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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One thought on “Post bake tasks: Group V valved sources

  1. Pingback: How to growth your first sample: Oxide remove | Dr. Faebian Bastiman

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