Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy

Abstract: Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells, transistors and optical sensors. Charge transport in these materials differs fundamentally from that in crystalline semiconductors owing to the lack of long-range order and high defect density. Despite the existence of well-established experimental techniques such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here, we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron–hole separation and charge trapping induced by the atomic disorder. Our findings demonstrate the rich dynamics of hot carrier transport in amorphous semiconductors that can be revealed by direct imaging based on SUEM.

Monte Carlo simulation of intersubband relaxation in wide, uniformly doped GaAs/AlxGa1-xAs quantum wells.

Abstract: Using an ensemble Monte Carlo simulation, we have investigated intersubband relaxation of photoexcited electrons in GaAs/Al x Ga 1−x As quantum wells having a subband separation smaller than the polar optical phonon energy. Intra- and intersubband scattering through polar optical phonons, acoustic phonons, ionized impurities, and electron-electron scattering are included in the simulation. A comparison is made to recent time-resolved pump and probe experiments performed on uniformly doped samples in which good agreement between theory and experiment is obtained. Our results show that the intersubband decay of electrons from the first excited subband into the ground subband is limited by ionized impurity scattering during the photoexcitation process. Polar optical phonon emission also contributes considerably to the electron decay and occurs from the thermal tail of the heated distribution function in both subbands. Intersubband scattering by intercarrier interaction plays a lesser role for the decay. The heating of the distribution functions is due to ionized impurity intersubband scattering and electron-electron intrasubband scattering, which convert potential energy of an electron into kinetic energy. These mechanisms drive both subbands rapidly towards a single quasiequilibrium distribution with a common electron temperature and chemical potential after the pulse is over. The cooling rate of this distribution function, which governs the intersubband decay, depends initially on the energy relaxation through polar optical phonons, whereas at much longer times acoustic phonon scattering predominates. Thus, the intersubband decay of electrons is nonexponential.

Shape‐resonant and many‐electron effects in the S 2p photoionization of SF6

Abstract: The core‐level photoexcitation and photoionization of SF6 were studied in the vicinity of the resonances below and above the S 2p threshold. The decay channels of the S 2p→6a1g discrete excitation were characterized, with decay leading mostly to valence‐shell satellites. The S 2p continuum data show an oscillatory asymmetry parameter β(S 2p) near threshold that is virtually identical to β(Si 2p) in SiF4. It also resembles—but differs from—theoretical curves for β(S 2p) in atomic sulfur and in SF6. Data at the feature assigned as an eg shape resonance indicate strong multielectron properties for this state, because a resonance in the S 2p satellite is observed at the same photon energy as the main‐line resonance. We propose a unified model which generally includes configuration interaction both in the continuum‐state manifold and between discrete doubly excited states and the continua, to explain this unexpected satellite behavior. Finally, the S(L2,3VV) Auger electron asymmetry parameter shows no signific...

Vacuum ultraviolet photochemistry of methane, silane and germane

Abstract: The photochemistry of jet-cooled CH4, SiH4 and GeH4 molecules following excitation at the Lyman-α wavelength (121.6 nm) has been investigated by high resolution photofragment translational spectroscopy methods. Complementary ab initio calculations of selected portions of the potential energy surfaces for the various components of the 1T2 and 3T2 excited states arising from the 3sa1←1t2 electron promotion are presented in the case of CH4. The form of the H atom recoil velocity distribution arising in the 121.6 nm photolysis of CH4 is rationalised in terms of initial excitation to both the 21A′ and 11A″ excited states (Jahn–Teller components of the degenerate 1T2 state), followed by a range of decay mechanisms. CH4(21A′) molecules can decay adiabatically, ia sequential extension of first one, then a second, C–H bond with eventual formation of two H atoms and CH2(a1A1) products, or after internal conversion (IC) to the ground state. The H + CH3() products resulting from the IC process display a recoil velocity distribution characterised by an anisotropy parameter β∽ + 2, implying that the fragmentation involves irreversible extension of the C–H bond along which the transition dipole points at the instant of photon absorption. Fragmentation of CH4(11A″) molecules to H + CH3() products proceeds ia intersystem crossing to the lowest 3A′ potential energy surface. The recoil anisotropy of these products (β∽ − 0.45) implies that this radiationless process also occurs on a timescale that is rapid compared to the parent rotational period. Both single H–C bond fission channels may yield CH3() products with such high levels of internal excitation that they are unstable with respect to further unimolecular decay; any H atoms that result from this secondary decay must contribute to the observed yield of slow H atoms with β∽0. All H atoms resulting from Lyman-α photolysis of both SiH4 and GeH4 have (low) kinetic energies and little or no recoil anisotropy, compatible with their being formed ia three body fragmentation to, primarily, H + H + SiH2/GeH2(1A1) products. Faster H atoms are evident in the total kinetic energy release (TKER) spectra obtained following 157.6 nm photoexcitation of SiH4, but the power dependence of this fast H atom signal implies that these arise as a result of a two photon process involving initial formation of SiH2 + H2 products and subsequent photolysis of the nascent silylene fragments.

Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity

Abstract: Extreme ultraviolet (XUV) transient reflectivity around the germanium M_(4,5) 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 ± 0.2 ps attributed to the X−L intervalley scattering and a hot hole relaxation rate of 600 ± 300 fs 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.