Index to NASA Tech Briefs, 1975

This index contains abstracts and four indexes--subject, personal author, originating Center, and Tech Brief number--for 1975 Tech Briefs

Amorphous Silicon Thin Film Transistor Fabrication and Models

One of the primary purposes of this research was to develop techniques to improve the quality of vacuum evaporated amorphous silicon (a-Si), i.e. lower the density of localized states in the mobility gap. The electron beam evaporation of amorphouss silicon and hydrogenation by ion implanting has proved promising. This technique permits independent control of amorphous silicon disorder and the hydrogenation level, thereby separating the process of hydrogenation from that of film deposition. Electrical measurement of field effect conductance changes was used as a probing tool to monitor changes in the properties of a-Si before and after hydrogenation. Field effect data was transcribed by a computer program to determine the density of localized states. Amorphous silicon films were prepared by electron beam evaporation of a high purity silicon onto the surface of a thermally oxidized crystalline silicon substrate. The films were deposited at a fixed rate in a high vacuum. Immediately after deposition, some films were subjected to in situ thermal anneal and some films were not. A Comparison of the results of these two eases revealed the porous nature of evaporated a-Si. Hydrogen incorporation into a- Si films was performed by ion implantation followed by a low temperature thermal activation of the hydrogen. After hydrogenation, a field effect conductance change of four orders of magnitude was observed on the devices which were not in situ thermally annealed. A comparison before and after hydrogenation demonstrates that almost three orders of magnitude reduction (from about 1022 to about 1019/cm3-eV) in the density of localized states near the Fermi level (N F/T) was achieved. Varying the hydrogen implantation dosage between lxlO16 to 1.5xl017/cm2, with all other sample preparation procedures fixed, caused a decrease in NF/T from 8.6xl020 to ixl019/cm3-eV. The effect of in situ thermal annealing prior to hydrogen implantation was also investigated. By performing a 400°C anneal for four hours immediately following film deposition the film porosity was greatly reduced. The film was then implanted with hydrogen to a total dose of lxl017/cm2. A field effect Conductance change of six orders of magnitude was observed which yielded a N|F/T of 4xl017/cm3-eV, approaching that of glow discharge produced films. The second purpose of the research was to develop modeling techniques for the a-Si:H TFT. Despite rapid progress in the TFT performence, [performance] the theoretical basis to determine static- and dynamic-characteristics of TFTs has not yet been determined mainly because the influence of the localized states on TFT operation is very complicated. The theoretical expression of drain current as a function of gate bias and drain voltage was derived. To use the theoretical expressions, the localized state density distribution N(E) must be known, A derived yet practical formula for the N(E) did not exist. A common way is to use the experiment of field effect conductance change to determine the N(E), With the data theoretical expressions the localized state density N(E) could be calculated by using a numerical technique, but it is cumbersome and connot [cannot]be determined uniquely. As a design tool for devices and circuits, a simple theory which can express concisely the TFT characteristics is very important. In this report, several models for N(E) are listed:. Approximate analyses for characteristics pf a-Si:H TFT are derived. In two special cases, i.e. uniform localized state density distribution and exponential localized distribution, some useful approximate expressions was obtained. Compared with the experiment data, the uniform density distribution of localized state model is a good approximate expression for a large density discribution [distribution] of localized states near Fermi level. The exponential model is a good approximate expression for lower density distribution of localized states near Fermi level

A Positive Ion Beamline for Space Qualification of Birefringent Materials

The constant advancements in spaceborne technology have provided an immense increase to the boundaries of human knowledge in a variety of research fields. As these continue, and new technologies arise, their suitability for deployment in the space environment must be assessed due to the harsh operating environment of space. One component of the space environment is intense radiation, specifically charged particle radiation, which can cause damage to a variety of system components. Effects include changes to electrical, structural, and optical properties, the latter of which is the focus of this work. A recently introduced technology to spaceborne imaging instrumentation is Acousto-Optic Tunable Filters. These devices use the Acousto-Optic effect and birefringent materials, such as tellurium dioxide and lithium niobate, to create narrow band image quality tunable filters. As common radiation damage effects include changes to transmittance, reflectance and absorbance of optical materials, as well as changes to the atomic structure causing changes to refractive indices and birefringence, radiation testing of these devices to assess long term performance is critical to further development of the technology for space applications. Radiation testing involves accelerated lifetime testing of materials under multiple years' worth of equivalent radiation in much shorter time frames (hours), using charged particle radiation provided by an ion accelerator. This work details the development of a positive ion accelerator and its use for radiation testing. The accelerator can provide beam energies from 5 - 20 keV, beam diameters of 0.8 - 2.5 cm and beam currents from 0.5 - 15 $\mu$A, all adjustable by user input settings. The system can also accommodate other ion species such as helium ions. The system was primarily used with proton radiation, due to its dominance in the solar wind and general space environment, to examine induced damage effects in silicon, quartz, lithium niobate and tellurium dioxide as a function of fluence (protons/$\text{cm}^2$). Measurements of transmittance, reflectance and absorbance, as well as an investigation with Raman spectroscopy, were completed for all materials at varying fluences. Comparison of results to those in the literature shows good agreement, however, not all results have comparable data available in the current literature. Results are used to assess space mission suitability and show that tellurium dioxide has the highest radiation resistance of the investigated materials

The Conference on High Temperature Electronics

The status of and directions for high temperature electronics research and development were evaluated. Major objectives were to (1) identify common user needs; (2) put into perspective the directions for future work; and (3) address the problem of bringing to practical fruition the results of these efforts. More than half of the presentations dealt with materials and devices, rather than circuits and systems. Conference session titles and an example of a paper presented in each session are (1) User requirements: High temperature electronics applications in space explorations; (2) Devices: Passive components for high temperature operation; (3) Circuits and systems: Process characteristics and design methods for a 300 degree QUAD or AMP; and (4) Packaging: Presently available energy supply for high temperature environment

Reliable Nanofabrication of Single-Crystal Diamond Photonic Nanostructures for Nanoscale Sensing

In this manuscript, we outline a reliable procedure to manufacture photonic nanostructures from single-crystal diamond (SCD). Photonic nanostructures, in our case SCD nanopillars on thin (< 1$\mu$m) platforms, are highly relevant for nanoscale sensing. The presented top-down procedure includes electron beam lithography (EBL) as well as reactive ion etching (RIE). Our method introduces a novel type of inter-layer, namely silicon, that significantly enhances the adhesion of hydrogen silsesquioxane (HSQ) electron beam resist to SCD and avoids sample charging during EBL. In contrast to previously used adhesion layers, our silicon layer can be removed using a highly-selective RIE step which is not damaging HSQ mask structures. We thus refine published nanofabrication processes to ease a higher process reliability especially in the light of the advancing commercialization of SCD sensor devices.Comment: v2: accepted for publication in Micromachines 2019, 10(11), 718; https://doi.org/10.3390/mi1011071

Femtosecond Laser Written Volumetric Diffractive Optical Elements And Their Applications

Since the first demonstration of femtosecond laser written waveguides in 1996, femtosecond laser direct writing (FLDW) has been providing a versatile means to fabricate embedded 3-D microstructures in transparent materials. The key mechanisms are nonlinear absorption processes that occur when a laser beam is tightly focused into a material and the intensity of the focused beam reaches the range creating enough free electrons to induce structural modification. One of the most useful features that can be exploited in fabricating photonic structures is the refractive index change which results from the localized energy deposition. The laser processing system for FLDW can be realized as a compact, desktop station, implemented by a laser source, a 3-D stage and focusing optics. Thus, FLDW can be readily adopted for the fabrication of the photonic devices. For instance, it has been widely employed in various areas of photonic device fabrication such as active and passive waveguides, couplers, gratings, opto-fluidics and similar applications. This dissertation describes the use of FLDW towards the fabrication of custom designed diffractive optical elements (DOE’s). These are important micro-optical elements that are building blocks in integrated optical devices including on-chip sensors and systems. The fabrication and characterization of laser direct written DOEs in different glass materials is investigated. The design and performance of a range of DOE’s is described, especially, laser-written embedded Fresnel zone plates and linear gratings. Their diffractive efficiency as a function of the fabrication parameters is discussed and an optimized fabrication process is realized. The potential of the micro-DOEs and their integration shown in this dissertation will impact on the fabrication of future on-chip devices involving customized iv DOEs that will serve great flexibility and multi-functional capability on sensing, imaging and beam shaping

Epitaxial growth of silicon on oxygen implanted substrates

The feasibility of growing epitaxial layers of silicon on silicon substrates with a buried oxide layer formed by the implantation of oxygen ions, has been studied. Conditions for epitaxial growth from a silane source in a reactor, built and commissioned as a part of the programme, have been established. Buried implanted oxide layers have been formed by high dose implantation of oxygen ions in silicon. The effects of dose at a given energy, energy for a given peak concentration, and temperature on the distribution profile of oxygen have been studied. An approximate Gaussian distribution is observed at doses contributing less than the stoichiometric requirement of oxygen for the formation of silicon dioxide. A saturation in the oxygen content is reached when the stoichiometric requirement is exceeded. A consequent reduction in the interface damage is also observed. Other parameters being equal, at higher substrate temperatures the interface damage is decreased. It has been attempted to optimise conditions for a dose of 1.4 x 1018 0+.cm-2 at 200 keY which provides the stoichiometric concentration only at the peak of the distribution. The epitaxial layers deposited on substrates maintained at 5500C during implantation have a crystalline quality comparable to those of layers on untreated substrates. Fabricated p-n junction diodes have low leakage currents and high breakdown voltages. The minority carrier lifetime is comparable to that in diodes processed similarly but without an implanted oxide layer