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We report on a simple and reproducible method for fabricating InGaN/GaN multi-quantum-well (MQW) nanorod light-emitting diodes (LEDs), prepared by combining a SiO 2 nanosphere lithography and dry-etch process. Focused-ion-beam (FIB)-deposited Pt was contacted to both ends of the nanorod LEDs, producing bright electroluminescence from the LEDs under forward bias conditions. The turn-on voltage in these nanorod LEDs was higher (13 V) than in companion thin film devices (3 V) and this can be attributed to the high contact resistance between the FIB-deposited Pt and nanorod LEDs and the damage induced by inductively-coupled plasma and Ga + -ions. Our method to obtain uniform MQW nanorod LEDs shows promise for improving the reproducibility of nano-optoelectronics.

The applicability of using Electron Beam Induced Current (EBIC) measurements on Schottky barriers to obtain the mean electron–hole pair creation energy in β-Ga2O3 is reported. It is shown that, when combined with Monte Carlo simulation, this approach yields for Si, GaN, and 4H–SiC a data set consistent with empirical expressions proposed earlier in the literature for many different semiconductors. The method is then applied to β-Ga2O3, where complications related to hole trapping in the material give rise to a strong gain in EBIC and have to be carefully treated and taken into account. When this is done, the mean electron–hole pair energy formation is found to be 15.6 eV, in reasonable agreement with the values predicted by empirical expressions. [ABSTRACT FROM AUTHOR]

The performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with either uncapped surfaces or with low-temperature (LT) GaN or SiO2 or SiNx as gate insulators is reported. The sheet carrier concentrations of AlGaN/GaN HFETs with any of these surface insulating layers are similar to each other and in each case about 50% higher than that in an AlGaN/GaN HFET with a free surface. This result is consistent with the insulator layers providing passivation of surface states that cause the depletion of the channel layer. Due to the closer lattice match with the AlGaN surface layer, the HFET with a LT-GaN layer as the gate insulator shows the best dc and rf device performance, demonstrating that this material is an effective insulator for nitride electronic devices. [ABSTRACT FROM AUTHOR]

Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. The mechanism for the observed magnetic behavior is complex and appears to depend on a number of factors, including Mn-Mn spacing, and carrier density and type. For example, in a simple Ruderman-Kittel-Kasuya-Yosida carrier-mediated exchange mechanism, the free-carrier/Mn ion interaction can be either ferromagnetic or antiferromagnetic depending on the separation of the Mn ions. Potential applications for ferromagnetic semiconductors and oxides include electrically controlled magnetic sensors and actuators, high-density ultralow-power memory and logic, spin-polarized light emitters for optical encoding, advanced optical switches and modulators and devices with integrated magnetic, electronic and optical functionality. [ABSTRACT FROM AUTHOR]

Presents as study that discussed how carbon and tin can be incorporated during growth by metalorganic molecular beam epitaxy in order to produce p-type n-type p-type semiconductor structures. Experimental procedure; Results and discussion; Conclusion.

Describes a single-energy, implant isolation scheme for thick semiconductor device structures such as heterojunction bipolar transistors. Advantages of III-V semiconductors over silicon-based technologies; Details on the experiment; Discussion on the results of the study.

Abstract: There has been significant recent interest in the use of surface-functionalized thin film and nanowire wide bandgap semiconductors, principally GaN, InN, ZnO and SiC, for sensing of gases, heavy metals, UV photons and biological molecules. For the detection of gases such as hydrogen, the semiconductors are typically coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions. The wide energy bandgap of these materials make them ideal for solar-blind UV detection, which can be of use for detecting fluorescence from biotoxins. The use of enzymes or adsorbed antibody layers on the semiconductor surface leads to highly specific detection of a broad range of antigens of interest in the medical and homeland security fields. We give examples of recent work showing sensitive detection of glucose, lactic acid, prostate cancer and breast cancer markers and the integration of the sensors with wireless data transmission systems to achieve robust, portable sensors. [Copyright &y& Elsevier]

Wide band gap semiconductors such as GaN and ZnO have continued to be at the forefront of spintronics research due to the demonstration of room-temperature ferromagnetism in these materials. A goal of this research is the fabrication of a spintronic device that may provide easy incorporation into existing GaN and ZnO technologies, higher integration density, and less power consumption that its electronic counterparts while achieving similar speeds. The progression of the ferromagnetic metals incorporated into GaN has moved from transition metals to rare earth metals such as the lanthanides. The properties of transition metal-doped ZnO thin films for the two most studied transition metal dopants, namely Mn and Co, are also discussed in this review. [ABSTRACT FROM AUTHOR]

Transparent dual-gate (DG) InGaZnO4 thin film transistors for OR logic operation were fabricated on a glass substrate. A 100-nm-thick SiO2 layer used as both top and bottom gate dielectrics was deposited by plasma enhance chemical vapor deposition at 200 °C. Compared to bottom gate, top gate thin film transistors (TFTs) exhibited better device performance with higher saturation mobility, drain current on-to-off ratio, lower threshold voltage, and subthreshold gate-voltage swing. This improved performance was mainly attributed to low process-induced damage or low parasitic capacitance between gate and source/drain and low parasitic resistance between channel and source/drain in top-contact TFT configuration (coplanar type). DG-mode TFTs showed saturation mobility of ∼16.9 cm2 V-1 s-1, drain current on-to-off ratio of ∼1×106, subthreshold gate-voltage swing of ∼0.33 V decade-1, and threshold voltage of ∼1.25 V. The results demonstrate that DG InGaZnO4 TFTs are effective in improving the device performance because the channel layer is modulated independently by a top or, bottom gate signal and are well suited for OR gate operation. [ABSTRACT FROM AUTHOR]

A robust method for creating Cu-plated through-wafer vias for AlGaN/GaN high electron mobility transistors on Si is reported. The initial 70 μm deep vias with diameters of 50 μm are created by deep Si reactive ion etching, followed by pulsed Cu electroplating and mechanical polishing to planarize the metal. This is an attractive approach for increasing the effective thermal conductivity of the composite substrate for very high power device applications. [ABSTRACT FROM AUTHOR]

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb-Sb-Ag-Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10-6 K-1 and 24 × 10-6 K-1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi2Te3. The CTE of the LAST specimens with a higher Ag content (Ag0.86Pb19Sb1.0Te20) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag0.43Pb18Sb1.2Te20). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the cast and hot-pressed LAST with the Ag0.86Pb19Sb1.0Te20 composition, whereas the CTE of the Ag0.43Pb18Sb1.2Te20 specimen increased monotonically with temperature. [ABSTRACT FROM AUTHOR]

In the open literature, reports of mechanical properties are limited for semiconducting thermoelectric materials, including the temperature dependence of elastic moduli. In this study, for both cast ingots and hot-pressed billets of Ag-, Sb-, Sn- and S-doped PbTe thermoelectric materials, resonant ultrasound spectroscopy (RUS) was utilized to determine the temperature dependence of elastic moduli, including Young's modulus, shear modulus and Poisson's ratio. This study is the first to determine the temperature-dependent elastic moduli for these PbTe-based thermoelectrics, and among the few determinations of elasticity of any thermoelectric material for temperatures above 300 K. The Young's modulus and Poisson's ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Young's modulus, E, versus temperature, T, relationship, E(T) = E0(1-bT), is consistent with predictions for materials in the range well above the Debye temperature. A nanoindentation study of Young's modulus on the specimen faces showed that both the cast and hot-pressed specimens were approximately elastically isotropic. [ABSTRACT FROM AUTHOR]

Abstract: Compositional changes (alloying) in metals, ceramics and semiconductors lead to changes in microstructure and mechanical properties that range from subtle to dramatic. In particular, hardness measurements have been widely used to track changes in mechanical properties as a function of composition. In this study, composition dependence of the Vickers indentation hardness, H, is examined for 17 specimens that include 15 different n-type compositions within the quaternary chalcogenide family of LAST (lead-antimony-silver-tellurium) high-temperature thermoelectric materials. In addition, nanoindentation measurements were made on six of the 17 specimens that were indented via Vickers indentation. The H values obtained independently by the microindentation (Vickers indentation) and nanoindentation techniques agreed very well. In this study, relationships from the literature for the parabolic dependence of H on the single compositional variable x for pseudo binaries AB1−x C x is successfully extended to describe the composition dependence of H for a true quaternary system. Possible implications of hardness change on fabrication and in-service reliability are also discussed. [Copyright &y& Elsevier]

The use of Ir diffusion barriers in Ni/Au-based Ohmic contacts to p-type CuCrO2 layers was investigated. A specific contact resistance of ~5 x 10-4 Ω cm² was achieved after annealing at 500°C for the Ir-containing contacts, and the contacts were rectifying for lower anneal temperatures. In this case, the contact resistance was basically independent of the measurement temperature, indicating that tunneling is the dominant transport mechanism in the contacts. The morphology for the Ir-containing contacts was still smooth at 500°C although Auger electron spectroscopy depth profiling showed that some of the nickel had diffused to the surface and had oxidized. Contacts annealed at 800°C showed that some copper and most of the nickel had diffused to the surface and oxidized. The presence of the Ir diffusion barrier does increase the thermal stability of the contacts by ~200°C compared to conventional Ni/Au contacts. By contrast, the use of other materials such as TaN, ZrN, and W2B5 as the diffusion barrier led to poorer thermal stability, with the contact resistance increasing sharply above 400°C. [ABSTRACT FROM AUTHOR]

Ir/Au Schottky contacts and Ti/Al/Jr/Au ohmic contacts on n-type GaN were investigated as a function of annealing temperature and compared to their more common Ni-based counterparts. The Jr/Au ohmic contacts on n-type GaN with n ≈ 1017 cm-3 exhibited barrier heights of 0.55 eV after annealing at 700°C and displayed less intermixing of the contact metals compared to Ni/Au. A minimum specific contact resistance of 1.6 x 10-6 Ωcm-2 was obtained for the ohmic contacts on n-type GaN with n ≈ 1018 cm-3 after annealing at 900°C. The measurement temperature dependence of contact resistance was similar for both Ti/Al/Ir/Au and Ti/Al/Ni/Au, suggesting the same transport mechanism was present in both types of contacts. The Ir-based ohmic contacts displayed superior thermal aging characteristics at 350°C. Auger electron spectroscopy showed that Ir is superior to Ni as a diffusion barrier at these moderate temperatures. [ABSTRACT FROM AUTHOR]

The optical and electrical properties and surface morphology of sputtered, E-beam deposited, and composite E-beam/sputtered indium-tin-oxide (ITO) films for metal-semiconductor-metal (MSM) photodetector applications were compared. The resistivity of sputtered ITO was almost 2 orders of magnitude less than that of E-beam-deposited ITO, while the resistivity of the composite films was only 15% higher than that of sputtered films. The transmittance at 1.55 μm of sputtered and composite ITO films was 83 and 77%, respectively, which was much higher than the value of 13% for the E-beam-deposited ITO. The composition of ITO films for both deposition techniques was very similar, as determined from Auger electron spectroscopy surface scans and depth profiles. The sputtered films showed much smoother surfaces and smaller grain size. lnGaAs-based MSM devices with ITO fingers have been fabricated and showed higher photo response than that of MSM devices with conventional Ti/Au fingers. [ABSTRACT FROM AUTHOR]

MgCaO offers promise as a gate dielectric for GaN-based metal-oxide-semiconductor (MOS) transistors, particularly in combination with environmentally stable capping layers such as Sc2O3. X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the valence band (ΔEv) of Mg0.5Ca0.5O/GaN heterostructures in which the MgCaO was grown by rf plasma-assisted molecular beam epitaxy on top of thick GaN templates on sapphire substrates. A value of ΔEv = 0.65 eV ± 0.10 eV was obtained by using the Ga 3d energy level as a reference. Given the bandgap of 7.45 eV for the MgCaO, we infer a conduction band offset ΔEC of 3.36 eV in the Mg0.5Ca0.5O system. [ABSTRACT FROM AUTHOR]

AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/TiB2/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB2/Au or Ni/TiB2/Au) and subjected to long-term annealing at 350°C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride-based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB2/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on Si on poly-SiC substrates formed by the Smart Cut™ process. The Smart Cut™ approach is an alternative solution to provide both a high resistivity and an excellent thermal conductivity template needed for power applications. Although the structure has not been optimized, devices with 0.7 μm gate length show breakdown voltage of >250 V, fT of 18 GHz, and fmax of 65 GHz. [ABSTRACT FROM AUTHOR]

The photon-emission efficiencies and photon indistinguishabilities of a single-photon source, which employs a cavity coupled with a quantum dot, are studied under above-band and resonant excitations. The results are obtained by solving master equations and by applying the quantum regression theorem. According to the study, the photon indistinguishability increases with the Purcell factor under resonant excitation, which is consistent with the increase in emission efficiency; however, these two figures of merit are inconsistent for the above-band excitation scheme. Moreover, the efficiencies, defined as the average photon number emitted in one excitation cycle, are almost the same for the two different excitation schemes, whereas the excitation power needed to reach that efficiency is much lower under resonant excitation than that for above-band excitation. These results will be helpful in improving the performances of the applications concerning indistinguishability and efficiency. [ABSTRACT FROM AUTHOR]

AlMnN and AlCrN have been synthesized by gas-source molecular beam epitaxy (GSMBE). Using optimized growth conditions and compositions, sccm films as determined by x-ray diffraction (XRD) and transmission electron microscopy, which also show room-temperature magnetic behavior were obtained for both materials. Chromium was found to produce greater magnetic ordering as evidenced by a higher technical saturation. The AlCrN also exhibited a higher remanent magnetization and a M versus T behavior more typical of ferromagnetism than that observed for AlMnN. These results suggest that Cr is a superior dopant for formation of AlN-based, dilute magnetic semiconductors. [ABSTRACT FROM AUTHOR]

BCl3/Ar and BCl3/N2 plasma chemistries were compared for patterning of InP, InAs, InSb, InGaAs, InGaAsP, and AlInAs. Under electron cyclotron resonance conditions etch rates in excess of 1 µm/min can be achieved at room temperature with low additional rf chuck power (150 W). The etch rates are similar for both chemistries, with smoother surface morphologies for BCl3/Ar. However, the surfaces are still approximately an order of magnitude rougher (as quantified by atomic force microscopy) than those obtained under the same conditions with Cl2/Ar. InP surfaces etched at high BCl3-to-Ar ratios have measurable concentrations of boron-and chlorine-containing residues. [ABSTRACT FROM AUTHOR]

Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from -3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect kidney injury molecule-1 (KIM-1), an important biomarker for early kidney injury detection. The gate region consisted of 5 nm gold deposited onto the AlGaN surface. The gold was conjugated to highly specific KIM-1 antibodies through a self-assembled monolayer of thioglycolic acid. The HEMT source-drain current showed a clear dependence on the KIM-1 concentration in phosphate-buffered saline solution. The limit of detection was 1 ng/ml using a 20×50 μm2 gate sensing area. This approach shows potential for both preclinical and clinical kidney injury diagnosis with accurate, rapid, noninvasive, and high throughput capabilities. [ABSTRACT FROM AUTHOR]

The use of TiB2-based Ohmic contacts on Pt-gate AlGaN/GaN heterostructure diode hydrogen sensors is shown to provide very stable operation for detection of 1% H2 in air under field conditions where temperature is allowed to vary. In contrast, the use of more conventional Ti/Al/Pt/Au Ohmic contacts led to higher background variations in current that affect the ultimate detection threshold of the sensors. Combined with a differential pair geometry that compares current from an active diode with Pt-gate contact and a passive diode with Ti/Au gate, the more stable TiB2-based Ohmic contacts reduce false alarms due to ambient temperature changes. The diodes exhibit a change in forward current of more than 1 mA at 1.5 V when 1% H2 is introduced into an air ambient. [ABSTRACT FROM AUTHOR]

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 -5.5 to -6.5 V 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 ∼4.5 cm2 V-1 s-1, lower than the Hall mobility of ∼17 cm2 V-1 s-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. [ABSTRACT FROM AUTHOR]

Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated. While the current-voltage characteristics from Ti/Au contacts showed back-to-back Schottky behavior, a specific contact resistance of ∼1×10-4 Ω cm2 was achieved by using Ni instead of Ti. The contact resistivity was fairly independent of measurement temperature, suggesting that tunneling is the dominant transport mechanism. The contact resistance remained practically constant upon annealing in the 100–400 °C range. Above 500 °C, the morphology became rough and the contact showed rectifying behavior. This degradation resulted from both the out-diffusion of oxygen and the in-diffusion of Ni and Au in CuCrO2. [ABSTRACT FROM AUTHOR]

An enhancement of ∼0.18 eV (an 18% increase) in Schottky barrier height was obtained for Au deposited at cryogenic temperatures on n-type GaN relative to conventional deposition at 300 K (barrier height of 1.0 eV). Enhancements of 0.04–0.11 eV were achieved for Au deposition on p-GaN under the same conditions. The increase in barrier height on n-GaN persists for annealing temperatures up to ∼200 °C. At higher annealing temperatures, both types of diodes show a deterioration in rectifying behavior. The reverse current of low temperature deposited diodes was approximately two orders of magnitude lower than conventional Au/n-GaN diodes. The ideality factor of the cryogenically processed n-type devices (∼1.06) was similar to that for room temperature diodes (1.13). This simple process method has potential for improving output resistance and power gain and lowering gate leakage current and noise in GaN-based transistors. [ABSTRACT FROM AUTHOR]

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 ∼383 nm as well as peaks at 430, 640, and 748 nm. The threshold bias for UV light emission was <3 V, corresponding to a current density of 6.08 A cm-2 through the PEDOT/PSS at 3 V. [ABSTRACT FROM AUTHOR]

N+ ion implantation at moderate doses (1013–1014 cm-2) into nominally undoped (n∼1017 cm-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 300 K and band-edge electroluminescence at 120 K (∼390 nm) 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 25 Ω due to the use of the conducting ZnO substrate. [ABSTRACT FROM AUTHOR]

The annealing temperature (25–800 °C) dependence of ohmic contact characteristics on p-GaN using a W2B/Ti/Au metallization scheme deposited by sputtering are reported. The contacts are rectifying in the as-deposited condition but become ohmic for annealing at >=500 °C. A minimum specific contact resistivity of 1.7×10-3 Ω cm-2 was obtained after annealing at 800 °C for 60 s. Higher annealing temperatures produced sharp increases in the resistivity of the GaN and irreproducible contact properties. However, the contact morphology was similar over the entire annealing range used here. Auger electron spectroscopy profiling showed the onset of Ti out-diffusion through the Au at 500 °C. By 800 °C the Ti was almost completely removed to the surface, where it became oxidized. These boride-based contacts have superior thermal stability to the more common Ni/Au, whose morphology degrades significantly above 500 °C. [ABSTRACT FROM AUTHOR]

A comparison of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn0.05Cd0.95O layers grown on ZnO buffer layers on GaN/sapphire templates showed a minimum contact resistivity of 2.3×10-4 Ω cm2 at 500 °C anneal temperature for Ti/Al/Pt/Au and 1.6×10-4 Ω cm2 at 450 °C for Ti/Al. The morphology of the Ti/Al/Pt/Au contacts showed much better thermal stability and remained smooth until at least 450 °C, whereas the Ti/Au contacts show a reacted appearance after 350 °C anneals. Auger electron spectroscopy depth profiling of the contact schemes as a function of anneal temperature suggests that the formation of TiOx phases that induce oxygen vacancies in the ZnCdO are responsible for the improved contact resistance after annealing in both types of metal schemes. [ABSTRACT FROM AUTHOR]

Electron-beam-deposited Ti/Au ohmic contacts on undoped (n∼1017 cm-3) bulk ZnO substrates exhibited as-deposited specific contact resistivity of 3×10-4 Ω cm2, regardless of the polarity (Zn face or O face) of the ZnO substrate. The annealing environment (air or N2) also had no significant effect on contact properties. The specific contact resistivity slightly decreased after annealing at 300 °C but started to increase above 350 °C. The measurement temperature dependence of specific contact resistivity revealed that the dominant current transport mechanism is field emission even in the moderately doped ZnO. As the annealing temperature increased, some voids were observed on the metal surface, possibly due to reaction of Ti/Au metallization and the evaporation of the oxygen from the ZnO substrate. [ABSTRACT FROM AUTHOR]

Pt-gated AlGaN/GaN high electron mobility transistors can be used as room-temperature hydrogen gas sensors at hydrogen concentrations as low as 100 ppm. A comparison of the changes in drain and gate current-voltage (I-V) characteristics with the introduction of 500 ppm H2 into the measurement ambient shows that monitoring the change in drain-source current provides a wider gate voltage operation range for maximum detection sensitivity and higher total current change than measuring the change in gate current. However, over a narrow gate voltage range, the relative sensitivity of detection by monitoring the gate current changes is up to an order of magnitude larger than that of drain-source current changes. In both cases, the changes are fully reversible in <2–3 min at 25 °C upon removal of the hydrogen from the ambient. [ABSTRACT FROM AUTHOR]

The transport properties of single InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of both length/square of radius ratio and temperature. The resistivity of the n-type InN nanowires with diameter >100 nm was measured by the transmission line method and the value was on the order of 4×10-4 Ω cm. The specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10-7 Ω cm2. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) both with and without a Au gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various halide ions. The polar nature of the halide ions leads to a change of surface charge in the gate region on the HEMT, producing a change in the surface potential at the semiconductor/liquid interface. HEMTs with a Au-gate electrode not only doubled the sensitivity of changing the channel conductance as compared to gateless HEMT, but also showed the opposite conductance behavior. When anions adsorbed on the Au, they produced a counter charge for electrovalence. These anions drag some counter ions from the bulk solution or create an image positive charge on the metal for the required neutrality. The gateless HEMTs can be used as sensors for a range of chemicals through appropriate modification with covalently bonded halide functional groups on the Au surface. This creates many possibilities to functionalize the surface for a wide range of integrated biological, chemical, and fluid monitoring sensors. [ABSTRACT FROM AUTHOR]

Zn0.9Mg0.1O/ZnO p-n junctions were grown by pulsed laser deposition at ≤500 °C on bulk n-type, (100), nonpolar, a-plane ZnO substrates. No postgrowth annealing was performed, with the P-doped ZnMgO showing p-type conductivity (hole density ∼1016 cm-3, mobility ∼6 cm2 V-1 s-1) in the as-grown state. Front-to-back p-n junctions were fabricated with Ni/Au used as the p-Ohmic contact and Ti/Au as the backside n-Ohmic contact. The p contacts showed improved characteristics after annealing up to 400 °C, but the n contacts were Ohmic as deposited. The junctions showed rectifying behavior up to 200 °C. The forward turn-on voltage was ∼6.5 V at 25 °C. The simple, low-temperature growth and processing sequence show the promise of ZnO for applications in transparent electronics and UV light emitters. [ABSTRACT FROM AUTHOR]

Single ZnO nanorods with Ohmic contacts at either end exhibit large changes in current upon exposing the surface region to polar liquids introduced through an integrated microchannel. The polar nature of the electrolyte introduced led to a change of surface charges on the nanorod, producing a change in surface potential at the semiconductor/liquid interface. The nanorods exhibit a linear change in conductance between pH 2 and 12 of 8.5 nS/pH in the dark and 20 nS/pH when illuminated with ultraviolet (365 nm) light. The nanorods show stable operation with a resolution of ∼0.1 pH over the entire pH range. The results indicate that ZnO nanorods may have applications in integrated chemical, gas, and fluid monitoring sensors. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source/drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN/GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of -(+)6.4×10-2 mS/bar for application of compressive (tensile) strain. The AlGaN/GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications. [ABSTRACT FROM AUTHOR]

Single ZnO nanorods with diameters of ∼130 nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25 to 150 °C, the resistivity of nanorods treated in H2 at 400 °C prior to measurement showed an activation energy of 0.089±0.02 eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp 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. [ABSTRACT FROM AUTHOR]

Pt contacted AlGaN/GaN high electron mobility transistors with Sc2O3 gate dielectrics show reversible changes in drain–source current upon exposure to H2-containing ambients, even at room temperature. The changes in current (as high as 3 mA for relatively low gate voltage and drain–source voltage) are approximately an order of magnitude larger than for Pt/GaN Schottky diodes and a factor of 5 larger than Sc2O3/AlGaN/GaN metal–oxide–semiconductor (MOS) diodes exposed under the same conditions. This shows the advantage of using a transistor structure in which the gain produces larger current changes upon exposure to hydrogen-containing ambients. The increase in current is the result of a decrease in effective barrier height of the MOS gate of 30–50 mV at 25 °C for 10% H2/90% N2 ambients relative to pure N2 and is due to catalytic dissociation of the H2 on the Pt contact, followed by diffusion to the Sc2O3/AlGaN interface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm-1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm-1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ohmic and Schottky contacts to p-type Zn[sub 0.9]Mg[sub 0.1]O are reported. The lowest specific contact resistivity of 3×10[sup -3] Ω cm[sup 2] was obtained for Ti/Au annealed at 600 °C for 30 s. Ni/Au was less thermally stable and showed severe degradation of contact morphology at this annealing temperature. Both Pt and Ti with Au overlayers showed rectifying characteristics on p-ZnMgO, with barrier heights of ∼0.55–0.56 eV and ideality factors of ∼1.9. Comparison of these results with the same metals on n-type ZnO indicates that high surface state densities play a significant role in determining the effective barrier height. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurement of the electrical characteristics of 250 devices on the same 2 in. diameter wafer shows that Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors (HEMTs) have lower average specific contact resistance after annealing at 850 °C for 30 s (4.6×10[sup -5] Ω cm[sup 2]) compared to more standard Ti/Al/Ni/Au contacts (2×10[sup -4] Ω cm[sup 2]). HEMTs with these Ir-based contacts also show average interdevice isolation currents approximately a factor of 2 lower, higher peak transconductance (134 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 V compared to 23 V) than the devices with Ni-based contacts. This Ir-based contact metallurgy looks promising for applications requiring extended thermal stability of the HEMTs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]