Interstitial Related Defects Research Articles

Abnormal red shift in photoluminescence emission of ZnO nanowires

In this study, ZnO nanowires (NWs) have been synthesized through a simple chemical method and their optical properties were investigated using UV–vis, photoluminescence (PL) and Raman spectroscopy. PL studies revealed the red shift in the near band edge (NBE) emission peak with increasing diameter from 170 nm to 560 nm. By taking the sample with diameter d = 170 nm as a reference, the value of this red shift was found to be varied from 55 meV to 149 meV with the increase in diameter. Furthermore, this red shift shows the direct relation with the width of extra Zinc interstitial (Zni) or metal clusters related defect depletion layer at the boundary walls of nanowires. In this paper, the explanation for this red shift in PL is established by band bending at the edges of NWs caused by the depletion layer of excess Zn related defects. The confinement of the electrons at the surface of NWs was also found to be responsible for this red shift.

M-center in low-energy electron irradiated 4H-SiC

We report on the low-energy electron irradiated 4H-SiC material studied by means of deep-level transient spectroscopy (DLTS) and Laplace-DLTS. Electron irradiation has introduced the following deep level defects: EH1 and EH3 previously assigned to carbon interstitial-related defects. We propose that EH1 and EH3 are identical to M1 and M3, also recently assigned to carbon interstitial defects, and assign them to Ci=(h) and Ci0 (h), respectively.

Formation of carbon interstitial-related defect levels by thermal injection of carbon into n-type 4H-SiC

Electrical properties of point defects in 4H-SiC have been studied extensively, but those related to carbon interstitials (Ci) have remained elusive until now. Indeed, when introduced via ion irradiation or implantation, signatures related to Ci observed by deep level transient spectroscopy tend to overlap with those of other primary defects, making the direct identification of Ci-related levels difficult. Recent literature has suggested to assign the so-called M center, often found in as-irradiated 4H-SiC, to charge state transitions of the Ci defect in different configurations. In this work, we have introduced excess carbon into low-doped n-type 150 μm thick 4H-SiC epilayers by thermal annealing, with a pyrolyzed carbon cap on the sample surface acting as a carbon source. Because the layers exhibited initially low concentrations of carbon vacancies ([VC]=1011cm), this enabled us to study the case of complete VC annihilation and formation of defects due to excess carbon, i.e., carbon interstitials Ci and their higher-order complexes. We report on the occurrence of several new levels upon C injection, which are likely Ci-related. Their properties are different from those found for the M center, which point toward a different microscopic identity of the detected levels. This suggests the existence of a rich variety of Ci-related defects. The study will also help generating new insights into the microscopic process of VC annihilation during carbon injection processes.

ZnO Nanosheet-Nanowire morphology tuning for Dye-sensitized solar cell applications

In this work, ZnO nanosheet-nanowire morphology tuning could be achieved by simply changing the NaOH concentration in a low-temperature chemical bath. Our method produces nano/microstructures ranging from nanorods/nanowires with a wide range of diameters (~20 nm-4 µm) and aspect ratios (~1–20) to nanosheets with different dimensions. Photoluminescence spectra were dominated by violet emission attributed to Zn interstitial related defects forming during the growth. Focusing on the specific surface area of the final product, 30–40 nm-thick nanosheets showed the best J-V performance and transport dynamics when implemented in dye-sensitized solar cells.

Photoluminescence studies of nitrogen-vacancy and silicon-vacancy centers transformation in CVD diamond

In this study, the low-temperature micro-photoluminescence (PL) technology was employed to investigate the transformation of nitrogen-vacancy (NV), silicon-vacancy (SiV) centers in diamond crystal. Results showed that the NV and SiV luminescence were controlled by electron irradiation followed by thermal annealing. Both centers vanished together with the emergence of neutral single vacancy (GR1 center) after 200 keV electron irradiation. Interstitial related defects and vacancies were activated to diffuse by annealing (above ∼400 and 700 °C, respectively). The vacancies migrated to be captured by N and Si atoms due to the strain fields around the atoms attracted vacancies, and the NV and SiV centers appeared again in the PL spectra. In addition, compared the annealing behavior with NV center, the new emission at 639.7 nm was attributed to the nitrogen combined with carbon interstitials.

Strong violet emission and optical power limiting properties of reduced graphene oxide/MoO3 synergistic composites

Herein, we report the synthesis of reduced graphene oxide/molybdenum oxide (rGO/MoO3) via a simple precipitation method to improve the optical nonlinearity of MoO3. The successful materialization of composites was confirmed through x-ray diffraction, Raman spectroscopy, and field emission scanning electron microscopy studies. Rietveld refinement was done for the prepared samples to study the structural analysis. The optical studies revealed strong UV absorption and strong violet emission under 330 nm excitation. The mechanism of violet, blue, and green emissions from MoO3 is proposed through molybdenum interstitial related defects. The variation of bandgap in rGO/MoO3 composites was explained by the graphene induced strain on MoO3. The phonon lifetime of each sample was calculated, and it was found to decrease with respect to the rGO concentration, which makes this composite material potentially applicable for several electronic and optical applications. Moreover, energy dependent optical power limiting properties of the prepared MoO3 and rGO/MoO3 nanocomposites were measured by open aperture z-scan using nanosecond Nd-YAG pulsed laser operating at 532 nm excitation. It is found that the rGO/MoO3 nanocomposites have better optical power limiting properties with a good two photon absorption coefficient of 9.0 × 10−10 m/W. This could be attributed to the efficient interfacial charge transfer between MoO3 and rGO.

Identification and control of defects in nitrogen-doped ZnO

ZnO is regarded as a wide band gap semiconductor with high-efficient luminescent properties due to the stable existence of excitons at room-temperature. However, the prominent opto-electronic properties have not been achieved in real application because the defects existing in the material are quite complex in terms of form and property. Also, the p-type doping of ZnO is well-known to be extremely hard. As a result, more and more recent works have shown that the properties of the ZnO material are of high-possibility to be determined by the defects in ZnO. For instance, in p-type doped ZnO, free holes could be a result of complex defects rather than the anticipated dopants. Therefore, it is quite important to investigate the forms, properties, and control of various defects in ZnO. In this article, we will discuss the properties and control of various intrinsic and extrinsic defects in ZnO by reviewing the recent progress on related studies. Firstly, the formation, form, and control technique of the zinc interstitial related donors will be discussed. It is shown that the zinc interstitial related defect is an important compensating source to holes in nitrogen-doped ZnO material. The defects could be identified and tracked quantitively by a combination of a batch of characterization methods, such as electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Raman backscattering spectroscopy. The defects could be controlled by appropriately setting the condition of growth and/or post thermal treatment. Secondly, the main origin of possible acceptors in nitrogen-doped ZnO material has been pointed out. Intrinsic vacancies in combination with the extrinsic dopants, nitrogen, could be possible acceptors with shallow activation energy. Meanwhile, zinc vacancy small clusters could also be a candidate responsible for the shallow states of acceptors in ZnO. Moreover, novel complex acceptors as proposed by first-principles theoretical calculations have been experimentally introduced and realized. Under O-rich condition during nitrogen doping of ZnO, nitrogen molecule (N2) or nitrogen-hydrogen (NH x ) complexes could be incorporated on the zinc site, having shallow activation energy to contribute holes. As a result, the origin of p-typeness realized in nitrogen-doped ZnO could be partially understood. Combining the photoluminescence characterization, the corresponding relation between the defects and the optical emission lines has been established. Such a “fingerprint” could be utilized to assist identify defects in ZnO. Thirdly, we will discuss the isovalent-acceptor (IA) co-doping scheme, like tellurium-nitrogen or sulfur-nitrogen co-doping in ZnO. Such a co-doping technique could be a feasible route for obtaining high-efficient p-type ZnO material by suppressing compensation and enhancing activation rate of free holes. Zinc interstitials related donor-like complexes could be well suppressed while the co-doped isovalent element is beneficial to stabilizing nitrogen, restoring strain mismatch, and elevating the valence band maximum (VBM). The elevated VBM can further facilitate the ionization of acceptors, making the p-typeness much prominent. At last, using the IA co-doping technique, we will show the realization of ZnO homojunction light-emitting diode made by IA-co-doped ZnO nano-rod on high-quality ZnO thin films at room temperature and other applications of ZnO material to devices, like high-performance non-volatile memory and broadband photo-detector. The current problem, perspective and outlook of the defects and p-type doping in ZnO have been briefly discussed at the end of the article.

Temperature dependence of optical centres in ultrapure diamond after 200 keV electron irradiation

In this work, we conduct a temperature-dependent study of optical centres in ultrapure diamond after 200 keV irradiation by photoluminescence spectra, which are performed in a temperature range of 80–200 K. The zero phonon lines (ZPLs) include sizes of 550.3 nm, 593 nm, and 741 nm (GR1 centre). With an increase in measurement temperature, the ZPLs shift to the lower energy side, together with intensity quenching and an increase in the full width at half maximum. These results are noted in models of lattice contraction and electron–phonon coupling, and their inhomogeneous and homogeneous broadening mechanisms are also carefully distinguished by the Voigt function. The 550.3 nm and 593 nm emissions present harder bonds than the GR1 centre, close thermal quenching energies to the interstitial-related defect, a lower thermal softness than that of the ideal diamond, weak phonon bands at each lower energy side, and different broadening mechanisms with the GR1 centre, indicating that the 550.3 nm and 593 nm lines are possibly interstitial-related.

Nitrogen interstitial defects in silicon. A quantum mechanical investigation of the structural, electronic and vibrational properties

The vibrational features of eight interstitial nitrogen related defects in silicon have been investigated at the first principles quantum mechanical level by using a periodic supercell approach, a hybrid functionals, an all electron Gaussian type basis set and the Crystal code. The list includes defects that will be indicated as Ni (one N atom forming a bridge between two Si atoms), Ni-Ns (one interstitial and one substitutional N atom linked to the same Si atom), Ni-Ni (two Ni defects linked to the same couple of silicon atoms) and Ni-Sii-Ni (two Ni defects linked to the same interstitial silicon atom). Four 〈0 0 1〉 split interstitial (dumbbell) defects have also been considered, in which one lattice atom splits in two, and as a result the two interstitial atoms are three fold coordinated: they are two N (indicated as IN-N), one N and one Si (IN-Si), one N and one C (IC-N). For comparison, also the case with two Si atoms (ISi-Si) has been included. Four of these eight defects have unpaired electrons, and have been described through the UHF (Unrestricted Hartree-Fock like) computational scheme. The local defect geometry and the charge and spin density distributions have been analyzed. For the first time, intensities of IR and Raman spectra were calculated along with the frequencies, and this is crucial for the comparison of theoretical simulations with experiments. All these defects present very characteristic features in their IR spectrum, dominated by one or two very intense peaks. It has been possible to find a simulated counterpart to each one of the five peaks reported by Stein in 1985 (Applied Physics Letters, 47, 1339), and then to establish a correspondence between the microscopic structure of the defects and the IR intense peaks. The Raman spectra are in all cases dominated by the perfect silicon peak at about 530 cm−1, and are then not very useful for the characterization of the defects.

Role of zinc interstitial defects in indium and magnesium codoped ZnO transparent conducting films

Transparent conducting In and Mg codoped ZnO (IMZO) thin films were deposited on quartz substrates at room temperature by radio frequency magnetron sputtering. The influence of Mg concentration (from 0 to 6 at.%) on the structural, optical and electrical properties of IMZO thin films was investigated in detail. All IMZO thin films have typical hexagonal wurtzite structure with a preferential orientation along the c-axis and show high average optical transmittance above 85% in the visible region. And, the optical band gap is tunable with the addition of the Mg dopants. The presence of indium favors energetically the formation of abundant zinc interstitial (Zni) related shallow donor defects, thus greatly increasing the carrier concentration in the film. However, it was found that the electrical conductivity decreased as the Mg content increased. The main origin is related to the density reduction of Zni-related defects. Furthermore, the electrical properties of IMZO thin films can be significantly improved by thermal diffusion of extrinsic Zn atoms which creates extra Zni-related defects that provide electron carriers. These efforts will facilitate the development of ZnO-based TCO thin films.

Fermi level unpinning of metal/p-type 4H-SiC interface by combination of sacrificial oxidation and hydrogen plasma treatment

An ideal metal/p-type 4H-SiC interface with a “free-pinned” Fermi level has been achieved by the combination of sacrificial thermal oxidation (SO) and hydrogen plasma treatment (HPT) on the SiC surface. It is found that the Fermi level pinning could be attributed to the contaminants and defects of the p-type 4H-SiC surface. According to the X ray photoelectron spectroscopy and deep-level transient spectroscopy results, the oxygen and carbon contaminants decreased after SO. However, high-density carbon interstitial-related defects were generated close to the valance band during oxidation. With the subsequent HPT, the chemical residues and detrimental carbon-induced defects were eliminated by the reaction with hydrogen atoms. The p-type 4H-SiC surface was chemically and electrically well saturated with the surface Fermi level position close to the bulk position. An analytical model for the elimination of surface contaminants and defects was proposed to reveal the underlying mechanism of Fermi level depinning.

STUDY OF NANOSTRUCTURES OF ZINC OXIDE SYNTHESIZED BY LOW-TEMPERATURE HYDROTHERMAL METHOD

Room temperature photoluminescence (PL) properties of vertically aligned and spindle-shaped, randomly oriented ZnO nanorods synthesized by using a low temperature hydrothermal method are studied. In air, the vertically aligned ZnO nanorods oriented mainly parallel to the luminescencerecording axis exhibited only one, very strong UV emission peak at 382 nm. This band is assigned to emission of free excitons. A new violet PL band near 400 nm arises with increasing angle between the nanorod growth direction and the luminescence-recording axis. The violet band also appears under UV illumination in vacuum and vanishes after exposure to air. The randomly oriented ZnO nanorods along with free exciton related PL band reveal a broad yellow-orange emission band around 590 nm. The violet band is attributed to Zn vacancy related defects or their complexes, while the yellow-orange emission band is ascribed to oxygen interstitial related defects.

Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites.

We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.

Damage production in silicon carbide by dual ion beams irradiation

Lattice damage and evolution in single crystalline 6H-SiC under Si + He successively dual ion beams irradiation is studied by using Raman spectroscopy, high resolution X-ray diffraction (HRXRD) and nano-indentation tests. Single Si and He ion irradiations are also performed for the comparison. The results of Raman spectra reveal that the damage level increases with the fluence. A normal strain profile along the ion path is generated due to ion irradiation induced dilation of lattices, contributing mainly by interstitial related defects. Moreover, Si and He ion implantation produced different types of defects. The damage and chemical bonding states are significantly changed after He atoms implanted in Si pre-irradiated samples. Si + He dual ion irradiations increase the damage level further, resulting in changes of the damage states because of complex defects interactions. The nano-hardness of irradiated SiC is combined results of hardening effects of some kinds of defects and the breakdown of covalent-bonds. The mechanical properties present significant differences between single Si, He and Si + He successively dual ion beam irradiations, due to defects evolution during the irradiation process.

Enhanced photophysical properties of plasmonic magnetic metal-alloyed semiconductor heterostructure nanocrystals: a case study for the Ag@Ni/Zn1-xMgxO system.

Understanding the effect of homovalent cation alloying in wide band gap ZnO and the formation of metal-semiconductor heterostructures is very important for maximisation of the photophysical properties of ZnO. Nearly monodisperse ZnO nanopyramid and Mg alloyed ZnO nanostructures have been successfully synthesized by one pot decomposition of metal stearate by using oleylamine both as activating and capping agent. The solid solubility of Mg(ii) ions in ZnO is limited to ∼30% without phase segregation. An interesting morphology change is found on increasing Mg alloying: from nanopyramids to self-assembled nanoflowers. The morphology change is explained by the oriented attachment process. The introduction of Mg into the ZnO matrix increases the band gap of the materials and also generates new zinc interstitial (Zni) and oxygen vacancy related defects. Plasmonic magnetic Ag@Ni core-shell (Ag as core and Ni as shell) nanocrystals are used as a seed material to synthesize Ag@Ni/Zn1-xMgxO complex heterostructures. Epitaxial growth is established between Ag(111) and ZnO(110) planes in the heterostructure. An epitaxial metal-semiconductor interface is very crucial for complete electron-hole (e-h) separation and enhancement of the exciton lifetime. The alloyed semiconductor-metal heterostructure is observed to be highly photocatalytically active for dye degradation as well as photodetection. Incorporation of magnetic Ni(0) makes the photocatalyst superparamagnetic at room temperature which is found to be helpful for catalyst regeneration.

Interstitial oxygen related defects and nanovoids in Au implanted a-SiO2 glass depth profiled by positron annihilation spectroscopy

Samples of amorphous silica were implanted with Au ions at an energy of 190 keV and fluences of ions cm−2and ions cm−2 at room temperature. The damage produced by ion implantation and its evolution with the thermal treatment at 800 °C for one hour in nitrogen atmosphere was depth profiled using three positron annihilation techniques: Doppler broadening spectroscopy, positron annihilation lifetime spectroscopy and coincidence Doppler broadening spectroscopy. Around the ion projected range of nm, a size reduction of the silica matrix intrinsic nanovoids points out a local densification of the material. Oxygen related defects were found to be present at depths four times the ion projected range, showing a high mobility of oxygen molecules from the densified and stressed region towards the bulk. The 800 °C thermal treatment leads to a recovery of the silica intrinsic nanovoids only in the deeper damaged region and the defect distribution, probed by positrons, shrinks around the ion projected range where the Au atoms aggregate. Open volume defects at the interface between Au and the amorphous matrix were evidenced in both the as implanted and in the thermal treated samples. A practically complete disappearance of the intrinsic nanovoids was observed around when the implantation fluence was increased by two orders of magnitude ( ions cm−2). In this case, the oxygen defects move to a depth five times larger than .

Identification of defect-related emissions in ZnO hybrid materials

ZnO hybrid materials with singly precipitated ZnO nanocrystals embedded in the glass surface were fabricated by melt-quenching method followed by the annealing process. A series of samples containing different densities and species of intrinsic defects were obtained under different annealing conditions in a controllable manner, which was an ideal platform to identify the complicated defect origins. By employing photoluminescence (PL), excitation-dependent PL, PL excitation (PLE), and Raman spectroscopy, the radiative transitions of visible emission bands at around 401, 490, and 528 nm were unambiguously involved with zinc interstitial-related defect levels as initial states, and the corresponding terminal states were suggested to be valence band, oxygen vacancies, and zinc vacancies, respectively. This study may deepen the fundamental understanding of defect-related emissions and physics in ZnO and benefit potential applications of ZnO hybrid materials in optoelectronics.

Synthesis of ZnO nanoparticles by a hybrid electrochemical-thermal method: influence of calcination temperature

ZnO nanoparticles (NPs) with size less than 100 nm were successfully prepared by a hybrid electrochemical-thermal method using metallic zinc and NaHCO3 without the use of any zinc salt, template or surfactant. The NPs were characterized by Fourier transform infra-red (FT-IR) spectroscopy, UV-visible spectroscopy, photoluminescence spectroscopy (PL), thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. UV-visible spectral analysis indicated that the particle size increased with increasing calcination temperature. The band gap (3.91-3.83 eV) was higher for synthesized ZnO NPs than their bulk counterparts (3.37 eV). The FT-IR spectra at different calcination temperatures showed the characteristic band for ZnO at 450 cm-1 to be prominent with increasing temperature due to the conversion of precursor into ZnO. The wurtzite hexagonal phase was confirmed by XRD analyses for ZnO NPs calcined at 700oC. The green photoluminescent emission from ZnO NPs at different calcination temperatures is considered to be originated from the oxygen vacancy or interstitial related defects in ZnO. SEM images clearly showed that the NPs are granular and of almost uniform size when calcined at higher temperatures. EDX spectra further confirmed the elemental composition and purity of ZnO obtained on calcination at 700oC. The NPs are well dispersed near or above calcination temperature of 700oC.Bangladesh J. Sci. Ind. Res. 50(1), 21-28, 2015

Tunable zinc interstitial related defects in ZnMgO and ZnCdO films

We report tunable band gap of ZnO thin films grown on quartz substrates by radio frequency magnetron sputtering. The zinc interstitial (Zni) defects in ZnO films were investigated by X-ray diffraction, Raman scattering, Auger spectra, first-principle calculations, and Hall measurement. Undoped ZnO film exhibits an anomalous Raman mode at 275 cm−1. We first report that 275 cm−1 mode also can be observed in ZnO films alloyed with Mg and Cd, whose Raman intensities, interestingly, decrease and increase with increasing Mg and Cd alloying content, respectively. Combined with the previous investigations, it is deduced that 275 cm−1 mode is attributed to Zni related defects, which is demonstrated by our further experiment and theoretical calculation. Consequently, the concentration of Zni related defects in ZnO can be tuned by alloying Mg and Cd impurity, which gives rise to different conductivity in ZnO films. These investigations help to further understand the controversial origin of the additional Raman mode at 275 cm−1 and also the natural n-type conductivity in ZnO.

Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?

Evidencing interstitial Zn related defect states inside the conduction band of Zn-rich ZnO nanorods.