1564
IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 11, NOVEMBER 2011
Low-Temperature, Ion Beam-Assisted SiC Thin Films With Antireflective ZnO Nanorod Arrays for High-Temperature Photodetection Wei-Cheng Lien, Dung-Sheng Tsai, Shu-Hsien Chiu, Debbie G. Senesky, Roya Maboudian, Albert P. Pisano, and Jr-Hau He
Abstract—This work demonstrates high-temperature operation of metal-semiconductor-metal photodetectors (MSM PDs) using low-temperature, ion beam-assisted deposition of nanocrystalline SiC thin films and hydrothermal synthesis of ZnO nanorod arrays (NRAs). Due to the incorporation of ZnO NRAs, the photo-to-dark current ratio of SiC MSM PDs is increased from 4.9 to 13.3 at 25 ◦ C and from 4.9 to 7.6 at 200 ◦ C. The enhancement in the sensitivity suggests that the ZnO NRAs could serve as an effective antireflective layer to guide more light into the SiC MSM PDs. This was confirmed through the characterization of reflectance measurements and finite-difference time-domain analysis. These results support the integration of nanocrystalline SiC thin films and ZnO NRAs for use in high-temperature photodetection applications. Index Terms—Nanowires, photodetectors (PDs), SiC, ZnO.
I. I NTRODUCTION
P
HOTODETECTORS (PDs) have drawn interest for use in combustion flame monitoring, pollution analyzers, and optical communication devices [1]. Among them, metalsemiconductor-metal (MSM) PDs offer high-speed operation and can be readily integrated with optoelectronic and microelectromechanical systems for signal detection [2]. Most operation environments require the PDs to work at high temperatures. However, conventional Si-based PDs, with narrow bandgap of 1.12 eV, are limited to low operation temperatures (below 125 ◦ C) due to generation of thermal carriers and significant shifts in the optical properties, leading to the deterioration in spectral response [3]. SiC, on the other hand, is a wide-bandgap material with excellent thermal and chemical stability as well as electrical, mechanical, and optical properties [4]. However, Manuscript received July 17, 2011; revised July 27, 2011; accepted August 2, 2011. Date of publication September 18, 2011; date of current version October 26, 2011. The research was supported by the National Science Council 99–2120-M-007–012, 99-2112-M-002–024-MY3 and 99-2622-E-002-019CC3. W.-C. Lien and D.-S. Tsai contributed equally to this work. The review of this letter was arranged by Editor P. K.-L. Yu. W.-C. Lien is with the Department of Electrical Engineering, & Institute of Photonics and Optoelectronics at National Taiwan University, Taipei 10617, Taiwan. He is also with Berkeley Sensor and Actuator Center, University of California, Berkeley, CA 94706 USA. D.-S. Tsai, S.-H. Chiu, and J. H. He are with Department of Electrical Engineering, & Institute of Photonics and Optoelectronics at National Taiwan University, Taipei 10617, Taiwan (e-mail:
[email protected]). D. G. Senesky, R. Maboudian, and A. P. Pisano are with the Berkeley Sensor and Actuator Center, University of California, Berkeley, CA 94706 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2164570
the high production costs of crystalline 4H- or 6H-SiC wafers and large thermal budget of chemical-vapor-deposited SiC thin films on Si substrates ranging from 800 ◦ C to 1600 ◦ C have hindered the development of SiC-based PDs and integration with CMOS processes [5]. In contrast, SiC thin films can be deposited with an ion beam-assisted method at low temperatures, ranging from room temperature to 430 ◦ C on Si substrates, offering the advantages of CMOS-compatible processing and reduced cost [6]. Moreover, there have been no reports on temperature-dependent characteristics for low-temperature, ion beam-assisted SiC PDs. Most PDs are fabricated with antireflection (AR) coatings to enhance the responsivity by reducing the surface reflection and allowing more light to reach the active region. A conventional single layer antireflection coating (ARC) works only in a limited spectral range for a specific angle of incidence (AOI), typically for a near normal incidence. Recently, it was reported that nanostructures exhibit superior broadband, omnidirectional AR characteristics [7]–[11]. Despite the numerous investigations on optical characterizations of AR nanostructures, more practical applications are required to demonstrate their feasibility. In this letter, we present MSM PDs employing nanocrystalline SiC films using ion beam-assisted methods and ZnO nanorod arrays (NRAs) using hydrothermal synthesis. SiC PDs with ZnO NRAs as an AR layer exhibit greatly enhanced responsivity, demonstrating a photodetection scheme with working temperatures as high as 200 ◦ C. We used reflectance measurements and finite-difference time-domain (FDTD) analysis to gain insight into light harvesting of ZnO NRAs. This study paves the way for photosensing applications in harsh, high-temperature conditions. II. E XPERIMENT Fig. 1 shows the schematic of the ZnO NRA/Au/SiC MSM PDs. A 600-nm-thick SiC thin film was first deposited by ion beam-assisted deposition on Si(100) substrates at 430 ◦ C and 3 × 10−6 Torr [6]. The MSM PDs were defined using photolithography with active areas of 500 × 158 µm2 and utilized 8-µm-wide, 150-µm-long interdigitated Au electrodes with 8-µm-wide spacing on the SiC/Si substrates. Au contacts were annealed at 600 ◦ C in N2 ambient. A 100-nm-thick ZnO seed layer was deposited onto the PDs using a RF magnetron sputtering with the power of 40 W at 5 × 10−3 Torr. ZnO NRAs were grown in an aqueous solution at 75 ◦ C [8].
0741-3106/$26.00 © 2011 IEEE
LIEN et al.: LOW-TEMPERATURE, ION BEAM-ASSISTED SIC THIN FILMS
Fig. 1. Cross-sectional schematic diagram of ZnO NRA/Au/SiC MSM PD. The ZnO NRAs are used as antireflection coatings and the nanocrystalline SiC semiconductor thin film is used to increase the operation temperature of the PD.
1565
Fig. 3. (a) I–V curves of SiC MSM PDs and ZnO NRA/ SiC MSM PDs measured in dark and under 532-nm illumination with the power density of 1.26 × 105 W/m2 at room temperature. (b) PDCR value as function of the temperature under a 10 V bias.
Fig. 2. (a) Cross-sectional SEM image of ZnO NRA/SiC layers. (b) Crosssectional high-resolution TEM image of SiC with the corresponding ED patterns. (c) The photoluminescence spectrum of the nanocrystalline SiC thin films.
III. R ESULTS AND D ISCUSSION Fig. 2(a) is a cross-sectional scanning electron microscopy (SEM) image of the ZnO NRAs on the SiC MSM PDs, showing that well-aligned ZnO NRAs with a length of approximately 1.25 µm and a diameter of approximately 50 nm were successfully grown on the SiC surfaces using the hydrothermal approach. The cross-sectional high-resolution transmission electron microscopy (TEM) image of SiC with its corresponding electron diffraction (ED) pattern is shown in Fig. 2(b). The weak ring of SiC ED pattern suggests that the SiC thin film is nanocrystalline with short-range order structures rather than completely amorphous phase [12]. The room-temperature photoluminescence measurement under 325-nm excitations further supports TEM and ED results as shown in Fig. 2(c). The two prominent peaks located at 455 nm (2.73 eV) and 545 nm (2.28 eV) correspond to the amorphous SiC phase [13] and the crystalline 3C-SiC phase, respectively [3]. The I–V curves of the SiC PDs in the dark and under 532-nm illumination at room temperature are shown in Fig. 3(a). It should be noted that the green light at 532 nm is not absorbed by wide-bandgap ZnO (3.3 eV). One can see that dark current values are low even at a high bias, which is desired for a satisfactory MSM PD. Clearly, SiC PDs possess a photosensing feature. By introducing ZnO NRAs as an AR layer guiding the photons efficiently into the SiC PDs to create more opportunities for photocarrier generation, the photocurrent can be enhanced. To quantitatively evaluate the temperature-dependent photosensing characteristics of the SiC PDs with ZnO NRAs, the sensitivity factor, taken as photo-to-dark current ratio (PDCR), is measured using the formula [14] PDCR = (Ip − Id )/Id
(1)
where Id is the dark current and Ip is the photocurrent under illumination. The response curve in Fig. 3(b) shows that the SiC PDs are capable of photosensing even at 200 ◦ C mainly due to the small levels of dark currents and high thermal stability of
Fig. 4. Reflectance spectra of (a) bare SiC MSM PDs and (b) ZnO NRA/SiC MSM PDs with a wide range of AOIs.
the SiC films at high temperatures. Generally, a further increase in temperature lowers the sensitivity factor of SiC PDs due to an increase in dark current at a higher temperature. This is due to generation of the thermal carriers at higher temperatures which cannot be completely eliminated. With increasing the operation temperature to 250 ◦ C, the PDCR value decreases to unity, indicating that the device no longer operates normally. One possible explanation is that the stress within the SiC increases due to the mismatch in the coefficients of thermal expansion between SiC and the underlying Si substrate as the temperature increases, causing the failure of the PDs [15]. Another explanation might be that the underneath Si substrate could provide extra leakage current paths which significantly increase the dark current at high temperatures making the PDCR decreases to unity. Further improvement of the crystallinity of SiC, stable metal contacts at high temperatures, or using the insulating substrates might be able to extend the working temperatures of SiC PDs. By introducing the ZnO NRA layers, the PDCR value of SiC PDs is increased from 4.9 to 13.3 at 25 ◦ C and from 4.9 to 7.6 at 200 ◦ C, demonstrating the feasibility of ZnO NRAs for light trapping at elevated temperatures. The reflection spectra of bare SiC MSM PDs without any ARC are shown in Fig. 4(a). The dip of reflection at 455 nm corresponds to the bandgap absorption of amorphous SiC. Fig. 4(b) confirms that the use of ZnO NRAs as an AR layer significantly reduces the reflection over a wide range of wavelengths. The reflectance of MSM PD with ZnO NRAs is decreased to below 2% in the UV region and 20% in the visible/NIR regions over a wide range of AOIs. The dip of reflection at 380 nm corresponds to the bandgap absorption of ZnO NRAs. To gain insight into light coupling into the SiC PDs, the steady-state distribution of electromagnetic fields was simulated by FDTD analysis based on Maxwell’s equations (Fig. 5), indicating the
1566
IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 11, NOVEMBER 2011
that the nanocrystalline SiC thin films incorporated with ZnO NRAs hold promise for the next-generation PDs for the operation in high-temperature conditions. R EFERENCES
Fig. 5. FDTD simulation of time-averaged and normalized TE electric field distribution at 532 nm (a) without and (b) with ZnO NRAs. The insets in (a) and (b) are the enlarged images at the top SiC surface.
time-averaged TE-polarized electric filed intensity distribution with PDs at 532 nm. The insets in Fig. 5(a) and (b) show that the ZnO NRAs guide propagating light efficiently across the interfaces by reducing the refractive index mismatch between air and SiC, and also widening the field distribution within the PDs by increasing the light scattering on the SiC surface. IV. C ONCLUSION In summary, the MSM PDs employing nanocrystalline SiC thin films from ion beam-assisted deposition were successfully fabricated and characterized with working temperatures up to 200 ◦ C. The use of ZnO NRAs as AR layers synthesized by a hydrothermal method increases the PDCR value of SiC MSM PDs from 4.9 to 13.3 at 25 ◦ C and from 4.9 to 7.6 at 200 ◦ C, which is also supported by reflectance measurements and FDTD simulations. Suitable vacuum seal technique or packaging engineering will be required to avoid the ambient humidity and the decomposition of ZnO NRAs. This work demonstrates
[1] E. Monroy, F. Omnes, and F. Calle, “Wide-bandgap semiconductor ultraviolet photodetectors,” Semicond. Sci. Technol., vol. 18, no. 4, pp. R33– R51, Apr. 2003. [2] S. M. Sze and K. K. Ng, Physics of Semiconductor Devices. Hoboken, NJ: Wiley, 2007, pp. 712–716. [3] A. Vijayakumar, R. M. Todi, and K. B. Sundaram, “Amorphous-SiCBNbased metal semiconductor metal photodetector for high-temperature applications,” IEEE Electron Device Lett., vol. 28, no. 8, pp. 713–715, Aug. 2007. [4] S. E. Saddow and A. Agarwal, Advances in Silicon Carbide Processing and Applications. Norwood, MA: Artech House, 2004, pp. 2–11. [5] W.-C. Lien, N. Ferralis, C. Carraro, and R. Maboudian, “Growth of epitaxial 3C-SiC films on Si(100) via low temperature SiC buffer layer,” Cryst. Growth Des., vol. 10, pp. 36–39, 2010. [6] D. G. Jones, R. G. Azevedo, M. W. Chan, A. P. Pisano, and M. B. J. Wijesundara, “Low temperature ion beam sputter deposition of amorphous silicon carbide for vacuum encapsulation,” in Proc. IEEE Int. Conf. Micro Electro Mech. Syst., Kobe, Japan, 2007, pp. 275–278. [7] J. Y. Huang, X. D. Wang, and Z. L. Wang, “Bio-inspired fabrication of antireflection nanostructures by replicating fly eyes,” Nanotechnology, vol. 19, no. 2, p. 025 602, Jan. 2008. [8] Y.-C. Chao, C.-Y. Chen, C.-A. Lin, Y.-A. Dai, and J.-H. He, “Antireflection effect of ZnO nanorod arrays,” J. Mater. Chem., vol. 20, pp. 8134– 8138, 2010. [9] Y. A. Dai, H. J. Chang, K. Y. Lai, C. A. Lin, R. J. Chung, G. R. Lin, and J. H. He, “Subwavelength Si nanowire arrays for self-cleaning antireflection coatings,” J. Mater. Chem., vol. 20, pp. 10 924–10 930, 2010. [10] Y. L. Chueh, Z. Fan, K. Takei, H. Ko, R. Kapadia, A. A. Rathore, N. Miller, K. Yu, M. Wu, E. E. Haller, and A. Javey, “Black Ge based on crystalline/amorphous core/shell nanoneedle arrays,” Nano Lett., vol. 10, no. 2, pp. 520–523, Feb. 2010. [11] G. J. Lin, K. Y. Lai, C. A. Lin, Y.-L. Lai, and J. H. He, “Efficiency enhancement of InGaN-based multiple quantum well solar cells employing antireflection ZnO nanorod arrays,” IEEE Electron Device Lett., vol. 32, no. 8, pp. 1104–1106, Aug. 2011. [12] S. L. Cheng, H. H. Lin, J. H. He, T. F. Chiang, C. H. Yu, and L. J. Chen, “Evolution of structural order in germanium ion-implanted amorphous silicon layers,” J. Appl. Phys., vol. 92, no. 2, pp. 910–913, Jul. 2002. [13] J. Xu, L. Yang, Y. Rui, J. Mei, X. Zhang, W. Li, Z. Ma, L. Xu, X. Huang, and K. Chen, “Photoluminescence characteristics from amorphous SiC thin films with various structures deposited at low temperature,” Solid State Commun., vol. 133, no. 9, pp. 565–568, Mar. 2005. [14] W.-R. Chang, Y.-K. Fang, S.-F. Ting, Y.-S. Tsair, C.-N. Chang, C.-Y. Lin, and S.-F. Chen, “The hetero-epitaxial SiCN/Si MSM photodetector for high-temperature deep-UV detecting applications,” IEEE Electron Device Lett., vol. 24, no. 9, pp. 565–567, Sep. 2003. [15] Y. Z. Chiou, “Failure mechanisms of GaN metal-semiconductor-metal photodetectors after stressing,” IEEE Trans. Device Mater. Rel., vol. 10, no. 1, pp. 82–86, Mar. 2010.
