The operation of multiquantum-well barriers

The multiquantum barrier (MQB), proposed by Iga et al in 1986, has been shown by several researchers to be an effective structure for improving the operating characteristics of laser diodes. These improvements include a reduction in the laser threshold current and increased characteristic temperatures. The operation of the MQB has been described as providing an increased barrier to electron overflow by reflecting high energy electrons trying to escape from the active region of the laser.This is achieved in a manner analogous to a Bragg reflector in optics. This thesis presents an investigation of the effectiveness of the MQB as an electron reflector. Numerical models have been developed for calculating the electron reflection due to MQB. Novel optical and electrical characterisation techniques have been used to try to measure an increase in barrier height due to the MQB in AlGaInP.It has been shown that the inclusion of MQB structures in bulk double heterostructure visible laser diodes can halve the threshold current above room temperature and the characteristic temperature of these lasers can be increased by up to 20K.These improvements are shown to occur in visible laser diodes even with the inclusion of theoretically ineffective MQB structures, hence the observed improvement in the characteristics of the laser diodes described above cannot be uniquely attributed to an increased barrier height due to enhance electron reflection. It is proposed here that the MQB improves the performance of laser diodes by proventing the diffusion of zinc into the active region of the laser. It is also proposed that the trapped zinc in the MQB region of the laser diode locally increases the p-type doping bringing the quasi-Fermi level for holes closer to the valence band edge thus increasing the barrier to electron overflow in the conduction band.

Tooth wear in Irish teenagers: a laboratory and epidemiological study

Aim: To investigate the characteristics, development and determinants of toothwear among Irish schoolchildren. Methods: A cross-sectional (examination at 16-years-old) and longitudinal (examinations at 5-,12-,14-years) study were conducted. Two indices were used to measure toothwear, children/parents completed a demographic profile and questionnaire on oral hygiene and dietary practices, health, and lifestyle in both studies. Saliva was collected from consenting 16-year-olds. The explanatory variables for the cross-sectional and longitudinal study were derived from children/parents responses. Differences in salivary profiles were determined for subsets; the protein concentration was determined with Bradford protein assay and protein carbonyl concentration (a protein oxidation marker) was determined spectrophotometrically. Gel-electrophoresis and mass spectrometry determined proteins and ion chromatography inorganic ions. Statistical significance was accepted at p<0.05. Results: At 16-years-old the prevalence of toothwear with dentine visible was 44%. No difference in salivary flow rates existed. In unstimulated saliva a higher mean, protein carbonyl (p<0.0001) and total calcium concentration (p<0.002) existed for the group with moderate toothwear. In stimulated saliva the moderate toothwear group had a lower mean protein concentration(p<0.0001). The 2-DE protein spots prepared for a sub-group differed between those with toothwear and without. Mass spectrometry, identified one of the different proteins as IgA. For 16-year-olds, the self-reported factors indicated that brushing after breakfast was associated with lower toothwear scores(p<0.03). Nail-biting, being asthmatic or reporting a dry mouth were associated with higher toothwear scores(all p<0.05). Eating an apple daily or less was associated with less toothwear(p<0.002). In the longitudinal study toothwear into dentine at age five or 12-years was associated with more toothwear at age 14(all p<0.05). Discussion: The results illustrate the multifactorial aetiology of toothwear. The biochemical and physical correlates of saliva with toothwear requires further research. Conclusion: The impact of previous toothwear, salivary, dietary and personal factors on toothwear in the early permanent dentition is demonstrated.