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Abstract
With low toxicity and high abundance of silicon, silicon nanocrystal (Si-NC) based white light-emitting device (WLED) is expected to be an alternative promising choice for general lighting in a cost-effective and environmentally friendly manner. Therefore, an all-inorganic Si-NC based WLED was reported for the first time in this paper. The active layer was made by mixing freestanding Si-NCs with hydrogen silsesquioxane (HSQ), followed by annealing and preparing the carrier transport layer and electrodes to complete the fabrication of an LED. Under forward biased condition, the electroluminescence (EL) spectrum of the LED showed a broadband spectrum. It was attributed to the mechanism of differential passivation of Si-NCs. The performance of LED could be optimized by modifying the annealing temperature and ratio of Si-NCs to HSQ in the active layer. The external quantum efficiency (EQE) peak of the Si WLED was 1.0% with a corresponding luminance of 225.8 cd/m2, and the onset voltage of the WLED was 2.9V. The chromaticity of the WLED indicated a warm white light emission.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, gallium nitride-based white light-emitting device (WLED) has become a mainstream light source for general lighting because of its high efficiency, stability in operation and broad spectral range [1–10]. However, gallium is a rare element with abundance of only ∼18 ppm and low annual outputs [11–13], and the surge in consumption of gallium nitride in high-power electronics and 5G applications may further accelerate the potential supply shortage [14–16]. Furthermore, rare earth materials were widely used as phosphors for white light emission [17–19]. Therefore, the sustainable development of general lighting in the future requires much less consumption of rare elements or even avoids using these elements. To address this issue, it is a potential solution to develop general lighting-used WLED based on abundant elements such as silicon (Si), which is the fundamental material in microelectronics and Si photonic industries [20–22] with abundance of ∼27.7%. As a matter of fact, tremendous efforts have been made in this respect. LEDs emitting yellow or red light based on colloidal silicon quantum dots and Si-NCs embedded in SiO2 have been reported [23–29]. The hybrid approach of using organic materials and Si quantum dots has been proven to give rise to white light emission via the mixture of red component from Si quantum dots and blue/green components from organic material(s) [26,30–32]. To date, EQE up to 3.5% was achieved from organic/Si quantum dots LED [30]. However, the instability of organic components under intense current injection may compromise the device performance in practical use. Therefore, developing an all-inorganic Si WLED with red, green and blue (RGB) emissions that are intrinsically from Si itself has become intriguing. In this work, a broadband all-inorganic Si WLED was made. The active layer was a composite thin film with high-density Si nanocrystals (Si-NCs) embedded in hydrogen silsesquioxane (HSQ), or termed Si-NCs:HSQ. The white light emission employed the mechanism of differential passivation of Si-NCs [33]. A zinc oxide (ZnO) layer was used as the electron transport layer (ETL) to enhance the light emission and surface roughening was performed to improve the light extraction. The chromaticity of the Si WLED could be tuned by adjusting the annealing temperature and Si-NC concentration. A maximal EQE of 1.0% was achieved with a luminance of 225.8 cd/m2 for the present configuration of Si WLED. To the best of our knowledge, it is the first demonstration of an all-inorganic Si WLED reported thus far.
2. Experimental section
2.1 Materials
Methyl isobutyl ketone (MIBK) solution of hydrogen silsesquioxane (HSQ) (Dow Corning, FOx 15), n-pentane (AR, Aladdin, 99.0%), hydrofluoric acid (metals basis, Aladdin, 49 wt. % in H2O, ≥ 99.99998%) were used for Si-NC preparation. Hydrogen peroxide (AR, Aladdin, 30 wt. % in H2O), Ag pellets (PVD, Zhongnuo, 99.99%), ZnO sputtering target (PVD, Zhongnuo, 99.99%), Al pellets (PVD, Zhongnuo, 99.99%), ITO pellets (PVD, Zhongnuo, 99.999%) and p-type Si wafer (4 inch × 500 µm, <100>, 0.5–1.0 Ω·cm, Suzhou Research Materials Microtech) were used for LED device fabrication.
2.2 Si-NC synthesis and active layer preparation
HSQ was heated in dryer oven to remove the solvent. The scraped solute was ground in a mortar. The obtained white powder was placed into a quartz crucible and transferred in an inert atmosphere to a high-temperature furnace. The sample was heated at the rate of 20 °C/min till 1100 °C for phase separation, and held at 1100 °C for 70 min to make Si atoms grow into nanocrystals in the 5% H2 and 95% N2 atmosphere. After that, the brown product was treated with hydro-ball-milling to decrease the size of particles. Then the suspension was filtered by filter paper with hole diameter of 1 µm, and the solid residue was collected by polytetrafluoroethylene beaker. The finely ground powder was etched by a mixture of 1:1:1 49% HF:H2O:ethanol solution for 90 min at room temperature to remove the surrounding SiO2. Once liberated, the hydride-terminated Si crystals were extracted into two 5 mL portions of pentane. The solution of Si-NCs was then cooled to -3.5 °C (same as the storing temperature of HSQ) and mixed with XR-1541-006 (solution with 6% HSQ) as the main ingredient of active layer at volume ratio of 1:1, 2:1, 3:1 and 4:1 for further use.
2.3 Surface roughening of Si wafer
The 4 inch p-type Si wafer was cut into pieces with the size of 2×2 cm2. Then piranha solution and diluted hydrofluoric acid were used to clean the square Si substrate. A thin layer of Ag (< 6 nm) was thermal evaporated onto the p-type Si to form metal nanoparticles. Metal catalyzed chemical corrosion occurred on the surface of substrate when put into the solution composed of deionized water, H2O2 and HF (10:5:1 in volume). The reaction was stopped at 300 s by taking out p-type Si and washed by deionized water immediately.
2.4 All inorganic Si WLED fabrication
A dosage of 80 µl mixture was spin-coated on nanoroughened Si substrate. The spin speed was firstly set at 500 rpm for 10 s and then increased to 4000 rpm for another 60 s for uniformity of film thickness. To solidify the film, the sample was heated in a muffle stove at 120 °C for 5 min, followed by thermal annealing in N2 atmosphere. Further annealing was carried out at different temperatures (400 °C, 700 °C and 900 °C) for 30 min under the protection of N2. Then, the Al layer was deposited on the back side of substrate and annealed for better contact. Afterwards, ZnO ETL was sputtered by vacuum magnetron sputtering system without substrate heating. ITO was then evaporated onto the front surface of sample as front electrode, followed by heating in nitrogen at 250 °C for 5 min.
2.5 Characterization
Transmission Electron Microscope (TEM) and high resolution TEM (HRTEM) images were recorded on a FEI system (G2 F20 S-Twin, Tecnai). Elemental analysis was made by using energy dispersive X-Ray spectroscopy (EDX, X-MaxN 80T, Oxford Instruments) that was attached to the TEM system. Photoluminescence (PL) measurement was performed at room temperature on a spectrophotometer (Hitachi, F-4500) with an excitation source of a 150 W Xe lamp. The exciting beam wavelength was 300 nm. To acquire photoluminescence quantum yield (PLQY), Si-NCs pentane solution was put into a cuvette and excited by 365 nm laser in an integrating sphere. Emitted and absorbed photons were detected by a calibrated spectrometer (QE-pro, Ocean Optics). The PLQY values were calculated using Suzuki’s method [34]. For the device performance test, our LED was driven by a source-meter (2400, Keithley). The EL spectra were measured with the same spectrophotometer (F-4500, Hitachi). The film thickness was measured by using cross sectional scanning electron microscopy (SEM) on a Hitachi system (Hitachi S4800) and step profiler (DektakXT, Bruker). Morphology of nanorouphened Si was acquired by atomic force microscopy (AFM) system (XE-100, Park systems). The Fermi level was detected by Kelvin Probe System (KP020, KPTechnology). UV–VIS spectrum was detected by double-beam UV-visible spectrophotometer (Beijing Purkinje General Instrument, TU-1900). Two source meter units (2400, Keithley and 2000, Keithley) linked to a calibrated silicon photodiode (PDA100A-EC, Thorlabs) were used to measure the current–voltage–brightness characteristics. To calculate EQE, the spatial emission from the LED was assumed as a Lambertian profile. A detail description of EQE calculation was available in Ref. [35].
3. Results and discussion
3.1 Si-NCs properties characterization
A schematic process to synthesize the optically active material is displayed in Fig. 1(a). Freestanding Si-NCs were prepared by annealing HSQ at 1100 °C in a forming gas of hydrogen and nitrogen, followed by chemical etching by hydrofluoric acid (HF) and extracting by n-pentane. Detailed information is available in the experimental section. The structure of the annealed HSQ thin film, or the composite thin film with Si-NCs embedded in SiO2 (Si-NCs:SiO2), was identified by TEM in Fig. 1(b). The distribution of Si-NCs was uniform and the average spacing between Si-NCs was ∼1 nm. Moreover, the HRTEM image showed distinct diffraction fringe of Si (111) with an interplanar crystal spacing of 3.1 Å in Fig. 1(b) inset, which is consistent with previous reports [30]. The average size of Si-NCs was 2.8 ± 0.4 nm according to Fig. 1(c). The EDX result as shown in Fig. 1(d) indicated the presence of oxygen and silicon. The observed carbon was from ion thinning process during the TEM sample preparation. Bright red emission from Si-NCs was visible under ultraviolet illumination. The absorption of Si-NCs was illustrated in Fig. 1(e). The PL spectrum of Si-NCs peaked at 680 nm in wavelength, as shown in Fig. 1(e), with a PLQY of 41.0%. Lifetime of Si-NCs:HSQ was acquired to be 17.5 ± 0.9 µs from PL decay curve in Fig. 1(f).
Fig. 1. Silicon nanocrystals film characteristics. (a) A schematic diagram of synthesis and extraction of Si-NCs. (b) TEM image of SiNCs:HSQ ; inset: HRTEM image of a single Si-NC. (c) Diameter distribution histogram of Si-NCs. (d) EDX signal of Si-NCs. (e) Emission (red) and absorption (blue) spectrum of Si-NC in pentane. (f) PL decay curve of Si-NCs:HSQ.
3.2. PL of active layer and mechanism of tunable luminescence
The active layer was a composite thin film with Si-NCs embedded in HSQ (Si-NCs:HSQ) after solidification annealing. It was basically composed of Si-NCs and Si oxides as indicated by EDX measurement as well. Figure 2(a) shows the normalized PL spectra of Si-NC from the Si-NCs:HSQ after solidification and annealing at different temperatures (Ts). The peak in the PL spectrum redshifted as T increased from 400 °C to 900 °C. It has been found both experimentally and theoretically that when Si-H bonds at surfaces of Si cluster are gradually replaced by the Si-O ones, the bandgap of Si cluster shrinks, and a continuous redshift is expected in the PL emission [36–40] (Fig. 2(b)).
Fig. 2. PL characteristics and mechanism. (a) PL spectra of HSQ:SiNCs layer annealed at different temperatures. (b) Variation of wavelength during Si = O and Si-H related interface modification of Si-NC; inset: illustration of the tunable PL emission mechanism. (c) FT-IR spectra of differently annealed active layers; inset: integrated peak intensity at different annealing temperatures. (d) PL spectra of active layer with increasing Si-NC concentration. The PL intensity increased steadily as expected with increasing number of Si-NCs.
In this work, the surface bonds changed with the increasing T, as shown by the FTIR data in Fig. 2(c). The observed IR absorption peaks at around 850, 880, 2100 and 1070 cm−1 were assigned to the vibrations of the Si–H rocking, Si–H bending, Si–H stretching, and Si–O–Si stretching modes, respectively [41,42]. When T increased, the intensity of Si-H bonds decreased, indicating dissociation of Si-H bonds and formation of Si-O ones, as shown in Fig. 2(c). The ratio of integral intensity of Si-O bonds to that of Si-H bonds increased with increasing T (Fig. 2(c) inset). The following equations describe the thermal decomposition process of HSQ [43].
During the annealing process, hydrogen was released, and Si-H bonds tended to form around Si-NCs at relatively low temperatures. However, when T increased above 400°C, Si-H bonds started to dissociate and the Si-O gradually dominated [44–46].
To enhance the PL intensity of Si-NC, heavier doping of Si-NCs in Si-NCs:HSQ was performed. Figure 2(d) shows the PL spectra as a function of volume ratios (Rs) of Si-NC solution to HSQ. Theoretically, PL emission with further higher intensity could be achievable at higher concentration of Si-NCs. However, for R = 4:1 and beyond, cracks were found on the surface of thin film in this work, which can result in poor contacts between the active layer and ETL and compromise for the performance of the electroluminescence (EL) device. Therefore, the following Si WLED device reported was based on the active layer with R = 3:1, unless otherwise specified.
3.3. Fabrication and characterization of all-inorganic Si WLEDs
Figure 3(a) depicts the structure of the Si WLED in this work. 80 nm-thick indium-tin-oxide (ITO) and 1.5 µm thick Al layers were used as cathode and anode, respectively. The thickness of the active film was 180 ± 10 nm. A 40 nm-thick ZnO layer was prepared by magnetron sputtering between the ITO and the active layer as ETL. The surface of p-type Si substrate was roughened by means of metal-assisted chemical etching [47], which can increase the tunneling current via enhanced inhomogeneous local electric field on the surface. Meanwhile, a rough surface reduces internal total reflection and thus improves the light extraction of the device [48–50]. Figure 3(b) shows the energy band diagram of the WLED with no bias voltage. The energy levels were drawn using parameters measured in this work and from literatures [51], and the detailed values are listed in Table 1.
Fig. 3. Device performance and EL emission. (a) Cross-sectional SEM image of the LED; inset: three-dimensional structure diagram of the LED. (b) The proposed energy diagram of the WLED device at zero field. (c) EL spectra of HSQ:SiNCs layer annealed at different temperature under same applied voltage. (d) Chromaticity of the light emission from differently annealed devices. (e) EL spectra of active layer with increasing Si-NC concentration. (f) EL intensity as a function of device driving voltage U, indicating the turn-on voltage; inset: EL spectra of LED at different bias voltages.
Table 1. Parameters of device band structure
Figure 3(c) depicts the EL counterparts to the PL spectra in Fig. 2(a) at the bias voltage of 20 V. The sample annealed at 400 °C showed the strongest luminescence at the same voltage. It could be explained that some surface bond-mediated conducting channels still existed at 400 °C, and with increasing T, those surface bonds started to dissociate, which made the charge transport more difficult. The difference between the PL and corresponding EL spectra arises from their different excitation processes, therefore Si-NCs with different sizes contribute to the light emission in different manners [52]. The devices exhibited emissions at the visible regime with Commission Internationale del’Eclairage (CIE) chromaticity coordinate at (0.35, 0.35), (0.37, 0.40) and (0.39, 0.41) for T = 400, 700 and 900°C, respectively (Fig. 3(d)). For the annealed device at 400 °C, the white light contained more blue components, which had a higher correlated color temperature (CCT ∼ 4900 K) than the other two and was cooler. The color rendering index was calculated to be 70.55.
Figure 3(e) depicts the EL spectra with maximal intensities for the samples with different Rs. The EL intensity increased steadily with increasing R value, until at R = 4:1, as cracks occurred on the thin film surface as well as poor charge injection and transport. Only 3:1 device was chosen for subsequent optoelectronic testing. In the cases of R = 2:1 and 3:1, EL intensity enhancement was larger than PL. The light emission enhancement by increasing Si-NC density could be attributed to the increased carrier mobility when more Si-NCs were doped. The slight difference in the EL peak position might be related to the variance in the applied bias voltage or/and the way to select Si-NCs for excitation for different Si-NC densities. Figure 3(f) plots the EL intensity as a function of bias voltage. The onset voltage could be estimated as low as 2.9 eV upon sudden growth of emission intensity [30,52–56]. In the inset, EL spectra for different bias voltages were illustrated. The blue shift of EL as bias voltage goes up was attributed to the improving of the electric field which leads to an injection of carriers in smaller clusters, enhancing shorter wavelength luminescence [57–59].
Figure 4(a) shows the current density-voltage (J-V) and luminance-voltage (L-V) curves of the Si WLED, respectively. A relatively low leakage current density of 0.27 mA/cm2 at -5 V was found for the device. Under forward bias, the current density and EL rose rapidly beyond the onset voltage of 2.9 V. The maximum brightness of the device was 225.8 cd/m2, which is close to the standard value for commercial display [60]. To investigate the current conduction mechanism, the J-V dependence was reformed into the form of ln(J/E2) -1/E, where E is the electric field that is proportional to V. The experimental data were then reproduced with the Fowler-Nordheim (F-N) tunneling model described as follows [61,62]:
The ln(J/E2) − 1/E equation was expressed as:Fig. 4. Optoelectronic characteristics of WLED. (a) Current density J (blue) and luminance L (yellow) as a function of driving voltage U. (b) Fowler–Nordheim plot of the device. The right dash line was fitted by the F-N equation. Inset: schematic diagram of F-N tunneling. (c) The device EQE as a function of luminance L; inset: pictures of a working WLED in the dark background at different luminance. (d) WLED emission decay under constant injection current.
4. Conclusion
In summary, freestanding Si-NCs with uniform particle size distribution was prepared. The active layer was made by mixing HSQ and Si-NCs, followed by annealing at moderately high temperatures. Broadband EL emission covering RGB regions was observed due to differential passivation of Si-NCs, and an all-inorganic Si WLED was achieved with a CCT of ∼ 4900 K. The Si substrate was roughened and a ZnO ETL layer was used to improve the performance of the WLED. The device featured a low onset voltage of 2.9 V, a peak EQE of 1.0% and luminance of up to 225.8 cd/m2. This first demonstration of an all-inorganic Si WLED provides a solution to overcome the challenge of huge consumption of rare elements.
Funding
Science and Technology Commission of Shanghai Municipality (16YF1400700, 18JC1411500); National Basic Research Program of China (973 Program) (2017YFA0303403, 2106YFC0201401); National Natural Science Foundation of China (51472051, 61705042).
Disclosures
The authors declare no conflict of interest.
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