Raman and IR Studies of GaN

Infrared reflection and Raman spectroscopies have been employed to derive zone centre and some zone boundary phonon energies in wurtzite and in zincblende GaN (TABLE 1) [1-10]. Phonon and coupled modes have been employed to characterise stress conditions and carrier densities in thin films and device structures. 

The wide spread of studied material has led to some uncertainty in phonon frequencies, especially of the LO modes. Recently, however, the coupling to plasmons in doped material and stress induced effects due to lattice mismatch with the substrate have been separated. Aj(LO) lies close to Eg in sapphire and has been confused in Raman experiments. In 2 pm GaN/sapphire (0001) [9] modes are within 1 cm 1 of values in bulk GaN (TABLE 1). 

Phonon modes in cubic GaN on GaAs (001) have been identified in Raman and infrared absorption (TABLE 2) [12,13], Relative intensities have been used to identify fractions of cubic or hexagonal inclusions [14]. Phonon modes of GaN with isotopically enriched 15N have been studied very recently [15]. 

The angular dependence of the quasi-TO v(qTO) mode between Aj and E geometries fulfils the relation v(qTO)2 = [v(Ei) cos ]2 + [v ) sin ]2 [16]. Phonon spectra in superlattices of cubic [17] and wurtzite [18] GaN/AIN have been calculated. In an interpretation of second order Raman spectroscopy the following modes have been assigned (TABLE 3) [19]. 

TABLE 3 Experimental Raman modes interpreted in second order Raman processes [19]. 

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A3.2 Raman and IR studies of GaN TABLE 2 Phonon modes in cubic zincblende GaN. 

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