How to growth your first sample: Oxide remove

Faebian Bastiman

Perhaps you have your shiny new MBE system just installed, or perhaps you have a second hand system that you have painstakingly recomissioned or perhaps you are a new PhD student revitalising a growth system after several years of inactivity, either way you will be faced with the same question: how do you grow your first sample?

First of all make sure you have a RHEED system installed and take the time to align it properly (Essential maintenance: RHEED align). RHEED is the fundamentally most useful tool to you at this stage. You will use it to discern many fundamental properties and calibration reference points. Next load your system with cell material and perform a bake (Essential maintenance MBE bakeout). Once your system is baked you can perform the numerous post bake tasks including outgassing your sources (Post bake tasks: Cell outgas) gathering flux data for all your sources (Post bake tasks: group III flux, group V flux and doping sources) and outgassing the manipulator stage (Post bake tasks: manipulator outgas).

Next you will need to physically load your first wafer through the fast entry lock (FEL) and, preferably, outgas it in the preparation chamber’s outgassing stage: typically 400°C for 1 hour for most III-V substrates with the exception of 300°C for InP. Whilst the substrate is outgassing you can set your cell temperatures and fluxes. One of the critically most important parameters for III-V MBE growth is the V:III flux ratio. Usually this is selected from legacy data, however with a new system you will have to make some assumptions. Note If you have a new system the manufacturer may have typical flux data for the sources with which you can better estimate the starting point.

Consider GaAs/GaAs(100) epitaxy.

First of all set your Ga cell to 975°C and measure the beam equivalent pressure (BEP) on the monitoring ion gauge (MIG) using the method in Little known MBE facts: Flux determination. Most Ga cells at 975°C will give a growth rate of 0.2 to 0.3 ML/s, the actual magnitude is irrelevant at moment, anywhere between 0.05 and 2 ML/s will do. 0.2 to 0.3 ML/s is a reasonable starting point.

Next consider the As flux. First set your As cracker at 850°C so you are predominantly creating Asand set your bulk to 350°C (leave the bulk for an hour to stabilise before you continue).  Then take your Ga BEP and multiply it by 25 and find this value of As BEP by varying the valve position. Hopefully it will be around 60-80% of the valve’s fully open position. If you cannot reach this BEP you will need to increase the As bulk temperature and wait an hour before taking more readings. In this case increase the As bulk temperature in 10°C steps.

For “good” MBE the As:Ga ratio will need to be very carefully tuned, however for establishing growth of your first sample the ratio need not be so precise. In most of the system I have operated I used a As2:Ga BEP ratio of 10:1, hence a value of 25:1 would result in over supply. In all honesty you can over supply at 100:1 and still grow. The most important thing is not to undersupply the atomic flux ratio. Undersupplying the As atomic flux to a value less than the Ga atomic flux will result in Ga droplets and will irrevocably damage your first sample. It is better therefore to err on the side of caution and oversupply the As.

Once the substrate has outgassed, retract the MIG and transfer the substrate to the growth chamber. You can transfer the sample once it is below 250°C. Set the substrate rotating and direct the RHEED spot to create a RHEED pattern. It should look similar to that of Figure 1a. Ramp the thermocouple temperature to 400°C at 1°C/s and leave it to stabilise for a few minutes. The RHEED pattern should not change at this stage. Next open the As valve to the position you found earlier and prepare to search for the oxide remove temperature. Oxide remove is a non-too-subtle evolution of the RHEED pattern from the occasional small streaks of Figure 1a to the clear, distinct and frequent features of Figure 1b. The transition takes place within a few seconds once the correct temperature has been reached. In order to find this temperature, first ramp the substrate to 550°C at 0.5°C/s. Once it has stabilised at this temperature for a few minutes continue to ramp up in 10°C steps at 0.5°C/s until you see the pattern shown in Figure 1b. This temperature is 590 ± 10 °C, though due to the discrepancy of the thermocouple and the actual substrate temperature it will happen at a different thermocouple temperature. Typically this is at a higher value, so do not be surprised if you have to go to even 750°C on the thermocouple to create 590°C on the substrate. However the discrepancy is usually smaller and in some cases the thermocouple may even read lower than the actual temperature!

Oxide remove RHEED

Figure 1: [100] and [-110] RHEED diffraction patterns 

Once you have found the oxide remove temperature, hold the substrate there for 10 minutes. You can even go 10-20°C higher at this stage without risking damage to a GaAs substrate. What is important is that you lower the thermocouple temperature to 10°C less than the oxide remove temperature before you start to grow.

The As flux has been irradiating the surface throughout, and now it is time to supply some Ga into the mix and see what happens. Open the Ga shutter for 10s and then close it again. The RHEED pattern should “improve” to look like the one in Figure 1c. Note the [110] azimuth looks remarkably 2x at this point, the [-110] azimuth may even look a little 4x to the trained eye. If you have no idea what I mean by 2x and 4x read Little known MBE facts: RHEED reconstructions. Note if the Ga flux is too low (less than 0.1ML/s) you may not see any change at this stage, similarly if it is too high you many skip straight over this “improvement” into the next stage (and so continue reading). Next open the Ga shutter for 30 seconds and then close it. The RHEED pattern should now “worsen” to look like that in Figure 1d. Yes I said worsen. The 2x will now have wavy second order rods, and the 4x will degrade into chevrons. This is completely normal and in fact natural. What you did with the first 10s of Ga is simply planarise the existing surface; supplying a few atoms here and there to fill in some of the spaces and widen the ML islands or terraces. What you have just done with the 30s Ga is actually start to grow, by which I mean sweeping the terraces across the surface (for step flow growth). What happens when these steps start flowing is that they encounter many little “pits” caused by the oxide remove and the steps start bunching around them. Ultimately what you do is roughen the surface at this point and the only way to recover is to eventually fill in the pit and allow the steps to flow unhindered on the surface. In order to do this simply open the Ga for 5 minutes and once closed again the RHEED should resemble the (2×4) shown in Figure 1e. Well done (!) you have successfully grown your first layer. Creating a (2×4) reconstruction is the goal here, and once done you can start calibrations. You can now grow for a full half an hour and then begin calibrating the V:III ratio (see Little Known MBE facts: Group V overpressure), the growth rate (see Little known MBE facts: RHEED oscillations (1)) and even the substrate temperature (see Little known MBE facts: Temperature determination and RHEED and Little known MBE facts: Making a static reconstruction map).

A few notes just in case this did not happen:

1)      If the 2x changed to a 4x (and the 4x to a 2x) during the periods when the Ga shutter was opened the As flux is too low (or the Ga flux is too high). You will need to either increase the As flux (I would suggest you double the BEP) or decrease the Ga cell temperature (I would suggest -25°C).

2)      If you are looking at a very spotty RHEED pattern you probably created Ga droplets, in this case the Ga flux was way too high. I would suggest you increase the As flux by a factor of 4 and try again (with a new substrate).

3)      If the RHEED pattern did not change (or only changed subtly) after opening the Ga shutter in each step, then the Ga flux is too low. Increase the Ga cell temperature by 25°C and try again (no need to change the sample, but you may need to increase your As flux accordingly).

Note: the psychedelic, high contrast RHEED images were captured by Safire RHEED software available from Createc.

Essential maintenance: Cryo pumps

Faebian Bastiman

Cryo pumps and ion pumps make an exceptionally good team for III-As MBE. Whilst the ion pump stoically and gradually maintains the UHV vacuum, cyro pumps provide that little extra pumping capacity to bring you from “good” high -10 mBar to “great” low -10 mbar. However the pumping capacity of a cryo pump degenerates with time, making them less and less effective. In fact from the moment they reach minimum temperature (~10K) every following moment sees them becoming less effective until their internal absorbing surfaces finally saturate. The efficient operating time will depend on the cryo pump volume and how aggressively you operate your MBE system. In a typical R&D III-As system the efficient operating cycle is usually 2-3 months.

But how to regenerate a cryo pump?

Well first of all you will need to shut the gate valve that connects the cryo pump to the growth chamber. After that you are free to turn off the compressor and power down the pump. However remember in so doing the pump will rapidly heat up and the internal pressure can exceed atmosphere by over 100 times. Clearly leaving the pump sealed on both ends is not a good idea. The pressure increase is of course not instantaneous, and happens more readily above 77K when N2 gas starts evaporating.

The next step involves connecting a dry scroll pump to the exhaust and opening the exhaust valve once the cyro pump reaches ~80K. The cryo pump can then be left in this state for several hours until the internal temperature equilibrates to the ambient. At this temperature most of the contaminants are highly volatile; however water and some of the other cryo absorbed matter may need a little more encouragement. The final step in regeneration is achieved by using heated dry N2 gas to raise the cryo pumps internal temperature to ~60°C. Indeed most cryo pumps have a double-exhaust to enable this function.

In order to perform this final step you will need (i) ultra-pure N2 from a cylinder (ii) a fully baked, ultra clean SS line from the cylinder to the cyro pump and (iii) a heat source. Since you are baking the line anyway, you can simply install heat wraps or tape on the line to heat the pipe to ~100°C and in so doing this will also heat the N2 passing through the pipe. A more elegant (and expensive) solution is to utilise an ultra-pure 316L SS inline N2 gas heater (from Heateflex). The N2 needs applying constantly during the ramp up and ramp down steps, with a hold time of about an hour.

Once the N2 heating cycle is completed the dry scroll pump can be operated to pull the cryo pump down to ~10-3 mBar. Whilst the cyro pump can be operated from this base pressure , you can significantly reduce early stage contamination by employing a turbo pumping station (like the Pfeiffer Hi-cube Classic N.B. avoid the Hi-cube Eco which does not have sufficient pump rate for this application) to bring the interior pressure down to ~10-7 mBar (N.B. It is the size of the roughing pump and not the turbo molecular pump that is the limiting factor).

A schematic of the regeneration system is shown below. Note the inclusion of the 3 way bypass valves for the pumping station to avoid flushing it with heated N2 gas during the final regeneration step. It is also possible to include an optional wide range gauge (WRG) on the cryo exhaust to monitor the internal pressure whilst refining the regeneration cycle’s timing.

Cryo pump regeration system

Then simply seal the exhaust, restart the compressor and wait for the internal temperature to reach ~10K. Once done you are free to open the gate valve and continue normal system operation. The entire regeneration cycle may take 5-6 hours. So rather than lose an entire day it is better to schedule it over night or over a weekend. How can you do this? Why by automating the process with the MBE control software of course. What is that you say? “Your software does not have this feature”. Why not?

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?

Post bake tasks: Group V valved sources

Faebian Bastiman

The realisation of the valved As cracker source for solid source MBE resulted in a step improvement in material purity and near instantaneous flux changes, complimented by a step reduction in high vapour pressure material wastage. The source (bulk) temperature is essentially fixed and the desired flux Is selected by operating a “needle” value. The flux is essentially zero when the value is sealed, meaning it can be indefinitely idled at operating temperature with no risk of depleting the source material. Earlier growth campaigns were often As charge limited to around 3-6 months, whereas a 500cc As cracker source can last >10 years on an R&D system. The As charge can also be outgassed to high temperature whilst the valve is near closed to remove any impurities (like oxides and hydrides) from the as received bulk charge. Finally the post valve cracker zone can be operated at ~650°C or >850°C to generate predominantly As4 or As2 respectively. Similar motivations have seen the realisation of P and Sb based valved sources.

In order to effectively utilise a valved source, the flux-valve position response must be characterised similar to a standard effusion cells’ flux-temperature response. Whilst the latter can be generally modelled by a basic Arrhenius characteristic (see Post bake tasks: Group III: Arrhenius plots) the response of the needle valve bears no relation to the evaporation energy of the controlled species. However… if you take the time to characterise the response of your needle valve, you will see it can be modelled by an Arrhenius behaviour of arbitrary fitting energy (Efit).

In order to do this you will need to generate a sufficient flux with your bulk zone. The actual temperature varies substantially depending on the material being evaporated and the desired maximum flux (when the valve is 100%). Actually 100% open should give ≥110% of the desired maximum flux in order to avoid operating in the less responsive 95-100% valve position regime. Once you have operated your source you can use legacy data to select the bulk temperature. For first time users a guideline operating temperature of the bulk to be used for binary growth is (white)P: 90°C, As: 350°C, Sb: 500°C.

Remember before you start heating your bulk zone, you must heat your cracker zone (and valve zone if present) to avoid material condensing in these sections. The cracker zone can be arbitrarily operated at 1000°C at this stage. Once the cracker has stabilised, you can heat your bulk material. The cracker may only take 15 minutes to stabilise from a ramp rate of 0.5°C/s, however the thermal response of the bulk means it typically needs ramping at 0.25°C/s max and will need several hours to stabilise at the final temperature. Furthermore, the bulk will need heating to ~25°C hotter than you wish to operate it for several hours first, followed by a purge of the outgassed impurities through a small opening (10-15%) of the valve and finally it will need cooling to operating temperature. Hence it may take an entire day to get the source ready to operate.

In order to record fluxes you will need to insert your monitoring ion gage (MIG) (aka beam flux monitor (BFM)) into the beam path. Then starting with the valve at 100% open, wait for the beam equivalent pressure (BEP) to stabilise. The stabilisation time will depend on the size of your system and the efficiency of your cyropanel cooling and pumping capacity, however you can expect to wait 5-10 minutes. For a normal effusion cell you would now close the shutter and sample the background flux and finally subtract the open flux from the background flux to give the actual flux (see Little known MBE facts: Flux determination). However for high vapour pressure elements like P and As this is somewhat moot since the BEP is typically 4-5 orders of magnitude larger than the background flux and any background there is will largely consist of the group V species. However, subtracting the background is still necessary, especially if your system is subject to temperature and pressure fluctuations (like from disconnecting and refilling a LN2 dewar). For completeness you can measure the background flux on the growth chamber’s background ion gauge (GIG). This will allow you to discern any day-to-day background flux changes. The MIG and GIG can be read simultaneous, and so there is no need to close the source. Subtracting the GIG BEP from the MIG BEP will give you a repeatable measure for the valved sources BEP.

To complete the flux measurements, simply close the value in 10% steps once again leaving the BEP to reach a steady state after each step. In order to reduce the settling time, you can “over close” the value by 10% for 10 seconds and then open it to the desired value, i.e. 100% >>> 80% >>> 90%. This will enable the background to recover faster, however the effectiveness of this step depends on the system dimensions and pumping capacity.

Plot your gathered BEP vs valve position data with your favourite software. It should resemble the data in Figure 1 below. In principle any curve can be reasonably approximated by a polynomial of sufficient order. In the data below a 3rd order polynomial was used, however polynomials are exceptionally inelegant and tends to exhibit large errors at the max and min positions.

Slide1

An alternative is to first note the valve’s BEP response is typically non-linear and may contain one or more “inflexion” points where the response of the valve is different, but repeatable upon passing through the inflexion point. In the data below this inflexion point is at ~30%. The data on either side of the inflexion point follows a Arrhenius behaviour (shown in blue and green alongside the original polynomial in red) which can be fitted by utilising the % data as °C data and plotting 1000/(273+°C) vs log(flux), as shown below in figure 2. Hence with care to define the inflexion point(s) one can use the same fitting equation that is used for effusion sources.

Slide2

Ultimately the fitting equation comes down to personal preference. The important outcome is that you can model the valve’s flux response and use it to predict source’s fluxes during normal operation. The characterisation of each source’s response is an essential reference point for all subsequent stages of the growth process.

Molecular Beam Epitaxy: Which Pump?

Faebian Bastiman

Invented in 1915 the diffusion pump is the king of UHV pumps. The principle is rather simple, a unidirectional jet of oil (yes oil, yes oil!) is sprayed inside the vacuum trapping particulates and sweeping them through the exhaust. Possessing low cost, no moving parts, a high pump rate for all gasses (up to 50kL/s) and capable of operating in the 10-2 to 10-10 mBar range they offer superior characteristics to other UHV pumps. Sadly they have two major disadvantages. Firstly, diffusion pumps passively back stream small quantities of oil (yes oil, yes oil!) into the vacuum chamber they are pumping. The amount of back stream oil is minimised by optimal pump design and employing traps, however for many users the idea of any oil vapour entering their vacuum system is tantamount to blasphemy. Secondly, diffusion pumps rely on fore pumping in the form of a rotary vane pump to create an outlet pressure of around 0.1 mBar. Failure of the fore pumping causes significant back streaming of oil (yes oil, yes oil!) into the vacuum system. Failure of water cooling results in a similar back streaming of oil. The absolute system wide contamination of several MBE reactors (due to failure of one of these systems) saw the popularity of the diffusion pump rapidly diminish in the early 1990s.

The cryogenic pump (cyro pump) is arguably the ultimate evolution of earlier sorption pumps. Both employ a cryogenically cooled interior coated with highly absorbing material to trap gasses. The sorption pump is cooled by submerging the pump’s housing in LN2. They were often employed as a roughing pump on earlier MBE system. Whilst cheap and reliable, sorption pumps are notoriously slow with a low pumping volume. Cyro pumps possess the same broad pumping range as the diffusion pump (10-2 to 10-10 mBar) however at severely reduced capacity (1-2k L/s depending on the material). They also possess significant mechanical displacement (usually by the means of a piston) that can induce vibrations within the MBE system. The cyro pumps cooling is achieved with a He compressor that doubles the cost of the cryo pumps. In multi system facilities, three individual cyro pumps can be run off a single compressor. Both cyro pump and compressor require a full service every 5-8 years. Furthermore, sorption and cyro pumps require regular regeneration owing to saturation of their absorbing material and hence neither can operate continuously. Compressor failure leads to a rapid increase in internal temperature and pressure, resulting in back streaming of molecules into the chamber. This is prevented via a dedicated gate valve, however with no exit the rapidly building internal pressure can cause a fracture of the pump housing and thoughtful implementation is needed to avoid damage.

The ion pump is perhaps the most iconic of all MBE pumps. Older ion pumps and their controllers required hefty initialisation currents (~18A) and lower but equally significant operation currents (~5A). Modern ion pumps and controllers require around 1A for continuous operation, meaning (i) they are very efficient and (ii) they can be backed up with a small, desktop uninterruptable power supply (UPS). Ion pumps can therefore safely hold a system’s vacuum in the event of a power failure. Uniquely ion pumps also operate as a pressure gauge (see MBE: Which Gauge?). Their pump capacity is however smaller than other UHV pumps (~200-500 L/s depending on the pump dimensions) and they require a start pressure around 10-5 mBar however they can create an ultimate pressure close to 10-11 mBar. Note that the low pump capacity means that this low pressure is not possible in the growth chamber, however can readily be achieved in the preparation or buffer chambers. Ion pump have the advantage that they are clean, need little maintenance, produce no vibrations and unlike the cryo and turbo pump they can effectively pump H2 and He. Hence they are perfect for standalone pumping of a preparation or buffer chamber, however they must be combined with a cyro pump and/or a turbo pump to achieve the UHV condition necessary for low background doping material.

Turbo molecular pumps (or simply turbo pumps) come in all shapes and sizes and can be employed for a variety of applications. Like the diffusion pump they require fore pumping in the form of a roughing pump on their exhaust in order to operate. The roughing pump can take various forms (rotary vane, diaphragm or dry scroll) though in the interest of maximum pump capacity and low contamination the scroll pump is the roughing pump of choice. The turbo and scroll combination handily spans the atmospheric pressure to HV to UHV range. Turbo pump capacities range from 70 L/s to 4.5k L/s depending on the pump dimensions and the bore of the flange they are connected through. Smaller capacity pumps are employed to regenerate cryo pumps and pump down fast entry locks (FELs), whereas the large capacity pumps help the ion and/or cyro pump on an MBE’s growth chamber. Turbo pumps are “through” pumps with similarities in operation to a jet engine, unlike ion and cryo pumps which are “trap” pumps. Some applications, like phosphorus, require the through pump capabilities of a turbo pump along with additional phosphorus trapping capabilities. Since the turbo pump operates by accelerating molecules the pump’s efficiency is directly proportional to the molecules mass.

Titanium sublimation pumps (TSPs) operate in a similar manner to cyro and sorption pumps. In the case of TSPs the adsorbing material is a thin film of highly reactive titanium that is evaporated by supplying 40-50A through a titanium filament and the condensing surface is the MBE reactors chamber wall. Once saturated, the film can be re applied with a further current heating cycle. Pumping effectiveness is also increased by cooling the chamber walls with LN2 or positioning the TSP near the main LN2 cyro panel. Since titanium is particularly reactive with CO, H2O and O2, it can significantly reduce background doping and hence it is recommended to fire the TSP particularly prior to oxide removal and during growth of high mobility materials.

The cyro panel is a compulsory constituent of all high quality MBE systems. LN2 (77K) is by far the most common cooling medium and MBE systems typically consume 150 – 200 L/day when the LN2 is delivered from an efficient combination of high vacuum lines and a phase separator (see MBE: LN2 system). However some users argue that a custom chiller until pumping silicone oil (193K) or antifreeze (233K) is perfectly viable in certain situations.

So which pump? The cyropanel and TSP form a very effective secondary pumping combination and should always be present. As for the main pump… honestly I would like to see some new development go into the design and auxiliary services of the diffusion pump. Redundant fore pump configurations employing a team of pumps and triple backing up the water cooling (see MBE: water cooling system) along with suitable interlocks could make diffusion pumps fool proof. Resurgence, of course, relies on consumer confidence. Sadly the alternative is to employ all 3 of the remaining pumps (cyro, ion and turbo) in order to achieve even close to the same pumping capacity. This triple combo easily increases the pumping cost by a factor of 5x and of course occupies a great footprint and imposes other run cost and maintenance overheads. Of course the more pumps the better the vacuum, however most R&D reactors can be adequately pumped with a combination of 2 of the 3 pumps on the growth chamber. The exact combination depends on what you are pumping: for As systems an ion pump and cryo pump make a good combination, for N plasma systems two cyropumps make a good combination, for P systems an ion pump and a turbo (with a P trap) are needed. For further information, a simple R&D pump configuration can be found in MBE Design and Build: Vacuum System.

Molecular Beam Epitaxy: Which Gauge?

Faebian Bastiman

We MBE growers are interested in measuring nothing. We have various pieces of equipment specifically purchased to measure the degree of “nothing” inside our vacuum system. There are a number of vacuum measuring gauges available. Each with a specific purpose and each suited to a specific job. The gauges are:

  1. Pirani (including the Super Bee from Instrutech)
  2. Capacitance Manometer (Baratron from MKS)
  3. Wide range gauges (Edwards / Pfeiffer)
  4. Ion gauges and controllers (Epimax)
  5. Ion pumps
  6. Quadrupole mass analyser (Pfeiffer Prisma)

Interestingly only Manometers are “true” pressure gauges, since they work with the force per unit area exerted by the gas and are therefore independent of the gas species. Unfortunately the manometer is rather delicate and can be damaged by exposing it to greater than atmospheric pressure. The manometer can be used in the FEL automatic pump down system (see Design and Build: FEL Automatic pump down and vent), so long as you take precautions to protect against over pressure. However for MBE purposes a manometer is actually surplus to requirements.

All the other gauges are “relative” pressure gauges, i.e. they are not independent of the gas species. This means that the pressure reading is not the true reading (even when it is displayed to 3 d.p. of accuracy!), it could be the true pressure if the species being detected were the same species used during calibration, however due to humidity, temperature and composition variation of the species this is unlikely to be the case. Thus with an ion gauge you are in fact not measuring the amount of the species in the vacuum, you are measuring the ionisation potential of the species. For example, this is why the same number of atoms/nm2/s flux register with very different beam equivalent pressures (mBar) for Ga and In. However for MBE purposes this relative pressure measurement is perfectly adequate.

The Pirani gauges was actually invented in 1906 by a German physicist called Marcello Pirani. The gauge relies on thermal conductivity (in the form of heat loss) from a heated filament into the carrier gas. Hence a better vacuum results in less heat loss. A similar principle is used in MOVPE and MOCVD to monitor the gas flows. The Pirani needs careful calibration and the reading is affected by the thermal properties of the gas being monitored. Standard Pirani gauges can be operated in the 1000 to 0.01 mBar (1 to 10-5 Bar) range and are therefore useful to monitor the pressure of backing pumps. A Pirani’s sensitivity, response time and accuracy are strongly affected by both their hardware design and software control. The Super Bee from Instrutech is the best Pirani gauge I have used. It includes an integrated controller with digital display and configurable relay trips. The Pirani filament has a lifespan of decades.

Wide Range Gauges (WRGs) are actually two gauges in one. They utilize a Pirani down to 0.1 mBar (10-4 Bar) and then “seamlessly” switch to an inverted magnetron from 0.1 to 10-9 mbar. In truth there is a small “grey” area when switching, but since we are not particularly interested in pressures around 0.1 mBar, we can live with it. The inverted magnetron is a cold cathode filament-free ion collector utilizing a high voltage (~4kV) that is automatically deactivated at high pressure. The WRG is a variation of the Penning Gauge invented by the Dutch physicist Frans Michel Penning in 1937. Unfortunately the gauge’s reluctance to start at UHV means that they are not useful to measure the PREP and GROWTH chamber pressures. They are however very useful to monitor the pressure of the FEL which is constantly cycled between atmosphere and UHV. Whilst a WRG can be operated for a decade or more, it is a good idea to service and recalibrate it every 2-3 years.

Ion Gauges (IGs) are hot cathode gauges. They are composed of 3 electrodes. Namely, a filament that acts as an electron source, a grid that accelerates the electrons and a collector to detect ionized gases generated by collisions between the electrons and atoms inside the vacuum (specifically inside the grid). All MBE ion gauge heads are similar, but not identical. Two electrode configurations exists, each with reflectional symmetry to the other, and each with a specific cable set. It is therefore worth clarifying which layout you have before reordering. The vacuum feed-through typically consists of an 11 pin circular plug, though again certain manufacturer’s support different controllers with different cables and different feed-through pin outs. The three electrodes consume 5 of the 11 pins, with 3 for dual filament and the central pin reserved for the collection. It is a good idea therefore to buy the ion gauge head from the same manufacturer as the ion gauge controller. Here I recommend Epimax throughout.

Tungsten (W) creates a very robust ion gauge filament that can be operated up to 100W (much like a light bulb) and in so doing it can actually be used as a light source inside the vacuum system! Unfortunately In and to a lesser extend Ga modify the work function of gauges fitted with W filaments, and so either Yttrium (Y) or Thorium (Th) coated  Iridium (Ir) filaments are more reliable/accurate in III-V systems (and in particular in beam flux monitors (BFMs)  (aka monitoring ion gauges (MIGs)). Sadly the Ir-filament cannot be run at such high power, and so does not make a practical light source. The Ir-filament can however be operated with a DC ion gauge controller (Epimax PVCx) to make them less sensitive to AC noise and hence give better stability. A DC operated ion gauge can detect from 0.01 to 10-12 mBar. This broad range is only permissible utilizing very small currents within the noise floor of AC units. The IG is therefore the preferred gauge for the PREP chamber, GROWTH chamber and the BFM. The lifespan of an IG depends on its usage. If you are measuring background pressure with a W filament it may last 10+ years, however its life is significant;y reduced if you do not shut it off and cool it down to room temperature before venting the system. If you are using the IG as a BFM/MIG then the filament will be constantly bombarded and eroded by various metal from the effusion cells and it may not last more than 6 months. Furthermore the entire IG head will become coated in III-V material creating a high impedance short circuit between the filament and grid. The rate of build up of material is of course proportional to your flux rates. Any III-V material condensed on the PTFE of the IG head can be etched away in 5 – 10 s  with a few drops of Aqua Regia. If you are careful, and rinse (with di-water then IPA ) and bake the IG head thoroughly you can reuse it for 10+ years.

Ion pumps are in fact vacuum gauges! Admittedly they only monitor the vacuum inside the pump itself, but once they have stabilized the ion impacts that are registered in the form of a ion current  can be readily converted into a pressure with some fitting factors that utilize the pump dimensions. This conversion is available as an option in modern ion pump controllers. Thus the ion pump is similar in operation to an ion gauge and can be used (as a backup) to monitor the pressure inside the PREP and GROWTH chambers.

Finally we have the Quadrupole mass analyser which is a form of quadrupole mass spectrometer (QMS). It consists of four parallel cylindrical rods (hence quadrupole) each making an electrical pair powered with radio frequency (RF) voltage. The rods then “filter” the ions based on modification of their trajectory due to selectivity of their mass-charge ratio. QMS typically have a detection range from 1 to 100 mass units and therefore can detect the concentrations of a broad range of useful species including: He (4), Water (18), N2 (28) and O2 (32) making them very useful for leak detection in the GROWTH chamber. The hardware design has a significant effect on sensitivity and resolution. The Prisma range from Pfeiffer are excellent QMS for MBE purposes.

MBE Design and Build: Vacuum system

Faebian Bastiman

An MBE system can appear rather complicated when viewed as a single entity, however it is much simpler to break the system into separate sub-systems (see MBE: System and Subsystems for a full description). This article discusses the basic structure of the vacuum system. Specific information on the specifications and features of each pump can be found in MBE: Which Pump?

An MBE system typically comprises 3 chambers: FEL, PREP and MBE. The FEL (fast entry lock) is regularly vented, unloaded, loaded and then pumped down. The PREP (preparation chamber) is typically maintained at UHV and only experiences contamination from outgassing wafers or from an adjacent chamber during sample transfer. The MBE (growth chamber) is constantly bombarded with elemental/molecular beams, along with O2 from the surface oxide. It also undergoes massive temperature changes when a cell is ramped from standby to operating temperature or when the LN2 is turned off or on.

Unsurprisingly the MBE chamber has a large pump demand. Some people employ a policy of “the more pumps the better”, however for a small III-V research reactor the pumping requirements can be met with an ion pump and a cryo pump team. The PREP chamber has the lowest pumping demand, and can be operated with a single ion pump only. The FEL is best operated with two stages of pumping. Firstly a dry scroll pump to pull the chamber from atmosphere (1000 mBar) down to ~0.005 mBar fairly rapidly, then secondly a turbo pump to reach UHV conditions. This basic MBE pump configuration is shown in Figure 1.

Slide1

The system is shown in an idle state. The rectangles with an “X” through them are UHV gate valves that are either red (closed) or green (open). Similarly the circles with an “X” though them are small UHV valves. The valves with a grey outline to the left of the image form the FEL’s pump down and vent sequence, which is discussed in more detail in MBE Design and build: Auto Pump Down. The “IG” refer to ion gauges used to measure UHV vacuum conditions.  THE FEL pressure is monitored with a wide range gauge (WRG) capable of monitoring from atmospheric pressure to 1E-9 mBar and hence conveniently does not need turning off during venting. The scroll’s inlet pressure is monitored by a Super Bee pressure gauge from InstuTech. MBE pressure monitoring options are discussed more fully in MBE: Which gauge?

In this configuration the scroll acts as a HV pump for the entire system. In can bring any chamber from atmosphere down to 0.005 mBar via:

  1. FEL: through the dedicated circular valve
  2. PREP: either through the FEL or via an optional dedicated valve
  3. MBE: through the cyro pump’s exhaust valve.

The turbo pump is used to create a UHV in the FEL. When venting the FEL, it is isolated on its inlet and exhaust and set to standby speed. The N2 valve is then opened and the FEL is vented to atmospheric pressure with ultrapure N2. Once the samples have been exchanged, the FEL is first drawn down to 0.005 mbar on the dedicated bypass valve, then the turbo is reengaged. In this way the turbo is effectively idled with no backing up during the sample change, which is ok for 5-10 minutes.

The PREP chamber is pumped via “Ion pump 1”. Ion pumps should never be turned off, and so when venting the PREP it is necessary to isolate the ion pump from the chamber via a gate valve. The PREP is typically vented to atmosphere and pumped down via the FEL’s valves, however an additional bypass to the scroll line can be added if desired.

In most configurations an ion pump and cryo pump team is perfectly adequate for the MBE chamber. The cyro pump, like the turbo pump, can be used to bring the base pressure down from 0.005 mBar after venting. The N2 on the cryo pump’s exhaust can be used to regenerate the cryo pump or vent the entire system depending on whether the cryo pump’s gate valve is open or closed. Again the ion pump on the MBE chamber should never be turned off, merely isolated via its gate valve during venting. Remember the LN2 cyro-cooling shroud is also effectively a “pump”, though for the sake of clarity I have omitted from this article.

If you are growing with P then you can include an additional turbo pump on the MBE chamber (Figure 2). Providing each turbo pump with its own scroll greatly simplifies the system pumping logistics. In this example the P trap is “in line” between the turbo pump exhaust and the scroll inlet.

Slide2

Post bake tasks: Dealing with dopants

Faebian Bastiman

After your bake out you can establish the relationship between temperature, flux and growth rate for your group III sources using Post bake tasks: Arrhenius plot. This is very useful, since it enables you to estimate the growth rate from a cell temperature to within 2%, that is just ± 0.02 ML/s when your target is 1ML/s. The estimate remains valid for a few weeks, until the cell is sufficiently depleted. After which you need to re-calibrate the temperature, flux and growth rate relationship, however only for a single value this time since in the relationship:

eqn arr crop

The values of E and k never change, and all you need to do is find the new value for A”. Actually what you are proving with this experiment is that MBE effusion sources obey the laws of thermodynamics. That is that the value for activation energy (E) in eV for a certain element is identical to established data, any small fluctuations are purely instabilities in the measurement instrument: the monitoring ion gauge head (MIG). Since effusion cells obey the laws of thermodynamics you can predict the flux (in atoms/nm2/s) for any cell once you have a single calibration point.

What about doping cells?

The problem with doping cells (like Si or Be) is that the actual flux is too low to register with a MIG. When pushing the Si cell to >1350°C to measure Si, you are probably simply measuring N from the decomposition of the PBN crucible. When you push the Be cell to 1100°C to measure Be, you are evaporating £1 of Be per second. Hardly worth it. The fact is you do not need to measure the Si and Be fluxes. Since effusion cells obey the laws of thermodynamics you know that Si and Be will evaporate with an activation energy of -4.11 eV and -3.10 eV respectively from established values in the literature (vapour pressure data from Wikipedia for example). This means you only need to establish a single calibration point for each cell and then use the value of A” for that cell to extrapolate to all doping values of interest.  Figure 1 shows some nominal flux values for In, Ga, Al, Be and Si each scaled to their A” (i.e. atoms/nm2/s/A”) for comparison.

dopants

Before you gather your own data, consider what doping is. Doping is growing a very dilute ternary. Think about it. GaAs has a lattice parameter (a) of 0.565338 nm. With 8 atoms per a3 for zincblende that is 4.423 x 1022 atoms/cm3. Half of them Ga, and half of the As. When you want a Si doping of 4 x 1018 atoms/cm-3 you in fact want to grow Ga0.99982Si0.00018As. This means that the Si flux in atoms/nm2/s can be established from knowledge of the attained doping level and Ga flux in atoms/nm2/s. The doping density depends on the magnitude of both the Ga flux and the Si flux. And so, you can double the doping by either doubling the Si flux, or halving the Ga flux (and hence halving the GaAs growth rate). It therefore useful to work out your Si and Be fluxes in terms of atoms/nm2/s so that are directly comparable to the Ga flux.

Your calibrated doping fluxes will only be valid until the cell starts to deplete, similar to the group III source. Handily you are utilizing x104 less dopant material than you are group III material. So the dopant cell temperature vs flux relationship should be stable to within 2% for x104 as long, ~192 years. Well perhaps this is a little hyperbolical, but it should be stable for 5-10 years.

To do all this you still need one calibration point. You can grow:

  1. a 1 – 2um thick doped GaAs layer on an undoped substrate and perform SIMS or Hall measurements
  2. a 1 – 2um thick doped GaAs layer on a doped substrate and perform SIMS or CV measurements with a CV profiler
  3. a p-i-n diode and perform CV measurements

If you possess doping cells, but none of these capabilities you will hopefully be able to find a collaborator who can provide a free one off calibration. If not, two commercially profiled samples per every 10 years will not break the bank. For SIMS profiling contact LSA.


MBE Design and Build: Bake out controller

Faebian Bastiman

The bake out runs infrequently, but has a huge power demand when operating. In MBE: AC Power, we discussed the power requirements for an MBE system. I suggested a 32A 3PNE line was needed to operate the system, whereas a dedicated 20A 3PNE line was suggested for the bake out. This article provides a design of the control system for the bake out utilising a dedicated 20A 3PNE line.

The bake out can be performed in two general ways:

  1. A box or tent with a combination of 2.5kW fan heaters and 1kW ceramic heaters
  2. With heat wraps or a heater jacket

The principle of the bake out is fairly simple: heat the system to the target temperature for X hours, then cool down. The heat and cool ramp rates should not exceed 1°C /min in order to avoid thermal stress. Furthermore the heat ramp should suspend if the pressure in the chamber being heated exceeds 5.0 x 10-6 mBar. The control hardware of choice for managing the temperature and pressure requirements of a bake out is the Epimax PVCx ion gauge controller. The PVCx is a multi-purpose tool: primarily it functions as an ion gauge controller, it has units of mBar, Pascal, Torr and Amps thus it further functions as a picoammeter for beam flux measurements, it can operate a pirani, be utilised as an 8 channel normally open/normally closed (NO/NC) relay/trip switch and control the bake out. All these controls and trip conditions can be configurable via either the PVCx menu or via serial comms.

The basic components of the bake out control system are shown in Figure 1. The MBE chamber (depicted in blue) has an ion gauge monitoring the internal pressure and a k-type thermocouple monitoring the external temperature. The entire chamber is surrounded by an insulating SS box or fibreglass tent. Two heaters are mounted inside the box/tent: Firstly a 2.5kW fan heater (BesTec) and secondly a 1KW ceramic heater (VG Scienta). The actual number of heaters will depend on the size of your system. The AC heater power is simply controlled via NO contactors rated at 32A with a 24Vdc coil (RS). The PVCx then actuates the contactors with its relays by modulating the external 24Vdc supply.

Slide1

The fan contactor is on whenever the bakeout is above ambient temperature (i.e. constantly during bakeout) whereas the heating contactors make and break several times a minute to regulate the temperature. The fans can therefore simply be controlled by an AC switch and manually turned on before and off after the bake (Figure 2).

Slide2

Alternatively the bakeout can be performed with heat wraps (VG Scienta) or custom made jackets (sadly currenty no know supplier).  The jacket essentially comprises a heater element attached to a fibreglass fabric covering an inner fibregalss insulating layer (essentially loft insulation) that is custom shaped to fit the contours of the system. The bake out is therefore more efficient since the heat is injected view conduction into the chamber’s metal body and the is more readily retained. The jacket can be attached more swiftly than the wraps which need to be laboriously assembled each bake out, or left as a permanent feature for convenience at the cost of aestheticism. It is a good idea to use some insulation, even if is just Al foil to cover the heat wraps to retain the heat. In this alternative bake out the PVCx simply makes and breaks the contactor to control the power to the heat wraps/jacket rather than the heaters (Figure 3).

Slide3

The actual heater configuration, power demand and wiring will depend on your system configuration. Figure 4 shows my wiring diagram for a custom built bakeout heater for a Riber 32P system. The system comprises a dedicated 20A 3PNE line, a rack mounted miniature circuit breaker (MCB) and fuse panel, a rack mounted 5 x contactor array, a DIN rail mounted distribution enclosure (FIBOX) mounted on the Riber 32P frame, 2 x 2.5kW heaters, 4 x 1kW heaters and 4 x 1kW heat wraps. The bake out controls the temperature and pressure of each chamber: MBE, preparation (PREP) and the fast entry lock (FEL) with 3 PVCx controllers. The rack mounted switching panel utilises 5 x 3-way switches to enable the system to be set into manual/automatic/off. Note: for clarity I did not include the EARTH connections (green), but any electrician reading this can rest assured all heaters, fans, racks and the MBE system itself were suitably earthed.

Slide4

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).