Correlation of Optical, Structural, and Compositional Properties with V

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Correlation of Optical, Structural and Compositional Properties with V-Pit Distribution in InGaN/GaN Multi-Quantum Wells Marvin Hartwig Zoellner, Gilbert André Chahine, Lise Lahourcade, Christian Mounir, Costanza Lucia Manganelli, Tobias U. Schulli, Ulrich Theodor Schwarz, Roland Zeisel, and Thomas Schroeder ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04431 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Correlation of Optical, Structural and Compositional Properties with V-Pit Distribution in InGaN/GaN Multi-Quantum Wells Marvin Hartwig Zoellner1,*, Gilbert André Chahine2,3, Lise Lahourcade4, Christian Mounir5, Costanza Lucia Manganelli1, Tobias Urs Schülli2, Ulrich Theodor Schwarz4,6, Roland Zeisel4 and Thomas Schroeder1,7

1

IHP- Leibniz-Institut für innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt

(Oder), Germany. 2

European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France.

3

Université Grenoble Alpes, CNRS, Grenoble INP, SIMAP, 38000 Grenoble, France.

4

OSRAM Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg, Germany.

5

Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-Köhler-

Allee 103, 79110 Freiburg, Germany. 6

Institute of Physics, Technische Universität Chemnitz, Reichenhainer Straße 70, 09126

Chemnitz, Germany. 7

Leibniz-Institut für Kristallzüchtung (IKZ), Max-Born Str.2, 12489 Berlin, Germany.

*Corresponding

Author: Tel.: +49 335 5625 637; Fax: +49 335 5625 681; E-mail-address:

[email protected] (M. H. Zoellner) 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract InGaN/GaN double heterostructures and multi-quantum wells have been successfully developed since more than 20 years for LED lightning applications. Recent developments show that stateof-the-art LEDs benefit from artificially generated V-pit defects. However, the control of structural and chemical properties plays a tremendous role. In this paper we report on the lateral distribution of V-pit defects and photoluminescence of InGaN/GaN multi quantum wells grown on thick GaN on patterned sapphire substrates. The synchrotron based scanning x-ray diffraction microscopy technique K-Map was employed to locally correlate these properties with the local tilt, strain and composition of the InGaN/GaN multi-quantum well. Compositional fluctuation is the main factor for the variation of photoluminescence intensity and broadening. In turn V-pit defects, align along small-angle grain boundaries, and their strain fields are identified as reason for promoting the InGaN segregation process on a microscale.

Keywords InGaN, MQW, Photoluminescence, V-pit, composition, strain, homogeneity, Scanning X-ray Diffraction Microscopy

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1. Introduction Since first high brightness GaN-based blue light-emitting diodes (LEDs) have been developed on c-plane sapphire substrates, large achievements were succeeded in terms of efficiency and expanding the spectral range.1-3 Nowadays, this enables a plethora of optoelectronic applications as for interior/exterior lightning and full colour displays.4 However, to further improve the performance of such devices it is required to increase external quantum efficiency (EQE), which is the product of light extraction efficiency (LEE) and internal quantum efficiency (IQE). The former one is mainly reduced by the lower total reflection at the GaN/sapphire interface but significant total reflection at the chip/air interface.5-7 The latter one is related to the high defect density up to ~1010 cm-2, caused by the release of strain induced by the lattice mismatch (~14%) of GaN on sapphire.8,9 This leads to non-radiative recombination and leakage paths and hence short carrier lifetime.10 Both issues can be addressed by using pre-patterned sapphire substrate (PSS), which is meanwhile commercialized by LED manufacturers.11,12 In this case, the light becomes scattered at the structured interface to the sapphire substrate adjusting the light path in such a way that it can surpass the small critical angle (24.6°) to escape the device. Furthermore, after initial vertical GaN growth the side facets cause epitaxial lateral overgrowth (ELOG) bending dislocations horizontally towards the sapphire pattern.13,14 Ternary InGaN alloy based double heterostructures with multi-quantum wells (MQW) are of tremendous importance to achieve high brightness and to engineer the band gap and thus the emitting wavelength. Although, threading dislocations (TDs) propagating through the GaN buffer layer to the InGaN/GaN MQW usually open up V-pits,15-17 these LEDs still feature efficient band edge light emission despite the high defect density.18 Because the InGaN quantum well is thinner on the semipolar facets a potential barrier is created around the V-pits, which suppresses the charge carrier diffusion to defect sites.19-21 In contrast, for near-UV-LED applications it is crucial 3 ACS Paragon Plus Environment


to reduce the defect density due to low In content.22,23 Furthermore, the random V-pit distribution is accompanied by structural and compositional inhomogeneities.24-27 This InN fluctuation is additionally interrelated by strain fluctuation. Quantum well potential minima induced by the composition as well as strain inhomogeneity are reported to cause charge carrier localization promoting radiative recombination.28 Likewise these inhomogeneities will influence the local optoelectronic activity in a complex way, which has been studied intensively. Local structural homogeneity of the atomic order has been investigated by Extended X-ray Absorption Fine Structure (EXAFS),29 while atom probe tomography (APT) and transmission electron microscopy (TEM) were applied to investigate the short range fluctuations on a nanoscale.30-33 However, long range structural fluctuations on a microscale, as indicated by micro-photoluminescence (PL) mappings,34-37 have not been studied intensively, e.g. by synchrotron based X-ray fluorescence (SR XRF).38 Modern synchrotrons are at the very heart of fundamental and applied research due to their superior brilliance and the improvement in focusing X-ray optics. The scanning x-ray diffraction (XRD) microscopy technique quicK-mapping (K-map), developed at the European Synchrotron Radiation Facility (ESRF) is ideally suited to non-destructively image with sub-micron resolution structural inhomogeneities even in buried heterostructures.39,40 Local lattice orientations and constants can be extracted enabling to quantify tilt, strain and stoichiometry variations in epitaxial semiconductors as done for SiGe buffer layers.41 In this paper we evaluate and disentangle the impact of strain and composition fluctuations on the optical performance measured by photoluminescence in InGaN/GaN MQW grown on thick GaN/PSS.


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2. Experimental setup Initially, a thick, partially Si doped GaN buffer layer (~6 μm) has been grown on patterned sapphire substrate (PSS) at 1080°C with Trimethylgallium (TMGa) and Ammonia (NH3) as precursor. The PSS features a hexagonal pattern of round-shaped structures with a height of 1.7 µm and a distance of 3 µm. Subsequently, a nine times InGaN/GaN MQW with an InN concentration of nominal 12% in the quantum well was deposited. The full heterostack was grown in a standard metalorganic vapour phase epitaxy (MOVPE) production tool. Afterwards, the sample was structured by reactive ion etching to create quadratic 10×10 μm2 mesas. This gives additionally an insight to which extend a mesa edge influences the structural properties of a LED, e.g. close to vertical interconnect access (VIA). The surface morphology featuring V-pits (diameter ~300 nm) was measured within a scan range of 7.5×7.5 μm2 with a Bruker ICON Atomic Force Microscope (AFM). The PL activity was mapped using a confocal setup,42 with a 405 nm laser excitation. Nano X-ray diffraction was carried out at beamline ID01 at the ESRF. The setup is visualized in figure 1a. Here, X-ray energy was tuned to 8.9 keV. The X-ray focussing optics comprised a 300 μm Fresnel zone plate (FZP) with a 60 nm outermost zone size, a 60 μm beam stop (BS) and a 50 μm pinhole as order sorting aperture (OSA). Resulting, the focal distance and focal depth were 129.2 mm and 51 μm, respectively. The beam diameter was verified to be ~150 nm (perpendicular to ring plane) times ~200 nm (in the ring plane). The lateral beam projection length within the 103.5 nm thick MQW is 79 nm for the 0006 and 47 nm for the 10-15 Bragg reflection and has thus no sensitive impact on spatial resolution. The step size of the piezo stage was set to 150 nm. The sample was aligned to diffract from the 0. order of the superlattice of the 0006 and 10-15 reflections. Sequently, the area of 10×10 μm2 was scanned at 61 different Omega angles and an angular step size of 0.005°. The diffraction signal was monitored with an integration time of 30 ms by a 2D Maxipix detector (4 chips, 516×516 pixel, 5 ACS Paragon Plus Environment


55 μm2 pixel size), which was 85 cm away from the sample resulting in a 0.004° reciprocal space binning by the detector. The X-ray Strain Orientation Calculation Software (XSOCS), including the python based package xrayutilities, was used to identify the centre of mass position in terms of scattering vector Q and its components qx, qy and qz at each real space x-y position.39,43

3. Results & Discussion 3.1 V-pit correlation with PL activity The surface morphology was scanned by AFM in a 7.5×7.5 μm2 region to avoid effects of the mesa edges on the image quality (figure 1b). In this area a random distribution of V-pits can be observed. These V-pits are potentially originating from threading dislocations forming smallangle grain boundaries which propagate laterally through the volume of the MQW and not just vertically as pure screw dislocations would do. Following, the whole 10x10 μm2 mesa structure was mapped by PL. Full width at half maximum (FWHM) and intensity of the PL signal are plotted in figure 2a and b, respectively. The width ranges from 111 to 152 meV. Broad spectral regions are forming a mesh-like structure. Tagging the V-pit positions by green circles, it can be seen that all are aligned along this mesh of broad PL spectra (figure 2a). However, other morphological features, such as grooves observed by AFM, do not show a strict congruence. Additionally, the mesh of broadened PL spectra (red dotted lines) and the pattern of V-pit positions (green open circles) is overlapped with the integrated PL intensity in figure 2b. Regions of almost no activity and regions of high PL activity (6.8×104 cps) can be identified. Again a good agreement of the distributions can be found. Summarizing, the maximum PL activity is mainly higher in regions with no V-pits and narrow FWHM. In the following we shed light on how these properties are correlated.


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Figure 1. (a) Experimental setup of the scanning nano X-ray diffraction technique. (b) 7.5×7.5 μm2 AFM image of the surface morphology.

Figure 2. 10×10 μm2 mapping of (a) FWHM of the PL signal (b) integrated PL intensity. Vdefects are marked by green open circles. Mesh of PL broadening is marked by red dotted lines.



3.2 Structural properties To obtain a deeper insight into the crystallographic structure of the InGaN/GaN MQW, scanning X-ray diffraction microscopy measurements have been performed at ID01/ESRF. The 0006 reflection was mapped across the 10×10 μm2 area to visualize the local tilt in figure 3. Initially, the maximum peak intensity of the 0. order 0006 reflection in 3D reciprocal space is shown (figure 3a). It can be clearly seen that the V-pits (black open circles) show a 1:1 correlation with low intensive spots on the map. This is based on the lower growth rate on the {10-11} facets of the V-pits resulting in less diffraction volume of the InGaN/GaN MQW. In figure 3b the relative tilt α exhibiting no directional representation is plotted by calculating 𝛼 = acos (𝑞𝑧/𝑄0006) assuming that the mosaic spread is homogenously distributed in all directions. It is noted that non-reasonable values much higher than 0.1° occur at the very edge due to the non-existing diffraction signal (blue area in figure 3a) and the arising random fitting position in reciprocal space. Thus, outermost pixels are excluded from the following discussions. The tilt, as far as ~1 μm away from the edge, still shows increased values ranging from 0.05° up to ~0.1° and is thus higher than at the centre of the structure, which is probably caused by elastic relaxation towards the mesa edge. Inside the mesa structure, the tilt ranges up to 0.05°. Most pixels show a tilt of 0.008°, while the average tilt amounts to 0.014° as plotted in the histogram of figure 3c. The Vpits are usually located at positions of small tilt values. This might be caused by an averaging effect since regions of different crystal orientation may contribute to the diffraction signal at these positions. The directional representation of the tilt is mapped in figure 3d. From the tilt and directional representation the average angle between neighboring lattice vectors (given by qx, qy and qz) was calculated to be 0.006°. In contrast, it turned out that at V-pit positions the planes are tilted in average by 0.008° in respect to neighboring matrix elements of the mappings. Thus, it turned out that the V-pits (white open circles) are mainly located close to border regions of 8 ACS Paragon Plus Environment

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different tilt angles. This also supports the thesis that V-pits are part of a dislocation network aligning along small-angle grain boundaries. However, we would like to emphasize that this network is not correlated to the mesh in PL mapping, ruling out that this is the reason for the broadening.

Figure 3. 10×10 μm2 mapping of (a) 0006 Bragg reflection intensity maximum (b) relative tilt value (c) histogram of relative tilt values (d) directional representation of the tilt. V-defects are marked by black and white open circles.



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3.3 Composition & Strain In addition to the 0006 reflection, the 10-15 reflection was mapped at the same position to disentangle the influence of strain and composition. However, the tilt, which is measured from the 0006 reflection, was subtracted from the 10-15 reflection, giving corrected qx and qy values. Furthermore, qz has to be corrected since in a superlattice the 0. order position is given by the average composition (𝑥𝑎𝑣) determined by composition (𝑥) and thickness (𝑡) of the InGaN well (𝑤 ) and GaN barrier (𝑏) thickness, respectively: 𝑥𝑎𝑣𝑇 = 𝑥𝑤𝑡𝑤 + 𝑥𝑏𝑡𝑤 with the InGaN/GaN periodicity 𝑇 = 𝑡𝑤 + 𝑡𝑏.44,45 The nominal thicknesses were assumed to have a constant ratio 𝑡𝑤 𝑡𝑏 = 0.4375. This assumption might not be absolutely valid inside the V-pits due to a complex interaction of different sticking coefficients or rather compositional fluctuation and strain field distribution. However, Hangleiter et al. expected and observed a similar scaling of the well and barrier growth rate.19 Thus, the in-plane (𝑎) and out-of-plane (𝑐) lattice constant from the InGaN alloy (𝐿) could be determined. Here, the parameters (𝑎0, 𝑐0) describe the non-strained lattice constants for the pure InN (𝐴) and GaN (𝐵) components. By applying Vegard`s law and a linear approximation for the Poisson´s ratio 𝜐(𝐿) = 𝑥𝜐(𝐴) + (1 ― 𝑥)𝜐(𝐵) the following cubic equation enables to calculate the composition 𝑥 from the InGaN well:46 𝑃𝑥3 +𝑄𝑥2 +𝑅𝑥 + 𝑆 = 0

with:

(1)

𝑃 = (𝜐(𝐴) ― 𝜐(𝐵))(𝑎0(𝐴) ― 𝑎0(𝐵))(𝑐0(𝐴) ― 𝑐0(𝐵)) 𝑄 = (1 + 𝜐(𝐵))(𝑎0(𝐴) ― 𝑎0(𝐵))(𝑐0(𝐴) ― 𝑐0(𝐵))

(2) (3)

+ (𝜐(𝐴) ― 𝜐(𝐵))[(𝑎0(𝐴) ― 𝑎0(𝐵))𝑐0(𝐵) + (𝑎0(𝐵) ― 𝑎(𝐿))(𝑐0(𝐴) ― 𝑐0(𝐵))] 𝑅 = (𝑎0(𝐴) ― 𝑎0(𝐵))[(1 + 𝜐(𝐵))𝑐0(𝐵) ― 𝑐(𝐿)] + (𝑐0(𝐴) ― 𝑐0(𝐵))[(1 + 𝜐(𝐵))𝑎0(𝐵) ― 𝜐(𝐵)𝑎(𝐿)] + (𝜐(𝐴) ― 𝜐(𝐵))(𝑎0(𝐵) ― 𝑎(𝐿))𝑐0(𝐵) 10 ACS Paragon Plus Environment

(4)

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(5)

𝑆 = (1 + 𝜐(𝐵))𝑎0(𝐵)𝑐0(𝐵) ―𝜐(𝐵)𝑎(𝐿)𝑐0(𝐵) ― 𝑎0(𝐵)𝑐(𝐿) Afterwards

the

in-plane

strain

from

the

InGaN

well

was

calculated

by

𝜀|| =

(𝑎(𝐿) ― 𝑎0(𝐿)) 𝑎0(𝐿). Performing these calculations at each position result in strain and composition mappings shown in figure 4. The strain map (figure 4a) visualizes that at the mesa edges strain relaxation occurs, which is probably released elastically. This explains as well the high tilt pointing away from the structure (figure 3). Thus, an asymmetric strain distribution towards lower strain (down to -1.1%) is resulting with an average value of -1.29 ± 0.4 % while the most measurement points show a strain of -1.33% (figure 4b). Additionally, V-pits show a strong spatial correlation with relaxation as 98% of the V-pit positions exhibit 0.02% less strain compared to the average strain value of the surrounding matrix elements of the strain map. This can be caused i) plastically as V-pits are part of threading dislocations with an edge component or ii) elastically as the V-pits have steep facets which are not anymore exclusively pinned to the inplane lattice but experience another compression out-of-plane from the side wall of the V-pit compensating the measured in-plane compression. In general, the compositional fluctuation mapping (figure 4c) shows a normal distribution with an average composition of 12.8 % and a standard deviation 0.4 % across the mesa (figure 4d). Although, V-pits are positioned as expected in regions of low InN content,19,27 the network identified by PL broadening (black dotted lines) shows an even better congruence giving a meshlike appearance of low InN concentration in the composition mapping (figure 4c). Here, it is noted that if the thickness ratio (𝑡𝑤 𝑡𝑏) would not be constant spotty artefacts would appear at Vpit positions, which does not occur. In fact, high composition regions show a blotchy accumulation, matching the high intensity regions in PL (Pearson coefficient 0.57). This correlation might be caused by carrier localization due to the higher In concentration and 11 ACS Paragon Plus Environment


resulting local potential minima. As a result, it might be that i) the compositional fluctuation is caused by defect formation or ii) vice versa the defect formation is caused by InGaN phase separation. For the latter case we would expect the V-pits and grains to appear in positions of high InN content exhibiting high strain values, which is not the case. Thus, we suggest the former case, where the local defect distribution is already given by the dislocation network in the thick GaN buffer and no additional relaxation/defect formation occurs in the InGaN/GaN MQW. Rather we suggest that the InN accumulates in regions of a nearly perfect lattice without V-pits, driven by local strain fluctuations as discussed above. Finally, we find a correlation of high strain values with high InN composition values quantified by a Pearson coefficient of 0.55. Although, composition and strain variations are quite similar they do not show a 1:1 matching. This becomes apparent considering that higher concentrations of InN lead to higher compressive strain since the InGaN well is pseudomorphically grown on the GaN barrier or rather the thick GaN buffer. However, since the elastic relaxation at the mesa edge is caused by structuring after the growth, this inhomogeneity is not reflected by the composition mapping.


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Figure 4. (a) 10×10 μm2 mapping of in-plane strain fluctuation and (b) corresponding strain statistic. V-defects are marked by green circles. (c) 10x10 μm2 mapping of composition fluctuation and (d) corresponding composition statistic. Mesh of PL broadening is marked by black dotted lines.



4. Conclusion In conclusion, the 0006 and 10-15 Bragg reflections from an InGaN/GaN MQW grown on thick GaN/PSS have been mapped by scanning X-Ray Diffraction Microscopy with sub-micrometer resolution. The local tilt, strain and composition have been extracted and compared with the V-pit distribution measured by AFM and the optical properties mapped by PL. Initially, by the tilt fluctuation and its directional representation it was shown that the V-pits are part of a small-angle grain boundary network. Additionally, it was shown that the fluctuation of PL broadening shows a mesh-like structure which fits the PL intensity mapping and the V-pit distribution. Finally, we see a correlation indicating that the V-pit distribution or rather the strain fluctuation drives the InGaN segregation process, which in turn increases the PL activity at high InN content sites.

Acknowledgement We wish to acknowledge the ESRF for beamtime.


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Correlation of Optical, Structural, and Compositional Properties with V

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