Tutorial 3 - MOCVD - bubblers (©Rensselaer Polytechnic Institute)
A bubbler is a container filled with a metal-organic (MO) precursor. For the growth of
III-V nitride semiconductors, there are usually five types of MO precursors, which are
trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMAl), triethylgallium
(TEGa), and bis-cyclopentadienylmagnesium (Cp2Mg).
As is shown in Fig. 1, the MO precursor is contained in the bubbler. The carrier gas (N2
or H2) flows into the bubbler, dissolves the MO precursor to form a saturated gas solution, and
flows out of the bubbler.
When installing a new bubbler or deactivating an old bubbler, the sequence for
opening/closing the inlet and outlet valve is very important. When opening the valves, the outlet
valve should be opened first, so that the gas pressure inside the bubbler can be released through
the outlet. If the inlet valve is opened first by mistake, the gas pressure inside the bubbler may
push the liquid MO precursor out of the bubbler through the inlet. This must never happen.
When closing the valves, the inlet valve should be closed first before closing the outlet valve, so
that the gas pressure inside the bubbler can be released through the outlet.
The vapor pressure of a MO precursor depends on the temperature. In order to keep a
constant vapor pressure inside the bubbler, the bubbler is placed in a bath whose temperature is
precisely controlled by a thermostat. The accuracy of the temperature control is usually 0.1°C.
Here are some additional facts:
• When installing a new bubbler, wait until the temperature of bubbler has stabilized.
• The bubbler housing is usually made from stainless steel. The interior is electro-polished to provide passivation so that it does not react with the MO precursor.
• Typical bath temperatures for MO precursors are:
| Name | TGMa | TEGa | TMA1 | TMIn | Cp2Mg |
| |
| Temperature (°C) | 0 | 17 | 17 | 17 | 17 |
• When installing a new bubbler, wait until the temperature of bubbler has stabilized.
The bath in which the bubbler is placed is 50% water mixed with 50% coolant (we
use
automotive antifreeze). The coolant is added to prevent metal corrosion and freezing for
bubblers kept at 0°C.
• TMGa, TEGa and TMAl are liquids, while TMIn and Cp2Mg are solids at the typical bath temperature.
• MO precursors are usually flammable and toxic.
• Typical MO mass of a bubbler is 50g, 100g, or 250g.
• MO precursors are expensive. Typical list prices for 100g MO are:
| Name | TGMa | TEGa | TMA1 | TMIn | Cp2Mg |
| |
| Price ($) | 1800 | 2700 | 2300 | 6400 | 2800 |
• As soon as the carrier-gas/MO-precursor solution leaves the bubbler, it is strongly diluted
by adding additional carrier gas. This is to prevent precipitation and possible deposition
of the MO precursor on the inside wall of the gas lines.
• The definitions of solute, solvent, and solution are as follows:
Solute: A substance dissolved in another substance, usually the component of a solution
present in the lesser amount. Examples: salt in water, where salt is the solute.
Solvent: The component of a solution that is present in the greatest amount. It is the
substance in which the solute is dissolved. Examples: The solvent for sea water is water.
The solvent for a MO precursor is nitrogen or hydrogen.
Solution: A homogeneous mixture of two or more substances, which may be solids,
liquids, gases, or a combination of them. Examples: saltwater (water plus salt) and air
(dry air plus water).