Thermal Redistribution Research Articles

Electron-hole asymmetry and two-state lasing in quantum dot lasers

We study the decrease of the ground-state output with increasing current in two-state quantum dot lasing. We show that the asymmetry in the thermal population redistribution breaks the symmetric dynamical evolution of the electron-hole pairs. This fully explains the transition from two-state to single-state lasing observed experimentally. The model also reproduces the temperature dependence of the two-state lasing.

Effect of carrier hopping and relaxing on photoluminescence line shape in self-organized InAs quantum dot heterostructures

A steady-state thermal model for carrier dynamics in self-organized InAs∕GaAs quantum dot heterostructure is presented. The effect of quantum-dot size distribution, density of states random population, thermal emission, retrapping, and relaxation are investigated. The temperature-dependent photoluminescence spectra obtained from two samples with different dot densities and size uniformities are simulated in this model and show a good agreement with experiment. The corresponding mechanisms of carrier dynamics related to the thermal redistribution and lateral transition of excited carriers as well as the observed phenomena resulting from these mechanisms, are discussed in detail.

Optical and transport properties of InAs/GaAs quantum dots emitting at 1.3 μm

We report the results of spatially resolved micro-photoluminescence on self-organized InAs/GaAs quantum dots. Our results suggest that carrier thermal redistribution is linked to the effects of carrier migration in these quantum dots. The higher migration length was observed at 150K, making the higher carrier redistribution between the energy levels.

Thermal redistribution of hydrogen and boron in SiO 2 in SiN-capped p-type MOSFET structures

The thermal behavior of boron and hydrogen in a SiN-capped p-type MOSFET was investigated in order to explain SiN-enhanced boron diffusion in SiO 2. Measured hydrogen profiles indicated that hydrogen in the SiN migrated into the gate oxide during annealing under nitrogen. When this structure was annealed under nitrogen, the boron diffusivity in its SiO 2 increased to levels more than two orders of magnitude greater than that reported in SiO 2 annealed under hydrogen without a SiN cap. Nevertheless, the activation energy for boron diffusion in SiO 2 in a SiN-capped sample was the same as that for boron diffusion in SiO 2 annealed under hydrogen. This indicates that SiN-enhanced boron diffusion can be explained by the model proposed for hydrogen-induced boron diffusion: hydrogen termination of defects reduces the activation energy for interstitial-mediated diffusion of boron in SiO 2. Anomalous boron diffusion in SiO 2 in SiN-capped samples is thought to result from the SiN film facilitating the incorporation of a large amount of hydrogen in the SiO 2.

Intersublevel Relaxation Dependence of Carrier Hopping in Self-Organized InAs Quantum Dot Heterostructures

The temperature dependence of the photoluminescence (PL) emission spectra of self-organized InAs/GaAs quantum dots (QDs) grown under different growth conditions in the range 20-300K has been investigated. Three InAs QD samples were grown on (100) 2º-tilted toward (111)A Si-doped GaAs substrates by metal-organic chemical vapour epitaxy (MOVPE), with various size uniformities and dot densities. Observing the measured PL spectra at 20K, the differences caused by size uniformities among the three samples were obvious. The PL spectra were simulated with rate equations, taking into account the carrier relaxation between the first excited state and the ground state. Interestingly, the calculated relaxation lifetimes at 20K were 198ps, 139ps and 54ps for the samples. The temperature dependent PL spectra were also simulated using the same model. Based on the calculated values of temperature dependent relaxation lifetimes, the differences in changes with temperature among the three samples are discussed. The results are consistent with the thermal redistribution effect.

Metalorganic chemical vapor deposition and spontaneous emission of self-assembled InAs quantum dots in open space and in a planar microcavity

Metal organic chemical vapor deposition of InAs quantum dots (QDs) and GaAs/AlAs planar microcavities with InAs QDs as active layer using tertiarybutylarsine (TBAs) as arsenic precursor is reported. The effects of crucial growth parameters on growth of the InAs QDs are studied. The spontaneous emission (SE) of InAs QDs in open space and in planar microcavties are investigated using Photoluminescence (PL) and time resolved PL spectra. The behaviors of PL and time resolved PL of InAs QDs in open space can be explained in terms of thermal redistribution of carriers and carrier transfer between quantum dots. The SE rate of InAs QDs can be enhanced in the presence of a microcavity.

Conversion of an electron–hole plasma into a high density excitonic state in ZnO epitaxial thin films

Time-resolved luminescence spectra of an exciton-exciton scattering process and an electron–hole plasma (EHP) were measured in ZnO epitaxial thin films by the optical Kerr gate method. At moderate excitation density below critical Mott density, stimulated emission of luminescence due to the exciton–exciton scattering process (P band) was observed. The P band shows a red shift and spectral narrowing in the first 1–2 ps after the excitation which reflects a thermal redistribution of excitons in the n = 1 excitonic state. On the other hand, at high excitation density above the critical Mott density, stimulated emission of the luminescence due to the EHP was observed. After thermalization of the EHP, the luminescence intensity of the EHP band decreases with time, and the EHP band converts to the P band after ∼10 ps through a reverse Mott transition. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains.

We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes.

A New Approach to Cutting Temperature Prediction Considering the Thermal Constriction Phenomenon in Multi-layer Coated Tools

A novel approach to the prediction of cutting temperature in multi-layer coated tools is presented. This approach considers the contact mechanics at asperity level and resulting thermal constriction resistance phenomenon. Micro-contact model was developed and the correlation between contact pressure and thermal constriction resistance of uncoated and multi-layer coated tools is established. The thermal interaction and redistribution of heat between the workpiece, the chip and the tool were analyzed, supported by FE model, which considers thermal characteristics of multi-layer coating. The results indicate that the tool coating may cause significant reduction in heat flowing into the tool.

The Effect of Lamps Radius on Thermal Stresses for Rapid Thermal Processing System

The concept of rapid thermal processing has many potential applications in microelectronics manufacturing, but the details of chamber design remains an active area of research. In this work the influence of lamps radius on the thermal stresses in a wafer during the cooling process is studied in detail. Since the equations governing the present thermal-elastic system are coupled in nature, the solution for the temperature and stresses must proceed simultaneously by using a fully implicit finite difference method. After the thermal stresses are obtained, the optimum lamps radii for various heights of the chamber under the constant power ramp-down control scheme are determined based on the maximum shear stress failure criterion. The shortest cooling time that can significantly reduce the thermal budget and dopant redistribution is also predicted by applying the maximum stress control scheme. The result obtained is useful in the design of a reliable rapid thermal processor based on a more practical consideration, thermal stress.

Wavelength selective charge storage in self-assembled InGaAs/GaAs quantum dots

We present a wavelength selective optical memory device based on optically-induced charge storage in an inhomogeneously broadened ensemble of InGaAs quantum dots. We show that electrons and holes can be efficiently and resonantly stored without thermal redistribution of charge between dots over timescales much longer than 25 μs at 10 K. Direct information on the absorption mechanisms are obtained for self-assembled dots. The maximum efficiency of the charge storage process is estimated to be close to unity.

Influence of the temperature on the carrier capture into self-assembled InAs/GaAs quantum dots

Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer.

Polarization-dependent spectroscopy of the near-bandgap excitonic emission in free standing GaN

ABSTRACTWe report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E┴c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.

Passive and active oxidation of Si(100) by atomic oxygen: a theoretical study of possible reaction mechanisms.

Reaction mechanisms for oxidation of the Si(100) surface by atomic oxygen were studied with high-level quantum mechanical methods in combination with a hybrid QM/MM (Quantum mechanics/Molecular Mechanics) method. Consistent with previous experimental and theoretical results, three structures, "back-bond", "on-dimer", and "dimer-bridge", are found to be the most stable initial surface products for O adsorption (and in the formation of SiO(2) films, i.e., passive oxidation). All of these structures have significant diradical character. In particular, the "dimer-bridge" is a singlet diradical. Although the ground state of the separated reactants, O+Si(100), is a triplet, once the O atom makes a chemical bond with the surface, the singlet potential energy surface is the ground state. With mild activation energy, these three surface products can be interconverted, illustrating the possibility of the thermal redistribution among the initial surface products. Two channels for SiO desorption (leading to etching, i.e., active oxidation) have been found, both of which start from the back-bond structure. These are referred to as the silicon-first (SF) and oxygen-first (OF) mechanisms. Both mechanisms require an 89.8 kcal/mol desorption barrier, in good agreement with the experimental estimates of 80-90 kcal/mol. "Secondary etching" channels occurring after initial etching may account for other lower experimental desorption barriers. The calculated 52.2 kcal/mol desorption barrier for one such secondary etching channel suggests that the great variation in reported experimental barriers for active oxidation may be due to these different active oxidation channels.

Carrier thermalization and activation within self-assembled InAs/AlAs quantum dot states

The carrier transfer and activation processes among lateral coupled InAs/AlAs quantum dots (QDs) are investigated based on the analysis of excitation power and temperature dependence of photoluminescence. An unusual red-shift of ground-state emissions, accompanied by a shrinkage of its line width, with increasing power density, is observed. It is explained by scattering and tunnelling of carriers, which result in a preferential capture of carriers into lateral coupled QDs and recombination from the lowest energy levels. The temperature-induced red-shift of the photon emissions from ground and excited states are attributed to the thermal activation and redistribution among these QD states.

Anomalous thermal redistribution of beryllium implanted in InGaAs: a possible interaction with extended defects

Phosphorus and beryllium have been coimplanted in InGaAs. Various anneals have been performed in the temperature range of 745–826 °C. The secondary ion mass spectrometry measurements have revealed anomalous redistribution of the beryllium dopant: “up-hill” Be diffusion profiles have been observed. The observations of as-implanted profiles suggest that these anomalous beryllium peaks are due to beryllium trapping by the extended defects generated by the P implantation. A good agreement has been obtained between the experimental data and simulated profiles.

Thermal redistribution and dephasing of localized excitons in CdSe/ZnSSe quantum island structures

Temperature-dependent photoluminescence measurements of CdSe/ZnSSe quantum island structures show a blueshift of the emission maximum and a significant broadening of the emission band above 50 K. The features originate from a thermal activation of localized excitons and a redistribution of the occupation within the inhomogeneous island ensemble. The presence of exciton states with different localization at comparable transition energies is indicated by a multiexponential decay of the four-wave mixing photon echo observed at low temperatures. Upon increasing the temperature above 50 K, the decay of the four-wave mixing signal changes into a monoexponential behavior. Polarization-dependent measurements unhide strong exciton-exciton interaction and signify the formation of biexcitons with biexciton binding energies around 12 meV. The features indicate the dominance of weakly localized quantum-well-like precursor states of CdSe islands on the high-energy side of the emission band.

Impact History of Eros: Craters and Boulders

Preliminary measurements of craters and boulders have been made in various locations on Eros from images acquired during the first nine months of NEAR Shoemaker's orbital mission, including the October 2000 low altitude flyover. (We offer some very preliminary, qualitative analysis of later LAF images and very high-resolution images obtained during NEAR's landing on 12 February 2001). Craters on Eros >100 m diameter closely resemble the saturated crater population of Ida; Eros is more heavily cratered than Gaspra but lacks the saturated giant craters of Mathilde. These craters and the other large-scale geological features were formed over a duration of very roughly 2 Gyr while Eros was in the main asteroid belt, between the time when its parent body was disrupted and Eros was injected into an Earth-approaching orbit (probably tens of Myr ago). Saturation equilibrium had been expected to shape Eros' crater population down to very small sizes, as on the lunar maria. However, craters <200 m diameter are instead progressively depleted toward smaller sizes and are a factor of ∼200 below empirical saturation at diameters of 4 m. Conversely, boulders and positive relief features (PRFs) rise rapidly in numbers (differential power-law index ∼−5) and those <10 m in size dominate the landscape at high resolutions. The pervasive boulders and minimal craters on Eros is radically different from the lunar surface at similar scales. This may be partly explained by a major depletion of meter-scale projectiles in the asteroid belt (due to the Yarkovsky Effect: Bell 2001), which thus form few small craters and destroy few boulders. Additionally, the small size and low gravity of Eros may result in redistribution or loss of ejecta due to seismic shaking, thus preferentially destroying small craters formed in such regolith. Possibly Eros has only a patchy, thin regolith of mobile fines; the smaller PRFs may then reflect exposures of fractured bedrock or piles of large ejecta blocks, which might further inhibit formation of craters <10 m in size. Eros may well have been largely detached dynamically and collisionally from the main asteroid belt for the past tens of Myr, in which case its cratering rate would have dropped by two orders of magnitude, perhaps enhancing the relative efficacy of other processes that would normally be negligible in competition with cratering. Such processes include thermal creep, electrostatic levitation and redistribution of fines, and space weathering (e.g., bombardment by micrometeorites and solar wind particles). Combined with other small-body responses to impact cratering (e.g., greater widespread distribution of bouldery ejecta), such processes may also help explain the unexpected small-scale character of geology on Eros. If there was a recent virtual hiatus in cratering of Eros (during which only craters <∼300 m diameter would be expected to have formed), space weathering may have reached maturity, thus explaining Eros' remarkable spectral homogeneity compared with Ida.

Magma storage prior to the 1912 eruption at Novarupta, Alaska

New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H2O+CO2 fluid saturation were determined for an andesite (58.7 wt% SiO2) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H2O-saturated conditions (i.e., PH2O=PTOT) in a negatively sloping region in T–P space from 930 °C/100 MPa to 960 °C/75 MPa with fO2~NNO+1. The H2O-saturated equilibration conditions of the dacite are constrained to a T–P region from 850 °C/50 MPa to 880 °C/25 MPa. If H2O-saturated, these magmas equilibrated at (and above) the level where co-erupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H2O + CO2) fluid for the same range in PH2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H2O-saturated rhyolite is consistent with experimental findings only if the intermediates have XH2O fl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.

Photoluminescence and Time-Resolved Photoluminescence Studies of Self-Assembled InAs Quantum Dots

AbstractPhotoluminescence (PL) spectra and time resolved PL from self-assembled InAs/GaAs quantum dots (QDs) grown by metal organic chemical vapor deposition are studied. A reduction in the emission linewidth with increasing temperature was observed at low temperature range and an increase in the linewidth at higher temperature. It was also observed that the variation of PL peak energy with temperature does not follow Varshni's equation. These anomalous behaviors of PL can be explained in term of thermal redistribution of carriers. It was also found that the PL decay time increases with photon wavelength, which is due to the carrier transfer between laterally coupled QDs.