Thermoelectric materials produce an electrical potential when exposed to a temperature gradient. The lack of moving parts makes thermoelectric devices quiet and reliable, and therefore often used for off-grid energy generation. When deposited in the form of thin films, the surface to volume ratio is far larger than for bulk materials, granting them some useful attributes due to quantum electron confinement and phonon-scattering effects. Mg_3Sb_xBi_2-x is a thermoelectric material with good properties close to room temperature. This thesis explores the transport properties of Mg_3Sb_xBi_2-x thin films deposited using dc magnetron sputtering. 

Mg_3Bi_2 films were synthesized between room temperature and 400 C -- a relatively low deposition temperature of 200 C proved to be necessary for single crystal growth to avoid loss of Mg due to its relatively high vapor pressure. Results from energy-dispersive X-ray spectroscopy and X-ray diffraction confirm the loss of Mg at 300 C and above, while at room temperature the film was polycrystalline. The epitaxy was confirmed using X-ray pole figures and computer simulation based on density functional theory. The thermoelectric properties were measured between room temperature and 200 C. Mg_3Bi_2 is a semimetal with low electrical resistivity. The power factor   , where S is the Seebeck coefficient and ρ is the electrical resistivity, had a peak value of 200 µWm^-1K^-2 at room temperature. Through measuring the carrier concentration and mobility, it has been confirmed that the decrease in power factor as the temperature increases is due to the bipolar effect -- the small bandgap of the material being insufficient to prevent the minority carrier excitation. The electrons and holes neutralize, decreasing the net current, and therefore limiting the Seebeck coefficient. Decreasing the bipolar effect may be possible by increasing the majority carrier concentration and enlarging the band gap of the material. 

Mg_3Sb_2 is a semiconductor -- it has significantly higher electrical resistivity and Seebeck coefficient as compared to Mg_3Bi_2. Alloying both materials together can result in a semiconducting material with appreciably lower thermal conductivity. This is especially important for Mg_3Bi_2 which as a semimetal has high thermal conductivity. Furthermore, the increase in Sb content increases the energy gap and shifts the bipolar effect, making the material more suitable for higher temperatures. It must be noted that low band gap is more desirable for room temperature materials than for high temperature thermoelectrics. Five Mg_3Sb_xBi_2-x samples were synthesized at 200 C with x ranging from 0.00 to 1.19. Sample characterization included composition, crystal structure and transport properties. An Sb peak has been observed in X-ray diffraction results for x ≥ 1. The precipitation of Sb due to minute deficiency of Mg in those samples can affect the properties -- the focus of the study became x < 1 instead. Higher Sb content results in a more polycrystalline structure, higher band gap, electrical resistivity and Seebeck coefficient. For room temperature implementations, high Bi content is advantageous. This work has prospects for further study of Mg_3Sb_xBi_2-x based thin films and their synthesis on flexible substrates, which are often sensitive to higher temperatures.

Termoelektriska material kan omvandla temperaturgradienter till elektrisk spänning. Termoelektriska anordningar innehåller inga rörliga delar och är därför tysta och pålitliga, de används därför ofta för att generera elektricitet utanför elnätet. Det höga förhållandet mellan yta och volym för tunna filmer gör att vissa förmånliga kvantmekaniska effekter kan utnyttjas. Mg_3Sb_xBi_2-x är ett termoelektriskt material med goda egenskaper nära rumstemperatur. I detta arbete undersöks syntes och karakterisering av tunna filmer av Mg_3Sb_xBi_2-x tillverkade genom dc magnetron sputtring. Epitaxiell tillväxt av icke-dopade tunna filmer av p-typ är av vikt för fördjupad förståelse av fundamentala fysikaliska processer relaterade till Mg_3Bi_2-baserade tunna filmer. 

Tunna filmer av Mg_3Bi_2 har producerats vid olika temperaturer mellan rumstemperatur och 400 C. En relativt låg temperatur, 200 C, visade sig vara nödvändig för att åstadkomma enkristaller, högre temperaturer ledde till förluster av Mg på grund av dess relativt höga ångtryck. Resultat från röntgenspektroskopi och röntgendiffraktion bekräftar förlust av Mg vid temperaturer över 300 C samt att filmen blev polykristallin vid deponering i rumstemperatur. Epitaxiell tillväxt bekräftades genom polfigurer och täthetsfunktionalteori modellering. De termoelektriska egenskaperna mättes mellan rumstemperatur och 200 C. Mg_3Bi_2 är en halvmetall med låg elektrisk resistans. Materialets så kallade effektfaktor   , där S är Seebeck-koefficienten och   är resistansen, nådde sitt högsta värde 200 µWm^-1K^-2  vid rumstemperatur. Genom att mäta koncentrationen av laddningsbärarna och dess mobilitet kunde det bekräftas att minskningen av effektfaktorn med ökande temperatur beror på den bipolära effekten. Detta yttrar sig genom att det smala bandgapet i materialet är inte tillräckligt för att undvika excitation av minoritetsbärare vid högre temperatur. Genom att öka bandgapet kan den bipolära effekten minskas. 

Mg_3Sb_2 är en halvledare och har därför betydligt högre resistans och Seebeck-koefficient än Mg_3Bi_2. Genom att legera dessa material med varandra fås ett halvledande material med betydligt lägre termisk ledningsförmåga. Detta är speciellt viktigt för Mg_3Bi_2, som är en halvmetall och därför har hög termisk ledningsförmåga. Genom att öka halten Sb ökas även bandgapet och motverkar därmed den bipolära effekten vilket gör materialet mer lämpat för högre temperaturer. Små bandgap är mer lämpade för rumstemperaturtillämpningar än för tillämpningar vid höga temperaturer. Fem prover av Mg_3Sb_xBi_2-x, där x ligger mellan 0.00 och 1.19, synteserades. Dess komposition, kristallstruktur och transportegenskaper karaktäriserades. En diffraktionstopp från Sb kunde observeras för x ≥ 1. Utfällningar av Sb på grund av små förluster av Mg i de proverna kan påverka de termoelektriska egenskaperna, studien fokusserades därför på x < 1 istället. En högre halt av Sb resulterar i en polykristallin film med högre bandgap, resistans och Seebeck-koefficient. För rumstemperaturtillämpningar är det fördelaktigt med en högre halt av Bi. Den här studien möjliggör vidare undersökningar av Mg_3Sb_xBi_2-x filmer deponerade på flexibla substrat som ofta är känsliga för höga temperaturer. 

Thermoelectric thin films are a promising power source for wearable devices, an alternative to batteries offering a continuous, long-lasting flow of electricity with improved mechanical flexibility compared to its bulk counterparts. They are quiet and reliable off-grid solution for generating electricity when exposed to a temperature difference. The exposure to numerous kinds of stress is expected for many applications. As a flexible device is deformed in various ways, its transport properties are required to remain relatively unaffected. Furthermore, depending on the synthesis method, intrinsic stress can be significant. Magnetron sputtering allows to control the magnitude of the intrinsic stress to a certain extent. Stress can be further influenced through adjusting variables such as the substrate, synthesis temperature and deposition rate. It can be also externally induced by bending or stretching the sample.

Despite numerous studies showing the potential benefits of controlling stress, it is often overlooked when reporting thermoelectric properties of a material. Depending on which aspect of stress is of interest, a suitable substrate can be chosen; one with minimal lattice mismatch for epitaxial growth, a selection based on the thermal expansion coefficients to vary the thermal stress, or a flexible substrate for researching the effects of tensile stress. Because thin films have small cross-sectional area relative to bulk materials, studying large stresses is possible applying relatively small forces. This makes them not only convenient to work with when it comes to creating microdevices and sensors for Internet of Things, but also the perfect objects to study effects of stress.

This work explores the effects of stress on the thermoelectric properties of Mg3Bi2-based thin films. To create a benchmark, a single crystal Mg3Bi2 thin film was synthesized, with its composition, structure and transport properties characterized. Later, small defects were introduced into the lattice by substituting some Bi atoms with Sb, creating polycrystalline films. The films had highest power factor around the room temperature, which became the focus of this work. The thermal stress was adjusted by selecting various substrates with wide range of thermal expansion coefficients, and synthesizing the Mg3Bi2 thin films at different temperatures - the estimated compressive stress reached up to 250 MPa on thermoplastic substrates, while the tensile stress went up to over 150 MPa on crystalline substrates. High compressive stress was observed to change the nature of major charge carriers from positive to negative. The measured power factor varied by four orders of magnitude due to stress, which could explain the wide range of reported values for same material systems in the literature.

This work also focuses on the thermomechanical stability of the amorphous TiNiSn thin films, which is flexible when amorphous, but brittle when crystalline. Unlike Mg3Bi2, this material is resistant to oxidation, and therefore suitable for studies even at high temperatures. The tensile force acting on the sample was altered at various temperatures, observing the changes in the structure of the material using synchrotron radiation. Up to 250◦C, the sample remained stable and virtually unchanged under tensile stress of approximately 3.2 GPa and below. The crystallization occurred at 300◦C when stress of 1.0 GPa was applied- which is over 200◦C below the reported crystallization temperature for TiNiSn without stress. The degree of the crystallization appears to be proportional to the applied stress and quite stable under constant forces. This is promising for creating crystalline-amorphous composites at lower temperatures, using stress as an additional control parameter. Altering the Sn content changes the stress required for inducing the crystallization.

The effects of stress were shown to be quite significant when it comes to thermoelectricity, as shown on Mg3Bi2 thin films. Furthermore, stress has allowed to observe crystallization in amorphous TiNiSn thin films at significantly lower temperature than otherwise required. It is often feasible to manipulate the stress, at least to a certain extent, by small alterations to the experiment, such as choosing a substrate and growth temperature. While at times it can be challenging or time consuming to measure the stress in the thin films, there are ways to estimate it. This work highlights various ways to induce, estimate and measure stress, along with the benefits of doing so.