D. A. Mantell

In situ observation of photochemical prenucleation and patterned metalorganic chemical vapor deposition

Photochemical activation of a silicon native oxide surface with triisobutylaluminum (TIBA) and subsequent patterned metalorganic chemical vapor deposition (MOCVD) is observed in situ with x‐ray photoelectron spectroscopy. At room temperature and in the presence of a low vapor pressure (10−7 Torr) of TIBA, an ArF excimer laser is used to photodissociate the metalorganic adsorbed on the surface. The deposit forms islands consisting of oxygen, carbon, and aluminum. The oxygen is included by the reduction of the native oxide surface and the aluminum by the photodissociation of the TIBA. When the surface is heated to 300 °C (with the laser off), this deposit nucleates MOCVD film growth. The unprocessed native oxide surface does not nucleate growth so the deposit grows in the pattern initially written with the laser. The source of the prenucleation barrier that inhibits growth on the native oxide is discussed.

The effect of disorder on excited state dynamics in organic molecular films

We have investigated the dynamics of highly excited electron states in thin films of N,N′-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP), a well-known organic photoconductor, with time-resolved photoemission spectroscopy. We observe an extremely fast relaxation process of photoexcitations with a typical lifetime of 30 fs in DPEP films and attribute it to a rapid internal conversion process from S2 and S1 levels to the vibrational manifolds of S1 and S0 states. Interestingly, the relaxation rate is almost twice as fast at low excitation energies in polycrystalline DPEP films as it is in less-ordered DPEP films. We explain this difference by fast transitions within the manifold of extended states that are shown to form in ordered DPEP films.

Time-resolved electron diffraction to study photoinduced molecular dynamics at single crystal surfaces

We present a detailed description of ultrafast electron diffraction and its applications to study photoinduced molecular dynamics at single crystal surfaces. Experimental investigations for a new design of an ultrashort pulsed laser activated electron gun ((tau) < 5 ps) for time- resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time-resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. The total experimental temporal resolution is discussed in terms of the electron pulse width and the time difference between an electron scattered at the front edge of the sample to an electron scattered at the trailing edge of the sample.

MORPHOLOGY AND EXCITED STATE DYNAMICS IN THIN ORGANIC MOLECULAR FILMS PROBED BY FEMTOSECOND TIME-RESOLVED PHOTOEMISSION SPECTROSCOPY

We have studied the lifetimes of the excited electron states of thin N,N′bis(phenethyl)perylene-3,4:9,10-bis(dicarboximide) (DPEP) films, prepared in situ, using femtosecond time-resolved photoemission spectroscopy. DPEP is an organic compound similar to photoreceptor materials widely used in many imaging applications. By controlling the evaporation conditions, we have been able to grow films of different morphologies, and found that the relaxation dynamics depends on the morphology. We have investigated two distinct films characterized by very different absorption spectra. We have found that for the film with absorption maximum at 500 nm, a typical lifetime is 55 fs at 2.1 eV above the highest occupied molecular orbital level. For the other film with absorption maximum at 630 nm, the relaxation rate is almost twice as fast, resulting in a lifetime of 30 fs at the same energy. We attribute the extremely short lifetimes to a rapid internal conversion process from S2 and S1 levels to the vibrational manifol...

Femtosecond photoemission study of electron relaxation in two-dimensional layered electron systems

By using the femtosecond two photon photoemission technique, we directly measured the lifetimes of photoexcited electrons in two layered materials, graphite and SnS2. The inverse lifetimes of photoexcited electrons in these two-dimensional layered materials exhibit linear energy dependence, i.e., 1/τ∝(E−EF), instead of quadratic, i.e., 1/τ∝(E−EF)2 as predicted by Fermi liquid theory. This can be explained with the layered electron gas theory, according to which the electron-plasmon interaction is strong, even for electrons at low excitation energies, due to the formation of plasmon band in layered electron systems. Furthermore, our observations in SnS2 also suggest that in layered semiconductors, photogenerated electron-hole pairs will recombine directly by emitting plasmons, and this causes the lifetimes of the conduction band electrons to be much shorter than those in three-dimensional semiconductors.

Reverse surface photovoltaic effects in GaAs surface quantum wells

We observed that GaAs surface quantum wells on undoped AlGaAs substrates experienced a reverse surface photovoltage (RSPV) that caused the bands to bend upward when illuminated by femtosecond laser pulses for time-resolved two-photon photoemission spectroscopy (TRPES). The RSPV effect is created by trapping of photoexcited electrons in the quantum well. Secondary illumination creates a surface photovoltage opposite in magnitude that flattens the bands. The secondary illumination does not affect the width of the energy spectrum or the ultrafast dynamics with respect to the conduction band minimum. A simple model based on the recombination current from the AlGaAs fits the data excellently.

Photogenerated hot electron dynamics at GaAs (100) surfaces

MBE grown GaAs (100) surface quantum wells were studied using time-resolved two-photon photoemission spectroscopy where the electron distribution was directly determined. Relaxation lifetimes from 50 to 450 fs were measured and were found to be dependent on the energy of the excited electron distribution. Radiative lifetimes at the semiconductor - liquid interface were obtained using time-correlated single-photon counting. From the concentration dependence and other studies, we estimate that the electron transfer cross section is comparable to the molecular cross sections, i.e. electron transfer approaches the adiabatic limit and conditions are optimal for hot electron capture. This work provides parameters to make the hot electron transfer channel competitive with electron relaxation enabling a future generation of solar cells exploiting hot electrons.

Femtosecond studies of carrier relaxation processes at single crystal metal surfaces

Femtosecond time resolved two photon photoemission and above- threshold photoemission (ATP) have been used to characterize the dynamics of photoexcited electrons at single crystal Cu surfaces. The two photon photoemission studies measure nonradiative relaxation pathways of electrons near the surface, and the ATP studies demonstrate that photoemission occurs even when using light that is far below the work function. These studies provide important information regarding the extent and duration of the interaction of photoexcited electrons with surface adsorbates.