Band Alignment at GaN/Single-Layer WSe2 Interface

TL;DR: The band alignment parameters determined here provide a route toward the integration of group III nitride semiconducting materials with transition metal dichalcogenides (TMDs) for designing and modeling of their heterojunction-based electronic and optoelectronic devices.

Abstract: We study the band discontinuity at the GaN/single-layer (SL) WSe2 heterointerface. The GaN thin layer is epitaxially grown by molecular beam epitaxy on chemically vapor deposited SL-WSe2/c-sapphire. We confirm that the WSe2 was formed as an SL from structural and optical analyses using atomic force microscopy, scanning transmission electron microscopy, micro-Raman, absorbance, and microphotoluminescence spectra. The determination of band offset parameters at the GaN/SL-WSe2 heterojunction is obtained by high-resolution X-ray photoelectron spectroscopy, electron affinities, and the electronic bandgap values of SL-WSe2 and GaN. The valence band and conduction band offset values are determined to be 2.25 ± 0.15 and 0.80 ± 0.15 eV, respectively, with type II band alignment. The band alignment parameters determined here provide a route toward the integration of group III nitride semiconducting materials with transition metal dichalcogenides (TMDs) for designing and modeling of their heterojunction-based electr...

Summary

1. INTRODUCTION

• In order to study the band discontinuity at the GaN/SL-WSe2 heterointerface, thin GaN layers were epitaxially grown on a CVD prepared SL-WSe2.
• The sustainability of SL-WSe2 during GaN growth is confirmed by the atomic force microscopy (AFM), microphotoluminescence (µPL) and Raman spectroscopies.
• In addition, the band offset parameters for GaN/SL-WSe2 heterojunction were determined using high-resolution x-ray photoelectron spectroscopy and electronic band gap values of respective constituent layers in the heterojunction.

2. EXPERIMENTAL SECTION

• The Agilent technologies 5400 atomic force microscopy was used in tapping mode to acquire the surface morphology of the samples.
• Highangle annular dark field scanning transmission electron microscopy (HAADF-STEM) was utilized by operating a probe-corrected FEI Titan at an acceleration voltage of 300 kV.
• Using Horiba Aramis room temperature (RT) µPL, the emission of GaN and WSe2 layers was deduced with excitation lines of He-Cd laser of 325 nm and He-Ne laser of 633 nm, respectively.
• The high-resolution spectra were collected within the limits of spatial resolution at a fixed analyzer pass energy of 20 eV.
• In absence of electron beam charge, the surface of samples was electrically connected with a clean copper (Cu) foil.

3. RESULTS AND DISCUSSION

• Furthermore, the measured CBO value is verified by the Anderson's affinity rule which is defined as the difference between electron affinity values of constituent semiconductors of heterojuction.
• 50 The electron affinity (the energy separation between vacuum level and CBM) values GaN  , 2 WSe SL.

Figures

Figure 5a (i and ii) present the W 4f core-level and valence band spectra for SL-WSe2. b(i and ii) show the spectra of Ga 2p and W 4f core-levels for GaN/SL-WSe2. c(i and ii) show Ga 2p corelevel and valence band spectra acquired on GaN epilayer. The peak positions of core-levels are shown in parentheses.

Figure 6 shows the schematic representation of type II band alignment at GaN/SL-WSe2 heterointerface.

Figure 2 (a and b) show the low and high magnification cross-sectional images of high-angle annular dark field - scanning transmission electron microscopy (HAADF-STEM) for sample B. (c) shows the STEM-EELS elemental color maps (W-Blue, Se-Green, Ga-Red, and Al-Yellow). Figure 2(d) represents the STEM-EDS elemental line profiles acquired across the GaN/WSe2/csapphire heterojunction.

Figure 3(a-d) show the micro-Raman spectra of samples A-C collected on WSe2, GaN/WSe2 and bulk like GaN samples, respectively. Here, He-Cd (325 nm) and He-Ne (633 nm) excitation sources (Eexc > Eg) were used to enhance the Raman signal based on the band gap of each material.

Figure 1 shows the AFM scans for (a) GaN epilayer, (b) WSe2 and (c) GaN/WSe2 samples, respectively. The line profile of 1(b) represents the thickness of WSe2 layer. The inset to Figure 1(b) is optical microscopy image of WSe2 sample. Figure 1(d) shows the HRXRD 2θ-ω scans of on-axis and off-axis reflections and the inset shows the respective ω-scans acquired on GaN epilayer.

Figure 4(a and b) show the micro-photoluminescence (right y-axis) and absorbance spectra (left y-axis) of GaN and WSe2 samples. The green colored PL peak in Figure 4(b) is for GaN/WSe2 sample.

Full text

Band Alignment at GaN/Single-Layer WSe2 Interface
Item Type Article
Authors Tangi, Malleswararao; Mishra, Pawan; Tseng, Chien-Chih; Ng,
Tien Khee; Hedhili, Mohamed N.; Anjum, Dalaver H.; Alias, Mohd
Sharizal; Wei, Nini; Li, Lain-Jong; Ooi, Boon S.
Citation Tangi M, Mishra P, Tseng C-C, Ng TK, Hedhili MN, et al. (2017)
Band Alignment at GaN/Single-Layer WSe2 Interface. ACS
Applied Materials & Interfaces 9: 9110–9117. Available: http://
dx.doi.org/10.1021/acsami.6b15370.
Eprint version Post-print
DOI 10.1021/acsami.6b15370
Publisher American Chemical Society (ACS)
Journal ACS Applied Materials & Interfaces
Rights This document is the Accepted Manuscript version of a Published
Work that appeared in final form in ACS Applied Materials &
Interfaces, copyright © American Chemical Society after peer
review and technical editing by the publisher. To access the
final edited and published work see http://pubs.acs.org/doi/
full/10.1021/acsami.6b15370.
Download date 10/08/2022 06:31:16
Link to Item http://hdl.handle.net/10754/623191

1
Band alignment at GaN/single-layer WSe
2
interface
Malleswararao Tangi
1
, Pawan Mishra
1
, Chien-Chih Tseng
2
, Tien Khee Ng
1
, Mohamed Nejib
Hedhili
3
, Dalaver H. Anjum
3
, Mohd Sharizal Alias
1
, Nini Wei
3
, Lain-Jong Li
2
, and Boon S.
Ooi
1*
1
Photonics Laboratory, Computer, Electrical, and Mathematical Sciences and Engineering
(CEMSE) Division, King Abdullah University of Science and Technology (KAUST), Thuwal
23955-6900, Saudi Arabia.
2
Physical Science and Engineering (PSE) Division, King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
3
Imaging and Characterization Laboratory, King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
ABSTRACT: We study the band discontinuity at the GaN/single-layer (SL) WSe
2
heterointerface.
The GaN thin layer is epitaxially grown by molecular beam epitaxy on chemically vapor deposited
SL-WSe
2
/c-sapphire. We confirm that the WSe
2
was formed as an SL from structural and optical
analyses using atomic force microscopy, scanning transmission electron microscopy, micro-
Raman, absorbance, and micro-photoluminescence spectra. The determination of band offset
parameters at the GaN/SL-WSe
2
heterojunction is obtained by high-resolution x-ray photoelectron
spectroscopy, electron affinities and the electronic bandgap values of SL-WSe
2
and GaN. The
valence band and conduction band offset values are determined to be 2.25±0.15 and 0.80±0.15 eV
respectively, with type II band alignment. The band alignment parameters determined here provide
a route towards the integration of group III nitride semiconducting materials with transition metal
dichalcogenides (TMDs) for designing and modeling of their heterojunction based electronic and
optoelectronic devices.
KEYWORDS: GaN, single layer WSe
2
, 3D/2D heterojunction, HRXPS, band alignment,
molecular beam epitaxy
*
Corresponding author electronic mail: boon.ooi@kaust.edu.sa

2
1. INTRODUCTION
Group-III nitrides, GaN as a primary material, are well demonstrated with enormous
applications in high efficiency electronic and optoelectronics devices such as high electron
mobility transistors, light emitting diodes, and laser diodes.
1–4
Recently, group VI transition metal
dichalcogenides (TMDs) in the form of MX
2
has emerged as a novel atomic layered material
system with challenging thermoelectric, electronic and optoelectronic properties.
5–8
Among the
TMDs, tungsten diselenide (WSe
2
) in a single layer form is of potential interest for such devices
owing to its direct bandgap of ≈1.65 eV and prominent transport properties.
9,10
GaN has small
lattice mismatch of ≈3.2%, ≈1.0% and ≈0.8% with majorly studied TMDs such as WSe
2
, tungsten
disulfide (WS
2
) and molybdenum disulfide (MoS
2
)
11
, respectively, in comparison to commonly
used substrates such as sapphire, SiC and Si.
12,13
Moreover, recent advances in the integration of
2D-layered materials with wide band gap group III nitride semiconductors is exciting due to
their variety of applications in high current tunnel diodes.
14,15
Several efforts were made to grow
GaN on closely lattice matched TMDs. Yamada et al. presented the growth of GaN on bulk MoS
2
by plasma-enhanced molecular beam epitaxy
16
. There were recent attempts to grow GaN on large
area MoS
2
and WS
2
layers,
11,17
layered MoS
2
on GaN epilayers
18
by chemical vapor deposition
(CVD) growth techniques and layer transferred p-MoS
2
on GaN
14
. Though the deposition of large
area single-layer WSe
2
on sapphire exhibiting direct bandgap has been demonstrated,
9,19
the
growth of GaN on such large area monolayered WSe
2
has not yet been explored.
So far, the band offset parameters were determined for either group III nitrides on various
other semiconducting materials or solely TMDs based dissimilar heterostructures using X-ray
photoemission spectroscopic core-levels evaluated with respect to the valence band maximum and
bandgap studies. Though, the band offset parameters (junction type: valence band offset- ΔE
v
&
conduction band offset- ΔE
c
) are measured for various heterojunctions in the literature.
20–25
Recently we have reported the growth of GaN on SL-MoS
2
and the band alignment parameters
(Type-II: 1.86±0.08 & 0.56±0.10 eV) for GaN/SL-MoS
2
hetero-interface.
26
The band offset
parameters (valence band offset (VBO)-ΔE
v
and conduction band offset (CBO)-ΔE
c
) and type of
junction by HRXPS for epitaxially formed GaN/SL-WSe
2
hetero-interface is yet to be
experimentally investigated. The determination of band offset parameters is required to study the
group III nitride/TMDs heterojunction based devices. These hybrid heterojunctions, exhibiting
type I and II band alignments, can be utilized as TMD quantum well based light emitting devices

3
and the tunnel diodes, respectively.
6,15
Thus, the growth of GaN/SL-WSe
2
and the determination
of band offsets are necessary to provide a route towards the integration of group III nitrides with
TMDs for designing the electronic and optoelectronic devices.
In this report, in order to study the band discontinuity at the GaN/SL-WSe
2
heterointerface,
thin GaN layers were epitaxially grown on a CVD prepared SL-WSe
2
. The sustainability of SL-
WSe
2
during GaN growth is confirmed by the atomic force microscopy (AFM), micro-
photoluminescence (µPL) and Raman spectroscopies. In addition, the band offset parameters for
GaN/SL-WSe
2
heterojunction were determined using high-resolution x-ray photoelectron
spectroscopy (HRXPS) and electronic band gap values of respective constituent layers in the
heterojunction.
2. EXPERIMENTAL SECTION
The growth experiments of GaN on WSe
2
/c-sapphire substrates were carried out by Veeco
930 Gen plasma assisted molecular beam epitaxy (PAMBE) system at substrate temperature of
500
º
C. The large area WSe
2
SLs were prepared on c-sapphire substrates using CVD and the details
can be found elsewhere
19
. The ion and a cryo pumps were employed to attain a base pressure of
3×10
-11
Torr and oxygen partial pressure <10
-11
Torr, as obtained by a residual gas analyzer (RGA).
The substrates were thermally outgassed in introduction chamber at 200
º
C for 30 mins, further
cleaning was carried out in preparation chamber at 300
º
C for 60 mins and in the growth chamber
at 400
º
C for 30 mins. For GaN growth, the nitrogen plasma source was operated with the flow
rate of 1 standard cubic centimeter per minute (sccm) and RF power of 300 W and Ga metal was
evaporated by standard Knudsen cell with beam equivalent pressure (BEP) value of 2.10×10
-8
Torr. Prior to this, thin AlN buffer layer was grown. The corresponding chamber pressure was
≈2.5×10
-5
Torr. The thickness of the GaN epilayer (sample C) is measured to be ≈500 nm. The
thickness of GaN layer in sample B was estimated to be 6±1 nm from growth rate calibrations.
The Agilent technologies 5400 atomic force microscopy was used in tapping mode to acquire the
surface morphology of the samples. Structural quality of GaN epilayer was investigated by CuK
α
High Resolution X-ray Diffraction (HRXRD) having four-bounce Ge(022) monochromator. High-
angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was utilized
by operating a probe-corrected FEI Titan at an acceleration voltage of 300 kV. A cross- sectional
TEM specimen of sample B was prepared by using FEI’s Helios Dual Beam focused ion beam

4
(FIB)/SEM equipped with an Omniprobe. Electron energy loss spectroscopy (EELS) acquisition
was performed by using Gatan’s GIF Quantum of Model 966. Using Horiba Aramis room
temperature (RT) µPL, the emission of GaN and WSe
2
layers was deduced with excitation lines
of He-Cd laser of 325 nm and He-Ne laser of 633 nm, respectively. Absorbance spectra were
acquired with a Shimadzu UV3600 spectrophotometer equipped with integrating sphere. The high-
resolution XPS measurements were carried out using a Kratos Axis Ultra DLD spectrometer
equipped with a monochromatic Al K
α
X-ray source (hν = 1486.6 eV) operating at 150 W, a multi-
channel plate and delay line detector under a vacuum of ~10
-9
mbar. The high-resolution spectra
were collected within the limits of spatial resolution at a fixed analyzer pass energy of 20 eV. In
order to eliminate the shifts in the HRXPS spectra associated with the surface charging effect, the
measurements were acquired both with and without electron beam charge compensation. For both
cases, no changes were observed in the determined band alignment. In absence of electron beam
charge, the surface of samples was electrically connected with a clean copper (Cu) foil. The
remnant binding energy shifts were referenced to the adventitious carbon (C 1s) signal.
27–29
In this
study, CVD grown SL-WSe
2
(sample A), MBE grown GaN on SL-WSe
2
(sample B) and GaN
epilayer (sample C) were used to determine the band offsets at GaN/SL-WSe
2
hetero-interface.
3. RESULTS AND DISCUSSION
To investigate the surface morphology, root mean square (rms) roughness of GaN/WSe2
(B) and GaN epilayer (C), and thickness of WSe
2
layer (A), AFM scans were carried out in tapping
mode. Figures 1(a-c) show the AFM surface topography scans collected on samples C, A and B,
respectively. The AFM images were obtained at different scales to view the fine features according
to surface smoothness of the formed films. Color bars on top of the respective images in Figure 1
show height contrast of the features. From Figure 1(a), the root mean square (rms) surface
roughness for GaN film is ≈2 nm. Moreover, AFM is one of the most commonly used technique
to determine the number of monolayers of TMDs samples. The thickness of the large area WSe
2
layer is found to be ≈0.76 nm from the line profile as shown in Figure 1(b) for sample A, which is
in agreement with the thickness of Se-W-Se single-layer.
19
The inset to Figure 1(b) displays the
optical microscopy image of large area WSe
2
layer. Figure 1(c) shows the AFM surface
morphology for sample B exhibiting surface rms roughness of ≈4 nm which is higher than that of
relaxed GaN epilayer (sample C) that results from the lattice mismatch between thin GaN and

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Q1. What are the contributions in "Band alignment at gan/single-layer wse2 interface" ?

The authors study the band discontinuity at the GaN/single-layer ( SL ) WSe2 heterointerface. The band alignment parameters determined here provide a route towards the integration of group III nitride semiconducting materials with transition metal dichalcogenides ( TMDs ) for designing and modeling of their heterojunction based electronic and optoelectronic devices. The authors confirm that the WSe2 was formed as an SL from structural and optical analyses using atomic force microscopy, scanning transmission electron microscopy, microRaman, absorbance, and micro-photoluminescence spectra.