription
Excitation Path of Rare Earth Ions in Nitride
International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai Excitation Path of Rare Earth Ions in Nitride Semiconductors A. Melouah, and M. Diaf Questions still remain about the fundamental understanding of the mechanisms underlying the excitation of rare-earth ions in this host. Abstract—In the last decade, great interest in rare-earth (RE) doped wide band gap semiconductors, which combine the electronic properties of semiconductors with the unique luminescence features of RE ions[1]. Rare earth doped nitrides show light emission when excited by photons with energy higher than their band gap (UV) or lower than it (visible). Narrow emission lines associated to the weak dependence of intensity on temperature put into evidence the possibility of realizing full colour display devices based on the nitrides using the intra-4f n transitions from the RE ions, Rare earth doped GaN is an interesting system in which an energy transfer can occur between the semiconductor and the internal 4f states of the rare earth ions where Eu3+ stands for the red emission, Er3+ for the green and Tm3+ for the blue one. GaN epilayers grown by MOCVD are implanted with Tm, Er and Eu. Optical properties of RE-doped GaN films are studied by using Er, Eu, and Tm-doped GaN in order to maximize ELD brightness and efficiency as well as to apply the results to real devices. Red emission at 621 nm from the 5D0→7F2 transition of Eu3+ has been obtained from GaN:Eu. Spectral photoluminescence (PL) studies are performed on Eu-doped GaN thin films. II. SAMPLES AND EXPERIMENTS Europium ions were implanted into nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) on c-plane sapphire. The implantation energy was 300 keV with the ion beam perpendicular to the surface channelled implantation. The implantation was performed at room temperature for all samples. Four different Eu implanted samples were investigated. The fluence was kept fixed at 1·1015 Eu/cm2 for samples 23–25 and was slightly lower at 7 · 1014 Eu/cm2 for sample 48. The main difference between the samples comes from the annealing treatment. Sample 48 was annealed for 120 s at 1000 °C in a rapid thermal annealing apparatus between graphite strips under flowing N2 gas using a piece of unimplanted GaN as a proximity cap to inhibit the out-diffusion of nitrogen from the surface. Samples 23–25 were prepared with a 10 nm thick AlN capping layer prior to implantation allowing the use of a high temperature annealing treatment without noticeable deterioration of the GaN crystal quality. The annealing temperatures were 1100 °C (sample 23), 1200 °C (sample 24) and 1300 °C (sample 25). The Eu implanted samples were mounted in an APD closed-cycle helium cryostat and cooled down to 12 K. PL studies were performed by exciting the Eu3+:GaN samples with a CW HeCd laser (λexc = 325 nm) and a CW Ar+ ion laser (λexc = 514 nm). Visible luminescence was recorded using a monochromator equipped with a PMT. The monochromator resolution was kept below 0.1 nm for all spectra. We first investigated the photoluminescence (PL) spectra of (Eu) transitions in Eu-implanted GaN samples at 12K to the room temperature. Under below-band gap excitation, two main Eu centers were identified. Some Eu centers are excited via local defects or impurities forming therefore various Eudefect complexes while other Eu centers are isolated without any defect in their vicinity. In case of above band gap excitation the exact nature of the excitation path leading to the excitation of rare-earth ions in GaN leads to think that Eu takes the substitutional site of the Gallium. The most commonly assumed mechanism involves Eu-defect complexes where the defect captures an electron-hole pair before Eu excitation. The exact nature of the defects mediating this Eu Keywords— Rare earth, energy transfer, photoluminescence, Gallium Nitride, Eu3+ ions. R I. INTRODUCTION ARE earth doped III-Nitrides semiconductors have been studied for the few last years because of the possibility to develop compact and efficient electroluminescence devices [1, 2,3,4] . Trivalent Europium ions are of special interest because they exhibit an atomic-like transition at especially 621nm which corresponds to the red luminescence of this rare earth. Electroluminescence devices based on Eu doped were reported however their efficiency was too low for practical applications. It was observed that the room temperature Eu3+ photoluminescence (PL) intensity strongly depends on the band gap of the host materials [5] . It was found that for larger band gap there is less detrimental temperature quenching of Eu 3+ PL occurring. Therefore, doping Eu3+ ions into wide gap semiconductors is a promising approach to overcome the thermal quenching of Eu PL. Europium doped GaN is being widely studied from cryogenic to elevated temperatures. A. Melouah is in Laboratoire de physique des lasers, de spectroscopie optique etd’optoélectronique (LAPLASO) Badji Mokhtar-Annaba University, PoBox12, 23000 Annaba Algeria (E-mail : [email protected]) M. Diaf is in Laboratoire de physique des lasers, de spectroscopie optique etd’optoélectronique (LAPLASO) Badji Mokhtar-Annaba University, PoBox12, 23000 Annaba Algeria (E-mail : [email protected]). 271 International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai excitation needed to be clearly identified. 5 D2 5 D1 5 D0 621nm 7 F2 Fig3 :The experimental Setup Fig 1 a) :Eu Spin Orbit Splitting III. RESULTS AND DISCUSSION: e Europium ions excited resonantly within the 4f configuration exhibit a specific emission spectrum composed of sharp lines which are characteristic of Stark to Stark sublevel transitions as already described in the literature [6]. Figure 4 shows that thus, the excitation path for above band gap radiation appears to be involving defects with characteristics similar to the Eu-defect complexes excited below band gap. This also suggests that Eu centers excited by above band gap radiation or directly within defect-complexes are affected the same way by luminescence quenching processes. The evolution of Eu luminescence as a function of the photon flow for the He-Cd excitation wavelength (λ=325nm) is displayed in Figure 4. The figure 5 exhibits a saturation behavior, which is very pronounced for this above band gap excitation. From this saturation curve it is possible to derive an absorption cross-section for the excitation within Eu-defect complexes or above band gap. It is to be noted that this excitation cross-section is not a true cross-section which by definition refers to an instantaneous process. Indeed, in case of indirect excitation the Eu excitation path may be very complicated and require several steps involving different species. CB HeCd Laser Trap level Excitation Of Europium Eu3 + 5 D0 621nm 7 h F2 VB Fig1b): Illustration of the proposed sequential mechanism for Eu excitation see text for the full description. A. Experimental procedure: The films Grown by atmospheric pressure metalorganic chemical deposition (MOCVD)were mounted on a two axis goniometer. Eu ions were implanted with an energy of 300 Kev into as-grown films at room temperature. The implantation dosage was either. 1015 or 7.5.1014 ions/cm2. During this operation, the GaN <0001> axis was either aligned with the ion beam (‘channeled’ implantation) or tilted by 10° (‘random’ implantation). Subsequently, the samples were annealed in a tube furnace at 1000 to 1300° C for 20mn under flowing N2. The Eu-implanted wafers were mounted in APD Liquid cryostat and cooled down to12K. Photoluminescence spectroscopy was performed at this and room temperatures by exciting GaN samples with the 325 nm line from a HeCd laser and an Argon laser. Infrared luminescence was recorded using a monochromator equipped with a thermo-electric cooled InGaAs photodiode. The monochromator resolution was kept below 0,6nm for all spectra. High photon flow Medium photon flow low photon flow 2,0 1,8 Intensity (a.u.) 1,6 TAnneal.=1300°C 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 6200 6205 6210 6215 6220 6225 6230 6235 Wavelength (A) Fig4:Evolution of Eu luminescence as a function of the photon flow 272 International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2012) Jan. 7-8, 2012 Dubai REFERENCES Normalized PL intensity 1,0 Tanneal.=1200°C exc=325nm 12K 0,8 [1] [2] G A.J Steckel and R. Birkham Appl. Phys. Lett. 73, 1700 (1998) H. Ennen, U. Kaufmann, G.S Pomrenke, J. Schneider, J. Windschif and A. Axmannn, J. Crystal Growth 64, 165 (1983) . [3] G. Pomrenke, P.B. Klein, and D.W. Langer, Rare earth Doped Semiconductors, Material Research Society Symposium Proceedings, Vol.301, Pittsburg, 1993. [4] S. Coffa, A. Polman, and R.N. Schwartz, , Rare earth Doped Semiconductors II, Material Research Society Symposium Proceedings, Vol.422, Pittsburg, PA, 1996. [5] P. N. Favennec, H.L. Haridon, D. Moutonnet, and Y. Le Guillou, Electr. Lett. 25, 718 (1989). [6] M.Pan, A.J Steckl Appl. Phys.Lett (83) N1 (2003) 9. [7] A.Oussif, M. Diaf, ABraud, J.L.Doualand, R. Moncorgé, CISGM3_3rd ICMSE ; Jijel 25-27 Mai 2004. [8] A.Braud, J.L. Doualan, R. Moncorge, B. Pipeleers, A.Vantomme, Mat. Sc. Eng. B 105 (2003). [9] S.Kim, S.J.Rhee, D.A.Turnbull, E.E. Reuter, X.Li, J.J.Coleman and S.G.Bishop, Appl. Phys. Lett., Vol.71 (2) (1997) 231. [10] S.Kim, S.J.Rhee, X.Li, J.J.Coleman and S.G.Bishop, , Appl. Phys. Lett., Vol.76 (17) (2000) 2403 0,6 0,4 det.=620,8nm det.=622,6nm 0,2 0,0 0 1x10 20 2x10 20 -1 3x10 20 4x10 -2 20 Photon flow (s cm ) Fig5: Saturation of theEu2 faster than Eu1. This question is addressed by means of a new setup combining two lasers. Free electrons are first created by a HeCd laser at 325nm while the second laser corresponds to a below band gap excitation(fig6) . The second laser has a dramatic influence on Eu PL intensity related to the above band gap excitation. The effect of this second laser on the excitation path is investigated. For the first time to our knowledge, we have clear evidence that the above band gap excitation of Eu ions involves carrier traps which act as mediators for the excitation towards the rare-earth ions. The relevant trap is also clearly identified. intensité PL(UA) 0.0008 E E E 0.0007 0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 6180 6190 6200 6210 6220 6230 6240 6250 6260 Longueur d'onde (A) Fig 6: Quenching PL with a dual excitation IV. CONCLUSION Infrared PL spectra recorded in Eu-implanted GaN at 12K to Room temperature enable us to identify two different Eucenters on one hand Eu centers excited by above band gap radiation and on the other hand Eu ions excited via local defects are undoubtedly very different. It shows that the excitation path from above band gap to Eu ions involves nearby defects which trap electron-hole pairs before Eu excitation. The exact nature of the defects mediating this Eu excitation still remains to be clearly identified. This issue represents the key question of our future work. 273 6270