Two-dimensional phase-engineered 1T′– and 2H–MoTe₂-based near-infrared photodetectors with ultra-fast response

Kim, DongHwan,Lee, Rochelle,Kim, Sung,Kim, TaeWan ELSEVIER SCIENCE 2019 JOURNAL OF ALLOYS AND COMPOUNDS Vol.789 No.-

<P><B>Abstract</B></P> <P>Two-dimensional (2D) crystal growth allows wafer-scale van der Waals epitaxial integration of transition metal dichalcogenides (TMDs) semiconductors onto a Si substrate. 2D MoTe<SUB>2</SUB> and WTe<SUB>2</SUB> among TMDs are considered as possible candidates for high-performance near-infrared photodetector, due to its relatively low band gap energy (0.8–1.1 eV). Herein, 2D MoTe<SUB>2</SUB> was selected for the development of high-performance visible–near-infrared (0.5–1.1 μm) photodetectors. Phase-engineered MoTe<SUB>2</SUB> films of four atomic layers were grown by metal–organic chemical vapor deposition on an 8-inch SiO<SUB>2</SUB>/Si substrate. 1T′ and 2H phase MoTe<SUB>2</SUB> films were verified by Raman spectra and scanning transmission electron microscopy. A fabricated 2H-MoTe<SUB>2</SUB>-based field-effect transistor (FET) was found to have p-type electrical transport with a mobility of 22.8 cm<SUP>2</SUP>/V·s—the fastest among all currently reported 2H phase films synthesized by bottom-up methods—and an on/off ratio of 1.3 × 10<SUP>4</SUP>. Moreover, a photodetector based on the 1T′ phase (semimetal) film exhibited excellent performance, including photoresponsivity as high as 62–109 mA/W at 500–1000 nm—a 900% enhancement over that with a 2H phase (p-type semiconductor) film—and extremely fast response (a rise time of 0.82 μs and a fall time of 7.29 μs at a wavelength of 1000 nm).</P> <P><B>Highlights</B></P> <P> <UL> <LI> 2D MoTe<SUB>2</SUB> were grown by metal–organic chemical vapor deposition on an 8-inch Si wafer. </LI> <LI> Phase-engineered (1T′ and 2H) MoTe<SUB>2</SUB> has been achieved by optimizing MOCVD growth conditions. </LI> <LI> 2D MoTe<SUB>2</SUB> was selected for the development of visible–near-infrared photodetectors. </LI> <LI> The responsivity of 1T′-MoTe<SUB>2</SUB>-based photodetector is about ten times higher than 2H phase. </LI> <LI> Ultra-fast response time of 0.82–7.29 μs and photoresponsivity of 62–109 mA/W at 500–1000 nm. </LI> </UL> </P>

MoTe₂ Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering

Ma, Rui,Zhang, Huairuo,Yoo, Youngdong,Degregorio, Zachary Patrick,Jin, Lun,Golani, Prafful,Ghasemi Azadani, Javad,Low, Tony,Johns, James E.,Bendersky, Leonid A.,Davydov, Albert V.,Koester, Steven J. American Chemical Society 2019 ACS NANO Vol.13 No.7

<P>The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe<SUB>2</SUB>) phase, metallic monoclinic (1T′) MoTe<SUB>2</SUB> phase, and their lateral homojunctions can be selectively synthesized <I><I>in situ</I></I> by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of <I>in situ</I>-grown lateral 2H/1T′ MoTe<SUB>2</SUB> homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe<SUB>2</SUB> grow out of a polycrystalline 1T′-MoTe<SUB>2</SUB> matrix. We further demonstrate the operation of MoTe<SUB>2</SUB> FETs made on these <I>in situ</I>-grown lateral homojunctions with 1T′ contacts. The use of a 1T′ phase as electrodes in MoTe<SUB>2</SUB> FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T′ electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T′ interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe<SUB>2</SUB> homojunction FETs for use in memory and logic circuity applications.</P> [FIG OMISSION]</BR>

Suppression of Interfacial Current Fluctuation in MoTe₂ Transistors with Different Dielectrics

Ji, Hyunjin,Joo, Min-Kyu,Yun, Yoojoo,Park, Ji-Hoon,Lee, Gwanmu,Moon, Byoung Hee,Yi, Hojoon,Suh, Dongseok,Lim, Seong Chu American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.29

<P>For transition metal dichalcogenides, the fluctuation of the channel current due to charged impurities is attributed to a large surface area and a thickness of a few nanometers. To investigate current variance at the interface of transistors, we obtain the low-frequency (LF) noise features of MoTe2 multilayer field-effect transistors with different dielectric environments. The LF noise properties are analyzed using the combined carrier mobility and carrier number fluctuation model which is additionally parametrized with an interfacial Coulomb-scattering parameter (a) that varies as a function of the accumulated carrier density (Nacc) and the location of the active channel layer of MoTe2. Our model shows good agreement with the current power spectral density (PSD) of MoTe2 devices from a low to high current range and indicates that the parameter a exhibits a stronger dependence on Nacc with an exponent -gamma, of -1.18 to approximately -1.64 for MoTe2 devices, compared with -0.5 for Si devices. The raised Coulomb scattering of the carriers, particularly for a low-current regime, is considered to be caused by the unique traits of layered semiconductors such as interlayer coupling and the charge distribution strongly affected by the device structure under a gate bias, which completely change the charge screening effect in MoTe2 multilayer. Comprehensive static and LF noise analyses of MoTe2 devices with our combined model reveal that a chemical-vapor deposited h-BN monolayer underneath MoTe2 channel and the Al2O3 passivation layer have a dissimilar contribution to the reduction of current fluctuation. The three-fold enhanced carrier mobility due to the h-BN is from the weakened carrier scattering at the gate dielectric interface and the additional 30% increase in carrier mobility by Al2O3 passivation is due to the reduced interface traps.</P>

Electrical metal contacts to atomically thin 2H-phase MoTe₂ grown by metal–organic chemical vapor deposition

Kim, TaeWan,Joung, DaeHwa,Park, Jonghoo ELSEVIER 2018 Current Applied Physics Vol.18 No.7

<P>We grow atomically thin molybdenum ditelluride (MoTe2) films on a SiO2/Si substrate by means of metal-organic chemical vapor deposition (MOCVD). Our Raman spectroscopy measurements reveal the formation of 2H-phase MoTe2 films. Further, transmission electron microscopy and X-ray photoelectron spectroscopy studies indicate a three-atomic-layer structure and the surface element composition of MoTe2 films. In this study, we mainly focus on the influence of metal contacts attached to the films on their electrical performance. We fabricate 2H-phase-MoTe2-based field-effect transistors (FETs) with various metal contacts such as titanium/gold, nickel and palladium, which present p-type semiconductor properties. We also examine the influence of the work functions of the contact metals on the electrical properties of three-atomic-layer-MoTe2-based FET devices. For a p-type MoTe2 semiconductor, higher work functions of the contact metals afford narrower Schottky barrier heights (SBHs) and eventually highly efficient carrier injection through the contacts.</P>

Static and Dynamic Performance of Complementary Inverters Based on Nanosheet α-MoTe₂ <i>p</i>-Channel and MoS₂ <i>n</i>-Channel Transistors

Pezeshki, Atiye,Hosseini Shokouh, Seyed Hossein,Jeon, Pyo Jin,Shackery, Iman,Kim, Jin Sung,Oh, Il-Kwon,Jun, Seong Chan,Kim, Hyungjun,Im, Seongil American Chemical Society 2016 ACS NANO Vol.10 No.1

<P>Molybdenum ditelluride (alpha-MoTe2) is an emerging transition-metal dichalcogenide (TMD) semiconductor that has been attracting attention due to its favorable optical and electronic properties. Field-effect transistors (FETs) based on few-layer alpha-MoTe2 nanosheets have previously shown ambipolar behavior with strong p-type and weak n-type conduction. We have employed a direct imprinting technique following mechanical nanosheet exfoliation to fabricate high-performance complementary inverters using alpha-MoTe2 as the semiconductor for the p-channel FETs and MoS2 as the semiconductor for the n-channel FETs. To avoid ambipolar behavior and produce alpha-MoTe2 FETs with dean p-channel characteristics, we have employed the high-workfunction metal platinum for the source and drain contacts. As a result, our alpha-MoTe2 nanosheet p-channel FETs show hole mobilities up to 20 cm(2)/(V s), on/off ratios up to 105, and a subthreshold slope of 255 mV/decade. For our complementary inverters composed of few-layer alpha-MoTe2 p-channel FETs and MoS2 n-channel FETs we have obtained voltage gains as high as 33, noise margins as high as 0.38 V-DD, a switching delay of 25 mu s, and a static power consumption of a few nanowatts.</P>

Electrical metal contacts to atomically thin 2H-phase MoTe2 grown by metal–organic chemical vapor deposition

김태완,정대화,박종후 한국물리학회 2018 Current Applied Physics Vol.18 No.7

We grow atomically thin molybdenum ditelluride (MoTe2) films on a SiO2/Si substrate by means of metal–organic chemical vapor deposition (MOCVD). Our Raman spectroscopy measurements reveal the formation of 2Hphase MoTe2 films. Further, transmission electron microscopy and X-ray photoelectron spectroscopy studies indicate a three-atomic-layer structure and the surface element composition of MoTe2 films. In this study, we mainly focus on the influence of metal contacts attached to the films on their electrical performance. We fabricate 2H-phase-MoTe2-based field-effect transistors (FETs) with various metal contacts such as titanium/gold, nickel and palladium, which present p-type semiconductor properties. We also examine the influence of the work functions of the contact metals on the electrical properties of three-atomic-layer-MoTe2-based FET devices. For a p-type MoTe2 semiconductor, higher work functions of the contact metals afford narrower Schottky barrier heights (SBHs) and eventually highly efficient carrier injection through the contacts.

Long-Range Lattice Engineering of MoTe₂ by a 2D Electride

Kim, Sera,Song, Seunghyun,Park, Jongho,Yu, Ho Sung,Cho, Suyeon,Kim, Dohyun,Baik, Jaeyoon,Choe, Duk-Hyun,Chang, K. J.,Lee, Young Hee,Kim, Sung Wng,Yang, Heejun American Chemical Society 2017 NANO LETTERS Vol.17 No.6

<P>Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to similar to 1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N](+)e to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 x 10(14) cm(2) and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metalsemiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N](+)e . The combination of 2D electrides and layered materials yields a novel material design in terms of doping and lattice engineering.</P>

Electrical and Optoelectrical Properties of 1T´-phase MoTe2 Atomic Layer Film

김동환,김태완 한국진공학회 2019 Applied Science and Convergence Technology Vol.28 No.5

Transition metal dichalcogenides with two-dimensional ultrathin film-type structures are suitable for applications in semiconductor devices owing to the presence of a direct band gap. Among these materials, tungsten ditelluride (WTe2) and molybdenum ditelluride (MoTe2) have potential applications in optical and electrical devices as they easily control the phase and exhibit unique electrical transport characteristics. In this study, 1T´-phase few-layers MoTe2 thin films were grown on a silicon dioxide/silicon substrate using a metal organic chemical vapor deposition system; the phase of the grown MoTe2 thin film was analyzed using Raman spectroscopy and few-layers MoTe2 were observed using atomic force microscopy. A high-performance photodetector and field effect transistor were fabricated and used to analyze the photocurrent density and electrical characteristics of the thin films. On comparing the current of the laser with and without illumination for a photodetector based on the 1T´ phase, the results indicated a significant increase in the photocurrent under illumination. This indicates the strong dependence of the few-layers MoTe2 thin film on laser power. Thus, the 1T´-phase few-layers MoTe2 thin film with a high photocurrent density and excellent electrical transport characteristics is considered a promising material for high-performance photodetectors.

Accurate analysis of Schottky barrier height in Au/2H-MoTe2 atomically thin film contact

Dong Min Kim,Sang-il Kim,TaeWan Kim 대한금속·재료학회 2021 ELECTRONIC MATERIALS LETTERS Vol.17 No.4

Contact between two-dimensional (2D) transition metal dichalcogenides and certain metals (Au, Pd, Ti, or Pt) results instrong Fermi-level pinning and high resistance. Furthermore, to construct a high-performance device, accurate measurementof the contact resistance and Schottky barrier height ( SBH ) is crucial. In this study, the SBH and the resistance of a 2H phasefew-layer molybdenum telluride (2H–MoTe 2 ) fi lm, which was grown by metal organic chemical vapor deposition, and a Ti/Au electrode were determined by analyzing the high-temperature thermal transmission properties and the transmission linemethod, respectively. The carrier mobility, ON/OFF ratio, SBH , and contact resistance of the few-layer MoTe 2 device, whichhas p-type behavior with Ti/Au electrode, were found to be 53.7 cm 2 V −1 s −1 , 3.44 × 10 6 , 163 meV, and 10.2 MΩ , respectively.

Schottky barrier diode based on multilayer MoTe2 and the gate control of the direction of rectification

You Young Gyu,Lee Chang Hoi,Choi Inchul,Jhang Sung Ho 한국물리학회 2021 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.78 No.8

We have investigated Schottky barrier diode based on MoTe2 with Au and Cr/Au asymmetric contacts. While many metals show strong Fermi level pinning close to the valence band of MoTe2 , one can change MoTe2-Au Schottky contact via simple thermal annealing. The Schottky diode showed a clear rectification operation with a rectification ratio of 10–100 and the ideality factor of ∼ 1.5. Utilising the ambipolar nature of MoTe2 , we could control the direction of rectification by changing the majority carrier with a gate voltage.