To summarize the key issues for SCs integrating Si QDs, the Si QD thin films require uniform QD size [21, 28], heavily-doped concentration [21], and smaller QD separation [28-30], that is, higher QD density, or a more suitable matrix material. So far, the Si QD thin films are commonly deposited by a silicon-rich oxide single-layer (SRO-SL) or a [silicon dioxide/silicon-rich oxide] multilayer ([SiO2/SRO]-ML) structure. The SRO-SL structure is an easy and quick deposition process for the Si QD thin films, however, it is hard to well control the QD size and uniformity and efficiently reduce the QD separation [31, 32]. Although the [SiO2/SRO]-ML structure promises the QD size control and separation reduction, shown as Fig. 1-15, the minimum thickness of 2 nm for the SiO2 barrier layers is required to prevent the excess Si atoms in SRO active layers from over-diffusing [11], which will still degrade the carrier transport efficiency [28-30]. Hence, significantly smaller QD separation or other more suitable matrix materials [33] are indispensable for the Si-based SCs development integrating Si QDs.
Fig. 1-15: Illustration of the Si QD thin film fabricated by using a [SiO2/SRO]-ML deposition structure.
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1-4.1 Si QD Thin Films Utilizing a Gradient Si-rich Oxide Multilayer (GSRO-ML) Structure
In order to further enhance the carrier transport efficiency, more QD separation reduction is required to significantly increase the carrier tunneling probability [28-30].
Fig. 1-16 shows an illustration of the carrier transport process for the Si QD thin films fabricated by using the traditional [SiO2/SRO]-ML and novel deposition structures.
The red lines mean the main carrier transport paths in both structures, and the thicker lines represent the larger tunneling probability. It indicates when the Si QD thin films with good QD size control and further QD separation reduction are successfully developed, the carrier tunneling probability will be significantly increased due to a narrower barrier width formation. Hence, a novel deposition structure, leading to the good QD size control and further QD separation reduction, can be very helpful for the Si-based SCs integrating Si QDs.
In this study, we propose to develop a novel deposition structure, gradient Si-rich oxide multilayer (GSRO-ML), for the Si QD thin films with good QD size control and QD separation minimization by co-sputtering deposition and high-temperature annealing methods, as shown in Fig. 1-17. Each GSRO period is composed of a highly SRO layer sandwiched between gradient SRO layers. In this proposed structure, the Si-rich atoms are anticipated to self-assemble and take the highly SRO regions as the crystallized centers during annealing due to the significant difference of Si/O composition, and the further reduced QD separation, also means the higher QD density, is expected to be formed after annealing.
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Fig. 1-16: Illustrations of the main carrier transport process in the Si QD thin films fabricated by using the (a) traditional [SiO2/SRO]-ML and (b) novel deposition structures. The thicker lines in (b) than that in (a) represent the larger tunneling probability obtained.
Fig. 1-17: Illustration of the Si QD thin film fabricated by using our proposed GSRO-ML deposition structure.
1-4.2 Si QD Thin Films Utilizing ZnO Matrix Material
In addition to develop the GSRO-ML deposition structure for the Si QD thin films using the Si-based dielectric matrix materials, we also focus on developing a novel and more suitable matrix material since the characteristics of matrix material can obviously influence the electro-optical properties of the Si QD thin films. Fig.
1-18 shows an illustration of the carrier transport process in the Si QD thin films utilizing the insulated and semi-conductive matrix materials. Being different from
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tunneling through QDs by using an insulated matrix material, more efficient transport paths may be contributed from using a semi-conductive matrix material for the Si QD thin films. Hence, we propose to embed Si QDs into ZnO thin films because ZnO has many desirable features to function as Si QDs’ matrix material, such as wide (~3.3 eV) and direct bandgap, high transparency, and highly tunable electrical properties [34].
And so far, only few materials can simultaneously possess these properties. Hence, ZnO can serve as the Si QDs’ matrix material for bandgap engineering, reduce the optical loss from matrix’s absorption, and efficiently enhance the carrier transport efficiency. Undoubtedly, there are many advantages to embed Si QDs in ZnO thin films for SCs application.
Fig. 1-18: Illustration of the main carrier transport process for Si QDs embedded in an (a) insulated or (b) semi-conductive matrix material. The lines with different colors represent the different carrier transport paths.
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1-5 Summary
The third generation SCs with high efficiency and low cost plays the indispensible roles for popularized developments. Si QD has the good ability on bandgap engineering to efficiently improve the mismatched photon energy loss, hence, the Si-based SCs integrating Si QDs can possess great potential on being as the third generation SCs. However, so far, the carrier transport efficiencies are still significantly limited by using the Si-based dielectric matrix materials due to their naturally high resistance properties. In this study, we propose to develop the novel Si QD thin films by utilizing a GSRO-ML deposition structure for further QD separation reduction and integrating with ZnO matrix material for better carrier transport path.
The apparatus, sample fabrication, and characteristics for our proposed novel Si QD thin films are investigated and discussed in next chapters.
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Chapter 2
Experimental Apparatus
In this chapter, the experimental apparatus for our samples fabrication and characteristics measurement for the novel Si QD thin films are introduced in detail and shown below.