On the surface morphology of thin alkali halide photocathode "lms

Transcription

1 Nuclear Instruments and Methods in Physics Research A 438 (1999) 409}414 On the surface morphology of thin alkali halide photocathode "lms T. Boutboul *, A. Breskin, R. Chechik, E. Klein, A. Braem, G. Lion, P. MineH Department of Particle Physics, The Weizmann Institute of Science, Rehovot, Israel Electron Microscope Center, The Weizmann Institute of Science, Rehovot, Israel EP Division, CERN, 1211 Geneva 23, Switzerland LPNHE, Ecole Polytechnique, IN2P3-CNRS, F Palaiseau Cedex, France Received 4 May 1999; accepted 10 July 1999 Abstract Thin alkali halide "lms are currently used as transmissive UV-photocathodes and as protecting layers for visible photocathodes. The surface morphology of 20 and 75 nm thick evaporated CsI, NaI and CsBr "lms was investigated by means of a scanning electron microscope, to which the samples were transferred, under vacuum, with practically no contact with air. It is shown that the "lm continuity, in particular that of NaI, is strongly a!ected by short exposure to moisture. CsI, which is the less hygroscopic material among the three, exhibits the most continuous structure Elsevier Science B.V. All rights reserved. 1. Introduction Semi-transparent CsI vacuum-deposited photocathodes, commonly utilized in vacuum photomultiplier tubes, were reported to have an optimal thickness of the order of 10}20 nm [1], in the 160}190 nm UV spectral range. Moreover, such thin evaporated alkali halide "lms, CsI, NaI or CsBr, are currently employed as protective layers for alkali antimonide visible photocathodes [2,3]. Recent microscopic observations of CsI evaporated "lms showed that the "lm deposition is initiated by the growth of small islands, which makes the "lm continuity questionable below thicknesses of a few tens of nm [4]. The uniformity of such photo- * Corresponding author. address: thierry.boutboul@cern.ch cathodes or coating layers is essential for reaching high photoemission yields or good protection. First examinations by means of scanning electron microscope (SEM) and atomic force microscope (AFM) of 10}20 nm thick CsI and NaI "lms, evaporated on CaF substrates, have revealed completely discontinuous, island-like structures. Considerable e!orts were then invested by us, both at the Weizmann Institute and at CERN, to obtain continuous thin evaporated "lms. Extensive deposition studies were performed by using either resistive evaporation or an electron gun, at a vacuum of &10 Torr and varying parameters like the evaporation rate, substrate material and its temperature. Metal-coated windows, necessary for photoemission applications [5], and polished silicon are among the substrate materials we have investigated. However, continuous 20 nm thick CsI or NaI "lms were never obtained in these studies; /99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 9 )

2 410 T. Boutboul et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 409}414 the obtained layers exhibited a quite similar morphology of dissiminated crystallites. It should be stressed that alkali halides are hygroscopic materials and that the samples were undoubtedly exposed to air, for a few minutes, before their transfer to the SEM, for surface analysis. This could have damaged the surface morphology of the examined "lms. In this work, we report on the results of extended investigations of the surface structure of CsI, NaI and CsBr "lms, transferred under vacuum to the surface analysis apparatus. The sensitivity of "lm structure to controlled amounts of humidity was also investigated. 2. Experimental procedures and results was then vented with dry argon, through a valve, and its bottom part was quickly removed. This was done while keeping the argon #ushing, thus minimizing the sample exposure to air. The SEM port was closed and the system was rapidly evacuated. This process allowed for examining the structure of the alkali halide "lms `as evaporateda with practically no contact with air. In order to investigate the in#uence of moisture on the "lm morphology, the SEM chamber was, in turn, vented with Ar containing a controlled amount of humidity SEM examinations The "rst step of the work was to deposit 75 nm thick CsI, CsBr and NaI "lms on the CaF /Cr 2.1. Sample preparation and transfer The alkali halide photocathodes, considered within this work, were prepared in the following way. A &3 nm thick Cr layer and the alkali halide "lm were successively deposited by resistive evaporation onto an optically polished CaF substrate, 12 mm in diameter. A substrate made of glass, coated with Au/Ni, was investigated as well in the case of CsI. The alkali halides were of an `ultrapurea grade (Johnson Matthey). Both evaporations were performed within the same vacuum cycle (&10 Torr), the alkali halide "lm deposition rate being around 0.5 nm/s. The distance of the samples to the evaporation source was 350 mm. The samples were thus transferred inside a small vacuum vessel into the SEM. The cover of this vessel was designed to be compatible with the SEM port. Prior to the deposition process, the photocathode substrate was "xed and electrically connected to the vessel cover, for minimizing upcharging during SEM examinations. Following the deposition, the sample vessel was closed by means of a linear manipulator lowering the cover down to it and pressing it gently against a Nytrile rubber O-ring. The evaporation vessel was then "lled with dry argon, tightly sealing the transfer vessel. This procedure permitted the photocathode transfer to the SEM, without any contact with air. During installation at the SEM chamber, the vessel cover was "rst "xed to the microscope. The transfer vessel Fig. 1. An SEM view of a 75 nm thick CsI "lm deposited on a CaF /Cr substrate: (a) `as evaporateda and (b) after venting the sample with Ar at 30% relative humidity for 1 min. The full scale is 5 μm.

3 T. Boutboul et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 409} substrates. The samples were then transferred to the SEM, as described above. As can be seen in Figs. 1a and 2a, CsI and CsBr "lms appear to constitute continuous layers, composed of &100}200 nm in diameter crystallites. Nevertheless, 75 nm thick NaI "lms exhibit a discontinuous structure of interconnected crystallites (Fig. 3a); the surface coverage was only around 60%. Exposing the CsI and CsBr photocathodes to argon at &30% relative humidity (RH), for a few minutes, has no apparent impact on the "lm morphology, as seen in Fig. 1b for CsI exposed to humid Ar for 4 min. A 24 h moisture exposure of the CsI "lm slightly alters the layer continuity. NaI "lms appeared to be much more moisture sensitive than both other materials. Exposing the NaI photocathode to Ar at 30% RH had indeed a drastic e!ect on the "lm constitution; it provided a clear `islanda structure, with the coalescence of the layer into separated &2}3 μm in size crystal grains, as can be seen in Fig. 3b. Exposing 75 nm thick CsI and CsBr samples to Ar at a higher RH (&80%) for 1 min also caused an impressive coalescence of the "lms into disseminated crystal grains up to 1 μm in size, as shown in Fig. 2b for CsBr. Thinner evaporated "lms were investigated as well. 20 nm thick CsI, CsBr and NaI were deposited onto CaF /Cr substrates, following the same procedure described in Section 2.1. When unexposed to moisture, the CsI layer is found to be fairly uniform and composed of rather small crystallites (&60 nm in diameter), as shown in Fig. 4a. Fig. 2. As Fig. 1, for a 75 nm thick CsBr "lm: (a) `as evaporateda and (b) after exposure to Ar at 80% relative humidity for 1 min. The full scale is 5 μm. Fig. 3. As Fig. 1, for a 75 nm thick NaI "lm: (a) `as evaporateda and (b) after exposure to Ar at 30% relative humidity for 1 min. The full scale is 50 μm.

4 412 T. Boutboul et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 409}414 Fig. 4. An SEM view of a 20 nm thick CsI "lm evaporated on a CaF /Cr substrate: (a) `as evaporateda and (b) after exposing the sample to ambient air for 9 min. The full scale is 5 μm. Fig. 5. An SEM view of a 20 nm thick CsBr "lm deposited on a CaF /Cr substrate: (a) `as evaporateda and (b) after exposure to Ar at 23% relative humidity, for 1 min, and to Ar at 90% for an additional minute. The full scale is 5 μm. Nevertheless, neither CsBr nor NaI `as evaporateda "lms appear to be fully continuous. Whereas such CsBr layers exhibit a morphology of interconnected crystallites with a surface coverage of around 70% (Fig. 5a), those of NaI show a structure of disseminated &0.5 μm crystallites (Fig. 6a). Exposing this NaI "lm to argon at 30% RH for 1 min had almost no in#uence on the "lm morphology (Fig. 6b). When the CsBr "lm is exposed to humid argon (23% RH), there is no impact on the "lm structure for an exposure of up to 10 min. Nevertheless, for an increased humidity (60%, 75% and 90% were investigated), there is a clear coalescence of the layer into &0.5 μm in size separated grains. This is clearly shown in Fig. 5b for a "lm exposed to 23% RH for 1 min and to 90% for an additional minute. Since the Ar humidity monitoring system was not available by the time the 20 nm thick CsI photocathodes were investigated, the moisture sensitivity of such "lms was only qualitatively checked by exposing the samples to ambient air during 9 min. As can be seen in Fig. 4b, the air exposure caused the coalescence of the layer into slightly larger crystallites (&200}300 nm in size) and somewhat a!ected its continuity. Finally, a 20 nm thick CsI "lm was evaporated on a substrate of glass, coated with Au/Ni. As shown in Fig. 7a, the layer appears to be quite uniform and fairly similar to that evaporated on CaF /Cr (Fig. 4a) with slightly smaller crystallites.

5 T. Boutboul et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 409} Fig. 6. An SEM view of a 20 nm thick NaI "lm evaporated on a CaF /Cr substrate: (a) `as evaporateda and (b) after exposure to Ar at 30% relative humidity, for 1 min. The full scale is 12 μm. Fig. 7. An SEM view of a 20 nm thick CsI "lm deposited on a glass substrate coated with Au/Ni: (a) `as evaporateda and (b) after exposure to Ar at 82% relative humidity for 1 min. The full scale is 5 μm. After exposing this "lm to Ar at 82% RH for 1 min, there is an obvious restructuration of the layer into discontinuous &200 nm grains (Fig. 7b). 3. Summary and discussion Within this work, the morphology of thin alkali halide "lms was investigated `as evaporateda and following an exposure to controlled moisture. For the three materials considered, the thicker "lms (i.e. 75 nm) were naturally found to be more continuous than thinner ones (20 nm). Only CsI appeared to deposit as a fully continuous "lm at both thicknesses; 20 nm thick CsBr "lms exhibited a structure of interconnected crystal grains. Neither 20 nm nor 75 nm thick NaI "lms appeared to be continuous; the 20 nm thick "lms even showed a clear structure of disseminated grains. The exposure to moisture a!ected di!erently the three materials. NaI undoubtedly appeared to be the most sensitive; a 1 min exposure to Ar with &30% RH was su$cient to completely destroy the "lm, causing its coalescence into separated grains. CsI and CsBr deposited "lms were found to be more moisture resistant than NaI photocathodes. Such "lms coalesce only at higher relative humidity. The pronounced hygroscopic character of NaI could explain the observed lack of continuity of `as evaporateda "lms. It could have resulted

6 414 T. Boutboul et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 409}414 from an exposure to a minute amount of air, during the short passage between the evaporation chamber and the microscope, though under argon #ow. In any case, NaI seems to be a more delicate material to be used for UV-photoemission and visible photocathode protection purposes. On the contrary, the continuous surface structure and the relatively low moisture sensitivity, demonstrated by CsI, con"rms this alkali halide as an adequate material for such applications. The process of coalescence of an evaporated "lm into grains, following its exposure to humidity, merits further consideration. We assume that the alkali halide forms a solid surface, quite dry and made of crystallites, when it is deposited on the substrate. During exposure to humid argon, there is a high probability for the water vapour to come into contact with the deposit, due to the "lm surface extension. The water vapour is thus adsorbed on the material surface, triggering the formation of another phase: a solution. A di!usion of solid alkali halide into the solution takes place at the interface between the two phases, due to concentration difference and according to Fick's law [6]. After the water is evaporated, the material tends to recrystallize. Since its a$nity to itself is naturally higher than that to the substrate material, the solid tends to repart itself on the substrate but in larger grains than before. This leads to the reconstruction of the "lm into larger disseminated crystallites, as observed. Acknowledgements The work was partially supported by the Israel Science Foundation. A. Breskin is the W.P. Reuther Professor in the peaceful use of atomic energy. References [1] C. Lu, K.T. McDonald, Nucl. Instr. and Meth. A 343 (1994) 135. [2] A. Breskin, A. Buzulutskov, R. Chechik, M. Prager, E. Shefer, Appl. Phys. Lett. 69 (1996) [3] A. Buzulutskov, E. Shefer, A. Breskin, R. Chechik, M. Prager, Nucl. Instr. and Meth. A 400 (1997) 173. [4] F. Piuz, Nucl. Instr. and Meth. A 371 (1996) 96. [5] T. Boutboul, A. Akkerman, A. Breskin, R. Chechik, J. Appl. Phys. 84 (1998) [6] M. Prutton, Introduction to Surface Physics, Oxford Science Publications, Clarendon Press, Oxford 1995, p. 154.