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Experimental data on the electroreflectance spectra of γ-irradiated epitaxial GaN films on sapphire are reported. The irradiation doses are 105−2 × 106 rad. The theoretical electroreflectance spectra calculated on the basis of a model of three types of transitions are in agreement with experimental data with reasonable accuracy. The energies and broadenings of the transitions derived in the context of the model give grounds to infer that, in the GaN films, there are internal stresses dependent on the γ-irradiation dose.

The role of hot carriers in enhancing the radiation resistance of GaAs solar cells has been investigated. The laser-pulse induced, short-circuit current response method was used to study solar cell degradation caused by radiation damage. Samples were subject to radiation doses in the range 1×10 14 –4×10 16 electron cm −2 and then probed with laser pulses with 7 ns duration and 3.7 eV energy. We have developed a non-stationary theory of minority carrier flow in the active region of the solar cell. The theory allows a description of the temporal evolution of the short-circuit current and the dependence on irradiation dose. The model agrees well with experimental results using a single fitting parameter. This parameter is the carrier capture cross-section of radiation center E5 in the p-region emitter of the solar cell. The value of the cross-section was determined to be 0.1×10 −12 cm 2 from the results under non-stationary condition. This is seven times lower than that deduced from the current–voltage characteristics in the dark for a similar solar cell. The difference can be explained by a strong reduction of capture cross-section with increased carrier energy. Our results suggest that the observed cross-section reduction is caused by carrier accumulation at energies comparable with the optical phonon energy.

The results of experimental study of radiation resistance of GaAs-based solar cells are presented. The solar cells were irradiated by 1 MeV electrons at room temperature with the fluence up to 4 × 10 16 electrons cm - 2 . The radiation influence on the dark current and short-circuit current under illumination was investigated both experimentally and theoretically. It is shown that the radiation-produced electron traps E5 and hole traps H1 are responsible for irradiation-induced degradation of such solar cells. The radiation tolerance of the basic parameters (the short-circuit current, the output power) of GaAs solar cells is primarily determined by the radiation damage in p-regions.

Room temperature current–voltage and noise measurements have been made before and after gamma irradiation on AlGaN/GaN high electron mobility transistors (HEMTs) grown on sapphire. The saturation current due to radiation-induced defects shows a nonlinear dependence on radiation dose. The deviation of the device parameters does not exceed 20% at highest radiation dose 109 Rad and the devices with larger gate lengths demonstrate a higher radiation hardness to the 60Co gamma rays. The noise spectra of devices after gamma irradiation follow the flicker noise (1/fγ) dependence with the exponent γ close to one. The Hooge parameter estimated for the HEMTs after gamma irradiation dose does not show a gate-bias dependence and increases by approximately three to five times at radiation dose 2 × 108 Rad. The analysis of noise spectra allowed to identify the major sources of noise generation in the investigated HEMT heterostructures.

Mechanisms of electron transport formation in 2D conducting channels of AlGaN/GaN heterostructures in extremely high electric fields at 4.2 K have been studied. Devices with a narrow constriction for the current flow demonstrate high-speed electron transport with an electron velocity of 6.8 × 107 cm/s. Such a velocity is more than two times higher than values reported for conventional semiconductors and about 15% smaller than the limit value predicted for GaN. Superior velocity is attained in the channel with considerable carrier reduction. The effect is related to a carrier runaway phenomenon. The results are in good agreement with theoretical predictions for GaN-based materials.

We report on the influence of low gamma irradiation (10^4 Gy) on the noise properties of individual carbon nanotube (CNT) field-effect transistors (FETs) with different gate configurations and two different dielectric layers, SiO2 and Al2O3. Before treatment, strong generation-recombination (GR) noise components are observed. These data are used to identify several charge traps related to dielectric layers of the FETs by determining their activation energy. Investigation of samples with a single SiO2 dielectric layer as well as with two dielectric layers allows us to separate traps for each of the two dielectric layers. We reveal that each charge trap level observed in the side gate operation splits into two levels in top gate operation due to a different potential profile along the CNT channel. After gamma irradiation, only reduced flicker noise is registered in the noise spectra, which indicates a decrease of the number of charge traps. The mobility, which is estimated to be larger than 2x10^4 cm2V-1s-1 at room temperature, decreases only slightly after radiation treatment demonstrating high radiation hardness of the CNTs. Finally, we study the influence of Schottky barriers at the metal-nanotube interface on the transport properties of FETs analyzing the behavior of the flicker noise component., 19 pages, 9 figures, 1 table, accepted for publication in "Nanotechnology" journal

Novel test structures fabricated on the basis of Si nanowires are the ultimate building blocks for future nanoelectronics and biological sensor applications. These structures have to be stable in operation. However, a great many factors influence stability, especially in a nanowire channel. In this contribution, we report on the transport and noise properties of nanowires with different channel lengths. We applied small-dose gamma irradiation treatment of the samples to obtain more stable operation of the test device structures. The structures under study were fabricated on the basis of SOI wafers using nanoimprint technology. High-quality nanowires were obtained by chemical wet etching with low defect density. In spite of relatively large lengths of the channels from 2µm to 22 µm, we found that normalized current noise spectral density decreases as a function of 1/(L2 ) in the samples with lengths below 7µm. Such dependence is characteristic of the contact contribution to the noise properties.At the same time, in relatively long samples the behavior was changed to 1/L dependence, demonstrating the priority of channel noise. The latter dependence was also registered for samples with small lengths after gamma irradiation, reflecting the improvement effect of the irradiation due to stress relaxation in the contact regions as well as increased stability of the structures and decreased scattering in the structure characteristics (Figure 1).An analysis of the random telegraph signal noise component, recorded for samples with small length before gamma treatment and reduced level after this treatment, confirms the obtained results. This demonstrates the positive effect of irradiation on the stability and reliability of the structure parameters.

We demonstrate field-effect transistors (FETs) fabricated on the basis of individual carbon nanotubes (CNTs) with top- and side-gate configurations using two different dielectric layers, SiO2 and Al2O3. Transport properties of the FETs are investigated using noise spectroscopy before and after low-dose gamma irradiation treatment (106 rad). A strong generation-recombination (GR) noise component observed before the treatment is used to identify several charge traps of two dielectric layers by their activation energy. In spite of reduced flicker noise level after gamma irradiation, the GR noise component almost disappears in the spectra after treatment. This indicates the reduction in the number of charge traps. We study the influence of the Schottky barriers at the metal-nanotube interface on the properties of FETs by analyzing the flicker noise component and identified the regime where the contact contribution is negligible and the conductivity is determined by the properties of CNT material.

In recent years, considerable attention has been focused on studies on the influence of gamma radiation treatments on the performance of semiconductor materials and devices, which are very important for spacecraft applications. Among the different objects studied, carbon-nanotube-based (CNT) structures are considered promising due to their unique properties. There are a number of publications concerning their microwave applications. For example, a single-electron transistor can be used as a highly sensitive electrometer based on the sequential tunneling of electrons in the Coulomb blockade regime [1]. In this article, the authors reported on their investigation of radio-frequency single-electron transistor (rf-SET) operation of single-walled CNT quantum dots in the strong tunneling regime. They reached a charge sensitivity of 2.3×10−6e/Hz1/2 over a bandwidth of 85 MHz at a temperature of 4.2K and a carrier frequency of 754.2 MHz. These results indicate a gain-bandwidth product of 3.7×1013 Hz(3/2)/e, which is one order of magnitude better than for conventional rf-SETs.

Single-walled carbon nanotube field-effect transistors (CNT-FETs) were characterized before and after gamma radiation treatment using noise spectroscopy. The results obtained demonstrate that in long channel CNT-FETs with a length of 10 mu m the contribution of contact regions can be neglected. Moreover, radiation treatment with doses of 1x10(6) and 2x10(6) rad allows a considerable decrease parallel to the nanotube parasitic conductivity and even the shift region with maximal conductivity to the voltage range of nearly zero gate voltage that improves the working point of the FETs. The Hooge parameters obtained before and after gamma radiation treatment with a dose of 1x10(6) rad are found to be about 5x10(-3). The parameters are comparable with typical values for conventional semiconductors.

Here we report the results of an experimental study of the thermal conductivity of GaN crystals doped by oxygen with concentrations of 4 × 1016, 2.6 × 1018 and 1.1 × 1020 cm−3, carried out in the temperature interval 7–318 K. We observed the highest thermal conductivity ever reported for GaN, 269 Wm–1 K–1 at 300 K, in the sample with the lowest oxygen content. This result is explained by the renormalization of GaN elastic constants, caused by the effect of spontaneous polarization. Results were analyzed using the Callaway model. The contribution of phonon scattering by free carriers in doped GaN crystals was considered for the first time. We show that free electrons reduce the thermal conductivity by up to 32%–42% at 300 K for a sample with a 1.1 × 1020 cm−3 of oxygen concentration.

Experimental results on high electric field transport of 2DEG in GaN/AlGaN heterostructures are presented. Used time resolution of 0.1ns allows observing new peculiarities in the high field current flow that are shown and discussed.

In this paper, we presented results of steady state and pulse measurement of AlGaN/GaN heterostructures up to ultra high electric fields accompanied by low frequency noise measurements. Hot electron relaxation process were analysed. The contact resistance was measured in low field (ohmic) region and taken into account when calculating the average electric field. The velocity-field characteristics in AlGaN/GaN heterostructures obtained by measurement of the current-voltage characteristics. The spectra of the normalised current noise for different values of electric field E measured at T=300 K for the device with channel length of 25 /spl mu/m and width of 100 /spl mu/m is studied through experimental results.

Effect of small dose gamma-irradiation on electrical characteristics of AlGaN/GaN high electron mobility transistors has been investigated. Decreasing of the leakage current and its noise has been registered after dose of 1×106 Rad. As-grown heterostructures used in further for the device fabrication have been examined after the same radiation treatment. The small dose radiation results are explained within a model that takes into account relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

High-quality Si nanowire field effect transistors (FETs) were fabricated using thermal nanoimprint and chemical wet etching technologies. FET structures of different lengths demonstrate high carrier mobility with values of about 750 cm2/Vs and low volume densities of active traps in the dielectric layers of 5 × 1017 cm−3 eV−1. We investigated the transport properties of these n-type channel structures using low-frequency noise spectroscopy before and after gamma radiation treatment. Before gamma irradiation, FET structures with lengths of less than 4 μm exhibited noise from contact regions with 1/(L2) dependence for the relative 1/f noise. After gamma radiation, the spectra reflected the priority of channel noise with 1/L dependence for all samples. The transport characteristics show that the fabricated nanowire FETs improved scalability, decreased parameter scattering, and increased stability after treatment. The results demonstrate that these nanowire FETs are promising for nanoelectronic and biosensor applications due to the cost-efficient technology and advanced performance of FETs with improved stability and reliability.

High-quality silicon nanowire (NW) field-effect transistors (FETs) were designed and fabricated. Features of transport and modulation phenomena of the structures were studied using a number of techniques, including noise spectroscopy. Using the 1/f noise component level, the values of the volume trap densities in gate dielectric are estimated to be around 1 × 1017 cm−3 eV−1. This result proves high quality of the investigated structures. Analysis of Lorentzian noise components of NW samples is used to characterize single trap and its parameters. A strong modulation of carrier concentration in the conducting channel under influence of even single carrier capture event has been revealed. Possibility of fine tuning of the transport properties of the sample with low-dose gamma irradiation has been shown. The gamma radiation treatment of the NW samples was applied as an effective technique to confirm the strong influence of trap charges on conductivity behavior in the channel of NW FETs. The results demonstrate that the modulation effects at the nanoscale enable effective changing of the channel conductivity by single capture events and thus are promising for future information technologies and ultra-sensitive single-molecular sensor applications.

We propose and demonstrate a coding-decoding procedure as an important step to realize one more Si-based photonic device. Low-fluence (

In this work we present steady-state characteristics and low-frequency noise spectra of AlGaN/GaN based high electron mobility transistors (HEMTs) exposed to gamma ray radiation. The devices with a variety of gate length (150-350 nm) and width (100-400 νm) were irradiated by 60Co gamma rays with doses in the range of 104-109 Rad and flux of 102 Rad/s. Dose dependencies of basic operating parameters of the transistors, such as saturation current (Isat), transconductance (gm), channel conductance (gc), and threshold voltage (VT) are analysed. Our study show that visible changes of above mentioned parameters are observed under relatively small doses (105 Rad) and strongly depend on the HEMT's topology. The transconductance decreases and threshold voltage becomes more negative for all devices while deviation of these parameters from its initial values does not exceed 20% at highest irradiation dose. At the same time variation of the channel conductance as well as saturation current depends to a high extent on the gate voltage (Vg). At |Vg| < |Vcr|, both Isat and gc show a reversal proportional to the cumulative dosage of radiation. However, at |Vg| > |Vcr|, drain saturation current and channel conductance increase with the cumulative dosage of radiation. The effect is more pronounced in short-length-gate devices.