Bandedge and Optical Functions of GaN

The large span of applications of GaN based semiconductors has focused attention on the properties of these compounds. The aim of this Datareview is to quote recent results on the bandedge of GaN. Also, since GaN will be the basis of blue-UV laser diodes, for instance as the guiding layer, the determination of its optical constants will be examined. 

A useful approach to the wurtzite valence band structure is the quasi-cubic model of J.J. Hop field [3], In this framework, the relative energies of the valence band maxima are  

A more detailed k.p description of the valence band dispersion, in particular beyond the quasi cubic model, can be found in [1,4,5,7,8,11,12], The valence bandedge separations are then given by  

TABLE 1 Experimental or theoretical values of the crystal field and spin-orbit coupling terms appearing in EQN (1) for wurtzite GaN estimated to be strain free. 

The preceding bandgap values are given for low (2 - 10 K) temperature (T). Of primary importance is the temperature dependence of the bandgaps, in particular their value at room temperature (RT). From the previous discussion, different T dependencies are expected, due to the different strain state of the samples studied. For GaN on sapphire, both the residual epitaxial strain and the thermoelastic strain are compressive, whereas for GaN on SiC, the thermoelastic part of the residual strain is tensile. This leads to very different temperature dependencies between low T and RT, as shown in FIGURE 1. Note that results different from those shown on FIGURE 1 have also been reported [8], 

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