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Abstract
In this work, an ultra-sensitive optical absorption technique based on Cavity Ring-Down Spectroscopy (CRDS) was employed to study the effects of UV treatment on the optical properties of ultra-thin indium tin oxide (ITO) films. The ITO films were submitted to UV treatment either after the deposition process or in-situ during the thin-film growth process. Different flow rates of oxygen in the vacuum chamber during film growth were also investigated. An ITO-coated glass substrate inserted in the CRDS cavity at a Brewster’s angle provided a ring-down time of about 1.6 µs, which enabled measurements of optical absorption loss as small as 3 × 10−6. To compare the effects of the UV film treatment, the CRDS technique was employed to measure the extinction coefficient for samples coated with and without the UV treatment. While the optical absorption data was being collected, the electrical resistivity was also simultaneously monitored. The post-deposition UV treatment was found to improve the optical transparency and the electrical performance of ITO film; the optical extinction coefficient of the ultra-thin ITO film is shown to decrease by about 24%. The in-situ UV treatment during growth is also shown to consistently increase the optical transparency of the ultra-thin ITO films and providing outstanding optical performance especially for high flow rates of oxygen during film growth. The electrical resistivity for oxygen flow rates in the range 0.6 - 1.4 sccm is also improved by the in-situ UV treatment, however it shows a sharp increase for oxygen flow rates beyond 1.4 sccm. The CRDS platform is demonstrated here to provide a highly accurate and sensitive methodology for measurement of minute optical absorption losses in ultra-thin films that typically cannot be precisely measured using other conventional spectrophotometric techniques.
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1. Introduction
It has been demonstrated that cavity ring-down spectroscopy (CRDS) is an effective technique for measurements of small absorption losses. This technique has been typically applied to gas-phase samples to measure extremely weak absorption lines [1–4]. A few attempts have been made to extend this technique to study solid samples, particularly to thin films. Pipino et al. developed evanescent-wave cavity ring-down spectroscopy (EW-CRDS) to study the adsorbed I2 molecules on the minicavity facets using absorption at the surface of a total internal reflection of a Pellin–Broca prism [5]. Richard Engeln et al. used CRDS to measure sensitively fundamental IR lines of a thin C60 film (20 - 30 nm), by placing the ZnSe window coated with a C60 film at normal incidence in an optical cavity [6]. Logunov et al. used CRDS to probe relatively thin (2 - 20 µm) polymer film samples in the telecommunication wavelength ranges of 1200 - 1650 nm to study the properties of thin films compared to thick bulk films [7]. Marcus et al. demonstrated the ability of the cavity ring-down technique in mid-infrared spectroscopy to sensitively record very small optical loss of thin films; by using a Brewster’s angle configuration they recorded loss in a C60 film on a BaF2 substrate with absorption sensitivities as small as 1 × 10−5 [8].
Optimizing the optical transparency and the electrical conductivity of indium tin oxide (ITO) thin films is important for developing various optoelectronic devices, including solar cells [9–11], organic light-emitting diodes (OLEDs) [12,13], liquid crystal displays (LCD) [14], flat panel displays (FPDs) [15], and spectroelectrochemical applications [16–18]. Many surface modification techniques have been developed for enhancing performance and stability of the ITO film, such as conventional thermal annealing (CTA) treatments [19–22], microwave annealing [23], chemical treatment [24], and plasma treatment [12,25]. Among those methods, UV treatment has been shown to enhance the work function of ITO film by increasing the oxygen content [26–29]. UV radiation decomposes molecular oxygen in the air to form ozone and atomic oxygen. Because ozone is a highly effective oxidizing agent, it reacts quickly with organic contaminants and free radical molecules on the ITO surface, which changes the film constituents (In, Sn, O) ratio, and increases the surface oxygen content, thereby enhancing the performance of the transparent electrode [30,31]. Most of the previous studies used UV treatment for relatively thick ITO films (above 100 nm). However, only a few studies have been reported for the ultra-thin film (e.g. below 30 nm) [27,28,30–32], where the optical and spectroscopic properties cannot be precisely measured by conventional transmission approaches, and advanced techniques are needed in these cases. Moreover, reflectance and transmittance measurements of thin films can be very difficult when these quantities approaches unity [33].
In this work, we report the use of CRDS as a powerful tool for the in-situ measurements of the optical properties of ITO ultra-thin films under UV surface treatment with a precision of better than 1 ppm. The ITO films were treated with UV radiation either after the film deposition or in-situ during the film deposition process. CRDS technique was employed to precisely measure the minute change in the film extinction coefficient, while the electrical resistivity of the ITO films was measured through the two-point probe method.
2. Experimental procedures and setup
2.1. Ultra-thin ITO film deposition
Before the film deposition process, the glass substrates were cleaned using triton soap, deionized (DI) water, acetone, and methanol sequentially for 15 min using an ultrasonic cleaner, and then dried with N2 gas. Ultra-thin ITO films were deposited using DC magnetron sputtering technique. A 3-inch target of (In2O3)90:(SnO)10 wt.% and 99.99% purity was used as a sputtering target. Before deposition, the chamber was pumped down until the pressure reached about 5 × 10−7 Pa. The glass substrates were placed in the sputtering chamber for a sufficiently long time (15 minutes) to reach the deposition temperature of 250 °C, the rotation speed of the substrate table was 20 rpm to create uniform films. Subsequently, the working gases were introduced into the deposition chamber, the high purity argon was fixed at 100 sccm and different flow rates of oxygen in the range of 0.6 - 4 sccm were used. A DC power of 230 W was applied and a deposition time of 20 s was used to grow ultra-thin films. Two sets of samples were prepared, one set with and another set without UV exposure during film growth. For those samples that were exposed to UV radiation during film growth, an UV radiation source was installed inside the vacuum chamber. An UVC LED lamp with a maximum emission wavelength at 275 nm and a power 3 mW was used. To protect the lamp from being coated, the lamp was placed inside an aluminum tube with a length of 2 inches and at angle of ${45^\circ }$ with a vertical plane, while illuminating the surface with the growing film.
2.2. Cavity ring down spectroscopy setup
A stable, high-finesse cavity was formed by two identical high reflectivity concave mirrors (R = 99.995% at 532 nm, 1-m radii of curvature, CRD Optics, Inc.) mounted in adjustable mounts (CRD Optics, Inc.), as illustrated in Fig. 1. The cavity length for this experiment was ste to 1 m for creating a confocal resonator. A pulsed Nd:YAG laser beam at 532 nm, with a repetition rate of 10 Hz, and a pulse width of 4 - 6 ns was launched into the cavity. The p-polarized light beam was chosen by guiding the laser beam through a half-wave plate (AHWP05M-600, Thorlabs) and a linear polarizer. The glass substrate coated with the ITO film was mounted on a 3-D translation stage (XR25P-K2, Thorlabs) with additional rotational capabilities. The laser beam was set to enter the resonator cavity and impinge the surface of the glass substrate at the Brewster’s angle of the air-glass interface. The light outcoupled from the cavity was focused into an optical fiber by a focusing lens and directed into a photomultiplier detector (PMT, H5783, Hamamatsu). The ring-down signal from the PMT was recorded using a digital oscilloscope (4 Gsa/s, 1 GHz bandwidth, DSO8104A Infiniium, Agilent). The data was transferred into a computer via a GPIB to ethernet interface connection (GPIB-ENET/1000, National Instruments, Austin, TX). The averaged ring-down traces (typically 16 laser pulses) were used to calculate the ring-down time τ, using least-squares fitting routine via a custom- LabVIEW program. For post-deposition UV treatment, an UV LED lamp was mounted at a distance of 1 cm from the ITO sample.
Fig. 1. Scheme of CRDS experimental setup, which includes the pulsed-laser ring-down cavity, optics to select p-polarized light, ITO ultra-thin film on Brewster’s angle geometry, optical detector, oscilloscope, and data acquisition system.
2.3. Optical properties measurements setup
For retrieving the thickness and optical constants of each ultra-thin ITO sample, the transmittance and reflectance versus incident angle for both p- and s-polarized light were measured. The schematic of the experimental setup used for those measurement is shown in Fig. 2. A CW laser diode (Shanghai Dream lasers Tech., SDL-532-150T) generated light at the wavelength of 532 nm and operated at 15 mW. The laser beam was passed through a half-wave plate and a linear polarizer to select the light polarization and was directed towards the ITO sample that was mounted on a rotation stage (Quadra-Chek 100) to precisely control the incident angle. The reflected and transmitted optical intensities at different angles of incidence were monitored by a power meter (Newport, model 1930C).
Fig. 2. Schematic representation of the experimental setup used to measure the transmittance and reflectance versus the incident angle, for s and p-polarized light, of ITO ultra-thin film.
3. Optical CRDS measurements of a thin film and its electrical resistivity
In the optical resonator shown in Fig. 1, the round trip optical loss factor ${e^{ - {\; }{\alpha _r}{\; }2{\; }L}}$ can be given by [3,34]:
The ring-down time can be written as:
where c is the speed of light. The change of absorbance introduced by the film during the UV treatment can be written as:The sheet resistance ${R_s}$ was measured in real time during UV exposure. This was performed by contacting the ITO film with multi-meter as a two-point probe using a conductive adhesive copper tape; to make proper electrical contact and reducing the contact resistance. For any three-dimensional conductor, the resistance can be used to obtain the resistivity according to the equation:
where $\rho $ is the resistivity, l and $w$ are the length and width, respectively, t is the thickness, and ${R_{s}}$ is the sheet resistance. Once the film thickness is measured, the resistivity $\rho $ can be calculated by multiplying the sheet resistance by the film thickness:4. Results and discussions
4.1 Optical constants of ultra-thin ITO films
To extract the real (n) and imaginary (k) parts of the refractive index, and the thickness (t) of the ITO film, a computer code was written in the Mathematica platform. The code is based on a transfer-matrix technique and an optimization algorithm available in the Mathematica library was used to search for the optical constants of the ITO film ($n,\; k,\; t)$ that the calculated transmittance and reflectance matches the corresponding measured experimental data. The measured reflectance and transmittance spectra, for p and s polarized light, are plotted and compared to the model generated using the optical constants obtained by the Mathematica code; an example is shown in Fig. 3.
Fig. 3. Reflectance and transmittance against incident angle for p and s polarized light. The black dots are experimental results, and the red solid line is the modeled result.
To find the best parameters ($n,\; k,\; t)$, the values of refractive index and thickness of the modeled ITO film were searched to find a model transmittance and reflectance spectra that most closely matches the experimental measurements. The fitting parameters were nair = 1.00, nglass = 1.5089, and $\lambda \; \; $= 532 nm. The code worked extremely well to extract the film thickness t and the real part of refractive index n, but it failed to properly retrieve the value of extinction coefficient ($k$) because the k values were very small and measurements in ultra-thin films with single-pass reflectance and transmittance are insufficiently accurate for this purpose. To overcome this hurdle, additional measurements using the CRDS technique were deployed. The cavity ring-down time was measured for each coated sample (with and without UV exposure, and for each O2 flow rate) and then the measured ring-down times were converted to a single-pass transmittance for p-polarized light at 532 nm using Eqs.2 and 3 (see Table S1 in Supplement 1). The values of the real part of refractive index and the thickness of each sample that were already extracted in the previous experiment were used and the same Mathematica program with one data point (transmittance at p-polarization at Brewster’s angle) was used to extract the extinction coefficient k of the films (see Table S2 and S3 in Supplement 1).
4.2 Post-deposition UV treatment of ultra-thin ITO films
A set of ultra-thin ITO samples were prepared on glass substrates without any UV radiation during the film growth and using the parameters already described; a flow rate of 0.6 sccm for oxygen was selected for this set of samples. Those samples were then measured by the CRDS technique before and during a post-deposition UV treatment. For the optical performance measurements, the ring-down time before and during UV exposure treatment was monitored in real time during 500 s by the CRDS platform. As reported in Fig. 4(a), the effect of the post-deposition UV treatment can be clearly seen; in the first 100 seconds the ring-down time increases rapidly with UV exposure and then reaches a steady state at a longer time indicating no further improvements on the film optical properties. This experimental result can be interpreted as the following: it is known that for ITO films the oxygen vacancies are the main scattering loss source of light. UV treatment works to oxidize the sub-oxides such as InOx and SnOx by adding O-2 ions into the ITO surface, which leads to decrease the oxygen vacancies and increase the film transmittance [35,36]. The change in the ring-down time measured before and during UV exposure were converted to the corresponding change in absorbance per pass using Eq. (3), as displayed in Fig. 4(b). The negative values of absorbance mean that UV exposure worked to increase the transparency of ITO film. Based on the method described in Section 4.1, the values of thickness, real part of refractive index, and extinction coefficient without and after UV exposure are listed in Table1. As summarized in Table 1, the optical measurements using the CRDS revealed that the extinction coefficient k of the ITO thin film reduced from 0.0053 to 0.0040, a reduction of 24%. We also found that our CRDS setup at the Brewster’s angle configuration is able to resolve a detectable absorption signal as small as 3 × 10−6.
Fig. 4. (a) Ring-down time data as measured by CRDS tool for an ultra-thin ITO film before and during a post-deposition UV treatment. (b) Absorbance per pass in the cavity as a function of total time during UV treatment.
Table 1. Real and imaginary parts of the refractive index and thickness of an ultra-thin ITO film, data for before and after post-deposition UV treatment
The sheet resistance was recorded in real time during a post-deposition UV treatment and was used to retrieve the resistivity using Eq. (5). As we observe in Fig. 5, the UV treatment improves as well the film conductivity, where the resistivity drops from ${\; }4.8 \times {10^{ - 3}}\;\ \mathrm{\Omega} \;\ \;\ \textrm{cm}\;\ to\;\ 2.2 \times {10^{ - 3}}\;\ \mathrm{\Omega} \;\ \;\ \textrm{cm}$.
Fig. 5. The variations of the electrical resistivity of ultra-thin ITO film in real time during post-deposition UV treatment.
4.3 In situ UV treatment of ultra-thin ITO film during the sputtering deposition process
A set of samples of ultra-thin ITO films were grown at different flow rates of oxygen in the vacuum chamber during deposition. Figure 6 compares samples grown with and without in-situ UV radiation during the film growth process. Figure 6(a) shows the measured ring-down times and Fig. 6(b) the corresponding extinction coefficient for the samples deposited with and without UV radiation, for different O2 flow rate, which varied from 0.6 sccm to 4 sccm. In general, it can be seen that the transmittance of ITO film increased (i.e., ring-down increases) and the extinction coefficient decreased when oxygen flow rate was increased. At low O2 flow rate, the oxidation of the ITO film surface is incomplete and the prepared film become anoxic, so the number of the oxygen vacancies goes up and this leads to a lower transmission (i.e., a higher extinction coefficient). Therefore, the higher O2 flow rate decreases the oxygen vacancies, which leads to a higher transmittance (i.e., a lower extinction coefficient) of the ITO film [36,37]. An extinction coefficient as low as $k\; = \; 0.0014$ at a higher oxygen flow rate (4 sccm) is reported here.
Fig. 6. As deposited measurements of the ring-down time (a) and the corresponding extinction coefficient (b) for ultra-thin ITO films. Samples with and without UV irradiation during film growth at different oxygen flow rates (from 0.6 sccm to 4 sccm).
By comparing the extinction coefficient from Section 4.2 obtained for an ITO film grown with an oxygen flow rate of 0.6 sccm and after a post-deposition UV treatment, which gave $k\; = \; 0.0040$, with an ITO film coated in the presence of UV radiation in the sputtering chamber at the same oxygen flow rate, we conclude that both UV treatments seems to provide an almost equivalent route for improving the transparency properties of an ultra-thin ITO film.
The electrical resistivity of the ITO films with and without UV during growth are plotted in Fig. 7 for different rates of oxygen flow. In general, it can be seen that ITO films showed a monotonic increase in their resistivity as the O2 flow rate is increased, whereas the films prepared under O2 flow rate from 0.6 to1.4 sccm have low electrical resistivity values and just a modest increase as the oxygen flow is increased. The results showed that the resistivity becomes very large ($1.2 \times {10^{ - 1}}\mathrm{\;\ \Omega \;\ \;\ cm}$) at 4 sccm, so it has been excluded from Fig. 7. Those results can be explained by the fact that as the O2 flow is increased, the number of the oxygen vacancies is decreased, which results in decreasing the charge carriers concentration and leads to a higher resistivity [28,35,38,39]. Furthermore, we notice discrete changes of the electrical resistivity of ITO films prepared with UV exposure during growth; at low O2 flow rate in the range of 0.6 - 1.4 sccm, the in-situ UV treatment slightly decreases the electrical resistivity with the minimum value of 2.4 $\times {10^{ - 3}}\;\ \mathrm{\Omega} \;\ \;\ \textrm{cm}$ at 0.6 sccm. Although the optical transmittance of the deposited samples increases significantly as the O2 flow rate reached 3 - 4 sccm, the resistivity shows a monotonic increase in their values; this can be attributed to a highly oxidant O3 atmosphere that possibly decreases the concentration of oxygen vacancies (decrease the carrier concentrations) in ITO, thereby contributing to raising the electrical resistivity [38,39]. By considering both the trade-off of the optical transmittance and the electrical resistivity, we conclude that the O2 flow rate in the range of 0.6 - 1.4 sccm seems to provide an optimum range for delivering ultra-thin ITO films with good optical transparency and low electrical resistivity.
Fig. 7. Electrical resistivity of ITO ultra-thin film samples as deposited with and without UV exposure under different oxygen flow rates.
5. Conclusions
The potential of the CRDS technique for the absorption loss detection in the ultra-thin ITO film was demonstrated. By using a Brewster’s angle configuration, we have been able to measure optical loss sensitivity as small as $3 \times {10^{ - 6}}\; $. The ultra-thin ITO films were either treated with a post-deposition UV treatment or under an UV irradiation during film growth. Based on the measurements of optical constants and cavity ring-down times, the post-deposition UV treatment was found to increase the optical efficiency of ITO films by about 24%. The results show in situ UV treatment in the sputtering chamber decreases the film resistivity in the O2 flow rate in the range of 0.6-1.4 sccm, but it increases drastically for the oxygen flow rate above 1.4 sccm. The results show that optimum values for reaching ITO films with good transmittance and low electrical resistivity are in the range of 0.6-1.4 sccm oxygen flow rates, where our data show that an UV treatment highly improves their optical performance.
Disclosures
The authors declare no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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