化学気相成長 (CVD) 法による カーボン薄膜の成長制御

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1 化学気相成長 (CVD) 法による カーボン薄膜の成長制御 吉村雅満豊田工業大学表面科学研究室 R. Tiwari, 鈴木誠也, 松岡佑樹, D. Nguyen 1. カーボンナノチューブーグラフェン複合体 2. グラフェン

2 Background & MoJvaJon Discovery of Nano Carbons Fullerene 1 (Kroto ら ) MWCNT* 2 (Iijima) SWCNT 3 (Iijima) Graphene 4 (Novoselov) CNT- Graphite Composite 5 (Kondo) * CNT: carbon nanotube (1) H. W. Kroto et al., Nature, 318 (1985) 162. (2) S. Iijima, Nature, 354 (1991) 56. (3) S. Iijima and T. Ichihashi, Nature, 363 (1993) 603. (4) K. S. Novoselov et al., Science, 306 (2004) 666. (5) D. Kondo et al., Appl. Phys. Express, 1 (2008)

3 Background & MoJvaJon Physical Materials E cond current Ther cond 材料導電率 S/m 電流密度 A/cm 2 熱伝導率 W /(m K) グラフェン Graphene ~5000 単層 SWCNT C N T ~3500 多層 MWCNT C N T ~3000 Silver 銀 Copper 銅 アルミニウム Aluminum Excellent electric and thermal conducjvity properjes of the composite, consisjng of graphenes and carbon nanotubes are expected ApplicaJon: Via- hole wiring of electronic circuitry, etc

4 Background & MoJvaJon Composite structure first revealed by Kondo 5 Graphene layer 5 MWCNT 5 Scanning Electron Microscope Transmission Electron Microscope (5) D. Kondo et al., Appl. Phys. Express, 1 (2008) *SEM: scanning electron microscopy **TEM: transmission electron microscopy

5 Background & MoJvaJon Composite structure revealed by Kondo 5 Interface between surface graphene and CNTs 5 Scanning Electron Microscope Transmission Electron Microscope Catalys Catalyst parjcles on the Jp of CNT (5) D. Kondo et al., Appl. Phys. Express, 1 (2008) *SEM: scanning electron microscopy **TEM: transmission electron microscopy Model of composite by Kondo 5

6 Background & MoJvaJon (5) D. Kondo et al., Appl. Phys. Express, 1 (2008) CVD growth of carbon films few nm cat cat sub CNT C Cat acjvajon Carbon source C C C CNT growth few100 nm ~ few µm cat cat Graphene C Cat acjvajon Carbon source C C C precipitajon graphene In- situ TEM observajon 11 cat 2.1~3.6 nm Cat acjvajon Carbon source C C C precipitajon graphite granulajon CNT growth cat sup sub

7 Background & MoJvaJon (13) K. Hata et al., Science 306 (2004) about the previous model How is carbon supplied into the catalyst under the graphite layers? In the Kondo s model, the catalyst is covered with graphite and CNTs Catalyst loses its acjvity when covered with amorphous carbon or graphite 13 Kondo s model Cat acjvajon Carbon source precipitajon granulajon CNT growth C C C cat graphite Carbons From Where?

8 Background & MoJvaJon (5) D. Kondo et al., Appl. Phys. Express, 1 (2008) * AFM: atomic force microscopy Ideal vs. Real Structure Surface Graphite Layers 5 Flat, higher Ordered Graphite cat Noncon@nuous (turbostra@c ) Structure of Surface Graphite Model proposed by Kondo 5 SEM by Kondo Our AFM image Rough par@cle- like morphology

9 Effect of Catalyst Materials (Fe, Co) How carbon atoms go into the surface graphite layer? assumpjon: Defects in the graphite layers have role as a path. Many carbon Few carbon IniJal growth of composite cat sub graphite Low crystallinity cat High crystallinity The crystal quality of surface graphite layers supposed to affect the growth of composite

10 Effect of Catalyst Materials (Fe, Co) How the carbon go into the surface graphite layer? assumpjon: defects in the graphite layers have role The purpose: Many carbon can go ü Crystallinity of precipitated carbon 20 Fe: five- and seven- membered rings Co: high quality Ø Comparison between Fe and Co catalyst (20) Y. Shibuta and S. Maruyama, Comp. Mater. Sci., 39 (2007) 842. Few carbon can go To clarify the carbon supply path into the IniJal growth cat cat of composite underlying layers sub graphite To clarify Low the crystalinity growth model High crystalinity of composites Fe 108 Co 108 Graphite precipitajon 20 (Shibuta s simulajon)

11 Experimental Methods Experiment p Substrate SiO 2 (50 nm)/si Suppor@ng layers: Al 2 O 3 RF- spuferig(al 2 O 3 target) Intensity [arb. unit] Raman Spectroscopy wavelength:532 nm D G catalyst: Fe, Co Arc- plasma deposijon thickness measurement by AFM (precision: ±0.1 nm) cat support substrate Raman shift [cm -1 ] D: defect- related peak, G: graphite peak I G /I D : estimate of crystallinity M. S. Dresselhaus et al., Phys. Rep., 409 (2005) 47.

12 Experimental Methods p Alcohol thermal Chemical Vapor DeposiJon (CVD) Carbon source: alcohol(ethanol) C 2 H 2 has been previously used Custom- made apparatus Gas- flow control growth Temperature of substrate ( C) T G CVD 3 min 30 s 10 min Time (min) P G Pressure in chamber (torr) Alcohol creajon T G : growth temperature (670 C) P G : growth pressure (150 torr)

13 Experimental Methods CVD growth of CNT by Alcohol CVD Cross- secjonal SEM image 30 µm 500 nm heater 300 µm * cat: Co, thickness: 2.0 nm, Temp 800C, Pressure 310 torr, 5 min growth

14 Effect of Catalyst Materials (Fe, Co) Growth of Carbon Films from Fe and Co Fe Co CNT Inter composite Composite growth at thinner region for Co than Fe Graphite formajon at thick Co

15 Effect of Catalyst Materials (Fe, Co) Crystallinity of composites grown from Fe and Co Fe Co H X t a l s Q I G /I D Intermediate Co Inter Fe graphite (Co) Crystallinity:Raman (I G /I D ) Fe (0.79) <Co (1.05) ü Consistent with the Shibuta s study Co precipitates higher quality of graphenes L Fe Co 鉄コバルト thickness [nm] CNT Inter composite

16 Effect of Catalyst Materials (Fe, Co) Growth at thick catalyst region CNT arrays under the graphite for Fe No CNT arrays under the graphite for Co Fe 15.8 nm Co 4.6 nm Fe Co 20 µm 20 µm Composite formajon Graphite formajon

17 Effect of Catalyst Materials (Fe, Co) Defect in Graphite is path or not? Fe: composite Many Carbon Co:graphite Few carbon IniJal stage Fe Low crystallinity graphite Co High crystallinity Fe Co CNT growth composite No CNT graphite Graphite defects is PATH of carbons

18 Background & MoJvaJon Real Structure of the composites Flat, higher ordered HOPG disordered rough Surface graphite cat cat Model proposed by Kondo 5 Our proposed structure CNT

19 Conclusions p Effect of Catalytic Material to the Synthesis of Graphite-capped Vertically Aligned Carbon Nanotube Arrays Iron catalyzed aligned CNT arrays at the overall thickness (2.0 ~ 15.8 nm for CNT arrays ~ composite), while cobalt catalyst lost its acjvity to synthesize aligned CNT arrays above 4.6 nm. CrystalliniJes of the CNTs arrays synthesized from thin iron and cobalt film were compared by Raman spectroscopy. The Co catalyzed CNTs showed befer crystallinity than Fe- catalyzed ones (GD rajo: 1.05 and 0.79 for cobalt and iron, respecjvely). Based on these results, defects in the graphijc forms are assumed as paths of the carbon supply to the catalyst. Since the cobalt catalyst precipitates higher crystalline graphite than iron, the cobalt deacjvates quickly because of suppression of carbon supply, resuljng in the formajon of graphite film.

20 グラフェンの特性 高移動度 ( cm 2 /V s) 1000cm 2 /V s for Si 0.3µm 弾道電子 スピン伝導 熱伝導度およびヤング率が大きい 高光透過率 (97.7%) 低欠陥濃度 (~1x10 10 cm -2 )

21 グラフェンのエレクトロニクスを中心とした応用 デバイス機能 ( 基礎物理 ) 電子 (n, p; 高移動度 零ギャップ 面配線 ) スピン ( 高スピン拡散 ジグザグエッジスピン ) 光 ( ノン ボルン オッペンハイマー ) 電子源 ( ) 複合材料 ( 強度 導電性 ) 材料 ( 基礎物性 ) 作製 成長技術 ( セロテープ :HOPG 熱処理 :SiC,CVD) ( 単層 複数層 平坦性 サイズ ) 物性値制御 ( 高移動度 高安定性高スピン拡散 ) デバイス作製 単層 複数層 基板サイズ 集積化 ( 均一性 ) エッジ構造制御 オーミック電極接合バンドギャップ制御 ドーピング保護膜 ミクロな評価 高精度計測 デバイス設計理論 ( 従来デバイスとの整合性 )

22 Introduction TTI SURFACE SCIENCE LAB. Large Size Single Crystal Domain in CVD Graphene CVD Graphene: polycrystalline, randomly oriented Low nucleation density è Large domains High nucleation density à Small domains Graphene on Pt Graphene on Cu 1mm L. Gao et al., Nature Commun. 3, 699 (2012). Z. Yan et al., ACS Nano 6, 9110 (2012). Toward wafer scale graphene growth nucleation mechanism is important

23 Introduction In This Talk TTI SURFACE SCIENCE LAB. u High pressure annealing of Cu before atmospheric CVD à Suppress nucleation of graphene u Discuss the nucleation mechanism

24 Experimental TTI SURFACE SCIENCE LAB. High Pressure Annealing and Atmospheric CVD pressure gauge Cu foil (100 µm,nilaco) quartz tube inner quartz tube Needle valve tubular furnace Rotary Pump ball valve CH 4 /Ar Ar H 2 exhaust High pressure annealing ~0.34 MPa, 1050 C H 2 /Ar, 5 h Atmospheric CVD growth ATM, 1050 C H 2 /Ar/CH 4 (10 ppm) Observation Ø SEM, AFM, Auger mapping

25 Results and discussion TTI SURFACE SCIENCE LAB. SEM Observation of Graphene on Cu Atmospheric Pressure Annealing 10 min 20 min 30 min 100 µm High Pressure Annealing 10 min 20 min 30 min Low nucleation density on high pressure (HP) annealed Cu

26 Results and Discussion TTI SURFACE SCIENCE LAB. AFM: #240 Graphene/Cu <Threefold Annealing> 20 um -20 um

27 Results and discussion TTI SURFACE SCIENCE LAB. High Pressure Annealing Reduced the Nucleation Nucleation density (1/mm 2 ) ATM High pressure Time (min) 3 times lower nucleation density on HP Cu

28 Results and discussion TTI SURFACE SCIENCE LAB. AFM Images of the Pretreated Cu Before Growth ATM 1.7 nm High pressure 5.0 nm ~100 nm 200 nm 200 nm -1.7 nm -5.0 nm The step terrace structure after the HP annealing indicates clean and smooth surface à High pressure suppressed evaporation of Cu

29 Results and discussion Impurity Particles on Cu TTI SURFACE SCIENCE LAB. ATM Impurity Particles 1.5 nm High pressure 7.0 nm Sample 500 nm -1.5 nm 2 µm -7.0 nm Density of particles Height Surface Density of graphene (1/μm 2 ) (nm) Structure (1/μm 2 ) ATM ± x10 2 HP ±5 Step terrace 2.5x10 2

30 Results and discussion TTI SURFACE SCIENCE LAB. Auger Mapping of the Impurity Particles Sample: 10 min CVD growth on ATM annealed Cu SEM C KLL Si KLL L S 10 µm Hexagonal graphenes Ø The particle contains Si Ø Large (L) particle was located on the center of graphene Ø Many small (S) particles were on Cu substrate The impurity particles would be from the quartz tube

31 Results and discussion Particle-Assisted Nucleation TTI SURFACE SCIENCE LAB. Quartz Quartz H 2 /Ar Deposition H 2 /Ar Diffusion Desorption SiO x Cu Cu Quartz Quartz Quartz Quartz Aggregation H 2 /Ar H 2 /Ar/CH 4 Cu Nucleation and Growth Cu Quartz Quartz

32 Results and discussion TTI SURFACE SCIENCE LAB. Electropolishing of Cu before annealing Electropolishing + HP 25.8 nm Sub-millimeter sized graphene ~ 0.5 mm 5 µm nm Terrace width: ~400 nm Density of par@cles: ~0.01 µm - 2 u Electropolishing provided large terrace and low density of impurity u Sub-millimeter graphene was grown through electropolishing and HP

33 Conclusion TTI SURFACE SCIENCE LAB. Conclusion u Effect of high pressure annealing of Cu on atmospheric CVD graphene growth was studied u Role of high pressure annealing is suppression of evaporation of Cu à step terrace surface formation u Auger imaging revealed that the Si containing particle was located on the center of graphene u Particle-assisted nucleation process is suggested u Combining electropolishing and high pressure annealing, submillimeter sized graphene growth was succeeded Thank you for listening