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Process-induced defects in electroplated Au interconnect metallization on GaAs devices were detected during the course of reliability testing. Abnormally high lognormal sigma values (σ > 0.7) indicated the existence of a bi-modal failure mechanism. A distinct early lifetime failure mode was observed along with the intrinsic electromigration metallization wear-out failure mode. Physical characterization of the electroplated Au film revealed as-deposited nanoscale voids. Elimination of these voids through process improvement as well as suggested mechanisms for the early failures are discussed.
In this work, we have reported the interface characterization of rf sputtered ZnO/HfO2 in thin film transistor structure by dc current-voltage and admittance spectroscopy. The interface state density (Dit) of 1013 eV−1cm−2 was extracted from the Gp/ω vs ω plot was comparable to value obtained from the subthreshold behavior. The grain boundary trap density (NGB) of 9.12×1012 cm−2 was estimated using Levinson’s model. The interface state density distribution below the conduction band edge shows a decreasing trend with energy below the conduction band edge. We also studied the impact of introducing MgO interfacial layer between ZnO and HfO2 interface as an approach towards decreasing the interface state density.
Strontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity.
In this paper, we discuss the characteristics of the InGaP/GaAs heterojunction phototransistors (HPTs) before and after the electrical stress at room temperature and assess the effectiveness of the emitter-ledge passivation. Although an electrical stress given to the phototransistors by keeping a collector current density of 37 A/cm2 for 1 hour at room temperature was too small to affect the room-temperature common-emitter current gain and photocurrent of both HPTs with and without the emitter-ledge passivation, they showed a significant decrease at 420 K due to the room-temperature electrical stress. Nevertheless, the room-temperature common-emitter current gain and photocurrent of the HPT with the emitter-ledge passivation were still higher than those of the HPT without the emitter-ledge passivation. The effectiveness of the emitter-ledge passivation against the electrical stress was more significant than that on the current gain in the dark. In addition to the electrical stress experiment, for a potential application of the InGaP/GaAs HPTs in space, we will irradiate the HPTs with 1-MeV electrons at the Japan Atomic Energy Agency. Both current gain and photocurrent decreased significantly after the electron irradiation. In contrast to the electrical stress, the damage due to the high-energy electron irradiation is a bulk-related phenomenon, and the emitter-ledge passivation does not seem to suppress the degradation.
The influence of threading dislocations (TDs) on the dry thermal oxidation of c-plane gallium nitride (GaN) is investigated for oxidation temperatures above 800°C. The transformation of GaN to gallium oxide (Ga2O3) is preferably found at TDs and grain boundaries, showing enhanced vertical oxidation, compared to defect free surface sites. Therefore, the increase in surface roughness commonly obtained upon oxidation is explained by an inhomogeneous chemical reactivity associated with those crystal defects. Additionally, annealing in an N2 atmosphere showed that also decomposition is favored at such chemically reactive spots. Comparison between decomposition and oxidation suggests that at temperatures above 950°C, the Ga2O3 formation is supported by the decomposition of GaN and subsequent oxidation of the metallic gallium.
Cu (In, Ga) Se2 (CIGS) thin-film solar cells are optimal solar cells for spacecraft, since they have high efficiency, lightweight, flexible and high radiation tolerance. The CIGS solar module without a coverglass to prevent degradation in space has been demonstrated with a small satellite and its electrical performance indicates no degradation as predicted from ground tests. However, the cells need to prevent the damages from other effects in space. The paper introduces some space environment tests and how to improve performance in CIGS cells for spacecraft.
Reliability and degradation processes in broad-area InGaAs-AlGaAs strained quantum well (QW) lasers are under intensive investigation because these lasers are the key components for fiber lasers and amplifiers that have found both industrial and military applications in recent years. Unlike single-mode lasers that were developed for high reliability telecom applications, broad-area lasers were mainly targeted for applications that require less stringent reliability of the lasers until recently. Especially, the lack of field reliability data is a concern for satellite communication systems where high reliability is required of lasers for long-term duration. For our present study, we addressed this concern by performing long-term life-tests of broad-area InGaAs-AlGaAs strained QW lasers and also by studying mechanisms that are responsible for catastrophic degradation of the lasers.
Cadmium Zinc Telluride (CZT) semiconductor crystal properties have been studied extensively with a focus on correlations to their radiation detector performance. The need for defect-free CZT crystal is imperative for optimal detector performance. Extended defects like Tellurium (Te) inclusions, twins, sub-grain boundaries, and dislocations are common defects found in CZT crystals; they alter the electrical properties and, therefore, the crystal's response to high energy radiation. In this research we studied the extended defects in CZT crystals from two separate ingots grown using the low-pressure Bridgman technique. We fabricated several detectors cut from wafers of two separate ingots by dicing, lapping, polishing, etching and applying gold metal contacts on the main surfaces of the crystals. Using infrared (IR) transmission microscope we analyzed the defects observed in the CZT detectors, showing three dimensional scans and plot size distributions of Te inclusions, twins and sub-grain boundaries observed in particular regions of the CZT detectors. We characterized electrical properties of the detectors by measuring bulk resistivity and detector response to gamma radiation. We observed that CZT detectors with more extended defects showed poor opto-electrical properties compared to detectors with fewer defects.
CZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, we are yet to overcome the technological difficulties in production of large size, defect-free CZT crystals. The most common problem is accumulation of Tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. We employed high energy transmission X-ray diffraction techniques to study the quality of the CdZnTe crystals grown by Bridgman Technique. Crystallinity and defects within two different growth set-ups, i.e. with and without choked seeding, were compared by imaging the crystal orientation topography with white beam X-ray diffraction topography (WBXDT). The X-ray diffraction topography results show high correlation with large-area infrared transmission images of the crystals. Grain boundaries that are highly decorated with Te inclusions are observed. Characteristic Te inclusion arrangements as a result of growth conditions are discussed. We also measured the electronic properties of the detectors fabricated from ingots grown using two Bridgman processes, and observed a reduction in electrical resistivity of choked-seeding-grown CdZnTe crystals. Our results show that although choked seeding technique holds a promise in the realization of high quality mono-crystalline CdZnTe, current growth parameters must be improved to obtain defect-free crystals. These results are helpful to attain optimal seeding process for Bridgman-growth of large single crystals of CdZnTe.
Yield shear stress dependence on dislocation density and crystal orientation was studied in bulk GaN crystals by nanoindentation examination. The yield shear stress decreased with increasing dislocation density which is estimated by dark spot density in cathodoluminescence, and it decreased with decreasing nanoindentation strain-rate. It reached and coincided at 11.5 GPa for both quasi-static deformed c-plane (0001) and m-plane (10-10) GaN. Taking into account theoretical Peierls–Nabarro stress and yield stress for each slip system, these phenomena were concluded to be an evidence of heterogeneous mechanism associated plastic deformation in GaN crystal. Transmission electron microscopy and molecular dynamics simulation also supported the mechanism with obtained r-plane (-1012) slip line right after plastic deformation, so called pop-in event. The agreement of the experimentally obtained atomic shuffle energy with the calculated twin boundary energy suggested that the nucleation of the local metastable twin boundary along the r-plane concentrated the indentation stress, leading to an r-plane slip. This nanoindentation examination is useful for the characterization of crystalline quality because the wafer mapping of the yield shear stress coincided the photoluminescence mapping which shows increase of emission efficiency due to reduction of non-radiative recombination process by dislocation.
CZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, technological difficulties in production of large size defect-free CZT crystals are yet to be overcome. The most common problem is accumulation of tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. The common work-around solutions are to fabricate pixelated detectors by either grouping together many small volume CZT crystals to act as individual detectors, or to deposit a pixelated grid of electrical contacts on a larger, but defective, crystal, and selectively collect charge. These solutions are satisfactory in an R&D environment, but are unsuitable for mass production and commercial development. Our modeling effort is aimed at quantifying the various contributions of tellurium inclusions in CZT crystals to the charge generation, transport, and collection, as a function of inclusions size, position, and concentration. We model the energy deposition of gamma photons in the sensitive volume of the detector using LANL’s MCNP code. The electron-hole pairs produced at the energy deposition sites are then transported through the defective crystal and collected as integral charge at the electrical contact sites using CERN’s Garfield software package. The size and position distribution of tellurium inclusions is modeled by sampling experimentally measured distributions of such inclusions on a variety of commercially-grown CZT crystals using IR microscopy and image processing software packages.
CuxO thin films have been deposited on a quartz substrate by reactive radio frequency (rf) magnetron sputtering at different target powers Pt (140-190 W) while keeping other growth process parameters fixed. Room-temperature photoluminescence (PL) measurements indicate considerable improvement of crystallinity for the films deposited at Pt>170 W, with most pronounced excitonic features being observed in the film grown using Pt=190 W. These results corroborate well with the surface morphology of the films, which was found more flat, smooth and homogeneous for Pt >170 W films in comparison with those deposited at lower powers.
The stability of green phosphorescent OLEDs with different structures was evaluated through constant-current stressing. Through the modifications of the ITO anode by different plasma treatments and the hole transport layer (HTL) by incorporating inorganic dopants, we proved that energy level misalignment at the ITO/HTL interface leads to localized joule heating, accelerating defect generation and luminescence decay. Pulsed current stressing was then employed to suppress the joule-heating effect so as to differentiate the thermal and nonthermal factors governing the device degradation. For OLEDs with a large energy barrier at the ITO/HTL interface, the effective lifetime was markedly increased under pulsed operation, whereas in OLEDs with an appropriate interfacial energy level alignment, pulsed stressing with 10% duty cycle only improved the effective half life by ∼15% as compared to continuous-wave stressing, indicating a minor role played by joule heating.
Pm-Si:H PIN and NIP solar cells structures grown using plasma enhanced chemical vapor deposition (PECVD) technique were analyzed during 400 hrs of light-soaking exposition. The evolution of the structural and optical properties was observed and characterized by Raman spectroscopy, spectroscopic ellipsometry. The effect observed is related to defects creation due to induced hydrogen diffusion, break of Si-H bonds and the generation of dangling bonds that causes less passivated films. The film microstructure, and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures presents a slight decrease and it is observed to be between 15 to 24% for the PIN and 5 to 10% for the NIP. The optical gap increases from 1.66 to 1.68 eV for the PIN structure while for the NIP no significant change is observed during light-soaking. Hydrogen diffusion during lights soaking generates a decrease on the absorption properties of the films which in turn is expected to reduce the device efficiency during operation. In this work we show that long range motion of hydrogen during light-soaking causes a hydrogen rearrangement on the film and microstructure changes. We determined that there is not an pronounced change on the film structure during prolonged light exposition related to the stability of the pm-Si:H films. The PIN structure properties are more affected during light soaking in comparison to the NIP structure which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation.
A single-grained Pb(Zr,Ti)O3 (PZT) was successfully grown for the gate dielectric of polycrystalline-silicon (poly-Si) thin-film transistor (TFT). The total structure was MoW/PZT/HfO2/poly-Si/glass. The giant single-grained PZT was obtained by controlling the artificial nucleation formed by Pt dots in a desirable location and enlarging the nucleated seed until it covers the poly-Si channel. The single-grained diameter size was 40 μm with a (100) dominated texture. The poly-Si memory device with single-grained PZT showed an excellent ferroelectric, electrical and reliability properties comparing with poly-Si memory device with poly-grained PZT. Moreover, eliminating the grain boundary in PZT film showed the fatigue and retention characteristics with only 1.1 % after 1013 cycles and 22 % after 1 month, respectively.
Cadmium Zinc Telluride (CZT), considered as a viable material for use in room temperature radiation detectors, has an undesired presence of tellurium inclusions in the bulk. Thermal treatment, in the form of annealing, has been utilized to test the viability of refining CZT into better detector material, either by the elimination of the tellurium inclusions or by the migration of the inclusions under a temperature gradient, but usually with a deterioration of electrical properties. We took infrared micrographs and current voltage (IV) characteristics of CZT samples prior to thermal treatment. We carried out 24-hour thermal treatments with a range of temperature from 100°C to 700°C to determine an optimal annealing temperature and to verify changes in the sizes, morphologies, and locations of the tellurium inclusions on the surfaces and within the crystal bulk of the CZT. The IV curves and resistivities prior to and after thermal treatments were compared, as were the infrared micrographs before and after annealing. Also, the changes in electrical properties of the samples with annealing conditions were compared against structural changes monitored at the same steps during the annealing process, in order to understand the effects of the thermal annealing to the radiation detector properties of the material. Correlations between the shape, size and position of inclusions and electrical properties of the material were attempted.
The physical mechanisms responsible for electrically-induced parametric degradation in GaN-based high electron mobility transistors are examined using a combination of experiments, device simulation, and first-principles defect analysis. A relatively simple formulation is developed under the assumption that the hot-electron scattering cross-section is independent of the electron energy. In this case, one can relate the change in defect concentration to the operational characteristics of a device, such as the spatial and energy distribution of electrons (electron temperature), electric field distribution, and electron energy loss to the lattice.
The reliability of InAlGaN multiple quantum well LEDs emitting around 308 nm has been investigated. The UV-B LEDs were stressed at constant current and current density, while the heat sink temperature was varied between 15°C and 80°C. The results reveal two different modes of the decrease of the optical power during aging. First, a fast reduction of the optical power within the first 100 h (mode 1) can be observed, followed by a slower degradation for operation times >100 h (mode 2). Mode 1 can be described as an initial degradation activation process which saturates after a certain time, whereas the second degradation mode can be described by a square-root time dependence of the optical power, suggesting a diffusion process to be involved. Both degradation modes are accompanied by changes of the I-V characteristic, particularly the reverse-bias leakage current and the drive voltage. Furthermore, the degradation behavior is strongly influenced by the temperature. Both, the maximum reduction of the optical power and the increase of the leakage current become stronger at higher temperatures.
The influence of the substrate temperature on the morphology and ordering of InGaAs quantum dots (QD), grown on GaAs (001) wafers by Molecular Beam Epitaxy (MBE) under As2 flux has been studied using Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and Photoluminescence (PL) measurements. The experimental results show that lateral and vertical orderings occur for temperatures greater than 520°C and that QDs self-organize in a 6-fold symmetry network on (001) surface for T=555°C. Vertical orderings of asymmetric QDs, along directions a few degrees off , are observed on a large scale and their formation is discussed.
AlGaN/GaN High Electron Mobility Transistors were exposed to 60Co gamma-irradiation to doses up to 300Gy. The impact of Compton- electron injection (due to gamma-irradiation) is studied through monitoring of minority carrier transport using Electron Beam Induced Current (EBIC) technique. Temperature dependent EBIC measurements were conducted on devices before and after exposure to the irradiation, which provide us with critical information on gamma-irradiation induced defects in the material. As a result of irradiation, minority carrier diffusion length increases significantly, with an accompanying decrease in the activation energy. This is consistent with the longer life time of minority carrier in the material’s valence band as a result of an internal electron injection and subsequent trapping of Compton electrons on neutral levels.