Increasing the mobility and power-electronics figure of merit of AlGaN with atomically thin AlN/GaN digital-alloy superlattices

At a Glance

Metadata	Details
Publication Date	2022-07-18
Journal	Applied Physics Letters
Authors	Nick Pant, Woncheol Lee, Nocona Sanders, Emmanouil Kioupakis
Institutions	University of Michigan
Citations	15
Analysis	Full AI Review Included

Executive Summary

Core Value Proposition: Atomically thin AlN/GaN superlattices (SLs), or digital alloys, are proposed as a superior active material for ultra-wide band-gap (UWBG) power electronics, eliminating the performance-limiting alloy scattering present in random AlGaN alloys.
Mobility Enhancement: The 1ML (one-monolayer) SL exhibits phonon-limited electron mobility (369 cm² V^-1 s^-1 lateral) that is 3 to 4 times greater than the total mobility of random Al_0.5Ga_0.5N alloys.
Record Figure of Merit: The 1ML SL achieves the highest modified Baliga Figure of Merit (MBFOM) (up to 11.4 GW/cm²) among all known UWBG semiconductors with experimentally demonstrated dopability (including GaN, 4H-SiC, beta-Ga₂O₃, cBN, and diamond).
Performance Gain: The MBFOM of the 1ML SL is 300-400% greater than the current state-of-the-art relaxed GaN technology.
Doping and Integration Advantage: SLs achieve high performance at a lower effective Al composition (50%), avoiding the severe doping inefficiency (DX transition) and high contact resistance issues associated with the Al-rich (>80% Al) random alloys required for comparable band gaps.
Feasibility: These chemically ordered nanostructures are compatible with existing industrial growth techniques (e.g., MBE, MOVPE) and are predicted to be experimentally feasible for thick stacks (~60 nm).

Technical Specifications

Parameter	Value	Unit	Context
1ML SL Band Gap (E_G)	4.8	eV	Theoretical quasiparticle value
1ML SL Vertical Mobility (µ_\|\|)	452	cm² V^-1 s^-1	Room temperature, phonon-limited
1ML SL Lateral Mobility (µ_⊥)	369	cm² V^-1 s^-1	Room temperature, phonon-limited
1ML SL Vertical MBFOM	11.4	GW/cm²	Highest calculated MBFOM
1ML SL Lateral MBFOM	9.3	GW/cm²	Highest calculated MBFOM
1ML SL Breakdown Field (F_br)	6.2	MV/cm	Estimated based on E_G scaling
1ML SL Effective Al Content	50	%	Equivalent composition (Al_0.5Ga_0.5N)
Random Al_0.5Ga_0.5N Mobility (Total)	115	cm² V^-1 s^-1	Total mobility (alloy + phonon scattering)
GaN Reference Mobility (Relaxed)	830	cm² V^-1 s^-1	Experimental value (Table 4)
MBFOM Comparison (1ML SL vs. GaN)	~400	% greater	Performance improvement for vertical transport
MBFOM Comparison (1ML SL vs. beta-Ga₂O₃)	~3	times larger	Factor increase in MBFOM
Conduction Band Offset (1ML SL/AlN)	1.0	eV	For integration with dielectrics
Estimated Critical Thickness (SL on AlN)	~60	nm	For pseudomorphic growth
Dopant Ionization Energy (E_D)	15	meV	Assumed for 50% Al content

Key Methodologies

The electronic structure and transport properties were determined using predictive first-principles calculations:

Structural Relaxation: Ground-state crystal structures (GaN, AlN, 1ML SL, 2ML SL) were relaxed by minimizing total energy, ensuring all forces were less than 10^-3 Ry/Bohr. All materials were pseudomorphically lattice-matched to the basal c-plane of AlN.
Electronic Structure Calculation: Band structure and phonon calculations were performed using Quantum ESPRESSO based on Density-Functional Theory (DFT) in the Local-Density Approximation (LDA).
Band Gap Correction: Many-body quasiparticle corrections were applied using the G₀W₀ approximation (BerkeleyGW) to obtain accurate band gaps and effective masses.
Electron-Phonon Coupling: Ab initio electron-phonon matrix elements were calculated using Density-Functional Perturbation Theory (DFPT) at the LDA level.
Phonon-Limited Mobility: The phonon-limited mobility was obtained by iteratively solving the linearized Boltzmann Transport Equation (BTE) using the EPW code, integrating all interband and intraband scattering processes across the Brillouin zone.
Alloy-Scattering Mobility: The alloy-scattering-limited mobility for random AlGaN was calculated using an in-house code in the relaxation-time approximation (RTA), valid due to the elastic nature of alloy scattering.
Modified BFOM Calculation: The MBFOM was calculated by multiplying the standard Baliga Figure of Merit (BFOM) by the dopant ionization ratio (η). The breakdown field (F_br) was estimated phenomenologically based on the band gap (F_br ∝ E_G²). The dopant ionization energy (E_D) was derived by empirically fitting experimental data for Si in AlGaN.

Commercial Applications

The atomically thin AlN/GaN superlattices are designed for high-performance, high-power applications in the UWBG semiconductor market:

Next-Generation Power Electronics: Active regions for high-voltage switches, diodes, and transistors (e.g., HEMTs, FETs).
High-Efficiency Power Conversion: Devices requiring minimal conduction losses and high tolerance to electric fields.
Electrical Infrastructure: Components for the future electrical grid, including high-voltage transmission and distribution systems.
Transportation Electrification: Power modules for electric rail, aviation, and high-power electric vehicle charging systems.
Thermal Management Critical Devices: Applications where high thermal conductivity is essential for performance and reliability (III-nitrides offer significantly higher thermal conductivity than competitors like beta-Ga₂O₃).
Ambipolar Devices: As III-nitrides support both n-type and p-type doping, these SLs are suitable for ambipolar high-power devices.

View Original Abstract

Alloy scattering in random AlGaN alloys drastically reduces the electron mobility and, therefore, the power-electronics figure of merit. As a result, Al compositions greater than 75% are required to obtain even a twofold increase in the Baliga figure of merit compared to GaN. However, beyond approximately 80% Al composition, donors in AlGaN undergo the DX transition, which makes impurity doping increasingly more difficult. Moreover, the contact resistance increases exponentially with the increase in Al content, and integration with dielectrics becomes difficult due to the upward shift of the conduction band. Atomically thin superlattices of AlN and GaN, also known as digital alloys, are known to grow experimentally under appropriate growth conditions. These chemically ordered nanostructures could offer significantly enhanced figure of merit compared to their random alloy counterparts due to the absence of alloy scattering, as well as better integration with contact metals and dielectrics. In this work, we investigate the electronic structure and phonon-limited electron mobility of atomically thin AlN/GaN digital-alloy superlattices using first-principles calculations based on density-functional and many-body perturbation theory. The bandgap of the atomically thin superlattices reaches 4.8 eV, and the in-plane (out-of-plane) mobility is 369 (452) cm2 V−1 s−1. Using the modified Baliga figure of merit that accounts for the dopant ionization energy, we demonstrate that atomically thin AlN/GaN superlattices with a monolayer sublattice periodicity have the highest modified Baliga figure of merit among several technologically relevant ultra-wide bandgap materials, including random AlGaN, β-Ga2O3, cBN, and diamond.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0097963

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