Luminescence spectroscopy of excitons in Zn1−xNixO oxides

Author’s Accepted Manuscript Luminescence spectroscopy of excitons in Zn1xNixO oxides V.N. Churmanov, V.I. Sokolov, V.A. Pustovarov, N.B. Gruzdev, V.Yu. Ivanov www.elsevier.com/locate/physb

PII: DOI: Reference:

S0921-4526(17)30861-X https://doi.org/10.1016/j.physb.2017.10.122 PHYSB310479

To appear in: Physica B: Physics of Condensed Matter Received date: 30 June 2017 Revised date: 22 October 2017 Accepted date: 30 October 2017 Cite this article as: V.N. Churmanov, V.I. Sokolov, V.A. Pustovarov, N.B. Gruzdev and V.Yu. Ivanov, Luminescence spectroscopy of excitons in Zn 1oxides, Physica B: Physics of Condensed Matter, xNixO https://doi.org/10.1016/j.physb.2017.10.122 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Luminescence spectroscopy of excitons in Zn1-xNixO oxides V.N. Churmanova1, V.I. Sokolovb, V.A. Pustovarova, N.B. Gruzdevb, V.Yu. Ivanova a

Ural Federal University, Mira Street 19,620002 Yekaterinburg, Russia M.N. Miheev Institute of Metal Physics of Ural Branch of Russian Academy of Science, S. Kovalevskaya Str. 18, 620990, Yekaterinburg, Russia b

[email protected] Abstracts The paper presents the results of the study of two narrow luminescence lines I1 and I2 at the energies of 3.339 and 3.393 eV respectively in solid-state solutions Zn1-xNixO. The method of time-resolved luminescence spectroscopy with sub-nanosecond time resolution upon XUV excitation allows us to make a comparative analysis of nature of discovered lines. We consider the origin of narrow lines I1 and I2 as a radiative recombination of different excitons in Zn1-xNixO: I1-line is Wannier-Mott exciton, I2-line – p-d charge-transfer exciton. Noticeable differences of Wannier-Mott and p-d exciton properties are thoroughly discussed.

Keywords: NiO, excitons, x-ray induced luminescence

1.

Introduction Transition metal oxides are perspective materials for the creation of various types of

optoelectronic and spintronic devices. But comprehension of the energy spectrum of oxide compounds with 3d-transition metals (for instance, NiO, CoO) remains one of the unresolved tasks of physics of binary oxides. There is still no coherent conception of the structure of energy spectrum NiO and CoO. Research of excitons is an essential part of a detailed survey of these materials. Solid solutions of these oxide compounds can give new information about excitons. Previous studies of solid solutions Zn1-xNixO [1-4] reveal two narrow lines I1 and I2 at the low-temperature (T = 8K) X-ray spectrum and obtain temperature dependences of these lines for solid solution Zn0.4Ni0.6O at the temperature range of 8–50 K. It is noted a strong influence of temperature on I1 and I2 lines (shift and broadening of lines, change of the relation of maximal intensities with increasing temperature) and it is showed that such behavior is similar to the influence of the temperature on donor and acceptor excitons [dn+1h] in II-VI compounds doped with 3d elements (for instance, ZnO: Ni; ZnSe: Ni) [5]. Discovered facts allowed us to assume an origin of narrow lines I1 and I2 as a radiative annihilation of p-d charge-transfer excitons. The 1

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difference between the photoluminescence (PL) decay kinetics of the lines for various temperatures led us to an idea of diverse physical nature of I1 and I2 lines. In this paper, we examine X-ray luminescence spectra of several solutions Zn1-xNixO in the region of I1 and I2 lines upon various photon excitation energies in the spectral region of absorption edges of lattice atoms. The main goal of this work is to survey possible mechanisms of charge-carrier relaxation and radiative recombination using luminescence spectroscopy with sub-nanosecond time resolution upon soft X-ray excitation in the range of resonant energies of lattice atoms. New theoretical data enables us to consider the origin of narrow lines I1 and I2 as a radiative annihilation of the [eh] exciton (Wannier-Mott exciton) and p-d charge-transfer exciton at NiO and ternary solid solutions Zn1-xNixO.

2. Experiment The PL measurements were made on the samples of NiO and solid solutions of Zn1-xNixO with rock salt crystal structure. As starting material we have used the commercially available powder of NiO (99%; Prolabo) and ZnO (99.99%; Alfa Aesar) which has been pressed into pellets under pressure of about 1250 bar and placed into gold capsules. Quenching experiments at 7.7 GPa and 1000–1100 K have been performed using a toroid-type high-pressure apparatus. Details of experimental technique and calibration are described elsewhere [6]. Electron microscopy analysis shows the samples to be dense poreless oxide ceramics with rock salt cubic structure and grain size of about 10–20 μm. The present study was carried out by the means of the low-temperature luminescence extremely ultraviolet (XUV) spectroscopy with the time resolution. The time-resolved PL spectra, as well as the PL decay kinetics under XUV excitation, have been measured at T = 8 K using synchrotron radiation (SR) from the BW3 beamline (HASYLAB (DESY), Hamburg). The SR from the undulator was further monochromatized by a Zeiss SX700 monochromator. The PL emission spectra were measured by a 0.4 m vacuum monochromator (Seya-Namioka scheme) equipped with a microchannel plate-photomultiplier (MCP 1645, Hamamatsu). We have recorded the time-resolved spectra in the integral window, fast and slow Δt-wide temporal windows delayed by δt relative to the excitation pulse front. The parameters of fast time window: δt = 0.1 ns, Δt = 5.7 ns. The parameters of slow time window: δt = 58 ns, Δt = 14 ns. The temporal resolution of the whole detection system was 250 ps. The temporary interval between SR excitation pulses is equal 96 ns. The PL emission spectra were not corrected to the transmission of the detection system. 2

3. Results and Discussion Figure 1 shows the results of the study of X-ray luminescence of Zn0.4Ni0.6O solid solution and NiO (on inset) upon selective excitation by XUV-photons in the region of ultrasoft X-ray 130 eV and 850 eV. Such excitation energy generates the electron at the conduction band and the hole in the inner shell. After that generated core hole goes up to the valence band as a result of x-ray fluorescence or Auger process. Finally, it is formed a hole at the top of the valence band and relaxed electron at the bottom of the conduction band. At the excitation energy of Eexc= 130 eV the probing depth of the synchrotron radiation amounts 0.02 μm, at Eexc= 850 eV it becomes much substantial with the depth of 0.43 μm [7]. In solid solutions NixZn1-xO, electron-hole pairs originate at a depth approximately 0.05 μm from the surface upon optical interband excitation [7]. Upon the optical excitation, the defectiveness has the biggest degree close to the surface and it diminishes significantly upon the excitation with the energy of Eexc= 850 eV. The decay of I1 line upon the excitation at 130 eV is slower (τ ~ 0.6 ns) than the decay of I2 line. The relaxation rate of core hole to the top of the valence band is estimated significantly less. The sub-nanosecond decay time of the obtained PL decay kinetics enables to attribute the origin of radiation recombination only to electrons and holes at the bottom of the conduction band and the top of the valence band, respectively. The process of excitation can also induce d-d charge transfer transitions with the creation of Ni+ (d 9) and Ni+3 (d 7) ions. d 9-states form the conduction band whereas d 7-states locate deeply at the valence band. The holes on the d-states will be raised to the valence band with the same times as generated upon

X-

ray excitation core holes. Thus d-d charge transfer transitions cannot appear at the sub-nanosecond or nanosecond radiation recombination. Figure 2 presents X-ray luminescence spectra of Zn0.8Ni0.2O solid solution for different excitation energy. One can clearly see that there is no energy shift of I1 and I2-lines for different compositions of Zn1-xNixO. Such experimental fact demonstrates that band gap energy doesn’t depend on the concentration of Zn-ions for Zn1-xNixO solid solutions. Such invariant behavior of energy gap is proven in recent results of EXAFS studies [8]. According to this work, the interatomic distances Ni–O are almost independent of the composition of the solid solutions Zn1-xNixO (0.2 ≤ x≤1.0) with the rock-salt crystal structure and, therefore, the electrostatic interaction for a Ni–O6 cluster is of the same type. That is why the energy states both for the d8 configuration of Ni2+ ion and the charge transfer d9+h should have similar values in solid solutions Zn1-xNixO. According to theoretical calculations [9, 10] of NiO and Zn1-xNixO band structure, valence and conduction bands are formed from hybridized O 2p and Ni 3d states. Figure 3 shows the theoretical results of orbitally resolved O 2p and Ni 3d spectral densities [10]. Here one can see that the top of the NiO valence band is formed from O 2p and Ni t2g states. The width of the valence 3

band is 7.5 eV. The bottom of conduction band consists of two parts: mainly Ni eg character and a minor contribution from O 2p state. The increase of the Zn concentration leads mainly to a reduction in the number of unoccupied 3d electrons originating from Ni. And, consequently, the height of the eg conduction band peak is decreased. Based on it, we consider the origin of narrow lines I1 and I2 as a radiative recombination of different excitons in Zn1-xNixO: the I1-line is [eh] Wannier-Mott exciton, the more intensive I2-line is p-d charge-transfer exciton. The [eh] exciton is a Coulomb bound formation with an electron at Ni p-conduction band [9,10] and a hole at the p-valence band, whereas p-d exciton is a Coulomb-bound formation with an electron at Ni eg-conduction band and p-hole at the valence band. Calculations of NiO energy spectrum supply information that the valence band is likely to be formed by the p-states of oxygen ions and the d-states of the nearest nickel ions, situated at a distance of 2.09 Å; and the valence band is seen to have a large energy width. The conduction band is formed by the weak overlap of the d-states of the neighboring nickel ions situated at a distance of 3.62 Å, and it has a significantly smaller width as compared to the valence band [10]. These facts indirectly confirm the experimental result, proving that the mobility of holes is greater than the mobility of electrons. At present, there are no reliable data available on the motion of electron upon the conduction band. After the interband transition, the ninth d-electron appears on the Ni1+ site, and the hole moves over the oxygen ions in the Coulomb field of a negatively charged center. We name such electron-hole pair on the NiO fundamental absorption edge as a p-d charge transfer exciton and denote it as {d9h}. The braces indicate the Coulomb attraction. Earlier in Ni-doped II-VI compounds (for example, ZnSe: Ni, ZnO: Ni), p-d charge transfer excitons (or acceptor excitons) [d9h] were actively studied. In that case, the ninth electron was localized at the impurity center, and the hole was moving in the Coulomb field of a negatively charged center [5, 11, 12]. The square brackets also indicate the Coulomb attraction. A very strong temperature effect on acceptor excitons has been established. Acceptor excitons [d9h] are clearly observable at a temperature T = 4.2 K and can hardly be detected at a temperature T = 50 K. In ZnSe the energy of shallow hydrogen-like acceptors is 120 meV. Hence the disappearance of acceptor exciton lines in ZnSe: Ni cannot be explained by the thermal broadening of acceptor excitons due to coupling with phonons. The abnormally strong temperature dependence of the acceptor exciton [d9h] is described by non-radiating tunneling annihilation in the model of configuration curves. Thermally stimulated tunneling occurs between the configuration curves of acceptor exciton and an appropriated d8 energy level with a small saddle-point energy ε0. The tunneling efficiency may depend on the number of the final states, their symmetry and multiplicity [5, 13]. In principle, similar considerations can be applied for describing a strong temperature dependence of the charge transfer excitons {d9h} in NiO. According to the calculations, there may be excited states 3T1g (3P) and 1T1g 4

(1G), existing near the energy of {d9h} exciton [14]. Therefore a probability of non-radiative annihilation of charge transfer excitons {d9h} increases and the lifetime of the exciton decreases, which leads to broadening and weakening the intensity of the observed line even at low temperature. This immanent characteristic of p-d charge transfer excitons {d9h}, which is caused by the presence of the partially filled 3d shell, makes these excitons significantly different from the Wannier-Mott excitons in the semiconductors with direct allowed transitions. P-d charge-transfer exciton quite easy transfers energy into d-shell due to so-called Auger recombination on defect [15] and the increase of temperature facilitates this process. To sum up the discussion of two discovered peaks, we are quite convinced to assert that Wannier-Mott exciton (I1-line) and p-d charge transfer exciton {d9h} (I2-line) have been detected in

NiO and Zn1-xNixO solid solutions. For better understanding and more distinct observations of the peaks, further spectroscopy investigations are required to be performed.

4. Conclusion In summary, the luminescence spectra of solid solutions Zn1-xNixO and NiO at T= 8 K under XUV excitation have been investigated in the spectral region of I1 and I2 lines. It is experimentally figured out that band gap energy doesn’t depend on the concentration of Zn-ions for Zn1-xNixO solid solutions. Note that this is an anomalous behavior for solid solutions of AIIBVI-group, where common behavior is changing of energy gap if to change the concentration of doped ions. On the basis of the experimental and theoretical data, we believe that narrow lines I1 and I2 in oxide compounds Zn1-xNixO originate due to the radiation recombination of WannierMott exciton and p-d charge-transfer exciton, respectively.

Acknowledgments The authors are grateful to V.I. Anisimov, A.V. Lukoyanov, M.A. Korotin and N.A. Skorikov for the detailed discussion of the problem of excitons in oxide compounds Zn1-xNixO. This work was performed within a state contract under the topics of «Electron» No. 01201463326 , with the partial financial support provided by the Ural Branch of Russian Academy of Sciences, Grant no. 15-9-2-46. The research was supported by the Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006. The reported study was funded by RFBR according to the research project No. 16-32-00354 mol_a. 5

References [1] V.I.Sokolov, V.A.Pustovarov, V.N.Churmanov, V.Yu.Ivanov, N.B.Gruzdev, P.S.Sokolov, A.N.Baranov, A.S. Moskvin, Unusual x-ray excited luminescence spectra of NiO suggest selftrapping of the d-d charge-transfer exciton, Phys. Rev. B. 86 (2012) 115128. [2] V.I. Sokolov, V.A. Pustovarov, V.Yu. Ivanov, N.B. Gruzdev, P.S. Sokolov, A.N. Baranov, The Influence of Temperature on Narrow I1 and I2 Lines in the Luminescence Spectrum of Ni0.6Zn0.4O, Optics and Spectroscopy. 116, 5 (2014) 798-801. [3] V.N. Churmanov, V.I. Sokolov, N.B. Gruzdev et al., Exciton lines in luminescence spectra of NixZn1-xO under inner shell excitation, Physics Procedia. 76 (2015) 120-124. [4] V.N. Churmanov, V.I. Sokolov, N.B. Gruzdev et al., p-d charge transfer excitons in Zn1xNixO under inner shell excitation, Physica Status Solidi C. 13, 7-9 (2016) 610-613. [5] V.I. Sokolov, Hydrogen-like excitations of 3d-transition-element impurities in semiconductors, Semiconductors. 28, 4 (1994) 329-343. [6] A.N. Baranov, P.S. Sokolov, O.O. Kurakevich, V.A. Tafeenko, D.Trots, V.L.Solozhenko, Synthesis of rock-salt MeO-ZnO solid solution (Me=Ni2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature, High Pressure Research. 28 (2008) 515-519. [7] X-Ray Interactions With Matter: http://henke.lbl.gov. [8] Yu. Babanov, D. Ponomarev, Yu. Salamatov, Interatomic distances for overlapping shells in disordered systems: model-less approach, J.of Physics: Conference Series. 430 (2013) 012118. [9] M.A. Korotin, Z.V. Pchelkina, N.A. Skorikov, E.Z. Kurmaev, V.I. Anisimov, The coherent potential approximation for strongly correlated systems: electronic structure and magnetic properties of NiO–ZnO solid solutions, J. Phys.: Condens. Matter. 26 (2014) 115501. [10] J. Kuneš, V.I. Anisimov, A.V. Lukoyanov, D. Vollhardt, Local correlations and hole doping In NiO: A dynamical mean-field study, Physical Review B.75 (2007) 165115. [11] K.A. Kikoin, V.N. Fleurov, Transition Metal Impurities in Semiconductors: Electronic Structure and Physical Properties, World Scientific, Singapore, 1994. [12] V.I. Sokolov, K.A. Kikoin, Excitons Bound to Impurities of 3d Elements in II-VI Compounds, Soviet Scientific Reviews/Section A, Physics Reviews. 12, 3 (1989) 147-285. [13] V.I. Sokolov, A.N. Mamedov, The discovering of tunneling annihilation of donor exciton in CdS: Ni, JETP Letters. 43 (1986) 237-240. [14] A. Fujimori, F. Minami, Valence-band photoemission and optical absorption in nickel compounds, Physical Review B. 30, 2 (1984) 957-971. [15] D. J. Robbins and P. J. Dean, The effects of core structure on radiative and non-radiative recombinations at metal ion substituents in semiconductors and phosphors, Advances in Physics. 27, 4 (1978) 499-532.

Figure 1. Time-resolved luminescence spectra of solid solution Zn0.4Ni0.6O (fast time window: δt1=0.1 ns, Δt1=5.7 ns and slow time window: δt2=58 ns, Δt2=14 ns) under excitation with

6

energies Eexc=130 eV (1) and 850 eV (2) at T= 8 K. Inset: luminescence spectrum of NiO (fast time window: δt=0.1 ns, Δt=20 ns) under excitation with energy Eexc=130 eV at T= 8 K.

Figure 2. Luminescence spectra of Zn0.8Ni0.2O (time-integrated spectra) under excitation with energy Eexc=130 eV (1) and 450 eV (2) at T= 8 K.

Figure 3. Orbitally resolved O 2p and Ni 3d spectral densities. Substantial spectral weight transfer

relative to the LDA results (see inset) is observed [9].

I1

1,0

I2

Ni0.6Zn0.4O

Intensity, arb.units

0,8

Intensity, arb.units

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NiO

I2

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0,0 3,30

1

0,4

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3,34

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Slow window Eexc=130 eV

2 3,32

3,36

Photon energy (eV)

I1

0,6

3,32

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Photon energy (eV)

7

3,42

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Intensity, arb. units

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I2

Zn0.8Ni0.2O

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I1

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1

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3,34 3,36 3,38 3,40 Photon energy (eV)

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