How to grow your first sample: As capping

Faebian Bastiman

Having a metallic As source offers us the ability to create a very useful protective epilayer: an As cap. Once you have grown a layer you may want to remove the sample from your system and for example transfer it into a neighbouring system or send it to a collaborator for analysis. As soon as you remove the sample from vacuum the surface will of course oxidise. Oxidation cannot be avoided, however if you deposit an As cap of sufficient thickness before you remove the sample only the As will oxidise and your III-V epilayer surface will remain perfectly intact. Once the sample is safely bask inside a vacuum system, one need only heat it to ~300 °C and the As sublimes. A quick look with RHEED will show the original reconstruction is even preserved.


How does one deposit an As cap?

You can deposit an As cap on any surface. In this example I will discuss depositing an As cap on GaAs(100). At the end of your epilayer growth you will typically observe an As-rich (2×4) reconstruction. If you want to know what a reconstruction is see my What is a reconstruction? post. Essentially to deposit a cap all you need to do is fully open the As and turn off the sample heating. However there are a few things to consider:

Firstly, As2 tends to create a better cap than As4, so wherever possible you should use As2.

Secondly, the As cap grows very slowing whilst the substrate is above 100°C and it is preferable to cool the substrate down to 0°C in order to rapidly form a cap.

Thirdly, not only As but every other background species will readily condense onto the substrate at 0 °C including the hydrocarbon, water vapour, oxides and other undesirables lurking inside your vacuum.

In order to ensure the highest purity of your surface, it is a good idea to pre-protect the GaAs by creating an even more As-rich reconstruction than the (2×4). The reconstruction of choice is called a c(4×4). The c(4×4) is simply created by holding the substrate at 500 – 540 °C  in a moderate As flux for several minutes.

Test it yourself. Align the RHEED along the [-110] azimuth so you can see the 4x of the (2×4) shown in Figure 1a and then lower your substrate temperature until the 2x pattern of the c(4×4) appear as shown in Figure 1b. You may be asking yourself why a c(4×4) reconstruction has a 2x pattern. Well if you look closely at the reconstruction in Figure 1b you will see it is not simply a 2x, it is a pair of 2xs. The 2x at the top is out of phase with the 2x at the bottom (i.e. the lines on the RHEED screen do not line up). The 2x at the bottom is in fact exactly half way between the 2x at the top.

As cap v2

Rotate the sample to the [110] azimuth. You will see the exact same pair of 2x reconstructions there too. The c(4×4) looks the same along both [110] and [-110] azimuths. What is going on? Well the reconstruction’s unit cell is not aligned to the same directions as the (2×4). It is in fact centred on the original [100] and [010] directions of the (001) surface, which means it is at 45° to the (2×4) reconstruction.

The c(4×4) is composed of 1.75 ML of As on Ga. That  is a full ML of As plus another ¾ of a ML of As on top of that. This is significantly more As than the ¾ of a ML of the (2×4) reconstruction. The As of the upper ¾ ML of the c(4×4) are back bonded onto the As of the lower full ML, meaning that the As on the upper layer “sees” no Ga at all. Which more importantly means that the Ga in the buried layer is protected from contamination. The As dimers of the upper ¾ ML  arrange  themselves 3 abreast in a “brickwork pattern” and it is this arrangement that gives the pair of 2xs on the RHEED screen.

The c(4×4) is therefore perfect to pre-protect the epilayer. Once it has formed, simply ramp the substrate down to 0 °C  (or as low as you can go) and watch the RHEED. To get a more even As cap you can start the substrate rotation again now.

Once the substrate gets down below 100 °C you should find that the 2x is replaced by the 1x pattern shown in Figure 1c. This is because a thin amorphous As layer now exists on top of the c(4×4) and all you can see are the bulk lattice rods. As the amorphous As layer gets thicker, the RHEED beam can no longer penetrate through to the substrate and the RHEED pattern becomes the amorphous haze shown in figure 1d. This is probably somewhere around 5 – 10 nm thick. It may take >30 minutes from the moment the ramp down was started to the time the amorphous pattern is completed, depending on the background sample heating and the rate of cooling you can create inside your particular MBE chamber.

The As growth rate is highly temperature dependent. You want a final thickness of about 15 nm and from experience this takes about an additional 5 minutes after the amorphous RHEED has appeared. In order to get a more accurate estimate of the As cap growth rate, you can first create the c(4×4), then ramp down to 400°C, then turn off the As flux and leave the sample to equilibriate at 0°C  for half an hour, then apply the As flux and monitor the time it takes to create the amorphous pattern, then triple it. If you want to be really accurate with your cap thickness I suggest you perform cross sectional SEM on the sample and extract the cap thickness.

The As cap is not only useful to protect your epilayer ex situ, but can be used to perform two temperature calibrations at ~300 and ~400 °C  (in order to do that see my Making a static reconstruction map post.

How to grow your first sample: (2×4)/(4×2) transition 

Faebian Bastiman

The (2×4)/(4×2) RHEED transition is a very useful flux calibration point, since it tells you when your As and Ga fluxes are equal. I described what a (2×4) and a (4×2) reconstruction are in my What is a reconstruction post. In this post I will explain how to spot the transition with RHEED.

First of all you need to produce a good starting (2×4) surface. In order to do that follow the steps in my Oxide remove post. With the Ga shutter closed and an As overpressure incident on the sample you should see the As-rich (2×4) reconstruction on your RHEED pattern (Figure 1a). How can you be sure this is a (2×4) and not a (4×2) in disguise?

You can be 99% certain that if you are annealing at a temperature around 15 °C below the oxide remove temperature and the As flux that you are supplying is sufficient to grow GaAs that this is a (2×4) reconstruction. You can test very simply by stopping the rotation so you can see the 4x on the [‑110] azimuth and reducing the temperature. Reduce the temperature until a 2x appears. Rotate to 90° to the [110] azimuth. Still 2x?

If yes, then this a c(4×4) and the early reconstruction was a (2×4). Return to the original temperature and retrieve your (2×4). Note if you want to know what a c(4×4) is read my As cap post.

If no, and you are looking at a 4x, then this is now a (2×4). Remain at this temperature.

If no, and you are also not looking at a 4x, then something is very, very wrong with your sample. Are you sure it is GaAs(100)?

Assuming you have found your (2×4) make sure it looks like the one in Figure 1. If not change your temperature by 5 – 10 °C and your As flux by 10-20% and try to get the best (2×4) you can. You will note that in Figure 1a on the [‑110] azimuth the 4x rods are fairly short and bright even if the 2nd order rods are not very clear. The 2x on the [110] azimuth is simply a 2x and cannot give you any quality information at this point.

Try opening the Ga shutter. Does your 4x change to look like Figure 1b? It should. The Ga flux breaks up the static (2×4) and creates some disorder. This disorder results in an elongation of the 4x rods and an overall decrease in the intensity. The 2x on the other hand does not really change. You can consider Figure 1b as the dynamic (2×4), where “dynamic” means “growing”.

RHEED Ga to As

We are about to search for the Ga-rich (4×2) pattern. In your first few attempts do not rotate the sample, but rather keep the RHEED pointing along the [110] azimuth and looking at the 2x pattern. During the search the 2x will change into a 4x on [110] and the 4x will change into a 2x on [-110]. You can see the Ga-rich reconstructions in Figure 1d.

The problem with performing the transition without rotation is that you are only seeing the transition at the point the RHEED beam is hitting with the Ga and As flux pattern present at that point. This As/Ga ratio can be very different from the one created whilst rotating, since when you rotate you create a more even flux across the wafer. On the other hand since both the starting and ending reconstructions have both a 2x and a 4x azimuth it is not so obvious when the transition has happened until you have some experience looking at it.

So with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen close the As shutter (or close your cracker valve if you do not have a shutter).

Am I serious?


Close the As shutter for a few seconds and watch the RHEED, it should quite rapidly turn into a 4x like Figure 1d. Once it does open the As shutter again and make sure the 2x returns. When the As flux is greater than the Ga flux, you see the 2x of the (2×4) when the Ga flux is greater than the As flux you see the 4x of the (4×2). In the extreme case we just witnessed the Ga was of course practically infinitely bigger than the As flux.

Ok step 2 is a little less brutal… with the rotation off, the Ga shutter open and the 2x on the [110] azimuth on the screen… half the As flux. Wait 15s. Did the 4x appear? If yes return to the original As flux, if no half the As flux again. What you are doing is performing a binary search. You take the lowest As flux you know gives you a 2x and the highest As flux you know gives you a 4x and you apply the average of the two and see if you get a 2x or a 4x. Always wait the same 15s between flux changes for consistency.  

You may notice on your search that the 2x creates a rather dim 3x rather than the desired 4x (Figure 1c). This is a mixed reconstruction between the (2×4) and (4×2) that many people overlook. It is weak on both [110] and [-110] azimuths and hence probably lacks the long range order of the (2×4) and (4×2) reconstructions. The low intensity makes it difficult to say whether it is a (3×1) or a (3×4) or a (3×6) and indeed the physical surface of the GaAs could be made up of domains of each.

Once you have found the maximum As flux that gives you a 4x on the [110] in 15s, return to the original As flux that gave you the 2x on the [110]. In the third step gradually reduce the As flux from the start value toward the value you found and watch the RHEED pattern evolve. Double check the As flux you believe gives you a 4x on the [110] is reproducible and get familiar with the process.

Finally set the substrate rotation to around 0.2 revolutions per second and perform the third step again. The As flux required to create the 4x on the [110] azimuth may be a little different this time. With the rotation on you will see the 2x on the [-110] azimuth for the first time. Take a closer look at the reconstructions in Figure 1d.

The dynamic 4x you get when you are Ga rich is much sharper than the dynamic 4x you get in Figure 1b. This is because that whilst the Ga breaks up the As-rich (2×4), the Ga only ever improves the Ga-rich (4×2). Hence the dynamic 4x of Figure 1d is similar to the static 4x in Figure 1a. Note too that the 2x in Figure 1d is very different. There exist some extra bright spots at the bottom of the image that are much dimmer in the 2x of Figure 1a and cannot be seen in Figure 1b.

Hopefully you are now familiar with the process and can readily ascertain the maximum As flux required to create a (4×2) reconstruction in 15s. This is called the “1 to 1” point. The point at which the As flux is approximately equal the Ga flux. You could argue that the Ga flux is slightly higher than the As flux, since the surface turns Ga rich; however they are approximately equal. More importantly this transition is very repeatable. You need to make sure you always do the test in the same way. i.e. you must always wait 15s and always do the test at this temperature. Otherwise the flux will vary from check to check. It is worthwhile observing that variation yourself.

Once you have found this point you can calculate your fluxes in the system independent units of atoms/nm2/s (see Flux determination post) and state the atomic fluxes in any journal articles you write.

How to growth your first sample: What is a reconstruction?

Faebian Bastiman

In this article I will explain what a reconstruction is and how it appears in a RHEED pattern without using terms like “Ewald sphere” and “reciprocal lattice rods”. To answer the question of what a reconstruction is, we must first consider why a surface reconstructs. Let’s use zinc blende GaAs(100) as an example.

The simplest lattice to imagine is a cubic lattice. The unit cell of a lattice is the smallest collection of atoms, that when repeated over and over again, can recreate the entire lattice. For a cubic lattice this is simply a cube with sides of length a and an atom on each corner (vertex). With 8 vertices it is easy to make the mistake that the unit cell comprises 8 atoms. Instead consider that each of those atoms overlaps into 7 neighbouring unit cells within a full lattice. Thus each vertex atom contributes 1/8 of an atom to each unit cell, and therefore each cubic unit cell comprises (on average) 1 atom.

The next two simplest lattices to image are the body centred cubic (bcc) and the face centred cubic (fcc). When comparing a bcc lattice to a cubic lattice, there exists an extra atom right at the centre of the cube. In this case a bcc unit cell comprises 2 atoms. Similarly a fcc unit cell has an extra atom on each face. In this case each of the 6 atoms on the faces are shared between 2 neighbouring unit cells. The fcc unit cell then comprises of 3 + 1 = 4 atoms.

Imagine a blue fcc lattice of As atoms and a green fcc lattice of Ga atoms. Now take the unit cells of each and place them side by side. Pick up the Ga fcc unit cell and push it into the As fcc unit it overlaps by 75% of its longest diagonal. You would have two cubes that would look like Figure 1a. If you remove any of the green atoms that are not inside the As fcc unit cell, and connect any of the nearest neighbouring As and Ga atoms with a red line: you have just created zinc blende GaAs!


 interleaved fcc  



 Figure 1: (a) Two interleaved fcc unit cells (b) zinc blende unit cell


Why the long winded explanation? Well on the one hand it is useful to know that a zinc blende unit cell can be considered as two interleaved fcc unit cells, since it saves us trying to image zinc blende without a framework. On the other hand, when you consider the {100} (i.e. 100 family of planes) you can simply consider the As (or Ga) fcc face.

N.B. A quick aside on parentheses: (100) is the 100 plane, {100} are family of identical planes e.g. (100), (010), (001). Similarly [100] is the 100 direction, <100> are the family of directions.

So consider the {100} planes, each plane is made up of multiple fcc faces tiled to make a pattern. During MBE growth GaAs is typically As terminated, so we will consider a surface made of As fcc faces repeated over and over. When you look at the {100} face of an fcc unit cell you see a 2D square consisting of 5 atoms (Figure 2a). However you can see that (in 2D) each of the corner atom overlap with the 3 neighbouring squares, and so each corner atoms only contributes ¼ of an atom to the square. When you add 4 x ¼ to the one in the centre of the face this gives 2 atoms per square. Since  the length of the sides of the square is the lattice constant, a, you can say that the {100} plane of a zinc blende semiconductor has 2 atoms per a2 and in doing so can work out the atomic surface density. Similarly you can add up the atoms inside a zinc blende unit cell and say there are 8 x 1/8 + 6 x ½ + 4 = 8 atoms per a3 and work out the atomic volume density.

 Slide1_crop  Slide2_crop



 Figure 2 (a) As atoms of {100} plane aligned to <100> (b) As atoms of {100} plane aligned to <110>

Consider the {100} plane of As atoms. The surface represents an abrupt end to the lattice. All the As atoms are bound to Ga atoms below, and would like to bind to Ga atoms above. However they cannot because there are no Ga atoms: the crystal is terminated at this plane. This means that every other As atom within the lattice is bonded to 4 Ga atoms, except the ones on the exposed surface. Or to put it a different way, all the As atoms in the crystal have 8 electrons in their outer shell except the surface atoms. With nothing else to bind to, the As atoms choose to bind to each other. The surface is the only point in the lattice where an As atom bonds to another As atom. The As atoms are then said to dimerise (i.e. create a dimer) and in so doing the surface is said to have reconstructed.

In fact not only do they always dimerise, they always dimerise in the same direction. They dimerise in the <-110> direction. They dimerise in this direction because their motion is restricted in the <110> direction by the Ga atoms below them and in order to dimerise the As atoms need to move closer to each other. If you take the plane described in Figure 2a and rotate it 45° you can identify a smaller square with 1 atom on each corner and sides with length a/√2 (Figure 2b).  The square’s sides run in the <110> and <-110> directions. The row of blue As atoms at the top of Figure 2b also includes the position of the green Ga atoms in the plane below. Note the Ga atoms appear in the <110> direction. In this way the As atoms motion is restricted in the <110> direction and they can only move in the <-110> direction. Conversely when the surface is Ga terminated, the As atoms lay in the <-110> direction so Ga always dimerises in the <110> direction. Thus the two species create reconstructions that are at right angles to each other.

When the As atoms dimerise they create a feature on the surface that reapeats with half the periodicity (or occupies twice the space) of a single As atom. A reconstruction is therefore similar to a unit cell, in that it is the smallest feature that when repeated can recreate the entire surface. If one could freeze the {100} plane before it reconstructed it would look like Figure 3a. One can then draw a square around the smallest repeating pattern and note that the sides of the square are equal to a/√2. To save us using the term (a/√2 x a/√2) we call this (1 x 1). Once the surface has dimerised it looks like Figure 3b. Now because the As atoms have moved together  the repeating pattern is a rectangle with lengths (2a/√2 x a/√2) or (2 x 1). This is then a (2 x 1) reconstruction.


 Slide3_crop 2  Slide4_crop 2



 Figure 3 (a) (1 x 1) reconstruction (b) (2 x 1) reconstruction


The fact is GaAs  does not  form a (2 x 1) reconstruction. The force of coulomb repulsion prevents several As dimers aligning side-by-side in the <110> direction. In fact it is energetically favourable for only 2 As dimers to exist side-by-side and the next two are missing. This creates a pattern with a repeat period of (2 x 4). Since the Ga reconstruction appears at right angles to the As reconstruction, the Ga reconstruction is a (4 x 2). These are the main two reconstructions and along with c(4 x 4) they are the most commonly created and most useful reconstructions. They depend on both the As/Ga flux ratio and substrate temperature and can be used to create a static reconstruction map and to define when the As/Ga ratio = 1.

In order to do this you use RHEED. RHEED is a very useful tool to an MBE grower. It allows you to see a reciprocal space representation of the real space surface reconstruction. Though the fact is you do not need to know you are creating a reciprocal space reconstruction or even that RHEED pattern is in reciprocal space. In order to achieve a practical working knowledge of RHEED you simply need to know three things:

  1. the substrate surface acts like a diffraction grating to electrons
  2. when something gets bigger in real space, it gets smaller in reciprocal space
  3. when something is n times as far apart in real space, it is 1/n times as far apart in reciprocal space

The RHEED pattern can then be considered a diffraction pattern. Typically you would align the RHEED beam with the <110> and <-110> directions and you would observe the diffraction pattern created by each. First consider the surface of Figure 3a. Regardless of whether  you view the surface in the <110> or the <-110> direction As atoms are evenly spaced. This would create a diffraction pattern consisting of only the primary (zero order) rods in both directions: a so called 1x (one-by) reconstruction. The RHEED pattern you would observe in each direction is shown diagrammatically in the figure. Since the real space (1 x 1) reconstruction spacing is proportional to a, the reciprocal space RHEED pattern spacing is proportional to 1/a. So when you look at a substrate with a bigger lattice constant, like InAs, the zero order RHEED pattern spacing is smaller.

Now consider Figure 3b. When you align the RHEED beam in the <-110> direction the pattern would be the same 1x, since the As atoms still possess the same spacing. However when you look in the <110> direction the pattern would appear as a 2x pattern. That is doubling the pattern in real space leads to a halving of the spacing between the RHEED pattern streaks in reciprocal space. Another way to think of it is a doubling of the number of streaks in the RHEED pattern. Note that the 2x occurs when you align the RHEED beam in the <110> direction (termed the <110> azimuth) and this indicates the atoms have rearranged (or moved) in the <-110> direction. This is because you are using the sample to create a diffraction grating at right angles to the electron beam.

So for the As rich (2 x 4) surface, the <110> azimuth is 2x, and the <-110> azimuth is 4x. Similarly for the Ga rich (4 x 2) surface, the <110> azimuth is 4x, and the <-110> azimuth is 2x.

How to growth your first sample: Oxide remove

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 Asand 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!

Oxide remove RHEED

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.