Chapter 3 - Fabrication II: Heterostructures and Devices Flashcards
Describe the process of making vertical heterostructures by colamination technique and stacking, using hBN and graphene as an example.
1) Mechanical exfoliation of hBN onto silica.
2) Mechanical exfoliation of graphene on top of PMMA and water soluable polymer layer. Dissolve this in DI water, and fish the hydrophobic PMMA membrane with graphene out, and adhere it to glass substrate.
3) Bring graphene and hBN in contact, wile heating substrate (to drive off water).
4) Dissolve the PMMA with acetone.
5) Rinse and repeat.
What is viscoelastic stamping?
Viscoelastic stamping is a method to create vertical heterostructures of 2D materials. In this process:
i) Flakes are transferred onto the stamp by exfoliation with tape.
ii) The stamp is turned upside down.
iii) The flake is alligned on top of the traget substrate.
iv) The stamp is pressed against the substrate.
v) The stamp is peeled off very slowly.
vi) Sucess! The flake has been transferred.
Name some advantages with the viscoelastic stamping process for making vertical heterostructures.
- Dry transfer process.
- No need for resist and solvents.
- no resist and solvent residues on the surface or in between the stacked layers.
- Position with an accuracy of approx. 1 µm (perhaps a disadvantage?)
What are the requirements for heteroepitaxy of lateral heterostructures?
- Identical lattice symmetry.
- Similar lattice constants (within 4%)
- Avoidance of passivation of edge sides after first growth step (no transfer to air or into second growth chamber).
What heterostructures are so far possible for lateral heterostructures?
MoS2, MoSe2, WS2, WSe2
Mo, W + S, Se
How would you grow lateral heterostructures containing W?
Suquential evaporation of WS2 and WSe2 in a furnace.
How would you grow lateral heterostructures containing Mo?
Use MoO3, S and Se as precursors. Place substrate directly on the top of the MoO3 vessel, facing down. Place S at the edge of the heated zone of the CVD reactor and Se in the adjacent place at the cold side. Switch Se and S for growth step 2.
What is formed when making a lateral heterostructure of WSe2 and WS2?
A lateral p-n-junction. WSe2 is intrinsically p-type, WS2 n-type. Contact each region separately by nanolithography. Diode-like electrical characteristic (rectifying behaviour in I-V measurements).
What does the term ‘mesa’ signify in nanopatterning?
‘Mesa’ is the entire active device region.
What is the lateral pattern size with e-beam lithography?
Larger than 10 nm. With standard methods larger than 30 nm (new in NanoLab claims 6 nm).
In a short sentence, what is RIE?
Reactive Ion Etching: ions are accelerated by E-field torwards sample surface, physical etching method.
