Tom Zimmermann | MSU - Academia.edu (original) (raw)

Papers by Tom Zimmermann

Research paper thumbnail of Molecular Beam Epitaxy Regrowth of Ohmics in Metal-face AlN/GaN Transistors

Selective-area regrowth of source/drain was studied for metal-face AlN/GaN high electron mobility... more Selective-area regrowth of source/drain was studied for metal-face AlN/GaN high electron mobility transistors (HEMTs) using molecular beam epitaxy. Heavily doped graded n-type InN/InGaN/GaN was regrown to reduce the source/drain contact resistance. For the 70 nm regrown material, a sheet resistance of ~ 100 Ω/sq and a contact resistance of ~ 0.2 Ω.mm were extracted using non-alloyed Ti-based metal contacts. But the ohmic contact resistance and sheet resistance of the HEMTs with regrown source and drain were found to be yet high using the current processes.

Research paper thumbnail of Surface termination, crystal size, and bonding-site density effects on diamond biosensing surfaces

Diamond and Related Materials

Abstract Diamond's properties, such as chemical stability and low biofouling rates, make it a... more Abstract Diamond's properties, such as chemical stability and low biofouling rates, make it an ideal material for developing more adequate biosensing technology for single-use bioreactors. We propose a simplistic approach to covalently functionalizing diamond and evaluating the effects of different variables on the sensitivity of the bioactive surface. We hypothesize that by maximizing sensitivity, we can maximize the signal-to-noise ratio of the biosignal. Three different methods of achieving an oxygenated surface on diamond with varying crystal sizes is investigated to see which results in the most active biointerface. Surface termination species were confirmed post‑oxygenation with X-ray Photoelectron Spectroscopy. The diamond was then functionalized with a biotinylated monolayer, which was treated with gold-conjugated streptavidin to quantify the available bonding sites. After complete biofunctionalization, detection sensitivity of IL-8 and antibody density were assessed with ELISA and electrochemical impedance spectroscopy for quantification of sites with functional antibodies. The contradicting correlations between diamond morphology and percentage of C O bonds lead us to conclude that the effectiveness of oxygen-termination method is independent of diamond morphology. Through quantifying bonding sites, it was found that both the number of available sites and active sites increased with increasing crystal size. This result is suspected to be directly correlated to the hybridization of the diamond-carbon material in that functional species prefer sp3 carbon over sp2. Therefore, a diamond film with the largest possible crystals is predicted to have a higher detection sensitivity in biosensing applications.

Research paper thumbnail of MBE-Grown Ultrashallow AlN/GaN HEMT Technology

ECS Meeting Abstracts

not Available.

Research paper thumbnail of Polarisationseffekte in Gruppe-(III)-Nitriden und deren Anwendung in p-Kanal FETs und elektromechanischen Strukturen

Galliumnitrid kristallisiert sowohl in der thermodynamisch stabilen hexagonalen und polaren Wurzi... more Galliumnitrid kristallisiert sowohl in der thermodynamisch stabilen hexagonalen und polaren Wurzitstruktur (α-GaN) als auch in der metastabilen, kubischen Modifikation, wobei durch die besseren Materialqualitäten des Wurzit-GaN´s, bedingt dadurch, daß das hexagonale GaN als einzige Modifikation phasenrein mit niedriger Defektdichte gewachsen werden kann, die hexagonale Modifikation eine dominierende Rolle spielt, so daß das bipolare Wurzit-Material eine besondere Herausforderung für das Design von unipolaren Bauelementen darstellt. Besonders für optoelektro-Abbildung 2.1: Linearisierte Bandlücke von (Al x Ga 1−x) y In 1−y N-Materialsystemen als Funktion der Gitterkonstanten nische Anwendungen, insbesondere Laser, erscheint das nichtpolare kubische GaN interessant, aber da die polaren Eigenschaften des Materials eine Schlüsselrolle für

Research paper thumbnail of Microfluidic enzymatic biosensing systems: A review

Biosensors & bioelectronics, Jan 28, 2015

Microfluidic biosensing systems with enzyme-based detection have been extensively studied in the ... more Microfluidic biosensing systems with enzyme-based detection have been extensively studied in the last years owing to features such as high specificity, a broad range of analytes and a high degree of automation. This review gives an overview of the most important factors associated with these systems. In the first part, frequently used immobilization protocols such as physisorption and covalent bonding and detection techniques such as amperometry and fluorescence measurements are discussed with respect to effort, lifetime and measurement range. The Michaelis-Menten model describing the kinetics of enzymatic reactions, the role of redox mediators and the limitations of the linear measurement range of enzymatic sensors are introduced. Several possibilities of extending the linear measurement range in microfluidic systems such as diffusion-limiting membranes and the flow injection setup are presented. Regarding the integration of enzymes into microfluidic systems during the fabrication ...

Research paper thumbnail of Top-down AlN/GaN enhancement- & depletion-mode nanoribbon HEMTs

2009 Device Research Conference, 2009

III-V Nitride HEMTs are currently being intensively investigated for both depletion-and enhanceme... more III-V Nitride HEMTs are currently being intensively investigated for both depletion-and enhancement mode operation at high powers and high frequencies. Among the various methods that can render the polarization-doped HEMTs enhancement-mode, the least investigated are those that exploit 3-D nanoscale geometrical electrostatic effects. Recently, bottom-up grown GaN nanowire MISFETs have shown respectable depletion-mode device performance [1]. However, the handling of isolated nanowires is technologically challenging. Here, we demonstrate that by combining conventional epitaxially grown AlN/GaN HEMT structures with top-down nanoribbon fabrication, both E-mode and D-mode HEMTs with high performance can be realized in a facile manner, and allow integration on the same substrates. In addition to the ease of realizing E-mode and D-mode devices, these top-down nanoribbon HEMTs also take advantage of the superior electrostatics of wrap-gates and quasi-1D charge transport for high performance. The AlN/GaN HEMT structures used here are grown by Molecular Beam Epitaxy. The polarization-induced 2DEGs exhibit densities in the ~10 13 /cm 2 range, and RT mobilities close to ~1300 cm 2 /Vs. Thin nanoribbons (see Figure 1) are fabricated by E-Beam Lithography and etching after the Ohmic contact formation. By controlling the nanoribbon-width w y , and by using a high-k-oxide/Ni/Au gate metal stack wrapped around the top and the side-walls of the nanoribbon, the conducting channel can either be present for wide channels, or absent due to sidewall depletion in narrow channels (Fig 1). Thus, by lithographic adjustment of the ribbonwidths we realize enhancement-and depletion-mode operation, enabling the integration of EDmode logic on a single chip. The inset in Fig. 2 (left) shows a SEM picture of 50 nm deep etched nanoribbons between the source and drain contacts. DC measurements on an AlN/GaN nanoribbon HFET with a 3 m long gate and w y~7 0 nm show a maximum output current density of ~ 500 mA/mm (Fig. 3 left) and a positive threshold voltage of V th = +0.3 V in the transfer-characteristics (Fig. 2 left). A subthreshold-slope of 80 mV/decade was measured on a 2 m gate device indicating superior gate electrostatics and promise for digital logic-circuits. DC-output current densities of~ 1000 mA/mm of a depletion-mode AlN/GaN HFET with a 2 m long gate and w y >200 nm integrated on the same wafer were demonstrated (Fig. 3 right). Thus, the sidewall-depletion of nanoribbons is highly effective in forming both E-and D-mode HEMTs. In addition to the device characteristics presented, the top-down AlN/GaN nanoribbon HEMTs also exhibit high on/off ratios (>10 4), which makes them highly attractive for digital operation at high temperatures. The geometry of the devices allows for array structures for increasing the current drive and possible vertical stacking of the polarization-induced channels. Further device optimization in the geometry and reduction of the access-and contact-resistance of the top-down nanoribbon HEMT structures will enable the design of the first integrated AlN/GaN nanoribbon HEMT logic devices.

Research paper thumbnail of CMOS potentiostat and sensor with multilayer membrane for wide range measurements of glucose concentrations

IEEE SENSORS 2014 Proceedings, 2014

ABSTRACT We report on an electrochemical measurement setup comprising a CMOS potentiostat and a g... more ABSTRACT We report on an electrochemical measurement setup comprising a CMOS potentiostat and a glucose sensor with a two-layer membrane as the first steps towards the development of an integrated in-situ sensor system for bioreactors. The potentiostat has a chip size of 2.1 × 2.5 mm2 and a linear current range from -220 nA to 240 nA with a linearity of R2 = 0.9995. For wide range measurements of glucose concentrations in cell culture media, electrodes functionalized with the enzyme glucose oxidase were spin-coated with membranes made from polydimethylsiloxane (PDMS). With these membranes, glucose concentrations up to 400 mM were measured with a linear measurement range up to 100 mM. For interferent elimination, cellulose acetate membranes were employed.

Research paper thumbnail of Synthesis, Properties and Applications of Ultrananocrystalline Diamond

NATO Science Series, 2005

Research paper thumbnail of Tom Zimmermann1, Yu Cao1, Guowang Li1, Gregory Snider1, Debdeep Jena* 1, Huili (Grace) Xing*

Research paper thumbnail of MOSFETs and TFETs

2:30 PM ILA-3 Student Paper Geometry dependent Tunnel FET performance - Dilemma of electrostatics... more 2:30 PM ILA-3 Student Paper Geometry dependent Tunnel FET performance - Dilemma of electrostatics vs. quantum confinement Y. Lu1.2, A. Seabaugh1.2, P. Fay1.2, SJ Koester3, SE Laux4, W. Haensch4, and SO Koswatta2.4, 1Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, USA, 2Midwest Institute for Nanoelectronics Discovery (MIND), University of Notre Dame, Notre Dame, Indiana, USA 3Electrical Engineering, University of Minnesota, Minneapolis, Minnesota, USA, and 41BM TJ Watson Research Center, Yorktown Heights, ...

Research paper thumbnail of Doktor-Ingenieurs

Research paper thumbnail of Development of Microwave and Terahertz Detectors Utilizing AlN/GaN High Electron Mobility Transistors

We report our work on development of microwave and terahertz detectors using AlN/GaN high electro... more We report our work on development of microwave and terahertz detectors using AlN/GaN high electron mobility transistors. Microwave measurements (f = 10-40 GHz) using AlN/GaN HEMTs as detectors have been performed and the results have shown that the device work in non-resonant mode at room temperature with a responsivity roughly proportional to f 2 at low frequencies. Measured responsivity as a function of gate bias also shows reasonable agreement with theory and published results. Initial calculation results show that an AlN/GaN HEMT with 0.15 um gate length works in the resonant mode when it is cooled down to 77K. The fundamental resonant frequency increases from 200 GHz to 3.2 THz with gate-to-source voltage swing of 0.01 V to 2.0 V. The drain-to-source voltage response also increases with increasing of the gate-to-source voltage swing. We plan to integrate the AlN/GaN HEMT devices broadband lens-coupled antennas and low-pass filters for tunable plasma wave THz detectors.

Research paper thumbnail of MBE-grown buffer with high breakdown voltage for nitride HEMTs on GaN template

2009 International Semiconductor Device Research Symposium, 2009

Abstract The paper presents a molecular beam epitaxial (MBE) grown buffer for nitride HEMT. The s... more Abstract The paper presents a molecular beam epitaxial (MBE) grown buffer for nitride HEMT. The samples were grown at the same thermocouple temperature of 660ÂC. GaN buffer layer was directly grown on the GaN template. In the secondary ion mass spectrometry (SIMS) measurements, unexpected silicon and oxygen impurity atoms at the template surface that caused the buffer leakage were identified.

Research paper thumbnail of Ultra-scaled AlN/GaN enhancement-& depletion-mode nanoribbon HEMTs

2009 International Semiconductor Device Research Symposium, 2009

ABSTRACT

Research paper thumbnail of GaN based piezo sensors

Conference Digest [Late News Papers volume included]Device Research Conference, 2004. 62nd DRC., 2004

ABSTRACT

Research paper thumbnail of Work-function engineering in novel high Al composition Al<inf>0.72</inf>Ga<inf>0.28</inf>N/AlN/GaN HEMTs

68th Device Research Conference, 2010

Abstract In this paper, device performance of high Al composition Al 0.72 Ga 0.28 N/AlN/GaN HEMTs... more Abstract In this paper, device performance of high Al composition Al 0.72 Ga 0.28 N/AlN/GaN HEMTs with ALD Al 2 O 3 dielectric is reported. Employing different gate metals, the threshold voltages were shifted by-0.8 V in 0.5-μm-long HEMTs, which indicates an unpinned Fermi level at the ALD Al 2 O 3/Al 0.72 Ga 0.28 N interface. With lower contact resistances, high Al AlGaN HEMTs are well suited for further lateral and vertical scaling to push towards high frequency E/D-mode performance.

Research paper thumbnail of 4-NM AlN BARRIER ALL BINARY HFET WITH SiNx GATE DIELECTRIC

Selected Topics in Electronics and Systems, 2009

Undoped AlN/GaN heterostructures, grown on sapphire by molecular beam epitaxy, exhibit very low s... more Undoped AlN/GaN heterostructures, grown on sapphire by molecular beam epitaxy, exhibit very low sheet resistances, ~ 150 Ohm/sq, resulting from the 2-dimensional electron gas situated underneath a 4 nm thin AlN barrier. This extraordinarily low sheet resistance is a result of high carrier mobility and concentration (~ 1200 cm 2 /Vs and ~ 3.5x10 13 cm-2 at room temperature), which is ~ 3 x smaller than that of the conventional AlGaN/GaN heterojunction field effect transistor (HFET) structures. Using a 5 nm SiN x deposited by plasma enhanced chemical vapor deposition as gate-dielectric, HFETs were fabricated using these all binary AlN/GaN heterostructures and the gate tunneling current was found to be efficiently suppressed. Output current densities of 1.7 A/mm and 2.1 A/mm, intrinsic transconductance of 455 mS/mm and 785 mS/mm, were achieved for 2 µm and 250 nm gate-length devices, respectively. Current gain cutoff frequency f T of 3.5 GHz and 60 GHz were measured on 2 µm and 250 nm gate-length devices, limited by the high ohmic contact resistance as well as the relatively long gate length in comparison to the electron mean free path under high electric fields.

Research paper thumbnail of Gigahertz operation of epitaxial graphene transistors

2009 Device Research Conference, 2009

The transport properties of graphene with saturation currents as high as 1.5 A/mm and electron mo... more The transport properties of graphene with saturation currents as high as 1.5 A/mm and electron mobilities of 15 000 cm 2 /Vs [1] place this material in the limelight as an attractive candidate of nextgeneration nanoelectronic devices. While DC performance has been extensively studied just a few small signal characterizations have been done so far. The widely observed high carrier mobility naturally focuses our attention towards high frequency performance of graphene based devices. Gigahertz operation of exfoliated [2] and epitaxial [3] graphene FETs has been reported recently. Here we present the high frequency performances of graphene devices based on epitaxially grown graphene on SiC substrates. The measured f T. L G product of 8 GHz. μm and f max of 16 GHz is in very good agreement with previously reported values [3]. Surprisingly, in spite of the much lower electron mobility and transconductance, these FETs demonstrate remarkable small signal performance comparable to the small signal performance of higher mobility exfoliated graphene devices. We use epitaxial graphene on Si-face 4H-SiC. According to AFM characterization and Raman measurements the graphene thickness is 1.9 layers in average over the wafer [4]. We patterned the graphene by optical lithography and etched it in O 2 plasma. Using the same resist-pattern we etch 100 nm deep the SiC in CF 4 plasma to facilitate the sticking of the metal contacts on the surface. Cr/Au source/drain contacts and e-beam evaporated Al 2 O 3 /Ti/Au top gate (t ox = 15 nm) have been deposited to form field effect transistors (FETs). For increased RF performance short gates were fabricated using e-beam lithography. The channel lengths of the devices range from 1-4 μm and the gate length is 1-2 μm in optically defined and 40-500 nm in e-beam patterned devices. Standard ground-signal-ground probing pads are lithographically realized for the gate and drain. Open structures were used to de-embed the signals of the parasitic padcapacitance. Hall measurements reveal μ ~ 200-500 cm 2 /Vs throughout the sample showing smooth variation and better performance in the middle of the wafer. The DC output characteristics of the devices were linear up to 5 V and slightly sublinear above 10 V. The drain current had a weak gate dependence as we swept the gate voltage between +/-2.5 V. The gate modulation decreased as we drove the devices into high source-drain bias range indicating a non-reversible degradation mechanism by high electrical field. The conductance of the devices was varying between 3-7 mS, the transconductance was 120-400 μS/mm, which is significantly lower than in case of exfoliated devices. Small signal performance of devices with gate-lengths between 2 μm and 0.5 μm on epitaxially grown graphene were measured. The used bias-conditions for the FETs are V DS = 10 V and V GS = 2 V. A deembedded current gain cutoff frequency f T of 4.1 GHz for devices with 2 μm long gates was achieved. For a graphene FET with a 0.5 μm long gate an exceptional high power gain cutoff frequency f max of 16 GHz is demonstrated. These preliminary results are very encouraging and demonstrate the potential of the epitaxially grown graphene devices for high-frequency applications as the quality of graphene on insulating substrates improve over time.

Research paper thumbnail of High performance E-mode InAlN/GaN HEMTs: Interface states from subthreshold slopes

68th Device Research Conference, 2010

Due to the high two-dimensional electron gas (2DEG) concentration and high temperature stability,... more Due to the high two-dimensional electron gas (2DEG) concentration and high temperature stability, lattice matched lnAIN/AIN/GaN high electron mobility transistors (HEMTs) have attracted tremendous amount of interest for high-power and high-frequency electronics [1-2]. Employing the gate recess technology, a popular way to develop enhancement-mode (E-mode) devices for digital and mixed signal applications, record high performance (output current density of 2 A/mm, extrinsic transconductance of 890 mS/mm, and fJmax of 95/135 GHz for 150-nm gate length) have been very recently reported on E-mode lnAlN HEMTs [3]. Temperature dependent characterization of the subthreshold slope (SS) can provide valuable information on the interface states and their distribution near the band edges. In this paper, we have performed the field-effect measurements on these gate-recessed E-mod InA IN HEMTs reported in Ref. 3, and extracted the interface states from the temperature dependent SS from 80 to 300 K. The InAIN/AIN/GaN device structure (Fig. 1 inset) consists of an 8.7-nm lnAlN barrier, a I-nm AIN spacer and a Fe-doped GaN semi-insulating buffer on SiC substrate by MOCV D. The 2DEG concentration

Research paper thumbnail of Fet THZ Detectors Operating in the Quantum Capacitance Limited Region

International Journal of High Speed Electronics and Systems, 2011

In this paper, we report our studies on field effect transistor (FET) THz detectors operating in ... more In this paper, we report our studies on field effect transistor (FET) THz detectors operating in the non-resonant mode based on the Dyakonov-Shur plasma wave detection theory, where the quantum capacitance dominates. The influence of quantum capacitance in detector response is theoretically developed and numerically simulated at low and high frequencies. Fundamental constraints in the upper frequency limit are also analyzed for FET THz detectors based on various materials, showing advantages of GaN for 8 - 20 THz applications. Experiments at microwave and THz frequencies have been carried out for GaN based devices showing agreement with the theory.

Research paper thumbnail of Molecular Beam Epitaxy Regrowth of Ohmics in Metal-face AlN/GaN Transistors

Selective-area regrowth of source/drain was studied for metal-face AlN/GaN high electron mobility... more Selective-area regrowth of source/drain was studied for metal-face AlN/GaN high electron mobility transistors (HEMTs) using molecular beam epitaxy. Heavily doped graded n-type InN/InGaN/GaN was regrown to reduce the source/drain contact resistance. For the 70 nm regrown material, a sheet resistance of ~ 100 Ω/sq and a contact resistance of ~ 0.2 Ω.mm were extracted using non-alloyed Ti-based metal contacts. But the ohmic contact resistance and sheet resistance of the HEMTs with regrown source and drain were found to be yet high using the current processes.

Research paper thumbnail of Surface termination, crystal size, and bonding-site density effects on diamond biosensing surfaces

Diamond and Related Materials

Abstract Diamond's properties, such as chemical stability and low biofouling rates, make it a... more Abstract Diamond's properties, such as chemical stability and low biofouling rates, make it an ideal material for developing more adequate biosensing technology for single-use bioreactors. We propose a simplistic approach to covalently functionalizing diamond and evaluating the effects of different variables on the sensitivity of the bioactive surface. We hypothesize that by maximizing sensitivity, we can maximize the signal-to-noise ratio of the biosignal. Three different methods of achieving an oxygenated surface on diamond with varying crystal sizes is investigated to see which results in the most active biointerface. Surface termination species were confirmed post‑oxygenation with X-ray Photoelectron Spectroscopy. The diamond was then functionalized with a biotinylated monolayer, which was treated with gold-conjugated streptavidin to quantify the available bonding sites. After complete biofunctionalization, detection sensitivity of IL-8 and antibody density were assessed with ELISA and electrochemical impedance spectroscopy for quantification of sites with functional antibodies. The contradicting correlations between diamond morphology and percentage of C O bonds lead us to conclude that the effectiveness of oxygen-termination method is independent of diamond morphology. Through quantifying bonding sites, it was found that both the number of available sites and active sites increased with increasing crystal size. This result is suspected to be directly correlated to the hybridization of the diamond-carbon material in that functional species prefer sp3 carbon over sp2. Therefore, a diamond film with the largest possible crystals is predicted to have a higher detection sensitivity in biosensing applications.

Research paper thumbnail of MBE-Grown Ultrashallow AlN/GaN HEMT Technology

ECS Meeting Abstracts

not Available.

Research paper thumbnail of Polarisationseffekte in Gruppe-(III)-Nitriden und deren Anwendung in p-Kanal FETs und elektromechanischen Strukturen

Galliumnitrid kristallisiert sowohl in der thermodynamisch stabilen hexagonalen und polaren Wurzi... more Galliumnitrid kristallisiert sowohl in der thermodynamisch stabilen hexagonalen und polaren Wurzitstruktur (α-GaN) als auch in der metastabilen, kubischen Modifikation, wobei durch die besseren Materialqualitäten des Wurzit-GaN´s, bedingt dadurch, daß das hexagonale GaN als einzige Modifikation phasenrein mit niedriger Defektdichte gewachsen werden kann, die hexagonale Modifikation eine dominierende Rolle spielt, so daß das bipolare Wurzit-Material eine besondere Herausforderung für das Design von unipolaren Bauelementen darstellt. Besonders für optoelektro-Abbildung 2.1: Linearisierte Bandlücke von (Al x Ga 1−x) y In 1−y N-Materialsystemen als Funktion der Gitterkonstanten nische Anwendungen, insbesondere Laser, erscheint das nichtpolare kubische GaN interessant, aber da die polaren Eigenschaften des Materials eine Schlüsselrolle für

Research paper thumbnail of Microfluidic enzymatic biosensing systems: A review

Biosensors & bioelectronics, Jan 28, 2015

Microfluidic biosensing systems with enzyme-based detection have been extensively studied in the ... more Microfluidic biosensing systems with enzyme-based detection have been extensively studied in the last years owing to features such as high specificity, a broad range of analytes and a high degree of automation. This review gives an overview of the most important factors associated with these systems. In the first part, frequently used immobilization protocols such as physisorption and covalent bonding and detection techniques such as amperometry and fluorescence measurements are discussed with respect to effort, lifetime and measurement range. The Michaelis-Menten model describing the kinetics of enzymatic reactions, the role of redox mediators and the limitations of the linear measurement range of enzymatic sensors are introduced. Several possibilities of extending the linear measurement range in microfluidic systems such as diffusion-limiting membranes and the flow injection setup are presented. Regarding the integration of enzymes into microfluidic systems during the fabrication ...

Research paper thumbnail of Top-down AlN/GaN enhancement- & depletion-mode nanoribbon HEMTs

2009 Device Research Conference, 2009

III-V Nitride HEMTs are currently being intensively investigated for both depletion-and enhanceme... more III-V Nitride HEMTs are currently being intensively investigated for both depletion-and enhancement mode operation at high powers and high frequencies. Among the various methods that can render the polarization-doped HEMTs enhancement-mode, the least investigated are those that exploit 3-D nanoscale geometrical electrostatic effects. Recently, bottom-up grown GaN nanowire MISFETs have shown respectable depletion-mode device performance [1]. However, the handling of isolated nanowires is technologically challenging. Here, we demonstrate that by combining conventional epitaxially grown AlN/GaN HEMT structures with top-down nanoribbon fabrication, both E-mode and D-mode HEMTs with high performance can be realized in a facile manner, and allow integration on the same substrates. In addition to the ease of realizing E-mode and D-mode devices, these top-down nanoribbon HEMTs also take advantage of the superior electrostatics of wrap-gates and quasi-1D charge transport for high performance. The AlN/GaN HEMT structures used here are grown by Molecular Beam Epitaxy. The polarization-induced 2DEGs exhibit densities in the ~10 13 /cm 2 range, and RT mobilities close to ~1300 cm 2 /Vs. Thin nanoribbons (see Figure 1) are fabricated by E-Beam Lithography and etching after the Ohmic contact formation. By controlling the nanoribbon-width w y , and by using a high-k-oxide/Ni/Au gate metal stack wrapped around the top and the side-walls of the nanoribbon, the conducting channel can either be present for wide channels, or absent due to sidewall depletion in narrow channels (Fig 1). Thus, by lithographic adjustment of the ribbonwidths we realize enhancement-and depletion-mode operation, enabling the integration of EDmode logic on a single chip. The inset in Fig. 2 (left) shows a SEM picture of 50 nm deep etched nanoribbons between the source and drain contacts. DC measurements on an AlN/GaN nanoribbon HFET with a 3 m long gate and w y~7 0 nm show a maximum output current density of ~ 500 mA/mm (Fig. 3 left) and a positive threshold voltage of V th = +0.3 V in the transfer-characteristics (Fig. 2 left). A subthreshold-slope of 80 mV/decade was measured on a 2 m gate device indicating superior gate electrostatics and promise for digital logic-circuits. DC-output current densities of~ 1000 mA/mm of a depletion-mode AlN/GaN HFET with a 2 m long gate and w y >200 nm integrated on the same wafer were demonstrated (Fig. 3 right). Thus, the sidewall-depletion of nanoribbons is highly effective in forming both E-and D-mode HEMTs. In addition to the device characteristics presented, the top-down AlN/GaN nanoribbon HEMTs also exhibit high on/off ratios (>10 4), which makes them highly attractive for digital operation at high temperatures. The geometry of the devices allows for array structures for increasing the current drive and possible vertical stacking of the polarization-induced channels. Further device optimization in the geometry and reduction of the access-and contact-resistance of the top-down nanoribbon HEMT structures will enable the design of the first integrated AlN/GaN nanoribbon HEMT logic devices.

Research paper thumbnail of CMOS potentiostat and sensor with multilayer membrane for wide range measurements of glucose concentrations

IEEE SENSORS 2014 Proceedings, 2014

ABSTRACT We report on an electrochemical measurement setup comprising a CMOS potentiostat and a g... more ABSTRACT We report on an electrochemical measurement setup comprising a CMOS potentiostat and a glucose sensor with a two-layer membrane as the first steps towards the development of an integrated in-situ sensor system for bioreactors. The potentiostat has a chip size of 2.1 × 2.5 mm2 and a linear current range from -220 nA to 240 nA with a linearity of R2 = 0.9995. For wide range measurements of glucose concentrations in cell culture media, electrodes functionalized with the enzyme glucose oxidase were spin-coated with membranes made from polydimethylsiloxane (PDMS). With these membranes, glucose concentrations up to 400 mM were measured with a linear measurement range up to 100 mM. For interferent elimination, cellulose acetate membranes were employed.

Research paper thumbnail of Synthesis, Properties and Applications of Ultrananocrystalline Diamond

NATO Science Series, 2005

Research paper thumbnail of Tom Zimmermann1, Yu Cao1, Guowang Li1, Gregory Snider1, Debdeep Jena* 1, Huili (Grace) Xing*

Research paper thumbnail of MOSFETs and TFETs

2:30 PM ILA-3 Student Paper Geometry dependent Tunnel FET performance - Dilemma of electrostatics... more 2:30 PM ILA-3 Student Paper Geometry dependent Tunnel FET performance - Dilemma of electrostatics vs. quantum confinement Y. Lu1.2, A. Seabaugh1.2, P. Fay1.2, SJ Koester3, SE Laux4, W. Haensch4, and SO Koswatta2.4, 1Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, USA, 2Midwest Institute for Nanoelectronics Discovery (MIND), University of Notre Dame, Notre Dame, Indiana, USA 3Electrical Engineering, University of Minnesota, Minneapolis, Minnesota, USA, and 41BM TJ Watson Research Center, Yorktown Heights, ...

Research paper thumbnail of Doktor-Ingenieurs

Research paper thumbnail of Development of Microwave and Terahertz Detectors Utilizing AlN/GaN High Electron Mobility Transistors

We report our work on development of microwave and terahertz detectors using AlN/GaN high electro... more We report our work on development of microwave and terahertz detectors using AlN/GaN high electron mobility transistors. Microwave measurements (f = 10-40 GHz) using AlN/GaN HEMTs as detectors have been performed and the results have shown that the device work in non-resonant mode at room temperature with a responsivity roughly proportional to f 2 at low frequencies. Measured responsivity as a function of gate bias also shows reasonable agreement with theory and published results. Initial calculation results show that an AlN/GaN HEMT with 0.15 um gate length works in the resonant mode when it is cooled down to 77K. The fundamental resonant frequency increases from 200 GHz to 3.2 THz with gate-to-source voltage swing of 0.01 V to 2.0 V. The drain-to-source voltage response also increases with increasing of the gate-to-source voltage swing. We plan to integrate the AlN/GaN HEMT devices broadband lens-coupled antennas and low-pass filters for tunable plasma wave THz detectors.

Research paper thumbnail of MBE-grown buffer with high breakdown voltage for nitride HEMTs on GaN template

2009 International Semiconductor Device Research Symposium, 2009

Abstract The paper presents a molecular beam epitaxial (MBE) grown buffer for nitride HEMT. The s... more Abstract The paper presents a molecular beam epitaxial (MBE) grown buffer for nitride HEMT. The samples were grown at the same thermocouple temperature of 660ÂC. GaN buffer layer was directly grown on the GaN template. In the secondary ion mass spectrometry (SIMS) measurements, unexpected silicon and oxygen impurity atoms at the template surface that caused the buffer leakage were identified.

Research paper thumbnail of Ultra-scaled AlN/GaN enhancement-& depletion-mode nanoribbon HEMTs

2009 International Semiconductor Device Research Symposium, 2009

ABSTRACT

Research paper thumbnail of GaN based piezo sensors

Conference Digest [Late News Papers volume included]Device Research Conference, 2004. 62nd DRC., 2004

ABSTRACT

Research paper thumbnail of Work-function engineering in novel high Al composition Al<inf>0.72</inf>Ga<inf>0.28</inf>N/AlN/GaN HEMTs

68th Device Research Conference, 2010

Abstract In this paper, device performance of high Al composition Al 0.72 Ga 0.28 N/AlN/GaN HEMTs... more Abstract In this paper, device performance of high Al composition Al 0.72 Ga 0.28 N/AlN/GaN HEMTs with ALD Al 2 O 3 dielectric is reported. Employing different gate metals, the threshold voltages were shifted by-0.8 V in 0.5-μm-long HEMTs, which indicates an unpinned Fermi level at the ALD Al 2 O 3/Al 0.72 Ga 0.28 N interface. With lower contact resistances, high Al AlGaN HEMTs are well suited for further lateral and vertical scaling to push towards high frequency E/D-mode performance.

Research paper thumbnail of 4-NM AlN BARRIER ALL BINARY HFET WITH SiNx GATE DIELECTRIC

Selected Topics in Electronics and Systems, 2009

Undoped AlN/GaN heterostructures, grown on sapphire by molecular beam epitaxy, exhibit very low s... more Undoped AlN/GaN heterostructures, grown on sapphire by molecular beam epitaxy, exhibit very low sheet resistances, ~ 150 Ohm/sq, resulting from the 2-dimensional electron gas situated underneath a 4 nm thin AlN barrier. This extraordinarily low sheet resistance is a result of high carrier mobility and concentration (~ 1200 cm 2 /Vs and ~ 3.5x10 13 cm-2 at room temperature), which is ~ 3 x smaller than that of the conventional AlGaN/GaN heterojunction field effect transistor (HFET) structures. Using a 5 nm SiN x deposited by plasma enhanced chemical vapor deposition as gate-dielectric, HFETs were fabricated using these all binary AlN/GaN heterostructures and the gate tunneling current was found to be efficiently suppressed. Output current densities of 1.7 A/mm and 2.1 A/mm, intrinsic transconductance of 455 mS/mm and 785 mS/mm, were achieved for 2 µm and 250 nm gate-length devices, respectively. Current gain cutoff frequency f T of 3.5 GHz and 60 GHz were measured on 2 µm and 250 nm gate-length devices, limited by the high ohmic contact resistance as well as the relatively long gate length in comparison to the electron mean free path under high electric fields.

Research paper thumbnail of Gigahertz operation of epitaxial graphene transistors

2009 Device Research Conference, 2009

The transport properties of graphene with saturation currents as high as 1.5 A/mm and electron mo... more The transport properties of graphene with saturation currents as high as 1.5 A/mm and electron mobilities of 15 000 cm 2 /Vs [1] place this material in the limelight as an attractive candidate of nextgeneration nanoelectronic devices. While DC performance has been extensively studied just a few small signal characterizations have been done so far. The widely observed high carrier mobility naturally focuses our attention towards high frequency performance of graphene based devices. Gigahertz operation of exfoliated [2] and epitaxial [3] graphene FETs has been reported recently. Here we present the high frequency performances of graphene devices based on epitaxially grown graphene on SiC substrates. The measured f T. L G product of 8 GHz. μm and f max of 16 GHz is in very good agreement with previously reported values [3]. Surprisingly, in spite of the much lower electron mobility and transconductance, these FETs demonstrate remarkable small signal performance comparable to the small signal performance of higher mobility exfoliated graphene devices. We use epitaxial graphene on Si-face 4H-SiC. According to AFM characterization and Raman measurements the graphene thickness is 1.9 layers in average over the wafer [4]. We patterned the graphene by optical lithography and etched it in O 2 plasma. Using the same resist-pattern we etch 100 nm deep the SiC in CF 4 plasma to facilitate the sticking of the metal contacts on the surface. Cr/Au source/drain contacts and e-beam evaporated Al 2 O 3 /Ti/Au top gate (t ox = 15 nm) have been deposited to form field effect transistors (FETs). For increased RF performance short gates were fabricated using e-beam lithography. The channel lengths of the devices range from 1-4 μm and the gate length is 1-2 μm in optically defined and 40-500 nm in e-beam patterned devices. Standard ground-signal-ground probing pads are lithographically realized for the gate and drain. Open structures were used to de-embed the signals of the parasitic padcapacitance. Hall measurements reveal μ ~ 200-500 cm 2 /Vs throughout the sample showing smooth variation and better performance in the middle of the wafer. The DC output characteristics of the devices were linear up to 5 V and slightly sublinear above 10 V. The drain current had a weak gate dependence as we swept the gate voltage between +/-2.5 V. The gate modulation decreased as we drove the devices into high source-drain bias range indicating a non-reversible degradation mechanism by high electrical field. The conductance of the devices was varying between 3-7 mS, the transconductance was 120-400 μS/mm, which is significantly lower than in case of exfoliated devices. Small signal performance of devices with gate-lengths between 2 μm and 0.5 μm on epitaxially grown graphene were measured. The used bias-conditions for the FETs are V DS = 10 V and V GS = 2 V. A deembedded current gain cutoff frequency f T of 4.1 GHz for devices with 2 μm long gates was achieved. For a graphene FET with a 0.5 μm long gate an exceptional high power gain cutoff frequency f max of 16 GHz is demonstrated. These preliminary results are very encouraging and demonstrate the potential of the epitaxially grown graphene devices for high-frequency applications as the quality of graphene on insulating substrates improve over time.

Research paper thumbnail of High performance E-mode InAlN/GaN HEMTs: Interface states from subthreshold slopes

68th Device Research Conference, 2010

Due to the high two-dimensional electron gas (2DEG) concentration and high temperature stability,... more Due to the high two-dimensional electron gas (2DEG) concentration and high temperature stability, lattice matched lnAIN/AIN/GaN high electron mobility transistors (HEMTs) have attracted tremendous amount of interest for high-power and high-frequency electronics [1-2]. Employing the gate recess technology, a popular way to develop enhancement-mode (E-mode) devices for digital and mixed signal applications, record high performance (output current density of 2 A/mm, extrinsic transconductance of 890 mS/mm, and fJmax of 95/135 GHz for 150-nm gate length) have been very recently reported on E-mode lnAlN HEMTs [3]. Temperature dependent characterization of the subthreshold slope (SS) can provide valuable information on the interface states and their distribution near the band edges. In this paper, we have performed the field-effect measurements on these gate-recessed E-mod InA IN HEMTs reported in Ref. 3, and extracted the interface states from the temperature dependent SS from 80 to 300 K. The InAIN/AIN/GaN device structure (Fig. 1 inset) consists of an 8.7-nm lnAlN barrier, a I-nm AIN spacer and a Fe-doped GaN semi-insulating buffer on SiC substrate by MOCV D. The 2DEG concentration

Research paper thumbnail of Fet THZ Detectors Operating in the Quantum Capacitance Limited Region

International Journal of High Speed Electronics and Systems, 2011

In this paper, we report our studies on field effect transistor (FET) THz detectors operating in ... more In this paper, we report our studies on field effect transistor (FET) THz detectors operating in the non-resonant mode based on the Dyakonov-Shur plasma wave detection theory, where the quantum capacitance dominates. The influence of quantum capacitance in detector response is theoretically developed and numerically simulated at low and high frequencies. Fundamental constraints in the upper frequency limit are also analyzed for FET THz detectors based on various materials, showing advantages of GaN for 8 - 20 THz applications. Experiments at microwave and THz frequencies have been carried out for GaN based devices showing agreement with the theory.