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p-Si core–shell nanowire photodiode based on well-ordered Si nanowire array with smooth surface
Materials Science in Semiconductor Processing 27 (2014) 297–302
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High efficiency n-ZnO/p-Si core–shell nanowire photodiode based on well-ordered Si nanowire array with smooth surface Kyung Yong Ko a, Hyemin Kang a, Jungkil Kim b, Woo Lee b, Hee Sung Lee c, Seongil Im c, Ji Yeon Kang d, Jae-Min Myoung d, Han-Gil Kim e, Soo-Hyun Kim e, Hyungjun Kim a,n a
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon, Republic of Korea c Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea d Department of Materials Science and Engineering, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea e School of Materials Science and Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si 712-749, Republic of Korea b
a r t i c l e i n f o
Keywords: Photodiode ZnO Anodic aluminum oxide Core–shell nanowire Atomic layer deposition
abstract A highly efficient n-ZnO/p-Si core–shell nanowire (NW) photodiode was fabricated using ZnO grown by atomic layer deposition (ALD) on a well-ordered Si NW array. Si NW arrays were prepared by metal-assisted chemical etching, for which a metal mesh with a wellorganized nanohole array was made using anodic aluminum oxide. This resulted in a good arrangement, smooth surface, and small diameter distribution of the Si NW array. Consequently, ZnO layers with various thicknesses from 15 to 30 nm were deposited by the ALD method. Because of the smooth surface of the well-ordered Si NWs yielding low surface roughness scattering, the resulting photodiode showed significantly improved device characteristics. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Among many types of ZnO-based optoelectronic devices, ZnO/Si heterojunction photodiodes have attracted great attention because of their broadband light absorption and easy fabrication [1]. However, conventional planar structure devices have limitations in terms of effective light capturing and electron–hole pair separation, resulting in low efficiency [2,3]. In regard of these aspects, core–shell nanowire structures can strengthen the weak points of the planar structure by reducing the reflection of injected light and improving efficiency based on positional carrier separation [4,5].
n
Corresponding author. E-mail address: [email protected] (H. Kim).
http://dx.doi.org/10.1016/j.mssp.2014.07.012 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
Atomic layer deposition (ALD) proceeds through only surface chemical reactions and produces films with good uniformity and conformality on complex nano-structures [6]. Recently, the fabrication of high-efficiency n-ZnO:N/p-Si NW core–shell photodiodes has been reported using ALD for the ZnO shell on the Si NW array [7,8]. In these reports, synthesis of the Si NW array was carried out by electroless etching. However, because of etching with a random Ag particle, the Si NWs were found to have a large diameter distribution (50–200 nm) and very high surface roughness (1–5 nm) [9,10], which inevitably resulted in poor electrical properties because of surface roughness (SR) scattering [11]. Thus, improvement of the inadequate quality of the Si NW array produced by electroless etching is required to reduce carrier loss. A previous report showed that the roughness of the surface of Si NWs formed by electroless etching can be reduced by repeated processes of rapid thermal oxidation
298
K.Y. Ko et al. / Materials Science in Semiconductor Processing 27 (2014) 297–302
(RTO) and hydrogen fluoride (HF) etching [10]. However, this method is time consuming and decreases the amount of Si NW. Recently, we reported that a well-aligned Si NW array with average diameter of 50 nm can be fabricated with an anodic aluminum oxide (AAO) membrane as a template for a metal mesh used for the metal-assisted chemical etching [12]. For optoelectronic devices based on thin films, the influence of a parameter such as film thickness on the electrical and optical properties is important; however, to date, there have been few reports on the effect on electrical and optical properties caused by the various shell thickness of the core–shell photodiode [13]. In this study, we fabricated well-aligned n-ZnO/p-Si NW photodiodes using ALD grown ZnO shell layer of various thickness and a well-ordered Si NW array with a smooth surface. The electrical properties of the n-ZnO/p-Si NW core–shell photodiodes have been described. Also, the optical properties of the ZnO/Si NW core–shell devices were characterized by spectral responsivity and photoluminescence.
2. Experimental procedures The detailed process for the fabrication of the metal mesh assisted Si NW array (denoted as SiNW_P) has been described in a previous report [12]. Fig. 1 shows the
schematic diagram of the n-ZnO/p-SiNW_P fabrication process. In brief, Au (5 nm) and Ag (15 nm) were sequentially sputtered onto an AAO membrane with a wellordered nanohole array and then the Au/Ag coated AAO membrane was floated on a NaOH etching solution to release the Au/Ag metal mesh from the AAO. After transferring the Au/Ag bi-layered metal mesh onto the Si(100) (hole concentration: 1015 cm 3, resistivity: 7–15 Ω cm) substrate, the metal mesh covered Si(100) was etched in HF solution, resulting in a well-ordered Si NW array. The length of the NWs was determined by etching time, while the diameter of the NWs was measured from the nanoholes with small distribution in the AAO template. The remaining Au/Ag bilayer was cleaned in a HNO3 solution. For comparative study, a Si NW array without metal mesh was also prepared (denoted as SiNW_N). Then, ZnO ALD was carried out at 150 1C in a lab-made cold wall type ALD system. The ALD growth cycle was composed of diethyl zinc (DEZ) dosing (1 s) – Ar purging (10 s) – H2O exposure (1 s) – Ar purging (10 s). Under this ALD condition, the growth rate of ZnO was 1.9 Å/cycle. Thus, 80, 120, and 160 cycles of ZnO ALD produced ZnO layers of 15, 22, and 30 nm thickness, respectively. For fabrication of photodiodes, an indium tin oxide (ITO) top electrode was sputtered on all of the n-ZnO/p-Si NW samples using a patterned shadow mask. A Cu plate attached to the bottom of the Si substrate by silver paste was used as the bottom contact.
Fig. 1. Schematic fabrication process of n-ZnO/p-Si NW photodiode: (a) sputtering of Ag, Au sequentially on AAO, (b) transfer the Au/Ag metal mesh onto Si (100), (c) Ag catalyzed metal-assisted chemical etching, (d) well-aligned p-Si NWs, (e) ZnO coating on p-Si NWs through the ALD process, and (f) contact formed n-ZnO/p-Si NWs photodiode.
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Fig. 2. Tilted SEM images of (a) ZnO(15 nm)/SiNW_P, (b) ZnO(22 nm)/SiNW_P, (c) ZnO(30 nm)/SiNW_P, (d) ZnO(30 nm)/SiNW_N, and the top of (e) ZnO (30 nm)/SiNW_P, and (f) ZnO(30 nm)/SiNW_N.
The morphology of the core-shell NWs was studied with field-emission scanning electron microscopy (FESEM) (JSM-6701F). In order to characterize the interface of the ZnO/Si core–shell NW, high resolution transmission electron microscopy (HRTEM) (Tecnai F20, electron beam 200 keV) was used. The distribution of elements in ZnO/Si NW was observed by energy-dispersive X-ray spectroscopy (EDX). The current density of the devices was measured using a probe station (Keithley 306 electrometer). Spectral responsivities of the photodiodes were measured in the wavelength range between 320 and 660 nm using a 500 W Hg(Xe)-arc lamp with a monochromator as a light source (Oriel Optical System). Photoluminescence (PL) of the core–shell structures was measured at room temperature using a 500 μm diameter beam of the He–Cd laser.
3. Results and discussion Fig. 2 shows SEM images of the core–shell n-ZnO/p-Si NW structures. With increasing thickness of the ALD grown ZnO (Fig. 2(a)–(c)), it is apparent that the packing density of the n-ZnO/p-SiNW_P core–shell structures also increases. The increase in packing density is due to the increase in ZnO shell thickness, not the increase in Si NW areal density as all of the Si NW arrays were prepared under the same condition. Normally, the Si NWs, and consequently the fabricated core–shell structures, tend to collapse because of the high aspect ratio of the NWs and the surface tension force exerted on the nanowires during the evaporation of liquid [14]. However, it is interesting to note that for SiNW_P with 30 nm thick ZnO shell, the core–shell NWs appeared to be periodic with an apparent
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Fig. 3. (a) TEM image of the n-ZnO/p-Si NW_P; (b) and (c) HRTEM images of the n-ZnO/p-SiNW_P; (d) HRTEM image of the ZnO shell and SAED pattern of polycrystalline ZnO and single crystal Si; (e) HAADF image of the n-ZnO/p-SiNW_P; (f)–(i) EDX elemental maps of Si, Zn and O in the HAADF image and (j) spatial distributions of the elements across the n-ZnO/p-Si core–shell NW after analysis of the nanoprobe EDX line-scan.
Fig. 4. I–V curves of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO(30 nm)/pSiNW_N (inset: current density versus cross-sectional area of ZnO shell).
close packing structure as shown in Fig. 2(c) and (e). This is because the distance between neighboring Si NWs, determined from the geometry of the nanohole array of the AAO template, was 60 nm, which is almost exactly twice the shell thickness [12]. This also confirms the good diameter uniformity of the Si devices and excellent ordering of the structure. Meanwhile, this kind of closed packed structure cannot be observed for the n-ZnO(30 nm)/pSiNW_N assembly (see Fig. 2(d) and (f)) because of the poor diameter uniformity and bad ordering of the NWs. The TEM image of the conformal n-ZnO/p-Si core–shell NW in Fig. 3(a) shows that the Si NWs prepared by a metal mesh with ordered holes have a smooth surface. A further conformality analysis of ZnO on SiNW_P (Fig. 3(a)) was confirmed in the supplementary information Fig. S1. Fig. 3(b) is the TEM image of the n-ZnO/p-SiNW_P interface which depicts conformally deposited ZnO on Si NW. We can observe in Fig. 3(c) that the ZnO shell (15 nm) is uniformly deposited on SiNW_P. As determined from the HRTEM and the selected area electron diffraction (SAED)
Fig. 5. Spectral responsivity of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO (30 nm)/p-SiNW_N from 660 nm to 320 nm.
pattern, in Fig. 3(d), the ZnO shell is of a polycrystalline structure and the core is composed of single crystalline Si. The high-angle annular dark-field (HAADF) image of n-ZnO/p-Si NW is shown in Fig. 3(e). As revealed in Fig. 3 (f)–(i), the EDX elemental mapping result confirms that silicon, zinc and oxygen exist in the core-shell nanowire. The spatial distributions of the elements across the n-ZnO/ p-SiNW_P (shown in Fig. 3(j)) were obtained by analyzing the nanoprobe EDX line-scan taken at the position marked by the solid line in Fig. 3(e). As shown in Fig. S2, the comparison of roughness difference between two types of core–shell NW was confirmed by the TEM images. According to the standard deviation data of core-shell NW surface roughness, the surface of the n-ZnO/pSiNW_N was rougher than that of the n-ZnO/p-SiNW_P (see Supplementary information for details). Fig. 4 represents the I–V characteristics of the photodiodes. For n-ZnO/p-SiNW_P devices, the current density increases with increasing ZnO shell thickness for both
K.Y. Ko et al. / Materials Science in Semiconductor Processing 27 (2014) 297–302
forward and reverse bias. Since the current flows along the vertically aligned core–shell NW, the increment of current density with increasing shell thickness is mainly attributed to the increased cross-sectional area of the ZnO shell. The inset of Fig. 4 presents the current density versus the cross-sectional area of the ZnO shells, showing linear increase in current density with the ZnO cross-sectional area at 10 V of forward bias. A similar increasing tendency is seen from 2 V to 10 V which has been detailed in the supplementary information (Fig. S3). On the other hand, the n-ZnO(30 nm)/p-SiNW_N photodiode has the lowest current density for both forward (1.1 mA/cm2: 10 V) and reverse bias. This significantly low current density can be attributed to the high surface roughness of SiNW_N, as mentioned previously [9]. Subsequently, the spectral responsivities of the devices were measured. As shown in Fig. 5, all samples display a similar trend for responsivity as a function of wavelength in the range of 320 and 660 nm [1,8]. With decreasing wavelength, the responsivity decreases because of the increasing light absorption coefficient of bulk Si, resulting in minimum responsivity at 400 nm [15]. Below 400 nm, the responsivity increases again because of the effective generation of photo carriers in the ZnO shell. Among these devices, the n-ZnO(30 nm)/p-SiNW_P photodiode has the highest responsivity in both visible and UV regions and has a maximum value ( 1.8 A/W) at 660 nm. The overall responsivity of the 15 and 22 nm thick ZnO shells is smaller than that of the 30 nm thick ZnO shell. The effect on optical properties of various shell thickness of the coreshell photodiode has been rarely reported, despite the fact that the influence of a parameter such as film thickness on the optical properties is extremely important on thin film based optoelectronic devices. In a previous report, the thickness of ZnO film affects to the optoelectronic properties that the bandgap is gradually decreased with increasing film thickness from 45 nm to 225 nm due to the decreasing film stress. Also, the increased thickness of ZnO film can lead to higher concentration of defects and thus to decreased resistivity [13]. However, under UV irradiation, photo-generated carrier can extinct in thick ZnO while transport to electrode from ZnO because of short penetration depth ( 40 nm) of UV light [1]. n-ZnO (30 nm)/p-SiNW_N has the lowest responsivity for overall wavelength compared to the other n-ZnO/p-SiNW_P photodiodes. As mentioned above, the SiNW_N has a very rough surface. Thus, the photo-generated carriers in the device suffer from high surface scattering resulting in poor responsivity [9]. To further understand the effect of shell thickness on responsivity, photoluminescence (PL) measurements were carried out at room temperature. PL analysis on core–shell nanowire heterojunctions as a function of shell thickness has seldom been reported, whereas the PL of ZnO nanowires has been intensively investigated [16–18]. As shown in Fig. 6, the n-ZnO/p-SiNW_P device structure shows a strong near band edge (NBE) peak at 380 nm [19], without any observable deep-level defects related to the PL intensity in the visible range. Also, no shift in peak position, such as the blue-shift, was observed when the diameter of the studied core-shell NW was more than 80 nm. In a
301
Fig. 6. PL intensity of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO(30 nm)/pSiNW_N from 320 nm to 660 nm.
previous report, the blue-shift in the NBE PL peak was observed for ZnO nanorods with diameter smaller than 40 nm [20]. In any case, it should be noted that the NBE peak intensity significantly decreases with increasing ZnO shell thickness. We attribute this decrease in PL intensity with increasing shell thickness to the positional carrier separation. In a previous report on the CdTe/ZnO core–shell device structure, the intensity of the NBE PL peak arising from the ZnO nanorods as well as the NBE PL peak of CdTe itself was almost completely suppressed after deposition of the 88 nm thick CdTe shell. This “quenching” of the NBE PL intensity was attributed to the reduction of electron–hole recombination caused by the separation of photogenerated carriers. The band alignment of ZnO/Si is similar to that of CdTe/ZnO [1,21]. Thus, we surmise that the same argument can be applied to the current PL results. Especially, the thickness of all n-ZnO/p-SiNW_P devices in this study was quite thin as compared to the CdTe/ZnO structure mentioned above. In this range of small thickness, an incomplete electron–hole recombination can produce observable PL intensity. In Fig. 6, the PL spectrum for n-ZnO(30 nm)/p-SiNW_N is shown. It is interesting that the NBE peak intensity is almost the same as that for n-ZnO(30 nm)/p-SiNW_P. Since the difference between n-ZnO(30 nm)/p-SiNW_N and n-ZnO(30 nm)/p-SiNW_P is the Si NW surface roughness, we can expect the carrier separation to not be much different in these two materials. Because of the carrier separation, the radiative recombination rate should also be similar, agreeing with the comparable PL intensity. Even with this charge separation, the responsivity of n-ZnO(30 nm)/p-SiNW_N is significantly lower than that of n-ZnO(30 nm)/p-SiNW_P, displaying the importance of surface roughness in terms of improvement of efficiency for the photodiode. 4. Conclusions An ordered Si NW array with smooth surface and small diameter distribution was prepared by metal-mesh assisted chemical etching. Then, ALD grown ZnO/Si NW
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photodiodes with various shell thickness (15 nm to 30 nm) were fabricated and the device properties characterized. The conformally deposited polycrystalline ZnO shell on Si NWs was confirmed by HRTEM, SAED pattern and EDX elemental mapping. The I–V characteristics showed proportional increase of current density with increase in shell thickness. Also, it was observed that with increasing ZnO thickness between 15 nm and 30 nm, the spectral responsivity steadily increased 2–3 times. This significantly improved device performance for an ordered device with a smooth interface as a result of the reduced surface scattering of the photo-generated carriers and effective carrier separation. Acknowledgments This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2013K000173). In addition, this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0028594) Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. mssp.2014.07.012.
References [1] I.-S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [2] C.-P. Chen, P.-H. Lin, Y.-J. Hung, S.-S. Hsu, L.-Y. Chen, Y.-W. Cheng, M.-Y. Ke, Y.-Y. Huang, C.-H. Chang, C.-H. Chiu, H.-C. Kuo, J. Huang, Nanotechnology 21 (2010) 215201.
[3] K. Wang, J.J. Chen, Z.M. Zeng, J. Tarr, W.L. Zhou, Y. Zhang, Y.F. Yan, C.S. Jiang, J. Pern, a. Mascarenhas, Appl. Phys. Lett. 96 (2010) 123105. [4] Y. Zhang, L.-W. Wang, A. Mascarenhas, Nano Lett. 7 (2007) 1264. [5] A. Nduwimana, R.N. Musin, A.M. Smith, X.-Q. Wang, Nano Lett. 8 (2008) 3341. [6] H. Kim, H.-B.-R. Lee, W.-J. Maeng, Thin Solid Films 517 (2009) 2563. [7] H.-D. Um, S.A. Moiz, K.-T. Park, J.-Y. Jung, S.-W. Jee, C.H. Ahn, D.C. Kim, H.K. Cho, D.-W. Kim, J.-H. Lee, Appl. Phys. Lett. 98 (2011) 033102. [8] H. Kang, J. Park, T. Choi, H. Jung, K.H. Lee, S. Im, H. Kim, Appl. Phys. Lett. 100 (2012) 041117. [9] A.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature 451 (2008) 163. [10] S. Lee, J. Koo, J. Seo, S. Kim, K. Lee, J. Nanopart., 2012. [11] C. Buran, M. Pala, IEEE Trans. Electron Devices 56 (2009) 2186. [12] J. Kim, H. Han, Y.H. Kim, S.-H. Choi, J.-C. Kim, W. Lee, ACS Nano 5 (2011) 3222. [13] T. Singh, T. Lehnen, T. Leuning, D. Sahu, S. Mathur, Appl. Surf. Sci. 289 (2014) 27. [14] Y. Liang, C. Zhen, D. Zou, D. Xu, J. Am. Chem. Soc. 126 (2004) 16338. [15] K. Rajkanan, R. Singh, J. Shewchun, Solid State Electron. 22 (1979) 793. [16] L.H. Quang, S.J. Chua, K. Ping Loh, E. Fitzgerald, J. Cryst. Growth 287 (2006) 157. [17] Y.H. Leung, A.B. Djurišić, Z.T. Liu, D. Li, M.H. Xie, W.K. Chan, J. Phys. Chem. Solids 69 (2008) 353. [18] Y.F. Hsu, Y.Y. Xi, A.B. Djurišić, W.K. Chan, Appl. Phys. Lett. 92 (2008) 133507. [19] I. Shalish, H. Temkin, V. Narayanamurti, Phys. Rev. B 69 (2004) 245401. [20] X. Wang, H. Zhu, Y. Xu, H. Wang, Y. Tao, S. Hark, ACS Nano 4 (2010). [21] J.B. You, X.W. Zhang, S.G. Zhang, H.R. Tan, J. Ying, Z.G. Yin, Q.S. Zhu, P.K. Chu, J. Appl. Phys. 107 (2010) 083701.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
High efficiency n-ZnO/p-Si core–shell nanowire photodiode based on well-ordered Si nanowire array with smooth surface Kyung Yong Ko a, Hyemin Kang a, Jungkil Kim b, Woo Lee b, Hee Sung Lee c, Seongil Im c, Ji Yeon Kang d, Jae-Min Myoung d, Han-Gil Kim e, Soo-Hyun Kim e, Hyungjun Kim a,n a
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon, Republic of Korea c Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea d Department of Materials Science and Engineering, Yonsei University, 50 Yonsei Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea e School of Materials Science and Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si 712-749, Republic of Korea b
a r t i c l e i n f o
Keywords: Photodiode ZnO Anodic aluminum oxide Core–shell nanowire Atomic layer deposition
abstract A highly efficient n-ZnO/p-Si core–shell nanowire (NW) photodiode was fabricated using ZnO grown by atomic layer deposition (ALD) on a well-ordered Si NW array. Si NW arrays were prepared by metal-assisted chemical etching, for which a metal mesh with a wellorganized nanohole array was made using anodic aluminum oxide. This resulted in a good arrangement, smooth surface, and small diameter distribution of the Si NW array. Consequently, ZnO layers with various thicknesses from 15 to 30 nm were deposited by the ALD method. Because of the smooth surface of the well-ordered Si NWs yielding low surface roughness scattering, the resulting photodiode showed significantly improved device characteristics. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Among many types of ZnO-based optoelectronic devices, ZnO/Si heterojunction photodiodes have attracted great attention because of their broadband light absorption and easy fabrication [1]. However, conventional planar structure devices have limitations in terms of effective light capturing and electron–hole pair separation, resulting in low efficiency [2,3]. In regard of these aspects, core–shell nanowire structures can strengthen the weak points of the planar structure by reducing the reflection of injected light and improving efficiency based on positional carrier separation [4,5].
n
Corresponding author. E-mail address: [email protected] (H. Kim).
http://dx.doi.org/10.1016/j.mssp.2014.07.012 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
Atomic layer deposition (ALD) proceeds through only surface chemical reactions and produces films with good uniformity and conformality on complex nano-structures [6]. Recently, the fabrication of high-efficiency n-ZnO:N/p-Si NW core–shell photodiodes has been reported using ALD for the ZnO shell on the Si NW array [7,8]. In these reports, synthesis of the Si NW array was carried out by electroless etching. However, because of etching with a random Ag particle, the Si NWs were found to have a large diameter distribution (50–200 nm) and very high surface roughness (1–5 nm) [9,10], which inevitably resulted in poor electrical properties because of surface roughness (SR) scattering [11]. Thus, improvement of the inadequate quality of the Si NW array produced by electroless etching is required to reduce carrier loss. A previous report showed that the roughness of the surface of Si NWs formed by electroless etching can be reduced by repeated processes of rapid thermal oxidation
298
K.Y. Ko et al. / Materials Science in Semiconductor Processing 27 (2014) 297–302
(RTO) and hydrogen fluoride (HF) etching [10]. However, this method is time consuming and decreases the amount of Si NW. Recently, we reported that a well-aligned Si NW array with average diameter of 50 nm can be fabricated with an anodic aluminum oxide (AAO) membrane as a template for a metal mesh used for the metal-assisted chemical etching [12]. For optoelectronic devices based on thin films, the influence of a parameter such as film thickness on the electrical and optical properties is important; however, to date, there have been few reports on the effect on electrical and optical properties caused by the various shell thickness of the core–shell photodiode [13]. In this study, we fabricated well-aligned n-ZnO/p-Si NW photodiodes using ALD grown ZnO shell layer of various thickness and a well-ordered Si NW array with a smooth surface. The electrical properties of the n-ZnO/p-Si NW core–shell photodiodes have been described. Also, the optical properties of the ZnO/Si NW core–shell devices were characterized by spectral responsivity and photoluminescence.
2. Experimental procedures The detailed process for the fabrication of the metal mesh assisted Si NW array (denoted as SiNW_P) has been described in a previous report [12]. Fig. 1 shows the
schematic diagram of the n-ZnO/p-SiNW_P fabrication process. In brief, Au (5 nm) and Ag (15 nm) were sequentially sputtered onto an AAO membrane with a wellordered nanohole array and then the Au/Ag coated AAO membrane was floated on a NaOH etching solution to release the Au/Ag metal mesh from the AAO. After transferring the Au/Ag bi-layered metal mesh onto the Si(100) (hole concentration: 1015 cm 3, resistivity: 7–15 Ω cm) substrate, the metal mesh covered Si(100) was etched in HF solution, resulting in a well-ordered Si NW array. The length of the NWs was determined by etching time, while the diameter of the NWs was measured from the nanoholes with small distribution in the AAO template. The remaining Au/Ag bilayer was cleaned in a HNO3 solution. For comparative study, a Si NW array without metal mesh was also prepared (denoted as SiNW_N). Then, ZnO ALD was carried out at 150 1C in a lab-made cold wall type ALD system. The ALD growth cycle was composed of diethyl zinc (DEZ) dosing (1 s) – Ar purging (10 s) – H2O exposure (1 s) – Ar purging (10 s). Under this ALD condition, the growth rate of ZnO was 1.9 Å/cycle. Thus, 80, 120, and 160 cycles of ZnO ALD produced ZnO layers of 15, 22, and 30 nm thickness, respectively. For fabrication of photodiodes, an indium tin oxide (ITO) top electrode was sputtered on all of the n-ZnO/p-Si NW samples using a patterned shadow mask. A Cu plate attached to the bottom of the Si substrate by silver paste was used as the bottom contact.
Fig. 1. Schematic fabrication process of n-ZnO/p-Si NW photodiode: (a) sputtering of Ag, Au sequentially on AAO, (b) transfer the Au/Ag metal mesh onto Si (100), (c) Ag catalyzed metal-assisted chemical etching, (d) well-aligned p-Si NWs, (e) ZnO coating on p-Si NWs through the ALD process, and (f) contact formed n-ZnO/p-Si NWs photodiode.
K.Y. Ko et al. / Materials Science in Semiconductor Processing 27 (2014) 297–302
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Fig. 2. Tilted SEM images of (a) ZnO(15 nm)/SiNW_P, (b) ZnO(22 nm)/SiNW_P, (c) ZnO(30 nm)/SiNW_P, (d) ZnO(30 nm)/SiNW_N, and the top of (e) ZnO (30 nm)/SiNW_P, and (f) ZnO(30 nm)/SiNW_N.
The morphology of the core-shell NWs was studied with field-emission scanning electron microscopy (FESEM) (JSM-6701F). In order to characterize the interface of the ZnO/Si core–shell NW, high resolution transmission electron microscopy (HRTEM) (Tecnai F20, electron beam 200 keV) was used. The distribution of elements in ZnO/Si NW was observed by energy-dispersive X-ray spectroscopy (EDX). The current density of the devices was measured using a probe station (Keithley 306 electrometer). Spectral responsivities of the photodiodes were measured in the wavelength range between 320 and 660 nm using a 500 W Hg(Xe)-arc lamp with a monochromator as a light source (Oriel Optical System). Photoluminescence (PL) of the core–shell structures was measured at room temperature using a 500 μm diameter beam of the He–Cd laser.
3. Results and discussion Fig. 2 shows SEM images of the core–shell n-ZnO/p-Si NW structures. With increasing thickness of the ALD grown ZnO (Fig. 2(a)–(c)), it is apparent that the packing density of the n-ZnO/p-SiNW_P core–shell structures also increases. The increase in packing density is due to the increase in ZnO shell thickness, not the increase in Si NW areal density as all of the Si NW arrays were prepared under the same condition. Normally, the Si NWs, and consequently the fabricated core–shell structures, tend to collapse because of the high aspect ratio of the NWs and the surface tension force exerted on the nanowires during the evaporation of liquid [14]. However, it is interesting to note that for SiNW_P with 30 nm thick ZnO shell, the core–shell NWs appeared to be periodic with an apparent
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Fig. 3. (a) TEM image of the n-ZnO/p-Si NW_P; (b) and (c) HRTEM images of the n-ZnO/p-SiNW_P; (d) HRTEM image of the ZnO shell and SAED pattern of polycrystalline ZnO and single crystal Si; (e) HAADF image of the n-ZnO/p-SiNW_P; (f)–(i) EDX elemental maps of Si, Zn and O in the HAADF image and (j) spatial distributions of the elements across the n-ZnO/p-Si core–shell NW after analysis of the nanoprobe EDX line-scan.
Fig. 4. I–V curves of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO(30 nm)/pSiNW_N (inset: current density versus cross-sectional area of ZnO shell).
close packing structure as shown in Fig. 2(c) and (e). This is because the distance between neighboring Si NWs, determined from the geometry of the nanohole array of the AAO template, was 60 nm, which is almost exactly twice the shell thickness [12]. This also confirms the good diameter uniformity of the Si devices and excellent ordering of the structure. Meanwhile, this kind of closed packed structure cannot be observed for the n-ZnO(30 nm)/pSiNW_N assembly (see Fig. 2(d) and (f)) because of the poor diameter uniformity and bad ordering of the NWs. The TEM image of the conformal n-ZnO/p-Si core–shell NW in Fig. 3(a) shows that the Si NWs prepared by a metal mesh with ordered holes have a smooth surface. A further conformality analysis of ZnO on SiNW_P (Fig. 3(a)) was confirmed in the supplementary information Fig. S1. Fig. 3(b) is the TEM image of the n-ZnO/p-SiNW_P interface which depicts conformally deposited ZnO on Si NW. We can observe in Fig. 3(c) that the ZnO shell (15 nm) is uniformly deposited on SiNW_P. As determined from the HRTEM and the selected area electron diffraction (SAED)
Fig. 5. Spectral responsivity of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO (30 nm)/p-SiNW_N from 660 nm to 320 nm.
pattern, in Fig. 3(d), the ZnO shell is of a polycrystalline structure and the core is composed of single crystalline Si. The high-angle annular dark-field (HAADF) image of n-ZnO/p-Si NW is shown in Fig. 3(e). As revealed in Fig. 3 (f)–(i), the EDX elemental mapping result confirms that silicon, zinc and oxygen exist in the core-shell nanowire. The spatial distributions of the elements across the n-ZnO/ p-SiNW_P (shown in Fig. 3(j)) were obtained by analyzing the nanoprobe EDX line-scan taken at the position marked by the solid line in Fig. 3(e). As shown in Fig. S2, the comparison of roughness difference between two types of core–shell NW was confirmed by the TEM images. According to the standard deviation data of core-shell NW surface roughness, the surface of the n-ZnO/pSiNW_N was rougher than that of the n-ZnO/p-SiNW_P (see Supplementary information for details). Fig. 4 represents the I–V characteristics of the photodiodes. For n-ZnO/p-SiNW_P devices, the current density increases with increasing ZnO shell thickness for both
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forward and reverse bias. Since the current flows along the vertically aligned core–shell NW, the increment of current density with increasing shell thickness is mainly attributed to the increased cross-sectional area of the ZnO shell. The inset of Fig. 4 presents the current density versus the cross-sectional area of the ZnO shells, showing linear increase in current density with the ZnO cross-sectional area at 10 V of forward bias. A similar increasing tendency is seen from 2 V to 10 V which has been detailed in the supplementary information (Fig. S3). On the other hand, the n-ZnO(30 nm)/p-SiNW_N photodiode has the lowest current density for both forward (1.1 mA/cm2: 10 V) and reverse bias. This significantly low current density can be attributed to the high surface roughness of SiNW_N, as mentioned previously [9]. Subsequently, the spectral responsivities of the devices were measured. As shown in Fig. 5, all samples display a similar trend for responsivity as a function of wavelength in the range of 320 and 660 nm [1,8]. With decreasing wavelength, the responsivity decreases because of the increasing light absorption coefficient of bulk Si, resulting in minimum responsivity at 400 nm [15]. Below 400 nm, the responsivity increases again because of the effective generation of photo carriers in the ZnO shell. Among these devices, the n-ZnO(30 nm)/p-SiNW_P photodiode has the highest responsivity in both visible and UV regions and has a maximum value ( 1.8 A/W) at 660 nm. The overall responsivity of the 15 and 22 nm thick ZnO shells is smaller than that of the 30 nm thick ZnO shell. The effect on optical properties of various shell thickness of the coreshell photodiode has been rarely reported, despite the fact that the influence of a parameter such as film thickness on the optical properties is extremely important on thin film based optoelectronic devices. In a previous report, the thickness of ZnO film affects to the optoelectronic properties that the bandgap is gradually decreased with increasing film thickness from 45 nm to 225 nm due to the decreasing film stress. Also, the increased thickness of ZnO film can lead to higher concentration of defects and thus to decreased resistivity [13]. However, under UV irradiation, photo-generated carrier can extinct in thick ZnO while transport to electrode from ZnO because of short penetration depth ( 40 nm) of UV light [1]. n-ZnO (30 nm)/p-SiNW_N has the lowest responsivity for overall wavelength compared to the other n-ZnO/p-SiNW_P photodiodes. As mentioned above, the SiNW_N has a very rough surface. Thus, the photo-generated carriers in the device suffer from high surface scattering resulting in poor responsivity [9]. To further understand the effect of shell thickness on responsivity, photoluminescence (PL) measurements were carried out at room temperature. PL analysis on core–shell nanowire heterojunctions as a function of shell thickness has seldom been reported, whereas the PL of ZnO nanowires has been intensively investigated [16–18]. As shown in Fig. 6, the n-ZnO/p-SiNW_P device structure shows a strong near band edge (NBE) peak at 380 nm [19], without any observable deep-level defects related to the PL intensity in the visible range. Also, no shift in peak position, such as the blue-shift, was observed when the diameter of the studied core-shell NW was more than 80 nm. In a
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Fig. 6. PL intensity of ZnO(30, 22, 15 nm)/p-SiNW_P and ZnO(30 nm)/pSiNW_N from 320 nm to 660 nm.
previous report, the blue-shift in the NBE PL peak was observed for ZnO nanorods with diameter smaller than 40 nm [20]. In any case, it should be noted that the NBE peak intensity significantly decreases with increasing ZnO shell thickness. We attribute this decrease in PL intensity with increasing shell thickness to the positional carrier separation. In a previous report on the CdTe/ZnO core–shell device structure, the intensity of the NBE PL peak arising from the ZnO nanorods as well as the NBE PL peak of CdTe itself was almost completely suppressed after deposition of the 88 nm thick CdTe shell. This “quenching” of the NBE PL intensity was attributed to the reduction of electron–hole recombination caused by the separation of photogenerated carriers. The band alignment of ZnO/Si is similar to that of CdTe/ZnO [1,21]. Thus, we surmise that the same argument can be applied to the current PL results. Especially, the thickness of all n-ZnO/p-SiNW_P devices in this study was quite thin as compared to the CdTe/ZnO structure mentioned above. In this range of small thickness, an incomplete electron–hole recombination can produce observable PL intensity. In Fig. 6, the PL spectrum for n-ZnO(30 nm)/p-SiNW_N is shown. It is interesting that the NBE peak intensity is almost the same as that for n-ZnO(30 nm)/p-SiNW_P. Since the difference between n-ZnO(30 nm)/p-SiNW_N and n-ZnO(30 nm)/p-SiNW_P is the Si NW surface roughness, we can expect the carrier separation to not be much different in these two materials. Because of the carrier separation, the radiative recombination rate should also be similar, agreeing with the comparable PL intensity. Even with this charge separation, the responsivity of n-ZnO(30 nm)/p-SiNW_N is significantly lower than that of n-ZnO(30 nm)/p-SiNW_P, displaying the importance of surface roughness in terms of improvement of efficiency for the photodiode. 4. Conclusions An ordered Si NW array with smooth surface and small diameter distribution was prepared by metal-mesh assisted chemical etching. Then, ALD grown ZnO/Si NW
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photodiodes with various shell thickness (15 nm to 30 nm) were fabricated and the device properties characterized. The conformally deposited polycrystalline ZnO shell on Si NWs was confirmed by HRTEM, SAED pattern and EDX elemental mapping. The I–V characteristics showed proportional increase of current density with increase in shell thickness. Also, it was observed that with increasing ZnO thickness between 15 nm and 30 nm, the spectral responsivity steadily increased 2–3 times. This significantly improved device performance for an ordered device with a smooth interface as a result of the reduced surface scattering of the photo-generated carriers and effective carrier separation. Acknowledgments This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2013K000173). In addition, this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0028594) Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j. mssp.2014.07.012.
References [1] I.-S. Jeong, J.H. Kim, S. Im, Appl. Phys. Lett. 83 (2003) 2946. [2] C.-P. Chen, P.-H. Lin, Y.-J. Hung, S.-S. Hsu, L.-Y. Chen, Y.-W. Cheng, M.-Y. Ke, Y.-Y. Huang, C.-H. Chang, C.-H. Chiu, H.-C. Kuo, J. Huang, Nanotechnology 21 (2010) 215201.
[3] K. Wang, J.J. Chen, Z.M. Zeng, J. Tarr, W.L. Zhou, Y. Zhang, Y.F. Yan, C.S. Jiang, J. Pern, a. Mascarenhas, Appl. Phys. Lett. 96 (2010) 123105. [4] Y. Zhang, L.-W. Wang, A. Mascarenhas, Nano Lett. 7 (2007) 1264. [5] A. Nduwimana, R.N. Musin, A.M. Smith, X.-Q. Wang, Nano Lett. 8 (2008) 3341. [6] H. Kim, H.-B.-R. Lee, W.-J. Maeng, Thin Solid Films 517 (2009) 2563. [7] H.-D. Um, S.A. Moiz, K.-T. Park, J.-Y. Jung, S.-W. Jee, C.H. Ahn, D.C. Kim, H.K. Cho, D.-W. Kim, J.-H. Lee, Appl. Phys. Lett. 98 (2011) 033102. [8] H. Kang, J. Park, T. Choi, H. Jung, K.H. Lee, S. Im, H. Kim, Appl. Phys. Lett. 100 (2012) 041117. [9] A.I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature 451 (2008) 163. [10] S. Lee, J. Koo, J. Seo, S. Kim, K. Lee, J. Nanopart., 2012. [11] C. Buran, M. Pala, IEEE Trans. Electron Devices 56 (2009) 2186. [12] J. Kim, H. Han, Y.H. Kim, S.-H. Choi, J.-C. Kim, W. Lee, ACS Nano 5 (2011) 3222. [13] T. Singh, T. Lehnen, T. Leuning, D. Sahu, S. Mathur, Appl. Surf. Sci. 289 (2014) 27. [14] Y. Liang, C. Zhen, D. Zou, D. Xu, J. Am. Chem. Soc. 126 (2004) 16338. [15] K. Rajkanan, R. Singh, J. Shewchun, Solid State Electron. 22 (1979) 793. [16] L.H. Quang, S.J. Chua, K. Ping Loh, E. Fitzgerald, J. Cryst. Growth 287 (2006) 157. [17] Y.H. Leung, A.B. Djurišić, Z.T. Liu, D. Li, M.H. Xie, W.K. Chan, J. Phys. Chem. Solids 69 (2008) 353. [18] Y.F. Hsu, Y.Y. Xi, A.B. Djurišić, W.K. Chan, Appl. Phys. Lett. 92 (2008) 133507. [19] I. Shalish, H. Temkin, V. Narayanamurti, Phys. Rev. B 69 (2004) 245401. [20] X. Wang, H. Zhu, Y. Xu, H. Wang, Y. Tao, S. Hark, ACS Nano 4 (2010). [21] J.B. You, X.W. Zhang, S.G. Zhang, H.R. Tan, J. Ying, Z.G. Yin, Q.S. Zhu, P.K. Chu, J. Appl. Phys. 107 (2010) 083701.