Low Bias Voltage Range Research Articles

Hybrid simulation of radio frequency biased inductively coupled Ar/O<sub>2</sub>/Cl<sub>2</sub> plasmas

In the etching process, a bias source is usually applied to the substrate of the inductively coupled plasma (ICP) to realize independent modulation of the ion energy and ion flux. In this work, a hybrid model, i.e. a global model combined bi-directionally with a fluid sheath model, is employed to investigate the plasma properties and ion energy distribution function (IEDF) in biased inductively coupled Ar/O<sub>2</sub>/Cl<sub>2</sub> plasmas. The results indicate that at a bias frequency of 2.26 MHz, the Cl<sup>–</sup> ion density and ClO<sup>+</sup> ion density first increase with bias voltage rising, and then they decrease, and finally they rise again, which is different from the densities of other charged species, such as O and Cl atoms. At the bias frequency of 13.56 MHz and 27.12 MHz, except Cl<sup>–</sup> and <inline-formula><tex-math id="M3">\begin{document}$ {\text{Cl}}_2^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231369_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231369_M3.png"/></alternatives></inline-formula> ions, the evolutions of other species densities with bias voltage are similar to the results at lower bias frequency. The evolution of the species densities with bias frequency depends on the bias voltage. For instance, in the low bias voltage range (< 200 V), the densities of charges species, O and Cl atoms increase with bias frequency increasing due to a significant increase in the heating of the plasma by the bias source. However, when the bias voltage is high, say, higher than 300 V, except <inline-formula><tex-math id="M4">\begin{document}$ {\text{Cl}}_2^ + $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231369_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20231369_M4.png"/></alternatives></inline-formula> and Cl<sup>–</sup> ions, the densities of other charged species, O and Cl atoms first decrease with bias frequency increasing and then they increase due to a decrease and then an increase in the heating of the plasma by the bias source. In addition, as the bias frequency increases, the peak separation of IEDF becomes narrow, the high energy peak and low energy peak approach each other and they almost merge into one peak at high bias frequency. The results obtained in this work are of significant importance in improving the etching process.

Electron conduction mechanisms in magnetic tunnel junctions fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier

CoFeB/ Si-Zn-Sn-O /CoFeB magnetic tunnel junctions (MTJs) have been fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier. In the low bias voltage range (up to ∼0.2 V), direct tunneling is found to be the dominant transport mechanism in MTJs. Tunneling conduction is further verified by simulation of tunnel current density and differential conductance using Simmon’s and Brinkmann model, respectively. Simulated results provided valuable insights into the barrier properties, including interfacial barrier height, thickness, and barrier asymmetry. Above the direct tunneling regime, electron transport in MTJs is governed by Pool Frenkel emission, which possibly arises due to the presence of high-density localized tail states below the conduction band of amorphous Si-Zn-Sn-O. Tunnelling magnetoresistance value of MTJs is found to be very low, which is attributed to the presence of various inelastic conduction channels. The results of this study might be useful to explore the potential of amorphous Si-Zn-Sn-O for fabricating low-resistive MTJ based spintronic devices.

Record high photoresponse observed in CdS-black phosphorous van der Waals heterojunction photodiode

Two-dimensional (2D) materials have recently received great attention for their atomic thin thickness and thus derived outstanding electrical, optical and optoelectronic properties. Moreover, the dangling-bond-free surfaces of 2D materials enable the direct integration of different materials with various properties through van der Waals (vdW) forces into vdW heterostructures, providing new opportunities for constructing new type devices with superior performances. In this study, we report the vertical assembly of n-type CdS and p-type BP into p-n junctions. The electrically tunable heterojunction device shows a high current rectifying ratio up to 8×103 at a low bias voltage range of ±1 V and an ideality factor of 1.5. More interestingly, the CdS/BP vdW heterojunction exhibits an ultra-high photoresponsivity up to 9.2×105 A W−1 and an ultra-high specific detectivity of 3.2×1013 Jones with a low bias voltage of 1.0 V, which is among the highest in the reported results of 2D heterostructures. While operated at a self-powered mode, the device also exhibits excellent photodetection performances with a high photoresponsivity of 0.27 A W−1 and a high external quantum efficiency of 76%. Time-resolved photoresponse characterizations indicate that the device possesses a fast response time of about 10 ms. The developed CdS/BP vdW heterojunctions will find potential applications in the next-generation nanoscale electronics and optoelectronics applications.

Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements

We demonstrate electrical spin injection and detection in n-type Ge (n-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length (λGe) of the Ge layer used (n ∼ 1 × 1019 cm−3) at 296 K is estimated to be ∼0.44 ± 0.02 µm. Room-temperature spin signals can be observed reproducibly in the low bias voltage range (≤0.7 V) for LSVs with relatively low resistance–area product (RA) values (≤1 kΩ µm2). This means that the Schottky tunnel contacts used here are more suitable than ferromagnet/MgO tunnel contacts (RA ≥ 100 kΩ µm2) for developing Ge spintronic applications.

Multiple p-n junction subwavelength gratings for transmission-mode electro-optic modulators

We propose a free-space electro-optic transmission modulator based on multiple p-n-junction semiconductor subwavelength gratings. The proposed device operates with a high-Q guided-mode resonance undergoing electro-optic resonance shift due to direct electrical control. Using rigorous electrical and optical modeling methods, we theoretically demonstrate a modulation depth of 84%, on-state efficiency 85%, and on-off extinction ratio of 19 dB at 1,550 nm wavelength under electrical control signals within a favorably low bias voltage range from −4 V to +1 V. This functionality operates in the transmission mode and sustainable in the high-speed operation regime up to a 10-GHz-scale modulation bandwidth in principle. The theoretical performance prediction is remarkably advantageous over plasmonic tunable metasurfaces in the power-efficiency and absolute modulation-depth aspects. Therefore, further experimental study is of great interest for creating practical-level metasurface components in various application areas.

Charge transport in light emitting devices based on colloidal quantum dots and a solution-processed nickel oxide layer.

We fabricated hybrid light emitting devices based on colloidal CdSe/ZnS core/shell quantum dots and a solution-processed NiO layer. The use of a sol-gel NiO layer as a hole injection layer (HIL) resulted in overall improvement in device operation compared to a control device with a more conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HIL. In particular, luminous efficiency increased substantially because of the suppression of excessive currents and became as large as 2.45 cd/A. To manifest the origin of current reduction, temperature- and electric field-dependent variations of currents with respect to bias voltages were investigated. In a low bias voltage range below the threshold for luminance turn-on, the Poole-Frenkel (PF) emission mechanism was responsible for the current-density variation. However, the space-charge-limited current modified with PF-type mobility ruled the current-density variation in high bias voltage range above the threshold.

$Ka$- and $U$-Band Tunable Bandpass Filters Using Ferroelectric Capacitors

This paper presents the realization of two second-order quasi-elliptic ferroelectric tunable bandpass filters for millimeter-wave applications in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Ka$</tex></formula> - and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$U$</tex></formula> -band, respectively. These filters include a coplanar-waveguide topology implemented on epitaxial barium–strontium–titanate-on-sapphire technology. The <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Ka$</tex></formula> -band filter has a frequency tuning of 2.4 GHz (33.6–36 GHz) at low biasing levels <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(&lt; {{\hbox {30 V}}})$</tex></formula> , a stable 3-dB bandwidth of about 9%, a loss of 6.2–4 dB, and a compact size compared to state-of-the-art tunable <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Ka$</tex></formula> -band filters using similar technology (footprint area reduced by 35%). The proposed 50-GHz quasi-elliptic tunable filter, realized for the first time in this technology, tunes from 48.1 up to 52.1 GHz (8.3%) within the same low bias voltage range. It has a 3-dB bandwidth of 9.5%–8.7% and insertion loss of 7.6–4.9 dB. Both filters show excellent return loss, better than 20 dB.

Structural and mechanical property of Si incorporated (Ti,Cr,Al)N coatings deposited by arc ion plating process

(Ti,Cr,Al,Si)N coatings with different Al + Si fractions (0.6 and 0.65) were deposited by an arc ion plating (AIP) apparatus that is equipped with the plasma-enhanced type arc cathode. The (Ti,Cr,Al,Si)N coatings were deposited under different substrate bias voltages and effect of the deposition parameter on the composition, structure and mechanical properties was investigated. X-ray diffraction measurements of the (Ti,Cr,Al,Si)N coating deposited under different substrate bias voltages revealed that formation of the hexagonal phase (Wurzite structure) was only limited to a relatively low bias voltage range of 20 to 30 V. Above this bias voltage, the crystal structure of the coatings was single-phased cubic rock-salt structure (B1 phase) independent of the bias voltage. Grain size of the coating was calculated from the full width of half maximum (FWHM) of the X-ray diffraction peak and it was smaller than the one of conventional (Ti,Al)N or (Ti,Cr,Al)N coating with a comparable Al fraction. The grain size estimated from the cross-sectional TEM observation was less than 10 nm. From the TEM observation, the coating was compositionally homogeneous and there was no evidence that the film had a phase separation such as Si-rich and -poor region. Hardness of the (Ti,Cr,Al,Si)N coating with Al + Si = 0.6 was in the range of 26 to 27 GPa independent of the substrate bias. (Ti,Cr,Al,Si)N coating with Al + Si = 0.65 showed slight increase in hardness from 24 to 27 GPa when the substrate bias was increased to more than 100 V. To evaluate the oxidation resistance, annealing tests in the air at 1000 °C were conducted and surface SEM observations revealed that surface of the conventional (Ti,Al)N and (Ti,Cr,Al)N was covered with coarse oxide grains enriched with TiO 2. Whereas only a dense but very thin protective oxide layer was observed in case of (Ti,Cr,Al,Si)N coating after the oxidation. These coatings were applied to the high-speed dry cutting tests against hardened D2 steel (HRC 60) and the result clearly indicated better performance of (Ti,Cr,Al,Si)N compared to the conventional coatings such as (Ti,Al)N and (Ti,Cr,Al)N.

Unusual oscillation in tunneling magnetoresistance near a quantum critical point in Sr3Ru2O7.

We performed single-electron tunneling measurements on bilayer ruthenate Sr3Ru2O7. We observe an unusual oscillation in tunneling magnetoresistance near the metamagnetic quantum phase transition. The characteristic features of this oscillation suggest that it is unrelated to traditional quantum oscillations caused by orbit quantization. In addition, tunneling spectra are found to change sharply in the low bias voltage range of V<2 mV near the transition field. These observations reveal that the Fermi surface of Sr3Ru2O7 changes in a surprising way as the system undergoes strong critical fluctuations.

A NOVEL MERON-INDUCED PSEUDOSPIN WAVE IN THE BILAYER QUANTUM HALL COHERENT STATE AND THE RESIDUAL ZERO-BIAS PEAK IN TUNNELING CONDUCTANCE

In the bilayer quantum Hall coherent state for νT deviating slightly from unity, we show that, instead of the global order parameter, the periodic domain structure accompanying the charged meron pairs spontaneously breaks the pseudospin U(1) rotational symmetry. The motion of meron pairs induces a novel pseudospin wave. The long-range order of the periodic domains in a low bias voltage range leads to the residual zero-bias peak in the tunneling conductance, which can be distinct from the pseudopsin Goldstone feature in a high bias voltage range.

Optimum Reflection-Type Phase Shifter Using (Ba,Sr)TiO3 Thin Film

In this paper, in order to obtain a large differential phase shift with a little change in applied voltage, a ferroelectric reflective load circuit has been designed on top of barium strontium titanate (Ba,Sr)TiO3 [BST] thin film. The design of the ferroelectric reflection-type phase shifter is based on a reflection theory of terminating circuit, which has a reflection-type analogue phase shifter with two ports terminated in symmetric phase-controllable reflective networks. To achieve large amounts of phase shift in low bias-voltage range, the effects of change of capacitance and transmission line connected with two coupled ports of a 3-dB 90° branch-line hybrid coupler have been investigated. A large phase shift with a small capacitance change in the parallel terminating circuit has been demonstrated in the paper.

Measurement of bias voltage dependence of local barrier height at constant tip-sample separation

We measured the bias voltage dependence of the apparent local barrier height (LBH) by STM on an Au (1 1 1) surface with an Au tip. The results under two different conditions, a constant tip-sample separation and a constant current, were compared in the low bias voltage range (−30 to −5 mV). Under constant separation, the LBH does not change with the bias voltage, indicating that the LBH is independent of the bias voltage. Under constant current, the LBH increases with increasing absolute value of the bias voltage due to the increase of the tip-sample separation with increasing absolute bias voltage. We found that the LBH depends only on the separation in the low bias voltage range.

Effect of radio-frequency bias voltage on the optical and structural properties of hydrogenated amorphous silicon carbide

Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films have been deposited using the electron cyclotron resonance chemical vapor deposition process under varying negative rf-bias voltage at the substrate. The optical and structural properties of these films are characterized using Rutherford backscattering spectroscopy, transmittance/reflectance spectrophotometry, photothermal deflection spectroscopy, Fourier transform infrared absorption, Raman scattering, and room temperature photoluminescence (PL). These films deposited using a gas mixture of silane, methane, and hydrogen at a constant gas flow ratio showed a slight increase in the carbon fraction x, but very obvious structural transformation, at increasing rf induced bias voltage from −20 to −120 V. Near stoichiometric a-Si1−xCx:H films with a carbon fraction x of almost 0.5 are achieved at low bias voltage range from −20 to −60 V. Visible PL with relatively low efficiency can be observed from such films at room temperature. For larger bias voltages from −80 to −120 V, slightly C-rich a-Si1−xCx:H films (x&amp;gt;0.5) with larger optical gaps are obtained. These films have relatively higher PL efficiency, and the relative quantum efficiency was also found to depend strongly on the optical gap. Structurally, it was found that there is an increase in the hydrogen content and carbon sp2 bonding in the films at larger bias voltages. The latter leads to an increase in the disorder in the films. The linear relationship observed between the Urbach energy E0 and B factor in the Tauc equation suggests that the local defects related to microstructural disorder resulting from alloying with carbon dominate the overall defect structure of the films. Substrate biasing is noted to be crucial for the formation of Si–C bonds, as deduced from the Raman scattering results.

Morphological features of diamond-like carbon films deposited by plasma-enhanced CVD

Diamond-like carbon (DLC) films have been deposited by asymmetric capacitively coupled radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system. The morphological features of DLC films depending on the methane pressure and substrate bias voltage have been studied systematically by atomic force microscopy (AFM). In low pressure and low bias voltage range, the DLC film is rough, due to some micro-particles formation in the film. With increasing pressure and bias voltage, the films become smooth and particle free. With increasing self-bias voltage, first the hardness of the films increases, reaches a maximum around 250 V, then decreases; the roughness of the films decreases greatly (below 250 V), then decreases slightly (above 250 V). With increasing gas pressure, the hardness of the films increases first, reaches a maximum in the range of 10–100 mTorr, then decreases; the roughness of the films decreases greatly, reaches a minimum (10–100 mTorr ), then increases. The mechanism of formation of the morphological features in the DLC films made under different deposition conditions is discussed.

Secondary Coulomb blockade gap in a four-island tunnel-junction array

In the ring-shaped tunnel-junction array with four islands, the secondary Coulomb blockade gap in a low bias-voltage range is observed in the I-V characteristics. We attribute its appearance to the unique topology of the array which induces up to two electrons to get trapped inside. We have analyzed the formation and destruction of the gap in terms of detailed single-electron tunneling processes. The negative differential resistance behavior when the thermal and quantum fluctuations are present is also studied.

Transport processes of electrons in MNOS structures

The transport processes of electrons in MNOS structures, especially in the SiO2 layer and the surface region of the Si substrate, have been investigated using p-channel MNOS transistors with a relatively thick SiO2 layer to avoid the complexity of two-carrier transport in the system. An induced junction technique utilizing the electron and hole separation properties of the transistor structure was used as a means of analysis. Theoretical derivation of the carrier multiplication factor, i.e., the number of electron-hole pairs produced by an electron entering the Si from the SiO2, is calculated. It is experimentally shown that an electron entering the Si from the SiO2 produces approximately one electron-hole pair in the low negative gate bias voltage range up to a critical voltage, and above this critical voltage the multiplication factor increases with increasing gate bias voltage. The former observation is in good agreement with the prediction of the theory, considering cascaded impact ionization of an electron with high energy in Si. The latter fact reveals that electrons in SiO2 become hot in the high voltage range. The critical voltage coincides with the value theoretically estimated using an LO phonon energy of 0.153 eV and an electron-phonon scattering length of 1.74 Å in SiO2. The mean free path of the electrons between scatterings by defects or other scattering centers in SiO2 is estimated to be about 30 Å by analyzing gate bias dependence of the multiplication phenomena.