P.W. Sadik

Hydrogen-selective sensing at room temperature with ZnO nanorods

The sensitivity for detecting hydrogen with multiple ZnO nanorods is found to be greatly enhanced by sputter-depositing clusters of Pd on the surface. The resulting structures show a change in room- temperature resistance upon exposure to hydrogen concentrations in N2 of 10–500ppm of approximately a factor of 5 larger than without Pd. Pd-coated ZnO nanorods detected hydrogen down to 2.6% at 10ppm and >4.2% at 500ppm H2 in N2 after a 10min exposure. There was no response at room temperature to O2. Approximately 95% of the initial ZnO conductance after exposure to hydrogen was recovered within 20s by exposing the nanorods to either air or pure O2. This rapid and easy recoverability make the Pd-coated nanorods suitable for practical applications in hydrogen-selective sensing at ppm levels at room temperature with <0.4mW power consumption.

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.

Structure and magnetism of cobalt-doped ZnO thin films

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions. Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at.%. The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice. Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry. Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component. Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD. These particles are well oriented with the ZnO crystal structure. By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~0.08??B/Co. Depositions in the presence of oxygen background gas at 0.02?mTorr decreases the magnetization. The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth. Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO. Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation. Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out.

Functionalizing Zn- and O-terminated ZnO with thiols

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

Room temperature deposited oxide p-n junction using p-type zinc-cobalt-oxide

Oxide semiconductors are attractive materials for thin-film electronics and optoelectronics due to compatibility with synthesis on large-area, inexpensive glass and flexible plastic substrate. However, development of thin-film electronics has been hampered by the limited number of semiconducting oxides that are p-type. Here, we report on the properties of zinc-cobalt-oxide (Zn–Co–O) films, deposited at room temperature using pulsed laser deposition, that exhibit p-type conduction. Films are deposited at room temperature in a background of oxygen using a polycrystalline ZnCo2O4 ablation target. The p-type conduction is confirmed by positive Seebeck coefficient and positive Hall coefficient. Both electrical resistivity and carrier density are dependent on oxygen background pressure used during deposition. Zn–Co–O films deposited in 50 mTorr oxygen pressure appear to be amorphous based on x-ray diffraction, and show an electrical conductivity as high as 21 S cm−1. Distinct rectifying current-voltage characte...

Ni∕Au Ohmic contacts to p-type Mg-doped CuCrO2 epitaxial layers

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 ∼1×10−4Ωcm2 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.

Comparative study of ZrN and Zr–Ge–N thin films as diffusion barriers for Cu metallization on Si

The diffusion barrier properties of Zr–Ge–N and ZrN thin films for Cu metallization on Si have been examined. Both Zr–Ge–N and ZrN thin films deposited at room temperature via reactive magnetron sputtering are amorphous. X-ray diffraction of annealed Cu/nitride/Si structures indicates that a 50nm thick ZrN film is an effective diffusion barrier for annealing at 600°C for 1h. In contrast, Zr–Ge–N failed as a diffusion barrier upon annealing at 600°C as evidenced by the appearance of copper silicide diffraction peaks. Cross-section transmission electron microscopy also showed the formation of Cu3Si crystallites at the annealed Zr–Ge–N barrier interfaces. For ZrN barriers annealed at 600°C, the integrity of the nitride/Si interface is preserved and the energy-dispersive spectrometry line scan showed no Cu diffusion through the barrier into Si substrate. The result indicated that ZrN is superior to Zr–Ge–N as a copper diffusion barrier on Si.

Reaction-Limited Wet Etching of CuCrO2

An (NH4) 2 Ce(N03) 6 (/HNO 3 /H 2 O solution was used to obtain wet etch rates at 25°C in the range 500-1000 A min -1 for CuCrO 2 grown on sapphire substrates by pulsed laser deposition. The etching was reaction-limited, with an activation energy of 11.9 kCal mol -1 . Under these conditions, the etching of ZnO grown in a similar fashion was much faster (∼5 μm min -1 ), providing highly selective removal of ZnO from CuCrO 2 . In addition, the conventional etchants for ZnO (HCl, HNO 3 , H 3 PO 4 ) did not etch the CuCrO 2 . Simple photoresist masking can be used with the (NH4)2Ce(NO 3 ) 6 /HNO 3 /H20 mixtures to allow patterning of device structures.