Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

/pdf/the-2018-gan-power-electronics-roadmap-2iunv619fh.pdf

The 2018 GaN power electronics roadmap

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

This letter reports high-voltage GaN field-effect transistors fabricated on Si substrates. A halide-based plasma treatment was performed to enable normally off operation. Atomic layer deposition of Al2O3 gate insulator was adopted to reduce the gate leakage current. Incorporation of multiple field plates, with one field plate connected to the gate electrode and two field plates connected to the source electrode successfully enabled a high breakdown voltage of 1200 V and low dynamic on-resistance at high-voltage operation.

1200-V Normally Off GaN-on-Si Field-Effect Transistors With Low Dynamic on -Resistance

We performed hybrid functional calculations of native point defects and dangling bonds (DBs) in α-Al2O3 to aid in the identification of charge-trap and fixed-charge centers in Al2O3/III-V metal-oxide-semiconductor structures. We find that Al vacancies (VAl) are deep acceptors with transition levels less than 2.6 eV above the valence band, whereas Al interstitials (Ali) are deep donors with transition levels within ∼2 eV of the conduction band. Oxygen vacancies (VO) introduce donor levels near midgap and an acceptor level at ∼1 eV below the conduction band, while oxygen interstitials (Oi) are deep acceptors, with a transition level near the mid gap. Taking into account the band offset between α-Al2O3 and III-V semiconductors, our results indicate that VO and Al DBs act as charge traps (possibly causing carrier leakage), while VAl, Ali, Oi, and O DBs act as fixed-charge centers in α-Al2O3/III-V metal-oxide-semiconductor structures.

https://www.mrl.ucsb.edu/~vandewalle/publications/JAP113,044501(2013)-Choi-Al2O3-dielectric.pdf

Native point defects and dangling bonds in α-Al2O3

Structural and XPS characterization of ALD Al2O3 coated porous silicon

A method for integrating epitaxial, metallic transition metal nitride (TMN) layers within a compound semiconductor device structure. The TMN layers have a similar crystal structure to relevant semiconductors of interest such as silicon carbide (SiC) and the Group III-Nitrides (III-Ns) such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and their various alloys. Additionally, the TMN layers have excellent thermal stability and can be deposited in situ with other semiconductor materials, allowing the TMN layers to be buried within the semiconductor device structure to create semiconductor/metal/semiconductor heterostructures and superlattices.

Epitaxial metallic transition metal nitride layers for compound semiconductor devices

Ultrawide bandgap (UWBG) semiconductors can have higher figure of merit values than existing materials for power and radio frequency (rf) devices and are increasingly viewed as the next-generation semiconductors in high-power/temperature electronics. Gallium oxide (Ga2O3) is one of the UWBG semiconductors and has five different known polymorphs. Recently, there has been great interest in the most stable (β-Ga2O3) and metastable (ɛ- and α-Ga2O3) polymorphs of Ga2O3. In this chapter, we discuss molecular beam epitaxy (MBE) growth and characterization of β- and ɛ-Ga2O3. After a brief introduction, the chapter discusses MBE equipment considerations, amorphous Ga2O3 for gate dielectrics, heteroepitaxial growth of Ga2O3, and then focuses on homoepitaxial growth and metastable phase stabilization by MBE in Section 2.2 . Section 2.3 discusses current and future prospects, and finally a summary is presented in Section 2.4 .

MBE growth and characterization of gallium oxide

The sensitivity of the surface morphology and microstructure of N-polar-oriented InAlN to variations in composition, temperature, and layer thickness for thin films grown by plasma-assisted molecular beam epitaxy (PAMBE) has been investigated. Lateral compositional inhomogeneity is present in N-rich InAlN films grown at low temperature, and phase segregation is exacerbated with increasing InN fraction. A smooth, step-flow surface morphology and elimination of compositional inhomogeneity can be achieved at a growth temperature 50 °C above the onset of In evaporation (650 °C). A GaN/AlN/GaN/200-nm InAlN heterostructure had a sheet charge density of 1.7 × 1013 cm−2 and no degradation in mobility (1760 cm2/V s) relative to 15-nm-thick InAlN layers. Demonstration of thick-barrier high-electron-mobility transistors with good direct-current characteristics shows that device quality, thick InAlN layers can be successfully grown by PAMBE.

Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy

This chapter reviews our recent efforts on growth, fabrication, and characterization of GaN/AlN resonant tunneling diodes (RTDs). Working GaN/AlN RTDs were successfully demonstrated, and they could function well under the flux of very high current densities (e.g., ∼431 kA/cm2) without thermal breakdown. The high-speed nature of these devices was confirmed through switching experiments, achieving a 10–90% switching time of ≈55 ps. A fmax calculation shows a small-signal oscillation with frequency up to 164 GHz is possible. Unlike InGaAs/AlAs RTDs, the peak-to-valley current ratios (PVCRs) of GaN/AlN RTDs remain ∼1.5. Through computer modeling, temperature measurements, and material diagnosis, we reveal that there could be stronger inelastic scattering processes contributing to the valley current other than the coherent tunneling in the GaN/AlN RTDs. The possible inelastic mechanisms include optical phonons, interface roughness, and dislocations. Thus, the growth of high-quality GaN/AlN heterostructures and the evolution of bulk GaN substrates are critical for getting better performance devices. Finally, unipolar electroluminescence, without the presence of p-type doping, was observed in GaN/AlN RTDs. The interband tunneling process, which generates holes for the optical recombination, is likely due to the strong electric fields originating from the polarization effects native to wurtzite heterostructures.

Fabrication and Characterization of GaN/AlN Resonant Tunneling Diodes

A process for fabricating a suspended microelectromechanical system (MEMS) structure comprising epitaxial semiconductor functional layers that are partially or completely suspended over a substrate. A sacrificial release layer and a functional device layer are formed on a substrate. The functional device layer is etched to form windows in the functional device layer defining an outline of a suspended MEMS device to be formed from the functional device layer. The sacrificial release layer is then etched with a selective release etchant to remove the sacrificial release layer underneath the functional layer in the area defined by the windows to form the suspended MEMS structure.

David J. Meyer

Papers

Epitaxial metallic transition metal nitride layers for compound semiconductor devices

MBE growth and characterization of gallium oxide

Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy

Fabrication and Characterization of GaN/AlN Resonant Tunneling Diodes

Method for fabricating suspended mems structures