A simple and inexpensive circuit for emission and capture deep level transient spectroscopy

A simple and inexpensive circuit for deep level transient spectroscopy is described, which allows rapid characterization of emission as well as capture activation energies of deep levels. This flexibility of making capture activation studies affords more information on defect morphology than the more standard emission activation studies. This is demonstrated by making a representative capture activation energy measurement on the EL6 level in undoped n-type GaAs of 0.484±0.005 eV. Also the spectrometer has shown better performance than earlier reported systems by its ability to resolve the side peaks of the EL6 level, for which emission activation energies of 0.29 and 0.4 eV are assigned. Constructed around a commercially available capacitance meter and pulse generator, the control circuitry is designed and developed using inexpensive and off-the-shelf integrated circuits. © 1996 American Institute of Physics.published_or_final_versio

The transformations of the EL6 deep level defect in n-GaAs: is EL6 a DX-like center?

Symposium Theme: Defect and impurity engineered semiconductors IIBased on the charge redistribution effect, as observed by the present authors, and the earlier reported large lattice relaxation and persistent photoconductivity phenomena associated with the EL6 defect seen in doped, undoped, semiinsulating(SI) and low temperature grown GaAs (LT-GaAs), it is suggested that this defect be classified as a DX-center. A tentative unified atomic model is proposed for all the native defects EL2, EL3, EL5, and EL6 observed in GaAs.published_or_final_versio

Inexpensive circuit for the measurement of capture cross section of deep level defects in semiconductors

A simple and inexpensive circuit to facilitate the direct measurement of capture cross section, when synchronized with a deep level transient spectroscopy system, is described. It avoids the most commonly encountered problem of loading and distortion of the bias (trap filling) pulses of nanosecond duration in the capture cross-section measurement. The capacitance meter, whose internal circuitry is responsible for the distortion, is connected and disconnected from the rest of the apparatus with the help of simple and low-cost reed relay switches featuring high operating speed and low contact resistance. Sharp bias pulses as small as 30 ns can successfully be applied to the sample with no observable distortion. Finally, a representative measurement is shown to demonstrate the simplicity and high performance of the circuit. © 1996 American Institute of Physics.published_or_final_versio

Nature of the bulk defects in GaAs through high-temperature quenching studies

Deep-level transient spectroscopy has been applied to n-type horizontal gradient freeze grown GaAs that has been subjected to thermal stressing (quenching) and varying degrees of arsenic outdiffusion during rapid thermal annealing. The concentrations and activation energies of the various deep donor levels have been monitored. As a result of the external excitations in the lattice due to the thermal stress (quenching), dramatic effects occur in the defect level structure that could be of importance to device technology. It is found that the native EL6 group of defects is nearly absent in rapid thermally annealed material, while the levels EL5 and EL8 appear with EL3 becoming a dominant level that could act as a recombination center. With the lengthening of annealing time and significant As outdiffusion, there is a general reduction of the EL2, EL3, and EL5 defect concentrations together with a complete removal of EL8. Moreover, the EL2 activation energy may be varied from 0.827 to 0.922 eV by controlling the level of As out-diffusion. These observations are discussed in terms of the As Ga-As i model of the EL2 defect and the V As-V Ga divacancy model for the EL6 group of defects. The EL3, EL5, EL8, and EL15 defect levels seen in samples subjected to rapid thermally quenching are attributed to the breakup of V As-As i Frenkel pair defects known to be present in the as-grown material.published_or_final_versio

DX-like properties of the EL6 defect family in GaAs

Capacitance-voltage characterization at different temperatures and emission and capture deep-level transient spectroscopy carried out on undoped n-type GaAs lend strong confirmation to the recent suggestion that the EL6 defect arises from a center that is DX-like in nature. The evidence comes from the observation of an anomalous filling pulse duration dependence of the peak intensities of three to four different EL6 sublevels, similar to that recently found for the DX center in Al xGa 1-xAs and attributed to the charge redistribution. In addition, capture transients reveal large capture barriers (0.2-0.3 eV), which are typical of a defect undergoing large lattice relaxation into a deep-lying state. These observations indicate that the EL6 defect center comprises of a center with three to four slightly different ground-state configurations, each one of which forms as a result of some bond-breaking atomic displacement on capture of a second electron at the defect site. The significance of this in understanding the microstructure for the EL6 center is briefly discussed.published_or_final_versio

Deep level traps in the extended tail region of boron-implanted n-type 6H-SiC

Deep traps in the boron extended tail region of ion implanted 6H-SiC pn junctions formed during annealing have been studied using deep level transient spectroscopy. Dramatically high concentrations of ∼1016 cm-3 of the D center have been observed through the unusual appearance of minority peaks in the majority carrier spectra. No evidence is found for any shallow boron acceptor in this region, but an induced hole trap Ih at EV+0.46 eV is found under cold implantation conditions. These results support the picture of the extended tail, rich in boron-vacancy complexes such as the D center, which forms as a result of vacancy enhanced indiffusion. The dominance of the electrically active D center in the depletion layer of the technologically important SiC pn junction diode suggests the need for further research in this area. © 1998 American Institute of Physics.published_or_final_versio

Giant Persistent Photoconductivity of the WO3 Nanowires in Vacuum Condition

A giant persistent photoconductivity (PPC) phenomenon has been observed in vacuum condition based on a single WO3 nanowire and presents some interesting results in the experiments. With the decay time lasting for 1 × 104 s, no obvious current change can be found in vacuum, and a decreasing current can be only observed in air condition. When the WO3 nanowires were coated with 200 nm SiO2 layer, the photoresponse almost disappeared. And the high bias and high electric field effect could not reduce the current in vacuum condition. These results show that the photoconductivity of WO3 nanowires is mainly related to the oxygen adsorption and desorption, and the semiconductor photoconductivity properties are very weak. The giant PPC effect in vacuum condition was caused by the absence of oxygen molecular. And the thermal effect combining with oxygen re-adsorption can reduce the intensity of PPC

Determination of an optimal dosing regimen for aspirin chemoprevention of 1,2-dimethylhydrazine-induced colon tumours in rats

In order to establish an optimal timing and duration of aspirin treatment in the chemoprevention of 1,2-dimethylhydrazine (DMH)-induced colon cancer in rats, colon tumours were induced using an established protocol and aspirin was given in the diet at 500 p.p.m. during various stages of colon carcinogenesis. Results indicate that only aspirin treatment throughout the entire carcinogenic period significantly reduced tumour incidence and volume whereas intermittent aspirin dosing increased tumour number and/or volume, suggesting that aspirin must be used for an extended period in order to gain any chemopreventive benefit. © 1999 Cancer Research Campaig

malERA: An updated research agenda for insecticide and drug resistance in malaria elimination and eradication

Resistance to first-line treatments for Plasmodium falciparum malaria and the insecticides used for Anopheles vector control are threatening malaria elimination efforts. Suboptimal responses to drugs and insecticides are both spreading geographically and emerging independently and are being seen at increasing intensities. Whilst resistance is unavoidable, its effects can be mitigated through resistance management practices, such as exposing the parasite or vector to more than one selective agent. Resistance contributed to the failure of the 20th century Global Malaria Eradication Programme, and yet the global response to this issue continues to be slow and poorly coordinated—too often, too little, too late. The Malaria Eradication Research Agenda (malERA) Refresh process convened a panel on resistance of both insecticides and antimalarial drugs. This paper outlines developments in the field over the past 5 years, highlights gaps in knowledge, and proposes a research agenda focused on managing resistance. A deeper understanding of the complex biological processes involved and how resistance is selected is needed, together with evidence of its public health impact. Resistance management will require improved use of entomological and parasitological data in decision making, and optimisation of the useful life of new and existing products through careful implementation, combination, and evaluation. A proactive, collaborative approach is needed from basic science and the development of new tools to programme and policy interventions that will ensure that the armamentarium of drugs and insecticides is sufficient to deal with the challenges of malaria control and its elimination

Voluntary exercise inhibits intestinal tumorigenesis in ApcMin/+ mice and azoxymethane/dextran sulfate sodium-treated mice

<p>Abstract</p> <p>Background</p> <p>Epidemiological studies suggest that physical activity reduces the risk of colon cancer in humans. Results from animal studies, however, are inconclusive. The present study investigated the effects of voluntary exercise on intestinal tumor formation in two different animal models, <it>Apc</it>^Min/+mice and azoxymethane (AOM)/dextran sulfate sodium (DSS)-treated mice.</p> <p>Methods</p> <p>In Experiments 1 and 2, five-week old female <it>Apc</it>^Min/+mice were either housed in regular cages or cages equipped with a running wheel for 6 weeks (for mice maintained on the AIN93G diet; Experiment 1) or 9 weeks (for mice on a high-fat diet; Experiment 2). In Experiment 3, male CF-1 mice at 6 weeks of age were given a dose of AOM (10 mg/kg body weight, i.p.) and, 12 days later, 1.5% DSS in drinking fluid for 1 week. The mice were then maintained on a high-fat diet and housed in regular cages or cages equipped with a running wheel for 16 weeks.</p> <p>Results</p> <p>In the <it>Apc</it>^Min/+mice maintained on either the AIN93G or the high-fat diet, voluntary exercise decreased the number of small intestinal tumors. In the AOM/DSS-treated mice maintained on a high-fat diet, voluntary exercise also decreased the number of colon tumors. In <it>Apc</it>^Min/+mice, voluntary exercise decreased the ratio of serum insulin like growth factor (IGF)-1 to IGF binding protein (BP)-3 levels. It also decreased prostaglandin E₂and nuclear ��-catenin levels, but increased E-cadherin levels in the tumors.</p> <p>Conclusion</p> <p>These results indicate hat voluntary exercise inhibited intestinal tumorigenesis in <it>Apc</it>^Min/+mice and AOM/DSS-treated mice, and the inhibitory effect is associated with decreased IGF-1/IGFBP-3 ratio, aberrant β-catenin signaling, and arachidonic acid metabolism.</p