Generation of high photocurrent in threedimensional silicon quantum dot superlattice fabricated by combining biotemplate and neutral beam etching for quantum dot solar cells
 Makoto Igarashi^{1, 2},
 Weiguo Hu^{1, 2},
 Mohammad Maksudur Rahman^{1, 2},
 Noritaka Usami^{3} and
 Seiji Samukawa^{1, 2, 4}Email author
DOI: 10.1186/1556276X8228
© Igarashi et al.; licensee Springer. 2013
Received: 7 March 2013
Accepted: 1 May 2013
Published: 15 May 2013
Abstract
We fabricated a threedimensional (3D) stacked Si nanodisk (SiND) array with a high aspect ratio and uniform size by using our advanced topdown technology consisting of biotemplate and neutral beam etching processes. We found from conductive atomic microscope measurements that conductivity became higher as the arrangement was changed from a single SiND to twodimensional (2D) and 3D arrays with the same matrix of SiC, i.e., the coupling of wave functions was changed. Moreover, our theoretical calculations suggested that the formation of minibands enhanced tunneling current, which well supported our experimental results. Further analysis indicated that four or more SiNDs basically maximized the advantage of minibands in our structure. However, it appeared that differences in miniband widths between 2D and 3D SiND arrays did not affect the enhancement of the optical absorption coefficient. Hence, high photocurrent could be observed in our SiND array with high photoabsorption and carrier conductivity due to the formation of 3D minibands.
Keywords
Si nanodisk Aspect ratio Photocurrent MinibandBackground
Quantum dot superlattices (QDSLs) have attracted a great deal of interest from both physical scientists and device researchers. Electron wave functions diffuse and overlap, which merge discrete quantum levels into minibands, with quantum dots approaching and forming a quasicrystal structure. This band rearrangement has significant applications for many novel optoelectronic/electronic devices [1–15]. For example, quantum dot solar cells, the most exciting photovoltaic device with more than 63% conversion efficiency, have to utilize minibands for carrier transport and additional optical transitions.
Ideal QDSLs present a great challenge to current nanotechnologies. Several technologies (e.g., chemical solution methods and molecular beam epitaxy (MBE)) have convincingly been used to fabricate relatively uniform quantum dots; however, very few technologies can finitely arrange QDs to form a quasicrystal structure. The welldeveloped MBE technology can only achieve very limited control on the direction of growth, which induces a mixed state with the wetting layer. The most direct idea is to develop a topdown nanotechnology. However, nanometerorder sizes exceed most light/electron beam limitations, and suitable masks seem impossible to create. The neutral beam (NB) etching and ferritin biotemplate we developed have recently brought about a great breakthrough in that we successfully fabricated twodimensional (2D) array Si nanodisks (SiNDs) with sub10 nm, high density (>10^{11} cm^{2}), and quasihexagonal crystallization [16–20].
Photovoltaic conversion efficiency was determined by light absorbance and carrier collection efficiency. Our previous work has proven that wave function coupling relaxes the selection rule to induce additional optical transitions [21, 22]. We first observed enhanced conductivity in 2D and threedimensional (3D) array SiNDs with a SiC matrix in this study. Moreover, we calculated electronic structures and current transport, which theoretically suggested that minibands enhanced conductivity, within envelope function theory and the Anderson Hamiltonian method. These enhanced optical and electrical properties indicated a potential application for the highly efficient quantum dot solar cells.
Methods
Results and discussion
Conclusions
We developed an advanced topdown technology to fabricate a stacked SiND array that had a high aspect ratio and was of uniform size. We found from cAFM measurements that conductivity increased as the arrangement was changed from a single SiND to 2D and 3D arrays with the same matrix of SiC. This enhancement was most likely due to the formation of minibands, as suggested by our theoretical calculations. Moreover, the change in outofplane minibands did not affect the absorption coefficient. This enhanced transport should work in the collection efficiency of high carriers in solar cells.
Abbreviations
 cAFM:

Conductive atomic force microscopy
 IV:

Currentvoltage
 MBE:

Molecular beam epitaxy
 ND:

Nanodisk
 QDSL:

Quantum dot superlattices.
Declarations
Acknowledgements
This work is supported by the Japan Science and Technology Agency (JST CREST) and the GrantinAid for Japan Society for the Promotion of Science (JSPS) Fellows.
Authors’ Affiliations
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