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Artificial diamond structure in crystalline silicon found to be a semiconductor for light

Optical computer now a step closer
Researchers from the FOM Foundation, the MESA⁺ Institute at the University of Twente, Philips and ASML have designed a completely new method for manufacturing 3D structures in silicon, which function as a semiconductor for light. They publish their results in two leading journals. The use of standard equipment makes the integration in silicon chips possible and that is vitally important for their application in a future optical computer.

One way of making computers far faster still is the production of three-dimensional computer chips. The technology currently used for the manufacture of computer chips is not particularly suitable for the manufacture of three-dimensional spatial structures. The structures on a chip are built up layer by layer, as a result of which the production of an extensive structure is a time-consuming and expensive job. In two recently published articles in the journals 'Advanced Functional Materials' and 'Journal of Vacuum Science and Technology' former FOM researchers Johanna van den Broek, Willem Tjerkstra, Léon Woldering and Willem Vos described with their colleagues how they could make a three-dimensional structure in crystalline silicon that consisted of an orthogonal regular pattern of pores that crossed each other perpendicularly. The pores formed a diamond structure that was found to behave as a semiconductor for light: a photonic crystal (see Figure 1).

Optical computer
An intriguing analogy exists between crystalline silicon/diamond and the new artificial diamond structure: crystalline silicon and diamond both have a forbidden energy band as a result of which electrons with these energies cannot travel through the crystal (the band gap). This forbidden energy band lies at the heart of how semiconductors work, and it is how current is switched on and off in chips. In an analogous manner the new artificial diamond structure was found to behave as a crystal for light, a 'photonic crystal', and was found to have a forbidden energy band for light. Therefore in theory, photonic crystals reflect almost 100% of light with wavelengths in the forbidden band.

The availability of three-dimensional photonic crystals with a band gap makes it possible to accurately control and even capture photons. The high reflection of the structures can be used to produce a 'cage for light' in a photonic crystal. Inside such a cage the light continues to ‘bounce’ backwards and forwards between the walls of the cage until it can once again escape from it, preferably after a 'push on the button'. This makes it possible to control optical bits instead of electronic bits, so the optical computer has now come a step closer. An optical computer can process far more data simultaneously than a conventional computer. As optical computers contain electronic components as well, it is important that the new manufacturing method is entirely compatible with the methods in the chip industry so that electronic and photonic components can be simply manufactured on a single chip.

This research was funded by NanoNed/STW, the FOM Foundation and NWO.

Appendix: More details about the process
The researchers made the three-dimensional nanostructures in two steps using an innovative process. First of all they made use of ASML's advanced deep-UV lithography: with the help of UV light, a gigantic camera projected the orthogonal pore structure on a layer of photoresist. This provided a mask with millions of minute pores. Then using a plasma etching process from the chip industry they etched very deep nano pores in a silicon wafer. The result of this can be seen in Figure 2.

The second step is crucial: on the thin side of the wafer the researchers applied a second mask with extreme accuracy and they then used a very unconventional method to produce the same pattern of holes in the mask as in the first step. The challenge lay in the requirement that the pattern on the mask could deviate by no more than 30 millionths of a millimetre (30 nanometres) and 0.5º from the optimum position and angle. This was necessary to ensure that the required diamond structure arose after etching. The pattern was applied using standard equipment from the chip industry. The second set of pores was etched in the same manner as the first set. By using standard equipment the electronics and the three-dimensional structures made in this new method can be integrated onto silicon chips.

The pores in the new three-dimensional structure form a diamond structure which is the same as the carbon atoms in the well-known gemstone diamond or the silicon atoms in crystalline silicon (see Figure 3). The new structure has been magnified about 2000 times compared to a diamond crystal. To be certain of their success, the researchers sacrificed a number of their structures by carefully opening these up so that the diamond structure could be accurately analysed for pore depth, pore diameter, pore shape and alignment. This confirmed that the structure formed in the wafer is indeed as shown by the dotted lines in Figure 1.

Optical reflecting measurements like those shown in Figure 4 confirmed that the new diamond structures behave as very good photonic crystals. The roughness of the surface and the thickness of the crystal currently limit the maximum reflectivity. The broad forbidden bands are found at infrared wavelengths which are used in the telecommunication industry (1330 and 1550 nm).

References
[1] J.M. van den Broek, L.A. Woldering, R.W. Tjerkstra, F.B. Segerink, I.D. Setija, and W.L. Vos, Inverse woodpile photonic band gap crystals with a cubic diamond-like structure made from single crystalline silicon, Advanced Functional Materials http://onlinelibrary.wiley.com/doi/10.1002/adfm.201101101/abstract
[2] R.W. Tjerkstra, L.A. Woldering, J.M. van den Broek, F. Roozeboom, I.D. Setija, and W.L. Vos, A method to pattern etch masks in two inclined planes for three-dimensional nano- and microfabrication, Journal of Vacuum Science and Technology B (soon online)
http://link.aip.org/link/?JVB/29/061604, DOI: 10.1116/1.3662000.

For more information, please contact:
dr. Willem Tjerkstra, +31 (0)53 489 53 90 or prof.dr. Willem Vos.

In addition, Dutch researchers from the University of Twente and FOM-Institute Amolf have for the first time demonstrated that the vacuum fluctuations of light are drastically reduced by a factor 10 in a photonic band gap crystal. In 1987 it has been predicted that in a photonic crystal with a band gap the fluctuations are inhibited.

Inhibited Spontaneous Emission of Quantum Dots Observed in a 3D Photonic Band Gap
M.D. Leistikow, A.P. Mosk, E. Yeganegi, S. R. Huisman, A. Lagendijk, and W.L. Vos, Physical Review Letters, volume 107, 193903 (4 November 2011).
URL: http://link.aps.org/doi/10.1103/PhysRevLett.107.193903
DOI: 10.1103/PhysRevLett.107.193903

Contact:

Figure 3. Model of a diamond structure

Model of a diamond structure superimposed on the artificial diamond structure (as in Figure 1). The red spheres indicate the atoms in the crystal lattice of diamond or silicon; the grey lines represent the bonds between atoms. The similarities between the diamond lattice and the artificial structure are clearly visible.
Figure 1. Electron microscope image

Electron microscope image of a three-dimensional diamond structure in silicon (encircled), consisting of identical pores in an orthogonal pattern that cross each other at an angle of 90º. The upper part of the figure shows the upper surface of the chip, in which a large quantity of pores have been etched. Top left a dust particle can be seen that ended up on the crystal after the process. In the side of the chip an orthogonal pattern of pores has been etched in a single go, perpendicular to the other pores. The scale at the top right is two micrometres, about 50 times as thin as a human hair.
Figure 2. Part of a silicon wafer

Part of a silicon wafer containing a square with millions of pores in the orthogonal structure that were etched during the first step of the new process. The blue colour is caused by the interference of light on the pore structure. In the second step the second pore structure is etched on the thin side of the wafer.
Figure 4. Optical reflectivity spectrum

Optical reflectivity spectrum of a photonic crystal with an artificial diamond structure in silicon. It can be clearly seen that the crystal strongly reflects light with wavelengths in the forbidden zone around 1550 nm: this confirms the outstanding quality of the crystal.