Applied Surface Science 258 (2012) 8002–8007
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructuring and doping of silicon with nanosecond laser pulses Xiaohong Li a,∗ , Liyang Chang a , Rong Qiu a , Cai Wen a , Zhihui Li b , Sifu Hu a a b
School of Science, Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Mianyang 621010, China Accelerator Center, Institute of High Energy Physics CAS, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 23 February 2012 Received in revised form 25 April 2012 Accepted 25 April 2012 Available online 11 May 2012 PACS: 61.80.Ba 79.20.Ds 68.35.bg
a b s t r a c t We microstructure and dope silicon surfaces in SF6 atmosphere using nanosecond Nd:YAG laser pulses. The effects of scanning speed and laser pulse energy on surface morphology, optical and electronic properties of laser treated silicon are studied. When the scanning speed is 0.2 mm/s and the laser energy is 290 mJ, the absorptance of microstructured silicon can reach 90% in the visible spectrum and 80% in the infrared spectrum. In addition, its Hall mobility is measured as about 600 cm2 V−1 s−1 . The electron diffraction shows that the irradiated silicon surface is crystalline and no disordered surface layer is found, which is good for optoelectronic applications. © 2012 Elsevier B.V. All rights reserved.
Keywords: Silicon Nanosecond laser pulse Microstructuring Doping Optical properties Electronic properties
1. Introduction Laser surface microstructuring has been shown to be a clean and versatile surface structuring method, and it is effective and applicable to almost all solid materials [1]. Surface microstructuring leads to interesting and useful new features of materials such as low reflection [2], high light and electron emittance [3–5], special wetting properties [6,7], which enlarge their application domain [8–10]. As one of the most important semiconductors, silicon is widely used in microelectronics and optoelectronics. Irradiation of a silicon surface with laser pulses in various atmospheres can not only microstructure but also dope it, changing its optical and electronic properties dramatically. It is well known that silicon cannot effectively absorb the near-infrared radiation due to its band gap of 1.12 eV. Structuring silicon with laser pulses in chalcogen contained environment can greatly enhance the optical absorption in a wide spectrum range, especially in near-infrared range, which is interested for surveillance, military, and energy device applications [11,12]. One reason for the optical absorption enhancement of laser structured silicon is that surface structures decrease the reflection of silicon, and another reason is that doping silicon with
∗ Corresponding author. E-mail address: li
[email protected] (X. Li). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.155
the chalcogen elements causes the mid-band absorption and makes the near-infrared absorption in silicon possible. This optical property is desirable for silicon to be used in optoelectronic devices, for example, photodectors [13,14] and solar cells [15,16]. Structuring silicon is mostly done with high repetition frequency femtosecond (fs) lasers [13–16] in which laser pulses are focused into a circular spot to scan the silicon surface so that a large structured area can be obtained. Femtosecond lasers with high repetition frequency usually have low pulse energy and then the focused spot diameter is around a hundred micrometers. Compared with femtosecond lasers, nanosecond (ns) lasers can provide much stronger pulses, and they are more easily available and much cheaper. If a nanosecond laser is used to microstructure and dope silicon, low cost and high efficiency can be expected. In this work, a Nd:YAG nanosecond pulsed laser with low repetition frequency (10 Hz) but high pulse energy (the maximum 500 mJ for the wavelength of 532 nm) is used to microstructure and dope monocrystalline silicon. With the advantage of high pulse energy, the YAG laser pulse can be focused by cylindrical lens into a rectangle which is more than one centimeter long and several millimeters wide in maximum. In this way, a 2 in. silicon wafer can be scanned in several minutes. All the scanning is done under sulfur hexafluoride atmosphere and the effects of scanning speed and laser pulse energy on morphology, optical and electronic properties, and crystallinity are studied.
X. Li et al. / Applied Surface Science 258 (2012) 8002–8007
8003
Fig. 1. SEM images of samples made with different scanning speeds: (a) and (b) 0.1 mm/s; (c) and (d) 0.2 mm/s; (e) and (f) 0.4 mm/s. The lefts and rights are the top and side view images, respectively. The incident laser pulse energy was kept at 290 mJ.
2. Experimental A Nd:YAG nanosecond laser is employed to provide laser pulses with duration 10 ns and central wavelength 532 nm. The laser pulse has Gaussian spatial profile and its repetition frequency is 10 Hz. The 4 in. 100 n-type silicon wafer with thickness 525 m is arsenic doped and its resistivity is 2–4 cm. One side of silicon wafers is mechanically polished and the other side is chemically polished. Before the experiments, the silicon wafers were cleaved into 25 mm × 25 mm plates and the plates were first cleaned with acetone, methanol and deionized water, then dipped in the hydrofluoric acid to remove native silicon dioxide from wafers and followed by a deionized water rinse. After blow drying with nitrogen gas, the plates were put in vacuum chamber filled with the SF6 atmosphere of 1 atm pressure. The whole vacuum chamber was fixed on the stage of a high precision computer controlled 3-dimensional electrical translation (Newport). The laser beam was expanded by using a convex lens with a long focal length of 2 m before it passed through a cylindrical lens with 40 cm focal length to be focused into a rectangle. The focused laser beam was normally incident onto the sample surface and the focal line in the experiments was 12 mm long and about 0.1 mm wide. A large area of structured surface is directly obtained by translating the samples relatively to the stationary laser beam along the horizontal direction. Changing scanning speed can produce different
microstructures with various depths. The incident laser fluence is controlled by changing the pulse energy. After irradiation, the reflection and transmission spectra were measured with UVVIS-NIR spectrometer (Shimadzu UV3150, Japan). The electrical properties were measured with four-point probe method and Hall effects. The surface morphology was observed using a Leica Cambridge S440 scanning electron microscope (SEM). The crosssectional transmission electron microscopy (TEM) observations were performed using a JEM-2010 LaB6 electron microscope with a spherical aberration coefficient of Cs = 0.5 mm and the point resolution of 0.194 nm. The sulfur content in irradiated silicon surfaces was studied by energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion The SEM images of the samples made with different scanning speeds are given in Fig. 1. The incident laser energy was kept at 290 mJ when the scanning speed varied. The left images are taken from the top of the samples and the rights from the side. Three types of morphologies are identified, namely, cavities, spherical particles and irregular protrusions. These structures are closely related with the mechanism of ns laser ablation of silicon. The ns laser ablation of solids can be classified into volumetric ablation and superficial ablation based on their thermal properties when the laser fluence is
8004
X. Li et al. / Applied Surface Science 258 (2012) 8002–8007
above their ablation threshold. The volumetric ablation is defined by the condition [17],
1 , ˛
(1)
where , and ˛ are the thermal diffusivity, the laser pulse length and the absorption coefficient, respectively. For superficial ablation, it is
1 . ˛
(2)
From these two equations we can see that the weakly absorbed laser wavelength will favor the volumetric ablation and the strongly absorbed laser wavelength will prefer the superficial ablation. In the volumetric ablation, the thick and hot liquid layer will form on the surface and the subsurface superheating will induce the subsurface volume boiling, which produces large liquid droplets and cavities at the same time. The sizes of cavities and resolidificated liquid droplets are both in the range of micrometers [18]. Compared with volumetric ablation, the liquid layer in superficial ablation is thin and the subsurface volume boiling is difficult to form, which results in a smooth and featureless surface. The absorption coefficients of silicon for three common wavelengths of a pulsed Nd:YAG laser, that is, 1064 nm (the fundamental wavelength), 532 nm (the wavelength of the second harmonic) and 355 nm (the wavelength of the third harmonic), are about 7.3 cm−1 , 6.7E + 03 cm−1 and
Fig. 2. A magnification side-view SEM image of the sample made with the scanning speed of 0.1 mm/s and the laser pulse energy of 290 mJ showing the spheres on the structured silicon surface.
1.05E + 06 cm−1 , respectively [19]. Our Nd:YAG laser has the laser pulse length of 10 ns and the silicon’s thermal diffusivity at 300 K is 0.8 cm2 /s. So the 1064 nm irradiation of silicon causes the volumetric ablation and 355 nm the superficial ablation. For the 532 nm
Fig. 3. SEM images of samples made with different incident laser pulse energies: (a) and (b) 173 mJ; (c) and (d) 333 mJ; (e) and (f) 425 mJ. The lefts and rights are the top and side view images, respectively. The scanning speed was kept at 0.3 mm/s.
X. Li et al. / Applied Surface Science 258 (2012) 8002–8007
Fig. 4. The absorption spectra of samples made with different scanning speeds.
√ irradiation is close to (1/˛) and Ref. [20] reported that the silicon surface morphology after 532 nm irradiation is similar with that of 355 irradiation and different from our results. This difference should come from the different atmospheres during the irradiation. The authors in Ref. [20] did the experiments in air or vacuum and our experiments were made in SF6 atmosphere. Our experiments performed in vacuum [21,22] obtained the same results as those in Ref. [20]. In the volumetric ablation, the particles produced by the violent ejection are usually sparsely dispersed on the surface, as shown in the images of Ref. [23] taken with a high speed framing camera. Besides this type of particles due to subsurface volume boiling, we can also see another type of particles in Fig. 2, the magnification of Fig. 1b. This type of particles likes to aggregate together into a cluster and has smaller size. They may redeposit from the laser induced vapor plume or resolidify from the melt. The slower the scanning speed, the more these particles. With increasing the scanning speed to 0.2 mm/s, the surface is more clean and the protruded structures like penguin can be clearly identified. These rounded-top microspikes are similar to the structures produced by femtosecond laser surface structuring in vacuum [16]. In the environment of sulfur hexafluoride, femtosecond laser surface structuring usually produces sharp conical microspikes [24]. Although the formation mechanism of these quasi-ordered microspikes is still not well understood, the melt flow and resolidification should be important for producing these protrusions [25,26]. Further increasing scanning speed induces shallow
Fig. 5. The absorption spectra of samples made with different incident laser energies.
8005
Fig. 6. Dependence of the sheet resistance of laser irradiated silicon on the scanning speed.
channels and smaller microspikes. The microspikes have to form after multiple-laser pulse irradiation but too many laser pulses will ruin the surface structures and make the surface smooth again [27,1]. The scanning speed is directly related with the average number of laser pulses per unit area N by N=
df , V
(3)
where d is the width of laser spot, f is the laser frequency and V is the scanning speed. The higher scanning speed decreases the average number of laser shots on the surface and produces the smaller microspikes. The formation of microspikes not only depends on the number of laser pulses, but also on the laser fluence. They just can form in a small range of laser fluence. Therefore, when the laser pulse energy changes from the low of 173 mJ to the high of 425 mJ, the SEM images in Fig. 3 show the heights of the microspikes on the silicon surface first become larger then smaller. The reflection and transmission spectra from 240 to 2400 nm were measured, then the absorption spectra are calculated using the relation A = 1 − R − T and given in Figs. 4 and 5. Here A, R and T represent absorbtance, reflectance and transmittance, respectively. In the range of scanning speed from 0.1 to 0.4 mm/s, the reflectance decreases with increasing the scanning speed and then the absorbtance increases. When the laser energy varies, the situation is a little different. When the laser pulse energy increases from 173 to 380 mJ, the reflectance decreases in the whole measured spectrum
Fig. 7. Dependence of the sheet resistance of laser irradiated silicon on the incident laser energy.
8006
X. Li et al. / Applied Surface Science 258 (2012) 8002–8007
Fig. 8. Bright-field transmission electron micrograph: cross section of ns-laser-formed microstructure. Inset: selected area electron diffraction pattern obtained from the tip of the microstructure, indicated by the ellipse area.
range. Further increasing the laser energy makes the reflectance a bit higher. The dependence of the absorbtance on laser pulse energy is consistent with the SEM results. The optical property of silicon is closely related with its surface structures. The surface-structureinduced roughness makes the multiple reflections possible and changes the specular reflection of the flat unstructured surface to the diffuse reflection of the laser structured surface. The more obvious the surface structure, the lower reflectance and the higher absorbtance. Since the high laser pulse energy degrades the surface structure and makes the silicon surface flatter, the reflectance of the samples irradiated with high pulse energy should be higher than that with low laser pulse energy. In order to obtain the extremely low reflectance, the special microstructure on the surface, especially the spike microstructure, is desirable. Although the deeper spike microstructure can benefit the optical properties, the larger surface area will increase minority carrier recombination in the surface and junction regions, which is disadvantageous for silicon optoelectronics applications. The sulfur hexafluoride gas is not only important for structuring the silicon but also responsible for doping the silicon. As a group of VI element, sulfur can give rise to two donor impurity levels. These mid-gap energy levels expand the silicon’s absorption to near-infrared light with the wavelength longer than 1100 nm as we have shown in Figs. 4 and 5 [28,29]. The sulfur doping in silicon surface also adjusts the surface electronic properties and hence we took the measurement of sheet resistance and Hall effect. Figs. 6 and 7 give the sheet resistance versus the scanning speed and the laser energy, respectively. The sheet resistance of the untreated silicon substrate is 57.3 /square and the sulfur doping decreases the sheet resistance. Two factors can contribute to the change of the sheet resistance. One is the surface structure. Another is the doping of sulfur. The surface structure usually causes a small increase in the sheet resistance due to the surface roughness and the worse contact behavior, that is roughness-induced resistivity [30]. Our samples treated by the laser pulses on vacuum or chemically etched obey this rule. When sulfur is incorporated into the silicon surface, the n-doping is expected since a sulfur atom has two more valence electrons than silicon and the increase in carrier density should be the reason for the drop in sheet resistance. Our van der Pauw Hall measurement proves that sulfur doping makes the n-type substrate silicon n+. At the higher input laser energy or the lower scanning speed, the sheet resistance decreases more and indicates more
sulfur incorporated in silicon. The Hall mobility of the untreated silicon is measured as 1032 cm2 V−1 s−1 and the treated silicon is around 600 cm2 V−1 s−1 . Since the low Hall mobility is not desired in optoelectronic applications, the preparation parameters for laser structured silicon should be carefully chosen so that the optical and electronic properties can be balanced in the specific applications. In order to obtain more crystallinity information, we performed the cross-sectional TEM measurement. The selected area electron diffraction (SAED) of Fig. 8 indicates that the structured surfaces are still crystalline but with the area defects and no disordered surface layer is found in the samples, which is different from the femtosecond laser structured silicon surface [31]. The results of EDS analyses shows the presence of sulfur element through to the depth of about 1 m in the surface of the protrusion. The more detailed results of TEM and EDS will be published later. 4. Conclusions Silicon surfaces are irradiated by scanning nanosecond laser pulses and the effects of scanning speed and laser pulse energy on optical and electronic properties of silicon are studied. The optical absorption can reach 90% at the visible spectrum and 80% at the infrared spectrum when the scanning speed is 0.2 mm/s, which means a 2 in. diameter silicon wafer can be surface-structured in about 10 min. The resistivity and Hall effect measurements show that sulfur is incorporated into the silicon surfaces and makes the n type silicon n+. TEM and SAED show that the laser irradiated silicon surfaces are still crystalline but have some defects which make the Hall mobility decrease to about 60% of the untreated silicon. Acknowledgement This work was partly supported by the National Natural Science Foundation of China under Grant no. 10875099. References [1] A. Kabashin, P. Delaporte, A. Pereira, D. Grojo, R. Torres, T. Sarnet, M. Sentis, Nanofabrication with pulsed lasers, Nanoscale Research Letters 5 (3) (2010) 454–463. [2] A. Vorobyev, C. Guo, Colorizing metals with femtosecond laser pulses, Applied Physics Letters 92 (2008) 041914.
X. Li et al. / Applied Surface Science 258 (2012) 8002–8007 [3] A. Vorobyev, V. Makin, C. Guo, Brighter light sources from black metal: significant increase in emission efficiency of incandescent light sources, Physical Review Letters 102 (23) (2009) 234301. [4] T. Hwang, A. Vorobyev, C. Guo, Surface-plasmon-enhanced photoelectron emission from nanostructure-covered periodic grooves on metals, Physical Review B 79 (8) (2009) 85425. [5] C. Wu, C. Crouch, L. Zhao, E. Mazur, Visible luminescence from silicon surfaces microstructured in air, Applied Physics Letters 81 (2002) 1999. [6] A.Y. Vorobyev, C. Guo, Metal pumps liquid uphill, Applied Physics Letters 94 (22) (2009) 224102. [7] B. Wu, M. Zhou, J. Li, X. Ye, G. Li, L. Cai, Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser, Applied Surface Science 256 (1) (2009) 61–66. [8] A. Said, D. Recht, J. Sullivan, J. Warrender, T. Buonassisi, P. Persans, M. Aziz, Extended infrared photoresponse and gain in chalcogen-supersaturated silicon photodiodes, Applied Physics Letters 99 (2011) 073503. [9] H. Lochbihler, Colored images generated by metallic sub-wavelength gratings, Optics Express 17 (14) (2009) 12189–12196. [10] H. Mei, C. Wang, J. Yao, Y. Chang, J. Cheng, Y. Zhu, S. Yin, C. Luo, Development of novel flexible black silicon, Optics Communications 284 (4) (2011) 1072–1075. [11] C. Wu, C. Crouch, L. Zhao, J. Carey, R. Younkin, J. Levinson, E. Mazur, R. Farrell, P. Gothoskar, A. Karger, Near-unity below-band-gap absorption by microstructured silicon, Applied Physics Letters 78 (2001) 1850. [12] M. Sheehy, B. Tull, C. Friend, E. Mazur, Chalcogen doping of silicon via intense femtosecond-laser irradiation, Materials Science and Engineering: B 137 (1–3) (2007) 289–294. [13] Z. Huang, J. Carey, M. Liu, X. Guo, E. Mazur, J. Campbell, Microstructured silicon photodetector, Applied Physics Letters 89 (2006) 033506. [14] M. Casalino, G. Coppola, M. Iodice, I. Rendina, L. Sirleto, Near-infrared subbandgap all-silicon photodectors: state of the art and perspectives, Sensors 10 (12) (2010) 10571–10600. [15] J. Yoo, I. Parm, U. Gangopadhyay, K. Kim, S. Dhungel, D. Mangalaraj, J. Yi, Black silicon layer formation for application in solar cells, Solar Energy Materials and Solar Cells 90 (18–19) (2006) 3085–3093. [16] M. Halbwax, T. Sarnet, P. Delaporte, M. Sentis, H. Etienne, F. Torregrosa, V. Vervisch, I. Perichaud, S. Martinuzzi, Micro and nano-structuration of silicon by femtosecond laser: application to silicon photovoltaic cells fabrication, Thin Solid Films 516 (20) (2008) 6791–6795. [17] V. Tokarev, A. Kaplan, Suppression of melt flows in laser ablation: application to clean laser processing, Journal of Physics D: Applied Physics 32 (1999) 1526.
8007
[18] V. Craciun, D. Craciun, Evidence for volume boiling during laser ablation of single crystalline targets, Applied Surface Science 138–139 (1999) 218–223. [19] http://www.virginiasemi.com/pdf/optical%20properties%20of%20silicon71502. pdf. [20] X. Zeng, X. Mao, R. Greif, R. Russo, Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon, Applied Physics A: Materials Science & Processing 80 (2) (2005) 237–241. [21] C. Yuan, X. Li, D. Tang, H. Yang, G. Li, Evolution of silicon surface microstructure induced by Nd:YAG nanosecond laser, Acta Physica Sinica 59 (10) (2010) 7015–7019. [22] H. Yang, X. Li, G. Li, C. Yuan, D. Tang, Q. Xu, R. Qin, J. Wang, Silicon surface microstructures created by 1064 nm Nd:YAG nanosecond laser, Acta Physica Sinica 60 (2) (2011) 027901. [23] T. Sano, M. Yanai, E. Ohmura, Y. Nomura, I. Miyamoto, A. Hirose, K. Kobayashi, Observation of laser-induced explosion of solid materials and correlation with theory, Applied Optics 13 (2) (1974) 274–279. [24] T. Her, R. Finlay, C. Wu, S. Deliwala, E. Mazur, Microstructuring of silicon with femtosecond laser pulses, Applied Physics Letters 73 (1998) 1673. [25] M. Shen, C. Crouch, J. Carey, R. Younkin, E. Mazur, M. Sheehy, C. Friend, Formation of regular arrays of silicon microspikes by femtosecond laser irradiation through a mask, Applied Physics Letters 82 (2003) 1715. [26] J. Eizenkop, I. Avrutsky, G. Auner, D. Georgiev, V. Chaudhary, Single pulse excimer laser nanostructuring of thin silicon films: nanosharp cones formation and a heat transfer problem, Journal of Applied Physics 101 (2007) 094301. [27] K. Nguyen, D. Abi-Saab, P. Basset, E. Richalot, F. Marty, D. Angelescu, Y. LeprinceWang, T. Bourouina, Black silicon with sub-percent reflectivity: influence of the 3d texturization geometry, in: Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), 2011 16th International, IEEE, 2011, pp. 354–357. [28] T. Hwang, A. Vorobyev, C. Guo, Enhanced efficiency of solar-driven thermoelectric generator with femtosecond laser-textured metals, Optics Express 19 (104) (2011) A824–A829. [29] T. Kim, J. Warrender, M. Aziz, Strong sub-band-gap infrared absorption in silicon supersaturated with sulfur, Applied Physics Letters 88 (2006) 241902. [30] H. Marom, M. Eizenberg, The effect of surface roughness on the resistivity increase in nanometric dimensions, Journal of Applied Physics 99 (2006) 2204349. [31] C. Crouch, J. Carey, J. Warrender, M. Aziz, E. Mazur, F. Génin, Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon, Applied Physics Letters 84 (2004) 1850.