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.

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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.


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.

Post bake tasks: Group III flux: Arrhenius plots

Faebian Bastiman

After a thorough cell outgas (Post-bake tasks: Cell outgas) you are ready to record the beam equivalent pressures (BEPs) of the group III cells. This is doubly useful as on the one hand it allows you to establish a working range for the cell and on the other it enables you to gather some quantitative data for growth rate estimation to within 2% (see below). Here we will focus on the Al, Ga and In cells. To collect your flux data first insert the monitoring ion gauge (MIG) into the beam path. Next ramp the cell to the starting temperature and allow it stabilize at that temperature. Typical temperatures for group III sources are given in the table of figure 1. It is good practice to first heat the cell to Toutgas for 30 minutes, then cool down to the Thigh value for a further 15 minutes and gather data in a descending temperature sequence. That way the cell is outgassed before use and before each subsequent reading. In is also good practice to double check each reading after a period of 5 minutes to ensure the cell is stable at the temperature of interest.

Slide1_crop

Use the method outlined in Little known MBE facts: Flux determination to obtain each flux value by subtracting the background flux from the BEP flux. Collect data in steps of 25 °C waiting 15 mintues each time for the cell to stabilize at the new temperature. Once the data is gathered plot it in your favourite graphical analysis software (here I use Origin Lab) and you should have data similar to that shown in figure 2a. Note the discrepancy in data for the Ga1 and Ga2 cells. This is caused by the slight difference in capacity and slight difference in the angle the atomic flux makes to the MIG. Regardless of the absolute value, the envelope (shape) of the curve is the same.

Slide2_crop

The envelope of Figure 2a describes the Arrhenius data for each of the sources. Using the modified Arrhenius equation in figure 3 and defining an appropriate fitting equation inside the Origin fitting tool, the constants A, E and C can be calculated for these particular cells. Unfortunately small nuances in the fitting can lead to significantly different values for A, B and C in the modified Arrhenius. To make things simpler we can use the far right approximate (a basic Arrhenius), where A” is variable, E is the activation energy of the element, k is the Boltzmann constant (in eV = 8.6173E-5) and T is the absolute temperature (in K). Instead of plotting the basic Arrhenius, we can plot log(flux) vs 1000/T  and create a nice linear plot for simple linear fitting: y = mx + c. The full formula and working is shown in Figure 3.

For example:

Assume we plot log(flux) vs 1000/T for the Al cell, where the flux is the BEP flux in mBar, the log is in base e (i.e. natural log) and T is in K. We get a value for the slope (m) = -34.11 and the intercept (c) of 8.92. The intercept is our value for log(A”) in Figure 3, but the slope needs converting into an activation energy. To do this we need to multiply it by 1000 (because we plotted 1000/T) and then multiply it by the Boltzmann constant, k. The value comes out at -2.94 eV. It is negative because we are using y = c – (-)mx for our fit in the second equation of Figure 3.

The values for E should come out to be the activation energies from the literature, and, together with the log(A”) value, we can now predict the flux (in mBar) for a given cell temperature using the third equation in Figure 3. Some typical values for E and log(A”) are shown in the  table of Figure 3.

Group 3 Arr 2

 

 

 

Since the flux reading is directly proportional to the growth rate we have a direct means of setting the growth rate. To do that you will need to find out what your value for F is in the table of Figure 3 using the method outlines in Little known MBE facts: Growth rate and flux. Since the flux calculated by the value in Figure 3 comes out in mBar, F has units of atoms/nm2/s/mBar. The values for F in this case are very large since 1 x 10-7 mBar is around 1 atom/nm2/s. This is another good reason to work in nA for BEP since 1 nA is around 1 atom/nm2/s and the numbers are therefore more convenient (see Note below). You can now change the BEP from the system dependent values of nA/mBar to the system independent values of atoms/nm2/s. 

The cell ‘s flux is only stable for a short period of time owing to (i) consumption of material, (ii) material degassing and (iii) redistribution of the material inside the cell. Hence the flux data gathered will only be valid for a short period. It is good to refresh the flux data once a week. This can be done automatically with the appropriate software (see MBE Dreams: Software).

Note: The fluxes in this article were gathered on an EPIMAX PVCi, and hence are in units of mBar. The ion gauge had a tungsten (W) filament and the controller was operated with an emission current of 1mA and 19% sensitivity.  The new EPIMAX PVCx displays the collector current in the unit of nA in addition to pressure values. The units of nA and mBar follow a simple linear proportional correlation, the unit of choice is therefore simply user preference. The unit of nA is somewhat nicer to handle since the gathered fluxes will be in the 0.1 to 100 nA range and you can dispense with the obligatory 10-9 to 10-6 needed to express mBar.

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.