New concepts for efficient light absorption in nanoscale metal-semiconductor-metal photodetectors are analyzed from both theoretical and experimental point of view. They are based on sub-wavelength metallic gratings which allows light confinement in tiny volumes (< 100 nm) close to electrodes (< 100 nm). Two photodetector structures are proposed: (i) a resonant-cavity-enhanced subwavelength metal-semiconductor-metal photodetector, and (ii) a nanoscale metal-semiconductor grating photodetector. External quantum efficiency as high as 9 % has been obtained in 40 x 100 nm2 cross-section GaAs wires, limited by fabrication technology. These results show promising features for highly efficient and ultrafast photodetectors.
We summarize recent experiments probing photon statistics and collective interactions amongst individual z-oriented semiconducting polymer nanostructures. We show that oriented nanostructures from single molecules of a conducting polymer act as highly robust room-temperature single-photon sources. These results suggest the feasibility of an inexpensive "push-button" technology for polymer-based single-photon sources in photonic-based quantum information processing applications. In addition, fluorescence lifetime measurements on pairs of uniformly z-oriented polymer nanostructures that reveal far-field coherent coupling persisting on a distance scale of several optical wavelengths. Farfield photonic coupling between pairs of oriented luminescent polymer nanostructures is manifested by an oscillatory modulation in the fluorescence decay rate as a function of interparticle distance (both enhancement and inhibition of spontaneous luminescence relative to isolated particles), that results from modification of the vacuum field at the position of the probe dipole by the presence of the second radiating dipole.
We have studied the coupling between the InAs quantum dots and coupled quantum wells strained by the InAs quantum dots (i.e. coupled-quantum-well dots) by measuring photoluminescence spectra. By comparing the photoluminescence spectra of the coupled-quantum-well dots in which either the narrow well or wide well is grown next to the InAs quantum dots, we have evidenced the spatially inhomogeneous strains applied by the InAs quantum dots on the coupled quantum wells. Moreover, we have also investigated the three coupling regimes, i.e. when the dot level is higher than, lower than, and aligned to the lowest quantum-well transition. Based on our measurements, we conclude that the optical properties for the coupled-quantum-well dots have been significantly improved over the quantum dots.
The ground and excited state luminescent transitions in self-organized InAs/InP(001) quantum islands (QIs) grown in two different matrices (In0.52Al0.48As and InP), have been studied by cw photoluminescence (PL) and time resolved photoluminescence (TRPL). PL excitation (PLE) shows that the multi-component PL spectrum measured for the InAs/InAlAs QIs is associated to ground and related excited state transitions of QIs having monolayer-height fluctuation whereas for InAs/InP QIs the multi-component PL spectrum is only due to one ground state and their related excited states. This attribution is confirmed by the recombination life times measured by TRPL which are in the 1.2-1.4 ns range for the ground state transitions and in the 90-600 ps range for the excited state transitions.
Silicon defect and nanocrystal related white and red electroluminescences (EL) of Si-rich SiO2 based on metal-oxide-semiconductor (MOS) diode using transparent electrode contact are reported. The 500nm-thick Si-rich SiO2 film on n-type Si substrate is synthesized using multi-recipe Si-ion-implantation or plasma enhanced chemical vapor deposition (PECVD). After 1100°C annealing for 3 hrs, the PL of Si-ion-implanted sample at 415 nm and 455 nm contributed by the weak-oxygen bond and neutral oxygen vacancy defects is observed. The white-light EL spectrum was observed at reverse bias, which originates from the tunneling and recombination intermediate state of SiO2:Si+ at a threshold current and voltage of 1.56 mA and 9.6 V, respectively. The maximum EL power of 110 nW is obtained at biased voltage of 25 V. The linear relationship between the optical power and injection current with a corresponding slope of 2.16 uW/A is obtained. The 4-nm nanocrystallite silicon (nc-Si) is precipitated in the 240nm-thick PECVD grown silicon-rich SiO2 film annealed at 1100°C for 30 min with Indium-tin-oxide (ITO) of 0.8 mm in diameter, which contributes PL at 760 nm. The peak wavelength of the EL spectra coincides well with the PL. The threshold current and voltage are 86 V and 1.08 uA, respectively. The power-current (P-I) slope is determined as 697 uW/A. The carrier injection mechanism is dominated by Fowler-Nordheim(F-N) tunneling.
The future of nanoscience and nanotechnology depends critically on the advance of micro- and nanofabrication technologies. Nanoimprint is an emerging lithographic technique that promises highthroughput patterning of nanostructures with simple equipmental setups. Based on the mechanical embossing principle, nanoimprint technique can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scatterings in other conventional techniques. Nanoimprint can not only create resist patterns as in lithography, but can also imprint functional structures in polymers. This property can be exploited in many applications in the area of photonics and biotechnology. This article reviews basic principles of nanoimprint and some of the recent applications in this field.
Structures of tunnel pairs consisting of InGaAs quantum well (QW) and self-assembled InAs quantum dots (QDs) were employed to improve gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the influence of the relative position of ground states (GS) energies of QW and QDs as well as structure design on the properties of tunnel structures and the characteristics of multi-layer lasers. QDs on QW structures with different GS relative separation were grown by variation of In concentration in QWs with fixed growth process of QDs. An 1160 nm edge-emitting lasers with 4 pairs of QDs-on-QW as active medium showed higher (than in a similar multilayer QD lasers) maximum saturated gain, 26 cm-1, with low minimum threshold current density Jth = 95 A/cm2. First attempt of a triple-pair tunnel QW-on-QDs laser emitting at 1125 nm exhibited a saturated modal gain more than 50 cm-1. These lasers demonstrated broad multi-peak emission spectra with minimum threshold current density Jth = 255 A/cm2 and with lasing from intermediate states between QDs and QW GS transitions. RF small signal modulation characteristics of the 3x(QW-on-QDs) lasers were measured. From the damping factor and resonance frequency dependence, maximum possible 3dB cut-off frequency for this QW-on-QD media can be estimated as 13 GHz.
Growing a lattice-mismatched, dislocation-free epitaxial film on Si has been a challenge for many years. Herein, we exploit nanoscale heterojunction engineering to grow high-quality Ge epilayer on Si. A 1.2-nm-thick chemical SiO2 film is produced on Si in a H2O2 and H2SO4 solution. When the chemically oxidized Si substrate is exposed to Ge molecular beam, relatively uniform-size nanoscale seed pads form in the oxide layer and "touchdown" on the underlying Si substrate. Although the touchdown location is random, the seed pad growth is self-limiting to 7 nm in size. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO2, and the seeds coalesce to form an epilayer. The Ge epilayer is characterized by high-resolution, cross-sectional as well as plan-view transmission electron microscopy, Raman spectroscopy, and etch-pit density (EPD). The cross-sectional TEM images reveal that the Ge epilayer is free of dislocation network and that the epilayer is fully relaxed at 2 nm from the heterojunction. The Raman shift of Ge optical phonon mode exactly matches that of relaxed bulk Ge, further supporting that the epilayer is fully relaxed. The cross-sectional TEM images, however, show that stacking faults exist near the Ge-SiO2 interface. A small fraction (~4x10-3%) of these stacking faults propagate to the epilayer surface. The plan-view TEM sampling provides an estimate on the density of stacking faults (SF) at approximately 106 cm-2 and threading dislocations (TD) far below 106 cm-2. The SF and TD propagating to the surface form etch pits, when immersed in a solution containing HF, HNO3, glacial acetic acid, and I2. The total EPD, as a statistically more reliable estimate on SF and TD than the plan-view TEM, is consistently less than 2x106 cm-2, where SFs constitute 99 %, and TDs constitute 1 %. That is, the TD density is ~105 cm-2 as a conservative upper bound. The reduction of strain density near the Ge-Si heterojunction, leading to high quality Ge epilayer, is attributed to (1) a high density (~1011 cm-2) of nanoscale Ge seed pads interspaced by 2- to 12-nm-wide SiO2 patches and (2) the SiO2 patches serving as artificially introduced dislocation centers. Burgers circuit around each SiO2 patch results in b = (1/2). We have also determined that the surface mechanism responsible for the selective growth of Ge on Si over SiO2 is the high desorption rate of Ge adspecies based on their low desorption activation energy of 42 ± 3 kJ/mol.
Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular beam epitaxy (MBE). During stacking the SL template self-organizes into a highly ordered two-dimensional (In,Ga)As and, thus, strain field modulation on a mesoscopic length scale, constituting a Turing pattern in solid state. InAs QDs preferentially grow on top of the SL template nodes due to local strain recognition, forming a lattice of separated groups of closely spaced ordered QDs. The SL template and InAs QD growth conditions like number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers are studied in detail for optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy (AFM). The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution X-ray diffraction (XRD). Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature dependent PL measurements exhibit an unusual behavior of the full-width at half-maximum, indicating carrier redistribution solely within the QD molecules.
Coupling between InGaAs/GaAs quantum dots is investigated using differential transmission spectroscopy. Degenerate measurements show an initial carrier relaxation time that is relatively independent of carrier density. Two-color pump-probe techniques are used to spectrally resolve the carrier dynamics, revealing transfer between quantum dots and a homogeneous linewidth of 12 nm at room temperature. The time constant for carrier escape is shown to increase from 35 ps at room temperature to 130 ps at 230 K. We then employ a rate equation model to simulate the performance of a semiconductor optical amplifier with QDs as the active region.
We describe the realization and characterization of nanoscale light emitters comprising one or few semiconductor quantum dots (QDs) in the active region. These devices are intended for use as single-photon sources in fiber-based quantum cryptography systems. The epitaxial growth of low-density QDs emitting at 1300 nm is described, and emission from single QDs is demonstrated through micro-photoluminescence measurements. The realization of QD light-emitting diodes (LEDs) having a submicrometer active area is reported, based on oxidized current apertures. Finally, the integration of nanoscale current injection with high-quality factor optical cavities is described, in view of obtaining efficient single-photon LEDs.
Thin films of glass doped with PbTe quantum dots were successfully fabricated. The semiconducting quantum dots were grown by laser ablation of a PbTe target (99.99%) using the second harmonic of a Q-Switched Quantel Nd:YAG laser under high purity argon atmosphere. The glass matrix was fabricated by a plasma chemical vapor deposition method using vapor of tetramethoxysilane (TMOS) as precursor. The QD's and the glass matrix were alternately deposited onto a Si (100) wafer for 60 cycles. Cross-section TEM image clearly showed QD's layer well separated from each other with glass matrix layers. The influence of the ablation time on the size distribution of the quantum dots is studied. HRTEM revealed anisotropy in the size of the QD's: they were about 9nm in the high and 3-5 in diameter. Furthermore HRTEM studies revealed that the QD's basically growth in the (200) and (220) directions. The thickness of the glass matrix layer was about 20 nm. Absorption, photo luminescence and relaxation time of the multilayer were also measured.
We produced a PbTe quantum dot core doped optical fiber with tellurite glasses intended to be used in highly nonlinear ultrafast optical devices capable to operate at the optical communication window at 1300 and 1500 nm wavelength region. Attenuation peaks of the optical fiber depends on the heat treatment time as expected for dots growth and covered the whole mid infrared region near 1500 nm. The optical fiber preform was made with the rod-in-tube method and the fiber was produced with a 4 m high Heatway drawing tower. The optical fiber core can be heavily doped because tellurite glasses solubility for PbTe quantum dots is order of magnitude higher than borosilicate and phosphate glasses, for example. In order to match all the requirements for core-clad optical fibers we studied undoped and doped tellurite glasses optical and thermophysical properties as a function of the glass composition. We also followed the growth kinetics of the quantum dots by High Resolution Transmission Electron Microscopy in the bulk glass matrix and the optical fiber.
Commercially available colloidal semiconductor quantum dots are employed to produce an electron beam sensitive PMMA-quantum-dot (QD) positive composite via pre-polymerization. In PMMA-QD composite, the QDs are stabilized in rapidly formed oligomer matrices to prevent cluster, and the complete polymerization of the PMMA-QD composite is achieved by commonly used polymerization. The properties of the composites are measured and compared with the QDs in original colloidal solution. Patterning of the composites by direct write electron beam and UV optical lithography shows its promising applications in optoelectronics.
CQED has been an active field of research for the last three decades, describing the intimacy of the interaction of radiation and matter in small volumes λ3, and have demonstrated that modifies not only the nature of this interaction, but also atomic characteristic that often were thought intrinsic, such as spontaneous emission or the very quantum nature of the interaction. These conditions have acquired quite an importance for the current activity on Quantum computation and Information. However, most of that activity has been developed in the conceptual and experimental framework of atomic systems. There is evidence that such features also occurs in Quantum Dots. We compare the Dicke and Jaynes Cummings dynamics of atoms described by the Hamiltonian of Quantum Dots, and develop the SU(2) perturbative approach in the regimen where the excitation number (atoms + photons) is larger than the number of atoms L. We exhibit the dynamical detuning produced by the Forster interaction.
The correlations between the photoluminescence (PL) wavelength, integrated intensity, peak intensity, and FWHM with laser diode performance such as the maximum gain, injection efficiency, and transparency current density are studied in this work. The primary outcome is that the variation in PL intensity within a wafer originates primarily from differences in the radiative and non-radiative recombination rates and not from dot density variation. PL generated from 980 nm wavelength pumping appears to give more consistent data in assessing the optical quality of quantum dots that emit in the 1300 nm from the ground state.
Field-induced Stark effects in Ag-coated CdS quantum dot structures are presented. We observe clear exciton peaks due to the quantum confinement effect and the surface plasmon effect in Ag-coated CdS quantum dot fabricated by gamma ray irradiation method. In addition, we observed also a dominant red-shift of the CdS exciton absorption peak as the Wannier Stark effect, implying the strong local field effect in the metal-coated semiconductor composite quantum dot system. The Stark shift of the exciton peak is investigated as a function of the local field for different silver thickness and various sizes of quantum dots based on the effective-mass Hamiltonian using the numerical-matrix-diagonalization method.
Spin-polarized light-emitting diodes and lasers are a promising technology for future high-speed optical communications with enhanced bandwidth and security. In such devices, circularly-polarized emission results from radiative recombination of spin-polarized carriers, which are injected from either a ferromagnetic metallic contact or magnetic semiconductor. Here we discuss the epitaxial growth and application of III-Mn-V diluted magnetic semiconductors and their nanostructures as injector layers in spin-polarized, surface-emitting diodes and lasers. A high polarization efficiency of 30% at 4.5 K is demonstrated in spin-LEDs having GaMnAs spin-injector layers and high-temperature operation (T < 180 K) is observed in spin-LEDs having a GaAs spin injector embedded with Mn-doped InAs quantum dot nano-magnets. Spin-polarized vertical cavity surface-emitting lasers having thin GaMnAs spin-injector layers have also been investigated.