On the nature of the effect of adsorbed oxygen on the excitonic photoluminescence of ZnO

Journal of Luminescence 195 (2018) 153–158

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On the nature of the effect of adsorbed oxygen on the excitonic photoluminescence of ZnO

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V.V. Titov, A.A. Lisachenko , I.Kh. Akopyan, M.E. Labzowskaya, B.V. Novikov Saint-Petersburg State University, V.A. Fock Institute of Physics, Ul’yanovskaya str., 1, Saint-Petersburg 198504, Russia

A R T I C L E I N F O

A B S T R A C T

Keywords: Zinc oxide Powders and ZnO/Si ALD films UV–VIS Photoluminescence Oxygen thermo/photo-desorption Superficial 2D-quantum well Mass-spectrometry

The mechanism of the effect of adsorbed oxygen on photoluminescence (PL) of ZnO powders and ZnO/Si ALD films in the UV–VIS regions was studied, simultaneously with the in-situ UV photoelectron spectroscopy (UPS) and mass spectrometry (MS) measurements. We have found a drastic (up to 10 times) increase of the exciton PL along with a slight (by ~ 1.5 times) increase of green PL induced by thermo-reducing of ZnO surface in H2 or CO atmosphere or by a UV–VIS photo-reducing in the vacuum. The reaction products H2O, CO2 and photo-desorbed O2 were registered by the MS. According to UPS (8.43 eV), the change in PL is accompanied by a significant change in the surface dipole value δ without shift of the Fermi level EF or of the band bending VS. We believe that the slow surface states interact with the adsorbed oxygen and thus create a surface 2D-quantum well whose internal field destroys the excitons. The internal fast surface states not interacting with the slow ones provide a pinning of the Fermi level and the stability of the band bending value. The adsorbed oxygen also affects the surface defects thus reducing the VIS luminescence of ZnO.

1. Introduction ZnO is a multifunctional material widely used in a number of high tech areas: in optoelectronics, in solar to electrical or chemical energy conversion, in photocatalysis, in thermal control coatings of space stations [1–4]. ZnO can be used also as an active antireflection coating which provides a photocatalytic cleaning of the surface of solar cells, thus improving their efficiency and increasing their lifetime in the outdoor applications. The ZnO/Si heterostructures are used in photodetectors, UV-range photodiodes, components of solar cells and lightemitting diodes [5,6]. A unique ~ 60 meV strong bond of the excitons in ZnO enables one to develop optoelectronic devices operating at room temperature (RT), and raises the question of creating the exciton based lasers. In this regard, the importance of studying the ZnO exciton PL in the UV and VIS (defects and impurities) regions is undoubted. Earlier we investigated the features of low-temperature exciton PL in ALD films and single crystals of ZnO at high excitation levels [7]. It was shown that optical and PL characteristics weakly depend on the properties of ZnO/Si interfaces but strongly depend on the processes at the ZnO/gas phase interface [8]. The importance of the exciton localization processes in the surface potential fluctuations was shown [7]. In [9] the role of gas phase in the formation of the PL spectrum of ZnO in the exciton and "green" regions of the spectrum was established. A number of works [10–13] reveal the dependence of the PL

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spectrum and intensity on the environment. In particular, it is shown that the exciton PL is strongly influenced by the gas oxygen [14–16]. A priori, one associates the changes in the exciton and visible luminescence with the variation of the band bending, although, as to our knowledge, it was not demonstrated by relevant direct experiments. In [17] the correlation of PL intensity with the value of the contact voltage by Kelvin probe was carried out. However, the contact voltage value depends not only on the band bending but also on the surface dipole value. The aim of the present work is to reveal the mechanism of a gasphase oxygen effect on the exciton and "green" PL of ZnO at RT. For this purpose, comprehensive studies were carried out in three phases – the solid, adsorbed and gas phases, using a set of complementary experimental methods. In particular, the use of UPS (8.43 eV) monitoring allowed us to separate the two possible mechanisms corresponding to the contribution of the band bending and the dipole value which both contribute to the contact voltage. The work was carried out at low excitation levels with a minimal disturbance of the studied system. 2. Experimental For comparative studies we used meso- and nano-structured powders of high-purity (< 10−4% of contaminants) n-type ZnO and ZnO films (5–500 nm) deposited on Si wafers by atomic layer deposition

Corresponding author. E-mail address: [email protected] (A.A. Lisachenko).

https://doi.org/10.1016/j.jlumin.2017.11.022 Received 19 July 2017; Received in revised form 11 November 2017; Accepted 13 November 2017 Available online 16 November 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

Journal of Luminescence 195 (2018) 153–158

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allowed us to monitor the values of the thermoelectric work function φТ, and to separate the contributions of the dipole component δ and of the band bending VS (see Fig. 8). Another set-up allowed us to study the PL spectra of powders simultaneously with the MS analysis of the gas phase. The influence of the gas phase on the PL spectra was investigated in situ in a quartz reactor connected to the systems of UHV and of pure (99.999%) gas admission, as well as to the mass spectrometer Hiden HAL201 RC. The design of the reactor allowed us to perform the analysis of the gas composition above the sample in both quasi-static and flow-through regimes. A spectro-fluorimeter Cary Eclipse FL1110M012 at low excitation levels (the pulse amplitude < 40 mW/cm2, the average power < 15 μW/cm2) was used. The carrier concentrations were measured in situ by the Van-derPauw method in 100 nm-thick ZnO ALD film subjected to the same heat treatments as the powders. In the measuring scheme a magnetic field of 0.56 T was applied, and a Bridge Technology Ecopia HMS-3000 was used for read-outs and result calculations. The oxidized film had a too high resistance to obtain a reliable result, but the reduced film was easy to measure, giving the Hall-effect carrier densities of about 1·1018 cm−3, and the Hall-effect mobility of 18 cm2/V.

Fig. 1. Microphotograph of ZnO powders, typical examples of conglomerates.

[18] (ALD). Before the experiments, the samples were heated at 450 °C in an oxygen (99.999%) flow until the oxidation traces of the native impurities completely disappeared in the outgoing flow (oxidized samples). It was followed by a heating in UHV for 2 h (reducing of samples). A characterization of both samples was carried out and the results were compared. SEM microphotographs were obtained using a SEM-FIB Zeiss 1540-XB Cross Beam microscope with the spatial resolution of about 1 nm. The result of the SEM analysis of the initial ZnO powder structure is given in Fig. 1. The described treatments did not make any significant changes of the tested parameters. The powder consists of conglomerates with the size of up to 10 µm composed of nano-crystallites with the size of 20–500 nm stretched along the C axis. The specific surface area determined by the BET method was 20.3 m2/ g. XRD studies in symmetrical θ/2θ configuration were performed using a Bruker D8 DISCOVER high resolution diffractometer. The XRD measurements have shown that the powder consists of 100% wurtzite phase (Fig. 2). For XPS and UPS (21.2, 40.8 eV) a Thermo Fisher Scientific Escalab 250Xi spectrometer was used. The details of the methods and the results of ZnO films characterization by the methods of XPS, SEM, UPS (21.2 eV), and XRD prior to the experiments are given in [18]. The PL emission and excitation spectra at RT in air were analyzed by a spectrofluorimeter HORIBA Fluorolog-3 FL3-22iHR-1479C-4611-FL, using a xenon lamp (W = 1.1 mW/cm2 at λ = 330 nm) for excitation. For the in situ investigations we have adapted the MS of the gas and the adsorbed phases [19] combined with the UPS (8.43 eV) [20]. This

3. Results and discussion The description of ALD ZnO films deposited on n-Si and p-Si wafers used in comparative experiments is given in [18]. According to high-resolution sections of the XPS spectra, the O1s spectrum is a superposition of two signals at 530.7 eV and 532.6 eV. The first peak corresponds to oxygen atoms in the bulk of the ZnO lattice, and the second one originates from О atoms in Zn–ОН groups on the surface of the ZnO film [9]. The spectrum lacks a ~ 533.8 eV line belonging to adsorbed water [21]. This is expected, due to the preheating of the sample up to 450 °C. But it is the adsorption of water that was used by the authors [21] to modify the surface band bending. The Zn2p spectrum (Fig. 3) reveals two broadly resolved features at 1045.3 eV (Zn2p1/2) and 1022.2 eV (Zn2p3/2) corresponding to zinc atoms in the ZnO lattice [22–24]. No peak of C = O structures was observed, indicating no carbonate impurities in the sample. This is also confirmed by the XRD spectrum (Fig. 2), according to which the sample is composed of 100% zincite. As we have shown earlier, the spectrum in nearband PL of powder at T = 77 K is similar to the PL spectrum of a ZnO single crystal wurtzite, i.e. a free exciton emission (FX), the phonon replicas of free excitons (1LO, 2LO), the emission of the excitons localized at neutral donors (D0Xn) [25]. The peculiarities of the powder are the weaker emission of free excitons compared to the phonon replicas (1LO, 2LO) and the lines of bound excitons. Fig. 4 shows the PL spectra of ZnO powder at T = 77 K and the comparative spectra of ALD films and powders taken at RT. At T = 77 K the structure of the exciton emission is presented. The 375 and 384 nm peaks relate respectively to the first and to the second phonon replica of a free exciton band in the hexagonal wurtzite. The energy of the free exciton line allows us to determine the width of the band gap equal to 3.43 eV, which corresponds to that of a ZnO crystal. Taking into account also the results of SEM and XRD, we can consider our samples as crystals having no signs of confinement. A weak broad band in the green spectral region with the maximum at ~ 500 nm due to the intrinsic defects of ZnO [12,26,27] is also observed. At RT the main peak for powders is located at 385 nm, while for the films it is at 377 nm. It is established that the line broadens and shifts with the increase of the film thickness (and hence the increase of the crystallite size), and at d = 500 nm the maximum is shifted to 385 nm with a shoulder at 377 nm. At RT a relative contribution of the green band to the spectrum is significantly higher for the powder than for the films. We believe that the change in the band form and the shift of the maximum toward longer wavelengths are caused by a contribution of

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Fig. 2. XRD spectrum of ZnO powder sample. Vertical bars next to 2θ axis mark the positions of the strongest zincite reflections.

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Fig. 3. XPS detail scans of O 1s and Zn 2p peaks of ZnO films.

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Lum. intensity, a.u.

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Fig. 6. The influence of the gas phase on the PL of ZnO powder at RT. 1 – an initially oxidized sample; 2 – the same sample after heating in H2 at 650 K; 3 – heating of the basic sample in CO at 650 K; 4– sample 1 after photo-desorption of O2; 5 – samples 2, 3, 4 after admission of O2 at RT. The spectra are offset vertically for clarity.

Fig. 4. PL spectra at low excitation level at λex = 340 nm. 1 – ZnO powder at T = 77 K; 2 – powder at RT; 3 – ZnO/n-Si(111) d = 50 nm film at RT.

the bulk defects states. The excitation spectra of the green PL (Fig. 5) completely coincide for powders and films for λ ≤ 380 nm, i.e. in the region of fundamental ZnO absorption, where the absorption coefficient reaches k ~ 105 cm−1. Starting from λ ≥ 380 nm the k value decreases, reaching at 400 nm the value of k ~ 103 cm−1. This leads to the increase of the excited layer thickness in powder and consequently to the increase of PL excitation efficiency in powders.

3.1. Effect of surface reduction on PL intensity The influence of the gas phase on the PL of ZnO is shown in Fig. 6. As it is seen in the figure, the thermal reduction of the oxidized ZnO powder (Fig. 6, curve 1) in H2 atmosphere at 650 K results in the increase by more than one order of magnitude in the intensity of excitonic PL, and by the factor of ~ 1.5 in the intensity of the green band (Fig. 6, curve 2). The subsequent admission of oxygen at RT completely removes the induced effect, and the spectrum returns back to its initial form (Fig. 6, curve 5). The increase of PL after thermal reduction of the sample in H2 atmosphere could be probably caused by the creation of the active Hcenters. Therefore, we have carried out an additional study of the sample reduction in CO atmosphere. Heating in CO at 650 K also increases the intensity of both bands (see Fig. 6, curve 3). The fact that the magnitude of the effect is lower compared to H2 can be naturally explained by a greater reducing activity of hydrogen. The desorption of the products of superficial oxygen reactions on ZnO, i.e. H2O and CO2 into the gas phase during heating of the powder in H2 and CO respectively, has been established by the MS. The admission of oxygen at RT completely removes the effect induced by heating in H2 and CO (Fig. 6, curve 5). Thus, the effect has the same nature in both cases: it is the result of the removal of adsorbed and superficial oxygen. The surface nature of the process follows from the fact that the induced effect is completely eliminated by oxygen adsorption at RT, when the oxygen diffusion into the volume can be completely neglected. Chang et al. [28] also investigated the influence of the reduction on

Fig. 5. PL excitation spectra at RT of: 1, 2 – powder ZnO, λlum = 435, 520 nm correspondingly; 3, 4 – film d = 500 nm on n-Si(111) at λlum = 455, 505 nm correspondingly.

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previously shown [29] that the minimal power for creation of oxygen vacancies in ZnO is 106 W/cm2 for λ = 354 nm. Thus the oxygen vacancies could appear when PL was excited by a nitrogen pulsed laser and we had shown the importance of exciton localization processes in the surface potential fluctuations [7]. However the excitation power used in the present experiments was much lower.

PL. They observed a drastic drop of green band when the annealing temperatures exceeded 870 K. This is probably the temperature required to create defects in the bulk. The films deposited on various substrates: p-Si(100), n-Si(111), SiOx/Si have not revealed any significant difference in PL characteristics at RT and at low excitation levels. Only in the case of SiOx/Si wafer the relative contribution of the green PL was ~ 1.5 times higher than that one on the surfaces cleaned of the oxide. Thus the ZnO/Si interface has an insignificant influence on PL of ZnO.

3.4. The Fermi level pinning in the samples The effect can also be caused by changes in the value of the surface potential. The change in the intensity of the exciton luminescence of ZnO might be attributed to the change in the surface band bending, which governs the separation of electrons and holes, and thus affect their recombination rate. The adsorption of electron-donor or electron-acceptor molecules can change the value of band bending [21]. However we do not know any works in which the study of the PL spectra intensity was combined with the in situ measurements of the surface band bending values. It can be directed both up and down, depending on the sample preparation method and the subsequent treatments of the surface. At sufficiently high field strengths in the Schottky barrier the coupling within an exciton is broken and a "dead zone" for the excitons can be formed. The field intensity being sufficient for the exciton ioion nization Fexc is found from [31]:

3.2. Oxygen photodesorption Hence, the surface reducing can be obtained both by heating and also by UV irradiation of ZnO in vacuum. In the latter case the oxygen photo-desorption is registered by the MS. The reduction potential of electron at the bottom of the ZnO conduction band is higher than the electrochemical potential of O2 / O2−, and so the electrons are transferred from the conduction band to neutral adsorbed oxygen, forming the superoxide radical ions O−2 . In the dark an equilibrium distribution of charged species is established on the surface: − O2(gas) + e ↔ O−2(ads) + e ↔ O22(−ads) ↔ O−(surf ) + O−(surf ) + 2e ↔ 2O2(bulk )

During irradiation of ZnO the electron-hole pairs are generated. In vacuum, i.e. in the absence of oxygen in the gas phase, the holes shift the equilibrium on the surface to the left, resulting in the photo-desorption of oxygen into the vacuum:

O−2(ads) + h → O2(gas) ;

2

μ ⎞ −3 ion ion ε FH Fexc =⎛ ⎝ m0 ⎠ ⎜

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where FHion = 2.57·109 V/cm is the F value needed for the H atom ionization, μ is the exciton reduced mass, ε is the static dielectric constant of ZnO, m0 being the electron mass. One obtains.

2O−(surf ) + 2h → O2(gas) .

In the present work we analyzed the photo-desorption on films and on powders together with UPS and PL measurements (Fig. 7). The results obtained indicate an important role of removing of ZnO surface oxygen in PL increase. Consider some possible mechanisms of this influence.

ion Fexc = 2, 46⋅105

V . cm

The number of oxygen molecules desorbed in our experiments under the UV irradiation of powder exceeded 1·1011 mol/cm2. According to the Hall effect measurements the electron concentration in the bulk of ZnO was about ~ 1018 cm−3. For such concentration the Poisson equation gives the value of the ΔVs decrease due to photodesorption that might reduce the intensity of the field at the surface F by the value of ΔVs ~ 1·105 V/cm down to the value under the exciton ionization. At the same time, it is found that a sufficiently high density of surface states providing local energy states near the Fermi level can stabilize EF during the adsorption [30]. This effect of the EF pinning is observed at the concentrations of surface levels ~ 1012 сm−2. In order to analyze the evolution of the surface electronic structure we used a common UPS (21.2 eV) method. We tested it with the Thermo Fisher Scientific Escalab 250Xi spectrometer. Unfortunately, its sensitivity was insufficient to detect the density of surface states in the band gap near EF. To better understand the effect of electrophysical parameters on the PL we used an original UPS (8.43 eV) spectrometer [20]. A high aperture value of our spectrometer reaching 34° along with a high signal-tonoise ratio ≥ 5·104 allowed us to obtain the spectra of surface states in the forbidden band at the level of 1010 cm−2 that is impossible to get by standard UPS(21.2, 40.8 eV) instruments. The combination of PL and UPS measurements allowed us to trace the dependence of the PL characteristics on the electrical parameters shown in Fig. 8. The experimental results are presented in the form of UPS (8.43 eV) density of occupied states (DOS) spectra superimposed on the energy diagram of the sample. Three points of the spectrum are essential to construct the model. 1) The beginning of the spectrum, corresponding to a photoemission from the highest occupied level, which coincides in our case with the position of the Fermi level and gives the value of φТ. The flat part of the spectrum corresponds to a photoemission from the surface states in the band gap. 2) The start of growth in photoemission intensity determines the valence band edge and gives the value of φph.

3.3. The bulk or the surface? The observed oxygen evolution may be caused by the surface effects and also by the formation of oxygen vacancies in the bulk. However, the full reversibility of the effect during oxygen admission onto a reduced sample at RT indicates a superficial nature of the effect since the diffusion of oxygen into the volume is negligible at RT. The superficial nature of the ZnO reducing at T < 650 K is confirmed by the results of [28], where the irreversibility of the recovery in the atmosphere is found only at T > 670 K. The surface or volume vacancies cannot be responsible for the increase of the PL intensity. Indeed we have

Fig. 7. Oxygen photo-desorption from the UV irradiated ZnO powder. “+hν” and “−hν” mark the start and the end of the irradiation.

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"core-shell" nanostructure ZnO-ZnO1-δ is formed on the surface of the illuminated ZnO resulting in the downward band bending (Fig. 8). Subsequent adsorption of oxygen leads to the formation of a 2D quantum well, which manifests itself in the emergence of a dipole layer δ. Yet the decrease in the dipole value caused by the oxygen desorption indicates the destruction of quantum well lying on the outer side of the surface states. This model agree with the well-known ideas about the coexistence of two subsystems of electronic states on the semiconductor surface – "fast" internals and "slow" externals ones [36]. We believe that it is the field created in this quantum well that is responsible for the decrease in the intensity of the exciton PL. Thus it follows from the UPS results (Fig. 8) that the thermal and photo-reducing of the samples in our experiments affect only the superficial layer. Indeed, an oxygen adsorption at RT, when the diffusion from the volume is completely excluded, completely restores the original state of the sample. It avoids any explanation of the PL quenching by the creation of bulk defects, causing fluctuations of the electric field, which act as centers of excitons decay. The role of the intrinsic surface states of ZnO located above the top of the valence band and below the bottom of the conduction band in the formation of the PL spectra in UV and in the red regions of the spectrum was established in [37]. The authors suggest that the surface states could provide the potential opportunity to obtain a p-type surface conductivity in ZnO. Note that Allen et al. [38] have pointed to the influence of intrinsic surface states of ZnO on Schottky barrier parameters as well as on the Fermi level pinning on ZnO surface in ZnO-metal junctions. The pinning of the Fermi level determined in our work excludes the possibility of the increase of PL due to a decrease of the surface band bending. This band bending in its turn decreases the probability of ionizing the excitons. It is known that in high-quality single crystals Vs can be changed by a gas adsorption [21]. However, in real polycrystalline dispersed samples the pinning of the Fermi level is possible [30]. Based on the above experimental results we believe that the nature of the oxygen effect on the exciton PL includes an oxygen adsorption that creates a 2D surface quantum well which in its turn is responsible for the dipole component δ. At ZnO and TiO2 surfaces we have previously studied [34] 2D quantum wells which were observed also in [35]. The quantum 2D-sheet field is created by surface positive charges and the adsorbed oxygen that has drawn the electrons from the surface states. The field of a quantum well does not penetrate into the sample bulk, because it is screened by the surface states responsible for pinning the Fermi level. It is in the field of the quantum well where the ionization of the exciton occurs, leading to a drop in the excitonic PL. The hole causes a breaking of the chemical ZnO-O2 bond followed by oxygen photodesorption. In its turn the oxygen photo-desorption diminishes this non-radiative decay channel, leaving more excitons to yield a luminescence.

Fig. 8. Energy structure of ZnO. VB –valence band, CB – conduction band, VL1, VL2 vacuum levels before and after oxygen desorption, EF – Fermi level, VS– band bending, δ – dipole component, φТ – thermoelectric work function, DOS – density of occupied states, hν – photon energy 8.43 eV, Emax – maximum depth of energy probing. N(E) – density of occupied states according to UPS spectra. UPS of: oxidized – 1, reduced – 2, after oxygen photodesorption – 3.

The surface potential is formed by the Schottky barrier. 3) The value Emax gives the deepest level of energy obtainable for the given photon energy (8.43 eV). The first peaks of spectra are due to primary non-scattered electrons and give the DOS in the top of the valence band. The second peaks are formed by secondary scattered electrons and do not reflect the actual state density. However, their lower edges allow one to control the value of the dipole layer and its evolution in the course of treatments. We have found that in the case of thermo- and photo- reduction of films and powders of the n-type ZnO the Fermi level is not shifted, while φТ is reduced by ~ 0.5 eV. As it is seen from the energy scheme (see Fig. 8):

ΔφT = ΔVS + Δδ . The calculations show (see above) that the strength of the electrical field F created in this case is not sufficient for the ionization of excitons through the whole thickness of the film or powder. The UV irradiation decreases the value of φТ and sharply increases the UV PL intensity at the same time. Note that the reduction of φТ induced by the UV illumination occurs only for the account of δ even when the change of the φТ value exceeds 0.5 eV (Fig. 8, curve 3). Earlier we investigated the effect of oxygen photodesorption on the electrophysical parameters of ZnO [32,33]. It was shown [32] that photodesorption can occur through two channels. 1). The UV-generated holes in ZnO bulk are drawn out by the band bending field onto the surface and discharge the negatively charged oxygen molecules and atoms. As a result, a negative charge on the surface decreases, that leads to the decrease of the band bending and thus to the increase of the exciton luminescence. 2). In the second photodesorption channel the irradiation leads to the transition of an electron from the intrinsic surface states to the conduction band, followed by its filling with an electron of the adsorbed oxygen. The result is a breaking of oxygensurface bond and the oxygen desorption. The high density of the intrinsic surface states of ZnO ~ 1012 cm−2 V−1 changes the photodesorption scenario [33]. Such DOS is sufficient to pin the Fermi level on the surface and to stabilize the band bending value. In this case the oxygen photodesorption does not affect the efficiency of the bulk holes transfer onto the surface. It is such DOS that was in our sample. It was found [34,35] that a submonolayer

4. Conclusion We studied the spectra of PL emission and excitation of powders and films in the exciton and green regions at RT and at low levels of excitation. A comparison of the emission and excitation spectra of ALD films and powders of ZnO was performed. It was found that the gas phase strongly modifies the intensity and the shape of the spectra, while the ZnO/Si interface weakly affects them. The reduction of oxidized films and powders in H2 leads to an increase of the exciton band intensity by one order of magnitude, and of the green band intensity by the factor of about 3/2. A similar effect is produced by heating in CO. The desorption of the ZnO reduction products (H2O and CO2) during the heating of the powder in H2 and CO respectively has been established by the MS. The admission of oxygen at RT completely removes the effect induced by heating in H2 and CO. Thus, the effect has the 157

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same nature in both cases: it is the result of the removal of adsorbed and superficial oxygen. The surface nature of the process follows from the fact that the induced effect is completely eliminated by the oxygen adsorption at RT. At a subsequent irradiation in vacuum the oxygen photo-desorption is detected by the mass-spectrometry, and the PL intensity increases in both spectral regions. A similar behavior of composites and powders shows that the excitonic PL is mainly influenced by the processes on the ZnO-(gas phase) interface. The thermo- or photoreduced surface layer forms a superficial 2D well in the field of which the excitons decay.

[18]

[19]

[20]

Acknowledgements [21]

The research was supported by “Nanocomposite”, “Physical Methods of Surface Investigation”, “X-ray Diffraction Centre” and “Nanophotonics” centres of St. Petersburg State University.

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