Showing papers in "Journal of Vacuum Science & Technology B in 2019"

Review Article: Synthesis, properties, and applications of fluorescent diamond particles.

Abstract: Diamond particles containing color centers—fluorescent crystallographic defects embedded within the diamond lattice—outperform other classes of fluorophores by providing a combination of unmatched photostability, intriguing coupled magneto-optical properties, intrinsic biocompatibility, and outstanding mechanical and chemical robustness. This exceptional combination of properties positions fluorescent diamond particles as unique fluorophores with emerging applications in a variety of fields, including bioimaging, ultrasensitive metrology at the nanoscale, fluorescent tags in industrial applications, and even potentially as magnetic resonance imaging contrast agents. However, production of fluorescent nanodiamond (FND) is nontrivial, since it requires irradiation with high-energy particles to displace carbon atoms and create vacancies—a primary constituent in the majority color centers. In this review, centrally focused on material developments, major steps of FND production are discussed with emphasis on current challenges in the field and possible solutions. The authors demonstrate how the combination of fluorescent spectroscopy and electron paramagnetic resonance provides valuable insight into the types of radiation-induced defects formed and their evolution upon thermal annealing, thereby guiding FND performance optimization. A recent breakthrough process allowing for production of fluorescent diamond particles with vibrant blue, green, and red fluorescence is also discussed. Finally, the authors conclude with demonstrations of a few FND applications in the life science arena and in industry.

Review Article: Atomic layer deposition of optoelectronic materials

Abstract: Optoelectronic materials can source, detect, and control light wavelengths ranging from gamma and x rays to ultraviolet, visible, and infrared regions. Optoelectronic devices are usually systems that transduce electricity to optical signal or vice versa. Optoelectronic devices include many modern necessities such as lamps, displays, lasers, solar cells, and various photodetectors. Some important research topics in the field of optoelectronics materials are development of new materials, new technologies for fabricating materials, and design of device structures. Atomic layer deposition (ALD) is a technology that was developed in the early 1970s for manufacturing high-quality luminescent and dielectric films to be used in AC-driven thin film electroluminescent (TFEL) displays. Monochromic yellow-black displays based on a ZnS:Mn luminescent layer have been manufactured industrially using ALD since the mid-1980s. Multicolor displays (green-yellow-red) were successfully realized by filtering the broad emission band of ZnS:Mn or adding another luminescent material, e.g., green-emitting ZnS:Tb or SrS:Ce. However, applicable full-color AC TFEL devices could not be developed because of the lack of an efficient deep blue-emitting phosphor. Currently, the most promising application area in TFEL displays is transparent displays, which are commonly used in various vehicles. In the mid-1980s, epitaxial III-V semiconductors were studied using ALD. It was shown that manufacturing real epitaxial [atomic layer epitaxy (ALE)] films is possible for different III (Al, Ga, In) and V (N, P, As) materials. The advantages of ALE processing compared to more traditional metalorganic chemical vapor deposition or molecular beam epitaxy methods have remained low, however, and ALE is not used on a large scale. Research continues to be carried out using ALE, especially with nitride films. Thin film solar cells have continuously received attention in ALD research. ALD films may be used as both an absorber (CdTe, SnS) and a passivation [In2S3, Zn(O,S)] material. However, in the solar cell field, the real industrial-level use is in passivation of silicon cells. Thin ALD Al2O3 film effectively passivates all types of silicon cells and improves their efficiency. Transition metal dichalcogenides are emerging 2D materials that have potential uses as channel materials in field-effect transistors, as well as phototransistors and other optoelectronic devices. The problem with achieving large-scale use of these 2D materials is the lack of a scalable, low-temperature process for fabricating high-quality, large-area films. ALD is proposed as a solution for these limitations. This review covers all of these ALD applications in detail.Optoelectronic materials can source, detect, and control light wavelengths ranging from gamma and x rays to ultraviolet, visible, and infrared regions. Optoelectronic devices are usually systems that transduce electricity to optical signal or vice versa. Optoelectronic devices include many modern necessities such as lamps, displays, lasers, solar cells, and various photodetectors. Some important research topics in the field of optoelectronics materials are development of new materials, new technologies for fabricating materials, and design of device structures. Atomic layer deposition (ALD) is a technology that was developed in the early 1970s for manufacturing high-quality luminescent and dielectric films to be used in AC-driven thin film electroluminescent (TFEL) displays. Monochromic yellow-black displays based on a ZnS:Mn luminescent layer have been manufactured industrially using ALD since the mid-1980s. Multicolor displays (green-yellow-red) were successfully realized by filtering the broad emission...

Polarization switching kinetics of the ferroelectric Al-doped HfO2 thin films prepared by atomic layer deposition with different ozone doses

Abstract: Ferroelectric switching kinetics of the Al-doped HfO2 (Al:HfO2) thin films prepared by the atomic layer deposition process were investigated by varying the dose time of oxygen precursor (O3). When the O3 dose time was reduced to 3 s, the Al:HfO2 films exhibited an enhanced remnant polarization (2Pr) of 10.2 μC/cm2 due to the suppression of the monoclinic phase and the increase in the ratio of oxygen vacancy. Double-pulse switching and the Kolmogorov–Avrami–Ishibashi model were used to obtain detailed quantitative information on the switching kinetics of the Al:HfO2 films. The estimated values of switching time and activation energy showed the strong dependence of O3 dose. This suggests that the O3 dose condition can be a key control parameter to modulate the ferroelectric polarization switching dynamics of the Al:HfO2 thin films.

Electronic structure and dielectric function of Mn-Bi-Te layered compounds

Abstract: A comparative study of the electronic and optical properties of Mn-Bi-Te layered compounds was carried out using spectroscopic ellipsometry (SE) over a photon energy range of 0.7–6.5 eV at room temperature and density functional theory (DFT)-based first-principle calculations within the general gradient approximation with Hubbard like correction (GGA+U) and allowance for a spin-orbital coupling. The total energies of the above compounds in ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations are obtained by taking the long-range van der Waals interaction into account. The stability of the AFM state of MnBi2Te4 and MnBi4Te7 over the corresponding FM counterpart is disclosed. The SE-based and calculated dielectric functions are compared. It is shown that interband optical transitions in the accessed photon energy range mainly occur between Mn 3d + Te 5p states of the valence band and Bi 6p + Te 5p with a small admixture of Mn 3d states of the conduction band.A comparative study of the electronic and optical properties of Mn-Bi-Te layered compounds was carried out using spectroscopic ellipsometry (SE) over a photon energy range of 0.7–6.5 eV at room temperature and density functional theory (DFT)-based first-principle calculations within the general gradient approximation with Hubbard like correction (GGA+U) and allowance for a spin-orbital coupling. The total energies of the above compounds in ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations are obtained by taking the long-range van der Waals interaction into account. The stability of the AFM state of MnBi2Te4 and MnBi4Te7 over the corresponding FM counterpart is disclosed. The SE-based and calculated dielectric functions are compared. It is shown that interband optical transitions in the accessed photon energy range mainly occur between Mn 3d + Te 5p states of the valence band and Bi 6p + Te 5p with a small admixture of Mn 3d states of the conduction band.

CuAl2 thin films as a low-resistivity interconnect material for advanced semiconductor devices

Abstract: New interconnect materials that have a low line resistivity are required to address issues associated with the increased resistivity due to the aggressive downscaling of future semiconductor devices. In this work, CuAl2 thin films are investigated as a potential material for liner- and barrier-free interconnect applications. The results show that CuAl2 blanket films adhere well to and do not undergo interdiffusion with SiO2, as well as having a favorable size effect of resistivity. Furthermore, the filling of CuAl2 in narrow low-k trenches is investigated, and an excellent gap-filling performance is registered. These features suggest that CuAl2 is a promising alternative to Cu that does not require any additional liner or barrier layers for feature sizes less than 10 nm.

Characterization of ferroelectric hafnium/zirconium oxide solid solutions deposited by reactive magnetron sputtering

Abstract: The room temperature deposition of 10 nm-thick ferroelectric hafnium/zirconium oxide, ( Hf , Zr ) O 2, thin solid films is achieved with a single hafnium/zirconium, Hf/Zr, alloy target by reactive magnetron sputtering. After rapid thermal annealing (RTA), crystallization of our samples is analyzed by grazing incidence x-ray diffraction. Changing the pressure inside the chamber during deposition leads to grow amorphous or monoclinic phase (m-phase). The authors demonstrate that if the ( Hf , Zr ) O 2 films are crystallized in the m-phase after deposition, no ferroelectric/orthorhombic phase can be obtained further. On the contrary, when the as-deposited film is amorphous, the ferroelectric/orthorhombic phase appears after the RTA.The room temperature deposition of 10 nm-thick ferroelectric hafnium/zirconium oxide, ( Hf , Zr ) O 2, thin solid films is achieved with a single hafnium/zirconium, Hf/Zr, alloy target by reactive magnetron sputtering. After rapid thermal annealing (RTA), crystallization of our samples is analyzed by grazing incidence x-ray diffraction. Changing the pressure inside the chamber during deposition leads to grow amorphous or monoclinic phase (m-phase). The authors demonstrate that if the ( Hf , Zr ) O 2 films are crystallized in the m-phase after deposition, no ferroelectric/orthorhombic phase can be obtained further. On the contrary, when the as-deposited film is amorphous, the ferroelectric/orthorhombic phase appears after the RTA.

Diffusion of implanted Ge and Sn in β-Ga2O3

Abstract: The n-type dopants, Ge and Sn, were implanted into bulk (−201) β-Ga2O3 at multiple energies (60, 100, 200 keV) and total doses of ∼1014 cm−2 and annealed at 1100 °C for 10–120 s under either O2 or N2 ambients. The Ge-implanted samples showed almost complete recovery of the initial damage band under these conditions, with the disordered region decreasing from 130 to 17 nm after 1100 °C anneals. Fitting of secondary ion mass spectrometry profiles was used to obtain the diffusivity of both Ge and Sn, with values at 1100 °C of 1.05 × 10−11 cm s−1 for Ge and 2.7 × 10−13 cm s−1 for Sn for annealing under O2 ambients. Some of the dopant is lost to the surface during these anneals, with a surface outgas rate of 1–3 × 10−7 s−1. By sharp contrast, the redistribution of both dopants was almost completely suppressed during annealing in N2 ambients under the same conditions, showing the strong influence of point defects on dopant diffusivity of these implanted dopants in β-Ga2O3.

Forward bias degradation and thermal simulations of vertical geometry β-Ga2O3 Schottky rectifiers

Abstract: Vertical geometry β-Ga2O3 Schottky rectifiers of various sizes were deliberately stressed at a high forward current density level until a sudden decrease of reverse bias breakdown voltage was observed. The diodes were fabricated on an Sn-doped (n = 3.6 × 1018 cm−3) (001) β-Ga2O3 single crystal substrate with a 10 μm epilayer grown by halide vapor phase epitaxy with a carrier concentration of 3.5 × 1016 cm−3. The forward bias stressing caused reverse breakdown degradation and thermally induced failure on both the Ni/Au Schottky contact and the epitaxial layer due to the low thermal conductivity of Ga2O3. The resulting temperature distributions at forward bias under different current conditions were simulated using 3D finite element analysis. The temperature profile at the surface during the rectifier turn-on period shows a strong dependence with crystalline orientation, evidenced by infrared camera measurements. The maximum junction temperature rise occurs at the center of the metal contact and is in the range of 270–350 °C.

Isotropic silicon etch characteristics in a purely inductively coupled SF6 plasma

Abstract: The characteristics of isotropic etching of silicon in a purely inductively coupled SF 6 plasma are quantitatively studied. Since the etch results are strongly dependent on mask features, the authors investigated both large area and narrow trench etch characteristics. Circles of diameter 500 μm were used as a proxy for unpatterned surfaces and etched for different durations to establish the material etch rate and surface roughness. The average etch rate using the chosen recipe was found to be 2.27 μm/min. Arrays of narrow trenches ranging from 8 to 28 μm were also etched to analyze the effect of trench size on etch rate and degree of anisotropy. The etch rate of the trenches was found to strongly decrease with decreasing trench width. The results demonstrate that isotropic SF 6 etch can be readily used as a replacement for more exotic silicon vapor phase etch chemistries such as XeF 2.The characteristics of isotropic etching of silicon in a purely inductively coupled SF 6 plasma are quantitatively studied. Since the etch results are strongly dependent on mask features, the authors investigated both large area and narrow trench etch characteristics. Circles of diameter 500 μm were used as a proxy for unpatterned surfaces and etched for different durations to establish the material etch rate and surface roughness. The average etch rate using the chosen recipe was found to be 2.27 μm/min. Arrays of narrow trenches ranging from 8 to 28 μm were also etched to analyze the effect of trench size on etch rate and degree of anisotropy. The etch rate of the trenches was found to strongly decrease with decreasing trench width. The results demonstrate that isotropic SF 6 etch can be readily used as a replacement for more exotic silicon vapor phase etch chemistries such as XeF 2.

Study of damage generation induced by focused helium ion beam in silicon

Abstract: Helium ion microscope (HIM) has presented an outstanding ability to image and nanofabricate thin film and two-dimensional materials with high precision. However, the concomitant damage and implantation induced by focused helium ion beam should influence the imaging quality and nanomachining efficiency inevitably, especially for bulk samples. In this work, the authors investigated the generation process of damages at nanoscales in single crystalline bulk silicon caused by ions implantation in HIM using transmission electron microscopy. The dependence of implantation and damage on ion dose, ion energy, and beam current was also discussed and analyzed. It was found that the damage should be originated from the local defects caused by ion implantation and the crystal structure could be gradually destroyed and transform into amorphous silicon with the generation and growth of subsurface nanobubbles as ion dose increased. The local concentration of implanted helium ion was found as a universal factor to impact on the damage level and the size of nanobubbles directly. These findings not only shed lights on the effective imaging and nanofabrication of HIM but also provide a further understanding in the nuclear irradiation area.Helium ion microscope (HIM) has presented an outstanding ability to image and nanofabricate thin film and two-dimensional materials with high precision. However, the concomitant damage and implantation induced by focused helium ion beam should influence the imaging quality and nanomachining efficiency inevitably, especially for bulk samples. In this work, the authors investigated the generation process of damages at nanoscales in single crystalline bulk silicon caused by ions implantation in HIM using transmission electron microscopy. The dependence of implantation and damage on ion dose, ion energy, and beam current was also discussed and analyzed. It was found that the damage should be originated from the local defects caused by ion implantation and the crystal structure could be gradually destroyed and transform into amorphous silicon with the generation and growth of subsurface nanobubbles as ion dose increased. The local concentration of implanted helium ion was found as a universal factor to impact ...

Next generation of extreme-resolution electron beam lithography

Abstract: In top down nanofabrication research facilities around the world, the direct-write high-resolution patterning tool of choice is overwhelmingly electron beam lithography. Remarkably small features can be written in a variety of polymeric resists [V. R. Manfrinato et al., Nano Lett. 14, 4406 (2014); V. R. Manfrinato, A. Stein, L. Zhang, Y. Nam, K. G. Yager, E. A. Stach, and C. T. Black, Nano Lett. 17, 4562 (2017)]. However, this technology, which in this article the authors will refer to as conventional electron beam lithography (CEBL), is reaching its practical resolution and precision limits [V. R. Manfrinato et al., Nano Lett. 14, 4406 (2014)]. Hydrogen depassivation lithography (HDL) [J. N. Randall, J. W. Lyding, S. Schmucker, J. R. Von Ehr, J. Ballard, R. Saini, and Y. Ding, J. Vac. Sci. Technol. B 27, 2764 (2009); J. N. Randall, J. B. Ballard, J. W. Lyding, S. Schmucker, J. R. Von Ehr, R. Saini, H. Xu, and Y. Ding, Microelectron. Eng. 87, 955 (2010)] is a different version of electron beam lithography that is not limited in resolution and precision in the way that CEBL is. It uses a cold field emitter, a scanning tunneling microscope (STM) tip, to deliver a small spot of electrons on a Si (100) 2 × 1 H-passivated surface to expose a self-developing resist that is a monolayer of H adsorbed to the Si surface. Subnanometer features [S. Chen, H. Xu, K. E. J. Goh, L. Liu, and J. N. Randall, Nanotechnology 23, 275301 (2012)], and even the removal of single H atoms can be routinely accomplished [M. A. Walsh and M. C. Hersam, Annu. Rev. Phys. Chem. 60, 193 (2009)]. It is known that the H desorption process at low biases is a multielectron process [E. Foley, A. Kam, J. Lyding, and P. Avouris, Phys. Rev. Lett. 80, 1336 (1998)], but the tunneling distribution of the electrons from the STM tip to the Si surface lattice is not known. The authors have developed a simple model that demonstrates that the combination of two highly nonlinear processes creates a much higher contrast exposure mechanism than CEBL. Currently, HDL has been used almost exclusively on the Si (100) surface and has a limited number of pattern transfer techniques including Si and Ge patterned epitaxy, selective atomic layer deposition of TiO2 followed by reactive ion etching [J. B. Ballard, T. W. Sisson, J. H. G. Owen, W. R. Owen, E. Fuchs, J. Alexander, J. N. Randall, and J. R. Von Ehr, J. Vac. Sci. Technol. B 31, 06FC01 (2013)], and selective deposition of dopant atoms for quantum devices and materials [Workshop on 2D Quantum MetaMaterials held at NIST, Gaithersburg, MD, April 25–26, 2018, edited by J. Owen and W. P. Kirk]. While the throughput of HDL is very low, going parallel in a big way appears promising [J. N. Randall, J. H. G. Owen, J. Lake, R. Saini, E. Fuchs, M. Mahdavi, S. O. R. Moheimani, and B. C. Schaefer, J. Vac. Sci. Technol. B 36, 6 (2018)]. However, the most exciting aspect of HDL is its atomic-scale resolution and precision, which is key to nanoscale research. The authors see HDL emerging as the ultimate high-resolution patterning tool in top down nanofabrication research facilities.In top down nanofabrication research facilities around the world, the direct-write high-resolution patterning tool of choice is overwhelmingly electron beam lithography. Remarkably small features can be written in a variety of polymeric resists [V. R. Manfrinato et al., Nano Lett. 14, 4406 (2014); V. R. Manfrinato, A. Stein, L. Zhang, Y. Nam, K. G. Yager, E. A. Stach, and C. T. Black, Nano Lett. 17, 4562 (2017)]. However, this technology, which in this article the authors will refer to as conventional electron beam lithography (CEBL), is reaching its practical resolution and precision limits [V. R. Manfrinato et al., Nano Lett. 14, 4406 (2014)]. Hydrogen depassivation lithography (HDL) [J. N. Randall, J. W. Lyding, S. Schmucker, J. R. Von Ehr, J. Ballard, R. Saini, and Y. Ding, J. Vac. Sci. Technol. B 27, 2764 (2009); J. N. Randall, J. B. Ballard, J. W. Lyding, S. Schmucker, J. R. Von Ehr, R. Saini, H. Xu, and Y. Ding, Microelectron. Eng. 87, 955 (2010)] is a different version of electron beam lithography...

Linear and nonlinear filtering of spectra

Abstract: The authors investigate linear and nonlinear methods of reducing noise while preserving information in spectra, optical and otherwise. The optimum linear and nonlinear approaches are Gauss–Hermite and maximum-entropy, respectively. However, intelligent processing still requires an initial assessment of the data in reciprocal space.The authors investigate linear and nonlinear methods of reducing noise while preserving information in spectra, optical and otherwise. The optimum linear and nonlinear approaches are Gauss–Hermite and maximum-entropy, respectively. However, intelligent processing still requires an initial assessment of the data in reciprocal space.

Effect of thermal annealing for W/β-Ga2O3 Schottky diodes up to 600 °C

Abstract: The electrical and structural properties of sputter-deposited W Schottky contacts with Au overlayers on n-type Ga2O3 are found to be basically stable up to 500 °C. The reverse leakage in diode structures increases markedly (factor of 2) for higher temperature annealing of 550–600 °C. The sputter deposition process introduces near-surface damage that reduces the Schottky barrier height in the as-deposited state (0.71 eV), but this increases to 0.81 eV after a 60 s anneal at 500 °C. This is significantly lower than conventional Ni/Au (1.07 eV), but W is much more thermally stable, as evidenced by Auger electron spectroscopy of the contact and interfacial region and the minimal change in contact morphology. The contacts are used to demonstrate 1.2 A switching of forward current to −300 V reverse bias with a reverse recovery time of 100 ns and a dI/dt value of 2.14 A/μs. The on/off current ratios were ≥106 at −100 V reverse bias, and the power figure-of-merit was 14.4 MW cm−2.

Interface trap characterization of AlN/GaN heterostructure with Al2O3, HfO2, and HfO2/Al2O3 dielectrics

Abstract: The interfacial properties of AlN/GaN heterostructures with different dielectric layers such as Al2O3, HfO2, and HfO2/Al2O3 prepared by atomic layer deposition were investigated. Interface state density versus energy level plots obtained from the Terman method revealed the peculiar peaks at ∼0.25 eV for the samples with Al2O3 and HfO2/Al2O3 and at ∼0.52 eV for the sample with HfO2, associated with nitrogen vacancy-related defects. According to the parallel conductance method, both the interface and border traps were observed for the sample with Al2O3. However, the border traps were not observed with including an HfO2 layer. The lowest interface trap density and the reverse leakage current were obtained for the sample with an HfO2/Al2O3 bilayer. Analysis of x-ray photoelectron spectroscopy spectra obtained from the HfO2 layers showed the formation of Hf–Al–O bonding for the sample with HfO2 while such formation was not observed for the sample with HfO2/Al2O3. These results indicate the superior interfacial quality of AlN/GaN heterostructures with an HfO2/Al2O3 bilayer.

Correlation between sputter deposition parameters and I-V characteristics in double-barrier memristive devices

Abstract: Sputter deposition is one of the most important techniques for the fabrication of memristive devices. It allows us to adjust the concentration of defects within the fabricated metal-oxide thin film layers. The defect concentration is important for those memristive devices whose resistance changes during device operation due to the drift of ions within the active layer while an electric field is applied. Reversible change of the resistance is an important property for devices used in neuromorphic circuits to emulate synaptic behavior. These novel bioinspired hardware architectures are ascertained in terms of advantageous features such as lower power dissipation and improved cognitive capabilities compared to state-of-the-art digital electronics. Thus, memristive devices are intensively studied with regard to neuromorphic analog systems. Double-barrier memristive devices with the layer sequence Nb/Al/Al2O3/NbOx/Au are promising candidates to emulate analog synaptic behavior in hardware. Here, the niobium oxide acts as the active layer, in which charged defects can drift due to an applied electric field causing analog resistive switching. In this publication, crucial parameters of the process plasma for thin film deposition, such as floating potential, electron temperature, and the energy flux to the substrate, are correlated with the I-V characteristics of the individual memristive devices. The results from plasma diagnostics are combined with microscopic and simulation methods. Strong differences in the oxidation state of the niobium oxide layers were found by transmission electron microscopy. Furthermore, kinetic Monte Carlo simulations indicate the impact of the defect concentration within the NbOx layer on the I-V hysteresis. The findings may enable a new pathway for the development of plasma-engineered memristive devices tailored for specific application.

Brilliant mid-infrared ellipsometry and polarimetry of thin films: Toward laboratory applications with laser based techniques

Abstract: Tunable quantum cascade lasers (QCLs) have recently been introduced as mid-infrared (mid-IR) sources for spectroscopic ellipsometric and polarimetric setups. QCLs, with their unique properties with respect to coherence and brilliance in either pulsed or continuous-wave operation, are opening up numerous new possibilities for laboratory and industrial applications. In this review, the authors will focus on thin-film characterization techniques like ellipsometric and nanopolarimetric methods and summarize related state-of-the-art techniques in this rapidly developing field. These methods are highly relevant for optical, electronical, and biomedical applications and allow detailed structural analyses regarding band properties, spectra–structure correlations, and material anisotropy. Compared to classical Fourier-transform-IR spectroscopy, thin-film sensitivity can be achieved at high spectral and spatial resolution (<0.5 cm−1, <150 μm). Measurement times are reducible by several orders of magnitude into the millisecond and microsecond range with laser-based polarimetric setups involving modulation or single-shot concepts. Thus, mid-IR ellipsometric and polarimetric hyperspectral imaging can be performed on the time scale of minutes. For mid-IR ellipsometric imaging, thickness and structure information become simultaneously accessible at spatial resolutions of a few 100 μm and possibly even at the micrometer scale by the integration of microscopic concepts. With the atomic force microscopy-infrared spectroscopy based nanopolarimetric approach, anisotropy in the absorption properties can be investigated with lateral resolutions beyond the diffraction limit, reaching a few 10 nm.

Gas equilibration gas modulation refractometry for assessment of pressure with sub-ppm precision

Abstract: Gas modulation refractometry (GAMOR) is a methodology that, by performing repeated reference assessments with the measurement cavity being evacuated while the reference cavity is held at a constant ...

Electrostatic field enhancement on end-caps of cylindrical field-emitters

Abstract: The apex field enhancement factor (AFEF) γ a of a cylindrical emitter depends sensitively on its end-cap geometry. The hemispherical end-cap is well studied due to its simplicity, but, in general, a cylindrical emitter may terminate in a variety of end-cap shapes. It is well known that the AFEF depends on the ratio h / R a, where h is the total height of the emitter and R a is the apex radius of curvature. The authors show here that there can be a large variation in γ a as the end-cap geometry is altered while keeping h / R a fixed. They carry out a systematic numerical study and determine an approximate formula for γ a in terms of measurable end-cap geometry parameters such as its height H, the radius of the cylinder R, and the apex radius of curvature R a. They show that the formula is robust and can predict the net field emission current with errors generally less than 40 %.The apex field enhancement factor (AFEF) γ a of a cylindrical emitter depends sensitively on its end-cap geometry. The hemispherical end-cap is well studied due to its simplicity, but, in general, a cylindrical emitter may terminate in a variety of end-cap shapes. It is well known that the AFEF depends on the ratio h / R a, where h is the total height of the emitter and R a is the apex radius of curvature. The authors show here that there can be a large variation in γ a as the end-cap geometry is altered while keeping h / R a fixed. They carry out a systematic numerical study and determine an approximate formula for γ a in terms of measurable end-cap geometry parameters such as its height H, the radius of the cylinder R, and the apex radius of curvature R a. They show that the formula is robust and can predict the net field emission current with errors generally less than 40 %.

Nanostructures of type-II topological Dirac semimetal NiTe2

Abstract: Many transition metal dichalcogenides have been predicted and verified experimentally to exhibit topological semimetallic behavior due to symmetry breaking. NiTe2 is predicted to belong to an interesting class of materials: type-II topological semimetal. Here, we report the growth, structural, and magnetic properties of polycrystalline NiTe2 nanostructures synthesized using a two-step solvothermal technique. Nanostructures of NiTe2 crystalize in a hexagonal CdI2-type structure (space group P 3 ¯ m 1) with lattice parameters a = b = 3.85 A and c = 5.26 A. NiTe2 nanostructures exhibit paramagnetic behavior at room temperature and display a large increase in magnetization below 30 K. These results will certainly pave the way to fully understand one- and two-dimensional NiTe2 for topological behavior that can be useful for novel device applications.

Assessing stability of metal tellurides as alternative photomask materials for extreme ultraviolet lithography

Abstract: Tellurium (Te) is one of the elements with highest extinction coefficient κ at the 13.5 nm extreme-ultraviolet (EUV) wavelength. It is being considered as an alternative absorber material for binary photomasks in EUV lithography. The absorber material is required to remain chemically stable during EUV exposure, at elevated temperatures up to 150 °C, during mask cleaning, and in the low hydrogen pressure environment that is present in the EUV scanner. However, Te is known to react with oxygen and hydrogen, forming less EUV absorbing TeO2 and more volatile H2Te, respectively. Since the melting temperature of Te is only 449.5 °C at normal pressure, alloying Te with a more stable metal might result in a high κ material that will remain thermally and chemically stable over a wider range of operating conditions. In this paper, the authors report on the stability assessment of metal telluride (M-Te) alloys for the EUV absorber material. They combined Te with high κ metals, noble metals, and etchable metals. High κ and noble M-Te materials are both thermally more stable than etchable M-Te, but they cannot be patterned easily for use in an EUV photomask. High κ M-Te exhibits polycrystal morphology at room temperature compared to quasiamorphous noble M-Te though both can crystallize at a higher temperature. Hydrogen stability and cleaning solution stability of M-Te materials are improved considerably compared to Te, but their higher surface reactivity cannot be completely mitigated without the addition of an inert capping layer. Furthermore, etchable M-Te alloys are easily oxidized during deposition, resulting in lower electron density and hence lower κ. Nevertheless, M-Te alloys may be a way to stabilize Te for usage as the EUV absorber material.

Why converting field emission voltages to macroscopic fields before making a Fowler-Nordheim plot has often led to spurious characterization results

Abstract: An important parameter used to characterize large-area field electron emitters (LAFEs) is the characteristic apex field enhancement factor γC. This parameter is normally extracted from the slope of a Fowler-Nordheim (FN) plot. Several years ago, the development of an “orthodoxy test” allowed a sample of 19 published FN plots relating to LAFEs to be tested, and it was found that about 40% of the related papers were reporting spuriously high values for γC. In technological papers relating to LAFE characterization, the common practice is to preconvert the measured voltage into an (apparent) value of the macroscopic field before making and analyzing an FN plot. This paper suggests that the cause of the “spurious field enhancement factor value” problem is the widespread use of a preconversion equation that is defective (for example, not compatible with ordinary electrical circuit theory) when it is applied to so-called “nonideal” field emission devices/systems. Many real devices/systems are nonideal. The author argues that FN plots should be made using raw experimental current-voltage data, that an orthodoxy test should be applied to the resulting FN plot before any more-detailed analysis, and that (in view of growing concerns over the reliability of published “scientific” results) reviewers should scrutinize field emission materials characterization papers with enhanced care.

Fabrication and optical properties of regularly arranged GaN-based nanocolumns on Si substrate

Abstract: Nanocolumn light-emitting diodes (LEDs) are expected to achieve the monolithic integration of the three primary-color micro-LEDs for micro-LED displays. From the viewpoints of low cost and large-area substrates, a technology for the regular arrangement of nanocolumns on Si substrates is required. The improvement of GaN nanocolumns on Si would be an important advance for the preparation of high efficiency optical devices. In this paper, the effects of column diameter and height on the photoluminescence (PL) and cathodoluminescence properties of GaN nanocolumn arrays were investigated. The PL intensity of the 700-nm high (tall) nanocolumn was three times stronger than that of the 350-nm high (short) nanocolumn. Although the PL intensity decreased dramatically with an increasing diameter for the shorter nanocolumns, it retained its high value (up to 220 nm) for the taller GaN nanocolumns. For the latter specimens, a decrease in the number of emitting nanocolumns, which would reduce emission efficiency, was suppressed by the dislocation filtering effect. Moreover, yellow luminescence was suppressed for taller nanocolumns. In the low-temperature-PL spectra, the peak observed at 3.41 eV, related to a stacking fault, increased with diameter regardless of height. These results indicate that the appropriate design of column height and diameter is of considerable importance for obtaining high efficiency emissions. Finally, InGaN/GaN quantum wells were fabricated on the regularly arranged GaN nanocolumn platform. Blue, green, and red (RGB) emission colors with no significant change in emission intensity were observed. These results constitute an important step toward the monolithic integration of RGB micro-LEDs.

Magneto-optical confirmation of Landau level splitting in a GaN/AlGaN 2DEG grown on bulk GaN

Abstract: Landau level splitting in a two-dimensional electron gas (2DEG) confined in an ultrapure GaN/AlGaN heterostructure grown by molecular beam epitaxy on bulk GaN is verified spectroscopically. The Landau level fan reconstructed from magneto-photoluminescence (PL) data yields an effective mass of 0.24m0 for the 2D electrons. Narrow excitonic PL line widths < 100 μeV, an atomically flat surface of the layer stack, as well as the absence of the 2DEG in the dark environment, are important ancillary experimental findings while focusing on magneto-PL investigations of the heterostructure. Simultaneously recorded Shubnikov-de Haas and magneto-PL intensity oscillations under steady UV illumination exhibit an identical frequency and allow for two independent ways of determining the 2D density.Landau level splitting in a two-dimensional electron gas (2DEG) confined in an ultrapure GaN/AlGaN heterostructure grown by molecular beam epitaxy on bulk GaN is verified spectroscopically. The Landau level fan reconstructed from magneto-photoluminescence (PL) data yields an effective mass of 0.24m0 for the 2D electrons. Narrow excitonic PL line widths < 100 μeV, an atomically flat surface of the layer stack, as well as the absence of the 2DEG in the dark environment, are important ancillary experimental findings while focusing on magneto-PL investigations of the heterostructure. Simultaneously recorded Shubnikov-de Haas and magneto-PL intensity oscillations under steady UV illumination exhibit an identical frequency and allow for two independent ways of determining the 2D density.

Characterizing profile tilt of nanoscale deep-etched gratings via x-ray diffraction

Abstract: The authors report the development of fast, nondestructive, and high accuracy metrology for the characterization of profile tilt relative to the surface normal in nanoscale gratings using x-ray diffraction. Gratings were illuminated with a collimated x-ray beam (Cu Kα), similar to variable-angle small-angle x-ray scattering, to record changes of diffraction efficiency (DE) as a function of incidence angle. Simulations using scalar diffraction theory and rigorous coupled wave analysis predict extrema (0th order DE minimized, ±1st order DE maximized) when local grating bars are parallel to the incident x-ray beam. The surface normal was measured independently by reflecting a laser beam from the grating surface. The independent measurements using x rays and laser beams were referenced to each other via a slit reference plane to characterize the bar tilt angle relative to the surface normal. The fast x-ray measurement can be repeated at arbitrary points to study the spatial variation of the bar tilt angle across large gratings. Two test gratings etched with different deep reactive-ion etch chambers were prepared to investigate the performance of the proposed method. The authors report a repeatability of <0.01° and an accuracy of ∼0.08° with a fast scan speed (total integration time of 108 s to scan a line across ∼55 mm large grating samples at an interval of ∼2 mm). High spatial resolution (<50 μm) can be easily achieved at the expense of speed by limiting the incident x-ray spot size. This process is applicable to any periodic nanostructure as long as x-ray diffraction is well modeled.The authors report the development of fast, nondestructive, and high accuracy metrology for the characterization of profile tilt relative to the surface normal in nanoscale gratings using x-ray diffraction. Gratings were illuminated with a collimated x-ray beam (Cu Kα), similar to variable-angle small-angle x-ray scattering, to record changes of diffraction efficiency (DE) as a function of incidence angle. Simulations using scalar diffraction theory and rigorous coupled wave analysis predict extrema (0th order DE minimized, ±1st order DE maximized) when local grating bars are parallel to the incident x-ray beam. The surface normal was measured independently by reflecting a laser beam from the grating surface. The independent measurements using x rays and laser beams were referenced to each other via a slit reference plane to characterize the bar tilt angle relative to the surface normal. The fast x-ray measurement can be repeated at arbitrary points to study the spatial variation of the bar tilt angle acr...

Patterning of high-aspect-ratio nanogratings using phase-locked two-beam fiber-optic interference lithography

Abstract: Patterning high-aspect-ratio gratings by the phase-locked two-beam fiber-optic interference lithography (2-FOIL) is numerically and experimentally investigated in this paper. The Dill model is applied in the numerical simulation to understand the effects of an exposure dose and pattern contrast on the exposed photoresist grating profiles. Exposure experiments on the authors’ home-built 2-FOIL setup are conducted to demonstrate the suitability for manipulating the linewidth of photoresist gratings by tuning the exposure dose to achieve high aspect ratios over 6 at high pattern contrast thanks to the phase-locking mechanism. The high-aspect-ratio photoresist gratings serve as an excellent etching mask for the subsequent pattern transfer into underlying silicon substrates for high-aspect-ratio silicon gratings. Using these high-aspect-ratio silicon gratings as the nanoimprint mold, a square nanomesh is demonstrated by means of the multiple-step nanoimprint lithography. The authors’ work demonstrates that the proposed phase-locked 2-FOIL system enables high pattern contrast under long exposure duration, making it a suitable tool for fabricating high-aspect-ratio grating structures.Patterning high-aspect-ratio gratings by the phase-locked two-beam fiber-optic interference lithography (2-FOIL) is numerically and experimentally investigated in this paper. The Dill model is applied in the numerical simulation to understand the effects of an exposure dose and pattern contrast on the exposed photoresist grating profiles. Exposure experiments on the authors’ home-built 2-FOIL setup are conducted to demonstrate the suitability for manipulating the linewidth of photoresist gratings by tuning the exposure dose to achieve high aspect ratios over 6 at high pattern contrast thanks to the phase-locking mechanism. The high-aspect-ratio photoresist gratings serve as an excellent etching mask for the subsequent pattern transfer into underlying silicon substrates for high-aspect-ratio silicon gratings. Using these high-aspect-ratio silicon gratings as the nanoimprint mold, a square nanomesh is demonstrated by means of the multiple-step nanoimprint lithography. The authors’ work demonstrates that the...

Electron backscatter diffraction of Nb3Sn coated niobium: Revealing structure as a function of depth

Abstract: Over the last two decades, advances in Electron Backscatter Diffraction (EBSD) have moved the technique from a research tool to an essential characterization technique in many fields of material research. EBSD is the best suited technique for determining structure as a function of depth. This characterization is critically important but has been previously absent from Nb3Sn efforts. While EBSD is the technique of choice, obtaining quality data can be difficult. Sample preparation in particular is nontrivial. Here, we summarize the general principles of EBSD, discuss specific sample preparation techniques for Nb3Sn coated SRF cavity materials, and give examples of how EBSD is being used to understand fundamental growth mechanisms for Nb3Sn coatings.Over the last two decades, advances in Electron Backscatter Diffraction (EBSD) have moved the technique from a research tool to an essential characterization technique in many fields of material research. EBSD is the best suited technique for determining structure as a function of depth. This characterization is critically important but has been previously absent from Nb3Sn efforts. While EBSD is the technique of choice, obtaining quality data can be difficult. Sample preparation in particular is nontrivial. Here, we summarize the general principles of EBSD, discuss specific sample preparation techniques for Nb3Sn coated SRF cavity materials, and give examples of how EBSD is being used to understand fundamental growth mechanisms for Nb3Sn coatings.

Transparent conductive oxide-based architectures for the electrical modulation of the optical response: A spectroscopic ellipsometry study

Abstract: Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of ch...

Optical characterization of inhomogeneous thin films containing transition layers using the combined method of spectroscopic ellipsometry and spectroscopic reflectometry based on multiple-beam interference model

Abstract: This paper presents the results of the optical characterization of inhomogeneous thin films of polymer-like SiO x C y H z and non-stoichiometric silicon nitride SiN x. An efficient method combining variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at the near-normal incidence based on the multiple-beam interference model is utilized for this optical characterization. The multiple-beam interference model allows us to quickly evaluate the values of ellipsometric parameters and reflectance of the inhomogeneous thin films, which exhibit general profiles of their optical constants. The spectral dependencies of the optical constants of the inhomogeneous SiO x C y H z and SiN x thin films are determined using the Campi–Coriasso dispersion model. The profiles of the optical constants of these films can also be determined. Furthermore, the transition layers at the lower boundaries of the characterized films are also taken into account. Spectral dependencies of the optical constants of these transition layers are also determined using the Campi–Coriasso dispersion model.This paper presents the results of the optical characterization of inhomogeneous thin films of polymer-like SiO x C y H z and non-stoichiometric silicon nitride SiN x. An efficient method combining variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at the near-normal incidence based on the multiple-beam interference model is utilized for this optical characterization. The multiple-beam interference model allows us to quickly evaluate the values of ellipsometric parameters and reflectance of the inhomogeneous thin films, which exhibit general profiles of their optical constants. The spectral dependencies of the optical constants of the inhomogeneous SiO x C y H z and SiN x thin films are determined using the Campi–Coriasso dispersion model. The profiles of the optical constants of these films can also be determined. Furthermore, the transition layers at the lower boundaries of the characterized films are also taken into account. Spectral dependencies o...

Detection of DNA homopolymer with graphene nanopore

Abstract: Graphene nanopores hold great potential for applications such as molecular detection and DNA sequencing. Here, the authors present a fast and controllable way to fabricate sub-5-nm nanopores on a graphene membrane, with a process including two steps: (i) sputtering a large nanopore using a conventional, focused ion beam; and (ii) shrinking the large nanopore to under 5 nm using a scanning electron microscope. Conductance measurements confirm that the electron-beam-induced deposition of hydrocarbons not only shrinks the diameter of the nanopore but also increases its length. Furthermore, the authors report that using a salt gradient across the nanopore allows the detection of 3 nucleotide “C” and 3 nucleotide “G” homopolymer DNA strands based on differences in their physical dimensions.

Electron beam lithography on curved or tilted surfaces: Simulations and experiments

Abstract: There is a growing interest for patterning on curved or tilted surfaces using electron beam lithography. Computational proximity correction techniques are well established for flat surfaces and perpendicular exposure, but for curved and tilted surfaces adjustments are needed as the dose distribution is no longer cylindrically symmetric with respect to the surface normal. A graphical processing unit -accelerated 3D Monte Carlo simulation, based on first-principle scattering models, is used to simulate the asymmetric dose distribution. Based on that, an approximate adjustment is made to an existing high-performance proximity effect correction (PEC) algorithm aimed at the correct exposure of a pattern of nanowires on a 17° tilted surface. It was experimentally verified that using the adjusted PEC indeed leads to a more uniform exposure on tilted surfaces.