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.

Nanowires: Fourth NW sample

Faebian Bastiman

For our 4^th NW sample we took a little logical leap of faith based on our observations of the 3^rd NW sample and the trend established by the first 3 in general. The decision was made to increase the growth temperature by 40°C AND to double the As flux compared to the 3^rd NW sample. The results can be seen in Figure 1.

a	b
Figure 1: SEM of 4th NW sample

 

The NWs are now around 1000 nm in length but possess a slight small (30nm) diameter. They are only subtly reverse taper (ice cream cone shaped). The NW density has also increased accompanied by a reduction in the parasitic 2D growth compared to the first 3 samples. There are however many tilted NWs, and that is something we wish to eliminate.

On the whole we have taken a big step in the right direction. The result it getting close to our desired NW shape and density profile, whilst the parasitic growth is reducing. Where to go next? Well, it seems that the amount of parasitic growth is directly related to growth temperature, so in the first instance it would be good to increase the growth temperature by 20°C and see what happens. In doing so we will also need to increase the As flux, since the required As flux is strongly related to the growth temperature, especially at these elevated temperatures (>600°C). On the other hand, we have grown 4 samples and not hit an “edge” yet. An edge is a condition that marks the boundary of a NW variable map. Edges are as even more useful that NW data points at this moment, because they define the boundary conditions (max and min): Tgrow, As flux, growth rate, oxide anneal temperature. This concept is explored more in the article Nanowires: Edgework.

For out 5^th sample, let’s increase the growth temperature by 20°C but keep the As flux constant. In so doing we should reach a condition where As is limited AND parasitic growth is reduced/eliminated. The result could be interest. You can see how the 5^th sample turned by reading the following article: Nanowires: Fifth NW sample.

Nanowires: Third NW sample

Faebian Bastiman

After comparing the results from the first and second NW samples, the decision was made to further increase the As flux. The SEM results (Figure 1) indicate that the NWs have indeed doubled in length compared to the first sample. A good indication that the As flux is still slightly deficient. The only way to test this is to further increase the As flux and see if the wires continue to lengthen, whilst maintaining the same diameter. A shorter, broader wire could constitute more NW volume (and hence more GaAs going into the NW). Total GaAs growth is a combination of NW length, area and 2D parasitic growth. Great care must therefore be taken to evaluate the III:V deposition rates.

a	b
Figure 1: SEM image of 3rd NW sample

The 3 samples thus far have something in common. They all have significant parasitic growth. Since we have established that the As flux was too low, we can now assume that the growth temperature is also too low. When one increases the growth temperature, one must also increase the V:III ratio in order to balance As loss. Rather than continue to plod along toward the correct conditions we are going to take a logical leap of faith. We will increase the growth temperature by 40°C AND once more double the As flux. Why do we feel justified doing this? We are justified because…

1. We suspect we are growing too low in temperature
2. The As flux must be increased whenever the growth temperature is increased
3. Even if we fail to grow NWs we gain a data point, a boundary condition. We find a point at which NWs DO NOT form. This is very valuable at the moment, perhaps even more valuable than continuing to produce NWs.

The other possibility for the large amounts of parasitic growth is that the oxide is too thick. To address this we will need to try annealing the substrate at higher temperatures. The images also show a significant number of tilted NWs. This is also a good indication that the oxide is too thick. However, being systematic is also sometimes time consuming. You cannot change to many variables at once or your results will not be closely related enough for you to logically plan your next step. You can of course randomly stab at the problem until you obtain the result you want, however in so doing you display all the scientific prowess of a trained monkey. Let’s try to be clever about this. If we can establish a good algorithm to optimize our samples that is applicable to ALL MBE optimization problems, we can significantly reduce our workload and feel confident that we can expand into ANY and EVERY material combination. We reach the level of experts and more importantly we actually know what we are doing! You can see how our logical leap of faith turned in the following article: Nanowires: 4^th NW sample.

Nanowires: Second NW sample

Faebian Bastiman

After the successful nucleation of NWs, optimisation can begin. The course to optimisation is identical, regardless of the quality of the NW in the first NW sample. Our first NW sample revealed some tell tale features that we would like to address (see Nanowires: First NW sample for more details).

To begin the optimisation, we must first state our desires: I would like (i) to minimise parasitic 2D growth, (ii) to grow straight (non-tapered) wires, (iii) to grow ZB (non-WZ) wires (iv) to grow long (>800 nm) NWs, (v) to grow narrow (10 – 100 nm diameter) NWs, (vi) to achieve a higher (preferably controllable) NW number density.

The variables we have to play with are fairly limited. We have (a) oxide conditioning temperature, (b) growth temperature, (c) III:V ratio and (d) the absolute Ga flux .  These latter 3 represents the Archetypal MBE variable set. It gives us a 3D variable space to step through. For now we can ignore (d) and focus on the first 3 variables. Later, once (a) is established and fixed, we can eliminate it as a variable and concentrate on (d). Once (a), (b), (c) and (d) have been explored we can do more exotic things, like using a different III:V ratio during the nucleation c.f. steady state growth for example. For now we will keep it simple: We do this by fixing two of the three variables (e.g. (a) and (b)), and systematically varying the third (c) over a suitable range. Then we select the best results and fix (c) and (a), and vary (b) over a suitable range. An so on until we have mapped out a wide area and defined some upper and lower limits. Sadly the variables are not independent, particularly every (b) will have an optimal (c).

In order to begin we need to take a logical first step and analyse the result to see if we are travelling in the right direction. In this particular case the first NW sample resulted in NWs of an average 375 nm length. We hypothesised that the As was deficient and decided to increase the As by 33%, whilst keeping all other parameters the same.

The result of our experiment can be seen in Figure 1. In this sample the NWs are on average 500nm long – which is indeed 133% the length. We can conclude that As was deficient and hence can be confident in the taking the decision to increase the As flux further for the next sample. In fact let’s double the As flux compared to the first NW sample.You can find out what happened in the following article: Nanowires: Third NW sample.

a	b
Figure 1: SEM images of GaAs NWs grown self-seeded on Si(111) epiready substrate

Nanowires: First NW sample

Faebian Bastiman

After the initial system calibration (see Nanowires: Pre campaign calibrations) we are ready for our first attempt at self-seeded GaAs NW on Si(111). In this particular case the As flux was tuned to be approximately equal to the Ga flux of 0.11ML/s (0.69 atoms/nm²/s). The epiready Si(111) wafer was loaded into the sample holder (platen) with no ex situ processing. The platen was then placed inside the fast entry lock (FEL) and the FEL was outgassed to 125°C for 3 hours to remove moisture. The platen was then transferred to the preparation chamber’s outgas stage and heated to 400°C for 2 hours to outgas the platen and sample prior to growth. Finally the platen was transferred to the growth chamber, ready to grow.

Well not quite ready to grow. The sample is a single side polished 275 µm thick P-doped n+ Si(111) from CrysTec. It comes in a box of 25, in a batch of 8 boxes all processed at the same time, in the same way from the same ingot. Before NW growth the oxide needs “conditioning” in a very specific manner. The actual process is not well understood, but probably the oxide reduces (in a REDOX reaction) and likely reduces in overall oxide thickness. It is possible small “pin holes” open up in the oxide. Oxide conditioning is critical and whilst it is reproducible in wafers from the same batch, it varies slightly from batch to batch and significantly from supplier to supplier. Hence it is a good idea to order 200 or so wafers from a single supplier and from a single batch. You can then use 10 or so wafers trying to condition the oxide, and know that once it is established the same method can be used on the remaining 190 wafers. The difference is due to changes in both the doping (which affects the absolute substrate temperature vs thermocouple temperature) and also the composition of the native Si oxide (which will alter the temperature required to condition the oxide).

For this particular Si(111) substrate the oxide was conditioned at 655°C on the pyrometer, which equates to 675°C on the thermocouple. This same thermocouple reading corresponded to 630°C on the pyrometer on a GaAs(100) wafer. After conditioning for 20 minutes, the Si(111) substrate was cooled to 580°C on the pyrometer (592°C at the thermocouple) and the As valve was opened to double the required valve position for 2 minutes. The As valve was then closed to the required valve position and left to stabilize for a further 10 minutes. At this point the RHEED pattern displayed Kikuchi lines and a single 1x spot on the major azimuths zero order diffraction bars.

The Ga cell was then opened and after 19 seconds… The RHEED pattern revealed the twinned zinc blende (ZB) pattern affiliated with ZB NWs. We had NWs on the first attempt: A combination of good fortune and good calibration. In order to prevent disruption from rotation stops, the images of the RHEED pattern were only recorded after 30 minutes, during sample cooling and upon the cessation of growth. The twined ZB RHEED pattern is shown in Figure 1. The upper spot corresponds to the NW’s ZB structure, the lower spot corresponds to its rotated twin. The rotated twin occurs around a stacking fault. A third spot located between the two ZB spots denotes the presence of “significant” wurtzite (WZ) regions. As you can see no significant WZ is present. So we are also fortunate to have stumbled across the parameters to grow primarily ZB phase GaAs NWs. What else does the RHEED tell us?

Figure 1: Twinned ZB NW RHEED pattern after 30 minutes of NW growth

Well the nucleation time was around 19s, which is relatively short. This means the oxide is suitably conditioned and the As:Ga ratio is suitably close to 1:1. The nucleation time can be optimized later by increasing the oxide conditioning anneal temperature by 10 to 20°C and adjusting the As:Ga ratio. First one should focus on optimising the NW growth.

One thing that was noted between 5-10 minutes of NW growth was the presence of additional spots between the Si 1x and the NW twinned ZB spots. These are indicative of “parasitic” 2D growth on the oxide. Probably due to the growth temperature being too low or on an incorrect Ga:As ratio. One cannot be sure whether it is Ga or As deficient at this point.

Lastly the twined ZB spots show elongation perpendicular to the Si 1x pattern. This is scattering from the {110} NW side facets. Hence we can assume that the facets are well formed and the NWs will be straight, and hence probably hexagonal.

All that is left to do is to cool the sample down and see what SEM images can tell us. To preserve the Ga droplet on the NW the sample was cooled to 100 °C at 200 °C/min. First the Ga and As shutters were closed simultaneously, then the As valve was immediately closed, then the sample cooled immediately thereafter. If the sample is held under an As flux, the Ga droplet is “consumed” and replaced with heavily faulted (possibly WZ) single crystalline GaAs.

The sample was cleaved along two parallel (111) planes and mounted for SEM imaging using silver paste. An edge on image is shown in Figure 2a with a tilted image shown in figure 2b. There is clearly a lot of parasitic growth, largely in the form of Ga droplets on the surface. Also the wires are very short. 0.11ML/s planar growth calibrated on GaAs(100) equates to around 800nm long NW after 30 minutes of growth. These wires are only 400nm long. The wires are also reverse tapered (ice-cream cone shaped) with a large Ga droplet on top. All these 4 points indicate that the As flux is too low.

a	b
Figure 2: SEM images of GaAs NWs grown self-seeded on Si(111) epiready substrate

In order to perform a systematic study the As flux will be increased by 33% and the substrate temperature will be held constant for the 2^nd NW sample. The results should be NW of a similar density but with 133% the length and similar degree of parasitic growth. You can see how the second sample turned in the following article: Nanowire:  Second NW sample.

Nanowires: Pre campaign calibrations

Faebian Bastiman

Before attempting the first self-seeded GaAs nanowire growth on epiready Si(111)-n, the MBE reactor was thoroughly calibrated. The Group III cells were outgassed (see Post-bake tasks: Cell outgas) and the beam equivalent pressure (BEP) data for each cell was obtained (see Post bake tasks: Group III flux: Arrhenius plots). The BEP data for the As valved cracker source was similarly obtained (see Post bake tasks: As cracker flux). Growth was then performed on an undoped GaAs(100) sample in order to put some meaning to the arbitrary mBar or nA values. The mBar values were converted to ML/s and atoms/nm²/s using the method outlined in Little known MBE facts: Flux determination and the articles linked therein.

GaAs self seeded NW growth is very sensitive to substrate temperature and the As:Ga flux ratio. The actual growth rate is not so important, somewhere around 0.12 ± 0.03 ML/s is a good starting value. This equates to 0.75 ± 0.19 atoms/nm²/s. The sample temperature was monitored by a pyrometer at normal incidence to the substrate. The generated undoped GaAs(100) static reconstruction map (see Little known MBE facts: Making a static reconstruction map) was used to verify the pyrometer reading to with ±10°C and the emissivity of the pyrometer was adjusted to read this absolute value. The emissivity equalled 0.65 in this case.

The flux ratio was established at a substrate temperature of 580°C. At this temperature only 0.05 ML/s (0.31 atoms/nm²/s) of As is required to maintain a (2×4) reconstruction. With the Ga cell set to 0.11 ML/s (0.69 atoms/nm²/s) the As:Ga ratio was tuned such that the RHEED pattern immediately turned (3×6) upon opening the Ga shutter AND THEN turned to (4×2) after around 15 seconds on Ga deposition. This is arguably around 1:1 for this substrate temperature, perhaps slightly Ga rich in fact, but a good starting point for self-seeded NW  growth nonetheless.

With all that done, we are ready to attempt our first self-seeded GaAs NW growth on Si(111) –n epiready substrates. Importantly here for a reference the pyrometer emissivity is not changed (even though the emissivity of Si is indeed different to GaAs). The BEP values of the established 1:1 fluxes were checked and recorded for later reference and the Si(111)-n (CrysTec Si(111) n type (Phosphorus doped), 275um thick SS polished) sample was loaded. You can find out how the first NW sample proceeded in the following article: Nanowires: First NW sample.