Optical absorption on localized levels in gallium arsenide

The extrinsic optical absorption associated with defects in GaAs has been studied between 4 and 300°K in the range from 0.6 to 1.5 eV. Two bands in particular are investigated: the first located around 0.9 eV is associated with chromium; the second at 1.2 eV, present in all materials, is probably related to some native defects such as gallium vacancies. The variations of the energy and of the shape of these two bands versus temperature demonstrate that they cannot come from transitions between extrinsic levels and the energy bands of the perfect crystal. The absorption curves of each line appear to be Gaussian at every temperature. As a consequence of lattice coupling, the width of these bands increases greatly with temperature according to a law: $W\left(T\right)=W\left(0\right)\mathrm{}$ ${\left[coth\right(\frac{h\nu g}{2kT}\left)\right]}^{\frac{1}{2}}$. Finally, it is shown that a configurational-coordinate diagram may be applied to these two centers. On this basis, we give the values of the quantum energy of vibration of the centers in their ground states $h\nu g:13\mathrm{}$ meV for the chromium center, and 15 meV for the center at 1.2 eV. For this last one, we propose a configurational-coordinate diagram assuming the parabolic approximation to be valid. These results are discussed and compared with those obtained by photoconductivity or photoluminescence in GaAs, taking into account the important Stokes shift resulting from the lattice coupling we mentioned above. In particular, it is shown that the center absorbing at 1.2 eV is quite different of that emitting at 1.2 eV, i.e., the $V_{\mathrm{Ga}}:D$ complex for which Williams has already proposed a configurational-coordinate diagram.