Why Porous III-V Materials and What are the Challenges?

As it was already mentioned in Chapter 1 Porous silicon the investigation of porous Si
was initially triggered by the interesting luminescence properties of nanoporous Si structures.

3.1 Why Porous III-V Materials and What are the Challenges?

As it was already mentioned in Chapter 1 - Porous silicon , the investigation of porous Si was initially triggered by the interesting luminescence properties of nanoporous Si structures. However, at a first glance this argument appears to be irrelevant for III-V compounds, due to the fact, that nearly all representatives of this group of semiconductors show efficient luminescence in their bulk form, i.e. without being porous. The good emission properties of III-V compounds are mainly determined by the fact that the majority of them are direct semiconductors and therefore the radiative recombination of electron-hole pairs is much more efficient as compared with bulk Si, which is an indirect semiconductor. The "majority" was intentionally underlined, because gallium phosphide (GaP), for example, is an indirect semiconductor like Si, nevertheless using special doping techniques it is possible to make GaP a suitable material for light emitting devices as well.

In spite of these counterarguments, soon after the discovery of porous Si, porous III-Vs started to be investigated as well. Paradoxically and against any expectations the main driving force for investigating porous III-Vs were the optical properties as in the case of Si. The scientific community never stops to look for better and more efficient materials. Therefore, porous III-Vs started to be investigated with the aim of discovering new optical properties in addition to the already known from the bulk III-Vs.

III-V compounds offer much more possibilities in this respect as compared with Si, due to the fact that the shift from elementary to compound semiconductors entails a major crystallographic modification and offers the possibility to change their chemical composition. For example, by combining Al with GaAs at certain proportions, a different semiconductor is obtained with a different electronic band gap. Consequently the properties of each of these "mixtures" can be extended by making them porous. Thus, when saying III-V compounds it is not meant only the most known representatives of this group like GaAs, InP, GaP or GaN. Porous III-Vs represent a much larger semiconductor family, including their alloys with other materials. This multitude of materials in their bulk and porous form opens the way for new exciting physical properties.

Although the number of publications devoted to porous III-V compounds is much smaller than that devoted to porous Si, some important properties of porous III-V structures have been reported. In particular, the following new findings merit attention:



  • A sharp increase in intensity of the near-band-edge photo luminescence (PL ) in anodically etched GaP along with the emergence of blue and ultraviolet luminescence [22, 23, 24];
  • A strongly enhanced photo response was observed during pore formation in n-GaP electrodes by anodic etching in sulphuric acid solution [25, 26];
  • Evidence has been found for birefringence in porous InP at wavelengths suitable for optical communication systems [27];
  • Porosity-induced modification of the phonon spectrum was observed in GaP, GaAs and InP [28, 29, 30];
  • A very efficient optical second harmonic generation (SHG ) was observed in porous GaP membranes [31].


However, in order to make use of these interesting properties an exact control of the morphology of porous structures on III-V compounds is necessary. Two primary challenges can be identified in this regard: a) the improvement of the so called nucleation stage of pores and b) the improvement of the stability of pore growth into the depth of the substrate.

The optimization of nucleation etching conditions should be correlated with the etching conditions during the stable pore growth. This means that if the conditions for stable pore growth are applied before the nucleation is completed over the entire sample, then the pores which are not 100 % nucleated will die out. Or vice versa, if the conditions for stable pore growth are applied too late, then branching of pores can play a significant role in destroying the uniformity of the structures.



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