Optical and magnetic properties of GaN epilayers implanted with ytterbium

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Optical and magnetic properties of GaN epilayers implanted with ytterbium W.M. Jadwisienczak1, J. Wang1, H. Tanaka1, J. Wu2, R. Palai2, H. Huhtinen3, A. Anders4 (1. School of EECS, Ohio University, Athens OH, 45701, U.S.A; 2. Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, Puerto Rico, 00931-3343,U.S.A; 3. Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, Turku, FIN-20014, Finland; 4. Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, U.S.A) Received 31 August 2010; revised 21 November 2010

Abstract: We have studied the optical and magnetic properties of ytterbium implanted GaN epilayer grown on (0001) sapphire by metalorganic chemical vapor by deposition (MOCVD). Samples were implanted at room temperature with Yb ions at dose 4×1015 cm–2 and energy of 150 keV. The implanted samples were annealed at 1000 °C in N2 at atmospheric pressure to recover implantation damages. The photoluminescence (PL), PL excitation (PLE), and PL kinetics have been studied with continuous and pulse photo-excitations in 360–1100 nm spectral range at different temperatures. The characteristic Yb3+ ion emission spectra were observed in the spectral range between 970–1050 nm. Theoretical fittings of the experimental PL temperature and PL kinetics data suggest that Yb3+ ions are involved in at least two major luminescence centers. The PLE spectra indicate that excitation of the Yb3+ ion occurs via electron-hole pair generation and complex processes. Magnetization versus magnetic field curves shows an enhancement of magnetic order for Yb-implanted samples in 5 K to 300 K temperature range. The Yb-implanted GaN sample showing weak ferromagnetic behavior was compared with the ferromagnetic in situ doped GaYbN material. Keywords: luminescence; III-nitrides; rare earth ions; defects; ferromagnetism; rare earths

Rare-earth (RE) elements play an important role in many functional materials and exhibit interesting optical and magnetic properties. Despite of recent technological advances the electronic structure of the RE centers affecting these properties is still not well understood. During the last decade III-nitride (III-N) semiconductors doped with lanthanide ions have been extensively investigated for optoelectronic applications[1]. Furthermore, RE-doped III-Ns, especially Gddoped GaN, shown interesting magnetic behavior and are currently considered as prospective diluted magnetic semiconductors for spintronic devices[2,3]. The observed magnetism in these materials is partially due to highly localized 4f electrons making the direct f-f interactions between the neighboring RE3+ ions very weak. Also, the short/long range interactions between structural defects, defect clusters or impurities, and RE ions are considered as contributing to observed ferromagnetism in RE-doped GaN[4–6]. In the past Yb-doped III-V semiconductors were extensively investigated. Those studies proved to be useful in better understanding of other III-V semiconductors doped with RE3+ ions[7,8]. Among investigated RE doped III-Ns, ytterbium has been the least studied[9]. Recently, it was proposed that Yb3+ ions coexist in the YbGa site and VN-RE complex in the GaN host[10,11], which is not yet well understood. These results call for further investigations since the nature of the Yb3+ ion centers, as well as other RE3+ ions centers in III-Ns, their local environment and short/long distance interactions strongly affect optical properties of RE3+ ions and contribute to the

observed magnetic behavior.

1 Experimental The GaN material investigated here was 1.7 µm thick GaN grown on (0001) Sapphire by metal-organic chemical vapor deposition (MOCVD). Samples were implanted with Yb ions with energy not exceeding 150 keV and total dose up to 4×1015 cm–2 [12]. The simulated range and peak concentration, calculated using Pearson distributions, are: ~3.28 at.% at 12 nm and ~0.015 at.% at 70 nm, respectively. Samples were given isochronal thermal treatment in the nitrogen gas environment at 1000 ºC for 10 min under atmospheric pressure for achieving, specifically in this work, the high optical activity of Yb3+ ions. All spectroscopic characterizations were done for annealed samples unless mentioned otherwise. Photoluminescence (PL), and PL excitation (PLE) spectra were measured as a function of temperature using the experimental systems described elsewhere[13]. Due to the low excitation intensity available for the PLE measurement (~100 μW/cm2 at 370 nm) the ytterbium luminescence signal was detected by a photomultiplier Hamamatsu R316-02 equipped with a Melles-Griot interference filter model 03FII525 without any other dispersion element. PL kinetics was measured using 0.6 ms square pulses generated by modulated cw HeCd laser at 1 kHz and a photon counting system with a turbo-multichannel scaler Turbo-MCS with 5 ns channel dwell time and no delay between channels. Mag-

Foundation item: Project supported by the 1804 Fund grant of Ohio University and the US Department of Energy (DE-AC02-05CH11231) Corresponding author: W.M. Jadwisienczak (E-mail: [email protected]; Tel.: +1-7405932067) DOI: 10.1016/S1002-0721(09)60234-9

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netic properties were measured using superconducting quantum interference device (SQUID) from room temperature to down to 5 K. We used the in situ doped GaYbN epilayer grown on (0001) sapphire by plasma assisted molecular beam epitaxy (MBE) as a reference material in magnetic studies[14].

2 Results and discussion Fig. 1 shows the PL spectra of undoped and Yb implanted GaN epilayers at different temperatures. The reference PL spectrum shows excitonic peak at 355.3 nm and intrinsic/defects related bands at 435 and 550 nm at 12 K, respectively. The low temperature PL spectra of GaN:Yb3+ epilayer show two broad bands at 538 nm (yellow band) and at 675–700 nm (red band). The intensity of these bands decreases monotonically with different rates when ambient temperature increases. Recently, it has been reported that the yellow band in GaN is due to the VGa-ON complex and/or to intrinsic impurities, VGa and VGa-shallow donor complexes[15]. The red emission band observed only in GaN:Yb3+ samples is most probably associated with characteristic Yb3+ ion-implantation-induced defects. The yellow band peak position red-shifts by 106 meV, whereas the red band peak position blue-shifts by 157 meV, respectively. We believe that the yellow band is due to intrinsic impurities like O and C typical for the chemistry of MOCVD growth mode and related structural defects. The larger magnitude of the red band blue-shift as well as its stronger temperature dependence substantiate that it is associated with Yb-induced defects. The emission lines between 950–1050 nm correspond to transitions between the spin-orbit levels 2F5/2→2F7/2 of Yb3+

Fig. 1 PL spectra of reference (curve (1)) and Yb3+ ion implanted GaN epilayer (curves (2) and (3)) at different temperatures. Dashed curves represent components of the deconvoluted PL spectra showing yellow and red bands. Insert shows temperature dependence of Iem for Yb3+ ion fitted to the equation IPL=I1+I2 where I1,2= A1,2(1+B1,2exp(–εPL1,2/kBT))–1 where A, B are fitting parameters, εPL1,2 is the thermal activation energy, kB is the Boltzmann constant and T is temperature, respectively.

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(f13) ion and interactions between the lattice vibrations localized modes and pure electronic transitions[9]. The insert in Fig. 1 shows the temperature dependence of the integrated PL emission intensity (Iem) defined as the area under the Yb3+ ion emission lines in the temperature range 12 K to 300 K. The observed temperature dependence of Iem is nonlinear and shows moderate luminescence quenching at 300 K. It is seen that the PL Iem curve shows a weak step-like feature at 140– 160 K. The Iem temperature dependence curve was fitted by two-term exponential function (see Fig. 1 caption) yielding reasonable good fitting results. The estimated thermal quenching energies εPL1,2 derived for Yb3+ ion are 19.3 meV and 107 meV. This finding indicates that at least two different optically active Yb3+ ion centers are involved in generating the recorded PL spectra. Fig. 2 shows the low temperature PLE spectrum of the GaN:Yb3+ sample. The Iex curve is the intensity of excitation light as a function of wavelength obtained from the excitation source. The PLE spectrum was monitored at ~1 000 nm corresponding to the maximum transmission band of the interference filter used for filtering the PL signal. The filter FWHM of 40 nm conveyed almost the entire emission band of the Yb3+ ion defining the resolution of measured PLE spectrum. It is seen that the PLE spectrum has two distinct excitation bands at 357 nm and 473 nm, respectively. The band at 357 nm corresponds to the above band gap excitation and abruptly decreases above this wavelength. The second dominant band at 473 nm with ~50 nm FWHM and a few low intensity excitation bands between 500 nm and 800 nm, respectively, correspond to the energy levels in forbidden gap of GaN. They are most probably induced by defects, both structural and Yb-implantation related, and/or intrinsic impurities. Thus, relying on collected PLE spectrum we conclude that the energy transfer between the GaN host and Yb3+ ion proceeds through the electron-hole pair generation and subsequent complex energy migration through defect/impurity recombination centers not necessarily in the vicinity of Yb3+ ion. It should be stressed out here that the resonant excitation through defect/impurity centers requires substantial excitation energy dissipation and

Fig. 2 Uncorrected excitation spectrum of Yb3+ ion monitored at 1 000 nm (1), the excitation intensity as the function of wavelength (2), PL spectrum of GaN:Yb3+ (3) and spectral response of the interference filter used for detecting of the PLE spectrum (4).

W. M. Jadwisienczak et al., Optical and magnetic properties of GaN epilayers implanted with ytterbium

probably involves multi-phonon relaxation mechanisms. Fig. 3 shows PL decay curves recorded for the reference GaN sample (curve (1)) and GaN:Yb3+ (curve (2)) measured at 11 K. The curve (3) in Fig. 3 is the fitting to the function shown in the Fig. 3 caption. It is seen that reasonable good fitting was obtained for an initial and middle sections of the PL decay curve using the two-term exponential function, however, the PL decay long-time tail is not reproduced correctly. This indicates that at least two dominant Yb3+ ion centers are present in the studied samples along with some minority centers as shown by the measured PLE spectrum. Fig. 4 shows the field dependence of the magnetization for reference GaN and GaN:Yb3+ after annealing step measured at 5 and 300 K, respectively. Magnetization of the reference GaN was first investigated to eliminate the possibility of

Fig. 3 PL decay curve measured for reference GaN (1) and GaN:Yb3+ at 11 K (2). Open symbols line represents fitting to IPL=A1exp(–t/τ1)+A2exp(–t/τ2) where A1,2 are fitting parameters, t is time and τ1,2 are radiative decays times. The estimated PL decay times are: 37 μs and 172 μs, respectively.

Fig. 4 Magnetization loops obtained for Yb-implanted GaN annealed at 1000 °C (□) and for as-grown reference GaN sample (●) at 5 K (a) and 300 K (b)

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spurious transition metal impurities contribution for the magnetic response. As can be seen, GaN has a weak magnetic signal. Typically, the undoped GaN/sapphire (0001) samples do not show any magnetic signal beyond pure diamagnetism, therefore the observed weak magnetic signal from undoped GaN epilayer may be from the contamination and structural defects. It is known that doping with nonmagnetic atoms strongly affects the ferromagnetism in the III-Ns[16]. Therefore, the observed magnetization might have originated from free carriers-mediated processes if the concentration of unintentional impurities/defects is sufficiently high. When Yb3+ ions are implanted the magnetic signal increases due to overall contributions from the host and Yb3+ ions. It should be pointed out here that the observed magnetic signal is relatively weak ferromagnetic-type (or super-paramagnetic-type) without coercive force. Comparing the magnetic results shown in Fig. 4 one can conclude that the Yb3+ ions may change the effective magnetic signal, presumably due to their large ionic radius straining the GaN crystal locally[5]. The effective magnetic moment expected for trivalent free Yb3+ ions is 4.54μB[17]. This value can be affected by some of the ytterbium occupying different local sites. Although Yb3+ ions predominantly occupy Ga sites, there exists a fraction of Yb3+ ions involved in YbGa-VN complexes in these samples[11]. The difference in local ytterbium surroundings affects the short-range magnetic ordering in the system and change the effect of crystal field splitting on the energy levels of the Yb3+ ions. We have not observed clear magnetization saturation for Yb-implanted sample even in a high magnetic field (see Fig. 4), indicating a weak spin coupling in this material. Also, as can be seen, there is no strong diamagnetic behavior at low temperature as observed in many transition metals doped diluted magnetic semiconductors. Fig. 5 shows the zero-field-cooled (ZFC) and fieldcooled (FC) magnetization of undoped and GaN:Yb3+ thin films. We divided the magnetic value by the volume of Yb implanted sample (area×implanted ions penetration range) and presented the values in units [A/m]. One can observe that magnetization value is clearly higher (~3 times) in film implanted with Yb. The magnetic signal level is slightly

Fig. 5 Magnetization from GaN:Yb3+ annealed at 1000 °C for ZFC (□), for FC (■) and reference GaN epilayer for ZFC (○), for FC (●), respectively

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decreasing up to 300 K and the small difference between ZFC and FC curves can be seen, especially below 100 K. The GaN:Yb3+ sample annealing temperature of 1000 ºC was optimal for optical activation of Yb3+ ions; however, it is 100 ºC higher than for typically reported magnetic properties of Gd-implanted GaN material[4,18]. It was shown that for GaN:Gd the lower anneal temperature may produce a lower magnetization through incomplete annealing of the implantation induced damages. Therefore, an optimal anneal temperature may be different for RE implanted GaN if they are intentionally optimized for optoelectronic or spintronic applications. Above observations emphasize the importance of the GaN host matrix for the overall magnetic properties of GaN:RE3+. Presented SQUID measurements confirmed the suggestion that the magnetic signal from the RE dopant atom itself, in our case Yb3+ ion is small highlighting the role of magnetic contribution from the GaN host. At the present we do not have sufficient evidence letting us unambiguously exclude phase separation or metallic clusters (Yb-Yb) which could be responsible for exchange interactions and different Yb-Yb coupling schemes especially at the Yb ion peak concentration in studied samples (3.3 at.% at 12 nm). However, it was demonstrated for GaN:Gd grown by MBE that such condition does not occur within a few percent of the overall doping level[5]. Recently, we have shown that the GaYbN epilayers grown in situ on (0001) sapphire by plasma-assisted MBE exhibit strong magnetic hysteresis loops at 300 K[14]. These materials revealed very good optical quality. Room temperature ferromagnetism was confirmed by the hysteretic behavior, observed coercive force and saturation magnetization, and lack of the Yb phase segregation. We should mention that the samples studied here have significant differences compared to the samples investigated in Ref. [14]. Samples studied in the present work are of 1.7 μm thick and grown by MOCVD and implanted with Yb, whereas samples studied in Ref. [14] were 50 nm thick in situ grown by MBE, respectively. Taking into account the specific details of each sample growth mode we assume that the MOCVD samples are highly relaxed whereas the MBE samples are strained. Consequently, the spontaneous electric polarization induced by the internal strain, much stronger in the MBE epilayer than in MOCVD epilayer, seems to play a significant role in stabilizing magnetic ordering forcing domains to locked a particular direction[19,20]. This in turn induces variable long-range interactions contributing to magnetization enhancement in studied Yb-doped GaN samples.

3 Conclusions Measured PL luminescence spectra confirmed that Yb3+ ions are predominantly in trivalent state with tetrahedral coordination, occupying primarily Ga sites. The presence of second major Yb3+ ion site (VN-RE complex) was proposed to explain the PL temperature and PL decay dependences. The PLE spectroscopy showed that the energy transfer process between GaN host and Yb3+ ion is complex. The energy

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migration mechanism between initial and final state proceeds through structural and implantation-induced defects and impurities inducing localized energy levels in forbidden gap. Temperature dependent magnetic studies showed that the magnetic order of GaN epilayers is enhanced by implanted Yb ions. The observed magnetic behavior in Yb-implanted GaN sample was compared with in situ doped GaYbN material indicating the internal strain due to material growth mode affectively affects magnetization. This in turn confirms that the RE3+ ions unlikely by themselves induce the strong ferromagnetism in III-Ns at room temperature. Acknowledgements: The research at Ohio University was supported by the 1804 Fund grant. AA acknowledges support by the US Department of Energy under contract No. DE-AC02-05CH11231.

References: [1] Steckl A J, Zavada J M. Optoelectronic properties and applications of rare-earth-doped GaN. MRS Bull., 1999, 24: 30. [2] Dhar S, Kammemeier T, Ney A, Perez L, Ploog K H, Melnikov A, Wieck A D. Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN. Appl. Phys. Lett., 2006, 89: 062503. [3] Choi S W, Zhou Y K, Emura S, Lee X J, Teraguchi N, Suzuki A, Asahi H. Magnetic, optical and electrical properties of GaN and AlN doped with rare-earth element Gd. Phys. Stat. Sol. (c), 2006, 3: 2250. [4] Khaderbad M A, Dhar S, Perez L, Ploog K H, Melnikov A, Wieck A D. Effect of annealing on the magnetic properties of Gd focused ion beam implanted GaN. Appl. Phys. Lett., 2007, 91: 072514. [5] Ney A, Kammermeier T, Manuel E, Ney V, Dhar S, Ploog K H, Wilhelm F, Rogalev A. Element specific investigations of the structural and magnetic properties of Gd:GaN. Appl. Phys. Lett., 2007, 90: 252515. [6] Wang R, Steckl A J, Nepal N, Zavada J M. Electrical and magnetic properties of GaN codoped with Eu and Si. J. Appl. Phys., 2010, 107: 013901. [7] Taguchi A, Taniguch M, Takahei K, Excitation and relaxation of Yb3+ in InPAs and InP. Appl. Phys. Lett., 1992, 60: 965. [8] Lozykowski H J, Alshawa A K, Brown I G. Kinetics and quenching mechanisms of photoluminescence in Yb-doped InP. J. Appl. Phys., 1994, 76: 4836. [9] Jadwisienczak W M, Lozykowski H J. Optical properties of Yb ions in GaN epilayer. Opt. Mater., 2003, 23: 175. [10] Sanna S, Schmidt W G, Frauenheim T, Gerstmann U, Rareearth defect pairs in GaN:LDA+U calculations. Phys. Rev. B, 2009, 80: 104120. [11] Koubaa T, Dammak M, Kammoun M, Jadwisienczak W M, Lozykowski H J. Crystal field and Zeeman parameters of substitutional Yb ion in GaN. J. Alloys Compd., 2010, 496: 56. [12] Anders A. Ion charge state distributions of vacuum arc plasmas: The origin of species. Phys. Rev. E, 1997, 55: 969. [13] Lozykowski H J, Jadwisienczak W M. Thermal quenching of luminescence and isovalent trap model for rare-earth-ion-

W. M. Jadwisienczak et al., Optical and magnetic properties of GaN epilayers implanted with ytterbium doped AlN. Phys. Stat. Sol. (b), 2007, 244: 2109. [14] Wu J, Rivera A, Martinez A, Palai R, Liu K, Shur M, Yue L, He X, Binek C, Huhtinen H, Jadwisienczak W M. Int. J. High Speed Elect Syst., 2010 (in print). [15] Sedhain A, Li J, Lin J Y, Jiang H X, Nature of deep center emissions in GaN. Appl. Phys. Lett., 2010, 96: 151902. [16] Litvinov V I, Dugaev V K. Room-temperature ferromagnetism in dielectric GaN (Gd). App. Phys. Lett., 2009, 94: 212506. [17] Kittel C. Introduction to Solid State Physics, 8th Ed., Berkeley: John Wiley & Sons, Inc. 1995. [18] Han S Y, Hite J, Thaler G T, Frazier R M, Abernathy C R,

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Pearton S J, Choi S K, Lee W O, Park Y D, Zavada J M, Gwilliam R. Effect of Gd implantation on the structural and magnetic properties of GaN and AlN. App. Phys. Lett., 2006, 88: 042102. [19] Lo F Y, Melnikov A, Reuter D, Wieck A D, Ney V, Kammermeier T, Ney A, Schormann J, Potthast S, As D J, Lischka K. Magnetic and structural properties of Gd-implanted zincblende GaN. Appl. Phys. Lett., 2007, 90: 262505. [20] Palai R, Huhtinen H, Scott J F, Katiyar R S. Observation of spin-glass-like behavior in SrRuO3 epitaxial thin films. Phys. Rev. B, 2009, 79: 104413.