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.
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.
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