2D material photo-detector gain and responsivity control and enhancement through induced interface trap doping

A method for controlling any of a responsivity, response time, and trap characteristics of a two-dimensional (2D) material on a self-assembled monolayers (SAMs) device, the method including modifying a surface of an oxide substrate, in an atomic scale, to create the 2D material, wherein the modifying the surface includes modifying a level of impurities trapped in the surface and a doping level of the surface, and forming charge carrier traps at the surface, wherein a capture rate and an emission rate of the charge carrier is influenced by an exposure to a light signal, and wherein the exposure to the light signal further changes the doping level of the surface.

BACKGROUND

Technical Field

The embodiments herein generally relate to photo-detector materials, and more particularly to gain and responsivity control in two-dimensional (2D) photo-detectors.

Description of the Related Art

The study of 2D materials, covering a wide spectrum of physics and applications, has inspired attention. The broad range of bandgap properties, strong light-matter interactions, significant spin-orbit entanglement, suitable electronic transport properties, and effective plasmonic interactions are among several explored research areas that motivate 2D optoelectronic applications. However, a comprehensive understanding of the 2D material device properties and light-matter interactions are important to any progress in this field. For example, it is desirable to be able to control gain and responsivity of a 2D photo-detector.

SUMMARY

In view of the foregoing, an embodiment herein provides a method for controlling any of a responsivity, response time, and trap characteristics of a 2D material on a self-assembled monolayers (SAMs) device, the method comprising modifying a surface of an oxide substrate, in an atomic scale, to create the 2D material, wherein the modifying of the surface comprises modifying a level of impurities trapped in the surface and a doping level of the surface, and forming charge carrier traps at the surface, wherein a capture rate and an emission rate of the charge carrier is influenced by an exposure to a light signal, and wherein the exposure to the light signal further changes the doping level of the surface.

The modification of the doping level may comprise changing a barrier height at an interface contact of the oxide substrate, wherein the changing of the barrier height may comprise a first charge trapping at a surface of the oxide substrate. The oxide substrate may comprise silicon oxide, and the silicon oxide may be grown on a silicon layer, wherein the modification of the doping level may further comprise a second charge trapping at a surface of the silicon layer.

The silicon layer and the silicon oxide may be exposed to the light signal in a visible frequency range, wherein the exposure to the light signal results in the change in the barrier height at a contact of the silicon layer and the silicon oxide, and wherein the barrier height change amplifies a current in the silicon layer and the silicon oxide. A responsivity of a photo-detector made using the 2D device may be greater or equal to approximately 4.5×103A/W at approximately 7 V.

An embodiment herein provides a device comprising a semiconductor layer and an oxide layer grown on the semiconductor layer, wherein a surface of the oxide layer is modified in an atomic scale by modification of impurities trapped in the surface and a doping level of the surface, and forming charge carrier traps at the surface, wherein a capture rate and an emission rate of the charge carrier is influenced by an exposure to a light signal, wherein the exposure to the light signal further changes the doping level of the surface.

The modification of the doping level may comprise changing a barrier height at an interface contact of the oxide layer, and wherein the changing of the barrier height may comprise a first charge trapping at a surface of the oxide layer. The oxide layer may be grown on a semiconductor layer, and wherein the modification of doping level may further comprise a second charge trapping at a surface of the semiconductor layer. The semiconductor layer and the oxide layer may be exposed to the light signal in a visible frequency range, wherein the exposure to the light signal may result in the change in the barrier height at a contact of the semiconductor layer and the oxide layer, and wherein the barrier height change may amplify a current in the semiconductor layer and the oxide layer. A responsivity of a photo-detector made using the device may be greater or equal to approximately 4.5×103A/W at approximately 7 V. The device may be any of a photo-detector, memory device, and a quantum computational device.

An embodiment herein provides a device comprising a silicon oxide layer, wherein a surface of the silicon oxide layer is treated by applying oxygen plasma for approximately five minutes, and the surface is modified by applying thiol-based organosilanes, and a molybdenum disulfide layer grown on the silicon oxide layer using chemical vapor deposition. The oxygen plasma may form hydroxyl on the silicon oxide layer, wherein the silicon oxide layer may be placed with a drop of organosilane precursor in a vacuum desiccator under approximate vacuum conditions of approximately 5 mBar at approximately 60° C. for approximately 2 hours.

A contact angle of the silicon oxide layer may be greater than or equal to approximately 65 degrees. The thiol-based organosilanes may comprise thiol-terminated (3-Mercaptopropyl) methyl dimethoxysilane (CH3Si(OCH3)2(c)H2(c)H2(c)H2SH). The silicon oxide layer may be grown on a baseline pristine and approximately 300 nm thick thermally oxidized P-type Si substrates. The device may be a photo-detector and the spacing between the E12gand the A1gRaman peaks of the device is approximately 20 cm−1. The device may further comprise source and drain metal contacts connected to the molybdenum disulfide layer, wherein the metal contacts may be etched using an e-beam evaporation at approximately 5×10−6Torr. The metal contacts may be approximately 3 nm to approximately 50 nm thick Ti/Au. The device may be any of a photo-detector, memory device, and a quantum computational device.

DETAILED DESCRIPTION

The photo-detector and phototransistor properties of devices made using transition metal dichalcogenides (TMIDs) show promising photo-detection efficiency and responsivities. The embodiments herein provide photocurrent generation and utilize device properties and incorporate the role of interfaces.

The large surface area and lack of dangling bonds in 2D materials may make their interfaces with 3D materials in device platforms unique. These interfaces may include the interface of the 2D materials with their substrate and the interface of the 2D materials and metal contacts. Substrate interface interactions may result in interfacial charge carrier scattering and carrier mobility changes in 2D materials. They can also result in strong exciton localizations and affect the photoluminescence response. Furthermore, interfacial design can be used to control and modify the 2D device properties.

An embodiment herein uses the charge carrier traps in 3D nanomaterials as a tool for control of photo-detector properties. For instance, ultrasensitive photo-detectors made of quantum dots with deliberately designed surface trap functionalization may be used in photo-detectors made of low dimensional material systems. The dynamic behavior of contact interface traps and the nature of charge carrier capture and escape can modify the contact barrier and potential levels in the device and alter its properties. Therefore, the design of traps and controlling their properties may be used in tailoring the properties of nanomaterial based photo-detectors. Since 2D materials have large surface-to-volume ratios, interface properties have high importance in their optoelectronics.

Some the embodiments herein provide interface engineering in 2D atomic layers that can be used to selectively dope, induce carrier trapping, and modify the device properties in these materials. By creating traps and trap base doping, the embodiments herein control the contact barrier and thus responsivity of 2D photo-detectors.

Interface engineering strategies of the embodiments herein may be used for doping purposes and common patterned assembly strategies and can be used to localize the doping. The strategies in the embodiments herein can be used to develop sensors, photo-detectors, memory devices, and applications in logic circuits. Additionally, they can be used for systematic trapping of charge and photo-carriers. Combined with sufficient external control and actuation using other technologies such as magnetisms and plasmonics devices, the embodiments herein can be used for applications in any of quantum computing, quantum sensing, and control of quantum capacitance.

In an embodiment herein, the interfaces in devices made of two-dimensional materials such as molybdenum disulfide (MoS2) can effectively control their optoelectronic performance. The embodiments herein provide for utilizing the role of substrate interfaces on the photo-detector properties of MoS2devices and utilizing its photocurrent properties on both silicon oxide (SiO2) and self-assembled monolayer-modified substrates. In some embodiments, while the photoresponsivity of the devices may be enhanced through control of device interfaces, response times may moderately be compromised. This trade-off may be due to the changes in the electrical contact resistance at the device metal-semiconductor interface. The embodiments herein demonstrate that the formation of charge carrier traps at the interface can dominate the device photoresponse properties. The capture and emission rates of deeply trapped charge carriers in the substrate-semiconductor-metal regions are strongly influenced by exposure to light and can dynamically dope the contact regions and thus perturb the photo-detector properties. As a result, interface-modified photo-detectors have significantly lower dark-currents and higher on-currents. Through interfacial design, the embodiments herein achieve a device responsivity of 4.5×103A/W at 7 V, which is indicative of the large signal gain in the devices and exemplifying an important design strategy that enables highly responsive two-dimensional photo-detectors.

Referring now to the drawings, and more particularly toFIGS. 1 through 10, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.

FIG. 1is a schematic diagram illustrating use of self-assembled monolayers (SAMs)100for modification of oxide substrates surfaces (SiO2layer102), grown on a silicon layer (Si layer104), to tailor and change the interface properties of the SiO2layer102or using 2D material atomic layers in electronic and optical devices. In an embodiment, a variety of organosilanes SAMs106, such as any compounds containing any of —OH, —SH, —NH2, —CF2, and —CH3, may be used to create different interfaces which can cause to create modification of impurities trapped electronic/photonic properties and doping levels of the 2D atomic layered SiO2102.

Because of such electronic/photonics changes, the embodiments herein can tailor device properties of the 2D atomic layered materials. The doping level may change due to charge trapping at the metal contact as well as the oxide interface when the device is exposed with the light in the visible range, and results in changes in the barrier height at the contact and amplifies the on-current. Thus, one can dynamically tune/control the photocurrent and photoresponsivity properties of 2D photo-detectors by selecting/switching with a specific desired SAMs even for only one given 2D material. These affects may be used to control and design approaches for tuning of electronic and photonic responsivity in the 2D materials. In an embodiment, similar strategies can be implemented to locally or globally dope device areas for memory applications. The control of photo-carrier traps may also have new applications in the field of quantum computing as controlling and modulation their properties allow for new computational means.

FIG. 2, with reference toFIG. 1, is a schematic diagram illustrating a device200, according to an embodiment herein. An embodiment herein uses the role of interfaces on the photo-detector properties of single-layered molybdenum disulfide (MoS2)202. An embodiment herein may control the interface properties of the substrate204by controlling the interfacial chemistry. Comparing the photo-detector properties of MoS2devices on a SiO2substrate versus a thiol-modified substrate, the embodiments herein demonstrate that interfaces have an important role in controlling the responsivity and response time of the 2D photo-detectors. Two connectors206,208are electronically coupled together using the MoS2layer202, and are used to measure photo detective properties of the MoS2layer202.

FIG. 3A, with reference toFIGS. 1 and 2, is a schematic diagram of device300for electronic and optoelectronic characterization of a 2D photoelectric, according to an embodiment herein. The device300may include a substrate302, and connectors304,306on the substrate302. The connectors304,306may be electronically coupled to each other via photo-detector308. In an embodiment herein, the photo-detector308comprises a MoS2layer.FIG. 3B, with reference toFIGS. 1 through 3A, is a schematic diagram illustrating an enlarged image of a channel310between the connectors306and308, according to an embodiment herein. In an exemplary embodiment, the length of the channel310is between approximately 0.2 μm to approximately 3 μm. The measurements indicated on the figures herein are exemplary embodiments and are not limiting.

The measurement results described herein indicate that while the photosensitivity of the photo-detectors, such as the MoS2layer202, can be enhanced through interface engineering, the device response time may be slightly compromised. This trade-off may be attributed to changes in the electrical contact barrier at the metal-semiconductor junction. Therefore, interface modification may result in deep interfacial traps at the vicinity of metal contacts, the photo-induced doping effects of which dramatically reduce the contact resistance and result in a major gain in photo-detection. However, the resultant dynamic of capture and emission rates may lead to low device response times. These results explain an important mechanism occurring in the contact regions of 2D photo-detectors and establish guidelines for the design of highly sensitive 2D optoelectronics.

The control of interfaces may be a strong tool for modifying the properties of device300based on 2D materials. One approach for modification of interfaces as provided by the embodiments herein is the application of SAMs. The diversity in the chemistry, MOSFET compatibility, and functionality of SAMs are characteristics that make them an attractive choice for such applications.

An embodiment herein uses thiol-based organosilanes as a diverse class of SAMs for modification of SiO2interfaces. In an exemplary embodiment herein, thiol-terminated (3-Mercaptopropyl) methyldimethoxysilane (CH3Si(OCH3)2(c)H2(c)H2(c)H2SH) is used to modify the interface chemistry of SiO2substrates. In an example, to prepare the SAMs, an embodiment herein first treats the SiO2substrates with oxygen plasma for approximately five minutes to remove all organic residues and promote the formation of dense surface hydroxyl groups. Next, the embodiments herein place the substrates and a drop of organosilane precursor in a vacuum desiccator under moderate vacuum conditions (˜5 mBar at approximately 60° C.) for roughly 2 hours. The prepared —SH/SiO2has a significantly larger contact angle (˜65 degrees) as compared to pristine SiO2(˜40 degrees), representative of the enhanced hydrophobicity of the SAM-modified surface.

FIG. 4A, with reference toFIGS. 1 through 3B, is a diagram illustrating x-ray photoelectron spectra (XPS) in photoelectron count per binding energy, acquired from the as-grown SAMs according to an embodiment herein.FIG. 4Aillustrates a doublet peak, S2p3/2(sulfur 2p orbitals)402at 163 eV and S2p1/2404at 164 eV, an overall fit406obtained by adding the double peaks S2p3/2and S2p1/2, and a spectrum408acquired by the measurements herein, associated with the unbound S2p sulfur bonds in the thiolate chemistry and further support the observation of interface modifications in the embodiments herein. The graph inFIG. 4Ais obtained using the SAMs modified surfaces using XPS Al Kαx-ray radiation.

In an embodiment herein, MoS2samples are prepared using a chemical vapor deposition (CVD) technique and transferred to the —SH/SiO2substrates as well as to baseline, pristine 300 nm thick thermally oxidized P-type Si substrates. The transfer process may include coating the CVD-grown MoS2with Poly(methyl methacrylate) (PMMA) and releasing it from the substrate in a potassium hydroxide (KoH) solution. The PMMA/MoS2samples may be transferred to the desired substrates and the PMMA film removed using acetone solution. The SAMs modified interface may be referred to herein as MoS2—SH/SiO2and may use the MoS2—SiO2to describe a bare SiO2interface. The samples may then be characterized using Raman and photoluminescence (PL) spectroscopy.

FIG. 4B, with reference toFIGS. 1 through 4A, is a graph illustrating Raman and PL spectra acquired from CVD MoS2on SiO2/Si substrates, according to the embodiments herein. The spacing between the E12gand A1gRaman peaks, ˜20 cm−1, and the intense PL spectrum are indicative of the high quality and monolayer nature of the MoS2samples. An embodiment herein uses e-beam lithography and oxygen plasma etch to pattern source/drain metal contacts and channel MoS2geometry to yield bottom-gated transistors on both the baseline and SAM-modified substrates. An embodiment herein use e-beam evaporation at ˜5×10−6Torr to deposit 3 nm/50 nm thick Ti/Au metals as contact metal. Referred toFIGS. 3A and 3B, the device300may include single- and multi-channel length designs suitable for typical field-effect characterization, optoelectronic measurements, and contact resistance estimation using the transmission line method (TLM). The width of the device300may varies between 1-3 μm and the channel lengths vary between 0.2-3 μm.

To perform optoelectronic measurements on the MoS2sample, an embodiment herein uses a 532 nm laser with a beam diameter of 150 μm. The beam may be centered in such a way that the entire active device300, including the 2D material, is in the incident light region. The laser power may be adjusted using an acoustic modulator and perform photocurrent measurements. First, the current-voltage characteristics may be measured for device300(40 μm wide with 3 μm channel lengths) in dark and illuminated conditions with an emphasis on understanding the influence of interface changes. An incident laser power of 3.4 μW may be used for these experiments.

FIG. 5A, with reference toFIGS. 1 through 4B, illustrates experimental results indicating a distinction between the behavior of devices made on SiO2and —SH/SiO2. An increase in the conductance of the device300, roughly by 4 orders of magnitude, is indicative of a high device response in MoS2—SH/SiO2samples. These results, as compared to only one order of magnitude change in conductance for similar devices on SiO2, exemplify a major difference in optical detection properties brought about by the SAM surface modification. In these experiments, the dark current in MoS2—SH/SiO2devices measured at 0.1 V was close to one order of magnitude smaller than MoS2—SiO2devices. Meanwhile, when illuminated, the current in MoS2—SH/SiO2devices increased to roughly two orders of magnitude larger than that of MoS2—SiO2devices. The lower dark current, as well as the large photocurrent generation in MoS2—SH/SiO2devices, contributes significantly to the differences seen in the device300characteristics.

FIG. 5B, with reference toFIGS. 1 through 5A, illustrates results that further show the significant role of interface traps on the photo-detector characteristics of the device300, and presents measurement of the photoresponse and light power dependency of the photocurrent. At lower power, the photocurrent rapidly increases in both MoS2—SiO2and MoS2—SH/SiO2interface conditions, with the changes leveling out at higher powers. The photocurrent is steadily close to two orders of magnitude larger in MoS2—SH/SiO2interface conditions as compared to MoS2—SiO2. The device300responsivity for varied interfaces is calculated to further compare the performance from these two interface conditions.

The responsivity at wavelength (λ),

Rλ=IPP
(IPbeing the photocurrent and P the incident light power) is a measure of the photo-detector's effectiveness in converting light power to electrical current. As illustrated inFIG. 5C, the responsivity in the device300sharply increases to a maximum and then gradually decreases as the incident power is increased. Owing to the direct proportionality to the photocurrent, the photoresponse in MoS2—SH/SiO2devices also exhibits an increase of two orders of magnitude over that of MoS2—SiO2devices. The photocurrent and, as a result, responsivity of the device300is also highly dependent on the applied bias voltage. The embodiments herein explain these effects by measuring the responsivity as a function Vdsup to 7 V as illustrated inFIG. 5D. The measurements for MoS2—SH/SiO2samples show high values for responsivity in MoS2devices, with at least one order of magnitude enhancement. External quantum efficiency is related to the responsivity of the device300through

E.Q.E.=Rλ⁢hcλ⁢⁢q,
where h is the plank constant, c is the speed of light, and q is the electron charge. The external quantum efficiencies in the device300reach maximum values close to 1000% and 10000%, for MoS2—SiO2and MoS2—SH/SiO2devices, respectively. Since the absorption in 2D materials is small, the extraordinary interface-dependent gain in device300that results in their large responsivities and quantum efficiencies needs to be explained.

The embodiments herein further examine the temporal properties of the detector device300to improve understanding of the response time properties. A distinct feature of the current-voltage behavior in the device300is the consistent dependence on history of exposure to light; the device300with many hours of exposure to light shows higher conductance even if left in dark environments. The slow recovery of the device300conductance to values before exposure to light suggests that a slow dynamic process is involved.FIG. 6A, with reference toFIGS. 1 through 5D, is a graph illustrating measurement of the dark current-voltage characteristics of the device300before and after 30 min of exposure to 3.4 μW light. The conductivity increases by several orders of magnitude and remains high for a period greater than 18 hours. These effects are more prominent in the MoS2—SH/SiO2samples. Time resolved photocurrent measurements are also performed to examine the photoresponse of the device300at a modulation frequency of 2 Hz.

FIGS. 6B and 6C, with reference toFIGS. 1 through 6A, illustrate five cycles of the measurements on lower panels604and608, as well as a close-up of one representative cycle on upper panels602and606, for further analysis. As is evident, there is a gradual increase in the average signal measurements in MoS2—SH/SiO2, and the measurements are slightly noisy. However, this gradual increase in current is not as dramatic in the MoS2—SiO2devices. The distinct interface dependent behavior of rise and fall times in the device300signal resembles what was observed inFIG. 6A. The MoS2—SH/SiO2devices show slower rise and fall times as compared to MoS2—SiO2devices. Additionally, the long tail in both rise and fall characteristics suggest the involvement of two mechanisms of signal generation that needs to be determined. The net effect of these two mechanisms results in rise times of 140 ms and 96 ms for MoS2—SH/SiO2and MoS2—SiO2, respectively. The fall times for these conditions amount to 186 ms and 151 ms for MoS2—SH and MoS2—SiO2, respectively.

FIG. 6D, with reference toFIGS. 1 through 6C, is a diagram illustrating the dynamic behavior of the signal in a MoS2—SH/SiO2sample by measuring the time dependency of current. After exposing the device300to 30 minutes of 3.4 μW light the current changes are measured over time at varied gate voltages in dark conditions, as presented inFIG. 6D.

FIG. 6Dfurther illustrates that initially the signal drops quickly followed by a gradual and lengthy tail with zero and positive gate voltages. This behavior leads to a long recovery time of the device300to its original state as depicted inFIG. 6A. The measurements for the zero and positive 80V gate voltages are shown in the top panel ofFIG. 6D. The measured current has been manually shifted, by subtracting a constant value from the positively gated signal, to match the starting currents in the two devices and allow for a better comparison between the different voltage conditions. As a result, the magnitude of the signal is arbitrary, but the changes represent the distinct time-dependent behavior of the current and its relation to gate voltage.

The faster decay of the signal in the positive vs. zero gate voltage conditions and the continuous increase in the signal under the negatively gated conditions (in this case −80V), presents a contrasting behavior. As the decay and recombination of excitonic pairs in a free excitonic system are in the picosecond time scales, a different mechanism with long life times should be responsible for the observed behavior. The trapping of the carriers at the interface and its resultant modification of the contact resistance in device300is a common observation in nanomaterial-based photo-detectors that leads to increased gain and photoresponsivity. Formation of such donor type interface electron traps at the junction and the vicinity of the metal-semiconductor-SAMs are postulated to be the primary source of interaction that govern the photo-detector properties in the device300. Exposing the contact regions to light excites the trapped electrons in these sites and dopes the material, which amounts to significant changes in the contact barrier properties. To present this more clearly, some of the embodiments herein focus on the contact resistance properties of the device300.

To understand these results and the origin of changes in the properties of device300some of the embodiments herein look at the general electronic transport and device properties of MoS2on SiO2and —SH/SiO2surfaces and assess some of the main distinctions resulting from the interface alterations.FIGS. 7A through 7D, with reference toFIGS. 1 through 6D, are graphs illustrating the transfer curves acquired for varied drain804-source802(FIG. 8A) voltages for MoS2—SiO2and MoS2—SH/SiO2, respectively. As illustrated inFIGS. 7A through 7D, a clear change in threshold voltage, subthreshold behavior, and hysteresis of the device300are observable.FIGS. 7A through 7Dillustrate that the threshold voltages estimated from the transfer curves at Vds=0.1V are −1V and 11V for MoS2on SiO2and on —SH/SiO2, respectively. The subthreshold swings estimated for these devices also vary from 6V/decade in MoS2—SiO2to 4V/decade in MoS2—SH/SiO2. The relatively high subthreshold swings are attributed to the limited gate control in these devices based on the 300 nm SiO2bottom gates. Even still, the lower subthreshold swing values in —SH/SiO2supported devices is indicative of the passivation role of the SAM for reducing interface trap charge states and the significant modification of band tails that extend into the MoS2band gap. The trap states at the substrate interface, resulting from point defects and 2D interfaces, can degrade both the mobility and switching characteristics of the device300. The hysteresis may be significantly larger for the MoS2—SH/SiO2samples, at 23.7 V compared to the MoS2—SiO2with 2.5 V. A field-induced change in the interface states of the metal-semiconductor may be a reason for the observed hysteresis.

The output curves shown inFIGS. 7C and 7Dfurther distinguish the impact of the SAM-modified interface on MoS2device performance. The zero back gate voltage sheet resistances are roughly two orders of magnitude larger in MoS2—SH/SiO2samples. The sheet resistance of a MoS2—SiO2device is 2 MΩ/□ while the MoS2—SH/SiO2sample has a sheet resistance of 371 MΩ/□. The slight nonlinearity in the IV characteristics of the MoS2—SH/SiO2samples at low fields compared to MoS2—SiO2samples suggests a difference in the electron transport at the metal-semiconductor junctions. From these experiments, an embodiment herein provides that the observed interface-dependent differences in the properties of device300are highly related to the role of contact barrier properties.

FIG. 8A, with reference toFIGS. 1 through 7D, is a schematic diagram illustrating energy band diagrams of the devices highlighting four relevant conditions at the metal-semiconductor contact. At the contact, the Fermi energy level of the metal (Au) is lower than the bottom of MoS2conduction band energy which results in bending of the bands and an electric field that depletes regions of the material of free electrons. This forms the contact barrier shown in Part I ofFIG. 8A. If a bias is applied across the device300, the electric field will slightly modify the barrier (Part II). By applying a gate voltage, the barrier width will change as shown in Parts III and IV. Therefore, to explain the role of traps on the contact resistance of device300, the doping effects and the dynamic nature of trap capture and emission need to be considered. It is known that the doping of contact regions will result in contact resistance changes that follow the relationship (1):

Rc=exp⁡[2⁢ɛs⁢m*h⁢(ϕbnND)](1)
where NDis the doping density, εsis the relative semiconductor (MoS2) permittivity, m* is the effective carrier mass, h is the Plank constant, and ϕbnis the barrier potential. In the photo-detector devices herein, emission of the deeply trapped electrons is promoted by the illuminated condition. This would result in a significant reduction of the contact resistance due to a trap induced increase in the doping density. The process of electron capture and emission after exposure to light and the transient doping of the contact barrier control the time dependent properties of device300. Electrons are captured and emitted from the trap sites using a phonon assisted process and depend on the capture and emission activation energies. The capture and emission rates, 1/τcand 1/τe, are related to several important trap, oxide gate, and semiconductor properties and can be expressed using the following relationship:

Ln⁡(τcτe)=-1kT⁡[(ECOX-ET)-(EC-EF)-ϕ0+q⁢⁢ψs+q⁢xTTox⁢(Vgs-VFB-ψs)](2)
where τcand τeare the capture and emissions times, ECoxis the conduction band edge of the oxide (—SH/SiO2), ETis the trap energy, ECis the conduction band edge of MoS2, EFis the Fermi energy of the source802or the drain804, ϕ0is the difference between the electron affinities of MoS2and —SH/SiO2, ψsis the amount of band bending, xTis the position of the trap measured from the MoS2—SH/SiO2interface, Toxis the oxide thickness, Vgsis the gate source802voltage, and VFBis the flat band voltage.

FIG. 8B, with reference toFIGS. 1 through 8A, is a schematic diagram illustrating an energy band diagram, according to an embodiment herein. This relationship describes the application of a positive or negative gate voltage that would have opposite effects on the trapping time properties and contact barrier. Here, τeis weakly dependent on the gate voltage. A positive gate voltage would therefore decrease the capture time while a negative gate voltage will increase it, having a direct effect on the dynamic nature of the contact resistance of device300. In dark conditions, the traps will capture electrons and enforce a low doping level regime at the junction. Compared to the low trap density of MoS2—SiO2devices, this will result in higher contact resistance and explains the lower conductance in MoS2—SH/SiO2devices (FIG. 6A).

Exposing the samples to light excites the electrons into the conduction band and dopes the contact regions, thereby lowering the contact resistance. This explains the significant photocurrent generation in MoS2—SH/SiO2devices (FIG. 6A). In transition between the dark and light conditions, the contact resistance and properties of device300are subject to the competing carrier capture and emission and the diffusion of the trap sites into and out of the contact barrier regions. In zero gate voltage conditions, exposure to light will gradually excite the traps. The planar electric fields will also gradually diffuse more traps into the contact barrier region. In dark conditions, the traps will slowly capture the free carriers and result in a gradual increase in the contact resistance (FIG. 7Dtop panel). If a positive gate voltage is applied, the trapping capture times, τc, will decrease and the electron capture rates will increase.

The net effect will result in a rapid increase in the contact resistance and a drop in the signal of device300(FIG. 7Dtop panel). With an applied negative gate voltage, the τcincreases and the capture rate decreases. These effects, in addition to the field induced diffusion of traps, contribute to the contact barrier changes. Migration of defects in 2D materials may be use to explain memristor device characteristics in MoS2devices with varied concentrations of sulfur vacancies. These results suggest that point defects can migrate in these materials from high density defect regions, such as grain boundaries, in sufficiently large enough electric fields (˜1 MV/m).

The field assisted diffusion of the traps into the contact regions contribute to the initial contact resistance decrease in device300. Over time the diffusion of traps into the contact regions is balanced by a decrease in the electron capture and the contact resistance and current reach an equilibrium state in device300(FIG. 7Dbottom panel).

FIGS. 8C through 8D, with reference toFIGS. 1 through 8B, are diagrams confirming the above-described propositions and comprehend the role of metal-semiconductor-oxide interfaces in the device300according to the embodiments herein, and assess the contact resistances in the device300using Transfer Length Measurements (TLM). According to these measurements, the contact resistance before light exposure for MoS2—SH/SiO2and MoS2—SiO2is 3400 kΩ·μm and 140 kΩ·μm, respectively. Immediately after exposure to light, the contact resistances measured for the same devices changes to 3 kΩ·μm and 25 kΩ·μm for MoS2—SH/SiO2and MoS2—SiO2devices, respectively (FIG. 8D).

This indicates that the contact resistance for both interface conditions is dependent on the exposure to light. This is especially true for the MoS2—SH/SiO2devices where the contact resistance is found to decrease by three orders of magnitude, which would explain the significant increase in photosensitivity of the devices. The sheet resistances of the channel material (slope of the TLM fit) in devices made on different interfaces before and after light exposure can be compared.

The sheet resistances for MoS2—SH/SiO2and MoS2—SiO2devices before exposure to light are 80KΩ/□ and 8KΩ/□, respectively. After exposure to light, the sheet resistance decrease slightly to 73KΩ/□ and 6KΩ/□ for MoS2—SH/SiO2and MoS2—SiO2samples, respectively. The difference between the sheet resistances for each interface can be attributed, in addition to expected sample to sample changes, to the doping levels resulting from charge transfer from the 2D material to the SAMs. The small changes to the sheet resistance of the device300before and after exposure to light are an additional confirmation of the importance of contact resistance to the behavior of device300. The results described herein directly link the photocurrent characteristics of MoS2—SH/SiO2devices to their dynamic contact resistance properties.

It is evident that the greater part of the gain in the photocurrent of the devices is a result of the higher contact resistance change in MoS2—SH/SiO2devices. The low dark currents in MoS2—SH/SiO2devices combined with significantly higher photocurrent due to doping induced changes in the device contact resistance results in the significant photoresponse in these devices.

The mechanisms provided by the embodiments herein also explain the behavior of responsivity as the incident power increases. The general behavior of the photo-detector is markedly influenced by the magnitude of the interface trap population near the contact regions. The population of excited trap states is closely related to the trapping and de-trapping time constants as well as the incident light intensity. As the intensity of light increases, the population of excited traps increases. The excited trap population reaches equilibrium at higher intensities where most traps have been excited. This results in a gradual decrease in the responsivity of the photo-detectors as the contact resistance changes slow down because of the saturation in the doping level.

FIG. 9, with reference toFIGS. 1 through 8D, is a diagram used to examine the trap characteristics by measuring the transfer curve threshold voltages, acquired from the device300after every measurement at different incident power, according to an embodiment herein. The magnitude of threshold voltage changes is significantly larger in the MoS2—SH/SiO2as compared to MoS2—SiO2devices. The changes in the threshold voltage of the device300relative to the dark current devices may be attributed to doping. Since the sheet resistance of the channel material does not change much after exposure to light, the threshold voltage changes can be attributed to the increase in contact region doping. The magnitude of doping can be estimated by measuring the changes in the threshold voltage (nT=εoεΔVT/te, where εoand ε are the permittivities of free space and SiO2, respectively; e is the electron charge; t is the thickness SiO2layer (285 nm); and ΔVTis the threshold voltage change). The ΔVTestimated in the dark and light condition at incident powers close to saturation of threshold voltages, can be used to roughly estimate the trap population. The estimated number of traps in MoS2—SH/SiO2and MoS2—SiO2interfaces is 6.1×1016and 2.8×1016, respectively. It may be concluded that the higher number of traps in MoS2—SH/SiO2, by roughly a factor of two, enables the markedly high responsivity in the photo-detectors made on this interface.

Self-assembled monolayers are a powerful tool for controlling oxide interfaces and can be used to modify the interface properties of 2D MoS2devices. The influence of interface interactions is found to be related to the defect energy states they generate in the bandgap of a 2D material. The shallow and deep states lead to trapping of the free carriers and affect a variety of device properties. The results described herein demonstrate that substrate interface properties have a direct role in the photo-detector properties of 2D MoS2devices. The embodiments herein show that the excitation and trapping of carriers at the interface in the affinity of the metal-semiconductor contact regions can dope and modify its barrier properties. These effects slowly change the contact resistance of the device300after exposure to light. The net effect of the interactions results in extremely high device responsivities but also somewhat compromises their response speeds. Therefore, the control of trap properties at the interfaces of 2D materials is a highly effective design strategy and a substantial tool for control of their device properties.

In the embodiments herein, the photo-detection characterization is performed using a controlled environment probe station with a 1-inch fused silica optical window for sample illumination and a vacuum capability of 10−6Torr. A 543 nm wavelength He/Ne laser can be used as the light source. An acoustic optical modulator (AOM) can be used for laser intensity stabilization, control, and modulation (for light pulse generation). The turn-on and turn-off time constants of the couple laser-AOM are measured to be less than 150 ns. After passing through the AOM, a laser is directed to the optical window and focused onto the sample placed inside the vacuum probe station. In an embodiment herein, the device300is powered with a Keithley 2634B dual-channel source meter unit connected to the probe station with a triaxial cable for low-noise measurement. The read-out currents are amplified by a Stanford Research Systems (SR570) low-noise current amplifier and recorded by a 10 MHz bandwidth Teletronix oscilloscope.

FIG. 10, with reference toFIGS. 1 through 9, illustrates a flow diagram illustrating a method1000for controlling any of a responsivity, response time, and trap characteristics of a 2D material on a SAMs device300according to an embodiment herein. The method1000may include modifying (1002) a surface of an oxide substrate, in an atomic scale, to create the 2D material, wherein the modifying of the surface may include modifying (1004) a level of impurities trapped in the surface and a doping level of the surface. The method1000may further comprise forming (1006) charge carrier traps at the surface, wherein a capture rate and an emission rate of the charge carrier is influenced by an exposure to a light signal, wherein the exposure to the light signal may further change the doping level of the surface.

An embodiment herein provides an interface based approach that allows for the control of photocarrier trapping and doping in the 2D material. The selective doping at the contacts leads to significant changes in contact barrier properties and results in photoinduced doping. The large and controllable on-currents result in large photoresponsivities. Additionally, the nature of trapping and doping and the possibility of their selective area implemention, as presented in the embodiments herein, provides immense access to specialized device design. The embodiments herein can be used to improve upon conventional device concepts with enhanced properties, selectivity, and time response properties. These properties are important building blocks of many applications such as photo-detectors, sensors, and computing modules.