Photoreflectance and surface photovoltage spectroscopy of beryllium-doped GaAs/AlAs multiple quantum wells

We present an optical study of beryllium delta-doped GaAs/AlAs multiple quantum well (QW) structures designed for sensing terahertz (THz) radiation. Photoreflectance (PR), surface photovoltage (SPV), and wavelength-modulated differential surface photovoltage (DSPV) spectra were measured in the structures with QW widths ranging from 3 to 20 nm and doping densities from 2×10(10) to 5×10(12) cm(–2) at room temperature. The PR spectra displayed Franz-Keldysh oscillations which enabled an estimation of the electric-field strength of ~20 kV/cm at the sample surface. By analyzing the SPV spectra we have determined that a buried interface rather than the sample surface mainly governs the SPV effect. The DSPV spectra revealed sharp features associated with excitonic interband transitions which energies were found to be in a good agreement with those calculated including the nonparabolicity of the energy bands. The dependence of the exciton linewidth broadening on the well width and the quantum index has shown that an average half monolayer well width fluctuations is mostly predominant broadening mechanism for QWs thinner than 10 nm. The line broadening in lightly doped QWs, thicker than 10 nm, was found to arise from thermal broadening with the contribution from Stark broadening due to random electric fields of the ionized impurities in the structures. We finally consider the possible influence of strong internal electric fields, QW imperfections, and doping level on the operation of THz sensors fabricated using the studied structures. © 2005 American Institute of Physic

Energy levels in polarization superlattices: a comparison of continuum strain models

A theoretical model for the energy levels in polarization superlattices is presented. The model includes the effect of strain on the local polarization-induced electric fields and the subsequent effect on the energy levels. Two continuum strain models are contrasted. One is the standard strain model derived from Hooke's law that is typically used to calculate energy levels in polarization superlattices and quantum wells. The other is a fully-coupled strain model derived from the thermodynamic equation of state for piezoelectric materials. The latter is more complete and applicable to strongly piezoelectric materials where corrections to the standard model are significant. The underlying theory has been applied to AlGaN/GaN superlattices and quantum wells. It is found that the fully-coupled strain model yields very different electric fields from the standard model. The calculated intersubband transition energies are shifted by approximately 5 -- 19 meV, depending on the structure. Thus from a device standpoint, the effect of applying the fully-coupled model produces a very measurable shift in the peak wavelength. This result has implications for the design of AlGaN/GaN optical switches.Comment: Revtex

Graphene under hydrostatic pressure

In-situ high pressure Raman spectroscopy is used to study monolayer, bilayer and few-layer graphene samples supported on silicon in a diamond anvil cell to 3.5 GPa. The results show that monolayer graphene adheres to the silicon substrate under compressive stress. A clear trend in this behaviour as a function of graphene sample thickness is observed. We also study unsupported graphene samples in a diamond anvil cell to 8 GPa, and show that the properties of graphene under compression are intrinsically similar to graphite. Our results demonstrate the differing effects of uniaxial and biaxial strain on the electronic bandstructure.Comment: Accepted in Physical Review B with minor change

Ethnic Minorities and their Health Needs: Crisis of Perception and Behaviours

There is considerable evidence to suggest that racial and ethnic disparities exist in the provision of emergency and wider healthcare. The importance of collecting patient ethnic data has received attention in literature across the world and eliminating ethnic and racial health equalities is one of the primary aims of healthcare providers internationally. The poor health status of certain racial and ethnic groups has been well documented. The improvement of racial and ethnic disparities in healthcare is at the forefront of many public health agendas. This article addresses important policy, practice, and cultural issues confronted by the pre-hospital emergency care setup. This aspect of care plays a unique role in the healthcare safety net in providing a service to a very diverse population, including members of ethnic and racial minorities. Competent decision making by the emergency care practitioners requires patient-specific information and the health provider's prior medical knowledge and clinical training. The article reviews the current ethnicity trends in the UK along with international evidence linking ethnicity and health inequalities. The study argues that serious difficulties will arise between the health provider and the patient if they come from different backgrounds and therefore experience difficulties in cross-cultural communication. This adversely impacts on the quality of diagnostic and clinical decision making for minority patients. The article offers few strategies to address health inequalities in emergency care and concludes by arguing that much more needs to be done to ensure that we are hearing the voices of more diverse groups, groups who are often excluded from engagement through barriers such as language or mobility difficulties