W.E. Collins

Enhanced photoresponse in ZnO nanowires decorated with CdTe quantum dot

The photoresponse of ZnO nanowires irradiated with photons having energies below the band gap of ZnO (3.4eV) was studied before and after deposition of CdTe quantum dots via a pulsed electron-beam technique. The small amount of deposited CdTe did not increase the dark current of the samples. However, a substantial increase in the steady state photocurrent was observed after CdTe deposition suggesting a clear photosensitization effect. Results revealed that CdTe influences the photoconductivity transients of ZnO by minimizing its interaction with oxygen in air as well as providing additional traps that serve to increase the photocurrent time constant.

Increased short circuit current in organic photovoltaic using high-surface area electrode based on ZnO nanowires decorated with CdTe quantum dots

A photosensitized high-surface area transparent electrode has been employed to increase the short circuit current of a photovoltaic device with a blend of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) as the active layer. This is achieved by directly growing ZnO nanowires on indium tin oxide (ITO) film via a physical vapor method. The nanowire surface is then decorated with CdTe quantum dots by pulsed electron-beam deposition (PED). The nanowires alone provided a 20-fold increase in the short circuit current under visible light illumination. This was further increased by a factor of approximately 1.5 by the photosensitization effect of CdTe, which has an optical absorption of up to 820 nm.

Photoconductivity of CdTe nanocrystal films in a simple multilayer device structure

The photoconductivity of CdTe nanocrystal films was investigated by employing a ZnO/CdTe/In multilayer device structure. CdTe was deposited on a ZnO electrode by a pulsed electron-beam technique at argon background gas pressures of 9, 13 and 17 mTorr. Using two photo-excitation sources (visible and near-infrared), the device with the CdTe deposited at 17 mTorr demonstrated the highest photocurrent to dark current ratio, suggesting the highest quantum efficiency among the three different devices. It also demonstrated the highest short circuit photocurrent and the fastest photocurrent decay. These results are attributed to the formation of more nanocrystals at 17 mTorr with enhanced optoelectronic properties.

Evaluation of metal-free carbon nanotubes formed by SiC thermal decomposition

Thermal decomposition on SiC surface at high temperature and at low residual oxygen pressure yields a thin film of metal-free carbon nanotubes (CNTs) aligned on the SiC. In this study, CNT films on carbon-face 4H-SiC are produced under vacuum from 10−3to10−7Torr and at temperatures ranging from 1400to1700°C. Ex situ spectroscopic ellipsometry (SE) technique is applied to investigate the structure of the CNT films. It is found that two thin interfacial layers on the top and bottom of the CNTs are crucial in modeling the film structure from the SE data. We suggest a five layer structural model that consists of a CNT film as the main component, two interfacial layers, an amorphous carbon layer, and a surface roughness layer. This structural model based on the SE measurements is confirmed by cross-section transmission electron microscopy and can be applied for all CNT/SiC structures grown at various pressures and temperatures. At the growth pressure of 10−5Torr, the CNT layer on SiC consists of 30%–50% CNTs, ...

Role of Oxygen in Growth of Carbon Nanotubes on SiC

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.

Annealing behavior of tin implanted silica

Abstract Tin ions were implanted into silica substrates at 275 keV. The samples were annealed at 300–1000°C. The as-implanted and annealed samples were studied by the infrared specular reflectance technique. Kramers–Kronig transformation (KKT) was carried out on the reflectance spectra to obtain n, k, transverse optical (TO), and longitudinal optical (LO) spectra. Based on these spectra, we can understand the effects of ion implantation and annealing on the Si–O–Si bond angle, Si–O–Si bond breaking, and density change of implanted silica.

Influence of background gas pressure charging potential and target distance on the spot size ablated by single pulsed electron beam

The area of spot ablated by single pulsed electron beam has been measured as a function of pulsed electron deposition (PED) parameters. These parameters are background gas pressure, charging potential and target distance. A 50 nm thick film of silver on glass is used as ablation target in order to create a clear spot that can be measured. The spot area is found to reflect the power density of the pulsed electron beam and it can be varied by adjusting the PED parameters. These results are useful in the application of PED for material fabrication.

Infrared Reflectance Study of 3C-SiC Grown on Si by Chemical Vapor Deposition

We have performed a combined investigation of experiment and theory on the infrared reflectance from cubic SiC grown on Si by chemical vapor deposition. A damping behavior of the interference fringes away from the reststrahlen band and a dip or notch within the “flat top” are observed from some samples while they does not occur in high quality 3C-SiC/Si samples. The former is interpreted due to an interfacial transition layer existed between SiC-Si and a rough surface, while the latter can be demonstrated by a three-component effective medium model.

Ohmic Contacts on p-Type SiC Using Al/C Films

Al based alloys, such as Ti/Al, are commonly used for ohmic contacts on p-type SiC. The interfacial structures of a metal alloy film on SiC are very complicated after annealing. Al is considered as the key element responsible for forming ohmic contacts on p-type SiC, and reacts with C from SiC and forms Al4C3 and Si during annealing. In this study, we have investigated ohmic contact formation of a single component Al4C3 film on p-type SiC. Based on the stoichiometric formation of Al4C3 between Al and C at high temperatures, several samples with various Al/C mole ratios have been examined for ohmic contact formation after different annealing temperatures. Carbon rich and stoichiometric Al4C3 films form ohmic contacts on p-type 4H-SiC (~2.8 x1018 cm-3 ) after annealing at 800 and 900°C. X-ray diffraction (XRD) data have shown that a single component Al4C3 is formed when an ohmic contact on p-type SiC is activated. Al/SiC, as the control sample, does not form ohmic contacts under the same conditions. This study reveals that Al4C3 can be responsible for forming ohmic contacts on p-type SiC. However, its chemical instability requires that the secondary metal is necessary to form stable ohmic contacts when Albased films are used.

Optical Characterization of ZnO Materials Grown by Modified Melt Growth Technique

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.