Molecular Beam Epitaxy: Dream Machine

Faebian Bastiman

I want to start this article with a question. What do you not like about your MBE system? Take a moment and think. I am certain at least one problem will immediately materialise within your mind. Indeed, after teaching MBE operation to a number of PhD students, I constantly meet the question “Why?” “Why is it like that?”, “Why is that so slow?”, “Why is that not automatic?”, “Why is that not controlled?” and possibly my favourite: “Why do I have to come in on a Sunday evening to do that?” The reason MBE is “like that” is that we, the users, are “putting up with it”.

There has been tremendous advancement in MOCVD/MOVPE in recent decades because there has been a demand from industry. I think it is high time we, the researchers, made similar demands of MBE. Industry and academia are very different environments, but both adhere to a basic principle that I term “money-time” duality.

Consider a typical R&D MBE system. Sample transfer is an art form. Growth rate calibration is tedious and laborious. Instability hinders the systematic. Maintenance is complicated, laborious and hinders productivity. Throughput is manpower not machine limited. One skilled operator can probably grow two good samples a day. Research progress is, inevitably, slow. Bear in mind one system is probably facilitating several projects, collaborations and multiple characterisation studies.

Let’s analyse the above paragraph. I have actually operated a few MBE systems where I drew a great sense of achievement from actually successfully transferring a sample to its destination. Should sample transfer be the most challenging and rewarding activity in MBE operation? No! No it should not. So let’s automate that straight away. Automatic sample transfer is a must. It also opens up the possibility for batch processing: the execution of 12 samples without user interaction. Detailed systematic studies take days rather than months. Productivity has increased 1000%. Excellent, what next?

Growth rate calibrations are time consuming. I have operated a system where it was basically 90% of my job to monitor the cell fluxes, recipe writing the other 10%. So let’s automate that. Automatic cell flux tuning before every recipe negates the need to have user intervention and reduces instability. The growth rate can be calibrated in several ways. Until recently I considered RHEED the most valuable, now I believe in situ reflectivity to be even more powerful. With reflectivity feedback, sample temperate, layer thickness and composition can be controlled to <1% deviation. Suddenly MBE is highly systematic and stable. So let’s have automatic in situ reflectivity controlling the sample recipes. How about a dream software suite to go with out dream MBE system? Yes please, next?

Maintenance will always be a problem in MBE operation, but it need not be a crippling blow. Part of the maintenance responsibility falls to the user establishing good MBE practice but equally part of the responsibility falls to the manufacturer. There are several items that fall into the category “ease of maintenance”. Ten minutes (or perhaps an hour) talking to an experienced technician would highlight hundreds of issues that could be designed out. Maintenance should simply be regular, straightforward and swift. Let’s pay attention to the people who have spent their entire lives maintaining and fixing MBE system and incorporate their ideas, ok, what else?

One of the hidden problems is unit cost and this certainly limits the number of MBE research groups. Skill is not a limiting factor. A good teacher can teach the basics of MBE operation in about two weeks. A good student can become autonomous in about 3 months. Of course here we have a dangerous situation: if the system is fully manual, the process will take longer but the student will gain a deep knowledge of MBE. On the contrary, if the system is fully automatic the process will be much swifter, however the MBE knowledge gained will be more superficial. It is therefore always best to learn on a fully manual system. The automatic system is however essential for any serious research. This “dream machine” is not a wild fantasy. Everything mentioned in this article already exists. It is simply a matter of putting the pieces together.

So the final and most important question: How much will this system cost? Well before I answer, I will ask you: what is the cost of not having it? The answer is fewer research groups, less samples, less science, less progress, less understanding, more downtime, more maintenance, more frustration, more money wasted maintaining out of date machinery. The problem with the dream machine is not the £100k of parts; it is the £400k we pay to have those parts assembled. Much money can be saved by stripping the useful bits off old systems: the pumps, valves, cells, flanges, nuts and bolts and the racks. What we need is to invest a little time and effort into creating a compact, inexpensive stainless steel chamber and supporting frame integrated into automation software. A nice RHEED, reflectivity system and an As cracker. The substrate heater need only accommodate 2” or ¼” of 3” wafers maximum, we are performing R&D after all. 10 cell ports are plenty, by the time you have grown 360 samples in 1 month you will have thoroughly exhausted the cells, thoroughly explored one material system and can load a different combination of cells for the next month’s work.

How much does the system cost?

£500,000 new

Growing 4320 world class samples a year with less than 15 minutes work a day?

Priceless

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4 thoughts on “Molecular Beam Epitaxy: Dream Machine

  1. Pingback: Molecular Beam Epitaxy: Initial Outlay | Dr. Faebian Bastiman

  2. Pingback: Essential maintenance: Cell material regeneration | Dr. Faebian Bastiman

  3. Pingback: MBE Dreams: Dream Software | Dr. Faebian Bastiman

  4. Pingback: Facility Setup: A room with a view | Dr. Faebian Bastiman

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