Structure and Strain Relaxation of GaN Nanorods Grown on

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C: Physical Processes in Nanomaterials and Nanostructures

Structure and Strain Relaxation of GaN Nanorods Grown on Homoepitaxial Surface via Controlling Irregular Mask Chang-Hsun Huang, Anahita Pakzad, Wei-I Lee, and Yi-Chia Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10254 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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The Journal of Physical Chemistry

Structure and Strain Relaxation of GaN Nanorods grown on Homoepitaxial Surface via Controlling Irregular Mask Chang-Hsun Huang,† Anahita Pakzad,‡ Wei-I Lee,† and Yi-Chia Chou*, †

†

Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan

‡

Gatan Inc. 5794 W. Las Positas Blvd., Pleasanton CA, U.S.A.

*email: [email protected]

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ABSTRACT We propose an uncomplex and low cost approach using an irregular mask for growing GaN nanorods (NRs) bottom up on a free-standing (FS) GaN substrate through hydride vapor phase epitaxy (HVPE). The irregular mask consists of uncoalesced SiO2 islands deposited by plasma-enhanced chemical vapor deposition to isolate growth. The selection of mask amount is investigated to achieve reasonable NR density (high coverage), morphology (flat sidewalls and uniform diameters), and lattice quality (single crystalline; better quality than as-grown layer under same growth ambient). Using this growth approach with appropriate parameter, we successfully synthesize high-coverage of uncoalesced NRs on a homoepitaxial surface in a short growth duration. The morphology, density, and growth rate are controlled by adjusting V/III ratios. The CL and PL measurements of GaN shows luminescence was obtained at near 3.4 eV while structure grown with masks but contained defect signals when grown without mask. The residual strain relaxation within the GaN NRs has been confirmed using Raman spectrum and STEM strain mapping. Moreover, we demonstrate the strain distribution between different crystalline quality and lattice orientation in GaN NR. The method provides a quick and inexpensive method for future nanofabrication consideration.


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INTRODUCTION Gallium Nitride (GaN), with wide direct bandgap of 3.4 eV and high carrier mobility, is well known for its excellent optical and electrical properties which are promising for optoelectronic and electronic device applications.1-4 One-dimensional GaN nanostructures with low defect density permit more efficient strain relaxation and exhibit higher electrical properties compared with thin GaN layer.5,6 GaN nanorods (NRs) with high surface-to-volume ratios can be used to fabricate core-shell structures as covered with multiple quantum wells (MQWs).7,8 Furthermore, c-oriented GaN NRs provide non-polar or semi-polar facets for reducing or eliminating the quantum confined Stark effect (QCSE),9,10 which has a negative impact on light emission efficiency and lowering emission intensity of optical devices. Several publications have reported on controlling the structural growth and quality of InGaN/GaN MQW nanostructures for realizing nitride devices with highly favorable electrical and optical properties.11,12 III-V nanostructures can be synthesized through various methods, such as vapor-liquid-solid method (VLS),13,14 oxide-assisted method,15 laser-assisted catalytic growth,16 nano-imprinting,17 and lithography mask-assisted method.18 To date, the growth of GaN nanostructures can be accomplished through molecular beam epitaxy (MBE),19 metal organic chemical vapor deposition (MOCVD),14,18 and hydride vapor phase epitaxy (HVPE).20,21 In the latter, HVPE has been utilized to fabricate GaN bulk films owing to its near- atmosphere working pressure, which results in a rapid growth rate and a relatively low cost on vacuum system setup.22 However, few GaN nanostructure growth are reported using HVPE due to the 3 ACS Paragon Plus Environment


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difficulty of precise control on microstructure at high growth speed.21 On the other hand, for growing low defects GaN nanostructures, the heteroepitaxial growth such as GaN grown on SiC or sapphire brings concerns regarding the lattice mismatch and different thermal expansion coefficient between epitaxial GaN and substrate. The heteroepitaxial growth induces strain-related polarization in the GaN nanorods which may alter the intrinsic electrical and optical properties. Such strain-control issues are challenging in heteroepitaxial GaN growth.23 In this regard, homoepitaxy provides another promising thought for low strain and low defect GaN nanostructure growth. In this paper, we introduce an irregular mask on free-standing (FS) GaN substrate and grow GaN NRs on it using HVPE. It allows the growth of high density of GaN NRs without elaborated lithography. Thus the homoepitaxial growth takes place following HVPE process which allows the control of various growth parameters24 to achieve desired morphology of nanostructures and to understand the correlation with the corresponding optical properties.


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EXPERIMENT To prepare FS GaN substrates, a GaN thick film with c-orientation was grown on sapphire using a home-built horizontal HVPE system at growth temperature of 1050 °C. The high quality FS GaN substrate was obtained by laser lift-off to separate the GaN and sapphire, and followed by chemical clean with acetone and isopropanol. The thickness of FS GaN substrate is fabricated of 300±10 μm. Then, small amount of SiO2 was deposited on the FS GaN through plasma-enhanced chemical vapor deposition (PECVD; Plasmalab 80Plus) to create SiO2 islands on the substrate that serve as the irregular masks. For SiO2 island deposition, silane (SiH4) and N2O were used as precursor of 9 and 710 sccm, respectively. The process of SiO2 island formation was executed under RF power of 25 W and pressure of 1000 mTorr. GaN NRs were grown on the SiO2-coated FS GaN substrate using the HVPE. After nitridation for 20 min, GaCl and NH3 vapors were utilized as sources of the group III and V precursors, respectively. The GaCl vapor was synthesized by flowing HCl vapor through molten Ga at 850 °C. GaN NRs were grown at V/III ratios among 30-100 for 5 min and at a relatively low GaN growth temperature, 850 °C, under N2 carrier gas. The total pressure during growth was maintained at 700 Torr. The samples were cooled in N2 ambient after growth. The samples were investigated and characterized using JEOL JSM-6500F field-emission scanning electron microscopy (FE-SEM), room-temperature cathodoluminescence (CL), JEOL TEM-2100F high resolution transmission electron microscopy (HRTEM), and Bede D1 X-ray diffraction (XRD). The strain 5 ACS Paragon Plus Environment


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mappings were carried out in JEM-F200 scanning transmission electron microscopy (STEM) using a parallel beam with a convergent angle of ~1 mrad for post strain analysis. The HRTEM and CL samples were prepared by transforming GaN NRs onto a copper mesh with a lacey carbon film for imaging and characterization. In addition, low-temperature (10 K) photoluminescence (PL) measurements were conducted using a 325 nm He-Cd laser source, and room-temperature Raman spectroscopy (LABRAM HR 800 UV) was conducted using a 514 nm Ar+ laser source. The measurements in CL spectroscopy is under wavelength resolution of 0.5 nm and electron beam energy of 15 kV with operating current at 8x109

A. For PL spectroscopy, the resolution is 0.03 meV and the beam energy is 50 mW. To manifest the

polarity of the facets of GaN NRs, the GaN NRs were etched in 2M KOH solution at 80 °C for 2 min.


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RESULTS AND DISCUSSION

Figure 1. The (a) plane-view and (b) 3D morphology of AFM image of SiO2 islands deposited by PECVD in a 4 s duration on GaN substrate. (c) The 3D morphology of SiO2 islands redeposited with the same duration on same GaN substrate. The former SiO2 islands on GaN substrate are etched by BOE, and the SiO2 islands are redeposited under same duration and recipe by PECVD for reproducibility. (d-f) SEM images showing the tilted view of the morphology of GaN grown on FS GaN with SiO2 islands as mask at V/III=30, 43, and 60, respectively. (g) NR density as a function versus V/III ratio. (h) Height (black squares) and diameter (blue squares) distributions of NR as a function of V/III ratio. 7 ACS Paragon Plus Environment


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For compound materials growth, V/III ratio is one of the leading parameters for precise control on atomic details and orientations of materials especially for 1D nanostructures.13 In this paper, for noncatalyzed growth, the amount of the SiO2 predeposition before growth of NRs as the mask is another key parameter; it has to be enough for isolating the GaN NR growth (Figure 1a and b) but not too much to keep the density of NRs high enough. The SiO2 islands exist good reproducibility in short deposition duration on GaN substrate (Figure 1a-c). According to the Stranski–Krastanov growth, the SiO2 formation is converted from island into layer while prolonging the deposition duration (see Figure S1 in SI). Here we investigate the growth of GaN NRs at different V/III ratios with nitrogen supply fixed. Figure 1d-f presents SEM images of GaN grown on FS GaN with SiO2 mask predeposition time of 4 s (approximately forming 3nm-thick SiO2 with island form) with different V/III ratios. The morphology of the GaN NRs varies with the V/III ratio as the density and uniformity reach an ideal saturation at V/III=43. For comparison, for those grown on SiO2 predeposition of 3 s, the varying V/III ratios does not influence GaN NR formation (see Figure S2a in SI); this is because the SiO2 is too less to isolate GaN NR growth. While the SiO2 predeposition duration increases to 6 s, the isolation of GaN NRs is more obvious (see Figure S2b in SI). If the SiO2 is too much, it will terminate the surface for GaN epitaxial growth. The GaN NRs exhibit good uniformity on shape (uniform diameter, similar height, and vertical to the substrate) and density (good coverage on the substrate) when V/III=43 (Figure 1e). The changes of V/III ratio could modify the surface polarity thus changes the growth orientation.24 When V/III goes lower 8 ACS Paragon Plus Environment

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to 30 (Figure 1d), the lateral growth becomes more favorable so GaN do not grow vertically, instead it tends to grow two dimensionally as films. When V/III increases from 43 to 60 (Figure 1f), GaN grows vertically at reasonable rates (average growth is ~0.2 µm/min in Figure 1f) but the density becomes lower and less uniform in shape. The quantitative measurements on NR density and morphology are shown in Figure 1g and h, which will be discussed later.

Figure 2. Orientation and polarity analysis of GaN NRs grown at V/III = 43 using HRTEM, XRD, and SEM. (a, b) TEM images of two GaN NRs with (0001) and {101� 2} facets at the top. (c, d) HRTEM image and selected area diffraction patterns of (b, a), respectively. (e) XRD spectrum of GaN NRs grown on FS substrate which shows the texture of them. (f) SEM image of a GaN NR after KOH wet etching for 2 min



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at 80 °C. The {101� 2} facets of GaN NR were etched away, but the (0001) facet is still present after KOH treatment.

Figure 2 shows the details of the facets and orientations of the GaN NRs from Figure 1b where the growth took place at V/III=43. The rod height and diameter are approximately 1 μm and 400-600 nm, respectively. The GaN NRs are single crystalline with [0001] growth orientation. They contain {0001} and {101� 2} facets at their tops and {11� 00} sidewalls (Figure 2 a, b, and d). A few NRs have slightly angle shifted facets at the bottom as shown in Figure 2a. Figure 2c,d presents the HRTEM images of the GaN NRs from Figure 2b, a, respectively, along [101� 0] viewing direction. HRTEM images and the diffraction patterns show that the GaN NRs are single crystalline with c-orientation as growth direction. The angle between {101� 2} and (0001) facets is 43.2o as shown in Figure 2d. Besides, the XRD spectrum also

explicates that the GaN NRs have c-orientation as preferred orientation, which is depicted at 2θ = 34.5° and 72.9° as (0002) and (0004) GaN peaks in Figure 2e, respectively. Besides, stacking faults have been observed at the edge of the NRs as shown in Figure 2 c and d, which may due to the thickness change and the relative low growth temperature for GaN.25 Figure 1g and h represent the plot of NR density versus V/III ratio and the height and diameter

distributions of NRs versus V/III ratio, respectively. The measurement in Figure 1g and h are from images shown in Figure 1 and Figure S3. When shifting V/III from 30, 43, to 60, the morphology of NRs with c-


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plane sidewalls and (0001) growth orientation are shown at each experiment but the density of them vary. At the medium region of V/III, e.q. V/III = 43, the density is the highest but it went down when switching the V/III ratio either up or down. The SiO2 mask makes the substrate for growth discontinue. The amount has to be enough for isolating NR growth and not too much to achieve the density that is not too low. Another important factor for density control is V and III precursors, which affect the growth conditions. In our experiment, the precursor for V (NH3) is fixed and the precursor of III (GaCl) is changing while changing V/III ratio. At higher V/III, it preferred forming N-polarity planes (the plane of 11� 01 or 112� 2) so the sidewall shown in Figure 1f is not as good as shown in Figure 1e. For NRs grown at V/III = 60, the faceted angle with c-plane at the top of the rods contains 43.2° and at the sidewall of the rods contains 78° (inset in Figure 1f). Further analyses for facet formation of GaN NR are discussed in SI.



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Figure 3. CL spectra of the grown GaN and the substrate GaN. Blue curve is from GaN NRs grown at V/III = 43; red curve is from GaN grown without oxide patterns at V/III = 43 (it exhibits as a thin film); black curve is from the FS GaN substrate for comparison. The inset shows CL spectra of a single NR from the same sample. Figure 3 presents the CL spectra of GaN NRs grown with irregular mask (from Figure 1b with the optimized growth condition), GaN grown without irregular mask that reveals as thin films, and FS GaN for comparison. The near-band-edge (NBE) emission peak of the GaN NRs maintains at 3.48 eV (356 nm) and the full width at half maximum (FWHM) of it is identical to that of the standard GaN from FS GaN substrate. The intensity from NRs is lower due to the geometry of samples. For comparison, the CL measurement of GaN grown on FS GaN without the irregular mask is shown by the red curve in Figure 3. 12 ACS Paragon Plus Environment

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The FWHM of the CL spectrum is wider than the standard, and the NBE emission has slight red shift and significant reduction in intensity. It strongly indicates that the oxide mask is effective to confine the NR growth. The inset of Figure 3 shows the CL spectrum of a single GaN NR, which possesses the NBE peak at 3.50 eV (354 nm). Note that The CL spectrum shows near-band emission (NBE) measured over one micrometer deep of GaN.26 The height of our GaN nanorod is about 1 micrometer, so the GaN substrate contributes the CL signal. Regarding the variation of CL spectrum of single nanorod, we measured three nanorods and the FWHMs of them are 90, 110, and 106 nm. The NBE peaks of three nanorods are consistent and all located at 354 nm. The spectrum is detected from the m-axis (the propagation vector of detector beam is perpendicular to the c-axis) at room temperature. The wavelength of NBE peak is slightly lower in the CL spectrum of single and ensemble NRs (the inset of Figure 3), and this result is ascribed to different detection orientation reported in few publications.27,28 The panchromatic CL mapping of the GaN NRs are shown (see Figure S4 in SI), indicating that the GaN NRs possess uniform optical intensity.



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Figure 4. Normalized PL spectra of samples with GaN grown with and without irregular oxide mask and FS GaN substrate at 10 K. The blue curve is from GaN NRs grown with irregular mask at V/III = 43, the red curve is from GaN grown without irregular mask at V/III = 43, and the black curve is from the FS GaN substrate for comparison.

Figure 4 shows normalized PL spectra measured at 10 K to reduce thermal disturbance. The FWHM of the PL spectra from GaN NRs grown with irregular mask (84 meV) is a bit wider than that of the as grown FS GaN (35 meV). NBE emission peak of the GaN NRs grown with irregular mask, 3.43 eV (361 nm), shows a red shift from that of FS GaN, 3.47 eV (357 nm) which is slightly away from standard 3.40 eV, mainly because of donor-bound excitons (DBE).29 Note that a sharp peak at 3.29 eV (377 nm) is shown in the spectra of the FS GaN, that is caused by the recombination of neutral donor–acceptor pairs (DAPs).


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Weak yellow luminescence (YL; 2.2 eV, 564 nm) and ultraviolet luminescence (UVL; 3.3 eV, 376 nm) bands are observed from the GaN NRs grown with irregular mask. The UVL band is speculated to be generated by the DAP band. The DAP band has been attributed to defects such as SiGa and ON, which induce hole and electron recombination for deep acceptors and shallow donors, respectively.30 The YL band may attribute to point defects, such as VGa, or the VGaON complex in unintentionally doped GaN and threading dislocations.31-33 In addition, CN and the CNON complex are two possible candidates involved in YL band generation,29 but are not less applicable in the experimental conditions. Hence the serial results indicate that the appearance of the YL band and the broadening of the DAP band, both of which emerge in the PL spectrum of GaN NRs, is caused by the incorporation of oxygen in low vacuum environment of our HVPE system. Regarding the UVL band from the GaN grown without irregular mask, some reports classify that 3.41 eV, 3.34 eV, and 3.29 eV peaks are caused by three types of basal stacking faults in wurtzite GaN, which are related to I1, I2, and E, respectively.34,35 The peaks of LO phonon replicas of DBE (3.47 eV), located at 3.38 eV (DBE-LO) and 3.29 eV (DBE-2LO), are also observed in low temperature PL spectra.29,36 In addition, the peak at 3.25 eV is ascribed to the recombination of DAP and e-A bands.37 These multiple peaks are consistent with those expected in low temperature HVPE growth, where the GaN layer is fabricated with relatively higher defect density (see Figure S5 in SI). Thus, the nanostructure of GaN has a better optical performance than thin layer under same V/III ratio during growth. 15 ACS Paragon Plus Environment


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Figure 5. Raman spectra and mapping of GaN NRs grown on FS GaN with 6 s SiO2 predoposition. (a) Optical microscopic image of the junction between GaN NRs and the SiO2-coated FS substrate. (b, c) Raman mapping of (a) for E2 (high) and A1 (LO) phonon peaks, respectively. (d) Raman spectra of regrown GaN and the FS GaN substrate (as described in Figure 4). The inset shows the enlarged E2 (high) peak.

Figure 5a depicts an optical microscope image of the GaN NRs grown on the SiO2-coated FS substrate with 6 s SiO2 pre-deposition (more SiO2 than we deposited for typical NR growth experiment


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for better observation in optical microscopy) where the dark black dots represent the GaN NRs. Figure 5b and c present the Raman mapping of GaN NRs for the same location [E2 (high) and A1 (LO) phonon peaks, respectively] depicted in Figure 5a. The consequence of these intensities is consistent with the result in Figure 3e. In order to analyze the residual strain in the regrown GaN, Raman spectrum is carried out. Figure 5d shows the Raman spectra, captured under different conditions, of the GaN thin layer grown without irregular mask (red), NRs grown with irregular mask (pink), and FS substrate (blue). The Raman spectra of GaN NRs and the thin layer (Figure 5d) clarify that the Raman peaks are at 420, 532, 653, and 733 cm1

. The 532 and 733 cm-1 peak are attributed to the photon vibration frequency of A1 (TO) and A1 (LO) in

wurtzite GaN, respectively. As Figure 5d shows, the E2 (high) phonon peak remains constant in GaN NRs and FS substrate with 566.6 cm-1, indicating strain free after NR growth. While the A1 (TO) peak at 532 cm-1 appears in the Raman spectrum of GaN NRs, the peak position does not shift and is consistent with that of the GaN thin layer. The E2 (high) phonon peak slightly shifts to 567.6 cm-1 in the GaN thin layer, however, whereas the A1 (LO) peak does not shift. This result clarifies that the blue shift of E2 (high) peak is attributed to compressive stress and that it can be explained by the following equation:38 σ=

∆𝜔𝜔 (𝑐𝑐𝑐𝑐−1 𝐺𝐺𝑃𝑃𝑎𝑎−1 ) 4.3

where σ and ∆𝜔𝜔 are related to the compressive stress and the Raman shift from the stress-free peak position, respectively. The compressive stress in the GaN thin layer is calculated to be 0.23 GPa, which



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confirms the presence of residual stress exists in the homoepitaxial film regrown under low temperature. Moreover, this clearly points out that the GaN NRs grown with irregular mask are wurtzite and the rod structures release residual strain effectively compared with thin layer grown without irregular mask under same growth condition. The A1 (LO) peak is intensively influenced by electron and hole concentrations in wurtzite GaN. However, the intensity of A1 (LO) is relatively reduced after regrowth (Figure 5b), indicating that the NRs and regrown thin layer possess a larger free carrier concentration.39 The disparity of A1 (LO) peak between GaN NRs and thin layer is not obvious, while the free carrier concentration is mainly induced by the growth condition. The slight 420 cm-1 peak is attributed to acoustic phonon overtone, which is explained for ruined translated symmetries happened in single crystalline wurtzite GaN.40 A trifling shoulder near 653 cm-1 corresponds to the surface optical (SO) mode observed in polar GaN. Besides, the SO mode is located between A1 (TO) and A1 (LO) peaks and is related to the TO phonons and the dielectric constant of the material. The SO mode is strongly attributed to surface roughness and also high surface-to-volume ratio nanostructures.41


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Figure 6. STEM stain mapping and X-ray rocking curve of a single GaN NR grown on FS GaN. (a) A STEM image of a GaN NR grown at V/III = 43. (b) The reference diffraction pattern used to calculate the strain maps of (a). The diffraction spot u and v represent the c- and a-planes, respectively. (c, d) STEM strain mapping of (a) along y and x directions, respectively. (e, f) X-ray rocking curves of (002) and (102) planes of GaN NRs, respectively. We utilize STEM stain mapping to analyze strain field distribution inside GaN NR (see SI for 19 ACS Paragon Plus Environment


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methods). Figure 6a depicts a single GaN NR grown with irregular mask at V/III = 43 and the reference diffraction pattern used to calculate the strain maps is shown in Figure 6b. It presents two diffraction orientations where spots u and v depicts the c- and a-orientations, respectively. The strain mappings calculated in y- and x-directions are described in Figure 6c and 6d, where the positive values mean expansion from the averaged lattice distance and the negative values mean compression. Note that the precision of NBED strain mapping is about 0.1%, which includes the effects of dynamical scattering.42 The fringes seen in the maps are from the change in the rod thickness, but the strain variation along the axial direction much larger (> 5%) than the error of this method, and is consistent with findings from our XRD results. The strain is more uniform along c-orientation but with a thin layer of the shell strained, as shown in Figure 6c. Nevertheless, the NR is under tension at the bottom but under compression at the tip along the a-orientation. The strain is slightly higher at the shell as shown in Figure 6d. In addition, the top of NR is exposed to a slight compressive strain. We expect it from the cooling process of our experiment, turning off the HCl flow and then ammonia flow in sequence, operated with changing V/III manually. Besides, the strain field distribution is correlated with different morphology of NR as shown in a previous simulation; NR with sharp top possesses a larger strain field at the center.43 The strain variation in the NR, especially the compression at the shell, can potentially alter electron mobility and useful for power devices such as High electron mobility transistor (HEMT). X-ray rocking curve (XRC) provides information about crystalline quality of the grown GaN. The 20 ACS Paragon Plus Environment

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XRC of (002) and (102) peak is shown in Figure 6e and 6f, respectively. The details of Figure 6e showing (002) peak corresponds to the lattice quality of c-lattice. Figure 6f shows the (102) peak which is contributed from c- and a- lattice, so we compare it with the (002) peak to evaluate the quality of a-lattice. Comparing with the FWHM [161 arcsec for (002) and 92 arcsec for (102)] of the signal from FS GaN, the wider FWHM [343 arcsec for (002) and 279 arcsec for (102)] indicates the less perfect of a- and c- lattices. From the strain mapping, tensile stain exhibits at the bottom of the GaN NR so the FS substrate is subjected to a slight compressive strain. According to the XRC data, we deduce that the strain comes from the different growth T for GaN NRs and the FS GaN substrate: the FS GaN substrate was fabricated at higher T (1050 oC) to achieve high quality crystal, but the growth T for NRs is lowers (850 oC).44 Due to the anisotropic thermal expansion coefficient of GaN,45 the more distributed strain field along a-orientation than along c-orientation is expected. Besides, the FWHM of (102) peak increases more than that of (002) peak, which is an agreement with TEM strain mapping. Note that NR coverage is dependent on the growth temperature where the coverage was lower at higher temperature growth and the NRs easily coalesced at lower temperature growth.

SUMMARY In summary, GaN NRs with large non-polar and semi-polar surfaces have been successfully grown on FS GaN substrate using irregular mask by HVPE. We discuss the effects of precursor amount and V/III 21 ACS Paragon Plus Environment


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ratios on density, growth rate, and morphology of NRs. By adjusting theV/III ratios, the growth of GaN NRs can be controlled to achieve vertical, straight, and c-oriented GaN NRs. In addition, the density of oxide masks is critical in isolating the nucleation and growth of GaN NRs; it cannot be too much to cover the entire GaN surface and cannot be too less to allow rod structure growth. Based on the comparison of growth at the same condition but with (formed NRs) and without (formed films) masks, the one with oxide masks (formed GaN NRs) exhibits better performance in optical property including PL and CL. Thus, we confirm the efficient strain relaxation of the GaN NRs grown with irregular mask via Raman spectrum. The STEM strain mapping of the GaN NR is shown for confirmation.

ACLNOWLEDGEMENT We acknowledge funding from the Ministry of Science and Technology of Taiwan under Grant No. MOST-104-2112-M-009-015-MY3 and MOST-107-2636-M-009-002, and core facility support in NCTU from MOST. This work is partially supported by the Ministry of Education through the SPROUT ProjectCenter for Smart Semiconductor Technologies of National Chiao Tung University, Taiwan. We greatly thank Dr. Masahiro Kawasaki for helpful discussion.


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REFERENCES (1) Nakamura, S. The Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser Diodes. Science 1998, 281, 956-961. (2) Songmuang, R.; Katsaros, G.; Monroy, E.; Spathis, P.; Bougerol, C.; Mongillo, M.; De Franceschi, S. Quantum Transport in GaN/AlN Double-Barrier Heterostructure Nanowires. Nano Lett. 2010, 10, 3545-3550. (3) Krishnamoorthy, S.; Kent, T. F.; Yang, J.; Park, P. S.; Myers, R. C.; Rajan, S. GdN NanoislandBased GaN Tunnel Junctions. Nano Lett. 2013, 13, 2570-2575. (4) Nguyen, H. P. T.; Cui, K.; Zhang, S.; Djavid, M.; Korinek, A.; Botton, G. A.; Mi, Z. Controlling Electron Overflow in Phosphor-Free InGaN/GaN Nanowire White Light-Emitting Diodes. Nano Lett. 2012, 12, 1317-1323. (5) Hersee, S. D.; Rishinaramangalam, A. K.; Fairchild, M. N.; Zhang, L.; Varangis, P. Threading Defect Elimination in GaN Nanowires. J. Mater. Res. 2011, 26, 2293-2298. (6) Parkinson, P.; Joyce, H. J.; Gao, Q.; Tan, H. H.; Zhang, X.; Zou, J.; Jagadish, C.; Herz, L. M.; Johnston, M. B. Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core-Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy. Nano Lett. 2009, 9, 3349-3353. (7) Caccamo, L.; Hartmann, J.; Fàbrega, C.; Estradé, S.; Lilienkamp, G.; Prades, J. D.; Hoffmann, M. W. G.; Ledig, J.; Wagner, A.; Wang, X. et al. Band Engineered Epitaxial 3D GaN-InGaN Core–Shell 23 ACS Paragon Plus Environment


Page 24 of 30

Rod Arrays as an Advanced Photoanode for Visible-Light-Driven Water Splitting. ACS Appl. Mater. Interfaces. 2014, 6, 2235-2240. (8) Messanvi, A.; Zhang, H.; Neplokh, V.; Julien, F. H.; Bayle, F.; Foldyna, M.; Bougerol, C.; Gautier, E.; Babichev, A.; Durand, C. et al. Investigation of Photovoltaic Properties of Single Core–Shell GaN/InGaN Wires. ACS Appl. Mater. Interfaces. 2015, 7, 21898-21906. (9) Waltereit, P.; Brandt, O.; Trampert, A.; Grahn, H. T.; Menniger, J.; Ramsteiner, M.; Reiche, M.; Ploog, K. H. Nitride Semiconductors Free of Electrostatic Fields for Efficient White Light-Emitting Diodes. Nature 2000, 406, 865-868. (10) Chichibu, S. F.; Abare, A. C.; Minsky, M. S.; Keller, S.; Fleischer, S. B.; Bowers, J. E.; Hu, E.; Mishra, U. K.; Coldren, L. A.; Sota, T. Effective Band Gap Inhomogeneity and Piezoelectric Field in InGaN/GaN Multiquantum Well Structures. Appl. Phys. Lett. 1998, 73, 2006-2008. (11) Tchoulfian, P.; Donatini, F.; Levy, F.; Dussaigne, A.; Ferret, P.; Pernot, J. Direct Imaging of p– n Junction in Core–Shell GaN Wires. Nano Lett. 2014, 14, 3491-3498. (12) Yan, L.; Jahangir, S.; Wight, S. A.; Nikoobakht, B.; Bhattacharya, P.; Millunchick, J. M. Structural and Optical Properties of Disc-in-Wire InGaN/GaN LEDs. Nano Lett. 2015, 15, 1535-1539. (13) Chou, Y. C.; Hillerich, K.; Tersoff, J.; Reuter, M. C.; Dick, K. A.; Ross, F. M. Atomic-Scale Variability and Control of III-V Nanowire Growth Kinetics. Science 2014, 343, 281-284. (14) Kuykendall, T.; Pauzauskie, P.; Lee, S.; Zhang, Y.; Goldberger, J.; Yang, P. Metalorganic 24 ACS Paragon Plus Environment

Page 25 of 30 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


Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections. Nano Lett. 2003, 3, 1063-1066. (15) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. Microstructures of Gallium Nitride Nanowires Synthesized by Oxide-Assisted Method. Chem. Phys. Lett. 2001, 345, 377-380. (16) Duan, X.; Lieber, C. M. Laser-Assisted Catalytic Growth of Single Crystal GaN Nanowires. J. Am. Chem. Soc. 2000, 122, 188-189. (17) Mårtensson, T.; Carlberg, P.; Borgström, M.; Montelius, L.; Seifert, W.; Samuelson, L. Nanowire Arrays Defined by Nanoimprint Lithography. Nano Lett. 2004, 4, 699-702. (18) Hersee, S. D.; Sun, X.; Wang, X. The Controlled Growth of GaN Nanowires. Nano Lett. 2006, 6, 1808-1811. (19) Calarco, R.; Meijers, R. J.; Debnath, R. K.; Stoica, T.; Sutter, E.; Lüth, H. Nucleation and Growth of GaN Nanowires on Si (111) Performed by Molecular Beam Epitaxy. Nano Lett. 2007, 7, 22482251. (20) Avit, G.; Lekhal, K.; André, Y.; Bougerol, C.; Réveret, F.; Leymarie, J., Gil, E.; Monier, G.; Castelluci, D.; Trassoudaine, A. Ultralong and Defect-Free GaN Nanowires Grown by the HVPE Process. Nano Lett. 2014, 14, 559-562. (21) Kim, H. M.; Kim, D. S.; Park, Y. S.; Kim, D. Y.; Kang, T. W.; Chung, K. S. Growth of GaN Nanorods by a Hydride Vapor Phase Epitaxy Method. Adv. Mater. 2002, 14, 991-993. 25 ACS Paragon Plus Environment


Page 26 of 30

(22) Morkoç, H. Handbook of Nitride Semiconductors and Devices, Materials Properties, Physics and Growth (Vol. 1). John Wiley & Sons. 2009. (23) Zhu, D.; Wallis, D. J.; Humphreys, C. J. Prospects of III-nitride Optoelectronics Grown on Si. Rep. Prog. Phys. 2013, 76, 106501. (24) Wu, K. L.; Chou, Y.; Su, C. C., Yang; C. C., Lee; W. I.; Chou, Y. C. Controlling Bottom-Up Rapid Growth of Single Crystalline Gallium Nitride Nanowires on Silicon. Sci Rep. 2017, 7, 17942. (25) Chèze, C.; Geelhaar, L.; Brandt, O.; Weber, W. M.; Riechert, H.; Münch, S.; Rothemund, R.; Reitzenstein, S.; Forchel, A.; Kehagias, T. et al. Direct Comparison of Catalyst-Free and Catalyst-Induced GaN Nanowires. Nano Res. 2010, 3, 528-536. (26) Fleischer, K.; Toth, M.; Phillips, M. R.; Zou, J.; Li, G.; Chua, S. J. Depth Profiling of GaN by Cathodoluminescence Microanalysis. Appl. Phys. Lett. 1999, 74, 1114-1116. (27) Shubina, T. V.; Paskova, T.; Toropov, A. A.; Ivanov, S. V.; Monemar, B. Polarized Microphotoluminescence and Reflectance Spectroscopy of GaN with k Perpendicular to c: Strongly πPolarized Line Near the A Exciton. Phys. Rev. B 2002, 65, 075212. (28) Paskov, P. P.; Paskova, T.; Holtz, P. O.; Monemar, B. Polarized Photoluminescence Study of Free and Bound Excitons in Free-Standing GaN. Phys. Rev. B 2004, 70, 035210. (29) Reshchikov, M. A.; Morkoc, H. Luminescence Properties of Defects in GaN. J. Appl. Phys. 2005, 97, 061301. 26 ACS Paragon Plus Environment

Page 27 of 30 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


(30) Toth, M.; Fleischer, K.; Phillips, M. R. Direct Experimental Evidence for the Role of Oxygen in the Luminescent Properties of GaN. Phys. Rev. B 1999, 59, 1575. (31) Suski, T.; Perlin, P.; Teisseyre, H.; Leszczyński, M.; Grzegory, I.; Jun, J.; Boćkowski, M.; Porowski, M.; Moustakas, T. D. Mechanism of Yellow Luminescence in GaN. Appl. Phys. Lett. 1995, 67, 2188-2190. (32) Sahoo, P.; Oliveira, D. S.; Cotta, M. A.; Dhara, S.; Dash, S.; Tyagi, A. K.; Raj, B. Enhanced Surface Potential Variation on Nanoprotrusions of GaN Microbelt as a Probe for Humidity Sensing. J. Phys. Chem. C 2011, 115, 5863-5867. (33) Lee, S. M.; Belkhir, M. A.; Zhu, X. Y.; Lee, Y. H.; Hwang, Y. G.; Frauenheim, T. Electronic Structures of GaN Edge Dislocations. Phys. Rev. B 2000, 61, 16033. (34) Urban, A.; Müller, M.; Karbaum, C.; Schmidt, G.; Veit, P.; Malindretos, J.; Bertram, F., Christen, J.; Rizzi, A. Optical Emission of Individual GaN Nanocolumns Analyzed with High Spatial Resolution. Nano Lett. 2015, 15, 5105-5109. (35) Lähnemann, J.; Jahn, U.; Brandt, O.; Flissikowski, T.; Dogan, P.; Grahn, H. T. Luminescence Associated with Stacking Faults in GaN. J. Phys. D: Appl. Phys. 2014, 47, 423001. (36) Watanabe, K.; Nagata, T.; Wakayama, Y.; Sekiguchi, T.; Erdélyi, R.; Volk, J. Band-Gap Deformation Potential and Elasticity Limit of Semiconductor Free-Standing Nanorods Characterized in Situ by Scanning Electron Microscope–Cathodoluminescence Nanospectroscopy. ACS Nano 2015, 9, 27 ACS Paragon Plus Environment


Page 28 of 30

2989-3001. (37) Pittet, P.; Lu, G. N.; Galvan, J. M.; Bluet, J. M.; Anas, I.; Giraud, J. Y.; Balosso, J. PL Characterization of GaN Scintillator for Radioluminescence-Based Dosimetry. Opt. Mater. 2009, 31, 1421-1424. (38) Kisielowski, C.; Krüger, J.; Ruvimov, S.; Suski, T.; Ager III, J. W.; Jones, E.; Weber, Z. L.; Rubin, M.; Weber, E. R.; Bremser, M.D.; Davis, R. F. Strain-Related Phenomena in GaN Thin Films. Phys. Rev. B 1996, 54, 17745. (39) Kozawa, T.; Kachi, T.; Kano, H.; Taga, Y.; Hashimoto, M.; Koide, N.; Manabe, K. Raman Scattering from LO Phonon‐Plasmon Coupled Modes in Gallium Nitride. J. Appl. Phys. 1994, 75, 10981101. (40) Siegle, H.; Kaczmarczyk, G.; Filippidis, L.; Litvinchuk, A. P.; Hoffmann, A.; Thomsen, C. ZoneBoundary Phonons in Hexagonal and Cubic GaN. Phys. Rev. B 1997, 55, 7000. (41) Bhattacharya, S.; Datta, A.; Dhara, S.; Chakravorty, D. Surface Optical Raman Modes in GaN Nanoribbons. J. Raman Spectrosc. 2011, 42, 429-433. (42) Cooper, D.; Denneulin, T.; Bernier, N.; Béché, A.; Rouvière, J. L. Strain Mapping of Semiconductor Specimens with nm-Scale Resolution in a Transmission Electron Microscope. Micron 2016, 80, 145-165. (43) Ko, S. M.; Kim, J. H.; Ko, Y. H.; Chang, Y. H.; Kim, Y. H.; Yoon, J.; Lee, J.Y.; Cho, Y. H. Growth 28 ACS Paragon Plus Environment

Page 29 of 30 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


Mechanism of Catalyst-Free and Mask-Free Heteroepitaxial GaN Submicrometer- and Micrometer-Sized Rods under Biaxial Strain: Variation of Surface Energy and Adatom Kinetics. Cryst. Growth Des. 2012, 12, 3838-3844. (44) Consonni, V.; Hanke, M.; Knelangen, M.; Geelhaar, L.; Trampert, A.; Riechert, H. Nucleation Mechanisms of Self-Induced GaN Nanowires Grown on an Amorphous Interlayer. Phys. Rev. B 2011, 83, 035310. (45) Roder, C.; Einfeldt, S.; Figge, S.; Hommel, D. Temperature Dependence of the Thermal Expansion of GaN. Phys. Rev. B 2005, 72, 085218.



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Structure and Strain Relaxation of GaN Nanorods Grown on

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