This paper attempts to review the field of research into light emission from porous silicon. The driving force behind such research is the tantalizing goal of adding optoelectronic functions to the already impressive array of electronic functions provided by silicon-based devices. A silicon technology with included light emission would move even closer to complete dominance of the electronics market. After several years of research effort. the fundamental mechanisms of light emission are still not completely resolved. This is not surprising: porous silicon has many attributes of a new and complex material, and its study requires a truly interdisciplinary effort involving electrochemistry, surface science, structural and chemical microscopy on the atomic scale and detailed optical spectroscopy. This paper tries to connect these various threads; inevitably what emerges will only serve as a rather selective 'snapshot' of a still developing and often perplexing field.
Both Nevin (1969) and Shimp (1966) found on different choice procedures that pigeons equate (match) the proportion of their choices to the proportion of reinforcers each choice delivers. Their results differed in terms of the order of successive choices: Shimp found pigeons ordered successive choices so as to maximize the reinforcement rate, whereas Nevin found no evidence of such an ordering. Experiment 1 replicated both studies and found in both: (a) matching relations and (b) sequential dependencies of choice that corresponded with Shimp's maximizing prediction. The next three experiments studied the order of choices in three other choice procedures: (a) concurrent variable-interval schedules with a changeover delay, (b) concurrent variable-interval schedules without a changeover delay, and (c) concurrent-chains schedules. In all of these procedures, control of choice at the level of the response sequence was evident. The major features of the data from all four experiments were attributed to two molecular processes: response perseveration and reinforcement maximization. This evidence for a microstructure of choice suggests that the molar matching law is not isomorphic with the molecular processes governing concurrent performances.In a study by Herrnstein (1961), pigeons chose between two response keys, each associated with an independent variable-interval (VI) schedule of food reinforcement. A changeover delay (COD) that specified a minimum interval during which reinforcement was unavailable following a switch of choice-between keys was used to minimize interaction between these schedules. Herrnstein found with several pairs of VI schedules that the proportion of responses to a key (responses to a key divided by response total to both keys) equaled or "matched" the proportion of reinforcements that that
Suspended graphene has been studied by STM for the first time. Atomic resolution on mono- and bi-layer graphene samples has been obtained after ridding the graphene surface of contamination via high-temperature annealing. Static local corrugations (ripples) have been observed on both types of structures.
The trivacancy (V 3) in silicon has been recently shown to be a bistable center in the neutral charge state, with a fourfold-coordinated configuration, V 3[FFC], lower in energy than the (110) planar one. Transformations of the V 3 defect between different configurations, its diffusion, and disappearance upon isochronal and isothermal annealing of electron-irradiated Si:O crystals are reported from joint deep level transient spectroscopy measurements and first-principles density-functional calculations. Activation energies and respective mechanisms for V 3 transformation from the (110) planar configuration to the fourfold-coordinated structure have been determined. The annealing studies demonstrate that V 3 is mobile in Si at T>200C and in oxygen-rich material can be trapped by interstitial oxygen atoms so resulting in the appearance of V 3O complexes. The calculations suggest that V 3 motion takes place via consecutive FFC/planar transformation steps. The activation energy for the long-range diffusion of the V 3 center has been derived and agrees with atomic motion barrier from the calculations. © 2012 American Physical Society
We have recently found that the silicon trivacancy (V3) is a bistable defect that can occur in fourfold coordinated and (110) planar configurations for both the neutral and singly negative charge states [V. P. Markevich et al., Phys. Rev. B 80, 235207 (2009)]. Acceptor levels of V3 in both these configurations have been determined. It has also been shown that at T > 200 °C, the interaction of mobile trivacancies with interstitial oxygen atoms results in the formation of V3O complex with the first and second acceptor levels at Ec −0.46 and −0.34 eV. In the present work we identify donor levels arising from V3 and V3O complexes by means of deep level transient spectroscopy (DLTS) and high‐resolution Laplace DLTS on n+p silicon structures irradiated with 6 MeV electrons, combined with density functional modeling studies. It is found that both defects possess two donor levels in the (110) planar configurations. First donor levels at Ev +0.19 and +0.235 eV, and the second donor levels at Ev +0.105 and +0.12 eV are found for the V3 and V3O complexes, respectively.
Unintentionally doped gallium antimonide has been grown by molecular-beam epitaxy on gallium arsenide and gallium antimonide. Substrate temperatures in the range 480 to 620 °C and antimony to gallium flux ratios from 0.65 : 1 to 6.5 : 1 have been investigated. The deposition conditions have been related to growth morphology and to the electrical and optical properties of the epitaxial films. A strong correlation has been found between the quality of the layers and the degree of excess antimony flux; the best material in terms of both optical and electrical properties was obtained with the minimum antimony stable growth at a particular substrate temperature. All the material exhibited residual p-type behavior. The lowest hole concentration achieved was 7.8×1015 cm−3 with a corresponding room-temperature mobility of 950 cm2/V s. The narrowest PL (photoluminescence) features observed were peaks associated with bound exciton transitions with half-widths of 2–3 meV.
We report strongly enhanced Tb photoluminescence (PL) from a doped titania xerogel deposited on mesoporous anodic alumina. Transmission electron microscopy has confirmed the presence of the xerogel inside the pores of alumina films of up to several microns thickness. The optical bands attributed to D35→Fi7 (i=3,4,5), D45→Fj7 (j=3,4,5,6) transitions of Tb3+, with a predominant band at 2.28 eV, are investigated between 6 and 300 K. At all temperatures, the Tb PL increases and the bands become narrower with an increase in the Tb concentration in the xerogel, the number of deposited layers employed and the thickness of the porous alumina film.
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