A team of researchers from China and Singapore have developed a self-powered photodetector capable of being used in a variety of applications, such as astronomical investigations, communications and chemical analysis to mention a few.
Photodetectors generally need an external voltage to provide the driving force that will help to separate and measure photo-generated electrons comprising of the detection.
The research team, headed by Junling Wang and Le Wang at Nanyang Technological University in Singapore, eliminated this requirement by coming up with a novel, stable and sensitive photodetector based on a semiconducting junction known as a GdNiO3/Nb-doped SrTiO3 (GNO/NSTO) p-n heterojunction.
The driving force for efficiently separating photo-generated carriers is provided by an inherent electric field at the GNO/NSTO interface, thus preventing the need for an external power source.
In addition to its self-powered feature, Wang and his research team report altering the material properties in order to attain broad sensitivity. For these compounds, most of the research conducted till now concentrated on analyzing the origin of metal-insulator transition, but these researchers took a different approach.
The properties of perovskite nickelates, referring to the specific class of solar cell materials in which this structure falls, are extremely sensitive to oxygen content. This sensitivity allows excellent tuning of the final electronic structures by varying the oxygen environment during the process of film deposition (constructing the heterojunction).
“Our work is novel and confirms that nickelates films have tunable band gaps with changing of the oxygen vacancy concentration, which makes them ideal as light absorbing materials in optoelectronic devices,” said Wang. “Using the self-powered photodetector we designed, we study its photo responsivity using light sources with different wavelengths, with significant photo-response appearing when the light wavelength decreases to 650 nanometers.” Wang said.
The process of establishing the appropriate energy structure or band structure available to electrons of the 10 nm thick GNO films stands out to be a major challenge in producing this photodetector.
“To obtain the band structures, we used both spectroscopic ellipsometry measurements and ultraviolet photoelectron spectroscopy (UPS) measurements,” said Wang. The researchers were able to plot the work functions and energy levels of the different components in the devices by using the deduced values for the optical bandgap from these measurements together with known values and limits for GNO films.
The researchers hope to carry out investigations for more materials with similar features. “One of the remarkable features of nickelates […] is the dependence of their physical properties on the chosen rare earth element,” said Wang. “Thus far, we have only studied GdNiO3 film, but besides that we can also investigate other “R”-NiO3 films where “R” can be Nd (neodymium), Sm (animony), Er (erbium) and Lu (lutetium) and study their potential applications in the photodetector.”
Additionally, the team plans to enhance the performance of the photodetector by including an insulating SrTiO3 (STO) layer inserted between the NSTO substrate and GdNiO3 film.
This novel work has immense potential for applications using optoelectronic devices. “We believe that this paper will stimulate further studies and enlarge the potential applications of systems based on nickelates,” said Wang.