C. Jacob

Silicon—a new substrate for GaN growth

Generally, GaN-based devices are grown on silicon carbide or sapphire substrates. But these substrates are costly and insulating in nature and also are not available in large diameter. Silicon can meet the requirements for a low cost and conducting substrate and will enable integration of optoelectronic or high power electronic devices with Si based electronics. But the main problem that hinders the rapid development of GaN devices based on silicon is the thermal mismatch of GaN and Si, which generates cracks. In 1998, the first MBE grown GaN based LED on Si was made and now the quality of material grown on silicon is comparable to that on sapphire substrate. It is only a question of time before Si based GaN devices appear on the market. This article is a review of the latest developments in GaN based devices on silicon.

CVD growth and characterization of 3C-SiC thin films

Cubic silicon carbide (3C-SiC) thin films were grown on (100) and (111) Si substrates by CVD technique using hexamethyldisilane (HMDS) as the source material in a resistance heated furnace. HMDS was used as the single source for both Si and C though propane was available for the preliminary carbonization. For selective epitaxial growth, patterned Si (100) substrates were used. The effect of different growth parameters such as substrate orientation, growth temperature, precursor concentration, etc on growth was examined to improve the film quality. The surface morphology, microstructure and crystallinity of grown films were studied using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS).

The effect of Fe and Ni catalysts on the growth of multiwalled carbon nanotubes using chemical vapor deposition

The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is no trace of carbide phases in both the cases. Low frequency Raman analysis of the bamboo-like and filled CNTs confirms the presence of radial breathing modes (RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is higher than that from Ni catalyst as demonstrated by the high frequency Raman analysis. Simple models for the growth of bamboo-like and tubular catalyst filled nanotubes are proposed.

An atomic force microscopy and optical microscopy study of various shaped void formation and reduction in 3C-SiC films grown on Si using chemical vapor deposition

The formation of various uncommon shaped voids along with regular triangular and square voids in the epitaxial 3C-SiC films on Si has been investigated by optical microscopy and atomic force microscopy. Heteroepitaxial growth of 3C-SiC films on Si (001) and (111) substrates has been performed using hexamethyldisilane in a resistance-heated chemical vapor deposition reactor. The influence of the orientation of the Si substrate in determining the shape of the voids has clearly been observed. In addition, the growth period and the growth-temperature have been considered as the major parameters to control the size, density and shape of the voids. Generally, voids are faceted along {111} planes, but depending upon growth conditions, other facets with higher surface energy have also been observed. Finally the size and density of the voids are remarkably reduced, by suitable growth technique.

Studies on Ru/3C-SiC Schottky Junctions for High Temperature Hydrogen Sensors

The sensor response behavior of the Ru/3C-SiC (epilayer on Si substrate) Schottky junctions was studied at different temperatures in presence of hydrogen gas with concentrations varying from 5,000 ppm to 20,000 ppm. The output signal of the sensors, the response time, and the reversibility were investigated from the transient response characteristics of the sensors. The sensor parameters were found to improve with higher operating temperature, up to 400°C, above which they degrade due to the influence of thermally generated carriers in the Si substrate. The sensitivity was found to be a function of applied bias across the junction. As compared to the Pd/3C-SiC junctions, the Ru/3C-SiC Schottky sensors showed better resolution in the hydrogen concentration range between 10.000 and 20.000 ppm. Also, the Ru/3C-SiC sensors showed better reversibility. The secondary ion mass spectrometry study indicates that the Ru/3C-SiC Schottky sensors can be operated at 400°C without any significant degradation of the interface.

Effect of growth temperature on the CVD grown Fe filled multi-walled carbon nanotubes using a modified photoresist

Fe filled carbon nanotubes were synthesized by atmospheric pressure chemical vapor deposition using a simple mixture of iron(III) acetylacetonate (Fe(acac){sub 3}) with a conventional photoresist and the effect of growth temperature (550-950 {sup o}C) on Fe filled nanotubes has been studied. Scanning electron microscopy results show that, as the growth temperature increases from 550 to 950 {sup o}C, the average diameter of the nanotubes increases while their number density decreases. High resolution transmission electron microscopy along with energy dispersive X-ray investigation shows that the nanotubes have a multi-walled structure with partial Fe filling for all growth temperatures. The graphitic nature of the nanotubes was observed via X-ray diffraction pattern. Raman analysis demonstrates that the degree of graphitization of the carbon nanotubes depends upon the growth temperature.

SIMS and RBS study of thermally annealed Pd/β-SiC interfaces

The Pd/β-SiC interfaces were studied using secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (RBS). This was done with the intent of clarifying any reaction or inter-diffusion at the interface upon prolonged annealing at different temperatures in air. SIMS study indicates that the interface is stable up to 400 °C for at least 12 h. However, at 800 °C, the interface was completely degraded with significant inter-diffusion of palladium and silicon. The RBS study confirms the SIMS observations.

Ohmic contacts to 3C-SiC for Schottky diode gas sensors

Abstract The basic objective of this work was to investigate a suitable ohmic metallization to 3C-SiC epitaxial layer with relatively low process temperature, which is important for on-chip circuit integration. In this study, the ohmic contacts are associated with the catalytic-metal/SiC Schottky diode gas sensors operable at 400 °C. Therefore, the contacts must be reliable for efficient operation of the sensors at this temperature in air ambient. Several metallization schemes were studied and the observed electrical characteristics were correlated to the interface properties by secondary ion mass spectrometry. Nickel contacts to moderately doped (∼10 17 cm −3 ) 3C-SiC epilayer did not exhibit linear current–voltage relationship even after annealing at 600 °C. On the other hand, aluminum exhibited considerably low intercontact resistance in the as-deposited condition. But with increase in annealing temperature the current flow between a couple of Al contacts was impeded presumably due to very high affinity of this metal for oxygen. Titanium exhibited good ohmic properties after annealing at 400 °C. However, higher temperature (600 °C) annealing in air ambient (intended to examine its stability limit) caused surface oxidation of the Ti layer. This problem was minimized by deposition of 100 nm Au overlayer (on top of 400 nm Ti) which would increase the surface conductivity of the ohmic contacts and ensure stable wire bonding.

Carbon nanotube synthesis from propane decomposition on a pre-treated Ni overlayer

Growth of carbon nanotubes (CNTs) was performed by atmospheric pressure chemical vapour deposition (APCVD) of propane on Si(111) with a pre-treated Ni overlayer acting as a catalyst. Prior to the growth of CNTs, a thin film of Ni was deposited on Si(111) substrate by evaporation and heat treated at 900°C. The growth of nanotubes was carried out at 850°C using propane as a source of carbon. Distribution of the catalyst particles over the Si substrate was analysed before and after heat treatment by atomic force microscopy (AFM). The X-ray diffraction (XRD) pattern of the grown material revealed that they are graphitic in nature. Field emission scanning electron microscopy (FESEM) was used to investigate the growth process and it was found that a catalytic particle was always situated at the tip of the tube thus implying a tip growth mechanism. Evidence for the presence of radial breathing mode from multi-wall nanotubes (MWNTs) in the grown sample was obtained from micro-Raman analysis. Finally, high-resolution transmission electron microscopic (HRTEM) analysis confirmed that the graphene layers of the CNTs are well ordered with typical 0·34 nm spacing.

Pre-heating effect on the catalytic growth of partially filled carbon nanotubes by chemical vapor deposition.

The surface reconstruction of the Fe catalyst films due to high temperature processing in hydrogen prior to nanotube nucleation and its effect on the growth morphologies of partially filled carbon nanotubes (CNTs) synthesized using atmospheric pressure chemical vapor deposition (APCVD) of propane was investigated. Results show that pre-heating of the catalyst film deeply influences the particle size distribution, which governs the growth morphologies of the corresponding CNTs. The distribution of the catalyst particles over the Si substrate was analyzed before and after the heat treatment by atomic force microscopy (AFM) which reveals that heat treatment causes clusters of catalyst to coalesce and form macroscopic islands. The X-ray diffraction (XRD) pattern of the grown material indicates that they are graphitic in nature. Scanning electron microscopy (SEM) analysis suggested that the growth density strongly depends on the pre heat treatment of the Fe catalyst film. Multiwalled CNTs with partial catalyst filling were observed via high-resolution transmission electron microscopy (HRTEM) measurements. The degree of graphitization of the CNTs also depends on the pre heating as demonstrated by Raman analysis. A simple model for the growth of partially catalyst filled nanotubes is proposed.