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argon gas mixture of Al-doped ZnO thin films with improved electrical and optical properties
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Sequential PLD in oxygen/argon gas mixture of Al-doped ZnO thin films with improved electrical and optical properties Tudor Coman a , Daniel Timpu b , Valentin Nica c,d , Catalin Vitelaru e , Alicia Petronela Rambu f , George Stoian g , Mihaela Olaru b , Cristian Ursu b,∗ a
“Gheorghe Asachi” Technical University of Iasi, Blvd. Mangeron no. 64, 700029 Iasi, Romania Polymer Materials Physics Laboratory, “Petru Poni” Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, 700487 Iasi, Romania c Faculty of Physics, “Al. I. Cuza” University, 11 Carol I Blvd., 700506 Iasi, Romania d Department of Physical Chemistry, Saarland University, 66123 Saarbrucecken, Germany e National Institute for Optoelectronics, 409 Atomistilor St., Magurele-Bucharest, P.O. Box MG 05, Romania f Research Center on Advanced Materials and Technologies, Sciences Department, “Al. I. Cuza” University, 11 Carol I Blvd., 700506 Iasi, Romania g National Institute of Research and Development for Technical Physics, 47 Mangeron Blvd., 700050 Iasi, Romania b
a r t i c l e
i n f o
Article history: Received 9 May 2016 Received in revised form 9 December 2016 Accepted 11 January 2017 Available online xxx Keywords: Al-doped ZnO thin films Sequential PLD Room temperature deposition
a b s t r a c t Highly conductive transparent Al-doped ZnO (AZO) thin films were obtained at room temperature through sequential PLD (SPLD) from Zn and Al metallic targets in an oxygen/argon gas mixture. We have investigated the structural, electrical and optical properties as a function of the oxygen/argon pressure ratio in the chamber. The measured Hall carrier concentration was found to increase with argon injection from 1.3 × 1020 to 6.7 × 1020 cm−3 , while the laser shots ratio for Al/Zn targets ablation was kept constant. This increase was attributed to an enhancement of the substitution doping into the ZnO lattice. The argon injection also leads to an increase of the Hall mobility up to 20 cm2 V−1 s−1 , attributed to a reduction of interstitial-type defects. Thus, the approach of using an oxygen/argon gas mixture during SPLD from metallic targets allows obtaining at room temperature AZO samples with high optical transmittance (about 90%) and low electrical resistivity (down to 5.1 × 10−4 cm). © 2017 Elsevier B.V. All rights reserved.
1. Introduction As a versatile material with numerous applications, zinc oxide continues to attract the interest of researchers. When deposited as thin films, some of its common uses include gas [1] and ultraviolet radiation [2] sensors. It is also part of a group of oxide semiconductors investigated for making dye-sensitized solar cells [3]. For the past decade and a half it has been the subject of investigations as one of the most promising diluted magnetic semiconductors [4], with a theoretically high Curie temperature, as predicted by Dietl et al. [5]. It is well known that ZnO belongs to the class of transparent conductive oxides (TCO), which means it could be used as an electrode in devices such as LED/OLEDs or solar cells. For this purpose, it is most often doped with aluminum in order to upgrade its performances. Currently, the market is dominated by materials like Indium doped Tin Oxide (ITO), which until recently have shown superior properties. The ongoing efforts aiming to improve
∗ Corresponding author. E-mail address: [email protected] (C. Ursu).
the competitiveness of Al-doped ZnO (AZO) are usually motivated by the availability of all atomic components, low cost and reduced toxicity, as opposed to that of ITO. Obtaining AZO films with good electrical and optical properties at room temperature, on heat sensitive substrates such as polymers, for flexible and transparent electronics, still remains a challenge [6]. There are only scarce reports, focused exclusively on the study of impurity doped ZnO thin films deposition at room temperature, regardless of the substrate or doping element used [7]. Most of the studies are focused on material fabrication in a relatively low temperature range of 100–200 ◦ C [8], and at elevated temperatures [9,10]. This approach is necessary mainly because the particles arriving at the substrate need a high mobility for arranging themselves in a crystallographic state with a high degree of order, which is known to influence the electrical and optical properties. Among the various techniques proposed for AZO thin films fabrication, such as magnetron sputtering [7], PLD [11] or sol-gel [12], PLD is one of the most promising for obtaining high quality AZO thin films. The main advantages are the lower crystallization temperature needed to obtain a highly oriented structure and the flexibility it offers for choosing the optimum experimental parameters [13–15]. Never-
http://dx.doi.org/10.1016/j.apsusc.2017.01.102 0169-4332/© 2017 Elsevier B.V. All rights reserved.
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Table 1 Experimental deposition parameters of AZO samples in oxygen and in the oxygen/argon gas mixture. Oxygen samples PT [Pa]=PO2 [Pa] Oxygen/argon samples PO2 [Pa] (PT [Pa] = 4.6) O5.4 Ar0 O4.6 Ar0 O3 Ar0 O1 Ar0
5.4 4.6 3.0 1.0
O1.6 Ar3.0 O1.4 Ar3.2 O1.0 Ar3.6
1.6 1.4 1.0
theless, an excessive momentum transfer from the highly energetic laser ablation particles, having several hundreds of eV in the initial stage of plume expansion [16], could damage the lattice structure, having direct repercussions on the electrical properties of the growing film. It is therefore important to be able to control the energy of the arriving species, to obtain the desired properties without damaging the thin film structure or the substrate. Impurity doped ZnO films with a low electrical resistivity could be obtained by increasing at the same time both the mobility and the carrier concentration. In practice, this is difficult to achieve. A large number of donors supplying free carriers will result in a large number of ionized impurities, thus limiting the mobility due to scattering processes [17]. Thus, highly conductive AZO thin films can be obtained by assuring an adequate high number of free electrons while increasing the mobility. A high carrier concentration can be achieved as a result of doping and the creation of additional oxygen vacancies. When doping ZnO, the conductivity is enhanced following the replacement [18] of the zinc lattice ions with impurity elements (substitution doping), but this is accomplished by most of the methods which employ the heating of the substrate. In a previous work we have demonstrated that polar and nonpolar Al-doped ZnO thin films could be fabricated at room temperature by SPLD from metallic targets in an oxygen atmosphere [19]. Preferential crystalline orientations along the (001) or (110) axes were obtained by altering, through a combination of dopant concentration and deposition pressure, the surface diffusion of the particles leading to the film growth. However, the electrical and optical properties (lowest resistivity of 5.4 × 10−2 cm and transmittance less than 80%) were unsuitable for use the obtained materials as transparent electrodes. In this paper, we report an improvement over our previous results. Highly conductive transparent AZO thin films were obtained by using an oxygen/argon gas mixture instead of pure oxygen inside the chamber. The increase of both the carrier concentration and mobility was achieved, and AZO samples with ∼90% optical transmittance in the visible domain and low electrical resistivity (about 5 × 10−4 · cm) were obtained. The results demonstrate that the SPLD of AZO thin films from metallic targets in an oxygen/argon gas mixture leads to better electrical and optical properties. Moreover, having been obtained at room temperature, the material is an excellent candidate to be used for transparent and flexible electronics. 2. Experimental details AZO thin films were obtained at room temperature on glass substrate through sequential pulsed laser ablation from individual Zn and Al metallic targets in an oxygen/argon gas mixture. More details on the sequential PLD technique are given in our previous work [19]. The energy source for ablation was a KrF excimer laser (248 nm wavelength, 20 ns pulse duration). An ablation scheme of 30/5 laser shots on the Zn/Al targets, repeated 381 times, was used for depositing all the samples. The pumping speed was maintained the same in all cases, while the gas flow rates were adjusted for each experiment, as described below. Reference AZO samples were deposited in a pure oxygen atmosphere with deposition pressures ranging from 1 to 5.4 Pa (denoted by Ox Ar0 in Table 1). For the
rest of the samples, argon was injected in the chamber in various amounts, aiming to achieve oxygen partial pressures of 1.6, 1.4 and 1 Pa for a constant total deposition pressure of 4.6 Pa. The comparison was made for two distinct experimental situations, namely (i) using the same total pressure and different oxygen flow rates (to compensate for argon addition), and (ii) using the same oxygen flow and different total pressures (resulting from the argon addition). This approach allows one to distinguish between effects induced by changes of the oxygen content in the deposition environment (constant pressure) and the effect of changing the deposition pressure (constant oxygen partial pressure), respectively. The glass slides used as substrates were conditioned prior to deposition by ultrasonic cleaning for 10 min in distilled water, then rinsing in ethanol and acetone, being finally dried by blowing dry nitrogen on the surface. The deposition chamber was evacuated to a base pressure of 0.1 mPa and pre-ablation of both metallic ® targets (zinc with 99.95% purity, Goodfellow TM and aluminum with 99.95% purity, Nilaco Corporation) for a number of 3000 laser shots was performed to avoid film contamination from both the residual gas and surface impurities. The target to substrate distance of 4 cm, the laser shot frequency of 5 Hz and the laser fluence of 3 J/cm2 were kept constant during all samples deposition. The obtained samples were investigated structurally by Wide Angle X Ray Diffraction − WAXD, on a Diffractometer D8 ADVANCE (Bruker AXS, Germany) using the Cu-Ka radiation (l = 0.1541 nm), a parallel beam with Gobel mirror and a Dynamic Scintillation detector. All the diffractograms were registered at room temperature in Bragg-Brentano geometry over the range 2 = 30–60◦ with scanning angle rate of 0.02◦ and a 1s/step count time. The optical and electrical properties were subsequently analyzed by using ultraviolet–visible near-infrared spectroscopy − UV/Vis/NIR (Evolution 300, Thermo Scientific) and a Hall measurement system (HMS 3000, Ecopia) in Van der Pauw configuration, respectively. For electrical investigations ohmic contacts were deposited by PLD from indium metallic targets at high vacuum (0.1 mPa) on the four corners on the square samples of 15 × 15 mm2 area. Prior to the electrical measurements, the thickness was determined by sputtering the AZO samples until reaching the substrate with a focused ion beam (Carl Zeiss NEON 40 EsB CrossBeam) and subjecting them to scanning electron microscopy (SEM JEOL JSM 6390) analysis in section view. Top side scanning images were obtained also by SEM for determining the surface morphology of the samples. The deposition rates were determined in real time by using a quartz crystal microbalance – QCM (Q-pod from Inficon) equipped with 6 MHz quartz crystal oscillator and gold coated sensor. While using the Zn target will result in the deposition of ZnO, we assumed that the ablation of Al will lead to the deposition of Al2 O3 . In addition, we used this data together with knowledge of the ablation pattern (the number of pulses used for aluminum and zinc target ablation in a sequence) to obtain a rough initial estimation of the Al/Zn atomic ratio in the film. Considering that PLD deposited layers of ZnO and Al2 O3 have the same densities as the bulk material, the doping concentration can be expressed as: Al (%) = 19
rAl2 O3 rZnO
(1)
where rAl2O3 and rZnO are the deposition rates expressed in identical thickness/time units. An LS 55 system from Perkin Elmer was used to study the photoluminescence properties at room temperature by using a 320 nm excitation wavelength from a pulsed UV xenon source. 3. Results and discussions Our approach of using metallic targets instead of ceramics for AZO thin films PLD facilitates the obtaining of desired doping con-
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Table 2 Structural parameters derived from Rietveld analyses, thicknesses and optical band gap of the SPLD AZO samples.
Fig. 1. Representative morphology and thickness determination by SEM-FIB (sample O1.6 A3.0 ).
Samples
c (Å)
D [nm]
GOFa
Thickness [nm]
BG [eV]
O5.4 Ar0 O4.6 Ar0 O1.6 Ar3.0 O1.4 Ar3.2 O1.0 Ar3.6 O3 Ar0 O1 Ar0
5.2313 5.2433 5.2072 5.1899 5.2170 5.2651 –
21.4 32.2 30.9 28.4 34.6 28.6 –
1.16 1.48 1.25 1.09 1.04 1.33 –
346 367 296 272 265 383 405
3.43 3.67 3.83 3.85 3.68 3.72 3.88
a
GOF – goodness of fitting.
Fig. 2. Structural features revealed by X-ray diffraction of AZO thin films as a function of a) oxygen deposition pressure and b) oxygen partial pressure.
centrations but could also lead to the formation of particulates on the film surface. These thermally derived droplets that may result from the ablation process could affect the quality of the resulting materials and, consequently, the performances of the device in which they may be used. Thus, in order to avoid that, we have decreased the laser fluence down to 3 J/cm2 . SEM scans showed particulate-free surfaces for the deposited AZO thin films. Also, a granular morphology was evidenced for all the samples, with an example of this type of morphology given in Fig. 1. The X-ray patterns of reference samples (represented in Fig. 2a) deposited in a pure oxygen atmosphere reveal a typical ZnO wurtzite structure. At deposition pressures higher than 1 Pa, there is a dominant (002) diffraction peak that shifts toward lower 2 values as the oxygen pressure decreases. Other orientations like (100), (101) and (110) are also present, but have very low intensities. Generally, the growth of the ZnO thin films on amorphous substrates takes place preferentially, with the c-axis perpendicular to the substrate regardless of the method used, due to minimization of the surface energy density [20,21]. As the oxygen pressure decreases from 5.4 Pa to 4.6 Pa, the crystalline quality of the sample is enhanced as revealed by larger crystallite sizes, D (see Table 2). Further decreasing the deposition pressure, leads to a deterioration of the crystalline quality and for 1 Pa oxygen pressure only a broad asymmetric peak in the region 2 = 55–56◦ can be observed, suggesting the presence of an additional phase beside ZnO in the composition of the sample [19]. In Fig. 2b are presented the X-ray patterns of the samples deposited in oxygen/argon mixture. Unlike the case of AZO deposition in an oxygen only atmosphere, both (002) peak position and lattice constant c of the samples don’t vary monotonically with the oxygen partial pressure (PO2 ) and show a reverse trend. The (002) peak shifts to higher 2 values when lowering PO2 from 4.6 Pa to 1.4 Pa. Then, the preferential orientation vanishes when PO2 is
Fig. 3. Influence of argon addition on QCM determined deposition rates during AZO thin films deposition by SPLD technique.
further lowered to 1 Pa. This behavior is equivalent to a lattice constant variation from 5.2433 Å (sample O4.6 Ar0 ) down to 5.1899 Å (O1.4 Ar3.2 ) then back up to 5.2170 Å (O1.0 Ar3.6 ). The values of the lattice constant are known to depend on the doping concentration. Despite using the same procedure for depositing all the samples, the Zn/Al ratio in the film may vary due to the gas composition, which affects the deposition rate of species from the Zn and Al plasmas. Using PLD, Ojeda et al. [22] deposited Lax Ca1−x MnO3 in Ar. They found that the film composition was different from that of the target and attributed the changes to a stronger scattering of the lighter species, which eventually leads to a decrease of the light atom concentration in the deposited film. Oh et al. [23] have demonstrated that the laser produced Al plasma exhibits distinct radial expansion behavior with or without an Ar flow in the chamber. In our case, when the oxygen/argon gas mixture is used, Al atoms are likely to be scattered more efficiently by heavier Ar atoms on their way to the substrate. It is expected that higher ratios between the argon and oxygen partial pressures will result in more significant changes of the doping concentration in the AZO films. To verify the scattering hypothesis, we used QCM measurements to determine the deposition rates from Al and Zn targets ablation for a constant oxygen partial pressure PO2 of 1 Pa and different Ar partial pressures. As presented in Fig. 3, the deposition rates of both ZnO and Al2 O3 drop with argon injection. When calculating the percentage of aluminum relative to zinc (following the method detailed in the experimental section, equation 1), a steady decrease from 2.8% to 2% was found as a result of the incremental addition of argon. We have already shown in a previous work [19] that SPLD samples of ZnO doped with Al exhibit defects such as interstitials, which lead to values of the lattice constant c that are larger than those of
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Fig. 5. a) Normalized PL spectra of AZO samples deposited in oxygen and oxygen/argon gas mixture and b) deconvoluted PL spectrum for O1.6 A3.0 sample.
Fig. 4. Hall determined electrical properties: a) resistivity, b) carrier concentration and c) mobility as a function of gas composition. The dotted lines link the values for the samples in the oxygen/argon mixture to the corresponding reference film grown in oxygen, at identical total pressure.
undoped ZnO. Thus, the initial decrease of c that occurs when the inert gas is injected may be explained through a lowering of the number of Al interstitials. Introducing more argon, the interstitials are diminished until only Al ions substituting for Zn remain in the film, and the constant lattice reaches a minimum (the Al atomic
radius being lower than that of Zn). Further injection of argon leads to a decrease of the substitutional dopant concentration and the constant lattice begins to increase toward the value characteristic of undoped ZnO. Next, we evaluated the suitability of our samples as potential transparent electrodes, and we started by analyzing the influence of argon addition on the electrical performance of the obtained SPLD AZO thin films. In Fig. 4 are presented the variation of the resistivity, concentration and mobility of the charge carriers as a function of the oxygen partial pressure. Following argon injection, we notice a considerable decrease of the resistivity with respect to the reference samples obtained in a pure oxygen atmosphere. The resistivity for the reference sample deposited for an oxygen deposition pressure of 4.6 Pa is 2.4 × 10−3 cm, while the argon addition up to an oxygen partial pressure of 1.6 Pa leads to a decrease of the resistivity to a minimum of 5.1 × 10−4 cm. To clearly evidence the effect of argon addition on the electrical resistivity we will separate below the discussion regarding the carrier concentration and Hall mobility. Looking at Fig. 4b, one common feature of the AZO films grown in the oxygen/argon mixture is a higher concentration of charge carriers than in the reference samples. The samples obtained in the oxygen/argon mixture have an average ne ∼ 6 × 1020 cm−3 , with only small variations between them. This value of ne is close to that found for the sample O1 Ar0 obtained at the lowest oxygen pressure (i.e., 6.6 × 1020 cm−3 ), suggesting that a low oxygen content is the cause of the increase of ne . One assumption would be that the additional free electrons are the result of an increased number of donor type intrinsic defects like oxygen vacancies or zinc interstitials (as revealed by photoluminescence analyses; the corresponding data are presented in Fig. 5). Although the experimental data supports the existence of both VO and Zni , the total defect concentration required to inject ∼4 × 1020 cm−3 free electrons would be very high and similar to the dopant concentration. As it is generally assumed that native defect concentrations are much lower that extrinsic impurity concentrations, the higher number of carriers must be related to changes in the Al content. The extra free charges may come from a change of the doping mode. Concomitantly with the reduction of the Al content in the sample (as revealed from QCM measurements), the addition of argon alters the ratio between the number of Al atoms substituting Zn and those placed in interstitial positions. Lackner [24] also observed that the crystallinity of PLD
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ized impurity scattering as the dominant mechanism. The mobility, in this case, is given by the formula [28]:
=
Fig. 6. UV–vis spectra and Tauc plot for direct band gap determination of AZO thin films deposited in: a) oxygen and b) oxygen/argon gas mixture.
deposited AZO thin films was improved by injecting a supplementary argon gas allowing the growth of thermodynamically more stable orientations, such as (002). As additional electrons are injected in the conduction band, the increase of the carrier concentration is directly reflected on the measured optical band gap (i.e., the Burstein–Moss shift [25]). The optical band gap (BG) calculated from the transmission spectra (the corresponding data are presented in Fig. 6) by using the Tauc plot method [26] shows an increase with the oxygen depletion in the deposition chamber (see corresponding values in Table 2). The resistivity also depends on the carrier mobility, which varies with the argon/oxygen ratio, as it can be seen in Fig. 4c. Generally, the mobility may be affected by several scattering processes that can take place inside the material. The interface and surface scattering, which become important at low film thicknesses, can be neglected since the thickness of all our samples is in the range 200–400 nm. Grain boundary scattering is also negligible, since the mean free path of electrons, L given by the formula [27]:
L = h/ (2e) 3n/
1/3
(3)
is about 3 nm, much smaller than the average crystallite size. Here, h is Planck’s constant, e is the electron charge, n is the carrier concentration and is the carrier mobility. Thus, electrons will easily tunnel through the inter-grain potential barrier when their concentration is higher than 1020 cm−3 . This leaves the ion-
3ε2r ε20 h3 e3 m∗2 Z 2
ln 1 +
1 32/3 1/3 εr ε0 h2 n1/3 2m∗ e2
2
(4)
where h, e and ε0 are known physical constants, Z is the electric charge of one impurity, which we consider to be +1, corresponding to singly ionized Al donors in ZnO. The dielectric constant εr is 7.9. For the values of the effective electron mass, m*, we referred to Young et al. [29]. They determined the values for Al-doped ZnO and found that m* increases with carrier concentration. Based on their results, a value of ∼0.5 me can be extrapolated for densities of 6 × 1020 cm−3 , as is the case with most of our samples. By using this equation for our samples the calculated mobility values are higher than the measured ones by a factor between 1.1 and 3.8. This could be explained by the existence of scattering centers with higher charge, for example doubly ionized aluminum. Since trivalent Al replacing bivalent Zn in the lattice sites will result in singly charged Al+ , Al2+ can only be present as interstitials. The experimentally determined mobility is generally higher for samples obtained in the oxygen/argon mixture, sometimes approaching the values calculated if considering only +1 charged impurities. This supports the previously proposed explanation that introducing argon in the deposition chamber reduces the number of interstitial-type defects and favors substitution doping. We have already talked about point defects like Al interstitials and oxygen vacancies in our samples. To gain more insight into the structural defects and their influence on the physical properties of the AZO thin films, room temperature photoluminescence (PL) measurements were performed. In Fig. 5 are presented the PL spectra (normalized to the thickness) of the reference sample obtained in pure oxygen (O4.6 A0 ) and of the samples obtained for different oxygen partial pressures. The PL emission from the AZO samples is divided into two spectral regions: a UV region (with two characteristic peaks, 360 nm and 388 nm attributed to the free-exciton recombination near the band edge and electron transition from the conduction band (CB) to single ionized oxygen vacancy, respectively) and a visible emission from the deep levels attributed to different defects such as oxygen vacancies and interstitial Zn. The deconvolution of the visible part of the PL spectrum (Fig. 5b) identified several components assigned as follows: 416 nm − transition from Zn interstitial levels to the valence band (VB), 438 nm – transition between ionized Zn interstitial defect levels and the VB, 455 nm – radiative recombination of electrons from the doubly ionized Zn vacancy levels with holes in the VB, 480 nm – transition from the CB to oxygen vacancies levels and 520 nm – originating from the radiative recombination of holes given by the deep level oxygen vacancy with electrons from the CB minimum [30]. It can be clearly observed that when the oxygen partial pressure drops down to 1.6 Pa there is an enhancement of the crystalline quality as reflected by the increase of the UV emission at 360 nm (Fig. 5). Further depletion in oxygen of the deposition environment to a partial pressure of 1.4 Pa leads to a decrease of the intensity ratio between the exciton emission band and visible band, following the introduction of the oxygen vacancies. When the oxygen partial pressure reaches the value of 1 Pa, the intensity of the visible band that is related to the defects becomes dominant, describing a deterioration of the crystalline quality of the film. These findings are in good agreement with the previously presented XRD and Hall data which show that with argon injection the (002) preferential orientation changes to a polycrystalline phase with randomly oriented crystallites, while the charge carrier mobility falls below 10 cm2 /Vs. The best ratio between exciton to visible bands emission, encoun-
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tered for an oxygen partial pressure of 1.6 Pa, is reflected by the best electrical and optical properties among the investigated samples. As discussed above, the addition of the inert gas in the deposition chamber leads to an increased degree of substitution of zinc into the lattice. Thus, there is an increase of the crystalline quality of the films (as was observed by structural analyses) and an enhancement of the electrical performances (as both carrier concentration and mobility increase). In the same time, the introduction of inert gas lead to the emergence of supplementary oxygen vacancies that favored the development of Zn interstitials (as evidenced from the discussions on PL data) and ultimately damage the structural quality and electrical performances of the AZO films. It is clear that the deposition environment alters the chemical composition of the samples and hence may be successfully used as new control parameter for the optimization of film properties in room temperature SPLD of AZO thin films. Since AZO aims to be an alternative to high cost ITO material, the optical properties are particularly important. In Figs. 6 are presented the transmission spectra for the AZO samples deposited in oxygen and in the oxygen/argon mixture. The degree of transparency (the mean optical transmittance calculated between 400 and 800 nm) for the samples deposited in the oxygen/argon mixture (for example, sample O1.6 Ar3.0 has ∼90%,) is lower than that of the oxygen reference sample (O4.6 Ar0 with 93%). A low transmittance in the visible domain for semiconducting oxide thin films is often the consequence of a non-stoichiometric composition. Since it occurs at low oxygen deposition pressures, oxygen vacancies are considered responsible, with higher number of such defects leading to an increase of light scattering [31]. Although the AZO films deposited in gas mixture are thinner (see Table 2) with respect to the reference sample at the same total pressure, the increased number of oxygen vacancies is decisive, leading to a decrease of the transmittance. However, when compared to the sample O1 Ar0 , the transmittance values are higher in the oxygen/argon mixture, even if the oxygen pressure is the same in both cases, namely 1 Pa (see Fig. 6). This unusual high transmittance of the oxygen/argon samples at low oxygen partial pressure is due to the considerable decrease of the deposition rate. A higher amount of inert gas means fewer Zn and Al particles arrive at the substrate, as confirmed by the thickness determined from SEM-FIB (see Table 2) and by the deposition rate estimated with QCM as well (see Fig. 3). Therefore, fewer oxygen atoms will be required in order to grow stoichiometric samples. Hence, the samples deposited in the oxygen/argon gas mixture have unexpectedly high transmittance values even if the oxygen partial pressure is low. Along with preventing large stoichiometric deviations through a supplementary scattering of ablated particles, the argon addition also leads to an enhancement of the substitutional doping. Thus, the use of the oxygen/argon gas mixture translates into the introduction of a new control parameter, capable of finely regulating the electrical resistivity, almost independently of the optical transmittance.
4. Conclusions We have demonstrated that the use of a mixed oxygen/argon working environment during room temperature SPLD of Al-doped ZnO thin films considerably improves the electrical characteristics of the obtained materials. The argon addition leads to a lower electrical resistivity (i.e., 5.1 × 10−4 cm) compared to the usual practice of deposition in pure oxygen (i.e., 2.4 × 10−3 cm) due to an enhancement of the substitutional doping during AZO thin films deposition. The mean optical transmittance in the visible domain is not affected by the argon gas injection, since the stoichiometric deviation is prevented in this situation by the reduction of the Al and Zn
deposition fluxes. The optimum deposition conditions have been identified to be an oxygen partial pressure of 1.6 Pa coupled with a total deposition pressure of 4.6 Pa, providing simultaneously the best optical and electrical properties (a high transmittance of ∼90% and a resistivity of 5.1 × 10−4 cm). The presented results demonstrate that by using an oxygen/argon gas mixture in room temperature SPLD from metallic targets of AZO thin films, the electrical and optical properties are considerably improved. This approach may be successfully used for applications involving temperature sensitive substrates in transparent and flexible electronics. Acknowledgments This work was financially supported by the Romanian National Authority for Scientific Research, CNCS – UEFISCDI grant, project number PN-II-PT-PCCA-2013-4-1861> (contract number 272/2014) and by Core Program, under the support of ANCSI, project nr. PN 16.40.01.02. References [1] C.D. Lokhande, P.M. Gondkar, Rajaram S. Mane, V.R. Shinde, Sung-Hwan Han, CBD grown ZnO-based gas sensors and dye-sensitized solar cells, J. Alloy Compd. 475 (2009) 304–311. [2] S.K. Shaikh, S.I. Inamdar, V.V. Ganbavle, K.Y. Rajpure, Chemical bath deposited ZnO thin film based UV photoconductive detector, J. Alloy Compd. 664 (2016) 242–249. [3] G. Perez-Hernandez, A. Vega-Poot, I. Perez-Juarez, J.M. Camacho, O. Ares, V. Rejon, J.L. Pena, G. Oskam, Effect of a compact ZnO interlayer on the performance of ZnO-based dye-sensitized solar cells, Sol. Energy Mater. Sol. C 100 (2012) 21–26. [4] S.L. Ou, H.R. Liu, S.Y. Wang, D.S. Wuu, Co-doped ZnO dilute magnetic semiconductor thin films by pulsed laser deposition: Excellent transmittance, low resistivity and high mobility, J. Alloy Compd. 663 (2016) 107–115. [5] T. Dietl, H. Ohno, F. Matsukura, Hole-mediated ferromagnetism in tetrahedrally coordinated semiconductors, Phys. Rev. B 63 (2001) 195205. [6] T. Minami, Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822–5828. [7] D.K. Kim, H.B. Kim, Room temperature deposition of Al-doped ZnO thin films on glass by RF magnetron sputtering under different Ar gas pressure, J. Alloy Compd. 509 (2011) 421–425. [8] T. Minami, Y. Ohtani, T. Miyata, T. Kuboi, Transparent conducting Al-doped ZnO thin films prepared by magnetron sputtering with dc and rf powers applied in combination, J. Vac. Sci. Technol. A 25 (2007) 1172. [9] Y. Zhao, Y. Jiang, Y. Fang, The influence of substrate temperature on ZnO thin films prepared by PLD technique, J. Cryst. Growth 307 (2007) 278–282. [10] R.S. Ajimsha, Amit K. Das, P. Misra, M.P. Joshi, L.M. Kukreja, R. Kumar, T.K. Sharma, S.M. Oak, Observation of low resistivity and high mobility in Ga doped ZnO thin films grown by buffer assisted pulsed laser deposition, J. Alloy Compd. 638 (2015) 55–58. [11] C. Weigand, R. Crisp, C. Ladam, T. Furtak, R. Collins, J. Grepstad, H. Weman, Electrical, optical and structural properties of Al-doped ZnO thin films grown on GaAs(111)B substrates by pulsed laser deposition, Thin Solid Films 545 (2013) 124–129. [12] K.E. Lee, M. Wang, E.J. Kim, S.H. Han, Structural, electrical and optical properties of sol-gel AZO thin films, Curr. Appl. Phys. 9 (2009) 683–687. [13] X.Q. Gu, L.P. Zhu, L. Cao, Z.Z. Ye, H.P. He, P.K. Chu, Optical and electrical properties of ZnO:Al thin films synthesized by low-pressure pulsed laser deposition, Mater. Sci. Semicon. Proc. 14 (2011) 48–51. [14] M.G. Tsoutsouva, C.N. Panagopoulos, M. Kompitsas, Laser energy density, structure and properties of pulsed-laser deposited zinc oxide films, Appl. Surf. Sci. 257 (2011) 6314–6319. [15] S.Y. Kim, J.H. Lee, J.J. Kim, Y.W. Heo, Effects of temperature target/substrate distance, and background pressure on growth of ZnO nanorods by pulsed laser deposition, J. Nanosci. Nanotechnol. 14 (2014) 9020–9024. [16] C. Ursu, G.O. Pompilian, S. Gurlui, P. Nica, M. Agop, M. Dudeck, C. Focsa, Al2 O3 ceramics under high-fluence irradiation: plasma plume dynamics through space- and time-resolved optical emission spectroscopy, Appl. Phys. A-Mater. 101 (2010) 153–159. [17] J.T. Wang, X.L. Shi, W.W. Liu, X.H. Zhong, J.N. Wang, L. Pyrah, K.D. Sanderson, P.M. Ramsey, M. Hirata, K. Tsuri, Influence of preferred orientation on the electrical conductivity of fluorine-doped tin oxide films, Sci. Rep. 4 (2014) 3679. [18] Z. Pan, X. Tian, S. Wu, C. Xiao, Z. Li, J. Deng, G. Hu, Z. Wei, Effects of Al and Sn dopants on the structural and optical properties of ZnO thin films, Supperlatice Microst. 54 (2013) 107–117.
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Sequential PLD in oxygen/argon gas mixture of Al-doped ZnO thin films with improved electrical and optical properties Tudor Coman a , Daniel Timpu b , Valentin Nica c,d , Catalin Vitelaru e , Alicia Petronela Rambu f , George Stoian g , Mihaela Olaru b , Cristian Ursu b,∗ a
“Gheorghe Asachi” Technical University of Iasi, Blvd. Mangeron no. 64, 700029 Iasi, Romania Polymer Materials Physics Laboratory, “Petru Poni” Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, 700487 Iasi, Romania c Faculty of Physics, “Al. I. Cuza” University, 11 Carol I Blvd., 700506 Iasi, Romania d Department of Physical Chemistry, Saarland University, 66123 Saarbrucecken, Germany e National Institute for Optoelectronics, 409 Atomistilor St., Magurele-Bucharest, P.O. Box MG 05, Romania f Research Center on Advanced Materials and Technologies, Sciences Department, “Al. I. Cuza” University, 11 Carol I Blvd., 700506 Iasi, Romania g National Institute of Research and Development for Technical Physics, 47 Mangeron Blvd., 700050 Iasi, Romania b
a r t i c l e
i n f o
Article history: Received 9 May 2016 Received in revised form 9 December 2016 Accepted 11 January 2017 Available online xxx Keywords: Al-doped ZnO thin films Sequential PLD Room temperature deposition
a b s t r a c t Highly conductive transparent Al-doped ZnO (AZO) thin films were obtained at room temperature through sequential PLD (SPLD) from Zn and Al metallic targets in an oxygen/argon gas mixture. We have investigated the structural, electrical and optical properties as a function of the oxygen/argon pressure ratio in the chamber. The measured Hall carrier concentration was found to increase with argon injection from 1.3 × 1020 to 6.7 × 1020 cm−3 , while the laser shots ratio for Al/Zn targets ablation was kept constant. This increase was attributed to an enhancement of the substitution doping into the ZnO lattice. The argon injection also leads to an increase of the Hall mobility up to 20 cm2 V−1 s−1 , attributed to a reduction of interstitial-type defects. Thus, the approach of using an oxygen/argon gas mixture during SPLD from metallic targets allows obtaining at room temperature AZO samples with high optical transmittance (about 90%) and low electrical resistivity (down to 5.1 × 10−4 cm). © 2017 Elsevier B.V. All rights reserved.
1. Introduction As a versatile material with numerous applications, zinc oxide continues to attract the interest of researchers. When deposited as thin films, some of its common uses include gas [1] and ultraviolet radiation [2] sensors. It is also part of a group of oxide semiconductors investigated for making dye-sensitized solar cells [3]. For the past decade and a half it has been the subject of investigations as one of the most promising diluted magnetic semiconductors [4], with a theoretically high Curie temperature, as predicted by Dietl et al. [5]. It is well known that ZnO belongs to the class of transparent conductive oxides (TCO), which means it could be used as an electrode in devices such as LED/OLEDs or solar cells. For this purpose, it is most often doped with aluminum in order to upgrade its performances. Currently, the market is dominated by materials like Indium doped Tin Oxide (ITO), which until recently have shown superior properties. The ongoing efforts aiming to improve
∗ Corresponding author. E-mail address: [email protected] (C. Ursu).
the competitiveness of Al-doped ZnO (AZO) are usually motivated by the availability of all atomic components, low cost and reduced toxicity, as opposed to that of ITO. Obtaining AZO films with good electrical and optical properties at room temperature, on heat sensitive substrates such as polymers, for flexible and transparent electronics, still remains a challenge [6]. There are only scarce reports, focused exclusively on the study of impurity doped ZnO thin films deposition at room temperature, regardless of the substrate or doping element used [7]. Most of the studies are focused on material fabrication in a relatively low temperature range of 100–200 ◦ C [8], and at elevated temperatures [9,10]. This approach is necessary mainly because the particles arriving at the substrate need a high mobility for arranging themselves in a crystallographic state with a high degree of order, which is known to influence the electrical and optical properties. Among the various techniques proposed for AZO thin films fabrication, such as magnetron sputtering [7], PLD [11] or sol-gel [12], PLD is one of the most promising for obtaining high quality AZO thin films. The main advantages are the lower crystallization temperature needed to obtain a highly oriented structure and the flexibility it offers for choosing the optimum experimental parameters [13–15]. Never-
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Table 1 Experimental deposition parameters of AZO samples in oxygen and in the oxygen/argon gas mixture. Oxygen samples PT [Pa]=PO2 [Pa] Oxygen/argon samples PO2 [Pa] (PT [Pa] = 4.6) O5.4 Ar0 O4.6 Ar0 O3 Ar0 O1 Ar0
5.4 4.6 3.0 1.0
O1.6 Ar3.0 O1.4 Ar3.2 O1.0 Ar3.6
1.6 1.4 1.0
theless, an excessive momentum transfer from the highly energetic laser ablation particles, having several hundreds of eV in the initial stage of plume expansion [16], could damage the lattice structure, having direct repercussions on the electrical properties of the growing film. It is therefore important to be able to control the energy of the arriving species, to obtain the desired properties without damaging the thin film structure or the substrate. Impurity doped ZnO films with a low electrical resistivity could be obtained by increasing at the same time both the mobility and the carrier concentration. In practice, this is difficult to achieve. A large number of donors supplying free carriers will result in a large number of ionized impurities, thus limiting the mobility due to scattering processes [17]. Thus, highly conductive AZO thin films can be obtained by assuring an adequate high number of free electrons while increasing the mobility. A high carrier concentration can be achieved as a result of doping and the creation of additional oxygen vacancies. When doping ZnO, the conductivity is enhanced following the replacement [18] of the zinc lattice ions with impurity elements (substitution doping), but this is accomplished by most of the methods which employ the heating of the substrate. In a previous work we have demonstrated that polar and nonpolar Al-doped ZnO thin films could be fabricated at room temperature by SPLD from metallic targets in an oxygen atmosphere [19]. Preferential crystalline orientations along the (001) or (110) axes were obtained by altering, through a combination of dopant concentration and deposition pressure, the surface diffusion of the particles leading to the film growth. However, the electrical and optical properties (lowest resistivity of 5.4 × 10−2 cm and transmittance less than 80%) were unsuitable for use the obtained materials as transparent electrodes. In this paper, we report an improvement over our previous results. Highly conductive transparent AZO thin films were obtained by using an oxygen/argon gas mixture instead of pure oxygen inside the chamber. The increase of both the carrier concentration and mobility was achieved, and AZO samples with ∼90% optical transmittance in the visible domain and low electrical resistivity (about 5 × 10−4 · cm) were obtained. The results demonstrate that the SPLD of AZO thin films from metallic targets in an oxygen/argon gas mixture leads to better electrical and optical properties. Moreover, having been obtained at room temperature, the material is an excellent candidate to be used for transparent and flexible electronics. 2. Experimental details AZO thin films were obtained at room temperature on glass substrate through sequential pulsed laser ablation from individual Zn and Al metallic targets in an oxygen/argon gas mixture. More details on the sequential PLD technique are given in our previous work [19]. The energy source for ablation was a KrF excimer laser (248 nm wavelength, 20 ns pulse duration). An ablation scheme of 30/5 laser shots on the Zn/Al targets, repeated 381 times, was used for depositing all the samples. The pumping speed was maintained the same in all cases, while the gas flow rates were adjusted for each experiment, as described below. Reference AZO samples were deposited in a pure oxygen atmosphere with deposition pressures ranging from 1 to 5.4 Pa (denoted by Ox Ar0 in Table 1). For the
rest of the samples, argon was injected in the chamber in various amounts, aiming to achieve oxygen partial pressures of 1.6, 1.4 and 1 Pa for a constant total deposition pressure of 4.6 Pa. The comparison was made for two distinct experimental situations, namely (i) using the same total pressure and different oxygen flow rates (to compensate for argon addition), and (ii) using the same oxygen flow and different total pressures (resulting from the argon addition). This approach allows one to distinguish between effects induced by changes of the oxygen content in the deposition environment (constant pressure) and the effect of changing the deposition pressure (constant oxygen partial pressure), respectively. The glass slides used as substrates were conditioned prior to deposition by ultrasonic cleaning for 10 min in distilled water, then rinsing in ethanol and acetone, being finally dried by blowing dry nitrogen on the surface. The deposition chamber was evacuated to a base pressure of 0.1 mPa and pre-ablation of both metallic ® targets (zinc with 99.95% purity, Goodfellow TM and aluminum with 99.95% purity, Nilaco Corporation) for a number of 3000 laser shots was performed to avoid film contamination from both the residual gas and surface impurities. The target to substrate distance of 4 cm, the laser shot frequency of 5 Hz and the laser fluence of 3 J/cm2 were kept constant during all samples deposition. The obtained samples were investigated structurally by Wide Angle X Ray Diffraction − WAXD, on a Diffractometer D8 ADVANCE (Bruker AXS, Germany) using the Cu-Ka radiation (l = 0.1541 nm), a parallel beam with Gobel mirror and a Dynamic Scintillation detector. All the diffractograms were registered at room temperature in Bragg-Brentano geometry over the range 2 = 30–60◦ with scanning angle rate of 0.02◦ and a 1s/step count time. The optical and electrical properties were subsequently analyzed by using ultraviolet–visible near-infrared spectroscopy − UV/Vis/NIR (Evolution 300, Thermo Scientific) and a Hall measurement system (HMS 3000, Ecopia) in Van der Pauw configuration, respectively. For electrical investigations ohmic contacts were deposited by PLD from indium metallic targets at high vacuum (0.1 mPa) on the four corners on the square samples of 15 × 15 mm2 area. Prior to the electrical measurements, the thickness was determined by sputtering the AZO samples until reaching the substrate with a focused ion beam (Carl Zeiss NEON 40 EsB CrossBeam) and subjecting them to scanning electron microscopy (SEM JEOL JSM 6390) analysis in section view. Top side scanning images were obtained also by SEM for determining the surface morphology of the samples. The deposition rates were determined in real time by using a quartz crystal microbalance – QCM (Q-pod from Inficon) equipped with 6 MHz quartz crystal oscillator and gold coated sensor. While using the Zn target will result in the deposition of ZnO, we assumed that the ablation of Al will lead to the deposition of Al2 O3 . In addition, we used this data together with knowledge of the ablation pattern (the number of pulses used for aluminum and zinc target ablation in a sequence) to obtain a rough initial estimation of the Al/Zn atomic ratio in the film. Considering that PLD deposited layers of ZnO and Al2 O3 have the same densities as the bulk material, the doping concentration can be expressed as: Al (%) = 19
rAl2 O3 rZnO
(1)
where rAl2O3 and rZnO are the deposition rates expressed in identical thickness/time units. An LS 55 system from Perkin Elmer was used to study the photoluminescence properties at room temperature by using a 320 nm excitation wavelength from a pulsed UV xenon source. 3. Results and discussions Our approach of using metallic targets instead of ceramics for AZO thin films PLD facilitates the obtaining of desired doping con-
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Table 2 Structural parameters derived from Rietveld analyses, thicknesses and optical band gap of the SPLD AZO samples.
Fig. 1. Representative morphology and thickness determination by SEM-FIB (sample O1.6 A3.0 ).
Samples
c (Å)
D [nm]
GOFa
Thickness [nm]
BG [eV]
O5.4 Ar0 O4.6 Ar0 O1.6 Ar3.0 O1.4 Ar3.2 O1.0 Ar3.6 O3 Ar0 O1 Ar0
5.2313 5.2433 5.2072 5.1899 5.2170 5.2651 –
21.4 32.2 30.9 28.4 34.6 28.6 –
1.16 1.48 1.25 1.09 1.04 1.33 –
346 367 296 272 265 383 405
3.43 3.67 3.83 3.85 3.68 3.72 3.88
a
GOF – goodness of fitting.
Fig. 2. Structural features revealed by X-ray diffraction of AZO thin films as a function of a) oxygen deposition pressure and b) oxygen partial pressure.
centrations but could also lead to the formation of particulates on the film surface. These thermally derived droplets that may result from the ablation process could affect the quality of the resulting materials and, consequently, the performances of the device in which they may be used. Thus, in order to avoid that, we have decreased the laser fluence down to 3 J/cm2 . SEM scans showed particulate-free surfaces for the deposited AZO thin films. Also, a granular morphology was evidenced for all the samples, with an example of this type of morphology given in Fig. 1. The X-ray patterns of reference samples (represented in Fig. 2a) deposited in a pure oxygen atmosphere reveal a typical ZnO wurtzite structure. At deposition pressures higher than 1 Pa, there is a dominant (002) diffraction peak that shifts toward lower 2 values as the oxygen pressure decreases. Other orientations like (100), (101) and (110) are also present, but have very low intensities. Generally, the growth of the ZnO thin films on amorphous substrates takes place preferentially, with the c-axis perpendicular to the substrate regardless of the method used, due to minimization of the surface energy density [20,21]. As the oxygen pressure decreases from 5.4 Pa to 4.6 Pa, the crystalline quality of the sample is enhanced as revealed by larger crystallite sizes, D (see Table 2). Further decreasing the deposition pressure, leads to a deterioration of the crystalline quality and for 1 Pa oxygen pressure only a broad asymmetric peak in the region 2 = 55–56◦ can be observed, suggesting the presence of an additional phase beside ZnO in the composition of the sample [19]. In Fig. 2b are presented the X-ray patterns of the samples deposited in oxygen/argon mixture. Unlike the case of AZO deposition in an oxygen only atmosphere, both (002) peak position and lattice constant c of the samples don’t vary monotonically with the oxygen partial pressure (PO2 ) and show a reverse trend. The (002) peak shifts to higher 2 values when lowering PO2 from 4.6 Pa to 1.4 Pa. Then, the preferential orientation vanishes when PO2 is
Fig. 3. Influence of argon addition on QCM determined deposition rates during AZO thin films deposition by SPLD technique.
further lowered to 1 Pa. This behavior is equivalent to a lattice constant variation from 5.2433 Å (sample O4.6 Ar0 ) down to 5.1899 Å (O1.4 Ar3.2 ) then back up to 5.2170 Å (O1.0 Ar3.6 ). The values of the lattice constant are known to depend on the doping concentration. Despite using the same procedure for depositing all the samples, the Zn/Al ratio in the film may vary due to the gas composition, which affects the deposition rate of species from the Zn and Al plasmas. Using PLD, Ojeda et al. [22] deposited Lax Ca1−x MnO3 in Ar. They found that the film composition was different from that of the target and attributed the changes to a stronger scattering of the lighter species, which eventually leads to a decrease of the light atom concentration in the deposited film. Oh et al. [23] have demonstrated that the laser produced Al plasma exhibits distinct radial expansion behavior with or without an Ar flow in the chamber. In our case, when the oxygen/argon gas mixture is used, Al atoms are likely to be scattered more efficiently by heavier Ar atoms on their way to the substrate. It is expected that higher ratios between the argon and oxygen partial pressures will result in more significant changes of the doping concentration in the AZO films. To verify the scattering hypothesis, we used QCM measurements to determine the deposition rates from Al and Zn targets ablation for a constant oxygen partial pressure PO2 of 1 Pa and different Ar partial pressures. As presented in Fig. 3, the deposition rates of both ZnO and Al2 O3 drop with argon injection. When calculating the percentage of aluminum relative to zinc (following the method detailed in the experimental section, equation 1), a steady decrease from 2.8% to 2% was found as a result of the incremental addition of argon. We have already shown in a previous work [19] that SPLD samples of ZnO doped with Al exhibit defects such as interstitials, which lead to values of the lattice constant c that are larger than those of
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Fig. 5. a) Normalized PL spectra of AZO samples deposited in oxygen and oxygen/argon gas mixture and b) deconvoluted PL spectrum for O1.6 A3.0 sample.
Fig. 4. Hall determined electrical properties: a) resistivity, b) carrier concentration and c) mobility as a function of gas composition. The dotted lines link the values for the samples in the oxygen/argon mixture to the corresponding reference film grown in oxygen, at identical total pressure.
undoped ZnO. Thus, the initial decrease of c that occurs when the inert gas is injected may be explained through a lowering of the number of Al interstitials. Introducing more argon, the interstitials are diminished until only Al ions substituting for Zn remain in the film, and the constant lattice reaches a minimum (the Al atomic
radius being lower than that of Zn). Further injection of argon leads to a decrease of the substitutional dopant concentration and the constant lattice begins to increase toward the value characteristic of undoped ZnO. Next, we evaluated the suitability of our samples as potential transparent electrodes, and we started by analyzing the influence of argon addition on the electrical performance of the obtained SPLD AZO thin films. In Fig. 4 are presented the variation of the resistivity, concentration and mobility of the charge carriers as a function of the oxygen partial pressure. Following argon injection, we notice a considerable decrease of the resistivity with respect to the reference samples obtained in a pure oxygen atmosphere. The resistivity for the reference sample deposited for an oxygen deposition pressure of 4.6 Pa is 2.4 × 10−3 cm, while the argon addition up to an oxygen partial pressure of 1.6 Pa leads to a decrease of the resistivity to a minimum of 5.1 × 10−4 cm. To clearly evidence the effect of argon addition on the electrical resistivity we will separate below the discussion regarding the carrier concentration and Hall mobility. Looking at Fig. 4b, one common feature of the AZO films grown in the oxygen/argon mixture is a higher concentration of charge carriers than in the reference samples. The samples obtained in the oxygen/argon mixture have an average ne ∼ 6 × 1020 cm−3 , with only small variations between them. This value of ne is close to that found for the sample O1 Ar0 obtained at the lowest oxygen pressure (i.e., 6.6 × 1020 cm−3 ), suggesting that a low oxygen content is the cause of the increase of ne . One assumption would be that the additional free electrons are the result of an increased number of donor type intrinsic defects like oxygen vacancies or zinc interstitials (as revealed by photoluminescence analyses; the corresponding data are presented in Fig. 5). Although the experimental data supports the existence of both VO and Zni , the total defect concentration required to inject ∼4 × 1020 cm−3 free electrons would be very high and similar to the dopant concentration. As it is generally assumed that native defect concentrations are much lower that extrinsic impurity concentrations, the higher number of carriers must be related to changes in the Al content. The extra free charges may come from a change of the doping mode. Concomitantly with the reduction of the Al content in the sample (as revealed from QCM measurements), the addition of argon alters the ratio between the number of Al atoms substituting Zn and those placed in interstitial positions. Lackner [24] also observed that the crystallinity of PLD
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ized impurity scattering as the dominant mechanism. The mobility, in this case, is given by the formula [28]:
=
Fig. 6. UV–vis spectra and Tauc plot for direct band gap determination of AZO thin films deposited in: a) oxygen and b) oxygen/argon gas mixture.
deposited AZO thin films was improved by injecting a supplementary argon gas allowing the growth of thermodynamically more stable orientations, such as (002). As additional electrons are injected in the conduction band, the increase of the carrier concentration is directly reflected on the measured optical band gap (i.e., the Burstein–Moss shift [25]). The optical band gap (BG) calculated from the transmission spectra (the corresponding data are presented in Fig. 6) by using the Tauc plot method [26] shows an increase with the oxygen depletion in the deposition chamber (see corresponding values in Table 2). The resistivity also depends on the carrier mobility, which varies with the argon/oxygen ratio, as it can be seen in Fig. 4c. Generally, the mobility may be affected by several scattering processes that can take place inside the material. The interface and surface scattering, which become important at low film thicknesses, can be neglected since the thickness of all our samples is in the range 200–400 nm. Grain boundary scattering is also negligible, since the mean free path of electrons, L given by the formula [27]:
L = h/ (2e) 3n/
1/3
(3)
is about 3 nm, much smaller than the average crystallite size. Here, h is Planck’s constant, e is the electron charge, n is the carrier concentration and is the carrier mobility. Thus, electrons will easily tunnel through the inter-grain potential barrier when their concentration is higher than 1020 cm−3 . This leaves the ion-
3ε2r ε20 h3 e3 m∗2 Z 2
ln 1 +
1 32/3 1/3 εr ε0 h2 n1/3 2m∗ e2
2
(4)
where h, e and ε0 are known physical constants, Z is the electric charge of one impurity, which we consider to be +1, corresponding to singly ionized Al donors in ZnO. The dielectric constant εr is 7.9. For the values of the effective electron mass, m*, we referred to Young et al. [29]. They determined the values for Al-doped ZnO and found that m* increases with carrier concentration. Based on their results, a value of ∼0.5 me can be extrapolated for densities of 6 × 1020 cm−3 , as is the case with most of our samples. By using this equation for our samples the calculated mobility values are higher than the measured ones by a factor between 1.1 and 3.8. This could be explained by the existence of scattering centers with higher charge, for example doubly ionized aluminum. Since trivalent Al replacing bivalent Zn in the lattice sites will result in singly charged Al+ , Al2+ can only be present as interstitials. The experimentally determined mobility is generally higher for samples obtained in the oxygen/argon mixture, sometimes approaching the values calculated if considering only +1 charged impurities. This supports the previously proposed explanation that introducing argon in the deposition chamber reduces the number of interstitial-type defects and favors substitution doping. We have already talked about point defects like Al interstitials and oxygen vacancies in our samples. To gain more insight into the structural defects and their influence on the physical properties of the AZO thin films, room temperature photoluminescence (PL) measurements were performed. In Fig. 5 are presented the PL spectra (normalized to the thickness) of the reference sample obtained in pure oxygen (O4.6 A0 ) and of the samples obtained for different oxygen partial pressures. The PL emission from the AZO samples is divided into two spectral regions: a UV region (with two characteristic peaks, 360 nm and 388 nm attributed to the free-exciton recombination near the band edge and electron transition from the conduction band (CB) to single ionized oxygen vacancy, respectively) and a visible emission from the deep levels attributed to different defects such as oxygen vacancies and interstitial Zn. The deconvolution of the visible part of the PL spectrum (Fig. 5b) identified several components assigned as follows: 416 nm − transition from Zn interstitial levels to the valence band (VB), 438 nm – transition between ionized Zn interstitial defect levels and the VB, 455 nm – radiative recombination of electrons from the doubly ionized Zn vacancy levels with holes in the VB, 480 nm – transition from the CB to oxygen vacancies levels and 520 nm – originating from the radiative recombination of holes given by the deep level oxygen vacancy with electrons from the CB minimum [30]. It can be clearly observed that when the oxygen partial pressure drops down to 1.6 Pa there is an enhancement of the crystalline quality as reflected by the increase of the UV emission at 360 nm (Fig. 5). Further depletion in oxygen of the deposition environment to a partial pressure of 1.4 Pa leads to a decrease of the intensity ratio between the exciton emission band and visible band, following the introduction of the oxygen vacancies. When the oxygen partial pressure reaches the value of 1 Pa, the intensity of the visible band that is related to the defects becomes dominant, describing a deterioration of the crystalline quality of the film. These findings are in good agreement with the previously presented XRD and Hall data which show that with argon injection the (002) preferential orientation changes to a polycrystalline phase with randomly oriented crystallites, while the charge carrier mobility falls below 10 cm2 /Vs. The best ratio between exciton to visible bands emission, encoun-
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tered for an oxygen partial pressure of 1.6 Pa, is reflected by the best electrical and optical properties among the investigated samples. As discussed above, the addition of the inert gas in the deposition chamber leads to an increased degree of substitution of zinc into the lattice. Thus, there is an increase of the crystalline quality of the films (as was observed by structural analyses) and an enhancement of the electrical performances (as both carrier concentration and mobility increase). In the same time, the introduction of inert gas lead to the emergence of supplementary oxygen vacancies that favored the development of Zn interstitials (as evidenced from the discussions on PL data) and ultimately damage the structural quality and electrical performances of the AZO films. It is clear that the deposition environment alters the chemical composition of the samples and hence may be successfully used as new control parameter for the optimization of film properties in room temperature SPLD of AZO thin films. Since AZO aims to be an alternative to high cost ITO material, the optical properties are particularly important. In Figs. 6 are presented the transmission spectra for the AZO samples deposited in oxygen and in the oxygen/argon mixture. The degree of transparency (the mean optical transmittance calculated between 400 and 800 nm) for the samples deposited in the oxygen/argon mixture (for example, sample O1.6 Ar3.0 has ∼90%,) is lower than that of the oxygen reference sample (O4.6 Ar0 with 93%). A low transmittance in the visible domain for semiconducting oxide thin films is often the consequence of a non-stoichiometric composition. Since it occurs at low oxygen deposition pressures, oxygen vacancies are considered responsible, with higher number of such defects leading to an increase of light scattering [31]. Although the AZO films deposited in gas mixture are thinner (see Table 2) with respect to the reference sample at the same total pressure, the increased number of oxygen vacancies is decisive, leading to a decrease of the transmittance. However, when compared to the sample O1 Ar0 , the transmittance values are higher in the oxygen/argon mixture, even if the oxygen pressure is the same in both cases, namely 1 Pa (see Fig. 6). This unusual high transmittance of the oxygen/argon samples at low oxygen partial pressure is due to the considerable decrease of the deposition rate. A higher amount of inert gas means fewer Zn and Al particles arrive at the substrate, as confirmed by the thickness determined from SEM-FIB (see Table 2) and by the deposition rate estimated with QCM as well (see Fig. 3). Therefore, fewer oxygen atoms will be required in order to grow stoichiometric samples. Hence, the samples deposited in the oxygen/argon gas mixture have unexpectedly high transmittance values even if the oxygen partial pressure is low. Along with preventing large stoichiometric deviations through a supplementary scattering of ablated particles, the argon addition also leads to an enhancement of the substitutional doping. Thus, the use of the oxygen/argon gas mixture translates into the introduction of a new control parameter, capable of finely regulating the electrical resistivity, almost independently of the optical transmittance.
4. Conclusions We have demonstrated that the use of a mixed oxygen/argon working environment during room temperature SPLD of Al-doped ZnO thin films considerably improves the electrical characteristics of the obtained materials. The argon addition leads to a lower electrical resistivity (i.e., 5.1 × 10−4 cm) compared to the usual practice of deposition in pure oxygen (i.e., 2.4 × 10−3 cm) due to an enhancement of the substitutional doping during AZO thin films deposition. The mean optical transmittance in the visible domain is not affected by the argon gas injection, since the stoichiometric deviation is prevented in this situation by the reduction of the Al and Zn
deposition fluxes. The optimum deposition conditions have been identified to be an oxygen partial pressure of 1.6 Pa coupled with a total deposition pressure of 4.6 Pa, providing simultaneously the best optical and electrical properties (a high transmittance of ∼90% and a resistivity of 5.1 × 10−4 cm). The presented results demonstrate that by using an oxygen/argon gas mixture in room temperature SPLD from metallic targets of AZO thin films, the electrical and optical properties are considerably improved. This approach may be successfully used for applications involving temperature sensitive substrates in transparent and flexible electronics. Acknowledgments This work was financially supported by the Romanian National Authority for Scientific Research, CNCS – UEFISCDI grant, project number PN-II-PT-PCCA-2013-4-1861> (contract number 272/2014) and by Core Program, under the support of ANCSI, project nr. PN 16.40.01.02. References [1] C.D. Lokhande, P.M. Gondkar, Rajaram S. Mane, V.R. Shinde, Sung-Hwan Han, CBD grown ZnO-based gas sensors and dye-sensitized solar cells, J. Alloy Compd. 475 (2009) 304–311. [2] S.K. Shaikh, S.I. Inamdar, V.V. Ganbavle, K.Y. Rajpure, Chemical bath deposited ZnO thin film based UV photoconductive detector, J. Alloy Compd. 664 (2016) 242–249. [3] G. Perez-Hernandez, A. Vega-Poot, I. Perez-Juarez, J.M. Camacho, O. Ares, V. Rejon, J.L. Pena, G. Oskam, Effect of a compact ZnO interlayer on the performance of ZnO-based dye-sensitized solar cells, Sol. Energy Mater. Sol. C 100 (2012) 21–26. [4] S.L. Ou, H.R. Liu, S.Y. Wang, D.S. Wuu, Co-doped ZnO dilute magnetic semiconductor thin films by pulsed laser deposition: Excellent transmittance, low resistivity and high mobility, J. Alloy Compd. 663 (2016) 107–115. [5] T. Dietl, H. Ohno, F. Matsukura, Hole-mediated ferromagnetism in tetrahedrally coordinated semiconductors, Phys. Rev. B 63 (2001) 195205. [6] T. Minami, Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822–5828. [7] D.K. Kim, H.B. Kim, Room temperature deposition of Al-doped ZnO thin films on glass by RF magnetron sputtering under different Ar gas pressure, J. Alloy Compd. 509 (2011) 421–425. [8] T. Minami, Y. Ohtani, T. Miyata, T. Kuboi, Transparent conducting Al-doped ZnO thin films prepared by magnetron sputtering with dc and rf powers applied in combination, J. Vac. Sci. Technol. A 25 (2007) 1172. [9] Y. Zhao, Y. Jiang, Y. Fang, The influence of substrate temperature on ZnO thin films prepared by PLD technique, J. Cryst. Growth 307 (2007) 278–282. [10] R.S. Ajimsha, Amit K. Das, P. Misra, M.P. Joshi, L.M. Kukreja, R. Kumar, T.K. Sharma, S.M. Oak, Observation of low resistivity and high mobility in Ga doped ZnO thin films grown by buffer assisted pulsed laser deposition, J. Alloy Compd. 638 (2015) 55–58. [11] C. Weigand, R. Crisp, C. Ladam, T. Furtak, R. Collins, J. Grepstad, H. Weman, Electrical, optical and structural properties of Al-doped ZnO thin films grown on GaAs(111)B substrates by pulsed laser deposition, Thin Solid Films 545 (2013) 124–129. [12] K.E. Lee, M. Wang, E.J. Kim, S.H. Han, Structural, electrical and optical properties of sol-gel AZO thin films, Curr. Appl. Phys. 9 (2009) 683–687. [13] X.Q. Gu, L.P. Zhu, L. Cao, Z.Z. Ye, H.P. He, P.K. Chu, Optical and electrical properties of ZnO:Al thin films synthesized by low-pressure pulsed laser deposition, Mater. Sci. Semicon. Proc. 14 (2011) 48–51. [14] M.G. Tsoutsouva, C.N. Panagopoulos, M. Kompitsas, Laser energy density, structure and properties of pulsed-laser deposited zinc oxide films, Appl. Surf. Sci. 257 (2011) 6314–6319. [15] S.Y. Kim, J.H. Lee, J.J. Kim, Y.W. Heo, Effects of temperature target/substrate distance, and background pressure on growth of ZnO nanorods by pulsed laser deposition, J. Nanosci. Nanotechnol. 14 (2014) 9020–9024. [16] C. Ursu, G.O. Pompilian, S. Gurlui, P. Nica, M. Agop, M. Dudeck, C. Focsa, Al2 O3 ceramics under high-fluence irradiation: plasma plume dynamics through space- and time-resolved optical emission spectroscopy, Appl. Phys. A-Mater. 101 (2010) 153–159. [17] J.T. Wang, X.L. Shi, W.W. Liu, X.H. Zhong, J.N. Wang, L. Pyrah, K.D. Sanderson, P.M. Ramsey, M. Hirata, K. Tsuri, Influence of preferred orientation on the electrical conductivity of fluorine-doped tin oxide films, Sci. Rep. 4 (2014) 3679. [18] Z. Pan, X. Tian, S. Wu, C. Xiao, Z. Li, J. Deng, G. Hu, Z. Wei, Effects of Al and Sn dopants on the structural and optical properties of ZnO thin films, Supperlatice Microst. 54 (2013) 107–117.
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