Hole-based memory operation in an InAs/GaAs quantum dot heterostructure

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 95, 242114 (2009) and may be found at https://doi.org/10.1063/1.3275758.We present an InAs/GaAs quantum dot (QD) memory structure with all-electrical data access which uses holes as charge carriers. Charging and discharging of the QDs are clearly controlled by a gate voltage. The stored information is read-out by a two-dimensional hole gas underneath the QD-layer. Time resolved drain-current-measurements demonstrate the memory operation. Present write times are 80 ns

Temperature and electric field dependence of the carrier emission processes in a quantum dot-based memory structure

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 94, 042108 (2009) and may be found at https://doi.org/10.1063/1.3076126.Hole emission processes from self-organized GaAs0.4Sb0.6/GaAs quantum dots embedded in a - diode are studied by capacitance-voltage spectroscopy. The method introduced allows the investigation of the temperature and electric field dependence of carrier emission with time constants from below nanoseconds up to thousands of seconds. Different emission processes are clearly distinguished, such as tunneling, phonon-assisted tunneling, and thermal activation, each important for quantum-dot-based memory structures. The erase time was determined to 1.5 ms for an electric field of about 200 kV/cm. At 500 kV/cm, 10 ns are predicted sufficient for fast erasing

A write time of 6ns for quantum dot–based memory structures

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 92, 092108 (2008) and may be found at https://doi.org/10.1063/1.2890731.The concept of a memory device based on self-organized quantum dots (QDs) is presented, enabling extremely fast write times, limited only by the charge carrier relaxation time being in the picosecond range. For a first device structure with embedded InAs∕GaAs QDs, a write time of 6ns is demonstrated. A similar structure containing GaSb∕GaAs QDs shows a write time of 14ns. These write times are independent of the localization energy (e.g., storage time) of the charge carriers and at the moment are limited only by the experimental setup and the parasitic cutoff frequency of the low pass of the device.EC/FP6/500101/EU/Self-Assembled semiconductor Nanostructures for new Devices in photonics and Electronics/SANDI

A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients

Positive outcomes: validity, reliability and responsiveness of a novel person-centred outcome measure for people with HIV

Objectives Despite successful treatment, people living with HIV experience persisting and burdensome multidimensional problems. We aimed to assess the validity, reliability and responsiveness of Positive Outcomes, a patient-reported outcome measure for use in clinical practice. Methods In all, 1392 outpatients in five European countries self-completed Positive Outcomes, PAM-13 (patient empowerment), PROQOL-HIV (quality of life) and FRAIL (frailty) at baseline and 12 months. Analysis assessed: (a) validity (structural, convergent and divergent, discriminant); (b) reliability (internal consistency, test-retest); and (c) responsiveness. Results An interpretable four-factor structure was identified: ‘emotional wellbeing’, ‘interpersonal and sexual wellbeing’, ‘socioeconomic wellbeing’ and ‘physical wellbeing’. Moderate to strong convergent validity was found for three subscales of Positive Outcomes and PROQOL (ρ = −0.481 to −0.618, all p < 0.001). Divergent validity was found for total scores with weak ρ (−0.295, p < 0.001). Discriminant validity was confirmed with worse Positive Outcomes score associated with increasing odds of worse FRAIL group (4.81-fold, p < 0.001) and PAM-13 level (2.28-fold, p < 0.001). Internal consistency for total Positive Outcomes and its factors exceeded the conservative α threshold of 0.6. Test-retest reliability was established: those with stable PAM-13 and FRAIL scores also reported median Positive Outcomes change of 0. Improved PROQOL-HIV score baseline to 12 months was associated with improved Positive Outcomes score (r = −0.44, p < 0.001). Conclusions Positive Outcomes face and content validity was previously established, and the remaining validity, reliability and responsiveness properties are now demonstrated. The items within the brief 22-item tool are designed to be actionable by health and social care professionals to facilitate the goal of person-centred care

A new approach on the oxygen isotope micro analysis of diatom silica with a laser-fluorination based mass spectrometry unit

The analysis of oxygen isotopes from diatom silica in sediment cores has reached importance for reconstructing the paleoclimate and is especially valuable in non-carbonate lakes of cold regions, where no other bioindicators such as ostracods and foraminifera are available. Analyses so far were performed in marine (e.g. Shemesh et al. 1992) as well as in lacustrine environments (e.g. Leng et al. 2004). A new approach for samples in sub-mg range has been developed to provide a better chronological resolution and to expand the method to periods where less biogenic silica is available (e. g. samples corresponding to MIS 2-4). After extracting the diatoms from sediment cores with various preparation steps including wet chemistry, sieving and heavy liquid separation a minimum of ~700µg fine material from 5 g of wet sample is obtained. The pure sample (degree of purity: &#8805; 99%) is melted into a bead to eliminate the water vapour and reduce the surface to avoid an explosion of the material. The used periphery of the mass spectrometer (PDZ Europa 20-20) consists of a laser fluorination system operated under vacuum. The bead is reacted with a focused CO2 laser under BrF5 atmosphere within a stainless steel-chamber. The non-oxygen gas components are trapped in a -150°C cold trap, whereas oxygen passes on to a molecular sieve cooled with liquid nitrogen. It is then transferred to the mass spectrometer and compared with a reference standard of known isotopic composition.Specially designed software and a video camera are used to survey and record the process in the reaction chamber and allow an automised, remote operation. This guarantees maximum safety as the instrument is installed in two different rooms, with the reagent (BrF5), reaction chamber and the laser unit arranged under a hood. Tests on standard material (NBS 28, Campolungo) showed a standard deviation <0.2. Sample material from Lake Elgygytgyn (core LZ 1024) will be analysed and a &#948;18O curve of the last 280.000 years will be generated to add a strong climate proxy to the various analysis performed so far. The Lake lies inside a meteorite impact crater formed approximately 3.6 million years ago and hence offers a unique option to fill the spatial gap of locations in the Arctic where paleoclimate reconstructions are sparse. Former drilling operations show that the lake could contain the longest, most continuous terrestrial record of past climate change in the entire Arctic back to the time of impact (Brigham-Grette, 2006).The analysis aims on the planktonic Cyclotella ocellata-complex which is persistent through a variety of climate conditions and present throughout the core (Cherapanova, 2006). Another goal is to examine a possible species-dependent fractionation by comparing the mentioned complex with Pliocaenicus costatus var. Sibiricus in the holocene.The expected results will be the base for studying the climate history using stable isotopes in lacustrine diatoms of the whole 300 m sedimentary sequence on sediment cores at Lake Elgygygtgyn to be drilled within the frame of the ICDP in 2009.Keywords: paleoclimate; laser; fluorination; mass spectrometry; oxygen isotopes; diatom silica

Oxygen isotope micro analysis of diatom silica from Elgygytgyn Crater Lake, NE Russia

The analysis of oxygen isotopes from diatom silica in sediment cores has reached importance for reconstructing the paleoclimate and is especially valuable in non-carbonate lakes of cold regions, where no other bioindicators are present. A new approach for samples in sub-mg range has been developed to provide a better chronological resolution and to expand the method to periods where less biogenic silica is available. Sample material from Lake Elgygytgyn will be analysed and a &#948;18O curve of the last 280.000 years will be generated to add a strong climate proxy to the various analysis performed so far. The Lake lies inside a meteorite impact crater of about 18 km in diameter which was formed approximately 3.6 million years ago. 50 streams are draining into this cold, oligotrophic lake from within the crater rim whereas only one outlet stream is present. Hence, the lake offers a unique option to fill the spatial gap of locations in the Arctic where paleoclimate reconstructions are rare. Former drilling operations show that the lake could contain the longest, most continuous terrestrial record of past climate change in the entire Arctic back to the time of impact (Brigham-Grette, 2006).The analysis aims on the planktonic Cyclotella ocellata-complex which is persistent through a variety of climate conditions and present throughout the core (Cherapanova, 2006). Another goal is to examine a possible species-dependent fractionation by comparing the mentioned complex with Pliocaenicus costatus var. Sibiricus in the holocene. After extracting the diatoms from sediment cores with various preparation steps a minimum of ~700µg fine material from 5 g of wet sample is required. A bead is melted and reacted with a CO2 laser under BrF5 atmosphere. The oxygen is then transferred to the mass spectrometer and compared with a reference standard of known isotopic composition. Specially designed software and a video camera are used to survey and record the process in the reaction chamber and allow an automised, remote operation. Tests on standard material (NBS 28, Campolungo) showed a standard deviation <0.2. First results will be presented at the meeting. The expected results will be the base for studying the climate history using stable isotopes in lacustrine diatoms of the whole 300 m sedimentary sequence on sediment cores at Lake Elgygygtgyn to be drilled within the frame of the ICDP in 2009

Linking structural and electronic properties of high-purity self-assembled GaSb/GaAs quantum dots

We present structural, electrical, and theoretical investigations of self-assembled type-II GaSb/GaAs quantum dots (QDs) grown by molecular beam epitaxy. Using cross-sectional scanning tunneling microscopy (X-STM) the morphology of the QDs is determined. The QDs are of high purity (similar to 100% GaSb content) and have most likely the shape of a truncated pyramid. The average heights of the QDs are 4-6 nm with average base lengths between 9 and 14 nm. Samples with a QD layer embedded into a pn-diode structure are studied with deep-level transient spectroscopy (DLTS), yielding a hole localization energy in the QDs of 609 meV. Based on the X-STM results the electronic structure of the QDs is calculated using 8-band k.p theory. The theoretical localization energies are found to be in good agreement with the DLTS results. Our results also allow us to estimate how variations in size and shape of the dots influence the hole localization energy

The structural, electronic and optical properties of GaSb/GaAs nanostructures for charge-based memory

The potential for GaSb nanostructures embedded in GaAs to operate as charge-based memory elements at room temperature is introduced and explored. Cross-sectional scanning-tunnelling microscopy is employed to directly probe and optimize the growth of nanostructures by molecular beam epitaxy. The results of structural analysis are combined with electrical measurements made with deep-level transient spectroscopy, showing excellent agreement with theoretical calculations which model the electronic structure of the nanostructures using 8-band k.p theory. Hole-localization energies exceeding 600 meV in quantum dots and near-100% material contrast between GaSb-rich quantum rings (QRs) and the surrounding GaAs matrix are revealed (no intermixing). Optical measurements confirm the depth of the hole localization, and demonstrate substantially lower inhomogeneous broadening than has previously been reported. Multiple peaks are partially resolved in ensemble photoluminescence of GaSb/GaAs QRs, and are attributed to charge states from discrete numbers of confined holes