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Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a "phonon bottleneck,"a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. Verifying the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited-state electronic and lattice dynamics in polycrystalline silicon nanoparticles following 800 nm photoexcitation, which excites carriers with 0.35 ± 0.03 eV excess energy above the Δ1 conduction band minimum. The nanoparticles have nominal 100 nm diameters with crystalline grain sizes of about ∼16 nm. The extracted carrier?phonon and phonon?phonon relaxation times of the nanoparticles are compared to those for a silicon (100) single-crystal thin film at similar carrier densities (2 × 1019 cm?3for the nanoparticles and 6 × 1019 cm?3for the film). The measured carrier? phonon and phonon?phonon scattering lifetimes for the polycrystalline nanoparticles are 870 ± 40 fs and 17.5 ± 0.3 ps, respectively, versus 195 ± 20 fs and 8.1 ± 0.2 ps, respectively, for the silicon thin film. The reduced scattering rates observed in the nanoparticles are consistent with the phonon bottleneck hypothesis.

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M2,3 edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening (σ) and a red shift (ωs) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase (ΔT) is linearly proportional to the Gaussian broadening factor σ, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640±80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ωs∝ΔT1.5. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a

Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

The complex-valued index of refraction of germanium in the extreme ultraviolet (XUV) is measured by multiangle reflectance of synchrotron radiation. The resulting index of refraction is higher resolution than previously measured values. It reveals new structures attributed to transitions from the 3d-core orbitals to the Σc5,2 and the Xc5,2 conduction bands. Additionally, it is shown that the problem of total external reflection, which renders multi-angle reflectance measurements insensitive to the complex-valued refractive index at grazing incidence, can be overcome by employing measurements at angles of incidence away from the critical angle.

Transient extreme ultraviolet (XUV) spectroscopy probes core level transitions to unoccupied valence and conduction band states. Uncertainty remains as to what degree the core-hole created by the XUV transition modifies the measurement of photoexcited electron and hole energies. Here, the Si L 2,3 edge is measured after photoexcitation of electrons to the δ, L, and σ valleys of Si(100). The measured changes in the XUV transition probability do not energetically agree with the increasing electron photoexcitation energy. The data experimentally confirm that, for the Si L 2,3 edge, the time-dependent electron and hole energies are partially obscured by the core-hole perturbation. A model based on many-body approximations and the Bethe-Salpeter equation is successfully used to predict the core-hole modification of the final transition density of states in terms of both electronic and structural dynamics. The resulting fit time constants match the excited-state electron thermalization time and the intervalley electron-phonon, intravalley electron-phonon, and phonon-phonon scattering times previously measured in silicon. The outlined approach is a more comprehensive framework for interpreting transient XUV absorption spectra in photoexcited semiconductors.

Extreme ultraviolet (XUV) transient reflectivity around the germanium M4,5 edge (3d core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of 3.2±0.2ps attributed to the X-L intervalley scattering and a hot hole relaxation rate of 600±300fs ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4-5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of 66 with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment (66-70 for the range of XUV energies used. This work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.

A metal-semiconductor heterojunction is investigated by Auger and photoelectron spectroscopy to characterize the structural and electronic properties of the metallic film and to obtain the time-resolved electronic response induced by femtosecond laser excitation of the semiconductor material. The 3.5 monolayer (ML) Zn films deposited on p-type Si(100) at liquid nitrogen temperature grows in a layer-by-layer fashion. Electronic structure measurements by extreme ultraviolet (XUV) photoelectron spectroscopy indicate that the films are metallic in nature, creating a Schottky barrier at the 3.5 ML Zn/p-Si(100) interface. Utilizing a 35 fs, 800 nm pump pulse at a pump intensity of (2.5-6) × 109 W/cm2 to excite the Si and a time-delayed extreme ultraviolet pulse to probe the Zn, we observed large transient surface photovoltage shifts of 0.3-2.2 eV at carrier densities of (1.5-4.5) × 1020 cm-3. Three shifts are determined the Zn 3d core level, the photoemission onset, and the metallic Fermi level. The photovoltages increase with laser excitation intensity, and the Zn 3d core level exhibits the largest binding energy shifts due to pronounced screening of the core level. The large observed shifts are rationalized on the basis of the energetics of band flattening and carrier accumulation in the metallic layer of the Zn/p-Si(100) heterojunction at high carrier densities. The observed carrier recombination dynamics are biexponential in character, with similar time constants for both the Zn 3d and photoemission onset binding energy shifts. The Zn 3d core level shifts are also found to be sensitive to the electron temperature. These results show that core-level photoemission can be used to monitor valence electron dynamics, allowing separation of charge dynamics in heterojunctions and solids composed of multiple elements.