Dead-layer thickness

The depletion width can play a role in analyte-induced modulation of the semiconductor PL [4]. As molecules adsorb onto the surface of the semiconductor, the dead-layer thickness can change, resulting in what can be described as a luminescent litmus test When Lewis bases adsorb onto the semiconductor surface, they donate electron density to the solid, which decreases the electric field and thus decreases the dead-layer thickness. The reduction in D causes an enhancement in the PL intensity from the semiconductor. Figures 2a and 2b present typical PL enhancements observed from an etched n-CdSe substrate Relative to a nitrogen reference ambient, adsorption of the Lewis bases ammonia and trimethylamine cause a reversible increase in PL intensity. In contrast, when Lewis acids adsorb onto the surface, they can withdraw additional electron density, causing the electric field to increase and the PL intensity to decrease. Such effects have been observed with gases like sulfur dioxide [5]. 

The quantitative form of the dead-layer model relates PL intensity to dead-layer thickness, which is assumed to approximate the depletion width W [6,7] ... 

Photoluminescence experiments with ni-V wafers of Ino.5o(Gao.9o A1o.io)o.5oP were conducted 114]. The Lewis basic gaseous analytes ammonia, methylamine, dimethyl amine, and trimethylamine all yielded reversible PL enhancements. The Lewis acid sulfur dioxide, in contrast, caused reversible quenching of the semiconductor s PL intensity. These PL intensity changes were consistent with analyte-induced modifications of the dead-layer thickness. 

Fig. 9.21 Change of the dead-layer thickness Dd = Di -D2 upon exposure of Pd to hydrogen in the photoluminescent Schottky junction...

In this expression, whose origin is indicated in Fig. 4, PL] and PL2 are the PL intensities in N2 and amine ambients AD is the corresponding change in dead-layer thickness (equated with the change in depletion width) and a = a + B, where a and B are the absorptivities for the exciting and emitted light this treatment assumes that the surface recombination velocity is either very large or insensitive to the introduction of the amine (1). 

Monte Carlo methods demand a detailed knowledge of the detector geometry and construction. This information is not always available, that from the manufacturer being a nominal or estimated value. It is common, therefore, to read that Monte Carlo methods need fine tuning with modification of parameters such as dead layer thickness and even detector diameter to make the model fit experimentally determined data. In some cases, people have resorted to X-raying their detector within its cooled encapsulation in order to measure the true detector size under operating conditions. 

Parameters, such as dead layer thickness, cannot be measured. Often, mathematical models have to adjust them empirically in such a way as to make the model fit practical measurements. 

Normal coordinate to (2) Layer thickness Dead time... 

In 1984, the AMPTE mission launched the first carbon-foil TOF-MS into space, which would have been the second, had the Challenger shuttle disaster not delayed the Ulysses launch until 1991 (Fig. 11.2) [23]. The photons were filtered out by a traditional blackened deflection system, which directed the ions toward the 2 p,g/cm2 thick foil mounted on an 85% transparent grid almost a square centimeter in area. The grid provided the support needed to survive the launch. The foil thickness permitted >2keV/nuc ions to pass through and hit a SSD some 10 cm away. To ensure that the ions made it through the foil and also through the dead layer on the SSD (caused by the upper electrode), the foil and the entire TOF section were floated at 20 kV to post-accelerate the ions. Electrons sputtered off the carbon foil became the start, whereas electrons sputtered off the SSD became the stop pulse for the TOF. 

Fig. 4. The dead-layer model for analyzing changes in PL intensity for two states, a) and b). As indicated in the figure, state a) corresponds to the PL intensity in a N2 ambient and state b) corresponds to the PL intensity in the presence of a gaseous amine. The symbols CB and VB represent the solid s conduction and valence band edges, respectively. For each state, the PL intensity is proportional to the amount of incident light (intensity Iq absorptivity a ) absorbed beyond the nonemissive layer whose thickness is D. The ratio of the two PL Intensities leads to eq. 1.

The window defect is due to energy loss in the dead layer (window) of the front surface of the detector. It can be obtained from the thickness of the window and the stopping power of the ion. The thickness of the window can... 

The lack of sensitivity of solid-state detectors to lower energy particles is due to the dead layer of the detector. In the high resistivity detectors, tltis dead layer is due to the thickness of the electrode which can be on the order of 1 fim. Charge carriers generated by particles that do not have sufficient energy to penetrate significantly beyond this silicon layer are trapped there. 

Figure 6. a.Time dependence of polymer layer thickness 8h (logarithmic time scale) during desorption into pure solvent for 6 different PEO samples. Molecular weights are indicated in the figure. At t = 0 injection of pure solvent on a polymer layer initially saturated at c= 100 m starts. Initial parts of these curves are dashed since they are affected by instrumental artifacts (dead volume). 

The depletion region of the semiconductor is not only an insulating layer but also relatively non-emissive and hence commonly referred to as a dead layer . If we oversimplify the semiconductor interface into a near-surface non-emissive zone and an underlying emissive zone, then the adsorption-induced PL behavior is quantitatively related to changes in the thickness of the dead layer according to... 

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