Alan Gallagher

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm. The confocal apparatus permits the distribution of “on” and “off” times (i.e., periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales. In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, i.e., P(τoff)∝1/τoff1+α. Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states. This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implicatio...

“On”/“off” fluorescence intermittency of single semiconductor quantum dots

Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level a...

Production of high-quality amorphous silicon films by evaporative silane surface decomposition

High‐quality hydrogenated amorphous silicon films (a‐Si:H) have been produced by decomposition of low‐pressure silane gas on a very hot surface with deposition on a nearby, typically 210 °C substrate. A high‐temperature tungsten filament provides the surface for heterogeneous thermal decomposition of the low‐pressure silane and subsequent evaporation of atomic silicon and hydrogen. These evaporated species (primarily) induce a‐Si:H growth on nearby substrates which are temperature controlled using a novel substrate holder. The light and dark conductivities, optical band gap, deposition rates, and light‐soaking effects of preliminary films are reported. The decomposition‐evaporation process has been examined using a mass spectrometer to directly detect the decomposition rate and the evaporated radical species. Based on this data and other information, a simplified model for the deposition process is suggested. The excellent film quality and the attributes of the deposition process make this technique, whic...

Total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6

The total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6 have been measured for electron energies from threshold to 300 eV. Comparisons are made to earlier measurements.

Mono‐ and disilicon radicals in silane and silane‐argon dc discharges

Measurements of monosilicon (SiHn) and disilicon (Si2Hn) radicals at the cathode surface of dc discharges in silane and silane‐argon mixtures are reported. Silyl radical density per decomposed silane was constant for fixed flow conditions over a range of powers and silane‐argon ratios. The relative densities for other monosilicon radicals SiHn/SiH3 decreased with increased fraction of silane in silane‐argon mixtures. The density of disilicon radicals was observed to be comparable to some of the monosilicon radicals, with Si2H2 and Si2H4 the dominant Si2Hn species. Formation and destruction reactions are discussed for these radicals, disilane, and the deposited film. We deduce that disilane is formed primarily on surfaces and that sputtering is a significant source for radicals near the cathode.

Neutral radical deposition from silane discharges

The fractional contributions of the various SiHn radicals (n=0–3) to deposition are calculated for low‐power, pure‐silane rf and dc discharges. This is done using a radical diffusion plus reaction equation, combined with current knowledge of SiH4 dissociation fractionation, of SiHn+SiH4 reactions, and of the distributed source of radicals. The conclusion reached is that more than 98% of neutral radical deposition is by SiH3 for typical deposition pressures (>100 mT at 240 °C). The effect of SiH3+SiH3 reactions at higher power is also evaluated using an estimated reaction rate coefficient (k3). The resulting loss in deposition rate is given as a function of film growth rate and of k3.

Radical species in argon‐silane discharges

SiHn radical densities at the surface of discharges in Ar‐SiH4 mixtures have been measured by low‐energy, electron‐collisional ionization and mass spectrometer detection of SiH+n. The principal radical seen at the substrate surface of a dc proximity discharge is SiH3.

Near-field fluorescence imaging by localized field enhancement near a sharp probe tip

This work describes near-field investigations of luminescent nanosamples based on monitoring fluorescence due to the enhanced field around a laser-illuminated probe tip. These fluorescence effects are investigated as a function of probe-sample separation, which identify a strong, spatially localized (≈7 nm) enhancement of the incident laser field in the vicinity of the probe tip. From a model fit to the fluorescence data, the localized enhancement of the electric field is estimated to be >tenfold, which predicts a significant increase in localized excitation intensity (>100-fold) for fluorescence imaging of molecular size samples.

Reaction mechanism and kinetics of silane pyrolysis on a hydrogenated amorphous silicon surface

Three regimes of pressure and temperature are identified in which silane pyrolysis has distinctly different initial kinetics: in two regimes the initial reactions are heterogeneous and in the third regime it is homogeneous. We report here a preliminary model for the heterogeneous reaction regime where the decomposition rate is nearly independent of pressure. In the model the silicon surface is saturated with hydrogen and hence is nonreactive. The rate limiting step for silane decomposition is the creation of reactive surface sites by release of hydrogen. These reactive sites are refilled by decomposition of SiH4 or reincorporation of H2. A new adsorbed state of SiH4 is proposed which is bound to the surface by a three‐center bond. After making some simplifications to the full model the kinetics are solved for static‐ and flowing‐gas hot wall reactor experiments. The implications of the proposed reactions for the other two pyrolysis regimes and for silane discharges are briefly discussed.

Mechanisms influencing ''hot-wire'' deposition of hydrogenated amorphous silicon

Intrinsic hydrogenated amorphous silicon (a-Si:H) has been deposited using a hot tungsten filament in pure silane to drive the deposition chemistry—the “hot-wire” deposition method. The electronic and infrared properties of the film have been measured as a function of deposition parameters, leading to three principal conclusions. First, to obtain a high quality material, the Si atoms evaporated from the filament (distance L from the substrate) must react with silane (density ns) before reaching the substrate; this requires nsL greater than a critical value. Second, radical-radical reactions cause deterioration of film properties at high values of G(nsL),3 where G is the film growth rate; this requires G(nsL)3 less than a critical value. Finally, the film quality is a function of G, and as G is increased the substrate temperature must be correspondingly increased to obtain high film quality. By optimizing these parameters, we have produced films with excellent electronic properties (e.g., ambipolar diffusi...