Anti-reflective nano- and micro-structures on 4H-SiC for photodiodes
© Kang et al; licensee Springer. 2011
Received: 10 October 2010
Accepted: 18 March 2011
Published: 18 March 2011
In this study, nano-scale honeycomb-shaped structures with anti-reflection properties were successfully formed on SiC. The surface of 4H-SiC wafer after a conventional photolithography process was etched by inductively coupled plasma. We demonstrate that the reflection characteristic of the fabricated photodiodes has significantly reduced by 55% compared with the reference devices. As a result, the optical response I illumination/I dark of the 4H-SiC photodiodes were enhanced up to 178%, which can be ascribed primarily to the improved light trapping in the proposed nano-scale texturing.
Up to now, silicon (Si) has been the dominant material for high-efficiency solar cells. However, Si-based devices perform well only under the limited conditions of relatively low temperatures and power ranges. Alternatively, in the research on wide-bandgap semiconductors, silicon carbide (SiC) has shown considerable potential for both high-power and optoelectronic devices . SiC exhibits a wide-bandgap (3.26 eV) and superior thermal properties, which are advantageous for high-temperature applications and solar energy conversion . However, polished SiC surfaces have a natural reflectivity with a strong spectral dependence. The reflectivity is inevitably high (20-40%), due to the high refractive index of n = 2.7-3.5 of SiC . The optical losses associated with the reflectance of incident radiation are among the most important factors limiting the efficiency of a solar cell . Therefore, photovoltaic cells normally require special surface structures or materials, which can reduce reflectance. A common solution is utilization of antireflection coatings based on interference, such as transparent layers of SiO2 and Al2O3 . However, such coatings worked only in a limited spectral range, and more efficient reflection reduction in a broad spectral range has been achieved by surface texturing, which can normally be accomplished by wet or dry etching. In principle, the wet etching of SiC can be done only with molten KOH at over 500°C, which is not a practical method. For that reason, dry etching with fluorine species, such as SF6, and CF4, is considered as the desirable method to form the textured surface of SiC .
In this article, we report a method for forming nano-scale-textured structures on 4H-SiC surfaces so as to reduce the surface reflectance of SiC. An inductively coupled plasma (ICP) etching was employed to form the structures, and the performance of the SiC photodiode cells was compared to that of reference cells without surface nano-scale texturing.
Results and discussion
where p represents the probability that, depending on the location on the rough surface, the incident photon is either absorbed with probability factor a, or reflected with a probability factor of r = 1 - a. As the surface roughness increases, the reflectance decreases, since more photons are absorbed. Similarly, as the RMS values of the nano-honeycomb structures increases, the reflectance spectral dependence decreases because of the textured surface effect on the light trapping. It can be seen from the values of reflectance for 4H-SiC with different texturing structures that the nano-honeycomb structures exhibit clearly improved anti-reflective properties.
Comparison of the Schottky-type ultraviolet photodiode properties for different structures
I dark (A)
I illumination (A)
1.37 × 10-11
5.55 × 10-8
1.41 × 10-11
6.32 × 10-8
1.94 × 10-11
2.18 × 10-7
In summary, we proposed a method for fabricating nano-scale-textured structures on 4H-SiC surfaces to reduce reflection. After a conventional photolithography process to form the nano-honeycomb structures, the surface of 4H-SiC wafer was etched by ICP using a SF6 + O2 gas mixture. We demonstrated that the reflectance of the nano-honeycomb structures has significantly reduced by 55% compared with the reference cell. The reflectance was reduced because the roughness of the surface was increased. As a result, an optical response (I illumination/I dark) was increased by 178% for the nano-honeycomb structures, and an improved photocurrent was obtained from the subsequently fabricated 4H-SiC photo-diodes. The textured surface resulted in the reduction in reflectivity, which indicated that the amount of absorbed light increased because of efficient light trapping. It has been shown that the nano-honeycomb structures have proven as effective anti-reflective surface structures, which may open opportunities for the design of efficient photovoltaic cells on 4H-SiC.
This study was supported by the "System IC2010" project and "Survey of high efficiency power devices and inverter system for power grid" project of Korea Ministry of Knowledge Economy, by the National Research Foundation of Korea Grant funded by the Korean Government 2010-0011022, and by a Research Grant from Kwangwoon University in 2011.
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