Diluted Magnetic Semiconductors

Kun. Chen

Supervisor: Jing Li, Songyou Wang

Department of Optical Science and Engineering, Fudan University

Abstract:

The concept of Diluted Magnetic Semiconductors (DMS) was raised in 1960s. DMS are formed by replacing some of the nonmagnetic cations in II-VI, III-V, II-V or IV-VI group semiconductors (such as GaAs, GaN, GaIn, ZnO, InP) with magnetic cations such as Fe, Co, Ni and Mn. Great potential in applications has been excavated of DMS in non-volatile memory, spin valve, spin-FET, spin LED and so on. Charge and spin-the two intrinsic characteristics of electrons-have been researched and applied in a non-interference way for a pretty long time. DMS utilize the two properties simultaneously, existing the new branch of science-Semiconductor Spintronics, which has received a lot of attention. Origin of the magnetism in DMS has been studied in many cases.

This paper presents the history of DMS and spintronics. Several typical examples are involved as well as the inner principles. The situation of study on DMS is carried out for a better understanding. The prospect of this subject is also presented.

Keywords: diluted magnetic semiconductor, spintronics.

References:

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2.       Feng-Xian Jiang, Hong-Xiao Xu, et al. Applied Physics Letters 96, 052503 (2010).

3.       A. J. Behan, A. Mokhtari, et al. Physical Review Letters 100, 047206 (2008).

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Blue luminescent silicon nanocrystals prepared by ns pulsed laser ablation in water

Yanli Li

Supervisor:Jiada Wu

Abstract

To improve the light emission of silicon is the foundation for the development of silicon-based optoelectronic device and the realization of silicon-based optoelectronic integration. To build the nanostructure is an important method for the enhancement of silicon luminescence.

Si-ncs/ SiO₂ can be made by many different approaches and the most widely used one is based on phase separation and recrystallization. Sub-stoichiometric silica films （SiO_x）were deposited by different methods and annealed at 1100℃in nitrogen ambient to form Si-ncs/ SiO₂ . Our group have fabricated Si-ncs/ SiO₂ by reactive pulsed laser deposition with subsequent thermal annealing in an inert nitrogen atmosphere. Strong red PL band was observed at room temperature from Si nanocrystals with an average diameter of about 5nm at 325nm light excitation.

Here I report on a simple and cost-effective procedure to prepare blue luminescent colloidal silicon nanocrystals (Si-ncs) in de-ionized water at room temperature and atmospheric pressure by nanosecond pulsed laser ablation. At low laser fluences well-separated and spherical Si-ncs aggregates are formed. The fluence increase leads to fragmentation of spherical aggregates and the generation of finer Si-ncs with quantum confinement size effect. Aging in de-ionized water for several weeks improved the PL intensity. At least three weeks of aging are necessary for observation of broad blue room temperature PL with a maximum centered at 430nm.

It has been demonstrated that the confinement of plasma expansion by water results in an amplified shock pressure impact and a higher ablation rate. Oxide surrounding the Si-ncs provides suitable and stable surface passivation, resulting in proper exciton localization, and favorably offers a dielectric shell for nanoscale optoelectronic devices. The blue PL band intensity can be enhanced by increasing aging time and laser fluence. Preparing homogenous solid sample and further enhancing PL intensity is further experiments.

参考文献

[1]Vladimir Svrcek, Takeshi Sasaki, Applied Physics Letters 89, 213113(2006).

[2] Vladimir Svrcek, Takeshi Sasaki, Applied Physics B 94,133(2009).

[3]Vladimir Svrcek, Takeshi Sasaki, Yoshiki Shimizu, Journal of Laser Micro/Nanoengineering 2, 15(2007).

[4] V. Švrcek, T. Sasaki, T. Shimizu, N. Koshizaki, Journal Applied Physics 103, 023101 (2008)

[5] Vladimir Svrcek,  Journal of Laser Micro/Nanoengineering 5, 103(2010).

[6]Vladimir Svrcek, Davide Mariotti, and Michio Kondo, Optics Express 17,520(2009).

[7] Vladimir Svrcek, Takeshi Sasaki, Yoshiki Shimizu, Journal of Applied Physics， 103,023101(2008).

Dynamic study of ultrafast charge transfer in organic semiconductor

Wenping Guo

Supervisor: Haibin Zhao

Abstract

Solution-processed photovoltacics based on organic semiconductor materials represent a promising route to devices that can be processed inexpensively at ambient conditions and can be deposited with printing and coating techniques at large area substrates. However a significant limitation on the efficiency of such organic semiconductor solar cell is that absorption is mainly limited to visible wavelengths so that a significant fraction of the solar emission energy, occurring at longer wavelengths, cannot be harvested. To circumvent this limitation improving the efficiency of solar cell many efforts have been carried out on researching for low bandgap polymer materials studied using ultrafast transient absorption by pump-probe technology. Pump-probe experiments show photo-excitations to relax via an initial ultrafast decay while longer-lived photoinduced absorption is attributed to charge transfer exciton formation. The spectroscopy of transient absorption will help us to understand the processes of exciton generation, exciton recombination and change transfer in materials for choosing the right materials to improve the efficiency of the solar cell.

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