Influence of Deposition Techniques on CsI Photocathode Properties

1. Outline • Influence of the deposition technique on the properties of CsIphotocathodes (PCs) • Preliminary results on diamondPCs • Concluding remarks 2

2. Thermal evaporation (Joule effect) more utilised technique for the CsI thin film deposition Electron beam evaporationtechniqueused at TUM for HADES Ion beam sputtering (IBS)technique explored in our laboratory for the first time for the CsI thin film deposition Best parameters for the CsI film deposition by means of IBS are: Current beam of Energy beam of 50 mA 700 eV Deposition technique 3

3. Sample stability against aging due to humid air exposure Comparisonbetween the QE (%) of CsI photocathodes grown with two different techniques: thermal evaporation and IBS in our laboratories, without post-deposition thermal annealing QE (%) IBS< QE (%) thermal evaporation CsI photocathodes deposited by IBS seem to be more stable after 24 h air exposure than the evaporated ones. 4

4. Crystalline structure (XRD) 5

5. (A)crystallographic orientation(110) (B)crystallographic orientation(200) Deposition parameters n.d.: not detected 6

6. Quartz substrate covered with aTi/Au layer 3D AFM images of CsI films deposited with two different technique on Ti/Au substrate Ra = 1.29 nm Ra = 1.29 nm 3D AFM image Film deposited bythermal evaporation Film deposited byIBS Ra = 12.9 nm Ra = 14.8 nm 3D AFM image 3D AFM image

7. PCB substrate Quartz substrate covered withan Al layer Ra = 4.55 nm 3D AFM images of substrates with different roughness Ra Ra = 12.48 nm 3D AFM image 3D AFM image Peenedquartz substrate covered with an Au layer Ra = 350 nm 3D AFM image 8

8. QE (a.u.) vs. the substrate roughness The QE (a.u.) of CsI PCs deposited by IBSfollows the surface average roughness Ra ofsubstrates The QE (a.u.) of CsI PCs deposited by thermal evaporationseems to be independent from the surface average roughness Ra ofsubstrates, but……………………… 9

9. 3D AFM images of CsI films deposited with two different technique on Peened quartz substrate Peenedquartz substrate covered with an Au layer Ra = 350 nm 3D AFM image Film deposited byIBS Film deposited bythermal evaporation Ra = 283.90 nm Ra = 103.04 nm 3D AFM image 3D AFM image

10. Quartz substrate covered withan Al layer Ra = 4.55 nm 3D AFM images of CsI films deposited with two different technique on Al substrate 3D AFM image Film deposited bythermal evaporation Film deposited byIBS Ra = 39.82 nm Ra = 67.49 nm 3D AFM image 3D AFM image

11. Quartz substrate covered with aPCBlayer 3D AFM images of CsI films deposited by two different technique on PCB substrate Ra = 12.48 nm 3D AFM image Film deposited bythermal evaporation Film deposited byIBS Ra = 24.09 nm Ra = 13.54 nm 3D AFM image 3D AFM image

12. hv UV Photons hv hv hv hv  Electron photoexcitement regions FILM SUBSTRATO SUBSTRATE SUBSTRATE (a) Film deposited by thermal evaporation (b) Film deposited by IBS Effective reduction factor of the absorptionlength: n is the refractive index of CsI is the angle between the surface and the direction of the incident radiation FILM Model of a CsI film morphology deposited on the same substrate by the two different techniques In case (a) there is an enhancement of the maximum efficiency of photoemission for reflective PCs : Q = intrinsic QE T = probability that an electron that reach the surface can escape (T  1) L = escape length  = optical absorptionlength 10

13. Polymer material: PET

14. Progress in diamond films for the realization of UV photocathodes PHOTOCATHODE = key element of many detection systems Since many years the scientific research has been devoted to the study of materials for the PC production, depending on the spectral range of detection. For the UV range, PCs manufactured with alternativematerials with respect to CsI have to present the following properties: • quantum efficiency comparable to that of CsIPCs; exposure to high photon or ion flux • high stability for exposure in air 12

15. Properties Diamond CsI Density (g/cm3) 3.51 4.51 Bandgap EG (eV) 5.5 6.2 Electron affinity  (eV) < 1 eV (or negative) 0.1 Resistivity ( cm) 1013-1016 1010-1011 Optical transparency Broad from the deep (225 nm) UV to the far IR region From UV to far IR Stability for: - Air exposure - UV photon exposure Elevate Elevate Scarce Scarce Comparison diamond-CsI 13

16. F. J. Himpsel e al., Phys.Rev.B 20 (1979) 624 Electron affinity (PEA, NEA) Band energy for: a) Positive electron affinity (PEA) b) “Effective” negative electron affinity (NEA) due to Cs layer and its dipole layer c) “True”negative electron affinity(NEA)systems, typical of boron-doped natural diamond 14

17. @  = 150 nm QE (%) = 0.7% A.S. Tremsin* and O.H.W. Siegmund @  = 1500 Å QE (%) = 0.2 % Proceedings SPIE, vol. 4139, San Diego, California (2000) QE (%) of an amourphous diamondfilm deposited by means of IBS Our results (IBSdiamondfilm– Bari -) Literature (POLYCRYSTALLINE film) 19

18. (d) Si substrate traited with diamaond polwder (c) Si substrate traited with SiC powder (a) Sisubstrate nottraited (b) Si substrate traited with Al2O3 powder Substrates used Silicon (Si) substrates were used for the diamand film deposition because of their cubic crystalline structure, as that of diamond. Beforeproceeding to the deposition of diamond film, it is important to treat the surface of Si substrate with diamond powder in ultrasonic bath. Si not treated, in fact, presents: • low density of nucleation centres (104 cm-2) due to the high surface energy of diamond, the big mismatch between Si and diamond and the low probability of nucleation precursor sticking. Si traited with diamond powder presents: • high density of nucleation centres (1011 cm-2) At the LIMHP of Paris, nanocrystalline diamand films with different percentage of graphitewere deposited by MPECVD on quartz substrate too. 15

19. CH4/H2 plasma discharge conditions, adopted in experiment of diamond deposition are: 1. reactor UHV coupled to a microwave generator (2.45 GHz) 2. CH4 highly diluited in H2 (CH4 < 4%) 3. high deposition temperature (750-900 °C) 4. high microwave input power (0.45-2.5 kW) 5. high pressure (10-200 mbar) MPECVD:microwave plasma enhanced chemical vapour deposition Techniques of deposition for diamond films Poly and nanocrystalline diamond films were prepared by MPECVD, at the LIMHP (Laboratoire d’Ingénierie des Materiaux et des Hautes Pressions) - CNRS-UPR- Paris. Amorphous diamond films were prepared by IBS, at the Thin FilmLaboratory of Bari, starting from a carbon target. 16

20. AFM images of poly and nanocrystalline diamond films (MPECVD - LIMHP) NANOCRYSTALLINE POLYCRYSTALLINE Ra = 16.44 nm Ra = 48.83 nm Average GRAIN size ≥ 0.5-1 m Average GRAIN size ≤250-500 nm 17

21. @  = 150 nm QE (%) = 5  30 % Literature A.S. Tremsin* and O.H.W. Siegmund @  = 1500 Å QE (%) = 0.2 % Proceedings SPIE, vol. 4139, San Diego, California (2000) Comparison with literature Our results (MPECVDdiamond film – Paris-) 18

22. Aging due to air exposure Comparison between the RQE of a CsI PC, deposited by thermal evaporation, and a nanographitic (NG) diamond PC, deposited by MPECVD. The diamondPC presents a loweraging withrespect to the CsI one. 20

23. Concluding remarks • The study of the depositiontechniqueinfluence on morphological, structural and photoemissive properties of CsI PCs, indicates that the evaporated ones have a higher QE, and suggests to increase the substrate microroughness in order to enhance the photoyield of sputtered ones. • A modelof surfacemorphology has been also presented in order to explain the higherphotoemission of evaporatedPCs than that of PCs grown by IBS. • On the basis of the preliminary results ondiamondPCs we look forward to applying them to UVphotondetectors, because of their higherstability in air with respect to that of the detectors based on CsI PCs.

24. Thank you for your attention and see YOU at RICH 2006

27. Photon-aging (UV flux : 107 photons/mm2sec) Beforeexposure After exposure Photoelectron UVphoton Grains Substrate Channelingmechanism