Quantum-confined States Research Articles

III-nitride semiconductors for intersubband optoelectronics: a review

III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5–10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented.

Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through “doping;” however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.

Response of Semiconductor Nanocrystals to Extremely Energetic Excitation

Using a combination of transient photoluminescence and transient cathodoluminescence (trCL) we, for the first time, identify and quantify the distribution of electronic excitations in colloidal semiconductor nanocrystals (NCs) under high-energy excitation. Specifically, we compare the temporally and spectrally resolved radiative recombination produced following excitation with 3.1 eV, subpicosecond photon pulses, or with ionizing radiation in the form of 20 keV picosecond electron pulses. Using this approach, we derive excitation branching ratios produced in the scenario of energetic excitation of NCs typical of X-ray, neutron, or gamma-ray detectors. Resultant trCL spectra and dynamics for CdSe NCs indicate that all observable emission can be attributed to recombination between states within the quantum-confined nanostructure with particularly significant yields of trions and multiexcitons produced by carrier multiplication. Our observations offer direct insight into the transduction of atomic excitation into quantum-confined states within NCs, explain that the root cause of poor performance in previous scintillation studies arises from efficient nonradiative Auger recombination, and suggest routes for improved detector materials.

Absorption of Light in Positron and Electron States in Quasi-Zero-Dimensional Nanosystems

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of positron and electron in spherical nanovoids of metals and semiconductors. In the framework of dipole approximation it is shown that the oscillator forces and dipole moments for the transitions involving one-particle states in nanovoids are rather large, exceeding the corresponding typical parameters of bulk metal and semiconductor materials. It is established that giant optical absorption cross sections in the quasi-zero-dimensional systems to enable to use these nanosystems as new efficient absorbing nanomaterials for infra-red radiation.

Tunable electroluminescence from polymer-passivated 3C-SiC quantum dot thin films

Strong room-temperature photoluminescence in the wavelength range of 400-540 nm is achieved from a thin film containing polymer encapsulated 3C-SiC quantum dots (QDs). The QD surface is completely passivated by the polymer so that the QD film possesses voltage-tunable electroluminescence. The electroluminescence spectrum blue-shifts from 490 to 460 nm when the applied voltage is increased from 5 to 10 V. The tunable electroluminescence is attributed to carrier recombination in the core quantum confinement states as a result of Pool-Frenkel emission. Owing to the environmental friendliness, nontoxicity, easy fabrication, and stability, the polymer passivated 3C-SiC QD thin films have promising applications.

Ruderman-Kittel-Kasuya-Yosida interaction between diluted magnetic semiconductor quantum dots embedded in semiconductor

Using the Keldysh Green's function method, we calculated the Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interaction between the localized spins of two diluted magnetic semiconductor (DMS) quantum dots embedded in a two-dimensional semiconductor. It was shown that the RKKY interaction is strongly dependent on the hybridization between the quantum-confined states in DMS quantum dots and the band in the semiconductor, the discrete energy levels in DMS quantum dots and the carrier density of the semiconductor. Since the carrier energy levels in DMS quantum dots may be adjusted by the applied voltage gates, the RKKY interaction of the present system is gate-controllable. It provides an alternate way to get the controllable RKKY magnetic interaction in semiconductor nanostructures for potential application in quantum information processing.

Robust Surface Doping of Bi2Se3 by Rubidium Intercalation

Rubidium adsorption on the surface of the topological insulator Bi(2)Se(3) is found to induce a strong downward band bending, leading to the appearance of a quantum-confined two-dimensional electron gas state (2DEG) in the conduction band. The 2DEG shows a strong Rashba-type spin-orbit splitting, and it has previously been pointed out that this has relevance to nanoscale spintronics devices. The adsorption of Rb atoms, on the other hand, renders the surface very reactive, and exposure to oxygen leads to a rapid degrading of the 2DEG. We show that intercalating the Rb atoms, presumably into the van der Waals gaps in the quintuple layer structure of Bi(2)Se(3), drastically reduces the surface reactivity while not affecting the promising electronic structure. The intercalation process is observed above room temperature and accelerated with increasing initial Rb coverage, an effect that is ascribed to the Coulomb interaction between the charged Rb ions. Coulomb repulsion is also thought to be responsible for a uniform distribution of Rb on the surface.

A Compact Model of Quantum Electron Density at the Subthreshold Region for Double-Gate Junctionless Transistors

A compact model of quantum electron density at the subthreshold region is derived for junctionless (JL) double-gate (DG) FETs. The proposed quantum model is obtained under two different quantum confinement conditions. One is for a case of a thick channel and a heavily doped channel, where quantum confinement effects (QCEs) are modeled by a 1-D quantum harmonic oscillator. The other is for a case of a thin channel, where QCEs are modeled by the use of a 1-D quantum well surrounded by high potential barriers and an energy correction term coming from the depletion charge. It is shown that, regardless of the channel thickness, the quantum confinement is higher in JL than in inversion-mode (IM) DG FETs. However, for a thin channel, the quantum threshold voltage shift is less severe in JL than in IM DG FETs. The proposed model gives an analytical expression for the threshold voltage shift due to QCEs, which can be used as a quantum correction term for compact modeling.

Photoluminescence of oxidized porous silicon treated by sodium borohydride aqueous solution

Alkaline solutions are usually used to remove porous layer from porous silicon. However, novel star-like photoluminescent oxidized porous silicon (PS) particles have been successfully fabricated in sodium borohydride (NaBH4) solution in this work. It is interesting that porous layer has been oxidized in NaBH4 alkaline solution, rather than being removed. Silicon oxide is formed due to the constant reactions of silicon nanocrystals and NaOH. Then these oxides are adjusted to star-like morphology by the pressure of H2. Both NaOH and H2 are generated from the hydrolysis of NaBH4. Oxidized PS shows bright orange photoluminescence (PL) at 550nm. This emission can be simply tuned into red or blue by illumination of high pressure mercury lamp or high temperature treatment. The PL of NaBH4-treated PS is originated from quantum confinement and surface state of silicon nanocrystallites.

Quantum-Confined Electronic States in Atomically Well-Defined Graphene Nanostructures

Despite the enormous interest in the properties of graphene and the potential of graphene nanostructures in electronic applications, the study of quantum-confined states in atomically well-defined graphene nanostructures remains an experimental challenge. Here, we study graphene quantum dots (GQDs) with well-defined edges in the zigzag direction, grown by chemical vapor deposition on an Ir(111) substrate by low-temperature scanning tunneling microscopy and spectroscopy. We measure the atomic structure and local density of states of individual GQDs as a function of their size and shape in the range from a couple of nanometers up to ca. 20 nm. The results can be quantitatively modeled by a relativistic wave equation and atomistic tight-binding calculations. The observed states are analogous to the solutions of the textbook "particle-in-a-box" problem applied to relativistic massless fermions.

Lifetime-Enhanced Transport in Silicon due to Spin and Valley Blockade

We report the observation of lifetime-enhanced transport (LET) based on perpendicular valleys in silicon by transport spectroscopy measurements of a two-electron system in a silicon transistor. The LET is manifested as a peculiar current step in the stability diagram due to a forbidden transition between an excited state and any of the lower energy states due to perpendicular valley (and spin) configurations, offering an additional current path. By employing a detailed temperature dependence study in combination with a rate equation model, we estimate the lifetime of this particular state to exceed 48 ns. The two-electron spin-valley configurations of all relevant confined quantum states in our device were obtained by a large-scale atomistic tight-binding simulation. The LET acts as a signature of the complicated valley physics in silicon: a feature that becomes increasingly important in silicon quantum devices.

Visible electroluminescence from hybrid colloidal silicon quantum dot-organic light-emitting diodes

We demonstrate hybrid colloidal silicon quantum dot (SiQD)-organic light-emitting diodes with electroluminescence (EL) in the visible wavelengths. The device using blue photoluminescence (PL) SiQDs as emitters shows multiple EL peaks which are attributed to carrier recombination in the core quantum confinement states, the hole-transport-layer and the surface trap states, respectively. However, the red PL SiQD device shows a single EL peak consistent with the PL peak. These findings are in agreement with the previous report that large Stokes shift were observed for oxidized blue emission SiQDs due to oxide states while red emission SiQDs show negligible PL shift after oxidation.

Fabrication of porous nanostructured lead chalcogenide semiconductors for modern thermoelectric and optoelectronic applications

Porous structures of lead chalcogenides with porosities of 20-68% and pore dimensions as small as 7-20 nm were fabricated using an anodic electrochemical etching technique applied to epitaxial PbTe and PbSe films on Si. Anodized lead selenide demonstrated two basic types of porous morphology: quasiporous noncontinuous layers and hierarchical porous layers. Lead telluride had a more typical mesoporous morphology, with pores propagating at an angle of 35° to surface, which corresponds to the <100< directions in the epitaxial films and is promising for the fabrication of photonic crystals with Yablonovite-like structure. The sizes of PbTe nanocrystallites in porous layers with high porosity were calculated to be 26 nm, which indicates that quantum confinement conditions are realized in this material. Such low-dimensional morphology of porous lead telluride is prospective for the fabrication of microscale thermoelectric devices with high ZT.

Theory and simulation of photogeneration and transport in Si-SiO x superlattice absorbers

Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime.

Multiscale Modeling of a Quantum Dot Heterostructure

ABSTRACTA multiscale approach was adopted for the calculation of confined states in self-assembled semiconductor quantum dots (QDs). While results close to experimental data have been obtained with a combination of atomistic strain and tight-binding (TB) electronic structure description for the confined quantum states in the QD, the TB calculation requires substantial computational resources. To alleviate this problem an integrated approach was adopted to compute the energy states from a continuum 8-band k.p Hamiltonian under the influence of an atomistic strain field. Such multiscale simulations yield a roughly six-fold faster simulation. Atomic-resolution strain is added to the k.p Hamiltonian through interpolation onto a coarser continuum grid. Sufficient numerical accuracy is obtained by the multiscale approach. Optical transition wavelengths are within 7% of the corresponding TB results with a proper splitting of p-type sub-bands. The systematically lower emission wavelengths in k.p are attributable to an underestimation of the coupling between the conduction and valence bands.

Exciton−Plasmon Interactions in Metal−Semiconductor Nanostructures

The complementary optical properties of metal and semiconductor nanostructures make them attractive components for many applications that require controlled flow of electromagnetic energy on the nanometer length scale. When combined into heterostructures, the nanometer-scale vicinity of the two material systems leads to interactions between quantum-confined electronic states in semiconductor nanostructures and dielectric-confined electromagnetic modes in the metal counterparts. Such exciton−plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, creation of new excitations in the strong coupling regime, and increase of optical nonlinearities. With the advancement of novel fabrication techniques, the functionalities of metal−semiconductor nanostructures will be further increased for better control of optical properties and energy flows on nanometer length and femtosecond time scales.

Robust states in semiconductor quantum dot molecules

Semiconductor quantum dots coherently driven by pulsed laser are fundamental physical systems which allow studying the dynamical properties of confined quantum states. These systems are attractive candidates for a solid-state qubit, which open the possibility for several investigations in quantum information processing. In this work we study the effects of a specific decoherence process, the spontaneous emission of excitonic states, in a quantum dot molecule. We model our system considering a three-level Hamiltonian and solve the corresponding master equation in the Lindblad form. Our results show that the spontaneous emission associated with the direct exciton helps to build up a robust indirect exciton state. This robustness against decoherence allows potential applications in quantum memories and quantum gate architectures. We further investigate several regimes of physical parameters, showing that this process is easily controlled by tuning of external fields.

Charge spectroscopy of Si nanocrystallites embedded in a SiO2 matrix

In the present work we have determined the electronic levels in systems of Si nanocrystallites (NCs) embedded in the insulating matrix of silicon dioxide, SiO2, by employing the charge deep-level transient spectroscopy (Q-DLTS) technique. We have clearly shown that these levels are associated with the NCs. Correspondingly, we suggest that the levels that we found are associated mainly with two quantum confinement energies, 0.14 and 0.19 eV. These energies are shown to be consistent with the corresponding theoretical estimates for the presently studied Si–NCs/SiO2 systems. The fact that these levels are almost fixed for the various samples studied suggests the importance of the bulk-surface coupling under quantum confinement conditions.

Charging Dynamics in Electrically Pumped Quantum Wells

Injection pumping of quantum-well (QW) lasers can lead to charging of the active region due to the different electron and hole masses. To investigate the significance and physical consequences of this effect, the consequences of electrical pumping and the associated charging phenomena are investigated microscopically. The analysis reveals the necessity to include the extended barrier states energetically above the band edge in addition to the quantum-confined well states. It is shown that the barrier-state populations compensate the QW charging. The replenishing of stimulated carrier recombination under CW laser operation causes persistent charge oscillations at the plasma frequency.

Room-temperature solution route to free-standing SiO 2-capped Si nanocrystals with green luminescence

We report a new solution route for the preparation of SiO 2-capped silicon nanocrystals (Si NCs). The Si NCs terminated with SiO 2 are fully characterized by transmission electron microscopy, X-ray diffraction, UV–vis absorption, photoluminescence decay and Fourier transform infrared spectra. The photoluminescence spectra reveal that the Si NCs solution emits green luminescence at 535 and 578 nm excited at 490 nm. The origin of the green luminescence of Si NCs is studied. Our theoretical calculations reveal that the green emission is due to the surface related localized states of self-trapped excitons rather than the purely quantum-confined states, which agrees well with the experimental results. A self-trapped exciton model is proposed to take into account the stepwise localization of electron and hole at the Si–SiO 2 interface. From the localization energies the effective Bohr radii of the localized electrons and holes are estimated to be about 1.71 and 1.57 nm, respectively.