Diversity of contributions leading to the nominally n–type behavior of ZnO films obtained by low temperature Atomic Layer Deposition

Accepted Manuscript Diversity of contributions leading to the nominally n–type behavior of ZnO films obtained by low temperature Atomic Layer Deposition Tomasz A. Krajewski, Penka Terziyska, Grzegorz Luka, Elzbieta Lusakowska, Rafal Jakiela, Emil S. Vlakhov, Elzbieta Guziewicz PII:

S0925-8388(17)32935-3

DOI:

10.1016/j.jallcom.2017.08.206

Reference:

JALCOM 42963

To appear in:

Journal of Alloys and Compounds

Received Date: 4 April 2017 Revised Date:

17 August 2017

Accepted Date: 21 August 2017

Please cite this article as: T.A. Krajewski, P. Terziyska, G. Luka, E. Lusakowska, R. Jakiela, E.S. Vlakhov, E. Guziewicz, Diversity of contributions leading to the nominally n–type behavior of ZnO films obtained by low temperature Atomic Layer Deposition, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.206. 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 proof before it is published in its final 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.

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“Diversity of contributions leading to the nominally n–type behavior of ZnO films obtained by low temperature Atomic Layer Deposition”

Tomasz A. Krajewski1,♦, Penka Terziyska2, Grzegorz Luka1, Elzbieta Lusakowska1,

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Rafal Jakiela1, Emil S. Vlakhov2, Elzbieta Guziewicz1 Institute of Physics, Polish Academy of Sciences,

Aleja Lotnikow 32/46, PL–02668 Warsaw, Poland

Institute of Solid State Physics, Bulgarian Academy of Sciences,

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72 Tzarigradsko Chaussee Blvd, 1784 Sofia, Bulgaria

Abstract

This paper discusses possible contributions to the nominal n–type behavior of ZnO films grown by the Atomic Layer Deposition (ALD). The room–temperature photoluminescence

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(RT PL) and Secondary Ion Mass Spectroscopy (SIMS) investigations suggest an important role played by the zinc–related defects in their electrical behavior. This is also supported by

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increasing electron concentration, which changes from 1.7×1017 cm-3 to 8.6×1019 cm-3 with increasing ZnO growth temperature.

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The ellipsometry studies show that the absorption edge shifts with increasing growth temperature from 3.27 eV to 3.34 eV, indicating the self–compensation and the Burstein– Moss effect, whereas the maximal extinction coefficient k remains at the level of k = 0.5. After the rapid thermal processing (RTP) in oxygen and nitrogen–rich atmosphere significant changes in the defect–related RT PL were observed, suggesting the contributions of defects of miscellaneous origin in different spectral range, i.e., RTP annealing in N2 results in the defect–related luminescence peaked at 520 – 560 nm, i.e. 2.23 – 2.38 eV (ascribed to ♦

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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ACCEPTED MANUSCRIPT the defects involving oxygen vacancies), whereas similar treatment performed in O2 atmosphere activates the luminescence in the vicinity of 650 – 730 nm, i.e. 1.70 – 1.90 eV, where the zinc–related (presumably VZn) defects are efficient radiative centers.

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Keywords: semiconductors, electronic transport, (photo)luminescence, point defects, impurities in semiconductors, thin films

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I. Introduction

Zinc oxide (ZnO) is a very prospective material for electronics and optoelectronics. As for

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many of its predicted applications the control of ZnO electrical properties is crucial, a lot of scientific reports discusses possible reasons of the high n-type conductivity of this material. It is worth noting that the electron (n) concentration reported for ZnO fabricated by different techniques is 10 – 15 orders of magnitude higher than its intrinsic value, derived directly from

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the energy gap (ni ≈ 106 cm-3 for Eg ≈ 3.3 eV at T = 300 K). Therefore, it is important to control the level of n, as various electronic usages require different electrical properties of ZnO. For example, when used in the p–n or Schottky junctions, it ought to exhibit low n value

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(below 1017 cm-3) in combination with high (above 10 cm2V-1s-1) electron mobility. This allows to get low leakage and high driving current of the structure, respectively [1]. On the

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other hand, if applied as a transparent conductive electrode, ZnO should reveal both: high optical transparency (above 80%) and low resistivity (ρ ~ 10-4 Ωcm) corresponding to n ~ 1020 cm-3 [2 – 5]. Provided these requirements are fulfilled, ZnO can replace Indium–Tin– Oxide (ITO), which becomes more expensive due to the scarcity and increasing price of indium [6]. There are many theories that predict the contribution of different donors to the observed free electron concentration in ZnO. One of the most commonly (theoretically and

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ACCEPTED MANUSCRIPT experimentally) reported shallow donor in ZnO is the atomic hydrogen [7 – 9]. Due to the small enough activation energy, hydrogen atoms can locate in the interstitial positions of the ZnO lattice, as discussed e.g. in the review by Pearton et al. in 2003 [9]. In turn, other authors ascribe the role of dominant shallow donors to the zinc interstitials (Zni) in ZnO [10, 11]. The

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oxygen vacancies (VO), despite their low formation energy, are rather deep donors. However, they may act as the efficient compensation centers, hampering the p–type ZnO doping [12]. This was experimentally observed e.g. by Look et al. [13]. The authors proposed that the

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complex Zni–NO may be the shallow donor in ZnO with the activation energy of about 30 meV.

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Another candidate for the dominant donor in ZnO is the zinc antisite (ZnO). However, there is a debate concerning the stability of this defect as ZnO can split into the Zni and VO constituents [12, 14 – 16]. According to the theoretical calculations discussed in Ref. [12], high formation energy of such defects as oxygen interstitials (Oi) and oxygen antisites (OZn)

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makes them less probable to affect the conductivity of zinc oxide in equilibrium conditions. Moreover, the donor levels introduced by Oi ((0/+) and (+/++)) are located below the ZnO valence band maximum (VBM), whereas OZn is an acceptor–type defect.

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In the present work we discuss the role played by relevant donors in the polycrystalline ZnO thin films obtained at low temperature (100 – 200°C) by the Atomic Layer Deposition

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(ALD) as this topic is still regarded to be one of the most important issues in terms of the possible ZnO applications in the modern electronic devices, where stable and fully controllable electrical properties of this material are highly demanded.

II. Experimental details ZnO layers were grown in the low temperature range (between 100°C and 200°C) by the ALD process in the Cambridge NanoTech Savannah100 reactor using a double exchange reaction (see Eq. (1) below) involving diethylzinc and deionized water precursors: 3

ACCEPTED MANUSCRIPT Zn(C2H5)2 + H2O → ZnO + 2C2H6.

(1)

The precursors’ pulse/chamber purging time was the same for every ZnO deposition, i.e.: for DEZn: 20ms/8s and for H2O: 20ms/20s, respectively. The exposition time was not applied. Thus, the growth temperature remained the only factor distinguishing the given ALD

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process. The ZnO films were deposited on three different types of substrates: on Si and quartz (SiO2) used for electrical and optical investigations as well as on glass (as a reference substrate). However, the films that underwent the spectroscopic ellipsometry studies were

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only the ones deposited on Si. The examined layers were obtained at temperatures between 100°C and 200°C. Each time 1000 cycles was applied in the ALD process, resulting in the

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ZnO film thickness of about 135 – 180 nm. Noteworthy, no particular differences were noticed between the films grown at the same temperature on different substrates, which were uniformly covered with a ZnO layer exhibiting similar thickness.

The advantage of using so reactive organic compounds as precursors is their high vapor

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pressure (>10 Torr at 300 K), which allows to decrease the growth temperature down to 100°C. This enables to obtain films with low n concentration [17]. The electrical data were acquired at room temperature (RT) from the classical Hall effect

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measurements in the Van der Pauw configuration using a RH2035 PhysTech GmbH system equipped with a permanent magnet producing the magnetic field B = 0.426T. This setup

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allowed to detect electron concentration in the range of 1014 cm-3 – 1022 cm-3. The samples’ size was about 10 mm×10 mm. Electrical ohmic contacts were evaporated in vacuum involving a bilayer of Ti/Au (100 Å/400 Å thick). The CM2203 spectrometer with a Xe lamp serving as an excitation source was used for the RT photoluminescence (PL) investigations. The RT PL spectra were collected in the wavelength range of λem = 340 nm – 820 nm (after excitation with λex = 300 nm). The topography of ZnO layers was analyzed with the Digital Instruments III SPM MultiMode Atomic Force Microscopy system, whereas the ellipsometry

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ACCEPTED MANUSCRIPT measurements were performed using a Woollam M2000D rotating compensator spectroscopic ellipsometer with a wavelength range from 193 nm to 1000 nm in reflection mode. The involved data acquisition and analysis software was CompleteEASE 5.10 J. A. Woollam Co., Inc. The spectroscopic ellipsometry data of Ψ and ∆ were taken at room temperature at angles

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of incidence of 65°, 70° and 75°, using focusing optics that reduces the diameter of the spot to few hundred microns. The SIMS depth profiles of ZnO layers were collected with the IMS6F Cameca system. The annealing experiments in N2–rich as well as O2–rich atmospheres were

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carried out at 700°C (1 min.) in the dedicated AccuThermo AW610 furnace produced by the

III. Results and discussion

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Allwin21 Corporation.

3.1. Composition studies of the ALD–ZnO films. Influence on electrical parameters All the ZnO films subjected to the studies described in this paper are polycrystalline ones.

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They reveal (10.0), (10.1), (00.2) and (11.0) crystallographic orientations as derived from the XRD measurements (not shown here). Figure 1. presents an example of the AFM images obtained for the layers grown at 100 °C and 200 °C. Generally, it was observed that the

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surface roughness expressed by the Root Mean Square (RMS) parameter decreases with the increasing process temperature from 100 °C (corresponding to the roughness of about 9.5 nm,

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see Fig. 1a.) to 200 °C (1.1 nm, as presented in Fig. 1b.). Moreover, the films obtained at higher temperatures show a tendency to grow with the c axis perpendicular to the substrate surface (thus along the (00.2) direction). Further comments on this issue, combined with the extensive analysis of changes occurring in the XRD patterns, related to the ZnO growth temperature and such ALD parameters as pulsing and purging times, one can find in the papers by Kowalik et al. [18, 19]. Noteworthy, similar situation was reported by Lim and Lee in Ref. [20].

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Fig. 1. (Color online.) The AFM images of ZnO layers grown at 100°C (a) and 200°C (b). The decrease in surface roughness is from about 9.5 nm to 1.1 nm, respectively. The square–shaped AFM scan covers 4 µm2.

Importantly, improvement of the ZnO crystallography observed for the layers deposited at

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increased temperature (discussed also in Ref. [19]) affects the ZnO electrical parameters as

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well. Confirming this, Figure 2. presents the relations between the grain size (lower X–axis scale) calculated from the XRD measurements using the Scherrer’s formula – see e.g. Ref. [5], the value of RMS parameter (upper X–axis scale) and the free electron Hall mobility

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measured at RT.

Fig. 2. (Color online.) The as grown ZnO electron mobility dependence on the film surface roughness (RMS values as derived from the AFM studies – top scale) and grain size (as calculated from the XRD measurements – bottom scale). The films were obtained at the indicated temperatures.

As all the obtained layers reveal similar thickness, which is for them determined by the applied number of the ALD cycles, the data from Fig. 2. allow us to postulate that the grain boundaries–related scattering is one of the main factors limiting the Hall mobility. Same mechanism has been mentioned in Ref. [21] for the ZnO films grown by MetalOrganic Chemical Vapor Deposition (MOCVD) and independently in Ref. [22] for the magnetron sputtered ZnO:Al layers. Nevertheless, the fact that in case of the studied ZnO films an 6

ACCEPTED MANUSCRIPT increase of only 1 nm in their grain diameter in combination with its growth temperature difference of 20°C results in one order of magnitude higher electron mobility (µ ~2 → ~20 cm2V-1s-1) suggests that this factor is not the only one affecting the value of µ. Such a conclusion is justified basing on the supplemental analysis of the XRD spectra

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obtained from the ZnO films grown at 110°C and 130°C (not shown here). In general, they contain three main peaks related to the (10.0), (00.2) and (10.1) orientations. Interestingly, the differences between their intensity are not very pronounced. Thus, it is assumed that the

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carrier mobility is influenced also by other (e.g. chemical) phenomena, such as changes either in the hydrogen content in ZnO or in the oxygen–to–zinc ratio. Our previous investigations

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show that both of these values drop radically when the film growth temperature increases from 110°C to 130°C [23]. However, the scattering mechanism depends also on the carriers’ concentration. If n exceeds the threshold value for the metal – insulator transition, which for ZnO is estimated to be n ≈ 9.4×1018 cm-3 at RT [24], tunneling dominates the electron

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transport through grain boundaries. For such n values potential barriers at the grains are thin enough (only a few angstroms) to allow efficient tunneling. Additionally, the donors’ wavefunctions begin to overlap, forming an ‘impurity band’ [25, 26]. Such a situation may

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occur in case of highly conductive ZnO films, as observed for the layers grown at elevated

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temperature, e.g. 200°C (see Refs. [22, 27] and the discussion presented in Section 3.2.3). Interestingly, the ALD–ZnO films reveal an anti–correlation between the hydrogen concentration in the layers and the n value as derived from the SIMS profiles. This, as we have already reported in another studies [27, 28], indicates that hydrogen does not play the dominant role as shallow donor in the examined films. The respective data presented therein suggest that the increase in electron concentration observed for the samples obtained at higher temperatures may be related to their slight nonstoichiometry, namely, with increasing ZnO

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ACCEPTED MANUSCRIPT deposition temperature the contribution of zinc in the films becomes higher. Likely, a part of Zn atoms is located in the interstitial positions as well, becoming a shallow donor [27]. Surprisingly, despite a high level of n, we do not observe any indication of Zni clustering as it is presented in Ref. [29] for the case of Zn–doped GaAs. Otherwise, the clusters should

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be likely detected in the SEM or TEM analysis of the layers (not shown here).

3.2. Optical characterization of the ZnO layers. The influence of RTP annealing on

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defects and electrical conductivity 3.2.1 Ellipsometry studies

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The spectroscopic ellipsometry data of Ψ and ∆ were analyzed using a three–layer model consisting of a silicon substrate covered with a 2.7 nm thick SiO2 native oxide as a first layer, ALD–ZnO film as a main layer and, eventually, a rough surface layer (see the sketch

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presented in Fig. 3., similar to the approach presented e.g. in Ref. [30]).

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Fig. 3. The optical model used to represent the samples, consisting of silicon substrate, covered with thin native silicon oxide layer, main ALD–ZnO layer and the top rough surface layer. (similarly to the approach presented e.g. in Ref. [30]). The sketch is not depicted in scale.

The interfaces between the different layers were assumed to be abrupt and the layers – homogeneous in depth. The optical constants of ALD–ZnO film were regarded to be isotropic.

For the detailed analysis the tabulated values of optical constants of the silicon substrate and the silicon native oxide from the CompleteEASE® software database were used. To determine the thicknesses and the optical constants of ALD–ZnO layers, the following fit procedure was involved: first the Ψ and ∆ in the part of the spectrum where the ZnO layer

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ACCEPTED MANUSCRIPT is transparent were fitted using the Cauchy model in order to determine the thickness of ZnO films. Second, the b–spline was applied in order to extend the fit to the absorbing area of the film, keeping the thickness of ZnO layers constant, as determined from the transparent part of the spectrum. Finally, the b-spline layer was parametrized with the general oscillator model

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consisting of one PSemi-M0 and two Gaussian oscillators [31]. The rough layer for all samples was modeled by Bruggeman’s Effective Medium Approximation (EMA) of 50 % voids and 50 % bulk material [31]. The thickness and the surface roughness of each ZnO film

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as determined from the fits, together with the optical band gap (discussed further on) are given in Table 1.

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Growth Layer Optical Layer thickness MSE temperature roughness bandgap width (nm) (oC) (nm) (eV) 100 162.24 15.93 3.27 4.345 110 168.80 17.09 3.27 4.930 130 179.99 8.98 3.29 4.766 160 164.34 4.23 3.31 5.244 200 136.69 2.2 3.34 4.691 Table 1. Thickness, roughness and optical bandgap of the ZnO layers as obtained from the ellipsometric analysis.

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The experimental Ψ and ∆ values for ZnO sample obtained at 100°C together with the fitted theoretical model are depicted in Fig. 4. The Mean Square Error (MSE) factors of the fits for all samples are in the range of 4.3 to 5.2, which is an indication that the model

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represents the experimental Ψ and ∆ very well.

Fig. 4. (Color online.) Spectroscopic ellipsometry spectrum for sample ALD–ZnO layer deposited at 100°C, measured at the angle of incidence of 70°. The black empty circles denote experimental Ψ values, whereas the red empty triangles – experimental ∆. The theoretical model is depicted with solid lines.

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ACCEPTED MANUSCRIPT 3.2.2 Optical constants and bandgap width assessment The optical constants of each ZnO layer as determined from the fits are presented in Fig. 5. for the as grown ZnO layers deposited on Si substrates. The noticeable shift of refractive index (nr) peak towards the higher energies is observed with increasing ALD

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process temperature. For the ZnO layers deposited above 130°C the measured free electron concentration remains above the metal–insulator transition threshold value (as mentioned above and listed in Table 2. that will be discussed in details later on). In this case the Fermi

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level is situated in the conduction band, which causes a shift of the optical absorption edge towards the higher energies. This effect is known as a Burstein–Moss phenomenon [32].

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The peak values of nr increase for the ZnO layers deposited at temperatures from 100°C to 160°C. This may be attributed to the improvement of the film crystallinity and/or reducing the defects’ concentration with increasing deposition temperature. Similar behavior has been reported in Ref. [33]. The slightly lower maximal value of refractive index for the ZnO layer

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deposited at 200°C stems likely from the high electron concentration. Obviously, the extinction coefficient k is close to zero in the transparent region (below the ZnO absorption

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edge).

Fig. 5. (Color online.) Refractive index and extinction coefficient of the ZnO films deposited at different temperatures, from 100°C to 200°C (as indicated).

The optical bandgap (Eg) of examined ZnO films was estimated from the Tauc plots (see Fig. 6a.), using the relation of α ∝ hν − E g hv valid for the direct bandgap semiconductors

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ACCEPTED MANUSCRIPT [34]. Extrapolation of the corresponding linear part of (α hν ) (hν ) plot to (α hν ) = 0 yields 2

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then the estimated value of the optical band gap of each ZnO layer. The dispersion of absorption coefficient α was determined from the dispersion of

found by fitting the experimental data of Ψ and ∆.

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extinction coefficient k (λ) using the dependence of α = 4π k λ , whereas the k parameter was

The assessed optical bandgap of ZnO film deposited at 100°C as determined from the

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corresponding Tauc plot is 3.27 eV. Comparing this with the same parameter obtained for the ALD–ZnO deposited at 200°C one can notice that in the latter case Eg is approximately

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each ZnO layer are gathered in Table 1.

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70 meV wider (3.34 eV) than the previous value. The remaining widths of optical bandgap for

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Fig. 6. (Color online.) (a): Tauc plot for the ALD–ZnO films deposited at different (indicated) temperatures. The dashed lines show linear fits to the experimental data. The intercept at (αhν)2 = 0 estimates the optical bandgap (Eg) of the ZnO film. (b): Linear approximation of the n2/3 versus Eg dependence proving the existing Burstein–Moss effect in the examined set of ZnO layers (see e.g. Ref. [32]).

The observed shift of the absorption edge is generally attributed to the Burstein–Moss effect (as also confirmed by the linear Eg versus n2/3 dependence depicted in Fig. 6b.) caused by filling of the lower states of the conduction band with electrons. Thus, the noticed phenomenon may be assigned to the larger free electron concentration in the examined ZnO films occurring for the layers deposited at higher temperatures. This is likely related to the higher amount of Zni defects (shallow donors), revealing an activation energy of about 30 –

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ACCEPTED MANUSCRIPT 40 meV as obtained from the low-temperature PL investigations discussed in our previous work [27]. In order to verify this statement the profound examinations of RT defect–related photoluminescence will be discussed in the next part of the paper. The appropriate

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experiments were carried out both for the as grown ALD–ZnO films and after exposing them to the RTP process at 700°C (60 sec.) in nitrogen- and oxygen–rich conditions.

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3.2.3 Defect–related room–temperature photoluminescence studies in polycrystalline ZnO The RT PL was measured for ZnO layers obtained on a quartz (SiO2) substrate,

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demonstrating that the intensity of defect–related emission anti–correlates with the carrier concentration. This can be seen in Fig. 7., where the respective PL spectra collected for the as grown films are depicted. The anti–correlation is accompanied by an increase in the carriers mobility, which indicates that a decreased electron concentration in the samples grown at

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lower temperatures is related to a compensation effect caused by the deep defects (see Table 2.). In Ref. [35] it was claimed that the oxygen vacancy (VO) induces PL emission in the green/yellow spectral range. It was postulated that VO are deep donors in ZnO, which pin

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the Fermi level at approximately 700 meV below the conduction band (CB) minimum. The explanation given in Refs. [36, 37] remains consistent with the results of RT PL investigations

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presented in the current work.

Additionally, with increasing the ALD process temperature a rise in intensity of the near band edge luminescence (the one peaked at about 3.25 eV, see Fig. 7.) is noticed. This is seen for all the examined films, confirming the improving structural quality of ZnO films obtained at increased temperatures.

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Fig. 7. (Color online.) The RT PL spectra of ZnO/SiO2 films at the temperatures between 100°C and 200°C. As can be seen the defect–related photoluminescence anti–correlates with the layer growth temperature. The corresponding RT electron concentration as derived from the Hall effect measurements is gathered in Table 2.

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However, in order to shed more light on the problem of physical origin of the defect–

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related PL a set of Rapid Thermal Processing (RTP) experiments was carried out at 700°C in the nitrogen and oxygen–rich atmosphere for 60 seconds, after which the RT PL spectra were acquired and compared with the ones obtained for the as grown ZnO layers. The spectra are qualitatively similar for the films deposited at the temperature below 130°C, as it is shown in Fig. 8a. Above this temperature, the relation between the intensity of the defect–related PL

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peaks changes (see Figs. 8b. and 8c.), as will be discussed later on. Additionally, Table 2. gathers the pre– and after RTP electrical characteristics of the films for comparison.

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As demonstrated in Fig. 8a., intensity of the broad defect–related emission in the ALD– ZnO films deposited at the temperatures below 130°C (around 100°C) decreases as a result of

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the RTP annealing. This effect is slightly more pronounced in case of the process performed in oxygen–rich conditions. Moreover, as a result of annealing in O2 the defect–related PL likely consisting of two dominating subbands (peaked at about 1.9 eV and 2.3 eV) becomes more intensive in the red emission area (lower energies), as compared to the one of the as grown samples. This may indicate the presence of zinc vacancy–related defects in the annealed films [11, 38]. Simultaneously, the green emission in the vicinity of 2.25 – 2.3 eV (540 – 550 nm) is quenched. This observation suggests that the oxygen vacancies are compensated as a result of annealing process. The noticeable changes in the reciprocal

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ACCEPTED MANUSCRIPT relation between the mentioned subbands occur also if nitrogen is used for annealing, however, this atmosphere does not affect the intensity of a 2.3 eV subband so evidently. In turn, the slightly different situation was remarked after annealing the layers obtained at 130°C (see Fig. 8b.). Here, the RTP process carried out in the N2 atmosphere resulted in a substantial

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increase in the intensity of the luminescence peaked at about 2.3 eV, likely revealing an important contribution of oxygen vacancy–related defects to this part of the PL spectrum. This remark is confirmed by the fact that (as presented in Fig. 8c.) for the layers grown at

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200°C the luminescence in this area decreases again as a result of RTP process in oxygen. In this case the emission peak related to defects shows a more pronounced red emission,

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indicating relevant changes in the zinc sublattice (as mentioned above)..

The assumptions that a deep level emission (DLE) peaked at 2.3 eV refers to the defects involving oxygen vacancies are additionally supported by the corresponding XPS and RBS studies published by us previously in Ref. [23]. They indicate that the ALD–ZnO layers

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deposited at 100°C contain more oxygen than the ones obtained at 130°C. This could explain

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the weaker DLE in the vicinity of 2.3 eV for the films grown at 100°C after annealing in N2.

Fig. 8. (Color online.) The RT PL spectra of ZnO/SiO2 films obtained at 100°C (a), 130°C (b) and 200°C (c). The results obtained for as grown films (depicted in each panel with black solid lines) are compared with the ones acquired for the films subjected to the RTP annealing for 1 min. at 700°C in N2 (red dotted lines) and O2 (olive dashed lines).

Interestingly, very similar observations were described by Murmu et al. [39] as well as Kennedy et al. [40], who have performed the PL studies of hydrothermally grown ZnO crystals implanted with Gd ions and subsequently annealed in the temperature range of 650 – 750°C in vacuum and oxygen–rich conditions. As it has been noticed, the conducted

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ACCEPTED MANUSCRIPT annealing resulted in the red–shift of the defect-related emission, while the green luminescence was substantially suppressed, indicating the presence of oxygen vacancy– related defects in ZnO. In turn, the unmasked (in parallel) yellow/orange emission has been ascribed to the modification of zinc sublattice. These considerations find the reflection in the

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corresponding electrical data presented in Table 2. Namely, the RTP process carried out in O2 is not only accompanied by a drastic decrease in the electron concentration (reaching three orders of magnitude when compared to the as grown films) but also a substantial drop of their

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mobility is noticed (µ does not exceed ~6.5 cm2V-1s-1). On the contrary, annealing in N2 causes much weaker (if any) drop in electron concentration (up to ~1.5 order of magnitude),

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whereas the mobility increases remarkably (up to ~25 cm2V-1s-1 for the ZnO film grown at 130°C). One of the reasons for this behavior may be a decreased hydrogen content in the layers observed after annealing. Such a conclusion is also supported by the findings described in Refs. [41, 42], which discuss the role of hydrogen interstitials (Hi) and hydrogen – oxygen

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vacancy (H–VO) complexes in the increasing ZnO resistivity caused by removal of these defects through annealing in air. The supplemental SIMS investigations performed by us on the ZnO films after the RTP processes indicate that the annealing in O2 atmosphere decreases

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the amount of adsorbed hydrogen molecules on the ZnO surface. This may explain a relevant

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(reaching one order of magnitude) decrease in H2 concentration that was observed as a result of thermal treatment (from approximately 0.2 – 0.9% before RTP to about 0.02 – 0.04% after the process). Additionally, a much more pronounced DLE band in the vicinity of 2.3 eV occurring for the film obtained at 130°C after its annealing in nitrogen (see Fig. 8b.) is followed by a substantial decrease in electron concentration. To a certain extent similar behavior was observed for ZnO films deposited by radio frequency (RF) magnetron sputtering, as reported by Hamad et al. [43]. This could, in turn, be a hint that there exists another defect, likely introducing deep acceptor states, partially compensating n–type

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ACCEPTED MANUSCRIPT behavior of ZnO and exhibiting a tendency to be active in the zinc sublattice of the examined ALD–ZnO layers. The approach focused on its identification will be based on the deconvolution of the defect–related band of the RT PL spectra collected from the as grown as well as RTP–annealed films.

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Growth As grown layers After RTP at 700°C in N2 After RTP at 700°C in O2 temp./film n n n µ µ µ -3 -3 2 -1 -1 2 -1 -1 thickness (cm-3) (cm ) (cm ) (cm V s ) (cm V s ) (cm2V-1s-1) (nm) 100°C/162 2.88 3.80 1.44 1.70×1017 8.68×1015 3.09×1016 18 18 1.46 2.50 1.00 110°C/170 2.18×10 1.42×1017 1.77×10 20.91 25.03 5.40 130°C/178 7.51×1017 3.88×1016 2.13×1019 19 18 22.60 14.00 2.49 160°C/165 5.74×10 3.63×1016 3.63×10 19 19 18 200°C/136 26.43 2.72 6.51 8.58×10 4.53×10 3.87×10 Table 2. The influence of RTP annealing in nitrogen and oxygen atmospheres at 700°C within 1 min. on the electrical parameters of ZnO films deposited on SiO2 substrate. The process results in substantially decreased electron concentration and their Hall mobility for both gases used.

Nevertheless, it should be stressed that the presented studies prove the complexity of the relations between the optical and electrical response gathered from the examined ZnO films before and after their thermal treatment. As will be seen right below (in Figs. 9. – 11.),

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a proper deconvolution of the cumulative defect–related PL band of the ALD–ZnO layers requires the use of 4 – 5 Gaussian components, pointing out that there exist many defect states of different origin in the examined layers. This makes the analysis of their radiative behavior

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a challenging problem.

The data presented in Figs. 9. – 11. show the RT PL spectra acquired from the ZnO/SiO2

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films deposited at 100°C, 130°C as well as 160°C. The percentage contribution of different defect levels in the as grown layers to the cumulative fit peak analyzed within the energy range between 1.53 eV≤ E ≤ 2.75 eV is depicted and compared with the similar studies carried out after thermal treatment of these layers in nitrogen and oxygen–rich conditions (700°C, 1 min.). To obtain the appropriate energy values of trap levels in the bandgap, the multi–peak Gaussian fit was used in each spectrum case, whereas their contributions to the cumulative peak (adding up to 100%) were obtained by integrating the given Gaussian within the above–mentioned energy limits. Having a look at these figures in combination with 16

ACCEPTED MANUSCRIPT Fig. 8., one can easily notice that for the case of ZnO/SiO2 layers obtained at the lowest temperature (100°C) RTP annealing in both atmospheres results in the evident decrease of the defect–related luminescence with respect to the spectrum obtained for the as grown film. Obtaining the films at slightly elevated temperatures (130°C and 160°C) leads to the more

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intensive luminescence between 1.53 and 2.75 eV after both kinds of thermal treatment and finally, the growth temperature as high as 200°C activates the luminescence at low energies after annealing in oxygen. However, the most relevant features are the noticeable changes in

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the reciprocal intensity relation observed in the two main subbands of the wide DLE peak induced by the RTP processes. Generally, the RTP carried out in nitrogen–rich conditions

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enhances its higher energy (about 2.3 eV) component on the contrary to the same treatment in oxygen, which causes more pronounced luminescence at longer wavelengths (activating the lower, about 1.9 eV, energy subband).

As can be seen for the layers obtained at 100°C and 130°C (see the first two panels from

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the top in Figs 9. and 10.) RTP process in nitrogen substantially enhances the PL intensity in the vicinity of 2.23 eV and 2.38 eV, which total contribution to the analyzed spectrum range increases from approximately 20.1% to 37.6% and from 30% to 96.4% (for ZnO deposited at

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100°C and 130°C, respectively – see the peaks marked as P3–P5 in the corresponding spectra). According to Refs. [44 – 46] this emission may be attributed to the presence of certain defects

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in the oxygen sublattice of ZnO, i.e. VO and OZn. A very similar effect is visible for the layer obtained at 160°C. This, in turn, proves that despite being regarded as a deep donor in ZnO, oxygen vacancy may partially affect its free carrier concentration, especially that as listed in Table 2., the electron density drop after annealing ZnO in N2 is definitely lower than in the case of similar process carried out in oxygen. What concerns the role of antisite OZn acceptor type defect, due to its high formation energy its influence on the electrical parameters is rather limited [12].

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Fig. 9. (Color online.) RT PL spectra acquired from the ZnO/SiO2 films deposited at 100°C. The percentage contribution of different defect levels to the cumulative fit peak (thick orange line) within the energy range between 1.53 eV≤ E ≤ 2.75 eV is shown for as grown samples (top panel), layers after RTP annealing in N2 (700°C, 60 sec.) – middle panel and after RTP annealing in O2 (700°C, 60 sec.) – bottom panel.

Fig. 10. (Color online.) RT PL spectra acquired from the ZnO/SiO2 films deposited at 130°C. The percentage contribution of different defect levels to the cumulative fit peak (thick orange line) within the energy range between 1.53 eV≤ E ≤ 2.75 eV is shown for as grown samples (top panel), layers after RTP annealing in N2 (700°C, 60 sec.) – middle panel and after RTP annealing in O2 (700°C, 60 sec.) – bottom panel.

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Fig. 11. (Color online.) RT PL spectra acquired from the ZnO/SiO2 films deposited at 160°C. The percentage contribution of different defect levels to the cumulative fit peak (thick orange line) within the energy range between 1.53 eV≤ E ≤ 2.75 eV is shown for as grown samples (top panel), layers after RTP annealing in N2 (700°C, 60 sec.) – middle panel and after RTP annealing in O2 (700°C, 60 sec.) – bottom panel.

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ACCEPTED MANUSCRIPT Concerning the post–growth RTP annealing in oxygen (see the top and bottom panels in Figs. 9. – 11.), it can be stated that on the contrary to the situation described above, such treatment induces pronounced luminescence coming from the subband peaked at lower energy (1.70 – 1.71 eV), whereas the previously mentioned emission located between 2.23 and

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2.38 eV becomes either less intensive or even quenched. The latter is particularly visible for the layers grown at 130°C, in which the contribution of these two defect states to the whole analyzed part of the PL spectrum decreases from approximately 30% to 14.7% (see peaks P3

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and P4 in the corresponding panels of Fig. 10.). Simultaneously, in the PL of the layers obtained at 100°C as well as at 160°C, a substantial contribution of defect state exhibiting the

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energy of 1.70 – 1.71 eV appears (taking about 49.5% and 48.2% of the analyzed spectrum range, respectively – see the area of peak P1 in the bottom panels of Figs. 9. and 11.), ascribed to the active role of VZn acceptor–type defect in ZnO [44, 47]. Such a behavior suggests that the RTP–O2 process effectively fills in the oxygen vacancy deep donor states and enhances

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the role of VZn acceptors in the compensation phenomena. Additionally, it definitely affects the luminescence likely originating from the deep donor–like state of VO+ (E ~ 2.00 eV), which is metastable in ZnO [48, 49]. These observations are confirmed by the parallel

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electrical investigations (see Table 2.) indicating a decrease in the electron concentration in ALD–ZnO after the RTP process in O2, reaching three orders of magnitude as compared to

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the as grown films. Thus, thermal treatment in the well adjusted conditions is beneficial for tuning the electrical properties of ZnO for electronic applications. The complete list of defect states detected in the ALD–ZnO/SiO2 films examined in the current work is presented in Tables 3., 4. and 5. This is done in order to shed a possibly full light on the discussed problem having, however, in mind that the role of native defects in ZnO is still under debate and requires more systematic studies. This is particularly relevant, especially that in parallel to the states revealing the energies of about 1.61 – 1.63 eV as well

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ACCEPTED MANUSCRIPT as 2.56 eV related to the neutral oxygen vacancy [45, 46] and singly ionized zinc vacancy [44], respectively (not discussed in details in the present paper), the tables below contain some trap levels, e.g. E ~ 1.80 – 1.86 eV, which origin has not been cogently clarified so far.

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Growth ~1.62eV ~1.72eV ~1.80eV ~2.00eV ~2.16eV ~2.22eV ~2.36eV ~2.41eV ~2.56eV temp. 100°C (1.86) (1.97) (2.22) 110°C (1.62) (1.95) (2.23) (2.33) 130°C (1.68) (1.97) (2.22) 160°C (1.63) (2.01) (2.23) 200°C (1.61) (2.01) (2.14) (2.23) Table 3. The defect levels as found from the RTPL studies of the as grown ALD–ZnO/SiO2 thin films deposited within the temperature range of 100°C – 200°C. As the emission energy in some cases is difficult to be ascribed to the given and previously identified defect unambiguously it is written in the middle, between the two columns.

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Growth ~1.62eV ~1.72eV ~1.80eV ~2.00eV ~2.16eV ~2.22eV ~2.36eV ~2.41eV ~2.56eV temp. 100°C (1.80) (1.99) (2.23) (2.37) 110°C (1.71) (1.99) (2.23) (2.36) 130°C (1.61) (2.04) (2.23) (2.41) 160°C (2.11) (2.23) (2.38) (2.56) 200°C (1.61) (2.00) (2.23) Table 4. The defect levels as found from the RTPL studies of the ALD–ZnO/SiO2 thin films deposited within the temperature range of 100°C – 200°C and subjected to the RTP annealing process in nitrogen–rich conditions (700°C, 1 min.). As the emission energy in some cases is difficult to be ascribed to the given and previously identified defect unambiguously it is written in the middle, between the two columns.

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Growth ~1.62eV ~1.72eV ~1.80eV ~2.00eV ~2.16eV ~2.22eV ~2.36eV ~2.41eV ~2.56eV temp. 100°C (1.71) (1.95) (2.21) 110°C (1.66) (1.93) (2.23) (2.34) 130°C (1.62) (1.89) (2.23) (2.37) 160°C (1.70) (1.96) (2.22) 200°C (1.67) (1.91) (2.23) Table 5. The defect levels as found from the RTPL studies of the ALD–ZnO/SiO2 thin films deposited within the temperature range of 100°C – 200°C and subjected to the RTP annealing process in oxygen–rich conditions (700°C, 1 min.). As the emission energy in some cases is difficult to be ascribed to the given and previously identified defect unambiguously it is written in the middle, between the two columns.

IV.

Summary and conclusions

Concluding this work concerning the electrical and optical properties of ALD–ZnO thin films obtained in the low temperature range between 100°C and 200°C it is justified to claim that the relevant role in terms of their electrical conductivity is likely played by such defects as oxygen and zinc vacancies. This was verified by the throughout examination of changes in their RT PL spectra induced by the RTP annealing in oxygen– and nitrogen–rich atmosphere (700°C, 1 min.). As it was observed, not only decreases this treatment the free electron 21

ACCEPTED MANUSCRIPT concentration in the layers (up to three orders of magnitude when compared to the as grown films) but also modifies the PL intensity coming from the contributing subbands that form the defect–related part of the PL spectrum. This happens dependently on the applied annealing conditions.

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Analyzing the contributions to the RT PL spectrum within 1.53 eV ≤ E ≤ 2.75 eV leads to the conclusion that the RTP annealing in N2 generally results in the more pronounced DLE from the subband in the higher energy range (approximately 2.23 – 2.38 eV, i.e. ~520 –

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560 nm), where the response qualitatively attributed to the presence of oxygen vacancies in ZnO dominates the spectrum. On the contrary, RTP process carried out in the oxygen–rich

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atmosphere effectively quenches the green luminescence and activates (enhances) the emission processes in the lower energy range (about 1.7 – 1.9 eV, i.e. ~650 – 730 nm), where mainly the VZn trap states give the contribution to the measured PL. However, taking into account that three to five contributions are needed to properly

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deconvolute the defect PL band, the situation concerning defects in the ALD–ZnO thin films deposited in the temperature range between 100°C and 200°C is quite complicated. The high electron concentration observed in the layers grown at 200°C seems to be caused by the

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presence of Zni shallow donor as indicated both by the ellipsometric studies discussed in the

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present work and proving the existence of Burstein–Moss effect as well as by the previously published temperature–dependent PL investigations [27] yielding the donor activation energy of about 30 – 40 meV.

In turn, the low electron concentration measured for the ZnO films obtained at lower temperatures (100°C) is likely a result of two phenomena. The first one is a decreased amount of zinc interstitials. The second one is a compensation effect caused by acceptor–like defects attributed to zinc vacancies as indicated by a low carrier mobility and enhanced defect PL observed in these films.

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ACCEPTED MANUSCRIPT Acknowledgements The work was supported through the project of the National Science Centre of Poland (NCN) – decision number DEC-2013/09/D/ST5/03879 (T.A.K. and G.L.), by the Polish National Centre for Research and Development (NCBiR) through the project PBS2/A5/34/2013 (E.G.)

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as well as by the EU 7th FP REGPOT project INERA (GA3 16309).

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Abstract graphics

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Fig. 0. (Color online.) Comparison of the RT PL spectrum of as grown ALD-ZnO film deposited at 100°C (1) with the one obtained after RTP process performed on this layer in oxygen-rich conditions (700°C, 60 sec.) (2). The presented multi-Gaussian deconvolution illustrates the enhanced role of VZn-related radiative centers occurring after RTP annealing in oxygen.

Journal of Alloys and Compounds – bullet points highlights
ZnO films with low electron concentration were grown by the Atomic Layer Deposition;
Structural optical and electrical properties of the as grown ZnO were investigated;

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Photoluminescence results from as grown and RTP-annealed ZnO films were analyzed;
Basing on the PL studies the relevant defects in ZnO films have been identified;

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The influence of defects on the properties of the ALD-ZnO films has been discussed;

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ACCEPTED MANUSCRIPT Journal of Alloys and Compounds – bullet points highlights
ZnO films with low electron concentration were grown by the Atomic Layer Deposition;
Structural optical and electrical properties of the as grown ZnO were investigated;
Photoluminescence results from as grown and RTP-annealed ZnO films were analyzed;

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The influence of defects on the properties of the ALD-ZnO films has been discussed;

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Basing on the PL studies the relevant defects in ZnO films have been identified;

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ACCEPTED MANUSCRIPT Journal of Alloys and Compounds – Prime Novelty Statement (amended after Reviewer #1 suggestions) Hereby, we confirm that the current manuscript entitled “Diversity of contributions leading to the nominally n–type behavior of ZnO films obtained by low temperature Atomic Layer Deposition”, by Tomasz A. Krajewski et al., which is being submitted for publication in

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“Journal of Alloys and Compounds” contains the original work of all authors that has not been published previously in any other journal (except in the form of an abstract).

Furthermore, the manuscript is not under consideration for publication anywhere. Prior to the submission it has been approved by all authors and tacitly or explicitly by the responsible

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authorities where the work was carried out. If accepted, the paper will not be published elsewhere in the same form, in English or in any other language without the written permission obtained from the copyright–holder. This statement concerns both electronic and

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in–print publication.

The submitted paper is devoted to the discussion of possible contributions leading to the nominal n-type behavior of zinc oxide (ZnO) films. The investigated layers were obtained in the low-temperature (100°C ≤ Tg ≤ 200°C) Atomic Layer Deposition (ALD) process from diethylzinc (DEZn) and deionized water precursors.

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Using a variety of experimental techniques the morphological and electrical parameters of ZnO films have been investigated in combination with their composition analysis and optical response, especially in the defect-related, i.e. 1.53 eV ≤ E ≤ 2.75 eV range. Judging from the performed experiments, we attributed the higher free electron

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concentration in ZnO that occurs with increasing ALD process temperature to the presence of shallow donors, such as zinc interstitials (Zni) or likely their complexes in this material.

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Importantly, the performed RTP annealing in oxygen and nitrogen-rich atmosphere (700°C, 60 sec.) has shown that the changes in electrical parameters may also be ascribed to the active role of complexes involving deep defects such as oxygen- (VO) or zinc- (VZn) vacancies Having in mind that the debate concerning the defect states in ZnO and their role in electrical and optical behavior of this material still draws an attention, we believe that the present paper may be scientifically significant for the Community of Readers of “Journal of Alloys and Compounds”. Tomasz A. Krajewski, Ph.D. Institute of Physics, Polish Academy of Sciences Warsaw/Poland

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