Essential maintenance: Cell material regeneration

Faebian Bastiman

A simple means of ensuring the purity of your cells’ source material is to include a mini regeneration cycle at the start of each day. Under some circumstances you may operate your MBE system 24/7 and never actually cool down your sources. Indeed in the ideal world of batch processing and in situ growth monitoring with a dream machine (see MBE: Dream machine) an MBE system could be run non-stop for 3 months. However even under this utopia of operation you do not need all the sources to be hot all of the time, doing so is simply too wasteful.

When you cool down a cell, any impurities in the system can condense on the shutter, cell divider, water-cooling jacket and worse still alloy with the source material. Simply reheating your cell to operating temperature and growing will result in the cell outgassing onto your sample at the start of the growth. To avoid this you simply need to “over heat” the cell by ~25°C for half an hour at the start of each day. The impurities will outgas more readily at the higher temperature, and upon cooling to operating temperature the source will be ultra-pure once more. This extra step also has the added bonus of reducing the settling time between the cell’s thermocouple registering the operating temperature and the cell’s source material equilibrating at the desired flux.

One’s first thought is: “this is going to take up my valuable MBE time”. Quench that thought immediately! Consider instead the ability to set a “wake up” time for the MBE system at say 0700. The system will then execute a recipe to ramp all the cells to outgas temperature, outgas all the cells for 30 minutes and then ramp them down to operating temperature. At around 0800 the system can then start a flux check on each of the cells and tune the beam equivalent pressure (BEP) to the desired values. At around 0845 the auto flux tuning is complete and the machine sits waiting patiently for you to grow your first sample. My software does this, what you need to ask yourself is: why doesn’t yours?

Essential Maintenance: Pump down

Faebian Bastiman

After your maintenance cycle the time has come to pump the system down and start recovering the ultra-high vacuum (UHV) conditions necessary for MBE operation. In principle this simply involves turning on the pumps, but you can perform a few specific steps to greatly improve your pre-bake base pressure.

After maintenance your system has likely been exposed to atmospheric pressure for several hours. Water vapour contamination is unavoidable, though of course can be reduced by situating your MBE system in an air conditioned laboratory. Once the last flange is sealed (see Essential Maintenance: A how to guide for tips on MBE maintenance) the system interior is completely isolated with an air tight seal. Completely isolated from atmosphere, but sadly still full of atmosphere.

The first step is to pump that out. Dry scroll pumps should be used throughout on an MBE system. They have an excellent pumping rate and virtually zero possibility of contamination. The Edward’s nXDS is a good choice. Enable the ballast valve to enhance the pump’s handling of moist atmosphere and pump through until the Super Bee pressure gauge on the scroll pump’s inlet reaches 0.05 mBar. Now turn off the pump and vent the MBE system with N2.

What?! We have just pumped the system down and now you want me to vent it again? Are you sure? Yes I do. You just pumped out moist atmosphere from an air tight system, and now you are going to vent with dry, ultra pure N2. What will happen? Evaporation.

Evaporation is a wonderfully useful natural process for an MBE operator. Evaporation will transport moisture from the chamber moist inner surfaces into the dry N2 gas. Not all, but a significant fraction. The evaporation rate will depend on:

  1. The concentration of water in the N2: which is zero at the beginning
  2. Flow rate: initially high as the N2 penetrates the vacuum, but constatly dropping
  3. Pressure: evaporation happens faster at lower pressure
  4. Temperature: evaporation happens faster at higher temperature

Simply let the N2 in until the Super Bee reads 500 mBar (remember the process happens faster at lower pressure). Wait a minute. Then pump the chamber down with the scroll pump again. Once it has reached 0.05 mBar, turn the pump off and flush it with dry N2 again. On the second pump down, open the As and P cracker’s needle valve manually a whole turn. The bulk should have been protected from atmosphere when you shut the valve during Essential Maintenance: System Venting. It should therefore be free from moisture, but there is always a chance the valve has a little leak. To be safe, we will pump down the bulk with the scroll pump now.  To increase the effectiveness of the N2 flush you may consider heating the N2 gas to ~50°C. This can be highly regulated with an inline gas heater or poorly regulated by baking the SS N2 pipework to 50-70°C with heat wraps during the flushing phase.

Repeat whole the pump down and vent  process 5 times.

Finally once the system has been pumped down for the last time, start up your turbo pump. After 15 minutes, turn on your ion gauge to 0.1mA emission and check your pressure. It should be in the 10-6 mBar range. Having used the maintenance cycle as an opportunity to regenerate your cyro pump, it should now be operating and ready to take over too. Open the valve to the cryo and let it assist the turbo for a further hour. With the turbo and cyro pumping the growth chamber the pressure will quickly drop into the 10-7 mBar range. Then open the gate valve to the ion pump (which was never turned off) and leave the system over night.

What you should find is that in the morning the background pressure is ~5.0 x 10-8 mBar. That is a very impressive pressure before a bakeout. This is in fact at least 10x lower than without the repeated N2 purge step. If your system is not in the mid-high 10-8 mBar range you will need to leak test. Leak test either with an ex situ external leak tester or an in situ quadrupole mass analyser. Follow the instructions in Essential Maintenance: Leak Detection to identify the cause of the leak.

Once the pressure is <10-7 mBar you are ready to start powering up all the sub-systems you turned off in order to vent. First, turn on all your ion gauges to 0.1mA emission current. Next, double check all the PID controllers for your effusion sources are set to lower than ambient temperature, then turn all the PSUs on. Current limit all the PSUs to 2A to protect them during the effusion cell filament’s initial ramp up from cold. With a ramp rate of 0.1 °C /s ramp the sources (with the exception of the As bulk) to 100°C. This will drive off moisture. The pressure will inevitably rise for a few hours until the moisture is purged through the pumps. At the same time you can heat your substrate heater to 100°C too.

You are now ready to bake the system (see Essential Maintenance: Bakeout).

Essential maintenance: System venting

Faebian Bastiman When the time has come to perform maintenance on your MBE system you will need to raise the internal pressure to atmospheric pressure. You can vent the system at several points:

  1. Through the fast entry lock (FEL)
  2. Through a turbo molecular pump
  3. Through a cryopump
  4. Through a dedicated venting valve

Before you start to vent, take a moment to consider where you need to vent. Do you need to vent the growth chamber and the preparation? Or do you need to only vent the growth chamber? When venting through the FEL you need to vent the entire system in series with the FEL-preparation and preparation-growth chamber valves open? Alternatively you can vent through a pump or dedicated value on the growth chamber.  Most modern turbos support a venting feature. Pfeiffer’s certainly possess this feature. You need to selected the correct menu options, turn off the backing line scroll pump and once the speed drops below a pre-described value the vent process starts automatically. The cyro pump vent requires a little more effort: First you need to turned it off and allow it to reach room temperature, all the while pumping it down with a scroll pump (regeneration), and after that a small turbo or Pfeiffer Hi-cube to get a good base pressure. Only then can you open the gate valve that connects the pump to the main chamber and finally flush the cryo and chamber with N2 gas from the cryo’s exhaust valve. The third alternative is simply using a spare flange as a deidicated venting flange. It only needs to be a small flange, and you would have an adapter that drops to a ¼” Swagelok fitting. This is the simplest method. The next question is with what to vent. I would recommend hi purity N2 from a cylinder that is routed through a baked SS line (see MBE: N2/Gas system). Alternatively you can use LN2 boil off N2, but in that case it is more difficult to bake down the whole line to ensure the purity of the gas. Either way the N2 venting line needs regulating down to 1.2bar. Never use dry filtered air from a compressor to vent your system! It simply not pure enough! Venting then involves a generic, multi-step procedure:

  1. First ensure all your cells are at room temperature, except the Ga which should be at 50°C . Remember the As and P cracker valves need to be fractionally open whilst heating/cooling the cracker head. Always cool the bulk down first, then the cracker heat to avoid condensing material on the valve.
  2. Next current limit the Ga PSU to 2.00A and turn all the other PSUs off. This both saves power and gives peace of mind.
  3. Seal the As and P needle valves manually now to avoid contaminating the bulk.
  4. Next turn off the ion gauges and allow the filaments to cool: 30 minutes.
  5. Close the valves to the pumps.
    1. The ion pump can be left running (ion pumps should in fact never be turned off)
    2. Take the opportunity to regenerate your cryo
    3. Switch off the turbo pump and the backing scroll pump
    4. Close all valves to the pumps (unless you are venting through a pump)
    5. Vent
    6. Watch the Ga cell’s temperature and make sure it stays over 45°C

Venting can be dangerous. An uncontrolled vent can expose your system to dangerous pressures that cause damage to glass viewports, bellows and delicate internal items (like RHEED filaments). Which is why we regulate the N2 line to 1.2 bar. How many bars of pressure can an MBE system safely take? Well it is predominately a vacuum system so anywhere between 0 and 1. On the other hand you do not want to only partially vent it and then break a vent seal only to have atmosphere rush in at an alarming pace. The best way to vent is to monitor the internal pressure with a vacuum gauge. The Super Bee from Instrutech Inc  can monitor pressures from 0.001 to 1500 mbar. You can place one atop each of your backing scroll pumps in place of the more common pirani gauges. Then you can simply valve off the scroll pump, but leave the Super Bee open to the internal pressure via the Turbo’s gate valve. Let in the N2 and watch the Super Bee’s pressure reading slowly climb. Stop when you reach 1050 mBar, either manually or automatically using the Super Bee’s pressure trip digital output signal (the configuration is shown in MBE: Auto-FEL vent/pump down). What to do once the chamber is vented? There are 2 schools of thought:

  1. Keep the N2 flowing at all times to prevent excess moisture entering the system whilst you are conducting maintenance
  2. Turn off the N2 gas it has done its job

Both are valid, but remember you cannot stop moisture entering your system with 1.2 Bar N2. You would need to have a much greater rate of flow, particularly when opening a flange greater than a CF-38. That is a lot of N2 waste gas. It is therefore more economical and practical to simply turn off the N2 and leave it off. You can quickly get rid of the moisture with the pump down technique described in Essential maintenance: Pump down. Now the system is at atmosphere you can get started on all those maintenance jobs in your maintenance log. The general dos and don’ts are outlined in Essential Maintenance: A how to guide. Once done pump the system down using: Essential maintenance: Pump down and prepare for the bake out (see Essential Maintenance: Bakeout).

Post bake tasks: Manipulator outgas

Faebian Bastiman

One of the most overlooked outgas items are the sample heaters: the outgas stage in the preparation chamber and the manipulator (sample heater) in the growth chamber. Let’s review: You have just opened the entire system to atmosphere, replenished the source material and fixed the many little problems, pumped the system down and baked it and finally outgassed all the sources (see Post bake tasks: Cell outgas). After that you put in your sample, heat it up to oxide remove and grow, right?

Wrong! The heaters need just as thorough an outgas as you would give the cells. Let’s face it they are much closer to the substrate. Failure to outgas the heaters will delay your return to optimum material quality by a week or more. A thorough outgas will mean your first sample is already 90% of the way there. The second sample should be 100% of the quality of your best before you came down for maintenance. So how do you save a week or two at the start of your growth campaign?

First, load a sample plate (platen) with either a 2” Ta disc or a 2” Si substrate in place of your usual III-V substrate. These dummy samples are very useful as the temperature limit is >2000°C for Ta (>1200°C for Si), and therefore way beyond the range of your III-V heater. The procedure to outgas the heaters is nigh on identical to that established for outgassing new, cleaned platens (see Essential Maintenance: Sample holders). In brief all you need to do is load the dummy platen into the heater stage and approach the maximum temperatures, gradually, and monitor the background pressure. Keep the pressure in the low 10-7 mBar range. If the pressure rises too swiftly back off the temperature and hold it until the pressure recovers. Aim to hold each heater stage at maximum temperature for an hour. The process may take a whole day, though you can run it in parallel with your cell outgassing so no real time is lost…and viola! An instant return to fully quality. This procedure was used to produce both excellent mobility in InSb on GaAs and excellent optical quality from InGaAs SQW in a GaAs/AlGaAs DBR.

Suddenly the dreaded “MBE downtime” seems a lot more manageable.

Essential Maintenance: Crucible cracking

Faebian Bastiman

Pyrolytic Boron Nitride (PBN) crucibles for MBE are actually grown in a process not too dissimilar from III-V MOCVD deposition. The pyrolytic process involves depositing crystalline ceramic boron nitride on a carbon mold (mandrel). The PBN crucibles are therefore very high purity, nonporous and possess smooth sidewall (no pores). The PBN is stable up to around 1400°C and is resistive to HF acid.

Each III, V and dopant element has its own particular relationship with PBN, some of them are neutral conscientious objectors, other treat PBN with open hostility! The following article highlights the necessary steps to protect your PBN crucibles from the common III-V MBE elements.

First we have the friendly elements: P, As, In and Be. None of these elements pose any risk to PBN and as such the sources can be operated without additional consideration for the PBN-element relationship and interactions. In is the only nice group III. Aside from passively “spitting” during operation it is essentially neutral. A cell operated with a large thermal gradient from top to bottom (hot-lip) helps reduce the buildup of In near the cell’s orifice and prevents spitting affecting the substrate surface.

In contrast Al, Ga, Si, Sb and Bi each require additional care when situated inside a PBN crucible. Al can readily shatter PBN. The situation is unique amongst III-V elements and pertains to Al’s ability to wet PBN. Liquid Al placed in the bottom of a PBN crucible gradually “creeps” up the sidewalls. The cell must be operated with a cold-lip to prevent Al climbing out of the cell and shorting the heater filaments. Ga and In remain at the bottom of the crucible, thus as the material is depleted the flux drops over time for a given temperature. The thin and complete Al coverage provides a large and constant surface area, such that Al cells do not suffer from flux drops for a given temperature. This can make it difficult to predict when an Al cell is running out. Finally Al has a very different thermal expansion rate to PBN. The wet PBN cannot contract as fast as its Al film, hence cooling Al too rapidly from standby (850°C) to solid (660°C) can readily shatter the crucible. It is common practice to utilize very, very slow temperature ramps when cooling through this range. For example 0.05 °C/min, equating to ~63 hours. Once the cell falls below ~600°C, and you can be certain the Al is solid, a faster ramp can be used. It is therefore a very good idea to fully deplete the Al cell before cooling it down. Moreover, it is good practice to replace the PBN crucible every time the cell is cooled. A double walled crucible or a crucible insert offer a buffer against the damaging effect of cooling Al too rapidly. You can further buffer against damage by having the Al cell on an uninterruptable power supply (UPS), having a backup DC power supply unit (PSU) that automatically takes over if the primary supply fails and by utilising a backup water cooling system rather than simply crashing the cell temperature upon primary water cooling failure (see MBE: water cooling system).

The next dangerous element is Ga. Ga, like Si, Sb and Bi (and the common compound water), possesses a lower density (they expand) upon freezing. Ga decreases in density by around 3.1% upon freezing. There are only 4 elements that expand upon freezing and in III-V MBE we encounter them all. What luck!  Liquid Ga sitting at the bottom of a crucible first freezes at its exposed upper surface creating a nice air tight cap against the PBN, then as the main Ga liquid freezes it expands sideways cracking the PBN like freezing water in a pipe. Luckily Ga freezes at around 29°C, this means that holding the cell at 50°C at all times (even during maintenance and air exposure) protects the PBN against the damaging effects of freezing Ga. Ga cells can be protected in an identical manner to Al cells, however you can also utilize a “water-heater-loop” on the Ga cell to maintain 50°C by supplying 50°C water through the cell’s water cooling element in the event of a DC PSU failure or other electrical power fault.

I avoided discussing Si along with Ga, because Si is more dangerous. Not only does it freeze and expand (decreasing in density by a massive 10%), it also etches PBN when liquid. The simple solution to both problems is to keep the Si solid. Si melts at 1414°C, which is inside the “danger zone” for PBN temperature operation. In some circumstances it is advised to melt Si on the first usage, for example when wanting to utilize a “downward facing” cell port, Si can first be melted in an “upward facing” position and upon freezing will remain rigidly fixed inside the PBN crucible even when facing vertically downwards and physically shaken. The etching effects of liquid Si on PBN will be minor for quick, one time melting, however this does preclude PBN crucibles from being utilized with high Si deposition rates. In those cases either an e-beam or a Si-filament source are required. The best advice I can offer is to only outgas the Si cell to 1275°C maximum, avoid melting the Si and hence avoid any dangerous effects.

Next we have Bi. Bi decreases in density by a moderate 2.8%. Not as dangerous as Ga, but still posing the risk of crucible cracking. It also freezes at 271°C, so there is no chance to keep it melted during maintenance. The best practice here is to be careful. Never over load your crucible. You will be amazed how long Bi actually lasts. A 10 – 20g charge at the bottom of a crucible out lasts all other III-V elements; largely because it is only used in dilute quantities in III-V epitaxy. It is good practice to fully deplete the Bi source before cooling. This means accurately predicting your Bi usage to coincide with maintenance periods. Using rounded bottomed crucibles is also good practice, as they seem less susceptible to cracking. Finally, like Al, go through the freezing point slowly. I use 0.05°C/min when cooling from 325 to 250°C.

Finally we have Sb. Sb presents the smallest density decrease upon freezing. A mere 2.6%. Luckily it has a relatedly large vapour pressure whilst solid. A valved Sb source produces a suitable Sb flux for III-Sb epitaxy with the Sb bulk held at 500°C. This is safely distant from the melting-freezing point of 630°C. The best advice for Sb, like Si, is therefore to avoid melting it altogether and thereby avoid any dangerous situation.

Essential Maintenance: Sample holders

Faebian Bastiman

In III-V MBE we direct a molecular beam at a heated substrate. The substrate is located within a sample holder (platen) for ease of mounting and manipulation within the MBE system. Hence whilst intentional epitaxial deposition is taking place upon the substrate, any exposed metal will partake in unintentional amorphous deposition. The shiny new platen soon becomes coated in a blue/purple film. This film readily adsorbs moisture and readily oxidizes in air, thus it is a significant source of contamination to an MBE system. The substrate and platen are typically outgassed to around 400°C in a buffer chamber. A clean platen and epiready substrate need only 30 minutes to reach the mid 10-9 mBar range. After some 30 growth runs, the same substrate and platen take 90 minutes. Clearly the epiready substrate still requires 30 minutes, but the contaminated platen needs 90 minutes. The thicker the film, the longer the outgas procedure.  It seems we are wasting time outgassing our platen and in so doing we are unnecessarily contaminating our vacuum system. Also the III-V layer significantly alters the platen’s thermal response and actually lowers the maximum temperature you can achieve! What to do?

Most platens are composed of Molybdenum (Mo). Occasionally stainless steel (SS) may be utilized for low temperature (<550°C) substrates (InAs, InP, InSb) to save on material and machining costs, but Mo is standard. Mo is a very robust element, which means it can be rather aggressively cleaned without worrying about damaging it with chemical etching. Outlined below is a procedure to remove all III-V deposits from Mo platens. Note full breathing apparatus is advised and work should only be performed under a fume hood!

Firstly you will need two relatively coarse grades of wet & dry abrasive sheets. Automotive wet and dry is sufficient. I use 80 and 120 grit. Using a container of DI-water, submerge the contaminated platen and then rub it with the 80 grit. During this phase you can touch the platen with your gloved hands. 80 grit cuts through the III-V material relatively swiftly, whilst only inflicting minor scratches on the Mo platen. Working with the platen submerged in water will minimize III-V dust, however if you prefer not to work underwater regularly wet the platen to reduce the depth and number of scratches. Once all the material is removed, sand down the scratches with the 120 grit. Repeat with 240 grit if you want to eliminate all scratches. Finally thoroughly rinse the platen with DI-water and dispose of the III-V contaminated water by your facilities established practices. From this point on only handle the platen with clean tweezers.

Next you will need to submerge the platen in undiluted (38%) HCl for several hours. The Mo is impervious to HCl, though any wet & dry residue will be etched away. I deliberately avoid using Aqua Regia since this tarnishes the Mo. After the etching, rinse again with DI-water and this time blow dry with dry N2 gas. Finally soak the platen in isopropanol (IPA) for 5 minutes and blow dry for the last time.

You should hold in your tweezers a very shiny, silvery Mo platen free from aqueous IPA residue. Before you place the platen in your MBE system you need to bake off the dried on IPA residue. To do this place the platen in a 120°C baking cupboard overnight. Once removed and cooled the platen can be stored in a desiccator until needed.

The final phase is to outgas the platen in vacuo. It is best to load an appropriately sized tantalum (Ta) disc in place of a wafer. A Si wafer is a viable alternative to Ta. The Ta/Si is required in order to outgas the platen to the maximum heater temperature, which is around 100-200°C hotter than one would typically take a III-V wafer in an MBE system. Outgas the platen in two stages. First heat it to the maximum you can reach in the system’s preparation chamber outgas stage (~600°C), then take the platen to the maximum operation temperature of the growth chamber’s sample heating stage (~800°C). The maximum temperature will vary from system to system. It should however be at least 100°C higher than you would otherwise normally grow. When outgassing the platen keep an eye on the background pressure and avoid letting it exceed the mid 10-7 mBar range. Aim to hold the platen at maximum temperature for an hour. The entire procedure may take 8 hours per platen to complete. I perform the procedure manually the first few times, then write a simple recipe of substrate temperature ramps to perform the outgas automatically overnight thereafter.

Each platen may need cleaning as often as once a month or as infrequent as once a year. It depends on the system usage and the cocktail of materials deposited. Keep an eye out for the telltale lengthening of normal outgassing times. It is good practice to purchase a number of spare platens to have a few clean ones in reserve whilst the contaminated ones are “out for cleaning”.

Essential Maintenance: The maintenance cycle

Faebian Bastiman

MBE operation should be a constant cycle of growth campaigns and regular, scheduled maintenance periods. I put the word should in italics because, unless you are careful, maintenance can enter the cycle of reactionary quick fixes that inevitably lead to extended downtime. So how do we avoid this undesirable maintenance cycle? We need to put in some extra effort and employ some good operation/maintenance practices: fault logging, planning and regular servicing and keep a stock of spare parts.

Fault logging is highly underrated, but in retrospect it is one of the fundamentally most useful aspects of maintenance. It can be as simple as a quick note in a logbook at the end of each day to (hopefully) say “everything is ok” or (sadly) to note the occurrence of a new fault. I like to keep a spread sheet log that lists the problem, the date it was detected, actions taken and the date it was fixed. The log is also a useful reference during maintenance periods, since it is essentially a task list comprising all the little jobs that need to be completed.

Some problems are minor irritations, some are ex situ and can be repaired at your leisure and (again sadly) some are serious and require the premature interruption of the growth campaign. It is the latter type of problem that we would like to avoid. That is where two crucial aspects of good maintenance practice come in: planning and regular servicing.

Every part of the system has a finite lifetime. The service intervals are (i) on-going, (ii) once per campaign (which can be 3 – 12 months depending on your system, configuration and usage), (iii) annual, (iv) biennial, (v) once every 5 years and (vi) once every 10 years.

The on-going maintenance tasks are those that do not require the system to be opened up, repaired and baked, but are nevertheless essential. This includes cleaning the sample holders (see Essential Maintenance: Sample holders), regenerating the cryo pumps (see Essential Maintenance: Cryo pumps) and outgassing the cells after the weekend off (see Essential Maintenance: Cell material regeneration).

The once-per-campaign maintenance tasks should be undertaken during regular, scheduled maintenance periods before each growth campaign. The tasks are:

  • Replace Al and Bi crucibles (or replacing the PBN crucible inserts)
  • Replenishing effusion cell source material
  • Cleaning shutters (depends on system)
  • Cleaning viewports (depends on system)

It is a good idea to try to anticipate and time your material usage so everything runs out at the same time. This is easier said than done and experience (as always) plays a big part. It is a good start to put as little Al and Bi as possible, since it is best to fully deplete this sources each campaign (see Essential Maintenance: Crucible cracking for more details). Of course your other choice is to simply switch your alloy composition until all your cells are down. For example once my Ga cells are depleted I grow some InAs/InAs(100) pins diode  detectors for a colleague.

Annual and biennial tasks are in some ways system dependent and hence it is up to the user to decide what good practice is for their own particular circumstances. The tasks are:

  • LN2 cryo-panel cleaning/regeneration
  • Cleaning the substrate heater/manipulator
  • Inspect RHEED screen As coverage
  • Clean MIG ion gauge head (see Essential Maintenance: MIG head)
  • Replace any broken TSP filaments  (as necessary)
  • Replace broken ion gauge filaments (as necessary)
  • Service scroll pumps

The LN2 cyro panel regeneration is a neglected service item. In an ideal world each III-V MBE machine would possess two panels, employed with a “one to wash against the other” philosophy. I personally cleaned my Omicron MBE-STM cyro-panel once a year. This is good practice because the As layer is still relatively thin and can be brushed away. If you are going to service your cyro-panel once every 5 years you will need to have the whole thing shipped and dipped (see Essential Maintenance: Cryo-panel).

Arsenic gets everywhere. A thin film starts to build up on the growth chamber interior, despite the presence of shutters, and that means it starts to grow where it is not wanted. In particular this is the RHEED screen, MIG and the manipulator metal work. The latter point can result in poor RHEED access to the sample and eventually no RHEED at all.

5 year service items are often forgotten simply because they are 5 year service items. A lot can happen in 5 years, the entire composition of a research group could well change. Here are some notes to leave for your successors:

  • Inspect, clean or replace rubber seals on load lock and gate valves
  • Service UHV pumps
  • Service Al and Si cells

Lengthier service items are:

  • Other effusion cell replacement or service
  • Ion pump replacement or service
  • Software upgrade
  • Rack hardware service/upgrade
  • Clean and service RHEED gun

The UHV pumps are probably the most neglected regular service items. Oil-and-ball-bearing based turbo pumps require a return to manufacturer and full service once every 5 years, those with magnetic bearings can be operated up to 10 years. Ion pumps too require regenerating/replacing every 8-10 years.

It is a good idea to have your Al and Si cells serviced once every 5 years, since they are high operating temperature cells. The other effusion  cells will typically need servicing once every 10-15 years. The best practice for effusion cells is to buy ones with separate water cooling jackets and shutters, and hence the cell itself is simple to replace or repair. As crackers can operate 20+ years, but they will need refilling once every 5-15 years depending on your usage. The cost of a full service on an As cracker is around 65% of the price of a new one.

Finally let’s consider our stock of spare parts. Obviously the safest way to operate is to have two of every item; though for financial or space reasons this is impractical. You can buffer against some problems by entering into a service contract with the manufacturer whereby effusion cells and pumps are serviced on a rolling-replacement basis. It is however good practice to standardise your parts as much as possible and keep in stock:

  • One of each kind of crucible
  • One of each kind of bellows
  • Numerous gaskets of various sizes
  • Blanking flanges of various sizes
  • Cell source material and an As charge
  • Ion gauge heads and filaments
  • TSP filaments
  • A RHEED filament
  • A RHEED screen
  • A gate valve seal kit for each type of gate valve on your system
  • Manipulator/outgas stage filament
  • Multiple shutter blades
  • Cell parts (should you decide to service them yourself)
  • DC power supplies
  • A water chiller pump
  • An ion gauge controller

Clearly maintenance should not be taken lightly. It is a large part of MBE operation; however by establishing good maintenance practice it need not be overwhelming or full of unexpected surprises. And most importantly, good maintenance practice forms the foundation upon which one can establish very fruitful growth campaigns with minimal disruption.

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

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.