Little known MBE facts: High Purity Material

Faebian Bastiman

I was asked recently what purity of material a user should use in their MBE system. Here the grower was referring to the number of “N’s”. The N of the material is a reference to its purity and is actually the number of nines: Material that is 99% pure and 1% impure is termed 2N. 99.5% is termed 2N5, the “2N5” tells you it is at least half way to being 3N. Of course this does not tell you what the impurities are specifically, however you can assume they are made up of all the things you do not want in your III-V thin film layers (Zn, Cu, Fe, Sn, Hg, Ca, Te, etc) and a few III-V elements that you do not need to worry too much about (you are, after all, growing III-Vs anyway).The purity of material will ultimately determine the quality of your thin films, since any impurities in your cell material will likely incorporate as unintentional dopants in your layers.

If you are growing metals (for example MnAs) you are not too worried about unintentional doping and you may use a maximum of 5N5 material (99.9995%). If you are growing electronic or opto-electronic grade semiconductors (for example GaAs) you will want higher purity. Ultimately the highest you can get commercially is 9N (99.9999999%), however I would not recommend you all rush to the shops and buy such expensive material for general research. 9N is exponentially more expensive than 8N, 8N is exponentially more expensive than 7N. Similarly the ultimate background doping you can achieve with 9N is an order of magnitude lower than with 8N, and the same applies when comparing 8N to 7N. Before I suggest the appropriate material quality, let’s do a thought exercise with GaAs…

GaAs has a lattice constant of 0.565338 nm, and therefore an atomic density of 4.42 x 1022 cm-3. When opto-electronics people (specifically detector people) talk about background doping requirements they say that 1015 cm-3 is already good.  This means they would like the unintentional doping level to be 5×107 times lower than the atomic density. In this case 7N5 would be the appropriate choice for you group III and V material. One could argue that you should use 7N5 for all materials and that replacing a specific material with 8N is a waste of money. However this “all or nothing” philosophy is not really justified since unintentional doping is accumulative, and hence replacing your group IIIs with 8N might reduce your background doping from high 1015 to low 1015. Thus 7N5 to 8N is a good choice for opto-electronic research, and 9N is only for exceptional high mobility cases or world record attempts. Moreover, if you are simply experimenting with a new opto-electronic alloy and not so interested in device quality at this stage 6N5 is acceptable. Note that the source material is only one factor in your ultimate achievable material quality, for more information read my Optimum Quality post.

Finally consider the dopant material. When we dope a semiconductor we typically dope in the 1016 to 1019 cm-3 range depending on the application. At 1019 cm-3 the dopant is around 104 times lower in atomic density than the III or V species. This means the doping fluxes are around 104 times lower that the group III and V fluxes and hence any impurities introduced into your system from the dopant source will also be 104 times lower. Add to this that when you dope the alloy you are “trying” to add impurities and you can say that 6N is already a very good grade for dopant material and 5N may even be suitable.

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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.

Amazing.

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?

Yes.

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.