Characteristics of the surface
microstructures in thick InGaN layers
on GaN
Y. El Gmili,1 G. Orsal,2,3 K. Pantzas,1,4 A. Ahaitouf,1 T. Moudakir,1
S. Gautier,2,3 G. Patriarche,5 D. Troadec,6 J. P. Salvestrini,2,3
and A. Ougazzaden1,4,∗
1 UMI
2958, Georgia Tech-CNRS, 2 Rue Marconi, 57070, Metz, France
de Lorraine, LMOPS, EA4423, 2 Rue Edouard Belin, 57070 Metz, France
3 Supélec, LMOPS, EA4423, 2 Rue Edouard Belin, 57070 Metz, France
4 Georgia Institute of Technology, Georgia Tech Lorraine, 2 Rue Marconi, 57070 Metz, France
5 LPN CNRS, UPR, Route de Nozay, F-91460 Marcoussis, France
6 Université des Sciences et Technologies de Lille, IEMN, UMR 8520, 59000 Lille, France
2 Université
∗ abdallah.ougazzaden@ece.gatech.edu
Abstract:
This paper focuses on a comparative study of optical, morphological, microstructural and microcompositional properties of typical
InGaN samples which exhibit V-defects but also two additional surface
defects features, referred to as inclusion#1 (Ic1) and inclusion#2 (Ic2). HRXRD, AFM, SEM, STEM and EDX are used to characterize such defects.
Furthermore, hyperspectral mapping, spot mode and depth-resolved CL
measurements provided useful informations on the optical emission properties and microstructure. The main characteristic of Ic1 luminescence peak is
a decrease in intensity and no obvious shift in the CL peak position when going from the outside to the middle of such defect. More interesting was Ic2
which is shown to be local 3D top surface In-rich InGaN domains embedded
in an homogeneous InGaN matrix. In fact, this study pointed out that close
to the interface GaN/InGaN, it exists a 30 nm thick fully strained InGaN
layer with constant indium incorporation. As the growth proceeds spatial
fluctuation of the In content is observed and local In-rich 3D domains are
shown to emerge systematically around threading dislocations terminations.
© 2013 Optical Society of America
OCIS codes: (160.4760) Optical properties; (310.6860) Thin films, optical properties.
References and links
1. C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra, “High quantum efficiency
InGaN/GaN solar cells with 2.95 eV band gap,” Appl. Phys. Lett. 93, 143502 (2008).
2. E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M. Farrell, S. Keller, S. DenBaars, U.
Mishra, S. Nakamura, J. Speck, and C. Weisbuch, “High internal and external quantum efficiency InGaN/GaN
solar cells,” Appl. Phys. Lett. 98, 021102 (2011).
3. J. R. Lang, C. J. Neufeld, C. A. Hurni, S. C. Cruz, E. Matioli, U. K. Mishra, and J. S. Speck, “High external
quantum efficiency and fill-factor InGaN/GaN heterojunction solar cells grown by NH3-based molecular beam
epitaxy,” Appl. Phys. Lett. 98, 131115 (2011).
4. X.-M. Cai, S.-W. Zeng, and B.-P. Zhang, “Fabrication and characterization of InGaN p-i-n homojunction solar
cell,” Appl. Phys. Lett. 95, 173504 (2009).
5. J. Zhang and N. Tansu, “Optical gain and laser characteristics of InGaN quantum wells on ternary InGaN substrates,” IEEE Photon. J. 5, 2600111 (2013).
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1111
6. J. Zhang and N. Tansu, “Improvement in spontaneous emission rates for InGaN quantum wells on ternary InGaN
substrate for light-emitting diodes,” J. Appl. Phys. 110, 113110 (2011).
7. P. S. Hsu, M. T. Hardy, F. Wu, I. Koslow, E. C. Young, A. E. Romanov, K. Fujito, D. F. Feezell, S. P. DenBaars,
J. S. Speck, and S. Nakamura, “444.9nm semipolar (1122) laser diode grown on an intentionally stress relaxed
InGaN waveguiding layer,” Appl. Phys. Lett. 100, 021104 (2012).
8. G. B. Stringfellow, “Microstructures produced during the epitaxial growth of InGaN alloys,” J. Cryst. Growth
312, 735–749 (2010).
9. P. A. Ponce, S. Srinivasan, A. Bell, L. Geng, R. Liu, M. Stevens, J. Cai, H. Omiya, H. Marui, and S. Tanaka,
“Microstructure and electronic properties of InGaN alloys,” Phys. Status Solidi B 240, 273–284 (2003).
10. F. Bertram, S. Srinivasan, R. Liu, L. Geng, F. A. Ponce, T. Riemann, J. Christen, S. Tanaka, H. Omiya, and Y.
Nakagawa, “Spatial variation of luminescence of InGaN alloys measured by highly-spatially-resolved scanning
cathodoluminescence,” Mater. Sci. Eng. B 93, 19–23 (2002).
11. J. Bruckbauer, P. R. Edwards, T. Wang, and R. W. Martin, “High resolution cathodoluminescence hyperspectral
imaging of surface features in InGaN/GaN multiple quantum well structures,” Appl. Phys. Lett. 98, 141908
(2011).
12. M. Senthil Kumar, Y. S. Lee, J. Y. Park, S. J. Chung, C. H. Hong, and E. K. Suh, “Surface morphological studies
of green InGaN/GaN multi-quantum wells grown by using MOCVD,” Mater. Chem. Phys. 113, 192–195 (2009).
13. D. I. Florescu, S. M. Ting, J. C. Ramer, D. S. Lee, V. N. Merai, A. Parkeh, D. Lu, E. A. Armour, and L. Chernyak,
“Investigation of V-Defects and embedded inclusions in InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition on (0001) sapphire,” Appl. Phys. Lett. 83, 33–35 (2003).
14. S. Gautier, C. Sartel, S. Ould-Saad, J. Martin, A. Sirenko, and A. Ougazzaden, “GaN materials growth by
MOVPE in a new-design reactor using DMHy and NH3,” J. Cryst. Growth 298, 428–432 (2007).
15. D. Drouin, A. Ral Couture, D. Joly, X. Tastet, V. Aimez, and R. Gauvin, “CASINO V2.42—a fast and easy-to-use
modeling tool for scanning electron microscopy and microanalysis users,” Scanning 29, 92–101 (2007).
16. K. Pantzas, G. Patriarche, G. Orsal, S. Gautier, T. Moudakir, M. Abid, V. Gorge, Z. Djebbour, P. L. Voss, and
A. Ougazzaden, “Investigation of a relaxation mechanism specific to InGaN for improved MOVPE growth of
nitride solar cell materials,” Phys. Status Solidi A 209(1), 25–28 (2012).
17. K. Pantzas, G. Patriarche, D. Troadec, S. Gautier, T. Moudakir, S. Suresh, L. Largeau, O. Mauguin, P. L. Voss,
and A. Ougazzaden, “Nanometer-scale, quantitative composition mappings of InGaN layers from a combination
of scanning transmission electron microscopy and energy dispersive x-ray spectroscopy,” Nanotechnology 23,
455707 (2012).
18. S. Pereira, M. R. Correia, E. Pereira, C. Trager-Cowan, F. Sweeney, K. P. ODonnell, E. Alves, N. Franco, and A.
D. Sequeira, “Structural and optical properties of InGaN/GaN layers close to the critical layer thickness,” Appl.
Phys. Lett. 81, 1207–1209 (2002).
1.
Introduction
The tunability of the fundamental gap of indium gallium nitride (InGaN) across the full visible
spectrum has led to the development of a variety of optoelectronic devices that use this alloy,
including blue, green, red, and white light-emitting diodes, blue and green laser diodes, and
solar cells [1–4]. Recent works have also shown the potential of using the InGaN material
as substrates for reducing the charge separation effect especially for high performance laser
diode emitting in the range of yellow to green spectral regions [5–7]. Still, substantial work is
required to better understand the link between material quality and the optical and electronic
properties of InGaN epilayers, in order to improve the performances of these kind of devices. A
particular aspect of InGaN alloys is the presence of compositional fluctuations, both predicted
by theory and observed experimentally [8]. Ponce et al. [9] and Bertram et al. [10], in particular,
observed macroscopic inclusions in SEM micrographs of InGaN epilayers thicker than 100 nm
and with compositions ranging between 10 % and 32 %. The inclusions were systematically
found to have a higher indium concentration and to luminesce at a different wavelength than
the surrounding InGaN matrix they were embedded in. Inclusions with similar properties have
also been reported for InGaN/GaN multiple quantum well (MQW) stacks [11–13]. While these
inclusions have been thoroughly described in the above-mentionned papers, an explanation
to the mechanism behind their formation, the origin of their difference with the surrounding
matrix, and their impact on the growth of InGaN epilayers has yet to be proposed.
The present paper focuses on the InGaN epilayers that present similar inclusions. A com-
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1112
bined investigation of the morphological, structural, and optical properties of these inclusions
using scanning electron microscopy (SEM), atomic force microscopy (AFM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), bright field STEM
(BF-STEM), energy-dispersive x-ray spectroscopy (EDX), and cathodoluminescnence (CL) is
proposed. The inclusions are shown to systematically surround threading dislocations already
present in the GaN buffer. Two distinct types of inclusion are clearly identified and are shown to
correspond to distinct steps in a localized transition from two-dimensional to three-dimensional
growth. A mechanism that describes these localized transitions is proposed.
2.
Experiment
The InGaN epilayers discussed in the present study were grown in a T-shape reactor by MOVPE
[14]. Nitrogen (N2 ) was used as the carrier gas and trimethylgallium (TMGa), trimethylindium
(TMIn), and ammonia (NH3 ) were used as precursors to elementary gallium, indium, and nitrogen. All epilayers were grown at 800 ◦ C, with a reactor pressure of 100 Torr. The ratio of TMIn
to the sum of TMIn and TMGa in the vapour phase, TMIn/III, was 33.3 %, and the V/III ratio
was 6500. The epilayers were 77 nm thick and were grown on 3.5 μ m thick GaN/sapphire templates. High-resolution x-ray diffraction was used to determine the indium content and strain
state of the InGaN epilayers. These were obtained by combining symmetric (00.2) plane ω -2θ
scans and reciprocal space maps (RSMs) of the asymmetric (11.4) plane. The surface morphology of the epilayers was investigated using SEM and AFM. Sections of the samples were
prepared for observation in a STEM using focused ion beam (FIB) etching. To preserve the
sample surface during the FIB etching process, the samples were coated with a 50 nm thick
carbon layer, followed by a 150 nm thick layer of Si3 N4 . The sections were oriented along the
1 1 2 0 zone axis. HAADF-STEM, BF-STEM, and EDX experiments were then performed in
a dedicated, aberration-corrected JEOL 2200FS microscope working at 200 kV with a probe
current of 150 pA and a probe size of 0.12 nm at the FWHM.
The optical emission properties were investigated by CL which provide the possibility to analyze microscopic areas of the layer surface and to investigate in-depth the samples by adjusting
the electron beam energy from 0.5 to 25 keV. The room temperature CL measurements were
performed in a digital scanning electron microscope (SEM) (Zeiss supraT M 55VP). The CL
emission is detected via a parabolic mirror collector and analyzed by a spectrometer (iHR320)
with a focal length of 320 mm using 1200 grooves/mm grating and a spectral resolution of 0.06
nm. The signal is then registered by a HORIBA JOBIN YVON Instr., 1024 x 256 Symphony
charge-coupled Liquid N2 -cooled with a CCD camera device. Hyperspectral mapping was performed by synchronizing the scanning of the electron beam to the spectrometer. A complete CL
spectrum is obtained for each point on the sample and two-dimensional maps is reconstructed
from specific spectral features.
3.
Results and discussion
The InGaN layer is shown to be strained on GaN, as InGaN diffraction spot is located close
along the vertical line corresponding to pseudomorphic InGaN on GaN (Fig. 1a). In addition,
the (00.2) XRD ω -2θ scan revealed several diffraction satellites which fit well with 11.6 % indium composition and 77 nm InGaN layer thickness as shown in Fig. 1b. SEM and AFM surface
images revealed a 2D morphology, as shown in Figs. 2a and 2b. The root mean square (RMS)
roughness obtained for a 5 x 5 μ m2 surface is of around 2 nm. Despite the good macroscopic
properties of the alloy mentioned above, one can clearly observe two distinct microscopic surface defects features which mostly form close loop with or without hillocks inside (Fig. 2a). For
clarity, both types of defects are labeled hereafter as inclusion#1 (Ic1) and inclusion#2 (Ic2),
respectively. The AFM profiles taken along a line across the two different types of inclusion
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1113
Fig. 1. (a) (11.4) reciprocal space map and (b) (00.2) XRD ω -2θ scan and simulation fitted
by X’Pert Epitaxy software.
reveal that Ic2 are very rough as compared to Ic1 (Fig. 2b). Such surface morphology has already been reported by Bertram et al. [10] for single InGaN films and by Bruckbauer et al. and
Kumar et al. [11, 12] for InGaN/GaN MQW structures.
Fig. 2. (a) Typical SEM and (b) 5 x 5 μ m2 AFM surface images. The solid lines on the 2D
AFM image correspond to the AFM profile of Ic1 and Ic2 reported in the inset.
The HAADF-STEM and BF-STEM cross sectional images revealed that inclusions are systematically centered around threading dislocation terminations, as shown in Fig. 3. The average
diameter of these 3D domains is around 400 nm in agreement with SEM and AFM measurements.
HAADF-STEM and EDX were performed to investigate the composition of the different
regions of the sample. Between inclusions, the layer is homogeneous with an indium content of
around 12 % (Fig. 4). In Fig. 5a, the rough 3D domains (Ic2) show HAADF contrast variations
indicating of the indium fluctuation content. EDX measurements, taken for several lines along
the growth direction reveal two regions of different indium content (Fig. 5b). For the first 30
nm beyond the InGaN/GaN interface (InGaN#1) the composition is constant and around 12
%. Close to the surface, the indium incorporation increases especially for surface pyramidal
features (L5). As expected, directions other than 0 0 0 2 tend to incorporate more indium. In
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1114
Fig. 3. (a) Cross-section HAADF-STEM and (b) BF-STEM images.
addition, a slight depletion of indium in the surrounding of V-defect (L6, L7 and L8) is observed
in agreement with the results of Ponce et al. [9].
Fig. 4. (a) Cross-section HAADF-STEM image between inclusions and (b) corresponding
indium content measured by EDX through line scan L1 and L2.
Twenty local CL spectra were taken along a line across each type of inclusion to study their
luminescence properties. For clarity, only the most representative ones are presented in Figs.
6a and 6b. The main characteristic of Ic1 luminescence is a decrease in intensity and no shift
of the CL InGaN peak from the edge to the middle of such inclusion (Fig. 6a). The latter
observation indicates no significant variation of the strain and indium composition in agreement
with the results of Kumar et al. [12]. On the contrary, Bruckbauer et al. [11] have observed
emission inside the inclusion to be more intense and redshifted as compared to the InGaN
matrix. In Fig. 6b, CL measurements taken from the outside to the middle of Ic2 show a drastic
change in luminescence behavior: the main InGaN CL peak wavelength decrease in intensity
and is redshifted by 15 nm (108 meV). On the other hand, additional InGaN peaks are observed
at higher wavelength for spectra taken at the edge and the middle of Ic2. The shift or the
splitting of the CL peak wavelength can be explained in terms of strain relaxation and/or indium
composition fluctuation. We attributed Ic2 to microscopic relaxed regions, as local 3D growth is
observed. In addition, the large splitting of the 432 nm and 481 nm CL peaks with the emission
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1115
Fig. 5. (a) Cross-section HAADF-STEM image for an area containing 3D domains (Ic2)
and (b) corresponding indium content measured by EDX through lines scan L3 to L9.
wavelength at around 410 nm cannot be explained only by strain relaxation but by an increase
of the In content.
Fig. 6. Most representatives local CL spectra taken for twenty points along a line across the
different surface defects features: (a) Ic1 and (b) Ic2 (the inset shows the line scan and the
location of the four spots).
Refering to CL spectra shown in Fig. 6b, we have recorded CL hyperspectral mapping of Ic2.
Green, blue, and red colors reported in Fig. 7 correspond to normalized luminescence intensity
in the range of 392-424 nm, 426-454 nm and 457-507 nm, respectively. The luminescence
behavior clearly confirms higher indium content inside Ic2 as compared to the InGaN matrix,
in agreement with EDX measurements. Furthermore, series of mapping reveal that the indium
composition changes from one inclusion to the other and seems to increase with their in-plane
size.
Depth-resolved CL measurements were also performed for Ic2, as shown in Fig. 8. The
electron energy beam was varied from 4 keV to 5 keV which corresponds, according to the
Monte Carlo simulation [15], to calculated sample depths of maximum energy loss of 23 nm
and 33 nm, respectively. For the surrounding planar region, the InGaN CL peak position (410
nm) does not depend on the acceleration voltage (Fig. 8a). At the Ic2 position, the 410 nm
CL peak is only observed as the electron beam energy increases to 5 keV (Fig. 8b). Thus, Ic2
originates from the InGaN top surface layer and is attributed to the presence of In-rich areas
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1116
Fig. 7. CL hyperspectral mapping taken for 10 x 10 spots with an integration time of
2s/point and an acceleration voltage of 4 keV. Green, blue, and red colors correspond to
normalized luminescence intensity for wavelength λ = 407 nm, λ = 432 nm and λ = 477
nm, with a peak bandwidth of 32 nm, 28 nm and 50 nm, respectively.
embedded in a lower InGaN composition matrix.
Fig. 8. Most representative local CL spectra taken at 4 keV and 5 keV electron beam energy:
(a) outside and (b) inside of Ic2.
From our observation the formation of Ic2 can be explained as follows. After the first 30
nm, spatial fluctuation of the composition is observed and local In-rich 3D domains are shown
to emerge systematically around threading dislocations terminations. Indeed, indium adatoms
are prone to accumulate around V-defects and additional facets appear. Directions other than
0 0 0 2 enhance the growth, leading to In-rich pyramidal features. A slight depletion of the
indium composition in the neighborhood of Ic2 was also observed. We assume Ic1 to be the first
step in the formation of Ic2. Finally, we believe that the In-rich local 3D regions contribute to
the transition from two-dimensional to three-dimensional growth observed in thicker epilayers
[16–18].
4.
Conclusion
In the present work, we have reported on the luminescence properties of InGaN surface defects
features. Especially, the local and spatial variations of indium composition and microstructure
through inclusions were investigated by STEM, EDX and cathodoluminescence in scanning and
spot modes. Between inclusions and within the first 30 nm beyond the InGaN/GaN interface,
the film is shown to be homogeneous both in strain and composition. As the growth proceeds,
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1117
3D pyramidal In-rich domains emerge around threading dislocations terminations leading to
a slight depletion on indium in their neighborhood. We believe inclusions to correspond to
distinct steps in a localized transition from two-dimensional to three-dimensional growth.
Acknowledgment
This study has been funded by the ANR Habisol 2009, project NewPVonGlass (grant no. ANR08-HABISOL-020-1)
#185910 - $15.00 USD
(C) 2013 OSA
Received 25 Feb 2013; revised 3 Apr 2013; accepted 3 Apr 2013; published 17 Jul 2013
1 August 2013 | Vol. 3, No. 8 | DOI:10.1364/OME.3.001111 | OPTICAL MATERIALS EXPRESS 1118