# Cleaving Semiconductors for (Blue) Lasers Diodes

| The module is based on the paper of Kuramta et al: in FUJITSU Sci. Tech. J. 342 (1998). p.191. The authors provide data about cleavage planes in semiconductors.
|  | The preferred cleavage planes of a semiconductor are not as clear-cut as it seems. As everybody know who just once dropped a {100} Si wafer knows, the fracture plans are the {110} planes; a large part of the literature, however, including the paper given above, insists that it should be {111}.
|  | Be that as it may, here we take the planes given in the article.
From the Ref. given above Comparison always to GaN
| GaN as a semiconductor technology material only comes as a thin layer on a substrate other than GaN since there simply are no usable GaN single crystals
|  | As we know, if we grow thin layers with different lattices, we have to watch out for misfit dislocations. It is important to look for substrates with a lattice constant as similar as possible to that of the thin layer to be grown. The table above shows the lattice mismatch of prospective substrates to GaN and thus gives a guideline.
| If we want to make a Laser diode form the thin film, we have a few more requirements besides "just" avoiding misfit dislocations as best as we can:
|  | The substrate should have a high electrical and thermal conductivity. The first property would make it easier to supply the large current densities we need to operate a Laser diode, the second to remove efficiently the heat generated during operation.
|  | The whole stack of substrate and layers should cleave nicely on a well-defined and very flat plane because the two relevant surfaces obtained by cleavage will serve as the mirrors of the Faby-Perot resonator we need for a Laser. Now look at the possible cleavage relations:
From the Ref. given above
| Summing up: There is no ideal substrate - you have to find the optimal compromise once more if you wan to make the blue Laser diode.

With frame

9.2.2 Laser Diodes

© H. Föll (Semiconductor Technology - Script)