+ \item \begin{itemize}
+ \item Photon momentum insufficient.
+ Momentum contribution of phonon (lattice vibration) required.\\
+ $\Rightarrow$ Probability of transition very small.
+ \item Recombination energy of electron-hole pairs most probably
+ released as vibrational energy of the lattice.\\
+ $\Rightarrow$ Only direct band gap semiconductors suitable for
+ effective photon generation.
+ \end{itemize}