Donor-acceptor Pair Emission Research Articles

A unified treatment of the theory of donor-acceptor pair recombination emission

A general formalism for treating donor-acceptor pair emission is developed which highlights important differences between the spectra resulting from a random distribution of the donors and acceptors, and that resulting from a preferentially paired distribution. Application of the analysis to the donor-acceptor pair emission band observed in diamond shows that the properties reported are consistent with a marked preferential pairing of the donors and acceptors.

Variation of photoluminescence with carrier concentration in GaN

Photoluminescence measurements are reported on undoped and on Ge doped single crystal epitaxial layers of GaN, having room temperature carrier concentrations (electrons) in the ranges 1.8 × 10 19 to 6.3 × 10 19cm −3 and 2.8 × 10 18 to 1.5 × 10 19cm −3 respectively. Both the previously reported donor-acceptor pair emission and a higher energy emission were observed in all samples. The intensity of the former greatly exceeded that of the latter in the undoped crystals of lower carrier concentration as it did in the Ge-doped crystals. The high energy luminescence peak was found to shift from 3.45 eV to 3.53 eV (77°K) according to increasing carrier concentration. This was found to be consistent with a model of band to band recombination across a band gap of 3.50 eV (1.6°K) for crystals with carrier concentrations in the range n > ∼ 5 × 10 18 cm -3, a Burstein shift mechanism being responsible for the observed shift of the luminescence peak.

Luminescence Due to Oxygen and Self-Activated Centers in Zinc Telluride

Red emissions in ZnTe containing In and O are described. In addition to the isoelectronic oxygen emission, emission due to indium appears at a wavelength longer than 7500A at low temperatures. The emission changes its peak position with indium content. The emission at high indium concentrations is thought to be the self-activated emission in ZnTe, since it shows characteristics of donor-acceptor pair emission and is accompanied by thermal glow. The self-activated emission at low temperatures is excited efficiently by the host absorption, in contrast to the oxygen emission. This difference seems to reflect large difference of capture cross sections of these two quite different centers for free carriers. Structures below the band gap energy in the excitation spectrum of the self-activated emission are apparently related to the levels of Zn vacancies and In impurities. In this spectrum, a dip is seen at an energy corresponding to the sum of energies of a free exciton and a LO phonon.

Mechanism of the Green-Copper Luminescence in ZnS Crystals. I. Direct Evidence for the Pair Emission Mechanism

Using a ZnS:Cu, Al crystal grown from the melt, precise measurements were made at liquid helium temperature on the relationship between the decay rate of the green-copper luminescence and those of two kinds of photoinduced absorptions, one due to holes created at copper acceptors, and the other due to electrons trapped at aluminum donors. It was found that the decay rates of the two kinds of absorptions are equal to each other, and are about one half of that of the luminescence. This fact clearly incicates that the green-copper luminescence is caused by the recombination of electrons trapped at aluminum donors with holes trapped at copper acceptors according to the bimolecular kinetics, and provides the direct experimental evidence for the conclusion of the donor-acceptor pair emission mechanism for this luminescence presented in the previous investigations.

Mechanism of the Green-Copper Luminescence in ZnS Crystals. II. Polarization Characteristics

The polarization characteristics of the green-copper luminescence were investigated at liquid helium temperature using a melt-grown ZnS:Cu, Al crystal with cubic structure involving stacking faults. In accord with the results of the previous study at liquid nitrogen temperature, the luminescence is polarized preferentially perpendicular to the [111] c axis. The origin of this polarization is attributed to the crystal field with trigonal nature of the host lattice, and the polarization characteristics are interpreted on the basis of the donor-acceptor pair emission mechanism. With increasing temperature from liquid helium temperature, I ⊥ decreases while I || increases keeping I = I ⊥ + I || constant, I ⊥ and I || being the intensities of luminescence polarized perpendicular and parallel to the [111] c axis. These facts are accounted for, if one takes the thermal excitation of holes associated with copper acceptors into account.

The theory of the energy shift of donor—Acceptor pair emission with varying excitation intensity

The theory of the effect of varying the excitation intensity on the position of the emission peak of the broad structureless band (attributed to donor-acceptor pair recombination) is investigated. In particular, effects resulting from impurity banding, preferential pairing, excited state donor emission and variation of the donor-acceptor pair capture cross-section with varying distance of separation are described.

Mechanism of broad-band luminescences in ZnS phosphors—II. Characteristics of pair emission type luminescences

As to the Green-Cu, Blue-Ag and self-activated luminescences in ZnS phosphors, which have been elucidated in Part I to be interpreted by the donor-acceptor pair emission mechanism, decay characteristics are investigated in detail. The manners of decay are measured at 4.2°K for several photon energies of the entire emission band. The decay persists for up to 1000 sec. All the decay curves obey the relation of power law, and the decay rate becomes faster with increasing photon energy in the emission band. From some arguments, it is shown that these facts lead one to definitely conclude the pair emission mechanism. The features of these pair emission bands in ZnS are discussed. In case of the Green-Cu luminescence, the correlation of the long persistent decay with the infrared absorption due to aluminum coactivator as well as that due to the ionized state of copper is discussed, and is pointed out as being very strong evidence for the conclusion. Further, some discussions and interpretations are presented about the polarization properties of these luminescences and also about the manners of the temperature-dependence of the emission spectra.

Mechanism of broad-band luminescences in ZnS phosphors—I. Spectrum shift during decay and with excitation intensity

The measurements of spectrum shift during decay and with excitation intensity are made for the broad-band luminescences in ZnS phosphors due to Ib activator and IIIb or VIIb coactivator, in order to investigate the luminescence mechanism. The Green-Cu Blue-Ag luminescences show remarkable spectrum shifts; namely, the emission peak shifts to lower energies during decay and also with decreasing excitation intensity. It is concluded that these luminescences are due to the so-called donor-acceptor pair emission mechanism, assuming that the donor and acceptor states involved are created, respectively, by the coactivator and activator. The self-activated luminescence shows a gradual spectrum shift at liquid helium temperature, although it shows a somewhat irregular shift at liquid nitrogen temperature. It seems that one has to assume the pair emission mechanism also for this luminescence. The Blue-Cu, Red-Cu and Red-Cu.In luminescences show no spectrum shifts. It is concluded, together with the knowledge already obtained from other measurements, that the transitions for these luminescences occur between two localized levels confined within the associated centers.