Quilting Semiconductors

September 26, 2014

Semiconductor electronics is based on the principle that electrons behave differently in different materials. Sometimes the difference is just small concentrations of different impurities in the same material. One example is silicon, for which small additions of an element with excess electrons, called a donor, will make an N-type material, while adding an element deficient in electrons, called an acceptor, will make a P-type material.

For silicon, arsenic is a typical donor, having five valence electrons to silicon's four, while boron, with three valence electrons, is a typical acceptor. Joining N- and P-type materials results in a diode that allows current to flow in one direction, only. Because of an energy difference seen by electrons in these two materials, electrons will flow from the cathode (N-type) to the anode (P-type), but not in the reverse direction.

Semiconductor diode P-N junction and symbol. (Illustration by the author, rendered using Inkscape.)

When there are different materials on opposite sides of the junction, the structure is called a heterojunction. Heterojunctions are useful in the fabrication of semiconductor lasers, high speed transistors, and high speed optical detectors. Just as in the standard junction case, these heterojunctions are made by putting one material layer over another. Now, a team of scientists from the University of Washington (Seattle, Washington), the University of Warwick (Coventry, UK), and the University of Hong Kong (Hong Kong, China) have made side-by-side junctions of atomically thin materials; that is, they made lateral heterostructures. They made a semiconductor quilt.[1-2]

I've always enjoyed looking at quilts. Quilts had a utilitarian purpose in the past, since sewing together discarded clothing remnants into a bed covering was a useful task. Today, they're more likely to be found on a wall as decoration than on a bed. The lateral heterojunctions were a quilt of molybdenum diselenide, MoSe₂, and tungsten diselenide, WSe₂.[1-2]

We had a quilt like this in our house. It's apparently a traditional style. (Photograph by Brooke Raymond, via Wikimedia Commons.)

Molybdenum diselenide and tungsten diselenide are similar to graphene in that they produce monolayers with strong in-plane bonding, but weak layer-to-layer bonding by van der Waals forces, only. Like graphene, these materials appear to have some very useful electronic properties

Tungsten diselenide, in particular, seems to be more useful than graphene, as a number of recent journal articles illustrate.[3-8] I wrote about tungsten diselenide in a recent article (Light-Emitting Diodes of Tungsten Diselenide, March 17, 2014). Unlike graphene, these two selenides are direct bandgap semiconductors, which means that they will function as light emitters.

A layer of molybdenum disulfide, a close cousin to molybdenum diselenide and tungsten diselenide. (Via arXiv Preprint Server).

These lateral heterojunctions could be grown without lattice defects because the two selenides have the same crystal structure.[1-2] Says senior author of the study, Xiaodong Xu, an assistant professor of physics, and also materials science and engineering, at the University of Washington,

"Heterojunctions are fundamental elements of electronic and photonic devices... Our experimental demonstration of such junctions between two-dimensional materials should enable new kinds of transistors, LEDs, nanolasers, and solar cells to be developed for highly integrated electronic and optical circuits within a single atomic plane."[2]

The process for fabrication was not difficult. Hydrogen transport chemical vapor deposition was done at 900°C, first a partial layer of one selenide was produced on a substrate. This appeared as small triangular islands of two-dimensional crystal. Then the same was done for the other selenide, whose atoms filled the voids between these patches to produce the heterojunction quilt.[2]

Right, an optical micrograph of a lateral heterostructure layer in which the two selenides are distinguished by color. Left, a high-resolution scanning transmission electron microscopy (STEM) image showing a seam with atomic precision. (Right image by the University of Washington. Left image by the University of Warwick.)[2]

As explained by Sanfeng Wu, a Ph.D. candidate in physics at the University of Washington and one of the lead authors,

"This is a scalable technique... Because the materials have different properties, they evaporate and separate at different times automatically. The second material forms around the first triangle that just previously formed. That's why these lattices are so beautifully connected."[2]

The heterojunction growth, itself, took about five minutes, with an additional two hours for heating and cooling time.[2] Using optical microscopy, the lateral heterojunctions showed enhanced photoluminescence, which demonstrates their optoelectronic potential.[1-2] Atomically resolved scanning transmission electron microscopy at the University of Warwick showed that the two selenides join together in an undistorted hexagonal lattice with a graduation of molybdenum and tungsten atoms across the boundary region.[1-2]

Photoluminescence emission intensity map of a molybdenum diselenide-tungsten diselenide lateral heterostructure. The photoluminescence illustrates the potential optoelectronic applications of the material. (University of Washington image.)[2]

Such lateral heterojunctions could enable in-plane transistors and diodes, all of which could be integrated within one atomically thin layer.[1-2] Funding for this research came from the U.S. Department of Energy, the Clean Energy Institute at the University of Washington, the Research Grant Council of Hong Kong, the University Grants Committee of Hong Kong, the Croucher Foundation, the Science City Research Alliance and the Higher Education Funding Council for England's Strategic Development Fund.[2]

References:

1. Chunming Huang, Sanfeng Wu, Ana M. Sanchez, Jonathan J. P. Peters, Richard Beanland, Jason S. Ross, Pasqual Rivera, Wang Yao, David H. Cobden , and Xiaodong Xu, "Lateral heterojunctions within monolayer MoSe2–WSe2 semiconductors," Nature Materials, Article no. 4064 (August 24, 2014), doi:10.1038/nmat4064.
2. Michelle Ma, "Scientists craft atomically seamless, thinnest-possible semiconductor junctions," University of Washington Press Release, August 26, 201.
3. Jason S. Ross, Philip Klement, Aaron M. Jones, Nirmal J. Ghimire, Jiaqiang Yan, D. G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David H. Cobden and Xiaodong Xu, "Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p–n junctions," Nature Nanotechnology (March 9, 2014), doi:10.1038/nnano.2014.26.
4. Jason S. Ross, Philip Klement, Aaron M. Jones, Nirmal J. Ghimire, Jiaqiang Yan, D. G. Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David H. Cobden and Xiaodong Xu, "Electrically tunable excitonic light-emitting diodes based on monolayer WSe2 p–n junctions," arXiv Preprint Server, March 4, 2014.
5. Britton W. H. Baugher, Hugh O. H. Churchill, Yafang Yang and Pablo Jarillo-Herrero, "Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide," Nature Nanotechnology (March 9, 2014), doi:10.1038/nnano.2014.25.
6. Andreas Pospischil, Marco M. Furchi and Thomas Mueller, "Solar-energy conversion and light emission in an atomic monolayer p–n diode," Nature Nanotechnology (March 9, 2014), doi:10.1038/nnano.2014.14.
7. David L. Chandler, "Two-dimensional material shows promise for optoelectronics," MIT Press Release, March 10, 2014.
8. Michelle Ma, "Scientists build thinnest-possible LEDs to be stronger, more energy efficient," University of Washington Press Release, March 10, 2014.