Photonic crystal slabs with hexagonal air holes fabricated by selective area metal organic vapor phase epitaxy

Photonic crystal slabs with hexagonal air holes fabricated by selective area metal organic vapor phase epitaxy

Sensors and Actuators A 133 (2007) 288–293 Photonic crystal slabs with hexagonal air holes fabricated by selective area metal organic vapor phase epi...

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Sensors and Actuators A 133 (2007) 288–293

Photonic crystal slabs with hexagonal air holes fabricated by selective area metal organic vapor phase epitaxy L. Yang ∗ , J. Motohisa, J. Takeda, T. Fukui Research Center for Integrated Quantum Electronics (RCIQE) and Graduate School of Information Science and Technology, Hokkaido University, North 13 West 8, Sapporo 060-8628, Japan Received 8 July 2005; received in revised form 12 March 2006; accepted 10 May 2006 Available online 26 July 2006

Abstract It is shown that the photonic crystal slab (PCS) with hexagonal air holes has band gaps in the guided mode spectrum, which can be compared to that of the PCS with circular air holes, thus it is also a good candidate to be used for the PC devices. The PC with hexagonal air holes and a = 0.5 ␮m and r = 0.15 ␮m was fabricated successfully by selective area metal organic vapor phase epitaxy (SA-MOVPE). The vertical and smooth sidewalls are formed and the uniformity is very good. The same process was also used to fabricate a hexagonal air hole array with the width of 0.1 ␮m successfully. The air-bridge PCS with hexagonal air holes and a = 0.3 ␮m and r = 0.09 ␮m was also fabricated successfully by SA-MOVPE. Further optimization of the growth conditions for the sacrificial layer and the selective etching of the GaAs cap layer is also needed. Our experimental results indicate that SA-MOVPE is a promising method for fabricating PC devices and photonic nanostructures. © 2006 Elsevier B.V. All rights reserved. Keywords: SA-MOVPE; Photonic crystal slabs; Hexagonal air holes; Gap map in the guided mode spectrum; Air-bridge structure

1. Introduction Recently, photonic crystals (PCs), artificial optical materials with periodically modulated refractive indices, have attracted increasing attention. Although great improvements have been achieved since the first suggestion of the concept of PCs, there is still a long way to go before their actual application. For example, the lowest reported propagation loss for silicon on insulator (SOI) PC waveguides is about 0.6 dB/mm [1], which is about two orders of magnitude larger than that of the conventional SOI waveguides [2,3]. The lowest reported propagation loss for GaAs PC waveguides is about 0.76 dB/mm [4], which is still one order of magnitude larger than that of the conventional GaAs waveguides [5]. The pulsed lasing of the PC microlaser at the low temperature and optical pumping was first reported by Painter at 1999 and its continuous lasing at the room temperature and optical pumping was also reported [6,7]. Recently, electrically driven pulsed lasing at the room temperature has been reported by Park [8]. However, there are still no reports on its electrically



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driven continuous lasing at room temperature. The main reason originates from the imperfect fabrication technologies used for the PC devices. Most of the reported PC devices were fabricated using electron beam (EB) lithography and dry etching method. Therefore the deviation of the periodicity due to hole misplacement and the hole size nonuniformity in EB lithography, which affect the band gaps of the PCs, and the process-induced damage and relatively rough air–dielectric interfaces, which greatly increase the propagation loss of PC waveguides and the threshold current of laser diodes, are unavoidable. Until now, the fabrication of the perfect PC structures is still a challenge. We have to optimize the existing fabrication processes or find other new fabrication processes. On the one hand, we have to develop higher-resolution EB lithography or other lithography technologies [9] in order to form high-quality pattern. On the other hand, we have to further optimize the dry etching process [10–12] or try other intrinsically low-damage fabrication methods such as wet etching and/or selective area epitaxy [13–17]. The shapes of the holes or pillars fabricated by these two methods depend not only on the pattern formed on the material surface but also on the orientation of the crystal plane. Most semiconductor materials have diamond, zincblende or wurtzite crystal structures. For these

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materials, if the (1 1 1) plane is used as the substrate and {1 1¯ 0} planes are used as the sidewalls, hexagonal air holes or dielectric pillars can be formed. Actually, selective area metal organic vapor phase epitaxy (SA-MOVPE) has been used successfully to fabricate several types of uniform hexagonal semiconductor pillars on GaAs (1 1 1) and InP (1 1 1) substrates [14,15]. However, the shapes of the hexagonal air holes fabricated by this method deform from the designed structures and become irregular due to ¯ and <2¯ 1 1> directhe overgrowth at the six corners along <2 1¯ 1> tions [16,17]. It is believed that the overgrowth at the six corners can be controlled by the optimization of the growth conditions. In this paper, we will show the new development on the fabrication of the air-bridge PC slabs (PCS) with hexagonal air holes by SA-MOVPE. The band diagrams of the PCSs with hexagonal air holes were calculated by plane wave expansion method (PWE) with super cell method. It is found that it has band gaps in the guided mode spectrum that can be compared to that of the PCS with circular air holes. By optimizing the growth temperature and the partial pressures of trimethylgallium (TMG) and arsine (AsH3 ), we have successfully fabricated the PCs with hexagonal air holes and vertical and smooth sidewalls. A process was proposed and used to fabricate the air-bridge PCSs with hexagonal air holes. The fabricated air holes are hexagonal, uniform and have smooth and vertical sidewalls.

2. Theoretical calculation Before the discussion on the experimental results, we would like to discuss the possibility of the use of the PCS with hexagonal air holes for PC devices and compare its gap map in the guided mode spectrum to that of the PCS with circular air holes. Triangular lattice was selected as the studied object because such a structure has much wider band gaps than square and hexagonal lattices. The refractive index of the GaAs is 3.374 at 1.55 ␮m. Both the upper and lower claddings are air (nair = 1), which corresponds to the air-bridge PCS. The slab thickness (t) was selected to be 0.6a so that the designed PCS has maximum band gaps in the guided mode spectrum, where a is the lattice constant. The PWE method with the supercell method was used to calculate the band diagrams of the PCSs with hexagonal or circular air holes [18–21]. A supercell with 6a was used in the vertical direction, which is sufficiently thick to prevent the coupling of the optical fields in the neighboring supercells. The grid point number (Na × Nb × Nc , where Na , Nb and Nc are the grid point numbers along the lattice vectors a៝ and b៝ and the vertical vector c៝ ) used in the calculation is 16 × 16 × 128, which is sufficient to give a reasonably accurate result in a reasonable computation time. Fig. 1(a) and (b) show the calculated gap maps in the guided mode spectrum for the PCSs with hexagonal and circular air holes. Due to the presence of a horizontal symmetry plane bisecting the slab, the guided modes can be classified into even or odd modes according to whether they are even or odd with respect to reflections through this plane. Since these even and odd modes have strong similarities with TE and TM modes in 2D PCs, we also call them TE-like modes and TM-like modes. We can see that there are five band gaps in the guided mode spectrum for

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Fig. 1. (a) The calculated gap map in the guided mode spectrum for the PCS with hexagonal air holes and (b) that for the PCS with circular air holes. The TE-like modes are shown in solid lines “—” and the TM-like modes shown in dotted lines “. . .”

both two types of PCSs, three of which are for TE-like modes (shown in solid lines “—”) and the other two for TM-like modes (shown in dotted lines “. . .”). Compared to the first band gap in the guided mode spectrum of the PCS with circular air holes, that of the PCS with hexagonal air holes shifts to the higher frequency. This behavior can be explained on the basis of the electromagnetic variational theorem [20]. The PCS with circular air holes has a smaller volume of air holes than does the PCS with hexagonal air holes with the same values of r and t. Note that here r is the radius of the circular air holes or the half width of the hexagonal air holes. Thus the optical field penetrating into the air holes is larger for the PCS with hexagonal air holes than for the PCS with circular air holes, which leads to an increase in the frequency of the first band gap in the guided mode spectrum. The PCS with hexagonal air holes has the first band gap in the guided mode spectrum that can be compared to that of the PCS with circular air holes, hence it is also suitable to be used for PC devices. 3. Experimental results Due to the structural symmetry of GaAs material, if (1 1 1)B plane is used as the substrate, there are six vertical {1¯ 1 0} facets, which gives rise to the possible formation of the hexagonal air holes. SA-MOVPE is thought to be the most promising method in realizing such structures because faceting technology can be used in this method and process-induced damages and contamination can be avoided. Before the discussion on the fabrication of the air-bridge PCSs with hexagonal air holes by SA-MOVPE, we would like to firstly discuss the possibility of the fabrication of the PCs with hexagonal air holes directly on the n-type GaAs (1 1 1)B substrate. The fabrication procedure for the PCs with hexagonal air holes on the n-type GaAs (1 1 1)B substrate is shown here. First, ZEP520 resist was spin-coated on the n-type GaAs (1 1 1)B substrate and the designed pattern was formed on it by EB lithography. Then about 30 nm thick SiO2 was deposited by plasma sputtering and the pattern formed on the resist was

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Fig. 2. (a) SEM image for the patterned n-type GaAs (1 1 1)B substrate after the lift-off process. (b and c) SEM images of the fabricated PC with hexagonal air holes and a = 0.5 ␮m and r = 0.15 ␮m. (d) SEM image of the cross section of the fabricated PC with hexagonal air holes and a = 0.5 ␮m and r = 0.15 ␮m. (e) SEM image of the fabricated line-defect PC waveguide with hexagonal air holes and a = 0.5 ␮m and r = 0.15 ␮m. (f) SEM image for the fabricated PC with hexagonal air holes and a = 0.3 ␮m and r = 0.09 ␮m. (g) SEM image for the fabricated PC with hexagonal air holes and a = 0.2 ␮m and r = 0.05 ␮m. (h) SEM image for the fabricated PC microcavity with hexagonal air holes and a = 0.2 ␮m and r = 0.05 ␮m.

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transferred onto the SiO2 layer by the lift-off process. Finally, about 0.2 ␮m thick GaAs layer was grown on the exposed region of the n-type GaAs (1 1 1)B substrate by SA-MOVPE. Fig. 2(a) shows the scanning electron microscope (SEM) image of the patterned n-type GaAs (1 1 1)B substrate after the lift-off process. The lattice constant was selected to be 0.5 ␮m and the width of the hexagonal air holes 0.3 ␮m so that the designed PC can work at the optical communication wavelength (1.55 ␮m). For the growth of hexagonal semiconductor pillars, hexagonal pillars can be formed even when the masked pattern becomes circular due to the limit resolution of EB lithography and wet etching [14,15]. This self-healing growth mode facilitates the growth of semiconductor hexagonal pillars. However, there is not such a self-healing growth characteristics in the growth of hexagonal air holes, so we should optimize the fabrication process of the masked pattern for the growth of the air holes. It can be seen that uniform hexagonal SiO2 masked pattern is formed, which supplies a premise and a base to the growth of high-quality hexagonal air holes. The MOVPE growth was carried out in a horizontal lowpressure MOVPE system. The source materials were TMG and 5% AsH3 in hydrogen. The working pressure was fixed at 76 Torr. Patterned n-type GaAs (1 1 1)B substrates were heated from the room temperature to a thermal cleaning temperature of 650 ◦ C for 5 min and then fixed at a growth temperature. It is well known that the epitaxial growth rate is dependent on the crystallographic orientation. For example, The GaAs (1 1 1)B facet tends to grow under higher growth temperature and lower AsH3 partial pressure, while the GaAs {1 1¯ 0} facets tend to grow under lower growth temperature and higher AsH3 partial pressure. In order to prevent the lateral growth in the <1 1¯ 0> directions and enhance the growth rate in [1 1 1] direction, the growth temperature was selected to be 800 ◦ C and the partial pressure of AsH3 1.3 × 10−5 atm. The partial pressure of TMG was selected to be 1.3 × 10−6 atm. In such a growth condition, the growth rate is very low (about 1.8 nm/min), which is very important for the formation of the facets. Fig. 2(b) and (c) show the SEM images for the fabricated PCs with hexagonal air holes.

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From these figures, we can see that the PCs with hexagonal air holes have been fabricated successfully. The uniformity of the fabricated PCs is very good. The formation of the hexagonal air holes also indicates that facets are formed. Fig. 2(d) shows the SEM image for the cross section of the fabricated PC with hexagonal air holes. Note that the width of the hexagonal air holes in the SEM image is less than 0.3 ␮m just because the cleaved direction is along the <1¯ 1¯ 2> direction and the cleaved position approaches to the side edge. We find that the sidewalls are smooth and vertical, which is important in reducing the propagation loss of the PC waveguides and the threshold current of the PC laser diodes because inclined air holes will enhance the coupling of the TE-like waveguide modes and the TM-like slab modes and thus increase the propagation loss of the PC waveguides [22], and the rough sidewalls will enhance the surface recombination and thus increase the threshold current of the PC laser diodes. Fig. 2(e) shows the SEM image for the linedefect PC waveguide with hexagonal air holes. We can see that the interface between the air and the line-defect region is very smooth, which indicates that the propagation loss of the PC waveguide devices fabricated by SA-MOVPE can be reduced greatly because smooth interface can reduce the scattering loss. Theoretical results indicate that by adjusting the width of the line defect and the size of the hexagonal air holes along the line defect, a guided mode with a bandwidth as high as 9.1% of its central frequency can be reached [18]. Our experimental results together with the related theoretical calculations indicate that SA-MOVPE growth can be used to fabricate the PC waveguide devices with low propagation loss and large bandwidths and thus also can be used to fabricate complex PC integrated optical circuits. Systematic studies on the optical properties of the PC waveguide devices fabricated by SA-MOVPE are left for the future. In the above, we have discussed the possibility of the fabrication of the PCs with hexagonal air holes working in the optical communication wavelength by SA-MOVPE. However, sometimes we also need to fabricate the PCs with hexagonal air holes working at about 1.0 ␮m or less. In order to testify whether our

Fig. 3. Fabrication procedure for the air-bridge PCSs with hexagonal air holes.

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Fig. 4. (a) and (b) SEM images of the fabricated air-bridge PCS with hexagonal air holes and a = 0.3 ␮m and r = 0.09 ␮m.

process can be used for such structures, we designed two PC patterns with a = 0.3 ␮m, r = 0.09 ␮m and a = 0.2 ␮m, r = 0.05 ␮m, respectively. Fig. 2(f–h) show the SEM images for the fabricated PCs with hexagonal air holes and different structural parameters. We can see that hexagonal air hole arrays with vertical and smooth facets are formed and their uniformities are very good. From Fig. 2(h), we can see that a ring PC microcavity has been fabricated successfully. We think that a hexagonal air hole array with the width less than 0.1 ␮m can also be fabricated using the above fabrication process. As discussed in the above, uniform PCs with hexagonal air holes and vertical and smooth sidewalls can be fabricated successfully on the n-type GaAs (1 1 1)B substrate by SA-MOVPE. However, in such a structure, due to the inexistence of the indexdifference between the core region and the GaAs substrate, the PC slab modes are located above the light-line of the GaAs substrate, thus great radiation loss to the GaAs substrate is unavoidable. For the actual application of the above process for the PC devices, the confinement to the optical field in the vertical direction should be introduced. To achieve this aim, a sacrificial layer needs to be introduced, which can be used as the substrate for the high-quality growth of GaAs and selectively etched without damaging the formed GaAs PC patterns. AlGaAs material is the best choice for such a purpose. A proposed fabrication procedure for the air-bridge PCSs with hexagonal air holes is shown in Fig. 3. It is started with an epitaxial growth of GaAs buffer layer, Al0.5 Ga0.5 As sacrificial layer and GaAs cap layer on the GaAs (1 1 1)B substrate. Next, ZEP520 resist was spincoated on the epitaxially grown GaAs cap layer and the designed pattern was formed on it by EB lithography and selective etching of the GaAs cap layer. Then about 30 nm thick SiO2 was deposited by plasma sputtering and the pattern formed on the resist was transferred onto the SiO2 layer by the lift-off process. After the formation of the SiO2 mask, about 0.2 ␮m thick GaAs layer was grown on the exposed region by SA-MOVPE. Finally, the remaining SiO2 and the underlying Al0.5 Ga0.5 As layer were selectively etched by hydrofluoric acid solution to form the airbridge structure. Compared to the growth directly on the GaAs (1 1 1)B substrate, there are two technological challenges for the fabrication of the air-bridge GaAs PCS with hexagonal air holes. The first one is about the growth of Al0.5 Ga0.5 As sacrificial layer. In order to grow high-quality Al0.5 Ga0.5 As sacrificial

layer, the growth temperature was set to be 850 ◦ C. The partial pressure is 1.3 × 10−5 Torr for AsH3 , 3.2 × 10−6 Torr for TMG and 1.2 × 10−6 Torr for TMA. It should be pointed out that there are still some bubbles in the sacrificial layer and further optimization to the growth condition for Al0.5 Ga0.5 As is needed. The second one is about the selective etching process of the GaAs cap layer. H3 PO4 :H2 O2 :H2 O = 1.5:0.5:37.5 was used for the selective etching of GaAs cap layer. Fig. 4(a) and (b) show the SEM images for the fabricated air-bridge PCSs with hexagonal air holes and a = 0.3 ␮m and r = 0.09 ␮m. It can be seen that almost the same uniform hexagonal air holes with vertical and smooth sidewalls are formed. We also find that the undercut cladding GaAs is not perforated completely, which can be removed completed by increasing the etching time. Further optimization of the fabrication process is also needed. 4. Conclusion In summary, the calculated results indicate that the PCS with hexagonal air holes has band gaps in the guided mode spectrum that can be compared to that of the PCS with circular air holes, hence it is also a good candidate to be used for the PC devices. The PC with hexagonal air holes with a = 0.5 ␮m and r = 0.15 ␮m was fabricated successfully by SA-MOVPE. Vertical and smooth facets are formed and the uniformity is very good. The same process was also used to fabricate hexagonal air hole arrays with the width of 0.1 ␮m successfully. The air-bridge PCS with hexagonal air holes and a = 0.3 ␮m and 0.09 ␮m was also fabricated by SA-MOVPE. The hexagonal air holes are very uniform and the sidewalls are very smooth and vertical. Further optimization of the growth condition for the sacrificial layer and the selective etching process of GaAs cap layer is needed. Our experimental results indicate that SA-MOVPE growth is proved to be a promising method for fabricating PC devices and photonic nanostructures. Acknowledgments The authors thank Mr. Akamatsu for technical support in MOVPE growth. This work is partly financially supported by a Grant-in-Aid for Scientific Research and is also supported by the Japan Society of Promotion of Science (JSPS).

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Biographies Lin Yang received his BS degree in physics from Henan Normal University, Xinxiang, China, in 1997, MS degree in condensed matter physics from the Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China, in 2000, and PhD degree in microelectronics and solid state electronics from the Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China, in 2003. Now he is a JSPS postdoctoral fellow of Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo, Japan. His current research interests are photonic nanostructures, photonic crystals, waveguide devices and optical integrated circuits. Junichi Motohisa received his BS and PhD degrees from the University of Tokyo, Tokyo, Japan, in 1986 and 1993, respectively, specializing the transport properties of in-plane superlattices and quantum wires. He is currently an Associate Professor with the Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo. His research interests include the fabrication and characterization of nanostructures utilizing metal organic vapor phase epitaxial growth, and their application to electronic and photonic devices. Junichiro Takeda received his BS, MS and PhD degrees from Hokkaido University, Sapporo, Japan, in 2001, 2003 and 2006, respectively. Now he is a JSPS postdoctoral fellow of Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo, Japan. His current research interests are fabrication and characterization of compound semiconductor nanostructures and photonic crystals utilizing selective area metal organic vapor phase epitaxy. Takashi Fukui received his BS and MS degrees in applied physics and PhD degree in engineering from Hokkaido University, Sapporo, Japan in 1973, 1975 and 1983, respectively. From 1975 to 1991, Prof. Fukui was with NTT Basic Research Laboratories, Musashino, Tokyo. He was actively involved in the crystal growth of III–V compound semiconductor materials, especially for quantum structures. He was a pioneer in the formation of GaAs/InAs monolayer superlattices, GaAs quantum wires and quantum dots by metal organic chemical vapor deposition. In 1991, T. Fukui joined the Research Center for Interface Quantum Electronics (from 2001, Research Center for Integrated Quantum Electronics), Hokkaido University as Professor. His area of research encompasses the formation of quantum nanostructures and their application. He was instrumental in developing the MOVPE growth facility. By his extensive research he was successful in forming the InGaAs quantum wire lasers, GaAs single electron transistors and their logic circuits. His research interest also includes studies of optical and transport properties of GaAs and InAs quantum dots and related structures.