Fabrication of High-aspect-ratio Nanohole Arrays on GaN Surface by Using Wet-chemical-assisted Femtosecond Laser Ablation

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1 JLMN-Journal of Laser Micro/Nanoengineering Vol. 6, No. 1, 11 Fabrication of High-aspect-ratio Nanohole Arrays on GaN Surface by Using Wet-chemical-assisted Femtosecond Laser Ablation Seisuke Nakashima 1*, Koji Sugioka 1, Takuma Ito 1,, Hiroshi Takai, and Katsumi Midorikawa 1 1 RIKEN- Advanced Science Institute, -1 Hirosawa, Wako, Saitama , Japan seisuke@riken.jp Tokyo Denki University, - Nishiki-machi, Kanda, Chiyoda-ku, Tokyo 11-54, Japan A periodic array of nanoholes wi high aspect ratios was successfully fabricated by using a multi-scan laser irradiation technique, in which each position in e array is irradiated one time per scan wi a femtosecond laser pulse. This technique can easily avoid interference to e laser pulses by micro-bubbles generated in e wet-chemical-assisted ablation process. As a result, we obtained nano-sized ablation holes wi an aspect ratio of 1.6, which is much higher an at possible wi single pulse ablation. The dep of e ablation craters formed by multi-scan irradiation technique was found to saturate at higher pulse number, presumably due to e limited dep of focus. We also revealed at e ablated dep strongly depends on e refractive index of e medium facing e GaN substrate. In fact, compared to ablation in air followed by HCl etching, e wet-chemicalassisted ablation process enhances e aspect ratio by about 3 % because of e high refractive index of e HCl solution. In addition, by controlling e synchronization of e laser pulses and e motion of e piezo-stage, an almost perfectly periodic arrangement of nanoholes was fabricated on e GaN substrate. The technique developed in is study is likely to make a significant contribution towards realization of two-dimensional photonic crystal structures on GaN-based blue lightemitting diodes. Keywords: Femtosecond laser ablation, Wet-chemical process, Gallium nitride, Nanofabrication, High-aspect-ratio nanohole 1. Introduction Development of a precise and efficient technique for fabricating nanostructures in wide band gap GaN is challenging issue because of e high ermal stability and chemical durability of e material. At present, electron beam liography followed by plasma etching [1, ] is commonly used to produce high-precision two-dimensional (D) surface structures on GaN, and features as small as 15 nm have been fabricated. However, due to e complexity of e procedures, it suffers from high production costs and has low roughput, and is still difficult to implement in mass production. Alough several oer approaches have been studied, ey do not offer furer advantages over e electron beam liography technique [3-6]. In contrast, laser ablation processes offer a different approach to micro/nanofabrication of GaN [7-9]. Recently we have developed a wet-chemical-assisted femtosecond (fs) laser ablation meod and successfully fabricated nanosized craters wi a higher quality an wi e conventional ablation by using e second-harmonic of nearinfrared fs laser pulses [1, 11]. Since fs laser ablation is a direct and simple process, it is expected to be applicable to flexible fabrication of D periodic structures, allowing high efficiency and high roughput in production environment [1, 13]. This technique could even be applied to nanofabrication of GaN-based light-emitting diodes (LEDs). Such LEDs are widely used for many applications everywhere such as traffic lights, liquid crystal display backlighting, full color displays, and oer general lighting purposes. At is time, however, blue LEDs suffer from e serious problem of low light extraction efficiency, which is attributable to a large amount of total internal reflection at e interface between GaN (n=.5) and air (n=1). For example, e light extraction efficiency for a single GaN /air interface is less an 1 % despite e fact at e internal quantum efficiency of e LED is higher an 9 %. It has been proposed at integration of a D photonic crystal pattern of air holes on e device surface would be an effective means of enhancing e extraction efficiency [14-16]. In such a structure, total internal reflection is suppressed, and inplane lateral guide modes of light are prohibited; e resulting vertical waveguiding effect increases photon emission in e off-plane direction. To maximize e effect of e photonic crystal structure, processing accuracy is of primary importance, since it is necessary to produce nanoholes wi high aspect ratios. In e present study, we attempted to fabricate simple D photonic crystal structures consisting of arrays of high-aspect-ratio nanoholes using wet-chemical-assisted fs laser ablation. To achieve e required aspect ratio, multi-scan irradiation wi a pulsecontrolled fs-laser was carried out.. Experimental The single-crystal GaN substrates (Kyma Technologies, Inc.) utilized in is work were 47 μm ick and exhibited n-type conduction. The chemically and mechanically polished Ga-face was subjected to e laser irradiation. We utilized an fs laser (Clark-MXR, CPA-1) at emits pulses wi wids of 15 fs [full wid at half maximum (FWHM)], a waveleng of 387 nm (SHG) produced by a 15

2 JLMN-Journal of Laser Micro/Nanoengineering Vol. 6, No. 1, 11 Fig. 1 Pulse energy dependence of diameter ( ) and dep ( ) in ablation craters formed by single-pulse irradiation using wetchemical-assisted fs laser ablation. Inset is e variation of e aspect ratio ( ). BBO crystal, and a repetition rate of 1 khz. A pulse picker integrated Pockels cell was used to control e repetition rate. The laser beam was collimated to a diameter of 3 mm and was en focused onto e substrate surface by a 1 microscope objective lens wi a numerical aperture of.9 and wi a working distance of 1. mm. We used two kinds of fabrication techniques. The first was wetchemical-assisted fs laser ablation, in which e laser beam was focused directly onto a substrate immersed in a hydrochloric acid (HCl) solution. The ickness of e acid solution on e substrate surface was maintained at 63 μm. A two-step processing meod, i. e., fs-laser irradiation in air followed by HCl treatment, was also carried out for comparison, alough we have previously reported at is meod was less suitable for high-quality fabrication an wet-chemical-assisted ablation [11]. The substrate was placed on an XYZ stage scanned by piezo actuators, which enables synchronization of e laser pulses wi e stage, allowing e position of e irradiated spot to be precisely controlled. The characteristics of e ablation craters formed by e laser irradiation were investigated by atomic force microscopy (AFM), and cross-sectional profiles were used to determine e effect of irradiation conditions on e crater deps and FWHM diameters. 3. Results and discussions Figure 1 shows e pulse-energy dependence of e dep and diameter of ablation craters formed by singlepulse irradiation using wet-chemical-assisted fs laser ablation. It can be seen at e diameter continuously increases wi pulse energy, whereas e dep becomes saturated due to multiphoton absorption by e GaN [17]. Hence, e aspect ratio is limited to a maximum value of about. as shown in e inset of Fig. 1. The lattice constants of photonic crystal structures are corresponding to e emission waveleng of LEDs; 47 nm for general blue LEDs. In addition, some reports suggested at enhancement of e light extraction efficiency Fig. (a) AFM images of nanohole arrays formed by multiscan irradiation meod using wet-chemical-assisted ablation in HCl solution at a pulse energy of 1 nj for various pulse numbers, and (b) cross-sectional profiles of nanoholes for each pulse number. was realized by e D arrays of nanoholes wi e diameter of approximately 15 nm [, 16]. In e case of a p- GaN layer wi a ickness of nm, e aspect ratio must en be greater an 1.3. Therefore, to increase e etched dep and ereby e aspect ratio, we employed multipulse ablation. The simplest meods would be to apply successive pulses to e same position, before moving e laser spot to e next position. The number of pulses could be controlled by changing e open time of e shutter. However, when e ablation reaction takes place in a liquid solution, which is e case for wet-chemical-assisted ablation, e micro-bubbles generated by e preceding pulses interfere wi subsequent pulses, and e fabrication efficiency is drastically decreased. If e time interval between e pulses was longer an e time scale for disappearing of bubbles, such interference would be minimal. Alough is could be achieved by using namely using a low repetition rate of pulses, e fabrication time would inevitably become much longer. To avoid is drawback, we developed a multi-scan irradiation technique, in which a series of sequential scans is performed and each position is irradiated wi a single laser pulse during each scan. This provides sufficient time for e disappearance of microbubbles generated by e preceeding pulses due to selfpressurization effects, so at each pulse can successfully reach e substrate surface. In addition, high speed processing is feasible because, in principle, it is not necessary to decrease e repetition rate. 16

3 JLMN-Journal of Laser Micro/Nanoengineering Vol. 6, No. 1, 11 Figure (a) shows AFM images of nanohole arrays formed by multi-scan irradiation using wet-chemicalassisted ablation in a HCl solution at a pulse energy of 1 nj. The number of scans, and hence e pulse number was 1, 3, 1 and 5, respectively. Pulse overlap at each position is well controlled due to e high positioning accuracy of e piezo stage. The cross-sectional profiles shown in Fig. (b) indicate at e ablated dep increases wi pulse number, in strong contrast to e dependence on pulse energy for single-pulse ablation (Fig. 1). This implies at highly efficient fabrication can be achieved using e multiscan irradiation meod. In is case, e time interval between e scans was set to 4 second, which was sufficiently and much longer an e disappearance time of microbubbles. Alough clarification of e disappearance time is so important, it is too difficult to measure it accurately at present, which is because of e low quality of monitoring image. For comparison, Fig. 3 shows (a) AFM images and (b) cross-sectional profiles of nanohole arrays formed at e same pulse energy using e two-step processing meod. The pulse-overlap accuracy appears to be quite high even for higher pulse number. In addition, e cross-sectional shapes are similar to ose for wet-chemical-assisted ablation. This analogy between e two meods is not observed in e case of single-pulse ablation. Alough e question of reason is still open at present, it is clear at e multi-scan irradiation meod can significantly increase e ablation dep during eier one-or two-step processing. Figure 4 shows variations in hole diameter and dep as a function of pulse number for e wet-chemical-assisted Fig. 4 Pulse number dependence of (a) diameter and (b) dep of nanoholes formed using e multi-scan irradiation meod in e case of wet-chemical-assisted fs laser ablation ( and ) and two-step processing ( and ). ablation and e two-step processing meods. In bo cases, ere seems to be very little dependence of e diameter on e pulse number. We speculate at ermal accumulation was minimized because of e pseudo-lowrepetition-rate pulse. Meanwhile, e hole dep is seen to increase for pulse number up to 1 or, but en begins to saturate. It is interesting to note at e saturated dep at higher pulse number differs between e two meods. In Fig. 5, e dependence of aspect ratio on e pulse number is plotted. For bo meods, e aspect ratio is seen to initially increase and en become saturated at around 1 or pulses, even ough e dep and diameter have not been completely saturated yet. This indicates e similar ablation rates in bo directions of diameter and Fig. 3 (a) AFM images of nanohole arrays formed by multiscan irradiation meod using two-step processing meod at a pulse energy of 1 nj for various pulse numbers, and (b) crosssectional profiles of nanoholes for each pulse number. Fig. 5 Aspect ratios as a function of pulse number for e wetchemical-assisted ablation (red closed circles) and two-step processing (blue closed circles). 17

4 JLMN-Journal of Laser Micro/Nanoengineering Vol. 6, No. 1, 11 dep. This is presumably due to e same intensity of incident beam at every point on e surface of an ablation crater, namely, bottom or lateral surface. The saturated ratios are 1.6 for wet-chemical-assisted ablation and 1. for e two-step process. These values are significantly higher an e best aspect ratio obtainable wi single-pulse ablation. More notable is e fact at e saturated value for wet-chemical-assisted ablation is 3 % higher an at for ablation in air. This reason for is is ought to be e following. In e multi-scan irradiation meod, when e first pulse irradiates e surface of e GaN substrate, an ablation reaction occurs just at e surface. Almost simultaneously, e Ga-rich phase remaining on e surface of e nano-crater is chemically removed by e acid solution. After a sufficient amount of time, e micro-bubbles disappear and e next laser pulse arrives in e subsequent scan. During e second pulse, ablation occurs at e new surface cropped out by e preceding ablation process. These steps are repeated for each subsequent scan. However, after pulses or more, e ablation reaction progresses no furer because e beam intensity at e bottom of e hole becomes smaller an e reshold intensity for ablation due to e limited dep of focus of e laser beam. Therefore, it is speculated at e final shape of e ablation hole after pulses is determined by e spatial intensity distribution of e laser beam. In e r-z coordinate system shown in Fig. 6, an incident laser beam wi an intensity of I is irradiated in e z direction and focused on e origin. If e intensity distribution of e incident laser beam along e r axis is described as Gaussian, e intensity at an arbitrary point, I(r, z), can be expressed as r I( r, z) = I ( z)exp ( z). (1) ω I (z) denotes e intensity at e center of e beam and is calculated as I ( z) = I πω( z), where ω(z) is e beam Fig. 6 Beam radius (dotted line) and isointensity curves for various intensities (solid lines). In particular, e isointensity curve for e ablation reshold intensity represents e ablated region formed by e multi-scan irradiation technique. radius. Then, for any given intensity, e relationship between z and r can be obtained, leading to closed isointensity curves for different beam intensities, as shown in Fig. 6. At every point in is line, namely, bottom or lateral surface of ablation craters, e beam intensity is constant and ablation rate would be similar, as stated above. If e intensity is taken to be e ablation reshold intensity I, e obtained isointensity curve represents e ablated region produced by e multi-scan meod as ω( z) I πω( z) r = ln. () I By substituting e beam radius, ω ( z ) = ω 1+ ( Z Z R ), where Z R represents e Rayleigh leng and is expressed as Z R = πω n λ, into equation (), e dep Z and radius r of e ablation holes can be calculated as πω n I Z = 1, (3) λ πω I and r 1 πω I = ω ln. (4) I It is interesting to note at while e dep depends on e refractive index, n, of e medium facing e GaN substrate, e radius is independent of is parameter. As a result, e aspect ratio strongly depends on e refractive index of e medium as πω n I 1 πω I Aspect ratio = 1 ln. (5) λ πω I I Based on e difference in e refractive index between air (~1) and HCl acid solution (~1.3), it can be estimated at a 3 % higher aspect ratio can be achieved wi wetchemical-assisted ablation an wi e two-step processing meod. This estimation agrees well wi e experimental results. At present, it is quite difficult to determine e reshold intensity of ablation accurately. As a preliminary calculation, we assume at e reshold is e minimum energy for ablation reaction. In e experiment using multi-scan irradiation meod, e pulse energy is 1 nj and e minimum energy is 7.5 nj as can be seen in Fig. 1. Spot size, ω is calculated using e equation, ω =. 61 λ NA M. Since M is 1.3, ω would be 341 nm. From e equations (3)-(5), e diameter and dep can be estimated as 6 and 41 nm, indicating at e aspect ratio is Alough is estimation is not enough rigorous in terms of insufficiency of reshold estimation, e estimated values are very close to e experimental values. 4. Conclusions For e purpose of producing a D photonic crystal on a GaN LED, we attempted to fabricate periodic arrays of high-aspect-ratio nanoholes on GaN substrates. By controlling e synchronization of e piezo-stage wi e laser pulses, an almost perfectly tetragonal array of nanoholes was fabricated using wet-chemical-assisted fs laser ablation. Nanoholes wi high aspect ratios (~1.6) were successfully produced using multi-scan irradiation meod. This result clearly demonstrates at e multi-scan meod is more suitable for fabrication of high-aspect-ratio nano- 18

5 JLMN-Journal of Laser Micro/Nanoengineering Vol. 6, No. 1, 11 holes compared wi e single-pulse ablation process. From a comparison wi two-step processing meod, we speculated at e intensity distribution of e focused laser beam determines e final shape of nanoholes formed by multi-scan irradiation. It was found at aspect ratio significantly depends on e refractive index of e medium facing e GaN substrate. This means at e use of an acid solution wi a higher refractive index will lead to furer enhancement of e aspect ratio. References [1] C. Meier, K. Hennessy, E. D. Haberer, R. Sharma, Y.- S. Choi, K. McGroddy, S. Keller, S. P. DenBaars, S. Nakamura, and E. L. Hu, Appl. Phys. Lett., 88, (6) (Journals) []. K. Kim, J. Choi, S. C. Jeon, J. S. Kim, and H. M. Lee, Appl. Phys. Lett., 9, (7) (Journals) [3] 3. Z. S. Zhang, B. Zhang, J. Xu, K. Xu. Z. J. Yang, Z. X. Qin, T. J. Yu, and D. P. Yu, Appl. Phys. Lett., 88, (6) (Journals) [4] 4. M.-K. Kwon, J.-Y. Kim, I.-K. Park, K. S. Kim, G.- Y. Jung, S.-J. Park, J. W. Kim, and Y. C. Kim, Appl. Phys. Lett., 9, (8) (Journals) [5] 5. W. N. Ng, C. H. Leung, P. T. Lai, and H. W. Choi, Nanotechnology, 19, (8) 553. (Journals) [6] 6. T. A. Truong, L.M. Campos, E. Matioli, I. Meinel, C. J. Hawker, and P. M. Pertroff, Appl. Phys. Lett., 94, (9) 311. (Journals) [7] 7. J. Zhang, K. Sugioka, S. Wada, H. Tashiro, K. Toyoda, and K. Midorikawa, Appl. Surf. Sci., 17, (1998) 793. (Journals) [8] 8. T. Akane, K. Sugioka, and K. Midorikawa, Appl. Phys. A, 69, (1999) S39. (Journals) [9] 9. K. Obata, K. Sugioka, K. Midorikawa, T. Inamura, and H. Takai, Appl. Phys. A, 8, (6) 479. (Journals) [1] 1. S. Nakashima, K. Sugioka, K. Midorikawa, Appl. Surf. Sci., 55, (9) 977. (Journals) [11] 11. S. Nakashima, K. Sugioka, K. Midorikawa, J. Laser Micro/Nanoeng., 5, (1) 1. (Journals) [1] 1. K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao: Appl. Phys. Lett., 71 (1997) 339. (Journals) [13] 13. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao: Phys. Rev. Lett., 91, (3) (Journals) [14] 14. S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and E. F. Schubert, Phys. Rev. Lett., 78, (1997) 394. (Journals) [15] 15. E. Yablonobitch, T. J. Gmitter, and R. Bhat, Phys. Rev. Lett., 61, (1988) 546. (Journals) [16] 16. T. N. Oder, H. S. Kim, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett., 84, (4) 466. (Journals) [17] 17. S. Nakashima, K. Sugioka, K. Midorikawa, Appl. Phys. A, (1) in press. (Journals) (Received: August 9, 1, Accepted: December 1, 1) 19