Shallow Donor Levels Research Articles

Determination of activation behavior in annealed Al–N codoped ZnO Films

Al–N codoped ZnO films prepared via cosputtering technology were postannealed at 450 °C for 30 min under ambient vacuum and nitrogen. The extrinsic impurities in these annealed samples, resulting in evolutions on the carriers and radiation emissions, were investigated through their photoluminescence spectra and Hall-effect measurements. It was found that the donor-acceptor-pair emission was related to the VZn–AlZn transition at 2.86 eV and predominated over the defect-transition luminescence in the room-temperature photoluminescence (RTPL) spectrum of the vacuum-annealed sample, which possessed a high electron carrier concentration. With the help of temperature-dependent Hall measurements, a shallow donor level corresponding to Al on the Zn site (AlZn) was derived as EC − (51 ± 4) meV. In contrast, the defect-transition luminescence in the RTPL spectrum of the nitrogen-annealed Al–N codoped ZnO film, showing p-type conduction with a hole concentration of 1018 cm−3, was dominated by the VO–NO deep level emission at approximately 1.87 eV. The estimated acceptor level corresponding to the N on the O site (NO) was EV + (149 ± 6) meV. The binding energy and the activation energy associated with the NO acceptor were also determined by the low-temperature photoluminescence and temperature-dependent PL spectra.

Insertion of Zn atoms into Cu 3N lattice: Structural distortion and modification of electronic properties

Nanocrystalline Cu 3NZn x compound films were synthesized using reactive magnetron sputtering of metal targets. Up to a Zn content of 5.44 at.%, the deposits exhibit a satisfactory crystallinity that the X-ray diffraction patterns show distinct (0 0 1) and (0 0 2) reflections characteristic of the intrinsic Cu 3N lattice. The slightly enlarged lattice constant suggests insertion of zinc atoms to the center of cells of primitive Cu 3N lattice. All the ternary deposits exhibit an n-typed conductivity. Electrical resistivity at room temperature drops by three orders of magnitude with increasing zinc concentration from 0 to 5.44 at.%, and accordingly the activation energy for electrical conduction decreases from 21.9 to 14.1 meV, indicating the presence of shallow donor levels in doped samples. The electronic transport is governed by thermal activation in lightly doped samples, whereas in samples with a zinc concentration of 5.44 at.% or higher it is instead dominated by a hopping mechanism at low temperatures (<50 K).

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.

Hydrogen in red ZnO – defects or lattice expansion

The effect of hydrogen on ZnO while annealing at 1100 °C with Zn vapour or Ti metal is studied. ZnO turns red only when hydrogen is present in combination with oxygen deficiency. The coloration is reversible at 1100 °C by removing hydrogen either by evacuating or by gettering hydrogen with Ti at low temperature while annealing the ZnO at >950 °C. The latter is a simple method to lower the hydrogen impurity concentration. The annealing data are consistent with an activation energy >1.5 eV to reverse the coloration. Hall data on red ZnO reveal only a shallow donor level around 45 meV, too low for the colour change. Temperature dependent X-ray diffraction reveals a lattice expansion due to the presence of hydrogen which is sufficient to explain the colour change.

On the resonant donor level in n-CdTe according to data on electron transport under hydrostatic pressure

Results of quantitative analysis of experimental data on baric (under the hydrostatic pressure to P = 2.5 GPa and T = 300 K) and temperature (in the temperature range of 15–300 K at atmospheric pressure) dependences of the Hall coefficient and electrical conductivity of bulk n-CdTe crystals with the electron concentration of 1015–1017 cm−3 at T = 300 K are presented. The four-level model is used and included deep donor levels arranged in the band gap and in the continuous spectrum of the conduction band and shallow donor and acceptor levels. The location of the donor levels and pressure coefficients of energy gaps between them and the edge of the conduction band are determined.

Efficient field emission and optical properties of In-doped cadmium sulphide nanopens and nanopencils

Quasi-aligned indium (In)-doped cadmium sulphide (CdS) nanostructures synthesised by the gold (Au) metal-catalysed vapour–liquid–solid growth method on silicon (Si) substrates under different experiment conditions are reported. X-ray diffraction analysis on the as-prepared samples indicated the nanostructures as ‘wurtzite-type’ CdS crystal structures. Characterisation of the morphology and structure revealed the growth of two different geometries; nanopens and nanopencils with tip diameters ranging from 50–100 nm which can be varied through experimental conditions. Electron field emission measurements on the indium-doped quasi-aligned nanopencils and nanopens exhibit low turn-on electric fields of 5.5 and 4.5 V/µm (at the current density of 0.01 mA/cm2) for nanopens and nanopencils, respectively. This low turn-on field can be attributed to the sharp tip and higher indium doping level in the nanostrcutures. In addition, the indium doping into the CdS lattice was analysed by low-temperature photoluminescence spectroscopy (LT-PL). Temperature-dependent PL measurements showed that the PL spectra have three emission peaks at 9 K, which can be attributed to band edge free exciton emission and shallow donor levels donor–acceptor pair exciton emission because of doping. The result presents the promising application of these materials in the field of optoelectronics as efficient electron field emitters.

MOCVD法制备Cu掺杂ZnO薄膜

Un-doped and Cu-doped ZnO thin films were grown on c-Al2O3 substrate by metalorganic chemical vapor deposition(MOCVD) system.The way of changing the temperature of Cu(tmhd)2 source bottle was used to change the saturated vapor pressure of Cu(tmhd)2,in the meantime to change the quantity of Cu(tmhd)2 carried into the the reaction chamber.Then the content of Cu in ZnO films was tested to study the effects of Cu doping on the luminescent properties of ZnO,and explore the process technique and condition of controlling Cu valence in ZnO films.X-ray diffraction(XRD) show un-doped ZnO and ZnO∶Cu samples have good c-axis preferred orientation growth.The Cu content of these samples were measured by X-ray photoelectron spectroscopy(XPS).The content of Cu in ZnO films were not increased when the Cu(tmhd)2 source temperature was increased,which indicated that increasing the Cu(tmhd)2 source temperature did not lead to any increase of the Cu-doping level,and the solubility of Cu ions in ZnO is quite low.Low-temperature photoluminescence(PL) measurements show a strong and broad blue-violet(BV) light emission peak ranging from 2.8 to 3.3 eV,which could be assigned to donor-acceptor pair(DAP) recombination(between unknown shallow donor level and Cu-related acceptor level).Temperature dependent PL show a red-shift of the BV emission with increasing the temperature from 10 to 180 K,and a new weak shoulder peak appearing at BV emission's high-energy side at 80 K,which could be assigned to eA0.Moreover,the decrease of Cu content in ZnO∶Cu film resulted in quenching the intensity of BV emission red-shift of BV emission's peak position.This phenomenon is interesting and rarely reported in the previous works.However,we still need further study to clarify that if it is the electron transition between Zni-related donor energy level and valence band that causes the BV emission.

Study of composition dependent structural, optical, and magnetic properties of Cu-doped Zn1−xCdxS nanoparticles

Cu-doped Zn1−xCdxS nanoparticles were synthesized by coprecipitation technique in ice bath at 280 K. The band gap energy of Zn1−xCdxS:Cu nanoparticles can be tuned to a lower energy by increasing the Cd content, indicating the formation of the alloyed nanoparticles. The alloy structure is further supported by the systematic shifting of characteristic x-ray diffraction peaks to lower angles with increase in Cd content. Systematic copper doping induces a red shift in the energy band gap of Zn0.9Cd0.1S:Cu nanoparticles with increase in copper concentration. Cu-doped Zn0.9Cd0.1S nanoparticles were found to have ferromagnetic nature at 5 K whereas undoped particles were found to be diamagnetic. Green luminescence further proves proper doping of Cu into the ZnCdS matrix. It is believed that the green luminescence originates from the recombination between the shallow donor level (sulfur vacancy) and the t2 level of Cu2+. This method provides an inexpensive and simple procedure to produce ternary ZnCdS:Cu nanoparticles with tunable optical properties via changing Cd and/or Cu concentrations.

Impurity doping inSiO2: Formation energies and defect levels from first-principles calculations

Substitutional doping properties of silicon dioxide are investigated systematically with ab initio calculations. The dopants include a range of elements from group-III, group-V, and group-VII. We find that ${\text{SiO}}_{2}$ has a relatively symmetric doping profile in terms of its ionization energies, i.e., relatively shallow acceptor levels and donor levels are both predicted, despite its wide experimental band gap of 9.65 eV. The best candidates for $p$-type and $n$-type doping are ${\text{Al}}_{\text{Si}}$ and ${\text{P}}_{\text{Si}}$ with calculated ionization energy of 0.86 eV and 0.74 eV, respectively, both being less than 10% of the total band gap. Larger doping asymmetry exists in terms of the impurity formation energy: under optimum (O-rich) growth conditions, the shallowest acceptor, ${\text{Al}}_{\text{Si}}$, and donor, ${\text{P}}_{\text{Si}}$, have formation energies of 1.75 eV and 3.05 eV, respectively. These results provide theoretical insights on how to make this previously considered absolute insulator work as a wide-gap semiconductor at elevated temperatures.

Third generation biosensing matrix based on Fe-implanted ZnO thin film

Third generation biosensor based on Fe-implanted ZnO (Fe-ZnO) thin film has been demonstrated. Implantation of Fe in rf-sputtered ZnO thin film introduces redox center along with shallow donor level and thereby enhance its electron transfer property. Glucose oxidase (GOx), chosen as model enzyme, has been immobilized on the surface of the matrix. Cyclic voltammetry and photometric assay show that the prepared bioelectrode, GOx/Fe-ZnO/ITO/Glass is sensitive to the glucose concentration with enhanced response of 0.326 μA mM−1 cm−2 and low Km of 2.76 mM. The results show promising application of Fe-implanted ZnO thin film as an attractive matrix for third generation biosensing.

On the Nature and Behavior of Li Atoms in Si: A First Principles Study

On the basis of density functional theory calculations, we present the bonding and dynamic behavior of Li atoms in crystalline Si and how the incorporation of Li atoms affects the structure and stability of the host Si lattice. Our calculations clearly evidence that the inserted Li atom energetically prefers a tetrahedral interstitial site while exhibiting a shallow donor level. Because of their positive ionization, the interactions between neutral Li impurities are repulsive, suggesting that they favorably remain isolated, rather than clustered. We also find that the charge transferred from neutral Li is largely localized within the first nearest Si atoms, thereby effectively screening the positively ionized Li. In addition, our electronic structure analysis highlights that the charge transfer leads to a significant weakening of nearby Si−Si bonds by filling the antibonding sp3 states of Si. The mobility of Li interstitials is also estimated in the neutral and positive charge states.

Czochralski growth and characterization of β‐Ga2O3 single crystals

AbstractTransparent semiconducting β‐Ga2O3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga2O3. Thermodynamic calculations on different atmospheres containing CO2, Ar and O2 reveal that CO2 growth atmosphere combined with overpressure significantly decreases evaporation of volatile Ga2O3 species without any harm to iridium crucible. It has been found that CO2, besides providing high oxygen concentration at high temperatures, is also acting as a minor reducing agent for Ga2O3. Different coloration of obtained crystals as well as optical and electrical properties are directly correlated with growth conditions (atmosphere, pressure and temperature gradients), but not with residual impurities. Typical electrical properties of the n‐type β‐Ga2O3 crystals at room temperature are: ϱ = 0.1 – 0.3 Ωcm, µn,Hall = 110 ‐ 150 cm2V‐1s‐1, nHall = 2 ‐ 6 × 1017 cm‐3 and EIonisation = 30 ‐ 40 meV. A decrease of transmission in the IR‐region is directly correlated with the free carrier concentration and can be effectively modulated by the dynamic growth atmosphere. Electron paramagnetic resonance (EPR) spectra exhibit an isotropic shallow donor level and anisotropic defect level. According to differential thermal analysis (DTA) measurements, there is substantially no mass change of β‐Ga2O3 crystals below 1200 °C (i.e. no decomposition) under oxidizing or neutral atmosphere, while the mass gradually decreases with temperature above 1200 °C. High resolution transmission electron microscopy (HRTEM) images at atomic resolution show the presence of vacancies, which can be attributed to Ga or O sites, and interstitials, which can likely be attributed to Ga atoms. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Unveiling the defect levels in SnS thin films for photovoltaic applications using photoluminescence technique

AbstractExploiting the potential of the material for the photovoltaic applications requires an extensive defect level analysis, mandatory. Photoluminescence (PL) technique was employed to probe the defect levels in p‐SnS thin films deposited using chemical spray pyrolysis (CSP) technique. Three PL emissions were recorded at 1.09, 0.76, and 0.75 eV. Systematic investigations performed, focusing the 1.09 eV emission, revealed that the shoulder at 1.093 eV gets completely quenched beyond 110 K. From this study, we could identify a bound exciton associated to a shallow donor level whose activation energy was calculated to be 20 meV from Arrhenius plot. By studying the variation of PL intensity with excitation power, we could zero‐in that the emission at 1.09 eV was a donor–acceptor pair (DAP) transition. Knowing the band gap to be 1.33 eV, we could identify a deep acceptor at 0.22 eV above valence band. The band structure deduced from the present analysis is depicted in the abstract figure. magnified image

INFLUENCE OF pH VALUE ON THE STRUCTURES AND OPTICAL PROPERTIES OFZnONANOCRYSTALLINE SYNTHESIZED WITH A SOLUTION PROCESS

Three types of hexagonal wurtzite ZnO nanostructures namely nanorods, nanodisks and nanoflowers were prepared in three different pH value solutions. The results indicated that the concentration of H+ions in the solution would greatly affect the growth behaviors of ZnO crystal. In addition to a broad ultraviolet emission found on the photoluminescence spectra, two weaker peaks corresponding to the blue emission of ZnO were observed at 2.75 and 2.68 eV, which probably originate from the electron transition from the shallow donor level of interstitial Zn to the valence band of ZnO .

Role of point defects on conductivity, magnetism and optical properties in In 2O 3

In order to assess the effects of point defects including transition metal doping on its electronic structure, the self-consistent band structure of the transparent oxide In 2O 3 (in the Ia 3 structure) has been calculated with oxygen vacancies, oxygen and indium interstitial atoms and several transition metal dopants using density functional theory based first principles calculations. An oxygen vacancy alone does not act as a strong native donor but when combined with interstitial indium and (substitutional) transition metal doping, shallow donor levels close to the conduction band are formed. Spin polarized calculations show measurable magnetism in some of the transition metal doped systems while the dielectric functions indicate whether such systems remain transparent among other things.

Donor-donor binding in In2O3: Engineering shallow donor levels

Using first-principles band structure methods, we investigate the interactions between different donors in In2O3. Through the formation energy and transition energy level calculations, we find that an oxygen-vacancy creates a deep donor level, while an indium-interstitial or a tin-dopant induces a shallow donor level. The coupling between these donor levels gives rise to even shallower donor levels and leads to a significant reduction in their formation energies. Based on the analysis of the PBE0-corrected band structure and the molecular-orbital bonding diagram, we demonstrate these effects of donor–donor binding. In addition, total energy calculations show that these defect pairs tend to be more stable with respect to the isolated defects due to their negative binding energies. Thus, we may design shallow donor levels to enhance the electrical conductivity via the donor–donor binding.

Ionization behavior of deep level donors in passive film formed on surface of stainless steel in 5% salt solution

Deep level donor's ionization behavior of passive film formed on the surface of stainless steel was investigated by Mott-Schottky plots. It is indicated that transformation process of deep level donors' ionization behavior of passive film on surface of stainless steel can be divided into 4 stages with rising immersion time. At the initial immersion stage (10 min), Fe(Ⅱ) located in the octahedral sites of the unit cell is not ionized and the deep level does not appear in Mott-Schottky plots. At the second stage (9−38 h), Fe(Ⅱ) located in the octahedral sites starts to be ionized, which results in deep level donors' generation and density of deep level donors almost is constant with augmenting immersion time but the thickness of space charge layer is more and more thicker with rising immersion time. At the third stage (48 h−12 d), density of deep level donors rises with increasing immersion time and the thickness of passive films space charge layer decreases. At last stage (above 23 d), both the space charge layer's thickness and density of deep level donors are no longer changed with increasing immersion time. In the overall immersion stage, the shallow level donors' density is invariable all the time. The mechanism of deep level donor's ionization can be the generation of metal vacancies, which results in crystal lattice's aberration and the aberration energy urges the ionization of Fe(Ⅱ) in octahedral sites.

Stability of diatomic hydrogen in oxygen‐deficient ZnO

AbstractHere, we report a comparative analysis of the stability of different configurations of diatomic H and an oxygen vacancy, VO, in O‐deficient ZnO. It is found that in O deficient environment two spatially separated H, one occupying the interstitial site Hi and the other one substituting the O atom HO, which form shallow donor levels, can be energetically more favorable than the ${\rm H}_2^* $ complex and other possible locations of H and VO.

Oxygen Deficiency and Hydrogen Turn ZnO Red

The effect of hydrogen on ZnO while annealing at 1370 K under oxygen-poor conditions with excess Zn vapor or Ti metal is studied. ZnO turns red only when hydrogen is present in a complex with oxygen vacancies. A practical method is described to remove hydrogen from ZnO in a sealed ampoule and to bind it to Ti metal. Hydrogen coupled to an oxygen vacancy is the simplest defect to explain the observations. The coloration is reversible at 1370 K by adding or removing hydrogen, consistent with an activation energy >1.5 eV. In red ZnO Hall data show a shallow donor level around 45 meV.

Growth and characterization of non-polar ZnO thin films by pulsed laser deposition

Non-polar ZnO thin films were fabricated on r-plane sapphire substrates by pulsed laser deposition at various temperatures from 100 to 500 °C. The effects of the substrate temperature on structural, morphological and optical properties of the films were investigated. Based on the X-ray diffraction analysis, the ZnO thin films grown at 300, 400 and 500 °C exhibited the non-polar (a-plane) orientation and those deposited below 300 °C exhibited polar (c-plane) orientation. In the optical properties of non-polar ZnO films, there were two photoluminescence peaks detected. The peaks (near-band edge emission, blue emission) are due to electron transitions from band-to-band and shallow donor level to valence band, respectively.