A multiple beam apparatus has been constructed to facilitate the study of ion-enhanced fluorine chemistry on undoped polysilicon and silicon dioxide surfaces by allowing the fluxes of fluorine (F) atoms and argon (Ar+) ions to be independently varied over several orders of magnitude. The chemical nature of the etching surfaces has been investigated following the vacuum transfer of the sample dies to an adjoining x-ray photoelectron spectroscopy facility. The etching ‘‘enhancement’’ effect of normally incident Ar+ ions has been quantified over a wide range of ion energy through the use of Kaufman and electron cyclotron resonance-type ion sources. The increase in per ion etching yield of fluorine saturated silicon and silicon dioxide surfaces with increasing ion energy (Eion) was found to scale as (Eion1/2−Eth1/2), where Eth is the etching threshold energy for the process. Simple near-surface site occupation models have been proposed for the quantification of the ion-enhanced etching kinetics in these systems. Acceptable agreement has been found in comparison of these Ar+/F etching model predictions with similar Ar+/XeF2 studies reported in the literature, as well as with etching rate measurements made in F-based plasmas of gases such as SF6 and NF3.

A heated semiconductor light-emitting diode at low forward bias voltage V

Electric dipole radiation can be controlled by coherent optical feedback, as has previously been studied by modulating the photonic environment for point dipoles placed both in optical cavities 1-3 and near metal mirrors 4,5 . In experiments involving fluorescent molecules 4,5 , trapped ions 6,7 and quantum dots 8 the point nature of the dipole, its sub-unity quantum efficiency, and decoherence rate conspire to severely limit any change in total linewidth. Here we show that the transverse coherence of exciton emission in the monolayer twodimensional (2D) material MoSe 2 removes many of the fundamental physical limitations present in previous experiments. The coherent interaction between excitons and a photonic mode localized between the MoSe 2 and a nearby planar mirror depends interferometrically on mirror position, enabling full control over the radiative coupling rate from near-zero to 1.8 meV and a corresponding change in exciton total linewidth from 0.9 to 2.3 meV. The highly radiatively broadened exciton resonance (a ratio of up to 3 : 1 in our samples) necessary to observe this modulation is made possible by recent advances in 2D materials sample fabrication 9-11 . Our method of mirror translation is free of any coupling to strain or DC electric field in the monolayer, which allows a fundamental study of this photonic effect. The weak coherent driving field in our experiments yields a mean excitation occupation number of ∼10 −3 such that our experiments correspond to probing radiative reaction in the regime of perturbative quantum electrodynamics 12 . This system will serve as a testbed for exploring new excitonic physics 13 and quantum nonlinear optical effects 14,15 .The transition metal dichalcogenides (TMDs) MoSe 2 and MoS 2 become direct band gap semiconductors when isolated in monolayer form [16][17][18] , transferring a significant fraction of the interband spectral weight to a strong and spectrally narrow excitonic resonance 19,20 . Coherence 21 , spin-valley interactions 22,23 , strain effects 24 , many-body electron physics 25-27 and engineered confinement 28,29 have all been studied using TMD excitons.Monolayer and few-layer TMDs were first prepared by mechanical exfoliation 16,17,30 and were typically n-doped and inhomogeneously broadened by substrate roughness. By adding electrostatic control via a gate, the semiconductor could be made neutral 26,27 . Encapsulation of TMDs in atomically flat hBN (hexagonal Boron Nitride) has enabled further improvements 9-11 . While some residual imperfections persist 31-34 , sample qualities sufficient to manifest quantum coherent effects are now achievable.Modifying the electromagnetic environment by using a mirror to engineer the local photonic density of states can affect the radiative decay rate of a dipole 1,5 . In addition to those involving fluorescent molecules 4,5 , trapped ions 6,7 and quantum dots 8 , similar studies have been conducted with surface plasmon-polaritons 35 and with an acoustic gong 36 . For a perfect 0D dipole placed near a perfectly...

The reactive ion etching of silicon by fluorine in high-aspect ratio features was modeled to assess the relative importance of reactant transport on etching rate at the bottom of rectangular trenches. The flux of ions to the feature bottom was found by summing two components: ions arriving directly from the plasma and ions reflected from the sidewalls before reaching the bottom. The transport of neutral reactants within the feature was modeled with diffuse scattering and reaction rates following the kinetics reported by Gray et al. [J. Vac. Sci. Technol. B 11, 1243 (1993)] at all surfaces. The etching rate was found to depend most strongly upon the ion flux under typical process conditions, because of relatively low fluorine reaction probability and low reactant depletion within the feature. Reactant transport limitations are expected to be more important under conditions of low fluorine to ion flux ratio, high-substrate temperature, and high-ion energy.

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of monolayer MoS2 for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second-harmonic generation in monolayer MoS2, and find that the thirdharmonic process is efficient only for linearly polarized input light while the third-order two-photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer MoS2 excitonic structure a promising resource for coherent nonlinear photonics.