Beyond the Transistor
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Image courtesy of the University of Illinois at Urbana-Champaign
Schematic of a GaAs 3D photonic crystal (blue) containing an InGaAs light-emitting layer (red). The structure is lithographically patterned into the form of a cylindrical mesa with a ring electrode on the top surface (gold).
The invention of the transistor—and semiconductors generally—revolutionized technology by providing unprecedented control over the flow of electricity, ultimately making possible everything from today's smart phones and iPods to the Internet and supercomputers. These days a new class of device is emerging that could provide similar control over the flow of both electricity and light—with important potential applications for solar energy and a wide range of other fields.
The new devices, known as "photonic crystals," are, in a sense, kissing cousins of today's semiconductors. They have the power to precisely channel the flow not only of electrons—the stuff of electricity—but also of photons, or wave-particles of light.
First widely discussed in the late 1980s, fully functional photonic crystals have until recently existed more in the minds of theorists than in the real world. Simple one- and two-dimensional versions of these devices have been built, and some of these relatively simple devices are even sold commercially.
Three-dimensional photonic crystals have also been attempted, but until recently the devices constructed responded only to light, not to electricity.
The challenge has been to fashion a full-fledged device that (1) could precisely channel both electrons and photons and (2) could function in all three dimensions. The physics of these devices has been well understood; multiple designs have been proposed. The challenge has been in the fabrication.
Now, for the first time, a team of researchers from two DOE Energy Frontier Research Centers (EFRCs) has created a true three-dimensional "optoelectronic" photonic crystal that functions as a light-emitting diode (LED). In addition, the researchers have done so using standard industrial semiconductor fabrication techniques. The work was reported in Nature Materials.
The effort was led by Paul V. Braun, Professor of Materials Science and Engineering at the University of Illinois at Urbana-Champaign (Illinois). Braun is a principal investigator in the Light-Material Interactions in Energy Conversion EFRC (LMI-EFRC), led by the California Institute of Technology (Caltech), with Illinois as a partner. The collaboration was also supported by the Center for Energy Nanoscience EFRC, led by the University of Southern California. The DOE Office of Science established 46 EFRCs at universities, national laboratories, and other institutions around the nation in 2009 to accelerate basic research on energy.
A Transformative Breakthrough
“The potential exists for . . . whole circuits where relatively cool photons could replace hot electrons as carriers of energy and information. The last could prove important for the future of supercomputing, as semiconductor-based designs run up against ultimate limits of energy and heat.”
Braun's research represents an important milestone. Already we have seen how fiber optic cables—which simply transport light—have transformed our global communications culture, using photons to move massive quantities of information over vast distances with great precision at light speed.
A whole range of energy-producing and energy-consuming devices could be transformed by the far more sophisticated technology of photonic crystals: the potential exists for super-efficient solar cells, solid-state lighting that requires minimal power, super-efficient lasers that guide light with unprecedented exactness, and eventually perhaps whole circuits where relatively cool photons could replace hot electrons as carriers of energy and information. The last could prove important for the future of supercomputing, as semiconductor-based designs run up against ultimate limits of energy and heat.
The central goal here, as in so much energy research, is efficiency. Whether the focus is on solar energy or solid-state lighting, one is trying to maximize production or use of energy by avoiding energy losses through lack of light absorption, waste heat, and so forth. To do so requires ever more precise control over the flow of electrons and photons and over their interactions within the material.
Photonic crystals function on close analogy with traditional electronic semiconductors; they behave toward photons pretty much the way semiconductors behave toward electrons. In both cases, the effects are the result of waves interacting with crystalline structures in particular ways at different energy levels.
Semiconductors, as their name implies, are materials that do and don't conduct electricity. Typically, at very low energy levels or voltages, semiconductors act as insulators. You need to reach a certain voltage level for the semiconductor to begin conducting. That voltage level is known as the semiconductor's "band gap." Silicon, for example, has a band gap of 1.11 electron volts at a temperature of around 84 degrees Fahrenheit (the band gap typically drops as the temperature rises). Electrons with an energy lower than 1.11 volt cannot move through Silicon at that temperature. At those low voltages, silicon insulates rather than conducts.
A key idea behind photonic crystals has been to create a "photonic band gap"—that is, to devise a material that would conduct light at certain energy levels or wavelengths and block it at others. Why would one want to do that? The point is to exercise more precise control over interactions of electrons and photons. Essentially, you're trying to prevent uncontrolled activity that leads to energy loss. For example, you may wish to prevent something called "spontaneous emission," or an electron randomly losing energy and ejecting a photon. Electrons will do that. However, if the energy of the photon lies within the photonic band gap, the photon simply has nowhere to go in the photonic crystal material, and so spontaneous emission is prevented.
The latter concept was the starting point for Eli Yablonovich, one of two theorists, along with Sajeev John, to propose the concept of the photonic crystal, in a pair of seminal articles that the two physicists wrote separately in 1987. Yablonovich is currently at the University of California, Berkeley, and also a principal investigator in LMI-EFRC. John is at the University of Toronto.
Since then, theorists have produced a substantial body of work proposing various photonic crystal devices. In principle, they are not difficult to design, since all forces involved obey Maxwell's equations. The trick has been to build one.
Enlarge Photo
Image courtesy of the University of Illinois at Urbana-Champaign
Electrically pumped emission from the 3D GaAs photonic crystal LED at various drive currents. The electroluminescence intensity increases linearly with current.
A Bottom-Up Approach
Previous three-dimensional photonic crystals were made using a top-down approach where microscopic size holes, rods, and other features are etched or drilled into a chunk of material. This can introduce flaws into the surface, disrupting the conversion between electrical and optical signals and cause less efficient transfer of light. The top-down method made crystals that were only optically active, or able to direct light, not electronically active, meaning they cannot turn electricity to light and vice versa. Although these are useful, they are difficult to construct and only a small range of shapes can be introduced into the material.
Braun's group instead took a bottom-up approach by growing a single crystal by a process called epitaxy. Epitaxial growth is commonly used in industry to construct two-dimensional semiconductors, like those found in electronics. The group used a three-dimensional template filled with tiny spheres packed together and then encased the spheres with deposited gallium arsenide (GaAs), a widely used semiconductor material, through the template, filling the gaps between the spheres. After the template is filled, the researchers chemically remove the spheres, leaving behind a single crystal with the complex geometric structure of the template.
"The key discovery here was that we grew single-crystal semiconductor through this complex template," said Braun. "Gallium arsenide wants to grow as a film on the substrate from the bottom up, but it runs into the template and goes around it. It's almost as though the template is filling up with water. As long as you keep growing GaAs, it keeps filling the template from the bottom up until you reach the top surface."
This process eliminates many of the defects introduced by top-down fabrication methods plus has the advantage of using any template, creating endless options for shapes and possibilities.
The epitaxial approach also has the advantage of easily creating heterostructures, or structures that contain layers with different electrical properties or chemical compositions. To do this, the scientists partially fill the template with the vapor precursors for GaAs and then briefly switch to another material that is deposited on top of the GaAs. Then the stream is switched back to grow GaAs, much like a multi-layer cake with a thin layer of frosting between the two cake layers. Heterostructures are important for making lasers like those in CD and DVD players and allow for more diversity in the construction of other devices, like solar cells and LED lights. Using this approach, Braun and colleagues showed how their method has the ability to develop heterostructures, building a three-dimensional photonic crystal LED—the first such working device.
"The work from Paul's group beautifully illustrates the LMI-EFRC vision to sculpt and mold the flow of light in materials—in this case by molding the materials themselves," said LMI-EFRC Director Harry Atwater, who is also Howard Hughes Professor and Professor of Applied Physics and Materials Science at Caltech.
The Braun group is now working to optimize the process for specific applications, most notably building a better solar cell by altering the structure or by using other semiconductor materials. This innovative process could open whole new avenues for harvesting light and more efficiently converting it to electricity, as well as better ways of delivering that energy to the consumer.
—Dawn Adin and Patrick Glynn, DOE Office of Science, Dawn.Adin@science.doe.gov and Patrick.Glynn@science.doe.gov
Funding
DOE Office of Science, Office of Basic Energy Sciences
Publication
E. C. Nelson et al. "Epitaxial growth of three-dimensionally architectured optoelectronic devices," Nature Materials 10, 676 (2011)
