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Atomic hydrogen, produced by thermal dissociation of H2 molecules inside a hot tungsten capillary, is shown to be an efficient tool for multiple recleaning of degraded surfaces of high quantum efficiency transmission-mode GaAs photocathodes within an ultrahigh vacuum (UHV) multichamber photoelectron gun. Ultraviolet quantum yield photoemission spectroscopy has been used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure. For photocathodes grown by the liquid-phase epitaxy technique, the quantum efficiency is found to be stable at about 20% over a large number of atomic hydrogen cleaning cycles. A slow degradation of the quantum efficiency is observed for photocathodes grown by metal-organic chemical vapor deposition, although they reached a higher initial quantum efficiency of about 30%–35%. Study of the spatial distributions of photoluminescence intensity on these photocathodes proved that this overall degradation is likely due to insertion of a dislocation network into the mechanically strained photocathode heterostructures during multiple heating cycles and is not due to the atomic hydrogen treatment itself. [ABSTRACT FROM AUTHOR]

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O[sub 2] or NF[sub 3] in an extremely high vacuum setup (base pressure below 10[sup -12] mbar). Quantum efficiencies of 20%-25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to approx. 100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spontaneous changes in photoemission properties of a р-GaN(Cs)-vacuum interface with effective negative electronic affinity induced by rearrangements of its atomic structure have been studied for the first time. The optimum Сs coating that ensures both the maximum photoelectron escape probability and its stability has been found. A thermodynamic model has been proposed to escape the relation of the photoemission properties of the р-GaN(Cs)-vacuum interface to its free energy and entropy. [ABSTRACT FROM AUTHOR]

It has been found experimentally that the probability of emitting electrons from p-GaAs(Cs,O) to vacuum in the presence of a magnetic field depends on the sign of the circular polarization of exciting light. The main cause of this effect is the jump in the electron g-factor at the semiconductor-vacuum interface (from g* = −0.44 in GaAs to g 0 = 2 in vacuum). Owing to the jump in the electron g-factor, the effective electron affinity depends on the mutual orientation of optically oriented electrons and the magnetic field and this dependence results in the spin-dependent photoemission. [ABSTRACT FROM AUTHOR]

The photoemission of electrons from a p[sup +]-GaAs surface with negative electron affinity was studied experimentally at 4.2 K. A narrow peak and its phonon replicas were observed in the distribution of emitted electrons over the energies of longitudinal motion. These replicas are caused by elastic and inelastic electron tunneling from the bottom of the dimensional quantization band in the near-surface spatial-charge region through the potential barrier of the (Cs,O) activating coverage with emission of LO phonons. The measured position of the peak corresponding to elastically tunneling electrons is close to the calculated one. © 2000 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A method to map out the energy distribution N(E[sub ||],E[sub ⊥]) of an electron beam as a function of the longitudinal (E[sub ||]) and transverse (E[sub ⊥]) energy has been developed and applied to study the photoemission process from GaAs(Cs, O) at 90 K. The method proceeds by "marking" electrons with fixed longitudinal energy E[sub ||][sup b] and a subsequent measurement of the associated differential transverse energy distribution N[sub ⊥](E[sub ||][sup b],E[sub ⊥]), applying an adiabatic magnetic compression technique. The complete energy distribution N(E[sub ||],E[sub ⊥]) of electrons from a GaAs(Cs, O) photocathode obtained by a stepwise variation of E[sub ||][sup b] provides details about the transfer of electrons through the GaAs(Cs, O)-vacuum interface and demonstrates that not only electron energy loss, but also elastic electron scattering is of crucial importance in the escape process. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]