Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction

Article abstract of DOI:10.1002/adma.201503945

Laser-assisted phosphorus doping is demonstrated on ultrathin transition-metal dichalcogenides (TMDCs) including n-type MoS₂ and p-type WSe₂. Temporal and spatial control of the doping is achieved by varying the laser irradiation power and time, demonstrating wide tunability and high site selectivity with high stability. The laser-assisted doping method may enable a new avenue for functionalizing TMDCs for customized nanodevice applications.

Full text of DOI:10.1002/adma.201503945

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Site Selective Doping of Ultrathin Metal Dichalcogenides
by Laser-Assisted Reaction
Eunpa Kim, Changhyun Ko, Kyunghoon Kim, Yabin Chen, Joonki Suh, Sang-Gil Ryu,
Kedi Wu, Xiuqing Meng, Aslihan Suslu, Sefaattin Tongay, Junqiao W u , *
and Costas P. Grigoropoulos*
2D transition metal dichalcogenide (TMDC) devices exhibit
exceptional characteristics that are particularly suitable for next
generation optoelectronic and electronic device applications.[1–3]
They are excellent candidate materials for transistors,[4–6] photo-
dectors,[7,8] electroluminescent devices,[9] and sensors.[10–14] For
most of these applications, doping is needed to tune the free
carrier type and density. However, previous efforts have mostly
focused on doping TMDCs by means of charge transfer from
adsorbed molecules,[15–19] electrostatic,[20,21] or physisorption
gating,[22] defect engineering,[23,24] and substitutional doping
during growth.[25] Here we report a versatile method for widely
tunable, site-specific doping of ultrathin TMDCs (MoS2 and
WSe2) through focused laser irradiation in a phosphine envi-
ronment. The p-type doping with phosphorus is localized and
selective, and the doping level is widely tunable by varying the
duration and intensity of laser irradiation. We show the doping
tendency in both n-type and p-type TMDCs by photolumines-
cence mapping and device characterization. This study there-
fore establishes a new paradigm for digitally controlled and
air-stable doping of TMDCs for high-quality, and high-fidelity
nanodevices.
Recently, TMDCs with the general chemical formula of
MX2 (M = Mo and W; X = S, Se, and Te) have attracted much
interest owing to their finite direct band gaps, rich excitonic
dynamics, and valley polarization (valleytronics) associated with
the broken inversion symmetry. These layered semiconductors,
composed of vertically stacked layers held together by van der
Waals interactions, are emerging alternatives to silicon-based
electronics.[26] Despite the potential in electronics and opto-
electronics, reliable and stable processing methods are needed
for transition to practical applications.[27] More specifically,
controlled doping of semiconductors is vital for integration
into devices. Especially, for nanoscale devices using ultrathin
TMDCs as an ultrathin body in devices, in order to minimize
random dopant fluctuation and ensure device performance
reproducibility, site-specific doping with precise doping level
control becomes essential. Our method provides a systematic
approach to this problem, demonstrating superior, in situ con-
trol of the spatial distribution and doping level of TMDCs.
The laser doping method has been demonstrated to produce
superior device characteristics compared with other conven-
tional methods such as implantation or diffusion.[28,29] A sche-
matic diagram of the laser-assisted doping process is shown in
Figure 1a. The laser serves two major functions: (i) creation of
sulfur vacancies in the TMDC materials and (ii) simultaneous
dissociation of the dopant molecules. The released dopant mol-
ecules are then incorporated into the vacancy sites. Phosphine
(PH3) was introduced as a p-type dopant precursor for ultrathin
TMDCs where phosphorus occupies sulfur sites, in contrast to
its use for n-type doping of silicon. The PH3 doping of Si has
been extensively studied both experimentally and through theo-
retical modeling.[30–32] Despite differences between silicon and
TMDCs, it is reasonable to adopt the vacancy mechanism in the
present study. Considering that the dissociation temperature
of the TMDCs is in the range of 1200–1400 K, a laser power
slightly below this range is enough for breaking the PH3 mol-
ecules whose dissociation temperature is 685 K.
Dr. E. Kim, Dr. S.-G. Ryu, Prof. C. P. Grigoropoulos
Laser Thermal Lab
Department of Mechanical Engineering
University of California
Berkeley, CA 94720, USA
E-mail: cgrigoro@berkeley.edu
Dr. C. Ko, Dr. Y. Chen, J. Suh,
Dr. X. Meng, Prof. J. Wu
Department of Materials Science and Engineering
University of California
Berkeley, CA 94720, USA
E-mail: wuj@berkeley.edu
Prof. K. Kim
School of Mechanical Engineering
Sungkyunkwan University
Suwon 440-746, South Korea
K. Wu, Dr. X. Meng, Dr. A. Suslu, Prof. S. Tongay
School for Engineering of Matter, Transport and Energy
Arizona State University
Tempe, AZ 85287, USA
Figure 1b shows the optical image of mechanically exfoliated
monolayer and five-layer MoS2 fl akes. The monolayer MoS2
flake in Figure 1c is identified with a thickness of ≈0.7 nm.[4]
The photoluminescence (PL) map shown in Figure 1d is taken
from the laser-irradiated region indicated in Figure 1c.[33]
The PL spectra of monolayer MoS2 before and after the laser-
assisted phosphorus doping are shown in Figure 2a. The peak
PL intensity of the laser-doped area is approximately one order
of magnitude greater than that of the as-exfoliated monolayer
Dr. X. Meng
Research Center for Light Emitting Diodes (LED)
Zhejiang Normal University
Jinhua 321004, P. R. China
Prof. J. Wu
Division of Materials Sciences and Engineering
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
DOI: 10.1002/adma.201503945
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width half maximum (FWHM)). The PL peak
in monolayer WSe2 is red-shifted by 45 meV.
The PL intensity and position changes are
explained by the introduction of additional
free holes with the phosphorus doping.
These free holes turn the original, already
p-type WSe2 fl akes into more p-type, thus
favoring the formation and recombination
of positive trions (X+, e–h–h clusters) over
those of excitons (X, e–h pair). Similar effects
were also found in another p-type TMDC,
WS2 (see the Supporting Information). We
note that the laser doping process does not
cause detrimental structural damage to these
TMDCs under the given conditions. To iden-
tify the structural damage after laser-assisted
doping of MoS2, we conducted Raman map-
ping prior to single spot Raman spectrum
acquisition (Figure 2a,b, inset). The Raman
and PL mapping area was ≈10 × 10 µm2,
corresponding to ≈400 spots separated by
0.5 µm steps per mapping image. This proce-
dure helps confirm that point data are indeed
taken from the laser reaction spot center.
We analyze the effects of different laser
processing parameters (Figure 2c,d). It is
known that the laser-assisted thermochem-
ical processing provides a convenient means
for systematically and rapidly accessing a
wide parameter space in a single sample con-
figuration.[34–36] The enhancement of the PL
peak intensity of monolayer MoS2 at a fixed
laser power exhibits an approximately linear
dependence on the laser illumination time
Figure 1. a) Schematic diagram of the laser-assisted doping method. b) Optical image of as-
prepared monolayer MoS2 on SiO2/Si substrates. c) Atomic force microscopy (AFM) image of
the zoomed area in (b). Its thickness is around 0.7 nm, in good agreement with the thickness
of monolayer MoS2. The circle in (c) is the laser spot area in the laser doping. d) PL mapping
of the zoomed area in (c) that clearly shows the PL intensity enhancement of the laser-assisted
doped area.
MoS2. The spectral distribution of the PL also sharpens upon
the laser phosphorus doping while the peak energy of the
PL spectra in monolayer MoS2 is blue-shifted by ≈32 meV.
The PL peak positions are consistent with the energies of the
exciton peak (X; ≈1.88 eV, e–h pair) and the negative trion peak
(X−; ≈1.84 eV, e–e–h clusters).[20] For the as-exfoliated mon-
olayer, the PL contribution of the negative trion peak (X−) is
higher than that of the exciton peak (X), demonstrating that
the material is n-type in accordance with the previous reports,
presumably owing to omnipresent sulfur vacancies.[25] How-
ever, in the PL spectra of monolayer MoS2 after phosphorus
laser doping, the X peak dominates the X− peak contribution,
indicating addition of free holes to the system to improve parity
between the densities of free electrons and holes. Hence, the
blue shift of the PL spectrum is consistent with the hole doping
effect of phosphorus that acts as an acceptor in MoS2, resulting
in a compensation of the originally n-type MoS2.
In contrast to the intrinsically n-type MoS2, WSe2 is known
to be intrinsically p-type.[15] Such a phosphorus doping is
expected to enhance the free hole density in WSe2. Figure 2b
shows the PL spectra of monolayer WSe2, before and after the
laser-assisted phosphorus doping. The as-exfoliated monolayer
WSe2 exhibits a direct band gap at 1.67 eV. The PL intensity
of doped WSe2 is drastically reduced after the laser doping. In
addition, the spectral shape of the peak broadens (by 12% in full
(Figure 2c, inset), which implies monotonically increasing hole
doping over the laser illumination time. At a fixed laser power
of 200 mW and exposure times of 10, 60, and 120 s with the
PL intensity was enhanced by 1.6, 5, and 10 times, respectively.
On the other hand, as can be seen in Figure 2d, the enhance-
ment of PL intensity with respect to the laser power shows a
threshold behavior (Figure 2d, inset). Namely, the PL intensity
greatly increases from that of the as-exfoliated sample as the
laser power increases from 200 to 250 mW (from 6 to 35 times,
respectively), but decreases as the laser power increases to
300 mW. This implies the existence of an optimal laser power,
around 250 mW, at the fixed irradiation time of 60 s. Decom-
position of the monolayer MoS2 at the laser power of 300 mW
was detected by contact-mode AFM. Presumably, irradiation
with the laser at the power of 300 mW raises the local tempera-
ture in excess of the decomposition temperature of monolayer
MoS2, which should be ≈900 K at the given experimental condi-
tions considering the substrate type, the environment gas spe-
cies, and the gas pressure.
The Gaussian profile of the laser beam intensity is expected
to generate a spatially varying temperature distribution at the
irradiation spot. Although various reports have been published
on optical and thermal properties of TMDC materials,[37–40]
there is still insufficient information to enable accurate predic-
tion of the induced temperature in our laser doping process.
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Figure 2. a) PL spectra of as-exfoliated and optimally (250 mW for 60 s, laser-doped spot center) laser-doped monolayer MoS2. Inset: Raman spectrum
of as-exfoliated and laser-doped monolayer MoS2. b) The same spectra for monolayer WSe2 (optimal condition is 150 mV for 60 s). c) PL change of
monolayer MoS2 with different laser doping time at a fixed laser power of 200 mW. The inset shows the peak PL intensity values as a function of the
laser doping time. d) PL spectra of monolayer MoS2 with different laser doping power at a fixed laser doping time of 60 s. The inset shows the peak
PL intensity values as a function of the laser power.
The maximum PL intensities across the laser-doped regions in
the monolayers MoS2 (Figure 3a, inset) and WSe2 (Figure 3b,
inset) are plotted in Figure 3a,b. These distributions could be
fitted to a Gaussian profile, suggesting that the hole doping
concentration increases with doping temperature, which maxi-
mizes at the center of the laser reaction spot. The FWHM of
the PL intensity profile was ≈2.5 µm, which is in agreement
with the focal spot size of ≈2.3 µm, considering the 10× with a
numerical aperture (NA) of 0.28 objective lens and the 532 nm
laser light wavelength. Figure 3c, inset, shows that for MoS2, the
PL gradually shifts weight from X− to X emission as the reac-
tion spot is scanned from the edge to the center. In contrast, the
WSe2 shows a transition from X to X+ (Figure 3d, inset).
Reliable performance of devices requires stable doping of the
active materials. Indeed, previous studies have already achieved
both n-type and p-type dopings by surface charge transfer
mechanisms, mostly through chemical physisorption.[15]
However, physisorption doping is unstable and decays almost
completely within an hour,[17] or is retained for longer periods
only if the doping environment is maintained or protected.[22]
Among the various doping methods, substitutional doping of
foreign elements is an effective and stable doping strategy for
TMDCs.[25] In our work, the laser was used to create sulfur
vacancies and locally heat up the material to crack the pre-
cursor molecules, enabling the substitution of sulfur with phos-
phorus. The PL data in Figure 4a show that the laser-assisted
doping is irreversible and stable, even after exposure in air for a
month. In order to test the doping stability, we also studied the
effect of pure laser annealing. In this case, the monolayer MoS2
was laser irradiated in the absence of the dopant gas at the laser
power of 250 mW and an exposure time of 60 s (i.e., exactly
the same conditions used for doping). As shown in Figure 4b,
the PL intensity of monolayer MoS2 increased right after the
laser annealing to a level higher by an order of magnitude than
the incurred by laser doping, possibly due to suppression of
nonradiative recombination of excitons at defect sites created
by the laser annealing.[24] However, the PL intensity drastically
dropped after one week. Within two weeks, the PL intensity
receded to the original level prior to laser annealing, indicating
instability of the laser annealing effect. Presence of the PH3
molecules is, therefore, needed for stable doping.
In parallel to strong modification of the luminescent proper-
ties, the laser-assisted doping also causes drastic effects on elec-
trical properties of the ultrathin TMDCs. This was investigated
using field-effect transistor (FET) devices incorporating the
locally doped TMDC layers as current channels. As displayed
in Figure 5a,d, multiple electrodes were patterned onto TMDC
bilayer flakes, covering both pristine and laser-doped regions. A
voltage applied through the back gate provided additional mod-
ulation of carrier density in the TMDC channel. In the undoped
regions, n-type (p-type) FET behavior was observed with on–off
ratio exceeding five orders of magnitude from the MoS2 (WSe2)
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Figure 3. The PL peak intensity line profile of laser-assisted doping monolayer a) MoS2 and b) WSe2 at the reaction spot. Inset images show the PL
peak intensity-mapping image and optical microscopy image of the laser-doped region. Scale bars are 2 and 5 µm, respectively. Variation of the PL
intensity within the single spot of laser-doped monolayers c) MoS2 and d) WSe2. The inset shows the PL mapping image. The spacing between each
spot is 500 nm.
bilayers. The FET characteristics, i.e., source–drain current
(ISD) versus back gate voltage (VG) curves, of devices fabricated
at different distances from the center of the laser-doped region
change in a manner consistent with the spatial variation of PL
across the same region, as summarized in Figure 5b,e. The
phosphorus doping suppresses the n-type characteristics of the
MoS2 and enhances the p-type characteristics of the WSe2, with
increase in conductance as well as shift of threshold voltage.
The hysteresis features of FET loops are likely due to water
molecule absorption on the surface of the TMDCs, considering
that all electrical measurements were conducted under ambient
conditions.[41] In addition, the ISD versus source–drain voltage
(VSD) curves obtained from the MoS2 and WSe2 channels in
Figure 5c,f, respectively, further confirm introduction of accep-
tors by the laser treatment.
In summary, laser-assisted phosphorus doping was demon-
strated on ultrathin (monolayer and bilayer) TMDCs including
n-type MoS2 and p-type WSe2. This approach effectively intro-
duces electronically active phosphorus atoms into the TMDCs.
The precise level, temporal and spatial controls of the doping
are achieved by varying the laser irradiation power and time,
demonstrating wide tunability and high site selectivity. Future
Figure 4. Long-term room-temperature stability of laser-doped monolayer MoS2. a) PL spectra demonstrating the stability of the monolayer MoS2
laser-assisted doped with phosphorus and b) laser annealed with the same laser power and time but under exposure to oxygen and water molecules
in ambient air. c) The peak PL intensity of the laser-doped and laser-annealed monolayer MoS2 varying over time in ambient.
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Figure 5. a,d) Bilayer MoS2 and WSe2 FET devices. The flakes are outlined by the blue dashed boxes and the laser-irradiated areas are denoted by the red
dashed circles. b,e) Source–drain current (ISD) versus back gate voltage (VG) curves recorded from the devices in panels (a) and (d) with a bias voltage
of 500 mV. c,f) Source–drain current (ISD) versus source–drain voltage (VSD) curves recorded from the doped (Dev 2, 4) and undoped (Dev 1, 3) devices.
stage (Aerotech), by a 10× objective lens (Mitutoyo, 0.28 NA) for laser
delivery.
investigation of the choice of dopant, the dopant concentration,
and the contact engineering should be conducted. The high
stability and effectiveness of the laser-assisted doping method
combined with the site selectivity and tunability demonstrated
here may enable a new avenue for functionalizing TMDCs for
customized nanodevice applications.
Micro-Raman/PL Measurements: A commercially available micro-PL
and Raman measurement system (Renishaw) was equipped with an
Ar-ion laser (488 nm) and 1200, 2400 l mm–1 grating for PL and Raman,
respectively. A 100× objective lens (0.95 NA) was used, and the focused
laser spot size on the sample was around 0.5 µm. Typical excitation laser
power was around 25 µW to avoid heating, damaging, and nonlinear
optical effects. Typical measurement integration time was 10 s. All
measurements were performed at room temperature.
Nano-Auger Electron Spectroscopy (Nano-AES): Nano-AES was
conducted to investigated the chemical properties of locally laser-
assisted doped regions using an Oxford/Omicron system where a field
emission gun is equipped enabling a spatial resolution of ≈10 nm at an
ultrahigh vacuum of pressure ≈10−10 mbar.
Experimental Section
TMDCs Sample Preparation, MoS2: Single and bilayer MoS2 fl akes
were mechanically exfoliated from commercially available crystals of
MoS2 (SPI).
WS2 and WSe2: Single crystals of WX2 were grown by vapor transport
technique using iodine as the transport agent to facilitate material
growth using a two-step process. In the first step, polycrystalline WSe2
(WS2) powders were synthesized at 1065 °C (1110 °C) for 3 d in a
single-zone furnace using tungsten and selenium (sulfur) high-purity
(99.995%) powders (–325 mesh) with a molar ratio of 1:2.02 0.01
in evacuated quartz tubes. Samples were cooled down to room
temperature at 50 °C h–1. About 2.5 g of the WSe2 (WS2) and I2 granule
(3 mg cc–1) were introduced into the quartz tubes. The quartz tube was
evacuated to 2 × 10−6 Torr and sealed. Tubes were placed in a three-zone
furnace and the furnace was heated to 1030 °C, 1000 °C, and 940 °C
(1070 °C, 1030 °C, and 980 °C), respectively. Powders were located in
the hot zone (Tprecursor = 1030 °C and 1070 °C for WS2) and synthesized
crystals were collected in the cold zone (Tgrowth = 940 °C and 980 °C
for WS2). The total duration of the growth was set to 3–4 weeks and
eventually cooled down to room temperature at the rate of 50 °C h–1.
Laser Doping System: The laser-assisted doping experiments were
carried out in a dedicated apparatus. The system is equipped with a
multichannel gas panel for doping gas supply (10 sccm of 1% PH3)
with 400 sccm Ar dilution gas and is equipped with vacuum chambers
pumped to 10−4 Torr by a dry pump before the total pressure of 400 Torr
for laser doping reaction. The laser source already in place was a highly
stable continuous wave 532 nm laser (diode-pumped solid state laser,
5 W, Sprout-DTM, Lighthouse photonics) focused onto the sample in
the vacuum reaction chamber, which was placed on high precision XY
Device Fabrication and Electrical Characterization: Exfoliated 2D
crystals were patterned into rectangular patches by XeF2 etching
subsequent to standard electron beam lithography (EBL). Then, after
local laser doping, polymethylmethacrylate resist masks generated by
EBL were used to deposit metal electrode channels of Pt/Au and Ti/Au
(10 nm/70 nm) on the WSe2 and MoS2 layers, respectively, by e-beam
evaporation followed by the lift-off process. Back-gated FET experiments
were conducted on the probes station equipped with two Keithley 617
programmable electrometers, an NI BNC-2090 A/D board, and a current
amplifier, under the ambient conditions.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
C.K. and K.K. contributed equally to this work. The authors thank
Taegyun Park for his assistance in sample preparation. Support from
the US Air Force Office of Scientific Research AFOSR/AOARD under
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grant FA2386-13-4123 is gratefully acknowledged. This work was also
partially supported by the National Science Foundation under Grant
No. DMR-1306601. Nano-Auger experiment at the Molecular Foundry
was supported by the Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231. The laser-induced doping was conducted on the LACVD
apparatus in the UC Berkeley Marvell Nanofabrication Laboratory.
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