J.A.W. van der Does de Bye

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.

Donor-acceptor pair spectra in 6H and 4H SiC doped with nitrogen and aluminium

Abstract We investigated the low temperature luminescence spectra of 6H and 4H SiC doped with nitrogen and aluminium. The investigation involved photoluminescence spectra, time-resolved cathodoluminescence spectra and decay measurements. From the observation of (i) a detailed line structure in the photoluminescence spectra, (ii) a narrowing of the bands in the time-resolved spectra with increasing delay time and (iii) a non-exponential decay, it is concluded that the spectra characteristic for material doped with N and Al are due to donor-acceptor pair recombination. From the decay measurements the parameter W max in the relation describing the transition probability of the pairs is estimated to be 3x10 5 s -1 . It is further concluded, that the spectra of both polytypes involve two zero-phonon bands and their phonon replicas. We show that an interpretation in terms of N as the donor and Al as the acceptor is compatible with existing information about these impurities. Comparison of the spectral data with other information suggests that the existence of two zero-phonon bands is due to the difference in ionization energy between N donors on “hexagonal” and “cubic” sites respectively.

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 role of acceptors in the luminescence of n-type GaAs

Abstract From the time behaviour of the edge- and lower-energy luminescence in n-type GaAs it was concluded that single trapping of excited holes by acceptors occurs in all samples investigated. Short pulses of 600 kV electrons were used for the excitation. The relaxation times of the luminescence were measured as functions of temperature. Using the single trapping model it was possible to determine the rate constants for the trapping of free holes and for the subsequent radiative recombination with electrons. These rate constants vary with the ionization energy of the acceptors involved. The nature of these acceptors is unknown. They may be due to different kinds of defects. For the “hydrogenic” value of the ionization energy the rate constant for the radiative recombination agrees with the theoretical value given by Dumke. The trapping rate constant obtained was, however, found to be larger by several orders of magnitude than the value given by a radiative trapping theory similar to that used for hydrogen ions in an electron gas. Acceptor levels deeper than “hydrogenic” levels were found to trap more strongly. The holes at the deeper levels have a lower probability of radiative recombination. The radiationless recombination in n-GaAs is strong. It involves the free holes.

Decay of the Ce3+ luminescence of LaMgAl11O19 : Ce3+ and of CeMgAl11O19 activated with Tb3+ or Eu3+

Abstract Decay measurements have been performed on the Ce3+ luminescence of LaMgAl11O19 : Ce3+ and of CeMgAl11O19 activated with Tb3+. For all samples, the decay was found to be nearly exponential over at least two decades, and the decay time constant was practically the same, being ≈20 ns. From these results and from quantum efficiency data of the Ce3+ luminescence, conclusions are drawn about the various energy transfers and quenching interactions occurring in these compounds.

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.

Minority carrier lifetime in p-type gallium phosphide

Abstract The decay time of the luminescence involving bound minority carriers shortens above some temperature with increasing temperature owing to thermal quenching. However, above some higher temperature there will be a plateau since the luminescence decay cannot be faster than the decay of the excited free minority carriers. A simple recombination model identifies the plateau decay time with the minority carrier lifetime (τ Min ) as caused by the deep centres (“killers”). Values for the plateau decay time measured on GaP(Zn,O) (red emission) and GaP(N) (green emission) lie between 5 and 80 nsecs.

Quantum Efficiency Measurements Using a 2π‐Geometry Elliptic Mirror

For measurements of photoluminescence and electroluminescence an alternative is described to the well known, but rather inefficient, integrating sphere. The new instrument consists mainly of a deep elliptic mirror with 2 π geometry with respect to the sample. Sample and photodetector are positioned at the two focal points.

Models for energy transfer and their application to aluminate phosphors

Abstract Energy transfer efficiencies and decays of the luminescence in aluminate phosphors are discussed as a function of the activator (A) and sensitizer (S) concentration. A model with continuously variable SA distance and a model with discrete SA distances and site occupancies have been compared. Examples are given in the Ce 1-υy Tb υy ] 0.92 O 0.08 MgAl 11.13 O 19 and Ba 1-z Eu z Mg 1-υy Mn υy Al 10 O 17 phosphor systems. The consideration of the sensitizer decay curves leads to discrimination between discrete and continuum models for energy transfer, even in cases where efficiency data only do not allow such a distinction.