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 As2 and 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!
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.
Dr. Faebian Bastiman 