Nils Weimann

In this work, a systematic study for silicon doping of InGaAs via the metalorganic di‐tert‐butylsilane (DTBSi) as a non‐gaseous alternative to common gaseous precursors in a vertical close‐coupled showerhead metalorganic vapor phase epitaxy (MOVPE) reactor is conducted. At a growth‐temperature of 620 °C, doping concentrations up to n = 2.5 × 10 ¹⁹ cm ⁻³ and electron mobility up to μ _n = 1831 cm ² /Vs are achieved. It is found that, in the used reactor geometry, the doping process is mainly limited by the low decomposition of DTBSi, causing low incorporation efficiency compared to disilane. By increasing the reactor pressure from 100 to 200 mbar, it is possible to reduce the growth temperature by 40 K while achieving a doping concentration of n = 2.8 × 10 ¹⁹ cm ⁻³ with an electron mobility of μ _n = 1631 cm ² /Vs. A similar, although less pronounced effect can be observed when increasing the total carrier‐gas flow into the reactor. Increasing growth rate and V/III ratio are also found to enhance the dopant incorporation.

Terahertz (THz) technology provides precise monitoring capabilities in dynamic environments, offering unique insights into insect habitats. Our study focuses on environmental monitoring of European honey bees (Apis mellifera) through a combination of measurements and simulations. Initially, the dielectric material properties of honey bee body parts are characterized across the spectral range of 1–500 GHz to collect heterogeneous empirical data. To extend the study, honey bee mockups made from polyamide 12 (PA12) and epoxy resin are employed and validated as effective substitutes for real bees through comparative scattering analyses. The research further explores radar cross-section (RCS), imaging, and spectral properties using advanced THz technologies, including resonant tunneling diodes (RTDs) operating at 250 GHz and THz time-domain spectroscopy (THz-TDS) for frequencies exceeding 250 GHz. High-resolution imaging, utilizing a 450 GHz bandwidth, captures intricate anatomical features of both real and 3D-printed bees, showcasing the potential of THz technology for detailed environmental monitoring. Finally, simulations at 300 GHz assess the dosimetry and feasibility of non-invasive, continuous monitoring approaches based on the heterogeneous honey bee model.

This work presents a comprehensive analysis of indium phosphide (InP) triple-barrier resonant tunneling diodes (TB-RTDs) operating as direct terahertz (THz) detectors at zero bias. Through analytical derivation, the influence of device dimensions and of current-voltage curvature on voltage responsivity and noise equivalent power (NEP) is explored, and theoretical expressions for diode sensitivity are derived. On-wafer measurements of two scaled TB-RTDs with top contact areas of 0.5 ${\mu }\mathrm{m}^{2}$ and 1 ${\mu }\mathrm{m}^{2}$ are conducted, followed by a comparative analysis, including harmonic-balance simulation results based on a self-developed TB-RTD nonlinear model. The measurements reveal that the responsivity scales with device area, as predicted by the theory, with a peak responsivity of 2123V/W at 340GHz for the TB-RTD, and above 1200V/W across the entire WR2 band (330–500GHz) for the smaller 0.5 ${\mu }\mathrm{m}^{2}$ area device. The NEP values do not exceed 3.5 $\mathrm{pW}/{\sqrt\mathrm{{Hz}}}$ and ${2}\mathrm{pW/{\sqrt{\rm{Hz}}}}$ for the 1 ${\mu }\mathrm{m}^{2}$ and 0.5 ${\mu }\mathrm{m}^{2}$ devices, respectively, with the lowest measured NEP being ${1.15}\mathrm{pW/{\sqrt{\rm{Hz}}}}$ for the 0.5 ${\mu }\mathrm{m}^{2}$ device. These sensitivity values place the TB-RTD at a level comparable with the state-of-the-art THz direct detectors operating at room temperature. The investigation offers a clear picture of the intrinsic performance of TB-RTD operating at zero bias, with a detailed overview of the on-wafer measurement setup, power characterization method, and detector figures of merit, highlighting the potential of TB-RTDs as compact, power-efficient, and ultra-sensitive direct THz detectors.

In GaN‐on‐Si nanowire integration schemes, where the nanowires are contacted through the substrate, the interfaces between silicon substrate, AlN buffer layer, and GaN nanowire are of high interest. Herein, analysis by scanning transmission electron microscopy (STEM) of GaN nanowires grown on Si(111) by metal–organic vapor phase epitaxy using a polarity‐ and site‐controlled growth method is presented. This method is based on prestructuring the substrate, ex situ oxidation of the surface, and in situ oxide layer desorption. First, an AlN layer is grown to prevent melt‐back etching. Samples are prepared for STEM by focused ion beam cutting. STEM measurements in three different regions reveal that the AlN buffer material is polycrystalline. The degree of polycrystallinity is found to depend on the observed region: the highest degree exists at the field area between nanowires and the lowest at the sidewall of the Si‐pillars. On the sidewall, the ‐direction of the AlN grain is tilted by 30.1° with regard to the Si{111} sidewall plane, leading to reduced lattice mismatch at this location. The growth of GaN is competing with diffusion of Ga atoms through grain boundaries. The dominant mechanism is dependent on the region, leading to site‐controlled growth of nanowires.

This work shows the hetero-integration of an InP resonant tunneling diode (RTD) on a SiGe-BiCMOS mm-Wave integrated circuit (MMIC) for near-field wireless fundamental injection locking. We observe injection locking within a locking range of 26 GHz and a total power during injection locking of $-$ 3.4 dBm. The SiGe-based local oscillator (LO) is integrated with a frequency doubler and an on-chip patch antenna that operates between 220 and 247 GHz, providing a maximum output power of $-$ 7 dBm. The LO is near-field coupled to an InP RTD oscillator integrated into a slot antenna which reaches a free running maximum output power of $-$ 6.2 dBm. The chip-to-chip integration is carried out through flip-chip bonding.

Material characterization in the terahertz range is an interesting topic of research due to its great applications in material science, health monitoring, and security applications. Advances in terahertz generation, detection, and data acquisition have contributed to improved bandwidth, signal power, and signal-to-noise ratio. This enables advanced material characterization methods such as ellipsometry, which has been little explored in the terahertz frequency range, yet. Here, we introduce a comparison between material characterization with terahertz time-domain spectroscopy in transmission geometry and ellipsometry reflection geometry. Terahertz ellipsometry images were taken, showing spatially resolved refractive index estimation in the far field and higher image quality compared to single-polarization imaging.

In this paper, we study terahertz (THz) simultaneous wireless information and power transfer (SWIPT) systems. Since coherent information detection is challenging at THz frequencies and Schottky diodes may not be efficient for THz energy harvesting (EH), we propose a novel THz SWIPT system design that employs unipolar amplitude shift keying (ASK) modulation at the transmitter (TX) and a resonant-tunnelling diode (RTD)-based EH circuit at the receiver (RX) to extract both information and power from the received signal. Furthermore, we propose a novel model for the dependence of the instantaneous output power of the RTD-based RX on the instantaneous received power, which is based on a non-linear and non-monotonic piecewise function, whose parameters are adjusted to fit circuit simulation results. To determine the information rate-harvested power tradeoff of the considered THz SWIPT system, we derive the distribution of the transmit signal that maximizes the mutual information between the transmit and received signals subject to constraints on the required average harvested power at the RX and the peak signal amplitude at the TX. Since the computational complexity needed for maximization of the mutual information may be infeasible for real-time THz SWIPT systems, we derive low-complexity suboptimal input signal distributions that maximize an achievable information rate numerically and in closed form for high and low required average harvested powers, respectively. Furthermore, based on the obtained results, we propose a suboptimal closed-form distribution of the transmit signal which can also guarantee a desired harvested power at the RX. Our simulation results show that while the proposed EH model can capture the non-monotonicity of RTD-based EH circuits in the THz band, baseline linear and non-linear EH models, developed for Schottky-diode-based EH circuits, can not. Furthermore, we demonstrate that a lower reverse current flow and a higher breakdown voltage of the employed RTD are preferable when the input signal power at the RX is low and high, respectively. We also show that all proposed input distributions yield practically identical SWIPT system performance. Moreover, we reveal that the information rate-harvested power tradeoff of THz SWIPT systems is determined by the peak amplitude of the TX signal and the maximum instantaneous harvested power for low and high received signal powers, respectively. Finally, we compare the proposed THz SWIPT system with two baseline schemes and confirm that the RX circuit parameters, mathematical EH models, and optimal transmit signal distributions have to be carefully designed to achieve high performance in THz SWIPT systems.