III-V semiconductors are the preferred choice for RF, Power and space/ defense applications. Though expensive, they offer many advantages over silicon like higher power handling capacity, break down voltage etc.
Fig.1 Typical HEMT structure with 2DEG at AlGaN/GaN interface
We are familiar with the concept of two-dimensional electron gas(2DEG) in III-V semiconductors which form the basis of High Electron Mobility Transistor(HEMT).
The 2DEG is created due to a triangular quantum well formation at the heterojunction(between semiconductor materials) causing quantum confinement.
Fig.2 Typical HEMT structure with 2DE G at AlGaN/GaN interface
As ionized impurities are absent(to induce electrons), the scattering is minimal, leading to higher mobility(hence current) versus Si-based devices.The electron density in case of III-Nitride 2DEG is about ∼5 × 1013 cm−2.
In 2004, the turf of III-V semiconductors was challenged when researchers found the presence of high-density 2DEG at an oxide interface(LaAlO3 & SrTiO3) which exceeded the values seen in III-Nitride 2DEG. Similarly, GdTiO3-SrTiO3 heterostructure grown on an oxide substrate has demonstrated 2DEG electron density >3 × 1014 cm−2 . Devices based on these oxides have the potential of being used as sensors, Field Effect Transistors(FETs) etc.
Fig.3 Band diagram of STO-LAO interface with 2DEG
The only hindrance to realizing the full potential of oxides for Optronics application is the current use of single crystal oxide substrates for material/film growth.
For commercial viability, we would like to use single crystal substrates like silicon(001) or (111) with some demonstrations already being made for adoption
- LTO-STO heterostructure on Si(100) with a 2DEG charge density of 8.9 × 1014 cm−2
- GdTiO3-SrTiO3 on Si(001) with a 2DEG charge density of ∼9 × 1013 cm−2
Taking another step in this direction, researchers at Yale University have added a new substrate for the growth of oxides – GaAs(III-V material). They have been able to grow GdTiO3-SrTiO3 heterostructure on GaAs wafer using MBE. The electron density obtained at the GTO–STO interface is ∼2 × 1014 cm−2 .
In the future, post optimization and scaling, the devices fabricated on these films can act as the baseline for the next-generation electronic and optical devices.
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