Showing papers by "Stephen J. Pearton published in 2011"

Oxygen sensors made by monolayer graphene under room temperature

Abstract: The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O2) at room temperature. The monolayer graphene, grown by chemical vapor deposition with perfect uniformity within 1 cm × 1 cm will attach O2 molecules and enhance the hole conductivity, which will lead to a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O2 concentration and the detection limit of the simple O2 sensor was 1.25% in volume ratio.

Hydrogen detection using platinum coated graphene grown on SiC

Abstract: The characteristics of hydrogen detection using epitaxial graphene covered with platinum are reported. The multi-layered graphene was grown by chemical vapor deposition (CVD) on a Si-polar 4H-SiC substrate. Surface morphology was characterized by scanning electron and atomic force microscopy. Current–voltage measurements and real-time monitoring of the current flow through the graphene/platinum device were used to confirm the response to hydrogen gas. The background temperature was varied from room temperature to 175 °C in order to measure the activation energy of hydrogen detection.

Chemical Etch Characteristics of N-Face and Ga-Face GaN by Phosphoric Acid and Potassium Hydroxide Solutions

Abstract: We report the chemical etching characteristics of Ga-face and N-face GaN using phosphoric acid (H3PO4) or potassium hydroxide (KOH) solutions. Hexagonal pyramids, which consisted of the {10-1-1} planes, were present on the N-face after KOH (2M, 100°C) etching. By contrast, using the H3PO4 (85 wt.%, 100°C) solutions, the nitrogen surface of GaN showed dodecagonal pyramids. Dodecagonal and hexagonal pyramids repeatedly appear on the etched surface when using the H3PO4 or KOH solutions, respectively. A low concentration of H3PO4 (H3PO4 : deionized water = 1:32, 1:64) produced a roughened surface with coexistence of dodecagonal and hexagonal pyramids. The photoluminescence (PL) intensity of the etched surfaces significantly increased due to multiple scattering events compared to the non-etched surface. Thus, the etching techniques developed in this study were shown to improve the light extraction efficiency of light emitting diodes (LEDs), avoiding the damage to the GaN typically created by plasma etching methods.

Effect of Coated Platinum Thickness on Hydrogen Detection Sensitivity of Graphene-Based Sensors

Abstract: The effect of Pt metal thickness on the hydrogen sensing sensitivity of Pt-coated, multi-layered graphene grown by chemical vapor deposition on Si-polar 4H-SiC, was investigated. As-grown graphene samples and graphene samples coated with 1 or 4 nm thick Pt films were used in this study. Compared to graphene without platinum, significantly improved hydrogen detection sensitivity was observed with the addition of platinum films. The highest hydrogen sensitivity was observed with the graphene sensor with 1 nm platinum coating. The platinum coated graphene sensor also showed good selectivity for hydrogen detection over methane, ammonia, oxygen, and nitrogen oxide.

Experimental study of graphitic nanoribbon films for ammonia sensing

Abstract: We fabricate and study the ammonia sensing properties of graphitic nanoribbon films consisting of multi-layer graphene nanoribbons. These films show very good sensitivity to parts-per-million (ppm) level concentrations of ammonia, which is further enhanced by platinum functionalization, resulting in a relative resistance response of ∼70% when exposed to 50 ppm ammonia. In addition, the sensing response exhibits excellent repeatability and full recovery in air. We also study in detail the dependence of the sensing response on ammonia concentration and temperature. We find that the relative resistance response of the graphitic nanoribbon films shows a power-law dependence on the ammonia concentration, which can be explained based on the Freundlich isotherm. The activation energy obtained from an Arrhenius plot of the temperature-dependent measurements is ∼50 meV, which is consistent with the theoretical calculations of the adsorption energies of ammonia on large graphene sheets and nanoribbons. Their simple and low-cost fabrication process and good sensing response open up the possibility of using graphitic nanoribbon films for large-scale sensing applications.

Comparison of hole traps in n-GaN grown by hydride vapor phase epitaxy, metal organic chemical vapor deposition, and epitaxial lateral overgrowth

Abstract: Optical deep level spectroscopy (ODLTS) and microcathodoluminescence (MCL) spectra were measured for a large group of n-GaN samples grown via metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth (ELOG), or hydride vapor phase epitaxy (HVPE). In the MOCVD and ELOG samples, the ionization energy of dominant hole traps H1 was dependent on the excitation conditions and was ∼0.9 eV for high injection levels providing saturation of the ODLTS peak magnitude. The trap concentration increased with increasing Si donor concentration and correlated with the yellow band intensity in the MCL spectra. For the HVPE samples, the hole trap spectra were radically different from the MOCVD case: four hole traps—H2, H3, H4, and H5—with activation energies of 0.55, 0.65, 0.85, and 1.2 eV, respectively, were detected. In the MCL spectra, a broad green band that peaked near 2.5 eV was observed in addition to the usual yellow luminescence near 2.3 eV. This green band was attributed to the transitions involv...

Electric-Field-Driven Degradation in off-State Step-Stressed AlGaN/GaN High-Electron Mobility Transistors

Abstract: The critical degradation voltage of AlGaN/GaN high-electron mobility transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1-0.17 μm), drain bias voltage, and source/drain-gate contact distance. Devices with different gate lengths and gate-drain distances were found to exhibit the onset of degradation at different source-drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field driven. The degradation field was calculated to be ~ 1.8 MV/cm by Automatically Tuned Linear Algebra Software simulations. Transmission electron microscopy imaging showed creation of defects under the gate after dc stress.

Role of nonradiative recombination centers and extended defects in nonpolar GaN on light emission efficiency

Abstract: The correlation of integrated microcathodoluminescence efficiency with crystalline quality and deep trap density of nonpolar GaN films grown by metal organic chemical vapor deposition on semi-insulating 6H-m-SiC or r-sapphire is analyzed. The results suggest a strong influence of nonradiative recombination centers whose concentration decreases with decreased density of extended defects. Electron traps with energy levels at Ec−0.6 eV and which pin the Fermi level in films with high defect density are the most likely candidates for the decrease in light emission efficiency in nonpolar GaN.

Effect of source field plate on the characteristics of off-state, step-stressed AlGaN/GaN high electron mobility transistors

Abstract: The effects of source field plates on AlGaN/GaN high electron mobility transistor reliability under off-state stress conditions were investigated using step-stress cycling. The source field plate enhanced the drain breakdown voltage from 55 to 155 V and the critical voltage for off-state gate stress from 40 to 65 V, relative to devices without the field plate. Transmission electron microscopy was used to examine the degradation of the gate contacts. The presence of pits that appeared on both source and drain sides of the gate edges was attributed to the inverse piezoelectric effect. In addition, a thin oxide layer was observed between the Ni gate contact and the AlGaN layer, and both Ni and oxygen had diffused into the AlGaN layer. After step-stress cycling, additional threading dislocations were observed.

Characterization of the gate oxide of an AlGaN/GaN high electron mobility transistor

Abstract: A subnanometer thick interfacial oxide layer present between the Ni/Au gate metal stack and semiconducting epilayers of an AlGaN/GaN high electron mobility transistor was characterized using high-angle annular dark-field scanning transmission electron microscopy and laser-assisted atom probe tomography. It was revealed that the oxide is composed of distinct Ni-oxide-rich and Al-oxide-rich layers with no Ga-oxide detected. The results provide information that is of potential importance in determining failure mechanisms and improving reliability of AlGaN/GaN high electron mobility transistors.

Effect of Source Field Plate on the Characteristics of Off-State, Step-Stressed AlGaN/GaN High Electron Mobility Transistors

Abstract: The effects of source field plates on AlGaN/GaN high electron mobility transistor reliability under off-state stress conditions were investigated using step-stress cycling. The source field plate enhanced the drain breakdown voltage from 55 to 155 V and the critical voltage for off-state gate stress from 40 to 65 V, relative to devices without the field plate. Transmission electron microscopy was used to examine the degradation of the gate contacts. The presence of pits that appeared on both source and drain sides of the gate edges was attributed to the inverse piezoelectric effect. In addition, a thin oxide layer was observed between the Ni gate contact and the AlGaN layer, and both Ni and oxygen had diffused into the AlGaN layer. After step-stress cycling, additional threading dislocations were observed.

High sensitivity carbon monoxide sensors made by zinc oxide modified gated GaN/AlGaN high electron mobility transistors under room temperature

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) with zinc oxide (ZnO) nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD) with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change in surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm in the open cavity with continuous gas flow using a 50×50 μm2 gate sensing area.

Semiconductor Device-Based Sensors for Gas, Chemical, and Biomedical Applications

Abstract: AlGaN/GaN Sensors for Direct Monitoring of Nerve Cell Response to Inhibitors, I. Cimalla, M. Gebinoga, A. Schober, V. Polyakov, V. Lebedev, and V. Cimalla Recent Advances in Wide Bandgap Semiconductor Biological and Gas Sensors, B.H. Chu, C.Y. Chang, S.J. Pearton, J. Lin, and F. Ren Advances in Hydrogen Gas Sensor Technology and Implementation in Wireless Sensor Networks, T.J. Anderson, Y.L. Wang, B.H. Chu, S.J. Pearton, J. Lin and F. Ren InN-Based Chemical Sensors, Y.-H. Chang, Y.-S. Lu, and J. A. Yeh, Y.-L. Hong, H.-M. Lee, and S. Gwo ZnO Thin Film and Nanowire Based Sensor Applications, Y.W. Heo, S.J. Pearton, D.P. Norton and F. Ren Bio-Affinity Sensors Based on MOS Field-Effect Transistors, D. Landheer, W.R. McKinnon, W.H. Jiang, G. Lopinski, G. Dubey, N.G. Tarr , M.W. Shinwari, and M.J. Deen MEMS-Based Optical Chemical Sensors, H. Xie and Z.M Qi

Improvement of Off-State Stress Critical Voltage by Using Pt-Gated AlGaN/GaN High Electron Mobility Transistors

Abstract: By replacing the commonly used Ni/Au gate metallization with Pt/Ti/Au, the critical voltage for degradation of AlGaN/GaN High Electron Mobility Transistors (HEMTs) during off-state biasing stress was significantly increased. The typical critical voltage for the HEMTs with Ni/Au gate metallization was around -55V. By sharp contrast, no critical voltage was observed for the HEMTs with Pt/Ti/Au gate metallization, even up -100V, which was the instrumental limitation in this experiment. Both Schottky forward and reverse gate characteristics of the Ni/Au degraded once the gate voltage passed the critical voltage of -55V. There was no degradation exhibited for the HEMTs with Pt/Ti/Au gate metallization.

10 MeV electrons irradiation effects in variously doped n-GaN

Abstract: We studied 10 MeV electron irradiation effects in a group of n-GaN films grown by standard metalorganic chemical vapor deposition (MOCVD) and by epitaxial lateral overgrowth (ELOG) techniques. The samples were either undoped or Si-doped, so that the shallow donor concentrations ranged from 1014 cm−3 to 3 × 1018 cm−3. It was found that electron irradiation led to the compensation of n-type conductivity and that the carrier removal rate substantially increased with an increase in the starting donor concentration. For the MOCVD samples, it was observed that the main compensating defect introduced by electrons was a 0.15 eV electron trap detected by admittance spectroscopy. Once the Fermi level crossed the level of these traps two other centers with activation energies of 0.2 and 1 eV were found to contribute to the compensation, so that after high doses, the Fermi level in moderately doped samples was pinned near Ec −1 eV. In ELOG samples the 0.15 eV electron traps were not detected. Instead only the 0.2 and...

Finite-element simulations of the effect of device design on channel temperature for AlGaN/GaN high electron mobility transistors

Abstract: The effect of device design on maximum channel temperature for AlGaN/GaN high electron mobility transistors was investigated through finite element thermal simulations for dissipated power densities up to 5 W mm−1. The effects of substrate material, die size, and number of gate fingers were examined, as well as the divergence between maximum channel temperatures for two dimensional and three dimensional simulations. The maximum temperature increased as the die size decreased beyond a critical distance from the gates. The critical distance was dependent on the substrate material, with SiC requiring the smallest critical distance. At the maximum dissipated power, the temperature increased above ambient more than 300 °C for devices on sapphire substrates compared to an increase of 44 °C for devices on SiC substrates. As the number of gate fingers increased, the maximum channel temperature also increased, and a temperature gradient was observed along the gate width as well as between gate fingers.

ZnO, GaN, and InN Functionalized Nanowires for Sensing and Photonics Applications

Abstract: A review is given of recent work in use of functionalized ZnO, GaN, and InN nanowires (NWs) as hydrogen gas and biological sensors, and the use of ZnO NWs with polymers in hybrid LEDs. For the biological sensors, specific examples of the detection of lactic acid and glucose by ZnO NWs are detailed. We also discuss self-consistent solutions of a coupled quantum-drift diffusion equation with a 3-D Poisson equation for simulations of the effects of sensing molecules on the surface of the ZnO NWs on the resulting conductance of the wires. The change in conductance is found to be a strong function of the channel length and diameter of the NWs and the simulations can be used as a method to optimize the sensor performance.

Effects of proton irradiation on dc characteristics of InAlN/GaN high electron mobility transistors

Abstract: The effects of proton irradiation on the dc characteristics of InAlN/GaN high electron mobility transistors were investigated. In this study we used 5 MeV protons with doses varying from 2 × 1011 to 2 × 1015 cm−2. The transfer resistance and contact resistivity suffered more degradation as compared to the sheet resistance. With irradiation at the highest dose of 2 × 1015 cm−2, both forward- and reverse-bias gate currents were increased after proton irradiation. A negative threshold-shift and reduction of the saturation drain current were also observed as a result of radiation-induced carrier scattering and carrier removal. Devices irradiated with doses of 2 × 1011 to 2 × 1015 cm−2 exhibited minimal degradation of the saturation drain current and extrinsic transconductance. These results show that InAlN/GaN high electron mobility transistors are attractive for space-based applications when high-energy proton fluxes are present.

Deep electron and hole traps in neutron transmutation doped n-GaN

Abstract: In neutron transmutation doped n-GaN, the electrical properties are found to be dominated not by shallow Ge donors produced by interaction of thermal neutrons with Ga, but by electron traps at 0.45 or 0.2 eV. The traps switch from the former to the latter when the anneal temperature increased from 800 to 1000 °C. The concentrations of both traps rose linearly with neutron fluence and were close to the concentration of Ge donors, suggesting they are Ge complexed with different radiation defects. The authors note the similarity of the properties of these traps to the properties of the dominant electron traps in as-irradiated n-GaN. They also observed prominent hole traps with a level near Ev+1.2 eV. These traps were not detected in virgin or as-irradiated samples. The concentration of the 1.2 eV hole traps increased linearly with neutron fluence, and these traps were assigned to Ga vacancy complexes with oxygen.

Annealing temperature dependence of Ohmic contact resistance and morphology on InAlN/GaN high electron mobility transistor structures

Abstract: Ti/Al/Ni/Au Ohmic contact metallization on InAlN/GaN heterostructures both with and without a thin GaN cap layer was annealed at different temperatures. The minimum transfer resistance for the contacts of 0.65 Ω mm (specific contact resistivity of 2×10−5 Ω cm2) was achieved after 800 °C annealing for structures without the GaN cap, while those with the cap exhibited their lowest resistance at higher temperatures. The contact morphology showed considerable roughening by 750 °C but the carrier mobility was stable until annealing temperatures of 850 °C. Diffuse scattering experiments showed that the morphological roughness of the InAlN/GaN interface increased as a result of annealing at these temperatures and the data were consistent with outdiffusion of Ga into the InAlN. Unpassivated high electron mobility transistors with a gate dimension of 0.7×180 μm2 were fabricated using these contacts and showed a maximum drain current of 1.3 A/mm and an extrinsic transconductance of 366 mS/mm. The presence of the G...

Comparison of passivation layers for AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors require surface passivation layers to reduce the effects of surface traps between the gate and drain contacts. These traps lead to the creation of a virtual gate and the associated collapse of drain current under rf conditions. The authors have investigated three different materials for passivation layers, namely thin (7.5 nm) Al2O3 and HfO2 deposited with an atomic layer deposition system and conventional, thick (200 nm) plasma enhanced chemically vapor deposited SiNX. The latter is found to be the most effective in reducing drain current loss during gate lag measurements in both single and double pulse mode, but also reduces fT and fMAX through additional parasitic capacitance.

Large-area suspended graphene on GaN nanopillars

Abstract: The authors have demonstrated large-area suspended graphene on GaN nanopillars predefined by nanosphere lithography and inductively coupled plasma etching. The graphene was successfully suspended over large areas without ripples and corrugations. Scanning electron microscopy, atomic force microscopy and micro-Raman spectroscopy were used to characterize the properties of the suspended graphene on nanopillars. The thermal properties of the suspended and supported graphene were investigated by varying the underlying GaN nanopilllar geometries from flat-top to sharp-cone morphologies and heating the resulting structures via irradiation with laser powers of 1.53 mW, 8.03 mW, and 16.19 mW. The heat transfer was effective even when the contact area between the suspended graphene and the supporting substrate was small, due to the high thermal conductivities of graphene and GaN. The extremely high thermal conductivity of the graphene can improve the thermal management in GaN-based high power electronic and optoel...

Effect of temperature on CO detection sensitivity of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors

Abstract: The carbon monoxide (CO) detection sensitivities of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors were measured over a range of temperatures from 25–150 °C. Once the sensor was exposed to the CO-containing ambient, the drain current, I, of the high electron mobility transistors increased due to chemisorbed oxygen on the ZnO surface reacting with CO, forming CO2 and releasing electrons to the oxide surface. Although the sensor could detect CO as low as 100 ppm at room temperature, the detection sensitivity, ΔI/I, was only around 0.23%. By increasing the sensor temperature to 150 °C, the detection sensitivity was improved by a factor of over 30% to 7.5%.

Investigating the effect of off-state stress on trap densities in AlGaN/GaN high electron mobility transistors

Abstract: The temperature dependence of sub-threshold drain current versus gate voltage at a constant drain bias voltage were used to determine the trap densities in AlGaN/GaN high electron mobility transistors (HEMTs) before and after the off-state stress. Two different trap densities were obtained for the measurements conducted at 300–493 K and 493–573 K, respectively. The trap density at the lower temperature range almost doubled from 1.64 × 1012 to 3.3 × 1012/cm2–eV after a critical voltage for degradation of HEMTs was reached during the off-state drain voltage step-stress. The trap density at the higher temperature range only slightly increased from 8.1 × 1012 and 9.2× 1012/cm2–eV after the device stress. The trap densities were also strongly dependent on drain bias voltage; measurements conducted at higher drain bias voltages exhibited larger trap density due to more hot electrons generated at these conditions.

Effects of silicon nitride passivation on isolation-blocking voltage in algan/gan high electron mobility transistors

Abstract: The effects of plasma enhanced vapor deposited silicon nitride (SiNx) passivation layer thickness and the spacing between the contact windows openings in the SiNx layer on the isolation-blocking voltage of nitrogen ion implanted AlGaN/GaN high electron mobility transistors were studied. The isolation-blocking voltage was proportional to the thickness of the SiNx passivation layer. Early breakdown was observed for the samples without thick enough SiNx due to surface breakdown. The device was permanently damaged after the occurrence of this early breakdown. The dependence of the isolation-blocking voltage on the SiNx thickness was also modeled and the general trends of the simulated results were in good agreement with the experiment data. The effect of rf power used for depositing the SiNx layer on the isolation-blocking voltage was also studied. Ion bombardments during the SiNx deposition could cause the reduction of breakdown voltage. By employing optimized SiNx passivation conditions, a saturation drain ...