| ZnO and its alloys have extensive prospects in the applications of optoelectronic devices such as ultra-violet (UV)/blue light-emitting diodes and laser-diodes due to wide direct band-gaps. Though having the same crystal structure as GaN together with the similar cell parameters and band-gap, ZnO exhibits a much larger excitonic binding energy than GaN, so it is easier to realize effective stimulated radiation at room temperature (RT) and higher temperatures, accompanied by a low threshold voltage. By introducing Cd content into ZnO, Zn1-xCdxO alloy semiconductors can be formed, and the luminescence of the alloys can be red-shifted to blue, even green light spectra from UV spectrum. More important, in the soluble range, the crystal structures and cell parameters of Zn1-xCdxO alloys change very little, this is advantageous for preparing ZnO/Zn1-xCdxO heterojunction, superlattice and quantum wells etc. Therefore, it is of great significance to investigate on Zn1-xCdxO alloy semiconductors.Presently, studies on Zn1-xCdxO films were very little, and the crystal quality of the prepared films was very poor, analogical to amorphous state or multi-phase states. Our research group, i.e., Professor Ye Zhi-Zhen group of state key laboratory of silicon materials is one of the earliest groups engaging in ZnO film studies. Till now, we have made great progresses in the growth of ZnO film, ZnO p-doping and investigations on ZnMgO alloy semiconductors etc. Considering that ZnCdO alloy semiconductor also has potentially great values in ZnO-based semiconductor devices, so we decided to carry out this research, now we outline it as follows:By the dc reactive magnetron sputtering method, we have deposited Zn1-xCdxO (0≤x≤1.0) films on Si(111), sapphire and glass substrates, and systemically investigated the effects of growth parameters such as substrate temperatures, compositions on the film crystal structures. Results show that the optimal growth parameters for depositing Zn1-xCdxO (0≤x≤0.6) films are: substrate temperatures of 450℃ , sputtering powers of 40-60 W, sputtering pressures of 3~5 Pa with the argon to oxygen ratios of 1:4. Moreover, we noted that for x≤0.6, the films are of single-phases with highly c-axes preferred orientations; for x=0.8, distinct phase separations were observed; for x=l .0, the film also has a highly preferred orientation.For the first time, we successfully obtained the PL spectra of the alloy films, and testified that the luminescence arose from single phased crystals. With increasing Cd contents, the PL peaks of the films are red-shifted to 414 nm (x=0.6) from 376 nm (x=0), it establishes a good foundation for developing ZnO-based blue light devices. Theoretically we investigated the band-gap engineering of the alloy film in detail, and gave a formula between band-gap (Eg) of alloy film and Cd content (x), Eg(x)=3.29664-1.21687x+1.25539x2 (0≤x≤0.53). However, the relationship between the band-gap and the Cd content accords with Vegard law, c(x)=0.5229+0.00357x. Furthermore, we put forward an elementary model to explain the green-yellow luminescence from the film, and considered that it is mainly related to electron transitions from the single ionized oxygen vacancies (V0+) to Zn, Cd vacancies (Vzn-, VCd-), or oxygen interstices (00, other intrinsic defects such as Znj, Cdj, Ozn, OCd etc, may also play minor roles in the green-yellow emissions.Determining film compositions plays fundamental and important rules in studying alloy films. By XPS measurements, we observed that the Cd/Zn ratios in the Zn1-xCdxO (0≤x≤1.0) films were almost the same as in targets, and the Zn, Cd atoms were all in oxidized states. By SIMS measurements, we found a phenomenon that the Cd content can easily accumulate at the interface between the film and Si substrate, however, the Zn and O components can distribute uniformly with the film depth variations, and the Zn and Cd components have nearly no segregations at the film surface.We found that the electrical resistivity of the Zn1-xCdxO (0≤x≤...
|
PDF Full Text Request