High Mobility Multiple-Channel AlScN/GaN Heterostructures

TL;DR

This work addresses the persistent challenge of achieving low sheet resistance without sacrificing mobility in GaN-based transistors by leveraging AlScN as a barrier and introducing GaN/AlN interlayers to spatially separate the 2DEG from the AlScN. It employs molecular beam epitaxy to create both single- and multi-channel AlScN/GaN heterostructures with strain-balanced design, and uses structural (RSM, AFM) and electrical (Hall effect) measurements supplemented by Schrödinger-Poisson band calculations to validate the design. The results show that single-channel structures can reach mobilities up to ~4160 cm^2/Vs at 77 K with RT sheet resistances as low as ~170 Ω/□, and that multi-channel stacks achieve ultra-low sheet resistances of ~65 Ω/□ for three channels and ~45 Ω/□ for five channels at room temperature, dropping to ~21 Ω/□ and ~13 Ω/□ at 2 K, respectively, with carrier densities approaching 10^14 cm^-2. These findings place AlScN/GaN multi-channel heterostructures on par with state-of-the-art AlN/GaN, AlGaN/GaN, and AlInN/GaN platforms, offering a scalable pathway to high-speed, high-power GaN electronics and potential cryogenic device applications.

Abstract

Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy, and demonstrate the lowest sheet resistance among AlScN-based systems reported to date. Assorted schemes of GaN/AlN interlayers are first introduced in single-channel structures between AlScN and GaN to improve conductivity, increasing electron mobility up to $1370$ cm$^{2}$/V$\cdot$s at 300 K and $4160$ cm$^{2}$/V$\cdot$s at 77 K, reducing the sheet resistance down to 170 $Ω/\square$ and 70 $Ω/\square$ respectively. These improvements are then leveraged in multi-channel heterostructures, reaching sheet resistances of 65 $Ω/\square$ for three channels and 45 $Ω/\square$ for five channels at 300 K, further reduced to 21 $Ω/\square$ and 13 $Ω/\square$ at 2 K, respectively, confirming the presence of multiple 2DEGs. Structural characterization indicates pseudomorphic growth with smooth surfaces, while partial barrier relaxation and surface roughening are observed at high scandium content, with no impact on mobility. This first demonstration of ultra-low sheet resistance multi-channel AlScN/GaN heterostructures places AlScN on par with state-of-the-art multi-channel Al(In)N/GaN systems, showcasing its capacity to advance existing and enable new high-speed, high-power electronic devices.

Figures (10)

• Figure 1: (a) Cross-sectional schematic and (b) energy band diagram of a single-channel AlScN/GaN heterostructure.
• Figure 2: AlScN barrier thickness and composition required for strain balance in AlScN/AlN heterostructures strained on GaN, calculated by Eq. \ref{['strain-balance']}. Open circles serve as examples of strain-balanced configurations.
• Figure 3: Experimentally measured sheet resistance of single-channel Al$_{0.88}$Sc$_{0.12}$N/GaN heterostructures at 300 K (solid) and 77 K (dashed) with assorted interlayer (IL) configurations. Sheet carrier density and mobility are listed in Table \ref{['SC-Hall-table']}.
• Figure 4: Temperature-dependent (a) sheet carrier density, (b) Hall mobility, and (c) sheet resistance of single-channel Al$_{0.88}$Sc$_{0.12}$N/GaN samples SC1, SC5 and SC9.
• Figure 5: (a) Cross-sectional schematic and (b) energy band diagram of a multi-channel AlScN/GaN heterostructure with a 1 nm GaN/2 nm AlN interlayer.