Lars Voss

Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods

A comparison is made of the sensitivities for detecting hydrogen with Pt-coated single ZnO nanorods and thin films of various thicknesses (20–350 nm). The Pt-coated single nanorods show a current response of approximately a factor of 3 larger at room temperature upon exposure to 500ppmH2 in N2 than the thin films of ZnO. The power consumption with both types of sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the μW range. The optimum ZnO thin film thickness under our conditions was between 40–170 nm, with the hydrogen sensitivity falling off outside this range. The nanorod sensors show a slower recovery in air after hydrogen exposure than the thin films, but exhibit a faster response to hydrogen, consistent with the notion that the former adsorb relatively more hydrogen on their surface. Both ZnO thin and nanorods cannot detect oxygen.

Characterization of bulk GaN rectifiers for hydrogen gas sensing

Pd and Pt Schottky diodes were fabricated on free-standing 2-in.-diameter GaN substrates prepared by a combination of hydride vapor phase epitaxy of ∼350μm onto sapphire, substrate removal and subsequent growth of 3μm of epi GaN by metalorganic chemical vapor deposition. Vertical diodes with Ti∕Al∕Pt∕Au back contacts annealed at 850°C for 30s showed excellent rectification with an on/off ratio of ∼100 at 1.5V∕−10V. Both forward turn-on and reverse breakdown voltages showed negative temperature coefficients. Pd and Pt diodes showed detection of 10ppm H2 in N2 at 25°C, with fast (<10s) recovery times upon removal of hydrogen from the measurement ambient. The Pt showed higher detection sensitivity than Pd. Detection of C2H4 and C2H6 required much higher temperatures (∼450°C) and concentrations (10%) of the gases in N2 than hydrogen detection.

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

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.

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

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.

Improved long-term thermal stability of InGaN∕GaN multiple quantum well light-emitting diodes using TiB2- and Ir-based p-Ohmic contacts

InGaN∕GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni∕Au∕TiB2∕Ti∕Au or Ni∕Au∕Ir∕Au p-Ohmic contacts and annealed at 200 and 350°C for 45days. By comparison with companion devices with conventional Ni∕Au Ohmic contacts fabricated on the same wafer, MQW-LEDs with TiB2- and Ir-based Ohmic metallization schemes showed superior long-term thermal stability as judged by the change in turn-on voltage, leakage current, and output power, a promising result for applications where reliable operation at high temperature is required.

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

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

Electrical Performance of GaN Schottky Rectifiers on Si Substrates

Schottky rectifiers fabricated on GaN layers grown on 4 in. diam Si substrates show breakdown voltages (V R ) of ∼ 300 V at room temperature, on-state resistances (R ON ) of 40 mΩ cm 2 , and figure-of-merit (V B ) 2 /R ON of 2.25 MW/cm 2 . The reverse current is thermally activated with activation energies in the range 0.3-0.4 eV and is proportional to contact perimeter at reverse biases up to ∼100 V. This approach provides a low-cost alternative to GaN rectifiers on sapphire or SiC substrates while still maintaining good breakdown characteristics.

Improved thermally stable ohmic contacts on p-GaN based on W2B

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 1.7×10−3Ωcm−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.

AlGaN∕GaN high electron mobility transistors on Si∕SiO2/poly-SiC substrates

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 0.7μm gate length show breakdown voltage of >250V, fT of 18GHz, and fmax of 65GHz.

Increased Schottky barrier heights for Au on n- and p-type GaN using cryogenic metal deposition

An enhancement of ∼0.18eV (an 18% increase) in Schottky barrier height was obtained for Au deposited at cryogenic temperatures on n-type GaN relative to conventional deposition at 300K (barrier height of 1.0eV). Enhancements of 0.04–0.11eV 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.