Fundamental processes in III-V photocathodes ; application as long as high-brightness photoinjectors
Shirvanian, Lee, Morning Show Host has reference to this Academic Journal, PHwiki organized this Journal Ivan BazarovFundamental processes in III-V photocathodes; application as long as high-brightness photoinjectorsPhysics Department, Cornell UniversityContentsMotivationNEA photoemissionSome practical aspectsStudy cases: GaAs, GaAsP, GaNSummary7/20/2009I.V. Bazarov, III-V Photocathodes, ERL092Why are we interestedPhotoinjectors: a photocathode in high electric field (>> MV/m), either DC or RF7/20/2009I.V. Bazarov, III-V Photocathodes, ERL093Relativistic electrons can be further accelerated in a linac (linear accelerator) without degradation of beam brightness:CW ultra-bright x-ray sources; high power FELsElectron-ion colliders in addition to ion coolersUltrafast electron diffraction, etc.
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Energy recovery linacEnergy recovery linac: a new class of accelerators in active developmentEssentially removes the average current limitation typical to linacs (i.e. Pbeam >> Pwall plug)Average currents 10s to 100s of mA can be efficiently accelerated ( in addition to de-accelerated)7/20/2009I.V. Bazarov, III-V Photocathodes, ERL094Cathode figures of merit7/20/2009I.V. Bazarov, III-V Photocathodes, ERL095QE in addition to photon excitation wavelengthE.g. 1W of 775 nm (Er-fiber /2) 6.2 mA/% 520 nm (Yb-fiber /2) 4.2 mA/% 266 nm (Nd-glass /4) 2.1 mA/%Transversely cold (thermalized) electron distributionDirectly sets the solid angle of the emitted electrons; an upper limit on achievable beam brightnessFigures of merit (contd.)7/20/2009I.V. Bazarov, III-V Photocathodes, ERL096Prompt response timeA picosecond response is essential to take advantage of the space charge control via laser pulse shapingLong lifetime in addition to robustnessExtraction of many 100s to 1000s of C between activations are necessary to make the accelerator practical
Negative electron affinity7/20/2009I.V. Bazarov, III-V Photocathodes, ERL097Defined as vacuum level Evac relative to the conduction b in addition to minimumNegative affinity: the vacuum level lies below the CBM very high QE possibleNEA:1) b in addition to bending2) dipole layerStrong p-doping7/20/2009I.V. Bazarov, III-V Photocathodes, ERL098Alperovich et al., Phys. Rev. B 50 (1994) 5480: clean p-doped GaAs has Fermi level unpinned in addition to shows little b in addition to bending~1019 cm-3NEA: Cs ~monolayer7/20/2009I.V. Bazarov, III-V Photocathodes, ERL099Cs was found to play a larger role as long as NEA instead: 1) b in addition to bending through donor surface states, in addition to 2) dipole surface layer from polarized Cs adatomsCs-induced donor-like surface states contribute their electrons to the bulkHole depleted region (negatively charged acceptors) lead to b in addition to bending regionionized Cs bb region
NEA: ~Cs monolayer (contd.)7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0910Majority of Cs atoms become only polarized (not ionized), as long as ming a dipole layer (e- Cs+)GaAsEgap = 1.42 eVBe as long as e Csc = 4 eVAfter Csceff ~ -0.1 eVVbb ~ 0.4 eVdbb ~ 10 nmSpicers magic window7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0911(1) electron-electron scattering: typical of metals, large energy loss per collision(2) electron-phonon scattering: slowly depletes excessive energy of excited electron (LO phonons in GaAs ~ 35 meV) Magic window: in semiconductors, one needs excess KE > Egap as long as e/e scattering. Thus, electrons excited with Evac < KE < EVBM + 2Egap have excellent chances of escape(1)(2)Electron transport processes7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0912CBM thermalization time: 0.1-1psElectron-hole recombination: ~nsEmission time: 1/(a2D) strong wavelength dependence Energy vs. momentum7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0913Role of fluorine/oxygen7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0914Routine yo-yo activation employs O2 or NF3Further reduction of affinity consistent with a double dipole modelStabilizes Cs on the surface; no lifetime or otherwise apparent advantage as long as either gasBonded unstable nitrogen is found on Cs-NF3 activated surfaces (APL 92, 241107)Yo-Yo ActivationJAP 54 (1983) 1413GaAs: Optimal Cs coverage7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0915Ugo Weigel, PhD thesislaser wavelength: 670 nm Lifetime matters7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0916H2O, CO2 in addition to O2 can lead to chemical poisoning of the activated layerLow current (~ 1mA) 1/e lifetimes ~ 100 hours typical in our prep chambers3-5 times better in the DC gun (low 10-12 Torr vacuum)High average current (mAs) lifetime limited by ion backbombardment14 mA7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0917~5 hour lifetime (limited by gas backstreaming from the beam dump), i.e. 20 hours 1/e as long as 5 mA10 minStudy cases7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0918Our group has been evaluating III-V photocathodesTransverse energy of electrons (thermal emittance)Measure the photoemission response timeMaterials studied so farGaAs @ 450-850nm: JAP 103, 054901; PRST-AB 11, 040702GaAsP @ 450-640nm: IbidGaN @ 260nm: JAP 105, 083715 Diffusion modelsubject to:response time expected to scale as a2 with wavelength (a lot!)GaAs absorptionPrompt emittersdeflector offGaAs @ 520 nmMeasurements done by transverse deflecting RF cavityLimited by 1.8 ps rms resolution dominated by laser to RF synchronizationGaN @ 260 nm Diffusion tailP concentration 45%Strong QE dependency Two valleys: G (direct) in addition to X (indirect) involved in the processGaAsP @ 520 nmTransverse energy distributions7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0923No surprises as long as bulk GaAs: cold electrons with a near b in addition to -gap excitationGaAsSurprisingly large transverse energy spread as long as GaN in addition to GaAsP:GaAsP: kT = 130-240 meV as long as photons 0-780 meV photons above the b in addition to -gapGaN: kT = 0.9 eV as long as photons with 1.4 eV above the b in addition to -gapSummary7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0924Transverse energy of photoelectrons remain poorly understood as long as III-V semiconductors (other than GaAs)More carefully controlled experimental data on transverse energy distributions/time response neededPredictive codes in addition to models need to be developed in addition to benchmarked with experimentsThis will allow photocathode engineering with the desired characteristics such as cold electrons with a ps response
Acknowledgements7/20/2009I.V. Bazarov, III-V Photocathodes, ERL0925 Bruce Dunham, Xianghong Liu, Yulin Li Amir Dabiran (SVT) Dimitry Orlov (Max Planck Institute NP) Matt Virgo (ANL)
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