Chemically etched ultrahigh-Q wedge-resonator on a silicon chip

Ultrahigh-Q optical resonators are being studied across a wide range of fields, including quantum information, nonlinear optics, cavity optomechanics and telecommunications. Here, we demonstrate a new resonator with a record Q-factor of 875 million for on-chip devices. The fabrication of our device avoids the requirement for a specialized processing step, which in microtoroid resonators8 has made it difficult to control their size and achieve millimetre- and centimetre-scale diameters. Attaining these sizes is important in applications such as microcombs and potentially also in rotation sensing. As an application of size control, stimulated Brillouin lasers incorporating our device are demonstrated. The resonators not only set a new benchmark for the Q-factor on a chip, but also provide, for the first time, full compatibility of this important device class with conventional semiconductor processing. This feature will greatly expand the range of possible ‘system on a chip’ functions enabled by ultrahigh-Q devices

DFT Study of Water Adsorption and Decomposition on a Ga-Rich GaP(001)(2×4) Surface

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2×4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2×4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2×4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2×4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2×4) unit cell. Our results support a mechanism in which (1) the first step is the molecular adsorption, with the water molecule forming a Lewis acid–Lewis base bond to the sp^2 Ga atom of either the first-layer Ga–P mixed dimer or the second layer Ga–Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H_2O to form the HO/H decomposition state in which the hydroxyl moiety bonds with surface sp^2 Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the O/2H decomposition state, in which the oxygen moiety forms bridged Ga–O–Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp^2 Ga atoms. (4) We find that driving off the hydrogen as H_2 leads to the surface oxide state, bridged Ga–O–Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H_2O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes

Probing Surface Chemistry at an Atomic Level; Decomposition of 1-Propanethiol on GaP(001)(2×4) Investigated by STM, XPS, and DFT

The adsorption and decomposition mechanisms for 1-propanethiol on a Ga-rich GaP(001) (2 × 4) surface are investigated at an atomic level using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Using a combination of experimental and theoretical tools, we probe the detailed structures and energetics of a series of reaction intermediates in the thermal decomposition pathway from 130 to 773 K. At 130 K, the propanethiolate adsorbates are observed at the edge gallium sites, with the thiolate–Ga bonding configuration maintained up to 473 K. Further decomposition produces two new surface features, Ga–S–Ga and P-propyl species at 573 K. Finally, S-induced (1 × 1) and (2 × 1) reconstructions are observed at 673–773 K, which are reportedly associated with arrays of surface Ga–S–Ga bonds and subsurface diffusion of S. To understand the observed site-selectivity on the hydrogen dissociation of the thiol molecule at 130 K, the two most likely dissociation pathways (Ga–P vs Ga–Ga dimer sites) are investigated using DFT Gibbs energy calculations. While the theory predicts the kinetic advantage for the dissociation reaction occurring on the Ga–P dimer (Lewis acid–base combination), we only observed dissociation products on the Ga–Ga dimer (Lewis acid). The DFT calculations clarify that the reversible thiolate diffusion along the Ga dimer row prevents recombinative desorption, which is probable on the Ga–P dimer. Together with experimental and theoretical results, we suggest a thermal decomposition mechanism for the thiol molecule with atomic-level structural details

Impact of Surface Roughness in Measuring Optoelectronic Characteristics of Thin-Film Solar Cells

Microstructural properties of thin-film absorber layers play a vital role in developing high-performance solar cells. Scanning probe microscopy is frequently used for measuring spatially inhomogeneous properties of thin-film solar cells. While powerful, the nanoscale probe can be sensitive to the roughness of samples, introducing convoluted signals and unintended artifacts into the measurement. Here, we apply a glancing-angle focused ion beam (FIB) technique to reduce the surface roughness of CdTe while preserving the subsurface optoelectronic properties of the solar cells. We compare the nanoscale optoelectronic properties before and after the FIB polishing. Simultaneously collected Kelvin-probe force microscopy (KPFM) and atomic force microscopy (AFM) images show that the contact potential difference (CPD) of CdTe pristine (peak-to-valley roughness of approximately 600 nm) follows the topography. In contrast, the CPD map of polished CdTe (roughness of approximately 20 nm) is independent of the surface roughness. We demonstrate the smooth CdTe surface also enables high-resolution photoluminescence (PL) imaging at a resolution much smaller than individual grains (< 1 micrometer). Our finite-difference time-domain (FDTD) simulations illustrate how the local light excitation interacts with CdTe surfaces. Our work supports low-angle FIB polishing can be beneficial in studying buried sub-microstructural properties of thin-film solar cells with care for possible ion-beam damage near the surface.Comment: 4 pages, 4 figure

Probing Surface Chemistry at an Atomic Level; Decomposition of 1-Propanethiol on GaP(001)(2×4) Investigated by STM, XPS, and DFT

The adsorption and decomposition mechanisms for 1-propanethiol on a Ga-rich GaP(001) (2 × 4) surface are investigated at an atomic level using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Using a combination of experimental and theoretical tools, we probe the detailed structures and energetics of a series of reaction intermediates in the thermal decomposition pathway from 130 to 773 K. At 130 K, the propanethiolate adsorbates are observed at the edge gallium sites, with the thiolate–Ga bonding configuration maintained up to 473 K. Further decomposition produces two new surface features, Ga–S–Ga and P-propyl species at 573 K. Finally, S-induced (1 × 1) and (2 × 1) reconstructions are observed at 673–773 K, which are reportedly associated with arrays of surface Ga–S–Ga bonds and subsurface diffusion of S. To understand the observed site-selectivity on the hydrogen dissociation of the thiol molecule at 130 K, the two most likely dissociation pathways (Ga–P vs Ga–Ga dimer sites) are investigated using DFT Gibbs energy calculations. While the theory predicts the kinetic advantage for the dissociation reaction occurring on the Ga–P dimer (Lewis acid–base combination), we only observed dissociation products on the Ga–Ga dimer (Lewis acid). The DFT calculations clarify that the reversible thiolate diffusion along the Ga dimer row prevents recombinative desorption, which is probable on the Ga–P dimer. Together with experimental and theoretical results, we suggest a thermal decomposition mechanism for the thiol molecule with atomic-level structural details

Growth Mechanism and Electronic Structure of Zn_3P_2 on the Ga-Rich GaAs(001) Surface

The growth of epitaxial Zn_3P_2 films on III–V substrates unlocks a promising pathway toward high-efficiency, earth-abundant photovoltaic devices fabricated on reusable, single-crystal templates. The detailed chemical, structural, and electronic properties of the surface and interface of pseudomorphic Zn_3P_2 epilayers grown on GaAs(001) were investigated using scanning tunneling microscopy/spectroscopy and high-resolution X-ray photoelectron spectroscopy. Two interesting features of the growth process were observed: (1) vapor-phase P4 first reacts with the Ga-rich GaAs surface to form an interfacial GaP layer with a thickness of several monolayers, and (2) a P-rich amorphous overlayer is present during the entire film growth process, beneath which a highly ordered Zn_3P_2 crystalline phase is precipitated. These features were corroborated by transmission electron micrographs of the Zn_3P_2/GaAs interface as well as density functional theory calculations of P reactions with the GaAs surface. Finally, the valence-band offset between the crystalline Zn_3P_2 epilayer and the GaAs substrate was determined to be ΔE_V = 1.0 ± 0.1 eV, indicating the formation of a hole-depletion layer at the substrate surface which may inhibit formation of an ohmic contact

Structure, Chemistry, and Energetics of Organic and Inorganic Adsorbates on Ga-rich GaAs and GaP(00l) Surfaces

The work described in this dissertation includes fundamental investigations into three surface processes, namely inorganic film growth, water-induced oxidation, and organic functionalization/passivation, on the GaP and GaAs(001) surfaces. The techniques used to carry out this work include scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Atomic structure, electronic structure, reaction mechanisms, and energetics related to these surface processes are discussed at atomic or molecular levels. First, we investigate epitaxial Zn3P2 films grown on the Ga-rich GaAs(001)(6×6) surface. The film growth mechanism, electronic properties, and atomic structure of the Zn3P2/GaAs(001) system are discussed based on experimental and theoretical observations. We discover that a P-rich amorphous layer covers the crystalline Zn3P2 film during and after growth. We also propose more accurate picture of the GaP interfacial layer between Zn3P2 and GaAs, based on the atomic structure, chemical bonding, band diagram, and P-replacement energetics, than was previously anticipated. Second, DFT calculations are carried out in order to understand water-induced oxidation mechanisms on the Ga-rich GaP(001)(2×4) surface. Structural and energetic information of every step in the gaseous water-induced GaP oxidation reactions are elucidated at the atomic level in great detail. We explore all reasonable ground states involved in most of the possible adsorption and decomposition pathways. We also investigate structures and energies of the transition states in the first hydrogen dissociation of a water molecule on the (2×4) surface. Finally, adsorption structures and thermal decomposition reactions of 1-propanethiol on the Ga-rich GaP(001)(2×4) surface are investigated using high resolution STM, XPS, and DFT simulations. We elucidate adsorption locations and their associated atomic structures of a single 1-propanethiol molecule on the (2×4) surface as a function of annealing temperature. DFT calculations are carried out to optimize ground state structures and search transition states. XPS is used to investigate variations of the chemical bonding nature and coverage of the adsorbate species.</p

DFT Study of Water Adsorption and Decomposition on a Ga-Rich GaP(001)(2×4) Surface

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2×4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2×4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2×4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2×4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2×4) unit cell. Our results support a mechanism in which (1) the first step is the <i>molecular adsorption</i>, with the water molecule forming a Lewis acid–Lewis base bond to the sp² Ga atom of either the first-layer Ga–P mixed dimer or the second layer Ga–Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H₂O to form the <i>HO/H decomposition state</i> in which the hydroxyl moiety bonds with surface sp² Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the <i>O/2H decomposition state</i>, in which the oxygen moiety forms bridged Ga–O–Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp² Ga atoms. (4) We find that driving off the hydrogen as H₂ leads to the <i>surface oxide state</i>, bridged Ga–O–Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H₂O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes