Abstract

Photoemission diode standards for accurately measuring monochromatic ultraviolet light intensity (3000 Å–1100 Å) are described that are also blind to visible light (λ > 3600 Å). The standard uses an opaque photocathode of Cs2Te and is unique because of its combination of thinness (19 mm), high sensitivity (Q.E. > 10%), time stability, and uniformity of response. Design criteria, construction methods, and difficulties overcome in obtaining a stable, unform, high yield photocathode responses are discussed. Cs2Te is discussed in terms of a model for high yield photoemitters.

© 1973 Optical Society of America

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References

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  1. A. J. Blodgett, Ph.D. dissertation, Stanford University, 1965; Dissertation Abstr. 26, 4754 (1966) [Order No. 65-12745].
    [PubMed]
  2. E. Taft, L. Apker, J. Opt. Soc. Am. 43, 81 (1953).
    [CrossRef]
  3. A. H. Sommer, Photoemissive Materials (Wiley, New York, 1968).
  4. L. R. Canfield (private communication).
  5. R. Y. Koyama, Ph.D. dissertation, Stanford University, 1969; Dissertation Abstr. Intern. 30, 5654B (1970) [Order No. 70-10478].
  6. R. S. Bauer, Ph.D. dissertation, Stanford University, 1970; Dissertation Abstr. Intern. 32, 1150B (1971) [Order No. 71-19646].
  7. J. P. Causse, IRE Trans. Nuclear Sci. NS-9, 90 (June1962).
    [CrossRef]
  8. The photomultiplier quantum efficiency shown is EMR photomultiplier (model 541F-08-18) of Electro-Mechanical Research, Inc., Princeton, N.J. The curve matches current specifications and is from J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy (Wiley, New York, 1967), p. 225.
  9. Interestingly enough, the areas of nonuniformity and of normal material have spectral responses of the same shape, as first noted by L. R. Canfield,4 but the areas of nonuniformity peak lower at 6.8 eV and have a higher response at the minimum near 8.5 eV than normal material. (Note the middle region of cell 118 in Fig. 6.) In addition, for the latest fourteen cells we have made we have observed a direct correlation between a higher yield at the minimum near 8.5 eV and a higher visible response near 2.5 eV. Along with these observations, we may speculate that the visible response is emission from filled states up to an electron volt above the valence band maximum, possibly caused by nonstoichometry of Cs2Te, which would be most reasonably due to excess Te. If this is the case, the regions of nonuniformity with their relatively higher yields near 8.5 eV may simply be regions of considerable nonstoichiometry.
  10. W. E. Spicer, F. Wooten, Proc. IEEE 51, 1119 (1963); W. E. Spicer, J. Appl. Phys. 31, 2077 (1960).
    [CrossRef]
  11. The electron affinity EA defined here is actually the effective electron affinity found from EA = ET − EG, since any changes of the “real” value due to band bending at the surface are difficult to determine.
  12. N. V. Smith, G. B. Fisher, Phys. Rev. B3, 3662 (1971).
  13. The threshold for possible simultaneous emission of scattered primaries and secondary electrons occurs at 2ET or about 7 eV, but this process does not appear to be significant in this energy range.
  14. W. E. Spicer, Phys. Rev. 112, 114 (1958); C. N. Berglund, W. E. Spicer, Phys. Rev. 136, A1030 (1964); Phys. Rev. 136, A1044 (1964).
    [CrossRef]

1971 (1)

N. V. Smith, G. B. Fisher, Phys. Rev. B3, 3662 (1971).

1963 (1)

W. E. Spicer, F. Wooten, Proc. IEEE 51, 1119 (1963); W. E. Spicer, J. Appl. Phys. 31, 2077 (1960).
[CrossRef]

1962 (1)

J. P. Causse, IRE Trans. Nuclear Sci. NS-9, 90 (June1962).
[CrossRef]

1958 (1)

W. E. Spicer, Phys. Rev. 112, 114 (1958); C. N. Berglund, W. E. Spicer, Phys. Rev. 136, A1030 (1964); Phys. Rev. 136, A1044 (1964).
[CrossRef]

1953 (1)

Apker, L.

Bauer, R. S.

R. S. Bauer, Ph.D. dissertation, Stanford University, 1970; Dissertation Abstr. Intern. 32, 1150B (1971) [Order No. 71-19646].

Blodgett, A. J.

A. J. Blodgett, Ph.D. dissertation, Stanford University, 1965; Dissertation Abstr. 26, 4754 (1966) [Order No. 65-12745].
[PubMed]

Canfield, L. R.

L. R. Canfield (private communication).

Causse, J. P.

J. P. Causse, IRE Trans. Nuclear Sci. NS-9, 90 (June1962).
[CrossRef]

Fisher, G. B.

N. V. Smith, G. B. Fisher, Phys. Rev. B3, 3662 (1971).

Koyama, R. Y.

R. Y. Koyama, Ph.D. dissertation, Stanford University, 1969; Dissertation Abstr. Intern. 30, 5654B (1970) [Order No. 70-10478].

Samson, J. A. R.

The photomultiplier quantum efficiency shown is EMR photomultiplier (model 541F-08-18) of Electro-Mechanical Research, Inc., Princeton, N.J. The curve matches current specifications and is from J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy (Wiley, New York, 1967), p. 225.

Smith, N. V.

N. V. Smith, G. B. Fisher, Phys. Rev. B3, 3662 (1971).

Sommer, A. H.

A. H. Sommer, Photoemissive Materials (Wiley, New York, 1968).

Spicer, W. E.

W. E. Spicer, F. Wooten, Proc. IEEE 51, 1119 (1963); W. E. Spicer, J. Appl. Phys. 31, 2077 (1960).
[CrossRef]

W. E. Spicer, Phys. Rev. 112, 114 (1958); C. N. Berglund, W. E. Spicer, Phys. Rev. 136, A1030 (1964); Phys. Rev. 136, A1044 (1964).
[CrossRef]

Taft, E.

Wooten, F.

W. E. Spicer, F. Wooten, Proc. IEEE 51, 1119 (1963); W. E. Spicer, J. Appl. Phys. 31, 2077 (1960).
[CrossRef]

IRE Trans. Nuclear Sci. (1)

J. P. Causse, IRE Trans. Nuclear Sci. NS-9, 90 (June1962).
[CrossRef]

J. Opt. Soc. Am. (1)

Phys. Rev. (2)

N. V. Smith, G. B. Fisher, Phys. Rev. B3, 3662 (1971).

W. E. Spicer, Phys. Rev. 112, 114 (1958); C. N. Berglund, W. E. Spicer, Phys. Rev. 136, A1030 (1964); Phys. Rev. 136, A1044 (1964).
[CrossRef]

Proc. IEEE (1)

W. E. Spicer, F. Wooten, Proc. IEEE 51, 1119 (1963); W. E. Spicer, J. Appl. Phys. 31, 2077 (1960).
[CrossRef]

Other (9)

The electron affinity EA defined here is actually the effective electron affinity found from EA = ET − EG, since any changes of the “real” value due to band bending at the surface are difficult to determine.

The photomultiplier quantum efficiency shown is EMR photomultiplier (model 541F-08-18) of Electro-Mechanical Research, Inc., Princeton, N.J. The curve matches current specifications and is from J. A. R. Samson, Techniques of Vacuum Ultraviolet Spectroscopy (Wiley, New York, 1967), p. 225.

Interestingly enough, the areas of nonuniformity and of normal material have spectral responses of the same shape, as first noted by L. R. Canfield,4 but the areas of nonuniformity peak lower at 6.8 eV and have a higher response at the minimum near 8.5 eV than normal material. (Note the middle region of cell 118 in Fig. 6.) In addition, for the latest fourteen cells we have made we have observed a direct correlation between a higher yield at the minimum near 8.5 eV and a higher visible response near 2.5 eV. Along with these observations, we may speculate that the visible response is emission from filled states up to an electron volt above the valence band maximum, possibly caused by nonstoichometry of Cs2Te, which would be most reasonably due to excess Te. If this is the case, the regions of nonuniformity with their relatively higher yields near 8.5 eV may simply be regions of considerable nonstoichiometry.

A. H. Sommer, Photoemissive Materials (Wiley, New York, 1968).

L. R. Canfield (private communication).

R. Y. Koyama, Ph.D. dissertation, Stanford University, 1969; Dissertation Abstr. Intern. 30, 5654B (1970) [Order No. 70-10478].

R. S. Bauer, Ph.D. dissertation, Stanford University, 1970; Dissertation Abstr. Intern. 32, 1150B (1971) [Order No. 71-19646].

A. J. Blodgett, Ph.D. dissertation, Stanford University, 1965; Dissertation Abstr. 26, 4754 (1966) [Order No. 65-12745].
[PubMed]

The threshold for possible simultaneous emission of scattered primaries and secondary electrons occurs at 2ET or about 7 eV, but this process does not appear to be significant in this energy range.

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Figures (9)

Fig. 1
Fig. 1

The spectral distribution of the photoelectric yield of both a Cs2Te photodiode standard and a Cs3Sb phoodiode standard. The Cs2Te cell has a MgF2 window; the Cs3Sb cell has a LiF window. Note that the Cs2Te–MgF2 cell is blind to visible light.

Fig. 2
Fig. 2

Cross section of a photodiode standard. The circuit used during photelectric yield measurements is also shown.

Fig. 3
Fig. 3

View of photodiode standard in its mounting ring for use with a McPherson vacuum monochromator. Both the photodiode and the quartz filter shown may be moved in to intercept the entire light beam or be drawn out of it completely.

Fig. 4
Fig. 4

Schematic view of the apparatus used to activate the standard cells.

Fig. 5
Fig. 5

The spectral distribution of the photoelectric yield of an early (108) and a mote recent (119) Cs2Te–MgF2 photodiode. Note that the more recent cell, which was exposed to more cesium and then baked for an hour during processing, has a sharp threshold. This directly contradicts the data of Taft and Apker (Ref. 2), also shown. In their work, curve II was reported to be from a surface containing excess Cs, while curve I was exposed to less Cs.

Fig. 6
Fig. 6

The spectral distribution of photoelectric yield for a Cs2Te–MgF2 photodiode standard that uses an opaque photocathode. Note that the peak response is more than 105 times the response below 3 eV Also shown is the spectral response of an EMR photomultiplier that employs a semitransparent Cs2Te cathode on a LiF window (Ref. 8).

Fig. 7
Fig. 7

Transmission of MgF2 windows after different processing treatments. The bake at 550°C in a water-free atmosphere followed by polishing nearly brings back the original transmission.

Fig. 8
Fig. 8

Uniformity of the photoelectric yield measured for two Cs2Te–MgF2 standards at 8.8 eV, a photon energy at which great nonuniformity can be found. Cell 118 was an early standard made using an In2Te charge. Cell 140 is a more recent standard made by rapidly evaporating pure Te, giving a much improved uniformity of response. Each curve was measured at the National Bureau of Standards and is normalized to its maximum.

Fig. 9
Fig. 9

(a) Schematic energy level diagram of a semiconductor photoemitter where EGEA. For electron energies between 2EG and ET the escape depths will be large, being limited only by lattice scattering. Minimum threshold energies of electron-electron scattering (2EG) and of secondary electron emission (EG + ET) are shown. (b) The photoelectric yield of a typical Cs2Te–MgF2 standard showing the association between its structure and the model discussed. The transmission of a MgF2 window is plotted relative to the right ordinate.

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