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Abstract
Micro-LED has attracted tremendous attention as next-generation display, but InGaN red-green-blue (RGB) based high-efficiency micro-LEDs, especially red InGaN micro-LED, face significant challenges and the optoelectronic performance is inevitably affected by environmental factors such as varying temperature and operating current density. Here, we demonstrated the RGB InGaN micro-LEDs, and investigated the effects of temperature and current density for the InGaN RGB micro-LED display. We found that temperature increase can lead to the changes of electrical characteristics, the shifts in electroluminescence spectra, the increase of full width at half maximum and the decreases of light output power, external quantum efficiency, power efficiency, and ambient contrast ratios, while current density increase can also give rise to different changing trends of the varieties of parameters mentioned just above for the RGB micro-LED display, creating great challenges for its application in practical scenarios. Despite of the varying electrical and optical charateristics, relatively high and stable colour gamut of the RGB display can be maintained under changing temperature and current density. Based on the results above, mechanisms on the temperature and current density effects were analyzed in detail, which would be helpful to predict the parameters change of micro-LED display caused by temperature and current density, and provided guidance for improving the performance of InGaN micro-LED display in the future.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, micro light-emitting diode (LED) has attracted tremendous attention as a disruptive technology for display due to the high brightness, high contrast, fast response, long lifetime and low power consumption [1,2]. The traditional LEDs are mainly applied in lighting with a typical current density of 100 A/cm$^{2}$, while the current density of micro-LED for display is typically lower than 1 A/cm$^{2}$ [3]. And as the size decreases, the efficiency of micro-LED at low current density decreases significantly, which is mainly attributed to the increase of sidewall defects [4]. The lighting application of LED needs to find a suitable white light point in the International Commission on Illumination (CIE), while the micro-LED for display applications usually requires a larger color gamut. And the operating voltage, light output power (LOP), external quantum efficiency (EQE), power efficiency, electroluminescence (EL) wavelength, full width at half maximum (FWHM), and ambient contrast ratio (ACR) of micro-LEDs change dramatically with the changes of temperature and current density, resulting in drastic changes in the performance of the primary display micro-LEDs [5]. For example, there are significant temperature differences between morning and noon, winter and summer, leading to the electrical characteristics change of micro-LEDs. In addition, assuming the constant-voltage drive and pulse width modulation (PWM), the temperature increase can cause the driving current density increase, and then the great changes of the LOP, EQE, peak wavelength, chromaticity coordinate and ACR of the micro-LEDs. Among the scattered reports, it was revealed that the display characteristics of R, G and B primary LEDs are quite different, e.g. with the increase of temperature and current density, the InGaN blue primary LED show decreases in the peak EQE and blue shifts in EL wavelength [6–8], while for the red primary LED, which is usually based on AlGaInP, the EQE decline and color drift are much more pronounced [8,9]. Related research shows that the InGaN red micro-LED is more robust at a high temperature, and the thermal droop of output power in AlGaInP red micro-LED is severer [10]. Up to now, comprehensive research of temperature and current density effects on RGB micro-LED display has not been reported.
What is more, the red AlGaInP red LED exhibits obvious efficiency reduction after being fabricated into micro-LED due to high density of defects induced during the fabrication process [11], and has poor thermal stability at high temperature [12]. Moreover, the different properties between the AlGaInP LEDs and InGaN LEDs could lead to the mismatch of angular distribution for micro-LED display [13]. Therefore, AlGaInP and InGaN based hybrid red, green and blue mini or micro-LED display have great difficulties to achieve good perfomances. The research on the InGaN red micro-LED is highly desired with great significance [14–16] for the integrating of whole InGaN red, green and blue full-color display, the performance of which can also be affected by different current density and varying temperature. The current techniques to realize InGaN RGB micro-LED full-color display mainly include transfer printing technology [17,18], color conversion technology [19] and epitaxial growth technology [20]. Transfer printing technology can ensure high efficiency of RGB micro-LED, and integration with complementary metal-oxide-semiconductor (CMOS) to achieve high pixel density full color display.
In this work, we systematically investigated the characteristics of InGaN RGB micro-LED display at varying temperature and current density. In particular, the InGaN red micro-LED on silicon substrate was adopted here, considering that the silicon substrate has the advantages of large size, low cost and easy transferability. In addition, high-efficiency InGaN long-wavelength red LEDs can be prepared on the silicon substrates [16]. Our study found that with the increase of temperature, the optoelectronic characteristics such as LOP, EQE, power efficiency, and ACR all decrease, which brings great challenges to RGB micro-LED display. The EL peak wavelength and FWHM also vary with temperature and current density, causing different shifts in the chromaticity coordinates, but maintaining a high color gamut in CIE 1931. The results in our works are of great significance for analyzing and improving the characteristics of InGaN RGB micro-LED display.
2. Fabrication and characterizations of the RGB micro-LEDs
Typical top-emitting InGaN micro-LED schematic diagram is shown in Fig. 1(a), where the substrates of the green and the blue micro-LEDs are sapphires, and it is silicon for the red micro-LED. Each micro-LED includes a GaN buffer layer, a n-GaN layer, an InGaN/GaN multi-quantum wells (MQWs) layer, an AlGaN electron blocking layer (EBL), and a p-GaN layer. The epitaxial layer of the red micro-LED also includes a heavily Mg-doped p-GaN layer. During the chip process, a 40 nm-thick indium tin oxide (ITO) thin film was deposited on the top of the p-GaN as the current spreading layer. Then, standard photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) were employed to define mesa structures. After rapid thermal annealing at 550$^{\circ }$C to form a better ohmic contact, a 300 nm SiO$_2$ passivation layer was deposited by plasma-enhanced chemical vapor deposition (PECVD). Here, the SiO$_2$ passivation layer was defined by HF-based wet etching. Finally, Ti/Au (50/250 nm) bilayer was deposited by magnetron sputtering as the metal electrodes. The detailed fabrication process for the micro-LEDs can be referred to our previous work [21]. Figure 1(b) is the field emission scanning electron microscopy (FESEM) image of a blue micro-LED with pixel diameter of 80 $\mu$m. Figure 1(c) illustrates the typical emission spectra and light emission images of the RGB micro-LEDs ($\lambda _{peak}$=679 nm, 525 nm and 443 nm) with the current density of 5 A/$cm^{2}$ at 298 K. Relatively good brightness uniformity is observed for each RGB micro-LEDs, indicating their good suitabilities for display application.
Fig. 1. (a) Schematic illustration for chip structure of the RGB micro-LEDs. (b) The FESEM image of a blue micro-LED with pixel diameter of 80 $\mu$m. (c) The emission spectra for the RGB micro-LEDs at 5 A/$cm^{2}$ and 298 K. Insets: The optical microscope images of light emission for the RGB micro-LEDs.
Optoelectronic characterizations were conducted on a micro-probe platform. The RGB micro-LEDs were pasted on a Peltier plate, and the temperature was monitored by a thermocouple probe in real-time. The optical power meter (Thorlabs PM100D) was placed at a certain distance above the measured pixel to collect the LOP of the luminous pixel, which was calibrated by an integrating sphere. Temperature-dependent EL spectra of the RGB micro-LEDs were obtained using the spectrometer (Ocean Optics USB4000).
3. Results and discussion
The I-V characteristics of the proposed RGB micro-LEDs measured with temperature ranging from 285 K to 357 K are illustrated in Figs. 2(a)-(c). The leakage currents of the RGB micro-LEDs are as low as an order of nA, demonstrating the high quality of the micro-LEDs, and the leakage current changes with temperature hardly affect the display characteristics of micro-LEDs. Fast nonlinear increasing currents with voltages are observed in the low current density range, and the corresponding operating voltages are varied, which is strongly related to the band diagram of the LED structure, especially the difference in InGaN/GaN MQWs. For example, at the point corresponding to the current of $5\times 10^{-8}$ A and the temperature of 298 K, the voltages of the RGB micro-LEDs are 1.83 V, 1.93 V and 2.39 V, respectively. The phenomenon can be explained by drift diffusion and recombination model, as well as the generation recombination process in the MQWs [21]. Thus, the difference in InGaN band gap leads to the voltage variation in this low current range, i.e. the red micro-LED has the lowest voltage and the blue micro-LED has the highest voltage. However, as the current increases to larger values, it approximately increases linearly with voltage, and when the current is up to $5.0\times 10^{-5}$ A, the voltages are 2.629 V, 2.235 V and 2.601 V at the temperature of 298 K for the RGB micro-LEDs, respectively. The series resistances dominate [22], which are 52.58 k$\Omega$, 44.7 k$\Omega$ and 52.02 k$\Omega$ for the RGB micro-LEDs. This means that the epilayer’s quality and device fabrication of the red micro-LED need to be further improved to reduce the series resistance.
Fig. 2. Temperature-dependent logarithm current versus voltage curves of (a) the red, (b) the green and (c) the blue micro-LEDs, respectively.
What is more, it is also observed that the current increases with the temperature for all the micro-LEDs under the same operating voltage. The increase in hole concentrations with temperature is one of the key mechanisms, which has been detailed in previous references [22,23]. The obvious temperature effects may cause great challenges for practical application of the micro-LED display. Hence, the constant-current drive and PWM are preferred to adjust the duty cycles for the micro-LEDs to obtain uniform brightness, and to effectively avoid the impact of operating voltage fluctuations. What is more, even with constant-current drive, the optoelectronic properties such as efficiency and brightness of RGB micro-LEDs change to varying degrees at different temperature, thus the temperature effects on the performance of micro-LEDs create more challenges to the driving design of micro-LED display.
The temperature-dependent LOP densities versus current densities of the micro-LEDs at varied temperatures are shown in Figs. 3(a)-(c). It is observed that the LOP densities of the RGB micro-LEDs decrease by 32$\%$, 14.4$\%$ and 14$\%$ at 100 A/$cm^{2}$ and more by 33$\%$, 36.9$\%$ and 25.7$\%$ at 1 A/$cm^{2}$, respectively, when the temperature increases from 285 K to 357 K. Here, the temperature-dependent LOP can be described by the phenomenological equation [24].
where LOP$_{298K}$ and T$_{ch}$ are the LOP of micro-LED at 298 K and the characteristic temperature, respectively. We can extract the T$_{ch}$ of the RGB micro-LEDs which are 215 K, 158 K and 215 K at the current density of 1 A/$cm^{2}$, and 272 K, 428 K and 272 K at 100 A/$cm^{2}$, respectively. As shown in Figs. 3(d)-(f), the EQEs at 298 K of the RGB micro-LEDs are 0.08$\%$, 11.9$\%$ and 15.7$\%$ at 1 A/$cm^{2}$, and 0.1$\%$, 8.4$\%$ and 15.5$\%$ at 100 A/$cm^{2}$, respectively. The EQEs decrease by 33$\%$, 37$\%$ and 26$\%$ at 1 A/$cm^{2}$, and by 32$\%$, 14$\%$ and 14$\%$ at 100 A/$cm^{2}$, respectively, with the temperature changing from 285 K to 357 K. Generally, a high T$_{ch}$ implies weak temperature dependence which is desirable. The T$_{ch}$ is mainly influenced by the Shockley-Read-Hall (SRH) recombination rate at low current density, e.g. 1 A/$cm^{2}$ [24]. The SRH related non-radiative recombination has great temperature dependence at low current density, resulting in a strong temperature-dependent effect of the EQEs. As the current density increases to 100 A/$cm^{2}$, the radiative recombination and Auger recombination rates begin to dominate. These are consistent with the aforementioned LOP density and EQE decreasing trends. The decreased LOP would result in lowered brightness as temperature increases. Micro-LED display mainly works at low current density, e.g. 1 A/$cm^{2}$, so it is important to reduce the defect density, including bulk and sidewall defects, which improves not only the efficiency of micro-LED, but also the brightness stability of the micro-LED display.Fig. 3. Temperature-dependent (a-c) LOP density, (d-f) EQE, and (g-i) power efficiency curves at different current densities of the RGB micro-LEDs, respectively.
For InGaN RGB micro-LED display, the greatest challenge is how to improve the efficiency of the red micro-LED. It is worth noting that the EQE of the red micro-LED is relatively low, which can be attributed to the high defect density, the strong quantum-confine Stark effect (QCSE) generated in the InGaN/GaN QWs [25] and the part of light absorptions by the silicon substrate [25], etc. The related researches of red micro-LEDs on sapphire substrates also have been conducted and studied. Zhuang et al. demonstrated 630-nm InGaN red micro-LED ($20\times 20 \mu$m) on c-plane-patterned sapphire substrate with an EQE of 0.18$\%$ at 50 A/$cm^{2}$ [15]. Dussaigne et al. demonstrated 625-nm InGaN red micro-LED (10 $\mu$m) with an EQE of 0.14$\%$ at 8 A/$cm^{2}$ [26]. Huang et al. achieved a maximum EQE of 5.02$\%$ for InGaN red micro-LED on c-plane-patterned sapphire substrate through the superlattice structure, atomic layer deposition passivation and distributed Bragg reflector [27]. To improve the EQE, first and foremost is to improve the quality of the red-micro-LED epilayers. Moreover, the lift-off of silicon substrate and device transfer technologies for the red micro-LED can also be employed to improve the EQE [28]. We have been optimizing the device structure, e.g. removed the silicon substrate and then transferred the InGaN red micro-LEDs onto another substrate with a reflector structure in our previous work, greatly improving the EQE of InGaN micro-LEDs [29].
When the micro-LEDs are applied to display, it is necessary to analyze their power efficiency, which is defined as the ratio of luminance intensity over the power consumption (cd/W) [30]:
where $\eta$ stands for the power efficiency, $P_{light}$ and $P_{LED}$ are the LOP and power consumption of the RGB micro-LEDs, respectively, $v_{(\lambda )}$ is the visual function ($v_{555 nm}$ =1), $\alpha$ is the conversion efficiency from luminance intensity to luminous flux. The power efficiencies of RGB micro-LEDs are shown in Figs. 3(g)-(i). At the temperature of 298 K, when the current density is 1 A/$cm^{2}$, the corresponding power efficiencies of the RGB micro-LEDs are 0.001 cd/W, 19 cd/W and 1.4 cd/W, and when the current density is 100 A/$cm^{2}$, they are 0.014 cd/W, 12 cd/W and 1.3 cd/W. Compared to traditional displays such as OLED ($\eta _{green}$=12.1 cd/W, $\eta _{blue}$= 0.7 cd/W) and LCD ($\eta _{green}$= 7.4 cd/W, $\eta _{blue}$ =1.9 cd/W) in low current density applications (such as smart watch, mobile phone, etc.), the power efficiencies of the green and the blue micro-LEDs have certain advantages [31]. However, the power efficiency of the red micro-LED is relatively low due to the weak LOP, worse than the LCD ($\eta _{red}$= 2.2 cd/W) and the OLED ($\eta _{red}$= 2.5 cd/W). In addition, unlike the green and the blue micro-LEDs , the power efficiency of the red micro-LED increases with the increase of current density. The reason is that the obvious blue-shift in wavelength increases the visual function. What is more, from Figs. 3(g)-(i), the power efficiencies of all the micro-LEDs decrease obviously with the increase of temperature, which can be related to the corresponding decrease of the EQEs.The wavelength distribution and FWHM of the RGB three primary colors determine the display quality. Here, we discuss the temperature-dependent EL spectra (peak wavelength and FWHM) of the RGB micro-LEDs, as shown in Figs. 4(a)-(c). There are certain redshifts in the peak wavelengths of the RGB micro-LEDs at 1 A/$cm^{2}$ (1 nm, 4 nm and 5 nm, respectively) with the temperature changing from 285 K to 357 K, which is mainly due to the bandgap shrinkage caused by self-heating [29]. In addition, the peak wavelengths of the RGB micro-LEDs show varying degrees of blue-shifts (48 nm, 19 nm and 2 nm, respectively) with the current density increasing from 0.5 A/$cm^{2}$ to 100 A/$cm^{2}$ at 298 K, which are mainly due to the screening of the QCSE and band filling effect in the InGaN/GaN QWs [32]. The largest value of the red micro-LED is mainly attributed to its highest indium content InGaN/GaN QWs, and thus the strongest QCSE. For micro-LED display, the peak wavelength shifts will cause color drift to affect the color gamut and color deviation to a certain extent.
Fig. 4. (a-c) Peak wavelengths and (d-f) FWHMs of the RGB micro-LEDs with the temperature from 285 K to 357 K under different current densities.
The FWHMs of the RGB micro-LEDs with the temperature varying from 285 K to 357 K under different current densities are shown in Figs. 4(d)-(f). The FWHMs at 1 A/$cm^{2}$ and 298 K are 78 nm, 35 nm, and 15 nm, respectively. FWHMs scaled by 10 nm for the red micro-LED, and 6 nm for both the green and the blue micro-LEDs at 1 A/$cm^{2}$ with the temperature from 285 K to 357 K. The main reason for the largest FWHM increase of the red micro-LED is that the indium content in the QWs is the highest and not uniform, leading to a stronger exciton localized state effect [33]. The temperature dependent changes of FWHMs for all micro-LEDs are mainly due to the thermal excitation of electrons and broadening of the density of states [34]. The smaller FWHM would cause the color purer and thus a larger color gamut, so the RGB micro-LEDs need to be further optimized to obtain smaller FWHM for display.
To better describe the influence of the changing wavelengths on color gamut, the chromaticity coordinate changes of the RGB micro-LEDs under different temperatures at 1 A/$cm^{2}$ in CIE 1931 are illustrated in Fig. 5(a), it can be seen that with the increase of temperature, the values of the CIE x of the red and the green micro-LEDs increase, while the blue micro-LED decreases. At the same time, the values of the CIE y of the red and the green micro-LEDs decrease, while the blue micro-LED increases. According to the calculation of Heron’s formula, as the temperature increases, the color gamut increases from 85$\%$ NTSC to 94$\%$ NTSC, demonstrating that the color gamut becomes larger as the temperature increases, even though the FWHMs of the RGB micro-LEDs increase. Compared with QD-LCD ( 140$\%$ NTSC) and OLED ( 100$\%$ NTSC) [35], the color gamut of the RGB micro-LEDs in this study is slightly insufficient, mainly due to the large FWHM of the RGB micro-LEDs, so further improvements are needed, such as optimizing the structure of QWs and epi the growth technique of layers, to achieve high quantum efficiency emissions with suitable RGB peak wavelengths and narrow emission spectra [20]. In particular, the peak wavelength of the red micro-LED at low current density needs to be optimized to around 630-650 nm, which is more suitable for display. Figure 5(b) shows the chromaticity coordinate changes of the RGB micro-LEDs under different current density at 357 K. As the current density increases, the color coordinates of the RGB micro-LEDs show changes with accordance to the blue-shift of the peak wavelength, but the color gamut keeps a constant value of 94$\%$ NTSC. Generally, a wider color gamut will make the display look more realistic and vivid, but the other parameters such as color deviation, color uniformity, etc. also need to be considered. Therefore, obvious wavelength shifts and large FWHM are not conducive to high-quality stable display with high color gamut.
Fig. 5. Color gamut in CIE 1931 of RGB micro-LEDs (a) with different ambient temperatures at 1 A/$cm^{2}$, (b) with different current densities at 357 K.
ACR with different ambient light illuminance under different temperature is crucial to display application, which will directly affect the visual clarity and color of display. The ACR can be expressed as follows [36]: <numdisp3/> where $L_{on}$, $I_{am}$, $R_{L}$ and $L_{off}$ represent on-state luminance, ambient light illuminance, luminous reflectance, and off-state luminance, respectively. We assume that the InGaN RGB micro-LED display is completed, and the corresponding ACR calculation is mainly based on the reflectivity of the glass. Here we assume$R_L$ = 4$\%$ according to ref. [36], and in the actual micro-LED display, the effect of RGB micro-LEDs surface reflectivity on ACR needs to be taken into account for actual display in the industry. We estimated that the surface reflectivity of the RGB micro-LEDs with ITO film and Ti/Au surface in this study is about 17$\%$ according to the references [37,38], contrast ratio (CR)=$10^{6}$:1 ($L_{off}$ =0), the images displayed by different $L_{on}$ and $I_{am}$ were simulated [36]. The ACR versus temperature curves are shown in Figs. 6(a)-(c). The ACRs of the RGB micro-LEDs are about $10^{1}$, $10^{3}$ and $10^{4}$ at 1 A/$cm^{2}$ and 298 K, respectively, with the ambient luminance of 200 lux. To a certain extent, the blue and the green micro-LEDs can well meet the needs of high contrast of indoor display (ACR indoor is no more than $10^{3}$) [36]. However, the red micro-LED has a small ACR due to the low brightness. In addition, it can be seen that the ACR shows a weak decrease with the increase of temperature for each ambient luminance. The ACR versus current density curves with different ambient luminance at 298 K are shown in Figs. 6(d)-(f). It can be seen that the ACRs of the RGB micro-LEDs increase with the increase of current density for each ambient luminance, which is consistent with the variation trend of LOP density as the increase of current density. For RGB micro-LED indoor display, the ACR of the blue and the green micro-LEDs has clear advantages over LCD and OLED [39], but is relatively small for the red micro-LED due to the low brightness. Thus it is disired to further improve the efficiency of the red micro-LED to improve the competitiveness of ACR for RGB micro-LED display. Furthermore, we further tested the effect of temperature and current density on the optoelectronic properties of RGB micro-LED displays at higher temperatures (368 K and 383 K). The results indicate that the influence of temperature and current density effect on display at higher temperature is consistent with the trend analyzed in the text. Also, The digital displays of "01234" based on the green micro-LED arrays with a pixel diameter of 40 $\mu$m at 5 A/$cm^{2}$ as shown in Fig. 7 have been achieved. The peak wavelength is 525 nm and the uniform display brightness demonstrates the potential of micro-LED display.
Fig. 6. The ACR versus temperature curves with different ambient luminance at 1 A/$cm^{2}$ of (a) the red, (b) the green and (c) the blue micro-LEDs. The ACR versus current density curves of (d) the red, (e) the green and (f) the blue micro-LEDs with different ambient luminance under the temperature of 298 K.
Fig. 7. a typical digital displays of "01234" at 5 A/$cm^{2}$ based on the green micro-LED arrays with a pixel diameter of 40 $\mu$m.
4. Conclusions
In summary, the temperature and current density effects of InGaN RGB micro-LED display have been systematically investigated, and the main display characteristics of RGB micro-LEDs under different temperatures at 1 A/$cm^{2}$ are summarized as shown in Table 1. The temperature effects could lead to the electrical characteristics change of RGB micro-LEDs, and when the driving voltage is constant, the current density increases with the increase of temperature. The related optoelectronic characteristics including LOP, EQE, power efficiency and ACR, etc. show varying degrees of decline with the increase of temperature, resulting in the degradation of display quality. The EL peak wavelengths and FWHMs show slight shifts with varying temperature and current density, leading to the drift of chromaticity coordinates, but sustaining a high color gamut in CIE 1931. In addition, the InGaN red micro-LED is still the largest constraint in the entire RGB micro-LED display, and its LOP, EQE and power efficiency need to be improved by nearly two orders to be comparable to OLED or LCD. Moreover, the FWHM of the red micro-LED should be optimized to at least half of the current value to obtain a higher color gamut. Therefore, improving epilayer’s quality to reduce crystal defects, optimizing the structure of QWs to improve radiative recombination rate, and designing the structure of micro-LEDs such as flip-chip to enhance the heat dissipation and improve the temperature stability, can effectively promote the performance of InGaN RGB micro-LED display.
Table 1. Summary of the main display characteristics of RGB micro-LEDs under different ambient temperatures at 1 A/$cm^{2}$.
Funding
National Key Research and Development Program of China (2021YFB3601000, 2021YFB3601003, 2021YFE0105300); National Natural Science Foundation of China (61974031); Science and Technology Commission of Shanghai Municipality (21511101303); Leading-edge 247 Technology Program of Natural Science Foundation of Jiangsu Province (BE2021008-2).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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