Influence of photon recycling on the photoemission from GaAs-photocathode 1 H.E. Scheibler, S.A. Rozkov, V.V. Bakin, S.N. Kosolobov, A.S. Terekhov Rzhanov Institute of Semiconductor Physics Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia L.B. Jones, T.C.Q. Noakes & B.L. Militsyn STFC ASTeC, Daresbury, Warrington, United Kingdom 1. Introduction. 2. Diffusion of photoelectrons in GaAs-photocathodes. 3. Special heterostructure for the investigation of photon recycling in GaAs-photocathodes. 4. The influence of photon recycling on the properties of photocathodes for accelerator applications. 5. Conclusions. EWPPAA

2 Photoemission from GaAs-photocathodes  VAC FF CC VV z  semiconductor vacuum EWPPAA ħ  ≥  g Three step model of photoemission 1. Photoexcitation 2. Transport to the surface 3. Escape to vacuum

3 Diffusion of photoelectrons EWPPAA ħ  ex Al 1-x Ga x As buffer layer Glass p-GaAs active layer SiO ħ  lum if g ex (z) = A×exp(z/B), then one can find n(z) in the analytic form With photon recycling this intergo-differential equation has no solution in its analytic form photon recycling (readsorbed recombination radiation)

4 Influence of photon recycling (PR) on diffusion EWPPAA W.P. Dumke, Phys. Rev. 105, 139 (1957). T. Kuriyama et. al, Jpn. J. Appl. Phys. 16, 465 (1977). A. Lastras-Martinez, J. Appl. Phys. 50, 4156 (1979). M. Ettenberg, Appl. Phys. Lett. 30, 207 (1977). O. van Roos, J. Appl. Phys. 54, 2495 (1983). R.L. Bell, Negative electron affinity devices (1973). G.A. Antypas, L.W. James, J. Appl. Phys. 41, 2165 (1970). G.A. Allen, J. Phys. D 4, 308 (1971). J.C. Richard, Acta Electron. 16, 245 (1973). Diffusion of electrons in photocathodes Diffusion with photon recycling (readsorbed recombination radiation) Photon recycling was not taken into consideration!

5 Influence of photon recycling (PR) on diffusion EWPPAA Diffusion with photon recycling can be represented approximately as a changing of the diffusion length [A. Lastras-Martinez, O. van Roos]  – lifetime of electron; D – diffusion coefficient of electrons; - diffusion length.  rad – radiative lifetime;  nr – non-radiative lifetime; Internal quantum efficiency of radiative recombination Effective diffusion length is equal to If  = 0.9 then

6 Heterostructure for PR research EWPPAA ħ  ex Al 1-x Ga x As Glass SiO How does PR influence the diffusion of electrons in GaAs-photocathodes? p-GaAs (2) Al 1-x Ga x As p-GaAs (1) J ph electrolyte

7 Experiment EWPPAA This oscillation period corresponds to an etching of the GaAs by:  h = /(2n) = 980 nm / (2  3.53) = 139 nm Electrolyte - H 3 PO 4 :H 2 O 2 :H 2 O = 3:1:50; GaAs etching rate 40÷60 nm/min Time, min p-GaAs (2) p-GaAs (1)

8 Experiment EWPPAA Calculation: Monte Carlo simulations. Parameters: D = 40 cm 2 /s; S buf =1×10 4 cm/s; L d = 2.0  m;  = 0.8. = 633 nm p = 1×10 19 cm -3 fitted

9 Influence of photon recycling on diffusion EWPPAA Transmisson mode GaAs-photocathode: QE dependence on the active layer thickness  = 0.8  Absorption of luminescence photons by free holes was taken into account in our calculations! By our estimates, 13% of luminescence photons are absorbed by free holes in the bulk.  *=0.87×  = 0.70

Photocathodes for accelerator applications 10 EWPPAA Main parameters: 1. Quantum efficiency; 2. Lifetime; 3. Emittance; 4. Time response. dominated by process of escape to vacuum depended on electrons transport

Photocathodes for accelerator applications 11 EWPPAA Photon recycling was not taken into consideration. Thin (<1  m) active layer - no problem, but thick (>1  m) - ? D from literature can be D*

12 EWPPAA PR!  =850 nm; d ac =1200 nm – the absorption of light in the substrate was approximately equal to the absorption of light in the active layer Photocathodes for accelerator applications Special heterostructure. Photon recycling was not taken into consideration.

 Photon recycling is an essential mechanism in the transport of electrons to the emitting surface in GaAs–photocathodes.  The concentration profile of electrons in GaAs–photocathodes with reasonable active layer thicknesses can be described using the diffusive model without the inclusion of photon recycling, but only with an effective value for the diffusion length.  Do not forget about photon recycling! Summary 13 EWPPAA

14 GaAs p =10 19 cm -3 , cm -1 I lun, arb. units

15 EWPPAA Photocathode nm standard photocathode photocathode based on special structure