Friday, May 16, 2014

Laser enhancement - A path to defect free diamond growth?

In this post I'll take a look at an article titled "Control of crystallographic orientation in diamond synthesis through laser resonant vibrational excitation of precursor molecules" published recently in Scientific Reports.  In the article, Zhi Qiang Xie and coworkers describe the potential benefits of using a laser to introduce resonant vibrations of molecules during the growth of diamond films. How can laser light improve the growth process, and does this technique hold promise for improving diamond growth in general?

File:Laser play.jpg
Lasers! (Image from Wikipedia)  
You may be aware that it is possible to create diamond using lab equipment instead of mining it where it occurs naturally in the earth, but it may surprise you to learn how many ways there actually are to grow diamond in a lab. The one that is most interesting to me, as someone interested in diamond electronics, is a method called Chemical Vapor Deposition, or CVD.  The basic idea is to take molecules with carbon and hydrogen in them (which are not so surprisingly called hydrocarbons), and then excite them with some form of energy to form more reactive species.  Those excited molecules then react with a surface, adding carbons one at a time to an existing piece of diamond.  If that existing diamond is already a single crystal, the new diamond grown on top (with the exception of some defects that may grow) will be a part of that same single crystal.  Growth can also be done on existing diamond that is in the form of tiny nanodiamond crystals (in case you missed it, nanodiamonds were introduced in the last Blingtronics post).  Using nanodiamonds spread onto a surface, a diamond film can be grown as the individual nanodiamonds grow into larger (but still small) diamond crystals, bonded together strongly at the interfaces between the crystals. When the tiny crystals grow large enough, this kind of diamond is called polycrystalline diamond (which means "many crystals") and the interfaces between the crystal "grains" are called grain boundaries.

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This single crystal diamond was grown by CVD (from Wikipedia)  

There are a number of different kinds of reactors that can perform CVD reactions. The ones I am most familiar with, from using them at Michigan State, are microwave plasma reactors.  In a future post I hope to describe the MSU reactors better, but for now I'll just briefly mention some of the most common types of microwave reactors, just noting here that there are other kinds, and the one I have used is not one of the ones pictured below.

This Figure,  from [2], shows four common types of CVD reactor: (a) Hot Filament Reactor  (b) 'NIRIM' type Microwave Plasma Reactor (c) 'ASTEX' type Microwave Plasma Reactor (d) DC Arc Jet Plasma Torch Reactor 

The main distinction between these reactors is the method they use for splitting gasses into more reactive molecules that can then grow into diamond.  For example, in a hot filament reactor, like the one with the cross section up there labeled "a," the hydrocarbons are split into more reactive molecules by a filament, which is like the glowing thread you see inside an incandescent bulb.  The next two reactors shown use microwaves (just like in a microwave oven) for the splitting.  The last reactor shown is a bit different from the first three.  The first three reactors use a pump (like a strong vacuum) to remove most of the air from a chamber, leaving only hydrogen and the hydrocarbons gas, (like methane or acetylene) which are allowed into the reactor at a controlled rate.  On the other hand, the reactor in the fourth image, labeled (d), doesn't use a strong vacuum pump to make sure that the reactive diamond growing molecules are the only gasses around the sample.  It is a DC Arc Jet Plasma torch style reactor, and in this style the gasses at the point of growth are controlled by using a much higher flow rate of gasses, as a "jet" onto the sample as it's being grown.  Because the jet is strong, it doesn't need to be in an ultra pure environment pumped free of other air molecules like the previous reactors, and can even be operated outside of a chamber.  In this week's article, that's exactly the kind of system the authors used to grow their diamond, a plasma jet operated in air, and in this setup, they've modified it with a laser.

Diagram of the setup of the Arc Jet diamond deposition system, with a CO2 laser added to enhance the growth process, Figure from [1]  
So what does the laser do? Well, remember how I said that the reactor works by splitting the gasses into more reactive molecules? Basically the energy knocks some atoms off of a molecule, which gives the molecule new sites to bond to other things.  Molecules can also behave in different ways if they are excited by a resonant frequency.  It's kind of like that trick where a resonant sound frequency can break a wine glass.  Here's a nice video and explanation of how that works, (which I found here):

By using exactly the right frequency, the glass moves in sync with the sound waves.  In a similar way, molecules can move in resonant modes when they are hit with light waves of exactly the right frequency. In fact, there are a number of different types of these modes, where the hydrogens on a hydrocarbon move in a patterned way with each other: 
Vibrational modes of the methylene group, from the UC Davis Chem Wiki
So how do you find these exact light frequencies that make the hydrogens go into resonances like those? Remember  in the glass breaking video how he found the resonant frequency of the glass by hitting it and listening to it's natural vibration pitch? Something similar works for light waves and molecules.   Xie and coworkers shined laser light of different frequencies through the arc jet, and saw how much or how little was absorbed by the molecules.  If a lot was absorbed, it meant they were resonating with it.  The two peaks they found (10.532 μm and 10.22 μm) both correspond to a wagging mode (it's the fourth one up there!) of one of the hydrocarbons that is made in the splitting process in the diamond deposition reactor (the CH2- molecule).

The absorption for different laser wavelengths, from [1]
Xie and coworkers described in depth how these resonances might affect the diamond growth (which you should totally read about in their paper for more info if you're interested), and they looked to see if shining laser light of the resonant frequencies they found into the arc jet during deposition would change how the diamond grew.   They tried four different frequencies: a control with no laser, 10.22 μm (one of the wagging modes), 10.333 μm (no particular resonance) and 10.532 μm (the other wagging mode).  They grew polycrystalline diamond, which again means that lots of little diamond crystals will grow together into a rough film of connected individual crystals.  So did it work? Did the laser make the reactive molecules spin around more and give better crystals in the diamond as they grew?

polycrystalline diamond films grown with different enhancement laser wavelengths, from [1]
The short answer is, yes!  Both the 10.333 μm enhanced and the "no laser" films look more or less the same, showing that any change in the other two diamond films isn't just due to shining a laser into the jet.  The other two films do look different, especially in the bottom row, which were grown for 60 minutes instead of the 15 minutes in the top row.  The crystals in the  10.532 μm film look much bigger.  More interestingly, the crystals in the film in the 10.22 μm enhanced film look like they are all flat on top.  This means that the atoms in that film are all aligned in such a way that they show a particular crystal face.  That face is named the (100) face by crystallographers (totally a real job), and although I'm not going to get in depth about crystallography in this post, it's probably enough to know that the flat top surfaces are named (100), and that the fact that they are the (100) ones is a good thing.  For example, polycrystalline diamond with (100) faces on the surface is better for optics than other types of polycrystalline diamond, and so the ability to control the surface orientation to produce (100) polycrystalline diamond is a pretty neat advance in this field.  They also did the trick with the 10.22 μm laser to enhance single crystal growth, and got a single crystal diamond with virtually no defects on the surface.  A neat trick for sure!  Although these authors credit a lot of the improvement they saw from the laser enhancement to molecules that contain oxygen (which is not as possible in other diamond reactor designs), the increased energy of the right molecules by laser enhancement could, in theory, hold the key for defect free growth in other types of reactors as well.

So, as for our original questions, I think I've shown how laser light could improve the growth process, and that maybe this technique could have some potential for improving diamond growth in general! [1] Xie, Z., Bai, J., Zhou, Y., Gao, Y., Park, J., Guillemet, T., Jiang, L., Zeng, X., & Lu, Y. (2014). Control of crystallographic orientation in diamond synthesis through laser resonant vibrational excitation of precursor molecules Scientific Reports, 4 DOI: 10.1038/srep0458
[2] May, P. (2000). Diamond thin films: a 21st-century material Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 358 (1766), 473-495 DOI: 10.1098/rsta.2000.0542

What did you think of the post? Love it or hate it? I got it all wrong?  At last something's been explained so you understand it? What would you like to see more or less of? Please feel free to leave a comment!

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