This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Nuclear Instruments and Methods in Physics Research B 268 (2010) Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: High temperature annealing of Europium implanted AlN K. Lorenz a,b, *, S. Magalhães a,c, E. Alves a,b, M. Peres c, T. Monteiro c, A.J. Neves c, M. Boćkowski d a UFA, Instituto Tecnológico e Nuclear, Estrada Nacional 10, Sacavém, Portugal b CFN da Universidade de Lisboa, Av. Prof. Gama Pinto 2, Lisboa, Portugal c Departamento de Física and I3N, Universidade de Aveiro, Aveiro, Portugal d Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, Poland article info abstract Article history: Received 23 September 2009 Received in revised form 29 March 2010 Available online 7 May 2010 Keywords: Aluminium Nitride Rare Earth Implantation Rutherford Backscattering Spectrometry Photoluminescence AlN was implanted with 300 kev Eu ions within a wide fluence range from to at/cm 2. The damage build-up was investigated by Rutherford Backscattering/Channelling. Sigmoidal shaped damage build-up curves indicate efficient dynamic annealing. A regime with low damage increase for fluences below at/cm 2 is followed by a strong increase for intermediate fluences. For the highest fluences the damage curve rises slowly until a buried amorphous layer is formed. High temperature annealing was performed in nitrogen atmospheres at low pressure (1300 C, 10 5 Pa) or at ultra-high pressure (1450 C, 10 9 Pa). Implantation damage was found to be extremely stable and annealing only resulted in slight structural recovery. For high fluences out-diffusion of Eu is observed during annealing. Nevertheless, photoluminescence (PL) measurements show intense Eu-related red light emission for all samples with higher PL intensity for the high temperature high pressure annealing. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction 2. Experimental details Rare Earth (RE) doped group-iii nitride semiconductors (GaN, AlN, InN and their alloys) attract much research interest due to their unique optical properties with narrow and temperature stable emissions ranging from the infra-red to the ultraviolet. Prototypes of electroluminescent devices based on RE doped GaN that emit in all primary colours have been reported recently [1]. As host for REs, AlN with its large band gap allows energetically high lying RE levels to be exploited. Furthermore, decreased thermal quenching of the luminescence is expected for wide band gap semiconductors [2]. Emissions in the visible, infra-red and ultra-violet from RE doped AlN were reported for AlN:RE doped in situ during epitaxial growth or RF sputtering as well as by ion implantation [3 9]. The implantation damage build-up in AlN was studied by several groups that reported strong dynamic annealing effects and high amorphisation levels [10 13]. However, implantation damage was found to be very stable and negligible crystal recovery was achieved for rapid thermal annealing at 1000 C [10]. In this work we study the structural and optical properties of Eu implanted AlN films and the impact of high temperature annealing up to 1450 C. * Corresponding author at: UFA, Instituto Tecnológico e Nuclear, Estrada Nacional 10, Sacavém, Portugal. Tel.: ; fax: address: lorenz@itn.pt (K. Lorenz). AlN films grown by halide vapour phase epitaxy (HVPE) 1 on ( ) sapphire substrates were implanted with 300 kev Eu ions to fluences from to at/cm 2 at room temperature (RT). During the implantation the samples surface normal was tilted by 10 away from the ion beam direction in order to minimise channelling effects [13]. Post-implant annealing was performed in a tube furnace in low nitrogen overpressure of 10 5 Pa (hereafter named low pressure (LP) annealing) at 1300 C for 20 min; the samples were protected by an unimplanted piece of AlN placed face to face as a proximity cap to inhibit out-diffusion of nitrogen from the surface. Other samples were annealed for 30 min in a high pressure chamber with 10 9 Pa of nitrogen pressure (in the following named high pressure (HP) annealing) at 1450 and the samples surfaces were covered by AlN powder. Rutherford Backscattering/Channelling (RBS/C) studies were performed with a 1 mm diameter collimated beam of 2 MeV He + ions. The backscattered particles were detected at 140 o and close to 180 o with respect to the incoming beam direction using silicon surface barrier detectors with resolutions of 13 and 16 kev, respectively, located in the standard IBM geometry. PL measurements were carried out at 77 K with a 325 nm cw He Cd laser line and the excitation power density was typically less than 0.6 W cm 2. The luminescence was dispersed by a Spex 1704 monochromator 1 HVPE material was purchased from Technologies and Devices International Inc., Plum Orchard Drive, Silver Spring, MD 20904, USA X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi: /j.nimb

3 2908 K. Lorenz et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) (1 m, 1200 mm 1 ) equipped with a cooled Hamamatsu R928 photomultiplier. 3. Results and discussion Fig. 1 shows the Al-signal of selected RBS/C spectra for samples implanted to different fluences. The increase of implantation damage with fluence is reflected in the continuous increase of the backscattering yield in the h i aligned spectra. The two random spectra shown correspond to implantation fluences of and at/cm 2. Note that for the higher fluence the shape of the Al-barrier in the implanted region (around channel 405) is strongly influenced by the high Eu-concentration (8% in the maximum of the profile) leading to the decrease of Al-counts compared to the random spectrum of the lower fluence sample. For the highest fluence the aligned spectrum reaches the random level in the region close to the end of range of the implanted ions revealing the complete break-down of single crystalline order and suggesting the formation of a buried amorphous layer. The defect profiles (not shown) were extracted using the DICADA code [14] to account for de-channelling of the alpha particles. Fig. 2 presents the relative defect level in the Al-sublattice at the maximum of the defect profile as a function of the implantation fluence. The damage build-up proceeds slowly for fluences below at/cm 2 indicative of strong dynamic annealing due to effective diffusion and/or annihilation of point defects. For intermediate fluences between and at/cm 2 a strong increase of lattice damage is observed. For higher fluences the defect level rises slowly until reaching unity. In earlier studies, different behaviours were reported for this high fluence regime. Kucheyev et al., for 300 kev Au-implantation at 20 and 196 C, describe a gradual increase of the damage level with the fluence until the random level but the slowing down of defect production rate as seen here for fluences above at/cm 2 is not observed [10]. Jiang et al., on the other hand, report a saturation of damage below the random level upon implantation of high fluences of 1 MeV Au at 145 K [12]. The slow but gradual increase of damage build-up in the high fluence regime for Eu implantation may be influenced by chemical effects. Indeed, TEM analysis in our high fluence implanted samples showed the formation of an amorphous layer in the region of maximum Euconcentration [13]. In contrast to this, Au-implantation did not lead to amorphisation even for samples in which RBS/C suggested the break-down of crystalline order [10]. Fig. 1. Random and h0001i aligned RBS/C spectra after implantation of 300 kev Eu ions into AlN to different fluences. The aligned spectrum of an as-grown sample is shown for comparison. Only the region of the Al-signal is shown. Fig. 2. The maximum relative defect density in the Al-sublattice as a function of the implantation fluence. It is interesting to compare the present results to Eu implantation into the related semiconductor GaN under similar conditions [15]. The damage build-up curve for GaN is very similar with low damage accumulation for low fluences and a strong damage increase in the intermediate fluence regime. This strong increase of damage, however, starts at lower fluences in the case of GaN. Wendler et al. explained the radiation hardness of AlN with the strong bond strength of the material which they correlate with an atomic force constant determined from the long-wave optical constants [11]. Implantation damage in the bulk of GaN saturates for high fluences, however, strong erosion of the heavily damaged material is also observed. Furthermore, implanting GaN at RT leads to enhanced surface damage which is not the case for AlN where the maximum damage level is found close to the end of range of the implanted Eu. In the case of RE implanted GaN, HP annealing at 1200 and 1450 C resulted in a complete recovery of the crystal and no diffusion of the RE was observed [16,17]. In contrast to these results, RBS/C spectra taken after annealing of AlN at 1300 C at LP and 1450 C at HP reveal only little recovery of the crystal and no significant difference is observed for the two annealing conditions (Fig. 3a c). For the highest fluence studied after annealing ( at/cm 2 ) the increase of the Al-signal in the implanted region (in both random and aligned spectra) indicates the diffusion of Al-interstitials towards the implanted region eventually accompanied by nitrogen loss from the sample. Furthermore, a strong loss of Eu is observed after annealing for high fluence samples. Fig. 3(d f) shows the Eu signals of the random spectra before and after annealing. Only for the lowest fluence does the Eu-profile stay unchanged while for higher fluences a strong loss of Eu is observed for both annealing conditions with higher out-diffusion for the high temperature HP annealing. The Eu areal densities retained in the samples after annealing are summarized in Table 1. Despite the strong lattice damage and Eu out-diffusion for high fluences all samples show red Eu-related PL already in the as-implanted state (not shown). After annealing the PL intensity increases strongly. Fig. 4 shows the PL spectra in the region of the strongest Eu 3+ emission lines at 624 nm arising from the 5 D 0? 7 F 2 intra-ionic 4f 6 transition after HP annealing at 1450 C. The PL intensity is seen to increase when the fluence is raised from 1.2 to at/cm 2 but decreases again strongly for high fluences due to the strong lattice damage and the out-diffusion of Eu. The inset in Fig. 4 shows the integrated Eu PL intensity around 624 nm for both HP and LP samples. No correlation of PL intensity and Eu-concentration is observed pointing to the strong influence

4 K. Lorenz et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) Fig. 3. Left side: random and aligned RBS/C spectra after implantation of 300 kev Eu ions into AlN to different fluences before and after LP annealing at 1300 C or HP annealing at 1450 C. The aligned spectrum of an as-grown sample is shown for comparison. The random spectra shown correspond to the as-implanted samples except for Fig. 3c where the random spectrum after HP annealing was chosen. The random spectrum of the as-implanted sample is shown in Fig. 1. Right side: the Eu signal of random RBS spectra after implantation of 300 kev Eu ions into AlN to different fluences before and after LP annealing at 1300 C or HP annealing at 1450 C. Table 1 The nominal implantation fluences and the Eu areal densities retained in the samples after HP and LP annealing. Nominal fluence Retained Eu:HP Retained Eu:LP Despite the significant difference in damage and Eu-diffusion in the studied samples, the PL spectra show the same lineshape suggesting that the optically active RE ions are located in the same microscopic environment. Eu is known to be incorporated preferentially on near-substitutional Al-sites [8,9]. The optically active site is probably related to substitutional Eu while Eu ions incorporated in highly defective regions of the crystal or in clusters remain optically inactive. of implantation damage on PL properties. However, for the highest fluence concentration quenching may also be an issue. In Fig. 5 the PL spectra after LP and HP annealing are compared for two fluences revealing slightly higher PL intensities for the high temperature and high pressure annealing. Fig. 4. PL at 77 K of AlN implanted to different Eu-fluences and annealed at 1450 C in high nitrogen pressure. The inset shows the integrated PL intensity around 624 nm as a function of the Eu areal density retained in the sample after LP and HP annealing. Fig. 5. Comparison of 77 K PL spectra of AlN implanted to different Eu-fluences and annealed either at 1450 C in high or at 1300 C in low nitrogen pressure.

5 2910 K. Lorenz et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) Our results show that for optimum optical activation moderate RE fluences should be used and the implantation damage should be minimised already during the implantation since post-implant annealing of damage is difficult. In the case of GaN this could be realized by implantation at high temperatures or using channelled implantation with the beam entering the sample along the c-axis [15,18]. In fact, low fluence channelled implantation into AlN resulted in reduced lattice damage and strong Eu emission with very low temperature quenching [9,13]. 4. Conclusions The implantation damage build-up and high temperature annealing of Eu implanted AlN was investigated in a wide fluence range. AlN was found to be very resistant to radiation damage. However, implantation damage is extremely stable at temperatures up to 1450 C. Furthermore, Eu out-diffusion during annealing was observed for high Eu concentrations. Eu-related red light emission was achieved in all samples with highest PL intensities for moderate fluences. The data suggests that the same Eu-related emitting centre is observed in all samples despite strongly varying damage levels and Eu concentrations. Acknowledgements We acknowledge the support by FCT, Portugal(POCI/FIS/57550/ 2004, PTDC/FIS/66262/2006, PTDC/CTM/100756/2008, SFRH/BD/ 45774/2008, SFRH/BD/44635/2008 and Ciência 2007). References [1] A.J. Steckl, J.C. Heikenfeld, D.S. Lee, M.J. Garter, C.C. Baker, Y. Wang, R. Jones, IEEE J. Sel. Top. Quant. Electron. 8 (2002) 749. [2] P.N. Favennec, H. L Haridon, M. Salvi, D. Moutonnet, Y.L. Guillou, Electron. Lett. 25 (1989) 718. [3] J.D. MacKenzie, C.R. Abernathy, S.J. Pearton, U. Hömmerich, X. Wu, R.N. Schwartz, R.G. Wilson, J.M. Zavada, Appl. Phys. Lett. 69 (1996) [4] V.I. Dimitrova, P.G. Van Patten, H.H. Richardson, M.E. Kordesch, Appl. Phys. Lett. 77 (2000) 478. [5] U. Vetter, J. Zenneck, H. Hofsäss, Appl. Phys. Lett. 83 (2003) [6] H.J. Lozykowski, W.M. Jadwisienczak, Phys. Stat. Sol. B 244 (2007) [7] N. Nepal, J.M. Zavada, D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 93 (2008) [8] K. Lorenz, E. Alves, T. Monteiro, M.J. Soares, P.J.M. Smulders, Nucl. Instrum. Methods Phys. Res. B 242 (2006) 307. [9] M. Peres, A. Cruz, M.J. Soares, A.J. Neves, T. Monteiro, K. Lorenz, E. Alves, Superlattices Microstruct. 40 (2006) 537. [10] S.O. Kucheyev, J.S. Williams, J. Zou, C. Jagadish, M. Pophristic, S. Guo, I.T. Ferguson, M.O. Manasreh, J. Appl. Phys. 92 (2002) [11] E. Wendler, W. Wesch, Nucl. Instrum. Methods Phys. Res. B 242 (2006) 562. [12] W. Jiang, I.-T. Bae, W.J. Weber, J. Phys.: Condens. Matter 19 (2007) [13] K. Lorenz, E. Alves, F. Gloux, P. Ruterana, M. Peres, A.J. Neves, T. Monteiro, J. Appl. Phys. 107 (2010) [14] K. Gärtner, K. Hehl, G. Schlotzhauer, Nucl. Instrum. Methods 216 (1983) 275; K. Gärtner, K. Hehl, G. Schlotzhauer, Nucl. Instrum. Methods Phys. Res. B 4 (1984) 55; K. Gärtner, K. Hehl, G. Schlotzhauer, Nucl. Instrum. Methods Res. B 4 (1984) 63; K. Gärtner, K. Hehl, Nucl. Instrum. Methods Phys. Res. B 12 (1985) 205; K. Gärtner, Nucl. Instrum. Methods Phys. Res. B 132 (1997) 147; K. Gärtner, Nucl. Instrum. Methods Phys. Res. B 227 (2005) 522. [15] K. Lorenz, N.P. Barradas, E. Alves, I.S. Roqan, E. Nogales, R.W. Martin, K.P. O Donnell, F. Gloux, P. Ruterana, J. Phys. D: Appl. Phys. 42 (2009) [16] T. Monteiro, J. Soares, M.R. Correia, E. Alves, J. Appl. Phys. 89 (2001) [17] I.S. Roqan, K.P. O Donnell, R.W. Martin, P.R. Edwards, S.F. Song, A. Vantomme, K. Lorenz, E. Alves, M. Boćkowski, Phys. Rev. B 81 (2010) [18] B. Pipeleers, S.M. Hogg, A. Vantomme, J. Appl. Phys. 98 (2005)