Ground State Of Impurity Research Articles

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.

Spin splitting of X-valley-related donor impurity states in an AlAs barrier

We use magnetotunneling spectroscopy to observe the spin splitting of the ground state of an X-valley-related Si-donor impurity in an AlAs barrier. We determine the absolute magnitude of the effective Zeeman spin splitting factors of the impurity ground state to be g$_{I}$= 2.2 $\pm $ 0.1. We also investigate the spatial form of the electron wave function of the donor ground state, which is anisotropic in the growth plane.

Off-centering of hydrogenic impurities in quantum dots

We report exact numerically calculated ground state and binding energies of a hydrogenic donor impurity confined everywhere inside a spherical quantum dot (QD) surrounded by air or a vacuum. Finite spatial steplike potentials allowing the electronic density to partially leak outside the QD are considered. This model faces a divergence produced by the self-polarization potential at the position of the dielectric mismatch. We bypass it by replacing the edge steplike dielectric mismatch by a continuous variation within an extremely thin layer at this edge. A comprehensive study of several confining factors influencing electronic and binding energies is carried out and a highly nonadditive interplay is found. Our calculations show that within both the strong and weak confinement regimes we may be faced with three different behavior regimes. We call them low, intermediate, and high. In the low and intermediate behaviors, the mass, polarization, and self-polarization effects exert a very strong influence on the electron density distribution, so that perturbational estimations of the binding energy may not be appropriate even in the strong confinement regime. These low and intermediate behavior regimes are responsible for binding energy profiles not being monotonously decreasing vs off-centering. It is even theoretically possible to design systems with off-centering independent binding energies.

The electric field dependence of a donor impurity in graded GaAs quantum wires

The effect of the electric field on the binding energy of the ground state of a shallow donor impurity in a graded GaAs quantum-well wire (GQWW) was investigated. The electric field was applied parallel to the symmetry axes of the wire. Within the effective mass approximation, we calculated the binding energy of the donor impurity by a variational method as a function of the wire dimension, applied electric field, and donor impurity position. We show that changes in the donor binding energy in GQWWs strongly depend not only on the quantum confinement, but also on the direction of the electric field and on the impurity position. We also compared our results with those for the square quantum-well wire (SQWW). The results we obtained describe the behavior of impurities in both square and graded quantum wires.

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ∼5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ∼1.99 THz (8.2 meV) and ∼2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ∼3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ∼17 nW per 1 W of the input power.

The Photoionization Cross-Section of Impurities in Quantum Dots

In quantum dot structures, photoionization has been considered as an optical transition from the impurity ground state to the conduction subbands. Using a variational approach, we have calculated the photon energy dependence of the photoionization cross-section for a hydrogenic donor impurity in an infinite barrier GaAs quantum dot as a function of the sizes of the dot and the impurity position. The results we have obtained show that the photoionization cross-section is strongly affected by the quantum size effects and the position of the impurity and its overall shape seems to be a signature of the quantum dot system.

New mechanism of impurity conduction in lightly doped crystalline noncompensated silicon

It is found that the plastic deformation of lightly doped crystalline silicon samples (N<6×1016 cm−3) with a low compensation (K∼3×10−2) gives rise to nonohmic conduction σM in electric fields that differs radically from conventional hopping conduction via the ground states of impurities (σ3). The values of σM can exceed values of σ3 by a factor of 103−105. The value of σM and its dependence on the electric (E) and magnetic (H) fields can be controlled by varying the density of dislocations and the mode of thermal sample treatment. A strong anisotropy of σM is observed in samples with oriented dislocations: the conductivities along and across dislocations can differ by a factor of 104. The results are explained by the occurrence of conduction via the H−-like states of impurities concentrated in the vicinity of dislocations. The levels of these states lie between the upper and the lower impurity Hubbard bands.

NMR investigation of CdF 2 : Ga

113Cd nuclear spin-lattice relaxation measurements have been carried out over the temperature range 200–500 K on semiconducting crystals of CdF 2 : Ga and co-doped CdF 2 : Ga,Y. The relaxation rates 1/ T 1 in both crystals are found to be enhanced by two to three orders of magnitude relative to undoped CdF 2. We attribute this enhanced relaxation to the effects of thermally excited conduction electrons. The rates in the Y co-doped crystal are systematically about a factor of two higher than are found in CdF 2. Analysis of the temperature dependence yields a value Δ E=141±10 meV for both CdF 2 : Ga and CdF 2 : Ga,Y. This result is unexpected in light of the value Δ E=185±10 meV found previously for CdF 2 : In, and the expectation that the binding energies of the DX-like ground state of the trivalent impurity should lie deeper for CdF 2 : Ga.

A method of measuring the contribution of the image potential energy of the shallow impurity ground state in a quantum well

We show that the binding energy of the shallow impurity (at the center of the quantum well) ground state is strongly reduced by the presence of a metallic mirror at a few effective Bohr radii from the quantum well. For a given depth of the quantum well, we find that the absolute value of the image potential energy without the metallic mirror is equal to that with a fixed metallic mirror while a certain width of the quantum well is met. Hence, it is proposed that the binding energy of the shallow impurity ground state in the quantum well with and without the metallic mirror can be separately measured by the variation of the width of quantum well. The contribution of the image potential energy to the binding energy of the shallow impurity ground state may then be deduced.

Hydrogenic impurity ground state in quantum well: the envelope function revisited

We present a new variationnal method for calculating the ground state energy of an electron bound to an impurity located in a quantum well. This method relies on an envelope function which is determined exactly from a formal minimization procedure. The obtained energies are lower by as much as 10% than the ones found by the widely used free electron envelope function. Their large width limits are reached with exponentially small corrections as they should. We also find that, except for narrow wells, the shape of these exact envelope functions strongly depends on the impurity position, being consequently quite different from the usual free electron ones. In order to discuss the improvements brought by our new procedure in the most striking way, we have used a model semiconductor quantum well with infinite barrier height and simplified band structure. Extensions can be made to finite barrier and more realistic band structures, following the same technique.

Giant negative magnetoresistance in semiconductors doped by multiply charged deep impurities

A giant negative magnetoresistance has been observed in bulk germanium doped with multiply charged deep impurities. Applying a magnetic field the resistance may decrease exponentially at any orientation of the field. A drop of the resistance as much as about 10000% has been measured at 6 T. The effect is attributed to the spin splitting of impurity ground state with a very large g-factor in the order of several tens depending on impurity.

Magnetic field effects on the nonohmic impurity conduction of uncompensated crystalline silicon

The impurity conduction of a series of crystalline silicon samples with the concentration of major impurity N ≈ 3 × 1016 cm−3 and with a varied, but very small, compensation K was measured as a function of the electric field E in various magnetic fields H-σ(H, E). It was found that, at K 1 (negative magnetoresistance). With increasing E, these inequalities are simultaneously reversed (positive nonohmicity and positive magnetoresistance). It is suggested that both negative and positive nonohmicities are due to electron transitions in electric fields from impurity ground states to states in the Mott-Hubbard gap.

The effect of a spatially dependent effective mass on hydrogenic impurity binding energy in a finite parabolic quantum well with a magnetic field

The effect of a spatially dependent effective mass in a finite GaAs/AlxGa1−xAs parabolic quantum well with a magnetic field on hydrogenic impurity ground state (1s) binding energies, and lowly excited state (2p±-like) has been calculated, respectively, as a function of well width and magnetic field and impurity position by using the one-dimensional method. We compare our results with square quantum wells and without the effect of spatially dependent effective mass result. The physical meaning of the wave function square is discussed.

Observation of electronic Raman scattering from Mg-doped wurtzite GaN

Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy and metal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (full width at half maximum≅15 cm−1) observed at around 841 cm−1 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-doped GaN. Our experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about one-half of its binding energy.

Model for the Mn acceptor in GaAs

We present a model based on the Baldereschi–Lipari effective mass theory and the sp–d exchange interaction between the valence band and the Mn d electrons (3 d 5). It fits in with the available experimental data and yields a relationship between the exchange-induced splitting of the 1S 3/2 impurity ground state and the valence band exchange constant N 0 β. By using the recently reported value of the splitting, deduced from infrared spectroscopy, and fitting the known ground state binding energy, we obtain N 0β≃−0.9 eV. We also study the two-hole bound state in the Hartree self-consistent approximation and account for the absence of Mn-associated bound exciton and discuss the sign anomalies in the magneto-optical data in dilute Ga 1− x Mn x As.

Excited state impurity band conductivity in Y 2(SiO 4)O : Ce 3+

To properly describe the optical properties of doped insulators, one must include the host valence and conduction bands in addition to the energy levels of the impurity ion. Thus, the impurity-host system can be treated in a donor–acceptor model, similar to the semiconductor approach. Here we report another parallel between doped semiconductors and insulators: the formation of an impurity band. In Y 2(SiO 4)O : Ce 3+, the lowest 5d band of the Ce 3+ ion lies about 500 meV below the conduction band. We observe conductivity in heavily doped samples while exciting the lowest 4f–5d transition. This effect is attributed to the formation of an impurity sub-band. In contrast to the semiconductor case, where the impurity ground state levels form a sub-band due to the large Bohr radii of the impurities, the Ce 3+ ground state 4f wave functions are very compact and only for the more extended 5d states is an impurity band found.

Phonon spectroscopy of D − band tails of shallow impurity in Ge

We have investigated uniaxial stress dependence of phonon absorption in intermediately doped bulk Ge by means of heat pulse technique. Acoustical transparency for LA and FTA phonon modes increased and further saturated with increasing stress. It is supposed that observed phonon absorption is due to phonon assisted electron transitions from impurity ground state to D − band tails. Corresponding calculations of acoustical transparency agree with experiment and allow extraction of D − band parameters. The results are also in good agreement with the experimentally obtained stress dependence of activation energy.

Electronic Raman Scattering from Mg-Doped Wurtzite GaN

AbstractElectronic Raman scattering experiments have been carried out on both MBE and MOCVD-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (FWHM ≅ 15cm−1) observed for the first time at around 841cm−1 isattributed to the electronic Raman scattering from neutral Mg impurities in Mg-dopedGaN. From the analysis of the temperature-dependence of this electronic Ramanscattering signal binding energy of the Mg impurities in wurtzite GaN has been found to be Eb ≅ 172 ± 20meV. These experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about 3/5 of its binding energy.

Interimpurity light absorption in thin wires of III-V-type semiconductors

A study has been performed of interimpurity light absorption in weakly doped thin wires of III–V type semiconductors with Kane charge-carrier dispersion law with the one-dimensional random distribution of the impurities taken into account. The dependence of the absorption curve is analyzed: the location of the absorption maxima corresponding to transitions from the acceptor ground level to the ground level and first excited level of the donor as well as their edges and half-widths are found. It is shown that the nonparabolicity of the charge-carrier dispersion law leading to transitions between the impurity ground states causes an abrupt detachment of this absorption line from the remaining lines.

Phonon Absorption by D− Band Tails in Ge:Sb

Uniaxial stress dependence of acoustical phonon absorption in intermediately doped Ge:Sb has been studied using heat pulse technique. Abruptly decreasing LA and FTA phonon scattering in stress interval of 3−7 · 108dyn/cm2with further saturation up to 1.9 · 109dyn/cm2was observed. Results obtained from phonon measurements correlate with conductivity activation energy behavior on stress. Acoustical transparency stress dependence is assumed to be connected with phonon assisted electron transitions from impurity ground state D 0 to D−band.