10/06Workshop on High QE Milan Diamond Amplified Photocathodes Theory and Characterization John Smedley

10/06Workshop on High QE Milan Collaborators I. Ben-Zvi A. Burrill X. Chang J. Grimes T. Rao Z. Segelov Q. Wu R. Hemley, CI, Washington Z. Liu, CI, Washington S. Krasnicki CI, Washington K. Jenson, NRL, Washington J. Yater, NRL, Washington R. Stone, Rutgers U J. Bohon, Case Western U P. Stephens, SUNY SB

10/06Workshop on High QE Milan Overview Electron transport in diamond –Field –Temperature –Recombination –Grain boundaries –Band bending and NEA surfaces Diamond characterization Gain measurements

10/06Workshop on High QE Milan Electron Transport in Diamond Energy Filled States Empty States Primary e - penetrate < 1μm into diamond Lose energy via e - -e - scattering Excite e - into conduction band Some e- and holes will recombine (probability based on current and drift velocity) Secondary e - lose energy via e - -e - and e - -phonon scattering Eventually, e - reaches the bottom of the conduction band Holes drift toward metal layer, e - into diamond Some e - are trapped Most drift to vacuum side Trapped e - modify field in diamond Trap EgEg EaEa Hydrogen termination lowers electron affinity In some cases, E a can be negative Then Ф< E g, e - that reach the surface will be emitted

10/06Workshop on High QE Milan Field in the diamond is a critical parameter –Field should be high enough for e - velocity to saturate –Field should be low enough that the e - energy stays near the conduction band minimum Monte Carlo Simulations –Watanabe et al, J. of Applied Physics, (2004) –At an electric field of 2-4 MV/m, velocity ~ 10 5 m/s while energy is still <0.1 eV above minimum –For E>100MV/m, cascade secondary electrons will be produced Electric Field and Drift Velocity

10/06Workshop on High QE Milan Temperature Dependence

10/06Workshop on High QE Milan e - -hole Recombination Recombination occurs primarily in initial region of drift, where e - and hole densities overlap Probability of recombination increases quadraticly with primary current density Probability decreases with increasing electron and hole velocity Simple model provides insight into gain at low fields, temperature dependence and current dependence

10/06Workshop on High QE Milan 300K, 4keV Primary

10/06Workshop on High QE Milan 85K, 4keV Primary

10/06Workshop on High QE Milan Effect of C in grain boundaries Nebel, Semicond. Sci. Technol. 18 S1 (2003) e - trapped in the empty states Shift E Fermi toward conduction band Electron conduction along grain, and injection into diamond Leads to “dark” current while suppressing “photo- current”

10/06Workshop on High QE Milan Surface effects Bands bend near surfaces Carriers can be trapped in surface states NEA surfaces bind e - outside surface Field near surface can be significantly different than bulk Prins, Semicond. Sci. Technol. 18 S125 (2003)

10/06Workshop on High QE Milan Diamond Characterization Crystalline Orientation –X-ray diffraction for bulk orientation –Electron diffraction for surface orientation –NEA depends on surface crystalline structure –Thinning may effect surface Impurity Content –Raman for graphite content –Photoluminescence for impurities, traps –FTIR to study nitrogen and hydrogen –Mass-spectrometry is an option (destructive) –Even ppb impurity levels are critical 1 ppb = /cm 3 = potential charge traps

10/06Workshop on High QE Milan Diamond Characterization Photoconductivity –Measure mobility –Time resolved measures lifetime Electron energy spectrum –Measure electron affinity –Relevant for thermal emittance –Easiest in reflection mode w/o bias Gain Measurements –Transmission mode isolates bulk effects –Emission mode adds NEA surface effects

10/06Workshop on High QE Milan Powder X-ray Diffraction Most Interesting for Polycrystalline Diamonds Standard Electronic Grade Predominantly 110, 211 & Enhanced Polycrystalline Natural Diamond was 110 Single crystal CVD was 100

10/06Workshop on High QE Milan Raman/Photoluminescence

10/06Workshop on High QE Milan IR Spectroscopy (FTIR)

10/06Workshop on High QE Milan Secondary Electron Yield Measurements Ongoing work a NRL –J. Yater and A. Shih, J. Applied Physics (2000), (2005) Primarily reflection mode Boron-doped diamonds, Natural and CVD H or Cs termination Measured Secondary Electron Yield (SEY) and Energy Distribution Curves (EDC)

10/06Workshop on High QE Milan Natural Diamond, 100 orientationNatural Diamond, 111 orientation SEY EDC SEY EDC

10/06Workshop on High QE Milan CVD Diamond, polycrystalline SEY EDC Implications Crystalline orientation affects NEA surface Diamond quality affects Secondary Electron Yield Electron energy at emission is <1 eV SEY of 100+, with 3 keV primaries

10/06Workshop on High QE Milan Experimental Arrangement Primary Secondary Kapton Diamond A V2V2 I2I2 A V1V1 I1I1 Two types of measurements Transmission – both sides metallized Emission – one side metallized, one side hydrogenated

10/06Workshop on High QE Milan 4 KeV, nA 3 KeV nA 2 KeV, nA SEY Natural Diamond, Transmission Mode

10/06Workshop on High QE Milan SEY Natural Diamond, Transmission Mode, 81K High gain at low Gradient!

10/06Workshop on High QE Milan Gain for CVD Detector Grade Transmission Mode

10/06Workshop on High QE Milan Gain for CVD Detector Grade Emission Mode

10/06Workshop on High QE Milan Conclusions Electron drift in diamond can be modeled with Monte-Carlo –For 3MV/m field in diamond, velocity is near saturated while electron energy is sill near conduction band minimum –Regions of high field may lead to cascade secondary production Recombination limits gain at low fields and high densities –Less important at low temperatures due to increased mobilities With proper preparation, emission gain of >300 with small emission threshold. H termination can provide a robust, NEA surface for electron emission. e - affinity is affected by surface crystalline structure. Trap density in diamond affects gain A little NEA goes a long way –Further decrease in electron affinity increases energy spread –May increase dark current and reduce field in diamond Still a lot to do!

10/06Workshop on High QE Milan Still to do Test other diamonds, including some 30μm thick Understand differences between transmission gain and emission gain Demonstrate gain with I p = 10mA Temporal and energy profile of emitted electrons Characterize impurities and crystalline orientation Determine the proper method of metallization Develop transparent photocathode (LDRD 2006) Capsule fabrication and polishing diamond to 30μm are significant “engineering” challenges

10/06Workshop on High QE Milan Modification of 1.3 GHz gun for testing diamond capsules

10/06Workshop on High QE Milan SRF Gun modified to accept Choke joint with diamond

10/06Workshop on High QE Milan Measurements with Type II a Natural diamond

10/06Workshop on High QE Milan High Average Current Linacs Applications: –Linac-based light sources –Free-electron lasers –Electron-cooling for RHIC Challenges –Energy recovery –Large laser power required, even for high QE Cathodes:

10/06Workshop on High QE Milan Photoinjector

10/06Workshop on High QE Milan BNL SEEP Project Goals –Identify suitable diamond- tested 4 types of diamond: natural, synthetic optical, electronic, single crystal Best so far- natural and electronic grade –Determine optimal metallic coating –Measure gain in transmission mode >200 –Measure Temperature dependence of gain No effect –Measure gain in emission >30 –Measure time behavior and EDCs –Capsule fabrication –Test in RF injector