Zni Defects Research Articles

Investigation of Structural Electrophysical Parameters of ZnO al Films for Electronics

In this paper, the effect of the introduction of aluminum in concentrations 1—3 at was investigated. % in semiconductor nanostructured zinc oxide films obtained by sol-gel method. Based on the results of scanning electron microscopy, it was found that all samples have a branched structure, while there is a tendency for the branches to enlarge with an increase in the concentration of the modifier. Studies of optical transmission spectra have shown that the samples are transparent in the visible radiation range; the calculation of the optical band gap showed an increase in the amorphization of the material, which we associate with the for-mation of impurity levels formed by aluminum cations overlapping with the band conductivity, as well as an increase in the concentration of intrinsic point electrically active defects. Analysis of the results of the dependence of electrical conductivity on temperature showed its semiconductor nature and the presence of two regions with the following activation energies: the first region is a low—temperature region with low energy in the range of 35—75 meV; the second region is a high—temperature region with energy in the range of 0.35— 0.45 eV. We associate the low activation energy with donor levels of interstitial zinc Zni defects. At the same time, with an increase in the concentration of aluminum, the value of the activation energy of this level decreases, which is associated with the substitution of zinc atoms with aluminum and their displacement into the internode. The analysis of the electrical conductivity of the studied films in the range of 20...400 °C shows its maximum value at the concentration of the alloying impurity in 1 at. %. Thus, this composition is optimal from the point of view of obtaining transparent conductive films.

Combined Effects of Ultraviolet Irradiation and Magnetic Field on the Properties of Dip-coated ZnO thin films

In this study, four ZnO thin films were deposited on FTO substrates using the sol–gel dip coating method to examine their microstructural, morphological, and optical properties through various techniques. Three of them were subjected to ultraviolet (UV) light, magnetic field (MF), and a combination of UV and MF during deposition, referred as ZnO: UV, ZnO: MF, and ZnO: (UV+MF), respectively. The results obtained showed that the simultaneous UV and MF exposure improved the crystallinity and surface homogeneity of the as-deposited film. Moreover, ZnO: (UV+MF) film exhibited an average transparency of 80% in the visible region and a high optical bandgap (3.67 eV). Room-temperature photoluminescence (PL) spectra revealed a weak UV emission and a strong violet emission peaks for all films. However, the violet emission intensity being lower in ZnO: UV and ZnO: MF films due to a reduction in zinc interstitials (Zni) defects, The simultaneous UV and MF exposure did not reduce Zni defects, and the violet emission intensity was almost identical to that of the untreated film. These findings suggest that the ZnO: (UV+MF) film can be a promising candidate for the development of ultraviolet and violet lasers and light-emitting diodes.

Defects control and origins of blue and green emissions in sol-gel ZnO thin films

ZnO has broad application prospects in the field of LED lighting and display. However, the unclear luminescence mechanism limits the improvement of luminous efficiency and the realization of controllable luminescence. Herein, intrinsic defects (Zni and VO) in sol-gel ZnO thin films were controlled by post-annealed in different atmospheres (air, N2 and vacuum). The effect of annealing atmospheres on the intrinsic defects were quantitatively studied by XRD, Raman, XPS and EPR spectra, and the relationship between intrinsic defects and visible emissions were systematically investigated by excitation-dependent PL spectra under above bandgap and below bandgap excitations. Results show that N2-annealed ZnO film with more Zni defects and fewer Vo defects exhibits much stronger blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). Zinc interstitials states (including Zni and ex-Zni states) are the initial states of the electronic transitions for the blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). In addition, transitions from the conduction band to the VO level also emit a green peak at 525 nm, which can only be observed under above bandgap excitation. This work contributes to a better understanding of defect-induced blue and green emission in ZnO, which will help to achieve controllable luminescence and facilitate its commercial application as a luminescent material.

Tailoring electronics structure, electrical and magnetic properties of synthesized transition metal (Ni)-doped ZnO thin film

Improving Zinc Oxide (ZnO) conductivity is in dire need due to its major shortcoming of high resistivity as a result of low carrier concentration. In this work, ZnO was doped with a transition metal, Nickel (Ni) in different concentrations (1 mol%, 2 mol%, 3 mol%, and 4 mol%) and the influence of Ni concentrations in ZnO thin film in term of electrical, electronic structure as well as magnetic properties were investigated. ZnO thin films with Ni-doping were successfully produced via a sol-gel spin coating method. High-Resolution Transmission Electron Microscopy (HR-TEM) image observed for 3 mol% Ni:ZnO thin film shows clear and coherent lattice fringes in hexagonal shape with the amorphous structure inside. The resistivity of Ni:ZnO films show to decrease with the addition of Ni-doping down to 1.7 × 10−1 Ω cm, whilst the conductivity and the carrier concentration improved from 0.28 Sm-1 to 5.87 Sm-1 and 2.23 × 1014 cm−3 to 485.14 × 1014 cm−3 respectively. Thermal dependence activation energy (Ea) of ZnO thin film is found to be 1.3 eV and increases up to 15.1 eV above 373 K. X-ray Photoemission Spectroscopy/Ultraviolet Photoemission Spectroscopy (XPS/UPS) spectra indicate that Ni-doped ZnO induced more surface defects and native defects in the ZnO system. In addition to that, photoluminescence spectra show that Vo and Zni induced shallow donor in the system. Finally, via Vibrating Sample Magnetometery (VSM) measurement, it is revealed that the ferromagnetism of Ni:ZnO might be induced by the co-existence of Vo and Zni defects. These results may open an attractive path to tailor Ni-doping in ZnO system to act as potential candidates for the optoelectronic and spintronic applications.

Structural, morphological, magnetic and electrical properties of Ni-doped ZnO nanoparticles synthesized by co-precipitation method

Weak ferromagnetic behaviour was obtained in a systematic way at room temperature by doping of ZnO with nickel (Zn1-xNixO, where x = 0.00, 0.05, 0.15 and 0.20). The obtained results were correlated with conductivity and impedance studies. Diamagnetic to ferromagnetic change was observed with increased concentration of nickel. X-ray diffraction analysis confirmed wurtzite ZnO structure of prepared nanopowders while microstrain was increased with nickel concentration. Incorporation of nickel in ZnO structure was confirmed using EDAX analysis, while FTIR spectroscopy provided further information on functional groups. Transmission electron microscopy images showed that the particle sizes are in the range of 12-20 nm, and scanning electron microscopy analyses that grain size decreases with increase in nickel concentration. Photo luminescence studies confirmed the presence of VO and Zni defects in the prepared samples. It was concluded that the defect induced strain, grain boundaries and lower particle sizes are the reasons for weak ferromagnetic behaviour of the investigated samples.

Synthetic and effect of annealing on the luminescent properties of ZnO nanowire

ZnO nanowires were successfully fabricated by using the hydrothermal method in the existence of the poly ethylene glycol (PEG) with the molecular weight of 200. The experimental results exhibit that the ZnO nanowires with the diameter of ~30nm and the length of ten micrometers. PL spectra show a weak ultraviolet emission and an intense broad visible emission band for as-grown and annealed samples. These visible emission bands exhibit red-shifts from green (545nm) to yellow (580nm) and blue-shifts from yellow (580nm) to green (520nm) by annealing at aerobic or anaerobic environment, it indicates that the defect types are changed by annealed at different environment. Additionally, the red-shifts (520nm) and blue-shifts (580) can match up the bimodal lorentzian fitting (520nm and 583nm) of as-growth, which suggest that the visible emission band (545nm) is closely related to oxygen defects. The oxygen atomic can enter into the crystal lattice of ZnO and decrease the oxygen vacancy in air or oxygen, whereas, more oxygen vacancy defects is gave rise in vacuum annealed. We guess the energy levels of the intrinsic defects in ZnO nanowire maybe like that: the electrons of Zni defects compound with holes of VZn, Oi and OZn levels and conform to the yellow emission, and the green emission corresponds to the electron transition from the association defects deep donor level to the valence band.

Structural, optical, and magnetic properties of Ni doped ZnO nanoparticles: Correlation of magnetic moment with defect density

We report on the room temperature ferromagnetism in the Zn1−xNixO (x=0, 0.03 and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline, wurtzite ZnO structure for the 3% Ni doped ZnO NPs for higher milling time. HRTEM lattice image and SAED pattern show that the doped NPs are single crystalline with a d-spacing of 2.47Å corresponding to the (101) plane. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirm the presence of Ni ions inside the ZnO matrix with 2+ valance state. Room temperature magnetic measurements exhibit the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) and coercive field (Hc) of 260Oe. Micro-Raman studies illustrate doping/disorder induced additional Raman modes at ∼547, 574cm−1 in addition to 437cm−1 peak of pure ZnO. Photoluminescence (PL) spectra and UV–vis absorption measurements demonstrate some modification in the band edge emission and absorption characteristics, respectively. PL spectra also show defect related strong visible emission, which is believed to play a significant role in the FM ordering. These observations highlight the effect of changing defect density on the observed ferromagnetic moment values for the as synthesized Zn1−xNixO NPs. Magnetic interaction is quantitatively analyzed and explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Ni ions and OV, Zni defects. Systemic studies on the structural, magnetic, and optical properties reveal that both the nature of the defects as well as Ni2+ ions are significant ingredients behind attaining high moment as well as high ordering temperature in Ni doped ZnO NPs.

The electronic structures and optical properties of yttrium-doped zinc oxide with zinc interstitial defects calculated by first-principles

The geometric structures of ZnO, yttrium doped ZnO (YZO), ZnO with Zn interstitial (ZIO) and yttrium doped ZnO with Zn interstitial (YZIO) systems are investigated using the first-principle method based on density functional theory. The results show that doping yttrium ions can decrease the formation energy of Zni defects adjacent to yttrium ions. The electronic structures and absorption spectra of these systems are further obtained after revising the value of bandgap. Compared with ZnO, YZO and ZIO systems, there is an impurity level (Er) near the center of the forbidden band in YZIO, which will enhance the luminescence peak in yellow region. Besides, the bandgap of YZIO will increase with the increase of yttrium doping concentration, which suggests that the UV emission of YZIO would have a blue shift. In addition, the absorption property of YZIO system is also improved in both the visible and UV regions, which can be used in converting solar energy into electricity.

The origin of the ∼274 cm−1 additional Raman mode induced by the incorporation of N dopants and a feasible route to achieve p-type ZnO:N thin films

Nitrogen doped ZnO films (ZnO:N) were deposited on quartz glass substrates by radio-frequency magnetron sputtering technique with various N2 flow rate mixed with Ar. Raman measurements indicate that the intensity of ∼274cm−1 mode of ZnO:N films exhibits an anomalous variation, which neither depends on N2 flow rate nor on NO acceptor content based on X-ray photoelectron spectroscopy (XPS) analysis. Combined with defect formation energy calculations, it is demonstrated that the ∼274cm−1 mode is attributed to Zni defects which can be increased by the incorporation of NO acceptors, but suppressed by the presence of (N2)O double donors. XPS and optical absorption spectra suggest that ZnO:N film prepared under specific N2 flow rate (Ar:N2=3:1), has high concentration of acceptor NO with shallow states and the absence of shallow donor (N2)O defects, could be most likely to achieve p-type conductivity. But, at the same time, such specific ZnO:N film is the presence of shallow donor Zni defects bonded with NO acceptors. Performing density functional calculations in conjunction with the climbing image nudged elastic band method shows that Zni could be dissociated from Zni–NO complexes by post-annealing treatment and NO acceptors would be activated to p-type ZnO:N. This is confirmed by our further Hall investigation, indicating that p-type ZnO:N can be achieved by choosing appropriate post-annealing treatment.

First-principles calculations of electronic structure and optical properties of Boron-doped ZnO with intrinsic defects

This study adopted first-principles calculations to evaluate the effects of intrinsic defects on the electronic structure and optical properties of Boron-doped ZnO (BZO). Four types of defect were considered: non-defective (BZn), Zn vacancies (VZn), O vacancies (VO), and interstitial Zn (Zni). Calculations of formation energy illustrate that O-rich conditions tend to induce VZn, while O-poor conditions tend to induce VO and Zni. With respect to electric properties, VZn defects in BZO decrease carrier concentration as well as mobility, which consequently decreases the conductivity of BZO. The existence of VO or Zni defects in BZO leads to n-type conductive characteristics and increases the optical band gap. The existence of Zni defects in BZO also increases the effective mass, which decreases the mobility and conductivity of BZO. As for the optical properties, the introduction of VZn to BZO leads to an increase in transmittance in the visible light region, but a decrease in the UV region. The introduction of intrinsic VO and Zni defects to BZO leads to a significant decrease in transmittance in the visible as well as UV regions. The calculated results were also compared with experimental data from the literature.

Defect evolution on the optical properties of H+-implanted ZnO whiskers

The interplay of intentionally doped H impurity with native defects as well as its influence on the microstructure and optical properties of ZnO whiskers was systematically studied by varying the H+ implantation fluencies from 1 × 1014 to 1 × 1016 ions cm−2. Higher dose of H+ implantation resulted in the serious deterioration of the crystallinity and introduction of more nonradiative VO and Zni defects in the energy gap, as revealed by the X-ray diffraction (XRD), absorption spectra, Raman scattering, photoluminescence and electronic paramagnetic resonance (EPR) measurements. We demonstrate that H passivation of oxygen vacancies is in terms of forming V+0 defects rather than H+0 while deactivation of zinc vacancy related acceptor defects is through the process of hydrogen filling. The V+0 and ionized Zni defects are suggested to give rise to the H+-implantation induced blue band optical absorption.

Ferromagnetism in Li doped ZnO nanoparticles: The role of interstitial Li

ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. Both interstitial Zn (Zni) defects and Zn-O bonds were determined from the Zn LMM Auger peaks; however, the variation of these with Li concentrations was not large. Oxygen vacancies (Vo) concentrations are estimated to be relatively constant over the entire Li concentration range. We relate the Lii and Zni defects to the formation and stabilization of Zn vacancies and thus stabilizing the p-type ferromagnetism predicted for cation (zinc) vacancy in the ZnO type oxides.

Stress-dependent band gap shift and quenching of defects in Al-doped ZnO films

Al-doped ZnO (AZO) films were deposited on quartz substrates by the ultrasonically assisted chemical vapour deposition technique. The undoped ZnO film was found to be subjected to a stress which increases initially up to 3% Al doping, and then a slight decrease was observed for 5% Al doping. The band gap of AZO shows a blue shift up to 3% of Al doping as compared with the undoped ZnO. The blue shift in the band gap of the AZO films cannot be understood in the framework of Burstein–Moss shift and has been attributed to an increase in the stress present in the film. The photoluminescence spectrum of the undoped ZnO film shows a wide peak in the visible region which is suppressed with a small red shift after Al doping in the ZnO film. A detailed analysis of photoluminescence of ZnO and AZO films indicates suppression of zinc interstitials (Zni) and oxygen vacancies (VO) and creation of oxygen interstitial (Oi) defects after Al doping in ZnO films. X-ray photoelectron spectroscopy study also reveals suppression of oxygen vacancy related defects after Al doping in the ZnO film. The presence of Al in the ZnO matrix seems to change the defect equilibria leading to a suppression of Zni and VO and enhancement of Oi defects. The suppression of Zni defects is correlated with the increase in stress in Al-doped ZnO films.

Effects of Grain Boundary Barrier in ZnO/Si Heterostructure

The influence of ZnO microstructure on electrical barriers is investigated using capacitance-voltage (C – V), current-voltage (I – V) and deep level transient spectroscopy (DLTS) measurements. A deep level center located at Ec – 0.24 eV obtained by DLTS in the ZnO films is an intrinsic defect related to Zni. The surface states in the ZnO grains that have acceptor behavior of capturing electrons from Zni defects result in the formation of grain barriers. In addition, we find that the current transport is dominated by grain barriers after annealing at 600° C at O2 ambient. With the increment of the annealing temperature, the current transport mechanism of ZnO/Si heterostructure is mainly dominated by thermo-emission.

Electrical deactivation of interstitial Zn in heteroepitaxial InP by hydrogen and its effect on electronic properties

Hydrogen passivation of InP layers grown on lattice-mismatched substrates can achieve thermally stable deactivation of dislocation-related deep levels, making this a promising process for improving the performance of heteroepitaxial InP space solar cells. However, in addition to dislocation-related defects, interstitial Zn (Zni) defects that are characteristic of Zn-doped InP and which form deep donor states within the InP band gap, are important considerations for optimizing the electronic quality of these layers. Here, we show that hydrogen forms complexes with and deactivates Zni donor states within Zn-doped InP grown by metalorganic chemical vapor deposition. A combination of photoluminescence (PL), electrochemical capacitance–voltage dopant profiling, secondary ion mass spectroscopy and current–voltage (I–V) measurements are applied to a set of samples receiving systematic hydrogenation and annealing treatments. We find that the deactivation of Zni deep donors, as detected by monitoring the evolution of the donor–acceptor transition using PL measurements, causes an increase of ∼50% in the net acceptor concentration of heavily Zn-doped heteroepitaxial InP by elimination of the acceptor compensation effect due to active Zni donors. Analysis of I–V characteristics indicates that Zni passivation sharply reduces depletion region recombination and shunt currents within heteroepitaxial diodes, causing an increase in the diode turn-on voltage from 680 to 960 mV. Subsequent annealing above 500 °C reactivates the Zni defects, resulting in a systematic increase in doping compensation as well as a decrease in VTO toward the original, as-grown value. A study of the reactivation kinetics for the H–Zni complex reveals a greater thermal stability than that of H–Zn acceptor complexes but less than that of H-dislocation complexes in InP, with an estimated dissociation energy for the H–Zni complex of 2.3 eV. While these effects are observed for both homoepitaxial and heteroepitaxial Zn-doped layers, the effect is far more pronounced for the heteroepitaxial layers due to the relatively high Zni concentration in the latter.