Estudio De Mercado

High-resolution XPS measurements were performed using a Scienta ESCA300 spec- trometer at the National Centre for Electron Spectroscopy and Surface analysis, Daresbury Laboratory, UK. The spectrometer consists of a high intensity rotating anode Al-Kα x-ray source, a 7-crystal monochromator, a 300 mm mean radius hemi-

spherical electron energy analyser, and a multi-channel CCD detection system, as shown schematically in Fig. 3.4. The main features of the system are summarised in this section. More details about the spectrometer and its performance can be found

3.1. Photoemission spectroscopy 39

high-power fine focus electron gun

300mm diameter rotating anode 7 crystal x-ray monochromator Electrostatic electron lens 300mm radius electostatic analyser CCD camera sample

Figure 3.4: Schematic of the Scienta ESCA-300 spectrometer, adapted from Ref. [85].

in Refs. [85, 86].

The rotating anode x-ray source consists of an electron gun focussed on an Al band around the edge of a water-cooled titanium alloy disc, which rotates at∼10000 revolutions per minute. The emitted x-rays (predominantly Al-Kα,hν = 1486.6 eV)

pass into the monochromator chamber through a thin beryllium window which pre- vents any secondary high-energy electrons from reaching the monochromator crys- tals. Bremsstrahlung radiation and satellite x-ray lines are removed by monochro- mation using the (10¯10) plane of seven α-quartz crystals, arranged on a Rowland circle of 650 mm diameter. A combination of the finite width of the electron beam on

3.1. Photoemission spectroscopy 40

the anode and the diffraction width of theα-quartz crystals yields a total linewidth of the x-ray source of ∼0.26 eV [86]. The whole monochromator chamber is tem- perature stabilized by a quartz lamp to prevent changes in the lattice spacing of the monochromator crystals in order to maintain a constant photon energy. The monochromator chamber is separated from the analysis chamber by a thin Al win- dow in order to maintain a good vacuum in the analysis chamber and prevent any secondary electrons from the x-ray source impinging on the sample. The sample is mounted on a precision four-axis manipulator. The geometry is arranged so that the emission angle to the analyser can be varied from normal to grazing angles, while still maintaining x-ray illumination of the sample. The footprint of the x-ray spot on the sample is ∼4×0.5 mm2_.

If a sample is insulating, the emission of photoelectrons leaves the surface of the sample positively charged, which can both shift and distort the XPS peaks, making analysis impossible. To neutralise this charge, the sample can be irradiated with low energy electrons simultaneously to the photon illumination, correcting the spectral distortion and much of the peak shifts, although care must be taken with the exact energy referencing of the spectra [87]. A low energy (up to 9 eV) electron flood gun is incorporated into the analysis chamber, allowing such charge compensation to be performed. Note, many of the samples investigated here are conducting but were grown on insulating substrates. For these samples, an electrical contact was made to the top of the sample to ensure that it was properly grounded.

Emitted photoelectrons are collected by a five-element electrostatic electron lens and focussed onto the entrance slit of a 300 mm mean radius, 100 mm inner electrode gap, hemispherical electron energy analyser. The analyser can be operated with a pass energy of 20 to 1000 eV, with eight different slit widths from 0.2 mm to 4 mm. All spectra presented in this work were recorded with 0.8 mm slits at a pass energy of 150 eV, giving a good compromise between resolution and count rates. During each set of experiments, the Fermi edge of an ion-bombarded silver reference sample was measured. This was fitted by a Gaussian-broadened Fermi function, from which the Fermi level position and instrumental resolution were calibrated.

3.1. Photoemission spectroscopy 41

The instrumental resolution determined in this way was 0.40±0.05 eV, derived from the convolution of the analyser broadening and the natural linewidth of the x-ray source.

The multichannel electron detector contains two 40 mm diameter microchan- nel plates, of which the electron cascade from the second plate is incident on a phosphor screen, imaged by a CCD camera. The position on the CCD array is then converted into energy and the total counts accumulated by a computer.

Samples are mounted on stubs, and transferred using wobble sticks and rack and pinion railway transfer systems. Samples are introduced into the system via a fast entry load lock, pumped by a turbomolecular pump to a base pressure of

∼10−6 _{mbar. There is also a preparation chamber, where samples can be annealed}

by radiative heating or electron-beam heating. The temperature is calibrated by a thermocouple spot-welded close to the sample stub position. The preparation chamber also has ports where an ion gun and a thermal gas cracker can be installed for sample preparation. The spectrometer and preparation chambers are all pumped by separate oil diffusion pumps with liquid nitrogen cooled traps. The base pressure of the analyser and analysis chambers is ∼5×10−10 _{mbar, and the preparation}

chamber ∼1×10−9 _mbar.