Improvement of optical transmittance and electrical properties for the Si quantum dot-embedded ZnO thin film
© Kuo et al.; licensee Springer. 2013
Received: 4 September 2013
Accepted: 2 October 2013
Published: 23 October 2013
A Si quantum dot (QD)-embedded ZnO thin film is successfully fabricated on a p-type Si substrate using a ZnO/Si multilayer structure. Its optical transmittance is largely improved when increasing the annealing temperature, owing to the phase transformation from amorphous to nanocrystalline Si QDs embedded in the ZnO matrix. The sample annealed at 700°C exhibits not only high optical transmittance in the long-wavelength range but also better electrical properties including low resistivity, small turn-on voltage, and high rectification ratio. By using ZnO as the QDs’ matrix, the carrier transport is dominated by the multistep tunneling mechanism, the same as in a n-ZnO/p-Si heterojunction diode, which clearly differs from that using the traditional matrix materials. Hence, the carriers transport mainly in the ZnO matrix, not through the Si QDs. The unusual transport mechanism using ZnO as matrix promises the great potential for optoelectronic devices integrating Si QDs.
KeywordsSi quantum dot ZnO thin film Transport mechanism
Recently, Si quantum dots (QDs) embedded in traditional Si-based dielectric matrix materials like SiO2 and Si3N4 have been extensively studied and successfully applied to various optoelectronic devices [1–3], owing to their many unique characteristics such as highly tunable bandgap and better optical properties [4–6]. In particular, Si QD is persistently considered as a candidate for next-generation light emitters in Si photonics because of its greatly improved internal and external quantum efficiencies [7, 8]. To further improve the device performance, utilization of Si-rich Si-based dielectric materials as Si QDs’ matrices has also been developed [9, 10]. A suitable matrix material for Si QDs is very important for better device performance. We propose to embed Si QDs into a ZnO thin film because ZnO has many desirable features to function as Si QDs’ matrix material, e.g., wide and direct bandgap, high transparency, and highly tunable electrical properties . Hence, ZnO can serve as the Si QDs’ matrix to achieve bandgap engineering, reduce the optical loss from the matrix’s absorption, and efficiently enhance the carrier transport efficiency for optoelectronic device application. The fabrication and fundamental optical properties of the Si QD-embedded ZnO thin films have been reported in our previous works [12, 13]. In this study, improvement of optical transmittance and electrical properties of the Si QD-embedded ZnO thin films is investigated and discussed.
The ZnO/Si multilayer (ML) thin films with 20 bilayers are deposited on p-type Si (100) substrates or fused quartzes at room temperature using the radio-frequency (RF) magnetron sputtering method. The sputtering powers of ZnO and Si are fixed at 75 and 110 W, and the effective thicknesses of each ZnO and Si layer are fixed at 5 and 3 nm, respectively. After deposition, the ZnO/Si ML thin films are annealed at 500°C, 600°C, 700°C, or 800°C for 30 min in N2 environment. For electrical measurements, 100-nm-thick Al and Ni metal layers are deposited on the top and bottom surfaces of devices as electrodes using a thermal coater. The Raman spectra are measured using a 488-nm diode-pumped solid-state laser (HORIBA LabRam HR, HORIBA, Kyoto, Japan). The X-ray diffraction (XRD) patterns are examined by a Bede-D1 X-ray diffractometer with Cu Kα radiation (Bede Scientific, Engelwood, CO, USA). The transmittance spectra are obtained using a UV–vis-NIR spectrophotometer (Hitachi U-4100, Hitachi Ltd., Chiyoda, Tokyo, Japan). The cross-sectional morphologies are observed by a JSM-6500 F field-emission scanning electron microscope (SEM; JEOL Ltd., Akishima, Tokyo, Japan). The current–voltage (I-V) curves are measured using an Agilent E5270B precision measurement mainframe (Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
In summary, we successfully fabricate a nc-Si QD-embedded ZnO thin film on a p-Si substrate using a ZnO/Si ML deposition structure. Our results indicate that the optical transmittance can be largely enhanced by increasing Tann owing to the phase transformation of a- to nc-Si QDs embedded in the ZnO matrix, and up to about 90% transmittance in the long-λ range under a Tann higher than 700°C is obtained. The Si QD-embedded ZnO thin film annealed at 700°C exhibits good diode behavior with a large rectification ratio of approximately 103 at ±5 V and significantly lower resistivity than that using the SiO2 matrix material (104 times improvement). From temperature-dependent I-V curves, we find that the carriers transport mainly via the ZnO matrix, not through Si QDs, which is dominated by the multistep tunneling mechanism as in the n-ZnO/p-Si HJ diode. The unique transport mechanism differing from those using the traditional Si-based dielectric matrix materials can lead to much better carrier transport efficiency and electrical properties. Hence, we show that the Si QD thin film using the ZnO matrix material is very advantageous and has potential for optoelectronics device application.
This work is supported by Taiwan’s National Science Council (NSC) under contract number NSC-101-3113-P-009-004. The authors would like to thank the help from the Center for Nano Science and Technology (CNST) of National Chiao Tung University and National Nano Device Laboratories (NDL) in Taiwan.
- Anopchenko A, Marconi A, Wang M, Pucker G, Bellutti P, Pavesi L: Graded-size Si quantum dot ensembles for efficient light-emitting diodes. Appl Phys Lett 2011, 99: 181108. 10.1063/1.3658625View Article
- Lin GR, Lin CJ, Lin CK, Chou LJ, Chueh YL: Oxygen defect and Si nanocrystal dependent white-light and near-infrared electroluminescence of Si-implanted and plasma-enhanced chemical-vapor deposition-grown Si-rich SiO2. J Appl Phys 2005, 97: 094306. 10.1063/1.1886274View Article
- Perez-Wurfl I, Hao X, Gentle A, Kim DH, Conibeer G, Green MA: Si nanocrystal p-i-n diodes fabricated on quartz substrates for third generation solar cell applications. Appl Phys Lett 2009, 95: 153506. 10.1063/1.3240882View Article
- Garoufalis CS, Zdetsis AD: High level ab initio calculations of the optical gap of small silicon quantum dots. Phys Rev Lett 2001, 87: 276402.View Article
- Mirabella S, Agosta R, Franzò G, Crupi I, Miritello M, Savio RL, Stefano MAD, Marco SD, Simone F, Terrasi A: Light absorption in silicon quantum dots embedded in silica. J Appl Phys 2009, 106: 103505. 10.1063/1.3259430View Article
- Kang Z, Liu Y, Tsang CHA, Ma DDD, Fan X, Wong NB, Lee ST: Water-soluble silicon quantum dots with wavelength-tunable photoluminescence. Adv Mater 2009, 21: 661–664. 10.1002/adma.200801642View Article
- Lin GR, Lin CJ, Kuo HC: Improving carrier transport and light emission in a silicon-nanocrystal based MOS light-emitting diode on silicon nanopillar array. Appl Phys Lett 2007, 91: 093122. 10.1063/1.2778352View Article
- Cheng CH, Lien YC, Wu CL, Lin GR: Mutlicolor electroluminescent Si quantum dots embedded in SiOx thin film MOSLED with 2.4% external quantum efficiency. Opt Express 2013, 21: 391–403. 10.1364/OE.21.000391View Article
- Lin GR, Pai YH, Lin CT, Chen CC: Comparison on the electroluminescence of Si-rich SiNx and SiOx based light-emitting diodes. Appl Phys Lett 2010, 96: 263514. 10.1063/1.3459144View Article
- Conibeer G, Green MA, Konig D, Perez-Wurfl I, Huang S, Hao X, Di D, Shi L, Shrestha S, Puthen-Veetil B, So Y, Zhang B, Wan Z: Silicon quantum dot based solar cells: addressing the issues of doping, voltage and current transport. Prog Photovolt Res Appl 2011, 19: 813–824. 10.1002/pip.1045View Article
- Özgür Ü, Alivov YI, Liu C, Teke A, Reshnikov MA, Dogan S, Avrutin V, Cho SJ, Morkoç H: A comprehensive review of ZnO materials and devices. J Appl Phys 2005, 98: 041301. 10.1063/1.1992666View Article
- Kuo KY, Hsu SW, Chuang WL, Lee PT: Formation of nano-crystalline Si quantum dots in ZnO thin-films using a ZnO/Si multilayer structure. Mater Lett 2012, 68: 463–465.View Article
- Kuo KY, Hsu SW, Huang PR, Chuang WL, Liu CC, Lee PT: Optical properties and sub-bandgap formation of nano-crystalline Si quantum dots embedded ZnO thin film. Opt Express 2012, 20: 10470–10475. 10.1364/OE.20.010470View Article
- Cheng Q, Tam E, Xu S, Ostrikov KK: Si quantum dots embedded in an amorphous SiC matrix: nanophase control by non-equilibrium plasma hydrogenation. Nanoscale 2010, 2: 594–600. 10.1039/b9nr00371aView Article
- You JB, Zhang XW, Fan YM, Yin ZG, Cai PF, Chen NF: Effect of deposition conditions on optical and electrical properties of ZnO films prepared by pulsed laser deposition. Appl Surf Sci 2002, 197–198: 363–367.
- Sundaram KB, Khan A: Characterization and optimization of zinc oxide films by r.f. magnetron sputtering. Thin Solid Films 1997, 295: 87–91. 10.1016/S0040-6090(96)09274-7View Article
- Hao XJ, Cho EC, Scardera G, Shen YS, Bellet-Amalric E, Bellet D, Conibeer G, Green MA: Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells. Sol Energy Mater Sol Cells 2009, 93: 1524–1530. 10.1016/j.solmat.2009.04.002View Article
- Di D, Xu H, Perez-Wurfl I, Green MA, Conibeer G: Improved nanocrystal formation, quantum confinement and carrier transport properties of doped Si quantum dot superlattices for third generation photovoltaics. Res Appl: Prog Photovolt 2013, 21: 569–577.
- Lee JD, Park CY, Kim HS, Lee JJ, Choo YG: A study of conduction of ZnO film/p-Si heterojunction fabricated by photoinduced electrodeposition under illumination. J Phys D Appl Phys 2010, 43: 365403. 10.1088/0022-3727/43/36/365403View Article
- Mridha S, Basak D: Ultraviolet and visible photoresponse properties of n-ZnO/p-Si heterojunction. J Appl Phys 2007, 101: 083102. 10.1063/1.2724808View Article
- Zebbar N, Kheireddine Y, Mokeddem K, Hafdallah A, Kechouane M, Aida MS: Structural, optical and electrical properties of n-ZnO/p-Si heterojunction prepared by ultrasonic spray. Mater Sci Semicond Process 2011, 14: 229–234. 10.1016/j.mssp.2011.03.001View Article
- Zhang Y, Xu J, Lin B, Fu Z, Zhong S, Liu C, Zhang Z: Fabrication and electrical characterization of nanocrystalline ZnO/Si heterojunctions. Appl Surf Sci 2006, 252: 3449–3453. 10.1016/j.apsusc.2005.04.053View Article
- Dhananjay , Nagaraju J, Krupanidhi SB: Investigations on zinc oxide thin films grown on Si (100) by thermal oxidation. Mater Sci Eng B 2007, 137: 126–130. 10.1016/j.mseb.2006.11.007View Article
- Osinniy V, Lysgaard S, Kolkovsky V, Pankratov V, Larsen AN: Vertical charge-carrier transport in Si nanocrystal/SiO2 multilayer structures. Nanotechnology 2009, 20: 195201. 10.1088/0957-4484/20/19/195201View Article
- Veettil BP: Modelling and characterization of carrier transport through nanostructures. PhD thesis. University of New South Wales, School of Photovoltaic and Renewable Energy Engineering; 2012.
- Fangsuwannarak T: Electronic and optical characterisations of silicon quantum dots and its applications in solar cells. PhD thesis. University of New South Wales, Centre of Excellence for Advanced Silicon Photovoltaics and Photonics; 2007.
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.