Arthur Hotzel

Temperature dependence of surface state lifetimes, dephasing rates and binding energies on Cu(111) studied with time-resolved photoemission

Abstract The ultrafast electron dynamics of surface states on Cu(111) is investigated as a function of temperature between 25 K and 460 K employing time-resolved two-photon photoemission (2PPE) spectroscopy. Analysis of the thermally-induced energy shift of the unoccupied n = 1 image potential state based on a multiple reflection model allows a precise determination of the position of the upper edge ( L 6+ ) of the sp-gap in the (111)-directin( E L 6+ (T) − E F = 4.15eV− (0.26meV/K) T ). We find that the lifetime of the n = 1 image state decreases from22 ± 3 fs at 25 K to14 ± 3 fs at 350 K. This is attributed to the increasing penetration depth of the image state wave function into the bulk at higher temperatures, where the image state crosses the band edge. The phonon contribution to the electronic dephasing of the n = 0 surface state and the n = 1 image state on Cu(111) is determined from their temperature-dependent linewidths using three-level optical Bloch equations and is found to correlate with their wave function overlap with bulk states.

Dynamics of photoexcited electrons in metals studied with time-resolved two-photon photoemission

Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.

CAN WE CONTROL LIFETIMES OF ELECTRONIC STATES AT SURFACES BY ADSORBATE RESONANCES

Abstract Femtosecond time-resolved two-photon photoemission is used to study the excited electronic states of a Cu(111) surface covered with a monolayer of physisorbed O 2 on top of one to five Xe spacer layers. The spectra are dominated by a state 4.0 eV above E F whose lifetime depends strongly on the number of Xe spacer layers. The lifetime decreases with increasing number of spacer layers from (650±30) fs for one Xe layer to (90±10) fs for five layers. To explain this trend we propose a model where the image potential states couple to a negative-ion resonance of the O 2 molecule.

FEMTOSECOND TIME-RESOLVED PHOTOEMISSION OF ELECTRON DYNAMICS IN SURFACE RYDBERG STATES

Femtosecond time-resolved photoelectron spectroscopy provides a unique tool to study the dynamics of optically excited electrons at surfaces directly in the time domain. We present a new model for two-photon photoelectron spectroscopy from surface and image potential (or Rydberg) states which is based on density matrix theory. The formalism accounts for the influence of both energy and phase relaxation on experimental spectra and thus permits the study of the nature of inelastic and elastic scattering processes at surfaces in more detail. The analysis of experimental data employing the proposed model reveals a new mechanism for optical excitation of electrons to normally unoccupied states at surfaces which is feasible due to the influence of electronic dephasing. We discuss the nature of different relaxation channels with respect to our studies of image state dynamics on the bare and Xe or Kr covered Cu(111) surfaces.

Controlled modification of surface state lifetimes by physisorbed adsorbates

Femtosecond time-resolved two-photon photoemission is used to study the influence of physisorbed xenon and oxygen adlayers on the lifetime of image potential states and interfacial quantum well states on Cu(111). Adsorption of 0 to 3 layers of Xe leads to a pronounced increase of the n equals 1 image state lifetime from 22 fs to 300 fs, respectively. However, for adsorbate heterostructures consisting of one monolayer (ML) O2 on top of Xe spacer layers with variable thickness it is found that the lifetime of an oxygen induced quantum well state (0.35 eV below Evac) decreases from 650 fs to 90 fs when the number of spacer layers is raised from 1 ML to 5 ML. The results can be semiquantitatively reproduced by a model calculation which accounts for the modified image potential due to the Xe and O2 adlayers. The changes of the lifetimes are explained by the differences in the penetration of the excited state wave function into the Cu substrate.

Multilayer reticles: advantages and challenges for 28nm chip making

Chip manufacturing with multilayer reticles offers the possibility to reduce reticle cost at the expense of scanner throughput, and is therefore an attractive option for small-volume production and test chips. Since 2010, GLOBALFOUNDRIES Fab 1 uses this option for the 28nm IP shuttles and test chips offered to their customers for development and advance testing of their products. This paper discusses the advantages and challenges of this approach and the practical experience gained during implementation. One issue that must be considered is the influence of the small image field and the asymmetric reticle illumination on the lithographic key parameters, namely layer to layer overlay. Theoretical considerations and experimental data concerning the effects of lens distortion, lens heating, and reticle heating on overlay performance are presented, and concepts to address the specific challenges of multilayer reticles for high-end chip production are discussed.