Study of defect levels in CdTe using thermoelectric effect spectroscopy

We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (E_th) and trapping cross section (σ_th) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.     

Repeating of positive and negative high electric field stress and corresponding thermal post-stress annealing of the n-channel power VDMOSFETs

We have subjected n-channel power VDMOSFETs to a positive and negative high electric field stress (HEFS) followed by biased annealing at 150 °C. Stress-induced defects have been monitored using midgap-subthreshold and charge-pumping techniques, the use of which in tandem has enabled an insight into behaviours of fixed and switching traps in the gate oxide and oxide/silicon interface. The repetition of the stress/annealing sequence has resulted in some quantitative but no qualitative differences in response compared to the original sequence. We have observed complex kinetics of different types the stress-induced defects during post-HEFS annealing, including an intriguing latent buildup of “true” interface traps. Comparison of post-HEFS and post-irradiation annealing data has indicated differences of the nature of defects induced by Fowler-Nordheim injection and irradiation in our samples.     

Nondestructive analysis of structural defects in wide bandgap II-VI heterostructures

X-ray scattering measurements of wide bandgap II-VI heterostructures show that stacking faults (which nucleate defects that are responsible for optical degradation of light emitting diodes and lasers) introduce significant levels of diffuse scattering near Bragg reflections of both the epitaxial layers and the GaAs substrate. We employed triple axis x-ray diffraction techniques to investigate stackingfault diffuse scattering and used cathodoluminescence and transmission electron microscopy to independently measure the stacking fault density. For comparison, double axis scans from the same samples were largely incapable of detecting the presence of these defects. Measurements taken at different azimuthal positions exhibit different levels of diffuse scattering and the diffuse scattering intensity is related to the stacking fault intensity in each <110> direction, which suggests that this technique can provide a non-destructive assessment of defects present in these systems. For some samples, the ZnSe buffer layer exhibited a tilt with respect to the substrate along a <110> direction; this tilt was greater than the tilt which would be attributed to growing a strained layer on the slightly miscut substrates which were used here.     

Impact of strain and source/drain engineering on the low frequency noise behaviour in n-channel tri-gate FinFETs

The impact of strain-engineering on the low frequency of n-channel tri-gate FinFETs fabricated on silicon on insulator (SOI) substrates noise is reported. The work is first focused on the study of nFinFETs with a standard structure and with strain-engineered channel structures, using either global or local straining techniques, or a combination of both. A carrier number fluctuation dominant flicker noise has been observed for all devices. Whereas no clear correlation between the applied strain techniques and the 1/f noise level has been found, an unusual noise spectral density was observed for the devices with selective epitaxial grown (SEG) source and drain regions. This unusual noise behaviour has been investigated for different fin widths (0.15 μm up to 3 μm) and different temperature conditions (150 K up to 300 K). An empirical model is proposed in order to explain this unusual noise behaviour. Moreover, two Lorentzians attributed to defects in the depletion region of the silicon fin were observed, and energy level and cross-section of these defects were estimated.     

Thermoelectric Effect Spectroscopy and Photoluminescence of High-Resistivity CdTe:In

Thermoelectric effect spectroscopy and photoluminescence techniques were used to study the defect levels in samples from three crystals of CdTe:In grown by the vertical gradient freeze method. The main goal of the investigation was to study defects, which strongly trap charge carriers or act as recombination centers in order to eliminate them from the technological process. The main difference among detecting and non-detecting samples was the absence of electron traps with a very high capture cross-section and energy 0.6 eV to 0.7 eV, which act as lifetime killers even at low concentrations. Recently published ab initio calculations show a complex of Te antisite and Cd vacancy within this energy range.     

Relaxation of defects in amorphous barium titanate thin films

Relaxation of electrical defects in amorphous barium titanate thin films was studied by the thermally stimulated current (TSC) technique. We were able to detect at least three relaxation peaks in the TSC spectra. One is associated with activation energy of 0.65 eV and is possibly related to electronic trap levels below the conduction band. Another one is associated with activation energy close to 1 eV corresponding to the migration of positively charged oxygen vacancies within the films.     

Imaging Dislocations in Single-Crystal SrTiO<Subscript>3</Subscript> Substrates by Electron Channeling

Electron channeling contrast imaging (ECCI) using a conventional pole-mounted backscatter detector in a commercial scanning electron microscope (SEM) has been implemented to analyze extended defects. In-grown extended defects in bulk SrTiO₃ (001) substrates such as dislocation loops, subsurface dislocations parallel to the surface, and surface-penetrating dislocations have been distinguished by ECCI. The techniques of dislocation-selective etching and ECCI have been compared side-by-side, where surface-penetrating dislocations have been shown to have one-to-one correspondence with ECCI spot features. g·b and g·b × u have been implemented for subsurface portions of intrinsic dislocations, being identified as screw dislocations with Burgers vector in the 〈100〉 direction.     

Carbon-related defects in microelectronics

In the present study electrically active carbon and hydrogen-related (CH) defects, which can act as strong recombination centers in high power devices and CMOS photodetectors, are investigated in n-type Si. Several different CH-related defects are observed by using the deep level transient spectroscopy (DLTS) technique on hydrogenated Si samples with different oxygen content. The concentration of these defects is determined as low as 10¹²–10¹³ cm^− 3. By comparing samples with different O, C, and H concentrations the origin of the CH-related defects is derived. We show that the concentration of the electrically inactive substitutional C can be estimated by a comparison of the depth profiles of the electrically active CH-related defects in a sample with those in a reference sample which has an identical oxygen and known carbon content. This approach is applicable even for concentrations of substitutional C lower than 10¹⁵ cm^− 3.     

Investigation of electrically active defects in amorphous barium titanate thin films

DC and AC electrical properties of amorphous barium titanate thin film capacitors have been investigated as a function of temperature. A clear correlation is found between the temperature dependence of DC leakage currents and the temperature variation of the AC loss peaks, showing that these measurement techniques are probing the same electrical defects. Using either of these two techniques in amorphous barium titanate, we were able to detect oxygen vacancies diffusion with activation energy around 1 eV, and electron traps at 0.3 and 0.4 eV.     

Spin-coated PMMA films

Polymethylmethacrylate (PMMA) spin-coated thin films are commonly used as resist films in micro/nanofabrication processes. By using atomic force microscopy (AFM) imaging, scratching lithography and force–distance curves spectroscopy, the spin coating and post-processing conditions were determined, for obtaining films whose surface morphology appears featureless or is dominated by pinholes and other surface defects. Featureless appear the surfaces of films spin coated at 8 krpm from a 1.25% solution on silicon substrates and postbaked at 200 °C for 2 min on a hot plate, while surface defects in the form of large circular pits with diameters between 10 and 20 μm and depth of ∼2 nm dominate the surface morphologies of films spin coated at 7 krpm on glass substrates from a 2% solution and postbaked either at 200 °C for 2 min on a hot plate or at 170 °C for 30 min in an oven. Surface defects in the form of pinholes appear on the surfaces of films spin coated at 8 krpm on silicon substrates from a 1.25% solution (thickness of ∼8 nm) and postbaked at 170 °C for 60 min in an oven or left in a low vacuum chamber for a few days. The implication of the different film properties—depending on the preparation parameters—in lithographic techniques is explained and discussed in the paper.     

Comparative Studies of Carrier Dynamics in 3C-SiC Layers Grown on Si and 4H-SiC Substrates

Time-resolved nonlinear optical techniques were applied to determine the electronic parameters of cubic silicon carbide layers. Carrier lifetime, τ, and mobility, μ, were measured in a free-standing wafer grown on undulant Si and an epitaxial layer grown by hot-wall chemical vapor deposition (CVD) on a nominally on-axis 4H-SiC substrate. Nonequilibrium carrier dynamics was monitored in the 80 K to 800 K range by using a picosecond free carrier grating and free carrier absorption techniques. Correlation of τ(T) and μ _a(T) dependencies was explained by the strong contribution of diffusion-limited recombination on extended defects in the layers. A lower defect density in the epitaxial layer on 4H-SiC was confirmed by a carrier lifetime of 100 ns, being ~4 times longer than that in free-standing 3C.     

Time dependent breakdown characteristics of parylene dielectric in metal–insulator–metal capacitors

In this work, we present reliability results of MIM (Metal–Insulator–Metal) capacitors fabricated with parylene as the dielectric, deposited at room temperature. We have evaluated the time dependent dielectric breakdown (TDDB) of parylene-based MIM capacitors as a function of constant DC voltage stress, area and dielectric thickness of the capacitor. Mean-time-to-failure (MTTF) of parylene evaluated at different stress voltages shows a power law distribution over the applied voltage range and device area, with MTTF driven by the number of defects. Defect density in the parylene capacitors is also reported and is calculated to be ～1.2 × 10³ defects/cm².     

DLTS characterization of defects in GaN induced by electron beam exposure

The deep level transient spectroscopy (DLTS) technique was used to investigate the effects of electron beam exposure (EBE) on n-GaN. A defect with activation energy of 0.12 eV and capture cross section of 8.0×10^–16 cm² was induced by the exposure. The defect was similar to defects induced by other irradiation techniques such as proton, electron, and gamma irradiation. In comparison to GaN, the EBE induced defects in other materials such as Si and SiC are similar to those induced by other irradiation methods.     

Comparison of the surface morphology and microstructure of in situ and ex situ derived YBa2Cu3O7−x thin films

The surface morphology and microstructure of in situ and ex situ derived YBa₂Cu₃O_7−x (YBCO) thin films have been investigated. In situ films were deposited by single-target off-axis sputtering and three-target co-sputtering. Ex situ films were derived by metalorganic deposition (MOD) of trifluoroacetate precursors. Surface defects resulting from mixed a-axis and c-axis orientation as well as secondary phases have been identified in these films. Despite these defects, films with excellent electrical properties have been formed. However, defects interfere with film patterning and the fabrication of multi-layered structures. Several secondary phase precipitates have been identified, including CuO, Y₂O₃, Cu-Ba-O, and Y₂Cu₂O₅. Secondary phases resulting from a lack of stoichiometry can be eliminated by direct composition control in the MOD and three-target sputtering techniques, and by composition control through the application of an externally applied magnetic field in single-target off-axis sputtering. Secondary phases caused by contamination were also identified: Cr-Ba-O in off-axis sputtering, resulting from contamination by the oxidized heater block; and BaSO₄ in MOD, resulting from gas phase impurities. These results suggest that cation composition control is not sufficient to prevent the formation of secondary phases and that small levels of contamination may prevent phasepure material from being formed.     

Quantitative and Depth-Resolved Investigation of Deep-Level Defects in InGaN/GaN Heterostructures

Deep-level defects in In_0.17Ga_0.83N/In_0.02Ga_0.98N/p-GaN:Mg heterostructures were studied using deep-level optical spectroscopy (DLOS). Depth-resolved DLOS was achieved by exploiting the polarization-induced electric fields to discriminate among defects located in the In_0.17Ga_0.83N and the In_0.02Ga_0.98N regions. Growth conditions for the In _x Ga_1−x N layers were nominally the same as those in InGaN/GaN multi-quantum-well (MQW) structures, so the defect states reported here are expected to be active in MQW regions. Thus, this work provides important insight into defects that are likely to influence MQW radiative efficiency. In_0.17Ga_0.83N-related bandgap states were observed at E _v + 1.60 eV and E _v + 2.59 eV, where E _v is the valence-band maximum, compared with levels at E _v + 1.85 eV, E _v + 2.51 eV, and E _v + 3.30 eV in the In_0.02Ga_0.98N region. A lighted capacitance–voltage technique was used to determine the areal density of deep states. The possible origins of the associated defects are considered along with their potential roles in light-emitting diodes.     

Investigation of deep level defects in copper irradiated bipolar junction transistor

Commercial bipolar junction transistor (2N 2219A, npn) irradiated with 150 MeV Cu¹¹⁺-ions with fluence of the order 10¹² ions cm^?2, is studied for radiation induced gain degradation and deep level defects. I–V measurements are made to study the gain degradation as a function of ion fluence. The properties such as activation energy, trap concentration and capture cross-section of deep levels are studied by deep level transient spectroscopy (DLTS). Minority carrier trap levels with energies ranging from E_C ? 0.164 eV to E_C ? 0.695 eV are observed in the base–collector junction of the transistor. Majority carrier trap levels are also observed with energies ranging from E_V + 0.203 eV to E_V + 0.526 eV. The irradiated transistor is subjected to isothermal and isochronal annealing. The defects are seen to anneal above 350 °C. The defects generated in the base region of the transistor by displacement damage appear to be responsible for transistor gain degradation.     

Performance of 1 eV GaNAsSb‐based photovoltaic cell on Si substrate at different growth temperatures

We report the performance of a 1 eV GaNAsSb photovoltaic cell grown on Si/Si–Ge substrate using molecular beam epitaxy at different growth temperatures. The sample grown at 420°C showed the highest energy conversion efficiency, with a short circuit current of 18 mA/cm² and open circuit voltage of 0.53 V. With different growth temperature, performance of the cells degrade, which is attributed to the increase of nitrogen‐related defects and the decrease of antimony incorporation at higher growth temperature. Copyright © 2017 John Wiley & Sons, Ltd.     

Large Tolerance of Lasing Properties to Impurity Defects in GaAs(Sb)-AlGaAs Core-Shell Nanowire Lasers

GaAs-AlGaAs based nanowire (NW) lasers hold great potential for on-chip photonic applications, where lasing metrics have steadily improved over the years by optimizing resonator design and surface passivation methods. The factor that will ultimately limit the performance will depend on material properties, such as native- or impurity-induced point defects and their impact on non-radiative recombination. Here, the role of impurity-induced point defects on the lasing performance of low-threshold GaAs(Sb)-AlGaAs NW-lasers is evaluated, particularly by exploring Si-dopants and their associated vacancy complexes. Si-induced point defects and their self-compensating nature are identified using correlated atom probe tomography, resonant Raman scattering, and photoluminescence experiments. Under pulsed optical excitation the lasing threshold is remarkably low (<10 µJ cm⁻²) and insensitive to impurity defects over a wide range of Si doping densities, while excess doping ([Si]>10¹⁹ cm⁻³) imposes increased threshold at low temperature. These characteristics coincide with increased Shockley-Read-Hall recombination, reflected by shorter carrier lifetimes, and reduced internal quantum efficiencies (IQE) . Remarkably, despite the lower IQE the presence of self-compensating Si-vacancy defects provides an improved temperature stability in lasing threshold with higher characteristic temperature and room-temperature lasing. These findings highlight an overall large tolerance of lasing metrics to impurity defects in GaAs-AlGaAs based NW-lasers.     

Solid-state solutions of copper indium disulfide and zinc indium tetrasulfide: Growth,crystallography and opto-electronic properties

Copper indium disulfide–zinc indium tetrasulfide solid solutions with different contents of zinc indium tetrasulfide, ranging from 4 to 16 mol% were grown by the horizontal modification of the Bridgman–Stockbarger method. Their structural properties were investigated by the X-ray analysis. Spectral dependences of their photoconductivity were analyzed at T≈30 K. In the single crystals with 8–12 mol% of zinc indium tetrasulfide the induced photoconduction phenomenon was observed. It could be explained by the model of three recombination and trapping centers with different capture cross sections. Indium vacancies V_In or substitutional defects Cu_In are possibly the fast recombination centers; meanwhile copper vacancies V_Cu act as the slow recombination centers. The presence of electrically active shallow defects was confirmed by the measurements of the temperature dependences of electrical conductivity and thermally stimulated currents of these samples with n-type conductivity.     

Electrical characterization of defects introduced during metallization processes in n-type germanium

We have studied the defects introduced in n-type Ge during electron beam deposition (EBD) and sputter deposition (SD) by deep-level transient spectroscopy (DLTS) and evaluated their influence on the rectification quality of Schottky contacts by current–voltage (I–V) measurements. I–V measurements demonstrated that the quality of sputter-deposited diodes are poorer than those of diodes formed by EBD. The highest quality Schottky diodes were formed by resistive evaporation that introduced no defects in Ge. In the case of EBD of metals the main defect introduced during metallization was the V–Sb complex, also introduced during by electron irradiation. The concentrations of the EBD-induced defects depend on the metal used: metals that required a higher electron beam intensity to evaporate, e.g. Ru, resulted in larger defect concentrations than metals requiring lower electron beam intensity, e.g. Au. All the EBD-induced defects can be removed by annealing at temperatures above 325 °C. Sputter deposition introduces several electrically active defects near the surface of Ge. All these defects have also been observed after high-energy electron irradiation. However, the V–Sb centre introduced by EBD was not observed after sputter deposition. Annealing at 250 °C in Ar removed all the defects introduced during sputter deposition.