Selective-area growth of doped GaN nanorods by pulsed-mode MOCVD: Effect of Si and Mg dopants

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1 Phy. Statu Solidi B, (2017) / DOI /pb Selective-area growth of doped GaN nanorod by puled-mode MOCVD: Effect of Si and Mg dopant phyica tatu olidi baic olid tate phyic Si-Young Bae *,1, Kaddour Lekhal **,1, Ho-Jun Lee 2, Jung-Wook Min 3, Dong-Seon Lee 3, Yohio Honda 1, and Hirohi Amano 1,4 1 Intitute of Material and Sytem for Sutainability, Nagoya Univerity, Nagoya, Aichi , Japan 2 Department of Electrical Engineering and Computer Science, Nagoya Univerity, Nagoya, Aichi , Japan 3 School of Electrical Engineering and Computer Science, Gwangju Intitute of Science and Technology, Gwangju, Korea 4 Akaaki Reearch Center, Nagoya Univerity, Nagoya, Aichi , Japan Received 30 October 2016, revied 6 March 2017, accepted 23 March 2017 Publihed online 13 April 2017 Keyword doping, GaN, MOCVD, nanorod, puled-mode growth, elective epitaxy * Correponding author: iyoubae@gmail.com, Phone: þ , Fax: þ ** lekhal.kaddour@yahoo.fr, Phone: þ , Fax: þ Injecting current with a uniform carrier concentration i important for application with three-dimenional architecture uch a vertical power device or diplay. In III-nitride nanotructure, dopant not only incorporate differently depending on the urface orientation but can alo eriouly affect the kinetic equilibrium hape of the nanorod. Herein, we report elective-area growth of doped GaN nanorod grown by puled-mode metalorganic chemical vapor depoition. Two dopant, Si and Mg, were employed a donor and acceptor atom, repectively, for a mono-doping approach. Furthermore, a mixed flow of Si and Mg wa upplied for a co-doping approach. We compared the morphological effect and growth rate of each doped GaN nanorod array. Then, we propoed appropriate growth mechanim for the doped GaN nanorod on the bai of our tructural characterization. Thee reult might extend the morphological functionality of GaN nanorod by including doping and may alo provide an appropriate foundation for the deign of nanotructure-baed electronic or photonic device. 1 Introduction Selective-area growth of III-nitride nanotructure ha gained coniderable reearch interet for nanocale electronic and photonic application [1, 2]. Compared with the vapor liquid olid method, the elfcatalytic behavior of elective-area growth can provide poitional and morphological control of three-dimenional (3D) building block with le contamination [3]. In addition, the polarity of a electively grown nanotructure can be controlled uniformly by following that of the mother template, e.g., a already demontrated for Ga-polar GaN nanorod [4]. High-quality crytal are alo a primary feature of III-nitride nanotructure owing to dilocation bending and filtering a well a train relaxation [5, 6]. Furthermore, for device application uch a vertical power device and diplay, high apect ratio of III-nitride nanorod array can provide lower current denitie per unit chip ize, large active volume, and piezoelectric-field-free nonpolar urface [7]. Until now, mot reearch on the growth of III-nitride nanotructure ha focued on demontrating rod-like hape, i.e., nanorod and nanowire, becaue uch architecture form the bai of axial and radial 3D device. The growth of o-called core GaN nanorod, in particular, utilize Si flow to dope GaN emiconductor; the Si flow critically affect the morphology and growth rate during metalorganic vapor depoition (MOCVD) with a continuou ource flow [8]. A higher Si flow rate are upplied, a higher Si concentration or SiN x paivation occur at the idewall of the GaN nanorod, thereby enhancing the axial growth rate and uppreing the lateral growth rate [9 11]. Thi i beneficial for high apect ratio nanorod; however, a Si doping concentration that i too high may degrade the crytal quality and optical propertie of GaN nanorod [12]. Converely, le ha been reported on the effect of acceptor, e.g., Mg, on the facet-dependent growth rate and

2 phyica p tatu olidi b (2 of 7) S.-Y. Bae et al.: Selective-area growth of doped GaN nanorod by puled-mode MOCVD crytal quality for elective-area growth of GaN nanorod. Becaue high Mg flow rate lead to coniderable lateral growth behavior, the growth of highly elongated p-type GaN nanorod may be difficult to achieve in a elective epitaxial manner uing MOCVD [13]. In addition, the growth of the outermot p-gan hell layer around the n-gan core tructure (i.e., p-on-n type contact) commonly reult in a change of the equilibrium hape from nonpolar to emipolar plane a well a in huge thickne variation between the bottom and the top region [14]. Becaue dopant incorporation i in part governed by the urface orientation, controlled growth of the p-type GaN hell or core tructure may enable uniform current injection in 3D device [15]. It i alo anticipated that the operation mode of electronic device can be elected owing to the availability of the p-type core tructure [16]. One poible olution to prevent the lateral growth of Mg-doped GaN nanorod i to co-dope the material with both Si and Mg. In principle, co-doping offer everal benefit to III V emiconductor, uch a enhanced olubilitie, lowered ionization energie, and controlled carrier mobilitie and concentration [17]. In particular, native donor with a nitrogen vacancy or it complexe, which limit p-type doping, can be uppreed by everal mechanim uch a complex formation, decreae of compenation, increae in acceptor incorporation, and creening effect [17]. A a conequence, the effectivene of co-doping ha already been demontrated in both n- and p-type GaN epilayer with Si and Mg dopant [18, 19]. However, co-doping ha rarely been employed in GaN nanorod. Motivated by the above iue, we here report the morphological evolution of GaN nanorod with Si and Mg dopant. To grow highly elongated GaN nanorod, we have employed puled-mode MOCVD [20, 21]. By comparing the growth behavior of GaN nanorod with either monodoping or co-doping, we are able to propoe the mechanim that may govern the growth of the doped GaN nanorod. 2 Experimental AlN epitaxial layer of 90-nm thickne were prepared on Si(111) ubtrate for the mother template. For the making layer for elective-area growth, a 30-nm-thick SiO 2 layer wa depoited by radio frequency magnetron puttering. A polymer-reit hole array wa formed uing thermal nanoimprinting and ubequently tranferred to the SiO 2 layer by reactive ion etching with a CF 4 ga mixture. The diameter and pitch of the SiO 2 hole array were 460 and 1380 nm, repectively. The fabricated template were loaded into the MOCVD reactor for puled-mode growth. A trimethylgallium (TMG) flow rate of 15 ccm (78 mmol min 1 ) and ammonia (NH 3 ) flow rate of 5 lm ( mmol min 1 ) were utilized a the Ga and N ource, repectively. The valve operation during one cycle in puled mode compried four equential tep: TMG ON (5 ), purge (1 ), NH 3 ON (15 ), and purge (1 ). In total, there were 150 cycle. The carrier ga wa H 2. The growth temperature and reactor preure were fixed at Table 1 Experimental et and the flow rate range of the dopant for doping the GaN nanorod. experimental et dopant controlled parameter flow range (ccm) I Si Si(CH 3 ) II Mg EtCp 2 Mg III Si þ Mg Si(CH 3 ) (50 a ) EtCp 2 Mg a Si(CH 3 ) 4 flow of 50 ccm wa only applied for the Mg/Si flow ratio of C and 200 Torr, repectively. Further detail of the puled-mode growth procedure have been reported previouly [20, 21]. It hould be noted that there wa a continuou flow of the dopant ource during the growth of the GaN nanorod in all doping procee. For Si and Mg doping, Si(CH 3 ) 4 and EtCp 2 Mg were continuouly injected into the reactor regardle of the puled-mode operation of the TMG and NH 3 ource. Table 1 how the experimental et produced in thi tudy, where experiment I and II adopted mono-doping with Si and Mg dopant, repectively, wherea co-doping with two dopant wa ued for experiment III. The urface morphology and ize of the nanotructure were characterized by field-emiion canning electron microcopy (FE-SEM; Hitachi SU70). To acquire the tatitical ditribution of the nanotructure diameter and height, the image proceing program ImageJ wa ued [22]. Their cro-ectional microtructure were analyzed uing canning tranmiion electron microcopy (STEM; HD-2700) after preparing liced pecimen with a focued ion beam ytem. 3 Reult and dicuion 3.1 Effect of Si doping Figure 1 how tilted-view (panel (a) (d)) and ide-view (panel (e) (h)) SEM image of nanotructure grown with a Si(CH 3 ) 4 flow rate range of ccm. Thi correpond to the experimental et I in Table 1. Overall, the reult how almot no difference in the kinetic equilibrium hape, with hexagonal pyramidal hape in the top region. Conidering the tilt angle of the hexagonal pyramid in the ide view how that mot of the top urface are compoed of emipolar 1012g plane with an incline of 528. At the maximum flow rate of 100 ccm, we partially oberved an exceptional elongation of the GaN nanorod. In thi cae, the top urface not only conited of emipolar 1011g plane with an incline of 618 but the plane were alo aniotropically hifted to one ide. In addition, flat top urface were often oberved on the elongated nanorod. However, in mot cae, nonpolar idewall and emipolar top urface were oberved, indicating the increaed energetic tability of thee urface during puled-mode growth compared with that of axial c-plane urface. The ditribution of the vertical and lateral length of the nanorod i plotted in Fig. 2. The lateral length (diameter) were almot contant, wherea the vertical

3 Original Paper Phy. Statu Solidi B (2017) (3 of 7) Figure 1 (a d) Tilted-view and (e h) ide-view of SEM image of GaN nanorod doped uing Si(CH 3 ) 4 flow rate in the range of ccm. length gradually increaed a the Si(CH 3 ) 4 flow wa increaed. Surpriingly, the diameter of the nanorod were almot comparable to the diameter of a SiO 2 hole, indicating that lateral growth wa almot completely uppreed under certain growth condition. The increae of the vertical growth rate with increaing Si flow rate i in agreement with imilar behavior reported previouly [8, 9]. It i noteworthy that a lower V/III ratio hould generally be adopted for growing nanorod via MOCVD compared with for normal GaN epilayer [8, 23, 24]. The Ga-rich condition not only uppree trong H 2 paivation on the emipolar plane, but alo lead to the ready formation of Ga-bilayer, indicating enhanced growth along the axial c-direction [4, 25]. Furthermore, a theoretical model of the urface recontruction predicted that the formation of Ga-bilayer could be further promoted with Si-rich condition [25]. The growth behavior in thi tudy with Si-doping are thu in good agreement with the theoretical prediction. In the Figure 2 Ditribution of the vertical and lateral length of GaN nanorod doped with Si(CH 3 ) 4 flow rate in the range of ccm. following ection, we focu on dicuing the exceptional elongation of the heavily Si-doped GaN. 3.2 Effect of Mg doping Figure 3 how tilted-view (panel (a) (d)) and ide-view (panel (e) (h)) SEM image of nanotructure grown with an EtCp 2 Mg flow rate range of ccm, i.e., for the experimental et II in Table 1. The kinetic equilibrium hape of the top region gradually changed from being hexagonal pyramid to being flat urface. The hexagonal pyramid and truncated hexagonal pyramid were compoed of emipolar 1012g plane with an incline of 528. In the SEM image in Fig. 3, laterally enhanced growth feature reported previouly were not prominent depite a viible flattening of the top urface [13]. Figure 4 how the ditribution of the vertical and lateral length of nanorod doped with an EtCp 2 Mg flow rate range of ccm. Indeed, the lateral growth rate wa nearly contant, a wa the cae for the Si-doped GaN nanorod reported in the previou ection. The average height of Mg-doped GaN nanorod wa lightly increaed, but the increae wa not higher than that of the Si-doped GaN nanorod. Thi mean that the Mg flow rate doe not affect the axial growth rate of GaN nanorod a much a that of the Si dopant. It hould be highlighted that the nanorod height variation wa larget at the maximum EtCp 2 Mg flow rate of 1000 (Fig. 4). To undertand the morphological evolution of the top region and the fluctuation of the axial growth rate, we need to refer the theoretical model of the urface recontruction, which depend on the urface orientation. Akiyama et al. [26] reported that H-terminated urface with ubtitutional Mg (4N H þ Mg Ga ) are eaily tabilized over a wide chemical potential range compared with polar (0001) urface. In elective epitaxy, thee reaction uppre trong H 2 paivation of the emipolar plane, thereby increaing the growth rate of the emipolar plane. Hence, a flattened c-plane top urface appearing on the GaN nanorod with a high Mg flow rate i a reult of the competition of the growth velocity among kinetic Wulff

4 phyica p tatu olidi b (4 of 7) S.-Y. Bae et al.: Selective-area growth of doped GaN nanorod by puled-mode MOCVD Figure 3 (a d) Tilted-view and (e h) ide-view SEM image of GaN nanorod doped with EtCp 2 Mg uing a flow rate range of ccm. plane [27]. However, the reaon for the broad diperion of the nanorod height with a high Mg flow rate i not clearly undertood yet. We believe that the aniotropic growth rate of each emipolar plane i mainly affected by the variation of the axial growth rate ince the oberved emipolar urface were not equipped with complete ixfold ymmetry, a hown in Fig. 3(e) and (f). The light increae of the vertical growth rate might be caued by the formation of the Ga-bilayer under a high Mg flow rate a the c-plane urface tabilized [26, 28]. 3.3 Effect of Mg and Si co-doping In the lat two ection, we reported that a high Si flow rate increae the vertical growth rate of GaN nanorod with almot imilar kinetic equilibrium hape being formed in the top region, wherea a high Mg flow rate caued a flattening of the top urface with an almot contant growth rate. For practical optoelectronic application, taller nanorod are favorable a they produce more efficient device by lowering the current Figure 4 Ditribution of vertical and lateral length of GaN nanorod doped with EtCp 2 Mg flow rate in the range of ccm. denity and expanding the active area [7, 29]. Hence, to retain the high vertical growth rate during the growth of codoped GaN nanorod, the EtCp 2 Mg flow rate wa adjuted while fixing the Si(CH 3 ) 4 flow rate to it maximum of 100 ccm (ee alo experimental et III in Table 1). Figure 5 how tilted-view (panel (a) (d)) and ide-view (panel (e) (h)) SEM image for a Mg/Si flow rate ratio range of We oberved that the kinetic equilibrium hape in the top region were compoed of emipolar 1012g plane except for under a Mg/Si flow rate of 1 (Fig. 5(b) and (f)). Unlike the cae in which a flat top urface wa oberved for the maximum Mg flow rate of 1000 ccm (Fig. 3(d)), the codoped GaN nanorod diplayed a flat-topped urface only with a mall Mg flow rate of 100 ccm. We alo intermittently found exceptional elongation of the GaN nanorod with low-content Mg co-doping (Fig. 5(e)), imilar to that found for the heavily Si-doped GaN nanorod. A detailed ditribution of the vertical and lateral length of the GaN nanorod doped under a mixed flow of EtCp 2 Mg and Si(CH 3 ) 4 i preented in Fig. 6. The diameter of all co-doped GaN nanorod were almot contant, a wa the cae for the Si- and Mg-doped GaN nanorod. Hence, we can conclude that the diameter of the GaN nanorod grown by puled-mode MOCVD i not ignificantly affected by the dopant, indicating almot complete uppreion of the lateral growth. However, the average height of the GaN nanorod gradually decreaed a the Mg flow rate increaed. The height variation wa alo reduced by increaing the Mg flow rate. Thi indicate that the formation of Ga bilayer wa uppreed ince both Si and Mg ubtitute the ame Ga ite. In other word, the vertical growth rate wa reduced owing to the Si and Mg atom occupying the Ga ite. Hence, flat top urface produced with a Mg flow rate of 100 ccm can be oberved in Fig. 5(b); thee tructure formed more eaily with co-doping compared with for pure Mg-doping. We alo oberved mall tower-like nanotructure around the top region (Fig. 5(d)). Thi might be attributed to econdary nucleation on the GaN nanorod

5 Original Paper Phy. Statu Solidi B (2017) (5 of 7) Figure 5 (a d) Tilted-view and (e h) ide-view SEM image of GaN nanorod doped with a mixed flow of EtCp 2 Mg and Si(CH 3 ) 4. Note that the x- and y-value indicate the actual flow rate of EtCp 2 Mg and Si(CH 3 ) 4, repectively, where of Mg:Si ¼ x:y. owing to an exce of Mg atom, which reult in Mg precipitate and defect complexe [30, 31]. To ummarize, exce Mg uppree the exceptional elongation of the GaN nanorod that can be achieved for GaN nanorod grown under a high Si flow rate. 3.4 Polarity and atomic tructure Let u now conider the polarity of the doped GaN nanorod. Since we grew the GaN nanorod on Al-polar AlN on Si(111) template, the upper GaN nanorod maintain a metal polarity in general, i.e., a Ga polarity. In addition, the ixfold emipolar plane of the top region commonly indicate Gapolar GaN. However, it i neceary to invetigate the polarity of the flat top urface of the GaN nanorod becaue uch flat urface are likely to partially or fully include N polarity [4, 32]. In particular, N polarity can eaily be ditinguihed from common Ga polarity by KOH wet etching owing to their etch electivity [33]. We dipped the Figure 6 Ditribution of the vertical and lateral length of GaN nanorod doped with a mixed flow of EtCp 2 Mg and Si(CH 3 ) 4. Note that the Mg/Si flow rate ratio wa adjuted within the range of grown GaN nanorod into 5 mol KOH olution at 42 8C for 3 min. Figure 7 how tilted-view SEM image of the GaN nanorod after the KOH treatment. No etching wa oberved on the undoped and Mg-doped GaN nanorod in Fig. 7(a) and (b), repectively, indicating Ga-polar GaN nanorod. Although there wa a poibility that the polarity would be inverted at high Mg concentration, we did not oberve uch a behavior [34]. Converely, partially etched region were oberved for the exceptionally elongated GaN nanorod produced under a high Si flow rate (Fig. 3(c)). Thi confirm that one-dimenional inverion domain critically affect the exceptional elongation and aniotropic ymmetry of the top region. Figure 4(d) how the morphology of the KOHetched GaN nanorod fabricated uing co-doping. Depite uing the maximum Si flow rate of 100 ccm, there were no etched region with Mg co-doping, indicating that the formation of inverion domain wa uppreed. There have been everal report regarding the enhanced axial growth rate with Si flow that attribute it to SiN x paivation on the idewall of the GaN nanorod [9, 11]. To probe whether SiN x wa indeed formed on the idewall of the nanorod, we tudied STEM image of heavily Si-doped GaN nanorod. Figure 8 preent a bright field STEM image of a ingle Si-doped GaN nanorod (left) and an enlarged image of it idewall (right). Depite the clear view of the atomically tacked tructure, no SiN x paivation layer wa evident on the idewall of the GaN nanorod. Although the SiN x layer wa not clearly oberved in thi tudy, there i high probability that a high denity of Si atom i preent on nanorod idewall [10]. Wang et al. [8] alo reported that when a higher Si flow i applied to the GaN nanorod, a higher axial growth rate can be obtained irrepective of the formation of the SiN x layer. Hence, the enhanced axial growth rate demontrated in thi tudy when a large amount of Si dopant wa ued may be attributed to the high denity of Si atom on the idewall, which are motly inviible in TEM image. On the other hand, there have been everal report that GaN nanorod have height variation provided

6 phyica p tatu olidi b (6 of 7) S.-Y. Bae et al.: Selective-area growth of doped GaN nanorod by puled-mode MOCVD Figure 7 Tilted-view SEM image of (a) undoped and (b) Mg-doped nanorod produced with an EtCp 2 Mg flow rate of 1000 ccm. Panel (c) how nanorod grown with Si-doping with a Si(CH 3 ) 4 flow rate of 100 ccm, and panel (d) how the co-doped GaN nanorod after KOH treatment. that they include inverion domain [4, 35]. Becaue the formation energie of the inverion domain are different from the urrounding GaN matrix, dilocation and grain boundarie are energetically favorable [36]. Thee defect generally act a nucleation ite during crytal growth. In ummary, a high flow of Si dopant trongly influence the axial growth rate, wherea inverion domain may be reponible for the extraordinarily elongated nanorod. 4 Concluion In thi paper, we tudied the growth behavior of doped GaN nanorod by varying the dopant. Depite the large range of pure and or mixed dopant flow rate that we upplied during growth, the diameter of the electively grown GaN nanorod were nearly contant in puled-mode MOCVD. We briefly ummarize the feature of the doped GaN nanorod a follow. (i) For Si doping, the height of the rod gradually increaed and the height variation increaed a the Si flow rate wa increaed; thi wa poibly due to the formation of Ga-bilayer. Meanwhile, the ultra-long nanorod were formed by axial polarity inverion. (ii) Converely, higher Mg flow rate affected the equilibrium hape of the nanorod top region and caued height variation intead of promoting vertical growth; additionally, the top urface wa flattened at high Mg flow rate. (iii) Si and Mg co-doping not only maintained the higher axial growth rate, but alo effectively reduced the height variation. Although many more tudie are required to characterize the optical and electrical propertie of the doped GaN nanorod, thee reult ugget Figure 8 Bright field STEM image of a Si-doped GaN nanorod (left) and an enlarged image of it idewall (right). that there i cope for tuning the morphology of the nanorod, their height, and equilibrium hape while reducing ize variation by carefully electing the doping condition. Therefore, thi approach might contribute to the deign of nanocale vertical GaN p n diode with uniform carrier injection. It may alo provide experimental data to upplement theoretical tudie related to impuritydependent urface recontruction. Acknowledgement Thi work wa upported by JSPS and CNRS under the Japan France Reearch Cooperative Program and the Nobel Reearch Center (Amano LED Reearch Center) Project through a grant provided by GIST in Reference [1] K. Tomioka, T. Tanaka, S. Hara, K. Hiruma, and T. Fukui, IEEE J. Sel. Top. Quantum Electron. 17, 1112 (2011). [2] R. Yan, D. Garga, and P. Yang, Nature Photon. 3, 569 (2009). [3] J. Noboriaka, J. Motohia, and T. Fukui, Appl. Phy. Lett. 86, (2005). [4] S. F. Li, S. Fuendling, X. Wang, S. Merzch, M. A. M. Al- Suleiman, J. D. Wei, H.-H. Wehmann, A. Waag, W. Bergbauer, and M. Straburg, Cryt. Growth De. 11, 1573 (2011). [5] R. Colby, Z. Liang, I. H. Wildeon, D. A. Ewoldt, T. D. Sand, R. E. Garcıa, and E. A. Stach, Nano Lett. 10, 1568 (2010). [6] K. Kihino and S. Ihizawa, Nanotechnology 26, (2015). [7] A. Waag, X. Wang, S. F undling, J. Ledig, M. Erenburg, R. Neumann, M. Al Suleiman, S. Merzch, J. Wei, S. Li, H. H. Wehmann, W. Bergbauer, M. Straßburg, A. Trampert, U. Jahn, and H. Riechert, Phy. Statu Solidi C 8, 2296 (2011). [8] X. Wang, S. Li, M. S. Mohajerani, J. Ledig, H.-H. Wehmann, M. Mandl, M. Straburg, U. Steegm uller, U. Jahn, J. L ahnemann, H. Riechert, I. Griffith, D. Chern, and A. Waag, Cryt. Growth De. 13, 3475 (2013). [9] C. Tearek, M. Heilmann, E. Butzen, A. Haab, H. Hardtdegen, C. Dieker, E. Spiecker, and S. Chritianen, Cryt. Growth De. 14, 1486 (2014). [10] Z. Fang, E. Robin, E. Roza-Jimenez, A. Cro, F. Donatini, N. Mollard, J. Pernot, and B. Daudin, Nano Lett. 15, 6794 (2015). [11] R. K oter, J.-S. Hwang, C. Durand, D. L. S. Dang, and J. Eymery, Nanotechnology 21, (2009). [12] X. Wang, J. Hartmann, M. Mandl, M. S. Mohajerani, H.-H. Wehmann, M. Straburg, and A. Waag, J. Appl. Phy. 115, (2014).

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