Silicon strained layers grown on GaP(001) by molecular beam epitaxy

FWN – Publicaties zonder aanstelling Universiteit Leide

Quantitative considerations in medium energy ion scattering depth profiling analysis of nanolayers

The high depth resolution capability of medium energy ion scattering (MEIS) is becoming increasingly relevant to the characterisation of nanolayers in e.g. microelectronics. In this paper we examine the attainable quantitative accuracy of MEIS depth profiling. Transparent but reliable analytical calculations are used to illustrate what can ultimately be achieved for dilute impurities in a silicon matrix and the significant element-dependence of the depth scale, for instance, is illustrated this way. Furthermore, the signal intensity-to-concentration conversion and its dependence on the depth of scattering is addressed. Notably, deviations from the Rutherford scattering cross section due to screening effects resulting in a non-coulombic interaction potential and the reduction of the yield owing to neutralization of the exiting, backscattered H+ and He+ projectiles are evaluated. The former mainly affects the scattering off heavy target atoms while the latter is most severe for scattering off light target atoms and can be less accurately predicted. However, a pragmatic approach employing an extensive data set of measured ion fractions for both H+ and He+ ions scattered off a range of surfaces, allows its parameterization. This has enabled the combination of both effects, which provides essential information regarding the yield dependence both on the projectile energy and the mass of the scattering atom. Although, absolute quantification, especially when using He+, may not always be achievable, relative quantification in which the sum of all species in a layer add up to 100%, is generally possible. This conclusion is supported by the provision of some examples of MEIS derived depth profiles of nanolayers. Finally, the relative benefits of either using H+ or He+ ions are briefly considered

Transient field g-factor measurements on the 21+ states of 32S and 34S

Transient field integral precession measurements have been performed on the first-excited Jπ = 2+ states of 32S and 34S with the IMPAC technique on recoil into magnetized iron single-crystal frames. The results were analysed with an empirical parametrization of the field. This yields g-factors of g = +0.47 ± 0.09 and +0.50 ± 0.08 for 32S and 34S, respectively. In the present cases the influence of static magnetic hyperfine fields is negligible due to the short mean lives for 32S and 34S of 0.23 and 0.46 ps, respectively. Various complex model calculations yield g-factors in good agreement with experiment. Measured g-factors, including the present data, for light self-conjugate and neutron-excess Tz = 1 nuclei are also briefly discussed. The measured value of the g-factors for 32S confirms the empirical description of the transient field

Age-related impairments and influence of visual feedback when learning to stand with unexpected sensorimotor delays

Background: While standing upright, the brain must accurately accommodate for delays between sensory feedback and self-generated motor commands. Natural aging may limit adaptation to sensorimotor delays due to age-related decline in sensory acuity, neuromuscular capacity and cognitive function. This study examined balance learning in young and older adults as they stood with robot-induced sensorimotor delays.Methods: A cohort of community dwelling young (mean = 23.6 years, N = 20) and older adults (mean = 70.1 years, N = 20) participated in this balance learning study. Participants stood on a robotic balance simulator which was used to artificially impose a 250 ms delay into their control of standing. Young and older adults practiced to balance with the imposed delay either with or without visual feedback (i.e., eyes open or closed), resulting in four training groups. We assessed their balance behavior and performance (i.e., variability in postural sway and ability to maintain upright posture) before, during and after training. We further evaluated whether training benefits gained in one visual condition transferred to the untrained condition.Results: All participants, regardless of age or visual training condition, improved their balance performance through training to stand with the imposed delay. Compared to young adults, however, older adults had larger postural oscillations at all stages of the experiments, exhibited less relative learning to balance with the delay and had slower rates of balance improvement. Visual feedback was not required to learn to stand with the imposed delay, but it had a modest effect on the amount of time participants could remain upright. For all groups, balance improvements gained from training in one visual condition transferred to the untrained visual condition.Conclusion: Our study reveals that while advanced age partially impairs balance learning, the older nervous system maintains the ability to recalibrate motor control to stand with initially destabilizing sensorimotor delays under differing visual feedback conditions.Biomechatronics & Human-Machine ControlBiomechanical Engineerin

Sharp boron spikes in silicon grown at reduced and atmospheric pressure by fast-gas-switching CVD

Boron doping spikes in Si were grown by fast-gas-switching CVD at 800 and 850°C using Si2H6 and B2H6 in 0.03, 0.1 and 1 atm H2 as the carrier gas. The B2H6 doping gas was added for 2 s by two methods, namely during growth or as a flush while the Si2H6 flow was interrupted. High-resolution SIMS measurements have revealed extremely sharp and highly concentrated dopant profiles. Peak B concentrations up to 5×1021 cm-3 and, at 1 atm H2, a FWHM of 3 nm were obtained. Electrical measurements show that for B-spikes having a FWHM value of 4–5 nm, a sheet resistivity of as low as 580 O/ \Box can be obtained