1. Introduction

Silicon-based light emitting diodes (LEDs) potentially enable low-cost optoelectronics for applications of inter-/intra-chip optical interconnects, monolithic bio-sensors, illumination, backlight, etc [1,2]. Silicon (Si) in buck theoretically permits inefficient infrared light emission. Such an issue is proposed to be overcome using quantum-confined nanocrystals or nanoclusters. Silicon dioxide (SiO₂) film with Si nanocrystals (SiNCs) embedded is studied as Si-based LEDs, excited to emit light with an external quantum efficiency of up to 0.03% [3–5]. The emission spectrum also falls in the near-infrared region with the energy bandgap of 1.4-1.8 eV even for tiny SiNCs of 1 nm in size, confined by oxygen-related surface states. To extend the emission spectrum of the SiNCs embedded SiO₂ films, rare-earth dopants are studied to shift the emission photon energy either to 0.8 eV by Er³⁺ or to 2 eV by Eu³⁺ [6,7]. Nevertheless, the fabrication of the SiNCs embedded SiO₂ film involves high temperature post-annealing of over 1000°C. Such a high temperature is incompatible with the manufacturing requirements of very-large-scale-integrated (VLSI) technologies. Research of metal oxide films with the direct bandgap of over 3 eV, including ZnO and SnO₂, are conducted for Si-based LEDs for achieving broad emission spectrum. Grown on silicon substrates, ZnO and SnO₂ typically form in polycrystalline structures, impeding emission efficiency because of defect-related radiative recombination at grain boundaries [8,9].

In this study, Si-based out-of-plane light emitting heterostructures comprised of silicon nanowire, silver oxide and silver nanodendrite (SiNW/Ag_xO/AgND) are proposed for low-cost visible light emitters with large area manufacturing capability, which has the potential to be compatible with VLSI due to low-temperature process flows. The fabrication utilizes cost-effective processes, including natural lithography and silver nanoparticles (AgNPs) assisted wet chemical treatments for nanopatterning. The optical and electronic properties of electroluminescence (EL) of the heterostructure device are characterized under biases.

2. Experiment

Figure 1 shows the schematic diagram and the FESEM photo (Carl Zeizz, Ultra-55) of the SiNW/Ag_xO/AgND heterostructure device fabricated on a 4” p-type (100) silicon wafer with a resistivity of 4 ohm-cm. The contact pad firstly was defined using photolithography, followed by a thermal evaporation of Al/Ti onto the Si substrate. After a lift-off process, the device electrode was annealed at 420°C for 30 min to achieve an ohmic contact between Si and Al/Ti. Besides, the formation of heterostructures was conducted using AgNPs-assisted wet chemical etching where the unprotected Si region was exposed to a mixed etchant of DI water, HF (49%), H₂O₂ (30%) and AgNO₃ (0.34%) at 20°C to produce silicon nanowires (SiNWs) [10–12]. The concentration of AgNO₃ determines the SiNW diameter and the AgND geometry. The H₂O₂ concentration dominates the reaction rate of heterostructure formation. After wet chemical processes, the orientation of SiNWs was perpendicular to the Si substrate. The AgNDs were formed/precipitated atop and in conjunction with the SiNWs due to the chemical reduction of silver ions by H₂O₂. The AgNDs were randomly distributed and constituted a network of conductive paths on the top surface. The SiNWs were measured to be 3-5 μm in length and 50-300 nm in diameter. The AgNDs were found to be 1-10 μm in length and 100-200 nm in diameter. Upon exposure to air, natively induced silver oxide surrounded the surface of AgNDs containing the vicinity of conjunction of SiNWs and AgNDs, bringing out SiNW/Ag_xO/AgND heterostructures.

 

Fig. 1 The schematic diagram and FESEM of the SiNW/Ag_xO/AgND heterostructure device based on a silicon substrate. After nanopatterning using AgNPs-assisted wet chemical etching, AgNDs network was formed/precipitated atop in conjunction with SiNWs. The heterostructure was biased to investigate the electroluminescence phenomenon.

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The SiNW/Ag_xO/AgND heterostructure device was biased via attaching two tungsten probes with a 300 nm probe tip onto the Al/Ti contact pad and the AgNDs network atop the SiNWs. The p-type Si was connected to the positive terminal and the AgND was connected to the negative terminal. Modulation of dc bias across the device was used to electrically activate the heterostructures and to investigate the EL phenomenon. Visible EL was captured by an optical microscope (Olympus BX-51) associated with a CCD camera (Olympus c5060). Spectra were further characterized by an optical fiber (Ocean Optics QP1000-2-UV/VIS, NA of 0.22) along with a UV-VIS CCD spectrometer (BWTek, BRC111A).

3. Results and discussion

The SiNW/Ag_xO/AgND heterostructure device was investigated by I-V measurement and spectrum characterization. The I-V characterization of the heterostructure device is shown in Fig. 2 where the thick red line and the thin blue line represent the response before/after the electro-activation process. The heterostructures had no emission of lights unless being treated by the electro-activation process. The activation process is clearly depicted by the thick red line. When voltage applied lowered than 40 V, the resistance of the heterostructure device was estimated to be ~10⁴ ohm predominantly due to the existence of the insulating silver oxide (Ag_xO) in the conductive path [13]. When the applied voltage was approaching 40 V, the high electric field crossing the heterostructures causes the avalanche-type breakdown of the intrinsic dielectric Ag_xO. At the onset of conduction, the hot electrons driven under extremely high electric field obtained sufficient kinetic energy to break down the bonding between Ag and O, generating a lot of electron-hole pairs and causing the partially thermal decomposition of the dielectric Ag_xO [14]. The large current (~60 mA) passing through the Ag_xO started to migrate and to aggregate the Ag molecules to form Ag nanoclusters in Ag_xO layer, followed with light emission from heterostructures. Once electrically activated, the device was switched to the rest state (V = 0 V). The heterostructure device started to enter light emitting region at a lower threshold voltage (V_th) of 20 V while the device resistance immediately dropped to ~300 ohms with two to three orders of magnitude smaller than the initial resistance. The previous electro-activation process generated the percolation networks of tunneling paths composed of randomly-distributed Ag nanoclusters in Ag_xO which would electrically connect Ag nanodendrites and Si nanorods and efficiently decreased the resistance. The current saturation occurred to the applied voltages over 48 V. With further increasing the bias to 60 V, the sparks crossing at high voltage caused by the instant ionization of the lattice would damage the heterostructures by the Joule heating.

 

Fig. 2 The I-V characterization of the SiNW/Ag_xO/AgND heterostructure device before and after electro-activation. The red thick line shows the activation process. When the voltage approached 40 V, the heterostructure was activated via passing through large current of 60 mA, followed with light emission from the heterostructure. After activation, the device entered light emitting region at a lower threshold voltage of 20 V.

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Figure 3 displays the sequential CCD images of light emission from an activated SiNW/Ag_xO/AgND heterostructure device with a dc voltage of 30V. The images were snapshotted with a time interval of 1s. Some emission sites at the heterostructures started to illuminate due to sufficient radiative electron-hole recombination. There were many dynamic multicolored emission sites showing colors from dark red, yellow to orange and green, etc. The colors of the light spots spanning the visible spectrum may result from the diameter or shape of the Ag nanoclusters [15]. The size of the light spots was about 3~5 μm. Some light spots continued illuminating for seconds, followed by the decay of the EL intensity due to the limited available excitons for recombination. Other light spots were dynamic blinking with different frequencies due to intermittent carrier injections.

 

Fig. 3 The sequential CCD images of light illumination from the activated heterostructure device snapshotted with a time interval of 1s from (a) to (d). The device was biased at 30 V. Multicolored illumination spots were blinking with different frequencies. The colors primarily contain dark red, yellow to orange and green, etc.

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Figure 4(a) shows the EL spectra of an activated heterostructure device under dc voltages of 30, 40 and 50 V. The EL intensity increased with the enhancement of the dc bias. The EL spectra of the device fell in a visible–NIR range of 400-850 nm. In Fig. 4(b), three primary intensity peaks at 50 V were fitted using Gaussian distribution by MATLAB and had the wavelengths of 665 nm (dark red), 607 nm (yellow to orange) and 547 nm (green), corresponding to the photon energies of 1.87, 2.05 and 2.27 eV, respectively. The intensity peaks were primarily attributed to the multicolored emission spots as shown in Fig. 3. The device yielded unstable as the biased voltage approached 60 V because the large current would melt, redefine the interface between AgND and SiNW and burn out the emission site by Joule heating. The operation bias across the lighting emitting device is better less than 50 V.

 

Fig. 4 (a) The EL spectra of the heterostructure device at bias voltages of 30, 40 and 50V. (b) Three major emission peaks from the spectrum at 50 V were fitted using Gaussian distribution, showing the wavelengths of 665 nm (dark red), 607 nm (yellow to orange) and 547 nm (green), respectively. These emission peaks correspond to the multicolored emission spots shown in Fig. 3.

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The mechanism of light emission for the SiNW/Ag_xO/AgND heterostructure device requires further investigation. From the viewpoint of size dimension, neither SiNWs nor AgNDs are able to emit light. The diameter of the SiNWs in the heterostructure falls in a range of 50-300 nm, much larger than the quantum confinement size of 3 nm for the visible EL in the SiNWs [16]. The SiNWs in the device seems to be unable to excite visible EL. Further, the diameter of the AgNDs in the devices is about 100-200 nm, beyond the size of the Ag nanoclusters for luminescence. Both the SiNWs and the AgNDs are mainly reckoned for conducting current in the device. There exists a possible cause of light emitting by Ag nanoclusters embedded within silver oxide. An in-plane silver/silver oxide/silver (Ag/Ag_xO/Ag) heterostructure on glass substrates was reported to emit visible light. The light emission attributes electrical excitation in the visible luminescent Ag nanoclusters which function as quantum wells in Ag_xO junctions [13]. The light emission for our SiNW/Ag_xO/AgND heterostructures may result from Ag nanoclusters embedded in the Ag_xO layer. The embedded Ag nanoclusters are formed during the electro-activation step by electromigration and constitute the percolation network of the tunneling paths in the Ag_xO layer. The Ag nanoclusters as the tunneling sites have current conducted by carrier jumping through randomly distributed Ag nanoclusters in Ag_xO layer [17]. Due to the feature of Ag nanoclusters (approximately 2-8 atoms), the magnificent electric field (about 10⁹ V/m) crossing the Ag nanoclusters extracts the electron from the Ag nanoclusters, followed by the electron-hole recombination that contributes to the radiative emission. The emitted photon energies are consistent with the excitonic transition of the Ag nanoclusters with the sizes of less than eight atoms [15,18].

Our proposed out-of-plane light emitting heterostructure device developed from the in-plane silver nanodot heterostructures provides the positioning capability of light emitting heterostructures and obtains the high fill factor on a large area manufacture. The light emission sites along the tunneling paths in Ag_xO layer are natively confined at the conjunctions between SiNWs and AgNDs network. Besides, the fill factor of the light emission sites strongly depends on the density of conjunctions between SiNWs and AgNDs network. Both the positioning capability and the fill factor can be further improved using a monolayer of a polystyrene (PS) sphere array on the Si substrate as etching mask which not only precisely positions SiNWs but also narrows down the pitch of SiNWs under AgNPs-assisted etching [19]. In the current stage, the nanoscale Si-based light emitting heterostructures proposed are fabricated using inexpensive natural lithography instead of costly E-beam lithography and the cost-effective AgNPs-assisted chemical etching, enabling potentially low-cost visible light emitting sources with large area manufacturing capability.

4. Conclusion

With the treatment of AgNPs-assisted wet chemical etching, the nanoscale SiNW/Ag_xO/AgND heterostructures are formed on the top of out-of-plane SiNWs and start to emit light after an electro-activation process. The EL spectrum of the device falls in a visible–NIR range of 400-850 nm with three dominant intensity maxima at the wavelengths of 665 nm (dark red), 607 nm (yellow to orange) and 547 nm (green). The heterostructure device proposed paves a way to eliminate the use of relatively high cost direct band gap materials for the potential optoelectronic applications, such as optical interconnections and displays. Compared to the in-plane silver nanodot heterostructures, the out-of-plane heterostructures are capable of being precisely positioned and providing a high-fill factor Si-based light emitting displays. Further, the dependence of the SiNW and AgND dimensions on electro-activation, the decrease of the threshold voltage of the device and the cause of the light emitting mechanism will be investigated in the near future.