Electric field enhancement of escape probability on negative-electron-affinity surfaces J. R. Howorth, A. L. Harmer, E. W. L. Trawny, R. Holtom, and C. J. R. Sheppard English Electric Valve Company. Limited. Chelmsford. England (Received 23 April 1973; in final form 29 May 1973) Electric field enhancement of photoemission from negative-electron-affinity surfaces on silicon and GaAs has been studied. It is shown that the electric field increases the escape probability and does not change the spectral response of negative-electron-affinity surfaces. The results are explained by assuming a simple surface potential barrier together with work function lowering by the Schottky effect. A large external electric field enhances the photo- emission from the S-20 and other alkali metal photo- cathodes. 1,2 The effect is wavelength dependent and is much more pronounced at long wavelengths, where the photoelectrons have only a small probability of over- coming the positive electron affinity of the photocathode surface. The effect has been exploited in proximity- focused image tubes, where the focus field is also used to increase the quantum yield of the photocathode. 3 It has also been observed by Kohn4 that the emission cur- rent from a negative-electron-affinity (NEA) silicon cold cathode is field dependent, and this was deduced to be a field enhancement effect. This letter reports a study of field enhancement ef- fects on NEA surfaces of silicon and GaAs. Each sur- face was cleaned by conventional techniques and cesiated and Oxidized to peak white-light sensitivity as a reflec- tion photocathode. 5 An electric field was applied between the sample and a phosphor screen to allow emission uniformity to be studied. The phosphor screen had an aluminized backing to prevent optical feedback from the phosphor. Changes in the photoemission were studied as a function of variable electric field strength, up to 1 kV/mm. For all NEA surfaces of silicon and GaAs the field increased the photoemission by changing the escape probability, but did not affect th'e shape of the spectral response curve. This is shown in Fig. 1 for a sample of silicon (boron doping, 5X10 18 cm- 3 ) activated to an initial white-light sensitivity of 150 IlA/lm. It can be seen that an electric field of 600 V/mm increases the sensitivity to 250 IlA/l m . Similar results were obtained for GaAs. Th e change in escape probability as a function of electric field strength (Fig. 2) was derived from spectral response measurements of the photoemission. Curves a, b, and c in Fig. 2 are for a silicon surface (p-type doping, activated to white-light sensitivities of 11, 62, and 370 IlA/lm, respectively. Curves d and e are for a GaAs surface (p -type doping, 1X1Q19 cm- 3 ) activated to 120 and 250 IlA/lm, respec- tively. Curves f and g are for more lightly doped GaAs (p-type doping, 7x10 17 cm- 3 ) activated to 30 and 60 IlA/lm, respectively. The experimental results may be explained by assum- ing that a simple step potential exists at the vacuum surface (see, for example, Fisher et al. 6) and that photoelectrons are scattered in a bent-band region be- low the surface. Photoelectrons created in the bulk of the semiconductor become thermalized in the conduction 123 Appl. Phys. Lett., Vol. 23, No. 3, 1 August 1973 band minimum with a Boltzmann -type energy distribution characterized by a temperature T= 300 OK. As the elec- trons diffuse towards the surface, electron -phonon scattering occurs in the bent-band region, which modi- fies their energy distribution. This new energy distribu- tion may be calculated by using a solution to the trans- port equation given by Bartelink, Moll, and Meyer. 7 Re- sults of such a calculation by Kressel et al. 8 for GaAs agree favorably with their measured electron energy distribution. Similar calculations and measurements have not yet been performed on silicon. However, by assuming a phonon scattering length of 63 A 9 and by assuming that for a doping level of 5x10 18 cm- 3 the bent- band region extends for 200 A, it is calculated that ap- prOximately 95% of the electrons will be scattered· in the bent-band region. Thus, the energy distribution of electrons arriving at the photoemissive surface will have a peak denSity below the bottom of the bulk conduc- tion band and will have a high -energy tail above the bottom of the bulk conduction band with a Boltzmann distribution. The high -energy tail comprises electrons passing through the bent-band region without scattering. The application of a large external field, E, reduces the work function of a surface by Il.</> according to tb,e Schottky equation 1l.</>=(qE/4rrEo)1/2, (1) where q is the electronic charge and Eo is the vacuum - ",,-, "'-.. '-- -- hig . field lOOV/rM .1 o yield electrons per absorbed Photon 0 .UI 0 1---- 0.00 .4 0.5 0.6 ........ " / ........... " field "'''' \. \ \\ \\ 0.7 0.8 0.9 1.0 1.1 wavelength {iJ m) FIG. 1. Effect of fl.eld enhancement on the spectral response curve for silicon. p-type doping, 5 x 10 18 cm- 3 • Copyright © 1973 American Institute of Physics 123