The micro-light-emitting diode (micro-LED) has attracted tremendous attention due to its high brightness, good stability, fast response time, high modulation bandwidth, and low power consumption. It has great potential in advanced display fields, such as argument reality (AR), head-up display (HUD), and virtual reality (VR), as well as large-screen televisions and applications in visible light communication (VLC)^[1–13]. In addition, micro-LEDs have the advantages of wide spectrum resources, which is expected to solve the problem of radio-frequency (RF) communication service growth and spectrum shortage^[14–17]. It is worth noting that achieving a high pixel-per-inch (PPI) micro-LED display with high resolution is one of the key challenges and requires the pixel size of micro-LED to be increasingly smaller. Also, the reduction of pixel size has been reported to be an effective approach to increasing the modulation bandwidth of micro-LEDs in VLC applications due to the resistance-capacitance (RC) constant changes^[18]. However, related studies have shown that the reduction of the pixel size of the micro-LED will affect the optoelectronic characteristics for high-quality display, which are mainly reflected in current-voltage (I-V) characteristic, light output power (LOP), external quantum efficiency (EQE), electroluminescence (EL) spectrum, and modulation bandwidth^[14]. As the pixel size decreases to the micron level, the EQE of the micro-LED decreases, resulting from the unexpected surface recombination and the sidewall damage of the mesa from etching, which creates sidewall defects to serve as centers for nonradiative recombination^[2]. The increase of nonradiative recombination leads to the decrease of recombination efficiency for the micro-LED, causing the degradation of performance in display. The reduction of the pixel size will also greatly reduce the LOP, which leads to the low signal-to-noise ratio (SNR) for the micro-LED-based VLC system, further weakening the communication performance. Therefore, it is important and necessary to comprehensively investigate the relationship between the pixel size and the performance of micro-LEDs in display and VLC.

In addition, the sapphire substrate has been widely used for commercial GaN-based micro-LEDs. However, it has low thermal conductivity and can cause a strong self-heating effect for the sapphire-substrate micro-LEDs at high current density, limiting the intrinsic performance of the micro-LED^[19]. Also, there will be a high thermal mismatch and severe light crosstalk between the sapphire and the silicon-based complementary metal oxide semiconductor (CMOS) backplanes when using micro-LEDs on sapphire substrates for flip-chip micro-displays^[20]. Moreover, removing the sapphire substrate requires the laser lift-off process, which is relatively high-cost. Compared with sapphire substrates, silicon substrates show the advantages of good electrical and thermal conductivity, compatibility with silicon-based CMOS processes, and the availability of large-area substrates at low cost with high quality. Also, it is easy for the silicon substrates to be removed for device transfer through wet etching^[21]. Additionally, in contrast to the commonly used method of combining a blue-emitting LED with a phosphor to generate white light, using a violet-emitting LED to excite red, green, and blue phosphors offers distinct benefits^[22]. These benefits include much superior color rendering indices and the absence of a direct blue component^[23], which has proved to be disruptive to the human circadian rhythm^[24]. Previous studies mainly focus on the physical mechanisms of the size-dependent characteristics of micro-LEDs and their applications in display or VLC. The size-dependent characteristics of micro-LEDs on different substrates (particularly on sapphire versus silicon substrates) as well as violet micro-LEDs for simultaneous display and wireless optical communication applications remain insufficiently explored.

In this work, GaN-based blue micro-LEDs on a silicon substrate and violet micro-LEDs on a sapphire substrate with different sizes were fabricated. Then, the investigation of the LOP versus current versus voltage (L-I-V) characteristics, the EQE, the EL spectrum, the modulation bandwidth, and the communication data rate concerning the pixel size of the micro-LEDs and the temperature-dependency of LOP was carried out. Furthermore, the effects of the changes in pixel size on the performance of micro-LEDs in display and VLC applications were systematically analyzed. This study is capable of providing a theoretical foundation for determining the optimized size of the micro-LED for better performance in display and VLC applications.

The violet and blue LED wafers were grown on sapphire and silicon substrates, respectively, by metal-organic chemical vapor deposition (MOCVD), and the micro-LED structure mainly includes a GaN buffer layer, an n-doped GaN layer, an InGaN/GaN MQWs layer, an AlGaN electron block layer (EBL), and a p-doped GaN cap layer. An indium tin oxide (ITO) thin film was deposited on the surface of the p-GaN layer by electron beam evaporation as a current spreading layer with low resistivity and high transmittance, as shown in Fig. 1. Then, photolithography and inductively coupled plasma (ICP) etching were used to define the mesa of micro-LEDs with various sizes, and rapid thermal annealing was used to form an ohmic contact. Next, plasma-enhanced chemical vapor deposition (PECVD) was applied to deposit SiO2 as a passivation layer, and ICP etching was used to form an aperture on the micro-LED mesa. Additionally, a Ti/Au (50/250 nm) bilayer was deposited as n/p-pad through magnetron sputtering. The detailed fabrication process for the micro-LEDs can be referred to in our previous work^[25].

Optoelectronic characteristics of micro-LEDs were measured on an optical bench system. The I-V curve measurements were recorded with a sourcemeter (Keithley 2614B), and the LOP was collected by an optical power meter consisting of a standard photodiode power sensor (Thorlabs S120VC) and consoles (Thorlabs PM100D). A micro-spectrometer (Ocean Optics USB4000) was applied to measure the EL spectrum. Frequency response characteristics of micro-LEDs were measured through a vector network analyzer (Pico VNA106). Additionally, bit error ratio (BER) and eye pattern were measured by a signal quality analyzer (MP1800A) and an oscilloscope (86100 A).

Fundamental optoelectronic characteristics, including I-V and light output power-current (L-I) characteristics, were carried out first. Size-dependent I-V characteristics of blue and violet micro-LEDs are illustrated in Figs. 2(a) and 2(d). It can be seen that the I-V characteristics show strongly size-dependent behavior, and the current has a positive correlation with the pixel size under the same forward bias. The currents of blue micro-LEDs with diameters of 10 and 80 µm under 5 V forward bias are 0.81 and 13.38 mA, respectively. For violet micro-LEDs, the currents of devices with diameters of 10 and 80 µm under 5 V forward bias are 5.9 and 35.59 mA, respectively. This suggests that the micro-LED with a smaller size has higher resistance, as a result of a smaller active region area. Leakage currents for both blue and violet micro-LEDs are as low as an order of nA under the reverse bias, demonstrating fewer sidewall defects during etching, better insulation passivation in the process of fabrication, and higher quality of the micro-LED epilayers^[26].

In addition, size-dependent L-I characteristics of the blue and violet micro-LEDs are shown in Figs. 2(b) and 2(e). Note that the LOP presents a positive correlation with the pixel size under the same current. The LOPs for the blue micro-LEDs with diameters of 10 and 80 µm are 80 and 630 µW (at 10 mA), respectively. For violet micro-LEDs with diameters of 10 and 80 µm, the LOPs are 753 and 1347 µW (at 10 mA), respectively. This may be caused by a larger area of the active region for the micro-LED with a larger size. Additionally, the LOP of blue micro-LEDs on the silicon substrate tends to be lower than that of the violet micro-LEDs on the sapphire substrate, which may result from the absorption of blue light by the silicon substrate^[27]. It is necessary to minimize the absorption of light by the silicon substrate, such as inserting a reflector to reflect the light downward from the active region and removing the silicon substrate during the chip fabrication process to improve the LOP and luminous efficiency of the micro-LEDs on the silicon substrate in display and communication applications^[27].

Further investigation of the EQE of blue and violet micro-LEDs was carried out. Jmax-EQE is the current density where the maximum EQE occurred. As shown in Figs. 2(c) and 2(f), both blue and violet micro-LEDs show decreases in maximum EQE as the pixel size decreases as well as shifts in Jmax-EQE to a higher current density. The maximum EQEs of the blue micro-LEDs with diameters of 10 and 80 µm are 1.57% (at 72A/cm2) and 3.50% (at 20A/cm2), respectively. The maximum EQEs of the violet micro-LEDs with diameters of 10 and 80 µm are 2.43% (at 100A/cm2) and 4.52% (at 18A/cm2), respectively. The decrease in maximum EQE for micro-LEDs with smaller pixel sizes is attributed to lower internal quantum efficiency (IQE)^[28], which is caused by the increase of nonradiative recombination resulting from the increased sidewall degradation. Also, the increased sidewall degradation leads to an increase in the sidewall defect Shockley–Read–Hall (SRH) recombination, causing the shifts in Jmax-EQE to a higher current density^[29]. In addition, EQEs of micro-LEDs with various sizes follow approximately the same trend of droop at high current density. This phenomenon is related to the Auger recombination, which is independent of the pixel size^[30]. To improve the device performance and further support the development of a high-resolution micro-LED display, sidewall passivation and wet chemical treatment can be used to minimize the effect of sidewall damage.

To further analyze the shifts of the EL wavelength for blue and violet micro-LEDs with the reduction of pixel size, the peak wavelength versus the current density curves have been measured. As shown in Figs. 3(a) and 3(c), the peak wavelength shows a trend of blue shift as the current density increases. The peak wavelengths of the blue micro-LEDs with diameters of 60 and 80 µm show a shift of 14 nm, while the blue micro-LEDs with diameters of 10, 20, 40, and 50 µm show a shift of 13 nm as the current density increases from 1 to 2400A/cm2 [Fig. 3(a)]. The peak wavelengths of the violet micro-LEDs with diameters of 60 and 80 µm show a shift of 3 nm, while the violet micro-LEDs with diameters of 10, 20, 40, and 50 µm show a shift of 1 nm as the current density increases from 1 to 2400A/cm2 [Fig. 3(c)]. The blue shift is caused by the quantum-confined stark effect (QCSE) in the active region and the band-filling effect with increases in current density^[31].

Additionally, to measure the color purity of the EL emissions, the full width at half-maximum (FWHM) versus the current density curves of the blue and violet micro-LEDs were tested, as shown in Figs. 3(b) and 3(d). Broadening of the FWHM is observed as the current density increases. The FWHMs of the blue micro-LEDs with diameters of 10, 20, 50, 60, and 80 µm demonstrate a broadening of 13 nm, while the blue micro-LED with a diameter of 40 µm demonstrates a broadening of 8 nm as the current density increases from 1 to 2400A/cm2 [Fig. 3(b)]. The FWHMs of the violet micro-LEDs with diameters of 10, 20, 40, 50, and 60 µm demonstrate a broadening of 7 nm, while the violet micro-LED with a diameter of 80 µm demonstrates a broadening of 10 nm as the current density increases from 1 to 2400A/cm2 [Fig. 3(d)]. The dominance of the band-filling effect at higher current densities causes the broadening of FWHM^[32]. The micro-LED with a diameter of 80 µm tends to have a more obvious shift of wavelength and broadening of the FWHM with the increase of current density. The larger broadening of the FWHM in the micro-LED with the larger pixel size is attributed to the aggravated band-filling effect in the MQWs and serious self-heating effect, which results in higher operating temperatures than the micro-LED with a smaller pixel size^[33]. In display applications, a smaller FWHM can result in a color purer and thus a larger color gamut^[26]. Therefore, it is necessary to further optimize the MQW structure to improve the luminous efficiency and spectral stability to achieve a high-quality stable display.

Usually, micro-LEDs mainly work in display applications with low current densities. The EL emission images of the blue and violet micro-LEDs with different sizes at low current densities ranging from 0.1 to 10A/cm2 are illustrated in Figs. 4(a) and 4(c). It can be found that luminous intensity shows strong current density and pixel size dependencies, decreasing with the decrease of the current density and the shrinkage of the pixel size. Also, it has been observed that micro-LEDs with diameters of 10 and 20 µm exhibit no luminous intensity at the current density of 0.1A/cm2. Increases in the surface-to-volume ratio with decreases in the pixel size lead to more surface states, influencing the efficiency of charge carrier recombination and ultimately decreasing the luminous intensity. Additionally, defects caused by the etching process can serve as centers for non-radiative recombination at the edge of the active region and affect luminous intensity^[34].

To better describe the effect of the changes of wavelength on the color gamut, the chromaticity coordinates of the blue and violet micro-LEDs at different current densities in the CIE 1931 color space are shown in Figs. 4(b) and 4(d). Figure 4(b) displays that the chromaticity coordinates of the blue micro-LED with a diameter of 80 µm move from (0.151, 0.024) to (0.157, 0.019) as the current density increases from 1 to 2400A/cm2. For the 80 µm violet micro-LED, the chromaticity coordinates move from (0.340, 0.318) to (0.273, 0.208), as shown in Fig. 4(d). It is interesting that the chromaticity coordinates of the blue micro-LED gradually shift and curve to the right-bottom corner of the CIE 1931 color space, while the violet micro-LED shifts and curves toward the left-bottom corner of the CIE 1931 color space with the current density increasing from 1 to 2400A/cm2. The differences in the chromaticity coordinate shift of the blue and violet micro-LEDs are associated with different degrees of the spectral blue shift. The shift of the chromaticity coordinates of one color can result in a change in the color gamut of display applications. To compromise between the efficiency and the color gamut, micro-LEDs should be driven by current density corresponding to the maximum EQE^[31].

The changes in temperatures may also affect the optoelectronic characteristics of micro-LEDs. To investigate the stability and reliability of micro-LEDs at different temperatures, temperature-dependent LOP characteristics were also measured, as shown in Fig. 5. The decline of the LOP with the increase of temperature is similar for the blue and violet micro-LEDs. While the temperature increases from 300 to 350 K, the LOP of the blue micro-LED with a diameter of 80 µm drops about 5.6% at the current density of 10A/cm2 and drops about 1.9% at the current density of 2400A/cm2 in Fig. 5(a). Figure 5(b) shows that the LOP of the violet micro-LED with a diameter of 80 µm drops about 13.93% at the current density of 10A/cm2 and drops about 11.49% at the current density of 2400A/cm2 as the temperature increases from 300 to 350 K. The results in Fig. 5 indicate that the LOP at the lower current density decreases more significantly as the temperature increases. Also, the LOP of blue micro-LEDs shows less temperature dependence than that of the violet micro-LEDs. The SRH-related non-radiative recombination plays a relatively significant role in total recombination and has great temperature dependence at the low current density, leading to a strong temperature-dependent effect of the LOPs^[35]. However, radiative recombination and Auger recombination rates begin to dominate with the increases in current density, causing less temperature dependence on the LOP^[36]. Current density should be optimized according to the actual temperature to reduce decreases in LOPs.

In addition, the performance of the blue and violet micro-LEDs in the VLC application was tested. Modulation bandwidth is one of the essential parameters in the VLC system. To measure the modulation bandwidth of the micro-LEDs, a network analyzer and a high-speed photodetector were used. The extracted −3dB optical modulation bandwidth versus the current density curves of the micro-LEDs with different sizes are shown in Figs. 6(a) and 6(b). It can be seen that the modulation bandwidth increases as the current density increases, which may result from decreases in carrier lifetime at higher current density^[37,38]. Also, it is found that the modulation bandwidth increases with the decrease of the pixel size at the same current density, and the achievable maximum modulation bandwidth of the micro-LED with a smaller size is higher than that of the micro-LED with a larger size. The modulation bandwidths of the blue micro-LEDs with diameters of 10, 20, 40, 50, 60, and 80 µm are 191, 174, 156, 146, 139, and 131 MHz (at 800A/cm2), respectively. The maximum modulation bandwidths of the blue micro-LEDs with diameters of 10 and 80 µm are 360 MHz (at 2400A/cm2) and 163 MHz (at 1200A/cm2), respectively. The maximum modulation bandwidths of the violet micro-LEDs with diameters of 10 and 80 µm are 336 MHz (at 2400A/cm2) and 168 MHz (at 1400A/cm2), respectively. The increases in the modulation bandwidth with the decreases in the pixel size can be explained by the decreases in RC time constant and differential carrier lifetime^[39]. Also, micro-LEDs with smaller sizes can operate at higher current densities due to their better current spreading and heat dissipation capability^[40], which contributes to achieving higher modulation bandwidths compared to larger micro-LEDs. It should be noted that the micro-LEDs on the silicon substrate exhibit higher modulation bandwidth than that of the sapphire, which may be due to the much higher thermal conductivity of the silicon. Therefore, it is necessary to improve the heat dissipation for micro-LEDs on the sapphire substrate, such as using high thermal conductivity materials as heat sinks, optimizing thermal interface materials, and adopting a pulse driving method to improve the performance in VLC applications.

The communication capability of blue and violet micro-LEDs as VLC transmitters was further measured, using on-off keying (OOK) modulation. Measurements were conducted at optimized driven current density and peak-to-peak voltage (Vpp) under the transmission distance of 0.3 m. The red dashed line represents the forward error correction (FEC) threshold of 3.8×10−3. It can be seen that the maximum data rates of the blue and violet micro-LEDs with a diameter of 20 µm are 1.032 Gbps with a BER of 3.71×10−3 and 1.347 Gbps with a BER of 3.74×10−3, respectively. For blue and violet micro-LEDs with a diameter of 10 µm, the maximum data rates are 0.890 Gbps with a BER of 3.68×10−3 and 1.095 Gbps with a BER of 3.78×10−3, respectively. They are all lower than the FEC limit of 3.8×10−3 BER required for free-error data transmission^[41]. Notably, it is found that the micro-LED with a diameter of 20 µm exhibits the highest data rate. Although the micro-LED with a diameter of 10 µm has the highest modulation bandwidth (Fig. 6), the LOP of the micro-LED with a diameter of 10 µm is much lower than that of the device with a diameter of 20 µm [Figs. 2(b) and 2(e)] due to the self-heating effect at a high current density. Both modulation bandwidth and LOP are extremely important in the actual high-speed VLC system^[40]. For VLC applications, the micro-LED with a diameter of 20 µm can achieve the balance between the modulation bandwidth and the LOP. Additionally, for the realization of long-distance and high-speed optical communication, series-biased micro-LED arrays or parallel-biased micro-LED arrays can be used to achieve higher LOP while simultaneously maintaining high modulation bandwidths. Figures 7(c) and 7(d) show the captured eye diagrams of the blue and violet micro-LEDs with a diameter of 20 µm at a data rate of 610 Mbps with a BER of 4.58×10−5, and 680 Mbps with a BER of 7.87×10−6 respectively. The eye opens clearly, indicating good performance of micro-LEDs operating as transmitters in VLC^[38].

In order to easily compare the performance across pixel sizes and understand the trade-offs, the key metrics for micro-LEDs in both VLC and display applications have been systematically summarized in Tables 1 and 2. These results can provide theoretical guidance for optimizing the pixel size of micro-LEDs to improve the performance in display and wireless optical communication applications.

To summarize, the impacts of pixel size on fundamental optoelectronic properties, display corresponding characteristics, and VLC performance of GaN-based blue and violet micro-LEDs on silicon and sapphire substrates have been investigated, respectively. The differences in I-V characteristics of the micro-LEDs with different sizes are caused by the differences in the area of the active region. The LOP and the EQE show varying degrees of decline with the decrease in pixel size and also show temperature dependency, causing the degradation of display quality and communication performance. Measurement of the EL spectrum, including peak wavelength and FWHM, reveals that micro-LEDs with smaller sizes may be selected for a high-quality stable display. The shift of chromaticity coordinates with increasing current density can cause a change in the color gamut of display applications. In addition, micro-LEDs with a diameter of 10 µm have the highest modulation bandwidth, but micro-LEDs with a diameter of 20 µm exhibit the highest data rate, which implies the importance of balancing between modulation bandwidth and LOP when designing micro-LEDs for VLC applications. This work could provide theoretical guidance for improving the performance of micro-LED-based display and wireless optical communication by changing the pixel size of the micro-LED in the future.