Photoemitters Based on Glass - ITO Structures
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direction or even produce secondary electrons in cascade multiplication. Due to complexity of
the interaction between the primary beam and the internal field, an investigation of the electron emission with the internal field but without a primary beam was also performed.
In the ref. [6, 7] it was shown that illumination of the glasses by polarized light leads to increase of dichroism, birefringence and optical anisotropy. Moreover the performed investigations allowed to predict that the photoinduced anisotropy favors also a noncentrosymmetry in electron charge density distribution. Therefore one can suppose that the nonlinear optical methods, particularly optical second harmonic generation (SHG) can be a sensitive tool for detection degree of photostructural changes. EXPERIMENT The applied samples were similar to those used in the study of the field-induced SEE. Two conducting and transparent films of ln20 3:Sn (ITO) were evaporated on the both sides of a microscopic cover glass 16x16x0.2 mm. One film of the thickness 10รท20 nm was the emitting surface. The other, of thickness 1.tm, was polarized in order to create an internal field (field electrode). Evaporation was made by reactive ion sputtering on both sides of the glass substrate. This method ensures high uniformity of the surface layers, reliability of data and relatively low dusting. A key point of the method consists in bombardment of a solid target by ions of high energy. The target has been consisted from In-Sn alloy (80% In + 20% Sn). The surface resistance was varied within 1-50 Q/cm. Electric and optical properties of these films are described in [8, 9]. The measurements were performed at the pressure of about 2x 10-6 Pa. The sample as well as an electron energy analyzer and electron multiplier were placed in the vacuum. The schematic diagram of the apparatus is shown in Fig. 1. An internal field which favored electron emission into vacuum was made by applying polarizing voltage Up.,, from the interval from -2000 V to 0 V, to the field electrode. Appropriate operational conditions for the electron multiplier were received by acceleration of electrons between the emitting film and the multiplier, i.e. voltage Up = -200V at the emitting film and grounded entrance of the multiplier. Depending on the kind of performed measurements, grids 3 and 4 of the energy analyzer were either grounded or polarized by negative analyzing voltage Ua. For retarding field analysis, the voltage applied to grids 3 and 4 was changed from Ua to 0 V with respect to the emitting film. Then only those electrons which could force the retarding barrier eUa could get into the multiplier. When the investigation aimed to register all emitted electrons, grids 3 and 4 were grounded to enable free electrons flow into the multiplier. The electrons accelerated to the energy eUp create voltage pulses in the multiplier which are recorded in the multichannel pulse amplitude analyzer (Tristan 1024). The multiplier is joined to preamplifier which adjusts its parameters to the pulse analyzer. Th
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