Wet Etching of Silicon Germanium Nanowires

The aim of this project is to understand the wet etching mechanism of SiGe at the nanoscale. We will use the etching of SiGe nanowires as a case study and we will optimize the etchant solution in order to achieve high selectivity over Si, preserve the shape of SiGe nanowire (isotropic etching) and control their diameter below the threshold of ten nanometers.Transistors are the devices used to control the flow of current across electronic devices. Transistors’ working principle is analogous to how a valve controls the flow of water: applying voltage to a transistor enables to regulate the flow of current. When the voltage is turned off, the transistor prevents the current from flowing and subsequently the device stops working. This is rather similar to valve opening/closing and consequentially increasing/decreasing the water flow. But why are transistors so important? Transistors are the building blocks of logic devices used for computation and information storage, such as our computers and phones. More transistors mean faster processing power which improves your device’s performance and working speed. So, if you want a faster computer with greater computing power, more transistors are needed. Nowadays, the technology of transistors has gone a long way. 20 years ago the transistors were about 250 nanometers. The smallest transistors available today are only about 10 nanometers and any electronic device that you used today has several billion transistors. And to fit more transistors on the same chip, smaller transistors are needed. The next question you might be asking is what is the problem with making the transistors smaller? “Look at the data! In the past 20 years we have reduced the transistor size 25 times! Just continue with what we have been doing”. Yes, but if only things were so simple. The problem that we have to face is the current shortage between the transistors elements which will turn a transistor into a “never-closing valve with water always flowing”. The shortened transistors will always allow the flow of current, causing huge power losses. The transistors smaller than 7 nanometers will experience that problem, so producing transistors smaller than 7 nanometers will just create an energy loss problem instead of solving a space problem. To conclude, we have reached a physical barrier that doesn’t allow us to reduce the size of transistors using the traditional approach. Therefore, a new solution has been proposed. Similar to how humans build skyscrapers when there is little space on the ground left, transistors which were previously build next to each other are now going to be built on top of each other. The new transistors are called vertical transistors. Even though the transistors reach their size limitation, we will be able to fit a lot more of them on a chip. Now, instead of having planar transistors or “single-storey transistor houses”, we will have vertical transistors or “transistor skyscrapers” all over the chip. Of course, a skyscraper is just an analogy. In reality, the vertical transistors have a cylindrical shape. Scientists call these cylinders nanowires, as their diameter is only a few nanometers. In order to fit as many nanowires as possible, we need to reduce their size after they have been manufactured. The material used for the nanowires is silicon germanium and the process of reducing the nanowire size is called etching which is what the thesis focused on. Our research group carried out experiments in order to understand the mechanism of silicon germanium etching. We have proposed a model which describes the etching regimes and under what conditions they occur. Furthermore, we described when SiGe nanowires are etched isotropically so the shape of the nanowire is preserved. We have also studied the selectivity of the used etchant solutions over silicon which is the measure of how well the etchant solution etches SiGe and leaves Si untouched

Estimation of relative biological effectiveness of 225Ac compared to 177Lu during [225Ac]Ac-PSMA and [177Lu]Lu-PSMA radiopharmaceutical therapy using TOPAS/TOPAS-nBio/MEDRAS

Abstract Aim Over recent years, [225Ac]Ac-PSMA and [177Lu]Lu-PSMA radiopharmaceutical therapy have evolved as a promising treatment option for advanced prostate cancer. Especially for alpha particle emitter treatments, there is still a need for improving dosimetry, which requires accurate values of relative biological effectiveness (RBE). To achieve that, consideration of DNA damages in the cell nucleus and knowledge of the energy deposition in the location of the DNA at the nanometer scale are required. Monte Carlo particle track structure simulations provide access to interactions at this level. The aim of this study was to estimate the RBE of 225Ac compared to 177Lu. The initial damage distribution after radionuclide decay and the residual damage after DNA repair were considered. Methods This study employed the TOol for PArtcile Simulation (TOPAS) based on the Geant4 simulation toolkit. Simulation of the nuclear DNA and damage scoring were performed using the TOPAS-nBio extension of TOPAS. DNA repair was modeled utilizing the Python-based program MEDRAS (Mechanistic DNA Repair and Survival). Five different cell geometries of equal volume and two radionuclide internalization assumptions as well as two cell arrangement scenarios were investigated. The radionuclide activity (number of source points) was adopted based on SPECT images of patients undergoing the above-mentioned therapies. Results Based on the simulated dose–effect curves, the RBE of 225Ac compared to 177Lu was determined in a wide range of absorbed doses to the nucleus. In the case of spherical geometry, 3D cell arrangement and full radionuclide internalization, the RBE based on the initial damage had a constant value of approximately 2.14. Accounting for damage repair resulted in RBE values ranging between 9.38 and 1.46 for 225Ac absorbed doses to the nucleus between 0 and 50 Gy, respectively. Conclusion In this work, the consideration of DNA repair of the damage from [225Ac]Ac-PSMA and [177Lu]Lu-PSMA revealed a dose dependency of the RBE. Hence, this work suggested that DNA repair is an important aspect to understand response to different radiation qualities

Image-based dosimetry for [225Ac]Ac-PSMA-I&T therapy and the effect of daughter-specific pharmacokinetics

Purpose Although( 221)Fr and Bi-213 have sufficient gamma emission probabilities, quantitative SPECT after [Ac-225]Ac-PSMA-I&T therapy remains challenging due to low therapeutic activities. Furthermore, Fr-221 and Bi-213 may underlie a different pharmacokinetics due to alpha recoil. We conducted a quantitative SPECT study and a urine analysis to investigate the pharmacokinetics of Fr-221 and Bi-213 and the impact on image-based lesion and kidney dosimetry. Methods Five patients (7.7 +/- 0.2 MBq [Ac-225]Ac-PSMA-I&T) underwent an abdominal SPECT/CT (1 h) at 24 and 48 h (Siemens Symbia T2, high-energy collimator, 440 keV/218 keV (width 20%), 78 keV (width 50%)). Quantitative SPECT was reconstructed using MAP-EM with attenuation and transmission-dependent scatter corrections and resolution modelling. Time-activity curves for kidneys (CT-based) and lesions (80% isocontour 24 h) were fitted mono-exponentially. Urine samples collected along with each SPECT/CT were measured in a gamma counter until secular equilibrium was reached. Results Mean kidney and lesion effective half-lives were as follows: Bi-213, 27 +/- 6/38 +/- 10 h;Fr-221, 24 +/- 6/38 +/- 11 h;78 keV, 23 +/- 7/39 +/- 13 h. The Bi-213-to-Fr-221 kidney SUV ratio increased by an average of 9% from 24 to 48 h. Urine analysis revealed an increasing Bi-213-to-Ac-225 ratio (24 h, 0.98 +/- 0.15;48 h, 1.08 +/- 0.09). Mean kidney and lesion absorbed doses were 0.17 +/- 0.06 and 0.36 +/- 0.1 Sv(RBE=5)/MBq using Fr-221 and Bi-213 SPECT images, compared to 0.16 +/- 0.05/0.18 +/- 0.06 and 0.36 +/- 0.1/0.38 +/- 0.1 SvRBE=5/MBq considering either the( 221)Fr or 213Bi SPECT. Conclusion SPECT/CT imaging and urine analysis showed minor differences of up to 10% in the daughter-specific pharmacokinetics. These variances had a minimal impact on the lesion and kidney dosimetry which remained within 8%