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[Emi Kano](https://orcid.org/0000-0001-6134-4980), [Jun Uzuhashi](https://orcid.org/0000-0003-2023-8158), Koki Kobayashi, Kosuke Ishikawa, Kyosuke Sawabe, Tetsuo Narita, Kacper Sierakowski, Michal Bockowski, [Tadakatsu Ohkubo](https://orcid.org/0000-0003-3548-1951), Tetsu Kachi, [Nobuyuki Ikarashi](https://orcid.org/0000-0002-5575-5780)

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[Impact of Sequential N Ion Implantation on Extended Defects and Mg Distribution in Mg Ion‐Implanted GaN](https://mdr.nims.go.jp/datasets/7933b0f5-c47a-4194-95a9-a69730ba250a)

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Impact of Sequential N Ion Implantation on Extended Defects and Mg Distribution in Mg Ion‐Implanted GaNImpact of Sequential N Ion Implantation on ExtendedDefects and Mg Distribution in Mg Ion-Implanted GaNEmi Kano,* Jun Uzuhashi, Koki Kobayashi, Kosuke Ishikawa, Kyosuke Sawabe,Tetsuo Narita, Kacper Sierakowski, Michal Bockowski, Tadakatsu Ohkubo, Tetsu Kachi,and Nobuyuki Ikarashi1. IntroductionVertical GaN power devices have been developed as high-efficiency switching devices for high-power applications.[1–4]To unleash the full potential of such devices, selective area p-typedoping is required, and Mg ion implanta-tion processes have been developed for thispurpose.[4–9]A long-standing challenge with ionimplantation doping for semiconductordevices is how to control the dopantdistribution.[10–14] The outward diffusionof the dopants during postimplantationannealing lowers the dopant concentrationin the implanted region and changes theirconcentration profile, which can be a severerestriction when designing device struc-tures. Moreover, when fabricating the GaNdevices, controlling the Mg distribution is amajor challenge. A significant decrease inthe Mg concentration in the ion-implantedregion has been reported, especially in thehigh-concentration doping of Mg (typically,Mg concentration of about 1� 1019 cm�3or more)[8,9,15,16] that is used for forming the contact in the devi-ces. The diffusion of Mg follows the diffusion equation andthe distribution of Mg after 1300 °C annealing is documentedin ref. [9].Previous studies have shown that sequential N ion implanta-tion after Mg ion implantation suppresses the lowering of theMg concentration in the Mg-implanted region even afterannealing.[16–18] This is presumably because, according to posi-tron annihilation spectroscopy analysis, Mg atoms are distrib-uted in accordance with the vacancies produced by the N ionimplantation, thereby suppressing the outward diffusion ofMg. The previous analyses, however, did not take into accountthe effect of extended defects formed during annealing, whichattract Mg atoms and can affect the Mg concentration profile.[9,19]Transmission electron microscopy (TEM) analyses have indi-cated that the interstitial-type dislocation loops are formed in theinitial stage of annealing and the vacancy-type dislocation loopsare formed in the later stage.[19,20] The analyses have also indi-cated that, as the annealing duration increases, the small dislo-cation loops disappear (i.e., defect density decreases), while thesize of the large dislocation loops increases.[21–23] The results alsosuggest that the evolution of the extended defects is dominatedby the thermal diffusion of native defects.[19]The purpose of our current study is to reveal the impact of Mgand N sequential ion implantation on the Mg distribution andacceptor formation after annealing, as well as the underlyingmechanisms. To this end, we prepared Mg ion-implanted GaNcrystals with and without N implantation and analyzed theE. Kano, T. Kachi, N. IkarashiInstitute of Materials and Systems for SustainabilityNagoya UniversityNagoya, Aichi 464-8601, JapanE-mail: kano@imass.nagoya-u.ac.jpJ. Uzuhashi, T. OhkuboNational Institute for Materials ScienceTsukuba 305-0047, JapanK. Kobayashi, K. Ishikawa, K. Sawabe, M. BockowskiDepartment of ElectronicsGraduate School of EngineeringNagoya UniversityNagoya, Aichi 464-8603, JapanT. NaritaToyota Central R&D Labs., Inc.Nagakute, Aichi 480-1192, JapanK. Sierakowski, M. BockowskiInstitute of High Pressure Physics Polish Academy of SciencesSokolowska 29/37, 01-142 Warsaw, PolandThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/pssr.202400074.DOI: 10.1002/pssr.202400074In Mg ion implantation doping of GaN, sequential N ion implantation reportedlychanges Mg concentrations in the Mg ion-implanted region and the underlyingregion after activation annealing. The impact of sequential N ion implantationon defects and Mg distribution after postimplantation annealing is investigated.The atomic-resolution analyses show that, in the Mg ion-implanted region,the N ion implantation increases the concentration of MgGa. It is thus concludedthat the Mg soluble in GaN by Mg ion implantation is increased by N ionimplantation. The rest of the Mg atoms agglomerate to form clusters on theextended defects, and their concentration is also increased by the N implantation.The coarsening of extended defects is suppressed by the N ion implantation:the defects in the MgþN-implanted sample are nanoscale interstitial-typedefects, and they do not grow or annihilate after annealing. This indicates that theN implantation changes the concentrations of interstitials.RESEARCH ARTICLEwww.pss-rapid.comPhys. Status Solidi RRL 2024, 18, 2400074 2400074 (1 of 6) © 2024 Wiley-VCH GmbHmailto:kano@imass.nagoya-u.ac.jphttps://doi.org/10.1002/pssr.202400074http://www.pss-rapid.comhttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fpssr.202400074&domain=pdf&date_stamp=2024-04-22crystallographic defects and Mg distribution at an atomic resolu-tion using TEM and atom probe tomography (APT). The concen-tration of ion-implanted Mg and N was 1� 1019 cm�3. Ourresults show that the defects in these samples differ significantly.Furthermore, in the Mg-implanted region, the sequential Nimplantation increased both the number of Mg atoms distributeduniformly and the number of Mg atoms agglomerated to formclusters. The role of the defects in increasing the amount of theMg atoms is discussed.2. ResultsThe Mg ion-implanted GaN samples with and without sequentialN implantation are referred to in the following as MgþN-implanted and Mg-implanted samples, respectively. The samplepreparation conditions are the same for both samples exceptfor the N ion implantation. Undoped GaN epitaxial films weregrown by metalorganic vapor phase epitaxy on free-standingGaN (0001) substrates. Mg ion implantation was performed togenerate a 300 nm-thick box-shaped concentration profile witha concentration of 1� 1019 cm�3. The depth profile of the Mgconcentration in the as-implanted sample is shown in Figure 1.N ion implantation was performed to generate the 300 nm-thickbox-shaped profile with a concentration of 1� 1019 cm�3. Weused the SRIM program to design the N concentration profile.[24]Annealing was performed in N2 atmosphere of 1.0 GPa at1300 °C for 30min. This annealing condition is reportedlyenough for activating Mg atoms doped by ion implantation.[25]The structure and density of the extended defects were exam-ined by annular dark-field (ADF) scanning TEM (STEM) at anacceleration voltage of 200 kV. The inner and outer angles ofthe ADF detector were set to 20 and 60mrad, respectively. Foratomic scale resolution, we used the high-angle ADF (HAADF)condition, where the inner and outer angles were set to 50 and150mrad, respectively. TEM specimens were prepared bymechanical thinning followed by Ar-ion milling. The TEM spec-imen thicknesses were determined by convergent beam electrondiffraction (CBED) analysis using the Mbfit program.[26] Thedetails of the thickness determination are provided in the supple-mental material. Secondary ion mass spectrometry (SIMS) anal-ysis was utilized to measure the distribution of Mg and H. In thepresent annealing process, H reportedly diffuses into GaN fromthe ambient to form a stable complex with MgGa, and the con-centration of H matches that of MgGa.[9,27] MgGa, a Mg atom atthe Ga site, reportedly acts as an acceptor in GaN.[28] Three-dimensional Mg distributions were examined by APT. APTspecimens were prepared using a focused ion beam milling,where the low energy ion beam at 2 keV was utilized in the finalstep to reduce the damage during the milling. APT measure-ments were performed with 250 kHz 355 nm UV laser pulsingat a specimen temperature of 30 K. The laser pulse energy was setto 10 fJ. Frequency distribution analysis was used to examine thelocal fluctuations in Mg concentration.[24,29]The SIMS results of the Mg concentrations measured in thesamples are shown in Figure 1. The measured H concentrations,which represent the MgGa concentrations, are also shown. A Mgconcentration peak is observed in the ion implantation region(150–250 nm depth) of the MgþN-implanted sample. The Mgconcentration in the ion-implanted region is higher for theMgþN-implanted sample than for Mg-implanted sample, whichis consistent with previous reports.[16,17] Moreover, we found thatthe H concentration averaged over the depths from 150 to250 nm is higher in the MgþN-implanted sample than in theMg-implanted sample, with concentrations of 2.1� 1018 and1.2� 1018 cm�3, respectively (Table 1). This result demonstratesthat the sequential N implantation significantly increases theconcentration of MgGa in the Mg ion-implanted region.ADF–STEM images of the Mg-implanted and MgþN-implanted samples are shown in Figure 2a,b, respectively. Inboth micrographs. the incident beam direction is <1100> andthe TEM specimen thickness is 150 nm. The small bright dotsand circles are the extended defects produced by the ion implan-tation and subsequent annealing. In the Mg-implanted sample,small bright dots and circles with 50 to 100 nm diameters wereobserved. The loops reportedly form during defect coarsen-ing.[21,30] In contrast, in the MgþN-implanted sample, we founda high density of small bright dots but no circles. These findingsindicate that the coarsening is suppressed in the MgþN-implanted sample.Figure 1. Concentration depth profiles of Mg and H after annealing. Theprofile in the as-implanted sample is also indicated. In the ion-implantedregion (to the depth of 300 nm from the surface), both the total Mg con-centration and H concentration are higher in the MgþN-implanted sam-ple (red solid and dotted lines) than in the Mg-implanted sample (blacksolid and dotted lines).Table 1. Concentrations of H and Mg. The concentrations of MgGa (orH) in the MgþN-implanted and Mg-implanted samples were measuredby SIMS. The concentrations of uniformly distributed Mg and of Mgforming clusters were estimated from APT.MgþN [1018 cm�3] Mg [1018 cm�3]H 2.1 1.2Mg (uniformly distributed) 1.7 1.0Mg (forming clusters) 2.5–4.4 0.8–2.5www.advancedsciencenews.com www.pss-rapid.comPhys. Status Solidi RRL 2024, 18, 2400074 2400074 (2 of 6) © 2024 Wiley-VCH GmbH 18626270, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssr.202400074 by National Institute For, Wiley Online Library on [19/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.pss-rapid.comThe structural details of the defect in the MgþN-implantedsample are shown in Figure 2c–d. Two types of defects with typ-ical sizes of 3–10 nm are observed. Figure 2(c-1) and (c-2), respec-tively, shows the ADF–STEM and HAADF–STEM images of thedefect denoted as type 1. In the ADF–STEM image, the defect isobserved as a bright area with a central neck (arrowed). TheHAADF–STEM image, where each bright dot shows an individ-ual Ga atomic column, indicates that an extra atomic plane isinserted at the position indicated by the edge dislocation sym-bols. The Burgers circuit shows that the Burgers vector of thedefect is b= 12 <0001>. These results indicate that the defectis an interstitial-type dislocation loop with an extra GaN c-plane.In addition, the bright area in Figure 2(c-1) represents thechanges in the diffraction condition caused by the extra c-plane.Figure 2(d-1) and (d-2) respectively show the ADF–STEM andHAADF–STEM images of the defect denoted as type 2. In theADF–STEM image, the defect is seen as a bright region dividedinto two sides by the dark region (arrowed). The Burgers circuitindicated on the HAADF–STEM image shows that b= 13<1120>. These results indicate that the defect is aninterstitial-type dislocation loop with an extra GaN a-plane.The interstitial-type dislocation loops with an extra a-plane ora c-plane are also found in Mg ion-implanted GaN (without Nimplantation) in the initial stage of annealing at 1300 °C(<30min).[19,20] However, they grow and annihilate as theannealing progresses and are barely found at 30min.[19] In con-trast, the defects in the MgþN-implanted sample remain unan-nihilated even after annealing for 30min.The volume density of the dislocation loops was4.2� 1.1� 1016 cm�3 in the depth range of 150–250 nm fromthe sample surface. In this measurement, we counted the num-ber of dislocation loops in a 100 nm square of the cross-sectionalADF–STEM images at a TEM specimen thickness of 50 nm. Athinner specimen is suitable for observing small defects sincethe background intensity of a TEM image decreases as theTEM specimen thickness decreases. In contrast, the numberof defects per measurement area decreases as the specimenthickness decreases, thus increasing the standard deviation ofthe measured number of defects. Therefore, we opted to use50 nm-thick TEM specimens for defect density measurement,and dislocation loops with an observed image size of 3 nm orlarger were counted. The standard deviation is mainly due tothe variation in the number of dislocation loops per measure-ment area.We examined the Mg distribution in the MgþN sample at anatomic resolution. Figure 3a shows the ADF–STEM image and(b) shows the Mg atom map by APT of the same specimen area.The bright dots in the STEM image show the dislocation loopsand the green dots in the atom map show Mg atoms. The arrowsin the STEM image and the atommap indicate the same positionin the APT specimen, showing that Mg atoms agglomerate toform clusters at dislocation loops. Close-ups of the Mg atommap at type 1 and 2 defects are shown in Figure 3c,d, respec-tively. The atom maps show that the Mg clusters were flatdoughnut-shaped and that the clusters at the type 1 defects wereon the c-plane (i.e., (0001) plane) while those at the type 2 defectsFigure 2. a) Cross-sectional ADF–STEM image of Mg-implanted sample and b) MgþN-implanted sample. Bright circles and nanoscale dots are defectsproduced by the ion implantation and subsequent annealing. In (b), only nanoscale dots were observed. c) ADF–STEM and HAADF–STEM images of type1 and d) type 2 defects in MgþN-implanted samples, respectively. In (c-1), the defect is observed as a bright area with a central neck (arrowed). TheBurgers circuit in (c-2) shows b= 12<0001> and the edge dislocation symbols indicate an extra c-plane. In (d-1), the defect is seen as a bright regiondivided into two sides by the dark region (arrowed). The Burgers circuit in (d-2) shows b= 13 <1120>, indicating that an extra a-plane is added to thedefect.www.advancedsciencenews.com www.pss-rapid.comPhys. Status Solidi RRL 2024, 18, 2400074 2400074 (3 of 6) © 2024 Wiley-VCH GmbH 18626270, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssr.202400074 by National Institute For, Wiley Online Library on [19/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.pss-rapid.comwere on the a-plane (i.e., (1120) plane). These results indicate thatMg atoms agglomerate around the edges of the extra planes ofthe dislocation loops and that the strain fields generated by theextra planes attract Mg atoms to form clusters, as discussed inour previous work.[19] The APT results show that the averagenumber of Mg atoms per flat doughnut-shaped cluster was 56.5.Using the APT data, we classified Mg atoms in the Mg ion-implanted region as either clustered or uniformly distributed.Frequency distribution analysis was performed to estimate theconcentration of uniformly distributed Mg atoms (see the sup-plemental material for details). The results are listed inTable 1. The concentration of Mg atoms forming clusters isobtained by subtracting that of uniformly distributed Mg atomsfrom that of total Mg atoms estimated from APT analysis. Theconcentration of the Mg atoms forming clusters varied slightlydepending on the measurement area. This is mainly due to thesmall measurement volume of APT (typically 1� 105 nm3); thatis, the number of Mg clusters, which affects the amount of theMg atoms in the area, varies from one measurement area toanother. The results in Table 1 indicate that the concentrationof Mg atoms forming clusters is larger in the MgþN-implantedsample than in the Mg-implanted sample. Moreover, the concen-tration of uniformly distributed Mg atoms is larger in theMgþN-implanted sample than in the Mg-implanted sample.This strongly suggests that the concentration of Mg soluble inGaN is higher in the MgþN-implanted sample compared tothe Mg-implanted sample.3. DiscussionThese SIMS and APT analyses indicate that the concentration ofH is comparable to that of uniformly distributed Mg atoms ineach sample (Table 1). This suggests that the uniformly distrib-uted Mg atoms occupy Ga sites (MgGa) and that only a small partof Mg atoms in the clusters form stable complexes with H atoms.More importantly, the concentration of H in the MgþN-implanted sample is remarkably higher than that in theMg-implanted sample. Previous studies have indicated thatthe concentration of Mg soluble in GaN in doping by Mg ionimplantation is 1.3� 1018 cm�3 after postimplantation anneal-ing,[15] which is comparable to the concentration of MgGa andthat of uniformly distributed Mg atoms in the Mg-implantedsample in the present experiment. Thus, these results indicatethat the sequential N ion implantation increases the concentra-tion of Mg soluble in GaN in doping by Mg ion implantation.Since MgGa forms through the reaction between a Ga vacancyand a Mg atom, and since the N ion implantation forms Gavacancies in addition to those produced by Mg implantation,[17]the increased Ga vacancy concentration in theMgþN-implantedsample could be a cause for the increased MgGa concentration inthe sample.Next, we estimated the percentage of Mg atoms agglomeratedon the defects in the Mg forming cluster. The number of Mgatoms agglomerated at the dislocation loops in the MgþN sam-ple is given by the product of the dislocation loop density(4.2� 1016 cm�3 in Figure 2b) and the number of Mg atomsper dislocation loop (56.5 in Figure 3); that is, 2.4� 1018 cm�3.This indicates that the amount of Mg atoms agglomerated at thedislocation loops accounts for more than half of that of the Mgatoms forming clusters (2.5–4.4� 1018 cm�3). In addition, dislo-cation loops that are smaller than those examined in our experi-ment (smaller than 3 nm) are also accompanied by a strain fieldand would cause Mg cluster formation. Therefore, we can con-clude that the Mg agglomeration at the dislocation loops is themain cause for the Mg cluster formation in the MgþN-implanted sample.Our TEM analysis shows that small interstitial-type dislocationloops are observed in the MgþN-implanted sample. This meansthat coarsening, i.e., annihilation and growth, of the dislocationloops was significantly suppressed. The annihilation and growthof the interstitial-type dislocation loops depend directly on theconcentration of interstitials around the dislocation loops.[22,23]Namely, since the dislocation loop has an extra GaN a- or c-plane(Figure 2), the dislocation loop annihilates (grows) when bothconcentrations of Ni and Gai around the dislocation loop arebelow (above) the equilibrium concentration of the dislocationloop. Thus, the results observed in the MgþN sample suggestthat either the concentration of Ni or Gai is above the equilibriumconcentration and that the other concentration is below it.Therefore, the results in Figure 2 indicate that the N ion implan-tation changed the concentrations of Ni and Gai in theMg-implanted sample to suppress the annihilation and growthFigure 3. a) ADF–STEM and b) Mg atom map by APT of the same speci-men area of the MgþN-implanted sample. Green dots in b) show Mgatoms. Arrows indicate the same specimen position, showing that Mgforms clusters at dislocation loops. Close-ups of Mg clusters viewed inthe orthogonal directions: (c-1), (c-2) cluster at type 1 defect and (d-1),(d-2) cluster at type 2 defect. Clusters are flat doughnut-shaped, andthe cluster at the type 1 defect is on the c-plane while that at the type2 defect is on the a-plane.www.advancedsciencenews.com www.pss-rapid.comPhys. Status Solidi RRL 2024, 18, 2400074 2400074 (4 of 6) © 2024 Wiley-VCH GmbH 18626270, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/pssr.202400074 by National Institute For, Wiley Online Library on [19/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.pss-rapid.comof the nanoscale dislocation loops. The TEM analysis also showsthat no vacancy-type dislocation loops were found in the MgþN-implanted sample. We previously reported that a GaN a-plane ismissing in a vacancy-type dislocation loop in Mg ion-implantedGaN,[20] which indicates that the same number of VGa and VN areneeded to form the vacancy-type dislocation loop. Furthermore,the sequential N ion implantation reportedly reduces the concen-tration of VN,[18] while it increases the concentration of VGa in theMg ion-implanted GaN.[16,17] Thus, in the present experiment,the sequential N implantation might reduce the concentrationof VN to suppress the formation of vacancy-type dislocation loops.4. ConclusionWe investigated the impact of sequential N ion implantation ondefects and Mg distribution in Mg ion-implanted GaN after post-implantation annealing at 1300 °C for 30min. The concentrationof ion-implanted Mg and N was 1� 1019 cm�3. Atomic-resolutionanalyses performed in the Mg ion-implanted region showed thatthe concentrations of both uniformly distributed Mg atoms andMgGa are increased by the N ion implantation. In addition, theseconcentrations coincide with each other in the MgþN-implanted and Mg-implanted samples. We thus conclude thatthe Mg soluble in GaN by Mg ion implantation was increasedby the N ion implantation. The increased solubility can be attrib-uted to the increased VGa by the N implantation. Our analysesalso indicate that the rest of the Mg atoms formed clusters onthe extended defects and that the amount of these Mg atomswas increased by the N ion implantation. We found that thecoarsening of extended defects was suppressed by the N ionimplantation: the defects in the MgþN-implanted sample werenanoscale interstitial-type defects, and they did not grow or anni-hilate after annealing. This indicates that the N implantationchanged the concentrations of interstitials, which in turn affectedthe coarsening of the defects. Furthermore, no vacancy-typeextended defects were observed in the MgþN-implanted sam-ple, which suggests that the concentration of VN is reduced bythe N implantation.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThe authors would like to thank Prof. Koh Saito of Nagoya University andProf. Kenji Tsuda of Tohoku University for their invaluable discussions onthe CBED measurement of specimen thickness. Part of this work was sup-ported by the MEXT “Program for Research and Development of Next-Generation Semiconductor to Realize Energy-Saving Society” (JPJ005357)and the “Program for Creation of Innovative Core Technology for PowerElectronics” (JPJ009777).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.KeywordsGaN, Mg, N ion-implantation, transmission electron microscopyReceived: March 7, 2024Revised: April 7, 2024Published online: April 22, 2024[1] T. Oka, T. Ina, Y. Ueno, J. Nishii, Appl. Phys. Express 2015, 8, 6.[2] R. Tanaka, S. Takashima, K. Ueno, H. Matsuyama, M. Edo, Jpn. J.Appl. Phys. 2020, 59, SGGD02.[3] J. Liu, M. Xiao, R. Zhang, S. Pidaparthi, H. Cui, A. Edwards,M. Craven, L. Baubutr, C. Drowley, Y. Zhang, IEEE Trans. ElectronDevices 2021, 68, 2025.[4] T. Kachi, T. Narita, H. Sakurai, M. Matys, K. Kataoka, K. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.pss-rapid.com Impact of Sequential N Ion Implantation on Extended Defects and Mg Distribution in Mg Ion-Implanted GaN 1. Introduction 2. Results 3. Discussion 4. Conclusion