F. Hartmann

Universal and reconfigurable logic gates in a compact three-terminal resonant tunneling diode

Submicron-sized mesas of resonant tunneling diodes (RTDs) with split drain contacts have been realized and the current-voltage characteristics have been studied in the bistable regime at room temperature. Dynamically biased, the RTDs show noise-triggered firing of spikelike signals and can act as reconfigurable universal logic gates for small voltage changes of a few millivolt at the input branches. These observations are interpreted in terms of a stochastic nonlinear processes. The logic gate operation shows gain for the fired-signal bursts with transconductance slopes exceeding the thermal limit. The RTD junction can be easily integrated to arrays of multiple inputs and have thus the potential to mimic neurons in nanoelectronic circuits.

GaAs/AlGaAs resonant tunneling diodes with a GaInNAs absorption layer for telecommunication light sensing

Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the AlGaAs tunnel barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 μm wavelength leads to a pronounced photo-effect with a sensitivities of around 103 A/W.

Cavity-enhanced resonant tunneling photodetector at telecommunication wavelengths

An AlGaAs/GaAs double barrier resonant tunneling diode (RTD) with a nearby lattice-matched GaInNAs absorption layer was integrated into an optical cavity consisting of five and seven GaAs/AlAs layers to demonstrate cavity enhanced photodetection at the telecommunication wavelength 1.3 μm. The samples were grown by molecular beam epitaxy and RTD-mesas with ring-shaped contacts were fabricated. Electrical and optical properties were investigated at room temperature. The detector shows maximum photocurrent for the optical resonance at a wavelength of 1.29 μm. At resonance a high sensitivity of 3.1×104 A/W and a response up to several pA per photon at room temperature were found.

Nanowatt logic stochastic resonance in branched resonant tunneling diodes

The authors have fabricated branched resonant tunneling diodes (RTDs). Using two branches as inputs, universal logic-gate operation was investigated as a function of noise added to the input signal. The difference in the output voltage, used as a measure for logic operation, shows a peak in the noise power characteristic associated with logic stochastic resonance. The split RTD allows morphing between universal logic-gates solely controlled by the noise level with power differences in the nanowatt range.

Nanothermometer Based on Resonant Tunneling Diodes: From Cryogenic to Room Temperatures

Sensor miniaturization together with broadening temperature sensing range are fundamental challenges in nanothermometry. By exploiting a large temperature-dependent screening effect observed in a resonant tunneling diode in sequence with a GaInNAs/GaAs quantum well, we present a low dimensional, wide range, and high sensitive nanothermometer. This sensor shows a large threshold voltage shift of the bistable switching of more than 4.5 V for a temperature raise from 4.5 to 295 K, with a linear voltage-temperature response of 19.2 mV K(-1), and a temperature uncertainty in the millikelvin (mK) range. Also, when we monitor the electroluminescence emission spectrum, an optical read-out control of the thermometer is provided. The combination of electrical and optical read-outs together with the sensor architecture excel the device as a thermometer with the capability of noninvasive temperature sensing, high local resolution, and sensitivity.

Light-induced stochastic resonance in a nanoscale resonant-tunneling diode

Resonant tunneling diodes (RTDs) have been often invoked as a primary nanoelectronic device candidate for cellular neural network physical implementation and for mimicking biological neuronlike behaviors. In this letter we report on the light-induced behavior of trench-etched RTDs capable of undergoing complex stochastic dynamics where electronic noise and light can cooperate for reproducing the stochastic resonance phenomenon previously observed in real biological neurons. The experimental measurements presented here add a missing piece to the quest for the optimal mimicking of bio-neural simulation by improving the functionality and thus the potential role of nanoscale RTDs.

Light sensitive memristor with bi-directional and wavelength-dependent conductance control

We report the optical control of localized charge on positioned quantum dots in an electro-photo-sensitive memristor. Interband absorption processes in the quantum dot barrier matrix lead to photo-generated electron-hole-pairs that, depending on the applied bias voltage, charge or discharge the quantum dots and hence decrease or increase the conductance. Wavelength-dependent conductance control is observed by illumination with red and infrared light, which leads to charging via interband and discharging via intraband absorption. The presented memristor enables optical conductance control and may thus be considered for sensory applications in artificial neural networks as light-sensitive synapses or optically tunable memories.

From micro- to nanomagnetic dots: evolution of the eigenmode spectrum on reducing the lateral size

Brillouin light scattering experiments and micromagnetic simulations have been exploited to investigate the spectrum of thermally excited magnetic eigenmodes in 10nm-thick elliptical Permalloy dots, when the longer axis D is scaled down from about 1000 to 100nm. It is shown that for D larger than about 200nm the characteristics of the spin-wave eigenmodes are dominated by dipolar energy, while for D in the range of about 100 to 200nm exchange energy effects cause qualitative and quantitative differences in the spin-wave spectrum. In this ‘mesoscopic’ regime, the usual classification scheme, involving one fundamental mode with large average magnetization and many other modes collected in families with specific symmetries, no longer holds. Rather, one finds the simultaneous presence of two modes with ‘fundamental’ character, i.e. with a significant and comparable value of the average dynamical magnetization: the former is at larger frequency and has its maximum amplitude at the dot’s centre, while the latter occurs at lower frequency and is localized at the dot’s edges. Interestingly, the maximum intensity swaps from the higher frequency mode to the lower frequency one, just when the dot size is reduced from about 200 to 100nm. This is relevant in view of the exploitation of nanodots for the design of nanomagnetic devices with lateral dimensions in the above interval, such as memory cells, logic gates, reading heads and spin-torque oscillators.

Room temperature operation of GaSb-based resonant tunneling diodes by prewell injection

We present room temperature resonant tunneling of GaSb/AlAsSb double barrier resonant tunneling diodes with pseudomorphically grown prewell emitter structures comprising the ternary compound semiconductors GaInSb and GaAsSb. At room temperature, resonant tunneling is absent for diode structures without prewell emitters. The incorporation of Ga0.84In0.16Sb and GaAs0.05Sb0.95 prewell emitters leads to room temperature resonant tunneling with peak-to-valley current ratios of 1.45 and 1.36, respectively. The room temperature operation is attributed to the enhanced Γ-L-valley energy separation and consequently depopulation of L-valley states in the conduction band of the ternary compound emitter prewell with respect to bulk GaSb.

A Memristive Pascaline

The original Pascaline was a mechanical calculator able to sum and subtract integers. It encodes information in the angles of the mechanical components (such as wheels and cylinders), is aided by gravity, and performs the calculations. Here, we show that such a concept can be realized in electronics using memory elements such as memristive systems. By using memristive emulators, we have demonstrated experimentally the memcomputing version of the mechanical Pascaline, capable of processing and storing the numerical results in the multiple levels of each memristive element. Our result is the first experimental demonstration of multidigit arithmetic with multilevel memory devices that further emphasize the versatility and potential of memristive systems for future on-chip information processing and storage architectures.