Quaternary alloy ScAlGaN: A promising strategy to improve the quality of ScAlN

Abstract

ScAlN is an emerging ultrawide bandgap semiconductor for next-generation radio frequency electronic devices. Here, we show that the material quality of ScAlN grown by molecular beam epitaxy can be drastically improved by alloying with Ga. The resulting quaternary alloy ScAlGaN exhibits a single-phase wurtzite structure, atomically smooth surface, high crystal quality, sharp interface, and low impurity concentration. Most significantly, oxygen impurity incorporation in ScAlGaN is found to be three to four orders of magnitude lower compared to that for ScAlN grown on AlN templates utilizing a similar Sc source. We further demonstrate that ScAlGaN/GaN superlattices exhibit clear periodicity with sharp interfaces. Moreover, GaN high electron mobility transistors with high sheet electron density and high mobility have been realized using ScAlGaN as a barrier. This work provides a viable approach for achieving high-quality Sc-III-N semiconductors that were not previously possible and further offers additional dimensions for bandgap, polarization, interface, strain, and quantum engineering.

Self-Rolled-Up Aluminum Nitride-Based 3D Architectures Enabled by Record-High Differential Stress

“…Based on the above equations, it is clear that the AlN layer is under a small tensile strain as the lattice constant of AlN is smaller (∼19%) than that of Si and the thermal expansion coefficient (CTE) of AlN is larger (∼47%) than that of Si . However, in the Al 1– x Sc x N/AlN stack, the lattice parameter (along the a -axis) and the thermal expansion coefficient of the MBE-grown Al 1– x Sc x N increase with increasing Sc composition. ,, Therefore, based on the lattice-mismatch strain, the Al 1– x Sc x N top layer should be under biaxial compression (approx. −11 GPa ( x = 0.3)) relative to AlN, but interestingly, the Al 1– x Sc x N/AlN stack rolls upward and the diameter decreases as the Sc composition is increased .…”

“…Besides stress-engineering, our CMOS-compatible planar processing technique is versatile enough to integrate heterogeneous materials into the S-RuM structures. Incorporating other functional materials, such as SiN x and Al x Sc 1– x N, together with AlN to fabricate 3D tubular structures can give rise to another degree of freedom toward furthering the performance in the field of high-electron-mobility transistors (HEMTs), − piezoelectric and ferroelectric devices, and on-chip nonlinear optics Figure a–c shows schematics of the heterogeneous bilayer stacks that we rolled up to cover a range of diameters.…”

“…58 However, in the Al 1−x Sc x N/AlN stack, the lattice parameter (along the aaxis) and the thermal expansion coefficient of the MBE-grown Al 1−x Sc x N increase with increasing Sc composition. 8,52,53 Therefore, based on the lattice-mismatch strain, the Al 1−x Sc x N top layer should be under biaxial compression (approx. −11 GPa (x = 0.3)) relative to AlN, but interestingly, the Al 1−x Sc x N/AlN stack rolls upward and the diameter decreases as the Sc composition is increased.…”

“…59 This indicates that the delta stress originating due to E f and α AlN − α AlScN is increasingly positive as we move vertically above the Al 1−x Sc x N/AlN stack and is much greater than the calculated value of +5.9 GPa (x = 0.3). 52,53 Note that our experiments are limited to a Sc composition of x = 0.3, as beyond that, the thermal-expansion mismatch tends to exceed the fracture toughness of the Al 1−x Sc x N film, and the film starts to crack. 8 To better understand the embedded strain, we used COMSOL Multiphysics to simulate the diameter of the rolled-up Al range of the Young's modulus.…”

An Epitaxial Ferroelectric ScAlN/GaN Heterostructure Memory

“…By fitting the I – V curves and tuning the carrier concentration of the GaN semiconductor electrode, the conductance modulation has been explained as electrical polarization engineering at the heterostructure interface. The memory effect exhibited weak dependence on operation temperature, maintaining a maximum rectifying ratio of ≈10 even at 670 K. With further optimization of the growth conditions, [ 63 ] electrode materials, device structures and especially the ferroelectric layer thickness, Sc‐III‐N‐based memory devices with low operation voltage, high rectifying ratio, long retention time, and good endurance resistance comparable to other state‐of‐the‐art memory devices are possible. These results suggest that ScAlN‐based ferroelectric/III‐nitride heterostructures can be potential candidates for ferroelectric‐resistive memory, and are anticipated to open the route toward next‐generation memristors and all nitride‐based monolithic integrated logic circuits for power‐efficient applications and harsh environments.…”