Spectral Ellipsometry Research Articles

The effects of carbon incorporation on the refractive index of PECVD silicon oxide layers

Silicon oxide (SiOx) has many applications, including as a low-refractive index material. Plasma enhanced chemical vapor deposition (PECVD) processes are facile, low temperature routes to produce thin SiOx layers. A route to decrease the refractive index of SiOx films is to increase the layer porosity although maintaining structural and optical stability remains challenging. Organic carbon-containing sacrificial layers have been shown to modify the growth and resulting structure of PECVD SiOx layers. In this work, we study the effect of adding methane (CH4) to the standard SiOx process gas mixture (silane and nitrous oxide) and varying deposition temperatures and microwave power in an industrial-scale, microwave PECVD reactor. Spectral ellipsometry was used to measure the optical properties of deposited layers, Fourier-transformed infrared (FTIR) spectroscopy to determine bonding and the layer porosity, and optical emission spectroscopy to characterize the plasma. We propose two regimes characterized by whether adding CH4 increases or decreases the refractive index and porosity of deposited layers compared to SiOx layers grown under standard conditions. However, the magnitude of the effect of adding CH4 was not large and would not find industrial application. Furthermore, the deposited layers’ refractive indices increased over time, indicating that the effects of adding CH4 to the process gas mixture were not stable. To help explain our results and to provide guidance for future efforts to better control the refractive index of PECVD SiOx layers via carbon incorporation while maintaining layer stability, we propose possible growth pathways for our layers considering both plasma reactions and surface processes.

In Situ Hydrogen Plasma Exposure for Varying the Stoichiometry of Atomic Layer Deposited Niobium Oxide Films for Use in Neuromorphic Computing Applications.

Niobium oxide (NbOx) materials of various compositions are of interest for neuromorphic systems that rely on memristive device behavior. In this study, we vary the composition of NbOx thin films deposited via atomic layer deposition (ALD) by incorporating one or more in situ hydrogen plasma exposure steps during the ALD supercycle. Films with compositions ranging from Nb2O5 to NbO2 were deposited, with film composition dependent on the duration of the plasma exposure step, the number of plasma exposure steps per ALD supercycle, and the hydrogen content of the plasma. The chemical and optical properties of the ALD NbOx films were probed using spectral ellipsometry, X-ray photoelectron spectroscopy, and optical transmission spectroscopy. Two-terminal electrical devices fabricated from ALD Nb2O5 and NbO2 thin films exhibited memristive switching behavior, with switching in the NbO2 devices achieved without a high-field electroforming step. The ability to controllably tune the composition of ALD-grown NbOx films opens new opportunities for realizing a variety of device structures relevant for neuromorphic computing and other emerging electronic and optoelectronic applications.

Ellipsometric and Spectrometric Studies of (Ga0.2In0.8)2Se3 Thin Film

Thermal evaporation technique is used to deposite (Ga0,2In0,8)2Se3 thin films. The refractive index and extinction coefficient dispersions are obtained from the spectral ellipsometry measurements. The dispersion of the refractive index is described in the framework of the Wemple–Di Domenico model. The optical transmission spectra of a (Ga0,2In0,8)2Se3 thin film are studied in the temperature range 77–300 K. The temperature behavior of the Urbach absorption edge, as well as the temperature dependences of the energy pseudogap and Urbach energy, are investigated. The influence of different types of disordering on the optical absorption edge of a (Ga0,2In0,8)2Se3 thin film is discussed. Optical parameters of a (Ga0,2In0,8)2Se3 thin film and a single crystal are compared.

Elastic Properties of GaN and AlN Films Formed on SiC/Si Hybrid Substrate, a Porous Basis

The nanoindentation method was used to study the elastic properties of gallium nitride and aluminum nitride films grown on nanoscale silicon carbide on silicon (SiC/Si), a new type of substrate. The values of the Young’s modulus of epitaxial films of such wide-gap semiconductors as GaN and AlN, grown on substrates SiC/Si. were determined for the first time. It was experimentally established using the nanoindentation method that the Young’s modulus of the GaN epitaxial layer on SiC/Si is 265 GPa, and that of the AlN film is 223 GPa. Using atomic force microscopy and spectral ellipsometry, the structural characteristics of gallium nitride and aluminum nitride films have been studied. The thicknesses of the films and the roughness of their surface are determined.

Optical Characteristics of Parallel-Sided Quartz Plates at Brewster Angles Determined by Spectral Ellipsometry

A method is proposed for determining the optical characteristics of glass plates after physicochemical surface treatment by measuring spectra of ellipsometric angles at incidence angles equal to Brewster’s angle. The dispersion dependence of the refractive index could be determined and the surface layer thickness during the plate manufacturing process could be estimated using the coordinates of the ellipsometric-angle spectral minima. Refractive-index spectra calculated by the Brewster formula for KU-1 optical quartz plates in the non-absorbing region were in satisfactory agreement with spectra determined by numerical methods in the DeltaPsi2 software of a UVISEL 2 spectral ellipsometer (Horiba). The relative error of a refractive-index determination of the quartz plates by the proposed method was ≤0.1%.

Formation of SnO and SnO2 phases during the annealing of SnO(x) films obtained by molecular beam epitaxy

SnO and SnO2 films were obtained on the SiO2 surface by the molecular-beam epitaxy method. The initial films are in the polycrystalline phase. The annealing of SnO(x) films at a temperature of 300 °C resulted in the formation of the tetragonal SnO phase. Three vibration modes Eg, A1g, and B1g with the frequencies of SnO bond vibrations of 113, 211 and ~360 cm−1, respectively, which correspond to the SnO phase, were first observed by the Raman spectroscopy method. The orthorhombic SnO2 films were obtained by increasing the annealing temperature to 500 °C. Based on the valence band XPS (X-ray photoelectron spectroscopy) spectrum, several features with the binding energy approximately 5 eV, 7.5 eV and 11 eV, which are the same with the valence band of SnO2, were identified. The refractive index and absorption coefficient were investigated by the spectral ellipsometry technique. The high absorption coefficients correspond to the high Sn content. The film dielectric properties were revealed at the temperature higher than 300 °C. The refractive index values lie in the range of 1.5–2.6 for the visible spectral region. The pronounced absorption edges at 2.85 eV and 3.6 eV corresponding to those of stannous oxide (SnO) and stannic oxide (SnO2) were observed. The photoluminescence (PL) from the SnO(x) films was observed at room temperature. The increase of the annealing temperature resulted in the increase of PL intensity. Such PL intensity behavior is likely due to the Sn nanoislands.

A Mask Based on a Si Epitaxial Layer for the Self-Catalytic Nanowire Growth on GaAs(111)B and GaAs(100) Substrates

GaAs nanowires (NWs) were generated on the surface of GaAs(111)B and GaAs(100) substrates from molecular fluxes by the self-catalytic growth method. A mask for NW growth was fabricated by oxidizing the epitaxial silicon layer that was grown on a substrate surface by the molecular beam epitaxy (MBE) method. Silicon was oxidized in purified air without moving the structures out of the vacuum system of the MBE apparatus. The process of Si/GaAs heterostructure oxidation was investigated using single-wave and spectral ellipsometry. The oxidized silicon surface morphology was studied by the atomic force microscopy methods. The scanning electronic microscopy method was used to examine the samples with NWs. The NW density was about 2.6 × 107 and 3 × 107 cm–2 for (111)B and (100), respectively.

Optical dielectric constants of single crystalline silver films in the long wavelength range

Optical dielectric constants are critical to modeling the electronic and optical properties of materials. Silver, as a noble metal with low loss, has been extensively investigated. The recently developed epitaxial growths of single crystalline Ag on dielectric substrates have prompted efforts to characterize their intrinsic optical dielectric function. In this paper, we report spectral ellipsometry measurements and analysis of a thick, epitaxially-grown, single-crystalline Ag film. We focus on the range of 0.18 – 1.0 eV or 1.24 – 7 µm, an energy and wavelength range that has not been examined previously using epitaxial films. We compare the extracted dielectric constants and the predicted optical performances with previous measurements. The loss is appreciably lower than the widely quoted Palik’s optical constants (i.e., up to a factor of 2) in the infrared frequency range. The improved knowledge of fundamental optical properties of the high-quality epitaxial Ag film will have a broad impact on simulations and practical applications based on Ag in the long wavelength range.

Atomic Layer Deposition of Lithium-Silicon-Tin Oxide Nanofilms for High Performance Thin Film Batteries Anodes

Tin oxide is the most promising material for thin film anodes of Li-ion batteries due to its cycling performance and high theoretical capacity. It is assumed that lithium-tin oxide can demonstrate even higher performance. Lithium-silicon-tin oxide nanofilms were prepared by atomic layer deposition (ALD), using the lithium bis (trimethylsilyl) amide (LiHMDS), tetraethyltin (TET) as a metal containing reagents and ozone or water or oxygen plasma as counter-reactants. Monocrystalline silicon (100) and stainless steel (316SS) were used as supports. The thicknesses of the nanofilms were measured by spectral ellipsometry (SE) and scanning electron microscopy (SEM). It was found that oxygen plasma is the most optimal ALD counter-reactant. The composition and structure were studied by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD). The nanofilms contain silicon as impurity, whose source is the ALD precursor (LiHMDS). The nanofilms deposited on stainless steel have shown the high Coulombic efficiency (99.1-99.8%) and cycling performance at a relatively high voltage (0.01 to 2.0V).

Оптические свойства, зонная структура и проводимость межфазной границы раздела гетероструктуры 3C-SiC(111)/Si(111), выращенной методом замещения атомов

Spectral ellipsometry in the photon energy range 0.5–9.3 eV was used to study epitaxial films of single-crystal silicon carbide of a cubic polytype 3C-SiC of 20–120 nm grown from silicon Si by atom substitution method. It was found that at the 3C-SiC (111) / Si (111) interface a thin intermediate layer is formed with a dielectric function corresponding to semimetal. This result is confirmed by quantum chemical modeling of the properties of the 3C-SiC (111) / Si (111) interface. It was shown that the conductivity of the layer is associated with p-electrons of Si atoms of SiC at the interface of the phases which are as far as possible from the Si atoms of the substrate.

Ab initio и экспериментальное исследование электронных, оптических и колебательных свойств CdGa-=SUB=-2-=/SUB=-Te-=SUB=-4-=/SUB=-

The electronic, optical, and lattice vibrational properties of CdGa2Te4 were studied experimentally using spectral ellipsometry, Raman scattering (Raman scattering) and infrared (IR) spectroscopy, as well as theoretically using the density functional theory (DFT). Eight Raman-active modes and twelve IR-active modes were detected and identified from consideration of the point group symmetry. Based on the analysis of the electronic spectrum and the density of states, the nature of the chemical bond in this semiconductor compound is determined. The theoretically calculated results are compared with the experimental data of the present work and with the results of experimental data available in the literature obtained by infrared spectroscopy and Raman scattering.

Маска на основе эпитаксиального слоя Si для самокаталитического роста нитевидных нанокристаллов на подложках GaAs (111)B и (100)

GaAs nanowire (NW) self-catalyzed growth on GaAs (111) B and GaAs (100) substrates was carried out by molecular beam epitaxy. A mask for the self-catalyzed NW growth was created by oxidizing an epitaxial silicon layer grown on the GaAs surface by molecular beam epitaxy (MBE). Silicon oxidation was realized in an atmosphere of purified air under normal conditions without moving the structures out from the vacuum system volume of the molecular beam epitaxy chamber. The oxidation process of a silicon layer was studied using single-wave and spectral ellipsometry and the surface morphology of oxidized silicon was studied by atomic force microscopy. Substrates with NWs were studied by scanning electron microscopy. The NW density was demonstrated to be 2.6•107 cm-2 and 3•107 cm-2 for (111)B and (100), respectively.

Низкотемпературный рост кубической фазы CdS методом атомно-слоевого осаждения на гибридных подложках SiC/Si

A new method has been developed for the epitaxial growth of cadmium sulfide CdS films in a metastable cubic phase on silicon substrates with a buffer layer of epitaxial silicon carbide by atomic layer deposition. This CdS phase is achieved due to the low growth temperature (~ 180 ℃). The cubic phase was identified by both X-ray diffraction (XRD) and spectral ellipsometry due to the fact that the main peak of CdS absorption is split into two peaks in the hexagonal phase (4.9 eV and 5.4 eV) and is a singlet in the cubic phase (5.1 eV).

Nanostructured amorphous gallium phosphide on silica for nonlinear and ultrafast nanophotonics.

Nanophotonics based on high refractive index dielectrics relies on appreciable contrast between the indices of designed nanostructures and their immediate surrounding, which can be achieved by the growth of thin films on low-index substrates. Here we propose the use of high index amorphous gallium phosphide (a-GaP), fabricated by radio-frequency sputter deposition, on top of a low refractive index glass substrate and thoroughly examine its nanophotonic properties. Spectral ellipsometry of the amorphous material demonstrates the optical properties to be considerably close to crystalline gallium phosphide (c-GaP), with low-loss transparency for wavelengths longer than 650 nm. When nanostructured into nanopatches, the second harmonic (SH) response of an individual a-GaP patch is characterized to be more than two orders of magnitude larger than the as-deposited unstructured film, with an anapole-like resonant behavior. Numerical simulations are in good agreement with the experimental results over a large spectral and geometrical range. Furthermore, by studying individual a-GaP nanopatches through non-degenerate pump-probe spectroscopy with sub-10 fs pulses, we find a more than 5% ultrafast modulation of the reflectivity that is accompanied by a slower decaying free carrier contribution, caused by absorption. Our investigations reveal a potential for a-GaP as an adequate inexpensive and CMOS-compatible material for nonlinear nanophotonic applications as well as for photocatalysis.

Исследование возможности получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними с помощью процессов ионно-лучевого и реактивного ионно-лучевого осаждения

В настоящее время для получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними в производстве интегральных микросхем применяются процессы атомно-слоевого осаждения и магнетронного распыления. Однако существуют процессы ионно-лучевого и реактивного ионно-лучевого осаждения, которые в основном используются для получения многослойных оптических покрытий. Исследована возможность получения структур с нанометровыми толщинами слоев и резкими границами раздела между ними в процессах ионно-лучевого и реактивного ионно-лучевого осаждения. Исследования проведены методами времяпролетной вторичной ионной масс-спектрометрии и спектральной эллипcометрии. На примере структуры Ta (3 нм)/Nb (3 нм)/Ta (3 нм) впервые показано, что процесс ионно-лучевого осаждения позволяет формировать структуры с нанометровыми толщинами слоев и резкими границами раздела между ними. Тогда как в процессе реактивного ионно-лучевого осаждения структуры Nb (3 нм)/Ta2O5(3 нм)/Nb (3 нм) происходит окисление на всю толщину металлического слоя, следующего за слоем оксида металла, за счет ионов, атомов и молекул кислорода, содержащихся в ионном пучке.

Исследования островковых пленок нитрида галлия на сапфировых подложках методами растровой электронной микроскопии и спектральной эллипсометрии

Методами растровой электронной микроскопии (РЭМ) и спектральной эллипсометрии проведены исследования зародышевых слоев нитрида галлия (GaN) субстананометровой толщины, нанесенных при различных режимах в процессе атомно-слоевой эпитаксии (АСЭ) из триэтил галлия и аммиака на сапфировые подложки. Полученные зародышевые слои представляют собой островковые пленки с различной степенью покрытия поверхности подложек, состоящие из кристаллитов GaN, имеющих различные размеры и средние толщины в диапазоне 10–40 нм. Разработана программа обработки РЭМ-изображений, позволяющая проводить количественную оценку площади частиц, включений, фаз, присутствующих в пленках и на поверхности подложек. Показано, что методика обработки спектральных эллипсометрических измерений островковых пленок из кристаллитов GaN на сапфировых подложках по модели Максвелла–Гарнетта выявляет такую же тенденцию в площади покрытия подложек пленками, что и обработка РЭМ-изображений. Разработанные программа и методика позволили определить оптимальный режим АСЭ зародышевых слоев GaN на сапфировых подложках для формирования высококачественных HEMT-структур из шести реализованных режимов. Они могут быть также эффективно использованы при исследовании любых островковых пленок, слоев с включениями физических и химических фаз и систем коллоидных частиц, применяемых в процессе формирования микроэлектронных структур.

Study of Elastic Properties of SiC Films Synthesized on Si Substrates by the Method of Atomic Substitution

The elastic properties of nanoscale silicon carbide film grown on a silicon substrate by the method of atomic substitution were studied. The Young modulus of nanoscale silicon carbide was for the first time measured by nanoindentation method. The structural characteristics of silicon carbide film on silicon were studied by the optical profilometry and spectral ellipsometry; the roughness and thickness of film were m-easured.

Study of Structural Modification of Composites with Ge Nanoclusters by Optical and Electron Microscopy Methods

Composites consisting of Ge nanoclusters embedded in GeO2 matrix were modified by selective removal of the germanium dioxide in deionized water or HF. Thin (up to 200 nm) and thick (300−1500 nm) GeO2{Ge-NCs} heterolayers were studied before and after the etching using Raman spectroscopy, scanning and spectral ellipsometry, scanning electron microscopy. It was found that a stable skeletal framework from agglomerated Ge nanoparticles (amorphous or crystalline) was formed after the etching of thin GeO2{Ge-NCs} heterolayers. When removal the GeO2 matrix from a thick GeO2{Ge-NCs} heterolayer, released Ge nanoclusters were arranged in a vertically ordered chains.

Features of the synthesis of lithium-based ternary oxide nanofilms by atomic layer deposition with LHMDS for thin-film LIBs

The present study is devoted to the development of perspective cathode (LiNiO2) and anode (lithiated SnO2) of solid-state lithium-ion batteries with the use of atomic layer deposition. The program of synthesis included a different number (ratio, 1/1–1/3) of lithium and metal oxide (Ni, Sn) deposition cycles in one supercycle. Lithium hexamethyldisilazide (LiHMDS), bis(cyclopentadienyl)nickel (II) (NiCp2) and tetraethyl tin (TET) were used as metal-containing reagents while oxygen remote plasma was used as a counter reagent. The deposition was conducted at 300 °C on the silicon surface and stainless-steel substrates. The obtained films were annealed at 400–700 °C for 10 min. Spectral ellipsometry, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray reflectometry (XRR), X-ray diffraction (XRD) were used to characterize samples. The presence of silicon and nitrogen atoms was found in films which were not observed during deposition of lithium oxide and nickel oxide structures at the same synthesis conditions. The electrochemical behavior of synthesized Li-Ni-O and Li-Sn-O electrodes were studied.

Monitoring subwavelength grating structures for vertical-cavity surface-emitting laser applications by spectroscopic ellipsometry

GaAs based vertical cavity surface emitting lasers (VCSELs) have one of the fastest growing markets due to their numerous applications in imaging technology, optical sensors, and interconnects. Stable, single-mode operation of these laser diodes is often achieved by forming subwavelength structures on the surface of the GaAs semiconductor. Quick and preferably noncontact inspection of the formed nanostructures is desired during the fabrication process. Nanostructure characterization by spectral ellipsometry-based metrologies has become an indispensable tool in the semiconductor industry. An advanced method of ellipsometry is the application of Mueller-matrix ellipsometry, which enables the characterization of structure details difficult to measure or not reachable by using standard ellipsometry measurements. In this paper, the authors present the results of nanostructure characterization by model-based dimension metrology using spectral ellipsometry and Mueller-matrix spectral ellipsometry of line gratings formed on GaAs substrates during the process of VCSEL fabrication.