The erratic p-type conductivity in nitrogen-doped ZnO film is still under investigation and has been debated up to now. In this study, the authors have studied the effect of rapid thermal process (RTP) on the properties of N-doped ZnO films grown by metal-organic chemical vapor deposition. Hall-effect measurements show that the sample is of p-type as the RTP temperature is lower than 350 °C while, as the RTP temperature increased up to 550 °C or higher, the conduction-type of the sample changed to be n-type. Correspondingly, obvious D and G peaks, which are related to graphite clusters, are observed to increase their intensity with RTP temperature, indicating that interstitial or substitutional carbon atoms may migrate to form carbon clusters in the grain boundary during RTP. RTP is also found to lead to significant changes on the photoluminescence of the samples, with enhanced visible emissions observed as RTP temperature increased. Similar changes are observed on the intensity ratios of the D over G peaks and the visible emission around 600 nm over the near-band-edge emission. This indicates that besides zinc vacancy (VZn) and oxygen vacancy (VO), which are popularly ascribed as the origins of the visible emissions around 500 and 550 nm, carbon clusters may be a possible origin of the visible emission around 600 nm. Finally, carbon clusters formed in the grain boundary are also supposed to at least partly be responsible for the type transition caused by RTP.
The fundamental optical properties of Ga-doped ZnO films grown by metalorganic chemical vapor deposition were investigated by room-temperature transmittance and photoluminescence (PL) spectroscopy. The Burstein–Moss (BM) shift of the absorption edge energy is observed at the carrier concentration up to 2.47×1019cm−3. The absorption edges are fitted to a comprehensive model based on the electronic energy-band structure near critical points plus relevant discrete and continuum excitonic effects, taking account of the Fermi-level filling factor. The theoretical calculation for BM effect is in good agreement with the experimental facts, considering the nonparabolic nature of conduction-band and band-gap renormalization (BGR) effects. Meanwhile, the monotonic redshift of the near-band-gap emission detected by PL measurements has also been observed with increasing free-carrier concentration, which is attributed to the BGR effects, and can be fitted by an n1∕3 power law with a BGR coefficient of 1.3×10−5meVcm.
The carrier recombination processes in p-type ZnO epilayers with P monodoping and In–P codoping have been studied by temperature-dependent photoluminescence spectroscopy. Good correlations were observed between carrier recombination and acceptor and donor energy levels. The exciton transition feature of acceptor-bound excitons (3.350eV), the free electron-acceptor emission (3.315eV), and the donor-acceptor-pair emission (3.246eV) exhibited different carrier recombination associated various defect complexes. The origins of two broad emissions at ∼2.99 and ∼2.89eV were found to be due to different photoelectron radiative transitions associated with deep level acceptors (isolated Zn vacancies). The acceptor-bound energies for P monodoped and In–P codoped epilayers ∼195 and ∼127meV, respectively. The small binding energy is helpful for acceptor ionization at room temperature, resulting in a high hole concentration in the codoped epilayer.
The electrochemical oxidation mechanism of vitamin E(VE) on pyrolytic graphite electrode (PGE) was studied by cyclic voltammetry. The results of experiment showed that the oxidative mechanism of VE was a quasi-reversible ECE process controlled by diffusion on PGE and the same on platinum electrode (PtE). The results of absorption spectroelectrochemistry experiment verified the oxidative mechanism of VE. It is clear that the oxidation of VE is a quasi-reversible 2e process to a divalent cation and followed by hydrolysis reaction to the quinone or semi-quinone compounds in acidic solution.
In this Letter, we report on the evolution of electronic properties governed by epitaxial misfit strain in cubic In2O3 epilayers grown on sapphire. At elevated growth temperature, the competition between the film/substrate lattice mismatch and the thermal expansion mismatch alters the macroscopic biaxial strain from compressive to tensile. Simultaneously, the electron concentration is tuned from degeneration to non-degeneration density below the Mott criterion. The observed surface electron accumulation and metal-insulator transition result from the oxygen deficiency formed at low growth temperature, while high-temperature epitaxy is favorable to achieve remarkably enhanced mobility. The effective strain-property coupling suggests that the improved oxygen stoichiometry and the Fermi level movement controlled by the biaxial strains are responsible for the Mott transition. The strain-mediated reduction of the electron effective mass contributes to the enhanced intrinsic mobility in tensile-strained In2O3 epilayers. These results highlight that strain engineering is an effective stimulus to manipulate the transport properties of oxide semiconductors with improved performance and unexpected functionalities.
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