Letter pubs.acs.org/NanoLett
Optical Emission of Individual GaN Nanocolumns Analyzed with High Spatial Resolution A. Urban,† M. Müller,*,‡ C. Karbaum,‡ G. Schmidt,‡ P. Veit,‡ J. Malindretos,† F. Bertram,‡ J. Christen,‡ and A. Rizzi† †
IV. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, 39106 Magdeburg, Germany
‡
ABSTRACT: Selective area growth has been applied to fabricate a homogeneous array of GaN nanocolumns (NC) with high crystal quality. The structural and optical properties of single NCs have been investigated at the nanometer-scale by transmission electron microscopy (TEM) and highly spatially resolved cathodoluminescence (CL) spectroscopy performed in a scanning transmission electron microscope (STEM) at liquid helium temperatures. TEM cross-section analysis reveals excellent structural properties of the GaN NCs. Sporadically, isolated basal plane stacking faults (BSF) can be found resulting in a remarkably low BSF density in the almost entire NC ensemble. Both, defect-free NCs and NCs with few BSFs have been investigated. The low defect density within the NCs allows the characterization of individual BSFs, which is of high interest for studying their optical properties. Direct nanometer-scale correlation of the CL and STEM data clearly exhibits a spatial correlation of the emission at 360.6 nm (3.438 eV) with the location of basal plane stacking faults of type I1. KEYWORDS: Gallium-nitride, nanocolumn, selective area growth, cathodoluminescence spectroscopy, scanning transmission electron microscopy, basal plane stacking fault
N
Using scanning transmission electron microscopy cathodoluminescence (STEM-CL)) at low-temperature (15 K), we present in the following a direct correlation of the optical properties with the actual real crystalline structure of ordered GaN NCs on nanometer-scale. We show for the first time the application of optimized nanopreparation of selected nanowires by focused ion beam for analytical STEM-CL. The advantage of highly spatially resolved cathodoluminenscence spectroscopy in a scanning transmission electron microscope at elevated temperatures on nanowire heterostructures has been shown by several groups.15,16 Wurtzite GaN nanocolumns with Ga-polarity have been obtained by selective area growth (SAG) on commercially available metal−organic vapor phase epitaxy (MOVPE) GaN(0001)/Al2O3 templates in a Veeco GenII MBE system equipped with an Unibulb RF-plasma source for nitrogen and effusion cells for the group III elements. A molybdenum mask was used for the selective area growth. A 10 nm thin Mo layer was deposited by electron beam evaporation onto the bare, degreased substrates at a low deposition rate of 1 Å s−1, which yields a smooth mask surface. Regular arrays of circular apertures were patterned by electron beam lithography (EBL) and a selective reactive ion etching step using a mixture of SF6 and Ar gases with an etch rate of Mo of about 3 Å s−1. The substrate temperature measured by a thermocouple was T =
itride-based nanocolumns (NC) are a promising material system for designing novel highly efficient optoelectronic devices.1,2 In comparison to planar structures, the optically active area of the device can be drastically increased by growing core−shell heterostructures on nanocolumns.3 The growth of heterostructures on top of the non- or semipolar facets of the nanocolumns reduces the negative impact of polarization fields (quantum-confined-Stark effect), leading to a higher efficiency.4 The growth of GaN NCs succeeds in a high crystal quality and a low density of line and extended defects as shown by previous studies.5,6 In particular, the low defect density of basal plane stacking faults (BSF) in NCs in comparison with heteroepitaxially grown a-plane or m-plane GaN layers gives access to the direct characterization of individual BSFs, which is of high interest to better understand the formation of these two-dimensional defects and the growth process of the nanocolumns. Rebane et al. have proposed that BSFs in GaN form quantum well-like structures for excitons.7 It has been shown that the presence of BSF I1 results in luminescence peaks ranging from 3.40−3.42 eV.8−10 Similar luminescence peaks have been found for stacking faults in GaN nanowires.11,12 Furthermore, time-resolved PL experiments on GaN nanowire ensembles proved that type I1 basal plane stacking faults indeed form quantum wells. Due to the activation energy of excitons bounded at stacking faults it is essential to cool below 60 K to study the luminenscence properties of stacking faults.13 In particular, the hole localization energy of 7 meV leads to thermally activated detrapping of excitons and quenches the emission of the stacking faults above 60 K.14 © XXXX American Chemical Society
Received: April 2, 2015 Revised: July 10, 2015
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DOI: 10.1021/acs.nanolett.5b01278 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters 780 °C. The nitrogen supply was ϕN = 6.2 nm/min, calibrated in equivalent growth rate of GaN layers under slightly Ga-rich conditions. For SAG N-rich growth conditions have been used by reducing the Ga-flux by a factor of nine relative to its value at stoichiometry. A small NC ensemble was prepared in cross-section for transmission electron microscopy (TEM) and STEM-CL using an optimized focused ion beam (FIB) preparation approach. First, the nanocolumns have been embedded in a protective Si3N4 matrix by plasma enhanced chemical vapor deposition. In a second step a cross-sectional lamella was milled from the ensemble using 30 keV Ga+ ions by a conventional FIB approach (FEI Nova 600 NanoLab). The 1.5 μm thick lamella was then further thinned down to about 250 nm with the FIB by iteratively milling its front- and backside. Consequently, the process ends up with a single, fully Si3N4-covered row of nanocolumns centered inside the TEM-lamella, as schematically shown in Figure 1. Postprocessing of the lamella by low
liquid nitrogen cooled back-illuminated Si-CCD for parallel CL detection is available to record a complete CL spectrum at each pixel, resulting in a four-dimensional data set ICL(x,y,λ). The STEM acceleration voltage is optimized to 80 kV minimizing sample damage and preventing luminescence degradation. More detailed information about the method can be found elsewhere.19 The morphology of the selectively grown nanocolumns from one of the investigated arrays is shown in the scanning electron microscopy (SEM) images in Figure 1. The nominal aperture diameter and pitch on the SAG template were 100 and 500 nm, respectively. The average nanocolumn diameter after 3 h of growth was 200 nm. The predominant growth direction is along [0001] and the Ga-polar nanocolumns show nonpolar sidewalls ({1100} mplane, as well as {1120} a-plane segments) with semipolar rplane {1102} surfaces at the tip. Due to the Ga diffusionassisted growth mechanism, the local growth conditions at the beginning of growth are expected to be more Ga-rich as compared to the nominal growth conditions.20,21 However, due to a smaller Ga diffusion length on the nanocolumn sidewalls21−23 as compared to the Ga diffusion length on the Mo layer, N-rich growth conditions are restored at the tip of the NC during growth. Under the applied growth conditions the Ga diffusion length on the Mo-mask has been estimated to be around 430 nm. The pencil-tip morphology20,24,25 of the Ga-polar nanocolumns grown by MBE under N-rich conditions has been explained by surface energy arguments within the framework of a semiquantitative broken-bond model.20 More recently the crystal shape of wurtzite GaN nanostructures has been described based on ab initio calculations.26 Two representative types of nanocolumns from the ensemble are shown in Figure 2. Although the nanocolumns are grown
Figure 1. SEM micrographs of the selectively grown Ga-polar nanocolumns. A bird’s eye view of the investigated array is shown to the left (a); a single row of nanocolumns was selected for further investigation (see text), as schematically indicated. The morphology of the nanocolumns is shown to the right in bird’s eye view and top-view geometry (b,c). The diameter of the NC shown in (b) and (c) is 200 nm.
energy (2−0.5 keV) Ar+ ions was applied to further reduce the thickness and to remove the amorphous surface layer, which is expected to be detrimental for the optical emission.17 TEM images of SAG GaN NCs were acquired using a Philips CM200-FEG-UT operated at 200 kV. The luminescence properties of individual GaN nanocolumns have been directly studied by a one by one comparison of the cathodoluminescence and scanning transmission electron microscopy. The CL-detection unit is integrated in a FEI STEM Tecnai F20 equipped with a liquid helium stage (T < 15 K) and a light collecting parabolic mirror. In STEM mode the electron beam is convergently focused into a spot and either kept at a single position or scanned over the region of interest in the sample. The high-angle annular dark field contrast (HAADF) is used for STEM imaging. The HAADF contrast is mainly given by the dominating Rutherford scattering of the partially screened atomic nuclei and the incoherence of the scattered electrons.18 Simultaneously, the emitted CL is focused by a retractable parabolically shaped aluminum mirror onto the entrance slit of the grating monochromator MonoCL4 (Gatan) and is detected by a Peltier-cooled GaAs (Cs) photomultiplier. Alternatively, a
Figure 2. TEM micrographs of two representative Ga-polar GaN nanocolumns in bright- and dark-field. The zone axis for (a) and (b) was [1120]; the DF images have been recorded with g = (1100) . The nanocolumn in (a) is free of extended defects; the one in (b) shows some BSFs as marked by the white triangle in the DF image.
homoepitaxially on GaN(0001) template, the occurrence of threading dislocations (TDs) is observed at the template interface. However, since the TDs are eliminated due to dislocation bending to the side surfaces,5 we will focus in this letter on the properties of the nanocolumn area well above the substrate interface. The nanocolumns have been investigated in [1120] zone axis geometry for which extended defects such as basal plane B
DOI: 10.1021/acs.nanolett.5b01278 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 3. HAADF image (a) of the ordered GaN nanocolumns grown on GaN/sapphire template simultaneously recorded with a low temperature panchromatic STEM-CL mapping (b). The CL intensity image exhibits the most intense emission from the GaN nanocolumns. In several GaN nanocolumns the highest CL intensity can be observed in the vicinity of extended defects.
temperature panchromatic STEM-CL mapping of a single nanocolumn (position 1) is displayed in Figure 4. The HAADF
stacking faults can be imaged. While the nanocolumn in Figure 2a appears to be free of extended defects, a clear contrast due to the presence of several BSFs appears in the dark-field image of another nanocolumn is shown in Figure 2b. Four types of BSFs are reported in the wurtzite structure: I1, I2, E,27 and the theoretically predicted type I3.28 The I1 and I2 types can be discerned in TEM by using two-beam dark field (DF) conditions, so that g = (1100).8,29 The BSF of type E is invisible for the used DF imaging conditions. Accordingly, the DF image in Figure 2b exhibits the presence of BSFs of types I1 and I2 in the nanocolumn. Figure 3a shows the cross-sectional STEM image in HAADF contrast. The STEM image clearly reveals the GaN buffer layer in the bottom part and the Mo-mask in bright contrast. On top of the GaN template, we observe the growth of ordered nanocolumns with a mean diameter of around 200 nm, which corresponds to the SEM image in Figure 1. The pencil-like NCs are embedded in a Si3N4 matrix, which is covered by Pt. The bright contrasts within several NCs perpendicular to the growth direction show up extended defects such as basal plane stacking faults due to the dechanneling effect of the scattered electrons. The local change of lattice periodicity by the BSFs leads to an increased angular range of scattered electrons resulting in a higher HAADF-intensity.30 The direct correlation of the HAADF image with the simultaneously recorded panchromatic CL intensity mapping at 15 K in Figure 3b indicates the highest CL intensity exclusively from the GaN NCs. Only weak emission from the GaN template is observed. For all NCs, a reduction of the CL intensity toward the NC tip and the sidewalls is observed. Furthermore, a modulation of the CL intensity can be seen for NCs, which contain extended defects. The spatially averaged CL spectrum of the NCs (not shown) exhibits an intense near-band-edge emission (NBE) at 356 nm (3.48 eV) followed by an emission of the BSF type I1 at 361 nm (3.43 eV) and a broad defect-related luminescence band between 365 and 405 nm (3.06−3.34 eV). The small GaN NC ensemble in Figure 3a reveals different structural and optical emission properties for the individual nanocolumns, concerning the number of stacking faults and emission energies. Hence, under the applied growth conditions quite far from equilibrium, kinetics may be locally different for each NC. In order to get a direct correlation between the structural and optical properties of these defects, investigations on single nanocolumns were performed. The positions of the investigated nanocolumns are marked in Figure 3b. The low
Figure 4. STEM image (a) of a single defect-free GaN nanocolumn and the simultaneously recorded panchromatic CL mapping at 15 K (b). The CL intensity image shows a homogeneous luminescence distribution exclusively from the GaN nanocolumn. The spectrally and spatially resolved CL linescan (c) reveals a dominant near-band-edge emission.
image in Figure 4a shows a defect-free nanocolumn. A reduction of the CL intensity toward the surface of the nanocolumn is observed. The diffusion of the generated carriers to the sidewalls results in nonradiative recombination processes at the nonpolar and semipolar surfaces. Furthermore, at the tip the excitation volume is reduced due to the pyramidal geometry. A similar intensity reduction is observed at the C
DOI: 10.1021/acs.nanolett.5b01278 Nano Lett. XXXX, XXX, XXX−XXX
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the crystal surface.31 Unlike the defect-free nanocolumn, the most intense emission in the panchromatic CL intensity image originates from the vicinity of the lower two defects. Moving toward the tip, the typically observed decrease of the CL intensity is detected. A CL spectrum linescan was performed to analyze the spectral characteristics of the defects. Figure 5c shows the normalized CL linescan from the GaN template to the nanocolumn tip. As a guide to the eye the positions of the defects are marked with white arrows. In the very base region mainly NBE emission appears. At about x = 100 nm, an additional emission at 360.6 nm (3.438 eV) appears in the CL linescan. Here, the generated excess carriers are able to diffuse to the BSFs resulting in an onset of the emission. A comparison of the CL and STEM data clearly reveals a spatial correlation of the defect related luminescence with the location of the stacking fault. Concurrently, the GaN NBE intensity reduces in growth direction and vanishes above 200 nm. According to literature the emission at 360.6 nm (3.438 eV) is attributed to the recombination of excitons bound to basal plane stacking faults of type I1.8,32 Similar has been found for aplane and m-plane GaN.10,33 Basal plane stacking faults lead to cubic insertions in wurzite GaN. For this type of basal plane stacking, the formation of a thin quantum well with type-II band alignment has been predicted by Stampfl et al.28 While exciting hexagonal material several ten nanometers below the quantum well-like heterostructure, the diffusion of the generated carriers leads to an onset of the BSF luminenscence. Probing the NC further in the direction toward the tip, a slight redshift of 11 meV is revealed in the emission above the middle stacking fault (Figure 5c). As BSFs represent ideal crystal phase quantum wells, thickness and compositional fluctuations, which would affect the transition energy, cannot occur. Therefore, a possible explanation for the appearance of two different components of BSF I1 emission could be the influence of polarization fields and strain. Moreover, Bastek et al. 9 have found different emission energies of BSF I 1 luminescence due to different sizes of the stacking faults. In our case, the topmost I1 stacking fault is formed in the tip region terminated by the semipolar facets, in contrast to the lower BSFs (Figure 5a). So, both effects could be responsible for the reduced emission energy. In summary, we have demonstrated for the first time the direct correlation of the optical and structural properties of selectively grown GaN NCs on a nanometer-scale by LHetemperature cathodoluminescence spectroscopy in a transmission electron microscope. The selective area growth of regular NC arrays allows the preparation of a small representative NC ensemble (∼10 NCs) for the STEM-CL experiments. Both, a defect-free NC and NCs with basal plane stacking faults have been investigated. In particular, we were able to resolve the emission of individual basal plane stacking faults in GaN nanocolumns allowing a detailed study of their optical properties. The investigated GaN NCs show the presence of type I1 BSFs, which have the lowest formation energy in the wurtzite structure.28
base of the nanocolumns and is assigned to carrier diffusion into the GaN template leading to nonradiative recombination. For a detailed investigation of the spectral evolution in growth direction a CL spectrum linescan (Figure 4c) was performed from the GaN template to the tip of the nanocolumn. The color coded CL intensity is plotted as a function of position and emission wavelength. Only the GaN near-band-edge (NBE) emission at 356 nm is observed with decreasing intensity toward the tip. The spectrum linescan proves that the nanocolumn is in fact free of optically active extended defects. In the spatially integrated spectrum of the nanocolumn in (Figure 4c) we observe exclusively the GaN NBE emission. No emission at the spectral positions of BSFs can be found. The GaN nanocolumn at position 2 from the ensemble in Figure 3b reveals the presence of a few basal plane stacking faults. The STEM image (Figure 5) exhibits the presence of
Figure 5. STEM image (a) of a single nanocolumn exhibits three basal plane stacking faults in bright contrast. The corresponding panchromatic CL intensity image (b) indicates the highest intensity in the vicinity of the defects. The CL spectrum lines (c) along the nanocolumn show the NBE emission in the bottom part. In the vicinity of the first basal plane stacking faults an emission at about 360.6 nm can be found.
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three extended defects as bright contrast parallel to the basal plane. The bottom two defects are separated by 45 nm and are located in the region terminated by nonpolar side facets. The top defect, which is separated by 95 nm from the middle one, is located in the tip region enclosed by the semipolar side facets. As already seen in Figure 2, the BSFs are not bound by partial dislocations as known from GaN bulk samples but terminate at
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
A.U. and M.M. contributed equally to this work. D
DOI: 10.1021/acs.nanolett.5b01278 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Notes
(26) Li, H.; Geelhaar, L.; Riechert, H.; Draxl, C. arXiv.org 1411.4839, 2014. (27) Blank, H.; Delavignette, P.; Gevers, R.; Amelinckx, S. Phys. Status Solidi B 1964, 7, 747. (28) Stampfl, C.; de Walle, C. G. V. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, R15052. (29) Mei, J.; Srinivasan, S.; Liu, R.; Ponce, F. A.; Narukawa, Y.; Mukai, T. Appl. Phys. Lett. 2006, 88, 141912. (30) Cowley, J.; Huang, Y. Ultramicroscopy 1992, 40, 171−180. (31) Liliental-Weber, Z.; Kisielowski, C.; Ruvimov, S.; Chen, Y.; Washburn, J.; Grzegory, I.; Bockowski, M.; Jun, J.; Porowski, S. J. Electron. Mater. 1996, 25, 1545−1550. (32) Lähnemann, J.; Brandt, O.; Jahn, U.; Pfueller, C.; Roder, C.; Dogan, P.; Grosse, F.; Belabbes, A.; Bechstedt, F.; Trampert, A.; Geelhaar, L. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 081302. (33) Khromov, S.; Monemar, B.; Avrutin, V.; Morkoc, H.; Hultman, L.; Pozina, G. Appl. Phys. Lett. 2013, 103, 192101.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the German Research Foundation DFG. In particular, we thank the SFB 602 ”Komplexe Strukturen in kondensierter Materie”, the SFB 787: ”Halbleiter-Nanophotonik”, the major research program INST272/148-1, and the ”Materials World Network” program for financial support. Furthermore, we thank the European Union Seventh Framework Program under “Nanowiring”, grant agreement no. 265073.
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