LED (LED lighting)

2022: Russian scientists have proposed a way to reduce LEDs without losing efficiency

Russian scientists at MISIS University, together with colleagues from South Korea, have come close to understanding the processes that impede the further miniaturization of devices based on wide-band semiconductors, for example, LEDs for LED displays. This was reported to TAdviser on December 20, 2022 by representatives of MISIS. The researchers suggest that the problem can be solved with additional surface treatment.

μLED LED mounted in cryostat holder

In the production of LEDs or high-power transistors, it is difficult to dispense with wide-band semiconductors, which are generally capable of operating at higher voltages and temperatures than narrow-band ones such as silicon. Power electronics of shirokoson semiconductors can be orders of magnitude more compact than power electronics made of silicon, not losing in terms of device parameters, and optical devices in the blue region of the spectrum can only be made on wide-band semiconductors, the researchers said.

One of the main tasks facing a scientist in the development of LEDs and transistors is to reduce devices without losing efficiency. Minimizing LEDs will create higher density and energy efficiency displays, for example, this is important for VR technologies where high-resolution displays are needed. Reducing the transistors, in turn, will allow more components to be placed on the substrate, thereby reducing the unit cost of the device.

"One of the key parameters for high-power transistors is resistance when RON is on. For silicon (Si) and gallium oxide (β-Ga2O3), they differ by 4,000 times in favor of a wide-band material. Physically, this means that we can make the transistor 4000 times smaller from the β-Ga2O3 and with characteristics that are not inferior to Si, or leave the dimensions the same and supply 60 times more voltage to the transistor. For LEDs, everything is a little easier. The smaller the LED, the greater the current density that causes the device to emit light. It turns out that the smaller the LED, the brighter it will glow up to fundamental physical limitations, "said Anton Vasiliev, co-author of the study, engineer of the scientific project of the laboratory of ultra-shearing semiconductors at NITU MISIS.

It is not difficult to make a device of small size, it is more difficult to preserve its characteristics, reducing the size (radiation efficiency for LEDs or transmitted power for a transistor), since miniaturization increases the effect of the semiconductor surface on the device's characteristics.

Nitrogen cryostat, which allows the test samples to be cooled to -196 ° C

Russian scientists, together with colleagues from Goryeo University in Seoul, studied the problem of falling efficiency during miniaturization of μLED LEDs used, for example, in the production of flat-panel displays, and associated it with defects formed on the side walls of the structure of materials.

The μLED samples for the study were grown by precipitation of organometallic compounds from a gaseous phase with diameters from 100 μm to 10 μm.

"In the course of our work, we learned a lot about these structures. For example, that on the points of light in the LED screen - pixels <30 μм диаметром значительно падает интенсивность и время спада излучения. Это указывало на постепенное уменьшение излучательной рекомбинации в структуре (уменьшение эффективности светодиода) с уменьшением диаметра пикселей, а зависимость интенсивности от диаметра говорила о довольно сильном вкладе в этот процесс поверхности образца, которая а) набирает дефекты в процессе травления и б) вносит всё более существенный вклад в свойства светодиода из-за приближения боковых стенок микро-СД (уменьшение диаметра в процессе травления) к активной области излучения пикселя», – поделился Антон Васильев.

To confirm the theory about the contribution of the surface to the deterioration of the effectiveness of the device, scientists conducted a study of the properties of defects for the entire line of samples. The researchers found an increase in the concentration of defects that were associated with surface damage by dry etching during LED production.

"Etching works like this: the flow of ions knocks atoms off the surface of the material, so you can mask important areas and form pixels of different diameters. But the smaller we want to make a pixel, the stronger the contribution of surface defects to the properties of the LED, and the share of non-radiative recombination increases, this leads to a drop in efficiency for small LEDs, "added Anton Vasiliev.

Anton Vasiliev, co-author of the study, engineer of the scientific project of the laboratory of ultra-shirokoson semiconductors NUST MISIS

The researchers suggest that the problem of falling the efficiency of μLED LEDs can be solved by additional etching in potassium hydroxide (KOH) in order to chemically remove most of the defective layer, annealing at a higher temperature (raise it from 700˚C to 900˚C) and passivation, in which the broken bonds on the surface of the material are closed with an Al2O3 layer. All this will reduce the concentration of active recombination centers and suppress their participation in non-radiative processes.

However, to clarify options for solving the problems of miniaturization of devices, scientists have to conduct further research. In the future, scientists plan to continue to study the properties of defects in μLED LEDs caused by dry etching, and, together with a group from Goryeo University, try to solve the problem by annealing and passivation or look for other ways to achieve maximum efficiency of micro-LEDs.

The study was carried out as part of the national project "Science and Universities." The results of the work are published in the scientific journal Alloys and compositions.

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