Emission current from metals

Table 4. Electron Emission Current from Metals at Elevated Temperatures...

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

Fig. 4.8. Field-emission spectrum of Mo(lOO). The quantity displayed, Jf, is the ratio between the observed field-emission current and the prediction based on a free-electron model, Eq. (4.20). As shown, the field-emission spectrum of Mo(lOO) near the Fermi level is substantially different from a free-electron-metal behavior. (After Weng, 1977.)...

At present, very high ultimate values of tunnel emission flow density can be obtained ( 10 ° A cm-2). The tunnel emission of electrons from metals is widely used in modern technology in developing various devices where high currents or intensive electron beams are required. 

Bilayered polysilane LEDs have been obtained by inserting a SiOx thin layer between the cathode and a Wurtz synthesized PMPS emitter film.94 The SiOx layers were prepared by 02 plasma treatment of the PMPS film surfaces. It was found that the external quantum efficiency was significantly enhanced by this treatment. This enhancement has been attributed to an increased electron injection via tunneling, resulting in a reduced hole current caused by the blocking effect of the thin SiOx layer. The weak visible emission observed from single-layer polysilane LEDs is almost completely eliminated. It was concluded that the visible emission is caused by the erosion of the PMPS surfaces due to the collision with hot metal particles during the vacuum deposition of the cathode, and this erosion process is avoided by the SiOx layer. 

Figure 159 EL spectra (a) and EL transients (b) for SL ITO/ Polymer/Metal LEDs (1) ITO/MEH-PPV/A1 (2) ITO/MEH-PPV/ MEH-PPV/(Mg/Ag) (3) ITO/M3EH-PPV/(Mg/Ag). EL spectra are recorded at different voltage pulse amplitudes. Characteristic cathode metal emission lines are indicated along with the position of broad band-red shifted emission maxima from the polymers. For chemical meaning of MEH-PPV and of M3EH-PPV, see Figs. 106 and 108. The transient current under a lps pulse (4) and transient response of EL polymer and cathode metal emission spectral regions at two different pulse amplitudes, 4.3 MV/cm (5) and 6.6MV/cm (6) have been measured on ITO/MEH-PPV/A1 device. EL transient are normalized to the maximum intensity of the polymer emission. After Ref. [472]. Copyright 2000 American Institute of Physics.

The multilayered structure and electroluminescent mechanism of OLEDs is illustrated in Figure 4.45. Depending on whether small organic molecules or long repeating-unit polymers are used (Figure 4.46), the diodes are referred to as OLEDs or PLEDs, respectively. Under positive current, electrons and holes are injected into the emissive layer from opposite directions - from the cathode and anode, respectively. The metal... 

Based on our research laboratory s observations of MEF from metals other than silver, MEC from chromium, coppo, nickel and zinc was also studied. Figure 1. 12 shows the enhancement factors for green chemiluminescence from diese metals with various thicknesses. A typical enhancement factor of 2-3-fold is observed from all metal surfaces, vdiich implies that chemically excited states can couple to these plasmon resonant metal particles. It is interesting to note that the chemiluminescence emission is dependent on the amount of reactants in the solution and diminishes once one of the reactants is used up. To test viiether the remainder of the inactive chemiluminescent dye can be excited with an external light source and still emit luminescence, additional experiments were undertaken where the chemiluminescence solution was excited with a laso at 473 nm. Interestingly, the inactive chemiluminescence dye can be optically excited and still emit luminescence with enhancement factors similar to the chemically excited conditions being observed. A detailed investigation of MEC from different metals is currently underway and will be reported in due course. 

The condition necessary for our heated probe is that the emission current be small compared to the probe current. Thermionic emission from a metal is given by... 

Fig. 30. Electron emission from metals with differently shaped surface barriers. j—current density in amp/em1, F—field in volts/A. Calculations for cf> = tf>2 = 4.5 ev, tj> j = 5.5 ev width of conduction band = 0.4 ev d = 2.5A. Slopes of F-N curves identical to better than 1%.

When the current flows, metal atoms are sputtered from the cathode into the area within and in front of the cup. Collisions with the neon or argon ions cause a proportion of the metal atoms to become excited and emit their characteristic radiation. The choice of filler gas depends on the element lead, iron, and nickel perform far better with neon than with argon however, neon is not suitable with some elements, such as lithium and arsenic, because a strong neon emission line is close to the best resonance line. For many elements, there is little to choose between the two gases. 

The last possibility, which corresponds to field emission in crystalline semiconductors, is a much less likely current path in a-Si H Schottky barriers under forward bias. Since the Fermi energy is near midgap, there is a relatively low density of states and the states are localized. Thus there are few states that can field emit into the metal. In reverse bias, field-emission breakdown can be observed because the emission is from the metal Fermi energy into the a-Si H conduction band. Measurements and an analysis have been... 

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