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The Semiconductor Industry Association (SIA) today announced worldwide sales of semiconductors reached $31.0 billion for the month of November 2016, an increase of 7.4 percent compared to the November 2015 total of $28.9 billion and 2.0 percent more than the October 2016 total of 30.4 billion. November marked the market’s largest year-to-year growth since January 2015. All monthly sales numbers are compiled by the World Semiconductor Trade Statistics (WSTS) organization and represent a three-month moving average.

“Global semiconductor sales continued to pick up steam in November, increasing at the highest rate in almost two years and nearly pulling even with the year-to-date total from the same point in 2015,” said John Neuffer, president and CEO, Semiconductor Industry Association. “The Chinese market continues to stand out, growing nearly 16 percent year-to-year to lead all regional markets. As 2016 draws to a close, the global semiconductor market appears likely to roughly match annual sales from 2015 and is well-positioned for a solid start to 2017.”

Month-to-month sales increased modestly across all regions: the Americas (3.3 percent), China (2.7 percent), Europe (2.5 percent), Asia Pacific/All Other (0.7 percent), and Japan (0.4 percent). Year-to-year sales increased in China (15.8 percent), Japan (8.2 percent), Asia Pacific/All Other (4.8 percent), and the Americas (3.2 percent), but fell slightly in Europe (-1.6 percent).

Traditional computer memory, known as DRAM, uses electric fields to store information. In DRAM, the presence or absence of an electric charge is indicated either by number 1 or number 0. Unfortunately, this type of information storage is transient and information is lost when the computer is turned off. Newer types of memory, MRAM and FRAM, use long-lasting ferromagnetism and ferroelectricity to store information. However, no technology thus far combines the two.

To address this challenge, a group of scientists led by Prof. Masaki Azuma from the Laboratory for Materials and Structures at Tokyo Institute of Technology, along with associate Prof. Hajime Hojo at Kyushu University previously at Tokyo Tech, Prof. Ko Mibu at Nagoya Institute of Technology and five other researchers demonstrated the multiferroic nature of a thin film of BiFe1?xCoxO3 (BFCO). Multiferroic materials exhibit both ferromagnetism and ferroelectricity. These are expected to be used as multiple-state memory devices. Furthermore, if the two orders are strongly coupled and the magnetization can be reversed by applying an external electric field, the material should work as a form of low power consumption magnetic memory.

Previous scientists had speculated that ferroelectric BFO thin film, a close relative of BFCO, might be ferromagnetic as well, but they were thwarted by the presence of magnetic impurity. Prof. M. Azuma’s team successfully synthesized pure, thin films of BFCO by using pulsed laser deposition to perform epitaxial growth on a SrTiO3 (STO) substrate. They then conducted a series of tests to show that BFCO is both ferroelectric and ferromagnetic at room temperature. They manipulated the direction of ferroelectric polarization by applying an electric field, and showed that the low-temperature cychloidal spin structure, essentially the same as that of BiFeO3, changes to a collinear one with ferromagnetism at room temperature.

In the future, the scientists hope to realize electrical control of ferromagnetism, which could be applied in low power consumption, non-volatile memory devices.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences have made the world’s smallest radio receiver – built out of an assembly of atomic-scale defects in pink diamonds.

This tiny radio — whose building blocks are the size of two atoms — can withstand extremely harsh environments and is biocompatible, meaning it could work anywhere from a probe on Venus to a pacemaker in a human heart.

The research was led by Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at SEAS, and his graduate student Linbo Shao and published in Physical Review Applied.

The radio uses tiny imperfections in diamonds called nitrogen-vacancy (NV) centers. To make NV centers, researchers replace one carbon atom in a diamond crystal with a nitrogen atom and remove a neighboring atom — creating a system that is essentially a nitrogen atom with a hole next to it. NV centers can be used to emit single photons or detect very weak magnetic fields. They have photoluminescent properties, meaning they can convert information into light, making them powerful and promising systems for quantum computing, phontonics and sensing.

Radios have five basic components — a power source, a receiver, a transducer to convert the high-frequency electromagnetic signal in the air to a low-frequency current, speaker or headphones to convert the current to sound and a tuner.

In the Harvard device, electrons in diamond NV centers are powered, or pumped, by green light emitted from a laser. These electrons are sensitive to electromagnetic fields, including the waves used in FM radio, for example. When NV center receives radio waves it converts them and emits the audio signal as red light. A common photodiode converts that light into a current, which is then converted to sound through a simple speaker or headphone.

An electromagnet creates a strong magnetic field around the diamond, which can be used to change the radio station, tuning the receiving frequency of the NV centers.

Shao and Loncar used billions of NV centers in order to boost the signal, but the radio works with a single NV center, emitting one photon at a time, rather than a stream of light.

The radio is extremely resilient, thanks to the inherent strength of diamond. The team successfully played music at 350 degrees Celsius — about 660 Fahrenheit.

“Diamonds have these unique properties,” said Loncar. “This radio would be able to operate in space, in harsh environments and even the human body, as diamonds are biocompatible.”

Graphene, a material that could usher in the next generation of electronic and energy devices, could be closer than ever to mass production, thanks to microwaves.

A new study by an international team of researchers from UNIST and Rutgers University has proved that it is now possible to produce high quality graphene, using a microwave oven. The team reports that this new technique may have solved some of graphene’s difficult manufacturing problems. The findings of the research have been published in the September issue of the prestigious journal Science.

Reducing graphene oxide sheets (prGON) into pristine graphene, using 1-to-2 second pulses of microwaves. Credit: UNIST

Reducing graphene oxide sheets (prGON) into pristine graphene, using 1-to-2 second pulses of microwaves. Credit: UNIST

This study was jointly conducted by Dr. Jieun Yang, an alumna of UNIST, Prof. Hyeon Suk Shin (School of Natural Science) of UNIST, Prof. Hu Young Jeon (School of Natural Science) of UNIST, Prof. Manish Chhowalla of Rutgers University, and five other researchers from Rutgers University, New Brunswick, NJ, United States.

Graphene comes from a base material of graphite, the cheap material in the ‘lead’ of pencils. The structure of graphite consists of many flat layers of graphene sheets. One of the most promising ways to achieve large quantities of graphene is to exfoliate graphite into individual graphene sheets by using chemicals. However, the oxygen exposure during the process may cause some inevitable side reactions, as it can ultimately be very damaging to the individual graphene layers.

Indeed, oxygen distorts the pristine atomic structure of graphene and degrades its properties. Therefore, removing oxygen from graphene oxide to obtain high-quality graphene has been a significant challenge over the past two decades for the scientific community working on graphene.

Dr. Yang and her research team have discovered that baking the exfoliated graphene oxide for just 1-to-2 second pulses of microwaves, can eliminate virtually all of the oxygen from graphene oxides.

“The partially reduced graphene oxides absorb microwave energy, produced inside a microwave oven ,” says Dr. Yang, the lead author of the study. She adds, “This not only efficiently eliminates oxygen functional groups from graphene oxides, but is also capable of rearranging defective graphene films.”

The results indicate that the new graphene exibits substantially reduced oxygen concentration of 4% much lower than the currently existing graphene with an oxygen content in the range of 15% to 25%.

Prof. Shin states, “Countries around the world, such as South Korea, U.S., England, and China have been investing heavily in research for the affordable, mass commercialization of graphene.”

He adds, “The current method for mass-producing high-quality graphene lacks reproducibility, but holds huge untapped market potential. Therefore, securing the fundamental technology for mass production of graphene is an extremely important matter in terms of commercializing future promising industries.”

The study’s co-author, Prof. Manish Chhowalla is an associate chair in the Department of Materials Science and Engineering in Rutgers’ School of Engineering and Director of the Rutgers Institute for Advanced Materials, Devices and Nanotechnology. Prof. Chhowalla has been working on a joint research project with Prof. Shin and Prof. Jeon of UNIST. Dr. Jieun Yang, a former student of Prof. Shin is now working as a post-doctoral associate in Chhowalla’s group at Rutgers University.

The Semiconductor Industry Association (SIA), representing U.S. leadership in semiconductor manufacturing, design, and research, today announced worldwide sales of semiconductors reached $30.5 billion for the month of October 2016, an increase of 3.4 percent from last month’s total of $29.5 billion and 5.1 percent higher than the October 2015 total of $29.0 billion. All monthly sales numbers are compiled by the World Semiconductor Trade Statistics (WSTS) organization and represent a three-month moving average. Additionally, a new WSTS industry forecast projects roughly flat annual semiconductor sales in 2016, followed by slight market growth in 2017 and 2018.

“The global semiconductor market has rebounded in recent months, with October marking the largest year-to-year sales increase since March 2015,” said John Neuffer, president and CEO, Semiconductor Industry Association. “Sales increased compared to last month across all regional markets and nearly every major semiconductor product category. Meanwhile, the latest industry forecast has been revised upward and now calls for flat annual sales in 2016 and small increases in 2017 and 2018. All told, the industry is well-positioned for a strong close to 2016.

Regionally, year-to-year sales increased in China (14.0 percent), Japan (7.2 percent), Asia Pacific/All Other (1.9 percent), and the Americas (0.1 percent), but decreased in Europe (-3.0 percent). Compared with last month, sales were up across all regional markets: the Americas (6.5 percent), China (3.2 percent), Japan (3.0 percent), Europe (2.2 percent), and Asia Pacific/All Other (2.0 percent).

Additionally, SIA today endorsed the WSTS Autumn 2016 global semiconductor sales forecast, which projects the industry’s worldwide sales will be $335.0 billion in 2016, a 0.1 percent decrease from the 2015 sales total. WSTS projects a year-to-year increase in Japan (3.2 percent) and Asia Pacific (2.5 percent), with decreases expected in Europe (-4.9 percent) and the Americas (-6.5 percent). Among major semiconductor product categories, WSTS forecasts growth in 2016 for sensors (22.6 percent), discretes (4.2 percent), analog (4.8 percent) and MOS micro ICs (2.3 percent), which include microprocessors and microcontrollers.

Beyond 2016, the semiconductor market is expected to grow at a modest pace across all regions. WSTS forecasts 3.3 percent growth globally for 2017 ($346.1 billion in total sales) and 2.3 percent growth for 2018 ($354.0 billion). WSTS tabulates its semi-annual industry forecast by convening an extensive group of global semiconductor companies that provide accurate and timely indicators of semiconductor trends.

All our smart phones have shiny flat AMOLED displays. Behind each single pixel of these displays hide at least two silicon transistors which were mass-manufactured using laser annealing technologies. While the traditional methods to make them uses temperatures above 1,000°C, the laser technique reaches the same results at low temperatures even on plastic substrates (melting temperature below 300°C). Interestingly, a similar procedure can be used to generate crystals of graphene. Graphene is a strong and thin nano-material made of carbon, its electric and heat-conductive properties have attracted the attention of scientists worldwide.

High-resolution transmission electron microscopy shows that after just one laser pulse of 30 nanoseconds, the silicon carbide (SiC) substrate is melted and separates into a carbon and a silicon layer. More pulses cause the carbon layer to organize into graphene and the silicon to leave as gas. Credit: IBS

High-resolution transmission electron microscopy shows that after just one laser pulse of 30 nanoseconds, the silicon carbide (SiC) substrate is melted and separates into a carbon and a silicon layer. More pulses cause the carbon layer to organize into graphene and the silicon to leave as gas. Credit: IBS

Prof. KEON Jae Lee’s research group at the Center for Multidimensional Carbon Materials within the Institute for Basic Science (IBS) and Prof. CHOI Sung-Yool’s team at KAIST discovered graphene synthesis mechanism using laser-induced solid-state phase separation of single-crystal silicon carbide (SiC). This study, available on Nature Communications, clarifies how this laser technology can separate a complex compound (SiC) into its ultrathin elements of carbon and silicon.

Although several fundamental studies understood the effect of excimer lasers in transforming elemental materials like silicon, the laser interaction with more complex compounds like SiC has rarely been studied due to the complexity of compound phase transition and ultra-short processing time.

With high resolution microscope images and molecular dynamic simulations, scientists found that a single-pulse irradiation of xenon chloride excimer laser of 30 nanoseconds melts SiC, leading to the separation of a liquid SiC layer, a disordered carbon layer with graphitic domains (about 2.5 nm thick) on top surface and a polycrystalline silicon layer (about 5 nm) below carbon layer. Giving additional pulses causes the sublimation of the separated silicon, while the disordered carbon layer is transformed into a multilayer graphene.

“This research shows that the laser material interaction technology can be a powerful tool for next generation of two dimensional nanomaterials,” said Prof. Keon. Prof. Choi added: “Using laser-induced phase separation of complex compounds, new types of two dimensional materials can be synthesized in the future.” IBS Prof. Keon is affiliated with the School of Materials Science and Engineering, KAIST and Prof. Choi with the School of Electrical Engineering and Graphene Research Center, KAIST.

They’re flexible, cheap to produce and simple to make – which is why perovskites are the hottest new material in solar cell design. And now, engineers at Australia’s University of New South Wales in Sydney have smashed the trendy new compound’s world efficiency record.

Dr. Anita Ho-Baillie, a Senior Research Fellow at the Australian Centre for Advanced Photovoltaics at UNSW, with the new perovskite cell. Credit: Rob Largent/UNSW

Dr. Anita Ho-Baillie, a Senior Research Fellow at the Australian Centre for Advanced Photovoltaics at UNSW, with the new perovskite cell. Credit: Rob Largent/UNSW

Speaking at the Asia-Pacific Solar Research Conference in Canberra on Friday 2 December, Anita Ho-Baillie, a Senior Research Fellow at the Australian Centre for Advanced Photovoltaics (ACAP), announced that her team at UNSW has achieved the highest efficiency rating with the largest perovskite solar cells to date.

The 12.1% efficiency rating was for a 16 cm2 perovskite solar cell, the largest single perovskite photovoltaic cell certified with the highest energy conversion efficiency, and was independently confirmed by the international testing centre Newport Corp, in Bozeman, Montana. The new cell is at least 10 times bigger than the current certified high-efficiency perovskite solar cells on record.

Her team has also achieved an 18% efficiency rating on a 1.2 cm2 single perovskite cell, and an 11.5% for a 16 cm2 four-cell perovskite mini-module, both independently certified by Newport.

“This is a very hot area of research, with many teams competing to advance photovoltaic design,” said Ho-Baillie. “Perovskites came out of nowhere in 2009, with an efficiency rating of 3.8%, and have since grown in leaps and bounds. These results place UNSW amongst the best groups in the world producing state-of-the-art high-performance perovskite solar cells. And I think we can get to 24% within a year or so.”

Perovskite is a structured compound, where a hybrid organic-inorganic lead or tin halide-based material acts as the light-harvesting active layer. They are the fastest-advancing solar technology to date, and are attractive because the compound is cheap to produce and simple to manufacture, and can even be sprayed onto surfaces.

“The versatility of solution deposition of perovskite makes it possible to spray-coat, print or paint on solar cells,” said Ho-Baillie. “The diversity of chemical compositions also allows cells be transparent, or made of different colours. Imagine being able to cover every surface of buildings, devices and cars with solar cells.”

Most of the world’s commercial solar cells are made from a refined, highly purified silicon crystal and, like the most efficient commercial silicon cells (known as PERC cells and invented at UNSW), need to be baked above 800°C in multiple high-temperature steps. Perovskites, on the other hand, are made at low temperatures and 200 times thinner than silicon cells.

But although perovskites hold much promise for cost-effective solar energy, they are currently prone to fluctuating temperatures and moisture, making them last only a few months without protection. Along with every other team in the world, Ho-Baillie’s is trying to extend its durability. Thanks to what engineers learned from more than 40 years of work with layered silicon, they’re are confident they can extend this.

Nevertheless, there are many existing applications where even disposable low-cost, high-efficiency solar cells could be attractive, such as use in disaster response, device charging and lighting in electricity-poor regions of the world. Perovskite solar cells also have the highest power to weight ratio amongst viable photovoltaic technologies.

“We will capitalise on the advantages of perovskites and continue to tackle issues important for commercialisation, like scaling to larger areas and improving cell durability,” said Martin Green, Director of the ACAP and Ho-Baillie’s mentor. The project’s goal is to lift perovskite solar cell efficiency to 26%.

The research is part of a collaboration backed by $3.6 million in funding through the Australian Renewable Energy Agency’s (ARENA) ‘solar excellence’ initiative. ARENA’s CEO Ivor Frischknecht said the achievement demonstrated the importance of supporting early stage renewable energy technologies: “In the future, this world-leading R&D could deliver efficiency wins for households and businesses through rooftop solar as well as for big solar projects like those being advanced through ARENA’s investment in large-scale solar.”

To make a perovskite solar cells, engineers grow crystals into a structure known as ‘perovskite’, named after Lev Perovski, the Russian mineralogist who discovered it. They first dissolve a selection of compounds in a liquid to make the ‘ink’, then deposit this on a specialised glass which can conduct electricity. When the ink dries, it leaves behind a thin film that crystallises on top of the glass when mild heat is applied, resulting in a thin layer of perovskite crystals.

The tricky part is growing a thin film of perovskite crystals so the resulting solar cell absorbs a maximum amount of light. Worldwide, engineers are working to create smooth and regular layers of perovskite with large crystal grain sizes in order to increase photovoltaic yields.

Ho-Baillie, who obtained her PhD at UNSW in 2004, is a former chief engineer for Solar Sailor, an Australian company which integrates solar cells into purpose-designed commercial marine ferries which currently ply waterways in Sydney, Shanghai and Hong Kong.

Researchers at the NYU Tandon School of Engineering have pioneered a method for growing an atomic scale electronic material at the highest quality ever reported. In a paper published in Applied Physics Letters, Assistant Professor of Electrical and Computer Engineering Davood Shahrjerdi and doctoral student Abdullah Alharbi detail a technique for synthesizing large sheets of high-performing monolayer tungsten disulfide, a synthetic material with a wide range of electronic and optoelectronic applications.

“We developed a custom reactor for growing this material using a routine technique called chemical vapor deposition. We made some subtle and yet critical changes to improve the design of the reactor and the growth process itself, and we were thrilled to discover that we could produce the highest quality monolayer tungsten disulfide reported in the literature,” said Shahrjerdi. “It’s a critical step toward enabling the kind of research necessary for developing next-generation transistors, wearable electronics, and even flexible biomedical devices.”

The promise of two-dimensional electronic materials has tantalized researchers for more than a decade, since the first such material — graphene — was experimentally discovered. Also called “monolayer” materials, graphene and similar two-dimensional materials are a mere one atom in thickness, several hundred thousand times thinner than a sheet of paper. These materials boast major advantages over silicon — namely unmatched flexibility, strength, and conductivity — but developing practical applications for their use has been challenging.

Graphene (a single layer of carbon) has been explored for electronic switches (transistors), but its lack of an energy band gap poses difficulties for semiconductor applications. “You can’t turn off the graphene transistors,” explained Shahrjerdi. Unlike graphene, tungsten disulfide has a sizeable energy band gap. It also displays exciting new properties: When the number of atomic layers increases, the band gap becomes tunable, and at monolayer thickness it can strongly absorb and emit light, making it ideal for applications in optoelectronics, sensing, and flexible electronics.

Efforts to develop applications for monolayer materials are often plagued by imperfections in the material itself — impurities and structural disorders that can compromise the movement of charge carriers in the semiconductor (carrier mobility). Shahrjerdi and his student succeeded in reducing the structural disorders by omitting the growth promoters and using nitrogen as a carrier gas rather than a more common choice, argon.

Shahrjerdi noted that comprehensive testing of their material revealed the highest values recorded thus far for carrier mobility in monolayer tungsten disulfide. “It’s a very exciting development for those of us doing research in this field,” he said.

van der Pauw measurements with a parameter analyzer are examined followed by a look at Hall effects measurements.

BY MARY ANNE TUPTA, Keithley Instruments Product Line at Tektronix, Cleveland, OH

Semiconductor material research and device testing often involves determining the resistivity and Hall mobility of a sample. The resistivity of a particular semiconductor material primarily depends on the bulk doping used. In a device, the resistivity can affect the capacitance, the series resistance, and the threshold voltage, so it’s important to perform this measurement carefully and accurately.

The resistivity of the semiconductor material is often determined using a four-point probe or Kelvin technique where two of the probes are used to source current and the other two probes are used to measure voltage. Using four probes eliminates measurement errors due to probe resistance, spreading resistance under each probe, and contact resis- tance between each metal probe and the semiconductor material. Because a high impedance voltmeter draws little current, the voltage drops are very small.

One useful Kelvin technique for determining the resistivity of a semiconductor material is the van der Pauw (vdp) method using a parameter analyzer with high input impedance and accurate low current sourcing. This article first looks at van der Pauw measurements with a parameter analyzer followed by a look at Hall effects measurements.

van der Pauw resistivity measurements

The van der Pauw method involves applying a current and measuring voltage using four small contacts on the circumference of a flat, arbitrarily shaped sample of uniform thickness. This method is particularly useful for measuring very small samples because geometric spacing of the contacts is unimportant, meaning that effects due to a sample’s size are irrelevant.

Using this method, the resistivity is derived from a total of eight measurements that are made around the periphery of the sample using the configurations shown in FIGURE 1.

FIGURE 1. van der Pauw resistivity conventions.

FIGURE 1. van der Pauw resistivity conventions.

Once all the voltage measurements are taken, two values of resistivity, ρA and ρB, are derived as follows:

Equation 1

 

where: ρA and ρB are volume resistivities in ohm-cm

ts is the sample thickness in cm

V1–V8 represents the voltages measured by the voltmeter

I is the current through the sample in amperes

fA and fB are geometrical factors based on sample symmetry. They are related to the two resistance ratios QA and QB as shown in the following equations (fA = fB = 1 for perfect symmetry).

QA and QB are calculated using the measured voltages as follows:

Equation 2

Also, Q and f are related as follows:

Equation 3

A plot of this function is shown in FIGURE 2. The value of f can be found from this plot once Q has been calculated.

FIGURE 2. Plot of f vs. Q.

FIGURE 2. Plot of f vs. Q.

Once ρA and ρB are known, the average resistivity (ρAVG) can be determined as follows:

Equation 4

The electrical measurements for determining van der Pauw resistivity require a current source and a voltmeter. To automate measurements, it’s possible to use a programmable switch to switch the current source and the voltmeter to all sides of the sample. However, a parameter analyzer offers greater efficiency.

A parameter analyzer with four source measure units (SMU) and four preamps (for high resistance measurements) is well-suited for performing van der Pauw resis- tivity measurements, and enables measurements of resistances greater than 1012Ω. A key advantage is that each SMU instrument can be configured as a current source or as a voltmeter with no external switching required. This eliminates leakage and offsets errors caused by mechanical switches as well as the need for additional instruments and programming.

For high resistance materials, a current source that can output very small current with a high output impedance is necessary. A differential electrometer with high input impedance is required to minimize loading effects on the sample.

Each terminal of the sample is connected to one SMU instrument, so a parameter analyzer with four SMU instruments is required. A diagram of how the four SMUs are configured for each of the tests is shown in FIGURE 3. For each test, three of the SMU instruments are configured as a current bias and a voltmeter. One of the SMUs applies the test current and the other two SMUs are used as high impedance voltmeters with a test current of zero amps on a low current range (typically 1nA range). The fourth SMU instrument is set to common. The voltage difference is calculated between the two SMU instruments set up as high impedance voltmeters. This measurement setup is duplicated around the sample, with each of the four SMU instruments changing functions in each of the four tests. The test current and voltage differences between the terminals from the four tests are used to calculate resistivity.

FIGURE 3. SMU Instrument Configurations for van der Pauw Measurements.

FIGURE 3. SMU Instrument Configurations for van der Pauw Measurements.

For high resistance samples, it’s necessary to determine the settling time of the measurement. This is done by sourcing current into two terminals of the sample and measuring the voltage difference between the other two terminals. The settling time can be determined by graphing the voltage difference versus the time of the measurement. A timing graph of a very high resistance material is shown in FIGURE 4. Note that settling time needs to be determined every time for different materials; however, it’s not necessary for low resistance materials since they have a short settling time.

FIGURE 4. Voltage vs. time graph of a very high resistance sample.

FIGURE 4. Voltage vs. time graph of a very high resistance sample.

Hall voltage measurements

Hall effect measurements are important to semiconductor material characterization because from the Hall voltage, the conductivity type, carrier density, and mobility can be derived. With an applied magnetic field, the Hall voltage can be measured using the configurations shown in FIGURE 5.

FIGURE 5. Hall voltage measurement configurations.

FIGURE 5. Hall voltage measurement configurations.

With a positive magnetic field, B, current is applied between terminals 1 and 3, and the voltage drop (V2–4+) is measured between terminals 2 and 4. When the current is reversed, the voltage drop (V4–2+) is measured. Next, current is applied between terminals 2 and 4, and the voltage drop (V1–3+) between terminals 1 and 3 is measured. Then the current is reversed and the voltage (V3–1+) is measured again.

Then the magnetic field, B, is reversed and the procedure is repeated again, measuring the four voltages: (V2–4–), (V4–2–), (V1–3–), and (V3–1–).

From the eight Hall voltage measurements, the average Hall coefficient can be calculated as follows:

Equation 5

where: RHC and RHD are Hall coefficients in cm3/C

ts is the sample thickness in cm

V represents the voltages measured by the voltmeter

I is the current through the sample in amperes

B is the magnetic flux in Vs/cm2 (1 Vs/cm2 = 108 gauss)

Once RHC and RHD have been calculated, the average Hall coefficient (RHAVG) can be determined as follows:

Equation 6

From the resistivity (ρAVG) and the Hall coefficient (RHAVG), the mobility (μH) can be calculated:

Equation 7

For successful resistivity measurements, potential sources of errors need to be considered. Here are the errors sources you are most likely to encounter.

Electrostatic Interference — Electrostatic interference occurs when an electrically charged object is brought near an uncharged object. Usually, the effects of the interference are not noticeable because the charge dissi- pates rapidly at low resistance levels. However, high resis- tance materials do not allow the charge to decay quickly and unstable measurements may result. The erroneous readings may be due to either DC or AC electrostatic fields.

To minimize the effects of these fields, an electrostatic shield can be built to enclose the sensitive circuitry. The shield should be made from a conductive material and connected to the low impedance (FORCE LO) terminal of the test instrument. The cabling in the circuit must also be shielded.

Leakage Current — For high resistance samples, leakage current may degrade measurements. The leakage current is due to the insulation resistance of the cables, probes, and test fixturing.

Leakage current may be minimized by using good quality insulators, by reducing humidity, and by using guarding.

A guard is a conductor connected to a low impedance point in the circuit that is nearly at the same potential as the high impedance lead being guarded. Using triax cabling and fixturing will ensure that the high impedance terminal of the sample is guarded. The guard connection will also reduce measurement time since the cable capacitance will no longer affect the time constant of the measurement.

Light — Currents generated by photoconductive effects can degrade measurements, especially on high resistance samples. To prevent this, the sample should be placed in a dark chamber.

Temperature — Thermoelectric voltages may also affect measurement accuracy. Temperature gradients may result if the sample temperature is not uniform. Thermoelectric voltages may also be generated from sample heating caused by the source current. Heating from the source current will more likely affect low resistance samples, because a higher test current is needed to make the voltage measure- ments easier. Temperature fluctuations in the laboratory environment may also affect measurements. Because semiconductors have a relatively large temperature coeffi- cient, temperature variations in the laboratory may need to be compensated for by using correction factors.

Carrier Injection — To prevent minority/majority carrier injection from influencing resistivity measurements, the voltage difference between the two voltage sensing terminals should be kept at less than 100mV, ideally 25mV, since the thermal voltage, kt/q, is approximately 26mV. The test current should be kept as low as possible without affecting the measurement precision.

Conclusion

The van der Pauw technique in conjunction with a parameter analyzer is a proven method for determining the resistivity of very small samples because geometric spacing of the contacts is unimportant. Hall effect measurements are important to semiconductor material characterization for determining conductivity type, carrier density, and mobility. Some parameter analyzers may include built-in configurable tests that include the necessary calculations.

For successful measurements, it’s important to consider potential sources of error including electronics interference, leakage current and environmental factor such as light and temperature. Resistivity can impact the characteristics of a device, serving as reminder of the importance of making accurate and repeatable measurements.

Gallium nitride (GaN) has emerged as one of the most important and widely used semiconducting materials. Its optoelectronic and mechanical properties make it ideal for a variety of applications, including light-emitting diodes (LEDs), high-temperature transistors, sensors and biocompatible electronic implants in humans.

In 2014, three Japanese scientists won the Nobel Prize in physics for discovering GaN’s critical role in generating blue LED light, which is required, in combination with red and green light, to produce white LED light sources.

Now, four Lehigh engineers have reported a previously unknown property for GaN: Its wear resistance approaches that of diamonds and promises to open up applications in touch screens, space vehicles and radio-frequency microelectromechanical systems (RF MEMS), all of which require high-speed, high-vibration technology.

The researchers reported their findings in August in Applied Physics Letters (APL) in an article titled “Ultralow wear of gallium nitride.” The article’s authors are Guosong Zeng, a Ph.D. candidate in mechanical engineering; Nelson Tansu, Daniel E. ’39 and Patricia M. Smith Endowed Chair Professor in the Electrical and Computer Engineering department, and Director of the Center for Photonics and Nanoelectronics (CPN); Brandon A. Krick, assistant professor of mechanical engineering and mechanics; and Chee-Keong Tan ’16 Ph.D., now assistant professor of electrical and computer engineering at Clarkson University.

GaN’s electronic and optical properties have been studied extensively for several decades, said Zeng, the lead author of the APL article, but virtually no studies have been done of its tribological properties, that is, its resistance to the mechanical wear imposed by reciprocated sliding.

“Our group is the first to investigate the wear performance of GaN,” said Zeng. “We have found that its wear rate approaches that of diamonds, the hardest material known.”

Wear rate is expressed in negative cubic millimeters of Newton meters (Nm). The rate for chalk, which has virtually no wear resistance, is on the order of 10 2 mm3/Nm, while that of diamonds is between 10-9 and 10-10, making diamonds eight orders of magnitude more wear resistant than chalk. The rate for GaN ranges from 10¬-7 to 10-9, approaching the wear resistance of diamonds and three to five orders of magnitude more wear resistant than silicon (10-4).

The Lehigh researchers measured the wear rate and friction coefficients of GaN using a custom microtribometer to perform dry sliding wear experiments. They were surprised by the results.

“When performing wear measurements of unknown materials,” they wrote in APL, “we typically slide for 1,000 cycles, then measure the wear scars; [these] experiments had to be increased to 30,000 reciprocating cycles to be measurable with our optical profilometer.

“The large range in wear rates (about two orders of magnitude)…can provide insight into the wear mechanisms of GaN.”

That range in wear resistance, the researchers said, is caused by several factors, including environment, crystallographic direction and, especially, humidity.

“The first time we observed the ultralow wear rate of GaN was in winter,” said Zeng. “These results could not be replicated in summer, when the material’s wear rate increased by two orders of magnitude.”

To determine how the higher summer humidity was affecting GaN’s wear performance, the researchers put their tribometer in a glove box that can be backfilled with either nitrogen or humid air.

“We observed that as we increased the humidity inside the glove box, we also increased the wear rate of GaN,” said Zeng.

Zeng gave a presentation about the Lehigh project in October at the International Workshop on Nitride Semiconductors (IWN 2016) in Orlando, Florida. The session at which he spoke was titled “Wear of Nitride Materials and Properties of GaN-based structures.” Zeng was one of seven presenters at the session and the only one to discuss the wear properties of GaN and other III-Nitride materials.

Tansu, who has studied GaN for more than a decade, and Krick, a tribology expert, became curious about GaN’s wear performance several years ago when they discussed their research projects after a Lehigh faculty meeting.

“Nelson asked me if anyone had ever investigated the friction and wear properties of gallium nitride,” said Krick, “and I said I didn’t know. We checked later and found a wide-open field.”

Tansu said the group’s discovery of GaN’s hardness and wear performance could have a dramatic effect on the electronic and digital device industries. In a device such as a smartphone, he said, the electronic components are housed underneath a protective coating of glass or sapphire. This poses potential compatibility problems which could be avoided by using GaN.

“The wear resistance of GaN,” said Tansu, “gives us the opportunity to replace the multiple layers in a typical semiconductor device with one layer made of a material that has excellent optical and electrical properties and is wear-resistant as well.

“Using GaN, you can build an entire device in a platform without multiple layers of technologies. You can integrate electronics, light sensors and light emitters and still have a mechanically robust device. This will open up a new paradigm for designing devices. And because GaN can be made very thin and still strong, it will accelerate the move to flexible electronics.”

In addition to its unexpectedly good wear performance, said Zeng, GaN also has a favorable radiation hardness, which is an important property for the solar cells that power space vehicles. In outer space, these solar cells encounter large quantities of very fine cosmic dust, along with x-rays and gamma rays, and thus require a wear-resistant coating, which in turn needs to be compatible with the cell’s electronic circuitry. GaN provides the necessary hardness without introducing compatibility issues with the circuitry.

The Lehigh group has begun collaborating with Bruce E. Koel, a surface chemistry expert and professor of chemical and biological engineering at Princeton University, to gain a better understanding of the interaction of GaN and water under contact. Koel was formerly a chemistry professor and vice president for research and graduate studies at Lehigh.

To determine the evolution of wear with GaN, the group has subjected GaN to stresses by running slide tests in which the slide distance and the corresponding number of cycles are varied. The group then uses an x-ray photoelectron spectrometer (XPS), which can identify the elemental composition of the first 12 nanometers of a surface, to scan the unworn surface of the GaN, the scar created by the slide machine, and the wear particles deposited by the slide machine on either side of the scar.

The group plans next to use aberration-corrected transmission electron microscopy to examine the lattice of atoms beneath the scar. Meanwhile, they will simulate a test in which the lattice is strained with water in order to observe the variations caused by deforming energy.

“This is a very new experiment,” said Zeng. “It will enable us to see dynamic surface chemistry by watching the chemical reaction that results when you apply shear, tensile or compressive pressure to the surface of GaN.”