Abstract
We demonstrate the development, performance, and application of a GaN-based micro-light emitting diode ( $\mu$ LED) array sharing a common p-electrode (anode), and with individually addressable n-electrodes (cathodes). Compared to conventional GaN-based LED arrays, this array design employs a reversed structure of common and individual electrodes, which makes it innovative and compatible with n-type metal-oxide-semiconductor (NMOS) transistor-based drivers for faster modulation. Excellent performance characteristics are illustrated by an example array emitting at 450 nm. At a current density of 17.7 kA/cm $^2$ in direct-current operation, the optical power and small signal electrical-to-optical modulation bandwidth of a single $\mu$ LED element with 24 $\mu$ m diameter are over 2.0 mW and 440 MHz, respectively. The optimized fabrication process also ensures a high yield of working $\mu$ LED elements per array and excellent element-to-element uniformity of electrical/optical characteristics. Results on visible light communication are presented as an application of an array integrated with an NMOS driver. Data transmission at several hundred Mb/s without bit error is achieved for single- and multiple- $\mu$ LED-element operations, under an on – off -keying modulation scheme. Transmission of stepped sawtooth waveforms is also demonstrated to confirm that the $\mu$ LED elements can transmit discrete multilevel signals.
Highlights
Gan-Based light emitting diodes (LEDs), which have achieved great importance in conventional chip formats as indicators and in solid-state lighting, can be fabricated into arrays of micro-scale LED elements with lateral dimensions of less than 100 μm [1]
This observation further indicates that the electrical performance of a GaN-based μLED element is dominated by the series-resistance from the metal-contact to p-type GaN which motivates our work on Pd metal contact to p-type GaN as discussed in early part
A deep etch down to the sapphire substrate is necessary to define separate GaN mesas within which each μLED element is fabricated. This step leads to a relatively deep gap, or trench, between adjacent GaN mesas, which has no counterpart in conventional μLED array process flows. We have found this deep gap has a great influence on the μLED element yield, and reference to Fig. 1(b) will show how dielectric and metal layers must be deposited over the exposed substrate in this region
Summary
Gan-Based light emitting diodes (LEDs), which have achieved great importance in conventional chip formats as indicators and in solid-state lighting, can be fabricated into arrays of micro-scale LED elements with lateral dimensions of less than 100 μm [1]. This, in turn, leads to a larger area requirement on the chip and a larger capacitance, further reducing the operating speed of PMOS transistors [18] These factors are highly disadvantageous for μLED array applications, since the achievable density of driver cells on a chip, and the modulation speed supplied from the CMOS, are both limited. Following these considerations, it is advantageous if the series resistance difference between μLED elements can be minimized and, simultaneously, NMOS drivers used. Compared with a conventional μLED array, this design employs a reversed common and individual electrode structure, which minimises the series-resistance differences from conductive paths and provides compatibility with NMOS transistor-based CMOS drivers. Transmission of stepped sawtooth waveforms is studied to illustrate the capability of this integrated system to transmit optical signals with discretely varying intensity
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