Many-body carrier interaction effects in quantum wells

Based on calculations within the density functional theory and an analysis of low-temperature photoluminescence spectra, the structure of electron–hole liquid in shallow Si/Si_1 – _ x Ge_ x Si (100) quantum wells 5 nm wide with germanium content x = 3–5.5% is studied. It is shown that the energy of quasi-two-dimensional electron–hole liquid localized in quantum wells for this composition range as a function of carrier concentration exhibits two local minima. The position of the deeper (major) minimum depends on the quantum well design and controls properties of quasi-two-dimensional electron–hole liquid at low temperatures. For the series of Si/Si_1 – _ x Ge_ x Si quantum wells, modification of properties of electron–hole liquid was experimentally shown, which can be interpreted as a change of the major minimum due to an increases in the germanium concentration in the Si_1 – _ x Ge_ x layer. The effect of the multicomponent composition (electrons, light and heavy holes) of the electron–hole liquid on low-temperature photoluminescence spectra of Si/Si_1 ‒ _ x Ge_ x Si quantum wells is discussed.

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are very similar to those of Si quantum wells sandwiched by SiO2 layers. The size-dependence of PL properties and resonantly excited PL spectra of SiCb-capped Si dots and wells indicate that excitons are localized near the interface between the crystalline Si core and surface oxide layer, and the strong coupling of electronic and vibrational excitations causes the broad PL spectrum. The exciton localization plays an essential role in the efficient luminescence process in nanoscale Si/SiO2 systems.

An intense laser field effect on the intersubband transitions in quantum wells is theoretically investigated within the framework of the effective-mass approximation. Results obtained show that intersubbband optical transitions and energy levels in quantum wells can be significantly modified and controlled by an intense laser field.

Quantum well lasers provide a number of options for altering the emission wavelength. The effective bandgap energy of these devices is a function of well width, well composition, and, to a lesser extent, barrier composition. In addition, there are multiple confined energy levels in quantum wells typically used in semiconductor laser applications. Usually, the gain provided by the first quantized state (n=1) is sufficient to overcome the cavity losses and the lasing emission occurs at an energy corresponding to the separation of the first confined energy level in the conduction band and the first confined energy level in the valence band. By altering the cavity losses, the laser may be forced to lase on the next highest confined energy level transition. This level has a higher density of states and therefore a higher potential gain if populated with electrons. This second quantized state or n=2 lasing is significantly shifted in wavelength from the n=1 lasing due to the large separation of confined energy levels.

Nonresonant carrier tunneling dynamics in GaAs/AlGaAs double quantum wells as a function of carrier density was investigated by using time‐resolved pump and probe measurements. At low temperatures, tunneling time increases with increasing carrier density. At high temperatures, no dependence of tunneling time on carrier density was shown. This unique behavior is well explained by the thermodynamic effect be‐ tween excitons and free carriers as a function of carrier density on the tunneling process. At low temperatures, a larger fraction of excitons at a higher carrier density, which have a relatively slow tunneling time in contrast to free electrons, leads to a slow tunneling time. The independence of the tunneling time at high temperatures is attributed to the insensitivity of the exciton population to the increased carrier density. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The guiding effect of InGaAs quantum wells in GaAs- and InP-based semiconductor lasers has been studied theoretically and experimentally. The results demonstrate that such waveguides can be effectively used in laser structures with a large refractive index difference between the quantum well material and semiconductor matrix and a large number of quantum wells (e.g. in InP-based structures).

Cyclotron resonance of two-dimensional electrons in a Si quantum well

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si1 − x Ge x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si1 − x Ge x layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T + and biexcitons, in the exciton gas.

We report the first observation of photoluminescence from electron-hole plasmas in Si/Si0.8Ge0.2/Si quantum wells. While at liquid helium temperature, luminescence due to shallow bound excitons is observed. At 77 K electron-hole plasma (EHP) luminescence dominates the spectra over a wide range of pump powers. Convolution of the occupied electron and hole densities of states gives an excellent fit to the photoluminescence line shape. A band-gap reduction of up to 15 meV at high carrier densities is observed for wide quantum wells, but no such shift is detected for narrow quantum wells.

Several models of oxygenated and hydrogenated surfaces of Si quantum wells and Si nanocrystals of variable shapes have been constructed in order to assess curvature effects on energy gaps due to the three Si-suboxides. Si-suboxides in partially oxydized models of nanocrystals of spherical shapes (or quantum dots) are shown to reduce energy gaps compared to the hydrogenated nanocrystals, consistent with previous results in the literature. This trend is shown to be reversed when planar interfaces are formed in Si-NC inside SiO2 thin films, as in Si quantum wells. At planar interfaces (or surfaces) the electronic charge density is shown to become extended and to distribute among the Si-suboxides, thus generating along these planar interfaces extended, or delocalized, states. This delocalization of the electronic states then increase the energy gap compared to equivalent hydrogenated interfaces (or surfaces). Determination of geometric effects of curvature on the band gaps are based on density functional theory (DFT) using the real-space numerical approach implemented in the ``Pseudopotential Algorithm for Real-Space Electronic Structure'' (PARSEC) program. A method for measuring interface effects due to Si-suboxides in Si-NC in SiO2 is also suggested: by comparing photoluminescence (PL) between as-grown and annealed Si-NC samples. The DFT calculations suggest that blueshift of more than 0.2 eV of the PL should be observed in as-grown samples having Si-suboxides at their planar interfaces, in comparison to annealed samples above 950 degree Celcius that have fewer or no Si-suboxides once annealed, providing that the bulk of Si-NC remains unaltered.

Magnetron sputter epitaxy of [formula omitted] heterostructures

Strong electron intersubband infrared absorption is observed for Sb δ-doped Si/Si1−xGex multiple quantum well structures grown on (110) Si substrates. The intersubband absorption is shown to be allowed for both the optical field components perpendicular and parallel to the quantum wells due to the tilted ellipsoidal of constant energy surfaces. About 90% infrared absorption is measured by a Fourier transform infrared spectrometer using a waveguide structure with 10 internal reflections. For various samples used in experiments, absorption peaks ranging from 4.9 to 5.8 μm are observed. The peak energy is shown to be tunable by changing the Ge composition in the Si1−xGex barriers and the doping concentration in the Si quantum wells.

A single-frequency admittance spectroscopy technique is presented to determine the band discontinuity of the Si/Si1−xGex/Si single quantum well. The accuracy of determining the activation energy for thermionic emission of holes over the Si barrier is improved as compared with the multiple-frequency admittance spectroscopy measurement. To derive the band offset from the activation energy, the Fermi energy with respect to the band edge is calculated more exactly by solving the Poisson equation which accounts for the carrier transfer at the heterointerface. The accuracy of this technique is verified by the reconstruction of the temperature dependence of capacitance and conductance by the computer simulation.

Photoluminescence (PL) of Si1-x Gex /Si quantum wells with abrupt interfaces formed by segregant-assisted growth (SAG) is reported. It was found that SAG-grown samples show spectral red-shift compared to those grown by conventional solid-source molecular beam epitaxy due to the suppression of Ge surface segregation during growth. Crystal quality of SAG-grown samples is discussed based on the excitation variation of PL intensity.

Si 1− xGe x/Si valence band offset determination using current–voltage characteristics