Abstract:
A plasma-based processing method includes depositing a transition metal dichalcogenide (TMDC) material onto a substrate. The TMDC material is plasma treated in an oxygen containing ambient to oxidize the TMDC material to form oxidized dielectric TMDC material. The oxidized dielectric TMDC material has a higher electrical resistivity as compared an electrical resistivity of the TMDC material before the plasma treating, typically &gt;10 3  times greater.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Provisional Application Ser. No. 61/979,714 entitled “PLASMA TREATED SEMICONDUCTOR DICHALCOGENIDE MATERIALS AND DEVICES THEREFROM”, filed on Apr. 15, 2014, which is herein incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    Disclosed embodiments relate to semiconductor transition metal dichalcogenide (TMDC) materials and devices including such materials. 
       BACKGROUND 
       [0003]    While graphene has a very high carrier mobility, the lack of a bandgap limits the application of graphene in nanoelectronic and optical devices. Recently single layer and few layer MoS 2  sheets have been receiving significant attention due to their tunable band gap from 1.2 eV in the bulk to 1.8 eV in a single layer. 
         [0004]    Field effect transistors (FETs) based on single layer MoS 2  have shown fast current switching with a mobility of about 250 cm 2 /Vs. In addition, phototransistor, chemical sensor, photovoltaic devices, and integrated circuits have been demonstrated based on MoS 2 . 
         [0005]    Significant progress has been made in modifying and controlling the intrinsic properties of MoS 2 . The ability to modulate the electronic and optical properties further widen the applications of 2D MoS 2  and may open up a new era in solid state electronics and opto-electronics. However, such modification of physical properties is of significant challenge as it requires controllably tuning the material properties. 
       SUMMARY 
       [0006]    This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter&#39;s scope. 
         [0007]    Disclosed embodiments include plasma oxidation methods for selectively oxidizing a single layer or a few layer 2D semiconductor transition metal dichalcogenide (TMDC) material to reduce its electrical conductivity. As defined herein, a semiconductor is a material having an electrical conductivity between that of an electrical conductor such as the metal copper and that of an electrical insulator (or dielectric) such as a silica, with a 25° C. resistivity of between 10 7  and 10 −3  ohm-cm. 
         [0008]    In one particular embodiment, a MoS 2  flake material is changed by plasma processing from a semiconductor to insulator (dielectric). Although the TMDC material is described herein as being MoS 2 , the TMDC material can be other materials provided they are semiconductors and can be oxidized by disclosed plasma processing including oxygen to raise the resistivity of the material to that of an insulator. For example, the TMDC can also generally comprise MoSe 2 , WS 2 , WSe 2 , In 2 Se 3 , or GaTe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is a cross sectional schematic diagram of an example back-gated field effect transistor (FET) that includes a disclosed oxidized dielectric TMDC layer, according to an example embodiment. 
           [0010]      FIG. 1B  shows the output characteristics of a FET including a single layer of oxidized dielectric MoS 2  at different back gate voltages (V G ) ranging from −60V to 40V with a voltage interval of 20 V, while  FIG. 1C  shows transfer characteristics of the single layer MoS 2  device. 
           [0011]      FIG. 2A  shows gate dependence of the source drain current (I DS ) for an example MoS 2 -based FET, where the curve corresponds to plasma exposure times of 0, 2, 3, 4, 5, 6, and 7 sec, respectively.  FIG. 2B  shows the effect of plasma exposure on the ON current (at V G =40V) and mobility of few layers MoS 2 -based FET. 
           [0012]      FIG. 3A  is an I DS  vs. V DS  characteristics curve for the few layers MoS 2 -based FET at different plasma exposure.  FIG. 3B  shows the resistance of the FET as a function of plasma exposure time. The line shown is the linear fit of the logarithmic electrical resistance as a function of exposure duration. 
           [0013]      FIG. 4A  is a Raman spectra of pristine MoS 2  and plasma etched MoS 2  obtained with a 532 nm excitation wavelength. While MoS 2  modes were conserved, new Raman peaks corresponding to Mo—O bonds in MoO 3  were measured in the flake after exposure.  FIG. 4B  shows PL spectra of the flake showing strong response of the monolayer and total PL quenching after plasma treatment. 
           [0014]      FIG. 5  is a X-ray photoelectron spectroscopy (XPS) spectrum of Mo (3d) and S(2 s) core levels for pristine (lower panel) and plasma treated (upper panel) MoS 2  flakes. 
           [0015]      FIG. 6A  shows a structural model of electrical property tuning via defect engineering in MoS 2  single layer as a function of oxygen plasma exposure time, while  FIG. 6B  shows the change in energy band diagram of multilayer MoS 2 -based FET with plasma exposure time. 
           [0016]      FIGS. 7A-D  show the effect of plasma exposure time on the mobility with  FIG. 7A  for 1 layer,  FIG. 7B  for 4 layers, and  FIG. 7C  for an 8 layer MoS 2  device.  FIG. 7D  is a plot of Mobility/Layer vs. Plasma time/Layer for 1 layer, 4 layer and an 8 layer FET and it was found that the curves for the different devices essentially collapse into same line. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein. 
         [0018]    One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure. 
         [0019]    Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5. 
         [0020]    Mechanisms are described below that are believed to explain the observed phenomena provided by disclosed embodiments. Although the mechanisms described herein are believed to be accurate, disclosed embodiments may be practiced independent of the particular mechanism(s) that may be operable. 
         [0021]    Disclosed embodiments recognize for semiconductor TMDC materials such as MoS 2  to be used as a building block for lateral devices such as lateral FETs, it is necessary to develop a technique for the fabrication of tunable electrical insulator regions that can be rendered “on demand” tunnel region(s). Disclosed plasma oxidation methods provide a relatively simple approach enabling the fabrication of lateral FETs where a controllable (selectable) dielectric TMDC region can be created on the surface of a two-dimensional (2D) semiconductor TMDC material, in contrast to known heterostructures in a vertical geometry where 2D materials of different bandgaps are placed on top of each other. The 2D TMDC material is generally a layer that is one (1) to eight (8) atomic layers thick, and disclosed plasma treating generally oxidizes regions in all the layer(s) of the TMDC material. For MoS 2  1 atomic layer corresponds to about 0.9 nms thick, and 8 atomic layers corresponds to about 6 nms thick. 
         [0022]    MoS 2  is a 2D TMDC material representing a new classes of materials, generally having good electrical, mechanical and optical properties. Disclosed embodiments include oxygen plasma techniques for tuning the electrical properties of the layer(s) of TMDC materials such as MoS 2  flakes from being a semiconductor to an insulator. Such controlled changes to the electrical properties of TMDC materials such as MoS 2  is expected to be of significant importance for a variety of nano-electronic device applications such as FETs, sensors, diodes and quantum devices. 
         [0023]    In one embodiment, a single layer or multilayer MoS 2 -based FET device is formed using disclosed methods. In one specific embodiment, MoS 2  deposited unto a substrate is then exposed to an oxygen plasma (e.g., O 2 :Ar mixture of 20%:80% by volume) treatment for different time durations. It has been shown that the mobility, ON current and electrical resistance of a single layer and multilayer MoS 2  FET varies exponentially by up to four orders of magnitude with the plasma exposure time. Raman and XPS study of the MoS 2  flakes that were exposed to O 2  plasma reveal dominant MoO 3  peaks. It is believed while exposed to oxygen plasma, energetic oxygen molecules interact with MoS 2  to create MoO 3  rich defect regions which are insulating (dielectric). The area coverage of the defect region increases with increasing exposure time. This effect can be exploited in fabricating lateral TMDC-based FETs without the need to pattern the TMDC layer. 
         [0024]      FIG. 1A  is a cross sectional diagram of an example back-gated FET (FET)  100  including an oxidized dielectric TMDC layer  116  including defects regions (defects)  116   a  therein comprising molybdenum trioxide, MoO 3 , according to an example embodiment. The TMDC layer  116  that is not shown as defects  116   a  provides the active layer for HFET which has the geometry of a conventional lateral FET. FET  100  comprises a TMDC layer  116  on a gate dielectric layer  122  on an electrically conductive substrate  112 . The gate dielectric layer  122  may comprise silicon nitride (SiN), silicon dioxide (SiO 2 ), Boron Nitride (BN), aluminum oxide (Al 2 O 3 ), or hafnium oxide (HfO 2 ), among other suitable insulators. 
         [0025]    As the substrate  112  is used as a back-gate for the FET  100 , the substrate  112  may comprise highly doped (n+ or p+) Si, typically doped to at least 5×10 18  cm −3 , or another electrically conductive substrate material. Contacting respective sides of the oxidized dielectric TMDC layer  116  is a source contact  114  and a drain contact  115  which each provide a low resistance Ohmic contact. The source contact  114  and a drain contact  115  may also be on the oxidized dielectric TMDC layer  116 . The source contact  114  and drain contact  115  can comprise a metal or metal alloy, such as gold (Au), Nickel (Ni), or Scandium (Sc). 
         [0026]    Regarding operation of FET  100 , a gate bias (Vg) is applied to the substrate  112  relative to the source. A source-drain bias (Vds) is then applied between source contact  114  and a drain contact  115 , such as Vds=100 mV. A positive gate voltage induce charges in TMDC  116  and changes the current (Id). The oxygen plasma create defects  116   a  being MoO 3  regions, which create bottlenecks for current flow. FET  100  is turned ON and OFF by applying a bias to the substrate  112  acting as a back gate. At Vgs=0, the FET  100  may not be fully OFF as shown in  FIG. 1C  described below. 
         [0027]    A method of fabricating TMDC-based lateral FET can include the following steps. A TMDC layer (e.g. MoS 2 )  116  is deposited on an electrically conductive substrate  112  by mechanical exfoliation, chemical vapor deposition (CVD) or an epitaxial processes. In mechanically exfoliation a bulk TMDC crystal is exfoliated to generate flakes that are deposited onto the substrate  112 . Another approach for depositing the TMDC layer is using an atomic layer deposition (ALD) process at about 300° C., such as a CVD process, but process control for the ALD process may be limited as compared to exfoliation. Then metal (ohmic) source and drain contacts  114  and  115  are formed on opposite ends of the TMDC layer  116  for providing the source and drain contact. The device is then exposed to a disclosed oxygen gas plasma and a defect induced oxidized dielectric TMDC layer  116  is formed including defects  116   a.    
       Examples 
       [0028]    Disclosed embodiments of the invention are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way. 
       Device Fabrication: 
       [0029]    Example devices were fabricated using single layer MoS 2  flakes that were mechanically exfoliated from a commercially available crystal of molybdenite (SPI Supplies Brand, Natural Molybdenite) using an adhesive tape micromechanical cleavage technique and deposited on a highly doped Si substrate capped with a thermally grown 250 nm thick layer of SiO 2 . Before MoS 2  deposition, the Si/SiO 2  wafers were cleaned using oxygen plasma followed by rinsing in acetone and isopropyl alcohol. 
         [0030]    An atomic force microscopy (AFM) height profile indicated a MoS 2  thickness of 0.9 nm, corresponding to a single layer. The number of MoS 2  layers was further confirmed by a Raman study. Laser power was kept below 1 mW in order to avoid any damage to the MoS 2  flake and on the other hand sufficient to obtain a good signal to noise ratio. Two prominent peaks at E2g and A1 g corresponding to in-plane and out-of-plane vibrations of atoms were identified separated by a Raman shift Δ=19.46 cm −1  confirms the single layer nature of the flake. Standard electron beam lithography (EBL) was used to pattern metal contacts corresponding to source contact  114  and drain contact  115  on the MoS 2  flakes. Other metal contact techniques can also be used such as optical lithography. 
         [0031]    For the fabrication of a MoS 2  FET. first a double layer electron beam resist, methyl methacrylate/poly(methyl methacrylate) (MMA/PMMA), was spun on the substrate and baked, followed by e-beam exposure and development in (1:3) methyl isobutyl ketone:isopropyl alcohol (MIBK:IPA). After defining the electrodes (source contact  114  and drain contact  115 ), 35 nm Au was deposited by thermal evaporation, followed by liftoff in acetone. 
         [0032]    The electron transport measurements of the MoS 2  FET were performed in a probe station at ambient condition using a Keithley 2400 source meter and a DL instruments  1211  current preamplifier interfaced with LabView program. The measurements were performed before and after each oxygen plasma treatment. The plasma treatment on the MoS 2  FETs was carried out using a commercial (Plasma Etch, PE-500) plasma chamber at a power of 100 W operating at 50 kHz. During plasma exposure, the pressure within the plasma chamber was held at 1 Torr and a gas mixture of Oxygen (20%) and Argon (80%) flow at a constant rate of 15 sccm. Each time, the sample was exposed for 5 sec and the electron transport measurements were repeated. 
       Results and Discussions: 
       [0033]      FIG. 1B  shows the output characteristics (I D  vs. V DS ) for different back-gate voltages (V BG ) for FET  100  varying from −20 to 40 V (bottom to top) with a step size of 10 V. An increase of drain current with gate voltage indicates n-type FET behavior.  FIG. 1C  shows the transfer characteristics (drain-current I DS  as a function of back-gate voltage V G ) for FET  100  measured at a fixed source-drain bias voltage V DS =100 mV for the as-fabricated FET  100 . The I DS  increased by several orders of magnitude with the increase V G , demonstrating n-type FET behavior. The current ON-OFF ratio of the device was found to be ˜10 4 . The field effect mobility of FET  100  can be calculated using the relation μ=(L/WC G ×V DS ) (dI DS /dV G ), μ=(L 2 /C G ×V DS )×(dI D /dV C ) where L is the channel length, W is the channel width and C G =∈ 0 ∈ r /d is the capacitance between the substrate  112  (gate) and gate dielectric  122  (SiO 2 ), C c =(2π∈L)/ln(2 h/r) with ∈ r ˜3.9∈ 0  being the effective dielectric constant of SiO 2 , and d (=250 nm) is the silicon oxide thickness. The mobility of the FET  100  was calculated to be 6 cm 2 /Vs. 
         [0034]      FIG. 2A  shows the transfer characteristic at VDS=100 mV of FET  100  after each plasma exposure. Several interesting behaviors can be seen from this transfer characteristic curve. First, the drain current at all gate voltages decreases with an increase of oxygen plasma exposure. This can be more clearly seen in  FIG. 2B  (right axis) where there is a plot of the ON-current at V G =40 V in a semi-log scale. The drain current was ˜285 nA for the as fabricated sample, which decreased exponentially with time to value of less than 20 pA, a drop of more than four orders of magnitude, after only a total of 6 s plasma exposure time. After a 6 s exposure, the current become negligibly small. The rapid drop of current with oxygen plasma exposure evidences electrons are getting trapped in defect regions of the MoS 2  and the trapped states are increasing with increasing plasma exposure. The mobility of the device after each plasma exposure is calculated from the I−V G  curves in  FIG. 2A  and is plotted in  FIG. 2B  (left axis) in a semi-log scale. Like the ON-current behavior, the mobility also drops exponentially from 6 cm 2 /Vs for as fabricated sample to 4×10 −4  cm 2 /Vs, after a 6 s plasma exposure. Similar to ON-current, the decrease of mobility is also more than four orders magnitude with plasma exposure. 
         [0035]      FIG. 3A  shows the dependence of current-voltage FET characteristics upon plasma exposure time. The I DS −V DS  graph of the device at V G =40 V is plotted in  FIG. 3A  for different plasma exposure times. It is observed that at all exposure time the I DS −V DS  curves are linear around the zero bias representing Ohmic behavior.  FIG. 3B  demonstrates the dependence of resistance on the plasma exposure time. A large variation in resistance is observed after plasma exposure. The linear fit of the logarithmic resistance as a function of time indicates that the resistance increases exponentially upon plasma exposure. Similar changes in resistance were also observed for other gate voltages. 
         [0036]    Similar device characteristics were obtained on two other single layer FETs. To explore the physical mechanism responsible for the observed change in electronic transport properties, Raman spectroscopy, X-ray photospectroscopy (XPS) and photoluminescence (PL) characterization of the pristine and plasma treated MoS 2  flakes were performed. Raman spectroscopy is a powerful tool to investigate changes in composition of 2D materials. The Raman signature of the pristine flake and the plasma treated monolayer were compared. 
         [0037]      FIG. 4A  shows the Raman spectra of a representative single layer MoS 2  flake before and after 6 s of oxygen plasma treatment. The two Raman peak corresponding to E 2g   1  (˜385 cm −1 ) and A g   1  (˜410 cm −1 ) modes, characteristic of MoS 2  observed in the pristine flake, clearly decrease in amplitude after treatment. Interestingly, E 2g   1  (in plane) is severely affected as a result of the treatment, while A g   1  shifts only of 3 cm −1  with a strong amplitude decrease (6 times) and a significant broadening. Finally the disappearance of the LAM mode at 450 cm −1  also confirms the disruption of the MoS 2  lattice during oxygen plasma treatment. On the other hand, the apparition of other peaks observed in the 150-400 rel. cm −1  range indicate the formation of Mo—O bonds in the system at 180 cm −1  corresponding to the B 2g  vibrational mode of MoO 3 , 225 cm −1  corresponding to the B 3g  mode. 
         [0038]    Hence the electronic properties were explored using PL spectroscopy. As a result of oxygen plasma treatment, the PL of MoS 2  is fully quenched after 6 s of treatment (see  FIG. 4B ). Photoluminescence involves exciting excess electron-hole pairs optically with energy of the incident photons higher than the band gap of the semiconductor sample. The emitted radiation is a result of the radiative recombination in the sample. For single layer MoS 2 , the PL distribution is centered around 1.84 eV (often referred to as exciton A1), with a second peak with lower amplitude around 1.0 eV (often referred to as exciton B1). A decrease in PL intensity, as seen in  FIG. 4B  indicates competing non-radiative processes removing excess carriers in the system. The results are indication of a reorganization of the excess carriers in presence of MoO 3 . 
         [0039]      FIG. 5  show the XPS spectra of pristine MoS 2  and plasma treated MoS 2  respectively. Three prominent peaks were observed at energies 227 eV, 229.7 eV, and 233.1 eV in pristine MoS 2  sample, origin of which has been attributed to binding energy of S 2 s, Mo 3d 5/2  and Mo 3d 3/2  electrons in Mo—S bond of the MoS 2  crystal respectively. All these peaks were also found at same binding energies for the plasma treated sample, however an additional peak at energy 236.4 eV could be observed, corresponding to the higher oxidation state Mo +6 . This new peak further confirms the presence of MoO 3  in the plasma treated sample. 
         [0040]    Based on the Raman, XPS and PL study, the following mechanism is proposed to explain the electrical property evolution of MoS 2  due to disclosed plasma treatment. During plasma treatment high energetic charge particles bombard on MoS 2  surface. Since S has much smaller mass compared to Mo, they move from the lattice site and lattice vacancies are created. Because of the excess oxygen supplied by the plasma, oxidation takes place at the defect sites in the surface. The oxidation process can be described as 2MoS 2 +70 2 =2MoO 3 +4SO 2 . The work function of MoO 3  is ˜6.8 eV which is larger than that of MoS 2  (4.5 to 5.2 eV). As a result of this large work function difference, electron transfer can occur from MoS 2  to MoO 3  resulting in hole doping in MoS 2 . In addition, MoO 3  has a bandgap of 3.2 eV-3.8 eV, making it electrically insulating in nature. Therefore, the creation of MoO 3  in MoS 2  hinders the charge transport. An exponential increase in resistance ( FIGS. 3A , B) is indicative of percolation conduction mechanism in MoS 2  upon plasma exposure. This evidences that with increase of plasma exposure time, MoO 3  rich region increases in MoS 2  which creates significant distortion of lattice ( FIG. 6A ). Those lattice distortions create tunnel barriers (see  FIG. 6B ) inside the 2D materials which reduces the charge carrier transport through the device. The tunnel barrier height increases with the exposure time, and for high exposure of time completely cease the transport. 
         [0041]    In order to investigate whether or not similar electronic property tuning can occur in multilayered MoS 2  flakes, devices were fabricated with 4 layer and 8 layer MoS 2  flakes. The results are summarized in  FIGS. 7A-D  ( FIG. 7A  for 1 layer,  FIG. 7B  for 4 layers and  FIG. 7C  for 8 layers). It can be seen that the plasma exposure time needed to see changes in mobility (also in current) increases with increasing MoS 2  thickness. For example, in single layer sample, a mobility of less than 0.001 cm 2 /Vs was achieved in 6 sec while similar mobility change occurred in 8 layer sample in 60 sec. It was found that the time required to see changes in mobility linearly scale with the number of layer. This is shown in  FIG. 7D  where there is a plot the mobility/layer with time/layer. This demonstrates that a controlled tuning of electronic properties from single layer and multilayer TMDC sample such as MoS 2  is possible in a predictive way. 
         [0042]    While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the Disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents. 
         [0043]    Although disclosed embodiments have been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. While a particular feature may have been disclosed with respect to only one of several implementations, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.