Study On Chalcogenide Thin Films With Chemical Method

Thermoelectric cooling has broad applications because of its stable, silence, non-contamination, low-cost and long life characters. However, the most used chalcogenides for thermoelectric cooling in human life just have a max ZT value of 1. The conversion efficiency of the coolers made up by these materials is much lower than compression refrigerators. Theory proves that, the conversion efficiency can catch up with home refrigerators if the ZT value goes up to 3. It is surely that the environment-friendly thermoelectric cooling will be broadly in use in each aspect of the society. Recently, with the development of new materials and nanotechnology, thermoelectric materials are now a hot field again in materials research. However, some of these materials are not suitable for cooling because of their lower ZT value under room temperature and low temperature. Although there are some materials have higher ZT value than traditional ones, it is still not suitable for the growing modern technology on the request of thermoelectric coolers.The cooling efficiency should be high under room temperature and low temperature in microelectronics, optoelectronics, laser technology and other high-tech fields. The coolers should be in mini size and could be integrated. Traditional thermoelectric materials cannot meet these demands. In recent years, theories and experiments said that, thermoelectric nanostructures, which contain thermoelectric superlattices, quantum wires and quantum dots, have much higher qualities than bulk materials. The research on how to enhance the thermoelectric properties was began from 1999 in overseas, and has made some progress these years. What they focused on is chalcogenide thermoelectric materials, such as (Bi,Sb)2(Se,Te)3 system materials. The (Bi,Sb)2(Se,Te)3 based chalcogenide materials are one of the most traditional materials groups, which also have the best property under room temperature. The max ZT value of the materials can goes up to 1, and most of the cooling parts are made of (Bi,Sb)₂(Se,Te)₃ materials. The synthesis method of chalcogenide thermoelectric thin films and superlattice is based on physical methods, such as Molecular Beam Epitaxy, MOCVD, and PLD and so on. The shortcoming of these methods is that, the equipment cost very much, longer preparation period, and cannot precisely control the component of the layers. So, there should be a new low cost method, which can quickly synthesize chalcongenide thermoelectric thin films and superlattices. And people can take deep research on superlattie structure, properties and the relations between them. So, how to get the better performance thermoelectric superlattice materials is a serious problem in thermoelectric materials research.Spin coating is a normal method for prepare thin films in industry. It is broadly used in synthesize inorganic optical thin films (such as monitor), ITO, organic optical thin films. So to speak, this method is suitable for synthesize organic thin films and oxide thin films. Here are the reasons:First dissolve the organic materials in the solvent, and then the organic thin film is made after the solvent evaporation. The oxide films are synthesized from the calcination of sol film or salt solutions. What our study is aim on chalcogenides. These materials cannot be made by the methods just mentioned. So, we proposed a spin coating-co-reduction method to synthesize chalcogenide thermoelectric thin films and superlattices. The key point of the method is:dissolve the source materials into organic or inorganic solution, spin-coat the solution, reduce the dried films under a reducing condition, then the final chalcogenide thin films are synthesized. Spin coat different films on each other we can get quantum wells and superlattices.The main parts of the study are:1. After deep research on spin coating and reduction, we chose Bi(NO₃)₃·5H₂O, Sb₂(OCH₂CH₂O)₃, TeO₂, SeO₂ and Pb(NO₃)₂ as the raw materials, ethylene glycol as the solvent, N2H4 as the reducing agent. Bi, Sb, Te, Se and Pb thin films have been synthesized respectively. The XRD result of the Bi film illustrated that, the precursor concentration have an impact on the habit of the film orientation. 2. Bismuth onion structured nanospheres with the same structure as carbon onions have been synthesized. The nanospheres were synthesized through a hydrothermal method using bismuth hydroxide and silicon wafer as reactants. The high pressure in the reaction system played an important role for the formation of bismuth onion structure. The experiments also indicated that the sample has property of capacity.3. "Spin coating-co-reduction" method has been invented. It is a new chemical method contains both spin coating process and reduction process. Bi₂Te₃ thin films have been synthesized through the new method. The XRD, SEM and HRTEM results indicated that, Bi₂Te₃ thin films are made up of nano plates. The nano plates are in shape of hexagons, and with the size of 100-200nm. The nano plates can lie down on the substrate by controlling of the precursor concentration or the spin velocity. Then the substrate was covered by the nanoplates, and formed thin film. The thin film has a c-axis preferential direction. The mechanism for synthesis of Bi₂Te₃ thin film is as follows:Bi(NO₃)3 was decomposed into Bi₂O₃ and gas in the drying process; When the precursor film is heated in a reduction atmosphere, both Bi₂O₃ and TeO₂ are reduced to elemental atoms of Bi and Te; The reduced atoms are very active, and they react with each other and form stable Bi₂Te₃ compound, and finally formed a thin film.4. Bi₂Te₃ thin films have been grown on different substrates. The mismatches between Bi₂Te₃ thin film and Si, LiNbO₂, Al2O₃ were calculated. The results indicates that Si (111), LiNbO₃ (104) and Al2O₃ (006) are the best choice as substrates. Highly c-axis oriented thin films have been synthesized on these substrates. A silicon substrate coated with Au has also been taken as a substrate, the Bi₂Te₃ thin film grown well on it. The thin film also has a c-axis orientation on Au. The precursor were also been studied, the results indicated that propylene glycol is a better choice than ethylene glycol. That is because Bi₂Te₃ thin films grown well if propylene was taken as the solvent.5. Sb₂Te₃ and Bi₂Se₃ thin films were synthesized in order to explore the range of applicability of the method. It is found the same morphology as Bi₂Te₃, both of the two thin films grown along the c-axis. The AFM images of Sb₂Te₃ indicated that the sapphire substrate have a force on the growth on Sb₂Te₃ nano plates. The force restricted the formation direction in plane level of the nano plates. The methanol was taken as the solvent for Bi₂Se₃ based precursor. Because of SeO₂ can be easily evaporated, a two-step method was introduced into the reduction process. First, reduce the wet film under lower temperature, which is 120℃. Then pre-reduce the sample again under 230℃, which is a higher temperature. After that, a c-axis thin film can be synthesized.6. Double layered structure sample was studied. Take Bi₂Se₃ and Sb₂Te₃ as the sample. First, a Sb₂Te₃ thin film was deposited on sapphire substrate, the thin films grown along c-axis. Second, Bi₂Se₃ thin film was deposited on Sb₂Te₃ thin film. Both of the two films are totally matched. The XRD results show that, both of the two thin films grown along c-axis. It is also found that, the peaks of Bi₂Se₃ all shift to the lower angle. The reason is that, the residual stress between the interfaces makes the Bi₂Se₃ lattice constant goes larger.7. The electrical property of Bi₂Te₃ has been tested. A result shown that, if the precursor concentration stayed stable, when the spinning speed went higher, the resistivity would go higher; if the spinning speed stayed stable, when the precursor concentration went higher, the resistivity would go lower. Bi₂Te₃ thin film is N-type semiconductor.In conclusion, the study is based on chalcogenide thermoelectric materials. A new chemical method, "Spin coating-co-reduction" method was invented. Bi₂Te₃, Sb₂Te₃ and Bi₂Se₃ highly oriented thin films were synthesized through this method. All of the experiments are studied for the synthesis of superlattice.