Observation of Quantum Size Effect from Silicon Nanowall
© The Author(s). 2016
Received: 26 July 2016
Accepted: 21 November 2016
Published: 29 November 2016
We developed a fabrication technique of very thin silicon nanowall structures. The minimum width of the fabricated silicon nanowall structures was about 3 nm. This thinnest region of the silicon nanowall structures was investigated by using cathode luminescence and ultraviolet photoelectron spectroscopy (UPS). The UPS measurements revealed that the density of states (DOS) of the thinnest region showed a stepwise shape which is completely different from that of the bulk Si. Theoretical analysis clearly demonstrated that this change of the DOS shape was due to the quantum size effect.
KeywordsSilicon nanowall Quantum size effect Ultraviolet photoelectron spectroscopy Cathode luminescence
Multi-junction solar cells consisting of materials with different band gaps are one of the options to overcome the conversion efficiency limit of single junction solar cells . Crystalline silicon (Si) is the most promising material for the bottom cell of a tandem solar cell. Recently, a material for the top cell has been widely studied [2, 3]. Si nanowire and nanowall are one of the options for the top cell material. The band gaps of Si nanowire and nanowall can be varied by changing their diameter or width owing to the quantum size effect , and there is the potential for high efficiency all-Si tandem solar cells. In a previous research, Si nanowires were mainly used for light-trapping structure of Si-based solar cells. In this case, the size of Si nanowires was micrometer or submicrometer range which corresponds to the wavelength of visible and infrared light [5–10]. In order to apply nanostructured Si for the top cell of all-Si tandem solar cells, it is important to reduce the size to less than 5 nm  to utilize quantum size effect. Therefore, techniques to fabricate extremely thin Si nanowire or nanowall are important to realize all-Si tandem solar cells.
Fabrication processes of nanostructured Si (Si nanowire or Si nanowall) are roughly divided into two types: top-down and bottom-up, i.e., etching of bulk Si [12–16] and growing Si nanowire on a substrate . The advantage of the top-down process is the easy control of the direction of nanostructured Si. The starting material of this method is a Si wafer; therefore, material quality is also high enough. The typical top-down process consists of a mask patterning and anisotropic etching. The arrangement of nanostructured Si can be controlled by mask patterning. By the combination of mask patterning, e.g., nanoimprint and photolithography, and anisotropic etching, e.g., metal-assisted chemical etching (MACE) [18–20] and reactive ion etching (RIE), various processes are selectable. We have developed a device integration process of Si nanowire with a diameter of 30 nm using silica nanoparticle dispersion and MACE , and confirmed the photovoltaic power generation of the axial-junction Si nanowire solar cell . However, the diameter of the Si nanowires was not thin enough to utilize the quantum size effect.
In this work, we succeeded to fabricate very thin Si nanowall by the combination of an etching process and a slimming process using thermal oxidation. The minimum width of the Si nanowall was 3 nm. We also investigate to confirm the quantum size effect of the Si nanowall. Si nanowall confines the carriers in one dimension; therefore, a smaller size is required to utilize the quantum size effect than Si nanowire. This is one of the disadvantages of Si nanowall; however, the Si nanowall is much stronger than Si nanowire from the viewpoint of mechanical strength. In addition, the light absorption of Si nanowall is greater than that of Si nanowire . Therefore, it is important to confirm the quantum size effect of Si nanowall. In previous works, photoluminescence (PL) and scanning tunneling spectroscopy (STS) were used for confirming the quantum size effect of nanostructured Si. The PL method can measure the band gap and has been used for analysis of nanodot  and nanoporous structures . The PL measurement includes undesirable signals such as signals from interface defects and requires high density of nanostructured Si to detect signals related to the quantum size effect. The STS method can measure the local density of states (DOS) and has been used for the analysis of single Si nanowire . However, it requires an atomically flat measurement surface and is difficult to measure Si nanowire and nanowall vertical to the substrate. Therefore, we investigated our Si nanowall by using cathode luminescence (CL) and ultraviolet photoelectron spectroscopy (UPS).
Results and discussions
In order to confirm the quantum size effect, we also analyzed the slimmed Si nanowall by UPS. A helium discharge tube was used as the light source and UV light with energy of 40.8 eV was irradiated to the tips of the Si nanowall. The kinetic energy of the photoelectrons emitted from a sample is influenced by the work function and the binding energy. Therefore, an UPS spectrum reflects the density of states in the valence band . The most important advantage of UPS is high surface sensitivity. The maximum kinetic energy of electrons in this measurement is 40.8 eV, which corresponds to the mean free path of electrons less than 1 nm . This indicates that the UPS can only measure the DOS of the surface of the sample. Therefore, we can selectively detect the UPS signal of the tips of the Si nanowall if we can prepare the sample in which the tips are located at the surface.
We investigated properties of an extremely thin Si nanowall in which the width of the thinnest region was 3 nm. We found that CL measurement is not suitable to detect the quantum size effect due to the undesirable luminescence caused by the diffusion of injected electrons and the influence of the oxide layer. We also fabricated a slimmed Si nanowall without the oxide layer and measured it by UPS. When the width of Si nanowall was 3 nm, the change of the DOS structure in the valence band was observed. According to the comparison between the experimental DOS structure and the theoretical quantum levels, we concluded that this change in the DOS is caused by the quantum size effect.
A part of this work was performed under management of JST supported by the MEXT FUTURE-PV Innovation.
A part of this work was supported by the “Nanotechnology Platform” (project No.12024046) of the MEXT, Japan.
DK designed the study and wrote the initial draft of the manuscript. SY, MH, and YI contributed to sample fabrication and CL measurement. AT, MT, SM, and MK contributed to the interpretation of the data and reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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