 Nano Commentary
 Open Access
 Published:
Generation of high photocurrent in threedimensional silicon quantum dot superlattice fabricated by combining biotemplate and neutral beam etching for quantum dot solar cells
Nanoscale Research Letters volume 8, Article number: 228 (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.
Background
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
The fabrication of the 3D SiND array was based on biotemplate and NB processes. Figure 1 schematically illustrates the fabrication flow, which started with (Figure 1a) a 2nmthick SiC film and 4nmthick polySi being deposited alternately four times on the ndoped Si substrate using a highvacuum sputtering system and electron beam evaporation. Then a 3nmthick SiO_{2} layer was fabricated as a surface oxide (called NBOSiO_{2} after this) by the NB oxidation process we developed at a low temperature of 300°C [16]. Figure 1b has a 2D array of biotemplate molecules (ListeriaDps) that was deposited on the surface of the NBOSiO_{2}. Figure 1c shows the biotemplate protein shell that was removed by annealing it in an oxygen atmosphere to obtain a 2D array of iron cores as a uniform mask for the etching process. Figure 1d shows the etching process that was carried out with nitrogen trifluoride gas/hydrogen radical treatment (NF_{3} treatment) to remove the surface SiO_{2}, which was carried out with NB etching to remove the polySi. Here we performed a onestep etching and found a wellaligned vertical etching profile due to high etching selectivity between the iron cores and etched material and the low selectivity of 1.3 between Si and SiC. The etching process has been detailed elsewhere [17–19]. Figure 1e shows that the iron cores were then removed by HCl wet cleaning, and then the remaining surface SiO_{2} was removed by NF_{3} treatment. Figure 1f shows that the SiC was deposited between pillars, which were stacked SiNDs, by the sputtering system. The diameter, space between NDs, and average ND centertoND center distance corresponded to 6.4, 2.3, and 8.7 nm in the structure. The size distribution of the SiNDs was less than 10% for all samples [19, 21]. We prepared three types of SiND arrangements, as seen in Figure 2: separated SiNDs as a single QD, a 2D array of SiNDs as a 2D QDSL, and a 3D array of SiNDs as a 3D QDSL. The electrical conductivity and optical absorption in QDSLs were methodically, experimentally, and theoretically investigated with these samples to study the effect of wave function coupling between QDs.
Results and discussion
Conductive atomic force microscopy (cAFM) has been used to investigate conductivity, as seen in Figure 3. Changing the matrix from SiO_{2} to SiC greatly increases current (I) and decreases threshold voltage (V), according to comparisons of the 2D arrays of SiNDs. Although a primary factor should be macroconductivity differences between SiC and SiO_{2}, one cause is minibands that enhance conductivity, which was revealed in a later theoretical simulation. More significantly, conductivity became higher as the arrangement was changed from a single SiND to 2D and 3D arrays with the same matrix of SiC, i.e., the coupling of wave functions was changed. Note that conductivity in the 3D array was higher than that in the 2D array, even though the total thickness of the QDSL expanded. These results indicate that the formation of minibands both inplane and outofplane (vertically) might enhance carrier conductivity in QDSLs.
We considered resonant tunneling to be a theoretical mechanism that could explain our experimental results on the basis of these results. Therefore, we theoretically investigated enhanced conductivity due to the formation of minibands. Our developed topdown nanotechnology achieved great flexibility in designing parts for the quantum structure, such as the independently controllable diameter and thickness, high aspect ratio, and different matrix materials. The finite element method duly described the complex quantum structures. The electronic structure and wave function within envelope function theory are presented as.
Here we mainly took into consideration the matrix material, realistic geometry structure, and number of stacking layers. The results are presented in Figure 4. A distinct feature is that electron wave functions are more strongly confined in the SiNDs in the SiO_{2} matrix due to the higher band offset of the Si/SiO_{2} interface. Thus, they resulted in higher quantum levels. In addition, stronger confinement means weaker coupling of the wave function and narrower minibands in the same geometry alignment. By stacking our NDs from one layer to ten layers, the miniband in Figure 5 gradually broadens, and at around four to six layers, the miniband width seems to saturate. The probability of the wave function diffusing into the barrier exponentially reduces with distance, which indicates that wave function coupling exponentially saturates as the number of layers increases. Perhaps four or sixlayer NDs are sufficient to maximize the advantage of minibands.
Chang et al. [23] considered interdot coupling with the Anderson Hamiltonian model to deduce tunneling current density as
Here E(k_{ xy }) is related to the energy discrepancy, t, due to inplane ND coupling E(k_{ xy }) = 2t[cos(k_{ x }R) + cos(k_{ y }R)]. We simulated the IV properties of our structures with this. The results are in Figure 6. The calculated results also revealed that the wider minibands in the SiC matrix resulted in better transport properties than those in the SiO_{2} matrix. A simplified, but not too obscure, explanation is that the formation of minibands broadens the resonance levels to increase jointstate density. Carrier transport in this twobarrier structure mainly depends on resonant tunneling. Moreover, if the Coulomb blockade effect is neglected, the tunneling jointstate density in Equation 2 can be simplified as a parabola function with a resonant peak at ~E_{0}– E(k_{ xy }). The formation of minibands broadens the resonant peak to allow more states to approach maximum, which results in enhanced current. Thus, wider minibands mean a higher current density and lower threshold voltage, as can be seen in the SiNDs in the SiC matrix. In addition, the 2D array of SiNDs in the SiC matrix has a lower miniband level, E_{0}, which also shifts the IV curves to a lower threshold voltage. This tendency closely matches that in our experimental results, and due to the larger tunneling resistance in the SiO_{2} interlayer (C_{ t }), the threshold voltage (V) is further increased in realistic IV curves. Moreover, conductivity in the 2D and 3D arrays of SiNDs was enhanced due to the same mechanism that broadened the wave functions and formed wider minibands. As these were also very consistent with the trend in our experimental results, they clarified that the formation of minibands both inplane and outofplane could enhance carrier transport in QDSLs. Enhanced conductivity is very important for electronic/optoelectronic devices, which indicates high charge injection efficiency in lasers and carrier collection efficiency in solar cells.
Optical absorption was then investigated by measuring the transmittance of samples using ultravioletvisiblenearinfrared spectroscopy. Our previous work demonstrated that the formation of minibands perpendicular to incident light could enhance photon absorption, i.e., 2D minibands could improve the absorption coefficient in the 2D array of SiNDs [21, 22]. Therefore, we investigated what effect 3D minibands had on optical absorption in this study. Figure 7 shows the absorption coefficients in the 2D and 3D SiND array samples prepared on transparent quartz substrates. The absorption coefficient in the 3D array was almost the same as that in the 2D array, and the calculated bandgap energy of both samples was 2.2 eV. Moreover, the change in the miniband width between the samples should be 3.85 meV, as shown in Figure 5 (0.95 meV in single layer and 4.80 meV in four layers). Therefore, it seems that the change of 3.85 meV in the miniband width is not sufficiently large to affect photon absorption.
Finally, we fabricated a p^{++}in Si solar cell with a 3D array of SiNDs as an absorption layer, as shown in Figure 8, and measured the amount of possible photocurrent generated from the SiND layers where the high doping density (>10^{20} cm^{3}) of the p^{++}Si substrate prevented photocurrent from being generated inside the substrate itself. Here we found that the generated shortcircuit current density from the p^{++}in solar cell was 2 mA/cm^{2}, where the largest possible photocurrent generated in the SiND layers and nSi emitter was estimated to be 3.5 mA/cm^{2} for the former and 1.0 mA/cm^{2} for the latter [22]. Since 1 mA/cm^{2} is the highest possible value for photocurrent from the nSi emitter according to this estimate, the actual value should be lower than the calculated value. Therefore, we found that out of the total photocurrent of 2 mA/cm^{2}, much more of it (>1 mA/cm^{2}) was contributed to by SiND. This confirms that most of the observed photocurrent originated from the carrier generated at the SiND itself because of high photoabsorption and carrier conductivity due to the formation of 3D minibands in our SiND array.
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.
References
Luque A, Marti A: Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels. Phys Rev Lett 1997, 78: 5014. 10.1103/PhysRevLett.78.5014
Konakov AA, Burdov VA: Optical gap of silicon crystallites embedded in various wideband amorphous matrices: role of environment. J Phys Condens Matter 2010, 22: 215301. 10.1088/09538984/22/21/215301
Thamdrup LH, Persson F, Bruus H, Kristensen A, Flyvbjerg H: Experimental investigation of bubble formation during capillary filling of SiO_{2} nanoslits. Appl Phys Lett 2007, 91: 163505. 10.1063/1.2801397
Conibeer G, Green MA, Corkish R, Cho Y, Cho EC, Jiang CW, Fangsuwannarak T, Pink E, Huang Y, Puzzer T, Trupke T, Richards B, Shalav A, Lin KL: Silicon nanostructures for third generation photovoltaic solar cells. Thin Solid Films 2006, 511–512: 654.
Cho EC, Park S, Hao X, Song D, Conibeer G, Park SC, Green MA: Silicon quantum dot/crystalline silicon solar cells. Nanotechnol 2008, 19: 245201. 10.1088/09574484/19/24/245201
Conibeer G, Green MA, Cho EC, König D, Cho YH, Fangsuwannarak T, Scardera G, Pink E, Huang Y, Puzzer T, Huang S, Song D, Flynn C, Park S, Hao X, Mansfield D: Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films 2008, 516: 6748. 10.1016/j.tsf.2007.12.096
Nuryadi R, Ikeda H, Ishikawa Y, Tabe M: Ambipolar Coulomb blockade characteristics in a twodimensional Si multidot device. IEEE Trans Nanotechnol 2003, 2: 231. 10.1109/TNANO.2003.820788
Cordan AS, Leroy Y, Goltzene A, Pepin A, View C, Mejias M, Launois H: Temperature behavior of multiple tunnel junction devices based on disordered dot arrays. J Appl Phys 2000, 87: 345. 10.1063/1.371867
Uchida K, Koga J, Ohba R, Takagi SI, Toriumi A: Silicon singleelectron tunneling device fabricated in an undulated ultrathin silicononinsulator film. J Appl Phys 2001, 90: 3551. 10.1063/1.1392959
Macucci M, Gattobigio M, Bonci L, Iannaccone G, Prins FE, Single C, Wetekam G, Kern DP: A QCA cell in silicononinsulator technology: theory and experiment. Superlattices Microstruct 2003, 34: 205. 10.1016/j.spmi.2004.03.010
Lent CS, Tougaw PD: A device architecture for computing with quantum dots. Proc IEEE 1997, 85: 541. 10.1109/5.573740
Nassiopoulou AG, Olzierski A, Tsoi E, Berbezier I, Karmous A: Ge quantum dot memory structure with laterally ordered highly dense arrays of Ge dots. J Nanosci Nanotechnol 2007, 7: 316.
Pothier H, Lafarge P, Urbina C, Esteve D, Devoret MH: Singleelectron pump based on charging effects. Europhys Lett 1992, 17: 249. 10.1209/02955075/17/3/011
Shin M, Lee S, Park KW: The study of a singleelectron memory cell using coupled multiple tunneljunction arrays. Nanotechnol 2001, 12: 178. 10.1088/09574484/12/2/322
Hirvi KP, Paalanen MA, Pekola JP: Numerical investigation of one‒dimensional tunnel junction arrays at temperatures above the Coulomb blockade regime. J Appl Phys 1996, 80: 256. 10.1063/1.362813
Igarashi M, Tsukamoto R, Huang CH, Yamashita I, Samukawa S: Direct fabrication of uniform and high density sub10nm etching mask using ferritin molecules on Si and GaAs surface for actual quantumdot superlattice. Appl Phys Express 2011, 4: 015202. 10.1143/APEX.4.015202
Huang CH, Igarashi M, Woné M, Uraoka Y, Fuyuki T, Takeguchi M, Yamashita I, Samukawa S: Twodimensional Sinanodisk array fabricated using bionanoprocess and neutral beam etching for realistic quantum effect devices. Jpn J Appl Phys 2009, 48: 04C187. 10.1143/JJAP.48.04C187
Huang CH, Igarashi M, Horita S, Takeguchi M, Uraoka Y, Fuyuki T, Yamashita I, Samukawa S: Novel Si nanodisk fabricated by biotemplate and defectfree neutral beam etching for solar cell application. Jpn J Appl Phys 2010, 49: 04DL16. 10.1143/JJAP.49.04DL16
Huang CH, Wang XY, Igarashi M, Murayama A, Okada Y, Yamashita I, Samukawa S: Optical absorption characteristic of highly ordered and dense twodimensional array of silicon nanodiscs. Nanotechnol 2011, 22: 105301. 10.1088/09574484/22/10/105301
Hirano R, Miyamoto S, Yonemoto M, Samukawa S, Sawano K, Shiraki Y, Itoh KM: Roomtemperature observation of size effects in photoluminescence of Si_{0.8}Ge_{0.2}/Si nanocolumns prepared by neutral beam etching. Appl Phys Express 2012, 5: 082004. 10.1143/APEX.5.082004
Budiman MF, Hu W, Igarashi M, Tsukamoto R, Isoda T, Itoh KM, Yamashita I, Murayama A, Okada Y, Samukawa S: Control of optical bandgap energy and optical absorption coefficient by geometric parameters in sub10 nm siliconnanodisc array structure. Nanotechnol 2012, 23: 065302. 10.1088/09574484/23/6/065302
Igarashi M, Budiman MF, Pan W, Hu W, Tamura Y, Syazwan ME, Usami N, Samukawa S: Effects of formation of minibands in twodimensional array of silicon nanodisks with SiC interlayer for quantum dot solar cells. Nanotechnol 2013, 24: 015301. 10.1088/09574484/24/1/015301
Kuo DMT, Guo GY, Chang YC: Tunneling current through a quantum dot array. Appl Phys Lett 2001, 79: 3851. 10.1063/1.1420775
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.
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
MI and SS conceived and designed the experiment, fabricated the silicon nanodisk samples, performed electrical and optical measurements, analyzed these data, and wrote the paper. MMR and NU fabricated the solar cell structures and analyzed the IV data. WH performed the theoretical calculations. All authors discussed the results, commented on the manuscript, and read and approved the final version.
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License ( https://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Igarashi, M., Hu, W., Rahman, M.M. et al. Generation of high photocurrent in threedimensional silicon quantum dot superlattice fabricated by combining biotemplate and neutral beam etching for quantum dot solar cells. Nanoscale Res Lett 8, 228 (2013). https://doi.org/10.1186/1556276X8228
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1556276X8228
Keywords
 Si nanodisk
 Aspect ratio
 Photocurrent
 Miniband