## Nov 19, 2017

### Researchers image perfectly smooth side-surfaces of 3-D silicon crystals with a scanning tunneling microscope

Figure.1. A schematics of a Si(110) sample with a Kapton film mask: dry etching from the (110) top-surface and STM-tip approaching to the (-111) side-surface. Credit: Osaka University

A research collaboration between Osaka University and the Nara Institute of Science and Technology for the first time used scanning tunneling microscopy (STM) to create images of atomically flat side-surfaces of 3-D silicon crystals. This work helps semiconductor manufacturers continue to innovate while producing smaller, faster, and more energy-efficient computer chips for computers and smartphones

Our computers and smartphones each are loaded with millions of tiny . The processing speed of these devices has increased dramatically over time as the number of transistors that can fit on a single   continues to increase. Based on Moore's Law, the number of transistors per chip will double about every 2 years, and in this area it seems to be holding up. To keep up this pace of rapid innovation, computer manufacturers are continually on the lookout for new methods to make each transistor ever smaller.

Figure.3. Spatial-derivative STM images with 200x200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Current microprocessors are made by adding patterns of circuits to flat silicon wafers. A novel way to cram more transistors in the same space is to fabricate 3-D-structures. Fin-type field effect transistors (FETs) are named as such because they have fin-like silicon structures that extend into the air, off the surface of the chip. However, this new method requires a silicon crystal with a perfectly flat top and side-surfaces, instead of just the top surface, as with current devices. Designing the next generation of chips will require new knowledge of the atomic structures of the side-surfaces.

Figure.3. Spatial-derivative STM images with 200x200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Now, researchers at Osaka University and the Nara Institute of Science and Technology report that they have used STM to image the side-surface of a silicon crystal for the first time. STM is a powerful technique that allows the locations of the individual  atoms to be seen. By passing a sharp tip very close to the sample, electrons can jump across the gap and create an electrical current. The microscope monitored this current, and determined the location of the atoms in the sample.
"Our study is a big first step toward the atomically resolved evaluation of transistors designed to have 3-D-shapes," study coauthor Azusa Hattori says.
To make the side-surfaces as smooth as possible, the researchers first treated the crystals with a process called reactive ion etching. Coauthor Hidekazu Tanaka says, "Our ability to directly look at the side-surfaces using STM proves that we can make artificial 3-D structures with near-perfect atomic surface ordering."
Source:PHY

## Keywords

Etching;
Single crystal;
Wafer;

Source:ScienceDirect

## Oct 22, 2017

### Pushing the limit of thin-film absorption in solar and water-splitting applications

Credit: Rensselaer Polytechnic Institute

A silicon solar cell harvests the energy of the sun as light travels down through light-absorbent silicon. To reduce weight and cost, solar cells are thin, and while silicon absorbs visible light well, it captures less than half of the light in the near-infrared spectrum, which makes up one-third of the sun's energy. The depth of the material limits absorption. But what if light within the cell could be channeled horizontally so that silicon could absorb its energy along the width of the cell rather than its depth?

With such an advance in mind, Shawn-Yu Lin, professor of physics, applied physics, and astronomy at Rensselaer Polytechnic Institute, has built a nanostructure whose crystal lattice bends  as it enters the material and directs it in a path parallel to the surface, known as "parallel to interface refraction." The structure is built of overlapping nanotubes and resembles a three-dimensional grid made of Lincoln Logs. Photonic nanocrystals built using his process enable extreme "light trapping" and could have applications from thin-film solar cells to photo-chemical functions like sensing and water splitting.
"These results prove that this effect exists, that if you follow my guidelines of simple cubic symmetry, you can bend light by 90 degrees. The  forces light to bend in a deterministic manner, rather than random scattering or surface effects," said Lin. "This is a new type of light-matter interaction that lies at the heart of what light trapping is intended to do."
In experimental results, which appear in Scientific Reports, Lin created a photonic crystal using , a material with weak  absorption, to prove the success of the effect. Results using a 900-nanometer thick titanium dioxide photonic nanocrystal showed absorption enhanced by one to two orders of magnitude greater than a reference film of the same material for some regions. Lin built the nanocrystal – first in silicon, now in titanium dioxide – based on the theoretical predictions of his collaborator, Sajeev John, a physicist at the University of Toronto.
Light trapping describes the process of confining light to a given space, usually with the intent of converting it to other forms of energy. In one approach, materials are designed to slow light, so that it spends more time in the material. In the approach Lin used, the light is bent away from its given path, making it travel a longer distance within the material, in this case, the full width of a titanium dioxide wafer.
Light always bends somewhat as it enters a material with a different optical index, something easily seen as light enters water. In water, and many other , the light bends only slightly. The arrangement of atoms in the titanium dioxide crystal Lin created matches the scale of visible light wavelengths, scattering light at multiple points in space simultaneously as it moves into the lattice. As a consequence, light cannot move as it does through space or any continuous medium. Instead, it is bent at an obtuse angle – a phenomenon known as "negative refraction" – and channeled along the width of the material.
To manipulate the flow of visible light, with wavelengths ranging from 400 to 700 nanometers, Lin pioneered a method of building a perfectly symmetrical cube of nanotubes to match the scale of the light. First, a layer of titanium dioxide is deposited on a substrate. Then, a thin layer of chromium dioxide is deposited to serve as a mask for a photolithographic process that etches lines into the titanium dioxide. Once completed, a solvent is used to remove the remaining chromium dioxide, completing the first layer of "logs." To build additional layers, a layer of silicon dioxide is deposited to fill the cavities between the logs, the surface is polished flat to the top of the first layer, and the entire process is repeated at a precise 90-degree angle from the first layer.
One layer of the material is less than 1 millionth of a meter – or micron – thick, but was produced in wafers 100 millimeters wide, giving the material as much as 100,000 times the space to absorb light.
"This discovery proves a huge enhancement in path length when using a material that has a very low absorption. Its discovery changes the name of the game from vertically absorbed, to horizontally absorbed in a super thin structure," said Lin.
Lin and John were joined in their research by Rensselaer postdoctoral research associates Brian J. Frey and Ping Kuang, and Mei-Li Hsieh of the National Chiao-Tung University in Tiawan, and Jian-Hua Jiang, of Soochow University in China. "Effectively infinite optical path-length created using a simple cubic photonic crystal for extreme light trapping" is published in Scientific Reports.
Lin's research fulfills The New Polytechnic, an emerging paradigm for higher education which recognizes that global challenges and opportunities are so great they cannot be adequately addressed by even the most talented person working alone. Rensselaer serves as a crossroads for collaboration—working with partners across disciplines, sectors, and geographic regions—to address complex global challenges, using the most advanced tools and technologies, many of which are developed at Rensselaer. Research at Rensselaer addresses some of the world's most pressing technological challenges—from energy security and sustainable development to biotechnology and human health. The New Polytechnic is transformative in the global impact of research, in its innovative pedagogy, and in the lives of students at Rensselaer.
SourcePHYS

### Monocrystal Introduced World’s First 350 Kg KY Sapphire Crystal

Monocrystal, a Russian manufacturer specializing in synthetic sapphire growing and processing, recently demonstrated the world’s first 350 kg KY sapphire crystal.

(Monocrystal/ LEDinside)

The 350 kg crystal is a part of Monocrystal’s technological roadmap, aimed at enabling higher crystal uniformity, more efficient large diameter ingot throughput for LED, and size-sensitive optical applications.

Low bubble content, which is crucial for ultra-large sapphire products, has successfully been achieved with the 350 kg crystal.

Another objective of the roadmap is to move Monocrystal’s sapphire supply reliability to a new level, which is now of paramount importance, since major LED makers are increasing their capacities.

“Our ongoing “Extra Large Stress Free” initiative is in response to challenging conditions on our main market: sapphire for LEDs. Extra-large crystals enable high crystal uniformity across a 6” wafer surface and guarantee uniform wavelength distribution. Our LED customers are able to ramp up their production securely, having Monocrystal as a reliable source,” Monocrystal’s CEO Oleg Kachalov commented.

“We also have received a very positive feedback from the non-LED market since the introduction of our extra-large crystals in 2015. Working closely with our partners, we have already enabled several promising large-size applications. We are confident that our new 350 kg crystals will allow greater flexibility of our customers’ designs and further expand the scope of sapphire use,” Monocrystal’s VP Sales Mikhail Berest added.

Keywords:LED,Monocrystal,sapphire crystal,

Source:LEDinside

## Sep 6, 2017

### Researchers find novel technique for tuning the color of LED light emission

Credit: Foto Ruhrgebiet / fotolia.com

The color of the light emitted by an LED can be tuned by altering the size of their semiconductor crystals. LMU researchers have now found a clever and economical way of doing just that, which lends itself to industrial-scale production.

Unlike our old friend the incandescent lightbulb,  (or LEDs) produce light of a defined color within the spectral range from the infrared to the ultraviolet. The exact wavelength of the emission is determined by the chemical composition of the  employed, which is the crucial component of these devices. In the case of some semi-conducting materials, the color can also be tuned by appropriately modifying the size of the crystals of which the light-emitting layer is composed. In crystals with dimensions on the order of a few nanometers, quantum mechanical effects begin to make themselves felt.

LMU researchers in collaboration with colleagues at the University of Linz (Austria) have now developed a method for the production of semi-conducting nanocrystals of defined size based on the cheap mineral oxide known as perovskite. These crystals are extremely stable, which ensures that the LEDs exhibit high color fidelity – an important criterion of quality. Moreover, the resulting semiconductors can be printed on suitable surfaces, and are thus predestined for the manufacture of LEDs for use in displays.

The crucial element in the new method is a thin wafer, only a few nanometers thick, which is patterned like a waffle. The depressions serve as tiny reaction vessels, whose shape and volume ultimately determine the final size of the nanocrystals. "Optimal measurements of the size of the crystals were obtained using a fine beam of high-energy X-radiation at the Deutsche Elektronen-Synchrotron (DESY) in Hamburg", says LMU researcher Dr. Bert Nickel, member of the Nanosystems Initiative Munich (NIM), a Cluster of Excellence.

Moreover, the wafers are produced by means of an economical electrochemical process, and can be fashioned directly into LEDs. "Our nanostructure oxide layers also prevent contact between the  and deleterious environmental factors such as free oxygen and water, which would otherwise limit the working lifetime of the LEDs," as Dr. Martin Kaltenbrunner of the Johannes Kepler University in Linz explains. In the next step, we want to enhance the efficiency of these diodes further, and explore their potential for use in other applications, such as flexible displays.

Source:PHYS

## Keywords

Silicon
Scratch test
Fracture
Anisotropy
Finite element analysis

Source:ScienceDirect

## Abstract

The use of III-nitride-based light-emitting diodes (LEDs) is now widespread in applications such as indicator lamps, display panels, backlighting for liquid-crystal display TVs and computer screens, traffic lights, etc. To meet the huge market demand and lower the manufacturing cost, the LED industry is moving fast from 2 inch to 4 inch and recently to 6 inch wafer sizes. Although Al2O3(sapphire) and SiC remain the dominant substrate materials for the epitaxy of nitride LEDs, the use of large Si substrates attracts great interest because Si wafers are readily available in large diameters at low cost. In addition, such wafers are compatible with existing processing lines for 6 inch and larger wafers commonly used in the electronics industry. During the last decade, much exciting progress has been achieved in improving the performance of GaN-on-Si devices. In this contribution, the status and prospects of III-nitride optoelectronics grown on Si substrates are reviewed. The issues involved in the growth of GaN-based LED structures on Si and possible solutions are outlined, together with a brief introduction to some novel in situ and ex situ monitoring/characterization tools, which are especially useful for the growth of GaN-on-Si structures.
Source:IOPscience

## Abstract

The unique properties of diamond have stimulated the study of and search for its applications in many fields, including optics, optoelectronics, electronics, biology, and electrochemistry. Whereas chemical vapor deposition allows the growth of polycrystalline diamond plates more than 200 mm in diameter, most current diamond application technologies require large-size (25 mm and more) single-crystal diamond substrates or films suitable for the photolithography process. This is quite a challenge, because the largest diamond crystals currently available are 10 mm or less in size. This review examines three promising approaches to fabricating large-size diamond single crystals: growing large-size single crystals, the deposition of heteroepitaxial diamond films on single-crystal substrates, and the preparation of composite diamond substrates.
Source:IOPscience

## Abstract

A versatile and reliable approach is created to fabricate wafer-scale colloidal crystal that consists of a monolayer of hexagonally close-packed polystyrene (PS) spheres. Making wafer-scale colloidal crystal is usually challenging, and it lacks a general theoretical guidance for experimental approaches. To obtain the optimal conditions for self-assembly, a systematic statistical design and analysis method is utilized here, which applies the pick-the-winner rule. This new method combines spin-coating and thermal treatment, and introduces a mixture of glycol and ethanol as a dispersion system to assist self-assembly. By controlling the parameters of self-assembly, we improve the quality of colloidal crystal and reduce the effect of noise on the experiment. To our best knowledge, we are first to pave this path to harvest colloidal crystals. Importantly, a theoretical analysis using an energy landscape base on our process is also developed to provide insights into the PS spheres' self-assembly.
Keywords:Crystal wafer,

Source:IEEE

## Abstract

The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.
Keywords:single crystals,β-Ga2O3,
Source: iopscience

## Abstract

A small Fresnel lens array was diamond turned in a single crystal (0 0 1) InSb wafer using a half-radius negative rake angle (−25°) single-point diamond tool. The machined array consisted of three concave Fresnel lenses cut under different machining sequences. The Fresnel lens profiles were designed to operate in the paraxial domain having a quadratic phase distribution. The sample was examined by scanning electron microscopy and an optical profilometer. Optical profilometry was also used to measure the surface roughness of the machined surface. Ductile ribbon-like chips were observed on the cutting tool rake face. No signs of cutting edge wear was observed on the diamond tool. The machined surface presented an amorphous phase probed by micro Raman spectroscopy. A successful heat treatment of annealing was carried out to recover the crystalline phase on the machined surface. The results indicated that it is possible to perform a 'mechanical lithography' process in single crystal semiconductors.
Keywords:single crystal,
Source:Iopscience