A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated. © 2005 American Institute of Physics.published_or_final_versio

Choi, HW

Chua, SJ

Dawson, MD

Edwards, PR

Jeon, CW

Liu, C

Martin, RW

Tripathy, S

Watson, IM

Applied Physics Letters

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated

Choi, H.W.

Jeon, C.W.

Liu, C.

Watson, I.M.

Dawson, M.D.

Edwards, P.R.

Martin, R.W.

Tripathy, S.

Chua, S.J.

Name not available

InGaN nano-ring structures for high-efficiency light emitting diodes

Choi, H. W.

Jeon, C. W.

Watson, I. M.

Dawson, M. D.

Edwards, P. R.

Martin, R. W.

Chua, S. J.

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryChoi, H.W. and Jeon, C.W. and Liu, C. and Watson, I.M. and Dawson, M.D. and Edwards, P.R. andMartin, R.W. and Tripathy, S. and Chua, S.J. (2005) InGaN nano-ring structures for high-efficiencylight emitting diodes. Applied Physics Letters, 86 (2). ISSN 0003-6951Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/InGaN nano-ring structures for high-efficiency light emitting diodesH. W. Choi,a) C. W. Jeon, C. Liu, I. M. Watson, and M. D. DawsonInstitute of Photonics, University of Strathclyde, Glasgow G4 ONW, United KingdomP. R. Edwards and R. W. MartinDepartment of Physics, University of Strathclyde, Glasgow G4 0NG, United KingdomS. Tripathy and S. J. ChuaInstitute of Materials Research and Engineering, 3 Research Link, Singapore 117602(Received 6 August 2004; accepted 9 November 2004; published online 30 December 2004)A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlledring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ringstructures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm−1 Raman shifthas been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nmblueshift observed in the cathodoluminescence spectra can be attributed to band filling and/orscreening of the piezoelectric field. A light emitting diode based on this geometry has beendemonstrated. © 2005 American Institute of Physics. [DOI: 10.1063/1.1849439]Various forms of the InGaN-based microscale light emit-ting diodes (LEDs) have been reported in recent years,1,2 allof which indicate that an enhanced efficiency can beachieved with such microstructures. The overall externalquantum efficiency of an InGaN/GaN LED is heavily depen-dent on the internal quantum efficiency and the extractionefficiency. However, the presence of internal piezoelectricfields and spontaneous polarization limits the quantum effi-ciency to approximately 30%3 due to quantum-confinedStark effects,4 while the high refractive index of nitride ma-terials means that most of the light emitted from the activeregion remains trapped in the mesa structure due to totalinternal reflection at the LED-air interface.5 A significant in-crease in extraction efficiency has already been demonstratedin the current generation of micro-LEDs, where the dimen-sion of an individual element is typically larger than 4 mm.This is due to a higher surface area to light generation arearatio and reduced absorption. However, in order to achieve areduction of the piezoelectric field in the InGaN quantumwells, the dimension of the micro-LED should be furtherreduced. In fact, Demangeot et al. observed significant strainrelaxation in InGaN microstructures of less than 1 mmthrough micro-Raman scattering,6 suggesting that micro-LEDs should be scaled down to submicron dimensions inorder to benefit from an increase in internal quantum effi-ciency.In this letter, we report on the fabrication of nanoscaleInGaN ring structures using standard microfabrication tech-niques. The objective of fabricating this structure is to pro-duce a LED with high extraction efficiency and internalquantum efficiency. This can be achieved if partial or com-plete strain relaxation takes place in the nano-ring structures,leading to a reduction in the piezoelectric field, together withan increased surface area to volume ratio. A threefold in-crease in light output can be expected. The fabricated ringstructures, along with the as-grown wafer, microdisks (withdiameter of 12 mm) and microrings (with internal/externaldiameters of 12/20 mm) are investigated by Raman spectros-copy and cathodoluminescence (CL) to detect any degree ofstrain relaxation. A LED based on an array of nano-rings hasbeen demonstrated.Disk structures are patterned using AZ1805 photoresistas a masking material and standard photolithography tech-niques (using a 365 nm excitation source). The ring struc-tures are spontaneously formed during photoresist develop-ment. They are subsequently transferred to the nitridematerial by inductively coupled plasma (ICP) etching in anSTS Multiplex ICP system. The sample used consist of athree-period InGaN (3 nm)/GaN (7 nm) multi-quantum well(MQW) structure grown by metalorganic chemical vapordeposition, with emission wavelength targeted for 470 nm.The extent of strain relaxation in the structures is evaluatedby Raman spectroscopy using the 514 nm line of an Ar+ laseras an excition source focused to a spot of less than 1 mm.The scattered light was detected in the backscattering geom-etry with a Jobin-Yvon T64000 triple-grating spectrmeterwith a LN2-cooled charge coupled device detector. The ringstructures were also examined by cathodoluminescence (CL)in an electron probe microanalyzer (Cameca SX100).The ring structures are designed to have an internal di-ameter of 500 nm and a wall width of about 300 nm. Such apattern cannot be imaged directly using 365 nm lithographytechniques. Instead, 1.5 mm microdisk structures are pat-terned onto photoresist. Upon development, a ring structureis spontaneously formed. The evolution of the ring structureduring the development process is illustrated in the atomicforce microscopy (AFM) images in Fig. 1. This phenomenonmay be explained by the Fresnel diffraction effect.7 The mi-crodisk array pattern on the mask, apart from playing the roleof image transfer, also acts as a three-dimensional diffractiongrating. Consider the cross section of the microdisk arraypattern on the photomask as shown in Fig. 2(a). This is es-sentially a two-dimensional N-slit diffraction grating with aslit width of 2 mm (separation of the microdisks) and a slitspacing of 1.5 mm (diameter of the microdisks). During lightexposure, the photoresist-coated sample is held in proximityto the photomask, as depicted in Fig. 2(b). When monochro-matic light (of 365 nm) passes through it, a diffraction pat-a)Author to whom correspondence should be addressed; present address:Department of Electrical and Electronic Engineering, University of HongKong, Pokfulam Road, Hong Kong; electronic mail: hwchoi@eee.hku.hkAPPLIED PHYSICS LETTERS 86, 021101 (2005)0003-6951/2005/86(2)/021101/3/$22.50 © 2005 American Institute of Physics86, 021101-1Downloaded 10 Jan 2005 to 130.159.248.44. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsptern is formed. The simulated intensity plot of the diffractionpattern is shown in Fig. 2(c). While the maxima occur at thecenter of the slits, secondary peaks are observed at positionscorresponding to the center of the microdisks. These second-ary peaks are a result of first and higher order diffractionpatterns. As a result, the centers of the microdisk regions areexposed and a hole is spontaneously formed upon develop-ment. The intensity of the secondary peaks is naturally lowerthan those of the primary peaks; this explains the lower rateof development of the aperture in the center of the microdiskcompared to the external exposed regions. The nano-ringphotoresist pattern is subsequently transferred onto the GaNmaterial by ICP etching using a combination of Cl2/Ar asetchant gases. The etching was terminated just beneath theMQW layers, ensuring that strain relaxation can occur in thelight emission region. A cross-section image of an individualnano-ring obtained from AFM scanning is illustrated in theinset of Fig. 3. The nano-ring has an internal diameter of 500nm and a wall width of 300 nm.The fabricated ring structures are subjected to examina-tions by Raman spectroscopy. The E2 mode frequency fromthe Raman spectra is used to monitor the strain component inthe structure, since it is much more sensitive to strain com-pared to other Raman modes due to its independence fromthe free carrier concentration. The E2 line will shift towardsthe higher-frequency region with increasing compressivestrain by 2.9 cm−1/GPa.8 Raman spectra taken from the as-grown sample, nano-ring, microring and microdisk structuresare illustrated in Fig. 3. Apart from the nano-ring structure,the spectra have peaks at 568.4 cm−1, typical of compres-sively strained as-grown GaN. This is consistent with thefindings of Demangeot et al., where no significant shift wasobserved for reactive-ion etched pillars with dimensionsgreater than 1 mm. However, a significant shift was recordedin the spectra of the nano-ring structure. In fact, its E2 modecenter frequency of 567.3 cm−1 was close to that of a spec-trum measured from a piece of free-standing GaN (whichFIG. 3. Raman spectra for the as-grown, microdisk, microring and nano-ring structures. A cross-sectional profile of a single nano-ring element isshown in the inset of the figure.FIG. 1. (Color online) AFM images s10310 mmd illustrating the evolution of the ring structure during photoresist development at (a) t=15 s and (b) t=30 s. The ring pattern array transferred onto the nitride material by ICP etching is shown in (c).FIG. 2. (a) Schematic image of the microdisk circular mask pattern used. (b)The photomask acting as a diffraction grating during exposure. (c) Simu-lated intensity plot of the diffraction pattern.021101-2 Choi et al. Appl. Phys. Lett. 86, 021101 (2005)Downloaded 10 Jan 2005 to 130.159.248.44. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspcan be assumed to be strain free). We thus conclude thatsignificant strain relaxation has taken place in the formationof the nano-ring structures. The fabricated structures werealso excited with a 20 keV–1 nA electron beam, and thecollected CL spectra are shown in Fig. 4. A consistent blue-shift was observed as the dimension of the structures isscaled down, reaching a maximum of 9 nm from the nano-ring structure compared to the as-grown sample. The blue-shift can be attributed to band-filling effects and/or screeningof the piezoelectric field by the carriers, as the current den-sities in the various structures increased as the dimensionreduces.A LED adopting the nano-ring structure has been fabri-cated. The light emission region consists of a high-densityarray of nano-ring mesa structures. Ni/Au ohmic contacts aredeposited onto the p-type GaN to allow for current injection.However, the metallization must not create a shortage path-way between the n-type and p-type regions. In order toachieve this, the “self-aligned” contact opening process isadopted, which is also employed for the fabrication of mi-croring LEDs. Details of the fabrication process can be foundin Refs. 2 and 9. A microphotograph of light emission fromthe device is shown in Fig. 5. Each pixel of light representslight emission from a single nano-ring element. At present,not all the elements are operational. This is attributed to thecomplexity of the fabrication process, especially the diffi-cultly in establishing an ohmic contact to the individual sub-micron elements. Nevertheless, this represents a demonstra-tion of electroluminescence from a nano-scale MQWstructure formed by structuring using microfabricationtechniques.In summary, the fabrication of nano-ring structures withan internal diameter of 500 nm and a wall width of 300 nmhas been reported. The mechanism of spontaneous assemblyof the ring structures making use of Fresnel diffraction ef-fects is proposed. Substantial strain relaxation in the struc-tures has been observed through Raman spectroscopy. Blue-shifts in the CL spectra have also been observed, attributed toband-filling effects and/or screening of the piezoelectricfields. A LED based on the nano-ring structure has also beendemonstrated.1S. X. Jin, J. Li, J. Y. Lin and H. X. Jiang, Appl. Phys. Lett. 77, 3236(2000).2H. W. Choi, C. W. Jeon, M. D. Dawson, P. R. Edwards, and R. W. Martin,IEEE Photonics Technol. Lett. 15, 510 (2003).3M. R. Krames, J. Bhat, D. Collins, N. F. Gardner, W. Gotz, C. H. Lowery,M. Ludowise, P. S. Martin, G. Mueller, R. Mueller-Mach, S. Rudaz, D. A.Steigerwald, S. A. Stockman and J. J. Wierer, Phys. Status Solidi A 192,237 (2002).4S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 69,4188 (1996).5H. X. Jiang and J. Y. Lin, Crit. Rev. Solid State Mater. Sci. 28, 131(2003).6F. Demangeot, J. Gleize, J. Frandon, M. A. Renucci, M. Kuball, D. Pey-rade, L. Manin-Ferlazzo, Y. Chen, and N. Grandjean, J. Appl. Phys. 91,6520 (2002).7S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era (Lattice,Sunset Beach, CA, 2000), Vol. 1, p. 560.8F. Demangeot, J. Frandon, M. A. Renucci, O. Briot, B. Gil, and R. L.Aulumbard, Solid State Commun. 100, 207 (1996).9H. W. Choi, C. W. Jeon, and M. D. Dawson, Appl. Phys. Lett. 83, 4483(2003).FIG. 4. Normalized CL spectra of the as-grown, microdisk, microring andnano-ring structures.FIG. 5. (Color online) Microphotograph of light emission from a submicronring LED. Each pixel of light represents light emission from a single nano-ring element.021101-3 Choi et al. Appl. Phys. Lett. 86, 021101 (2005)Downloaded 10 Jan 2005 to 130.159.248.44. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Title InGaN nano-ring structures for high-efficiency light emittingdiodesAuthor(s) Choi, HW; Jeon, CW; Liu, C; Watson, IM; Dawson, MD; Edwards,PR; Martin, RW; Tripathy, S; Chua, SJCitation Applied Physics Letters, 2005, v. 86 n. 2, p. 021101-1-021101-3Issued Date 2005URL http://hdl.handle.net/10722/42697Rights Creative Commons: Attribution 3.0 Hong Kong LicenseInGaN nano-ring structures for high-efficiency light emitting diodesH. W. Choi,a) C. W. Jeon, C. Liu, I. M. Watson, and M. D. DawsonInstitute of Photonics, University of Strathclyde, Glasgow G4 ONW, United KingdomP. R. Edwards and R. W. MartinDepartment of Physics, University of Strathclyde, Glasgow G4 0NG, United KingdomS. Tripathy and S. J. ChuaInstitute of Materials Research and Engineering, 3 Research Link, Singapore 117602(Received 6 August 2004; accepted 9 November 2004; published online 30 December 2004)A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlledring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ringstructures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm−1 Raman shifthas been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nmblueshift observed in the cathodoluminescence spectra can be attributed to band filling and/orscreening of the piezoelectric field. A light emitting diode based on this geometry has beendemonstrated. © 2005 American Institute of Physics. [DOI: 10.1063/1.1849439]Various forms of the InGaN-based microscale light emit-ting diodes (LEDs) have been reported in recent years,1,2 allof which indicate that an enhanced efficiency can beachieved with such microstructures. The overall externalquantum efficiency of an InGaN/GaN LED is heavily depen-dent on the internal quantum efficiency and the extractionefficiency. However, the presence of internal piezoelectricfields and spontaneous polarization limits the quantum effi-ciency to approximately 30%3 due to quantum-confinedStark effects,4 while the high refractive index of nitride ma-terials means that most of the light emitted from the activeregion remains trapped in the mesa structure due to totalinternal reflection at the LED-air interface.5 A significant in-crease in extraction efficiency has already been demonstratedin the current generation of micro-LEDs, where the dimen-sion of an individual element is typically larger than 4 mm.This is due to a higher surface area to light generation arearatio and reduced absorption. However, in order to achieve areduction of the piezoelectric field in the InGaN quantumwells, the dimension of the micro-LED should be furtherreduced. In fact, Demangeot et al. observed significant strainrelaxation in InGaN microstructures of less than 1 mmthrough micro-Raman scattering,6 suggesting that micro-LEDs should be scaled down to submicron dimensions inorder to benefit from an increase in internal quantum effi-ciency.In this letter, we report on the fabrication of nanoscaleInGaN ring structures using standard microfabrication tech-niques. The objective of fabricating this structure is to pro-duce a LED with high extraction efficiency and internalquantum efficiency. This can be achieved if partial or com-plete strain relaxation takes place in the nano-ring structures,leading to a reduction in the piezoelectric field, together withan increased surface area to volume ratio. A threefold in-crease in light output can be expected. The fabricated ringstructures, along with the as-grown wafer, microdisks (withdiameter of 12 mm) and microrings (with internal/externaldiameters of 12/20 mm) are investigated by Raman spectros-copy and cathodoluminescence (CL) to detect any degree ofstrain relaxation. A LED based on an array of nano-rings hasbeen demonstrated.Disk structures are patterned using AZ1805 photoresistas a masking material and standard photolithography tech-niques (using a 365 nm excitation source). The ring struc-tures are spontaneously formed during photoresist develop-ment. They are subsequently transferred to the nitridematerial by inductively coupled plasma (ICP) etching in anSTS Multiplex ICP system. The sample used consist of athree-period InGaN (3 nm)/GaN (7 nm) multi-quantum well(MQW) structure grown by metalorganic chemical vapordeposition, with emission wavelength targeted for 470 nm.The extent of strain relaxation in the structures is evaluatedby Raman spectroscopy using the 514 nm line of an Ar+ laseras an excition source focused to a spot of less than 1 mm.The scattered light was detected in the backscattering geom-etry with a Jobin-Yvon T64000 triple-grating spectrmeterwith a LN2-cooled charge coupled device detector. The ringstructures were also examined by cathodoluminescence (CL)in an electron probe microanalyzer (Cameca SX100).The ring structures are designed to have an internal di-ameter of 500 nm and a wall width of about 300 nm. Such apattern cannot be imaged directly using 365 nm lithographytechniques. Instead, 1.5 mm microdisk structures are pat-terned onto photoresist. Upon development, a ring structureis spontaneously formed. The evolution of the ring structureduring the development process is illustrated in the atomicforce microscopy (AFM) images in Fig. 1. This phenomenonmay be explained by the Fresnel diffraction effect.7 The mi-crodisk array pattern on the mask, apart from playing the roleof image transfer, also acts as a three-dimensional diffractiongrating. Consider the cross section of the microdisk arraypattern on the photomask as shown in Fig. 2(a). This is es-sentially a two-dimensional N-slit diffraction grating with aslit width of 2 mm (separation of the microdisks) and a slitspacing of 1.5 mm (diameter of the microdisks). During lightexposure, the photoresist-coated sample is held in proximityto the photomask, as depicted in Fig. 2(b). When monochro-matic light (of 365 nm) passes through it, a diffraction pat-a)Author to whom correspondence should be addressed; present address:Department of Electrical and Electronic Engineering, University of HongKong, Pokfulam Road, Hong Kong; electronic mail: hwchoi@eee.hku.hkAPPLIED PHYSICS LETTERS 86, 021101 (2005)0003-6951/2005/86(2)/021101/3/$22.50 © 2005 American Institute of Physics86, 021101-1Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsptern is formed. The simulated intensity plot of the diffractionpattern is shown in Fig. 2(c). While the maxima occur at thecenter of the slits, secondary peaks are observed at positionscorresponding to the center of the microdisks. These second-ary peaks are a result of first and higher order diffractionpatterns. As a result, the centers of the microdisk regions areexposed and a hole is spontaneously formed upon develop-ment. The intensity of the secondary peaks is naturally lowerthan those of the primary peaks; this explains the lower rateof development of the aperture in the center of the microdiskcompared to the external exposed regions. The nano-ringphotoresist pattern is subsequently transferred onto the GaNmaterial by ICP etching using a combination of Cl2/Ar asetchant gases. The etching was terminated just beneath theMQW layers, ensuring that strain relaxation can occur in thelight emission region. A cross-section image of an individualnano-ring obtained from AFM scanning is illustrated in theinset of Fig. 3. The nano-ring has an internal diameter of 500nm and a wall width of 300 nm.The fabricated ring structures are subjected to examina-tions by Raman spectroscopy. The E2 mode frequency fromthe Raman spectra is used to monitor the strain component inthe structure, since it is much more sensitive to strain com-pared to other Raman modes due to its independence fromthe free carrier concentration. The E2 line will shift towardsthe higher-frequency region with increasing compressivestrain by 2.9 cm−1/GPa.8 Raman spectra taken from the as-grown sample, nano-ring, microring and microdisk structuresare illustrated in Fig. 3. Apart from the nano-ring structure,the spectra have peaks at 568.4 cm−1, typical of compres-sively strained as-grown GaN. This is consistent with thefindings of Demangeot et al., where no significant shift wasobserved for reactive-ion etched pillars with dimensionsgreater than 1 mm. However, a significant shift was recordedin the spectra of the nano-ring structure. In fact, its E2 modecenter frequency of 567.3 cm−1 was close to that of a spec-trum measured from a piece of free-standing GaN (whichFIG. 3. Raman spectra for the as-grown, microdisk, microring and nano-ring structures. A cross-sectional profile of a single nano-ring element isshown in the inset of the figure.FIG. 1. (Color online) AFM images s10310 mmd illustrating the evolution of the ring structure during photoresist development at (a) t=15 s and (b) t=30 s. The ring pattern array transferred onto the nitride material by ICP etching is shown in (c).FIG. 2. (a) Schematic image of the microdisk circular mask pattern used. (b)The photomask acting as a diffraction grating during exposure. (c) Simu-lated intensity plot of the diffraction pattern.021101-2 Choi et al. Appl. Phys. Lett. 86, 021101 (2005)Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspcan be assumed to be strain free). We thus conclude thatsignificant strain relaxation has taken place in the formationof the nano-ring structures. The fabricated structures werealso excited with a 20 keV–1 nA electron beam, and thecollected CL spectra are shown in Fig. 4. A consistent blue-shift was observed as the dimension of the structures isscaled down, reaching a maximum of 9 nm from the nano-ring structure compared to the as-grown sample. The blue-shift can be attributed to band-filling effects and/or screeningof the piezoelectric field by the carriers, as the current den-sities in the various structures increased as the dimensionreduces.A LED adopting the nano-ring structure has been fabri-cated. The light emission region consists of a high-densityarray of nano-ring mesa structures. Ni/Au ohmic contacts aredeposited onto the p-type GaN to allow for current injection.However, the metallization must not create a shortage path-way between the n-type and p-type regions. In order toachieve this, the “self-aligned” contact opening process isadopted, which is also employed for the fabrication of mi-croring LEDs. Details of the fabrication process can be foundin Refs. 2 and 9. A microphotograph of light emission fromthe device is shown in Fig. 5. Each pixel of light representslight emission from a single nano-ring element. At present,not all the elements are operational. This is attributed to thecomplexity of the fabrication process, especially the diffi-cultly in establishing an ohmic contact to the individual sub-micron elements. Nevertheless, this represents a demonstra-tion of electroluminescence from a nano-scale MQWstructure formed by structuring using microfabricationtechniques.In summary, the fabrication of nano-ring structures withan internal diameter of 500 nm and a wall width of 300 nmhas been reported. The mechanism of spontaneous assemblyof the ring structures making use of Fresnel diffraction ef-fects is proposed. Substantial strain relaxation in the struc-tures has been observed through Raman spectroscopy. Blue-shifts in the CL spectra have also been observed, attributed toband-filling effects and/or screening of the piezoelectricfields. A LED based on the nano-ring structure has also beendemonstrated.1S. X. Jin, J. Li, J. Y. Lin and H. X. Jiang, Appl. Phys. Lett. 77, 3236(2000).2H. W. Choi, C. W. Jeon, M. D. Dawson, P. R. Edwards, and R. W. Martin,IEEE Photonics Technol. Lett. 15, 510 (2003).3M. R. Krames, J. Bhat, D. Collins, N. F. Gardner, W. Gotz, C. H. Lowery,M. Ludowise, P. S. Martin, G. Mueller, R. Mueller-Mach, S. Rudaz, D. A.Steigerwald, S. A. Stockman and J. J. Wierer, Phys. Status Solidi A 192,237 (2002).4S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 69,4188 (1996).5H. X. Jiang and J. Y. Lin, Crit. Rev. Solid State Mater. Sci. 28, 131(2003).6F. Demangeot, J. Gleize, J. Frandon, M. A. Renucci, M. Kuball, D. Pey-rade, L. Manin-Ferlazzo, Y. Chen, and N. Grandjean, J. Appl. Phys. 91,6520 (2002).7S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era (Lattice,Sunset Beach, CA, 2000), Vol. 1, p. 560.8F. Demangeot, J. Frandon, M. A. Renucci, O. Briot, B. Gil, and R. L.Aulumbard, Solid State Commun. 100, 207 (1996).9H. W. Choi, C. W. Jeon, and M. D. Dawson, Appl. Phys. Lett. 83, 4483(2003).FIG. 4. Normalized CL spectra of the as-grown, microdisk, microring andnano-ring structures.FIG. 5. (Color online) Microphotograph of light emission from a submicronring LED. Each pixel of light represents light emission from a single nano-ring element.021101-3 Choi et al. Appl. Phys. Lett. 86, 021101 (2005)Downloaded 08 Nov 2006 to 147.8.21.97. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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InGaN nano-ring structures for high-efficiency light emitting diodes

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