RM and TM Modes

It is more difficult to fabricate semitransparent than opaque devices because the thickness of the sensitive layer is an added critical parameter not important in the opaque structure. (Improper thickness in the semitransparent device would limit both the range of useful spectral response and overall quantum efficiency [5.29].) Most new photoemissive surfaces, particularly NBA, are now investigated initially in their opaque ( reflection mode , RM , or front illuminated ) form, which usually offers the highest initial optical and electron-emissive quantum efficiency, and only after determining optimum thickness and optimum fabrication methods are new surfaces produced in the semitransparent ( transparent , transmission mode , TM , or back illuminated ) form. Eventually both forms are offered commercially in several variants, each optimized for selected parameters such as low cost, peak response at a specific 

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Considered first in this chapter are the physical principles of operation applicable to both classes of photoemissive surface, to the extent to which they are understood. The operation and construction of classical devices are then examined in detail, using two extreme cases, (CsSb) and (AgCsO) -S —1, as examples. NEA devices are then considered. These are discussed in somewhat greater depth than classical devices inasmuch as these recently developed detectors are less likely to be familiar to the user. One example of NEA operation, NEA GaAs, is the simplest structure and is discussed in detail for both RM and TM modes. Other IR-sensitive emitters, including those using layers of complicated quaternary compound semiconductor alloys such as InGaAsP, are then briefly summarized. The chapter is concluded with a summary of device-to-device trade-offs in classical and NEA devices. 

Before considering other possible TM or RMIR N E A photoemitters, we discuss in this subsection the fabrication of NEA GaAs and the general factors required to optimize RM mode photocathodes [5.46,47, 83, 89 -91], plus the additional special constraints of TM mode fabrication [5.14, 15, 89, 91, 92-96]. 

SEA should be determined, mainly, by the A-mode aerosol. The latter is characterized by substantially constant size distributions for weak dust loads Tm < 0.4 M-m, a = 2.2 for moderate and heavy dust loads rm = 0.45 Hm, tJ = 2.1 (rm is the mean radius of particles in the number lognormal distribution cH the dispersion of distribution). 

A Digital Instruments Scanning Probe Microscope was used in the tapping mode for all AFM measurements as seen schematically in Figure 3.2-3. A silicon nitride tip was driven with a ratio of 3.0V (RMS) to 1.8V (Set point). At least three height and phase images were generated on a 5.0 im, 2.0 im, or a 1.0 tm square scale for each sample listed in Table 3.2-3. In addition, the difference in elevation for two specifically selected points on each polymer brush sample were measured. 

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