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Long- and short-period nanostructure formation on semiconductor surfaces at different ambient conditions R. A. Ganeev,1,2,3,* M. Baba,1,4 T. Ozaki,2 and H. Kuroda1,4 1
The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan 2 Institut national de la recherche scientifique, Centre Énergie, Matériaux et Télécommunications, 1650 Lionel-Boulet, Varennes, Québec J3X 1S2, Canada 3 Institute of Electronics, Uzbekistan Academy of Sciences, Akademgorodok, 33, Dormon Yoli Street, Tashkent 100125, Uzbekistan 4 Faculty of Medicine, Saitama Medical University, 38 Hongou, Moro, Moroyama, Iruma, Saitama 350-0495, Japan *Corresponding author:
[email protected] Received January 6, 2010; revised February 14, 2010; accepted March 1, 2010; posted March 1, 2010 (Doc. ID 122360); published April 28, 2010 We present the results of studies of nanoripples formation during interaction of the 800 nm, 120, and 35 fs pulses with semiconductor surfaces. Simultaneous appearance of the ripples with the period 共700 nm兲 close to the wavelength of interacting radiation and considerably smaller period 共180 nm兲 was achieved. We discuss the experimental conditions for the formation of these nanoripples (incidence angle, polarization, number of shots, etc.). We show a decisive role of surrounding medium on the quality of nanoripples formation. The selforganization of high-quality nanoripples was clearly shown in the case of dense surrounding medium (methanol), while in the case of insufficient amount of surrounding material (i.e., at different vacuum conditions), the quality of ripples considerably decreased. © 2010 Optical Society of America OCIS codes: 220.4241, 200.4740.
1. INTRODUCTION Nanostructure formation is one interesting phenomenon that appears during the interaction of ultrashort laser pulses with semiconductors. Initially, the reported distances between the ripples were in the range of the wavelength of femtosecond laser (⬃800 nm in most cases when the Ti:sapphire lasers were used for ablation of semiconductors). These structures dubbed as “laser-induced periodic surface structures” (LIPSS), or “nanoripples” received attention due to both the fundamental problem of ultrashort laser–matter interaction and the practical point of view. The nanoripple formation was associated with the formation of diffraction gratings with nanosized distance between the grooves, interference between the laser radiation and the plasmon-polariton waves created during initial random surface heterogeneities [1], Bosecondensation [2], self-organization [3], Coulomb explosion [4], influence of second-harmonic generation [5], local field enhancement [6], etc. Recent observation of the LIPSS with the period in the range of 130– 200 nm (i.e., of order of / 4 – / 6) using the same lasers [5,7,8] received much attention due to the uncertain nature of the formation of these subwavelength structures. Various approaches intended to explain the appearance of the subwavelength ripples were similar to those applied for the description of much broader structures [1–3,9,10]. The subwavelength LIPSS (SWLIPSS) on the semiconductors have demonstrated both the nar0740-3224/10/051077-6/$15.00
rower period between the ripples and other peculiarities among which one can distinguish the influence of polarization, semiconductor bandgap, angle of interaction, etc., on the structural properties and quality of nanoripples. Further understanding of the nature of these structures requires additional studies, since no definitive mechanisms leading to this process has been recognized by the community. The aim of our studies was to analyze the peculiarities of this process by varying experimental conditions of the formation of ordinary and subwavelength LIPSS. In this paper, we demonstrate the influence of above-mentioned characteristics (i.e., polarization of femtosecond radiation, bandgap of semiconductors, angle of interaction of the laser radiation with the surface, etc.) on the variation of nanoripple pattern, when one can distinguish the regimes of formation of the broad and/or narrow periodical structures, as well as nanoholes and nanodots, on the surfaces of various semiconductors. We also carry out the LIPSS formation at different ambient conditions (vacuum, atmospheric air at different pressures, and liquid). We show that presence of dense surrounding medium supports the formation of high-quality LIPSS.
2. EXPERIMENTAL SETUP We used the radiation of Ti:sapphire lasers (wavelength = 800 nm, pulse durations 120 fs and 35 fs, pulse energy © 2010 Optical Society of America
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up to E = 0.5 mJ) to create the nanostructures on the surfaces of semiconductors. The radiation was focused by a 150 mm focal length spherical lens (or, in some cases, a cylindrical lens) at the normal angle of incidence on the GaAs, InAs, Si, Ge, ZnSe, GaN, and ZnO polished strips. The fluence of laser radiation on the surfaces of semiconductors was varied in the range of 0.1– 0.5 J cm−2. At the pulse repetition rate of 10 Hz and the pulse energy on target surface adjusted to approximately 0.05 mJ, the irradiation of the surface was carried out during 0.1– 2 min. The size of the ablated surface was varied in the range of 200– 400 m. After irradiation, the sample was rinsed in acetone and ethanol and finally rinsed in de-ionized water to remove plume deposition. In some cases, the nanoripple formation started to appear during the first few (5–20) shots. The influence of polarization of femtosecond radiation, semiconductor bandgap, pulse energy, number of shots, etc., on the formation of LIPSS was examined. We also compared the appearance of ripples in the cases of normal incidence and 76° angle of interaction. The structure of ripples was analyzed using the scanned electron microscope (SEM) JEOL JSN-5600. Most of these studies were carried out at atmospheric air conditions. We also created the LIPSS on the semiconductors placed in methanol and vacuum chamber. In the former case, different residual air pressures were used during LIPSS formation.
3. EXPERIMENTAL RESULTS A. Ordinary and Subwavelength Nanoripples Formation The absorption of femtosecond light plays a decisive role in variation of the morphology of the surface. After irra-
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diation with sufficient intense femtosecond pulses, bond weakening and breaking occurs in the affected zone. The rearrangement of the crystal lattice starts after exceeding the ablation threshold of the surfaces. The next shots led to further absorption, movement of defects, and creation of new bonds between the atoms, which leads to formation of grains and ripples depending on the laser intensity. Formation of surface waves and interaction with electromagnetic wave leads to stabilization of the ripple-like structure. During these studies we observed various types of ripples, dots, and holes. The first few shots produce sparsely and randomly distributed nanostructures. These intermediate structures will further excite propagating plane surface plasmons that interfere with the laser wave. This interference will, finally, result in the permanent extended periodic structures. After a few hundred shots, at the intensities of laser radiation on the surface of 1 ⫻ 1012 W cm−2, the nanoripples were obtained for almost all samples. In most cases the distance between ripples 共750– 650 nm兲 was close to the wavelength of laser radiation. The LIPSS mostly appeared in the area close to the edges of ablated spots. The central part of the ablated surface was considerably destroyed after multiple shots at the same spot and did not show the regular structure (Fig. 1(a)), while the edges of laser craters consisted of the nanoripples (Fig. 1(b)). The comparison of the structures in the case of linear and circular polarization of laser radiation is presented in Figs. 1(c) and 1(d). The characteristic pattern of the 650 nm LIPSS produced by linearly polarized 120 fs pulses (Fig. 1(c)) transformed to the regular nanodot-like pattern in the case of circularly polarized laser radiation
Fig. 1. SEM images of the (a) central part of ablated spot of GaAs wafer and (b) peripheral area of the same spot. The ablated structures of silicon wafer are presented in the cases of (c) linear and (d) circular polarization.
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(Fig. 1(d)). The sizes of these nanodots were varied between 100 and 130 nm. Note that application of circular polarization led to appearance of both the chaotic pattern and dotted structure with the characteristic sizes a few times less than the wavelength of laser radiation. In some cases we also observed the chaotically oriented nanoripples. In order to verify the polarization dependence of the LIPSS, we rotated the polarization of femtosecond radiation by introducing a half-wave plate. We found the rotation of the ripples in such a way that the ripples and field polarization were orthogonal to each other. This feature was underlined in most of the previous reports (see, for example, [2,5,9,11]), while, in some cases, it was reported that the direction of ripples coincided with the electric vector of electromagnetic wave [12,13]. For some samples, we observed a simultaneous appearance of the two patterns of nanoripples with different periods (Fig. 2). One can see the SWLIPSS with the characteristic distances between the ripples of ⬃ / 5 − / 4, as well as ordinary ripples similar to previously reported patterns (starting from the pioneered work of Birnbaum [14] with the ripple period in the range of the wavelength of laser radiation). As we mentioned in the introduction section, the SWLIPSS alone have already been reported by several groups and were observed at different experimental conditions. There are various interesting applications of these subwavelength structures, which could be useful for production of the periodic gratings with the distance between grooves considerably less than the wavelength of laser radiation, spectroscopy, nonlinear optics,
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enhancement of surface area and growth of catalytic reactions near such structured surfaces, writing of information using nanorippled surfaces, etc. In our case, we observed the SWLIPSS with the shortest period of 180 nm. In the case of the interaction of laser radiation with surfaces at the angles of interaction close to the Brewster’s angles for these semiconductors (70°–80°), we observed some exotic structures, which showed the prolonged lines of nanoholes, cracks, etc. The period of these structures 共d兲 depended on the angle of interaction 共兲 and polarization of laser radiation. For p-polarized light, it depends as dp = / 共1 + sin 兲, while for s-polarized radiation, this dependence is presented as ds = / cos . We carried out the laser ablation of Si, SiC, and GaAs wafers at the angle of interaction of = 76° and found the distance between nanoripples of 400– 500 nm, which was close to the expected distance between the lines in the case of p-polarized laser pulses. We studied various semiconductors and found that the period and quality of LIPSS depended on the bandgap of these samples. In most cases, our results corroborated with those of previously reported studies. In particular, it was reported that the subwavelength LIPSS appear at the photon energies 共Eph兲 less than the bandgap 共Ebg兲 of the semiconductor [15]. This feature was confirmed in the case of SiC (Eph = 1.56 eV, EbgSiC = 3.37 eV) and ZnO (EbgZnO = 3.2 eV), while the long-period and chaotic patterns appeared in most cases of the ablation of GaAs (EbgGaAs = 1.43 eV), Si (EbgSi = 1.12 eV), InAs (EbgInAs = 0.35 eV), and Ge (EbgGe = 0.66 eV) at equal experimental conditions. At the same time one has to note that, in some
Fig. 2. (a–c) Three SEM images of the ripples on the SiC surface showing different periods (730 and 180 nm). (d) High resolution SEM of the SWLIPSS produced on the SiC surface.
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cases, we did not observe the SWLIPSS on the surface of ZnSe (EbgZnSe = 2.35 eV). The “quality” of nanoripple formation on the surfaces of GaN, InAs, and Ge was worse compared with SiC, GaAs, ZnO, and Si. The ablation of ZnSe led to the appearance of a chaotic structure using the circular polarization of laser radiation, while sparse ripples appeared in the case of linear polarization. The ablation of SiC led to the appearance of clearly visible 180– 230 nm SWLIPSS. ZnO demonstrated the ripples with the period of 300– 400 nm. In some cases, we observed two sets of orthogonal ripples with different period on the surface of this semiconductor (ZnO), as well as the femtosecond laser-induced periodic structures covered by random nanostructures. The ablation of Ge also caused the appearance of ripples in both directions, i.e., orthogonally and in parallel to the laser polarization. We also used the cylindrical focusing lens to obtain the extended periodic structures. Figure 3(a) shows the chain of nanoholes produced using the cylindrical focusing of femtosecond pulses on the surface of silicon waver. Another structure was observed in the case of ablation of the GaAs using cylindrical focusing (Fig. 3(b)). In that case the distance between the strips was about 2 m. B. Nanoripple Formation at Different Ambient Conditions The quality of LIPSS formation was analyzed at different ambient conditions. We produced the nanoripples by irradiating the silicon strips placed in methanol and a vacuum chamber. In the former case, four different re-
Fig. 3. Nanoripple formation of the surfaces using the cylindrical focusing of femtosecond radiation. (a) The chain of nanoholes produced on the surface of silicon waver. (b) Nanoripple structure observed in the case of ablation of the GaAs using the cylindrical lens.
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sidual air pressures were used during LIPPS formation (10−1, 1, 100, and 760 torr). Figure 4 shows different patterns of nanoripples formed at equal conditions of laser irradiation (35 fs, 10 Hz pulse repetition rate, 200 shots). In the case of dense surrounding medium (methanol), we observed the high-quality ripples with flat tops (Figs. 4(a) and 4(b)). In the case of atmospheric air conditions 共760 torr兲 in a vacuum chamber, the ripples were also well defined, though less pronounced (Fig. 4(c)). With decrease of surrounding gas pressure 共100 torr兲, the ripples became less configured (Fig. 4(d)), while at moderate vacuum conditions (1 and 10−1 torr), the LIPSS formation almost stopped (Figs. 4(e) and 4(f)). These observations point out a decisive role of surrounding medium on the quality of ripple formation. One can assume that surrounding the medium prevents the material’s chaotic movement during laser ablation. It prevents the chaotic movement of ablated material during first initial shots and serves for fixation of surface waves. It is clearly seen in Figs. 4(a) and 4(b) that the flat, top waves became “frozen” at the conditions when a dense medium (methanol) surrounds the area of laser ablation.
4. DISCUSSION When femtosecond pulses are used, thermal and mechanical forces have no effect because those forces associated with heat flow or material viscosity are unimportant over distances of the order of wavelength or the absorp-
Fig. 4. SEM images of the nanostructures formed on the silicon wafer at different conditions of surrounding medium. (a) methanol; (b) the same pattern at higher resolution; (c) air, 760 torr; (d) air, 100 torr; (e) air, 1 torr; (f) vacuum, 10−1 torr.
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tion depth, which are typically of the order of micrometers. Upon femtosecond laser irradiation, the high electronic excitation leads to a nonthermal, ultrafast phase change, which occurs within less than 1 ps. The periodic structures then exhibit typical signature of pressureinduced transformation. The cooling time is an important parameter, which influences the ripple-like pattern formation on the surfaces of semiconductors. In semiconductors, this parameter is of order of 10−10 − 10–9 s [2]. The typical thermal conductivity of semiconductors is of the order of 0.5共T / 300兲 W cm−1 K−1 [16]. In the meantime, the energy relaxation time in semiconductors is 10−12 – 10−11 s [17]. This difference just shows that ablation time, which involves the displacement of particles, is considerably longer than laser interaction and energy relaxation after absorption. As for thermal conductivity for the ablation process, for example in methanol as a surrounding medium, its role seems insignificant taking into account the pulse duration of laser radiation. The sample quality (defect density, surface roughness) and specific processing conditions (pulse fluence and number of pulses) can change the pattern on nanoripples. Since the intensity of surface plasmons shows its maximum at the surface of material, the quality of ripples depends on the roughness of the surface. In the case of ablation in liquid, the roughness of the surface becomes less important due to pressure of ambient medium. So, at equal conditions, the ablation in a dense medium allows one to achieve a better quality ripple. We used a CCD camera and optical microscope for checking the quality and roughness of semiconductors before and after femtosecond laser ablation. We illuminated the ablated area by white light and checked the reflectivity of the area of ablation. The initial CCD images of unablated surfaces were smooth, and the inclined incident light had a high reflection ratio, especially on the surfaces of high-refractive-index semiconductors. The ablated areas have shown another pattern. The central parts of ablated spots, which were ablated at higher peak intensity of femtosecond laser, had a decreased reflection due to increased scattering from the crater walls. The edges of ablated spots showed an increased reflectivity due to creation of nanoripples. The color of these parts was yellow and red due to preferential reflection of these parts of the white-light spectrum from the ripples. It is known that surface roughness causes an increase in the modulus of the surface plasmon wave vector [18], and this will correspond to an increase in the real part of the refractive index. An increased real part of the refractive index for propagating surface plasmons will cause a reduced LIPSS period. In some previous reports, the period between nanoripples in SWLIPSS was associated with the generation of a second harmonic on the surface [19,20]. In those reports, the period was defined by the relation of d = / 2n. Here n is the refractive index of the semiconductor. In our case this parameter was equal to 120 and 200 nm for GaAs and ZnO, respectively. At the same time, the experimentally observed measurements of this parameter were 700 and 380 nm. Lack of coincidence between the calculated and observed periods of ripples points out the diffi-
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culties of this hypothesis for the description of our observations of both the ordinary and subwavelength LIPSS. Previously, fluid confinement has been reported to produce substantially subwavelength structures for femtosecond irradiation of semiconductors. In particular, in air a 700 nm ripple period was observed in [21] with = 800 nm pulses, while a periodicity of 100 nm was formed in silicon in water. As the laser ablation is conducted in air, the ablated plume cannot expand as rapidly as those in a vacuum chamber, which will induce an instantaneous high-energy and high-pressure region in the laser focus [22]. Therefore, the nanoripples grow very rapidly. The same can be said about the ablation in more dense media (i.e., liquids). The extremely short pulse duration of femtosecond lasers allows one to deposit a highdensity energy with minimal thermal effect into a restricted volume of the target material.
5. CONCLUSIONS We carried out systematic studies of the influence of ambient conditions on nanoparticle formation and their quality. We have shown that, for a dense surrounding medium, the formed nanoripples show better shape and stability. We have also demonstrated the formation of two sets of nanoripples (ordinary LIPSS and SWLIPSS) at similar conditions. In particular, we reported the studies of the influence of polarization of the femtosecond radiation, bandgap of semiconductors, angle of interaction of the laser radiation with the surface, etc., on the variation of nanoripple pattern, when one can distinguish the regimes of the formation of ordinary and subwavelength LIPSS, as well as some exotic patterns (nanoholes and nanodots). We found a unique type of laser-induced periodic surface structure when simultaneous appearance of regular structures of different periods was observed. In the whole, this phenomenon shows the example of self-organization in the system, which initially does not demonstrate the structured directions. Such self-organization was clearly shown in the case of a dense surrounding medium (methanol), while in the case of insufficient amount of surrounding material (i.e., at different air-pressure conditions), the quality of ripples was considerably decreased. These observations point out the ambient conditions at which one can achieve high-quality LIPSS formation.
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