Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Nanos tructured GaN on Silicon Fabricated by Electrochemical and Laser-induced Etching Asmiet Ramizy 1 , Z. Hassan 1 *and Khalid Omar 1 1 Neon-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Abstract-Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes. The etched samples exhibited dramatic increase in photoluminescence intensity as compared to the as-grown samples. The Raman spectra also displayed stronger intensity peaks which were shifted and broadened as a function of etching parameters. Wide-gap III–V nitride semiconductors such as GaN are most promising for blue or ultraviolet (UV)-emitting devices. For the fabrication of GaN nanostructures-based devices, it is important to control the size of nanocrystals as well as to develop a reliable means of monitoring the size distributions of the nanocrystallites in these luminescent materials. In recent years, processing techniques for III– V nitrides nanostructures have been successfully established, especially for crystal growth, while the most suitable etching method is not still concrete because of the excellent chemical stability and high hardness of these compounds. Plasma etching [1] and reactive ion etching (RIE) [2, 3] have mainly been applied so far to etching of III–V nitride crystals. With these processes, however, damage by ion or plasma bombardment is a serious problem. In this work, the fabrication of nanostructured porous GaN by electrochemical etching (Figure 1) and laser-induced etching (Figure 2) have been attempted. Laser processing has the advantages of not causing damage or contamination, as well as of special selectivity with high resolution and high efficiency; however, there are few reports on the laser processing of III–V nitride crystals. The studies on the fundamental properties of these nanostructures are very important due to their unique structural and optical properties relative to the bulk form of the corresponding materia l. Figure 1. The electrochemical etching set-up GaN thin films were grown on n-type Si (111) substrate using Veeco model Gen II molecular beam epitaxy (MBE) system. The GaN samples with (0001) orientation, carrier concentration of 2.1 10 19 cm -3 , and thickness of 0.47 um were placed in an electrolyte solution with ethanol 99.999%: HF40% (4:1), and applied with current density of 75mA/cm 2 for the electrochemical etching, and for the laser-induced etching, power density of 12 W/cm 2 fro m a laser diode (=635 nm, 1.90 eV) was applied. The etching duration was 12 min. The as-grown samp les exh ibited relatively surface morphology. Electrochemical etching resulted in the formation of pores structures with different sizes and shape. The etched surface became hexagonal, and pores structures are confined to smaller size. In addition, the pores walls were very thin with some short thin tips at the top. On the other hand, the surface morphology of the sample obtained after laser induced etching process shows deep and extremely thin pores. Photoluminescence (PL) spectra showed blue shift luminescence relative to the as-grown sample. The band edge emission wavelength shifted from 362.0 to 335.0 nm for the electrochemically etched sample, and to 346.5 for the laser-induced etched sample. The average diameter of the GaN crystallites was about 7-10 n m, as determined fro m the PL data. The Raman spectra for the etched samples revealed shifted and broadened peaks relative to the as grown GaN which can be attributed to the quantum confinement effects on electronic wave function of the GaN nanocrystals. In summary, GaN nanostrutures have been fabricated by two different etching techniques. The quantum confinement effects are considered to control the mechanism of the lu minescence in the nanocrystallites. This study was supported by FRGS grant and Universiti Sains Malaysia. *Corresponding author: zai@usm.my [1] S.A. Smith, C.A. Wolden, M.D. Bremser, A.D. Hanser, R.F. Davis, W.V. Lampert. Appl. Phys. Lett. 71, 3631 (1997) [2] D. Basak, M. Verdú, M.T. Montojo, M.A. Sánchez-Garcia, F.J. Sánchez, E. Munõz, E. Calleja. Semicond. Sci. Technol. 12, 1654 (1997) [3] J. B. Fedison, T. P. Chow, H. Lu, I. B. Bhat, J. Electrochem. Soc. 144, L221 (1997) Figure 2. The laser-induced etching set-up 6th Nanoscience and Nanotechnology Conference, zmir, 2010 353
Poster Session, Tuesday, June 15 Theme A1 - B702 Effect of Porosity on the Characteristics of GaN Grown on Sapphire A. Mahmood 1,2 , Z. Hassan 1 *,F.K. Yam 1 S.K. , Mohd Bakhori 1 and L.S. Chuah 3 1 Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia 2 Department of Applied Sciences, Universiti Teknologi MARA, 13500 Permatang Pauh, Penang, Malaysia 3 Physics Section, School of Distance Education, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Abstract-We investigated the structural and optical characteristics of porous GaN obtained by UV assisted electrochemical etching. SEM micrographs indicated that the average pore size for samples was around 0.16 . Photoluminescence (PL) measurements revealed that the near band edge peak of all the porous samples were red-shifted. Raman spectra exhibited the shift of E 2 (high) to the lower frequency for porous samples. The preparation of porous semiconductors has attracted a great deal of research interest in recent years, primarily due to the potential for intentional engineering of properties not readily obtained in the corresponding crystalline precursors as well as the potential applications in optoelectronics, chemical and biochemical sensing [1-4]. When porosity is formed, these materials exhibit various special optical features, for instance, the shift of bandgap [5], luminescence intensity enhancement [6], as well as photoresponse improvement [7]. To date, porous silicon (Si) receives enormous attention and has been investigated most intensively; however the instability of physical properties has prevented it from large scale applications [8]. Thus, this leads to the development of other porous semiconductors, for instance, the wide bandgap materials such as GaN [2]. In this work, porous GaN layers were prepared by ultraviolet (UV) assisted electrochemical etching method using unintentionally doped (UID) n-type GaN films grown on sapphire (0001) substrate with GaN thickness of Platinum (Pt) wire was used as a cathode electrode. The wafer was cleaved into few pieces and subsequently dipped into 2 % concentration of KOH electrolyte under illumination of 500 W UV lamp for various anodization duration and applied voltages. Table 1 shows the anodization conditions for preparation of porous GaN samples. Table 1. Anodization condition for the samples Sample KOH Voltage Duration concent rat ion wt . % (V) (min) SU1 2 15 15 SU2 2 20 20 SU3 2 20 30 SU4 2 30 20 SU5 2 30 30 Scanning electron microscopy (SEM) images of the porous GaN samples generated under different conditions were shown in Figure 1. From the SEM micrographs, the pore size of all the samples was found to be varied widely, and different shapes could be observed. For the SU1 and SU2 samples, the etching was in the initial stage; pores started to form, and the size of the pores were relatively small, therefore mostly circular shaped structures were observed. For SU3, SU4 and SU5 samples, the surface became relatively rough. SEM images revealed that the average pore size for samples were around 0.16 34 samples was found to be influenced significantly by the anodization duration and the change of applied voltage. The size of the pores increased with the increase of the anodization duration. Furthermore, it can be observed that the pores are not distributed uniformly. On the other hand, it is interesting to note that the porous GaN prepared by the electrochemical etching method does not always produce similar surface morphology. (a) (b) (c) (e) Figure 1. SEM images of different samples. (a) as grown, (b) SU1, (c) SU2, (d) SU3, (e) SU4 and (f) SU5. Photoluminescence (PL) measurements revealed that the near band edge peak of all the porous samples were redshifted; moreover, the PL intensity enhancement was observed in the porous samples. The red shift was also ascribed to the relaxation of the compressive stress in the porous samples. Raman spectra exhibited the shift of E 2 (high) to the lower frequency for porous samples. In contrast, the forbidden modes, A 1 transverse optical (TO) and E 1 (TO) phonon modes were present in Raman spectra only for SU3, SU4 and SU5. In summary, the studies showed that porosity could influence the structural and optical properties of the GaN. Support from FRGS, USM Postgraduate Grant and Universiti Teknologi MARA is gratefully acknowledged. *Corresponding author: zai@usm.my [1] H.Sohn, S. Letant, M.J. Sailor, W.C. Trogler, J. Am. Chem. Soc. 122, 5399 (2000) [2] V.S.Y. Lin, K. Motesharei, K.P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278, 840 (1997) [3] L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990) [4] A.G. Cullis, L.T. Canham, P. D.J. Calcott, J. Appl. Phys. 82, 909 (1997) [5] X. Li, Y.-W. Kim, P.W. Bohn and I. Adesida, Appl. Phys. Lett. 20, 980 (2002) [6] M.A. Steven-Kaleeff, I.M. Tiginyanu, S. Langa, H. Foll and H.L. Hartnagel, J. Appl. Phys. 89, 2560 (2001) [7] M. Mynbaeva, N. Bazhenov, K. Mynbaev, E. Evstropov, S.E. Saddow, Y. Koshka and Y. Melnik, Phys. Status Solidi B 228, 589 (2001) [8] P.M. Fauchet, L. Tsybeskov, C. Peng, S.P. Duttagupta, J. Von Behren, Y. Kostoulas, J.M.V. Vandyshev and K.D. Hirschman, IEEE J. Sel. Top. Quantum Electron. 1, 1126 (1995) (d) (f) 6th Nanoscience and Nanotechnology Conference, zmir, 2010 354
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