Growth Of VA-VIA Group Compound Semiconductor Thermoelectric Nanofilms Via The Route Of Electrochemical Atomic Layer Epitaxy

Low dimensional or nanoscale thermoelectric materials,which are very promising in many fields such as integrated circuit, microelectronics and optoelectronics,attract much attention in recent years. However, their current methods for preparation are obsessed by many problems such as very expensive precursors, high operating temperature, complicated growing techniques and toxic byproducts. In this dissertation, a new method named as electrochemical atomic layer epitaxy (ECALE), which originates from a combination of electrochemistry deposition and atomic layer epitaxy, in which elemental atomic layers that make up the compound are alternatively under-potentially deposited (UPD) to form the compound nanofilms, is firstly applied to growth Bi₂Te₃ and Sb₂Te₃ VA-VIA compound semiconductor thermoelectric nanofilms. Various methods and techniques, such as XRD, EDX, SEM/FESEM, EPMA, XPS, FTIR and electrochemical analysis are used to study the under-potential deposition behavior of Bi, Sb and Te on substrate and each other, and characterize the structure and composition of nanofilms elaborately. Effects of depositing conditions on film growth are studied and depositing parameters are optimized; ECALE depositing thermodynamics and growing mechanism of nanofilm thermoelectric materials are studied and analyzed. To our knowledge it is the first time that bismuth telluride and antimony telluride compound nanofilms are grown via the route of ECALE and the relevant depositing mechanism is studied and discussed. It is a very meaningful scientific exploration and innovation to carry out this research, and it will be of great importance in developing new techniques and enriching basic theories for thermoelectric materials. Thermoelectric effects, the applications and efficiency of thermoelectric materials have been introduced firstly in section one of this dissertation. Progress in thermoelectric materials has been reviewed in detail. The preparation techniques for thermoelectric nanofilms were surveyed. Some fundamentals of UPD and the characteristic of ECALE were analyzed. Based on the above work, the purpose and significance of this thesis have been pointed out. An automatic ECALE deposition equipment was designed and developed. The pivotal parts such as the automated solution change system; the automated electrochemical thin-layer flow deposition reactor and the electrochemistry measurement system were expatiated. In section two, the electrochemical aspects of the component elements of bismuth telluride and antimony telluride VA-VIA group compound semiconductor UPD on different substrate such as Pt, cold rolled Ag and polycrystal Au substrates and each other are carefully investigated by cyclic voltammetry, anodic potentiodynamic scan and coulometry. The optimal technical conditions are determined for the ECALE process of Bi2Te3 and Sb2Te3 on various substrates. ECALE growing processes for Bi2Te3 and Sb2Te3 nanofilms on various substrates and their thermodynamics mechanism are studied and discussed. Composition and morphology and structure control of deposits are analyzed. Preliminary theoretical investigations have been carried out on thermodynamics of the ECALE process. The effective way of depositing Bi2Te3 on Pt is determined. Bi2Te3 thin films were elaborated by combination of the oxidative Te UPD with the reductive Bi UPD process for the first time. The dependence of the deposit on the potential adjustment slope for depositing the first 30 atomic layers of Bi and Te is examined as well, and the optimal potential adjustment slope is determined as ?6 mV/p. Based on thermodynamic equations, the optimal range of pH over which it is possible to dissolve bismuth and tellurium, meanwhile to protect the structure of the silver surface from forming a silver oxide layer is obtained. It is found that predeposition of a little bismuth can effectively prevent silver from surface oxidation. Vice versa, the presence of the silver oxide layer can inhibit the deposition of Bi. The cyclic voltammetric difference between the UPDs on a naked substrate and an element covered substrate is studied. The UPD thermodynamics of Bi and Te on cold rolled silver is expatiated. Cyclic voltammograms were used to analyze the electrochemical aspects of tellurium and bismuth UPD on Au. The electrochemical aspects of Te UPD on the Bi-covered Au showed that the UPD peak at 0.23V is essentially Te UPD on the Au substrate, while the UPD peak at 0^-0.3V corresponds with the interaction of Te atoms with the adsorbed Bi atoms. Integrated Faradaic charges from the anodic stripping currents were used to determine the deposition potentials for each element. By using buffers, similar concentration and pH for the deposition of bismuth and tellurium, an optimized deposition program is developed. FESEM studies showed the deposits were conformal with the Au substrate suggesting an epitaxial growth mechanism has occurred. ECALE growing process of Sb2Te3 nanofilms on Pt substrates arestudied. A 19nm interface layer develops between the Pt substrate and the growing semiconductor is attained for the first time. The potential adjusting program which is necessary to obtain a steady state deposition condition is set up. The optical energy band gap of the Sb2Te3 compound nanofilms deposited in this work is blueshifted to a higher value in comparison with that of bulk single crystal Sb2Te3 compound. The bluesift of optical band gap may be attributed to the nanocrystalline structure in the Sb2Te3 deposits. Based on the equilibrium thermodynamics, a theoretical calculated model for the submonolayer coverage and the eletrosorption valency is put forward. The equilibrium double layer properties of submonolayer deposition are analyzed. An equation of submonolayer equilibrium potential is settled in comprehensive consideration of UPD submonolayer properties. The formalism differs from traditional Nernst equation which implicitly chooses the activity of the bulk UPD metal as the standard state of unit activity. A theoretical relation has been set up between underpotential and coverage. Preliminary effort has been given to the relationship between the electrosorption valency and underpotential.