The Study Of Surface States And Cleaning Technique On GaAs Photocathode

Negative electron affinity gallium arsenide (GaAs) photocathode as a core component of the third generation low-light-level (LLL) imaging intensifier, has many virtues, such as high quantum efficiency, low dark current, concentrated electrons energy distribution and angle distribution, adjustable long-wave threshold, great potential to extend the long-wave spectral response, and so on. The sensitivity of GaAs photocathode after activated by Cs-O directly influences the performance of the three generation image intensifier. In order to achieve a NEA state and higher photocurrent, photocathode should be cleaned before activated by Cs-O. It is important to carry out the study of surface states and cleaning technique on GaAs photocathode. This paper is mainly research for GaAs photocathode’s surface state, combined with the actual Cs-O activation photocurrent curve to establish a three-dipole model based on the dual-dipole model. To validate the model need for the GaAs surface treatment, and how to estimate GaAs surface state will be verification premise. Designed a series of analysis experiments to test the effect of high temperature cleaning technology, by measuring GaAs surface element types, distribution and content measurement, and combined with Ar ion etching for the two-dimensional distribution of the depth direction using X-ray photoelectron spectroscopy (XPS). The three dipole model was verified by the photocurrent curves activated of three types of GaAs materials from different manufacturers. The suitable temperature of high temperature surface cleaning teehnolegy could improve the GaAs photocathode sensitivity after Cs-O activation.Based on Spicer’s "Three-step Model" about photoemission, we studied the photoemission theory of GaAs photocathode thoroughly. The existing models have been studied, such as heterojunction model, double dipole layer model, the weak nuclear force effect of cesium, amorphous surface model, group model, and so on. Then a new surface model has been established by combining with the actual photocurrent curves of activation process. The surface barrier formation process of GaAs photocathode was simulated according to the variation of photocurrent while the photocathode was activated and the spectra response was in-situ tested. The two-dipole model has been amended to establish a three dipole model. It was considered that the photocathode surface barrier formed by three kind of dipole layers, the first dipole layer was composed of GaAs (Zn)--Cs+dipole, the second dipole layer was composed of Cs-O-Cs dipole, and the third dipole layer was composed of GaAs-O-Cs dipole, the second and third dipole layer embedded in the first dipole layer, the three layers overlapping together formed the piecewise uniform electric field. The accurate distribution of the three dipoles and the mutual relationship still remains to be explored.In order to study the surface state and verify the three dipole model, we must obtain a clean GaAs surface with less structural defects, and require a special surface cleaning processing method. In order to determine the effect of GaAs surface thermal cleaning process from the GaAs surface state, we designed experiments to test and analysis it. Using the X-ray Photoelectron Spectroscopy (XPS) to get the qualitative and quantitative analysis of the surface of photocathode the element types, distribution and content, when cleaning the surface before and after. Combined with Ar ion etching, two-dimensional depth direction distribution is attained. By the types and relative contents of elements on GaAs surface at thermal cleaning process before and after, the atomic arrangement can be simulated. By GaAs materials from three different manufacturers activated in different devices, the three dipole model was verified with variation of photocurrent.In this paper, it is important to understand the Cs-O activation mechanism of GaAs material and the surface state after high temperature cleaning and improve photocathode sensitivity through the research on GaAs surface state and cleaning technology. The investigation has significance to produce of high performance GaAs photocathode.