Preparation before growth

 Substrate annealing

Degassing the substrate usually takes at least several hours. Tiancong degases HOPG for 6h using 600 C. Shujie degas SiC overnight at 650 C. One way to tell whether the substrate surface is cleaned is by the sharpness and brightness of its RHEED pattern.

Start RHEED

Evaporation source degassing and flux calibration

Start degassing the sources using evaporation parameters 1-2h before evaporation with shutters closed. Then, when you are ready for growth, simply open shutters.

Measure Source flux with QCM and record the numbers. These are important growth parameters that have to be tracked for each growth. For 1T’-WTe2, Tiancong uses W 0.05 A/min and Te 7 A/min. The Te:W flux ratio needs to be 20-100 in order to have enough Te on the surface, because Te does not stick to substrate at all at this temperature. W flux is used to control the growth rate. Assuming all W atoms stick to QCM and substrate, one can calculate the growth rate.

Growth

General remarks

MBE growth works best when the number of elements in the to-be-grown compound is small (1 or 2 is bad, 3 can already be difficult).

Growth substrates selection

Growth parameters and growth recipe

During MBE growth, all you can control are flux of atoms/molecules and sample temperature. Essentially, you can imagine each of them being a time-dependent function and your growth recipe is nothing but a set of functions during the growth time.  

The simplest and the most common growth recipe is to keep all functions (substrate temperature and flux) independent of time, i.e., you wait for substrate temperature to be stable, open all shutters at the same time, and after growth, close all shutters at the same time and then ramp down the substrate temperature.

However, there are other growth tricks. One common trick is called growth interruption, where the source shutters are turned off after the growth of one layer from a certain period of time. The idea is that the just-grown layer can reorganize itself through different processes before it is capped by a subsequent layer. This is often used in III-V semiconductor growth.

What can act as nucleation centers during growth

1. Dirt on the substrates.
2. Dirt from sample plates, especially if they were used for previous growth of other materials. When you heat up the sample, the previously-adsorbed atoms/molecules can come off the substrates and stick to the substrate surface. This problem is especially severe if you have used the sample plate for materials of a high vapor pressure (such as Se), but now would like to use it for growth of a low vapor pressure material (such as Te). The simple solution is to use different plates for different samples. The other way is to clean the sample plates before growth, for example by Br water solutions or filing the top surface off.
3. Dirt from evaporation source.  
4. Dirt from vacuum.
   In general, since the dirt absorption rate is near constant, the slower your growth is (the longer time the growth takes), the more dirt the surface will absorb.

Growth rate

What do evaporated atoms/molecules do on surfaces

They do not stick to the surface and desorb.
They absorb and react with surface.
They absorb, do not react with surface, and form an amorphous structure on their own.
They absorb, do not react with surface, and form a crystalline structure on their own.

If there are several types of molecules/atoms co-evaporating onto a surface, in addition to doing their own things, they can also react with each other and form amorphous or crystalline compound structures. The desired result of MBE growth is such crystalline compound structures.

Post-annealing

Post-annealing is usually performed to enhance sample quality through one of the following processes:

1. Fill in the vacancies.

For example, TMDs are usually post-annealed while keeping the Se or Te flux. This can cause Se or Te to find the vacancies and form bonding with transition metal atoms. (The Se or Te vacancies tend to occur during growth because Se or Te has a much higher vapor pressure than transition metals and can easily fly away.)

2. Smoothen sample surface due to diffusion.

For example, after sputtering gold, we post-anneal gold to let gold atoms diffuse and form smoother surfaces.