The optimization of (Cs,O) activation of NEA photocathode

As one of the main direction of developing negative electron affinity (NEA) photocathode with higher response performance, the research work on (Cs,O) activation technique of NEA photocathode has been carried. At present the main adoptive activation technique is the "high-low temperature" two-step activation. Although NEA photocathode with high sensitivity has been already prepared by use of this activation method, the research work of activation technique is still semi-experienced and semi-quantitative and therefore the lack of the theoretic guidance of further optimization of activation technique is existent. It is owing to the lack of effective on-line measurement means, and there is not enough useful information obtained during activation. As a result clear and, reasonable explanations for mechanism of activation technique and the details of NEA formation during activation have been inadequate. In this paper, the on-line measurement technology of spectral response and photocurrent are firstly used to research on the "high-low temperature" two-step (Cs,O) activation process of NEA photocathode. A series of GaAs slices were activated and different activation manners were adopted by changing Cs and O exposure coverage in activation cycles. On-line spectral response curves and the dependent curves of photocurrent with (Cs,O) activation cycles were measured during each different activation process, and the variations of photoemission performance parameters of photocathodes, which include threshold wavelength, peak response, peak wavelength, sensitivity and surface escape probability with (Cs,O) deposition cycles were calculated according to the measured curves. By analysis of the relations between different "high-low temperature" activation technique and sensitivity and stability of NEA photocathode, the optimized (Cs,O) activation technique was obtained and the relations between "high temperature" activation and "low temperature" activation was also discovered in this paper. In view of the interaction of GaAs surface and (Cs,O) overlaying atoms and the surface analysis results of GaAs(Cs,O) surface, the formation of NEA surface during activation and mechanisms "high-low temperature" activation were further explored. It was concluded that the surface of NEA ph- otocathode is composed of GaAs-O interface barrier and Cs-O layer, and the purpose of optimization of activation technique is to make an optimized GaAs-O interface barrier with enough small layer thickness and uniform distribution of GaAs-O bonding, and to make an optimized Cs-O layer with sufficient numbers of Cs-O dipoles at a thinner Cs-O layer. The reasons why higher sensitivity can be achieved by "low-temperature" activation compared to "high-temperature" activation are that optimized GaAs-O interface barrier is obtained by low-temperature heat cleaning and the optimized Cs-O layer is also sequentially obtained at the existence of GaAs-O barrier.