Novel GaN surface treatment for improved microLED efficiency

The display industry is constantly evolving and developing new, better technologies in the quest for improved visual experience and reduced power consumption. So far, consumer electronics such as smartphones have been driving this technology race for the brightest and highest resolution displays.

Today, the situation is not much different; while OLED and QD technology dominates the market, manufacturers are looking for the next generation displays that will deliver the best performance and meet challenging demands set by the booming applications such as VR/AR.

Many display makers place their bets on microLEDs because of their overwhelming properties such as excellent pixel density and brightness, extremely low power consumption, higher switching speed, and wider color spectrum, while completely lacking the viewing angle and uniformity issues of LCD. This makes the microLED-based displays nearly ideal solution for a wide variety of applications.

Despite their undeniable benefits, microLEDs are extremely difficult and expensive to produce, preventing the technology from entering the mass market. It has been reported that microLED screens currently available on the market have yields as low as 30%, resulting in display prices ranging hundreds of thousands of dollars. One of the main issues arises during assembly and testing of millions of chips; another one is related to the quality of the chip itself. While there are ways to deal with the former problem, the latter one has been lacking an optimal solution.

MicroLEDs are made of compound semiconductor materials that are difficult to process. Additionally, these materials have a tendency to oxidize aggressively, which results in a high density of defects at the atomic level on their surfaces. As the chips are taken down to micron size and the surface/volume ratio increases, the effect of these defects becomes more prominent. The defects lead to detrimental mechanisms, such as high rate of surface recombination, imposing severe limitations to the microLED efficiency. This becomes especially harmful when the size of the microLED is comparable to the carrier diffusion length on compound semiconductor materials.

Differences in surface quality obtained during multiple steps of the manufacturing process have a direct impact on the final performance level of each chip, resulting in potentially poor quality homogeneity across the wafer. Comptek’s solution boosts chip performance and improves homogeneity across the wafer, eliminating the need for expensive chip binning.

Most commonly, microLEDs are manufactured using GaN-based chips. Despite its overwhelming semiconductor properties, GaN is a challenging material to process. Apart from the difficulty in obtaining high-quality epitaxy with extremely low variations in thickness and wavelength, there is another challenge linked to GaN properties - the tight crystalline bond strengths in group III-nitride materials. They make the material chemically inert and difficult to etch.

Etching is a fundamental part of the microLED manufacturing process. The technique is used to create isolations, mesas, trenches, and contact recesses, and, later on, it is used to remove defective layers formed on surfaces of the chips. Obtaining good quality, well-defined etched surfaces at atomic level is extremely hard due to ion-induced damage created during dry-etching processes. Restoring GaN surfaces requires strong chemistry and high-temperature regimes.

Comptek Solutions’ Kontrox™ is a powerful passivation technology that greatly reduces surface defect states density in III-V materials. It can be applied to both the sidewalls and the surfaces of microLEDs reducing the surface recombination mechanism and, as a result, boosting energy efficiency, illumination, and performance uniformity of the microLED chips. Outstanding results can be achieved even for such challenging material as GaN.

To reduce structural damage and prepare the surface for proper passivation, we have developed an effective pretreatment for GaN microLEDs; this preparation process helps to avoid the ion-induced damage and obtain morphologically well-specified semiconductor surfaces. This pretreatment contributes to the overall performance of our novel passivation technology, which can almost double the efficiency of GaN microLEDs. Moreover, the same brightness levels can be achieved at much lower current densities - up to 30 times - meaning a huge improvement in power efficiency.

Unlike common approaches that require KOH based chemistries at high temperature, we implement wet-etching that uses commercially available mild chemistry at room temperature. It removes contamination on the GaN surface, leaving a very well-specified surface morphology with particular crystalline orientations on the microLED sidewalls. Our approach represents an effective and more environmentally friendly solution.

Figure 1: SEM images of GaN LED sidewalls after Comptek´s surface preparation process, showing well defined crystallographic structures in the different layers of the chip

Moreover, with our technology, the performance uniformity of the microLED chips within the wafer is hugely improved, as initially poorly performing chips show greater improvement compared to those with better characteristics. This helps to decrease the amount of rejected parts and raise manufacturing yields, enabling crucial cost reductions. This is an important leap towards the large-scale manufacturing of microLED displays.

Figure 2: Relative External Quantum Efficiency (EQE) measurements in 5µm GaN µLEDs vs current density.

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