Mg Films Grown by Pulsed Laser Deposition as Photocathodes: QE and surface adsorbates L. Cultrera INFN – National Laboratories of Frascati

Aknowledgements INFN – National Laboratories of Frascati  F. Tazzioli  G. Gatti INFN and Physics Department of Lecce University  A. Perrone  P. Miglietta

Pulsed Laser Deposition System Typical deposition parameters:  Laser: XeCl* ( =308nm,  =30ns)  Fluence: J/cm 2  Target: 99.99% pure Mg  Substrate: polished Cu or Si(100)  UHV pressure: Pa In order to increase the deposition rate the UHV chamber may be filled with pure He with pressure ranging up to 5 Pa:  Mg film with 2.5  m thickness have been grown PLD should ensure high purity coating with strong adhesion to the substrate.

Laser Cleaning and QE measurement set-up: Transfer Line and Signal Acquisition

Laser Cleaning and QE measurement set-up: Laser Cleaning Here is reported the 2D map of the cumulative laser energy distribution during one of the laser cleaning process. The map is calculated summing the 2D energy distribution of each laser pulse during the scanning of the laser beam over the area interested by the cleaning process as recorded on the virtual cathode using a CCD triggered camera. The total area of this map is about 2.4x 2.4 mm 2. Laser parameters during cleaning  x =  y =300  m Power density= 2-3 GW/cm 2 Energy density=  J/mm 2

QE measures on PLD grown samples Mg film covered with graphite protective layer  PLD has been performed in UHV ( 5x10 -6 Pa)  Mg layer has 200 nm thickness  C layer has 60 nm thickness Mg film covered with native oxide layer  PLD has been performed in He (5 Pa)  Mg layer has 2500 nm thickness

Mg films covered with graphite protective layer: QE vs. laser cleaning passes

Mg films covered with graphite protective layer: SEM and EDX Graphite still covers ~80% of irradiated areas. QE of the film may be higher if graphite layer is fully removed.

Mg films covered with graphite protective layer: Diffraction pattern arises from the anodic grid EXPERIMENTSIMULATION

Mg films covered with native oxide layer: QE vs. laser cleaning passes Quantum Efficiency of the sample increases with the number of laser cleaning passes. The high value of QE achieved in the case of naturally oxidized coating demonstrates the high purity of the grown Mg film.

Mg film covered with native oxide layer: SEM of a Mg film deposited on Cu Before Cleaning After Cleaning

Mg films covered with native oxide layer: SEM of a Mg film deposited on Si(100)

Mg films covered with native oxide layer: QE map after laser cleaning The measured QE maps over the irradiated areas reveal that the uniformity of the emission is within a rms spread of 15%

Mg films covered with native oxide layer: QE vs. Time We expected to measure only the decrease of the QE due to progressive oxidation of pure Mg layers. On the contrary an initial strong increase of the quantum yield has been measured during the first hours just after the laser cleaning treatment. Similar observations were reported in literature. 1 The increase of QE was stimulated by means of low pressure oxygen flux. [1] Q. Yuan et al., J. Vac. Sci. Technol. B, 21, 2830 (2003)

QE vs. Time: Effect of Oxygen Adsorption Oxygen contamination leads to a decrease of Mg work function. 1 The higher is the 0 2 partial pressure, the lower is the QE rise time but also lower is the maximum yield. These results agree with numerical calculations that show that a decrease of work function can be associated to initial stages of oxidation. Strong oxidation lead to formation of rocksalt structure and to increased work function. 2 [1] Q. Yuan et al., J. Vac. Sci. Technol. B, 21, 2830 (2003) [2] E. Shroder, Applied Physics Report, (2003)

Mass Spectra of Residual Gases In our experimental condition the most part of the residual gas in UHV chamber is due to hydrogen. O 2 pressure is lower than torr. May hydrogen have a role in QE variation?

Mg work function (back in time up to 1935!): effect of oxygen or hydrogen adsorption While continuous oxygen adsorption from Mg layers results in a decrease of the QE due to the increase of the work function, hydrogen adsorption does not seem to show this detrimental effect: hydrogen adsorption leads to higher QE through the lowering of the work function. 1 [1] R.J. Cashman and W.S. Huxford, Phys. Rev., 48, 734 (1935)

Surface dipole modification of the work function Scenario Due to the intrinsic difficulties to pump out hydrogen from the UHV chamber, at the working pressures (5x10 -7 Pa) the most part of residual gases is constituted of H 2. At lower partial pressure water vapor is also present. The formation of Mg-H bonds with a negative dipole lowers the vacuum level allowing the increase of the QE. During QE measurement UV photons may induce photodissociation of Mg-H bonds (2.0 eV) If oxidation of Mg layer takes place no photodissociation can occur because Mg-O binding energy (>5 eV) is higher than photon energy (4.67 eV), moreover the deep oxygen contamination increases the surface barrier with a consequent decrease of the measured QE.

Conclusions Further QE measurement associated with laser cleaning should use geometric configurations that avoid the use of obstacles, as grid, along laser path. Looking at residual gases mass spectra correlation with respect to quantum efficiency may be helpful to better understand the physical adsorption processes related to QE variation during time. Moving towards shorter laser wavelength (5 th harmonic of nm with photon energy of 5.82 eV) may be beneficial for Mg cathodes (high QE and photodissociation of Mg-O bonds). Tests of promising PLD grown film in a real rf-gun are being performed and should give indications about the surface structure effect in high electric field (adhesion, dark current…).

Yttrium

Mg films on Si (100)

Mg QE vs. photons energy