Raman analysis of phonon lifetimes in AlN and GaN of wurtzite structure

Raman analyses of the lifetimes of phonons in GaN and AlN crystallites of wurtzite structure are presented. In order to ensure the accuracy of the measurement of the phonon lifetimes, an experimental procedure to eliminate the broadening due to the finite slit width was performed. The lifetime analyses indicate that the phonon lifetimes in AlN as well as in GaN fall into two main time regimes: a relatively long time of the $E_2^1$ mode and much shorter times of the $E_2^2,$ $E1\left(\mathrm{TO}\right)\mathrm{},$ and $A1\left(\mathrm{TO}\right)\mathrm{}$ modes. The lifetimes of the $E_2^1,$ $E_2^2,$ $E1\left(\mathrm{TO}\right)\mathrm{},$ $A1\left(\mathrm{TO}\right)\mathrm{},$ and $A1\left(\mathrm{LO}\right)\mathrm{}$ modes of an high-quality AlN crystallite are 4.4, 0.83, 0.91, 0.76, and 0.45 ps, respectively. Moreover, the lifetime of the $A1\left(\mathrm{LO}\right)\mathrm{}$ mode found in this study is consistent with the current phonon-decay model of that mode in wurtzite structure materials. The lifetimes of $E_2^1,$ $E_2^2,$ $E1\left(\mathrm{TO}\right)\mathrm{},$ and $A1\left(\mathrm{TO}\right)\mathrm{}$ of a GaN crystallite were found to be 10.1, 1.4, 0.95, and 0.46 ps, respectively. The $A1\left(\mathrm{LO}\right)\mathrm{}$ mode in the GaN was not observed and its absence is attributed to plasmon damping. The lifetime shortening due to impurities was also studied: the lifetimes of the Raman modes of an AlN crystallite, which contains about two orders of magnitude more Si and C impurities relative to the concentration of the high-quality crystallite were found to be 50% shorter.