7.1 Epitaxy
213
Currently, more advanced growth-modes are applied to achieve location-specific
growth and size control [10] or even lateral or vertical heterostructures [11–13].
Under certain growth conditions, even certain (high-)symmetry configurations of
homobilayers can be favoured [14] based on how the grown monolayer is chemically
terminated [15]. TEM high-resolution images and diffraction patterns could verify
such targeted alignment (not shown or discussed here), as addressed for instance in
the Supporting Information section of [14].
Some studies even indicated that, while the quality of CVD-grown large-area
monolayers may not have been as good as that of mechanically exfoliated flakes,
the optical properties on the same substrate did not differ much. However, growthinduced strain and defects can alter the charge-carrier lifetimes and the spectral
features [7].
For example, CVD growth of WSe 2 monolayers in the study of [7] was achieved
using a tungsten source carrier chip (5 nm WO 3
3 thin film on 90 nm SiO 2 ) and a
sapphire substrate. The tungsten source chip was covered by the sapphire growth
substrate in a face-to-face contact configuration. The sample was loaded into the
center of a 2” diameter and 24” long quartz tube (comparable to the sketch in Fig. 7.2),
and a ceramic boat with 1 g of selenium powder was located upstream in the quartz
tube. After loading, the ambient gas of the tube was purged out by a mechanical pump.
At a typical base pressure of 10 mTorr, the furnace was heated to 750
◦ C at a specific
ramping rate (13 min
−1 ) and the temperature held at 750
◦ C for 4 min. Afterwards,
the temperature was raised to 850
◦ C at the same ramping rate. 20 sccm of Ar gas was
introduced at 500
◦ C during the temperature increase to reduce moisture inside of
the tube, and the flow was ended at 500
◦ C with decreasing temperatures. During the
process, hydrogen gas was supplied to improve WO 3 reduction temperature upwards
from 700
◦ C to 600
◦ C temperature downwards. During growth, a 1.6 Torr pressure
was maintained in the furnace. After 20 min at 850
◦ C, the furnace was cooled down
to room temperature naturally. For the resulting optical properties of the obtained
samples, see e.g. [7, 16].
Naturally, the outcome of a CVD growth process depends on various parameters and is based on profound long-term experience. However, as examples from
the literature reveal [17], CVD-grown monolayers are not perfectly monocrystalline
with large flakes exhibiting numerous grain boundaries (cf. [18] on imaging secondharmonic generation studies) and are full of defects, mainly chalcogen vacancies (cf.
[19] on the control of point defects in TMDCs). Commonly, said from the author’s
experience, monolayers of large domain size are very rare, and full-monolayer coverage of substrates comes at the cost of polycrystallinity. Moreover, monolayers often
exhibit bilayer and few-layer features on top, if the growth process is not optimised
for (nearly-pure) monolayer yield, and in some cases even carry clusters of material
spot-wise on the ultrathin sheets.
In contrast, preferentially-grown WS 2 homobilayers can be synthesised via a twostep low-pressure chemical-vapour deposition (LPCVD) process [14]. In general,
monolayers obtained from the first growth step will provide seeding sites for another
monolayer of the same material to grow on top during the second run, resulting in
3 Tungsten trioxide.
213
Currently, more advanced growth-modes are applied to achieve location-specific
growth and size control [10] or even lateral or vertical heterostructures [11–13].
Under certain growth conditions, even certain (high-)symmetry configurations of
homobilayers can be favoured [14] based on how the grown monolayer is chemically
terminated [15]. TEM high-resolution images and diffraction patterns could verify
such targeted alignment (not shown or discussed here), as addressed for instance in
the Supporting Information section of [14].
Some studies even indicated that, while the quality of CVD-grown large-area
monolayers may not have been as good as that of mechanically exfoliated flakes,
the optical properties on the same substrate did not differ much. However, growthinduced strain and defects can alter the charge-carrier lifetimes and the spectral
features [7].
For example, CVD growth of WSe 2 monolayers in the study of [7] was achieved
using a tungsten source carrier chip (5 nm WO 3
3 thin film on 90 nm SiO 2 ) and a
sapphire substrate. The tungsten source chip was covered by the sapphire growth
substrate in a face-to-face contact configuration. The sample was loaded into the
center of a 2” diameter and 24” long quartz tube (comparable to the sketch in Fig. 7.2),
and a ceramic boat with 1 g of selenium powder was located upstream in the quartz
tube. After loading, the ambient gas of the tube was purged out by a mechanical pump.
At a typical base pressure of 10 mTorr, the furnace was heated to 750
◦ C at a specific
ramping rate (13 min
−1 ) and the temperature held at 750
◦ C for 4 min. Afterwards,
the temperature was raised to 850
◦ C at the same ramping rate. 20 sccm of Ar gas was
introduced at 500
◦ C during the temperature increase to reduce moisture inside of
the tube, and the flow was ended at 500
◦ C with decreasing temperatures. During the
process, hydrogen gas was supplied to improve WO 3 reduction temperature upwards
from 700
◦ C to 600
◦ C temperature downwards. During growth, a 1.6 Torr pressure
was maintained in the furnace. After 20 min at 850
◦ C, the furnace was cooled down
to room temperature naturally. For the resulting optical properties of the obtained
samples, see e.g. [7, 16].
Naturally, the outcome of a CVD growth process depends on various parameters and is based on profound long-term experience. However, as examples from
the literature reveal [17], CVD-grown monolayers are not perfectly monocrystalline
with large flakes exhibiting numerous grain boundaries (cf. [18] on imaging secondharmonic generation studies) and are full of defects, mainly chalcogen vacancies (cf.
[19] on the control of point defects in TMDCs). Commonly, said from the author’s
experience, monolayers of large domain size are very rare, and full-monolayer coverage of substrates comes at the cost of polycrystallinity. Moreover, monolayers often
exhibit bilayer and few-layer features on top, if the growth process is not optimised
for (nearly-pure) monolayer yield, and in some cases even carry clusters of material
spot-wise on the ultrathin sheets.
In contrast, preferentially-grown WS 2 homobilayers can be synthesised via a twostep low-pressure chemical-vapour deposition (LPCVD) process [14]. In general,
monolayers obtained from the first growth step will provide seeding sites for another
monolayer of the same material to grow on top during the second run, resulting in
3 Tungsten trioxide.