The state of Cs on negative electron affinity surfaces

364

Applied Surface Science 33/34 (1988) 364-369 North-Holland, Amsterdam

T H E S T A T E O F Cs O N N E G A T I V E E L E C T R O N A F F I N I T Y S U R F A C E S Masahiro M I Y A O , Tatsuaki W A D A , Toshiyuki N I T F A and Minoru HAGINO Research Institute of Electronics, Shizuoka Universi O, 3-5 1 Johoku, ttamamatsu 432, Japan

Received 23 August 1987; accepted for publication 15 October 1987

We examine the flash desorption of Cs from NEA surfaces, and measure the workfunction change during NEA activation. The lifetime changes on NEA photocathodes under the operation of decay and reactivation cycles are also measured. The thickness and structure of the Cs/O layer are discussed. We discard the heterojunction model. The workfunction is changed not by the state of the Cs/O layer but by the amount of ionic Cs adsorbed on the top surface.

1. Introduction The a d s o r p t i o n of Cs on a s e m i c o n d u c t o r surface creates a very interesting system. The electron affinity of this surface becomes negative. In these materials some of the electrons excited into the c o n d u c t i o n b a n d can be emitted into the v a c u u m through the surface. Such surfaces can act as good photoemitters. U n f o r t u n a t e l y , the sensitivity of the N E A surface is strongly affected by the residual gases. These effects may arise from the change of state of the Cs a n d oxygen adsorbed on the surface. We must fully u n d e r s t a n d the state of the adsorbed Cs in order to make highly d u r a b l e photocathodes. F r o m the studies of m a n y authors, we can infer a detailed picture of adsorbed Cs on a s e m i c o n d u c t o r surface. However, the structure of the C s / O layers on the N E A surface is not fully understood. Three models have been proposed, namely, the heterojunction, the dipole, a n d the cluster models [1 ]. In this work, we discuss the activated layer of N E A , a n d discard the heterojunction model.

2. Experiment N EA from Cs tube or process. amount

surfaces are prepared by the yo-yo method. Cs atoms are generated getters. Oxygen is leaked into the v a c u u m system through a heated Ag a v a c u u m leak valve. N E A activation is achieved by the following First, the Cs is adsorbed on the surface until s a t u r a t i o n occurs. The of saturated Cs on the surface is measured to be a b o u t 5.4 × 1() 14

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M. Miyao et al. / State of Cs on NEA surfaces

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a t o m s / c m 2 using a Cs ion gun. Next, oxygen is adsorbed until the best condition is indicated by photocurrent or workfunction measurements. The above cesiation and oxidation processes are repeated several times until the photocurrent does not increase anymore. The experiments are done by three methods. The first is measuring the Cs flash desorption from the NEA GaP surfaces. The GaP sample is mounted just in front of the QMS analyzer. A Cs getter is placed between the sample and the QMS. After the activation, the sample temperature is increased linearly from room temperature up to 700°C. We chose the rate of temperature increase to be about 0.6 ° C / s or 1.7 ° C / s . The desorbed Cs is measured by the ion current of the QMS. The total amount of desorbed Cs is calculated from the time integration of the ion current. The total adsorbed oxygen is also calculated from the vacuum vapor pressure. The second method is measuring the workfunction change of GaP during cesiation and oxidation. The cesiation and oxidization processes are monitored by the photocurrent excited by white light illumination. Workfunction changes are measured by the contact potential method using an electron beam. We used LEED optics. These experiments are done in a vacuum of 1.3 × 10 -8 Pa. The third type of experiment involves measurement of the lifetime of the photocurrent emitted from the NEA GaAs. GaAs samples are activated whilst monitoring the photocurrent resulting from illumination by H e - N e laser light. All operations are carried out under a vacuum of 2.0 x 10 -8 Pa or lower pressure. The emission current is decreased by the adsorption of residual gases. The decayed surfaces are reactivated by the adsorption of Cs again and again.

3. Results 3.1. Flash desorption

The flash desorption spectra for Cs from NEA surfaces for various thicknesses of the C s / O layers are shown in fig. 1. Suffixes in the figure show the number of activation-cesiation cycles used for the NEA activation. In this figure, the total amounts of oxygen exposure are also noted. From this figure it is seen that the spectra are essentially the same, despite the change in thickness of the C s / O layers. The only difference is the height increase of the peaks which appear near 200 and 670 ° C. In fig. 2, the amounts of adsorbed Cs and oxygen contained on the NEA surfaces are shown versus the cesiation-activation cycles. The composition ratios of O / C s in the surface layers are shown in fig. 3. From these results, the overlayer consists of Cs20, and there is always one half a monolayer (2.7 x 1014 a t o m s / c m 2) of excess Cs on the surface.

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M. Miyao et al. / State of Cs on NEA surfaces

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3.2. Work function change The decrease of workfunction from the initial clean GaP surfaces are shown in fig. 4. These values are obtained after the cesiation and the oxidization. In the figure, total values of the oxygen exposure are also indicated as a function of the cesiation and oxidation cycles. The workfunction decreases monotonically. The workfunction saturates at - 3 . 9 eV after five cesiation-oxidation

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M. M~vao et al. / State of Cs on N E A surfaces

3.3. Photoemission lifetime The quantum efficiency of N E A photoemitters decreases during the operation. These decayed surfaces can be reactivated by cesiation alone. In fig. 5, the quantum efficiency of N E A surfaces is shown. The upper set of points are measured just after activation or re-activation. The lower set of points are measured after 16 h of operation under 0.6 mW H e - N e light illumination. The decayed surface is re-activated sufficiently by the cesiation. Nevertheless, its decay becomes more rapid. This means that the lifetime of the N E A surface becomes shorter when the re-activations are repeated. In fig. 6 the lifetime ol the N E A surface is plotted against the number of cesiation and oxidation cycles. The last data points in figs. 5 and 6 are the result achieved when the cesiation is entirely carried out using a few times of full Cs adsorption. By this method, the lifetime of the N E A surface is completely recovered. These surfaces show a longer lifetime than that of the freshly activated surface.

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M. Miyao et al. / State of Cs on NEA surfaces

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4. Discussion The Cs flash desorption spectra from the optimum activated N E A G a P surface show the lack of Cs desorption between room temperature and 120 o C. The Cs desorbed at this temperature is in the metallic state. The Cs which desorbed at this temperature begins desorbing over 600 o C when the oxygen is adsorbed. This means that oxygen reacts only with the excess metallic Cs, and does not react with the ionic state of Cs when the N E A activation is carried out successively. The contribution of the ionic Cs on the N E A surface does not change during the oxidation. The C s / O overlayer is made from a few monolayers of Cs and oxygen. The composition ratio of these layers is the same as for Cs20. There is also half a monolayer of excess Cs over its surface. Only the Cs20 layer increases during the activation cycle. The growth rate of the Cs20 layer by the cesiation oxidation is about 0.2-0.3 monolayer/cycle. The reduction of the workfunction by the activation cycle saturates over the fourth cycle. The estimated thickness of this layer is only 0.8-1.0 monolayer. On the other hand, the photoemission quantum efficiency m a x i m u m appears at the second or third (GAP) or at about the tenth (GaAs) cycle. The decayed surfaces, caused by the adsorption of residual gases, can be revived m a n y times by further cesiation. However, the lifetime becomes shorter. This problem is overcome by applying sufficient cesiation. The lifetime shortening, caused by repeated re-activation, shows that the decay of the quantum efficiency is not only by oxidation by the residual gases but also by interdiffusion of Cs into the oxide layer. From the above results, the ionized Cs adsorbed on the Cs20 is the main factor responsible for lowering the workfunction of N E A surfaces. There seems to be little direction contribution of the Cs20 layers to the workfunction decline. The C s / O layer thickness has an effect on the workfunction through changing the ionization efficiency of Cs on the top layer.

Acknowledgments This work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and a Joint J a p a n - U S A Collaboration Program in High Energy Physics.

References [1] See, for example: C.Y. Su, W.E. Spicer and I. Lindau, J. Appl. Phys. 54 (1983) 1413; D.C. Rodway and M.B. Allenson, J. Phys. D 19 (1986) 1353; M.B. Besanqon, R. Landels and J. Jupille, J. Vacuum Sci. Technol. A 5 (1987) 2025.