1. Strong infrared electroluminescence from black silicon
at room temperature
Quan Lü, C Liang, Li Zhao, and Zuimin Jiang
State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures
(Ministry of Education) and Department of Physics, Fudan University, Shanghai , China
Significant efforts have been devoted to the development of light emitters based on Si. However bulk Si is a poor light emitter due to its indirect band gap.
Black silicon (b-Si) has been receiving a great deal of attention, but very few mentioned electroluminescence (EL) from b-Si or structures of similarity to b-Si.
The EL spectra exhibit only the light emission band centered at 0.78 eV (~1.6 μm) which was attributed to the D1-related EL and do not contain any peaks that are typically observed from crystalline Si.
I. Sample fabrication
III. Temperature dependence of the integrated EL intensity
An fs laser beam as a light source, which provided 800 nm, 125 fs laser pulses with a repetition rate of 1 kHz, for the irradiation on the Si wafer surface to form a forest of spikes on it in a chamber with 70 kPa SF6 gas.
In order to enhance the luminescence, the b-Si samples were processed with RTA treatment for 120 s in forming gas (H2:N2=5%:95%) at a temperature of 800 °C.
For the front electrode of the LEDs, a 750 nm thick ITO film was deposited on the surfaces of the spikes by DC magnetic sputtering at 200 °C. The sputtering atmosphere was comprised of Ar and O2 (Ar:O2=3:1).
After that, an indium pad was used as a contact, while a 150 nm thick aluminum layer was deposited onto the reverse of the substrate as the back electrode.
The device was processed with RTA treatment for 120 s in forming gas (H2:N2=5%:95%) at 300 °C to construct the ohmic contacts.
FIG. 3. (a) Integrated EL intensity as a function of applied forward voltage at temperatures 20, 80, 200 and 300 K. (b) Integrated EL intensity as a function of temperature at a fixed forward current density of 0.85 A/cm2. The inset shows the integrated PL intensity as a function of temperature.
The EL intensity is hugely enhanced when the temperature increases from 20 K to RT.
When temperature increased, the carriers will thermally be excited to cross the barrier and be trapped by the energy levels for the luminescence.
IV. Current density dependence of the integrated EL intensity
FIG. 4. D1-related integrated EL intensity as a function of the current density; both in logarithmic scale measured at temperatures of 30, 90, 180, 220, 260, and 300 K. Data for 300 K was measured twice, the second time having an extended current density range from 0.35 to 1.05 A/cm2. A fitting for the 300 K plot was attempted, with a power-law function, , where m is power exponent, is the integrated EL intensity and J is the current density. The m values were found to be 9.2 at low current densities, 8.3 at middle current densities, and 6.1 at high current densities. For low temperature, the m value looks to be even larger than that of RT.
FIG. 1. RT current density-voltage (J-V) curve of the LEDs. The inset is the schematic of the LED device. The sample area is 1 cm2.
II. Effect of thermal annealing to enhancement of EL intensity
When the forward bias voltage increases (i.e. the injected current density increases), the quasi-Fermi level of electrons will be lifted up in respect to the conduction band in the p-n junction region, the injected electrons in the conduction have a larger population at higher energy levels, which favors the electrons to cross the barrier and be trapped by the energy levels for the luminescence. This would also cause the strong super-linear behavior in the current density dependence of EL intensity.
V. Conclusion
We have demonstrated a simple, one-step fabrication method of producing LED devices based on b-Si.
The integrated intensity of D1-related EL increased monotonically and significantly as the measured temperature increased, implying that our LED device could possibly be applicable as RT light emitters for optical fiber communication systems.
The integrated EL intensity as a function of current density at RT was fitted with a power-law function, , with m value being 8.3 at middle current densities.
FIG. 2. RT EL spectra from the n+-p junction diode of (a) b-Si annealed at 800 °C and (b) unannealed b-Si for different forward bias voltages, respectively.
High temperature thermal annealing is a key factor for producing strong infrared EL in b-Si, which is in accordance with our previous research on PL of b-Si.