Tungsten-disulphide-based heterojunction photodetector

Abstract

Two-dimensional (2D) materials have realized significant new applications in photonics, electronics, and optoelectronics. Among these materials is tungsten disulphide ( WS 2 ), which is a 2D material that shows excellent optoelectronic properties, tunable/sizable bandgap in the visible range, and good absorption. A polycrystalline WS 2 thin film is successfully deposited on a substrate using radio frequency magnetron sputtering at room temperature. The x-ray diffraction pattern reveals two hexagonal structured peaks along the (100) and (110) planes. Energy-dispersive x-ray spectroscopy reveals a non-stoichiometric WS 2 film with 1.25 ratio of S/W for a 156.3 nm thick film, while Raman shifts are observed at the E 2 g 1 and A 1 g phonon modes located at 350.70 cm − 1 and 415.60 cm − 1 , respectively. A sandwiched heterojunction photodetector device is successfully fabricated and illuminated within the violet range at 441 nm and 10 V of bias voltage. The maximum photocurrent values are calculated as 0.95 μA, while the responsivity is observed at 169.3 mA W − 1 and detectivity 1.48 × 10 8 Jones at illuminated power of 0.6124 μm. These results highlight the adaptability of the present technique for large-scale applications as well as the flexibility to promote development of advanced optoelectronic devices.

Full text

1
Tungsten Disulphide based Heterojunction
Photodetector
HARITH AHMAD1,2, HAROON RASHID1, MOHAMMAD FAIZAL ISMAIL1 AND
KAVINTHERAN THAMBIRATNAM1,*
1Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia
2Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 60115,
Indonesia
*kavintheran@gmail.com
Abstract: Two-dimensional (2D) materials have realized significant new applications in
photonics, electronics and optoelectronics. Among these materials, tungsten disulphide
(WS2), which is a 2D material that shows excellent optoelectronic properties, tunable/sizable
bandgap in the visible range and good absorption. A polycrystalline WS2 thin film is
successfully deposited on a substrate using radio frequency magnetron sputtering at room
temperature. X-ray diffraction pattern reveals two hexagonal structured peaks along the (100)
and (110) planes. Energy-dispersive X-ray spectroscopy reveals a non-stoichiometric WS2
film with 1.25 ratio of S/W for a 156.3 nm thick film, while Raman shifts are observed at the
E1
2g and A1g phonon modes located at 350.70 cm-1 and 415.60 cm-1 respectively. A
sandwiched heterojunction photodetector device was successfully fabricated and illuminated
within the violet range at 441 nm and 10 V of bias voltage. The maximum photocurrent
values are calculated as 0.95 µA, while the responsivity was observed 169.3 mAW-1 and
detectivity 1.48 x 108 Jones. at illuminated power of 0.6124 µm. These results highlight the
adaptability of the present technique for large scale applications as well as the flexibility to
promote the development of advanced optoelectronic devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) nanomaterials such as graphene have recently received significant
attention from the semiconductor industry due to their unique optical and electrical properties,
such as strong interaction with photons in a wide energy range and high carrier mobility in a
broad spectrum [1]. However, graphene based field effect transistors (FETs) did not garner its
projected market share because of low ON/OFF ratios arising from the lack of a bandgap in
this material. These spurred research efforts into other 2D materials such as transition metal
dichalcogenides (TMDCs) that would be able to overcome this limitation. TMDCs in
particular are of interest due to their atomic structure of MX2 where M is a transition metal
atom such as Mo or W and X is a chalcogenide atom such as S or Se [2,3]. The X-M-X layer
materials are covalently bonded to the six chalcogenides neighbouring them within the same
plane with a very weak van der Waals force. There gives 2D nanomaterials based on TMDCs
its unique characteristics such as surfaces without any dangling bonds [4]. It is also possible
to raise vertical hetero-structures using different 2D materials without the conventional lattice
mismatch due to the sheets having different lattice constants that are only bonded by a weak
van der Waals force as in layered materials. As a result, even atomically thin layers interact
with light strongly such as monolayer tungsten disulphide (WS2) being able to absorb roughly
10 % at exitonic resonances, covering a very wide electromagnetic spectrum because of their
assorted electronic properties [3]. Furthermore, recent research has demonstrated that WS2 has
very strong absorption [5]. In this manner, WS2 has substantial potential for high-responsivity
photo-detecting applications [6,7], acting as a sandwiching material between metal
heterojunction contacts for the fabrication of high performance devices [8].

In this w
o
of radio freq
u
analysed usi
n
emission sc
a
compositiona
the WS
2
thin
thin film is
p
hotodetecto
r
analysed. Th
e
are also deter
m
2. Method
o
The fabricati
o
two phases.
I
substrate. Th
e
acetone and
t
minutes and
p
articles fro
m
surface of t
h
temperature
(
p
ower of 50
W
(p-Si:B) wa
f
thicknesses
o
substrate. Th
e
followed by
D
surface with
n
on top of a h
each other u
s
WS
2
/SiO
2
/p-
S
Fi
g
XRD me
a
D8 Advance
(
voltage and
4
to 80° with a
film was stu
d
dispersive x-
r
o
rk, a WS
2
thi
n
u
ency (RF) m
a
n
g X-ray diffra
c
a
nning electr
o
l
analysis and
film in order
t
sandwiched
r
and its opto
e
e
photocurrent
,
m
ined.
o
logy
o
n of
t
he thin
I
n the first st
a
e
SLG substr
a
t
hen methano
l
finally dried
u
m
the surface o
f
h
e cleaned S
L
(
RT). The dep
o
W
under high
v
f
er together
w
o
f 1000 micr
o
e
p-Si:B/SiO
2
D
I water for 2
n
o impurities.
otplate for 15
s
ing a paper c
l
S
i:B structure.
F
. 1. Schemati
c
a
surement of t
h
(
Cu K-Alpha
1
4
0 mA current.
step size of 0.
0
ied by means
o
r
ay spectrosco
p
n
film is succe
s
a
gnetron sputt
e
c
tion (XRD)
w
o
n microsco
p
Raman spect
r
t
o investigate
a
in the hete
r
e
lectronic pro
p
,
responsivity,
film WS
2
b
as
e
a
ge, WS
2
thi
n
a
te is cleaned
i
l
again for 1
0
u
sing pure ni
t
f
the substrate.
L
G substrate
o
sition proces
s
v
acuum condit
i
w
ith a therm
a
o
ns and 1.5
m
substrate was
0 minutes an
d
Afterwards, b
minutes at a
t
l
ip to form a
h
F
igure 1 show
c
diagram of thin
f
h
e WS
2
thin fi
1
) equipment a
t
The XRD pa
t
0
25° over a 0.
1
o
f FESEM of
J
p
y (EDX) wa
s
s
sfully deposit
e
e
ring. The str
u
w
hile the surfa
c
p
y (FESEM).
r
oscopy is use
d
a
n in-depth pr
o
r
ojunction str
u
p
erties device
directivity an
d
e
d heterojunct
i
n
film is dep
o
i
n an ultrason
i
0
minutes eac
h
t
rogen (N
2
) g
a
Subsequently
,
using RF m
a
s
was carried
i
ons. At the sa
m
a
lly oxidised
m
icrons respec
t
cleaned ultras
o
d
then dried w
i
oth the SLG/
W
t
emperature o
f
h
eterojunction
s the schemati
c
f
ilm WS
2
b
ased he
lm was perfor
m
t
RT with a w
a
t
tern was reco
r
1
s exposure ti
J
EOL
b
rand w
i
s
executed to
e
d on top of s
o
u
ctural proper
t
c
e morpholog
y
Energy dis
p
d
to fingerpri
n
o
file analysis.
u
cture of a
under differe
d external qu
a
i
on photodete
c
o
sited onto a
s
ic bath in the
h
, then de-ion
i
a
s to remove
,
the thin WS
2
a
gnetron sput
t
out over a pe
r
m
e time, a bo
r
silicon diox
i
tively was pr
o
nically with
a
i
th N
2
gas as
b
W
S
2
and p-Si:
B
f
100 °C and
s
photodetecto
r
c
diagram of t
h
e
terojunction phot
o
r
med using a
B
a
velength of 0
.
r
ded over a sc
a
i
me. The morp
h
ith
m
odel nu
m
investigate co
m
o
da lime glass
b
t
ies of the thi
n
y
is obtained u
p
ersive X-ra
y
n
t the phonon
m
Subsequently
,
SLG/WS
2
–p-
S
e
nt illuminatio
n
a
ntum efficien
c
c
tor can be di
v
s
oda lime gla
sequence of
m
ised (DI) wat
e
any undesire
d
film is deposi
t
t
ering system
r
iod of 60 mi
n
r
on doped p-ty
p
i
de (SiO
2
) l
a
epared as a
s
a
cetone for 30
b
efore to ensu
r
B
/SiO
2
films
a
s
andwiched ti
g
r
device with
a
h
e fabricated
d
o
detector device
B
ruker AXS
G
.
15406 nm un
d
a
n range of 2θ
hology of the
d
m
ber JSM7600
F
m
positional a
n
2
b
y means
n
film are
sing field
y
(EDX)
m
odes of
,
the WS
2
S
i:B/SiO
2
n
powers
c
y (EQE)
v
ided into
ss (SLG)
m
ethanol,
e
r for 20
d
residual
t
ed on the
at room
n
utes at a
p
e silicon
a
yer with
s
econdary
minutes,
r
e a clean
a
re placed
g
htly with
a
SLG/n-
d
evice.
G
ermany’s
d
er 40 kV
from 20°
d
eposited
F
. Energy
n
alysis of

WS
2
thin fil
m
532 nm equi
p
of the fabric
a
range at 441
p
hotodetecto
r
p
erformed us
i
p
ower of the
l
as frequency
30 MHz syn
t
b
ased respo
n
oscilloscope.
3. Results
Fig. 2
p
rovi
d
seen that the
WS
2
film ex
h
and the (110
)
1398. The lat
t
with a hexag
o
p
lane was ob
s
p
hases corre
s
p
urity of the
s
The surfa
c
in Fig. 3 (a),
The thicknes
s
spectrum of
W
presence of t
u
was calculat
e
deficiency of
m
with Oxford
p
ped with Ren
i
a
ted WS
2
b
ase
d
nm for illumi
n
r
was mainta
i
i
ng a Keithley
l
aser source w
a
m
odulation at
t
hesized funct
i
n
ses of the d
e
All optoelectr
o
& Discussi
o
es the XRD p
crys
t
allites o
f
h
ibits only two
)
p
lane at 2θ
=
t
ice parameter
s
o
nal crystal s
t
s
erved to be 3
s
ponding to i
m
s
ample.
F
i
c
e morpholog
y
and shows th
e
s
of the depo
s
W
S
2
thin film
o
u
ngsten (W) a
n
e
d to be 1.25,
S is attributed
instrument. R
a
i
shaw inVia a
t
d
heterojuncti
o
n
ation. The d
i
i
ned at 2 c
m
’s 2410 – 110
0
a
s tuned by v
a
1 Hz, 5 Hz, 1
0
i
on generator
(
e
vice are obta
o
nic measure
m
o
n
attern of the
s
f
WS
2
mainta
i
diffraction pe
=
60.82° whic
h
s
of the sampl
e
t
ructure of 2H
times higher
t
m
purities are
s
i
g. 2. XRD pa
y
image obtain
e
e
polycrystalli
n
s
ited film is
m
o
n the other h
n
d sulphur (S)
confirming t
h
to thermal de
c
a
man spectra
w
t
25 mW laser
o
n photodetect
o
i
stance betwee
m
. The curren
t
0
V – Source
M
a
rying the inpu
t
0
Hz, 100 Hz,
(
SFG) from S
t
ined using a
m
ents are carri
e
s
puttered WS
2
i
n hexagonal
c
ak
s correspon
d
h
is in agreem
e
e
are determin
[9,10]. The i
n
t
han the secon
d
s
een at the c
u
ttern of deposited
e
d by FESEM
n
e growth of
W
m
easured to be
and is shown
i
elements in th
e
h
e non-stoichi
o
c
omposition [1
w
as obtained a
t
power. The o
p
o
r was charac
t
e
n laser sourc
e
t
– voltage (
M
eter® betwe
e
t
voltages fro
m
300 Hz and 5
0
t
anford Resea
r
Yokogawa
D
e
d out at room
t
thin film. Fr
o
crystal struct
u
d
ing to the (1
0
e
nt with the J
C
n
ed to be a=0.3
n
tensity of fir
s
d
peak along t
h
u
rrent resoluti
o
WS
2
thin film
fo
r
deposited
W
S
2
with a su
r
e
approximatel
y
in Fig. 3(b), a
n
e thin film. T
h
o
metric natur
e
2].
t
RT by using
p
toelectronic
p
t
erized within
t
e
and the surf
a
(
IV) measure
m
e
n -20 V to +
2
m
0.0 V to 2.7
V
00 Hz using a
r
ch Systems.
T
D
LM2054 mix
e
temperature.
o
m the figure,
u
re. The
p
oly
c
0
0)
p
lane at 2θ
C
PDS card n
u
3
15 nm and c=
s
t peak along
h
e (110) plan
e
o
n, indicating
WS
2
thin film
r
face coverage
y 156.3 nm.
T
a
nd clearly ind
i
h
e normalized
S
e
of the WS
2
f
3
a laser of
p
roperties
t
he violet
a
ce of the
m
ent was
2
0 V. The
V
as well
DS345 –
T
he time-
e
d signal
it can be
c
rystalline
= 34.28°
u
mber 84-
1.227 nm
the (100)
e
[11]. No
the high
is shown
of 80 %.
T
he EDX
i
cates the
S
/W ratio
f
ilm. The

Fig. 4 sh
o
EDX analysi
s
while the pre
s
Fig. 5
chan
g
The Ram
a
b
e seen that
t
Fig. 3. (a)
F
Fig. 4.
o
ws the eleme
s
. The presenc
e
s
ence of W an
d
. Raman spe
c
g
e in the material
s
a
n spectra of t
h
t
here are two
F
ESEM surface i
m
Elemental co
m
ntal composit
i
e
of C, O, Na,
d
S confirms t
h
c
tra of WS
2
thin
s
ize
h
e WS
2
thin fi
l
dominant pea
k
m
age and (b) ED
X
m
position of sput
t
i
on of the sp
u
Mg, Si and
C
h
e deposited
W
film. The shift i
n
l
m sample is
g
k
s arising fro
m
X
spectrum of WS
2
t
ered WS
2
thin fil
m
u
ttered WS
2
th
C
a are attribut
e
W
S
2
layer.
n
the vibration
m
g
iven in Fig. 5.
m
the weak v
a
2
thin film
m
in film as ob
t
e
d to the SLG
m
ode indicates th
e
.
From the fig
u
a
n der Waals
i
4
t
ained via
substrate,
e
u
re, it can
i
nterlayer

forces which
from optical
p
while the A
1
g
from Fig. 5 t
h
415.60 cm
-1
r
the findings
o
WS
2
is obser
v
process [17].
The opt
o
heterojunctio
n
illuminated c
o
277.9 µW to
and the frequ
e
V to 20 V an
d
observed that
as photodete
c
WS
2
and p-S
i
trend can be
the increase
i
277.9 µW at
b
SLG/n-WS
2
/
S
forward bias
r
Fig.
6
and i
l
b
otto
m
heter
o
The phot
o
SLG/n-WS
2
/
S
Equation 1:
where I
ph
the dark curr
where the las
the laser po
w
of the incide
n
interesting c
h
distressed latt
i
p
honon E
1
2g
a
n
g
mode is cau
s
h
at both the E
1
r
espectively, c
o
o
f previous lit
e
v
ed [15,16], a
o
electronic
c
n
photodetec
t
o
nditions. The
obtain the IV
c
e
ncy is set to
1
d
shown in Fi
g
under increas
i
c
tor with the 4
4
layers can be
observed fro
m
i
n laser powe
r
b
ias voltage.
T
S
iO
2
/p-Si:B
ph
r
egion and ne
g
6
. (a) IV curv
e
l
luminated condit
i
m
r
ight, (b) Pa
r
o
junction photode
t
o
current used
S
iO
2
/p-Si:B
ph
is photocurre
n
ent. Significa
n
er power hea
v
w
er indicates a
n
n
t photons to
b
h
aracteristic u
i
ce vibration a
n
n
d A
1g
modes.
s
ed by the ou
t
1
2g
and A
1g
p
h
o
o
nfirming the
e
rature [14]. I
n
n
d could be a
c
haracteristics
t
or is invest
i
laser powers
a
c
urves as sho
w
1
Hz. The IV
c
g
. 6 (a) with l
o
i
ng illuminati
o
4
1 nm light so
u
observed, eve
n
m
the partially
r
from its dar
k
T
hese IV curve
s
h
otodetector,
w
g
atively in the
r
e
s of SLG
/
n-WS
2
/
i
ons at various p
o
r
tially magnified
t
ecto
r
with voltag
e
to weigh th
e
h
otodetector
u





n
t, I
illumination
is
n
t changes in
v
ily influences
n
arrowed dep
l
b
e converted i
n
sed to evalu
a
n
d bonding be
t
The E
1
2g
p
ho
n
t
-of-
p
lane vib
r
o
non modes o
growth of the
n
certain cases
result of the
s
of the
W
i
gated using
a
re varied at
0
w
n in Fig. 6.
T
c
urve was me
a
garithmic coo
r
n from dark c
o
u
rce. Good Sc
h
n
though the I
V
magnified log
a
k
current to 0.
6
s
also indicate
w
hich is its
a
r
everse bias re
g
/
SiO
2
/p-Si:B hete
r
o
wers with the c
u
logarithmic IV
e
bias from 5 V to
e
photo-respo
n
u
nder various




the current un
the photocurr
e
the photocurr
e
l
etion region a
t
n
to a photocur
r
a
te the perfor
m
t
ween layers.
T
n
on mode is a
n
r
ations of the
f WS
2
are loc
a
WS
2
layer [1
3
s
, a blue-shift
i
strain encoun
t
W
S
2
based
S
IV measure
m
0
.6124 µW, 13
T
he bias voltag
a
sured betwee
n
r
dinates on th
e
o
nditions, the
d
h
ottky barrier
f
V
curves are n
o
arithmic IV c
u
6
124 µW, 13.
a unique feat
u
a
bility to res
p
g
ion when un
d
r
ojunction photod
e
u
rves in logarith
m
curve of SLG/
n
11 V with norma
l
n
se characteri
conditions c
a


n
der illuminate
d
e
nt can be ob
s
e
nt by the rise
t the p-n junc
t
r
ent [18]. The
m
ance of ph
o
T
he two peaks
n
in-
p
lane opt
i
S atoms. It is
a
ted at 350.70
3
], and augur
w
i
n the in-
p
lan
e
t
ered during t
h
S
LG/n-WS
2
/Si
O
m
ent under
d
.54 µW, 115.
8
g
e is maintaine
d
n
a voltage ra
n
e
bottom right.
d
evice express
i
f
unction betw
e
o
n-linear. An i
n
u
rves in Fig.
6
54 µW, 115.8
u
re of the hete
r
p
onds positive
l
d
er illuminatio
n
e
tecto
r
under dar
k
m
ic coordinates a
t
n
-WS
2
/SiO
2
/p-Si:
B
l
curve on top lef
t
stics of heter
a
n be calcul
a
d
conditions a
n
s
erved from
F
in power. Inc
r
t
ion, which all
o
responsivity
i
o
todetector an
d
5
originate
i
cal mode
apparent
cm
-1
and
w
ell with
e
mode of
h
e growth
O
2
/p-Si:B
d
ark and
8
µW and
d
at 10 V
n
ge of -20
It can be
i
vely acts
e
en the n-
n
creasing
6
(b) with
µW and
r
ojunction
l
y in the
n
.
k
t
B
r
ojunction
a
ted from
(1)
n
d I
dark
is
F
ig. 7 (a),
r
ement in
o
ws most
i
s another
d
can be

measured as
i
area of light i
n
Fig.
7
SLG/
n
EQE
o
It can be
p
ower of the
l
p
ower of 0.
6
response at 4
4
separation o
f
light is illum
i
p-Si and n-
W
are accelerat
e
symbolizes t
h
impact to the
can be under
s
where D
*
sensing area
a
1.48 x 10
8
Jo
n
The EQE
the IV curves
where h i
illumination
w
heterojunctio
n
the fabricate
d
compares to
energy and c
a
Figure 8
p
hotodetecto
r
i
n Equation 2,
n
cident [19]:
. (a) Power d
e
n
-WS
2
/SiO
2
/p-Si:
B
o
f heterojunction
p
observed fro
m
l
aser. The ma
x
6
124 µW. Th
i
4
1 nm withou
t
f
electron-hole
i
nated at the s
u
W
S
2
regions. In
e
d towards th
e
h
e ability of th
e
total noise co
m
s
tood from Eq
u
is the detect
i
a
nd q is the c
h
n
es for an inci
d
of the SLG/
n
under forwar
d
s Plank const
a
w
avelength at
n
photodetect
o
d
device whe
r
higher illumi
n
a
rrier recombi
n
shows the t
i
r
under 441 n
m
where R is re
s



e
pendent photocu
r
B
under illuminat
i
p
hotodetecto
r
SL
G
m
Fig. 7 (a)
t
x
imum value o
f
i
s is a proper
t
t
any external
pairs (EHP)
f
u
rface of phot
o
the forward b
i
e
n region an
e
detector to d
e
m
es from Pois
s
u
ation 3 as foll
o

∗


i
vity and is d
i
h
arge of an ele
c
d
ent light inte
n
n
-WS
2
/SiO
2
/p-
S
d
bias conditio
n


a
nt, c is the v
e
441 nm. A h
i
o
r device. Fig.
r
e a low laser
n
ation power
s
n
ation due to t
h
i
me-dependen
t
m
laser illumi
n
s
ponsivity, P
il
l






r
rent and responsi
v
i
on at 441 nm, (
b
G
/n-WS
2
/SiO
2
/p-S
i
hat the respo
n
f
R is estimate
t
y of the san
d
power source
.
f
rom charge
c
o
detector, the i
n
i
as region, the
d contribute
t
e
tect a weak o
p
s
on noise fro
m
o
ws:


2

i
rectly proport
c
tron. The ma
x
n
sity of 0.6124
S
i:B heteroju
n
n
s [22] and gi
v



e
locity of ligh
t
i
gh EQE valu
e
7 (b) shows
t
illumination
s
. This sugge
s
h
e low EQE v
a
t
current res
p
n
ation at vario
u
l
uminated
is the l
a
v
ity of heterojunc
t
b
) Power depend
e
i:B under illumin
a
n
sivity decrea
s
e
d as 169.3 m
A
d
wiched phot
o
.
The in-
b
uilt
e
c
arrier recomb
i
n
cident photo
n
photo-generat
e
t
o photocurre
n
ptical signal,
a
m
the dark curr
e
t
ional to R, S
x
imum value
o
µm.
n
ction photode
t
v
en as:
t, e is charge
e
is an indicat
o
t
he power dep
e
power demo
n
s
ts inadequat
e
a
lue (<1).
p
onse of san
d
u
s laser powe
r
a
ser power an
d
tion photodetecto
r
e
nt detectivity an
d
a
tion at 441 n
m
s
es with the i
n
A
W
-1
for an ill
u
o
detector to
d
e
lectric field
c
i
nation [20].
W
n
s generate E
H
ed holes in th
e
n
t [18]. The
d
a
ssuming that
t
e
nt [21]. The
d
is the effecti
v
o
f D
*
is preme
t
ecto
r
is obtai
n
of an electro
n
o
r of a highly
endence of th
e
n
strates a hig
h
e
absorption
o
d
wiched heter
r
s of 0.6124 µ
W
6
d
A is the
(2)
r
d
n
creasing
u
mination
d
etect the
c
auses the
W
hen the
H
Ps in the
e
p region
d
etectivity
t
he actual
d
etectivity
(3)
v
e photo-
ditated to
n
ed from
(4)
n
and λ is
sensitive
e
EQE of
h
EQE as
o
f photon
r
ojunction
W, 13.54

µW, 115.8 µ
W
in Table 1.
Fig. 8
illum
i
Tab
Figure 9
s
range at 441
n
at 10 V
b
ias
frequencies i
s
W
and 277.9
µ
. Time depe
n
i
nation at 441 nm
w
l
e 1 Rise and f
a
Power (µ
W
0.6124
13.54
115.8
277.9
277.9
s
hows the
t
ime
n
m with vario
u
voltage and
l
s
also tabulate
d
µ
W at 10 V bi
a
n
dent current resp
w
ith different po
w
a
ll time of hetero
j
W
) Frequency (
H
1
5
10
100
300
500
dependent cu
r
u
s frequencies
l
aser power 2
7
d
in Table 1.
a
s voltage. Th
e
onse of photodet
w
ers at bias voltag
e
j
unction photod
e
H
z) Rise Time (
-
15.53
16.03
15.76
2.83
1.42
0.27
0.22
0.20
r
rent response
i.e., 1 Hz, 5 H
z
7
7.9 µW. The
e
rise times an
d
t
ector within the
e
and modulation
f
e
tector illuminat
e
(
m.s) Fall Time
-
15.76
16.63
16.27
3.95
3.12
2.26
1.06
0.67
of the
p
hotod
e
z, 10 Hz, 100
H
e
rise times a
n
d fall times is
violet range ligh
t
f
requency of 1 H
z
e
d under 441 nm
(m.s)
e
tector within
H
z, 300 Hz a
n
n
d fall times
a
7
tabula
t
ed
t
z
the violet
n
d 500 Hz
a
t various

8
Fig. 9. Time dependent current response of photodetector within the violet range at 441 nm
with different frequencies at bias voltage and laser power 277.9 µW
4. Conclusion
A WS2 thin film sandwiched SLG/n-WS2/SiO2/p-Si:B heterojunction photodetector is
successfully fabricated by RF magnetron sputtering. XRD pattern reveals a polycrystalline
WS2 film with two diffraction peaks corresponding to (100) plane at 2θ = 34.28° and (110)
plane at 2θ = 60.82° with hexagonal structure. FESEM reveals nano-flake like structures in
the 156.3 nm thick layer of non-stoichiometric WS2 film with S/W ratio of 1.25. Raman shifts
are found at the E1
2g and A1g phonon modes located at 350.70 cm-1 and 415.60 cm-1
respectively. The optoelectronic characteristics show evidence of Schottky barrier behaviour
in the sandwiched photodetector with 10 V bias voltage and the ability to work in both the
forward and reverse bias regions. The maximum Iph was obtained at higher laser power as
0.95 µA. The maximum value of R was calculated as 169.3 mAW-1 and D* was observed
1.48 x 108 Jones for an incident light intensity of 0.6124 µm. Overall, these results highlight
the technological potential of TMDC sandwiched photodetectors for practical applications in
next-generation high performance optoelectronics.
Funding
Ministry of Higher Education (MoHE), Malaysia (LRGS (2015) NGOD/UM/KPT, GA 010-
2014 (ULUNG)); University of Malaya (RU 001-2017).
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