B. Henderson

Luminescence decay in disordered low‐dimensional semiconductors

The luminescence decay of excitons in disordered low‐dimensional semiconductors with quantum confinement is shown experimentally to be characterized by a nonexponential profile and an absence of spectral diffusion. We are able to describe this luminescence as a hopping‐assisted recombination using the correlation function approach to nondispersive transport developed by H. Scher, M. F. Shlesinger, and J. T. Bendler [Phys. Today 41, 26 (1991)]. We suggest a simple derivation of analytical functions which accurately describe the anomalous luminescence decays of disordered II‐VI superlattices and of porous silicon, and show that this model includes exponential and Kohlrausch [Pogg. Ann. Phys. 119, 352 (1863)] (stretched‐exponential) relaxations as special cases.

Disorder and the optical spectroscopy of Cr3+-doped glasses: I. Silicate glasses

This paper reports optical absorption and luminescence studies of Cr3+-doped silicate glasses with compositions chosen to vary the octahedral crystal-field splitting at the Cr3+ site. The extensive site-to-site disorder in the glasses is signalled by the inhomogeneous broadening of radiative transitions and by non-exponential fluorescence decay patterns. Laser-excited luminescence shows that the octahedral crystal-field splitting Dq varies between the different sites occupied by Cr3+ ions. Both R-line and 4T2 to 4A2 emissions are inhomogeneously broadened by site-to-site disorder. Fluorescence line narrowing (FLN) of the R line gives a particularly dramatic demonstration of the site multiplicity. Measurement of the width of the FLN component of the R line as a function of temperature has been used to probe the density of two-level vibrational systems coupled to the electronic levels of the Cr3+ ion.

The MOCVD growth without prereaction of ZnSe and ZnS layers

Abstract It is shown that the liquid adduct dimethylzinc-triethylamine (Me 2 Zn(NEt 3 ) 2 ) can be used as the zinc source to grow at low temperatures single crystal layers, principally of ZnSe but also of ZnS, by MOCVD without any significant prereaction of the constituent reactants. The high quality and uniformity of the ZnSe layers produced is demonstrated by a low residual carrier concentration, high mobility, an absence of deep-centre photoluminescence and a narrow width for X-ray rocking curves.

Spin-dependent recombination in irradiated Si/SiO2 device structures

We report studies of spin‐dependent recombination at the Si/SiO2 interface in electron irradiated (100) and (111) p‐channel metal‐oxide‐silicon field‐effect transistors and metal‐oxide‐silicon wafers. Electron spin resonance transitions on the Pb center increase the recombination current at the Si/SiO2 interface by 2–3 parts in 104. The results are interpreted using a model involving the recombination of electrons and holes at Pb centers with which they are spatially correlated.

Characterization of Cr3+ centres in LiNbO3 using fluorescence line narrowing

The optical spectroscopic properties of Cr3+ in LiNbO3 have been investigated using optical absorption, fluorescence, fluorescence line narrowing (FLN), selective excitation and radiative lifetime measurements. The present results are compared with data obtained using other techniques including electron spin resonance (ESR) and electron nuclear double resonance (ENDOR), and shown to be consistent with there being five Cr3+ centres. The predominant centre is due to substitution at the Li+ site, which provides the weak crystal field in which Cr3+ ions emit into a broad 4T2 to 4A2 band. Using FLN, two centres experiencing a stronger crystal field are shown to be due to substitution at this same site, probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest-neighbour cation shell. The addition of Mg2+ ions to LiNbO3 appears not to create new Cr3+ complexes: rather it modifies the nature of the disorder, thereby changing the relative concentrations of the different centres. To this extent it becomes possible to identify two other lightly occupied Cr3+ centres where substitution is at the Nb5+ site. This total of five Cr3+ centres in LiNbO3 gives a consistent interpretation of all the available spectroscopic evidence, including ESR and ENDOR, while pointing to the difficulties of theoretical work in sites where the differences in the energy levels of Cr3+ ions are too small to be distinguished by the available models.

Potential laser gain media with the stoichiometric formula RETiNbO6

Abstract The laser heated pedestal growth (LHPG) technique has been used to grow miniature crystals of the mixed niobates, RETiNbO 6 with RE = Nd, Pr and Er, having typical dimensions of 0.5 mm diameter and 10 mm in length. The Nd and Pr compounds grow in the aeschynite structure, whereas the Er compound grows with the euxenite structure, The crystals grown by the LHPG technique were characterised by measurement of their X-ray diffraction patterns and optical absorption and photoluminescence spectra. The latter show very high absorption coefficients for the rare earth ions and strong luminescence signals. Brief reports are presented of these spectra, and discussed in terms of the potential of RETiNbO 6 single crystals as laser gain media.

DISORDER AND THE OPTICAL SPECTROSCOPY OF CR3+-DOPED GLASSES. II, GLASSES WITH HIGH AND LOW LIGAND FIELDS

For pt. I see ibid., vol.3, p.1915 (1991). The paper reports spectroscopic studies of Cr3+ ions in fluorozirconate. fluoride, borate and telluride glasses in which the average strength of the octahedral crystal field Dq/B varies from 1.6 to 2.4. The emission spectra are shown to be inhomogeneously broadened by disorder. Non-exponential decay of the emission intensity following pulsed excitation is another signature of the extensive disorder in these glasses. The large density of states of two-level systems is shown to account for the larger-than-expected homogeneous width of the R line in lithium borate and potassium borate glasses.

Vibronic structure in the photoluminescence spectrum of Cr3+ ions in garnets

The low temperature photoluminescence from Cr3+ ions in Y3Al5O12 (YAG), Y3Ga5O12 (YGG), Gd3Sc2Ga3O12 (GSGG ) and Gd3Sc2Al3O12 (GSAG) have different characteristic lineshapes. YAG and YGG emit entirely in the R-lines with associated sidebands, whereas for GSGG and GSAG the emission is a melange of R-line processes and broad 4T∗2g→4A2g bands. This paper compares the experimental spectra of Cr3+ ions in these garnets in terms of the symmetry and energies of the vibrational models coupled to the electronic states. In GSGG, we have used fluorescence line narrowing (FLN) to probe the broadening of the vibronic structure introduced by compositional disorder.

Crystal growth and optical characterisation of Cr3+-doped YAl3(BO3)4

Abstract This paper reports the high temperature top-seeded solution growth (TSSG) of large single crystals of Cr 3+ -doped YAl 3 (BO 3 ) 4 from a K 2 Mo 3 O 10 B 2 O 3 flux cooled at a rate of 2–4 K per day in the temperature range 950–1060°C, using a crystal rotation rate of 4–6 rpm. Optical absorption measurements at T = 300 K reveal broad bands associated with transitions from the 4 A 2 ground state of Cr 3+ to the 4 T 2 and 4 T 1 excited states. The doublet R lines and their one-phonon sideband are clearly resolved at room temperature in both absorption and luminescence spectra: the splitting of the R lines demonstrates the effect of the even-parity trigonal distortions of their environment on the 4 A 2 and 2 E levels of the Cr 3+ ions.

The growth and optical assessment of Cr3+-doped RX(BO3)4 crystals with R = Y, Gd; X = Al, Sc

Abstract This paper reports the high temperature top-seeded solution growth (TSSG) of Cr 3+ -doped mixed borate crystals having the general formula RX 3 (BO 3 ) 4 , with R 3+ = Y 3+ or Gd 3+ and X 3+ = Al 3+ or Sc 3+ . Both YAl 3 (BO 3 ) 4 (YAB) and GdAl 3 (BO 3 ) 4 (GAB) have been grown from a mixed K 2 Mo 3 O 10 B 2 O 3 flux, whereas YSc 3 (BO 3 ) 4 (YSB) and GdSc 3 (BO 3 ) 4 (GSB) are grown from K 2 Mo 3 O 10 Li 2 B 4 O 7 mixtures. Typical crystals ranging in volume from 60 mm 3 to 14 cm 3 have been grown, with Cr 3+ concentrations of order 0.2–5.0 at%. Measurements of the optical absorption and luminescence spectra show that in YAl 3 (BO 3 ) 4 and GdAl 3 (BO 3 ) 4 the Cr 3+ ions occupy strong crystal field sites whereas in YSc 3 (BO 3 ) 4 and GdSc 3 (BO 3 ) 4 these ions occupy weak field sites. The significance of this observation is that the Cr 3+ ions in weak field sites emit via the broadband 4 T 2 → 4 A 2 transitions, with obvious potential as tunable laser gain media.