Showing papers by "Nils Weimann published in 2020"

Toward Mobile Integrated Electronic Systems at THz Frequencies

Abstract: This paper discusses advances related to the integration of future mobile electronic THz systems. Without claiming to provide a comprehensive review of this surging research area, the authors gathered research on selected topics that are expected to be of relevance for the future exploration of components for practical mobile THz imaging and sensing applications. First, a brief technology review of integrated mobile THz components is given. Advances in III-V technology, silicon technology, and resonant-tunneling diodes (RTD) are discussed. Based on an RTD source and a SiGe-HBT direct detector, low-cost and compact computed tomography is presented for volumetric continuous-wave imaging at around 300 GHz. Moreover, aspects of system integration of mobile THz MIMO radars are discussed. Thereby, a novel phase-locked loop concept utilizing a high-stability yttrium-iron-garnet-tuned oscillator to synthesize ultra-stable reference mmWave signals is shown, and an adaptive self-interference cancellation algorithm for THz MIMO in the digital domain based on Kalman filter theory is proposed.

Subharmonic Injection Locking for Phase and Frequency Control of RTD-Based THz Oscillator

Abstract: Phase and frequency control of resonant tunneling diode (RTD) based terahertz oscillators are major challenges in realizing coherent signal sources for arrayed applications, such as spatial power combining, beam steering, or multi-in multi-out systems. In this letter, we demonstrate frequency locking and control of an RTD oscillating at f 0 ∼ 550 GHz, via radiative injection of a weak sinusoidal subharmonic signal at f 0/2. Precise frequency control, within the locking range of around 2 GHz, is demonstrated. A peak output power enhancement of 14 dB in the whole locking range, compared to the free running oscillator, is achieved. Furthermore, occurrence of phase locking is identified by the spectral linewidth reduction, quantifiable in the full-width at half-maximum parameter. A signal linewidth of 490 Hz was achieved in locked operation.

Spatially controlled VLS epitaxy of gallium arsenide nanowires on gallium nitride layers

Abstract: We present Au catalyzed p-GaAs nanowire growth on n-GaN layers as a possible method to grow an arsenide on a nitride compound semiconductor by metal organic vapor phase epitaxy. The GaAs growth position, the nanowire density and the nanowire growth direction are controlled by a combination of vapor–liquid–solid growth and selective area epitaxy. Thus, a spatially controlled nanowire growth is attained, which is mandatory for device fabrication. The growth position is defined by lithographically positioned Au discs on n-GaN. By adapting the growth conditions (QTBAs, presaturation) the nanowire density is optimized. Lateral and vertical anisotropic nanowire growth is attained through VLS growth in structured SiOx openings. Critical technological parameters for successful control of the growth direction are the positioning of the Au catalyst in relation to the SiOx mask, the size of the eutectic in relation to the opening dimensions, and the SiOx thickness. These results lead to distinct pn-junction positions and adjustable nanowire growth dimensions and directions.

Large-Signal Modelling of sub-THz InP Triple-Barrier Resonant Tunneling Diodes

Abstract: A large-signal equivalent circuit model is developed for ultra-high frequency signal generation and detection provided by an InP triple barrier resonant tunneling diode. On-wafer DC and S-parameter measurements on 0.5 um2 and 1 um2 area devices were made from 20 MHz to 67 GHz. The bias dependent measurement data are utilized to extract the parameters of a compact RF model which accurately describes the static and dynamic behavior of the triple barrier resonant tunneling under zero bias and forward bias condition.

InP HBT technology for THz applications

Abstract: InP-based electronic technologies are well suited for THz applications due to the combination of high electron velocity and breakdown field in this material system. Today's highest frequency circuits are built from InP HEMT devices, exceeding 1 THz application frequency. The InP heterojunction bipolar transistor (InP HBT) has high potential for further development in the THz frequency range through scaling and process development. A lack of circuit complexity in InP electronic circuit technology compared to CMOS can be mitigated by heterointegration. The concept of vertical integration is of particularly high interest for THz phased arrays.

The accurate predictions of THz quantum currents requires a new displacement current coefficient instead of the traditional transmission one

Abstract: Modeling of quantum devices is still based on the original idea of Landauer that the macroscopic (DC) conductance of electron devices can be related to the (microscopic) transmission coefficient of electrons. In this paper we propose a simple model that captures the role of the particle and displacement currents in quantum electron devices working at THz by substituting the traditional transmission coefficient by a new displacement current coefficient. In particular, our model is used to compute the total current of a Resonant Tunnelling Diode (RTD) device under AC conditions. The new model, based on a time-dependent approach, is firstly shown to reproduce the DC behaviour of the Landauer model. Later on, at input frequencies higher than 500 GHz, large differences between the two models are observed. In particular, unexpected high frequency behavior is observed in simulations with an input signal up to 2 THz.

A systematic study of Ga- and N-polar GaN nanowire–shell growth by metal organic vapor phase epitaxy

Abstract: Metal organic vapor-phase epitaxy of GaN shells on N- and Ga-polar nanowires on AlN/Si(111) templates has been studied in detail. A polarity-dependent epitaxial optimization of nitride-based core–shell structures is necessary to attain the desired shell shape. On N-polar wires, a maximal shell length has been achieved using N2, only, as a carrier gas, while the length decreases by substitution of N2 with H2. A strong impact of the NW growth template polarity has been observed, which has to be considered to attain the desired shell shape. On Ga-polar wires under pure N2, an exclusive coverage of the wire tip occurs. Shell growth and an increasing shell length are obtained by injecting increased H2 flows. The semi-polar {101} and polar (000) planes have been identified as the facets that limit the vertical shell length growth evolution on the N- and Ga-polar core–shell structures, respectively. Meanwhile, the m-planar lateral growth mode is found to be identical for both types of polarities. The data are used to set up a growth model that includes the facet-dependent termination, carrier-gas dependent H-passivation, Ga-adatom length and Ga-adlayer formation, and the thereby adjusted three-dimensional growth and shell shape for both polarities. The attained insights and the developed technology allow the epitaxy of homogeneous complex crystal architectures, mandatory for optimized nitride core–shell NW-based devices.

Design of a 300 GHz externally Injection-Lockable Push-Push Oscillator for Beam Steering Applications

Abstract: A design for a 300 GHz reflection type push-push oscillator including an input for an external injection locking signal is proposed. The oscillator is based on substrate transferred InP double heterojunction bipolar transistors. The intrinsic performance of the circuit is evaluated in Harmonic-Balance (HB) simulations. Furthermore, the circuit behavior under the influence of an injected signal is investigated in transient simulations and the key parameters for the locking range are derived. Additionally, a comparison of one-sided and differential injection of the locking signal is made and it is shown that the one-sided injection of the locking signal is a viable approach for the here proposed oscillator.

Modeling the Noise of Transferred-Substrate InP DHBTs at Highest Frequencies

Abstract: This paper investigates noise modeling of transferred-substrate indium phosphide double heterobipolar transistors (InP DUBTs). It is shown that the shot noise of these devices exhibits a pronounced correlation which allows for a reliable extrapolation of the noise performance based on standard noise measurement at lower frequencies, or even on the knowledge of small-signal model parameters alone.