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Terbium-doped silicon-rich silicon oxide films were deposited and hydrogenated at elevated temperatures. The influence of hydrogenation on defects-mediated non-radiative recombination of excited Tb 3+ ions was investigated by means of photoluminescence and photoluminescence decay measurements. An increase in photoluminescence intensity and photoluminescence decay time was observed upon hydrogenation for the main 5 D 4 – 7 F 5 transition of Tb 3+ ions. This observation was ascribed to saturation of non-radiative recombination defect centers with hydrogen. It was found that the number of emitted photons increases upon passivation as a result of two effects: (1) a reduction of the non-radiative recombination rate, and (2) optical activation of new Tb 3+ emitters. Based on the obtained results and literature data, Förster energy-transfer was suggested as the interaction responsible for the non-radiative coupling between Tb 3+ ions and defects. [ABSTRACT FROM AUTHOR]

In this work, silicon-rich silicon oxide films of different stoichiometry were annealed at high temperatures in order to obtain silicon nanocrystals embedded in silica. The low-frequency Raman scattering has been observed and related to acoustic phonons confined in these nanocrystals. It has been found that this scattering consists of two modes: one at a lower frequency and one at a higher frequency. The depolarization ratios for these modes were determined, showing that the lower frequency mode is depolarized and the higher frequency mode is polarized. It has been also found that under specific conditions of film preparation the product of mode frequency and nanocrystal diameter is scale invariant. Finally, it has been shown that the confined acoustic phonon frequencies do not simply depend on the nanocrystal size alone, but also on the Si concentration in the film itself. This effect has been ascribed to the accelerated nucleation and enhanced crystallization occurring in the films deposited with higher Si content. [ABSTRACT FROM AUTHOR]

Silicon-rich silicon oxide films, with various Si concentrations, were deposited by plasma enhanced chemical vapor deposition and annealed at 1100 °C in order to form silicon nanocrystals. For these films, it has been found that the absorption edge shifts as a function of the nanocrystal size due to the quantum confinement of exciton. This result showed that the size-related effects are present in the investigated films. Next, we examined the influence of the nanocrystal size on the vibrational modes. In this case, the Raman line related to silicon nanocrystals significantly down-shifts as a function of the nanocrystal size, which is against the predictions of the phonon confinement model. It has been shown that this effect is due to stress exerted on the nanocrystals. It has been also found that this stress changes from compressive to tensile, and the stress character depends on the film stoichiometry. [ABSTRACT FROM AUTHOR]

Cadmium telluride films deposited on amorphous substrates exhibit a grain structure characterized by [111]-oriented grains, but where the in-plane grain orientation is randomized due to the absence of epitaxy. Here, we explore the viability of promoting an in-plane grain alignment through graphoepitaxy. Fifteen different substrate surface textures were fabricated using focused ion beam lithography. This approach allows for the side-by-side deposition of surface textures where both the areal extent and depth of the surface features are varied in a systematic manner. CdTe films deposited overtop these textures show grain structures with dramatic variations, revealing that particular length scales have the most pronounced effect on the grain structure. [ABSTRACT FROM AUTHOR]

The small lattice mismatch and sixfold symmetry offered by the (0001) planes of sapphire make it an ideal substrate candidate for the deposition of (111) CdTe films. There, however, exists a wide disparity in film quality among various researchers with both single crystal and highly twinned, multidomain films being reported. We have developed a pulsed laser deposition process that enables us to deposit nearly single-domain (111) CdTe films exhibiting excellent surface morphology. Such films are deposited on as-received sapphire substrates in vacuum conditions where oxygen is readily available. If, however, film deposition is preceded by the deposition of a submonolayer of aluminum prior to film growth then a secondary CdTe domain emerges with an in-plane orientation having a 180°-in-plane offset from the first domain. These multidomain films show poor crystallographic and morphological properties, similar to what has been reported elsewhere. It is concluded that the singly terminated (0001) sapphire substrates are a prerequisite for the deposition of high-quality (111) CdTe films. [ABSTRACT FROM AUTHOR]

Two major secondary phases have been revealed in the ceramics based on the complex perovskites Ba(Co1/3Nb2/3)O3: a Ba-rich compound Ba8CoNb6O24 with the layered perovskite structure, and a Nb-rich compound with the structure of tetragonal tungsten bronze and the composition close to Ba6CoNb9O30. The presence of a small amount of the Ba-rich phase is accompanied with the rise in the quality factor (Q) of the material whereas the high amount is deleterious to Q. In contrast, the presence of even a small amount of Nb-rich phase always results in a sharp fall in the Q-factor. The ways of tuning temperature coefficient τf in Ba(Co1/3Nb2/3)O3 ceramics have been discussed. [ABSTRACT FROM AUTHOR]

Abstract: The (100) SrTiO3 substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate''s unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (100) SrTiO3. The as-received substrate gives rise to a [111] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film''s grain structure. For this case, a [211] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film''s grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges. [Copyright &y& Elsevier]

Abstract: In contrast to simple ternary perovskites (e.g., SrTiO3 and BaTiO3), the defect chemistry model of complex barium perovskites with general formula Ba()O3 has yet to be developed. In an attempt to gain an insight into a point defect chemistry of these compounds, we present first results of electron spin resonance, positron annihilation spectroscopy and dielectric spectroscopy (10–100GHz) of Ba()O3, where B′=Mg, Zn, Ni, Co and B″=Nb and Ta. Both extrinsic and intrinsic defects were found in these materials. A Mn ion impurity serves as an excellent sensor for the early stages of the B-site cation disorder and oxygen activity. Based on the several Ba()O3 examples, we demonstrate that Schottky disorder plays a crucial role in the high temperature phase stability and dielectric loss of Ba()O3 perovskites. [Copyright &y& Elsevier]

We describe and characterize a multichannel modular room temperature photoluminescence spectroscopy system. This low cost instrument offers minimization of size and complexity as well as good flexibility and acceptable spectral resolution. The system employs an efficient flexible front end optics and a sensitive spectrometer with a charge-coupled device array detector. The spectrometer has no moving parts and is more robust than a scanning system. The scientific motivation was to enable the photoluminescence study of various silicon photonics structures. Typical applications are presented for SiOx (x<2) films. It is demonstrated that high-quality steady state photoluminescence data with excellent signal to noise enhancement capability can be delivered besides the ability to perform simultaneous multiwavelength measurements in one shot. This instrument is shown to be useful for evaluating semiconductor wafers, including those intended for light emitting structures from silicon-based photonic crystals. The design, construction, calibration, and the unique features of this system are presented, and performance tests of a prototype are discussed. [ABSTRACT FROM AUTHOR]

Quantum well (QW) band gap modification has been carried out in 1.55 μm InGaAsP-based laser structures using a silicon dioxide cap (SiO2) and rapid thermal annealing (RTA). Migration of the semiconductor atoms across the QW interface changes the profile of QW from a square well to a rounded well. The modification of the profile causes the shift of energy levels in the QW. In consequence, a blue shift of the emission peak is observed in photoluminescence (PL). In this paper, the contactless electromodulation technique called photoreflectance (PR), which is an absorption-like experiment, has been used. This method allows the detection of excited states transitions, as well as the fundamental one, in low-dimensional structures. In PR experiment, different energy shifts are observed for different confined state transitions reflecting the character of the changes in the well profile. The role of the microwave power of ECR-PECVD process is analysed and its impact on the blue shift of both ground and excited states is discussed in this paper. It is shown that the value of the blue shift can be controlled by the microwave power. [Copyright &y& Elsevier]

The quantum well intermixing (QWI) of 1.55 μm laser structure through thermal treatment, utilising various cap layers, has been investigated by both photoluminescence (PL, emission-like) and photoreflectance (PR, absorption-like) experiments. A blue shift of the QW ground state transition has been observed for post-grown-modified laser structures. The influence of the cap stoichiometry on QWI has been analysed. It has been found that the magnitude of the blue shift strongly depends on the stoichiometry of the dielectric film. In PR, besides the blue shift of the fundamental transition, a blue shift of the excited state transitions has been observed. The blue shift is evidently stronger for the ground state transition than for the higher energy ones. The character of the recombination process at room temperature has been found as free carrier recombination for both as-grown- and post-grown-modified laser structures. [Copyright &y& Elsevier]

• Low loss germanium oxide grown through ion-assisted alternating current sputtering. • Sputtering in the hysteresis regime affects film loss. • High uniformity across large area. • Low losses are achievable without the need for post-deposition process steps. • Propagation losses of 0.1 dB/cm for 638 nm were obtained for slab waveguide. We report low optical loss germanium oxide (GeO 2) thin films which have been deposited by low pressure ion-assisted alternating current dual magnetron sputtering of germanium targets in an oxygen plasma environment. The germanium oxide films ranging from 0.7- to 1.0-µm-thick were fabricated at low temperature and high deposition rates of 6–38 nm/min on silicon and thermally oxidized silicon substrates. The refractive index of the films was determined through variable angle spectroscopic ellipsometry and found to be 1.6051 on average at a wavelength of 638 nm. The films were shown to be near stoichiometric through Rutherford backscattering spectroscopy analysis and displayed sub-nanometer roughness when measured with atomic force microscopy. High peak-to-valley uniformity on a 3-inch substrate with relative deviation of 0.8% was achieved. We characterized the attenuation of the GeO 2 thin films from visible to near-infrared wavelengths and observed it to be as low as 0.1 dB/cm at 638 nm for deposition rates of 8 nm/min. This deposition technique provides complementary metal-oxide-semiconductor compatible GeO 2 thin films suitable for integrated photonics applications and is promising for the fabrication of other dielectric waveguide materials. [ABSTRACT FROM AUTHOR]

Abstract: In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er3+ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy. Room temperature emission bands centered at ∼1.54 and at 0.75μm have been obtained for all samples. The most intense emission band at ∼1.54μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality. [Copyright &y& Elsevier]

Photoreflectance spectroscopy has been used to study interdiffusion effects of dielectric-capped, rapid-thermal-annealed InGaAsP-based quantum well laser structures grown by gas source molecular beam epitaxy. Post-growth modification-induced changes of the quantum well shape influence its energy levels. For the processed laser structures a blue shift of ground and excited state transitions has been observed. It has been found that the energy difference between the two lowest heavy hole levels decreases approximately linearly with the blue shift of the ground state transition. [Copyright &y& Elsevier]

Supercontinuum generation (SCG) is an important nonlinear optical process enabling broadband light sources for many applications, for which silicon nitride (Si 3 N 4 ) has emerged as a leading on-chip platform. To achieve suitable group velocity dispersion and high confinement for broadband SCG the Si 3 N 4 waveguide layer used is typically thick (>∼700 nm), which can lead to high stress and cracks unless specialized processing steps are used. Here, we report on efficient octave-spanning SCG in a thinner moderate-confinement 400-nm Si 3 N 4 platform using a highly nonlinear tellurium oxide (TeO 2 ) coating. An octave supercontinuum spanning from 0.89 to 2.11 µm is achieved at a low peak power of 258 W using a 100-fs laser centered at 1565 nm. Our numerical simulations agree well with the experimental results giving a nonlinear parameter of 2.5 ± 0.5 W -1 m -1 , an increase by a factor of 2.5, when coating the Si 3 N 4 waveguide with a TeO 2 film. This work demonstrates highly efficient SCG via effective dispersion engineering and an enhanced nonlinearity in CMOS-compatible hybrid TeO 2 -Si 3 N 4 waveguides and a promising route to monolithically integrated nonlinear, linear, and active functionalities on a single silicon photonic chip.

In this article, we report a Si/Ge waveguide phototransistor with high responsivity and low dark current under low bias voltages, due to an engineered electric field distribution. The photodetector consists of n-i-p-i-n doping regions and shows a responsivity of 606 A/W at 1 V bias, and 1032 A/W at 2.8V bias with an input optical power of -50 dBm, and dark current of 4 µA and 42 µA respectively. This is achieved by placing two p + -doped regions in the silicon slab region beneath the Ge epitaxial layer. A measured small signal -3 dB bandwidth of 1.5 GHz with a -80 dBc/Hz phase noise response at 1 KHz frequency offset were demonstrated experimentally.

A new integrated deposition system taking advantage of magnetron sputtering and electron cyclotron-plasma enhanced chemical vapour deposition (IMS ECR-PECVD) is presented that mitigates the drawbacks of each fabrication system. This tailor-made system provides users with highly homogeneous and pure thin films with less undesired hydrogen and well-controlled rare-earth concentration compared to existing methods of rare-earth doping, such as metalorganic powders, sputtering, and ion implantation. We established the first comprehensive report on the deposition parameters of argon flow and sputtering power to achieve desired rare-earth concentrations in a wide composition range of terbium (Tb) doped-silicon oxide (Tb:SiO x ) matrices including silicon-rich ( x  < 2), oxygen-rich ( x  > 2), and stoichiometric silicon oxide ( x  = 2). The deposition parameters to fabricate crystalline structure (Tb 2 Si 2 O 7 ) in oxygen-rich samples are reported where Tb ions are optically active. IMS ECR-PECVD pushes the solubility limit of the rare-earth dopant in silicon films to 17 at.% for the desired future nanophotonic devices., Supplementary Information: The online version contains supplementary material available at 10.1557/s43578-023-01207-2., Competing Interests: Competing interestAuthors have no conflict of interest to declare., (© Crown 2023.)

We report on the design, fabrication and characterization of subwavelength grating metamaterial waveguides coated with tellurium oxide. The structures are first fabricated using a standard CMOS compatible process on a silicon-on-insulator platform. Amorphous tellurium oxide top cladding material is then deposited via post-process RF magnetron sputtering. The photonic bandstructure is controlled by adjustment of the device geometry, opening a wide range of operating regimes, including subwavelength propagation, slow light and the photonic bandgap, for various wavelength bands within the 1550 nm telecommunications window. Propagation loss of 1.0 ± 0.1 dB/mm is reported for the tellurium oxide-cladded device, compared to 1.5 ± 0.1 dB/mm propagation loss reported for the silicon dioxide-cladded reference structure. This is the first time that a high-index (n > 2) oxide cladding has been demonstrated for subwavelength grating metamaterial waveguides, thus introducing a new material platform for on-chip integrated optics.

Amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were grown using electron cyclotron resonance chemical vapour deposition using a mixture of methane, nitrogen, and silane as precursors. The origin of the variation of macroscopic properties such as hardness (H), elastic modulus (E), photoluminescence (PL), and the optical band gap was investigated through their correlation with the microscopic features of a-SiCN:H thin films as a function of the process parameters, including the deposition temperature and methane gas flow rate. From a microstructural perspective, the thin films were investigated using x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection, transmission electron microscopy, and x-ray diffraction. It is verified that an increase of the substrate temperature resulted in the substitution of hydrogen atoms mainly by carbon atoms, causing the density of the silicon carbide-related structures to increase in the amorphous structure of the a-SiCN:H thin films. Hardness and elastic modulus were found to increase with the deposition temperature and decreased with an increase of the methane gas flow during the deposition, resulting in higher carbon content in the films. The observed changes are ascribed to the reduced density of the weak hydrogen terminated bonds and the variation of the relative bond density of Si-C to Si-N bonds. In addition, the thin films were depth profiled using a slow positron beam to investigate the role of vacancies. The observed increase of the positronium formation with increasing deposition temperature was found to correlate with the variation of PL, where an enhancement of the visible emission originating from carbon-related defects was observed. A set of optimized process parameters to fabricate a-SiCN:H thin films with improved visible emission and hardness properties is suggested.

Amorphous sub-nanometre Tb-doped SiOxNy/SiO2 superlattices were fabricated by means of alternating deposition of 0.7 nm thick Tb-doped SiOxNy layers and of 0.9 nm thick SiO2 barrier layers in an electron-cyclotron-resonance plasma enhanced chemical vapour deposition system with in situ Tb-doping capability. High resolution transmission electron microscopy images showed a well-preserved superlattice morphology after annealing at a high temperature of 1000 °C. In addition, transparent indium tin oxide (ITO) electrodes were deposited by electron beam evaporation using a shadow mask approach to allow for the optoelectronic characterization of superlattices. Tb(3+) luminescent spectral features were obtained using three different excitation sources: UV laser excitation (photoluminescence (PL)), under a bias voltage (electroluminescence (EL)) and under a highly energetic electron beam (cathodoluminescence (CL)). All techniques displayed Tb(3+) inner transitions belonging to (5)D4 levels except for the CL spectrum, in which (5)D3 transition levels were also observed. Two competing mechanisms were proposed to explain the spectral differences observed between PL (or EL) and CL excitation: the population rate of the (5)D3 state and the non-radiative relaxation rate of the (5)D3-(5)D4 transition due to a resonant OH-mode. Moreover, the large number of interfaces (trapping sites) that electrons have to get through was identified as the main reason for observing a bulk-limited charge transport mechanism governed by Poole-Frenkel conduction in the J-V characteristic. Finally, a linear EL-J dependence was measured, with independent spectral shape and an EL onset voltage as low as 6.7 V. These amorphous sub-nanometre superlattices are meant to provide low-cost solutions in different areas including sensing, photovoltaics or photonics.

Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er3+ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.

Silicon nanoclusters (Si-ncs) embedded in silicon nitride films have been studied to determine the effects that deposition and processing parameters have on their growth, luminescent properties, and electronic structure. Luminescence was observed from Si-ncs formed in silicon-rich silicon nitride films with a broad range of compositions and grown using three different types of chemical vapour deposition systems. Photoluminescence (PL) experiments revealed broad, tunable emissions with peaks ranging from the near-infrared across the full visible spectrum. The emission energy was highly dependent on the film composition and changed only slightly with annealing temperature and time, which primarily affected the emission intensity. The PL spectra from films annealed for duration of times ranging from 2 s to 2 h at 600 and 800°C indicated a fast initial formation and growth of nanoclusters in the first few seconds of annealing followed by a slow, but steady growth as annealing time was further increased. X-ray absorption near edge structure at the Si K- and L3,2-edges exhibited composition-dependent phase separation and structural re-ordering of the Si-ncs and silicon nitride host matrix under different post-deposition annealing conditions and generally supported the trends observed in the PL spectra.

A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl(2)O(4) substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.

The dependence of the plasmon field enhancement of the nonradiative relaxation rate of the band gap electrons in vertically aligned CdTe-Au core-shell nanorods on the plasmonic gold nanoshell thickness is examined. Increasing the thickness of the gold nanoshell from 15 to 26 nm is found to change the decay curve from being nonexponential and anisotropic to one that is fully exponential and isotropic (i.e., independent of the nanorod orientation with respect to the exciting light polarization direction). Analysis of the kinetics of the possible electronic relaxation enhancement mechanisms is carried out, and DDA simulated properties of the induced plasmonic field of the thin and thick gold nanoshells are determined. On the basis of the conclusions of these treatments and the experimental results, it is concluded that by increasing the nanoshell thickness the relaxation processes evolve from multiple enhancement mechanisms, dominated by highly anisotropic Auger processes, to mechanism(s) involving first-order excited electron ejection process(es). The former is shown to give rise to nonexponential anisotropic decays in the dipolar plasmon field of the thin nanoshell, while the latter exhibits an exponential isotropic decay in the unpolarized plasmonic field of the thick nanoshell.

We studied the anisotropy of the influence of plasmonic fields, arising from the optical excitation of a gold nanoshell plasmon absorption at 770 nm, on the lifetime of the bandgap state of the CdTe core in vertically aligned CdTe-Au core-shell nanorods. The previously observed decrease in the lifetime was studied as a function of the tilt angle between the long axis of the nanorod and the electric field polarization direction of the plasmon inducing exciting light. It is observed that the strongest enhancement to the exciton relaxation rate occurs when the two axes are parallel to one another. These results are discussed in terms of the coupling between the exciton transition moment of the CdTe rod and the electric field polarization direction of the gold nanoshell plasmon at 770 nm, which was determined from theoretical modeling based on the discrete dipole approximation.

The intense electromagnetic fields of plasmonic nanoparticles, resulting from the excitation of their localized surface plasmon oscillations, are known to enhance radiative processes. Their effect on the nonradiative electronic processes, however, is not as well-documented. Here, we report on the enhancement of the nonradiative electronic relaxation rates in CdTe nanowires upon the addition of a thin gold nanoshell, especially at excitation energies overlapping with those of the surface plasmon oscillations. Some possible mechanisms by which localized surface plasmon fields can enhance nonradiative relaxation processes of any quantized electronic excitations are proposed.

Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived from an unusual catalytically driven growth mode. This growth mode could only proceed when the surface of the substrate was corrupted with an alcohol layer, although the role of the corruption was not fully understood. Here, we present a study detailing the remarkable role that this substrate surface alteration plays in the development of CdTe nanowires; it dramatically improves the size uniformity and largely eliminates lateral growth. These effects are demonstrated to arise from the altered surface's ability to limit Ostwald ripening of the catalytic seed material and by providing a surface unable to promote the epitaxial relationship needed to sustain a lateral growth mode. The axial growth of the CdTe nanowires is found to be exclusively driven through the direct impingement of adatoms onto the catalytic seeds leading to a self-limiting wire height associated with the sublimation of material from the sidewall facets. The work presented furthers the development of the mechanisms needed to promote high quality substrate-based vertically aligned CdTe nanowires. With our present understanding of the growth mechanism being a combination of selective area epitaxy and a catalytically driven vapour-liquid-solid growth mode, these results also raise the intriguing possibility of employing this growth mode in other material systems in an effort to produce superior nanowires.

We describe the integration of optically pumped silicon nanocrystals (Si-ncs) embedded in SiO(2) with low loss silicon nitride slab waveguides. An emission waveguide containing Si-ncs with a broad band emission centered at 850 nm, together with a low loss transmission silicon nitride waveguide forms a two section device. The waveguides are fabricated via the deposition of SiO(x) and silicon nitride using ECR-PECVD. Incorporation of hydrogen through annealing, while beneficial to emission from the Si-ncs, is found to increase material absorption in silicon nitride. This is reconciled by annealing at low temperature. This work shows clearly the potential for this material system as a means for the integration of optical emission and waveguiding using a wholly VLSI compatible processing technology. We further suggest that immediate applications exist in particular in the field of evanescent sensing.