Synthesis and physical properties of impurity doped ZnO materials

Abstract

ZnO is becoming more and more popular as third-generation semiconductor for its wide band gap (~3.4 eV) and large exciton binding energy (60meV) at room temperature, which permits efficient excitonic emission processes, therefore ZnO has gereat development potential in the field of optoelectronic devices. In addition, ZnO has a high melting point, high thermal and chemical stability. ZnO single crystal thin film can be obtained at the high temperature at certain condition, so it can greatly reduce the defects formed in ZnO. Furthermore, ZnO is abundant, cheap, innoxious, easy to be prepared and with potential commercial value. To realize the ZnO-based device applications, an imperative issue is to fabricate the high quality n- and p-type ZnO. While ZnO is naturally n-type conductivity due to the various native defects, which is the difficulty in achieving high-quality stable p-type ZnO. Powder, ceramic and thin film n- and p-type ZnO-based samples are made by various methods including sol-gel, solid phase and pulsed laser deposition (PLD) methods. The physical properties of the samples are characterized by X-ray diffraction (XRD), Field-Emission Scanning Electron Microscope (FE-SEM), Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Hall test instruments. A special phenomenon is observed during the investigation that the Li-doped thin films show ferromagnetism at room temperature. The detailed contents and innovations would be introduced next. For n-type ZnO, Al is chosen as the dopant element in research. Ceramics and thin films are prepared to do the investigation with various doping compositions (Zn1−xAlxO, x = 0.02-0.2). The XRD patterns show the Al is well doped into the host material and higher reaction temperature may improve the composition. The good target ceramics also affect the quality of the thin films. Except the substrate peak, there is only the (002) c-axis direction peaks are observed from the XRD patterns of thin films. The flat surface and low roughness thin films are observed from FE-SEM and AFM images. The optical transmittance of the thin film is good, and the carrier concentration of electrons of Zn1-xAlxO film is as high as 4×1021 cm-3. Both the powder and ceramic Li-doped ZnO show no impurity peak from the XRD patterns. The reaction temperature and time would affect the morphology of the powder which is made by sol-gel method. pH value should be another considerable condition during the experiment. All of the samples show the signature dielectric property follow the changeable test conditions. For p-type Li doped ZnO thin film, conditions has been changed including different substrates, temperatures and various dopant concentration. Thin films are grown on the Al2O3(0001) substrate at 600 ℃ with the Li concentration from 2% to 18%. SEM and AFM images show the surface conditions of the thin films while the optical absorption studies in the wavelength range 200-900 nm revealed an increase in the band gap of the Li-doped ZnO films from 3.19 to 3.41 eV. And during the Zn1−xLixO thin films for x=0.01, 0.05 and 0.10 on Pt (111) /Ti/SiO2/Si substrate under 500 ℃, 5 at% and 10 at% Li doped ZnO thin films show p-type behavior by Hall effect testing results. The existence of defects such as Lii (interstitial Li) and LiZn (substitutional Li on the Zn site) should be considered in the host materials. The stabilization of p-type thin films is also discussed in defects theoretically. In further research, the structural, electric and magnetic properties of ZnO:Li thin films for 2%, 5%, 8% and 10% Li-doped thin films which are prepared on Pt(111)/TiO2/Si/SiO2 substrates by pulsed laser deposition are reported. Lattice parameters and Zn-O bond lengths are calculated from X-ray diffraction (XRD) results. The 8% ZnO:Li thin film show room temperature ferromagnetism and the hysteresis loop is observed. In particular, the native point defects in ZnO (such as Zni, Vo, Vzn) and Li-related defects (Lii and Lizn) are analyzed by X-ray photoelectron spectroscopy (XPS). Zni and Vo defects give indirect evidences, while the photoluminescence results show circumstantial proof of the existence of VZn, LiZn and VZn defects play a vital role in p-type films and ferromagnetism, respectively. Moreover, the Hall effect results also corroborate the formation and stabilization of efficiency factors and thus stabilizing the p-type ferromagnetism predicted for cation vacancy in ZnO thin film.