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An important aspect of our making of vertical quantum dots is to etch away a thin GaAs cap layer.

Unfortunately, traditional chemical (wet) etching usually comes with anisotropic etching profile, which is undesirable
for the smaller quantum dots that we want.

For example, using H2O/H2O2/H2SO4 wet etching, depending on the crystal axes, the side walls can be very different:

On the right, GaAs etching profile of sulfuric acid etch, with the mask patterned along the (1,-1,0) crystal axis. (Using MIT CMSE's Jeol 6400 SEM, which is kind of old, but still a lovely machine.)

Same etchant and conditions, but only the mask is now along the (1,1,0) axis. See the little "hat" on top of the mesa. (Again, using the JEOL 6400.)

An alternative is to use reactive ion etching (RIE). I'm currently working on converting a plasma asher of our group's to perform this task.

A Halocarbon 22 (HCFC 22) plasma is used as our principle etchant. The florine ions generated from Halocarbon 22 conveniently
provides AlGaAs etching selectivity needed for our vertical dots (no gas mixing required).

The etching profile of the Halocarbon plasma on GaAs is also known to be rather smooth, with a outward slight slope. (See photo on the right, obtained using MIT CMSE's JEOL 6320FV, a very nice field emission SEM, and permits us to get resolution 10x better than the old JEOL 6400) Hopefully, the slope is negligible for my dots... fingers crossed...:)

More details to come soon.