Formation of Hybrid Molecules Composed of Ga Metal Particle in

The formation of hybrid molecules composed of a pair of a Ga metal particle and a semiconductor quantum ring (QR) is demonstrated by using molecular b...
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Formation of Hybrid Molecules Composed of Ga Metal Particle in Direct Contact with InGaAs Semiconductor Quantum Ring Jihoon H. Lee, Zhiming M. Wang,* Kimberly Sablon, and Gregory J. Salamo Materials Research Science and Engineering Center, UniVersity of Arkansas, FayetteVille, Arkansas 72701

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 690–694

ReceiVed May 22, 2007; ReVised Manuscript ReceiVed October 9, 2007

ABSTRACT: We demonstrate the formation of hybrid molecules composed of a pair of a metal particle and a semiconductor quantum ring (QR) by using molecular beam epitaxy on a GaAs (100) surface. To form three-dimensional semiconductor InGaAs QRs with a hole in the center of the structure, a thin layer (10 monolayer) of GaAs was applied on InAs quantum dots, which transformed the distribution of surface free energy. These InGaAs QRs were then used as a template for the localization of Ga metal particles; there was one metal particle on a QR. By choosing the optimal surface temperature (most favorable thermal energy), the surface diffusion of Ga adatoms was effectively enhanced. This, in turn, allowed the localization of Ga metal particles on InGaAs QRs, consequently forming hybrid molecules. This study helps understand the formation of various types of hybrid molecules that will be used in applications in optoelectronics.

1. Introduction The study of and progress in nanotechnology, more specifically in nanostructures, are expected to provide new insights into the next-generation-device applications, as well as into the fundamental understanding of the physics involved. The potential impact of these nanostructure-based devices could be very large, and thus, the designing, engineering, and synthesis of nanostructures have received tremendous attention from several research fields. New fabrication approaches have been developed to control the optical and electronic properties of nanostructures, and several applications have been demonstrated, such as nearfield optical microscopy,1 subwavelength photonics,2 focusing of light,3 tagging,4 and molecular sensing.5,6 Recently, it has been demonstrated that the photoluminescence (PL) intensity is significantly enhanced on surface plasmons (SPs) that are excited on thin metallic films or nanoparticles, because of the coupling of recombining states with SPs at a metallic interface.7–10 More recently, the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles were theoretically studied by Govorov and Zhang et al.11,12 These molecules are anticipated to demonstrate novel optical properties, including heating and melting processes of surrounding matrix and absorption spectrum broadening and shifting. These novel optical properties can be induced by plasmon resonance through strong coupling and coherent and incoherent interactions of these hybrid molecules.11,12 To take advantage of these unique properties of the nanostructures, one of the key challenges in nanotechnology is the ability to manipulate and assemble these nanoscale structures. Although there have been numerous efforts on the synthesis of singular nanostructures, the study and the development of the assembly and manipulation of nanostructures have been somewhat deficient. Without these abilities, it will not be possible to take full advantage of these unique properties of the nanostructures can be hindered. In this paper, we report on the formation by molecular beam epitaxy (MBE) of hybrid molecules composed of Ga metal particles in direct contact with InGaAs quantum rings (QRs). This was accomplished by selecting the most favorable thermal * Corresponding author. E-mail address: [email protected].

energy, that is, an optimal surface temperature. Under these conditions, the surface diffusion of Ga adatoms was effectively enhanced. This, in turn, permitted the localization of Ga particles on InGaAs QRs, resulting in the formation of hybrid molecules. In the initial step, InAs quantum dots (QDs) were formed on a GaAs (100) surface through the Stransky-Krastanov (S-K) growth model.13–17 In the subsequent step, the formation of QRs, namely the transformation from QDs, was performed. Threedimensional (3D) semiconductor ring-shaped nanocrystals, with a size of a few nanometers to tens of nanometers, are called QRs. Because electrons and holes can be confined at a discrete energy level, a phenomenon known as quantum confinement, it is possible to design and engineer the bandgap structure by controlling the size of the nanostructures. The transformation process was achieved by applying a thin layer of GaAs in order to change the allocation of surface energy matrix, and thus, the QDs were transformed into QRs. These QRs were used as a template for the localization of Ga metal particles. Finally, Ga metal particles were deposited to form hybrid molecules in direct contact with these QRs.

2. Experimental Section In this work, samples were grown on epitaxy-ready GaAs (100) surface by solid source MBE. Samples were mounted on a Mo holder and degassed for 0.5 h at 350 °C. Then, the degassed sample holder was introduced into a growth chamber. To grow a defect-free GaAs buffer layer, surface oxide was thermally desorbed at 610 °C under a beam equivalent pressure (BEP) of nearly 6 µTorr of As4 flux. Under a highly overpressured As4 flux (a BEP of 6.4 µTorr), a 500 nm GaAs buffer layer was grown, and a 10 min growth interruption was applied to stabilize the GaAs buffered surface matrix at 600 °C. The nominal growth rate of GaAs was 0.7 monolayer per second (ML/s), and the growth rate was deduced by using an in situ reflexion high-energy electron diffraction (RHEED) system. For InAs QD growth shown in Figures 1a and 2a, the surface temperature was reduced to 520 °C, and 1.6 ML of InAs was deposited with 30 s of growth interruption right after QD relaxation. We observed the transition of RHEED pattern from streaky to spotty during QD formation, which indicates a 3D transition of the surface matrix because of coherent strain relaxation. This process is theoretically depicted by the S-K growth model.13–17 Subsequently, the surface temperature was reduced to 440 °C in order to fabricate QRs, shown in Figures 1b and 2b. These QRs were fabricated by applying 10 monolayers (MLs) of GaAs on previously grown QDs, shown in Figure 1a. The QRs were then used as a template for further

10.1021/cg0704706 CCC: $40.75  2008 American Chemical Society Published on Web 01/03/2008

Formation of Ga-InGaAs Semiconductor QR Hybrids

Crystal Growth & Design, Vol. 8, No. 2, 2008 691 reduced directly to 300 °C to form Ga metal particles. Prior to applying an atomic beam of Ga, the growth was paused to reach a background pressure of ∼5 × 10-9 Torr to avoid the effect of background As pressure on Ga metal particle formation. These Ga metal particles are known to be extremely reactive in foreign materials such as As.18–20 Three monolayers of Ga (an equivalent amount of GaAs when As4 flux is supplied) was applied directly on the planar GaAs surface. For the samples in Figure 4, the As valve was completely closed at 440 °C to discontinue the As4 flux after the formation of QR templates, as seen in Figure 1b. Subsequently, the substrate temperature was decreased to 250 and 300 °C for the deposition of Ga metal particles. In fact, we systematically varied the amount of Ga deposition and substrate temperature to examine the morphologic interaction between QRs and Ga metal particles. After the termination of each growth, the surface temperature was quenched under the same growth condition as the last step for each sample. For morphologic surface analysis, an atomic force microscope (AFM) in air was used.

3. Results and Discussion

Figure 1. Transition from InAs QDs to InGaAs QRs. Figures are 1 × 1 µm area of plane-view AFM images. (a) AFM image of InAs QDs with 1.6 ML deposition on GaAs (100) surface. (b) AFM image of QRs formed by the deposition of 10 ML of GaAs on InAs QDs shown in (a). Height scale bar is located next to each figure. Lateral scale bar (100 nm) in (a) applies to Figures 1b and 2a.

Figure 2. Cross-sectional line profiles of a QD and a QR. Line profiles show the shape evolution from QDs to QRs after deposition of 10 ML of GaAs on QDs. The insets are 200 × 200 nm of plane-view of AFM images, and the two insets of individual quantum structure show how the line profiles were taken. growth of hybrid molecules. For the Ga metal particle sample on planar GaAs surface, shown in Figure 3, arsenic flux was discontinued at 500 °C after GaAs buffer growth, and then, the substrate temperature was

In order to fabricate hybrid molecules composed of a pair of Ga metal particles and QRs, the first step was to form QDs. Figure 1a shows InAs QD formation on a GaAs (100) surface with 1.6 ML deposition at a surface temperature of 520 °C. The density of QDs was ∼1.8 × 1010/cm2, and the average size of QDs was 15 nm in height and 42 nm in diameter at the base of the QDs. The S-K growth model13–17 facilitates the formation mechanism of QDs, namely the coherent strain relaxation, by enlarging the surface area to equilibrate the surface energy. Strain relaxation of pure InAs on GaAs is induced by a sufficient lattice mismatch between InAs and GaAs (7.2%) and is known to take place at ∼1.6 ML on a GaAs (100) surface.15 InAs, GaN, and InP are some of the most extensively studied QD material systems, and these efforts led to novel QD applications.21–23 Subsequently, fabricating hybrid molecules requires the transformation of QDs into 3D QRs. Figure 1b shows the formation of InGaAs QRs at a surface temperature of 440 °C. Figure 2 provides a more detailed perspective of the transition from QDs to QRs with cross-sectional line profiles and insets of a QD and a QR. The transition from QDs to QRs was induced by applying a thin layer of GaAs (10 ML). Indeed, the resulting structure consisted of InGaAs because of intermixing near the interface and diffusion. The conversion process is very complex, but the driving force of this interesting shape transition of QDs to QRs is typically attributed to the redistribution of the surface free energy.27–39 When a thin GaAs capping layer is applied to QDs, the change in the surface energy matrix drives materials outward from the center of the QDs, thus forming QRs with a hole in the center of the structure. QRs have also been fabricated through several growth approaches, namely a thin layer (a few nanometers) GaAs capping on QDs,32–36 postgrowth annealing with application of a thin GaAs layer,38,39 self-assembled growth based on S-K model,40,41 and droplet-assisted methods.42–44 At first glance, the QRs are elongated along the [011j] direction, as clearly seen in Figures 1b and 2b. This indicates that the diffusion during the transition from QDs to QRs was highly anisotropic along both directions. This can be due to the anisotropic nature of the GaAs (100) surface.24,25 Hence, the dimensions of the QRs are highly asymmetric along different directions, namely the height and length along [011] and [011j]. We provide more details on the geometry of InGaAs: (i) The average height of the QRs was 5 nm along [011j] and 2 nm along [011]. (ii) The length of the QRs was 110 nm along [011j] and 45 nm along [011] at the base of the QRs. (iii) During the shape transition of QDs to QRs, the ratio of reduction in height was 3 along [011j] and 7.5 along [011]. (iv) The ratio of increase

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Figure 3. AFM image and cross-sectional line profile of Ga droplet formed on GaAs (100) surface: 3 ML Ga deposition (an equivalent amount of GaAs when As4 flux is supplied) at 300 °C. (a) 1 × 1 µm area of plane-view. (b) Inset with line profile directions.

in length was 2.6 along [011j], whereas the length stayed nearly the same along [011], 45 nm for QRs and 42 nm for QDs. These observations confirm the anisotropic nature of this surface, along both [011j] and [011]. Indeed, the asymmetry and the noncircular shape of the QRs may be due to the anisotropic surface diffusion of the Ga and In adatoms toward [011j] and [011] during the transformation of QDs, induced by the asymmetric (2 × 4) surface reconstruction of the GaAs (100).24,25 Now, the QR template to house Ga metal particles is ready for the preparation of hybrid molecules. The following discussion will focus on the interaction of Ga metal particles with a planar GaAs (100) surface. Figure 3 shows the formation of Ga metal particles on a GaAs (100) planar surface with 3 ML deposition at the substrate temperature of 300 °C. The Ga metal particles were randomly distributed over the surface. The density of Ga particles was ∼0.73 × 1010/cm2, and the average size of Ga particles was 11.5 nm in height and 50 nm in diameter at the bottom of the particles, as seen in Figure 3b. These Ga metal particles are also known as Ga droplets, and they are formed in the absence of As atoms. The formation of metallic droplets on a GaAs surface is based on the Volmer-Webber growth model.26 When metal atoms such as Ga, In, and Al are applied on GaAs surfaces, the equilibrium state of the surface free energy on these surfaces is reached when these metal atoms are brought together. This can also be explained in terms of the binding energy between atoms: the binding energy between metal atoms is greater than that between metal atoms and the GaAs surface, thus forming metal particles. In addition, these metal atoms are very reactive in aforeign environment; thus, when As atoms are subsequently introduced into a chamber, these metal particles interact with As atoms and form semiconductors such as GaAs, InAs, and AlAs. This process is known as crystallization and arsenization.19 By using this approach, a variety of semiconductor quantum structures have been fabricated, such as QDs,27 QD molecules,19 QRs, and various configurations of quantum structures.20,25,28 So far, we have discussed the formation of QR-template and Ga metal particles. Now, let us discuss how these metal particles interact with QR-template samples.

Figure 4. Localized formation of Ga metal particles on InGaAs QRs (hybrid molecules). Both (a) and (b) are plane-view of AFM images (1 × 1 µm area). (a) 3 ML of Ga deposition (an equivalent amount of GaAs when As4 flux is supplied) at a surface temperature of 250 °C. (b) 3 ML of Ga at 300 °C. Scale bar (100 nm) in (b) applies to (a) and (b).

We applied Ga atoms on the QRs in the absence of AS atoms, and the results are shown in Figures 4 and 5. Figure 4a shows the 3 ML deposition of Ga on a QR template at 250 °C, and Figure 4b shows the same amount of Ga deposition at a higher substrate temperature, 300 °C. First of all, with the same amount of Ga deposition (3 ML), metal particles were successfully paired with underlying InGaAs QRs at 300 °C, making wonderful hybrid molecules, whereas these metal particles were still rather randomly distributed at the lower substrate temperature of 250 °C. The percentage of paired Ga metal particles with InGaAs QRs at the growth temperature of 300 °C was over 90%, although some of the QRs did not have Ga particles. At the surface temperature of 250 °C, most Ga metal particles were formed on the planar GaAs (100) surface. A total of 40% of the Ga metal particles were paired with InGaAs QRs, whereas the rest of the Ga particles were found on the planar GaAs (100) surface at the surface temperature of 250 °C. The average size of Ga particles grown at 300 °C was 18 nm in height and 58 nm in diameter, as shown in Figure 5c. For those Ga metal particles formed on the planar GaAs (100) surface at 250 °C, as seen in Figure 5a, the average size was 9.5 nm in height and 37 nm in diameter, whereas the average size of Ga particles formed on QRs was 12 nm in height and 48 nm in diameter, as shown in Figure 5b. When comparing the average size of Ga metal particles paired with InGaAs QRs at both growth temperatures, we found that the average height was nearly 200%

Formation of Ga-InGaAs Semiconductor QR Hybrids

Figure 5. Cross-sectional line profiles of Ga metal particles on a QR and a GaAs (100) surface at different growth temperatures. The insets are 200 × 200 nm of plane-view of AFM images and show the Ga metal particles and where the line profiles were taken.

taller and the diameter was 57% wider at 300 °C than at 250 °C. It is understandable that the size of Ga metal particles grown at a higher surface temperature is larger because a higher substrate temperature (higher thermal energy) provides a longer range surface diffusion of adatoms (surface atoms), thus resulting in a larger size of particles. This also matches our observations on the size and density of Ga metal particles on a GaAs (100) surface as well as on high index surfaces.29 Nevertheless, it is interesting that the sizes of the droplets differ even at the same growth temperature of 250 °C. Those Ga metal particles paired with QRs were much larger: 26% taller in height and 29% wider in diameter. The ratio of difference in size was in the same range, ∼27.5%. This size difference at the same growth temperature, namely at 250 °C, can be due to the higher step density on the sidewalls of the holes within the QRs. GaAs (100) is characterized as a singular surface without any surface misorientation, namely miscut. The singular GaAs surface when the miscut is below and/or near 10° is known as a vicinal surface. When the miscut is larger than 10°, the misorientation along [011] is a type-A surface, that is GaAs (311)A and (711)A surfaces, and the miscut along [011j] is called a type-B surface, that is GaAs (511)B and (711)B surfaces.30 The height of ML steps on GaAs (100) is known to be 2.82 Å, and more ML steps are found as the surface misorientation increases. Therefore, we measured the average slope angles on sidewalls of the holes of QRs to confirm our speculation on the higher step density on the sidewalls of QRs, and we obtained a value of 22°. When more ML steps are present, the likelihood of atom incorporation is relatively higher on these locations with higher density of ML steps. As a result, under identical growth conditions, the nanostructures grown on the surface with a higher density of ML steps result in rapid incorporation of adatoms. This can potentially explain the size difference of Ga metal particles.24

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Also, the densities of Ga metal particles of the two samples at 250 and 300 °C were quite different. The density of Ga particles formed at 250 °C was ∼1.03 × 1010/cm2, and that of Ga particles formed at 300 °C was ∼0.52 × 1010/cm2. The density of Ga particles formed at 250 °C was nearly 200% higher than that of Ga particles formed at 300 °C. It is a universal consensus that particles formed at relatively lower temperature have a relatively higher density.29 This behavior of the density of Ga metal particles also agrees with the previously explained behavior of the size distribution of Ga metal particles at two different growth temperatures. That is, the density increases as the size decreases with the change of surface temperatures and vice versa, when other conditions are kept the same. When comparing the densities of Ga metal particles for the same amount of deposition (3 ML) at the same growth temperature (300 °C), we find that the density of Ga particles grown on planar GaAs (100), Figure 3, is ∼0.73 × 1010/cm2, which is still higher (by 40%) than that of particles grown on the QR template. This becomes even clearer with the comparison of the sizes of Ga metal particles grown on these two different surfaces, planar surface and QR template, with the same amount of Ga deposition at the same surface temperature. The average height of Ga particles grown on QR template was 57% taller, whereas their diameter was only 16% wider. Typically, the dimensional increase of Ga metal particles progresses in a moderate range for height and diameter, but here, the hole of QRs already defined the diameter of Ga particles. This size difference can be attributed to the high step density on the sidewalls of QRs; that is, the density of Ga particles on the QR template was lower because of the relatively rapid incorporation of atoms on the sidewalls of QRs, specifically the hole of QRs. To summarize these observations on size and density of Ga metal particles under various conditions, there appears to be a clear correlation between the size and density of Ga metal particles under surface conditions such as planar surface and QR-template surface, and the size and density of Ga metal particles change when the growth environment is changed. So far, we have discussed the size and density of Ga metal particles in terms of the interaction with ML steps. Next, we need to determine the cause of localization. We have already discussed the high incorporation rate of atoms on highly misoriented surfaces. Therefore, the first possible explanation might be related to the geometry. Under our growth conditions, the surface temperature played a major role in confining the Ga metal particles on the QRs. As seen in Figure 4, the Ga metal particles were randomly distributed on the QR template at 250 °C, whereas these particles were paired with QRs at 300 °C. Although it is not shown here, the coupling of particles and QRs was hindered above 300 °C. This can be explained in terms of the surface energy and surface diffusion. Simply put, the diffusion of atoms has to overcome the incorporation energy to be randomly distributed over the surface. The diffusion length of surface atoms here is directly related to the thermal energy. With too low thermal energy, such as at 200 °C (not shown in this article), all of the Ga particles were observed to be formed randomly over the surface, regardless of the surface condition. At the slightly higher surface temperature of 250 °C, the diffusion of atoms was enhanced but still not enough to confine all the atoms in the hole of the QRs. Therefore, some of the Ga particles were paired, and the others were not paired at this growth temperature (40% paired and 60% not paired). At the right surface temperature, diffusion is sufficient to bring most of the atoms to the highly misoriented surface, but it is not large

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enough to overcome the incorporation energy on the step edges, thus forming hybrid molecules. At the high end, the thermal energy is so large that diffusion finally overcomes the incorporation energy, leading to a random formation again. Another consideration for the localization of Ga metal particles on QRs is the surface chemistry. As discussed, the InGaAs QRs were formed from InAs QDs, and the rest of the surface was GaAs. The domain of materials on the surface is inhomogeneous. The content of In is very high on QRs and very small or even zero elsewhere. Therefore, when Ga atoms are applied on this surface, the interaction between Ga atoms and the surface can potentially affect their diffusion and localization.

4. Conclusions To summarize, hybrid molecules consisting of a pair of a Ga metal particle and a semiconductor InGaAs QR were formed by MBE on a GaAs (100) surface. The QR template was fabricated by modifying the distribution of the surface free energy by applying a thin GaAs layer on previously formed InAs QDs. The size and the density of the droplets were discussed systematically to describe the potential reasons for the localization of Ga metal particles on InGaAs QRs. The relatively higher density of ML steps on the sidewalls of the holes of the QRs was identified as the potential cause of the localization of Ga metal particles on InGaAs QRs. The difference in surface chemistry with inhomogeneous domains of materials was discussed as another possibility for the localization of Ga metal particles. The technique used for epitaxial hybrid molecules can be used to form different types of metal elements, such as Al or In, and provides information on the interaction between metal particles and semiconductor structures. Therefore, this study can significantly improve the understanding of the formation of various types of hybrid molecules consisting of metal particles and semiconductor nanostructures, and consequently, it can find applications in optoelectronics. Acknowledgment. The authors acknowledge the financial support of the NSF (through Grant DMR-0520550).

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