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[Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), [Jun Chen](https://orcid.org/0000-0003-4272-2653), Ryo Tanaka, Shinya Takashima, Masaharu Edo, [Tadakatsu Ohkubo](https://orcid.org/0000-0003-3548-1951), Takashi Sekiguchi

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Jun Uzuhashi, Jun Chen, Ryo Tanaka, Shinya Takashima, Masaharu Edo, Tadakatsu Ohkubo, Takashi Sekiguchi; Effect of sequential N ion implantation in the formation of a shallow Mg-implanted p-type GaN layer. J. Appl. Phys. 7 August 2024; 136 (5): 055702. and may be found at https://doi.org/10.1063/5.0216601[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Effect of sequential N ion implantation in the formation of a shallow Mg-implanted p-type GaN layer](https://mdr.nims.go.jp/datasets/ab768a0e-41d0-4d86-8017-508b60e81b02)

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Effect of sequential N ion implantation in the formation of a shallow Mg-implanted p-type GaN layer Jun Uzuhashi1,2*, Jun Chen1*, Ryo Tanaka3, Shinya Takashima3, Masaharu Edo3, Tadakatsu Ohkubo1, and Takashi Sekiguchi1,2 1National Institute for Materials Science, Tsukuba, 305-0047, Japan 2University of Tsukuba, Tsukuba 305-8577, Japan 3Advanced Technology Laboratory, Fuji Electric Co., Ltd., Hino, Tokyo 191-8502, Japan * Authors to whom correspondence should be addressed:UZUHASHI.Jun@nims.go.jp and CHEN.Jun@nims.go.jp Keywords: gallium nitride, ion implantation, transmission electron microscopy, atom probe tomography, cathodoluminescence Abstract An area-selectable Mg doping via ion implantation (I/I) is essential to realize gallium nitride (GaN) based power switching devices. Conventional post-implantation annealing forms considerable defects in the GaN, resulting in extremely low activation efficiency. The recent invention of ultra-high-pressure annealing (UHPA) has substantially improved the p-type activation efficiency, however, the UHPA causes an unexpected Mg diffusion. Thus, both annealing processes resulted in a much lower Mg concentration in the GaN matrix than the Mg dose. In this study, the effect of a sequential N I/I for p-type Mg-implanted GaN was investigated by the correlative cathodoluminescence (CL), transfer length method (TLM), scanning transmission electron microscopy (STEM), and atom probe tomography (APT) analyses. APT results have revealed that the sequential N I/I can successfully maintain the Mg concentration in the GaN matrix in the higher range of 1018 cm-3 or more. Our investigation suggests that sequential N I/I is a promising technique to maintain the Mg concentration higher and improve the p-type activation efficiency.  mailto:UZUHASHI.Jun@nims.go.jpmailto:CHEN.Jun@nims.go.jpIntroduction Gallium nitride (GaN) has attracted by its capability for the next-generation devices in high-power switching applications [1-2]. Owing to the recent improvements in the GaN crystal growth technique [3], the demonstrations of vertical-type GaN devices have been reported [4-12]. For application in high-power switching, the metal oxide semiconductor (MOS) gate driving is desired, thus, several studies of MOS field-effect transistors (MOSFETs) in both lateral and vertical structures were reported [13-22]. Considering practicality and reliability, it is essential to precisely control area selective conduction types by ion implantation (I/I) technique. However, forming p-type GaN by Mg I/I is still a critical problem. The Mg I/I unavoidably introduce defects into GaN, resulting in low activation efficiency compared with Mg doping via epitaxial growth [23]. A high-temperature post-implantation annealing is commonly carried out to recover these defects and activate Mg atoms [24], however, it results in an inefficient p-type activation due to the formation of defects [23-28]. Recently, the ultra-high-pressure annealing (UHPA) technique substantially improved the Mg activation efficiency [28-37]. However, a significant Mg diffusion is reported as one of the features of the UHPA [28-37]. An efficient p-type activation with high Mg concentration is required for ohmic contact formation to realize the GaN-based MOSFETs [22], however, both annealing techniques drastically decrease Mg concentration in the GaN matrix due to Mg clustering (conventional annealing) or Mg diffusion (UHPA) [26-28]. To overcome these problems, the sequential N I/I technique has been recently reported [38-42]. In this work, the effect of a sequential N I/I technique on shallow Mg I/I layer was studied by using correlative analysis techniques; secondary ion mass spectrometry (SIMS), cathodoluminescence (CL), transfer length method (TLM), scanning transmission electron microscopy (STEM), and atom probe tomography (APT) analyses. These results have clarified how the sequential N I/I technique affects the formation of defects, Mg distribution, and p-type activation. Experimental methods A 4 µm thick homoepitaxial n-GaN layer was grown by metal-organic chemical vapor deposition (MOCVD) on a [0001] oriented free-standing GaN substrate. The GaN substrate was prepared by the standard hydride vapor phase epitaxy (HVPE) technique [43]. Mg and N ions were shallowly implanted on the n-GaN epitaxial layer in 7 dgrees with twist 15 degrees at room temperature with different doses at 25 keV for Mg and 15 keV for N. Four dose levels ranging from 0.9×1014 to 5.4×1014cm-2 were designed. Samples with different dose levels are referred to as Mg(n)+N(m), here, “n” and “m” are numbers. "1" represents a dose of 0.9×1014 cm-2, "2" is twice that amount, 1.8×1014 cm-2, "3" is 3.6×1014 cm-2, and "4" is 5.4×1014 cm-2. For example, the sample name "Mg(2)+N(4)" indicates a dose of 1.8×1014 cm-2 for Mg and 5.4×1014 cm-2 for N. Table 1 shows the sequential Mg and N I/I GaN samples used in this study. Four samples named as Mg(1)+N(1), Mg(2)+N(2), Mg(2)+N(4) and Mg(4)+N(4) were selected from a variety of combinations. Thereafter, conventional post-implantation annealing under an N2 atmosphere at 1573 K for 5 min was performed with a 300 nm AlN encapsulation cap by a sputtering method to prevent GaN dissociation. Finally, the AlN cap was chemically removed. Details can be found at Kumar et al. [26]. Mg concentrations as a function of depth were analyzed by SIMS using O2+ ions as primary ions with a beam energy of 5 keV. The CL measurements were carried out by a HORIBA MP32 CL system attached to a Hitachi SU6600 field emission scanning electron microscope at 2 keV with a specimen temperature of 80 K. Considering the penetration depth of the electron beam, the CL result with 2 keV corresponds to the emission from ~40 nm depth. In addition, an angle cutting cross-sectional sample preparation method was carried out at 3 keV to reveal the depth distribution of the donor-acceptor-pair (DAP) emissions [44]. TLM measurement was done to determine the I-V characteristics. Ni (50 nm) / Au (150 nm) were used as an anode electrode, and N2 gas annealing was performed at 723K for 4 minutes [22]. STEM and APT techniques were applied for the microstructural investigations. STEM observations were carried out by using a Thermo Fisher Scientific Titan G2 80-200. APT analyses were carried out using a CAMECA LEAP 5000XS. Both STEM and APT specimens were prepared by using a focused ion beam (FIB) with a scanning electron microscopy (SEM) system, Thermo Fisher Scientific Helios 5UX, with a 2 keV final cleaning [45, 46]. APT measurements were carried out with 10 fJ UV laser pulsing at a specimen temperature of 30 K. It should be noted that 1×1019 cm-3 corresponds to 0.0114 at% in APT, and the APT detection limit of Mg with this measuring condition is a higher range of 1017 cm-3.  Samples Mg dose (cm-2) N dose (cm-2) SIMS & APT TLM CL [MgSIMS] (cm-3) [Mgmatrix] (cm-3) Mg clusters (nm-3) Rs (Ω/□) ρc (Ωcm2) DAP (count/s) GL (count/s) Mg(1)+N(1) 0.9×1014 0.9×1014 7.0×1018 4.9×1018 0.8×10-3 3.0×105 0.20 72.8 29.0 Mg(2)+N(2) 1.8×1014 1.8×1014 2.0×1019 8.6×1018 0.7×10-3 1.5×105 0.0076 119.3 6.6 Mg(2)+N(4) 1.8×1014 5.4×1014 3.0×1019 8.0×1018 1.5×10-3 N/A N/A 1.8 2.5 Mg(4)+N(4) 5.4×1014 5.4×1014 9.0×1019 1.6×1019 2.0×10-3 2.7×105 0.37 26.1 4.8 Table 1.  Sequential Mg and N I/I GaN samples used in this study. The implantation doses of Mg and N, the concentrations of Mg and cluster extracted from SIMS and APT measurements, the sheet resistivity Rs and contact resistance ρc based on TLM, and the CL intensity of donor-acceptor pair (DAP) and green luminescence (GL) at ~40 nm depth are listed.  (N/A: not available) Results and Discussion Figure 1 shows the Mg concentration profiles measured by SIMS. Fig. 1(a) shows the profile down to a depth of 800 nm, and Fig. 1(b) shows magnified profiles near the sample surfaces. It should be noted that the profiles below 35 nm were intentionally excluded to rule out uncertainties such as surface roughness, and as-implanted Mg and N profiles is drawn as dotted-lines by the stopping and range of ions in matter (SRIM) simulation in the case of 1.8×1014 cm-2 dose. At the depth of 40 nm, the Mg concentration measured by SIMS (hereafter refered to as [MgSIMS]) was maintained at approximately 7.0×1018 cm-3 for the sample Mg(1)+N(1), 2.0×1019 cm-3 for Mg(2)+N(2), 3.0×1019 cm-3 for Mg(2)+N(4), and 9.0×1019 cm-3 for Mg(4)+N(4). The [MgSIMS] values of Mg(2)+N(2) and Mg(2)+N(4) were almost identical. However, the magnitude of Mg diffusion in the depth direction varied depending on the dose conditions. The Mg diffusion was inhibited by increasing the N dose. From Fig. 1(a), it can be found that the diffusion depth of Mg was in the order of Mg(1)+N(1) (~600 nm) > Mg(2)+N(2) (~400 nm) > Mg(4)+N(4) (~300 nm) > Mg(2)+N(4) based on Fick’s law [47, 48]. In the case of the Mg(2)+N(4), the diffusion of Mg is less obvious compared with the other samples. The effectiveness in suppressing the Mg diffusion by the sequential N I/I (called “vacancy-guided Mg redistribution technique”) was reported by previous studies [38, 40].  Figure 1. Mg concentration profiles measured by SIMS. (a) Mg profiles up to 800 nm depth, and (b) magnified profiles near the sample surfaces indicated purple box in (a). As-implanted Mg and N profiles is drawn as dotted-lines by the stopping and range of ions in matter (SRIM) simulation in the case of 1.8×1014 cm-2 dose. Figure 2(a-d) shows the angle-cutting cross-sectional CL images of the donor-acceptor-pair (DAP) emission (2.7~3.3 eV) of the four samples. It was found that the DAP luminescence depth became shallower as the N dose increased. Since DAP emission is associated with Mg activation [27, 28], the depth of DAP luminescence was in good agreement with the Mg diffusion observed by SIMS. Fig. 2(e) shows the CL spectra of green luminescence (GL) (2.1~2.5 eV) and DAP emission (2.7~3.3 eV), which were taken at an accelerating voltage of 2 keV, corresponding to the electron range of ~ 40 nm depth. When the dose was Mg(1)+N(1), the CL intensity of GL remained strong. On the other hand, the Mg(2)+N(2) sample showed a weakened GL emission and an enhanced DAP emission. The GL emission was considered to be associated with the presence of N vacancy-type defects (VN) [38-40, 49, 50]; therefore, the CL results suggest that it is effective in suppressing the number of VN by the sequential N I/I technique. However, if the doses of Mg or N were further increased, the DAP intensity decreased, suggesting that a higher N dose may induce non-radiative centers.  Figure 2. Angle-cutting cross-sectional CL images of the donor-acceptor-pair (DAP) emission (2.7~3.3 eV) of (a) Mg(1)+N(1), (b) Mg(2)+N(2), (c) Mg(2)+N(4), and (d) Mg(4)+N(4) samples. Note that the tilt correction in the depth direction was applied for CL images. (e) The CL intensity of the green band (2.1~2.5 eV) and the DAP emission (2.7~3.3 eV) at 2 keV. Figure 3 shows the results of the TLM measurement. The values of sheet resistance Rs (Ω/□) and contact resistance ρc (Ωcm2) are written in the graphs. Since Schottky-like curves were observed in all the samples, the values were calculated by using a method to evaluate the nonlinear curves reported by Piotrzkowski et al. [51]. The Mg(2)+N(2) sample has the best properties, the lowest Rs and ρc (Fig. 3(b)), and the others have worse properties. Tanaka et al. reported the only Mg I/I sample showing almost no current flow [22], therefore, it can be concluded that the sequential N I/I technique has improved the p-type activation. However, the properties are again degraded if the N dose becomes higher than Mg(2)+N(2) (Fig. 3(c) and (d)). Almost no current flow was observed in the Mg(2)+N(4). The samples with Mg(2)+N(1), Mg(2)+N(3), Mg(3)+N(3) conditions also show worse properties compared with Mg(2)+N(2) (see Suppl. Fig. 1). In this study, the Mg(2)+N(2) was somehow in the sweet spot. The results of TLM and CL measurements agree with each other, that is, the Mg(2)+N(2) shows the best properties in Rs and ρc (TLM), the highest DAP, and the lowest GL emissions (CL). Also, the Mg(2)+N(4) shows almost no current flow in TLM, and no DAP emissions by CL analysis. Figure 3. The results of the TLM measurement of shallow Mg and N I/I samples. (a) Mg(1)+N(1), (b) Mg(2)+N(2), (c) Mg(2)+N(4), and (d) Mg(4)+N(4). To reveal the relationship between the properties and the microstructures of samples, STEM and APT were applied. Figure 4 shows the cross-sectional low-angle annular dark field (LAADF)-STEM images. Distortion caused by the vacancy clusters, or the Mg clustering in the GaN gives a bright contrast in the LAADF-STEM image as reported previously [26-29, 40, 41]. The low-magnification images are shown in Fig. 4(a-d), and a large number of tiny bright dots are observed in all the samples. The typical defect in higher magnification is shown in Fig. 4(e). These defects are the extrinsic stacking fault on the c-plane highlighted with pink-colored marks as reported previously [26, 29, 33]. From the low magnification images, the number densities of these defects seem to be increasing with the increase of  N dose. Figure 4. (a-d) Low-magnification cross-sectional LAADF-STEM images. (e) Typical defect in higher magnification. Pink-colored marks indicates the extrinsic stacking fault on the c-plane. Figure 5 shows the results obtained by APT. To avoid complications, the three-dimensional Mg distributions of only the Mg(2)+N(2) sample are shown in Fig. 5(a) and (b). Fig. 5(a) shows the Mg distribution up to ~100 nm depth, and one of the typical Mg clusters is shown in Fig. 5(b). Disk-shaped Mg clusters on the c-plane were formed in all the samples. The extrinsic stacking fault on the c-plane observed by STEM corresponds to these disk-shaped Mg clusters. To compare with the SIMS results, the Mg concentrations in the GaN matrix (hereafter, refer to [Mgmatrix]) at 40 nm depth were calculated by excluding Mg clusters from APT data. A threshold of 0.3 at% Mg was used here. The results of [Mgmatrix] are as below; ~4.9×1018 cm-3  (Mg(1)+N(1)), ~8.6×1018 cm-3 (Mg(2)+N(2)), ~8.0×1018 cm-3  (Mg(2)+N(4)), and ~1.6×1019 cm-3 (Mg(4)+N(4)) as summarized in Fig. 5(c). APT analysis eveals that the increase in Mg dose keeps the [Mgmatrix] increasing. Both the [MgSIMS] and the [Mgmatrix] are in the order of the Mg doses. At the same time, the increase of Mg and N I/I doses strongly enhances the formation of Mg clusters as the calculated number densities of Mg clusters shown in Fig. 5(d). Between Mg(1)+N(1) and Mg(2)+N(2) samples, there is no significant difference in the number density (0.8×10-3 nm-3 for Mg(1)+N(1), and 0.7×10-3 nm-3 for Mg(2)+N(2)). On the other hand, the number density increases to 1.5×10-3 nm-3 for Mg(2)+N(4), and 2.0×10-3 nm-3 for Mg(4)+N(4).   Figure 5. (a) Three-dimensional Mg atoms distributions up to ~100 nm depth, and (b) typical Mg clusters. (c) Calculated Mg concentrations of [MgSIMS] and [Mgmatrix] from SIMS and APT results at ~40 nm depth. (d) The number density of Mg clusters calculated from 30 to 50 nm depth. Table 1 is a summary of SIMS, APT, TLM, and CL data taken inside the I/I layer with a depth of ~ 40 nm, including the Mg concentrations measured by SIMS and APT, the number density of Mg clusters, the sheet resistivity Rs and contact resistance ρc, as well as the CL intensity of donor-acceptor pair (DAP) and green luminescence (GL). To discuss the effect of sequential N I/I, two types of comparison are made. One type of comparison is the effect of increasing Mg and N doses while maintaining the same ratio. Another comparison is the effect of increasing the N dose while keeping the Mg dose fixed at 1.8×1014 cm-2.  For the comparison of samples with simultaneous increases in Mg and N doses  (Mg(1)+N(1), Mg(2)+N(2), and Mg(4)+N(4)), the increase in Mg dose successfully made both [MgSIMS] and [Mgmatrix] higher. This result is consistent with the reports by Uedono et al. and Kano et al. that the trapping of implanted Mg ions in vacancies was enhanced by the sequential N I/I [40, 42]. There was no significant difference in the number density of Mg clusters between the Mg(1)+N(1) and Mg(2)+N(2) samples; whereas the [Mgmatrix] became higher in the Mg(2)+N(2) sample. On another hand, strong GL emission observed in the Mg(1)+N(1) sample indicated the presence of a high density of VN defects [38, 39, 41], whereas in the Mg(2)+N(2) sample, the intensity of GL emission was considerably reduced, suggesting that VN defects were suppressed. We can conclude that the higher [Mgmatrix] and lower N vacancy-type defects in Mg(2)+N(2) are the main reasons for the improved electrical properties. However, the number density of Mg clusters in the Mg(4)+N(4) sample was more than twice that of the Mg(1)+N(1) and Mg(2)+N(2) samples. It has been reported the formation of Mg clusters strongly suppressed the DAP emissions associated with p-type activation [27, 28, 52]. Thus, the TLM property and DAP emissions of the Mg(4)+N(4) sample are considered to be degraded because of an increase in Mg-enriched defects.  In another comparison of samples where only the N dose increased (Mg(2)+N(2) and  Mg(2)+N(4)), there was no significant difference in [MgSIMS] and [Mgmatrix] due to the same Mg dose. Nevertheless, the Mg(2)+N(4) sample showed almost no current flow in TLM and very weak DAP emission in CL. We have investigated a series of samples with the Mg dose fixed at 1.8 × 1014 cm-2 and N dose varied from 0.9 × 1014 (Mg(2)+N(1)), 1.8 × 1014 (Mg(2)+N(2)), 3.6 × 1014 (Mg(2)+N(3)) to 5.4 × 1014 cm-2 (Mg(4)+N(4)) (see Suppl. Table 1). In the Mg(2)+N(1) sample, DAP emission was strong; however, GL emission was also strong due to insufficient N I/I resulting in still high-density VN defects; in the Mg(2)+N(3) and Mg(2)+N(4) samples, both DAP and GL emissions extremely decreased. Sakurai et al. reported that an excessive N I/I reduced the entire CL intensity, indicating an increase in the nonradiative recombination centers (NRCs) [39]. Uedono et al. reported that sequential N I/I increased not only the concentration of vacancy-type defects but also other types of defects such as dislocation loops, Mg clusters, and stacking faults [41]. Thus, the worst properties of TLM and CL of Mg(2)+N(4) sample are probably due to the introduction of numerous NRCs owing to excess N I/I [53]. This comparison strongly indicates that the Mg/N dose ratio changes the balance between the vacancy concentration and the size of vacancy clusters, the formation of stacking faults and Mg clusters. Conclusions In summary, it was revealed that the sequential N I/I dose technique has both positive and negative effects on p-type activation; the positive effect is to make [Mgmatrix] higher and to reduce VN, and the negative effect is enhancing the formation of Mg clusters and the introduction of vacancies, resulting in suppressing the p-type activation efficiency. An insufficient N dose cannot suppress the VN, resulting in strong GL emission; however, an excessive N I/I introduces numerous NRCs originating from vacancy-type defects, resulting in weak emission in entire CL spectra. Here, we can conclude that optimizing the I/I condition is a trade-off between the Mg concentration in the GaN matrix and the number density of these defects. Our investigations have clarified that the optimized sequential N I/I condition enables the Mg concentration to maintain higher and improves the p-type activation efficiency.   Supplementary Material Suppl. Fig. 1. The results of the TLM measurement of shallow Mg and N I/I samples. Suppl. Table 1. Sequential Mg and N I/I GaN samples used in this study. The implantation doses of Mg and N, the concentrations of Mg and cluster extracted from SIMS and APT measurements, and the CL intensity of donor-acceptor pair (DAP) and green luminescence (GL) at ~40 nm depth are listed. Acknowledgments The support for this research was provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through its Program for Creation of Innovative Core Technology for Power Electronics Grant No. JPJ009777. The authors thank Kyoko Suzuki for her technical support of STEM/APT analyses.  References 1. T.P. Chow and R. Tyagi, IEEE Trans. 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