Little known MBE facts: In growth rate

Faebian Bastiman

InGaAs QW and InAs QDs are popular active regions in opto-electronic devices. In Little Known MBE facts: RHEED oscillations (2) a concept was introduced to establish the In growth rate from a known Ga growth rate but what about an accurate and independent determination of the In growth rate? Well, conveniently there are several options.

The first method is simple. All you need is a rather expensive InAs(100) substrate and its lattice parameter: 6.05840 Å. RHEED oscillations can be performed for In on InAs in an identical manner as for Ga on GaAs. The required As flux for maximum RHEED oscillations for any given ML/s growth rate should be ~87% the As flux needed for GaAs; this maintains the 1.6:1 ratio on InAs. If you don’t know why read One thing with InAs is that the demand for As over pressure rises significantly with increased substrate temperature. So if you need more than ~1.6:1 it implies that the substrate temperature is too high.

The second method uses the idea of adding and subtracting RHEED oscillation growth rates for ternaries introduced in Litte Known MBE facts: RHEED oscillations (2). In this way the growth rate of GaAs and InGaAs can be used to find the InAs growth rate. To test, try growing a QW of In0.06Ga0.94As of 10nm total thickness and employ a little maths and logic: In0.06Ga0.94As is effectively 9.4 nm of GaAs and 0.6 nm of InAs deposited at the same time. If you use a GaAs growth rate of 0.94 nm/s (for example), the whole QW should take 10 seconds to grow. An InAs growth rate of 0.6 nm/s will then give you the desired thickness and composition. If not, and you are certain about your GaAs growth rate, the InAs growth rate is wrong. As an added check of composition-thickness, a simple 3QW structure can be grown as shown in Figure 1 (below). The RT PL peak wavelength varies as a function of both InGaAs thickness and composition: e.g. [In] = 6%, 10nm QW should give RT PL ~900nm.

The third method is very useful for very low growth rates. It involves using the S-K transition which occurs when ~1.8ML of InAs is deposited on GaAs(100). There is an abrupt RHEED transition from streaks to spots which is highlighted in figure 2 (below). The actual S-K transition is a little imprecise since it relies heavily on temperature (particularly where non-unity In sticking coefficients are in question). It also is less accurate at higher growth rates, since the exact moment of the transition is subjective and at >0.25ML/s the timing depends on how accurate you are with your stopwatch. You will certainly want to make 3 attempts and average the times. Luckily 1.8ML of InAs can be readily flushed from GaAs with a brief (5-10 minutes) anneal at ~600°C under As flux and then you can cool the sample back to ~500°C and try again.

The fourth option is a clever trick you can play if you have access to a phosphorus source. The III-V alloy In1-xGaxP is lattice matched to GaAs at room temperature for x = ~50%. This little fact means you can grow 250nm of In0.5Ga0.5P/GaAs(100) and then analyse the exact composition with XRD peak splitting. When the Ga and In growth rates are identical the composition will be exactly In0.5Ga0.5P (just make sure In sticking is unity by growing around 480°C). Also if you are using RHEED oscillations you can accurately determine your Ga growth rate with GaAs RHEED oscillations, then work out your InGaP growth rate from RHEED oscillations AND discover the optimum III:V ratio for phosphorus growth at the same time. The fact that one method utilises 250nm of material and the other utilises 30ML means you can also get an idea of how those pesky shutter transients are affecting things.

The fifth and final option is a little super-lattice (SL) on an InAs(100) or a GaSb(100) substrate. Ideally, you will need access to antimony (Sb) and that way you can grow an GaSbAs/InAs(100) or a InAsSb/GaSb(100) lattice matched SL. Lattice matching at room temperature means the substrate peak and SL-average-composition peak coincide and will only see the satellite peaks. You can also utilise binary super-lattice InAs/GaSb but you may get strange effects from the interfaces unless you know what you are doing (more on that later). This technique relies on prior knowledge of the Ga growth rate (again) but can also provide interfacial and structural quality information.

The actual method(s) you use is largely a matter of personal preference and intended application. If you are trying to work out the growth rates so you can grow InGaP/GaAs(100) lattice matched bulk it makes little sense to utilise a InAs/GaSb SL to work out the growth rates. The nice thing is that with so many ways to double-check the actual growth rate should (in theory) be very accurate indeed.