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High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition
Accepted Manuscript High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition Man-Ling Lin, Jheng-Ming Huang, Ching-Shun Ku, Chih-Ming Lin, Hsin-Yi Lee, JenhYih Juang PII:
S0925-8388(17)32936-5
DOI:
10.1016/j.jallcom.2017.08.207
Reference:
JALCOM 42964
To appear in:
Journal of Alloys and Compounds
Received Date: 20 June 2017 Revised Date:
16 August 2017
Accepted Date: 21 August 2017
Please cite this article as: M.-L. Lin, J.-M. Huang, C.-S. Ku, C.-M. Lin, H.-Y. Lee, J.-Y. Juang, High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.207. 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.
ACCEPTEDconductive MANUSCRIPT Al-doped ZnO thin High mobility transparent
films by atomic layer deposition Man-Ling Lin1, Jheng-Ming Huang2, Ching-Shun Ku3, Chih-Ming Lin4, Hsin-Yi Lee3＊, and Jenh-Yih Juang 1＊ Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
2
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
3
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
4
Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan
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Abstract
The effects of growth temperature on the microstructure, transport and optoelectronic properties of a series of Al-doped ZnO (AZO) films with thickness of ~30 nm deposited on polished silicon-(100) and glass substrates by the atomic layer deposition (ALD) were investigated. By adopting an in-situ doping growth
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scheme the critical length effect associated with adjacent Al2O3 layers commonly encountered in previous ALD growth schemes was avoided and effective Al-doping was achieved with the growth temperature ranging from 100 °C to 300 °C. Experimental results showed that, in general, increasing the growth
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temperature would result in much improved film crystallinity and carrier mobility, with the average transmittance in the visible wavelength range being exceeding 95% in all cases. In particular, for AZO films
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grown at 300°C, an unprecedented mobility of 136 cm2V-1s-1 was obtained, comparing to the typical values of 50∼60 cm2V-1s-1 reported previously. The resistivity of these 300°C films (ρ ≈ 6×10-4 Ω-cm), nevertheless, is slightly higher than that of some highly-doped ZnO (ρ ≈ 2∼4×10-4 Ω-cm) prepared by sputtering methods. The secondary ion mass spectroscopy (SIMS) analyses revealed that hydrogen incorporation is the key in reducing the charge trap density and, hence, resulting in much enhanced carrier mobility. The present results promise a keen competitiveness of AZO with the indium tin oxide (ITO) film for thin-film-transistor (TFT) as well as in photovoltaic device applications. Keywords: Semiconductors, Atomic Layer Deposition, Electric Transport, Optical Properties. *Corresponding authors: E-mail: [email protected] (H. Y. Lee); [email protected] (J. Y. Juang)
1. Introduction
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Transparent conductive oxides (TCO) are among the critical keys enabling the rapid progress of the information technology industries owing to their wide variety of attractive physical properties, such as high mobility, low resistivity, high transmittance in visible region, highly transparent in infrared (IR) range [1]. Even more importantly, most of these prominent properties are tunable by doping, like in semiconductors.
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Presently, TCO films derived by a wide array of preparation methods and materials have already been ubiquitously used for various applications, including organic light-emitting diodes [2], flat panel displays [3], as well as photovoltaic devices [4, 5]. Perhaps, one of the most widely used TCO materials is the indium tin
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oxide (ITO). In addition to its high conductivity and optical transparency in visible wavelengths, the relatively lower processing temperature (~250 °C) [6] has made ITO the indispensable transparent electrodes
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that evidently led to the flourishing development in organic and flexible electronic devices witnessed over the past decades [7]. However, due to the scarcity, toxicity, and high production cost of indium [8], recently, exploring indium-free alternatives has become one of the hotly pursued research topics in this field. Al-doped ZnO (AZO) stands out as one of the most commonly cited alternative materials for replacing
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ITO because of its abundance, non-toxicity, and low cost [7]. Previously, AZO thin films had been grown by various deposition techniques, including pulse laser deposition [9], sputtering [10], and metal organic chemical vapor deposition [11]. However, these deposition methods require either high growth temperature
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(> 400 °C) or high post-annealing temperature (> 500 °C), which not only would limit the choice of suitable substrates but also often lead to performance degradation of the devices. In general, higher growth
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temperatures would result in increased dopant solubility and hence carrier concentration, nevertheless, it is usually accompanied by substantial reductions in carrier mobility and optical transparency. Among others, effects such as the intra-granular scattering resulting from charged-impurity and strain-induced lattice disruption, as well as, inter-granular scattering due to refinement of grain size and the presence of second phases segregated at grain boundaries, are the common causes responsible for the mobility degradation [12]. Obviously, in addition to increasing the doping (carrier) concentration, controlling the film microstructure is also of essential importance for obtaining desired transport and optical properties of TCO films. In this respect, atomic layer deposition (ALD) with low growth temperature is more advantageous for depositing high-quality AZO thin film because of its viability of controlling the film thickness and composition at the
monolayer level, as well as the capability of obtaining large-area uniformity and satisfactory reproducibility
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[13]. In this study, we report the electrical and optical properties of ALD–AZO films obtained with a novel in-situ doping scheme. The effects of growth temperature on the crystalline structure, optoelectronic properties and, in particular, the much enhanced carrier mobility of the resulting films is addressed.
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2. Experimental procedures The AZO films investigated in the present study were grown on polished silicon-(100) and glass substrates by the flow-rate interruption ALD method at temperatures varying in the range of 100~300 °C. To
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carry out the flow-rate interruption scheme, we installed a quick ALD diaphragm valve (Swagelok) between the reactor and the pump station to retain the precursor in the reactor chamber, which has been shown to
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substantially increase the reaction rate at low temperatures [14]. Briefly, in this study, the film growth procedures were as followings. Diethylzinc [Zn(C2H5)2] (DEZn), trimethylaluminum [Al(CH3)3] (TMA) and DI water (H2O) were adopted as the starting precursors for zinc, aluminum, and oxygen, respectively. For DEZn and H2O precursors, the time of pulse and stock durations were 50 ms and 2 s, respectively. N2 was
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used as the purge gas with a set pressure of 6×10-2 Torr. The purge and pumping periods were 3 s and 8 s, respectively. According to the process optimized in the previous study [15], the 5:1 frequency ratio of Zn-O and Zn-Al-O cycles could result in highly conductive AZO thin films. Thus, in each ALD cycle, 5 layers of
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Zn-O were deposited followed by adding 1 layer of Zn-Al-O, to serve as the in-situ doping layer for promoting the uniformity of Al-doping in the ZnO matrix. All samples investigated in this study were
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obtained with a total of 30 ALD cycles. The total thickness of the films varies from ∼20-30 nm depending on the growth temperature.
The structural characteristics of the AZO films were examined with a synchrotron x-ray source.
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high-resolution x-ray scattering experiments were performed at wiggler beamline BL-17B1 in the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. Incident x-rays were focused vertically with a mirror and were made monochromatic with energy 8 keV by a Si (111) double-crystal monochromator. Using slits in two pairs between the sample and the detector, the typical scattering vector resolution in the vertical scattering plane was set to ~ 1×10-3 nm-1 in these experiments. The microstructure and composition of samples were characterized using transmission electron microscopy (TEM, JEOL
JEM-ARM200F Japan) operating at voltage of 200 kV and energy dispersive X-ray spectroscopy (EDS),
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respectively. The optical properties of representative AZO films were characterized by photoluminescence (PL) using a He-Cd laser as the light source. The excitation wavelength and output power of the He-Cd laser were 325 nm and 4 mW, respectively. The Hall effect measurements were carried out to determine the electrical and transport properties of the AZO samples using a National Instruments PXI-1024Q AC system
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with an applied magnetic field of 0.68 T. The transmittance was measured using a Hitachi U-3010 UV-Visible Scanning Spectrophotometer. The depth profile of the film composition was examined by
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secondary ion mass spectroscopy (SIMS).
3. Results and discussion
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Figure 1 shows the XRD results of the AZO films grown on Si(100) substrates by the ALD processes described above at various growth temperatures, ranging from 100 °C to 300 °C. The results clearly indicate that all films preserve the hexagonal wurtzite structure (space group P63mc) characteristics of the parent ZnO [16]. Nevertheless, it is also evident from Figure 1 that the film microstructure can be very different as
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the growth temperature varies. Namely, for films grown at temperatures below 200 °C, the microstructure appears to be of more randomly distributed polycrystalline structure, as suggested by the more evenly distributed intensity between the (100), (002), and (101) crystallographic orientations. At the temperature
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below 200 °C, the c-axis lattice constant of AZO/Si films calculated from XRD peak of 2θ = 34.66° is 5.17 Å, which is indeed slightly smaller than 5.20 Å for bulk wurtzite ZnO [17]. Since the ionic size of Al3+
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(0.53 Å) is substantially smaller than that of Zn2+ (0.74 Å) [18], one expects to see noticeable change in lattice constant if the doped Al occupies the lattice site of Zn substitutionally [19]. The fact that the position of the diffraction peaks, hence the lattice constants, for films deposited below 200 °C remained essentially unchanged implies that the Al-doping might not be fully prevailed at temperatures lower than 250 °C [20]. Nonetheless, when the growth temperature was further increased to 300 °C, the position of the (002) peak shifts noticeably to higher diffraction angle of 2θ = 34.82°, corresponding to a c-axis lattice constant of 5.15 Å, indicating substantial increase of Al-doping, which is in line with the observation made by Musat et al. [21]. It is also noted that, the films deposited at 250 and 300 °C appeared to grow preferentially along the c-axis with much improved crystallinity, as indicated by the sharpened (002) diffraction peak with higher
intensity. Moreover, the position of the (002) diffraction peak has evidently shifted to higher diffraction
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angles, indicating a reduction of the c-axis lattice constant caused by substitutional Al-doping. We believe that, at higher growth temperatures, the strong polarity between the Zn2+ and O2− ions stacking along the c-axis could have helped the c-axis preferred orientation growth seen here. Finally, we note that previous reports indicated that the ALD-derived ZnO thin films grown at lower temperatures or with doping often
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exhibited a tendency of growing with the a-axis preferred orientation [22], which is not observed in the present study. The differences are presumably due to the detailed growth conditions, such as temperature, substrates, total film thickness, and even the growth system used. It is noted that in [22] the growth-doping
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scheme was more “traditional” and the AZO films were grown on glass substrate with a total thickness of ~ 200 nm. Whereas, in our case, the AZO films used for TEM characterizations were grown on Si substrates
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by the in-situ doping scheme with a thickness of only ~30 nm (see below).
In order to obtain more detailed microstructural information and its correlation with the optoelectronic properties of the obtained AZO films, cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was performed. Figure 2 shows the representative X-HRTEM images and cross-sectional
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height profile for the AZO films grown at 150 °C and 300 °C, respectively. It can be seen that all films are essentially growing on a native SiOx layer of ∼2 nm-thick. Nevertheless, at the growth temperature of 300 °C, the SiOx layer appears to be much blurred, presumably due to the enhanced inter-diffusion at elevated
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temperatures. The morphology of the nano-crystallites also appears to evolve from the equiaxed feature (Fig. 2(a)) to more columnar characteristic (Fig. 2(b)) as the growth temperature was increased from 150°C to
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300°C, which is in line with the more c-axis preferred growth nature for films grown at higher substrate temperatures seen in the XRD results shown in Figure 1. Moreover, both film are having about the same thickness, which is in contrast to the temperature-dependent growth rate of ZnO films grown by ALD reported by previous studies. Oh et al. [22] reported that the growth rate of ZnO films decreased monotonically from 0.177 nm/cycle to 0.132 nm/cycle when the growth temperature was increased from 150 °C to 250 °C. On the other hand, Li et al. [23] reported that the growth rate increases with temperature for temperatures below 200 °C and then decreases with further increase in growth temperature. These comparisons indicate that the reported temperature-dependent growth rate is sensitive to the systems and detailed conditions used by each research group. Since the growth rate of ZnO is very much dependent on
the surface density of O-H bonding groups [24], thus the details of growth system and conditions can affect
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the eventual results. Furthermore, the total film thickness in the abovementioned studies was approaching 200 nm, which is much thicker than the present ones and may not reflect the real growth rate in the early stages of ALD processes. The next important question to be asked is whether or not the Al ions are successfully doped on the lattice site of Zn. We will address this issue in the following with the related
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optoelectronic properties. Figure 3 shows the room-temperature PL spectra for the ALD AZO thin films grown at various temperatures. It is clear that only the UV region emissions are observed and no emissions in the visible
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range associated with various crystalline defects, surface states and impurities are seen. This is indicative that the visible range emissions are either nearly completely suppressed due to the absence of the
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abovementioned defects in the present films [25] or quenched by some yet to be identified mechanism. Nevertheless, a closer examination on the PL spectra does give rise to distinctive details for films grown at different temperatures. The band gap energies derived from near band edge (NBE) peaks are 3.46 eV and 3.31 eV corresponding to an Al concentration of 3.4 % [12] and 1% [26], respectively. Using the Vegard’s
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law , = 1 − , + , and take , = 3.37 eV and , = 6.40 eV reported previously, the expected energy band gap of the 3.4% sample is consistent with those of the 250 °C and 300 °C films. In contrast, it is obvious that the Vegard’s law alone is inadequate to account for that of the 1%
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AZO. Figure 4 summarizes some of the results reported in literature to illustrate that most of the measured energy gap values in AZO samples prepared by various processes are, in fact, largely falling short of that
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expected from the Vegard’s law. As pointed out by Bernard and Zunger [27], due to complicated factors such as volume deformation, charge exchange and the relaxation of the anion-cation bond lengths brought about by alloying, one often needs to modify the Vegard’s law with a bowing parameter to give: , = 1 − , + , − 1 − [28]. In the present case, the bowing parameters of 0.34 eV and 8.11 eV are obtained for Al concentration of 3.4% and 1%, respectively. The significantly larger bowing parameter for lower doping concentration has also been observed in GaAsN alloys [29] and was attributed to the localized impurity-like states formed near the band edges. Namely, in diluted GaAs1-xNx alloys, the valence band is strongly localized on the As sublattice and the conduction band is strongly localized on the nitrogen sublattice. We believe that due to the large disparity in ionic size (Al3+∼0.53Å; Zn2+∼0.74Å) similar
effects may also prevail in the present case. Nevertheless, further investigations are needed to clarify this
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issue. Back to the results shown in Figure 3, it is clear that the PL spectra can be roughly sorted into two groups; one has the emission at energies (∼ 3.46 eV) higher than the intrinsic energy gap of 3.37 eV for pure ZnO [30]; the other has the emission at energies ranging from 3.32 ± 0.01 eV, which is slightly lower than
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3.37 eV. Moreover, the emission intensity of the former is consistently lower than that of the latter. It is generally conceived that both improved crystalline quality and size confinement effects can lead to substantial intensity enhancement in the UV emission spectrum of various ZnO nanostructures [31, 32]. By
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comparing the microstructures displayed in Figure 2(a) and 2(b) for films grown at 150 °C and 300 °C, respectively, it is suggestive that the higher PL emission intensity observed for the 100 °C and 200 °C films
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might be due to the similar size-confinement effect. Thus, sub-band-gap emissions in films grown at 100-200 °C may arise from the involvement of exciton binding energy, which may not be as significant in films consisted of larger crystallites (250-300°C films).
On the other hand, perhaps the most remarkable results of the present study are concerning the
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significant improvement in the transport properties of the AZO films obtained by the proposed in-situ doping scheme. Figure 5 shows the resistivity and mobility as a function of deposition temperature for the AZO films investigated in the present study. As is evident from Figure 5, the resistivity of the present AZO
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films drops progressively from ∼4×10-2 Ω−cm to ∼6×10-4 Ω−cm as the growth temperature is increased from 100 °C to 300 °C. According to the Burstein-Moss effect [33], the bandgap energy should increase with the ⁄!
, where n is the carrier density. Since the carrier density due to Al-doping is
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carrier density at a rate of
insensitive to the energy band gap (see the inset of Figure 5), one can assume that in-situ doping has been effective even at very low deposition temperatures and the changes in resistivity are mainly resulted from the microstructures. Within the context of this scenario, the precipitous drop in film resistivity at 150°C could thus be attributed to the amorphous to crystalline transition of the film microstructure. Whereas, the continuous reduction in film resistivity at higher deposition temperatures is associated with the progressive enhancement of crystallinity as well as the growth of grain size, both are known to be effective in reducing the scattering of carrier transport [34-37]. More remarkably, within the same deposition temperature range, the carrier mobility has drastically improved from ∼4 cm2V−1s−1 for amorphous 50°C film, to ∼10-20
cm2V−1s−1 for films deposited below ACCEPTED 200°C, and finally reaching 136 cm2V-1s-1 for films grown at 300°C. MANUSCRIPT Comparing with the mobility values reported previously in the literature (e.g. Banerjee et al. [36] reported a maximum value of 17.7 cm2V-1s-1 for 100 nm-thick AZO films deposited at 150 °C; Ahn et al. [38] reported a mobility of ~17 cm2V-1s-1 for 200 nm-thick AZO films grown at 100-300 °C; Luka et al. [39] reported a maximum mobility 21.8 cm2V-1s-1 for 200 nm-thick AZO films at 200 °C; Li et al. [23] reported a maximum
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mobility ~7 cm2V-1s-1 for 90 nm-thick AZO films at 150 °C), this value of mobility is rather significant in terms of application potential.
In particular, considering that the present film thickness is only in the order of few tens of nanometers
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higher transparency can be expected, which, in turn, is promising for various applications. Indeed, as displayed in Figure 6, the optical transmittance spectra taken in the wavelength range of 280-800 nm for
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AZO films grown at different temperatures show that all samples have a strong absorption in the UV region and high transmittances in the visible region. The average transmittance in the wavelength range between 400-800 nm exceeds 95 % (see the inset of Figure 6). The seemingly scattered behaviors near the absorption edge of 360~370 nm for films deposited at various temperatures are attributed to the interfacial roughness
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between the film and glass substrates and the differences in the film microstructure. In any case, the present results have evidently shown substantially better transmittance than those reported in literatures for ITO thin films or ALD-derived AZO films that did not use the flow-rate interrupted scheme [40-44].
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In order to clarify the underlying mechanism responsible for the unprecedented enhancement in the electric transport performance obtained in the present study, secondary ion mass spectroscopy (SIMS) depth
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profiles of Al, Zn, O, H, C, and Si for the two representative AZO films deposited at 200°C and 250°C were performed. This is also the temperature range where the significant change in mobility was observed (Figure 5). As shown in Figure 7, a rather uniform distribution of Al is evident in both AZO films, indicating that uniform Al doping is indeed achieved in the present in-situ doping process scheme. Moreover, the 250°C film is having a higher relative concentration than that of the 200°C films, confirming our previous conjectures that higher growth temperatures would result in more efficient doping. It is also noted that unlike other elements, the Al concentration distribution appears to exhibit a slight periodic fluctuations, presumably due to the in-situ doping scheme adopted in this study. However, the periodicity seems to be greater than the diffusion length of Al in bulk ZnO within the similar temperature range [45]. It is noted that
Luka et al. [46] also reported periodic distribution of Al in their AZO films by SIMS measurements,
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indicating that in ALD processes the diffusivity of individual atomic species are significantly faster than in the case of bulk diffusion. In addition, the SIMS results also clearly indicate that the relative concentration of H-atom near the surface (0-10 nm) in the 250°C sample is substantially larger than that of the 200°C sample. It is suggestive that incorporation of H in filling up the oxygen vacancies might have played an
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important role in reducing the density of grain barrier traps and giving rise to the much enhanced carrier mobility obtained in the present study.
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4. Conclusions
In summary, a series of ultrathin Al-doped ZnO films were prepared at various growth temperatures by
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atomic layer deposition with an in-situ doping deposition scheme. The film microstructure showed a progressive transition from amorphous to equiaxed polycrystalline to (002)-oriented columnar structure as the growth temperature was changed from 100 °C to 300 °C. The energy gap of the films revealed by the photoluminescence measurements indicated that the band-gap widening and associated near-band-edge
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emissions have involved alloying effect of Al2O3 and ZnO, instead of purely due to the Burstein–Moss effect and band-gap renormalization effects. According to the modified Vegard’s law, bowing parameters of 0.34 eV and 8.11 eV were obtained for the Al concentration of 3.4% and 1%, respectively. The UV-Vis
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measurements revealed that all samples are having above 95% transmittance, which are higher than that of most ITO films. More importantly, the AZO film grown at 300 °C displayed an unprecedented high mobility
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of 136 cm2V-1s-1 and a resistivity of 6×10-4 Ω-cm, which have significant implication for serving as the transparent conducting oxide in oxide thin-film-transistors (TFTs) applications. In particular, the advantages of monolayer-control and high aspect ratio unique to ALD process offer outstanding opportunities in overcoming the size effect expected for future TFTs. Finally, based on the XPS and SIMS results, the incorporation of hydrogen into the AZO films during ALD processes have played an important role in giving rise to the high mobility observed.
Acknowledgements This work was financially supported by the Ministry of Science and Technology, Taiwan, R.O.C. under grant
numbers: MOST 103-2112-M009-015-MY3 and MOST 101-2221-E-213-001-MY3. J. Y. Juang is also
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partially supported by the MOE-ATU program operated at NCTU.
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Physical Review Letters, 76 (1996) 664-667. [30] W.Y. Liang, A.D. Yoffe, Transmission Spectra of ZnO Single Crystals, Physical Review Letters, 20 (1968) 59-62. [31] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach, Appl. Phys. Lett., 78 (2001) 407-409. [32] Y.J. Xing, Z.H. Xi, Z.Q. Xue, X.D. Zhang, J.H. Song, R.M. Wang, J. Xu, Y. Song, S.L. Zhang, D.P. Yu, Optical properties of the ZnO nanotubes synthesized via vapor phase growth, Appl. Phys. Lett., 83 (2003) 1689-1691. [33] T.S. Moss, The Interpretation of the Properties of Indium Antimonide, Proceedings of the Physical Society. Section B, 67 (1954) 775. [34] S.-M. Park, T. Ikegami, K. Ebihara, P.-K. Shin, Structure and properties of transparent conductive doped ZnO films by pulsed laser deposition, Applied Surface Science, 253 (2006) 1522-1527. [35] Z. Baji, Z. Lábadi, Z.E. Horváth, I. Bársony, Structure and morphology of aluminium doped Zinc-oxide
layers prepared by atomic layer deposition, Thin Solid Films, 520 (2012) 4703-4706. ACCEPTED MANUSCRIPT [36] P. Banerjee, W.-J. Lee, K.-R. Bae, S.B. Lee, G.W. Rubloff, Structural, electrical, and optical properties of atomic layer deposition Al-doped ZnO films, J. Appl. Phys., 108 (2010) 043504. [37] P. Genevée, F. Donsanti, G. Renou, D. Lincot, Study of the aluminum doping of zinc oxide films prepared by atomic layer deposition at low temperature, Applied Surface Science, 264 (2013) 464-469. [38] C.H. Ahn, S.Y. Lee, H.K. Cho, Influence of growth temperature on the electrical and structural
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characteristics of conductive Al-doped ZnO thin films grown by atomic layer deposition, Thin Solid Films, 545 (2013) 106-110. [39] G. Luka, T.A. Krajewski, B.S. Witkowski, G. Wisz, I.S. Virt, E. Guziewicz, M. Godlewski, Aluminum-doped zinc oxide films grown by atomic layer deposition for transparent electrode applications, Journal of
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Materials Science: Materials in Electronics, 22 (2011) 1810-1815. [40] T. Ocal, S. Yusuf, A. Gulnur, O. Lutfi, High quality ITO thin films grown by dc and RF sputtering without oxygen, Journal of Physics D: Applied Physics, 43 (2010) 055402. [41] D. Kudryashov, A. Gudovskikh, K. Zelentsov, Low temperature growth of ITO transparent conductive
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oxide layers in oxygen-free environment by RF magnetron sputtering, Journal of Physics: Conference Series, 461 (2013) 012021. [42] Z. Ghorannevis, E. Akbarnejad, M. Ghoranneviss, Structural and morphological properties of ITO thin films grown by magnetron sputtering, Journal of Theoretical and Applied Physics, 9 (2015) 285-290. [43] M. Marikkannan, M. Subramanian, J. Mayandi, M. Tanemura, V. Vishnukanthan, J.M. Pearce, Effect of ambient combinations of argon, oxygen, and hydrogen on the properties of DC magnetron sputtered indium tin oxide films, AIP Advances, 5 (2015) 017128.
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[44] T. Dhakal, A.S. Nandur, R. Christian, P. Vasekar, S. Desu, C. Westgate, D.I. Koukis, D.J. Arenas, D.B. Tanner, Transmittance from visible to mid infra-red in AZO films grown by atomic layer deposition system, Solar Energy, 86 (2012) 1306-1312. [45] J.-M. Huang, C.-S. Ku, C.-M. Lin, S.-Y. Chen, H.-Y. Lee, In situ Al-doped ZnO films by atomic layer
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deposition with an interrupted flow, Mater. Chem. Phys., 165 (2015) 245-252. [46] G. Luka, L. Wachnicki, B.S. Witkowski, T.A. Krajewski, R. Jakiela, E. Guziewicz, M. Godlewski, The
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uniformity of Al distribution in aluminum-doped zinc oxide films grown by atomic layer deposition, Materials Science and Engineering: B, 176 (2011) 237-241.
Figure captions
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Figure 1. XRD spectra of the AZO films deposited at 100, 150, 200, 250 and 300 °C, respectively. The insets shows the calculated c-axis lattice constant as a function of deposition temperature (right) and the ZnO (002) diffraction peak with expanded scale (left), indicating that the lattice starts to exhibit apparent changes for T >200 °C.
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Figure 2. High-resolution cross-sectional transmission electron microscope (HR-XTEM) images of the AZO thin films grown at (a) 150 °C and (b) 300 °C. Notice the significant improvement in the crystallinity of the AZO grains when the growth temperature is raised to 300 °C. (c) and (d) are the height profiles along the
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vertical green lines indicated, revealing the thickness of SiOx layers.
Figure 3. Room-temperature PL spectra of the AZO films deposited on Si (100) substrates. The inset shows
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that the near band edge emission exhibits an abrupt change as the deposition temperature was raised above 250°C.
Figure 4. Summary of energy gaps obtained for AZO films with different Al doping concentrations. Figure 5. The electrical properties of the AZO films grown at the various temperatures. The inset shows the
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Figure 6. UV-Vis spectra of the AZO films grown on glass. The inset shows the average transmittance of visible region from 400-800nm.
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Figure 7. SIMS results showing the depth profiles of elements Zn, Al, O, H, C, and Si for AZO films
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ACCEPTED MANUSCRIPT (1) Effective Al-doping in ZnO achieved by in-situ doping ALD scheme. (2) Unprecedented carrier mobility of 136 cm2V-1S-1 was obtained.
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(3) All AZO films exhibited over 95% transmittance over wide spectral range.
S0925-8388(17)32936-5
DOI:
10.1016/j.jallcom.2017.08.207
Reference:
JALCOM 42964
To appear in:
Journal of Alloys and Compounds
Received Date: 20 June 2017 Revised Date:
16 August 2017
Accepted Date: 21 August 2017
Please cite this article as: M.-L. Lin, J.-M. Huang, C.-S. Ku, C.-M. Lin, H.-Y. Lee, J.-Y. Juang, High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.207. 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.
ACCEPTEDconductive MANUSCRIPT Al-doped ZnO thin High mobility transparent
films by atomic layer deposition Man-Ling Lin1, Jheng-Ming Huang2, Ching-Shun Ku3, Chih-Ming Lin4, Hsin-Yi Lee3＊, and Jenh-Yih Juang 1＊ Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
2
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
3
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
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Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan
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Abstract
The effects of growth temperature on the microstructure, transport and optoelectronic properties of a series of Al-doped ZnO (AZO) films with thickness of ~30 nm deposited on polished silicon-(100) and glass substrates by the atomic layer deposition (ALD) were investigated. By adopting an in-situ doping growth
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scheme the critical length effect associated with adjacent Al2O3 layers commonly encountered in previous ALD growth schemes was avoided and effective Al-doping was achieved with the growth temperature ranging from 100 °C to 300 °C. Experimental results showed that, in general, increasing the growth
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temperature would result in much improved film crystallinity and carrier mobility, with the average transmittance in the visible wavelength range being exceeding 95% in all cases. In particular, for AZO films
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grown at 300°C, an unprecedented mobility of 136 cm2V-1s-1 was obtained, comparing to the typical values of 50∼60 cm2V-1s-1 reported previously. The resistivity of these 300°C films (ρ ≈ 6×10-4 Ω-cm), nevertheless, is slightly higher than that of some highly-doped ZnO (ρ ≈ 2∼4×10-4 Ω-cm) prepared by sputtering methods. The secondary ion mass spectroscopy (SIMS) analyses revealed that hydrogen incorporation is the key in reducing the charge trap density and, hence, resulting in much enhanced carrier mobility. The present results promise a keen competitiveness of AZO with the indium tin oxide (ITO) film for thin-film-transistor (TFT) as well as in photovoltaic device applications. Keywords: Semiconductors, Atomic Layer Deposition, Electric Transport, Optical Properties. *Corresponding authors: E-mail: [email protected] (H. Y. Lee); [email protected] (J. Y. Juang)
1. Introduction
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Transparent conductive oxides (TCO) are among the critical keys enabling the rapid progress of the information technology industries owing to their wide variety of attractive physical properties, such as high mobility, low resistivity, high transmittance in visible region, highly transparent in infrared (IR) range [1]. Even more importantly, most of these prominent properties are tunable by doping, like in semiconductors.
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Presently, TCO films derived by a wide array of preparation methods and materials have already been ubiquitously used for various applications, including organic light-emitting diodes [2], flat panel displays [3], as well as photovoltaic devices [4, 5]. Perhaps, one of the most widely used TCO materials is the indium tin
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oxide (ITO). In addition to its high conductivity and optical transparency in visible wavelengths, the relatively lower processing temperature (~250 °C) [6] has made ITO the indispensable transparent electrodes
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that evidently led to the flourishing development in organic and flexible electronic devices witnessed over the past decades [7]. However, due to the scarcity, toxicity, and high production cost of indium [8], recently, exploring indium-free alternatives has become one of the hotly pursued research topics in this field. Al-doped ZnO (AZO) stands out as one of the most commonly cited alternative materials for replacing
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ITO because of its abundance, non-toxicity, and low cost [7]. Previously, AZO thin films had been grown by various deposition techniques, including pulse laser deposition [9], sputtering [10], and metal organic chemical vapor deposition [11]. However, these deposition methods require either high growth temperature
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(> 400 °C) or high post-annealing temperature (> 500 °C), which not only would limit the choice of suitable substrates but also often lead to performance degradation of the devices. In general, higher growth
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temperatures would result in increased dopant solubility and hence carrier concentration, nevertheless, it is usually accompanied by substantial reductions in carrier mobility and optical transparency. Among others, effects such as the intra-granular scattering resulting from charged-impurity and strain-induced lattice disruption, as well as, inter-granular scattering due to refinement of grain size and the presence of second phases segregated at grain boundaries, are the common causes responsible for the mobility degradation [12]. Obviously, in addition to increasing the doping (carrier) concentration, controlling the film microstructure is also of essential importance for obtaining desired transport and optical properties of TCO films. In this respect, atomic layer deposition (ALD) with low growth temperature is more advantageous for depositing high-quality AZO thin film because of its viability of controlling the film thickness and composition at the
monolayer level, as well as the capability of obtaining large-area uniformity and satisfactory reproducibility
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[13]. In this study, we report the electrical and optical properties of ALD–AZO films obtained with a novel in-situ doping scheme. The effects of growth temperature on the crystalline structure, optoelectronic properties and, in particular, the much enhanced carrier mobility of the resulting films is addressed.
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2. Experimental procedures The AZO films investigated in the present study were grown on polished silicon-(100) and glass substrates by the flow-rate interruption ALD method at temperatures varying in the range of 100~300 °C. To
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carry out the flow-rate interruption scheme, we installed a quick ALD diaphragm valve (Swagelok) between the reactor and the pump station to retain the precursor in the reactor chamber, which has been shown to
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substantially increase the reaction rate at low temperatures [14]. Briefly, in this study, the film growth procedures were as followings. Diethylzinc [Zn(C2H5)2] (DEZn), trimethylaluminum [Al(CH3)3] (TMA) and DI water (H2O) were adopted as the starting precursors for zinc, aluminum, and oxygen, respectively. For DEZn and H2O precursors, the time of pulse and stock durations were 50 ms and 2 s, respectively. N2 was
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used as the purge gas with a set pressure of 6×10-2 Torr. The purge and pumping periods were 3 s and 8 s, respectively. According to the process optimized in the previous study [15], the 5:1 frequency ratio of Zn-O and Zn-Al-O cycles could result in highly conductive AZO thin films. Thus, in each ALD cycle, 5 layers of
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Zn-O were deposited followed by adding 1 layer of Zn-Al-O, to serve as the in-situ doping layer for promoting the uniformity of Al-doping in the ZnO matrix. All samples investigated in this study were
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obtained with a total of 30 ALD cycles. The total thickness of the films varies from ∼20-30 nm depending on the growth temperature.
The structural characteristics of the AZO films were examined with a synchrotron x-ray source.
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high-resolution x-ray scattering experiments were performed at wiggler beamline BL-17B1 in the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. Incident x-rays were focused vertically with a mirror and were made monochromatic with energy 8 keV by a Si (111) double-crystal monochromator. Using slits in two pairs between the sample and the detector, the typical scattering vector resolution in the vertical scattering plane was set to ~ 1×10-3 nm-1 in these experiments. The microstructure and composition of samples were characterized using transmission electron microscopy (TEM, JEOL
JEM-ARM200F Japan) operating at voltage of 200 kV and energy dispersive X-ray spectroscopy (EDS),
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respectively. The optical properties of representative AZO films were characterized by photoluminescence (PL) using a He-Cd laser as the light source. The excitation wavelength and output power of the He-Cd laser were 325 nm and 4 mW, respectively. The Hall effect measurements were carried out to determine the electrical and transport properties of the AZO samples using a National Instruments PXI-1024Q AC system
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with an applied magnetic field of 0.68 T. The transmittance was measured using a Hitachi U-3010 UV-Visible Scanning Spectrophotometer. The depth profile of the film composition was examined by
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secondary ion mass spectroscopy (SIMS).
3. Results and discussion
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Figure 1 shows the XRD results of the AZO films grown on Si(100) substrates by the ALD processes described above at various growth temperatures, ranging from 100 °C to 300 °C. The results clearly indicate that all films preserve the hexagonal wurtzite structure (space group P63mc) characteristics of the parent ZnO [16]. Nevertheless, it is also evident from Figure 1 that the film microstructure can be very different as
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the growth temperature varies. Namely, for films grown at temperatures below 200 °C, the microstructure appears to be of more randomly distributed polycrystalline structure, as suggested by the more evenly distributed intensity between the (100), (002), and (101) crystallographic orientations. At the temperature
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below 200 °C, the c-axis lattice constant of AZO/Si films calculated from XRD peak of 2θ = 34.66° is 5.17 Å, which is indeed slightly smaller than 5.20 Å for bulk wurtzite ZnO [17]. Since the ionic size of Al3+
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(0.53 Å) is substantially smaller than that of Zn2+ (0.74 Å) [18], one expects to see noticeable change in lattice constant if the doped Al occupies the lattice site of Zn substitutionally [19]. The fact that the position of the diffraction peaks, hence the lattice constants, for films deposited below 200 °C remained essentially unchanged implies that the Al-doping might not be fully prevailed at temperatures lower than 250 °C [20]. Nonetheless, when the growth temperature was further increased to 300 °C, the position of the (002) peak shifts noticeably to higher diffraction angle of 2θ = 34.82°, corresponding to a c-axis lattice constant of 5.15 Å, indicating substantial increase of Al-doping, which is in line with the observation made by Musat et al. [21]. It is also noted that, the films deposited at 250 and 300 °C appeared to grow preferentially along the c-axis with much improved crystallinity, as indicated by the sharpened (002) diffraction peak with higher
intensity. Moreover, the position of the (002) diffraction peak has evidently shifted to higher diffraction
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angles, indicating a reduction of the c-axis lattice constant caused by substitutional Al-doping. We believe that, at higher growth temperatures, the strong polarity between the Zn2+ and O2− ions stacking along the c-axis could have helped the c-axis preferred orientation growth seen here. Finally, we note that previous reports indicated that the ALD-derived ZnO thin films grown at lower temperatures or with doping often
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exhibited a tendency of growing with the a-axis preferred orientation [22], which is not observed in the present study. The differences are presumably due to the detailed growth conditions, such as temperature, substrates, total film thickness, and even the growth system used. It is noted that in [22] the growth-doping
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scheme was more “traditional” and the AZO films were grown on glass substrate with a total thickness of ~ 200 nm. Whereas, in our case, the AZO films used for TEM characterizations were grown on Si substrates
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by the in-situ doping scheme with a thickness of only ~30 nm (see below).
In order to obtain more detailed microstructural information and its correlation with the optoelectronic properties of the obtained AZO films, cross-sectional high-resolution transmission electron microscopy (X-HRTEM) was performed. Figure 2 shows the representative X-HRTEM images and cross-sectional
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height profile for the AZO films grown at 150 °C and 300 °C, respectively. It can be seen that all films are essentially growing on a native SiOx layer of ∼2 nm-thick. Nevertheless, at the growth temperature of 300 °C, the SiOx layer appears to be much blurred, presumably due to the enhanced inter-diffusion at elevated
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temperatures. The morphology of the nano-crystallites also appears to evolve from the equiaxed feature (Fig. 2(a)) to more columnar characteristic (Fig. 2(b)) as the growth temperature was increased from 150°C to
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300°C, which is in line with the more c-axis preferred growth nature for films grown at higher substrate temperatures seen in the XRD results shown in Figure 1. Moreover, both film are having about the same thickness, which is in contrast to the temperature-dependent growth rate of ZnO films grown by ALD reported by previous studies. Oh et al. [22] reported that the growth rate of ZnO films decreased monotonically from 0.177 nm/cycle to 0.132 nm/cycle when the growth temperature was increased from 150 °C to 250 °C. On the other hand, Li et al. [23] reported that the growth rate increases with temperature for temperatures below 200 °C and then decreases with further increase in growth temperature. These comparisons indicate that the reported temperature-dependent growth rate is sensitive to the systems and detailed conditions used by each research group. Since the growth rate of ZnO is very much dependent on
the surface density of O-H bonding groups [24], thus the details of growth system and conditions can affect
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the eventual results. Furthermore, the total film thickness in the abovementioned studies was approaching 200 nm, which is much thicker than the present ones and may not reflect the real growth rate in the early stages of ALD processes. The next important question to be asked is whether or not the Al ions are successfully doped on the lattice site of Zn. We will address this issue in the following with the related
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optoelectronic properties. Figure 3 shows the room-temperature PL spectra for the ALD AZO thin films grown at various temperatures. It is clear that only the UV region emissions are observed and no emissions in the visible
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range associated with various crystalline defects, surface states and impurities are seen. This is indicative that the visible range emissions are either nearly completely suppressed due to the absence of the
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abovementioned defects in the present films [25] or quenched by some yet to be identified mechanism. Nevertheless, a closer examination on the PL spectra does give rise to distinctive details for films grown at different temperatures. The band gap energies derived from near band edge (NBE) peaks are 3.46 eV and 3.31 eV corresponding to an Al concentration of 3.4 % [12] and 1% [26], respectively. Using the Vegard’s
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law , = 1 − , + , and take , = 3.37 eV and , = 6.40 eV reported previously, the expected energy band gap of the 3.4% sample is consistent with those of the 250 °C and 300 °C films. In contrast, it is obvious that the Vegard’s law alone is inadequate to account for that of the 1%
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AZO. Figure 4 summarizes some of the results reported in literature to illustrate that most of the measured energy gap values in AZO samples prepared by various processes are, in fact, largely falling short of that
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expected from the Vegard’s law. As pointed out by Bernard and Zunger [27], due to complicated factors such as volume deformation, charge exchange and the relaxation of the anion-cation bond lengths brought about by alloying, one often needs to modify the Vegard’s law with a bowing parameter to give: , = 1 − , + , − 1 − [28]. In the present case, the bowing parameters of 0.34 eV and 8.11 eV are obtained for Al concentration of 3.4% and 1%, respectively. The significantly larger bowing parameter for lower doping concentration has also been observed in GaAsN alloys [29] and was attributed to the localized impurity-like states formed near the band edges. Namely, in diluted GaAs1-xNx alloys, the valence band is strongly localized on the As sublattice and the conduction band is strongly localized on the nitrogen sublattice. We believe that due to the large disparity in ionic size (Al3+∼0.53Å; Zn2+∼0.74Å) similar
effects may also prevail in the present case. Nevertheless, further investigations are needed to clarify this
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issue. Back to the results shown in Figure 3, it is clear that the PL spectra can be roughly sorted into two groups; one has the emission at energies (∼ 3.46 eV) higher than the intrinsic energy gap of 3.37 eV for pure ZnO [30]; the other has the emission at energies ranging from 3.32 ± 0.01 eV, which is slightly lower than
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3.37 eV. Moreover, the emission intensity of the former is consistently lower than that of the latter. It is generally conceived that both improved crystalline quality and size confinement effects can lead to substantial intensity enhancement in the UV emission spectrum of various ZnO nanostructures [31, 32]. By
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comparing the microstructures displayed in Figure 2(a) and 2(b) for films grown at 150 °C and 300 °C, respectively, it is suggestive that the higher PL emission intensity observed for the 100 °C and 200 °C films
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might be due to the similar size-confinement effect. Thus, sub-band-gap emissions in films grown at 100-200 °C may arise from the involvement of exciton binding energy, which may not be as significant in films consisted of larger crystallites (250-300°C films).
On the other hand, perhaps the most remarkable results of the present study are concerning the
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significant improvement in the transport properties of the AZO films obtained by the proposed in-situ doping scheme. Figure 5 shows the resistivity and mobility as a function of deposition temperature for the AZO films investigated in the present study. As is evident from Figure 5, the resistivity of the present AZO
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films drops progressively from ∼4×10-2 Ω−cm to ∼6×10-4 Ω−cm as the growth temperature is increased from 100 °C to 300 °C. According to the Burstein-Moss effect [33], the bandgap energy should increase with the ⁄!
, where n is the carrier density. Since the carrier density due to Al-doping is
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carrier density at a rate of
insensitive to the energy band gap (see the inset of Figure 5), one can assume that in-situ doping has been effective even at very low deposition temperatures and the changes in resistivity are mainly resulted from the microstructures. Within the context of this scenario, the precipitous drop in film resistivity at 150°C could thus be attributed to the amorphous to crystalline transition of the film microstructure. Whereas, the continuous reduction in film resistivity at higher deposition temperatures is associated with the progressive enhancement of crystallinity as well as the growth of grain size, both are known to be effective in reducing the scattering of carrier transport [34-37]. More remarkably, within the same deposition temperature range, the carrier mobility has drastically improved from ∼4 cm2V−1s−1 for amorphous 50°C film, to ∼10-20
cm2V−1s−1 for films deposited below ACCEPTED 200°C, and finally reaching 136 cm2V-1s-1 for films grown at 300°C. MANUSCRIPT Comparing with the mobility values reported previously in the literature (e.g. Banerjee et al. [36] reported a maximum value of 17.7 cm2V-1s-1 for 100 nm-thick AZO films deposited at 150 °C; Ahn et al. [38] reported a mobility of ~17 cm2V-1s-1 for 200 nm-thick AZO films grown at 100-300 °C; Luka et al. [39] reported a maximum mobility 21.8 cm2V-1s-1 for 200 nm-thick AZO films at 200 °C; Li et al. [23] reported a maximum
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mobility ~7 cm2V-1s-1 for 90 nm-thick AZO films at 150 °C), this value of mobility is rather significant in terms of application potential.
In particular, considering that the present film thickness is only in the order of few tens of nanometers
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higher transparency can be expected, which, in turn, is promising for various applications. Indeed, as displayed in Figure 6, the optical transmittance spectra taken in the wavelength range of 280-800 nm for
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AZO films grown at different temperatures show that all samples have a strong absorption in the UV region and high transmittances in the visible region. The average transmittance in the wavelength range between 400-800 nm exceeds 95 % (see the inset of Figure 6). The seemingly scattered behaviors near the absorption edge of 360~370 nm for films deposited at various temperatures are attributed to the interfacial roughness
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between the film and glass substrates and the differences in the film microstructure. In any case, the present results have evidently shown substantially better transmittance than those reported in literatures for ITO thin films or ALD-derived AZO films that did not use the flow-rate interrupted scheme [40-44].
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In order to clarify the underlying mechanism responsible for the unprecedented enhancement in the electric transport performance obtained in the present study, secondary ion mass spectroscopy (SIMS) depth
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profiles of Al, Zn, O, H, C, and Si for the two representative AZO films deposited at 200°C and 250°C were performed. This is also the temperature range where the significant change in mobility was observed (Figure 5). As shown in Figure 7, a rather uniform distribution of Al is evident in both AZO films, indicating that uniform Al doping is indeed achieved in the present in-situ doping process scheme. Moreover, the 250°C film is having a higher relative concentration than that of the 200°C films, confirming our previous conjectures that higher growth temperatures would result in more efficient doping. It is also noted that unlike other elements, the Al concentration distribution appears to exhibit a slight periodic fluctuations, presumably due to the in-situ doping scheme adopted in this study. However, the periodicity seems to be greater than the diffusion length of Al in bulk ZnO within the similar temperature range [45]. It is noted that
Luka et al. [46] also reported periodic distribution of Al in their AZO films by SIMS measurements,
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indicating that in ALD processes the diffusivity of individual atomic species are significantly faster than in the case of bulk diffusion. In addition, the SIMS results also clearly indicate that the relative concentration of H-atom near the surface (0-10 nm) in the 250°C sample is substantially larger than that of the 200°C sample. It is suggestive that incorporation of H in filling up the oxygen vacancies might have played an
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important role in reducing the density of grain barrier traps and giving rise to the much enhanced carrier mobility obtained in the present study.
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4. Conclusions
In summary, a series of ultrathin Al-doped ZnO films were prepared at various growth temperatures by
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atomic layer deposition with an in-situ doping deposition scheme. The film microstructure showed a progressive transition from amorphous to equiaxed polycrystalline to (002)-oriented columnar structure as the growth temperature was changed from 100 °C to 300 °C. The energy gap of the films revealed by the photoluminescence measurements indicated that the band-gap widening and associated near-band-edge
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emissions have involved alloying effect of Al2O3 and ZnO, instead of purely due to the Burstein–Moss effect and band-gap renormalization effects. According to the modified Vegard’s law, bowing parameters of 0.34 eV and 8.11 eV were obtained for the Al concentration of 3.4% and 1%, respectively. The UV-Vis
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measurements revealed that all samples are having above 95% transmittance, which are higher than that of most ITO films. More importantly, the AZO film grown at 300 °C displayed an unprecedented high mobility
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of 136 cm2V-1s-1 and a resistivity of 6×10-4 Ω-cm, which have significant implication for serving as the transparent conducting oxide in oxide thin-film-transistors (TFTs) applications. In particular, the advantages of monolayer-control and high aspect ratio unique to ALD process offer outstanding opportunities in overcoming the size effect expected for future TFTs. Finally, based on the XPS and SIMS results, the incorporation of hydrogen into the AZO films during ALD processes have played an important role in giving rise to the high mobility observed.
Acknowledgements This work was financially supported by the Ministry of Science and Technology, Taiwan, R.O.C. under grant
numbers: MOST 103-2112-M009-015-MY3 and MOST 101-2221-E-213-001-MY3. J. Y. Juang is also
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partially supported by the MOE-ATU program operated at NCTU.
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Figure captions
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Figure 1. XRD spectra of the AZO films deposited at 100, 150, 200, 250 and 300 °C, respectively. The insets shows the calculated c-axis lattice constant as a function of deposition temperature (right) and the ZnO (002) diffraction peak with expanded scale (left), indicating that the lattice starts to exhibit apparent changes for T >200 °C.
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Figure 2. High-resolution cross-sectional transmission electron microscope (HR-XTEM) images of the AZO thin films grown at (a) 150 °C and (b) 300 °C. Notice the significant improvement in the crystallinity of the AZO grains when the growth temperature is raised to 300 °C. (c) and (d) are the height profiles along the
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vertical green lines indicated, revealing the thickness of SiOx layers.
Figure 3. Room-temperature PL spectra of the AZO films deposited on Si (100) substrates. The inset shows
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that the near band edge emission exhibits an abrupt change as the deposition temperature was raised above 250°C.
Figure 4. Summary of energy gaps obtained for AZO films with different Al doping concentrations. Figure 5. The electrical properties of the AZO films grown at the various temperatures. The inset shows the
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carrier concentration in the respective AZO films.
Figure 6. UV-Vis spectra of the AZO films grown on glass. The inset shows the average transmittance of visible region from 400-800nm.
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Figure 7. SIMS results showing the depth profiles of elements Zn, Al, O, H, C, and Si for AZO films
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deposited at 200 °C and 250 °C.
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5.18
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Eg,AZO=(1-x)Eg,ZnO+xEg,Al O 2
Experiment data of Kim et al. Experiment data of Kuo et al. Experiment data of You et al. Our data
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ACCEPTED MANUSCRIPT (1) Effective Al-doping in ZnO achieved by in-situ doping ALD scheme. (2) Unprecedented carrier mobility of 136 cm2V-1S-1 was obtained.
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(3) All AZO films exhibited over 95% transmittance over wide spectral range.