Defect-related Mechanism Research Articles

Performance assessment of dual material vertical TFET based on staggered heterojunction of GaSb-Si

In this research paper, a new III/V–Si-based TFET is proposed. This device benefits from a doping-less tunneling junction. By using a vertical structure, the issue of scaling is effectively addressed. Instead of using inductive metal, a cladding layer is utilized in our device, which turns the intrinsic source into a p+ region. This helps to overcome the challenges related to silicide formation. We use the Silvaco ATLAS simulator to conduct all the numerical simulations. The impact of defect-related mechanisms on our device performance is thoroughly evaluated. We demonstrate that implementing a large underlap between the gate and drain region in a vertical configuration can effectively solve ambipolar conduction. Parameters such as Ion = 10.1 μA/μm, Ion/Ioff = 1.1 × 1011, and fT = 50.1 GHz at VGS = 0.5V indicate that our device is an excellent option for low-power applications.

Structure and physical properties of hydrogenated (Co + Al)-doped ZnO films: Comparative study with co-doped ZnO films

(Co + Al)-doped ZnO films have been synthesized by the RF magnetron sputtering. Films of this composition have first been obtained in mixed atmosphere of Ar + H2. High hydrogen concentration of 20–50% has been used together with high enough substrate temperature of 450 °C. The used technological conditions affected the morphology, chemical composition, optical, electric, and magnetic properties of the films to an even more than in the case of Co-doped ZnO films synthesized under the same conditions and studied earlier. The films exhibit ferromagnetic behavior at room temperature with much greater magnetization and magneto-optical activity compared to the Co-doped films. At the same time, the hydrogenated films show an increase in electric conductivity in comparison with samples synthesized in the atmosphere of Ar + O2. The magnetic nature of the hydrogenated films has been associated with the defect-related mechanism.

Low temperature and magnetic field dependent ferroelectric behaviour of four-layered Aurivillius Bi5FeTi3O15 film prepared by spin coating

Thin films of four–layered Aurivillius Bi5FeTi3O15 compound were prepared using spin coating method. Phase purity of the sample is confirmed via X–ray diffraction and Raman spectroscopy measurements. Ferroelectricity in the prepared film is established from polarization versus electric field (P–E) loop measurements. The remnant and saturation polarization obtained from low temperature P–E measurements reveal a peculiar temperature dependence viz., decreasing on lowering the temperature. Apart from this, a development of an internal field in the system is observed towards lower temperatures which is reflected as biasing in the loops. Further, temperature dependent leakage current measurements have been performed and the analysis into conduction mechanisms shed light into the possible defect related mechanisms responsible for the aforementioned peculiar low temperature ferroelectric behavior. Also, the possibility of multiferroicity in the prepared film is explored with external magnetic field dependent P–E measurements and the results show very weak coupling between polarization and magnetic field.

Photoconductivity study of spray pyrolized pure and Cu-modified SrTiO3 thin films

The SrTiO3 (STO) and Cu-STO nanostructured thin films were prepared using the spray pyrolysis technique. So prepared films were characterized for structural, electrical and optical and photoconductivity properties. The FE-SEM showed with inhomogeneous grain growth. Scherrer formula used to calculate particle size. The band gap, activation energies are estimated. AFM results were analyzed to estimate roughness. Absorbance and band gaps are calculated. The tungsten 100 W bulb was used to irradiate the films. The nature of I-V characteristics of the films changes with the ranges of applied bias. The effect of electric field and intensity relies photoconductivity of the films. The photo generated current Ip and dark current Id varies according to the power law I α Vm with the applied field and intensity of illumination, due to the defect related mechanism including both recombination centres and traps. The variation in linearity is attributed to flow of trap limited as well as space charge limited current inside the material. Doping of Cu led to the introduction of a high concentration of oxygen vacancies on the surface of the STO lattice. Consequently, to maintain the regional charge balance of STO, Cu2+ could be substituted for Sr2+. Moreover, oxygen vacancies induced localized states into the band gap of the modified STO and acted as photoinduced charge traps, thus promoting the photoconductivity process. The photoconductivity of Cu modified films was found to be more than that of pure STO thin films. This study will have significant impact on the use of Cu-STO in optoelectronics devices.

Numerical Device Modeling, Analysis, and Optimization of Extended-SWIR HgCdTe Infrared Detectors

Imaging in the extended short-wavelength infrared (eSWIR) spectral band (1.7–3.0 μm) for astronomy applications is an area of significant interest. However, these applications require infrared detectors with extremely low dark current (less than 0.01 electrons per pixel per second for certain applications). In these detectors, sources of dark current that may limit the overall system performance are fundamental and/or defect-related mechanisms. Non-optimized growth/device processing may present material point defects within the HgCdTe bandgap leading to Shockley–Read–Hall dominated dark current. While realizing contributions to the dark current from only fundamental mechanisms should be the goal for attaining optimal device performance, it may not be readily feasible with current technology and/or resources. In this regard, the U.S. Army Research Laboratory performed physics-based, two- and three-dimensional numerical modeling of HgCdTe photovoltaic infrared detectors designed for operation in the eSWIR spectral band. The underlying impetus for this capability and study originates with a desire to reach fundamental performance limits via intelligent device design.

Investigation on room temperature photoluminescence of pure and aluminum doped zinc oxide nanoparticles

Abstract Pure and aluminum doped zinc oxide nanoparticles were prepared by soft chemical method. The prepared nanoparticles were characterized by XRD, SEM-EDAX, UV-Vis, PL and FT-IR studies. XRD patterns revealed that the nanoparticles were crystallized in hexagonal wurtzite structure with an average particle size of 19 nm to 26 nm. The surface morphology was explored using SEM micrographs. The incorporation of aluminum was confirmed by EDAX and FT-IR studies. The band gaps of the particles were found from 3.48 eV to 3.53 eV through UV-Vis spectral studies. The defect related mechanism was investigated using PL measurements. The chemical functional groups in FT-IR spectra proved the formation of pure and aluminum doped zinc oxide nanoparticles.

Ferromagnetic and photoluminescence properties of Cu-doped ZnO nanorods by radio frequency magnetron sputtering

Zn0.92Cu0.08O nanorods array were synthesized by radio frequency magnetron sputtering deposition at different substrate temperatures. No extra catalysts were used during synthesis. The structural and photoluminescence properties were investigated. Results showed that the ferromagnetism (FM) observed at 20 °C were an intrinsic property of Cu-doped ZnO nanorods. With the increase in substrate temperature, an improved crystallinity along with the increase in oxygen vacancies was observed in Zn0.92Cu0.08O nanorods, which caused an enhancement of magnetic moment in Zn0.92Cu0.08O nanorods. The experimental results showed that the interaction between substitutional CuZn in a divalent charge and oxygen vacancies played an important role in the origin of ferromagnetism in Zn0.92Cu0.08O nanorods. The observed FM is explained in terms of the defect related mechanism.

Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO nanoparticles

AbstractIn the present paper, ZnO nanoparticles (NPs) with particle size of 20–50 nm have been synthesized by hydrothermal method. UV-visible absorption spectra of ZnO nanoparticles show absorption edge at 372 nm, which is blue-shifted as compared to bulk ZnO. Photoluminescence (PL) and photoconductive device characteristics, including field response, light intensity response, rise and decay time response, and spectral response have been studied systematically. The photoluminescence spectra of these ZnO nanoparticles exhibited different emission peaks at 396 nm, 416 nm, 445 nm, 481 nm, and 524 nm. The photoconductivity spectra of ZnO nanoparticles are studied in the UV-visible spectral region (366–691 nm). In spectral response curve of ZnO NPs, the wavelength dependence of the photocurrent is very close to the absorption and photoluminescence spectra. The photo generated current, Ipc = (Itotal - Idark) and dark current Idc varies according to the power law with the applied field IpcαVr and with the intensity of illumination IpcαIL r, due to the defect related mechanism including both recombination centers and traps. The ZnO NPs is found to have deep trap of 0.96 eV, very close to green band emission. The photo and dark conductivities of ZnO NPs have been measured using thick film of powder without any binder.

Effect of substrate temperature on the room-temperature ferromagnetism of Cu-doped ZnO films

Wurtzite structure ZnO films doped with ~2 at% Cu were deposited at substrate temperatures ( T s) from 350 to 600 °C by helicon magnetron sputtering. All the films exhibited room-temperature (RT) ferromagnetism (FM) and the maximum saturation magnetization ( M s) was 1.2 emu/cm 3 (~0.15 μ B/Cu). Cu ions were mainly in a divalent state as identified by X-ray photoelectron spectroscopy. FM tended to increase with decreasing T s, and vacuum annealing enhanced the M s. These results suggested that oxygen vacancies and/or zinc interstitials might contribute to the ferromagnetic performance. Thus, the observed FM was explained in terms of the defect related mechanism.

Investigation of nanocrystalline Pbs/ n-Si heterostructures for optoelectronic applications

The nano-crystalline PbS/ n-Si heterostructure was studied using photocurrent spectra, I– V and C– V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size.

Multi-walled BN nanotubes synthesized by carbon-free method

Pure multi-walled BN nanotubes were synthesized via a carbon-free chemical vapor deposition process using boron and gallium oxide mixture as reaction precursor. Transmission electron microscopy was used to investigate their structure, morphology and defects. The wall deformation, dependent on tube diameter, was observed and explained in terms of strain relaxation of bond rotation. Opposed to carbon nanotubes, bending of BN nanotubes typically results in fracture at their concave side. Ring defect-related mechanism was proposed to interpret the fracture. The ring defects also result in the formation of a nanocone with 300° disclination. The nanocones end up with BN nanotubes exhibiting the small innermost shell ∼0.4 nm in diameter.

Photoluminescence and its excitation mechanisms in Si wires and dots

Investigations of photoluminescence, its temperature dependence, Raman scattering and IR absorption spectra were done for the study of the photoluminescence mechanism in porous silicon and Si enriched silicon oxide films. Red (1.6-1.7 eV) and orange (1.9-2.2 eV) photoluminescence bands are observed in both objects. Comparative investigations indicate that an oxide defect related mechanism is involved in the emission of PL bands in Si wires and silicon oxide films. Photoluminescence excitation mechanisms are discussed as well.

The Influence of Defects on Compatibility and Yield of the HfO2-PolySilicon Gate Stack for CMOS Integration

ABSTRACTHafnium-based dielectrics are under wide consideration for high-K gate dielectric applications. Since the gate electrode typically used in CMOS integration consists of polysilicon with n- or p-type dopants, compatibility of the HfO2layer with the polySi deposition and dopant activation steps is critical. Capacitors were fabricated with HfO2films deposited by ALD and MOCVD, and using polysilicon gate electrodes deposited by CVD processes. These devices showed leakage failures with yields that were observed to depend on the area, dielectric thickness and annealing conditions during the process. Investigation of the root cause of these leakage failures suggested that the leakage failures may be caused by a defect-related mechanism. The implication of this is that the leakage occurs at localized ‘defect’ sites rather than broadly through the HfO2layer. Emission microscopy analysis and physical characterization of the HfO2film were used to corroborate the proposed model. Defect density was observed to be strongly dependent on the processing of the dielectric film. In order to make Hf-based dielectric stacks compatible with polysilicon for conventional CMOS transistor integration with acceptable yield, further postdeposition treatment may be necessary to eliminate or cure the defects.

The Influence of Defects on Compatibility and Yield of the HfO2-PolySilicon Gate Stack for CMOS Integration

ABSTRACTHafnium-based dielectrics are under wide consideration for high-K gate dielectric applications. Since the gate electrode typically used in CMOS integration consists of polysilicon with n- or p-type dopants, compatibility of the HfO2layer with the polySi deposition and dopant activation steps is critical. Capacitors were fabricated with HfO2films deposited by ALD and MOCVD, and using polysilicon gate electrodes deposited by CVD processes. These devices showed leakage failures with yields that were observed to depend on the area, dielectric thickness and annealing conditions during the process. Investigation of the root cause of these leakage failures suggested that the leakage failures may be caused by a defect-related mechanism. The implication of this is that the leakage occurs at localized ‘defect’ sites rather than broadly through the HfO2layer. Emission microscopy analysis and physical characterization of the HfO2film were used to corroborate the proposed model. Defect density was observed to be strongly dependent on the processing of the dielectric film. In order to make Hf-based dielectric stacks compatible with polysilicon for conventional CMOS transistor integration with acceptable yield, further postdeposition treatment may be necessary to eliminate or cure the defects.

Comparative investigation of photoluminescence of silicon wire structures and silicon oxide films

Photoluminescence spectra and their dependence on temperature as well as Raman scattering spectra and Atomic Force Microscopy investigations have been used to study the peculiarities of the red photoluminescence band in low-dimensional Si structures, such as porous silicon and silicon oxide films. It has been shown that the red photoluminescence band of porous silicon is complex and can be decomposed into two elementary bands. It was discovered that elementary band intensities depend very much on surface morphology of porous silicon. The same positions of the photoluminescence bands are also observed in silicon oxide films for different oxide composition. Comparative investigation of the PL temperature dependences in porous silicon and silicon oxide films indicates that silicon-oxide defect related mechanisms of some elementary photoluminescence bands are involved.

Defect related photoluminescence in Si wires

Photoluminescence spectra and their dependence on the temperature have been used to study the peculiarities of the red photoluminescence in low-dimensional Si structures, such as porous silicon and silicon oxide films with an admixture of silicon. It has been shown that red photoluminescence band of Si wires is complex and can be decomposed into two elementary bands. Practically the same positions of photoluminescence bands are observed in silicon oxide films. Comparative investigation of photoluminescence temperature dependence in Si wires and silicon oxide indicates that oxide defect related mechanisms for photoluminescence bands are involved. The photoluminescence excitation mechanisms in both objects are discussed as well.

Luminescence and X-ray diffraction studies of Ge nanocrystals in amorphous silicon oxide

Ge nanocrystals embedded in a-SiO 2 films have been synthesized by rapid thermal annealing. Under excitation sources of different wavelengths, three photoluminescence (PL) peaks at 1.8, 2.2 and 3.1 eV were observed. A forming gas anneal on the samples resulted in a reduction of the intensity of the PL peaks. Studies of the annealed samples by X-ray diffraction revealed the formation of GeO 2 and Ge nanocrystals in the rapid thermal annealed system. The origin of the PL peaks is suggested to be due to defect related mechanism.

Defect mechanism of photoinduced superconductivity in YBa2Cu3O6+x.

Although persistent photoconductivity and photoinduced superconductivity have been experimentally observed in oxygen-deficient and superconducting $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$, the mechanism which is responsible for these effects is still under debate. The possibility of a defect related mechanism in laser-deposited thin films of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ is studied using four-point resistivity and photoluminescence techniques. It is found that there is a correlation between the photoluminescence spectrum and the wavelength dependence of the photoinduced conductivity. Peaks in the luminescence spectrum change intensity upon initiation of the photoinduced state. These results support an oxygen vacancy defect model of photoinduced persistent conductivity and superconductivity in which the oxygen vacancies act as weakly luminescent $F$-centers under illumination.

Raman spectra of hydrogen-treated optical fibres

Chemical reactions in a Ge-P-silicate optical fibre exposed to hydrogen at 100 bar and 150°C for times up to 120 h are studied by Raman spectroscopy. Hydride and hydroxyl groups are formed, as Si-H and P-OH. The reaction kinetics are consistent with a defect-related mechanism. The saturation concentration of hydroxyl is about 103 ppm.

Enhanced diffusion in Si and Ge by light ion implantation

This paper presents theoretical and experimental results of investigations of impurity-atom diffusion enhancement in Si and Ge crystals under light ion bombardment. Using existing ion range and displacement mechanism theories, a continuity equation for bombardment-generated defects is written and solved to give an expression for the steady-state excess defect concentration as a function of depth in the crystal. With the aid of existing diffusion theory, this is interpreted as a spatially varying atomic diffusivity which is a function of the bombarding beam parameters, temperature, the atomic species, and an experimentally measurable defect diffusion length. This spatially varying diffusivity, inserted in an appropriate diffusion equation, determines the evolution of the distribution of any impurity species diffusing by a defect-related mechanism. Some numerical solutions of this equation and some analytic solutions of interest are given, and a comparison of theory and experiment is presented. Among the results of this comparison are (i) defect diffusion length measurements of 0.27 μ in p-type Ge, and 0.086 μ in n-type Si, (ii) a measured yield of approximately 25 mobile defects/proton in either Ge or Si, mostly clustered at the end of the proton's track. A simple method is presented for the experimental measurement of the required defect diffusion length. Impurity concentration profiles of the proton-enhanced diffusion of P31 in Ge and B in Si were measured, and experimental checks of several theoretical predictions were carried out. There are some indications of a diffusivity saturation at high beam currents in Si which may result from the generation of immobile defect clusters. Impurity diffusivity enhancement by a factor of greater than 105 was observed in Si. Several physical parameter values of interest are calculated. Transmission electron microscopy measurements were undertaken to observe the effects of the bombardment on crystalline order in Si, and V-I and C-V characteristics of p-n diodes produced using enhanced diffusion of B in Si were examined. The uses of bombardment-enhanced diffusion as a device fabrication tool are discussed, and some unresolved topics are indicated.