Improving conductivity of AlGaN-based ultraviolet devices using heated-pressurized water

The development of high-powered ultraviolet (UV) light sources has become important in various industries, such as sterilization, biotechnology, medicine, and chemical curing processes. In recent years, rapid advancements have been made in UV-light emitting diodes and UV-lasers, particularly those based on aluminum gallium nitride (AlGaN) semiconductors. However, these devices require a high electrical current, which is impeded by the insulating nature of sapphire and AlN substrates used in these semiconductors.

Consequently, a method for exfoliating or removing the insulating layer is crucial. To address this problem, a team of researchers from Japan, led by Professor Motoaki Iwaya from the Department of Materials Science and Engineering at Meijo University, developed a method for exfoliating the insulating substrates from AlGaN semiconductors using heated and pressurized water. In a recent study, they have now conducted a series of experiments to explain the mechanism behind this method. Their findings were published in Volume 16 of the journal Applied Physics Express on 11 October 2023.

“We have established a method that can exfoliate substrates and is easy to install in semiconductor processes and uses readily available water. In addition, it is extremely impactful for researchers working on AlGaN UV semiconductor light emitting devices to understand the exfoliation mechanism and know that it is possible to achieve exfoliation of large-diameter semiconductor wafers using this method,” points out Prof. Iwaya.

The team investigated the dependence of the reaction of AlN and AlGaN on different crystallographic planes that exist in crystalline materials during exfoliation using heated-pressurized water in an autoclave. The crystallographic planes reveal how the atoms or molecules are lined up in a material, shedding light on its mechanical, electrical, magnetic, optical, and other functional properties.

This involved the examination of four distinct samples of AlN, each characterized by different crystallographic planes, along with two additional samples of AlGaN on AlN, with a focus on specific a- and m-crystallographic planes, respectively. The heated-pressurized water was applied to the samples in a specific way to keep them under saturated vapor pressure.

Following the exfoliation treatment, the researchers analyzed the samples using cross-section scanning electron microscopy and quantified their properties using X-ray photoelectron spectroscopy. Their experiments revealed that the reaction of AlN and AlGaN during exfoliation was highly dependent on the orientation of crystallographic planes. They identified the main exfoliation mechanism as the etching of a- and m-planes with heated-pressurized water. Notably, there was no reaction in the +c-plane and an AlOOH-altered layer was formed in the –c-plane.

The proposed method demonstrated applicability across different types of AlGaN semiconductors with varying AlN compositions and proved suitable for semiconductor wafers with large diameters.

“The findings of the study have the potential to further propel the development of high-power UV-light emitting semiconductor devices, which can enable new applications in various industries that remain untapped until now,” remarks Prof. Iwaya, further emphasizing the importance of their work.

Going ahead, these findings can enhance our understanding of high-powered UV-light emitting devices and help in the realization of such high-powered lasers to replace the harmonics of existing gas and solid-state lasers. Here’s wishing the team kudos for their contribution towards advancing the development of such devices for a better future!