Jul 26, 2017

New method to make gallium arsenide solar cells

New method to make gallium arsenide solar cells
Image of a printed GaAs solar cell with a size ~10 x 10 mm2 on a glass substrate, with simple, metal grid contacts. Image copyright: Nature, DOI:doi:10.1038/nature09054

(PhysOrg.com) -- A new "transfer-printing" method of making light-sensitive semiconductors could make solar cells, night-vision cameras, and a range of other devices much more efficient, and could transform the solar industry.

Scientists at the University of Illinois at Urbana-Champaign have developed a new and cheaper way of producing microchips of  (GaAs), a compound semiconductor that responds to light. Gallium arsenide is about twice as effective as silicon in converting incident solar radiation to light, with a theoretical conversion rate of up to 40 percent, and has for that reason been used in solar cells in space crafts.

The problem with GaAs is its expense and the need for wafers to be grown in precisely controlled conditions. The wafers are sliced for use, but only the surfaces are used and the rest is essentially wasted. Now the Illinois research team, led by materials scientist John Rogers, has developed an alternative and potentially much more cost-effective technique involving growing stacks of layers of GaAs alternating with aluminum arsenide (AlAs).

When the stack is complete, the scientists then chemically etch away the AlAs layers using hydrofluoric acid, leaving the films of GaAs, which they then peel off and stamp onto another substrate such as glass, silicon, or plastic using a silicon-based soft rubber stamp. Rogers and his colleagues have been working on perfecting the technique for around ten years.
Semiconductor manufacturing technique holds promise for solar energy
This is a flexible array of gallium arsenide solar cells. Gallium arsenide and other compound semiconductors are more efficient than the more commonly used silicon. Credit: John Rogers

They have learned that if they press the stamp on the stack and lift it quickly it picks up only the top film. They then transfer the GaAs to the substrate by stamping it onto the surface and peeling the stamp back slowly. They could then build the devices such as , semiconductor field effect transistors and , and near-infrared imaging devices on the substrates. The method yields large quantities of high quality GaAs films, leaving the original wafer for reuse to grow more films.

Using their technique, which is described in the journal Nature, the researchers succeeded in mass-producing tiny solar cells about 500 micrometers in diameter, and they also produced components for mobile phones and infrared-imaging devices.

Rogers said GaAs has a great deal of potential in the future, and the team is now developing commercially viable  that will be able to generate electricity for about $1 per watt.
Semiconductor manufacturing technique holds promise for solar energy
A pile of gallium arsenide solar cells is manufactured in stacks and then peeled apart layer by layer. They can be integrated into a number of electronic devices. Credit: John Rogers

More information: Jongseung Yoon, GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies, Nature, Volume: 465, Pages: 329-333, Date published: 20 May 2010, DOI:doi:10.1038/nature09054

Source: Phys
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Jul 17, 2017

Thinking thin brings new layering and thermal abilities to the semiconductor industry

This image shows a thick bulk gallium nitride (GaN) crystal wafer (2 inches in diameter) with a GaN film in the foreground fabricated by controlled spalling (its film thickness is ~20 microns or 1/5th the thickness of a sheet of paper. Credit: Bedell/IBM Research

What would a simple technique to remove thin layers from otherwise thick, rigid 
semiconductor crystals mean for the semiconductor industry? This concept has been actively explored for years, as integrated circuits made on thin layers hold promise for developments including improved thermal characteristics, lightweight stackability and a high degree of flexibility compared to conventionally thick substrates.


In a significant advance, a research group from IBM successfully applied their new "controlled spalling" layer transfer technique to gallium nitride (GaN) crystals, a prevalent semiconductor material, and created a pathway for producing many layers from a single substrate.

As they report in the Journal of Applied Physics, controlled spalling can be used to produce thin layers from thick GaN crystals without causing crystalline damage. The technique also makes it possible to measure basic physical properties of the material system, like strain-induced optical effects and fracture toughness, which are otherwise difficult to measure.

Single-crystal GaN wafers are extremely expensive, where just one 2-inch wafer can cost thousands of dollars, so having more layers means getting more value out of each wafer. Thinner layers also provide performance advantages for power electronics, since it offers lower electrical resistance and heat is easier to remove.

"Our approach to thin film removal is intriguing because it's based on fracture," said Stephen W. Bedell, research staff member at IBM Research and one of the paper's authors. "First, we first deposit a nickel layer onto the surface of the material we want to remove. This nickel layer is under tensile strength—think drumhead. Then we simply roll a layer of tape onto the nickel, hold the substrate down so it can't move, and then peel the tape off. When we do this, the stressed nickel layer creates a crack in the underlying material that goes down into the substrate and then travels parallel to the surface."

Their method boils down to simply peeling off the tape, nickel layer and a thin layer of the substrate material stuck to the nickel.

"A good analogy of how remarkable this process is can be made with a pane of glass," Bedell said. "We're breaking the glass in the long direction, so instead of a bunch of broken glass shards, we're left with two full sheets of glass. We can control how much of the surface is removed by adjusting the thickness of the nickel layer. Because the entire process is done at room temperature, we can even do this on finished circuits and devices, rendering them flexible."

The group's work is noteworthy for multiple reasons. For starters, it's by far the simplest method of transferring thin layers from thick substrates. And it may well be the only layer transfer method that's materially agnostic.
The same 20-micron spalled GaN film, demonstrating the film's flexibility. Credit: Bedell/IBM Research

"We've already demonstrated the transfer of silicon, germanium, gallium arsenide, gallium nitride/sapphire, and even amorphous materials like glass, and it can be applied at nearly any time in the fabrication flow, from starting materials to partially or fully finished circuits," Bedell said.

Turning a parlor trick into a reliable process, working to ensure that this approach would be a consistent technique for crack-free transfer, led to surprises along the way.

"The basic mechanism of substrate spalling fracture started out as a 
materials science problem," he said. "It was known that metallic film deposition would often lead to cracking of the underlying substrate, which is considered a bad thing. But we found that this was a metastable phenomenon, meaning that we could deposit a thick enough layer to crack the substrate, but thin enough so that it didn't crack on its own—it just needed a crack to get started."

Their next discovery was how to make the crack initiation consistent and reliable. While there are many ways to generate a crack—laser, chemical etching, thermal, mechanical, etc.—it turns out that the simplest way, according to Bedell, is to terminate the thickness of the nickel layer very abruptly near the edge of the 
substrate.

"This creates a large stress discontinuity at the edge of the 
nickel film so that once the tape is applied, a small pull on the tape consistently initiates the crack in that region," he said.

Though it may not be obvious, 
gallium nitride is a vital material to our everyday lives. It's the underlying material used to fabricate blue, and now white, LEDs (for which the 2014 Nobel Prize in physics was awarded) as well as for high-power, high-voltage electronics. It may also prove useful for inherent biocompatibility, which when combined with control spalling may permit ultrathin bioelectronics or implantable sensors.

"Controlled spalling has already been used to create extremely lightweight, high-efficiency GaAs-based solar cells for aerospace applications and flexible state-of-the-art circuits," Bedell said.

The group is now working with research partners to fabricate high-voltage GaN devices using this approach. "We've also had great interaction with many of the GaN technology leaders through the Department of Energy's ARPA-E SWITCHES program and hope to use controlled spalling to enable novel devices through future partnerships," Bedell said.

Explore further: SOI wafers are suitable substrates for gallium nitride crystals
More information: S. W. Bedell et al, Layer transfer of bulk gallium nitride by controlled spalling, Journal of Applied Physics (2017). DOI: 10.1063/1.4986646 
Journal reference: Journal of Applied Physics 

Source: Phys
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Jul 9, 2017

Wafer-scale processing technology for monolithically integrated GaSb thermophotovoltaic device array on semi-insulating GaAs substrate

Abstract

This paper presents the entire fabrication and processing steps necessary for wafer scale monolithic integration of series interconnected GaSb devices grown on semi-insulating GaAs substrates. A device array has been fabricated on complete 50 mm (2 inch) diameter wafer using standard photolithography, wet chemical selective etching, dielectric deposition and single-sided metallization. For proof of concept of the wafer-scale feasibility of this process, six large-area series interconnected GaSb p–n junction thermophotovoltaic cells with each cell consisting of 24 small-area devices have been fabricated and characterized for its electrical connectivity. The fabrication process presented in this paper can be used for optoelectronic and electronic device technologies based on GaSb and related antimonide based compound semiconductors.
Keywords:GaAs,
Source: iopscience
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Jul 7, 2017

Direct-bonded four-junction GaAs solar cells*

Abstract

Direct wafer bonding technology is able to integrate two smooth wafers and thus can be used in fabricating III–V multijunction solar cells with lattice mismatch. In order to monolithically interconnect between the GaInP/GaAs and InGaAsP/InGaAs subcells, the bonded GaAs/InP heterojunction must be a highly conductive ohmic junction or a tunnel junction. Three types of bonding interfaces were designed by tuning the conduction type and doping elements of GaAs and InP. The electrical properties of p-GaAs (Zn doped)/n-InP (Si doped), p-GaAs (C doped)/n-InP (Si doped) and n-GaAs (Si doped)/n-InP (Si doped) bonded heterojunctions were analyzed from the I–Vcharacteristics. The wafer bonding process was investigated by improving the quality of the sample surface and optimizing the bonding parameters such as bonding temperature, bonding pressure, bonding time and so on. Finally, GaInP/GaAs/InGaAsP/InGaAs 4-junction solar cells have been prepared by a direct wafer bonding technique with the high efficiency of 34.14% at the AM0 condition (1 Sun).
Keywords:GaAs,
Source: iopscience
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