Photoluminescence from low temperature grown InAs/GaAs quantum dots

The authors investigated a set of self-assembled InAs/GaAs quantum dots (QDs) formed by mol. beam epitaxy at low temp. (LT, 250 DegC) and postgrowth annealing. A QD photoluminescence (PL) peak around 1.01 eV was obsd. The PL efficiency quickly quenches between 6 and 40 K due to the tunneling out of the QD into traps within the GaAs barrier. The PL efficiency increases by a factor of 45-280 when exciting below the GaAs band gap, directly into the InAs QD layer. This points towards good optical quality QDs, which are embedded in a LT-GaAs barrier with a high trapping efficiency. [on SciFinder (R)

Wavelength controlled multilayer-stacked linear InAs quantum dot arrays on InGaAsP/InP(100) by self-organized anisotropic strain engineering : a self-ordered quantum dot crystal

Multilayer-stacked linear InAs quantum dot (QD) arrays are created on InAs/InGaAsP superlattice templates formed by self-organized anisotropic strain engineering on InP (100) substrates in chemical beam epitaxy. Stacking of the QD arrays with identical emission wavelength in the 1.55 µm region at room temperature is achieved through the insertion of ultrathin GaAs interlayers beneath the QDs with increasing interlayer thickness in successive layers. The increment in the GaAs interlayer thickness compensates the QD size/wavelength increase during strain correlated stacking. This is the demonstration of a three-dimensionally self-ordered QD crystal with fully controlled structural and optical properties

Formation of InAs quantum dot arrays on GaAs (100) by self-organized anisotropic strain engineering of a (In,Ga)As superlattice template

We demonstrate the formation of well-defined InAs quantum dot (QD) arrays by self-organized engineering of anisotropic strain in a (In,Ga)As/GaAs superlattice (SL). Due to the accumulation and improvement of the uniformity of the strain-field modulation along [011], formation of InAs QD arrays along [0-11] with 140 nm lateral periodicity is clearly observed on the SL template when the number of SL periods is larger than ten. By enhancing the In adatom surface migration length at low growth rates, clear arrays of single InAs QDs are obtained. The QD arrays exhibit strong photoluminescence efficiency that is not reduced compared to that from InAs QD layers on GaAs. Hence, ordering by self-organized anisotropic strain engineering maintains the high structural quality of InAs QD

Effect of annealing on formation of self-assembled (In,Ga)As quantum wires on GaAs (100) by molecular beam epitaxy

The role of annealing for (In,Ga)As self-organized quantum wire (QWR) formation on GaAs (100) during growth of (In,Ga)As/GaAs superlattice (SL) structures is studied by X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. XRD and AFM evidence that annealing after the supply of each layer of elongated (In,Ga)As quantum dots (QDs) in the SL is the crucial process for QWR formation. We conclude that during annealing, the shape anisotropy of the QDs is enhanced due to anisotropic mass transport and the QDs become connected along the [0-11] direction. Strain reduction by In desorption, revealed by XRD and PL, which accompanies this process, then results in well defined, uniform QWR arrays by repetition in SL growt

Carrier dynamics of LT InAs/GaAs QDs using time resolved differential reflectivity

We present a Time Resolved Differential Reflectivity (TRDR) study of LT (low temperature grown) Stransky - Krastanov InAs/GaAs Quantum Dots (QDs) grown using molecular beam epitaxy. The photoluminescence (PL) spectrum shows a QD-peak around 1200nm. In the TRDR measurements we observe an initial fast decay (80ps) followed by a much slower decay of about 800ps. The strong temperature dependence of the PL-signal is not observed in the reflectivity signal. This leads us to conclude that the electrons are trapped at a fast rate by As antisite defects while the hole decay dynamics take place at a slower rate, which is also monitored in TRDR

Wavelength tuning of InAs/InP quantum dots: Control of As/P surface exchange reaction

Wavelength tuning of single and vertically stacked InAs quantum dot [QD] layers embedded inInGaAsP/InP [100] grown by metal organic vapor-phase epitaxy is achieved by controlling theAs/P surface exchange reaction during InAs deposition. The As/P exchange reaction is suppressedfor decreased QD growth temperature and group V-III flow ratio, reducing the QD size andphotoluminescence [PL]emission wavelength. The As/P exchange reaction and QD PL wavelengthare then reproducibly controlled by the thickness of an ultrathin [0¿2 ML] GaAs interlayerunderneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs toquantum dashes at low group V-III flow ratio. Temperature dependent PL measurements revealexcellent optical properties of the QDs up to room temperature with PL peak wavelengths in thetechnologically important 1.55 ¿region for telecom applications. Widely stacked QD layers arereproduced with identical PL emission to increase the active volume, while closely stacked QDlayers reveal a systematic PL redshift and linewidth reduction due to vertical electronic couplingwhich is proven by the linear polarization of the cleaved-side PL changing from in plane toisotropic. ¿ 2006 American Vacuum Society

Cost-effectiveness of medically assisted reproduction or expectant management for unexplained subfertility:when to start treatment?

STUDY QUESTION Over a time period of 3 years, which order of expectant management (EM), IUI with ovarian stimulation (IUI-OS) and IVF is the most cost-effective for couples with unexplained subfertility with the female age below 38 years? SUMMARY ANSWER If a live birth is considered worth Euro32 000 or less, 2 years of EM followed by IVF was the most cost-effective, whereas above Euro32 000 this was 1 year of EM, 1 year of IUI-OS and then 1 year of IVF. WHAT IS KNOWN ALREADY IUI-OS and IVF are commonly used fertility treatments for unexplained subfertility although many couples can conceive naturally, as no identifiable barrier to conception could be found by definition. Few countries have guidelines on when to proceed with medically assisted reproduction (MAR), mostly based on the expected probability of live birth after treatment, but there is a lack of evidence to support the strategies proposed by these guidelines. The increased uptake of IUI-OS and IVF over the past decades and costs related to reimbursement of these treatments are pressing concerns to health service providers. For MAR to remain affordable, sustainable and a responsible use of public funds, guidance is needed on the cost-effectiveness of treatment strategies for unexplained subfertility, including EM. STUDY DESIGN, SIZE, DURATION We developed a decision analytic Markov model that follows couples with unexplained subfertility of which the woman is under 38 years of age for a time period of 3 years from completion of the fertility workup onwards. We divided the time axis of 3 years into three separate periods, each comprising 1 year. The model was based on contemporary evidence, most notably the dynamic prediction model for natural conception, which was combined with MAR treatment effects from a network meta-analysis on randomized controlled trials. We changed the order of options for managing unexplained subfertility for the 1 year periods to yield five different treatment policies in total: IVF-EM-EM (immediate IVF), EM-IVF-EM (delayed IVF), EM-EM-IVF (postponed IVF), IUIOS-IVF-EM (immediate IUI-OS) and EM-IUIOS-IVF (delayed IUI-OS). PARTICIPANTS/MATERIALS, SETTING, METHODS The main outcomes per policy over the 3-year period were the probability of live birth, the average treatment and delivery costs, the probability of multiple pregnancy, the incremental cost-effectiveness ratio (ICER) and finally, which policy yields the highest net benefit in which costs for a policy were deducted from the health effects, i.e. live births gained. We chose the Dutch societal perspective, but the model can be easily modified for other locations or other perspectives. The probability of live birth after EM was taken from the dynamic prediction model for natural conception and updated for Years 2 and 3. The relative effects of IUI-OS and IVF in terms of odds ratios, taken from the network meta-analysis, were applied to the probability of live birth after EM. We applied standard discounting procedures for economic analyses for Years 2 and 3. The uncertainty around effectiveness, costs and other parameters was assessed by probabilistic sensitivity analysis in which we drew values from distributions and repeated this procedure 20 000 times. In addition, we changed model assumptions to assess their influence on our results. MAIN RESULTS AND THE ROLE OF CHANCE From IVF-EM-EM to EM-IUIOS-IVF, the probability of live birth varied from approximately 54-64% and the average costs from approximately Euro4000 to Euro9000. The policies IVF-EM-EM and EM-IVF-EM were dominated by EM-EM-IVF as the latter yielded a higher cumulative probability of live birth at a lower cost. The policy IUIOS-IVF-EM was dominated by EM-IUIOS-IVF as the latter yielded a higher cumulative probability of live birth at a lower cost. After removal of policies that were dominated, the ICER for EM-IUIOS-IVF was approximately Euro31 000 compared to EM-EM-IVF. The range of ICER values between the lowest 25% and highest 75% of simulation replications was broad. The net benefit curve showed that when we assume a live birth to be worth approximately Euro20 000 or less, the policy EM-EM-IVF had the highest probability to achieve the highest net benefit. Between Euro20 000 and Euro50 000 monetary value per live birth, it was uncertain whether EM-EM-IVF was better than EM-IUIOS-IVF, with the turning point of Euro32 000. When we assume a monetary value per live birth over Euro50 000, the policy with the highest probability to achieve the highest net benefit was EM-IUIOS-IVF. Results for subgroups with different baseline prognoses showed the same policies dominated and the same two policies that were the most likely to achieve the highest net benefit but at different threshold values for the assumed monetary value per live birth. LIMITATIONS, REASONS FOR CAUTION Our model focused on population level and was thus based on average costs for the average number of cycles conducted. We also based the model on a number of key assumptions. We changed model assumptions to assess the influence of these assumptions on our results. The change in relative effectiveness of IVF over time was found to be highly influential on results and their interpretation. WIDER IMPLICATIONS OF THE FINDINGS EM-EM-IVF and EM-IUIOS-IVF followed by IVF were the most cost-effective policies. The choice depends on the monetary value assigned to a live birth. The results of our study can be used in discussions between clinicians, couples and policy makers to decide on a sustainable treatment protocol based on the probability of live birth, the costs and the limitations of MAR treatment. STUDY FUNDING/COMPETING INTEREST(S) This work was supported by the ZonMw Doelmatigheidsonderzoek (80-85200-98-91072). The funder had no role in the design, conduct or reporting of this work. B.W.M. is supported by a NHMRC Practitioner Fellowship (GNT1082548). B.W.M. reports consultancy for ObsEva, Merck KGaA and Guerbet and travel and research support from ObsEva, Merck and Guerbet. TRIAL REGISTRATION NUMBER N/A

External validation of a dynamic prediction model for repeated predictions of natural conception over time

This work was supported by a Chief Scientist Office postdoctoral training fellowship in health services research and health of the public research (ref PDF/12/06). There are no conflicts of interest.Peer reviewedPostprin

IVF for unexplained subfertility : whom should we treat?

Acknowledgements The authors would like to thank Tenovus Scotland and the Amsterdam Reproduction & Development Research Group for funding this project. We acknowledge the data management support of the Grampian Data Safe Haven (DaSH) and the associated financial support of NHS Research Scotland, through NHS Grampian investment in the Grampian DaSH. For more information, visit the DaSH website http://www.abdn.ac.uk/iahs/facilities/grampian-data-safe-haven.php. Funding Tenovus Scotland [grant G17.04], travel was supported by the Amsterdam Reproduction & Development Research Group [grant V.000296 to RvE].Peer reviewedPostprin

Self-assembled InAs quantum dots formed by molecular beam epitaxy at low temperature and postgrowth annealing

Self-assembled InAs quantum dots are grown at low temperature (LT) by molecular beam epitaxy (MBE) on GaAs substrates. The growth is in situ monitored by reflection high-energy electron diffraction, and ex situ evaluated by atomic force microscopy for the morphological properties, and by high-resolution x-ray diffraction for the structural properties. While two monolayers as-grown LT (250 degrees C) InAs layers exhibit shallow mounds due to the low adatom migration length at low temperature, well-developed InAs dots are formed after postgrowth annealing above 450 degrees C. The structural quality of the LT GaAs matrix grown on top and of the embedded InAs dot layer is improved when a 3 nm GaAs interlayer is deposited (at 480 degrees C) on the InAs dots and subsequently annealed at 580 degrees C before LT GaAs overgrowth. These high structural quality LT-grown InAs dots are considered for applications in high-speed optical modulators and switches operating at low power by combining the high optical nonlinearity of quantum dots with the ultrafast optical response provided by LT growth in MB