Showing papers in "Journal of Vacuum Science & Technology B in 2020"

Spectral ellipsometry of monolayer transition metal dichalcogenides: Analysis of excitonic peaks in dispersion

Abstract: Optical constants of monolayers MoS2 and WS2 were measured by spectroscopic ellipsometry in the spectral range 365–1700 nm. Analysis of ellipsometry spectra was carried out, taking into account the excitonic nature of dispersion. In the considered wavelength range, excitons originate from different interband transitions. As a result, they can be described via Tauc–Lorentz oscillators, one for each exciton. The scalability and universality of such an approach could be applied for the extended wavelength range and to the other transition metal dichalcogenides.Optical constants of monolayers MoS2 and WS2 were measured by spectroscopic ellipsometry in the spectral range 365–1700 nm. Analysis of ellipsometry spectra was carried out, taking into account the excitonic nature of dispersion. In the considered wavelength range, excitons originate from different interband transitions. As a result, they can be described via Tauc–Lorentz oscillators, one for each exciton. The scalability and universality of such an approach could be applied for the extended wavelength range and to the other transition metal dichalcogenides.

Polyphosphazene polymers: The next generation of biomaterials for regenerative engineering and therapeutic drug delivery.

Abstract: The demand for new biomaterials in several biomedical applications, such as regenerative engineering and drug delivery, has increased over the past two decades due to emerging technological advances in biomedicine. Degradable polymeric biomaterials continue to play a significant role as scaffolding materials and drug devices. Polyphosphazene platform is a subject of broad interest, as it presents an avenue for attaining versatile polymeric materials with excellent structure and property tunability, and high functional diversity. Macromolecular substitution enables the facile attachment of different organic groups and drug molecules to the polyphosphazene backbone for the development of a broad class of materials. These materials are more biocompatible than traditional biomaterials, mixable with other clinically relevant polymers to obtain new materials and exhibit unique erosion with near-neutral degradation products. Hence, polyphosphazene represents the next generation of biomaterials. In this review, the authors systematically discuss the synthetic design, structure-property relationships, and the promising potentials of polyphosphazenes in regenerative engineering and drug delivery.

Empirical modeling and Monte Carlo simulation of secondary electron yield reduction of laser drilled microporous gold surfaces

Abstract: This work investigates secondary electron yield (SEY) mitigation from a metal surface with a microporous array fabricated using the laser drilling technique. We propose a general empirical model to fit the experimentally measured SEY of a flat gold surface for normal and oblique incidences of primary electrons. Using this empirical model, we develop a two-dimensional Monte Carlo (MC) simulation scheme to determine the effective SEY of a microporous array. It is found that the SEY from a porous surface is significantly reduced compared to that of the flat surface. By taking into account all the generations of secondary electrons inside a well, our MC results are found to be in very good agreement with the experimental data. The dependence of the SEY on the aspect ratio of the micropores and porosity of the surface is examined. A simple empirical formula has been proposed to evaluate the effective SEY of the gold microporous array for pores of arbitrary aspect ratios.This work investigates secondary electron yield (SEY) mitigation from a metal surface with a microporous array fabricated using the laser drilling technique. We propose a general empirical model to fit the experimentally measured SEY of a flat gold surface for normal and oblique incidences of primary electrons. Using this empirical model, we develop a two-dimensional Monte Carlo (MC) simulation scheme to determine the effective SEY of a microporous array. It is found that the SEY from a porous surface is significantly reduced compared to that of the flat surface. By taking into account all the generations of secondary electrons inside a well, our MC results are found to be in very good agreement with the experimental data. The dependence of the SEY on the aspect ratio of the micropores and porosity of the surface is examined. A simple empirical formula has been proposed to evaluate the effective SEY of the ...

Angular dependence of secondary electron yield from microporous gold surfaces

Abstract: We report exhaustive measurements of the secondary electron yield (SEY) from a gold film containing an array of micropores as a function of the angle of incidence of the primary electrons. The SEY measurements are in good agreement with Monte-Carlo (MC) simulations. A highly accurate empirical fit to the SEY data as a function of the incident electron impact angle is also proposed. In this study, the micropores have aspect ratios (ratio of pore height over pore diameter) ranging from about 1.5 to 3.5. The effect of the pore array density (porosity) and pore aspect ratio is analyzed in greater detail. It is found that increasing the pore aspect ratio and porosity leads to a sharp reduction in the total SEY in agreement with MC simulations.

Highly uniform silicon field emitter arrays fabricated using a trilevel resist process

Abstract: The authors report silicon field emitter arrays (FEAs) that were fabricated using a trilevel resist process and are highly uniform. The authors explored the current sensitivity of FEAs to tip radius variation using different tip radius distributions and show that reducing the tip radius dispersion is an effective alternative to increasing the resistance of a current limiter for achieving uniform emission current. In order to reduce the tip radius dispersion, the authors use a trilevel resist process to increase the uniformity of the array of dots used as the etching mask for forming the silicon tips. SEM images show that they were able to reduce the standard deviation of the dot diameter by 60% using a trilevel resist process instead of a single layer resist process. Device characterization showed that the FEAs have a very narrow range of slopes, b FN, extracted from the Fowler–Nordheim plot, indicating that the field emitters within the FEA are highly uniform.The authors report silicon field emitter arrays (FEAs) that were fabricated using a trilevel resist process and are highly uniform. The authors explored the current sensitivity of FEAs to tip radius variation using different tip radius distributions and show that reducing the tip radius dispersion is an effective alternative to increasing the resistance of a current limiter for achieving uniform emission current. In order to reduce the tip radius dispersion, the authors use a trilevel resist process to increase the uniformity of the array of dots used as the etching mask for forming the silicon tips. SEM images show that they were able to reduce the standard deviation of the dot diameter by 60% using a trilevel resist process instead of a single layer resist process. Device characterization showed that the FEAs have a very narrow range of slopes, b FN, extracted from the Fowler–Nordheim plot, indicating that the field emitters within the FEA are highly uniform.

Mid-IR and UV-Vis-NIR Mueller matrix ellipsometry characterization of tunable hyperbolic metamaterials based on self-assembled carbon nanotubes

Abstract: Mueller matrix ellipsometry over the wide spectral range from the mid-IR to UV is applied to characterize the dielectric function tensor for films of densely packed single-walled carbon nanotubes aligned in the surface plane. These films optically act as metamaterials with an in-plane anisotropic, bulk effective medium response. A parameterized oscillator model is developed to describe electronic interband transitions, π − π ⋆ plasmon resonances, and free-carrier absorption. Wide ranges of hyperbolic dispersion are observed and exceptional tuneability of the hyperbolic ranges is demonstrated by comparing results for a film of aligned but unordered carbon nanotubes with a film fabricated under optimized conditions to achieve hexagonally close-packed alignment of the nanotubes. The effect of doping on the optical properties and hyperbolic range is discussed.

Stability of TiO2-coated ZnO photocatalytic thin films for photodegradation of methylene blue

Abstract: Investigations on the stability of titanium dioxide ( TiO 2)-coated zinc oxide (ZnO) thin films upon repeated uses for methylene blue (MB) degradation were conducted. Photocorrosion of ZnO, upon exposure to light in aqueous media, can affect the photocatalytic performance due to loss of material. Hence, coating with a more stable metal oxide was seen as a way to suppress the effects of photocorrosion. In this study, homogeneous wurtzite ZnO nanostructured thin films were obtained from thermal oxidation of sputter-deposited Zn films on glass substrates. TiO 2 was subsequently deposited onto the ZnO nanostructured thin films using a reactive magnetron sputtering system in an admixture of argon and oxygen gases. After deposition, the thin films were annealed at 500 °C for 1 h. The photocatalytic efficiency and stability of the thin films were investigated after multiple degradation cycles. The addition of a TiO 2 film increased the surface roughness and blueshifted the absorption edge of the ZnO thin films. The coated films obtained up to 94.3% degradation efficiency of MB after a 180-min exposure cycle using a solar light simulator. After three cycles, degradation efficiency decreased for the uncoated ZnO photocatalysts. Analysis of the MB solution after one degradation cycle revealed the presence of Zn 2 + ions attributed to the effects of photocorrosion. Higher Zn 2 + concentrations were observed when the ZnO surface is uncoated. This study showed that the addition of a thin, antiphotocorrosion material such as TiO 2 layer decreased the dissolution of ZnO caused by photocorrosion without a significant reduction in the photodegradation efficiency.

Extending the metal-induced gap state model of Schottky barriers

Abstract: Fermi level pinning at Schottky barriers strongly limits the minimization of contact resistances in devices and thereby limits the scaling of modern Si electronic devices, so it is useful to understand the full range of behaviors of Schottky barriers. The authors find that some semiconductor interfaces with compound metals like silicides have apparently weaker Fermi level pinning. This occurs as these metals have an underlying covalent skeleton, whose interfaces with semiconductors lead to miscoordinated defect sites that create additional localized interface states that go beyond the standard metal-induced gap states (MIGSs) model of Schottky barriers. This causes a stronger dependence of Schottky barrier height on the metal and on interface orientation. These states are argued to be an additional component needed to extend the MIGS model.

Nondestructive characterization of nanoscale subsurface features fabricated by selective etching of multilayered nanowire test structures using Mueller matrix spectroscopic ellipsometry based scatterometry

Abstract: Nondestructive measurement of three-dimensional subsurface features remains one of the most difficult and unmet challenges faced during the fabrication of three-dimensional transistor architectures, especially nanosheet and nanowire based field effect transistors. The most critical fabrication step is the selective etching of silicon-germanium subsurface layers. The resulting shape and dimensions of the remaining Si(1 − x)Gex structure strongly impacts further processing steps and ultimately the electrical performance of gate-all-around transistors, thus creating the need for accurate inline metrology. In order to demonstrate the ability to characterize this etch, nanowire test structures made from Si(1 − x)Gex/Si/Si(1 − x)Gex/Si/Si(1 − x)Gex/Si multilayers have been characterized using Mueller matrix spectroscopic ellipsometry based scatterometry. Transmission electron microscopy images were used to corroborate the authors’ scatterometry measurements. Here, they successfully demonstrate the ability to measure the Si(1 − x)Gex etch, providing an industrially viable technique for inline three-dimensional metrology.

Electron dynamics during the reignition of pulsed capacitively-coupled radio-frequency discharges

Abstract: The authors report on phase resolved optical emission spectroscopy (PROES) measurements of pulsed capacitive coupled plasmas (CCPs) through argon. The PROES results indicate that under some conditions, the electron heating mechanism can be changed substantially from that dominant in continuous CCPs. The normally dominant α heating mode of electropositive plasmas can be aided by a drift-ambipolar (DA) heating mode during the early portion of the reignition. The DA heating mode is ordinarily only found in electronegative discharges. The authors found that Ar discharges pulsed at 10 kHz only exhibited the α heating mode throughout the reignition process, while those pulsed at 0.1 kHz exhibited a mixed α and DA heating mode during the reignition. The differences in the two heating modes cause substantial differences in the spatial pattern of the light emission from the plasma in addition to an overshoot in the light emission intensity.

Inkjet-printed MoS2-based field-effect transistors with graphene and hexagonal boron nitride inks

Abstract: Field-effect transistors (FETs) are powerful devices in the semiconducting electronics industry and their manufacturing forms the basis of countless electronic devices. Most contemporary FETs rely on inorganic materials, mainly silicon that uses conventional photolithography, etching, and deposition techniques in sophisticated and expensive clean-room environments. An alternative route to fabricating FETs is via inkjet printing that offers the possibility of mass production and working with additively manufactured, low-cost materials, to form high functionality devices with applications in a wide array of fields. Although the inkjet-printed electrode-based sensor is widely reported, the number of all inkjet-printed FETs is still limited. Here, the authors report the design, fabrication, and characterization of an all inkjet-printed FET. Two-dimensional layered materials, such as electrically conducting graphene, semiconducting molybdenum disulfide (MoS2), and dielectric-hexagonal boron nitride (hBN), were used to construct the printed FET on an Si/SiO2 substrate. Here, the authors also present the annealing temperature analysis of the drop-cast hBN ink, which provided a clear outlook toward the printed dielectric layer fabrication of the transistor. To have an idea of the leakage current of the FET, the authors inkjet-printed a simple capacitor device first with graphene and hBN inks, which was characterized by using the small-signal impedance technique, capacitance-frequency (C-F), and capacitance-voltage (C-V), where the change in C was measured from F ∼ 1 kHz up to 5 MHz. At low frequency, ∼1 KHz, the maximum capacitance ∼36 pF was found at 20 V.

Absolute pressure and gas species identification with an optically levitated rotor

Abstract: The authors describe a novel variety of spinning-rotor vacuum gauge in which the rotor is a ∼ 4.7 − μ m − diameter silica microsphere, optically levitated. A rotating electrostatic field is used to apply torque to the permanent electric dipole moment of the silica microsphere and control its rotational degrees of freedom. When released from a driving field, the microsphere’s angular velocity decays exponentially with a damping time inversely proportional to the residual gas pressure and dependent on gas composition. The gauge is calibrated by measuring the rotor mass with electrostatic co-levitation and assuming a spherical shape, confirmed separately, and uniform density. The gauge is cross-checked against a capacitance manometer by observing the torsional drag due to a number of different gas species. The techniques presented can be used to perform absolute vacuum measurements localized in space, owing to the small dimensions of the microsphere and the ability to translate the optical trap in three dimensions, as well as measurements in magnetic field environments. In addition, the dynamics of the microsphere, paired with a calibrated vacuum gauge, can be used to measure the effective molecular mass of a gas mixture without the need for ionization and at pressures up to approximately 1 mbar.The authors describe a novel variety of spinning-rotor vacuum gauge in which the rotor is a ∼ 4.7 − μ m − diameter silica microsphere, optically levitated. A rotating electrostatic field is used to apply torque to the permanent electric dipole moment of the silica microsphere and control its rotational degrees of freedom. When released from a driving field, the microsphere’s angular velocity decays exponentially with a damping time inversely proportional to the residual gas pressure and dependent on gas composition. The gauge is calibrated by measuring the rotor mass with electrostatic co-levitation and assuming a spherical shape, confirmed separately, and uniform density. The gauge is cross-checked against a capacitance manometer by observing the torsional drag due to a number of different gas species. The techniques presented can be used to perform absolute vacuum measurements localized in space, owing to the small dimensions of the microsphere and the ability to translate the optical trap in three ...

First-principles study of clean tungsten surface work function under electric field

Abstract: The effect of an external electric field on the work functions of clean tungsten (W) surfaces, W (100), W (110), and W (111) has been investigated using first-principles calculations based on density functional theory. By applying an electric field from 0 up to 0.3 V/A, the effective and local work functions can be determined for comparison after employing five different pseudopotentials. It is found that as the electric field increases, the work functions of tungsten surfaces reduce accordingly. A reduction of work function can be as large as ∼0.35 eV. Based on these calculations, a new scaling law of work function reduction due to the charge transfer near the metal/vacuum interface caused by an external electric field is obtained. In addition, the local work function is found to be closely related to the charge density distribution. With this approach, field emission properties of metals can be better understood and described.

One-dimensional simulation of Couette–Poiseuille flow with variable cross section for the full range of gas rarefaction

Abstract: Clearance mass flows are a major loss mechanism in dry running rotatory positive displacement vacuum pumps. Therefore, a detailed knowledge of the clearance mass flow is crucial to calculate the operation of those pumps. The small clearance heights and the large pressure range of such pumps require a wide range of gas rarefaction parameters to be taken into account. The flow in the clearance can be described as a combined Couette–Poiseuille flow with variable cross section. This is typically done by solving the Stokes equation, but especially at high gas rarefaction parameters, the inertia cannot be neglected any more, which can lead to choking of the flow. A one-dimensional approach for the compressible fluid flow was provided by Shapiro. It is shown that this approach can be carried over for the rarefied gas flow. The problem is solved in bounds for the constant total temperature and compared to experimental investigations by varying the pressure ratio and the circumferential speed of the clearance boundary in a wide range of gas rarefaction parameters.

Sol-gel synthesized indium tin oxide as a transparent conducting oxide with solution-processed black phosphorus for its integration into solar-cells

Abstract: In this work, indium tin oxide (ITO) thin films were synthesized using solgel processing with a mixture of InCl3, methanol, and SnCl2, where the solutions were spin coated onto glass substrates. The maximum transmittance of the ITO thin film in the visible region was found to be ∼75% for films annealed at 650 °C, where plasma treatment of the substrate was found to aid in the large-area continuity and homogeneity over the glass substrates compared to films annealed at lower temperatures. Two-dimensional (2D), semiconducting black phosphorus (BP) dispersions were then prepared by liquid exfoliation, where the black phosphorus bulk crystals were finely ground inside a glove box and dissolved in N-cyclohexyl-2-pyrrolidone. Following further treatment, the BP solution dispersions were drop cast onto the transparent ITO thin films to form heterostructures toward transparent electronics and future solar cell applications. Direct electrical probing of the black phosphorus revealed that it was electrically conducting and the currents measured were large on the order of a few microampere at ∼20 V. Raman and photoluminescence measurements on the black phosphorus revealed that the flakes ranged in thickness from few-layers up to bulk. Few-layer black phosphorus can be distinguished from the bulk through the red-shift of the Ag1, Bg2, and Ag2 peaks for bulk black phosphorus flakes compared to the few-layers’ black phosphorus flakes. Electrical measurements made in the heterostructure interfaces showed a higher magnitude of currents at the black phosphorus interface compared to the bare ITO film. The combined architecture of black phosphorus on ITO thin films shows promise in its use for transparent electronics, which can also serve as a stepping stone for future solar cell platforms.

Thermal degradation behavior of self-assembled monolayer surfactant on silicon substrate

Abstract: In nanoimprint lithography, a release agent on the mold surface is usually necessary for easy demolding between the mold and the imprinted (thermal) resist. In this work, the thermal stability of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) monolayers is studied using x-ray photoelectron spectroscopy. The FDTS monolayers are deposited on Si (100) substrates via vapor phase reactions. Significant fluorine desorption of the monolayers is observed for samples annealed at 250 and 300 °C in air. The fluorine coverage decreases as a function of annealing time at a given annealing temperature. The desorption is proposed to be dependent on the monolayer packing details and may be influenced by the intermolecular heat transfer. Removal of the CF3 groups is found to be faster than that of the CF2 group as evidenced by the CF2/CF3 peak area ratios that increase with the annealing time. Sessile drop water contact angle and fluorine coverage evolution results show that the estimated useful coating lifetime is 180 min when the samples are annealed at 300 °C and ∼560 min when annealed at 250 °C. The peak position of the binding energy of the F 1s spectral line is related to the monolayer fluorine coverage and it may be a result of the interactions between the molecular chain and the negatively charged silicon substrate. Furthermore, nearly no chain desorption is detected for samples annealed in an inert environment, which may be attributed to the elimination of reactive oxygen and moisture molecules. The thermal degradation behaviors in ambient and inert atmosphere provide useful information for designing a nanoimprint process for the commercial manufacturing of polymeric microstructure and nanostructure.

Dry etching strategy of spin-transfer-torque magnetic random access memory: A review

Abstract: The spin-based memory, spin transfer torque-magnetic random access memory (STT-MRAM), has the potential to enhance the power efficiency of high density memory systems. Its desirable characteristics include nonvolatility, fast operation, and long endurance. However, dry etching of MRAM structures remains a challenge as the industry is ramping up its production. In this paper, we explore the etching strategies that have been used to etch the MRAM structures. Several etching techniques have been developed to attain optimal device performance. These are reactive ion etching, time modulated plasma etching, atomic layer etching, and ion beam etching. Sidewall profile, sidewall contamination or damage, redeposition, selectivity, and noncorrosiveness are the main factors to consider while selecting the best etching methods. This paper starts with the fundamentals of MRAM reading, writing, and storing principles and finishes with the current approaches to solve the etch challenges. For etching, the most commonly used magnetic materials such as CoFeB, CoFe, and NiFe are covered in this article.

Electrostatic shielding versus anode-proximity effect in large area field emitters

Abstract: Field emission of electrons crucially depends on the enhancement of the local electric field around nanotips. The enhancement is maximum when individual emitter-tips are well separated. As the distance between two or more nanotips decreases, the field enhancement at individual tips reduces due to the shielding effect. The anode-proximity effect acts in quite the opposite way, increasing the local field as the anode is brought closer to the emitter. For isolated emitters, this effect is pronounced when the anode is at a distance less than three times the height of the emitter. It is shown here that for a large area field emitter (LAFE), the anode-proximity effect increases dramatically and can counterbalance shielding effects to a large extent. Also, it is significant even when the anode is far away. The apex field enhancement factor for an LAFE in the presence of an anode is derived using the line charge model. It is found to explain the observations well and can accurately predict the apex enhancement factors. The results are supported by numerical studies using comsol multiphysics.

Field emission from nanotubes and flakes of transition metal dichalcogenides

Abstract: Transition metal dichalcogenides such as MoS2 and WS2 are low-dimensional semiconductor materials. MoS2 and WS2 nanotubes and flakes were grown by a chemical transport reaction under a temperature gradient. I2 was used as a transport agent for previously synthesized MoS2 and WS2, respectively. These multilayered nanotubes are indirect bandgap semiconductors with a bandgap depending on their diameter. WS2 flakes were prepared by the sulfurization of thin WOx flakes. To increase the field enhancement of such low-dimensional structures by a higher aspect ratio, two approaches were examined: (a) the MoS2 and WS2 nanotubes were attached individually by a focused ion beam with Pt on dry etched n-type Si pillars and (b) the WS2 flakes were grown directly on the surface of the (n-type and p-type) Si pillars. Integral field emission measurements were performed in a diode configuration with a 50 μm mica spacer in a vacuum chamber at pressures of about 10−9 mbar. At a voltage of 900 V (18 MV/m), the integral emission current from the nanotubes is up to 11 μA for the lateral mounted MoS2 and about 1.3 μA (1.0 μA) for the upright mounted WS2 (MoS2). The onset voltage for a current of 1 nA is about 550 V for MoS2 and 500 V for WS2, respectively. The voltage conversion factor is in the range of 6 × 104–8 × 104 cm−1 for the nanotubes. The mounted MoS2 flakes show a field emission current of about 6 μA at 18 MV/m in contrast to the directly grown WS2 flakes, which show a pronounced saturation regime and, therefore, a lower emission current of about 0.5 μA is reached at 1500 V (25 MV/m). The WS2 flakes show a two times higher (1 × 105 cm−1) voltage conversion factor in comparison to the MoS2 flakes (5 × 104 cm−1). The extracted characteristics of the current-limiting part show a difference in the behavior of the extracted current-limiting characteristics between the lateral (linear) and upright mounted (exponential) nanotubes and the MoS2 flakes. In contrast, the WS2 flakes show charge carrier depletion effects.

Schottky conjecture and beyond

Abstract: The “Schottky conjecture” deals with the electrostatic field enhancement at the tip of compound structures such as a hemiellipsoid on top of a hemisphere. For such a 2-primitive compound structure, the apex field enhancement factor γ a ( C ) is conjectured to be multiplicative ( γ a ( C ) = γ a ( 1 ) γ a ( 2 )), provided the structure at the base (labeled 1, e.g., the hemisphere) is much larger than the structure on top (referred to as crown and labeled 2, e.g., the hemiellipsoid). The author first demonstrates numerically that, for generic smooth structures, the conjecture holds in the limiting sense when the apex radius of curvature of the primitive-base R a ( 1 ) is much larger than the height of the crown h 2 (i.e., h 2 / R a ( 1 ) → 0). If the condition is somewhat relaxed, the author shows that it is the electric field above the primitive-base (i.e., in the absence of the crown), averaged over the height of the crown, that gets magnified instead of the field at the apex of the primitive-base. This observation leads to the corrected Schottky conjecture (CSC), which, for 2-primitive structures, reads as γ a ( C ) ≃ ⟨ γ a ( 1 ) ⟩ γ a ( 2 ), where ⟨ ⋅ ⟩ denotes the average value over the height of the crown. For small protrusions ( h 2 / h 1 typically less than 0.2), ⟨ γ a ( 1 ) ⟩ can be approximately determined using the line charge model so that γ a ( C ) ≃ γ a ( 1 ) γ a ( 2 ) ( 2 R a ( 1 ) / h 2 ) ln ⁡ ( 1 + h 2 / 2 R a ( 1 ) ). The error is found to be within 1 % for h 2 / R a ( 1 ) < 0.05, increasing to about 3 % (or less) for h 2 / R a ( 1 ) = 0.1 and bounded below 5% for h 2 / R a ( 1 ) as large as 0.5. The CSC is also found to give good results for 3-primitive compound structures. The relevance of the CSC for field emission is discussed.The “Schottky conjecture” deals with the electrostatic field enhancement at the tip of compound structures such as a hemiellipsoid on top of a hemisphere. For such a 2-primitive compound structure, the apex field enhancement factor γ a ( C ) is conjectured to be multiplicative ( γ a ( C ) = γ a ( 1 ) γ a ( 2 )), provided the structure at the base (labeled 1, e.g., the hemisphere) is much larger than the structure on top (referred to as crown and labeled 2, e.g., the hemiellipsoid). The author first demonstrates numerically that, for generic smooth structures, the conjecture holds in the limiting sense when the apex radius of curvature of the primitive-base R a ( 1 ) is much larger than the height of the crown h 2 (i.e., h 2 / R a ( 1 ) → 0). If the condition is somewhat relaxed, the author shows that it is the electric field above the primitive-base (i.e., in the absence of the crown), averaged over the height of the crown, that gets magnified instead of the field at the apex of the...

Field emission microscopy of carbon nanotube fibers: evaluating and interpreting spatial emission

Abstract: In this work, the authors quantify field emission properties of cathodes made from carbon nanotube (CNT) fibers. The cathodes were arranged in different configurations to determine the effect of cathode geometry on the emission properties. Various geometries were investigated including (1) flat cut fiber tip, (2) folded fiber, (3) looped fiber, and (4) fibers wound around a cylinder. The authors employ a custom field emission microscope to quantify I-V characteristics in combination with laterally resolved field-dependent electron emission area. Additionally, they look at the very early emission stages, first when a CNT fiber is turned on for the first time, which is then followed by multiple ramp-up/down runs. Upon the first turn on, all fibers demonstrated limited and discrete emission area. During ramping runs, all CNT fibers underwent multiple (minor and/or major) breakdowns, which improved emission properties in that turn-on field decreased and field enhancement factor and emission area both increased. It is proposed that breakdowns are responsible for removing initially undesirable emission sites caused by stray fibers higher than average. This initial breakdown process gives way to a larger emission area that is created when the CNT fiber subcomponents unfold and align with the electric field. The authors' results form the basis for careful evaluation of CNT fiber cathodes for dc or low frequency pulsed power systems in which large uniform area emission is required or for narrow beam high frequency applications in which high brightness is a must.

Nanoscale etching of perovskite oxides for field effect transistor applications

Abstract: The etching of epitaxially grown perovskite oxide BaSnO3 (BSO) and BaTiO3 (BTO) thin films is studied using Cl-based (BCl3/Ar) and F-based (CF4/Ar) plasma chemistries in an inductively coupled plasma reactive ion etching (ICP-RIE) system for the development of field effect transistors (FETs). It is found that the BCl3/Ar process has a time-independent and a higher etch rate and creates a smooth etched surface, while the etch rate of BSO and BTO in CF4/Ar plasma decreases with the etching time duration. For the BCl3/Ar etching process, the etch rate increases with both ion density and ion energy, suggesting the combination of chemical plasma etching and physical ion sputtering mechanisms. Using the Cl-based etching process, BaSnO3 and BaTiO3 heterojunction FETs are developed. The devices with a gate length of 1.5 μm have a saturation current density of 287.6 mA/mm, a maximum transconductance of gm = 91.3 mS/mm, an FET mobility of 45.3 cm2/V s, and a threshold voltage of −1.75 V. The etching processes developed in this work will enable further development of perovskite oxide heterostructure electronic devices.The etching of epitaxially grown perovskite oxide BaSnO3 (BSO) and BaTiO3 (BTO) thin films is studied using Cl-based (BCl3/Ar) and F-based (CF4/Ar) plasma chemistries in an inductively coupled plasma reactive ion etching (ICP-RIE) system for the development of field effect transistors (FETs). It is found that the BCl3/Ar process has a time-independent and a higher etch rate and creates a smooth etched surface, while the etch rate of BSO and BTO in CF4/Ar plasma decreases with the etching time duration. For the BCl3/Ar etching process, the etch rate increases with both ion density and ion energy, suggesting the combination of chemical plasma etching and physical ion sputtering mechanisms. Using the Cl-based etching process, BaSnO3 and BaTiO3 heterojunction FETs are developed. The devices with a gate length of 1.5 μm have a saturation current density of 287.6 mA/mm, a maximum transcond...

Effect of proton irradiation on the mobility of two-dimensional electron in AlGaN/AlN/GaN high electron mobility transistors at low temperature

Abstract: The authors simulated the damage caused by proton irradiation to the device and analyzed the effect of proton irradiation on two-dimensional electron mobility taking various scattering mechanisms into account. Proton-irradiation simulation of the AlGaN/AlN/GaN HEMT device was carried out to obtain the irradiation simulation results by using SRIM software. Then, considering various scattering mechanisms, the authors established a model to simulate two-dimensional electron mobility under different proton energy and irradiation doses at low temperature. The theoretical data show that proton irradiation significantly decreased the mobility of a two-dimensional electron in a GaN-based HEMT at low temperature.

Synthesis of TiN/N-doped TiO2 composite films as visible light active photocatalyst

Abstract: Titanium nitride/nitrogen-doped titanium oxide (TiN/N-doped TiO 2) composite films were synthesized for visible light photodegradation applications. Thin films of TiN were sputter-deposited on precleaned glass substrates in an admixture of argon and nitrogen gases. The grown TiN films were subsequently oxidized in air at 350 °C at 15, 30, and 60 min. Raman spectral analysis revealed the formation of TiO 2 with anatase structure at 15 min and transitioned to the rutile structure at longer oxidation times. X-ray photoelectron spectral analysis revealed the formation of N-doped TiO 2 from the oxidized Ti. Visible light-induced photodegradation of methylene blue as test analyte showed 30% removal efficiency after exposure to visible light after 2.5 h. The highest degradation efficiency was observed when the anatase phase of TiO 2 is the dominant phase in the film. Moreover, N-doping realized the visible light sensitivity of TiO 2. This makes the composite film ideal for solar light-driven photodegradation of organic contaminants in wastewater.

Comparison of macroscopic and microscopic emission characteristics of large area field emitters based on carbon nanotubes and graphene

Abstract: Nanostructured multitip surfaces have sufficient potential to obtain the high emission currents necessary to develop stable and noninertial sources of free electrons with increased levels of permissible currents. The key to understanding the processes of formation and stability of macroscopic emission currents from these large area field emitters (LAFEs) is assessing the local characteristics of individual emission sites. Herein, a method for determining the local emission characteristics of nanoscale emission sites is developed via processing the glow pattern data and a system for rapidly recording the current–voltage characteristics of LAFEs.

Procedure for robust assessment of cavity deformation in Fabry─Pérot based refractometers

Abstract: A novel procedure for a robust assessment of cavity deformation in Fabry–Perot (FP) refractometers is presented. It is based on scrutinizing the difference between two pressures: one assessed by the uncharacterized refractometer and the other provided by an external pressure reference system, at a series of set pressures for two gases with dissimilar refractivity (here, He and N 2). By fitting linear functions to these responses and extracting their slopes, it is possible to construct two physical entities of importance: one representing the cavity deformation and the other comprising a combination of the systematic errors of a multitude of physical entities, viz., those of the assessed temperature, the assessed or estimated penetration depth of the mirror, the molar polarizabilities, and the set pressure. This provides a robust assessment of cavity deformation with small amounts of uncertainties. A thorough mathematical description of the procedure is presented that serves as a basis for the evaluation of the basic properties and features of the procedure. The analysis indicates that the cavity deformation assessments are independent of systematic errors in both the reference pressure and the assessment of gas temperature and when the gas modulation refractometry methodology is used that they are insensitive to gas leakages and outgassing into the system. It also shows that when a high-precision (sub-ppm) refractometer is characterized according to the procedure, when high purity gases are used, the uncertainty in the deformation contributes to the uncertainty in the assessment of pressure of N 2 with solely a fraction (13%) of the uncertainty of its molar polarizability, presently to a level of a few ppm. This implies, in practice, that cavity deformation is no longer a limiting factor in FP-based refractometer assessments of pressure of N 2.

nanolithography toolbox—Simplifying the design complexity of microfluidic chips

Abstract: Microfluidic devices typically require complex shapes such as funnels, spirals, splitters, channels with different widths, or customized objects of arbitrary complexity with a smooth transition between these elements. Device layouts are generally designed by software developed for the design of integrated circuits or by general computer-aided design drawing tools. Both methods have their limitations, making these tasks time consuming. Here, a script-based, time-effective method to generate the layout of various microfluidic chips with complex geometries is presented. The present work uses the nanolithography toolbox (NT), a platform-independent software package, which employs parameterized fundamental blocks (cells) to create microscale and nanoscale structures. In order to demonstrate the functionality and efficiency of the NT, a few classical microfluidic devices were designed using the NT and then fabricated in glass/silicon using standard microfabrication techniques and in poly(dimethylsiloxane) using soft lithography as well as more complex techniques used for flow-through calorimetry. In addition, the functionality of a few of the fabricated devices was tested. The powerful method proposed allows the creation of microfluidic devices with complex layouts in an easy way, simplifying the design process and improving design efficiency. Thus, it holds great potential for broad applications in microfluidic device design.

Effects of mask material conductivity on lateral undercut etching in silicon nano-pillar fabrication

Abstract: High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contact. It is also reported that a higher bias power leads to less undercut than a lower one, due to increased and more directional physical bombardment by ions.High aspect ratio silicon structures have gained significant interest due to their vast applications. Minimal lateral etch under the mask is essential to achieve such high aspect ratio structures. Previously, the authors reported that chromium oxide is better than metallic chromium as a hard mask for silicon etching in terms of etch rate and selectivity to resist during mask structure fabrication. Here, it is reported that a metal oxide etch mask also gives less lateral etch than a metal etch mask. Following mask structure fabrication by electron beam lithography and lift-off, silicon was etched using a nonswitching (i.e., SF6 and C4F8 gases simultaneously injected into a chamber) pseudo-Bosch process. The amount of lateral etching right underneath the mask is less (roughly half) for Cr2O3 and Al2O3 masks than Cr or Al masks. One plausible explanation for the difference is the metal-assisted plasma etching effect where the metal catalyzes the chemical reaction by injecting holes into the silicon in contac...

Quantum and transport lifetimes in optically induced GaN/AlGaN 2DEGs grown on bulk GaN

Abstract: A two-dimensional electron gas (2DEG) is absent in ultrapure GaN/Al0.06Ga0.94N heterostructures grown by molecular beam epitaxy on bulk GaN at 300 K and in the dark. However, such a 2DEG can be generated by UV illumination and persists at low temperature after blanking the light. Under steady UV illumination as well as under persistence conditions, pronounced quantum transport with Shubnikov–de Haas oscillations commencing below 2 T is observed. The low temperature 2DEG mobility amounts to only ∼20 000 cm2/V s, which is much lower than predicted for the dominant scattering mechanisms in GaN/AlGaN heterostructures grown on GaN with low threading dislocation density. A rather small ratio of the transport and quantum lifetimes τt/τq of ∼10 points at elastic scattering events limiting both the transport and quantum lifetimes.

Atomic spectroscopy and laser frequency stabilization with scalable micrometer and sub-micrometer vapor cells

Abstract: We report on the atomic spectroscopy and laser frequency stabilization using a new type of a miniaturized glass vapor cell with a scalable thickness varying from 500 nm up to 8 μm. The cell is fabricated by lithography and etching techniques in a Pyrex glass substrate, followed by anodic bonding. It is filled with rubidium vapor using a distillation procedure. This simple and cost-effective fabrication method provides an attractive and compact solution for atomic cells, with applications in quantum metrology, sensing, communication, and light-vapor manipulations at the subwavelength scale. Using the fabricated cell, we have performed fluorescence and transmission spectroscopy of the Rubidium D2 line and observed sub-Doppler broadened lines. As an example, for a potential application, we have used the fabricated cell to demonstrate the stabilization of a 780 nm diode laser to the level about 10−10 in fractional frequency.