Growth of Catalyst-Free Hexagonal Pyramid-Like Inn Nanocolumns on Nitrided Si(111) Substrates Via Radio-Frequency Metal–Organic Molecular Beam Epitaxy

crystals

Article Growth of Catalyst-Free Hexagonal Pyramid-Like InN Nanocolumns on Nitrided Si(111) Substrates via Radio-Frequency Metal–Organic Molecular Beam Epitaxy

Wei-Chun Chen 1,* , Tung-Yuan Yu 2, Fang-I Lai 3, Hung-Pin Chen 1, Yu-Wei Lin 1 and Shou-Yi Kuo 4,5,* 1 Taiwan Instrument Research Institute, National Applied Research Laboratories, 20. R&D Rd. VI, Hsinchu Science Park, Hsinchu 30076, Taiwan; [email protected] (H.-P.C.); [email protected] (Y.-W.L.) 2 Taiwan Semiconductor Research Institute, National Applied Research Laboratories, 26, Prosperity Road I, Hsinchu Science Park, Hsinchu 30078, Taiwan; [email protected] 3 Electrical Engineering Program C, Yuan-Ze University, 135 Yuan-Tung Road, Chung-Li 32003, Taiwan; ﬁ[email protected] 4 Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Taoyuan 33302, Taiwan 5 Department of Urology, Chang Gung Memorial Hospital, Linkou, No.5, Fuxing Street, Kwei-Shan, Taoyuan 333, Taiwan * Correspondence: [email protected] (W.-C.C.); [email protected] (S.-Y.K.); Tel.: +886-3577-9911 (ext. 646) (W.-C.C.); +886-3211-8800 (ext. 3351) (S.-Y.K.) Received: 1 May 2019; Accepted: 3 June 2019; Published: 5 June 2019

Abstract: Hexagonal pyramid-like InN nanocolumns were grown on Si(111) substrates via radio-frequency (RF) metal–organic molecular beam epitaxy (MOMBE) together with a substrate nitridation process. The metal–organic precursor served as a group-III source for the growth of InN nanocolumns. The nitridation of Si(111) under flowing N2 RF plasma and the MOMBE growth of InN nanocolumns on the nitrided Si(111) substrates were investigated along with the effects of growth temperature on the structural, optical, and chemical properties of the InN nanocolumns. Based on X-ray diffraction analysis, highly <0001>-oriented, hexagonal InN nanocolumns were grown on the nitride Si(111) substrates. To evaluate the alignment of arrays, the deviation angles of the InN nanocolumns were measured using scanning electron microscopy. Transmission electron microscopy analysis indicated that the InN nanocolumns were single-phase wurtzite crystals having preferred orientations along the c-axis. Raman spectroscopy confirmed the hexagonal structures of the deposited InN nanocolumns.

Keywords: InN nanocolumns; nitridation; nitrided Si(111)

1. Introduction Nanocolumns (NCs) have proven eﬀective for lateral stress relaxation; thus, they exhibit low defect densities in III-nitride heterostructures. This property is particularly beneﬁcial for systems with large lattice mismatch, such as indium nitride (InN), which is typically grown on foreign substrates. InN is an interesting and potentially important III-nitride semiconductor material with superior electronic transport properties [1]. InN has attracted considerable research attention concerning the development of optoelectronic devices (e.g., full-color light-emitting diodes, solar cells, and optochemical sensors [2]) because of its room temperature direct optical band gap in the near-infrared region (Eg = 0.7 eV) [3]. Moreover, the electron mobility of InN can exceed 2100 cm2/(V s) with a residual carrier concentration ·

Crystals 2019, 9, 291; doi:10.3390/cryst9060291 www.mdpi.com/journal/crystals Crystals 2019, 9, 291 2 of 11 approaching 8 1017 cm 3 at room temperature [4]. However, the relatively low decomposition × − temperature (<600 ◦C) in the III-nitride materials [5], large lattice mismatch with universal substrates, and impurity-prone surface of InN hinder the development of InN-related devices. Furthermore, the narrow growth window of InN (between 500 ◦C and 550 ◦C) limits device processing. InN nanocolumns have been grown on Si(111) substrates with buffer layers such as Si3N4 [6], GaN, low-temperature InN [7], AlN/-Si3N4 double-layers [8], InN/-Si3N4 double-layers [9], and InGaN [10]. Al2O3 has been widely used as a substrate for heteroepitaxial InN growth despite its insulating property and large a-axis lattice mismatch with InN (~25%). Si(111) is a more suitable substrate for InN growth because it has a smaller lattice mismatch with InN (~8%), better doping properties, and greater thermal conductivity compared to Al2O3. β-Si3N4 has a lattice constant of 7.602 Å along its a-axis [11], which is only slightly larger than twice the lattice constant of InN (a = 3.548 Å). If twice the lattice constant of InN is considered a nominal lattice constant, the lattice mismatch between Si and InN will be small. InN nanocolumns have been grown using various methods, including radio-frequency (RF) molecular beam epitaxy (MBE) [12], RF sputtering [13], and metal–organic chemical vapor deposition (MOCVD) [14]. However, because of the low dissociation efficiency of ammonia, these methods require large amounts of ammonia to obtain the flux of active nitrogen required for producing high-quality nitride. Although substantial progress has been made in the development of plasma sources to mitigate this nitrogen problem, the InN growth rates of the above methods are still lower than that achieved via metal–organic molecular beam epitaxy (MOCVD). RF-MOMBE, which is performed in a growth 9 chamber with a load lock and a base pressure of ~10− Torr, combines the characteristics of both MBE and MOCVD and is suitable for the mass production of InN. In the RF-MOMBE growth of InN, a precursor such as trimethylaluminum or trimethylindium (TMIn) serves as the group-III precursor, and atomic nitrogen generated by RF plasma is the source of N. Due to low dissociation temperature and high N2 equilibrium pressure over InN, plasma-assisted systems are thought to be suitable for InN growth. Compared to the MOCVD growth of epitaxial nitride films, RF-MOMBE generally allows for a lower growth temperature [15]. In our previous study, the RF-MOMBE growth temperatures of InN-related alloys were all lower than the corresponding MOCVD growth temperatures [16]. Despite the promising characteristics of RF-MOMBE, few studies have reported the RF-MOMBE growth of hexagonal pyramid-like InN nanocolumns. Therefore, in this study, we focused on nitride growth at low temperature, including AlN growth at 800–900 ◦C, InN growth at 450–550 ◦C, and GaN growth at 750–850 ◦C. According to previous studies, InN materials grown via RF-MOMBE have often been deposited often sapphire substrates. Silicon (Si) is a promising material for the growth of group-III–V materials. The good thermal conductivity of Si is especially interesting for electronic applications, including low-cost, high-brightness light-emitting diodes, and high-power chips. However, few studies have grown InN nanostructures on Si(1111) substrates via RF-MOMBE. Nitride thin films could be suitable buffer layers for the growth of InN on Si wafers and other lattice-mismatched group-III–V heteroepitaxial systems. Previous studies have shown that InN grown on nitrided Si(111) substrates exhibits good crystallinity. Kumar et al. reported that InN films deposited onto Si substrates with different buffer layers exhibited high crystallinity and wurtzite lattice structures; furthermore, the InN/-Si3N4 double buffer layer exhibited a minimum full-width at half-maximum (FWHM) of the E2(high) mode [9]. Segura-Ruiz et al. [17] reported the growth of misoriented and tilted InN nanorods on Si(111) substrates with amorphous SixNy buffer layers. However, Park et al. [18] the deposition of porous SixNy buffer layers with numerous nanometer-sized holes under certain experimental conditions; such a porous buffer layer could be a good template for the low-temperature deposition of nitride layers. Nishikawa et al. [19] reported that both substrate nitridation and growth initiation via In droplet formation are essential for the growth of InN nanocolumns with excellent c-axis orientation. InN nanostructures such as nanowires, nanotubes, and nanocolumns have attracted considerable attention because the Crystals 2019, 9, 291 3 of 11 quantum confinement effect may yield novel functions and improved performance [20]. Lee et al. [21] reported the predeposition of In before the growth of InN nanocolumns; InN nanocolumns were obtained by forming droplets of Au + In solid solution on Au-coated substrates following by nitriding these droplets. In this study, hexagonal pyramid-like InN nanocolumns were grown on nitrided Si(111) substrates via catalyst-free RF-MOMBE in conjunction with substrate nitridation. The substrate was nitrided at 900 ◦C for 60 min. InN nanocolumns were subsequently grown on the nitrided Si(111) substrate at 500–540 ◦C. The effects of growth temperature on the structural, optical, and electrical properties of the InN nanocolumns were then investigated.

2. Materials and Methods InN nanocolumns were grown on nitrided mirror-like Si(111) wafers as substrates. The growth 9 chamber was evacuated to a base pressure of 10− Torr using a turbomolecular pump. TMIn, which served as the group-III precursor, was delivered to the growth chamber by heating the metal–organic source in the absence of a carrier gas. Active nitrogen radicals were supplied by a RF plasma source (13.56 MHz). The Si(111) wafers were cleaned on a wet bench using the standard Radio Corporation of America process. The wafers were then dipped in buffered oxide etching solution for 5 min to remove native surface oxides. Subsequently, the substrates were placed in the growth chamber and thermally cleaned at 900 ◦C for 90 min under ultrahigh vacuum to remove any oxide residues in preparation for nitridation and InN growth. The Si(111) substrates were nitrided at 900 ◦C for 60 min and then cooled to 500–540 ◦C for InN growth. The N2 (99.9999%) flow rate was 1 sccm, and the RF plasma power was 300 W. The dynamic pressure of the growth chamber during deposition was approximately 2 10 5 Torr. × − The structural properties of the InN nanocolumns were characterized by X-ray diffraction (XRD; Bruker D8 ADVANCE, AXS GmbH, Karlsruhe, Germany) with a Cu-Kα X-ray source. The surface morphologies of the InN nanocolumns were examined by field-emission scanning electron microscopy (FE-SEM; Hitachi S-4300 FE-SEM, Hitachi, Ltd, Chiyoda-ku, Japan), while the InN microstructure was analyzed by field-emission transmission electron microscopy (FE-TEM; JEOL ARM200F, JEOL Ltd, Toyoko, Japan) with an electron voltage of 200 kV. Cross-sectional TEM specimens were prepared using a dual-beam focused ion beam (Seiko SII-3050; manufactured by FEI, Hillsboro, OR, USA). Raman spectroscopy (HR800; Horiba Jobin Yvon Labram, Bensheim, Germany) was performed at room temperature in backscattering geometry using the 514.5-nm Ar+ laser line for excitation. X-ray photoelectron spectroscopy (VG ESCA/XPS Theta Probe; Thermo Fisher Scientific Inc., Waltham, MA, USA) with Al Kα (hν = 1486.6 eV) radiation was used to characterize the bonding characteristics of the elements in the films.

3. Results and Discussion Figure1 shows the XRD patterns of the InN nanocolumns grown at various temperatures on the nitrided Si(111) substrates. All patterns show the peaks of Si(111) (2θ = 28.44◦) and Si(222) (2θ = 58.86◦), a strong InN(0002) peak (2θ = 31.36◦), a weak InN(10–13) peak (2θ = 56.9◦), and the InN(0004) peak (2θ = 65.4◦). The XRD results indicate the formation of highly <0001>-oriented hexagonal InN nanocolumns on the nitrided Si(111) substrates, similar to the results of Feng et al. [6]. The peaks at 2θ = 44.1◦ and 64.4◦ are attributed to the XRD sample stage. CrystalsCrystals2019 2019,,9 9,, 291x FOR PEER REVIEW 44 of 1111

* XRD sample stage Si(111) Si(222) InN(0002) :Si(111) β

K o InN(1013) 540 C * * InN(0004)

530 oC

520 oC Intensity (a.u.) Intensity

500 oC

20 25 30 35 40 45 50 55 60 65 70 75 2 Theta (degree)

Figure 1. X-ray diffraction (XRD) θ–2θ diffraction patterns of InN grown on nitrided Si(111) substrates atFigure different 1. X-ray temperatures diffraction from (XRD) 500 θ◦–2Cθ to diffraction 540 ◦C. patterns of InN grown on nitrided Si(111) substrates at different temperatures from 500 °C to 540 °C. The c-axis lattice parameter of the InN nanocolumns was calculated to be approximately 0.57 nm, in agreementThe c-axis with lattice the valueparameter reported of the for InN bulknanocolumn crystal [22s was]. The calculated results indicate to be approximately stress relaxation 0.57 in thenm, InN in nanocolumnsagreement with after the growth. value Thereported FWHM for values InN ofbulk the crystal XRD peaks [22]. wereThe determinedresults indicate from thestressω scansrelaxation of the InN(0002)in the InN reflection nanocolumns (results after not shown). growth. The The FWHM FWHM values values of the of samples the XRD grown peaks at 500 were◦C, 520determined◦C, 530 ◦ C,from and the 540 ω◦ scansC were of approximately the InN(0002) 366,reflection 360, 389, (res andults 480not arcmin,shown). respectively, The FWHM indicating values of thatthe samples the broad grown peaks at can 500 be °C, attributed 520 °C, 530 to the °C, high and defect540 °C densities were approximately and amorphous 366, In–O 360, 389, compounds and 480 inarcmin, the InN respectively, nanocolumns. indicating The crystalline that the qualitybroad peak wass slightly can be betterattributed in the to InN the nanocolumnshigh defect densities grown atand 520 amorphous◦C compared In–O to compounds those grown in at the 500 InN◦C, nanocolumns. implying that The the crystalline growth temperature quality was was slightly within better the reactivein the InN regime. nanocolumns grown at 520 °C compared to those grown at 500 °C, implying that the growthFigure temperature2 presents was SEM within micrographs the reactive showing regime. the surface morphologies and cross-sections of the InN nanocolumns.Figure 2 presents All ofSEM the micrographs InN nanocolumns showing grew the preferentially surface morphologies along the < 0001and >cross-sectionsdirection. Growth of the beganInN nanocolumns. from a thin polycrystalline All of the InN InN nucleationnanocolumns layer gr onew top preferentially of the nitrided along Si3N 4theultrathin <0001> bu direction.ffer layer. TheseGrowth results began reflect from thea thin fact polycrystalline that the InN nanocolumns InN nucleation were layer grown on top via of chemical the nitrided reaction Si3N from4 ultrathin TMIn andbuffer RF layer. N2 plasma. These Theresults average reflect growth the fact rate that of the the InN InN nanocolumns nanocolumns were was approximately grown via chemical 0.8 µm reaction/h. The InNfrom growth TMIn and rate RF achieved N2 plasma. by RF-MOMBE The average is growth higher thanrate of that the of InN conventional nanocolumns MBE. was In approximately this study, the InN0.8 μ nanocolumnsm/h. The InN exhibited growth rate three-dimensional achieved by RF-MOMBE (3D) features is higher with varying than that nanometer-scale of conventional structures MBE. In andthis study, shapes the ranging InN nanocolumns from pyramid-like exhibited columns three-dimensional to pyramids (3D) depending features with on growth varying temperature. nanometer- Thescale plane-view structures SEMand shapes images ranging show that from aligned, pyramid-like self-assembled columns InN to pyramids nanocolumns depending with diameters on growth of 60temperature. to 120 nm formed The plane-view uniformly SEM over images the entire show substrate that aligned, at a high self-assembled density of approximately InN nanocolumns 1010 cm with2 at growthdiameters temperatures of 60 to 120 of 500nm to formed 540 ◦C. Theuniformly densities over of InNthe nanocolumnsentire substrate grown at a athigh 500 ◦densityC, 520 ◦ C,of 530approximatelyC, and 540 10C10 were cm2 approximatelyat growth temperatures 5.3 1010 cm of 2500, 4.8 to 5401010 °C.cm 2The, 3.7 densities1010 cm of2, andInN 1.3nanocolumns1010 cm2, ◦ ◦ × × × × respectively.grown at 500 Thus, °C, 520 the °C, density 530 °C, decreased and 540 °C with were increasing approximately growth 5.3 temperature. × 1010 cm2, 4.8 The × density,1010 cm2, size,3.7 × and1010 shapecm2, and of the1.3 × InN 1010 nanocolumns cm2, respectively. were Thus, mainly the determined density decrease by thed with diffusion increasing of In adatoms. growth temperature. During the initialThe density, stages ofsize, the and growth shape process, of the InN the formationnanocolumns of TMIn were and mainly/or In determined droplets enhanced by the diffusion nucleation of atIn specificadatoms. locations, During the resulting initial instages columnar of the growthgrowth [process,23]. Thus, the since formation the di ffofusion TMIn of and/or In increases In droplets with temperature,enhanced nucleation the density at specific of InN nucleilocations, was resulting insufficient in columnar for columnar growth growth [23]. at Thus, high since temperature. the diffusion The largeof In columnincreases diameter with temperature, can likely be the attributed density of to InN the large nuclei seed was size insufficient and low densityfor columnar of nuclei growth during at high temperature. The large column diameter can likely be attributed to the large seed size and low

Crystals 2019, 9, 291 5 of 11 Crystals 2019, 9, x FOR PEER REVIEW 5 of 11 thedensity initial of growth nuclei stageduring at highthe initial growth growth temperature, stage at which high growth gave rise temperature, to a high lateral which growth gaverate rise andto a crackinghigh lateral of the growth TMIn rate precursor and cracking (i.e., reaction of the of TMIn TMIn precursor with nitrogen), (i.e., reaction leading of to TMIn large grainwith size.nitrogen), The typicalleading nanocolumn to large grain height size. was The approximately typical nano 240column nm. Theheight nanocolumn was approximately diameter increased 240 nm. from The 60nanocolumn to 130 nm as diameter the growth increased temperature from 60 was to increased130 nm as from the growth 500 ◦C totemperature 540 ◦C. The was InN increased nanocolumns from grown500 °C atto 500 540◦ °C.C and The 520 InN◦ Cnanocolumns were similar grown in size. at Some 500 °C of and the 520 nanocolumns °C were simila werer orientedin size. Some along of the the cnanocolumns-axis without tapering,were oriented whereas along others the werec-axis tilted without with tapering, respect to wherea the substrates others normal. were tilted When with the growthrespect temperature to the substrate was furthernormal. increased, When the nanocolumns growth temperature with diameters was further and heights increased, of approximately nanocolumns 80with and diameters 220 nm (530 and◦ heightsC) or 130 of nmapproximately and 240 nm 80 (540 and◦C), 220 respectively, nm (530 °C) wereor 130 observed. nm and 240 The nm deviation (540 °C), anglesrespectively, of theInN were nanocolumns observed. The grown deviation at 500 angles◦C, 520 of◦ theC, andInN 530nanocolumns◦C were determined grown at 500 to be °C, 75 520◦, 75 °C,◦, andand 80530◦, respectively.°C were determined The height:diameter to be 75°, 75°, ( hand/d) ratio80°, respectively. of the nanocolumns The height:diameter decreased as ( theh/d) growth ratio of temperaturethe nanocolumns increased decreased to 540 as◦ C.the The growth growth temperat temperatureure increased was shown to 540 to°C. a Theffect growth the diff temperatureusion of In adatomswas shown along to nanorodaffect the sidewalls diffusion [ 16of ].In Since adatoms the Inalong diffusion nanorod length sidewa increaseslls [16]. with Since temperature, the In diffusion high temperaturelength increases leads with to the temperatur decompositione, high of temperature In–N. In our leads previous to the study, decomposition continuous InNof In–N. films In were our grownprevious via RF-MOMBEstudy, continuous at various InN temperatures films were grown in the rangevia RF-MOMBE of 500–540 ◦atC. various Therefore, temperatures the above results in the demonstraterange of 500–540 that the°C. Therefore, size, morphology, the above and results optimal demonstrate growth temperature that the size, of morphology, InN nanocolumns and optimal were influencedgrowth temperature by the nitrided of InN Si(111) nanocolumns substrate. The were results influenced are similar by tothe those nitrided observed Si(111) for InNsubstrate. nanorods The grownresults via are MBE similar [24 ].to those observed for InN nanorods grown via MBE [24].

FigureFigure 2. 2.Plane-view Plane-view and and cross-sectional cross-sectional SEM SEM images images of of InN InN grown grown on on nitrided nitrided Si(111) Si(111) substrates substrates at at didifferentﬀerent temperatures temperatures in in the the range range of of 500 500 to to 540 540◦ C.°C.

TEMTEM was was used used to to characterize characterize the the microstructures microstructures of of the the InN InN nanocolumns nanocolumns and and nitrided nitrided Si SixNxNyy.. Cross-sectionalCross-sectional TEM TEM specimens specimens were were prepared prepared from from the the grown grown layers. layers. The The dark-ﬁeld dark-field TEM TEM image image of of a nitrided SixNy ultra layer (Figure 3a) clearly shows a rough surface, which was likely caused by plasma-beam damage to the SixNy/Si(111) surface. The observed V-like holes are consistent with the

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Crystals 2019, 9, x FOR PEER REVIEW 6 of 11 a nitrided SixNy ultra layer (Figure3a) clearly shows a rough surface, which was likely caused by

Nplasma-beam2 plasma-beam damage damaging to the Sia xnonstoichiometricNy/Si(111) surface. silicon The observed nitride V-likelayer holes[25]. Figure are consistent 3b shows with a thecross- N2 sectionalplasma-beam bright-field damaging TEM a nonstoichiometric (XTEM) image of siliconInN nanocolumns nitride layer grown [25]. Figure on a 3nitridedb shows Si(111) a cross-sectional substrate atbright-ﬁeld 520 °C. The TEM electron (XTEM) beam image incident of InN direction nanocolumns in Figure grown 3b is parallel on a nitrided to the Si(111)[ 1 120 ] substrateInN direction, at 520 while◦C. theThe vertical electron axis beam is incidentparallel directionto the [0001] in FigureInN direction.3b is parallel Similar to the to [the1120 SE]InNM direction,results, the while XTEM the images vertical indicateaxis is parallel a rough to InN the[0001] surfaceInN anddirection. V-like Similarholes in to th thee interface SEM results, between the XTEMInN and images the Si indicate substrate. a rough For InNInN nanocolumns surface and V-like grown holes on the in the V-like interface holes, between the crysta InNls likely and the grew Si substrate.along the sidewalls For InN nanocolumns of the holes, resultinggrown on in the a deviation V-like holes, in growth the crystals angle. likelyAs a result, grew the along initial the growth sidewalls of adjacent of the holes, InN nanocolumns resulting in a occurreddeviation in in the growth voids angle. between As a result,the substrate the initial and growth InN nanocolumns. of adjacent InN A nanocolumns previous study occurred found in that the amorphousvoids between SixN they can substrate develop and gradually InN nanocolumns. on bare Si(111) A previous before studythe growth found of that InN amorphous [26]. Thus, Si xtheNy epitaxialcan develop relation gradually is lost on on bare the Si(111)nitrided before area, the resu growthlting in of misoriented InN [26]. Thus, nanorods the epitaxial or a coarse-grained relation is lost structureon the nitrided on Si(111). area, resulting in misoriented nanorods or a coarse-grained structure on Si(111).

FigureFigure 3. ((a)) Cross-sectional bright-ﬁeldbright-field TEM TEM (XTEM) (XTEM) image image of of nitrided nitrided Si(111). Si(111). (b) ( XTEMb) XTEM image image of InN of InNnanocolumns nanocolumns grown grown on a nitridedon a nitrided Si(111) substrate.Si(111) substrate. (c) XTEM (c image) XTEM of theimage top of athe InN top nanocolumn. of a InN nanocolumn.(d) High-resolution (d) High-resolution scanning electron scanning microscopy electron (HRTEM) microscopy image of(HRTEM) the area inimage c indicated of the byarea the in red c indicateddashed circle. by the The red inset dashed in d shows circle. the The FFT patterninset in of d the shows wurtzite the InNFFT nanocolumn. pattern of the (e) HRTEMwurtzite lattice InN nanocolumn.image of the area(e) HRTEM in d indicated lattice image by the of red the arrow. area in d indicated by the red arrow.

FigureFigure 33c–ec–e showsshows aa high-resolutionhigh-resolution TEMTEM (HRTEM)(HRTEM) imageimage ofof thethe toptop ofof anan InNInN nanocolumnnanocolumn alongalong the the [ [1112020]] directiondirection ofof InNInN alongalong withwith thethe correspondingcorresponding FFTFFT pattern and lattice image. The The resultsresults indicate indicate that that no no metallic metallic In In was was presen presentt on on the the top top of of the InN nanocolumn, and and the the InN InN nanocolumnsnanocolumns were were single single-phase-phase hexagonal hexagonal crysta crystalsls with with preferred preferred orientations along along the c-axis. As As shownshown in in Figure Figure 3e,3e, the the InN InN surface surface is is the the (0002) (0002) plane plane of ofInN, InN, and and direct direct measurement measurement based based on the on latticethe lattice image image yielded yielded a lattice a lattice parameter parameter of ofc =c 5.7= 5.7 Å. Å. These These results results are are in in good good agreement agreement with with previouslypreviously reported XRD results [27]. [27]. TheThe compositional compositional depth depth profile proﬁle and and binding binding ener energygy of of the the InN InN nanocolu nanocolumnsmns grown grown at at 520 520 °C◦C werewere evaluated by XPS XPS (Figure (Figure 44).). Figure4 4aa showsshows thethe variationvariation inin thethe atomicatomic ratiosratios ofof silicon,silicon, carbon,carbon, nitrogen,nitrogen, indium, indium, and and oxygen oxygen along along the the etching etching dire directionction from from the the atmosphere atmosphere to to the the InN interface towardtoward the the InN/Si(111) InN/Si(111) interface. interface. The The layer layer structur structuree of of InN InN on on nitrided nitrided Si(111) Si(111) was was clearly clearly observed. observed. TheThe elemental elemental concentrations concentrations of of indium indium and and nitrogen nitrogen were were slightly slightly reduced reduced in in the the layer layer moving moving in in thethe etching etching direction. direction. The The carbon carbon atomic atomic concentration concentration decreased decreased rapidl rapidlyy after after the initial the initial etching etching step and then remained nearly constant. Unintentional oxygen incorporation occurred because of the impurities remaining in the vacuum background, and a high concentration of oxygen was detected near the interface due to oxygen incorporation. Specht et al. [28] reported similar results InN grown

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Crystals 2019, 9, x FOR PEER REVIEW 7 of 11 step and then remained nearly constant. Unintentional oxygen incorporation occurred because of the viaimpurities MBE. High remaining concentration in the vacuum of oxygen background, was also probably and a high caused concentration by voids of between oxygen the was substrate detected andnear InN the interfacenanocolumns. due to These oxygen results incorporation. are in good Specht agreement et al. [28 with] reported the TEM similar images. results However, InN grown no diffractionvia MBE. High spots concentration corresponding of oxygento crystalline was also oxygen-related probably caused impurities by voids (e.g., between In theoxide) substrate were observedand InN nanocolumns.in the selected area These electron results diffraction are in good patterns; agreement thus, withthe oxygen the TEM might images. exist as However, impurities no ordi ffpointraction defects spots in corresponding the film. Furthermore, to crystalline XPS oxygen-relatedindicated a low impurities concentration (e.g., of In carbon oxide) on were the observedsurfaces ofin the selectedInN nanocolumns, area electron which diffraction may have patterns; originated thus, the from oxygen the TMIn might precursor exist as impurities and/or adsorbed or point carbon.defects inIwao the et film. al. [29] Furthermore, indicated that XPS carbon indicated wa as lowincorporated concentration as a background of carbon on element the surfaces during of the the plasma-assistedInN nanocolumns, metal–organic which may have molecular originated beam from epit theaxy TMIn (PA-MOMBE) precursor andgrowth/or adsorbed of InN, although carbon. Iwao the propertieset al. [29] indicated were improved that carbon due to was they incorporated prefer to form as a the background CO and/or element CO2 with during oxygen the atoms plasma-assisted that have slightlymetal–organic high concentration. molecular beam Oxygen epitaxy contamination (PA-MOMBE) was growth previously of InN, reported although in theInN properties grown via were RF- MOMBEimproved [30]. due to they prefer to form the CO and/or CO2 with oxygen atoms that have slightly high concentration. Oxygen contamination was previously reported in InN grown via RF-MOMBE [30].

Figure 4. (a) XPS depth proﬁle and (b) In3d spectra of InN nanocolumns grown on a nitrided Si(111) Figure 4. (a) XPS depth profile and (b) In3d spectra of InN nanocolumns grown on a nitrided Si(111) substrate at 520 ◦C. substrate at 520 °C.

Figure 4b shows the In3d XPS spectra of InN nanocolumns grown at 520 °C obtained from 440 to 458 eV at various depths of the film. Overlapping peaks were deconvoluted using XPSpeak41

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Crystals 2019, 9, x FOR PEER REVIEW 8 of 11 Figure4b shows the In3d XPS spectra of InN nanocolumns grown at 520 ◦C obtained from 440 to 458 eV at various depths of the film. Overlapping peaks were deconvoluted using XPSpeak41 software. software. Based on the XPS spectra, the binding energies (BEs) of In3d3/2 and In3d5/2 were determined Based on the XPS spectra, the binding energies (BEs) of In3d and In3d were determined to be to be 451.2 and 443.6 eV, respectively. This indicates that the difference3/2 in 5the/2 BEs of the In3d3/2 and 451.2 and 443.6 eV, respectively. This indicates that the difference in the BEs of the In3d and In3d In3d5/2 spin–orbit doublet was almost constant at 7.6 eV with an intensity ratio of 4:3, 3in/2 agreement5/2 withspin–orbit the literature doublet [31]. was The almost spectrum constant corresponding at 7.6 eV with to an the intensity surface ratioof the of InN 4:3, innanocolumn agreement shows with the a broadliterature peak [31 at]. 444.2 The spectrumeV and a correspondingshoulder peak to at the 444.6 surface eV, which of the InNare assigned nanocolumn to the shows BEs of a broad In–N peak[32] at 444.2 eV and a shoulder peak at 444.6 eV, which are assigned to the BEs of In–N [32] and In O and In2O3 (BE = 444.6–444.9 eV) [33], respectively. This may be related to the absorption of H2O 2on3 the(BE InN= 444.6–444.9 surface. Specht eV) [et33 al.], respectively.[28] reported Thissimilar may results be related for InN to grown the absorption via MBE. of H2O on the InN surface.The SpechtRaman etspectrum al. [28] reported of the wurtzite similar resultsInN nanoco for InNlumns grown on viaa nitrided MBE. Si(111) substrate (Figure 5) exhibitsThe Raman E2(high) spectrum peaks at of 489 the wurtziteand 590 InNcm−1nanocolumns for the A1(LO) on mode a nitrided under Si(111) 488 nm substrate excitation. (Figure The5) 1 Ramanexhibits peak E2(high) positions peaks of at the 489 A and1(LO) 590 and cm −E2(LO)for the modes A1(LO) of modethe InN under nanocolumns 488 nm excitation. in this study The Raman agree wellpeak with positions previously of the reported A1(LO) and values, E2(LO) and modes the narrow of the linewidth InN nanocolumns is similar in to this that study of high-quality agree well withInN filmspreviously [34,35]. reported However, values, the andstrain-free the narrow Raman linewidth frequencies is similar of the to E that2(high) of high-quality high and A InN1(LO) films modes [34,35 of]. InNHowever, remain the uncertain, strain-free and Raman the reported frequencies values ofvary the widely E2(high) from high 483 and to 495 A1(LO) cm−1 modesfor the E of2(high) InN remain mode 1 anduncertain, 580–596 and cm− the1 for reported the A1(LO) values mode vary [36]. widely from 483 to 495 cm− for the E2(high) mode and 1 580–596 cm− for the A1(LO) mode [36].

540oC Si E high 2 A (LO) 1

530oC Si E high 2 A (LO) 1

520oC Si

high

Intensity (a.u.) E 2 A (LO) 1

500oC Si E high 2 A (LO) 1

350 400 450 500 550 600 650 700 Wavelength (cm-1)

FigureFigure 5. 5. RoomRoom temperature temperature Raman Raman spectra spectra of of InN InN nano nanocolumnscolumns on on a a nitrided nitrided Si(111) Si(111) substrate. substrate.

4. Conclusions

Crystals 2019, 9, 291 9 of 11

4. Conclusions We fabricated hexagonal InN nanocolumns on nitrided Si(111) substrates via RF-MOMBE and studied the effect of growth temperature on the structural, morphological, optical, and compositional properties of the InN nanocolumns. The XRD results indicated that highly <0001>-oriented hexagonal InN nanocolumns were grown on the nitride Si(111) substrates. FESEM analysis demonstrated that the diameters of the InN nanocolumns on nitrided Si(111) depended strongly on the growth temperature. Hexagonal, pyramid-like 3D InN nanocolumns with high density were observed rather than one-dimensional InN nanowires. The Si3N4 buffer layer appeared to aid the self-assembly of the InN nanocolumns. The lattice parameter observed by HRTEM (c = 5.7 Å) agrees well with that of wurtzite-structured InN. The surface of the nitrided Si(111) substrate was clearly rough, likely as a result of damage from the RF plasma beam. The XPS results showed that oxygen was incorporated into the InN nanocolumns. The observation of the A1(LO) and E2(high) phonon modes of InN in the Raman spectrum confirmed the hexagonal structures of the InN nanocolumns. These findings indicate that the nitrided Si(111) substrate is essential for engineering the growth of InN on Si wafers. This substrate may also be applicable for other lattice-mismatched group-III–V heteroepitaxial systems. Nanocolumn structures containing heterojunctions could find applications in highly confined optoelectronic and electronic devices.

Author Contributions: W.-C.C. carried out most of the experimental work, including the preparation and characterization of InN nanocolumns and the drafting of the manuscript. T.-Y.Y. carried out the high-resolution XPS measurements. F.-I.L. and H.-P.C. supported the analysis of the InN samples. Y.-W.L. performed the XRD measurements. S.-Y.K. revised the manuscript. All authors read and approved the manuscript. Funding: This research was funded by the Ministry of Science and Technology, grant number MOST 107-2111-E-492-001 and MOST 107-2622-E-492-012-CC3 Acknowledgments: The authors would like to thank L.L. Wei carried out the high-resolution TEM analysis. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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