A.T. Vink

Fine structure in the low temperature luminescence of Zn2SiO4:Mn and Mg4Ta2O9:Mn

Abstract The luminescence of powder samples of the well-known green-emitting Zn 2 SiO 4 :Mn and the red-emitting Mg 4 Ta 2 O 9 :Mn phosphor shows a considerable fine structure at 4° K in appropriately prepared samples containing a sufficiently low Mn concentration. For (Zn 1- x Mn x ) 2 SiO 4 (0.0005⩽ x ⩽0.05) two sharp lines were found which are interpreted as due to zero phonon transitions between the 4 T 1 and 6 A 1 levels of Mn 2+ ions on the two crystallographically different zinc sites. The remaining structure is ascribed to vibronic sidebands. The decay times of the luminescence bands associated with the two sites differ; they are 12 and 15 ms for the high and low energy bands respectively. The experimental results of Vlam are confirmed by our data. In addition some (Zn 1- y Be y ) 2 SiO 4 :Mn (0.025⩽ y ⩽1) samples were investigated. In Mg 4 Ta 2 O 9 :Mn two zero phonon lines could be identified, indicating that in this material Mn 2+ is distributed over two inequivalent Mg sites. Most of the phonon replicas were found at intervals of 15 meV. Raman scattering experiments showed that this energy corresponds to one of the lattice vibrations. The decay time of this luminescence band is 1.0 ms.

The dielectric constant of GaP from a refined analysis of donor-acceptor pair luminescence, and the deviation of the pair energy from the coulomb law

Abstract It is shown that, by using computer simulated spectra, the fine structure observed in donor- acceptor pair luminescence can be analysed up to large pair distances R . For GaP, R is about 70 A. Results of an application of this analysis to new accurate measurements on O P -C P and S P -C P pairs are presented. From the spectra involving the deep donor O p , the static dielectric constant ϵ is calculated to be 11.02 ± 0.05 at 1.6 °K. This value of ϵ can account for the experimental R dependence of the O P -C P pair energy, as calculated from the Coulomb interaction in the pairs, for R ⪆ 17 A. The deviation of pair energy from the Coulomb law is determined for O P -C P as well as for the shallow S P -C P pairs. For short distances the energy of O P -C P pairs is larger than calculated from the Coulomb law, contrary to the finding for shallow pairs, for example S P -C P . The deviations from the Coulomb law for these pairs are discussed by considering a model due to Williams and Mehrkam and the van der Waals polarization energy. It is shown that the known treatments do not provide a satisfactorily theoretical explanation for these deviations. A theoretical approach along the lines of that of Williams and Mehrkam, but by taking full acount of polarization seems to be the most promising one. Finally, a new tabulation of donor and acceptor ionization energies is presented. It is partly based on the new data on ϵ, O P -C P and S P -C P pairs and, in addition, on S P -Zn Ga and S P -Cd Ga pairs.

Luminescence and absorption of Tb3+in mo·Al2O3·B2O3·Tb2O3 glasses

Abstract Quantum yields of the green Tb luminescence for 254 nm excitation of glass compositions in the system MO·Al 2 O 3 · B 2 O 3 ·Tb 2 O 3 ( M = Mg , Ca , Sr , Ba and Zn ) were studied in relation to absorption and excitation spectra. Yields as high as 80% were observed. The Tb 4f-5d absorption maximum ranges from 218 to 232 nm, always at a longer wavelength than the glass matrix absorption. The yield strongly depends on the spectral position of the 4f-5d absorption, due to competing impurity absorption at 254 nm.

Characteristic infrared luminescence in GaP due to Mn

Abstract The observation at low temperatures of a structured characteristic luminescence in Mn-doped GaP is reported. This luminescence, centred at ≈ 1.34 eV, is attributed to the 4 T 1 → 6 A 1 internal transition of Mn 2+ (3d 5 ) on a Ga site. Some aspects of the fine structure are discussed, notably the observation of a localized vibration of Mn and the possible presence of a dynamic Jahn-Teller effect.

Optical properties of excitons bound to neutral SiGa-donors in GaP and the degeneracy of the SiGa-donor ground state

Abstract Low temperature near band-edge absorption and luminescence spectra of Si-doped n-type GaP are reported. Electrical and optical evidence is presented that these spectra are due to the creation and decay of excitons bound to neutral Si donors on Ga sites. Several zero-phonon transitions, each with strong replications by momentum conserving phonons, were observed in both absorption and luminescence. From these measurements it is concluded that the ground state of the SiGa donor is split into two sublevels 0.6 meV apart. Crystals containing a sufficiently low concentration of S to be suitable for a reliable analysis of temperature-dependent Hall-effect measurements were selected by combining 300°K Hall-effect data with low temperature S-exciton absorption data. The electrical measurements support the identification of Si as the main shallow donor. Moreover, it is concluded from these measurements that the degeneracy of the ground state of a Ga-site donor is 3 times that of a P-site donor, in agreement with theoretical expectations.

The kinetics of donor-acceptor pair transitions with strong phonon coupling in GaP

Donor-acceptor pair luminescence in GaP was studied by time-resolved spectroscopy, by measuring and analysing the integral band decay over many decades of intensity and time and by measuring and analysing the temperature dependence of the luminescence intensity. We report here the results obtained with some or all of these techniques for pairs involving deep as well as shallow centres. Those with a deep centre are: SP-SiP and SiGa-SiP, in which the acceptor is the deep centre, and OP-CP and OP-ZnGa, in which the donor is the deep centre. These pairs all have a broad, phonon-dominated luminescence band. The pairs involving shallow centres include SiGa-CP, SP-CP and TeP-CP. Due to the different way of momentum conservation in these pairs, those involving a donor on a Ga-site have strong phonon co-operation and weak zero-phonon (ZP) transitions, whereas those with a donor on a P-site have strong ZP transitions. With time-resolved spectroscopy, well-resolved structure, due to a ZP pair band and its phonon replicas, is obtained for nearly all pairs. A comparison is made with the structure obtained by measuring in the stationary state at very low excitation densities. Some trends in the strength of phonon co-operation are noted. In the case of SiGa-CP pairs, sharp replicas of ZP pair lines with a momentum-conserving phonon are also resolved. Using the integral band decay technique and results from time-resolved spectroscopy we have determined the transition probabilities for the total luminescence band as well as for the ZP band for nearly all pairs mentioned. The influence of incomplete saturation of the pairs during excitation with a focussed beam of an argon-ion laser is discussed and approximately corrected for. Extrapolated to zero pair separation, the total transition probability ranges from ∽ 105s-1 for SiGa-CP pairs to ∽ 15 × 105s-1 for OP-CP and OP-ZnGa pairs. The ZP probability ranges from ∽ 4 × 103s-1 for SiGa-CP and SiGa-SiP pairs to ∽ 4 × 105s-1 for SP-CP pairs. The transition probability for pairs in GaP is discussed in relation to the site occupied by the donor and the depth of the centres involved. The relatively high dope concentrations used in some cases, ∽ 1 to 2 × 1018cm-3, are discussed briefly in relation to “concentration quenching”. The temperature dependence of the luminescence intensity of SP-SiP pairs is satisfactorily analysed with a simple linear model. The same model is applied to similar literature data on the SP-CP pair luminescence. Using in addition data on the average pair transition probability, trapping rate constants for hole capture by the SiP and CP acceptors were calculated to be ∽ 10-9 cm3s-1. Evidence is presented that the trapping rate constant for holes by the ZnGa-OP complex is about 2 × 10-9 cm3s-1.

Kinetics of green and red-orange pair luminescence in GaP

Decays of the donor-acceptor pair luminescence have been measured as a function of the pair distance R at liquid He temperatures using a pulsed laser beam excitation and applying gated integration techniques to display the decays. The decay time constants were derived from the exponential decays below R ≈ 20 A (on the resolved pair lines) and from the major components of the non-exponential decays at larger R. The decay time depends exponentially on R, the dependence increasing in the “green” pair sequence (S-C), (S-Zn) and (S-Cd) with the characteristic lengths 9.1 A, 7.7 A and 7.0 A. For the “red-orange” pairs (S-Si) no such length could be derived owning to the strong phonon replication. The radiative transition probabilities extrapolated to zero R are ≈ 5 × 105 sec-1 for the “green” pairs and ≈ 105 sec-1 (rough estimate) for the “red-orange” pairs. The zero-phonon transition probabilities were found to be ≈ 4 × 105 sec-1 and ≈ 3 × 103 sec-1 respectively. Possible causes of decay distortion above R ≈ 20 A are discussed as well as thermal effects.

Low temperature luminescence in GaP at very low excitation densities

Abstract By using very low excitation densities in low temperature luminescence studies on GaP very narrow donor-acceptor pair bands were obtained. This has resulted in clear phonon replication patterns, including new replications, of the no-phonon pair bands in ‘green pair’ luminescence involving donors at P and Ga-sites. In the ‘red-orange’ luminescence involving the deep Sip-acceptor structure is observed, which is identified with the no-phonon pair band and its phonon replicas.

The dependence of the radiative transition probability of donor-acceptor pairs on pair separation

Abstract The probability W ( R ) for radiative donor-acceptor pair transitions is generally assumed to decrease exponentially with increasing pair separation R , with a characteristic length a . When one centre in the pair is deep and strongly localized and the other one is shallow, a equals half the Bohr radius of the shallower centre. For pairs in GaP involving two shallow centres we have previously reported that a exceeds half this Bohr radius, and have suggested this to be due to the spatial extent of the deeper centre. In the present paper we investigate this point, both experimentally, and by considering Novotnys calculations — for direct transitions — on the R -dependence of W ( R ), in which the Bohr radii of both donor and acceptor are parameters. According to these calculations, W ( R ) is not exponential in the case of two shallow centres. In a limited range of R , W ( R ) is approximately exponential, however, and theoretical “effective values” of a can be obtained. These effective values are compared with experimental data on a for the pair series S P -Si P , S P -Cd Ga , S P -Zn Ga and S P -C P in the indirect semiconductor GaP; a amounts to ∽ 5.4, 7.0, 7.7 and 9.1 A, respectively. The comparison shows that the difference found between the experimental a and half the Bohr radius of the less localized centre in the pair can indeed be explained by the influence on a of the spatial extent of the more localized centre. This findings implies a non-exponential W ( R ) for pairs with two shallow centres. This conclusion depends critically on the values used for the Bohr radii of the acceptors, however. These are not accurately known, due to insufficient knowledge of m ∗ h , one of the quantities needed in calculating the radii. We used m ∗ h = 0.36 m 0 to 0.40 m 0 , corresponding to the normally used hydrogen-model energy of 40–45 meV. An alternative explanation for the experimental values of a is that in reality m ∗ h is significantly lower than the 0.36 m 0 to 0.40 m 0 used, leading to larger acceptor Bohr radii and, in the extreme case, to no influence on a of the more localized centre and to an exponential W ( R ). In order to decide between the two possible explanations, additional experiments are presented. These include an accurate comparison of a between S P -C P and O P -C P as well as between S P -Zn Ga and O P -Zn Ga pairs. This is done by analysing the intergral band decay and time-resolved spectra of these pairs. The conclusions for all pairs mentioned are: (i) The Bohr radius of the acceptors involved is significantly larger than calculated from an effective-mass energy of 40–45 meV; they correspond to a hydrogen-model energy of 28 meV ( m ∗ h = 0.25 m 0 ). (ii) There is in first approximation no influence on the shape of W ( R ) of the spatial extent of the more localized centre. (iii) The experimental results are well described by an exponential W ( R ) with a equal to half the Bohr radius of the less localized centre of the pair.

ESR of a Mn-S nearest neighbour complex in GaP

Abstract ESR measurements on a manganese-sulphur nearest neighbour complex having axial symmetry along 〈111〉 axes are reported. The spin Hamiltonian parameters are found to be g = 2.002 ± 0.001, A = (-57 ±1) × 10 -4 cm -1 , D = (-306 ± 2) × 10 -4 cm -1 and ( a − F ) = (+12 ±2) × 10 -4 cm -1 . The formation of these complexes is discussed. briefly.