Showing papers by "Stephen J. Pearton published in 2007"

ZnO Doped With Transition Metal Ions

Abstract: Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches

Room temperature deposited indium zinc oxide thin film transistors

Abstract: Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5to−6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5cm2V−1s−1, lower than the Hall mobility of ∼17cm2V−1s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.

Enzymatic glucose detection using ZnO nanorods on the gate region of AlGaN∕GaN high electron mobility transistors

Abstract: ZnO nanorod-gated AlGaN∕GaN high electron mobility transistors (HEMTs) are demonstrated for the detection of glucose. A ZnO nanorod array was selectively grown on the gate area using low temperature hydrothermal decomposition to immobilize glucose oxidase (GOx). The one-dimensional ZnO nanorods provide a large effective surface area with high surface-to-volume ratio and provide a favorable environment for the immobilization of GOx. The AlGaN∕GaN HEMT drain-source current showed a rapid response of less than 5s when target glucose in a buffer with a pH value of 7.4 was added to the GOx immobilized on the ZnO nanorod surface. We could detect a wide range of concentrations from 0.5nMto125μM. The sensor exhibited a linear range from 0.5nMto14.5μM and an experiment limit of detection of 0.5nM. This demonstrates the possibility of using AlGaN∕GaN HEMTs for noninvasive exhaled breath condensate based glucose detection of diabetic application.

Functionalizing Zn- and O-terminated ZnO with thiols

Abstract: We have investigated the adsorption of dodecanethiol on zinc- and oxygen-terminated ZnO surfaces. Strong enthalpic adsorption is demonstrated by the stability of sulfur on both ZnO surfaces for temperatures up to 400°C. The minimal presence of the S 2p3∕2 170eV peak suggests absorption of the sulfur as an unoxidized thiol. The results indicate a higher surface coverage of the thiol on the zinc-terminated surface. Evidence from reflection high energy electron diffraction measurements for the surface ordering after thiol treatment of the oxygen-terminated ZnO surface suggests that the dodecanethiol molecules can adsorb in a highly ordered manner. These results further open the possibility for biofunctionalization of ZnO for biosensing applications.

Ferromagnetism in Transition-Metal Doped ZnO

Abstract: ZnO is an attractive candidate for spintronics studies because of its potential for exhibiting high Curie temperatures and the relative lack of ferromagnetic second phases in the material. In this paper, we review experimental results on transition-metal (TM) doping of ZnO and the current state of theories for ferromagnetism. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field that has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analog. We have been unable to detect optical spin polarization in ZnO.

Fast electrical detection of Hg(II) ions with AlGaN∕GaN high electron mobility transistors

Abstract: Bare Au gated and thioglycolic acid functionalized Au-gated AlGaN∕GaN high electron mobility transistors (HEMTs) were used to detect mercury (II) ions. Fast detection of less than 5s was achieved for thioglycolic acid functionalized sensors. This is the shortest response time ever reported for mercury detection. Thioglycolic acid functionalized Au-gated AlGaN∕GaN HEMT based sensors showed 2.5 times larger response than bare Au-gated based sensors. The sensors were able to detect mercury (II) ion concentration as low as 10−7M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions. The dimensions of the active area of the sensor and the entire sensor chip are 50×50μm2 and 1×5mm2, respectively. Therefore, portable, fast response, and wireless based heavy metal ion detectors can be realized with AlGaN∕GaN HEMT based sensors.

Low temperature (<100 °C) patterned growth of ZnO nanorod arrays on Si

Abstract: Micropatterned growth of ZnO nanorod arrays on silicon substrates using a low temperature aqueous method is demonstrated. ZnO nanocrystals were used as seeds for producing well-aligned, wurtzite ZnO nanorod arrays with spin coating. The shape of the ZnO nanorods was sensitive to the orientation of substrate as well as the molar composition of the chemical precursors. X-ray diffraction was used to investigate the effect of the substrate orientation on the crystal structure of the patterned grown ZnO nanorod arrays, showing that growth on (100) Si was faster than on (111) Si, with the nanorods growing preferentially in the ⟨001⟩ direction. Photoluminescence of the patterned ZnO nanorod arrays showed good optical quality with near band-edge emission at 3.24eV, showing the promise of this approach for micropatterned optical device applications using low temperature synthesis and the conventional lithography.

Fast neutron irradiation effects in n-GaN

Abstract: The electrical properties and deep level spectra in undoped n-GaN films irradiated by fast neutrons are reported. The electron removal rate was ∼5cm−1, and the dominant deep states introduced by neutron damage were electron traps with activation energy of 0.75eV. For high doses of 1.7×1017–1018cm−2 the material becomes semi-insulating n-type with the Fermi level pinned near Ec−0.85eV. Deep level spectra are dominated by electron traps with activation energy of 0.75eV, close to the energy of the Fermi level pinning in heavily irradiated material. Neutron irradiation also introduces a high density of centers giving rise to strong persistent photocapacitance. The observed phenomena are explained under the assumption that the dominant defects in neutron irradiated GaN are disordered regions produced by high-energy recoil atoms.

Spatial variations of doping and lifetime in epitaxial laterally overgrown GaN

Abstract: Spatial variations of donor concentration and diffusion length/lifetime were studied for epitaxial laterally overgrown undoped n-GaN samples by electron beam induced current (EBIC) and microcathodoluminescence (MCL). The dependence of the EBIC signal collection efficiency on the probing beam accelerating voltage shows that the local electron concentration is three times lower and the local lifetime about twice as high in the laterally overgrown regions compared to the regions grown in the SiO2 mask windows. Band edge MCL profiling shows that the lifetime difference could be an order of magnitude higher. EBIC scans along the SiO2 stripes in the low dislocation density overgrown regions show long propagation distances of holes in the quasineutral part of the structure, explained by the existence of a potential profile forming a trough for transport of holes while spatially separating nonequilibrium carriers.

Magnetization dependence on carrier doping in epitaxial ZnO thin films co-doped with Mn and P

Abstract: The magnetic and transport properties of Mn-doped ZnO thin films co-doped with P are examined. Superconducting quantum interference device magnetometry measurements indicate that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by P doping. In particular, under conditions where the acceptor dopants are activated leading to a decrease in free-electron density, magnetization is enhanced. The result is consistent with hole-mediated ferromagnetism in Mn-doped ZnO, in which bound acceptors mediate the ferromagnetic ordering. Increasing the electron density decreases the acceptor concentration, thus quenching the ferromagnetic exchange. This result is important in understanding ferromagnetism in transition metal doped semiconductors for spintronic devices.

Behavior of rapid thermal annealed ZnO:P films grown by pulsed laser deposition

Abstract: The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of antisite PZn defects. Rapid thermal annealing yielded a carrier-type conversion from n- to p-type for the ZnO:P films grown at ∼700 °C; samples grown at substantially lower or higher temperatures tended to remain n-type even after the thermal annealing process. The properties and behavior of the n-to-p conversion are most consistent with the formation of PZn‐2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5−1.0 at. % were considered, with evidence for P segregation in the higher phosphorus concentrations.

Neutron radiation effects in epitaxially laterally overgrown GaN films

Abstract: Neutron radiation effects were studied in undoped n-GaN films grown by epitaxial lateral overgrowth (ELOG). The irradiation leads to carrier removal and introduces deep electron traps with activation energy 0.8 eV and 1 eV. After the application of doses exceeding 1017 cm−2, the material becomes semi-insulating n-type, with the Fermi level pinned near the level of the deeper electron trap. These features are similar to those previously observed for neutron irradiated undoped n-GaN prepared by standard metal–organic chemical vapor deposition (MOCVD). However, the average carrier removal rate and the deep center introduction rate in ELOG samples is about five-times lower than in MOCVD samples. Studies of electron beam induced current (EBIC) show that the changes in the concentration of charged centers are a minimum in the low-dislocation-density laterally overgrown regions and radiation-induced damage propagates inside these laterally overgrown areas from their boundary with the high-dislocation-density GaN in the windows of the ELOG mask.

Optical and magnetic behavior of erbium-doped GaN epilayers grown by metal-organic chemical vapor deposition

Abstract: The authors report on the optical and magnetic properties of GaN epilayers, grown by metal-organic chemical vapor deposition, with in situ Er doping at concentrations up to ∼1021cm−3. Using ultraviolet laser excitation, all samples exhibited photoluminescence near 1540nm with the integrated intensity approximately proportional to the Er concentration. Data from superconducting quantum interference device measurements indicated room temperature ferromagnetic ordering in all Er-doped GaN epilayers. The saturation magnetization in these samples also followed a nearly linear fit to the Er concentration. X-ray diffraction spectra did not reveal evidence of any second phases over this range of Er concentrations.

Mechanism for radiative recombination in ZnCdO alloys

Abstract: Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content. © 2007 American Institute of Physics.

Persistent photoconductivity in p-type ZnO(N) grown by molecular beam epitaxy

Abstract: Current transport mechanisms and persistent photoconductivity effects were studied in nitrogen-doped ZnO films grown by molecular beam epitaxy having p-type or n-type conductivity at 25°C. In both types of samples the current flow is determined by the n-type channels surrounded by higher resistivity regions. The persistent photoconductivity wave form is reasonably described by the stretched-exponents-type expression, with only a slight temperature dependence of the characteristic decay time. The persistent photocurrent decay process is greatly accelerated by infrared illumination (threshold energy of the photons ∼1.4eV). The results suggest that the Fermi level in the higher resistivity regions is pinned near Ev+1.9±0.1eV and the height of the potential barrier for electrons in the n-type channels is around 1.4±0.1eV.

Properties of Fe-doped, thick, freestanding GaN crystals grown by hydride vapor phase epitaxy

Abstract: The electrical properties, deep level spectra, optical transmission, and luminescence spectra were measured on freestanding GaN crystals grown by hydride vapor phase epitaxy. The samples are semi-insulating n type with room temperature resistivity of 3.8×109Ωcm and high electron mobility of 715cm2∕Vs. The Fermi level in these samples is pinned by a Fe-related level near Ec−0.57eV that could be due to the Fe2+∕Fe3+ transition. This level manifests itself also as a strong blue luminescence band peaked near 2.85eV. An additional Fe-related band with optical threshold near 1.6eV is observed in optical transmission spectra. The samples are paramagnetic, suggesting an absence of significant Fe precipitation.

Effect of Si Co Doping on Ferromagnetic Properties of GaGdN

Abstract: Single-phase GaGdN and GaGdN:Si films were grown on sapphire substrates by gas source molecular beam epitaxy using solid Gd, Ga, and Si sources and active nitrogen derived from a RF nitrogen plasma source. The undoped films were highly resistive films but became conductive with the addition of Si. Superconducting quantum interference device magnetometry indicated room-temperature ferromagnetism in both types of materials. Structural defects had a strong influence on the magnetic ordering of the material, as seen in a drastic reduction of magnetic moment with degrading crystalline quality. Magnetization of the codoped film increased with Si content, reaching levels higher than that of the undoped material. The Gd-doped AlN films grown in a similar fashion also displayed Curie temperatures above room temperature.

Lattice vibrational properties of ZnMgO grown by pulsed laser deposition

Abstract: Vibrational modes were studied in n-type and p-type ZnMgO films doped with P (Mg composition of 7at.%) grown by pulsed laser deposition on sapphire. The characteristic phonon frequencies were deduced from the analysis of IR reflectance measured by Fourier-transform spectroscopy. From comparison with similarly grown ZnO (P) films, Mg incorporation reduced the frequency of TO phonons by 14.5cm−1 and introduced two Mg related modes near 530 and 969cm−1. The first is likely to belong to the local vibrational mode of substitutional Mg, and the second is tentatively attributed to strongly lattice relaxed off-center Mg atoms. In addition, it was found that Mg incorporation triggers the formation of two phonon bands with characteristic frequencies of 501 and 634cm−1 that most likely belong to lattice defects.

Semi-Insulating, Fe-Doped Buffer Layers Grown by Molecular Beam Epitaxy

Abstract: Semi-insulating GaN(Fe) films grown by molecular beam epitaxy (MBE) were characterized by measuring electrical properties, deep-level spectra, Fe distribution profiles, microcathodoluminescence (MCL) spectra, electron-beam-induced current, and MCL imaging. The films were high-quality GaN(Fe) with Fe concentration from ∼3 X 10 16 to ∼ 3 X 10 17 cm -3 . The resistivity of GaN(Fe) buffers was >10 5 fl cm, with the Fermi level pinned near E c 0.5 eV. The buffer quality was characterized for Si-doped GaN and AlGaN/GaN transistor structures grown by MBE on GaN(Fe). In contrast to the reported results for growth by metallorganic chemical vapor deposition, Fe distribution profiles did not show long tails extending into the layers grown on top of GaN(Fe). No features attributed to Fe were observed in lightly doped n-GaN grown on GaN(Fe) buffers. The n-GaN films showed electron mobility of >500 cm 2 /V s. AlGaN/GaN transistor structures grown on GaN(Fe) buffers showed two-dimensional electron gas mobility > 1900 cm 2 /V s at 300 K, with a sheet density ∼1 X 10 13 cm -2 and a good pinch-off and a low interdevice leakage.

ZnO-Based Cyclodextrin Sensor Using Immobilized Polydiacetylene Vesicles

Abstract: We report that polydiacetylenes (PDAs) vesicles were successfully immobilized and chemisorbed on single crystal ZnO surfaces. Immobilized PDAs on ZnO were found to be sensitive to temperature and selectively sensitive to α- and γ-cyclodextrins. This approach is attractive for the on-chip integration of various types of sensors, ultraviolet light emitting diodes, and transparent electronics with PDAs.

Influence of redeposition on the plasma etching dynamics

Abstract: This work reports on measurements of the degree of redeposition of sputtered species during the etching of platinum (Pt), barium-strontium-titanate (BST), strontium-bismuth-tantalate (SBT), and photoresist (PR) in a high-density argon plasma. While PR exhibits a redeposition-free behavior, the degree of redeposition of Pt, BST, and SBT species increases from 10% to 95% as the argon pressure increases from 0.5to10mTorr. These results are in good agreement with the predictions of a simple model accounting for the backscattering of sputtered species following their interaction with the gas phase. Based on these results and using other experimental data reported in the literature, it is further demonstrated that, depending on the plasma etching conditions, redeposition effects can induce misinterpretation of the etch rate data.

Ohmic contacts to p-type GaN based on TaN, TiN, and ZrN

Abstract: Ohmic contacts to p-GaN using a Ni∕Au∕X∕Ti∕Au metallization scheme, where X is TaN, TiN, or ZrN, are reported. The dependence of the contact properties on annealing temperature (25–1000°C) in N2 is examined. For annealing temperatures greater than 500°C, the contacts display Ohmic characteristics and reach a minimum of about 2×10−4Ωcm2 after annealing at 700°C for 60s in a N2 ambient. The specific contact resistance is stable on annealing up to at least 1000°C. However, at high temperatures the morphology of the contacts are very rough and there is a large degree of intermixing between the metallic layers. The thermal stability of these contacts are superior as compared to conventional Ni∕Au, which display poor characteristics at annealing temperatures greater than 500°C.

Cathodoluminescence studies of carrier concentration dependence for the electron-irradiation effects in p-GaN

Abstract: Electron-irradiation increase of nonequilibrium carrier lifetime was studied as a function of hole concentration in Mg-doped GaN. Temperature-dependent cathodoluminescence (CL) studies yielded activation energies of 344, 326, 237, and 197meV for samples with hole concentrations of 2×1016, 9×1016, 3×1018, and 7×1018cm−3, respectively. The systematic decay of activation energy with carrier concentration was found to be consistent with Mg acceptors, indicating the involvement of the latter levels in irradiation-induced lifetime changes.

Role of grain boundaries in ZnO nanowire field-effect transistors

Abstract: ZnO nanowires have attracted strong interest for potential nanoelectronics, optoelectronics, and nanosensor applications The role of grain boundaries (GBs) in ZnO nanowire transistors is examined by solving a two-dimensional Schrodinger equation in the nanowire cross section, coupled to a drift-diffusion equation along the nanowire We show that a GB results in a potential barrier with the thickness determined by the gate insulator thickness and the height determined by the number of the trap states at the GB The GB leads to a decrease of the source-drain current because the voltage drop at the GB reduces the electric field at other channel positions The on current depends on the nanowire diameter nonmonotonically due to two competing mechanisms Increasing the number of GBs in the channel decreases both the on current and off current When the total number of GBs is small, its effect on the I‐V characteristics can be phenomenologically viewed as an increase of the threshold voltage When the total num