byKai-Ling Liang 1,2,Wei-Hung Kuo 1,Chien-Chung Lin 1,3,* andYen-Hsiang Fang 1,*
1Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute, Hsinchu 31057, Taiwan
2Graduate Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
3Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan*
Authors to whom correspondence should be addressed.
Micromachines 2023, 14(3), 589; https://doi.org/10.3390/mi14030589
Received: 13 February 2023 / Revised: 25 February 2023 / Accepted: 27 February 2023 / Published: 28 February 2023
(This article belongs to the Special Issue Micro-Light Emitting Diode: From Chips to Applications)
Abstract
Colloidal CdSe/ZnS quantum dots (QD) enhanced micro-LEDs with sizes varying from 10 to 100 μm were fabricated and measured. The direct photolithography of quantum-dot-contained photoresists can place this color conversion layer on the top of an InGaN-based micro-LED and have a high throughput and semiconductor-grade precision. Both the uncoated and coated devices were characterized, and we determined that much higher brightness of a QD-enhanced micro-LED under the same current level was observed when compared to its AlGaInP counterpart. The color stability across the device sizes and injection currents were also examined. QD LEDs show low redshift of emission wavelength, which was recorded within 1 nm in some devices, with increasing current density from 1 to 300 A/cm2. On the other hand, the light conversion efficiency (LCE) of QD-enhanced micro-LEDs was detected to decrease under the high current density or when the device is small. The angular intensities of QD-enhanced micro-LEDs were measured and compared with blue devices. With the help of the black matrix and omnidirectional light emission of colloidal QD, we observed that the angular intensities of the red and blue colors are close to Lambertian distribution, which can lead to a low color shift in all angles. From our study, the QD-enhanced micro-LEDs can effectively increase the brightness, the color stability, and the angular color match, and thus play a promising role in future micro-display technology.
Keywords: quantum dots; micro-LED; micro-display; angular color
1. Introduction
Micro-LED has been regarded as a prospective display technology in the next generation because of its outstanding features of high scalability, high brightness, high contrast, fast response, and good stability [1,2]. In pursuit of high-quality display using micro-LEDs, the potential solution will provide high density pixels with vivid colors [3]. However, both of them are not easy targets to be achieved. Mass transfer and epitaxial growth for fabricating RGB micro-LEDs become extremely difficult as the chip size shrinks and the pixel density grows [4]. To make the situation worse, the external quantum efficiency (EQE) of the micro-LEDs tends to deteriorate as we reduce the size of the device [5,6,7,8]. Therefore, quantum dots (QD) color converting method, which can mitigate some of the aforementioned problems, has become one of the attractive options that can absorb blue light to generate color-transferred red and green light [9,10,11,12]. In addition to sustained quantum efficiency, the extra QD layer can provide an extra light source in the visible light communications [13,14], in which the modulated visible photons can be used for data transmission and provide an alternative solution of current Wi-Fi scheme.
The size-dependent EQE has been widely investigated [5,15,16]. In general, the InGaN micro-LEDs showed less inclination to decay when its size is reduced [7]. With its emission blue photons, the InGaN micro-LEDs is a natural choice as the color conversion light source [17]. On the other hand, blue shift of emission wavelength is detected with increasing injected current in InGaN micro-LEDs because of band filling effect, while red shift is observed in AlGaInP due to the self-heating effect [18]. The different behaviors of InGaN and AlGaInP red micro-LEDs can lead to undesirable change in the color gamut coverage which can directly affect the color quality of the display. The QD, on the other hand, is famous for their wavelength stability across various conditions [19].
In previous study [20], angular color shift problem of mixed RGB colors was detected in micro-LED displays due to the mismatched angular distribution between AlGaInP red and InGaN blue/green micro-LEDs. This is due to the different refractive indices of the two different material systems and the sidewall emission of the devices. The angular shift problems have been discussed in some micro-LED articles [21,22], but were seldom mentioned in QD on micro-LED structure. Hence, in this study, we will first place the QD composite on top of the device, which could have a stronger thermal influence on these nanocrystals. The size effect on the QD-enhanced micro-LEDs can also be observed via various devices with dimensions of 100 × 100, 50 × 50, 25 × 25 and 10 × 10 μm2. We then analyzed the angular distribution of blue and red QD micro-LEDs with size of 100 × 100, 50 × 50 and 25 × 25 μm2. We hope this photonic characterization can be helpful for the further integration of QD with the micro-LEDs.
2. Materials and Methods
A series of blue micro-LEDs and red QD micro-LEDs with size of 100 × 100, 50 × 50, 25 × 25, and 10 × 10 μm2 were fabricated from commercial 4-inch InGaN/GaN blue epitaxial wafers grown on sapphire substrates by a metal-organo chemical vapor deposition (MOCVD) system. For blue light micro-LEDs process, a layer of indium tin oxide (ITO) was deposited onto the wafer as the ohmic contact layer of p-type GaN. The mesa pixels of 100 × 100, 50 × 50, 25 × 25 and 10 × 10 μm2 were then defined by photolithography process, and followed by an etching process of inductively coupled plasms-reactive ion etch (ICP-RIE). After dry etching, a 100 nm dielectric layer of Si3N4 was deposited as a passivation layer by plasma enhanced chemical vapor deposition (PECVD). Then, the n-contacts and p-contacts were opened by ICP-RIE, and followed by Ti/Au layer was deposition with thickness of 100 nm. Next, a layer of black matrix (BM) was covered with 1 μm of black photoresist. The light-emitting areas were opened with the same size as micro-LEDs’ sizes by photolithography process. The n-contact pads and p-contact pads were also opened in this step as conductive electrodes. The finished blue light micro-LED structure is shown as Scheme 1a.
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