I. Electrochemistry and Electrodeposition
11. Precursor formation and characterisation
15.3. Characterisation techniques
Figure 15.2.: Typical annealing temperature profile measured at the graphite box. The controller temperature is indicated by the dotted line.
15.3. Characterisation techniques
A general introduction to SEM, EDX, XRD, and ICP-MS is given in chapter 3. This chapter will discuss specific issues of the techniques related to this part of the thesis and further introduce the principles of AES and EBSD.
15.3.1. Ex-situ and in-situ XRD
The samples measured by ex-situ XRD in chapter 19.2.1 have been annealed in the oven described in the previous section 15.2. The standard conditions of 100 mg Se powder and a H2/N2 background
gas pressure of 10 mbar have been applied. The temperature of the actual annealing step was varied between 250 °C and 550 °C. Afterwards the annealed absorbers were analysed in a θ − 2θ configured XRD set-up (section 3.2).
For in-situ XRD measurements the samples were annealed in a special sample stage which could be mounted to the XRD goniometer (figure 15.3). Grazing incidence configuration with an incidence angle of 4 ° was used to record the diffractograms (section 3.2). The sample was covered by a thin graphite dome which is transparent to X-rays, but causes additional reflections at 28.6 ° and 26.1 °. The graphite dome allows to maintain an inert atmosphere during annealing. The pressure is adjusted by an overpressure valve opening at 250 mbar above atmosphere. No elemental Se has been added in the annealing to avoid contamination and reaction of the stage. The temperature was linearly ramped up from 40 − 50 °C to 550 °C within 77 min. 20 scans of each 4 min were taken during the heat up process, i.e. during each scan the temperature continuously increased by 27 − 28 °C. Afterwards the sample was kept at 550 °C for 30 min. Finally the sample was cooled down to 50 °C again within 58 min. Also during the constant temperature step and the cooling a new scan was recorded every 4 min.
15.3.2. ICP-MS impurity measurements
The trace cation impurities in several samples have been studied by ICP-MS. The samples can be categorised to the following classes:
1. absorbers prepared by PVD in a 3-stage process [130]
15. Experimental
X‐ray source
detector
graphite dome
N
2gas
air cooling
overpressure valveheater
thermocouple
sample holder
sample
Figure 15.3.: Set-up for in-situ XRD measurements 3. precursor stacks and single layers prepared by electrodeposition
All samples have been prepared on soda-lime glass (SLG) and boro-aluminosilicate glass (BAG). The measurement of trace cation impurities requires a more careful sample preparation than for a simple determination of film composition. Especially a long contact between the glass substrate and the acids during the dissolution process of the samples must be avoided. Otherwise the dissolution of the glass substrate increases the impurity level. The dissolution rate of Na from soda-lime glass is about 3 µg/ cm2· h
at pH 2 and is therefore a significant contribution to a thin film sample of 1.4 mg/cm2 [136, 137]. In case of the finished absorbers the absorber layer was scraped with a
highly pure Cu-plate. The obtained powder was afterwards dissolved in acid to obtain a liquid sample for ICP-MS. Scraping was not successful for samples containing an indium selenide layer, because the latter adheres strongly to the Mo substrate. These precursor stacks have been dipped with the sample side into aqua regia. As soon as the precursor and the Mo had dissolved the remaining glass substrate was taken out again. The dissolved samples were processed by ICP-MS as described in section 3.3. 5 blank solutions have been processed in the same way like the sample solutions and defined the background contamination level of the process. Also Mo coated glass substrates without sample coating have been processed to determine the impurity caused by the dissolved Mo layer. The background contamination level was subtracted from the masses measured by ICP-MS. For the precursor stacks and single layers also the Mo impurities have been subtracted.
Although ICP-MS has a detection limit of ∼ 1 ppt for many elements [138], the quantification limit can be much less. Its actual value depends on the element, the preparation procedure, concentration limits to avoid instrument contamination, etc. In the recent procedure especially alkali metals have a high quantification limit of 12 µg/sample. Nevertheless it is still possible to discuss these results qualitatively as long as they are significantly above the background contamination level.
15.3.3. Auger-Electron Spectroscopy (AES)
Auger-Electron Spectroscopy (AES) is a surface sensitive technique to measure the composition of samples. An electron beam is focused on the sample and removes electrons from the inner shells of the atoms. The remaining hole can be filled again by the relaxation of another electron from an outer shell. The energy that is released in the relaxation process can be emitted as X-ray photon (principle of EDX). Another possibility is the exciton of an Auger electron from an outer shell (figure 15.4). The energy of the Auger electron depends on the energies of the atomic shells and is therefore characteristic for each element. The typical energy of the Auger electrons is 20 − 2000 eV. Taking into account the mean free path of electrons in a solid only electrons generated 0.5 − 5 nm away from the surface can escape the sample [52]. In order to obtain depth profiles the sample is ablated by sputtering with Ar ions. The ablation is paused to record the AES spectra.
The AES depth profiles in this thesis (figures 16.1a, 16.1b, 19.3) have been recorded with a primary electron beam of 10 kV acceleration voltage and 1 nA current. The sample was sputtered with 3 kV
15.3. Characterisation techniques
Ar+ ions and a current of 3 µA on an area of ∼ 1 mm2. The sputtering was done at an angle
of about 45 ° to avoid roughening of the sample. The sensitivity factors of each elemental in the composition analysis have been determined on a PVD CuInSe2reference sample (Cu-rich grown and
KCN-etched).
a)
b)
0 eV
E
Figure 15.4.: Auger effect: a) An electron vacancy is created at an inner shell. b) When the vacancy is filled again from an outer shell, an Auger electron is emitted that carries the energy from the relaxation process.
15.3.4. Electron Backscatter Diffraction (EBSD)
Electron Backscatter Diffraction (EBSD) is a technique to obtain microscopic resolved information on the structure of a material [139]. Similar to XRD it can be used for phase identification. In this thesis it has been applied to measure the orientation of crystallites (figure 18.3). EBSD measurements can be performed in an extended SEM set-up. The electron beam is diffracted on the sample and a diffraction pattern is obtained on an area detector (figure 15.5). The orientation of the crystallite at the spot of measurement can be calculated from the pattern. The sample is scanned by the electron beam which results in an orientation map (figure 18.3, coloured map). A primary electron voltage of 15 kV and a current of 5 nA were used to record the EBSD maps.
While SEM cross section pictures can be taken on a simple breaking edge, EBSD maps require a more careful sample preparation. The samples have been mechanically polished, afterwards ion polished and in the end coated with a thin carbon film to ensure electrical conductivity [140].