Classical Photoemissive Surfaces

Cesium antimonide as a photoemitter [5.43,51] has found wide application in commercial photodetection devices. Tubes with (CsSb) on several opaque and semitransparent substrates, and with various window materials, have been given several S -numbers including S-4, 5, 11, 13, 17, and 19 [5.52]. The physical properties of this material have also been rather extensively investigated through measurements of, for example, a( ), Eq, <)>, and defect characteristics [5.53]. We have therefore chosen it as the first example of a classical photoemissive surface. 

NEA surfaces differ from classical photoemissive surfaces in that conduction band electrons require no excess thermal energy above to escape. It is a cold electron emission device, while in classical positive electron affinity surfaces a small barrier is present at the surface and either hot electron escape or tunneling of thermalized electrons is required for emission. NEA and classical electron emission are contrasted in Fig. 5.8 [5.67]. Part (a) is a classical emitter, (b) is a p-type semiconductor treated to obtain NEA, and (c) is an n-type semiconductor similarly treated but without attaining NEA. (Many details of this figure are clarified in later sections.)... 

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. 

In order to obtain useful photoemission at photon energies lower than the threshold of Cs, it would seem necessary simply to find a metallic compound alloy in which the electron work function is lower than the 2eV elemental minimum (but there is none), or to find a semiconductor where Ef + X<2eV. This search has led to the discovery of the classical photoemissive compounds. The binary compound with the lowest electron affinity is cesium antimonide, CsjSb, where X=0.45 eV [5.39]. Here Eq 1.6 eV for a threshold c Eq + X of about 2 eV. For the more complex substance (NaKCsSb), which is not actually a true compound but is rather (NaKSb) with a surface skin of (NaKCsSb) [5.40],... 

The negative electron affinity (NEA) photocathode is the most recent and generally the highest-performance photoemissive surface discovered to date [5.4,11-21]. Its operation is an extension of the same physical principles which apply to all other photoemissive surfaces, differing only in the means of obtaining low electron affinity and in the specific factors which determine photo yield. The principles of operation are so similar to those of classical emitters that the first NEA surface was completely predicted by theoretical extension of classical principles prior to its first experimental fabrication [5.66]. 

The third photoemission step corresponds to the escape of the excited electron into the vacuum. The classical picture due to Fowler (1931) is used with the three-step model. The momentum of the excited electron is resolved into components parallel and perpendicular to the surface. The motion through the surface in the perpendicular direction is opposed by retarding forces whose action is described by... 

These terms represent a bulk effect, a classical surface potential effect, and a surface field effect, respectively. Selection rules are stated through which these various contributions to the net photocurrent can be resolved. A complete theoretical description of photoemission, therefore, will enable resolution of each of these components as functions of the angles and j8), the available light intensity with depth, and the substrate epitaxy. This theory is discussed in detail in Ref. 51 note also Chapters 4 and 6 in this book. 

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