Showing papers by "David J. Meyer published in 2018"

GaN/NbN epitaxial semiconductor/superconductor heterostructures.

Abstract: 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.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures

Abstract: Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.0074% at 18.8 mA. In contrast to traditional GaN light emitters, p-type doping and p-contacts are completely avoided, and instead, holes are created in the GaN on the emitter side of the tunneling structure by direct interband (that is, Zener) tunneling from the valence band to the conduction band on the collector side. The Zener tunneling is enhanced by the high electric fields (~5 × 106 V cm−1) created by the notably large polarization-induced sheet charge at the interfaces between the AlN and GaN. Narrow-band ultraviolet electroluminescence has been observed from unipolar GaN/AlN heterostructures. Ever since the realization of GaN light-emitting diodes in the early 1990s, GaN photonics has been growing steadily, but a significant bottleneck has been p-type GaN contacts, which are hard to grow and to realize uniform injection of carriers. Now, Paul Berger of Ohio State University and co-workers have developed devices that eliminate the need for p-type GaN doping by using a bipolar tunneling scheme for charge injection. The light emission of the devices was centered on a wavelength of 360 nanometers and had a narrow spectral width of less than 16 nanometers. While the devices had an optical power on the microwatt scale and a low quantum efficiency, the team anticipates that their performance can be improved by subsequent optimization.

431 kA/cm 2 peak tunneling current density in GaN/AlN resonant tunneling diodes

Abstract: We report on the design and fabrication of high current density GaN/AlN double barrier resonant tunneling diodes grown via plasma assisted molecular-beam epitaxy on bulk GaN substrates. A quantum-transport solver was used to model and optimize designs with high levels of doping and ultra-thin AlN barriers. The devices displayed repeatable room temperature negative differential resistance with peak-to-valley current ratios ranging from 1.20 to 1.60. A maximum peak tunneling current density (Jp) of 431 kA/cm2 was observed. Cross-gap near-UV (370–385 nm) electroluminescence (EL) was observed above +6 V when holes, generated from a polarization induced Zener tunneling effect, recombine with electrons in the emitter region. Analysis of temperature dependent measurements, thermal resistance, and the measured EL spectra revealed the presence of severe self-heating effects.

X-Band Power and Linearity Performance of Compositionally Graded AlGaN Channel Transistors

Abstract: We report on the power and linearity performance of metal organic chemical vapor deposition grown AlGaN channel polarization-graded field-effect transistor (PolFET). The fabricated transistors with 3-D electron channels showed nearly flat transconductance profiles. Maximum ${f}_{T}$ and ${f}_{\text {max}}$ of 23 and 65 GHz were measured for 0.7- $\mu \text{m}$ gate-length transistors, corresponding to an ${f}_{T}$ - ${L}_{G}$ product of 16.2 GHz $\cdot \mu \text{m}$ . Load-pull measurement at 10 GHz revealed a maximum output power of 2 W/mm with a maximum small signal gain of 16 dB. Two-tone measurement at 10 GHz showed an OIP3 of 33 dBm for 150- $\mu \text{m}$ device width and a corresponding linearity figure-of-merit OIP3/ $\text{P}_{\text {DC}}$ of 3.4 dB. These results suggest that PolFETs could be useful for high-frequency applications requiring high linearity.

Steep Sub-Boltzmann Switching in AlGaN/GaN Phase-FETs With ALD VO 2

Abstract: We report the first demonstration of ultralow leakage AlGaN/GaN MOS-HEMTs on silicon substrates loaded at the source with atomic layer deposition (ALD)-grown VO2 metal–insulator transition (MIT) material resistors. The resulting GaN phase-transition FET (phase-FET) shows steep sub-Boltzmann switching in its transfer characteristics in both sweep directions with slopes of ~9.9 and ~28.2 mV/decade at 60 °C. The control MOS-HEMTs and the phase-FETs show more than 12-order on/off ratio with ultralow off-state leakage. Because of the MIT, the drain current and the transconductance also show unconventional behavior. This first demonstration of ultralow leakage steep switching in GaN phase-FETs using integration-friendly ALD VO2 opens the door to introducing new functionalities in nitride low-power digital devices, microwave circuits, photonic devices, and power electronics in the GaN-on-silicon platform.

Device Modeling of Graded III-N HEMTs for Improved Linearity

Abstract: When GaN HEMTs are used in power amplifier applications, their performance falls well short of ideal due to power-gain roll-off that results from having a peaked transconductance characteristic. A promising design solution involves compositionally grading the channel, and we here formulate a numerical device model to explore this approach that couples linear electroelasticity, diffusion-drift transport with new mobility models, and density-gradient theory. Lumped modeling of the large-signal behavior is also developed to explore the power amplifier performance. Preliminary results presented here indicate that the graded-channel idea has value, especially for gate lengths greater than about 100nm.

Scandium Aluminum Nitride as an Emerging Material for High Power Transistors

Abstract: Incorporating a novel ultra-wide bandgap material, ScAlN, as the layer material in III-nitride highelectron-mobility transistors has the potential to improve output power at millimeter wave frequencies Lattice-matched ScAlN can be grown by molecular beam epitaxy with high interfacial quality and phase purity Varying the ScAlN barrier thickness from 3–25 nm results in sheet charge densities of 20–32 × 1013 cm−2and electron mobilities as high as 1060 cm 2 /V·s

Selective oxidation of transition metal nitride layers within compound semiconductor device structures

Abstract: Methods for integrating transition metal oxide (TMO) layers into a compound semiconductor device structure via selective oxidation of transition metal nitride (TMN) layers within the structure.

Noise Measurements of High-Speed, Light-Emitting GaN Resonant-Tunneling Diodes

Abstract: We report here the first RF noise measurements on two designs of n-doped GaN/AlN double-barrier resonant tunneling diodes (RTDs), each having a room-temperature negative differential resistance (NDR) and also strong near-UV light emission. The measurements are made with a standard, un-isolated RF receiver and calibration is made using a substitution-resistor/hot-cold radiometric technique which works in the positive differential resistance (PDR) region but not the NDR region. A high-quality InGaAs/AlAs double-barrier RTD is used as a control sample and displays shot noise suppression down to $\Gamma\approx$0.5 in the PDR region, as expected. The GaN/AlN RTDs display both shot-noise enhancement and suppression in the PDR regions, but no obvious sign of sudden shot-noise enhancement in the threshold bias region of light emission. This supports the hypothesis that the holes required for light emission are created by electronic (Zener) interband tunneling, not impact ionization. Further the minimum shot-noise factor of $\Gamma\sim$ 0.34 suggests that the GaN/AlN RTDs are acting like triple-barrier devices.