For experiments where very strong or even enhanced terahertz fields are useful and Fabry-
Perot interference is acceptable, the epitaxial transfer process is extremely useful. This
process was introduced to us by Garrett Cole of Crystalline Mirror Solutions.
Instructions
Before going into the clean room:
• Sketch the wafer geometry, including which direction the flat was. Lapping will remove all reference points, and the sample may break in two.
• Lap down all samples to 150 microns thick. • Dice sapphire to appropriate size(s).
Once in the clean room:
• Clean 1165-Acetone-Isopropanol-DI water-dry N2.
• Do an O2 descum, 100 W, 1 minute to finish cleaning the sample.
• Take a full set of pictures of the growth side of the sample and record the size if any pieces broke off during lapping.
• Heat up on hot plate to 130◦ deg C to finish drying. Silicon makes a good wafer
carrier for this process because thermal contact is occasionally important.
• Put large wax piece on the growth side of the sample, then put the whole wafer carrier on the hot plate, still at 130◦ C. The wax takes a few minutes to melt. If more is needed, remove the wafer to cool so that if the wax falls of the tweezer at
the wrong time, it wont mess up the sample. Take this step slowly as removing
black wax is difficult and very dirty.
• After melting fully, drop the temperature to 100◦ C in 5◦ C increments. Once the
temperature gets to 100◦ C, cover with an aluminum boat and let anneal for an hour.
• Decrease temperature to 80◦ C in 5◦ C increments.
• Lay/balance the silicon carrier wafer or glass slide over the wax. If it is off to one side, do your best to right it while the wax is still soft. Needs to be > 11/4” to use
with vacuum evaporator.
• Drop the temperature to 50-60◦ C in 5◦ C increments, then remove from the hot
plate and let cool completely. The samples should be glued to the wafer carrier
with black wax with the lapped sides exposed
• Prepare 30-100:1 H20:NH4OH and 30:1 H2O2:NH4OH solutions (200:5 and 300:10
mL, respectively, works nicely). We call the latter Garrett Cole’s etch.
• Etch in water-ammonium hydroxide solution for 30 seconds, then immediately place in peroxide-ammonium hydroxide solution.
• Etch until the substrate is removed. It will be obvious as the entire sample will be very smooth. This takes about 90 minutes for 150 microns. Occasionally a white
scum builds up on the substrate, just swab it off in situ.
• While etching, clean a sapphire substrate on a solvent spinner if possible (500-1000 rpm, but substrate may be too small) following this order: (tergitol+swab)-DI-IPA-
DI, Nomarsky microscopy inspect and pictures, repeat until very clean, no dust at
all. Some dots may remain that (often) get annihilated in the descum. Then 200 W
300 mtorr O2 descum the sapphire wafer, 2 minutes. Double check with Nomarsky
microscopy to ensure cleanliness.
• Once peroxide-ammonium hydroxide etch is complete, take pictures, etch in buffered HF until the color is very uniform, no interference fringes, 3 minutes. Every 30-45
seconds, rinse in DI water. Then take pictures of the entire sample.
• After HF, check if any weird dust/filaments are there. If so, wash in AZ400K in a clean beaker, could possibly take 5 minutes. Be patient here, this dust prevents
good Van der Waals contact.
• Add a very small drop of water to the exposed epi, then place the sapphire on top. It works best if the sapphire is appropriately sized. Be very careful that it doesn’t
slide off at first. Wick away any drops of water that are squeezed out with a clean
wipe. Press firmly with the wooden end of a qtip, and you should see Fabry-Perots
with the wipe.
• IF IF IF there is an obvious bubble (of water, not air), it is maybe possible to squeeze the bubble out by covering with a Berkshire wipe and using a qtip like a
rolling pin.
• Dont touch them after that, let the water evaporate. Takes about 2 hours when pulling rough vacuum, or a day or two in air.
• In aluminum boat, dissolve black wax in chloroform. May take a few fresh chlo- roform baths to get it all. Make sure to dispose of the used chloroform solution
safely.
• Once no more black wax is visible, IPA-DI-chloroform-IPA-Ace-IPA-DI- gentle blow dry to clean
There are several improvements that could be made to this process. Using etching to
define the transferred region would be very useful. Dicing introduces far too many micro-
cracks at the edges to be useful, and cleaving is imprecise and can induce microcracks at
the initiating corner. By defining a sample size with a photolithography mask, samples
will be much more uniform and less likely to be severely damaged at the edges. Also, it
may be possible to skip lapping entirely by simply etching the back surface faster. Also,
to ensure an extremely smooth epitaxial bonding interface, more etch stop layers could be
added in growth. A triple layered structure, 73.5% AlGaAs-GaAs-73.5% AlGaAs would