Faebian Bastiman
Pyrolytic Boron Nitride (PBN) crucibles for MBE are actually grown in a process not too dissimilar from III-V MOCVD deposition. The pyrolytic process involves depositing crystalline ceramic boron nitride on a carbon mold (mandrel). The PBN crucibles are therefore very high purity, nonporous and possess smooth sidewall (no pores). The PBN is stable up to around 1400°C and is resistive to HF acid.
Each III, V and dopant element has its own particular relationship with PBN, some of them are neutral conscientious objectors, other treat PBN with open hostility! The following article highlights the necessary steps to protect your PBN crucibles from the common III-V MBE elements.
First we have the friendly elements: P, As, In and Be. None of these elements pose any risk to PBN and as such the sources can be operated without additional consideration for the PBN-element relationship and interactions. In is the only nice group III. Aside from passively “spitting” during operation it is essentially neutral. A cell operated with a large thermal gradient from top to bottom (hot-lip) helps reduce the buildup of In near the cell’s orifice and prevents spitting affecting the substrate surface.
In contrast Al, Ga, Si, Sb and Bi each require additional care when situated inside a PBN crucible. Al can readily shatter PBN. The situation is unique amongst III-V elements and pertains to Al’s ability to wet PBN. Liquid Al placed in the bottom of a PBN crucible gradually “creeps” up the sidewalls. The cell must be operated with a cold-lip to prevent Al climbing out of the cell and shorting the heater filaments. Ga and In remain at the bottom of the crucible, thus as the material is depleted the flux drops over time for a given temperature. The thin and complete Al coverage provides a large and constant surface area, such that Al cells do not suffer from flux drops for a given temperature. This can make it difficult to predict when an Al cell is running out. Finally Al has a very different thermal expansion rate to PBN. The wet PBN cannot contract as fast as its Al film, hence cooling Al too rapidly from standby (850°C) to solid (660°C) can readily shatter the crucible. It is common practice to utilize very, very slow temperature ramps when cooling through this range. For example 0.05 °C/min, equating to ~63 hours. Once the cell falls below ~600°C, and you can be certain the Al is solid, a faster ramp can be used. It is therefore a very good idea to fully deplete the Al cell before cooling it down. Moreover, it is good practice to replace the PBN crucible every time the cell is cooled. A double walled crucible or a crucible insert offer a buffer against the damaging effect of cooling Al too rapidly. You can further buffer against damage by having the Al cell on an uninterruptable power supply (UPS), having a backup DC power supply unit (PSU) that automatically takes over if the primary supply fails and by utilising a backup water cooling system rather than simply crashing the cell temperature upon primary water cooling failure (see MBE: water cooling system).
The next dangerous element is Ga. Ga, like Si, Sb and Bi (and the common compound water), possesses a lower density (they expand) upon freezing. Ga decreases in density by around 3.1% upon freezing. There are only 4 elements that expand upon freezing and in III-V MBE we encounter them all. What luck! Liquid Ga sitting at the bottom of a crucible first freezes at its exposed upper surface creating a nice air tight cap against the PBN, then as the main Ga liquid freezes it expands sideways cracking the PBN like freezing water in a pipe. Luckily Ga freezes at around 29°C, this means that holding the cell at 50°C at all times (even during maintenance and air exposure) protects the PBN against the damaging effects of freezing Ga. Ga cells can be protected in an identical manner to Al cells, however you can also utilize a “water-heater-loop” on the Ga cell to maintain 50°C by supplying 50°C water through the cell’s water cooling element in the event of a DC PSU failure or other electrical power fault.
I avoided discussing Si along with Ga, because Si is more dangerous. Not only does it freeze and expand (decreasing in density by a massive 10%), it also etches PBN when liquid. The simple solution to both problems is to keep the Si solid. Si melts at 1414°C, which is inside the “danger zone” for PBN temperature operation. In some circumstances it is advised to melt Si on the first usage, for example when wanting to utilize a “downward facing” cell port, Si can first be melted in an “upward facing” position and upon freezing will remain rigidly fixed inside the PBN crucible even when facing vertically downwards and physically shaken. The etching effects of liquid Si on PBN will be minor for quick, one time melting, however this does preclude PBN crucibles from being utilized with high Si deposition rates. In those cases either an e-beam or a Si-filament source are required. The best advice I can offer is to only outgas the Si cell to 1275°C maximum, avoid melting the Si and hence avoid any dangerous effects.
Next we have Bi. Bi decreases in density by a moderate 2.8%. Not as dangerous as Ga, but still posing the risk of crucible cracking. It also freezes at 271°C, so there is no chance to keep it melted during maintenance. The best practice here is to be careful. Never over load your crucible. You will be amazed how long Bi actually lasts. A 10 – 20g charge at the bottom of a crucible out lasts all other III-V elements; largely because it is only used in dilute quantities in III-V epitaxy. It is good practice to fully deplete the Bi source before cooling. This means accurately predicting your Bi usage to coincide with maintenance periods. Using rounded bottomed crucibles is also good practice, as they seem less susceptible to cracking. Finally, like Al, go through the freezing point slowly. I use 0.05°C/min when cooling from 325 to 250°C.
Finally we have Sb. Sb presents the smallest density decrease upon freezing. A mere 2.6%. Luckily it has a relatedly large vapour pressure whilst solid. A valved Sb source produces a suitable Sb flux for III-Sb epitaxy with the Sb bulk held at 500°C. This is safely distant from the melting-freezing point of 630°C. The best advice for Sb, like Si, is therefore to avoid melting it altogether and thereby avoid any dangerous situation.
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