University of Groningen

Growth and nanostructure of tellurides for optoelectronic, thermoelectric and phase-changeapplicationsVermeulen, Paul Alexander

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45 *This chapter is adapted from: Vermeulen P.A., Momand J., Kooi, B.J., Pulsed laser deposition of epitaxial tungsten-telluride heterostructures. Submitted.

Chapter 4. Pulsed laser deposition of

epitaxial tungsten-telluride

heterostructures.

Biaxially textured WTe2 films are grown by exploiting vdWaals epitaxy at low temperatures, using Pulsed Laser Deposition.

4.1 Abstract Transition-Metal Di-Chalcogenides (TMDCs) are incorporated in nanoscale

devices for their exceptional properties such as a direct bandgap and strong light

interaction at monolayer thickness. TMDC crystals are grown using high-

temperature processes like Chemical Vapour Deposition or Bridgeman growth.

The mono- or few-layer films required for devices are usually manually

exfoliated. However for industrial applications it is crucial to develop growth

methods using van der Waals epitaxy. This requires large scale and low

temperature deposition of TMDCs. Furthermore, while monolayer TMDCs are of

huge interest, designing heterostructures of 2D-bonded materials provides even

more opportunity to tune functional properties. We report the growth of single-

crystal-like WTe2-Bi2Te3 epitaxial multilayers on various substrates, using a

single-step low temperature Pulsed Laser Deposition process.

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

46

4.2 Introduction TMDCs have a highly anisotropic layered structure and are often studied in

mono- or bilayer form.1 Due to the highly dissimilar nature of the component

elements unusual properties arise, such as giant magnetoresistance and

superconductivity, as well as a semiconductor-metal structural transition for

WTe2.2–4 Furthermore, monolayers have a direct band gap, allowing efficient use as

transistors and applications in optics as emitter/detector elements.5 One

monolayer consists of three atomic planes: |X-M-X|, where M is a transition metal

like Mo or W, and X is a chalcogenide like S or Te. The bonding within the triplet

layers is predominantly covalent and strong, whereas the bonding between them is

weak, because it is based on van der Waals (vdW) bonding. TMDCs therefore

belong to the 2D-(bonded) materials. For Mo- and W-dichalcogenides two stable

unit cell structures often denoted as 2H (hexagonal) and 1T´ (monoclinic), have

been reported.6 The structure and concomitant functional properties seem to

depend both on the specific TMDC and the preparation method.7–9 Exploiting the

weak bonding between monolayers, TMDCs are often incorporated in so-called

vdW heterostructures, where they are combined with other 2D-materials such as

graphene, hBN or tetradymites. This allows a vast range of nanoscale device

applications such as spintronics, LED and (light)-field sensor applications.10–12 For

these applications, TMDCs are usually prepared using CVD and Bridgeman

techniques, and then exfoliated.9 While crystal quality is high, manual alignment

(position and rotation) of the constituent monolayers is crucial for the functional

properties of the system.13,14 A direct deposition technique removes the need for

manual alignment and covers a larger area, which is essential for industrial

implementation.

Due to the weak vdW bonding, 2D-materials with highly different bond length

and -symmetry can be grown into a stable heterostructure. Surprisingly, the

individual layers still show strongly preferred in- and out-of-plane crystal

orientations, allowing growth of biaxially textured single-crystal-like films.15,16 In

fact, promising demonstrations of WTe2 and MoS2/MoSe2 growth using molecular

beam epitaxy (MBE) have been reported using a vdW substrate such as graphene or

tetradymites.17–22 While the rather costly MBE requires careful tuning of many

interdependent system parameters, Pulsed Laser Deposition (PLD) promises to

yield large-area stoichiometric films of comparable quality with more ease of use,

lower cost, and monolayer thickness control.23 Furthermore, most cited TMDC

deposition processes involve high-temperature (> 400 °C) growth which severely

limits their applicability in standard Si wafer integration. Promise for the low-

temperature PLD growth of TMDC thin films is found in literature, however.7,24–27

In this report we describe how to grow biaxially textured WTe2 monolayers at low

temperature using Pulsed Laser Deposition.

4.3 Experimental

47

4.3 Experimental Thin tungsten-telluride films were grown using a PLD system produced by

TSST. A KrF excimer laser (248 nm) was used with a spot size of 1.3 mm2 at a 45°

incidence angle, operated at 1 Hz. The target-substrate distance was 4 cm and the

background pressure was below 10-7 mBar. Growth was monitored using a high-

pressure Reflective High-Energy Electron Diffraction (RHEED) system, operated at

30 kV. Single-Crystal (SC) targets of WTe2 were obtained from HQ-Graphene, and

a target made from sintered powders (SP) of W and Te was obtained from K-tech

Supplies Ltd..

Cross-sections of crystalline films grown on mica were prepared using a FEI

Helios G4 Focused Ion Beam (FIB) system. Samples were analyzed using several

electron microscopes: an FEI Nova NanoSEM 650 with EDS and CBS detectors,

FEI Themis-Z, JEOL 2010 and 2010F TEM systems.

4.4 Results All stages of the deposition were investigated to obtain a full understanding of

the growth process. First the ablation is investigated by using Scanning Electron

Microscopy and Electron Dispersive Spectroscopy (SEM-EDS) to analyze the

elemental composition and structure of the ablation targets. Next, the plasma

transport onto the substrate is characterized by varying process conditions during

deposition and obtaining the stoichiometry and crystallinity using Transmission

Electron Microscopy (TEM) and EDS. Finally, we optimize substrates to grow a

textured crystalline film, using RHEED and TEM to characterize the texture.

Ablation-Target analysis

The SC and SP targets of WTe2 were investigated using SEM-EDS after several

depositions. EDS was performed within and outside the ablation track for direct

comparison. The results are shown in table 1.

The EDS scans show that the ablated track is still close to stoichiometric WTe2,

albeit with some slight W-enrichment. The trend is consistent for both SC and SP

targets. Figure 1 shows a composition-sensitive Back-Scattered Electron (BSE)

image of the ablation track on the SC target. The track looks relatively smooth and

no phase segregation is observed. Outside the ablation track globules of

redeposited Te are observed, which indicates that slight post-pulse Te evaporation

(and redeposition) takes place. This process reduces the Te content within the

ablated area over time. The Te evaporation effects seem comparable for both

powder and single-crystal targets. We conclude that the WTe2 targets ablate

stoichiometrically (at a fluence of 1 Jcm2). Targets needs to be grazed periodically

however, to avoid worsening the evaporation effect over time (as is illustrated in

the Appendix).

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

48

At.% Te Powder Single Crystal

Pristine 64.8 66.0

Ablated 60.8 61.5 Table 1. SEM-EDS analysis of the powder- and single-crystal targets, at.% Te

measured on a pristine area and an ablated area. The measurement error is on

the order of a few at.%.

Figure 1. a.) Back-Scattered Electron (BSE) scan of the SC WTe2 target. The

ablation track on the bottom shows uniform small features, the darker globular

structures outside the ablation track consist of redeposited tellurium. The

diagonal line is a crystalline step edge which is preserved across many

depositions. b.) Higher magnification BSE scan of the deposition track. The

contrast is formed due to roughness, the scan shows no phase segregation.

Plasma transport and deposition

The fluence (irradiated laser energy per unit area) and substrate temperature

were optimized for both SC and SP targets. A copper mesh grid coated with

amorphous carbon film for TEM analysis was used as a substrate. This allowed for

TEM-EDS analysis which is more accurate for thin films than SEM-EDS analysis.

The films were generally deposited at room temperature to avoid re-evaporation. A

typical TEM bright-field image is shown in the Appendix. The films are generally

deposited in the amorphous phase, but some crystalline droplets or splashes of W

are found as well. Figure 2 shows the atomic composition of the films for varying

fluence and substrate temperatures. The process pressure was kept at 10-7 mBar.

Increasing this pressure to 0.12 mBar, the nominal value for Bi2Te3 films, reduced

the Te-content by ~20 at.% which was deemed undesirable (see Appendix). While

for increased fluence the Te content continuously reduces, increasing substrate

temperature shows an almost discontinuous step around 300 °C.

4.4 Results

49

Figure 2. a.) The fluence is varied to find the optimum ablation energy to obtain

stoichiometric WTe2 at 30 °C. Composition is measured using TEM-EDS on

continuous-carbon TEM grids. b.) The substrate temperature is varied and Te

content is measured. Excessive evaporation starts around 300 °C. Only slight

difference is observed between single crystal and powder targets.

We conclude from the reduced Te content at higher fluence and temperatures,

that Te is a more volatile element, as was also indicated by the evaporation

observed on the target (figure 1a). The deposition parameters show a wide

operating window is available: fluence should be kept below 1.5 Jcm-2 (at 30 °C),

and below 250 °C (at 0.8 Jcm2). Below these settings a stoichiometric or Te-rich

film is deposited. We expect that depositing a slightly Te-rich film may be

favourable, since excess Te may be easily evaporated to regain stoichiometry.

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

50

Film growth

Since no films grown on the amorphous carbon films used to obtain the data in

figure 2 crystallized, it is clear that a crystalline substrate is needed as a seed crystal

for crystalline growth. We have used a biaxially textured Bi2Te3 layer, deposited on

mica, as a seed crystal (See Appendix). The WTe2 growth conditions were obtained

from the previous section: 0.8 Jcm-2, 210 °C , 10-7

mBar. RHEED analysis of the

growth of a thick WTe2 film is shown in figure 3a. Initially, the bright streaks of

Bi2Te3 are clearly visible (red arrows). After growth of 0.5 ML WTe2, new streaks

start appearing (orange arrows) while the Bi2Te3 intensity diminishes rapidly. This

pattern looks as shown in figure 3b, and 3c when rotated by 30°. After 2-3 ML of

WTe2, the streaks start to fade again, and the RHEED pattern starts to resemble

figure 3d. When no Bi2Te3 seed layer is used, the WTe2 deposition immediately

shows a pattern such as shown in figure 3d. The wide rings indicate WTe2 is in an

amorphous or nanocrystalline state. To investigate the atomic structure of WTe2, a

2 ML crystalline film grown on Bi2Te3 is rotated into two zone axes 30° apart.

Intensity profiles across both RHEED patterns were taken. The peak spacings

correspond well to the literature values for the lattice plane spacing in the [100]

and [010] zone axes of 1T´-WTe2.

Figure 3. a.) RHEED trace during deposition of WTe2 on Bi2Te3. The bright

streaks of the Bi2Te3 [12-30] zone axis (red) fade during the first ML growth. The

WTe2 streaks appear after 0.5 ML (purple) and disappear after 2 ML. bcd.)

RHEED pattern of the 1T´-WTe2 crystal structure in the [100] and [010] zone

axes and amorphous/nanocrystalline phase. efg.) Schematic view of three WTe2

zone axes observed in this work. h.) Intensity profiles of the RHEED patterns in

bc. The peaks correspond to the spacings in ef. The red arrows indicate faint

Bi2Te3 peaks, which shows that the Bi2Te3-WTe2 films grow epitaxially matched to

the vdWaals substrate.

4.4 Results

51

For post-deposition structural analysis, a plan-view TEM sample was made by

growing on a thin (electron-transparent) amorphous SiO2 membrane. A 5 nm WTe2

layer was deposited on an out-of-plane textured Bi2Te3 seed (7 nm). The bright-

field TEM and large area diffraction (1.2 μm2) are shown in figure 4a. The film

looks homogeneous but contains voids (light spots), and some grain tilt is observed

(darker areas). Radial intensity profiles are taken from the diffraction pattern and

compared to a textured Bi2Te3 film on SiO2, and an untextured crystalline WTe2

film grown on amorphous carbon (see Appendix). All peaks of the bilayer film on

amorphous SiO2 can be ascribed to out-of-plane textured WTe2 and Bi2Te3.

The atomic composition was obtained by performing EDS on the same area as

the diffraction pattern, and repeated for another site. We obtain a normalized

composition of Bi18W22Te60 (at.%). Assuming stoichiometric layers of Bi2Te3 and

WTe2, based on the Bi and W content, we expected Bi16W20Te64, which means the

film is 4 at.% Te-deficient, and therefore almost stoichiometric within the error of

the EDS system. Since both layers of the film contain Te, no statement can be made

regarding the stoichiometry of the sublayers.

A cross-section of a Bi2Te3-WTe2 heterostructure was investigated using

HAADF-STEM (figure 4c) to show the capability of epitaxially incorporating WTe2

within a multilayered device structure. The heterostructure contains a single and a

double bilayer WTe2. The number of pulses is linearly related to the deposition rate,

and is estimated as 286 pulses/nm for WTe2. The quintuple-layers of Bi2Te3 and

triplet layers of WTe2 as well as the vdWaals gaps separating them are clearly

resolved. By using a high-magnification HAADF-DPC STEM scan, the atomic

structure of the WTe2 and Bi2Te3 layers is resolved. From the alternating Te-atom

positions it is clear that the WTe2-1T´ phase was grown, as already indicated by

RHEED. The Bi2Te3 and WTe2 are epitaxially matched and possess a common zone

axis, however the WTe2 layers are not fully flat but show some waviness. This effect

has also been observed for other growth methods, and is due to the lattice

mismatch between Bi2Te3 and WTe2 and the weak interlayer bonding.20 The

vdWaals gap between Bi2Te3 and WTe2 is blurred in some spots, indicating the

occurrence of limited intermixing. We note that even after deposition of 4 ML of

WTe2 an epitaxial relation is maintained with the subsequent Bi2Te3 layer. This

indicates that even when the RHEED pattern is extremely weak, the crystal quality

is still sufficient for enabling heterostructure growth.

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

52

Figure 4. a.) Plan-view bright-field TEM of a Bi2Te3-WTe2 heterostructure. White

spots in the film are voids, while the darker areas indicate stronger scattering

contrast caused by misaligned grains. A diffraction pattern taken from a 1.25 μm

diameter area is shown in the inset. Sharp rings are observed and the radial

intensity profile is plotted in b). The profiles for textured Bi2Te3, untextured WTe2,

and a textured Bi2Te3-WTe2 heterostructure (from a) are plotted. For the

heterostructure only c-axis texture peaks were observed for both WTe2 and Bi2Te3.

c) Cross section of a multilayered Bi2Te3-WTe2 film grown on mica. The vdWaals

gaps in both materials can be clearly distinguished.

4.5 Discussion

53

4.5 Discussion We have shown it is possible to grow biaxially textured WTe2 with monolayer

control at low temperatures, using PLD. However, the films can only be grown

using crystalline targets and using Bi2Te3 seed layers.

Since the powder target does not contain WTe2 phases, it has to overcome an

energy barrier not present for the single crystal target. For CdTe it has been shown

that the ablation/evaporation mainly produces non-reactive Te2 monomer units,

which can be cracked using higher (laser) energy.20,28 However this effect was not

observed by us; even when fluence was increased to 5 Jcm-2 no crystallization was

observed. It seems likely however that the enthalpy of mixing of the highly

dissimilar W and Te atomic species is relatively low, and therefore using a SP will

not produce the desired structures.

Concerning the Bi2Te3 seed layers, two mechanisms may contribute to the

successful epitaxy of WTe2. Since the lattice parameter is highly dissimilar (20%), it

seems unlikely the Bi2Te3 provides a lattice matching condition directly. However,

since both compounds share bond symmetry and an outer Te layer, compatibility is

higher than for a mica-WTe2 interface. Furthermore, as shown in figure 3, the

WTe2 crystal quality deteriorates as the film increases in thickness. This indicates

the direct proximity of the Bi2Te3 is necessary, which indicates intermixing or

scavenging effects might play a role. Since the WTe2 films stay roughly

stoichiometric however, it seems more likely the crystal structure deteriorates due

to increasing waviness of the WTe2 sublayers for increasing sublayer thickness.

4.6 Conclusions The growth of single-crystal-like WTe2 films at low temperatures (210 °C) using

Pulsed Laser Deposition was demonstrated. We have elucidated the ablation and

deposition processes, and found it is crucial to use a single-crystal target and a

Bi2Te3-type seed layer. We analyzed the growth dynamics using RHEED and TEM.

Furthermore, we showed cross-sectional TEM analysis of mono- or few-layer WTe2

heterostructures grown using the principle of van der Waals epitaxy. Using the

presented recipe, WTe2 growth using PLD may be implemented for further

research into functional properties and devices.

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

54

4.7 Literature 1 D. JARIWALA, V. K. SANGWAN, L. J. LAUHON, T. J. MARKS AND M. C. HERSAM, ACS NANO, 2014, 8,

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HALDOLAARACHCHIGE, M. HIRSCHBERGER, N. P. ONG AND R. J. CAVA, NATURE, 2014, 514, 205–208.

4 Y. WANG, E. LIU, H. LIU, Y. PAN, L. ZHANG, J. ZENG, Y. FU, M. WANG, K. XU, Z. HUANG, Z. WANG, H. Z. LU, D. XING, B. WANG, X. WAN AND F. MIAO, NAT. COMMUN., 2016, 7, 1–6.

5 Q. H. WANG, K. KALANTAR-ZADEH, A. KIS, J. N. COLEMAN AND M. S. STRANO, NAT. NANOTECHNOL., 2012, 7, 699–712.

6 K. A. N. DUERLOO, Y. LI AND E. J. REED, NAT. COMMUN., 2014, 5, 1–9. 7 T. A. J. LOH AND D. H. C. CHUA, J. PHYS. CHEM. C, 2015, 119, 27496–27504. 8 T. A. EMPANTE, Y. ZHOU, V. KLEE, A. E. NGUYEN, I. H. LU, M. D. VALENTIN, S. A. NAGHIBI

ALVILLAR, E. PRECIADO, A. J. BERGES, C. S. MERIDA, M. GOMEZ, S. BOBEK, M. ISARRARAZ, E. J. REED AND L. BARTELS, ACS NANO, 2017, 11, 900–905.

9 Z. CAI, B. LIU, X. ZOU AND H.-M. CHENG, CHEM. REV., 2018, ACS.CHEMREV.7B00536. 10 A K. GEIM AND I. V GRIGORIEVA, NATURE, 2013, 499, 419–25. 11 K. S. NOVOSELOV, A. MISHCHENKO, A. CARVALHO, A. H. C. NETO AND O. ROAD, SCIENCE (80-. ).,

2016, 353, 461. 12 S. J. MCDONNELL AND R. M. WALLACE, THIN SOLID FILMS, 2016, 616, 482–501. 13 C. R. WOODS, L. BRITNELL, A. ECKMANN, R. S. MA, J. C. LU, H. M. GUO, X. LIN, G. L. YU, Y. CAO, R.

V. GORBACHEV, A. V. KRETININ, J. PARK, L. A. PONOMARENKO, M. I. KATSNELSON, Y. N. GORNOSTYREV, K. WATANABE, T. TANIGUCHI, C. CASIRAGHI, H.-J. GAO, A. K. GEIM AND K. S. NOVOSELOV, NAT. PHYS., 2014, 10, 451–456.

14 H. KUMAR, L. DONG AND V. B. SHENOY, SCI. REP., 2016, 5, 21516. 15 A. KOMA, K. UENO AND K. SAIKI, J. CRYST. GROWTH, 1991, 111, 1029–1032. 16 A. KOMA, THIN SOLID FILMS, 1992, 216, 72–76. 17 S. VISHWANATH, X. LIU, S. ROUVIMOV, L. BASILE, N. LU, A. AZCATL, K. MAGNO, R. M. WALLACE,

M. KIM, J.-C. IDROBO, J. K. FURDYNA, D. JENA AND H. G. XING, J. MATER. RES., 2016, 31, 900–910.

18 H. C. DIAZ, R. CHAGHI, Y. MA AND M. BATZILL, 2D MATER., 2015, 2, 044010. 19 E. XENOGIANNOPOULOU, P. TSIPAS, K. E. ARETOULI, D. TSOUTSOU, S. A. GIAMINI, C. BAZIOTI, G. P.

DIMITRAKOPULOS, P. KOMNINOU, S. BREMS, C. HUYGHEBAERT, I. P. RADU AND A. DIMOULAS, NANOSCALE, 2015, 7, 7896–7905.

20 L. A. WALSH, R. YUE, Q. WANG, A. T. BARTON, R. ADDOU, C. M. SMYTH, H. ZHU, J. KIM, L. COLOMBO, M. J. KIM, R. M. WALLACE AND C. L. HINKLE, 2D MATER., 2017, 4, 025044.

21 S. TANG, C. ZHANG, DI. WONG, Z. PEDRAMRAZI, H. Z. TSAI, C. JIA, B. MORITZ, M. CLAASSEN, H. RYU, S. KAHN, J. JIANG, H. YAN, M. HASHIMOTO, D. LU, R. G. MOORE, C. C. HWANG, C. HWANG, Z. HUSSAIN, Y. CHEN, M. M. UGEDA, Z. LIU, X. XIE, T. P. DEVEREAUX, M. F. CROMMIE, S. K. MO AND

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4.8 Appendix

55

4.8 Appendix

Target ablation track analysis

The main text shows that after a few depositions the targets show some

roughness. When it is not grazed, this roughness is exacerbated on each subsequent

distribution, creating an often-seen pillar structure, where one element is

preferentially evaporated, in this case the tellurium. The effects can be clearly seen

in figure S1.

Figure S1. a.) Single-crystal target after extended ablation without grazing.

b.)The pillar shows extensive W-enrichment: the white features are pure W.

Fluence and temperature tuning on amorphous carbon TEM-grids

A typical bright-field view of a WTe2 film on amorphous carbon is shown in

figure S2. The films are typically homogeneous and smooth, except for small

crystalline W droplets. The films are amorphous, unless one grows on a broken and

wrinkled carbon grid. Then the film crystallizes in a circular area around the hole,

presumably due to the higher availability of nucleation sites within the wrinkled

film.

Figure S2. a.) An amorphous WTe2 film as deposited on carbon film. b.) SAED of

the area shown in a. The broad rings indicate an amorphous or nanocrystalline

film.

Chapter 4. Pulsed laser deposition of epitaxial tungsten-telluride heterostructures.

56

Influence of process gas on the WTe2 deposition.

A WTe2 deposition was performed at higher chamber pressure than normal,

using a fluence of 0.8 Jcm2 at room temperature. The results are shown in table S1.

The Te content dropped by 20 at.%, which we speculate is due to the lower mass of

Te allowing it to be scattered by the process gas more easily than W.

Ar Pressure (mBar)

At.% Te

10-2

57

10-7

78

Table S1. Atomic concentration Te for high and low process pressure. Te is

preferentially stopped from reaching the substrate.

Use of Mica-Bi2Te3 seed crystal.

Two WTe2 films were grown using a mica substrate and single-crystal target.

For one film, a layer Bi2Te3 was deposited. The RHEED in figure S3 shows the

WTe2 film is amorphous or highly disordered without this seed layer. Figure S4

shows sharp interfaces with both mica and WTe2 are formed by the Bi2Te3.

Figure S3. RHEED patterns of substrate (mica), seed crystal (Bi2Te3) and WTe2

layer. It shows the use of a seed crystal is essential for obtaining an ordered WTe2

film. Furthermore, when Bi2Te3 is used the figure shows epitaxial alignment of all

sublayers.

4.8 Appendix

57

Figure S4. HAADF-STEM image showing the muscovite mica substrate, Bi2Te3

seed crystal, and one monolayer of WTe2. Another block of Bi2Te3 was grown on

top. The individual layers have grown fully epitaxially, and the vdWaals gaps are

clearly resolved in all materials.

EDS Analysis of the bilayer on SiO2 window

EDs analysis was performed on a bilayer of Bi2Te3 and WTe2. To obtain the

atomic composition of this film, the background (Si, C, Cu, O) was removed from

the total composition. Two measurements on different film areas were performed.

Due to the extremely low thickness, the film signal intensity is quite low.

Nevertheless, both measurements closely agree. The normalized intensity is

calculated, as well as the nominal composition of this WTe2-Bi2Te3 bilayer system.

At.% Area 1 Area 2 stdev avg Normalized 100% Normalized & Stoichiometric

Te 11.31 10.21 0.78 10.76 60.42 64.00

W 4.10 3.74 0.26 3.92 22.00 20.01

Bi 3.25 3.01 0.18 3.13 17.58 15.99

Table S2. Atomic content of the bilayer constituent species. The composition is

measured in two distinct areas. Both scans show similar composition within the

fitting area of the EDS algorithm. The average normalized compositions are

calculated, as well as the stoichiometric composition closest to the observed

composition.

58