Large-capacity integration of optical functional nanowires on the entire surface of a silicon wafer — Expectations for applications such as high-power solar cells Hokkaido University et al.
On February 7, 2023, Hokkaido University announced that a joint research group with the University of Tokyo and Ehime University had succeeded in large-capacity integration of gallium arsenide-based semiconductor nanowires with excellent light-emitting/light-receiving functions over the entire surface of a silicon wafer. .
Group III-V compound semiconductors, which combine elements from groups III and V of the periodic table, are used in LEDs, lasers, sensors, etc. due to their photoelectric conversion function and high electron mobility.
III-V crystals are usually difficult to epitaxially grow on silicon substrates due to the difference in thermal expansion coefficients of the constituent layers. However, epitaxial growth is possible for nanowires that form needle-like crystals through small openings.
Methods such as metal-organic vapor phase epitaxy and molecular beam epitaxy for large-scale synthesis of nanowires require lithography and pre-formation of fine patterns for the formation of micro-apertures for wire formation, as well as crystal formation conditions. It was generally considered unsuitable due to its limitations.
The research team used a commercially available 2-inch silicon (111) wafer as a substrate to fabricate nanowires using molecular beam epitaxy.
Gallium droplets were formed on a silicon wafer in order to use gallium itself, which is a constituent element used to grow gallium arsenide crystals, as a catalyst to promote the nucleation and growth of crystals.
As a result of examining the temperature of the substrate where the crystals are formed, the vapor pressure of the metal element to be supplied, the pressure inside the device, etc., it was found that high-density nanowires were uniformly formed over the entire surface of the wafer under appropriate conditions.
In addition, after forming the gallium arsenide nanowire, it was confirmed that by covering the surroundings with aluminum gallium arsenide, which has a high light and electron confinement effect, the electrons in the gallium arsenide part inside do not flow out to the outside and function well.
In addition, the structure was designed so that the naturally oxidized film acts as a protective layer on the surface of the wire exposed to air. It protects the wire surface for a long time and contributes to the improvement of optical properties.
The wires formed this time are GaAs/AlGaAs core-shell nanowires, in which an outer shell of aluminum gallium arsenide covers an inner core of gallium arsenide. About 6 µm in length and about 250 nm in diameter. The density is approximately 50 million lines/cm 2 , which is equivalent to approximately 700 million lines in the entire 2-inch substrate. In the future, it will also be possible to form device structures.
Due to light scattering and absorption by the nanowires, the reflectance of visible wavelengths is less than 2%, and the original mirror surface of the silicon wafer changes to appear black. In addition to having an emission intensity equal to or higher than that of a commercially available gallium arsenide substrate, the emission wavelength became uniform over the entire wafer.
In the nanowire group, an efficient light absorption promoting effect was also obtained. It is expected to contribute to increasing the output of solar cells and adding inexpensive and high-performance optical functions to silicon technology.