Facility Setup: A room with a view

Faebian Bastiman

One of the questions I get asked frequently is: does an MBE system need to be in a cleanroom? My answer is: no, but it needs to be in a clean room. What do I mean? Well the room must be designed in a way that is easy to clean, and must be kept clean. The ideas are summed up very nicely in this blog post from Hutchins and Hutchins.

So what is the difference between a clean room and a cleanroom? What does an ideal MBE lab look like? Well consider the classic laboratory (and I am not talking about Dr Frankenstein’s castle’s highest tower complete with lightning rod) I am talking about the classic white walls, floors and ceilings, plethora of air vents and cold fluorescent light. Ghastly!

The white surfaces and ventilation can be maintained, but one of the first things you should consider is to flood the room with natural light. This of course means windows. Not only is sunlight a free and available mood enhancer, it also increases productivity and assists is healthy sleep patterns. Everyone, even scientist chained to their MBE systems, deserve natural light. Furthermore strong sunlight is an excellent means of detecting airborne or surface dust, and prompting you to do some cleaning.

The next thing to consider is that the dirtiest thing in the room is the human being, so one should limit the room one wishes to keep clean to human exposure. The MBE lab is also generally unpleasantly noisy due to the various pumps maintaining UHV. The best thing to do is therefore to divide the floor space into an office and a lab area. I direct your attention to figure 1 which represents an example MBE layout in a corner building location.

Figure 1: An example MBE lab

Note the light blue rectangles are windows and run the length of the upper and right edges. The corridor at the lower edge of the image has a door shown in purple that leads into the yellow office area. The office area is equipped with a desk that houses the MBE system’s control PC and has a further internal window where one can view the MBE system’s racks. The office should perhaps be air-conditioned to a desired temperature (e.g. 22 °C) but need not be filtered or humidity controlled.

The dark green space directly above the office is the dressing area. Here the user removes their external footwear and replaces them with non-shedding, clean footwear. One should also wear a knee length clean room coat (and if one is going to load/unload samples a hair net and face mask). The floor to the dressing room should be coated in sticky mats and there should be a sink for washing hands on entry and exit.

The light green lab is entered primarily through the dressing room, though the lower windowed wall should be removable to enable the MBE system to be installed in the first instance. The lab is where one should focus their intentions to create cleanliness. The room should be air-conditioned (e.g. 22 °C) to counteract the heat load from the racks. The air conditioning could also consist of low level filtering to reduce the typical class 1,000,000 or ISO 9 in a typical office room to 100,000 or ISO 8 to reduce cleaning frequency, but this is not essential for MBE research. Additionally one could consider employing humidity control (down to 40%) to reduce moisture intake into system during sample loading and maintenance. Most importantly the floors, walls and ceiling must be constructed from non-shedding material that is easy to clean with a solvent wipe. This is liable to be the highest expense, especially the ceiling tiles. The MBE system itself should be situated on the centre of the lab with at least a metre free on all sides. Ideally, the rack should be installed next to the system to avoid passing cables between the two. All cables should feed down from the ceiling above the rack and not across the floor, the same goes for the services (process gas, cooling water, cryo pump He, LN2) all should be fed down from the ceiling. The floor around the system can then be easily cleaned and there are no obstacles in the way that prevent dust being drawn into the low level extracting ventilation on the lab’s walls.

The lab requires a laminar flow fume cupboard for exchanging samples or performing maintenance. The remainder of the available wall space can be used for essential and convenient storage of bake out panels, substrates, ultra-pure metals, gaskets, flanges, spare cells, etc. Spare cells could optionally be stored on an outgas rig if there is sufficient space.

The lab should be free from paper, pencils, books, ink etc. All of that should be stored in the office that houses the MBE system’s operator. Ultimately, there are only three reasons to enter the lab space:

1. Load or unload samples
2. Clean
3. Perform system maintenance

If the MBE system has automatic sample transfer and can execute batches of samples (see my Dream Machine blog post), one need only enter the room for a few minutes every other day. In doing so one need only clean the lab once per week. It should be vacuumed with a high efficiency particulate air (HEPA) filtered vacuum cleaner and all surfaces should be wiped down with appropriate solvents. In most cases the system maintenance should be of the scheduled kind and should be followed by the most exhaustive cleaning. Maintaining a slightly higher differential pressure in the lab compared to adjoining rooms will also reduce particle intake on ingress.

Note the LN2 phase separator to provide cryo cooling is not shown in the figure, but is intended to be installed in the ceiling space above the MBE system.

The final room to the left of the figure is the service corridor. This houses any of the services that create particles or dirt. Particularly one may wish to install the air compressor for pneumatic gas, the He compressor for the cryo pump, the water chiller for cell cooling and a pair of N2 cylinders for ultra clean process gas. One can also include an AC power distribution panel and a large roughing pump that augments any turbo pumps on the system. The service corridor need not be located on the same floor as the lab, and could be a full service area that includes the facility’s backup generator, UPS system and end line trap for the roughing pump’s exhaust.

The result is a very clean, easy to maintain lab space where cutting edge opto-electronic and electronic semiconductor research can take place that does not cost the earth.

Little known MBE facts: High Purity Material

Faebian Bastiman

I was asked recently what purity of material a user should use in their MBE system. Here the grower was referring to the number of “N’s”. The N of the material is a reference to its purity and is actually the number of nines: Material that is 99% pure and 1% impure is termed 2N. 99.5% is termed 2N5, the “2N5” tells you it is at least half way to being 3N. Of course this does not tell you what the impurities are specifically, however you can assume they are made up of all the things you do not want in your III-V thin film layers (Zn, Cu, Fe, Sn, Hg, Ca, Te, etc) and a few III-V elements that you do not need to worry too much about (you are, after all, growing III-Vs anyway).The purity of material will ultimately determine the quality of your thin films, since any impurities in your cell material will likely incorporate as unintentional dopants in your layers.

If you are growing metals (for example MnAs) you are not too worried about unintentional doping and you may use a maximum of 5N5 material (99.9995%). If you are growing electronic or opto-electronic grade semiconductors (for example GaAs) you will want higher purity. Ultimately the highest you can get commercially is 9N (99.9999999%), however I would not recommend you all rush to the shops and buy such expensive material for general research. 9N is exponentially more expensive than 8N, 8N is exponentially more expensive than 7N. Similarly the ultimate background doping you can achieve with 9N is an order of magnitude lower than with 8N, and the same applies when comparing 8N to 7N. Before I suggest the appropriate material quality, let’s do a thought exercise with GaAs…

GaAs has a lattice constant of 0.565338 nm, and therefore an atomic density of 4.42 x 10²² cm^-3. When opto-electronics people (specifically detector people) talk about background doping requirements they say that 10¹⁵ cm^-3 is already good.  This means they would like the unintentional doping level to be 5×10⁷ times lower than the atomic density. In this case 7N5 would be the appropriate choice for you group III and V material. One could argue that you should use 7N5 for all materials and that replacing a specific material with 8N is a waste of money. However this “all or nothing” philosophy is not really justified since unintentional doping is accumulative, and hence replacing your group IIIs with 8N might reduce your background doping from high 10¹⁵ to low 10¹⁵. Thus 7N5 to 8N is a good choice for opto-electronic research, and 9N is only for exceptional high mobility cases or world record attempts. Moreover, if you are simply experimenting with a new opto-electronic alloy and not so interested in device quality at this stage 6N5 is acceptable. Note that the source material is only one factor in your ultimate achievable material quality, for more information read my Optimum Quality post.

Finally consider the dopant material. When we dope a semiconductor we typically dope in the 10¹⁶ to 10¹⁹ cm^-3 range depending on the application. At 10¹⁹ cm^-3 the dopant is around 10⁴ times lower in atomic density than the III or V species. This means the doping fluxes are around 10⁴ times lower that the group III and V fluxes and hence any impurities introduced into your system from the dopant source will also be 10⁴ times lower. Add to this that when you dope the alloy you are “trying” to add impurities and you can say that 6N is already a very good grade for dopant material and 5N may even be suitable.

MBE Dreams: Dream Software

Faebian Bastiman

Returning to the theme of “Dream MBE”, I would like to spend a moment discussing molecular beam epitaxy software. Off the shelf MBE software is passable at best. Ok, so it can open and close shutters, perform temperature ramp ramps and execute a sequence of commands in some form of recipe: This is, when all is said and done, the bare minimum. When you spend some time actually considering the features you would like in MBE software, you quickly realise nothing on the market really hits the spot. So… I have spent the past six months writing my own MBE control software: EPIC™.

You cannot create the world’s greatest MBE software in six months, however you can implement an exciting list of features and lay the foundation for fantastic things to follow. Beyond the bare minimum functionality, the features of EPIC™ are:

1. Comprehensive, optimised software-hardware interface (SHI)
2. <1 millisecond to process a shutter request
3. Full device parameter explorer and editor
4. Ability to monitor PID loops, pyrometers, plasma sources, pressure gauges, pumps, valves, motors, services (LN2, water flow, gas pressures) status and other hardware (e.g. reflectivity data) all from a central, grouped, tabulated status screen
5. Associate any number of physical devices with an individual control loop
6. Ramp to temperature set point or output power for PID controllers
7. Flexible, simple script recipe with full control flow, local and global variables, unrestricted parallel editing and execution including “edit on the fly” and access to any parameter from any device
8. A colour co-ordinated message box
9. Wake-up and sleep scheduling
10. Substrate location and status tracking
11. Automatic flux scheduling, gathering and Arrhenius/polynomial fitting
12. Atomic flux, growth rate, doping and composition calculation tools
13. Material usage tracking and cell fill level estimation
14. Two data logging modes: SPAM (log at fixed interval) and SMART (log only on change)
15. Watchdogs warnings and actions
16. Integrated simulation mode with full hardware emulation

Ok let’s analyse that numbered list. (1) SHI is a fantastic interface with every device possessing its own read and write thread. SHI enables (2) and (3) by its very nature. Not every process requires less than 1 ms shutter response, though since it exists by default I thought I might as well include it in my list. The device explorer is also a nice feature, since the front panel menu on most devices can be a pain.

The status screen (4) is very comprehensive and the layout facilitates ease of reading. A quick glance can reassure you that every part of the MBE system is ok (or equally quickly warn you when it isn’t!). All necessary items are interactive, giving the ability to perform ramps and open/close single/multiple shutters. The ability to associate any number of devices with a control loop (5) is very useful for plasma sources when they may have: a mass flow controller, a RF power controller, a main shutter, several valves and an optical monitor. So too is performing power ramps (6) for certain control loops, like carbon sources.

The recipe script (7) is essentially its own programming language complete with interpreter. Each recipe executes on its own thread and all commands run to 1 ms precision. The control flow allows you to step through each line individually one-by-one or allows the recipe to run freely, you can pause any line, go to any line from any line, skip lines, stop the recipe at any point and edit the recipe on the fly (only when paused for safety) and the recipe checks every line for errors on edit. Local and global variables allow parallel recipes to control complicated processes. Whereas access to any device parameter gives the ability to do almost anything, including in situ reflectivity control (see Dream Machine).

The message box (9) simply keeps you informed with a variety of colour co-ordinated, time stamped status updates including “Recipe X completed”, “Ramp X started”, “Sample X moved to position Y” or the less pleasant but equally useful “device X failed to respond”.

Wake up and sleep is fantastic (9). One can sleep much better knowing the machine will wake up at 06:30, do all the flux checks for the day and wait for you to stroll in at 09:00. Equally, the sleep mode prevents that troublesome “did I ramp down the Gallium before I left?” since it sets the system to a predefined standby state.

When you have more than a handful of substrates and/or more than a single user it is very useful to have a substrate tracking screen (10). Each substrate can be individually tagged with a name, a colour co-ordinated status (new / degassed / grown) and its current location in the system. The substrate tracking paves the way for extended functionality such as automatic run logging, automatic growth sheet creation, automatic sample transfer and batch sample processing (all of which are on my to do list).

Any software that purports to convert effusion cell temperature into growth rate is just plain lying! Beam equivalent pressure (BEP) is typically what you measure and atomic flux in atoms/nm²/s is what you get. Growth rate is simply something you induce depending on the atomic density of your substrate. Software that gathers flux data for you (11) and plots it on a graph is a welcome start. A one click fitting option ranging from simple Arrhenius to 12^th order polynomial fitting is sufficient to handle all source and opens the door for much, much more. The first step is enabling conversion from BEP to atomic flux. Once done you can use a growth rate and doping calculator tool that updates to the correct doping density and growth rate for any substrate (12). You can then “ramp to doping density” or “ramp to growth rate” rather than “ramp to set point”. You can estimate the real time composition during growth and calculate the composition ahead of time for a given cell temperature combination. Perhaps my personal favourite is that you can integrate the flux with time every single second and track the cell material usage, display it on a simply bar chart and estimate the fill level of each of your sources to 1%. All of these flux related features are present in the current version.

Of course you want to log all relevant data values (14) and the option to log only on change prevents large data files being created when the system is idle (over a weekend). You also want to monitor the instantaneous data values and be warned when they are outside of an “OK” range. Moreover, for certain values you would like the software to take an action on your behalf to prevent damage (15). Finally you may want to run the software in “off line” mode to “try before you buy” or even to “test a recipe before you run it for real” and in these circumstances the simulation mode is very handy.

Q: What could possibly be missing?

A: Access to the source code.

The reasons for open source MBE software are almost as numerous as the feature list itself, so let’s stick to the two most important ones:

1. Add device drivers for any hardware
2. Add/edit/remove content to the user interface

Adding device drivers is somewhat essential. The system hardware changes over time as the system is upgraded or re-commissioned for a new purpose. The current EPIC™ has a generic driver template that can be used to create a new device driver in around 30 minutes. The next step is to make that driver template available to all and to use device driver DLLs. Fairly simple.

The user interface is a very personal thing. The software-hardware interface (SHI) is rather universal once done right, but the user interface is a matter of taste. Script recipes are very powerful but perhaps not for everyone. Tabulated data makes for fast referencing but perhaps diagrams are more your thing. Etc. Etc. Etc. There are three solutions beyond making the software open source:

1. Create a software development kit (SDK) for EPIC™ to allow everyone to customise it with their own desired features
2. Allow SHI to be a stand-alone data acquisition tool that can be used either via windows API or via a socket interface to allow the user to create their own graphic user interface or web application
3. Provide customisation on demand and provide a tailor made solution for each user

Happily EPIC™ will have all three.

One final question: Where can I try the latest EPIC™ beta demo version?

Here.

How to grow your first sample: (2×4)/(4×2) transition

Faebian Bastiman

The (2×4)/(4×2) RHEED transition is a very useful flux calibration point, since it tells you when your As and Ga fluxes are equal. I described what a (2×4) and a (4×2) reconstruction are in my What is a reconstruction post. In this post I will explain how to spot the transition with RHEED.

First of all you need to produce a good starting (2×4) surface. In order to do that follow the steps in my Oxide remove post. With the Ga shutter closed and an As overpressure incident on the sample you should see the As-rich (2×4) reconstruction on your RHEED pattern (Figure 1a). How can you be sure this is a (2×4) and not a (4×2) in disguise?

You can be 99% certain that if you are annealing at a temperature around 15 °C below the oxide remove temperature and the As flux that you are supplying is sufficient to grow GaAs that this is a (2×4) reconstruction. You can test very simply by stopping the rotation so you can see the 4x on the [‑110] azimuth and reducing the temperature. Reduce the temperature until a 2x appears. Rotate to 90° to the [110] azimuth. Still 2x?

If yes, then this a c(4×4) and the early reconstruction was a (2×4). Return to the original temperature and retrieve your (2×4). Note if you want to know what a c(4×4) is read my As cap post.

If no, and you are looking at a 4x, then this is now a (2×4). Remain at this temperature.

If no, and you are also not looking at a 4x, then something is very, very wrong with your sample. Are you sure it is GaAs(100)?

Assuming you have found your (2×4) make sure it looks like the one in Figure 1. If not change your temperature by 5 – 10 °C and your As flux by 10-20% and try to get the best (2×4) you can. You will note that in Figure 1a on the [‑110] azimuth the 4x rods are fairly short and bright even if the 2^nd order rods are not very clear. The 2x on the [110] azimuth is simply a 2x and cannot give you any quality information at this point.

Try opening the Ga shutter. Does your 4x change to look like Figure 1b? It should. The Ga flux breaks up the static (2×4) and creates some disorder. This disorder results in an elongation of the 4x rods and an overall decrease in the intensity. The 2x on the other hand does not really change. You can consider Figure 1b as the dynamic (2×4), where “dynamic” means “growing”.

We are about to search for the Ga-rich (4×2) pattern. In your first few attempts do not rotate the sample, but rather keep the RHEED pointing along the [110] azimuth and looking at the 2x pattern. During the search the 2x will change into a 4x on [110] and the 4x will change into a 2x on [-110]. You can see the Ga-rich reconstructions in Figure 1d.

The problem with performing the transition without rotation is that you are only seeing the transition at the point the RHEED beam is hitting with the Ga and As flux pattern present at that point. This As/Ga ratio can be very different from the one created whilst rotating, since when you rotate you create a more even flux across the wafer. On the other hand since both the starting and ending reconstructions have both a 2x and a 4x azimuth it is not so obvious when the transition has happened until you have some experience looking at it.

So with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen close the As shutter (or close your cracker valve if you do not have a shutter).

Am I serious?

Yes.

Close the As shutter for a few seconds and watch the RHEED, it should quite rapidly turn into a 4x like Figure 1d. Once it does open the As shutter again and make sure the 2x returns. When the As flux is greater than the Ga flux, you see the 2x of the (2×4) when the Ga flux is greater than the As flux you see the 4x of the (4×2). In the extreme case we just witnessed the Ga was of course practically infinitely bigger than the As flux.

Ok step 2 is a little less brutal… with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen… half the As flux. Wait 15s. Did the 4x appear? If yes return to the original As flux, if no half the As flux again. What you are doing is performing a binary search. You take the lowest As flux you know gives you a 2x and the highest As flux you know gives you a 4x and you apply the average of the two and see if you get a 2x or a 4x. Always wait the same 15s between flux changes for consistency.  

You may notice on your search that the 2x creates a rather dim 3x rather than the desired 4x (Figure 1c). This is a mixed reconstruction between the (2×4) and (4×2) that many people overlook. It is weak on both [110] and [-110] azimuths and hence probably lacks the long range order of the (2×4) and (4×2) reconstructions. The low intensity makes it difficult to say whether it is a (3×1) or a (3×4) or a (3×6) and indeed the physical surface of the GaAs could be made up of domains of each.

Once you have found the maximum As flux that gives you a 4x on the [110] in 15s, return to the original As flux that gave you the 2x on the [110]. In the third step gradually reduce the As flux from the start value toward the value you found and watch the RHEED pattern evolve. Double check the As flux you believe gives you a 4x on the [110] is reproducible and get familiar with the process.

Finally set the substrate rotation to around 0.2 revolutions per second and perform the third step again. The As flux required to create the 4x on the [110] azimuth may be a little different this time. With the rotation on you will see the 2x on the [-110] azimuth for the first time. Take a closer look at the reconstructions in Figure 1d.

The dynamic 4x you get when you are Ga rich is much sharper than the dynamic 4x you get in Figure 1b. This is because that whilst the Ga breaks up the As-rich (2×4), the Ga only ever improves the Ga-rich (4×2). Hence the dynamic 4x of Figure 1d is similar to the static 4x in Figure 1a. Note too that the 2x in Figure 1d is very different. There exist some extra bright spots at the bottom of the image that are much dimmer in the 2x of Figure 1a and cannot be seen in Figure 1b.

Hopefully you are now familiar with the process and can readily ascertain the maximum As flux required to create a (4×2) reconstruction in 15s. This is called the “1 to 1” point. The point at which the As flux is approximately equal the Ga flux. You could argue that the Ga flux is slightly higher than the As flux, since the surface turns Ga rich; however they are approximately equal. More importantly this transition is very repeatable. You need to make sure you always do the test in the same way. i.e. you must always wait 15s and always do the test at this temperature. Otherwise the flux will vary from check to check. It is worthwhile observing that variation yourself.

Once you have found this point you can calculate your fluxes in the system independent units of atoms/nm²/s (see Flux determination post) and state the atomic fluxes in any journal articles you write.

MBE Dreams: Efficiency of Effusion Cells

Faebian Bastiman

Have you ever wondered what the material delivery efficiency of standard MBE effusion cells actually is? We typically spend on average £2k a year on source materials and £1.5k a year on substrates. We slice our substrates into 11.4 x 11.8 mm² pieces and get 12 from a 2” wafer. This means our substrate usage efficiency is around 80%. This is the price we pay for cleaving square pieces out of circular substrates. This means we are using £1.2k out of our £1.5k of wafers and losing some £300 a year directly into the recycling bin.

But what about our £2k of cell material?

Well we buy a 240g charge of As every 3 years. This means we load 1.93 x 10²⁴ atoms into our system every 3 years. We grow on average 1 µm a day, on a ~10 x 10 mm² active area, 280 days a year for 3 years. This is 84 mm³ of GaAs or 1.86 x 10²¹ As atoms. That means out of 1.93 x 10²⁴ As atoms we put into our system only 1.86 x 10²¹ actually end where we want them i.e. in our epilayers. That is 0.1%. Ouch, 99.9% material wasted!

Ok, just breathe. We know As is pretty wasteful, it is so gaseous that it goes everywhere. What about our group III’s?

Well we buy a 30g charge of Ga every 6 months. This means we place 2.6 x 10²³ Ga atoms in our small 10cc effusion cells every 6 months. We still grow the same 1 µm a day, 87% of which is Ga. We grow this for 140 days in 6 months on our same ~10 x 10 mm² active area. This is 12 mm³ of GaAs or 2.7 x 10²⁰ Ga atoms. That is also an efficiency of 0.1%.

So out of our £2k a year on material we are only using £2 usefully. The other £1.998k ends up plastered all over the chamber walls, caking shutter blades and gathering in a large pool at the lowest point on the system.

Can we do anything to improve the situation? Sadly for standard effusion cells the answers is: very little.

We could reduce the cell-substrate distance. In the extreme case you could have the sample at the cell orifice, however the temperature rise when opening the shutter would be horrendous and group III cells tend to spit material too. The cell-sample distance is related to the number of sources, and assuming you want 10 cells and a pyrometer pointed towards your sample the current distances are already the lowest possible.

We could reduce the cell’s orifice with a Ta aperture. Effusion cells are designed to create a uniform flux across a certain sample area however they also spray material non-uniformly in almost every direction. In our case the sample area is 4x less than intended, so we could potentially achieve 4x the efficiency (a jaw dropping 0.4%) by reducing the orifice. The minimum aperture is related to the required flux uniformity, and in most cases this is optimized at the time of manufacture too.

We could also lower the deposition rate. There are some fixed time intervals for MBE, like the time the shutters are closed whilst the oxide is being removed and whilst samples are being transferred. When you grow at 0.1 ML/s you are losing 10x less material than when you grow at 1 ML/s in these fixed times. Of course some times lowering the deposition rate does not help. When you are growing a 5 nm / 5 nm GaAs/AlAs superlattice, for example, it does matter what the deposition rate is since (assuming  the growth rates of the two cells are identical) the atomic equivalent of 5nm of Ga will be deposited on the Ga shutter whilst it is closed during the 5nm AlAs layer, and vice versa. The efficiency increase depends on the ratio of growth time to preparation time. If we grow a 100 period 5 nm / 5 nm superlattice at 0.11 ML/s we waste 0.37x less material than if we grow at 0.55 ML/s but the total sample growth is increased by 1.9x. This comes down to throughput, the data in this article is gathered for a deposition rate of 0.55 ML/s, 0.000001 ML/s is very efficient but… 

With the orifice and the 0.11 ML/s growth rate the efficiency of a Ga cell would be 1%. This is certainly better, but still extremely wasteful.  

The final option is to used valved sources. Whilst valved sources are not particularly effective for high vapour pressure elements like As and P, Sb can be delivered with much higher efficiency because it behaves more like an Architypal molecular beam than a gas source. Valved sources are pretty standard for group Vs but what about group IIIs? Well e-Science are currently introducing their Valved Titan effusion cells for Ga and In. A valved source has two distinct advantages over a standard cell. On the one hand the material wastage is significantly reduced, since the cell can be idled hot with the valve closed. On the other hand, varying the valve position can enable instantaneous changes in deposition rate. A fully valve sourced III-V MBE system is certainly amongst my MBE dreams.

Molecular Beam Epitaxy: LN2 system

Faebian Bastiman

LN2 usage can represent the major expense in LN2 operation. Designing an efficient LN2 cooling system for an MBE operation is more involved than a simple fluid transport system. At one end the system should comprise a large dedicated external storage tank and the other end consists of the cryo panel of the MBE system. There are two main loss mechanisms that must be minimised in order to transport the LN2 from one end of the system to the other.

Firstly there is the compound effect of connection loss (via heat loss) and N2 gas heating. The heat loss results in evaporation of LN2 into N2 within the pipework. The actual LN2 lost at the point of heat loss is fractional compared to the “heating” the “warm” N2 gas causes to the “cold” LN2. This may seem like an odd concept, but the N2 gas is indeed hot compared to the LN2. The poorly insulated connection that resulted in heat loss is localised at a single point on the system, however the N2 gas permeates the entire pipework system and constantly converts LN2 to N2, which in turn exacerbates the problem. In order to minimise this loss mechanism the entire transport system, including valves, pipework and connections, must be vacuum insulated. Moreover, the integrity of the vacuum insulations must be regularly checked and maintained.

The second form of loss is flash loss. Flash loss is a foreign concept to those outside the LN2 community. It is a loss induced by reducing the pressure of the LN2. The change in pressure causes a fraction of the LN2 to “flash off” as N2. The reason for the dedicated tank is now clear: The tank must store the LN2 as close to atmospheric pressure as possible. A small pressure must be maintained in order to ensure the LN2 can reach the destination with the intended flow rate. The lower the LN2 storage pressure, the lower the flash loss.

The presence of N2 and LN2 in the system is termed “two phase flow”. Two phase flow implies the cooling is inefficient since the LN2-cyro panel contact area is reduced in the presence of N2 gas. Hence a “phase separator” is typically employed in order to separate the gas and liquid phases and ensure only LN2 reaches the cryo panel. The phase separator stores the LN2 at atmospheric pressure (the most efficient pressure) and uses gravity to feed LN2 into the cryo panel. It is important to remember that N2 gas will be generated inside the cryo panel. The cryo panel is the only “intended” heat loss present on the system. Additional triaxial pipes ensure that the N2 gas generated in the cryo panel can leave without interrupting the LN2 entering the cryo panel. Thus the cryo panel is optimally cooled and achieves optimum efficiency resulting in an optimal vacuum and (hopefully) optimal samples.

The ideal LN2 system would therefore comprise:

1. a low pressure LN2 tank
2. a vacuum insulated tank connection valve
3. vacuum insulated pipework
4. a vacuum insulated phase separator connection valve
5. an atmospheric pressure phase separator
6. a vacuum insulated triax LN2 feed hose and a vacuum insulated return hose from the phase separator to the cryo panel
7. the cryo panel
8. a stainless steel N2 gas exhaust line from the phase separator to the outside world

The volume of the tank (1) will depend on your individual usage (typically 100 – 250L /system/day) and the frequency of your deliveries. Expect to pay £10-20k for the tank. The valves (2 and 4) are generally included. The pipework (3) varies by manufacturer (~£300/m) and typically requires a small, dedicated pump to ensure vacuum integrity. The phase separator (5) is around £16k and the hoses (6) are around £5k. The entire system will therefore cost around £50k. Which may seem like a lot, but to put it in perspective it roughly equates to the amount of money you would waste in the first two years of operation of an inefficient system. All that remains is to choose your manufacturer: VBC, VBS, DeMaCo or PECO.

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

Little known MBE facts: MBE crystal growth optimisation

Faebian Bastiman

In the first instance MBE appears to be a rather complicated affair involving pumps, LN2, valves, fluxes and vacuums. A high purity vacuum and effusion sources are essential and require great attention,  however when an MBE system is well maintained and operated the problem of MBE growth optimisation becomes trivially two dimensional: A III:V MBE grower ultimately only has control over two parameters (1) the growth temperature and (2) the III:V flux ratio.

Most miscible materials can therefore be optimised within less than 10 sample runs. Take GaAs/GaAs(100) as a thought exercise. A simple assessment of a direct band gap semiconductor like GaAs is a room temperature PL investigation. To differentiate between the epilayer and the substrate we will want to confine carriers and possibly shift the wavelength away from the ~872nm of bulk GaAs at RT. A suitable test structure is shown in figure 1 (below) with a RT PL of ~810nm.

The first thing to do is discern a favourable starting point. The oxide remove temperature is a good starting temperature for optimal GaAs growth (~580°C) and a flux ratio that gives a slightly weak 2×4 pattern and >20 RHEED oscillations before damping is a good flux. The flux ratio can also be estimated by (carefully and quickly) finding the dynamic (2×4)/(4×2) transition by dropping the As flux for a given growth rate. This corresponds to an As:Ga  of ~1:1. A good starting point is ~1.6:1, hence the As beam equivalent pressure of the 1:1 can be simply multiplied by 1.6.

Once these basic conditions have been found, a bench mark test structure can be obtained. Then the optimisation can begin. Increase and decrease the As flux by ~10-15% and increase and decrease the growth temperature by 10-20°C. Which of these 5 is the best? Which of these 5 gives the brightest PL with the narrowest FWHM? The cycle can then be repeated until no further improvement in RT PL is observed.

A myriad of III-Vs can be optimised either directly (by RT PL) or indirectly (in the case of AlGaAs) by the effect they have on RT PL of the confined layer. The only encountered notable exception is GaAsBi. Where the miscibility, ordering and phase separation are just some of the added complexity. GaAsBi is actually an excellent system to study as it possesses every complication to MBE growth. The idea being once you can grow bismide you can grow anything.