eV Centre Research Articles

Luminescence bands in natural brown diamonds

We report on new luminescence bands exhibited by natural brown/mauve diamonds. We describe detailed absorption and luminescence measurements in the visible spectral region. The structured bands with ZPL at 2.424 eV, 2.114 eV, 1.819 eV and 1.707 eV, and a broad band with a maximum at 1.8 eV are shown to be the main components of the visible luminescence. Bands with ZPL at 2.114 eV, 1.819 eV and 1.707 eV are shown to correspond to forbidden transitions with lifetimes of the order of msec. Energy absorbed by the 2.424 eV centre is transferred into the 1.819 eV centre.

Contrast in triple bound excitons in copper-doped gallium phosphide

The bound exciton (BE) recombination no-phonon lines at 1.911 eV and 2.177 eV in copper doped gallium phosphide have been studied under uniaxial stress. The 1.911 eV centre exhibits splitting characteristic of a BE at an axial centre having rhombohedral I symmetry. The electronic degeneracy within each centre is not reduced, in agreement with ODMR and Zeeman spectroscopy. By contrast, the stress splitting of the 2.177 eV (COL) no-phonon line indicates a pure trigonal centre with additional electronic degeneracy in the initial BE state characteristic of an E to A transition with no splitting under (111) stress. The difference is attributed to the character of the electron which is deduced to be in a valley-orbit E state for the 2.177 eV BE, but has normal A character for the 1.911 eV BE. This difference may account for the failure of ODMR studies on the 2.177 eV system while the 1.911 eV BE shows very strong ODMR signal strength, previously incomprehensible since both BE behave as near isotropic spin triplets.

Excitation and decay of H4 luminescence in diamond

Photoluminescence and cathodoluminescence decay time measurements have been carried out for the H4 (2.499 eV) centre in diamond. The photoluminescence decay time increases progressively from approximately= 19 to 23 ns as the energy of the exciting light is increased from 2.6 to 4.0 eV, and the cathodoluminescence decay curves are non-exponential. Luminescence excitation measurements reveal a previously unreported absorption band peaking at about 3.2 eV in the ultraviolet spectral region; absorption of light in this band produces H4 luminescence. The range of decay times noted above is believed to be due, at least in part, to transitions from the higher excited levels via the 2.499 eV excited state. The decay time of 19+or-1 ns measured with photoexcitation at 2.6 eV shows no significant variation with temperature, and this represents the most likely value of the radiative lifetime of the 2.499 eV centre.

Luminescence decay time of the 1.945 eV centre in type Ib diamond

The luminescence decay time of the 1.945 eV centre has been determined as 13+or-0.5 ns for natural type Ib diamonds. For synthetic diamonds the value is slightly lower, probably because of competitive non-radiative recombination in these relatively impure crystals. The similarity of the decay time to that for the H3 centre further supports the model proposed for the vacancy-enhanced aggregation of nitrogen in diamond. No temperature dependence of the 1.945 eV decay time has been detected in the range 77-700K, and this result is not inconsistent with the energy level scheme used to interpret ESR data for the 1.945 eV centre.

The 1.40 eV and 2.56 eV centres in synthetic diamond

Cathodoluminescence in the 1.40 eV and 2.56 eV systems is observed from most nitrogen-containing synthetic diamonds. The authors show that there is a tendency for the 2.56 eV luminescence to be strong when the 1.4 eV luminescence is strong, and that both are strongest for diamonds grown in the presence of nickel. Absorption due to the 1.4 eV centre has been observed for the first time, using diamonds grown with a nickel catalyst-solvent. Temperature-dependence measurements confirm that the 1.4 eV zero-phonon doublet is due to a split ground state, and show that the 2.56 eV system has a complex excited state structure with a group of four well defined levels at 2.5634, 2.5618, 2.5602 and 2.5587 eV and a diffuse level consisting of one excited state, or two or more overlapping states at approximately 2.588 eV. No absorption has been detected in the 2.56 eV system, and in most diamonds the 1.4 eV absorption is too weak to detect. This indicates that the centres are present in low concentrations, and the circumstantial evidence suggests that they both involve nickel complexes. The latter conclusion is strongly supported by recently reported studies on diamonds implanted with nickel.

The mobility of γ-ray induced defects in reverse biased germanium n+p diodes

Abstract The motion of γ-induced defect centres in reverse biased germanium n+p diodes has been observed using deep level transient spectroscopy. The concentration profile of the Ev + 0.38 eV centre is presented as a function of the duration of the electric field application. A mobility of 6.4 ± 3.4 × 1014 cm2/v.sec at 25°C was obtained for this level, whilst a value of 2.2 ± 1.3 × 10−14 cm2/v.sec at 25°C was obtained for the Ev + 0.18 eV centre.

Photoconductivity in n-type GaAs:O associated with the deep level at 0.4 eV

The photoconductivity of GaAs:O (2 Omega cm at 300K) has been studied between 90-200K using a pulsed HF gas laser, in addition to conventional steady-state and chopped-light techniques. The substantial power available has been exploited to study photoresponse up to saturation level. Besides confirming the existence of a broadened trap distribution of width 160 meV below the conduction band, the authors have determined for the 0.4 eV centre the density NT, the volume capture rate cn, the absolute photoionisation cross section sigma n, the effective radius of the centre rT*, the Huang-Rhys factor S, the Franck-Condon shift dFC, the optical ionisation energy ET and its dependence on temperature. They are NT=1.4*1014 cm-3, cn=1.3*10-8 cm3 s-1 at 100K, sigma n=7.1*10-18 cm2 at 100K and at 0.4682 eV, rT*=11 AA, S=3.5, dFC=0.11 eV, ET=0.420-7*10-5 TeV. The spectral dependence of sigma n agrees with a new model by Ridley (1980) rather than with the Lucovsky model, and this new model is exploited in the derivation of the above quantities. It is pointed out that the Franck-Condon shift may be more a property of the host crystal than of the defect.

Optical studies of the 2.367 eV vibronic absorption system in irradiated type I b diamond

A vibronic centre with a zero phonon line at 2.367 eV is produced in type I b diamond by electron irradiation. The absorption system associated with the centre is strongly photochromic, and at low temperature (less than 200 K) may be bleached to less than 10% of its original strength; as the 2.367 eV system is bleached an absorption band with a zero phonon line at 1.979 eV is produced. The process may be reversed by warming the sample to room temperature. The temperature dependence of the thermal recovery process has been analysed to obtain the associated activation energy, and the temperature dependences of the intensities of the 1.979 and 2.367 eV zero phonon lines have been investigated in the temperature range 5–100 K. Analysis of the data shows that both zero phonon lines are due to transitions from split ground states, and the splittings and relative degeneracies of the levels in the two ground states have been determined. No photoconductivity associated with absorption at the 2.367 eV centre has been detected, suggesting that the bleaching phenomena are not due to a change of charge state of the centre or a charge transfer process. It is therefore proposed that the 1.979 and 2.367 eV centres are the same defect in two different orientations with respect to the diamond lattice. The acti­vation energy associated with the thermal recovery of the 2.367 eV system after bleaching may then be identified with the reorientation energy of 1.979 eV centres. The 2.367 eV centre is unstable on thermal annealing, being virtually destroyed at 200 °C. A probable model for the centre is a substitutional nitrogen–interstitial complex, involving at least two interstitials.

Energy loss spectra for vibro-rotational molecular excitation in an impulsive spectator model

Inelastic collisions between ionized atoms and molecules within the energy range 1-25 eV (centre of mass system) are treated in an impulsive quantum-mechanical spectator model. The model allows extensive calculations of the energy distribution over the final rotational and vibrational states. Qualitative information on the dependence of these 'energy loss spectra' on the collision energy and on the scattering angle is obtained from numerical calculations, based on an idealized model of the system N2+Li+. Comparison is also made with experimental data for H2+H+ scattering.

Measurement of differential cross sections for individual rotational quantum transitions in the scattering of Li+ by H2 at E c.m.= 0.6 eV

Individual rotational quantum transitions have been resolved in the energy loss spectra of Li+ ions after scattering by H2 molecules. Time of flight measurements at 0.6 eV centre of mass energy yield differential inelastic cross sections for the j= 0 →j= 2 and j= 1 →j= 3 rotational transitions relative to the differential elastic cross section (j= 0 →j= 0, j= 1→j= 1) at centre of mass angles between 14 and 32°.

The effect of nitrogen impurity on the annealing of radiation damage in diamond

Impurity nitrogen in type Ia diamond is known to trap radiation damage centres during annealing. Most of the nitrogen is also known to be present in two forms, labelled A and B from the absorption peaks produced by them in the range 1000-1332 cm-1. In this paper, the effect of these different forms of nitrogen on the annealing of radiation damage is considered. The visible absorption spectra of 41 annealed diamonds have been compared with their infrared spectra. The results suggest that the A form of nitrogen produces the 2.463 eV centre, while the B form gives the 2.499 eV centre, by trapping a radiation damage centre during annealing. The trapping centres compete with each other for the damage centres. In natural type Ib diamond (containing paramagnetic nitrogen), the 640nm (1.945 eV) centre is produced by the annealing, in agreement with du Preez (1965).

Photoluminescence associated with the 1.673, 1.944 and 2.498 eV centres in diamond

Measurements are reported of photoluminescence from three defect centres in diamond which have characteristic absorption and emission lines at 1.673 eV, 1.944 eV and 2.498 eV respectively. When excited with plane polarized light the emission from the 1.673 eV centre was found to be unpolarized in agreement with a centre possessing tetrahedral symmetry. Analysis of the polarization of emission of the other centres suggests that the symmetry of the 1.944 eV centre is either monoclinic I or trigonal, and that of the 2.498 eV centre is monoclinic I. A comparison of the absorption and excitation spectra indicates that the lines at 2.887 eV and 2.954 eV and a broad absorption extending from 2.8 eV to higher energies are all associated with the 1.673 eV defect.

Electron mobility and edge emission in CdS crystals

Drift mobility experiments on undoped CdS crystals showed that in some specimens the generated electrons interact predominantly with a level of centres 0.16 eV below the conduction band. Such crystals often give rise to strong green luminescence at 77°K. It is shown that moderate heat treatment (350°C) leads to a progressive reduction in the density of the 0.16 eV centres and also quenches the emission. The electron transport at the lower temperatures is now partly controlled by the group of shallower levels investigated previously by Spear and Mort. It is concluded that the 0.16 eV centre is not directly associated with the luminescent transition.