时 间： 2013年5月9日下午4点

地 点： 四川大学物理馆三楼学术报告厅

报告题目：Spin degree of freedom in organic semiconductors and spin valves, and charge transport in amorphous zinc tin oxide

报告人： Dr.Leander Schulz(美国德克萨斯大学奥斯汀分校)

摘要:

The presentation will be divided into two sections: The first part will deal with the spin degree of freedom in organic semiconductors (OSC) and organic-metallic spin valves. In the second part, recent results of charge transport measurements in thin-film transistors based on inorganic zinc tin oxide (ZTO) are presented.

Using the spin property of charge carriers for electronic applications has been the goal and, to a large extent, a dream for a large community ranging from physicists to electrical engineers. Since organic spintronics is a fairly young research field, many fundamental questions are still unanswered or at least contentious. For instance, what is/are the spin relaxation mechanism(s) in OSC? What are the spin relaxation rates? With respect to spin valves, is spin-polarized current injected through pin holes or the entire surface? The list of questions is very long. I will try to answer some of these questions.

After a brief introduction to organic spintronics, I will show how the electron spin relaxation rate in organic semiconductors can be determined with the so-called muon spin relaxation technique [1]. By employing another branch of muon spectroscopy, the low-energy muon spin rotation technique, the depth-resolved measurement of the spin polarization of injected carriers at buried interfaces in fully functional organic-based spin valves was realized [2]. The spin coherence length is determined from the spatial dependence of the spin polarization of the electrons that are injected away from the metal electrode into the organic semiconductor [2]. Eventually, I will present how the sign of the spin polarization of charge carriers that move across a metal-organic interface was modified by inserting an additional simple polar layer. This inversion of the spin polarization is confirmed by macroscopic magneto-transport measurements [3].

In the second part, transparent amorphous semiconductor oxides will be discussed, a material class that attracted attention because of its application in transparent electronics. An example of these semiconducting oxides is the wide band gap n-type semiconductor zinc tin oxide (ZTO) whose mobility can exceed 25 cm2/Vs despite its amorphous state [4].

From temperature-dependent characteristics of field-effect transistors that are based on ZTO, the temperature-dependent mobility and consequently information about the nature of the charge carrier transport in these devices are obtained [4,5]. At sufficiently high mobilities and gate bias, a mobility edge that separates thermally activated and band-like transport is observed. For low-mobility regimes and samples, charge transport is dominated by thermal excitation. This transport region is well described by the multiple-trap and release model. Furthermore, I will discuss the determination of the trap-release energies and the different mobilities (2T vs. 4T, linear vs. saturation) of such a FET [5].

By partially combining the two research fields, the talk will be concluded by outlining possible research directions in the future.

[1] L. Schulz et al., Physical Review B 84, 085209 (2011).

[2] A. J. Drew et al., Nature Materials 8, 109 (2009).

[3] L. Schulz et al., Nature Materials 10, 39–44 (2011).

[4] C.-G. Lee and A. Dodabalapur, Appl. Phys. Lett. 96, 243501 (2010).

[5] L. Schulz et al., submitted to Applied Physics Letters (2013).