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A multi-color solid-state display structure has been fabricated and characterized, which is based on phase-change material Ge2Sb2Te5 (GST) and conducting transparent material indium tin oxide (ITO). The significant influence of ITO is investigated by experiments as well as simulations. We found that the ITO layer in fact plays a significant or even a dominate role in the color change of GST-based multi-layer solid state display structure. Multi-color modulation can be achieved by changing the phase state of both ITO and GST. Based on those results, better color presentation can be realized by improving the design of this new solid-state display.
© 2017 Optical Society of America
1. Introduction
Fast digital information display devices are playing an increasingly significant role in many applications such as computers, smart windows [1], electronic skin [2] and wearable devices. In the past decades, the liquid crystal display (LCD) is widely used due to its good display quality and rapid response. However, conventional display technologies have many limitations, including low energy efficiency, lack of flexibility, high manufacturing cost etc. Display technologies with low power, flexibility, large area and low cost are highly wanted. Among them, the electrophoretic display (EPD) [3] and the electrowetting display (EWD) [4] are based on the motion of charged droplets in response to the electric field. EPDs meet with success in commercialization due to their capabilities of display retention and good sunlight legibility. Both EPDs and EWDs are energy-efficient reflective displays without backlight, which is valuable in rollable devices [5, 6] and E-paper. Another display method, called the electrochromic displays (ECDs) [7], with a variety of device structures, are mostly based on the optical response to a bistable and reversible structure variation by electronic stimulus. ECDs stand out due to the “memory effect”, large-area fabrication and considerable switching cycles, which are widely used in large smart windows, rear-view mirrors [8], etc. The solid-state display, as a newly-developed display method, refers primarily to optical coatings [9, 10]. Based on strong interference effects, multi-layer optical coatings feature high compatibility with flexible applications and good legibility under sunlight, but suffer from limited and unchangeable color presentation.
Recently, a phase-change material-based dynamic optical interference coating structure has been developed by Oxford University, which is reported to be promising for low-power, retentive, flexible, and high-resolution displays [11–13]. The reported structure contains a top indium tin oxide (ITO) layer, an ultra-thin sandwiched Ge2Sb2Te5 (GST) film and a bottom ITO layer on a reflective Pt surface (the structure can be termed as IGIP). Solid GST alloy can switch between amorphous state and two crystalline states induced by joule heating, and thus modulate its electrical and optical property repeatedly [14–16]. The amorphous GST material transforms to face-centered cubic crystalline state (FCC) when annealed at around 220 °C and further turns to hexagonal crystalline state (HEX) when the thermal temperature increases to 400°C [17]. A quick quenching process over 640°C can reverse the process and re-amorphize the GST layer [18]. Along with the phase switch, both the electrical property [14, 19] and optical property [20–22] of GST change drastically. According to the results of the Oxford group [11–13], in such an layered IGIP structure, the phase-change material GST is demonstrated to be able to modulate the color dynamically and maintain the display with low energy consumption. However, the articles attribute the color modulation entirely to the GST while the role of ITO in color change in such structure has not been studied in the literature.
In this paper, effects of the ITO layer in IGIP are investigated by experiments, as well as simulations. The Fabry-Perot-type strong interference effect [9] in a single-layer ITO film coated on Pt surface (IP) is studied first. The color variation due to the crystallization of ITO after thermal treatment is demonstrated. With improved preparation technics of IGIP structure and careful design of experiments, the contribution of ITO as well as GST in overall color change is measured and analyzed. Then the influence of ITO on color modulation is compared with the influence of GST quantitatively. Based on those results, we have further improved the design of this new solid-state display structure for better color presentation.
2. Experiments
Two types of samples are fabricated at room temperature by using a sputtering method (sputtering system MSP-6600 by Beijing Jinshengweina Technology). Structure A: a thin layer of ITO film is deposited on a reflective Pt coated silicon substrates (refer to IP samples in this work), and Structure B: a multi-layered IGIP structure which is essentially fabricated by depositing a GST layer and then another ITO layer on top of an IP structure. Figure 1 shows the schematic of two structures. The thickness of ITO in IP samples are either 70, 120, 180, or 230 nm, while all the IGIP samples consist of a 10 nm ITO layer, a 7 nm GST layer, a 180 nm ITO layer, and a 100 nm Pt layer from top to bottom.
Fig. 1 Schematic representation of (a) ITO/Pt (IP) sample and (b) ITO/GST/ITO/Pt (IGIP) sample. At the interface of every two adjacent layers, two arrows represent the reflection and penetration of incident light.
After deposition, IP samples with 180-nm ITO layer are divided into three groups, and then are subjected to a thermal treatment by staying at room temperature of 25°C (no anneal), anneal at 220°C, and anneal at 400°C respectively for 5 minutes in N2 atmosphere using a rapid thermal process. Before and after this thermal treatment, the visible reflection spectrums of all samples are measured by a variable angle spectroscopic ellipsometer (VASE by J.A. Woollam Company).
In order to analyze the independent color contributions of sandwiched GST layer and bottom ITO layer in IGIP structure and separate the influence of these two layers, some of those annealed IP samples are subjected to a second cycle of deposition and anneal: a 7-nm thin GST layer and then a top 10-nm ITO layer (IG) are successively deposited on top of the IP structures. Then these samples are divided into another three groups, and are subjected to another thermal treatment at 25°C (no anneal), 220°C and 400°C respectively. Various optical properties of these samples are measured.
We also apply a numerical method, reported in a previous work [23], to simulate colors and analyze samples with a variety of different structures.
3. Results and discussions
A. Optical property of as-deposited IP and influence of ITO thickness
Figure 2(a) shows the observed colors of four types of IP samples, of which the ITO thickness is 70, 120, 180, and 230 nm, respectively. Four distinguishable colors are obvious under sunlight. The measured reflection spectrums of IP are plotted in Fig. 2(b), which also show four patterns. The above colors and spectrums of as-deposited IP samples demonstrate the significant role of ITO thickness in the resulting color of the IP structures.
Fig. 2 Varying colors of four as-deposited IP (ITO/Pt) samples. (a) Image of 4 types of IP samples under natural sunlight of which the ITO thickness is 70, 120, 180, 230 nm, respectively. (b) The measured reflection spectrums of IP samples in visible wavelength.
The above observation and results suggest that ITO in fact also plays a significant role in the color of the entire stack of IGIP structure, and cannot be treated oversimply as a transparent layer. This phenomenon can be explained by the thin-film interference of the IP structure, as shown schematically in Fig. 1(a). Since the thickness of ITO is a critical parameter in thin-film interference, it has a very large impact on the color of as-deposited IP structure due to the strong thin-film interference.
B. Effect of thermal treatment on ITO thin films in IP structures
It has been reported and demonstrated that certain thermal treatments applied on ITO thin films can lead to structure variation of ITO from amorphous to poly-crystalline state [24–27]. As a result of annealing, both electrical and optical properties of ITO will change, and so do the colors of IP structure.
The visual appearance of Structure A (IP) under sunlight is investigated before and after annealed at different temperatures. Figure 3(a) shows an obvious color variation of IPs from bright violet to warm pink, which corresponds to as-deposited sample, sample after annealed at 150°C, 220°C, and 400°C respectively. The thickness of ITO in these samples is 180 nm.
Fig. 3 (a) Visual Appearance of IP structures after annealed at 25°C, 150°C, 220°C, 400°C for 5 minutes are observed under natural sunlight. For each temperature, the thermal process is applied once (left) or three times (right) to check the dependence of color on annealing times. The thickness of ITO layer is 180 nm. (b) Refractive index and extinction coefficient of as-deposited and annealed ITO at 220°C and 400°C.
The color does not change until the annealing temperature reaches 150°C. Consistent with the curves of optical constants, the color of IPs fades as the temperature arises. Once the color is changed after first anneal, it is stable, and additional anneals at the same temperature will not change the color further. As shown in Fig. 3(a), the color after first anneal is very close to the color after three times of anneals at the same temperature.
To fully understand the effort of thermal treatment of ITO on the color of IP structure, a monolayer ITO film is deposited on glass substrate, instead of Pt-coated silicon substrates, in order to conduct the optical measurement properly, and is then annealed at different temperatures. The measured optical constants of as-deposited and annealed ITO are plotted in Fig. 3(b). The thickness of ITO film is determined to be 320 nm. As shown in the figure, the optical constants of ITO annealed at 220°C decreases in visible wavelength compared with its as-deposited state, and a higher annealing temperature at 400°C further decreases its optical constants. The measured optical constants are in good agreement with literature [25].
These experimental results reveal the optical property variation of ITO after thermal process: the ability of reflection weakens relatively as annealing temperature raises in visible wavelength, and the extinction coefficient decreases in short wavelength means the sample absorb less light in 300-500 nm wavelength, indicating the transparency of the material is strengthened [27]. As the decrease of refraction index and extinction coefficient after anneal process, the film turns from bright violet to warm pink.
C. Color contribution of GST layer and bottom ITO layer
The above results show that the crystallization of ITO has an important effect on the change of optical property of interference coatings. We suppose that this effect would contribute to color change in IGIP structures.
To further understand the relative influence of those two layers of GST and ITO, we investigate colors of the IGIP structures under different experimental conditions. The detailed experiment contains two cycles of depositing and anneal processes: (1) A 180-nm ITO layer is deposited on Pt coated substrates (IP structure). The structures are divided into three groups, and are subjected to the first thermal treatment at room temperatures of 25°C (no anneal), anneal at 220°C and 400°C respectively. The temperature of first anneal is expressed by the value x. (2) A 7-nm thin GST layer and then a top 10-nm ITO layer (IG) are successively deposited on top of the IP structure, resulting in a complete IGIP structure. The samples are divided into another three groups, and are subjected to another thermal treatment at 25°C, 220°C and 400°C respectively. The temperature of the second anneal is expressed by the value y.
The final colors of these IGIP samples are shown in Fig. 4(b). Each sample in the figure can be described as [IG (IP)x]y. In all of these samples, x (the temperature of the first anneal of IP structure) is always less or equal than y (the temperature of the second anneal of IGIP structure), therefore after the first thermal treatment for the IP structure, the color of bottom ITO layer has become stable already, and won’t change again during the second thermal treatment which is dedicated to introduce change to the IG structure only without changing the optical property of IP structure. By this way, we can separate the color contribution of IP layers and IG layers. As explained above, colors in Fig. 4(a) is for the IP structures without IG, which shows the annealing effect on optical property of bottom ITO film only. These colors are the same as colors of IP structures in Fig. 3(a).
Fig. 4 (a) Visual appearance of IP samples with identical structure and process as those in Fig. 3. (b)~(f) Contrastive colors of [IG(IP)x]y structures after two cycles of deposition and anneal processes, where x refers to the temperature of first anneal on IP structure before deposition of IG, and y is the temperature of second anneal on entire IGIP structure. The value t is the thickness of bottom ITO layer. (b) Images of IGIP samples under natural sunlight, exhibiting color change caused by crystallization of ITO and GST. (c) Simulated colors of IGIPs by numerical method with identical structure as (b), which agree well with experimental colors in (b). (d) (e) and (f) More simulated colors of IGIPs with different value of t.
Visible appearance in Fig. 4 reveals the contrastive color changes. Compared with colors in Fig. 4(a), the colors in 1st row of Fig. 4(b) show clearly that the bottom ITO layer contributes to the final color of IGIP samples. The gray appearance of amorphous GST layer changes the color of the structures, due to the weak reflection of amorphous GST in visible wavelength [23].
In Fig. 4(b), the three colors of [IG (IP)x]25 structures in the 1st row as well as the two colors of [IG (IP)x]220 in 2nd row indicate that the crystallization of bottom ITO layer has an effective and significant influence on colors of the entire IGIP structure. In each row, the GST layers share the same phase state but ITO layers are different in phase structure, so the color variation in each row are caused by ITO. These color changes are agreed with colors of IP structures in Fig. 3(a), which turn from intense violet to warm pink after thermal processes.
The colors change between 1st row and 2nd row is caused by the phase change of GST from amorphous to FCC state, while the variation between 2nd and 3rd row is due to another crystallization from FCC to HEX state. These results suggest that GST film acts like a color switch, transforming from a slight-gray filter to a more transparent film.
These experimental results indicate that, by incorporating the role of ITO in color modulation, and combining with GST, we can design IGIP structure with more distinguishable colors than the works previously reported in the literature. For example, by modulating the phase of GST and ITO, four distinguishable colors of IGIP are obtained under sunlight in Fig. 4(b).
To confirm ITO’s ability to modulate color, we have also simulated the corresponding IGIP structures. Figure 4(c) shows the simulated color results of IGIPs, which agree well with experimental results in Fig. 4(b). We have further simulated several IGIP structures with different thicknesses. More colors of IGIP structures are displayed in Fig. 4 (d), 4(e) and 4(f), in which the thickness of bottom ITO layer is 70nm, 120nm and 230nm respectively. Colors including blue, deep blue, violet, yellow, greenish yellow, brown, grass green, greenish blue are shown, which show the potential for three primary colors or other basic color system.
D. Quantitative analysis of color contribution
In an attempt to compare GST and ITO layer quantitatively, we have calculated the percentage change in reflectivity of some IGIP samples to analyze the individual influence of IG (GST) and IP (ITO) structure. The measured spectrums of relevant IGIP samples are plotted in Fig. 5(a). The three curves of [IG (IP)400]y suggest the influence of crystallization of GST while those of [IG (IP)x]25 indicate the effect of crystallization of ITO in the color change of IGIP. The curves, plotted in Fig. 5(b) refer to the absolute values of percentage change in reflectivity of [IG (IP)x]y caused by annealed GST (∆R1 and ∆R2) and annealed ITO (∆R3 and ∆R4).
Fig. 5 The measured spectrums and absolute values of percentage change in reflectivity of relevant IGIP structures. (a) The spectrums of [IG (IP)400]y and [IG (IP)x]25, referring to the 3rd column and 1st row in Fig. 4(b). (b) Absolute values of percentage change in reflectivity of [IG (IP)x]y are calculated by formula ∆R (%) = (Rafter anneal-Rbefore anneal)/Rbefore anneal × 100%. ∆R1 and ∆R2 indicate the influence of GST, comparing the variation of reflectivity from [IG (IP)400]25 to [IG (IP)400]220 and then to [IG (IP)400]400. ∆R3 and ∆R4 indicate the influence of ITO, comparing the variation of reflectivity from [IG (IP)25]25 to [IG (IP)220]25 and then to [IG (IP)400]25.
To measure the color change of concerned film, we define the average of ∆R as index δ. After the first anneal for GST, δ reaches 43.20%, and 13.27% after the second anneal, while for ITO the first δ is 71.76% and second 118.12%. The average change of ∆R for ITO is larger than that for GST.
Although the average of ∆R may not be able to reflect the influence of concerned layer accurately, the large percentage of ITO seems to suggest that ITO plays a more important role in color modulation in IGIP structure, compared with GST. We can also get more detailed comparison at different wavelength from spectrum.
With those new discovery and understanding of the important role of ITO layer in color modulation in IGIP structure, we can now have more effective methods to design and manufacture IGIP-based display devices. Several parameters can be amended to produce a display with more color variation: (1) changing the optimized thickness of bottom ITO layer, (2) choosing transparent material other than ITO, (3) modulating the thickness and multi-level phase change of GST layer.
4. Conclusion
In summary, we demonstrate the strong interference effect in ITO/Pt (IP) structures and investigate the optical property variation of IPs due to the thermic crystallization of ITO. We found that ITO layer in fact plays an important role in color change of the ITO/GST/ITO/Pt structure (IGIP), which is a new discovery not thoroughly studied in previous literatures. The average of absolute values of percentage change in reflectivity caused by annealed ITO can reach as high as 118.12%, while the value is 43.20% by GST. By modulating the phase of both ITO and GST successively, four or more colors can be realized in one single multi-layer display structure, which is a more advanced approach to design display compared with those reported in the literature. These results will pave the way for the development of this GST-based promising solid-state displays technology featuring flexibility, low power consumption and high resolution.
Acknowledgments
The authors acknowledge with gratitude support from the Collaborative Innovation Center of Suzhou Nano Science and Technology.
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