Effects of Laser Annealing Parameters on Optical and Electrical Properties of ITO/Metallic Glass Alloy Bi-layer Films
© Lin et al. 2015
Received: 10 April 2015
Accepted: 16 June 2015
Published: 30 June 2015
AgAlMg (AAM) films with three different atomic percentage compositions are prepared, namely, Ag12Al62Mg26 (denoted as A1AM), Ag22Al46Mg32 (denoted as A2AM), and Ag36Al25Mg39 (denoted as A3AM). In addition, the AAM films are deposited with four different thicknesses, i.e., 3, 6, 9, and 12 nm. The indium-tin oxide thickness is assigned a constant value of 30 nm in every case. The results show that the optical transmittance of the AAM/IAAM films improves (i.e., increases) with a reducing AAM film thickness, while the electrical resistivity improves (i.e., reduces) with an increasing film thickness. It is shown that the IA2AM film with an AMM thickness of 9 nm yields the optimal compromise between the optical transmittance and the electrical resistivity. The as-deposited IAAM films are found to have optical transmittance and electric resistivity values of 65 % and 90 Ω/□, respectively. The IA2AM films are annealed using a near-infrared laser at different pulse energies with a wavelength of 1064 nm and repetition rates ranging from 100 ~ 400 kHz. For both films, the optical and electrical properties are enhanced as the pulse energy increases to a certain critical value due to a transition from an amorphous microstructure to a crystalline structure. Given a repetition rate of 400 kHz and a pulse energy of 1.03 μJ, the optical transmittance and sheet resistance of the IAAM film are found to be 80 % and 15 Ω/□, respectively. The corresponding value of the Haacke figure of merit changed from 0.15 × 10−3 to 7.16 × 10−3 Ω−1 due to the optimal laser annealing conditions.
Transparent conducting oxide (TCO) films have a low electrical resistivity and a high optical transmittance in the visible range and are therefore widely used for such electronic applications as flat panel displays, organic light-emitting diodes (OLEDs), cholesteric liquid crystal displays, touch panels, and solar cells. Because of the various TCO films available, indium-tin oxide (ITO) is one of the most commonly applied for optoelectronic devices due to its excellent electrical and optical properties and its ease of preparation using physical vapor deposition techniques [1–5]. However, to minimize the sheet resistance, the thickness of the ITO film should exceed 100 nm [6, 7]. Consequently, the use of pure ITO films for optoelectronic devices is cost prohibitive. To address this problem, the literature contains many proposals for minimizing the ITO cost by means of ITO-metal layer-ITO sandwich structures. Typically, these structures are based on the use of highly conductive metals such as silver or copper as the middle layer. In general, the results have shown that a metal layer thickness of around 5 ~ 20 nm yields both good optical transmittance in the visible light range and a high conductivity [8–13].
Bulk metallic glasses (BMGs) possess an amorphous structure and have many advantageous properties, including high strength, excellent hardness, and anticorrosion. Therefore, BMGs have an extensive range of potential applications in the industrial, electronics, and biomedicine fields. For example, Inoue  examined the mechanical, chemical, and magnetic properties of BMGs produced using various casting processes. In addition, three empirical rules for BMGs were proposed, namely (1) multi-component structures consisting of at least three elements, (2) atomic size mismatches of at least 12 % among the three main elements, and (3) negative heats of mixing among the three main elements. Various thin film metallic glasses (TFMGs) have been developed in recent years. These TFMGs exhibit a lower surface roughness than pure metallic films  and have many unique properties, including high strength, mechanical properties, and physical properties [16–18]. Notably, metallic glass films have high nucleation rate and a thickness much lower than that of the metal layers used in traditional ITO sandwich structures. Consequently, the use of TFMGs to realize thin transparent conductor films with excellent transparency and conductivity properties has attracted growing interest in recent years.
It is well known that the optical and electrical properties of thin films can be improved through annealing [19–21]. As a result, the literature contains various proposals for the localized annealing of TCO films using pulsed laser systems [22–26]. For example, Lin and Hsu  patterned ITO thin-film electrodes using a high-repetition-rate fiber laser and showed that the residual stress reduced as the repetition rate increased.
Lee et al.  deposited an ITO/ZrCu bi-layer film on a PET substrate using a magnetron sputtering technique. It was shown that a continuous and smooth ZrCu layer with a thickness of less than 6 nm could be obtained given an appropriate choice of sputtering conditions. Moreover, the ZrCu film had an optical transmittance of 73 % for incident light with a wavelength of 550 nm. However, ZrCu metallic glass films have a relatively high sheet resistance. Accordingly, in the present study, the ZrCu film is replaced with a metallic glass alloy film comprising three low-resistivity components, namely, silver (Ag), aluminum (Al), and magnesium (Mg). The study considers both AgAlMg (AAM) monolithic films and ITO/AgAlMg (IAAM) bi-layer films. The investigation focuses particularly on the effects of the AAM composition and AAM film thickness on the optical transmittance and electric resistivity of the monolithic and bi-layer structures. The as-deposited AAM and IAAM films are then annealed using a fiber laser with pulse repetition rates ranging from 100 to ~400 kHz. The optimal laser annealing parameters are determined by evaluating the electrical, optical, and structural properties of the various samples using four-point probe technique, spectrophotometry, scanning electron microscopy (SEM), and X-ray diffraction (XRD).
The AAM and IAAM films were deposited on glass substrates using a magnetron sputtering system (Kao Duen Co.). The glass substrates were purchased from Nippon Electric Glass Co. and had a thickness of 0.7 mm and an optical transmittance of 90 % for an incident wavelength of 550 nm. The IAAM films were deposited by means of a co-sputtering technique using an ITO target with a composition of 90 wt.% In2O3 and 10 wt.% SnO2, and pure Ag, Al, and Mg targets, respectively. All of the targets had a diameter of 2 in.. The sputtering process was performed using an Ar flow rate of 30 sccm, a base pressure of 2 × 10−6 Torr, and a working pressure of 5 mTorr. In synthesizing the IAAM films, AgAlMg films with a thickness of 3 ~ 12 nm were deposited on the glass substrate, and an ITO layer with a thickness of 30 nm was then sputtered on the metallics glass (MG) layer. To evaluate the effects of the composition of the MG layer on the optical and electrical properties of the AAM and IAAM films, the discharge power of the Ag target was varied in the range of 25 to 37 W in order to obtain three different AAM films, namely, Ag12Al62Mg26 (denoted as A1AM), Ag22Al46Mg32 (denoted as A2AM), and Ag36Al25Mg39 (denoted as A3AM). Note that the composition is specified in terms of the atomic percentage in every case.
where D is the laser spot diameter, and S is the length of two successive laser spots and is dependent on the repetition rate and the scanning speed. For the laser spot diameter, repetition rates, and scanning speeds used in the present annealing tests, the overlapping rate was determined to be around 99 %. The morphologies and compositions of the as-deposited and annealed AAM and IAAM films were examined by SEM (JSM-7600F) and in-plane XRD (Bruker D8 Advance). The sheet resistance was measured using a four-point probe (SR-H1000C). Finally, the optical transmittance was measured over the range of 200 ~ 1100 nm using a UV-vis-IR spectrophotometer (Lambda 35, PerkinElmer).
Results and Discussion
where T is the transmittance (expressed in percentage terms) and R is the sheet resistance (expressed in units of Ω/□). Considering the application of the annealed IA2AM films to the display field, the transmittance at 550 nm light wavelength is used. Substituting the transmittance and sheet resistance values of the as-deposited IA2AM and annealed sample with a repetition rate of 400 kHz and a pulse energy of 0.91 μJ (i.e., 80 % and 15 Ω/□, respectively) into Eq. (3), the figure of merit is found to be 0.15 × 10−3 to 7.16 × 10−3 Ω−1, respectively. This high value of ΨTC confirms the optimality of the laser annealing conditions.
Monolithic AAM metallic glass films and bi-layer ITO/AAM metallic glass films have been deposited on glass substrates using a magnetron sputtering process. The investigation has focused specifically on the effects of the Ag content (12 ~ 36 at.%) and AAM film thickness (3 ~ 12 nm) on the optical transmittance and sheet resistance of the synthesized samples. With increasing AAM film thickness from 3 to 12 nm, for both films, the optical transmittance increases with a reducing film thickness, whereas the electrical resistance reduces with an increasing film thickness. Moreover, for a constant film thickness, the optical transmittance and electrical resistance both reduce as the Ag content is increased. The results have shown that an IAAM film comprising an ITO layer with a thickness of 30 nm and an Ag22Al46Mg32 film with a thickness of 9 nm provides the optimal compromise between the optical and electrical properties of the film. The as-deposited films have been annealed using a pulsed fiber laser system with various repetition rates and pulse energies. The results have shown that for both films (AAM and IAAM), the amorphous structure of the as-deposited samples transforms to a crystalline structure as the pulse energy is increased. However, as the pulse energy is increased beyond a certain critical value, the excessive energy input causes significant damage of the film surface. It has been shown that the crystalline structure improves the electrical and optical properties of the annealed samples compared to those of the amorphous as-deposited films. For the A2AM film, the electrical resistivity and optical transmittance have optimal values of 21 Ω/□ and 70 % given a repetition rate of 150 kHz and a pulse energy of 1.02 μJ. Similarly, for the IA2AM film, the optimal values of the electrical resistivity and optical transmittance are 15 Ω/□ and 80 %, respectively, given a repetition rate of 400 kHz and a pulse energy of 0.91 μJ. For both films, the electrical resistivity increases significantly as the pulse energy is increased beyond the corresponding threshold value due to the damage caused to the film surface. Overall, the results presented in this study show that the addition of a 30-nm ITO film to a thin (9 nm) AAM metallic glass alloy structure yields a significant improvement in the electrical and optical properties of the thin-film structure. The corresponding value of the Haacke figure of merit changed from 0.15 × 10−3 to 7.16 × 10−3 Ω−1 due to the optimal laser annealing conditions. Thus, the present IAAM bi-layer structure represents an ideal solution for a wide variety of TCO applications.
The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan under Grant No. MOST 103-2221-E-020-010 and 103-2120-M-110-004.
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