Influence of the substrate surface texture on the photon-sensitivity stability of CsI thin film photocathodes

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Abstract

A study on the influence of the substrate morphology on the photoemission properties of caesium iodide (CsI) thin film photocathodes, in the range 150–200 nm, has been performed. Various types of conductive substrates, patterned by colloidal lithography, have been compared to the standard printed circuit board (PCB), used for the ALICE experiment at CERN [M.A. Nitti, et al., Nucl. Instr. and Meth. A 523 (2004) 323.]. A correlation between the substrate surface texture and the photoemission stability of the films has been demonstrated.

The combination of colloidal lithography and plasma etching, or physical evaporation, allows to create on substrates arrays of nanostructures whose shape and pitch can be controlled by changing some parameters during the patterning process.

In order to be comparable with the CsI photoelectron escape length and to preserve the substrate morphology in the film, a layer of 20 nm has been deposited on all the samples.

Scanning electron microscopy (SEM) investigations of the colloidal lithography patterned (CLP) substrates have been performed.

Atomic force microscopy (AFM) topographic images of the CsI thin film evaporated on PCB and CLP substrates have also been acquired and compared, showing a clear difference in the surface texture.

An ageing test, consisting of an air exposure with a relative humidity of about 45% for 24 h, resulted in a higher quantum efficiency stability of textured CsI thin film photocathodes evaporated on nanostructured substrates with respect to those grown on standard PCB ones.

Introduction

Caesium iodide (CsI) is widely used in several nuclear physics experiments as photoemissive material for the ultraviolet (UV) thin film photocathode (PC) preparation. However, the main drawback of this material is its high hygroscopicity [1]. Many parameters have been studied in order to enhance the CsI PC stability in air, attracting great attention in the scientific world.

Results concerning 24 h air exposure of CsI thin film PCs evaporated on various conductive materials with surfaces nanostructured by colloidal lithography, other than on a standard printed circuit board (PCB) substrate, are presented in this paper. This ageing test evidences a correlation existing between the substrate surface texture and the photoemission stability of the CsI films.

Section snippets

Substrates preparation

The patterning of n-type conductive silicon (n-Si) and stainless steel (SS) substrates was performed by colloidal lithography.

A physical nanomask was obtained by exploiting the spontaneous self-assembly of polystyrene-sulphated nanoparticles (PS-np, 500±50 nm in diameter) on these substrates. Colloidal suspension of PS-np was spin-coated for this purpose [2], [3], [4], [5]. The pattern of the colloidal template was transferred into the surface either by plasma etching processes (Fig. 1a) or by

Topographic observation

In Fig. 2 the SEM/SE image of the patterned Si LPHT substrate after the lift-off is shown .

The SEM characterization demonstrated a successful transferring of the polystyrene-sulphated nanoparticles pattern.

QE analysis

The PC sensitivity was determined by comparing the electric current measured by the PC and by a NIST—calibrated reference photodiode (PD). The absolute QE value at 150 nm of the CsI thin film PCs evaporated on the patterned substrates was found to be very similar to that of the PC deposited on

Conclusions

The CsI thin film PCs evaporated on the patterned substrates, with respect to those grown on the no-patterned one (PCB), show a similar absolute QE, but an enhanced photon-sensitivity stability in air.

The result concerning the quantum efficiency can be well explained on the basis of a simple photoemission model [8]. An island-grown film, with respect to an uniform one, has a greater photosensitive area because of the greater grain effective area. Moreover, since the grain size was chosen to be

Acknowledgment

Mr. G. Casamassima is acknowledged for his precious technical assistance.

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