Host Band Structure Research Articles

Performance tweaking of Co3O4 UV radiation detector via zirconium dopant microstructural and photo-electronic engineering

Incorporation of dopant impurities in semiconducting metal oxide nanostructures has been presumed to increase the pace at which photo-induced holes and electrons separate due to possible bandgap engineering and oxygen vacancy modulations. In this study, we have adopted an electrodeposition technique using a three-electrode configuration for the preparation of Co3O4 thin film samples and investigated the effect of Zr as a dopant ion on the UV radiation sensing performance of nanostructured Co3O4 host material via energy band engineering and microstructural modulation. Consistency in optical energy band gap measurements of the deposited Zr-doped Co3O4 (Zr@Co3O4) was unveiled with the aid of two models: Tauc’s and absorption spectrum fitting (ASF) methods. Ag/Zr@Co3O4/ITO/glass UV radiation detector was fabricated and a sample with 3 % Zr-dopant content demonstrated high UV-365 nm detector performance: detectivity, 1.2 ×1011 Jones, and responsivity, 1818 mA W−1, with high cycling stability, attributable to the host's dopant microstructural tailoring, and the enhancement in its electronic charge carrier number and their scattering potentials within the host's band structure, thanks to the synergistic impacts of dopant ion incorporation and UV irradiation.

N - to p -type conductivity transition and band-gap renormalization in ZnO:(Cu+Te) codoped films

The natural conductivity of as-grown ZnO is $n$-type. It has been challenging to produce stable $p$-type material, which has delayed possible technological applications. In this work, the conductivity transformation from $n$- to $p$-type ZnO films deposited by sputtering was followed as a function of copper and tellurium concentrations and of the substrate temperature during growth (room temperature, $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}, 250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and $350{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$). The nominal codopant concentrations ranged from 1 to 12 at %. For the minimum concentration, compensation effects yielded highly resistive ZnO. Nonetheless, by tailoring the concentration of Cu and Te, it was possible to vary the resistivity $({10}^{3}--{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm})$, mobility $(\ensuremath{\sim}{10}^{\ensuremath{-}2}--{10}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s})$, and free-hole density $({10}^{15}--{10}^{20}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3})$ of $p$-type ZnO grown at $250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Besides modifying the electrical properties, codoping changed the host band structure significantly, producing a band-gap renormalization from 3.2 eV (UV) to 1.8 eV (red). This control over the band gap is advantageous for applications where controllable photon absorption or emission are sought. Experimentally, films were stable for a period of at least six months. Band-gap engineered $p$-type ZnO:(Cu+Te) films open the possibility for the fabrication of all-ZnO optoelectronic devices such as homojunction solar cells and/or light-emitting diodes.

Locating impurity and defect levels in the host band gap by first-principles calculations: Pure and Ce3+-doped YAlO3

The structural and electronic properties of the YAlO3 crystal (neat, Ce3+-doped and with antisite defects) were calculated from the first-principles. Comparison of the calculated electronic band structures of the ideal YAlO3 host with the YAlO3:Ce3+ and YAlO3 (antisite) cases allowed to determine the Ce3+ ground 4f state to be at 3.5 eV above the valence band top, in very good agreement with the Dorenbos model prediction. The complete energy level scheme of the YAlO3:Ce3+ crystal was obtained, with the cerium crystal field split 4f and 5d levels superimposed onto the host's band structure. It was shown that the yttrium antisite ions produce localized defect levels 0.87 eV below the conduction band, which enables the antisite defects to serve as the trap centers for the electron excitation energy in this host.

Synthesis and spectroscopic characterization of strontium magnesium silicate phosphors for white afterglow applications

Eu 2+ and Dy 3+ ion–activated strontium magnesium silicate phosphors were prepared using a modified solid-state method and two silicon sources: silicon oxide and silicon nitride. Dy 3+ ions were deliberately used to form various trap levels in the host band structure and thus achieve long-lasting performance of the silicate host. The coexistence of Sr 2 MgSi 2 O 7 and Sr 3 MgSi 2 O 8 phases was observed in the X-ray diffraction patterns obtained for phosphors synthesized from various silicon oxide to silicon nitride molar ratios. Photoluminescence spectra contained two broad bands at 464 nm (blue) and 540 nm (yellow) for the same host owing to 4f 7 –4f 6 5d transitions of Eu 2+ ions. The phosphors prepared using higher percentages of silicon nitride exhibited white light emission with chromaticity of (0.28, 0.32). A persistent white afterglow decay time of 10 s was determined using time-resolved spectroscopy for the phosphors prepared using high percentages of silicon nitride. A low-temperature thermoluminescence peak with a trap depth of 0.49 eV and lifetime of 140 s was found to be responsible for the afterglow properties of the strontium magnesium silicate phosphors. The present study demonstrated the suitability of using the obtained silicate phosphors for near-white-afterglow applications.

Regulating Electronic Structure of Metal-Organic Frameworks for Improved Photocatalytic Activity Through Trace of Molecular Doping

Doping, introducing trace of impurity into a material to alter its original electrical or optical properties, constitutes the very basis for the great number of electronic devices pervaded in material sciences. Unfortunately, this strategy has yet to be applied in metal-organic frameworks (MOFs). In this study, we for the first time demonstrate that doping trace of organic molecules into MOFs dramatically alters the electric structure and optical property of the bulky material without disturbing the original frameworks as confirmed by both powder and single-crystal x-ray diffraction. This method is applicable to various dopants and other types of MOFs. Theoretical calculation suggested that the dopant introduces an impurity level in the host band structure, which greatly narrows the band gap and boosts the photogenerated electron-hole separation. Therefore, a photocatalytic inert MOFs can achieve excellent activity for water splitting though a trace of molecular doping. These findings presented here provide a powerful yet facile strategy toward regulating the electronic band structure of MOFs for improved catalytic activity without necessarily changing the original chemical components, topology, and porosity.

Luminescence tuning of Ce3+, Pr3+ activated (Y,Gd)AGG system by band gap engineering and energy transfer

Tuning of phosphor luminescence properties, including the emission energy/intensity and thermal stability, is an important way to develop superior luminescent materials for diverse applications. In this work, we discuss the effect of band gap engineering and energy transfer on the luminescence properties of Ce3+ or Pr3+ doped (Y,Gd)AGG systems, and analyze the underlying reasons for their different phenomena. By using VUV-UV excitation spectra and constructing VRBE schemes, the changes of host band structure, 5d excited level energies and emission thermal stability of Ce3+ and Pr3+ with the incorporation of Gd3+ ions were studied. In addition, the energy transfer dynamics was also investigated in terms of the luminescence decay curves. This work demonstrates a way to tune phosphor luminescence properties by combining band gap engineering and energy transfer tailoring and provides an inspiring discussion on the different results of Gd3+ doping on the Ce3+ and Pr3+ emissions.

A short review of theoretical and empirical models for characterization of optical materials doped with the transition metal and rare earth ions

In this paper, a brief retrospective review of the main developments in crystal field theory is provided. We have examined how different crystal field models are applied to solve the problems that arise in the spectroscopy of optically active ions. Attention is focused on the joint application of crystal field and density functional theory (DFT) based models, which takes advantages of strong features of both individual approaches and allows for obtaining a complementary picture of the electronic properties of a doped crystal with impurity energy levels superimposed onto the host band structure.

Spectroscopic perspective on the interplay between electronic and magnetic properties of magnetically doped topological insulators

We combine low energy muon spin rotation (LE-$\mu$SR) and soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study the magnetic and electronic properties of magnetically doped topological insulators, (Bi,Sb)$_2$Te$_3$. We find that one achieves a full magnetic volume fraction in samples of (V/Cr)$_x$(Bi,Sb)$_{2-x}$Te$_3$ at doping levels x $\gtrsim$ 0.16. The observed magnetic transition is not sharp in temperature indicating a gradual magnetic ordering. We find that the evolution of magnetic ordering is consistent with formation of ferromagnetic islands which increase in number and/or volume with decreasing temperature. Resonant ARPES at the V $L_3$ edge reveals a nondispersing impurity band close to the Fermi level as well as V weight integrated into the host band structure. Calculations within the coherent potential approximation of the V contribution to the spectral function confirm that this impurity band is caused by V in substitutional sites. The implications of our results on the observation of the quantum anomalous Hall effect at mK temperatures are discussed.

Validity of the rigid band approximation in the study of the thermopower of narrow band gap semiconductors

Theoretical studies of thermoelectric properties using ab initio electronic structure calculations help not only to understand existing experimental data but also to predict new materials which can be potentially good thermoelectrics. However, in these studies it is inevitable to employ some approximations. It is therefore important to verify their reliability. To this end, we have investigated the validity of the rigid band approximation (RBA), commonly used in calculating the thermopower ($S$) in doped (sometimes heavily) narrow band gap semiconductors. We have considered two important systems: half-Heusler HfCoSb and PbTe. We calculate band structures of pure and doped systems [using quasiperiodic approximation (QPA)] by employing the density-functional method. We then use Boltzmann transport theory to calculate the thermopower using both RBA and the band structure with QPA. We find that band structures do not change significantly when isovalent impurities are present excepting in specific cases. However, charged impurities (relevant to the doping case) providing carriers can change the host band structure appreciably. We find that impurities in general remove existing degeneracies which tend to reduce the RBA value of $\ensuremath{\mid}\phantom{\rule{-0.16em}{0ex}}S\phantom{\rule{-0.16em}{0ex}}\ensuremath{\mid}$. The reduction is significant in both HfCoSb and PbTe when charged defects are present.

Ferromagnetic nature of (Ga, Cr)As epilayers revealed by magnetic circular dichroism

A systematic study of magnetic circular dichroism (MCD) was carried out in a wave length range 500–960 nm for (Ga1−x, Crx)As epilayers with x=2.38% and 4.59% grown by the low temperature molecular beam epitaxy (LT-MBE) technique. Hysteresis characteristics showed up indeed in the magnetic field dependence of both MCD and magnetization measured by the superconductor quantum interference device (SQUID). The Curie temperature of the (Ga1−x, Crx)As epilayer was determined to be about 12 K by the Arrott approach. The present result provides evidences that there is strong coupling of the Cr spins to the GaAs host band structure in (Ga1−x, Crx)As samples. That affects the critical point of the semiconductor host, and makes the magnetization behavior in a plot of M∼B/T (magnetic field divided by temperature) substantially different from standard superparamagnetism.

Possibled0ferromagnetism in MgO doped with nitrogen

We study the possibility of ${d}^{0}$ ferromagnetism in the compound MgO doped with nitrogen (N). The Haldane-Anderson impurity model is formulated within the tight-binding approximation for determining the host band structure and the impurity-host hybridization. Using the quantum Monte Carlo technique, we observe a finite local moment for an N impurity, and long-range ferromagnetic correlations between two N impurities. The ferromagnetic correlations are strongly influenced by the impurity bound state. When the ferromagnetic correlation between a pair of impurities is mapped onto the isotropic Heisenberg model for two spin-1/2 particles, the effective exchange constant ${J}_{12}$ is found to increase with increasing temperature. Similar temperature dependence of ${J}_{12}$ is also obtained in other diluted magnetic semiconductors, such as zincblende ZnO doped with Mn. The temperature dependence of ${J}_{12}$ suggests that the mapping of the full Hamiltonian onto the spin Hamiltonian cannot fully describe the magnetic correlations for the diluted magnetic semiconductors at least in the limit of low impurity spin.

Inter-impurity and impurity–host magnetic correlations in semiconductors with low-density transition–metal impurities

Experiments on (Ga,Mn)As in the low-doping insulating phase have shown evidence for the presence of an impurity band at 110 meV above the valence band. The motivation of this paper is to investigate the role of the impurity band in determining the magnetic correlations in the low-doping regime of the dilute magnetic semiconductors. For this purpose, we present results on the Haldane–Anderson model of transition–metal impurities in a semiconductor host, which were obtained by using the Hirsch–Fye quantum Monte Carlo (QMC) algorithm. In particular, we present results on the impurity–impurity and impurity–host magnetic correlations in two- and three-dimensional semiconductors with quadratic band dispersions. In addition, we use the tight-binding approximation with experimentally determined parameters to obtain the host band structure and the impurity–host hybridization for Mn impurities in GaAs. When the chemical potential is located between the top of the valence band and the impurity bound state (IBS), the impurities exhibit ferromagnetic (FM) correlations with the longest range. We show that these FM correlations are generated by the antiferromagnetic coupling of the host electronic spins to the impurity magnetic moment. Finally, we obtain an IBS energy of 100 meV, which is consistent with the experimental value of 110 meV, by combining the QMC technique with the tight-binding approach for an Mn impurity in GaAs.

Crystal structure effect on the ferromagnetic correlations in ZnO with magnetic impurities

We study the ferromagnetism in the compound (Zn,Mn)O within the Haldane–Anderson impurity model by using the quantum Monte Carlo technique and the tight-binding approximation for determining the host band structure and the impurity-host hybridization. This computational approach allows us to determine how the host crystal structure influences the impurity bound state, which plays an important role in the development of the ferromagnetic (FM) correlations between the impurities. We find that the FM correlations are strongly influenced by the crystal structure. In particular, in p-type (Zn,Mn)O, we observe the development of FM correlations with an extended range at low temperatures for wurtzite and zinc-blende crystal structures. However, for the rocksalt structure, no FM correlations are observed between the impurities. In addition, in n-type ZnO with magnetic impurities, the impurity bound state and FM correlations are not found.

Disorder-enhanced spin polarization in diluted magnetic semiconductors

We present a theoretical study of diluted magnetic semiconductors that includes spin-orbit coupling within a realistic host band structure and treats explicitly the effects of disorder due to randomly substituted Mn ions. While spin-orbit coupling reduces the spin polarization by mixing different spin states in the valence bands, we find that disorder from Mn ions enhances the spin polarization due to formation of ferromagnetic impurity clusters and impurity bound states. The disorder leads to large effects on the hole carriers which form impurity bands as well as hybridizing with the valence band. For Mn doping $0.01\ensuremath{\lesssim}x\ensuremath{\lesssim}0.04$, the system is metallic with a large effective mass and low mobility.

Influence of band structure effects on domain-wall resistance in diluted ferromagnetic semiconductors

Intrinsic domain-wall resistance (DWR) in (Ga,Mn)As is studied theoretically and compared to experimental results. The recently developed model of spin transport in diluted ferromagnetic semiconductors [Van Dorpe et al., Phys. Rev. B 72, 205322 (2005)] is employed. The model combines the disorder-free Landauer-B\"uttiker formalism with the tight-binding description of the host band structure. The obtained results show how much the spherical $4\ifmmode\times\else\texttimes\fi{}4$ $\mathbit{k}\mathbit{p}$ model [Nguyen, Shchelushkin, and Brataas, Phys. Rev. Lett. 97, 136603 (2006)] overestimates DWR in the adiabatic limit, and reveal the dependence of DWR on the magnetization profile and crystallographic orientation of the wall.

Ground state splitting ofS8rare earth ions in semiconductors

We propose a new mechanism leading to the ground state splitting for the rare earth ^8S ions in semiconductor crystals. The resulting splitting is due to three effects, the first is the intra atomic 4f-5d spin-spin interaction, the second one is the spin - orbit interaction for 5d electrons and the third one is their hybridization with the valence band states of semiconductors host. The resulting splitting significantly depends on the relative position of 5d level with respect to semiconductor host band structure. We also discuss different model, already known in the literature, which is also based on ion - band states hybridization. For both models, as an example, we present results of numerical calculations for rare earth ion in IV-VI semiconductor PbTe.

Impurity perturbation to the host band structure and recoil of the impurity state

At sufficiently high doping levels, the impurities in a semiconductor are expected to perturb the host band structure, and the perturbed host is then expected to alter the impurity state from that of the dilute limit (a recoil). Despite many decades of studies on impurities, it has been impossible to simultaneously and accurately track the evolution of the host band structure and the impurity state. The isoelectronically doped system provides a unique opportunity to track this evolution. GaAs:N, as a prototype system, has been investigated both experimentally and theoretically for this purpose.

Relating Localized Electronic States to Host Band Structure In Rare-Earth-Activated Optical Materials

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Charles W. Thiel, Herve Cruguel, Huasheng Wu, Yongchen Sun, Gerald J. Lapeyre, Rufus L. Cone, Randy W. Equall, and Roger M. Macfarlane, "Relating Localized Electronic States to Host Band Structure In Rare-Earth-Activated Optical Materials," Optics & Photonics News 12(12), 64-64 (2001) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Nonradiative processes involving rare-earth impurities in nanostructures

Doping nanostructures with rare earth (RE) impurities offers a new way of engineering light emitting devices (LED). However, the proximity of interfaces opens new channels of nonradiative processes detrimental for the operation of LEDs. Here we discuss the influence of host band-structure on nonradiative processes at surfaces, as well as the RE autoionization across the interface. The effect of a spatial confinement on the radiative processes is also discussed.

Electronic structure near the band gap of heavily nitrogen doped GaAs and GaP

AbstractIsoelectronic impurity nitrogen atoms have been found to generate a series of localized states in GaP and GaAs. These states can be either bound (within the band gap) or resonant (above the band gap) when in the dilute doping limit (roughly < 1019 cm−3 for GaP and < 1018 cm−3 for GaAs). With increasing nitrogen doping level, a shift of the absorption edge from the binary band gap has been observed for the so-called GaPN or GaAsN alloy. We discuss the similarity and dissimilarity between the two systems in the following aspects: (1) How does the nitrogen doping perturb the host band structure? (2) How do the nitrogen bound states evolve with increasing nitrogen doping level? (3) What are the dominant contributors to the band edge absorption? And (4) does a universal model exist for GaPN and GaAsN? Other issues that will be discussed are: how does one define the band gap for these materials, and what is the relevance of various theoretical band structure calculations to the experimentally measured parameters.