Modulation-doped Semiconductor Quantum Wells Research Articles

半导体量子阱中传播效应对子带间拉比振荡的影响

We investigate intersubband Rabi flopping in modulation-doped semiconductor quantum wells with and without the propagation effects, respectively. It is shown that propagation effects have a larger impact on Rabi flopping than the nonlinearities rooted from electron-electron interactions in multiple quantum wells. By using ultrashort \pi pulses, an almost complete population inversion exists if the propagation effects are not considered; while no complete population inversion occurs in the presence of propagation effects. Furthermore, the magnitude of the impact of propagation effects may be controlled by varying the carrier density.

An Oxide Semiconductor Nanoparticle in an Aqueous Medium: A Surface Charge Density Investigation

The surface charge density in semiconductor metal oxide quantum dots dispersed in alkaline aqueous colloids was obtained by solving the three-dimensional Schrödinger and Poisson's equations. The calculation was carried out self-consistently within the frame of the finite difference method. Surface charge density, in the range of 0.1−0.3 C/m2, was obtained for spherical ZnO nanoparticles in an aqueous medium at pH around 11. The calculated surface charge density, in very good agreement with the average value reported in the literature (0.2 C/m2), is obtained as long as a proton-transfer mechanism through the semiconductor−electrolyte interface takes place. The calculated band energy profile for water-based semiconductor colloidal quantum dots, is very much similar to the band energy profile found in symmetric modulation-doped semiconductor quantum wells.

Energy levels in metal oxide semiconductor quantum dots in water-based colloids

The three-dimensional Schrödinger and Poisson’s equations are used to calculate the conduction band profile, energy levels, and Fermi energy of negatively charged semiconductor quantum dots. The calculation is carried out self-consistently within the frame of the finite-difference method. Assuming the effective mass of the proton at the semiconductor–electrolyte interface, we found the conduction band profile for the spherical ZnO quantum dots dispersed as aqueous colloids very similar to the conduction band profile of symmetric modulation-doped semiconductor quantum wells. The energy levels and Fermi energy of the spherical ZnO quantum dots are obtained as a function of the band offset at the semiconductor–electrolyte interface. A comparison of the energy levels for negatively charged and uncharged quantum dots is used as an alternative explanation of the observed reversible blue shift in the absorption spectrum of semiconductor colloids under illumination.

Excitons in the quasi-two-dimensional electron gas.

The response of modulation-doped semiconductor quantum wells to a band-gap light field is discussed with special attention paid to excitons and screening. Absorption and luminescence spectra are calculated in a mean-field approximation, taking into account the confinement and finite mass of the particles, finite temperatures, and external magnetic fields. Bound excitonic states are found to dominate the low-temperature luminescence spectra for densities up to \ensuremath{\simeq}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. At higher densities the excitonic state can be recovered by applying a magnetic field. The reappearance of the excitonic state is accompanied by an abrupt blueshift and increased oscillator strength of the luminescence. An efficient method to solve the zero-field Bethe-Salpeter integral equation in two dimensions is based on the application of a small auxiliary magnetic field.

Theory of optical spectra in a magnetic field in doped semiconductor quantum wells: Impurity-induced broadening and transitions.

The effect of carrier-impurity interactions on luminescence- and excitation-spectroscopy line shapes and the Landau-level spectral density in a strong quantizing magnetic field is examined in modulation-doped semiconductor quantum wells. The line-shape function is obtained by summing the ladder diagrams,'' extending our previous one-rung'' approximation. Apart from yielding a line broadening, the carrier-impurity interaction is found to induce off-diagonal transitions (ODT) ({ital n}{r arrow}{ital n}{prime}; {ital n}{prime}{ne}{ital n}) between the Landau levels in the conduction and valence bands, breaking the usual {ital n}{r arrow}{ital n} selection rule. Here the first and second integers indicate the Landau quantum numbers in the conduction (valence) and valence (conduction) bands, respectively, for luminescence (excitation), for example, in an {ital n}-type system. The Landau-level spectral density (essential for obtaining the line-shape functions) is investigated by a self-consistent Born approximation which includes inter-Landau-level impurity scattering. The theory is applied to an {ital n}-type strained In{sub {ital x}}Ga{sub 1{minus}{ital x}}As/GaAs quantum well, where optical transitions arise between the conduction band and the strain-split in-plane light-hole'' band. For excitation spectra, the theory predicts that ODT introduce lines below the usual {ital n}{sub {ital F}}{r arrow}n{sub F} threshold transition as well as satellite lines between the usual main {ital n}{rmore » arrow}{ital n} lines above the threshold (i.e., {ital n}{ge}{ital n}{sub {ital F}}). Here {ital n}{sub {ital F}} is the quantum number of the lowest-lying empty or partially filled conduction-band Landau level. The luminescence line shape is dominated by ODT 1, 2,...{r arrow}0 (in addition to the main 0{r arrow}0 transition) at low temperatures and by the usual {ital n}{r arrow}{ital n} transitions at high temperatures.« less

Theory of photoabsorption in modulation-doped semiconductor quantum wells.

A systematic study of the electronic and optical properties of modulation-doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells is undertaken. We consider cases in which the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barriers are doped either n or p type and the GaAs wells are filled with a gas of free carriers. The electronic band structure, exciton binding energy, exciton oscillator strength, and interband absorption are studied as functions of well width and doping concentration. A multiband effective-mass method is used which takes coupling between heavy- and light-hole states into account. In our model for excitons we include effects of valence-subband nonparabolicity, free-carrier screening, and the k dependence of optical matrix elements. The interband optical absorption is obtained using Fermi's golden rule with a correction made for the screened final-state interaction. Our theoretical results are compared with available experimental data.

Band-mixing effect on the emission spectrum of modulation-doped semiconductor quantum wells.

Emisssion spectra of n-type modulation-doped semiconductor quantum wells are calculated in a multivalley effective-mass model which includes the valence-band mixing. The many-body effect is taken into account by consideration of the single electron-hole pair excitations and the shakeup process. The resulting emission spectra for both parallel and perpendicular polarizations are found in reasonable agreement with the reported experimental data.

Absorption spectra of modulation-doped semiconductor quantum wells

Absorption spectra of modulation‐doped GaAs–Ga1−xAlxAs semiconductor quantum wells are studied with the use of a multiband effective mass theory in which coupling of the heavy and light hole valence bands is taken into account. We consider the case in which the (GaAl)As barrier is doped with acceptors and the quantum well is populated with holes. Interband optical absorption for vertical transitions are obtained using Fermi’s golden rule in the envelope function approximation explicitly taking into account the variation of the optical matrix elements with k. Two dimensional excitonic effects are also included in a model similar to that of Shinada and Sugano but including the k dependence of the optical matrix elements. We find that the mixing of heavy and light hole states of both symmetric and antisymmetric envelope functions gives rise to direct optical transitions between all pairs of valence and conduction subbands in violation of the Δn=0 selection rule.