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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Essential calibration: Post-bake optical test structure

Faebian Bastiman

After a bake out, thorough cell outgas and growth rate calibration the material quality of the MBE system must be determined and assessed. On a III-As system you would want to assess the quality of GaAs and InAs and/or InGaAs and AlGaAs. A 6nm GaAs QW with RT PL of 820 nm is a good optical test layer since it is easy to differentiate between the substrate and the well. An In15Ga85As QW has an RT PL of ~950nm and is sufficiently longer in wavelength that it does not interfere with the GaAs peaks. 2.5 ML (i.e a total of 15.645 In atoms/nm2 on GaAs) InAs QDs can be tailored to give a variety of wavelengths. If grown at ~490-500°C with a growth rate of 0.017 ML/s (i.e. 0.106 In atoms/nm2/s) the RT PL wavelength should be around 1250nm. In fact, these 3 wavelengths are sufficiently distant such that with a little AlGaAs confinement you can put them all in one structure. An example structure is shown in figure 1 below:

post bake jpg 2

The total growth time is 80 minutes including temperature ramps and As flux changes. The GaAs should  be grown around 570°C, the InGaAs around 540°C and the InAs around 500°C. Once grown the sample surface should have a slightly purple hue and of course should be shiny. The RT PL should be taken from 800 – 1300 nm and should resemble figure 2 below. Remember this is the first sample after bake out, but after a thorough cell outgas the quality should already be very good.

 otc PL

Figure 2

The peak at ~1250nm is the InAs QDs and the peak at 955 is the InGaAs QWs. Both of these are excellent intensities, around 33% of the best values obtained at these wavelengths on this system. This is an indirect indication that the Al40Ga60As is good quality, since poor AlGaAs cladding around the QDs would introduce non-radiative recombination centres. The 3 x InGaAs QW are expected to be brighter than the single InAs QD layer. But  what has happened to the GaAs QWs?

Well the insert shows the scan limited from 750 to 885nm, and the GaAs wells are there. The structure is excited with a 650nm diode laser, which excites carriers and stimulates radiative recombination in all structures. The 820nm GaAs QW PL light must first travel up through the structure and escape before it can be detected. Since it is shorter in wavelength (higher in energy) than the InGaAs and InAs above it, a fraction is reabsorbed. In this case that fraction is around 99%. Thus the GaAs does not seem to emit at all compared to the InGaAs. Hence to actually achieve comparable intensities from all the active layers the InGaAs QW must be reduced from 3 to 1, or the GaAs QW must be placed above the InGaAs in the structure.

Post-bake tasks: Cell outgas

Faebian Bastiman

After a standard 48 hour bake out, an MBE system needs a thorough outgas. Once the system has cooled to around 75°C it is a good idea to fire up the titanium sublimation pumps (TSP) and turn the ion pumps to maximum voltage. The background pressure is liable to soar into the 10-6 mBar range, but should return to low 10-10 mBar in a few hours. Once the system is at room temperature again, the cells can be outgassed.

To begin you do not need water cooling or LN2. Simply heat the cells as outlined in Phase 1 in the table below.

outgas table crop

There will be a pressure spike as the cells’ outer bodies outgas. Ga will already have melted during the bakeout and In will join it at 150°C. Note that Bi should not be melted (271°C) in this stage since we are about to return to 90°C on all cells to apply water cooling and avoid having the water boil in the cooling shrouds.

Once the cells have reached their Phase 2 values water cooling can be applied. Check for leaks and ensure an adequate flow is reaching the cells and then swiftly proceed to the values in Phase 3. The LN2 cooling can now be applied to the cooing shroud and after an hour the background pressure should reach a low 10-10 mBar value.  Now the cell material can be outgassed.

It is a good idea to do each cell in turn, since this allows an individual cell’s effect on the background pressure to be monitored. There is no particular order, save Bi should be the penultimate cell and Al the last. This is because once these cells are “hot” they cannot be returned to room temperature without changing the crucible. Of course we hope the other cells will not fail during the outgas and we will not need to open the system to fix the problem, but we must prepare for the worst.

Approach the ultimate outgas temperature slowly whilst monitoring the background pressure. The pressure should not exceed the 10-7 mBar range at any time. Each cell typically takes several hours to reach the final temperature and should be held for an hour before being cooled to operation or standby values. The cracker part of a group V cell must be heated before the bulk, it is a good idea to leave the cracker at the outgas temperature whilst outgassing the bulk, then drop the bulk to operation values, and finally lower the outgas temperature of the cracker to operation values. The actual values depend on the exact source and vary by manufacturer. As a good rule of thumb you should outgas the cells 25 to 50°C hotter than they will ultimately be operated.  The nitrogen and phosphorus cells are speciality cells and even a general outgas procedure cannot be stated.

Let’s fix: Cleaning an Al cell shutter

Rob Richards

When we leave the lab we make sure that everything is in its standby state and we set up an automated outgas (or “morning initialization” as we call it). This usually means that the cells are clean and ready to use for the next day. However, one Friday morning we came in to find that the water cooling system had broken; water was not being pumped through the cells and was boiling in the pipes, causing the cells to heat up and outgas into the chamber. Having failed to restart the water cooler we crashed the cell temperatures to prevent them from being damaged. When we took the machine apart (it needed restocking with gallium and indium anyway) we found the aluminium shutter looking like this.

The sizable blob of aluminium which had accumulated on the shutter had clearly reacted with the outgassed material from the cells and now looked pretty disgusting!

The first step in the cleaning process was to remove as much of the aluminium as possible with a craft knife. After this the shutter looked significantly better but still not quite good enough to be used again. The remaining aluminium had to be etched away, so the shutter was placed in a beaker of room temperature HCl. The reaction was very quick and after ten minutes the shutter looked like this. Nice and clean and ready to be put back into the system!

Essential maintenance: MBE bake out

Danuta Mendes

To improve the vacuum in the main chamber after essential maintenance a bake-out is traditionally performed on an MBE machine.  The contaminants and moisture adsorbed on the side walls of the main chamber will take longer to desorb and pumped out at room temperature. After using the repeated N2 purge technique outlined in Essential Maintenance: Pump down,  it is entirely possible to recover the vacuum with normal, sustained operation of an MBE system. The process is however fast-tracked by heating the machine for a period of time in excess of 100°C. The result is a lower background pressure and good vacuum within the system. Instructions on how to build your own bake out system can be found in MBE Design and build : Bake out controller.

Our Omicron MBE-STM has been baked in two stages: a mini bake using heater coils, coiled around the main chamber (Fig 1) and a more comprehensive bake by enclosing in a custom built box composed of thermally insulated stainless steel panels (Fig 2).  It is important to short RHEED gun power pins with Al foil static build during the bake out. It is best to regulate the bake out ramp from room temperature to bake out temperature with a PID controller set to 1°C/min. You can use a faster ramp (2°C/min max) and cover the view ports with Al foil in order to buffer against the additional thermal stresses, but an unregulated ramp is not recommended.

The mini bake shown in Fig 1 is more flexible but less comprehensive that the full bake (Fig 2). Heater coils can be wrapped around the chambers that need baking out without the need of unplugging electrical parts or plastic tubing which are used for water cooling the effusion cells. A custom designed bake out jacket with built in heater filaments would be a MBE wardrobe worth investing in 🙂

A full bake out using thermally insulated stainless steel panels (Fig 2) is both temperature and pressure dependent. It is possible to achieve the same vacuum by baking a machine at 200°C for 24 hours, 125°C for 48 hours or 100°C for 60 hours. The maximum bake out temperature is limited by the temperature sensitive parts in the machine. The lowest pressure achievable within a “used” III-As MBE chamber during the bake-out is of the order of 10-7 mBar which can be achieved in ~50 hours at 125°C and typically consists of  As vapour pressure.

  

 The full bake out  provides a better vacuum as the heat is more uniformly distributed. However, these are more cumbersome to put together and require removal of all electrical and thermally sensitive connections. It is essential to bake the chamber long enough till the moisture and contaminants adsorbed onto the side walls have sufficient thermal energy to be desorbed and eventually pumped out. Since the vapour pressure within the system has been reduced it is possible to reach ultra-high vacuum and good growth conditions which is essential for good epitaxy !