Polarity Control in 3D GaN Structures Grown by Selective Area

Article pubs.acs.org/crystal

Polarity Control in 3D GaN Structures Grown by Selective Area MOVPE Xue Wang,* Shunfeng Li, Sönke Fündling, Jiandong Wei, Milena Erenburg, Hergo-H. Wehmann, and Andreas Waag Institut für Halbleitertechnik, TU Braunschweig, Hans-Sommer-Straße 66, 38106 Braunschweig, Germany

Werner Bergbauer and Martin Strassburg Osram Opto Semiconductors GmbH, Leibnizstraße 4, 93055 Regensburg, Germany

Uwe Jahn and Henning Riechert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany ABSTRACT: During the metal organic vapor phase epitaxy of GaN microcolumns, both nitrogen- and gallium-polar GaN in the same structures could be detected on patterned SiOx/sapphire templates. To clarify its origin, the spatial distribution of surface polarity has been analyzed by both Kelvin probe force microscopy and selective etching techniques. A new “truncated pyramid + column” growth method was developed to effectively avoid the formation of mixed polarity and realize selective area growth of single N-polar GaN columns. The strong yellow luminescence in mixed polarity structures can substantially be eliminated in single N-polar core−shell lightemitting diodes.

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INTRODUCTION Three dimensional (3D) GaN microcolumns have recently gained increased attention for applications in optoelectronic and electronic devices.1 This is due to the low defect density and reduced influence of lattice and thermal mismatch with the substrate. In comparison to conventional planar film light emitting diodes (LEDs), the active region of 3D column LED systems can be much larger when using a core−shell geometry with high aspect ratio.2,3 A further advantage is that the InGaN/ GaN multi quantum wells (MQW) mainly grow on nonpolar m-plane side walls. The benefits of employing nonpolar or semipolar orientations for LED fabrication have been extensively demonstrated in recent years. Traditional MQWs grown on the polar c-plane suffer from the quantum-confined Stark effect (QCSE)4 which leads to a strong reduction in the oscillator strength, while nonpolar surfaces eliminate the QCSE and the increased transition probability using nonpolar orientations may lead to higher light output power or lower threshold current densities.5 Ga-polar GaN microcolumns have been successfully realized in the past by plasma-assisted molecular beam epitaxy (PAMBE)6,7 as well as by using a pulsed metal organic vapor phase epitaxy (MOVPE) on Ga-polar GaN templates.8 However, 3D GaN columns with a mixed polarity (both Nand Ga-polarity) were often reported in the literature when © 2012 American Chemical Society

employing a more conventional MOVPE continuous growth mode on nitrided patterned SiOx/sapphire or SiNx/sapphire templates.9−11 Mixed polarity leads to inversion domain boundaries (IDB) between the Ga- and N-polar parts, which are expected to cause high reverse leakage currents12 and hence to reduce the efficiency of LEDs containing IDBs. Therefore, single polar GaN columns are a prerequisite for high efficiency 3D core−shell LEDs. In addition, understanding the origin of the occurrence of a mixed polarity will lead to a deeper understanding of 3D GaN MOCVD growth mechanisms in general, which is important for further developing 3D GaN light-emitting structures. We have recently suggested that polarity plays an important role for the growth mechanisms occurring during the growth of 3D GaN microcolumns, determining substantially the shape of the 3D objects.11 For N-polar GaN column growth under hydrogen carrier gas, (0001̅) c-plane and {11̅00} m-planes can be easily formed.11 This is in contrast to Ga-polar GaN growth, where {11̅01} r-planes generally occur, leading to pyramidally shaped objects.11 Due to the passivation effect of hydrogen on r-planes,13 vertical sidewalls are difficult to produce in this case. Received: February 3, 2012 Revised: March 29, 2012 Published: April 2, 2012 2552

dx.doi.org/10.1021/cg3001637 | Cryst. Growth Des. 2012, 12, 2552−2556

Crystal Growth & Design

Article

Figure 1. Three 0°-tilted FESEM images of (a) GaN nucleation seed, where the dashed hexagon indicates the contour of the aperture in the SiOx layer; (b) GaN column grown on the nucleation seed shown in (a); (c) the GaN column shown in (b) after 5 min of etching in hot KOH (the arrows indicate Ga-polar domains); (d) GaN columns array before and (e) after 30 min of hot KOH etching. growth time was increased so as to obtain truncated pyramids with single N-polar top surfaces. The V/III ratio was kept at 100. For 3D core−shell LEDs, the growth conditions of n-doped GaN core have been described above, MQWs and p-doped GaN shell on the n-doped GaN columns were realized with same growth conditions as for layer LED growth. The QWs growth was performed at 720 °C and 600 hPa for 120 s; p-doped GaN was grown at 1000 °C and 250 hPa for 900 s. To study the evolution of mixed polarity in GaN columns, the polarities of GaN nucleation seeds and columns have been analyzed by etching the samples in a 2 M KOH solution at 80 °C20 as well as spatially resolved photoassisted Kelvin probe force microscopy. The morphology of all samples was characterized by a Zeiss supra 35 field emission scanning electron microscope (FESEM) with an acceleration voltage of 2 kV. The optical properties of mixed polar and single Npolar core−shell LEDs were investigated using room temperature cathodoluminescence and photoluminescence.

Since polarity drastically modifies the growth mechanisms, we developed analytical techniques for determining the polarity in 3D structures with high spatial resolution, e.g. surface photovoltage Kelvin probe force microscopy (KPFM)14 and hot KOH etching.10 In this paper, we report on the origin of mixed polarity in our 3D structures, including a new “truncated pyramid + column” approach which can effectively avoid the formation of mixed polarity. We demonstrate that our understanding of the formation of mixed polar structures leads to a reproducible fabrication of single N-polar GaN columns. Additionally, cathodoluminescence (CL) and photoluminescence (PL) spectroscopy results of single N-polar and mixed polar core− shell LEDs will be presented.

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EXPERIMENTS

RESULTS AND DISCUSSION Figure 1a shows a FESEM image of nucleation seed after 100 s growth time. The dashed hexagon marks the contour of the aperture. The subsequent vertical growth of the GaN column started on the c-facet nucleation seed (cf. Figure 1b). The polarity domains in GaN columns preserve the orientation of their original nucleation seeds. The polarity of the GaN columns was examined by etching the samples in a 2 M KOH solution at 80 °C for 30 min.20 After the KOH etching, Gapolar domains remain intact while N-polar domains display hillock like surfaces (cf. Figure.1c). This etching result proves that inside the dashed hexagon the seed is N-polar and outside of the aperture Ga-polar material regularly occurs. We conclude that the mixed polarity emerges even during the first nucleation step. The most likely reason for the occurrence of Ga-polar GaN seems to be the additional nucleation seeds on the SiOx mask. The nucleation step was performed at a low growth pressure and a low flow ratio of H2/N2 carrier gas which leads to a low vertical and high lateral growth rate of both polarities.10,18,21 We have observed that for both polarities the top surface c-plane grows slower than the other facets at described nucleation growth condition. As expected by Wulff’s theory,22 the c-facets of Ga-polar and N-polar domains appear during the nucleation step. The FESEM images of an ensemble of mixed polar GaN columns before and after 30 min KOH etching are presented in Figures 1d and 1e.

For all the growth experiments, we have used patterned c-plane sapphire substrates with 0.25° off-cut relative to the m-plane, covered by a 30 nm thick SiOx layer. The thin SiOx layer was deposited by plasma-enhanced chemical vapor deposition (PECVD). Photolithography was applied to pattern the surface. Thus, an array of hexagonal openings was etched in the SiOx layer using an inductively coupled plasma (ICP) etcher. The diameter of the hexagonal apertures in the SiO2 mask varied from 200 nm to 5 μm and the pitch ranged from 1 to 10 μm. The GaN columns were grown in a vertical 3 × 2 in. FT Thomas Swan MOVPE system, using Trimethylgallium (TMGa) and Ammonia (NH3) as precursors. Before growth, an in situ thermal baking and nitridation of the templates were employed. The nitridation process enables N-polar GaN growth in the openings.15 The growth of GaN columns was performed in two steps. With the mixed polar 3D GaN column growth, first, to ensure a high selectivity of nucleation, a nucleation step was performed at 1000 °C and a H2/ N2 (mixed carrier gas) flow rate ratio of 1:1. In the second step, in order to enhance the vertical growth of GaN columns, the growth temperature and the flow rate ratio were increased to 1080 °C and 2:1, respectively.16−18 In addition, silane was injected into the reactor during the column growth for n-type doping. The V/III ratio was kept at 100 in both steps (more experimental details can be found in ref 11). The growth time of n-doped GaN columns was 1800 s. In order to get a selective and homogeneous growth of nucleation seeds, silane as an antisurfactant was not used in the nucleation step.19 Therefore, two steps growth is necessary. To achieve the growth of truncated pyramids, which will later be used for the newly suggested “truncated pyramid + column” approach, the temperature in the nucleation step was reduced to 960 °C and the 2553



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polarity in GaN columns. Figure.3 schematically shows this two-step approach. The first step is the growth of truncated

The spatial distribution of surface polarity in a GaN column (after 20 min KOH etching and not etched down to sapphire) was analyzed by photoassisted KPFM according to the method described in ref 22. The evolution of surface photovoltage (SPV) of both polarity domains in a mixed polar GaN column is presented in Figure 2 and the topographic image of the GaN

Figure 3. Schematic representation of the “truncated pyramid + column” growth approach: (a) Growth of GaN truncated pyramids with single N-polar top surface; (b) Growth of single N-polar GaN columns.

pyramids at a reduced growth temperature of 960 °C (cf. Figure 3a). It has been reported that with decreasing temperature the growth rate of the r-planes of Ga-polar GaN decreases and the growth rate in ⟨0001⟩ and directions increases.21,28 Under our growth conditions (at a relatively low temperature of 960 °C), we have observed that the growth rate of the r-planes of Ga-polar seeds is smaller than that of the other two facets. According to Wulff’s theory,22 the shape of a 3D crystal is determined by mainly the crystal facets with smaller growth rates. Therefore, during the growth, the {11̅01} r-planes of the Ga-polar nucleation seeds around the apertures appear, while finally their Ga-polar top facet (0001) and the mfacets {11̅00} disappear. Inside the opening the seeds are Npolar with a flat surface. After a certain growth time, the truncated pyramids are formed with pure N-polar top surfaces. The second step is the column growth on top of the truncated pyramid [cf. Figure 3(b)]. The {11̅01} r-plane of the Ga-polar nucleation seed is a slow growing plane,11 since this plane is passivated with N−H bonds.13 Under these growth conditions, the GaN columns grow exclusively on the N-polar c-facets of the seeds not on the r-plane of the Ga-polar additional nucleation seeds, which leads to GaN columns of single Npolarity. Figure 4a shows an FESEM image of a truncated pyramid with a single N-polar top surface after the first growth step. Figure 4b shows a single N-polar column. The diameter of the column is larger than the diameter of the truncated pyramid cplane surface, which is caused by lateral growth of the m-planes. Figure 4c presents the images of an ensemble of single N-polar GaN columns after etching for 15 min in hot KOH. The GaN columns were completely etched away and the Ga-polar nucleation collars around the apertures still remain on the SiOx mask. It is clearly visible that there are more Ga-polar nucleation seeds around the aperture at the lower nucleation temperature deposited than there were at 1000 °C (cf. Figure 1e). On average, more than 80% of an aperture’s perimeter is filled with Ga-polar nucleations at 960 °C. However, the Gapolar domains did not grow upward in the GaN columns, as explained above. The single N-polarity of these GaN columns was confirmed by hot KOH etching as well as by KPFM.14 Core−shell LED structures have been grown on both types of samples, i. e. the mixed polarity ones as well as those of

Figure 2. Evolution of surface photovoltage of the surface of Ga- and N-polar domains in one KOH etched GaN column under 360 nm UV light illumination at UV light intensity of 7 μW/cm2. Insert: Topographic image of the etched GaN column.

column is shown in the insert. The initial SPV signals for both polarities rise up simultaneously, after switching on UV light with intensity of 7 μW/cm2. During 150s continuous illumination, the SPV signal of the lower GaN surface which was etched down by KOH decreases rapidly in a logarithmic way while the SPV of the surface of the remaining domain decreases at a very slow rate. This SPV behavior resembles the measurement results on GaN layers with Ga- and N-polarities, indicating that the etched down surface is N-polar and the remaining one is Ga-polar, which is in agreement with the KOH etching result.23 It is quite surprising, that Ga- and N-polar domains show the same height within a column, in contrast to the dramatically different growth rates observed for N- and Ga-polar surfaces in an epitaxial lateral overgrowth (ELOG) experiment.24 Equal growth rates of both GaN polarity domains have also been found under pure N2 carrier gas flow GaN layer growth, and have been thought to originate from a mass-transport-limited growth.25 A strong indication of this transport-limited growth regime in GaN layer growth is a constant growth rate at temperatures above 900 °C and under pure N2 carrier gas which is thought to be a very important condition.26 However, in the present work, a H2 to N2 mixture of 2:1 as carrier gas was used to promote vertical growth of GaN columns and the GaN growth rate decreases distinctly with increasing temperature between 980 and 1100 °C, possibly due to the formation and desorption of GaHx from the growing surface.27 Therefore, we conclude that the same growth rate of both types of domains in one GaN column can be reached even outside the transportlimited regime, the origin of that behavior being unclear at the moment. More work has to be done to explain this result and clarify the growth process. Now, a new “truncated pyramid + column” growth method will be suggested in order to suppress the formation of mixed 2554



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core−shell LEDs show a pronounced yellow band emission (YL). This is in contrast to the single N-polar core−shell quantum well structures, where the yellow band is practically absent. In difference of the single N-polar core−shell LEDs, mixed polar LEDs still contain IDBs and Ga-polar domains besides N-polar domains. The IDBs and corresponding dislocations in the mixed polar LEDs are one of the sources of the YL;30 in Ga-polar domains, on the other hand, it is widely believed that Ga vacancies (VGa) or related complexes can generate YL.31 As for N-polar GaN, Q. Sun et.al. and D. Du et.al. have observed that N-polar GaN layers show much lower YL than Ga-polar layers,32,33 which could be explained by a larger migration length of Ga atoms on a N-polar surface,34,35 by which the formation of VGa in N-polar GaN is reduced. In the present work, GaN columns were grown at a low V/III ratio which creates a local stoichiometry richer in Gallium, so that the formation of VGa will have been further reduced in the Npolar domains. We believe that this is the reasons that the single N-polar core−shell LEDs show weak YL.

Figure 4. Three 0°-tilted FESEM images of (a) GaN truncated pyramid after first nucleation growth step, (b) single N-polar GaN column, and (c) array of single N-polar GaN columns after 15 min etching in hot KOH.

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CONCLUSION In conclusion, it has been shown that both Ga- and N-polarity can occur during the 3D growth of GaN into selective SiOx/ sapphire mask patterns. In the discussed case of an InGaN/ GaN core−shell LED structures, the occurrence of Ga-polar material leads to IDBs and a strong yellow band emission. Single N-polar GaN column growth has been realized using a new “truncated pyramid + column” growth approach. This method can eliminate the formation of Ga-polar c-facets during the nucleation on the SiOx mask material, which is probably the main cause of the mixed polarity observed in previous attempts to grow N-polar GaN columns. Single N-polar core−shell LEDs have been demonstrated to exhibit only very weak YL in both CL and PL spectra in comparison to mixed polar LEDs.

single N-polarity. After the n-doped GaN column growth, a 5fold InGaN/GaN MQW and a p-doped GaN layer are grown as a shell on both the m-plane and the c-plane facets. Room temperature (RT) cathodoluminescence monochromatic intensity maps were taken at a wavelength of 400 nm and using an acceleration voltage of 8 kV. Figure 5a and b show a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 0049-531-3913784. Fax: 0049-531-3915844. Notes

The authors declare no competing financial interest.

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Figure 5. Color-coded RT CL maps superimposed on FESEM images of (a) single N-polar and (b) mixed polar core−shell LEDs, recorded for the side wall InGaN/GaN MQW emission at 400 nm. (c) Normalized CL and PL spectra at room temperature for single N-polar core−shell LEDs and mixed polar core−shell LEDs; these spectra are vertically shifted for clarity.

ACKNOWLEDGMENTS This work has been funded partly by the German ministry of education and research (BMBF) within the Project “Monalisa” (Project No. 01BL0811) and partly by the European Community within the FP 7 project “SMASH” (Project No. 228999). We are grateful to D. Rümmler, A. Schmidt, B. Matheis and M. A. M. Al-Suleiman for the substrate preparation and M. Schilling and F. Ludwig for giving us the possibility to use their FESEM. We would also like to thank for the support by the Braunschweig International Graduate School of Metrology (IGSM).

superposition of such CL maps on corresponding SEM images of single N-polar and mixed polar GaN column core−shell LEDs, respectively. At this wavelength, the InGaN/GaN MQW emission from the m-planes is clearly visible. In a c-plane layer LED which was grown with the same growth parameters of the InGaN/GaN MQWs for core−shell LEDs, the thicknesses of InGaN QWs and GaN barriers are about 2.3 and 5.4 nm, respectively, and the indium concentration in the QWs is about 14%. However, it is known that thicknesses and In concentration in the columns depend on their geometry.29 RT CL and PL spectra of single N-polar and mixed polar GaN core−shell LEDs are shown in Figure 5c. The “absolute” PL intensity of single N-polar and mixed polar 3D LEDs are comparable for the 360 nm-450 nm region. The mixed polar

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REFERENCES

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Polarity Control in 3D GaN Structures Grown by Selective Area

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