J. J. Davies

Photoluminescence of excitons bound at Te isoelectronic traps in ZnSe

ZnSe1−xTex belongs to the interesting class of semiconductor alloys in which Te acts both as an isoelectronic trap and as a constituent of the alloy. We have grown ZnSe single crystals having various concentrations of Te atoms from a mixture of Te and Se solutions. We observe an emission band having phonon structure and peaking at 2.67 eV in slightly Te-doped specimens (x < 0.008), and a broad band at 2.48 eV in rather heavily Te-doped specimens (x ≤ 0.046). These two bands can be attributed respectively to recombination of excitons bound at Te1 atoms and Ten (n ≥ 2) clusters. The results are interpreted in terms of intermediate electron-phonon coupling, and relevant optical processes are also discussed.

Growth and assessment of CdS and CdSe layers produced on GaAs by metalorganic chemical vapour deposition

Abstract CdS and CdSe layers have been grown successfully on GaAs by MOCVD. The surface morphology, RHEED patterns and photo-luminescence properties of the layers were studied for different substrate orientations. It was found that the CdS grew hexagonally on the (111)A substrate face, whilst the layers produced on (100), (110) and (111)B faces were a mixture of cubic and hexagonal phases. Layers of CdSe grown on GaAs were also a mixture of cubic and hexagonal phases for substrate faces other than (111)A, for which the layer was predominantly cubic. The change in photoluminescence with growth temperature was investigated and for CdS the best growth temperature, as measured by the width of the bound exciton emission, was 350 ° C. No evidence of large scale diffusion of Ga or As from the substrate was found.

ODMR studies of recombination emission in II–VI compounds

Abstract For II–VI materials Optically Detected Magnetic Resonance can be used: (i) to identify donor-acceptor and triplet state recombination emission bands; (ii) to identify the centres participating in such recombination; (iii) to determine the energies of these centres relative to the band edges; (iv) to study energy transfer and competitive recombination processes; and (v) to study exchange and hyperfine interactions and thus to obtain information about the wavefunctions of the centres involved in the recombination. The article explains how these investigations are carried out and illustrates the applications by reference to specific cases. A summary of published work on ODMR of II–VI compounds is given in an appendix.

CdS/CdSe strained layer superlattices grown by MOCVD

The authors report the first growth of superlattices in the wurtzite CdS/CdSe system. The superlattices were grown by metal-organic chemical vapour deposition on GaAs (111) substrates. Transmission electron microscopy shows the superlattices to be of good quality with abrupt interfaces. The layers were purely hexagonal with no twinning. Low-temperature photoluminescence of the superlattices was studied for various layer thicknesses. All the structures showed a broad intense near-infrared emission which shifts to higher energy for the thinnest layers. In addition, thicker layers showed a narrower red emission at 690 nm. The photoluminescence spectra provide preliminary evidence that the structure is of type II.

Origin of the red luminescence in Mg-doped GaN

Optically detected magnetic resonance and positron annihilation spectroscopy experiments have been employed to study magnesium-doped GaN layers grown by metal-organic vapor phase epitaxy. As the Mg doping level is changed, the combined experiments reveal a strong correlation between the vacancy concentrations and the intensity of the red photoluminescence band at 1.8eV. The analysis provides strong evidence that the emission is due to recombination in which electrons both from effective-mass donors and from deeper donors recombine with deep centers, the deep centers being vacancy-related defects.

MBE growth of CdTe and Cd1−xMnxTe layers and multilayers on InSb substrates

Abstract Single and multiple layers of CdTe and CdMnTe, including MQW structures with well and barrier thicknesses down to 20 A, have been grown by MBE on (001) InSb. The broadening of the excitonic photoluminescence lines in single layers is consistent with current theories of statistical alloy fluctuations. For multiple thin layer structures the PL spectra show quantum confinement effects. The DCXRD rocking curve data show that both the single and multiple layers are of high structural quality.

Photoluminescence studies of type IIa and nitrogen doped CVD diamond

Abstract Photoluminescence (PL) studies were carried out on CVD, type IIa and high purity HPHT diamond samples irradiated with electrons of energies between 150 and 300 KeV; near threshold energies for carbon displacement. The majority of PL spectra are obtained using a 488-nm lasing line, with samples cooled to approximately 7 K. Of particular interest is the behaviour of the self-interstitial related centre, 3H, at 503.5 nm. The centre is particularly sensitive; its formation varies significantly with dose and dose rate and is severely quenched with incident laser power in excess of 10 mW. 3H is the dominant centre in highly doped (50–100 ppm) nitrogen samples, for doses between (10 19 –10 20 ) el/cm 2 , but reduces with higher doses. In lower nitrogen (few ppm) samples, the centre is considerably weaker after equivalent doses, comparable to the Raman line. In type IIa crystals, creation of 3H varies considerably from sample to sample. Upon annealing, 3H is at an optimum between 310 and 330 °C for type IIa diamonds and vanishes by 400 °C. Indications show these temperatures increase slightly as nitrogen content is increased. Migration of the centre well outside the irradiated area is frequent, tens of microns after irradiation and hundreds of microns post annealing. Other centres of interest include GR1, the neutral diamond vacancy, which is found to be created linearly with dose and be rate independent. Using 325 and 457.9 nm lines the TR12 centre was studied. It has a strong dose rate dependence, growing as dose rate raised to a power of approximately 2 and is unaffected by annealing up to 700 °C. A 244-nm line was used to study the 5RL centre and contrary to some reports was observed in samples containing approximately 0.1 ppm of nitrogen. PL provides an extremely sensitive way of measuring the nitrogen concentration in diamond, to levels of less than 0.1 ppm. The problem remains how to obtain an accurate measurement.

Interpretation of the temperature-dependent behaviour of the emission from isoelectronic tellurium centres in epitaxial ZnSe1-xTex

The emission from isoelectronic tellurium centres in metal-organic chemical vapour deposition ZnSe1-xTex has been investigated with photoluminescence and photoluminescence excitation spectroscopy. The emission is dominated by two bands, previously called the S1 and S2 bands and attributed to excitonic emission at isolated tellurium ions or tellurium ion pairs or clusters. The authors account for the temperature-dependent behaviour of these bands with a model in which the tellurium isoelectronic centres behave as spherical potential wells which trap holes via low lying resonant states in the valence band. Changes in intensity of the emission when both bands are observed together have been satisfactorily accounted for within the framework of this model and indicate that the S1 emission results from exciton recombination at tellurium pairs whilst the S2 emission results from recombination at tellurium triplets or larger clusters.

Optically-detected magnetic resonance studies of II–VI compounds

Abstract ODMR has proved to be a very successful technique for studying recombination processes in II–VI semiconductors. The increase in resolution compared with optical spectroscopy can exceed five orders of magnitude and often makes possible precise identification of the centres that participate in the recombination. Accurate determination of the structural and magnetic properties of defects and impurities is possible and the interactions between recombining centres can be investigated. The purpose of the paper is to describe the general principles of ODMR, with particular emphasis on recent developments for II–VI compounds and their alloys.

An ODMR investigation of silver doped ZnSe

ODMR (optically detected magnetic resonance) is now a well established method of identifying impurity centres in light emitting semiconductors, and has the advantage over EPR in that it permits direct correlation of the defect centre with a particular emission band. By using ODMR, we have shown that the strong green emission band peaking at 2.35 ± 0.02 eV (525 nm) in ZnSe:Ag is due to recombination between shallow donors and medium-deep acceptors which lie 0.31 ± 0.05 eV above the valence band. The ODMR spectrum consists of a strong isotropic donor resonance with a g value of 1.108 ± 0.001, and a set of anisotropic acceptor resonances at lower fields. The centre responsible for these acceptor resonances displays orthorhombic symmetry, and has |gz| = 2.410 ± 0.006, |gy| = 1.958 ± 0.006 and |gx| = 1.845 ± 0.006. The direction of gy is [110], whereas gz and gx are tilted away from the [111] and [112] directions respectively by 6.5° ± 0.5°. Hyperfine structure corresponding to a nuclear spin of 1/2 is also observed with |Az| = (30 ±8) ×10-4cm-1, |Ay| ⩽ 10×10-4cm-1 and |Ax| = (30±8)×10-4cm-1. The centre proposed to explain thes measurements is, in the paramagnetic state, an acceptor-like complex [Ag2+Zn−Se2-−Ag+i]+, in which the substitutional and interstitial ions are separated by the selenium ion.