Brian P. Downey

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

Epitaxial metallic β-Nb2N films grown by MBE on hexagonal SiC substrates

RF-plasma MBE was used to epitaxially grow 4- to 100-nm-thick metallic β-Nb2N thin films on hexagonal SiC substrates. When the N/Nb flux ratios are greater than one, the most critical parameter for high-quality β-Nb2N is the substrate temperature. The X-ray characterization of films grown between 775 and 850 °C demonstrates β-Nb2N phase formation. The (0002) and X-ray diffraction measurements of a β-Nb2N film grown at 850 °C reveal a 0.68% lattice mismatch to the 6H-SiC substrate. This suggests that β-Nb2N can be used for high-quality metal/semiconductor heterostructures that cannot be fabricated at present.

Large-Signal RF Performance of Nanocrystalline Diamond Coated AlGaN/GaN High Electron Mobility Transistors

In this split-wafer study, we have compared the dc, pulsed, small and large signal RF electrical performance of nanocrystalline diamond (NCD) coated AlGaN/GaN high electron mobility transistors (HEMTs) to reference devices with silicon nitride passivation only. The NCD-coated HEMTs were observed to outperform reference devices in transconductance, large-signal gain, output power density, and power-added efficiency at 4 GHz. The measured improvements were suspected to be related to reduced dispersion and lower source access resistance afforded by the NCD film.

Epitaxial Lift-Off and Transfer of III-N Materials and Devices from SiC Substrates

In this paper, electrical characterization results of N-polar GaN high-electron-mobility transistors that have been released from a 6H-SiC wafer and manually transferred to a Si wafer using a novel epitaxial lift-off (ELO) technique are presented. This recently developed ELO method uses a thin sacrificial layer of Nb2N, a hexagonal epitaxial conductor with less than 1% lattice mismatch to 4H- and 6H-SiC, to serve as the template for III-N device heterostructure growth. Measured results of transferred devices indicate that electron transport properties and low power density electrical performance are nominally unchanged relative to values measured before release. This technique has several advantages over competing ELO techniques, such as the well-known smart cut method, including bonding-ready released material with atomically-smooth backsides (≤ 0.5 nm rms), easy substrate reclaim with indefinite recycling potential, and a transfer process that can be performed after full front-side device processing and yield screening has been completed.

Characterization of molecular beam epitaxy grown β-Nb2N films and AlN/β-Nb2N heterojunctions on 6H-SiC substrates

β-Nb2N films and AlN/β-Nb2N heterojunctions were grown by molecular beam epitaxy (MBE) on 6H-SiC. The epitaxial nature and β-Nb2N phase were determined by symmetric and asymmetric high-resolution X-ray diffraction (HRXRD) measurements, and were confirmed by grazing incidence diffraction measurements using synchrotron photons. Measured lattice parameters and the in-plane stress of β-Nb2N on 6H-SiC were c = 5.0194 A, a = 3.0558 A, and 0.2 GPa, respectively. The HRXRD, transmission electron microscopy, and Raman spectroscopy revealing epitaxial growth of AlN/β-Nb2N heterojunctions have identical orientations with the substrate, abrupt interfaces, and bi-axial stress of 0.88 GPa, respectively. The current finding opens up possibilities for the next generation of high-power devices that cannot be fabricated at present.

Thermally reflowed ZEP 520A for gate length reduction and profile rounding in T-gate fabrication

The characteristics of thermally reflowed ZEP 520A-7 (ZEP), a resist commonly used in electron beam lithography, are presented for use as a gate stem resist layer in T-gate process development. As-developed ZEP lines possess a resist sidewall profile that displays varying amounts of undercut, which are determined by the conditions used to expose the line. The authors find that after thermal reflow, the top of the ZEP profile becomes substantially rounded in shape, mitigating “metal cathedraling” problems, a yield-affecting issue that becomes more pronounced as the gate length is reduced. In addition to profile rounding, a linewidth reduction of over 100 nm is observed, and this process has been used to produce gate lengths in the 30–40 nm range. The changes in feature size and the final profile shape depend on the as-developed sidewall angle. Additionally, the ZEP reflow process saturates after a certain amount of time, so reproducibility is not hindered by a lack of precise control in timing. As larger l...

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.

Microstructure of Ti/Al/Ni/Au ohmic contacts for N-polar GaN/AlGaN high electron mobility transistor devices

The microstructure of Ti/Al/Ni/Au ohmic contacts on N-polar GaN/AlGaN high electron mobility transistor heterostructures annealed from 800 °C to 900 °C has been studied using transmission electron microscopy and associated analytical techniques. Two ohmic metal stacks with different Ti/Al/Ni/Au layer thicknesses (20/200/40/50 nm and 20/100/10/50 nm) have been examined. Samples with low ohmic contact resistance after annealing were found to have two common characteristics: (1) the top GaN channel layer had completely reacted with Ti metal to form a polycrystalline TiN layer and (2) a ∼5 nm-thick Au-rich layer was present near the TiN/AlGaN interface. Possible conduction mechanisms related to the presence of Au in low ohmic contact resistance samples are discussed.

Demonstration of GaN HyperFETs with ALD VO 2

Owing to strong electron-electron interactions, transition metal oxide materials can exhibit multiple phases with vastly different electronic, magnetic, structural, and thermal properties. Reversible control of the transitions between these phases by electronic means can give rise to completely novel devices which can provide new functionalities and help to overcome limits of traditional semiconductor devices [1, 2]. VO2 is a transition metal oxide material that exhibits a metal-insulator transition (MIT) at a temperature of ~67 C [3]. Recently, by coupling VO2 to the source of traditional semiconductor MOSFET devices, hybrid-phase-transition-FET (hyper-FET) devices were demonstrated [4]. These HyperFETs showed steep switching slope less than the room-temperature Boltzmann switching limit of ~60 mV/dec [4]. GaN based electronics has emerged as an enabler of high-speed and high-power RF and microwave electronics [5], and is currently being investigated intensively for next-generation high-voltage power electronics [6,7], as well as steep-switching based low-power digital electronics [8]. In this work, we combine ALD-grown VO2 with III-Nitride high-electron mobility transistors (HEMTs) to realize GaN-VO2 HyperFETs, demonstrating steep-switching behavior in a platform that is amenable to integration and scaling.

GaN/NbN epitaxial semiconductor/superconductor heterostructures

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor—an electronic gain element—to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance—a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.