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Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency >99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding ( f × Q ) and phonon lifetimes up to 1.36 × 10 17  Hz and 500 µs respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits. Acoustic resonators may find application for qubit coupling in compact quantum information and processing systems. Here the authors show a multi-phonon source with high quality factors and long phonon lifetimes via epitaxial high-overtone bulk acoustic resonators.

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. Group III/nitride semiconductors have been grown epitaxially on the superconductor niobium nitride, allowing the superconductor’s macroscopic quantum effects to be combined with the semiconductors’ electronic, photonic and piezoelectric properties. Mix and match The perfect epitaxial growth of one crystalline semiconductor on another is a fundamental feature of many high-performance electronic and optoelectronic devices. Rusen Yan and colleagues demonstrate that a similar level of epitaxial integration can be achieved between the group III nitride semiconductors and the superconducting nitride metal NbN x . This ability to grow highly ordered, high-quality semiconducting structures directly on a crystalline superconductor provides a route for exploring a host of new device possibilities that combine the properties of the two subsystems.

This paper presents a new and comprehensive equivalent circuit model for high overtone bulk acoustic resonators (HBARs). HBARs demonstrate several very sharp resonance modes distributed nearly periodically over a very wide frequency range. This spectrum response of HBARs offers unique advantages but poses significant modeling challenges. The proposed circuit incorporates and models the unique physical components of the HBAR: piezoelectric transducer, substrate (a perfectly periodic multimode cavity), piezoelectric coupling, and critically, the imperfectly matched transducer-substrate interface which imparts characteristic aperiodicity to the HBAR mode spectrum. By judicious use of fixed, periodic, or tightly constrained virtual lumped-element branches, and sets of branches, the model retains clear and intuitive links to the physical device, while reducing the complexity needed for fitting dense, broadband datasets. We demonstrate the validity and power of this model by simultaneously fitting measured data for 61 modes of a GaN/NbN/sapphire HBAR over a span of 1 GHz, and extracting modal parameters such as quality factors and coupling coefficients. We show that this new model is compact and yet scalable: by leveraging the inherent internal relationships in an HBAR, the model can be easily expanded to include multiple transducer overtones and envelopes, multiple distinct transducers, and spurious modes. In addition to fitting measured datasets, the new model can also be used to easily analyze various perturbations to the nominal state of the HBAR. We expect the new model to be useful for the design of classical HBAR-based oscillators, filters, and sensors, and for the integration of HBARs into quantum circuits.

The UL-94 Vertical Burning Flammability Test (UL-94V) is used to measure flammability characteristics of plastic materials. The results of the test allow for plastic materials to be separated into classification categories. These categories will be discussed and related to fire phenomena. Simulations of the test have allowed for the development of general flame height and heat flux correlations. We believe these are independent of the actual solid fuels. In addition, the heat flux from the ignition burner, a specified premixed flame, has been measured. These data provide the basis for assessing fire behavior of materials using their fire properties such as heat of combustion, heat of gasification, ignition temperature, and thermal properties. Criteria for ignition, sustained burning, and flame spread are determined. These outcomes are then related to the UL-94V classification categories. An analysis of melting is also considered in order to assess the flaming drip aspect of the test.

ScN alloyed AlN (ScxAl1-xN, ScAlN) is a wurtzite semiconductor with attractive ferroelectric, dielectric, piezoelectric, and optical properties. Here, we show that ScAlN films (with x spanning 0.18 to 0.36) contain nanoscale Sc-rich clusters which maintain the wurtzite crystal structure. While both molecular beam epitaxy (MBE) and sputter deposited Sc0.3Al0.7N films show Sc clustering, the degree of clustering is significantly stronger for the MBE-grown film, offering an explanation for some of the discrepancies between MBE-grown and sputtered films reported in the literature. Moreover, the MBE-grown Sc0.3Al0.7N film exhibits a dispersive and anomalously large dielectric permittivity, roughly double that of sputtered Sc0.3Al0.7N. We attribute this result to the Sc-rich clusters locally reaching x ~ 0.5 and approaching the predicted ferroelectric-to-paraelectric phase transition, resulting in a giant (local) enhancement in permittivity. The Sc-rich clusters should similarly affect the piezoelectric, optical, and ferroelectric responses, suggesting cluster-engineering as a means to tailor ScAlNs functional properties.

The materials properties of SiC, such as wide bandgap, high breakdown electric field, and good thermal conductivity, make it an appealing option for high temperature and high power applications. The replacement of Si devices with SiC components could lead to a reduction in device size, weight, complexity, and cooling requirements along with an increase in device efficiency. One area of concern under high temperature or high current operation is the stability of the ohmic contacts. Ohmic contact degradation can cause an increase in parasitic resistance, which can diminish device performance. While contact studies have primarily focused on the high temperature stability of ohmic contacts to SiC, different failure mechanisms may arise under high current density stressing due to the influence of electromigration. In addition, preferential degradation may occur at the anode or cathode due to the directionality of current flow, known as a polarity effect. The failure mechanisms of ohmic contacts to p-type SiC under high current density stressing are explored. Complementary materials characterization techniques were used to analyze contact degradation, particularly the use of cross-sections prepared by focused ion beam for imaging using field emission scanning electron microscopy and elemental analysis using Auger electron spectroscopy. Initially the degradation of commonly studied Ni and Al-based contacts was investigated under continuous DC current. The contact metallization included a bond pad consisting of a TiW diffusion barrier and thick Au overlayer. The Ni contacts were found to degrade due to the growth of voids within the ohmic contact layer, which were initially produced during the high temperature Ni/SiC ohmic contact anneal. The Al-based contacts degraded due to the movement of Al from the ohmic contact layer to the surface of the Au bond pad, and the movement of Au into the ohmic contact layer from the bond pad. The inequality of Al and Au fluxes generated voiding within the ohmic contact layer causing a large increase in contact resistance. A bottom to top approach was used to develop a more robust contact structure based on the failure mechanisms of the Ni and Al-based contacts. Contacts utilizing a Pd layer contacting the SiC were found to provide a lower specific contact resistance (ρc) and improved stability under current stressing. A Pd/Ti contact was introduced that when annealed under a N2 atmosphere produced a robust TiN layer at the surface of the contact. The ρ c of the Pd/Ti contact was (4.7±1.7)×10−6 Ω cm2, compared to the Ni and Al-based contacts, all of which had a ρc of greater than 10−5 Ω cm2. The Pd/Ti contacts were able to withstand higher currents than the Ni or Al-based contacts under continuous DC current stressing. The degradation mechanism of the Pd/Ti contacts depended on whether the current was pulsed or continuous. Under continuous DC stressing, Au from the bond pad diffused through the TiW barrier and into the ohmic contact region leading to severe intermixing and voiding. Under pulsed DC stressing, voiding at the Au/TiW interface occurred caused by the electromigration of Au. The different degradation mechanisms were related to the temperature during stressing, as the temperature of the continuous DC stressed contacts exceeded 649°C at failure, while the peak temperature of the pulsed DC contacts was between 316°C and 371°C, using 5 µs pulses and a 10% duty cycle. Finally, the lowest ρc, (1.4±0.6)×10 −6 Ω cm2, was attained with a Pd/Ti/Pt contact, which also possessed a very smooth surface morphology, especially compared to the conventional Ti/Al contact. The Pd/Ti/Pt contacts were also shown to be more stable under continuous DC lateral current stressing than the Ti/Al contacts. A polarity effect on temperature was observed during stressing with the temperature of the cathode being higher than the anode, likely due to carrier recombination at the cathode for the p-type material. The increased temperature caused preferential degradation of the cathode of both the Pd/Ti/Pt and Ti/Al contacts. Under continuous DC stressing, degradation of the Pd/Ti/Pt contacts was characterized by voiding and intermixing at the leading edge of the cathode. The temperature of the Pd/Ti/Pt contacts during stressing was reduced under pulsed DC current, and voiding was observed between the Au bond pad and the ohmic contact. Mechanical stresses and thermal cycling were suspected to have produced the voiding under pulsed DC stressing.

Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency > 99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding fxQ products and phonon lifetimes up to 1.36 x 10^17 Hz and 500 microseconds respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits.

We present a comprehensive study of dielectric properties including complex permittivity, loss, and leakage of high-ScN-fraction ScAlN thin films grown using molecular beam epitaxy (MBE). Dielectric spectroscopy is carried out on high-ScN-fraction (30%-40% ScN fraction) samples from 20 Hz to 10 GHz. We find that real permittivity ' increases significantly with increasing ScN fraction; a trend confirmed by density functional theory. Further, ' is strongly dispersive with frequency and increasing ScN fraction, with values for Sc0.4Al0.6N varying from 150 down to 60 with increasing frequency. Loss, dispersion, and DC leakage current correspondingly increase with ScN fraction. The high ' and strongly dispersive behavior in MBE ScAlN are not observed in a sputter-deposited ScAlN control with a similar ScN fraction, highlighting fundamental differences between films produced by the two deposition methods. Microscopy and spectroscopy analyses are carried out on MBE- and sputter-deposited samples to compare microstructure, alloy, and dopant concentration.

We report a method to obtain insights into lower thermal conductivity of \\beta-Ga2O3 thin films grown by molecular beam epitaxy (MBE) on c-plane sapphire and 4H-SiC substrates. We compare experimental values against the numerical predictions to decipher the effect of boundary scattering and defects in thin-films. We used time domain thermoreflectance (TDTR) to perform the experiments, density functional theory and the Boltzmann transport equation for thermal conductivity calculations, and the diffuse mismatch model for TBC predictions. The experimental thermal conductivities were approximately 3 times smaller than those calculated for perfect Ga2O3 crystals of similar size. When considering the presence of grain boundaries, gallium and oxygen vacancies, and stacking faults in the calculations, the crystals that present around 1% of gallium vacancies and a density of stacking faults of 106 faults/cm were the ones whose thermal conductivities were closer to the experimental results. Our analysis suggests the level of different types of defects present in the Ga2O3 crystal that could be used to improve the quality of MBE-grown samples by reducing these defects and thereby produce materials with higher thermal conductivities.

A dual-channel AlN/GaN/AlN/GaN high electron mobility transistor (HEMT) architecture is proposed, simulated, and demonstrated that suppresses gate lag due to surface-originated trapped charge. Dual two-dimensional electron gas (2DEG) channels are utilized such that the top 2DEG serves as an equipotential that screens potential fluctuations resulting from surface trapped charge. The bottom channel serves as the transistor's modulated channel. Two device modeling approaches have been performed as a means to guide the device design and to elucidate the relationship between the design and performance metrics. The modeling efforts include a self-consistent Poisson-Schrodinger solution for electrostatic simulation as well as hydrodynamic three-dimensional device modeling for three-dimensional electrostatics, steady-state, and transient simulations. Experimental results validated the HEMT design whereby homo-epitaxial growth on free-standing GaN substrates and fabrication of same-wafer dual-channel and recessed-gate AlN/GaN HEMTs have been demonstrated. Notable pulsed-gate performance has been achieved by the fabricated HEMTs through a gate lag ratio of 0.86 with minimal drain current collapse while maintaining high levels of dc and rf performance.