ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 117, № 2, с. 332-352
ЛАЗЕРЫ И ИХ ПРИМЕНЕНИЕ
OPTICAL MODIFICATION OF SEMICONDUCTOR SURFACES THROUGH THE NANORIPPLES FORMATION USING ULTRASHORT LASER PULSES:
© 2014 г. R. A. Ganeev
Saitama Medical University, Saitama 350-0495, Japan E-mail: firstname.lastname@example.org Received December 13, 2013
We review the studies of nanoripples formation on various semiconductor species using ultrashort pulses and different experimental conditions. We present recent studies in this field and discuss the fabrication of 2D periodic nanostructures formed on the surface of semiconductor crystals applying a method of two-beam interference of femtosecond laser. Further, we discuss the short-period ripples formation and present the studies of the semiconductors with different bandgaps potentially suitable for generation of subwavelength nanoripples. We also show the method of formation of the nanoholes and nanodots on the surface of different semiconductors. Ripples formation using different laser parameters and ambient media is analyzed with the objective of identifying conditions of forming short-period nanoripples. Finally, we discuss the formation of extended homogenoous laser-induced periodic surface structures using few-cycle laser pulses.
1. INTRODUCTION: MOTIVATION OF NANORIPPLES FORMATION STUDIES
The future of nanotechnology ultimately rests on the controllable fabrication, integration, and stability of nanoscale devices. Nanofeatures synthesized within and on solid surfaces hold promise for unprecedented functionality. However, understanding of the fundamental phenomena leading to the formation, stability, and morphological evolution of nanoscale features is lacking and still under debate. There are many theories on the subject, however, two types of formation mechanisms seen to be dominant. They are (a) resonant mechanisms, which are based on electromagnetic aspects, for example periodic energy deposition due to roughness of surface, plasmon excitation on surface or wake plasmon excitation and (b) non-resonant mechanisms, more related with thermal consequences of the irradiation of the target by the laser, such as capillary waves formed in the melted layer.
Nanosized ripples, or nanoripples, have been observed since 1960's  during pulsed laser irradiation of semiconductors. Since the appearance of femtosecond lasers nanoripples has been observed on metals, semiconductors, and dielectrics. The set of resonant mechanisms leading to formation of ripple is defined by the strong link between ripple periodicity and laser wavelength [2, 3]. It includes the excitation of surface electromagnetic wave such as surface plasmon polari-ton, and surface waves excited by an isolated defect or surface roughness, especially under femtosecond irradiation. One can note the appearance at these condi-
tions, together with nanoripples, the nanodots, nanowires, nanoholes, and irregular nanostructures.
Some images of nanoripples and other structures and shown in the following figures. Periodic nanoripples in the surface and subsurface layers in ZnO irradiated by 1300 nm femtosecond laser pulses are presented in Fig. 1 . High spatial frequency periodic structures induced on metal surfaces by scanned femtosecond laser pulses are presented in Figs. 2 and 3 .
The interest in nanoripples is defined by potential applications in building microfluidic channels, changing the color of materials, modifying local electrical properties, building sub-diffraction-limit optical diffraction gratings, detection of materials with increased sensitivity using surface-enhanced Raman scattering, bio-sensing, water contamination monitoring, explosive detection, detection of organic molecules, etc. The interest in such nanostructures is also stimulated by a number of other circumstances. Along with fundamental problems associated with the appearance of nanoripples on the surface of irradiated objects, there arise a number of interesting possibilities of using these structures for solving some problems of spectroscopy and nonlinear optics, for enlargement of the surface area of the object, which may increase the rate of catalytic reactions in the vicinity of such surfaces, etc. An interesting proposal in this direction is to write information on such gratings (like on compact-discs, but with much higher density). For implementation of the latter proposal, one should consider feasibility of both one-dimensional method or recording of the grating and its two-dimensional analog. For this purpose, one has to perform experiments with crossed femtosecond
Fig. 1. SEM images of ablation area irradiated by 1300 nm laser for (a) 0.86 J/cm2, 20 pulses; (b) 0.86 J/cm2, 50 pulses, (c) 0.86 J/cm2, 900 pulses; (d) 0.19 J/cm2, 2000 pulses; (e) 0.19 J/cm2, 20000 pulses; and (f) 0.23 J/cm2, 90000 pulses. The arrow in (d) shows the laser polarization .
beams. Furthermore, the use of semiconductors with different bandgap widths makes it possible to select such structures both from the viewpoint of possibility of reproduction of the stripes and from the viewpoint of their dimensional characteristics. An additional motivation for these studies is related to possibility of finding more favorable technological regimes for generation of nanostripes, e.g., at large angles of incidence of the light upon the surface.
The use of femtosecond laser pulses for material modification has a number of advantages over longer pulses, such as smaller heat-affected zone and a reduction of the plasma screening effect . The precision of laser machining with femtosecond pulses is generally better than that produced by longer-pulse lasers. Surface patterning has a variety of potential ap-
plications, for example where increased surface area is desired. Surface roughening could be used to improve adhesion of other materials . A textured surface can have high optical absorption and in some applications could be used as an alternative to anti-reflecting coatings . Pointed structures can be used as the fieldemission sources [7—11]. Additionally, long-range ordering is relevant to the use of nanostructures in electronic materials applications . Thus, both scientific and practical goals led to growing interest the formation and analysis of these structures.
Several models and theories for the formation of nanostructures exist in the literature (based on surface plasmons, surface instability, Bose-Einstein condensation etc.) ; however, we do not analyze them since the main task of this review is to show the experimental evidences of the nanoripples formation in different conditions. We do not pretend on the completeness of this review due to large amount of studies on that matter. Because of the enormous complexity of the physical and chemical mechanisms, of course no review of this length can claim to be absolutely complete. It is for sure that there are some more aspects not included in the paper and the mechanisms are still controversially discussed (e.g. the nature and role of accumulation effects related to dislocations and thermal energy deposition). A presentation and comparison of all existing approaches would already require a thicker book.
In this review paper, we show some examples of experimental studies of the long- and short-period nan-oripple formation of different species using ultrashort pulses. In Sec. 2, we discuss the peculiarities of nan-oripple formation. Fabrication of two-dimensional periodic nanostructures by two-beam interference femtosecond pulses is analyzed in Sec. 3. In Sec. 4, we show the methods of short-period formation of semiconductor surfaces. Different periodic nanostructures on semiconductors are discussed in Sec. 5. Nanoripple formation from ultrashort laser pulse irradiation of semiconductors of different bandgaps is analyzed in Sec. 6. In Sec. 7, we discuss the extended homogeneous nanoripple formation during interaction of few-cycle pulses with a moving silicon wafer. Finally, in Sec. 8, we present a summary of the studies of short-period structure formation.
Fig. 2. SEM images ofthe scanned lines on the surface of stainless steel by using different scanning speeds: (a) 2.0 mm/s, (b) 1.0 mm/s, (c) 0.5 mm/s, and (d) 0.25 mm/s. The laser fluence was fixed at 0.16 J/cm2 .
Fig. 3. SEM image of the LIPSS formed on the surface of nickel by scanning femtosecond laser beam with a fluence of 0.16 J/cm2 and a scanning speed of 2.0 mm/s .
2. PECULIARITIES ON NANORIPPLES FORMATION
The formation of parallel grooves on laser-irradiated surfaces was first reported by Birnbaum  in 1965. In several subsequent investigations using neodymi-um-doped lasers and CO2 lasers, rippled with periods roughly equal to the laser wavelength were observed. Formation was typically attributed to the interaction between incident and scattered radiation. Similar laser-induced periodic surface structures (LIPSS or "ripples") have since been observed on a wide variety of materials from a range of laser sources, and their formation processes have been explained. These structures consist of parallel ripples, and, for semiconductors, generally form lines aligned perpendicular to the polarization of the incident laser radiation. For irradiation at normal incidence, they usually have a period roughly equal to the wavelength of the incident laser light in air.
Within the past few years, many groups have observed structures using femtosecond pulse irradiation whose periods were substantially shorter than the irradiation wavelength. In some investigations involving multiple laser wavelengths, the presence of such structures depended on the irradiating wavelength. For femtosecond pulse irradiation, substantially sub-wavelength periods, which formed only when the incident photon energy was less than the bandgap energy of the material, were reported , lines running perpen
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