Showing papers by "Stephen J. Pearton published in 2008"

Structure and magnetism of cobalt-doped ZnO thin films

Abstract: The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at% The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD These particles are well oriented with the ZnO crystal structure By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~008??B/Co Depositions in the presence of oxygen background gas at 002?mTorr decreases the magnetization The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out

Electrical detection of biomaterials using AlGaN/GaN high electron mobility transistors

Abstract: Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography, chemiluminescence, selected ion flow tube, and mass spectroscopy, have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specifi...

High performance indium gallium zinc oxide thin film transistors fabricated on polyethylene terephthalate substrates

Abstract: High-performance amorphous (α−) InGaZnO-based thin film transistors (TFTs) were fabricated on flexible polyethylene terephthalate substrates coated with indium oxide (In2O3) films. The InGaZnO films were deposited by rf magnetron sputtering with the presence of O2 at room temperature. The n-type carrier concentration of InGaZnO film was ∼2×1017 cm−3. The bottom-gate-type TFTs with SiO2 or SiNx gate dielectric operated in enhancement mode with good electrical characteristics: saturation mobility 11.5 cm2 V−1 s−1 for SiO2 and 12.1 cm2 V−1 s−1 for SiNx gate dielectrics and drain current on-to-off ratio >105. TFTs with SiNx gate dielectric exhibited better performance than those with SiO2. This is attributed to the relatively high dielectric constant (i.e., high-k material) of SiNx. After more than 500 h aging time at room temperature, the saturation mobility of the TFTs with SiO2 gate dielectric was comparable to the as-fabricated value and the threshold voltage shift was 150 mV.

Room temperature hydrogen detection using Pd-coated GaN nanowires

Abstract: Multiple GaN nanowires produced by thermal chemical vapor deposition were employed as gas sensors for detection of hydrogen at concentrations from 200–1500 ppm in N2 at 300 K. Palladium coating of the wires improved the sensitivity by a factor of up to 11 at low ppm concentrations relative to uncoated controls. The GaN nanowires showed relative responses of ∼7.4% at 200 ppm and ∼9.1% at 1500 ppm H2 in N2 after a 10 min exposure. Upon removal of hydrogen from the measurement ambient, ∼90% of the initial GaN conductance was recovered within 2 min. Temperature dependent measurements showed a larger relative response and shorter response time at elevated temperature. The adsorption activation energy of the sensor was 2.2 kcal mol−1 at 3000 ppm H2 in N2. These sensors exhibit low power consumption (<0.6 mW) at 300 K.

High-Performance Indium Gallium Zinc Oxide Transparent Thin-Film Transistors Fabricated by Radio-Frequency Sputtering

Abstract: Thin-film transistors (TFTs) based on amorphous indium gallium zinc oxide (a-IGZO) were fabricated by radio-frequency magnetron sputtering on glass substrates. The TFT device structure was a bottom-gate type, consisting of indium zinc oxide and HfO 2 as electrodes (gate, source, and drain) and gate dielectric, respectively. The resistivity of the a-IGZO channel layer was ∼1 Ω cm. TFTs with a 6 μm gate length and 100 μm gate width displayed a saturation mobility of ∼7.2 cm 2 V -1 s -1 , a threshold voltage of 0.44 V, a drain current on-off ratio of ∼10 5 , and subthreshold gate-voltage swing of ∼ 0.25 V decade -1 . After 1000 h aging time at room temperature, the saturation mobility remained almost constant while the threshold voltage shift was as small as 460 mV. The IGZO TFTs based on HfO 2 gate dielectrics sputtered near room temperature were found to be good candidates for applications on organic flexible substrates.

Phosphorus doped ZnO light emitting diodes fabricated via pulsed laser deposition

Abstract: ZnO-based light emitting diodes were fabricated on c-plane sapphire using ZnO:P∕Zn0.9Mg0.1O∕ZnO∕Zn0.9Mg0.1O∕ZnO:Ga p-i-n heterostructures. The p-i-n heterojunction diodes are rectifying and show light emission under forward bias. The electroluminescence spectra shows deep level emission at low bias, but near band edge ultraviolet emission at high voltage bias. A decrease in leakage currents in as-fabricated structures was achieved via low temperature oxygen annealing.

Synthesis and microstructure of vertically aligned ZnO nanowires grown by high-pressure-assisted pulsed-laser deposition

Abstract: The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen background pressures of 500-mTorr results in nanowire diameters as small as 50–90 nm, with diameters largely determined by growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The deep-level emission is believed caused by oxygen vancancies formed during growth.

c-erbB-2 sensing using AlGaN∕GaN high electron mobility transistors for breast cancer detection

Abstract: Antibody-functionalized, Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect c-erbB-2, which is a breast cancer marker. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when target c-erbB-2 antigen in a buffer at clinically relevant concentrations was added to the antibody-immobilized surface. We could detect a range of concentrations from 16.7to0.25μg∕ml. These results clearly demonstrate the promise of portable electronic biological sensors based on AlGaN∕GaN HEMTs for breast cancer screening.

Toward conductive traces: Dip Pen Nanolithography® of silver nanoparticle-based inks

Abstract: Low cost, direct writing of conductive traces is highly desired for applications in nanoelectronics, photonics, circuit repair, flexible electronics, and nanoparticle-based gas detection. The unique ability of Dip Pen Nanolithography (DPN®) to direct write a variety of materials onto suitable surfaces with nanoscale resolution and area-specific patterning is leveraged in this work. We present a direct-write approach toward creating traces with commercially available silver nanoparticle (AgNP)-based inks using DPN. In this work we demonstrate submicron AgNP feature creation together with a discussion on the ink transport mechanism.

Wireless hydrogen sensor network using AlGaN/GaN high electron mobility transistor differential diode sensors

Abstract: We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and AlGaN/GaN high electron mobility transistor (HEMTs) differential sensing diodes as the sensing devices. The active device in the differential pair is coated with 10 nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti/Au. Our sensors have a wide range of detection from ppm levels to ∼30%, with the added advantages of a very rapid response time within a couple of seconds, and rapid recovery. The sensors have shown good stability for more than 18 months in an outdoor field test. Currently, the wireless sensing system consists of six wireless sensor nodes and a base station. The wireless sensor node consists of a sensor, a power management system with back-up batteries in case of power outages and a wireless transceiver. The base station consists of a high sensitivity receiver and an in-house developed intelligent monitoring software that does basic data logging and tracking of each individual sensor. The software defines and implements the monitoring states, transitions, and actions of the hydrogen sensor network. Also, the software is able to warn the user of potential sensor failure, power outages and network failures through cell phone network and Internet. Real-time responses of the sensors are displayed through a web site on the Internet. ( http://ren.che.ufl.edu/app/default.aspx ).

Low Hg(II) ion concentration electrical detection with AlGaN/GaN high electron mobility transistors

Abstract: Thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect mercury(II) ions. The drain current of the HEMT sensors monotonically increased with the mercury(II) ion concentration from 1.5 × 10−8 to 4 × 10−8 M. The drain current reached equilibrium around 15–20 s after the concentrated Hg ion solution added to the gate area of the HEMT sensors. The effectiveness of the thioglycolic acid functionalization was evaluated with a surface contact angle study. The results suggest that portable, fast response, and wireless-based heavy metal ion detectors can be realized with AlGaN/GaN HEMT-based sensors.

Enzyme-based lactic acid detection using AlGaN /GaN high electron mobility transistors with ZnO nanorods grown on the gate region

Abstract: The detection of lactic acid with ZnO nanorod-gated AlGaN∕GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167nM to 139μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN∕GaN HEMT based sensors.

ZnO and Related Materials for Sensors and Light-Emitting Diodes

Abstract: ZnO is an attractive material for ultraviolet (UV) light emitters and detectors, and transparent thin-film transistors. It is also readily synthesized in the form of nanostructures. In this paper we discuss the fabrication of a number of ZnO thin-film and nanowire devices, including transistors, diodes, and UV and pH sensors. ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with surface-absorbed species. The large surface area of the nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for high-density sensor arrays.

Dielectric passivation effects on ZnO light emitting diodes

Abstract: Plasma-enhanced chemical vapor-deposited SiO2 and SiNx were used to passivate ZnO heterojunction light emitting diodes (LEDs). Postdielectric deposition annealing was critical in obtaining good LED electrical and optical characteristics. No diode characteristics or light emission was observed unless the structures were annealed at 350°C after fabrication. Annealed diodes showed a band-edge electroluminescence (EL) (385nm) and a broad defect band with a peak at 930nm at room temperature. The SiO2 and SiNx had very different passivation effects in terms of the electrical and EL characteristics of the LEDs. After annealing, the SiO2 passivated ZnO LEDs showed diode I-V characteristics and emitted light. However, the annealed SiNx-passivated ZnO LEDs showed leaky diode characteristics and no light emission. We attribute these differences to the role of hydrogen on the LEDs.

Botulinum toxin detection using AlGaN∕GaN high electron mobility transistors

Abstract: Antibody-functionalized, Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng∕ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN∕GaN HEMTs for botulinum toxin detection.

Low-temperature-fabricated InGaZnO4 thin film transistors on polyimide clean-room tape

Abstract: Amorphous (α-)InGaZnO4 thin film transistors (TFTs) were fabricated on polyimide clean-room tape at low temperature (<100 °C). The α-InGaZnO4 films with an n-type carrier concentration of ∼1016 cm−3 were deposited by rf-magnetron sputtering in a mixed ambient of Ar/O2. The bottom-gate-type TFTs showed good saturation mobility (∼5.3 cm2 V−1 s−1), drain current on-to-off ratio of approximately 105, threshold voltage of 1.1 V, and subthreshold gate-voltage swing of 0.55 V decade−1. These results were comparable to those of the same oxide TFTs that we have fabricated on either glass or polyethylene terephthalate substrates. The results demonstrate that even polyimide clean-room tape can be an appropriate substrate for inexpensive-flexible-adhesive-transparent electronic devices.

Selective-hydrogen sensing at room temperature with Pt-coated InN nanobelts

Abstract: The hydrogen sensing characteristics of multiple InN nanobelts grown by metalorganic chemical vapor deposition were investigated. Pt-coated InN sensors could selectively detect hydrogen at the tens of ppm level at 25 °C, while uncoated InN showed no detectable change in current when exposed to hydrogen under the same conditions. Upon exposure to various concentrations of hydrogen (20–300 ppm) in N2 ambient, the relative resistance change increased from 1.2% at 20 ppm H2 to 4% at 300 ppm H2. Approximately 90% of the initial InN resistance was recovered within 2 min by exposing the nanobelts to air. Temperature-dependent measurements showed larger resistance change and faster response at high temperature compared to those at room temperature due to increase in catalytic dissociation rate of H2 as well as diffusion rate of atomic hydrogen into the Pt/InN interface. The Pt-coated InN nanobelt sensors were operated at low power levels (∼0.5 mW).

Effects of laterally overgrown n-GaN thickness on defect and deep level concentrations

Abstract: The effects of the layer thickness and of Si doping on the dislocation type and density, electron concentration, and deep trap spectra were studied for epitaxially laterally overgrown (ELOG) GaN films with the ELOG region thickness varying from 6to12μm. Electron beam induced current imaging shows that for the thickest layers, the major part of the threading dislocations are filtered out while for thinner films they bend, but do not go out of play. The concentration of residual donors and major electron traps is found to decrease with increasing the film thickness. Si doping suppresses the concentration of the main electron trap with activation energy of 0.6eV and enhances the concentration of the main hole trap at Ev+0.85eV.

Donor nonuniformity in undoped and Si doped n-GaN prepared by epitaxial lateral overgrowth

Abstract: Local donor concentrations were measured in the regions of lateral overgrowth and in the normal vertical growth regions of n-GaN films prepared by epitaxial lateral overgrowth (ELOG). The films were doped with Si to various concentrations. The local donor densities were determined from measurements of the collection efficiency dependence of the electron beam induced current (EBIC) on the energy of the probing electron beam. This dependence was compared with the results of theoretical modeling using the local donor density and diffusion length of charge carriers as fitting parameters. The results show that the donor concentration in the ELOG regions is systematically more than two times lower than the concentration in the vertical growth regions in the gaps of the SiO2 mask used for selective growth. The observed difference is ascribed to the anisotropy of the Si incorporation efficiency. Comparison of these EBIC results with the results of capacitance-voltage profiling obtained on large area Schottky diod...

Carrier concentration dependence of Ti∕Au specific contact resistance on n-type amorphous indium zinc oxide thin films

Abstract: Ti(200A)∕Au(800A) Ohmic contacts to n-type amorphous indium zinc oxide (IZO) films with carrier concentrations of (1×1015)–(5×1020)cm−3 showed as-deposited specific contact resistances in the range of (3×10−1)–(1×10−4)Ωcm2. Postgrowth annealing from 200to500°C resulted in significant improvement in contact resistances due to increase of the carrier concentration in the near surface region of IZO layer, which can be attributed to the formation of Ti–O alloy phases that induce oxygen vacancies in the IZO. After annealing at 500°C, the lowest contact resistance of 8×10−6Ωcm2 was achieved in the sample with carrier concentration of 5×1020cm−3. Temperature dependent measurement showed that tunneling was dominant transport mechanism in the contacts on the most highly doped films (n∼5×1020cm−3) and thermionic emission on the most lightly doped films (n∼1×1015cm−3).

Role of Gate Oxide in AlGaN/GaN High-Electron-Mobility Transistor pH Sensors

Abstract: We report on a comparison of different gate oxides for AlGaN/GaN high-electron-mobility transistor (HEMT) pH sensors. The HEMTs show a linear increase in drain-source current as the pH of the electrolyte solutions introduced to the gate region is decreased. Three different gate oxides were examined, namely the native oxide on the AlGaN surface, a UV-ozone-induced oxide and an Sc2O3 gate deposited by molecular beam epitaxy. The Sc2O3 produced superior results in terms of resolution in measuring small changes in pH. The devices with Sc2O3 in the gate region exhibited a linear change in current between pH 3 and 10 of 37 μA/pH with a resolution of <0.1 pH over the entire pH range. In contrast, the native oxide devices showed a larger change in current, ∼70 μA/pH, but with a degraded resolution of ∼0.4 pH. Results for the UV-ozone oxide were intermediate in resolution, 0.2 pH. These HEMTs have promise for detecting pH changes in biological samples and can be readily integrated into a standard package for wireless data transmission.

Detection of chloride ions using an integrated Ag∕AgCl electrode with AlGaN∕GaN high electron mobility transistors

Abstract: AlGaN∕GaN high electron mobility transistors (HEMTs) with an Ag∕AgCl gate exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chloride (Cl−) ion. The Ag∕AgCl gate electrode, prepared by potentiostatic anodization, changes electrical potential when it encounters Cl− ions. This gate potential changes lead to a change of surface charge in the gate region of the HEMT, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMT channel. These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increasing the drain current of the HEMT. The HEMT source-drain current was highly dependent on Cl− ion concentration. The limit of detection achieved was 1×10−8M using a 20×50μm2 gate sensing area.

Electrical and structural properties of AlN/GaN and AlGaN/GaN heterojunctions

Abstract: The electrical and structural properties of AlN/GaN heterostructures grown by molecular beam epitaxy on sapphire are compared with those of AlGaN/GaN heterostructures. The structural characteristics as assessed by x-ray diffraction show little difference but the electron density in the two-dimensional electron gas is about twice higher for AlN/GaN structures with only slightly lower mobility than in AlGaN/GaN. By proper choice of the Fe doping in GaN(Fe) and the thickness of unintentionally doped GaN layers, the composite buffer of the structure can be made semi-insulating. The current through the AlN/GaN structures is determined by tunneling through the AlN barrier and is much higher than that for AlGaN/GaN films due to the lower thickness of AlN compared to AlGaN. Increasing the thickness of AlN from 3 to 4 nm decreases the leakage current by about an order of magnitude.

RF Characteristics of Room-Temperature-Deposited, Small Gate Dimension Indium Zinc Oxide TFTs

Abstract: Depletion-mode indium zinc oxide channel thin film transistors (TFTs) with gate dimension of 1 X 200 μm and drain-to-source distance of 2.5 μm were fabricated on glass substrates using radio frequency magnetron sputtering deposition at room temperature. Plasma-enhanced chemical vapor deposited SiN x was used as the gate insulator. The threshold voltage was around -2.5 V. Saturation current density at zero gate bias voltage was 2 mA/mm, and a maximum transconductance of 7.5 mS/mm was obtained at V ds = 3 V. The drain current on-to-off ratio was > 10 5 . The maximum field effect mobility measured in the saturation region was ∼ 14.5 cm 2 V -1 s -1 . A unity current gain cutoff frequency, f T , and maximum frequency of oscillation, f max of 180 and 155 MHz, respectively, were obtained. The equivalent device parameters were extracted by fitting the measured s parameters to obtain the intrinsic transconductance, drain resistance, drain-source resistance, transit time, and gate-drain and gate-source capacitance.

GaN, ZnO and InN nanowires and devices.

Abstract: A brief review is given of recent developments in wide bandgap semiconductor nanowire synthesis and devices fabricated on these nanostructures. There is strong interest in these devices for applications in UV detection, gas sensors and transparent electronics.

Electrical properties of GaN (Fe) buffers for AlGaN∕GaN high electron mobility transistor structures

Abstract: The electrical properties of AlGaN∕GaN high electron mobility transistor structures grown on composite GaN(Fe)∕GaN buffers by molecular beam epitaxy were reported. The concentration of Fe in the GaN(Fe) layer ranged from 8×1016to3×1017cm−3 as established by secondary ion mass spectrometry. The thickness of the undoped GaN layer of the buffer was varied from 2.2to4.1μm. For thinner buffers and higher Fe concentration, the buffer was semi-insulating, with the Fermi level pinned near Ec-0.57eV. For thicker buffers and lower Fe concentration, the top part of the buffer was conducting. Admittance spectra measured in conducting buffers also showed a prominent contribution from Ec-(055–0.6)eV electron traps. Despite the universal prominence of these traps in all our films, the behavior of their concentration with Fe doping and with increased distance from the GaN (Fe)∕GaN boundary is not compatible with the assumption that they are due to substitutional Fe acceptors. Possible compensation mechanisms in the studi...

Effects of hydrogen on the optical properties of ZnCdO∕ZnO quantum wells grown by molecular beam epitaxy

Abstract: Temperature-dependent cw- and time-resolved photoluminescence (PL), as well as optically detected magnetic resonance (ODMR) measurements are employed to evaluate effects of deuterium (2H) doping on optical properties of ZnCdO∕ZnO quantum well structures grown by molecular beam epitaxy. It is shown that incorporation of H2 from a remote plasma causes a substantial improvement in radiative efficiency of the investigated structures. Based on transient PL measurements, the observed improvements are attributed to efficient passivation by hydrogen of competing nonradiative recombination centers via defects. This conclusion is confirmed from the ODMR studies.

SiC via fabrication for wide-band-gap high electron mobility transistor/microwave monolithic integrated circuit devices

Abstract: A process for achieving high yield of SiC through wafer via holes without trenching or micromasking and with excellent electrical connection after subsequent metal plating across full wafers was developed for use in high electron mobility transistors (HEMTs) and microwave monolithic integrated circuits (MMICs) using an inductively coupled plasma etch. Consideration was given to the choice of wafer platen, hard mask, gas chemistry, surface treatments, and plasma parameters in order to achieve an acceptable etch rate while at the same time minimizing trenching and micromasking that can harm via yield. In addition, the issue of wafer thickness variation and etch nonuniformity leading to punch through of Au pads at the bottom of the vias was addressed by the addition of a metal layer to the front side of the wafer. The etch rate achieved for 25% of a 2 in. diameter wafer is approximately 3800 A/min while demonstrating acceptable levels of trenching and micromasking with little or no Au punch through. The fina...