ARTICLE pubs.acs.org/crystal
Polarity and Its Influence on Growth Mechanism during MOVPE Growth of GaN Sub-micrometer Rods S. F. Li,*,† S. Fuendling,† X. Wang,† S. Merzsch,† M. A. M. Al-Suleiman,† J. D. Wei,† H.-H. Wehmann,† and A. Waag† †
Institut f€ur Halbleitertechnik, TU Braunschweig, Hans-Sommer-Strasse 66, 38106 Braunschweig, Germany
W. Bergbauer†,‡ and M. Strassburg‡ ‡
Osram Opto Semiconductors GmbH, Leibnizstrasse 4, 93055 Regensburg, Germany
bS Supporting Information ABSTRACT: The influence of polarity during the MOVPE growth of GaN based sub-micrometer (sub-μm) rods has mostly been neglected up to now. In this paper we demonstrate that the surface polarity plays a crucial role for the morphology of the GaN sub-μm rods. Based on the differences between N-polar and Ga-polar surfaces, a model is suggested to explain the influence of various parameters on the morphology of GaN sub-μm rods for the first time. For Ga-polar GaN, the {10-11} r-planes, similar to N-polar (000-1) c-planes, are terminated by nitrogen atoms. These N-terminated surfaces can be passivated by hydrogen, which leads to a stable surface with low growth rates and therefore tends to keep a pyramidal shape with stable r-planes. For N-polar GaN, {10-1-1} r-planes can be modified by H2 etching, leading to the formation of more stable {1-100} m-planes, hence supporting the formation of sub-μm rods with vertical sidewalls.
’ INTRODUCTION In recent years, III-nitride nano-/sub-micrometer rods have gained great interest due to promising applications in future nano-optoelectronics and nanophotonics.1-3 The epitaxial growth of GaN nano-/sub-micrometer rods has the advantage of reduction of threading dislocation density due to their high aspect ratio and large surface-to-volume ratio.4 The small footprint of nano-/sub-micrometer rods helps to relieve the lattice and thermal expansion mismatch, allowing the growth of nanorods on strongly lattice-mismatched and large area substrates. The three-dimensional geometry of nano-/sub-micrometer rods can be used to control the light extraction, modifying the external quantum efficiency. Moreover, core/shell structures are proposed and have been demonstrated to increase the volume of the active region by growth of InGaN quantum wells on the whole cylindrical surface of the rods.5 In this case, polarization fields are absent on the side surfaces (m-planes) of GaN nano-/submicrometer rods grown along the c-axis, which helps to improve the emission efficiency and wavelength stability. Up to now, self-organized GaN nanorods have been extensively investigated.6-9 Light emitting diodes (LEDs) based on self-organized GaN nanorods were already demonstrated by several groups.6,7 Besides that, detailed studies are ongoing to gain complete understanding of the growth mechanisms of selforganized GaN nanorods. Recently, it has been claimed that the vapor-liquid-solid (VLS) growth mode which is normally r 2011 American Chemical Society
found in other semiconductor nanorods is not likely to be the growth mechanism in molecular beam epitaxy (MBE) growth.8 Although numerous achievements were obtained on the basis of self-organized GaN nanorods, control of the emission wavelength is a challenge since the emission wavelength strongly depends on, for example, the size, position, and alloy composition of the QWs within the nanorods. In self-organized GaN nanorod ensembles, there is usually a quite broad statistical distribution of these properties. Selective area growth (SAG) of GaN nanorods on patterned substrates, e.g. patterned SiO2/ GaN1 or Ti/GaN,2 is proposed to circumvent these problems. However, there are only limited reports on successful selective area growth of GaN nanorods by metal organic vapor phase epitaxy (MOVPE). This is due to the difficulties in achieving nanostructures with vertical sidewalls by MOVPE, in contrast to MBE growth. Although a pulsed growth mode was shown to successfully lead to GaN nanorods on a patterned SiO2/GaN template by MOVPE,1 understanding of the growth mechanism is still poor. In particular, this would help to develop a continuous-flow growth mode closer to the regular growth windows used today for LED production.
Received: November 19, 2010 Revised: February 10, 2011 Published: March 07, 2011 1573
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Crystal Growth & Design Comprehensive studies on the influence of the polarity on GaN nano-/sub-micrometer rod growth are not published yet even though it is known that the polarity of GaN plays an important role concerning the reactivity and stability of the surfaces.9,10 Recently, we demonstrated N-face GaN nanorods grown by selective area MOPVE growth in a continuous-flux growth mode.11,12 Still, the influence of GaN polarity on the nano-/sub-micrometer rod growth and growth mechanism behind it remains unclear. In this paper, we report our work on continuous-flux selective area growth of GaN sub-micrometer (sub-μm) rods on patterned SiO2/sapphire, SiO2/GaN/sapphire, and SiO2/bulk GaN templates. The effects of the GaN polarity and growth conditions on the GaN sub-μm rod growth are analyzed and discussed. A growth model is proposed to explain the dependency of the morphology on the polarity and the influence of growth conditions, especially the H2 concentration in the carrier gas. In this work, we use growth templates with feature sizes of both 1 μm and 400 nm for convenience of template preparation and characterization. The same results have been observed for our GaN nanorods with diameters down to 250 nm on the same kind of templates and using the same growth parameters.11,12
’ EXPERIMENTS Various growth templates were used for the growth experiments: patterned SiO2/sapphire, SiO2/GaN/sapphire, and SiO2/N-polar bulk GaN substrates. In order to enforce selective area growth, SiO2 layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) with 30 nm thickness. Subsequently, photolithography or nanoimprint lithography was adopted to transfer the pattern onto the SiO2 covered substrates, followed by inductively coupled plasma etching (Sentech 500C) to open the holes in the SiO2 mask layer. The patterns were composed by hexagonal-shaped openings with different diameter and interhole distances. The diameter of the opening varies from 200 to 2000 nm, and the distances between the holes are from 400 to 5000 nm, depending on the desired pattern layout. After chemical cleaning with acetone and isopropanol, the templates were introduced into a 3 in. 2 in. Thomas Swan MOVPE system for GaN sub-μm rod growth. For SiO2/sapphire templates, a thermal baking step was employed at 1100 °C for 20 min in a H2 carrier environment. After baking, a nitridation step was performed on the growth templates at temperature of about 1080 °C with 4000 sccm NH3 and N2 carrier gas. For patterned SiO2/GaN/sapphire templates, the growth temperature was directly ramped up to 1080 °C with 4000 sccm NH3 flow in order to protect the GaN surface from decomposition. Afterward, the trimethylgallium (TMGa) flow was introduced into the reactor for GaN sub-μm rod growth. The growth was in a continuous flow mode. The V/III ratio was kept at about 100. This is a value of about one-order less than the one used in our normal GaN layer growth. The flow ratio of H2/N2 carrier gas was kept as 2:1 for all the samples investigated in this paper. The GaN sub-μm rods were doped with Si with a concentration equivalent to about 1 1018 cm-3 in a GaN layer growth. The growth time was kept constant at 1200 s in all the investigated samples. All other growth parameters are the same as for the samples grown on patterned SiO2/ sapphire templates. GaN sub-μm rod growth was also performed on double-side polished bulk GaN templates. The bulk GaN substrate was produced by Lumilog with a thickness of 465 μm and an n-type unintentional background doping of about 1017 cm-3 . The same mask layer deposition and pattern transfer processes were applied to the N-polar side of these bulk GaN templates. The growth process was the same as that on patterned SiO2/ GaN/sapphire templates.
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To verify the polarity of the top surface of the GaN sub-μm rods, the as-grown GaN sub-μm rod samples were wet chemically etched in an aqueous 5 M KOH solution at 80 °C for 50 min. The surface morphology of all samples was characterized by a Zeiss supra 35 field emission scanning electron microscope (FESEM) with an acceleration voltage of 2 kV.
’ RESULTS In order to elucidate the influence of surface polarity, both patterned N-polar bulk GaN substrate and GaN/sapphire templates, which act as Ga-polar “quasi-substrates” were employed for selective area MOVPE growth of GaN sub-μm rods. The GaN/sapphire templates have smooth and featureless surfaces. Kelvin probe force microscopy (KPFM) measurements confirmed the Ga-polarity of the surfaces.13 The typical morphology of GaN sub-μm rods grown on bulk N-polar GaN substrate and Ga-polar GaN/sapphire quasi-substrate is presented in parts a and b, respectively, of Figure 1. The growth parameters are kept the same for these two samples. However, a distinct difference in GaN sub-μm rod morphology is found in these two images. For the GaN sub-μm rods grown on patterned N-polar GaN substrate, a clear sub-μm rod structure with vertical sidewalls and a diameter of about 1.5-2.0 μm can be observed in Figure 1a, with a top part having the shape of a truncated pyramid. Although the aspect ratio is relatively low of about 1.5, this is substantially different from GaN grown on Ga-polar quasi-substrate (cf. typical FESEM image in Figure 1b). There, the GaN growth on patterned SiO2/GaN templates shows only pyramidal GaN structures with dimensions close to the pattern size. The inset (image in the lower right corner) shows the enlarged cross-section view of a typical pyramidal structure. The angle between the bottom (0001) surface and the side edge between two adjacent planes is 58.4°, indicating that these planes are {10-11} planes, which fits our former investigations.14 They are stable planes, which limit the final morphology of the GaN crystals.15 N-polar GaN sub-μm rods can also be grown directly on patterned SiO2/sapphire templates. Figure 1c shows a typical SEM image of GaN sub-μm rods grown on this kind of template with the same growth parameters as were used in the samples shown in Figure 1a and b, except that there is a nitridation step prior to the GaN growth in order to achieve N-polarity. GaN sub-μm rods with aspect ratio of about 2-4 are achieved for the growth on the patterned SiO2/sapphire templates. This higher aspect ratio in comparison to GaN sub-μm rods grown on patterned N-polar bulk substrate may be due to the higher surface temperature during sub-μm rod growth, since the bulk GaN template is double-side polished, which probably leads a reduced heat transfer from the susceptor to the sample. The same growth parameters were employed for GaN nanorod growth on templates with opening size of 400 nm. The morphology is shown in Figure 1d. A typical GaN nanorod has a measured diameter of about 460 nm and an aspect ratio of 12. An inhomogeneity in GaN nanorod height can be seen in this image, probably due to the influence of different pattern geometry. Further adjustments on growth parameters are necessary to improve the height homogeneity on this pattern. Many investigations of GaN directly grown on nitrided sapphire templates show that this process results in an N-polar surface.16 It is known that KOH only etches N-polar GaN but does not significantly attack Ga-polar GaN.17 To determine the polarity 1574
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Figure 1. Comparison of GaN sub-μm rods grown on different templates with the same growth parameters. The images were obtained by tilting the samples 30° with respect to the sample normal. (a) Patterned SiO2 covered N-polar bulk GaN template. (b) Patterned SiO2/Ga-polar GaN/sapphire template; GaN nanostructures typically show a pyramidal shape. Inset: Detailed side view of one structure. (c) Patterned SiO2/sapphire templates. (d) Patterned SiO2/sapphire templates with smaller opening size of 400 nm; the measured rod shows a diameter of about 460 nm and a height of about 5.6 μm, yielding in aspect ratios up to 12.
Figure 2. GaN sub-μm rods on patterned SiO2/sapphire after wet chemical etching. The etching was performed in hot KOH solution for 50 min. The nominal diameter of the openings and the distance between openings of the mask are 1 and 5 μm, respectively.
of the sub-μm rods, wet chemical etching was performed on asgrown GaN sub-μm rods with an aqueous KOH solution at 80 °C for 50 min. Figure 2 presents the FESEM image of GaN sub-μm rods on patterned SiO2/sapphire templates after wet chemical etching. All the GaN sub-μm rods were either completely etched away or removed mostly, with only a small amount of remnant left. The remaining parts are from Ga-polar GaN parasitically nucleated and grown on the rim of the SiO2 mask, proved by KPFM and wet chemical etching experiments. Detailed investigations of GaN polarity will be published elsewhere. However, already this result proves that the GaN sub-μm rods grown on sapphire with a prior nitridation step have N-polar top surfaces. We would like to stress that the same etching results were observed on smaller GaN nanorods of 400 nm diameter and the same growth methods performed in an earlier work,12 proving that occurrence of N-polar GaN sub-μm rods by this method is independent of the pattern
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Figure 3. Schematic drawings of GaN atomic structure. (a) GaN pyramidal structure with Ga-polar top surface. (b) GaN pyramidal structure with N-polar top surface. (c) N-polar GaN pyramidal structure etched by hydrogen. Dashed lines indicate the more stable (1-100) Mplane. The detailed surface reconstructions are not included in this schematic drawing.
size and growth system. Scanning KPFM measurements also show that our GaN sub-μm rods have N-polar top surfaces.18 Moreover, since GaN sub-μm rods were grown on highly lattice-mismatched SiO2/sapphire templates, there could be an argument that rather the strain may determine the vertical growth than the polarity. Our growth results on unstrained N-polar and Ga-polar quasi-substrates indicate, however, that strain does not play a critical role for the development of GaN sub-μm rods. In general, the polarity determines the vertical growth, thus forming GaN sub-μm rods. In MOVPE growth, the carrier gas plays an important role in defining the surface quality.19 Our recent work shows that the H2/N2 ratio of the carrier gas can effectively shape the morphology of GaN nano-/sub-μm rods.11,12 With pure N2 as carrier gas, only pyramidal structures were observed, terminated with six (10-11) planes (r-plane) (cf. Figure 1a in ref 11). When the H2/N2 ratio was increased to 1:2, a clear GaN nano-/sub-μm rod growth occurred with well developed {1-100} side facets (cf. Figure 1b in ref 11). A higher H2 fraction also leads to the suppression of coalescence of the individual GaN nano-/sub-μm rods. When the H2/N2 ratio was further increased to 2:1, the height of GaN nano-/sub-μm rods further increased (cf. Figure 1c in ref 11 and Figure 4 in ref 12). Growth of GaN nano-/subμm rods with higher H2/N2 ratios or even pure H2 gas does not further increase the height of the GaN nano-/sub-μm rods. Instead, the filling of the openings became more inhomogeneous; for example, some holes were not filled with GaN. The detailed results can be found in refs 11 and 12.
’ DISCUSSION AND GROWTH MODEL We knew from previous experiments that the polarity and carrier gas mixture are important parameters to obtain c-oriented 1575
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Crystal Growth & Design GaN sub-μm rods on patterned SiO2/sapphire templates. These results invoke us to discuss the growth mechanism of GaN subμm rods on patterned substrates. The influence of polarity on the III-nitride layer growth has been reported in numerous publications. Jain et al. showed that InN growth with high V/III ratio shows a flat surface for N-polar InN and a pyramidal morphology for In-polar InN.20 It has been pointed out by several works that hydrogen can passivate the N-terminated surface.21-23 A theoretical investigation from Northrup et al. suggested that, in the presence of hydrogen, a 3(N-H) surface reconstruction forms a passivation layer on the (000-1) surface, which explains the lower growth rate on the N-polar surface compared to the Ga-polar surface.22 Figure 3a and b depicts the atomic structure of GaN with Ga-polar and N-polar surfaces as a ball-and-stick model. The r-planes {10-11} or {10-1-1} are also shown in these two drawings. A careful look at Figure 3a and b reveals that {10-11} surfaces are also terminated with N atoms. Feenstra et al.23 reported that an H-terminated structure also exists on {10-11} r-plane surfaces, which can passivate the r-plane surfaces. In MOVPE growth ambient, there are plenty of hydrogen atoms from the carrier gas, decomposition of NH3, and other sources. Thus, we believe that {10-11} r-planes during Ga-polar GaN sub-μm rod growth are passivated with N-H bonds, which leads to stable {10-11} r-planes and hence a low growth rate on these planes (indicated in Figure 3a). According to the Wulff growth theory,24 the low growth rate planes remain after growth, thus leading to a pyramidal shape of the structures. In contrast, the {10-1-1} r-plane surfaces occurring in an N-polar GaN sub-μm rod are terminated by Ga atoms (see Figure 3b), which may lead to a different stability of surface atoms. This difference can explain the different morphologies (pyramidal for Ga-polar and vertical facets for N-polar) occurring during the growth of GaN subμm rods of different polarity. Besides, a one-order lower V/III ratio than the value for our typical GaN layer growth was used during the growth of our GaN nanostructures. A low V/III ratio (or higher Ga flux) is suggested to suppress the hydrogen passivation according to ref 22, supporting higher growth rates on c-planes of N-polar GaN. Many publications report that the H2 carrier gas can also effectively etch the GaN via Ga-H and N-H formation at elevated temperature; see, for example, refs 25 and 26. Figure 3c demonstrates schematically how the H atoms etch the r-plane of N-polar GaN via Ga-H and N-H or even NH3 formation. Similar to the c-plane surface of Ga-polar GaN, the {10-1-1} r-plane is terminated with Ga atoms. The density of dangling bonds of r-planes is 16.0 nm-2, which is significantly higher than on m-planes (12.1 nm-2).15 So the atoms in r-surfaces are supposed to be less stable as compared to atoms in m-surfaces due to less bonds to the bulk of the crystal. As a result, hydrogen atoms are suggested to be able to easily attack the surface Ga atoms on the r-plane at high temperature, especially at those sites that have only two bonds to internal N atoms and remove them from the surface. On the other hand, the {1-100} m-planes (indicated as dashed lines in Figure 3c) have less dangling bonds and, thus, should be thermodynamically more stable. Consequently, the r-planes area can be reduced by hydrogen etching and GaN sub-μm rods with more stable {1-100} side planes grow up. At the same time, the diameter of the GaN sub-μm rods is reduced due to etching. Moreover, an increasing hydrogen concentration also enhances the etching of parasitic nuclei on the masked area. Thus, the coalescence of
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adjacent GaN sub-μm rods can be suppressed, as shown in Figure 1 of ref 11. Further work to elucidate the surface atomic structure of r-plane surfaces during Ga- and N-polar GaN MOVPE growth is necessary to clarify the different influences. In general, the differences in the surface atomic structure make the growth of Ga-polar GaN sub-μm rods more difficult than the growth of N-polar GaN sub-μm rods. This result is corroborated by experience from epitaxial lateral overgrowth experiments (ELOG) of GaN on a-plane GaN templates.27 For the same growth conditions, the lateral Ga-polar surface exhibits a pyramidal structure, whereas the N-polar surface has a flat appearance with much lower growth rate. It is worthwhile to point out that these experiments only show that under hydrogen carrier gas the N-polar GaN sub-μm rod growth is “easier” than Ga-polar GaN sub-μm rod growth. Nevertheless, it is possible to achieve Ga-polar GaN sub-μm rods by MOVPE growth if one can modify the r-plane surface stability under hydrogen exposure. From theoretical considerations,22 by exposing the H passivated surface to a Ga flux, it is possible that the 3(N-H) passivation surface will become unstable, which may also be an explanation why pulsed growth works for GaN nanorods formation on patterned GaN templates (supposed to be Ga-polar).1
’ CONCLUSION In summary, we achieved MOVPE growth of N-polar GaN sub-μm rods in a continuous flow mode, in contrast to other approaches, which are often far away from standard growth conditions. The influence of the GaN polarity on the growth of sub-μm rods and its interdependence with other growth parameters, such as hydrogen concentration, is presented. A model taking into account the etching and passivation effect of hydrogen on different facets is suggested to explain the observed experimental results. Surface polarity, in combination with hydrogen in the carrier gas mixture, is the key parameter to achieve vertical growth and, thus, the formation of GaN sub-μm rods. In contrast, GaN grown on patterned Ga-polar GaN layers on sapphire shows a pyramidal structure. No vertical growth was observed in this case. These phenomena are explained by the passivation effect of hydrogen on N-terminated surfaces, e.g. GaN (000-1) and {1011} planes. The hydrogen-passivated {10-11} r-planes are low growth rate facets which tend to maintain their shape during growth. In contrast, for N-polar GaN, the {10-1-1} r-planes are terminated with Ga atoms, which are relatively easier to be attacked by hydrogen, forming stable {1-100} m-planes, thus leading to well-defined GaN sub-μm rods. ’ ASSOCIATED CONTENT
bS
Supporting Information. SEM image of etched Ga-polar GaN truncated pyramidal structures by hot KOH solution and KPFM measurement results showing the polarity of our GaN submicrometer rods grown on patterned SiO2/sapphire. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Institute of Semiconductor Technology, Braunschweig University of Technology, Hans-Sommer-Strasse 66, 38106 Braunschweig, Germany. E-mail:
[email protected]. Telephone: 0049-531-3913806. 1576
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’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the German ministry of education and research (BMBF) within the project “Monalisa” (Project No. 01BL0811) and by the European Community within the FP 7 project “SMASH” (Project No. 228999). We thank D. R€ummler, A. Schmidt, and B. Matheis for the substrate preparation and M. Schilling and F. Ludwig for giving us the possibility to use their FESEM. Work has partly been performed in the Joint Optical Metrology Centre Braunschweig (JOMC).
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