Low threshold voltage blue light emitting diodes based on thulium doped gallium oxides

Lightening, which accounts for about 20% of total energy consumption globally, is an indispensable integral part of human lives. Generally, light sources need to be stable, safe, efficient and most importantly, environmentally friendly. Among the available light sources, light-emitting diodes (LEDs) is fundamental. Currently, white LEDs are the most commonly used in various lighting applications. There are two main ways of obtaining white light emission; one by coating blue LEDs with yellow emitting phosphors and second by fabricating a combination of red, green and blue light. Contrastingly, high-resolution displays require full-color red, blue and green LEDs to achieve broad and bright color combinations.

Presently, most studies on high-resolution displays are primarily concentrated on micro-LEDs. For example, InGaN/GaN-based materials with various structures such as quantum wells have been grown on single substrates to achieve full-color micro-LEDs. Unfortunately, the emission wavelengths of these structures are highly susceptible to various environmental conditions that degrade their bandgap, presenting a serious problem. Recently, rare earth ions doped wide bandgap semiconductors have drawn significant research attention as a potential alternative luminescent material for addressing this fundamental problem because their narrow emission line and emission wavelengths are rarely affected by temperature.

Using gallium oxides (Ga₂O₃) doped with rare-earth ions as the host materials, green and red LEDs based on Er-Ga₂O₃ and Eu-Ga₂O₃ respectively, as well as a combination of red and green light have been successfully fabricated in the previous studies. Additionally, blue light emission has been found to be independent of temperature and of significant importance in obtaining white or full-color LEDs. Nevertheless, blue LEDs based on Ga₂O₃ as the host material are yet to be reported despite their potential importance in developing solid-state LEDs.

On this account, researchers of Prof. Guo’s group from Saga University in Japan reported the fabrication of low threshold voltage blue LEDs based on thulium-doped gallium oxides (Tm-Ga₂O₃). In their approach, Tm-Ga₂O₃ films were deposited on silicon substrates via pulsed laser deposition technique. The structure and electrical performance of the fabricated Tm-Ga₂O₃/Si LEDs, including the electroluminescence (EL) spectra, was investigated at various voltage-currents and compared to those of Tm-GaN/Si devices. The work is currently published in the research journal, Applied Physics Express.

The research team reportedly observed blue emissions with naked eyes at 476 nm and 460 nm wavelengths from the fabricated LEDs. The current-voltage characteristics measurements revealed that the blue emissions were observed at a lower threshold voltage of 6.3V, relatively lower than that of Tm-GaN/Si devices. The blue emissions were attributed to ¹G₄–³H₆ and ¹D₂–³F₄ transitions. Furthermore, combining the red and green emissions obtained from Eu-Ga₂O₃ and Er-Ga₂O₃, respectively, the authors reported the strong blue emission could realize full-color LEDs with a single Ga₂O₃ host.

In summary, LEDs based on Tm-GaO₃ films were successfully fabricated by depositing the films on silicon substrates. As a result, blue emissions from the Tm-Ga₂O₃/Si LEDs and their and EL characteristics were reported. Compared with the existing host materials like GaN and ZnO, Ga₂O₃ exhibit a wider bandgap, suggesting that it is a promising host material when doped with rare earth elements. Using semiconductor materials with a wide bandgap can enhance the luminous efficiency of rare earth ion dopants. In a statement to Advances in Engineering, Prof. Guo said their study pave the way for more efficient integration of Ga₂O₃ based full-color LEDs with conventional Si technology.