Sandra Pralgauskaite

Reliability and low-frequency noise measurements of InGaAsP multiple quantum well buried-heterostructure lasers

Analysis of low-frequency noise characteristics is used to investigate the behavior of gain-coupled buried-heterostructure lasers. Devices which exhibit threshold current degradation during electrical and thermal overstress, as well as stable devices, are examined. In addition to examining the magnitudes of the optical and electrical noise as functions of current and frequency, the correlation between the electrical and optical noise is examined. It is shown that unreliable lasers exhibited white electrical and optical noises with negative correlation between them at threshold while the reliable devices did not. While the root cause of these characteristic noise differences is as of yet unknown, the present results suggest the potential utility of noise measurements as a means of predicting laser reliability and the development of low-cost screening techniques.

InGaAs bow-tie diodes for terahertz imaging: low frequency noise characterisation

Noise characteristics of bow-tie InGaAs diodes in forward and backward directions were measured in frequency range from 10 Hz to 20 kHz at room temperature. It was found that the spectral density of voltage fluctuations changes with frequency approximately as 1/f, indicating that origin of noise is superposition of generation and recombination processes in defects of the structure. It was determined that the dependence of spectral density of current fluctuation on current in backward and forward directions at different frequencies exhibits asymmetry which reflects non-uniform electric field distribution within the structure. As both noise and sensitivity increase with bias current, optimal conditions for device operation are discussed.

Noise characteristics and radiation spectra of multimode MQW laser diodes during mode-hopping effect

Comprehensive investigation of noise (optical and electrical fluctuations and cross-correlation factor between them) and radiation spectrum characteristics have been carried out for InGaAsP/InP Fabry-Perot (FP) multiple quantum well laser diodes (LDs). Mode-hopping occurs at particular operation conditions (injection current and temperature) in FP LD operation. LD radiation spectrum is outspread and intensive highly correlated optical and electrical fluctuations are observed during mode-hopping. Physical processes during mode-hopping have 1 µs - 1 ms characteristic times and are caused by generation-recombination and carrier capture processes in centers formed by defects in barrier layer.

Optical and electrical low-frequency noise of ridge waveguide InGaAsP/InP MQW lasers

A detail study of both the optical and electrical low- frequency noise spectra and their correlation factor of graded-index separate-confinement-heterostructure, multiple quantum well strained-layer Fabri-Perot and distributed- feedback InGaAsP/InP laser diodes has been carried out under the wide current and temperature ranges. A particular attention was concentrated to the investigation of optical and electrical fluctuations due to the mode hopping effect, which was observed at specific forward currents and temperatures. Both electrical and optical noises at mode hopping areas have the Lorentzian type spectrum, and are very strongly correlated and very sensitive to temperature and facet reflectivity.

Dynamic characteristics of gain-coupled InGaAsP laser diodes and their reliability

Dynamic characteristics of partly gain-coupled multiple-quantum-well InGaAsP distributed feedback laser diodes have been investigated. Basic dynamic parameters (differential gain, K-factor, differential carrier lifetime) were found out. It is shown that the increase of coupling coefficient results not only in both oscillation and damping frequency increasing, but also in time-resolved frequency chirp oscillation decreasing. Relation between K-factor and differential carrier lifetime and their correlation with deferential gain indicates essential differential gain role in dynamic parameter dispersion.

Low frequency noise spectroscopy of multi-walled carbon nanotubes composites

Resistivity and low frequency (10 Hz – 20 kHz) noise characteristics of composite materials with multi-walled carbon nanotubes (MWCNTs) have been carried out over wide voltage and temperature (73–380 K) ranges. Composite materials with epoxy-grafted (MWCNTs surface covered by liquid epoxy resin) and amino-grafted (MWCNTs surface covered by polyethylene polyamine) MWCNTs were investigated. Low-frequency noise spectra of investigated materials comprise of 1/fα-type components, what indicates that conduction is caused by Motts hopping between carbon nanoparticles controlled by charge carrier capture and release processes in localized states between them. MWCNTs surface treatment by polyethylene polyamine leads to the larger density of surface states and at the same time more intensive low frequency fluctuations.

Thermal noise-limited sensitivity of FET-based terahertz detectors

Here we present a detailed study on estimation of noise-dependent parameters, such as signal-to-noise ratio (SNR) or noise equivalent power (NEP), of field-effect-transistor based terahertz detectors (TeraFETs). Commonly, these parameters are estimated from a well-known assumption, that detectors performance is limited by the thermal noise of transistors channel. However, practice shows that the influence of other noise sources or transient effects is considerable. We summarize TeraFET noise measurements performed on different material systems based transistors, such as AlGaN/GaN, AlGaAs/GaAs, silicon CMOS, and monolayer graphene. We have achieved a good agreement between thermal noise and measured data. However, attention has to be paid to gate leakage currents and slow defect charging and discharging effects, which can strongly influence TeraFETs performance estimation.

Mid-infrared laser diodes investigation via low-frequency noise spectroscopy and electrical derivative characteristics

Electrical noise and electrical derivative (ED) characteristics of different heterostructure GaSb-based type-I mid-infrared laser diodes (LDs) are investigated in order to find a relation between these characteristics. The measurement results demonstrate that the manner of electrical noise voltage spectral density dependence on forward current in subthreshold region could be predicted from ED characteristics. Low-frequency noise spectroscopy, a sensitive diagnostic tool, during the LDs aging experiment reveals major changes in cross-correlation between electrical and optical fluctuations attributed to slight LDs degradation and also a non-optimum LD facet coating process as an additional noise source.

Low frequency noise spectroscopy and threshold characteristics of laser diodes

Electrical and optical noise characteristics of multiple-quantum-well distributed feedback InGaAsP laser diodes (LD) are investigated validating low-frequency noise spectroscopy as a tool for the laser diode reliability and quality evaluation. A special attention was paid to the interpretation of cross-correlation coefficient between electrical and optical fluctuations. Negative cross-correlation coefficient at the threshold could be potentially used to identify unreliable lasers. We demonstrate correlation method for estimation of the electrical noise part correlated to the optical fluctuations, what is related to the defect presence in the LD active layer. Also changes of other characteristics at the lasing threshold are presented: electrical derivative, conductance, and capacitance, providing more information about processes in laser diodes and helping to identify threshold correctly.

Low frequency noise characteristics of BaTiO 3 -NiFe 2 O 4 composites at phase transition

In this work we present detailed investigation of low frequency noise characteristics of BaTiO3-NiFe2O4 ceramic composites of different percentage of NiFe2O4. At the particular temperatures (that gets under the broad maximum of dielectric permittivity observed around 300 K) very intensive few separate peaks of random telegraph noise signal were observed. It shows that phase transition in the investigated materials goes through separate stages instead of the smeared out one. Characteristic times of the physical processes during the phase transition are close to one millisecond, and activation energy is around (0,34-0,49) eV.