Observations of conduction-band structure of 4H- and 6H-SiC

Ballistic electron-emission spectroscopy (BEES) and photoluminescence are used to study conduction-band structure related transport properties of the 4H and 6H polytypes of SiC. A secondary energy threshold at 2.7 eV is observed in the BEES spectrum of 4H-SiC, in good agreement with a value of 2.8 eV deduced from reported ab initio calculations. The results from 6H-SiC, are suggested to be influenced by transport properties of other polytype inclusions, also supported by band-edge transitions evident in 6H-SiC photoluminescence spectra

Electroluminescence from single nanowires by tunnel injection: an experimental study

We present a hybrid light-emitting diode structure composed of an n-type gallium nitride nanowire on a p-type silicon substrate in which current is injected along the length of the nanowire. The device emits ultraviolet light under both bias polarities. Tunnel-injection of holes from the p-type substrate (under forward bias) and from the metal (under reverse bias) through thin native oxide barriers consistently explains the observed electroluminescence behaviour. This work shows that the standard p-n junction model is generally not applicable to this kind of device structure.Comment: 6 pages, 6 figure

Epitaxial Catalyst-Free Growth of InN Nanorods onc-Plane Sapphire

We report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H–InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with thec-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor–liquid–solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV

Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification

We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution

Hall photovoltage deep-level spectroscopy of GaN films

Spectroscopy of photoinduced changes in semiconductor Hall voltage is proposed as a method to characterize deep levels. An analytical expression for the Hall coefficient as a function of the charge trapped at grain boundaries is derived. The experimental Hall voltage is demonstrated by measuring thin films of GaN grown on sapphire and is shown to be consistent with the model. The Hall voltage spectrum is correlated to spectra from three other deep-level spectroscopies: photoluminescence, photoconductivity, and surface photovoltage, obtained under the same conditions from the same sample. Comparing spectra from the various spectroscopies shows that the yellow-luminescence-related deep states in GaN are charged in equilibrium and discharged by the exciting photons, and suggests that the blue-luminescence-related states are deep donors positioned 2.8 eV above the valence band and neutral in equilibrium