The rapid development of semiconductor technology offers a wide possibility of various systems implementation. In particular, light-emitting diodes (LEDs) have vast areas of applications including displays, automotive light and traffic signals. This can be attributed to their inherent physical characteristics that enable their efficient and reliable operation at both low current and voltages as well as severe conditions such as extremely high temperatures. Among the available light-emitting diodes, high-quality red-light light-emitting diodes are highly preferred for various applications. They are generally grown on lattice-matched GaAs substrate using metal-organic chemical vapor deposition. Unfortunately, the use of GaAs substrates for the fabrication of light-emitting diodes limits the light extraction efficiency of mostly the red and yellow light-emitting diodes due to the high light absorption nature of the material. Therefore, the development of alternative and efficient methods for fabrication of light-emitting diodes are highly desirable.
Recently, wafer bonding technology has been adopted for light-emitting diodes fabrication. It involves replacing the light absorbing GaAs substrate to enhance the efficiency. Additionally, the high brightness of the wafer is realized by transferring the epilayers to Si substrate and CuW metal substrates despite the difference in the thermal coefficients. The substrates are thereafter exposed to thinning action. Moreover, thinning of CuW substrates has remained a challenge. In a recently published literature, AlGaInP light-emitting diodes with patterned copper substrates have been fabricated. This, however, does not offer a perfect solution to the aforementioned challenges due to the cracking of the epilayers during thermal annealing processes.
To this note, researchers at the National Chiao Tung University led by Professor Ray-Hua Horng developed a new metal copper-invar-copper substrate based on the wafer bonding and epilayer transferring technologies. The copper-invar-copper substrate comprised of three layers: the top layer made of 20µm copper, middle Invar layer of 64µm and bottom layer made of 20µm copper. Additionally, the Invar layer was made up of iron and nickel metals mixed in the ratio 7:3. The authors computed the thermal expansion coefficient of the copper-invar-copper substrate and compared it to that of the GaAs substrate and AlGaInP epilayers. Eventually, the authors evaluated the performances of the resulting light-emitting diode packages with the copper-invar-copper substrate. The work is currently published in the research journal, Optics Express.
The authors observed that the coefficients of the copper-invar-copper substrate were approximately similar to that of GaAs substrate and AlGaInP epilayers. On the other hand, the high thermal conductivity of the copper-invar-copper substrate highly contributed to the excellent performance of the resulting light-emitting diodes. This closely followed a low redshift and high output power phenomenon. According to the authors, the copper-invar-copper substrate required a thorough cleaning by acid to improve the mechanical properties of the bonding material.
In summary, Professor Ray-Hua Horng and her research team successfully designed a novel composite metal substrate for the fabrication of thin-film AlGaInP light-emitting diode devices. Using the copper-invar-copper substrate to replace the original GaAs substrate prevented absorption of red light and by improving on the heat dissipation. Thus, the resulting light-emitting diodes exhibited normal electrical and optical properties. Considering that the epilayer could be transferred to the copper-invar-copper substrate without the need for thinning, copper-invar-copper is considered a promising substrate for the fabrication of high-efficiency thin-film light-emitting diodes with vertical electrodes for numerus applications.
Horng, R., Sinha, S., Lee, C., Feng, H., Chung, C., & Tu, C. (2019). Composite metal substrate for thin film AlGaInP LED applications. Optics Express, 27(8), A397.