Kinetics of radiative recombinations in GaSe and influence of Cu doping on the luminescence spectra

Spontaneous photoluminescence (PL) spectra of Cu-doped and undoped $\epsilon$-GaSe have been investigated in the temperature range from 80 to 300 K and at low laser-excitation intensity ($P$) from ${10}^{-3\mathrm{}}$ to 10 W ${\mathrm{cm}}^{-2\mathrm{}}$. The main modification of the spectra in doped crystals with respect to those of undoped samples is the appearance of two bands in the extrinsic part of the PL spectrum and centered at 655 and 678 nm, respectively. The luminescence at energies below the excitonic recombinations (extrinsic bands) is enhanced by doping. Also indirect free- and bound-excitonic lines are also strongly influenced by the impurity concentration; in fact, their emission intensity, which depends linearly on $P$ in undoped crystals, shows a quadratic dependence in doped samples. The temperature dependence of the PL spectra gives the thermal activation energy of some extrinsic bands, which results equal the ionization energy of the acceptor levels involved in the extrinsic transitions. A simple kinetic model of the radiative recombination is proposed; it accounts for the experimental data of the excitation intensity dependence of the free- and bound-excitonic lines. This model can also explain the different temperature dependence of the PL intensity of these lines: linear for the free-excitonic emissions, exponential for the bound-excitonic recombinations. Some radiative transitions from donor levels located in the energy gap of GaSe are analyzed and a scheme of donor and acceptor states involved in the PL spectra is proposed.