J.J. Davies

CdS/CdSe intrinsic Stark superlattices

Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain‐induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band‐gap tunability and the space‐charge effects ...

Ga2Te3 and tellurium interfacial layers in ZnTe/GaSb heterostructures studied by Raman scattering

Raman spectroscopy of ZnTe layers grown by molecular beam epitaxy on (100) GaSb is reported. When the laser excitation is above the band gap of the ZnTe, scattering is observed only from the ZnTe LO mode and overtones. With excitation below the ZnTe band gap, a series of low frequency peaks is observed. By comparison with bulk data these peaks are identified as originating from Ga2Te3 and Te present at the GaSb/ZnTe interface. We conclude that the presence of this interface material may degrade the layer quality and give rise to the anomalously large strain previously reported for such epilayers.

Photoluminescence studies of CdS/CdSe wurtzite superlattices; Evidence for large piezoelectric effects

Abstract Strained-layer superlattices of wurtzite CdS/CdSe were grown on (111)A GaAs. The photoluminescence shows a single intense emission band which shifts to lower energy in longer period structures. This shift, which cannot be accounted for by the usual Kronig-Penney model, provides direct evidence for the presence of piezoelectric fields in the structures.

Raman studies of phosphorus-doped ZnSe

Abstract Raman spectroscopy of ZnSe crystals doped with phosphorus and gallium is described. The data are analysed in the context of previously reported optically detected magnetic resonance experiments which showed that substitutional phosphorus forms a deep acceptor 0.7 eV above the valence band. A Raman signal at 375 cm -1 corresponding to the T 2 vibrational mode of a centre of T d symmetry is ascribed to the ionized state of the phosphorus acceptor. The results are consistent with the magnetic resonance data, which show that the phosphorus is not associated with a second impurity or defect.

Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn 1 − x Mn x S e / Z n S e quantum disk samples of different radii

Quantum disc structures (with diameters of 200 and 100 nm) were prepared from a

Spin-flip Raman scattering studies of doped epitaxial zinc selenide

{\mathrm{Zn}}_{0.72}{\mathrm{Mn}}_{0.28}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}

Spin‐flip Raman scattering from shallow and deep donor centers in nitrogen‐doped p‐type zinc selenide

single quantum well structure by electron beam lithography followed by an etching procedure that combined dry and wet etching techniques. The quantum disc structures and the parent structure were studied by photoluminescence and photoluminescence excitation spectroscopy. For the light-hole excitons in the quantum well region, shifts of the energy positions are observed following fabrication of the discs, confirming that strain relaxation occurs in the pillars. The light-hole exciton lines also sharpen following disc fabrication: this is due to an interplay between strain effects (related to dislocations) and the lateral size of the discs. A further consequence of the small lateral sizes of the discs is that the intensity of the donor-bound exciton emission from the disc is found to decrease with the disc radius. These size-related effects occur before the disc radius is reduced to dimensions necessary for lateral quantum confinement to occur but will remain important when the discs are made small enough to be considered as quantum dots.

Atmospheric pressure metalorganic chemical vapour deposition growth and optical studies of ZnSe1−xTex thin film alloys

Raman scattering has contributed to the understanding of semiconductor materials at a fundamental level and has also proved powerful in the characterization of semiconductors, their alloys and heterostructures. This brief review will indicate some of the ways in which Raman spectroscopy has been applied in the study of II-VI semiconductors, with special attention to recent examples relevant to the application of ZnSe-based materials in optoelectronics. In particular, studies of electron spin-flip Raman scattering in p-type ZnSe will be discussed which show the presence of two distinct donor centres in highly doped p-type ZnSe:N; the two donors can be distinguished in spin-flip Raman scattering experiments by their different g-values. The resonance behaviour of the two Raman signals confirms that they are associated with two different donor centres and reveals a correlation between the localisation energy of an exciton at the neutral donor centre and the binding energy of the electron to the donor ion. Directions for future experiments will be indicated.

The metalorganic chemical vapour deposition and photoluminescence of tellurium-doped ZnS/CdS: Te strained layer superlattices

The net acceptor concentration in p‐type ZnSe doped with increasing amounts of nitrogen is believed to be limited by the formation of a compensating donor species at a depth of about 45–55 meV beneath the conduction band. We report spin‐flip Raman scattering from these and other donor‐like centers in specimens produced by nitrogen radical doping during molecular beam epitaxial growth. The experiments show that the main compensating donor center introduced by nitrogen doping has a g value of 1.36±0.07, in agreement with the interpretation of previous optically detected magnetic resonance spectra from nitrogen‐doped layers.

Spectroscopic evidence for piezoelectric effects in wurtzite CdS/CdSe strained-layer superlattices

Abstract We report on the growth of ZnSe 1−x Te x layers on (100) oriented GaAs substrates by atmospheric pressure metal-organic chemical vapour deposition (MOCVD) with a new zinc adduct dimethylzinc-triethylamine. Optical studies were confined to samples with a low tellurium (Te) concentration, x