Due to quantum confinement of the carriers along the 'thin' direction of the 2D material (i.e. out of the plane), the electronic structure of the same compound in the bulk form or as a single monolayer can be quite different. For example, MoS₂ is a semiconductor in both bulk and monolayer forms, but it shows a direct band gap (KK transition) as a monolayer while it has an indirect character (ΓK) in the bulk<sup>72</sup>. Note that the opposite behavior is found for the InSe semiconductor, with a direct band gap in the bulk form turning indirect at the monolayer limit<sup>73</sup>. Other compounds, such as monolayer WTe₂, can exhibit peculiar electronic properties at the monolayer limit, such as bidimensional topological insulator states<sup>74</sup>.

### 5.1.2 Layered crystals, van der Waals gap and allotropes

In such anisotropic crystals, assembled from both covalent (in plane) and van der Waals (out of plane) interactions, it is important to evaluate the respective strength of the different cohesion forces.

<figure><img src="image_90.png" /><figcaption>FIG. 1. The energy content of covalent and two van der Waals bonds vs the distance between atoms (Ref. 19).</figcaption></figure>

<table><caption>TABLE II. Comparison of the energy content of various bond types (Refs. 6 and 19).</caption><thead><tr><th>Bond type</th><th>Energy content (kJ/mol)</th></tr></thead><tbody><tr><td>Ionic bonds</td><td>590-1050</td></tr><tr><td>Covalent bonds</td><td>563-710</td></tr><tr><td>Metallic bonds</td><td>113-347</td></tr><tr><td>Van der Waals (intermolecular) bonds</td><td></td></tr><tr><td>Hydrogen bonds with fluorine</td><td>&lt;42</td></tr><tr><td>Hydrogen bonds without fluorine</td><td>10-26</td></tr><tr><td>Other dipole-dipole bonds</td><td>4-21</td></tr><tr><td>Dipole-induce dipole bonds</td><td>&lt;2</td></tr><tr><td>Dispersion bonds</td><td>0.08-42</td></tr></tbody></table>

**Figure 5.2:** Energies of the different types of bonds as a function the distance, from Niklaus *et al.* (with further reference to 'C. Nobel, *Industrial Adhesives Handbook*, Casco Nobel, Fredensborg, Denmark, 1992' as Ref19).

FIG. 5.2, taken from Niklaus *et al.*<sup>75</sup>, shows the respective potential wells for various interactions. Similarly to covalent interactions, van der Waals forces are repulsive at short range and attractive at longer range. Their potential well is also much shallower than that of covalent bonds, but it extends over a much longer distance, about 2 to 4 Å. Above 5 Å the interaction is considered minimal. The energy required to break van der Waals bonds is at least one order of magnitude weaker than for most ionic or covalent bonds (see Table II in FIG. 5.2).

This near-universal description of various dipole-dipole (van der Waals) interactions with a unique potential well situated about 3 Å from the origin sets a particular vertical spacing which is common to many van der Waals solids. FIG. 5.3 shows the vertical spacing of several layered van der Waals solids of increasingly complex crystal structures:

* 1 monolayer is a single atomic plane : hexagonal graphite, hexagonal BN

* 1 monolayer is 3 sub-layers with tetrahedral coordination : hexagonal WSe₂

* 1 monolayer is 4 sub-layers with mixed tetrahedral and octahedral coordination : trigonal In₂Se₃

<footnote n="72"><sup>72</sup>K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz. "Atomically ThinMoS2: A New Direct-Gap Semiconductor". *Physical Review Letters* 105.13 (2010). DOI: 10.1103/physrevlett.105.136805.</footnote>

<footnote n="73"><sup>73</sup>G. W. Mudd, M. R. Molas, X. Chen, V. Zólyomi, K. Nogajewski, Z. R. Kudrynskyi, Z. D. Kovalyuk, G. Yusa, O. Makarovsky, L. Eaves, M. Potemski, V. I. Fal'ko, and A. Patanè. "The direct-to-indirect band gap crossover in two-dimensional van der Waals Indium Selenide crystals". *Scientific Reports* 6.1 (2016). DOI: 10.1038/srep39619.</footnote>

<footnote n="74"><sup>74</sup>Z. Fei, T. Palomaki, S. Wu, W. Zhao, X. Cai, B. Sun, P. Nguyen, J. Finney, X. Xu, and D. H. Cobden. "Edge conduction in monolayer WTe2". *Nature Physics* 13.7 (2017), pp. 677-682. DOI: 10.1038/nphys4091.</footnote>

<footnote n="75"><sup>75</sup>F. Niklaus, G. Stemme, J. -Q. Lu, and R. J. Gutmann. "Adhesive wafer bonding". *Journal of Applied Physics* 99.3 (2006), p. 031101. DOI: 10.1063/1.2168512.</footnote>
<figure><img src="image_91.png" /><figcaption>**Figure 5.3:** Van der Waals gaps in layered van der Waals crystals of increasing monolayer complexity: (a) hexagonal graphite, (b) hexagonal boron nitride, (c) hexagonal WSe₂, (d) trigonal In₂Se₃.</figcaption></figure>

The van der Waals *gap*, defined as the vertical spacing between the terminating atoms of adjacent monolayers, does not vary significantly between many 2D materials and remains close to ~3Å, despite differences in chemical nature (C,BN) or crystal structure between the various monolayers (WSe₂, In₂Se₃).

<figure><img src="image_92.png" /><figcaption>**Figure 5.4:** Bernal (ABA) and rhombohedral (ABC) stacking in few-layer graphene, from Warner *et al*.</figcaption></figure>

Although the van der Waals interaction is considered ‘weak’ (see Table FIG. 5.2), all layered van der Waals solids studied here have well-defined crystal structures.

As expected from the covalent bonds, the respective positions of all atoms inside a monolayer are well-defined.

In general, the same is true for the relative positions of each monolayer, leading to standard structure determination of the bulk layered van der Waals crystals by conventional XRD or TEM.

The effect of the ‘weak’ van der Waals bonds appears in the form of allotropes of the crystal structure, which are often close polytypes and only differ in nature by small in plane translations between individual monolayers.

A well-known example is the Rhombohedral (ABC) stacking of few-layer graphene, which only differs from the conventional Bernal stacking (ABA) by a small transation of the third layer(see FIG. 5.4 from Warner *et al*.<fnref n="76" />). Rhombohedral few-layer ‘graphene’ is experimentally obtained on some particular SiC substrates<fnref n="77" /> and recent calculations indicates that its may be the most stable form of ‘few-layer graphene’<fnref n="78" />.

## 5.2. Van der Waals epitaxy

### 5.2.1 Concept

For covalent heterostructures in excess of the critical thickness, misfit dislocations are introduced as missing or extra half-plane of atoms, to absorb the lattice mismatch at the heterointerface. Due to the small thickness of the epilayer (FIG. 5.5.a, grey) compared to the underlying substrate (FIG. 5.5.a, blue), the misfit dislocation (MD) segments tend to localize at the heteroepitaxial interface and may extend to the active region (FIG. 5.5.a, green) as threading dislocation (TD) segments.

<footnote n="76">⁷⁶J. H. Warner, M. Mukai, and A. I. Kirkland. "Atomic Structure of ABC Rhombohedral Stacked Trilayer Graphene". *ACS Nano* 6.6 (2012), pp. 5680–5686. DOI: 10.1021/nn3017926.</footnote>

<footnote n="77">⁷⁷D. Pierucci, H. Sediri, M. Hajlaoui, J.-C. Girard, T. Brumme, M. Calandra, E. Velez-Fort, G. Patriarche, M. G. Silly, G. Ferro, V. Soulière, M. Marangolo, F. Sirotti, F. Mauri, and A. Ouerghi. "Evidence for Flat Bands near the Fermi Level in Epitaxial Rhombohedral Multilayer Graphene". *ACS Nano* 9.5 (2015), pp. 5432–5439. DOI: 10.1021/acsnano.5b01239.</footnote>

<footnote n="78">⁷⁸R. Guerrero-Avilés, M. Pelc, F. R. Geisenhof, R. T. Weitz, and A. Ayuela. "Rhombohedral trilayer graphene is more stable than its Bernal counterpart". *Nanoscale* 14.43 (2022), pp. 16295–16302. DOI: 10.1039/d2nr01985j.</footnote>
<figure><img src="image_93.png" /><figcaption>**Figure 5.5:** (a) Conventional heteroepitaxy on mismatched substrate. (b) Heteroepitaxy on a van der Waals substrate. (c) Generalized van der Waals heteroepitaxy between van der Waals substrate, epilayer and active layer.</figcaption></figure>

In the presence of the van der Waals gap (see FIG. 5.3), the concept of misfit dislocation still holds mathematically but the weaker cohesion force out of plane allows for different relaxation mechanisms.

For a layered heterostructure, the interaction strength between the two lattices depends on the width of the van der Waals gap. For low values of van der Waals gap (~ 3 Å), the two lattices may strain, translate or rotate to assemble into a low energy configurations, as in any homogeneous layered bulk crystal (see FIG. 5.3). However, any excessive in plane strain can also be adapted by widening the van der Waals gap, which directly controls the interaction strength between the two lattices.

In theory, it is thus possible to adapt arbitrary lattice mismatch, while maintaining a precise and definite in plane alignment (FIG. 5.5.b). A first review by Koma<fnref n="79" /> reports the van der Waals epitaxy of NbSe₂ over MoS₂ with ~9% lattice mismatch (both materials being layered van der Waals crystals similar to hexagonal WSe₂, see FIG. 5.3). The NbSe₂ directly relaxes at the initial stages of growth (without extended defect). An in plane alignment of the two lattices is observed<fnref n="80" />.

Such van der Waals heteroepitaxy is not limited to substrate-epilayer configuration and one can consider multiple van der Waals heterostructures with mismatched materials stacked out of plane in a defect-free manner, but still precisely aligned in plane by van der Waals interactions (FIG. 5.5.c).

### 5.2.2 Case study: WSe₂ on Se-terminated GaAs(111)B

It is sometime possible to transform a covalent substrate into its quasi-van der Waals equivalent by passivating all the dangling bonds of its exposed surface. For III-V substrates, the pioneering work of Koma *et al.* first makes use of sulfur to create a GaS-like van der Waals termination of the GaAs(111)B surface<fnref n="81" />, before moving to selenium and a GaSe-like termination<fnref n="82" />.

Such Se-passivation of the GaAs(111)B surface is depicted FIG. 5.6.(a-c). Due to the compatible symmetries and comparable in plane lattice parameters between hexagonal GaSe ($a_{hexa}^{GaSe} = 3.74Å$) and the (3) surface lattice formed by the top Ga atoms of the (111)B plane ($a_{hexa}^{GaAs(111)} = a_{cubic}/\sqrt{2} \approx 3.99Å$), it is possible to covalently terminate most Ga dangling bonds with Se. In the simplified picture of FIG. 5.6.(c), no dangling bond is left, effectively transforming the covalent GaAs(111)B surface into its quasi-van der Waals GaSe counterpart.

<footnote n="79">⁷⁹A. Koma. "Van der Waals epitaxy—a new epitaxial growth method for a highly lattice-mismatched system". *Thin Solid Films* 216.1 (1992), pp. 72–76. DOI: 10.1016/0040-6090(92)90872-9.</footnote>

<footnote n="80">⁸⁰Several polytypes of NbSe₂ are reported, hexagonal (WSe₂-like) and monoclinic (WTe₂-like). We can assume reference to hexagonal NbSe₂ in the initial work from Koma.</footnote>

<footnote n="81">⁸¹K. Ueno, T. Shimada, K. Saiki, and A. Koma. "Heteroepitaxial growth of layered transition metal dichalcogenides on sulfur-terminated GaAs{111} surfaces". *Applied Physics Letters* 56.4 (1990), pp. 327–329. DOI: 10.1063/1.102817.</footnote>

<footnote n="82">⁸²K. Ueno, H. Abe, K. Saiki, and A. Koma. "Heteroepitaxy of Layered Semiconductor GaSe on a GaAs(111)B Surface". *Japanese Journal of Applied Physics* 30.Part 2, No. 8A (1991), pp. L1352–L1354. DOI: 10.1143/jjap.30.11352.</footnote>
<figure><img src="image_94.png" /><figcaption>**Figure 5.6:** (a) Structure of the layered van der Waals crystal GaSe. (b) Crystal structure of Ga-terminated covalent GaAs(111)B. (c) Se-termination of the GaAs(111)B surface. (d) Quasi van der Waals epitaxy of WSe₂ on Se-GaAs(111)B. (e-f) Corresponding atomically resolved STEM images, on a sample obtained by MBE growth in the group of M. Jamet (CEA Grenoble). (adapted from [Pierucci2022])</figcaption></figure>

FIG. 5.6. (d-f) shows a recent example [Pierucci2022] with the quasi-van der Waals epitaxy of a WSe₂ monolayer (see WSe₂ crystal structure in FIG. 5.3.c) obtained by MBE (CEA Grenoble, M. Jamet) over a Se-terminated quasi-van der Waals GaAs(111)B surface.

The axes of the WSe₂ hexagonal lattice are aligned with those from the hexagonal lattice formed by the terminating Ga atoms of the Se-GaAs(111)B surface, despite the huge lattice mismatch (~17%, $a_{\text{WSe}_2}^{\text{hexa}} = 3.28$ Å, $a_{\text{GaAs}(111)}^{\text{hexa}} \approx 3.99$ Å). The WSe₂ material is only strain-free within the STEM experimental limit (maximum ~2–3%), but we can consider it fully relaxed in a first approximation. In this framework, a coincidence lattice may be considered with 6 GaAs surface units matching the spacing of 7 WSe₂ (~4% mismatch).

<figure><img src="image_95.png" /><figcaption>**Figure 5.7:** Experimental determination of the electronic band structure using ARPES. (a) valence bands of GaAs showing the light holes (LH), heavy hole (HH) and spin orbit split (SO) bands at Γ. (b) Valence band of WSe₂ monolayer, showing the direct band gap at K. (c) Theoretical (dashed lines) and experimental band structure of monolayer WSe₂. (adapted from [Pierucci2022])</figcaption></figure>

FIG. 5.7 shows the experimental dispersion of the valence band obtained by synchrotron Angle Resolved Photoemission Spectroscopy (ARPES) on the WSe₂/Se-GaAs(111)B sample of FIG. 5.6. The contributions from the GaAs substrate (FIG. 5.7.a) and WSe₂ monolayer (FIG. 5.7.b) are clearly distinguished. The experimental band structure of WSe₂ matches that calculated for a free-standing (fully relaxed) WSe₂ monolayer, shifted in energy by charge transfer from the Se-GaAs substrate (FIG. 5.7.c). This confirms the quasi-van der Waals epitaxy of WSe₂, with a very limited impact of the substrate lattice through epitaxial strain, while still maintaining a precise crystallographic alignment in and out of plane between substrate and epilayer.
5.2.3 Moiré superlattices

While the concept of van der Waals epitaxy can be traced back to the 1990s (see §5.2.4), the current focus on bi-dimensional materials has certainly renewed its interest. An interesting extension of van der Waals heterostructure is the fabrication of moiré superlattices, formed by slight in plane misalignments of the respective unit cells, minor differences of the in plane lattice parameters or any combination thereof.

<figure><img src="image_96.png" /><figcaption>**Figure 5.8:** Example of moiré superlattices in 2D materials. [Left] Twisted boron nitride bilayer. (a) Theoretical moiré to the orthorhombic position with honeycomb lattices, (b) experimental result, (c) theoretical moiré close to orthorhombic position, (d) theoretical moiré close to hexagonal position. Adapted from Woods *et al.* [Right] Twisted WSe₂ bilayer close to the hexagonal high symmetry position, without and with lattice reconstruction observed by scanning tunneling microscopy (STM), from Li *et al.*</figcaption></figure>

Such controlled angular misorientation between the two lattices has lead to rich physics, including supra-
conducting bilayer graphene at ‘magic’ in plane angles<sup>83</sup>, moiré excitons in WSe<sub>2</sub>/WS<sub>2</sub><sup>84</sup>, moiré flat bands in
twisted bilayer WSe<sub>2</sub> (Li *et al.*<sup>85</sup>) and strong electrostatic potential in twisted BN bilayers (Woods *et al.*<sup>86</sup>).

The base concept is illustrated in FIG. 5.8 using the data from Woods *et al.* For two BN monolayers with angular misalignment close to the orthorhombic position (FIG. 5.8.left.a-c), the two lattices arrange into a moiré supercell (FIG. 5.8.left.a), which contains all the possible stackings that are accessible by a simple *translation* between the two lattices. For orthorhombic BN, we adopt the same notation as for the graphene allotropes (see §5.1.2) and the moiré supercell contains all three high symmetry combinations with A as the bottom layer: AA, AB and AC=BA (FIG. 5.8.left.a).

If the supercell area is large enough (i.e. at low twist angles), we will observe a ‘surface’ reconstruction which tends to maximize the area covered by the low energy stackings. For BN, the AA configuration is so unfavorable that its extension is reduced to a point (FIG. 5.8.left.c) and it is not visible in the experiment (FIG. 5.8.left.b). Conversely, the formation energies of two other high symmetry stacking AB and AC=BA are much lower and equal, which leads to a moiré supercell reconstruction formed by alternating AB and BA triangles (FIG. 5.8.left.c).

Li *et al.* observe similar reconstructions for twisted WSe₂ bilayers (FIG. 5.8.right) close to the hexagonal stacking. For large twist angles with respect to the reference hexagonal stacking (60-54.1=4.9°), the moiré supercell is too small and the surface does not reconstruct. However, the van der Waals gap still adapts locally to each different stacking combination. At lower twist angles (60-58.4=1.6°), the supercell is larger and the bilayer reconstructs. Here, AA' is the most stable configuration (see FIG. 5.8.left.d), and tends to dominate the

<footnote n="83"><sup>83</sup>Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero. "Unconventional superconductivity in magic-angle graphene superlattices". *Nature* 556.7699 (2018), pp. 43–50. DOI: 10.1038/nature26160.</footnote>

<footnote n="84"><sup>84</sup>C. Jin, E. C. Regan, A. Yan, M. I. B. Utama, D. Wang, S. Zhao, Y. Qin, S. Yang, Z. Zheng, S. Shi, K. Watanabe, T. Taniguchi, S. Tongay, A. Zettl, and F. Wang. "Observation of moiré excitons in WSe2/WS2 heterostructure superlattices". *Nature* 567.7746 (2019), pp. 76–80. DOI: 10.1038/s41586-019-0976-y.</footnote>

<footnote n="85"><sup>85</sup>E. Li, J.-X. Hu, X. Feng, Z. Zhou, L. An, K. T. Law, N. Wang, and N. Lin. "Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe2". *Nature Communications* 12.1 (2021). DOI: 10.1038/s41467-021-25924-6.</footnote>

<footnote n="86"><sup>86</sup>C. R. Woods, P. Ares, H. Nevison-Andrews, M. J. Holwill, R. Fabregas, F. Guinea, A. K. Geim, K. S. Novoselov, N. R. Walet, and L. Fumagalli. "Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride". *Nature Communications* 12.1 (2021). DOI: 10.1038/s41467-020-20667-2.</footnote>
surface area (noted ‘B’ in FIG. 5.8.right).

### 5.2.4 Limits of van der Waals epitaxy

The previous sections illustrate the behavior of single or few layer van der Waals material interacting with other van der Waals (§5.2.3) and or quasi-van der Waals (§5.2.2) surfaces. In light of Chapter 3, dealing with the oriented crystal growth on amorphous surfaces, it is also interesting to investigate the growth of such layered van der Waals materials on amorphous substrates.

In Ahn *et al.*<sup>87</sup>, the growth of mono- and bi-layer WSe<sub>2</sub> monocrystalline flakes is performed by Chemical Vapor Deposition (CVD) on amorphous SiO<sub>2</sub> and several crystalline substrates: (0001) Al<sub>2</sub>O<sub>3</sub> (0001) AlN and (100) SrTiO<sub>3</sub>. No in plane epitaxial relationship is reported between the WSe<sub>2</sub> and the various (crystalline) substrates. However, the out of plane direction is always the *c* axis of the WSe<sub>2</sub> hexagonal crystal, for all types of substrate (crystalline and amorphous).

<figure><img src="image_97.png" /><figcaption>**Figure 5.9:** (a) Growth process of WSe<sub>2</sub> on thermal mismatched substrate. (b) Room-temperature photoluminescence (PL) of WSe<sub>2</sub> monolayer on various substrate. (c) Thermal mismatch between WSe<sub>2</sub> and several substrate. (adapted from Ahn *et al.*).</figcaption></figure>

FIG. 5.9.b shows the shift of the WSe<sub>2</sub> room-temperature PL peak wavelength depending on the chosen substrate. These various red and blue shifts correlate with the thermal mismatch of the corresponding support with WSe<sub>2</sub> (FIG. 5.9.c), relative to the growth temperature (825°C).

We observe here a conventional thermal mismatch phenomenon (FIG. 5.9.a), in which the WSe<sub>2</sub> layer is nominally relaxed during the growth, at high temperature. The *thermal* strain appears during the cooldown to room temperature, as the (thick) substrate forces its thermal expansion over the (thin) WSe<sub>2</sub> layer. The resulting in plane strain in the layer (up to 1% in Ahn *et al.* ) is visible in the PL spectra and disappears after the mechanical transfer of the WSe<sub>2</sub> crystals to another support.

The existence of such thermal strain implies the ‘anchoring’ of the layered van der Waals crystal to the substrate, despite the van der Waals gap (see §5.2.1). Defects in WSe<sub>2</sub>, such as dangling bonds or Se vacancies are possible candidates. Note that this phenomenon occurs indifferently on crystalline and amorphous (SiO<sub>2</sub>) supports. Ahn *et al.* report that a ‘rapid’ thermal quenching is required to obtain a reproducible strain state and that ‘slow’ cooling can result in partial relaxation.

<footnote n="87"><sup>87</sup>G. H. Ahn, M. Amani, H. Rasool, D.-H. Lien, J. P. Mastandrea, J. W. A. III, M. Dubey, D. C. Chrzan, A. M. Minor, and A. Javey. “Strain-engineered growth of two-dimensional materials”. *Nature Communications* 8.1 (2017). DOI: 10.1038/s41467-017-00516-5.</footnote>
## 5.3. Current work : 3R and 2H polytypes of bi-layer WSe₂

### 5.3.1 CVD growth of WSe₂ mono- and bi-layer on amorphous SiO₂ substrates

As in Ahn *et al.* (see §5.2.4), we perform at C2N the growth of layered van der Waals solid WSe₂ on amorphous SiO₂ substrates by CVD. The WSe₂ monocrystals obtained are oriented, with the *c* axis of the hexagonal crystal normal to the flat amorphous SiO₂ surface.

Note that this is the same orientation as the hexagonal GaN nanowires grown by MBE on SiO₂ (see §3.4 page 35).

<figure><img src="image_98.png" /><figcaption>**Figure 5.10:** (a) Experimental setup for the chemical vapor deposition growth of WSe₂. (b) Optical image of a WSe₂ monolayer, as grown on a SiO₂(283nm)/Si substrate. (*Phd thesis of Gaia di Berardino, unpublished data.*)</figcaption></figure>

The FIG. 5.10.a briefly presents the chemical vapor deposition (CVD) system used to grow the WSe₂ crystals. The reactor configuration is horizontal and classifies as a hot wall CVD (quartz tube). Elemental Se and tungsten oxide WO₃ are used as Se and W precursors, respectively. The carrier gas is Ar (~100 sccm), with some extra hydrogen (~10-50 sccm). The typical operating pressure is a 2-5 mbar and the growth duration is 15-30 min.

Due to the strong changes in optical index between air, SiO₂, Si and WSe₂, it is possible to distinguish monolayers of WSe₂ (height ≈ 6.45 Å) by standard optical microscopy (white light illumination) on specific SiO₂(283nm)/Si substrates. Our WSe₂ monolayers are triangular monocrystals, a few tens of microns wide (FIG. 5.10.b).

<figure><img src="image_99.png" /><figcaption>**Figure 5.11:** Optical microscopy of WSe₂ bilayers. (a) 2H, AA' stacking. (b) 3R, AB stacking. (*Master2 internship of Tamazouzt Chennit, unpublished data.*)</figcaption></figure>

In addition to monolayers, WSe₂ bilayers (height ≈ 12.9 Å) can be obtained on the same SiO₂ substrates in various polytypes. FIG. 5.11 shows the two most common configurations, which correspond to the two lowest formation energies: hexagonal 2H AA' stacking (FIG. 5.11.a) and rhombohedral 3R AB stacking (FIG. 5.11.b). The main difference is the rotation (0° or 60°) of the second WSe₂ layer with respect to the base WSe₂. Note that most bulk WSe₂ crystals are 2H, so that studies based on exfoliation have mostly focused on 2H WSe₂ bilayers, while the 3R WSe₂ structures are mostly obtained by CVD or MBE growth.
### 5.3.2 Crystal structure and optical signatures of bi-layer WSe$_{2}$ polytypes

The atomic structure of the 2H and 3R WSe$_{2}$ bilayer polytypes (see Fig. 5.11) is detailed in Fig. 5.12.

<figure><img src="image_100.png" /><figcaption>**Figure 5.12:** Atomic structure of (a) 2H WSe$_{2}$ bilayer (AA' stacking) and (b) 3R WSe$_{2}$ polytype (AB stacking). (Phd thesis of Gaia di Berardino, unpublished data.)</figcaption></figure>

The most stable polytype of bilayer WSe$_{2}$ is often referred to as 2H, or more precisely as AA' (Fig. 5.12.a). This bilayer structure simply consists in a single repeat of standard unit cell of the bulk WSe$_{2}$ hexagonal phase. The formation energy of the alternative 3R AB stacking is slightly higher$^{88}$ and the corresponding structure is formed by the first two layers of the hypothetical 3R rhombohedral phase of WSe$_{2}$ (ABC stacking).

Experimentally, the crystal phase can be unambiguously assessed from atomically resolved STEM-HAADF images taken along the 0001 zone axis, showing either empty or centered hexagons for AA' or AB, respectively.

<figure><img src="image_101.png" /><figcaption>**Figure 5.13:** (a) Atomically resolved STEM-HAADF image of a single WSe$_{2}$ monolayer and corresponding HAADF intensity profile. (b) Similar analysis for a 3R WSe$_{2}$ bilayer, with a centered hexagonal structure. (Phd thesis of Gaia di Berardino, unpublished data.)</figcaption></figure>

<footnote>$^{88}$J. He, K. Hummer, and C. Franchini. "Stacking effects on the electronic and optical properties of bilayer transition metal dichalcogenides MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$". *Physical Review B* 89.7 (2014), p. 075409. DOI: 10.1103/physrevb.89.075409.</footnote>
FIG. 5.13 compares two atomically resolved STEM-HAADF images obtained from CVD-grown WSe₂ transferred on TEM grids. For the WSe₂ monolayer (FIG. 5.13.a), the hexagonal lattice is well visible with unequal intensities at the hexagon summits which correspond to individual W and superposed Se₂ atoms, respectively. A hypothetical 2H bilayer would show the same hexagonal structure but with equal intensities at each summit of the hexagon (see FIG. 5.12.a). The 3R bilayer polytype presents a centered hexagon structure, as quantified by the matching HAADF intensity profile (FIG. 5.13.b).

<figure><img src="image_102.png" /><figcaption>**Figure 5.14:** (a) Room temperature photoluminescence of 2H, 3R bi- and mono-layer WSe₂. (b) Corresponding room temperature Raman characterization (532 nm laser).
*(Phd thesis of Gaia di Berardino, unpublished data.)*</figcaption></figure>

FIG. 5.14.a presents the room temperature photoluminescence (PL) of mono- and bi-layer WSe₂ crystals, in 3R and 2H configurations. The transition from direct (monolayer) to indirect (bilayer) band gap is marked by a sharp decline (7x) in absolute PL intensity. The two WSe₂ bilayers have different emission peak energies and linewidths, hinting at different electronic band structures.

Room-temperature Raman spectroscopy can also distinguish between WSe₂ mono- and bi-layers, with the specific absence of the *B*₂g line for monolayers (FIG. 5.14.b). The main difference between 3R and 2H WSe₂ polytypes is the intensity ratio between the near-superposed *A*₁g and *E*₂g Raman modes (~250 cm⁻¹).

## 5.4. Future work : WSeTe alloys

In addition to the layering polytype (i.e. the stacking order and relative positioning between monolayers of the same crystal structure), it is also possible to change the crystal phase of a given monolayer, moving from the standard hexagonal structure (graphene, boron nitride, WSe₂) to less symmetric crystals.

### 5.4.1 Crystal structure of WTe₂

Given the close proximity of chalcogens Se (Z=34) and Te (Z=52), one could expect WSe₂ and WTe₂ to share the same crystal structure. However, apart from the common laminar (van der Waals) structure, all their other structural and electronic properties are vastly different. The most stable crystal structure of monolayer WTe₂ is not hexagonal (1H) but monoclinic (distorted trigonal 1T'), which turns WTe₂ into a semi-metal with rather peculiar properties.

We see here that the Gard<fnref n="89" /> polytype notations of §5.3.2 (3R, 2H) now shows an ambiguous proximity with the polymorphic crystal phase notations (1H, 1T', 1T). In the current literature, notation "1X" usually refers to the crystal phase of individual monolayer, while "nX" (n>1) usually denotes a particular polytype (typically made of several monolayers which all share the *same* crystal phase).

<footnote n="89">⁸⁹A. Guinier, G. B. Bokij, K. Boll-Dornberger, J. M. Cowley, S. Đurović, H. Jagodzinski, P. Krishna, P. M. de Wolff, B. B. Zvyagin, D. E. Cox, P. Goodman, T. Hahn, K. Kuchitsu, and S. C. Abrahams. "Nomenclature of polytype structures. Report of the International Union of Crystallography, Ad hoc Committee on the Nomenclature of Disordered, Modulated and Polytype Structures". *Acta Crystallographica Section A Foundations of Crystallography* 40.4 (1984), pp. 399-404. DOI: 10.1107/s0108767384000842.</footnote>
<figure><img src="image_103.png" /><figcaption>**Figure 5.15:** (a-b) Cross section and top view of hexagonal (1H) WSe₂ monolayer. (c-d) Cross section and top view of distorted trigonal (1T') WTe₂ monolayer. (F. Oehler, personal notes)</figcaption></figure>

FIG. 5.15.(a-b) shows the standard hexagonal structure of a single WSe₂ monolayer. The non-primitive orthorhombic unit cell (FIG. 5.15.b dotted gray line) is preferred to the standard hexagonal representation to better show the proximity with the 1T' phase of WTe₂. FIG. 5.15.(c-d) shows the crystal structure of 1T' WTe₂ in matching azimuth with respect to 1H WSe₂. Due to the non-equivalent crystal structures, the axis naming scheme follows different conventions, with the exception of *c* being the 'out-of-plane' direction for both WSe₂ and WTe₂. Still, the primitive 1T' WTe₂ unit cell is a close sibling of the non-primitive 1H WSe₂ orthorhombic cell.

The distortion from 1T (regular trigonal) to 1T' (distorted trigonal) is visible as local W-W bond shortening along the *a* direction of the WTe₂ unit cell (FIG. 5.15.c-d). This distortion creates an anisotropy in the plane of the monolayer, with parallel zig-zag W-W chains (short W-W distance) aligned along the *b* direction. Such in-plane anisotropy is relatively common in 2D materials and can be found in several other layered van-der-Waals solids, notably black phosphorus<fnref n="90" />.

### 5.4.2 Electronic structure of WTe₂

In the case of 1T' WTe₂, the electronic band structure at room temperature resembles that of a semi-metal. However, intriguing properties are revealed at low temperature. As shown by Tang *et al.*<fnref n="91" />, the valence and conduction bands should cross at Γ (FIG. 5.16) but the strong spin orbit coupling results in an anti-crossing and the opening of a small topological gap (45-55 meV). This effectively creates a 2D topological insulator, characterized by an insulating bulk 2D phase and (topologically protected) helical spin-polarized edge currents. This peculiar 2D topological character is lost with increasing thickness<fnref n="92" />: 3ML 1T' WTe₂ behaves as a metal at low temperature, whereas the case of 2ML 1T' WTe₂ is unclear.

<footnote n="90">⁹⁰F. Xia, H. Wang, and Y. Jia. "Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics". *Nature Communications* 5.1 (2014). DOI: 10.1038/ncomms5458.</footnote>

<footnote n="91">⁹¹S. Tang, C. Zhang, D. 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 Z.-X. Shen. "Quantum spin Hall state in monolayer 1T'WTe2". *Nature Physics* 13.7 (2017), pp. 683–687. DOI: 10.1038/nphys4174.</footnote>

<footnote n="92">⁹²Z. Fei, T. Palomaki, S. Wu, W. Zhao, X. Cai, B. Sun, P. Nguyen, J. Finney, X. Xu, and D. H. Cobden. "Edge conduction in monolayer WTe2". *Nature Physics* 13.7 (2017), pp. 677–682. DOI: 10.1038/nphys4091.</footnote>
Note that strong spin orbit coupling (SOC) is a major feature of many TMD compounds, including the (trivial) electronic band structure of direct-band gap of MoS₂ or WSe₂ monolayers, as seen in the calculations of Alidoust et al.<fnref n="93" /> (FIG. 5.17). The SOC effect is particularly visible at K, with a large energy splitting of the valence bands (V1-V2) of several hundreds of meV (~450 meV for WSe₂ [Pierucci2022]).

<figure><img src="image_104.png" /><figcaption>**Figure 5.16:** Electronic band structure of 1T' WTe₂, showing the origin of the 2D topological band gap at Γ. (from Tang *et al*.)</figcaption></figure>

<figure><img src="image_105.png" /><figcaption>**Figure 5.17:** Effect of the spin orbit coupling (SOC) on the (trivial) band structure of hexagonal (1H) MoS₂ monolayer. Note the energy splitting of the spin-polarized valence bands (V1,V2) at K. (from Alidoust *et al*.).</figcaption></figure>

<footnote n="93">⁹³N. Alidoust, G. Bian, S.-Y. Xu, R. Sankar, M. Neupane, C. Liu, I. Belopolski, D.-X. Qu, J. D. Denlinger, F.-C. Chou, and M. Z. Hasan. "Observation of monolayer valence band spin-orbit effect and induced quantum well states in MoX2". *Nature Communications* 5.1 (2014). DOI: 10.1038/ncomms5673.</footnote>
5.4.3 Crystal and electronic structure of ternary WSeTe alloy

At the frontier between 1H WSe₂ and 1T' WTe₂, the ternary alloy WSeTe sits at a rather unique position. The random alloy WSe₂xTe₂(1-x) has been synthesized in the bulk form over the whole composition range (x ∈ [0, 1]) and exfoliated down to monolayer thickness. As shown by Yu et al.<fnref n="94" />, the alloy at mid-composition WSeTe (x ≈ 0.5) changes abruptly its crystal structure from 1H (WSe₂ -like) to 1T' (WTe₂ -like), in the monolayer as well as in the bulk form (FIG. 5.18.a-d).

<figure><img src="image_106.png" /><figcaption>**Figure 5.18:** (a) WSe₂xTe₂(1-x) bulk and exfoliated monolayer over the full composition range. (b-d) Crystal structure and chemical composition. (e) Room temperature PL from direct band gap 2H WSe₂xTe₂(1-x) alloys. (f) Corresponding variation of the PL peak position with the alloy content x. (adapted from Yu *et al*.).</figcaption></figure>

This structural change is associated to a brutal transition in the electronic band structure, which is par-
ticularly visible in the optical properties of WSe₂xTe₂(₁₋ₓ) monolayers. Similarly to 1H monolayer WSe₂ (see
FIG. 5.14.a), 1H ternary monolayers from the Se-rich region (WSe₂xTe₂(₁₋ₓ), x > 0.5) also display a direct
band gap character, with an intense PL emission at room temperature. Conversely, 1T' monolayer from the
Te-rich domain (WSe₂xTe₂(₁₋ₓ), x < 0.5) behave mostly as semi-metals. As shown FIG. 5.18.(e-f), the electronic
transition at x ~ 0.5 is very sharp, and the optical gap (Eₓ~0.5 ~ 1.4 eV, 880 nm) drops directly to ~ 0 eV for
x > 0.5, as soon as the crystal structure changes from 1H to 1T'.

**5.4.4 Growth challenge of WSeTe alloys**

As of today, there are many reports of the CVD growth of binary WSe₂ and WTe₂ but we could find no data
on CVD-grown ternary WSe₂xTe₂(1-x).

Barton *et al.*<fnref n="95" /> describe the growth of WSe₂xTe₂(₁₋ₓ) by MBE, but had to face strong experimental challenges. In contrast to the homogeneous bulk growth of WSe₂xTe₂(₁₋ₓ) over the complete range (x ∈ [0, 1]) (see §5.4.3 above), Barton *et al.* report a large miscibility gap between xlower ≈ 0.16 and xupper ≈ 0.79. This discrepancy is possibly related to the different growth temperatures used in these techniques (~ 900°C for bulk vapor transport and ~ 250°C for MBE).

<footnote n="94">⁹⁴P. Yu, J. Lin, L. Sun, Q. L. Le, X. Yu, G. Gao, C.-H. Hsu, D. Wu, T.-R. Chang, Q. Zeng, F. Liu, Q. J. Wang, H.-T. Jeng, H. Lin, A. Trampert, Z. Shen, K. Suenaga, and Z. Liu. "Metal-Semiconductor Phase-Transition in WSe2(1-x)Te2x Monolayer". *Advanced Materials* 29.4 (2016), p. 1603991. DOI: 10.1002/adma.201603991.</footnote>

<footnote n="95">⁹⁵A. T. Barton, R. Yue, L. A. Walsh, G. Zhou, C. Cormier, C. M. Smyth, R. Addou, L. Colombo, R. M. Wallace, and C. L. Hinkle. "WSe(2-x)Tex alloys grown by molecular beam epitaxy". *2D Materials* 6.4 (2019), p. 045027. DOI: 10.1088/2053-1583/ab334d.</footnote>
<figure><img src="image_107.png" /><figcaption>**Figure 5.19:** Schematics of ternary growth, sulfurization and tellurisation steps. (from Wang *et al.*).</figcaption></figure>

Wang *et al.*<fnref n="96" /> report the growth of the closely related ternary compound WS₂ₓTe₂(₁-x) (Sulfur-Tellurium) by CVD (at 850°C). There are some indications of a miscibility gap, but also some post-growth sulfurization or tellurization treatment to obtain well-controlled WSe₂ₓTe₂(₁-x) monolayers (FIG. 5.19). Although the details are not fully understood, Wang *et al.* are able to obtain alloy content over the full composition range (x ∈ [0, 1]), which is promising for the proposed WSe₂ₓTe₂(₁-x) (Selenium-Tellurium) alloys.

### 5.4.5 Janus WSeTe monolayers

In addition to the random alloys of §5.4.4 (FIG. 5.20.a), it is possible to construct yet another ternary alloy in the form of a Janus monolayer. In this particular configuration, the top Se sub-layer has been fully replaced by its Te counterpart, forming a new asymmetrical crystal unit "SeWTe" (FIG. 5.20.b).

<figure><img src="image_108.png" /><figcaption>**Figure 5.20:** (a) Random alloy WSeTe monolayer. (b) Janus WSeTe monolayer. (c) Extract from Yao *et al.* showing the calculated electronic dispersion of hexagonal Janus WSeTe monolayer, without (left) and with (center) spin orbit coupling. At K and K', the valleys are spin ↑ and ↓ polarized, respectively. At Γ, we find instead a helical spin texture (right).</figcaption></figure>

The electronic structure of such Janus WSeTe monolayer is detailed by Yao *et al.*<fnref n="97" />.

<footnote n="96">⁹⁶Z. Wang, J. Sun, H. Wang, Y. Lei, Y. Xie, G. Wang, Y. Zhao, X. Li, H. Xu, X. Yang, L. Feng, and X. Ma. "2H/1T' phase WS2(1-x)Te2x alloys grown by chemical vapor deposition with tunable band structures". *Applied Surface Science* 504 (2020), p. 144371. DOI: 10.1016/j.apsusc.2019.144371.</footnote>

<footnote n="97">⁹⁷Q.-F. Yao, J. Cai, W.-Y. Tong, S.-J. Gong, J.-Q. Wang, X. Wan, C.-G. Duan, and J. H. Chu. "Manipulation of the large Rashba spin splitting in polar two-dimensional transition-metal dichalcogenides". *Physical Review B* 95.16 (2017). DOI: 10.1103/physrevb.95.165401.</footnote>
Compared to random alloys WSeTe, Janus WSeTe (1H) monolayers also present the usual ‘giant’ spin orbit splitting at *K*/*K'* between spin up and down polarized bands, similar to many other 1H TMD binaries (see FIG. 5.17). However, the WSeTe Janus structure breaks an additional vertical symmetry, so that the monolayer now hosts an out of plane electric polarization field *P* (FIG. 5.20.b). This gives rise to further band modification at Γ, but this time with an helical spin texture in the plane (FIG. 5.20.c.right). Therefore the spin polarization of the topmost valence each bands varies from out of plane at *K* and *K'*, to in plane at Γ. Janus 1H WSeTe monolayers are expected to show one of the largest Rashba energies (*E*<sub>R</sub>) and momentum offsets (*k*<sub>R</sub>) at Γ, due to the strong inner polarization field (*P*) out of plane. These large values of Rashba splitting are interesting and even compare favorably with recent materials (such as BiTeI monolayer) with potential application in future spintronic devices for which high energy differences between spin-polarized bands are critical<sup>98</sup>.

Note that another form of Rashba effect can affect random ternary TMD alloy monolayers, with spontaneous
in plane polarization fields leading to unidirectional spin up/down splitting at Γ [Zribi2022]. This ‘out of
plane’ (for the spin) Rashba effect is different of the ‘conventional’ (in plane, for the spin) Rashba effect shown
FIG. 5.20.c.

Still, Janus monolayers are notoriously difficult to fabricate and there are only a few reports of successful Janus monolayer fabrication in the MoSSe system, based on post-growth selenization of MoS₂<sup>99</sup> or sulfurization of MoSe₂<sup>100</sup>.

**5.5. Conclusion**

Using mechanical exfoliation, the thickness dependencies of many layered van der Waals crystals have been investigated, revealing rich physics at the single monolayer limit. Moving from mechanical exfoliation to the direct growth of 2D materials by CVD or MBE is an important motivation for the crystal growth community, to prepare the fabrication of future devices and test the scalability of the growth processes.

In this context of this memoir, van der Waals epitaxy (Chapter 5) is yet another way to obtain oriented
crystal growth while limiting defects on mismatched substrates.

It is characterized by the following properties:

* The out of plane orientation of the grown material is strongly influenced by the lamellar structure of layered van der Waals crystals. On a flat surface, the lamellae often orient parallel to the starting surface, near-independently of its nature (amorphous, crystalline, polar, non-polar, van der Waals).

* In the case of crystalline substrates, van der Waals growth may proceed epitaxially. Significant in plane lattice mismatches (over several percents) can be accommodated by extending the out of plane van der Waals gap, while retaining the overall epitaxial relationship.

* Selected covalent substrates can be transformed in situ to expose a quasi-van der Waals surface, which offers many synergies between (quasi-) van der Waals and conventional planar epitaxy.

However van der Waals epitaxy also brings specific new challenges:

* The physical mechanism which allows van der Waals heterostructures to escape lattice mismatch is also at the origin of many crystal polytypes. Here, the weaker binding along the out of plane direction allows some relative movements between high symmetry positions in the plane, so that selecting and maintaining a particular crystal phase during growth may be experimentally challenging.

* In the case of simultaneous nucleations on mismatched crystalline substrates, the lateral coherence length may be limited. As each crystallite adapts the lattice mismatch in its own way, in plane mosaicity and high densities of grain boundaries may be created directly at the first interface, despite a well-defined epitaxial relationship between the two crystals.

<footnote n="98"><sup>98</sup>R. Li, Y. Cheng, and W. Huang. "Recent Progress of Janus 2D Transition Metal Chalcogenides: From Theory to Experiments". *Small* 14.45 (2018), p. 1802091. DOI: 10.1002/smll.201802091.</footnote>

<footnote n="99">⁹⁹A.-Y. Lu, H. Zhu, J. Xiao, C.-P. Chuu, Y. Han, M.-H. Chiu, C.-C. Cheng, C.-W. Yang, K.-H. Wei, Y. Yang, Y. Wang, D. Sokaras, D. Nordlund, P. Yang, D. A. Muller, M.-Y. Chou, X. Zhang, and L.-J. Li. "Janus monolayers of transition metal dichalcogenides". *Nature Nanotechnology* 12.8 (2017), pp. 744–749. DOI: 10.1038/nnano.2017.100.</footnote>

<footnote n="100">¹⁰⁰J. Zhang, S. Jia, I. Kholmanov, L. Dong, D. Er, W. Chen, H. Guo, Z. Jin, V. B. Shenoy, L. Shi, and J. Lou. "Janus Monolayer Transition-Metal Dichalcogenides". *ACS Nano* 11.8 (2017), pp. 8192–8198. DOI: 10.1021/acsnano.7b03186.</footnote>
Final remarks

Relative to commercial applications, planar epitaxial techniques using monocrystallines substrates are by far the most widespread. As of today, active layers of IV-IV, III-V, III-N or II-VI semiconductors are all mostly obtained by heteroepitaxy on mismatched buffer layers, and their planar growth follows the established limitations regarding critical thickness, lattice mismatch, thermal mismatch, polarity and symmetry.

My attempts at the oriented growth of crystalline semiconductors on amorphous substrates are also quite
limited. Strong crystal textures are obtained, in and out of the plane, but the coalescence of individual grains
always creates a huge density of defects in the final material.

The removal of the coalescence step by growing well-separated individual nanostructures may be seen as the
simple downscaling of (planar) selective area growth. However nanostructure epitaxy hosts new paradigms for
strain relaxation so that high quality material can be obtained, in near total independence of the substrate.
Even better, heteroepitaxial shells of arbitrary thickness can be grown strain-free on nanowire core below a
certain radius, which is simply not possible in planar geometry. Still, it is difficult to control the simultaneous
axial and radial growth processes, which often results in compositional inhomogeneities (alloys, doping), in the
form of unintentional axial or radial gradients.

The replacement of the covalent bonds at the initial heterointerface by van der Waals interactions is
probably the least disruptive concept investigated in this work. Here, the lattice mismatch may be adapted
freely by widening the van der Waals gap, with the promise of dislocation free layers and yet perfect epitaxial
relationship. An additional strong point of van der Waals epitaxy is the possible synergy between the traditional
(covalent) and van der Waals approaches to integrate new planar materials and lamellar semiconductors in
existing fabrication processes.

Scientifically, the rich physic phenomena obtained by slight misalignments, 180° rotations or small translations
between individual van der Waals materials is rather astounding. Controlling and engineering these many
orientations during or after growth is likely to be a stimulating challenge for the 2D materials community.
Chapter 6

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  <tr>
    <td>[Tang2015]</td>
    <td>F. Tang, J. S. Barnard, T. Zhu, F. Oehler, M. J. Kappers, and R. A. Oliver. "Microstructural dependency of optical properties of m-plane InGaN multiple quantum wells grown on 2°misoriented bulk GaN substrates". Applied Physics Letters 107.8 (2015), p. 082104. DOI: 10.1063/1.4928723.</td>
  </tr>
  <tr>
    <td>[Tong2022]</td>
    <td>C. Tong, A. Delamarre, R. D. Lépinau, A. Scaccabarozzi, F. Oehler, J.-C. Harmand, S. Collin, and A. Cattoni. "GaAs/GaInP nanowire solar cell on Si with state-of-the-art VOC and quasi-Fermi level splitting". Nanoscale (2022). DOI: 10.1039/d2nr02652j.</td>
  </tr>
</table>
[Zhu2016]
T. Zhu, D. Gachet, F. Tang, W. Y. Fu, F. Oehler, M. J. Kappers, P. Dawson, C. J. Humphreys, and R. A. Oliver. "Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence". *Applied Physics Letters* 109.23 (2016), p. 232103. doi: 10.1063/1.4971366.

[Zribi2022]
J. Zribi, D. Pierucci, F. Bisti, B. Zheng, J. Avila, L. Khalil, C. Ernandes, julien chaste, F. Oehler, M. Pala, T. Maroutian, I. Hermes, E. Lhuillier, A. Pan, and A. Ouerghi. "Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2x alloy". *Nanotechnology* (2022). doi: 10.1088/1361-6528/aca0f6.
# Chapter 7

# Curriculum Vitæ & Publications

## 7.1. Curriculum vitæ

Fabrice OEHLER

15/04/1983

fabrice.oehler@universite-paris-saclay.fr

Chargé de Recherche Classe Normale
Centre National de la Recherche Scientifique
Section 08

Centre de Nanoscience et Nanotechnologie (C2N) UMR9001
10 Boulevard Thomas Gobert
91120 Palaiseau, France

+33 (0) 1 70 27 04 70

https://mat2d.c2n.universite-paris-saclay.fr/en/

### 1. Academic Titles

* 2001, Baccalauréat général série S, mention Très Bien
  *Lycée la Martinière Monplaisir, Lyon, France*

* 2006, Engineer degree "Ecole Supérieure de Physique Chimie Industrielle de la ville de Paris"
  *ESPCI, Paris, France*

* 2007, Master of Science, "Ecole Nationale Supérieure", specialized in non-linear physics
  *ENS-Lyon, Lyon, France*

### 2. Research training and Titles

* 2007-2010, PhD thesis "Etude et compréhension des mécanismes de croissance catalysés des nanofils de silicium obtenus par Dépôt Chimique en phase Vapeur" supervised by Thierry BARON and Pascal GENTILE (jury : M. GENDRY, B. CAUSSAT, F. GLAS, P. FERRET)
  *CEA-Grenoble, Université Joseph Fourier, Grenoble, France*

* 2011-2013, Post-doctoral study, MOCVD growth of (Al,In)GaN structures for LEDs
  *Université de Cambridge, Material Dpt., GaN center, Cambridge, Royaume-Uni*

* 2013-2014, Post-doctoral study, MBE growth of III-V nanowires (InP, InAs, GaP, GaAs)
  *Laboratoire de Photonique et Nanostructures, Marcoussis, France*

### 3. Professional positions

* 2014, Recruited at CNRS, Chargé de Recherche 2ème classe, section 08
  *Laboratoire de Photonique et Nanostructures (LPN), Marcoussis, France*

* 2014-2020 ELPHYSE team at the LPN lab then C2N lab :

* Crystal growth, supervising work related to epitaxial III-V nanostructures obtained by MBE

* Alternative substrate fabrication using Metal Induced Crystallization
    *Centre de Nanosciences et Nanotechnologies (C2N), Palaiseau, France*

* 2020-2023 MAT2D team at the C2N lab :

* Crystal growth, 2D materials, transition metal dichalcogenides (WSe₂) by CVD
    *Centre de Nanosciences et Nanotechnologies (C2N), Palaiseau, France*
4. Working at the lab

* Responsible for the website of C2N 2019 - 2021

* Member of teams "Wet bench" and "Air treatment" at the C2N cleanroom, 2020 - 2023

* Elected member (researcher) at the C2N Lab council C2N, 2020 - ...

* Elected member (researcher) Comité Local d'Hygiène et Sécurité (CLHSCT) du C2N, 2020 - ...

5. Supervision

* Phd D. PELATI, 2015-2019 [main supervisor 90%]
  "Elaboration of GaAs solar cells based on textured substrates on glass"

* Phd R. de LEPINAU, 2017-2020 [co-supervisor 30%]
  "GaAs-on-Si solar cells based on nanowire arrays grown by molecular beam epitaxy"

* Phd A. PISHCHAGIN, 2017-2021 [co-supervisor 30%]
  "Diameter engineering in III-V nanowire heterostructures - Experiments and modelling"

* Phd G. di BERARDINO, 2022-2025 [main supervisor 100%]
  "CVD growth of phase switching 2D material WSeTe"

* Post-doctoral A. SCACCABAROZZI, 2017-2019 [co-supervisor 30%]
  "MBE growth of GaAs nanowire arrays for solar cells"

* Post-doctoral J. TANG, 2017-2018 [co-supervisor 30%]
  "MBE growth of quantum dots embedded in III-V nanowire"

* Post-doctoral C. HIMWAS, 2017-2019 [co-supervisor 30%]
  "GaAsP nanowires growth by MBE"

6. Scientific publications

More than 70 published works in peer-reviewed journals, including 9 Nano Letters, 3 PRB and 7 APL.
See up to date profiles: ORCID 0000-0003-1020-160X and Google Scholar.

7.2. Publication list with supervised PhD or post-doc

7.2.1 Publication with supervised PhD

Note: PhD names are listed in red color, supervised post-docs are marked using purple.

[Pishchagin2021] A. Pishchagin, F. Glas, G. Patriarche, A. Cattoni, J.-C. Harmand, and F. Oehler. "Dynamics of Droplet Consumption in Vapor-Liquid-Solid III-V Nanowire Growth". *Crystal Growth & Design* (2021). DOI: 10.1021/acs.cgd.1c00504.

[Pelati2018] D. Pelati, O. Mauguin, L. Largeau, F. Brisset, F. Glas, and F. Oehler. "Kinetics and crystal texture improvements on thin germanium layers obtained by aluminium induced crystallization using oxygen plasma". *Surface and Coatings Technology* 343 (2018), pp. 121-126. DOI: 10.1016/j.surfcoat.2017.10.034.

[Pelati2020] D. Pelati, G. Patriarche, L. Largeau, O. Mauguin, L. Travers, F. Brisset, F. Glas, and F. Oehler. "Microstructure of GaAs thin films grown on glass using Ge seed layers fabricated by aluminium induced crystallization". *Thin Solid Films* 694 (2020), p. 137737. DOI: 10.1016/j.tsf.2019.137737.

[Pelati2017] D. Pelati, G. Patriarche, O. Mauguin, L. Largeau, F. Brisset, F. Glas, and F. Oehler. "In Situ Optical Monitoring of New Pathways in the Metal-Induced Crystallization of Amorphous Ge". *Crystal Growth & Design* 17.11 (2017), pp. 5783-5789. DOI: 10.1021/acs.cgd.7b00799.

[Pelati2019] D. Pelati, G. Patriarche, O. Mauguin, L. Largeau, L. Travers, F. Brisset, F. Glas, and F. Oehler. "GaAs (1 1 1) epilayers grown by MBE on Ge (1 1 1): Twin reduction and polarity". *Journal of Crystal Growth* 519 (2019), pp. 84-90. DOI: 10.1016/j.jcrysgro.2019.05.006.
[Lepinau2019] R. Lépinau, A. Scaccabarozzi, G. Patriarche, L. Travers, S. Collin, A. Cattoni, and F. Oehler. "Evidence and control of unintentional As-rich shells in GaAs1-xPx nanowires". *Nanotechnology* 30.29 (2019), p. 294003. DOI: 10.1088/1361-6528/ab14c1.

7.2.2 Publications with supervised post-doc

Note: PhD names are listed in red color, supervised post-docs are marked using purple.

[Chen2021] H.-L. Chen, A. Scaccabarozzi, R. Lépinau, F. Oehler, A. Lemaître, J.-C. Harmand, A. Cattoni, and S. Collin. "Quantitative Assessment of Carrier Density by Cathodoluminescence. I. GaAs Thin Films and Modeling". *Physical Review Applied* 15.2 (2021), p. 024006. DOI: 10.1103/physrevapplied.15.024006.

[Chen2017] H.-L. Chen, C. Himwas, A. Scaccabarozzi, P. Rale, F. Oehler, A. Lemaître, L. Lombez, J.-F. Guillemoles, M. Tchernycheva, J.-C. Harmand, A. Cattoni, and S. Collin. "Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence". *Nano Letters* 17.11 (2017), pp. 6667-6675. DOI: 10.1021/acs.nanolett.7b02620.

[Chen2021a] H.-L. Chen, R. Lépinau, A. Scaccabarozzi, F. Oehler, J.-C. Harmand, A. Cattoni, and S. Collin. "Quantitative Assessment of Carrier Density by Cathodoluminescence. II. GaAs Nanowires". *Physical Review Applied* 15.2 (2021), p. 024007. DOI: 10.1103/physrevapplied.15.024007.

[Oehler2018] F. Oehler, A. Cattoni, A. Scaccabarozzi, G. Patriarche, F. Glas, and J.-C. Harmand. "Measuring and Modeling the Growth Dynamics of Self-Catalyzed GaP Nanowire Arrays". *Nano Letters* 18.2 (2018), pp. 701-708. DOI: 10.1021/acs.nanolett.7b03695.

[Scaccabarozzi2020] A. Scaccabarozzi, A. Cattoni, G. Patriarche, L. Travers, S. Collin, J.-C. Harmand, F. Glas, and F. Oehler. "Stable and high yield growth of GaP and In0.2Ga0.8As nanowire arrays using In as a catalyst". *Nanoscale* (2020). DOI: 10.1039/d0nr04139d.

[Himwas2017] C. Himwas, S. Collin, P. Rale, N. Chauvin, G. Patriarche, F. Oehler, F. H. Julien, L. Travers, J.-C. Harmand, and M. Tchernycheva. "In situ passivation of GaAsP nanowires". *Nanotechnology* 28.49 (2017), p. 495707. DOI: 10.1088/1361-6528/aa9533.

[Himwas2019] C. Himwas, S. Collin, H.-L. Chen, G. Patriarche, F. Oehler, L. Travers, O. Saket, F. Julien, J.-C. Harmand, and M. Tchernycheva. "Correlated optical and structural analyses of individual GaAsP/GaP core-shell nanowires". *Nanotechnology* (2019). DOI: 10.1088/1361-6528/ab1760.

[Saket2020a] O. Saket, C. Himwas, A. Cattoni, F. Oehler, F. Bayle, S. Collin, L. Travers, A. Babichev, F. H. Julien, J. C. Harmand, and M. Tchernycheva. "Influence of surface passivation on the electrical properties of p-i-n GaAsP nanowires". *Applied Physics Letters* 117.12 (2020), p. 123104. DOI: 10.1063/5.0022157.

[Saket2020] O. Saket, C. Himwas, V. Piazza, F. Bayle, A. Cattoni, F. Oehler, G. Patriarche, L. Travers, S. Collin, F. H. Julien, J.-C. Harmand, and M. Tchernycheva. "Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications". *Nanotechnology* 31.14 (2020), p. 145708. DOI: 10.1088/1361-6528/ab62c9.
# Appendix A

## Complete list of publications by year

### 2023

[Chiout2023] A. Chiout, C. Brochard-Richard, L. Marty, N. Bendiab, M.-Q. Zhao, A. T. C. Johnson, F. Oehler, A. Ouerghi, and J. Chaste. "Extreme mechanical tunability in suspended MoS2 resonator controlled by Joule heating". npj 2D Materials and Applications 7.1 (2023). DOI: 10.1038/s41699-023-00383-3.

[Khalil2023a] L. Khalil, C. Ernandes, J. Avila, A. Rousseau, P. Dudin, N. D. Zhigadlo, G. Cassabois, B. Gil, F. Oehler, J. Chaste, and A. Ouerghi. "High p doped and robust band structure in Mg-doped hexagonal boron nitride". Nanoscale Advances (2023). DOI: 10.1039/d2na00843b.

[Khalil2023] L. Khalil, D. Pierucci, E. Velez-Fort, J. Avila, C. Vergnaud, P. Dudin, F. Oehler, J. Chaste, M. Jamet, E. Lhuillier, M. Pala, and A. Ouerghi. "Hybridization and localized flat band in the WSe2/MoSe2 heterobilayer". Nanotechnology 34.4 (2023), p. 045702. DOI: 10.1088/1361-6528/ac9abe.

### 2022

[Khalil2022] L. Khalil, P. M. Forcella, G. Kremer, F. Bisti, J. Chaste, J.-C. Girard, F. Oehler, M. Pala, J.-F. Dayen, D. Logoteta, M. Goerbig, F. Bertran, P. L. Fèvre, E. Lhuillier, J. Rault, D. Pierucci, G. Profeta, and A. Ouerghi. "Alpha-As2Te3 as a platform for the exploration of the electronic band structure of single layer beta-tellurene". Physical Review B 106.12 (2022), p. 125152. DOI: 10.1103/physrevb.106.125152.

[Pierucci2022] D. Pierucci, A. Mahmoudi, M. Silly, F. Bisti, F. Oehler, G. Patriarche, F. Bonell, A. Marty, C. Vergnaud, M. Jamet, H. Boukari, E. Lhuillier, M. Pala, and A. Ouerghi. "Evidence for highly p-type doping and type II band alignment in large scale monolayer WSe2/Se-terminated GaAs heterojunction grown by molecular beam epitaxy". Nanoscale 14.15 (2022), pp. 5859-5868. DOI: 10.1039/d2nr00458e.

[Tong2022] C. Tong, A. Delamarre, R. D. Lépinau, A. Scaccabarozzi, F. Oehler, J.-C. Harmand, S. Collin, and A. Cattoni. "GaAs/GaInP nanowire solar cell on Si with state-of-the-art VOC and quasi-Fermi level splitting". Nanoscale (2022). DOI: 10.1039/d2nr02652j.

[Zribi2022] J. Zribi, D. Pierucci, F. Bisti, B. Zheng, J. Avila, L. Khalil, C. Ernandes, Julien Chaste, F. Oehler, M. Pala, T. Maroutian, I. Hermes, E. Lhuillier, A. Pan, and A. Ouerghi. "Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2x alloy". Nanotechnology (2022). DOI: 10.1088/1361-6528/aca0f6.
2021

[Chen2021a] H.-L. Chen, R. D. Lépinau, A. Scaccabarozzi, F. Oehler, J.-C. Harmand, A. Cattoni, and S. Collin. "Quantitative Assessment of Carrier Density by Cathodoluminescence. II. GaAs Nanowires". *Physical Review Applied* 15.2 (2021), p. 024007. DOI: 10.1103/physrevapplied.15.024007.

[Chen2021] H.-L. Chen, A. Scaccabarozzi, R. D. Lépinau, F. Oehler, A. Lemaître, J.-C. Harmand, A. Cattoni, and S. Collin. "Quantitative Assessment of Carrier Density by Cathodoluminescence. I. GaAs Thin Films and Modeling". *Physical Review Applied* 15.2 (2021), p. 024006. DOI: 10.1103/physrevapplied.15.024006.

[Chiout2021] A. Chiout, F. Correia, M.-Q. Zhao, A. T. C. Johnson, D. Pierucci, F. Oehler, A. Ouerghi, and J. Chaste. "Multi-order phononic frequency comb generation within a MoS2 electromechanical resonator". *Applied Physics Letters* 119.17 (2021), p. 173102. DOI: 10.1063/5.0059015.

[Ernandes2021] C. Ernandes, L. Khalil, H. Almabrouk, D. Pierucci, B. Zheng, J. Avila, P. Dudin, J. Chaste, F. Oehler, M. Pala, F. Bisti, T. Brulé, E. Lhuillier, A. Pan, and A. Ouerghi. "Indirect to direct band gap crossover in two-dimensional WS2(1-x)Se2x alloys". *npj 2D Materials and Applications* 5.1 (2021). DOI: 10.1038/s41699-020-00187-9.

[Ernandes2021a] C. Ernandes, L. Khalil, H. Henck, M.-Q. Zhao, J. Chaste, F. Oehler, A. T. C. Johnson, M. C. Asensio, D. Pierucci, M. Pala, J. Avila, and A. Ouerghi. "Strain and Spin-Orbit Coupling Engineering in Twisted WS2/Graphene Heterobilayer". *Nanomaterials* 11.11 (2021), p. 2921. DOI: 10.3390/nano11112921.

[Khalil2021] L. Khalil, J.-C. Girard, D. Pierucci, F. Bisti, J. Chaste, F. Oehler, C. Gréboval, U. N. Noumbé, J.-F. Dayen, D. Logoteta, G. Patriarche, J. Rault, M. Pala, E. Lhuillier, and A. Ouerghi. "Electronic band gap of van der Waals alpha-As2Te3 crystals". *Applied Physics Letters* 119.4 (2021), p. 043103. DOI: 10.1063/5.0058291.

[Pishchagin2021] A. Pishchagin, F. Glas, G. Patriarche, A. Cattoni, J.-C. Harmand, and F. Oehler. "Dynamics of Droplet Consumption in Vapor-Liquid-Solid III-V Nanowire Growth". *Crystal Growth & Design* (2021). DOI: 10.1021/acs.cgd.1c00504.

2020

[Pelati2020] D. Pelati, G. Patriarche, L. Largeau, O. Mauguin, L. Travers, F. Brisset, F. Glas, and F. Oehler. "Microstructure of GaAs thin films grown on glass using Ge seed layers fabricated by aluminium induced crystallization". *Thin Solid Films* 694 (2020), p. 137737. DOI: 10.1016/j.tsf.2019.137737.

[Saket2020a] O. Saket, C. Himwas, A. Cattoni, F. Oehler, F. Bayle, S. Collin, L. Travers, A. Babichev, F. H. Julien, J. C. Harmand, and M. Tchernycheva. "Influence of surface passivation on the electrical properties of p-i-n GaAsP nanowires". *Applied Physics Letters* 117.12 (2020), p. 123104. DOI: 10.1063/5.0022157.

[Saket2020] O. Saket, C. Himwas, V. Piazza, F. Bayle, A. Cattoni, F. Oehler, G. Patriarche, L. Travers, S. Collin, F. H. Julien, J.-C. Harmand, and M. Tchernycheva. "Nanoscale electrical analyses of axial-junction GaAsP nanowires for solar cell applications". *Nanotechnology* 31.14 (2020), p. 145708. DOI: 10.1088/1361-6528/ab62c9.

[Scaccabarozzi2020] A. Scaccabarozzi, A. Cattoni, G. Patriarche, L. Travers, S. Collin, J.-C. Harmand, F. Glas, and F. Oehler. "Stable and high yield growth of GaP and In0.2Ga0.8As nanowire arrays using In as a catalyst". *Nanoscale* (2020). DOI: 10.1039/d0nr04139d.

[Zribi2020] J. Zribi, L. Khalil, J. Avila, J. Chaste, H. Henck, F. Oehler, B. Gil, S. Liu, J. H. Edgar, C. Giorgetti, Y. J. Dappe, E. Lhuillier, G. Cassabois, A. Ouerghi, and D. Pierucci. "Structural and electronic transitions in few layers of isotopically pure hexagonal boron nitride". *Physical Review B* 102.11 (2020). DOI: 10.1103/physrevb.102.115141.
2019

[Himwas2019]
C. Himwas, S. Collin, H.-L. Chen, G. Patriarche, F. Oehler, L. Travers, O. Saket, F. Julien, J.-C. Harmand, and M. Tchernycheva. "Correlated optical and structural analyses of individual GaAsP/GaP core-shell nanowires". *Nanotechnology* (2019). DOI: 10.1088/1361-6528/ab1760.

[Lepinau2019]
R. de Lépinau, A. Scaccabarozzi, G. Patriarche, L. Travers, S. Collin, A. Cattoni, and F. Oehler. "Evidence and control of unintentional As-rich shells in GaAs1-xPx nanowires". *Nanotechnology* 30.29 (2019), p. 294003. DOI: 10.1088/1361-6528/ab14c1.

[Mancini2019]
L. Mancini, M. Morassi, C. Sinito, O. Brandt, L. Geelhaar, H.-G. Song, Y.-H. Cho, N. Guan, A. Cavanna, J. Njeim, A. Madouri, C. Barbier, L. Largeau, A. Babichev, F. H. Julien, L. Travers, F. Oehler, N. Gogneau, J.-C. Harmand, and M. Tchernycheva. "Optical properties of GaN nanowires grown on chemical vapor deposited-graphene". *Nanotechnology* 30.21 (2019), p. 214005. DOI: 10.1088/1361-6528/ab0570.

[Pelati2019]
D. Pelati, G. Patriarche, O. Mauguin, L. Largeau, L. Travers, F. Brisset, F. Glas, and F. Oehler. "GaAs (1 1 1) epilayers grown by MBE on Ge (1 1 1): Twin reduction and polarity". *Journal of Crystal Growth* 519 (2019), pp. 84-90. DOI: 10.1016/j.jcrysgro.2019.05.006.

[Tang2019]
F. Tang, T. Zhu, W.-Y. Fu, F. Oehler, S. Zhang, J. T. Griffiths, C. Humphreys, T. L. Martin, P. A. J. Bagot, M. P. Moody, S. K. Patra, S. Schulz, P. Dawson, S. Church, J. Jacobs, and R. A. Oliver. "Insight into the impact of atomic- and nano-scale indium distributions on the optical properties of InGaN/GaN quantum well structures grown on m-plane freestanding GaN substrates". *Journal of Applied Physics* 125.22 (2019), p. 225704. DOI: 10.1063/1.5097411.

2018

[Khalid2018]
N. Khalid, J.-Y. Kim, A. Ionescu, T. Hussain, F. Oehler, T. Zhu, R. A. Oliver, I. Farrer, R. Ahmad, and C. H. W. Barnes. "Structure and magnetic properties of an epitaxial Fe(110)/MgO(111)/GaN(0001) heterostructure". *Journal of Applied Physics* 123.10 (2018), p. 103901. DOI: 10.1063/1.5022433.

[Morassi2018]
M. Morassi, L. Largeau, F. Oehler, H.-G. Song, L. Travers, F. H. Julien, J.-C. Harmand, Y.-H. Cho, F. Glas, M. Tchernycheva, and N. Gogneau. "Morphology Tailoring and Growth Mechanism of Indium-Rich InGaN/GaN Axial Nanowire Heterostructures by Plasma-Assisted Molecular Beam Epitaxy". *Crystal Growth & Design* 18.4 (2018), pp. 2545-2554. DOI: 10.1021/acs.cgd.8b00150.

[Oehler2018]
F. Oehler, A. Cattoni, A. Scaccabarozzi, G. Patriarche, F. Glas, and J.-C. Harmand. "Measuring and Modeling the Growth Dynamics of Self-Catalyzed GaP Nanowire Arrays". *Nano Letters* 18.2 (2018), pp. 701-708. DOI: 10.1021/acs.nanolett.7b03695.

[Pelati2018]
D. Pelati, O. Mauguin, L. Largeau, F. Brisset, F. Glas, and F. Oehler. "Kinetics and crystal texture improvements on thin germanium layers obtained by aluminium induced crystallization using oxygen plasma". *Surface and Coatings Technology* 343 (2018), pp. 121-126. DOI: 10.1016/j.surfcoat.2017.10.034.

[Robin2018]
Y. Robin, M. Pristovsek, H. Amano, F. Oehler, R. A. Oliver, and C. J. Humphreys. "What is red? On the chromaticity of orange-red InGaN/GaN based LEDs". *Journal of Applied Physics* 124.18 (2018), p. 183102. DOI: 10.1063/1.5047240.
2017

[Chen2017] H.-L. Chen, C. Himwas, A. Scaccabarozzi, P. Rale, F. Oehler, A. Lemaître, L. Lombez, J.-F. Guillemoles, M. Tchernycheva, J.-C. Harmand, A. Cattoni, and S. Collin. "Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence". *Nano Letters* 17.11 (2017), pp. 6667-6675. DOI: 10.1021/acs.nanolett.7b02620.

[Henck2017] H. Henck, Z. B. Aziza, O. Zill, D. Pierucci, C. H. Naylor, M. G. Silly, N. Gogneau, F. Oehler, S. Collin, J. Brault, F. Sirotti, F. Bertran, P. L. Fèvre, S. Berciaud, A. T. C. Johnson, E. Lhuillier, J. E. Rault, and A. Ouerghi. "Interface dipole and band bending in the hybrid p-n heterojunction MoS2/GaN(0001)". *Physical Review B* 96.11 (2017), p. 115312. DOI: 10.1103/physrevb.96.115312.

[Himwas2017] C. Himwas, S. Collin, P. Rale, N. Chauvin, G. Patriarche, F. Oehler, F. H. Julien, L. Travers, J.-C. Harmand, and M. Tchernycheva. "In situ passivation of GaAsP nanowires". *Nanotechnology* 28.49 (2017), p. 495707. DOI: 10.1088/1361-6528/aa9533.

[Humphreys2017] C. Humphreys, J. Griffiths, F. Tang, F. Oehler, S. Findlay, C. Zheng, J. Etheridge, T. Martin, P. Bagot, M. Moody, D. Sutherland, P. Dawson, S. Schulz, S. Zhang, W. Fu, T. Zhu, M. Kappers, and R. Oliver. "The atomic structure of polar and non-polar InGaN quantum wells and the green gap problem". *Ultramicroscopy* 176 (2017), pp. 93-98. DOI: 10.1016/j.ultramic.2017.01.019.

[Kim2017] J.-Y. Kim, A. Ionescu, R. Mansell, I. Farrer, F. Oehler, C. J. Kinane, J. F. K. Cooper, N.-J. Steinke, S. Langridge, R. Stankiewicz, C. J. Humphreys, R. P. Cowburn, S. N. Holmes, and C. H. W. Barnes. "Structural and magnetic properties of ultra-thin Fe films on metal-organic chemical vapour deposited GaN(0001)". *Journal of Applied Physics* 121.4 (2017), p. 043904. DOI: 10.1063/1.4973956.

[Pelati2017] D. Pelati, G. Patriarche, O. Mauguin, L. Largeau, F. Brisset, F. Glas, and F. Oehler. "In Situ Optical Monitoring of New Pathways in the Metal-Induced Crystallization of Amorphous Ge". *Crystal Growth & Design* 17.11 (2017), pp. 5783-5789. DOI: 10.1021/acs.cgd.7b00799.

2016

[Griffiths2016] J. T. Griffiths, F. Oehler, F. Tang, S. Zhang, W. Y. Fu, T. Zhu, S. D. Findlay, C. Zheng, J. Etheridge, T. L. Martin, P. A. J. Bagot, M. P. Moody, D. Sutherland, P. Dawson, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "The microstructure of non-polar a-plane (11-20) InGaN quantum wells". *Journal of Applied Physics* 119.17 (2016), p. 175703. DOI: 10.1063/1.4948299.

[Kumaresan2016a] V. Kumaresan, L. Largeau, F. Oehler, H. Zhang, O. Mauguin, F. Glas, N. Gogneau, M. Tchernycheva, and J.-C. Harmand. "Self-induced growth of vertical GaN nanowires on silica". *Nanotechnology* 27.13 (2016), p. 135602. DOI: 10.1088/0957-4484/27/13/135602.

[Kumaresan2016] V. Kumaresan, L. Largeau, A. Madouri, F. Glas, H. Zhang, F. Oehler, A. Cavanna, A. Babichev, L. Travers, N. Gogneau, M. Tchernycheva, and J.-C. Harmand. "Epi-taxy of GaN Nanowires on Graphene". *Nano Letters* 16.8 (2016), pp. 4895-4902. DOI: 10.1021/acs.nanolett.6b01453.

[Kundys2016] D. Kundys, D. Sutherland, M. J. Davies, F. Oehler, J. Griffiths, P. Dawson, M. J. Kappers, C. J. Humphreys, S. Schulz, F. Tang, and R. A. Oliver. "A study of the optical and polarisation properties of InGaN/GaN multiple quantum wells grown on a-plane and m-plane GaN substrates". *Science and Technology of Advanced Materials* 17.1 (2016), pp. 736-743. DOI: 10.1080/14686996.2016.1244474.
[Priante2016]
G. Priante, F. Glas, G. Patriarche, K. Pantzas, F. Oehler, and J.-C. Harmand. "Sharpening the Interfaces of Axial Heterostructures in Self-Catalyzed AlGaAs Nanowires: Experiment and Theory". *Nano Letters* 16.3 (2016), pp. 1917-1924. DOI: 10.1021/acs.nanolett.5b05121.

[Pristovsek2016]
M. Pristovsek, Y. Han, T. Zhu, F. Oehler, F. Tang, R. A. Oliver, C. J. Humphreys, D. Tytko, P.-P. Choi, D. Raabe, F. Brunner, and M. Weyers. "Structural and optical properties of (11-2) InGaN quantum wells compared to (0001) and (11-20)". *Semiconductor Science and Technology* 31.8 (2016), p. 085007. DOI: 10.1088/0268-1242/31/8/085007.

[Schulz2016]
S. Schulz, D. S. P. Tanner, E. P. O'Reilly, M. A. Caro, F. Tang, J. T. Griffiths, F. Oehler, M. J. Kappers, R. A. Oliver, C. J. Humphreys, D. Sutherland, M. J. Davies, and P. Dawson. "Theoretical and experimental analysis of the photoluminescence and photoluminescence excitation spectroscopy spectra of m-plane InGaN/GaN quantum wells". *Applied Physics Letters* 109.22 (2016), p. 223102. DOI: 10.1063/1.4968591.

[Zhu2016]
T. Zhu, D. Gachet, F. Tang, W. Y. Fu, F. Oehler, M. J. Kappers, P. Dawson, C. J. Humphreys, and R. A. Oliver. "Local carrier recombination and associated dynamics in m-plane InGaN/GaN quantum wells probed by picosecond cathodoluminescence". *Applied Physics Letters* 109.23 (2016), p. 232103. DOI: 10.1063/1.4971366.

2015

[Priante2015]
G. Priante, G. Patriarche, F. Oehler, F. Glas, and J.-C. Harmand. "Abrupt GaP/GaAs Interfaces in Self-Catalyzed Nanowires". *Nano Letters* 15.9 (2015), pp. 6036-6041. DOI: 10.1021/acs.nanolett.5b02224.

[Reid2015]
B. P. L. Reid, C. Kocher, T. Zhu, F. Oehler, C. C. S. Chan, R. A. Oliver, and R. A. Taylor. "Non-polar InGaN quantum dot emission with crystal-axis oriented linear polarization". *Applied Physics Letters* 106.17 (2015), p. 171108. DOI: 10.1063/1.4919656.

[Schulz2015]
S. Schulz, D. P. Tanner, E. P. O'Reilly, M. A. Caro, T. L. Martin, P. A. J. Bagot, M. P. Moody, F. Tang, J. T. Griffiths, F. Oehler, M. J. Kappers, R. A. Oliver, C. J. Humphreys, D. Sutherland, M. J. Davies, and P. Dawson. "Structural, electronic, and optical properties of m-plane InGaN/GaN quantum wells: Insights from experiment and atomistic theory". *Physical Review B* 92.23 (2015). DOI: 10.1103/physrevb.92.235419.

[Sutherland2015]
D. Sutherland, T. Zhu, J. T. Griffiths, F. Tang, P. Dawson, D. Kundys, F. Oehler, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "Optical studies of non-polar m-plane (1100) InGaN/GaN multi-quantum wells grown on freestanding bulk GaN". *physica status solidi (b)* 252.5 (2015), pp. 965-970. DOI: 10.1002/pssb.201451563.

[Tang2015]
F. Tang, J. S. Barnard, T. Zhu, F. Oehler, M. J. Kappers, and R. A. Oliver. "Microstructural dependency of optical properties of m-plane InGaN multiple quantum wells grown on 2°misoriented bulk GaN substrates". *Applied Physics Letters* 107.8 (2015), p. 082104. DOI: 10.1063/1.4928723.

[Tang2015a]
F. Tang, T. Zhu, F. Oehler, W. Y. Fu, J. T. Griffiths, F. C.-P. Massabuau, M. J. Kappers, T. L. Martin, P. A. J. Bagot, M. P. Moody, and R. A. Oliver. "Indium clustering in a-plane InGaN quantum wells as evidenced by atom probe tomography". *Applied Physics Letters* 106.7 (2015), p. 072104. DOI: 10.1063/1.4909514.

[Wallace2015]
M. J. Wallace, P. R. Edwards, M. J. Kappers, M. A. Hopkins, F. Oehler, S. Sivaraya, R. A. Oliver, C. J. Humphreys, D. W. E. Allsopp, and R. W. Martin. "Effect of the barrier growth mode on the luminescence and conductivity micron scale uniformity of InGaN light emitting diodes". *Journal of Applied Physics* 117.11 (2015), p. 115705. DOI: 10.1063/1.4915628.
2014

[Badcock2014] T. J. Badcock, P. Dawson, M. J. Davies, M. J. Kappers, F. C.-P. Massabuau, F. Oehler, R. A. Oliver, and C. J. Humphreys. "Low temperature carrier redistribution dynamics in InGaN/GaN quantum wells". *Journal of Applied Physics* 115.11 (2014), p. 113505. DOI: 10.1063/1.4868628.

[Davies2014] M. J. Davies, P. Dawson, F. C.-P. Massabuau, F. Oehler, R. A. Oliver, M. J. Kappers, T. J. Badcock, and C. J. Humphreys. "The effects of varying threading dislocation density on the optical properties of InGaN/GaN quantum wells": *physica status solidi (c)* 11.3-4 (2014), pp. 750-753. DOI: 10.1002/pssc.201300452.

[Dinh2014] D. V. Dinh, F. Oehler, V. Z. Zubialevich, M. J. Kappers, S. N. Alam, M. Caliebe, F. Scholtz, C. J. Humphreys, and P. J. Parbrook. "Comparative study of polar and semipolar (1122) InGaN layers grown by metalorganic vapour phase epitaxy". *Journal of Applied Physics* 116.15 (2014), p. 153505. DOI: 10.1063/1.4898569.

[Emery2014] R. M. Emery, T. Zhu, F. Oehler, B. Reid, R. A. Taylor, M. J. Kappers, and R. A. Oliver. "Non-polar (1120) InGaN quantum dots with short exciton lifetimes grown by metal-organic vapour phase epitaxy": *physica status solidi (c)* 11.3-4 (2014), pp. 698-701. DOI: 10.1002/pssc.201300525.

[Griffiths2014] J. T. Griffiths, T. Zhu, F. Oehler, R. M. Emery, W. Y. Fu, B. P. L. Reid, R. A. Taylor, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "Growth of non-polar (11-20) InGaN quantum dots by metal organic vapour phase epitaxy using a two temperature method": *APL Materials* 2.12 (2014), p. 126101. DOI: 10.1063/1.4904068.

[Kundys2014] D. Kundys, S. Schulz, F. Oehler, D. Sutherland, T. J. Badcock, P. Dawson, M. J. Kappers, R. A. Oliver, and C. J. Humphreys. "Polarized photoluminescence excitation spectroscopy of a-plane InGaN/GaN multiple quantum wells grown on r-plane sapphire". *Journal of Applied Physics* 115.11 (2014), p. 113106. DOI: 10.1063/1.4868692.

[Massabuau2014] F. C.-P. Massabuau, M. J. Davies, F. Oehler, S. K. Pamenter, E. J. Thrush, M. J. Kappers, A. Kovács, T. Williams, M. A. Hopkins, C. J. Humphreys, P. Dawson, R. E. Dunin-Borkowski, J. Etheridge, D. W. E. Allsopp, and R. A. Oliver. "The impact of trench defects in InGaN/GaN light emitting diodes and implications for the green gap problem": *Applied Physics Letters* 105.11 (2014), p. 112110. DOI: 10.1063/1.4896279.

[Massabuau2014a] F. C.-P. Massabuau, A. L. Fol, S. K. Pamenter, F. Oehler, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "The impact of growth parameters on trench defects in InGaN/GaN quantum wells": *physica status solidi (a)* 211.4 (2014), pp. 740-743. DOI: 10.1002/pssa.201300485.

[Oehler2014] F. Oehler, D. Sutherland, T. Zhu, R. Emery, T. Badcock, M. Kappers, C. Humphreys, P. Dawson, and R. Oliver. "Evaluation of growth methods for the heteroepitaxy of non-polar (1120) GAN on sapphire by MOVPE". *Journal of Crystal Growth* 408 (2014), pp. 32-41. DOI: 10.1016/j.jcrysgro.2014.09.009.

[Reid2014a] B. P. L. Reid, T. Zhu, C. C. S. Chan, C. Kocher, F. Oehler, R. Emery, M. J. Kappers, R. A. Oliver, and R. A. Taylor. "High temperature stability in non-polar (1120) InGaN quantum dots: Exciton and biexciton dynamics": *physica status solidi (c)* 11.3-4 (2014), pp. 702-705. DOI: 10.1002/pssc.201300666.

[Reid2014] B. P. L. Reid, C. Kocher, T. Zhu, F. Oehler, R. Emery, C. C. S. Chan, R. A. Oliver, and R. A. Taylor. "Observations of Rabi oscillations in a non-polar InGaN quantum dot": *Applied Physics Letters* 104.26 (2014), p. 263108. DOI: 10.1063/1.4886961.

[Rhode2014] S. L. Rhode, W. Y. Fu, M. A. Moram, F. C.-P. Massabuau, M. J. Kappers, C. McAleese, F. Oehler, C. J. Humphreys, R. O. Dusane, and S.-L. Sahonta. "Structure and strain relaxation effects of defects in InxGa1-xN epilayers": *Journal of Applied Physics* 116.10 (2014), p. 103513. DOI: 10.1063/1.4894688.
[Sutherland2014]
D. Sutherland, F. Oehler, T. Zhu, J. T. Griffiths, T. J. Badcock, P. Dawson, R. M. Emery, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "An investigation into defect reduction techniques for growth of non-polar GaN on sapphire". *physica status solidi (c)* 11.3-4 (2014), pp. 541-544. DOI: 10.1002/pssc.201300532.

[Wallace2014]
M. J. Wallace, P. R. Edwards, M. J. Kappers, M. A. Hopkins, F. Oehler, S. Sivaraya, D. W. E. Allsopp, R. A. Oliver, C. J. Humphreys, and R. W. Martin. "Bias dependence and correlation of the cathodoluminescence and electron beam induced current from an InGaN/GaN light emitting diode". *Journal of Applied Physics* 116.3 (2014), p. 033105. DOI: 10.1063/1.4890497.

## 2013

[Buttard2013]
D. Buttard, T. Schülli, P. Gentile, and F. Oehler. "Structural investigation of silicon nanowires with grazing incidence small angle X-ray scattering". *Micro & Nano Letters* 8.10 (2013), pp. 709-712. DOI: 10.1049/mnl.2013.0405.

[Massabuau2013]
F. C.-P. Massabuau, L. Trinh-Xuan, D. Lodié, E. J. Thrush, D. Zhu, F. Oehler, T. Zhu, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "Correlations between the morphology and emission properties of trench defects in InGaN/GaN quantum wells". *Journal of Applied Physics* 113.7 (2013), p. 073505. DOI: 10.1063/1.4792505.

[Oehler2013a]
F. Oehler, M. E. Vickers, M. J. Kappers, and R. A. Oliver. "Alloy content determination of fully strained and partially relaxed semi-polar group III-nitrides by x-ray diffraction". *Journal of Applied Physics* 114.5 (2013), p. 053520. DOI: 10.1063/1.4817422.

[Oehler2013]
F. Oehler, T. Zhu, S. Rhode, M. Kappers, C. Humphreys, and R. Oliver. "Surface morphology of homoepitaxial c-plane GaN: Hillocks and ridges". *Journal of Crystal Growth* 383 (2013), pp. 12-18. DOI: 10.1016/j.jcrysgro.2013.07.035.

[Oehler2013b]
F. Oehler, M. E. Vickers, M. J. Kappers, C. J. Humphreys, and R. A. Oliver. "Fundamentals of X-ray Diffraction Characterisation of Strain in GaN Based Compounds". *Japanese Journal of Applied Physics* 52.8S (2013), 08JB29. DOI: 10.7567/jjap.52.08jb29.

[Taylor2013]
E. Taylor, F. Fang, F. Oehler, P. R. Edwards, M. J. Kappers, K. Lorenz, E. Alves, C. McAleese, C. J. Humphreys, and R. W. Martin. "Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates". *Semiconductor Science and Technology* 28.6 (2013), p. 065011. DOI: 10.1088/0268-1242/28/6/065011.

[Wallis2013]
D. J. Wallis, D. Zhu, F. Oehler, S. P. Westwater, A. Pujol, and C. J. Humphreys. "Measuring the composition of AlGaN layers in GaN based structures grown on 150 mm Si substrates using (205) reciprocal space maps". *Semiconductor Science and Technology* 28.9 (2013), p. 094006. DOI: 10.1088/0268-1242/28/9/094006.

[Zhang2013]
Y. Zhang, M. Kappers, D. Zhu, F. Oehler, F. Gao, and C. Humphreys. "The effect of dislocations on the efficiency of InGaN/GaN solar cells". *Solar Energy Materials and Solar Cells* 117 (2013), pp. 279-284. DOI: 10.1016/j.solmat.2013.06.022.

[Zhu2013]
T. Zhu, F. Oehler, B. P. L. Reid, R. M. Emery, R. A. Taylor, M. J. Kappers, and R. A. Oliver. "Non-polar (11-20) InGaN quantum dots with short exciton lifetimes grown by metal-organic vapor phase epitaxy". *Applied Physics Letters* 102.25 (2013), p. 251905. DOI: 10.1063/1.4812345.

## 2012

[Gentile2012]
P. Gentile, A. Solanki, N. Pauc, F. Oehler, B. Salem, G. Rosaz, T. Baron, M. D. Hertog, and V. Calvo. "Effect of HCl on the doping and shape control of silicon nanowires". *Nanotechnology* 23.21 (2012), p. 215702. DOI: 10.1088/0957-4484/23/21/215702.
2011

[Demichel2011] O. Demichel, V. Calvo, P. Noé, B. Salem, P.-F. Fazzini, N. Pauc, F. Oehler, P. Gentile, and N. Magnea. "Quantum confinement effects and strain-induced band-gap energy shifts in core-shell Si-SiO2nanowires". *Physical Review B* 83.24 (2011). DOI: 10.1103/physrevb.83.245443.

[Hertog2011] M. I. den Hertog, C. Cayron, P. Gentile, F. Dhalluin, F. Oehler, T. Baron, and J. L. Rouviere. "Hidden defects in silicon nanowires". *Nanotechnology* 23.2 (2011), p. 025701. DOI: 10.1088/0957-4484/23/2/025701.

[Potie2011] A. Potié, T. Baron, F. Dhalluin, G. Rosaz, B. Salem, L. Latu-Romain, M.KGelschatz, P. Gentile, F. Oehler, L. Montès, J. Kreisel, and H. Roussel. "Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires". *Nanoscale Research Letters* 6.1 (2011). DOI: 10.1186/1556-276X-6-187.

2010

[Demichel2010] O. Demichel, V. Calvo, A. Besson, P. Noé, B. Salem, N. Pauc, F. Oehler, P. Gentile, and N. Magnea. "Surface Recombination Velocity Measurements of Efficiently Passivated Gold-Catalyzed Silicon Nanowires by a New Optical Method". *Nano Letters* 10.7 (2010), pp. 2323-2329. DOI: 10.1021/nl903166t.

[Hertog2010] M. I. den Hertog, J. L. Rouviere, H. Schmid, D. Cooper, M. T. Björk, H. Riel, F. Dhalluin, P. Gentile, P. Ferret, F. Oehler, T. Baron, P. Rivallin, S. Karg, and W. Riess. "Off axis holography of doped and intrinsic silicon nanowires: Interpretation and influence of fields in the vacuum". *Journal of Physics: Conference Series* 209 (2010), p. 012027. DOI: 10.1088/1742-6596/209/1/012027.

[Oehler2010] F. Oehler, P. Gentile, T. Baron, P. Ferret, M. D. Hertog, and J. Rouvière. "The Importance of the Radial Growth in the Faceting of Silicon Nanowires". *Nano Letters* 10.7 (2010), pp. 2335-2341. DOI: 10.1021/nl904081g.

2009

[Demichel2009a] O. Demichel, V. Calvo, N. Pauc, A. Besson, P. Noé, F. Oehler, P. Gentile, and N. Magnea. "Recombination Dynamics of Spatially Confined Electron-Hole System in Luminescent Gold Catalyzed Silicon Nanowires". *Nano Letters* 9.7 (2009), pp. 2575-2578. DOI: 10.1021/nl900739a.

[Demichel2009] O. Demichel, F. Oehler, V. Calvo, P. Noé, N. Pauc, P. Gentile, P. Ferret, T. Baron, and N. Magnea. "Photoluminescence of silicon nanowires obtained by epitaxial chemical vapor deposition". *Physica E: Low-dimensional Systems and Nanostructures* 41.6 (2009), pp. 963-965. DOI: 10.1016/j.physe.2008.08.054.

[Oehler2009] F. Oehler, P. Gentile, T. Baron, and P. Ferret. "The effects of HCl on silicon nanowire growth: surface chlorination and existence of a 'diffusion-limited minimum diameter'". *Nanotechnology* 20.47 (2009), p. 475307. DOI: 10.1088/0957-4484/20/47/475307.

[Oehler2009a] F. Oehler, P. Gentile, T. Baron, M. D. Hertog, J. Rouvière, and P. Ferret. "The morphology of silicon nanowires grown in the presence of trimethylaluminium". *Nanotechnology* 20.24 (2009), p. 245602. DOI: 10.1088/0957-4484/20/24/245602.
[Demichel2008] O. Demichel, F. Oehler, P. Noé, V. Calvo, N. Pauc, P. Gentile, T. Baron, D. Peyrade, and N. Magnea. "Photoluminescence of confined electron-hole plasma in core-shell silicon/silicon oxide nanowires". *Applied Physics Letters* 93.21 (2008), p. 213104. DOI: 10.1063/1.3021359.

[Hertog2008] M. I. den Hertog, J.-L. Rouviere, F. Dhalluin, P. J. Desré, P. Gentile, P. Ferret, F. Oehler, and T. Baron. "Control of Gold Surface Diffusion on Si Nanowires". *Nano Letters* 8.5 (2008), pp. 1544-1550. DOI: 10.1021/nl073356i.
**Titre** : Sur le fil de la croissance cristalline : epitaxie planaire, nanostructurée et van der Waals

**Mots clés** : semiconducteur, heteroepitaxie, nanofils, film mince, van der Waals

**Résumé** : L'hétéroepitaxie planaire est de loin la méthode la plus répandue pour la croissance monolithique des semiconducteurs actifs au cœur des dispositifs électroniques. Basée sur des substrats monocristallins, cette approche est sévèrement limitée par la faible compatibilité entre les divers matériaux, due aux désaccords de maille, d'expansion thermique, de symétrie ou de polarité.

Nous explorons ici trois approches différentes qui contournent certaines de ces limites: la croissance de cristaux orientés sur substrat amorphe, l'épitaxie de nanostructures individuelles et finalement la croissance van der Waals de semiconducteurs lamellaires.

**Title** : Along the edges of crystal growth : planar, nanostructured and van der Waals epitaxy

**Keywords** : semiconductor, heteroepitaxy, nanowire, planar, van der Waals

**Abstract** : Planar heteroepitaxial growth on monocrystallines substrates is by far the most widespread method to fabricate the active layers of semiconducting devices obtained using bottom-up techniques. Limitations regarding critical thickness, lattice mismatch, thermal mismatch, polarity or symmetry are well established, and they limit severely the possible choices of materials and heterostructures.

We investigate here three different approaches to circumvent some of these limits: first the oriented crystal growth on amorphous substrates, then the epitaxy of individual nanostructures and finally the van de Waals epitaxy of lamellar compounds.