Investigation On Preperation Of In-N Co-doping P-ZnO Thin Films And ZnO Relative Doping With Other Elements

Zinc oxide (ZnO) is a novel II-VI compound semiconductor with a wide direct bandgap and a hexagonal wurtzite structure. It is usually with (002) orientation due to the lower surface free energy of (002) plane. ZnO is a unique material that exhibits optoelectronic, piezoelectric and ferromagnetic multiple properties, as well as its versatile nanostructures. In particular, it is a potential candidate for applications in short-wavelength optoelectronic devices, including light emitting diodes (LEDs) and laser diodes (LDs), due to its direct wide-bandgap (3.37 eV) and high exciton binding energy (60 meV, cf. 25 meV for GaN), which will favor efficient excitonic emission processes at room temperature. To realize these device applications, an imperative issue is to fabricate both high-quality p-type and n-type ZnO films. However, like most wide bandgap semiconductors, ZnO has the "asymmetric doping" limitation. It can be an easily doped high-quality n-type, but it is difficult to dope p-type. So the greatest challenge in exploiting ZnO, as in the case of GaN, is in achieving p-type doping. In this work, we mainly concern about this issue, and moreover this work is also expended from it.1. The realization of In-N co-doping p-ZnO thin films and the fabrication of ZnO-based homo-structure p-n junctionp-type ZnO thin films have been realized by the In-N co-doping method by DC reactive magnetron sputtering. Secondary ion mass spectroscopy (SIMS) revealed that the nitrogen incorporation was enhanced by the presence of Indium in ZnO. The as-grown In-N co-doped ZnO film shows acceptable p-type behavior at room temperature with high film quality. To further understand the p-type properties, In-N co-doped ZnO films were deposited on different substrates. The lowest reliable room-temperature resistivity was found to be 3.12 Ωcm with a carrier concentration of 2.04×10¹⁸ cm^-3 and a Hall mobility of 0.979 cm²V^-1S^-1. The p-type behavior is stable and reproducible..The influence of indium concentrations on electrical properties of In-N co-doped ZnO thin films has been studied. Based on Hall-effect measurements and analyses, impurity scattering is the dominant mechanism determining the diminished mobility in ZnO with higher In concentration. X-ray photoelectron spectroscopy reveals that the presence of In enhances the solubility of N with the formation of In-N and Zn-N bonds. Theoptimal properties, namely resistivity of 16.1 Ω-cm and Hall mobility of 1.13 cm²V^-1s^-1, are obtained at an indium concentration of 0.14 at. %. The diffraction angle of co-doped ZnO is closest to that of un-doped ZnO.p-type ZnO thin films have been fabricated by In-N co-doping at temperatures between 490 to 580 °C. At a lower temperature, the carrier type is ambiguous, butp-type conductivity is obtained at intermediate temperatures. However, the samples become n-type at higher temperatures. A reasonable model was proposed to explain specific complex; the formation and dissociation of the complex explained the doping behavior as a function of temperature. The lowest room temperature resistivity is found to be 7.85 Ω-cm from the In-N co-doped p-type ZnO film produced on a buffer layer. The carrier concentration is 5.40×10¹⁷ cm^-3 and Hall mobility is 1.47 cm²V^-1s^-1. ZnO-based homo-structure p-n junctions comprising an n-ZnO: In layer on a p-ZnO: (In, N) layer on a buffer layer display apparent electrical rectification in repeated measurements thereby confirming the formation of a typical p-n junction. The ability to control p-type conductivity in In-N co-doped ZnO films by adjusting the growth temperature and using a buffer layer is important to the design of ZnO-based optoelectronic devices.2. Investigation on ZnO relative doping with other elementsLi-doped ZnO thin films were prepared on glass substrates by DC reactive magnetron sputtering, the influence of post-annealing temperature on the electrical, structural, and optical properties of the films is presented in this article. The optimal p-type conduction is achieved at the annealing temperature of 500 °C with a resistivity of 57 Ωcm, carrier concentration of 1.07×10¹⁷cm^-3 and Hall mobility of 1.03 cm²V^-1s^-1. For the ZnO: Li thin film annealed at 500 °C, the diffraction angle of the (002) peak is 34.43 °, which is almost the same to that of pure ZnO, implying that Li_zn can be stable in the ZnO: Li film due to Li incorporation. A conversion from p-type conduction to n-type at annealing temperature above 500 °C was also confirmed by Hall measurement. We suggest that the conversion may result from reevaporation of Li acceptor, better crystallinity, and aggravated creation of V_o at high annealing temperature. From the PL analysis, two acceptor levels located at ～140 and 260 meV above the valence band were identified. We attributed the 140 meV level to Li_zn acceptor.Zn_1-xCd_xO(x=0.1, 0.2) thin films with highly (002)-preferred orientations were deposited on glass and Si(111) substrates by DC reactive magnetron sputtering method inthe atmospheres with different Ar/O₂ ratios. The properties were investigated by XRD, optical absorption spectra, XPS, SEM and AFM. When the Ar/O₂ ratios change from 1:4 to 1:1, the FWHM of the films deposited on glass substrates of Zno.9Cdo.1O films decrease(from 0.36°) gradually and reach a minimum value of 0.29° at the ratio of 1:1; the band-gap(E_g) decrease(from 3.149eV) gradually and reach a minimum value of 3.099eV at the same ratio of 1:1.When the Ar/O₂ ratios continue to increase up to 2:1, the FWHM increase to 0.35°; the band-gap(E_g) increase to 3.114eV. The variations for Zn_0.8Cd_0.2O films are same to the Zno.9Cdo.1O films. A mechanism for the influence of Ar/O₂ gas ratios on the band-gap of Zn_1-xCd_xO thin films are proposed.