GaN power amplifiers based on AlGaN-GaN high electron mobility transistor (HEMT) technology are produced for defense applications that require high voltage, high power, and high efficiency operation. Many groups have reported various failure mechanisms in GaN HEMTs over the last two decades. However, the fact that no dominant failure mode and mechanism has been identified is a major concern for space applications. Earlier our group reported radiation effects of commercial high-power GaN HEMTs exposed to protons and heavy ions as well as failure mechanisms in RF GaN HEMTs fabricated per our design. For the present study, we continued to investigate RF AlGaN-GaN HEMTs grown on SiC substrate with Ni-Pd-Au Schottky contacts. Our GaN HEMTs had a Schottky gate length of 250 nm, a total gate width of 6 × 150 μm periphery, and a field plate. Our HEMTs were irradiated with 50 MeV protons at the Lawrence Berkeley National Laboratory, with three different doses of 42, 160, and 889 krad(Si) for this experiment. All our irradiated devices were exposed to protons while they were shorted. First, pre- and post-irradiation DC I-V characteristics were compared, and DC bias-temperature stress tests were performed on unirradiated HEMTs. Some HEMTs were thermally stressed as monitor samples for comparison. Second, we employed focused ion beam (FIB) techniques to prepare TEM specimens from degraded devices for defect and chemical analysis using a high resolution TEM. AFM techniques were also employed to study gate pitting.
This study analyzes the elevated temperature tensile results of SLM IN718 as a function of strain rate and test temperature in order to better understand the temporal and thermal aspects of environmental sensitivity. Fully heat-treated SLM samples are directly compared to wrought material and corresponding industry standards in order to provide a valuable perspective on the current state of SLM capabilities. It is found that SLM material tested across all conditions have inferior strength and ductility compared to wrought material of the same heat treatment. Strength variation is attributed to different sizes of the primary strengthening phase, γ’’, while ductility variation is caused by environmental sensitivity. SLM samples show evidence of brittle intergranular fracture, crack growth, and oxidized NbC particles on the fracture surface. These features are intensified with decreasing strain rate and increasing temperature. EBSD-generated misorientation maps and strain rate sensitivity calculations demonstrate that the mechanism of plastic deformation is similar between the two processing conditions but wrought material has a greater overall damage tolerance. Premature failure attributed to intergranular crack growth leads to poor ductility in SLM material. Faster strain rates and lower temperatures are shown to improve the ductility in SLM IN718 but despite this recovery it remains susceptible to environmental attack even in the extreme cases of the current study. Sources of environmental sensitivity and the degree to which they affect elevated temperature mechanical properties are discussed.
GaN high electron mobility transistors (HEMTs) on SiC substrates are produced for both commercial and defense applications that require high voltage, high power, and high efficiency operation. Although leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability, long-term reliability in the space environment remains a major concern due to unknown degradation mechanisms. For the present study, we continued to investigate degraded RF GaN HEMTs using micro-analytical techniques. Our RF AlGaN-GaN devices grown on SiC substrate had a Ni-Pd-Au Schottky gate length of 0.25 μm, a total gate width of 6 × 150 μm periphery, and a field plate. First, we performed DC bias-temperature stress tests on GaN HEMTs and some GaN HEMTs were thermally stressed as control samples. Second, we employed focused ion beam (FIB) to prepare TEM cross sections from degraded and control devices for defect analysis using a high resolution TEM. Pits, cracks, and pockets of palladium were found at the edge of the drain side of the gate. We report our detailed analysis results including our understanding on the out-diffusion of Pd as a potential degradation mechanism in our RF GaN HEMTs.
GaN high electron mobility transistors (HEMTs) on SiC substrates are produced for both commercial and defense applications that require high voltage, high power, and high efficiency operation. Although leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability, long-term reliability in the space environment remains a major concern due to unknown degradation mechanisms. For the present study, we investigated stressed/degraded RF GaN HEMTs using micro-analytical techniques. Our RF AlGaN-GaN devices grown on SiC substrate had a Ni-Pd-Au Schottky gate length of 0.25 μm, a total gate width of 6 × 150 μm periphery, and a field plate. First, we performed DC bias-temperature stress tests on GaN HEMTs and some GaN HEMTs were thermally stressed as monitor samples. Second, we employed focused ion beam (FIB) to prepare TEM cross sections from degraded and monitor devices for defect analysis using a high resolution TEM. Defects containing highly Pd-enriched features were found at the edge of the drain side of the gate. We present our detailed analysis results including our understanding on the out-diffusion of Pd as a potential degradation mechanism in our RF devices.
Leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability. However, long-term reliability in the space environment still remains a major concern due to a number of defects and traps as well as unknown degradation mechanisms. Thus, careful study of reliability and radiation effects of GaN HEMTs should be performed before GaN HEMT technology based solid state power amplifiers (SSPAs) are successfully deployed in space satellite systems. We studied both RF GaN HEMTs fabricated per our design and commercial high-power GaN HEMTs, both grown on SiC substrates. Our RF devices had a nominal Ni-Au Schottky gate length of 0.25 μm, a total gate width of 6 × 150 μm periphery, and a field plate, while high-power devices had a Ni-Au Schottky gate length of 0.4 μm, a total gate width of 10 × 350 μm periphery, and a field plate. First, DC and RF characteristics of RF devices were compared before and after they were aged under different conditions (DC and temperature). Focused-ion-beam was employed to prepare TEM cross sections from degraded devices for defect analysis using a high-resolution TEM. Also, we performed strain analysis on pristine and degraded devices using TEM-based techniques. Second, DC characteristics of high-power devices were compared before and after they were irradiated with protons and heavy ions. Some of devices were exposed while they were unbiased, DC biased, and DC and RF biased.
While various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure of plasma beam sputtered silver mirrors was used to investigate how the composition of the very thin adhesion layer changes the mechanism of corrosion feature growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above were varied. Optical measurements and microscopy, SEM, TEM, and EDS were used to characterize the compositional and chemical effects at the layer interfaces. Large circular corrosion features formed along the silver-chromium interfaces; the addition of nickel to the layers on either side of the silver limited the growth of these features, but resulted in the corrosive attack of the silver itself.
Electron BackScatter Diffraction (EBSD) is a relatively new, scanning electron microscope-based technique used to characterize microstructures and textures in crystalline metal and ceramic materials. Advances in SEM technology, especially the development of field emission SEMs, as well as in EBSD detector design have allowed characterization at the sub-10 nm level. This paper gives a basic introduction to the EBSD technique with applications on materials with microstructures on the micron and sub-micron scale. Automated EBSD mapping at these and other resolution levels is used to study texture, individual grain orientation, crystallography-based phase identification, grain size, grain shape, strain state, grain boundary character, area percentages of multiple phases in bulk samples, crystallography of facets and failure initiation sites, and other materials characteristics. Sample sectioning and polishing are often necessary for mapping microstructures in bulk samples, however as-grown structures such as thin films and interconnects are suitable for mapping as is, and “point & shoot” type analyses may be used on other unprepared samples in conjunction with SEM imaging for phase identification and basic crystallographic orientation studies. For micron-scale devices and components, EBSD-equipped dual beam FIBs are used to select cross-sectional planes of analysis with high precision.
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