Little known MBE facts: Flux determination

Faebian Bastiman

A flux monitor or monitoring ion gauge (MIG) is a very useful tool for MBE growth.  The principle is rather simple: insert an ion gauge head into the beam path and instead of monitoring the background pressure you can monitor the beam equivalent pressure (BEP). Each elemental species responds differently to ionisation and detection, and the angle of incidence and temperature difference also affect the final measured values. Handily any given cell is in a fixed position relative to the MIG. The ionisation potential is a constant and the effect of temperature differential is small over a 100°C range.

Gathering flux data is straight forward. For an effusion cell (e.g. Ga, In, Al or Bi) you simply need to vary the cell temperature over the range of interest and note down the flux when the shutter is open and when the shutter is closed. Typically the measurement may take 10-60s to stabilise. When you subtract the shutter closed “background” from the shutter open “measured” flux you end up with an “actual” flux for that cell temperature. The As and P cracker or Sb valved sources are quicker. With the source set to operating temperature you just need to step the valve position through the available range. Stabilisation times will vary and to some extent the background flux is moot since the background is predominantly the group V species being monitored.

The result so far is a system dependent BEP determination for each cell. This is useful for you, but not for anyone reading your publication and attempting to repeat the results. What you want is a system independent atomic flux determination. In order to achieve that you need to do a little more work.

The group III cells’ flux can be calibrated in terms of ML/s from RHEED oscillations using Little Known MBE facts: RHEED oscillations (1). This can be converted into atoms/nm2/s using the method in Little Known MBE facts: Growth rate and Flux. We now have a system independent flux in atoms/nm2/s for all our group III species, but what about group V?

Let’s take As as an example. Using the static map you created in Little known MBE facts: Making a static reconstruction map you can estimate the As flux that is necessary to retain a As-rich (2×4) reconstruction at a growth temperature of 580°C. In Little known MBE facts: Group V overpressure we called this flux AsAs. When growing GaAs we also need to balance the incident Ga flux with an additional As flux: AsGa. Thus the total As, Astotal = AsAs + AsGa. This enables a good number of RHEED oscillations (>30) and maintains a (2×4) during growth with very dim 2nd order rods. What happens if you reduce the As flux when growing? Well when you reduce the As to less than Astotal and open the Ga shutter for a few seconds the RHEED will be a slightly weaker (2×4). Close the Ga. Reduce the As flux further. Open Ga for 5s, check the RHEED and close the Ga. Eventually the RHEED will be (1×1), reducing the As further will result in a Ga rich (4×2) pattern (for more information see my (2×4)/(4×2) post). At the point where the (1×1) emerges the AsGa part of the Astotal flux and Ga flux are equal. If you now subtract AsAs that you estimate at this temperature from the Astotal that gave you a (1×1) you have a value of BEP that represents the atoms/nm2/s of As atoms that is exactly equal to the atoms/nm2/s of Ga. So if this growth rate is 1ML/s, that is 6.258 atoms/nm2/s of Ga and therefore also 6.258 atoms/nm2/s of As.

The atoms/nm2/s for P and Sb can be can be calculated using the method outlined for As above, providing of course you have III-P and III-Sb substrates to grow the binary on. The other method is to grow a dilute ternary i.e. GaAsP or GaAsSb and with knowledge of the composition and As fluxes you can estimate the P and Sb fluxes. The problem is you are estimating the flux in atoms/nm2/s “incorporated” as oppose to atoms/nm2/s “incident”. The lattice site competition and other complications that arise when growing group V ternaries mean the incident and incorporated are significantly different. Another means of checking is to “grow” a layer of pure group V, like Sb or Bi, at 0°C and assume the sticking coefficient is unity. The thickness of this layer can be measured with SEM and then you can estimate the incident flux utilising the atomic density of the amorphous film. Though clearly the pyrophoric nature of P precludes it from this kind of experiment.

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5 thoughts on “Little known MBE facts: Flux determination

  1. Pingback: Nanowires: Pre campaign calibrations | Dr. Faebian Bastiman

  2. Pingback: Post bake tasks: Group V valved sources | Dr. Faebian Bastiman

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

  4. Pingback: How to grow your first sample: (2×4)/(4×2) transition  | Dr. Faebian Bastiman

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