Materials Science Forum Vols. 527-529

Abstract: We report on the fabrication and testing of GaN resistive gas sensors for hydrogen detection. The Si-doped n-type GaN was grown by organometallic vapor phase epitaxy (OMVPE) on c-plane sapphire substrates. The device structure is simply a pair of metal ohmic contact pads. The sensors are sensitive to H2 gas over a wide range of concentration: the lowest concentration tested being ~0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. In the continuous operation mode with varying H2 concentration, a clear and sharp response was recorded with no memory effects during ramping up and down cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. The possible gas sensing mechanisms are still under investigation.

Abstract: 3C-(Si1-xC1-y)Gex+y ternary alloys were grown on 8.5° off axis 4H-SiC substrates by solid source molecular beam epitaxy in a temperature range between 750°C and 950°C. Energy dispersive X-ray (EDX) analysis revealed a decrease of the Ge incorporation versus substrate temperature. This effect is due to the fixed Si/Ge ratio during the epitaxial growth. The Ge distribution within the grown epitaxial layers was found to be nearly homogeneous. The investigations by atomic location by channeling enhanced microanalysis allowed the conclusion that Ge is located mainly at Si lattice sites.

Abstract: The epitaxial relationship of Si deposited on 3C-SiC was studied using both free standing 3C-SiC(100) material from Hoya and 3C-SiC thin layers deposited on Si(100) as substrates. The conditions of Si growth were varied depending on the substrate. When Si is deposited at 1000°C on (001) 3C-SiC, it is in perfect epitaxial relation with the SiC layer [100]Si//[100]SiC and [001]Si//[001]SiC. After a 20 ms flash lamp pulse on the same sample, which has the effect of fast melting of the Si top layer only, the defects in the Si are eliminated. Using free standing 3C-SiC, the deposition temperature was not limited by the Si melting point so that it was fixed at 1500°C in order to form a set of Si liquid droplets on the surface with diameters ranging from 5 to 20 μm. Surprisingly more than 60% of the Si droplets exhibit the epitaxial relation [110]Si//[001]SiC and [111]Si//[110]SiC after crystallization. The occurrence of this epitaxial relationship can be understood in terms of lattice mismatch reduction from 20% to 18.3%. The conditions of crystallization, most probably the cooling rate, seem to have a strong effect on Si orientation.

Abstract: ZnO bulk crystal wafers, undoped and doped with various impurities of Ga, Er, Co, Ho, Fe, Mn, and co-doped Mg-Li, have been prepared by a modified melt growth method, and characterized by optical techniques of Raman scattering, photoluminescence and UV-visible transmission. Their wurtzite structures were confirmed, with a small degree of crystalline imperfection. It is shown that with some dopants, such as, Co and Fe, the electronic energy gap is affected much less than the optical absorption gap. Computer analysis has helped greatly in obtaining useful information of the optical properties of the ZnO bulk materials.

Abstract: Heteroepitaxial n-ZnO films have been grown on commercial p-type 6H-SiC substrates by plasma-assisted molecular-beam epitaxy, and n-ZnO/p-SiC heterojunction mesa structures have been fabricated and their photoresponse properties have been studied. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2 x 10-4 A/cm2 at -10 V, a breakdown voltage greater than 20 V, a forward turn on voltage of ∼5 V, and a forward current of ∼2 A/cm2 at 8 V. Photosensitivity of the diodes, when illuminated from ZnO side, was studied at room temperature and photoresponsivity of as high as 0.045 A/W at -7.5 V reverse bias was observed for photon energies higher than 3.0 eV.

Abstract: Carbon nanotubes (CNTs) grown on SiC are metal-free, well-aligned, and with low structural defects. In this study, CNT formation on SiC is examined in high vacuum (10-5torr) and ultra-high vacuum (10-8torr). Multi-wall carbon nanotubes and graphitic structures are the main products on the SiC surface at 1400-1800°C in 10-5torr. Under ultra-high vacuum, the decomposition rate of SiC is much lower than in high vacuum, indicating that SiC is decomposed by oxidation reaction. Using X-ray photoelectron spectroscopy (XPS), the intensity of the O1s peak at 530.3 eV decreases with increasing take-off angle, indicating that this oxygen species exists on the walls of CNTs. The results show that oxygen with a low pressure not only oxidizes SiC, but also forms a highly thermally stable carbon-oxygen compound, and interacts with the CNTs at high temperatures.

Abstract: Aligned carbon nanotubes (CNT’s) are formed on the surface of silicon carbide (SiC) wafers during high temperature anneals. The exposed 4H SiC surface transforms into CNT’s for temperatures in the range of 1400-1700°C and under moderate vacuum conditions (10-2 – 10-5 torr). The rate of formation on the C-face (0001,‾) is about three times the rate on the Si-face (0001), but both rates increase with anneal temperature. SEM, TEM and Raman scattering measurements have confirmed the presence of both single-wall and multi-wall CNT’s. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. CNT formation is believed to be catalyzed by low concentrations of residual oxygen in the chamber. Subsequent I-V measurements provide insight into the electrical characteristics of the CNT’s and the SiC/CNT interface.

Abstract: Carbon nanotubes present interesting potential applications especially in nanoelectronics. Their electrical properties are known to be a function of their chirality. It happens that 1/3 of CNs are metallic and 2/3 are semiconductors. Narrow nanotubes are expected to be wide-band gap semiconductors. Several experimental results have shown that the thickness of a multi-wall nanotube along the axis can change, while the interlayer spacing remains fairly constant. These observations suggest the coexistence in the same tube of a scroll structure and a multi-wall nested tube. We explain this defect as a screw dislocation which by gliding transforms between these two forms. In this paper, we present a density functional theory study of the structure and energetics of screw dislocations in AA and ABC graphite, and we discuss their role in the scroll-to-nanotube transformation in multi-wall nanotubes.

Abstract: The formation of metal/diamond Ohmic contacts is essential to most electronic devices. In order to form a good Ohmic contact to diamond a carbide-forming metal such as Ti or Cr is necessary. In this study, Cr/Au contacts to heavily boron-doped single crystal CVD diamond were fabricated by subsequent deposition of Cr and Au. The surface morphology and specific contact resistance of diamond/Cr/Au contacts has been investigated. The reaction between the Cr metal and the diamond during annealing gives an improved specific contact resistance. However, this reaction also causes a significant change in the surface morphology. The surface morphology of singlecrystal diamond is shown to greatly influence the properties of metal contacts to diamond. Shearforce mode atomic force microscopy (AFM) investigations have been used to examine the diamond surface before metallization, and after removing the metal contact. The initial diamond surface was predominantly smooth, apart from some scratches from the polishing process. Surface RMS roughness values of around 0.4nm were found. Correlation between surface morphology and contact resistance has been found, with rougher surfaces exhibiting a barrier to conduction. An understanding of the contact formation process is an essential step in achieving high quality Ohmic contacts which are vital in the fabrication of high quality diamond devices.