ISSN 10637834, Physics of the Solid State, 2015, Vol. 57, No. 9, pp. 1865–1869. © Pleiades Publishing, Ltd., 2015. Original Russian Text © I.Kh. Akopyan, V.Yu. Davydov, M.E. Labzovskaya, A.A. Lisachenko, Ya.A. Mogunov, D.V. Nazarov, B.V. Novikov, A.I. Romanychev, A.Yu. Serov, A.N. Smirnov, V.V. Titov, N.G. Filosofov, 2015, published in Fizika Tverdogo Tela, 2015, Vol. 57, No. 9, pp. 1817–1821.
LOWDIMENSIONAL SYSTEMS
Photoluminescence Spectra of Thin ZnO Films Grown by ALD Technology I. Kh. Akopyana, V. Yu. Davydovb, M. E. Labzovskayaa, A. A. Lisachenkoa, Ya. A. Mogunova, D. V. Nazarovc, B. V. Novikova, *, A. I. Romanychevc, A. Yu. Serova, A. N. Smirnovb, V. V. Titova, and N. G. Filosofova a
St. Petersburg State University, Universitetskaya nab. 7–9, St. Petersburg, 199034 Russia * email:
[email protected] b Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia c Institute of Chemistry, St. Petersburg State University, Universitetskii pr. 26, Petrodvorets, St. Petersburg, 198504 Russia Received April 16, 2015
Abstract—The photoluminescence of ZnO films grown by atomic layer deposition (ALD) on silicon sub strates has been investigated. A new broad photoluminescence band has been revealed in the exciton region of the spectrum. The properties of the band in the spectra of the films with different crystallographic orienta tions of substrates have been studied in a wide temperature range at different excitation levels. A model describing the origin of the new band has been proposed. DOI: 10.1134/S1063783415090036
1. INTRODUCTION RT
A widebandgap semiconductor ZnO ( E g = 3.37 eV) has attracted much attention in the last years as a promising material for making various optoelec tronic devices operating in a shortwavelength spec trum region, in particular, for making highefficiency blue and ultraviolet light sources, lasers, solar battery units, photocatalysts, gas sensors, etc. [1, 2]. Zinc oxide has a number of advantages over other wide bandgap semiconductors actively studied nowadays (GaN, ZnSe): first of all, it possesses a high binding energy of exciton (60 meV), which assumes the possi bility of making ZnObased devices operating at room and higher temperatures. Thin films are of most interest for applications. Among the methods of making such films, atomic layer deposition (ALD) is relatively simple and allows growing homogeneous films of large area. This tech nique has shown much progress in the last years [2]. On the other hand, the optical properties of this type of films are poorly studied [3]. This work is aimed at studying the exciton photolu minescence and Raman spectra of nanocrystalline ZnO films grown by atomic layer deposition on silicon substrates. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Atomic layer deposition procedure consisted of 4 stages: injection of zinc diethyl Zn(C2H5)2, blowing
out the reactor by a nitrogen flux, injection of water vapor, and secondary cleaning the reactor by a nitro gen flux. The duration of each stage was 0.1–0.2 s with 0.5–2.0s intervals. The precursors, zinc diethyl vapor, O2, N2, and deionized water were deposited onto the pSi surface at temperatures of 180 and 240°C. We used silicon plates with the surface orienta tions (100) and (111). The thickness of the films under investigation was 3.5, 45, 100, and 440 nm. The film structure was analyzed by scanning electron micros copy (SEM) and Xray diffraction analysis. According to the SEM images, the films consisted of quite tightly packed ZnO crystallites with the characteristic sizes depending on the number of cycles and the synthesis temperature. All films had a hexagonal structure. Fig ure 1 shows the SEM images of two 100nm films on silicon plates with different crystallographic orienta tions. The films grown on the (111) surface at T = 180°C (Fig. 1a) were formed by rather tightly arranged columns with a diameter of about 20 nm and more ori ented perpendicular to the substrate surface. The films grown on the (100) surface (Fig. 1b) consisted of cha otically arranged irregular particles. The most perfect structures appeared at a growth temperature of 180°C on the (111) silicon substrates. The photoluminescence spectra were studied in the temperature range from 5 to 300 K with the excitation by He–Cd (λ = 325 nm) and nitrogen (λ = 337 nm) lasers. A Lomofotonika MDR2042 monochromator and a Janis Research Company closedcycle helium cryostat. The excitation density ranged from 4 to 1000 kW/cm2.
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1 Fig. 1. Scanning electron microscopy image of ZnO films grown by the atomic layer deposition (d = 100 nm, Tgr = 180°C) on the (a) (111) and (b) (100) Si substrate.
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The Raman scattering spectra were detected in the backscattering geometry at room temperature on a T6400 spectrometer equipped by a JobinYvonHor iba confocal microscope. A He–Cd laser (the radia tion wavelength of 325 nm) was used as an excitation source). The laser beam was focused to a spot with a diameter ~1–2 μm on the sample surface. A typical power density was no greater than 500 W/cm2 to avoid the effect of the laser radiation on the structure of the samples under investigation. Since the used excitation energy was higher than the band gap of bulk ZnO by more than 400 meV, the resonance conditions for the incident photons occurred, under which the leading contribution to the scattering cross section came from the Frölich mechanism of the electron–phonon inter action. 3. RESULTS AND DISCUSSION The photoluminescence spectrum of ZnO films grown by the ALD technology at T = 5 K and low exci tation levels resembles the spectra of single crystals and films known from literature [1, 2]. The most intense line comes from an exciton localized on the neutral donor D0(X). Phonon recurrences of free and bound excitons are also seen. The resonance emission
380 λ, nm
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Fig. 2. Luminescence spectra of ZnO films (d = 100 nm) on (curve 1) (111) and (curve 2) (100) Si substrates at T = 5 K.
of a free exciton is usually unseen. All lines in the spec tra of the films are broadened compared to the photo luminescence spectrum of single crystals. The photo luminescence spectrum of the film on the (111) Si substrate at the excitation by the He–Cd laser (W = 50 kW/cm2) is shown in Fig. 2 (curve 1). The domi nant structure at this excitation is the band of the first phonon recurrence of excitons. There is also a contin uous structureless background in the region of bound excitons and phonon recurrences. This background forms a broad asymmetric peak with a maximum at ~380 nm in the films grown by the ALD technology on the (100) substrate (Fig. 2, curve 2). A further increase in the excitation intensity results in a considerable transformation of the spectra of the films grown on both (100) and (111) substrates. Figure 3 illustrates the evolution of the photoluminescence spectrum of the 45nm ZnO film on the (111) sub strate. All bands in the photoluminescence spectra broaden with an increase in the excitation density and there appears a longwavelength luminescence tail. The structure of the spectrum is gradually smeared
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3 2 1
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Fig. 4. Photoluminescence spectra of a ZnO film (d = 45 nm) on a (100) Si substrate at T = 79 K and the excita tion density W = (1) 0.3, (2) 3, and (3) 30 kW/cm2 pro vided by a He–Cd laser.
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spectra of a 45nm film on a (100) substrate at T = 79 K with an increase in the excitation intensity.
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Fig. 3. Photoluminescence spectra (T = 5 K) of a ZnO film on a (111) Si substrate (d = 45 nm, Tgr = 240°C) at differ ent excitation densities provided by a He–Cd laser (W = 0.3 kW/cm2, curve 1) and a N2 laser (W = (2) 3 kW/cm2, (3) 100 kW/cm2, and (4) 1 MW/cm2.
out. As a result, a broad asymmetric band with a max imum near 380 nm, a pretty sharp blue edge and a flat red tail extending to 420 nm is formed (Fig. 3, curve 4). It was found that such a transformation takes place in the spectra of all ALD films under investigation. How ever, the intensity of the new band, the length of the red tail and the limits of the excitation density, at which the evolution of the spectrum occurs differ depending on the parameters of synthesis. The band is red shifted and looses intensity with an increase in temperature from 5 to 300 K; at the same time, the relative intensity of a free exciton increases and the respective peak becomes dominant in the pho toluminescence spectrum. An increase in the excitation intensity leads to a blue shift of the photoluminescence band and a decrease of its halfwidth. The length of the red tail does not increase in this case. Figure 4 illustrates the blue shift of the new band in the photoluminescence PHYSICS OF THE SOLID STATE
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It should be mentioned that the emergence of a structureless peak is reversible. The photolumines cence spectrum return to the initial form at the transi tion from the maximum excitation level to the initial (low) one. However, the integral photoluminescence intensity after strong optical irradiation appears to be much lower than the initial one. It takes several min utes to raise the signal to the original level. We studied Raman spectra, since they could pro vide additional information on the exciton–phonon interaction and the role of the surface, mechanical deformations and defects. Figure 5 shows the reso nance Raman spectra of 100 and 45nm ZnO films synthesized by atomic layer deposition and a reference hexagonal ZnO single crystal grown by hydrothermal synthesis. As is seen, the spectrum of the ZnO crystal manifests strong first, second and thirdorder multi phonon LO scattering. In the general case, the fre quency of a 1LO phonon ranges from 574 cm–1 (the LO phonon of the symmetry A1) to 583 cm–1 (the LO phonon of the symmetry E1) depending on the mutual orientation of the polarization vector of the excitation field and the direction of the optical axis of the crystal owing to the anisotropic character of shortrange forces in the uniaxial ZnO crystal [4]. In our case, the frequency of the 1LO mode in the spectrum of the sin gle crystal is 574 cm–1 corresponding to the phonon of the symmetry A1 (LO), which can be observed if the polarization vector of the excitation field of perpen dicular to the optical axis of the ZnO single crystal.
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ground in the region of the exciton photolumines cence, which was attributed to the broadening of spec tral lines [2]. 1LO 2LO 3LO 1 1' 2' 3'
2 3 340
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Fig. 5. (1'–3') Raman spectra at T = 300 K and (1–3) pho toluminescence spectra at T = 79 K of (1, 1') a 45nm ZnO film on a (100) Si substrate, (2, 2') a 100nm ZnO film on a (111) Si substrate and (3, 3') a ZnO single crystal.
The frequencies of the 1LO modes in the spectra of the 45 and 100nm film samples are 576 and 575.5 cm–1, respectively, and can be also attributed to the phonons with the symmetry A1 (LO). This implies that the opti cal axis in both film samples coincides with the growth direction of the films. At the same time, a small blue shift of the 1LO modes in the film samples should be attributed to the presence of elastic compression strains in the plane perpendicular to the growth direc tion of the ZnO film. However, the analysis of the spectral position of the phonon line E2 (high), which is most sensitive to deformations and information on which was obtained with the use of offresonance exci tation (we do not include these spectra in the present work), urges us to reject this assumption. The assump tion that the above shift of the 1LO phonon is caused by the manifestation of the coupled Plasmon–phonon mode, the position of which strongly depends on the density of carriers in the sample, seems more probable. In this case, the position of the 1LO phonon in the film samples should indicate a higher density of free carri ers in the films than in the single crystal. In addition to the LO scattering, the spectra of the film samples exhibit a feature with the maximum at 567 cm–1. The frequency of this feature is lower than the frequency of the A1 (LO) phonon in ZnO but higher than the frequency of the A1 (TO) phonon (380 cm–1). We attribute it to the manifestation of the surface optical phonon [5]. Switching to the discussion of the nature of the structureless photoluminescence band it should be mentioned that its spectral position, the dependence on temperature and intensity does not allow attribut ing it to the effects of high exciton density. These effects were studied many times in single crystals and epitaxial ZnO films [6] with the account of the laser effects, exciton–exciton interaction and the elec tron–hole plasma. Some works pointed out the back
We suggest that the new band is associated with the presence of charged defects in the films that appear during the film growth and with a change in their charge state under illumination by ultraviolet light. Most probably, this regards interstitial zinc atoms. A random spatial distribution of charged centers can lead to the formation of inhomogeneous electric fields creating states in the band gap, on which excitons are localized. A large width of the new luminescence band indicates a continuous spectrum of such states. Dow and Redfield [7] calculated the profile of the exciton absorption in the homogeneous electric field with the inclusion of the Frantz–Keldysh effect and showed the emergence of the exponential longwavelength absorption tail. They also showed that when the fields are stochastic and their magnitude varies in space, the exponential tail that appears in the homogeneous field is preserved under averaging over all configurations of the random field. The phenomenon is similar to that observed in disordered systems, where the fluctuating potential leads to the formation of tails of the density of states at the edges of the bands, on which excitons and carriers can be localized. It can be assumed that photoluminescence at high temperatures close to room temperature is contributed by the recombination of free electrons with acceptor like states [8]. In this case, the extended longwave length tail can be also observed and the blue shift can be associated with an increase in the Fermi energy in the conduction band. Presumably, both processes contribute to luminescence, yet with different effi ciencies at high and low temperatures. 4. CONCLUSIONS The ZnO films were synthesized by the ALD tech nology on silicon substrates with different crystallo graphic orientations. The morphology of the films was studied using scanning electron microscopy. It was shown that nanostructures of the 100nm films grown at 180°C on the substrates with the orientations (111) and (100) are different in shape: nanocolumns in the former case and irregular nanoparticles in the latter case. A new broad structureless band was revealed in the lowtemperature photoluminescence spectra. Its properties were investigated as functions of the excita tion intensity and temperature. The possibility of localization of excitons in the potential fluctuations induced by the inhomogeneous electric field and a possible contribution of radiation transitions from the conduction band to the acceptorlike states were dis cussed. A blue shift of the 1LO phonon owing to the manifestation of a coupled plasmon–phonon mode was revealed in the Raman spectra.
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ACKNOWLEDGMENTS We thank the staff of the Resource Centers “Inno vation Technologies of Composite Nanomaterials,” “XRay Diffraction Methods of Investigation,” and “Nanostructuring of Photoactive Materials” of the St. Petersburg State University. REFERENCES 1. C. Klingshirn, M. Grundmann, A. Hoffman, B. Meyer, and A. Waag, Phys. J. 5 (1), 33 (2008). 2. U. Ozgur, Ya. I. Alivov, C. Liu, A. Teke, M. A. Re shchikov, S. Dogan, V. Avrutin, S.J. Cho, and H. Mor kos, J. Appl. Phys. 98, 041301 (2005).
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3. T. Tynell and M. Kappinen, Semicond. Sci. Technol. 29, 1 (2014). 4. T. S. Damen, S. P. S. Porto, and B. Nell, Phys. Rev. 142, 570 (1966). 5. H. F. Liu, S. Tripathy, G. X. Hu, and H. Gong, J. Appl. Phys. 105, 053507 (2009). 6. C. Klingshirn, J. Fallert, H. Zhou, J. Sartor, J. Thiele, F. MaierFlaig, and H. Kalt, Phys. Status Solidi B 247, 1424 (2010). 7. G. D. Dow and D. Redfield, Phys. Rev. B: Solid State 1, 3358 (1970). 8. J. DeSheng, Y. Makita, K. Ploog, and H. J. Queisser, J. Appl. Phys. 53 (2), 999 (1982).
Translated by A. Safonov
