EAG is your R&D partner, working with aerospace and defense manufacturers and suppliers to create faster, safer and more reliable aircraft and components. We use the most advanced…
EAG is your R&D partner, working with aerospace and defense manufacturers and suppliers to create faster, safer and more reliable aircraft and components. We use the most advanced analytical methodologies to answer complex engineering, materials and manufacturing problems, from evaluating the purity of new materials to confirming the characteristics of ceramic matrix composites. For research, development and production, we work alongside your technical staff to formulate and troubleshoot the next generation of aerospace and defense materials.
For over 50 years, EAG’s scientists have applied advanced analytical techniques to solve tough problems and support R&D activities for manufacturers and users of a wide variety…
For over 50 years, EAG’s scientists have applied advanced analytical techniques to solve tough problems and support R&D activities for manufacturers and users of a wide variety of chemicals and materials. EAG offers reliable analytical results, development support for novel products, and answers to complex problems for manufacturers of chemicals used in metals, fibers, plastics, coatings, adhesives, construction materials, textiles, lubricants and more.
Consumer products manufacturers and partners turn to EAG to answer R&D questions, ensure supply chain quality, respond to regulatory requirements and ensure well-supported legal…
Consumer products manufacturers and partners turn to EAG to answer R&D questions, ensure supply chain quality, respond to regulatory requirements and ensure well-supported legal arguments. EAG scientists have helped all types of consumer products companies accelerate innovation, resolve manufacturing concerns and comply with evolving regulations.
Drive R&D productivity and keep pace with evolving environmental regulations.
From agrochemical development to evaluating innovative packaging solutions, EAG scientists support…
Drive R&D productivity and keep pace with evolving environmental regulations.
From agrochemical development to evaluating innovative packaging solutions, EAG scientists support the agriculture/food s industry with scientific expertise required to enhance shelf life, ensure product safety and understand environmental impact. Whether in need of food quality investigations or help supporting regulatory filing of new chemicals, ask EAG. WE KNOW HOW.
When a business’s brand and reputation is at stake, clients turn to EAG’s litigation support services for dependable scientific answers. Since 1959, our scientific expertise,…
When a business’s brand and reputation is at stake, clients turn to EAG’s litigation support services for dependable scientific answers. Since 1959, our scientific expertise, testing and testimony have supported legal strategies for intellectual property, product liability and insurance cases. Our scientists offer expertise in technology, life sciences, electronics, industrial and consumer products, having tested a broad array of materials for legal challenges. We understand the unique needs of legal projects – speed, effective communication, reliability and confidentiality.
From initial concept and prototypes to verification/validation, submission and launch, EAG Laboratories is your partner in medical device development. From implant…
From initial concept and prototypes to verification/validation, submission and launch, EAG Laboratories is your partner in medical device development. From implant biocompatibility studies to surface chemistry for contact lenses, we have the experience and techniques to characterize, test and assure purity of medical devices and their materials, components, products and packaging. By utilizing our unique expertise in materials sciences, chemistry, electronics and regulatory support, EAG empowers device companies to create new ways to heal and improve patient quality of life.
Overcome R&D roadblocks with technical and regulatory know-how. For over two decades, EAG scientists have helped solve complex issues for the pharmaceutical Industry. Using…
Overcome R&D roadblocks with technical and regulatory know-how. For over two decades, EAG scientists have helped solve complex issues for the pharmaceutical Industry. Using multidisciplinary science and engineering, we help improve product development efficiency, avoid regulatory setbacks and uncover sources of problematic manufacturing issues. From GLP and cGMP studies to contaminant and packaging investigations, the scientists of EAG support every phase of the product lifecycle.
For the high-tech industry, EAG is the leader in materials characterization, surface analysis, microscopy and electronics testing. Our scientists and engineers troubleshoot and…
For the high-tech industry, EAG is the leader in materials characterization, surface analysis, microscopy and electronics testing. Our scientists and engineers troubleshoot and solve issues with consumer electronics, chip packages, printed circuit boards, displays, sensors, high-speed communications and more. We meet the demanding needs of technology innovation, enabling our clients to have confidence in their research & development, supply chains, and ultimately their manufacturing processes
Pharmaceutical & Biopharmaceutical Development Services
EAG brings unparalleled expertise to the development and commercialization of small molecule drugs, biopharmaceuticals, antibody-drug conjugates (ADCs), drug-device combination…
EAG brings unparalleled expertise to the development and commercialization of small molecule drugs, biopharmaceuticals, antibody-drug conjugates (ADCs), drug-device combination products and other therapies. From designing IND-enabling studies to delivering full CMC analytical and QC support, we join your R&D team as a true partner. EAG scientists take time to understand both your commercial goals and the unique characteristics of your compound. We provide expert guidance to balance regulatory expectations with expediency and cost, and approach technical challenges with flexibility and resolve.
When it comes to understanding the physical structure, chemical properties and composition of materials, no scientific services company offers the breadth of experience, diversity…
When it comes to understanding the physical structure, chemical properties and composition of materials, no scientific services company offers the breadth of experience, diversity of analytical techniques or technical ingenuity of EAG. From polymers to composites, thin films to superalloys—we know how to leverage materials sciences to gain a competitive edge. At EAG, we don’t just perform testing, we drive commercial success—through thoughtfully designed investigations, technically superior analyses and expert interpretation of data.
Having helped develop the test methods that shape current regulatory guidelines, EAG chemists, biologists and toxicologists have evaluated the environmental impact of thousands of…
Having helped develop the test methods that shape current regulatory guidelines, EAG chemists, biologists and toxicologists have evaluated the environmental impact of thousands of active ingredients and formulations—from pesticides and pharmaceuticals to industrial chemicals and consumer products. Whether you are exploring “what if” scenarios, registering a new active ingredient or formulation, responding to a data call-in or seeking to understand the latest guidance, turn to EAG for technical excellence, sound advice, GLP-compliant study execution and expert interpretation.
Whether connecting the internet of things, guiding surgical lasers or powering the latest smart phone, integrated circuits and microelectronics touch nearly every aspect of human…
Whether connecting the internet of things, guiding surgical lasers or powering the latest smart phone, integrated circuits and microelectronics touch nearly every aspect of human life. In the world of technology, innovation and continuous improvement are imperatives—and being able to quickly and reliably test, debug, diagnose failures and take corrective action can make the difference between a doomed product launch and building a successful global brand. EAG offers you the world’s largest and most diverse collection of specialized analytical instrumentation, capacity to perform a variety of microelectronic tests in parallel, and the multi-disciplinary expertise required to draw true insight from data.
No contract service provider has more experience performing custom synthesis and producing isotopically labeled compounds to support product development in life science, chemical…
No contract service provider has more experience performing custom synthesis and producing isotopically labeled compounds to support product development in life science, chemical and related industries than we do. From 14C and 3H radiolabeled clinical trial materials synthesized under cGMP, to stable-labeled active ingredients for metabolism and environmental fate/effects testing, turn to EAG. We have extensive experience with multi-step and other complex synthesis projects, and our comprehensive, in-house analytical services ensure quick turnaround of purity and structural confirmation.
EAG combines biotechnology and protein characterization expertise with more than 50 years' experience analyzing chemical compounds in plant and environmental matrices to address…
EAG combines biotechnology and protein characterization expertise with more than 50 years’ experience analyzing chemical compounds in plant and environmental matrices to address the growing needs of the biotechnology crop industry. We offer a wide range of techniques required to fully characterize the event insertion and expressed proteins, as well as the various studies required to confirm the food, feed and environmental safety of products that represent the trait. From early-stage protein confirmation to GLP-compliant EDSP and allergenicity testing, we help you make faster, more informed development decisions and comply with evolving global regulations of genetically engineered crops.
When you need solid science and investigative engineering to address product failures, inform legal strategy, protect intellectual property or address product liability disputes,…
When you need solid science and investigative engineering to address product failures, inform legal strategy, protect intellectual property or address product liability disputes, turn to EAG. We’ve provided technical consulting, analysis and expert testimony for hundreds of cases involving the aerospace, transportation, medical device, electronics, industrial and consumer product industries. Our team of experts understands the legal process and your need for responsiveness, effective communication, scientifically defensible opinion and confidentiality. From professional consulting to data review to trial preparation and expert witness testimony, ask EAG.
Using an array of advanced separation techniques and innovative technology, we conduct highly precise analytical chromatography for various industries. Whether you want a closer…
Using an array of advanced separation techniques and innovative technology, we conduct highly precise analytical chromatography for various industries. Whether you want a closer look at the purity of your pharmaceutical or need to better understand an agrochemical’s components, EAG has the expertise to separate and evaluate any compound.
Need to evaluate the molecular structure of a compound or identify its origins? EAG knows how. With state-of-the-art tools, we can separate, vaporize and ionize the atoms and…
Need to evaluate the molecular structure of a compound or identify its origins? EAG knows how. With state-of-the-art tools, we can separate, vaporize and ionize the atoms and molecules in almost any pure or complex material to detect and obtain mass spectra of the components. We rely on decades of experience in mass spectrometry to provide our clients with precise analyses and the best detection limits.
EAG is a world leader in high-resolution imaging down to the atomic level. We offer unmatched analytical know-how, generating extremely detailed surface and near surface images…
EAG is a world leader in high-resolution imaging down to the atomic level. We offer unmatched analytical know-how, generating extremely detailed surface and near surface images for various industries, from consumer electronics to nanotechnology. Using state-of-the-art equipment and innovative techniques, we conduct expert imaging to aid in failure analysis, dimensional analysis, process characterization, particle identification and more. If you want to investigate a material with angstrom scale resolution, you can count on EAG to get the job done quickly and precisely.
EAG offers a vast array of spectroscopic techniques to clients in various industries, from defense contractors to technology pioneers. We combine unparalleled expertise and…
EAG offers a vast array of spectroscopic techniques to clients in various industries, from defense contractors to technology pioneers. We combine unparalleled expertise and methodology with cutting-edge technology to analyze your organic, inorganic, metallic and composite materials for identification, compositional, structural and contaminant information. Whether you need expert spectroscopic analysis to improve your production process or to surmount a technical challenge, EAG is up to the task.
Need to identify your unique material? Want to analyze the thermal properties of a sample, or measure the success of a process step? If it has to be done quickly and it has to be…
Need to identify your unique material? Want to analyze the thermal properties of a sample, or measure the success of a process step? If it has to be done quickly and it has to be done right, you can count on EAG. We offer a range of adaptable techniques and innovative methods to evaluate the physical and chemical characteristics of any compound. Our highly precise testing and analytical services will improve your production process, expedite R&D and help you conquer any technical challenge.
One of the most respected names in contract research and testing, EAG Laboratories is a global scientific services company operating at the intersection of science, technology and…
One of the most respected names in contract research and testing, EAG Laboratories is a global scientific services company operating at the intersection of science, technology and business. The scientists and engineers of EAG apply multi-disciplinary expertise, advanced analytical techniques and “we know how” resolve to answer complex questions that drive commerce around the world.
Science and technology transcend industry boundaries, and so does demand for EAG’s expertise. We partner with companies across a broad spectrum of high-tech, high-impact and…
Science and technology transcend industry boundaries, and so does demand for EAG’s expertise. We partner with companies across a broad spectrum of high-tech, high-impact and highly regulated industries. We help our customers innovate new and improved products, investigate manufacturing problems, perform advanced analyses to determine safety, efficacy and regulatory compliance, and protect their brands.
EAG’s corporate culture is firmly rooted in four guiding principles: “foster a growth mindset,” “find a better way,” “earn more loyal customers,” and “win…
EAG’s corporate culture is firmly rooted in four guiding principles: “foster a growth mindset,” “find a better way,” “earn more loyal customers,” and “win together.” Across all of our 20+ locations, you will find a true passion for science and the power of science to improve the world we live in. Hear what some of our ~1200 scientists, engineers and support personnel say about what it means to be part of EAG Laboratories.
EAG is growing, and we are always looking for talented, problem-solving oriented individuals to join our company. If you have a “we know how” spirit, we want to hear from you.…
EAG is growing, and we are always looking for talented, problem-solving oriented individuals to join our company. If you have a “we know how” spirit, we want to hear from you. Browse current openings now, and re-visit our careers page often.
From the buffer layer to the very top of the device, a novel form of SIMS can uncover troublesome impurities in GaN-on-silicon HEMTs
By Temel H. Buyuklimanli and Charles W. Magee
One of the devices attracting the most interest in the compound semiconductor manufacturing industry is the GaN-based HEMT (also known as GaN FET). Its capability to operate at high voltages and deliver high powers at microwave frequencies makes it an attractive candidate for deployment in base stations, and in a range of defence applications, including radar.
The impressive performance of this wide bandgap HEMT stems from the specific material properties associated with GaN. Compared to GaAs-based materials, which have also been used for generating power in the microwave region, GaN has a larger peak electron velocity; a higher thermal stability; and a larger band gap. All of these traits make GaN a very appropriate material for the channel of a HEMT. In such structures it is often paired with AlGaN to form a two-dimensional electron gas (2DEG), which lies at the very heart of the device, dictating its electrical characteristics.
Selecting the best substrate for the manufacture of GaN HEMTs is far from easy. From a performance perspective, GaN is ideal, because it ensures a perfect lattice match between the epilayers and the foundation. If such a substrate were practical, it would allow the use of a relatively thin buffer layer, because its role would only be to isolate the 2DEG from the substrate – it would not be needed to prevent high levels of defects in the active region of the device. But today GaN substrates cannot be grown in sufficiently large sizes to make their price commercially viable. Consequently, GaN HEMTs have to be grown on foreign substrates. One common choice is singlecrystal SiC, which combines good electrical and thermal conductivity with a lattice constant that is close to that of GaN – the difference is just 3 percent. However, while not as expensive as GaN, SiC is still pricey.
Figure 1. The PCOR-SIMS technique that has been pioneered by EAG Laboratories can be used to compare the centre and edge of a GaN-on-silicon HEMT. Note that although the profile provides accurate values for the aluminium and gallium atomic fractions and layer thicknesses, the 2DEG region of the structure is almost imperceptible at the surface. However, as shown in latter figures, its nature can be revealed by PCOR-SIMS.
A cheaper alternatives is sapphire, but it has a poor thermal conductivity − a disadvantage for high-power devices − and its lattice mismatch with GaN is 13 percent. Due to these drawbacks, a more popular low-cost option is silicon, which is now drawing a great deal of attention. Its great strength is that it has a large base of established manufacturing tools and processes. However, it also has its weaknesses, including a very large lattice mismatch with GaN that gives rise to a high density of defects in the epilayers. To prevent the defect density in the active layers from being so high that device performance is unacceptable, thick AlGaN buffer layers are inserted between the substrate and the 2DEG-forming layers.
Figure 2. Cross-sectional transmission electron microscopy images reveal the generation of surface pits (above) and the higher magnification of the details (below).
Scrutinising the HEMT
A characterisation technique that can reveal a great deal about GaN-on-silicon HEMTs is a variant of secondary ion mass spectrometry known as ‘Pointby- point CORrected’ SIMS, or PCOR-SIMS.
Figure 3. SIMS profiles enforce the importance of cleaning before measuring a profile of carbon.
We have developed this at EAG Laboratories. Compared to regular SIMS, it can determine layer thickness, composition and doping profile more accurately, because, at every data point, a calibration is undertaken with respect to alloy composition.
In the remainder of this article, we will take a journey through a GaN HEMT, or GaN FET, grown on 150 mm silicon, beginning with the buffer layer and finishing with the region around the 2DEG (see Figure 1). During this trip we will comment on the influence of pits on device profiling; uncover impurities that can hamper device performance; and understand the composition of the channel of the device.
We start our journey with the buffer layer, which is made of AlN. This material is not well lattice-matched to the underlying silicon, but it serves two important purposes: it provides an insulating layer that isolates those above from the substrate; and it acts as a seed layer, aiding the growth of subsequent layers of AlGaN with successively diminishing aluminium content. By decreasing the proportion of aluminium in AlGaN, the defect density is reduced to acceptable levels for subsequent growth of the GaN barrier.
Another insight provided by the PCOR-SIMS profile, shown in Figure 1, is that the bottom half of the GaN barrier layer is doped with carbon. This compensates for unintentional n-type doping by impurities (mainly silicon and oxygen) in the AlGaN buffer, and leads to an increase in the breakdown field strength. Unfortunately, at the edge of the wafer, the carbon doping in the GaN portion of the buffer is ten times higher than it is in the centre (Figure 1). This has considerable implications on the capability of the barrier layer to reduce the electric field, which varies across the 150 mm wafer.
The good news is that at a greater vertical distance above the substrate, where the growth of AlGaN begins, variations in thickness and doping level between the centre and the edge of the wafer are far smaller. It is important to monitor the level of carbon, as well as that of silicon and oxygen, because if carbon is excessively high, it will lead to leakage in the device.
According to reports, carbon doping is a major issue in the vicinity of the 2DEG, because it promotes a vertical leakage current. This is highly undesirable, because it degrades the carrier density and the carrier mobility of the 2DEG channel electrons, leading to an increase in dynamic on-resistance and current collapse. All these changes are to the detriment of device performance and reliability.
Despite the detrimental character of carbon doping on the 2DEG and the device properties, there are only a few reports that consider the residual carbon level in the active layers (the AlN spike and the AlGaN barrier layer). Perhaps this is because the measurements of carbon in this near-surface region are severely hampered by surface pits, which are always present, due to threading dislocations that originate deep in the buffer layer and reach the surface (see Figure 2).
In this region it is not easy to measure the carbon profile accurately. Carbon-containing species are adsorbed onto the air-exposed top surface, and are not entirely removed by the SIMS sputtering process until the entire pit is sputtered through. This leads to an artificially deep carbon profile, which can totally obscure the real carbon distribution in the 2DEG region.
To circumvent this problem, we have developed a proprietary surface-cleaning procedure that removes the vast majority of carbon from the surface, thus eliminating the deep tail of the carbon profile (see Figure 3). Thanks to this, true measurements are possible for the carbon concentration in the AlGaN barrier layer just above the 2DEG, as well as in the AlN spike just below it.
Iron and magnesium can also be used to dope the buffer layer. For these elements, SIMS provides very low detection limits. This is evident in Figure 4, which shows a peak in the iron profile just inside the GaN barrier. This peak is absent at the edge of the wafer, highlighting another difficultly in growing uniform layers across large substrates.
Figure 4. Iron and magnesium impurities are uncovered in HEMTs by SIMS measurements.
It is also important to control non-metallic impurities in the GaN barrier layer. The level of silicon in the GaN directly below the 2DEG must be as low as possible, because the device is designed to function in the absence of dopants. SIMS can be capable of silicon detection limits in the mid 1014 atom/cm3 range, which is low enough to see the troublesome 2 x 1015atom/cm3 silicon level just beneath the 2DEG in a GaN HEMT (see Figure 5). Another concern raised by this profile, shown in Figure 5, is the level of hydrogen in the GaN barrier. Hydrogen can have deleterious effects on device reliability, so it is critical to keep its levels as low as possible. In this case, PCOR-SIMS reveals a higher hydrogen-level in the carbon-doped portion of the barrier that is lightly elevated in carbon content. Presumably, this arises from the precursor used for carbon doping.
Figure 5. Low-detection-limit measurements by PCOR-SIMS reveal the presence of hydrogen and silicon in the GaN barrier layer of a GaN-on-silicon HEMT.
To the 2DEG
Near the surface of the structure is the twodimensional electron gas, which is responsible for current flow in the transistor. This 2DEG results from a conduction-band discontinuity between a thin, top, doped AlGaN layer and an undoped GaN layer. This creates a triangular quantum well that accumulates electrons. The active region is exceedingly thin, with a thickness of just 20-30 nm. Consequently, measuring this accurately by SIMS requires great care. However, with PCOR-SIMS, it is possible to identify the aluminium level in the top AlGaN layer, as well as the impurity levels of carbon, hydrogen, oxygen and silicon (see Figure 6).
Since this AlGaN layer is on the top of the structure, it is important to take steps to minimize the effects of surface contamination, which is always present on air-exposed surfaces. To achieve this, we use our proprietary surface-cleaning procedure to remove the carbon initially present on the sample surface. This enables a determination of the carbon doping level in the AlGaN layer of 1-2×1017 atoms/cm3, within the top 15 nm of the sample.
This particular measurement also yields another important piece of information for the device engineer – the thickness of the AlGaN barrier layer (see the inset to Figure 6). It is through this layer that the potential on the gate acts to control the electron density in the 2DEG, and thus the conductance of the device.
Figure 6. PCOR-SIMS can reveal the thickness of the channel, and the levels of various elements in this region.
Figure 7. By overlaying SIMS and cross-sectional transmission electron microscopy images, it is possible to produce a detailed analysis of the HEMT channel region, including an arbitrary conductivity change curve.
Another way to look at this region is to overlay the profiles of aluminium and carbon on a cross-sectional transmission electron microscopy image of the same region (see Figure 7). With this approach, it is possible to see the location of the AlN delta layer. Its role is to improve the carrier mobility in the 2DEG, by mitigating coulomb scattering from donors in the AlGaN. The micrograph reveals that the actual thickness of the AlN delta layer is correctly measured by the full-width-half maximum of the aluminium profile above the constant level in the AlGaN layer. However, there is a tail into the underlying GaN, due to surface pits (shown in Figure 3). A plot of the carbon concentration reveals the location of the doping with respect to the exact vertical location of the interface, which is shown by the aluminium profile. Note that transmission electron microscopy cannot be used to detect carbon, even with energy dispersive X-ray spectrometry or electron energy-loss spectroscopy.
Also shown in Figure 7 is a plot of conductivity. This is influenced by the instantaneous surface potential, from which can be inferred the surface conductivity. The 2DEG is formed just inside the GaN, at the depth at which sample conductivity recovers after passing through a zone of decreased conductivity, just inside the barrier layer where the holes accumulate. Our study of the HEMT, from the buffer to the 2DEG, show the tremendous capability that PCOR-SIMS has for determining accurate concentrations of matrix elements and dopants within GaN HEMTs. This technique can be used to optimise epitaxial layer growth, aid failure analysis, and thus support the growth of the GaN HEMT (GaN FET) industry.
The authors wish to thank Ozgur Celik, Wei Ou, Andrew Klump, Wei Zhao, Yun Qi, Jeffrey Serfass and Mike Salmon from EAG Laboratories
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