In situ X-ray photoelectron spectroscopy is applied to electrochemical lithiation/delithiation processes of an amorphous Si electrode sputter-deposited on a Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 solid electrolyte. After the first lithiation, a broad Li peak appears at the Si surface, and peaks corresponding to bulk Si and Si suboxide significantly shift to lower binding energy. The appearance of the Li peak and shift of the Si peaks confirm the formation of lithium-silicide and lithium-silicates due to the lithiation of Si and native suboxide. The composition of lithium-silicide is estimated to be Li 3.44 Si by quantitative analysis of electrochemical response and photoelectron spectra. Peak fitting analysis shows the formation of Li 2 O and Li 2 CO 3 due to side reactions. Upon the following delithiation, the peak corresponding to Li 3.44 Si phase shifts back to higher binding energy to form Li 0.15 Si phase, while lithium-silicates, Li 2 O, and Li 2 CO 3 remained as irreversible species. Thus, electrochemical reactions accompanied with lithiation/delithiation processes are successfully observed.
The Mg doping efficiency is found to be drastically enhanced in the p-GaN films grown on the free-standing GaN substrates by metal organic chemical vapor deposition. The free hole concentrations are five and ten times higher in the lightly and heavily Mg-doped homoepitaxial p-GaN, respectively, than those in the p-GaN-on-GaN/sapphires grown and activated at the same conditions although the Mg incorporation concentrations {[Mg]} are much lower. The indication of the p-type conductivity in the photoluminescence spectra at room temperature in p-GaN-on-GaN substrates is found to be the dominant ultraviolet luminescence band located at around 3.26 eV. This behavior is different in the heteroepitaxial p-GaN, for which the fingerprint of the p-type conductivity is the emergence of blue luminescence bands at around 2.9 eV. The markedly enhanced activation efficiency is attributed to the suppression of self-compensation centers in the high-quality homoepitaxial films. The Mg-Ga-O disordered layer, which is typically observed on the surface of p-GaN-on-GaN/sapphires due to the Mg diffusion along edge-type dislocations, is also inhibited on the homoepitaxial p-GaN film, which is beneficial for the stable operation of the vertical-type metal-oxide-semiconductor field effect transistors.
The electrochemical lithiation/delithiation in amorphous Si thin film electrodes deposited on a L 6.6 La 3 Zr 1.6 Ta 0.4 O 12 are dynamically analyzed by operando X-ray photoelectron spectroscopy. In the initial lithiation, the Si 2p peak corresponding to bulk Si 0 significantly shifts to a lower binding energy due to the formation of Li x Si and then monotonically with increasing capacity, i.e., the Li content x in Li x Si. When the lithiation stops at capacity of 2200 mAh g −1 (x = ∼2.3), the peak recovers monotonically to a higher binding energy throughout the successive delithiation. When Li is inserted into Li x Si up to 3400 mAh g −1 (x = 3.5), however, the peak drastically shifts in the capacity range of 1520−1920 mAh g −1 (x = 1.6−2.0) in the successive delithiation. This shift is attributed to the phase transition of crystalline Li 15 Si 4 formed in the preceding lithiation to the amorphous phase. The mechanism of initial lithiation/delithiation at each step is summarized on the basis of the state of charge, Li content x in Li x Si, and positions of XPS peaks.
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