Showing papers by "Stephen J. Pearton published in 2006"

Zinc oxide bulk, thin films and nanostructures : processing, properties and applications

Abstract: Foreword 1. Basic Properties and Applications of ZnO (V.A. Coleman and C. Jagadish) 2. Doping and Defects in ZnO (D.C. Look) 3. Synthesis and Characterization of Nitrogen-Doped ZnO Films Grown by MOCVD (T.J. Coutts, Xiaonan Li, T. Barnes, B. Keyes, C.L. Perkins, S.E. Asher, Shengbai Zhang, Su-Huai Wei and S. Limpijumnong) 4. Pulsed Laser Deposition of ZnO (V. Gupta and K. Sreenivas) 5. Optical Properties of ZnO and Related Alloys (U. Ozgur and H. Morkoc) 6. Minority Carrier Transport in ZnO and Related Materials (O. Lopatiuk, A. Osinsky and L. Chernyak) 7. Contacts to ZnO (Jae-Hong Lim and Seong-Ju Park) 8. Ion Implantation into ZnO (S.O. Kucheyev and C. Jagadish) 9. Advances in Processing of ZnO (K. Ip, S.J. Pearton, D.P. Norton and F. Ren) 10. Novel Nanostructures and Nanodevices of ZnO (Zhong Lin Wang) 11. ZnO/ZnMgO heterojunction FETs (M. Yano, K. Koike, S. Sasa and M. Inoue) 12.ZnO Thin Film Transistors (R. Hoffman) 13. ZnO Piezoelectric Devices (Yicheng Lu, N. Emanetoglu and Ying Chen) 14. Gas, Chemical and Biological Sensing with ZnO (Young-Woo Heo, F. Ren and D.P. Norton) 15. ZnO-Based Light Emitters (A. Osinsky and S. Karpov) 16. Ferromagnetism in ZnO Doped With Transition Metal Ions (D.P. Norton, S.J. Pearton, J.M. Zavada, W.M. Chen and I.A. Bouyanova)

Electroluminescence from ZnO nanowire/polymer composite p-n junction

Abstract: The characteristics of a hybrid p-n junction consisting of the hole-conducting polymer poly(3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT/PSS) and n-ZnO nanorods grown on an n-GaN layer on sapphire are reported. Spin coating of polystyrene was used to electrically isolate neighboring nanorods and a top layer of transparent conducting indium tin oxide (ITO) was used to contact the PEDOT/PSS. Multiple peaks are observed in the electroluminescence spectrum from the structure under forward bias, including ZnO band edge emission at ∼383nm as well as peaks at 430, 640, and 748nm. The threshold bias for UV light emission was <3V, corresponding to a current density of 6.08Acm−2 through the PEDOT/PSS at 3V.

ZnO spintronics and nanowire devices

Abstract: ZnO is a very promising material for spintronics applications, with many groups reporting room-temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during pulsed laser deposition (PLD), we find an inverse correlation between magnetization and electron density as controlled by Sn-doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for ferromagnetism include the bound magnetic polaron model or exchange that is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. The progress in ZnO nanowires is also reviewed. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for microlasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼−3 V, and a maximum transconductance of 0.3 mS/mm. Under ultraviolet (UV) illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼5 mS/mm. The channel mobility is estimated to be ∼3 cm2/Vss, comparable to that for thin film ZnO enhancement mode metal-oxide semiconductor field effect transistors (MOSFETs), and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. The Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25°C, very low reverse currents, and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25°C to 150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089 eV and was insensitive to ambient used. By contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. Sensitive pH sensors using single ZnO nanowires have also been fabricated.

Gallium Nitride Processing for Electronics, Sensors and Spintronics

Abstract: Advanced Processing of GaN for Electronic Devices Dry Etching of GaN and Related Materials Design and Fabrication of GaN High Power Rectifiers Chemical, Gas, Biological and Pressure Sensing Nitride-Based Spintronics Novel Insulators for GaN MOSFETs and AlGaN/GaN MOS-HEMTs

Electrical detection of deoxyribonucleic acid hybridization with AlGaN/GaN high electron mobility transistors

Abstract: Au-gated AlGaN∕GaN high electron mobility transistor (HEMT) structures were functionalized in the gate region with label-free 3′-thiol-modified oligonucleotides. This serves as a binding layer to the AlGaN surface for hybridization of matched target deoxyribonucleic acid (DNA). X-ray photoelectron spectroscopy shows the immobilization of thiol-modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected by electrical measurements. The HEMT drain-source current showed a clear decrease of 115μA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection approach for biological sensing.

Optical and magnetic properties of Eu-doped GaN

Abstract: GaN films were doped with Eu to a concentration of ∼0.12at.% during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were postannealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 3.0×10−7to5.4×10−7Torr. The maximum PL intensity was obtained at a Ga flux of 3.6×10−7Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ∼0.1–0.45emu∕cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all the growth conditions but these cannot account for the observed magnetic properties.

Effect of Gd Implantation on the Structural and Magnetic Properties of GaN and AlN

Abstract: Gd+ ions were implanted at total doses of 3-6x10(14) cm(2) into single-crystal GaN or AlN epilayers grown on sapphire substrates and annealed at 700-1000 degrees C. The implanted Gd showed no detectable diffusion in either material after annealing, as measured by secondary ion mass spectrometry, corresponding to a diffusion coefficient

Electrical transport properties of single GaN and InN nanowires

Abstract: The transport properties of single GaN and InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of temperature, annealing condition (for GaN) and length/square of radius ratio (for InN). The as-grown GaN nanowires were insulating and exhibited n-type conductivity (n ≈ 2×1017 cm−3, mobility of 30 cm2/V s) after annealing at 700°C. A simple fabrication process for GaN nanowire field-effect transistors on Si substrates was employed to measure the temperature dependence of resistance. The transport was dominated by tunneling in these annealed nanowires. InN nanowires showed resistivity on the order of 4×10−4 Ω cm and the specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10−7 Ω cm2. For In N nanowires with diameters <100 nm, the total resistance did not increase linearly with length/square of radius ratio but decreased exponentially, presumably due to more pronounced surface effect. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN nanowires.

Robust detection of hydrogen using differential AlGaN /GaN high electron mobility transistor sensing diodes

Abstract: The use of AlGaN∕GaN high electron mobility transistor (HEMT) differential sensing diodes is shown to provide robust detection of 1% H2 in air at 25°C. The active device in the differential pair is coated with 10nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti∕Au. The active diode in the pair shows an increase in forward current of several milliamperes at a bias voltage of 2.5V when exposed to 1% H2 in air. The HEMT diodes show a response approximately twice that of GaN Schottky diodes, due to the presence of piezoelectric and spontaneous polarization in the heterostructure. The use of the differential pair removes false alarms due to ambient temperature variations.

Ultraviolet photoluminescence from Gd-implanted AlN epilayers

Abstract: Deep ultraviolet emission from gadolinium (Gd)-implanted AlN thin films has been observed using photoluminescence (PL) spectroscopy. The AlN epilayers were ion implanted with Gd to a total dose of similar to 6x10(14) cm(-2). Using the output at 197 nm from a quadrupled Ti:sapphire laser, narrow PL emission was observed at 318 nm, characteristic of the trivalent Gd ion. A broader emission band, also centered at 318 nm, was measured with excitation at 263 nm. The PL emission intensity decreased by less than a factor of 3 over the sample temperature range of 10-300 K and decay transients were of the order of nanoseconds.

Carrier concentration dependence of acceptor activation energy in p-type ZnO

Abstract: The characteristics of an acceptor level in Sb-doped, p-type ZnO were studied using cathodoluminescence (CL) spectroscopy as a function of hole concentration. Variable-temperature CL measurements allowed us to estimate the activation energy of an Sb-related acceptor from temperature-induced decay of CL intensity. The values of activation energy of about 212±28, 175±20, 158±22, and 135±15meV were obtained for samples with carrier concentrations of 1.3×1017, 6.0×1017, 8.2×1017, and 1.3×1018cm−3, respectively. The involvement of acceptor levels is supported by the temperature-dependent hole concentration measurements. The possible origins of the strong temperature dependence are discussed.

Band-edge electroluminescence from N+-implanted bulk ZnO

Abstract: N+ ion implantation at moderate doses (1013–1014cm−2) into nominally undoped (n∼1017cm−3) bulk single-crystal ZnO substrates followed by annealing in the range 600–950°C was used to fabricate diodes that show visible luminescence at 300K and band-edge electroluminescence at 120K (∼390nm) under forward bias conditions. The current-voltage behavior of the diodes are characteristic of metal-insulator-semiconductor devices and suggest the implantation creates a more resistive region in the n‐ZnO in which holes are created by impact ionization during biasing, similar to the case of electroluminescence in ZnO varistors. The series resistance is only 25Ω due to the use of the conducting ZnO substrate.

Electrical properties of undoped bulk ZnO substrates

Abstract: Undoped bulk ZnO crystals obtained from Tokyo Denpa show either resistive behavior [(5×104)−(3×105) Ohm cm) or low n-type conductivity (n ⋍1014 cm−3) with mobilities in the latter case of 130–150 cm2/V sec. The variation in resistivity may be related to the thermal instability of Li that is present in the samples. The Fermi level is pinned by 90-meV shallow donors that are deeper than the 70 meV and hydrogen-related 35-meV shallow donors in Eagle Pitcher and Cermet substrates. In all three cases, 0.3-eV electron traps are very prominent, and in the Tokyo Denpa material they dominate the high-temperature capacitance-frequency characteristics. The concentration of these traps, on the order of 2×1015 cm−3, is about 20 times higher in the Tokyo Denpa ZnO compared to the two other materials. The other electron traps at Ec −0.2 eV commonly observed in undoped n-ZnO are not detected in conducting Tokyo Denpa ZnO samples, but they may be traps that pin the Fermi level in the more compensated high-resistivity samples.

Studies of minority carrier diffusion length increase in p-type ZnO:Sb

Abstract: Minority electron diffusion length was measured in p-type, Sb-doped ZnO as a function of temperature using the electron beam induced current technique. A thermally induced increase of electron diffusion length was determined to have an activation energy of 184±10meV. Irradiation with a low energy (5kV) electron beam also resulted in an increase of diffusion length with a similar activation energy (219±8meV). Both phenomena are suggested to involve a SbZn–2VZn acceptor complex. Saturation and relaxation dynamics of minority carrier diffusion length are explored. Details of a possible mechanism for diffusion length increase are presented.

System for hydrogen sensing

Abstract: Exemplary embodiments provide a self-powered wireless gas sensor system and a method for gas sensing using the system. The system can be used to detect and constantly track a presence of various gases including hydrogen, ozone and/or any hydrocarbon gas, and remotely transmit the sensing signal. The system can include a low power gas sensor that consumes less than about 30 nano-watts of power. As a result, the system can detect the presence of hydrogen at about 10 ppm. The sensor can also provide a fast response time of about 1-2 seconds. In various embodiments, the system can be physically small and packaged with all components assembled as a single compact unit.

Fermi level pinning in heavily neutron-irradiated GaN

Abstract: Undoped n-GaN grown by two different metallorganic chemical vapor deposition (MOCVD) techniques, standard MOCVD and epitaxial lateral overgrowth, and Mg-doped p-GaN prepared by hydride vapor phase epitaxy and molecular beam epitaxy were irradiated with fast reactor neutrons to the high fluence of 1018 cm−2. In such heavily irradiated samples the Fermi level is shown to be pinned in a narrow interval of Ec−(0.8−0.95) eV, irrespective of the starting sample properties. The Fermi level pinning position correlates with the measured Schottky barrier height in n-type GaN. The results are interpreted from the standpoint of the existence of the charge neutrality level in heavily disordered material. Based on published theoretical calculations and on deep level transient spectroscopy (measurements and lattice parameter measurements in irradiated material), it is proposed that the Fermi level could be pinned between the gallium-interstitial-related deep donors near Ec−0.8 eV and nitrogen-interstitial-related accept...

Neutron irradiation effects in p‐GaN

Abstract: Electrical properties, admittance, and microcathodoluminescence spectra are compared for p‐GaN samples grown by hydride vapor phase epitaxy (HVPE) and by molecular beam epitaxy (MBE). The former are characterized by a high 300K hole concentration and a weak temperature dependence of conductivity. The latter samples show strongly temperature-activated conductivity due to ionization of Mg acceptors. The main effects of neutron irradiation were similar for the p-HVPE and the p-MBE materials: a compensation of p-type conductivity starting with neutron fluences exceeding 2×1016cm−2 and conversion to high resistivity n type with the Fermi level pinned near Ec‐(0.8–0.9)eV after irradiation with high doses of 1018cm−2. For the heavily neutron irradiated p-HVPE samples, a strong increase was observed in the c-lattice parameter which indicates an important role for interstitial-type defects.

Effect of growth conditions on the magnetic characteristics of GaGdN

Abstract: GaGdN layers were grown by gas source molecular beam epitaxy with varying crystal quality and Gd concentrations as set by the Gd cell temperature. Magnetic measurements showed ferromagnetic behavior at room temperature, with the saturation magnetization dependent both on Gd concentration and crystalline quality. The Gd concentration was under the detection limit of secondary ion mass spectrometry, and estimated to be on the order of <1017at.∕cm3. As expected at this low dopant concentration, x-ray diffraction measurements showed the films to be single phase. Gd-doped samples codoped with Si to make them conducting with resistivity of 0.04Ωcm showed similar magnetic properties as Gd-doped films without addition of Si.

Low specific contact resistance Ti∕Au contacts on ZnO

Abstract: Ti∕Au Ohmic contacts on heavily Al-doped (n∼1019cm−3) n-ZnO produce low specific contact resistivity of 2.4×10−7Ωcm2 in the as-deposited condition and extremely low minimum values of 6×10−8Ωcm2 after annealing at 300°C.The contact resistance is independent of measurement temperature after low temperature anneals, suggesting that tunneling is the dominant transport mechanism in the contacts. The contact morphology roughens after annealing at 150°C and Auger electron spectroscopy depth profiling shows Zn outdiffusion through the metal and intermixing of Au and Ti. However, the morphology does not significantly worsen after anneals at 450°C. This metallization scheme looks very attractive for the n-electrode of ZnO-based light-emitting diode structures.

Effective temperature measurements of AlGaN/GaN-based HEMT under various load lines using micro-Raman technique

Abstract: We report on contact-free measurement of channel temperature of AlGaN/GaN HEMTs under different operation modes. Micro-Raman spectroscopy was successfully used to measure the temperature of operating HEMT devices with

Anisotropic x-ray absorption effects in the optical luminescence yield of ZnO nanostructures

Abstract: The authors have found that the directionality of the orbital populated following core-level x-ray absorption of a hexagonal nanostructure has a strong influence on the resulting optical luminescence yield spectra. For ZnO, there is an enhancement of the band gap exciton luminescence following O 1s to 2pz relative to 2px,y excitation. The defect luminescence O 1s excitation spectrum also shows sensitivity to the nature of the defect (surface or bulk).

Effects of Zn content on structural and transparent conducting properties of indium-zinc oxide films grown by rf magnetron sputtering

Abstract: Indium-zinc oxide (IZO) films were grown on glass substrates by rf magnetron sputtering using targets of 50mol% In2O3–50mol% In2O3(ZnO)3 and In2ZnkOk+3 (k=3, 4, 5, and 7) at room temperature and 300°C. The difference in Zn content between the films and the sputter targets varied with the growth temperature. The structural, electrical, and optical properties of the IZO films were investigated as a function of Zn content. The crystal structure of IZO films grown at room temperature changed from amorphous to crystalline at a Zn content (Zn∕(Zn+In)) of 68at.%. IZO films grown at 300°C using a target of 50% In2O3–50% In2O3(ZnO)3 had a Zn content of 40at.% and its x-ray diffraction peaks were matched with those of ITO. As the Zn content in IZO thin films grown at 300°C increased from 40to74at.%, the conductivity and optical band gap energy decreased.

Electrical and recombination properties and deep traps spectra in MOCVD ELOG GaN layers

Abstract: Electrical properties, deep levels spectra, spectra of microcathodoluminescence, diffusion lengths of minority carriers were studied in undoped GaN films grown by standard metallorganic chemical vapour deposition on sapphire and by the epitaxial lateral overgrowth technique (ELOG) It is shown that the total concentrations of shallow donors and of electron and hole traps are very considerably lower in the ELOG material compared to the ordinary MOCVD samples MCL and electron beam induced current EBIC imaging of ELOG samples suggests that the dislocation density in the overgrown regions is on the order of 106 cm–2 while in-between these regions it is similar to the dislocation density in the standard material, about 109 cm–2 Local diffusion lengths were shown to follow the same trend as dislocation densities (© 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

Schottky barrier height of boride-based rectifying contacts to p-GaN

Abstract: Schottky contact formation on p-GaN using a W2B-based metallization scheme was investigated using x-ray photoelectron spectroscopy (XPS), current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Schottky barrier height (SBH) determined from XPS is 2.7eV, whereas fitting of the I-V’s gives 1.2 and 3.8eV depending on the assumed mechanism of forward current flow. While the C-V’s and the measurement temperature dependence of the I-V’s support tunneling as being the dominant transport mechanism, this latter approach overestimates the true SBH of W2B∕p-GaN contacts due to the presence of an interfacial layer acting as an additional barrier to carrier transport.

Neutron irradiation effects in AlGaN/GaN heterojunctions

Abstract: It is shown that in neutron-irradiated undoped n-AlGaN/GaN heterojunctions, the mobility and sheet conductivity start to decrease only after exposure to doses higher than 10 15 cm −2 . The effects on mobility are mostly due to the introduction of additional scattering centers in the GaN buffer layer of the structures while there are only slight changes in the two-dimensional electron concentration. Electron traps with activation energy of 0.21, 0.35 and 0.45 eV were observed in the AlGaN barrier, hole traps with energies of 0.18, 0.2, 0.26, 0.7 and 1 eV were detected and could be located either in the AlGaN barrier or in the GaN buffer.

Si-diffused GaN for enhancement-mode GaN MOSFET on Si applications

Abstract: Si diffusion into GaN was studied as a function of encapsulant type (SiO2 or SiNx) and diffusion temperature. Using a SiO2 encapsulant, the Si diffusion exhibited an activation energy of 0.57 eV with a prefactor of 2.07×10−4 cm2 sec−1 in the temperature range 800–1,000°C. An enhancement-mode MgO/GaN-on-Si metal-oxide semiconductor field-effect transistor (MOSFET) was fabricated utilizing Si-diffused regions under the source and drain to provide an accumulated channel. The gate leakage through the undoped GaN was low enough for us to achieve good saturation behavior in the drain-current-voltage characteristics. The devices showed improved transconductance and drain current relative to previous devices with Si-implanted source/drain regions.