Progress in CsI & Diamond thin film photocathodes 5 th International Workshop on Ring Imaging Cherenkov Counters (RICH 2004) Playa del Carmen, Mexico Playa del Carmen, Mexico November 30-December 5, 2004 November 30-December 5, 2004 M. A. Nitti, E. Nappi, A. Valentini INFN Sezione di Bari, Via Amendola 173, Bari (Italy)

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Outline deposition technique  Influence of the deposition technique CsI photocathodes on the properties of CsI photocathodes (PCs) (PCs) diamond PCs  Preliminary results on diamond PCs  Concluding remarks 2

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Deposition technique Thermal evaporation (Joule effect) more utilised technique for the CsI thin film deposition Electron beam evaporation technique used 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 50 mA 700 eV3

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Sample stability against aging due to humid air exposure stable CsI photocathodes deposited by IBS seem to be more stable after 24 h air exposure than the evaporated ones. QE (%) IBS < QE (%) thermal evaporation Comparison between the QE (%) of CsI photocathodes grown with two different techniques: thermal evaporation and IBS in our laboratories, without post-deposition thermal annealing 4

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Crystalline structure (XRD) 5

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Deposition parameters Deposition mode Beam current (mA) Beam energy (eV) Growth rate (nm/s) QE (%) ( = 160 nm) (A) Grain size (nm) (B) Grain size (nm) ION BEAM SPUTTERING n.d. ION BEAM SPUTTERING n.d. ION BEAM ASSISTED SPUTTERING THERMAL EVAPORATION126.4n.d n.d.: not detected (A) crystallographic orientation(110) (A) crystallographic orientation (110) (B) crystallographic orientation(200) (B) crystallographic orientation (200)6

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, D AFM images of CsI films deposited with two different technique on Ti/Au substrate 3D AFM image Film deposited by thermal evaporation 3D AFM image R a = 1.29 nm R a = 12.9 nm Film deposited by IBS R a = 14.8 nm QuartzTi/Au Quartz substrate covered with a Ti/Au layer R a = 1.29 nm 3D AFM image

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, D AFM images of substrates with different roughness R a R a = nm R a = 350 nm R a = 4.55 nm Quartz substrate covered with an Al layer PCB PCB substrate Peenedquartz substrate covered with an Au layer Peened quartz substrate covered with an Au layer 8 3D AFM image

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 QE (a.u.) vs. the substrate roughness The QE (a.u.) of CsI PCs deposited by IBS follows the surface average roughness R a of substrates The QE (a.u.) of CsI PCs deposited by thermal evaporation seems to be independent from the surface average roughness R a of substrates, but……………………… 9

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 R a = nm Film deposited by IBS R a = nm 3D AFM image 3D AFM images of CsI films deposited with two different technique on Peened quartz substrate Peenedquartz substrate covered with an Au layer Peened quartz substrate covered with an Au layer R a = 350 nm 3D AFM image Film deposited by thermal evaporation

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, D AFM images of CsI films deposited with two different technique on Al substrate R a = 4.55 nm Quartz substrate covered with an Al layer R a = nm Film deposited by IBS R a = nm 3D AFM image Film deposited by thermal evaporation

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, D AFM images of CsI films deposited by two different technique on PCB substrate R a = nm Film deposited by IBS R a = nm QuartzPCB Quartz substrate covered with a PCB layer 3D AFM image Film deposited by thermal evaporation R a = nm 3D AFM image

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Model of a CsI film morphology deposited on the same substrate by the two different techniques (a) Film deposited by thermal evaporation (b) Film deposited byIBS Electron photoexcitement regions FILM SUBSTRATO hv FILM hv UV Photons SUBSTRATE  hv absorptionlength Effective reduction factor of the absorption length: n n is the refractive index of CsI   is the angle between the surface and the direction of the incident radiation (a) In case (a) there is an enhancement of the maximum efficiency of photoemission reflective for reflective PCs : Q = intrinsic QE T = probability that an electron that reach the surface can escape (T  1) L = escape length absorptionlength = optical absorption length 10

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Polymer material: PET

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Progress in diamond films for the realization of UV photocathodes PHOTOCATHODEdetection PHOTOCATHODE = key element of many detection systems materialsPCspectral range 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. UV PCsalternative For the UV range, PCs manufactured with alternative materials with respect to CsI have to present the following properties: comparableCsI PCs –quantum efficiency comparable to that of CsI PCs; exposure to high photon or ion flux stability –high stability for exposure in air 12

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Comparison diamond-CsI PropertiesDiamondCsI Density (g/cm 3 ) Bandgap E G (eV) Electron affinity  (eV) < 1 eV (or negative) 0.1 Resistivity (  cm) 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 Scarce 13

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Electron affinity (PEA, NEA) Band energy for: (PEA) a) Positive electron affinity (PEA) “Effective”(NEA) b) “Effective” negative electron affinity (NEA) due to Cs layer and its dipole layer “True”(NEA c) “True” negative electron affinity (NEA) systems, typical of boron-doped natural diamond F. J. Himpsel e al., Phys.Rev.B 20 (1979)

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 QE (%) of an amourphous diamond film deposited by means of IBS Proceedings SPIE, vol. 4139, San Diego, California (2000) A.S. Tremsin* and O.H.W. Siegmund Literature (POLYCRYSTALLINE = 1500 Å QE (%) = 0.2 % Our results (IBS diamond film – Bari = 150 nm QE (%) = 0.7 %19

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Substrates used Silicon (Si) substrates were used for the diamand film deposition because of their cubic crystalline structure, as that of diamond. Before Before proceeding to the deposition of diamond film, it is important to treat the surface of Si diamond powder ultrasonic bath. substrate with diamond powder in ultrasonic bath. Si not Si not treated, in fact, presents:  low density of nucleation centres (10 4 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 traiteddiamond powder Si traited with diamond powder presents:  high density of nucleation centres (10 11 cm -2 ) At the LIMHP of Paris, nanocrystalline diamand films with different percentage of graphite were deposited by MPECVD on quartz substrate too. not (a) Si substrate not traited(b) Si substrate traited Al 2 O 3 with Al 2 O 3 powder (c) Si substrate traited SiC with SiC powder (d) Si substrate traited diamaond polwder with diamaond polwder 15

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Techniques of deposition for diamond films PolynanocrystallineMPECVD Poly and nanocrystalline diamond films were prepared by MPECVD, at the LIMHP (Laboratoire d’Ingénierie des Materiaux et des Hautes Pressions) - CNRS-UPR- Paris. MPECVD: microwave plasma enhanced chemical vapour deposition chemical vapour deposition Amorphous IBS Amorphous diamond films were prepared by IBS, at the Thin Film Laboratory of Bari, starting from a carbon target. CH 4 /H 2 plasma discharge conditions, adopted in experiment of diamond deposition are: 1. reactor UHV coupled to a microwave generator (2.45 GHz) 2. CH 4 highly diluited in H 2 (CH 4 < 4%) 3. high deposition temperature ( °C) 4. high microwave input power ( kW) 5. high pressure ( mbar) 16

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 AFM images of poly and nanocrystalline diamond films (MPECVD - LIMHP) NANOCRYSTALLINE POLYCRYSTALLINE Average GRAIN size ≤ nm R a = nm R a = nm Average GRAIN size ≥  m 17

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Comparison with literature Proceedings SPIE, vol. 4139, San Diego, California (2000) A.S. Tremsin* and O.H.W. Siegmund Our results (MPECVD diamond film – Paris-) = 1500 Å QE (%) = 0.2 = 150 nm QE (%) = 5  30 %18

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Aging due to air exposure MPECVD Comparison between the RQE of a CsI PC, deposited by thermal evaporation, and a nanographitic (NG) diamond PC, deposited by MPECVD. The diamond PC presents a lower aging with respect to the CsI one. 20

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Concluding remarks deposition technique  The study of the deposition technique influence on CsI morphological, structural and photoemissive properties of CsI PCshigher QE PCs, indicates that the evaporated ones have a higher QE, and increasemicroroughness suggests to increase the substrate microroughness in order to enhance the photoyield of sputtered ones. model surfacemorphology  A model of surface morphology has been also presented in higherphotoemissionPCs order to explain the higher photoemission of evaporated PCs PCsIBS. than that of PCs grown by IBS. diamond PCs  On the basis of the preliminary results on diamond PCs UV photondetectors we look forward to applying them to UV photon detectors, higher stability in air because of their higher stability in air with respect to that of detectorsCsI PCs the detectors based on CsI PCs.

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Thank you for your attention and see YOU at RICH 2006

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004

5 th International Workshop on RICH, Playa del Carmen, Mexico 1 st December, 2004 Photon-aging (UV flux : 10 7 photons/mm 2  sec) Before exposure After exposure Channeling mechanism Substrate Photoelectron UV photon Grains