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A hitherto unknown butadiyne-linked BODIPY dimer (4) has been synthesized under the conventional Glaser coupling reaction conditions using the ethynyl-substituted pyrrolyl-BODIPY (2) prepared by gold-catalyzed C-H alkynylation. Due to the [Formula: see text]-extended structure of the BODIPY chromophore in 4, a characteristic broad absorption band in the near-infrared (NIR) region has emerged. Upon photoexcitation, the environment-dependent fluorescence spectral features were observed. The fluorescent response varied with solvents, viscosity, and temperature, which was found to originate from the conformational changes between the coplanar and twisted conformers in the otherwise rigid butadiyne-bridged dimer 4. Therefore, the BODIPY dimer 4 would be a potential NIR fluorescence material for use in the probes responding to the distinct viscous environment in biological tissues.

The synthesis of organometallic complexes of modified 26π-conjugated hexaphyrins with absorption and emission capabilities in the third near-infrared region (NIR-III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis-PdII and bis-PtII complexes of hexaphyrin via N-confusion modification led to substantial metal dπ -pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO-LUMO energy gap. A remarkable long-wavelength shift of the lowest S0 →S1 absorption beyond 1700 nm was achieved with the bis-PtII complex, t-Pt2 -3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t-Pt2 -3 potentially useful as a NIR-III PA contrast agent. The rigid bis-PdII complexes, t-Pd2 -3 and c-Pd2 -3, are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR-III-emissive and photoacoustic-response capabilities.

An expanded metalloporphyrin-based "black dye" Au-Pd oxohexaphyrin (AuPd-1) with absorption capabilities across the visible-to-near-infrared (NIR) range was synthesized. This black dye, AuPd-1, possessed efficient light-harvesting and photostable capabilities, which were indicative of superior photothermal (PT) conversion abilities. Encapsulation of AuPd-1 with a micellar nanocapsule resulted in a compound that demonstrated intense photoacoustic (PA) properties in the NIR region in water. This finding indicated how metal (d)-ligand (π) molecular orbital interactions in metalloporphyrins could aid in the design of visible-to-NIR light-harvesting black dyes for PT and PA applications.

Hybrid metal-organic frameworks (MOFs) with core/shell-like hierarchical structure comprised of zirconium metal and porphyrin (e.g., TPP) and its isomer, N-confused porphyrin (NCP), were synthesized through a seed-mediated reaction. The hierarchical structures of hybrid MOFs were characterized by the microscopic image analyses (e.g., scanning electron microscope (SEM), energy dispersive X-ray (EDX) spectrometry, and confocal laser scanning microscope (CLSM)). Taking advantage of the intrinsic light-harvesting properties of the porphyrin dye and the N-confused isomer, changing the core/shell layer structures of hybrid MOFs allows for tuning of the visible-to-near-infrared (NIR) absorption/emission characters, excited-state energy migrations, and photosensitization capabilities. The Förster energy transfer event occurring in the bulk MOF samples by photoexcitation enabled us to control the photoinduced singlet oxygen generation through the comprehensive light-harvesting ability of these hybrid porphyrinic MOFs. Therefore, implementation of a precisely designed porphyrin "substitute" into the MOF-based materials indeed provides a new mimic of the photosynthetic pigment system and should be potentially applicable for solar-light-driven devices.

A series of rhenium(i) dipyrrinato complexes (Re1-Re8) have been prepared and characterized; their crystal structures, phosphorescence and singlet oxygen generation studies are reported. The aromatic substituents, such as thienyl, p-bromophenyl, p-fluorophenyl, m-fluorophenyl, pentaflurophenyl, N-butylcarbazole, N-phenylcarbazole, and N-butylphenothiazine, are linked to the C5 position of Re-dipyrrinates. Varying the electronic nature of the substituents from electron donating (e.g., carbazole) to electron withdrawing (e.g., pentaflurophenyl) allowed the change in the structural, electrochemical, and spectroscopic properties of these complexes. In particular, the rhenium dipyrrinates showed phosphorescence in the near IR region with sufficiently longer triplet state lifetimes (τT = 9-29 μs). Also, a large Stokes shift (Δν = 5682-6957 cm-1) was witnessed for all the rhenium dipyrrinates. Triplet emission was reflected in the efficient singlet oxygen generation yields (ΦΔ ∼ 0.75-0.98) along with the distinct photo-stability. Density functional theory (DFT) calculations revealed that the electron density is spread over the dipyrrin unit in most complexes. Rhenium dipyrrinate having a phenothiazine substituent exhibited the smallest HOMO-LUMO band gap (2.820 eV) among all Re-complexes.

Triplet photosensitizers that generate singlet oxygen efficiently are attractive for applications such as photodynamic therapy (PDT). Extending the absorption band to a near-infrared (NIR) region (700 nm≈) with reasonable photostability is one of the major demands in the rational design of such sensitizers. We herein prepared a series of mono- and bis-palladium complexes (1-Pd-H2 , 2-Pd-H2 , 1-Pd-Pd, and 2-Pd-Pd) based on modified calix[6]phyrins as photosensitizers for singlet oxygen generation. These palladium complexes showed intense absorption profiles in the visible-to-NIR region (500-750 nm) depending on the number of central metals. Upon photoirradiation in the presence of 1,5-dihydroxynaphthalene (DHN) as a substrate for reactive oxygen species, the bis-palladium complexes generated singlet oxygen with high efficiency and excellent photostability. Singlet oxygen generation was confirmed from the characteristic spectral feature of the spin trapped complex in the EPR spectrum and the intact 1 O2 emission at 1270 nm.

Organoplatinum(II) complexes of calix[4]phyrin analogues, singly N-confused calix[4]phyrin (Pt-2), and doubly N-confused calix[4]phyrin (Pt-3), were synthesized and characterized. The explicit structures of these organoplatinum(II) complexes were elucidated by single-crystal X-ray diffraction and spectroscopic studies. The introduction of N-confused pyrrole rings to the parent calix[4]phyrin scaffold was found to have profound effects on the photophysical properties, such as the bathochromic shifts of both the absorption and phosphorescence maxima. The triplet excited state properties of these platinum complexes were analyzed by DFT calculations at the B3LYP level. The organoplatinum(II) complexes derived from the deformed scaffolds can serve as potent triplet sensitizers for singlet oxygen generation under aerobic conditions.

We have studied the electronic structure of the surface unoccupied band (SUB) of clean Ge(001)-$c(4\ifmmode\times\else\texttimes\fi{}2)$, with high energy and momentum resolution, by means of time- and angle-resolved two-photon photoelectron spectroscopy. The time evolution of photoelectron intensity images, measured as functions of energy and emission angle after photoexcitation with laser pulses (1.5 eV, 200 fs), provides a momentum space view of the relaxation pathways of surface excited electrons toward the bottom of the SUB. Surface excited electrons relax in several picoseconds along the strongly dispersive directions ($\overline{\mathrm{\ensuremath{\Gamma}}}\overline{{J}^{\ensuremath{'}}}$ and $\overline{\mathrm{\ensuremath{\Gamma}}}\overline{{{J}_{2}}^{\ensuremath{'}}}$) and then accumulate near the band bottom. Taking into account the ultrafast change of surface potential, possibly due to the spatial redistribution of nonthermal carriers generated by photoexcitation, an energy width of 0.22 eV was determined as the surface band gap, as well as the surface dispersion properties along three high-symmetry directions.

An attempt is made to generate a stream of heralded single photons using spontaneous parametric downconversion (SPDC) in type-II β-barium borate (BBO) pumped by a fiber-coupled light-emitting diode (LED). The hallmark interlocked circles develop only after removing the fluorescence background without correlation with pump polarization during spatial mode mapping of SPDC photons that allows otherwise diffuse circles to be projected distinctly in the image plane as photons with an identical wave vector from across the output area of BBO are collectively focused. Time-gating reveals a clear signature of photon antibunching, which promises a heralded single photon source fueled by commercially available LEDs.

Germanane (GeH) is a hydrogen-terminated layered crystal of germanium. We fabricated an electric double layer transistor (EDLT) on a GeH thin film on the Ge (111) substrate and investigated its electronic properties. The EDLT showed ambipolar transfer characteristics at 240 K. The sheet resistance Rsheet at null gate bias (VG = 0 V) exceeded 1 MΩ below 100 K, which indicates an insulating behavior. For VG = −2 V (hole accumulation), a gradual increase in Rsheet up to 10 kΩ was found upon decreasing temperature to 40 K, i.e., semiconducting or even weakly insulating. Remarkably, Rsheet for VG = 1 V (electron accumulation) decreased down to 3 kΩ upon decreasing temperature to 40 K, indicating metallic conduction. Accumulation of electrons (holes) was confirmed for VG > 1 V (VG < −1 V) by Hall effect measurements. Hall mobilities of electrons and holes, which increase with decreasing temperature, were 6500 cm2 V−1 s−1 and 570 cm2 V−1 s−1, respectively, at 120 K.

Complexation of novel multiply N-confused expanded calix[n]phyrins with boron difluoride afforded a new class of cyclic BODIPY (boron-dipyrromethene) arrays. The structures of circularly arranged BODIPY subunits linked in an N-confused fashion give rise to such photophysical properties unique to the macrocycles as redshifted emission wavelengths along with apparent large Stokes shifts, long emission lifetimes, and solid-state lasing. The DFT calculations support the size-dependent excited-state dynamics of the macrocycles.

Novel near-infrared phosphorescent iridium(III) complexes based on benzo-annulated N-linked corrole analogue (termed as benzonorrole) were synthesized. The structures of the complexes revealed octahedral coordination geometries involving an organometallic iridium–carbon bond with two external axial ligands. Interestingly, the iridium(III) complex exhibits near-infrared phosphorescence at room temperature at wavelengths beyond 900 nm. The significant redshift of the emission, as compared to the corrole congener, is originated from the ligand-centered triplet character. The fine-tuning of the photophysical properties of the complexes was achieved by introducing electron-donating and electron-withdrawing substituents on the axial pyridine ligands.

We studied the temperature dependence of high-density carrier dynamics in as-grown and thermally annealed Ge quantum dots (QDs) in silicon crystals. In as-grown and thermally annealed samples, photoluminescence (PL) intensity exhibited a power-law dependence on the excitation intensity and its power-law index was ~ 0.7 at low temperatures. With increasing measurement temperature, PL intensity decreased and the index of the power-law function increased up to ~ 1.5, in which carrier recombination dynamics is dominated by a single carrier trapping. Moreover, in thermally annealed QDs, the index of the power law increased more rapidly than as-grown QDs, suggesting that the carrier recombination dynamics drastically changed in thermally annealed QDs. Effects of Ge/Si interface on high-density carrier recombination process are discussed.

An attempt is made to mimic a system under the influence of nonradiative recombination (NR) by use of a Si 1 − x Ge x quantum well (QW) embedded in a Si 1 − x Ge x /Si strained-layer superlattice. Carrier transfer mediates the coupling between the first miniband of superlattice barrier and QW ground state, which is useful in setting up an artificial NR center based only on radiative channels. Steady-state and transient photoluminescence (PL) reproduces spectroscopic features shared by NR systems at large such as quadratic dependence of PL intensity on excitation power and prolonged decay time at increased pumping, which are quite opposite to radiative recombination.

We studied the Auger recombination in Si1-xGex /Si quantum wells using near-infrared photoluminescence (PL) spectroscopy. We found that the PL intensity under high-density excitation depends strongly on the width of the single quantum wells and that the Auger recombination is more pronounced for wide wells. This indicates that the Auger recombination rate depends on the spatial profile of the hole wave function in the Si1-xGex well layer. We discuss the method to control the nonradiative Auger recombination rate in coupled double quantum wells (CDQWs) by manipulating the wave functions; the rate is reduced by tuning the barrier width. In CDQWs voltage-biased on the surface, the PL intensity decrease with surface voltage, but the electric field dependence of the PL intensity is complicated under low-density-excitation condition. This unique electric-field dependence of the PL intensity is sensitive to the excitation intensity and the sample temperature. We tentatively attribute these PL behaviors to the appearance of the impact ionization under the low excitation intensity and high-surface-bias voltage conditions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Light amplifying characteristics of GaSb quantum dots embedded in Si grown on SOI were studied. Amplified spontaneous emission was observed at 6.6 K under pulsed visible laser excitation. Pump power dependence of on–off gain measured in a pump-and-probe configuration showed a threshold behavior, which indicates the relevance of a three level system. At 30 K, however, gain was diminished while absorption dominated, and the photoluminescence intensity was saturated at increased pump intensities. This indicates that the interface localized electron–hole pairs providing gain have detrapped out of the interface trap due to thermal excitation and that free carrier absorption dominates at high pump.

Voltage-controlled emission wavelength switching (VCEWS) is demonstrated in a light-emitting diode (LED) containing a strained Si1−xGex/Si double quantum well (DQW) in the active region. Impact ionization in a reverse-biased Si and SiGe allows the otherwise evasive bipolar VCEWS in LEDs containing type-II anti-electron QWs. A clear spectral dominance switch was observed upon changing the direction of the driving current where electroluminescence (EL) from the QW on the positive electrode side dominated. In transient EL, a 500 kHz bandwidth was obtained, indicating that the impact ionization controls the generation of the carriers.

The unique type-II potential lineup of pseudomorphic Si 1− x Ge x /Si quantum wells (QWs) allows a negative charge buildup in the near-surface region. As a result, diminished Shockley–Read–Hall (SRH) recombination is observed in the surface-side QW in a double QW geometry where charge imbalance due to the negative charge buildup plays a critical role. Spectral changes due to the charge redistribution under longitudinal electric fields indicate that the second QW on the substrate-side is key to the charge imbalance, and allows an enhanced internal quantum efficiency of radiative recombination in the surface-side QW.

Light amplifying characteristics of GaSb quantum dots embedded in Si were studied. Under visible pulsed laser excitation, a single-pass gain coefficient more than 10 dB cm−1 was obtained for planar waveguide geometry. The gain was found to decrease as the temperature increases, and eventually loss dominates beyond 25 K. All electrical operations of optical amplifier were successful with a gain value of 3 dB cm−1.

It is demonstrated that strained Si1−xGex/Si single quantum wells (SQWs) are capable of generating light pulses simply by controlling the population of only one type of carriers in the SQW through longitudinal electric fields. The unique band lineup of strained Si1−xGex/Si QWs allows loose confinement of electrons, which makes the electrons easy to be detrapped upon application of electric field. At a weak positive surface bias, an electron reservoir is formed in the near-surface region while holes stay in the QW so that electrons and holes are spatially apart and thereby luminescence signal is diminished. Upon removal of the bias voltage, the electrons that have built up in the near-surface are driven back to the QW, turning on luminescence. Triggered 50 ns light pulse train was generated with alternating bias voltage waveform for a single-shot excitation of electron–hole pairs at the outset.

We have studied the optical properties of compound semiconductor quantum dots (CSQDs) embedded in Si. Both photoluminescence and electroluminescence spectra were found to be associated with an inhomogeneously broadened band in the near-infrared. A long decay lifetime of luminescence was observed, which is in support of an indirect transition in both k - and real-space. Strong localization of electron–hole pairs was found to occur due to a deep potential well created by the built-in electric dipole at the III–V/Si interface. A Si-based light-emitting diode with GaSb-CSQDs in the active layer showed a high value of quantum efficiency. Light amplification was also observed under pulsed laser excitation.

Si-based trans-column semiconductor quantum dots (TCQDs) are created as a new class of Si-based self-assembling nanostructures in an attempt to realize an efficient Si-based light emitter with light amplifying characteristics. A high value of quantum efficiency has been obtained for EL at low temperature, and amplified spontaneous emission of interband radiative recombination has been observed under visible, pulsed laser excitation for GaSb TCQDs embedded in Si.

The morphological evolution of InAs and InSb quantum dots grown on Si (001) by molecular beam epitaxy was studied by reflection high-energy electron diffraction and scanning probe microscopy. For the InSb/Si system, the maximum density of 1�×�1010�cm–2 was achieved at a growth temperature of 200�°C with the average base length and dot height of 80 nm and 1 nm, respectively. A broad luminescence band extending over 1.1–1.4�μm was observed at low temperature from the In-based quantum dots embedded in Si. The decay lifetimes of luminescence were of the order of submicroseconds, which indicates an indirect transition both in real- and in k-space. Excessive thermal budget after the growth only resulted in the development of luminescence features characteristic of In-doped Si.

Surfactants in heteroepitaxy are catalytic elements that float up to the surface during growth to control the energetics/kinetics of adatoms. “Autosurfactants” are exceptional in that the surfactant action is self-contained without foreign species. So far, autosurfactants as surface smootheners are known. Here, we demonstrate a different class of autosurfactants as surface-segregation quenchers: Bi, a dopant with a strong surface-segregation tendency in Si, is utilized to lock otherwise elusive Bi adatoms themselves to the Si lattice underneath during molecular beam epitaxy. Quasi-1D δ-doping of Bi in Si up to 4 × 1020 cm−3 in terms of volume concentration is achieved.

Differential ghost imaging was attempted in time domain, i.e., temporal differential ghost imaging (TDGI), using pseudo-randomized light pulses and a temporal object consisting of no-return-to-zero bit patterns of varying duty. Evaluation of the signal-to-noise characteristics by taking account of errors due to false cross-correlation between the reference and the bucket detector readings indicates that the TDGI outperforms its non-differential counterpart, i.e., time-domain GI, in terms of consistently high and even duty-independent signal-to-noise ratios that are achieved. Dynamic local averaging helps save data recording without compromising the essential features of the TDGI., 9 pages, 4 figures

A composition window of x < 0.3 was found for which a good SiGe-on-insulator (SiGe-OI) substrate can be obtained by low-energy implantation of oxygen ions (25 keV O + ) onto a pseudomorphic Si 1-x Ge x /Si(001) alloy. Both the surface oxidation of SiGe during post-implant annealing and the thermally induced instability of SiGe alloys were found to be limiting factors influencing the composition window.

Epitaxial Si(Ge)/oxide semiconductor/wide-gap material systems have been developed as a new class of Si-based heterostructure. Optoelectronic device capabilities are highlighted, and also reported are preliminary results of the fabrication of Si/SiO2 cavity resonance photodiodes and SiGe-based wavelength-selective “near-surface” photodetectors in a metal–semiconductor–metal configuration.

Nanometer order SiGe-on-insulator substrates have become accessible by using the separation-by-implanted-oxygen (SIMOX) technique. Low energy (25 keV) oxygen ion (O+) implantation was performed on a strain-relieved Si0.82Ge0.18 substrate. The dose window for a SOI structure was found by cross-sectional imaging to be 2–2.5 × 1017 cm−2. The Ge composition of the top SiGe layer is controllable by post-implant annealing. Fabrication of SiGe-on-insulator substrates on a thick pseudomorphic Si0.82Ge0.18 grown on Si(1 0 0) is also demonstrated. The results hold promise for a low-cost and high-throughput SiGe-SIMOX.

Si quantum wires (QWRs) embedded in SiO2 are successfully fabricated by low-energy oxygen implantation on a V-groove patterned substrate. Si QWRs aligned to [1 1 0] appeared at the bottom-center of the V-groove. The [1 1 0] cross-section of the Si QWR is a hexagon encompassed by four Si {1 1 1} and two Si {0 0 1} lateral facets.

Anomalies are found in the near-band-edge luminescence properties of Δ 2 -light-hole indirect excitons in Si 1− y C y -based tensilely strained quantum wells (QWs). The experimental spectra exhibit a clear signature of phonon-assisted transitions on the lower energy side of the “no-phonon” transition, which indicates the relevance of “virtual” indirect valleys and in-plane k -dispersion, as opposed to the theoretical prediction that the zone-centered Δ 2 valleys take over the conduction band edge. Intervalley scattering between [0 0 1]-Δ 2 valleys and in-plane Δ 4 valleys is suggested as the underlying mechanism. On the other hand, the experimental evidence was found for “apparently enhanced” quantum-confined Stark red shifts for Si 1− y C y -based QWs. However, quantitative estimates are in conflict with the experimental results and predict a blue shift due to exciton weakening which masks the Stark effect as in the case of Δ 4 -heavy hole excitons in Si 1− x Ge x -based QWs.

It is found that the oscillator strength is spontaneously modulated in most of experimentally accessible ordered superlattices due to grown-in unintentional disorders. Such disorders cause the localization and the real-space separation of the electron and hole wavefunctions. As a result, an ordered superlattice is transformed into a disordered superlattice where the coherence of superlattice states is broken and the oscillator strength is diminished, which is found to be an ubiquitous effect regardless of the potential lineup. We discuss the limitation of ordered superlattices, and the physics and control of the wavefunctions and the oscillator strength are explored by using a deliberately disordered superlattice grown by molecular beam epitaxy (MBE).

Fabrication of epitaxial Si/SiO 2 low dimensional structures using in situ multiple low energy oxygen implantation in combination with Si molecular beam epitaxy is described, highlighting its potential for the control of quantum confinement structures. Highly-oriented Si nanodots and Si quantum wires embedded in SiO 2 are demonstrated as prominent examples and the formation mechanism of the epitaxial Si/SiO 2 bilayer is described. Successful growth of epi-ready Si/SiO 2 Bragg reflector is demonstrated in view of optoelectronic application.

Phonon-suppressed indirect-gap radiative recombination was observed from a type-II Ge/Si dot grown by gas source molecular beam epitaxy (MBE). The phonon suppression occurs due to the formation of an electronic dot and the phononless k-diagonal Δ t.c - Γ 25' interband recombination of three-dimensionally confined excitons is understood in the context of the elimination of momentum conservation. The dot confinement is established by large self-organized Stranski-Krastanow dots and the interface trapping due to hole space charge. The dot characteristics were confirmed by detailed measurements and the electronic dot precursor was found near the growth mode transition.

We report on photoluminescence studies of the quantum-confined Stark effect (QCS) in strained Si 1 − x Ge x Si type-I quantum wells (QWs) with symmetric confinement potential. Application of a weak longitudinal electric field leads to an upward shift of the free-exciton peaks due to a field-driven decrease of the exciton binding energies arising from the shallow electron confinement of SiGe QWs. The existence of the masked QCS has been separately confirmed by observing prolonged luminescence decay times with increasing electric field. The insertion of an asymmetric potential does not fully restore the QCS redshift, indicating that an enhancement in the exciton binding energy is necessary to establish the QCS redshift.

A new class of non-dispersive Si nanoparticle (SNP) system has been created on Si(1 0 0) wafers by low-energy oxygen co-implantation during Si MBE. Ordered, highly oriented Si nanoparticles embedded in SiO2 with dimensions d = 4–100 nm are observed. Transmission electron microscopy shows that SNPs are oriented preferentially to [1 0 0], indicative of epitaxial growth of SNP as compared to conventional dispersive SNPs. Individual SNP is confirmed to be a single crystal and exhibits unusual habits with (1 0 0) and (1 1 1) facets. Broad visible luminescence bands are observed at room temperature.

Photoluminescence (PL) study on lateral bandgap modulation in a Si 1-x Ge x /Si quantum well induced by self-assembled Ge islands which act as stressors is carried out. It is confirmed that excitons are confined in the modulated quantum structure (MQS) in compressively strained regions induced by the Ge islands. Redshift of PL peaks from MQSs does not show a monotonic change but complicated behavior, i.e., it increases up to Q = 3.7 ML and saturates above Q = 4.1 ML after once retrogressing with the increase in the Ge coverage. The results are explained in terms of elastic deformation due to Ge stressors and lattice relaxation in the Ge islands.

A detailed experimental study of the optical properties of separation by implanted oxygen (SIMOX) substrates is performed using photoluminescence techniques. The photoluminescence properties of SIMOX substrates are compared with those of Si substrates. The silicon-on-insulator (SOI) layer is found to be free of any strain. High-quality SiGe quantum wells are grown on SOI using gas source molecular beam epitaxy. It is found that the photoluminescence properties of these quantum wells on SOI differ significantly from those of the SiGe quantum wells grown on a Si substrate under identical growth conditions. Intense photoluminescence and higher operation temperature are obtained for quantum wells grown on SOI substrates. It is shown that growth of a few hundred A Si buffer layer on SOI is essential in order to achieve a high quality SiGe epitaxy on SOI. Finally, a SiO2/Si/SiO2 optical cavity is proposed on SIMOX substrates, and it is employed to further enhance the photoluminescence intensity of SiGe quantum ...

A novel Si nanoparticle (SNP) system of spatially coherent distribution was created by low energy oxygen ion implantation during Si molecular beam epitaxy. Highly oriented Si nanoparticles embedded in an SiO 2 matrix of dimensions d of 4–100 nm with log–normal size distribution are demonstrated. The single-crystal character of the SNPs is revealed by transmission electron microscopy and the preferred crystalline axis is oriented to the substrate normal, [100], retaining the epitaxial character compared with dispersive SNPs. The morphology of individual nanoparticles is significantly faceted towards [100] and [111] as opposed to regular polyhedra with the crystal habit of [311], [111] characteristic of thermodynamically stabilized dispersive SNPs. Visible luminescence bands are observed at room temperature.

Time-resolved photoluminescence (PL) of dislocation-related features (D1–D4) was studied in strain-relaxed molecular beam epitaxy SiGe/Si(100). Low-temperature decay transients are essentially non-exponential for all D bands. The D1, D2 bands are characterized by long ( τ >200 ns) decay times while short ( τ

Radiative recombination and relevant dynamic effects are studied in SiGe-based shallow quantum confinement systems where an inherently small barrier height controls the optoelectronic properties. Dynamic localization is demonstrated in a double quantum well carrying a pair of quantum wells with differential potential depths, leading to a remarkable observation of highly efficient “classical” carrier blocking due to an attractive potential as opposed to conventional repulsive potential blocking utilizing the wave character of carriers. The underlying mechanism is controlled by hole re-emission with increasing temperature. In addition, it is shown that shallow-confined electrons are susceptible to electric fields, thereby masking the Stark shifts in type-I quantum wells at low field regime and reducing decay lifetimes due to exciton field ionization. These are shown to arise from extended wavefunction of electrons assuming three-dimensional character. Furthermore, modulation of exciton luminescence due to excess background charges is shown to address the importance of screening in a high pump condition.

Anomalies have been observed in photoluminescence (PL) of pure-Ge/Si type-II coupled quantum wells (II-CQWs). Although the II-CQWs (LGe=1.5 ML) showed clear energy red-shift with decreasing Si spacer thickness (LSi) due to interwell coupling of the hole wave-function, an anomalous energy lowering was observed for smaller LSi (≤6 A). Moreover, PL intensity of the II-CQWs increased linearly with increasing excitation power, as opposed to a sublinear increase in type-II QWs due to exciton localization at the interfaces. These results seem to be the result of enhanced electron capture by the II-CQW potential profile. Furthermore, correlation of thickness fluctuation among the layers was observed by high-resolution transmission electron microscopy, which may be connected to the PL anomalies.

Evidence for the resonant capture of electrons into quantum wells (QW's) is demonstrated in the photoluminescence (PL) of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QW's doped with Be. We find that the PL intensity ratio between the free-to-bound and excitonic transitions exhibits a strong oscillation as a function of the QW width bearing a striking resemblance to the theoretical prediction of the electron capture rate. It is shown that this PL behavior reflects the buildup of excess negative charge in the QW arising from the different capture efficiencies of electrons and holes. \textcopyright{} 1996 The American Physical Society.

Highly efficient green luminescence was observed from a new class of quantum confined geometry referred to as neighboring confinement structure (NCS). The photoluminescence (PL) intensity of AlP GaP NCSs was even higher than that of a 300-period AlP GaP superlattice (SL), and the PL of the NCS exhibited much improved immunity against thermal quenching compared to the SLs. The luminescence origin of the NCS was confirmed from the well width dependence of the PL peak shift, and the main luminescence line was assigned to no-phonon from a phonon-resolved PL study.

We report on exciton diffusion dynamics in SiGe Si quantum wells (QWLs) on a V-groove patterned Si substrate. The decay time of excitons in the (111) facet QWLs was found to decrease with increasing temperature in contrast to the radiative lifetime of two-dimensional excitons. Reduction of the V-groove period leads to decrease of the decay time of the (111) facet QWL excitons. These behaviors are explained in terms of the competition between the radiative recombination and the exciton diffusion to the other SiGe parts with lower quantized energies. A quantitative analysis of diffusion length is performed by reproducing temporal profiles of the luminescence from the (111) QWLs.

Intense photoluminescence (PL) is observed from submonolayer Ge embedded in Si. The PL peak energies are found to decrease with increasing Ge coverage (Q), indicating quantum confinement. In the excitation power dependence, a new peak is found to develop on the lower energy side of the free exciton (FE) for Q < 0.5 monolayers (ML). The intensity of the new peak increases as the square of the FE, indicating that the observed emission is a two-exciton process. From temperature dependence and PL decay profiles, we conclude that the newly found feature is due to biexcitons. These observations are explained in terms of Ge build-up at the step edges, forming spontaneous quantum wires, as observed by plan-view transmission electron microscopy.