Scanning Electron Microscopy (SEM)

4.3.2.1 The instrument

A scanning electron microscope (JEOL 7000, Fig. 4.4) was used to study the surface morphology and chemistry of the films.

Figure 4.4 Scanning Electron Microscope JEOL 7000 at the University of Birmingham.

When the samples were very smooth and hence with low topological contrast, they were

4.3. Electron Microscopy 60

tilted to the maximum angle experimentally possible (5^◦) with respect to the incident electron beam and the working distance was decreased to 6 mm from 10 mm (default setting). The samples were all mounted on metallic SEM stubs with double-sided sticky tape. Silver paint was used to provide a path for the electrical current from the surface down to the metallic stub, thus preventing charging of the specimen. Prior to introducing the specimen into the SEM column, the surface of the specimen as well as that of the SEM stub was cleaned with ethanol. Spatial resolution can deteriorate drastically by contamination of the specimen, and/or apertures. Contamination results when the beam interacts with organic volatiles (from oil diffusion pumps, rubber vacuum seals, vacuum grease, fingerprints and samples) and causes them to polymerize. Polymerization occurs primarily where the beam is most intense (crossover points), such as at the apertures and above all, on the specimen.

4.3.2.2 EDX analysis

All the EDX analyses were performed on the JEOL 7000. INCA software was used to acquire and process the EDX spectra. The following X-ray lines were used for quantifi-cation: In L (3.2870 keV), Sn L (3.4440 keV) and O K (0.5249 keV). The overvoltage parameter, U=E₀/E_c must obviously exceed unity for any X-ray generation to take place.

(E₀ is the incident beam energy and E_c is the minimum energy for X-ray generation from a given line). In fact, a value of at least U=2 is desirable [134]. Hence, the microscope was operated at 7 kV resulting in a 96 µA current. This choice was motivated by Monte Carlo simulations of the electron trajectories inside the specimen and above all, the intensity of the detected oxygen X-ray versus depth at 7 kV, since the substrate also contains oxygen. The simulations were performed using CASINO version 2.42 [160].

The model consisted of an Indium Oxide (In₂O₃) layer (thickness 250 nm - chamber 1 and 350 nm - chamber 2) on top of a semi-infinite Alkaline Earth Boro-Aluminosilicate glass substrate. The densities were respectively 7.18 g/cm³ and 2.40 g/cm³. The initial beam diameter was 10 nm. The results are presented in Figs. 4.5 and 4.6.

The intensity of the detected oxygen X-ray versus depth at a beam voltage of 7 kV is presented in Fig. 4.7.

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Figure 4.5 Monte Carlo modelling of beam trajectories in an In2O3 layer with total thickness 250 nm on a glass substrate at an accelerating voltage of 7 kV. 300 trajectories are represented. Backscattered electrons are represented in red.

Figure 4.6 Monte Carlo modelling of beam trajectories in an In₂O₃ layer with a total thickness 350 nm on a glass substrate at an accelerating voltage of 7 kV. 300 trajectories are represented. Backscattered electrons are represented in red.

Figure 4.7 Detected X-ray intensity versus depth for oxygen (K line) at an accelerating voltage of 7 kV. Results are derived from previous Monte Carlo modelling for an In₂O₃ layer with total thickness of 350 nm on a glass substrate.

4.3. Electron Microscopy 62

The Monte Carlo simulations (Figs. 4.5 - 4.6) and Fig. 4.7 confirm that the influence of the substrate is minimal (in terms of oxygen contribution) at both 250 and 350 nm, provided an acceleration voltage of 7 kV is used. This was confirmed by the absence of Silicon peaks.

The working distance (the distance of the sample from the objective lens) was kept at 10 mm due to EDX detector - sample stage geometry constraints. A highly polished surface is required for accurate quantitative analysis, since surface roughness would cause undue random absorption of the generated X-ray signal, which is impossible to account for in the quantification procedure. Nonetheless, all our ITO thin films were sufficiently smooth (a few nanometres roughness) that such an effect could be neglected.

Prior to each measurement, the system was optimised (“quant optimisation” option in INCA software) since ambient temperature changes can alter the gain/exact peak positions of the system and the microscope beam current varies with time. It was performed via the acquisition of a spectrum from a pure titanium sample.

The default standard In, Sn and O peaks used during the quantification procedure (matching process) were replaced by In, Sn and O peaks acquired experimentally from InAs, pure Sn and Al₂O₃ standard specimens respectively. Hence, InAs, Sn and Al₂O₃ were used as chemical standards.

The bulk ITO target was not used as a chemical standard because its composition was not exactly known. Finally, when Sn is in solid solution in the In₂O₃ matrix, ITO can be written as In_2−xSn_xO₃. Hence the nominal (In+Sn)/O atomic ratio should be 0.66. With 10 wt.% of SnO₂ in the matrix, the nominal In/Sn atomic ratio was calculated to be 9.85.

4.3.2.3 EELS measurements

EELS measurements were performed with a Gatan 666 parallel electron energy-loss spectrometer attached to an FEI Tecnai F20 Schottky field emission gun transmission electron microscope, which offers an energy resolution of 0.8 eV when operating at 200 keV (University of Birmingham). Another set of EELS measurements was performed at Daresbury in the SuperSTEM laboratory (University of Liverpool). Measurement details are presented in the next two paragraphs.

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4.3.2.3.1 Measurements at Birmingham The spectrometer consists of a magnetic prism spectrometer where aberrations are corrected using quadrupole and sextupole lenses through the Gatan EL/P software (version 3.0). The microscope was operated in conventional diffraction mode. Only low loss spectra were acquired (0-50 eV). Operating in conventional diffraction mode presents two advantages:

• large enough area can be selected (up to a few hundreds of microns square)

• reduced problem associated with chromatic aberration in the objective and projec-tor lenses which can result in differing collection efficiencies at different energy losses [146].

• the acceptance angle β is easy to compute.

In diffraction mode, the spectrometer object is the demagnified image of the specimen in the final projector lens and so the spectrometer is said to be image coupled. The angle of acceptance at the specimen, β, is defined by the radius of the spectrometer aperture, R, which was selected to be 1 mm. For inellastically scattered electrons in conventional diffraction mode, β is defined as:

β = h_VR

h_AL (4.3)

where L is the camera length (for the viewing screen) and h_V and h_A are the distances from the projector lens crossover to the viewing screen and spectrometer entrance aperture respectively. In an FEI Tecnai F20, ^h_h^V

A=0.718. L was chosen to be 150 mm giving β=4.78 mrads ensuring the dipole selection rule to be verified.

The area from which the spectrum is taken is limited by the size of the selected area diffraction aperture (SAED). The smallest aperture was chosen (10 µm diameter). It is large enough to select several grains/crystallites and associated defects but not too large to select an area where the thickness would not be uniform resulting in less precise deconvolution/background removal procedures. However, the precision of the area selected using the SAED aperture is strongly affected by spherical and chromatic aberration in both the TEM objective and projector lenses. It was not an important issue in this work since “any” area containing a few tens of grains/defects could be selected. The extraction voltage was reduced down to 3800 V to reduce the electron energy spread and hence improve energy resolution. No drift or contamination problems were ancountered as the beam was spread (conventional diffraction mode). The smallest condenser aperture was generally used for several reasons: