Values of impedance, resistance, power and resistivity measured without presence of white light.
The low dimensional chalcogenide materials with high band gap of ~1.8 eV, specially molybdenum di-sulfide (MoS2), have been brought much attention in the material science community for their usage as semiconducting materials to fabricate low scaled electronic devices with high throughput and reliability, this includes also photovoltaic applications. In this chapter, experimental data for MoS2 material towards developing the next generation of high-efficiency solar cells is presented, which includes fabrication of ~100 nm homogeneous thin film over silicon di-oxide (SiO2) by using radio frequency sputtering at 275 W at high vacuum~10−9 from commercial MoS2 99.9% purity target. The films were studied by means of scanning and transmission electron microscopy with energy disperse spectroscopy, grazing incident low angle x-ray scattering, Raman spectroscopy, atomic force microscopy, atom probe tomography, electrical transport using four-point probe resistivity measurement as well mechanical properties utilizing nano-indentation with continuous stiffness mode (CSM) approach. The experimental results indicate a vertical growth direction at (101)-MoS2 crystallites with stacking values of 7-laminates along the (002)-basal plane; principal Raman vibrations at E12g at 378 cm−1 and A1g at 407 cm−1. The hardness and elastic modulus values of H = 10.5 ± 0.1 GPa and E = 136 ± 2 GPa were estimated by CSM method from 0 to 90 nm of indenter penetration; as well transport measurements from −3.5 V to +3.5 V indicating linear Ohmic behavior.
- thin film
- electron microscopy
- MoS2 sputtering
- elastic modulus
- x-ray diffraction
- electrical transport
- focus ion-beam
- atom probe tomography
Layered chalcogenide materials have been of high relevance since almost 40 years for their diverse applications such as tribology , chemical catalysis  and nowadays as semiconductors towards development of high-throughput and energy efficient transistors and devices [3, 4]. MoS2 is a two-dimensional material with a band gap ranging between 0.9 and 1.8 eV as calculated theoretically by first principles methods and as measured experimentally by Kam & Parkinson using photo-spectroscopy as a function of crystal orientation [5, 6]. The crystal structure of MoS2 is hexagonal with space group R3m (a = b = 3.16 Å and c = 18.41 Å), having d-bonded layers of S-Mo-S along a-b plane which are stacked along c-axis by weak Van der Waals forces with 6.2 Å of separation within layers . The crystal structure was studied using electron microscopy techniques as described by Chianelli et al. who were able to observe its layered structure . However, electron beam dosage during electron microscopy studies plays an important role to avoid any structural damage as described by Ponce et al. when using TEM technique who concluded high-resolution imaging at operational voltages near ~80 kV  to be possible. By in-situ TEM, Helveg et al. were able to synthesize small clusters of MoS2 from molybdenum oxide and hydrogen sulfide gases at beam radiation dosage of 100 e−/Å2s . The mechanical properties were studied by Casillas et al. achieving an atomistic observation of a resilient nature on MoS2 laminates at 8GPa of external applied pressure and its mechanical recovery during in-situ AFM on TEM sample holder . Applying atomic force microscopy (AFM), Bertolazzi et al. determined a Young modulus values of 270GPa ± 100GPa and fracture strength of 16~30GPa in MoS2 layers as suspended in patterned silicon wafers [12, 14], and Castellanos-Gomez et al. estimated an average Young modulus E = 330GPa in suspended MoS2 sheets over patterned silicon wafer . The mechanical properties were studied by density functional theory and molecular dynamics, Jiang et al. calculated a theoretical Poisson’s ratio value of v = 0.29 applying Stillinger-Weber potential . The reactive empirical bond-order (REBO) potential was used by Li et al. to understand structural effects at chemical bonding within S-Mo-S layers, their findings indicate induced vacancies on the basal plane can influence Poisson’s ratio values . The atom probe tomography enables the chemical understanding with three-dimensional spatial resolution and was applied to determine dopants, contamination and ionic distribution within semiconducting matrix , Singh et al. used APT technique to determine distribution of Ti over MoS2 matrix . Regarding electrical transport, Lia et al.  and Samuel et al.  performed transport electrical measurements encountering a linear ohmic behavior in MoS2. This chapter covers mechanical, electrical and microstructure characterization by electron microscopy, low angle x-ray, atom probe tomography and CSM-nanoindentation to obtain information about crystal growth, elastic modulus (E), hardness (H) and electrical transport on MoS2 films.
2. Experimental methods and results
2.1 RF sputtering
The Molybdenum di-Sulfide (MoS2) films were fabricated with a high vacuum Kurt J. Lesker© PVD 75 machine; applying RF-sputtering at a rate of 2.26 Å/sec at 275 W of plasma power over 4″-diameter silicon oxide (SiO2) wafers. The films were deposit from commercial MoS2 99.9% targets (Kurt J. Lesker). By using dwell time of 300 seconds a film thickness value of ~100 nm was achieved as indicated by profilometry measurements, Figure 1E.
2.2 Scanning electron microscopy
The film morphology and crystallographic structure were investigated using scanning and high-resolution transmission electron microscopy (SEM, TEM). SEM was performed in a Hitachi® SU5500 unit, equipped with Energy-dispersive X-ray spectroscopy (EDS) unit and operated at 30 kV with 8A of current to avoid surface damage on the film. Observations indicate a high-degree of porosity and vertically aligned MoS2 film matrix, as presented in Figure 2A–C which is in agreement with Kong et al. . EDS analysis reveals the two main signals that correspond to Sulfur-Kα and Molybdenum-Lα at 2.4 keV, as presented in Figure 2E, in agreement with Lince & Fleischauer .
2.3 Transmission electron microscopy and atom probe tomography
The microstructure of MoS2 thin matrix was also studied using Scanning Electron Transmission Microscopy (STEM) using a Cs-corrected 2200-JEOL, with STEM unit, equipped with a high-angle annular dark-field (HAADF) detector, X-Twin lenses and CCD camera. A lamella was prepared using Focus-Ion Beam model JEOL JEM 9320 at 30 kV and 25 mA, MoS2 film surface coated with gold and gallium. Atom Probe Tomography (APT) was performed on Cameca® LEAP 4000X high-resolution system in laser pulse mode (wavelength ~355 nm), measurements were taken at 60 K with evaporation rate of 0.5 and laser frequency of 100 kHz, laser beam was set to 70pJ/V, all data was reconstructed using IVAS© 3.6.10a package. The samples were prepared using focus ion beam FEI Strata dual-beam instrument coupled with micromanipulator Oxford® Omniprobe® 200 by lift-out method as described by Szász et al. . MoS2 film surface was protected using platinum layer, and cuts were done at 30 kV at 260pA gun power. Using both techniques, it was possible to determine chemical composition, and spatial resolution of S-Mo-S distribution along film matrix, stacking and orientation, as presented in Figure 3. In the image the top part corresponds to MoS2 and uppermost bright layer is due to gold gallium coating.
The atom probe tomography (APT) is a technique used to understand in a three-dimensional reconstruction with high-spatial resolution the chemical distribution and composition as indicated by Kelly & Miller . A sample is placed in the main APT chamber to undergo an ionizing evaporation process at a high electric field triggered by a laser pulse; the potential energy of an atom at the sample surface, as caused by the applied voltage on the sample neV, is converted into kinetic energy ~1/2mv2 in the vicinity of the tip. This relationship, in order to understand the mass-to charge-state ratio m/n of evaporated ions, is given by Eq. (1); with n as number of electrons removed from the ion, e electron charge (−1.62 × 10−19C), V total applied voltage, m is atomic mass and speed of atoms are given by conventional v = d/t, which is with good approximation constant, distance d and lastly t is the time of flight, as described by the schematic drawing taken from Kelly and Larson . Short laser pulses (<1 ns) are used for APT and can field evaporated for almost any material regardless of its electrical conductivity as described by Kellogg et al. .
Nowadays, usage of APT to survey spatial distribution of atomistic species in semiconducting devices like n-doped metal-oxide field effect transistors  and Singh et al. applied with high success to titanium-MoS2 and strontium oxide-MoS2 films . In this case, APT measurements were performed to understand the spatial distribution of MoS2 film matrix. Figure 4 illustrates the preparation of APT samples using a FIB (Figures 5–9).
2.4 Raman spectroscopy
The Raman spectroscopy was obtained using Alpha 300RA system equipped with a 532 nm Nd-YAG laser and a 100X 0.9 NA objective. The laser power was varied to avoid surface damage; with no additional sample preparation during study. Modes of vibration at E12g = 378 cm−1 and A1g = 407 cm−1 are indicators of sulfur vibrations caused by dangling bongs on S-Mo-S chemical structure as indicated schematically Figure 10 (insets).
2.5 Grazing incidence X-ray diffraction (GIXD)
X-ray diffraction was collected using a Panalytical X-Pert system with source of CuKαλ = 1.41 Å radiation. The grazing incidence angle was fixed at 0.5o with 20o < θ < 80o and step size of 0.02o with a graphite flat crystal monochromator, described by Liu et al. while characterizing same layers of MoS2  and presented in Figure 11.
2.6 Nanoscale mechanical properties
The nanoscale mechanical properties were evaluated to obtain Elastic modulus (E) and Hardness (H) of MoS2 thin films; this was possible using an Agilent nanoindenter model G200 coupled with a DCM II head instrument and Berkovich diamond indenter tip radius of 20 ± 5 nm, penetration depth limit of 400 nm, strain rate of 0.05 s−1, and harmonic displacement and frequency of 1 nm and 75 Hz, Poisson’s coefficient of ν = 0.22. The equipment was calibrated using a standard fused silica sample, under test parameters of C0 = 24.06, C1 = −184.31, C2 = 6532.04, C3 = −25482.45, and C5 = 19015.30 as constant area of contact for continuous stiffness method (CSM) as described in detail by Li et al. . All data was recorded by AFM Nano Vision© system attached to the nanoindenter system. The estimated values for hardness (H) and elastic modulus (E) were calculated using Eq. (2) to determine stiffness S, when comparing to silicon substrates (001) surface termination and applying a continuous stiffness method as described extensively by Pharr et al. .
In Eq. (2), ω is the excitation frequency, (Zo) displacement amplitude, (φ) phase angle, and (F0) is the excitation amplitude, all those values can be obtained if the machine parameters load-frame stiffness Kf and stiffness of springs (Ks) as well the mass m are known input values during nanoindentation test. The coating hardness of film Hf can be estimated using a work indentation model described by Eq. (3), having Hc as composed film/substrate hardness; Hs and Hf as substrate and film hardness, the constant k represents a fitting parameter determined experimentally from the variation of Hc with relative indentation depth (β = Hc/t).
The elastic modulus E can be estimated using Eq. (4); having Eeff as effective reduced elastic modulus of the system in array film/substrate, contact area is determinate by A as function of the penetration depth, ν is the Poisson ratio, t represents film thickness, and α is a parameter which depends on the material and the indenter geometry, in our case a pyramidal shape, as described by Domínguez-Rios et al.  and Hurtado-Macias et al. .
By using CSM method, it is was possible to estimate elastic modulus and hardness values as follows: Three regions of test are observed in the Figures 12 and 13, where region I is hardness values for MoS2 crystallites with penetration depth of 0–90 nm, having no influence from silicon oxide substrate and a hardness value of H = 6.0 ± 0.1 GPa and elastic modulus of E = 136 ± 2 GPa. The region II, which has a penetration deep of 90–120 nm both values of elastic modulus and hardness are increased, meaning a clear influence by silicon oxide substrate, as confirmed by profilometry a thin film thickness of ~105 nm (both insets of Figure 1). The region III with penetration depth of 120–150 nm represents a hardness and elastic modulus of silicon oxide substrate, which are in partial agreement with Malzbender & With  to whom performed similar experiment on SiO2 spin coated with methyltrimethoxysilane.
The obtained values for hardness and elastic modulus are smaller estimations when comparing with results as presented by Bertolazzi et al. [12, 14] for single layers of MoS2; we believe this occurs because of low dimension laminates can be stronger than stacking of MoS2 crystallites. The applied force was done over (001)-basal plane as suspended on patterned silicon holes [12, 14], and in this case indenter tip can sweep MoS2 crystallites over surface area. For that reason, our research team proceed to estimate film adherence by using AFM scratching technique in encountering a deformation 0.85 μm2 with a residual groove width 1 μm (a total groove height 125 nm and pile up height 40 nm), as presented in Figure 14, along with indentation sites completed to obtain elastic modulus and hardness values.
2.7 Electrical transport and resistivity
The electrical transport of the MoS2 film matrix was investigated using four-point probe method as indicated in Figure 15, equipped with Keithley 4200-SCS in applied voltage range from −3.5 to 3.5 V. The transport measurements were done at room temperature and by direct contact to the MoS2 film surface, no especial solder or metallic glue was used. Also, they were completed in the presence of light and dark conditions, the results indicate a linear Ohmic behavior, as presented in Figure 16 and resistivity values in Figure 17, from Tables 1 and 2 it was possible to determine values differences when white light is present, some authors refer this as photo-voltaic effect due to its intrinsic semiconductor nature of MoS2 [2, 32, 33].
|Size (m)||Impedance||Resistance||Power||Resistivity (Ω/m)|
|5 × 10−3||0.4064||1.41 × 106||0.0381||26.507|
|10 × 10−3||0.508||1.83 × 106||0.0381||27.473|
|15 × 10−3||0.6096||2.89 × 106||0.0254||24.106|
|Size (m)||Impedance||Resistance||Power||Resistivity (Ω/m)|
|5 × 10−3||0.4064||1.86 × 106||0.0381||35.047|
|10 × 10−3||0.508||2.46 × 106||0.0381||36.605|
|15 × 10−3||0.6096||3.92 × 106||0.0254||33.256|
3. Discussion and conclusion
By using radio frequency sputtering techniques at high-vacuum it was possible to fabricate MoS2 films with thickness of ~100 nm over pristine silicon oxide (SiO2) wafers. The film surface analysis was carried out using electron microscopy and spectroscopy techniques and results indicate molybdenum di-sulfide had a vertical crystallite growth as shown in Figures 2C and 5. Energy disperse confirms Sulfur-Kα (60%) and Molybdenum-Lα (40%) at 2.4 keV signal; and Raman spectroscopy modes of vibration at surface corresponding to E12g = 378 cm−1 and A1g = 407 cm−1. From high-resolution STEM it was possible to determine a degree of stacking between 7 layers along (002)-basal plane and to confirm vertical growth in agreement with Kong et al. , and APT preliminary measurements indicate a large quantity of sulfur and molybdenum with no grain boundaries or high impurities within film matrix for specific thin film growth using RF-sputtering conditions. From electrical transport measurements, it was possible to determine a linear Ohmic behavior and excitation when external visible light was on and off during four-point probe measurements as indicated by Figure 16, the resistivity values 26.5 Ω/m versus 35.0 Ω/m for off and on in external visible light, as possible caused by intrinsic semiconductor nature of MoS2 in agreement with [2, 32, 33]. The mechanical properties were also investigated as previously reported by Ramos et al.  for indenter penetrating film surface 0-90 nm it was possible to estimate hardness of H = 10.5 ± 0.1 GPa and elastic modulus E = 136 ± 2 GPa by the continuous stiffness method . It was concluded that MoS2 films are a promising semiconducting material candidate for large scale photovoltaic applications, due to low cost and reliable and straight forward approach for homogenous low dimension films and its possible chemical combination with other group VI semiconducting materials as indicated by Najmaei et al. [34, 35] and Matis et al.  and Paranjape et al. [37, 38, 39, 40, 41, 42].
The principal author thanks Universidad Autónoma de Ciudad Juárez for financial support by PIVA-2017 project titled: “Fabricación de plantillas metálico-orgánicas: Una ruta hacia celdas solares más eficientes” and PROFOCIE 2015. The Laboratorio Nacional de Nanotecnología of Centro de Investigación en Materiales Avanzados (CIMAV-Chihuahua) and ICNAM program of Kleberg Advanced Laboratory Center of University of Texas at San Antonio both for the usage of electron microscopy and characterization equipment and facilities. Part of this work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. We are grateful to the Karlsruhe Micro and Nano Facility (KNMF) of Karlsruhe Institute of Technology for usage of Atom Probe Tomography and FIB instruments, under proposal 2018–020-022609. To facultad de ciencias de la electronica of Benemerita Universidad Autónoma de Puebla for usage of four-point probe station and laboratory facilities.
Conflict of interest
Authors declare no conflict of interest.