Ali Javey - Academia.edu (original) (raw)

Papers by Ali Javey

Scientific reports, 2015

In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thicknes... more In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thickness modulation. Kelvin probe force microscopy is used to map the surface potential at the monolayer-multilayer heterojunction, and consequently the conduction band offset is extracted. Scanning photocurrent microscopy is performed to investigate the spatial photocurrent response along the length of the device including the source and the drain contacts as well as the monolayer-multilayer junction. The peak photocurrent is measured at the monolayer-multilayer interface, which is attributed to the formation of a type-I heterojunction. The work presents experimental and theoretical understanding of the band alignment and photoresponse of thickness modulated MoS2 junctions with important implications for exploring novel optoelectronic devices.

Nano, 2006

Abstract: This paper presents a review on our recent work on carbon nanotube field effect transis... more Abstract: This paper presents a review on our recent work on carbon nanotube field effect transistors, including the development of ohmic contacts, high-κ gate dielectric integration, chemical functionalization for conformal dielectric deposition and pushing the ...

IEEE International Electron Devices Meeting 2003, 2003

... Ali Javey, Qian Wang, Woong Kim, and Hongjie Dai* Department of Chemistry and Laboratory for ... more ... Ali Javey, Qian Wang, Woong Kim, and Hongjie Dai* Department of Chemistry and Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA. ... bilayer, Ni, MO, W and Pt, Pd is found tn form the most transparent contact for p-type FETs. ...

Proceedings of Technical Program - 2014 International Symposium on VLSI Technology, Systems and Application (VLSI-TSA), 2014

ABSTRACT We show that transmission line method, where a set of devices are used, does not always ... more ABSTRACT We show that transmission line method, where a set of devices are used, does not always correctly estimate series resistance of mechanically-exfoliated transition metal dichalcogenide MOSFETs. We calculate series resistance and carrier mobility from current-voltage characteristics of a single device. We show that series resistance should be considered for accurate mobility calculation even for long channel devices.

Journal of the American Chemical Society, Jan 4, 2014

Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wi... more Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of ~1.2 × 10(13) cm(-2), which corresponds to the degenerate doping limit for MoS2. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-...

ABSTRACT Here, we present the fabrication and electrical analysis of InAs/WSe2 van der Waals hete... more ABSTRACT Here, we present the fabrication and electrical analysis of InAs/WSe2 van der Waals heterojunction diodes formed by the transfer of ultrathin membranes of one material upon another. Notably, InAs and WSe2 are two materials with completely different crystal structures, which heterojunction is inconceivable with traditional epitaxial growth techniques. Clear rectification from the n-InAs/p-WSe2 junction (forward/reverse current ratio >106) is observed. A low reverse bias current <10−12A/μm2 and ideality factor of ∼1.1 were achieved, suggesting near-ideal electrically active interfaces.

Nano letters, 2005

Short channel ( approximately 80 nm) n-type single-walled carbon nanotube (SWNT) field-effect tra... more Short channel ( approximately 80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-kappa gate dielectrics (ALD HfO(2)) are obtained. For nanotubes with diameter approximately 1.6 nm and band gap approximately 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70 mV/decade, negligible ambipolar conduction, and high on/off ratios up to 10(6) at a bias voltage of 0.5 V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.

ACS Nano, 2014

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials f... more We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS2 is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >10(6), and an electron mobility of ∼33 cm(2)/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe2-MoS2 diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications.

Physical review letters, Jan 12, 2004

Single walled carbon nanotubes with Pd Ohmic contacts and lengths ranging from several microns do... more Single walled carbon nanotubes with Pd Ohmic contacts and lengths ranging from several microns down to 10 nm are investigated by electron transport experiments and theory. The mean-free path (MFP) for acoustic phonon scattering is estimated to be l(ap) approximately 300 nm, and that for optical phonon scattering is l(op) approximately 15 nm. Transport through very short (approximately 10 nm) nanotubes is free of significant acoustic and optical phonon scattering and thus ballistic and quasiballistic at the low- and high-bias voltage limits, respectively. High currents of up to 70 microA can flow through a short nanotube. Possible mechanisms for the eventual electrical breakdown of short nanotubes at high fields are discussed. The results presented here have important implications to high performance nanotube transistors and interconnects.

Nano letters, 2007

We report a general approach for three-dimensional (3D) multifunctional electronics based on the ... more We report a general approach for three-dimensional (3D) multifunctional electronics based on the layer-by-layer assembly of nanowire (NW) building blocks. Using germanium/silicon (Ge/Si) core/shell NWs as a representative example, ten vertically stacked layers of multi-NW field-effect transistors (FETs) were fabricated. Transport measurements demonstrate that the Ge/Si NW FETs have reproducible high-performance device characteristics within a given device layer, that the FET characteristics are not affected by sequential stacking, and importantly, that uniform performance is achieved in sequential layers 1 through 10 of the 3D structure. Five-layer single-NW FET structures were also prepared by printing Ge/Si NWs from lower density growth substrates, and transport measurements showed similar high-performance characteristics for the FETs in layers 1 and 5. In addition, 3D multifunctional circuitry was demonstrated on plastic substrates with sequential layers of inverter logical gates...

Vertically aligned uniform WO 3 nanorod film has been successfully synthesized by using chemical ... more Vertically aligned uniform WO 3 nanorod film has been successfully synthesized by using chemical vapor deposition (CVD) technique without any catalyst. X-ray diffraction (XRD), Raman spectrum, fieldemission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images indicate that the as-prepared WO 3 nanorod film is monoclinic phase and consists of densely-aligned single crystalline nanorods with diameters approximately 30-110 nm and lengths around 1 m. A room temperature photoluminescence-type humidity sensing device based on this WO 3 nanorod film integrated with Si supported substrate has been directly established to investigate their humidity sensing properties, which presents its high response, excellent linearity, quick response/recovery performance and reliable repeatability toward a very wide humidity range. Further comparison with the WO 3 control sample without oxygen vacancy or defect which has poor response demonstrates that oxygen vacancies in the structure play a pivotal role in the high response humidity sensing application.

ABSTRACT We introduce a new class of spin-on dopants composed of organic, dopant-containing polym... more ABSTRACT We introduce a new class of spin-on dopants composed of organic, dopant-containing polymers. These new dopants offer a hybrid between conventional inorganic spin-on dopants and a recently developed organic monolayer doping technique that affords unprecedented control and uniformity of doping profiles. We demonstrate the ability of polymer film doping to achieve both p-type and n-type silicon by using boron- and phosphorus-containing polymer films. Different doping mechanisms are observed for boron and phosphorus doping, which could be related to the specific chemistries of the polymers. Thus, there is an opportunity to further control doping in the future by tuning the polymer chemistry.

Scientific reports, 2015

In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thicknes... more In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thickness modulation. Kelvin probe force microscopy is used to map the surface potential at the monolayer-multilayer heterojunction, and consequently the conduction band offset is extracted. Scanning photocurrent microscopy is performed to investigate the spatial photocurrent response along the length of the device including the source and the drain contacts as well as the monolayer-multilayer junction. The peak photocurrent is measured at the monolayer-multilayer interface, which is attributed to the formation of a type-I heterojunction. The work presents experimental and theoretical understanding of the band alignment and photoresponse of thickness modulated MoS2 junctions with important implications for exploring novel optoelectronic devices.

Nano, 2006

Abstract: This paper presents a review on our recent work on carbon nanotube field effect transis... more Abstract: This paper presents a review on our recent work on carbon nanotube field effect transistors, including the development of ohmic contacts, high-κ gate dielectric integration, chemical functionalization for conformal dielectric deposition and pushing the ...

IEEE International Electron Devices Meeting 2003, 2003

... Ali Javey, Qian Wang, Woong Kim, and Hongjie Dai* Department of Chemistry and Laboratory for ... more ... Ali Javey, Qian Wang, Woong Kim, and Hongjie Dai* Department of Chemistry and Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA. ... bilayer, Ni, MO, W and Pt, Pd is found tn form the most transparent contact for p-type FETs. ...

Proceedings of Technical Program - 2014 International Symposium on VLSI Technology, Systems and Application (VLSI-TSA), 2014

ABSTRACT We show that transmission line method, where a set of devices are used, does not always ... more ABSTRACT We show that transmission line method, where a set of devices are used, does not always correctly estimate series resistance of mechanically-exfoliated transition metal dichalcogenide MOSFETs. We calculate series resistance and carrier mobility from current-voltage characteristics of a single device. We show that series resistance should be considered for accurate mobility calculation even for long channel devices.

Journal of the American Chemical Society, Jan 4, 2014

Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wi... more Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of ~1.2 × 10(13) cm(-2), which corresponds to the degenerate doping limit for MoS2. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-...

ABSTRACT Here, we present the fabrication and electrical analysis of InAs/WSe2 van der Waals hete... more ABSTRACT Here, we present the fabrication and electrical analysis of InAs/WSe2 van der Waals heterojunction diodes formed by the transfer of ultrathin membranes of one material upon another. Notably, InAs and WSe2 are two materials with completely different crystal structures, which heterojunction is inconceivable with traditional epitaxial growth techniques. Clear rectification from the n-InAs/p-WSe2 junction (forward/reverse current ratio >106) is observed. A low reverse bias current <10−12A/μm2 and ideality factor of ∼1.1 were achieved, suggesting near-ideal electrically active interfaces.

Nano letters, 2005

Short channel ( approximately 80 nm) n-type single-walled carbon nanotube (SWNT) field-effect tra... more Short channel ( approximately 80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-kappa gate dielectrics (ALD HfO(2)) are obtained. For nanotubes with diameter approximately 1.6 nm and band gap approximately 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70 mV/decade, negligible ambipolar conduction, and high on/off ratios up to 10(6) at a bias voltage of 0.5 V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.

ACS Nano, 2014

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials f... more We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS2 is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >10(6), and an electron mobility of ∼33 cm(2)/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe2-MoS2 diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications.

Physical review letters, Jan 12, 2004

Single walled carbon nanotubes with Pd Ohmic contacts and lengths ranging from several microns do... more Single walled carbon nanotubes with Pd Ohmic contacts and lengths ranging from several microns down to 10 nm are investigated by electron transport experiments and theory. The mean-free path (MFP) for acoustic phonon scattering is estimated to be l(ap) approximately 300 nm, and that for optical phonon scattering is l(op) approximately 15 nm. Transport through very short (approximately 10 nm) nanotubes is free of significant acoustic and optical phonon scattering and thus ballistic and quasiballistic at the low- and high-bias voltage limits, respectively. High currents of up to 70 microA can flow through a short nanotube. Possible mechanisms for the eventual electrical breakdown of short nanotubes at high fields are discussed. The results presented here have important implications to high performance nanotube transistors and interconnects.

Nano letters, 2007

We report a general approach for three-dimensional (3D) multifunctional electronics based on the ... more We report a general approach for three-dimensional (3D) multifunctional electronics based on the layer-by-layer assembly of nanowire (NW) building blocks. Using germanium/silicon (Ge/Si) core/shell NWs as a representative example, ten vertically stacked layers of multi-NW field-effect transistors (FETs) were fabricated. Transport measurements demonstrate that the Ge/Si NW FETs have reproducible high-performance device characteristics within a given device layer, that the FET characteristics are not affected by sequential stacking, and importantly, that uniform performance is achieved in sequential layers 1 through 10 of the 3D structure. Five-layer single-NW FET structures were also prepared by printing Ge/Si NWs from lower density growth substrates, and transport measurements showed similar high-performance characteristics for the FETs in layers 1 and 5. In addition, 3D multifunctional circuitry was demonstrated on plastic substrates with sequential layers of inverter logical gates...

Vertically aligned uniform WO 3 nanorod film has been successfully synthesized by using chemical ... more Vertically aligned uniform WO 3 nanorod film has been successfully synthesized by using chemical vapor deposition (CVD) technique without any catalyst. X-ray diffraction (XRD), Raman spectrum, fieldemission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images indicate that the as-prepared WO 3 nanorod film is monoclinic phase and consists of densely-aligned single crystalline nanorods with diameters approximately 30-110 nm and lengths around 1 m. A room temperature photoluminescence-type humidity sensing device based on this WO 3 nanorod film integrated with Si supported substrate has been directly established to investigate their humidity sensing properties, which presents its high response, excellent linearity, quick response/recovery performance and reliable repeatability toward a very wide humidity range. Further comparison with the WO 3 control sample without oxygen vacancy or defect which has poor response demonstrates that oxygen vacancies in the structure play a pivotal role in the high response humidity sensing application.

ABSTRACT We introduce a new class of spin-on dopants composed of organic, dopant-containing polym... more ABSTRACT We introduce a new class of spin-on dopants composed of organic, dopant-containing polymers. These new dopants offer a hybrid between conventional inorganic spin-on dopants and a recently developed organic monolayer doping technique that affords unprecedented control and uniformity of doping profiles. We demonstrate the ability of polymer film doping to achieve both p-type and n-type silicon by using boron- and phosphorus-containing polymer films. Different doping mechanisms are observed for boron and phosphorus doping, which could be related to the specific chemistries of the polymers. Thus, there is an opportunity to further control doping in the future by tuning the polymer chemistry.