Low Rf Power Density Research Articles

A Multiband, Polarization-Controlled Metasurface Absorber for Electromagnetic Energy Harvesting and Wireless Power Transfer

A multiband, polarization-controlled energy harvesting metasurface is presented in this article for harvesting electromagnetic energy in the Wi-Fi bands. Inspired by the split-ring resonator (SRR), a metasurface of SRRs is designed by nesting multiple SRRs together. The novelty of the design lies in the controllable multiband polarization and adjustable absorption frequency, so as to achieve the active modulation of energy harvesting on the metasurface at different frequencies. The metasurface is composed of a periodic array of SRRs unit cells, an impedance-matched rectifier, and a load. Metasurface arrays and rectifier integration are proposed to reduce power losses, while the channels formed by the array enhance energy density and improve the ability to capture energy. Moreover, the design for the rectifier is capable of rectifying RF energy from multiple Wi-Fi bands simultaneously at low input RF power density. The finite array of <inline-formula> <tex-math notation="LaTeX">$5\times $ </tex-math></inline-formula> 5 SRRs absorber is fabricated and tested. Measurements in an anechoic chamber show that the fabricated prototype reaches the efficiency of 66.5&#x0025; at 2.4 GHz, 40.6&#x0025; at 5.2 GHz, 35.6&#x0025; at 5.8 GHz under transverse electric (TE) polarization, and 38.3&#x0025; at 5.3 GHz under transverse magnetic (TM) polarization when the incident power density is greater than 66 <inline-formula> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula>/cm².

Photovoltaic and impedance spectroscopy characterization of single-junction a-Si:H p–i–n solar cells deposited by simple shadow masking techniques using PECVD

Hydrogenated amorphous silicon p–i–n solar cells with a 1 × 1 cm2 active surface area were fabricated using shadow masks on the 20 × 20 cm2 glass substrate coated with a fluorine-doped tin oxide film. The intrinsic, n-type hydrogenated amorphous silicon (a-Si:H), and p-type a-SiC:H thin films were deposited using plasma-enhanced chemical vapor deposition at 13.56 MHz plasma excitation frequency and on 20 × 20 cm2 and SnO2:F covered glass substrates. Low rf-power densities (less than 0.1 W/cm2) and substrate temperatures (less than 190 °C) were used for this purpose. Raman spectra of the films are dominated by a broad peak around 480 cm−1 that is the characteristic of the amorphous silicon network for all the three types of films. Scanning electron microscopy measurements revealed that the surface of the a-Si:H films deposited on SnO2:F-coated glass substrates (Asahi-VU) replicates the texture of the SnO2:F film. Spectroscopic ellipsometry spectra were analyzed with the Tauc–Lorentz dispersion model, and the results revealed that the optical gap of the intrinsic a-Si:H films is on the order of 1.7 eV, while that of the a-SiC:H is on the order of 1.8 eV. These results were further confirmed by optical transmission measurements. The highest efficiency obtained for solar cells prepared with shadow masking under our condition is on the order of 8.83% with a Voc of 0.856 V, a short circuit current density of 15.6 mA/cm2, and a fill factor of 66.07%. The obtained efficiency is slightly lower than the record efficiency obtained in this family of cells (10.3%) prepared by laser scribing because the low short-circuit current slightly lowers the fill factor. Impedance spectroscopy measurements were performed on the cells in the dark in the frequency range of 1 kHz–100 kHz. The analysis of impedance either in the Nyquist diagram or in the Bode diagram suggests a lumped circuit consisting of resistance Rs in series with a parallel combination of resistance Rp and capacitance Cp that account for the p–i–n structure. The value of Rp changed with the applied DC bias. The value of the series resistance agrees with the value obtained from the current–voltage characteristics of the cell.

Effects of annealing on wettability and physical properties of SnO thin films deposited at low RF power densities

The SnO thin films were deposited at low RF power densities by RF magnetron sputtering. According to XRD and XPS analyses, the SnO thin film comprised nanocrystalline orthorhombic SnO with a (110) orientation. Reducing RF power density resulted in better nanocrystallinity, changing hydrophobicity to hydrophilicity, and increasing the optical transmission in the UV–vis–NIR region. After annealing, the SnO thin film favored p-type conductivity and hydrophilicity. As the annealing temperature increased, the coexistence of nanocrystalline orthorhombic SnO and tetragonal SnO2 in the film clearly increased the optical transmission in the ultraviolet region. The SnO thin films after annealing at 500℃ in vacuum and N2 (200sccm) exhibited a higher hole mobility and a better optical selection in the ultraviolet region, respectively.

Medium Wave Energy Scavenging for Wireless Structural Health Monitoring Sensors

Energy scavenging presents a promising approach for powering the wireless sensors used for structural health monitoring. RF energy scavenging has recently been intensively studied due to the abundance of ambient RF energy. In this paper, an RF scavenger operating at the AM frequency band has been fabricated and tested. The scavenger is composed of an RF front end and an energy processing circuit. The RF front-end design has been optimized for operation in low RF power densities. The energy processing circuit, which consist of a boost converter and a control circuit with low power overhead, requires a minimum input power of -39 dBm for proper operation.

From amorphous to microcrystalline: Phase transition in rapid synthesis of hydrogenated silicon thin film in low frequency inductively coupled plasmas

Hydrogenated silicon (Si:H) thin films were fabricated on glass substrates by low frequency inductively coupled plasma-assisted chemical vapor deposition using a silane precursor with low hydrogen dilution at room temperature. The crystallinity and microstructure properties of the Si:H thin films deposited at different inductive radio-frequency (rf) power density were systematically studied by Raman spectroscopy, x-ray diffraction, and scanning electron microscopy. We found that at a low rf power density of 16.7 to 20.8 mW/cm3, the structure of silicon thin films evolves from a completely amorphous phase to an intermediate phase containing both amorphous and microcrystalline silicon. As the power density is increased to a moderate value of 25 mW/cm3, a highly crystallized (111)-preferred hydrogenated microcrystalline silicon (μc-Si:H) film featuring a vertically aligned cone-shaped structure, is emerging. Both the crystallinity and deposition rate exhibit a monotonic increase with the increase in the rf power density, reaching a maximum value of 85% and 1.07 nm/s, respectively, at a power density of 41.7 mW/cm3. Scanning electron microscopy reveals that continuous and dense μc-Si:H films with grain size of tens to hundreds nanometers can be achieved deterministically without the formation of amorphous incubation layer, and this is of great importance for synthesis of multilayer structures in p-i-n solar cells. The formation mechanism of the μc-Si:H films and the elimination of the amorphous incubation layer are explained in terms of the high electron density and the plasma-surface interactions.

High-Current Electron Beams for High-Power Free-Electron Masers Based on Two-Dimensional Periodic Lattices

High-power gigawatt-level radiation can be generated by the interaction of an electromagnetic wave and an annular electron beam with a transverse dimension much larger than the operating wavelength. The use of such a large-circumference annular beam allows the generation of high beam currents while also maintaining low space charge and RF power densities inside the interaction region. This circumvents the problems associated with potential depression in the beam channel and RF breakdown inside the oscillator. In this paper, we present the studies of high-current magnetically confined annular electron beams and discuss their production and transportation through a coaxial beam channel which formed the interaction region of a free-electron maser (FEM). The results from numerical simulations, using the software packages KARAT and MAGIC, are compared with the experimental measurements. The operation of a FEM, driven by a high-current annular electron beam, is presented, and the tunability of the maser, inside a frequency range defined by an input 2-D Bragg mirror, is demonstrated.

Performance enhancement of mc-Si solar cells due to synergetic effect of plasma texturization and SiN x:H AR coating

The present paper discusses the plausible physical processes dominant during plasma texturization of multicrystalline silicon (mc-Si) wafers, deposition of silicon nitride (SiN x ) antireflection (AR) coating and firing of contacts through it. During plasma texturization, it is observed that by using low RF power density and loading wafers on the ground electrode, the texturization process is dominated by chemical etching. The resulting surface of the wafer shows low-reflectivity (<10% in wavelength range 350–800 nm) and low-defect density leading to improved minority carrier lifetime. It is postulated that plasma-etched nanoscale structures accelerate the migration of hydrogen released during firing of contacts. As a result of these physical processes, an improvement up to ~2.4% in absolute efficiency of large area (~149 cm 2) multicrystalline silicon solar cells has been achieved.

Slow- and helicon-wave sustained discharges in HF/VHF bands of radio frequency

Helicon-wave sustained (HW) and slow-wave sustained (SW) discharges are studied in a whistler- or helicon-wave range of frequency using high-frequency (13.56 MHz) and very-high-frequency (50 and 144 MHz) bands of rf. For 13.56 and 50 MHz, in addition to capacitive-coupling (E) discharge and the HW discharge, the SW discharge is observed at an extremely low rf power and plasma density; lower than the E discharge region. For 144 MHz, no E discharges appear and the transition from the SW discharge to the HW discharge occurs. In the SW discharge, both the electromagnetic and electrostatic waves are observed. The results of two-dimensional wave code and analytic calculation indicate that the electrostatic component of the electric field parallel to a static magnetic field is dominant in plasma production in the SW discharge.

Plasma production by helicon and slow waves.

The observation of slow-wave sustained (SW) discharge in a whistler- or helicon-wave range of frequency is made using high-frequency and very-high-frequency bands of rf. The SW discharge occurs at an extremely low rf power and plasma density, which are lower than a capacitive-coupling discharge region.

Influence of deposition conditions on the thermal stability of ZnO:Al films grown by rf magnetron sputtering

The resistivity and the thermal stability of transparent conducting ZnO layers doped with aluminum have been correlated with the conditions of the sputtering process. Layers deposited at low rf power density (∼1.3 to 2.6 W/cm2) exhibit a low resistivity of 9×10−4 Ω cm, predominantly due to a high concentration of intrinsic donor type defects. These donors are compensated during annealing at high temperature in a vacuum; the low resistivity increases and the layers are not thermally stable. At rf power densities of ∼3.2 W/cm2 and more, the deposition conditions yield a high growth rate and the extrinsic aluminum dopant is incorporated on vacant cation sites. These substitutional donors are thermally stable therefore a low resistivity is retained after annealing at 550 °C.

00/01990 Multicrystalline silicon solar cells with porous silicon emitter

The present paper discusses the plausible physical processes dominant during plasma texturization of multicrystalline silicon (mc-Si) wafers, deposition of silicon nitride (SiNx) antireflection (AR) coating and firing of contacts through it. During plasma texturization, it is observed that by using low RF power density and loading wafers on the ground electrode, the texturization process is dominated by chemical etching. The resulting surface of the wafer shows low-reflectivity (<10% in wavelength range 350–800 nm) and low-defect density leading to improved minority carrier lifetime. It is postulated that plasma-etched nanoscale structures accelerate the migration of hydrogen released during firing of contacts. As a result of these physical processes, an improvement up to ~2.4% in absolute efficiency of large area (~149 cm2) multicrystalline silicon solar cells has been achieved.

Enhanced boron doping in amorphous and microcrystalline silicon by Ar dilution

Thin films of boron-doped silicon - hydrogen alloy materials have been deposited by plasma enhanced chemical vapour deposition using argon as the diluent gas. The effect of hydrogen and helium dilution on the film properties is also reported for comparison. Films deposited by dilution with argon showed a higher conductivity than those deposited with hydrogen or helium dilution at any gas phase doping and dilution level. Bonded boron content in the films was studied by measuring SiB with the help of secondary ion mass spectroscopy. The increase of conductivity even with a lowering of the SiB/Si ratio in the deposited films at high argon dilution indicates an enhancement of doping efficiency. Moreover, boron-doped microcrystalline samples were prepared by argon dilution at low rf power density () by decreasing the silane flow. The structural and electrical properties of the deposited films revealed a high degree of microcrystallinity. A higher deposition rate with less degradation of the transparent conducting oxide (TCO) coated substrates compared to hydrogen or helium dilution indicates compatibility of the argon dilution technique with solar cell fabrication processes.

Plasma Polymer Films for Dropwise Condensation of Steam

AbstractThin hydrophobic polymer films produced by radio frequency (rf) plasma decomposition of hexamethyldisiloxane (HMDSO) have been produced on flat metal (titanium, stainless steel, and copper/nickel) and silicon substrates. By using a statistical design methodology, the deposition conditions have been assessed for their control of the deposition rate, refractive index, water droplet contact angle, and longevity of dropwise condensation of steam. The films showing the best performance were those deposited at the slower rates, using low rf power density and high monomer flow rates, and having the lower refractive indices. There is some dependence on substrate material, with CuNi behaving worst, and titanium providing the best results of the three metals studied. This may be connected with the stability of an interfacial oxide layer. The steam immersion tests have already exceeded 7500 hours of continuous condensation, and many of the films are still producing excellent dropwise condensation. The reasons for these effects of preparation conditions and substrate material on performance are discussed with reference to the plasma chemical process.

Microwave probe diagnostic for the lower hybrid multijunction antenna on TdeV

On the TdeV tokamak a microwave probe diagnostic enables direct measurement of the electromagnetic fields in ten reduced waveguides of the lower hybrid current drive multijunction antenna. In each instrumented reduced waveguide, the local field is measured at two different locations by probes through coupling holes located in the narrow wall of the waveguides. The amplitude and phase of the signals are measured with a multichannel heterodyne circuit and are used to calculate the incident and reflected fields at the antenna mouth. The probes are under vacuum and they are bakeable up to the maximum operating temperature of the antenna (T=350 °C). They are calibrated at room temperature but the evolution of their characteristic with temperature is taken into account in the data analysis. Typical accuracies of the field measurements at the grill mouth are: ±9% for the amplitude and ±6° for the phase. The probe diagnostic has been operating reliably for the last two years and the probes do not appear to perturb the operation of the antenna nor to reduce its power handling capability. Comparisons of the probe measurements with calculations from the multijunction antenna modeling code SWAN show that the code is accurate for low rf power densities at the antenna mouth.

Control of Microstructure and Optoelectronic Properties of Si:H Films by Argon Dilution in Plasma-Enhanced Chemical Vapor Deposition from Silane

Structural and optoelectronic properties of thin films of silicon-hydrogen binary alloy (Si:H) deposited from silane and argon mixture in a rf glow discharge plasma have been studied for different argon dilutions and rf powers. It has been observed that with low rf power density ( 30-70 mW/cm3) increase of argon dilution up to 95% reduces the microstructure in the films, as determined from IR absorption spectra. Simultaneously, increase in refractive index and decrease in ESR spin density have been observed. Above 95% argon dilution or with higher rf powers, transmission electron microscopy (TEM) studies reveal a dominance of the columnar growth mechanism, and the optoelectronic properties of the films deteriorate. At 99% argon dilution, microcrystallites appear to form within columnar regions. Addition of a small amount of hydrogen to the silane-argon plasma improves the network significantly, which is manifested by the changes in the dark conductivities in the different rf power regimes.

Ellipsometric study of a-Si:H thin films deposited by square wave modulated rf glow discharge

Thin films of hydrogenated amorphous silicon (a-Si:H), deposited by square wave modulated (SQWM) rf silane discharges, have been studied through spectroscopic and real time phase modulated ellipsometry. The SQMW films obtained at low mean rf power density (19 mW/cm2) have shown smaller surface roughness than those obtained in standard continuous wave (cw) rf discharges. At higher rf powers (≥56 mW/cm2), different behaviors depending on the modulating frequency have been observed. On the one hand, at low modulating frequencies (&amp;lt;40 Hz), the SQWM films have shown a significant increase of porosity and surface roughness as compared to cw samples. On the other, at higher modulating frequencies, the material density and roughness have been found to be similar in SQWM and cw films. Furthermore, the deposition rate of the films show more pronounced increases with the modulating frequency as the rf power is increased. Experimental results are discussed in terms of plasma negative charged species which can be relatively abundant in high rf power discharges and cause significant effects on the deposited layers through polymers, clusters, and powder formation.

Hydrogen Dilution effects on Amorphous Hydrogenated Silicon (a-Si:H) Prepared by Glow Discharge Near Room Temperature

ABSTRACTWe report here the first phase of our attempt to deposit “device-quality” a-Si:H films by glow discharge near room temperature (50 °C). We have found that, at a low rf power density (40 mW/cm2) and high hydrogen/silane ratios (≥ 2), film quality is not uniquely determined by the substrate temperature, Ts. The microstructure of our low hydrogen content films as revealed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies is similar to that of “device-quality” films deposited at standard Ts. The next phase of our work is to ascertain whether or not similar IR and NMR characteristics between our low Ts films and the standard Ts films imply also similar opto-electronic properties.

Microcrystalline Silicon Films Produced by RF Magnetron Sputtering and the Effect of Diffrent Ambients on their Conductivity

AbstractResults on characterisation of undoped, μc-Si:H films prepared by rf magnetron sputtering technique are presented. Highly conducting films (10−3 Δ−cm−1) were obtained at fairly low rf power density (l.2W/cm2). Critical parameters for obtaining microcrystalline phase were identified. The effect of humid ambient on film properties was looked into.

Influence of Deposition Conditions on Properties of Hydrogenated Amorphous Silicon Prepared by RF Glow Discharge

The electrical and structural properties of amorphous silicon films prepared by pure silane rf glow dicharge have been measured as a function of the rf power, the substrate temperature, the gas flow rate and the gap between the two electrodes. The properties of the deposited films show a very different dependence on the rf power from those reported previously. The films prepared at high rf power density (1–2 W/cm2) reveal a photoconductivity comparable with that of device-grade films prepared at low rf power density (0.02 W/cm2). Increasing the gas flow rate increases the deposition rate up to 70 A/s at 2 W/cm2. The photoconductivity of films prepared at this high deposition rate is 1.2×10-5 (Ω·cm)1 at 600 nm, 1 mW/cm2. This paper discusses the deposition mechanism inducing these results.