Effects of interfacial barrier confinement and interfacial states

on the light emission of Si nanocrystals

Jiarong Chen

supervisor：Ming Lu

Light emission of Si is of vital importance for integrated optoelectronics and photonics based on modern Si technologies. The composite of Si nanocrystals (Si-nc’s) embedded in dielectric matrix such as SiO2, Si3N4, or SiC has been regarded as a promising material for Si light sources due to its robustness in strength, stable light emission and feature of stimulated emission. However, practical Si light sources, whether photoexcited or LED-like, are still unavailable after years of studies. One of the bottleneck problems lies in low light emission of Si-nc. To enhance the photoluminescence (PL) of Si-nc, various approaches have been proposed such as hydrogen passivation, Si-nc density modulation, and rare earth doping, for example. To promote the electroluminescence (EL) of Si-nc, efforts are mainly focused on designing novel channels for carrier tunneling, and lowering the interfacial barrier between Si-nc and its matrix by using small band gap-width dielectrics.

It has been known that some interfacial states are responsible for the light emission of Si-nc, while others act as non-radiative centers. The light emission of Si nanocrystal (Si-nc) depends not only on the Si-nc itself, but also on the interface between Si-nc and its matrix. Recently some work shows that when the interfacial barrier confinement is weakened, the PL intensity of Si-nc could be drastically reduced, no matter how high the Si-nc density is. However, systematic works about the effect of interfacial barrier confinement and that of interfacial states on the light emission of Si-nc are still rare.

Here I report on two interfacial effects, i.e., the effect of interfacial barrier confinement and that of interfacial states, on the photoluminescence (PL) of Si-nc in a systematic manner via designing Si-nc samples with different matrices composed of SiO2 or/and Si3N4, and monitoring their PL emissions as functions of exciting photon wavelengths, combined with electron occupation probability calculations. It has been demonstrated that to achieve a strong PL of Si-nc, dielectric matrix with wide band gap widths are necessary such as SiO2, but the interfacial state between Si-nc and matrix also plays an important role. To enhance the EL of Si-nc, attention should be paid not only to the carrier injection and transfer as usually done, but also to the interfacial effects.

参考文献：

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Photoinduced Spin Precession in Co2FeAl/(Ga,Mn)As bilayer with Low Power Excitation

Haochen Yuan

Supervisor: Haibin Zhao

Diluted magnetic semiconductors (DMS), in which magnetic impurities are artificially embedded into a semiconducting host, may allow the integration of the spin degree of freedom with semiconducting properties in a single material. As a paradigmatic case of DMS, (Ga,Mn)As has been investigated extensively in the past decades due to its promising potential in semiconductor spintronic devices[1]. However, the low Curie temperature (TC) of (Ga,Mn)As may hamper its practical application. The highest TC reported up to now is only 200 K[2], still well below room temperature. One of the promising ways to solve this problem is utilizing proximity effect at the interface of ferromagnetic metal/ferromagnetic semiconductor heterostructures. Recently, magnetic order and coupling in Fe/(Ga,Mn)As films were studied and proximity effect induced magnetic order were observed that extended more than 2 nm, even at room temperature[3]. On the other hand, cobalt-based Heusler alloys are desirable spintronic materials due to their high spin polarization, low Gilbert damping constant, and high TC. High-quality Heusler alloy Co2FeAl could be well epitaxied on GaAs (001). In 2013,  ferromagnetic interfacial interaction in the Co2FeAl/(Ga,Mn)As bilayer was observed and the Mn ions in a 1.36 nm thick (Ga,Mn)As layer remain spin polarized up to 400 K due to the magnetic proximity effect[4]. So spin dynamics research in Co2FeAl/(Ga,Mn)As bilayer will not only present direct evidence of interfacial coupling, but also reveal the  relationships between interfacial coupling and spin precession.

In this work, pronounced spin precessions are observed in Co2FeAl/(Ga,Mn)As bilayer under irradiation of ultrafast laser pulses with an extremely low energy density I of ~5 J/cm2 at room temperature. This is consistent with the fact that there is interfacial coupling between two layers. More importantly, that interfacial coupling is modulated by the ultrafast femtosecond laser triggering spin precession in Co2FeAl. Our findings will provide a new platform for designing room temperature ferromagnetic DMSs and suggest a pathway for ultrafast manipulation of ferromagnetic spin state in spintronic devices with low power density.

参考文献：

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[2]. L. Chen et al., Nano. Lett. 11, 2584 (2011).

[3]. F. Maccherozzi et al., Phys. Rev. Let. 101, 267201 (2008).

[4]. S. H. Nie et al., Phys. Rev. Let. 111,027203 (2013).

A Novel Balance Detector Specially Designed for Ultrafast Laser Pump-probe Measurement with Low Repetition Rate.

Zhe Zheng

Supervisor: Haibin Zhao

A novel balance detection method of low repetition rate ultrafast laser pump-probe technique is proposed here. The balance method aims at how to reduce the large mismatch differential signal caused by different responses in the time domain of the two photo diodes or other electronic components, as well as to raise the signal-noise ratio. Kennel of the new method is to reshape the photo-current generated by the sharp, delta function like laser pulses into a more smooth form. With the help of this pulse reshape technique, signal-noise ratio of the balance detector is raised about one hundred times more than normal cross polarization technique both theoretically and experimentally, while huge mismatch differential signal disappears as well. Moreover, gate integrator, a must in the conventional ultrafast measurement, can be deleted from our data collecting procedure, which will surely introduce a certain degree of noise.

参考文献：

[1]Christopher A. Werley, Stephanie M. Teo, and Keith A. Nelson, Review of scientific instrument 82,123108(2011).

[2]A. A. Rzhevsky et al, Physical review B 75, 224434 (2007).

[3]H. -G. Bach, Proc. Of SPIE Vol. 6014 60140B-1.

[4]A. Eschenlohr, nature material Vol. 12, 2013, DOL: 10.1038/NMAT354

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