TUNNELING ESCAPE TIME OF ELECTRONS THROUGH DOUBLE-BARRIER RESONANT-TUNNELING STRUCTURES

Tunneling escape of electrons from quantum wells (QWs) has systematically been studied in an arbitrarily multilayered heterostructures, both theoretically and experimentally. A wave packet method is developed to calculate the bias dependence of tunneling escape time (TET) in a three-barrier, two-well structure. Moreover, by considering the time variation of the band-edge profile in the escape transient, arising from the decay of injected electrons in QWs, we demonstrate that the actual escape time of certain amount of charge from QWs, instead of single electron, could be much longer than that for a single electron, say, by two orders of magnitude at resonance. The broadening of resonance may also be expected from the same mechanism before invoking various inhomogeneous and homogeneous broadening. To perform a close comparison between theory and experiment, we have developed a new method to measure TET by monitoring transient current response (TCR), stemming from tunneling escape of electrons out of QWs in a similar heterostructure. The time resolution achieved by this new method reaches to several tens ns, nearly three orders of magnitude faster than that by previous transient-capacitance spectroscopy (TCS). The measured TET shows an U-shaped, nonmonotonic dependence on bias, unambiguously indicating resonant tunneling escape of electrons from an emitter well through the DBRTS in the down-stream direction. The minimum value of TET obtained at resonance is accordance with charging effect and its time variation of injected electrons. A close comparison with the theory has been made to imply that the dynamic build-up of electrons in DBRTS might play an important role for a greatly suppressed tunneling escape rate in the vicinity of resonance.

Effect of Surface Potential Barrier on the Electron Energy Distribution of NEA Photocathodes

By calculating the energy distribution of electrons reaching the photocathode surface and solving the Schrodinger equation that describes the behavior of an electron tunneling through the surface potential barrier,we obtain an equation to calculate the emitted electron energy distribution of transmission-mode NEA GaAs photocathodes. Accord- ing to the equation,we study the effect of cathode surface potential barrier on the electron energy distribution and find a significant effect of the barrier-Ⅰ thickness or end height,especially the thickness,on the quantum efficiency of the cath- ode. Barrier Ⅱ has an effect on the electron energy spread, and an increase in the vacuum level will lead to a narrower electron energy spread while sacrificing a certain amount of cathode quantum efficiency. The equation is also used to fit the measured electron energy distribution curve of the transmission-mode cathode and the parameters of the surface barri- er are obtained from the fitting. The theoretical curve is in good agreement with the experimental curve.

Ti Schottky barrier diodes on n-type 6H-SiC

Using thermal evaporation, Ti/6H-SiC Schottky barrier diodes (SBD) were fabricated. They showed good rectification characteristics from room temperature to 200degreesC. At low current density. the current conduction mechanism follows the thermionic emission theory. These diodes demonstrated a low reverse leakage current of below 1 X 10(-4)Acm(-2). Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800V. In addition. these SBDs showed superior switching characteristics.

Asymmetric dark current in double barrier quantum well infrared photodetectors

Asymmetric dark current and photocurrent versus voltage characteristic in the Double Barrier Quantum Wells (DBQWs) photovoltaic infrared photodetector has been studied. A model based on asymmetric potential barriers was proposed. The asymmetric potential thick barrier, which due to the Si dopant segregation during growth makes a major contribution to the asymmetrical I-V characteristic, calculations based on our model agree well with experimental results. This work also confirms the potential use of this DBQWs for infrared photodetector with large responsivity and little dark current under negative bias.

Resonant tunneling of holes in GaMnAs-related double- barrier structures

Using the multiband quantum transmitting boundary method (MQTBM), hole resonant tunneling through AlGaAs/GaMnAs junctions is investigated theoretically. Because of band-edge splitting in the DMS layer, the current for holes with different spins are tuned in resonance at different biases. The bound levels of the "light" hole in the quantum well region turned out to be dominant in the tunneling channel for both "heavy" and "light" holes. The resonant tunneling structure can be used as a spin filter for holes for adjusting the Fermi energy and the thickness of the junctions.

Structural and photoelectric studies on double barrier quantum well infrared detectors

GaAs/AlAs/GaAlAs double barrier quantum well (DBQW) structures are employed for making the 3 similar to 5 mu m photovoltaic infrared (IR) detectors with a peak detectivity of 5x10(11) cmHz(1/2)/W at 80K. The double crystal x-ray diffraction is combined with synchrotron radiation x-ray analysis to determine the exact thickness of GaAs, AlAs and GaAlAs sublayers. The interband photovoltaic (PV) spect ra of the DBQW sample and the spectral response of the IR photocurrent of the devices are measured directly by edge excitation method, providing the information about spatial separation processes of photogenerated carriers in the multiquantum wells and the distribution of built-in field in the active region.

Interface delamination of the thermal barrier coating subjected to local heating

To investigate the possible failure modes of the thermal barrier coating (TBC) used to protect the scramjet combustion chamber, the local heating via laser beam irradiation was utilized to simulate the service condition of high thermal flux and high temperature gradient. Firstly, the experimental method and process were described and the typical fracture morphology of the TBC under test were provided. Then, the theoretical and finite element modeling were carried out to study the temperature, deformation and stresses of the specimen when the top ceramic coat was subjected to local heating, and to demonstrate the mechanism on the failure of the TBC. It is revealed that the interface delamination shall appear and ultimately lead to the failure of the TBC under such thermal loading of local quick heating. According to the outcome of this study, the driving force of the interface delamination is influenced greatly by the key structural parameters and performance matching. Moreover, by utilizing the rules of the effects of these parameters on the fracture driving force, there is some possibility for the designer to optimize the performances of the TBC.

Fusion dynamics of symmetric systems near barrier energies

Within the framework of the improved isospin-dependent quantum molecular dynamics (ImIQMD) model,he fusion dynamics of symmetric reaction systems are investigated systematically. Calculations show that the number of nucleon transfer in the neck region is appreciably dependent on the incident energies, but strongly on he reaction systems. A comparison of the neck dynamics is performed for the symmetric reactions 58Ni+58Niand 64Ni+64Ni at energies in the vicinity of the Coulomb barrier. An increase of the ratios of the neutron to proton in the neck region at initial collision stage is observed and obvious for the latter system, which reduces the fusion barrier of two colliding nuclei. The distribution of the dynamical fusion barriers and the fusion excitation functions are calculated and compared with the available experimental data.

Alterations of immune functions induced by 12C6+ion irradiation in mice

Purpose: To estimate the biological risks to the immune system of the type of space radiation, 12C6+, encountered by cosmonauts during long-term travel in space. Materials and methods: The Kun-Ming strain mice were whole-body irradiated by 12C6+ ion with 0, 0.01, 0.05, 0.075, 0.2, 0.3, 0.5, 0.75, 1 or 2 Gy, at a dose rate of 1 Gy/min. At 35 days after irradiation, the thymus and spleen weights were measured, the natural killer (NK) cells activity of spleen was determined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT), and the interferon-gamma (IFN-gamma) levels in serum and thymus were detected with enzyme-linked immunosorbent assays (ELISA). Results: The results showed that the thymus weight, IFN-gamma levels in serum and the activity of splenic NK-cells had significantly increased at a dose of 0.05 Gy. With further dose increase, the weight of spleen continued to increase but the weight of thymus, IFN-gamma level and NK-cells activity declined. Conclusions: These results suggest that the dose of 0.05 Gy irradiation has a stimulatory effect on mouse immunity; this effect declined with increasing dose.