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Tunnel junctions could have a greatimpact on gallium nitride andaluminum nitride-based devices such as light-emitting diodes and lasersby overcoming critical challenges related to hole injection and p-contacts.This paper demonstrates the use of GdN nanoislands to enhance interbandtunneling and hole injection into GaN p–n junctions by severalorders of magnitude, resulting in low tunnel junction specific resistivity(1.3 × 10–3Ω-cm2) comparedto the previous results in wide band gap semiconductors. Tunnel injectionof holes was confirmed by low-temperature operation of GaN p–njunction with a tunneling contact layer, and strong electroluminescencedown to 20 K. The low tunnel junction resistance combined with lowoptical absorption loss in GdN is very promising for incorporationin GaN-based light emitters. [ABSTRACT FROM AUTHOR]

Ultra-wide band gap semiconductor devices based on β-phase gallium oxide (Ga 2 O 3 ) offer the potential to achieve higher switching performance and efficiency and lower manufacturing cost than that of today's wide band gap power electronics. However, the most critical challenge to the commercialization of Ga 2 O 3 electronics is overheating, which impacts the device performance and reliability. We fabricated a Ga 2 O 3 /4H-SiC composite wafer using a fusion-bonding method. A low-temperature (≤600 °C) epitaxy and device processing scheme was developed to fabricate MOSFETs on the composite wafer. The low-temperature-grown epitaxial Ga 2 O 3 devices deliver high thermal performance (56% reduction in channel temperature) and a power figure of merit of (∼300 MW/cm 2 ), which is the highest among heterogeneously integrated Ga 2 O 3 devices reported to date. Simulations calibrated based on thermal characterization results of the Ga 2 O 3 -on-SiC MOSFET reveal that a Ga 2 O 3 /diamond composite wafer with a reduced Ga 2 O 3 thickness (∼1 μm) and a thinner bonding interlayer (<10 nm) can reduce the device thermal impedance to a level lower than that of today's GaN-on-SiC power switches.

Monoclinic β-Ga 2 O 3 , an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon-phonon coupling that dictates electron transport properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga 2 O 3 films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using ab initio based electron-phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm 2 V -1 s -1 followed by a surprising decrease with an increasing carrier density due to the plasmon-phonon coupling. The modeling and experimental data further reveal an important "antiscreening" (of electron-phonon interaction) effect arising from dynamic screening from the hybrid plasmon-phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm 2 V -1 s -1 is possible if high electron densities (>10 20 cm -3 ) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga 2 O 3 and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon-phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron-phonon interactions.

β-phase gallium oxide (Ga 2 O 3 ) is an emerging ultrawide bandgap (UWBG) semiconductor ( E G ∼ 4.8 eV), which promises generational improvements in the performance and manufacturing cost over today's commercial wide bandgap power electronics based on GaN and SiC. However, overheating has been identified as a major bottleneck to the performance and commercialization of Ga 2 O 3 device technologies. In this work, a novel Ga 2 O 3 /4H-SiC composite wafer with high heat transfer performance and an epi-ready surface finish has been developed using a fusion-bonding method. By taking advantage of low-temperature metalorganic vapor phase epitaxy, a Ga 2 O 3 epitaxial layer was successfully grown on the composite wafer while maintaining the structural integrity of the composite wafer without causing interface damage. An atomically smooth homoepitaxial film with a room-temperature Hall mobility of ∼94 cm 2 /Vs and a volume charge of ∼3 × 10 17 cm -3 was achieved at a growth temperature of 600 °C. Phonon transport across the Ga 2 O 3 /4H-SiC interface has been studied using frequency-domain thermoreflectance and a differential steady-state thermoreflectance approach. Scanning transmission electron microscopy analysis suggests that phonon transport across the Ga 2 O 3 /4H-SiC interface is dominated by the thickness of the SiN x bonding layer and an unintentionally formed SiO x interlayer. Extrinsic effects that impact the thermal conductivity of the 6.5 μm thick Ga 2 O 3 layer were studied via time-domain thermoreflectance. Thermal simulation was performed to estimate the improvement of the thermal performance of a hypothetical single-finger Ga 2 O 3 metal-semiconductor field-effect transistor fabricated on the composite substrate. This novel power transistor topology resulted in a ∼4.3× reduction in the junction-to-package device thermal resistance. Furthermore, an even more pronounced cooling effect is demonstrated when the composite wafer is implemented into the device design of practical multifinger devices. These innovations in device-level thermal management give promise to the full exploitation of the promising benefits of the UWBG material, which will lead to significant improvements in the power density and efficiency of power electronics over current state-of-the-art commercial devices.

Heteroepitaxy of β-phase gallium oxide (β-Ga 2 O 3 ) thin films on foreign substrates shows promise for the development of next-generation deep ultraviolet solar blind photodetectors and power electronic devices. In this work, the influences of the film thickness and crystallinity on the thermal conductivity of (2̅01)-oriented β-Ga 2 O 3 heteroepitaxial thin films were investigated. Unintentionally doped β-Ga 2 O 3 thin films were grown on c -plane sapphire substrates with off-axis angles of 0° and 6° toward ⟨112̅0⟩ via metal-organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the β-Ga 2 O 3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the β-Ga 2 O 3 films were measured via time-domain thermoreflectance. The interface quality was studied using scanning transmission electron microscopy. The measured thermal conductivities of the submicron-thick β-Ga 2 O 3 thin films were relatively low as compared to the intrinsic bulk value. The measured thin film thermal conductivities were compared with the Debye-Callaway model incorporating phononic parameters derived from first-principles calculations. The comparison suggests that the reduction in the thin film thermal conductivity can be partially attributed to the enhanced phonon-boundary scattering when the film thickness decreases. They were found to be a strong function of not only the layer thickness but also the film quality, resulting from growth on substrates with different offcut angles. Growth of β-Ga 2 O 3 films on 6° offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c -plane sapphire. However, the β-Ga 2 O 3 films grown on 6° offcut sapphire exhibit a lower thermal boundary conductance at the β-Ga 2 O 3 /sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown (2̅01)-oriented β-(Al x Ga 1- x ) 2 O 3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the β-(Al x Ga 1- x ) 2 O 3 films exhibit lower thermal conductivities (2.8-4.7 W/m·K) than the β-Ga 2 O 3 thin films. The dominance of the alloy disorder scattering in β-(Al x Ga 1- x ) 2 O 3 is further evidenced by the weak temperature dependence of the thermal conductivity. This work provides fundamental insight into the physical interactions that govern phonon transport within heteroepitaxially grown β-phase Ga 2 O 3 and (Al x Ga 1- x ) 2 O 3 thin films and lays the groundwork for the thermal modeling and design of β-Ga 2 O 3 electronic and optoelectronic devices.

Since the advent of the first computers, chemists have been at the forefront of using computers to understand and solve complex chemical problems. As the hardware and software have evolved, so have the theoretical and computational chemistry methods and algorithms. Parallel computers clearly changed the common computing paradigm in the late 1970s and 80s, and the field has again seen a paradigm shift with the advent of graphical processing units. This review explores the challenges and some of the solutions in transforming software from the terascale to the petascale and now to the upcoming exascale computers. While discussing the field in general, NWChem and its redesign, NWChemEx, will be highlighted as one of the early codesign projects to take advantage of massively parallel computers and emerging software standards to enable large scientific challenges to be tackled.

A novel parallel algorithm for noniterative multireference coupled cluster (MRCC) theories, which merges recently introduced reference-level parallelism (RLP) [Bhaskaran-Nair, K.; Brabec, J.; Aprà, E.; van Dam, H. J. J.; Pittner, J.; Kowalski, K. J. Chem. Phys.2012, 137, 094112] with the possibility of accelerating numerical calculations using graphics processing units (GPUs) is presented. We discuss the performance of this approach applied to the MRCCSD(T) method (iterative singles and doubles and perturbative triples), where the corrections due to triples are added to the diagonal elements of the MRCCSD effective Hamiltonian matrix. The performance of the combined RLP/GPU algorithm is illustrated on the example of the Brillouin-Wigner (BW) and Mukherjee (Mk) state-specific MRCCSD(T) formulations.

The details of the graphical processing unit (GPU) implementation of the most computationally intensive (T)-part of the recently introduced regularized CCSD(T) (Reg-CCSD(T)) method [ Kowalski , K. ; Valiev , M. J. Chem. Phys. 2009 , 131 , 234107 ] for calculating electronic energies of strongly correlated systems are discussed. Parallel tests performed for several molecular systems show very good scalability of the triples part of the Reg-CCSD(T) approach. We also discuss the performance of the Reg-CCSD(T) GPU implementation as a function of the parameters defining the partitioning of the spinorbital domain (tiling structure). The accuracy of the Reg-CCSD(T) method is illustrated on three examples: the methyfluoride molecule, dissociation of dodecane, and open-shell Spiro cation (5,5'(4H,4H')-spirobi[cyclopenta[c]pyrrole] 2,2',6,6'-tetrahydro cation), which is a frequently used model to study electron transfer processes. It is demonstrated that a simple regularization of the cluster amplitudes used in the noniterative corrections accounting for the effect of triply excited configurations significantly improves the accuracies of ground-state energies in the presence of strong quasidegeneracy effects. For methylfluoride, we compare the Reg-CCSD(T) results with the CR-CC(2,3) and CCSDT energies, whereas for Spiro cation we compare Reg-CCSD(T) results with the energies obtained with completely renormalized CCSD(T) method. Performance tests for the Spiro, dodecane, and uracil molecules are also discussed.