ATOMIC LAYER DEPOSITION OF 2D TRANSITION METAL DICHALOGENIDES

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1 ATOMIC LAYER DEPOSITION OF 2D TRANSITION METAL DICHALOGENIDES Annelies Delabie, M. Caymax, B. Groven, M. Heyne, K. Haesevoets, J. Meersschaut, T. Nuytten, H. Bender, T. Conard, P. Verdonck, S. Van Elshocht, M. Heyns, K. Barla, I. Radu, A. Thean Imec, Leuven, Belgium KU Leuven (University of Leuven), Leuven, Belgium Emerging Research, Materials and Processes Potential Solutions to Semiconductor Industry s challenges Semicon Europe 2015 Dresden, Oct

2 2D Transition Metal Dichalcogenides MX 2 M: group 4-7 2D MX 2 MoS 2 X Layered inorganic materials with versatile and adjustable properties Depending on M, X: insulating, metallic, semi-metallic, semiconducting Depending on the number of monolayers Complementary and superseding those of graphene 2

3 Semiconducting 2D MX 2 : nano-electronic applications FET Vertical TFET V G High-k MX 2 B V S MX 2 A I V D Radisavljevic et al., Nat. Mat., 12, (2013) Semiconducting 2D MX 2, e.g., MoS 2, WSe 2, MoSe 2 Significant bandgaps No dangling bonds at interfaces Reduced short channel effects for planar transistors Choice of bandgaps and band alignment hetero-stacks for 0 V 3

4 Semiconducting 2D MX 2 : nano-electronic applications Exfoliation proof of concept Industrial applications require deposition of few- or single-layer MX 2 on large area substrates Sulfurization of metals or metal oxides Molecular Beam Epitaxy (MBE) Chemical Vapor Deposition (CVD) Atomic Layer Deposition (ALD) High quality structure mobility Stoichiometric (X/M = 2), pure Monocrystalline 2D structure Uniform and continuous layers Monolayer controlled deposition From multi- to single-layer MX 2 hetero-stacks 4

5 MX 2 Chemical Vapor Deposition (CVD) MX 2 crystals (lateral size ~100µm) with optical and electrical properties comparable to exfoliation Challenges: Monolayer control on large area substrates High temperature MoS 2 by MoO 3 /S CVD 700ºC on Si/SiO 2 Van der Zande et al, Nature Materials, 12, 554, 2013 H. Wang et al, Nanoscale, 2014, 6, MoS 2 by MoCl 5 /H 2 S CVD 700ºC on J. Si/SiO Park et al, 2 Nanoscale, 7, 1308 (2015) K. Kang et al, Nature, 520, 656 (2015) 5

6 Scharf et al. Acta Materialia 54, 4731, 2006 Nandi et al., Electrochimica Acta 146, 706, 2014 Z. Jin et al., Nanoscale 2014 Tan et al., Nanoscale, 2014, 6, Browning et al., Mater. Res. Express, 2015, 2, MX 2 Atomic Layer Deposition (ALD) ALD principle: growth control on large area substrates at low temperature Feasibility of MX 2 ALD demonstrated, but quality of films currently inferior to CVD by WF 6 /H 2 S ALD Crystalline 2D at ºC Zn(Et) 2 enables deposition 10 at% Zn impurities MoS 2 by MoCl 5 /H 2 S ALD, 300ºC Control of # layers by cycles H 2 S anneal 800 C required MoS 2 by Mo(CO) 6 /H 2 S plasma, H 2 S or (CH 3 S) 2 (PE)ALD Amorphous or polycrystalline MoS 2 films deposited at C Crystallization anneal required

7 Missing insight: redox chemistry of MX 2 ALD +VI +V +IV +III +II +I 0 Reduction of M Oxidation of M H 2 S S + 2H + + H 2 S + 2e - S 2- + WF 6 2e - WH 2 ( i PrCp) 2 H 2 H 2 S, Zn(Et) 2, H 2 plasma, Si? MoS 2 H 2 S, H 2 S plasma, (CH 3 S) 2 H 2 S (CH 2 SH) 2,T H 2 S S MoF 6 MoCl 5 Mo(N(CH 3 ) 2 ) 4 Mo(thd) 3 Mo i PrCp(CO) Mo(CO) 6 2NO AVS 15 th international conference on ALD, June 28 July 1, Portland, US

8 Two 300mm processes for deposition Reducing agents Si sacrificial layer WF 6 + H 2 S H 2 plasma 1. Pulsed Chemical Vapor Deposition Al 2 O 3 2. Plasma Enhanced Atomic Layer Deposition Al 2 O 3 A. Delabie et al., Chem. Commun., 2015, DOI: 8

9 9 EQUIPMENT RC2 300mm ASM Eagle12_2 PECVD reactor WF 6 /H 2 S precursors 1 Torr T = 450 C

10 OUTLINE 1. Pulsed Chemical Vapor Deposition WF 6 + H 2 S Si sacrificial layer H 2 plasma Al 2 O 3 2. Plasma Enhanced Atomic Layer Deposition Al 2 O 3 A. Delabie et al., Chem. Commun., 2015, DOI: 1

11 W, S content (E+15at/cm 2 ) Sequential WF 6 /H 2 S reactions: no nucleation on dielectric substrates No deposition at C on several dielectric substrates (Al 2 O 3, SiO 2, Si 3 N 4 ) In line with observations of Scharf Number of cycles Scharf et al. Acta Materialia 54, 4731, 2006 W S Al 2 O 3 substrates RBS Al 2 O SiO ALD reaction cycle WF 6 H 2 S EVA C T ( C) W at/cm 2 (RBS) W at DL EVA C S at/cm 2 (RBS) S < DL < DL < DL 450 < DL < DL 11

12 12 Concept Si sacrificial layer approach W Si Monolayer control? thickness control by thickness Si layer Sacrificial Si Al 2 O 3 insulator Silicon wafer WF 6 /H 2 S Crystalline 2D formed by sequential reactions of WF 6 and H 2 S on Si substrate 2 WF 6(g) + 3 Si (s) 2W (s) + 3 SiF 4(g) W (s) + 2 H 2 S (g) (s) + 2 H 2(g) Al 2 O 3 insulator Silicon wafer

13 deposition enabled by sacrificial Si layer W content linearly increases with Si thickness for Si layers up to 6 nm (RBS) S/W = 1.9 for W content up to 6ML, thicker layers require higher T (RBS) Si consumed (XPS) 2D structure of (Raman Spectroscopy) 1-6nm MBD Si on 30nm Al 2 O 3, Si oxidation + HF clean, 14 cycles WF 6 /N 2 /H 2 S/N 2, 450ºC 13

14 deposition enabled by sacrificial Si layer 1-6nm MBD Si on 30nm Al 2 O 3, Si oxidation + HF clean, 14 cycles WF 6 /N 2 /H 2 S/N 2, 450ºC 2D structure of Polycrystalline, rather small grain size (5nm) and random crystal orientation 14

15 OUTLINE 1. Pulsed Chemical Vapor Deposition WF 6 + H 2 S Si sacrificial layer H 2 plasma Al 2 O 3 2. Plasma Enhanced Atomic Layer Deposition Al 2 O 3 A. Delabie et al., Chem. Commun., 2015, DOI: 1

16 W, S content (E+15 at/cm 2 ) PEALD enables nucleation on dielectric substrates e.g. Al 2 O 3 No nucleation/deposition for WF 6 /H 2 S ALD on Al 2 O 3 H 2 plasma in PEALD reaction cycle enables nucleation of W, C, 300W, PEALD S, C, 300W, PEALD W, C, ALD y = 0.06x S, C, ALD PEALD reaction cycle WF 6 H 2 plasma H 2 S EVA C EVA C EVA C y = 0.03x RBS Number of ALD cycles ALD reaction cycle WF 6 H 2 S EVA C EVA C 16

17 17 Self-limiting nature of WF 6, H 2 S reactions WF 6 and H 2 S reaction: W content shows saturation, indicating self-limiting nature of reactions as in conventional ALD Initial experiments: S/W ratio not repeatable - high H 2 plasma power (300W) 2-20s 10s, W 60s300ºC WF 6 H 2 plasma H 2 S EVAC EVAC EVAC

18 18 H 2 plasma conditions redox mechanism H 2 plasma power needs to be minimized to confine the reaction to the top surface only control partial reduction of W 6+ (WF 6 ) to W 4+ ( ) Too high plasma power: too low S content, poor stability of the films (prone WSto PEALD, 300ºC, 30nm a- 2 oxidation in air) Al 2 O 3 H 2 plasma W (+VI) F 6 W (+IV) F x 10s 10s, W 60s300ºC WF 6 H 2 plasma H 2 S EVAC EVAC EVAC H 2 S W (+IV) S 2 W (0) RBS Al 2 O 3 Si

19 19 H 2 plasma conditions redox mechanism H 2 plasma power needs to be minimized to confine the reaction to the top surface only control partial reduction of W 6+ (WF 6 ) to W 4+ ( ) Low plasma power: as deposited films show clear raman features of 10s 10s, W 60s300ºC WF 6 H 2 plasma H 2 S EVAC EVAC EVAC

20 cycles As deposited layers have 2D structure Thin layers (2-3 monolayers) 2D structure Low deposition T ( ºC) Absence of template or anneal Nano-crystalline, domain size ~10 nm 10s 10s,100W 60s WF 6 H 2 plasma H 2 S EVA C EVA C 300ºC EVA C 65 PEALD cycles 2-3 ML 300ºC, 100W a-al 2 O 3 Si 20

21 21 Parallel orientation of basal planes for 300ºC PEALD Low mean spread of basal plane orientation: FWHM = 0.14, XRD rocking curve for (002)-diffraction peak 3 ML, 300ºC, a- Al 2 O 3 (002) at 13.5 polycrystalline Al 2 O 3 Si

22 22 Crystal orientation is controlled by PEALD temperature In line with results for thermal WF 6 /H 2 S ALD with ZnEt 2 Scharf et al. Acta Materialia 54, 4731 (2006) PEALD 300ºC, 300W Mainly parallel orientation PEALD 450ºC, 300W More random orientation polycrystalline Al 2 O 3 Si polycrystalline Al 2 O 3 Si

23 W and S content (10 15 at/cm2) PEALD growth behavior 10s 10s, 100W 60s Substrate enhanced growth on Al 2 O 3 substrates 300ºC WF 6 H 2 plasma H 2 S EVAC EVAC EVAC Formation of 1 st monolayers occurs faster than bulk growth: 2D material basal planes are relatively inert Nucleation behavior can affect film structure (layer continuity, domain size, grain boundary structure) y = 0.053x y = 0.027x Number of ALD reaction cycles E 1 2g ~4ML ~2.5ML ~1.5ML ~1ML A 1g 23

24 24 Structure of 2D materials electronic properties Polycrystalline structure Crystal and domain size Inter-grain defects: Grain boundary structure: tilt and mirror twin grain structures, disconnected grains Contaminants, impurities, dangling bonds Intra-grain defects Point defects: impurities, M or X vacancies Mirror twin boundary ring defects Van der Zande et al, Nature Materials, 12, 554, 2013

25 Structural improvements impact anneal Increased domain size for PEALD by 900 C anneal As deposited 900 C anneal Al 2 O 3 Al 2 O 3 Si S i 25

26 Structural improvements impact substrate Significantly increased domain size for PEALD by deposition on sapphire substrates Decreased roughness at bottom interface Is MX 2 Atomic Layer Epitaxy feasible? Al 2 O 3 Si Sapphire 26

27 Conclusions CVD and ALD provide promising routes for wafer scale deposition of 2D materials 300mm substrates Reducing agents enable CVD and ALD from WF 6 and H 2 S precursors Si sacrificial layers for CVD H 2 plasma for PEALD with nano-crystalline 2D structure is obtained at low deposition temperature ( ºC) without the need of template or anneal Further structural improvements can be achieved by providing templates for epitaxial seeding and post-deposition anneal A. Delabie et al., Chem. Commun., 2015, DOI: /C5CC05272F 27

28 ASPIRE INVENT ACHIEVE