Resonant Donor Research Articles

Correlation of donor electrons in diluted magnetic semiconductors with iron

Substitutional iron in HgSe forms a resonant donor state whose energy is superimposed on the conduction band continuum. Above a certain doping level the system of Fe donors is only partially occupied by electrons, i.e. two charge states, Fe3+ and Fe2+, coexist, corresponding to the inhomogeneous mixed valence regime. Under these conditions the Coulomb repulsion between the donor electrons tends to keep them apart, leading to a correlation of their positions. The existence of the correlation results in a dramatic reduction of the rate of scattering by ionised impurity potentials. The same Coulomb interactions are responsible for the formation of the Coulomb gap in the one-particle density of impurity states, which in turn suppresses (otherwise very efficient) resonant scattering. As a result, the mobility and the Dingle temperature in HgSe:Fe can exceed those in HgSe doped with Ga to similar doping levels. A review is given of relevant experimental results obtained in HgSe:Fe, Hg1-xMnxSe:Fe and HgSe1-xTex:Fe by means of the Shubnikov-de Haas effect, resistivity and Hall voltage studies. Experimental findings are compared with results of numerical simulations and with a simple model calculation which accounts for short-range correlations.

Electron paramagnetic resonance and Coulomb gap in HgSe:Fe

The temperature and concentration dependence of the EPR absorption due to the Fe3+ state in HgSe is shown to be consistent with a Coulomb gap in the defect density of states. The latter is a consequence of inter-ion Coulomb interaction which renders a correlated ground state with short-range order of the partially ionised resonant donors. Possible mechanisms for a decrease of both the mobility and the EPR intensity for very high iron concentrations in this ordered, glass-like state are discussed.

Ionization potentials of non-metal monosulfides. Conjugation in radical cations containing Group IV elements

The first vertical ionization potentialsI(ns) of 69 monosulfides XSY (X, Y=H, Hal, organic, or heteroorganic substituent) are related to the inductive σI resonance (σR+) and polarizability (σα) constants of the substituents by dependences of theI(nS)=a+bΣσI+bΣσR+bΣσα type. TheI(ns) values are also affected by hyperconjugation which increases on going from XSH to XSY (Y≠H) compounds. The first calculations of the σR+ parameters characterizing the conjugation of Si-, Ge-, Sn-, and Pb-containing substituents with the S.+ radical cation center are reported. The reasons for weakening of resonance donor properties of heteroorganic substituents of the +M-type in the systems studied as compared to those of the same substituents in the corresponding aromatic radical cations are considered.

Some Advanced Microwave Techniques for the Investigation of Defect States in Semiconductors

The application of microwave techniques to the investigation of electronic defect properties relies on either of two principles: (i) resonant effects due to spin transitions (EPR) or (ii) non-resonant absorption due to free carriers, which originate from defects. Examples for both types are reviewed: The combination of ESR (i) with optical excitation allows in favourable cases characterization of deep defect levels with respect to their chemical nature and their local symmetry, and thresholds for photo-ionization and deionization can be obtained. Free carrier absorption (ii) can be utilized in order to monitor the emission of carriers from deep impurity states in a space charge region. This transient Microwave-Absorption Spectroscopy is shown to provide considerably higher bandwidth than conventional capacitance spectroscopy at comparable sensitivity. Finally, magneto-oscillations of the Shubnikov-de Haas type (iii) can be used for a precise determination of the free carrier concentration. In the zero gap compound HgSe: Fe, the simultaneous observation of the Fe3+-ESR and the magneto-oscillations allows investigation of the self-ionizing resonant donor level, Fe2+/3+.

Pressure-dependent measurements on n+ GaAs (Si, Sn): the effect of deep donor (DX) states on the electrical properties and persistent photoconductivity

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the mobility, free electron density (n) and the occupancy of the DX centre in heavily doped n-GaAs as a function of doping level and hydrostatic pressure. The results show that the DX centre produces a resonant donor level between the Γ - and L-conduction band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration mode measurements indicates that the DX level can be identified with SiGa. The level acts to pin the Fermi energy at electron concentrations around 1.8 × 1019 cm−3. Analysis of the results indicates that macroscopic charge separation is not responsible for persistent photoconductivity in these samples.

Fe-based semimagnetic semiconductors (invited)

Recently, the family of semimagnetic semiconductors (SMSC) or diluted magnetic semiconductors (DMS) typically involving manganese ions, i.e., Cd1−xMnxTe, Cd1−xMnxSe, Zn1−xMnxTe, Hg1−xMnxTe, was extended to include iron ions in selenium and tellurium II-VI compounds. Contrary to Mn-based DMS where d states of Mn are energetically superimposed on the valence-band continuum, for Cd1−xFexSe and Zn1−xFexSe the Fe2+(3d6) is a donor whose energy level lies deeply in the energy gap. On the other hand, for Hg1−v−xCdvFexSe with x≤0.15 and v≲0.40, the Fe2+ level is a resonant donor located in the conduction band. Also in the case of this material, the mobility of free electrons is surprisingly high, while the Dingle temperature is low. Because of the Coulomb interaction between the ionized donors Fe3+(3d5) at low T, there will appear a correlation of the positions of charges in this system. This leads to a kind of ‘‘condensation’’ of the charges in the system of donors and to their ‘‘crystallization’’ at even lower T (i.e., formation of a ‘‘charge superlattice’’ or a localized Wigner crystal of ionized donors). The space ordering of ionized donors dramatically influences the free-carrier scattering and consequently leads to high mobilities and low Dingle temperatures. Another implication of intersite repulsion is the existence of the Coulomb gap in the one-electron d-band density of states. We discuss some of the effects related to its formation. We shall also present some magnetic properties of these new DMS: the specific heat, magnetic susceptibility, and the magnetization.

Pressure-dependent studies of the DX centre in Si- and Sn-doped n +GaAs

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the effect of hydrostatic pressure on the free electron concentration, mobility, and the occupancy of the DX centre in MBE grown n +GaAs heavily doped with either Si or Sn. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The position and occupancy of the DX centre are calculated using Fermi-Dirac statistics. For the Si-doped samples comparison with local vibrational mode measurements indicate that the DX level can be identified with Si Ga.

Investigation of the DX center in heavily doped n-GaAs.

Shubnikov--de Haas and persistent-photoconductivity measurements are used to study the mobility, the free-electron density (n), and the occupancy of the DX center in heavily doped n-GaAs [Si,Sn] as a function of doping level and hydrostatic pressure. The results show that the DX center produces a resonant donor level between the \ensuremath{\Gamma} and L conduction-band minima at a concentration comparable with the doping level. For the Si-doped samples, comparison with local vibration-mode measurements indicates that the DX level can be identified with ${\mathrm{Si}}_{\mathrm{Ga}}$.

Reduction of charge-center scattering rate in Hg1-xFexSe.

The electrical resistivity, electron concentration, and mobility of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$Se are reported for 4.2<T<300 K and for 0.0001<x<0.12. The data are interpreted within an electronic band-structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground-state energy coincides with the conduction-band continuum. The electron-concentration data enable determination of the donor energy as a function of the temperature and crystal composition. Unexpectedly high values of the low-temperature electron mobility for \ensuremath{\sim}0.0003\ensuremath{\le}x\ensuremath{\le}0.01 indicate a reduction of the charged-impurity scattering rate at these temperatures. This effect is interpreted in terms of a spatial ordering of charges within the Fe-related impurity system.

Formation of a superlattice of ionized resonant donors or acceptors in semiconductors

In a semiconductor with high concentration of resonant donor states in the conduction band or of resonant acceptor states in the valence band, the Fermi level is pinned to the resonant state and only some of the resonant donors (or acceptors) are ionized. Due to the (screened) Coulomb interaction of the ionized centers, there occurs an ordering in space of these centers (made possible by the presence of neutral centers), even in the form of “crystallization”. It has a dramatic effect on scattering of free carriers. We compare these predictions with the recent data on Hg 1− x Fe x Se.

NMR studies of methylenebispyridinium derivatives

AbstractThe methylene proton chemical shifts of the magnified image system appear to be affected by the synergistic interactions of a combination of various effects. Although it is difficult to single out the origin, transmission and magnitude of these effects, the synergistic interactions pervade the molecule, causing a net displacement of the chemical shifts. The substituent‐produced chemical shifts show a clear trend in their dependence on the nature of the substituents and the Hammett parameters. Strong through‐conjugation between the aromatic nitrogen and resonance donor substituents in the para‐position is observed.

Location of the Fe2+(3d6) donor in the band structure of mixed crystals Hg1-vCdvSe

Several experimental methods-absorption, photo-emission, and transport measurements-were used to determine the energy position of the Fe2+(3d6) donor state in the band structure of the semi-magnetic semiconductor Hg1-v-xCdvFexSe. For v less than or equal to 0.40 the Fe2+(3d6) level is a resonant donor located in the conduction band.

II-VI Compounds with Fe - New Family of Semimagnetic Semiconductors

AbstractSeveral experimental methods: absorption, photoemission and transport measurements were used to determine the energy position of substitutional Fe2+ (3d6) donor state in the band structure of the semimagnetic semiconductor Hg1-v-xCdvFexSe for 0≤v≤0.7 and v+x=l, and x≤0.15. For v≤0.40, Fe2+(3d6) level is a resonant donor located in the conduction band. For v=O (HgSe) we obtain 230 meV for the position of Fe2+(3d6) level with respect to the bottom of the conduction band which coincides with the position of the Fermi level for electron concentration N ≅5x1018 cm-3. Surprisingly, the mobility of free electrons (T∼4.2K) is abnormally high and the Dingle temperature measured in quantum magnetoresistivity oscillations (SdH effect) and magnetooptical measurements is abnormally low. Because of the Coulomb interaction between the ionized donors, at low T, there will appear some correlation of their positions. This may lead to a kind of “liquefying” of the system of ions and to its “crystallisation” (i.e. formation of a superlattice or hyperlattice of ionized donors) at even lower T. The space-ordering of ionized donors influences dramatically the free-carrier scattering and correspondingly explains the high mobility and low Dingle temperature. Finally, we shall also present some magnetic properties of these new semimagnetic materials.

Electronic Transport Properties of Hg1-xFexSe

AbstractThe electrical resistivity, electron concentration, and mobility of Hg1-xFexSe are reported for 4.2K < T < 300K and for 0.0001 < x < 0.12. The data are interpreted within an electronic band structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground state energy coincides with the conduction band continuum. The electron concentration data enable determination of the value of the donor energy as a function of the temperature and the crystal composition. The low temperature electron mobility for ∼ 0.0003 ≤ x ≤ 0.01 is considerably higher than expected and indicates a reduction of the charged impurity scattering effects at low temperatures.

Observation of resonant impurity states in semiconductor quantum-well structures.

Resonant Raman scattering experiments on GaAs(Si doped)-${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum-well structures show transitions involving the ground state of the donors and narrow resonant donor states derived from higher conduction subbands. These new impurity-related features, which occur at slightly higher energies than the associated conduction intersubband excitations, have been studied as a function of power density, temperature, and well width.

Far-infrared two-photon intraband transitions in n-InSb

Two-photon cyclotron resonance and two-photon shallow donor transitions have been excited by a high power pulsed D 2O laser emitting λ = 119 μm and 66 μm laser lines at the same time. Transitions involving the absorption of two 119 μm photons or simultaneously one 119 μm and one 66 μm photon were observed. Two-photon selection rules are discussed by a rigorous treatment of the symmetry of the free electron Hamiltonian.

Applications of 95Mo NMR. 5—substituent effects in the 95Mo and 13C NMR spectra of benzyltricarbonyl(η5‐cyclopentadienyl)molybdenum(II) derivatives

AbstractA systematic study has been made of the effects of substituent induced chemical shifts in [(η5‐C5H5)(CO)3Mo(CH2C6H4R)] compounds. Both 95Mo and 13C NMR shifts in the aromatic ring are reported. The (η5‐C5H5)(CO)3MoCH2 group is a reasonably strong resonance donor (σR° = −0.21) and weak inductive donor (σI = −0.07). The molybdenum chemical shifts are extremely sensitive to the effects of distant substituents (range c. 40 ppm). Since the shift correlates well with substituent constants in this series, it is suggested that the chemical shift is controlled by the paramagnetic term for this spin 5/2 nucleus.

Transmission de l'effet de substituant à travers les systèmes insaturés-IV. Utilisation des relations de type Hammett avec les paramètres spectroscopiques i.r. et RMN pour la détermination des conformations de cétones α, β-éthyléniques aromatiques

The i.r. stretching frequencies of the carbonyl group and the chemical shift of the ethylenic proton in three series of α,β-unsaturated ketones, 3-substituted-5,5-dimethyl-2-cyclohexen-1-ones s-trans p-substituted-4-aryl-3-buten-2-ones and p-substituted-3-aryl-5,5-dimethyl-2-cyclo-hexen-1-ones, have been analysed using correlations with Hammett type substituent constants. The comparison of parameters ϱ of these correlations allows us to obtain quantitative information on the conformation of the last series of ketones. In this series, the dihedral angle between the aromatic ring and the plane of enonic system varies inversely with the resonance donor effect of substituent (0° for p-NH 2, 38° for NO 2).

High Magnetic Field Observation of the Resonance Donor States of S in InSb

AbstractElectrical transport measurements in InSb heavily doped with sulfur (n ≈ 5 × 1018 cm−3) are performed. At T = 4.2 K the Hall coefficient and transverse magnetoresistivity are measured as a function of pressure up to 2100 MPa and magnetic field up to 18 T. At the highest pressure, a decrease of the frequency and a decrease of the damping of Shubnikov‐de Haas oscillations are observed. These effects are explained as a result of the transfer of electrons from the conduction band to the resonance states of sulfur. At atmospheric pressure these states are 0.55 eV above the bottom of conduction band. In the lower pressure range, the experimental dependence of the effective Dingle temperature TD* = TD + Ti can be explained by the model in which the scattering by ionized impurities and short‐range potentials are taken into account. At the highest pressures a change of inhomogeneity of carrier concentration due to the transfer of electrons to the resonance states has to be considered.

Addition of carbon nucleophiles to arene-chromium complexes

The electrophilic reactivity of arenes coordinated to the chromium tricarbonyl unit has been developed into several distinct methods for coupling carbon nucleophiles with aromatic rings. Addition of the nucleophile produces stable η 5-cyclohexadienyl chromium complexes which can be oxidized to induce loss of the endo hydrogen and the metal, overall nucleophilic substitution for hydrogen. Alternatively, the intermediate can be protonated and the resulting cyclohexa-1,3-diene can be detached from the chromium, effecting nucleophilic addition with reduction of one double bond. If a halogen (F, Cl) is present as a ring substituent, and if the nucleophile can migrate about the arene ligand, then loss of halide can occur parallel with classical nucleophilic aromatic substitution for halogen in electron-deficient haloarenes. With substituted arenes, the regioselectivity of addition becomes important and is often very high. Particularly useful are strong resonance donor substituents (RO-, R 2N-, F-) where selectivity for meta attack is high. Indole provides an excellent example of selective activation, as the six-membered ring complexes selectively and is then susceptible to nucleophilic substitution, predominantly at the 4 and 7 positions. Substitution for halogen is a somewhat limited process and depends upon the nature of the nucleophile. Very reactive nucleophiles add to unsubstituted positions and are often slow to isomerize to the ipso position from which loss of halide can occur.