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The dependence of structural, electrical and optical properties on the composition of photochromic yttrium oxyhydride thin films
Materialia 6 (2019) 100307
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Full Length Article
The dependence of structural, electrical and optical properties on the composition of photochromic yttrium oxyhydride thin films Chang Chuan You a,∗, Trygve Mongstad b, Erik Stensrud Marstein a, Smagul Zh. Karazhanov a a b
Department for Solar Energy, Institute for Energy Technology, Kjeller NO-2027, Norway Solenergi FUSen AS, Oslo NO-0603, Norway
a r t i c l e
i n f o
Keywords: Photochromism Persistent photoconductivity Yttrium oxyhydride Reactive magnetron sputtering Optical properties Electrical properties
a b s t r a c t Thin films of yttrium oxyhydride (YHO) exhibit reversible light-induced resistivity and transmission changes at room temperature and ambient pressure. Establishing how the physical properties of YHO films are influenced by their chemical composition is an important challenge that can open the way toward practical applications. In this work, we have prepared YHO thin films with lateral gradient of oxygen and hydrogen concentrations by reactive magnetron sputtering deposition. This enables us to efficiently investigate the effect of changes in the composition of the films on their structural, electrical and optical properties. The as-deposited YHO film appeared to be black opaque and non-photochromic in the oxygen-poor part of the film, and it changed to yellow transparent and photochromic in the oxygen-rich part of the film. We report a gradual increase in the lattice constant with increasing oxygen content of the film, as revealed by grazing incidence X-ray diffraction measurements. Electrical resistivity measurements unveiled that persistent photoconductivity was strongly enhanced as the oxygen content decreased in the photochromic yellow transparent film. Analysis of the kinetics of the photochromic reaction indicated that the bleaching speed increased with increasing oxygen content in the photodarkened film. Unusual large persistent photochromism was discovered in the same yellow YHO film with lowest oxygen content which lasted for almost 10 days. Moreover, it was shown that the optical constants can be tuned by varying the oxygen content in the photochromic film as well.
1. Introduction The possibility of tailoring electrical and optical properties of rareearth and transition metal hydrides induced by hydrogenation and dehydrogenation has attracted extensive research interest due to their potential application in a wide range of technological devices [1–3]. These metal hydride films are typically capped with a thin Pd layer on the surface and thus are expected to be oxygen free. However, for uncapped thin films, they will get oxidized upon exposing to air. The oxidized yttrium hydride is a material of special interest because it is discovered recently to possess photochromic effect [4], i.e., exhibiting reversible color change as a result of the absorption of electromagnetic radiation. The photochromic effect of yttrium oxyhydride (YHO) is manifested by a remarkable change of optical transmission upon illumination in which the transparency of the film can be typically reduced from ∼ 90% to 40– 50% in the visible and infrared regions of the light spectrum. The photodarkened YHO film returns back to its initial yellow clear state upon
∗
Corresponding author. E-mail address: [email protected] (C.C. You).
dark annealing at ambient conditions. This switchable optical characteristic is attractive for potential applications such as energy-saving smart windows, ophthalmic lenses and sensors [5,6]. Photochromism of other rare-earth metal (Gd, Dy and Er) oxyhydrides has recently been reported as well [7]. These materials all belong to an emerging class of materials called mixed anion systems in which the hydride, oxide, and hydroxide anions share the same sites in the crystal lattice [8–10]. When YHO films are prepared by combining reactive magnetron sputtering with subsequent oxidation, material properties are observed to be dependent on the partial hydrogen pressure during film deposition. In particular, the oxygen concentration in such films appears to be inversely correlated with that of hydrogen [11] and a ternary phase diagram has recently been constructed for YH2−𝛿 O𝛿 in which 0.45 < 𝛿 < 1.5 yield stable photochromic compounds [12]. Moreover, the photochromic response is strongly affected by the amount of oxygen present in the film [13] and the optical band gap of YHO films can be engineered by controlling the deposition conditions [14]. Recent electron microscopy characterization revealed that the microstructure of photochromic YHO thin films is dominated by a columnar growth, in which the columns consist of multiple grains with an average grain size of 15 nm [15]. In order to fully exploit such materials for energy-saving smart window and optoelectronic applications, a thorough knowledge of the dependence of physical properties on the film composition is crucial. Hence, in the present study
https://doi.org/10.1016/j.mtla.2019.100307 Received 8 January 2019; Accepted 28 March 2019 Available online 5 April 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
C.C. You, T. Mongstad and E.S. Marstein et al.
Materialia 6 (2019) 100307
we aim to establish the relationship between the chemical composition of YHO thin films and their structural, electrical and optical properties, with focus on investigating the decay kinetics of photoconductive and photochromic responses after illumination. 2. Material and methods YHO thin films were prepared on glass substrates by using reactive magnetron sputtering deposition in a Leybold Optics A550V7 inline sputter-coating system. The glass substrate size was 76 × 26 mm2 . In order to achieve a lateral gradient in the film composition, the substrate carrier was placed in a static position in front of the Y target during film growth. The total gas flow of argon and hydrogen was set constant at 200 sccm with an Ar:H2 gas ratio of 4:1. The deposition pressure was 0.66 Pa. Note that oxygen gas was not used during the film growth. The main incorporation of oxygen into the film is believed to take place when the yttrium hydride film reacted with air right after it was taken out from the vacuum chamber [16]. More details of the deposition condition can be found elsewhere [13]. The film thickness was determined using a stylus surface profilometer. The structural properties of the films were investigated by performing grazing-incidence X-ray diffraction (GI-XRD) measurements using a Bruker D8 Discover X-ray diffractometer with a beam footprint chosen to be approximately 2 × 10 mm2 in the sample. The lateral spacing between each GI-XRD measurement was 5 mm. The electrical resistivity measurements were carried out after sample illumination using a standard four-point-probe setup. The van der Pauw resistivity measurements were performed on the bleached yellow transparent portion of the YHO sample in a Keithley 4200 Semiconductor characterization system. Prior to the van der Pauw resistivity measurements, four 5 × 5 mm2 Al metal contacts were deposited on the yellow film by thermal evaporation through a shadow mask. The resulting sample size was about 26 × 26 mm2 , which was electrically isolated from the rest of the film by gently scribing the film with a diamond pen. The optical properties of the thin films were characterized using an Ocean Optics QE65000 spectrometer equipped with an integrating sphere and deuterium and tungsten halogen light sources. The spot diameter of the incident light was ∼ 5 mm for the transmission and reflection measurements. In order to induce the photoconductive and photochromic effects, the samples were exposed to continuous illumination up to 24 h in a solar simulator from WACOM Electric with an intensity calibrated at ∼ 1000 W/m2 under ambient conditions. 3. Results and discussion 3.1. Structural properties Fig. 1 shows photographs of a representative YHO thin film sample with gradient chemical composition captured 2 h (top panel) and 57 days (bottom panel) after deposition. The red marker line depicts the original boundary between the non-photochromic black opaque and photochromic yellow transparent parts of the film right after it was removed from the sputtering chamber. After the same sample was exposed to oxygen in air in darkness at room temperature over several weeks, a narrow slice ( ∼ 10 mm in width) of the black opaque film close to the boundary was observed to be transformed permanently to yellow transparent due to oxidation. The relative lateral oxygen content obtained from a typical YHO film, expressed in terms of an average bulk [O]/[Y] ratio, was found to increase gradually from ∼ 0.25 in the black region to ∼ 0.65 in yellow region, as verified by time-of-flight energy elastic recoil detection analysis and nuclear reaction analysis in our previous study [13]. Fig. 2(a) shows GI-XRD diffractograms recorded on a YHO thin film sample at different measurement positions ranging from 5 to 70 mm. This sample has been stored in air in the dark for 90 days at the time of measurements. Four Bragg reflection peaks corresponding to (111), (200), (220) and (311) crystal orientations were identified. We note that
Fig. 1. Photographs of a representative YHO thin film sample captured 2 h (top panel) and 57 days (bottom panel) after deposition. The red line marks the original boundary between the black opaque and yellow transparent parts of the as-deposited film. We note that the same sample was exposed to oxygen in air in darkness at room temperature. As can be seen, a narrow slice of the black film close to the boundary was permanently transformed to yellow color due to oxidation.
the diffraction lines around 62∘ in Fig. 2(a) correspond to the (222) Bragg reflection peak. Mongstad et al. have previously shown that both black opaque and yellow transparent YHO thin films belong to the same space group Fm3̄ m with a fcc crystal structure [17]. As can be seen in Fig. 2(a), the (222) peak of the black film is more prominent as compared to the (222) peak in the yellow film consisting of an increased amount of oxygen, which can be ascribed to different preferential orientational growths of the grains. The XRD patterns suggest that there is more orientational growth in the ⟨100⟩-direction for the yellow film, whereas the ⟨111⟩-direction is preferred for the black film [17]. In Fig. 2(b) the average lattice constant, which was derived from these four reflection peaks, is plotted as a function of the measurement position. Note that the oxygen content in the film increases along the lateral direction from position 5–70 mm. It can be clearly seen that the average lattice constant increased gradually from ∼ 5.27 to 5.34 Å when the visual appearance of the YHO film changed from black opaque to yellow transparent as displayed by the color bars in the figure. This increase in lattice constant can be attributed to an increase of the oxygen content in the same film, leading to an expansion of the unit cell, in agreement with previous results obtained from different black or yellow YHO samples [17]. In addition, the average lattice constant of the transformed film at position 40 mm was found to be slightly smaller as compared to that of the adjacent yellow film with higher oxygen concentrations. 3.2. Electrical properties The photo-induced electrical resistivity of a YHO sample measured at different bleaching times up to 1056 h at measurement positions ranging from 5 to 70 mm is displayed in Fig. 3(a). Note that the sample was continuously illuminated for 24 h in a solar simulator before the resistivity measurements. During bleaching the sample was rested in air in darkness at room temperature and ambient pressure. The film thickness of this sample was measured to be ∼ 710 nm at the bottom left corner in the black region of the film and ∼ 835 nm at the top right corner in the yellow region, based on thickness measurements performed on a neighboring YHO sample sputter-deposited at the same time. The lowest resistivity in the black opaque film was recorded at position 5 mm with 𝜌 ∼ 10−2 Ω cm. However, more conductive black YHO films with a resistivity on the order of 10−3 Ω cm are achievable by lowering the oxygen content [17,18]. It is also evident from Fig. 3(a) that the resistivity of the non-photochromic black film was not noticeably affected by the long light exposure time. The photo-induced resistivity of the photochromic yellow film was found to vary by about one order of magnitude from
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Materialia 6 (2019) 100307
Fig. 2. (a) GI-XRD diffractograms recorded at measurement positions ranging from 5 to 70 mm on a YHO thin film 90 days after deposition. Four Bragg reflection peaks corresponding to the (111), (200), (220) and (311) crystal orientations were recorded. (b) Average lattice constant obtained at different measurement positions on the same YHO film consisting of black opaque, transformed and yellow transparent regions as depicted by the three horizontal color bars.
Fig. 3. (a) Photo-induced resistivity of a YHO sample measured at different bleaching times ranging from 0.1 to 1056 h at measurement positions of 5–70 mm after the illumination was turned off. (b) Photoconductivity as a function of bleaching time for varying oxygen contents obtained at different measurement positions. The solid lines are best fits to the experimental data.
𝜌 = 1.3 × 103 to 1.8 × 104 Ω cm as the measurement position was moved from 40 to 70 mm, as illustrated by the blue circle symbols. The main feature is that when going from the black to the yellow part of the illuminated film, the resistivity increased greatly by approximately six orders of magnitude with increasing oxygen content, which is similar to that reported for other related metal oxyhydrides such as GdOy Hx films [19]. Hence, the YHO film experienced a metallic-to-semiconducting transition which was also accompanied by large optical band gap opening as the oxygen content raised above a threshold [O]/[Y] value of ∼ 0.5 [13]. We note that the resistivity of the yellow transparent YHO film before illumination could not be determined by using our four-point-probe equipment because the maximum measurable resistivity was capped at ∼ 105 Ω cm for these films. Hence, in order to probe the resistivity of the completely bleached YHO film, we performed van der Pauw resistivity measurements instead. The average resistivity across the confined yellow YHO sample area was estimated at ∼ 1.8 × 106 Ω cm. Therefore, it should be feasible to achieve a modulation in electrical resistivity by at least two orders of magnitude upon illumination. For example, the resistivity could change from ∼ 1.8 × 106 in the bleached state to 1.8 × 104 Ω cm after light exposure at measurement position 70 mm. It is interesting to note that the photo-induced resistivity obtained at position 60 mm increased ∼ 600% from 1.1 × 104 to 6.2 × 104 Ω cm after a bleaching time of 26 h, whereas in the transformed film at position 40 mm the resistivity increased ∼ 200% from 1.3 × 103 to 2.4 × 103 Ω cm during the same time period, implying an enhanced ability to maintain the photo-induced conductance. Indeed, as displayed in Fig. 3(b), the changes of photoconductivity 𝜎 with respect to the bleaching time can be approximated by an exponential decay function: 𝜎(𝑡) ∼ 𝜎0 exp(−𝑡∕𝜏𝑃 ), where 𝜎 0 is the initial photoconductivity acquired right after the illumination was turned off, t is bleaching time, and 𝜏 P is decay time constant which can be viewed as the mean lifetime of 𝜎 0 . 𝜏 P was estimated to be 69, 41 24, 18, 16 and 15 h at measurement positions 40, 45, 50, 55, 60 and 65 mm, respectively, extracted
from the slopes corresponding to the best fits (solid lines) of the experimental data (symbols) presented in Fig. 3(b). This finding indicates that the strongest persistent photoconductivity (PPC) appeared in the photochromic yellow transparent film at position 40 mm containing least amount of oxygen as compared to the adjacent positions of 45–65 mm with higher oxygen contents. PPC in semiconductors may depend on several factors such as the potential barriers that separate photo-induced electrons and holes and inhibit recombination between them, types of traps or defect centers and their capture cross section for electrons and holes, and the probability of ejection of charge carriers to the conduction or valence band [20]. The photochromic yellow transparent YHO film is a semiconductor with an optical band gap of ∼ 2.5–3.0 eV depending on the oxygen content and the type of band gap: direct or indirect [13,21]. The electron-hole recombination might therefore take place through a recombination center between the conduction and valence band. Based on our film composition analysis [13], the average [O]/[Y] ratio at measurement position 60 mm is expected to be ∼ 30% larger than that obtained at measurement position 40 mm. One simple hypothesis is that such an increase of oxygen incorporation into the film might reduce the density of trap centers (e.g., anion vacancies [22] or vacancy-complexes) responsible for capturing, storing and ejection of excited free charge carriers, eventually leading to increased recombination of electron-hole pairs. Consequently, PPC is suppressed due to reduced effective lifetime of photo-generated charge carriers. Further study is required to elucidate the underlying mechanisms responsible for the oxygen-dependent PPC observed in YHO thin films. 3.3. Optical properties Establishing the composition-dependent optical constants is important for the potential application of YHO thin films. Fig. 4(a)–(d) shows optical transmission spectra (blue crosses) acquired from measurement positions of 40, 50, 60 and 70 mm on a YHO sample. The transmission,
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Materialia 6 (2019) 100307
Fig. 4. (a)–(d) Optical transmission spectra (blue crosses) obtained from measurement positions of 40, 50, 60 and 70 mm on a YHO sample, respectively. The solid red line is the best fit to the experimental data. d is the average optical film thickness derived from the model. (e) Refractive index n (symbols) as a function of wavelength 𝜆 obtained at measurement positions 40–70 mm. The solid lines are best fits to the data based on the general Cauchy model. (f) Extinction coefficient k obtained at measurement positions 40–70 mm.
T, can be described by an optical model reported by Swanepoel [23], in order to determine the thickness and optical constants of thin films. A reasonably good fit to the data can be obtained, as shown by the red solid lines. Here, d is the average optical film thickness derived from the model. As shown, d was calculated to be ∼ 916 ± 9, 909 ± 4, 888 ± 6 and 861 ± 8 nm at measurement positions of 40, 50, 60 and 70 mm, respectively. From the model, the refractive index at wavelengths corresponding to the minima and maxima of the interference fringes of the transmission spectrum can be calculated and is plotted in Fig. 4(e). The dependence of the refractive index, n, on the wavelength, 𝜆, can be determined by the general Cauchy model: 𝑛(𝜆) = 𝐵 + 𝐶∕𝜆2 + 𝐷∕𝜆4 , where B, C and D are fitting parameters. The four solid lines in Fig. 4(e) represent the best fits to the data points (symbols) acquired at different measurement positions on the YHO sample. As can be seen, when the oxygen content reduced in the film, an appreciable increase in the refractive index was observed. For instance, at 𝜆 = 630 nm, the refractive index was estimated at 2.22, 2.18, 2.15 and 2.10 at the measurement positions 40, 50, 60 and 70 mm, respectively. It should be pointed out that the Swanepoel model gave inaccurate results for wavelengths below ∼ 500 nm due to the fact that strong optical absorption in the material is taking
place, as can be seen in Fig. 4(a)–(d). The extinction coefficient, k, can be approximated by the following expression instead: 𝑘(𝜆) = 𝜆∕4𝜋𝑑 × [ ] ln (1 − 𝑅)∕𝑇 , where R is reflection [17]. As shown in Fig. 4(f), k was found to be largest in the photochromic film with lowest oxygen content at measurement position 40 mm, indicating strongest light absorption. We have previously reported the impact of varying oxygen content on the photochromic response of YHO thin films [13]. Here we turn our attention to investigate the kinetics of the photodarkening process and how fast such photodarkened films bleach back to their original yellow transparent state after light exposure. Fig. 5(a)–(d) displays optical transmission spectra acquired before and after illumination and at different bleaching times at measurement positions 40, 50, 60 and 70 mm. The black solid line represents the transmission obtained before illumination and the blue solid line shows the transmission acquired right after 24 h illumination. As can be seen, a substantial modulation in transmission was observed afterwards. Even after a long bleaching time of 1056 h, the transmission in the bleached film at measurement position 40 mm could not be fully recovered back to the initial values measured before illumination, as shown by the dark red solid line. However, optical measurements carried out at positions 50–70
C.C. You, T. Mongstad and E.S. Marstein et al.
Materialia 6 (2019) 100307
Fig. 5. (a)–(d) Transmission spectra acquired before (black solid line) and after illumination at increasing bleaching times up to 1056 h at measurement positions 40, 50, 60 and 70 mm on the same YHO sample. (e) Photochromic response (in terms of ⟨ΔT⟩ averaged over a wavelength range of 500–900 nm) plotted as a function of bleaching time obtained at different measurement positions. The film was continuously illuminated for 24 h prior to the measurements. The solid lines are the best fits to the experimental data. (f) Photochromic response as a function of illumination time at measurement positions of 40–70 mm. The solid lines are fits to the experimental data, which can be described by a logarithmic function.
mm containing more oxygen revealed that a nearly complete recovery of the original transmission was feasible even at a shorter bleaching time, as shown in Fig. 5 (b)–(d). Fig. 5(e) displays an overview of the change of the photochromic response as a function of the bleaching time. Note that the strength of the photochromic response can be characterized by an average optical contrast ⟨ΔT⟩, defined as the difference between the optical transmission (averaged over a wavelength range of 500–900 nm) acquired before and after illumination as follows: ⟨Δ𝑇 ⟩ = ⟨𝑇before ⟩ − ⟨𝑇after ⟩. The main finding is that the optical contrast diminishes in an exponential fashion with bleaching time, and can be well described by the following expression: ⟨ΔT⟩ ∼ exp(−𝑡∕𝜏𝐵 ), where t is bleaching time and 𝜏 B is bleaching time constant. Similar bleaching behavior of YHO thin films has also been reported by other group [7], although with different illumination conditions leading to different bleaching time constants. The solid lines corresponding to the various measurement positions in Fig. 5(e) represent the best fits to the experimental data (symbols) based on the least squares fitting method. The bleaching time constant was determined to be 238, 42, 29 and 26 h at measurement positions 40, 50, 60 and 70 mm, respectively. This result indicates that the bleaching speed was slowest in the transformed film
containing lowest amount of oxygen and fastest in the most oxygenrich region at measurement position 70 mm. We note that the bleaching speed is generally dependent on the illumination conditions such as light wavelength, intensity and duration used to generate the photochromic effect, in addition to the thickness and composition of YHO. Various environmental effects including the ambient pressure, humidity and temperature are expected to play a vital role in the bleaching process as well. The photochromic response obtained after 1, 5 and 24 h of illumination at measurement positions 40, 50, 60 and 70 mm is shown in Fig. 5(f). During the first hour of illumination, photodarkening was observed to be strongest in the transformed film at position 40 mm with lowest oxygen content and weakest in the most oxygen-rich region at position 70 mm, indicating that the coloration speed from the yellow to dark color transition increased with decreasing amount of oxygen. For the illumination time ranging from 1 to 24 h, the variation of the photochromic response with illumination time can be roughly fitted by a logarithmic function as shown by the solid lines, which represent the best fits to the experimental data. As can be seen from the slopes, the coloration speed was surprisingly fastest for the film with highest
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Materialia 6 (2019) 100307
oxygen content in the same thin film sample. Unusual large persistent photochromism was observed in the same yellow YHO film with lowest oxygen content which lasted for almost 10 days. The present findings demonstrate the possibility to tailor the switchable electrical and optical properties of YHO thin films by controlling the composition, which is interesting for potential utilization in technological applications such as energy-saving smart windows, optical data storage devices and sensors. Declaration of interest None. Acknowledgments Fig. 6. Bleaching time constant for photochromic response (𝜏 B ) and decay time constant for persistent photoconductivity (𝜏 P ) acquired at different measurement positions in the same YHO sample. The solid lines are a guide to the eye. The black opaque, transformed and yellow transparent regions are depicted by the three horizontal color bars.
This work was supported by the Norwegian Research Council through the FRINATEK projects 240477/F20 and 287545 and the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50. We also thank Dr. Vishnukanthan Venkatachalapathy for help with XRD measurements.
oxygen content at measurement position 70 mm. Thus, this finding suggests that both fast and slow coloration kinetics may be involved in the photochromic reaction depending on the total illumination time. Mongstad et al. have previously demonstrated that yellow transparent YHO thin films exhibited photochromism and PPC simultaneously through a series of light switching experiments [4]. Clearly, the photochromic effect is considered to be closely correlated to the PPC effect in this material. For example, recent ellipsometry measurements suggested that the photochromic effect could be associated with a gradual formation of metastable metallic domains in the semiconducting YHO lattice upon illumination [24]. It is probable that such metallic domains also give rise to an increased conductivity in the film, thus contributing to the PPC effect. Our present time-dependent resistivity and transmission data revealed that both the bleaching time constant for photochromic response (𝜏 B ) and the decay time constant for PPC (𝜏 P ) decreased with increasing oxygen content, as summarized in Fig. 6. Interestingly, 𝜏 B was observed to be larger than 𝜏 P for all examined measurement positions in the same yellow transparent YHO film, indicating that photochromism persisted longer than photoconductivity after the illumination was turned off. This finding implies that electronic mechanisms (e.g., recombination of photo-generated electron-hole pairs or decay of metastable metallic domains) might not be the only driving forces behind the aging of the photochromic effect, but structural changes such as lattice relaxation [25] and possible surface effects [26] may also be involved. It should be noted that the largest 𝜏 B of 238 h was obtained in the photochromic YHO film containing lowest oxygen content at measurement position 40 mm, as shown in Fig. 6. Such large persistent photochromic response may prove to be useful for optical memory applications. However, the origin of the observed persistent photochromism is currently not known, which deserves further investigations.
References
4. Conclusions We have fabricated yttrium oxyhydride thin films with gradient chemical composition by reactive magnetron sputtering deposition. We observed strong dependence of structural, electrical and optical properties on the varying oxygen content of YHO thin films. The visual appearance of the as-deposited YHO film changed from black opaque and non-photochromic in the oxygen-poor region of the film to yellow transparent and photochromic in the oxygen-rich region of the film. We have shown that the lattice constant and electrical resistivity increased with increasing oxygen content, whereas a reduction in the optical refractive index and extinction coefficient was observed simultaneously. Persistent photoconductivity was found to be strongly enhanced as the oxygen content decreased in the yellow transparent YHO film. Furthermore, the bleaching speed of photochromic response increased with increasing
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Full Length Article
The dependence of structural, electrical and optical properties on the composition of photochromic yttrium oxyhydride thin films Chang Chuan You a,∗, Trygve Mongstad b, Erik Stensrud Marstein a, Smagul Zh. Karazhanov a a b
Department for Solar Energy, Institute for Energy Technology, Kjeller NO-2027, Norway Solenergi FUSen AS, Oslo NO-0603, Norway
a r t i c l e
i n f o
Keywords: Photochromism Persistent photoconductivity Yttrium oxyhydride Reactive magnetron sputtering Optical properties Electrical properties
a b s t r a c t Thin films of yttrium oxyhydride (YHO) exhibit reversible light-induced resistivity and transmission changes at room temperature and ambient pressure. Establishing how the physical properties of YHO films are influenced by their chemical composition is an important challenge that can open the way toward practical applications. In this work, we have prepared YHO thin films with lateral gradient of oxygen and hydrogen concentrations by reactive magnetron sputtering deposition. This enables us to efficiently investigate the effect of changes in the composition of the films on their structural, electrical and optical properties. The as-deposited YHO film appeared to be black opaque and non-photochromic in the oxygen-poor part of the film, and it changed to yellow transparent and photochromic in the oxygen-rich part of the film. We report a gradual increase in the lattice constant with increasing oxygen content of the film, as revealed by grazing incidence X-ray diffraction measurements. Electrical resistivity measurements unveiled that persistent photoconductivity was strongly enhanced as the oxygen content decreased in the photochromic yellow transparent film. Analysis of the kinetics of the photochromic reaction indicated that the bleaching speed increased with increasing oxygen content in the photodarkened film. Unusual large persistent photochromism was discovered in the same yellow YHO film with lowest oxygen content which lasted for almost 10 days. Moreover, it was shown that the optical constants can be tuned by varying the oxygen content in the photochromic film as well.
1. Introduction The possibility of tailoring electrical and optical properties of rareearth and transition metal hydrides induced by hydrogenation and dehydrogenation has attracted extensive research interest due to their potential application in a wide range of technological devices [1–3]. These metal hydride films are typically capped with a thin Pd layer on the surface and thus are expected to be oxygen free. However, for uncapped thin films, they will get oxidized upon exposing to air. The oxidized yttrium hydride is a material of special interest because it is discovered recently to possess photochromic effect [4], i.e., exhibiting reversible color change as a result of the absorption of electromagnetic radiation. The photochromic effect of yttrium oxyhydride (YHO) is manifested by a remarkable change of optical transmission upon illumination in which the transparency of the film can be typically reduced from ∼ 90% to 40– 50% in the visible and infrared regions of the light spectrum. The photodarkened YHO film returns back to its initial yellow clear state upon
∗
Corresponding author. E-mail address: [email protected] (C.C. You).
dark annealing at ambient conditions. This switchable optical characteristic is attractive for potential applications such as energy-saving smart windows, ophthalmic lenses and sensors [5,6]. Photochromism of other rare-earth metal (Gd, Dy and Er) oxyhydrides has recently been reported as well [7]. These materials all belong to an emerging class of materials called mixed anion systems in which the hydride, oxide, and hydroxide anions share the same sites in the crystal lattice [8–10]. When YHO films are prepared by combining reactive magnetron sputtering with subsequent oxidation, material properties are observed to be dependent on the partial hydrogen pressure during film deposition. In particular, the oxygen concentration in such films appears to be inversely correlated with that of hydrogen [11] and a ternary phase diagram has recently been constructed for YH2−𝛿 O𝛿 in which 0.45 < 𝛿 < 1.5 yield stable photochromic compounds [12]. Moreover, the photochromic response is strongly affected by the amount of oxygen present in the film [13] and the optical band gap of YHO films can be engineered by controlling the deposition conditions [14]. Recent electron microscopy characterization revealed that the microstructure of photochromic YHO thin films is dominated by a columnar growth, in which the columns consist of multiple grains with an average grain size of 15 nm [15]. In order to fully exploit such materials for energy-saving smart window and optoelectronic applications, a thorough knowledge of the dependence of physical properties on the film composition is crucial. Hence, in the present study
https://doi.org/10.1016/j.mtla.2019.100307 Received 8 January 2019; Accepted 28 March 2019 Available online 5 April 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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we aim to establish the relationship between the chemical composition of YHO thin films and their structural, electrical and optical properties, with focus on investigating the decay kinetics of photoconductive and photochromic responses after illumination. 2. Material and methods YHO thin films were prepared on glass substrates by using reactive magnetron sputtering deposition in a Leybold Optics A550V7 inline sputter-coating system. The glass substrate size was 76 × 26 mm2 . In order to achieve a lateral gradient in the film composition, the substrate carrier was placed in a static position in front of the Y target during film growth. The total gas flow of argon and hydrogen was set constant at 200 sccm with an Ar:H2 gas ratio of 4:1. The deposition pressure was 0.66 Pa. Note that oxygen gas was not used during the film growth. The main incorporation of oxygen into the film is believed to take place when the yttrium hydride film reacted with air right after it was taken out from the vacuum chamber [16]. More details of the deposition condition can be found elsewhere [13]. The film thickness was determined using a stylus surface profilometer. The structural properties of the films were investigated by performing grazing-incidence X-ray diffraction (GI-XRD) measurements using a Bruker D8 Discover X-ray diffractometer with a beam footprint chosen to be approximately 2 × 10 mm2 in the sample. The lateral spacing between each GI-XRD measurement was 5 mm. The electrical resistivity measurements were carried out after sample illumination using a standard four-point-probe setup. The van der Pauw resistivity measurements were performed on the bleached yellow transparent portion of the YHO sample in a Keithley 4200 Semiconductor characterization system. Prior to the van der Pauw resistivity measurements, four 5 × 5 mm2 Al metal contacts were deposited on the yellow film by thermal evaporation through a shadow mask. The resulting sample size was about 26 × 26 mm2 , which was electrically isolated from the rest of the film by gently scribing the film with a diamond pen. The optical properties of the thin films were characterized using an Ocean Optics QE65000 spectrometer equipped with an integrating sphere and deuterium and tungsten halogen light sources. The spot diameter of the incident light was ∼ 5 mm for the transmission and reflection measurements. In order to induce the photoconductive and photochromic effects, the samples were exposed to continuous illumination up to 24 h in a solar simulator from WACOM Electric with an intensity calibrated at ∼ 1000 W/m2 under ambient conditions. 3. Results and discussion 3.1. Structural properties Fig. 1 shows photographs of a representative YHO thin film sample with gradient chemical composition captured 2 h (top panel) and 57 days (bottom panel) after deposition. The red marker line depicts the original boundary between the non-photochromic black opaque and photochromic yellow transparent parts of the film right after it was removed from the sputtering chamber. After the same sample was exposed to oxygen in air in darkness at room temperature over several weeks, a narrow slice ( ∼ 10 mm in width) of the black opaque film close to the boundary was observed to be transformed permanently to yellow transparent due to oxidation. The relative lateral oxygen content obtained from a typical YHO film, expressed in terms of an average bulk [O]/[Y] ratio, was found to increase gradually from ∼ 0.25 in the black region to ∼ 0.65 in yellow region, as verified by time-of-flight energy elastic recoil detection analysis and nuclear reaction analysis in our previous study [13]. Fig. 2(a) shows GI-XRD diffractograms recorded on a YHO thin film sample at different measurement positions ranging from 5 to 70 mm. This sample has been stored in air in the dark for 90 days at the time of measurements. Four Bragg reflection peaks corresponding to (111), (200), (220) and (311) crystal orientations were identified. We note that
Fig. 1. Photographs of a representative YHO thin film sample captured 2 h (top panel) and 57 days (bottom panel) after deposition. The red line marks the original boundary between the black opaque and yellow transparent parts of the as-deposited film. We note that the same sample was exposed to oxygen in air in darkness at room temperature. As can be seen, a narrow slice of the black film close to the boundary was permanently transformed to yellow color due to oxidation.
the diffraction lines around 62∘ in Fig. 2(a) correspond to the (222) Bragg reflection peak. Mongstad et al. have previously shown that both black opaque and yellow transparent YHO thin films belong to the same space group Fm3̄ m with a fcc crystal structure [17]. As can be seen in Fig. 2(a), the (222) peak of the black film is more prominent as compared to the (222) peak in the yellow film consisting of an increased amount of oxygen, which can be ascribed to different preferential orientational growths of the grains. The XRD patterns suggest that there is more orientational growth in the ⟨100⟩-direction for the yellow film, whereas the ⟨111⟩-direction is preferred for the black film [17]. In Fig. 2(b) the average lattice constant, which was derived from these four reflection peaks, is plotted as a function of the measurement position. Note that the oxygen content in the film increases along the lateral direction from position 5–70 mm. It can be clearly seen that the average lattice constant increased gradually from ∼ 5.27 to 5.34 Å when the visual appearance of the YHO film changed from black opaque to yellow transparent as displayed by the color bars in the figure. This increase in lattice constant can be attributed to an increase of the oxygen content in the same film, leading to an expansion of the unit cell, in agreement with previous results obtained from different black or yellow YHO samples [17]. In addition, the average lattice constant of the transformed film at position 40 mm was found to be slightly smaller as compared to that of the adjacent yellow film with higher oxygen concentrations. 3.2. Electrical properties The photo-induced electrical resistivity of a YHO sample measured at different bleaching times up to 1056 h at measurement positions ranging from 5 to 70 mm is displayed in Fig. 3(a). Note that the sample was continuously illuminated for 24 h in a solar simulator before the resistivity measurements. During bleaching the sample was rested in air in darkness at room temperature and ambient pressure. The film thickness of this sample was measured to be ∼ 710 nm at the bottom left corner in the black region of the film and ∼ 835 nm at the top right corner in the yellow region, based on thickness measurements performed on a neighboring YHO sample sputter-deposited at the same time. The lowest resistivity in the black opaque film was recorded at position 5 mm with 𝜌 ∼ 10−2 Ω cm. However, more conductive black YHO films with a resistivity on the order of 10−3 Ω cm are achievable by lowering the oxygen content [17,18]. It is also evident from Fig. 3(a) that the resistivity of the non-photochromic black film was not noticeably affected by the long light exposure time. The photo-induced resistivity of the photochromic yellow film was found to vary by about one order of magnitude from
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Fig. 2. (a) GI-XRD diffractograms recorded at measurement positions ranging from 5 to 70 mm on a YHO thin film 90 days after deposition. Four Bragg reflection peaks corresponding to the (111), (200), (220) and (311) crystal orientations were recorded. (b) Average lattice constant obtained at different measurement positions on the same YHO film consisting of black opaque, transformed and yellow transparent regions as depicted by the three horizontal color bars.
Fig. 3. (a) Photo-induced resistivity of a YHO sample measured at different bleaching times ranging from 0.1 to 1056 h at measurement positions of 5–70 mm after the illumination was turned off. (b) Photoconductivity as a function of bleaching time for varying oxygen contents obtained at different measurement positions. The solid lines are best fits to the experimental data.
𝜌 = 1.3 × 103 to 1.8 × 104 Ω cm as the measurement position was moved from 40 to 70 mm, as illustrated by the blue circle symbols. The main feature is that when going from the black to the yellow part of the illuminated film, the resistivity increased greatly by approximately six orders of magnitude with increasing oxygen content, which is similar to that reported for other related metal oxyhydrides such as GdOy Hx films [19]. Hence, the YHO film experienced a metallic-to-semiconducting transition which was also accompanied by large optical band gap opening as the oxygen content raised above a threshold [O]/[Y] value of ∼ 0.5 [13]. We note that the resistivity of the yellow transparent YHO film before illumination could not be determined by using our four-point-probe equipment because the maximum measurable resistivity was capped at ∼ 105 Ω cm for these films. Hence, in order to probe the resistivity of the completely bleached YHO film, we performed van der Pauw resistivity measurements instead. The average resistivity across the confined yellow YHO sample area was estimated at ∼ 1.8 × 106 Ω cm. Therefore, it should be feasible to achieve a modulation in electrical resistivity by at least two orders of magnitude upon illumination. For example, the resistivity could change from ∼ 1.8 × 106 in the bleached state to 1.8 × 104 Ω cm after light exposure at measurement position 70 mm. It is interesting to note that the photo-induced resistivity obtained at position 60 mm increased ∼ 600% from 1.1 × 104 to 6.2 × 104 Ω cm after a bleaching time of 26 h, whereas in the transformed film at position 40 mm the resistivity increased ∼ 200% from 1.3 × 103 to 2.4 × 103 Ω cm during the same time period, implying an enhanced ability to maintain the photo-induced conductance. Indeed, as displayed in Fig. 3(b), the changes of photoconductivity 𝜎 with respect to the bleaching time can be approximated by an exponential decay function: 𝜎(𝑡) ∼ 𝜎0 exp(−𝑡∕𝜏𝑃 ), where 𝜎 0 is the initial photoconductivity acquired right after the illumination was turned off, t is bleaching time, and 𝜏 P is decay time constant which can be viewed as the mean lifetime of 𝜎 0 . 𝜏 P was estimated to be 69, 41 24, 18, 16 and 15 h at measurement positions 40, 45, 50, 55, 60 and 65 mm, respectively, extracted
from the slopes corresponding to the best fits (solid lines) of the experimental data (symbols) presented in Fig. 3(b). This finding indicates that the strongest persistent photoconductivity (PPC) appeared in the photochromic yellow transparent film at position 40 mm containing least amount of oxygen as compared to the adjacent positions of 45–65 mm with higher oxygen contents. PPC in semiconductors may depend on several factors such as the potential barriers that separate photo-induced electrons and holes and inhibit recombination between them, types of traps or defect centers and their capture cross section for electrons and holes, and the probability of ejection of charge carriers to the conduction or valence band [20]. The photochromic yellow transparent YHO film is a semiconductor with an optical band gap of ∼ 2.5–3.0 eV depending on the oxygen content and the type of band gap: direct or indirect [13,21]. The electron-hole recombination might therefore take place through a recombination center between the conduction and valence band. Based on our film composition analysis [13], the average [O]/[Y] ratio at measurement position 60 mm is expected to be ∼ 30% larger than that obtained at measurement position 40 mm. One simple hypothesis is that such an increase of oxygen incorporation into the film might reduce the density of trap centers (e.g., anion vacancies [22] or vacancy-complexes) responsible for capturing, storing and ejection of excited free charge carriers, eventually leading to increased recombination of electron-hole pairs. Consequently, PPC is suppressed due to reduced effective lifetime of photo-generated charge carriers. Further study is required to elucidate the underlying mechanisms responsible for the oxygen-dependent PPC observed in YHO thin films. 3.3. Optical properties Establishing the composition-dependent optical constants is important for the potential application of YHO thin films. Fig. 4(a)–(d) shows optical transmission spectra (blue crosses) acquired from measurement positions of 40, 50, 60 and 70 mm on a YHO sample. The transmission,
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Fig. 4. (a)–(d) Optical transmission spectra (blue crosses) obtained from measurement positions of 40, 50, 60 and 70 mm on a YHO sample, respectively. The solid red line is the best fit to the experimental data. d is the average optical film thickness derived from the model. (e) Refractive index n (symbols) as a function of wavelength 𝜆 obtained at measurement positions 40–70 mm. The solid lines are best fits to the data based on the general Cauchy model. (f) Extinction coefficient k obtained at measurement positions 40–70 mm.
T, can be described by an optical model reported by Swanepoel [23], in order to determine the thickness and optical constants of thin films. A reasonably good fit to the data can be obtained, as shown by the red solid lines. Here, d is the average optical film thickness derived from the model. As shown, d was calculated to be ∼ 916 ± 9, 909 ± 4, 888 ± 6 and 861 ± 8 nm at measurement positions of 40, 50, 60 and 70 mm, respectively. From the model, the refractive index at wavelengths corresponding to the minima and maxima of the interference fringes of the transmission spectrum can be calculated and is plotted in Fig. 4(e). The dependence of the refractive index, n, on the wavelength, 𝜆, can be determined by the general Cauchy model: 𝑛(𝜆) = 𝐵 + 𝐶∕𝜆2 + 𝐷∕𝜆4 , where B, C and D are fitting parameters. The four solid lines in Fig. 4(e) represent the best fits to the data points (symbols) acquired at different measurement positions on the YHO sample. As can be seen, when the oxygen content reduced in the film, an appreciable increase in the refractive index was observed. For instance, at 𝜆 = 630 nm, the refractive index was estimated at 2.22, 2.18, 2.15 and 2.10 at the measurement positions 40, 50, 60 and 70 mm, respectively. It should be pointed out that the Swanepoel model gave inaccurate results for wavelengths below ∼ 500 nm due to the fact that strong optical absorption in the material is taking
place, as can be seen in Fig. 4(a)–(d). The extinction coefficient, k, can be approximated by the following expression instead: 𝑘(𝜆) = 𝜆∕4𝜋𝑑 × [ ] ln (1 − 𝑅)∕𝑇 , where R is reflection [17]. As shown in Fig. 4(f), k was found to be largest in the photochromic film with lowest oxygen content at measurement position 40 mm, indicating strongest light absorption. We have previously reported the impact of varying oxygen content on the photochromic response of YHO thin films [13]. Here we turn our attention to investigate the kinetics of the photodarkening process and how fast such photodarkened films bleach back to their original yellow transparent state after light exposure. Fig. 5(a)–(d) displays optical transmission spectra acquired before and after illumination and at different bleaching times at measurement positions 40, 50, 60 and 70 mm. The black solid line represents the transmission obtained before illumination and the blue solid line shows the transmission acquired right after 24 h illumination. As can be seen, a substantial modulation in transmission was observed afterwards. Even after a long bleaching time of 1056 h, the transmission in the bleached film at measurement position 40 mm could not be fully recovered back to the initial values measured before illumination, as shown by the dark red solid line. However, optical measurements carried out at positions 50–70
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Fig. 5. (a)–(d) Transmission spectra acquired before (black solid line) and after illumination at increasing bleaching times up to 1056 h at measurement positions 40, 50, 60 and 70 mm on the same YHO sample. (e) Photochromic response (in terms of ⟨ΔT⟩ averaged over a wavelength range of 500–900 nm) plotted as a function of bleaching time obtained at different measurement positions. The film was continuously illuminated for 24 h prior to the measurements. The solid lines are the best fits to the experimental data. (f) Photochromic response as a function of illumination time at measurement positions of 40–70 mm. The solid lines are fits to the experimental data, which can be described by a logarithmic function.
mm containing more oxygen revealed that a nearly complete recovery of the original transmission was feasible even at a shorter bleaching time, as shown in Fig. 5 (b)–(d). Fig. 5(e) displays an overview of the change of the photochromic response as a function of the bleaching time. Note that the strength of the photochromic response can be characterized by an average optical contrast ⟨ΔT⟩, defined as the difference between the optical transmission (averaged over a wavelength range of 500–900 nm) acquired before and after illumination as follows: ⟨Δ𝑇 ⟩ = ⟨𝑇before ⟩ − ⟨𝑇after ⟩. The main finding is that the optical contrast diminishes in an exponential fashion with bleaching time, and can be well described by the following expression: ⟨ΔT⟩ ∼ exp(−𝑡∕𝜏𝐵 ), where t is bleaching time and 𝜏 B is bleaching time constant. Similar bleaching behavior of YHO thin films has also been reported by other group [7], although with different illumination conditions leading to different bleaching time constants. The solid lines corresponding to the various measurement positions in Fig. 5(e) represent the best fits to the experimental data (symbols) based on the least squares fitting method. The bleaching time constant was determined to be 238, 42, 29 and 26 h at measurement positions 40, 50, 60 and 70 mm, respectively. This result indicates that the bleaching speed was slowest in the transformed film
containing lowest amount of oxygen and fastest in the most oxygenrich region at measurement position 70 mm. We note that the bleaching speed is generally dependent on the illumination conditions such as light wavelength, intensity and duration used to generate the photochromic effect, in addition to the thickness and composition of YHO. Various environmental effects including the ambient pressure, humidity and temperature are expected to play a vital role in the bleaching process as well. The photochromic response obtained after 1, 5 and 24 h of illumination at measurement positions 40, 50, 60 and 70 mm is shown in Fig. 5(f). During the first hour of illumination, photodarkening was observed to be strongest in the transformed film at position 40 mm with lowest oxygen content and weakest in the most oxygen-rich region at position 70 mm, indicating that the coloration speed from the yellow to dark color transition increased with decreasing amount of oxygen. For the illumination time ranging from 1 to 24 h, the variation of the photochromic response with illumination time can be roughly fitted by a logarithmic function as shown by the solid lines, which represent the best fits to the experimental data. As can be seen from the slopes, the coloration speed was surprisingly fastest for the film with highest
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oxygen content in the same thin film sample. Unusual large persistent photochromism was observed in the same yellow YHO film with lowest oxygen content which lasted for almost 10 days. The present findings demonstrate the possibility to tailor the switchable electrical and optical properties of YHO thin films by controlling the composition, which is interesting for potential utilization in technological applications such as energy-saving smart windows, optical data storage devices and sensors. Declaration of interest None. Acknowledgments Fig. 6. Bleaching time constant for photochromic response (𝜏 B ) and decay time constant for persistent photoconductivity (𝜏 P ) acquired at different measurement positions in the same YHO sample. The solid lines are a guide to the eye. The black opaque, transformed and yellow transparent regions are depicted by the three horizontal color bars.
This work was supported by the Norwegian Research Council through the FRINATEK projects 240477/F20 and 287545 and the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963/F50. We also thank Dr. Vishnukanthan Venkatachalapathy for help with XRD measurements.
oxygen content at measurement position 70 mm. Thus, this finding suggests that both fast and slow coloration kinetics may be involved in the photochromic reaction depending on the total illumination time. Mongstad et al. have previously demonstrated that yellow transparent YHO thin films exhibited photochromism and PPC simultaneously through a series of light switching experiments [4]. Clearly, the photochromic effect is considered to be closely correlated to the PPC effect in this material. For example, recent ellipsometry measurements suggested that the photochromic effect could be associated with a gradual formation of metastable metallic domains in the semiconducting YHO lattice upon illumination [24]. It is probable that such metallic domains also give rise to an increased conductivity in the film, thus contributing to the PPC effect. Our present time-dependent resistivity and transmission data revealed that both the bleaching time constant for photochromic response (𝜏 B ) and the decay time constant for PPC (𝜏 P ) decreased with increasing oxygen content, as summarized in Fig. 6. Interestingly, 𝜏 B was observed to be larger than 𝜏 P for all examined measurement positions in the same yellow transparent YHO film, indicating that photochromism persisted longer than photoconductivity after the illumination was turned off. This finding implies that electronic mechanisms (e.g., recombination of photo-generated electron-hole pairs or decay of metastable metallic domains) might not be the only driving forces behind the aging of the photochromic effect, but structural changes such as lattice relaxation [25] and possible surface effects [26] may also be involved. It should be noted that the largest 𝜏 B of 238 h was obtained in the photochromic YHO film containing lowest oxygen content at measurement position 40 mm, as shown in Fig. 6. Such large persistent photochromic response may prove to be useful for optical memory applications. However, the origin of the observed persistent photochromism is currently not known, which deserves further investigations.
References
4. Conclusions We have fabricated yttrium oxyhydride thin films with gradient chemical composition by reactive magnetron sputtering deposition. We observed strong dependence of structural, electrical and optical properties on the varying oxygen content of YHO thin films. The visual appearance of the as-deposited YHO film changed from black opaque and non-photochromic in the oxygen-poor region of the film to yellow transparent and photochromic in the oxygen-rich region of the film. We have shown that the lattice constant and electrical resistivity increased with increasing oxygen content, whereas a reduction in the optical refractive index and extinction coefficient was observed simultaneously. Persistent photoconductivity was found to be strongly enhanced as the oxygen content decreased in the yellow transparent YHO film. Furthermore, the bleaching speed of photochromic response increased with increasing
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