Photocathodes negative electron affinity

The ability to obtain single-photon counting using methods such as avalanche photon detectors and negative electron affinity photocathode photomultiphers has thus far been limited to the visible and infrared regions. The vertical QCD which utihzes a triple-well quantum (Q) dot system of the type illustrated in Fig. 9 offers a novel approach to sense THz radiation. Here, the detector is first primed into active-mode by tunnel injection into the top Q-dot (QDl) of the SES followed by an IR pulse that puts the electron into the middle Q-dot (QD2) of the THz-RDC. This electron will remain in the QD2 until a THz photon induces the electron s transition to QD3. Finally, an IR photon ejects the electron from QD3 thus resetting the detector. Since the electron injection into the QCD system and ejection from the detector are quick transitions, only the middle Q-dot (QD2) will be occupied for a significant period of time. Consequently, to successfully read-out the state of our triple Q-dot system one must be able to differentiate between the two possible states (1) if THz photons are present, the electron will quickly be ejected from the entire QCD system and no electrons will be present in QD2 or (2) if no THz photon is present, QD2 will remain occupied by an electron. 

Negative electron affinity (NEA) GaAs reflection photocathodes which exhibit sensitivities as high as 2150 pA/lm. 

Conventional Photocathodes. The photocathode controls the spectral properties of the phototube. Photocathodes can be divided into two categories, conventional ones and negative electron affinity ones. The energy band diagrams of... 

Fig. 2.9. Photoemissive process semiconductor photocathode having negative electron affinity...

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]. 

NEA photocathodes are photoconductive semiconductors whose surface has been treated to obtain a state of negative electron affinity . This state is reached when the energy level of an electron at the bottom of the conduction band in the bulk of the semiconductor is greater than the zero energy level of an electron in the vacuum. Hence an electron excited to the conduction band within the bulk can, if it travels to the activated surface without first recombining, energetically fall out of the photocathode into free space. 

Recently, a new type of photocathode has been developed that is based on pho-toconductive semiconductors whose surfaces have been treated to obtain a state of negative electron affinity (NBA) (Fig. 4.108). In this state an electron at the bottom of the conduction band inside the semiconductor has a higher energy than the zero energy of a free electron in vacuum [254]. When an electron is excited by absorption of a photon into such an energy level within the bulk, it may travel to... 

Figure 4.108 Level scheme for negative electron affinity photocathodes...

Devices with negative electron affinity (NEA) Material with negative electron affinity is used for photocathode, e.g., strongly doped semiconductor coated with cesium... 

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