D. Wolverson

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.

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

Comparison of multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity

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

Spin-flip Raman scattering studies of doped epitaxial 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.

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

Large-area high-density patterns of quantum dots with a diameter of 200 nm have been prepared from a series of four multiple quantum well structures of different well width (40 A, 60 A, 80 A and 100 A) by electron lithography followed by ion beam etching. Below-bandgap photomodulated reflectivity spectra of the quantum dot samples and the parent heterostructures were then recorded at 10 K and the spectra were fitted to extract the linewidths and the energy positions of the excitonic transitions in each sample. The fitted results are compared with calculations of the transition energies in which the different strain states in the samples are taken into account. We show that the main effect of the nanofabrication process is a change in the strain state of the quantum dot samples compared with the parent heterostructures. The quantum dot pillars turn out to be freestanding, whereas the heterostructures are in a good approximation strained to the ZnTe lattice constant. The lateral size of the dots is such that extra confinement effects are not expected or observed.

Magnetic-field induced type I to type II transition in multiple quantum well samples

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.

Spin-flip Raman scattering from electrons bound to donors in both wells and barriers of CdTe/Cd0.93Mn0.07Te heterostructures

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.

Spin-flip Raman spectroscopy of nitrogen acceptors in ZnSe layers with different biaxial strains

A series of four multiple quantum well samples with Mn concentrations x of about 7.5% and well widths ranging from 40 A to 100 A have been studied by below-bandgap photomodulated reflectivity (BPR) at a temperature of 1.5 K and in magnetic fields up to 6 T in the Faraday geometry. The band alignment in zero magnetic field is type I with the ZnTe being the quantum well material. The giant Zeeman splitting (due to the s, p - d exchange interaction between the ions and the free carriers) makes possible a magnetic-field induced transition of the band alignment from type I to type II: for the heavy-hole potential, the well region moves from the ZnTe layer to the layer when the magnetic field is increased sufficiently. This type I - type II transition can be monitored by the magnetic-field dependence of the excitonic transitions, particularly of those that involve the heavy-hole state, such as the excitonic transition in the quantum well. The magnetic field dependences of the heavy-hole excitonic transitions obtained from the BPR spectra were compared with calculations in which the effects of chemical valence band offset (VBO), strain, exciton binding energy, interface roughness and, most importantly, the enhanced paramagnetism at the interfaces were accounted for. It was found that a determination of the chemical VBO is independent of the state of the interface for the samples with wider well width ( A), but that effects of interface roughness cannot be neglected for smaller well widths (<50 A). Taking all these factors into account the best agreement between experiment and calculation was obtained for a chemical VBO of .

Spin-flip Raman scattering studies of post-growth annealed p-type nitrogen-doped zinc selenide

Abstract The electron spin-flip Raman (SFR) spectra of a series of CdTe/Cd0.93Mn0.07Te dilute magnetic multiple quantum well structures show two distinct lines, one caused by electrons bound to donors in the wells and the other by electrons under the combined influence of the well potential and the potential due to ionised donors in the barriers. The assignment was confirmed by a variational calculation. The investigation has provided a unique method of studying the interaction between electrons and donors at different locations in a quantum well structure.