Question: Quantum Dot Laser - Energy Band

[ulver48]

Hello

I am new here, so it's nice to meet you. I study quantum dot lasers and there is something I don't understand about the energy band diagram. Quantum dots are formed with the Stranski- Krastanov method. For example small InGaAs islands of quantum dots are created on a substrate of InGaAs which behaves as a quantum well . The resulting band diagram is shown in the following picture.

I also know that due to quantum confinement the energy gap of a material gets bigger. So if the InGaAs quantum well has a smaller energy gap than the quantum dot due to quantum confinement , why does it's energy gap in the above picture look bigger than the energy gap of the quantum dot ? Shouldn't the energy gap of the quantum dot be larger due to the relation below if the quantum dot and the quantum well are made from the same stuff? Maybe I don't understand band diagrams very well. Thanks for your time.

 

[ulver48]

Well I think that the quantum dot consists of InAs which has a lower energy gap than the InGaAs of the wetting layer, so that's the reason why that happens.

 

[Cthugha]

No, the wetting Layer also consists of InAs. It is the remainder of the thin InAs sheet, which did not form QDs. You may treat it as somehow similar to a QW, though.

However, you have neglected the exciton binding energy. While stronger confinement will increase the kinetic energy terms, the Coulomb interaction between electron and hole will increase as well due to their short separation and therefore the binding energy will increase. These effects compete and the sum of both terms determines the ground state energy of the QD.