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APPLIED PHYSICS LETTERS 87, 163121 共2005兲 Near-red emission from site-controlled pyramidal InGaN quantum dots V. Pérez-Solórzano,a兲 A. Gröning, and M. Jetter 4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany T. Riemann and J. Christen Institut für Experimentelle Physik, Otto-von-Guericke Universität, Universitätsplatz 2, 39160 Magdeburg, Germany 共Received 20 April 2005; accepted 24 August 2005; published online 14 October 2005兲 We have fabricated InGaN nanostructures on top of GaN hexagonal pyramids by selective metalorganic vapor-phase epitaxy. With this approach, we are able to exactly control the position of the emitting quantum dot, which is an essential requirement for functionalized single-photon emitters. The emission properties as well as the relaxation and recombination mechanisms were investigated using spectroscopic methods. Regions of different confinement were identified, with the photoluminescence emission from the InGaN quantum dots around 2.03 eV and a decay time of 1.4 ns. The constant temperature behavior of the radiative decay time confirms its zero-dimensional character. Spatially resolved cathodoluminescence measurements attribute this emission to the apex of the pyramid. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2108126兴 Local composition fluctuations acting as quantum dots 共QDs兲 in ternary InGaN alloys have been proposed to be responsible for the high efficiency of blue-green lightemitting devices.1 The growth of such QDs has been demonstrated via the Stranski-Krastanov growth mode2,3 and other routes.4 However, these methods do not allow the control of the position and even the size of the nanostructures, which is an essential requirement for applications such as singlephoton emitters. Photon antibunching observed from sitecontrolled InGaAs/ AlGaAs QDs grown by selective epitaxy on patterned substrates5 confirms this method as an alternative to self-assembly. For GaN hexagonal pyramids, optically pumped laser action6 and more than three different groups of optical resonance modes have been observed,7 making them suitable as high-finesse cavities via total internal reflection inside the pyramids. Recent studies investigating the optical properties of selectively grown 5 ⫻ InGaN / GaN QD layers8 showed narrow emission peaks 共⬃650 eV兲 originating in the pyramid apex with a decay lifetime of about 0.5 ns. In this Letter we report on the growth and characterization of site-controlled InGaN QDs located on GaN micropyramids. A SiO2 mask deposited on 1 m high-quality GaN grown by metalorganic vapor-phase epitaxy was patterned using electron-beam lithography. The subsequent overgrowth with GaN results in hexagonal pyramidal structures with facets formed by 兵101គ 1其 planes, arising from the wurzite structure. InGaN with a nominal thickness of 3 nm was deposited on the micropyramids, and afterwards a GaN barrier was grown under the same conditions. The growth method has been described in detail elsewhere.9 An InGaN quantum well grown under the same conditions on planar GaN has an In content of about 18%. Time-integrated and time-resolved single-photon counting photoluminescence 共PL兲 measurements were performed using a frequency-doubled Ti:sapphire laser at 325 nm 共200 fs pulse width兲 which was focused to a spot-size of approximately 20 m in a temperature-controlled cryostat. The PL spectra were collected using a 0.32 m Jobin-Yvon monochromator with a 1800 grooves/ mm grating and a fast multichannel plate photomultiplier tube. Cathodoluminescence 共CL兲 imaging was carried out in a scanning electron microscope 共SEM兲, operated at 5 keV and an electron-beam current of 400 pA, with a spatial resolution of 40 nm. CL spectra shown here have been taken under continuous-wave 共cw兲 excitation. Figure 1 shows the side-view SEM image of the completely overgrown sample under investigation. The pyramids are connected to the substrate only by the material deposited in the predefined windows. When they are filled, the lateral growth over the SiO2 mask begins with a much lower growth rate compared to the vertical direction, which allows the formation of structures of more than 2 m height. In the present case, holes with 1 m diameter and a separation between centers of 5 m were patterned. The selectivity of the SiO2 is also excellent, as we could not observe any material deposition apart from the openings. a兲 FIG. 1. SEM side view of the sample under investigation. Material deposition was observed in the openings of the SiO2 mask only. Author to whom correspondence should be addressed; electronic mail: [email protected] 0003-6951/2005/87共16兲/163121/3/$22.50 87, 163121-1 © 2005 American Institute of Physics Downloaded 07 Feb 2006 to 128.178.175.12. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 163121-2 Perez-Solorzano et al. Appl. Phys. Lett. 87, 163121 共2005兲 FIG. 2. PL spectra over the emission energy for different optical excitation powers. With increasing excitation power 共I0 = 0.05 mW兲 the low-energy part of the spectra saturates and peaks at the high-energy side occur. The lines are guides to the eyes. In Fig. 2 the power-dependent PL of this sample is shown. As the excitation density is increased, the emission peak always broadened, indicating a possible state filling effect.10 This signifies that if the ground state is completely filled, the injected carriers can reach states with higher energy, where a radiative recombination is also possible. If the excitation density is high enough, even states of the expected quantum wire or quantum well on the pyramid could be reached. Such an effect is clear evidence of the presence of zero-dimensional structures within the pyramid. Figure 3共a兲 shows the low-temperature time-integrated PL measured by an excitation power of 15 mW, displaying a wide emission peak at 2.17 eV. In principle, one would expect the formation of a quantum well on the side-walls of the pyramid, a quantum wire at its edges, and a quantum dot on the top as it has been observed for other material systems.11 Further optimization of the growth could make just one of these structures to be dominant in the emission. Considering this we identified three different well-defined peaks within the broad spectrum, with energies of 2.03, 2.20, and 2.35 eV. Time-resolved PL measurements at different temperatures were performed to obtain further information about the recombination behavior of our structures. The PL emission demonstrated a clear nonexponential decay. We fitted our data with a stretched exponential model for heavily disordered materials,12 I共t兲 = I0 exp关− 共t/*兲兴, where * is the time constant and  is the stretching parameter, giving a measure of the degree of disorder. This model fits in the case of the nitride material system because of the presence of piezoelectric fields, which have a large influence on the overlap of the electron-hole wave function through the quantum-confined Stark effect. For our investigated structures the value of  ranges between 0.7 and 0.9. This value is FIG. 3. 共a兲 Low-temperature time-integrated PL spectra consisting of three different emission peaks. 共b兲 Radiative decay time vs temperature of the low- and high-energy emissions from Fig. 3共a兲. The dashed-lines are fits, taking zero-dimensional excitonic behavior or two-dimensional excitonic behavior with transfer from the barriers into account. higher compared to the other published data for InGaN lowdimensional structures,13,14 and is a result of reduced piezoelectric fields in the pyramid structure.15 Figure 3共b兲 shows the variation of the radiative decay time with the temperature for the peaks at 2.03 eV 关zerodimensional 共0D兲兴 and 2.35 eV 关two-dimensional 共2D兲兴. Assuming that at 5 K only radiative emission is observed,16 the radiative decay times can be calculated from the experimental decay times, taking the intensities of the emission peaks into account. The decay time at T = 5 K for the peak at 2.03 eV is 1.5 ns, and for the peak at 2.35 eV we observed 0.9 ns. For the emission at 2.03 eV, the radiative decay time is independent of the temperature whereas the 2.35 eV luminescence peak varies by a factor of 10. It is well known that the radiative lifetime of a zero-dimensional exciton is nearly constant over temperature, while the two-dimensional excitonic lifetime varies linearly with the temperature.17 This ex- Downloaded 07 Feb 2006 to 128.178.175.12. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 163121-3 Appl. Phys. Lett. 87, 163121 共2005兲 Perez-Solorzano et al. plains the temperature dependence of the emission peak at 2.03 eV 共0D兲, but in order to clarify the behavior of the 2.35 eV luminescence peak, we have to take the transport of the carriers into the quantum well on the sidewalls of the pyramid into account. Assuming that the carriers in the quantum well and in the barriers are in thermal equilibrium during the recombination process and the number of emitted electrons and holes in the barriers are equal, the rate equations for this system describing the carrier dynamics in the low-injection regime can be solved, resulting in an effective lifetime,18 eff = ex 1 + C冑T exp关− 共EA/2kT兲兴 . ex is the temperature-dependent excitonic lifetime mentioned above. The constant C is containing the ratio ex / bar of the excitonic lifetime and the lifetime in the barrier, as well as the effective masses of the electrons and holes in the quantum well and barriers, respectively. EA represents the activation energy of the excitons out of the quantum well. With this model, we have achieved good agreement with our experimental data 关dashed lines in Fig. 3共b兲兴. In this lowinjection regime we can derive from the obtained value of 126 meV for EA a total confinement energy ⌬E of 252 meV, which is in the range of the band offset of a comparable InGaN / GaN quantum well.19 From C we have extracted the lifetime ratios ex / bar between 100 and 900. Presumably these large values are due to the presence of nonradiative decays channels that shorten the lifetime in the barriers. This was confirmed by examinations of the temperature behavior of the radiative and nonradiative decays 共not shown here兲. We found that the radiative decay dominates for both emissions at 2.03 and 2.35 eV for low temperatures up to 80 K. The constant temperature behavior of the radiative decay with long experimental decay times confirms the zerodimensional character of the low-energy emission peak. A tentative assignment attributes the peak at 2.2 eV to the onedimensional 共1D兲 structure 共quantum wire兲 as expected from the sample geometry. However, the temperature dependence of the radiative decay time does not shows unambiguously 1D character, probably due to the carrier transport processes between the structures of different dimensionality. A more detailed study of the pyramid edges is required in the future. In order to clarify the origin of these emissions within the pyramidal structure, spatially resolved CL measurements were performed. In Fig. 4 the CL intensity collected for two different wavelength ranges is displayed. One can clearly see that the emission between 490 and 510 nm is mainly coming from the sidewalls of the pyramid, whereas between 580 and 640 nm only emission from the apex of the pyramid is observable. This is in excellent agreement with our PL results where we could determine the zero-dimensional behavior around 610 nm. In summary, we have performed time-integrated, timeresolved, temperature-dependent photoluminescence, and FIG. 4. 共Color online兲 CL mapping of a single InGaN pyramid for two different wavelength ranges. Only the emission between 580 and 640 nm originates in the apex of the pyramid. spatially resolved cathodoluminescence measurements on site-controlled pyramidal InGaN quantum dots. We found a strong green emission 共around 2.35 eV兲 originating from the sidewalls of the pyramids, and also emission at 2.03 eV corresponding to a QD. The combination of PL and CL measurements supports the fact that an InGaN quantum dot is formed at the apex of the pyramid, having an emission wavelength in the orange-red spectral range. 1 S. Chichibu, T. Azuhata, T. Sota, and S. Nakamura, Appl. Phys. Lett. 69, 4188 共1996兲. 2 K. Tachibana, T. Someya, and Y. Arakawa, Appl. Phys. Lett. 74, 383 共1999兲. 3 Y. Kobayashi, V. Perez-Solorzano, J. Off, B. Kuhn, H. Gräbeldinger, H. Schweizer, and F. Scholz, J. Cryst. 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