Hengameh Allaf Navirian

Repetitive ultrafast melting of InSb as an x-ray timing diagnostic

We have demonstrated the possibility of using repetitive ultrafast melting of InSb as a timing diagnostic in connection with visible-light pump∕x-ray probe measurements at high-repetition-rate x-ray facilities. Although the sample was molten and regrown approximately 1×106 times, a distinct reduction in time-resolved x-ray reflectivity could be observed using a streak camera with a time resolution of 2.5ps. The time-resolved x-ray reflectivity displayed this distinct decrease despite the fact that the average reflectivity of the sample had fallen to approximately 50% of its original value due to accumulated damage from the prolonged laser exposure. The topography of the laser-exposed sample was mapped using an optical microscope, a stylus profilometer, photoelectron microscopy, and a scanning tunneling microscope. Although the surface of the sample is not flat following prolonged exposure at laser fluences above 15mJ∕cm2, the atomic scale structure regrows, and thus, regenerates the sample on a nanosecond...

Synchrotron-based ultrafast x-ray diffraction at high repetition rates

We present a setup for ultrafast x-ray diffraction (UXRD) based at the storage ring BESSY II, in particular, a pump laser that excites the sample using 250 fs laser-pulses at repetition rates ranging from 208 kHz to 1.25 MHz. We discuss issues connected to the high heat-load and spatio-temporal alignment strategies in the context of a UXRD experiment at high repetition rates. The spatial overlap between laser pump and x-ray probe pulse is obtained with 10 μm precision and transient lattice changes can be recorded with an accuracy of δa/a(0) = 10(-6). We also compare time-resolved x-ray diffraction signals from a laser excited LSMO/STO superlattice with phonon dynamics simulations. From the analysis we determine the x-ray pulse duration to 120 ps in standard operation mode and below 10 ps in low-α mode.

Nanoscale heat transport studied by high-resolution time-resolved x-ray diffraction

We report on synchrotron-based high-repetition rate ultrafast x-ray diffraction (UXRD) experiments monitoring the transport of heat from an epitaxial La0.7Sr0.3MnO3/SrTiO3 superlattice (SL) into the substrate on timescales from 100ps to 4µs. Transient thermal lattice expansion was determined with an accuracy of 10 7 , corresponding to a sensitivity to temperature changes down to 0.01K. We follow the heat flow within the SL and into the substrate after the impulsive laser heating leads to a small temperature rise of 1T = 6K. The transient lattice temperature can be simulated very well using the bulk heat conductivities. This contradicts the interpretation of previous UXRD measurements, which predicted a long-lasting expansion of SrRuO3 for more than 200ps. The disagreement could be resolved by assuming that the heat conductivity changes in the first hundred picoseconds.

Direct time-domain sampling of subterahertz coherent acoustic phonon spectra in SrTiO3 using ultrafast x-ray diffraction

We synthesize sub-THz longitudinal quasimonochromatic acoustic phonons in a SrTiO3 single crystal using a SrRuO3/SrTiO3 superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast x-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/omega(2) power law known from Akhiezers model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher q is discussed in the framework of nonlinear effects appearing due to the high amplitude of the synthesized phonons.

Subpicosecond hard x-ray streak camera using single-photon counting.

We have developed and characterized a hard x-ray accumulating streak camera that achieves subpicosecond time resolution by using single-photon counting. A high repetition rate of 2 kHz was achieved by use of a readout camera with built-in image processing capabilities. The effects of sweep jitter were removed by using a UV timing reference. The use of single-photon counting allows the camera to reach a high quantum efficiency by not limiting the divergence of the photoelectrons.

Thermoelastic study of nanolayered structures using time-resolved X-ray diffraction at high repetition rate

We investigate the thermoelastic response of a nanolayered sample composed of a metallic SrRuO3 electrode sandwiched between a ferroelectric Pb(Zr0.2Ti0.8)O3 film with negative thermal expansion and a SrTiO3 substrate. SrRuO3 is rapidly heated by fs-laser pulses with 208 kHz repetition rate. Diffraction of X-ray pulses derived from a synchrotron measures the transient out-of-plane lattice constant c of all three materials simultaneously from 120 ps to 5 μs with a relative accuracy up to Δc/c = 10−6. The in-plane propagation of sound is essential for understanding the delayed out-of-plane compression of Pb(Zr0.2Ti0.8)O3.

Time-resolved x-ray scattering from laser-molten indium antimonide.

We demonstrate a concept to study transient liquids with picosecond time-resolved x-ray scattering in a high-repetition-rate configuration. Femtosecond laser excitation of crystalline indium antimonide (InSb) induces ultrafast melting, which leads to a loss of the long-range order. The remaining local correlations of the liquid result in broad x-ray diffraction rings, which are measured as a function of delay time. After 2 ns the liquid structure factor shows close agreement with that of equilibrated liquid InSb. The measured decay of the liquid scattering intensity corresponds to the resolidification rate of 1 m/s in InSb.

Shortening x-ray pulses for pump-probe experiments at synchrotrons

We implemented an experimental scheme for ultrafast x-ray diffraction at storage rings based on a laser-driven Bragg-switch that shortens the x-ray pulses emitted from an undulator. The increased time-resolution is demonstrated by observing changes of intensity, position and width of the diffraction peaks of a La0.7 Sr0.3 MnO3/SrTiO3 superlattice sample after optical excitation, i.e., by quantitatively measuring the propagation of an expansion wave through the sample. These experimental transients with timescales of 35 to 60 ps evidence a reduction of the x-ray pulse duration by a factor of two.

Time-domain sampling of x-ray pulses using an ultrafast sample response

We employ the ultrafast response of a 15.4 nm thin SrRuO3 layer grown epitaxially on a SrTiO3 substrate to perform time-domain sampling of an x-ray pulse emitted from a synchrotron storage ring. Excitation of the sample with an ultrashort laser pulse triggers coherent expansion and compression waves in the thin layer, which turn the diffraction efficiency on and off at a fixed Bragg angle during 5 ps. This is significantly shorter than the duration of the synchrotron x-ray pulse of 100 ps. Cross-correlation measurements of the ultrafast sample response and the synchrotron x-ray pulse allow to reconstruct the x-ray pulse shape.

Influence of residual gas composition and background pressure in a multi-stage co-evaporation chamber on the quality of Cu(In, Ga)Se 2 thin films and their device performance

Thin film solar cells with Cu(In, Ga)Se2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies above 21%. Typical multi-stage co-evaporation chambers are MBE-like (ultra-)high vacuum systems with individual effusion sources for each element. Cleanliness of the process chamber and the background pressure during the co-evaporation process could be of importance for the chamber design and a fair comparison of production costs when comparing different PV/Chalcopyrite technologies. Here we study the influence of the background pressure quality on the electronic and structural properties of the deposited absorber layer. To achieve this, we analyzed the residual gas composition before and the background pressure during consecutive co-evaporation processes and investigate the effect of a combined cleaning (mechanical and electro-chemical) of the chamber walls together with a simple conditioning of the chamber after opening the chamber and re-filling the crucibles. Cleaning of the chamber yielded a significant reduction in carbon species and an overall lower base pressure. The background pressure during the process was reduced from ~6×10-6mbar (before cleaning with water cooling shroud) to 1*10-7 mbar (after cleaning with LN2 filled cooling shroud). The type and amounts of contaminants in the absorber layer are characterized by laser ablation inductively coupled plasma mass spectroscopy (LA ICP-MS). The impact of the process pressure on the growth of the CIGSe layer is analyzed with respect to preferential orientation (using XRD), grain-size (using SEM), in-depth elemental gradients (using GDOES) and the electronic quality (using TRPL, C-V). Analysis of completed solar cell devices shows that the absorber band-gap is hardly affected by the chamber conditions, whereas we see an improved collection of charge carriers generated by photons in the infra-red spectral range from the conditioned chamber, also resulting in slightly higher jsc. The major effect is an increase in median Voc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).Thin film solar cells with Cu(In,Ga)Se2 (CIGSe) absorbers prepared by co-evaporation reach efficiencies above 21% [1]. Typical multi-stage co-evaporation chambers are MBE-like (ultra-)high vacuum systems with individual effusion sources for each element. Cleanliness of the process chamber and the background pressure during the co-evaporation process could be of importance for the chamber design and a fair comparison of production costs when comparing different PV/Chalcopyrite technologies. Here we study the influence of the background pressure quality on the electronic and structural properties of the deposited absorber layer. To achieve this, we analyzed the residual gas composition before and the background pressure during consecutive co-evaporation processes and investigate the effect of a combined cleaning (mechanical and electro-chemical) of the chamber walls together with a simple conditioning of the chamber after opening the chamber and re-filling the crucibles. Cleaning of the chamber yielded a significant reduction in carbon species and an overall lower base pressure. The background pressure during the process was reduced from ∼6∗10−6 mbar (before cleaning with water cooling shroud) to 1∗10−7 mbar (after cleaning with LN2 filled cooling shroud). The type and amounts of contaminants in the absorber layer are characterized by laser ablation inductively coupled plasma mass spectroscopy (LA ICP-MS). The impact of the process pressure on the growth of the CIGSe layer is analyzed with respect to preferential orientation (using XRD), grain-size (using SEM), in-depth elemental gradients (using GDOES) and the electronic quality (using TRPL, C-V). Analysis of completed solar cell devices shows that the absorber band-gap is hardly affected by the chamber conditions, whereas we see an improved collection of charge carriers generated by photons in the infra-red spectral range from the conditioned chamber, also resulting in slightly higher jsc. The major effect is an increase in median Voc values from 585mV (before cleaning and conditioning) to 635mV (after cleaning and condition). The overall solar cell efficiency is increased by 18% (relative).