APPLIED PHYSICS LETTERS 88, 153120(2006) Visible range whispering-gallery mode in microdisk array based on size-controlled si nanocrystals Rong-Jun Zhang, Se-Young Seo, Alexey P. Milenin, Margit Zacharias, and Ulrich Gisele Mar-Planck-Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany (Received 8 November 2005; accepted 20 March 2006: published online 13 April 2006) Microdisks based on in-plane embedded size-controlled Si nanocrystal/SiO2 superlattices (Si-NCs/SiO SLs) were mass-fabricated arranged in well-ordered arrays. The microdisks were fabricated with size irregularity of below 0.2%o over large-scale areas. Overlappin whispering-gallery modes (WGM) of the visible nanocrystalline-silicon luminescence were observed. A comparison between analytical calculation and experimental results is reported. We found that only one axial and one radial WGM exist due to their thin disk thickness and birefringence characteristic of Si-NCS/Sio2 SLs, and that the mode spacing is 15 nm and 6 nm for microdisks with a diameter of 8.8 um and 23.7 um, respectively. The advantages of such size-controlled Si-NCS/SiO, embedded in microdisk arrays for Si-based photonic application will be discussed. 2006 American Institute of Physics. [DOI: 10.1063/1.2195712 Optical microdisks, which confine light to a small modal the microdisk. The thickness of the Sio and Sio2 layers was volume by resonant recirculation with low optical loss, 3 nm and 4 nm, respectively. Finally, an additional 50 nm have received considerable attention during the past years. buffer Sio, layer was added to protect the layered structure Si-based microphotonics have been a technical challenge The total film thickness including the top/bottom Sio with respect to mass production of highly integrated photo- layers is about 310 nm. The samples were then annealed at nic devices with the help of present mature complementary 1100C for 1 h under a N2 atmosphere to realize the metal-oxide-semiconductor technique. Photonic devices Si-NCs in the Sio2 matrix based on Si are expected at least partially to functionally Optical lithography was used to pattern various struc replace the present electronic devices. As novel examples of tures. The etching processes were undertaken using sequen- Si-based integrated photonics devices, the silicon optical tial anisotropic and isotropic dry plasma etching processes modulator and silicon Raman laser have been reported Thus, while the c-CAF8790% Ar plasma chemistry was used far. In past decades, the efficient visible luminescence at to etch anisotropically through the SiO2 layer with the room temperature from silicon nanocrystals(Si-NCs)was a embedded Si-NCs, the SF plasma was used to etch isotro- focus of interest because it might lead to the development of pically the Si substrate. 4 a si-based efficient light source 5.6 rece Finally, arrays of microdisks with diameters of 8.8 um Si-NCs, and a electroluminescent field-effect device based 23. 7 um, and 48.0 um were achieved. For comparison, an on Si-NCs, were demonstrated. 7. array of microsquares with a side length of 8.8 um was also In order to further decrease the overall size of a prepared. The microdisks were arranged in a hexagonal array Si-NC-based light source, more efficient device structures with side length of 13, 28, and 55 um, depending on the size need to be adopted. One candidate for such a structure is the of the microdisks with 8.8 um, 23. 7 um, and 48.0 um di- microdisk resonator, which has an excellent o value and ameter, respectively. For the microsquare arrays, we chose a many advantages for lasing which is realized by nearly infi- square lattice arrangement, with a lattice constant of nite light circulations of whispering-gallery modes(WGMs) 12.5 um. As a result, the peripheries of microdisks or within a small volume. Even though a high-o value and microsquares were separated from their neighboring ones lasing were already observed from silica microdisks, by at least -4 um, ensuring a sufficient separation to effec little has been reported demonstrating whispering-gallery tively exclude optical coupling between microdisks or modes of the visible luminescence from si-NCs in microsquares microdisk oom-temperature photoluminescence(PL) was mea- In this letter, we present the mass fabrication of micro- sured using liquid-nitrogen-cooled charge-coupled-device disk based on Si-NCs in an arrayed structure, and the camera attached to a single monochromator. The 488 nm line observation of WGMs from them. We will show the clear of an Ar laser with a power density of 6 W/cm? was used as emergence of WGMs in microdisk arrays. excitation source. The Si-NCs/SiO, SL of an unstructured The preparation of multilayers of size-controlled part, and the arrays of microdisks or microsquares were mea- Si-NCs in Sio, matrix was reported elsewhere. As a first sured under the same conditions. All spectra were corrected step, a 50 nm buffer Sio2 layer was evaporated on Si(100) for the wavelength depending sensitivity of the measuring wafers.Then, 30 periods of the SiO, /SiO2 superlattices system. Scanning electron microscopy (SEM) was performed (SLs)were evaporated on top as the active emitting using a JSEM 6300F. Figure 1(a) shows the SEM image of an array of micro- of Optical Science and Engineering, Fudan disks with a diameter of 8.8 um. The hexagonal arrangement of the microdisks can clearly be seen. No misshaped disk was found by the SEM investigationVisible range whispering-gallery mode in microdisk array based on size-controlled Si nanocrystals Rong-Jun Zhang,a
Se-Young Seo, Alexey P. Milenin, Margit Zacharias,b
and Ulrich Gösele Max-Planck-Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany
Received 8 November 2005; accepted 20 March 2006; published online 13 April 2006 Microdisks based on in-plane embedded size-controlled Si nanocrystal/SiO2 superlattices
Si-NCs/SiO2 SLs were mass-fabricated arranged in well-ordered arrays. The microdisks were fabricated with size irregularity of below 0.2% over large-scale areas. Overlapping whispering-gallery modes
WGM of the visible nanocrystalline-silicon luminescence were observed. A comparison between analytical calculation and experimental results is reported. We found that only one axial and one radial WGM exist due to their thin disk thickness and birefringence characteristic of Si-NCs/SiO2 SLs, and that the mode spacing is 15 nm and 6 nm for microdisks with a diameter of 8.8
m and 23.7
m, respectively. The advantages of such size-controlled Si-NCs/SiO2 embedded in microdisk arrays for Si-based photonic application will be discussed. © 2006 American Institute of Physics. DOI: 10.1063/1.2195712 Optical microdisks, which confine light to a small modal volume by resonant recirculation with low optical loss, have received considerable attention during the past years. Si-based microphotonics have been a technical challenge with respect to mass production of highly integrated photo￾nic devices with the help of present mature complementary metal-oxide-semiconductor technique.1 Photonic devices based on Si are expected at least partially to functionally replace the present electronic devices. As novel examples of Si-based integrated photonics devices, the silicon optical modulator and silicon Raman laser have been reported so far.2–4 In past decades, the efficient visible luminescence at room temperature from silicon nanocrystals
Si-NCs was a focus of interest because it might lead to the development of a Si-based efficient light source.5,6 Recently, optical gain in Si-NCs, and a electroluminescent field-effect device based on Si-NCs, were demonstrated.7,8 In order to further decrease the overall size of a Si-NC-based light source, more efficient device structures need to be adopted. One candidate for such a structure is the microdisk resonator, which has an excellent Q value and many advantages for lasing which is realized by nearly infi- nite light circulations of whispering-gallery modes
WGMs within a small volume.9 Even though a high-Q value and lasing were already observed from silica microdisks,10,11 little has been reported demonstrating whispering-gallery modes of the visible luminescence from Si-NCs in a microdisk.12 In this letter, we present the mass fabrication of micro￾disk based on Si-NCs in an arrayed structure, and the observation of WGMs from them. We will show the clear emergence of WGMs in microdisk arrays. The preparation of multilayers of size-controlled Si-NCs in SiO2 matrix was reported elsewhere.13 As a first step, a 50 nm buffer SiO2 layer was evaporated on Si
100 wafers. Then, 30 periods of the SiOx /SiO2 superlattices
SLs were evaporated on top as the active emitting part of the microdisk. The thickness of the SiOx and SiO2 layers was 3 nm and 4 nm, respectively. Finally, an additional 50 nm buffer SiO2 layer was added to protect the layered structure. The total film thickness including the top/bottom SiO2 layers is about 310 nm. The samples were then annealed at 1100 °C for 1 h under a N2 atmosphere to realize the Si-NCs in the SiO2 matrix. Optical lithography was used to pattern various struc￾tures. The etching processes were undertaken using sequen￾tial anisotropic and isotropic dry plasma etching processes. Thus, while the c-C4F8 / 90%Ar plasma chemistry was used to etch anisotropically through the SiO2 layer with the embedded Si-NCs, the SF6 plasma was used to etch isotro￾pically the Si substrate.14 Finally, arrays of microdisks with diameters of 8.8
m, 23.7
m, and 48.0
m were achieved. For comparison, an array of microsquares with a side length of 8.8
m was also prepared. The microdisks were arranged in a hexagonal array with side length of 13, 28, and 55
m, depending on the size of the microdisks with 8.8
m, 23.7
m, and 48.0
m di￾ameter, respectively. For the microsquare arrays, we chose a square lattice arrangement, with a lattice constant of 12.5
m. As a result, the peripheries of microdisks or microsquares were separated from their neighboring ones by at least 4
m, ensuring a sufficient separation to effec￾tively exclude optical coupling between microdisks or microsquares. Room-temperature photoluminescence
PL was mea￾sured using liquid-nitrogen-cooled charge-coupled-device camera attached to a single monochromator. The 488 nm line of an Ar laser with a power density of 6 W/cm2 was used as excitation source. The Si-NCs/SiO2 SL of an unstructured part, and the arrays of microdisks or microsquares were mea￾sured under the same conditions. All spectra were corrected for the wavelength depending sensitivity of the measuring system. Scanning electron microscopy
SEM was performed using a JSEM 6300F. Figure 1
a shows the SEM image of an array of micro￾disks with a diameter of 8.8
m. The hexagonal arrangement of the microdisks can clearly be seen. No defective/ misshaped disk was found by the SEM investigation over the a Present address: Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China. b Electronic mail: zacharia@mpi-halle.de APPLIED PHYSICS LETTERS 88, 153120
2006 0003-6951/2006/8815
/153120/3/$23.00 © 2006 American Institute of Physics 88, 153120-1 Downloaded 24 Apr 2006 to 149.220.35.134. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp