Local vibrational modes

With the improvement of epitaxial growth techniques, impressive advances in nitride-based device technology have been made leading to the development of high brightness light emitting diodes and blue laser diodes. However, many aspects of the physical properties of these materials that are not well understood. In particular, p-type doping is difficult to achieve in GaN and the role played by unintentional impurities and structural defects is being investigated.

Impurity atoms may show up in the vibrational spectrum of the host lattice as local vibrational modes (LVMs). The frequency of the LVMs is very sensitive to the impurity local environment and to the formation of complexes, and therefore Raman scattering of LVMs can provide useful information about the incorporation of impurities in GaN.

Figure 1 shows the in-plane x(yy) Raman spectra of MBE-grown, Mg-doped GaN, where LVMs can be clearly seen (frequencies given in red). The observation of these modes is a direct proof that Mg has been incorporated to the GaN lattice in Ga substitutional positions.

For the first time, we have observed on the same MBE-grown sample LVMs associated with Mg donors, Mg-H complexes [see Fig. 2 (a)], and C-H complexes [see Fig. 2 (b)]. Although commonly associated with the use of organic precursors in MOCVD growth techniques, carbon contamination is quite typical in MBE systems because any hot source (filaments) will produce a strong outgassing. The presence of H and C unintentional impurities, which may be detrimental to the achievement of p-type conductivity, can be readily monitored by means Raman scattering measurements.

Fuente:

http://www.ija.csic.es/labs/raman/node4.html

Gerald Soto, CRF 2010-1.

http://gerald-ees.blogspot.com/