New technique makes it easier to etch semiconductors

Creating semiconductor structures for high-end optoelectronic devices just got easier, thanks to University of Illinois researchers.

The team developed a method to chemically etch patterned arrays in the semiconductor gallium arsenide, used in solar cells, lasers, light emitting diodes (LEDs), field effect transistors (FETs), capacitors and sensors. Led by electrical and computer engineering professor Xiuling Li, the researchers describe their technique in the journal Nano Letters.

A semiconductor's physical properties can vary depending on its structure, so semiconductor wafers are etched into structures that tune their electrical and optical properties and connectivity before they are assembled into chips.

Semiconductors are commonly etched with two techniques: "Wet" etching uses a chemical solution to erode the semiconductor in all directions, while "dry" etching uses a directed beam of ions to bombard the surface, carving out a directed pattern. Such patterns are required for high-aspect-ratio nanostructures, or tiny shapes that have a large ratio of height to width. High-aspect-ratio structures are essential to many high-end optoelectronic device applications.

While silicon is the most ubiquitous material in semiconductor devices, materials in the III-V (pronounced three-five) group are more efficient in optoelectronic applications, such as solar cells or lasers.

Unfortunately, these materials can be difficult to dry etch, as the high-energy ion blasts damage the semiconductor's surface. III-V semiconductors are especially susceptible to damage.

To address this problem, Li and her group turned to metal-assisted chemical etching (MacEtch), a wet-etching approach they had previously developed for silicon. Unlike other wet methods, MacEtch works in one direction, from the top down. It is faster and less expensive than many dry etch techniques, according to Li. Her group revisited the MacEtch technique, optimizing the chemical solution and reaction conditions for the III-V semiconductor gallium arsenide (GaAs).

The process has two steps. First, a thin film of metal is patterned on the GaAs surface. Then, the semiconductor with the metal pattern is immersed in the MacEtch chemical solution. The metal catalyzes the reaction so that only the areas touching metal are etched away, and high-aspect-ratio structures are formed as the metal sinks into the wafer. When the etching is done, the metal can be cleaned from the surface without damaging it.

"It is a big deal to be able to etch GaAs this way," Li said. "The realization of high-aspect-ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of semiconductor lasers where surface grating is currently fabricated by dry etching, which is expensive and causes surface damage."

To create metal film patterns on the GaAs surface, Li's team used a patterning technique pioneered by John Rogers, the Lee J. Flory-Founder Chair and a professor of materials science and engineering at the U. of I. Their research teams joined forces to optimize the method, called soft lithography, for chemical compatibility while protecting the GaAs surface. Soft lithography is applied to the whole semiconductor wafer, as opposed to small segments, creating patterns over large areas -- without expensive optical equipment.

"The combination of soft lithography and MacEtch make the perfect combination to produce large-area, high-aspect-ratio III-V nanostructures in a low-cost fashion," said Li, who is affiliated with the Micro and Nanotechnology Laboratory, the Frederick Seitz Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at the U. of I.

Next, the researchers hope to further optimize conditions for GaAs etching and establish parameters for MacEtch of other III-V semiconductors. Then, they hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals.

"MacEtch is a universal method as long as the right condition for deferential etching with and without metal can be found," Li said.

The Department of Energy and the National Science Foundation supported this work.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by University of Illinois at Urbana-Champaign.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Matt DeJarld, Jae Cheol Shin, Winston Chern, Debashis Chanda, Karthik Balasundaram, John A. Rogers, Xiuling Li. Formation of High Aspect Ratio GaAs Nanostructures with Metal-Assisted Chemical Etching. Nano Letters, 2011; 11 (12): 5259 DOI: 10.1021/nl202708d

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

New X-ray technique for electronic structures: Ability to probe deep below material surfaces should be boon for nanoscale devices

ScienceDaily (Aug. 26, 2011) — The expression "beauty's only skin-deep" has often been applied to the chemistry of materials because so much action takes place at the surface. However, for many of the materials in today's high technologies, such as semiconductors and superconductors, once a device is fabricated it is the electronic structures below the surface, in the bulk of the material or in buried layers, that determine its effectiveness. For the past 30 years, one of the most valuable and widely used techniques for studying electronic structures has been ARPES -- Angle-Resolved PhotoEmission Spectroscopy. However, this technique primarily looks at surfaces.

Now, for the first time, bulk electronic structures have been opened to comparable scrutiny through a new variation of this standard called HARPES -- Hard x-ray Angle-Resolved PhotoEmission Spectroscopy -- whose development was led by researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab).

"HARPES should enable us to study the electronic structure of any new material in the bulk, with minimum effects of surface reactions or contamination," says physicist Charles Fadley who led the development of HARPES. "Our technique should also allow us to probe the buried layers and interfaces that are ubiquitous in nanoscale devices, and are key to smaller logic elements in electronics, novel memory architectures in spintronics, and more efficient energy conversion in such technologies as photovoltaic cells."

Fadley is a physicist who holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California (UC) Davis where he is a Distinguished Professor of Physics. He is also one of the world's foremost practitioners of photoelectron spectroscopy, a technique based on the photoelectric effect described in 1905 by Albert Einstein. When a beam of photons -- particles of light such as x-rays -- is flashed on a sample, energy is transferred from the photons to electrons, causing them to be emitted from the sample. By measuring the kinetic energy of these emitted photoelectrons and the angles at which they are ejected, scientists can learn much about the sample's electronic structure.

The successful demonstration of the HARPES technique has been reported in the journal Nature Materials in a paper titled "Probing bulk electronic structure with hard X-ray angle-resolved photoemission." Fadley was the senior author of this paper. The lead and corresponding author was Alexander Gray, a member of Fadley's UC Davis research group and also an affiliate with Berkeley Lab's Materials Sciences Division.

"The key to probing the bulk electronic structure is using hard x-rays, which are x-rays with sufficiently high photon energies to eject photoelectrons from deep beneath the surface of a solid material," Gray says. "High-energy photons impart high kinetic energies to the ejected photoelectrons, enabling them to travel longer distances within the solid. The result is that more of the signal originating from the bulk will be detected by the analyzer."

Whereas the typical ARPES experiment, using low energy or "soft" x-rays (10~100 eV photons), probes to a depth of less than 10 angstroms (a few layers of atoms), with their HARPES technique Fadley and Gray and their colleagues on this project were able to probe as deep as 60 Angstroms into the bulk of single crystals of tungsten and gallium-arsenide. Their achievement was made possible by a combination of third generation light sources capable of producing intense beams of hard x-rays, and an advanced electron spectrometer to measure energies and angles.

"While high-energy photons are needed to penetrate into the bulk, at high energies the photoemission intensity that carries information about the electronic band structure is drastically reduced by various factors, such as phonon effects and small photoelectric cross sections of the valence-band electron orbitals," Gray says. "However, HARPES measurements become possible with the advent of the third-generation synchrotron light sources and the development of hard x-ray monochromators and optics capable of focusing a highly intense x-ray beam into a very small measurement spot."

To demonstrate the capabilities of their HARPES technique, Fadley and Gray used a high intensity undulator beamline at the SPring8 synchrotron radiation facility in Hyogo, Japan, which is operated by the Japanese National Institute for Materials Sciences. The samples they worked with, tungsten and gallium arsenide, contain relatively heavy elements that have relatively small phonon effects (atomic vibrations) but to further reduce these effects the samples were cryo-cooled. By combining room temperature and cryo data, the researchers were able to correct for the influence of indirect transitions and photoelectron diffraction in their results.

"Having sufficient photons from the beamline was critical as was having a high energy resolution that required an undulator source and a special monochromator and a photoelectron spectrometer with both high throughput for intensity and a lens with angle-resolving capability," Fadley says.

Adds Gray, "Our HARPES technique not only provided us with information about the energies of the emitted photoelectrons, but also with information about the crystal momentum of electrons within the bulk solid. This extra dimension carries a vast amount of information regarding electronic, magnetic and structural properties of materials, and can be used for in-depth studies of such novel phenomena as high-temperature superconductivity and so-called Mott transitions from insulating to conducting states that might be used for logic switching in the future."

In the future, Fadley and Gray will be able to carry out HARPES experiments much closer to home. At Berkeley Lab's Advanced Light Source (ALS), the first of the world's third generation synchrotron radiation facilities, a new experimental chamber for beamline 9.3.1 is scheduled to open this fall that will provide unique hard x-ray angle-resolved photoemission capabilities.

Says Zahid Hussain, who manages the ALS Scientific Support group, "An additional hard x-ray photoemission spectroscopy chamber at beamline 9.3.1 will feature an ambient pressure high energy photoemission capability that will allow the study of energy related problems, such as batteries, fuel cells, and catalysis under in-situ and in-operando conditions. It will also enable depth-sensitive studies and make it possible to probe not only solid, but also gas and liquid interfaces. This will be the first such experimental facility in the world."

Co-authoring the Nature Materials paper with Fadley and Gray were Christian Papp, Shigenori Ueda, Benjamin Balke, Yoshiyuki Yamashita, Lukasz Plucinski, Jan Minár, Juergen Braun, Erik Ylvisaker, Claus Schneider, Warren Pickett, Hubert Ebert and Keisuke Kobayashi.

This research was supported in part by the DOE Office of Science.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

A. X. Gray, C. Papp, S. Ueda, B. Balke, Y. Yamashita, L. Plucinski, J. Minár, J. Braun, E. R. Ylvisaker, C. M. Schneider, W. E. Pickett, H. Ebert, K. Kobayashi, C. S. Fadley. Probing bulk electronic structure with hard X-ray angle-resolved photoemission. Nature Materials, 2011; DOI: 10.1038/nmat3089

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Down to the wire: Inexpensive technique for making high quality nanowire solar cells

ScienceDaily (Sep. 1, 2011) — Solar or photovoltaic cells represent one of the best possible technologies for providing an absolutely clean and virtually inexhaustible source of energy to power our civilization. However, for this dream to be realized, solar cells need to be made from inexpensive elements using low-cost, less energy-intensive processing chemistry, and they need to efficiently and cost-competitively convert sunlight into electricity.

A team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has now demonstrated two out of three of these requirements with a promising start on the third.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, led the development of a solution-based technique for fabricating core/shell nanowire solar cells using the semiconductors cadmium sulfide for the core and copper sulfide for the shell. These inexpensive and easy-to-make nanowire solar cells boasted open-circuit voltage and fill factor values superior to conventional planar solar cells. Together, the open-circuit voltage and fill factor determine the maximum energy that a solar cell can produce. In addition, the new nanowires also demonstrated an energy conversion efficiency of 5.4-percent, which is comparable to planar solar cells.

"This is the first time a solution based cation-exchange chemistry technique has been used for the production of high quality single-crystalline cadmium sulfide/copper sulfide core/shell nanowires," Yang says. "Our achievement, together with the increased light absorption we have previously demonstrated in nanowire arrays through light trapping, indicates that core/shell nanowires are truly promising for future solar cell technology."

Yang, who holds a joint appointment with the University of California (UC) Berkeley, is the corresponding author of a paper reporting this research that appears in the journal Nature Nanotechnology. The paper is titled "Solution-processed core-shell nanowires for efficient photovoltaic cells." Co-authoring this paper with Yang were Jinyao Tang, Ziyang Huo, Sarah Brittman and Hanwei Gao.

Typical solar cells today are made from ultra-pure single crystal silicon wafers that require about 100 micrometers in thickness of this very expensive material to absorb enough solar light. Furthermore, the high-level of crystal purification required makes the fabrication of even the simplest silicon-based planar solar cell a complex, energy-intensive and costly process.

A highly promising alternative would be semiconductor nanowires -- one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch up to the millimeter scale. Solar cells made from nanowires offer a number of advantages over conventional planar solar cells, including better charge separation and collection capabilities, plus they can be made from Earth abundant materials rather than highly processed silicon. To date, however, the lower efficiencies of nanowire-based solar cells have outweighed their benefits.

"Nanowire solar cells in the past have demonstrated fill factors and open-circuit voltages far inferior to those of their planar counterparts," Yang says. "Possible reasons for this poor performance include surface recombination and poor control over the quality of the p-n junctions when high-temperature doping processes are used."

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that function as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that function as a positive pole. When photons from the sun are absorbed, their energy is used to create electron-hole pairs, which are then separated at the p-n junction -- the interface between the two layers -- and collected as electricity.

About a year ago, working with silicon, Yang and members of his research group developed a relatively inexpensive way to replace the planar p-n junctions of conventional solar cells with a radial p-n junction, in which a layer of n-type silicon formed a shell around a p-type silicon nanowire core. This geometry effectively turned each individual nanowire into a photovoltaic cell and greatly improved the light-trapping capabilities of silicon-based photovoltaic thin films.

Now they have applied this strategy to the fabrication of core/shell nanowires using cadmium sulfide and copper sulfide, but this time using solution chemistry. These core/shell nanowires were prepared using a solution-based cation (negative ion) exchange reaction that was originally developed by chemist Paul Alivisatos and his research group to make quantum dots and nanorods. Alivisatos is now the director of Berkeley Lab, and UC Berkeley's Larry and Diane Bock Professor of Nanotechnology.

"The initial cadmium sulfide nanowires were synthesized by physical vapor transport using a vapor-liquid-solid (VLS) mechanism rather than wet chemistry, which gave us better quality material and greater physical length, but certainly they can also be made using solution process" Yang says. "The as-grown single-crystalline cadmium sulfide nanowires have diameters of between 100 and 400 nanometers and lengths up to 50 millimeters."

The cadmium sulfide nanowires were then dipped into a solution of copper chloride at a temperature of 50 degrees Celsius and kept there for 5 to 10 seconds. The cation exchange reaction converted the surface layer of the cadmium sulfide into a copper sulfide shell.

"The solution-based cation exchange reaction provides us with an easy, low-cost method to prepare high-quality hetero-epitaxial nanomaterials," Yang says. "Furthermore, it circumvents the difficulties of high-temperature doping and deposition for typical vapor phase production methods, which suggests much lower fabrication costs and better reproducibility. All we really need are beakers and flasks for this solution-based process. There's none of the high fabrication costs associated with gas-phase epitaxial chemical vapor deposition and molecular beam epitaxy, the techniques most used today to fabricate semiconductor nanowires."

Yang and his colleagues believe they can improve the energy conversion efficiency of their solar cell nanowires by increasing the amount of copper sulfide shell material. For their technology to be commercially viable, they need to reach an energy conversion efficiency of at least ten-percent.

This research was supported by the DOE Office of Science.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Jinyao Tang, Ziyang Huo, Sarah Brittman, Hanwei Gao, Peidong Yang. Solution-processed core–shell nanowires for efficient photovoltaic cells. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.139

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here