Electrochemical pore etching in Ge

While electrochemical pore etching in semiconductors has become a thriving field for research and applications in the past 15 years or so, little work has been done in Ge. Besides Si, Ge is the only semiconductor with a minority carrier diffusion length large enough to enable the use of backside illumination, which has proved to be the decisive “trick” for the formation of excellent macropores in Si. First experiments in Ge have been performed, but Ge proved to behave in rather unexpected ways—the large body of pore etching knowledge obtained with Si and the III–V semiconductors was not directly applicable to Ge. While no good pores could be produced in most previous endeavors including our own, we finally succeeded in producing deep pores in n-type but also in p-type Ge of various doping levels and crystal orientations. In particular, it is now possible to create “good” macropores in a uniform distribution. A host of new and not yet totally understood phenomena was discovered, including pores of various kinds and crystallography. The paper will shortly outline the results obtained and relate it to what is known from other semiconductors under comparable circumstances.