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Micron-scale single-crystal nanowires of metallic TaSe3, a material that forms -Ta-Se3-Ta-Se3- stacks separated from one another by a tubular van der Waals (vdW) gap, have been synthesized using chemical vapor deposition (CVD) on a SiO2/Si substrate, in a process compatible with semiconductor industry requirements. Their electrical resistivity was found unaffected by downscaling from the bulk to as little as 7 nm in width and height, in striking contrast to the resistivity of copper for the same dimensions. While the bulk resistivity of TaSe3 is substantially higher than that of bulk copper, at the nanometer scale the TaSe3 wires become competitive to similar-sized copper ones. Moreover, we find that the vdW TaSe3 nanowires sustain current densities in excess of 108 A/cm2 and feature an electromigration energy barrier twice that of copper. The results highlight the promise of quasi-one-dimensional transition metal trichalcogenides for electronic interconnect applications and the potential of van der Waals materials for downscaled electronics.

Proton radiation damage is an important failure mechanism for electronic devices in near-Earth orbits, deep space and high energy physics facilities. Protons can cause ionizing damage and atomic displacements, resulting in device degradation and malfunction. Shielding of electronics increases the weight and cost of the systems but does not eliminate destructive single events produced by energetic protons. Modern electronics based on semiconductors - even those specially designed for radiation hardness - remain highly susceptible to proton damage. Here we demonstrate that room temperature (RT) charge-density-wave (CDW) devices with quasi-two-dimensional (2D) 1T-TaS2 channels show remarkable immunity to bombardment with 1.8 MeV protons to a fluence of at least 10^14 H+cm^2. Current-voltage I-V characteristics of these 2D CDW devices do not change as a result of proton irradiation, in striking contrast to most conventional semiconductor devices or other 2D devices. Only negligible changes are found in the low-frequency noise spectra. The radiation immunity of these "all-metallic" CDW devices can be attributed to their two-terminal design, quasi-2D nature of the active channel, and high concentration of charge carriers in the utilized CDW phases. Such devices, capable of operating over a wide temperature range, can constitute a crucial segment of future electronics for space, particle accelerator and other radiation environments., 18 pages, 2 display items

We report measurements of low-frequency electronic noise in ordered superlattice, weakly-ordered and random-packed thin films of 6.5 nm PbSe quantum dots prepared using several different ligand chemistries. For all samples, the normalized noise spectral density of the dark current revealed a Lorentzian component, reminiscent of the generation-recombination noise, superimposed on the 1/f background (f is the frequency). An activation energy of 0.3 eV was extracted from the temperature dependence of the noise spectra. The noise level in the ordered films was lower than that in the weakly-ordered and random-packed films. A large variation in the magnitude of the noise spectral density was also observed in samples with different ligand treatments. The obtained results are important for application of colloidal quantum dot films in photodetectors., Comment: 24 pages, 6 figures and supplemental info

Wafer-scale MoS2 growth at arbitrary integer layer number is demonstrated by a technique based on the decomposition of carbon disulfide on a hot molybdenum filament, which yields volatile MoSx precursors that precipitate onto a heated wafer substrate. Colorimetric control of the growth process allows precise targeting of any integer layer number. The method is inherently free of particulate contamination, uses inexpensive reactants without the pyrophoricity common to metal–organic precursors, and does not rely on particular gas-flow profiles. Raman mapping and photoluminescence mapping, as well as imaging by electron microscopy, confirm the layer homogeneity and crystalline quality of the resultant material. Electrical characterization revealed microampere output current, outstanding device-to-device consistency, and exceptionally low noise level unparalleled even by the exfoliated material, while other transport properties are obscured by high-resistance contacts typical to MoS2 devices.

We investigated low-frequency noise in two-dimensional (2D) charge density wave (CDW) systems, 1T-TaS2 thin films, as they were driven from the nearly commensurate (NC) to incommensurate (IC) CDW phases by voltage and temperature stimuli. This study revealed that noise in 1T-TaS2 has two pronounced maxima at the bias voltages, which correspond to the onset of CDW sliding and the NC-to-IC phase transition. We observed unusual Lorentzian features and exceptionally strong noise dependence on electric bias and temperature, leading to the conclusion that electronic noise in 2D CDW systems has a unique physical origin different from known fundamental noise types. We argue that noise spectroscopy can serve as a useful tool for understanding electronic transport phenomena in 2D CDW materials characterized by coexistence of different phases and strong pinning.

Quasi-1D van der Waals materials, such as transition metal trichalcogenides, have strong covalent bonds in one direction and weaker bonds in cross-plane directions. They can be prepared as crystalline nanowires or nanoribbons consisting of 1D atomic threads, i.e., chains. We have examined the current carrying capacity of ZrTe3 nanoribbons using a set of structures fabricated by the shadow mask method. The bulk crystals were synthesized by the chemical vapor transport method and exfoliated onto Si/SiO2 substrates. It was found that ZrTe3 nanoribbons reveal an exceptionally high current density, on the order of ~100 MA/cm2, at the peak of the stressing DC current. The low-frequency noise was of 1/ ${f}$ type near room temperature ( ${f}$ is the frequency). The noise amplitude scaled with the resistance, following the trend established for other low-dimensional materials. The high current density in ZrTe3 can be attributed to the single-crystal nature of quasi-1D van der Waals materials.

The measured response of a monolithic microwave integrated circuit (MMIC) to the sub-terahertz (300 GHz) radiation is in qualitative agreement with the analytical theory of the overdamped plasmonic detection and has been used to establish and identify the integrated circuit faults. The bias and polarization dependences of the response measured between the different pins provide information about the faults location. In contrast to a more conventional terahertz and radio frequency imaging and testing techniques, this method relies on the electronic response with the resolution determined by the transistor size. Another advantage of this non-destructive non-contact approach is that the MMICs or VLSIs can be tested either biased or unbiased. Multiple transistors within the circuit or the entire circuit can be tested fast.

We report results of investigation of the low-frequency electronic excess noise in quasi-1D nanowires of TaSe3 capped with quasi-2D h-BN layers. Semimetallic TaSe3 is a quasi-1D van der Waals material with exceptionally high breakdown current density. It was found that TaSe3 nanowires have lower levels of the normalized noise spectral density, SI/I2, compared to carbon nanotubes and graphene (I is the current). The temperature-dependent measurements revealed that the low-frequency electronic 1/f noise becomes the 1/f2 type as temperature increases to ∼400 K, suggesting the onset of electromigration (f is the frequency). Using the Dutta–Horn random fluctuation model of the electronic noise in metals, we determined that the noise activation energy for quasi-1D TaSe3 nanowires is approximately EP ≈ 1.0 eV. In the framework of the empirical noise model for metallic interconnects, the extracted activation energy, related to electromigration is EA = 0.88 eV, consistent with that for Cu and Al interconnects. Our...

We report results regarding the electron transport in vertical quasi-two-dimensional (2D) layered 1T-TaS 2 charge-density-wave (CDW) devices. The low-frequency noise spectroscopy was used as a tool to study changes in the cross-plane electrical characteristics of the quasi-2D material below room temperature. The noise spectral density revealed strong peaks - changing by more than an order-of-magnitude - at the temperatures closely matching the electrical resistance steps. Some of the noise peaks appeared below the temperature of the commensurate to nearly-commensurate CDW transition, possibly indicating the presence of the debated hidden phase transitions. These results confirm the potential of the noise spectroscopy for investigations of electron transport and phase transitions in novel materials.

We report on the current oscillations in quasi-2D 1T-TaS2 charge-density-wave two-dimensional devices. The MHz-frequency range of the oscillations and the linear dependence of the frequency of the oscillations on the current closely resemble the narrow band noise, which was often observed in the classical bulk quasi-1D trichalcogenide charge-density-wave materials. In bulk quasi-1D materials, the narrow band noise was interpreted as direct evidence of charge-density-wave sliding. Despite the similarities, we argue that the nature of the MHz oscillations in 1T-TaS2 is different from the narrow band noise. Analysis of the biasing conditions and current indicates that the observed oscillations are related to the current instabilities due to the voltage-induced transition from the nearly commensurate to incommensurate charge-density-wave phase.

We describe the low-frequency current fluctuations, i.e. electronic noise, in quasi-one-dimensional ZrTe3 van der Waals nanoribbons, which have recently attracted attention owing to their extraordinary high current carrying capacity. Whereas the low-frequency noise spectral density, SI/I2, reveals 1/f behavior near room temperature, it is dominated by the Lorentzian bulges of the generation-recombination noise at low temperatures (I is the current and f is the frequency). Unexpectedly, the corner frequency of the observed Lorentzian peaks shows strong sensitivity to the applied source-drain bias. This dependence on electric field can be explained by the Frenkel-Poole effect in the scenario where the voltage drop happens predominantly on the defects, which block the quasi-1D conduction channels. We also have found that the activation energy of the characteristic frequencies of the G-R noise in quasi-1D ZrTe3 is defined primarily by the temperature dependence of the capture cross-section of the defects rather than by their energy position. These results are important for the application of quasi-1D van der Waals materials in ultimately downscaled electronics.

This work was supported, in part, by the Semiconductor Research Corporation (SRC) contract 2018-NM-2796: One-Dimensional Single-Crystal van-der-Waals Metals: Ultimately-Downscaled Interconnects with Exceptional Current-Carrying Capacity and Reliability, and by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE award EFRI-1433395: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications.

Magnonics is a rapidly developing subfield of spintronics, which deals with devices and circuits that utilize spin currents carried by magnons - quanta of spin waves. Magnon current, i.e. spin waves, can be used for information processing, sensing, and other applications. A possibility of using the amplitude and phase of magnons for sending signals via electrical insulators creates conditions for avoiding Ohmic losses, and achieving ultra-low power dissipation. Most of the envisioned magnonic logic devices are based on spin wave interference, where the minimum energy per operation is limited by the noise level. The sensitivity and selectivity of magnonic sensors is also limited by the low frequency noise. However, the fundamental question "do magnons make noise?" has not been answered yet. It is not known how noisy magnonic devices are compared to their electronic counterparts. Here we show that the low-frequency noise of magnonic devices is dominated by the random telegraph signal noise rather than 1/f noise - a striking contrast to electronic devices (f is a frequency). We found that the noise level of surface magnons depends strongly on the power level, increasing sharply at the on-set of nonlinear dissipation. The presence of the random telegraph signal noise indicates that the current fluctuations involve random discrete macro events. We anticipate that our results will help in developing the next generation of magnonic devices for information processing and sensing., Comment: 18 pages; 3 figures

Terahertz (THz) plasmonic field effect transistor sensors have emerged as prime contenders for applications in THz and sub-THz communications, biomedical sensing, and imaging. Noise is one of the most important factors determining their performance. We show that the conventional approach of using thermal noise of the device impedance for predicting the sensor performance is inadequate because it ignores the effect of the input circuit and (for very short, submicron devices) ignores a very strong effect of the ballistic or quasi-ballistic transport on the device performance.

We demonstrate that a field effect transistor (FET) operating as THz or far infrared sensor could be used as a sensitive spectrometer when exposed to a strong tunable local oscillator signal with the varying frequency and phase. The FET nonlinearity results in the detected signal being enhanced by the local oscillator power and proportional to the cosine of the phase difference between the local oscillator and the signal. In contrast to the conventional heterodyne regime, the output signal becomes independent of the local oscillator power at large local oscillator powers when the induced local oscillator voltage becomes large compared to the gate voltage swing at the operating point. The observed gain in this regime is over 90 and predicted maximum gain is on the order 100,000 to 500,000. This regime of the detector operation is very promising for applications in the terahertz interferometry and spectroscopy.

The noise mechanisms in graphene and MoS 2 are quite different. The noise characteristics of MoS 2 transistors could be described by the McWhorter model. The range of the trap densities responsible for the 1/f noise in MoS 2 extracted from the noise measurements is from ∼1018 eV−1cm−3 to ∼ 6×1020 eV−1cm−3. The smallest noise level and smallest trap density was found for multilayer MoS 2 transistors with the number of layer N>6. The noise level in graphene, in general, is much smaller than in MoS 2 transistors and does not comply with the McWhorter model. The low frequency noise in high quality graphene transistors might be relatively low and comparable to Si MOSFETs. The contacts have an important effect on the noise in graphene, but the dominant sources of noise in not aged devices is the channel. The gate voltage dependencies of noise in graphene indicate that the mobility fluctuations give the dominant contribution to noise. The noise measurements of electron irradiated graphene devices and measurements in magnetic field confirm this hypothesis. Both in MoS 2 and graphene, the noise decreases with the number, N, of the atomic layers. The 1/f noise in relatively thin graphene devices with N

We review results of our studies of the low-frequency electronic noise in quasi-1D TaSe 3 nanowires of. The semi-metallic TaSe 3 is a quasi-1D van der Waals material with an exceptionally high breakdown current density. Our noise studies showed that TaSe 3 nanowires have lower levels of the normalized noise spectral density, S I /I2, compared to carbon nanotubes and graphene. The temperature-dependent measurements revealed that the low-frequency 1/f noise becomes the 1/f2-type as temperature increases to ∼400 K, suggesting the onset of electromigration. Using the Dutta-Horn random fluctuation model of the electronic noise, we determined that the noise activation energy for quasi-1D TaSe 3 nanowires is approximately E P ≈1.0 eV. From the empirical noise model for interconnects, the extracted activation energy, related to electromigration, is E A =0.88 eV, consistent with that for Cu and Al interconnects. Our results suggest that TaSe 3 nanowires and similar systems have potential for ultimately downscaled local interconnect applications.

Owing to their ultimate surface-to-volume ratio two-dimensional (2D) van der Waals materials are candidates for flexible gas sensor applications. However, all demonstrated devices had relied on direct exposure of the active 2D channel to gases, which presents problems for their reliability and stability. We demonstrated, for the first time, selective gas sensing with molybdenum disulfide (MoS2) thin films transistors capped with a thin layer of hexagonal boron nitride ( $h$ -BN). The resistance change, $\Delta R/R$ , was used as a sensing parameter to detect chemical vapors. It was found that $h$ -BN dielectric passivation layer does not prevent gas detection via changes in the current in the MoS2 channel. The detection without direct contacting the channel with analyte molecules was achieved with $\Delta R/R$ ratio as high as $10^{\mathrm { {3}}}$ . In addition, we show that the use of $h$ -BN cap layers (thickness $H \sim 10$ nm) improves sensor stability and prevents degradation due to environmental and chemical exposure.

We report on the transport and low-frequency noise measurements of MoS2 thin-film transistors (TFTs) with thin (2–3 atomic layers) and thick (15–18 atomic layers) channels. The back-gated transistors made with the relatively thick MoS2 channels have advantages of the higher electron mobility and lower noise level. The normalized noise spectral density of the low-frequency 1/ $f$ noise in thick MoS2 transistors is of the same level as that in graphene. The MoS2 transistors with the atomically thin channels have substantially higher noise levels. It was established that, unlike in graphene devices, the noise characteristics of MoS2 transistors with thick channels (15–18 atomic planes) could be described by the McWhorter model. Our results indicate that the channel thickness optimization is crucial for practical applications of MoS2 TFTs.

We report on the current-carrying capacity of the nanowires made from the quasi-1D van der Waals metal tantalum triselenide capped with quasi-2D boron nitride. The chemical vapor transport method followed by chemical and mechanical exfoliation were used to fabricate the mm-long TaSe3 wires with the lateral dimensions in the 20 to 70 nm range. Electrical measurements establish that the TaSe3/h-BN nanowire heterostructures have a breakdown current density exceeding 10 MA cm(-2)-an order-of-magnitude higher than that for copper. Some devices exhibited an intriguing step-like breakdown, which can be explained by the atomic thread bundle structure of the nanowires. The quasi-1D single crystal nature of TaSe3 results in a low surface roughness and in the absence of the grain boundaries. These features can potentially enable the downscaling of the nanowires to lateral dimensions in a few-nm range. Our results suggest that quasi-1D van der Waals metals have potential for applications in the ultimately downscaled local interconnects.

Low frequency noise (LFN) characterization was performed on the HfSiON gate stacks fabricated with the SiON interfacial layers (ILs) and a La cap layer. The LFN data identified N and La related defects located in the IL/HK region.

We report on the low-frequency current fluctuations and electronic noise in thin-films made of Bi(2)Se(3) topological insulators. The films were prepared via the "graphene-like" mechanical exfoliation and used as the current conducting channels in the four- and two-contact devices. The thickness of the films ranged from ∼50 to 170 nm to avoid hybridization of the top and bottom electron surface states. Analysis of the resistance dependence on the film thickness indicates that the surface contribution to conductance is dominant in our samples. It was established that the current fluctuations have the noise spectrum close to the pure 1/f in the frequency range from 1 Hz to 10 kHz (f is the frequency). The relative noise amplitude S(I)/I(2) for the examined Bi(2)Se(3) films was increasing from ∼5 × 10(-8) to 5 × 10(-6) (1/Hz) as the resistance of the channels varied from ∼10(3) to 10(5) Ω. The obtained noise data is important for understanding electron transport through the surface and volume of topological insulators, and proposed applications of this class of materials. The results may help to develop a new method of noise reduction in electronic devices via the "scattering immune" transport through the surface states.

We report results of the experimental investigation of the low-frequency noise in graphene transistors. The graphene devices were measured in three-terminal configuration. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and the Hooge parameter αH ~10-3. Both top-gate and back-gate devices were studied. The analysis of the noise spectral-density dependence on the gate biases helped us to elucidate the noise sources in these devices. We compared the noise performance of graphene devices with that of carbon nanotube devices. It was determined that graphene devices works better than carbon nanotube devices in terms of the low-frequency noise. The obtained results are important for graphene electronic, communication and sensor applications.

The low frequency (1/f) noise characteristics of Schottky-gated strained-Si n-channel modulation doped field effect transistors have been investigated as a function of Ge concentration for different virtual substrates. The gate voltage dependence of the 1/f noise agrees well with the McWhorter carrier number fluctuations model. The trap density (extracted using a Ge dependent potential barrier height and tunnelling constant) is low in devices on thick virtual substrates (Nt = (2–6) � 10 16 cm � 3 eV � 1 ), and does not degrade with the increase of the Ge concentration from 30% to 40%. This trap density is the same for thin Helax virtual substrates (He + ions implanted thin substrate) but increases two orders of magnitude for thin low-temperature grown substrates.

Aging the LEDs by driving at high current, results in the decrease of optical power proportional to the reciprocal square route of stress time. With aging time, change in the current–voltage characteristics indicates decrease of the current at low voltage below the light emission threshold, decrease of the forward voltage drop at high currents and usually no change in the series resistance. No change in the peak wavelength and half bandwidth were found with aging. Low frequency noise measured at low and high currents either did not depend on aging time or decreased. No correlation between noise, the device power, and the rate of the power degradation were found. These results are in strong contrast to previous studies of longer wavelength GaN-based LEDs. The possible degradation mechanism is the diffusion of the Al atoms out from the p-type cladding layer and lowering of the cladding layer potential barrier as a result.

The 1/ f noise in insulated gate strained Si n-channel modulation doped field effect transistors (MOSMODFETs) and in control Si metal–oxide–semiconductor FETs (MOSFETs) has been studied at gate voltages below and above the threshold. All transistors have a deposited gate oxide of 20 nm and gate length of 0.5 µm. Mobilities extracted from the capacitance– and current–voltage characteristics were found between 580–700 cm2 V-1 s-1 for the MOSMODFETs, and between 300–400 cm2 V-1 s-1 for the Si MOSFETs. In spite of the difference in the mobility both FETs demonstrated identical noise characteristics. The 1/ f noise was found well described by the model of number of carriers fluctuations equally below and above threshold. The effective density of traps is ~5×1010 eV-1 cm-2 responsible for the noise was within the usual range reported before for regular n-channel Si MOSFETs and somewhat higher than for p-SiGe MODFETs.

Ballistic electron conduction offers a new possible approach to the development of low-power high-speed logic circuits [1-3]. Ultra-high mobility of graphene allows achieving the ballistic or near-ballistic transport regime at room temperature (RT) in devices with relatively long channels [4-5]. In this presentation, we show that heterostructure field-effect transistors (HFETs) with graphene channel encapsulated between two layers of hexagonal boron nitride (h-BN) combine extremely high electron mobility (∼36,000 cm2/Vs) with a strongly reduced 1/ f noise level (f is the frequency). Low-frequency 1/f noise hampers the operation of numerous devices and can be a major impediment to development of practical low-power low-voltage applications of graphene and other 2D van der Waals materials [6-8]. The low noise level is beneficial for the low-voltage electronic applications. The prototype h-BN-graphene-h-BN HFETs were fabricated using the mechanically exfoliated h-BN and graphene flakes on Si/SiO2 wafers. The viscoelastic materials adhered to glass slides were used as transparent stamps for the layer transfer. The stamps were spin coated with poly-propylene carbonate (PPC). A micromanipulator was used for careful positioning of h-BN flakes on the stamp over graphene flakes. Following this procedure, the h-BN layer was added to the stack resulting in a fully encapsulated graphene monolayer. The stack was then released onto target Si/SiO2 substrate by heating the stage to an appropriate temperature. The resulting heterostructure was etched to expose the edges of graphene and create one-dimensional Cr/Au (10/100 nm) electrical contacts. Figure 1 shows the schematics and an optical microscopy image of a representative device. Figure 2 (a) shows the current-voltage (I-V) characteristics of the representative HFET. Both the effective and field-effect mobility extractions gave consistent results showing the mobility of ∼36,000 cm2/Vs at the carrier concentration of 7×1011 cm2. Figure 2 (b) presents the normalized 1/f noise spectral density of the graphene encapsulated device. The channel-area normalized noise spectral density in BN-graphene-BN HFET is factor of ×5–×10 smaller than that in typical reference graphene FETs without the channel encapsulation. The observed strong noise reduction can be explained by screening of the traps in SiO 2 by the BN barrier. Other possible physical mechanisms and prospects of further noise suppression will be discussed at the presentation.

Raman spectroscopy was used to determine the number of atomic planes in the exfoliated graphene samples and verify the quality of the selected flakes (see Figure 2 (a)). The optical microscopy images of the exfoliated BN and graphene flakes, and the resulting heterostructure are shown in Figure 2 (b-c). The current-voltage (I–V) characteristics of a representative BN-graphene-BN HFET are shown in Figure 3 (a). Both the effective and field-effect mobility extractions gave consistent results and the room-temperature (RT) mobility was determined to be ∼30,000 cm2/Vs at 7·1011 cm−2. The normalized 1/f noise spectral density of the graphene encapsulated device is presented in Figure 3 (b). We found that the channel-area normalized noise spectral density in BN-graphene-BN HFET is factor of ×5 – ×10 smaller than that in typical reference graphene FETs without channel encapsulation. The observed strong noise reduction can be related to screening of the traps in SiO 2 by the BN barrier. Other possible physical mechanisms and prospects of further noise suppression will be discussed at the presentation.

The percolation threshold p 0 ≈ 0.6 is determined for monodisperse platinum nanostructures with 1.8-nm metallic particles deposited in a monolayer onto an insulating substrate through laser electrodispersion. It is shown that, in the “metallic” state (for p > p 0), both the magnitude of the noise and its temperature dependence are close to those of pure metallic Pt layers. The frequency dependence of the normalized noise power spectral density is described by the relationship S I /I 2 ∼ 1/f γ with the exponent γ close to unity. For current densities j ≥ 107−108 A/cm2, the noise power spectral density S I increases more rapidly with a further increase in the current as compared to I 2 because of the current generating excess defects. For p < p 0, the dependence of the conductivity σ on the temperature is adequately described by the standard relationship σ ∼ exp[−(T 0/T)1/2]. The normalized noise power spectral density S I /I 2 exceeds the corresponding value for a quasi-metallic structure by many orders of magnitude. The noise power spectral density S I is approximately proportional to the square of the current only for very low currents and increases steeply with a further increase in the current.

We report on the experiments about detection of the THz radiation by silicon FETs with nanometer gate lengths. The observed photo-responses measured as a function of the gate voltage are all in agreement with the predictions of the Dyakonov-Shur theory. The plasma wave parameters deduced from the experiments allow us to predict also the possibility of resonant detection in THz range by nanometer size silicon devices. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate that high power light-emitting diodes (LED’s) exhibit low-frequency noise characteristics that are clearly superior to those of quartz tungsten halogen lamps, the non-coherent light source most commonly employed when freedom from intensity variation is critical. Their extreme stability over tens of seconds (combined with readily selectable wavelength) makes high power LED’s ideal light sources for DC recording of optical changes, from living cells and tissues, that last more than a few hundred milliseconds. These optical signals ( � I/I0) may be intrinsic (light scattering, absorbance or fluorescence) or extrinsic (absorbance or fluorescence from probe molecules) and we show that changes as small as 8 × 10 −5 can be recorded without signal averaging when LED’s are used as monochromatic light sources. Here, rapid and slow changes in the intrinsic optical properties of mammalian peptidergic nerve terminals are used to illustrate the advantages of high power LED’s compared to filament bulbs. © 2004 Elsevier B.V. All rights reserved.

Low frequency fluctuations in light intensity of 340 nm and 280 nm GaN -based light emitting diodes (LEDs) are compared with noise properties of other commercially available UV and visible wavelength LEDs and halogen lamps. At low frequencies, LEDs can exhibit lower levels of noise than halogen lamps. An LED noise quality factor β is estimated for the UV LEDs.

AlGaN thin films and Schottky barrier Al 0.4 Ga 0.6 N diodes exhibit generation-recombination (GR) noise with activation energies of 0.8 - 1 eV. GR noise in AlGaN / GaN Heterostructure Field Effect transistors (HFETs) corresponds to activation energies in the range from 1 - 3 meV to 1 eV. No GR noise is observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN / InGaN / GaN Double Heterostructure Field Effect Transistors (DHFETs). Local levels responsible for the GR noise in HFETs and DHFETs might be located in AlGaN barrier layers.

Low-frequency noise in AlGaN/InGaN/GaN double heterostructure field effect transistors was measured at room and elevated temperatures as function of gate and drain voltages. Both 1/ f noise and generation–recombination (g–r) noises were observed. The Hooge parameter, α , was estimated to be close to 1×10 −3 . The activation energy for observed g–r noise was found to be E a ∼1.6 eV (the largest reported activation energy for GaN based devices). The measurements also confirmed that the double heterostructure provided superior carrier confinement in 2D channel even at high carrier concentrations.

The low frequency noise in GaN/AlGaN heterostructure field effect transistors (HFETs) was studied in the temperature range from 8 to 300 K. At gate biases close to the threshold, the noise came from the device region under the gate, and the Hooge parameter αch was inversely proportional to ns (α ~ 1/ns) in the entire temperature range. This dependence might be explained by electron tunnelling from the 2D gas into the traps in the adjoining GaN or AlGaN layers. At voltages close to zero, the ungated source-gate and gate-drain regions were responsible for the noise, and the Hooge constant was two orders of magnitude larger. This result is consistent with recent studies of the mechanism of the current collapse in GaN-based FETs. A notable contribution from the generation-recombination noise with activation energy 0.24 eV was observed in the temperature interval from 50 K to 150 K.

Low-frequency noise in Al 0.4 Ga 0.6 N thin films (50 nm) was measured at room and elevated temperatures as function of gate and drain voltages. Both 1/f noise and generation-recombination noise were observed. Hooge parameter, α, was estimated to be about 7. The activation energy for observed generation-recombination noise was found to be Ea ~ 1.0 eV . This activation energy is consistent with the activation energy observed for g-r noise in AlGaN/GaN HFETs.

We report on structural and electrical properties of CdS films chemically deposited at temperatures 60–70 °C from solutions containing cadmium citrate complex-ions and thiourea. We demonstrated the material deposition on a variety of flexible substrates, including viewfoils, cloth, and threads and fabricated devices on the deposited films including solar cells, stress sensors, and photoconductive sensors. Scanning electron microscopy and high resolution transmission electron microscopy data show that the films consist of nanocrystalline grains. The X-ray diffraction data identify crystalline CdS with hexagonal structure with a very high degree of crystallinity. The relative intensity of the peaks in the X-ray spectrum shows that the crystallites are oriented. The nanostructure of the CdS films results in a giant reproducible sensitivity to stress (tension) under UV illumination.

We report on experimental study of the low frequency noise in GaN-based Field Effect Transistors. In both GaN Metal Semiconductor Field Effect Transistors (MESFETs) and AlGaN/GaN Heterostructure Field Effect Transistors (HFETs), the main noise sources are located in the channel. Gate voltage dependence of noise in MESFETs complies with the Hooge formula and indicates the bulk origin of noise. The dependencies of the Hooge parameter, α, on sheet electron concentration ns in HFETs are extracted from measured drain current fluctuations taking into account the contact resistance, and the resistance of the ungated regions of the transistors. At low channel concentrations α is inversely proportional to ns (α ~ 1/ns). This dependence as well as the temperature dependence of noise might be explained by electron tunneling from the 2D gas into the traps in the bulk GaN or AlGaN.

The transient characteristics of GaN highly doped thin channel metal–semiconductor and metal-oxide–semiconductor field effect transistors were investigated by switching the transistors from the OFF to the ON state. Transient gated transmission line model measurements showed that the channel resistance under the gate remains constant and the current collapse effects are linked to the transient variations of the series source–gate and gate–drain resistances, similar to what was previously reported for GaN/AlGaN heterostructure field effect transistors. The transient temperature measurements revealed that trapping processes responsible for the transient behavior cannot be described by the activation mechanism.

The dependence of the 1/f noise on the 2D concentration in the channel of AlGaN/GaN heterostructure field effect transistors has been obtained taking into account the influence of the contact series resistance and the resistance of the ungated source–gate and gate–drain regions. At low channel concentrations, the Hooge parameter α increases with the decrease of the channel concentration ns approximately as 1/ns. This dependence can be explained by the tunnelling of electrons from 2D electron gas to the traps in GaN or AlGaN layers. At high channel concentrations, the electron spillover from the 2D-channel to a parallel low mobility, noisy ‘parasitic conduction channel’ leads to the increase of α with the concentration increase.

We present the theory of a field effect transistor (FET) operating as a THz or far infrared detector. We demonstrate that the detected signal is enhanced by orders of magnitudes by using a homodyne detection scheme involving the interference of a weak incoming signal and a strong signal of a local oscillator with the close frequency. We developed a theory valid for the arbitrary relationship between the amplitude of the local oscillator and the gate voltage swing. Remarkably, the response saturates at a high local oscillator intensity at the value which depends on the phase difference between the signals. The observed gain in this regime is over 100, and the predicted maximum gain in this operating regime is on the order of 105. These results show that a FET could be used as a sensitive spectrometer and/or interferometer when exposed to a strong tunable local oscillator signal with the varying frequencies and phases. This regime of the detector operation is very promising for the interferometric and spect...

We report on a transparent gate silicon MOS photo-impedance sensor, where a gated light sensitive semiconductor layer connects fixed capacitances. The resistance of the semiconductor and the capacitance of the MOS structure change with illumination. The frequency dispersion makes the coupling of these capacitances sensitive to light intensity extending the sensor dynamic range and tuning the sensitivity of the sensor. Our modeling results demonstrate advantages of this novel sensor in terms of sensitivity and dynamic range. The design and concept of this device could be extended to many other semiconductor materials, where frequency dispersion is related either to traps, or embedded nanoparticles or carrier generation processes.

We report on the effects of the frequency dispersion in light sensitive materials used in photoimpedance wireless sensors. An example of such a sensor is a gated semiconductor connecting two or more fixed capacitances. The impedance of the device under illumination is changed by the change in the photoresistance of the semiconductor layer and the change in the gate-semiconductor capacitance. We report on the design and simulation of the frequency dispersion of the impedance of this device in silicon and discuss the physics and device performance. We also evaluate the dynamic range and sensitivity of the wireless photoimpedance sensors and show their advantages for wireless sensing applications compared to more conventional light sensors.

The dependence of the 1/f noise on 2D electron concentration in the channel n Ch of AlGaN/GaN Heterostructure Field Effect Transistors and Metal Oxide Semiconductor Heterostructure Field Effect Transistors has been studied and compared. The dependencies of Hooge parameter αCh for the noise sources located in the channel of the transistors on sheet electron concentration are found identical for both types of devices. The increase of the Hooge parameter αCh with the decrease of the channel concentration observed in both types of devices confirms that the noise sources are located in the region under the gate in the AlGaN/GaN heterostructure and that electron tunneling from the 2D electron gas into the traps in GaN or AlGaN layers is a probable noise mechanism.

Local levels with a large activation energy E/sub a//spl sim/0.8-1.0 eV have been observed in low-frequency noise measurements of GaN/AlGaN heterostructure field effect transistors (HFETs and MOS-HFETs) grown on 4N-SiC substrates. The noise might come from the thin (30 nm) AlGaN barrier layer. The estimates of the level parameters based on this assumption resulted in reasonable values of capture cross section /spl sigma//sub n//spl ap/(10/sup -12/-10/sup -13/) cm/sup 2/ and trap concentration N/sub t//spl ap/5-10/sup 16/ cm/sup -3/.

We demonstrate the capability to control the optical polarization of room-temperature stimulated emissions (SEs) at 238–239 nm from optically pumped AlGaN multiple-quantum-well (MQW) heterostructures on bulk AlN. The results of structural and optical characterizations provided evidence that altering the strain state in the pseudomorphically grown MQW laser structures enabled the switching of the polarization direction of the SE from predominantly transverse electric (TE) at 238 nm to predominantly transverse magnetic (TM) at 239 nm. The SE observed at 238 nm represents the shortest peak wavelength with TE polarization yet reported for AlGaN materials grown on any type of substrate.

We report results of investigation of the low-frequency excess noise in device channels made from topological insulators - a new class of materials with a bulk insulating gap and conducting surface states. The thin-film Bi 2 Se 3 samples were prepared by the 'graphene-like' mechanical exfoliation from bulk crystals. The fabricated four-contact devices had linear current-voltage characteristics in the low-bias regime |V SD | < 0.1 V. The current fluctuations had the noise spectral density S I ~1/f for the frequency f< 10 kHz. The noise density S I followed the quadratic dependence on the drain-source current and changed from about ~10 -22 to 10 -18 A 2 /Hz as the current increases from ~10 -7 to 10 -5 A. The obtained data is important for planning transport experiments with topological insulators. We suggest that achieving the pure topological insulator phase with the current conduction through the 'protected' surface states can lead to noise reduction via suppression of certain scattering mechanisms. The latter has important implications for implementing the ultra-low-power and ultra-low-noise electronics.