Charge transport mechanism of Al/Bi2Te3/Al thin film devices

Results of dielectric and conduction properties of vacuum evaporated Bi₂Te₃ thin film capacitors (Al/Bi₂Te₃/Al) have been reported in the frequency range 12 Hz to 100 kHz at various temperatures (303–423 K). The variation of capacitance and dielectric constant was found to be temperature and frequency dependent. The capacitance of the film decrease with increasing temperature which may be due to the expansion of the lattice and also due to the excitation of charge carriers at the sites of imperfection. The dielectric constant decreases with frequency at all temperature. This can be attributed to an interfacial polarization caused by space charge. The prevailing ac conduction mechanism in these films is hopping type. The activation energies were evaluated for various thicknesses and it is found to be 0.016 and 0.014 eV for the frequencies 200 Hz and 1 kHz, respectively.     

High-resolution DLTS studies of vacancy-related defects in irradiated and in ion-implanted n-type silicon

Using high-resolution Laplace deep-level transient spectroscopy (DLTS), we have compared the electron emission characteristics of vacancy-related defects in silicon. The samples include material irradiated with high-energy protons, material implanted with a heavy ion and silicon irradiated with 2 MeV electrons. We show that in the proton- and electron-irradiated material the DLTS peak in the region of the (- -/-) state of the divacancy at E_c=0.23 eV contains only one feature. The DLTS peak at 250 K which contains the signal derived from the (-/0) state of the divacancy is much larger in ion-implanted silicon than in electron-irradiated silicon. The Laplace DLTS is able to resolve clearly the (-/0) divacancy state and the V–P defect, whereas conventional DLTS shows only a broad peak in that region.     

Model based precise analysis of the injection currents in Al/ZrO2/Al2O3/ZrO2/SiO2/Si structures for use in charge trapping non-volatile memory devices

Metal/insulator/Silicon (MIS) capacitors containing multilayered ZrO₂/Al₂O₃/ZrO₂/SiO₂ dielectric were investigated in order to evaluate the possibility of their application in charge trapping non-volatile memory devices. The ZrO₂/Al₂O₃/ZrO₂ stacks were deposited by reactive rf magnetron sputtering on 2.4 nm thick SiO₂ thermally grown on p-type Si substrate. C–V characteristics at room temperature and I–V characteristics recorded at temperatures ranging from 297 K to 393 K were analyzed by a comprehensive model previously developed. It has been found that Poole-Frenkel conduction in ZrO₂ layers occurs via traps energetically located at 0.86 eV and 1.39 eV below the bottom of the conduction band. These levels are identified as the first two oxygen vacancies related levels in ZrO₂, closest to its conduction band edge, whose theoretical values reported in literature are: 0.80 eV, for fourfold, and 1.23 eV, for threefold coordinated oxygen vacancies.     

Effect of annealing on structural,optical and electrical properties of pulse electrodeposited tin sulfide films

Polycrystalline tin sulfide (SnS) thin films were grown on conducting glass substrates by pulse electrodeposition. The effect of annealing on the physical properties such as structure, morphology, optical, and opto-electronic properties were evaluated to understand the effect of post-deposition treatment for SnS films. Annealing at temperatures higher than 250 °°C resulted in the formation of SnS₂ as a second phase, however, no significant grain growth or morphological changes were observed for films after annealing at 350 °C. A small change in band gap of 0.1 eV observed for films annealed at 350 °C was interpreted as due to the formation of SnS₂ rather than due to morphological changes. This interpretation was supported by X-ray diffractometry, scanning electron microscopy, and Raman spectral data. The electric conduction in the films is controlled by three shallow trap levels with activation energies 0.1, 0.05, and 0.03 eV. The trap with energy 0.03 eV disappeared after annealing at higher temperature, however, the other two traps were unaffected by annealing.     

CCD image sensor with compensated reset operation

A low voltage charge coupled device (CCD) image sensor has been developed by adjusting the electron potential barrier in the electron sensing structure. A charge injection to the gate dielectrics of a MOS transistor was utilized to optimize the electron potential level in the output structure. A DC bias generating circuit was added to the reset structure which sets reference voltage and holds the signal charge to be detected. The generated DC bias is added to the reset pulse to give an optimized voltage margin to the reset operation, and is controlled by adjustment of the threshold voltage of a MOS transistor in the circuit. By the pulse-type stress voltage applied to the gate, the electrons and holes were injected to the gate dielectrics, and the threshold voltage could be adjusted ranging from 0.2 V to 5.5 V, which is suitable for compensating the incomplete reset operation due to the process variation. The charges trapped in the silicon nitride lead to the positive and negative shift of the threshold voltage, and this phenomenon is explained by Poole–Frenkel conduction and Fowler–Nordheim conduction. A CCD image sensor with 492(H) × 510(V) pixels adopting this structure showed complete reset operation with the driving voltage of 3.0 V. The image taken with the image sensor utilizing this structure was not saturated to the illumination of 30 lux, that is, showed no image distortion.     

The effect of oxide layer thickness on the quantification of 1.5 MeV γ–radiation induced interface traps in the Ag/SiO2/Si MOS devices

This work presents the effect of varied thickness of oxide layer and radiation dose on electrical characteristics of Ag/SiO₂/Si MOS devices irradiated by 1.5 MeV γ–radiations of varied doses. SiO₂ layers of 50, 100, 150 and 200 nm thickness were grown on Si substrates using dry oxidation and exposed to radiation doses of 1, 10 and 100 kGy. The exposure to radiation resulted in generation of fixed charge centers and interface traps in the SiO₂ and at the Si/SiO₂ interface. Capacitance-conductance-voltage (C-G-V) and capacitance-conductance-frequency (C-G-f) measurements were performed at room temperature for all MOS devices to quantify the active traps and their lifetimes. It is shown that accumulation and minimum capacitances decreased as the thickness of SiO₂ layer increased. For the unexposed MOS devices, the flat band voltage V_FB decreased at a rate of −0.12 V/nm, density of active traps increased by 4.5 times and depletion capacitance C_DP, increased by 2.5 times with the increase of oxide layer thickness from 50 to 200 nm. The density of active traps showed strong dependence on the frequency of the applied signal and the thickness of the oxide layer. The MOS device with 200 nm thick oxide layer irradiated with 100 kGy showed density of active interface traps was high at 50 kHz and was 3.6×10¹⁰ eV⁻¹ cm⁻². The relaxation time of the interface traps also increased with the exposure of γ–radiation and reached to 9.8 µs at 32 kHz in 200 nm thick oxide MOS device exposed with a dose of 100 kGy. It was inferred that this was due to formation of continuum energy states within the band gap and activation of these defects depended on the thickness of oxide layer, applied reverse bias and the working frequency. The present study highlighted the role of thickness of oxide layer in radiation hard environments and that only at high frequency, radiation induced traps remain passivated due to long relaxation times.     

Preparation and characterization of phosphorus-doped silicon nanocrystals in SiCx films

Phosphorus (P)-doped silicon nanocrystals (Si-NCs) embedded in SiC matrix were prepared using magnetron sputtering and rapid thermal annealing with heavily P-doped Czochralski silicon as the doping target. The microstructure and electrical properties of the Si-NC thin films were characterized using transmission electron microscope, Raman spectroscopy and Hall measurement. It was observed that the microstructure changed from geometrically isolated Si-NCs to network Si-NCs with the annealing temperatures from 800 to 1200 °C. The evolution of microstructure led to the significant change of conductivity (10^?6 - 10¹ S cm^?1) in the Si_0.85C_0.15 thin films that possessing a fixed phosphorus concentration. A percolation threshold of crystalline-silicon (c-Si) content (30–40%) was found for the considerable increase of conductivity, where the carrier concentration dominated it. It suggested that the network Si-NCs not only increased the carrier mobility, but also boosted the carrier concentration. In addition, for the Si_0.85C_0.15 thin film with c-Si content above percolation threshold, the activate energy of conductivity could be lower than 70 meV and the work function lower than 4.10 eV.     

Temperature dependent resistivity study on zinc oxide and the role of defects

The temperature dependent (30–550 °C) resistivity of zinc oxide (ZnO) has been studied by the standard four probe resistivity method. The room-temperature resistivity of the sample is measured as 0.75 M Ωm. Resistivity versus temperature plot of the sample shows normal NTCR (negative temperature coefficient of resistance) behavior up to 300 °C. However, a crossover from NTCR to a PTCR (positive temperature coefficient of resistance) behavior is observed at ～300 °C. The origin of the PTCR behavior is explained with the defects present in the ZnO annealed up to 550 °C. Temperature dependent S-parameter (positron annihilation line-shape parameter) indicates the formation of oxygen vacancy like defects in this temperature region. At the PTCR region, the activation energy for the electron conduction is calculated ～2.6 eV. This value is very close to the theoretically predicted defect level energy of 2.0 eV for oxygen vacancies present in ZnO.     

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.     

Characteristics of RF reactive sputter-deposited Pt/SiO2/n-InGaN MOS Schottky diodes

All RF sputtering-deposited Pt/SiO₂/n-type indium gallium nitride (n-InGaN) metal–oxide–semiconductor (MOS) diodes were investigated before and after annealing at 400 °C. By scanning electron microscopy (SEM), the thickness of Pt, SiO_2, n-InGaN layer was measured to be ~250, 70, and 800 nm, respectively. AFM results also show that the grains become a little bigger after annealing, the surface topography of the as-deposited film was smoother with the rms roughness of 1.67 nm and had the slight increase of 1.92 nm for annealed sample. Electrical properties of MOS diodes have been determined by using the current–voltage (I–V) and capacitance–voltage (C–V) measurements. The results showed that Schottky barrier height (SBH) increased slightly to 0.69 eV (I–V) and 0.82 eV (C–V) after annealing at 400 °C for 15 min in N₂ ambient, compared to that of 0.67 eV (I–V) and 0.79 eV (C–V) for the as-deposited sample. There was the considerable improvement in the leakage current, dropped from 6.5×10⁻⁷ A for the as-deposited to 1.4×10⁻⁷ A for the 400 °C-annealed one. The annealed MOS Schottky diode had shown the higher SBH, lower leakage current, smaller ideality factor (n), and denser microstructure. In addition to the SBH, n, and series resistance (R_s) determined by Cheungs׳ and Norde methods, other parameters for MOS diodes tested at room temperature were also calculated by C–V measurement.     

Border traps and bias-temperature instabilities in MOS devices

An overview of the effects of border traps on device performance and reliability is presented for Si, Ge, SiGe, InGaAs, SiC, GaN, and carbon-based MOS devices that are subjected to bias-temperature stress, with or without exposure to ionizing radiation. Effective border-trap densities and/or energy distributions are estimated using capacitance-voltage hysteresis, low-frequency noise, charge pumping, and other electrical techniques that vary the time scale over which charge exchange between the semiconductor channel and near-interfacial dielectric. Oxygen vacancies and hydrogen impurity complexes are common border traps in a wide variety of systems subjected to bias-temperature stress. Charge trapping and emission tend to dominate observed bias-temperature instabilities for as-processed devices at higher oxide electric fields (> 4–6 MV/cm), and for irradiated devices. Hydrogen diffusion and reactions become relatively more significant in as-processed devices at lower electric fields (< 4–6 MV/cm).     

Optical and electrical characteristics of 17 keV X-rays exposed TiO2 films and Ag/TiO2/p-Si MOS device

This work presents the effect of varied doses of X-rays radiation on the Ag/TiO₂/p-Si MOS device. The device functionality was observed to depend strongly on the formation of an interfacial layer composed of SiO_x and TiO_y, which was confirmed by the spectroscopic ellipsometry. The XRD patterns showed that the as prepared TiO₂ films had an anatase phase and its exposure to varied doses of 17 keV X-rays resulted in the formation of minute rutile phase. In the X-rays exposed films, reduced Ti³⁺ state was not observed; however a fraction of Ti–O bonds disassociated and little oxygen vacancies were created. It was observed that the device performance was mainly influenced by the nature and composition of the interfacial layer formed at the TiO₂/Si interface. The spectroscopic ellipsometry was used to determine the refractive indices of the interfacial layer, which was 2.80 at λ=633 nm lying in between that of Si (3.87) and TiO₂ (2.11). The dc and frequency dependent electrical measurements showed that the interface defects (traps) were for both types of charge carriers. The presence of SiO_x was responsible for the creation of positive charge traps. The interface trap density and relaxation time (τ) were determined and analyzed by dc and frequency dependent (100 Hz–1 MHz) ac-electrical measurements. The appearance of peak in G/ω vs log (f) confirmed the presence of interface traps. The interface traps initially increased up to exposure of 10 kGy and then decreased at high dose due to compensation by the positive charge traps in SiO_x part of the interface layer. It was observed that large number of interface defects was active at low frequencies and reduced to a limiting value at high frequency. The values of relaxation time, τ ranged from 4.3±0.02×10⁻⁴ s at 0 V and 7.6±0.2×10⁻⁵ s at −1.0 V.     

Effect of electron irradiation on morphological,compositional and electrical properties of nanocluster carbon thin films grown using room temperature based cathodic arc process for large area microelectronics

The influence of 8 MeV electron beam bombardment on room temperature grown nanocluster carbon using cathodic arc process has been studied here. Atomic force microscopy (AFM) study shows that surface roughness varies with varying electron doses. High doses of electrons could causes thermal induce graphitization and morphological changes in the films. Raman spectroscopy analysis reveals that G-peak vary from 1555 cm⁻¹ to 1570 cm⁻¹ and D-peak varying from 1361 cm⁻¹ to 1365 cm⁻¹ indicating the disorderness and presence of both graphitic and diamond-like phases. Room temperature conductivity changes by two to three orders in magnitude. The conductivity in the films could be due to conduction of charge carriers through neighboring islands of conductive chains. Defect states calculated using the differential technique varies from 8 × 10¹⁷cm⁻³ eV⁻¹ to 1.5 × 10¹⁹ cm⁻³ eV⁻¹. Irradiation of nanocluster carbon thin films could be helpful to tune the electrical properties and defect densities of the nanocluster carbon films for various large area, flexible electronic and nano electronic applications.     

Effect of trap states on the electrical doping of organic semiconductors

We demonstrate that direct charge transfer (CT) from trap states of host molecules to the p-dopant molecules raises the doping effect of organic semiconductors (OS). Electrons of the trap states in 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) (E_HOMO = 6.1 eV) are directly transferred to the p-dopant, 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) (ELUMO = 5.4 eV). This doping process enhances the conductivity of doped OS by different ways from the ordinary doping mechanism of generating free hole carriers and filling trap states of doped OS. Trap density and trap energy are analysed by impedance spectroscopy and it is shown that the direct charge transfer from deep trap states of host to dopants enhances the hole mobility of doped OS and the I–V characteristics of hole only devices.     

Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.     

Optical and electrical measurement of FeSe2 thin films obtained at low temperature

FeSe₂ thin films were prepared at low temperature by thermal annealing at 350 °C during 6 h of sequentially evaporated iron and selenium films under selenium atmosphere. The structural, optical and electrical characteristics were investigated. The roughness of films (~76 nm) was confirmed by AFM images. Moreover, optical band gap of FeSe₂, which was evaluated as nearly 1.11 eV and confirmed by the electrical study which yielded a value in the order of 1.08 eV. The electrical conductivity, conduction mechanism, dielectric properties and relaxation model of theses thin films were studied using impedance spectroscopy technique in the frequency range 5 Hz–13 MHz under various temperatures (180–300 °C). Besides, complex impedance and AC conductivity have been investigated on the basis of frequency and temperature dependence.     

Intragap-trapped-carriers enhancement of the low-temperature delayed phosphorescence in Alq3

Time resolved electroluminescence of Alq₃ based organic light emitting devices was measured on a millisecond time scale. The millisecond delayed electroluminescence (DEL) was detected at 80 K and 26 K, while no long lived electroluminescence was observed at room temperature. The DEL spectrum consists of two components centered at ∼2.3 and ∼1.9 eV. The shape of the high energy band is similar to the prompt fluorescence emission peak, although a systematic red shift of about 30 meV is reproducibly found. The low-energy band position corresponds to the phosphorescence of Alq₃. The time decay of both bands is nonexponential, with the low energy band decaying slower. To explain dependencies of intensities of both components on the delay and the injection-current density a mechanism based on enhancement of the triplet-exciton radiative decay by carriers trapped in deep intragap traps is proposed.     

Evidence for causality between GaN RF HEMT degradation and the EC-0.57 eV trap in GaN

The degradation of industry-supplied GaN high electron mobility transistors (HEMTs) subjected to accelerated life testing (ALT) is directly related to increases in concentrations of two defects with trap energies of E_C-0.57 and E_C-0.75 eV. Pulsed I-V measurements and constant drain current deep level transient spectroscopy were employed to evaluate the quantitative impact of each trap. The trap concentration increases were only observed in devices that showed a 1 dB drop in output power and not the result of the ALT itself indicating that these traps and primarily the E_C-0.57 eV trap are responsible for the output power degradation. Increases from the E_C-0.57 eV level were responsible for 80% of the increased knee walkout while the E_C-0.75 eV contributed only 20%. These traps are located in the drain access region, likely in the GaN buffer, and cause increased knee walkout after the application of drain voltage.     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

Structural and optoelectronic properties of nanostructured TiO2 thin films with annealing

About 480 nm thick titanium oxide (TiO₂) thin films have been deposited by electron beam evaporation followed by annealing in air at 300–600 °C with a step of 100 °C for a period of 2 h. Optical, electrical and structural properties are studied as a function of annealing temperature. All the films are crystalline (having tetragonal anatase structure) with small amount of amorphous phase. Crystallinity of the films improves with annealing at elevated temperatures. XRD and FESEM results suggest that the films are composed of nanoparticles of 25–35 nm. Raman analysis and optical measurements suggest quantum confinement effects since Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk TiO₂ Optical band gap energy of the as-deposited TiO₂ film is 3.24 eV, which decreases to about 3.09 eV after annealing at 600 °C. Refractive index of the as-deposited TiO₂ film is 2.26, which increases to about 2.32 after annealing at 600 °C. However the films annealed at 500 °C present peculiar behavior as their band gap increases to the highest value of 3.27 eV whereas refractive index, RMS roughness and dc-resistance illustrate a drop as compared to all other films. Illumination to sunlight decreases the dc-resistance of the as-deposited and annealed films as compared to dark measurements possibly due to charge carrier enhancement by photon absorption.