Search

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.

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.

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)

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.

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.

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 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

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.

Rare earth doping (Er, Pr) was performed by Si during molecular beam epitaxy (MBE) and incorporation characteristics were studied using ex-situ secondary ion mass spectrometry. We observed considerable surface segregation for the growth temperature, 300–800°C, and kinetic aspect of Er and Pr segregation in the low temperature regime is demonstrated. An attempt is made to make a sharp doping profile using Sb-based heteroepitaxy.

The electric field dependence of photoluminescence is studied in Si-based quantum structures. A higher energy shift of luminescence peaks for fields applied along the growth axis has been observed in GaAs/Al x Ga 1-x As semiconductor superlattices and in superlattices involving SiGe materials. However, we have observed blue shifts not only in superlattices, but in quantum wells, especially at low fields. We attribute this observation to a large decrease of the exciton binding energy offset by the field-induced shifts of the band edge states. The variationally calculated values for the field-induced shifts are compared and discussed with the experimentally observed ones.

An anomalous peak shift was observed in photoluminescence (PL) of Si 1−x Ge x Si quantum wells (QW) with abrupt compositional interfaces formed by solid source molecular beam epitaxy (SSMBE) with atomic hydrogen (AH). It is found that MBE growth of QWs on Si(100) with AH leads to a considerable spectral blue-shift in conflict with the predicted red-shift associated with an abrupt interface. This was found to be caused by well width reduction due to selective etching of Si by AH during MBE. In contrast, strained Si 1−x Ge x Si QWs grown on Si(110) using AH revealed an apparent spectral red-shift in good agreement with the abrupt interface formation through the suppression of the Ge surface segregation with AH. The result seems to be explained as due to a reduced Si etch rate of Si on the Si(110) surface compared to Si(100).

Growth mode changeover during gas source molecular beam epitaxy of Ge on Si(100) is explored by photoluminescence (PL) spectroscopy with a subatomic layer resolution. By observing confinement effect and development of island-related emissions with increasing Ge coverage in Si/pure-Ge/Si quantum wells, the onset of the island formation is determined to be 3.7 monolayers (ML). Furthermore, the equilibrium critical thickness for island formation is found to be lower, i.e. 3.0 ML, by adopting growth interruption after the Ge growth. Intense quantum-confined PL is clearly observed at room temperature from the islanded Ge of quantum dot character, demonstrating the potential of the Ge islands as efficient light emitters.

Spontaneous emission from strained Si 1-x Ge x /Si quantum wells (QW) and allied structures is explored with primary focus on optical characterization in the time domain. Time-correlated single photon counting method is used with complementary steady-state analysis to solicit a wealth of otherwise evasive information on the basis of steady-state measurement alone. Two-dimensional nature of quantum-confined free excitons in strained Si 1-x Ge x /Si QWs, carrier dynamics associated with exciton localization in superlattices, and surface recombination kinetics in near surface quantum confined geometry are described to demonstrate the potential of combined use of time-resolved and steady-state luminescence analysis for characterization of indirect gap strained Si 1-x Ge x /Si QWs.

We report on the observation of a spatially-indirect transition in the photoluminescence (PL) excitation spectra of Be-doped GaAs/AlGaAs quantum wells (QWs). Using this spatially-indirect transition we determine the band offset ratio accurately. With Be acceptors in the QW, free-to-bound and bound-exciton transitions show up in PL in addition to free-exciton transition. As we vary the excitation photon energy, we find a sharp intensity changeover between the free-to-bound and the excitonic transitions at a certain photon energy. A systematic investigation of this energy as a function of the well width shows that this feature corresponds to the onset of a spatially-indirect transition from the valence-band top of the barrier to the n = 1 conduction subband in the QW. Based on the fact that the energy of this barrier-to-well transition critically depends on the valence-band offset, we have determined the conduction-band offset ratio as Qc = 0.62 with accuracy better than ΔQc = ±0.01.

We study a new aspect of the Berry-phase effect as the collaborative x-ray translation by a crystal with undulated deformation. The macroscopic translation was observed around the interface of a heteroepitaxial crystal deformed by quantum dots of $4.1$ germanium monolayers on a silicon substrate. The quantum dots formed a large local gradient of deformation at the interface, which triggered the x-ray translation into two directions. This effect provides a new probe for investigating the interfacial strain, and leads to a single-crystal beam splitter with parallel exit beams.

Luminescence properties associated with strain-induced band modification in SiGe/Si heterostructures such as quantum wells (QWS) and quantum wires (QWRS) are reviewed. Among several issues concerning formation of highly luminescent SiGe materials, surface segregation is shown to be the main cause of deteriorating interface integrity. To resolve the problem, a new technique called segregant-assisted growth (SAG) is proposed. SAG and gas-source MBE (GSMBE), which is considered to be quasi-SAG, are shown to provide high-quality SiGe/Si heterostructures with abrupt interfaces. Highly efficient band edge emissions are observed in not only type-1 but also spatially indirect type-2 QWS. The energy shift in QWS is discussed based upon the band modification due to surface segregation during growth. The ratio between the no-phonon (NP) peak and its phonon replica (TO) in the edge emission reflects the nature of the QWS formed with alloy materials. The coupling of QWS and evolution of superlattices are well understood based on the effective-mass approximation by precisely taking into account the band alignment. QWRS are well fabricated on V-groove patterned substrates and luminescent properties very different from those of QWS are observed. These findings indicate the high potential of SiGe/Si heterostructures, not only in scientific areas but also in device applications.

Successful observation is reported of the quantum‐confined Stark effect in electroluminescence (EL) under a ‘‘forward’’ electric‐field condition, with a current injected into a p‐i‐n diode containing Al0.3Ga0.7As/GaAs symmetric coupled double quantum wells (CDQWs). The Stark shift observed in EL and (under a reverse bias) in photoluminescence (PL), in both of which the transitions involved are between the electron and heavy‐hole lowest states, is found to be symmetrical with respect to the flatband bias, in agreement with calculations. The origin of the optical transitions between electron and light‐ or heavy‐hole states in the CDQWs is also clearly identified by a combination of PL and circularly polarized photoluminescence excitation spectra.

We report a new phenomenon concerning the photoluminescence (PL) from AlxGa1-xAs/GaAs (x = 0.31) quantum wells with Be-delta-doping at the well center. For excitation photon energy larger than the barrier band gap, the PL intensity ratio between the free-to-bound and the excitonic transitions was found to display a strong oscillation as a function of the well width. For “resonant” well widths, the free-to-bound transition was observed to be 2.3 times as strong as the excitonic transitions. We correlate this phenomenon with the electron capture efficiency from the barrier into the well, which also exhibits a similar oscillation as a function of the well width.

We report a new method, based on electrolytic deposition, to passivate GaAs surfaces electronically with thin organic films. This enables us to perform the surface treatment with a reduction in the surface recombination velocity and the formation of insulator layers at the same time, without an energetic process, such as evaporation, sputtering or growth from a plasma, which results in the production of damaged surface layers involving a high density of surface states. The film-forming material used has a moiety containing reactive sulfur (-S − ) that is assumed to bond directly to the GaAs surface, and long alkyl chains acting as an insulating part. Electronic investigations of the treated surface have revealed that the present method substantially improves the surface electronic properties of GaAs.

Luminescence characterization is presented on potential-modulated strained Si 1− x Ge x /Si(100) quantum wells. Interwell coupling was studied in a series of coupled quantum wells (CQW) with an intervening Si barrier for symmetric and asymmetric configurations. Systematic red shift of photoluminescence peak energy was observed in symmetric CQWs with decreasing Si barrier width centered at CQW. Interwell coupling was further evidenced in asymmetric CQWs luminescence as spectral dominance switch between the narrower and the wider wells, reflecting the tunneling-controlled carrier transfer. Evolution of Kronig-Penney superlattice state was observed as consistent luminescence peak shift with change in the structural parameters. Diffusion induced blue-shift of luminescence peak amounting to 22 meV was observed.

By means of “hybrid” Si molecular beam epitaxy (MBE), an n-type Si contact layer for an electroluminescent (EL) p-i-n diode was successfully regrown on a Si 1− x Ge x /Si single quantum well (SQW) layer. The starting undoped SQW layer was grown by gas-source MBE (GSMBE) using disilane (Si 2 H 6 ) and germane (GeH 4 ), and the n-Si contact layer was regrown by using solid-source MBE after transferring the sample through the air. A (2×1) reconstruction was observed on a GSMBE-prepared Si surface even after the sample was exposed to air for 15 h. Evidence of the excellent quality of the EL p-i-n device was provided by the sharpest emission lines, ≈ 5.5 meV, ever reported in the EL spectra of an SiGe system.

We have investigated the in-plane transport of photoexcited carriers and excitons in Al0.3Ga0.7As/GaAs quantum well structures as a function of excitation photon energy and temperature at low carrier density (108 cm-2). We have discovered a strong correlation between the diffusivity and the excitation photon energy; the exciton diffusivity becomes quenched as the excitation photon energy decreases closer to the heavy-hole exciton state. We have also found significant enhancement of diffusivity when a light-hole exciton state is resonantly excited. We attribute the enhancement to the difference between exciton scattering and free carrier scattering.

Anisotropic dislocation glide expected in a multilayer structure modulated with shear modulus was investigated in Si-Ge/Si systems with an intention to apply this concept to dislocation filters that are used to reduce misfit dislocations in hetero-epitaxial films. The applicability of this approach to metallic systems is discussed.

The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves from the thin‐film samples are analyzed considering various interferences including multiple reflections. The experimental results show a reduction of the magnitude of the second‐harmonic coefficient with increasing Al content in the AlGaAs system. This tendency is consistent with Miller’s rule.

n-Si/Si 1-x Ge x multiple quantum wells with «abrupt» heterointerfaces have been created by segregant-assisted growth (SAG) using Sb. Distinct well-width dependence of intersubband absorption energy in thinner quantum wells (L z =14-26 A) has been successfully observed. The well-width dependence of the absorption peak energy was in good agreement with a calculation based on the Kronig-Penney model where interfacial transience was taken to be shorter than than 0.4 nm. Present results show the potential power of SAG for creating «abrupt» Si/Si 1-x Ge x interfaces

Surface segregation of In atoms during molecular beam epitaxy (MBE) and its influence on the energy levels in InGaAs/GaAs quantum wells (QWs) were studied by secondary ion mass spectroscopy (SIMS) and photoluminescence (PL). A strong dependence of In surface segregation on the growth condition was found. The segregation length was observed to increase from 0.8 to 2.9 nm when the growth temperature was raised from 370 to 520°C. At the same time, noticeable peak energy shift was observed in the PL spectrum of the InGaAs/GaAs QWs. A model calculation taking into account In surface segregation yielded excellent agreement with the observed energy shift. These results clearly demonstrate that In surface segregation plays an important role in determining the energy levels in InGaAs/GaAs QWs.

Electroluminescence (EL) was observed above room temperature in p-type strained Si 0.65 Ge 0.35 /Si multiple quantum wells (MQWs) grown on Si(111) substrates by Si MBE. No-phonon band-edge luminescence and its transverse optical phonon replica were well resolved in the EL spectrum at 330 K. In contrast, a broad alloy band dominated the spectrum at lower temperatures, located approximately 100 meV below the band-edge state. Such alloy band emission was found to develop in QWs with a higher Ge content, x > 0.3. Both the emission of this alloy band and the quantized band-edge states were observed around 180 K. The emission intensity of the alloy band tended to saturate with increasing current injection, whereas the band-edge emission was found to increase superlinearly, suggesting the alloy band emission to be of localized excitonic origin.

Si⧸Si1-xGex interface transients were investigated with respect to the absuptness and smoothness by using sputter depth profiling (SIMS), X-ray photoemissions (XPS), photoluminescence and transmission electron microscopy. Ge segregation was confirmed by blue-shift of luminescence energy above what is expected in a square well potential, along with SIMS measurement. Ge segregation was found to persist even at 300°C or lower. In contrast, Ge segregation was quenched by either segregant-assisted growth using Sb and Ga or solid-phase regrowth of Si cap layer; thereby a sharp Si⧸Si1-xGex interface was obtained. However, low-temperature-grown samples required an extensive anneal to restore band-edge luminescence, indicating a lack of quality of crystallinity.

We present a systematic study of luminescence from strained SiGe/Si quantum well (QW) structures grown by gas source silicon molecular beam epitaxy (MBE) and the result is compared with luminescence from QWs grown by conventional solid source silicon MBE. Distinct excitonic photoluminescence (PL) was obtained both from single QW (SQW) and multiple QW (MQW) structures grown by either method. A systematic blue shift of PL energy was observed with decreasing well width and the well width dependence of PL emission energy was in good agreement with a square potential profile for QWs in the case of gas source MBE. In contrast, a considerable energy shift exceeding square potential eigenvalues was found for solid source MBE-grown QWs. We found that the asymmetric potential profile due to surface segragation of germanium atoms was responsible for this blue shift. Electroluminescence and PL data from (111)-oriented QWs are also presented.

High-quality cubic GaN epitaxial films have been successfully grown on (100) GaAs sunstrates by metalorganic vapor phase epitaxy (MOVPE) using 1,1-dimethylhydrazine (DMHy) as the nitrogen source material. The full width at half maximum (FWHM) of the X-ray diffraction profile of 0.65° has been obtained, which is the narrowest reported to date. The grown films were found to be made up of ridges with (111)B facets, with increasing film thickness. Raman peak characteristic of cubic symmetry has been obtained for the first time, showing that GaN films were of excellent quality.

Field-enhanced Stokes shifts (FES), i.e. a large downward shift of luminescence peak energy under transverse electric field in excess of quantum confined Stark shift, were observed in strained Si 1− y C y /Si quantum wells. The microscopic mechanism of FES is attributed to a carrier heating due to carrier-carrier scattering under transverse electric field, which allows carriers to relax down to lower energy sites by jumping over kinetic barriers in lateral directions in an inhomogeneous potential of fluctuated heterointerfaces.

A new class of silicon-based light-emitting diode is demonstrated using InSb-quantum-dot-embedded Si containing the emissive {311} rod-like defects (RLDs). A narrow peak centered at 1377 nm (900 meV) characteristic of the {311} RLDs was found to develop out of an otherwise broad background electroluminescence (EL) upon the application of electric fields in the growth direction. Such electric-field-active EL was observed up to 150 K with a slight downward shift of the peak energies, accompanied by an anomaly in the thermal roll-off of the EL intensity. Spectral variations with temperature and electric field indicate a switching of dominance between the closely correlated defect states that are responsible for the EL emission.