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International audience; BackgroundMicrobeam radiation therapy (MRT) is a treatment modality based on spatial fractionation of synchrotron generated x-rays into parallel, high dose, microbeams of a few microns width. MRT is still an under-development radiosurgery technique for which, promising preclinical results on brain tumors and epilepsy encourages its clinical transfer.PurposeA safe clinical transfer of MRT needs a specific treatment planning system (TPS) that provides accurate dose calculations in human patients, taking into account the MRT beams properties (high dose gradients, spatial fractionation, polarization effects). So far, the most advanced MRT treatment planning system, based on a hybrid dose calculation algorithm, is limited to a macroscopic rendering of the dose and does not account for the complex dose distribution inherent to MRT if delivered as conformal irradiations with multiple incidences. For overcoming these limitations, a multi-scale full Monte-Carlo calculation engine called penMRT has been developed and benchmarked against two general purpose Monte Carlo codes: penmain based on PENELOPE and Gate based on Geant4.MethodsPenMRT, is based on the PENELOPE (2018) Monte Carlo (MC) code, modified to take into account the voxelized geometry of the patients (CT-scans) and offering an adaptive micrometric dose calculation grid independent to the CT size, location and orientation. The implementation of the dynamic memory allocation in penMRT, makes the simulations feasible within a huge number of dose scoring bins. The possibility of using a source replication approach to simulate arrays of microbeams, and the parallelization using OpenMPI have been added to penMRT in order to increase the calculation speed for clinical usages. This engine can be implemented in a TPS as a dose calculation core.ResultsThe performance tests highlight the reliability of penMRT to be used for complex irradiation conditions in MRT. The benchmarking against a standard PENELOPE code did not show any significant difference for calculations in centimetric beams, for a single microbeam and for a microbeam array. The comparisons between penMRT and Gate as an independent MC code did not show any difference in the beam paths, whereas in valley regions, relative differences between the two codes rank from 1 to 7.5% which are probably due to the differences in physics lists that are used in these two codes. The reliability of the source replication approach has also been tested and validated with an underestimation of no more than 0.6% in low dose areas.ConclusionsGood agreements (a relative difference between 0 to 8%) were found when comparing calculated peak to valley dose ratio (PVDR) values using penMRT, for irradiations with a full microbeam array, with calculated values in the literature. The high-resolution calculated dose maps obtained with penMRT are used to extract differential and cumulative dose-volume histograms (DVHs) and analyze treatment plans with much finer metrics regarding the irradiation complexity. To our knowledge, these are the first high-resolution dose maps and associated DVHs ever obtained for cross-fired microbeams irradiation, which is bringing a significant added value to the field of treatment planning in spatially fractionated radiation therapy.

This paper presents full-quantum 3-D simulations predicting the electrical performance of nanowire tunnel-FETs based on III–V hetero-junctions. Our calculations exploit an eight-band $\mathrm {k}\cdot \mathrm {p}$ Hamiltonian within the nonequilibrium Green’s functions formalism and include phonon scattering. It is shown that the on-current of GaSb/InAs hetero-junction tunnel-FETs is limited by quantum confinement effects on the bandstructure induced by the small nanowire diameter necessary to preserve an optimal electrostatic integrity at short gate lengths. To circumvent this problem, additional on-current improvements with no substantial subthreshold swing degradation can be achieved by engineering the source region through the insertion of an InAs/GaSb/InAs quantum well along the transport direction. Such a design option is predicted to provide on/off-current ratios larger than $10^{7}$ even at $V_{DD}$ = 300 mV.

International audience; We propose to employ a grading of the molar fraction in the source region of III-V hetero-junction tunnel-FETs as a means to improve the on-current without degrading the subthreshold swing. Our full quantum simulations show that the molar-fraction grading increases the on-current by enlarging the hole wave function penetration from the source to the channel region. We also compare the performance of graded AlGaSb/InAs tunnel FETs and InAs MOSFETs and show that at V DS =0.3 V, the tunnel device can outperform the MOSFET in terms of both on-current and subthreshold slope.

This work presents a systematic design study of nanowire Tunnel-FETs at LG=17nm employing a 3D Poisson-NEGF solver based on a 8×8 k·p Hamiltonian and including phonon scattering. In particular: (a) we revisit the design of GaSb-InAs based hetero-junction tunnel-FETs showing that this system is unlikely to yield a broken bangap profile at the very narrow features necessary for a good sub-VT slope value; (b) we propose new design options for hetero-junction tunnel-FETs, relying on the use of strain and of a graded molar fraction (xM) in AlxMGa(1-xM)Sb, which improve remarkably on current preserving optimal sub-VT slopes; (c) we show that interface defects can frustrate any design strategy aiming at sub-VT slope values below 60mV/dec.