Remote Charges Research Articles

Spatial correlations of remote impurity charges: Mechanism responsible for the high mobility of a two-dimensional electron gas.

We have examined the electron mobility \ensuremath{\mu} as a function of the concentration of a two-dimensional electron gas n, in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures. Depending on the method used to vary n, different values of \ensuremath{\mu} can be obtained for the same n and in the same heterostructure. We suggest the origin of this effect lies in the interdonor interactions leading to spatial correlations of remote impurity charges. It results in a significant mobility enhancement at low temperatures. This finding demonstrates the necessity of taking into account many-body effects within the impurity system in different phenomena occurring in highly doped semiconductors.

Correlations of the remote impurity charges—A method of 2DEG mobility tuning in GaAs/AlGaAs heterostructures

Low temperature 2DEG mobility and carrier concentration measurements made on two modulation doped GaAs/AlGaAs heterostructures are described. We demonstrate that, depending on the method which is used to populate the metastable donor states of Si-remote impurity, different values of electron mobility can be obtained for the same 2DEG density and in the same heterostructure. Concept of the correlation in the spatial distribution of the impurity charges (i.e. positively and negatively charged states of Si donor), caused by the Coulomb interactions is employed to explain our experimental finding. The invoked effect induces a strong reduction of the 2DEG scattering and can produce the mobility enhancement by as much as a factor of two over its value with a random distribution of impurity charges.

Tuning of 2DEG Mobility by Modification in Ordering of Remote Impurity Charges in GaAs/AlGaAs Heterostructures

Tuning of 2DEG Mobility by Modification in Ordering of Remote Impurity Charges in GaAs/AlGaAs Heterostructures

Lattice models for the MSXα description of transition metal impurities in ionic crystals

The development of a near MSXα description of transition metal impurities in polar solids is described. Non-muffin-tin corrected MTXα calculations are performed for clusters comprising the impurity together with one or two shells of adjacent lattice ions stabilized by potentials calculated from lattice models of increasing sophistication. The method is illustrated by its application to Cr3+:MgO and Cu+:NaX (X=Cl,F). The calculations are much faster than Hartree–Fock (+configuration interaction) techniques and the method offers an attractive route for computing the potential surfaces and wave functions which determine the impurity related properties of the doped host. The representation of the confining lattice potential is shown to be of paramount importance. Clusters comprising the impurity plus the two nearest shells of lattice ions embedded in a lattice of adjacent Hartree–Fock–Slater ions and remote point charges are shown to give good quality potential surfaces, but only when lattice–cluster exchange effects are included.

Scattering mechanisms in inverted AlGaAs-GaAs heterostructures studied by magnetotransport measurements

We determined the single particle relaxation time τs and the classical transport scattering time τt out of Shubnikov-de Haas (SdH) measurements at a GaAs-AlGaAs-GaAs double heterostructure consisting of a normal and an inverted interface. As a result we find that even in inverted heterostructures the classical scattering time τt is an order of magnitude larger than the single particle relaxation time τs. From a comparison with theoretical calculations including scattering from remote impurities, interface charges, and interface roughness, we conclude that the mobility in inverted heterostructures at low temperatures is dominated by Coulomb scattering from remote dopants and interface charges in the barrier, rather than interface roughness scattering. Nevertheless, the mobility of the inverted interface turns out to be significantly lower than that of a normal interface.

On the Fermi-Thomas screening of remote charges in quantised inversion layers at low temperatures

The standard theory of Fermi-Thomas screening of remote impurities in quantised inversion layers uses the assumption that the difference between the energy of the ground state and the average potential energy in the layer is not altered by the presence of a remote impurity. Analysis shows that this assumption is not correct and that, as a consequence, the screening factor is smaller than is usually assumed. The results are illustrated for the case of GaAs inversion layers.

Electric field effects on the ground state hyperfine structure of hydrogenic atoms

The hyperfine structure of the ground state of the one-electron atom, with nuclear spin of 1/2, is discussed in terms of perturbation theory. The contact and dipolar hyperfine interactions plus the electric field effects from remote static charges are treated, performing exact summation over all discrete and continuum states, treating the second and third order mixed perturbation terms linear in the hyperfine Hamiltonian. Expressions for the energy terms, linear in the electric field gradient, quadratic in the electric field, and quadratic in the gradient are all derived, and their magnitudes evaluated for a simple field configuration.