Search Results (47)

Charge carrier traps due to crystal defects in GaN on Si HEMT devices are responsible for dynamic performance degradation, long-term reliability limitations, and peculiar failure modes. The behavior of traps depends on many variables including heterostructure quality, the specific device structure, and operating conditions. To study the short time dynamics of charge trapping and release on the threshold voltage shift and hysteresis of commercial normally off GaN HEMTs we use triangular 0–5 V gate voltage pulses in the μs to ms duration range. Measurements are performed for single pulses by varying pulse duration and for a train of a few pulses by varying their number. The results indicate that hysteresis and related threshold voltage shift occur after repeated pulses, suggesting an accumulation of trapped charges. However, for a triangular wave hysteresis vanishes, meaning that a dynamic balance between charge trapping and release is established in the device. This can be considered as a positive indicator of device robustness and reliability. The same method, used to measure the gate threshold voltage shift and dynamic R_ON after a 30 min off-state DC stress at V_DS = 55 V with a floating gate, highlights an appreciable performance degradation of the device. Full article

Thermal accumulation under high output power densities is one of the most significant challenges for GaN power devices. Diamond, with its ultra-high thermal conductivity, offers great potential for improving heat dissipation in high-power GaN devices. In this study, nanocrystalline diamond (NCD) passivated high-electron mobility transistors (HEMTs) based on AlGaN/GaN-on-Si heterostructures were fabricated with a gate length of 2 μm. The NCD film has a thickness of 250–383 nm and a uniform morphology with a grain size of mostly ~240 nm. Compared to the devices without NCD passivation, those devices with the NCD passivation layer show an increase in current density from 447 mA/mm to 555 mA/mm, a reduction in on-resistance from 20 Ω·mm to 13 Ω·mm, and a noticeable suppression of current degradation at high-drain voltages. Junction temperature measurements under varied output power densities reveal a 36% improvement in heat dissipation efficiency with the NCD passivation. These results fully demonstrate the promising potential of NCD for enhancing heat dissipation in high-power GaN devices. Full article

The energy band alignment of a stacked Si/GaN heterostructure was investigated using X-ray photoelectron spectroscopy (XPS) depth profiling, highlighting the influence of the amorphous interface region on the electronic properties. The crystalline Si/GaN pn heterostructure was formed by stacking a Si nanomembrane onto a GaN epi-substrate. The amorphous layer formed at the stacked Si/GaN interface altered the energy band of the stacked heterostructure and affected the injection of charge carriers across the junction interface region. This study revealed the interfacial upward energy band bending of the stacked Si/GaN heterostructure with surface potentials of 0.99 eV for GaN and 1.14 eV for Si, attributed to the formation of the amorphous interface. These findings challenge the conventional electron affinity model by accounting for interfacial bonding effects. Electrical measurements of the stacked Si/GaN pn heterostructure diode exhibited a rectifying behavior, consistent with the XPS-determined energy band alignment. The diode outperformed early design with a low leakage current density of 5 × 10⁻⁵ A/cm² and a small ideality factor of 1.22. This work underscores the critical role of the amorphous interface in determining energy band alignment and provides a robust methodology for optimizing the electronic performance of stacked heterostructures. The XPS-based approach can be extended to analyze and develop multi-layered bipolar devices. Full article

In this study, metal–silicide-based contacts to GaN-cap/AlGaN/AlN-spacer/GaN-on-Si heterostructure were investigated. Planar Schottky diodes with Cu-covered anodes comprising silicide layers of various metal–silicon (M–Si) compositions were fabricated and characterized in terms of their electrical parameters and thermal stability. The investigated contacts included Ti–Si, Ta–Si, Co–Si, Ni–Si, Pd–Si, Ir–Si, and Pt–Si layers. Reference diodes with pure Cu or Au/Ni anodes were also examined. To test the thermal stability, selected devices were subjected to subsequent annealing steps in vacuum at incremental temperatures up to 900 °C. The Cu/M–Si anodes showed significantly better thermal stability than the single-layer Cu contact, and in most cases exceeded the stability of the reference Au/Ni contact. The work functions of the sputtered thin layers were determined to support the discussion of the formation mechanism of the Schottky barrier. It was concluded that the barrier heights were dependent on the M–Si composition, although they were not dependent on the work function of the layers. An extended, unified Schottky barrier formation model served as the basis for explaining the complex electrical behavior of the devices under investigation. Full article

Developing cost-effective methods to synthesize large-size GaN films remains a challenge owing to the high dislocation density during heteroepitaxy. Herein, AlGaN/GaN HEMTs were grown on 6- and 8-inch Si(111) substrates using metal–organic chemical vapor deposition, and their basic properties and dislocation evolution characteristics were investigated thoroughly. With the insertion of a 100 nm thin AlGaN buffer layer, bow–warp analysis of the epitaxial wafers revealed excellent stress control for both the 6- and 8-inch wafers. HR-XRD and AFM analyses validated the high crystal quality and step-flow growth mode of GaN. Further, Hall measurements demonstrated the superior transport performance of AlGaN/GaN heterostructures. It is worth noting that dislocations tended to annihilate in the AlN nucleation layer, the thin AlGaN buffer layer, and the GaN buffer layer in the initial thickness range of 200–300 nm, which was indicated by ADF-STEM. To be specific, the heterointerfaces exhibited a significant effect on the annihilation of c-type (b = <0001>) dislocations, which led to the formation of dislocation loops. The thin inserted layers within the AlGaN buffer layer played a key role in promoting the annihilation of c-type dislocations, while they exerted less influence on a-type (b = 1/3<11$\overline{2}$0>) and (a+c)-type (b = 1/3<11$\overline{2}$3>) dislocations. Within an initial thickness of 200–300 nm in the GaN buffer layer, a-type and (a+c)-type dislocations underwent strong interactions, leading to considerable dislocation annihilation. In addition, the EELS results suggested that the V-shaped pits in the AlN nucleation layer were filled with the AlGaN thin layer with a low Al content. Full article

The high transport characteristics of AlGaN/GaN heterostructures are critical components for high-performance electronic and radio-frequency (RF) devices. We report the transport characteristics of AlGaN/GaN heterostructures grown on a high-resistivity (HR) Si(111) substrate, which are unevenly distributed in the central and edge regions of the wafer. The relationship between the composition, stress, and polarization effects was discussed, and the main factors affecting the concentration and mobility of two-dimensional electron gas (2DEG) were clarified. We further demonstrated that the mechanism of changes in polarization intensity and scattering originates from the uneven distribution of Al composition and stress in the AlGaN barrier layer during the growth process. Furthermore, our results provide an important guide on the significance of accomplishing 6 inch AlGaN/GaN HEMT with excellent properties for RF applications. Full article

In this work, a novel enhancement-mode GaN p-MISFET with a buried back gate (BBG) is proposed to improve the gate-to-channel modulation capability of a high drain current. By using the p-GaN/AlN/AlGaN/AlN double heterostructure, the buried 2DEG channel is tailored and connected to the top metal gate, which acts as a local back gate. Benefiting from the dual-gate structure (i.e., top metal gate and 2DEG BBG), the drain current of the p-MISFET is significantly improved from −2.1 (in the conv. device) to −9.1 mA/mm (in the BBG device). Moreover, the dual-gate design also bodes well for the gate to p-channel control; the subthreshold slope (SS) is substantially reduced from 148 to ~60 mV/dec, and such a low SS can be sustained for more than 3 decades. The back gate effect and the inherent hole compensation mechanism of the dual-gate structure are thoroughly studied by TCAD simulation, revealing their profound impact on enhancing the subthreshold and on-state characteristics in the BBG p-MISFET. Furthermore, the decent device performance of the proposed BBG p-MISFET is projected to the complementary logic inverters by mixed-mode simulation, showcasing excellent voltage transfer characteristics (VTCs) and dynamic switching behavior. The proposed BBG p-MISFET is promising for developing GaN-on-Si monolithically integrated complementary logic and power devices for high efficiency and compact GaN power IC. Full article

Two-dimensional materials are widely used as a new generation of functional materials for photovoltaic, photocatalyst, and nano-power devices. Strain engineering is a popular method to tune the properties of two-dimensional materials so that performances can be improved or more applications can be obtained. In this work, a two-dimensional heterostructure is constructed from SiC and GaN monolayers. Using first-principle calculations, the SiC/GaN heterostructure is stacked by a van der Waals interaction, acting as a semiconductor with an indirect bandgap of 3.331 eV. Importantly, the SiC/GaN heterostructure possesses a type-II band structure. Thus, the photogenerated electron and hole can be separated in the heterostructure as a potential photocatalyst for water splitting. Then, the external biaxial strain can decrease the bandgap of the SiC/GaN heterostructure. From pressure to tension, the SiC/GaN heterostructure realizes a transformation from a type-II to a type-I semiconductor. The strained SiC/GaN heterostructure also shows suitable band alignment to promote the redox of water splitting at pH 0 and 7. Moreover, the enhanced light-absorption properties further explain the SiC/GaN heterostructure’s potential as a photocatalyst and for nanoelectronics. Full article

Power electronics are becoming increasingly more important, as electrical energy constitutes 40% of the total primary energy usage in the USA and is expected to grow rapidly with the emergence of electric vehicles, renewable energy generation, and energy storage. New materials that are better suited for high-power applications are needed as the Si material limit is reached. Beta-phase gallium oxide (β-Ga₂O₃) is a promising ultra-wide-bandgap (UWBG) semiconductor for high-power and RF electronics due to its bandgap of 4.9 eV, large theoretical breakdown electric field of 8 MV cm⁻¹, and Baliga figure of merit of 3300, 3–10 times larger than that of SiC and GaN. Moreover, β-Ga₂O₃ is the only WBG material that can be grown from melt, making large, high-quality, dopable substrates at low costs feasible. Significant efforts in the high-quality epitaxial growth of β-Ga₂O₃ and β-(Al_xGa_1−x)₂O₃ heterostructures has led to high-performance devices for high-power and RF applications. In this report, we provide a comprehensive summary of the progress in β-Ga₂O₃ field-effect transistors (FETs) including a variety of transistor designs, channel materials, ohmic contact formations and improvements, gate dielectrics, and fabrication processes. Additionally, novel structures proposed through simulations and not yet realized in β-Ga₂O₃ are presented. Main issues such as defect characterization methods and relevant material preparation, thermal studies and management, and the lack of p-type doping with investigated alternatives are also discussed. Finally, major strategies and outlooks for commercial use will be outlined. Full article

The growth of nanoscale columnar Al_xGa_1-xN/AlN heterostructures on the surface of silicon substrates using plasma-activated nitrogen molecular-beam epitaxy was investigated in this work. Silicon substrates include atomic-smooth cSi substrate, Si substrate with a transition layer of porous silicon porSi/cSi and a hybrid substrate involving a silicon carbide layer grown with matched substitution of the atoms on the surface of porous silicon SiC/porSi/cSi. A complex analysis performed using a set of structural and spectroscopic techniques demonstrated that the epitaxial growth of the nuclear AlN layer on all types of the substrates in a N-enriched environment resulted in the formation of Al_xGa_1-xN/AlN heterostructures with a Ga-polar surface, which was realized only on the SiC/porSi/cSi substrate. The layer of Al_xGa_1-xN on cSi and porSi/cSi substrates was in the state of disordered alloy with an excess of gallium atom content. It was shown that a great difference in the lattice parameters of a substrate–film pair resulted not only in the appearance of a number of various defects but also in a considerable effect on the chemical process of the formation of the alloys, in our case, the Al_xGa_1-xN alloy. It was shown that nanoscale columns of Al_xGa_1-xN formed on SiC/porSi/cSi substrate were inclined relative to the c-axis, which was connected with the features of the formation of a SiC layer by the matched substitution of the atoms on the porous Si substrate, resulting in the formation of the inclined (111) SiC facets at the boundary of the (111) Si surface and pores in Si. Optical studies of the grown samples demonstrated that the optical band-to-band transition for the AlxGa1-xN alloy with E_g = 3.99 eVB was observed only for the heterostructure grown on the SiC/porSi/cSi substrate. A qualitative model is proposed to explain the difference in the formation of Al_xGa_1-xN layers on the substrates of cSi, porSi/cSi and SiC/porSi/cSi. The results obtained in our work demonstrate the availability of using SiC/porSi/cSi substrates for the integration of silicon technology and that used for the synthesis of nanoscale columnar Al_xGa_1-xN heterostructures using plasma-activated molecular-beam epitaxy with a nitrogen source. Full article

Systematic results of lattice dynamical calculations are reported as a function of m and n for the novel (SiC)_m/(GeC)_n superlattices (SLs) by exploiting a modified linear-chain model and a realistic rigid-ion model (RIM). A bond polarizability method is employed to simulate the Raman intensity profiles (RIPs) for both the ideal and graded (SiC)_10-Δ/(Si_0.5Ge_0.5C)_Δ/(GeC)_10-Δ/(Si_0.5Ge_0.5C)_Δ SLs. We have adopted a virtual-crystal approximation for describing the interfacial layer thickness, Δ (≡0, 1, 2, and 3 monolayers (MLs)) by selecting equal proportions of SiC and GeC layers. Systematic variation of Δ has initiated considerable upward (downward) shifts of GeC-(SiC)-like Raman peaks in the optical phonon frequency regions. Our simulated results of RIPs in SiC/GeC SLs are agreed reasonably well with the recent analyses of Raman scattering data on graded short-period GaN/AlN SLs. Maximum changes in the calculated optical phonons (up to ±~47 cm⁻¹) with Δ = 3, are proven effective for causing accidental degeneracies and instigating localization of atomic displacements at the transition regions of the SLs. Strong Δ-dependent enhancement of Raman intensity features in SiC/GeC are considered valuable for validating the interfacial constituents in other technologically important heterostructures. By incorporating RIM, we have also studied the phonon dispersions [${{{\mathsf{\omega }}_{}}_{\mathrm{j}}^{\mathrm{S}\mathrm{L}}}_{}\left(\overrightarrow{\mathbf{q}}\right)$] of (SiC)_m/(GeC)_n SLs along the growth [001] as well as in-plane [100], [110] directions [i.e., perpendicular to the growth]. In the acoustic mode regions, our results of ${{{\mathsf{\omega }}_{}}_{\mathrm{j}}^{\mathrm{S}\mathrm{L}}}_{}\left(\overrightarrow{\mathbf{q}}\right)$ have confirmed the formation of mini-gaps at the zone center and zone edges while providing strong evidences of the anti-crossing and phonon confinements. Besides examining the angular dependence of zone-center optical modes, the results of phonon folding, confinement, and anisotropic behavior in (SiC)_m/(GeC)_n are compared and contrasted very well with the recent first-principles calculations of (GaN)_m/(AlN)_n strained layer SLs. Full article

The high breakdown electric field, n-type doping capability, availability of high-quality substrates, and high Baliga’s figure of merit of Ga₂O₃ demonstrate its potential as a next-generation power semiconductor material. However, the thermal conductivity of Ga₂O₃ is lower than that of other wide-bandgap materials, resulting in the degradation of the electrical performance and reduced reliability of devices. The heterostructure formation on substrates with high thermal conductivity has been noted to facilitate heat dissipation in devices. In this work, Ga₂O₃ thin films with an Al₂O₃ interlayer were deposited on SiC substrates by radio frequency sputtering. Post-deposition annealing was performed at 900 °C for 1 h to crystallize the Ga₂O₃ thin films. The Auger electron spectroscopy depth profiles revealed the interdiffusion of the Ga and Al atoms at the Ga₂O₃/Al₂O₃ interface after annealing. The X-ray diffraction (XRD) results displayed improved crystallinity after annealing and adding the Al₂O₃ interlayer. The crystallite size increased from 5.72 to 8.09 nm as calculated by the Scherrer equation using the full width at half maximum (FWHM). The carrier mobility was enhanced from 5.31 to 28.39 cm² V⁻¹ s⁻¹ in the annealed Ga₂O₃ thin films on Al₂O₃/SiC. The transfer and output characteristics of the Ga₂O₃/SiC and Ga₂O₃/Al₂O₃/SiC back-gate transistors reflect the trend of the XRD and Hall measurement results. Therefore, this work demonstrated that the physical and electrical properties of the Ga₂O₃/SiC back-gate transistors can be improved by post-deposition annealing and the introduction of an Al₂O₃ interlayer. Full article

This paper proposes a new type of substrate for manufacturing LEDs based on AlInGaN heterostructures. Instead of depositing SiC layers on the surface of Si using the conventional method, a new method involving the coordinated substitution of atoms (MCSA) to form the SiC layer is proposed. This new approach enables the growth of epitaxial GaN layers with low defect content and facilitates transfer to any surface. The paper details the technology of manufacturing LEDs on SiC/Si substrates obtained by the MCSA and elaborates on the benefits of using these substrates in LED production. Additionally, the advantages of the growth interface between SiC and Si materials are discussed. Moreover, it is found that thinner SiC layers (<200 nm) contribute to the scattering of the LED’s own radiation in the heterostructure waveguide, which decreases its absorption by silicon. For flip-chip LEDs with the substrate removed, substrates with thicker SiC layers (~400 nm) and a growth porous layer of several microns at the SiC-Si interface is utilized to simplify Si substrate removal and enhance the manufacturing process’s cost-effectiveness. Full article

We fabricated superconducting and ferromagnetic nanostructures, which are intended for applications in transmission electron microscopy (TEM), in a commercial sample holder that can be cooled using liquid helium. Nanoscale superconducting quantum-interference devices (nanoSQUIDs) with sub-100 nm nanobridge Josephson junctions (nJJs) were prepared at a distance of ~300 nm from the edges of a 2 mm × 2 mm × 0.05 mm substrate. Thin-film TiN-NbN-TiN heterostructures were used to optimize the superconducting parameters and enhance the oxidation and corrosion resistance of nJJs and nanoSQUIDs. Non-hysteretic I(V) characteristics of nJJs, as well as peak-to-peak quantum oscillations in the V(B) characteristics of the nanoSQUIDs with an amplitude of up to ~20 µV, were obtained at a temperature ~5 K, which is suitable for operation in TEM. Electron-beam lithography, high-selectivity reactive ion etching with pure SF₆ gas, and a naturally created undercut in the Si substrate were used to prepare nanoSQUIDs on a SiN membrane within ~500 nm from the edge of the substrate. Permalloy nanodots with diameters down to ~100 nm were prepared on SiN membranes using three nanofabrication methods. High-resolution TEM revealed that permalloy films on a SiN buffer have a polycrystalline structure with an average grain dimension of approximately 5 nm and a lattice constant of ~0.36 nm. The M(H) dependences of the permalloy films were measured and revealed coercive fields of 2 and 10 G at 300 and 5 K, respectively. These technologies are promising for the fabrication of superconducting electronics based on nJJs and ferromagnetic nanostructures for operation in TEM. Full article

Non-recessed ohmic contact resistance (R_c) on ultrathin-barrier (UTB) AlGaN(<6 nm)/GaN heterostructure was effectively reduced to a low value of 0.16 Ω·mm. The method called the ‘ohmic-before-passivation’ process was adopted to eliminate the effects of fluorine plasma etching, in which an alloyed Ti/Al/Ni/Au ohmic metal stack was formed prior to passivation. The recovery of 2-D Electron Gas (2DEG) adjacent to the ohmic contact was enhanced by composite double-layer dielectric with AlN/SiN_x passivation. It is found that the separation between the recovered 2DEG and the ohmic contacting edge can be remarkably reduced, contributing to a reduced transfer length (L_T) and low R_c, as compared to that of ohmic contact to the AlGaN(~20 nm)/GaN heterostructure with a pre-ohmic recess process. Thermionic field emission is verified to be the dominant ohmic contact mechanism by temperature-dependent current-voltage measurements. The low on-resistance of 3.9 Ω·mm and the maximum current density of 750 mA/mm with V_g = 3 V were achieved on the devices with the optimized ohmic contact. The non-recessed ohmic contact with the ‘ohmic-before-passivation’ process is a promising strategy to optimize the performance of low-voltage GaN-based power devices. Full article