Investigations On The Photoelectric Properties Of Semiconducting Transition Metal Chalcogens Compounds

Transition Metal Chalcogens semiconductors own kinds of particular characteristics with the special nature of d orbitals in the transition metal element, such as ferroelectric, ferromagnetic, thermoelectric, magneto-electric coupling,photo-electric effect and so on. Therefore, they have attracted much attention and are widely studied not only in the conventional semiconductor communities but also the emerging lowdimensional material field. By doping impurities into the semiconductors or changing their structures, researchers can modify the electronic structures of semiconductors and then make them applicable with the changed optoelectronic properties. This thesis mainly investigated the p-type doping of wide bandgap Zn O, and the strong microwave absorbing mechanism of black Ti O2. Besides, we have studied the electronic structures of gadolinium chalcogenides with first principles calculations for predicting semiconductors for neutron detecting. The details are as follows:(1) Zn O is a good candidate for making ultraviolet laser with low threshold current. However, the difficulties in preparing stable low-resistance P type Zn O have limited the developing of Zn O material. In this work, we selected Zn O single crystal as substrate, and grew nitrogen doped Zn O films on Zn-polar face with Plasma-assisted molecular beam epitaxy(P-MBE). The X-ray photoelectron spectroscopy measurements indicated that Zn-polar Zn O substrate is better suited for N-doping than O-polar. Then we employed a so called “polarization-induced doping” method, i.e., grew a Mg compositionally graded N-doped Mg Zn O film. The built-in electronic polarization will promote the ionization of NO acceptor dopants. In this way, we successfully fabricated Zn O p-n junctions with excellent rectification property. Wavelength selective ultraviolet photodetector was realized with this device which can even work under 0 volt. We propose “polarization-induced doping” provided a viable option for making P type Zn O with Nitrogen-doping.(2) Black Ti O2 nanoparticles in the crystalline-core/amorphous-shell structures synthesized through hydrogenation exhibit the extraordinary capability of absorbing microwaves with reflection loss values reaching-49.0 d B(99.99999%). Such highly efficient microwave absorption is unusual, considering the facts that their sizes are much smaller than the microwave wavelengths and being a wide-band-gap semiconductor, pristine Ti O2 is normally inert to microwave. In this study, with a facile PLA technique, we synthesized the amorphous Ti O2 nanoparticles and compared their microwave response with the crystalline anatase ones and those core/shell structure black Ti O2 nanoparticles. Although also in black color, the amorphous Ti O2 nanoparticles have been found to perform behavior similar as the crystalline particles in absorbing microwaves in the range of 1.0-18.0 GHz. It elucidates that the contribution from the impurities or defects in amorphous region is indeed trivial in the overall microwave-absorbing performance of black Ti O2. Moreover, with the help of FEM calculations, we demonstrated the collective oscillations of accumulated interface charges, i.e. plasmons, can be evoked in resonance with microwaves at a reasonably charge density, and their strong electric field enhancement can explain highly efficient microwave absorption of black Ti O2. Therefore, the strong microwave absorption in black Ti O2 indeed originates from the synergy effect between its crystalline-core and amorphous-shell. These results in fact indicate that “disorder-engineering” illuminates a new way of making highly-efficient MAMs with oxide semiconductors.(3) By converting the energy of nuclear radiations to excited electrons and holes, semiconductor detectors have provided a highly efficient way for detecting them, such as photons or charged particles. However, for detecting the radiated neutrons, those conventional semiconductors hardly behave well, as few of them possess the enough capability for capturing these neutral particles. While the element of Gd has the highest nuclear cross section, here for searching proper neutron-detecting semiconductors, we investigate theoretically the Gd-chalcogenides whose electronic band structures have never been characterized clearly. Among them, we identify that g-phase Gd2Se3 should be the best candidate for neutron detecting since it possesses not only the right bandgap of 1.76 e V for devices working under room temperature but also the desired indirect gap nature for charge carriers surviving longer. We propose further that the semiconductor neutron detectors with single-neutron sensitivity can be realized with such a Gd-chalcogenide on condition that their crystals can be grown with good quality.