III−V semiconductors, such as InAs, with an ultrathin high-κ oxide layer have attracted a lot of interests in recent years as potential next-generation metal−oxide− semiconductor field-effect transistors, with increased speed and reduced power consumption. The deposition of the high-κ oxides is nowadays based on atomic layer deposition (ALD), which guarantees atomic precision and control over the dimensions. However, the chemistry and the reaction mechanism involved are still partially unknown. This study reports a detailed time-resolved analysis of the ALD of high-κ hafnium oxide (HfO x ) on InAs(100). We use ambient pressure X-ray photoemission spectroscopy and monitor the surface chemistry during the first ALD half-cycle, i.e., during the deposition of the metalorganic precursor. The removal of In and As native oxides, the adsorption of the Hf-containing precursor molecule, and the formation of HfO x are investigated simultaneously and quantitatively. In particular, we find that the generally used ligand exchange model has to be extended to a two-step model to properly describe the first half-cycle in ALD, which is crucial for the whole process. The observed reactions lead to a complete removal of the native oxide and the formation of a full monolayer of HfO x already during the first ALD half-cycle, with an interface consisting of In−O bonds. We demonstrate that a sufficiently long duration of the first half-cycle is essential for obtaining a high-quality InAs/HfO 2 interface.
Useful electronic, magnetic, and optical properties have been proposed and observed in thin films of Ti1−xMxO2 (M=Ta,Nb,V). In this work, we have studied phase formation for films of Ti1−xTaxO2 prepared by pulsed laser deposition. We show that substitutional Ta in TiO2 results in a different material system in terms of its electronic properties. Moss–Burstein shift is ruled out by comparing the electrical transport data of anatase and rutile TiO2. Vegard’s law fit to the blueshift data and the high energy optical reflectivity studies confirm the formation of an alloy with a distinct band structure.
Transitional metal doped ZnO is a good candidate for dilute magnetic semiconductors possessing high Curie temperature ferromagnetism. The local atomic configuration of dopant elements in ZnO is an important issue for understanding their ferromagnetic mechanism. In this work Co, Mn, and Cu doped ZnO nanoparticles with particle size of about 5 nm were prepared by the coprecipitation method. X-ray absorption fine structure spectra were measured at doppant metal K-edges for the as-prepared and calcinated samples. The results show significantly different local structural evolutions for various dopant element doping and heat treatment. Co-doped nanoparticles are stable up to high temperature calcinations, while Mn and Cu in ZnO exhibit complex interatomic diffusion and reduction behavior activated by modest calcinations, and this is explained by either a charge transfer from ZnO to doppant element or the reduction induced by thermal decomposition products of surfactants. Multiple scattering calculations were performed on Co substituted ZnO clusters to simulate the Co clustering in ZnO and its effect on the measured X-ray absorption fine structure spectra.
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