Low Grain Boundary Density Research Articles

Influence of morphology and structure geometry on NO 2 gas-sensing characteristics of SnO 2 nanostructures synthesized via a thermal evaporation method

SnO 2 microwires, nanowires and rice-shaped nanoparticles were synthesized by a thermal evaporation method. The diameters of microwire and nanowire were 2 μm and 50–100 nm, respectively, with approximately the same length (∼20 μm). The size of nanoparticles was about 100 nm. It was confirmed that the as-synthesized products have SnO 2 crystalline rutile structure. The sensing ability of SnO 2 particle and wire-like structure configured as gas sensors was measured. A comparison between the particle and wire-like structure sensors revealed that the latter have numerous advantages in terms of reliability and high sensitivity. Although its high surface-to-volume ratio, the nanoparticle sensor exhibited the lowest sensitivity. The high surface-to-volume ratio and low density of grain boundaries is the best way to improve the sensitivity of SnO 2 gas sensors, as in case of nanowire sensor which exhibited a dramatic improvement in sensitivity to NO 2 gas.

Thin metal films deposited at low temperature for optoelectronic device application

Thin metal layers play an important role in the development of electronic devices. Thin metal films deposited at low temperature (LT=77 K) have shown some unique properties which enhance device performance. The microstructure properties of thin metal films formed at room temperature (RT=300 K) and LT were investigated in this work. An insulating substrate was used for Au, Ag, and Al metal deposition. The surface morphology and microstructure of the metal films was studied by transmission electron spectroscopy (TEM) and atomic force microscopy (AFM). The resistance of the thin films was measured in situ as a function of film thickness and temperature. It was found that all LT thin films become electrically continuous at a much lower thickness than the RT films. Electrical measurements determined that the LT films demonstrated several orders of lower resistance compared to RT film at very thin (less than 100 Å) thickness, which could lead to the potential applications of these films on electronic and optoelectronic devices. It is observed by both TEM and AFM that the LT films showed much lower density of grain boundaries than the RT samples at the same thickness. This is consistent with the resistance measurement results.

Schottky barrier height and thermal stability of the NiAl/n-Ge/GaAs(001) interface

We have successfully grown epitaxial NiAl on single-crystal Ge interlayers, which were in turn grown on GaAs(001) substrates. The goal has been to use the single crystallinity of the NiAl overlayer with its characteristically low density of grain boundaries as a diffusion barrier to prevent outdiffusion of n-type dopant atoms from the Ge interlayer. Heavily As-doped Ge has proven effective in lowering the Schottky barrier height of GaAs substrates to 0.2–0.5 eV. The epitaxial overlayers and interfaces have been characterized in situ by means of x-ray photoemission, x-ray photoelectron diffraction, and low-energy electron diffraction. We have found that even at growth temperatures as low as 210 °C, outdiffusion of As from the Ge layer into the growing NiAl epifilm occurs, causing the barrier height to increase from ∼0.4 eV to ∼0.6 eV. Furthermore, limited Al atom indiffusion and Al–Ga cation exchange occur during the initial phase of NiAl growth, as does limited disruption of the Ge epilayer surface. Band alignment at the Ge/GaAs heterojunction is also perturbed as a result of the cross-diffusion that occurs, as evidenced by an ∼0.15 eV increase in the valence band offset. Annealing at 350 °C causes additional As loss from the Ge interlayer, a further barrier height increase, additional penetration of Al atoms to the Ge/GaAs interface, and enhanced Al–Ga cation exchange. Thus, it appears that such a system is of little value as a high-thermal-stability, low-barrier-height metal contact to GaAs. Interface degradation is dominated by interdiffusion and chemistry.