Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2
© Gardelis et al; licensee Springer. 2011
Received: 22 December 2010
Accepted: 16 March 2011
Published: 16 March 2011
In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.
Silicon nanocrystals (Si NCs) embedded in dielectric matrices such as silicon dioxide or silicon nitride have unique electrical and optical properties which are determined by quantum size and Coulomb blockade effects [1–3]. A significant consequence of the quantum size effect is the bandgap opening with decreasing NC size [4, 5]. This unique property of the Si NCs can be exploited in order to build absorbers for photovoltaic applications [6–12]. A fundamental problem with the existing silicon (Si) photovoltaics is that a significant part of the solar cell spectrum in the ultraviolet region, i.e., at energies much higher than the bandgap of silicon, is absorbed creating hot electrons and holes which relax to their respective band edges, losing their energy as heat through electron-phonon scattering and subsequent phonon emission. This effect poses a limit to the conversion efficiency of the cell. One way to increase the conversion efficiency beyond this limit is to use a tandem cell, i.e., a stack of absorber layers with different bandgaps to cover a larger range of the solar spectrum than a single bandgap absorber layer. These structures belong to the third generation of solar cells and are predicted to have an energy conversion efficiency limit of 60% .
The growth of very thin nanocrystalline (nc) Si films with thickness from 5 to 30 nm by low-pressure chemical vapor deposition (LPCVD) was reported by the authors previously . Films grown by this method have columnar structures and consist of a high density of Si NCs with a very narrow size distribution and arranged in a two-dimensional (2-D) array configuration . The size of the NCs in the z-direction is homogeneous in the whole film and equal to the film thickness, whereas in the x-y plane their size does not vary significantly with film thickness. In this work, we have grown similar nc-Si films on quartz substrates, in a range of thicknesses between 10 and 30 nm using LPCVD and subsequently oxidized them in order to form films containing Si NCs of controlled sizes embedded in a SiO2 matrix. The aim of this work is to use such films as absorbers in photovoltaic devices. We investigated their electrical and optical properties and measured photocurrent. We found that the electrical transport properties of the films were determined by tunneling of carriers through the SiO2 barriers between the Si NCs at low temperatures, whereas at higher temperatures by thermionic emission over these barriers. We also observed Coulomb blockade effects which persisted even above room temperature for the films containing the smaller Si NCs. In the case of the smaller NCs, photocurrent measurements as a function of energy showed similar dependence as that of the absorption, revealing strong absorption and photocurrent generation in visible and ultraviolet. Photoluminescence was observed only in the film which contained the smallest Si NCs (with sizes less than 2 nm), which were well isolated from each other. By comparing the photoluminescence and absorption spectra obtained from this film, we confirm the existence of an energy shift between photoluminescence (PL) and absorption, known as the Stokes shift. In addition, the PL energy is red-shifted compared with the corresponding energy predicted from the quantum confinement effect due to a pinning of the bandgap at Si NCs/SiO2 interfaces [15–24]. In this film, no photocurrent was observed.
where α(λ) is the absorption coefficient and x is the film thickness. Photoluminescence was excited by the 457.9-nm line of an Ar ion laser.
Results and discussion
The as-grown films had columnar structures and consisted of Si NCs, in a 2-D array configuration, with sizes in the z-direction equal to the film thickness. The lateral dimension (x-y plane) of the Si NCs was in all films between 12 and 13 nm with a narrow size Gaussian distribution which did not vary much with film thickness . Thermal oxidation reduced the thickness of the films and hence the vertical dimension of the Si NCs within the films. Oxidation has also reduced, to some extent, the lateral size of the Si NCs giving rise to thin SiO2 tunnel barriers between adjacent Si NCs.
Electrical transport measurements
Electrical measurements under illumination
In summary, we have investigated systematically the electrical, optical, and photocurrent properties of very thin films on quartz containing Si NCs in a 2-D configuration, with sizes in the range between less than 2 and 20 nm, embedded in a silicon dioxide matrix. Strong Coulomb blockade effects in the electric transport were observed, particularly in the films containing the smaller Si NCs. Absorption measurements showed an energy upshift of the energy bandgap of the Si NCs with decreasing size. Photocurrent spectra followed absorption, revealing that photocurrent is indeed due to electron hole generation within the Si NCs. Moreover, in films containing very small Si NCs (sizes <2 nm), separated by SiO2 barriers, strong quantum confinement effects were observed. Excitons generated by light absorption within the Si NCs were strongly localized, and no photocurrent was measured. In these films, exciton recombination by light emission was more probable than non-radiative recombination, resulting in light emission at room temperature. This systematic study confirms that Si NCs are interesting for use as better ultraviolet absorbers than bulk Si in photovoltaic devices. However, when strong confinement comes into play in the Si NCs, one should consider strong localization effects of the photo-generated excitons that result in the absence of photocurrent.
This work was financially supported by the EU research project FP6-ICT-I3 ANNA, contract no. 026134.
- Nassiopoulou AG: Silicon Nanocrystals in SiO2 Thin Layers. Encyclopedia of Nanoscience and Nanotechnology. Volume 9. Edited by: Nalwa HS. Valencia: American Scientific; 2004:793–813.Google Scholar
- Gardelis S, Nassiopoulou AG, Vouroutzis N, Frangis N: Effect of exciton migration on the light emission properties in silicon nanocrystal ensembles. J Appl Phys 2009, 105: 113509. 10.1063/1.3138811View ArticleGoogle Scholar
- Balberg I, Savir E, Jedrzejewski J, Nassiopoulou AG, Gardelis S: Fundamental transport processes in ensembles of silicon quantum dots. Phys Rev B 2007, 75: 235329. 10.1103/PhysRevB.75.235329View ArticleGoogle Scholar
- Matsumoto T, Suzuki J, Ohnuma M, Kanemitsu Y, Masumoto Y: Evidence of quantum size effect in nanocrystalline silicon by optical absorption. Phys Rev B 2001, 63: 195322. 10.1103/PhysRevB.63.195322View ArticleGoogle Scholar
- Zacharias M, Heitmann J, Scholz R, Kahler U, Schmidt M, Bläsing J: Size-controlled highly luminescent silicon nanocrystals: A SiO/SiO2 superlattice approach. Appl Phys Lett 2002, 80(4):661–663. 10.1063/1.1433906View ArticleGoogle Scholar
- Rölver R, Berghoff B, Bätzner D, Spangenberg B, Kurz H, Schmidt M, Stegemann B: Si/SiO2 multiple quantum wells for all silicon tandem cells: Conductivity and photocurrent measurements. Thin Solid Films 2006, 516: 6763.View ArticleGoogle Scholar
- Kirchartz T, Seino K, Wagner J-M, Rau U, Bechstedt F: Efficiency limits of Si/SiO2 quantum well solar cells from first-principles calculations. J Appl Phys 2009, 105: 104511. 10.1063/1.3132093View ArticleGoogle Scholar
- Conibeer G, Green M, Corkish R, Cho Y, Cho E-C, Jiang C-W, Fangsuwannarak T, Pink E, Huang Y, Puzzer T, Trupke T, Richards B, Shalav A, Lin K-L: Silicon nanostructures for third generation photovoltaic solar cells. Thin Solid Films 2006, 511–512: 654. (511–512 is the volume, therefore should be written as bold) (511-512 is the volume, therefore should be written as bold) 10.1016/j.tsf.2005.12.119View ArticleGoogle Scholar
- Cho E-C, Park S, Hao X, Song D, Conibeer G, Park S-C, Green MA: Silicon quantum dot/crystalline silicon solar cells. Nanotechnology 2008, 19: 245201. 10.1088/0957-4484/19/24/245201View ArticleGoogle Scholar
- Ficcadenti M, Pinto N, Morresi L, Murri R, Serenelli L, Tucci M, Falconieri M, Krasilnikova Sytchkova A, Grilli ML, Mittiga A, Izzi M, Pirozzi L, Jadkar SR: Si quantum dots for solar cell fabrication. Materials Science and Engineering B 2009, 159–160: 66. 10.1016/j.mseb.2008.10.054View ArticleGoogle Scholar
- Kim S-K, Cho C-H, Kim B-H, Park S-J, Lee JW: Electrical and optical characteristics of silicon nanocrystal solar cells. App Phys Lett 2009, 95: 143120. 10.1063/1.3242030View ArticleGoogle Scholar
- Stupca M, Alsalhi M, Saud TA, Almuhanna A, Nayfeh MH: Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticle. Appl Phys Lett 2007, 91: 063107. 10.1063/1.2766958View ArticleGoogle Scholar
- Ross RT, Nozik AJ: Efficiency of hot-carrier solar energy converters. J Appl Phys 1982, 53: 3813. 10.1063/1.331124View ArticleGoogle Scholar
- Lioutas ChB, Vouroutzis N, Tsiaoussis I, Frangis N, Gardelis S, Nassiopoulou AG: Columnar growth of ultra-thin nanocrystalline Si films on quartz by Low Pressure Chemical Vapor Deposition: accurate control of vertical size. Phys Stat Sol A 2008, 205: 2615. 10.1002/pssa.200880224View ArticleGoogle Scholar
- Lioudakis E, Othonos A, Nassiopoulou AG, Lioutas B, Frangis N: Influence of grain size on ultrafast carrier dynamics in thin nanocrystalline silicon films. Appl Phys Lett 2007, 90: 191114. 10.1063/1.2738383View ArticleGoogle Scholar
- Lioudakis E, Othonos A, Hadjisavvas GC, Kelires PC, Nassiopoulou AG: Quantum confinement and interface structure of Si nanocrystals of sizes 3–5 nm embedded in a-SiO2. Physica E 2007, 38: 128–134. 10.1016/j.physe.2006.12.020View ArticleGoogle Scholar
- Lupi M, Ossicini S: Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2: Beyond the quantum confinement effect. Phys Rev B 2005, 71: 035340. 10.1103/PhysRevB.71.035340View ArticleGoogle Scholar
- Puzder A, Williamson AJ, Grossman JC, Galli G: Surface Chemistry of Silicon Nanoclusters. Phys Rev Lett 2002, 88: 097401. 10.1103/PhysRevLett.88.097401View ArticleGoogle Scholar
- Ramos LE, Furthmüller J, Bechstedt F: Effect of backbond oxidation on silicon nanocrystallites. Phys Rev B 2004, 70: 033311. 10.1103/PhysRevB.70.033311View ArticleGoogle Scholar
- Vasiliev I, Chelikowsky JR, Martin RM: Surface oxidation effects on the optical properties of silicon nanocrystals. Phys Rev B 2002, 65: 121302(R).View ArticleGoogle Scholar
- Wolkin MV, Jorne J, Fauchet PM, Allan G, Delerue C: Electronic States and Luminescence in Porous Silicon Quantum Dots: The Role of Oxygen. Phys Rev Lett 1999, 82: 197. 10.1103/PhysRevLett.82.197View ArticleGoogle Scholar
- Lioudakis E, Antoniou A, Othonos A, Christofides C, Nassiopoulou AG, Lioutas ChB, Frangis N: The role of surface vibrations and quantum confinement effect to the optical properties of very thin nanocrystalline silicon films. J App Phys 2007, 102: 083534. 10.1063/1.2800269View ArticleGoogle Scholar
- Othonos A, Lioudakis E, Nassiopoulou AG: Surface-related states in oxidized silicon nanocrystals enhance carrier relaxation and inhibit Auger recombination. Nanoscale Res Lett 2008, 3: 315. 10.1007/s11671-008-9159-8View ArticleGoogle Scholar
- Lioudakis E, Othonos A, Nassiopoulou AG: Ultrafast transient photoinduced absorption in silicon nanocrystals: Coupling of oxygen-related states to quantized sublevels. Appl Phys Lett 2007, 90: 171103. 10.1063/1.2728756View ArticleGoogle Scholar
- Machida E, Uraoka Y, Fuyuki T, Kokawa R, Ito T, Ikenoue H: Characterization of local electrical properties of polycrystalline silicon thin films and hydrogen termination effect by conductive atomic force microscopy. Appl Phys Lett 2009, 94: 182104. 10.1063/1.3130210View ArticleGoogle Scholar
- Durrani ZAK, Rafiq MA: Electronic transport in silicon nanocrystals and nanochains. Microelectron Eng 2009, 86: 456. 10.1016/j.mee.2009.03.123View ArticleGoogle Scholar
- Kamiya T, Durrani ZAK, Ahmed H, Sameshima T, Furuta Y, Mizuta H, Lloyd N: Reduction of grain-boundary potential barrier height in polycrystalline silicon with hot H2O-vapor annealing probed using point-contact devices. J Vac Sci Technol B 2003, 21: 1000. 10.1116/1.1570849View ArticleGoogle Scholar
- Kawamura K, Kidera T, Nakajima A, Yokoyama S: Coulomb blockade effects and conduction mechanism in extremely thin polycrystalline-silicon wires. J Appl Phys 2002, 91: 5213. 10.1063/1.1464650View ArticleGoogle Scholar
- Ciurea ML, Teodorescu VS, Iancu V, Balberg I: Electronic transport in Si-SiO2 nanocomposite films. Chem Phys Lett 2006, 423: 225. 10.1016/j.cplett.2006.03.070View ArticleGoogle Scholar
- Geigenmüller U, Schön G: Single-Electron Effects in Arrays of Normal Tunnel Junctions. Europhys Lett 1989, 10: 765.View ArticleGoogle Scholar
- Middleton AA, Wingreen NS: Collective transport in arrays of small metallic dots. Phys Rev Lett 1993, 71: 3198. 10.1103/PhysRevLett.71.3198View ArticleGoogle Scholar
- Seto JYW: The electrical properties of polycrystalline silicon films. J Appl Phys 1975, 46: 5247. 10.1063/1.321593View ArticleGoogle Scholar
- Tarng ML: Carrier transport in oxygen-rich polycrystalline-silicon films. J Appl Phys 1978, 49: 4069. 10.1063/1.325367View ArticleGoogle Scholar
- Tringe JW, Plummer JD: Electrical and structural properties of polycrystalline silicon. J Appl Phys 2000, 87: 7913. 10.1063/1.373475View ArticleGoogle Scholar
- Grovenor CRM: Grain boundaries in semiconductors. J Phys C: Solid State Phys 1985, 18: 4079. 10.1088/0022-3719/18/21/008View ArticleGoogle Scholar
- Seager CH, Pike GE: Electron tunneling through GaAs grain boundaries. Appl Phys Lett 1982, 40: 471. 10.1063/1.93138View ArticleGoogle Scholar
- Manousiadis P, Gardelis S, Nassiopoulou AG, unpublished work
- De la Torre J, Souifi A, Poncet A, Bremond G, Guillot G, Garrido B, Morante JR: Ground and first excited states observed in silicon nanocrystals by photocurrent technique. Solid State Electron 2005, 49: 1112. 10.1016/j.sse.2005.04.013View ArticleGoogle Scholar
- Hosono H, Kajihara K, Suzuki T, Ikuta Y, Skuja L, Hirano M: Vacuum ultraviolet optical absorption band of non-bridging oxygen hole centers in SiO2 glass. Solid State Comm 2002, 122: 117. 10.1016/S0038-1098(02)00118-7View ArticleGoogle Scholar
- Mahdouani M, Bourguiga R, Jaziri S: Polaronic states in Si nanocrystals embedded in SiO2 matrix. Physica E 2008, 41: 228. 10.1016/j.physe.2008.07.018View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.