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1.
ZnO thin films and related optoelectronic
devices (2002-present)
ZnO is an attractive wide-bandgap (3.37eV at room temperature) oxide
semiconductor for its potential applications in short-wavelength
optoelectronic devices, such as ultraviolet (UV) detectors and UV
light-emitters. ZnO has the same wurtzite structure as GaN. Compared to
GaN, ZnO has a much larger exciton binding energy (~60 meV cf. ~25 meV),
which is favorable for fabricating low-threshold excitonic lasers.
Within one unit cell of wurtzite ZnO, there are two Zn atoms and two O
atoms. Their atomic position is: Zn: (0 0 0)，(1/3
2/3 1/2); O: (0 0 Z)，(1/3
2/3 1/2+Z) (where, Z=0.38 Å)
The
main barrier to get high quality ZnO thin films are the large mismatch
between Zno and the sapphire (or Si) substrate and the mixed polarity
which have great impact on the optoelectronic properties of ZnO-based
devices. Our focus is on the detailed and systematic investigation of
the relationship between growth condition and corresponding structure of
ZnO thin films prepared by MBE by SEM, XRD, TEM, HRTEM and Electron
Holography. Our aim is to find a better way preparing high quality ZnO
thin films (low density of defects and uniform polarity) and apply it to
fabricate related optoelectronic devices.
The research work is within the system of ZnO/buffer layer/substrate by
MBE, which involves
a) structures of the interface and the
effects of various buffer layers such as MgO, AlN and Ga.
b) the defect characteristics of ZnO
films.
c) the polarity study of ZnO films by
EELS, CBED and Electron Holography.
Highlight
We successfully measured the polarity of ZnO thin films by electron
holography.
Figure 1, Polarity results from the stacking sequence of the (0001)
atomic planes in wurtzite-type ZnO, that is, wurtzite zinc oxide has no
center-symmetry in the C-axis direction. We define that along the C-axis
direction, if zinc atom points to oxygen atom, the film is a zinc
polarity, i.e., [0001] polarity.
Figure 2, The
different type of the bounded charges in the surface of the ZnO film
indicates different spontaneous polarization in the film, thus different
polarity of ZnO film.
Figure 3,
The negative bounded charges (low potential) in the outer surface of ZnO
film indicates [0001] polarity (Zn polarity).
Collaborators:
Professor Qikun Xue
Tsinghua University ﹠ Institute of Physics,
Chinese Academy of Sciences
Professor Xiaolong Du
State Key Laboratory for Surface Physics
Institute of Physics, Chinese Academy of Sciences
2.
Magnetic Tunnel Junctions and related
spintronic devices
Magnetic tunnel junctions (MTJs) have promising potential applications
in spintronics devices such as magnetic random access memories (MRAMs),
magnetic read heads, and magnetic sensors. Therefore they are studied
extensively in these days.
 
Figure 1, This is the typical image of single barrier MTJ and the
corresponding schematic figure. The middle layer is the barrier layer,
generally Aluminum oxide (or MgO is preferred at present). The top and
bottom are ferromagnetic layers such as cobalt iron. Iridium Manganese
acts as antiferromagnetic pinning layer, as shown in the right schematic
diagram. The bottom layer is pinned by AFM, and the orientation of the
magnetic moments is fixed, but the top layer is free. So the orientation
of the magnetic moments of the top free layer can be changed from
parallel to anti-parallel by an external magnetic field. The tunnel
resistance for the parallel alignment (RP)is
normally lower than that for the anti-parallel (RAP)and
the ratio gives the magnitude of the TMR value:
TMR = (RAP - RP) /RP.
 
Why studying DBMTJs:
The double barrier MTJ has three advantages: a), A higher TMR value as
shown in theoretical works. The TMR value is more than two times of the
TMR value of single barrier. b), Be suitable for investigating the
spin-polarized electron coherent tunneling
c), TMR of DBMTJs decreases more slowly than that of SBMTJ as a function
of a bias voltage and has a higher V1/2 value and is more suitable for
application.
The TMR value and magnetic properties are strongly affected by the
quality of the barrier and the interfaces between ferromagnetic
electrodes. Our focus is on the study of barrier (AlOx layer) shapes and
microstructures in CoFe-based and CoFeB-based Double Barrier Magnetic
Tunneling Junctions
(DBMTJs) by HRTEMHigh Resolution Electron Microscopy and Electron
Holography and their effect on the magnetic properties of the DBMTJs.
Our object is to explore the right method acquiring the DBMTJ with both
high TMR and high V1/2 and meanwhile better understand the rich physics
phenomenon (such as oscillatory tunnel
magnetoresistance) in the DBMTJ.
Collaborator:
Professor Xiufeng Han
State Key Laboratory of Magnetism
Institute of Physics, Chinese Academy of Sciences
3.
Microstructures of defects in the structurally complex metallic alloy
phases
A phason is one of the most
characteristic defects resulting from atomic rearrangement in
quasicrystals. In the study of defects and plasticity of a complex alloy
phase (CMAP) ξ'- (Al-Pd-Mn) crystalline approximant of the corresponding
quasicrystal, new types of linear defect (phason line) and planar defect
(phason plane) were observed. The characteristic features of the defects
are that they appear as phason in quasicrystals. All of these
observations reveal that phason lines and, hence, their resulting phason
planes are quite important structural defects for the CMAPs.
The great achievement by Zhang’s
group is that the atomic structure determination of both individual phason lines and phason planes in the CMAP ξ'-Al–Pd–Mn
phase were determined by HREM observations and theoretical HREM
simulations. The results show that a representational atomic structural
models for phason planes in the ξ'-Al-Pd-Mn phase can be constructed by
introducing a shift between two parts of the perfect crystalline
structure using translation vector of
rac.
HREM simulations based on the structural model for both edge-on
and inclined types of phason lines agree well with the experimental
results. Taking into account the fact that the structural difference
between various curved phason planes is due to the variation in the
arrangement of individual phason lines.
First observation and systematic research of two-dimensional phason
defect-phason plane in the CMAP ξ-Al-Pd-Mn is presented using TEM. Base
on the study of two-dimensional planar defects (phason planes) in
related phase ξ'-Al-Pd-Mn, the atomic structure model of phason planes
in ξ -Al-Pd-Mn phase was determined by means of HREM and theoretical
HREM simulation. The related translation vector of phason plane in
ξ-Al-Pd-Mn phase is
rac.
A comparison of the geometrical arrangement and the order of motion of
phason planes observed in ξ -Al-Pd-Mn phase with those reported for
corresponding ξ'-Al-Pd-Mn phase shows that these two phases are
interconnected by phason defects.
 
The atomic structures of four CMAPs of -,
ξ1-, ξ2-, and ξ3- Al-Pd-Mn phases have
been determined by means of HREM study and theoretical structural
simulations base on the periodic stacking of two different types of
phason planes. The HREM simulations of the CMAP -phase
based on the structural model matches perfectly well with respect to the
experimental results, and the agreement were supported also by the
electron diffraction and X-ray powder diffraction simulations based on
the structural model.
 
The results show
that the structure model of periodic stacking of two types of phason
planes can be used to construct the atomic structure of related
superstructures of ξ’- and ξ-phase in the Al-Pd-Mn alloys.
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