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Black germanium produced by inductively coupled plasma etching
Materials Letters 94 (2013) 86–88
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Black germanium produced by inductively coupled plasma etching Sandra Schicho, Abdelatif Jaouad n, Christian Sellmer, Denis Morris, Vincent Aimez, Richard Are s Centre de Recherches en Nanofabrication et Nanocaracte´risation (CRN2), Institut Interdisciplinaire d’Innovation Technologique (3IT), Universite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J1K 2R1
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2012 Accepted 6 December 2012 Available online 20 December 2012
We report on a lithography-free self-organized inductively coupled plasma (ICP) etching process to manufacture nanostructured Ge surfaces with a strong absorbance in the visible and near-infrared wavelength region, so-called black germanium. Scanning electron micrographs reveal the presence of vertical tapered needles, a few tens of mm long and with a mean width of about 500 nm. The samples show very low reflectance and superhydrophobic characteristics that open up a wide range of electrooptical and microfluidic applications. Contrary to bulk Ge, black Ge shows a low temperature photoluminescence emission band at 650 nm, which can be a sign of quantum confinement or of recombination in GeOx-associated defects. Obtaining black Ge by ICP allows adjusting physical properties of the layer by controlling the plasma parameters. & 2012 Elsevier B.V. All rights reserved.
Keywords: Black Ge Superhydrophobicity ICP etching High-energy photoluminescence
1. Introduction Black silicon—a needle-like surface modification of crystalline silicon—was discovered as an initially unwanted side effect of RIE deep etching of silicon [1]. Today, its high absorbance is used for image sensors with increased sensitivity and thermal imaging cameras [2]. Furthermore, it is used in high-efficiency photodetectors [3,4] and terahertz technology [5]. Due to its similar properties similar applications can be considered for black germanium that would allow, e.g., to extend the detection range of photodetectors further into the infrared wavelength region. Conventional multilayer antireflective coatings (ARCs) for, e.g., solar cells are incident angle- and wavelength-dependent, and a stepped refractive index approach is used. Black Ge can potentially replace these complicated ARC stacks since it does not show these limitations. First silicon solar cells using black Si have already been demonstrated by NREL [6] and other laboratories [7,8]. Due to the fact that the low reflectance stretches over a wider wavelength region, nanostructured Ge could replace black Si as a more effective ARC. The superhydrophobicity of black Ge is worth mentioning ,which makes it appealing for microfluidic devices where a very low flow resistance is required for minimal energy consumption [9,10], and the large surface-to-volume ratio, which suggests an application as durable electrode for, e.g., lithium-ion batteries with prolonged lifetimes [11]. Already in 1978 Gilbert et al. reported on black Ge surfaces that were developed after etching sputtered non-crystalline Ge
n
Corresponding author. E-mail address: [email protected] (A. Jaouad).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.014
films in H2O2 [12]. Chueh et al. achieved nanoneedle-like, black appearing Ge surfaces by using a Ni-catalyzed vapor deposition process [13]. With both methods the nanoneedle structure is hard to control. With laser etching under SF6 atmosphere, conical microstructures with nanospikes on top were achieved by Nayak et al. Varying laser fluence and number of shots provides the potential to change the appearance, density or regularity of the features [14]. We present an ICP process to achieve black germanium with a high reproducibility. Two power supply sources (coil and platen) regulate the chemical etching by neutrals and radicals and physical etching by ions separately. Hence, a good control over the etched structure can be realized.
2. Experimental Small pieces of p-doped germanium were etched using a multiplex advanced silicon etcher (Surface Technology Systems). The used etching process is derived from the so-called BoschProcess for deep, anisotropic etching of Si and is based on the alternation of several etching and passivation cycles [15]. The parameters to achieve black Ge are summarized in Table 1. To evaluate the hydrophobicity, contact angle measurements were carried out by disposing a droplet of either water or methanol onto the Ge surface. A high resolution image was taken and the contact angle was determined by a software. Photoluminescence was measured at room temperature using a commercial Philips PLM-150 mapper. A Nd:YAG laser emitting at 1064 nm served as excitation source while the luminescence was detected by an InGaAs photodiode array. Additional temperature-dependent PL
S. Schicho et al. / Materials Letters 94 (2013) 86–88
measurements at 650 nm were carried out using a green diode laser (l ¼532 nm) as excitation source. The laser power was kept at 15 mW and a liquid nitrogen cooled Ge detector recorded the PL emission with lock-in technique.
3. Results and discussion After etching a polished Ge piece for 20 min (¼ 60 etching/ passivation cycles) the visual shiny surface changed to totally black after the process. Fig. 1a and b show a cross-sectional and top view SEM image, respectively. Nanoneedles with a height of about 30 mm and a width of a few hundred nm with a sharp tip are visible. Hence, the aspect ratio of this structure is much higher compared to the structure manufactured with laser etching. The reported conical microstructure was 10–15 mm high and 5 mm wide with a 1.5 mm high nanotip [14]. In Fig. 1a we also see that the vertical nanoneedles are arranged in a dense array with short distances, generally below 1 mm, between the needles. The total reflectivity R of black Ge was measured in the wavelength region between 300 and 1800 nm. In Fig. 2 the reflectivity spectra of a black Ge sample and an untreated reference sample are compared. It can be seen that the total reflectivity of the black Ge over the whole measured spectrum is significantly lower than the one of the reference. Up to a wavelength of 1400 nm the reflectivity lies below 2.5%, rising to only 13% at 1800 nm. Similar results were published previously [12,13]. The reduced reflectivity observed for black Ge is explained by a gradual change of the refractive index of the needle array medium from 1 to about 4, which correspond to the refractive index values for air and bulk Ge, respectively [16]. The reflectivity increases for both Ge samples above 1550 nm since the Ge samples become transparent below the direct band gap. Hence, light that is backscattered from the interface at the back side of the sample is detected [17,18]. For un-etched Ge a contact angle between a water droplet and the surface of 901 was found, whereas it was not possible to disperse a water droplet onto the surface of black Ge. The water droplet would touch the surface, but would not stick to it. Hence, we estimate a contact angle very close to 1801. It is reasonable to assume that this superhydrophobicity follows the Cassie–Baxter
87
model in which the liquid touches only the tips of a microstructured surface [19]. Since the nanoneedles are several tens of mm high and aligned in a dense array, they prevent the droplet from touching the base of the surface. With methanol, a much more volatile liquid than water, a static contact angle of 901 was measured. The optical properties of the black Ge have been investigated using photoluminescence (PL) measurements. Fig. 3 shows the PL spectra of the samples in the range between 1400 and 1800 nm. A peak is visible for the etched and reference samples at around 1615 nm, which corresponds to an energy value of 0.77 eV. This energy value matches well with the direct bandgap of crystalline Ge with a doping concentration of 5 1018 cm 3 [20]. Furthermore, the observed peak coincides with the threshold wavelength at which both Ge samples become transparent (Fig. 2). Photoluminescence measurements were then carried out in the wavelength region between 550 nm and 800 nm. Fig. 4 shows the PL spectra measured at temperatures ranging from 20 K to 300 K. A broad emission band is observed at 650 nm for the black Ge sample at all temperatures whereas no PL signal was found for the un-etched reference. Such a high-energy PL band was also observed for Si nanostructures and was attributed to quantum confinement [21–23]. Strong confinement of carriers in nanometer-size regions can give rise to an intense blue-shifted PL band. In the case of black Ge, the emission most probably originates from carrier recombination in the sharp tip’s region of the nanoneedles. Local fluctuation of the band gap energy seems to trap carriers in this high-energy band gap region, where they
Table 1 Parameter used during the etching. Cycle
t (s)
p (mTorr)
SF6-flow (sccm)
O2-flow (sccm)
C4F8-flow (sccm)]
Pcoil (W)
Pplaten (W)
Etch Passivation
13 7
30 20
130 –
13 –
– 85
800 600
5 –
Fig. 2. Reflectivity spectra of a black Ge sample (black straight curve) and a polished Ge wafer (red dashed curve) (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).
Fig. 1. (a) Cross-sectional and (b) top view of Ge etched for 20 min.
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S. Schicho et al. / Materials Letters 94 (2013) 86–88
increasing temperature as observed for black silicon [23]. Carriers get thermally activated out of the luminescent regions and diffuse towards non-radiative defect centers.
4. Conclusion Black germanium surfaces were accomplished by ICP etching with good reproducibility which also offers a high degree of controllability of the nanostructures. The samples show very low reflectivity and superhydrophobicity, opening up a wide range of applications. Photoluminescence measurements showed an emission band at the position of the direct bandgap of bulk Ge and an additional emission band at 650 nm (only for black Ge) which is explained by quantum confinement in the small tips of the nanoneedles or by recombination mechanisms involving GeOx defects. Fig. 3. Room-temperature PL spectra of a black and an un-etched Ge sample.
Fig. 4. Temperature-dependent PL spectra of the black Ge.
can recombine radiatively. Bound excitons at defect sites in the nanoneedles might add an extra confinement. The PL peak at 650 nm could also stem from recombination processes due to GeO2-associated defects [24]. The absence of the peak for the reference sample is then explained by a much smaller surface at which these oxide defects can occur. Due to thermal quenching the intensity of the PL band decreases with
References [1] Jansen H, de Boer M, Legtenberg R, Elwenspoek M. J Micromech Microeng 1995;5:115–20. [2] Pralle MU, Carey JE, Homayoon H, Alie S, Sickler J, Li X, et al. Proc SPIE 2010;7660:76600N–3N. [3] Koynov S, Brandt MS, Stutzmann M. Appl Phy Lett 2006;88:2031071–3. [4] Huang Z, Carey JE, Liu M, Guo X, Mazur E, Campbell JC. Appl Phys Lett 2006;89:33506. [5] Hoyer P, Theuer M, Beigang R, Kley EB. Appl Phys Lett 2008;93 091106-3. [6] Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM. Appl Phys Lett 2009;95:123501–3. [7] Toor F, Branz HM, Page MR, Jones KM, Yuan HC. Appl Phys Lett 2011;99: 103501–103503. [8] Koynov S, Brandt MS, Stutzmann M. Phys States Solidi (RRL) 2007;1:R53–5. [9] Kim J, Kim C. Proceedings of the 15th IEEE international conference on micro electro mechanical systems, 2002 pp. 479–82. [10] Choi C, Kim C. Phys Rev Lett 2006;96:066001–6. ¨ , Talalaev V, Nilsson B, Hagberg M, et al. [11] Kalem S, Werner P, Arthursson O Nanotechnology 2011;22:235307–8. [12] Gilbert LR, Messier R, Roy R. Thin Solid Films 1978;54:149–57. [13] Chueh YL, Fan Z, Takei K, Ko H, Kapadia R, Rathore AA, et al. Nano Lett 2010;10:520–3. [14] Nayak BK, Gupta MC, Kolanski KW. Nanotechnology 2007;18:195302–4. [15] Laermer F, Schilp A. WO/1994/014187 JP, US, European Patent Office, 06 23, 1994. [16] Lafuma A, Que´re´ D. Nat Mater 2003;2:457–60. [17] Leem JW, Song YM, Yu JS. Opt Express 2011;19:26308–17. [18] Kuo ML, Poxson DJ, Kim YS, Mont FW, Kim JK, Schubert EF, et al. Opt Lett 2008;33:2527–9. [19] Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546–51. [20] Jain SC, Roulston J. Solid State Electron 1991;34:453–65. [21] Medvid A, Dmytruk I, Onufrijevs P, Pundyuk I. Phys Stat Sol C 2007;6:3066–9. [22] Wang B, Leu PW. Nanotechnology 2012;23:194003–7. ¨ [23] Serpenguzel A, Kurt A, Inanc- I, Carey JE, Mazur E. J Nanophoton 2008;2:021770–9. [24] Choi WK, Ng V, Ng SP, Thio HH, Shen ZX, Li WS. J Appl Phys 1999;86: s1398–1403.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Black germanium produced by inductively coupled plasma etching Sandra Schicho, Abdelatif Jaouad n, Christian Sellmer, Denis Morris, Vincent Aimez, Richard Are s Centre de Recherches en Nanofabrication et Nanocaracte´risation (CRN2), Institut Interdisciplinaire d’Innovation Technologique (3IT), Universite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J1K 2R1
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 September 2012 Accepted 6 December 2012 Available online 20 December 2012
We report on a lithography-free self-organized inductively coupled plasma (ICP) etching process to manufacture nanostructured Ge surfaces with a strong absorbance in the visible and near-infrared wavelength region, so-called black germanium. Scanning electron micrographs reveal the presence of vertical tapered needles, a few tens of mm long and with a mean width of about 500 nm. The samples show very low reflectance and superhydrophobic characteristics that open up a wide range of electrooptical and microfluidic applications. Contrary to bulk Ge, black Ge shows a low temperature photoluminescence emission band at 650 nm, which can be a sign of quantum confinement or of recombination in GeOx-associated defects. Obtaining black Ge by ICP allows adjusting physical properties of the layer by controlling the plasma parameters. & 2012 Elsevier B.V. All rights reserved.
Keywords: Black Ge Superhydrophobicity ICP etching High-energy photoluminescence
1. Introduction Black silicon—a needle-like surface modification of crystalline silicon—was discovered as an initially unwanted side effect of RIE deep etching of silicon [1]. Today, its high absorbance is used for image sensors with increased sensitivity and thermal imaging cameras [2]. Furthermore, it is used in high-efficiency photodetectors [3,4] and terahertz technology [5]. Due to its similar properties similar applications can be considered for black germanium that would allow, e.g., to extend the detection range of photodetectors further into the infrared wavelength region. Conventional multilayer antireflective coatings (ARCs) for, e.g., solar cells are incident angle- and wavelength-dependent, and a stepped refractive index approach is used. Black Ge can potentially replace these complicated ARC stacks since it does not show these limitations. First silicon solar cells using black Si have already been demonstrated by NREL [6] and other laboratories [7,8]. Due to the fact that the low reflectance stretches over a wider wavelength region, nanostructured Ge could replace black Si as a more effective ARC. The superhydrophobicity of black Ge is worth mentioning ,which makes it appealing for microfluidic devices where a very low flow resistance is required for minimal energy consumption [9,10], and the large surface-to-volume ratio, which suggests an application as durable electrode for, e.g., lithium-ion batteries with prolonged lifetimes [11]. Already in 1978 Gilbert et al. reported on black Ge surfaces that were developed after etching sputtered non-crystalline Ge
n
Corresponding author. E-mail address: [email protected] (A. Jaouad).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.014
films in H2O2 [12]. Chueh et al. achieved nanoneedle-like, black appearing Ge surfaces by using a Ni-catalyzed vapor deposition process [13]. With both methods the nanoneedle structure is hard to control. With laser etching under SF6 atmosphere, conical microstructures with nanospikes on top were achieved by Nayak et al. Varying laser fluence and number of shots provides the potential to change the appearance, density or regularity of the features [14]. We present an ICP process to achieve black germanium with a high reproducibility. Two power supply sources (coil and platen) regulate the chemical etching by neutrals and radicals and physical etching by ions separately. Hence, a good control over the etched structure can be realized.
2. Experimental Small pieces of p-doped germanium were etched using a multiplex advanced silicon etcher (Surface Technology Systems). The used etching process is derived from the so-called BoschProcess for deep, anisotropic etching of Si and is based on the alternation of several etching and passivation cycles [15]. The parameters to achieve black Ge are summarized in Table 1. To evaluate the hydrophobicity, contact angle measurements were carried out by disposing a droplet of either water or methanol onto the Ge surface. A high resolution image was taken and the contact angle was determined by a software. Photoluminescence was measured at room temperature using a commercial Philips PLM-150 mapper. A Nd:YAG laser emitting at 1064 nm served as excitation source while the luminescence was detected by an InGaAs photodiode array. Additional temperature-dependent PL
S. Schicho et al. / Materials Letters 94 (2013) 86–88
measurements at 650 nm were carried out using a green diode laser (l ¼532 nm) as excitation source. The laser power was kept at 15 mW and a liquid nitrogen cooled Ge detector recorded the PL emission with lock-in technique.
3. Results and discussion After etching a polished Ge piece for 20 min (¼ 60 etching/ passivation cycles) the visual shiny surface changed to totally black after the process. Fig. 1a and b show a cross-sectional and top view SEM image, respectively. Nanoneedles with a height of about 30 mm and a width of a few hundred nm with a sharp tip are visible. Hence, the aspect ratio of this structure is much higher compared to the structure manufactured with laser etching. The reported conical microstructure was 10–15 mm high and 5 mm wide with a 1.5 mm high nanotip [14]. In Fig. 1a we also see that the vertical nanoneedles are arranged in a dense array with short distances, generally below 1 mm, between the needles. The total reflectivity R of black Ge was measured in the wavelength region between 300 and 1800 nm. In Fig. 2 the reflectivity spectra of a black Ge sample and an untreated reference sample are compared. It can be seen that the total reflectivity of the black Ge over the whole measured spectrum is significantly lower than the one of the reference. Up to a wavelength of 1400 nm the reflectivity lies below 2.5%, rising to only 13% at 1800 nm. Similar results were published previously [12,13]. The reduced reflectivity observed for black Ge is explained by a gradual change of the refractive index of the needle array medium from 1 to about 4, which correspond to the refractive index values for air and bulk Ge, respectively [16]. The reflectivity increases for both Ge samples above 1550 nm since the Ge samples become transparent below the direct band gap. Hence, light that is backscattered from the interface at the back side of the sample is detected [17,18]. For un-etched Ge a contact angle between a water droplet and the surface of 901 was found, whereas it was not possible to disperse a water droplet onto the surface of black Ge. The water droplet would touch the surface, but would not stick to it. Hence, we estimate a contact angle very close to 1801. It is reasonable to assume that this superhydrophobicity follows the Cassie–Baxter
87
model in which the liquid touches only the tips of a microstructured surface [19]. Since the nanoneedles are several tens of mm high and aligned in a dense array, they prevent the droplet from touching the base of the surface. With methanol, a much more volatile liquid than water, a static contact angle of 901 was measured. The optical properties of the black Ge have been investigated using photoluminescence (PL) measurements. Fig. 3 shows the PL spectra of the samples in the range between 1400 and 1800 nm. A peak is visible for the etched and reference samples at around 1615 nm, which corresponds to an energy value of 0.77 eV. This energy value matches well with the direct bandgap of crystalline Ge with a doping concentration of 5 1018 cm 3 [20]. Furthermore, the observed peak coincides with the threshold wavelength at which both Ge samples become transparent (Fig. 2). Photoluminescence measurements were then carried out in the wavelength region between 550 nm and 800 nm. Fig. 4 shows the PL spectra measured at temperatures ranging from 20 K to 300 K. A broad emission band is observed at 650 nm for the black Ge sample at all temperatures whereas no PL signal was found for the un-etched reference. Such a high-energy PL band was also observed for Si nanostructures and was attributed to quantum confinement [21–23]. Strong confinement of carriers in nanometer-size regions can give rise to an intense blue-shifted PL band. In the case of black Ge, the emission most probably originates from carrier recombination in the sharp tip’s region of the nanoneedles. Local fluctuation of the band gap energy seems to trap carriers in this high-energy band gap region, where they
Table 1 Parameter used during the etching. Cycle
t (s)
p (mTorr)
SF6-flow (sccm)
O2-flow (sccm)
C4F8-flow (sccm)]
Pcoil (W)
Pplaten (W)
Etch Passivation
13 7
30 20
130 –
13 –
– 85
800 600
5 –
Fig. 2. Reflectivity spectra of a black Ge sample (black straight curve) and a polished Ge wafer (red dashed curve) (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).
Fig. 1. (a) Cross-sectional and (b) top view of Ge etched for 20 min.
88
S. Schicho et al. / Materials Letters 94 (2013) 86–88
increasing temperature as observed for black silicon [23]. Carriers get thermally activated out of the luminescent regions and diffuse towards non-radiative defect centers.
4. Conclusion Black germanium surfaces were accomplished by ICP etching with good reproducibility which also offers a high degree of controllability of the nanostructures. The samples show very low reflectivity and superhydrophobicity, opening up a wide range of applications. Photoluminescence measurements showed an emission band at the position of the direct bandgap of bulk Ge and an additional emission band at 650 nm (only for black Ge) which is explained by quantum confinement in the small tips of the nanoneedles or by recombination mechanisms involving GeOx defects. Fig. 3. Room-temperature PL spectra of a black and an un-etched Ge sample.
Fig. 4. Temperature-dependent PL spectra of the black Ge.
can recombine radiatively. Bound excitons at defect sites in the nanoneedles might add an extra confinement. The PL peak at 650 nm could also stem from recombination processes due to GeO2-associated defects [24]. The absence of the peak for the reference sample is then explained by a much smaller surface at which these oxide defects can occur. Due to thermal quenching the intensity of the PL band decreases with
References [1] Jansen H, de Boer M, Legtenberg R, Elwenspoek M. J Micromech Microeng 1995;5:115–20. [2] Pralle MU, Carey JE, Homayoon H, Alie S, Sickler J, Li X, et al. Proc SPIE 2010;7660:76600N–3N. [3] Koynov S, Brandt MS, Stutzmann M. Appl Phy Lett 2006;88:2031071–3. [4] Huang Z, Carey JE, Liu M, Guo X, Mazur E, Campbell JC. Appl Phys Lett 2006;89:33506. [5] Hoyer P, Theuer M, Beigang R, Kley EB. Appl Phys Lett 2008;93 091106-3. [6] Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM. Appl Phys Lett 2009;95:123501–3. [7] Toor F, Branz HM, Page MR, Jones KM, Yuan HC. Appl Phys Lett 2011;99: 103501–103503. [8] Koynov S, Brandt MS, Stutzmann M. Phys States Solidi (RRL) 2007;1:R53–5. [9] Kim J, Kim C. Proceedings of the 15th IEEE international conference on micro electro mechanical systems, 2002 pp. 479–82. [10] Choi C, Kim C. Phys Rev Lett 2006;96:066001–6. ¨ , Talalaev V, Nilsson B, Hagberg M, et al. [11] Kalem S, Werner P, Arthursson O Nanotechnology 2011;22:235307–8. [12] Gilbert LR, Messier R, Roy R. Thin Solid Films 1978;54:149–57. [13] Chueh YL, Fan Z, Takei K, Ko H, Kapadia R, Rathore AA, et al. Nano Lett 2010;10:520–3. [14] Nayak BK, Gupta MC, Kolanski KW. Nanotechnology 2007;18:195302–4. [15] Laermer F, Schilp A. WO/1994/014187 JP, US, European Patent Office, 06 23, 1994. [16] Lafuma A, Que´re´ D. Nat Mater 2003;2:457–60. [17] Leem JW, Song YM, Yu JS. Opt Express 2011;19:26308–17. [18] Kuo ML, Poxson DJ, Kim YS, Mont FW, Kim JK, Schubert EF, et al. Opt Lett 2008;33:2527–9. [19] Cassie ABD, Baxter S. Trans Faraday Soc 1944;40:546–51. [20] Jain SC, Roulston J. Solid State Electron 1991;34:453–65. [21] Medvid A, Dmytruk I, Onufrijevs P, Pundyuk I. Phys Stat Sol C 2007;6:3066–9. [22] Wang B, Leu PW. Nanotechnology 2012;23:194003–7. ¨ [23] Serpenguzel A, Kurt A, Inanc- I, Carey JE, Mazur E. J Nanophoton 2008;2:021770–9. [24] Choi WK, Ng V, Ng SP, Thio HH, Shen ZX, Li WS. J Appl Phys 1999;86: s1398–1403.