Matthew T. Hardy

Charge control in N-polar InAlN high-electron-mobility transistors grown by plasma-assisted molecular beam epitaxy

N-polar InAlN-based high-electron-mobility transistors (HEMTs) have fundamental advantages relative to conventional Ga-polar AlGaN HEMTs for high frequency devices. An understanding of the epitaxial design space for controlling sheet carrier density (ns) and mobility (μ) is desirable to maximize power and frequency performance by improving breakdown voltage and reducing parasitic access resistance. In this work, the authors show that In0.17Al0.83N barrier thickness has a minimal impact on ns and μ, and an AlGaN cap layer decreases both ns and μ. Optimization of AlN and GaN interlayers can be used to maximize μ and set ns in the range of 1–3 × 1013 cm−2. The authors use this approach to demonstrate N-polar HEMTs grown on freestanding GaN substrates with sheet resistance Rs = 190 Ω/◻ and μ = 1400 cm2/V·s, leading to a maximum drain current density of 1.5 A/mm for HEMTs with a 5-μm source–drain spacing and Pt-based Schottky gates.

Surface preparation of freestanding GaN substrates for homoepitaxial GaN growth by rf-plasma MBE

The authors have investigated different methods for preparing the surfaces of freestanding, Ga-polar, hydride vapor-phase epitaxy grown GaN substrates to be used for homoepitaxial GaN growth by plasma-assisted molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy and secondary ion mass spectroscopy, respectively, were used to characterize the microstructure and to measure the concentrations of impurities unintentionally incorporated in the MBE-grown homoepitaxial GaN layers. Heating Ga-polar substrates to ∼1100 °C is as effective as a wet chemical clean for reducing impurity concentrations of oxygen, silicon, and carbon. The combination of an aggressive ex situ wet chemical clean with in situ Ga deposition and thermal desorption results in homoepitaxial GaN layer growth with very low residual impurity concentrations and without generating additional threading dislocations.

AlN/GaN/AlN resonant tunneling diodes grown by rf-plasma assisted molecular beam epitaxy on freestanding GaN

The authors report the growth by rf-plasma assisted molecular beam epitaxy of AlN/GaN/AlN resonant tunneling diodes which exhibit stable, repeatable, and hysteresis-free negative differential resistance (NDR) at room temperature for more than 1000 bias sweeps between −2.5 and +5.5 V. The device layers were grown on freestanding, Ga-polar GaN substrates grown by hydride vapor phase epitaxy and having a density of threading dislocations between 106 and 107 cm−2. The authors speculate that the repeatable NDR is facilitated by the low-dislocation density substrates.

Epitaxial ScAlN grown by molecular beam epitaxy on GaN and SiC substrates

ScxAl1-xN is a promising ultra-wide bandgap material with a variety of potential applications in electronic, optoelectronic, and acoustoelectric devices related to its large piezoelectric and spontaneous polarization coefficients. We demonstrate growth of ScxAl1-xN on GaN and SiC substrates using plasma-assisted molecular beam epitaxy with x = 0.14–0.24. For metal-rich growth conditions, mixed cubic and wurtzite phases formed, while excellent film quality was demonstrated under N-rich growth conditions at temperatures between 520 and 730 °C. An rms roughness as low as 0.7 nm and 0002 rocking curve full-width at half maximum as low as 265 arc sec were measured for a Sc0.16Al0.84 N film on GaN. To further demonstrate the quality of the ScAlN material, a high-electron-mobility transistor heterostructure with a Sc0.14Al0.86 N barrier, GaN/AlN interlayers, and a GaN buffer was grown on SiC, which showed the presence of a two-dimensional electron gas with a sheet charge density of 3.4 × 1013 cm−2 and a Hall mob...

XeF2 etching of epitaxial Nb2N for lift-off or micromachining of III-N materials and devices

This paper presents characterization of the effects of XeF2 vapor phase etching conditions on the lateral etch rate and etch uniformity of a sacrificial, epitaxial Nb2N layer grown between a III-N high-electron-mobility transistor heterostructure and a 6H-SiC substrate. To achieve uniform and repeatable lateral Nb2N removal, an etch temperature of 100 °C or higher was required, providing average etch rates ranging from 10 to 40 μm/min. A net compressive stress and positive strain gradient in the released III-N material were inferred from the buckling of clamped-clamped beams and the convex curvature of cantilever structures, respectively. XeF2 etching of epitaxial Nb2N sacrificial layers in III-N material structures allows for a highly selective, completely dry release process that is compatible with common micromachining and epitaxial lift-off techniques.

Plasma-assisted Molecular Beam Epitaxy of N-polar InAlN-barrier High-electron-mobility Transistors

Plasma-assisted molecular beam epitaxy is well suited for the epitaxial growth of III-nitride thin films and heterostructures with smooth, abrupt interfaces required for high-quality high-electron-mobility transistors (HEMTs). A procedure is presented for the growth of N-polar InAlN HEMTs, including wafer preparation and growth of buffer layers, the InAlN barrier layer, AlN and GaN interlayers and the GaN channel. Critical issues at each step of the process are identified, such as avoiding Ga accumulation in the GaN buffer, the role of temperature on InAlN compositional homogeneity, and the use of Ga flux during the AlN interlayer and the interrupt prior to GaN channel growth. Compositionally homogeneous N-polar InAlN thin films are demonstrated with surface root-mean-squared roughness as low as 0.19 nm and InAlN-based HEMT structures are reported having mobility as high as 1,750 cm2/V∙sec for devices with a sheet charge density of 1.7 x 1013 cm-2.

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

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.