A multilayered approach of Si/SiO to promote carrier transport in electroluminescence of Si nanocrystals
© Li et al; licensee Springer. 2012
Received: 16 November 2011
Accepted: 27 March 2012
Published: 27 March 2012
The electroluminescence (EL) and photoluminescence of Si nanocrystals (Si-nc) from multilayered samples of Si/SiO are investigated. Si-nc are formed within Si and SiO layers after furnace annealing. It is found that the presence of Si interlayers creates extra carrier paths for EL emission. A comparative study is further performed on a multilayered Si/SiO sample and a single-layered one with Si and SiO homogeneously mixed. Both samples have the same ratio of Si to O and the same contents of Si and O. The multilayered sample is found to have higher EL intensity, less turn-on voltage, lower resistance, and higher current efficiency than the single-layered one. The results indicate that Si interlayers in Si/SiO may act as carrier channels, which promote carrier transport and enhance the EL emission of Si-nc.
Development of highly luminescent Si is of vital importance for Si optoelectronics . At present, Si nanocrystals (Si-nc) embedded in SiO2, or SiO2:Si-nc, have been widely accepted as a promising material for Si light emission due to their stable light emission, robust structure, and stimulated emission feature (JZ and ML, unpublished data) [1–26]. Researchers have been focused on the enhancement of photoluminescence (PL) of Si from the SiO2:Si-nc [3, 6, 7, 11, 20, 24] and that of electroluminescence (EL) of Si [14–19, 21, 25]. One of the key problems regarding the enhancement of EL emission is how to promote the carrier (electron or hole) transport [21, 25, 26]. The Si3N4 :Si-nc has once been adopted since the bandgap width of Si3N4 is less than that of SiO2, so carrier transport within Si3N4 would become easier than that within SiO2 . For the same reason, lateral carrier injection has been suggested recently for the sample of SiO2:Si-nc/SiO2 . However, a matrix with a small bandgap width does not favor carrier confinement within Si-nc, which tends to degrade light emission of Si-nc. For instance, it is found that the PL intensity of Si-nc from amorphous Si:Si-nc/SiO2 is one order of magnitude less than that of SiO2:Si-nc/SiO2 (JZ and ML, unpublished data). In this work, we propose another approach to improve carrier transport, that is, an approach of multilayered structure of Si/SiO. After thermal annealing, Si-nc are formed in both Si and SiO layers. Through the investigation of PL and EL emissions from multilayered Si/SiO, it is found that Si interlayers help to facilitate carrier transport. Further comparative studies on a multilayered Si/SiO sample and a single-layered one, with Si and SiO homogeneously mixed, indicate that Si interlayers in Si/SiO act as carrier channels that promote carrier transport and enhance the EL emission of Si-nc.
Preparation of samples
Forty-layered Si/SiO samples were prepared by evaporating Si and SiO alternatively by electron-beam and resistance heating, respectively, onto p+-type Si substrates (0.5 to approximately 1 Ω·cm) in a vacuum chamber with a base pressure less than 5 × 10-5 Pa. An ex-situ furnace annealing in nitrogen atmosphere at 1,100°C was performed for 1 h to induce phase separation (JZ and ML, unpublished data) [3, 4, 7, 11] to form Si-nc. The thickness of SiO remained at 3.75 nm, and that of Si interlayer (dSi) was variable.
For a comparative study, we chose a 40-layered Si/SiO sample with thicknesses of SiO and Si layers being 3.75 and 1.00 nm, respectively, and a single-layered sample prepared by co-evaporating Si and SiO with the same ratio of Si to O and the same contents of Si and O as those of the multilayered sample. For further EL identification, a pure SiO2 sample with a thickness of 150 nm was prepared by electron beam evaporation, followed by furnace annealing at 1,100°C in nitrogen for 1 h.
For EL emission, Al was firstly evaporated on the backside of the p+-type Si substrate of the SiO2:Si-nc; then, the sample was annealed ex situ in nitrogen at 480°C for 10 min. A ring of Al electrode was finally evaporated onto the top surface of sample.
Monitoring, characterization and measurements of samples
Film thickness was monitored by a calibrated microbalance (MDC-360, Maxtek Digicom Limited, Shenzen, China). The PL spectra were recorded on a fluorescence spectrometer (F-4500, Hitachi High-Tech, Minato-ku, Tokyo, Japan) with a xenon lamp as an excitation source. The wavelength of the selected exciting beam was 300 nm. Forward-biased voltage was applied to the EL device. EL emission was also recorded with the spectrometer (Hitachi, F-4500). The biased voltage as well as voltage and current readings were provided by a source meter (2400, Kiethley Instruments Inc., Cleveland, OH, USA). High-resolution transmission electron microscopy (HRTEM) was measured on an electron microscope (G2 F20, Tecnai, Amsterdam, The Netherlands) to examine the formation of Si-nc. Reflectance spectra were measured with a spectrophotometer (UV-3101 PC, Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). In this study, all the samples refer to those that have been furnace-annealed at 1,100°C for 1 h in nitrogen.
Results and discussion
We report that a multilayered approach of Si/SiO can promote carrier transport for EL emission by establishing carrier channels made of Si interlayers. To promote carrier transport along the normal of the thin film surface within SiO2: Si-nc, one normally tries to align Si-nc closely along the direction of the electric field so that carrier tunneling can become easier. However, how to align the self-organized Si-nc remains an open problem so far. The approach of a multilayered structure of Si/SiO proposed here might, to some degree, circumvent this problem and help to enhance the EL emission of Si-nc.
This work was supported by the National Basic Research Program of China (973 Program) under grant number 2010CB933703 and the National Science Foundation of China under grant numbers 60878044 and 60638010.
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