Bias Voltage Vb Research Articles

A low noise capacitance to voltage readout circuit based on active feedback charge sensitive amplifier for micro-gyro.

Trans-impedance amplifier (TIA) based capacitance-voltage (C-V) readout circuits are widely used in micro-machined gyroscopes' control system. In this work, the noise performance of a TIA and charge sensitive amplifier (CSA) based C-V readout circuit is analyzed in detail. The theoretical derivation and simulation experiments carried out in this paper show that the thermal noise of high value feedback resistors in TIA dominates the output noise of the circuit, while a CSA-based readout circuit can overcome this shortcoming. Then, a CSA-based C-V readout circuit is designed with a 1.8V transistor on a 180nm complementary metal-oxide-semiconductor. This C-V-based readout circuit occupies an area of 0.039 mm2, which is smaller than the area of a 0603 package resistor. When 1V bias voltage (Vb) is added to the capacitors under test, the tested C-V gain of this circuit is as high as 225 dB, and the tested output noise of the circuit is less than 150 nV/Hz. Finally, the fabricated chip achieved a resolution of 840 zF/Hz at Vb as low as 1V. This CSA-based C-V readout circuit has more advantages in terms of noise and area over the TIA circuit and is more suitable for integration into the interface integrated circuit for micro-gyroscopes.

(Invited) Vertically Stacked Graphene Junction Diodes

Graphene is expected to be nanomaterial of the post-silicon era because of its numerous advantageous properties. The single crystal graphene thermally grown on SiC substrate is considered the most promising platform for future electric devices. Large area epitaxial graphene on SiC substrate has been successfully fabricated using high-temperature rapid thermal annealing [1]. In this study, we present vertically stacked graphene junction diodes fabricated using direct bonding technique [2–4]. Figure 1(a) shows the schematic of a stacked graphene junction diode. Two graphene layers on SiC are directly bonded to each other. Gold–coated contact pins are attached to the graphene surface to establish ohmic contact. The graphene samples and contact probes are fixed by acrylic molds. Strong nonlinear current–voltage (I-V) characteristics are observed for this simple device configuration, as shown in Fig. 1(b). In the low bias voltage (Vb) region, the resistance of the graphene diode is approximately 20 GΩ. The current abruptly increases when the bias voltage is increased to approximately 30 V. The two-terminal resistance at a high bias voltage (100 V) is approximately 21 kΩ. The vertically stacked graphene junction exhibits bidirectional thyristor-type characteristics with a high on-off ratio of approximately 106. The Fowler–Nordheim (F-N) tunneling phenomenon is observed in the sub-threshold region. The gap distance estimated from the F-N plot is approximately 6 nm, which can be attributed to the large off resistance. The Coulomb force generated by applying the bias voltage causes the two graphene layers to attract each other, resulting in an electrically conductive state. The junction voltage (Vj) of the device was measured using a four-terminal configuration. As shown in Fig. 1(c), negative resistance is observed in the low-resistance region. The effective internal resistance of the junction diode is reduced to approximately 450 Ω. The off resistance of the diodes is considerably modified by the initial distance between the graphene layers. However, the I-Vj characteristics in the negative resistance region are identical for each device. This result suggests that the cause of the negative resistance phenomena is closely related to the band structure of graphene. A vertically stacked graphene junction diode with a high on-off ratio was demonstrated. The main component of the diode comprised a pair of single-crystal graphene layers fabricated through direct bonding. The negative resistance of the graphene diode presents new possibilities for the application of functional devices, such as terahertz emitters.[1] T. Aritsuki, et al., Jpn. J. Appl. Phys. 55 (2016) 06GF03.[2] J. Du, eta al., Jpn. J. Appl. Phys. 58 (2019) SDDE01.[3] N. Murakami, et al., Jpn. J. Appl. Phys. 60 (2021) SCCD01.[4] M. Ohi, et al., Jpn. J. Appl. Phys. 62 (2023) to be published. Figure 1

Ultrafast ultraviolet laser-induced voltage of air

In this work, the time-scale laser-induced voltage (LIV) of air was investigated under KrF excimer laser irradiation. The LIV response was fitted by a sum of three exponential functions, where the fitted parameters were influenced by the bias voltage Vb and the laser energy Ein. Higher Vb produced significant LIV. An exponential relationship was observed between the Ein and LIV signal amplitude (Vp). After decreasing the adjustable resistor, the LIV trace was almost triangular and symmetrical due to the impedance effect with the response time which was completely limited by the laser pulse duration. As a potential application, the LIV of air presented a viable method for single-pulse laser detection.

Unveiling adsorption characteristics of BC5 monolayer: High electronic anisotropy and gas sensing performance

The exploitation of highly sensitive, power-efficient, robust and recoverable gas sensing devices has become a key tool for the detection of toxic gases in the environment. In this work, we theoretically elucidate the electronic properties of the BC5 monolayer and the gas-sensitive performance of the BC5-based gas sensor for organic and inorganic gases using density functional theory and non-equilibrium Green’s function method. The results indicate that SO2, CH4, and CH3OH are physisorption, whereas CO, NH3, NO, NO2, C2H2, C2H4 and HCHO are chemisorption on the BC5 monolayer. Then, the transport properties of the BC5-based nanodevice are sensitive to the adsorption of CO, NH3, NO, NO2, C2H2, C2H4 and HCHO, hence the BC5 monolayer can be used to construct the gas sensor for detecting these gas molecules. In addition, the pristine BC5-based nanodevice shows a strong anisotropic transport properties under low bias voltages (Vb), and the ratio of the maximum anisotropy is 1.94 when Vb is 0.1 V. In the meantime, the BC5-based gas sensor is more sensitive to CO and HCHO with the gas sensitivity reaching 1.76 and 2.56 at the bias of 0.1 V along the zigzag direction, respectively. Finally, the microscopic mechanisms of inorganic and organic gas molecules adsorbed on the BC5-based gas sensor are systematically revealed by presenting the respective scattering states at Fermi level. All these theoretical predictions might trigger a new avenue to expand the application of fresh carbon based nano-materials in designing next generation gas sensing devices.

Investigating the effect of bias voltage on the microstructural thermal stability and tribological performance of N-doped hydrogenated diamond-like carbon coatings

Here, N-doped hydrogenated diamond-like carbon (a-C:H:N) coatings were deposited onto 316 L stainless steel and silicon wafer substrates by applying plasma-enhanced chemical vapor deposition at different bias voltages (Vb). The coatings were annealed at 25–590 °C for 4 h. The structure and chemical bonding of the coatings were characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The residual stress, surface hardness, and tribological performance were investigated by using the Stoney method, a nanoindenter, and sliding against alumina. The fraction of the constituent bonds in the coating was controlled depending on Vb. The evolution of these bonds during annealing determined the thermal and mechanical performance of the a-C:H:N coatings, particularly the CHn group, sp2C bonds, and CO bonds (CO and CO). As Vb increased, the fraction of CHn groups, CO bonds, and CN bonds decreased, whereas that of sp2C increased. When annealed at above 430 °C, the breakage of the CHn groups and CN bonds accelerated the sp3 → sp2 transformation. The coating prepared at −500 V exhibited the highest hardness of 22.5 GPa and low residual stress of −0.57 GPa; after annealing at 590 °C for 4 h, a hardness of 15 GPa was maintained. The wear resistance was positively related to the surface hardness of the coatings, whereas the coefficient of friction was responsible for the fraction of sp3CHn, CO bonds, and sp2C bonds. The wear resistance of the coating deposited at −500 V was double that of the coating deposited at −740 V.

Numerical Simulations of Nanolayered Heterostructures of Nanopatterned Graphene and Vanadium Oxide for Mid-Infrared Photodetectors Operating Close to Room Temperature

We present the model of an ultrasensitive mid-infrared (mid-IR) photodetector consisting of a hybrid heterostructure made of nanopatterned graphene (NPG) and vanadium dioxide (VO2) which exhibits a large responsivity of R∼105 V/W, a detectivity exceeding D* ∼ 1010 Jones, and a sensitivity in terms of noise-equivalent power NEP ∼ 10 fW/Hz close to room temperature by taking advantage of the phase change of a thin VO2 film. Our proposed photodetector can reach an absorption of nearly 100% in monolayer graphene because of localized surface plasmons (LSPs) around the patterned circular holes. The geometry of the nanopattern and an electrostatic gate potential can be used to tune the absorption peak in the mid-IR regime between 3 and 12 μm. After the photon absorption by the LSPs in the NPG sheet, the phase change of VO2 from insulating to metallic phase is triggered, resulting in a current through the VO2 sheet due to the applied bias voltage Vb. The response time is about 1 ms, shorter than the detection times of current VO2 bolometers. With use of a gradient thickness of the VO2 layer, a linear dependence between input power Pinc of the incident light and the photocurrent Iph is achieved. Our envisioned mid-IR photodetector reaches detectivities of cryogenically cooled HgCdTe photodetectors and sensitivities larger than VO2 microbolometers while operating close to room temperature.

Piezoelectric MEMS Acoustic Transducer with Electrically-Tunable Resonant Frequency.

The paper presents a technique to obtain an electrically-tunable matching between the series and parallel resonant frequencies of a piezoelectric MEMS acoustic transducer to increase the effectiveness of acoustic emission/detection in voltage-mode driving and sensing. The piezoelectric MEMS transducer has been fabricated using the PiezoMUMPs technology, and it operates in a plate flexural mode exploiting a 6 mm × 6 mm doped silicon diaphragm with an aluminum nitride (AlN) piezoelectric layer deposited on top. The piezoelectric layer can be actuated by means of electrodes placed at the edges of the diaphragm above the AlN film. By applying an adjustable bias voltage Vb between two properly-connected electrodes and the doped silicon, the d31 mode in the AlN film has been exploited to electrically induce a planar static compressive or tensile stress in the diaphragm, depending on the sign of Vb, thus shifting its resonant frequency. The working principle has been first validated through an eigenfrequency analysis with an electrically induced prestress by means of 3D finite element modelling in COMSOL Multiphysics®. The first flexural mode of the unstressed diaphragm results at around 5.1 kHz. Then, the piezoelectric MEMS transducer has been experimentally tested in both receiver and transmitter modes. Experimental results have shown that the resonance can be electrically tuned in the range Vb = ±8 V with estimated tuning sensitivities of 8.7 ± 0.5 Hz/V and 7.8 ± 0.9 Hz/V in transmitter and receiver modes, respectively. A matching of the series and parallel resonant frequencies has been experimentally demonstrated in voltage-mode driving and sensing by applying Vb = 0 in transmission and Vb = −1.9 V in receiving, respectively, thereby obtaining the optimal acoustic emission and detection effectiveness at the same operating frequency.

Broadband High-Efficiency Millimeter-Wave Power Amplifiers in 22-nm CMOS FD-SOI with Fixed and Adaptive Biasing

The design of broadband highly-efficient millimeter-wave (mm-Wave) Power Amplifiers (PA) in 22-nm CMOS FD-SOI (fully depleted silicon-on-insulator) is discussed. One design uses fixed biasing while the other utilizes an adaptive biasing network to improve its Power-Added-Efficiency (PAE) in power backoff. These designs aim to cover the key Fifth-Generation (5G) FR2 (Frequency Range 2) band (e.g., from 24.25 GHz to 43.5 GHz). In measurement, the PAs obtain very broadband 3-dB BW, ranging from 19.1 GHz to 46.5 GHz for the fixed-bias design and 18.8 GHz to 41.9 GHz for the adaptively biased design. The adaptively biased design is able to achieve enhanced output 1-dB compression point (OP1dB) and PAE@P1dB because of its adaptive biasing network. Measurements on the fixed-biased PA at 24/28/37/39 GHz yield max. PAE of 22.2/22.4/19.7/19.8%, and POUT,SAT of 14.9/15.3/14.7/14.7 dBm, with OP1dB of 11.8/11.6/12/11.6 dBm, and PAE@P1dB of 14.1/13.4/14.5/13.5%. Measurements on the adaptively biased PA at 24/28/37/39 GHz yield max. PAE of 20.1/15.2/10.3/7.9%, and POUT,SAT of 14.6/14.2/12.5/11.2 dBm, with improved OP1dB of 13.1/12.3/12.1/11.2 dBm, and PAE@P1dB of 19.1/14.4/10.3/7.9%. Data from body bias voltage VB on the PA performance is also presented. These measurement results are compared with post-layout parasitic-extraction (PEX) simulations, and also against other novel silicon broadband mm-Wave PAs in literature.

Effects of bias voltage and substrate temperature on the mechanical properties and oxidation behavior of CrSiN films

Nanocomposite CrSiN films with a Si content of 14 at.% were fabricated through reactive direct current magnetron cosputtering. The Si content was determined based on the superior mechanical properties and oxidation resistance for CrSiN films reported in a previous study. Substrate bias voltage (Vb) and temperature (Ts) were the process variables for sputtering CrSiN films. The results indicated that the chemical composition (Cr34–37Si14N49–52) and crystalline phase (CrN) of CrSiN films were not affected by Vb or Ts. The application of Vb and Ts significantly increased the hardness of the films from 13.4 to 22.2 GPa, and Young's modulus increased from 223 to 299 GPa. The oxidation behavior of the films annealed at 800 °C in ambient air was identical, which was accompanied by the formation of an amorphous oxide layer and extruded Cr2O3 precipitations.

Mechanism of snake-like propagation for positive streamers in a meandering plasma plume excited by a positively biased sinusoidal voltage

Being able to generate a remote plasma plume, the atmospheric pressure plasma jet has become an indispensable tool for extensive application fields. A plasma plume usually has a straight column morphology, which results from straight-line or stochastic snake-like propagations of streamers. The snake-like propagation of streamers is unclear in the mechanism. In this paper, a meandering plume is generated for the first time downstream of an argon plasma jet excited by a positively biased sinusoidal voltage, which transits from a straight column plume with an increasing bias voltage (Vb). Results indicate that the wavelength of the meandering plume is inversely proportional to voltage frequency and directly proportional to the gas flow rate. For every voltage cycle, integrated light emission from the meandering plume presents one pulse with low Vb and two pulses with high Vb. Fast photography reveals that the meandering plume results from repeated snake-like propagations of positive streamers due to the influence of ion clouds. To confirm the existence of ion clouds, an opposite argon flow is introduced. With wiping off ion clouds in the argon stream, the meandering plume gradually turns back to a straight column with an increasing opposite flow rate. All these phenomena mentioned above have been explained qualitatively on the basis of ion clouds. These results may shed light on the streamers' dynamics in the atmospheric pressure plasma jet.

Influence of substrate bias and temperature on the crystallization of metallic NbTaTiVZr high-entropy alloy thin films

Metallic sputtered high-entropy alloy (HEA) thin films often result in amorphous structures, due to the film growth kinetics and the large atomic size mismatch of the constituent elements. In this paper, single-phase crystalline NbTaTiVZr HEA thin films were achieved by the appropriate choice of both the alloying elements and the synthesis conditions. Regarding the latter, substrate biasing prompts Ar+ ion irradiation during film growth, whereas substrate heating increases the adatom mobility, inducing specific structural modifications. The control of both variables eliminates traditional crystallization strategies, such as adding nitrogen to the gas mixture during film growth or post-thermal annealing of the as-deposited films. Therefore, we have investigated the relationship between the synthesis conditions, the structure, and the mechanical properties of a Nb20Ta26Ti22V16Zr16 HEA thin film, due to its potential application in the field of refractory coating materials. The as-deposited films prepared at 400 °C possessed a body-centered cubic (bcc) phase, and their preferential orientation changed according to the bias voltage value (Vb) chosen. Low energy ion irradiation (Vb ≈ –25 V) resulted in crystallite coarsening and surface roughening. On the other hand, higher negative bias voltages (Vb ≈ –75 V) led to lower growth rates, grain refining, and improved mechanical properties. In addition, the chemical states and composition were determined by X-ray photoelectron spectroscopy (XPS) and the HEA phase formation was predicted using empirical parameters and compared to the results obtained by the Calculation of Phase Diagrams (CALPHAD) approach.

Design of power-efficient CMOS based oscillator circuit with varactor tuning control

This paper presents a low-power, wide tuning range CMOS voltage-controlled oscillator with MCML (MOS current mode logic) differential delay cell. Voltage controlled oscillator (VCO) circuit is designed in TSMC 0.25 μm CMOS process. To achieve the broad frequency range concept of variable capacitance is employed in the proposed VCO circuit. Source/drain tuning voltage (Vtune) and body bias voltage (Vb) of I-MOS varactor are used to achieve variable capacitance at different I-MOS varactor widths (W). The dual control voltage of I-MOS varactor results in a tuning range from 0.528 GHz to 2.014 GHz. VCO's figure of merit (FoM) is 152.13 dBc/Hz with phase noise of −93.77 dBc/Hz at 1 MHz offset from the oscillation frequency. The proposed VCO dissipates maximum power of 3.127 mW.

Design and Analysis of an Afterpulsing Auto-Correction System for Single Photon Avalanche Diodes

In this work, an auto-afterpulsing correction system for single photon avalanche photodiode (SPAD) is proposed and implemented which can be used in real-time to minimize the afterpulsing effect in SPAD. The system is based on passive quenching circuit and retriggerable monostable multivibrator and can be used for any SPADs with different bias voltages (Vb). Two different SPADs are used to evaluate the proposed system, one is a planar SPAD fabricated in-house (Vb is around 26V) and the other one is a commercial reach-through SPAD (Vb is around 126V). Experimental results show that for both SPADs, the afterpulsing probabilities can be effectively reduced to below 0.5% from 5% and 4% respectively. Results also show that, with a proper time interval threshold, the SPAD’s afterpulsing probability can be maintained below 0.5% with the increasing of the bias voltage. The system developed is simple and can be used for effectively minimizing the afterpulsing effect for any SPADs that makes it very useful in low cost and compact SPAD based photon counting applications.

Ion-temperature determination with a baffled Langmuir probe.

An electrostatic Langmuir probe for real-time measurements of parameters in magnetized plasma is tested in fully ionized, barium, Q-machine plasma. The small-diameter, long-length, tungsten wire sensor, i.e., the probe tip, oriented with its cylindrical axis perpendicular to the magnetic field (B), is partially shielded by ceramic baffles, or masks, that form sensor-access slots between the baffles. Adjusting the azimuthal orientation of the slots, by rotating the probe about its cylindrical axis, changes the fraction of proximity gyro-orbiting electrons, relative to the fraction of proximity gyro-orbiting ions, that can access the recessed sensor along the magnetic field. Thus, the ratio between the electron and ion saturation currents, Ie sat and Ii sat, can be adjusted without having affected the probe bias voltage Vb. When optimally shielded (Ie sat/Ii sat=1), accurate, real-time measurements of space potential Vs can be acquired. When maximally shielded (Ie sat/Ii sat≪1), accurate, real-time measurements of ion temperature Ti can be acquired. Subtracting the floating potential Vf of an optimally shielded baffled probe from Vf of a maximally shielded baffled probe yields Ti (and its fluctuation phase) in real time.

Self-assembly of nanocrystalline@amorphous core-shell nanostructure in the TA19 alloy film to achieve high strength

Improving the strength of alloy materials and simultaneously retaining high toughness are strongly desirable by various engineering applications. Nanocrystalline@amorphous dual-phase nanostructure consisting of nanocrystalline-cores encapsulated with amorphous-shells has been regarded as one of the most effective routes to achieve high strength yet high toughness, which may overcome the limitation of either single-phase nanocrystalline alloys or single-phase metallic glasses because of reverse Hall-Petch effect or shear-band deformation. Herein, nanocrystalline@amorphous core-shell nanostructure has been successfully assembled in the sputtered TA19 alloy film only by regulating bias voltages (Vb), and then Vb induced evolution of microstructure and mechanical property was further investigated by combined experiments of X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), select area electron diffraction (SAED), transmission electron diffraction (TEM) and nanoindentation. The results show that the bombardment of energy ions can promote formation of amorphous zones in TA19 alloy film, successfully achieving ordered core-shell nanostructure at −120 V of Vb, while single-phase nanocrystalline occurs at Vb = −40 V and amorphous dominated structure appears at Vb = −200 V. Maximum hardness value of 11.2 GPa achieves at Vb = −120 V when the ordered core-shell nanostructure is formed with invisible shear bands around indenter, but more amorphous result in disappearance of core-shell nanostructure at Vb = −200 V, which worsens hardness, accompanying with presence of obvious shear bands around indenter. This novel core-shell-like structure succeeded in achieving high strength or hardness yet high toughness by providing strong resistance to hinder movements of both grains and shear bands due to large amount of nanocrystalline@amorphous interfaces. It is suggested that the Vb as a key factor controlling the core-shell-like structure of TA19 films, which may provide a new strategy to improve mechanical properties of alloy films.

Theoretical study of amplification of the tunnel current in the redox-mediated tunneling junctions for the symmetric and asymmetric electron-molecular coupling in the case of the adiabatic transport regime

Dependence of amplification A of the tunnel current in the electrochemical bridged tunneling junction on the bias voltage Vb, reorganization free energy Er, the broadening Δ of the electronic level of the redox molecule (RM) and intensity of the Debye screening in the tunneling gap is studied for the cases of the fully adiabatic and partially adiabatic electron transitions. We consider a one–level single RM within the spin-less model and assume the wide-band approximation for the electronic structure of the working electrodes. It is shown that amplification in the fully and partially adiabatic limits is appreciably smaller than that in the diabatic limit studied earlier [I.G. Medvedev, Electrochim. Acta 186 (2015) 613–623] due to the sufficiently large values of the broadening Δ. It is also shown that, in contrast to the diabatic limit, amplification depends weekly on the asymmetry of the coupling of the RM to the working electrodes. On the other hand, similar to the diabatic limit, amplification as a function of the bias voltage Vb approaches to relatively large values at large values of Vb in the case of the full Debye screening while, in the realistic case of the intermediate Debye screening, amplification increases at first with the increase of Vb, then reaches a maximum value Amax at some value Vb max and, when Vb farther increases, decreases to A = 1. The functions A(Vb) are calculated for different values of the parameters of the tunneling junction and it is shown that Amax increases with the increase of Er and the intensity of Debye screening and decrease with the increase of Δ. The approximate analytical expression which shows in the explicit form the dependence of A on the parameters of the system is presented.

Effect of bias voltage on microstructure and optical properties of Al2O3 thin films prepared by twin targets reactive high power impulse magnetron sputtering

Al2O3 thin films were deposited on silicon (100) and tin-doped indium oxide (ITO) substrates by twin targets reactive high power impulse magnetron sputtering (TTR-HiPIMS). Effects of the substrate bias voltage (Vb) on microstructure, surface morphology, chemical composition and optical transmittance of the deposited films were investigated by grazing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), X-ray photoelectrons spectroscopy (XPS) and Ultra violet-visible-near infrared (UV-visible-NIR) spectra, respectively. The AFM scans showed that increasing the bias voltage (Vb) from 0 to −40 V caused the weakly crystalline film to evolve into fully crystalline Al2O3 film at temperatures as low as 230 °C. However, when Vb reached −60 V, the crystallinity decreased. This result was also confirmed by GIXRD patterns, which showed that the best crystallinity of Al2O3 in γ phase was obtained for film deposited at a bias of −40 V. Besides, with the increase of Vb, the deposition rate decreased from 88 to 70 nm/h, and the O/Al ratio of the as-deposited films reduced from 1.53 to 1.44. The binary collision approximation (BCA) Monte-Carlo code SDTrimSP was also used to calculate the sputter yield of Al and Al2O3. It was found that the sputter yield of Al2O3 was about 1/8 of Al, which is supposed to be one of the main reasons for the observed low deposition rate. UV-vis-NIR transmittance spectra of Al2O3 films deposited under different bias voltages were evaluated after deposition on ITO substrates. The results revealed that all the as-deposited films had high optical transmittance (≈80%) in the wavelength range of 200–800 nm. Moreover, a marginal decrease of the transmittance was also observed with increasing Vb.

Deposition of AlN films for acoustic biosensors by deep oscillation magnetron sputtering: effect of bias voltage

The present study showed that Deep Oscillation Magnetron Sputtering (DOMS) has an excellent potential to produce AlN films for biosensing applications. However, careful control of the process parameters is required to attain the desired features. Al/AlN/Al films deposited via DOMS on Si wafers were extensively characterized to assess this potential. The resulting microstructures were examined by glancing angle x-ray diffraction, scanning electron microscopy coupled with energy dispersive microanalysis, atomic force microscopy, and x-ray photoelectron spectroscopy. The results indicated that there is a strong effect of the bias voltage (Vb) applied to the substrate on the structure of the Al/AlN/Al films and that it can be divided in two regimes. The low Vb regime produced the best features of the AlN films for the present application. These included a strong (002) orientation, low residual stress, and uniform surface roughness. In the high Vb regime, these features degraded. The effects were associated to the nature and energy of the impinging species as a function of Vb.

Analysis of magnetotransport properties and microstructure in current-perpendicular-to-plane pseudo spin-valves using Co2Fe(Ga0.5Ge0.5) Heusler alloy and Ag/Mg-Ti-O/Ag-based spacer

Current-perpendicular-to-plane magnetoresistance (CPP-MR) devices using a Co2Fe(Ga0.5Ge0.5) Heusler alloy as ferromagnetic electrodes and a Ag/Mg-Ti-O (MTO)/Ag tri-layer as a spacer are studied. A large voltage output of 16 mV with a magnetoresistance ratio of 46% and resistance-area product RpA in the parallel magnetization state of approximately 200 mΩ μm2 are obtained. The microstructure characterization reveals that upon annealing at 550 °C, nanoscale Ag channels are formed penetrating the MTO layer, which are responsible for the large MR output. At room temperature, for all the devices, RpA decreases with increasing the bias voltage Vb, which is contradictory to the conventional CPP-giant magnetoresistance devices with a current-confined-path structure, in which the RpA increases with increasing Vb. Such a Vb dependence of RpA is in accordance with the temperature dependence of RpA. These results suggest that the magneto-transport channels comprise semiconducting current confined paths, bringing new perspectives for the design and engineering of CPP-MR devices with conductive oxides-based multilayer spacers.

Insight into the structural evolution during TiN film growth via atomic resolution TEM

Although the microstructure evolution during TiN film growth has been extensively studied, how the atomic-resolution structure of the films changes with the deposition conditions remains unclear. Here, a series of TiN films were prepared on silicon substrates at different bias voltages (Vb) by magnetron sputtering. The microstructure and atomic resolution structures of the films were systematically investigated with transmission electron microscopy (TEM) and high resolution TEM (HRTEM), complemented by X-ray photoelectron spectroscopy measurement and density functional theory (DFT) calculations.With increasing the bias voltage, the morphology of columnar grain evolves, and grain size and dislocation density increase, and films become much dense. HRTEM observations reveal that the boundaries between two neighboring grains are dominated by low-angle grain boundaries in the case of floating bias conditions. Numerous twins surprisingly form in the TiN films under certain bias voltage. The initiation of twins in TiN films can be attributed to the reduction of stacking fault energy (SFE) caused by non-stoichiometry to a certain degree, i.e., N/Ti atomic ratio, as validated by theoretical calculations. Moreover, the mechanisms of hardness alterations under different bias-voltages are discussed.