Excitonic and Optical Confinement in Microwire Heterostructures with

Article pubs.acs.org/JPCC

Excitonic and Optical Confinement in Microwire Heterostructures with Nonpolar (Zn,Cd)O/(Mg,Zn)O Multiple Quantum Wells M. Lange, C. P. Dietrich, M. Lorenz, and M. Grundmann* Institut für Experimentelle Physik II, Universität Leipzig, Linnéstr. 5, D-04103 Leipzig, Germany ABSTRACT: Nonpolar (Zn,Cd)O/(Mg,Zn)O multiple quantum wells (MQWs) were deposited in radial direction on ZnO nanowires using pulsed-laser deposition. The heterostructures with diameters in the micro regime exhibit both excitonic and optical confinement. Making use of the quantum confinement effect, the QW-related emission was tuned between 2.46 and 3.35 eV. Because of the cavity properties of the resonator with hexagonal cross section, hexagonal whispering gallery modes (WGMs) are observed up to energies close to the (Mg,Zn)O bandgap, enabling a spatial and spectral overlap of the MQW luminescence and the WGM. The resonant WGM energies were reproduced by using resonant wave numbers from numerical calculations.

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INTRODUCTION Semiconductor nano- and microwires are ideal building blocks for miniaturized optoelectronic devices as result of their excellent optical and electrical properties.1 Nowadays, semiconductor technology cannot be imagined without the integration of heterostructures, as they increase the spectrum of possible devices and enable high efficiencies. By using heterostructures, the advantages of two materials can be combined or fundamental properties can even be added in multilayered heterostructures, as done, for example, in lightemitting diodes or semiconductor lasers. To realize photonic microwire devices, the incorporation of quantum wells (QWs) is an important step.2 Efficient devices additionally require an optimal light confinement, which in general is realized by Bragg mirrors that require a complex layer system. An alternative is given by whispering gallery mode (WGM) resonators, where the light is naturally confined due to total internal reflections inside the dielectric medium, allowing high-quality factors.3 ZnO exhibits a large oscillator strength and enhanced exciton stability up to temperatures well above room temperature, making it a perfect material for the realization of a roomtemperature polariton laser. Additionally, the fabrication of naturally built, perfectly shaped resonators with distinct cavity properties4 and quantum-well structures with emission energies in the visible spectral range5,6 is possible with ZnO and its ternary alloys. In the literature, nanowire (NW) heterostructures with (Zn,Cd)O QWs were reported by Cheng et al.7 However, their wire radii were already too small to observe any WGM in the visible or UV spectral range. Additionally, they coated ZnO NW arrays and were able to perform luminescence measurements only on the ensemble of wires. Because of high wire density, conclusions on the homogeneity could not be drawn. Dietrich et al.8 reported microwire heterostructures with (Zn,Cd)O QWs that showed distinct cavity effects, as proven by the observation of WGMs. Drawbacks of this approach are © 2013 American Chemical Society

the large diameter of the wire cores, causing absorption and refraction effects for energies only slightly below the ZnO bandgap and the manual preparation of the wires for the coating step. This makes a perfect alignment and therefore a homogeneous coating very challenging but manageable.8 In this study, we report on the excitonic and optical confinement in heterostructures with nonpolar (Zn,Cd)O/ (Mg,Zn)O MQWs, grown in the radial direction on NW side facets. A spatial and spectral overlap of the QW emission and WGM is proven. The luminescence of the samples was investigated to study the excitonic confinement, which made a tuning of the QW emission between 2.46 and 3.35 eV possible. A detailed and methodologically sound study is carried out on the optical confinement by reproducing the experimentally observed mode spectra using reasonable material parameters. The realized microwire heterostructure arrays with a low lateral wire density secure the homogeneity of the MQW shells and enable the simultaneous growth of a large number of structures but still allow studies on single wires.

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EXPERIMENTAL SECTION The microwire QW heterostructures were synthesized via a three-step pulsed-laser deposition (PLD) process using a highpressure PLD quartz chamber9 and a conventional PLD system.10 Compressed and sintered ceramic pellet targets of ZnO, Zn0.7Cd0.3O, and Zn0.88Mg0.12O were used. A ZnO buffer layer was deposited by PLD, and after a transfer the ZnO NWs were grown using high-pressure PLD. For further details of the NW growth see, for example, ref 11. In a third step, the ZnO NWs were coated with (Zn,Cd)O/ (Mg,Zn)O MQWs with four QWs by using conventional PLD. Received: February 20, 2013 Revised: April 3, 2013 Published: April 5, 2013 9020

dx.doi.org/10.1021/jp401809f | J. Phys. Chem. C 2013, 117, 9020−9024

The Journal of Physical Chemistry C

Article

To vary the thickness of the QW in the heterostructure, the number of pulses applied for the QW, which is proportional to the QW thickness, was varied, whereas the thickness of the barrier layer was kept constant. The NWs direct toward the plasma plume, and the substrate rotates for a better homogeneity of the coating process. To obtain a large Cd content in the QWs, the NWs were held at a temperature of ∼300 °C.5 Oxygen partial pressures of 2.6 Pa for (Zn,Cd)O and 0.2 Pa for (Mg,Zn)O were applied, respectively. During the MQW deposition, the NW diameter significantly increases to several hundred nanometers and a microwire quantum-well heterostructure is formed. (See Figure 1.) Figure 1c shows the

Figure 1. SEM pictures of a freestanding wire before (a) and after (b) MQW deposition. The insets show enlarged views of each wire. The images were taken at a 45° tilt. (c) Cross-sectional view of the microwire QW heterostructure that was cut by a focused ion beam. The wire was covered with Pt prior to the cut. Figure 2. (a) Room-temperature luminescence spectra of microwire heterostructures with different QW thicknesses. The arrows indicate the QW energy. (See the text.) The average (Mg,Zn)O emission energy is given by a dashed line. (b) Calculated QW thickness in dependence of the number of QW pulses. The dashed line shows a linear fit. (c) QW energy in dependence of the QW thickness. The dashed line shows the calculated curve.

cross section of the microwire QW heterostructure that was prepared by a focused ion beam after the wire was covered with Pt for a stabilization. The NW core and the MQW shell cannot be distinguished in the cross-sectional SEM pictures. The hexagonal cross section is, in general, maintained in the main part of the wire after the deposition of the MQW. However, the regions near the corners are not perfect because they are partially rounded or fringed, although the uncoated NWs show a perfect cross section (not shown). Additionally, no conformal coating of the wire was possible near the tip due to the directed growth process. Room-temperature cathodoluminescence (CL) has been excited by a CamScan CS44 scanning electron microscope (SEM) with an acceleration voltage of 5 kV. CL signals were spectrally dispersed by a 320 mm grid monochromator and a nitrogen-cooled, back-illuminated charged-coupled device. To study the luminescence along the c axis of the microwire heterostructures, we tilted the samples by 45° with respect to the electron beam. Field-emission scanning electron microscopy images were obtained using an FEI Nova 200 Nanolab.

sample to sample, as result of small variations of the Mgcontent. In addition, the samples exhibit a weak green emission band due to deep defect states centered at ∼2.35 eV that is typical for ZnO-based nanostructures grown by PLD.12 Obviously, most of the spectra are modulated by sharp peaks as result of the optical confinement in the wire acting as a dielectric resonator. From the mode spacing, it can be concluded that the observed eigenmodes of the resonator are WGM. For most of the wires, WGM are observed in the transparency region of the (Mg,Zn)O, the QW range, and the near-band-gap energy region of the barriers. Although the WGMs mask the QW emission in spectra of Figure 2, the maximum of the QW luminescence can be determined. For a more precise determination, spectra were taken below the tip where no or weaker WGMs are observed (not shown). The energy of the QW maximum is indicated by an arrow in Figure 2a and is plotted in dependence of the QW thickness in panel c. The QW energy was tuned over a wide spectral range between 2.46 and 3.35 eV. The QW thickness was extracted by comparing the experimental QW energies with theoretical ones, calculated with an effective mass-based envelope function approximation by a variational approach.13 The theoretical energies were calculated by a 1D QW calculation. The wire diameter is large enough so that additional quantum confinement effects due to the wire itself can be neglected. To determine the growth rate and therefore be able to calculate the QW thickness for the

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RESULTS AND DISCUSSION Luminescence measurements were performed in the main part of the wire below the tip. There, a conformal coating of the NW with the MQW shell was obtained. The homogeneity along the wire axis will be discussed more extensively below. Quantum confinement was proven by changing the QW thickness. (See Figure 2a.) The energy of the QW-related emission was tuned in almost the entire range between the emission energy of the barrier material and the bulk emission energy of the QW material of 2.46 eV.5 The luminescence of the (Mg,Zn)O barrier layer is observed at an energy of ∼3.45 eV corresponding to a Mg content of 0.075. Small fluctuations in the energy of the barrier-related emission band occur from 9021



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the reason why only weak WGMs are observed at the (Mg,Zn) O emission band. In Figure 3, the spectrum of a microwire heterostructure with a QW thickness of 0.6 nm and a Mg content of 0.04 is presented. To investigate the optical confinement, we followed the procedure first reported Czekalla et al.: 16 In the transparency region of (Mg,Zn)O, values for the refractive index and an estimate of the wire radius from SEM are applied to determine the interference order N and a more precise wire radius Ra,WGM for the WGM pathway by fitting the resonant energy. Up to energies of ∼2.7 eV (transparency region), the refractive index for thin films was used, which is given by a three-term Cauchy approximation:17

complete set of samples from the number of pulses, we plotted the QW thickness over the number of pulses applied for the QW. (See Figure 2b.) The parameters that were necessary for the calculations were approximated with the ZnO values as given in ref 11. For (Zn,Cd)O, they were calculated in a virtual crystal approximation, using values for ZnO and for wurtzite CdO.5,14 The exciton binding energy was determined after the method described by Leavitt and Little15 using the wave functions of the numerical calculations. In this regard, the values are scaled with the bulk value of the (Zn,Cd)O exciton binding energy under the assumption of a spherically symmetric hydrogen-like electron−hole function and an effective mass model. The bulk value of the (Zn,Cd)O exciton binding energy was calculated to be 41.3 meV, taking into account the change in the dielectric constant and the effective masses. The transition energy of the QW is plotted as function of the QW thickness as a dashed line in Figure 2c. The QW thickness in dependence of the number of pulses exhibits a linear relationship with only small deviations. (See Figure 2b.) Therefore, the final QW thickness is calculated using the number of pulses and the calculated growth rate. The growth rate was determined to be 1.1 nm per 100 pulses. A good agreement of the experimental and the calculated QW energies is obtained, proving the quantum confinement effect. In addition to the quantum confinement, optical confinement is a major feature of the microwire heterostructures, significantly influencing the emission characteristics. Sharp peaks of WGM are observed in the transparency region of (Mg,Zn)O. For most of the samples, WGMs are also observed in the QW region and the near-band-gap energy range. (See Figure 3.) This is a proof of spatial and spectral overlap of the QW emission with WGM. The strong increase in the absorption near the band edge of the (Mg,Zn)O barrier is

nr, (x , E) = 1.844 − 0.782·x + (1.177 − 2.933·x) × 10−2(E /eV)2 + (1.650 − 2.073·x) × 10−3(E /eV)4

(1)

However, the procedure of Czekalla et al. had to be varied because the diameter of the microwire heterostructures is too small to apply a plane-wave model for the energies of the resonances, as discussed, for example, in ref 18. Instead, we used tabulated values for the product of wave vector and microwire radius of the resonances given in ref 19.20 The wave numbers and therefore also the energies of the resonances for the different interference numbers are a function of the refractive index nr and the wire radius Ra. (See Figure 4.)

Figure 4. Resonant wave numbers Re(kR) of a hexagonal cavity. Gray data points and dashed curves present numerical solutions and linear interpolations after ref 19. Selected interference orders N are given at the right side. The open symbols refer to resonant wave numbers of microwire heterostructures with different QW thicknesses, as indicated.

For higher energies, where no data for the refractive index are available, N and Ra,WGM are applied to determine the refractive index point by point, using the resonant energies and the data of Figure 4. For the spectrum in Figure 3, the determined values of the refractive index are shown as data points in the lower part of the image. Additionally, data points for further wires are shown in Figure 4. The fitting parameters nr,∥ and Ra,WGM are shown in Figure 5 and in the inset, respectively. For a Mg content of 0.04 as well as for 0.075, the calculated values of the refractive index show a reasonable dependence on the energy. Because of the incorporation of Mg in ZnO, the values of the refractive

Figure 3. Luminescence spectrum for a QW thickness of 0.6 nm. The calculated resonant energies are shown as a dashed line and the interference orders are assigned. In the transparency region of the (Mg,Zn)O, thin film values of the refractive index nr,∥,TF were applied. Above 2.7 eV, the index of refraction nr,∥,WGM was calculated from the resonant energies. In the lower part of the Figure, the spectral dependence of the refractive index is plotted as a function of the energy. 9022



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Gabriele Ramm for PLD target preparation and gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft in the framework of Sonderforschungsbereich 762 “Functionality of Oxide Interfaces” (SFB 762), Graduate School “Leipzig School of Natural Sciences Building with Molecules and Nano-objects” (BuildMoNa) (GS185/1), and the European Social Fund in the framework of “ESF in Sachsen”.

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REFERENCES

(1) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Nanowire Electronic and Optoelectronic Devices. Mater. Today 2006, 9, 18−27. (2) (a) Sun, Y.; Cho, Y.-H.; Kim, H.-M.; Kang, T. W. High Efficiency and Brightness of Blue Light Emission from Dislocation-Free InGaN/ GaN Quantum Well Nanorod Arrays. Appl. Phys. Lett. 2005, 87, 093115-1−093115-3. (b) Björk, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. One-Dimensional Heterostructures in Semiconductor Nanowhiskers. Appl. Phys. Lett. 2002, 80, 1058− 1060. (3) Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Ultra-High-Q Toroid Microcavity on a Chip. Nature 2003, 421, 925− 928. (4) Dietrich, C. P.; Lange, M.; Sturm, C.; Schmidt-Grund, R.; Grundmann, M. One- and Two-Dimensional Cavity Modes in ZnO Microwires. New J. Phys. 2011, 13, 103021. (5) Lange, M.; Dietrich, C. P.; Brachwitz, K.; Stölzel, M.; Lorenz, M.; Grundmann, M. Visible Emission from ZnCdO/ZnO Multiple Quantum Wells. Phys. Status Solidi RRL 2012, 6, 31−33. (6) (a) Sadofev, S.; Kalusniak, S.; Puls, J.; Schäfer, P.; Blumstengel, S.; Henneberger, F. Visible-Wavelength Laser Action of ZnCdO/(Zn,Mg) O Multiple Quantum Well Structures. Appl. Phys. Lett. 2007, 91, 231103. (b) Yamamoto, K.; Adachi, M.; Tawara, T.; Go-toh, H.; Nakamura, A.; Temmyo, J. Synthesis and Characterization of ZnCdO/ ZnO Multiple Quantum Wells by Remote-Plasma-Enhanced MOCVD. J. Cryst. Growth 2010, 312, 1496−1499. (7) Cheng, C.; Liu, B.; Sie, E. J.; Zhou, W.; Zhang, J.; Gong, H.; Huan, C. H. A.; Sum, T. C.; Sun, H.; Fan, H. J. ZnCdO/ZnO Coaxial Multiple Quantum Well Nanowire Heterostructures and Optical Properties. J. Phys. Chem. C 2010, 114, 3863−3868. (8) Dietrich, C. P.; Lange, M.; Stölzel, M.; Grundmann, M. Microwire (Mg,Zn)O/ZnO and (Mg,Zn)O/(Cd,Zn)O Non-Polar Quantum Well Heterostructures for Cavity Applications. Appl. Phys. Lett. 2012, 100, 031110. (9) Lorenz, M.; Kaidashev, E. M.; Rahm, A.; Nobis, T.; Lenzner, J.; Wagner, G.; Spemann, D.; Hochmuth, H.; Grundmann, M. MgxZn1−xO (0 ≤ x < 0.2) Nanowire Arrays on Sapphire Grown by High-Pressure Pulsed-Laser Deposition. Appl. Phys. Lett. 2005, 86, 143113. (10) Lorenz, M. Pulsed Laser Deposition of ZnO-Based Thin Films. In Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells; Ellmer, K., Klein, A., Rech, B., Eds.; Springer Series in Materials Science; Springer: Berlin, Germany, 2008; Vol. 104, pp 305− 358. (11) Lange, M.; Dietrich, C. P.; Zúñiga-Pérez, J.; von Wenckstern, H.; Lorenz, M.; Grundmann, M. MgZnO/ZnO quantum well nanowire heterostructures with large confinement energies. J. Vac. Sci. Technol., A 2011, 29, 03A104. (12) Grundmann, M.; Wenckstern, H.; Pickenhain, R.; Nobis, T.; Rahm, A.; Lorenz, M. Electrical Properties of ZnO Thin Films and

Figure 5. Spectra of the refractive index for two Mg contents below the (Mg,Zn)O bandgap. The solid lines shows the values of (Mg,Zn) O thin films after ref 17, whereas the symbols refer to refractive indexes calculated from the resonant energies. The inset shows the wire radius determined from the WGM in dependence of the wire radius measured by SEM. The solid lines show slopes of 1 and 1.1, respectively.

index are smaller in the transparency region and the curves are shifted to higher energies as result of the increase in the bandgap energy. As proposed by Czekalla et al., the WGM provide a fast and simple technique to obtain the refractive index of (Mg,Zn)O shells on ZnO nanostructures, which are not accessible for macroscopic techniques like spectroscopic ellipsometry.16 Calculations of Ra,WGM for spectra at different positions along the wire axis make a study of the homogeneity of the coating along the wire axis possible. A relative variation in the final wire radius as small as 1.6% was calculated for a QW thickness of 1.5 nm (not shown). The agreement of the wire radius determined from the WGM and by SEM is fair. The WGM radius is on average 5% larger than the one measured by SEM. A slight overestimation of the theoretical radius was also reported in ref 19. Taking into account the uncertainty of the SEM measurements, the difference can be only partially explained. Additionally, the difference can also be caused by an underestimation of the refractive index, which is feasible due to the different growth conditions of thin film materials on which literature formulas for the refractive index are based.17

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CONCLUSIONS In summary, excitonic and optical confinement was reported for microwire nonpolar (Zn,Cd)O/(Mg,Zn)O MQW heterostructures. The quantum confinement effect was proven by samples with different QW thicknesses emitting in a range of QW energies between 2.46 and 3.35 eV. Optical confinement is clearly observed in the spectra of the heterostructures because they exhibit peaks due to WGM. The WGMs occur in the spectral range of the QW emission, proving a spatial and spectral overlap of the QW emission with the WGM. From the resonant energies, in particular, the mode spacing, it is concluded that hexagonal WGMs occur in the cavity. The properties of the optical cavity were investigated by reproducing the mode spectrum using reasonable fitting parameter. 9023



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Optical Properties of ZnO-Based Nanostructures. Superlattices Microstruct. 2005, 38, 317−328. (13) Harrison, P. Quantum Wells, Wires, and Dots: Theoretical and Computational Physics of Semiconductor Nanostructures, 1st ed.; John Wiley & Sons: Hoboken, NJ, 2000. (14) Schleife, A.; Fuchs, F.; Rödl, C.; Furthmüller, J.; Bechstedt, F. Band-Structure and Optical-Transition Parameters of Wurtzite MgO, ZnO, and CdO from Quasiparticle Calculations. Phys. Status Solidi B 2009, 246, 2150. (15) Leavitt, R. P.; Little, J. W. Simple Method for Calculating Exciton Binding Energies in Quantum-Confined Semiconductor Structures. Phys. Rev. B 1990, 42, 11774. (16) Czekalla, C.; Nobis, T.; Rahm, A.; Cao, B.; Zúñiga-Pérez, J.; Sturm, C.; Schmidt-Grund, R.; Lorenz, M.; Grundmann, M. Whispering Gallery Modes in Zinc Oxide Micro- and Nanowires. Phys. Status Solidi B 2010, 247, 1282−1293. (17) Schmidt, R.; Rheinlander, B.; Schubert, M.; Spemann, D.; Butz, T.; Lenzner, J.; Kaidashev, E. M.; Lorenz, M.; Rahm, A.; Semmelhack, H. C.; Grundmann, M. Dielectric Functions (1 to 5 eV) of Wurtzite MgxZn1−xO (x ≤ 0.29) Thin Films. Appl. Phys. Lett. 2003, 82, 2260. (18) (a) Nobis, T.; Kaidashev, E. M.; Rahm, A.; Lorenz, M.; Grundmann, M. Whispering Gallery Modes in Nanosized Dielectric Resonators with Hexagonal Cross Section. Phys. Rev. Lett. 2004, 93, 103903. (b) Wiersig, J. Hexagonal Dielectric Resonators and Microcrystal Lasers. Phys. Rev. A 2003, 67, 023807. (19) Nobis, T.; Grundmann, M. Low-Order Optical WhisperingGallery Modes in Hexagonal Nanocavities. Phys. Rev. A 2005, 72, 063806. (20) They were calculated by a semiclassical approximation based on pseudointegrable ray dynamics and boundary waves. It is applied to find solutions of the 2D Helmholtz equation.

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Excitonic and Optical Confinement in Microwire Heterostructures with

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