Two-dimensional Quantum Wells Research Articles

Some Unconventional Organic−Inorganic Hybrid Low-Dimensional Semiconductors and Related Light-Emitting Devices

Structural, optical, electrical, and optoelectronic properties, as well as prototype light-emitting devices, are being reported for the two-dimensional (quantum well) organic−inorganic hybrid semiconductor systems based on the organic cations CH3(CH2)7CH═CH(CH2)8NH3+ (OL) and on the inorganic networks formed out of MX42− anions, where M = Pb and X = I, Br, and Cl. These new crystalline compounds show extremely intense excitonic bands at chemically controllable positions in the ultraviolet−visible spectral region, with excitonic binding energies of more than 180 meV at room temperature. These novel properties render them as strong potential candidates in optoelectronic applications, comparable to the artificial low-dimensional systems. Prototype light-emitting devices (LED) based on the above-mentioned compounds exhibit naked eye electroluminescence, in some cases, semicontinuous operation, for the first time at room temperature. In addition, these devices operate at relatively low voltages and are readily realized. Because of the enhanced film-forming and degradation-resistant properties of the active materials, the device functionality is being exhibited for weeks. Thus, it is expected that this particular reported synthetic route of hybrid organic−inorganic semiconductors could provide cost-effective materials for novel optoelectronic devices. Moreover, the properties of some composite systems of the type (CH3NH3)n−1(OL)2PbnX3n+1 are being reported briefly.

One- and two-dimensional spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires

Spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires (CMNs) and type-I excitons in an InAs/InP single quantum well (SQW) was studied by means of time-resolved and spectrally resolved photoluminescence. InP/InAs/InP CMNs in hexagonal symmetry are made of six facets and six edges which work as two-dimensional quantum wells and one-dimensional quantum wires, respectively. At 5 K type-II excitons lose their energy in two stages. In the first stage, two-dimensional spectral diffusion takes place in the type-II quantum well region in CMNs similar to spectral diffusion of type-I excitons in the InAs/InP SQW. In the second stage, slower one-dimensional spectral diffusion takes place in the quantum wire region in CMNs. Acoustic-phonon-mediated migration of excitons to lower-energy-localized states leads to the spectral diffusion in two dimensions and one dimension.

A study of a hydrogen atom in a two-dimensional quantum well

A feature of a hydrogen atom confined in a two-dimensional quantum well in the presence of an electric field is investigated. The binding energy of the confined hydrogen atom is calculated as a function of the confined potential radius and as a function of the intensity of an applied electric field. It is shown that the binding energy and the oscillator strength of the confined hydrogen atom are highly dependent on the confined potential radius and the intensity of an applied electric field.

Magneto-thermoelectric effects in two-dimensional quantum well: role of short-range potential

We have studied transverse thermomagnetic effects in a quantum well (QW) with parabolic potential in the presence of a magnetic field parallel to the two-dimensional electron gas layer. The calculation was carried out for the case of elastic electron scattering on short-range potential for degenerate and non-degenerate electron gas. It is shown that the reviewed mechanism of charge carriers' relaxation is essential for the electroconductivity at low temperatures. In the quantum limit, the dependencies of the transverse Nernst–Ettingshausen coefficient and the thermopower on the magnetic field strength, the temperature and the carrier density are determined and analyzed. We have showed that the magnetothermopower is not determined by the entropy only, as is the case for bulk specimens.

Two-Dimensional Excitons and Photoluminescence Properties of the Organic/Inorganic (4-FC6H4C2H4NH3)2[PbI4] Nanomaterial

Single crystals of a hybrid organic/inorganic material with the formula (4-FC6H4C2H4NH3)2[PbI4] were synthesized and studied by X-ray diffraction, ellipsometry, and photoluminescence. The crystals consist of a self-assembled multilayer structure with a P21/a space group. The inorganic semiconductor sublattice is built up from corner-sharing PbI6 octahedra forming infinite two-dimensional quantum wells. The organic (4-FC6H4C2H4NH3)+ chains form the insulator barriers. A strong room temperature photoluminescence emission, due to excitons confined in the PbI quantum wells, is found around 2.34 eV. The exciton binding energy 540 meV is determined from the temperature dependence of the photoluminescence intensity and using a modified Arrhenius model, which accounts for the two-dimensional electronic structure. Calculations of the exciton binding energy based on a variational method indicate that the dielectric confinement effect is enhanced by the low value of the organic barrier dielectric constant. Moreover, it is shown that the temperature variation of the photoluminescence peak energy is well described by Varshni’s model, thus indicating the free character of the two-dimensional excitons. The temperature dependence of the photoluminescence line width is also analyzed and interpreted in terms of homogeneous broadening due to exciton−phonons interaction.

SURFACE POLARON SELF-ENERGY AND EFFECTIVE MASS IN A WURTZITE GaN NANOWIRE

The ground-state self-trapping energy and effective mass of surface polarons in a freestanding wurtzite GaN nanowire (NW) are studied using the second-order perturbation approach. Based on the dielectric continuum and Loudon's uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of surface optical (SO) modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations of the ground-state polaron self-trapping energy and the correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapping energy and the correction of effective mass is far larger than those in 1D GaAs NW systems. The reasons for this obvious difference in the two 1D structures can be attributed to the different electron–phonon coupling constants and electron effective masses of bulk material constituting the two types of 1D confined systems. Finally, the polaronic properties of the wurtzite 1D GaN NWs are compared with those of wurtzite GaN -based two-dimensional quantum wells. The physical origins leading to these characteristics and their distinction in the different-dimensionality systems is carefully analyzed.

Polaronic effects due to surface optical vibration modes in a freestanding cylindrical wurtzite nanowire

The ground-state polaron self-trapped energy and effective mass due to the surface optical (SO) phonon modes in a freestanding wurtzite GaN nanowire (NW) were studied by means of the Lee–Low–Pines variational approach. Based on the dielectric continuum and Loudon’s uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of SO modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations on the ground-state polaron self-trapped energy and correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapped energy and correction of effective mass are far larger than those in 1D GaAs NW systems. The reasons resulting in this obvious difference in the two 1D structures are mainly due to the different electron–phonon coupling constants and electron effective masses of bulk materials constituting the two types of 1D confined system. Finally, the polaronic properties of the wurtzite 1D GaN NWs have been compared with those of the wurtzite GaN-based two-dimensional quantum wells. The physical origination of these characteristics and their distinction in the different-dimensionality systems has been analyzed in depth.

Silicon–germanium nanostructures for on-chip optical interconnects

Silicon–germanium epitaxially grown on silicon in the form of two-dimensional (quantum wells) and three-dimensional (quantum dots) nanostructures exhibits photoluminescence and electroluminescence in the technologically important spectral range of 1.3–1.6 μm. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency, and a long carrier radiative lifetime. This paper summarizes recent progress in the understanding of carrier recombination in Si/SiGe nanostructures and presents a potential new route toward CMOS compatible light emitters for on-chip optical interconnects.

Photoluminescence properties of Pb 1−xSn xTe/CdTe quantum wells grown on (1 0 0)-oriented GaAs substrates by molecular beam epitaxy

This paper describes photoluminescence (PL) properties of Pb 1− x Sn x Te/CdTe quantum wells (QWs) grown on (1 0 0)-oriented GaAs substrates by molecular beam epitaxy. Despite the difference in crystal structure between these two tellurides, two-dimensional QWs with an abrupt heterointerface were formed at a low growth temperature of 220 °C. Highly efficient midinfrared PL was observed from the QWs even at room temperature. Control of the Sn content in QWs allowed tuning of the PL peak energy in a wide range of a 3–5 μm atmospheric window. Considering effects of both the strain-induced band deformation and the quantum confinement in QWs, the transition energy observed was found to agree well with theoretical calculation.

Influence of two-dimensional magnetic photonic quantum wells on resonant tunneling spectral character

The non-magnetic material closed photonic quantum well (PQW) and magnetic material PQW structures based on the non-magnetic material open PQW are proposed. The transmission spectra and the field distributions of these three PQW structures are calculated by finite-difference time-domain method, the quantized energy states are researched, and the feasibility of enhancing spectral intensity significantly by self-structure is disclosed. It is found that the optical transmittance of the magnetic PQW is close to 1, and the energy loss is less compared to non-magnetic PQW. Compared with the closed PQW structures, the device’s volume can be reduced, the degree of free regulation of the energy band project can be increased, and more photon bound states can be obtained. The results show that the open PQW is the traveling wave well, and its capability of capturing photons is weak. However, the closed PQW and the magnetic PQW are standing wave wells. Their capabilities for capturing photons are strong, while the light field gradient of the material PQW is bigger.

Atomlike, Hollow-Core–Bound Molecular Orbitals of C 60

The atomic electron orbitals that underlie molecular bonding originate from the central Coulomb potential of the atomic core. We used scanning tunneling microscopy and density functional theory to explore the relation between the nearly spherical shape and unoccupied electronic structure of buckminsterfullerene (C60) molecules adsorbed on copper surfaces. Besides the known pi* antibonding molecular orbitals of the carbon-atom framework, above 3.5 electron volts we found atomlike orbitals bound to the core of the hollow C60 cage. These "superatom" states hybridize like the s and p orbitals of hydrogen and alkali atoms into diatomic molecule-like dimers and free-electron bands of one-dimensional wires and two-dimensional quantum wells in C60 aggregates. We attribute the superatom states to the central potential binding an electron to its screening charge, a property expected for hollow-shell molecules derived from layered materials.

Quantum wave packet revival in two-dimensional circular quantum wells with position-dependent mass

We study quantum wave packet revivals on two-dimensional infinite circular quantum wells (CQWs) and circular quantum dots with position-dependent mass (PDM) envisaging a possible experimental realization. We consider CQWs with radially varying mass, addressing particularly the cases where M ( r ) ∝ r w with w = 1 , 2 , or −2. The two PDM Hamiltonians currently allowed by theory were analyzed and we were able to construct a strong theoretical argument favoring one of them.

Thermal and thermoelectric behavior of silicon-germanium quantum well structures

Tailoring thermoelectric materials for specific designs and applications has been gaining momentum during past three decades. Initially confined to conventional (bulk) framework an entirely new scenario emerged with inclusion of low-dimensional structures in the scheme of things. The paper examines the effect of size reduction on phonon and electron properties in two-dimensional (quantum well) structures with an aim to maximize thermoelectric performance. The formulation has been applied to silicon-germanium quantum wells with well width ranging from 50–500 A aimed at finding best alloy combination for thermoelectric applications.

Phonon modes and electron-phonon interactions in cylindrical quantum wires: Macroscopic and microscopic analyses for device applications

The macroscopic and microscopic analyses of phonon modes that has been done in past works for two-dimensional quantum wells are developed in this work for one-dimensional quantum wires. Inconsistencies and errors in past works on phonon modes in quantum size structures are discussed including the correct form of the Lorentz factor that should be used and the proper quantization of the phonon modes. Connections between the macroscopic and microscopic approaches are described. Longitudinal optical and surface longitudinal optical phonon modes are calculated. The free polarization Hamiltonian and electron-phonon interaction Hamiltonian are developed. The application of this analysis to the development of high operating temperature quantum wire infrared photodetectors is discussed.

Large image potential effects in Si∕SrTiO3 and Si∕HfO2 two-dimensional quantum well structures

Charge image effects on the confinement properties of Si∕SrTiO3 and Si∕HfO2 two-dimensional quantum wells are studied. The combination of strong dielectric mismatch and band offset of the layers gives rise to structured confinement potentials, which can trap carriers close to the interfaces in Si∕SrTiO3 but not in Si∕HfO2 two-dimensional quantum wells. The charge image blueshifts strongly (a few hundred meV) the carrier recombination energy, comparable to the shift related to the well width shortening due to actual graded interfaces.

VARIATIONAL CALCULATIONS OF CHARGED EXCITONS IN GaAs QUANTUM-WELL WIRES

The binding energy of positively and negatively charged excitons in GaAs quantum-well wires is calculated variationally as a function of the wire width by using a two-parameter wave function and a one-dimensional equivalent model. There is no artificial parameter added in our calculation. It is found that the binding energies are closely correlated to the sizes of the wire, and also that their magnitudes are greater than those in the two-dimensional quantum wells compared. In addition, we also calculate the average interparticle distance and the distribution of the wave function of exciton centre-of-mass as functions of the wires width. The results are discussed in detail.

Ge/Si self-assembled Islands for Photonics Applications

ABSTRACTWe first present an analysis of the band line-up in the case of SiGe/Si quantum wells and in the case of SiGe/Si self-assembled islands. The conduction and valence band diagrams are obtained from a 30 band k.p Hamiltonian which allows to describe simultaneously conduction and valence band states. The strain field is obtained from a microscopic valence force field theory. The band edge alignment is strongly dependent on the input parameters for this heterosystem. We determine the average valence band offset from photoluminescence measurements of heterostructures grown on relaxed SiGe buffer layers. A type II band line-up is calculated for all Ge compositions in the case of two-dimensional quantum wells and SiGe/Si self-assembled islands. The 30-band formalism allows the determination of the near-infrared interband recombination energy as a function of the self-assembled island structural parameters. We then present recent results obtained by embedding SiGe/Si self-assembled islands in two-dimensional photonic crystals. The photoluminescence of GeSi islands acts as an internal probe to characterize the optical properties of silicon-based two-dimensional photonic crystals designed for the near-infrared spectral range. Cavities, defect-free photonic crystals operated at the second Bragg order and two-dimensional photonic crystals fabricated on top of one-dimensional Bragg mirrors (2D + 1D) are described. We show that, in the case of 2D +1D structures, we can control the quality factor of optical modes at the second Bragg order by matching the resonance conditions and controlling the thickness of the layers. Photonic crystals with pure Ge layers are finally described.

Photoluminescence transitions of GaAs/AlGaAs asymmetric double-well structures

We measured photoluminescence emission spectra from GaAs/AlGaAs semiconductor heterostructures in which asymmetric two-dimensional quantum wells are separated by a thin barrier in the conduction band while the valence band has single square quantum well connected to the GaAs flat band with a thin barrier. In such structure the tunneling barrier generates two subband located energetically in close proximity in the conduction band while the valence band does not have such tunnel-coupled energy gap. Therefore, the photogenerated valence holes tend to move to GaAs flat band region. With low external excitation power, only the interwell optical transitions occur. However, this asymmetric quantum structure shows intense photoluminescence emission behavior with increasing external excitation power. With increasing laser power, simply the number of photogenerated holes near the interface increase and they recombine with the conduction electrons which shows non-linear emission behavior.

Resonant spin Hall conductance in quantum Hall systems lacking bulk and structural inversion symmetry

A previous work [Shen, Ma, Xie, and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on two-dimensional quantum wells with Rashba type spin-orbit interaction under a strong perpendicular magnetic field is generalized to include the Dresselhaus coupling. The Rashba coupling and the Dresselhaus coupling interplay with the Zeeman effect in opposing ways. The former tends to produce a resonant spin Hall effect at certain magnetic fields while the latter suppresses it. Due to the resonant spin Hall effect, the spin Hall current is highly nonohmic at low temperatures. The condition for the resonant spin Hall conductance in the presence of both Rashba and Dresselhaus couplings is derived using a perturbation method. In the presence of disorder, we argue that the resonant spin Hall conductance occurs when the two Zeeman split extended states near the Fermi level become degenerate due to the Rashba coupling, and that the quantized charge Hall conductance changes by $2{e}^{2}∕h$ instead of ${e}^{2}∕h$ as the magnetic field changes through the resonant field.

Variational Calculations of Neutral Bound Excitons in GaAsQuantum-Well Wires

The binding energy of an exciton bound to a neutral donor (D0,X)in GaAs quantum-well wires is calculated variationally as a function ofthe wire width for different positions of the impurity inside the wireby using a two-parameter wavefunction. There is no artificialparameter added in our calculation. The results we have obtained showthat the binding energies are closely correlated to the sizes of thewire, the impurity position, and also that their magnitudes are greaterthan those in the two-dimensional quantum wells compared. In addition,we also calculate the average interparticle distance as a function ofthe wire width. The results are discussed in detail.