**4.3. Vertically-stacked polychromatic LED**

Recent advances in the performance of high power LEDs have led to the development of novel illumination sources with added functionality and intelligence. These advanced technologies have attracted attention from both academia and industry due to the increasing demand for dynamic lighting not only for outdoor activities, such as accent and task lighting, stage and studio, but also in indoor activities, such as tunable interior mood lighting [20]. Numerous spectral conversion schemes have been adopted for polychromatic LEDs, such as phosphors [21], polymer dyes [22] and CdSe/CdS quantum dots [23] for color down-conversion. These schemes, however, inevitably suffer from energy loss due to Stokes shift in the conversion processes, as well as scattering losses associated with the particles. Another conversion-free approach involves the combination of three discrete LED chips of the primary colors red (R), green (G) and blue (B) arranged side-by-side on the same plane to generate a polychromatic spectrum. On the other hand, challenges exist in uniform color mixing, both spatially and angularly in such configurations [24]. K.N. Hui et al. [7,10] pioneered the stacking of TP-LED chips in that instead of placing the RGB chips onto the same plane, the individual LEDs were arranged in a vertically-stacked configuration to produce LED-stacks. The LED-stacks consisted of an InGaN/GaN blue LED (470 nm) stacked onto an InGaN/GaN green LED (520 nm), which is subsequently stacked onto an AlGaInP/GaAs red LED (650 nm). Such a stacking strategy ensures optimal color mixing and minimal absorption losses. Figure 8 provides a schematic diagram of the proposed device.

20 mA. Figure 7(c) and 7(e) shows the emission from the reference LED structure from two different angles. A ring of yellow light is clearly observed at the periphery of the circular chip, which was attributed to the relatively thicker coating on the sidewalls. This effect is suppressed with the TC-LED structure, as shown in Figure 7(d) and 7(f), where uniform

**Figure 7.** Operational images of the packaged devices: (a) and (b) are the circular reference LED and TC-LED; (c) and

Recent advances in the performance of high power LEDs have led to the development of novel illumination sources with added functionality and intelligence. These advanced technologies

(e) show the QD-coated circular reference LED, (d) and (f) show the QD-coated TC-LED.

**4.3. Vertically-stacked polychromatic LED**

white light emission can be observed from different angles.

148 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**Figure 8.** (a) SEM image of an LED-stack assembled from LED chips with a truncated pyramid geometry, (b) operation‐ al image of the LED-stacks, and (c) schematic diagram showing the mixing of light inside the LED-stacks.

Figure 9 shows the corresponding electroluminescence (EL) spectral data and optical emission images of the LED-stacks driven at the tested voltage combinations. A wide range of colors can be obtained from the stacked LEDs, which depend on the choice of the individual LED chip with a specific wavelength and spectral bandwidth. The proposed LED-stacks with a color tunable function have potential applications in high-resolution panel displays. Table 1 summarizes the combinations of an applied driving current for generating a range of colors. In this study, the overall performance of the packaged stacked LEDs device was measured in a calibrated 12-inch integrating sphere. The optical signal was channeled using an optical fiber to an optical spectrometer. At a total driving cur‐ rent of 20 mA, the LED-stacks produced a luminous efficacy of 33 lm/W, whereas the commercial RGB LEDs produced a luminous efficacy of 30 lm/W. The corresponding CIE coordinates, CRI and CCT values of the LED-stacks were (0.32, 0.33), 69, and 6300 K, respectively, which is a promising result for a prototype device.

**Figure 9.** (a)-(d) illustrates the electroluminescence spectrum of the various colors emitted by the LED-stacks. The in‐ serted images show the corresponding devices.


**Table 1.** Biased voltage (current) of the electroluminescence spectrum for Figure 9(a) to (d).

Figure 10 shows the CIE chromaticity as a function of the viewing angle of the LEDstacks and commercial RGB LED. The CIE coordinates of light emission collected at the normal incidence (0°) from the top of the LED-stacks and the commercial RGB LEDs were (0.32, 0.33) and (0.33, 0.33), respectively. As the angle of observation was increased from 0° to 70 in 10° increments, shifting of the CIE coordinate from the stacked LEDs was insignif‐ icant and the device emitted mixed white light. On the other hand, under the same test, the CIE coordinate of the commercial RGB LED shifted to a yellowish white. At an observation angle of 70°, the farthest shift of the CIE coordinate away from the initial CIE coordinates of the stacked LEDs was (0.29, 0.29), whereas the CIE coordinate of the commercial RGB LED were shifted to (0.41, 0.39). These results highlight the effectiveness of the vertical stacking of LEDs in achieving uniform color mixing.

**Figure 10.** Angular dependence of the CIE chromaticity on the LED-stacks and commercial RGB LED.

### **5. Summary**

generating a range of colors. In this study, the overall performance of the packaged stacked LEDs device was measured in a calibrated 12-inch integrating sphere. The optical signal was channeled using an optical fiber to an optical spectrometer. At a total driving cur‐ rent of 20 mA, the LED-stacks produced a luminous efficacy of 33 lm/W, whereas the commercial RGB LEDs produced a luminous efficacy of 30 lm/W. The corresponding CIE coordinates, CRI and CCT values of the LED-stacks were (0.32, 0.33), 69, and 6300 K,

**Figure 9.** (a)-(d) illustrates the electroluminescence spectrum of the various colors emitted by the LED-stacks. The in‐

**Curve Red LED Green LED Blue LED CIE (x,y)** (a) 2.57 V (2 mA) 2.70 V (10 mA) (0.18, 0.27) (b) 2.60 V (2 mA) 2.53 V (2 mA) (0.31, 0.13) (c) 2.97 V (5 mA) 2.61 V (2 mA) (0.46, 0.53) (d) 3.72 V (14 mA) 2.68 V (4 mA) 2.53 V (2 mA) (0.32, 0.33)

Figure 10 shows the CIE chromaticity as a function of the viewing angle of the LEDstacks and commercial RGB LED. The CIE coordinates of light emission collected at the normal incidence (0°) from the top of the LED-stacks and the commercial RGB LEDs were

**Table 1.** Biased voltage (current) of the electroluminescence spectrum for Figure 9(a) to (d).

serted images show the corresponding devices.

respectively, which is a promising result for a prototype device.

150 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

This chapter reviewed several studies of laser micromachining on GaN LEDs. Based on the knowledge of several key parameters, such as the scan cycle, scan speed, pulse energy, and offset focus of laser micromachining, laser micromachining is a feasible approach for obtaining high quality and performance GaN LEDs. On the other hand, a few laser micromachining applications have been addressed. Geometrically-shaped LEDs provides an effective way of enhancing the light extraction efficiency of LEDs. Angularly uniform white LEDs help improve the angular color uniformity of white LEDs. Vertically-stacked polychromatic LEDs can improve light extraction and has potential applications including high uniformity colortunable light sources and conversion-free white LED. The mass production of high light extraction efficiency LEDs, high angular color uniformity white LEDs and high functionality GaN-based LEDs may be realized in the near future when the laser micromachining approach is adopted widely.
