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The recently demonstrated semiconductor grafting approach allows one to create an abrupt, low interface-trap-density heterojunction between a high-quality p-type single-crystalline semiconductor (non-nitrides) with n-type GaN. However, due to the surface band-bending from GaN polarization, an energy barrier exists at the grafted heterojunction, which can impact the vertical charge carrier transport. Reducing the energy barrier height is essential for some advanced device development. In this work, we employed UV/O3 and O2 plasma to treat a Ga-polar GaN surface with/without an ultrathin (∼2 nm) ALD-Al2O3 coating and studied the effects of the treatments on surface band-bending. Through XPS measurements, it was found that the treatments can suppress the upward band-bending of the Ga-polar GaN by 0.11–0.39 eV. The XPS results also showed that UV/O3 treatment is an effective surface cleaning method with little surface modification, while O2 plasma causes a strong oxidation process that occurs inside the top layer GaN. [ABSTRACT FROM AUTHOR]

Beta-phase gallium oxide (β-Ga2O3) has exceptional electronic properties with vast potential in power and radio frequency electronics. Despite the excellent demonstrations of high-performance unipolar devices, the lack of effective p-type dopants in β-Ga2O3 has hindered the further development of Ga2O3-based bipolar devices. In this work, we applied the semiconductor grafting approach and fabricated monocrystalline Si/β-Ga2O3p–n heterojunctions, of which the characteristics were systematically studied. The heterojunctions demonstrated a diode rectification over 1.3 × 107 at ±2 V with a diode ideality factor of 1.13. Furthermore, capacitance–voltage (C–V) measurement showed frequency dispersion-free characteristics from 10 to 900 kHz. The interface defect density (Dit) was calculated as 1–3 × 1012/cm2 eV. Scanning transmission electron microscopy (STEM) and x-ray photoelectron spectroscopy (XPS) revealed that an ultrathin oxygen-rich layer existed on the Ga2O3 surface and later formed an ultrathin interfacial layer after bonding with Si. It is speculated that the excessive oxygen at the Ga2O3 surface enhanced the passivation of the Si dangling bonds and thus reduced Dit. This work improved our understanding of interface properties of the semiconductor grafting approach, providing useful guidance on the future development of Si/Ga2O3 heterojunction devices. [ABSTRACT FROM AUTHOR]

Recent demonstrations of grafted p-n junctions combining n-type GaN with p-type semiconductors have shown great potential in achieving lattice-mismatch epitaxy-like heterostructures. Ultrathin dielectrics deposited by atomic layer deposition (ALD) serve both as a double-sided surface passivation layer and a quantum tunneling layer. On the other hand, with excellent thermal, mechanical, and electrical properties, ZrO2 serves as a high-k gate dielectric material in multiple applications, which is also of potential interest to applications in grafted GaN-based heterostructures. In this sense, understanding the interfacial band parameters of ultrathin ALD-ZrO2 is of great importance. In this work, the band-bending of Ga-polar GaN with ultrathin ALD-ZrO2 was studied by x-ray photoelectron spectroscopy (XPS). This study demonstrated that ZrO2 can effectively suppress upward band-bending from 0.88 to 0.48 eV at five deposition cycles. The bandgap values of ALD-ZrO2 at different thicknesses were also carefully studied. [ABSTRACT FROM AUTHOR]

Microwave electronics have evolved from bulky waveguide and vacuum tube‐based format to smaller and lighter board‐based electronics, and then to the highly miniaturized, chip‐based solid‐state form. Spurred by the advent of high‐performance flexible inorganic single‐crystalline semiconductors, a new opportunity of technology evolution from rigid microwave electronics to flexible and/or stretchable microwave electronics has emerged. The new forms of microwave electronics are expected to provide novel functions to be achieved with advanced materials, device, and system design considerations and innovative fabrication methods. Over the last years, some microwave components, modules, and systems with operation frequency greater than or equal to 1 GHz in the flexible/stretchable form factors are demonstrated. These demonstrations have alluded to the prospects of the new stage of microwave electronics evolution. Herein, a comprehensive review of the microwave/flexible microwave electronics that are demonstrated up to date is provided. An introduction to the need of flexible/stretchable microwave electronics emphasizes the motivation and potential influences of this new growing field. This review also highlights important developments in transistors, diodes, various passives, antennas, circuits, and system demonstrations, all in flexible or stretchable forms. Finally, an outlook for specific technical gaps at both the fundamental and applications levels is presented. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an injection-locked broadband optoelectronic oscillator (OEO) to generate freely tunable phase-locked dual-frequency microwave signals. When two single-tone signals inside and outside the passband of the electrical broadband bandpass filter (BPF) are, respectively, injected into the OEO, a phase-locked dual-frequency microwave signal with ultra-low near-end side-mode spurs can be generated from the OEO cavity. Therefore, one frequency of the output signal is equal to the frequency of the injected signal within the BPF, and the other frequency is equal to the sum frequency or the differential frequency of two injected signals. By changing the frequencies of two injected signals, two frequencies of the generated dual-frequency microwave signal can be arbitrarily tuned over the passband of the BPF. In our experiment, freelytu nable phase-locked dual-frequency microwave signals within the frequency range of 8∼12 GHz can be generated, where the side-mode suppression ratios (SMSRs) are more than 90 dB during the frequency tuning.