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Chemical, vibrational and optical signatures of nitrogen in ZnO nanowires
Materials Science in Semiconductor Processing xx (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Chemical, vibrational and optical signatures of nitrogen in ZnO nanowires L. Zhua, S. Khachadorianb, A. Hoffmannb, M.R. Phillipsa, C. Ton-Thata, a b
⁎
School of Mathematical and Physical Sciences, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
A R T I C L E I N F O
A BS T RAC T
Keywords: ZnO nanowires Nitrogen doping Luminescence Raman modes X-ray absorption
ZnO nanowires with various concentrations of nitrogen molecules have been fabricated by remote plasma annealing. X-ray absorption near-edge spectroscopy (XANES) reveals that nitrogen exists mainly in two chemical states: atomic nitrogen substituting oxygen (NO) and molecular nitrogen (N2) weakly bound to the ZnO lattice; the latter state increases substantially with prolonged plasma time. Cathodoluminescence microanalysis of individual nanowires reveals a broad emission band at 3.24 eV at 10 K, attributable to the recombination of a shallow donor and a N2 acceptor state. The Raman modes at 547 and 580 cm−1 from the Ndoped nanowires are found to rise in proportion to the N2 concentration, indicating they are related to N2 molecules or defects caused by the incorporation of N2 in the nanowires.
1. Introduction One-dimensional ZnO nanowires are a promising material system for various applications in nano-optoelectronic devices, such as shortwavelength light emitting diodes, field emission transistors and ultraviolet lasers [1]. Utilization of nanowires requires a good understanding and control of their properties that are largely affected by intrinsic defects and impurities. The lack of reliable p-type material poses a major challenge to the realization of ZnO nanowire-based nanodevices. Group V elements, especially nitrogen which has a similar radius to oxygen, are believed to be the natural choice for achieving p-type conductivity in ZnO [2,3]. ZnO nanowires possess a large surface-tovolume ratio, which enables facile incorporation of nitrogen at a high concentration, making gas phase ion doping an effective doping method for ZnO nanostructures [4]. Furthermore, the use of nanowires allows control over the type of native surface defects prior to nitrogen incorporation, which can enhance its doping level [5]. Understanding the chemical states of nitrogen dopants and their signatures in ZnO is pivotal in investigating the physics and chemistry of p-type doping in this semiconductor. Previous studies have shown that atomic nitrogen substituting oxygen (NO) is a deep acceptor in ZnO, whereas N2 on an oxygen site (N2)O is regarded as a shallow donor which compensates the p-type doping [6–8]. However, the nature of the N-related acceptor state is complex and to date remains controversial. Recently, it has been proposed that N2 molecule could be accommodated at a Zn site [(N2)Zn], which is predicted to be a double shallow acceptor responsible for the experimentally observed donoracceptor-pair (DAP) transition in N-doped ZnO [9]. Our previous study ⁎
of N-doped nanowires using the X-ray absorption spectroscopy established that nitrogen molecules are formed prevalently during nitrogen plasma doping [10]. Nitrogen-related vibrational modes, in particular those appearing at 275, 510, 582, and 643 cm−1, have been ascribed to nitrogen doping [11–13]. These anomalous modes, on the other hand, were also observed in bulk ZnO doped with other elements such as Fe, Sb and Al [14], which further adds ambiguity to the assignment of nitrogen vibrational modes. The aim of this work is to investigate the relationships between the chemical states of nitrogen and their optical and Raman signatures in ZnO nanowires. It is clear that precise chemical characterization of nitrogen dopants is a prerequisite to the evaluation of ZnO Raman data. 2. Experimental details ZnO nanowires were grown on the a-sapphire substrate via the vapor phase transport method as described in detail previously [15]. Admixture of ZnO and graphite powder was used as the source material. To enhance nitrogen incorporation by vacancy doping [7], nanowires were grown under oxygen-deficient conditions, which lead to an intense defect emission band peaking at 2.47 eV, attributable to VO [16]. Nitrogen doping was conducted by annealing nanowires at 300 °C in radio-frequency (RF) nitrogen plasma with 230 V cathode bias. The combination of abundant nitrogen radicals and the lowtemperature annealing enabled the nanowires to be efficiently doped with nitrogen without significant damage to their crystal structure and homogeneous distribution of the dopants. The nanowires were characterized using an FEI Quanta 200 Scanning Electron Microscope
Corresponding author. E-mail address: [email protected] (C. Ton-That).
http://dx.doi.org/10.1016/j.mssp.2016.11.038 Received 11 September 2016; Received in revised form 16 November 2016; Accepted 23 November 2016 Available online xxxx 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhu, L., Materials Science in Semiconductor Processing (2016), http://dx.doi.org/10.1016/j.mssp.2016.11.038
Materials Science in Semiconductor Processing xx (xxxx) xxxx–xxxx
L. Zhu et al.
Fig. 1. SEM image of N-doped ZnO nanowires on an a-plane sapphire substrate. The nanowires exhibit a regular hexagonal cross section and a smooth c-plane tip surface. No changes in the nanowire morphology were observed after remote plasma annealing. Inset displays the SEM image of one nanowire with a diameter of ~100 nm.
(SEM) with attached cathodoluminescence (CL) spectrometer. X-ray Absorption Near Edge Spectroscopy (XANES), implemented in the Total Fluorescence Yield (TFY) mode, was performed on the Soft X-ray Spectroscopy beamline at the Australian Synchrotron. The incident Xray beam was parallel to the c-axis of the nanowires. The photon energy scale was calibrated against the Au 4f7/2 peak at 84 eV from a clean gold film in electrical contact with the sample. Raman spectroscopy was conducted in backscattering geometry using a LabRAM HR800 spectrometer (Horiba Jobin Yvon) with the 457.48 nm laser excitation line. The laser power on the sample was set at 1 mW. 3. Results and discussion Fig. 1 shows the N-doped ZnO nanowires grown on a-sapphire substrate. The single-crystal ZnO nanowires are preferentially grown along the crystallographic [0001] direction of the wurtzite crystal and possess hexagonal tip facets. The nanowires have a diameter in the range of 120–250 nm and the typical length varies from 1 to 1.5 µm. The inset of Fig. 1 is the close-up view of a truncated nanowire removed from the substrate. There were no observable changes in the nanowire morphology after the nitrogen doping. Fig. 2 shows the N K-edge XANES spectra of the undoped and N-doped ZnO nanowires, which represent resonant electron transitions from the N 1 s core state to the final unoccupied N-related states of p-symmetry [8]. The spectra can be decomposed into Lorentzian components corresponding to three chemical states of nitrogen: P1 at 400.1 eV, P2 at 400.7 eV and P3 at 404.5 eV. The solid lines represent the fitted results of the XANES data points. The main component P1, associated with atomic nitrogen substituting oxygen (NO) in the lattice, is the transition of a 1s electron to 2pπ* state in the hybridized N-Zn orbital [17,18]. The P2 component arises from the 1s→π* transition in the N-N bond [19], which is identified as the signature of N2 molecules. This N2-related resonance at 400.7 eV has previously been observed in N-doped ZnO [18], GaN [20] and InN [21]. XANES is highly sensitive to the bonding arrangement of dopants; the similar XANES characteristics of N2 in various compounds strongly suggest that this molecular species is bound weakly to the host material. The P3 energy is identical to the signature absorption peak of N2O gas and has been assigned as the N 1s to 2pσ* transition in N-O species [17], hence the P3 peak is attributable to atomic N at Zn sites (NZn). The N2 concentration grows substantially with plasma time, indicating more nitrogen present as molecular species, whereas NO and NZn concentrations are virtually unchanged (inset of Fig. 2). N2 has a lower formation energy than NO [9], and given the high-temperature ballistic transport of atoms in nanowires, the highly mobile N atoms are anticipated to form pairs with increasing doping concentration.
Fig. 2. N K-edge XANES spectra of the ZnO nanowires that were nitrogen plasma annealed for different time periods. No nitrogen was detectable in the as-grown nanowires (bottom spectrum). The spectra were fitted with three Lorentzian functions (labelled as P1, P2 and P3) associated with three nitrogen chemical states. The inset shows the XANES integrated intensities of P1, P2 and P3 states as a function of plasma time.
Fig. 3 displays the temperature-dependent CL spectra of N-doped ZnO nanowires at temperatures from 10 K to 300 K. The luminescence features of the as-grown (nominally undoped) nanowires are depicted in the inset with two narrow phonon replica peaks of free excitons (FXLO and FX-2LO), separated by the longitudinal optical phonon energy (73 meV). For the N-doped nanowires (120 mins plasma), the dominant feature is a broad, asymmetrical band at 3.24 eV in addition to the D°X emission at 3.35 eV. The luminescence band at 3.24 eV bears close resemblance in peak energy and shape to those observed in Zn-face cplane ZnO epilayers doped with a high concentration of nitrogen ( > 5×1019 cm−3) [22]. This band is relatively independent of temperature up to 40 K but quenched quickly as temperature is increased up to 140 K. Based on the XANES measurements in Fig. 2 and previous correlative investigations [10], this band can be ascribed to a donoracceptor pair (DAP) emission with the shallow acceptor being loosely bound N2. The asymmetrical shape and large half-width of the luminescence band arise from the superposition of the DAP zero phonon line and its phonon replica. This broad band, not previously reported in ZnO nanowires, has been observed in heavily compensated ZnO and ZnSe crystals and explained by fluctuating Coulomb potentials arising from high densities of ionized impurities [22,23]. The recombination energy of the DAP can be written as:
EDAP = Eg –(EA + ED ) − 2γ where Eg, EA and ED are the band gap, acceptor and donor binding energies, respectively, and γ is the amplitude of the potential fluctuations, γ ∝ (N A− + ND+)2/3. With this model, it is apparent increasing 2
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Fig. 4. Raman spectra of as-grown and N-doped ZnO nanowires normalized to the E2(high) mode, showing the monotonic enhancement of the 547 and 580 cm−1 peaks with increasing nitrogen incorporation. The intrinsic ZnO modes at 331, 377 and 437 cm−1 are virtually unaffected by the nitrogen doping. The inset displays the fitted spectrum in the region from 480 to 640 cm−1.
tentatively ascribed to the thermal decay of neutral N2 acceptor. Several scenarios are possible for the configuration of the molecule with increasing temperature: vibrational-rotational excited states or excited electronic states of N2. Fig. 4 shows the Raman spectra of the N-doped ZnO nanowires together with as-grown nanowires for comparison. The peaks at 331, 377, and 437 cm−1 correspond to intrinsic E2(high) – E2(low), A1(TO) and E2(high) modes of wurtzite ZnO [25]; while the peaks at 417 and 644 cm−1 can be attributed to the a-plane sapphire substrate [26]. The Raman modes appearing at 548 and 580 cm−1 are strongly enhanced with increasing nitrogen concentration, suggesting that these modes are related to nitrogen. Kaschner et al. [27] observed the 275 and 580 cm−1 Raman peaks in the ZnO film doped with nitrogen and assigned them, together with 644 cm−1, to nitrogen-related vibrational modes. Here, in contrast, the intensities of the 275 cm−1 and 644 cm−1 modes in the N-doped nanowires do not change with increasing nitrogen doping level. Let us now discuss the origin of the observed Raman modes in the nanowires. As shown in the inset of Fig. 2, prolonged plasma time increases the N2 concentration substantially, whereas the concentrations of NO and NZn species are almost unaffected. It is therefore anticipated that the enhanced Raman modes in the N-doped nanowires are somehow related to the formation of N2 acceptors in the nanowires. Fig. 5 shows the integrated Raman intensities, obtained from Lorentzian curve fitting to the Raman peaks after normalizing to the E2(high) mode, plotted as a function of N2 concentration. The Raman modes at 275 and 644 cm−1 are found to be independent of the nitrogen level, thus they are not related to nitrogen dopants or defects that are induced by the nitrogen doping. On the other hand, the intensities of the 547 and 580 cm−1 Raman modes increase in proportion to the N2 concentration. The Raman peak at 580 cm−1 (that is at a distance equal to the LO phonon energy of 72 meV from the laser excitation line) is close to the E1(LO) mode at 584 cm−1 of bulk ZnO [25]. A quasi-LO mode between E1(LO) and A1(LO) has been reported in randomly oriented ZnO nanowires [28], but it is unlikely to be the origin of the 580 cm−1 peak since our nanowires are strongly oriented along the c-axis orientation as revealed by X-ray diffraction (not shown). Although the E1(LO) Raman mode is forbidden in the backscattering geometry, impurity-induced scattering allows the detection of this LO phonon mode. Wang et al. [11] observed the broadening of the 575 cm−1 Raman peak with enhanced N+ implantation, which
Fig. 3. (a) Temperature-resolved CL spectra of N-doped ZnO nanowires (EB=5 keV, IB=1.2 nA). The spectra are vertically shifted for clarity. The dashed curves indicate the peak positions of D°X, FX-LO and DAP. The N-related DAP exhibits a significant redshift with respect to the D°X with increasing temperature from 10 K. Inset displays the typical luminescence features (D°X, FX-LO and FX-2LO) of the as-grown (undoped) ZnO nanowires. (b) The Arrhenius plot of the DAP emission yields an activation energy of 9.0 ± 0.4 meV for temperatures below 60 K and 20.0 ± 0.5 meV above 60 K.
temperature has significant effects on the shape and energy position of the DAP band. At higher temperatures, more acceptors and donors are thermally ionized, leading to a redshift of the DAP band with respect to the band-edge emission. Evidently, the DAP and D°X peaks are separated by 94 meV at 10 K, while at 100 K the separation is 116 meV (Fig. 3a). The temperature-resolved spectra show that the DAP loses its intensity rapidly with increasing temperature above 60 K. Fig. 3(b) displays the corresponding Arrhenius plot of the DAP emission intensity. The Arrhenius fitting of the data yields an activation energy of EA1=9.0 ± 0.4 meV for T < 60 K and EA2=20 ± 0.5 meV for T > 60 K. While the chemical origin of the shallow donor in the DAP is unclear at this stage, the exceptionally small EA1 could be related to the strength of the localization of electrons at donor-like surface defects caused by the plasma [24]. An activation energy of 20 ± 0.5 meV is 3
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attributed to strong Fröhlich interaction mediated by N2 acceptors. This mode could potentially be used as an indicator for nitrogen acceptors in ZnO nanostructures. Acknowledgement This research was supported under Australian Research Council’s Discovery Projects funding scheme (Project no. DP150103317). The synchrotron work was undertaken on the Soft X-ray Spectroscopy beamline at the Australian Synchrotron, Australia. We wish to acknowledge the technical assistance of Drs Bruce Cowie, Anton Tadich and Lars Thomsen. References [1] X.D. Wang, C.J. Summers, Z.L. Wang, Large-scale hexagonal-patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays, Nano Lett. 4 (3) (2004) 423–426. [2] G.D. Yuan, W.J. Zhang, J.S. Jie, X. Fan, J.A. Zapien, Y.H. Leung, L.B. Luo, P.F. Wang, C.S. Lee, S.T. Lee, p-type ZnO nanowire arrays, Nano Lett. 8 (8) (2008) 2591–2597. [3] L. Wang, N. Giles, Determination of the ionization energy of nitrogen acceptors in zinc oxide using photoluminescence spectroscopy, Appl. Phys. Lett. 84 (2004) 3049. [4] H.Q. Le, S. Tripathy, S.J. Chua, The influence of nitrogen plasma treatment on the lattice vibrational properties of hydrothermally grown ZnO nanorods, Appl. Phys. Lett. 92 (14) (2008) 141910. [5] W. Park, Y. Jun, S. Jung, G.C. Yi, Excitonic emissions observed in ZnO single crystal nanorods, Appl. Phys. Lett. 82 (2003) 964. [6] M. Tarun, M.Z. Iqbal, M. McCluskey, Nitrogen is a deep acceptor in ZnO, AIP Adv. 1 (2) (2011) 022105. [7] E.-C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Compensation mechanism for N acceptors in ZnO, Phys. Rev. B 64 (8) (2001) 085120. [8] P. Fons, H. Tampo, A.V. Kolobov, M. Ohkubo, S. Niki, J. Tominaga, R. Carboni, F. Boscherini, S. Friedrich, Direct observation of nitrogen location in molecular beam epitaxy grown nitrogen-doped ZnO, Phys. Rev. Lett. 96 (4) (2006) 45504. [9] W.R.L. Lambrecht, A. Boonchun, Identification of a N-related shallow acceptor and electron paramagnetic resonance center in ZnO: N2+ on the Zn site, Phys. Rev. B 87 (19) (2013) 195207. [10] C. Ton-That, L. Zhu, M.N. Lockrey, M.R. Phillips, B.C.C. Cowie, A. Tadich, L. Thomsen, S. Khachadorian, S. Schlichting, N. Jankowski, A. Hoffmann, Molecular nitrogen acceptors in ZnO nanowires induced by nitrogen plasma annealing, Phys. Rev. B 92 (2) (2015) 024103. [11] J.B. Wang, H.M. Zhong, Z.F. Li, W. Lu, Raman study of N+-implanted ZnO, Appl. Phys. Lett. 88 (10) (2006) 101913. [12] F. Friedrich, M. Gluba, N. Nickel, Identification of nitrogen and zinc related vibrational modes in ZnO, Appl. Phys. Lett. 95 (2009) 141903. [13] A. Kaschner, U. Haboeck, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H. Alves, D. Hofmann, Nitrogen-related local vibrational modes in ZnO: N, Appl. Phys. Lett. 80 (2002) 1909. [14] C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E. Kaidashev, M. Lorenz, M. Grundmann, Raman scattering in ZnO thin films doped with Fe, Sb, Al, Ga, and Li, Appl. Phys. Lett. 83 (2003) 1974. [15] C. Ton-That, M.R. Phillips, M. Foley, S.J. Moody, A.P.J. Stampfl, Surface electronic properties of ZnO nanoparticles, Appl. Phys. Lett. 92 (26) (2008) 261916. [16] C. Ton-That, L. Weston, M.R. Phillips, Characteristics of point defects in the green luminescence from Zn- and O-rich ZnO, Phys. Rev. B 86 (11) (2012) 115205. [17] C.W. Zou, X.D. Yan, J. Han, R.Q. Chen, W. Gao, J. Metson, Study of a nitrogendoped ZnO film with synchrotron radiation, Appl. Phys. Lett. 94 (17) (2009) 171903. [18] M. Petravic, P.N.K. Deenapanray, V.A. Coleman, C. Jagadish, K.J. Kim, B. Kim, K. Koike, S. Sasa, M. Inoue, M. Yano, Chemical states of nitrogen in ZnO studied by near-edge X-ray absorption fine structure and core-level photoemission spectroscopies, Surf. Sci. 600 (7) (2006) L81–L85. [19] G.R. Wight, C.E. Brion, K-shell excitations in NO and O2 by 2.5 keV electron impact, J. Electron Spectrosc. Relat. Phenom. 4 (4) (1974) 313–325. [20] B.J. Ruck, A. Koo, U.D. Lanke, F. Budde, S. Granville, H.J. Trodahl, A. Bittar, J.B. Metson, V.J. Kennedy, A. Markwitz, Quantitative study of molecular N2 trapped in disordered GaN:O films, Phys. Rev. B 70 (23) (2004) 235202. [21] A. Bozanic, Z. Majlinger, M. Petravic, Q. Gao, D. Llewellyn, C. Crotti, Y.W. Yang, Characterization of molecular nitrogen in III–V compound semiconductors by near-edge x-ray absorption fine structure and photoemission spectroscopies, J. Vac. Sci. Technol. A 26 (4) (2008) 592–596. [22] S. Lautenschlaeger, S. Eisermann, G. Haas, E.A. Zolnowski, M.N. Hofmann, A. Laufer, M. Pinnisch, B.K. Meyer, M.R. Wagner, J.S. Reparaz, G. Callsen, A. Hoffmann, A. Chernikov, S. Chatterjee, V. Bornwasser, M. Koch, Optical signatures of nitrogen acceptors in ZnO, Phys. Rev. B 85 (23) (2012) 235204. [23] E. Kurtz, S. Einfeldt, J. Nürnberger, S. Zerlauth, D. Hommel, G. Landwehr, p-Type Doping of ZnSe. On the Properties of Nitrogen in ZnSe:N, Phys. Status Solidi BBasic, Solid State Phys 187 (2) (1995) 393–399. [24] C. Ton-That, L.L.C. Lem, M.R. Phillips, F. Reisdorffer, J. Mevellec, T.P. Nguyen,
Fig. 5. Integrated intensities of the anomalous Raman modes as a function of N2 concentration in the nanowires. The 547 and 580 cm−1 modes increase in proportion to N2 concentration, while the intensities of the 275 and 644 cm−1 peaks are unchanged.
was attributed to the increased density of phonon states. The non-polar E2(high) phonon mode in ZnO arises from the deformation potential, while the polar E1(LO) mode is also subject to the Fröhlich carrierphonon interaction. Hence, the strong enhancement of the 580 cm−1 mode in the N-doped nanowires is attributable to the Fröhlich interaction that is mediated by N2 acceptors. An alternative explanation for the 580 cm−1 enhancement could be that it arises from a few of nanowires with a high defect density. However, this can be ruled out in our samples since the nanowires do not appear to become inferior in quality after the nitrogen doping; the full width at half maximum of the E2(high) mode remains at 6.08 ± 0.18 cm−1, independent of the doping level. It is noteworthy that the Raman mode 580 cm−1 enhances further after annealing the N-doped nanowires in N2 gas at 350 °C. The gas annealing also increases the portion of N2 in the nanowires, providing further evidence that this Raman mode is associated with N2. The Raman mode at 547 cm−1 also exhibits a significant rise in intensity with increasing N2 concentration in the ZnO nanowires. This mode exists in the undoped nanowires but grows and broadens with increasing doping level. Previously the mode at 552 cm−1 has been attributed to disorder-activated silent Raman 2B1(low) mode [29]. Our previous density functional theory calculations of the vibrational modes in N-doped ZnO revealed that the formation of N2 complexes increases the phonon density of states in the spectral range from 400 to 560 cm−1[30]. Considering the strong dependence of the 547 cm−1 mode on the N2 concentration, the enhancement and broadening of this peak in highly doped nanowires could be due to N2 complexes or intrinsic defects induced by the presence of abundant N2. Unequivocal assignment of this Raman mode requires additional study and is a topic for future work. 4. Conclusions In conclusion, we present experimental evidence for the chemical, vibrational and optical signatures of nitrogen in ZnO nanowires, which are doped with various N2 concentrations by remote plasma annealing. Nitrogen in the N-doped nanowires predominantly exists in two chemical states: atomic nitrogen substituting oxygen (NO) and N2 molecules. The CL emission from individual N-doped nanowires reveals a donor-acceptor pair emission at 3.24 eV with the acceptor being a N2 molecule. The intensity of the 580 cm−1 Raman mode shows the strongest enhancement with increasing N2 concentration and is 4
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Nanopart. Res. (2011) 1–6. [28] D.-H. Kim, G.-W. Lee, Y.-C. Kim, Interaction of zinc interstitial with oxygen vacancy in zinc oxide: An origin of n-type doping, Solid State Commun. 152 (18) (2012) 1711–1714. [29] F.J. Manjón, B. Marí, J. Serrano, A.H. Romero, Silent Raman modes in zinc oxide and related nitrides, J. Appl. Phys. 97 (5) (2005) 053516. [30] S. Khachadorian, R. Gillen, C. Ton-That, L.C. Zhu, J. Maultzsch, M.R. Phillips, A. Hoffmann, Revealing the origin of high-energy Raman local mode in nitrogen doped ZnO nanowires, Phys. Status Solidi Rapid Res. Lett. 10 (4) (2016) 334.
C. Nenstiel, A. Hoffmann, Shallow carrier traps in hydrothermal ZnO crystals, New J. Phys. 16 (8) (2014) 083040. [25] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence of Raman scattering in ZnO, Phys. Rev. B 75 (16) (2007) 165202. [26] G. Pezzotti, W. Zhu, Resolving stress tensor components in space from polarized Raman spectra: polycrystalline alumina, Phys. Chem. Chem. Phys. 17 (4) (2015) 2608–2627. [27] L. Bouzaïene, L. Sfaxi, M. Baira, H. Maaref, C. Bru-Chevallier, Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots, J.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Chemical, vibrational and optical signatures of nitrogen in ZnO nanowires L. Zhua, S. Khachadorianb, A. Hoffmannb, M.R. Phillipsa, C. Ton-Thata, a b
⁎
School of Mathematical and Physical Sciences, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
A R T I C L E I N F O
A BS T RAC T
Keywords: ZnO nanowires Nitrogen doping Luminescence Raman modes X-ray absorption
ZnO nanowires with various concentrations of nitrogen molecules have been fabricated by remote plasma annealing. X-ray absorption near-edge spectroscopy (XANES) reveals that nitrogen exists mainly in two chemical states: atomic nitrogen substituting oxygen (NO) and molecular nitrogen (N2) weakly bound to the ZnO lattice; the latter state increases substantially with prolonged plasma time. Cathodoluminescence microanalysis of individual nanowires reveals a broad emission band at 3.24 eV at 10 K, attributable to the recombination of a shallow donor and a N2 acceptor state. The Raman modes at 547 and 580 cm−1 from the Ndoped nanowires are found to rise in proportion to the N2 concentration, indicating they are related to N2 molecules or defects caused by the incorporation of N2 in the nanowires.
1. Introduction One-dimensional ZnO nanowires are a promising material system for various applications in nano-optoelectronic devices, such as shortwavelength light emitting diodes, field emission transistors and ultraviolet lasers [1]. Utilization of nanowires requires a good understanding and control of their properties that are largely affected by intrinsic defects and impurities. The lack of reliable p-type material poses a major challenge to the realization of ZnO nanowire-based nanodevices. Group V elements, especially nitrogen which has a similar radius to oxygen, are believed to be the natural choice for achieving p-type conductivity in ZnO [2,3]. ZnO nanowires possess a large surface-tovolume ratio, which enables facile incorporation of nitrogen at a high concentration, making gas phase ion doping an effective doping method for ZnO nanostructures [4]. Furthermore, the use of nanowires allows control over the type of native surface defects prior to nitrogen incorporation, which can enhance its doping level [5]. Understanding the chemical states of nitrogen dopants and their signatures in ZnO is pivotal in investigating the physics and chemistry of p-type doping in this semiconductor. Previous studies have shown that atomic nitrogen substituting oxygen (NO) is a deep acceptor in ZnO, whereas N2 on an oxygen site (N2)O is regarded as a shallow donor which compensates the p-type doping [6–8]. However, the nature of the N-related acceptor state is complex and to date remains controversial. Recently, it has been proposed that N2 molecule could be accommodated at a Zn site [(N2)Zn], which is predicted to be a double shallow acceptor responsible for the experimentally observed donoracceptor-pair (DAP) transition in N-doped ZnO [9]. Our previous study ⁎
of N-doped nanowires using the X-ray absorption spectroscopy established that nitrogen molecules are formed prevalently during nitrogen plasma doping [10]. Nitrogen-related vibrational modes, in particular those appearing at 275, 510, 582, and 643 cm−1, have been ascribed to nitrogen doping [11–13]. These anomalous modes, on the other hand, were also observed in bulk ZnO doped with other elements such as Fe, Sb and Al [14], which further adds ambiguity to the assignment of nitrogen vibrational modes. The aim of this work is to investigate the relationships between the chemical states of nitrogen and their optical and Raman signatures in ZnO nanowires. It is clear that precise chemical characterization of nitrogen dopants is a prerequisite to the evaluation of ZnO Raman data. 2. Experimental details ZnO nanowires were grown on the a-sapphire substrate via the vapor phase transport method as described in detail previously [15]. Admixture of ZnO and graphite powder was used as the source material. To enhance nitrogen incorporation by vacancy doping [7], nanowires were grown under oxygen-deficient conditions, which lead to an intense defect emission band peaking at 2.47 eV, attributable to VO [16]. Nitrogen doping was conducted by annealing nanowires at 300 °C in radio-frequency (RF) nitrogen plasma with 230 V cathode bias. The combination of abundant nitrogen radicals and the lowtemperature annealing enabled the nanowires to be efficiently doped with nitrogen without significant damage to their crystal structure and homogeneous distribution of the dopants. The nanowires were characterized using an FEI Quanta 200 Scanning Electron Microscope
Corresponding author. E-mail address: [email protected] (C. Ton-That).
http://dx.doi.org/10.1016/j.mssp.2016.11.038 Received 11 September 2016; Received in revised form 16 November 2016; Accepted 23 November 2016 Available online xxxx 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhu, L., Materials Science in Semiconductor Processing (2016), http://dx.doi.org/10.1016/j.mssp.2016.11.038
Materials Science in Semiconductor Processing xx (xxxx) xxxx–xxxx
L. Zhu et al.
Fig. 1. SEM image of N-doped ZnO nanowires on an a-plane sapphire substrate. The nanowires exhibit a regular hexagonal cross section and a smooth c-plane tip surface. No changes in the nanowire morphology were observed after remote plasma annealing. Inset displays the SEM image of one nanowire with a diameter of ~100 nm.
(SEM) with attached cathodoluminescence (CL) spectrometer. X-ray Absorption Near Edge Spectroscopy (XANES), implemented in the Total Fluorescence Yield (TFY) mode, was performed on the Soft X-ray Spectroscopy beamline at the Australian Synchrotron. The incident Xray beam was parallel to the c-axis of the nanowires. The photon energy scale was calibrated against the Au 4f7/2 peak at 84 eV from a clean gold film in electrical contact with the sample. Raman spectroscopy was conducted in backscattering geometry using a LabRAM HR800 spectrometer (Horiba Jobin Yvon) with the 457.48 nm laser excitation line. The laser power on the sample was set at 1 mW. 3. Results and discussion Fig. 1 shows the N-doped ZnO nanowires grown on a-sapphire substrate. The single-crystal ZnO nanowires are preferentially grown along the crystallographic [0001] direction of the wurtzite crystal and possess hexagonal tip facets. The nanowires have a diameter in the range of 120–250 nm and the typical length varies from 1 to 1.5 µm. The inset of Fig. 1 is the close-up view of a truncated nanowire removed from the substrate. There were no observable changes in the nanowire morphology after the nitrogen doping. Fig. 2 shows the N K-edge XANES spectra of the undoped and N-doped ZnO nanowires, which represent resonant electron transitions from the N 1 s core state to the final unoccupied N-related states of p-symmetry [8]. The spectra can be decomposed into Lorentzian components corresponding to three chemical states of nitrogen: P1 at 400.1 eV, P2 at 400.7 eV and P3 at 404.5 eV. The solid lines represent the fitted results of the XANES data points. The main component P1, associated with atomic nitrogen substituting oxygen (NO) in the lattice, is the transition of a 1s electron to 2pπ* state in the hybridized N-Zn orbital [17,18]. The P2 component arises from the 1s→π* transition in the N-N bond [19], which is identified as the signature of N2 molecules. This N2-related resonance at 400.7 eV has previously been observed in N-doped ZnO [18], GaN [20] and InN [21]. XANES is highly sensitive to the bonding arrangement of dopants; the similar XANES characteristics of N2 in various compounds strongly suggest that this molecular species is bound weakly to the host material. The P3 energy is identical to the signature absorption peak of N2O gas and has been assigned as the N 1s to 2pσ* transition in N-O species [17], hence the P3 peak is attributable to atomic N at Zn sites (NZn). The N2 concentration grows substantially with plasma time, indicating more nitrogen present as molecular species, whereas NO and NZn concentrations are virtually unchanged (inset of Fig. 2). N2 has a lower formation energy than NO [9], and given the high-temperature ballistic transport of atoms in nanowires, the highly mobile N atoms are anticipated to form pairs with increasing doping concentration.
Fig. 2. N K-edge XANES spectra of the ZnO nanowires that were nitrogen plasma annealed for different time periods. No nitrogen was detectable in the as-grown nanowires (bottom spectrum). The spectra were fitted with three Lorentzian functions (labelled as P1, P2 and P3) associated with three nitrogen chemical states. The inset shows the XANES integrated intensities of P1, P2 and P3 states as a function of plasma time.
Fig. 3 displays the temperature-dependent CL spectra of N-doped ZnO nanowires at temperatures from 10 K to 300 K. The luminescence features of the as-grown (nominally undoped) nanowires are depicted in the inset with two narrow phonon replica peaks of free excitons (FXLO and FX-2LO), separated by the longitudinal optical phonon energy (73 meV). For the N-doped nanowires (120 mins plasma), the dominant feature is a broad, asymmetrical band at 3.24 eV in addition to the D°X emission at 3.35 eV. The luminescence band at 3.24 eV bears close resemblance in peak energy and shape to those observed in Zn-face cplane ZnO epilayers doped with a high concentration of nitrogen ( > 5×1019 cm−3) [22]. This band is relatively independent of temperature up to 40 K but quenched quickly as temperature is increased up to 140 K. Based on the XANES measurements in Fig. 2 and previous correlative investigations [10], this band can be ascribed to a donoracceptor pair (DAP) emission with the shallow acceptor being loosely bound N2. The asymmetrical shape and large half-width of the luminescence band arise from the superposition of the DAP zero phonon line and its phonon replica. This broad band, not previously reported in ZnO nanowires, has been observed in heavily compensated ZnO and ZnSe crystals and explained by fluctuating Coulomb potentials arising from high densities of ionized impurities [22,23]. The recombination energy of the DAP can be written as:
EDAP = Eg –(EA + ED ) − 2γ where Eg, EA and ED are the band gap, acceptor and donor binding energies, respectively, and γ is the amplitude of the potential fluctuations, γ ∝ (N A− + ND+)2/3. With this model, it is apparent increasing 2
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Fig. 4. Raman spectra of as-grown and N-doped ZnO nanowires normalized to the E2(high) mode, showing the monotonic enhancement of the 547 and 580 cm−1 peaks with increasing nitrogen incorporation. The intrinsic ZnO modes at 331, 377 and 437 cm−1 are virtually unaffected by the nitrogen doping. The inset displays the fitted spectrum in the region from 480 to 640 cm−1.
tentatively ascribed to the thermal decay of neutral N2 acceptor. Several scenarios are possible for the configuration of the molecule with increasing temperature: vibrational-rotational excited states or excited electronic states of N2. Fig. 4 shows the Raman spectra of the N-doped ZnO nanowires together with as-grown nanowires for comparison. The peaks at 331, 377, and 437 cm−1 correspond to intrinsic E2(high) – E2(low), A1(TO) and E2(high) modes of wurtzite ZnO [25]; while the peaks at 417 and 644 cm−1 can be attributed to the a-plane sapphire substrate [26]. The Raman modes appearing at 548 and 580 cm−1 are strongly enhanced with increasing nitrogen concentration, suggesting that these modes are related to nitrogen. Kaschner et al. [27] observed the 275 and 580 cm−1 Raman peaks in the ZnO film doped with nitrogen and assigned them, together with 644 cm−1, to nitrogen-related vibrational modes. Here, in contrast, the intensities of the 275 cm−1 and 644 cm−1 modes in the N-doped nanowires do not change with increasing nitrogen doping level. Let us now discuss the origin of the observed Raman modes in the nanowires. As shown in the inset of Fig. 2, prolonged plasma time increases the N2 concentration substantially, whereas the concentrations of NO and NZn species are almost unaffected. It is therefore anticipated that the enhanced Raman modes in the N-doped nanowires are somehow related to the formation of N2 acceptors in the nanowires. Fig. 5 shows the integrated Raman intensities, obtained from Lorentzian curve fitting to the Raman peaks after normalizing to the E2(high) mode, plotted as a function of N2 concentration. The Raman modes at 275 and 644 cm−1 are found to be independent of the nitrogen level, thus they are not related to nitrogen dopants or defects that are induced by the nitrogen doping. On the other hand, the intensities of the 547 and 580 cm−1 Raman modes increase in proportion to the N2 concentration. The Raman peak at 580 cm−1 (that is at a distance equal to the LO phonon energy of 72 meV from the laser excitation line) is close to the E1(LO) mode at 584 cm−1 of bulk ZnO [25]. A quasi-LO mode between E1(LO) and A1(LO) has been reported in randomly oriented ZnO nanowires [28], but it is unlikely to be the origin of the 580 cm−1 peak since our nanowires are strongly oriented along the c-axis orientation as revealed by X-ray diffraction (not shown). Although the E1(LO) Raman mode is forbidden in the backscattering geometry, impurity-induced scattering allows the detection of this LO phonon mode. Wang et al. [11] observed the broadening of the 575 cm−1 Raman peak with enhanced N+ implantation, which
Fig. 3. (a) Temperature-resolved CL spectra of N-doped ZnO nanowires (EB=5 keV, IB=1.2 nA). The spectra are vertically shifted for clarity. The dashed curves indicate the peak positions of D°X, FX-LO and DAP. The N-related DAP exhibits a significant redshift with respect to the D°X with increasing temperature from 10 K. Inset displays the typical luminescence features (D°X, FX-LO and FX-2LO) of the as-grown (undoped) ZnO nanowires. (b) The Arrhenius plot of the DAP emission yields an activation energy of 9.0 ± 0.4 meV for temperatures below 60 K and 20.0 ± 0.5 meV above 60 K.
temperature has significant effects on the shape and energy position of the DAP band. At higher temperatures, more acceptors and donors are thermally ionized, leading to a redshift of the DAP band with respect to the band-edge emission. Evidently, the DAP and D°X peaks are separated by 94 meV at 10 K, while at 100 K the separation is 116 meV (Fig. 3a). The temperature-resolved spectra show that the DAP loses its intensity rapidly with increasing temperature above 60 K. Fig. 3(b) displays the corresponding Arrhenius plot of the DAP emission intensity. The Arrhenius fitting of the data yields an activation energy of EA1=9.0 ± 0.4 meV for T < 60 K and EA2=20 ± 0.5 meV for T > 60 K. While the chemical origin of the shallow donor in the DAP is unclear at this stage, the exceptionally small EA1 could be related to the strength of the localization of electrons at donor-like surface defects caused by the plasma [24]. An activation energy of 20 ± 0.5 meV is 3
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attributed to strong Fröhlich interaction mediated by N2 acceptors. This mode could potentially be used as an indicator for nitrogen acceptors in ZnO nanostructures. Acknowledgement This research was supported under Australian Research Council’s Discovery Projects funding scheme (Project no. DP150103317). The synchrotron work was undertaken on the Soft X-ray Spectroscopy beamline at the Australian Synchrotron, Australia. We wish to acknowledge the technical assistance of Drs Bruce Cowie, Anton Tadich and Lars Thomsen. References [1] X.D. Wang, C.J. Summers, Z.L. Wang, Large-scale hexagonal-patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays, Nano Lett. 4 (3) (2004) 423–426. [2] G.D. Yuan, W.J. Zhang, J.S. Jie, X. Fan, J.A. Zapien, Y.H. Leung, L.B. Luo, P.F. Wang, C.S. Lee, S.T. Lee, p-type ZnO nanowire arrays, Nano Lett. 8 (8) (2008) 2591–2597. [3] L. Wang, N. Giles, Determination of the ionization energy of nitrogen acceptors in zinc oxide using photoluminescence spectroscopy, Appl. Phys. Lett. 84 (2004) 3049. [4] H.Q. Le, S. Tripathy, S.J. Chua, The influence of nitrogen plasma treatment on the lattice vibrational properties of hydrothermally grown ZnO nanorods, Appl. Phys. Lett. 92 (14) (2008) 141910. [5] W. Park, Y. Jun, S. Jung, G.C. Yi, Excitonic emissions observed in ZnO single crystal nanorods, Appl. Phys. Lett. 82 (2003) 964. [6] M. Tarun, M.Z. Iqbal, M. McCluskey, Nitrogen is a deep acceptor in ZnO, AIP Adv. 1 (2) (2011) 022105. [7] E.-C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Compensation mechanism for N acceptors in ZnO, Phys. Rev. B 64 (8) (2001) 085120. [8] P. Fons, H. Tampo, A.V. Kolobov, M. Ohkubo, S. Niki, J. Tominaga, R. Carboni, F. Boscherini, S. Friedrich, Direct observation of nitrogen location in molecular beam epitaxy grown nitrogen-doped ZnO, Phys. Rev. Lett. 96 (4) (2006) 45504. [9] W.R.L. Lambrecht, A. Boonchun, Identification of a N-related shallow acceptor and electron paramagnetic resonance center in ZnO: N2+ on the Zn site, Phys. Rev. B 87 (19) (2013) 195207. [10] C. Ton-That, L. Zhu, M.N. Lockrey, M.R. Phillips, B.C.C. Cowie, A. Tadich, L. Thomsen, S. Khachadorian, S. Schlichting, N. Jankowski, A. Hoffmann, Molecular nitrogen acceptors in ZnO nanowires induced by nitrogen plasma annealing, Phys. Rev. B 92 (2) (2015) 024103. [11] J.B. Wang, H.M. Zhong, Z.F. Li, W. Lu, Raman study of N+-implanted ZnO, Appl. Phys. Lett. 88 (10) (2006) 101913. [12] F. Friedrich, M. Gluba, N. Nickel, Identification of nitrogen and zinc related vibrational modes in ZnO, Appl. Phys. Lett. 95 (2009) 141903. [13] A. Kaschner, U. Haboeck, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H. Alves, D. Hofmann, Nitrogen-related local vibrational modes in ZnO: N, Appl. Phys. Lett. 80 (2002) 1909. [14] C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E. Kaidashev, M. Lorenz, M. Grundmann, Raman scattering in ZnO thin films doped with Fe, Sb, Al, Ga, and Li, Appl. Phys. Lett. 83 (2003) 1974. [15] C. Ton-That, M.R. Phillips, M. Foley, S.J. Moody, A.P.J. Stampfl, Surface electronic properties of ZnO nanoparticles, Appl. Phys. Lett. 92 (26) (2008) 261916. [16] C. Ton-That, L. Weston, M.R. Phillips, Characteristics of point defects in the green luminescence from Zn- and O-rich ZnO, Phys. Rev. B 86 (11) (2012) 115205. [17] C.W. Zou, X.D. Yan, J. Han, R.Q. Chen, W. Gao, J. Metson, Study of a nitrogendoped ZnO film with synchrotron radiation, Appl. Phys. Lett. 94 (17) (2009) 171903. [18] M. Petravic, P.N.K. Deenapanray, V.A. Coleman, C. Jagadish, K.J. Kim, B. Kim, K. Koike, S. Sasa, M. Inoue, M. Yano, Chemical states of nitrogen in ZnO studied by near-edge X-ray absorption fine structure and core-level photoemission spectroscopies, Surf. Sci. 600 (7) (2006) L81–L85. [19] G.R. Wight, C.E. Brion, K-shell excitations in NO and O2 by 2.5 keV electron impact, J. Electron Spectrosc. Relat. Phenom. 4 (4) (1974) 313–325. [20] B.J. Ruck, A. Koo, U.D. Lanke, F. Budde, S. Granville, H.J. Trodahl, A. Bittar, J.B. Metson, V.J. Kennedy, A. Markwitz, Quantitative study of molecular N2 trapped in disordered GaN:O films, Phys. Rev. B 70 (23) (2004) 235202. [21] A. Bozanic, Z. Majlinger, M. Petravic, Q. Gao, D. Llewellyn, C. Crotti, Y.W. Yang, Characterization of molecular nitrogen in III–V compound semiconductors by near-edge x-ray absorption fine structure and photoemission spectroscopies, J. Vac. Sci. Technol. A 26 (4) (2008) 592–596. [22] S. Lautenschlaeger, S. Eisermann, G. Haas, E.A. Zolnowski, M.N. Hofmann, A. Laufer, M. Pinnisch, B.K. Meyer, M.R. Wagner, J.S. Reparaz, G. Callsen, A. Hoffmann, A. Chernikov, S. Chatterjee, V. Bornwasser, M. Koch, Optical signatures of nitrogen acceptors in ZnO, Phys. Rev. B 85 (23) (2012) 235204. [23] E. Kurtz, S. Einfeldt, J. Nürnberger, S. Zerlauth, D. Hommel, G. Landwehr, p-Type Doping of ZnSe. On the Properties of Nitrogen in ZnSe:N, Phys. Status Solidi BBasic, Solid State Phys 187 (2) (1995) 393–399. [24] C. Ton-That, L.L.C. Lem, M.R. Phillips, F. Reisdorffer, J. Mevellec, T.P. Nguyen,
Fig. 5. Integrated intensities of the anomalous Raman modes as a function of N2 concentration in the nanowires. The 547 and 580 cm−1 modes increase in proportion to N2 concentration, while the intensities of the 275 and 644 cm−1 peaks are unchanged.
was attributed to the increased density of phonon states. The non-polar E2(high) phonon mode in ZnO arises from the deformation potential, while the polar E1(LO) mode is also subject to the Fröhlich carrierphonon interaction. Hence, the strong enhancement of the 580 cm−1 mode in the N-doped nanowires is attributable to the Fröhlich interaction that is mediated by N2 acceptors. An alternative explanation for the 580 cm−1 enhancement could be that it arises from a few of nanowires with a high defect density. However, this can be ruled out in our samples since the nanowires do not appear to become inferior in quality after the nitrogen doping; the full width at half maximum of the E2(high) mode remains at 6.08 ± 0.18 cm−1, independent of the doping level. It is noteworthy that the Raman mode 580 cm−1 enhances further after annealing the N-doped nanowires in N2 gas at 350 °C. The gas annealing also increases the portion of N2 in the nanowires, providing further evidence that this Raman mode is associated with N2. The Raman mode at 547 cm−1 also exhibits a significant rise in intensity with increasing N2 concentration in the ZnO nanowires. This mode exists in the undoped nanowires but grows and broadens with increasing doping level. Previously the mode at 552 cm−1 has been attributed to disorder-activated silent Raman 2B1(low) mode [29]. Our previous density functional theory calculations of the vibrational modes in N-doped ZnO revealed that the formation of N2 complexes increases the phonon density of states in the spectral range from 400 to 560 cm−1[30]. Considering the strong dependence of the 547 cm−1 mode on the N2 concentration, the enhancement and broadening of this peak in highly doped nanowires could be due to N2 complexes or intrinsic defects induced by the presence of abundant N2. Unequivocal assignment of this Raman mode requires additional study and is a topic for future work. 4. Conclusions In conclusion, we present experimental evidence for the chemical, vibrational and optical signatures of nitrogen in ZnO nanowires, which are doped with various N2 concentrations by remote plasma annealing. Nitrogen in the N-doped nanowires predominantly exists in two chemical states: atomic nitrogen substituting oxygen (NO) and N2 molecules. The CL emission from individual N-doped nanowires reveals a donor-acceptor pair emission at 3.24 eV with the acceptor being a N2 molecule. The intensity of the 580 cm−1 Raman mode shows the strongest enhancement with increasing N2 concentration and is 4
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Nanopart. Res. (2011) 1–6. [28] D.-H. Kim, G.-W. Lee, Y.-C. Kim, Interaction of zinc interstitial with oxygen vacancy in zinc oxide: An origin of n-type doping, Solid State Commun. 152 (18) (2012) 1711–1714. [29] F.J. Manjón, B. Marí, J. Serrano, A.H. Romero, Silent Raman modes in zinc oxide and related nitrides, J. Appl. Phys. 97 (5) (2005) 053516. [30] S. Khachadorian, R. Gillen, C. Ton-That, L.C. Zhu, J. Maultzsch, M.R. Phillips, A. Hoffmann, Revealing the origin of high-energy Raman local mode in nitrogen doped ZnO nanowires, Phys. Status Solidi Rapid Res. Lett. 10 (4) (2016) 334.
C. Nenstiel, A. Hoffmann, Shallow carrier traps in hydrothermal ZnO crystals, New J. Phys. 16 (8) (2014) 083040. [25] R. Cuscó, E. Alarcón-Lladó, J. Ibáñez, L. Artús, J. Jiménez, B. Wang, M.J. Callahan, Temperature dependence of Raman scattering in ZnO, Phys. Rev. B 75 (16) (2007) 165202. [26] G. Pezzotti, W. Zhu, Resolving stress tensor components in space from polarized Raman spectra: polycrystalline alumina, Phys. Chem. Chem. Phys. 17 (4) (2015) 2608–2627. [27] L. Bouzaïene, L. Sfaxi, M. Baira, H. Maaref, C. Bru-Chevallier, Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots, J.
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