Category Archives: Process Materials

Insulating oxides are oxygen containing compounds that do not conduct electricity, but can sometimes form conductive interfaces when they’re layered together precisely. The conducting electrons at the interface form a two-dimensional electron gas (2DEG) which boasts exotic quantum properties that make the system potentially useful in electronics and photonics applications.

Researchers at Yale University have now grown a 2DEG system on gallium arsenide, a semiconductor that’s efficient in absorbing and emitting light. This development is promising for new electronic devices that interact with light, such as new kinds of transistors, superconducting switches and gas sensors.

“I see this as a building block for oxide electronics,” said Lior Kornblum, now of the Technion – Israel Institute of Technology, who describes the new research appearing this week in the Journal of Applied Physics, from AIP publishing.

Oxide 2DEGs were discovered in 2004. Researchers were surprised to find that sandwiching together two layers of some insulating oxides can generate conducting electrons that behave like a gas or liquid near the interface between the oxides and can transport information.

Researchers have previously observed 2DEGs with semiconductors, but oxide 2DEGs have much higher electron densities, making them promising candidates for some electronic applications. Oxide 2DEGs have interesting quantum properties, drawing interest in their fundamental properties as well. For example, the systems seem to exhibit a combination of magnetic behaviors and superconductivity.

Generally, it’s difficult to mass-produce oxide 2DEGs because only small pieces of the necessary oxide crystals are obtainable, Kornblum said. If, however, researchers can grow the oxides on large, commercially available semiconductor wafers, they can then scale up oxide 2DEGs for real-world applications. Growing oxide 2DEGs on semiconductors also allows researchers to better integrate the structures with conventional electronics. According to Kornblum, enabling the oxide electrons to interact with the electrons in the semiconductor could lead to new functionality and more types of devices.

The Yale team previously grew oxide 2DEGs on silicon wafers. In the new work, they successfully grew oxide 2DEGs on another important semiconductor, gallium arsenide, which proved to be more challenging.

Most semiconductors react with oxygen in the air and form a disordered surface layer, which must be removed before growing these oxides on the semiconductor. For silicon, removal is relatively easy — researchers heat the semiconductor in vacuum. This approach, however, doesn’t work well with gallium arsenide.

Instead, the research team coated a clean surface of a gallium arsenide wafer with a layer of arsenic. The arsenic protected the semiconductor’s surface from the air while they transferred the wafer into an instrument that grows oxides using a method called molecular beam epitaxy. This allows one material to grow on another while maintaining an ordered crystal structure across the interface.

Next, the researchers gently heated the wafer to evaporate the thin arsenic layer, exposing the pristine semiconductor surface beneath. They then grew an oxide called SrTiO3 on the gallium arsenide and, immediately after, another oxide layer of GdTiO3. This process formed a 2DEG between the oxides.

Gallium arsenide is but one of a whole class of materials called III-V semiconductors, and this work opens a path to integrate oxide 2DEGs with others.

“The ability to couple or to integrate these interesting oxide two-dimensional electron gases with gallium arsenide opens the way to devices that could benefit from the electrical and optical properties of the semiconductor,” Kornblum said. “This is a gateway material for other members of this family of semiconductors.”

A team of Russian, Czech and German researchers gained a new perspective on the properties of three materials of biological origin. Besides two reference materials with well-studied properties — serum albumin and cytochrome C — the researchers looked at the extracellular matrix of the Shewanella oneidensis MR-1 bacterium, which is used in biofuel cells. The team measured the materials’ dynamic conductivity and dielectric permittivity in a wide range of frequencies and temperatures. To interpret their findings, the researchers used theoretical approaches and concepts from condensed matter physics. The paper detailing the study was published in the journal Scientific Reports.

“So far, the formalism of condensed matter physics has only found limited use in classical biochemistry and biophysics. As a result, certain interesting effects evade our attention,” says Konstantin Motovilov, senior research scientist at the Laboratory of Terahertz Spectroscopy at Moscow Institute of Physics and Technology (MIPT). “When we do make use of this language, we acquire new ways of modeling observed phenomena and describing biological structures. In our paper, we characterize the behavior of proteins, considered as classical amorphous semiconductors, with the help of the formalism of condensed matter physics.”

Before discussing the study, here is a quick example of how solid-state physics explains the electrical properties of different materials.

There are in fact multiple mechanisms of electrical conductivity. For each, there is a corresponding theory that describes the properties of certain materials. For example, the conductivity in metals is adequately explained by the Drude theory. In the theory, there is no interaction between the conduction electrons, which are assumed to only occasionally collide with crystal lattice, impurities, and defects. Electrical conductivity is the inverse of electrical resistivity. Conductivity indicates how easy it is for an electric current to pass through a given material. Within the Drude model, this property does not depend strongly on frequency up to the frequency of the collisions between charge carriers and lattice or impurities. However, there is a large group of conductive materials that do not fit this description. Yet their behavior in an external electromagnetic field is quite interesting. Among them are glasses, ionic conductors, and amorphous semiconductors.

To qualitatively describe the electrical properties of such materials, another theory was proposed about 40 years ago by Andrzej Karol Jonscher, an English physicist. According to his theory, charge carriers — electrons, for example — can adequately be considered as free at room temperature, provided the alternating current frequency does not exceed several megahertz. Under these conditions, the Drude model is applicable and conductivity is nearly constant, i.e., it does not depend on the frequency of the external field. If, however, the frequency is higher, this description is no longer valid and there is an increase in conductivity proportional to a certain power — which is close to 0.8 — of frequency. The same effect is observed for materials that are gradually cooled, even if the frequency is kept constant.

Interestingly, different materials exhibit quite similar behavior in that regard. Moreover, if you restate the dependences — say, talk about the ratio between direct current (static) conductivity and alternating current conductivity, as opposed to conductivity as such — the relations for all materials turn out to be identical, revealing the so-called Universal Dielectric Response (UDR). This curious phenomenon was thoroughly investigated in a study that examined the conduction in glasses and other amorphous materials, offering new insights into their structure and properties.

The authors of the paper showed that Jonscher’s law for conductivity applies to three organic materials. Among them, two are well-known reference proteins: bovine serum albumin and bovine heart cytochrome C. Their structural, physical, and chemical properties have been investigated in detail, so the researchers used them as reference materials.

In addition, they examined the extracellular matrix and filaments (EMF) of the Shewanella oneidensis MR-1 bacterium, which can produce electricity in biological fuel cells. S. oneidensis has been used in many studies with a focus on alternative energy sources, so its electrical properties are of interest to both researchers and engineers. In 2010, a team of researchers based in the United States and Canada showed that the bacterium’s extracellular appendages behave a lot like p-type semiconductors. The electrical properties of S. oneidensis MR-1 have nevertheless not been studied in detail. The recently published paper is an attempt to remedy that.

The authors measured the conductivity of the materials, as well as the energy losses in a frequency range from 1 hertz to 1.5 terahertz, or trillion hertz, for temperatures from -260 to 40 degrees Celsius. (Strictly speaking, the energy losses are given by the imaginary part of the complex dielectric permittivity.) Next, the researchers measured the direct current conductivity of EMF for temperatures from zero to 40 C, as well as the temperature dependence of their heat capacity. For each of the three materials, water content and ion concentration were also determined.

To do this, the researchers pressed the substances into pellets using a 1-centimeter mold. They then applied electrodes to the faces of the pellets to pass alternating current through them in order to measure the electrical conductivity and dielectric permittivity of the materials in the 1-300 million hertz range. For higher frequencies, this approach does not work, so for the 30-1,500 gigahertz, or billion hertz, range, the team obtained the spectra of complex dielectric permittivity using quasioptical terahertz spectroscopy. No measurements were made in the intermediate frequency range.

It turned out that at room temperature, EMF conductivity is nearly constant, and when the frequency is increased above several million hertz, or several megahertz, the conductivity is proportional to a certain power — which is close to 1 — of the frequency. Cytochrome C did not exhibit such behavior unless the frequency was low and the temperature high. In the case of albumin, it was not observed at all. This suggests that different conductivity mechanisms are at play in these materials. It is likely that EMF has nearly free charges at room temperature — just like in the Drude model — whereas albumin does not have them and cytochrome C is a mixed bag.

The dependence observed by the researchers can be explained in terms of the individual properties of the materials. Both cytochrome C and albumin are regular proteins. Although these materials do have some free charges, these are not nearly as many as it would be necessary to justify the Drude model. Comparing the conductivity in EMF to that in metals (conductors) is more realistic, as free charges are more easily generated in these molecules. However, a comparison even more valid would be that with a solution of table salt, which has a high concentration of free ions.

Naturally, a complete description is more complex and would require us to take the water content of materials and other factors into account. For instance, because EMF contains significant amounts of loosely bound water, its conductivity grows quadratically at temperatures of about -250 C and frequencies on the order of 100 billion hertz (sub-terahertz terahertz range). Temperatures that low cause the bulk water in the material to freeze, and high frequencies mean that the dielectric properties resulting from water dipole dynamics become non-negligible. The other materials, too, exhibit deviations from Jonscher’s predictions, but they are not as dramatic.

The authors have thus clearly shown the powerful methodology and instrumentation of condensed matter physics to be effective for fundamental research into the electrodynamics of biological objects. The next step could involve the application to biomaterials research of the wide range of other theories and models that have been effectively used by the physics community for many decades.

Carbon nanotubes bound for electronics need to be as clean as possible to maximize their utility in next-generation nanoscale devices, and scientists at Rice and Swansea universities have found a way to remove contaminants from the nanotubes.

Rice chemist Andrew Barron, also a professor at Swansea in the United Kingdom, and his team have figured out how to get nanotubes clean and in the process discovered why the electrical properties of nanotubes have historically been so difficult to measure.

Scientists at Rice and Swansea universities have demonstrated that heating carbon nanotubes at high temperatures eliminates contaminants that make nanotubes difficult to test for conductivity. They found when measurements are taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlap, which scrambles the results. The plot shows the deviation when probes test conductivity from minus 1 to 1 volt at distances greater or less than 4 microns. Credit: Barron Research Group/Rice University

Scientists at Rice and Swansea universities have demonstrated that heating carbon nanotubes at high temperatures eliminates contaminants that make nanotubes difficult to test for conductivity. They found when measurements are taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlap, which scrambles the results. The plot shows the deviation when probes test conductivity from minus 1 to 1 volt at distances greater or less than 4 microns. Credit: Barron Research Group/Rice University

Like any normal wire, semiconducting nanotubes are progressively more resistant to current along their length. But over the years, conductivity measurements of nanotubes have been anything but consistent. The Rice-Swansea team wanted to know why.

“We are interested in the creation of nanotube-based conductors, and while people have been able to make wires, their conduction has not met expectations,” Barron said. “We wanted to determine the basic science behind the variability observed by other researchers.”

They discovered that hard-to-remove contaminants — leftover iron catalyst, carbon and water — could easily skew the results of conductivity tests. Burning those contaminants away, Barron said, creates new possibilities for carbon nanotubes in nanoscale electronics.

The new study appears in the American Chemical Society journal Nano Letters.

The researchers first made multiwalled carbon nanotubes between 40 and 200 nanometers in diameter and up to 30 microns long. They then either heated the nanotubes in a vacuum or bombarded them with argon ions to clean their surfaces.

They tested individual nanotubes the same way one would test any electrical conductor: by touching them with two probes to see how much current passes through the material from one tip to the other. In this case, tungsten probes were attached to a scanning tunneling microscope.

In clean nanotubes, resistance got progressively stronger as the distance increased, as it should. But the results were skewed when the probes encountered surface contaminants, which increased the electric field strength at the tip. And when measurements were taken within 4 microns of each other, regions of depleted conductivity caused by contaminants overlapped, which further scrambled the results.

“We think this is why there’s such inconsistency in the literature,” Barron said. “If nanotubes are to be the next-generation lightweight conductor, then consistent results, batch-to-batch and sample-to-sample, are needed for devices such as motors and generators as well as power systems.”

Heating the nanotubes in a vacuum above 200 degrees Celsius (392 degrees Fahrenheit) reduced surface contamination, but not enough to eliminate inconsistent results, they found. Argon ion bombardment also cleaned the tubes but led to an increase in defects that degrade conductivity.

Ultimately the researchers discovered vacuum annealing nanotubes at 500 degrees Celsius (932 Fahrenheit) reduced contamination enough to measure resistance accurately.

Barron said engineers who use nanotube fibers or films in devices currently modify the material through doping or other means to get the conductive properties they require. But if the source nanotubes are sufficiently decontaminated, they should be able to get the desired conductivity by simply putting their contacts in the right spot.

“A key result of our work is that if contacts on a nanotube are less than 1 micron apart, the electronic properties of the nanotube change from conductor to semiconductor, due to the presence of overlapping depletion zones, which shrink but are still present even in clean nanotubes,” Barron said.

“This has a potential limiting factor on the size of nanotube-based electronic devices,” he said. “Carbon-nanotube devices would be limited in how small they could become, so Moore’s Law would only apply to a point.”

Researchers at the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS), have shown that defects in monolayer molybdenum disulfide (MoS2) exhibit electrical switching, providing new insights into the electrical properties of this material. As MoS2 is one of the most promising 2D semiconductors, it is expected that these results will contribute to its future use in opto-electronics.

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

Defects can cause major changes in the properties of a material, leading to either desirable or unwanted effects. For example, petrochemical industry has long taken advantage of the catalytic activity of MoS2edges, characterized by the presence of a high concentration of defects, to produce petroleum products with reduced sulfur dioxide (SO2) emissions. On the other hand, having a pristine material is a must in electronics. Currently, silicon rules the industry, because it can be prepared in a virtually defect-free manner. In the case of MoS2, its suitability for electronic applications is currently limited by the presence of naturally occurring defects. So far, the precise link between these defects and the degraded properties of MoS2 has been an open question.

In IBS, a team of physicists, material scientists, and electrical engineers worked closely together to explore the electronic properties of sulfur vacancies in MoS2 monolayers, using a combination of atomic force microscopy (AFM) and noise analysis. The scientists used a metallic AFM tip to measure the noise signal, i.e., the variation of electrical current passing through a single layer of MoS2 placed on a metal substrate.

The most common defects in MoS2 are instances of missing single sulfur atoms, also known as sulfur monovacancies. In a perfect sample, each sulfur atom has two valence electrons that bind to two molybdenum electrons. However, where a sulfur atom is missing, these two molybdenum electrons are left unsaturated, defining the neutral state (0 state) of the defect. However, the team observed rapid switching events in their noise measurements, indicating the state of the vacancy switched between neutral (0 state) and charged (-1 state).

“The switching between 0 and -1 is happening continuously. While an electron resides at the vacancy for a while, it is missing from the current, such that we observe a current drop,” explains Michael Neumann, one of the co-first authors of the study. “This goes a long way towards understanding the known anomalies of MoS2, and it is very interesting that sulfur vacancies alone are enough to explain these anomalies, without requiring more complex defects.” According to the experiments and earlier calculations, two electrons can be also trapped at the vacancy (-2 state), but this does not seem to be energetically favored.

The new observation that sulfur vacancies can be charged (-1 and -2 states) sheds light on several MoS2 anomalies, including its reduced electron mobility observed in MoS2 monolayer samples: electrons move following the direction of an applied voltage, but get scattered by charged defects. “The -1 state is occupied around 50% of the time, which would lead to scattering of electrons, and thus explain why MoS2 has such poor mobility,” clarifies Neumann. Other MoS2 characteristics which can be explained by this study are the n-type doping of MoS2, and the unexpectedly large resistance at the MoS2-metal junction.

“This research opens up the possibility of developing a new noise nanospectroscopy device capable of mapping one or more defects on a nanoscale scale over a wide area of a 2D material,” concludes the corresponding author Young Hee Lee.

The full study is available on Nature Communications.

Researchers from North Carolina State University have found that the transfer of triplet excitons from nanomaterials to molecules also creates a feedback mechanism that returns some energy to the nanocrystal, causing it to photoluminesce on long time scales. The mechanism can be adjusted to control the amount of energy transfer, which could be useful in optoelectronic applications.

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Felix N. Castellano, Goodnight Innovation Distinguished Chair of Chemistry at NC State, had previously shown that semiconductor nanocrystals could transfer energy to molecules, thereby extending their excited state lifetimes long enough for them to be useful in photochemical reactions.

In a new contribution, Castellano and Cédric Mongin, a former postdoctoral researcher currently an assistant professor at École normale supérieure Paris-Saclay in France, have shown that not only does the transfer of triplet excitons extend excited state lifetimes, but also that some of the energy gets returned to the original nanomaterial in the process.

“When we looked at triplet exciton transfers from nanomaterials to molecules, we noticed that after the initial transfer the nanomaterial would still luminesce in a delayed fashion, which was unexpected,” says Castellano. “So we decided to find out what exactly was happening at the molecular level.”

Castellano and Mongin utilized cadmium selenide (CdSe) quantum dots as the nanomaterial and pyrenecarboxylic acid (PCA) as the acceptor molecule. At room temperature, they found that the close proximity of the relevant energy levels created a feedback mechanism that thermally repopulated the CdSe excited state, causing it to photoluminesce.

Taking the experiment one step further, the researchers then systematically varied the CdSe-PCA energy gap by changing the size of the nanocrystals. This resulted in predictable changes to the resultant excited state lifetimes. They also examined this process at different temperatures, yielding results consistent with a thermally activated energy transfer mechanism.

“Depending on relative energy separation, the system can be tuned to behave more like PCA or more like the CdSe nanoparticle,” says Castellano. “It’s a control dial for the system. We can make materials with unique photoluminescent properties simply by controlling the size of the nanoparticle and the temperature of the system.”

More materials for electronic applications could be identified, thanks to the discovery of a new metal-organic framework (MOF) that displays electrical semiconduction with a record high photoresponsivity, by a global research collaboration involving the University of Warwick.

Research published today in Nature Communications shows how high photoconductivity and semiconductor behaviour can be added to MOFs – which already have a huge international focus for their applications in gas storage, sensing and catalysis.

The new work, conducted by Universities in Brazil, the United Kingdom and France – including researchers at Warwick’s Department of Chemistry – found that the new MOF has a photoresponsivity of 2.5 × 105 A.W-1- the highest ever observed.

The MOF has been prepared using cobalt (II) ions and naphthalene diimides and acid as ligands. The structure shows anisotropic redox conduction, according to the directions of the crystal lattice. The conduction mechanism is sensitive to light, and may be modified or modulated according to the incident wavelength.

Photoactive and semiconducting MOFs are rare but desirable for electrical and photoelectrical devices.

These results are the first of this kind concerning MOFs and are the starting point for the possibility of discovery of even more functional materials, displaying properties suitable for practical applications.

The potential for use in electronic components and photoconversion devices, such as solar cells and photocatalysts provides a very exciting future for such materials.

Professor Richard Walton, from Warwick’s Department of Chemistry, commented:

“The material we have discovered paves the way for new applications of a topical family of materials in many areas ranging from technology to energy conversion. We illustrate how MOFs that combine organic and inorganic components can produce unique functional materials from readily available chemicals.

“Our work was underpinned by Warwick’s strengthening collaborative links with Brazilian universities and our exceptional equipment for materials analysis “

University of Alabama at Birmingham physicists have taken the first step in a five-year effort to create novel compounds that surpass diamonds in heat resistance and nearly rival them in hardness.

They are supported by a five-year, $20 million National Science Foundation award to create new materials and improve technologies using the fourth state of matter — plasma.

Plasma — unlike the other three states of matter, solid, liquid and gas — does not exist naturally on Earth. This ionized gaseous substance can be made by heating neutral gases. In the lab, Yogesh Vohra, a professor and university scholar in the UAB Department of Physics, uses plasma to create thin diamonds film. Such films have many potential uses, such as coatings to make artificial joints long-lasting or to maintain the sharpness of cutting tools, developing sensors for extreme environments or creating new super-hard materials.

To make a diamond film, Vohra and colleagues stream a mix of gases into a vacuum chamber, heating them with microwaves to create plasma. The low pressure in the chamber is equivalent to the atmosphere 14 miles above the Earth’s surface. After four hours, the vapor has deposited a thin diamond film on its target.

In a paper in the journal Materials, Vohra and colleagues in the UAB College of Arts and Sciences investigated how the addition of boron, while making a diamond film, changed properties of the diamond material.

It was already known that, if the gases are a mix of methane and hydrogen, the researchers get a microcrystalline diamond film made up of many tiny diamond crystals that average about 800 nanometers in size. If nitrogen is added to that gas mixture, the researchers get nanostructured diamond, made up of extremely tiny diamond crystals averaging just 60 nanometers in size.

In the present study, the Vohra team added boron, in the form of diborane, or B2H6, to the hydrogen/methane/nitrogen feed gas and found surprising results. The grain size in the diamond film abruptly increased from the 60-nanometer, nanostructured size seen with the hydrogen/methane/nitrogen feed gas to an 800-nanometer, microcrystalline size. Furthermore, this change occurred with just minute amounts of diborane, only 170 parts per million in the plasma.

Using optical emission spectroscopy and varying the amounts of diborane in the feed gas, Vohra’s group found that the diborane decreases the amounts of carbon-nitrogen radicals in the plasma. Thus, Vohra said, “our study has clearly identified the role of carbon-nitrogen species in the synthesis of nanostructured diamond and suppression of carbon-nitrogen species by addition of boron to the plasma.”

Since the addition of boron can also change the diamond film from a nonconductor into a semiconductor, the UAB results offer a new control of both diamond film grain size and electrical properties for various applications.

Over the next several years, Vohra and colleagues will probe the use of the microwave plasma chemical vapor deposition process to make thin films of boron carbides, boron nitrides and carbon-boron-nitrogen compounds, looking for compounds that survive heat better than diamonds and also have a diamond-like hardness. In the presence of oxygen, diamonds start to burn at about 1,100 degrees Fahrenheit.

A new technology enables dramatically lower thermal budget capability that is enabling to thermal processes like epitaxy, CVD and diffusion, without any semiconductor material consumption.

BY ROBERT PAGLIARO, RP Innovative Engineering Solutions, LLC, Mesa, AZ

As semiconductor based electronic devices have become smaller, faster, smarter, 3-dimensional, and multi-functional the methods and materials required to fabricate them demand novel approaches to be developed and implemented in the device manufacturing facilities. Amongst the most challenging requirements are the need to lower the thermal budgets of the front end thermal processes and to minimize the semiconductor material consumption that comes with the conventional oxidizing (hydrogen peroxide and ozone based chemistries) wet cleaning processes chemistries such as APM, HPM, SPM and SOM.

A novel wet surface preparation method that removes existing surface contamination and native oxide from semiconductor surfaces and then passivates them with a pristine and stable hydrogen passivated surface has been developed and commercialized by APET Co, Ltd. in a system called the TeraDox. This patented technology enables dramatically lower thermal budget capability that is enabling to thermal processes like epitaxy, CVD and diffusion, without any semiconductor material consumption.

The TeraDox system is an enhanced version of the APET FRD (HF etching, Rinse and Dry). The name TeraDox implies the ability to provide a process chemistry with < 1 ppb impurities, particularly dissolved oxygen, which allows for producing pristine and stable H-passivated semiconductor surfaces. Dilute HF and HCl (dHF and dHCl) are the etching chemistries used for removing the native and chemical oxides from Si, SiGe and Ge surfaces. The TeraDox system has a single vessel wet processor and a wafer transfer/drying hood that allows for a segue between the load, chemical fill, etch, insitu-rinse, dry and unload steps of the process sequence, while keeping the process chemistry and the wafers in a continuous ambient of ultra- pure N2. This equipment and process design eliminate the exposure of the wafers to air and minimizes gas perme- ation throughout the entire oxide removal and H-passiv- ation process sequence. These are all critical elements to achieving the best surface quality results. While there are a variety of important parameters towards achieving a pristine and stable H-passivated surface one of the most enabling ingredients to the APET TeraDox process and equipment IP is the PPT level degassing capability for the UPW and aqueous chemicals used in the H-passivation process. The unique UPW and chemical degassing apparatus require an optimized hardware configuration with membrane contactors and facilities used for the vacuum + UHP N2 sweep gas to achieve a DO degassing efficiency > 99.999%. This ultra-high degassing efficiency allows for a Dissolved Oxygen (DO) concen- tration capability of < 100 ppt.

It has been well proven and documented by multiple world-renowned surface scientists [1,2,3] since the late 1980s that the level of dissolved oxygen (DO), as well as other dissolved impurities (such as CO2, TOC, silica and N2), has a direct impact on the efficiency of H-passivation and the native oxide (initial and changing thickness vs. queue time) that follows the removal of native and chemical oxides from semicon- ductor surfaces. Queue time (Q-time) is the amount of time that the H-passivated wafer are exposed to air before being placed in an inert environment for the subsequent process step (epi, poly silicon, metal, ion implantation etc.). It can be seen in FIGURE 1 how native oxide regrowth occurs after HF treatment in air and UPW vs. exposure time [1].

Screen Shot 2017-12-06 at 12.26.01 PM

A similar DO vs. surface oxide and carbon relationship is also verified using encapsulated SIMS. This method uses dynamic SIMS to measure the amount of O, C that are trapped at the epi layer/silicon wafer interface. This has been a widely used characterization method to assess a pre-low temperature epi surface prepa- ration process’ hydrogen surface passivation quality since the early 90s. The typical epi cap is ~80-150nm and is deposited using a 650°C SiH4 source deposition process. The objective is to be able to minimize the thermal budget of the pre-deposition bake step which is required to remove any surface oxides and organics to allow perfect epitaxial deposition with no contami- nants or defects at the interface.

FIGURE 2 demonstrates how the encapsulated SIMS interface O (areal oxide density, AOD) using a 650°C SiH4 no bake Si deposition process is strongly dependent on the DO concentration. Three samples are depicted with different surface preparation conditions, a reference wafer with no surface preparation, a wafer dHF wet processedwith the UPW DO ~ 1ppb, and a wafer dHF wet processed with the DO ~0.1 ppb.

Screen Shot 2017-12-06 at 12.26.31 PM

It can be seen in FIGURE 3 how applying a 700C/80T/60s bake before a 650C Si deposition process with the UPW DO at 0.1ppb yields non-detectable O and C. This SIMS data info is relatively old (2010) but is still good for reference. The current APET TeraDox wet process capability can provide non-detectable O and C without a bake before the 650°C Si deposition process.

Screen Shot 2017-12-06 at 12.26.46 PM

As mentioned earlier, undesirable native oxide thickness increases with queue time on H-passivated Si, SiGe and Ge surfaces. So, it is important to minimize the Q-time between the H-passivation process and the subsequent process step, but the quality and stability of the H-passivation does need to accommodate practical queue times in a manufacturing environment. The H-passivation from the APET TeraDox process has proven to be stable enough for up to at least 8-hour Q-times for most low temperature process applications, which makes it suitable for most semiconductor device manufacturing facilities.

Aside from the low surface oxygen benefit from having ultra-low DO in this process there are other very important benefits to this as well. Having ultra-low DO prevents water marks, microroughness (faceting), bacterial contamination and material consumption. If there is no DO in the UPW or the etching chemistry then there is no competing mechanism to simultaneously oxidize and etch the semiconductor material during the oxide etch and insitu-rinse steps. If the surface is being oxidized/etched then orientation selective faceting will occur. Faceting leads to gener- ation a mix of mono-, di- and tri- hydride terminations on the different orientations of the semiconductor surface. An example is silicon (100), which if it is kept atomically smooth after the oxide is removed by HF, the surface will be dominated by di-hydride terminations. If the surface is faceted it will contain lower energy mono-hydride terminations. Higher energy hydride bonds lead to better surface stability while the lower energy hydride bonds make the surface less stable and will re-oxidize faster with Q-time.

So in general, the pristineness and the atomic smoothness of the semiconductor surface are what dictates the quality and stability of the H-passivating surface preparation process.

While the TeraDox process performance has continued to improve with the new innovations, the capabilities have surpassed the detection limits of conven- tional measurement methods like encapsulated SIMS characterization. Encapsulated SIMS also has a lot of drawbacks and limitations which make it an impractical process monitoring method in manufacturing facil- ities. The need to have a more sensitive measurement method that can measure “as processed” surfaces in a fast, real time and non-destructive manner had become an urgent requirement.

There are a variety of very good electrical and optical measurement methods that have been in use for many years, but most of them do not provide surface specific information directly. Surface parameters such as surface recombination velocity and lifetime (SRV and Ts) can be calculated relatively accurately using multiple step procedures by measurement methods such as uPCD, QSS-PC, PL and SPV. SRV (surface recombination velocity) and Ts (surface recombination lifetime) are extremely sensitive to surface contamination such as C, O metals and dopants as well as micro- roughness. This diverse sensitivity make it ideal for assessing surface preparation methods.

Until recently, only one measurement technique has been found that can measure the SRV and Teff (effective lifetime) of the surface directly and quickly on as processed H-passivated wafers. While doing a lot of research for the ideal measurement method to pair with the APET TeraDox H-passivation process, it was discovered that an enhanced version of the CADIPT department at the University of Toronto’s PCR-LIC technology, called Quantitative Lock-in Carrierog- raphy and Imaging (Q-LIC), could have the unique and enabling capabilities needed for this application. After completing an array of screening and optimization testing over the course of 8 months, the results have validated Q-LIC as an ideal measurement method for “as processed” H-passivated surfaces. In FIGURE 4, the plot demonstrates the SRV vs Q-time for four different wet cleans and an unprocessed control. The data shows strong evidence of the differentiation between different H-passivation methods (process and equipment), the level of DO in the wet process chemistry, and the dynamically changing surface state over time.

FIGURE 4. Q-LIC SRV measurements vs Q-time for four different HF last wet processes.

FIGURE 4. Q-LIC SRV measurements vs Q-time for four different HF last wet processes.

APET currently has five patents, related to this technology, integrated on the commercially available TeraDox wet process equipment, four of which include the use of vacuum/N2 sweep degassing with membrane contactors for both the UPW and chemical degassing.

The UPW degassing is done in a separate stand-alone module (called the APET Dox unit) that treats up to 60 lpm of UPW before going to the main unit. All Dox units are guaranteed to have DO < 1 ppb, but all of the units in use to date achieve < 200 ppt. The most recently installed Dox unit system has a base DO level of ~30-40 ppt. Aside from the importance of PPT level degassing of the UPW much attention has also been given towards the design and materials used in the entire TeraDox system to prevent gas permeation into the UPW supply and the process chemistry to achieve optimum H-passivation. The most recent TeraDox related patent that was issued to APET was for chemical degassing. The degassing of the HF and HCl are typically overlooked in this application. Typically, HF comes in ~48% and HCl in ~37% concentrations with the balance of these supplied mixtures is in DO saturated water. So even diluted etching chemistries of up to 400 (UPW) :1 (chemical) ratios will typically still produce a composite DO of > 3ppb in the process vessel, even if the UPW supply is degassed to 0 ppt. Having the unique chemical degassing capability to < 1ppb DO significant improves the overall performance of the H-passivation process. The chemical degassing apparatus is integrated into the HF and HCl chemical delivery lines inside the TeraDox system’s main unit.

In summary, APET has developed and commercialized a unique and enabling wet surface preparation technology, the TeraDox process and equipment, that can produce pristine and stable hydrogen passivated semiconductor surfaces. While there are several critical factors and innovations that enable the TeraDox’s unique process performance capabilities, the fully integrated “dry in/dry out” system design and the unique PPT level degassing of the process chemistries are the most facili- tating features on the TeraDox system.

Acknowledgement

A special thanks to Dr. Andreas Mandelis and his staff at the University of Toronto for their support in optimizing their Q-LIC system to provide data for this paper as well as demonstrating a suitable measurement method for the “as processed” H-passivation application.

References

1. M. Morita et al, J. Appl. Phys. 88 (3), 1 (1990)
2. A. Philipossian, J. Electrochem. Soc. 139 No. 10, 2956 (1992)
3. F. H. Li, M. K. Balazs, and S. Anderson, J. Electrochem. Soc. 152,
G669 (2005)

Smartphones and computers wouldn’t be nearly as useful without room for lots of apps, music and videos.

Devices tend to store that information in two ways: through electric fields (think of a flash drive) or through magnetic fields (like a computer’s spinning hard disk). Each method has advantages and disadvantages. However, in the future, our electronics could benefit from the best of each.

“There’s an interesting concept,” says Chang-Beom Eom, the Theodore H. Geballe Professor and Harvey D. Spangler Distinguished Professor of Materials Science and Engineering at the University of Wisconsin-Madison. “Can you cross-couple these two different ways to store information? Could we use an electric field to change the magnetic properties? Then you can have a low-power, multifunctional device. We call this a ‘magnetoelectric’ device.”

In research published recently in the journal Nature Communications, Eom and his collaborators describe not only their unique process for making a high-quality magnetoelectric material, but exactly how and why it works.

Physics graduate student Julian Irwin checks equipment in the lab of materials science and engineering Professor Chang-Beom Eom, where researchers have produced a material that could exhibit the best qualities of both solid-state and spinning disk digital storage. Credit: Sarah Page/UW-Madison College of Engineering

Physics graduate student Julian Irwin checks equipment in the lab of materials science and engineering Professor Chang-Beom Eom, where researchers have produced a material that could exhibit the best qualities of both solid-state and spinning disk digital storage. Credit: Sarah Page/UW-Madison College of Engineering

Magnetoelectric materials — which have both magnetic and electrical functionalities, or “orders” — already exist. Switching one functionality induces a change in the other.

“It’s called cross-coupling,” says Eom. “Yet, how they cross-couple is not clearly understood.”

Gaining that understanding, he says, requires studying how the magnetic properties change when an electric field is applied. Up to now, this has been difficult due to the complicated structure of most magnetoelectric materials.

In the past, says Eom, people studied magnetoelectric properties using very “complex” materials, or those that lack uniformity. In his approach, Eom simplified not only the research, but the material itself.

Drawing on his expertise in material growth, he developed a unique process, using atomic “steps,” to guide the growth of a homogenous, single-crystal thin film of bismuth ferrite. Atop that, he added cobalt, which is magnetic; on the bottom, he placed an electrode made of strontium ruthenate.

The bismuth ferrite material was important because it made it much easier for Eom to study the fundamental magnetoelectric cross-coupling.

“We found that in our work, because of our single domain, we could actually see what was going on using multiple probing, or imaging, techniques,” he says. “The mechanism is intrinsic. It’s reproducible — and that means you can make a device without any degradation, in a predictable way.”

To image the changing electric and magnetic properties switching in real time, Eom and his colleagues used the powerful synchrotron light sources at Argonne National Laboratory outside Chicago, and in Switzerland and the United Kingdom.

“When you switch it, the electrical field switches the electric polarization. If it’s ‘downward,’ it switches ‘upward,'” he says. “The coupling to the magnetic layer then changes its properties: a magnetoelectric storage device.”

That change in direction enables researchers to take the next steps needed to add programmable integrated circuits — the building blocks that are the foundation of our electronics — to the material.

While the homogenous material enabled Eom to answer important scientific questions about how magnetoelectric cross-coupling happens, it also could enable manufacturers to improve their electronics.

“Now we can design a much more effective, efficient and low-power device,” he says.

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future: While graphene – a one atom thin, honeycomb-shaped carbon layer – is a conductive material, it can become a semiconductor in the form of nanoribbons. This means that it has a sufficiently large energy or band gap in which no electron states can exist: it can be turned on and off – and thus may become a key component of nanotransistors.

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The smallest details in the atomic structure of these graphene bands, however, have massive effects on the size of the energy gap and thus on how well-suited nanoribbons are as components of transistors. On the one hand, the gap depends on the width of the graphene ribbons, while on the other hand it depends on the structure of the edges. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals, i.e. they are conductive, they become semiconductors with the armchair edge.

This poses a major challenge for the production of nanoribbons: If the ribbons are cut from a layer of graphene or made by cutting carbon nanotubes, the edges may be irregular and thus the graphene ribbons may not exhibit the desired electrical properties.

Creating a semiconductor with nine atoms

Empa researchers in collaboration with the Max Planck Institute for Polymer Research in Mainz and the University of California at Berkeley have now succeeded in growing ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. The specially prepared molecules are evaporated in an ultra-high vacuum for this purpose. After several process steps, they are combined like puzzle pieces on a gold base to form the desired nanoribbons of about one nanometer in width and up to 50 nanometers in length.

These structures, which can only be seen with a scanning tunneling microscope, now have a relatively large and, above all, precisely defined energy gap. This enabled the researchers to go one step further and integrate the graphene ribbons into nanotransistors. Initially, however, the first attempts were not very successful: Measurements showed that the difference in the current flow between the “ON” state (i.e. with applied voltage) and the “OFF” state (without applied voltage) was far too small. The problem was the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, it needed to be 50 nanometers thick, which in turn influenced the behavior of the electrons.

However, the researchers subsequently succeeded in massively reducing this layer by using hafnium oxide(HfO2) instead of silicon oxide as the dielectric material. The layer is therefore now only 1.5 nanometers thin and the “on”-current is orders of magnitudes higher.

Another problem was the incorporation of graphene ribbons into the transistor. In the future, the ribbons should no longer be located criss-cross on the transistor substrate, but rather aligned exactly along the transistor channel. This would significantly reduce the currently high level of non-functioning nanotransistors.