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Accueil du site > LPCNO > Séminaires > 2018 > Electron transfer through semiconductor-vacuum interface : photoemission from GaAs(Cs,O) with positive and negative electron affinity

Electron transfer through semiconductor-vacuum interface : photoemission from GaAs(Cs,O) with positive and negative electron affinity

Date : 03/05/2017 à 14:00

Titre : Electron transfer through semiconductor-vacuum interface : photoemission from GaAs(Cs,O) with positive and negative electron affinity

Intervenant : V. L. Alperovich

Provenance : Novosibirsk State University, Russia.

Salle : Salle de séminaire - LPCNO

Abstract : The problem of electron transfer through the interface between a solid and the vacuum, in other words, about transformation of a quasi-particle with Bloch wave function in a crystal into a free electron in vacuum, is important for quantitative description of the thermoelectronic emission and photoemission, and also the phenomena in which the reverse electron transfer from the vacuum in a crystal takes place, electron diffraction, electron energy loss spectroscopy electron beam induced currents. How to calculate correctly the probability of electron transfer through the interface ? Is it possible to use the envelope wave function approximation ? What are the correct boundary conditions ? What is the role of electron momentum and energy scattering ? Reliable and convincing answers to these questions are lacking even for the most carefully studied and practically important systems like p-GaAs(Cs,O) photocathodes. Adsorption of cesium and oxygen on the p-GaAs surface reduces the surface barrier to a state of negative effective electron affinity (NEA) χ*, at which the vacuum level is below the bottom of the conduction band in the bulk of GaAs. Due to the high quantum yield (up to 50%), p-GaAs(Cs,O) NEA photocathodes are widely used in the photomultipliers and sources of ultra-cold and spin-polarized electrons. Semiconductor surfaces with relatively small positive effective electron affinity χ* 0.2-0.4 eV have recently attracted attention anew due to the opportunity of increasing solar energy conversion efficiency using the photon-enhanced thermionic emission (PETE) [1]. The Cs/GaAs surface seems suitable for the PETE converters because of the optimal band gap in GaAs and of the opportunity to adjust the affinity by varying cesium coverage. However, realization of the potential advantages of PETE-converters based on the Cs/GaAs surface is restricted by its thermal instability and the smallness (≤10%) of the electron escape probability [2, 3]. To elucidate the emission mechanisms, which determine the electron escape probability, in this work we studied, by means of photoemission quantum yield spectroscopy, the transition from positive to negative electron affinity at the GaAs(Cs,O) surface [4]. A qualitatively different evolution of the escape probabilities was observed for "hot" (Pt) and "thermalized" (Ph) photoelectrons. It was found that Pt is a non-monotonic function of χ* and goes through a deep minimum Pt 1% near zero affinity. The minimum is apparently due to the effective capture of electrons from the bottom of the conduction band in the bulk of GaAs to the two-dimensional subband in the band bending region and to the necessity of thermal activation for emission from this subband at zero affinity. Further decrease of χ* to negative values leads to the increase of Pt due to the activation energy decrease. At sufficiently high modulus of negative χ*, electrons emit from the 2D subband into vacuum at no energy cost, and this provides high photoemission quantum yield of p-GaAs(Cs,O) NEA photocathodes. The prospects of PETE solar energy converters based on Cs/GaAs, GaAs(Cs,O) and other semiconductor surfaces with positive electron affinity are discussed.

[1] J.W.Schwede et al., Nat. Mater. 9 (2010) 762 ; Nature Commun. 4 (2013) 1576.
[2] A.G. Zhuravlev, A.S. Romanov, V.L. Alperovich, Appl. Phys. Lett. 105 (2014) 251602.
[3] A.G. Zhuravlev, V.L. Alperovich, Appl. Surf. Sci. 395 (2017) 3.
[4] A.G. Zhuravlev, V.S. Khoroshilov, V.L. Alperovich, JETP Letters 105 (2017) 686.