A Formation Mechanism of X Level due to an Indium-Carbon Dimer in Silicon

It is known that acceptor-carbon complexes have ionization energies less than those of the corresponding substitutional, separate acceptors in silicon. We present the formation mechanism for a shallower acceptor energy level called an X level that is due to an indium- carbon pair. Ab initio calculation methods were used to evaluate electronic structures and lattice relaxations of silicon with indium, carbon or a carbon-indium dimer. The results shows that the bonding interaction between the 5p orbitals of the indium atom and the 3sp orbitals of the silicon atoms bound with the indium atom mainly determines the ionization energy of the X level, and the ionic bonding interaction of the carbon atomic orbitals with the indium atomic orbitals in the X level enables the bonding interaction of the orbitals between the indium atom and the silicon atom to lower the corresponding indium acceptor level, and then to form the shallower X level.

Theoretical Studies on P-Type Conduction in (S,Cu) Co-Doped ZnO

Based on the first-principles calculations with density functional theory, the formation energy and electronic structure of (S, Cu) co-doped ZnO has been investigated, where the doping cases including related defects for Cu mono-doped, S-Cu co-doped, and S-2Cu co-doped ZnO are studied. The calculated results show that the formation energy of S-2Cu complex is lower than that of S-Cu complex under the O-rich condition. From the electronic structure, S-2Cu complex forms a peak of impurity state at the top of valence band. It was further found that heavy doping of Cu, not only enhances the acceptor concentration, but also leads to shallower acceptor energy level. Therefore, we concluded that S-2Cu complex is suitable for yielding better p-type conductivity in ZnO. The results are in good agreement with the experiment results.

Ab Initio Investigations of Electronic Structure and Optical Properties of Ag-F Codoped ZnO

Motivated by the widely discussed Ag doped ZnO and the lack of follow-up reports about the realization of p-n junctions, we calculated the electronic structures and optical properties of pure, Ag-doped and Ag-F codoped ZnO based on the density-functional theory. It was found that Ag doped ZnO shows p-type conduction character. But there are some unstable factors and self-compensations in this structure. We also calcualted the formation energy and ionization energy of the impurity for Ag-F codoped ZnO. It was found that incorporating the reactive donor F into Ag doped ZnO system, not only enhances the Ag acceptor solubility, but also gets a shallower Ag acceptor energy level in the band gap. In addition, we analyze the imaginary part of the dielectric function, reflectivity and absorption coefficient for pure ZnO and Ag-F codoped ZnO. Compared with the pure ZnO, the remarkable feature for Ag-F codoped ZnO is that there is a strong absorption in the visible-light region, which indicates that it could be taken as a potential candidate for a photocatalytic material.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

Characteristics of a phosphorus-doped p-type ZnO film by MBE

A solid-source GaP effusion cell was used to provide phosphorus dopants to achieve p-type ZnO with molecular-beam epitaxy (MBE). Room temperature (RT) Hall-effect measurements reveal that phosphorus-doped ZnO has a strong p-type conduction with a hole concentration of 6.5×1018 cm-3 and a hole mobility of 9.0 cm2/V s. X-ray diffraction measurements show a preferential growth orientation along <11-20> by θ-2θ scan and a tilt of ZnO (11-20) plane relative to the substrate surface by rocking curve and reciprocal space map. Photoluminescence (PL) spectra at 8.5 K show a dominant acceptor-bound exciton emission at 3.319 eV. The acceptor energy level of the phosphorus dopant is calculated to be 0.18 eV above the valence band from PL spectra, which is consistent with the temperature dependence of PL measurements.