Gregor Keller

High-Speed GaN/GaInN Nanowire Array Light-Emitting Diode on Silicon(111)

The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space.

Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation

We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradiation with swift heavy ions. Devices were prepared with exfoliated single layers of MoS2 and graphene, respectively. They were characterized before and after irradiation with 1.14 GeV U28+ ions using three different fluences. By electrical characterization, atomic force microscopy, and Raman spectroscopy, we show that the irradiation leads to significant changes of structural and electrical properties. At the highest fluence of 4 × 1011 ions/cm2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance.

Sensitive high frequency envelope detectors based on triple barrier resonant tunneling diodes

InP-based resonant tunneling diodes with symmetrical I/V-characteristics have shown their excellent high frequency performance for THz signal generation. We present a modification with an additional third barrier to create an unsymmetrical I/V-characteristic. With their large current densities and low capacitances these devices are promising candidates for zero bias high frequency envelope detectors. Based on simulations two layer stacks are grown by MBE technology. The fabricated devices were measured at dc- and high frequencies. First measurement results for the short circuit responsivity are discussed.

Analysis of terahertz zero bias detectors by using a triple-barrier resonant tunneling diode integrated with a self-complementary bow-tie antenna

Recently, heavy emitter doping rather than decreasing the barrier thickness has boosted the peak current density of resonant tunneling diodes (RTDs) above 1,000 kA/cm2. Based on this achievement very mature InP-based RTD with current densities above 500 kA/cm2 are nowadays the leading solid-state THz device [1, 2]. Here, we show that even triple-barrier RTD (TBRTD) devices now reach a current density in excess of 250 kA/cm2 making this element ideally suited for rectification [3] but now at THz frequencies. Figure 1 is the state of art of THz detection sensitivity of previously reported zero bias detectors. Focusing on such zero bias broadband THz detection, we have also been studying on a design policy for a μm-sized on-chip self-complementally antenna and especially we have reported basic performances of a bow-tie antenna[4,5] integrated with a conventional homogeneous semiconductor mesa structure. However, it was still limited studies considering neither of actual nonlinear devices and peripheral circuits.

Characterization and modeling of zero bias rf-detection diodes based on triple barrier resonant tunneling structures

InP-based resonant tunneling diodes with symmetrical I/V-characteristics have shown their excellent high frequency performance for THz signal generation. For signal detection we present a device with an additional third barrier to create an unsymmetrical I/V-characteristic. Sensitivity measurements are performed and further improvements by scaling of the active device area are discussed. To allow SPICE based circuit simulation an approach for a large signal model is presented.

Frequency locking of a free running resonant tunneling diode oscillator by wire-less sub-harmonic injection locking

Wire-less sub-harmonic injection locking of a free running oscillator is presented. The free running oscillator consists of an InP-based triple barrier resonant tunneling diode integrated in a slot antenna designed for oscillation at f0 from 31 GHz to 35 GHz. The wire-less sub-harmonic injection locking signal is provided by a horn antenna to the slot antenna resonator of the fundamental mode RTD oscillator. The injection signal is set to a frequency of fsub = f0/2. A frequency locking and control with a precise factor of 2 is experimentally demonstrated which is a proof of concept for wire-less sub harmonic injection locking of a free running oscillator.

Junction field-effect transistor based on GaAs core-shell nanowires

Nanowire FETs with all-around Junction-Gate are demonstrated using GaAs core-shell nanowires. The electrical properties of the n-channel Junction FET were determined by DC measurements. The radial pn-junctions show diode-type I-V characteristics. The output and transfer I-V characteristics exhibit good pinch-off, and hysteresis-free transient behavior. First devices with 190 nm nanowire channel diameter show a drain current of ID = 260 nA and a transconductance of gm = 300 nS.

Optimized RTD-HBT VCO design based on large signal transient simulations

This paper presents an optimized RTD-HBT single ended voltage controlled oscillator topology with increased tuning range and improved frequency stability at 20 GHz oscillation frequency. By connecting the RTD to the emitter of an HBT, a larger voltage swing at the parallel resonator compared to the direct connection can be achieved. In addition the RTD-capacitance influence on the oscillation frequency can be suppressed efficiently. Transient assisted harmonic balance simulations promise an increased oscillation power by 2 dB and a doubled tuning range of about 3.2 GHz compared to the conventional RTD-HBT circuits.