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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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The first and most frequently used was a device from MMR with a H-50 electrical controller and the K-20 temperature controller, the device uses a magnetic field of 14 Tesla and DC to power the magnets and measurements. In this device the sample was required to use the so called van der Pauw gemotry, which can be seen in Figure 2.4. In the MMR device the probes were not soldered directly unto the contacts which simplified operations but decreased the quality of the connection. The second device that was tried was from PPMS which supports an alternating magnetic field and AC-power use in the measurements. In this device the probes was soldered onto the contacts for better connection and a bridge type geometry was used to increase the sensitivity, which can be seen in Figure 2.5.

2.4.3 X-ray diffraction X-ray diffraction (XRD) was used to determine the crystal structure of the thin films. It is a fairly quick technique which can be used to determine which structure is present but also physical information such as distance between different atomic layers which also gives information about the dimensions of the unit cell.

XRD is a common technique to identify the structure of crystalline materials, using the theory of atom - x-ray and x-ray - x-ray interaction. When a material is illuminated by an x-ray source the energy intensive x-ray photons are reflected by crystal planes in the material. The photons then, depending on the structure of the material, interact with each other. The simplest way of describing this is by using Bragg’s Law which gives the diffraction conditions,

2dhkl sin(θ) = nλ (2.3)

Where dhkl is the distance between the atomic planes in the material, θ is the angle between the sample and the intensity peak, n is an integer and finally λ is the wavelength of the radiation used in the measurement. However, for a fuller understanding and predictions of intensities advanced Fourier analysis is required.

There are several different setups for XRD-analysis and in this report two slightly different setups were used: Grazing Incidence XRD and XRD. The difference in the setups is the angle of incidence for the x-ray beam, for the ordinary XRD the incidence angle was 5 degrees while in the GIXRD setup it was 0.5 degrees. The GIXRD setup is more appropriate for thin-films while XRD is more suitable for bulk materials, however for the setup with controllable temperature only the XRD setup was available. In this setup there was a sample holder made out of a Pt-Ru alloy, this sample holder was also crystalline so in these experiments several background peaks can be observed.

2.4.4 Energy dispersive x-ray spectroscopy To confirm the compositions of the films, Energy Dispersive X-ray Spectroscopy or simply EDS, was used because it is a fairly easy and well known method. The accuracy of EDS has been compared RBS measurements on Mg2 NiH4 -thin films and it has proven to give the same results [18]. The electron microscope used in this paper was a Hitachi S-4800 equipped with a Noran System SIX detector for EDS measurements.

In EDS high energy electrons are sent towards the atoms and kicks out electrons orbiting the cores in the inner shells, an electron in an outer shell then subsequently falls in to take its place.

In this process the electron looses potential energy which is emitted as x-ray radiation. The energy of the emitted photons depend on which atomic specie, from which shell it falls from and also to which shell. A schematic overview of the process can be found in Figure 2.8

Figure 2.8: Schematic overview of the processes involved in EDX.[22]

The perhaps biggest drawback of EDS is that the electronic configurations of many elements are quite similar, this leads to that the energy of the emitted X-rays are in some cases also quite similar. This means that if certain elements are present in a sample it’s impossible to quantify the amounts of these elements. The second major drawback is that lighter materials such as hydrogen are invisible in the EDS-spectra. However magnesium and nickel are fairly heavy and does not overlap significantly with other atoms which makes it possible to detect and quantify them.

2.4.5 Band gap calculations To calculate the band gap in the material, a method developed by E. Centurioni was used. It uses the transmission and reflection spectra with the refractive index of the material to calculate the actual absorption and band gap. To get the refractive index a technique called ellipsometry is used, however no such measurements were conducted in this work so previous ellipsometry measurements were used. Unfortunately, refractive index data have only been measured for certain magnesium contents and structures such as: a-Mg1.9 NiH4, a-Mg2.1 NiH4, c-Mg2.2 NiH4, a-Mg2.3 NiH4 and c-Mg2.4 NiH4. So to calculate the band gap for the compositions where no refractive index data were available the closest composition was chosen.

The algorithm in the program is based on the internal light flux in multilayered materials, instead of the light intensity which is used in other techniques. However in this work the program was used to calculate the band gap, to calculate the optical band gap the reflectance, absorption and refractive index data was used to calculate the so called Tauc plot. In the Tauc plot the linear section is identified and the optical band gap is calculated by letting the tangent of the linear intersect the x-axis.

More information about the method can be found in the paper ’Generalized matrix method for calculation of internal light energy flux in mixed coherent and incoherent multilayers’ by E.

Centurioni. [1] 2.4.6 Spectrophotometry Spectrophotometry was the technique used to obtain the absorption and reflection spectra of the deposited samples. This technique was used because it’s essentially the only way to obtain these spectra. It is also a very common technique in almost all research areas were optical properties are interesting. The technique is straight forward and simple: first two optical cords are mounted on the sample holder one connected to the light source and the other connected to the light detector. The next step is to create a reference, both without the light source and then a reference of the light source itself, now the sample is put in place and the transmission and reflection spectra is acquired simply by changing the location of the light source.

To treat the data the program ’Spectrasuite’ was used with an acquisition time of 0.3 s, and three averaged acquisitions for each result. The light sources used were a UV-VIS source and a infrared source the UV-VIS operated in the region 198-998 nm and the infrared 899-1719 nm.

3 Results

3.1 Synthesis of Mg2.xNiH4 Thin-films with three different magnesium contents were synthesized using reactive co-sputtering described in Chapter 2.3 with a Leybold sputter system A550v7. The different compositions were Mg2.0 NiH4, Mg2.2 NiH4 and Mg2.4 NiH4, the settings used to achieve the different compositions can be found in Table 3.1.

The hydrogen content was not experimentally determined in this thesis, but the nickel:hydrogen Table 3.1: General processing parameters for deposition of the different samples.

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ratio is assumed to be close to 1:4. This assumption is based on the fact that the obtained XRD patterns is consistent with Mg2 NiH4 and also due to the fact that computational studies shows that Mg2 NiH4 is the only stable configuration and if there is a lack of hydrogen atoms then there will be regions of Mg2 NiH4 and regions of metallic Mg2 Ni [5]. However it is possible that there could be small variations in the hydrogen concentration, but this and the possibility of metallic Mg2 Ni-regions will be discussed later. The thickness of the films were measured using a prolometer, KLA-Tencor alpha-step 200, to 400 nm for Mg2.0 NiH4, 400 nm Mg2.2 NiH4 and 410 nm for the Mg2.0 NiH4 sample.

Samples for electrical characterisation were made by the Mg2.0 NiH4 and Mg2.4 NiH4 samples, the aluminum contacts deposited was determined to have a thickness of 1 micrometer.

To confirm the compositions EDS was used. The results are presented in Table 3.2 as mean values of three measurements over different areas on the samples.

Figure 3.1: Image of the deposited Mg2.

0 NiH4 -thin films; A1 - As-deposited, A2 - Annealed at 220C and A3 - Annealed at 290C.

Figure 3.2: Image of the deposited Mg2.

2 NiH4 -thin films; B1 - As-deposited, B2 - Annealed at 220C and B3 - Annealed at 290C.

Figure 3.3: Image of the deposited Mg2.

4 NiH4 -thin films; C1 - As-deposited, C2 - Annealed at 220C and C3 - Annealed at 290C.

Table 3.2: EDS results for the Mg and Ni composition of the synthesized films.

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GIXRD results were obtained for all compositions, samples A1 - A3, B1, B2, C1 and C2 didn’t give any diffraction pattern, which is expected from previous research. These samples will be described to be amorphous, however measurements with TEM shows that the as deposited state is actually consistent of nanocrystals of the monoclinic structure [13]. The two diffraction patterns that were obtained from sample B3 and C3 are presented in Figure 3.4 and compared with litterature data in Table 3.4. Where 2theta simply is the diffraction angle multiplied by two.

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To investigate if any structure change occurs at higher temperatures XRD measurements of Mg2.4 NiH4 were conducted at 300C. One should note that in the XRD measurement a higher angle of incidence is used compared to GIXRD, which leads to that the peaks with higher indices should not be visible in the spectra. There is also a slight difference in the alignment so the XRD results at room temperature differ slightly from the GIXRD results at room temperature.

The sample holder leaves a significant background signal which is marked in the spectras.

Table 3.5: Comparison of obtained peaks at 300C and room temperature for Mg2.

4 NiH4 with reference data [29].

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Figure 3.5: Comparison of the XRD-patterns for Mg2.

4 NiH4 at 300C and at room temperature with the background from the sample holder marked.

holder unfortunately almost coincides with the peaks from the sample which makes it hard to evaluate the intensity of the signal from the sample.

3.3 Electrical properties of Mg2.0NiH4 and Mg2.4NiH4 It proved very difficult to get any Hall-results for the samples, attempts were made att two different setups but neither the MMS or the PPMS could give any clear results with regard to charge carrier and mobility. Attempts were also made on the bridge type geometry but the same problem appeared.

Table 3.6: Result of Hall-measurements performed on a crystalline Mg2.

4 NiH4 sample at 325K.

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Where d is the thickness of the sample and Rmeasured is the measured resistance.

Table 3.7: Measured resistivities of the deposited films in ohm*cm.

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The result in Table 3.7 shows that the resistance is strongly dependent on annealing temperature where the Mg2.0 NiH4 shows the most drastic increase upon heating going from the lowest to the highest resistivity. It seems also as if deviations from Mg2.0 results in increased resistivity. To get a feeling if these resistivities should be considered high or low among semiconductors one can compare them with the resistivities of silicon. Intrinsic, or pure, silicon have a resistivity of about 105 ohm*cm which decreases to 10−3 for heavily doped silicon. So the measured resistivities lies in that region except for the annealed Mg2.0 NiH4 samples.

3.4 Calculated bandgaps of the deposited thin films The band gaps were calculated for all compositions and also for the heated state.

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The band gap of the as-deposited states are roughly the same independent of magnesium content, and the general trend is that the band gap increases with annealing. Which is most strongly seen where there is a structure change from amorphous to crystalline. Upon heating the band gap seems to shrink compared to the state at room temperature.

3.5 The thermochromic effect in Mg2.4NiH4 When the Mg2.4 NiH4 samples with crystalline structure were heated up to around 200C a reversible change in color were observed, which can be seen in Figure 3.6. The change in transmission and reflection were also recorded, which is demonstrated in Figure 3.7 and 3.8.

Figure 3.6: Image of the Mg2.

4 NiH4 sample at different temperatures; A: Room temperature, B: 100C, C: 200C, D:300C

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Figure 3.8: Reflection spectra of Mg2.

4 NiH4 sample heated to 300C and at room temperature.

As seen the reflection spectra hardly changes upon heating, however the transmission spectra changes drastically upon heating which is seen as a color change in the samples and a narrower band gap in the band gap calculations.

3.6 Raman spectra results For the following Raman measurements the settings used can be found in Table 3.9.

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Below are spectra from two different regions of interest, first is the low energy region and the region were the translation modes, phonons and acoustic modes are found. To identify the peaks an article by Parker et al is referred to, one should however keep in mind that the reference peaks are from nanoparticles.

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From Figure 3.9, 3.10 and 3.11 one can see that the spectra from the as-deposited samples look very similar but is quite different from the reference spectra. No change is seen in the Mg2.0 NiH4 upon heating, however the Mg2.2 NiH4 and Mg2.4 NiH4 samples annealed at 290C is quite different and are more similar to the reference spectra while the same samples only annealed at 220C almost retains the spectra of the as deposited samples.

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