Yellow luminescence

Besides REE, broad spectral bands characterize the luminescence of zircon. They are structureless down to 4.6 K, which makes difficult the correct interpretation of the nature of the luminescent centers. Different suppositions are made in previous studies and even the question about a yellow luminescence connection with intrinsic or impurity defect remains open. For example, the yellow band ( C-band ) was ascribed to SiO -defects (Votyakov et al. 1993 Krasnobayev et al. 1988) while the same emission ( band VII ) was explained by impurity luminescence, namely by Yb " " created by radioactive reduction of Yb " " (Kempe et al. 2000). 

Time-resolved luminescence spectroscopy of zircon revealed luminescence lines, which maybe confidentially ascribed to a Eu center (Fig. 4.38d). Usually they are hidden by a broad band yellow emission of zircon and may be detected only with a long delay time using its much longer decay time compared to yellow luminescence. 

Luminescence of Er " in the emission spectra of zircon is very difficult to detect under UV excitation even by time-resolved spectroscopy. The reason is that it has a relatively short decay time similar to those of zircon yellow luminescence, which usually is much stronger than Er " fines. Visible excitation, which is not effective for broad band luminescence, allows the revealing of Er " " luminescence fines, using high-resolution steady-state spectroscopy (Fig. 4.38f). 

Figure 4.39a represents the liuninescence band detected in all investigated natural zircons. It has an excitation band peaking at 310 nm. The broad yellow band at 575 nm with a decay time of 25-35 ps represents classical zircon luminescence. The picture is not principally changed with different delay times and gate width. After heating the sample up to 700 °C the luminescence intensity is nearly the same, but after that it is strongly reduced and at 800 °C the yellow luminescence disappears. Luminescence spectra at 77 and 4.2 K are not substantially different. 

Fig. 5.65. Comparison of radiation-induced luminescence and EPR of different centers as a fimction of the heating temperature. Left integrated yellow liuninescence (upper) and EPR of TF, Yh, Nh and SiO (lower). Right yellow luminescence bands of different origin with different thermal stability (Gaft et al. 1986)...

Thus, pure samples of the complex formed crystals with varying luminescence. Most of the crystals exhibited the described yellow luminescence, but others displayed a bluish-white luminescence and a few of the crystals showed a pink luminescence. The two new forms of trinudear complex can be obtained by evaporation of dichloromethane solutions of the complex. They consisted of one triclinic form and one monoclinic form and both crystallized as colorless blocks, in contrast to the results with the original solvoluminescent hexagonal form that crystallized as colorless needles. These polymorphs differed in the packing of the nearly planar molecules and in the nature of the aurophilic interactions between the trinudear units (see Figure 6.16). 

W. D. Bancroft and H. B. Weiser point out that the blue luminescence of sodium is obtained without the yellow luminescence (i) when sodium salts are introduced into a flame of hydrogen in chlorine (ii) when metallic sodium bums slowly in oxygen, chlorine, or bromine (iii) when a sodium salt is fused (iv) when cathode rays act on sodium chloride (v) when anode rays first act on sodium chloride (vi) when one heats the coloured residue obtained by the action of anode rays or cathode rays on sodium chloride and (vii) when sodium chloride is precipitated rapidly from aq. soln. with hydrochloric acid or alcohol. The yellow luminescence of sodium is obtained, accompanied by the fainter blue luminescence (i) when a sodium salt is introduced into the Bunsen flame (ii) when sodium burns rapidly in oxygen, chlorine, or bromine and (iii) when canal rays act on sodium chloride. It is claimed that the yellow luminescence is obtained when sodium vapour is heated but it is very difficult to be certain that no burning takes place under these conditions. 

Multiple activation of zinc sulfide is also possible. Zinc-cadmium sulfide, doubly activated with silver and gold, which is used as a white-luminescing, one-component phosphor for monochromic cathode-ray tubes [5.334], and the yellow-luminescing ZnS Cu, Au, A1 phosphor, whose emission color corresponds to that of Zn, x Cdx S Cu [5.332], are known. 

Partially functionalized cyclopolysilanes recently attracted attention as model substances for siloxene and luminescent silicon. The yellow luminescent silicon is formed by the anodic oxidation of elemental silicon in HF-containing solutions and may be used for the development of silicon-based materials for light-emitting structures which could be integrated into optoelectronic devices77. Because the visible photoluminescence of... 

Colorless, non-luminescent solutions of [AuI C(NHMe)2 2](PF6)-0.5(acetone) become intensely luminescent when they are frozen in a liquid N2 bath [48]. Strikingly, the colors of the emission vary in different solvents and appear only after the solvent has frozen. The frozen acetonitrile solution produces a green-yellow luminescence, with dimethyl sulfoxide and pyridine the emission is different shades of blue, with acetone it is orange, but with dimethyl-formamide no luminescence is observed. The process is entirely reversible ... 

Most work has been performed on the yellow emission. The first ODMR in this area was by Glaser and co-workers [14-17], Two defects have been identified the effective mass donor, previously observed in EPR, and a deep donor (g = 1.989, g = 1.992). Glaser and co-workers have argued that the yellow luminescence is due to a two step process in which an electron is first transferred from a shallow donor to a deep ( 1 eV) double donor and then combines radiatively with a hole in a shallow acceptor to give the yellow luminescence. The argument is supported by results on n- [14-16] and prtype [15,16] samples (which support the double donor aspect of the argument) and the temporal evolution and excitation dependence of the ODMR [18]. 

FIGURE 6 Cathodoluminescence spectra obtained in the defective area of the crystal which appeared yellow in an optical microscope (a) and in the transparent area of the crystal (b). Note changes in the intensities of the yellow luminescence peak (2.4 eV) and band to band luminescence (3.5 eV). 

A8.7 Yellow luminescence in GaN A8.8 Hydrogen and acceptor compensation in GaN A8.9 3d transition metals in GaN and related compounds A8.10 Er-doped GaN and AIN... 

In n-type GaN the lowest-energy native defect is the gallium vacancy (Vg ), a triple acceptor. This defect plays a role in donor compensation (see Datareview A8.1), as well as in the frequently observed yellow luminescence (see Datareview A8.7). 

The presence of a shallow acceptor level in GaN has been attributed to C substituting on an N site by Fischer et al [7], In luminescence experiments on GaN from high temperature vapour phase epitaxy in a C-rich environment donor-acceptor and conduction-band-to-acceptor transitions have been distinguished in temperature dependent experiments. From the separation of both contributions an optical binding energy of 230 meV close to the value of effective mass type acceptors was obtained. Hole concentrations up to 3 x 1017 cm 3 were achieved by C doping with CCU by Abernathy et al [10], In addition Ogino and Aoki [17] proposed that the frequently observed yellow luminescence band around 550 nm should be related to a deep level of a C-Ga vacancy complex. The identification of this band, however, is still very controversial. 

The yellow luminescence (YL) in GaN is a broad luminescence band centred around 2.2 eV. The YL appears to be a universal feature it has been observed in bulk GaN crystallites as well as in epitaxial layers grown by different techniques. The intensity can vary over a wide range, with good samples exhibiting almost no YL. 

At this time the gallium vacancy (in isolated form or complexed with an impurity) appears to be the most likely source of the yellow luminescence. In this Datareview we will summarise the available evidence. 

FIGURE 1 Schematic illustration of levels involved in the yellow luminescence in GaN. Gallium vacancies introduce a deep acceptor level about 1.1 eV above the valence band Transitions between shallow donors and the deep acceptor level give rise to the YL. 

Strong theoretical as well as experimental evidence is now available identifying gallium vacancies as the source of the yellow luminescence in GaN. 

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