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«c SIMON LINDBERG, 2013 Cover: To the left is an image of a mono crystalline silicon solar cell [27], In the middle is an image of three samples of ...»

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To identify the presence of impurities in the films, but also to investigate the bonding present in the material, Raman spectroscopy measurements were conducted on all samples. To start with the spectra from the amorphous materials, they differ significantly from the literature spectra and also from the spectra from the crystalline samples. All of the spectra from the different amorphous samples look very similar, no clear change is visible which would indicate a change in the crystallite size. However there are two significant features that need to be discussed. The first feature is the very sharp peak at 182 cm−1 which is not present in the literature and is very sharp.

It was first assigned to the Mg-translation peak, but the sharpness of the peak led to a discussion of its origin, and after investigating different possibilities it was concluded to most probably be a plasma line artifact from the HeNe-laser. As such this peak does not belong to the sample itself but to the laser, so it has been excluded from any further discussions. The second interesting feature, or maybe more a lack of feature is the absence of the Ni-H bond peaks. This was very surprising and could first not be explained by other than that the NiH4 -complexes maybe wasn’t present in the material. But after discussing the question with the technician specialized in Raman spectroscopy at the department the issue of the magnitude of the band gap came into light.

The amorphous samples have band gaps between 1.53eV to 1.85eV which is less than the laser energy at 1.96eV. When the band gap is narrower than the laser this might lead to certain effects in the Raman spectra, due to the fact that the laser can excite electrons across the band gap. This is most probably the reason why the Ni-H is missing.

On the other hand, Raman spectra from the crystalline Mg2.4 NiH4 and Mg2.2 NiH4 samples are very similar and both are more or less consistent with the literature data. However there are still deviations and peak assignment is difficult in the low energy region, while the Ni-H peaks appear as expected. The band gap of the crystalline samples is in the region of 2.04eV to 2.15eV so the laser is unable to excite the electrons and the laser energy is very close to the excitation energy. These are conditions that leads to a so called Resonant Raman mode, where a strong enhancement of the Raman signals occurs. The theory behind this phenomenon can be found in the book ’Semiconductor Physics’ [7].

Since no clear peaks from oxides or other contaminations were detected in the Raman spectra of the deposited samples, two samples were exposed to distilled water to force the films to oxidize.

During the process, small gas bubbles formed on the surface and a film seemed to be formed at the surface, which changed the color of the film slightly. Then Raman spectra were obtained for both samples and they showed that in the crystalline sample the Ni-H peaks disappeared and in the sample from the amorphous sample exposed to water there was a new peak present at 550 cm−1. This could be a sign that when the films are oxidized in water the NiH4 complexes changes into NiO, however more investigations should be done before any conclusions can be made.

To summarize the results from the Raman measurements, the spectra of the amorphous materials show little resemblance with the previous measurements found in the literature. Most notably is the lack of peaks for the Ni-H bond, which would indicate that these bonds are missing in the material. However the relationship between the energy of the laser and the band gap plays an important role possibly enhancing the Raman signals, which in the case of the amorphous materials might lead to the fact that the Ni-H peaks are too weak. No presence of oxide bonds or other unwanted contaminations can be assured in any of the samples, this might be due to very small amount or that some of the impurities simply aren’t Raman active. As for the increase of resistivity in the Mg2.0 NiH4 samples; if it was due to a change in crystallite size, this should be seen in the Raman spectra if Mg2.0 NiH4 interacts the same way as silicon.

The last thing that will be discussed is the difference in band gap between the different compositions and structures, which is most striking in the Mg2.2 NiH4 and Mg2.4 NiH4 samples. This change in bad gap simply by going from amorphous, or nanocrystalline, to crystalline is not seen in other materials such as silicon. In silicon there is only a subtle change in the band gap between amorphous, nanocrystalline, microcrystalline and crystalline structures. So why is the change so big in this case? The answer probably lies in the fact that inside the crystallites in the amorphous samples the monoclinic structure is present. The monoclinic structure has a different band structure, and band gap compared to the cubic structure.

5 Conclusions and outlook Now it is time to draw some conclusions based on the results in this thesis and to answer the questions stated in the introduction. Let’s start with the deposition of the thin films, where three different compositions were synthesized: Mg2.4 NiH4, Mg2.2 NiH4 and Mg2.0 NiH4. The compositions were determined by EDS and showed to be quite uniform with small compositional variations. Two thin films with the composition Mg2.4 NiH4 and Mg2.2 NiH4 were successfully annealed and crystallized irreversibly to a monocrystalline cubic structure at 290C which confirms the results from previous work on Mgx NiH4 gradients. An intermediate orange state was also found when the Mg2.4 NiH4 and Mg2.2 NiH4 samples were heated to 220C, this state was found to be amorphous from XRD-measurements and is not described previously in the literature. However XRD-measurement showed no signs of structure change in the Mg2.0 NiH4 samples which confirms that an excess of magnesium atoms are required to achieve the crystalline structure.





The second question was regarding the presence of impurities and defects in the thin films, which were investigated with XRD and Raman spectroscopy. And we can conclude that no impurities in the form of hydroxides, MgO or NiO were found in the Raman spectras, but since not all peaks were identified to 100% it could be possible that other impurities such as metallic clusters or foreign atomic species could be present, but not identified in the spectra, so more research is required. However Raman measurements om thin films immersed in distilled water showed signs of being oxidized and possibly forming NiO, but again more research such as XRD-measurements on the oxidized films would be interesting. The results from the XRD measurements performed on the crystalline Mg2.4 NiH4 and Mg2.2 NiH4 samples was quite consistent with literature data, and the small change in the position of the peaks could be explained by a small change in the dimensions of the unit cell, which could indicate that there are some strain present in the material. The most important difference between the reference and the obtained data is a peak at 19 degrees which is present in our samples but not in the reference. This peak could be assigned to foreign compounds such as oxides present in the material, or structural defects. However more peaks should be present if there is another crystal structure mixed with the cubic Mg2 NiH4 structure. The peak also showed that it had some long ranging characteristics because it was not present in the XRD-data from measurements taken with a bigger incidence angle.

The final question on the structure is how the hydrogen atoms are distributed in the material.

Raman spectroscopy was used once again to investigate the presence of different bonds in the material. Ni-H bonds were detected in the two crystalline samples, but it is hard to say if the bonds change somehow with different magnesium content. However no Ni-H bonds were detected in the amorphous materials, and these Raman spectra differed significantly from the reference spectra. This was a quite unexpected result and can probably be explained by the relationship between the laser used and the magnitude of the band gap, where the band gap is smaller than the laser in the amorphous materials. But most likely they are bonded in a similar way as in the crystalline samples, however to confirm this a Raman measurements with weaker laser is required.

The next step was to investigate the physical properties, firstly the band gap of the different samples were calculated using transmission and reflection spectra from spectrophotometry. The result showed that the as deposited samples for all compositions had quite similar band gap with a slightly smaller band gap for the Mg2.0 NiH4 sample. The band gap for the Mg2.0 NiH4 sample were also almost unaffected by heating. That cannot be said about the Mg2.4 NiH4 and Mg2.2 NiH4 samples, upon annealing the band gap increased both in the samples annealed at 220C and 290C where the band gap of the orange samples were in between the as deposited samples and the crystalline samples. The band gap of the as deposited and crystallized samples is consistent with previous measurements. The big change in the band gap between the annealed Mg2.2 NiH4 and Mg2.4 NiH4 samples is probably explained by a change in the crystal structure from a monoclinic structure inside the crystallites in the as deposited structure to the cubic structure in the annealed samples.

What conclusions can be made regarding the resistivity to magnesium content and annealing process and how does it compare to previous measurements? The Mg2.0 NiH4 and Mg2.4 NiH4 samples showed very different characteristics, the Mg2.0 NiH4 sample measured a very low resistivity in the as deposited state which increased drastically upon heating and the highest resistivity measured was found in the Mg2.0 NiH4 sample annealed at 290C. For the Mg2.4 NiH4 sample the resistivity hardly changed upon annealing. The big change in resistivity for the Mg2.0 NiH4 sample could be explained by a change in size of the crystallites, however this should be seen in the Raman as broadening of the peaks but no such change is seen. Compared to previous data the resistivities are very high in all cases compared to both capped films investigated previously [4] and measurements done on uncapped gradients [13].

The Hall-measurements that were conducted in the thesis gave some contradicting and interesting results, essentially the Hall-current seems to be to weak to measure by the devices we used making it impossible to determine the sign of the charge carrier mobility and density. This is contradicting previous Hall-measurements conducted on Pd-capped thin films, which describes the Mg2.0 NiH4 as a heavily doped semiconductor.

During the annealing process of the samples we noticed that the crystalline Mg2.2 NiH4 and Mg2.4 NiH4 samples changed to a dark red color upon heating. This thermochromic effect was investigated with spectrophotometry and XRD analysis, the conclusions that can be made are that the change is reversible and that the band gap is decreased with almost 0.3eV when heated to 300C. The structure of the heated sample was measured with XRD to see if any change occurs, unfortunately there is a background signal from the sample holder which interferes with the peaks from the sample. However a broad peak appears at 17 degrees which cannot be identified and is not present in other measurements. The shape of the peak itself is quite broad and it is hard to say what could be the reason behind this peak, another attempt with a different setup would be interesting both to investigate the unidentified peak and to avoid the peaks from the sample holder. Hydrogen desorption was also ruled out to be the reason behind the color change since it is reversible. So the color change is probably due to a change in the positions of the hydrogen atoms, it would be interesting to investigate this further using neutron scattering on heated samples.

Which conclusions can be made on the relationship between resistivity and structure results?

The Raman spectra result do not give any indication on the presence of any impurities in the form of oxides or hydroxides, when it comes to metallic clusters or other impurities it is difficult to draw any conclusions due to the fact that some of the peaks were shifted when compared to the reference data. The peak at 19 degrees in the XRD results indicates that something unidentified is present in the structure, which would lead to an increase in resistivity either if it is from impurities or defects in the thin films. It is also most likely that the targets used in the deposition process have to low purity to successfully deposit semiconducting thin films and substitution defects could be fatal to the transport process and physical properties of the charge carriers without being detectable in Raman or XRD.

What is the outlook of the future research regarding Mg2 NiH4 ? From this thesis one might say that the possibility of introducing Mg2 NiH4 as a new semiconductor material in the solar cell industry is rather slim. However before ruling it out completely I would say that some issues need to be further investigated. First of all a the impurities in the material needs to be more thoroughly investigated, I would suggest that methods such as EXAFS and XPS should be used to see what unwanted atomic species are present in the material. Also I would suggest that a residual gas analyzer should be invested in for the sputtering system to actually now what is left in the chamber during the depositions. To detect structural defects further TEM measurements would be interesting. I also think it would be interesting to investigate the semiconducting properties of Mg2 NiH4 in the form of nanoparticles as a component in so called hybrid solar cells.

For better understanding of which role hydrogen plays in the material I would like to see neutron scattering experiments to be performed on samples in the form of thin films, both crystalline and amorphous. But also techniques such as FTIR and FT-Raman would be interesting to investigate the nature of the Ni-H bonds and how the structure of the material affects them.

The last part that would be interesting to investigate is the thermochromic effect seen in the Mg2.4 NiH4 material. In this case I would like to redo the temperature dependent XRD measurements, but also measure the transmission and reflection spectra of Mg2.4 NiH4 samples at lower temperatures. Temperature dependent Raman and spectrophotometry measurements would also be interesting to see what happens when the sample is cooled down or heated up.

References



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