Fig. 1. Process flow of the 5-step lithography for the micro-LED displays. The left side is the side view and the right side is the top view. The figure is not drawn to scale. (a) The initial state of the epi-wafer, (b) the current spreading region and mesa fabrication, (c) the pixel isolation, (d) the n-contact metal deposition, (e) the p-contact window formation, and (f) the p-contact metal deposition.

Fig. 2. Microscopic photograph of a 7×7  μm2 micro-LED array with one pixel turned on and an enlarged view of an active pixel picked out from the micro-LED arrays with mesas ranging from 7×7  μm2 to 100×100  μm2.

Fig. 3. Images of the 50 μm micro-LED array (48×48) in operation. Both Chinese characters and English characters are shown in the display.

Fig. 4. I-V-L characterization of the micro-LED arrays based on the measurement of a clustered 6×6 pixel array. (a) The I-V characteristics of the micro-LED arrays. The upper inset shows the photograph of the micro-LED arrays in full operation under 3 V bias. (b) The luminance as a function of current density of the micro-LED arrays. The upper inset shows the photos taken during the measurement. The lower inset plots the relation between the luminance of one single LED pixel (in different sizes) and the current density. (c) The EQE as a function of current density of the micro-LED arrays. The inset is the same curves plotted in a semi-log scale. (d) The selected cluster of 6×6 pixel array in each micro-LED device as the data source.

Fig. 5. (a) Schematic diagram of the transient characteristic measurement system. (b) The typical response of a micro-LED to a pulse excitation. (c) The 100 kHz pulse signal with 50% duty cycle and voltage varied from 2.75 to 3.25 V. (d)–(f) The normalized EL intensity of the micro-LED arrays as a function of time under 100 kHz pulse operation with applied voltage from 2.75 to 3.25 V. (The 100×100  μm2 micro-LED array under 3.25 V bias excitation is undermeasured due to the output limit of the pulse signal generator.)

Fig. 6. EL intensity of the 7×7  μm2 device array as a function of time under 1 MHz pulse operation. (a) 2.75 V, (b) 3.00 V, and (c) 3.25 V.

Fig. 7. Normalized luminance as a function of the duty cycle with various forward voltages and frequencies.

Fig. 8. Estimated EQE under different pulse operations with 1 MHz frequency and various duty cycles. (a) 2.75 V, (b) 3.00 V, and (c) 3.25 V.

Fig. 9. (a) Conventional PWM mode compared with short-pulse PWM mode. (b) The relative luminance of a 7 μm micro-LED generated within a 100 ns duration. (c) The short-pulse mode combined with overshootings.