Cu-doped GaN is a promising candidate for a nitride-based diluted magnetic semiconductor. Theoretical predictions show the possibility of ferromagnetism and high spinpolarization for certain arrangements of Cu atoms in the GaN lattice. Initial experimental results have already indicated ferromagnetism. However, the influence of structural defects on the ferromagnetic order in Cu-doped nitrides is not clear. Hence, the origin of the ferromagnetism is still under debate. We have used density functional theory (DFT) to verify previous theoretical predictions and to investigate the effects of the position of Cu atoms on the ferromagnetic properties. Our DFT calculations show high degrees of spin-polarization, independent of the arrangement of Cu atoms. Additionally, we have investigated the growth of Cu-doped GaN by molecular beam epitaxy. The influence of parameters, such as Cu to Ga ratio and growth temperature, on the structural and magnetic properties will be discussed

Fischer, Gerda

Ganz, P. R.

Schaadt, D. M.

Sürgers, Christoph

English

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Journal of Physics: Conference SeriesCu-doped GaN grown by molecular beam epitaxyTo cite this article: P R Ganz et al 2010 J. Phys.: Conf. Ser. 200 062006 View the article online for updates and enhancements.Related contentTopical ReviewA Bonanni-Room Temperature Ferromagnetism inGa1xHoxN (x=0.0 and 0.05) DilutedMagnetic Semiconductor Thin FilmsGhulam Murtaza Rai, Muhammad AzharIqbal, Yong-bing Xu et al.-Growth, luminescence and magneticproperties of GaN:Er semiconductor thinfilms grown by molecular beam epitaxyK Dasari, J Wu, H Huhtinen et al.-Recent citationsFerromagnetism in Cu-doped polar andnonpolar GaN surfacesRafael González-Hernández et al-Magnetic properties of Cu-doped GaNgrown by molecular beam epitaxyPhilipp R. Ganz et al-Structure and magnetic properties of Ni-doped AlN filmsD. Pan et al-This content was downloaded from IP address 129.13.72.197 on 24/05/2018 at 11:53Cu-doped GaN grown by molecular beam epitaxyP R Ganz1,2, C Sürgers1,3, G Fischer3 and D M Schaadt1,21 DFG-Center for Functional Nanostructures, Universität Karlsruhe (TH), 76128 Karlsruhe,Germany2 Institut für Angewandte Physik, Universität Karlsruhe (TH), 76128 Karlsruhe, Germany3 Physikalisches Institut, Universität Karlsruhe (TH), 76128 Karlsruhe, GermanyE-mail: philipp.ganz@physik.uni-karlsruhe.deAbstract. Cu-doped GaN is a promising candidate for a nitride-based diluted magneticsemiconductor. Theoretical predictions show the possibility of ferromagnetism and high spin-polarization for certain arrangements of Cu atoms in the GaN lattice. Initial experimental resultshave already indicated ferromagnetism. However, the influence of structural defects on theferromagnetic order in Cu-doped nitrides is not clear. Hence, the origin of the ferromagnetism isstill under debate. We have used density functional theory (DFT) to verify previous theoreticalpredictions and to investigate the effects of the position of Cu atoms on the ferromagneticproperties. Our DFT calculations show high degrees of spin-polarization, independent of thearrangement of Cu atoms. Additionally, we have investigated the growth of Cu-doped GaNby molecular beam epitaxy. The influence of parameters, such as Cu to Ga ratio and growthtemperature, on the structural and magnetic properties will be discussed.1. IntroductionGroup–III nitride semiconductor cover a large bandgap-area and have good thermal and chemicalstability. They are therefore interesting for many optoelectronic applications and devices. Thepossibility to synthesize a diluted magnetic semiconductor (DMS) based on group-III nitridesis in demand for the application area of spintronics. An ideal DMS like a conventional group-III nitride doped with a transition metal, should exhibit ferromagnetism at or above room-temperature. Indeed, theoretical [1, 2] and experimental [3, 4] results show room-temperatureferromagnetism in Mn- and Gd-doped GaN. However, this ferromagnetism could also be causedby clusters of Mn or Gd in the GaN host, because Mn and Gd are intrinsic ferromagneticelements. The formation of such clusters was already observed experimentally [5, 6].Copper (Cu) as a non-magnetic transition metal is a promising candidate for overcoming theproblem with magnetic secondary phases. Room-temperature ferromagnetism in Cu-doped ZnOwas proven theoretically [7] and experimentally [8]. The first density functional theory (DFT)calculations for Cu-doped nitrides by Wu et al. [9] suggested room-temperature ferromagnetismwith a 100 % spin-polarization of the conduction carriers and a total magnetization of 2.0 µBohrper Cu atom. DFT calculations for Cu-doped GaN by Rosa and Ahuja [10], showed only a weakferromagnetism, making the material insuitable for spintronic applications. The reason could befound in the Cu arragement in the GaN lattice. Two possible configurations with different Cu-Cudistances have been considered thus far: a ”far” configuration (Cu-Cu distance: 5.23 Å), whichis ferromagnetic in accordance with [9], and a ”close” configuration (Cu-Cu distance: 3.22 Å),which is lower in energy and shows only weak ferromagnetism. The first experimental resultsInternational Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 062006 doi:10.1088/1742-6596/200/6/062006c© 2010 IOP Publishing Ltd 1were reported on Cu-implanted GaN films [11]. However, the observed magnetic saturation(0.01 to 0.27 µBohr) was much lower than predicted by Wu et al. [9] and depended on theannealing temperature. Room-temperature ferromagnetism in Cu-implanted samples was alsofound by [12]. Yang et al. [12] and Hong [13] concluded, that vacancy-like defects should beconsidered in order to understand the observed magnetism. Furthermore, room-temperatureferromagnetism was also observed in Cu-doped GaN nanowires [14] which exhibit a single-crystalline and defect-free nature. A much higher magnetic saturation of 0.86 µBohr per Cuatom was detected in these nanowires compared to implanted films, but still much lower thantheoretically predicted.2. MethodsDFT calculations were performed using the WIEN2k code [15]. The properties were studiedon a 2 x 2 x 2 GaN supercell, with two Cu atoms substituting Ga sites. Other defects suchas vacancies or interstitial atoms were not consindered. For the calculation of the exchange-correlation potential, the general gradient approximation (GGA) within the PBE scheme [16]was used. A 7 x 7 x 4 k mesh was used to scan the irreducible Brillouin zone. The convergencecriteria was set to 1.4 meV for the total energy. All calculations were obtained for wurtziteGaN with the lattice constants a = 3.189 Å and c = 5.189 Å. After structural optimization,a marginal change in the lattice constants was observed. Cu-doped GaN films were grown onC -plane sapphire substrates by plasma assisted molecular beam epitaxy in a Riber compact21 MBE-sytem. For an improved backside heating in the chamber, the substrates were gluedwith Indium on Si(001) wafers. After outgasing the substrate in the loadlock chamber, a three-step-growth was performed. First, nitridation of the sapphire surface was made in an activatednitrogen atmosphere with a pressure of 2.4× 10-5 Torr. Second, a 25 nm buffer layer was grownat low substrate temperature and slightly Al rich conditions. Last, a Cu-doped GaN epitaxiallayer followed the buffer layer. This layer was grown at 750 ◦C thermocouple temperature fortwo hours. To promote the growth of a high-quality crystalline layer, a slow growth rate ofabout 70 nm per hour was selected. During growth, reflection high-energy electron diffraction(RHEED) was used to characterize the surface. The structural properties were analyzed withscanning electron microscopy (SEM). X-ray diffraction (XRD) with a Bruker D8 diffractometeryielded information about the crystalline quality and secondary phases. For a characterizationof the magnetic properties, a superconducting quantum interference device (SQUID) was used.3. Results and discussion3.1. DFT calculationsOur calculations were made for two Cu atoms in the supercell corresponding to a high Cuconcentration of 12.5 at.%. The Cu atoms were put on Ga sites. Figure 1 shows the electronicdensity of states (DOS) for the spin-up and spin-down channels for the ”far” configuration(Fig. 1a), corresponding to a Cu-Cu distance of 5.169 Å, and for the ”close” configuration(Fig. 1b), corresponding to a Cu-Cu distance of 2.986 Å, respectively. For both, a high spin-polarization of about 90 % is observed. For the ”far” configuration, we obtain a semiconductor-like DOS for the spin-up channel and a metallic behavior for the spin-down channel. The energydifference between these two spin states is about 600 meV. However, for the ”close” configuration,the energy difference is about 300 meV, much lower than for the ”far” configuration. There isalso a little change in the band arrangement. For the ”close” configuration, the spin-up statesare more in the valence band and the spin-down states are slightly above zero energy, whichcorresponds to the valence band edge.International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 062006 doi:10.1088/1742-6596/200/6/0620062Figure 1. Spin-up (top) and spin-down (bottom) for ”far” (a) and ”close” (b) Cu arrangement.3.2. Growth of Cu-doped GaN by molecular beam epitaxyAside from the calculations, we investigated the growth of Cu-doped GaN by plasma assistedmolecular beam epitaxy. The structural properties of C -plane Cu-doped GaN grown on C -planesapphire are presentetd in Figs. 2 and 3. The surface morphology was imaged by SEM as shownFigure 2. SEM picture andRHEED pattern after growth. Figure 3. ω–2θ scan.in Fig. 2. Needle-shaped islands are visible on top of the film. The RHEED pattern (insetin Fig. 2) indicates a flat surface of the Cu-doped GaN. Energy dispersive x-ray spectroscopy(EDX) was used to determine the composition of these islands. They have an average heightof about 100 nm and contain a high amount of copper and gallium, both in a concentration ofabout 50 %. Efforts to etch them away with 30 % HCl or 65 % HNO3 were not successful. NoCu could be detected in the layer using EDX. XRD measurements indicated no secondary phasein Cu-doped GaN. Figure 3 shows a strong peak at an angle of ω = 20.82◦ corresponding to theC -plane of the sapphire substrate. The full width at half maximum (FWHM) is about 0.0092◦.The peak at 17.23◦ is the C -plane peak (0002) of the GaN. The FWHM of GaN (about 0.0389◦)is broader but still reasonable for GaN. The lower quality of the GaN can have two reasons.First, the growth of Cu-doped GaN can be optimized with regard to the nitridation, the bufferand the epitaxial layer. A second reason for the lower quality could be the dopand. Cu maycause defects, that give rise to a lower crystalline quality. A third peak is observed at 17.89◦in Fig. 3. This peak is due to the AlN buffer layer and a thin AlGaN layer between the bufferand epitaxial layer, because the peak is shifted by to lower angles with respect to the pure AlNC -plane peak.International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 062006 doi:10.1088/1742-6596/200/6/06200633.3. Magnetic propertiesThe magnetic properties were measured by SQUID magnetometry for samples with varying Cuto Ga beam equivalent pressure (BEP) flux ratio. The growth was investigated for sampleswith BEP flux ratios from 0.9 % to 4.8 %. Figure 4a) presents the magnetization at roomFigure 4. a) Nominal magnetization as a function of external field for BEP flux ratios of 1.2 %(black) and 4.8 %(grey). The negative slope of M(H) at large fields is due to the diamagneticcontribution of the sapphire substrate. b) Magnetization as a function of temperature.temperature as a function of external magnetic field for two different Cu to Ga BEP flux ratios.The magnetic saturation is getting lower for a higher flux ratio. For the sample with a fluxratio of 1.2 %, the nominal magnetic saturation is calculated to 0.42 µBohr per Cu atom, altoughthe real moment may be much larger considering the large amount of Cu in the islands onthe film. The reduced magnetic saturation for higher ratios is attributed to the higher Cuallocation, because there was no change in other parameters. Fig. 4b) shows the magnetizationas a function of temperature, measured up to 400 K. The magnetization decreases slowly withincreasing temperature. Therefore, the Curie temperature is expected to be far above 400 K.We presented DFT calculations on Cu-doped GaN containing 12.5 % Cu atoms. All calcu-lations show a high degree of spin-polarization. A ferromagnetic behavior at room-temperaturewas also observed in epitaxially grown Cu-doped GaN. For a higher Cu to Ga BEP flux ratio,the saturation magnetization decreases.[1] Sato K and Katayama-Yoshida H 2001 Jpn. J. Appl. Phys. 2 Part 2 L485[2] Dalpian G M and Wei S H 2005 Phys. Rev. B 72 115201[3] Thaler G T et al. 2002 Appl. Phys. Lett. 80 3964[4] Dhar S, Perez L, Brandt O, Trampert A, Ploog K H, Keller J and Beschoten B 2005 Phys. Rev. B 72 24520[5] Baik J M, Jong H W, Kim J K and Lee J L 2003 Appl. Phys. Lett. 82 583[6] Dhar S, Brandt O, Trampert A, Däweritz L, Friedland K J, Ploog K H, Leller J, Beschoten B andGüntherodt G 2003 Appl. Phys. Lett. 82 2077[7] Feng X 2004 J. Phys.: Condens. Matter 16 4251[8] Buchholz D B, Chang R P H, Song J H and Ketterson J B 2005 Appl. Phys. Lett. 87 082504[9] Wu R Q, Peng G W, Liu L, Feng Y P, Huang Z G and Wu Q Y 2006 Appl. Phys. Lett. 89 062505[10] Rosa A L and Ahuja R 2007 Appl. Phys. Lett. 91 232109[11] Lee J-H et al. 2007 Appl. Phys. Lett. 90 032504[12] Yang X L, Chen Z T, Wang C D, Zhang Y, Pei X D, Yang Z J, Zhang G Y, Ding Z B, Wang K and Yao S D2009 Journ. Appl. Phys. 105 053910[13] Hong J 2008 Journ. Appl. Phys. 103 063907[14] Seong H-K, Kim J-Y, Kim J-J, Lee S-C, Kim S-R, Kim U, Park T-E and Choi H-J 2007 Nano Lett. 7 (11)3366[15] Blaha P, Schwarz K and Sorati P 1990 Comp. Phys. Communications 59 339[16] Perdew J P, Burke K and Ernzerhof M 1996 Phys. Rev. Lett. 77 3865International Conference on Magnetism (ICM 2009) IOP PublishingJournal of Physics: Conference Series 200 (2010) 062006 doi:10.1088/1742-6596/200/6/0620064

Cu-doped GaN grown by molecular beam epitaxy

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Cu-doped GaN grown by molecular beam epitaxy

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