Undoped ZnO Samples Research Articles

Influence of Co-doping on the structural, optical and magnetic properties of ZnO nanoparticles synthesized using auto-combustion method

In the present work, we have interested to understand the influence of cobalt doping on the various properties of ZnO nanoparticles, a series of samples were successfully synthesized using sol–gel auto-combustion method. The effects of Co doping on the structural and optical properties of ZnO:Co nanoparticles were investigated using X-ray diffraction (XRD), scanning electron microscopy, fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy, photoluminescence spectroscopy and vibrating sample magnetometer (VSM). With the sensitivity of the XRD instrument, the structural analyses on the undoped and Co-doped ZnO samples reveal the formation of polycrystalline hexagonal-wurtzite structure without any secondary phase. FTIR spectra confirm the formation of wurtzite structure of ZnO in the samples. The optical absorption spectra showed a red shift in the near band edge which indicates that Co2+ successfully incorporated into the Zn2+ lattice sites. The room temperature PL measurements show a strong UV emission centered at 392 nm (3.16 eV), ascribed to the near-band-edge emissions of ZnO and defect related emissions at 411 nm (violet luminescence), 449 nm (blue luminescence) and 627 nm (orange-red luminescence), respectively. Magnetic study using VSM reveals that all the samples are found to exhibit room temperature ferromagnetism.

Effects of Cu diffusion-doping on structural, optical, and magnetic properties of ZnO nanorod arrays grown by vapor phase transport method

Well-aligned ZnO nanorods were prepared by the vapor phase transport method on Si covered with a ZnO buffer layer. After the nanostructure growth, Cu was doped into the ZnO nanorods by diffusion at three different temperatures and for different times. Undoped and Cu diffusion-doped ZnO samples are highly textured, with the c axis of the wurtzite structure along the growth direction. The incorporation of Cu caused some slight changes in the nanorod alignment, although the wurtzite crystal structure was maintained. X-ray photoelectron spectroscopy measurements revealed that Cu ions were in a divalent state and substituted for the Zn2+ ions of the ZnO matrix. Photoluminescence results at 10 K indicate that the incorporation of copper leads to a relative increase of Cu-related structured green band deep level intensity. Magnetic measurements revealed that both undoped and Cu diffusion-doped ZnO samples exhibited room temperature ferromagnetism. It was also found that bound magnetic polarons play an important role in the appearance of room temperature ferromagnetism in Cu diffusion-doped ZnO nanorods.

Zinc Vacancy‐Induced Room‐Temperature Ferromagnetism in Undoped ZnO Thin Films

Undoped ZnO thin films are prepared by polymer‐assisted deposition (PAD) and treated by postannealing at different temperatures in oxygen or forming gases (95% Ar + 5% H2). All the samples exhibit ferromagnetism at room temperature (RT). SQUID and positron annihilation measurements show that post‐annealing treatments greatly enhance the magnetizations in undoped ZnO samples, and there is a positive correlation between the magnetization and zinc vacancies in the ZnO thin films. XPS measurements indicate that annealing also induces oxygen vacancies that have no direct relationship with ferromagnetism. Further analysis of the results suggests that the ferromagnetism in undoped ZnO is induced by Zn vacancies.

Growth and Characterization of Urea Doped p-Type ZnO Thin Film Grown by Pulsed Laser Deposition

In the present study we have used urea as the source for doping nitrogen in ZnO since the most successful acceptor type dopant is the group V element like nitrogen. The nitrogen doped ZnO films have been deposited on glass substrates using Pulsed Laser Deposition technique using 248 nm KrF laser at energy 300 mJ by varying the number of laser pulses with a repetition rate of 10 pulse/sec in vacuum (10-6 mbar) at a constant temperature of 300 °C. The XRD pattern confirms the formation of wurtzite structure of ZnO, which is polycrystalline in nature. We have also performed UV absorption spectroscopy and the band gap is found to be 3.4 eV. Resistivity of the film increases with the increase of thickness for the undoped ZnO samples where the carrier concentrations are found to be of the order of 1017 cm-3. The mobility of the as-grown film is found to be 24.9 cm2/V-s. After doping with nitrogen the carrier concentration drops to the order of 1015 cm-3 and the mobility becomes 1.5 cm2/V-s. The mobility slightly varies with thickness. The resistivity increases to 1.3 KΩ-cm and the film shows p-type behavior. The results are explained on the basis of the available theory.

Characterization of Undoped ZnO Films Post-Annealed by Using Helium Gas

In this study, we used helium gas for the rst time for annealing undoped ZnO lms at di erent temperatures in the range of 500 800 C and we report their structural, optical and electrical properties. All measurements were carried out at room temperature. The undoped ZnO samples exhibited a (002) preferential orientation with the c-axis perpendicular to the substrate, suggesting that samples of high quality were prepared. The PL spectra of the ZnO samples revealed a strong near-band-edge ultraviolet (UV) emission peak at 3.26 eV, a strong red emission peak at 1.91 eV and a weak green emission peak at 2.36 eV, which were attributed to a free electron-acceptor transition, a zinc interstitial and a single ionized oxygen vacancy, respectively. All peak strengths in the PL spectra increased with increasing post-annealing temperature. We also con rmed from PL and Hall measurements that all samples revealed n-type conductivity. With increasing post-annealing temperature, the electron carrier concentration and the strength of UV emission peak increased, suggesting that they are related to each other.

Band-gap engineering of Zn1−xGaxO nanopowders: Synthesis, structural and optical characterizations

We report the preparation and detailed structural and optical characterizations of single phase gallium doped ZnO nanopowders. A low temperature solution-based technique was developed to synthesize high-purity Zn1−xGaxO (x:0–0.05) nanopowders. Structural and optical characterization experiments were performed using x-ray diffraction (XRD), energy dispersive x-ray spectroscopy, scanning electron microscopy, photoluminescence spectroscopy, Raman spectroscopy, and optical transmission spectroscopy. Analysis of XRD data showed a maximum in the lattice volume for x=0.03 as the Ga concentration increases in the Zn1−xGaxO series. Optical transmission spectroscopy results also showed an initial increase in the band gap (Eg) followed by a decrease for x>0.04. A maximum band gap was observed for the Zn0.96Ga0.04O sample, which has a value of 3.42 eV compared to the band-gap value of 3.31 eV for the undoped ZnO sample. All Zn1−xGaxO nanopowder samples showed a broad photoluminescence spectrum with significant shift in peak positions compared to bulk ZnO sample. The broadening and shifting in luminescence are understood to be caused by the nanocrystalline nature of the material and the presence of deep-level surface defects. These effects were further investigated by Raman microscopy where an increase in the intensity of E1(TO) mode was observed accompanied by the appearance of a few additional Raman active modes.

Influence of Mn doping on the microstructure and optical property of ZnO

Undoped and Mn-doped ZnO samples with different percentage of Mn content (1, 2 and 3 mol%) were synthesized by a simple solvo-thermal method. We have studied the structural, chemical and optical properties of the samples by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, Fourier transform infrared (FTIR) spectroscopy and UV–VIS spectroscopy. The XRD spectra show that all the samples are hexagonal wurtzite structures. The lattice parameters calculated for the Mn-doped ZnO from the XRD pattern were found to be slightly larger than those of the undoped ZnO, which indicates substitution of Mn in ZnO lattice. SEM photograph shows that the grain size of undoped ZnO is bigger than the Mn-doped ZnOs, indicating hindrance of grain growth upon Mn doping. As the Mn doping increases the optical band gap decreases for the range of Mn doping reported here.

Influence of Al dopant on microstructure and optical properties of ZnO thin films prepared by sol–gel spin coating method

Zinc oxide thin films are prepared on c-plane sapphire substrates using the sol–gel spin coating method. After coating the samples were annealed at 873K. Effects of Al doping on the morphological, microstructural and optical properties were examined. Both the pure and Al doped ZnO samples are (0002) oriented. Undoped ZnO samples posses film-like morphology where as Al doping resulted in the formation of disconnected spherical grains. Cathodoluminescence spectroscopy measured at room temperature shows a strong ultra violet exciton emission around 382nm along with deep level green emission due to defects around 500nm for undoped ZnO film. Excitonic emission blue shifted to 380nm and increase in deep level emission were observed for Al doped ZnO samples possibly due to size effects.

Nonlinear I– V characteristics in doped ZnO based-ceramic varistor

Two similar sets of Zn 1− x Fe x O ceramic samples with various x values (0.00< x<0.50) are prepared by two different heat treatments. The first set is quenched from sintering temperature down to room temperature, and the second is left in the furnace and slowly cooled to room temperature. These samples are examined by using X-ray diffraction patterns (XRD), scanning electron microscope (SEM) and DC electrical measurements. XRD data indicate that the replacement of Zn 2+ ion by Fe 3+ ion does not influence the Wurtzite structure of ZnO samples, and other well-known peaks are formed. The results of SEM indicate that no secondary phases are formed at grain boundaries, supporting the XRD results. It is also apparent that the average grain size decreases with increasing Fe content up to x=0.10, followed by an increase at x>0.30. With increasing Fe content, a nonlinear region, obtained from I– V characteristics, clearly appears and is shifted to higher fields. Furthermore, the nonlinear coefficients are generally increased by adding Fe up to x=0.30, followed by a decrease at x=0.50. Moreover, the values of the barrier heights are also generally increased with Fe up to x=0.10 and kept unchanged with further Fe content. On the other hand, the electrical conductivities at room temperature are measured and their values are found to be decreasing with increasing Fe content, as compared to those of an undoped ZnO sample. Our results are discussed in terms of both point defects and intrinsic donors, which are produced by Fe doping in the ZnO ceramic system.

Growth and photoluminescence for undoped and N-doped ZnO grown on 6H-SiC substrate

Growth and photoluminescence (PL) spectra of undoped and N-doped ZnO grown on 6H-SiC substrate by plasma-assisted molecular beam epitaxy (P-MBE) method were especially studied. Compared with PL spectrum of undoped ZnO, a broadening of near-band-edge (NBE) emission was observed in N-doped samples at room temperature. The low-temperature (5 K) PL spectrum confirmed that some acceptor levels associated with N doping were presented in the N-doped ZnO sample. The NBE emission of undoped and N-doped ZnO samples grown on 6H-SiC showed a small blue-shift compared with that grown on Al 2O 3, it was relate to the change of band gap caused by stress. The intensity of dominant NBE emission from ZnO films grown on 6H-SiC is stronger and the FWHM is smaller than that grown on Al 2O 3 substrate. The deep-level emission can be observed on both samples, but the intensity ratio of NBE to deep-level emission is larger for ZnO samples grown on 6H-SiC.

Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy

Undoped and Ga-doped (3 wt %) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under an oxygen flow rate from 0 to 50 SCCM. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photothermal (PPT) spectroscopy, which is effective in observing a nonradiative transition process. The PPT peak around 2.5 eV was observed only for the undoped ZnO samples grown under a low oxygen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with the increase of the oxygen flow rates. No corresponding peak was found for the Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies, and it agrees with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the carrier concentration. Burstein-Moss effect and band-gap-narrowing effect in ZnO should be considered in the case of high carrier concentration. Comparing the experimental results with the theoretical predictions, we found out that the proposed PPT edge energy coincides well with the Fermi level EF.

Unstable luminescence in GaN and ZnO

We report observations of unstable photoluminescence (PL) in GaN and ZnO. Beginning with GaN, a broad blue band with a maximum at 3.02 eV and well-defined fine structure at its high-energy wing bleached under ultraviolet (UV) exposure at low temperatures. The bleaching of this band was accompanied by enhancement of a broad yellow band peaking at 2.2 eV. We also observed a very similar PL evolution in undoped bulk ZnO samples, where the intensity of the green band (at 2.6 eV) significantly decreased with concomitant enhancement of the yellow band (at 2.25 eV). The transformations are explained by recombination-enhanced defect reactions in these materials.

Dependence of oxygen flow rate on piezoelectric photothermal spectra of ZnO thin films grown by a reactive plasma deposition

Undoped and Ga-doped (3 wt.%) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under oxygen flow rate from 0 to 50 sccm. Piezoelectric photothermal spectroscopy (PPTS) was measured for characterizing the ZnO films from the viewpoint of nonradiative recombination processes. Strong peak due to the band edge transition was observed around 3.3 eV in all the samples at room temperature. Furthermore, a broad band of the PPT signal at 2.5 eV was observed for the undoped ZnO samples grown under low oxygen flow rate. This signal disappeared with increasing the oxygen flow rates and did not appear in the Ga-doped samples. Therefore, it is considered that this PPT signal at 2.5 eV was due to the electron transition from the oxygen vacancies.

Energy transfer from the host to Eu3+ in ZnO

Abstract Undoped, and Eu2O3,Li- and EuF3,Li-codoped ZnO samples were prepared through solid state chemical reaction. Sharp exciton emission lines in UV were detected at 10 K, and a strong green emission band at 500 nm were observed at both 10 K and room temperature in the undoped ZnO samples. Emission and excitation spectra revealed that the energy transfer from the host to Eu3+ was very weak in ZnO:1%Eu2O3,Li. In contrast, quite efficient energy transfer from the host to Eu3+ was observed in the samples of ZnO:1%EuF3,Li. Possible physical reasons of the enhancement of energy transfer are discussed.

Defect chemistry and electrical characteristics of undoped and Mn-doped ZnO

Undoped and Mn-doped ZnO samples were sintered at 1373, 1473, and 1573 K, for 2 h, in air, and then quenched to room temperature. Defect concentrations in these samples at the sintering temperatures and at room temperature and below were calculated using a refined defect model. Using the calculated electron concentrations and assuming a constant electron mobility of 100 cm2/Vs, conductivities at room temperature were calculated and compared with experimental ones. The agreement between the experimental and calculated conductivities is very good for all the samples of undoped and Mn-doped ZnO. In the Mn-doped ZnO case, the ionisation energy of the Mn defect in ZnO was estimated to be ∼2.0 eV. Using the experimental conductivities and the calculated electron concentrations, the electron mobilities were calculated between 70 and 300 K. The results show that the temperature dependence of the mobility in undoped ZnO is similar to that in ZnO single crystals observed in other works, and heavy Mn doping significantly reduces the electron mobility below room temperature probably due to impurity scattering. The role of Mn on the electrical conductivity of ZnO could be understood.