Xiuxun Han

Intersubband optical absorption in quantum dots-in-a-well heterostructures

The theoretical analysis of intersubband optical transitions for InAs/ InGaAs quantum dots-in-a-well ( DWELL ) detectors are performed in the framework of effective-mass envelope- function theory. In contrast to InAs/ GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR ) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window ( 8 - 12 mu m ) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data. (c) 2005 American Institute of Physics.

Spin-dependent tunneling through NiFe nanoparticles

Double magnetic tunnel junctions (DMTJs) have been fabricated using alumina barriers with NiFe particles (∼1.8u2002nm) embedded within. The junctions exhibit spin-dependent transport properties and Coulomb blockade effects. We study differences between control samples and the DMTJs; specifically I-V characteristics and tunnel magnetoresistance (TMR) versus bias voltage characteristics. Clear differences in the systems are evident: the DMTJ with NiFe particles shows a marked peak in TMR at low bias, whereas the dependence of TMR on bias is much weaker for the control MTJ without embedded particles. Hence the TMR at low bias is enhanced by the Coulomb blockade effects.

Magnetoresistance of nickel nanocontacts fabricated by different methods

Nickel nanocontacts have been fabricated by focused ion-beam (FIB) milling of e-beam patterned planar contacts, FIB milling of conical-shaped nanoperforations in a silicon nitride membrane, and nanoimprinting using an atomic force microscope. Their sizes ranged from 1 to 30 nm. Magnetoresistance of up to 3% is developed in a field of a few millitesla. This is interpreted in terms of ballistic magnetoresistance across a wide domain wall whose structure is determined by dipolar interactions at the contact.

Microstructure investigation on barrier shapes of double barrier magnetic tunnel junctions

Barrier shapes and its detailed microstructures in the double barrier magnetic tunnel junctions were intensively investigated by both high resolution transmission electron microscopy and electron holography. Two broad (>2nm) potential wells (i.e., shapes of AlOx layers) with slanted interfaces were observed in the electron hologram of the as-deposited samples. However, in the hologram of the annealed samples, two narrowed (down to 1.18nm) and almost equal (height) potential wells with sharp and steep interfaces were acquired. This indicates that the value of tunnel magnetoresistance can be increased from 12.8% to 29.4% at room temperature by annealing treatment where the sharpness and height of the barriers played a critical role.