Little Known MBE facts: RHEED oscillations (2)

Faebian Bastiman

So you have established your binary GaAs and AlAs growth rates using Little Known MBE facts: RHEED oscillations (1) and now your thoughts are moving to ternaries. The AlxGa1-xAs ternary is fully miscible. [Al] > ~85% are indirect gap materials. If you are using AlGaAs as a carrier confining cladding layer you may want [Al] from 30-40%. So how do we calculate our ternary growth rate?

Well conveniently algebra of epitaxy holds. First find your GaAs growth rate of 0.7ML/s and your AlAs growth rate of 0.3ML/s, separately. Then when you open the two cells’ shutters together you will get Al0.3Ga0.7As growing at 1ML/s. Just remember to suitably increase your As flux to ensure good RHEED oscillations for each measurement and good stoichiometric crystal growth.

On the other hand, you can approach the problem from an entirely different angle. In the growth of InGaAs (for example) you can first accurately determine your GaAs growth rate and then (at a suitably low temperature to ensure unitary In sticking coefficient: <540 °C but good adatom mobility: >500°C) you can add a little In and grow InxGa1-xAs. The resulting increase in growth rate will allow you to determine the InAs growth rate (GR) since:

GRInGaAs = GRGaAs +GRInAs

This conveniently means we you can accurately determine your growth rate and composition for any and all AlxGa1-xAs or InxGa1-xAs ternary alloys on a single sample within a matter of minutes. How very efficient of you.

11 thoughts on “Little Known MBE facts: RHEED oscillations (2)

  1. Pingback: Little known MBE facts: In growth rate | Dr. Faebian Bastiman

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  3. First of all, thank you Dr. Bastiman for your wonderful effort. This blog is very helpful, specially for the students.
    I have couple of questions.
    1.Let’s take your example of growing Al0.3Ga0.7As with a growth rate of 1ML/s. Now to grow the same layer at slower rate, say 0.5ML/s, should the GaAs and AlAs growth rate be half of the previous rate?
    2. If I want to grow multilayer AlxGa(1-x)As structure with different x value, can I do so by changing the Al temperature only while keeping the Ga temperature same for all layers or do I need to also change the Ga temperature suitably??
    Thanks in advance!!

    • To answer your two questions in turn:

      1) Yes, exactly. If you grow Al0.3Ga0.7As with Al at 0.3ML/s and Ga at 0.7ML/s to achieve a total 1ML/s, and you want to grow at a total of 0.5ML/s you would simply use Al at 0.15ML/s and Ga at 0.35ML/s. In order to grow optimum quality material you would also then reduce the As flux (roughly half) but it depends on the growth temperature.

      2) You can do this either way, it depends on exactly how you want to do it. If you want to retain a constant total growth rate of 0.5ML/s throughout, which would then enable you to use the same As flux throughout, you would need to change both the Al and Ga flux (this is the way I prefer) otherwise you can simply change the Al temperature and reduce or increase the As flux according to the new total growth rate.

      • Thank you Dr. Bastiman for your kind reply. Regarding my 2nd doubt, I actually want to grow few AlxGa(1-x)As structures keeping Al cell at different temperature and then perform HRXRD to know the actual Al concentration. For this purpose If I keep the Ga cell temperature same for all different Al cell temperatures, will that affect the crystal structure??
        On another note, when we grow InAs QD we use an optimized V/III ratio. Now during the growth we use different cells Ga, Al etc having different flux for different layers. So the ration of As:III material keeps changing from layer to layer. How it’ll affect the grown material if one keeps the As:III ration fixed for all the layers (by suitably adjusting As valve opening)?
        Thanks in advance!!

        • Dear Samuya,

          There is no problem fixing the Ga temperature and altering the Al temperature to achieve the desired composition. This is in fact the normal way to do it.

          As for the As:III ratio, there will always be an optimum As:III ratio for each alloy, for a given growth temperature and growth rate. Have a read of the blog post Little known MBE facts: Group V overpressure which explains the concept. It would be nice to simply be able to simply supply five times the As flux when growing at five times the growth rate, but that is not quite right and also depends on the growth temperature. In order to answer more fully, I would need to know the growth rates of each of the layers. For example if the total growth rate is changing from 0.5ML/s to 1ML/s you can simply double the As flux and be fairly confident the As:III ratio will be near optimum. If you are going from 1ML/s to 0.1ML/s simply cutting the As flux by a factor of 10 may not be the optimum As:III ratio. This is why I tend to alter BOTH the Al and Ga cell temperatures to achieve the desired composition (or indeed utilize 2 Ga cells) so the total growth rate can be kept constant (usually 0.6ML/s for me) and hence the As flux can be kept constant for all layers. So in conclusion it is best to utilize near optimum As:III flux for all layers, the effect on the growth of supplying sub-optimum As:III ratio depends on how far you are away from optimum. If the As is too low you will growth group III rich and probably end up with metallic droplets which kill the epitaxy, if the As is too high the layer quality (either optical or electrical) will be lower. The higher the As the lower the layer quality. Though as long as the As flux is not “too low” there is some margin for error and you can typically over supply the As up to 3x optimum without adversely effecting the quality too much.

          Faebian

  4. Hello Dr. Bastiman
    I’m trying to do our In-rate callibration just the way you described above but due to the large lattice mismatch between GaAs and InGaAs with higher In-concentrations there are just very few oscillations visible to determine the growth rate of In. Maybe we are out of range of the correct growth parameters. What V/III ratio would you recommend and what substrate temperature?
    Thanks in advance

    • You cannot use RHEED oscillations beyond the critical thickness, there is to much roughness. If you want to calibrate In with high concentrations you need to do so on lattice matched substrates. Pure InAs substrate is, of course the best to calibrate the In growth rate, since you can homo-epitaxially grow InAs on InAs. Otherwise you can grow InGaAs around 50-50 on InP substrates.

      • This was our second thought to do InAs calibration on InAs. But a InAs wafer does cost much more than a GaAs Wafer. So I tried to find some informations to InAs growth on InAs (because I knwo from Si-Rheed calibration on a Si-Substrate that it’s not trivial to do so. you have to be in a certain temperature range to really see oscillations because the thermodynamics of growth change very much). But I couldn’t find anything for InAs. Do you know that it works well? What substrate temp. would you propose?

        • Si growth on Si substrates on a III-V deposition system is typically limited by the surface oxide removal step which requires around 1250 °C on the substrate, otherwise you end up with surface roughness which inhibit oscillations. If you only intend to use the Si for doping you can do doping profiling for this.

          InAs growth on InAs works perfectly for RHEED oscillations. You need to oxide remove at 530°C and then grow the InAs at around 500 to 510 °C with a similar III/V ratio to GaAs.

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