Two-dimensional Quantum Wells Research Articles

Visible luminescence from silicon quantum dots and wells

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO 2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are similar to those of two-dimensional Si quantum wells sandwiched by SiO 2 layers. The size dependence of PL properties and resonantly excited PL spectra of SiO 2-capped Si nanocrystals indicate that excitons are localized near the nanocrystal surface and the strong coupling of localized excitons to surface oxide vibrations causes broad PL spectra in the visible spectral region. The localization of excitons near the interface between crystalline Si (c-Si) and surface oxide layer is attributed to efficient luminescence in nanoscale c-Si/SiO 2 systems such as zero-dimensional nanocrystals and two-dimensional quantum wells

Resonant Excitation Spectroscopy of Light-Emitting Silicon Nanostructures

AbstractWe have studied optical properties and electronic structures of zero-dimensional (OD) Si nanocrystals and two-dimensional (2D) Si quantum wells by means of resonant excitation spectroscopy. Resonantly excited luminescence spectra of 0D Si nanocrystals are sensitive to the surface structure. The size dependence of the photoluminescence (PL) properties and resonantly excited PL spectra of SiO2-capped Si nanocrystals indicate that excitons are localized at the interface between the c-Si and SiO2 surface layer. The TO-phonon related structure in resonantly excited luminescence is clearly observed in H-passivated Si nanocrystals. Hpassivated Si nanocrystals show their crystalline nature, while the oxidized Si nanocrystals show their disordered nature. Luminescence properties of Si/SiO2 nanocrystals are similar to those of Si/SiO2 quantum wells. The electronic structures and luminescence properties of Si nanostructures are discussed.

Two-dimensional excitonic emission in InAs submonolayers.

Photoluminescence (PL) and time-resolved photoluminescence (TRPL) were used to study optical emissions of ultrathin InAs layers with average layer thickness ranging from 1/12 to 1 ML grown on GaAs substrates. We have found that the inhomogeneous broadening of the PL from InAs layers can be well described by the quantum-well model with InAs islands coupling to each other and being regarded as a quasiwell. From the temperature dependence of the exciton linewidth, the exciton--LO-phonon scattering coefficient was found to be comparable to that in conventional two-dimensional quantum wells. In the TRPL measurements, the PL decay time increases linearly with temperature, which is a typical characteristic of free excitons in quantum wells. All these results indicate that the excitons localized in InAs exhibit two-dimensional properties of quantum wells, despite the topographical islandlike structure. \textcopyright{} 1996 The American Physical Society.

Coupling of Excitons in Parallel Organic and Semiconductor Quantum Wires

Abstract We study the resonance energy transfer between parallel neighboring organic and semiconductor quantum wires which is caused by the resonance dipole-dipole interaction between the wires. Due to this interaction, a new hybrid excited state arises which is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wavevectors along the wires and decays rather slowly with increasing inter-wire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with the increasing interwell distance and takes place only for nonzero excitonic wavevectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength which might be especially interesting for the nonlinear optics.

Compressibility of the electron gas: Analytical results for width effects within the Hartree-Fock approximation.

We present analytical results for the compressibility of the interacting electron gas within the Hartree-Fock approximation. For the three-dimensional and the ideally two-dimensional electron gas, the well-known results from the literature are found. For the finite width effects in the two-dimensional (quantum well) and the one-dimensional (quantum wire) electron gas, analytical results are described. The explicit form of the interaction potential between the charged carriers enters into the expression for the compressibility: finite-width effects reduce the corrections induced by interaction effects. Our predictions could be tested by capacitance measurements. \textcopyright{} 1996 The American Physical Society.

Intersubband far-infrared emission and magnetotransport of 2D hole gas in a strong in-plane electric field and transverse magnetic fields

We observe FIR emission and magnetotransport phenomena of modulation doped p- GaAs AlGaAs multi-quantum wells structures of different well width in a streaming regime. This FIR emission originates from radiative transitions between the upper quantum subbands, where hot holes are trapped by the emission of LO-phonons and the ground quantum state. The importance of the light-heavy states mixing with an increase of confinement effect is shown experimentally. The magnetoresistance in a strong in-plane electric field applied along the [110] crystallographic direction is found to be negative, which is explainedDby strong warping of the constant hole energy surfaces in two-dimensional quantum wells.

Variational Approach for Phonon Renormalization Effects in Photoexcited Quantum Wires and Quantum Wells

We investigate the effects of screening on polaronic corrections to the effective band edge in photoexcited quasi-one-dimensional GaAs quantum wires and two-dimensional quantum wells. We develop a variational method to calculate the polaron energy of a two-component plasma (electrons and holes) coupled to LO-phonons. Screening effects are incorporated within a dynamical scheme. We find that the screening effects and finite well width considerably reduce the polaron energy as the plasma density increases. Many-body corrections beyond the random-phase approximation is also considered.

Hybrid excitons in parallel organic and inorganic semiconducting quantum wires.

We study excited states in parallel neighboring organic and inorganic semiconducting quantum wires. Due to the resonance dipole-dipole interaction between the wires, the hybrid exciton is a mixture of Frenkel and Wannier-Mott excitons. For the transition dipole moments oriented perpendicular to the wires, the interwire hybridization strength is nonzero even for small exciton wave vectors along the wires and decays rather slowly with increasing interwire spacing. This is in contrast to two-dimensional quantum wells where the dipole-dipole coupling decays fast with increasing interwell distance and is nonzero only for nonzero excitonic wave vectors. The hybrid wire excitons possess both a large radius and a relatively large oscillator strength, which could be especially interesting with respect to applications in nonlinear optics.

Phase transition in a many-electron gas in a two-dimensional polar-semiconductor quantum well.

We investigate the problem of an electron gas interacting with longitudinal-optical phonons in a two-dimensional polar-semiconductor quantum well for the entire range of the coupling parameter using the Feynman path-integral method and the random-phase approximation. We show that at a critical value of the electron concentration an optical polaron will undergo a transition from the localized state to a mobile state.

Strong accumulation of As precipitates in low temperature InGaAs quantum wells grown by molecular beam epitaxy

Low temperature InGaAs strained quantum wells have been grown by molecular beam epitaxy and annealed at 600–900 °C for 10 min. For an optimized annealing condition, arsenic precipitates can be successfully confined in the InGaAs wells and completely depleted in the GaAs barriers. The strong accumulation of As precipitates shows that the phenomena are not due to the strain effect but may be explained by the difference of interfacial energy between precipitate and matrix. The ability to control the As precipitates into two-dimensional quantum wells in LT materials has unique applications in a wide variety of devices.

All-coupling many polaron gas in a two-dimensional semiconductor quantum well

Abstract The problem of an all-coupling many polaron gas in a two-dimensional semiconductor quantum well is studied for the first time using a variational method and the static random phase approximation. It is predicted that at a critical carrier density a two-dimensional optical polaron will make a sharp transition from the self-trapped state to a delocalized state.

Magnetic field-induced saddle point in GaAs/AlGaAs short period superlattices

We have measured interband magneto-absorption spectra of (GaAs) 6/ ( Al 0.51 Ga 0.49 As) 6 = (6,6) and ( GaAs) 14/( AlAs) 6 = (14,6) under pulsed high magnetic fields up to 150T. In sample (6,6), drastic broadening of the exciton peak due to a magnetic field-induced saddle point was observed when high magnetic fields were applied in the direction parallel to the layer planes and the cyclotron radius became smaller than the superlattice period. The line shape of the broadened exciton resonance was qualitatively understood as an effect of the Coulomb interaction between electrons and holes. The exciton binding energy of sample (6,6) was estimated as 7.0meV using the perturbational theory, which reflects an intermediate nature of the superlattice between bulk crystals and two-dimensional quantum wells.

An improved Lamb shift calculation for a bound polaron in a purely two-dimensional quantum well

Using an improved variational wave function the 2s excited state energy of a Coulomb impurity bound polaron in a purely two-dimensional quantum well is calculated and the excitation energies and the phonon-induced Lamb shift corrections are obtained for several polar materials.

Polaronic States Incorporating Effects of Phonon Confinement in a Rectangular Quantum Well Wire

The electron self-energy due to the interaction of the electron with the LO-phonon incorporating effects of phonon confinement in a rectangular quantum well wire has been calculated as a function of the size of the wire by the perturbative method within the framework of the effective mass approximation. The results show that in a very small wire the self-energy has a peak at the beginning and then increases gradually to the two-dimensional limit value as the wire expands in one direction while keeping fixed in the other direction. The peak values are about at the square cross sections. For larger wires, the self-energy increases monotonically without appearing peaks and approaches slowly to the two-dimensional limit values. The results also present that the self-energy is related to the size of the cross section rather than to the cross sectional area of the wire, and the absolute values of the self-energies are less than those in comparable two-dimensional quantum wells.

Dc current-voltage characteristics of a double-quantum-well field-effect resonant tunneling transistor

We reported the observation of steplike drain current and the negative transconductance, in a field-effect transistor. Our transistor has a double-quantum-well channel, and the quantum well close to the surface is depleted. We demonstrate that the novel transistor characteristics are from tunneling of electrons in between two-dimensional quantum wells. The observed current-voltage characteristic in gate tunneling current has a large peak-to-valley ratio (380–1), which is close to an idealized delta functionlike behavior.

Ground and excited states of a bound polaron in a purely two-dimensional quantum well

The bound-polaron problem in a purely two-dimensional quantum well is studied variationally for the entire range of the electron-phonon coupling constant and the Coulomb binding parameter. The ground-state energy, the average number of virtual phonons around the electron and the size of the polaron are calculated. A comparison made with the corresponding quantities for bulk crystals shows that the polaronic effects are more pronounced in two dimensions. The energies of the first two excited states are obtained and the phonon-induced Lamb shift corrections are computed for several polar materials.

Time-resolved spin dynamics in diluted magnetic semiconductor quantum wells

Ultrashort laser pulses were used to prepare charge-carrier populations of known initial energy and spin polarization in two-dimensional quantum wells of the diluted magnetic semiconductor Cd1−xMnxTe. The evolutions of the carrier spin polarization and of magnetization transferred to the magnetic ions were observed using time-resolved optical techniques.

Exchange-induced splitting of exciton energy levels in quantum wires.

Effects of the electron-hole exchange interaction on excitons in one-dimensional quantum wires are studied theoretically by performing a simplified calculation with a scaling argument. It is shown that the exchange parameters are drastically enhanced in narrow quantum wires compared to the bulk values. As expected, the variation of the exchange-induced splitting as a function of the wire size has the same qualitative behavior as those in comparable two-dimensional quantum wells but with much higher magnitudes.

Investigation of the 2D–3D transition of the band gap renormalization in GaAs

We have analyzed the band gap renormalization due to many body effects in electron-hole plasmas in GaAs in the transition region between two-dimensional quantum wells and bulk material. The band gap renormalization depends strongly on the well widths. A transition of the band gap renormalization from the two-dimensional behavior observed for quantum wells with widths L z <8 nm to the behavior of bulk GaAs occurs for well widths between 10 and 20 nm.

Disorder, screening, and quantum Hall oscillations.

The density of states (DOS) of electrons in two-dimensional (2D) quantum wells and the broadening of Landau levels (LL's) is evaluated. The electrons are assumed to interact with screened, charged impurities located at random in the material. The random location of the impurities leads to a disordered environment seen by the electrons. The screening of the impurities by the 2D electron gas is evaluated in a simple Thomas-Fermi model which depends on the DOS at the Fermi surface, n(${E}_{F}$). The DOS and screening are evaluated iteratively at each magnetic field value. We find that the self-consistent evaluation of the DOS and screening leads to broadening of the LL's and n(${E}_{F}$) that oscillates with B. The oscillating LL widths and n(${E}_{F}$) are compared with the data of Heitmann et al., Englert et al., Wang et al., and Smith et al. and reproduce all the observed values quite well. The chief adjustable parameter is the impurity concentration.