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We propose a scheme for efficient directional emission of laser-driven wakefields from a magnetized plasma. In the scheme, a laser–plasma interaction region is sandwiched between a pair of dielectric prisms of total internal reflection. The wakefields, propagating in the plasma at different angles to the laser beam, are cumulated by the prisms and radiated to free space in the direction of the laser beam. For magnetic fields

Ion implantation induced phase transformation and the crystal structure of a series of ion implanted β-Ga2O3 samples were studied using electron diffraction, high resolution transmission electron microscopy, and scanning transmission electron microscopy. In contrast to previous reports suggesting an ion implantation induced transformation to the orthorhombic κ-phase, we show that for 28Si+, 58Ni+, and stoichiometric 69Ga+/16O+-implantations, the monoclinic β-phase transforms to the cubic γ-phase. The γ-phase was confirmed for implantations over a range of fluences from 1014 to 1016 ions/cm2, indicating that the transformation is a general phenomenon for β-Ga2O3 due to strain accumulation and/or γ-Ga2O3 being energetically preferred over highly defective β-Ga2O3.

Conventional photodetectors (PDs) generally exhibit a unipolar photoresponse within their responsive spectral range. Different from the traditional PDs, we report here a broadband PD based on the MoS2/GaN/Si heterojunction that shows a unique phenomenon of wavelength selectivity through photocurrent polarity inversion. Overall, the device can differentiate the photons of the ultraviolet (UV)/visible region from that of the near infrared (NIR) region. This polarity inversion is explained with the help of the band diagram and the wavelength dependent photothermoelectric (PTE) effect in MoS2. The vertical transport characteristics of the MoS2/GaN/Si device exhibit a high spectral response in a broad range of wavelengths (300–1100 nm) in a self-biased mode. The maximum response of the device is found to be 23.81 A/W at a wavelength of 995 nm. Our results demonstrate a route for the development of PDs without filter that possess a lot of potential for the futuristic photonic devices.

The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.

A large linear magnetoresistance up to 600% at a magnetic field of 9 T was discovered in single HgSe:Co crystals with a low Co content (not exceeding 0.035 at. %). This was accompanied by a noticeable rise in relative magnetoresistance in comparison with undoped HgSe. An analysis of sample inhomogeneity is presented. Possible sources of linear magnetoresistance in HgSe:Co are discussed.

Heterojunction band offset parameters are critical for designing and fabricating junction-based devices as these parameters play a crucial role in determining the optical and electronic properties of a device. Herein, we report the band discontinuities at the MoS2/III-nitride (InN, GaN, and AlN) heterointerfaces. Few-layer MoS2 thin films are deposited by pulsed laser deposition on III-nitrides/c-sapphire substrates. Band offsets [valence band offset (VBO) and conduction band offset (CBO)] at the heterojunctions are determined by high-resolution x-ray photoelectron spectroscopy. The estimated band alignments are found to be type-I (VBO: 2.34 eV, CBO: 2.59 eV), type-II (VBO: 2.38 eV, CBO: 0.32 eV), and type-III (VBO: 2.23 eV, CBO: 2.87 eV) for MoS2/AlN, MoS2/GaN, and MoS2/InN, respectively. Such determination of the band offsets of 2D/3D heterojunctions paves a way to understand and design the futuristic photonic and electronic devices using these material systems.

Magnetoresistive properties of the single crystal of HgSe with a low electron concentration were studied in a wide range of temperatures and magnetic fields. Some fundamental parameters of the spectrum and scattering of electrons were experimentally determined. Two important features of magnetic transport were found—strong transverse magnetoresistance (MR) and negative longitudinal MR, which can indicate the existence of the topological phase of the Weyl semimetal (WSM) in HgSe. Taking this hypothesis into account, we suggest a modified band diagram of mercury selenide at low electron energies. The obtained results are essential for the deeper understanding of both physics of gapless semiconductors and WSMs—promising materials for various applications in electronics, spintronics, computer, and laser technologies.

Considering its high carrier mobility and high saturation velocity, a low-noise amplifier is thought of as being the most attractive analogue application of graphene field-effect transistors. The noise performance of graphene field-effect transistors at frequencies in the K-band remains unknown. In this work, the noise parameters of a graphene transistor are measured from 10 to 26 GHz and noise models are built with the data. The extrinsic minimum noise figure for a graphene transistor reached 1.5 dB, and the intrinsic minimum noise figure was as low as 0.8 dB at a frequency of 10 GHz, which were comparable with the results from tests on Si CMOS and started to approach those for GaAs and InP transistors. Considering the short development time, the current results are a significant step forward for graphene transistors and show their application potential in high-frequency electronics.

An oxide semiconductor changes its resistance with exposure of water molecules and is accepted to be governed by electronic and protonic conduction in low and high humid atmosphere, respectively, without any experimental evidences. Here, we report on the experimental evidence of a relative humidity (RH) dependent crossover, from an electronic to protonic conduction and its oscillatory behaviour in mesoporous SnO2. Interestingly, oscillatory conduction observed in the intermediate humidity range (70%–90% RH) lies in between two monotonic variations that substantiate the competitive adsorption and desorption processes of oxygen species and water molecules. In addition, we have shown that the conduction process can be tuned predominantly electronic or protonic by pre- and post-UV treatment. The conductance increases by 2–3 orders as the conduction changes from pure electronic to protonic, suggesting an insulator-to-metal like transition.

Optical rectennas for infrared energy harvesting commonly incorporate metal/double-insulator/metal diodes. Required diode characteristics include high responsivity and low resistance near zero bias with a sub-micron area, which have not been obtainable simultaneously. Diodes based on a new material set, Co/Co3O4/TiO2/Ti and an area of 0.071 μm2, provide a median maximum responsivity of 4.1 A/W, a median zero-bias responsivity of 1.2 A/W, and a median resistance of 14 kΩ. The highest performing diode has a maximum responsivity of 4.4 A/W, a zero-bias responsivity of 2.2 A/W, and a resistance of 18 kΩ.

We report on the fabrication and characterization of a Fabry-Perot microcavity enclosing a thin diamond membrane at cryogenic temperatures. The cavity is designed to enhance resonant emission of single nitrogen-vacancy centers by allowing spectral and spatial tuning while preserving the optical properties observed in bulk diamond. We demonstrate cavity finesse at cryogenic temperatures within the range of F=4000–12 000 and find a sub-nanometer cavity stability. Modeling shows that coupling nitrogen-vacancy centers to these cavities could lead to an increase in remote entanglement success rates by three orders of magnitude., Applied Physics Letters, 110 (17), ISSN:0003-6951, ISSN:1077-3118

Photoconduction (PC) properties in the ZnO films with the (110) nonpolar surface (a-plane) epitaxially grown by chemical vapor deposition on the LiGaO2 (010) substrates with low lattice mismatches (4.0% along the c-axis and 3.8% along the m-axis) have been studied. The structural and optical qualities of the epitaxial films have been characterized using theta-two theta and phi scans, X-ray diffraction, rocking curve, and photoluminescence measurements. The nonpolar ZnO film exhibits a near visible-blind ultraviolet photoresponse. The optimal photocurrent to dark current ratio (i.e., sensitivity) can reach 13360%. The responsivity of the a-plane ZnO photoconductor-type detector can also reach 17 AW−1, which is two to four orders of magnitude higher than those of the m-plane, a-plane, and r-plane photodiodes based on ZnO/ZnMgO quantum wells. The normalized gain at 2.9 cm2V−1 of the nonpolar film is also comparable with the optimal recorded value of the ZnO nanowires. In addition, the PC mechanism has also b...

Use of multilayer dielectric thin films provides opportunities to optimize properties for different applications. In this work, the changes in the leakage current in Ba0.8Sr0.2TiO3 thin films upon introduction of ZrO2 layers of different thicknesses are studied. Unusual changes in the leakage current and the transition field with variation in the number and thickness of the ZrO2 layers are observed. Profile of the oxygen concentration across the sample, as determined by x-ray photoelectron spectroscopy, shows that the oxygen diffusion length controls the modulation in the concentration of the charged oxygen vacancies, and hence the depletion layer thicknesses, at the interfaces. A qualitative model for the observed behavior is provided.

The electromechanical coupling coefficient K2 for surface acoustic waves propagating on c and a surfaces of bulk AlN single crystals has been measured using the S11-parameter method in the frequency range of 160–360 MHz. The extracted values of K2 are 0.11% and 0.47% for the c and a surfaces, respectively. By fitting our experimental data to our numerical simulation results, we have estimated piezoelectric constants, which are in a reasonable agreement with literature data. Our results are consistent with the negative sign of the e15 constant.

Pulsed-laser-deposited polycrystalline/amorphous films of mixed-valent manganites [La0.7Ca0.3MnO3 (LCMO), La0.5Sr0.5MnO3 (LSMO), La0.5Ba0.5MnO3 (LBMO), and (La0.6Pr0.4)0.67Ca0.33MnO3 (LPCMO)] grown at low temperature (450 °C) on single crystal (001) silicon substrate are evaluated for uncooled bolometric applications. It is shown that the temperature coefficient of resistance (TCR) and electrical noise (Sv) depend on the chemical composition. The optimum performance is found for LCMO with TCR of ∼7% K−1 and spectral noise ∼8.9×10−13 V2/Hz. In LBMO and LSMO the noise is much lower, but so is the TCR (1.5%–2% K−1). In LPCMO the TCR is high (∼10% K−1) and the noise is as well.

The effect of 200 MeV Ag ion irradiation on the temperature and field dependence of critical current density (JC) of high quality MgB2 thin films is studied. Substantial increase in JC is observed over a certain field range for the film irradiated at a dose of 1012 ions/cm2. Our analysis suggests that columnar defects are not formed under irradiation conditions used in these studies, which correspond to an electronic energy loss of about 16 keV/nm. Defects clusters are likely to be responsible for the observed improvement in JC.

Transparent conducting films of SnO2 are grown on single-crystal R-Al2O3 substrates by the pulsed electron beam deposition (PED) technique, and their electrical, optical, and microstructural properties are compared with the films grown by pulsed-laser deposition (PLD). In PED, the pulsed electron beam used for ablation is generated by a channel-spark system with discharge voltage of 12 kV, current ∼1 kA, and pulse duration of ∼100 ns. The PED films are highly oriented, show good epitaxy, and are highly transparent with transmittance of ∼80% in the visible and IR range. The measured band gap is close to 3.9 eV. It is shown that the PED film properties compare well with those of PLD films, with the 119Sn Mossbauer spectroscopy showing comparable local environments in the two.

An all-perovskite ferroelectric field-effect transistor with a ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) gate and a colossal magnetoresistive La0.8Ca0.2MnO3 (LCMO) channel has been successfully fabricated by pulsed-laser deposition on Si. A clear and square channel resistivity hysteresis loop, commensurate with the ferroelectric hysteresis loop of PZT, is observed. A maximum modulation of 20% after an electric field poling of 1.5×105 V/cm, and 50% under a magnetic field of 1 T, are achieved near the metal-insulator transition temperature of the LCMO channel. A data retention time of at least one day is measured. The effects of electric and magnetic fields on the LCMO channel resistance are discussed within the framework of phase separation scenario.

Zr-doped ZnO (ZZO) thin films have been investigated as an anode material, a potential alternative to indium tin oxide (ITO), for organic light emitting diode (OLED) devices. ZZO films have been deposited on glass substrates by pulsed laser deposition. The electrical and optical properties of these films were studied as a function of substrate temperature and oxygen pressure during deposition. For a 200-nm-thick ZZO film grown at 250 °C in 1 mTorr of oxygen, a resistivity of 5.6×10−4 Ω cm was measured and an average optical transmittance of 84% was measured in the visible range (400–700 nm). The ZZO films, grown at different oxygen pressures, were used as an anode contact for OLED devices. External electroluminescence quantum efficiencies (0.8%–0.9%) comparable to those (0.9%–1.0%) measured for control devices fabricated on commercial ITO anodes were obtained at high current densities (1000 A/m2). These results demonstrate that ZZO is a good anode material. In addition, it is an attractive alternative to ...

Photoluminescence (PL) properties of two GaN epilayers grown in identical conditions on substrates of Al face and N face of bulk single-crystal AlN are studied in the temperature range from 8 to 300 K under weak cw excitation and strong pulsed excitation up to the intensities when electron–hole heating and stimulated emission are observed. At low temperatures and pump intensities, PL of GaN on Al-face AlN is consistent with that of homoepitaxial Ga-face GaN, while GaN on N-face AlN exhibits features indicating the existence of tail localized states. At carrier densities high enough for band-to-band transitions to dominate, the room-temperature PL of GaN on N-face AlN is higher than that in GaN on Al-face AlN due to longer effective lifetime of photoexcited carriers.

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La1-xSrxTiO3-d). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (

The electron flow in large-area, space-charge-limited diodes is susceptible to transit time instability. The instability spatially and temporally modulates the electron beam and results in a wide, time-dependent, electron energy distribution and high-power rf emission. The electromagnetic waves that power the instability are guided and amplified in the anode–cathode (AK) region which acts as a parallel-plate transmission line with negative conductance. We investigated how to mitigate this instability in large-area rectangular electron diodes. We found that by slotting the rectangular cathode, along both its axes, with parallel grooves of predetermined depth and periodicity, the AK region was modified into a slow wave structure. The unstable electromagnetic waves are slowed down and attenuated to such an extent that the instability is eliminated and stable electron flow is achieved.