Edge emission and deep centre luminescence in zinc selenide

Abstract

The photoluminescent properties of a wide range of single crystals of zinc selenide have been examined at liquid nitrogen and helium temperatures. The crystals were grown either using a continuous flow process or in sealed capsules in controlled partial pressures of zinc and selenium. Crystals doped with a number of acceptor and donor-like impurities have been studied as have nominally pure samples. The object of the work was to determine whether there was any correlation between the observed edge emission spectra and the crystal growth conditions. Transmission and absorption spectra have also been investigated. At temperatures below 10 K the luminescence emission of ZnSe in the vicinity of the band gap consists of a series of sharp lines and broad bands. The sharp lines are identified as I(_1) and 1(_2) lines which are associated with the recombination of excitons bound to neutral acceptors and donors respectively. At least two different I(-1) lines and one 1(_2) line have been observed. Their origin is discussed. The broad band emission close to the band gap consists of pairs of series of bands. One component of the pair is the high energy series (HES) while the other is the low energy series (LES). The HES is associated with the recombination of a free electron and a bound hole. This leads to the zero phonon band of the series, the remaining members of the series are longitudinal optical phonon replicas. The LES is associated with distant pair recombination between electrons bound at shallow donors and holes bound at the same acceptors which are responsible for the HES. The LES also consists of a zero order member and phonon replicas. Three different pairs of high and low energy series have been observed. The appearance of particular pairs is determined by the way in which the crystal was grown rather than by the nature of the impurities added. From the spectral locations of the HES components three possible acceptor levels with ionisation energies of 0.095, 0.112 and 0.122 eV have been found. In undoped crystals the donor ionisation energy is 32 meV. Chlorine and aluminium impurities lead to donors with ionisation energies of 27 meV while the indium donor has an ionisation energy of 29 meV. The mean separation of the donors and acceptors contributing to the distant pair band is estimatedoto be of the order of 100Å. A number of luminescence bands associated with deep centres have also been studied, in particular in crystals doped with copper or manganese. The origin of these bands is discussed.