STEM is a combination of both SEM and TEM. It uses the scanning principle of SEM however through a thin sample and collects the transmitted electrons like a TEM. The advantage of this is that STEM can spatially correlate the signal unlike TEM and can be in higher resolution than that of SEM. STEM scans in raster pattern with a finely focused electron beam across the sample and collect serial signals which are then correlated with the beam position. This is very useful in determining the characteristic of a certain area. For example, EDX signal can be collected in the STEM mode, which can provide information on the composition distribution of the sample. Furthermore, STEM can also be used for highly dependent z-contrast imaging using the high-angle annular dark field (HAADF) mode. A combination of both the EDX and HAADF can provide detailed information on the concentration of elements present in the sample up to atomic level in an optimised condition.
3. Experimental techniques
3.5.2 EDX spectroscopy
As represented earlier in figure 3.5, X-rays can be produced by electron-sample interaction. X-rays are the inelastic scattering of electrons that can be triggered in two ways, the Bremsstrahlung (continuum) or the characteristic x-ray emission. Bremsstrahlung X-rays result from electrons that lose energy due to the deceleration when it interacts with an atom (figure 3.12 (a)). The emission is a continuous energy spectrum of X-ray photons as shown in figure 3.12(c). Characteristic X-rays are the X-ray emission caused by the transition of an electron from an inner shell towards the outer shell. This transition occurs due to the hole created in the first place by high-energy incident electron (figure 3.12(b)). The energy of the X-ray emission is equivalent to the energy level difference between the outer and inner shell electron involved in the transition. Hence, the characteristic X-ray emissions are seen as sharper peaks in the spectra with the wavelength or energy representing the energy of the atom from the shell or subshell concerned (figure 3.12 (c)). This can be then translated into determining the nature of the atom and inner shell that is involved in the process.
Figure 3.12: (a-b) The process of Bremsstrahlung and characteristic x-rays emission and c) an example of EDX spectrum characterizing the Bremsstrahlung and the characteristic x-ray emission energy/wavelength.
3.5.3 Qualitative and quantitative EDX analysis
A qualitative method offers a quick and an effective way to present the elements of the samples. In general, the qualitative method is straightforward, indicating elements of the samples by their characteristic X-ray peaks. This process can be automatically done with the
help of analysis software such as Analysis Station Software, which is used in this thesis. The intensity of the peak can be compared qualitatively from one sample to another as the value is close to the expected signal emitted from the sample. Qualitative analysis has been carried out in this thesis for determining the elements in the Au-particle. This is due to the fact that, it is difficult to have a standard reference for such alloy composition. As a general rule, the peak that will be considered has to be significant (above background level) and this usually has to be manually checked to be assured that the automatic qualitative system done by the software is correct. Figure 3.13 shows an example of the spectrum taken from a spot EDX analysis on the Au-particle alloy. From the spectrum it is clearly seen that there is no significant Ga peak, hence the Ga element shall not be considered to be present in this analysis. This step is important for element pre-selection for the quantitative analysis.
Figure 3.13: Representative of an EDX spectrum of a Au particle showing the selection of
In, Ga, As and Au (element selection are highlighted in green). The Ga and As peaks are by common sense should not be included in the analysis due to low counts almost levelled with the background level.
Quantitative analysis involves measuring the intensity of the spectral peaks corresponding to pre-selected elements for both samples and standards under known
operating conditions, calculating intensity ratios (k values), and converting these k values
into chemical concentration. The k values are the constant retrieved from the known
standard of two elements, A and B with the following equation : !!
3. Experimental techniques
The ratio of X-ray intensities, IA and IB for two elements A and B, is independent of thickness
and can be simply related to the corresponding weight or atomic fractions of the elements,
𝐶! and 𝐶! where 𝑘!" is a factor (the k value) that relates to the efficiency of X-ray
production and the detection of the X-ray. K values vary with microscope condition hence will be most accurate when measured in the same system as the sample to be analysed. This ratio method is only valid for thin samples, such as those used in TEM. In SEM, absorption and fluorescence effect has to be taken into account since the samples are thick and these effects are not negligible.
There are two methods of quantitative analysis; standardless quantitative and
standard quantitative. Standardless quantitative uses the k values provided by the standard
sample measured elsewhere in a different system provided by the analysis software. On the
other hand, standard quantitative requires one to determine the k values using a standard
sample measured by the same system with the same conditions. In this thesis, all the EDX
data are collected using the same STEM setup. The k values are determined using GaAs and
InAs nanowires as the standards. First, GaAs and InAs nanowires are statistically measured
by considering Ga (L), In(L) and As (L) peaks. Then the k values are determined (in Analysis
Station software) for various size (30 -250 nm) nanowires. The average k values are used
since they are found to be independent of the nanowire diameter as shown in figure 3.14. This result strengthens the fact that thin samples can be quantified without any absorption or fluorescence effect taken into account for samples up to 300 nm thick using 200 kV
electron beams. The average k value is used for InAs and GaAs with As as a reference point
for the entire InGaAs samples to ensure uniformity across all measurements. For InGaP, similar method was carried out using InP and GaP nanowires considering Ga (L), In(L) and P (K) peaks. The final composition of InGaP is determined using P as a reference. It is found that there is a random error of about ±4% for reference sample contributing to similar error for the calculated data. Random errors could come from non-uniformity of the sample shape, crystal structure and facets of the nanowires.
Figure 3.14: Graph of k values for a) GaAs and b) InAs nanowires versus diameter. Arsenic
is taken as a reference point for the calculation of the k values and the average k values for both InAs and GaAs are used for InGaAs alloy composition using Analysis Station software.