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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 21 — Oct. 21, 2013
  • pp: 25517–25525
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Blue-green emitting microdisks using low-temperature-grown ZnO on patterned silicon substrates

Marcel Ruth, Thomas Zentgraf, and Cedrik Meier  »View Author Affiliations


Optics Express, Vol. 21, Issue 21, pp. 25517-25525 (2013)
http://dx.doi.org/10.1364/OE.21.025517


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Abstract

Zinc oxide (ZnO) as an extremely bright emitter is an attractive material for photonic devices. However, devices made of epitaxially grown ZnO are difficult to fabricate due to the lack of selective etching processes. Here, we demonstrate that by a low-temperature growth process on pre-patterned silicon dioxide (SiO2) microdisks (MDs) high quality ZnO resonators are created. The devices exhibit whispering gallery modes (WGMs) over the blue-green part of the visible spectrum with quality factors exceeding Q = 3500, which are among the highest values reported in this material system so far. By deposition of SiO2 capping layers we find an enhanced coupling of the spontaneous emission from the active medium into the MDs, observed by sharp WGMs up to a radial quantum number of N = 3.

© 2013 Optical Society of America

1. Introduction

Photonic resonators play an important role as building blocks in integrated photonic circuits and in the field of quantum information technology. In comparison to resonators based on Fabry-Pérot interferences, circularly shaped microdisks (MDs) exhibit low losses, allowing to trap light on much longer timescales, resulting in higher quality factors, lower lasing thresholds and less demand of gain material. Therefore, MDs are ideal not only for fundamental research, e.g., in nonlinear optics, but also for micro-laser devices [1

1. M. Bürger, M. Ruth, S. Declair, J. Förstner, C. Meier, and D. J. As, “Whispering gallery modes in zinc-blende AlN microdisks containing non- polar GaN quantum dots,” Appl. Phys. Lett. 102(8), 081105 (2013). [CrossRef]

] with small spectral bandwidths and high efficiency. Furthermore, their usability as sensitive chemical detectors has been demonstrated [2

2. W. Fang, D. B. Buchholz, R. C. Bailey, J. T. Hupp, R. P. H. Chang, and H. Cao, “Detection of chemical species using ultraviolet microdisk lasers,” Appl. Phys. Lett. 85(17), 3666 (2004). [CrossRef]

]. These reports range from single-molecule analysis [3

3. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-Free, Single-Molecule Detection with Optical Microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]

] to medical and biological applications like virus detection [4

4. J. Zhu, S. K. Özdemir, L. He, D.-R. Chen, and L. Yang, “Single virus and nanoparticle size spectrometry by whispering-gallery-mode microcavities,” Opt. Express 19(17), 16195–16206 (2011). [CrossRef] [PubMed]

]. For a variety of applications in the visible (VIS) spectral range transparent materials with large band gaps are essential. Zinc oxide (ZnO) is a versatile material for electronic, optoelectronic and photonic devices in the ultraviolet (UV) spectral range due to its high emission efficiency [5

5. N. Xu, Y. Cui, Z. Hu, W. Yu, J. Sun, N. Xu, and J. Wu, “Photoluminescence and low-threshold lasing of ZnO nanorod arrays,” Opt. Express 20(14), 14857–14863 (2012). [CrossRef] [PubMed]

, 6

6. M. Ding, D. Zhao, B. Yao, S. e, Z. Guo, L. Zhang, and D. Shen, “The ultraviolet laser from individual ZnO microwire with quadrate cross section,” Opt. Express 20(13), 13657–13662 (2012). [CrossRef] [PubMed]

]. Intrinsic point defects in ZnO give rise to luminescence over a broad range in the VIS spectrum. This enables tunable resonant emission in the VIS spectral region with a single material system. Several reports have demonstrated “bottom-up” grown disk-like hexagonal flakes [7

7. D. J. Gargas, M. C. Moore, A. Ni, S.-W. Chang, Z. Zhang, S.-L. Chuang, and P. Yang, “Whispering Gallery Mode Lasing from Zinc Oxide Hexagonal Nanodisks,” ACS Nano 4(6), 3270–3276 (2010). [CrossRef] [PubMed]

, 8

8. S. S. Kim, Y.-J. Kim, G.-C. Yi, and H. Cheong, “Whispering-gallery-modelike resonance of luminescence from a single hexagonal ZnO microdisk,” J. Appl. Phys. 106(9), 094310 (2009). [CrossRef]

], as well as other resonant structures like needles [9

9. V. V. Ursaki, A. Burlacu, E. V. Rusu, V. Postolake, and I. M. Tiginyanu, “Whispering gallery modes and random lasing in ZnO microstructures,” J. Opt. A, Pure Appl. Opt. 11(7), 075001 (2009). [CrossRef]

] or self-assembled hexagonal microresonators [10

10. C. D. Dietrich, M. Lange, T. Böntgen, and M. Grundmann, “The corner effect in hexagonal whispering gallery microresonators,” Appl. Phys. Lett. 101(14), 141116 (2012). [CrossRef]

, 11

11. C. Czekalla, C. Sturm, R. Schmidt-Grund, B. Cao, M. Lorenz, and M. Grundmann, “Whispering gallery mode lasing in zinc oxide microwires,” Appl. Phys. Lett. 92(24), 241102 (2008). [CrossRef]

]. Recently, other examples for photonic devices based on self-organized ZnO devices have been reported, including lasing in ZnO nanosheets [12

12. K. Okazaki, D. Nakamura, M. Higashihata, P. Iyamperumal, and T. Okada, “Lasing characteristics of an optically pumped single ZnO nanosheet,” Opt. Express 19(21), 20389–20394 (2011). [CrossRef] [PubMed]

] and polariton confinement in ZnO nanocylinders [13

13. D. Xu, W. Liu, S. Zhang, X. Shen, and Z. Chen, “Three-dimensional confinement of polaritons in ZnO microcylinder,” Opt. Express 21(3), 3911–3916 (2013). [CrossRef] [PubMed]

].

However, up to date no fully suspended photonic devices based on single crystalline heterostructures have been demonstrated in the ZnO material system. This is due to the lack of selective dry or wet etching processes that are difficult to achieve in this material system [14

14. M. Mehta, M. Ruth, K. A. Piegdon, D. Krix, H. Nienhaus, and C. Meier, “Inductively coupled plasma reactive ion etching of bulk ZnO single crystal and molecular beam epitaxy grown ZnO films,” J. Vac. Sci. Technol. B 27(5), 2097 (2009). [CrossRef]

, 15

15. M. Mehta and C. Meier, “Controlled Etching Behavior of O-Polar and Zn-Polar ZnO Single Crystals,” J. Electrochem. Soc. 158(2), H119 (2011). [CrossRef]

]. Therefore, Liu and associates [16

16. X. Liu, W. Fang, Y. Huang, X. H. Wu, S. T. Ho, H. Cao, and R. P. H. Chang, “Optically pumped ultraviolet microdisk laser on a silicon substrate,” Appl. Phys. Lett. 84(14), 2488 (2004). [CrossRef]

] proposed the fabrication of ZnO MDs on pre-patterned silicon substrates by metal-organic vapor phase deposition (MOCVD) and demonstrated UV lasing in this system. Nevertheless, silicon (Si) and silica (SiO2) are due to the different lattice constants and thermal expansion coefficients not an ideal choice as a substrate for epitaxial growth of ZnO. Instead of two-dimensional growth, at lower growth temperatures typically the formation of polycrystalline films with small grains is observed. In spite of these fundamental challenges, silicon remains an interesting substrate for photonic ZnO-based devices as the Si planar technology is highly developed and the highest quality factors for photonic resonators so far have been achieved in the SiO2/Si system [17

17. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip,” Appl. Phys. Lett. 85(25), 6113 (2004). [CrossRef]

]. Several groups report on the successful growth directly on Si templates [18

18. J. J. Zhu, B. X. Lin, X. K. Sun, R. Yao, C. S. Shi, and Z. X. Fu, “Heteroepitaxy of ZnO film on Si(111) substrate using a 3C-SiC buffer layer,” Thin Solid Films 478(1-2), 218–222 (2005). [CrossRef]

] or by using buffer layers [19

19. W. Guo, A. Allenic, Y. B. Chen, X. Q. Pan, W. Tian, C. Adamo, and D. G. Schlom, “ZnO epitaxy on (111) Si using epitaxial Lu2O3 buffer layers,” Appl. Phys. Lett. 92(7), 072101 (2008). [CrossRef]

, 20

20. M. A. Gluba, N. H. Nickel, K. Hinrichs, and J. Rappich, “Improved passivation of the ZnO/Si interface by pulsed laser deposition,” J. Appl. Phys. 113(4), 043502 (2013). [CrossRef]

]. To overcome the problems with silicon substrates for ZnO growth, we have recently developed a low-temperature molecular beam epitaxy (MBE) growth process that leads to a very smooth, closed but polycrystalline ZnO layer of granular material on Si(111) [21

21. M. Ruth and C. Meier, “Scaling coefficient for three-dimensional grain coalescence of ZnO on Si(111),” Phys. Rev. B 86(22), 224108 (2012). [CrossRef]

, 22

22. P. Kröger, M. Ruth, N. Weber, and C. Meier, “Carrier localization in ZnO quantum wires,” Appl. Phys. Lett. 100(26), 263114 (2012). [CrossRef]

] and SiO2 [23

23. M. Ruth and C. Meier, “Structural enhancement of ZnO on SiO2 for photonic applications,” AIP Advances 3(7), 072114 (2013). [CrossRef]

]. The luminescence of these layers is dominated by an emission over a broad range of the visible spectrum centered at around 2.95 eV. Thus, devices with sharp resonances over the visible spectral range can be fabricated from LT-ZnO.

Here, we report on the structure and optical resonances of ZnO/SiO2-based photonic devices. The structural and optical properties of low-temperature (LT) grown ZnO/SiO2 MDs and the effects of a capping layer deposition on the devices are analyzed. The optical properties of the samples are investigated by confocal micro-photoluminescence (µPL), the structural properties by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD).

2. Experimental

Commercial Si(111) substrates are used as a starting material after a chemical cleaning procedure. In the first step, a 90 nm thick SiO2 layer is grown using thermal oxidation in a tube furnace at 1050° C under dry oxygen (O2) atmosphere. Using electron beam lithography (EBL), circles with diameters in the µm range are patterned into a positive resist on top of the oxide layer. Thereafter, 50 nm of aluminum (Al) are evaporated as a hard-mask layer. The EBL-defined patterns are transferred into the Al layer using a lift-off process. Subsequently, the structures are etched into the Si/SiO2 material by anisotropic etching in a dry reactive plasma. First the SiO2 layer is removed completely in a CHF3 based plasma. Afterwards, the structures are extended ~500 nm deep into the Si layer by a C4F8/SF6 process. SiO2 MDs on Si posts were then formed in a 80° C hot 1:1:4 C3H8O:KOH:H2O solution. As the KOH etching of silicon in the [100] equivalent directions is almost 1000 times faster than in the [111] direction, the post of the disk remains around 500 nm high. The finished SiO2 MDs are cleaned in a H2SO4:H2O2 solution at 80° C for 20 min followed by a rinse of water before the ZnO overgrowth. The ZnO films are grown in a vertical plasma-assisted MBE system, where oxygen radicals (O) are supplied via a radio-frequency plasma source (ν = 13.56 MHz, P = 300 W), while zinc (Zn) is provided by thermal evaporation using a double-zone effusion cell with a tantalum inset. Before growth, a pure oxygen plasma is used for additional cleaning and oxygen termination of the surface. At a substrate temperature of 150° C, ~55 nm ZnO were grown on the pre-patterned samples at an oxygen flow rate of 0.5 sccm and a Zn beam equivalent pressure of 1.4 × 10−7 Torr. Some of the samples have been capped by an additional SiO2 layer in order to study the coupling of LT-ZnO into a SiO2 membrane. This capping layer was deposited in a plasma enhanced chemical vapor deposition (PECVD) system using N2O and SiH4 at 300° C.

For the micro-photoluminescence experiments, single MDs have been excited using the 325 nm laser line of a continuous wave HeCd laser. The numerical aperture of the microscope objective was NA = 0.55, yielding a diffraction-limited focus diameter of d ≈0.7 μm with an excitation power density of 6.5 W/cm2. A spectral resolution of 0.09 nm is achieved in a Czerny-Turner monochromator with a focal length of 500 mm. The dispersed light was detected with a UV-enhanced charge-coupled device (CCD).

3. Results and discussion

The structural properties of the ~55 nm thin, LT grown ZnO films on thermal SiO2 are analyzed using AFM to obtain the surface morphology, while the crystal quality is investigated by cross-sectional SEM and XRD measurements.

As shown in Fig. 1(a)
Fig. 1 (a) 10 × 10µm2 AFM measurement, (b) cross-section SEM micrograph and (c) XRD ω/2θ scan of a 60 nm LT-grown ZnO layer on thermal SiO2 on Si(111).
, the surface of the ZnO film is smooth over a wide range of 10 × 10 μm2 revealing a RMS roughness of 0.5 nm. However, the ZnO film is polycrystalline and consists of many small grains as clearly visible in the cross-section SEM image in Fig. 1(b). Anyway, only the dominant ZnO(0002) XRD reflex is observed in a symmetric full range ω/2θ scan at around 34.4° in Fig. 1(c), while the FWHM of the corresponding rocking curve is 5.8° and therefore very broad. This indicates that the grains are highly oriented along the c-axis but slightly tilted against each other. Furthermore, an in-plane rotation of the single grains is observed by transmission electron microscopy (TEM, data not shown here). On the pre-patterned SiO2 MDs these films grow with almost vertical edges and a smooth and flat surface shape, although the granular structure is visible at the edges. Beneath the disks no ZnO is deposited due to the highly directional material flux, neither at the shaded Si(111) surface, nor at the bottom of the disk [see Fig. 2(a)
Fig. 2 SEM micrographs (a) of a 4 μm SiO2 microdisk after the LT-ZnO overgrowth and (b) of an overgrown microdisk additionally capped with 50 nm SiO2. The measurements were performed at a viewing angle of 77°.
]. After deposition of an additional SiO2 layer using PECVD, the surface exhibits a similar roughness than the LT-ZnO film while the edges appear smoother and slightly more tilted as visible in the SEM image in Fig. 2(b).

The SiO2 capping layer strongly affects the optical properties of the ZnO/SiO2 MDs, which are investigated by micro-photoluminescence (μPL) measurements. Figure 3
Fig. 3 PL spectra and illustrations of a 4 μm micro disk before (lower part, blue) and after the capping with 30 nm SiO2 by PECVD (upper part, red).
shows the room temperature μPL spectra of a 4 μm MD before (blue) and after the capping with 30 nm SiO2 (red). Both spectra are characterized by a broad emission in the visible spectral range centered at around 2.95 eV.

The observed blue-green emission from the LT-ZnO layer is very different from the well-known photoluminescence of single crystalline ZnO. The energy of the maximum intensity is clearly below the near band edge transition of 3.37 eV [24

24. Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç¸, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98, 041301 (2005).

] known from ZnO, which is also present in the spectra as a weak shoulder. It also does not fit to the typical yellow-green emission in ZnO that is attributed to intrinsic vacancies at around 2.5 eV [25

25. F. H. Leiter, H. R. Alves, A. Hofstaetter, D. M. Hofmann, and B. K. Meyer, “The oxygen vacancy as the origin of a green emission in undoped ZnO,” Phys. Status Solidi, B Basic Res. 226(1), R4–R5 (2001). [CrossRef]

, 26

26. A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, “First-principles study of native point defects in ZnO,” Phys. Rev. B 61(22), 15019–15027 (2000). [CrossRef]

]. Other intrinsic defects like interstitial Zni or Oi and antisite defects ZnO or OZn, according to recent theoretical calculations [27

27. A. Janotti and C. G. Van de Walle, “Native point defects in ZnO,” Phys. Rev. B 76(16), 165202 (2007). [CrossRef]

, 28

28. R. Ramprasad, H. Zhu, P. Rinke, and M. Scheffler, “New Perspective on Formation Energies and Energy Levels of Point Defects in Nonmetals,” Phys. Rev. Lett. 108(6), 066404 (2012). [CrossRef] [PubMed]

], do not have transition energies that could explain the blue-violet emission, too. As described above, the LT-grown ZnO film is polycrystalline with very small grain diameters of just a few nanometers. Therefore, we assign the recombination band at 2.95 eV to electron interface traps as suggested by Cordaro et al. [29

29. J. F. Cordaro, Y. Shim, and J. E. May, “Bulk electron traps in zinc oxide varistors,” J. Appl. Phys. 60(12), 4186 (1986). [CrossRef]

]. These recombinations are located at the grain boundaries, which represent a double Schottky-barrier and cause a depletion zone of some nanometers on both sides.

In Fig. 4
Fig. 4 High resolution PL spectra of a capped 4 µm MD. Different WGM types are distinguished by color and additional markers. The lower part shows the strong dispersion in the energy dependent mode spacing of the four different mode types.
the WGMs are further investigated by high resolution μPL measurements. A clear substructure of the different modes is observed illustrated by four different colors and corresponding spectral position markers underneath. The most prominent modes shown in red belong to the radial quantum number of N = 1 and are distributed over the entire investigated spectral region. These are followed by less intense WGMs with higher radial mode numbers N = 2 (green) and N = 3 (blue), localized closer to the center of the disk, where the light is absorbed by the remaining Si post. WGMs with the highest intensity are located on the low energy side of the emission curve due to the increased self-absorption at higher energies. Despite that, a fourth type of modes (orange) with very weak intensities is observed only at the high energy side, whose origin remains unclear. It should be noted that for N = 4 modes the radiative loss is higher than for modes of lower radial order, thus leading to lower quality factors for theses modes. The mode spacing versus mode energy is shown in the lower part of Fig. 4 for the four mode types. With increasing energy, the spacing between the N = 1 modes (red triangles) strongly decreases from around 54 meV at 2.4 eV to 39 meV at 3.0 eV. A similar behavior is also observed for the three other WGM types, indicating the dispersion of the underlying materials. As the refractive index of SiO2 is almost constant in this spectral region, the LT-ZnO layer is assumed to be the origin of the observed dispersion. Close to the main electronic resonance of the LT-ZnO at ELT-ZnO≈2.95 eV both the real and the imaginary part of the refractive index are increasing due to the electronic polarization which interacts with the optical fields.

The results are shown in Fig. 6
Fig. 6 FDTD-simulations of microdisks with d = 5 µm. (Upper part) Uncapped device consisting of 90 nm SiO2 and 50 nm LT-ZnO. (Lower part) Device after capping with 50 nm SiO2
. The |Hy(E)| spectra show the results of the broad pulse excitation simulation for the uncapped and the capped device. Moreover, the Hy-fields are plotted for the xz-plane through the center of the LT-ZnO layer and for a plane parallel to the y-axis for a selected mode.

For both types of devices a series of equidistant sharp resonances is found in the |Hy(E)| spectra. The peaks are caused by the WGMs localized in the periphery of the devices. In the simulations the resonances with a radial quantum number N = 1 dominate the spectra due to the position of the excitation source in the simulation, which was located close to the edge of the device so that these modes were preferably excited. With regard to the mode spacing, small differences between the capped device (ΔE = 64.1 meV) and the uncapped device (ΔE = 69.3 meV) are found. These can be explained with the better overlap of the field distribution with high-index material for the capped device, suggesting an improved confinement. This is in agreement with the simple approximation used above, from which the relation ΔE ~1/neff can be derived. The analysis of the field distribution supports these results. In both cases, the field maximum is located inside the LT-ZnO film. For the uncapped device, however, a large fraction of the field is not localized and thus subject to scattering and radiative loss. The fact that the capped device provides a much better confinement is also visible in the broad pulse computations, where an identical pulse has been used for both devices. In the case of the uncapped device, the field energy is found to decay with a rate of ~1 dB/ps (Q = 10 329), while the capped device only exhibits an energy decay rate of 0.069 dB/ps (Q = 12 800). This demonstrates that the confinement is significantly improved by the capping layer.

4. Conclusions

Acknowledgments

This work was supported by the BMBF via grant no. 03X5509 and the Deutsche Forschungsgemeinschaft (DFG) via the Research Training Group GRK 1464 “Micro- and Nanostructures for Optoelectronics and Photonics”.

References and links

1.

M. Bürger, M. Ruth, S. Declair, J. Förstner, C. Meier, and D. J. As, “Whispering gallery modes in zinc-blende AlN microdisks containing non- polar GaN quantum dots,” Appl. Phys. Lett. 102(8), 081105 (2013). [CrossRef]

2.

W. Fang, D. B. Buchholz, R. C. Bailey, J. T. Hupp, R. P. H. Chang, and H. Cao, “Detection of chemical species using ultraviolet microdisk lasers,” Appl. Phys. Lett. 85(17), 3666 (2004). [CrossRef]

3.

A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-Free, Single-Molecule Detection with Optical Microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]

4.

J. Zhu, S. K. Özdemir, L. He, D.-R. Chen, and L. Yang, “Single virus and nanoparticle size spectrometry by whispering-gallery-mode microcavities,” Opt. Express 19(17), 16195–16206 (2011). [CrossRef] [PubMed]

5.

N. Xu, Y. Cui, Z. Hu, W. Yu, J. Sun, N. Xu, and J. Wu, “Photoluminescence and low-threshold lasing of ZnO nanorod arrays,” Opt. Express 20(14), 14857–14863 (2012). [CrossRef] [PubMed]

6.

M. Ding, D. Zhao, B. Yao, S. e, Z. Guo, L. Zhang, and D. Shen, “The ultraviolet laser from individual ZnO microwire with quadrate cross section,” Opt. Express 20(13), 13657–13662 (2012). [CrossRef] [PubMed]

7.

D. J. Gargas, M. C. Moore, A. Ni, S.-W. Chang, Z. Zhang, S.-L. Chuang, and P. Yang, “Whispering Gallery Mode Lasing from Zinc Oxide Hexagonal Nanodisks,” ACS Nano 4(6), 3270–3276 (2010). [CrossRef] [PubMed]

8.

S. S. Kim, Y.-J. Kim, G.-C. Yi, and H. Cheong, “Whispering-gallery-modelike resonance of luminescence from a single hexagonal ZnO microdisk,” J. Appl. Phys. 106(9), 094310 (2009). [CrossRef]

9.

V. V. Ursaki, A. Burlacu, E. V. Rusu, V. Postolake, and I. M. Tiginyanu, “Whispering gallery modes and random lasing in ZnO microstructures,” J. Opt. A, Pure Appl. Opt. 11(7), 075001 (2009). [CrossRef]

10.

C. D. Dietrich, M. Lange, T. Böntgen, and M. Grundmann, “The corner effect in hexagonal whispering gallery microresonators,” Appl. Phys. Lett. 101(14), 141116 (2012). [CrossRef]

11.

C. Czekalla, C. Sturm, R. Schmidt-Grund, B. Cao, M. Lorenz, and M. Grundmann, “Whispering gallery mode lasing in zinc oxide microwires,” Appl. Phys. Lett. 92(24), 241102 (2008). [CrossRef]

12.

K. Okazaki, D. Nakamura, M. Higashihata, P. Iyamperumal, and T. Okada, “Lasing characteristics of an optically pumped single ZnO nanosheet,” Opt. Express 19(21), 20389–20394 (2011). [CrossRef] [PubMed]

13.

D. Xu, W. Liu, S. Zhang, X. Shen, and Z. Chen, “Three-dimensional confinement of polaritons in ZnO microcylinder,” Opt. Express 21(3), 3911–3916 (2013). [CrossRef] [PubMed]

14.

M. Mehta, M. Ruth, K. A. Piegdon, D. Krix, H. Nienhaus, and C. Meier, “Inductively coupled plasma reactive ion etching of bulk ZnO single crystal and molecular beam epitaxy grown ZnO films,” J. Vac. Sci. Technol. B 27(5), 2097 (2009). [CrossRef]

15.

M. Mehta and C. Meier, “Controlled Etching Behavior of O-Polar and Zn-Polar ZnO Single Crystals,” J. Electrochem. Soc. 158(2), H119 (2011). [CrossRef]

16.

X. Liu, W. Fang, Y. Huang, X. H. Wu, S. T. Ho, H. Cao, and R. P. H. Chang, “Optically pumped ultraviolet microdisk laser on a silicon substrate,” Appl. Phys. Lett. 84(14), 2488 (2004). [CrossRef]

17.

T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip,” Appl. Phys. Lett. 85(25), 6113 (2004). [CrossRef]

18.

J. J. Zhu, B. X. Lin, X. K. Sun, R. Yao, C. S. Shi, and Z. X. Fu, “Heteroepitaxy of ZnO film on Si(111) substrate using a 3C-SiC buffer layer,” Thin Solid Films 478(1-2), 218–222 (2005). [CrossRef]

19.

W. Guo, A. Allenic, Y. B. Chen, X. Q. Pan, W. Tian, C. Adamo, and D. G. Schlom, “ZnO epitaxy on (111) Si using epitaxial Lu2O3 buffer layers,” Appl. Phys. Lett. 92(7), 072101 (2008). [CrossRef]

20.

M. A. Gluba, N. H. Nickel, K. Hinrichs, and J. Rappich, “Improved passivation of the ZnO/Si interface by pulsed laser deposition,” J. Appl. Phys. 113(4), 043502 (2013). [CrossRef]

21.

M. Ruth and C. Meier, “Scaling coefficient for three-dimensional grain coalescence of ZnO on Si(111),” Phys. Rev. B 86(22), 224108 (2012). [CrossRef]

22.

P. Kröger, M. Ruth, N. Weber, and C. Meier, “Carrier localization in ZnO quantum wires,” Appl. Phys. Lett. 100(26), 263114 (2012). [CrossRef]

23.

M. Ruth and C. Meier, “Structural enhancement of ZnO on SiO2 for photonic applications,” AIP Advances 3(7), 072114 (2013). [CrossRef]

24.

Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç¸, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98, 041301 (2005).

25.

F. H. Leiter, H. R. Alves, A. Hofstaetter, D. M. Hofmann, and B. K. Meyer, “The oxygen vacancy as the origin of a green emission in undoped ZnO,” Phys. Status Solidi, B Basic Res. 226(1), R4–R5 (2001). [CrossRef]

26.

A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, “First-principles study of native point defects in ZnO,” Phys. Rev. B 61(22), 15019–15027 (2000). [CrossRef]

27.

A. Janotti and C. G. Van de Walle, “Native point defects in ZnO,” Phys. Rev. B 76(16), 165202 (2007). [CrossRef]

28.

R. Ramprasad, H. Zhu, P. Rinke, and M. Scheffler, “New Perspective on Formation Energies and Energy Levels of Point Defects in Nonmetals,” Phys. Rev. Lett. 108(6), 066404 (2012). [CrossRef] [PubMed]

29.

J. F. Cordaro, Y. Shim, and J. E. May, “Bulk electron traps in zinc oxide varistors,” J. Appl. Phys. 60(12), 4186 (1986). [CrossRef]

OCIS Codes
(230.5750) Optical devices : Resonators
(250.5230) Optoelectronics : Photoluminescence

ToC Category:
Optical Devices

History
Original Manuscript: July 24, 2013
Revised Manuscript: September 11, 2013
Manuscript Accepted: September 22, 2013
Published: October 18, 2013

Citation
Marcel Ruth, Thomas Zentgraf, and Cedrik Meier, "Blue-green emitting microdisks using low-temperature-grown ZnO on patterned silicon substrates," Opt. Express 21, 25517-25525 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-21-25517


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References

  1. M. Bürger, M. Ruth, S. Declair, J. Förstner, C. Meier, and D. J. As, “Whispering gallery modes in zinc-blende AlN microdisks containing non- polar GaN quantum dots,” Appl. Phys. Lett.102(8), 081105 (2013). [CrossRef]
  2. W. Fang, D. B. Buchholz, R. C. Bailey, J. T. Hupp, R. P. H. Chang, and H. Cao, “Detection of chemical species using ultraviolet microdisk lasers,” Appl. Phys. Lett.85(17), 3666 (2004). [CrossRef]
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