Enhanced photo-sensitivity through an increased light-trapping on Si by surface nano-structuring using MWCNT etch mask
© Hwang et al; licensee Springer. 2011
Received: 1 August 2011
Accepted: 31 October 2011
Published: 31 October 2011
We demonstrate an enhanced photo-sensitivity (PS) through an increased light-trapping using surface nano-structuring technique by inductively coupled plasma (ICP) etching on multi-walled carbon nanotube (MWCNT) etch masked Si with hexamethyl-disilazane (HMDS) dispersion. In order for a systematic comparison, four samples are prepared, respectively, by conventional photolithography and ICP etching using MWCNT as a etch mask. MWCNT-etched Si with HMDS dispersion shows the highest RMS roughness and the lowest reflectance of the four. Two test device structures are fabricated with active regions of bare-Si as a reference and MWCNT etch masked Si with HMDS dispersion. The increased light-trapping was most significant at mid-UV, somewhat less at visible and less noticeable at infrared. With an ICP-etched Si using CNT HMDS dispersion, PS is very sharply increased. This result can lead to applications in optoelectronics where the enhancement in light-trapping is important.
Light-trapping, in other word optical absorption, in such device applications as photovoltaics, light-emitting diodes, light sensors, photo-diodes, and transistors, plays an important role in their device functionality and in order to suppress reflection losses and increase conversion efficiency [1–10]. In general, approximately 30-40% of photons are reflected when incident on planar wafers. Theoretically, through an ideal light-trapping the length of optical path in a material with a refractive index of n can be enhanced by a factor of 4n 2, and thus the amount of photons that can be absorbed in a material can be significantly enhanced. Various light-trapping techniques therefore have been explored and developed to restrain the reflection losses and enhance optical absorption in various applications. For example, in photovoltaic applications a thin film known as an antireflection coating can be adapted, which has a refractive index that is intermediate between those of semiconductors (n s) and air (n 0) . TiO2 (n = 2.3), Ta2O5 (n = 2.25), Si3N4 (n = 2.0), Al2O3 (n = 1.85), SiO2 (n = 1.5), and MgF2 (n = 1.38) are widely known materials that can be used for antireflection coatings . On the other hand, surface patterning or texturing instead of planar substrates have been widely investigated and adapted in order to handle light-trapping in a more efficient way [12–14]. Patterns that can be used in surface texturing can be either regular or random. A regularly textured surface can be yield with various types of patterns [3, 5, 6] using conventional photolithography. Randomly textured surfaces have been demonstrated [4, 7–10] using SnO2, ZnO, Ag, glass, and plastics, which showed an improved spectral response in longer wavelengths. Meanwhile, CNTs have been widely proposed in composite materials to reinforce the mechanical strength and catalytic activities [15–21], which varies from metals, metal oxides, and ceramic composites to polymers. CNTs can also be used in various applications such as an emitter for the field effect displays, micro-supercapacitors, color fine-tuning, electrochemical sensors and hydrogen storage, etc. [22–29]. On the other hand, due to their superior mechanical strength, CNTs can be used as a etch mask in plasma dry-etching process . While etch masks through conventional photolithography process can generate micron-scale patterns, CNT etch mask technique can provide nanoscale surface patterns as observed by microscopy [31–33].
In this letter, we demonstrate an enhanced photo-sensitivity (PS) through an increased light-trapping on Si, which is achieved by increasing surface roughness and suppressing reflection losses through a surface nano-texturing using inductively coupled plasma (ICP) etching. Four Si samples are prepared using conventional photolithography and ICP etching using multi-walled carbon nanotubes (MWCNT). MWCNTs dispersed in hexamethyl-disilazane (HMDS) were used as an etch mask for the ICP etching. The highest RMS roughness with a value of 7.66 nm and thus the lowest reflectance by 41.5% at mid-UV are achieved with the ICP-etched Si using MWCNT etch mask. The increased light-trapping is most significant at mid-UV, less at visible, and finally somewhat insignificant at infrared region. Based on the RMS roughness and reflectance analyses, two I-V test device structures are fabricated. Device active region using bare-Si is set as a reference while ICP-etched Si using MWCNT etch mask is fabricated for a PS comparison. With an ICP-etched Si with HMDS dispersion, PS at UV illumination is very sharply increased through back-to-back Schottky-barriers.
In this experiment, in order to perform surface morphology, RMS roughness, and reflectance analyses, four Si samples were prepared: bare-Si (sample A), square trench-patterned Si using conventional wet chemical etching (sample B), ICP-etched Si using MWCNT-dispersion in isopropyl-alcohol (sample C), and another ICP-etched Si using MWCNT-dispersion in HMDS (sample D). For the preparation of sample A, a conventional cleaning procedure using acetone and methanol was performed. For sample B, a pattern area of 3 × 3 μm2 was fabricated using conventional photolithography and wet chemical etching; namely chemical cleaning using H2SO4:H2O2 = 1:1 and HF, photo-resist (PR) spin coating, baking, UV exposure and development, etc. After the patterning, 50 nm of Si was removed using HNO3:HF:DI = 100:3:40. For the preparation of ICP-etched Si, MWCNTs (MWCNTs) with a diameter of 10-15 nm were used (Hanhwa nanotech Co., Korea), which were grown using a chemical vapor deposition. For the preparation of one ICP-etched Si sample (sample C), initially approximately 13.5 mg of MWCNTs was dispersed in approximately 200 mL of isopropyl-alcohol. The mixed solution was then dropped on a Si surface and the sample was dried in air. Subsequently, the sample was heated on a hot-plate at 120°C for 2 min to fix the MWCNTs. For the preparation of the other ICP-etched Si (sample D), MWCNTs were dispersed in HMDS for an improved dispersion. For the samples C and D, 50 nm of Si was subsequently etched away using ICP dry-etching with an ambient gas mixture of SF6:O2 (20:4%) at a chamber pressure of 30 mTorr and RF power of 30 W. Followed by the dry-etching, samples were cleaned in a boiled acetone at 120°C for 5 min and in a methanol and finally rinsed in DI water (boiling temperature of acetone is 56°C). Based on the analyses, two test device structures were consequently fabricated using bare-Si and an ICP-etched Si using MWCNT dispersed in HMDS for a comparison of PS. An silicon on insulator (SOI) wafer was used for the device fabrication, which included 350 μm of p-type Si with a doping of 1017 cm-3, 100 nm of SiO2, and another 100 nm thick p-type Si at the top with a doping of 1017 cm-3. Both test structures were exactly the same except the active regions: bare-Si for one and ICP-etched Si for the other. For the contacts, Schottky-barrier of Cr/Ag (50/50 nm) was fabricated on the top layer using conventional photolithography and an e-beam evaporation at approximately 3 × 10-5 Torr at approximately 150°C. For the measurements of surface morphology, an atomic force microscope (AFM, N8 ARGOS, Bruker AXS Inc.) was used in air with a non-contact mode. Reflectance was measured using Avaspec-3648 and halogen and deuterium lamps were used for light sources. For the I-V characterization, Keithley Semiconductor Characterization System (SCS-4200) was used under dark and at illumination (approximately 200 nm) with a power density of approximately 137 mW/cm2.
Results and discussion
In conclusion, to test light-trapping of Si, four samples were prepared: bare-Si, square masked-pattern, MWCNT etch-masked Si with isopropyl-alcohol dispersion, and MWCNT etch-masked Si with HMDS dispersion. The MWCNT etch-masked Si with HMDS dispersion (sample D) showed the height RMS roughness and lowest reflectance as compared to the other three tested Si samples. The reflectance was most significant at mid-UV region and less significant at infrared. Based on the RMS roughness and reflectance experiments, two device structures were fabricated with active regions of bare-Si and CNT etch-masked Si and tested under dark and at UV illumination. While both devices showed a similar behavior indicating increased current at UV illumination, the PS indicating the ratio of the current change at UV-illumination over the current under dark much sharply increased with the test device of CNT etch-masked Si. The increased photo response can be due to the increased surface roughness and decreased reflectance and thus the increased light-trapping. This result can find applications in such devices as photovoltaics, light-emitting diodes, photo-diodes, and photo-transistors, where the light-tramping is important.
atomic force microscope
inductively coupled plasma
multi-walled carbon nanotube
This study was supported by the National Research Foundation of Korea Grant funded in part by the Korean Government (2010-0015360), (2011-0004804), and (2011-0030821). This research has been conducted in part by the research grant of Kwangwoon University in 2011. This research was supported in part by the MKE (The Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency) (NIPA-2011-C1090-1111-0002).
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