化合物半导体表面金属纳米粒子的形成与性质

First and foremost, I would like to express my heartfelt thanks to my thesis advisor  Professor Rachel S. Goldman for guidance and patience throughout my doctoral research. I  learned three precious lessons from her as follows. First, any theoretical prediction should be  experimentally proved. Second, we must be open to new ideas and ready to modify our opinions  if contradictory evidence emerges. Third, everything should be based on intellectual honesty.  These simple yet profound principles which she taught me resulted in this dissertation. I believe  that her enthusiasm and high standard for work will benefit me in the future.

Surface plasmon resonance (SPR) is the collective oscillation of electrons in  a solid or liquid induced by incident light.1  The resonance occurs when the frequency of incident  light matches the natural frequency of electrons oscillating against the restoring force of positive  nuclei, as shown in Fig. 1.1 Typical plasmonic materials are metals because of high free  electron density. Recently, SPR of metallic NPs on semiconductor surfaces have shown  significant promise for various applications including enhanced light emission, efficient solar  energy harvesting, high sensitivity biosensing, and negative refractive index metamaterials. It  has been shown that metallic NPs enable the generation of SPR in the ultraviolet, visible, and  infrared ranges, where SPR energies are inversely proportional to diameters of the metallic NPs,  as shown in Fig. 1.2. 

Existing methods for the fabrication of these metallic NPs require complex multi-level  processing such as lithography. Meanwhile, ion irradiation-induced surface NP formation  involves the self-assembly process for achieving dense arrays of metallic NPs on semiconductor  surfaces. In this thesis, we explore a new plasmonic material (Ga) fabricated via FIB irradiation  for enhancing energy conversion efficiencies of semiconductors. Here, we report formation and  properties of FIB-induced metallic NPs on compound semiconductor surfaces.

In particular, this thesis focuses on the influence of metallic NP on optical properties of  semiconductors, and therefore, it is important to discuss how SPR of metallic NPs is related to  the light emission efficiency of semiconductor surfaces. Figure 1.4(a) shows a typical PL spectra  taken from GaAs layers grown on a GaAs substrate in Goldman group Gen-II molecular beam  epitaxy system. To enhance the PL efficiency of semiconductors such as GaAs layers, SPR of  metallic NPs have been utilized to tailor light-matter interaction in metal-semiconductor  composites.1,13,17  The metallic NP-enhanced light emission efficiency of semiconductor surfaces  has been explained mainly in terms of enhanced spontaneous emission rate.13  Meanwhile, the  influence of surface metallic NPs on the incident light absorption efficiency of semiconductor  layers has been a remaining issue for the metallic NP SPR-enhanced light emission efficiency of  semiconductor surfaces. Figure 1.4(b) and 1.4(c) shows a band diagram and a schematic  illustration where the relative influences of metallic NP SPR on absorption and emission  processes in semiconductor layers are considered. One of this thesis study’s goals is studying the  influence of Ga NP diameter-dependent SPR on both absorption and emission processes.

Fig1.1

All FIB irradiation described in this thesis was carried out in a FEI NOVA 200 dual beam  workstation shown schematically in Fig. 2.1. The FIB system consists of an ion beam column,  an electron beam column, an energy dispersive x-ray, and secondary electron detectors, in a high  vacuum chamber with 10-5 torr base pressure. This system combines a high-resolution secondary  electron microscope for surface imaging, a Ga+ FIB for irradiation and patterning, and x-ray  microanalysis for characterization. During FIB irradiation, surface imaging and x-ray  microanalysis are achieved via the collection of secondary electrons and characteristic x-rays,  respectively.