Observation of Anisotropic Growth and Compositional Discontinuity in

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Observation of anisotropic growth and compositional discontinuity in AlGaN electron-blocking layers on GaN micro-rods Woo-Young Jung, Chan-Min Kwak, Jae-Bok Seol, Jin Kuen Park, Chan Gyung Park, and Young Kyu Jeong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01554 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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Crystal Growth & Design

Observation

of

anisotropic

growth

and

compositional

discontinuity in AlGaN electron-blocking layers on GaN microrods ,

Woo-Young Jung, †,⊥ Chan-Min Kwak,†,⊥ Jae-Bok Seol,‡ ⊥ Jin Kuen Park,§ Chan-Gyung Park †, ‡ Young Kyu Jeong∥,* †

Department of Materials Science and Engineering, POSTECH, Pohang 37673, South Korea

‡

National Institute for Nanomaterials Technology, POSTECH, Pohang 37673, South Korea

§

Department of Chemistry, Hankuk University of Foreign Studies, Yongin 449-791, South Korea

∥

Non-Ferrous Materials Group, KITECH, Gangneung 25440, South Korea

Corresponding Author *

Dr. Young K. Jeong, Tel: +82-10-9215-0639, E-mail: [email protected]

Author Contribution ⊥

These authors contributed to this work equally.

KEYWORDS : AlGaN layer; electron-blocking layer; layer separation; atom probe tomography

ACS Paragon Plus Environment

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ABSTRACT

In GaN micro-rods, phase separation of the AlGaN electron-blocking layer is an enormous obstacle for achieving high-efficiency light-emitting diodes, as this phenomenon negatively affects the device efficiency by inducing unwanted band-energy modulations. Here, we found that the AlGaN electron-blocking layer on each m-plane of the GaN micro-rod appears to be phase separated, and each electron-blocking layer has a different thickness and length. Our careful analysis based on atom probe tomography reveals that the Al distribution in AlGaN is not uniform and that Al-rich and Al-deficient regions are clearly present. In addition, the longer surface diffusion length of Ga adatoms, as compared to Al adatoms, and the different initial strain state of each m-plane in the GaN rods are deeply associated with the different growth rate and inhomogeneous Al composition of AlGaN, resulting in phase separation of the AlGaN electron-blocking layer. These atomic-scale observations in the structural and chemical composition of AlGaN grown on GaN micro-rods could provide expanded opportunities for building a wide range of high-quality AlGaN electron-blocking layers.

INTRODUCTION GaN-based light-emitting diodes (LEDs) have attracted increasing research interest as energy-efficient, environmentally benign light sources due to their tunable color emission and long-term stability with high efficiency.1-5 However, the GaN epilayer grown on a (0001) coriented sapphire substrate faces substantial challenges. Firstly, the presence of a large lattice misfit between the GaN and the substrate inevitably causes compressive strain in the GaN layer and produces a high density of threading dislocations by more than ~108−9 /cm2 at the


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GaN/substrate interfaces, thereby reducing the device efficiency.6-10 Secondly, strong piezoelectric and polarization fields along the [0001] polar direction can be incorporated into the interface, leading to a reduction in the internal quantum efficiency of the devices.8-12 To overcome these problems, intensive efforts have been made to modify the configuration of the GaN layers from a conventional epilayer to a three-dimensional micro-rod structure possessing six facets with a hexagonal symmetry.13-17 This micro-rod structure provides non-polar and semipolar sidewalls for growing polarization-free GaN layers on the c-oriented sapphire substrate, enabling reduced spontaneous polarization fields and improved light-extraction efficiency.18-21 Despite this progress, a lack of emission efficiency at high injection current densities, which is referred to as ‘efficiency droop’, is another major concern for device applications of GaN micro-rods.22 This undesired phenomenon is known to mainly arise from electron overflow into the p-type GaN layers, causing insufficient hole supply to the active region, thus eventually decreasing both the output light power and the internal quantum efficiency of the devices. To prevent the electron overflow, the AlGaN layers with a wide band-gap energy have been explored as an electron-blocking layer (EBL) on the GaN layers.23 The AlGaN-based EBL provides a large potential well to the free electrons, effectively confining them within multiple quantum wells (MQWs) that are composed of InGaN and GaN. However, the growth parameters of the AlGaN layers, particularly the Al composition, must be precisely adjusted for the proper functioning of the EBL. AlGaN is so metastable that its chemical composition can vary in both the lateral and vertical directions during growth. 24,25 Consequently, the compositional fluctuation in AlGaN causes the EBL to divide into several Al-rich and Al-deficient layers,20-22 which is referred to as a phase-separation phenomenon. 26-28 This low-quality EBL negatively affects the device efficiency by inducing unwanted band-energy modulations. Therefore, it is important to


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understand the growth mechanism regarding this unintended phenomenon for developing a highquality AlGaN layer that has a uniform Al distribution over the EBL. Meanwhile, unveiling the structural and compositional states of the AlGaN grown on GaN micro-rods at the atomic scale can further offer expanded opportunities for building a wide range of high-quality AlGaN layers. Working towards this goal, we prepared hexagonal GaN micro-rod structures, in which the InGaN/GaN MQWs and the AlGaN EBLs are laterally overgrown on all six m-plane facets of the micro-rods. In the structures, the AlGaN EBLs are found to consist of several separated layers with different Al compositions. Moreover, these EBLs have different thicknesses and lengths, indicating that the EBLs on all six m-plane facets grow asymmetrically. Based on the scanning transmission electron microscopy (STEM) and atom probe tomography (APT) results, we suggest a possible scenario regarding the phase separation of the EBLs and their asymmetric growth in the hexagonal GaN micro-rod structures.

EXPERIMENTAL SECTION The GaN micro-rods were grown on a c-plane GaN template by metal-organic chemical vapor deposition (MOCVD). The GaN template was prepared by depositing a 4-µm-thick n-GaN layer and an 80-nm-thick SiO2 window layer on a c-plane sapphire substrate. The SiO2 window layer that was periodically hole-patterned with a diameter of ~1 µm was used as a mask for the fabrication of the micro-rods. The GaN micro-rods grew on the patterned template along the (0001) direction and exhibited a hexagonal shape, delimited by a low growth rate to the (101ത0) m-plane with a (0001) plane as their top facet. Subsequently, the n-GaN buffer, InGaN/GaN


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MQWs, AlGaN EBLs and p-GaN layers were laterally overgrown on the m-planes facets of micro-rods to make the core-shell LED structures. The structural and compositional characteristics of the as-grown GaN core-shell structures were investigated using TEM and APT. TEM analysis was achieved using a JEM2100F (probe cs-corrected, JEOL) with STEM high-angle annular dark field (HAADF) imaging mode at an accelerating voltage of 200 kV. In the APT analysis, laser-assisted wide-angle tomographic AP (LAWATAPTM, CAMECA) was used in UV laser-pulse mode with an energy of 5 nJ at 30 K. For the TEM and APT analyses, a single micro-rod from the ensemble array was prepared by detaching it with a sharp knife. Then, an optimized focused ion beam (FIB, Helios Nano-Lab 600i) process was applied to thin the single micro-rod to electron transparency. The both TEM and APT samples were prepared using the lift-out method.29, 30 The FIB acceleration voltage was reduced from 20 to 1 kV in the last step to prevent surface degradation. In addition, low-energy milling at an acceleration voltage of 1 kV was used to minimize Ga-ion damage. Note that the FIB-thinned specimen was taken near the central part of the micro-rod (white dashed line in Figure 1a).

RESULTS AND DISCUSSION Figure 1(a) shows a scanning electron microscopy (SEM) image of the GaN core-shell micro-rod array and a single freestanding micro-rod detached from the substrate. The GaN micro-rods are found to have six m-plane facets with hexagonal symmetry. The height of the structure is approximately 4 µm, and the average diameter (distance between the m-planes facing each other) is approximately 2.25 µm. In addition, six different semi-polar facets and small


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remaining flat c-planes are located on top of the rods. Figure 1(b) shows the STEM-HAADF images of the InGaN/GaN MQWs and AlGaN EBL in the FIB-thinned GaN micro-rod. In this experiment, the EBLs were laterally overgrown with a single-step MOCVD process that is known to be more effective in achieving homogeneities of atomic compositions than a conventional two-step process. Thus, uniform Al content over the EBL was expected. However, as clearly shown in this image, the EBL grown on the In0.14Ga0.86N/GaN MQWs consists of several separated layers with different brightness contrasts. In addition, the separated layers on each m-plane appear to have different thicknesses and lengths. Figure 1(c) also shows the asymmetric EBL growths on the other m-planes of the same FIB-thinned sample. Since the STEM-HAADF measurement is known to be an informal tool to form atomic-resolution images where the contrast is directly related to the atomic number, we can imply that the brighter and darker layers in the EBL correspond to Al-deficient and Al-rich regions, respectively. To ensure that the phase separation of the EBL is truly associated with the inhomogeneity of Al content, APT measurements were employed to obtain compositional mapping. Using this technique, we could construct an atom map that is a point cloud of colored and shaded spheres representing the positions of different types of atoms. Figure 2(a) shows the three-dimensional (3D) atom map of Ga (green), Al (red), and In (purple) in the APT reconstruction obtained from the region of the EBL and MQWs in the FIB-machined GaN micro-rod. For visual clarity, 5.2 at.% of In was chosen as a threshold value to display the interfaces between the InGaN and the GaN. The compositional profiles of Ga, Al and In were also obtained across the interfaces between the EBL and the MQWs, and they are plotted in Figure 2(b). This result shows that the MQWs are composed of four pairs of InGaN/GaN, and each pair has a uniform thickness. Moreover, the interfaces between InGaN and GaN are nearly atomically flat, indicating a


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homogeneous In (brown lines) distribution in InGaN. In contrast, the Al distribution in the AlGaN EBL is non-uniform, and Al-rich (or Ga-deficient) and Al-deficient (or Ga-rich) regions are clearly distinguishable. The Al mole fraction (x) in the AlxGa1-xN EBL varies in the range of 0.8 to 0.2 with the highest value of 0.437. These results are consistent with the obtained STEMHAADF images shown in Figure 1(b)-(c). By correlating the concentration profile and the HAADF-STEM images, we conclude that the inhomogeneous Al distribution is mainly responsible for the EBL phase separation. The nature and origin of such EBL phase separation are still unclear. To gain detailed insight, we focused on the effect of the built-in residual strain in the EBL. Recently, the driving force for phase separation was shown to strongly depend on the magnitude of the strain induced by lattice mismatch between multilayered heteroepitaxial structures.26,

27

To experimentally

identify the effect of the residual tensile strain caused by the MQWs on the phase separation of the AlGaN EBL, we prepared other GaN micro-rod structures, in which the AlGaN EBLs are directly grown on GaN buffer layers without MQWs. Using these structures, we could eliminate the residual strain resulting from the MQWs that cause considerable tensile force due to the considerable lattice mismatch between InGaN and GaN. Figure 3(a) shows a HAADF-STEM image of the GaN micro-rod grown without MQWs. Interestingly, the EBL in this image appears to be a monolayer with a uniform thickness. Moreover, a relatively homogeneous Al distribution is observed, contrary to the EBL grown with the MQWs, as shown in Figure 3(b). In addition, we observe a huge difference in the average growth rate and Al composition of the EBL between the two samples grown with and without MQWs. The average thickness and Al mole fraction of the EBL grown without MQWs are ~88.7 nm and ~0.052, while the two average values of the EBL grown with MQWs are ~37.6 nm and 0.124, respectively. In other words, the EBL grown


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under a low tensile strain state has a fast growth rate with low Al content. In contrast, the EBL grown under a high tensile strain state grows slowly with a high Al content. These results reveal that the initial strain state plays an important role in determining the growth rate and Al composition of the EBL. Note that an abrupt increase in Al composition near a depth of ~90 nm can also be caused by the tensile strain at the AlGaN EBL/GaN interface. Regarding the initial strain state, we carefully suppose that the different residual strain on each m-plane is associated with the different thicknesses of the last quantum barrier (4th Qb) on each m-plane in the GaN micro-rod. As shown in Table S1, the standard deviation of the 4th Qb shows the highest value (~4.45) among the MQWs, indicating their large fluctuation in thickness. Considering that the 4th Qb acts as a strain-compensating layer to reduce the tensile stress arising from the underlying InGaN MQWs, the residual strain accumulated on each m-plane is expected to be different depending on the thickness of the 4th Qb. In addition to the initial strain state, the surface kinetic process, such as an adatom diffusion, has a great effect on the growth rate and composition homogeneity of the EBL. On the surface of the GaN micro-rods, Ga adatoms can diffuse a much longer distance (0.7~5 µm)31, 32 than the Al adatoms due to the weak Ga-N bonds. Considering the m-plane length ranges between 0.7 and 1 µm (Figure S1), Ga atoms can freely move around on the six m-plane surfaces due to their long diffusion length, as marked with the green dotted line in Figure 4(a). However, the Al atoms can hardly move to neighboring m-planes due to their short diffusion length (red dotted line).23,26 Assuming that the initial strain state on each m-plane is different because of the asymmetric growth of the GaN micro-rods [as shown in Figure 1(a)], Ga atoms are expected to diffuse onto m-planes that are under low tensile stress, since the growth of Ga atoms is energetically more favorable on planes that have similar lattice parameters as GaN. As a result,


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Ga-rich AlGaN layers preferentially grow on the m-planes that are under low tensile stress, simultaneously making Ga-deficient AlGaN layers on the neighboring m-planes that are under high tensile stress. Considering the long diffusion length of Ga atoms and the asymmetric initial strain distribution on each m-plane in the GaN micro-rods, we can suggest a possible model for the growth sequence of the EBLs. For example, Figure 4(b) represents the proposed growth sequence (as denoted with ①-⑨) of the EBL shown in Figure 1(b). Initially, the first EBL begins to grow on the last QB of the MQWs. In this stage, the growth rate is severely limited due to the residual tensile strain that results from the underlying MQW layer. Therefore, the first layer is expected to grow slowly with a relatively high Al composition. Additionally, the slow growth rate allows an Al source flux to be exposed for a long time, eventually providing an Alrich first EBL. Afterwards, we observe that the next EBL (②) in the m2-plane grows faster than the EBL in the m1-plane. This observation indicates that the m2-plane has lower initial residual strain than the m1-plane, and thereby, more Ga atoms move to the m2-plane, as schematically depicted in Figure 4(a). Therefore, the thick Ga-rich (Al-deficient) and thin Ga-deficient (Alrich) AlGaN layers asymmetrically grow on the m2-plane and m1-plane, respectively. In contrast, the third EBL (③) in the m1-plane grows faster than the EBL in the m2-plane, resulting in a thick Ga-rich layer in the m1-plane and a thin Ga-deficient EBL in the m2-plane. From stages ②-③, we can infer that the Ga-deficient AlGaN layer acts as a buffer for relaxing the underlying residual tensile stress and thus allows Ga atoms to gather on the strain-relaxed m1 plane to grow the thick Ga-rich AlGaN layer, leaving a thin Ga-deficient AlGaN layer on the


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neighboring m2-plane. In this manner, the Al-deficient (Ga-rich) and Al-rich (Ga-deficient) AlGaN layers alternatively grow on each m-plane throughout the EBL growth process, leading to phase separation in the EBL in the GaN core-shell micro-rods. CONCLUSIONS We found that the AlGaN EBL on each m-plane was phase separated, and each EBL had a different thickness and length. Our careful analysis based on APT measurements showed that the long diffusion length of Ga adatoms and the initial strain state in the EBL play considerable roles in determining the growth rate and Al composition of the EBL, resulting in phase separation of the AlGaN EBL.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Thickness of each layer on every six m-planes of the micro-rod, SEM image of micro-rod array, cross section HAADF-STEM images acquired at six different m-plane corners of a single microrod.

Acknowledgement This work has been conducted with the support of i) the Korea Institute of Industrial Technology (KITECH JJ-16-0001) and ii) the Hankuk University of Foreign Studies Research Fund of 2017


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Notes The authors declare no competing financial interest.

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FIGURES

(a)

(b)

(c) p-GaN

p-GaN

AlGaN EBL n-GaN Last Q b

MQWs

MQWs n-GaN 2 um

20 nm

Figure 1. (a) SEM image of the GaN core-shell micro-rod array, (b) cross-sectional STEMHAADF image of the InGaN/GaN MQWs and AlGaN EBL in the FIB-thinned GaN micro-rod, (c) cross-sectional STEM-HAADF image of the other m-planes of the same FIB-thinned sample. A single freestanding micro-rod was detached from the substrate and FIB-thinned for structural and compositional characterizations (white dashed line in Figure 1(a)). The white dashed lines in (b), (c) indicate the boundary lines in EBL


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Figure 2. APT analysis of AlGaN EBL and four pairs of InGaN/GaN MQWs in the FIB-thinned GaN micro-rod: (a) 3D atom map of N, Ga, Al and In in the APT reconstrcturtion using 5.2 at.% In isoconcentration envelopes, (b) the corresponding compositional profiles of Ga, Al and In across the interfaces between the EBL and the MQWs. Al and In atoms detected are displayed as red and purple dots.


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Figure 3. (a) HAADF-STEM image and (b) compositional (Al/Ga mole fraction) profile of the EBL directly grown on the n-GaN buffer layer without MQWs. The average thickness and Al mole fraction of the EBL grown without MQWs are ~88.7 nm and ~0.052, respectively, which is more than two times higher than those for the EBL grown with MQWs.


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(a)

Ga

(b)

Al Al-rich AlGaN

GaN

⑨ ⑦

p-GaN

⑧ ⑥ ⑤

④

High ε low ε

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② ①

low ε

Al-depleted AlGaN

③ Last Qb

10 nm

⑧ ⑦ ④ ⑥ ② ⑤ ①

③

Figure 4. Schematic diagrams showing (a) the growth of the Al-rich (Ga-deficient)/Al-deficient (Ga-rich) AlGaN layers on the m-planes depending on the surface strain states and (b) the suggested growth sequence of the AlGaN EBL shown in Figure 1(b). Thick and thin brown lines represent the Al-deficient and Al-rich AlGaN layers, respectively. The circled numbers (① - ⑨) indicate the growth sequence of the separated EBL.


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For Table of Contents Use Only

Observation

of

anisotropic

growth

and

compositional

discontinuity in AlGaN electron-blocking layers on GaN microrods Woo-Young Jung, †,⊥ Chan-Min Kwak,†,⊥ Jae-Bok Seol,‡,⊥ Jin Kuen Park,§ Chan-Gyung Park †, ‡ Young Kyu Jeong∥,*

The Al-deficient (Ga-rich) and Al-rich (Ga-deficient) AlGaN layers alternatively grow on each m-plane throughout the EBL growth process, leading to phase separation in the EBL in the GaN core-shell micro-rods.


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Observation of Anisotropic Growth and Compositional Discontinuity in

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