- Nano Express
- Open Access
Compensating the Degradation of Near-Infrared Absorption of Black Silicon Caused by Thermal Annealing
© Wang et al. 2016
- Received: 6 October 2015
- Accepted: 26 January 2016
- Published: 1 February 2016
We propose the use of thin Ag film deposition to remedy the degradation of near-infrared (NIR) absorption of black Si caused by high-temperature thermal annealing. A large amount of random and irregular Ag nanoparticles are formed on the microstructural surface of black Si after Ag film deposition, which compensates the degradation of NIR absorption of black Si caused by thermal annealing. The formation of Ag nanoparticles and their contributions to NIR absorption of black Si are discussed in detail.
- Infrared absorption
- Thermal annealing
- Microstructured silicon
- Ag nanoparticles
It is well known that Si is the most economical, technologically sophisticated material with the highest crystal quality among all semiconductors. However, due to its wide bandgap of 1.12 eV, Si has less absorption in the near-infrared (NIR) region with the wavelength longer than 1100 nm, which limits its optoelectronic applications within the range of visible to NIR (<1100 nm). Therefore, enhancing the NIR absorption with a longer wavelength has become a topic of great interest. This will extend the photoresponse of Si-based optoelectronic devices into the NIR region (>1100 nm) [1–4]. Recently, a type of Si named “black Si” fabricated by femtosecond (fs)-laser irradiation method has been reported [5, 6]. Black Si exhibits a very high absorption in not only the visible but also the NIR region extending to 2.5 μm because of the special surface microstructure and chalcogen hyperdoping induced by fs-laser processing.
The photodiode on black Si fabricated by fs-laser processing has been demonstrated [7, 8]. It can be even sensitive to the infrared wavelengths up to 1600 nm. Although black Si extends the photoelectric response range to a longer wavelength compared with Si, its photoelectric response intensity in the NIR region is still weak. One possible reason is that its NIR absorption is seriously degraded via thermal annealing process during device fabrication . It was reported that the NIR absorptance from 1200 to 2500 nm decreases to less than 50 % from 90 % after thermal annealing at 775 K due to the diffusion of sulfur ion hyperdoped by fs laser . However, thermal annealing processing associated with ohmic contact formation, defect passivation, or ion activation is a necessary step for photodiode fabrication. Therefore, remedying the degradation of NIR absorption caused by thermal annealing is required in order to improve the NIR photoelectric response of black Si. Newman et al. have attempted to solve the degradation of NIR absorption by annealing at a higher temperature . They showed that the amount of NIR absorption depended on the rate of postannealing cooling, suggesting a kinetically limited deactivation process. In their report, NIR absorption can be reactivated by annealing at a higher temperature between 1350 and 1550 K followed by fast cooling (>50 K/s) because hyperdoped-related defects responsible for NIR absorption are in equilibrium at a higher temperature in black Si.
In this study, we propose the use of thin Ag film deposition to remedy the degradation of NIR absorption of black Si caused by thermal annealing. We characterize the morphology of Ag nanoparticles formed on the microstructure surface after Ag deposition and analyzed its influence on the NIR absorption of black Si by theoretical simulation.
Black Si was fabricated on a 450-μm-thick single-side polished boron-doped Si (100) wafer with a resistivity of 10 Ω cm. A 1-kHz, 100-fs, and 800-nm Ti:sapphire laser was used for the laser processing. To fabricate a microstructural surface, cleaned Si wafer was placed on a translation stage in a vacuum chamber filled with high pure SF6 gas at a pressure of 5 × 104 Pa. The wafer was irradiated with a snakelike scanning in SF6 ambient by the fs laser with a fluence of approximately 0.75 J/cm2, leading to the formation of 2 × 2 cm2 black Si with a microstructural surface. Then, black Si was thermally treated at 825 K in a furnace with N2 ambient for 10 min. Thin Ag films were deposited on the microstructural surface by thermal evaporation at a rate of 0.5 Å/s. The surface morphology was characterized by a scanning electron microscope (SEM). The integrated reflectance (R) and transmittance (T) spectra between 500 and 2500 nm were measured in a spectrometer (PerkinElmer Lambda-1050) equipped with a 160-mm integrating sphere. The integrated absorptance (A) spectra were extracted through A = 1 − R − T.
It is known that thin-film kinetic growth is a random deposition process for evaporated materials. With film thickness increasing, continuous films will be gradually developed from discontinuous and island ultrathin films formed at the initial stage of deposition. Generally for Ag film deposition on a Si substrate, we have confirmed that continuous Ag films were formed when their thickness increases to 20 nm. However, in this study, random and irregular nanoparticles instead of continuous films are formed on the microstructure surface of black Si even though the thickness reaches 40 nm, which is shown in Fig. 3. This is mainly caused by the spikelike microstructure surface, resulting in an oblique deposition with a very large deposition angle. In this case, shadowing effect originates from oblique incident atoms being preferentially deposited on the hills of the surface [11–14], leading to the formation of random and irregular particles in Fig. 3b, c. That is also why Ag nanoparticles tend to gather on the tip positions of black Si surface in Fig. 3b, c. In addition, the nanostructures distributed on the spikelike microstructure surface in Fig. 3a further promote the shadowing effect. As a result, an amount of random and irregular Ag nanoparticles are formed even when the deposition thickness reaches 40 nm.
The proposed method in this study has two main advantages. First, thin Ag film deposition becomes an oblique deposition due to the microstructure surface of black Si, which induces that a large amount of Ag nanoparticles can be conveniently formed on black Si. Second, LSPR generated at Ag particles/Si interface is broadened by the random distribution, the irregular shape, and the different sizes of Ag particles, resulting in a broadband absorption enhancement. It is known that LSPR could enhance the photoelectric response by not only trapping photons [20–22] but also generating hot electrons . Thus, this study provides a promising way to boost the photoelectric response of black Si. However, metal nanoparticles may lead to some negative effects such as local heating, unintended light absorption, and electric field accumulation during biasing the device. Therefore, further studies are required for the contribution of Ag particles to the photoelectric response of black Si.
We have successfully remedied the degradation of NIR absorption of black Si by using thin Ag film deposition. The average absorptance in the whole NIR region from 1100 to 2500 nm was drastically increased from 60 to 80 % after 20-nm-thick Ag film deposition. Because the conical spike microstructure surface induces an oblique Ag deposition with a very large deposition angle, a large amount of random and irregular Ag nanoparticles with different sizes were formed on the microstructure surface due to the shadowing effect. It contributed to a broadband localized surface plasmon resonance, resulting in a broadband absorption enhancement in the whole NIR region from 1100 to 2500 nm.
The authors acknowledge the project supported by the National Natural Science Foundation of China (Nos. 61306125 and U1435210), the Science and Technology Innovation Project (Y3CX1SS143) of CIOMP, the Science and Technology Innovation Foundation of CAS (CXJJ-15Q071), and the Science and Technology Innovation Project of Jilin Province (Nos. Y3293UM130, 20130522147JH, and 20140101176JC).
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