Jani Kivioja - Academia.edu (original) (raw)

Papers by Jani Kivioja

2D Materials, 2016

We measure graphene coplanar waveguides from direct current (DC) to a frequency f=13.5 GHz and ... more We measure graphene coplanar waveguides from direct current (DC) to a frequency f=13.5 GHz and show that the apparent resistance (in the presence of parasitic impedances) has an 2 dependence (where f 2), but the intrinsic conductivity (without the influence of parasitic impedances) is frequency-independent. Consequently, in our devices the real part of the complex alternating current(AC) conductivity is the same as the DC value and the imaginary part is ∼0. The graphene channel is modeled as a parallel resistive-capacitive network with a frequency dependence identical to that of the Drude conductivity with momentum relaxation time ∼2.1 ps, highlighting the influence of AC electron transport on the electromagnetic properties of graphene. This can lead to optimized design of high-speed analog field-effect transistors, mixers, frequency doublers, low-noise amplifiers and radiation detectors.

La presente invention concerne un appareil comportant : une anode formee d'oxyde de graphene ... more La presente invention concerne un appareil comportant : une anode formee d'oxyde de graphene a partir d'un pH acide; une cathode a partir d'un pH superieur au pH acide de l'anode; et des collecteurs de charge deposes sur l'anode et la cathode.

Työssä tutkittiin GaN- ja AlGaN-ohutkalvonäytteissä esiintyviä hilavirheitä positroniannihilaatio... more Työssä tutkittiin GaN- ja AlGaN-ohutkalvonäytteissä esiintyviä hilavirheitä positroniannihilaatiospektroskopialla. Työssä käytettiin TKK:n Fysiikan laboratorion kahta hitaiden positronien suihkua ja positronien elinaikamittalaitteistoa. Työssä mitattiin positronien keskimääräinen elinaika kahdessa HVPE-kasvatetussa GaN-ohutkalvonäytteessä. Vapaan positronin elinajaksi GaN-hilassa saatiin 156 ps ja galliumvakanssiin loukkuuntuneen positronin elinajaksi 235 ps. Nämä ovat samat kuin aiemmin tutkituissa erilliskiteissä. Näytteiden ei havaittu sisältävän Ga-vakanssien lisäksi muita positroneja loukkuunnuttavia virheitä. Positronin loukkuuntumiskertoimen lämpötilariippuvuus määritettiin elinaikamittausten perusteella. Loukkuuntumiskertoimen havaittiin noudattavan T[1/2]-lämpötilariippuvuutta, mikä osoittaa galliumvakanssin olevan negatiivisesti varautunut. Mitattujen GaN-näytteiden dislokaatio- ja epäpuhtaustiheyden oli mitattu olevan suurempi lähellä substraatin ja ohutkalvon välistä raj...

Appareil (100) comprenant un capteur (110) comportant un film de detection (140) concu pour fourn... more Appareil (100) comprenant un capteur (110) comportant un film de detection (140) concu pour fournir un signal sur la base d'une respiration de l'utilisateur et un dispositif de commande (130) associe fonctionnellement au capteur (110). Le dispositif de commande (130) est concu pour recevoir le signal sur la base de la respiration de l'utilisateur.

L'invention porte sur un appareil, qui comprend un guide d'ondes a nanofil et une nanopar... more L'invention porte sur un appareil, qui comprend un guide d'ondes a nanofil et une nanoparticule, le guide d'ondes a nanofil etant configure de facon a transmettre un rayonnement le long de son axe longitudinal d'une premiere extremite a une seconde extremite, la nanoparticule etant positionnee au moins a la seconde extremite du guide d'ondes de facon a recevoir le rayonnement transmis pour l'utilisation en spectroscopie.

Physical Review B, 2005

We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structu... more We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structure. The island was connected to leads through SQUID (Superconducting Quantum Interference Device) loops. Synchronized flux and voltage signals were applied whereby the Josephson energies of the SQUIDs and the gate charge were tuned adiabatically. From the current-voltage characteristics one can see that the pumped current increases in 1e steps which is due to quasiparticle poisoning on the measurement time scale, but we argue that the transport of charge is due to Cooper pairs.

IEEE Transactions on Applied Superconductivity, 2009

This paper presents a full-wafer, resist-less process for parallel fabrication of sub-micron Al/A... more This paper presents a full-wafer, resist-less process for parallel fabrication of sub-micron Al/AlOx/Al superconducting tunnel junctions (STJs). A stencil is fabricated out of a Si wafer on which 200 nm low-stress SiN has been deposited. The STJ apertures in the SiN are defined by e-beam lithography on the front side of the wafer, and the Si is etched from the backside for opening SiN membranes. Using a customized full-wafer stencil aligner, the stencil is aligned and clamped to a substrate wafer which contains Au contact pads. The clamped system is placed in a thermal evaporator equipped with two Al sources, from which two Al evaporations are performed, with an intermediate in-situ oxidation step. Once out of the evaporator, the stencil is unclamped from the substrate and d.c. measurements are performed down to 0.3 K. One challenge of this technique is the clogging of the stencil, which is due to the material deposited on the side walls of the nanoapertures. The Al deposited on the stencil is removed using wet-etching, making thus the stencil reusable. This technique provides a cheaper and cleaner fabrication of STJs than present fabrication techniques, e.g. e-beam lithography. The advantages of this process are now being investigated in nanojunctions.

Applied Physics Letters, 2012

Close-packed monolayers of 20 nm Au nanoparticles are self-assembled at hexane/water interfaces a... more Close-packed monolayers of 20 nm Au nanoparticles are self-assembled at hexane/water interfaces and transferred to elastic substrates. Stretching the resulting nanoparticle mats provides active and reversible tuning of their plasmonic properties, with a clear polarization dependance. Both uniaxial and biaxial strains induce strong blue shifts in the plasmonic resonances. This matches theoretical simulations and indicates that plasmonic coupling at nanometer scale distances is responsible for the observed spectral tuning. Such stretch-tunable metal nanoparticle mats can be exploited for the development of optical devices, such as flexible colour filters and molecular sensors.

ACS Nano, 2009

Top-gated silicon nanowire transistors are fabricated by preparing all terminals (source, drain, ... more Top-gated silicon nanowire transistors are fabricated by preparing all terminals (source, drain, and gate) on top of the nanowire in a single step via dose-modulated e-beam lithography. This outperforms other time-consuming approaches requiring alignment of multiple patterns, where alignment tolerances impose a limit on device scaling. We use as gate dielectric the 10؊15 nm SiO 2 shell naturally formed during vapor-transport growth of Si nanowires, so the wires can be implemented into devices after synthesis without additional processing. This natural oxide shell has negligible leakage over the operating range. Our single-step patterning is a most practical route for realization of short-channel nanowire transistors and can be applied to a number of nanodevice geometries requiring nonequivalent electrodes.

ACS Nano, 2013

Sensors allow an electronic device to become a gateway between the digital and physical worlds, a... more Sensors allow an electronic device to become a gateway between the digital and physical worlds, and sensor materials with unprecedented performance can create new applications and new avenues for user interaction. Graphene oxide can be exploited in humidity and temperature sensors with a number of convenient features such as flexibility, transparency and suitability for large scale manufacturing. Here we show that the two-dimensional nature of graphene oxide and its super-permeability to water combine to enable humidity sensors with unprecedented response speed (~30ms response and recovery times). This opens the door to various applications, such as touch-less user interfaces, which we demonstrate with a 'whistling' recognition analysis. Graphene is only the first of a new class of two-dimensional (2D) crystals with unique properties, 1 which are being investigated and deployed with increasing vigor in various fields such as electronics and optoelectronics. 2-4 Moreover, functionalized graphene materials provide a further set of 2D materials with exceptional promise for biological and chemical sensors. 5-8

We have measured the thermal response of a superconductor-normal metal-superconductor (SINIS) tun... more We have measured the thermal response of a superconductor-normal metal-superconductor (SINIS) tunnel junction structure at substrate temperature ∼ 60 mK by directly heating the electron system in the normal metal island. In our structure, we ÿnd the response time is determined by the electron-phonon coupling in the electron temperature range 300 -600 mK. By using AC heating, the cut-o frequency caused by this response time has been measured, showing that SINIS structures operate as a thermometer up to a few MHz in this temperature range. ?

In this work four different mesoscopic superconducting devices have been experimentally studied: ... more In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current I_c of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal-Insulator-Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate. Josephson junction, phase diffusion, Cooper pair pump, NIS tunnel junction 165 951-22-7709-3

Graphene oxide has been known for its advantage for sensing water molecules. There are several pa... more Graphene oxide has been known for its advantage for sensing water molecules. There are several papers about using graphene oxide as humidity sensor. [1],[2] Graphene oxide shows very fast response and recovery time, and excellent sensitivity. But humidity sensing behavior of graphene oxide in low humidity environment is not yet described. Here, we tested humidity sensing characteristic of graphene oxide, especially in low humidity environment. Electrodes were deposited on substrate by E-beam evaporation and graphene oxide membranes were made by vacuum filtration and stamped on the substrate. We measured resistance between two electrodes with changing humidity environment. When graphene oxide was exposed less than 30% relative humidity, the resistance of graphene oxide increased almost exponentially and finally the resistivity went extremely high. In this presentation, we explain the main mechanism of this reason and suggest the compensating method to sense the low humidity.

2D Materials, 2016

We measure graphene coplanar waveguides from direct current (DC) to a frequency f=13.5 GHz and ... more We measure graphene coplanar waveguides from direct current (DC) to a frequency f=13.5 GHz and show that the apparent resistance (in the presence of parasitic impedances) has an 2 dependence (where f 2), but the intrinsic conductivity (without the influence of parasitic impedances) is frequency-independent. Consequently, in our devices the real part of the complex alternating current(AC) conductivity is the same as the DC value and the imaginary part is ∼0. The graphene channel is modeled as a parallel resistive-capacitive network with a frequency dependence identical to that of the Drude conductivity with momentum relaxation time ∼2.1 ps, highlighting the influence of AC electron transport on the electromagnetic properties of graphene. This can lead to optimized design of high-speed analog field-effect transistors, mixers, frequency doublers, low-noise amplifiers and radiation detectors.

La presente invention concerne un appareil comportant : une anode formee d'oxyde de graphene ... more La presente invention concerne un appareil comportant : une anode formee d'oxyde de graphene a partir d'un pH acide; une cathode a partir d'un pH superieur au pH acide de l'anode; et des collecteurs de charge deposes sur l'anode et la cathode.

Työssä tutkittiin GaN- ja AlGaN-ohutkalvonäytteissä esiintyviä hilavirheitä positroniannihilaatio... more Työssä tutkittiin GaN- ja AlGaN-ohutkalvonäytteissä esiintyviä hilavirheitä positroniannihilaatiospektroskopialla. Työssä käytettiin TKK:n Fysiikan laboratorion kahta hitaiden positronien suihkua ja positronien elinaikamittalaitteistoa. Työssä mitattiin positronien keskimääräinen elinaika kahdessa HVPE-kasvatetussa GaN-ohutkalvonäytteessä. Vapaan positronin elinajaksi GaN-hilassa saatiin 156 ps ja galliumvakanssiin loukkuuntuneen positronin elinajaksi 235 ps. Nämä ovat samat kuin aiemmin tutkituissa erilliskiteissä. Näytteiden ei havaittu sisältävän Ga-vakanssien lisäksi muita positroneja loukkuunnuttavia virheitä. Positronin loukkuuntumiskertoimen lämpötilariippuvuus määritettiin elinaikamittausten perusteella. Loukkuuntumiskertoimen havaittiin noudattavan T[1/2]-lämpötilariippuvuutta, mikä osoittaa galliumvakanssin olevan negatiivisesti varautunut. Mitattujen GaN-näytteiden dislokaatio- ja epäpuhtaustiheyden oli mitattu olevan suurempi lähellä substraatin ja ohutkalvon välistä raj...

Appareil (100) comprenant un capteur (110) comportant un film de detection (140) concu pour fourn... more Appareil (100) comprenant un capteur (110) comportant un film de detection (140) concu pour fournir un signal sur la base d'une respiration de l'utilisateur et un dispositif de commande (130) associe fonctionnellement au capteur (110). Le dispositif de commande (130) est concu pour recevoir le signal sur la base de la respiration de l'utilisateur.

L'invention porte sur un appareil, qui comprend un guide d'ondes a nanofil et une nanopar... more L'invention porte sur un appareil, qui comprend un guide d'ondes a nanofil et une nanoparticule, le guide d'ondes a nanofil etant configure de facon a transmettre un rayonnement le long de son axe longitudinal d'une premiere extremite a une seconde extremite, la nanoparticule etant positionnee au moins a la seconde extremite du guide d'ondes de facon a recevoir le rayonnement transmis pour l'utilisation en spectroscopie.

Physical Review B, 2005

We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structu... more We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structure. The island was connected to leads through SQUID (Superconducting Quantum Interference Device) loops. Synchronized flux and voltage signals were applied whereby the Josephson energies of the SQUIDs and the gate charge were tuned adiabatically. From the current-voltage characteristics one can see that the pumped current increases in 1e steps which is due to quasiparticle poisoning on the measurement time scale, but we argue that the transport of charge is due to Cooper pairs.

IEEE Transactions on Applied Superconductivity, 2009

This paper presents a full-wafer, resist-less process for parallel fabrication of sub-micron Al/A... more This paper presents a full-wafer, resist-less process for parallel fabrication of sub-micron Al/AlOx/Al superconducting tunnel junctions (STJs). A stencil is fabricated out of a Si wafer on which 200 nm low-stress SiN has been deposited. The STJ apertures in the SiN are defined by e-beam lithography on the front side of the wafer, and the Si is etched from the backside for opening SiN membranes. Using a customized full-wafer stencil aligner, the stencil is aligned and clamped to a substrate wafer which contains Au contact pads. The clamped system is placed in a thermal evaporator equipped with two Al sources, from which two Al evaporations are performed, with an intermediate in-situ oxidation step. Once out of the evaporator, the stencil is unclamped from the substrate and d.c. measurements are performed down to 0.3 K. One challenge of this technique is the clogging of the stencil, which is due to the material deposited on the side walls of the nanoapertures. The Al deposited on the stencil is removed using wet-etching, making thus the stencil reusable. This technique provides a cheaper and cleaner fabrication of STJs than present fabrication techniques, e.g. e-beam lithography. The advantages of this process are now being investigated in nanojunctions.

Applied Physics Letters, 2012

Close-packed monolayers of 20 nm Au nanoparticles are self-assembled at hexane/water interfaces a... more Close-packed monolayers of 20 nm Au nanoparticles are self-assembled at hexane/water interfaces and transferred to elastic substrates. Stretching the resulting nanoparticle mats provides active and reversible tuning of their plasmonic properties, with a clear polarization dependance. Both uniaxial and biaxial strains induce strong blue shifts in the plasmonic resonances. This matches theoretical simulations and indicates that plasmonic coupling at nanometer scale distances is responsible for the observed spectral tuning. Such stretch-tunable metal nanoparticle mats can be exploited for the development of optical devices, such as flexible colour filters and molecular sensors.

ACS Nano, 2009

Top-gated silicon nanowire transistors are fabricated by preparing all terminals (source, drain, ... more Top-gated silicon nanowire transistors are fabricated by preparing all terminals (source, drain, and gate) on top of the nanowire in a single step via dose-modulated e-beam lithography. This outperforms other time-consuming approaches requiring alignment of multiple patterns, where alignment tolerances impose a limit on device scaling. We use as gate dielectric the 10؊15 nm SiO 2 shell naturally formed during vapor-transport growth of Si nanowires, so the wires can be implemented into devices after synthesis without additional processing. This natural oxide shell has negligible leakage over the operating range. Our single-step patterning is a most practical route for realization of short-channel nanowire transistors and can be applied to a number of nanodevice geometries requiring nonequivalent electrodes.

ACS Nano, 2013

Sensors allow an electronic device to become a gateway between the digital and physical worlds, a... more Sensors allow an electronic device to become a gateway between the digital and physical worlds, and sensor materials with unprecedented performance can create new applications and new avenues for user interaction. Graphene oxide can be exploited in humidity and temperature sensors with a number of convenient features such as flexibility, transparency and suitability for large scale manufacturing. Here we show that the two-dimensional nature of graphene oxide and its super-permeability to water combine to enable humidity sensors with unprecedented response speed (~30ms response and recovery times). This opens the door to various applications, such as touch-less user interfaces, which we demonstrate with a 'whistling' recognition analysis. Graphene is only the first of a new class of two-dimensional (2D) crystals with unique properties, 1 which are being investigated and deployed with increasing vigor in various fields such as electronics and optoelectronics. 2-4 Moreover, functionalized graphene materials provide a further set of 2D materials with exceptional promise for biological and chemical sensors. 5-8

We have measured the thermal response of a superconductor-normal metal-superconductor (SINIS) tun... more We have measured the thermal response of a superconductor-normal metal-superconductor (SINIS) tunnel junction structure at substrate temperature ∼ 60 mK by directly heating the electron system in the normal metal island. In our structure, we ÿnd the response time is determined by the electron-phonon coupling in the electron temperature range 300 -600 mK. By using AC heating, the cut-o frequency caused by this response time has been measured, showing that SINIS structures operate as a thermometer up to a few MHz in this temperature range. ?

In this work four different mesoscopic superconducting devices have been experimentally studied: ... more In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current I_c of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal-Insulator-Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate. Josephson junction, phase diffusion, Cooper pair pump, NIS tunnel junction 165 951-22-7709-3

Graphene oxide has been known for its advantage for sensing water molecules. There are several pa... more Graphene oxide has been known for its advantage for sensing water molecules. There are several papers about using graphene oxide as humidity sensor. [1],[2] Graphene oxide shows very fast response and recovery time, and excellent sensitivity. But humidity sensing behavior of graphene oxide in low humidity environment is not yet described. Here, we tested humidity sensing characteristic of graphene oxide, especially in low humidity environment. Electrodes were deposited on substrate by E-beam evaporation and graphene oxide membranes were made by vacuum filtration and stamped on the substrate. We measured resistance between two electrodes with changing humidity environment. When graphene oxide was exposed less than 30% relative humidity, the resistance of graphene oxide increased almost exponentially and finally the resistivity went extremely high. In this presentation, we explain the main mechanism of this reason and suggest the compensating method to sense the low humidity.