Axial Inhomogeneity of Mg-Doped GaN Rods: A Strong Correlation

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Axial Inhomogeneity of Mg-doped GaN Rods: A Strong Correlation Among Componential, Electrical, and Optical Analyses Sunghan Choi, Hyun Gyu Song, Yang-Seok Yoo, Chulwon Lee, Kie Young Woo, Eunhyung Lee, Sungwon David Roh, and Yong-Hoon Cho ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00030 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Axial Inhomogeneity of Mg-doped GaN Rods: A Strong Correlation Among Componential, Electrical, and Optical Analyses Sunghan Choi1, Hyun Gyu Song1, Yang-Seok Yoo1, Chulwon Lee1, Kie Young Woo1, Eunhyung Lee2, Sungwon David Roh2, Yong-Hoon Cho1*

1

Department of Physics and KI for the NanoCentury, Korea Advanced Institute of Science and

Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea 2

Advanced Materials & Component Lab, R&D Center, LG Innotek, 46, Baumoe-ro 6-gil,

Seocho-gu, Seoul 06763, Republic of Korea

KEYWORDS Mg-doped GaN rod, axial inhomogeneity, componential analysis, electrical analysis, optical analysis, correlation

ACS Paragon Plus Environment

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ABSTRACT

We systematically characterized the inhomogeneous doping properties along the c-axis of Mgdoped p-GaN micro-rods. Axial variation of doping concentration and electrical resistance on the p-GaN rod were measured by time-of-flight secondary-ion-mass-spectrometry and 4-point probe measurements,

respectively.

Defects-related

optical

information

was

obtained

from

photoluminescence spectra together with Raman experiments revealing the change of crystal quality and strain along the rod. Based on a correlation of these analyses, we confirmed that Mg concentration decreased along the axial direction of the rod, leading to increasing electrical resistance. This axial Mg concentration change was revealed by green luminescence because the intensity of green luminescence sensitively varied with the doping density in both high-doping and low-doping rods. Interestingly, the entire resistances at the highly doped rods were higher than the lowly doped rods due to overall mobility degradation at the high-doping rods caused by scattering effect of increased Mg impurities and strain. All analyses provided complementary information on the p-type doping process and contribute to understanding the p-doping properties of GaN rod based photonic devices. Furthermore, our axially-resolved optical spectroscopic (photoluminescence and Raman) methods can provide a facile, fast and nondestructive way to estimate the axial doping and conductivity inhomogeneity of Mg doped pGaN rod without having complex, time-consuming and destructive structural and electrical measurements.


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Introduction GaN submicrometer- and micrometer-sized rods have several advantages over conventional GaN films in photonic device applications, including low dislocation density and higher light extraction efficiency.1-3 GaN rods with GaN/InGaN core/shell quantum wells (QW) have additional merits, such as a larger active area, and enhanced internal quantum efficiency due to the low polarization-induced field in the QWs.4-6 These advantages have been demonstrated in numerous photonic devices based on GaN micro-/nano-rods with core/shell QW grown and fabricated with the aid of now mature techniques.7-10 In addition, bulk GaN rods without core/shell QW are also important for several photonic applications. For instance, bulk GaN rods with p-doping has been utilized for new design LEDs called p-n cross nanowire LEDs,11 heterojunction photovoltaic cells,12 heterojunction photodiodes,13 and water splitting.14 To optimize the performance of optoelectronic devices based on GaN rods, it’s necessary to control the doping profile along the rod.15 However, unlike conventional films, there are a number of doping incorporation paths in GaN rods, which can result in the non-uniform distribution of dopants along the growth direction of a single rod.16 Therefore, understanding differences in dopant distribution and electrical properties at precise positions along the rod is critical for optimization of the abovementioned GaN rod based devices.17 For that reason, spatially resolved doping measurement methods that can evaluate changes in doping properties along a single rod are required.18 Achieving this in rod structures with the Hall-effect and capacitance-voltage measurement of the conventional methods used for film samples is very challenging due to the one-dimensional geometry.19 Therefore, it is necessary to employ other doping measurement methods that are specifically applicable to the rods.


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In addition, different characterization methods are needed for p-type and n-type GaN materials, since the properties of p-type dopants are considerably different from those of n-type dopants. The doping properties of n-GaN rods are typically analyzed from doping density and resistivity information. n-type doping in GaN is commonly controlled by the Si incorporation during GaN growth. Due to its high solubility, and the low donor activation energy of Si at room temperature,20 the free carrier concentration is almost the same as the doping concentration in nGaN. Therefore, we can directly estimate electrical properties such as mobility and free carrier densities from the resistivity and doping concentration information of n-GaN rods.19 However, p-GaN exhibits a low carrier concentration compared to dopant density because Mg, widely used as the p-dopant in GaN, shows high acceptor activation energy. In addition, the native point defects in GaN act as compensation defects, leading to further reductions in carrier density in p-GaN. A high Mg concentration in p-GaN generates additional compensation defects, leading to a reduction in carrier density as well as crystal quality degradation.17, 21 This results in an increase in resistivity in highly doped p-GaN. Therefore, not only doping concentration, resistivity and defect information, but also information about their inter-relationships is required to understand p-doping properties in Mg-doped GaN. In this work, we systematically investigated p-doping inhomogeneity along the c-axis of a single Mg-doped p-GaN micro-rod grown by metal-organic chemical vapor deposition (MOCVD), using a combination of spatially resolved componential, electrical, and optical measurements, and correlations between the measurement results. A quantitative profile of Mg dopant concentration was obtained using time-of-flight secondary-ion-mass-spectrometry (TOFSIMS), a destructive component analysis that can determine the value of Mg concentrations at different positions on a single p-GaN micro-rod. A 4-point probe measurement22-23 reports on the


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changes in electrical properties induced by the incorporation of inhomogeneous impurities in a single p-GaN micro-rod, by probing changes in electrical resistance along the rod. Spatialresolved micro-photoluminescence (PL) experiments were used to obtain the luminescence spectra of incorporated defects produced by Mg doping, and provide better understanding of the distribution of defects within the sample by comparing the intensity ratio of the green luminescence for different locations on the p-GaN micro-rod. Also, micro-Raman spectra along the rod showed the spatial changes of crystal quality and strain induced by Mg-doping. Finally, we comprehensively analyzed the change in axial p-doping properties of a single p-GaN microrod based on the complementary results of all experiments. Based on the correlation of these four measurement results, we understood how the variations in Mg concentration affect the changes in electrical properties along the rod.

Results and Discussion Growth of p-GaN micro-rods and characterization of Mg-doping concentration. Figure 1a illustrates the growth process of an ensemble of Mg-doped p-GaN micro-rods. Before the p-GaN micro-rods’ regrowth, a 50 nm-thick SiO2 mask layer was deposited on top of the n-GaN film and hexagonal arrays of hole patterns having a 1 µm diameter and 3 µm pitch were fabricated on the mask layer, using photolithography and dry etching process. Then p-type GaN micro-rods were regrown on the n-GaN layer by MOCVD with a continuous supply of Mg, using the same growth conditions as those employed in growing undoped GaN rods. To activate the acceptors in the p-GaN, the grown ensemble of p-GaN micro-rods was thermally annealed at 720 oC for 15 minutes in a nitrogen atmosphere. For comparative study, we prepared two ensembles of p-GaN


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micro-rods, labeled as type sample A and sample B, grown with different amounts of Mg dopants, at a ratio of 3:7, respectively. A bird’s eye view scanning electron microscopy (SEM) image of the grown ensemble of p-GaN micro-rods is shown in Figure 1b. Thanks to the selective area growth process enabled by the patterned SiO2 mask formed on the n-GaN film, an ensemble of p-GaN micro-rods with a uniform diameter of about 1.2 µm diameter were successfully grown on both samples, which had heights of about 10.7 and 11.1 µm for samples A and B, respectively (Supporting information, Figure S1). To study the structural details of the grown p-GaN micro-rods, high-resolution transmission electron microscopy (HR-TEM) images of a single micro-rod and its diffraction patterns from fast Fourier transform (FFT) were obtained, and are shown in Figure 1c and d, respectively. As indicated in Figure 1c, the separation between two adjacent lattice planes is 0.52 nm, confirming the lattice spacing of the GaN (0001) plane.24 It is clear from Figure 1c and d that the regrown p-GaN micro-rod have the single crystalline wurtzite structure. TOF-SIMS, a destructive component analysis technique for quantitatively evaluating the mass of each ion composing the rods,25 was performed to check the exact Mg dopant profile at specific positions on a single p-GaN rod. Figure 2a shows the mass spectrum of each ion contained in a single rod of sample A, dispersed on a Si substrate. The weak signal arising from Ga+ and Mg+ ions is clearly discernible from the high-intensity Si+ peak in the background since Ga+ and Mg+ ions have a different mass than the Si+ ion. From the spatial distribution of the Ga+ and Mg+ ions mapped on the rod images, as shown in Figure 2b and c respectively. The amount of Mg dopants distinctly varies over the c-axis of the rod in contrast to the uniform distribution of Ga+ ions. For quantitative measurement of the Mg doping profiles for the p-GaN rods, the Mg concentration along the rod was calculated from the line profile of Mg+/Ga+ ion intensity ratio in mapping


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images obtained from three rod specimens each of samples A and B. Figure 2d shows the averaged Mg concentrations calculated from three rods of samples A and B along the growth direction of the rods. It confirms that the quantity of Mg concentration from the bottom side and top side are 2.6×1019 cm-3 and 1.6×1019 cm-3 for sample A, and 9.6×1019 cm-3 and 5.8×1019 cm-3 for sample B. We found that a large amount of the Mg dopants resides close to the bottom of the rods, and sample B has a higher Mg concentration level than sample A for the entire rod. This doping concentration reduction along the growth direction of the rod caused by three dimensional structures was reported in n-doped GaN, and other III-V materials.18, 26 In addition, the Mg density at the bottom part (from 0 to 4 µm above the bottom) decreased faster than that of the top side (from 4 to 9 µm apart from the bottom). If Mg precursors are incorporated in the GaN via the surface diffusion and vapor diffusion processes during the GaN rods’ growth,27 a faster change in Mg concentration at the bottom of the rod can be interpreted as a strong surface diffusion process at this site. Also, Mg concentration fluctuation bottom side of sample B (from 0 to 1 µm apart from the bottom) is considered as saturation of Mg-doping in this site due to the high Mg source injection.28 Electrical measurement using 4-point probe method. The change in p-conductivity of the Mg-doped micro-rods was analyzed by measuring the local electrical resistance over the rods using a 4-point probe measurement. The 4-point probe measurement excludes the parasitic effect which originates from contact resistance, and only measures the resistance of the sample. As schematically shown in Figure 3a, multiple electrodes were attached to the m-plane of a single rod planarized by hydrogen silsesquioxane (HSQ) on the SiO2 coated Si substrate, and used for local I-V measurements. While the outer two electrodes inject electrical current into the rod, the inner two electrodes measure the voltage applied between these electrodes. By moving the four


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electrical probes and obtaining I-V curves along the rod, the electrical resistances for different sites of the rod were recorded. Figure 3b shows the top view SEM image of the electrodes on the rod. The patterned metal electrodes were well attached with uniform width and distance on the m-plane of the single rod without disconnection. Figure 3c shows the resistances as a function of distance from the bottom of the rod, for the three rods chosen from among samples A and B, respectively. The inset of Figure 3c shows the linear I-V curves taken between the two numbered electrodes on the p-GaN rods shown in Figure 3b. The value of resistance increased as we probed closer to the top of the rods, where Mg concentration is low, in both samples A and B. In conventional p-GaN films, hole density starts to diminish at a Mg concentration higher than 3×1019 cm-3 due to the self-compensation effect.29 However, negligible reduction in carrier concentration by self-compensation is expected in the pGaN rods, because the sites of high Mg concentration, even in highly doped sample B, had low resistance. It is worth noting that all of the resistances measured for sample A were lower than those of sample B ,in spite of sample B having the higher doping concentration. To understand this point, mobility in the p-GaN rods needs to be determined, and can be calculated from the following equation19:

1 1 = ( ) (1)

where µ is the mobility, e is the elementary charge, ρ is the resistivity of the p-GaN micro-rod, p =

is the hole carrier concentration, R is the resistance, S is the cross-sectional area of current, and l is the distance between the two electrodes. It is difficult to obtain the exact µ because the information of p is unknown and estimation of S is not straightforward due to the non-uniform distribution of dopants across the lateral dimension of the rod.19, 30 However, because of the


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negligible self-compensation effect in the p-GaN rods, the value of

, where is the Mg

concentration, would have linear trend with the µ. Therefore, we used the parameter =

as an alternative estimation of mobility in the rod. In Figure 3d, while both samples A and B have almost constant values of K at every location on the rods, sample A has larger values of K than those of sample B. This trend can be interpreted that the mobility change along the single pGaN rod is relatively small in the both samples A and B, whereas the overall mobility is much lower in the sample B. Optical characterizations using micro-PL and micro-Raman spectroscopy. A micro-PL experiment was conducted on a single p-GaN micro-rod at the temperature of 10 K to investigate the defect states introduced by Mg doping along its c-axis. (Supporting information, Figure S2) Figure 4a shows the normalized PL spectra collected over the entire region of a rod of interest to compare the characteristics of the Mg-doped p-GaN and un-doped GaN (u-GaN) micro-rods. For the u-GaN rod, a strong GaN near band edge (NBE) emission peaked at 3.50 eV and a broad, weak emission ranging from 2.24 eV to 2.45 eV, called green luminescence (GL), were observed. In contrast to the u-GaN rod, the PL spectra of the p-doped rods (samples A and sample B) feature a strong ultraviolet luminescence (UVL) ranging from 3.10 eV to 3.30 eV and increased GL. The pronounced UVL of the Mg-doped GaN micro-rod is common in conventional Mgdoped GaN films and known to originate from the acceptor related states introduced by Mg.31-32 According to M. A. Reshchikov et al., the GL bands in the GaN system are divided into GL peaked at 2.48 eV, GL2 peaked at 2.36 eV, and GL3 peaked at 2.51 eV.32 Since the peak energy of GL of the rod is close to that of GL2, we assume that the GL in the rod has the similar origin observed in GL2. The GL2 is reported in GaN samples grown under Ga-rich condition and its


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origin is related to defect states from the native defects.32-33 This GL band shows large PL efficiency reduction at high excitation power regime,32-33 which is also found in the p-GaN rod sample. (Supporting information, Figure S3). Among the native defects of the GL origin, nitrogen vacancies (VN) have the smallest formation energy and thus are easily made during GaN growth under Ga-rich condition.32 Also, the formation energy of VN is further reduced by Mg incorporation as Mg doping lowers Fermi level energy.33 Therefore, more VN states exist in the p-GaN micro-rods than the undoped ones, which is the possible reason for a higher GL portion for the entire PL spectrum of the p-doped samples. Figure 4b and c show the series of normalized PL emission spectra, scanned along the c-axis of the rod with 1 µm steps, for both samples A and B. The spectrum shape in the UVL changes depending on the position of samples A and B. However, it is difficult to get clear defect information from the UVL change because the spectral variation in the UVL is affected by not only the number of defect states but also their inhomogeneous distribution, which causes potential fluctuations.32, 34 In both samples A and B, however, the GL intensity decreased from the bottom to the top of the rod. Furthermore, the overall GL intensity of sample B was brighter than that of sample A. Hence, in accordance with the GL intensity changes, the density of VN states is expected to have difference between two samples and changes along the axial position. A micro-Raman spectroscopy was employed to estimate the crystal quality and strain change along the c-axis of a single p-GaN micro-rod at room temperature. We investigated the peak position and full width half maximum (FWHM) of E Raman mode, which indicate the information of strain and crystal quality of the sample.35-36 The peak position and FWHM of E Raman mode were extracted by fitting this mode to a Gaussian function. Figure 4d shows the Raman spectra of E measured at the middle site of a single u-GaN rod, sample A, and sample B.


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The measured E peak positions of samples A and B are 567.5 cm-1 and 568.0 cm-1, respectively and located to the right of the peak position of u-GaN rod (566.7 cm-1). Assuming that unstrained GaN has E peak position at 567.0 cm-1,35 The shift of E peak position to higher energies with higher Mg-doping implies the higher compressive strain due to the Mg incorporation on Gasite.35 The FWHM of E is 4.0 cm-1 for the both samples A and B, and 3.8 cm-1 for u-GaN rod. Since the linewidth broadening of E mode indicates defect incorporation,36 the larger FWHM of E of the p-GaN rods compared to the u-GaN rod case maybe be induced by Mg incorporation. According to R. Kirste et al., sudden broadening of FWHM of E happens in highly doped p-GaN film with the increased compensation defects.35 However, the increase of compensation defects induced by high Mg-doping may be negligible as both samples A and B has the same FWHM of E . To compare the crystal quality and strain along the rod, FWHM and peak position of E at different position averaged over the three rods each from samples A and B were measured as shown in Figure 4e and f, respectively. Although there is a little FWHM broadening at the bottom side of the sample B (z = 1.0 µm), the FWHM of E are considered as almost constant along the both samples A and B. Although Mg concentration differs from sample to sample and varies along the position, the crystal quality of two samples is similar and almost independent of the axial position in the rod. Although E peak of the sample B has higher energy than that of sample A, the peak positions of both samples A and B shifted little along the different position. Therefore, while the strain remains almost constant along the rod, highly doped sample B has larger compressive strain than sample A.


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Correlation of p-doping analysis among componential, electrical, and optical results. The correlations between all of the experimental results, from TOF-SIMS, 4-point probes, micro-PL, and micro-Raman measurement were systematically analyzed, as shown in Figure 5a. In the case of p-GaN rod growth, the formation energy of VN at a specific height in the rod is determined by the Fermi level energy at the same position in the grown rod structure. If the p-GaN growth process occurs layer-by-layer along the axial direction as shown in the left side of Figure 5a, the local doping concentration determines the Fermi level at the same position in the rod. Then it is possible to estimate the change in relative VN density from the Mg concentration measured by TOF-SIMS. Since VN is regarded as a major origin of the GL in the PL spectra of the Mg-doped rod, it is possible to correlate Mg concentration and GL intensity based on the quantitative relation between the VN density and GL intensity. The parameter K calculated from the Mg concentration and the resistance data estimates the mobility change of the Mg-doped rods. According to Figure 3d, while the value of K does not show any spatial variation along the rod, there is a large difference between samples A and B. Since the mobility is influenced by crystal quality and strain, the E mode in Raman spectra gives the information associated with the mobility.29, 37 Also, the peak position of E in Raman spectra shows similar spatial and sample dependence with K. Therefore, it is possible to understand the origin of the variation in K based on the correlation between the peak position of E and K. The relationship between VN and Mg concentration is expressed as the following equation29:

=

1 + 1 + ⁄ 3

!" $

%

%

1 + 1 + ⁄

!" $

%


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= ' ∙

%

(1 + )1 + *+ ⁄,- )%

(2)

where NV is the VN density, NMg is the Mg concentration, α =4/exp (34 ⁄56), and Nval is the temperature dependent valence band density of states. Also, β is the valence band degeneracy factor, EA is the thermal Mg-acceptor ionization energy defined as 160 meV, k is the Boltzmann constant, T is the temperature, coefficient A =

( 7 7899 ⁄:;< )= %99 >

, and NCC is the fitting parameter.

At the growth temperature (1300 K), β=5.1 and Nval=2.9×1020 cm-3.29 Then it is possible to speculate the relative VN density profile along the rod using TOF-SIMS data. Assuming sufficiently rapid cooling after growth, the VN density during and after growth is almost identical.29 The relationship between the GL emission intensity in the PL spectra measured at 10 K and VN density can be expressed as the following equation according to the rate equation model of doped samples38:

?@A = hν × c × QGH

= hν × c × ∆JKL ( + ∆)K4 K ≈ hν × c × ∆JKL ∆K4 K = N ∙ ,

(3)

where IPL is the PL intensity of the GL originated from VN, h is the Plank constant, ν is the photon frequency, c is the light speed, coefficient QGH

=∆JKL ( + ∆)K4 K , ∆n is the photon excited electron number, ∆p is the photon excited hole number, p is the carrier number activated from the acceptor states. Also, QD is the coefficient for the net transition from the conduction band to the electron capturing state, QA is the coefficient for the net transition from the valence band to the hole capturing state, QR is the recombination coefficient of electrons and holes,38 and coefficient B = hνc∆JKL ∆K4 K . At low temperature (10 K), p is negligible, leading to ∆p+p≈∆p. Here, both the QD and QA are considered to be different values in samples A and B


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because of the different PL intensity measured from the two samples. According to eq 3, the increase in the GL portion is proportional to the VN density. From eqs 2 and 3, the relationship between ?@A and NMg is expressed as the following equation.

?@A = ' ∙ N ∙

%

(1 + 1 + ⁄

!" )

%

(4)

The spatial variation in the calculated GL intensity from eq 4 is shown in the above of Figure 5b, and the GL portion extracted from micro-PL spectra is shown below. The averaged portions of GL at each position of the three rods each from samples A and B are extracted from spatially resolved micro-PL spectra. Here, GL portion is defined as the PL intensity integrated over the GL bands normalized by total area of the PL spectrum. The calculated GL intensity was obtained by Mg concentration of TOF-SIMS based on eq 4 and then normalized by the GL portion, of the position 1 µm above the bottom of samples A and sample B, respectively for comparison. The axial change in calculated GL intensity from Mg concentration measured by TOF-SIMS matches well with the change in the GL portion from micro-PL spectra. Therefore, the GL portion extracted from the PL spectra provides a good estimate of Mg concentration. In addition, it is possible to estimate resistance change along the rod by using the GL portion, since the Mg concentration reduction along the axial direction of the rod leads to increased resistance along the same direction of both samples A and B. In the point of view of the electrical conductivity, VN defects of the origin of GL act as selfcompensation sites. Reschikov et al. predicted that Mg doped p-GaN with strong UVL emission shows good electrical conductivity, whereas the p-GaN with strong GL emission results in poor conductivity.33 The GL emission of every position of samples A and B is about 2 orders weaker than UVL emission, which is interpreted as good electrical conductivity with a little self-


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compensation effect in both samples A and B. This interpretation well matches with aforementioned negligible self-compensation effect in samples A and B explained from the changes of resistance and E mode FWHM. In a GaN film, the mobility decreases with higher doping concentration because of increased scattering by the defects.29, 39 Since the difference of doping concentration between the samples A and B is larger than the doping change along the single rod, the difference of K is dominant between two samples, whereas spatially change of K is not dominant in both two samples. Also, as mentioned in Figure 4e, the same FWHM of E of the samples A and B reflects that the compensation defects induced by high Mg-doping is negligible. Therefore, the overall K degradation in sample B compared to sample A is considered as being affected dominantly by the scattering effect from Mg impurities on Ga sites, but negligibly by the scattering effect from the compensation defects. However, mobility of GaN can also be reduced by the strain due to a piezoelectric scattering.40-41 Figure 5c shows the spatially resolved K and the peak position of E along both samples. Qualitatively, the K and peak position of E are independent from spatial change in the rod, however, the sample dependence of K and peak position of E is reverse. Therefore, it is considered that an increase of strain in sample B results in higher piezoelectric scattering, leading to the decrease in K and increasing entire resistance in this sample. The quantitative relationship between the mobility and the various scattering effects would be studied further.

Conclusion In conclusion, we systematically studied a p-type doping inhomogeneity along the axial direction of Mg-doped p-GaN micro-rods grown by MOCVD. Reduction in Mg concentration


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along the axial direction of the rod was measured by TOF-SIMS. The spatial distribution of GL intensity is well matched with the Mg-dopant distribution, since VN, which is the one of the origin of the GL in the PL spectrum, sensitively changes under different Mg concentration. The resistance measured from 4 point probes measurement increased as closer to the top of the rods, where Mg concentration is low, in both samples. However, the entire resistances of the sample B are larger than the sample A. To explain this phenomena, the parameter K was used for an estimation of the mobility of samples A and B. The K is almost constant along the c-axis of the both rods. However, sample B showed smaller values of K than sample A. This result can be explained by the combination of increased impurity scattering by Mg dopants and piezoelectric scattering by strain. Therefore, the strong correlation of the four complementary measurements allows a consistent and comprehensive characterization of the p-doping properties of the rod. Doping inhomogeneity of Mg-doped GaN micro/nano-rods revealed in this paper can be utilized for rod-based tunable laser42 or high resolution imaging system43 with an appropriate doping control.Based on the correlation, we can estimate the axial inhomogeneity in doping and conductivity in the Mg doped p-GaN rods by using spatially-resolved optical spectroscopic methods (PL and Raman mapping). These optical analyses are non-destructive and does not require the time-consuming fabrication process, therefore, provides a facile and fast estimation about the axial doping and conductivity inhomogeneity. Moreover, our complementary study offers reliable feedback and diagnosis for Mg-doping in the p-GaN rods growth process, laying a cornerstone for the development of highly efficient rod based photonic devices.

Methods


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Structural and Componential analyses. The structural properties of the p-GaN micro-rods were observed using SEM (FEI Philps XL-30 SFEG) and HR-TEM (Titan cubed G2 60-300). The TOF-SIMS measurements were performed on single p-GaN rods dispersed on a Si (100) substrate using a TOF-SIMS system (ION-TOF GmbH, TOF-SIMS5). Mg+ and Ga+ ion mapping images of few tens nanometers thickness from the sample surface were obtained using a 2D mapping mode to analyze a 50 µm × 50 µm area under the conditions of a 30 keV Cs+ primary ion beam. Metal electrodes fabrication and resistance measurement. For the 4 point probe measurements, p-GaN rods were dispersed on a 100-nm thick SiO2 deposited Si (100) substrate. After dispersion, HSQ coating with thermal annealing, and diluted HF solution (3.3 %) based wet etching were conducted for planarization around the rod and exposure of the m-plane of the rod for the electrodes contact. The electrode patterns aligned on the single rod were then made with SEM (JEOL, JSM-6510) based e-beam lithography. 30 nm Ni and 200 nm Au layers were then deposited on the p-GaN rod by using an e-beam evaporator (Korea Vacuum, KVE TC500200) and the metal electrodes were formed by a lift-off process. For the ohmic contact to the p-GaN rod, metal electrodes were annealed at 550 oC for 5 minutes under atmosphere environment. The I-V curves of samples A and B were obtained from the 4 point probe measurements at room temperature using a manual probe station (KEITHLEY, 4200 SCS) at room temperature. Optical characterization. The optical properties of a single GaN rod were measured by using a micro-PL setup with an excitation of 325 nm He-Cd continuous wave laser. Before the measurement, the single p-GaN rod on Si (100) substrate was coated with HSQ and annealed at 400 oC for 1 hour under a nitrogen atmosphere to suppress the optical modes in the PL spectrum


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(Supporting Information, Figure S4). A microscope objective lens (Mitutoyo, ×80, N.A. = 0.55) was used for excitation of the single rod with an excitation area of approximately 15 um2. To cool the sample to 10 K, a cryo station system (Advanced Research System, CS204F-DMX-20OM) was used. The low temperature (10 K) PL spectra along the growth direction of the rod were measured with a monochromator (Acton, SP2500) with an arrayed charge-coupled device (CCD) of 400×1340 pixels (Princeton Instruments, PIXIS: 400) and optical image rotation by dove prism and motorized stage movement (Supporting Information, Figure S2). Micro-Raman spectra along a single rod dispersed on a sapphire substrate were obtained by using a microRaman measurement system (Bruker, SENTERRA) at room temperature. The sample was excited by microscope objective lens (OLYMPUS, ×100, N.A. = 0.90) with a 532 nm Nd-YAG laser at 10mW.

ASSOCIATED CONTENT Supporting Information. Mg-doped p-GaN rods morphology, micro-PL setup, Excitation power dependent PL experiments of a single p-GaN rod, confirmation of modes suppression by HSQ. AUTHOR INFORMATION

Corresponding Author *Tel: +82-42-350-2549; fax: +82-42-350-5549; e-mail: [email protected]

ORCID Yong-Hoon Cho : 0000-0002-7701-8562


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Author Contributions S.C. performed SEM, TEM, TOF-SIMS, and 4-point probe measurements and analyzed data. S.C., H.G.S., and Y.-S.Y. carried out micro-PL experiments and analyzed data. S.C. and C. L. measured and analyzed micro-Raman spectrum. S.C. and K.Y.W. carried out the fabrication process of the metal electrodes and the process optimization. E. L., and S. D. L. performed the growth of the p-GaN rods. Y.-H.C. conceived and supervised this project. S.C. and Y.-H.C. wrote the manuscript with contributions from all co-authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Dr. ChungHyun Park for discussion about the correlation of measurement results. The authors also appreciate LG Innotek for providing the rod samples and Dr. Wonho Kim for discussing sample growth condition. This work was supported by the National Research Foundation

(NRF)

grant

funded

by

the

Korea

government

(MSIP)

(NRF-

2016R1A2A1A05005320), and the Climate Change Research Hub of KAIST (Grant No. N11180103).

ABBREVIATIONS QW, quantum well; MOCVD, metal-organic chemical vapor deposition; TOF-SIMS, time of flight secondary ion mass spectrometry; PL, photoluminescence; SEM, scanning electron


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microscopy; HR-TEM, high resolution – transmission electron microscopy; FFT, fast Fourier transform; HSQ, hydrogen silsesquioxane; u-GaN, un-doped GaN; NBE, near band edge; GL, green luminescence; UVL, ultraviolet luminescence; VN, nitrogen vacancy; FWHM, full width half maximum; CCD, charge-coupled device;

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Figure 1. (a) Schematic of an ensemble of p-GaN micro-rods growth. (b) A bird’s-eye view SEM image of the p-GaN micro-rods ensemble. (c) HR-TEM image of a single p-GaN micro-rod. (d) Diffraction patterns of the p-GaN micro-rod HR-TEM image obtained by FFT.


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Figure 2. (a) SIMS spectrum of a single p-GaN rod. Inset: a magnified image of the SIMS spectrum near the Mg+ components. (b) Ga+ ion mapping image of Sample A. (c) Mg+ ion mapping image of Sample A. (d) Averaged Mg concentration change along the vertical direction of the doped p-GaN rods. The error bar indicates the standard deviation of the averaged Mg concentration at a specific height.


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Figure 3. (a) Schematic of resistance measurement along the single p-GaN rod using the 4 point probe method. (b) Top view SEM image of the electrodes on the p-GaN rod. (c) The spatially resolved resistance of both samples A and B. Inset: I-V measurement results of the pGaN rod as shown in (b) using the 4 point probe method. (d) The spatially resolved K of both samples A and B. In (c), (d), the blue symbols mean sample A, while the red symbols indicate sample B. Also, each empty, half-filled and full triangle symbol represents a data set measured from different rod samples.


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Figure 4. (a) Normalized PL spectra of the entire area of a single u-GaN rod, Sample A, and Sample B. (b) Spatially resolved normalized PL spectrum along a single rod from sample A. (c) Spatially resolved normalized PL spectrum along a single rod from sample A. (d) Normalized Raman spectra at the middle point of the rod of a single u-GaN rod, Sample A, and Sample B. (e) Averaged FWHM of E change along the axial direction of samples A and B. (f) Averaged peak position of E change along the axial direction of samples A and B. The error bars of (e), (f)


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indicate the standard deviation FWHM and peak position of E at a specific position, respectively.

Figure 5. (a) Schematic of the analysis flow. (b) The correlation between the calculated GL intensity from Mg concentration and the averaged GL portion of both samples A and B. (c) The correlation between the spatially resolved K and averaged peak position of E of both samples A


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and B. All error bars indicate the standard deviation calculated with three rods from samples A and B.


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79x37mm (300 x 300 DPI)


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Figure 1. (a) Schematic of an ensemble of p-GaN micro-rods growth. (b) A bird’s-eye view SEM image of the p-GaN micro-rods ensemble. (c) HR-TEM image of a single p-GaN micro-rod. (d) Diffraction patterns of the p-GaN micro-rod HR-TEM image obtained by FFT. 86x104mm (300 x 300 DPI)


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Figure 2. (a) SIMS spectrum of a single p-GaN rod. Inset: a magnified image of the SIMS spectrum near the Mg+ components. (b) Ga+ ion mapping image of Sample A. (c) Mg+ ion mapping image of Sample A. (d) Averaged Mg concentration change along the vertical direction of the doped p-GaN rods. The error bar indicates the standard deviation of the averaged Mg concentration at a specific height. 148x104mm (300 x 300 DPI)


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Figure 3. (a) Schematic of resistance measurement along the single p-GaN rod using the 4 point probe method. (b) Top view SEM image of the electrodes on the p-GaN rod. (c) The spatially resolved resistance of both samples A and B. Inset: I-V measurement results of the p-GaN rod as shown in (b) using the 4 point probe method. (d) The spatially resolved K of both samples A and B. In (c), (d), the blue symbols mean sample A, while the red symbols indicate sample B. Also, each empty, half-filled and full triangle symbol represents a data set measured from different rod samples. 177x108mm (300 x 300 DPI)


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Figure 4. (a) Normalized PL spectra of the entire area of a single u-GaN rod, Sample A, and Sample B. (b) Spatially resolved normalized PL spectrum along a single rod from sample A. (c) Spatially resolved normalized PL spectrum along a single rod from sample A. (d) Normalized Raman spectra at the middle point of the rod of a single u-GaN rod, Sample A, and Sample B. (e) Averaged FWHM of E2H change along the axial direction of samples A and B. (f) Averaged peak position of E2H change along the axial direction of samples A and B. The error bars of (e), (f) indicate the standard deviation FWHM and peak position of E2H at a specific position, respectively. 176x162mm (300 x 300 DPI)


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Figure 5. (a) Schematic of the analysis flow. (b) The correlation between the calculated GL intensity from Mg concentration and the averaged GL portion of both samples A and B. (c) The correlation between the spatially resolved K and averaged peak position of E2H of both samples A and B. All error bars indicate the standard deviation calculated with three rods from samples A and B. 174x169mm (300 x 300 DPI)


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Axial Inhomogeneity of Mg-Doped GaN Rods: A Strong Correlation

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