IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 5, MAY 2019 2263 The Impact of Luminous Properties of Red, Green, and Blue Mini-LEDs on the Color Gamut Weijie Guo , Nan Chen, Hao Lu, Changwen Su, Yue Lin , Guolong Chen, Yijun Lu , LiLi Zheng, Zhangbao Peng , Hao-Chung Kuo, Fellow, IEEE, Chih-Hao Lin, Tingzhu Wu , and Zhong Chen Abstract— Color gamut is of the paramount importance and virtual reality (VR) devices [2]. Nowadays, these two lead- in the display.Light–current–voltage(L–I–V ) characteristics ing display technologies are both pursuiting the high dynamic of red, green, and blue flip-chip mini-light-emitting diodes range (HDR) features [3]. Although the OLED display has (LEDs) (100 µm × 200 µm) are investigated at temperatures similar to the operational temperatures. Both the ideality been commercialized in portable devices, monitors, and tele- factor and the temperature dependence of external quantum visions [4], and begun attracting interest in flexible or foldable efficiency (EQE) suggest that the nonradiative loss in red devices, the LCD with mini-LED backlight is still among the mini-LED is higher. We also illustrate the intensive lateral most promising technologies for the next-generation flat dis- luminous intensity fluctuation for red mini-LED under the play due to the superiorities in low cost and mass production over-driving current by capturing the spatial emission map- ping. The influence of temperature and driving current on capabilities [3]–[5]. The mini-LED, with sizes ranging from the chromaticity coordinates of mini-LEDs is determined. 100 to 200 μm, serves as the light source in backlight of Furthermore, we propose a drive-current algorithm to maxi- LCD, showing the potential to replace the phosphor-converted mize the color gamut of the trichromatic mixed light at differ- white light LEDs (PC-LEDs) [6]. Benefiting from the tiny ent temperatures. The results indicate that, for the applica- pitches and the larger number of dimming zone, the red, tion of self-emitting mini-LED displays, the maximum color gamut is more sensitive to green emission, whereas enhanc- green, and blue (RGB) full color mini-LED array could also ing the radiative recombination in red mini-LED could con- achieve self-emitting displays if the angular resolution density tribute a lot to the improvement on net performance. is high enough [3]. In addition, being tiny also adapt mini- Index Terms— Displays, mini-light-emitting diode (LEDs), LEDs for thinner or curved displays, which is highly desirable color gamut. for smartphone applications [6]. As RGB LEDs have different thermal and luminous char- I. INTRODUCTION acteristics, the variation in one color can exert the obvious HE fact that the light emission of light-emitting diode change on chromatic properties of the mixed color [7]. The T(LED) can be controlled precisely facilitates the improve- understanding on the electrical-thermal-chromatic character- ments in performance of displays [1]. Liquid crystal display istics of RGB mini-LED chips is essential for the design (LCD) and organic LED (OLED) displays are two dominating of driver, mechanical structure and heat dissipation of self- technologies for TVs, smartphones, augmented reality (AR), emitting mini-LED displays [8]. In this paper, light–current–voltage (L–I–V ) characteristics Manuscript received January 3, 2019; revised March 4, 2019; accepted of RGB flip-chip type mini-LEDs at temperatures ranging March 17, 2019. Date of publication April 4, 2019; date of current version April 22, 2019. This work was supported in part by the National Natural from 300 to 380 K are determined experimentally. More- Science Foundation of China under Grant 61504112, Grant 11604285, over, the influence of driving current on the distribution of Grant 51605404, Grant 11504182, and Grant 11674054, in part by radiative recombination is also measured. The temperature the Science and Technology Project of Fujian Province under Grant 2018H6022, in part by the Natural Science Foundation of Fujian Province and current-injection dependences of the centroid wavelength, under Grant 2018J01103, in part by the Technological Innovation Project spectral width, and chromaticity coordinates for RGB mini- of Economic and Information Commission of Fujian Province, and in part LEDs are determined. We further discuss the influence of by the Strait Postdoctoral Foundation of Fujian Province. The review of this paper was arranged by Editor C. Surya. (Weijie Guo and Nan Chen these properties on color gamut of the trichromatic mixed light contribute equally to this work.) (Corresponding authors: Tingzhu Wu; and propose a drive-current algorithm to maximize the color Zhong Chen.) gamut at different temperatures. It is shown that color gamut W. Guo, N. Chen, H. Lu, C. Su, Y.Lin, G. Chen, Y.Lu, L. Zheng, Z. Peng, T. Wu, and Z. Chen are with the Fujian Engineering Research Cen- of the RGB mixed system is more sensitive to the luminous ter for Solid-State Lighting, Collaborative Innovation Center for Opto- properties of green mini-LED. Therefore, the selection and electronic Semiconductors and Efficient Devices, Department of Elec- working condition of green mini-LEDs is critical for the color tronic Science, Xiamen University, Xiamen 361005, China (e-mail: [email protected]; [email protected]). gamut of self-emitting mini-LED displays. H.-C. Kuo and C.-H. Lin are with the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. II. EXPERIMENTS Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. We use commercial flip-chip type red (AlGaInP), Digital Object Identifier 10.1109/TED.2019.2906321 green (InGaN), and blue (InGaN) mini-LEDs as samples. 0018-9383 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 2264 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 5, MAY 2019 Fig. 1. Illustration for chip structure of (a) red and (b) blue and green mini-LEDs. Fig. 3. Ideality factor of RGB mini-LEDs at the temperatures from 300 to 380 K. The ideality factors of red mini-LED reach 2.0. green and blue mini-LEDs suggest the existence of radiative recombination from localized states in these chips. As one of the commonly used parameters, the ideality factor (nideality) is associated with carrier transport and recombination in LED [10]. Fig. 3 illustrates the fitted nideality for mini-LEDs, Fig. 2. (a) Current–voltage–temperature characteristics of flip-chip type according to the relationship RGB mini-LEDs, measured at the temperatures from 300 to 380 K. (b) Typical shape of emission spectra for mini-LEDs at 300 K, the driving ∂ −1 µ µ µ e ln I current is 300 A for blue, 900 A for green, and 30 000 A for red, and nideality = (1) the insets are optical photographs of light emission from LEDs. kB T ∂V where e denotes the elementary charge, kB is the Boltzmann’s constant, and T is the temperature. nideality reach 2.05 ± Fig. 1 illustrates the structure of these mini-LEDs. Red mini- ± ± LEDs are grown on GaAs substrates originally and then the 0.07 for red mini-LEDs, 1.77 0.03 for green, and 1.39 epi layers are transferred to a sapphire substrate through the 0.04 for blue, indicating that the existence of SRH recombi- nation in green and blue, whereas the carrier loss is dominant bonding of an adhesive layer using the similar fabrication n process reported in literature [9]. Green and blue mini-LEDs in red. It has been reported that ideality of red LED gradually are grown on c-plane sapphire substrates. The sizes of these increases from 1.47 to 1.95 as the size decreases from 350 to 15 μm, due to the carrier loss through sidewall defects [10]. mini-LEDs are all 100 μm × 200 μm. We mount the samples on a temperature-controlled heat sink (TLTP-TEC, Talent, The red mini-LEDs with flip-chip structure experience more processing steps during fabrication, which may generate higher China) and measure the electrical and luminous properties of n them from 300 to 380 K. During the measurements, samples density of sidewall defect and cause higher ideality. are driven by a current source (Keithley 2400, Keithley Inc., All of the external quantum efficiency (EQE) curves of RGB mini-LEDs drop as a whole when the temperature USA), and a spectrometer (QE65 Pro, Ocean Optics) is utilized to capture optical spectra. The distributions of radiative recom- increases (Fig. 4). The current for maximum EQE of red μ 2 μ bination for flip-chip type mini-LEDs are measured by the reaches 30 000 A (150 A/cm ), far higher than the 900 A (4.5 A/cm2) for green and 300 μA(1.5A/cm2) for blue. The microscopic hyperspectral imaging system, which consists of a hyperspectral camera (GaiaField-F-V10, Shuanglihepu Inc., maximum EQE of red mini-LED at 380 K drop to 44% of that at 300 K, whereas the ratios are 87% for green and 93% for China) and a high-magnification metallographic microscope (MJ51, Mingmei Inc., China). blue [Fig. 4(d)]. The intensive decay of EQE with temperature for red mini-LED originates from the carrier loss as revealed by nideality (Fig. 3). III. RESULTS AND DISCUSSION The emission spectrum, which determines the chromaticity A. Temperature-Dependent L–I–V Characteristics coordinates, could be featured by dominant wavelength and I–V curves of RGB mini-LEDs at the temperature rang- full width at half maximum (FWHM). Due to the asym- ing from 300 to 380 K are illustrated in Fig. 2(a).The metric shape of emission spectra, we use centroid emission shifts of I–V curves with increasing temperature are similar wavelength to evaluate the variation of emission drifting at between green and blue mini-LEDs, both weaker than that different temperature and driving current (Fig.