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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 20887
Optical photoresponse of CuS–n-Si radial heterojunction with Si nanocone arrays fabricated by chemical etching† Ajit K. Katiyar, Arun Kumar Sinha, Santanu Manna, Rakesh Aluguri and Samit K. Ray* The paper deals with the fabrication of a p-CuS–n-Si nanocone heterojunction based highly sensitive broad band photodetector. Cone-like one dimensional Si nanostructures formed by metal assisted
Received 25th August 2013, Accepted 9th October 2013
chemical etching, with superior antireflection characteristics have been used as templates for fabrication
DOI: 10.1039/c3cp53603c
target material for the fabrication of p-CuS–n-Si nanocone heterojunctions via pulsed laser ablation. The
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detection is reported.
of the heterojunction. Covellite CuS material was synthesized by a simple chemical reaction for used as effect of surface texturing of Si (cone like nanostructure vs. planar) on spectral photoresponse and
1. Introduction One dimensional (1D) silicon nanostructures such as nanowires,1–8 nanocones,9 nanodomes10 and nanopyramids11 are of immense interest for their applications in nanoscale devices. The capability of making radial or core–shell heterojunctions using nanowire templates make them attractive for photovoltaic,12,13 photodetection14,15 and biosensing16 applications with high surface to volume ratio. Both top-down and bottom-up approaches, such as vapour–liquid–solid (VLS) growth, reactive ion etching (RIE), electrochemical etching and metal-assisted chemical etching (MACE) have been utilized to fabricate 1D Si nanostructures. MACE, one of the top-down methods, has recently become attractive to fabricate Si nanowires for various heterojunction based optoelectronic devices such as solar cells,17–20 light emitting diodes21 and photodetectors22,23 etc. Although heterojunction devices fabricated with MACE grown Si nanowires exhibit better photovoltaic performance than those with planar ones, the step coverage of vapour phase deposited material on very closely spaced nanowires is a critical concern to fabricate a device with high aspect ratio. The problem can be eliminated using cone-like nanostructures since the geometry may allow for conformal surface coverage for improved heterojunction devices.24 Furthermore, the ordered Si nanoconical array is reported to exhibit excellent antireflection
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur-721302, India. E-mail: [email protected] † Electronic supplementary information (ESI) available: The details of analytical instruments, preparation of CuS material, and subsequent characterizations using photoluminescence and Raman analyses are given in the supporting information. See DOI: 10.1039/c3cp53603c
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and superior absorption properties in the visible range of the optical spectra.25,26 The photo physics of heterojunctions with nanostructured Si and metal sulphide materials like CdS have been reported.15,23,27,28 However, only a few studies are available on the nanowire heterojunction of Si with Cu based binary compounds such as Cu2O, CuxS (x = 1–2), which are known to possess predominantly p-type conductivity. As an important non-toxic, earth abundant, highly p-doped semiconductor with its metal-like electrical conductivity, CuS has attracted much attention in recent years to fabricate wide range of optical and electrical devices.29–34 Yuan et al.35 used a CuS thin film as a p-type material in dye sensitized solar cells and achieved significant photoelectric response. Wu et al.36 reported the fabrication of an efficient flexible photovoltaic device based on a Schottky junction between CuS nanotubes and ITO film. The study of a ZnO–CuS nanorod core–shell n–p heterojunction behaved as a rectifying diode with an ultrafast photoresponse, indicating its probable application in photodiodes.37 So the p-CuS–n-Si nanocone heterojunction appears attractive for nanoscale optoelectronic devices. Here we report the synthesis of wafer scale, cone-like Si nanostructures with excellent antireflection properties using a simple and low cost wet chemical etching technique. The nanocone templates have been used to fabricate a radial p–n heterojunction by depositing p-CuS on nanotextured n-Si via pulsed laser deposition (PLD). The characteristics of the p-CuS–n-Si nanocone heterojunction for use in broad band photodetectors are reported.
2. Experimental Phosphorus doped n-type single side polished Si (100) substrates with resistivity 2–10 O cm were first degreased in acetone and
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isopropanol by sonication, followed by rinsing with deionized (DI) water (resistivity B18 MO cm). Thereafter it was cleaned in piranha solution containing H2SO4 (97%) and H2O2 (35%) in a volume ratio 3 : 1 for 15 min at room temperature followed by several rinses with DI water. To remove the native oxide layer formed by the piranha treatment, samples were etched with 2% HF aqueous solution for 2–3 min at room temperature. The resultant H-terminated fresh Si pieces were placed into a gold coating solution containing 4.8 M HF and 0.005 M HAuCl4. After dipping in solution for 1 min, a uniform layer of Au nanoparticles (Au NPs) was coated, which was then immersed in an etchant with optimized concentration of HF (4.8 M) and H2O2 (0.4 M) for formation of cone-like Si nanostructures. After 20 min etching in ambient, the samples were washed repeatedly with DI water followed by treating with aqua regia to dissolve the residual gold, resulting in deep black wafer surfaces. Finally, the wafers were washed with 2% HF to remove the oxide layer, cleaned with DI water, and dried under N2 flow. As prepared CuS powder (given in ESI 1†) was finely ground, pressed into pellets of one inch diameter and sintered at 200 1C in a flow of N2 gas for 10 hours (for use as PLD target). p-CuS–n-Si radial heterojunction was fabricated by depositing a CuS layer on Au assisted etched Si nanostructures using KrF pulsed laser (l = 248 nm) ablation (2000 shots at a shot frequency 10 Hz). Similar deposition was also carried out on bulk n-Si for use as the control sample. The distance between the target and substrate was kept fixed at 5 cm. By this method, a CuS film nearly 150 nm thick was deposited on nanotextured and planar n-Si. Finally to complete the device fabrication, a nearly 200 nm thick transparent and conducting Al doped ZnO (AZO) layer was deposited by the same PLD technique on both p-CuS/nanostructured n-Si and p-CuS/planar n-Si, which are hereafter referred to as the cone-like radial heterojunction device and control device, respectively. A thick aluminium layer was deposited by thermal evaporation onto the
backside of both the devices to form the bottom electrode. The fabricated Si nanostructures and p-CuS–n-Si heterojunction were characterised by different analytical instruments, the details of which are given in ESI 2.† The spectral photocurrent responses of the planar and CuS–n-Si nanocone heterojunctions were measured using a setup consisting of a broadband light source, a monochromator, a mechanical chopper (set to 120 Hz), and a lock-in amplifier (Stanford Research, SR 530). The current– voltage (I–V) characteristics of the heterojunctions were recorded using a Keithley semiconductor parameter analyzer (model no. 4200-SCS).
3. Results and discussion 3.1.
Characterization of Si nanostructures
Fig. 1(a) shows the cross-sectional field emission scanning electron microscopy (FESEM) image of the fabricated nanostructures on an n-Si wafer. Highly dense, vertically oriented cone-like nanostructures with different heights and bases are observed. From the high resolution FESEM and TEM image of a single Si nanocone shown in Fig. 1(b) and (c) respectively, it is clear that the surface of the Si cones is porous in nature. Typical dimensions of the nanocone bases are 500–800 nm with the sharp tips having a diameter of a few nm. The heights of the nanocones are observed to be in the range of 2–4 mm. Fig. 1(d) shows the cross-sectional FESEM image of the CuS film deposited on the Si nanocones via pulsed laser deposition. The micrograph indicates conformal coverage of the CuS film on the Si nanocones. However, the bunching of several nanocones is clearly observed. The cross-sectional TEM image of a single Si nanocone coated with CuS (shown in Fig. 1e) again indicates the conformal surface coverage of nanocrystalline CuS over Si nanocones. A closer look at the Si–CuS interface of the nanocone textured Si indicates the formation of a rough interface
Fig. 1 (a) Cross-sectional FESEM image of cone-like nanostructures on an n-Si wafer. (b) High resolution FESEM image of typical Si nanocones. (c) TEM image of a single Si nanocone. (d) Cross-sectional FESEM micrograph of a CuS–n-Si nanocone heterojunction. (e) Cross-sectional TEM image of a Single Si nanocone coated with CuS.
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Paper It may be noted that the proposed MACE method is superior to cost intensive fabrication using e-beam lithography and reactive ion etching for large scale production. Therefore Si nanocones could be promising templates for the fabrication of heterojunction photosensing and photovoltaic devices.
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3.2.
Fig. 2 (a) Reflectance spectra of polished Si surfaces and nanotextured Si over a broad spectral range (330–1500 nm). (b) UV-visible absorption spectrum of the CuS film. Inset shows the plot of (ahn)2 vs. hn of the CuS film. (c) XRD spectra of the as-synthesized CuS powder and film grown on n-Si by PLD using a pellet of synthesized CuS as the target. XRD spectrum of the CuS film was recorded at 21 grazing incidence angle.
due to the porous nature of the Si nanocone surface. The rough interface and the presence of surface defects for a nanotextured heterojunction may play an important role in the rectification behavior of the heterojunction which will be discussed later. To investigate the optical characteristics of Si nanocones, we have carried out temperature dependent photoluminescence (PL) experiments spanning from 10 K to room temperature (300 K) and spectra are shown in Fig. S1 of the ESI.† Orange-red luminescence is observed by the naked eye at 300 K upon illumination by a UV excitation pump of 325 nm. The observed PL is attributed to the porous nature of the nanostructured surface38–41 (which is also observed from the FESEM image). Further discussions related to the PL characteristics can be found in the ESI 3.† Fig. 2(a) shows the optical reflectance spectra of planar and nanocone Si surfaces. The nanocone textured Si shows an average reflectivity of less than 2% in the wavelength range 400 nm to 1000 nm in comparison to 33% for planar Si in the same spectral range. The reflectivity of the nanocone Si below 400 nm and in the IR region beyond 1000 nm is also significantly reduced compared to planar Si. This excellent antireflection property arises because the Si nanocones act as a subwavelength structured surface. Such a textured surface on Si provides an effective medium with a smooth transition of the refractive index from air to Si, because the fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density.42 The above results imply that the fabricated cone-like Si nanostructures exhibit enhanced absorption due to superior antireflection properties over a wide spectral range.
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Properties of deposited CuS films on n-Si
The optical properties of the CuS film have been studied through diffuse reflectance spectroscopy (DRS), which is shown in Fig. 2(b). The material absorbs the light over a wide spectral range. The CuS film also shows an increased absorption in the near-IR region, which is characteristic of the covellite phase of CuS.43 The band gap of the CuS material has been determined using the Tauc equation44 aEp = K(Ep Eg)1/2, where a is the absorption coefficient, K is a constant, Ep is the photon energy, and Eg is the band gap. A plot of (aEp)2 vs. Ep, shown in the inset of Fig. 2(b), based on the direct transition and at a = 0 gives the band gap energy of the CuS film as 2.19 eV. Since the extracted band gap from the absorption measurement is more than the bulk band gap value (1.85 eV), quantum confinement of CuS in the nanocrystalline CuS film, might be responsible.45 From the X-ray diffraction pattern of CuS–n-Si, the crystallite size has been extracted to be B10 nm by using the Debye–Scherrer formula. Guneri et al.46 and Sangamesha et al.47 have also reported the elevated bandgap value for nanocrystalline CuS formed within the deposited film. Copper sulphide has different stable phases such as covellite (CuS), anilite (Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.96S), and chalcocite (Cu2S).48 The phase of the as-synthesized copper sulphide bulk and thin film deposited on n-Si via pulsed laser deposition has been analyzed by X-ray diffraction (XRD), which is shown in Fig. 2(c). For the as-synthesized CuS powder, a number of distinguishable X-ray diffraction peaks corresponding to the crystallographic planes of CuS (100), (101), (102), (103), (006), (105), (107), (110), (108), (202), (203) and (116) are observed, which are classified due to the pure hexagonal covellite structure (JCPDS 00-006-0464) with space group P63/mmc.49 However the grown CuS film on Si shows the diffraction peaks due to the (101), (102), (103) and (100) planes correspond to the same structure. It may be noted that the XRD pattern of the film grown on Si has a preferred orientation along the (101) plane. The diffraction intensity of the CuS thin film is obviously much lower in comparison to that of the CuS powder, due to low film thickness (B150 nm). The covellite structure of the CuS film has been further confirmed from the Raman analysis49 with the corresponding figure and discussion available in the ESI 4.† The chemical state and purity of the CuS thin film have been analyzed by X-ray photoelectron spectroscopy (XPS) and the results are presented in Fig. 3. The presence of core level and Auger peaks for Cu and S with very low intensity peaks of C1s and O1s in the survey spectrum, without surface cleaning is shown in Fig. 3(a). This indicates the absence of impurity elements in the CuS film, except trace amounts of carbon and oxygen originating due to the adsorption of hydrocarbons on the surface from the environment. The high-resolution core level XPS spectra of Cu2p and S2p electrons are shown in
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Fig. 3 XPS spectra of the CuS film grown on an n-Si substrate by PLD using a pellet of synthesized CuS material as the target. (a) Survey spectrum, (b) High resolution spectrum of Cu2p electrons. (c) High resolution spectrum of S2p electrons.
Fig. 3(b) and (c), respectively. Two strong peaks for Cu2p spectra observed at 932.0 eV and 951.8 eV are assigned to the 2p3/2 and 2p1/2 levels, respectively. The binding energy peaks observed for S2p electrons at 162.0 and 163.1 eV are attributed to the S2p3/2 and S2p1/2 states, respectively, which confirms the formation of CuS.50 The atomic percentage ratio for Cu : S has been extracted using the sensitivity factor and the area under the curve of Cu2p and S2p peaks using the Shirley background correction and is found to be 53 : 47. This is very close to the stoichiometric ratio 1 : 1 for CuS.
4. Characteristics of the p-CuS–n-Si cone-like radial heterojunction The schematic diagram of the cone-like radial heterojunction device with Al/n-Si/CuS/AZO structure is shown in Fig. 4. Fig. 5(a) shows the semi-log plot of the current–voltage characteristics of the cone-like radial heterojunction device and control device in the dark and with under illumination of 100 mW cm2 white light.
Fig. 4
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Schematic diagram for cone-like Si nanostructure based device.
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Fig. 5 (a) The semi-log plots of the current–voltage characteristics of the conelike radial heterojunction (nanotextured) and control devices in the dark and under illumination of 100 mW cm2 white light. (b) Transient photoresponse by switching the light source (100 mW cm2) ‘on’ and ‘off’ for cone-like radial heterojunction and control devices at 2 V bias.
The results indicate the formation of a junction between the p-CuS film and n-Si. The dark current of the diodes is approximately 104 A at 2 V bias. This relatively high reverse current is mainly due to the defects in the films and rough interfaces grown by PLD.51 The diode ideality factor, which controls the forward current, extracted using J = Js exp(qV/ZKT), is found to be 3.8 and 8.2 for the control and cone-like radial heterojunction devices, respectively. The higher value of ideality factor in the nanocone heterojunction results in a reduced forward current as compared to the planar one. The rough interface and presence of a high density of defect states, generated at the time of chemical etching of the substrate in the CuS–n-Si nanocone heterojunction could be responsible for the large leakage current as well as the higher Z value in comparison to the planar device. The reverse current density of both the devices increases under illumination. Under illumination, the reverse current increases due to the generation and collection of photoexcited minority carriers. Photo ( Jph) to dark current ( Jd) density ratio, calculated using ( Jph Jd)/Jd, is found to be 398.0 at 2 V for the cone-like radial heterojunction device in
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comparison to 91.0 for the control Si device. A significant improvement in the photo-to-dark current density ratio in the cone-like radial heterojunction device is observed in comparison with the planar one. This indicates the formation of a superior radial p-CuS–n-Si heterojunction using cone-like templates. It may be noted that the diffusion length of minority carriers is comparable to the diameter of the Si nanocone, resulting in the efficient collection of photocarriers in the radial heterojunction. This results in a higher photocurrent density in the radial heterojunction as compared to the bulk one. The dynamic photo response of both the devices has been tested by turning on and off the white light source (100 mW cm2) at 2 V bias and the results are shown in Fig. 5(b). In addition to the high on/off ratio, the nanocone device clearly shows a faster response at the decay edge as compared to the planar one, whereas there is no significant difference in the rise time of the devices. This clearly indicates efficient extraction of photo-carriers in the radial heterojunction due to a higher built-in electric field as compared to the planar heterojunction. Fig. 5(b) also shows that the photoresponse of both the devices is very steady and reproducible over repeated cycles. This behaviour of p-CuS–n-Si radial heterojunction diodes under illumination indicates that nanostructured heterojunctions may be potentially attractive for photodetection and optical switching devices. Using the band gap value of CuS from the absorption measurement and taking the other parameters from the literature, we have calculated the valence and conduction band offset values for the p-CuS–n-Si nanocone heterojunction. A schematic band diagram (type-II band alignment) of the above device is presented in Fig. 6. In the case of n-Si, the band gap energy (Eg porous-Si) and electron affinity (wSi) are assumed to be 1.91 eV and 3.69 eV, respectively. The corresponding values for p-type CuS (covellite) are taken as 2.18 and 1.91 eV,52 respectively. There are two energy band offsets due to the different electron affinities and band gap values of Si and CuS. The conduction band offset (DEc) is found to be: DEc = wporous-Si wCuS = 1.77 eV
The responsivity (R) of a photodiode is expressed as the ratio of photocurrent density to the intensity of incident light corresponding to a particular wavelength Rl = Jph/Popt, where Jph is the photocurrent density and Popt is the intensity of incident light (of a particular wavelength l). The responsivity has been calculated by comparing the photocurrent of p-CuS–nSi heterojunctions with that of a standard Si photodiode using the relation: Rl ¼ RSi
Jl JSi
(3)
where Rl is the responsivity of the p-CuS–n-Si device, RSi is the responsivity of a standard Si photodiode, Jl is the photocurrent density of the p-CuS–n-Si device and JSi is the photocurrent density of a standard Si photodiode. Fig. 7(a) shows the spectral responsivity of a cone-like radial heterojunction along with a planar control device at zero applied bias. The fabricated heterojunction devices show a photo-response over the broad spectral range from 400 nm to 1100 nm. The maximum responsivity for the planar and nanostructured devices is observed as 0.16 A W1 and 0.32 A W1, respectively, nearly at 990 nm at zero applied bias. This corroborates the efficient separation of photo-generated electrons and holes in the depletion region by a built-in field even for the unbiased radial
(1)
And the valence band offset (DEv) is: DEv = DEc + Eg
Fig. 6
porous-Si
Eg
CuS
= 1.50 eV
(2)
Band diagram of the fabricated n-Si–p-CuS heterojunction device.
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Fig. 7 (a) Spectral responsivity of the cone-like radial heterojunction device and control device at zero bias. (b) EQE spectra and detectivity of the cone-like radial heterojunction and control devices at zero bias.
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heterojunction device. The peak responsivity value of our nanocone device at zero bias is much higher than that recently reported for radial heterojunctions fabricated on Si nanowires e.g., Si–CdS23 (0.1 A W1) and Si–AgInSe253 (0.11 A W1). The spectral response consists of several broad peaks, as due to the absorption region of CuS (B550 nm), porous nature of nanocones (B600–750 nm) and self absorption region of Si. The responsivity plots indicate the dominant effect of absorption due to Si over that of CuS, as responsivity is found to be higher at 990 nm than at 550 nm for both the devices. External quantum efficiency (EQE) of the photodiodes has been calculated using the relation: EQEð%Þ ¼
100 1240 Jph 100 1240 Rl ¼ l Popt l
(4)
From the EQE spectra shown in Fig. 7(b), it was noticed that the peak value of EQE is more than 40% for the nanostructured device, whereas it is 20% for the control device at zero bias. The EQE almost remains the same up to nearly 550 nm for both the devices, but after 550 nm there is a sharp increase in the EQE of the nanostructured device, indicating the effect of enhanced absorption of Si nanocones in this range. Obviously, the photocurrent is higher in the radial heterojunction because of the high absorption for Si nano-cones as compared to planar Si, as demonstrated in Fig. 2(a). The parameter characterizing normalized signal-to-noise performance of a detector is known as the detectivity (D). Considering the contribution of shot noise to the dark current as the major one, the detectivity for a particular wavelength can be expressed as: RðlÞ DðlÞ ¼ pffiffiffiffiffiffiffiffiffiffi 2qJd
(5)
where q is the charge of an electron and Jd is the dark current density. Detectivity values of the devices, extracted using the above eqn, are also presented in Fig. 7(b). The peak detectivity is found to be 2.7 1010 and 1.4 1010 cm Hz1/2 W1 for the cone-like radial heterojunction device and control device, respectively, under the illumination of 990 nm light. The detectivity value is relatively low because of the large dark current in heterojunctions. The detectivity is found to be higher for the cone-like radial heterojunction device as compared to the control one, due to the higher photo response from the radial nanocone heterojunction with the dark current values remaining almost in the same order. The behaviour of n-Si–p-CuS nanocone radial heterojunction diodes under illumination indicates that the device can be useful for photodetection applications. The heteroepitaxial growth of CuS on Si using a refined technique and the optimization of interfacial properties may lead to the significant improvement of responsivity and detectivity of the radial heterojunction devices for use in nanophotonic devices.
Conclusion In summary, we have successfully synthesized wafer scale, highly antireflective, cone-like nanostructures on n-Si wafers
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using a low cost, simple Au catalyzed chemical etching method. The formation of highly dense vertically oriented cone-like Si nanostructures with nanoporous surfaces is presented. The photodiode characteristics of the n-Si–p-CuS heterojunction on fabricated cone-like nanotextured Si have been studied. The peak responsivity of 0.32 A W1 with a maximum EQE of 40% is found for the nanostructured device at 990 nm with zero applied bias, showing significant improvement over the control planar device. The nanostructured device shows improved responsivity and EQE due to enhanced light absorption and efficient photo-carrier extraction in the nanocone radial heterojunction.
Acknowledgements This work was supported by DST SERI sponsored ‘‘NSH’’ project. The XPS facility of the DST ‘‘FIST’’ project is also gratefully acknowledged.
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36 C. Wu, Z. Zhang, Y. Wu, P. Lv, B. Nie, L. Luo, L. Wangi, J. Hu and J. Jie, Nanotechnology, 2013, 24, 045402. 37 S. Panigrahi and D. Basak, RSC Adv., 2012, 2, 11963–11968. 38 A. I. Hochbaum, D. Gargas, Y. J. Hwang and P. D. Yang, Nano Lett., 2009, 9, 3550–3554. 39 Y. Q. Qu, L. Liao, Y. J. Li, H. Zhang, Y. Huang and X. F. Duan, Nano Lett., 2009, 9, 4539–4543. 40 W. Chern, K. Hsu, I. S. Chun, B. P. D. Azeredo, N. Ahmed, K. H. Kim, J. Zuo, N. Fang, P. Ferreira and X. Li, Nano Lett., 2010, 10, 1582–1588. 41 L. Sun, H. He, C. Liu, Y. Lu and Z. Ye, CrystEngComm, 2011, 13, 2439–2444. 42 H. Sai, H. Fujii, K. Arafune, Y. Ohshit and M. Yamaguchi, Appl. Phys. Lett., 2006, 88, 201116. 43 E. J. Silvester, F. Grieser, B. A. Sexton and T. W. Healy, Langmuir, 1991, 7, 2917–2922. 44 J. Tauc, R. Grigorovici and A. Vancu, Phys. StatusSolidi, 1966, 15, 627–637. 45 J. W. Haus, H. S. Zhou, I. Honma and H. Komiyama, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 47, 315. 46 E. Guneri and A. Kariper, J. Alloys Compd., 2012, 516, 20–26. 47 M. A. Sangamesha, K. Pushpalatha, G. L. Shekar and S. Shamsundar, ISRN Nanomaterials, 2013, 829430. 48 Y. B. Chen, L. Chen and L. M. Wu, Cryst. Growth Des., 2008, 8, 2736–2740. 49 Z. Zhan, C. Liu, L. Zheng, G. Sun, B. Li and Q. Zhang, Phys. Chem. Chem. Phys., 2011, 13, 20471–20475. 50 Y. C. Zhang, T. Qiao and X. Y. Hu, J. Cryst. Growth, 2004, 268, 64–70. 51 M. Takenaka, K. Morii, M. Sugiyama, Y. Nakano and S. Takagi, Opt. Express, 2012, 20, 8718–8725. 52 S. Midda, N. C. Bera, I. Bhattacharyya and A. K. Das, THEOCHEM, 2006, 761, 17–20. 53 M. Kulakci, T. Colakoglu, B. Ozdemir, M. Parlak, H. E. Unalan and R. Turan, Nanotechnology, 2013, 24, 375203.
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Optical photoresponse of CuS–n-Si radial heterojunction with Si nanocone arrays fabricated by chemical etching† Ajit K. Katiyar, Arun Kumar Sinha, Santanu Manna, Rakesh Aluguri and Samit K. Ray* The paper deals with the fabrication of a p-CuS–n-Si nanocone heterojunction based highly sensitive broad band photodetector. Cone-like one dimensional Si nanostructures formed by metal assisted
Received 25th August 2013, Accepted 9th October 2013
chemical etching, with superior antireflection characteristics have been used as templates for fabrication
DOI: 10.1039/c3cp53603c
target material for the fabrication of p-CuS–n-Si nanocone heterojunctions via pulsed laser ablation. The
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detection is reported.
of the heterojunction. Covellite CuS material was synthesized by a simple chemical reaction for used as effect of surface texturing of Si (cone like nanostructure vs. planar) on spectral photoresponse and
1. Introduction One dimensional (1D) silicon nanostructures such as nanowires,1–8 nanocones,9 nanodomes10 and nanopyramids11 are of immense interest for their applications in nanoscale devices. The capability of making radial or core–shell heterojunctions using nanowire templates make them attractive for photovoltaic,12,13 photodetection14,15 and biosensing16 applications with high surface to volume ratio. Both top-down and bottom-up approaches, such as vapour–liquid–solid (VLS) growth, reactive ion etching (RIE), electrochemical etching and metal-assisted chemical etching (MACE) have been utilized to fabricate 1D Si nanostructures. MACE, one of the top-down methods, has recently become attractive to fabricate Si nanowires for various heterojunction based optoelectronic devices such as solar cells,17–20 light emitting diodes21 and photodetectors22,23 etc. Although heterojunction devices fabricated with MACE grown Si nanowires exhibit better photovoltaic performance than those with planar ones, the step coverage of vapour phase deposited material on very closely spaced nanowires is a critical concern to fabricate a device with high aspect ratio. The problem can be eliminated using cone-like nanostructures since the geometry may allow for conformal surface coverage for improved heterojunction devices.24 Furthermore, the ordered Si nanoconical array is reported to exhibit excellent antireflection
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur-721302, India. E-mail: [email protected] † Electronic supplementary information (ESI) available: The details of analytical instruments, preparation of CuS material, and subsequent characterizations using photoluminescence and Raman analyses are given in the supporting information. See DOI: 10.1039/c3cp53603c
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and superior absorption properties in the visible range of the optical spectra.25,26 The photo physics of heterojunctions with nanostructured Si and metal sulphide materials like CdS have been reported.15,23,27,28 However, only a few studies are available on the nanowire heterojunction of Si with Cu based binary compounds such as Cu2O, CuxS (x = 1–2), which are known to possess predominantly p-type conductivity. As an important non-toxic, earth abundant, highly p-doped semiconductor with its metal-like electrical conductivity, CuS has attracted much attention in recent years to fabricate wide range of optical and electrical devices.29–34 Yuan et al.35 used a CuS thin film as a p-type material in dye sensitized solar cells and achieved significant photoelectric response. Wu et al.36 reported the fabrication of an efficient flexible photovoltaic device based on a Schottky junction between CuS nanotubes and ITO film. The study of a ZnO–CuS nanorod core–shell n–p heterojunction behaved as a rectifying diode with an ultrafast photoresponse, indicating its probable application in photodiodes.37 So the p-CuS–n-Si nanocone heterojunction appears attractive for nanoscale optoelectronic devices. Here we report the synthesis of wafer scale, cone-like Si nanostructures with excellent antireflection properties using a simple and low cost wet chemical etching technique. The nanocone templates have been used to fabricate a radial p–n heterojunction by depositing p-CuS on nanotextured n-Si via pulsed laser deposition (PLD). The characteristics of the p-CuS–n-Si nanocone heterojunction for use in broad band photodetectors are reported.
2. Experimental Phosphorus doped n-type single side polished Si (100) substrates with resistivity 2–10 O cm were first degreased in acetone and
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isopropanol by sonication, followed by rinsing with deionized (DI) water (resistivity B18 MO cm). Thereafter it was cleaned in piranha solution containing H2SO4 (97%) and H2O2 (35%) in a volume ratio 3 : 1 for 15 min at room temperature followed by several rinses with DI water. To remove the native oxide layer formed by the piranha treatment, samples were etched with 2% HF aqueous solution for 2–3 min at room temperature. The resultant H-terminated fresh Si pieces were placed into a gold coating solution containing 4.8 M HF and 0.005 M HAuCl4. After dipping in solution for 1 min, a uniform layer of Au nanoparticles (Au NPs) was coated, which was then immersed in an etchant with optimized concentration of HF (4.8 M) and H2O2 (0.4 M) for formation of cone-like Si nanostructures. After 20 min etching in ambient, the samples were washed repeatedly with DI water followed by treating with aqua regia to dissolve the residual gold, resulting in deep black wafer surfaces. Finally, the wafers were washed with 2% HF to remove the oxide layer, cleaned with DI water, and dried under N2 flow. As prepared CuS powder (given in ESI 1†) was finely ground, pressed into pellets of one inch diameter and sintered at 200 1C in a flow of N2 gas for 10 hours (for use as PLD target). p-CuS–n-Si radial heterojunction was fabricated by depositing a CuS layer on Au assisted etched Si nanostructures using KrF pulsed laser (l = 248 nm) ablation (2000 shots at a shot frequency 10 Hz). Similar deposition was also carried out on bulk n-Si for use as the control sample. The distance between the target and substrate was kept fixed at 5 cm. By this method, a CuS film nearly 150 nm thick was deposited on nanotextured and planar n-Si. Finally to complete the device fabrication, a nearly 200 nm thick transparent and conducting Al doped ZnO (AZO) layer was deposited by the same PLD technique on both p-CuS/nanostructured n-Si and p-CuS/planar n-Si, which are hereafter referred to as the cone-like radial heterojunction device and control device, respectively. A thick aluminium layer was deposited by thermal evaporation onto the
backside of both the devices to form the bottom electrode. The fabricated Si nanostructures and p-CuS–n-Si heterojunction were characterised by different analytical instruments, the details of which are given in ESI 2.† The spectral photocurrent responses of the planar and CuS–n-Si nanocone heterojunctions were measured using a setup consisting of a broadband light source, a monochromator, a mechanical chopper (set to 120 Hz), and a lock-in amplifier (Stanford Research, SR 530). The current– voltage (I–V) characteristics of the heterojunctions were recorded using a Keithley semiconductor parameter analyzer (model no. 4200-SCS).
3. Results and discussion 3.1.
Characterization of Si nanostructures
Fig. 1(a) shows the cross-sectional field emission scanning electron microscopy (FESEM) image of the fabricated nanostructures on an n-Si wafer. Highly dense, vertically oriented cone-like nanostructures with different heights and bases are observed. From the high resolution FESEM and TEM image of a single Si nanocone shown in Fig. 1(b) and (c) respectively, it is clear that the surface of the Si cones is porous in nature. Typical dimensions of the nanocone bases are 500–800 nm with the sharp tips having a diameter of a few nm. The heights of the nanocones are observed to be in the range of 2–4 mm. Fig. 1(d) shows the cross-sectional FESEM image of the CuS film deposited on the Si nanocones via pulsed laser deposition. The micrograph indicates conformal coverage of the CuS film on the Si nanocones. However, the bunching of several nanocones is clearly observed. The cross-sectional TEM image of a single Si nanocone coated with CuS (shown in Fig. 1e) again indicates the conformal surface coverage of nanocrystalline CuS over Si nanocones. A closer look at the Si–CuS interface of the nanocone textured Si indicates the formation of a rough interface
Fig. 1 (a) Cross-sectional FESEM image of cone-like nanostructures on an n-Si wafer. (b) High resolution FESEM image of typical Si nanocones. (c) TEM image of a single Si nanocone. (d) Cross-sectional FESEM micrograph of a CuS–n-Si nanocone heterojunction. (e) Cross-sectional TEM image of a Single Si nanocone coated with CuS.
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Paper It may be noted that the proposed MACE method is superior to cost intensive fabrication using e-beam lithography and reactive ion etching for large scale production. Therefore Si nanocones could be promising templates for the fabrication of heterojunction photosensing and photovoltaic devices.
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3.2.
Fig. 2 (a) Reflectance spectra of polished Si surfaces and nanotextured Si over a broad spectral range (330–1500 nm). (b) UV-visible absorption spectrum of the CuS film. Inset shows the plot of (ahn)2 vs. hn of the CuS film. (c) XRD spectra of the as-synthesized CuS powder and film grown on n-Si by PLD using a pellet of synthesized CuS as the target. XRD spectrum of the CuS film was recorded at 21 grazing incidence angle.
due to the porous nature of the Si nanocone surface. The rough interface and the presence of surface defects for a nanotextured heterojunction may play an important role in the rectification behavior of the heterojunction which will be discussed later. To investigate the optical characteristics of Si nanocones, we have carried out temperature dependent photoluminescence (PL) experiments spanning from 10 K to room temperature (300 K) and spectra are shown in Fig. S1 of the ESI.† Orange-red luminescence is observed by the naked eye at 300 K upon illumination by a UV excitation pump of 325 nm. The observed PL is attributed to the porous nature of the nanostructured surface38–41 (which is also observed from the FESEM image). Further discussions related to the PL characteristics can be found in the ESI 3.† Fig. 2(a) shows the optical reflectance spectra of planar and nanocone Si surfaces. The nanocone textured Si shows an average reflectivity of less than 2% in the wavelength range 400 nm to 1000 nm in comparison to 33% for planar Si in the same spectral range. The reflectivity of the nanocone Si below 400 nm and in the IR region beyond 1000 nm is also significantly reduced compared to planar Si. This excellent antireflection property arises because the Si nanocones act as a subwavelength structured surface. Such a textured surface on Si provides an effective medium with a smooth transition of the refractive index from air to Si, because the fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density.42 The above results imply that the fabricated cone-like Si nanostructures exhibit enhanced absorption due to superior antireflection properties over a wide spectral range.
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Properties of deposited CuS films on n-Si
The optical properties of the CuS film have been studied through diffuse reflectance spectroscopy (DRS), which is shown in Fig. 2(b). The material absorbs the light over a wide spectral range. The CuS film also shows an increased absorption in the near-IR region, which is characteristic of the covellite phase of CuS.43 The band gap of the CuS material has been determined using the Tauc equation44 aEp = K(Ep Eg)1/2, where a is the absorption coefficient, K is a constant, Ep is the photon energy, and Eg is the band gap. A plot of (aEp)2 vs. Ep, shown in the inset of Fig. 2(b), based on the direct transition and at a = 0 gives the band gap energy of the CuS film as 2.19 eV. Since the extracted band gap from the absorption measurement is more than the bulk band gap value (1.85 eV), quantum confinement of CuS in the nanocrystalline CuS film, might be responsible.45 From the X-ray diffraction pattern of CuS–n-Si, the crystallite size has been extracted to be B10 nm by using the Debye–Scherrer formula. Guneri et al.46 and Sangamesha et al.47 have also reported the elevated bandgap value for nanocrystalline CuS formed within the deposited film. Copper sulphide has different stable phases such as covellite (CuS), anilite (Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.96S), and chalcocite (Cu2S).48 The phase of the as-synthesized copper sulphide bulk and thin film deposited on n-Si via pulsed laser deposition has been analyzed by X-ray diffraction (XRD), which is shown in Fig. 2(c). For the as-synthesized CuS powder, a number of distinguishable X-ray diffraction peaks corresponding to the crystallographic planes of CuS (100), (101), (102), (103), (006), (105), (107), (110), (108), (202), (203) and (116) are observed, which are classified due to the pure hexagonal covellite structure (JCPDS 00-006-0464) with space group P63/mmc.49 However the grown CuS film on Si shows the diffraction peaks due to the (101), (102), (103) and (100) planes correspond to the same structure. It may be noted that the XRD pattern of the film grown on Si has a preferred orientation along the (101) plane. The diffraction intensity of the CuS thin film is obviously much lower in comparison to that of the CuS powder, due to low film thickness (B150 nm). The covellite structure of the CuS film has been further confirmed from the Raman analysis49 with the corresponding figure and discussion available in the ESI 4.† The chemical state and purity of the CuS thin film have been analyzed by X-ray photoelectron spectroscopy (XPS) and the results are presented in Fig. 3. The presence of core level and Auger peaks for Cu and S with very low intensity peaks of C1s and O1s in the survey spectrum, without surface cleaning is shown in Fig. 3(a). This indicates the absence of impurity elements in the CuS film, except trace amounts of carbon and oxygen originating due to the adsorption of hydrocarbons on the surface from the environment. The high-resolution core level XPS spectra of Cu2p and S2p electrons are shown in
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Fig. 3 XPS spectra of the CuS film grown on an n-Si substrate by PLD using a pellet of synthesized CuS material as the target. (a) Survey spectrum, (b) High resolution spectrum of Cu2p electrons. (c) High resolution spectrum of S2p electrons.
Fig. 3(b) and (c), respectively. Two strong peaks for Cu2p spectra observed at 932.0 eV and 951.8 eV are assigned to the 2p3/2 and 2p1/2 levels, respectively. The binding energy peaks observed for S2p electrons at 162.0 and 163.1 eV are attributed to the S2p3/2 and S2p1/2 states, respectively, which confirms the formation of CuS.50 The atomic percentage ratio for Cu : S has been extracted using the sensitivity factor and the area under the curve of Cu2p and S2p peaks using the Shirley background correction and is found to be 53 : 47. This is very close to the stoichiometric ratio 1 : 1 for CuS.
4. Characteristics of the p-CuS–n-Si cone-like radial heterojunction The schematic diagram of the cone-like radial heterojunction device with Al/n-Si/CuS/AZO structure is shown in Fig. 4. Fig. 5(a) shows the semi-log plot of the current–voltage characteristics of the cone-like radial heterojunction device and control device in the dark and with under illumination of 100 mW cm2 white light.
Fig. 4
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Schematic diagram for cone-like Si nanostructure based device.
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Fig. 5 (a) The semi-log plots of the current–voltage characteristics of the conelike radial heterojunction (nanotextured) and control devices in the dark and under illumination of 100 mW cm2 white light. (b) Transient photoresponse by switching the light source (100 mW cm2) ‘on’ and ‘off’ for cone-like radial heterojunction and control devices at 2 V bias.
The results indicate the formation of a junction between the p-CuS film and n-Si. The dark current of the diodes is approximately 104 A at 2 V bias. This relatively high reverse current is mainly due to the defects in the films and rough interfaces grown by PLD.51 The diode ideality factor, which controls the forward current, extracted using J = Js exp(qV/ZKT), is found to be 3.8 and 8.2 for the control and cone-like radial heterojunction devices, respectively. The higher value of ideality factor in the nanocone heterojunction results in a reduced forward current as compared to the planar one. The rough interface and presence of a high density of defect states, generated at the time of chemical etching of the substrate in the CuS–n-Si nanocone heterojunction could be responsible for the large leakage current as well as the higher Z value in comparison to the planar device. The reverse current density of both the devices increases under illumination. Under illumination, the reverse current increases due to the generation and collection of photoexcited minority carriers. Photo ( Jph) to dark current ( Jd) density ratio, calculated using ( Jph Jd)/Jd, is found to be 398.0 at 2 V for the cone-like radial heterojunction device in
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comparison to 91.0 for the control Si device. A significant improvement in the photo-to-dark current density ratio in the cone-like radial heterojunction device is observed in comparison with the planar one. This indicates the formation of a superior radial p-CuS–n-Si heterojunction using cone-like templates. It may be noted that the diffusion length of minority carriers is comparable to the diameter of the Si nanocone, resulting in the efficient collection of photocarriers in the radial heterojunction. This results in a higher photocurrent density in the radial heterojunction as compared to the bulk one. The dynamic photo response of both the devices has been tested by turning on and off the white light source (100 mW cm2) at 2 V bias and the results are shown in Fig. 5(b). In addition to the high on/off ratio, the nanocone device clearly shows a faster response at the decay edge as compared to the planar one, whereas there is no significant difference in the rise time of the devices. This clearly indicates efficient extraction of photo-carriers in the radial heterojunction due to a higher built-in electric field as compared to the planar heterojunction. Fig. 5(b) also shows that the photoresponse of both the devices is very steady and reproducible over repeated cycles. This behaviour of p-CuS–n-Si radial heterojunction diodes under illumination indicates that nanostructured heterojunctions may be potentially attractive for photodetection and optical switching devices. Using the band gap value of CuS from the absorption measurement and taking the other parameters from the literature, we have calculated the valence and conduction band offset values for the p-CuS–n-Si nanocone heterojunction. A schematic band diagram (type-II band alignment) of the above device is presented in Fig. 6. In the case of n-Si, the band gap energy (Eg porous-Si) and electron affinity (wSi) are assumed to be 1.91 eV and 3.69 eV, respectively. The corresponding values for p-type CuS (covellite) are taken as 2.18 and 1.91 eV,52 respectively. There are two energy band offsets due to the different electron affinities and band gap values of Si and CuS. The conduction band offset (DEc) is found to be: DEc = wporous-Si wCuS = 1.77 eV
The responsivity (R) of a photodiode is expressed as the ratio of photocurrent density to the intensity of incident light corresponding to a particular wavelength Rl = Jph/Popt, where Jph is the photocurrent density and Popt is the intensity of incident light (of a particular wavelength l). The responsivity has been calculated by comparing the photocurrent of p-CuS–nSi heterojunctions with that of a standard Si photodiode using the relation: Rl ¼ RSi
Jl JSi
(3)
where Rl is the responsivity of the p-CuS–n-Si device, RSi is the responsivity of a standard Si photodiode, Jl is the photocurrent density of the p-CuS–n-Si device and JSi is the photocurrent density of a standard Si photodiode. Fig. 7(a) shows the spectral responsivity of a cone-like radial heterojunction along with a planar control device at zero applied bias. The fabricated heterojunction devices show a photo-response over the broad spectral range from 400 nm to 1100 nm. The maximum responsivity for the planar and nanostructured devices is observed as 0.16 A W1 and 0.32 A W1, respectively, nearly at 990 nm at zero applied bias. This corroborates the efficient separation of photo-generated electrons and holes in the depletion region by a built-in field even for the unbiased radial
(1)
And the valence band offset (DEv) is: DEv = DEc + Eg
Fig. 6
porous-Si
Eg
CuS
= 1.50 eV
(2)
Band diagram of the fabricated n-Si–p-CuS heterojunction device.
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Fig. 7 (a) Spectral responsivity of the cone-like radial heterojunction device and control device at zero bias. (b) EQE spectra and detectivity of the cone-like radial heterojunction and control devices at zero bias.
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heterojunction device. The peak responsivity value of our nanocone device at zero bias is much higher than that recently reported for radial heterojunctions fabricated on Si nanowires e.g., Si–CdS23 (0.1 A W1) and Si–AgInSe253 (0.11 A W1). The spectral response consists of several broad peaks, as due to the absorption region of CuS (B550 nm), porous nature of nanocones (B600–750 nm) and self absorption region of Si. The responsivity plots indicate the dominant effect of absorption due to Si over that of CuS, as responsivity is found to be higher at 990 nm than at 550 nm for both the devices. External quantum efficiency (EQE) of the photodiodes has been calculated using the relation: EQEð%Þ ¼
100 1240 Jph 100 1240 Rl ¼ l Popt l
(4)
From the EQE spectra shown in Fig. 7(b), it was noticed that the peak value of EQE is more than 40% for the nanostructured device, whereas it is 20% for the control device at zero bias. The EQE almost remains the same up to nearly 550 nm for both the devices, but after 550 nm there is a sharp increase in the EQE of the nanostructured device, indicating the effect of enhanced absorption of Si nanocones in this range. Obviously, the photocurrent is higher in the radial heterojunction because of the high absorption for Si nano-cones as compared to planar Si, as demonstrated in Fig. 2(a). The parameter characterizing normalized signal-to-noise performance of a detector is known as the detectivity (D). Considering the contribution of shot noise to the dark current as the major one, the detectivity for a particular wavelength can be expressed as: RðlÞ DðlÞ ¼ pffiffiffiffiffiffiffiffiffiffi 2qJd
(5)
where q is the charge of an electron and Jd is the dark current density. Detectivity values of the devices, extracted using the above eqn, are also presented in Fig. 7(b). The peak detectivity is found to be 2.7 1010 and 1.4 1010 cm Hz1/2 W1 for the cone-like radial heterojunction device and control device, respectively, under the illumination of 990 nm light. The detectivity value is relatively low because of the large dark current in heterojunctions. The detectivity is found to be higher for the cone-like radial heterojunction device as compared to the control one, due to the higher photo response from the radial nanocone heterojunction with the dark current values remaining almost in the same order. The behaviour of n-Si–p-CuS nanocone radial heterojunction diodes under illumination indicates that the device can be useful for photodetection applications. The heteroepitaxial growth of CuS on Si using a refined technique and the optimization of interfacial properties may lead to the significant improvement of responsivity and detectivity of the radial heterojunction devices for use in nanophotonic devices.
Conclusion In summary, we have successfully synthesized wafer scale, highly antireflective, cone-like nanostructures on n-Si wafers
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using a low cost, simple Au catalyzed chemical etching method. The formation of highly dense vertically oriented cone-like Si nanostructures with nanoporous surfaces is presented. The photodiode characteristics of the n-Si–p-CuS heterojunction on fabricated cone-like nanotextured Si have been studied. The peak responsivity of 0.32 A W1 with a maximum EQE of 40% is found for the nanostructured device at 990 nm with zero applied bias, showing significant improvement over the control planar device. The nanostructured device shows improved responsivity and EQE due to enhanced light absorption and efficient photo-carrier extraction in the nanocone radial heterojunction.
Acknowledgements This work was supported by DST SERI sponsored ‘‘NSH’’ project. The XPS facility of the DST ‘‘FIST’’ project is also gratefully acknowledged.
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