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Current Fluctuations in Single Barrier Vertical GaAs/AlAs/GaAs Tunneling Devices

100%

Przybytek J. , Baj M.

nr 2

221-226

We report the experimental results of the low temperature (T = 4.2 K) low-frequency current fluctuations measurements in the single-barrier resonant tunneling GaAs/AlAs/GaAs device with Siδ-doping in the center of the 10 nm thick AlAs barrier. The dimensions of the device were 200μm by 200μm. For the biasing voltages 0.1 V<|U|<1 V we observed the Fano factors between F = 0.7 and F = 0.95. We explain it by the existence of the trapping centers/imperfections/resonant levels inside the barrier participating in the transport for this range of voltages. Only for the smallest biasing voltages the Fano factor tends to F = 1, expected for a highly nontransparent barrier.

Observation of Thermally-Activated Electron Traps in GaAs/AlAs/GaAs Heterostructures in Low-Frequency Noise Measurements

61%

Przybytek J. , Stankiewicz R. , Gryglas-Borysiewicz M. , Baj M. , Cavanna A. , Faini G.

nr 5

723-725

During our investigations of tunneling process in thin 7 nm thick GaAs/AlAs/GaAs vertical single-barrier tunneling structure with Si δ-doping inside the barrier we have observed fluctuations of the tunneling current which exhibited large Lorentzian noise with intensity depending on biasing voltage. We have shown that Lorentzian noise originates from multilevel random telegraph noise of the small number of fluctuators which influence the tunneling process. Time-domain analysis of the current noise measured for temperatures between 4.2 K and 50 K enabled to determine the thermal activation energies of these fluctuators lying between 0.8 and 3 meV.

Visualisation of the extent of damage in a rat spinal cord injury model using MR Microscopy of the water diffusion tensor

61%

Krzyzak A. T. , Jasinski A. , Weglarz W. P. , Adamek D. , Sagnowski P. , Baj M.

nr 3

255-264

Magnetic Resonance Diffusion Tensor Imaging (DTI) of the control and traumatic injured spinal cord of a rat in vitro is reported. Experiments were performed on excised spinal cords from 10 Wistar rats, using a home-built 6.4 T MR microscope. MRI and histopathological results were compared. Presented results show that DTI of the spinal cord, perfused with formalin 10 minutes after the injury, can detect changes in water diffusion in white matter (WM) and in gray matter (GM), in areas extending well beyond the region of direct impact. Histology of neurons of the GM shows changes that can be attributed to ischemia. This is in agreement with the observed decrease of diffusion in the injured regions, which may be attributed to the cytotoxic edema due to ischemia. However, the diffusion changes in highly anisotropic WM seem to be caused by a direct action of mechanical force of impact, which significantly distorts the nerve fibers.

Spectroscopy of a Single Si Donor by the Resonant Tunnelling Experiment

61%

Baj M. , Gryglas M. , Jouault B. , Maude D. , Faini G. , Gennser U. , Cavanna A.

tom 110

nr 2

157-162

Resonant tunnelling via single impurities placed in a single barrier was experimentally studied. The typically observed structures in the current-voltage characteristics seem to be paired. Such a pair can be interpreted as ground and excited states of an impurity, related to X_{xy} and X_z valleys, which is in agreement with the results obtained for big mesas. However, it is not clear why X_{xy} states can be seen without any phonon participation.

Transport in GaAs/AlAs/GaAs [111] Tunnel Junctions

61%

Lewińska S. , Gryglas-Borysiewicz M. , Przybytek J. , Baj M. , Jouault B. , Gennser U. , Ouerghi A.

nr 5

606-608

Resonant tunneling in single-barrier GaAs/AlAs/GaAs junctions grown in [111] direction was studied for samples with different concentration of silicon δ-doping in AlAs. In the I(V) characteristics, measured at 4 K, two kinds of peaks were observed: related to resonant tunneling via donors states in the barrier, and through X-minimum quantum well subbands. The results are compared to those previously obtained for analogous samples grown along [001] direction. The investigations reveal different symmetry of donor states in both cases.