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First Principles Calculations in Exploring the Magnetism of Oxide-Based DMS
Published in Jiabao Yi, Sean Li, Functional Materials and Electronics, 2018
Hafnium oxide (HfO2), also known as hafnia, was introduced by Intel in 2007 as a replacement for silicon oxide as a gate insulator in field-effect transistors. It has a wide band gap of 5.3-5.7 eV [117]. HfO2 is isomorphic to ZrO2 and they share the same high-pressure phase transition sequences. HfO2 has a monoclinic structure with the P21/c space group [118], with the increase of the pressure, it then becomes orthorhombic-I (space group Pbca) [119], orthorhombic- II (space group Pnma). The orthorhombic structure is quite stable under 1800 K [120]. HfO2 is cubic with space group Fm3m at high temperature (above 2700 K), which transforms to the tetragonal form with space group P42/n at about 2570 K [121]. Typical structures for HfO2 are listed in Figure 7.11.. Among which, the monoclinic structure attracts the most interest in research.
Multilayer High-Reflectance Coatings
Published in H. Angus Macleod, Thin-Film Optical Filters, 2017
For more demanding applications, particularly where the coating may have to be exposed to a more aggressive environment, hard oxide layers would normally be chosen, silicon dioxide as low index and titanium dioxide, tantalum pentoxide, or niobium pentoxide as high-index materials. Hafnium oxide is frequently used as a high-index material when a high laser damage threshold is a requirement. Levels of absorption of less than 0.5% can be achieved with ease, 0.1% with some extra care and 0.001% with attention to detail. Still lower levels can be achieved and are required especially for the reflecting structures in more advanced narrowband filters, dealt with in a later chapter. In thermal evaporation, the oxide materials demand higher source temperatures, and the simple directly heated boat sources applicable to zinc sulfide and cryolite must be replaced by electron beam sources, described in more detail in a later chapter. Sputtering is an alternative process that has become very popular. Magnesium fluoride, the tough material much used in antireflection coatings for the visible region, has an attractive low index of refraction but does suffer from rather high intrinsic tensile stress and so can be a somewhat unreliable material in high-reflectance multilayers.
Basic properties mapping of anodic oxides in the hafnium–niobium–tantalum ternary system
Published in Science and Technology of Advanced Materials, 2018
Andrei Ionut Mardare, Cezarina Cela Mardare, Jan Philipp Kollender, Silvia Huber, Achim Walter Hassel
Hafnium, niobium and tantalum have similar electrochemical characteristics in that they are all valve metals. This classification and its name are based on their current rectification upon electric field reversal (hence, the name ‘valve’) during metals anodisation under high field conditions. The final anodic oxide thickness is proportional to the applied potential and oxide formation factors of 2.3, 2.6 and 1.8 nm V−1 were measured for Hf, Nb and Ta, respectively [11]. The oxides of the aforementioned pure metals have applications in various fields. Due to its high dielectric constant and excellent thermal stability, hafnium oxide is investigated mainly as a gate material for field effect transistors and supercapacitors, while hafnium oxynitride is studied as a catalyst for oxygen reduction reactions [12,13]. Additionally, Hf1−xTaxO2 based memristors with excellent bipolar resistive switching characteristics were recently demonstrated, which promote the use of mixed Hf and Ta oxides in modern electronics [14]. The spectrum of niobium oxide applications is broad, ranging from use in capacitors to applications in electrochromic devices, gas sensors and solar cells [15–18]. Applications of Ta2O5 are found in the same major areas. Tantalum oxides, usually applied in high power resistors and capacitors, started recently to be investigated for cathodes in fuel cells, lithiation support in batteries and counter electrodes in solar cells [19–22].