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

Technical Documentation & Analysis: AlN/GaN Superlattices for UWBG Power Electronics

Executive Summary

This research demonstrates that atomically thin AlN/GaN digital-alloy superlattices (SLs) are highly promising candidates for next-generation ultra-wide band-gap (UWBG) power electronics, achieving record performance figures through the elimination of alloy scattering.

Record Performance: The 1 Monolayer (ML) AlN / 1 ML GaN superlattice exhibits the highest Modified Baliga Figure of Merit (MBFOM) among all known UWBG semiconductors with experimentally demonstrated dopability.
Mobility Enhancement: The SL structure eliminates alloy scattering, resulting in a phonon-limited electron mobility 3.1x (in-plane) to 3.8x (out-of-plane) greater than that of random Al_0.5Ga_0.5N alloys.
Figure of Merit: The vertical-transport MBFOM for the 1ML SL is 11.4 GW/cm², representing a performance improvement of up to 400% compared to state-of-the-art GaN technology.
Diamond Comparison: The 1ML SL MBFOM is approximately 10,000 times greater than the MBFOM of diamond (1.1 x 10^-3 GW/cm²), highlighting the competitive landscape in UWBG materials.
Integration Advantage: The SLs offer lower specific contact resistance and better integration with dielectrics compared to high-Al-content random AlGaN alloys, addressing critical technological hurdles.
Feasibility: The structures are compatible with existing industrial growth techniques (MBE, MOCVD) and are estimated to be experimentally feasible with pseudomorphic stack thicknesses up to ~60 nm on AlN substrates.

Technical Specifications

Data extracted from Tables 3 and 4, focusing on the high-performance 1ML Superlattice and comparative UWBG materials.

Parameter	Value	Unit	Context
Material System	AlN/GaN Digital-Alloy Superlattice (1ML)	N/A	Pseudomorphically strained to AlN
Effective Al Composition	50	%	Achieves high performance while maintaining efficient doping
Electronic Band Gap (E_G)	4.8	eV	Calculated (Theory, this work)
In-Plane Mobility ($\mu_{\perp}$)	369	cm² V^-1 s^-1	Lateral transport (Room Temperature)
Out-of-Plane Mobility ($\mu_{\parallel}$)	452	cm² V^-1 s^-1	Vertical transport (Room Temperature)
Breakdown Field (F_br)	6.2	MV/cm	Estimated based on E_G scaling
Baliga Figure of Merit (BFOM)	22 (		) / 18 ($\perp$)
Modified BFOM (MBFOM)	11.4 (		) / 9.3 ($\perp$)
Diamond MBFOM (Reference)	1.1 x 10^-3	GW/cm²	Factor of ~10,000 lower than 1ML SL
Estimated Critical Thickness (t_crit)	~60	nm	Feasible thickness for pseudomorphic growth on AlN
Static Dielectric Constant ($\epsilon_{s, \parallel} / \epsilon_{0}$)	9.9	N/A	Out-of-plane (1ML SL)

Key Methodologies

The research relies on advanced computational physics to predict material performance, focusing on eliminating disorder effects inherent in random alloys.

First-Principles Calculations: Electronic structure and transport properties were investigated using Density-Functional Theory (DFT), Density-Functional Perturbation Theory (DFPT), and Many-Body Perturbation Theory (MBPT, specifically the G₀W₀ approximation) to ensure accurate band gaps and effective masses.
Phonon-Limited Mobility: Electron mobility was calculated by iteratively solving the linearized Boltzmann Transport Equation (BTE), incorporating ab initio electron-phonon matrix elements derived from DFPT.
Structural Simulation: Atomically thin AlN/GaN superlattices (1ML/1ML and 2ML/2ML) were modeled under pseudomorphic strain, lattice-matched to the basal c-plane of a bulk AlN substrate.
Relaxation Criteria: Ground-state crystal structures were determined by minimizing total energy, requiring forces to be less than 10^-3 Ry/Bohr and total energy convergence within 10^-4 Ry.
Figure of Merit Quantification: The Modified Baliga Figure of Merit (MBFOM) was used, which accounts for the critical factor of dopant ionization efficiency, calculated for a fixed electron density of 10¹⁸ cm^-3.
Alloy Scattering Mitigation: The enhanced mobility was attributed to the chemical ordering of the superlattice structure, which effectively removes the detrimental alloy scattering mechanism present in random AlGaN.

6CCVD Solutions & Capabilities

The development of high-performance UWBG devices, such as the AlN/GaN superlattices proposed in this work, necessitates advanced material substrates and precise fabrication capabilities. 6CCVD provides the critical foundation and customization required to realize these next-generation power electronics.

Applicable Materials for UWBG Device Integration

While the active material is AlN/GaN, the high power density and thermal requirements of UWBG devices demand superior thermal management, a core strength of 6CCVD’s diamond products.

6CCVD Material	Relevance to AlN/GaN Research	Key Specification
Optical Grade SCD	Thermal Management Substrate: Diamond offers the highest known thermal conductivity (~2000 W/mK), essential for dissipating heat generated by high-power AlN/GaN devices, especially since III-nitrides are already superior to $\beta$-Ga₂O₃ (20 W/mK).	SCD plates up to 500 µm thick; Ra < 1 nm polishing for direct heteroepitaxy or bonding.
Polycrystalline Diamond (PCD)	Large-Area Heat Spreading: Cost-effective solution for large-area power modules or wafers (e.g., 100mm or 125mm) requiring robust thermal dissipation.	Wafers/Plates up to 125mm diameter; Thickness up to 500 µm.
Boron-Doped Diamond (BDD)	Electrochemical/Contact Research: Although the paper focuses on Si-doping in AlGaN, BDD is a highly conductive UWBG material useful for advanced contact layers or electrochemical sensing related to UWBG material characterization.	Custom doping levels and thicknesses available.

Customization Potential for Superlattice Devices

The successful integration of AlN/GaN superlattices into functional devices requires precise material handling, thin-film deposition, and custom contacting schemes. 6CCVD’s in-house capabilities directly support these requirements:

Custom Metalization Services: The paper discusses the challenge of high specific contact resistance and the need for ohmic contacts (e.g., Ti- or V/Zr-based). 6CCVD offers internal metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu deposition, allowing researchers to prototype and test various contact schemes directly on their diamond or other substrate materials.
Precision Thin Films: The AlN/GaN SLs are thin films (~60 nm stack thickness). 6CCVD specializes in providing ultra-thin SCD and PCD films, starting at 0.1 µm (100 nm), ideal for advanced heterostructure research, bonding, or transfer processes.
Custom Dimensions and Polishing: 6CCVD provides custom dimensions and laser cutting services for plates and wafers up to 125mm (PCD). We guarantee ultra-smooth surfaces (Ra < 1 nm for SCD) necessary for high-quality epitaxial growth or direct integration with the AlN substrates used in this research.

Engineering Support

6CCVD’s in-house PhD team specializes in UWBG material science and thermal management solutions. We offer consultation services to researchers and engineers working on similar Ultra-Wide Band Gap Power Electronics projects. We can assist with:

Optimizing diamond substrate selection (SCD vs. PCD) based on thermal load and cost constraints.
Designing custom metal stacks for low-resistance ohmic contacts on UWBG materials.
Providing material specifications and characterization data (e.g., thermal conductivity, surface roughness, crystallographic orientation) critical for replicating or extending this superlattice research.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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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