1. Introduction

Over the past decades, there has been increasing interest in the improvement of light extraction for gallium nitride (GaN)-based light-emitting diodes (LEDs) which are an eco-friendly energy-saving device [1–4]. However, further enhancements are required for high-power and high-brightness applications. Among various strategies to obtain high-performance GaN-based LEDs, a flip-chip (FC) configuration has been developed to enhance the light extraction through the transparent sapphire substrate [5–7]. The FCLEDs with a planar sapphire surface still experience total internal reflection (TIL) losses at the interface between the sapphire and air. Many studies have been reported on the light output power improvement of FCLEDs by reducing the TIL losses using various directly-textured architectures including microlenses [6], microcones [7], and groove lines [8] at the outer surface of the sapphire substrate. Especially, the microtextures exhibit a strong light diffuse scattering, which can enhance the light extraction efficiency of LEDs [9–11]. Unfortunately, these directly-patterned structures were mostly fabricated by lithography patterning and etching processes, thus increasing the complexity and cost of the fabrication process. On the other hand, microsphere array coating using silicon oxide and titanium oxide also enhanced the light extraction efficiency of FCLEDs [12]. But, it is relatively difficult to obtain the uniform and large-area microsphere coated monolayer on the sapphire.

Recently, a soft imprint lithography (SIL) is widely employed to fabricate and transfer micro/nano patterns for a variety of applications such as optoelectronics, fluidic mechanics, and biologics because of its simple, fast, and inexpensive process as well as high throughput [13–19]. In most reports on the SIL, a conformable and elastomeric polydimethylsiloxane (PDMS) has been generally used as membranes [13], stamps [14–17], or substrates [18] as well as protective encapsulation layers against mechanical damage to the underlying devices [19,20]. In particular, an introduction of polymer layers with textured architectures into the outer sapphire surface in FCLEDs offers many advantages such as enhanced light extraction and simple one-step process of surface textured encapsulation [19]. Moreover, the textured polymer layers can be more easily realized compared to the patterned sapphires. Thus, this textured polymer layer might be very useful to boost the optical efficiency in optical and optoelectronic devices fabricated on transparent substrates. Besides, once master molds are made, they can be repeatedly used in the SIL process. Additionally, the large-scale fabrication techniques of master molds and polymer templates with micropatterns have been developed by roll-to-roll and roll-to-plate processes using the SIL, which would facilitate the mass-production of large-sized light-emitting panels. Also, the PDMS film can be strongly laminated on the surface of planar transparent substrates with detachability from it. Although some studies have been reported on the polymer layers with positive/negative microcones [10] and one-dimensional (1D) triangular and 2D taper-like microgratings [19] to increase the light output power of LEDs, there is little work on the use of elastomeric PDMS layers with inverted tetrahedron-pyramidal micropattern (ITPM) architectures for GaN-based green FCLEDs. In this work, we demonstrated the improvement of light output power in GaN-based green FCLEDs operating at 525 nm via the ITPM PDMS film as the light extraction enhancement and encapsulation layer. The ITPM on the surface of PDMS films was transferred from tetrahedron-pyramidal micro-patterned sapphire substrates (i.e., TPM PSSs) as a master mold by the SIL technique. Their light diffuse scattering properties were investigated. For a theoretical analysis of optical light propagation properties, the finite-difference time-domain (FDTD) simulation was also performed.

2. Experimental and numerical modeling details

Figure 1 shows the schematic illustration of the process steps for the fabrication of ITPM PDMS films from TPM PSSs by the SIL method. As shown in Fig. 1, the TPM PSSs consisting of 2D periodic hexagonal arrays (AND Co.), which were prepared by conventional photolithography and subsequent inductively coupled plasma (ICP) dry etching, were used as a master mold. To obtain a large diffuse light scattering at wavelengths around 525 nm in transmission, we chose tapered PM PSSs with the average period and height of 2.5 ± 0.1 μm and 1.5 ± 0.05 μm, respectively [9]. For the fabrication of ITPM PDMS films, the commercial Sylgard 184 (Dow Corning Co.) mixed PDMS solution with a ratio of 10:1 (base:agent) was poured on master molds, and then thermally cured at a temperature of 75 °C for 2 h. The PDMS films were carefully peeled off from the master molds, creating the ITPM PDMS. Meanwhile, the epitaxial layers of FCLEDs were grown on back-side polished (0001) sapphire substrates by a metal organic chemical vapor deposition. The device structure is composed of undoped GaN buffer layer, n-GaN layer, InGaN/GaN multiple quantum wells (MQWs), p-GaN layer, and ITO electrode. Ti/Al/Ni/Au layers were used as n-/p- contact pads, and then a silicon dioxide (SiO₂) film was deposited on the surface of FCLEDs for passivation by a plasma-enhanced chemical vapor deposition. For the formation of n-/p- bump patterns, photolithography and buffered oxide etchant etching processes were performed. After that, Ti/Al/AuSn layers were deposited as a reflector. In soldering processes, the Sn was electroplated on the patterned bump area. For measurements, the devices were bonded on a lead frame and then loaded on a metal printed circuit board with an Ag paste. More details of the structure and fabrication of FCLEDs used in this work can be found in our previous work [21]. Finally, the fabricated ITPM PDMS films were incorporated into FCLEDs as an encapsulation and protection layer of the outer surface of sapphire substrates. Scanning electron microscopy (SEM; LEO SUPRA 55, Carl Zeiss) observation was carried out to investigate surface morphologies and patterned profiles of the fabricated samples. The optical light scattering behaviors were evaluated by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere. The light-current-voltage (L-I-V) curves of devices, which were mounted on the Cu submount with a thermoelectric cooler, were characterized from the back-side emission (i.e., light escape through the sapphire) by using a probe station system. The optical and spectral properties of devices were estimated by the calibrated detector with an integrating sphere under continuous-wave (CW) mode at room temperature.

 

Fig. 1 Schematic illustration of the process steps for the fabrication of ITPM PDMS films from TPM PSSs by the SIL method.

Download Full Size | PDF

The theoretical analysis on optical light scattering behaviors of the ITPM PDMS layer was also studied by the FDTD method using a commercial software (FullWAVE, Rsoft Design Group). To design the numerical model in simulations, the inverted pyramidal structure on the PDMS film was represented by a periodic geometry in the Cartesian coordinate system using a scalar-valued function of two variables, f(x, z), for simplicity. We assumed that the incident light enters from the PDMS layer to air at normal incidence. The E_y, i.e., amplitude of y-polarized electric field, was calculated for the incident plane wave with a Gaussian beam profile that is normalized at λ = 525 nm. The period and depth of ITPM were kept at 2.5 μm and 1.5 μm, respectively. The thicknesses of PDMS film and sapphire substrate were set to be 200 and 120 μm, respectively. The refractive indices of the PDMS and sapphire used in this calculation were also assumed to be 1.43 and 1.76, respectively.

3. Results and discussion

Figure 2 shows the top- and side-view SEM images of (a) the TPM PSS master mold and (b) the fabricated ITPM PDMS film. Using the SIL method, the negatively tetrahedron-micropyramidal structures with 2D periodic hexagonal close-packed uniform arrays were relatively well transferred onto the surface of 200 μm-thick PDMS film from TPM PSS master molds without any distinct deformation and distortion, as shown in the SEM images of Fig. 2(a) and 2(b), thus forming the ITPM PDMS. For the formed ITPM on the PDMS film, the average period and depth were estimated to be ~2.5 ± 0.1 μm and ~1.5 ± 0.05 μm, respectively.

 

Fig. 2 Top- and side-view SEM images of (a) the TPM PSS master mold and (b) the fabricated ITPM PDMS film.

Download Full Size | PDF

Figure 3 shows (a) the measured diffuse transmittance spectra of the planar sapphire and the ITPM PDMS/sapphire and (b) the contour plots of electric field (E_y) distributions calculated by the FDTD for the incident light propagating from the corresponding samples to air at λ = 525 nm. Light diffraction phenomenon of the corresponding samples by using a green laser is also shown in the inset of Fig. 3(a). As shown in Fig. 3(a), for the sapphire with a planar surface, there are almost no diffused lights in the wavelength region of 400-650 nm. On the contrary, the introduction of the ITPM PDMS film into the sapphire (i.e., ITPM PDMS/sapphire) led to the strongly enhanced diffuse transmittance (T_D) at λ = 400-650 nm, especially, T_D ~53% at λ = 525 nm (i.e., T_D ~1% for the planar sapphire). This is attributed to the microscale pyramidal structured PDMS film with an average period of 2.5 ± 0.1 μm [9–11]. For a periodic structure in the transparent medium, when the light at a normal incident angle enters into the grating with a period of Λ, the angle of the transmitted diffraction waves, θ_t,m, in the m-th diffraction order (e.g., m = 0; ± 1; ± 2; …) is given by the grating equation of sinθ_t,m = mλ/Λn [22], where λ is the incident wavelength of light and n is the refractive index of the incident medium. Therefore, for the transparent materials including glasses, plastics, and sapphires, etc., with micro-periodic structures, a large diffuse transmission can be obtained. This strong light scattering property of the ITPM PDMS layer can be also confirmed from the photograph in the inset of Fig. 3(a). For the planar sapphire, there is almost no light diffraction while the ITPM PDMS/sapphire shows a large diffracted light distribution. Similarly, from the FDTD calculation in Fig. 3(b), the ITPM PDMS film with a period of 2.5 μm exhibits a strong light scattering with a wide angular spread and helps a light propagation across the interface between the PDMS and air. On the other hand, there are no scattered lights for the planar sapphire without any patterns. From these results, this strong light scattering effect would improve the light output power of FCLEDs.

 

Fig. 3 (a) Measured diffuse transmittance spectra of the planar sapphire (black symbol-line) and the ITPM PDMS/sapphire (red symbol-line) and (b) contour plots of E_y distributions calculated by the FDTD for the incident light propagating from the corresponding samples to air at λ = 525 nm. The inset of (a) shows the light diffraction phenomenon of the corresponding samples by using a green laser.

Download Full Size | PDF

To demonstrate the practical feasibility of the ITPM PDMS film with a strong light scattering property as an encapsulation and protection layer of outer sapphire substrates in FCLEDs, the device performance of GaN-based FCLEDs employed with the ITPM PDMS layer was investigated. Figure 4 shows (a) the electroluminescence (EL) spectra and (b) the L-I-V curves for GaN-based green FCLEDs with and without (w/o) the ITPM PDMS film under CW mode at 298 K. As shown in Fig. 4(a), the peak wavelength (λ_peak) of ~525 nm in EL spectra was significantly not shifted after laminating the ITPM PDMS layer on the sapphire of the FCLED. This indicates that the incorporation of the ITPM PDMS layer on GaN-based FCLEDs does not induce any irradiative defects or surface damages. Furthermore, the EL intensity of the FCLED with the ITPM PDMS layer was improved compared to the reference FCLED without the ITPM PDMS layer. The inset of Fig. 4(a) shows the emission image of the corresponding devices. The FCLED with the ITPM PDMS layer was brighter than the reference FCLED. In I-V curves of Fig. 4(b), for both the FCLEDs, there was no distinct electrical degradation. On the other hand, for the I-L characteristics, the FCLED with the ITPM PDMS layer exhibited an increment percentage of ~11.1% compared to the reference FCLED, indicating the light output power (P_out) of ~41.9 mW at an injection current (I_In) of 500 mA (i.e., P_out ~37.7 mW at I_In = 500 mA for the reference FCLED). The integration of the ITPM PDMS layer into the outer surface of the sapphire substrate in FCLEDs provides a gradient-refractive-index (GRIN) profile in constituent materials, i.e., air (n = 1)/PDMS (1.43)/sapphire (1.76), as well as the extension of light path length (i.e., light scattering effect) caused by the ITPM-textured surface [3,9,10,12]. For this reason, the TIL is efficiently suppressed, and more photons can be escaped through the ITPM PDMS layer, leading to the enhancement in the light extraction efficiency of FCLEDs. The angular far-field radiation distributions of FCLEDs were also explored to further investigate the effect of the ITPM PDMS layer on the improvement of the light extraction efficiency. In the inset of Fig. 4(b), it can be also observed that the emission intensity of the FCLED with the ITPM PDMS layer was increased in all directions due to the GRIN profile and the roughened surface via the ITPM PDMS layer. The largely-diffused surface by the ITPM PDMS layer enhances the dispersed angular distribution of the light, which exhibits the higher view angle of ~133° at full-width at half-maximum for the FCLED with the ITPM PDMS layer than that (i.e., ~128°) of the reference FCLED.

 

Fig. 4 (a) EL spectra at I_In = 350 mA and (b) L-I-V curves for GaN-based green FCLEDs with and without the ITPM PDMS layer under CW mode at 298 K. The emission images at I_In = 1 mA and the measured angular far-field radiation distributions at I_In = 350 mA of the corresponding devices are also shown in the insets of (a) and (b), respectively.

Download Full Size | PDF

4. Conclusion

We demonstrated the enahncement of the light output power in GaN-based green FCLEDs with the ITPM PDMS layer. The ITPM on the surface of the PDMS film was transfered from the TPM PSS by the facile and cost-effective SIL technique. The lamination of the ITPM PDMS film with a large T_D value of ~53% at λ = 525 nm (i.e., ITPM PDMS/sapphire structure) as the encapsulation and protection layer onto the outer surface of the sapphire substrate in FCLEDs led to the boosted light extraction efficiency, exhibiting the increment percentage of ~11.1% compared to the reference FCLED. Their EL intensity and far-filed radiation pattern were also improved at λ ~525 nm. These results suggest that the ITPM PDMS cover-layers with a strong light scattering property could be useful in high-performance GaN-based FCLED applications.