1.2.1 Chemical Properties of GaN
Since Johnson et al.  first synthesized GaN in 1932, a large body of information
has repeatedly indicated that GaN is an exceedingly stable compound exhibiting
significant hardness. It is this chemical stability at elevated temperatures combined
with its hardness that has made GaN an attractive material for protective coatings.
Moreover, owing to its wide energy bandgap, it is also an excellent candidate for device
operation at high temperatures and caustic environments. Although the hardnessmay
have initiated the interest inGaN, it is the excellent semiconducting features that have
piqued the attention of researchers. While the thermal stability ofGaN allows freedom
of high-temperature processing, the chemical stability ofGaNpresents a technological
challenge. Conventional wet etching techniques used in semiconductor processing
have not been as successful for GaN device fabrication. For example, Maruska and
Tietjen  reported that GaN is insoluble in H2O, acids, or bases at room
temperature, but does dissolve in hot alkali solutions at very slow rates. Pankove 
noted that GaN reacts with NaOH forming a GaOH layer on the surface and
prohibiting wet etching of GaN. To circumvent this difficulty, he developed an
electrolytic etching technique forGaN. Low-qualityGaN has been etched at reasonably
high rates in NaOH [142,143], H2SO4 , and H3PO4 [145–147]. Although these
etches are extremely useful for identifying defects and estimating their densities in
GaN films, they are not as useful for the fabrication of devices .Well-established
chemical etching processes do help for the device technology development, and the
status of these processes in the case of GaN can be found in Volume 2, Chapter 1.
Various dry etching processes reviewed by Mohammad et al.  and Pearton
et al.  are promising possibilities and are discussed in Volume 2, Chapter 1.