Temperature dependence of the electrical characteristics up to 370 K of amorphous In-Ga-ZnO thin film transistors

The temperature dependence in the typical temperature operating range from 300 K up to 370 K of the electrical characteristics of IGZO TFTs fabricated at temperatures not exceeding 200 °C is presented and modeled.It is seen that up to T = 330 K, the transfer curves show a parallel shift toward more negative voltages. In both subthreshold and above threshold regimes, the drain current shows Arrhenius-type dependence. In the latter case, for low temperatures, the activation energy is around 0.35 eV for V_GS = 10 V, reducing as V_GS is increased. The observed behavior is consistent with having the VRH transport mechanism as the predominant one in conduction.     

Temperature dependent lasing characteristics of InAs/InP(100) quantum dot laser

We report on the lasing characteristics of InAs/InP(100) quantum dots laser through changing the temperature under continuous-wave mode. Three lasing peaks are simultaneously observed at temperature of 80 K and the lasing order of each peak is unrelated with each other when injection current increases. Laser spectra obtained under fixed current for different temperatures show a drastic influence on their shape. A large spectral broadening is observed at low temperature, while the width of lasing spectra gradually narrows when the operating temperature increased. The lasing process of quantum dot laser is obviously different from that of a reference quantum well laser in the same wavelength region. In addition, very high wavelength stability of 0.088 nm/K in the temperature range of 80–300 K is obtained, which is 6.2 times better than that of reference quantum well laser.     

Temperature-dependent ambipolar electrical characteristics of pentacene-based thin-film transistors: The impact of opposite-sign charge carriers

The temperature-dependent electrical and charge transport characteristics of pentacene-based ambipolar thin-film transistors (TFTs) were investigated at temperatures ranging from 77 K to 300 K. At room temperature (RT), the pentacene-based TFTs exhibit balanced and high charge mobility with electron (μ_e) and hole (μ_h) mobilities, both at about 1.6 cm²/V s. However, at lower temperatures, higher switch-on voltage of n-channel operations, almost absent n-channel characteristics, and strong temperature dependence of μ_e indicated that electrons were more difficult to release from opposite-signed carriers than that of holes. We observed that μ_e and μ_h both followed an Arrhenius-type temperature dependence and exhibited two regimes with a transition temperature at approximately 210–230 K. At high temperatures, data were explained by a model in which charge transport was limited by a dual-carrier release and recombination process, which is an electric field-assisted thermal-activated procedure. At T < 210 K, the observed activation energy is in agreement with unipolar pentacene-based TFTs, suggesting a common multiple trapping and release process-dominated mechanism. Different temperature-induced characteristics between n- and p-channel operations are outlined, thereby providing important insights into the complexity of observing efficient electron transport in comparison with the hole of ambipolar TFTs.     

Theory of rf SQUIDS operating at 77 K

A comprehensive analytical theory of nonhysteretic rf SQUIDs operating in the adiabatic mode at elevated temperatures, around and above 77 K, is presented. When β≪1 (β is the hysteresis parameter) the theory is applicable also for rf SQUIDs operating in the nonadiabatic mode. In contrast to previous theories which are applicable only if βΓ≪1 (the case of small thermal fluctuations — 4 K), where Γ is the noise parameter, the present theory is valid for βΓ around unity or higher (the case of high thermal fluctuations, 77 K). A good qualitatively agreement with experimental data has been found. Based on this analytical investigation, the superiority at 77 K of rf SQUID over its counterpart, the dc SQUID, is theoretically predicted.     

Long-wave (10 μm) infrared light emitting diode device performance

Electroluminescence in the range of 6–12 μm is observed from an Sb-based type-II interband quantum cascade structure. The LED structure has 30 active/injection periods. We have studied both top-emitting and flip-chip mount bottom emitting LED devices. For room temperature operation, an increase, saturation and decrease in light output occur at successively higher injection currents. An increase of about 10 times in light output occurs when device is operated at 77 K compared to room temperature operation. This increase is attributed to reduced Auger non-radiative recombination at lower temperatures. The peak-emission wavelengths at room temperature and 80 K operation are 7 and 10 μm, respectively. These devices can be used for high-temperature simulation in an infrared scene generation experiment.     

Microwave properties and applications of Y–Ba–Cu–O thin films grown on various substrates

Microwave properties of YBa₂Cu₃O_7-δ (YBCO) films grown on (100) LaAlO₃ (LAO), (110) NdGaO₃ (NGO) and (001) SrLaAlO₄ (SLAO) substrates were studied in the form of a microstrip ring resonator at temperatures above 20 K. The YBCO resonator on a SLAO substrate showed microwave properties better than or comparable to other YBCO resonators on LAO substrates. For the YBCO resonators on LAO and SLAO substrates, both Q_U and f₀ appeared to decrease as the temperature was raised. Meanwhile the resonator on a NGO substrate showed different behaviors with Q_U showing a peak at ∼70 K, which are attributed to the unique temperature dependence of the loss tangent of the NGO substrate. An X-band oscillator with a YBCO ring resonator coupled to the circuit was prepared and its properties were investigated at low temperatures. The frequency of the oscillator signal appeared to change from 7.925 GHz at 30 K to 7.878 GHz at 77 K, which was mostly attributed to the change in f₀ of the YBCO ring resonator. The signal power appeared to be more than 4.5 mW at 30 K and 2.1 mW at 77 K, respectively. At 55 K, the frequency of the oscillator signal was 7.917 GHz with the 3 dB-linewidth of 450 Hz.     

Influence of In doping on the structural,optical and acetone sensing properties of ZnO nanoparticulate thin films

Indium-doped zinc oxide (ZnO) nanoparticle thin films were deposited on cleaned glass substrates by spray pyrolysis technique using zinc acetate dihydrate [Zn(CH₃COO)₂ 2H₂O] as a host precursor and indium chloride (InCl₃) as a dopant precursor. X-ray diffraction results show that all films are polycrystalline zinc oxide having hexagonal wurtzite structure. Upon In doping, the films exhibit reduced crystallinity as compared with the undoped film. The optical studies reveal that the samples have an optical band gap in the range 3.23–3.27 eV. Unlike the undoped film, the In-doped films have been found to have the normal dispersion for the wavelength range 450–550 nm. Among all the films investigated, the 1 at% In-doped film shows the maximum response 96.8% to 100 ppm of acetone in air at the operating temperature of 300 °C. Even at a lower concentration of 25 ppm, the response to acetone in this film has been found to be more than 90% at 300 °C, which is attributed to the smaller crystallite size of the film, leading to sufficient adsorption of the atmospheric oxygen on the film surface at the operating temperature of 300 °C. Furthermore, In-doped films show the faster response and recovery at higher operating temperatures. A possible reaction mechanism of acetone sensing has been explained.     

A comparative study of electrical and optical properties of InN and In0.48Ga0.52N

We present results of our studies concerning electrical and optical properties of In_0.48Ga_0.52N and InN. Hall measurement were carried out at temperatures between T=77 and 300 K. Photoluminescence (PL) spectrum in InN and In_0.48Ga_0.52N. InN has a single peak at 0.77 eV at 300 K. However, the PL in In_0.48Ga_0.52N has two peaks; a prominent peak at 1.16 eV and a smaller peak at 1.55 eV. These two peaks are attributed to Indium segregation corresponding to a high Indium concentration of 48% and a low concentration of 36%. High electric field measurements indicate that drift velocity that tends to saturate at around V_d=1.0×10⁷ cm/s at 77 K in InN at an electric field of F=12 kV/cm. However, in In_0.48Ga_0.52N the I–V curve is almost linear up to an electric field of F=45 kV/cm, where the drift velocity is V_d=1.39×10⁶ cm/s. At applied electric fields above this value a S-type negative differential resistance (NDR) is observed leading to an instability in the current and to the irreversible destruction of the sample.     

Cathodoluminescence investigations of vertically stacked,sub-μm arrays of sidewall quantum wires on patterned GaAs (311)A substrates

Vertically stacked arrays of GaAs/(AlGa)As sidewall quantum wires (Qwires) were successfully fabricated on GaAs (311)A substrates patterned with 0.5 μm-pitch gratings. The Qwires exhibit a lateral confinement potential as large as 210 meV and high luminescence efficiency up to 300 K. The distinct carrier transfer and loss mechanisms are studied by temperature dependent, spectrally and spatially resolved cathodoluminescence. Despite the almost perfect carrier capture in the Qwires, non-radiative recombination within the connecting quantum well (Qwell) regions usually cannot be neglected even at low temperatures. For temperatures approaching 300 K, reemission of carriers out of the Qwell into the vertical (AlGa)As barriers contributes increasingly to the reduction of the carrier transfer and luminescence efficiency of the Qwires without notable repopulation of the connecting Qwells.     

Charge transport and recombination in heterostructure organic light emitting transistors

Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300 K to 135 K. We study parameters such as carrier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and conversely the brightness decreases. These contrary effects can be explained by a higher recombination efficiency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs.     

The shear strength of nano-Ag sintered joints and the use of Ag interconnects in the design and manufacture of SiGe-based thermo-electric modules

Thermo-electric modules (TEMs) can be used to convert heat into electricity by utilizing the Seeback effect. It is now possible to buy BiTe thermo-electric modules that can operate up to temperatures of around 300 °C. However, many applications, such as the harvesting of excess gas turbine heat, may occur at higher temperatures. Therefore, new materials and manufacturing processes need to be developed to produce packaged TEMs that can operate at a maximum operating temperature of 650 °C. Two critical areas in the manufacture of a SiGe TEM are the choice and strength of materials used to both sintered joint the TE material to the rest of the module and the metal used for the interconnects. The interconnection material needs to be sufficiently strong to withstand large temperature fluctuations while maintaining a low contact resistance, as well as being compatible with the nano-Ag sintered joint. Shear force tests of the sintered thermo electrical leg material showed that the joints are brittle when sintered to W metallized AlN substrates are used and ductile fracture behavior when sintered to Cu metallized AlN substrates using the NanoTach K nano silver paste. Almost all of the joints were found to be brittle when using the NachTach X nano silver paste. Shear testing of the sintered joints showed that the X paste joints were variable in strength and stiffness, having a typical Young’s modulus between 10 and 100 MPa at room temperature. The K paste joints were stiffer, but had a similar strength as compared to the X paste joints.     

Insight into the charge transport and degradation mechanisms in organic transistors operating at elevated temperatures in air

Operational stability of organic devices at above-room-temperatures in ambient environment is of imminent practical importance. In this report, we have investigated the charge transport and degradation mechanisms in pentacene based organic field effect transistors (OFETs) operating in the temperatures ranging from 25 °C to 150 °C under ambient conditions. The thin film characterizations techniques (X-ray photoelectron spectroscopy, X-ray diffraction and atomic force microscopy) were used to establish the structural and chemical stability of pentacene thin films at temperatures up to 150 °C in ambient conditions. The electrical behavior of OFETs varies differently in different temperature bracket. Mobility, at temperatures below 110 °C, is found to be thermally activated in presence of traps and temperature independent in absence of traps. At temperatures above 110 °C mobility degrades due to polymorphism in pentacene or interfacial properties. The degradation of mobility is compensated with the decrease in threshold voltage at high temperatures and OFETs are operational at temperatures as high as 190 °C. 70 °C has been identified as the optimum temperature of operation for our OFETs where both device behavior and material properties are stable enough to ensure sustainable performance.     

Broadband nuclear magnetic resonance using DC SQUID amplifiers

We have constructed two pulsed NMR spectrometers in which the signal is coupled to the input coil of a low T_c DC SQUID using a superconducting flux transformer, yielding broadband response, with bandwidth determined by the SQUID electronics. A 50 kHz bandwidth commercial system has been used to observe free induction decay signals from platinum powder, bulk platinum, ³He gas and surface monolayers of ³He in the temperature range from 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal-to-noise ratio is as calculated with the noise dominated by flux noise in the SQUID in all samples but the bulk metal. A second system, which operates in flux-locked loop mode with bandwidth of 3.4 MHz using a SQUID with additional positive feedback, has been used to observe NMR signals from platinum powder at frequencies from 38 to 513 kHz and at a temperature of 4.2 K. The advantage of this technique in the study of systems with short T₂ at frequencies below 1 MHz is discussed. In addition we discuss the benefits of both broadband and tuned input circuits for NMR detection and we describe the performance of a spectrometer with a tuned input circuit which has been used to obtain signals at 1 MHz from platinum powder at 4.2 K and from ∼2 layers of ³He absorbed on a surface area of 0.11 m² at 1.7 K. The amplifier noise temperature is predicted to be 60 mK in the ³He experiment. This demonstrates the potential of the tuned set-up for measurements at low millikelvin temperatures on systems with low spin density and with T₂ greater than several hundred microseconds.     

Synthesis and characterization of EuBa2Ca2Cu3O9−x: The influence of temperature on dielectric properties and charge transport mechanism

High dielectric constant materials have a crucial importance for various microelectronic applications such as memory devices, supercapacitors etc. Among other insulators, perovskite structured oxide materials attract great interest not only for their high dielectric constants but also their unique electrical and magnetic properties such as superconductivity etc. From this point of view, a new Europium based copper oxide layered material with perovskite structure (EuBa₂Ca₂Cu₃O_9−x coded as Eu-1223) has been synthesized by solid state reaction method in this work. The physical and chemical properties of Eu-1223 have been determined by FTIR, SEM, XRF, XRD, TGA and DTA techniques. The influence of temperature on impedance and dielectric properties of Eu-1223 has been investigated by impedance spectroscopy measurements performed within the frequency interval of 5 Hz–13 MHz between 298 K and 408 K temperatures. It has been found that the Eu-1223 material has high dielectric constants at each temperature operated. In addition, Eu-1223 sample behaves as a colossal dielectric material up to 300 kHz for 408 K due to observation of dielectric constant values which are greater than 10³. Furthermore, it has been revealed that Eu-1223 material can be used as thermally sensitive resistors in electronic circuits due to its decreasing resistance with increasing temperature. Moreover, it has been observed that the relaxation frequency of the system shifts from 46.5 kHz (low frequency radio wave band) to 1.57 MHz (mid frequency radio wave band) as the temperature increasing from 298 K to 408 K. According to dc conductivity investigations, the variation of dc conductivity with the inverse of temperature satisfies linear relationship that indicates a thermally activated nearest neighbor hopping conduction. On the other hand, it has been determined that ac conductivity has frequency dependent relation which obeys ω^s for the high frequency region. Furthermore, the frequency exponent, s, which takes values between 0.7 and 0.4, shows a decreasing behavior with increasing temperature. In conclusion, ac charge transport mechanism has been predicted as correlated barrier hoping for Eu-1223.     

Responsivity and lifetime of resonant-cavity-enhanced HgCdTe detectors

Resonant-cavity-enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors have been modelled and fabricated based on these structures. Responsivity has been measured and shows a peak responsivity of 8 × 10⁴ V/W for a 50 × 50 μm² photoconductor at a temperature of 200 K. The measured responsivity shows some agreement with the modelled responsivity across the mid-wave infrared window (3–5 μm). The measured responsivity is limited by surface recombination, which limits the effective lifetime to ≈15 ns. The optical cut-off of the detector varies with temperature as modelled from 5.1 μm at 80 K to 4.4 μm at 250 K. There is strong agreement between modelled peak responsivity and measured peak responsivity with varying temperature from 80 to 300 K.     

Nano-gap micro-electro-mechanical bulk lateral resonators with high quality factors and low motional resistances on thin silicon-on-insulator

The design, fabrication and experimental investigation of 22–25 MHz fragmented-membrane MEM bulk lateral resonators (BLR) with 100 nm air-gaps on thin (1 and 6 μm) silicon-on-insulator (SOI) are reported. Quality factors as high as 120,000 and motional resistances of as little as 60 kΩ are measured under vacuum at room temperature, with 12 V DC bias and low AC power. The temperature influence on the resonance frequency and quality factor is studied and discussed between 80 K and 320 K. Significant quality factor increase and motional resistance reduction are reported at cryogenic temperature. The paper shows that high-quality factor MEM resonators can be integrated on partially depleted thin SOI, which can be a substrate of choice for the fabrication of future integrated hybrid MEMS–CMOS integrated circuits for communication applications.     

Measurements of n-InGaAs with MBE layers: Relevance of negative magnetoresistance in the investigated samples

Measurements of n_H were performed. n_H values showed a distinct increase at temperatures below ~90 K (1.1 μm n-InGaAs samples) and a decrease at temperatures below ~30 K (7 μm n-InGaAs samples), depending on the doping level. These trends might be related to the magnetoresistance (MR) of the n-InGaAs samples. The MR behavior of the n-InGaAs samples with respect to magnetic field and temperature was apparently dependent on the doping level. Two n-InGaAs samples, one of which had a thin InGaAs epilayer (1.1 μm) and the other with a thicker (7 μm) epilayer, showed interesting behavior at low temperature. Their behavior at magnetic fields of approximately −15000 to +15,000 G were determined. The resistivity ((ρ_G – ρ₀)/ρ₀) of the 1.1 μm sample was negative at temperatures lower than 30 K.     

Flash evaporated cadmium sulfide films

Cadmium sulfide (CdS) thin films were deposited by the flash evaporation technique onto glass substrates kept at temperatures in the range 30–300 °C. The source material was CdS powder synthesized in the laboratory. The films exhibited hexagonal structure with dislocation density and the stress decreased as the substrate temperature increased. An optical band gap of 2.39 eV was obtained for the films deposited at 300 °C. Raman spectra exhibited peaks corresponding to Longitudinal Optical phonons of CdS with the full width at half maximum decreasing with increase of substrate temperature. Room temperature resistivity values are lower than earlier reports on chemically deposited CdS films.     

A study on MIS Schottky diode based hydrogen sensor using La2O3 as gate insulator

In this work, a Metal–Insulator–Semiconductor (MIS) based Schottky-diode hydrogen sensor was fabricated with La₂O₃ as a gate insulator. The electrical properties (current–voltage characteristics, change in barrier height and sensitivity) and hydrogen sensing performance (dynamic response and response time) were examined from 25 °C to 300 °C and towards H₂ with different concentrations. The conduction mechanisms were explained in terms of Fowler–Nordheim tunneling (below 120 °C) and the Poole–Frenkel effect at temperatures (above 120 °C). The results show that at an operating temperature of 260 °C, the sensitivity of the device can reach a maximum value of 4.6 with respect to 10,000-ppm hydrogen gas and its response time was 20 s.     

Effect of the deposition rate on the phase transition in silver telluride thin films

Silver telluride thin films of thickness 50 nm have been deposited at different deposition rates on glass substrates at room temperature and at a pressure of 2×10⁻⁵ mbar. The electrical resistivity was measured in the temperature range 300–430 K. The temperature dependence of the electrical resistance of Ag₂Te thin films shows structural phase transition and coexistence of low temperature monoclinic phase and high temperature cubic phase. The effect of deposition rate on the phase transition and the electrical resistivity of silver telluride thin films in relation to carrier concentration and mobility are discussed.