For vertical GaN based transistors, Mg with a substantial doping concentration is used as Current Blocking Layer which needs to be activated. I am not sure how the activation thing works here. Can anyone please explain me what is activation and how is this activation achieved?
In general activating a metal means to remove its oxide (passivating) layer. Magnesium definitely has a strong tendency to form a MgO layer on its surface. There are several ways to remove it, for instance etching, remelting or a "simple" mechanical removal. You should do this in an inert atmosphere to avoid instant re-oxidation of course.
Thank you very much for your reply.. in extension, what would 1% active at RT for Mg doping mean??
In my limited knowledge, Mg in GaN form compounds with hydrogen, and this is why Mg in GaN does not work as an acceptor. Activation i.e. annealing removes the hydrogen, thus Mg atoms work as acceptors. This description is in the patent by Dr.Nakamura.
Formation of MgO does not occur because there is no oxygen in the process of GaN production.
Wait, Sebastian: This question is about activating a dopant material, not a metal! In GaN, Mg may act as a p-type dopant -- which means that just adding Mg to GaN is not sufficient to obtain p-type GaN. Only after activation, Mg-doped GaN shows the desired p-type conductivity.
Remember that for a long time, only n-type GaN existed (which is typical for a semiconductor with a large band-gap energy; see, e.g., also ZnO). It was really tricky to obtain p-type GaN (it took many years to get it, and there was a Nobel prize for it: http://onlinelibrary.wiley.com/doi/10.1002/andp.201500802/abstract; see also https://www.researchgate.net/publication/275588803), and one of those tricks is to introduce a processing step that makes the dopant material Mg electrically active, i.e. to make at least a certain fraction of the Mg atoms act as acceptors -- which requires a certain amount of energy, known as the activation energy. Here are some "historical" papers about it:
-- "P-Type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation (LEEBI)" (1989), http://iopscience.iop.org/article/10.1143/JJAP.28.L2112 (free article; see also https://www.researchgate.net/publication/234520327)
-- "Thermal Annealing Effects on P-Type Mg-Doped GaN Films" (1992), http://iopscience.iop.org/article/10.1143/JJAP.31.L139 (see also https://www.researchgate.net/publication/248677447)
Another tricky thing is that the growth mechanism used for GaN, e.g. MBE or MOVPE, also has an influence on the Mg incorporation and activation. Read more about it in, e.g., "Mg doping of GaN by molecular beam epitaxy" (2011), http://iopscience.iop.org/article/10.1088/0022-3727/44/13/135406 (see also https://www.researchgate.net/publication/253510648)
Article P-Type Conduction in Mg-Doped GaN Treated with Low-Energy El...
Article Thermal Annealing Effects on P-Type Mg-Doped GaN Films
Article Mg doping of GaN by molecular beam epitaxy
Article Growth of GaN on sapphire via low-temperature deposited buff...
Dear Jan-Martin, thanks for your explanation. I was not aware that there is a special meaning of "activation" in the field of doping. I just had a definition of "active Mg" in mind - coming from electrochemical point of view. Sorry for possibly misleading you, Ifat...and thanks again for correcting me Yoshiki and Jan-Martin!
Thank you very much Sir, Jan-Martin Wagner. It was of a great help to me. Thank you very much, Sir. Really Grateful.
Thanks to others for helping me out too. :)