The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of th... more The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of the most promising techniques for obtaining high quality large scale graphene for electronics applications. Although graphene grown on the C-face has high mobility, its growth under vacuum is too fast, not self limited and produces high concentration of crystalline defects. Therefore a precise control over the Si evaporation rate is required. We demonstrate a new method to reduce the growth rate and yield thin graphene layers with excellent thickness uniformity on the C-face of SiC in ultra high vacuum conditions. The sample is capped by another SiC substrate with a rectangular recess of about one micron depth on its surface which forms a partially open cavity between the surfaces. During the growth by high temperature annealing, silicon atoms sublimated from the capped sample are confined inside the cavity between the two substrates. The confined silicon vapor maintains a high partial pre...
We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacu... more We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacuum conditions. A stack of two SiC substrates comprising a half open cavity at the interface is used to partially confine the depleted Si atoms from the sample surface during the growth. We observe that this configuration significantly slows the graphene growth to easily controllable rates on C-face SiC in UHV environment. Results of low-energy electron diffractometry and Raman spectroscopy measurements on the samples grown with stacking configuration are compared to those of the samples grown by using bare UHV sublimation process.
Spatial evolution of the breakdown of the quantum Hall effect has been investigated quantitativel... more Spatial evolution of the breakdown of the quantum Hall effect has been investigated quantitatively using Hall devices with constrictions that provide current density modulation. Either the excitation or the relaxation profile of the hot carriers are probed by measuring the resistivity rho_xx(x) of the sample as a function of the distance along the drift direction. The relaxation distances measured for different mobility samples coincide with the mean free path l_mfp of the samples, revealing that the impurity scattering is involved in the breakdown [1]. On the other hand, the excitation profile could be explained by impurity mediated tunneling of electrons between the Landau levels [2]. In addition, the effect of current modulation in time domain has also been studied by the application of pulsed (t >= 1 ns) currents exceeding the critical value. It has been found that a minimum pulse duration is required before the electronic system abruptly starts to develop a dissipative state...
We have measured the generation and relaxation of hot electrons along their drift direction near ... more We have measured the generation and relaxation of hot electrons along their drift direction near the breakdown of the quantum Hall effect. The dissipative resistivity of the two-dimensional electron system (2DES) is measured as a function of the distance from the electron injection. By injecting hot electrons into a Hall bar at subcritical currents, the relaxation process was analyzed. The inelastic relaxation length was found strongly correlated with the elastic mean free path. Thus, we conclude that the relaxation is triggered by Coulomb scattering. The injection of electrons at supercritical current levels allowed to monitor the evolution of the breakdown along the drift direction. After a linear increase of the resistivity with the drifting distance, a rather steep increase of disspation occurs. The linear increase is attributed to tunneling between Landau levels, whereas the steep increase is likely due to an avalanche-like multiplication of hot electrons.
The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longit... more The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longitudinal resistance with an associated loss in the quantization of Hall voltage is the major obstacle against improving the resistance standard which is currently based on this effect. Graphene is inherently a 2D material and has an unusual band structure that allows the quantization of the Hall resistance even at room temperature. These unique properties of graphene make it a good candidate as a high precision metrological characterization tool for the quantum Hall resistance. The uncertainty in the quantum Hall resistance in graphene has been rapidly improving recently and graphene samples have already been shown to reach the precision of the current best 2DEG samples. In this talk, experimental results on the breakdown of the quantum Hall effect in graphene on SiOx is presented. In narrow graphene samples of 1 micrometer width, the charge inhomogeneity is quite prominent and strongly af...
Among several methods for production of Graphene, CVD method has the highest potential for large ... more Among several methods for production of Graphene, CVD method has the highest potential for large area production of graphene with adequate crystalline quality, especially for the optoelectronic applications. In order to achieve a better conductivity, a practical approach can be usage of multilayer graphene while the slight reduction of transparency can be in the acceptable limits. Accordingly, multilayer graphene seems to be closer to meet the conductivity-transparency requirement for a variety of applications ranging from solar cells to displays or touch screens. Although, low carbon solubility in the copper impose the growth of graphene to predominantly single layer film and this growth is mainly due to the surface adsorption of carbon atoms on the surface [1], However recently, it has been reported that multi-layer graphene films can be grown under certain CVD conditions [2]. Towards the objective of large area high quality graphene growth, with controllable thickness, in this wo...
The chemical vapor deposition (CVD) is a fairly available method for the synthesis of large area ... more The chemical vapor deposition (CVD) is a fairly available method for the synthesis of large area graphene films with high quality for industrial applications [1]. Transition metals including Ni, Cu, Pt, Ir and Pd have been used as a substrate for graphene growth [2]. Among them, nickel is a well-known transition metal as a substrates for graphene growth, due to the high carbon solubility and low lattice mismatch with graphene [3]. In CVD method, where variables of parameters, such as temperature, gas flow rate and pressure are involved [3], since many experiments need to be performed for reaching at the optimum conditions. The Taguchi method, which is a combination of mathematical and statistical analyzing system to optimize synthetic processes of nanostructured materials [4-5] In this work, Taguchi method is applied to the optimization of the synthesis of graphene by nickel catalyzed chemical vapor deposition (CVD) with the aim of high transparency and low sheet resistance of the g...
The time evolution of excited charge carriers near the breakdown of the quantum Hall effect is st... more The time evolution of excited charge carriers near the breakdown of the quantum Hall effect is studied within a simple two-Landau-level model. The model incorporates impurity-assisted inter-Landau-level tunneling as the main mechanism, however, with a characteristic length that is much smaller than the scale set by the width of a sample with homogeneous current distribution. Using the carrier drift velocity to convert time into position dependence, we find that the model can reproduce recent experimental results astonishingly well.
High tensile stress suspended structures are demanded for high mechanical quality factor applicat... more High tensile stress suspended structures are demanded for high mechanical quality factor applications. However, high tensile stress causes distortion of the original shapes by contracting, buckling, and bending the suspended structures. We demonstrate a method to compensate for the shape deformation of suspended structures due to intrinsic tensile stress after they are released. With a new design, the distance between two suspended structures after wet etch can easily be tuned by a single fabrication beyond the lithographic resolution limits. The technique is simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability with 2.4 nm standard error.
We developed a new fabrication technique for the realization of nanogaps using conventional litho... more We developed a new fabrication technique for the realization of nanogaps using conventional lithography and in situ controlled thermal evaporation. A 20-40 nm gap between two suspended metallic electrodes is shrunk down to about 1 nm using controlled thermal evaporation. It is demonstrated that with this technique rigid and stable metallic vacuum tunneling junctions can be consistently produced. The fabricated nanogaps were characterized by I-V measurements and their gap sizes and barrier heights were interrogated using the Simmons' model.
""We have analyzed the breakdown of the quantum Hall effect in 1 μm wide Hall devices fabricated ... more ""We have analyzed the breakdown of the quantum Hall effect in 1 μm wide Hall devices fabricated from an exfoliated monolayer graphene transferred on SiOx. We have observed that the deviation of the Hall resistance from its quantized value is weakly dependent on the longitudinal resistivity up to current density of 5 A/m, where the Hall resistance remains quantized even when the longitudinal resistance increases monotonously with the current. Then a collapse in the quantized resistance occurs while longitudinal resistance keeps its gradual increase. The exponential increase of the conductivity with respect to the current suggests impurity mediated inter-Landau level scattering as the mechanism of the breakdown. The results are interpreted as the strong variation of the breakdown behavior throughout the sample due to the randomly distributed scattering centers that
mediates the breakdown.""
Recent advances using in situ focused ion beam implantation during an MBE growth interruption hav... more Recent advances using in situ focused ion beam implantation during an MBE growth interruption have been exploited to fabricate planar GaAs hot electron structures without the need for shallow ohmic contacts. This novel fabrication route shows a very high yield and has been used to demonstrate a prototype high-frequency oscillator structure based on electron multiplication in the base layer. Existing
Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic elec... more Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic electron emission spectroscopy on Au/ GaAs͑100͒ by contacting the patterned metallic thin film with one tip and injecting ballistic electrons with another tip. The collector current spectra agree with a Monte-Carlo simulation based on modified planar tunneling theory. Our results suggest that it is possible to study nanoscale metal-semiconductor interfaces without the requirement of an externally-contacted continuous metal thin film.
... Hall sensors [2]) or microscopic probes which average over a large ensemble of vortices (eg, ... more ... Hall sensors [2]) or microscopic probes which average over a large ensemble of vortices (eg, neutron scattering [5] and muon spin rotation [6]). Clearly ... raised deliberately 5 mm above the sample surface to smear away the fine structure due to discrete vortices moving under the ...
The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of th... more The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of the most promising techniques for obtaining high quality large scale graphene for electronics applications. Although graphene grown on the C-face has high mobility, its growth under vacuum is too fast, not self limited and produces high concentration of crystalline defects. Therefore a precise control over the Si evaporation rate is required. We demonstrate a new method to reduce the growth rate and yield thin graphene layers with excellent thickness uniformity on the C-face of SiC in ultra high vacuum conditions. The sample is capped by another SiC substrate with a rectangular recess of about one micron depth on its surface which forms a partially open cavity between the surfaces. During the growth by high temperature annealing, silicon atoms sublimated from the capped sample are confined inside the cavity between the two substrates. The confined silicon vapor maintains a high partial pre...
We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacu... more We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacuum conditions. A stack of two SiC substrates comprising a half open cavity at the interface is used to partially confine the depleted Si atoms from the sample surface during the growth. We observe that this configuration significantly slows the graphene growth to easily controllable rates on C-face SiC in UHV environment. Results of low-energy electron diffractometry and Raman spectroscopy measurements on the samples grown with stacking configuration are compared to those of the samples grown by using bare UHV sublimation process.
Spatial evolution of the breakdown of the quantum Hall effect has been investigated quantitativel... more Spatial evolution of the breakdown of the quantum Hall effect has been investigated quantitatively using Hall devices with constrictions that provide current density modulation. Either the excitation or the relaxation profile of the hot carriers are probed by measuring the resistivity rho_xx(x) of the sample as a function of the distance along the drift direction. The relaxation distances measured for different mobility samples coincide with the mean free path l_mfp of the samples, revealing that the impurity scattering is involved in the breakdown [1]. On the other hand, the excitation profile could be explained by impurity mediated tunneling of electrons between the Landau levels [2]. In addition, the effect of current modulation in time domain has also been studied by the application of pulsed (t >= 1 ns) currents exceeding the critical value. It has been found that a minimum pulse duration is required before the electronic system abruptly starts to develop a dissipative state...
We have measured the generation and relaxation of hot electrons along their drift direction near ... more We have measured the generation and relaxation of hot electrons along their drift direction near the breakdown of the quantum Hall effect. The dissipative resistivity of the two-dimensional electron system (2DES) is measured as a function of the distance from the electron injection. By injecting hot electrons into a Hall bar at subcritical currents, the relaxation process was analyzed. The inelastic relaxation length was found strongly correlated with the elastic mean free path. Thus, we conclude that the relaxation is triggered by Coulomb scattering. The injection of electrons at supercritical current levels allowed to monitor the evolution of the breakdown along the drift direction. After a linear increase of the resistivity with the drifting distance, a rather steep increase of disspation occurs. The linear increase is attributed to tunneling between Landau levels, whereas the steep increase is likely due to an avalanche-like multiplication of hot electrons.
The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longit... more The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longitudinal resistance with an associated loss in the quantization of Hall voltage is the major obstacle against improving the resistance standard which is currently based on this effect. Graphene is inherently a 2D material and has an unusual band structure that allows the quantization of the Hall resistance even at room temperature. These unique properties of graphene make it a good candidate as a high precision metrological characterization tool for the quantum Hall resistance. The uncertainty in the quantum Hall resistance in graphene has been rapidly improving recently and graphene samples have already been shown to reach the precision of the current best 2DEG samples. In this talk, experimental results on the breakdown of the quantum Hall effect in graphene on SiOx is presented. In narrow graphene samples of 1 micrometer width, the charge inhomogeneity is quite prominent and strongly af...
Among several methods for production of Graphene, CVD method has the highest potential for large ... more Among several methods for production of Graphene, CVD method has the highest potential for large area production of graphene with adequate crystalline quality, especially for the optoelectronic applications. In order to achieve a better conductivity, a practical approach can be usage of multilayer graphene while the slight reduction of transparency can be in the acceptable limits. Accordingly, multilayer graphene seems to be closer to meet the conductivity-transparency requirement for a variety of applications ranging from solar cells to displays or touch screens. Although, low carbon solubility in the copper impose the growth of graphene to predominantly single layer film and this growth is mainly due to the surface adsorption of carbon atoms on the surface [1], However recently, it has been reported that multi-layer graphene films can be grown under certain CVD conditions [2]. Towards the objective of large area high quality graphene growth, with controllable thickness, in this wo...
The chemical vapor deposition (CVD) is a fairly available method for the synthesis of large area ... more The chemical vapor deposition (CVD) is a fairly available method for the synthesis of large area graphene films with high quality for industrial applications [1]. Transition metals including Ni, Cu, Pt, Ir and Pd have been used as a substrate for graphene growth [2]. Among them, nickel is a well-known transition metal as a substrates for graphene growth, due to the high carbon solubility and low lattice mismatch with graphene [3]. In CVD method, where variables of parameters, such as temperature, gas flow rate and pressure are involved [3], since many experiments need to be performed for reaching at the optimum conditions. The Taguchi method, which is a combination of mathematical and statistical analyzing system to optimize synthetic processes of nanostructured materials [4-5] In this work, Taguchi method is applied to the optimization of the synthesis of graphene by nickel catalyzed chemical vapor deposition (CVD) with the aim of high transparency and low sheet resistance of the g...
The time evolution of excited charge carriers near the breakdown of the quantum Hall effect is st... more The time evolution of excited charge carriers near the breakdown of the quantum Hall effect is studied within a simple two-Landau-level model. The model incorporates impurity-assisted inter-Landau-level tunneling as the main mechanism, however, with a characteristic length that is much smaller than the scale set by the width of a sample with homogeneous current distribution. Using the carrier drift velocity to convert time into position dependence, we find that the model can reproduce recent experimental results astonishingly well.
High tensile stress suspended structures are demanded for high mechanical quality factor applicat... more High tensile stress suspended structures are demanded for high mechanical quality factor applications. However, high tensile stress causes distortion of the original shapes by contracting, buckling, and bending the suspended structures. We demonstrate a method to compensate for the shape deformation of suspended structures due to intrinsic tensile stress after they are released. With a new design, the distance between two suspended structures after wet etch can easily be tuned by a single fabrication beyond the lithographic resolution limits. The technique is simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability with 2.4 nm standard error.
We developed a new fabrication technique for the realization of nanogaps using conventional litho... more We developed a new fabrication technique for the realization of nanogaps using conventional lithography and in situ controlled thermal evaporation. A 20-40 nm gap between two suspended metallic electrodes is shrunk down to about 1 nm using controlled thermal evaporation. It is demonstrated that with this technique rigid and stable metallic vacuum tunneling junctions can be consistently produced. The fabricated nanogaps were characterized by I-V measurements and their gap sizes and barrier heights were interrogated using the Simmons' model.
""We have analyzed the breakdown of the quantum Hall effect in 1 μm wide Hall devices fabricated ... more ""We have analyzed the breakdown of the quantum Hall effect in 1 μm wide Hall devices fabricated from an exfoliated monolayer graphene transferred on SiOx. We have observed that the deviation of the Hall resistance from its quantized value is weakly dependent on the longitudinal resistivity up to current density of 5 A/m, where the Hall resistance remains quantized even when the longitudinal resistance increases monotonously with the current. Then a collapse in the quantized resistance occurs while longitudinal resistance keeps its gradual increase. The exponential increase of the conductivity with respect to the current suggests impurity mediated inter-Landau level scattering as the mechanism of the breakdown. The results are interpreted as the strong variation of the breakdown behavior throughout the sample due to the randomly distributed scattering centers that
mediates the breakdown.""
Recent advances using in situ focused ion beam implantation during an MBE growth interruption hav... more Recent advances using in situ focused ion beam implantation during an MBE growth interruption have been exploited to fabricate planar GaAs hot electron structures without the need for shallow ohmic contacts. This novel fabrication route shows a very high yield and has been used to demonstrate a prototype high-frequency oscillator structure based on electron multiplication in the base layer. Existing
Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic elec... more Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic electron emission spectroscopy on Au/ GaAs͑100͒ by contacting the patterned metallic thin film with one tip and injecting ballistic electrons with another tip. The collector current spectra agree with a Monte-Carlo simulation based on modified planar tunneling theory. Our results suggest that it is possible to study nanoscale metal-semiconductor interfaces without the requirement of an externally-contacted continuous metal thin film.
... Hall sensors [2]) or microscopic probes which average over a large ensemble of vortices (eg, ... more ... Hall sensors [2]) or microscopic probes which average over a large ensemble of vortices (eg, neutron scattering [5] and muon spin rotation [6]). Clearly ... raised deliberately 5 mm above the sample surface to smear away the fine structure due to discrete vortices moving under the ...
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mediates the breakdown.""
mediates the breakdown.""