Interpretation of the Temperature-Dependent Transport Properties of GaN/Sapphire Films Grown by MBE and MOCVD

The implications of spontaneous polarization effects for carrier transport measurements in GaN

“…Gallium nitride can crystallize in either a zinc-blende or a wurtzite structure, and for the most commonly encountered situation of material grown epitaxially on sapphire, it is the wurtzite phase which dominates. Measurements of the dimensions of the unit cell of this phase [1] have shown that it departs from the expected c:a ratio of 8 3 (=1.6330) for perfectly hexagonally close-packed atoms, and has a value of 1.6262. Because of the partly ionic nature of the Ga-N bond, and the lack of inversion symmetry, this results in a net dipole moment across the cell.…”

Observation of thermally activated conduction at a GaN–sapphire interface

“…[1][2][3][4][5] The similarity between the low temperature carrier density and the thermally activated value obtained at 300 K ͑Ref. 3͒ suggests that the mechanism is ionization of electrons from an impurity band, 1,2 whereas measurements on layers of different thickness 4 and differential Hall profiling 5 have suggested that the metallic conduction occurs in a high carrier density interface layer close to the substrate.…”

“…The presence of a minimum at ϳ90 K is characteristic of samples containing parallel conduction mechanisms, namely, in the conduction band and in an undefined quasimetallic channel. [1][2][3][4][5] An important point to note from these results is that, apart from some scatter in the data at low temperature, there is essentially no change in the curve for the first four etches, which remove 2.05 m of GaN. This implies that ͑a͒ the material removed was making no contribution to the conductivity of the sample, i.e., it was insulating, ͑b͒ some portion of the remaining 0.65 m is thus responsible for all the conduction measured, and ͑c͒ this conductivity is thermally activated.…”

A study of the morphology of GaN seed layers on in situ deposited SixNy and its effect on properties of overgrown GaN epilayers

“…It is expected that the porous Si x N y layer will supply sufficient density of Si atoms to the bottom face of the overgrown GaN epilayer, which may result in formation of a degenerate impurity band. Furthermore, this kind of V-shaped carrier concentration profile has been observed in typical GaN thin films grown on c-plane sapphire substrates [16][17][18]. In case of GaN thin films grown on sapphire, the V-shape carrier concentration profile with a minimum at about 90 K has been attributed to a parallel conduction through the conduction band and a quasimetallic degenerate channel [16][17][18][19][20][21].…”

“…Furthermore, this kind of V-shaped carrier concentration profile has been observed in typical GaN thin films grown on c-plane sapphire substrates [16][17][18]. In case of GaN thin films grown on sapphire, the V-shape carrier concentration profile with a minimum at about 90 K has been attributed to a parallel conduction through the conduction band and a quasimetallic degenerate channel [16][17][18][19][20][21]. The conduction through the quasimetallic channel can be related to an impurity band in a highly defective GaN/sapphire interface region which is heavily doped by oxygen outdiffusion from the sapphire substrate [21,22].…”