Na

Photocathodes on Polycrystalline CsI/Na Y. ARAMAKI Horikawacho Works, Toshiba Corporation, Kawasaki, Japan

INTRODUCTION It is difficult to make semitransparent photocathodes on polycrystalline CsI/Na (hereafter denoted p-CsI/Na) having both high sensitivity and long life. When a photocathode is actually made on p-CsI/Na, the photocurrent, which is initially high, often decreases during operation. There is little doubt that the cause of this reduction in photocurrent is an interaction between the substrate and the photocathode. This report describes Auger analysis of two improved types of photocathode.

AN IMPROVED PHOTOCATHODE

Methods of Photocathode Production on p-CsrjNa This report describes two ways of producing photocathodes with high sensitivity and long life on p-CsI/Na substrates. Basically, the processing of these photocathodes consists of two steps. The first step is to create a base layer on an intermediate layer just under a photoelectron conversion layer. This is done by introducing abundant alkali metal vapour; the process is called doping. The second step is to make the photoelectron conversion layer. The two ways of producing the photocathode differ in the first process. The first step of one method is introduction of abundant alkali metal vapour to the p-CsI/Na. This process, called doping, is different from that used in forming conventional photocathodes. In order to make K-Cs-Sb bi-alkali photocathodes, potassium or other alkali metals are used for doping. The potassium is introduced in a heated dispenser at a constant substrate temperature and the system is evacuated. The alkali metal is fully absorbed in the substrate whilst the photocurrent is monitored. After doping with the alkali metal, the photoelectron conversion layer is formed. Antimony is deposited first. The antimony deposition is followed by the introduction of potassium and then caesium. The photocathode is completed by further depositions of antimony and caesium in turn. 323 ADVANCES I N ELECTRONICS AND ELECTRON PHYSICS VOL. 74

Copyright Q 1988 Academic Press Limited All rights of reproduction in any form reserved

ISBN 0-12-014674-6

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Y. ARAMAKI

Another method for bi-alkali photocathodes will be described briefly. An oxide layer of alkali metal is first formed on the substrate, and then the photoelectron conversion layer is formed. The oxide layer of alkali metal is made as follows. As described above, alkali metal is introduced while keeping the pre-determined temperature of the substrate constant and while evacuating. The composition of each oxide layer is variable according to which alkali metal is introduced and whether it is made of composite layers. It is then essential that, after deposition of alkali metal on the substrate is completed, oxygen is introduced to fully oxidize the alkali metal. After oxidation of the alkali metal, a photoelectron conversion layer is formed by the method described above.

The Intermediate Layer The photocathode was made on an intermediate layer created by vacuum evaporation onto the p-CsI/Na. One type of intermediate layer was composed of A1203 and In203,and the other was made in In203alone. The A1203 was deposited in order to cover discontinuities in the p-CsI/Na which make it unsuitable as a substrate for the photocathode. AUGERANALYSIS OF IMPROVED PHOTOCATHODES The operating conditions of the Augur apparatus were as follows: base Torr; beam diameter, 0.5 pm; beam current, vacuum pressure, I x 0.1 pA; sputtering gas, xenon; sputtering rate, 3 A min-' (for Si02). Data were obtained from Auger analysis by sputter etching of the photocathode surface.

Auger Analysis of Photocathode Surfaces In carrying out Auger analysis of the photocathode surface great care was taken to minimize the movement of alkali metal by using only low probe beam currents. Figure 1 shows the results of Auger analysis conducted on the surfaces of three types of photocathodes. It can be seen that, in the case of the alkali metal oxide photocathode, the spectrum of carbon was hardly detected. This is believed to have been due to oxygen introduced in making the photocathode. Each spectrum in Fig. 1 is different in the peak height of each element. Another feature of these spectra is that sodium was detected despite its being separated from the p-CsI/Na by the intermediate layer.

PHOTOCATHODES ON POLYCRYSTALLINE CSI/NA

325

Distribution of Elements on Surface

The Auger analysis probe spot was scanned over the surface of the photocathode in order to obtain information on the distribution of elements over the surface. Figure 2 shows the results of a composition analysis of the photocathode surfaces in one direction. As was shown by the spectra in Fig. 1, each distribution is characterized by the ratio of alkali metals to Sb. Fig. 2(a) shows that Cs is two orders of magnitude higher than K and Sb. This is abnormal for conventional glass substrate photocathodes. In Fig. 2(6) and (c), the ratios of alkali metals to Sb seem acceptable. These facts are also clear from the averages and standard deviations calculated on the assumption of normal distributions and shown in Table I. In Fig. 2, Na was detected although it had not been introduced. In Fig 2(b), Na is present only where A1 exists, which suggests some correlation between these two elements. Correlation coefficients of elements were obtained. Values

t

L

I 0

0 0

I000

E l e c t r o n E n e r g y I eV

1

I

2000

FIG.I . Auger spectra of photocathodes: (a)alkali-metal-richphotocathode; (6) oxidized alkali metal photocathode on A1203 and 111203;(c) oxidized alkali metal photocathode on InzO3.

.

a

0

Distance

Distance

(b) FIG. 2. Elemental distribution in photocathodes: (a) alkali-metal-rich photocathode; ( b ) oxidized alkali metal photocathode on A1203 and In203.

327

PHOTOCATHODES ON POLYCRYSTALLINE CSI/NA

(c)

'

I

0.1

I

I

1

I

I

Distance

FIG.2. Elemental distribution in photocathodes: (c) oxidized alkali metal photocathodes on 111203.

were derived for each element together with data on the depth profile; these are shown in Table 11. As is evident in Table 11, there is a close correlation between A1 and Na and it seems that Na has diffused from substrate of p-CsI/Na. The diffusion coefficient of Na in A1203 is known to be large (Freer, 1980), which is consistent with this observation. TABLE I Calibrated average peak values ( f )and deviations (3) of elements Element Photocathode

R

Sb

cs

K

Na

0.0429 0.138

42.0 1.67

0.123 0.296

71.4 7.84

Potassium-rich layer base

d

Potassium oxide layer base

8 B

17.2 11.5

f 6

18.3 12.5

(In203+A1203)

Potassium oxide layer base

1.08 1.91 52.0 25.6

14.1 1.77

13.7 12.4

12.3 10.9

69.2 57.6

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TABLE I1 Relationship between elements Element Photocathode

Analysis direction

Potassium-rich layer base

Surface Depth

Potassium oxide layer base (In203+AlzOd

Surface Depth

@

~($1~($1

-@ 0 o(--@) @

Potassium oxide layer base (In204

Surface Depth

@ @

0

0

Sb-K

Sb-Cs

Sb-Na

x

X

X

x

X

X

O(@)x

X

x(@)

0 0

Na-In

Na-A1 X

X

@

-@

~

-@

@ Reliability 99%; 0reliability 95%; x low or no reliability; - negative (others are positive). Data in parentheses were obtained from other specimens.

Profile in the Depth Direction The distributions of elements in the photocathode were also measured in the depth direction. Results are shown in Fig. 3(a), (b) and (c).

0

(a)

10

20

30

Sputtering Time (min.)

FIG.3. Depth profile of photocathodes: (a) alkali-metal-rich photocathodes.

t

L

0 (b)

1

10

I

I

20

30

Sputtering Time (rnin.)

t s

. vo

-

0

i --

I

I

Sb

In

I

FIG.3. Depth profile of photocathodes:(b) oxidized alkali metal photocathode on A1203 and (c) oxidized alkali metal photocathodes on InzO,.

In203;

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Y. ARAMAKI

Alakali metal rich layer photocathode

Figure 3(a) shows that there is more Cs than Sb and that it is present not only on the surface of the intermediate layer but also deep down. There is not so much K as Cs, but it is present at a greater depth than Sb. Na is abundant. Oxidized alkali metal layer photocathode

This photocathode consists of an oxide layer under the photoelectron conversion layer, the composition of which varies in accordance with the type of intermediate layer. The photocathode was applied to both types of intermediate layers. In either case, the ratio of Cs to Sb is near to unity and is smaller than in the case of alkali-metal-rich layer photocathode shown in Fig. ] ( a ) . PHOTOCATHODE SENSITIVITY

If the sensitivity of the alkali-metal-rich photocathode formed on an intermediate layer consisting of layers of A1203 and In203 was taken as 1, the sensitivity of the oxidized alkali metal photocathode formed on a double layer of A1203 and In203 was 1.5, while that of the oxidized alkali metal photocathode on a single intermediate layer of In203was 1.8. These sensitivity values might change with conditions of formation of the intermediate layers. CONCLUSION

In this experiment we prepared two kinds of K-Cs-Sb photocathodes on polycrystalline CsI/Na and carried out an Auger analysis. In one type of photocathode the ratio of Cs to Sb is high and the Cs forms a deep layer, while in the other type the alkali metal oxide is formed under a photoelectron conversion layer and the ratio of K to Sb is high, while that of Cs to Sb is low. In the presence of A1203,Na diffuses easily towards the photocathode from the p-CsI/Na substrate. ACKNOWLEDGEMENTS The author wishes to acknowledge the contribution made by Mr Y. Ueda and Mr T. Suzuki who carried out the Auger analysis.

REFERENCE Freer, R.(1980). J. Marer. Sci. 15, 803-824