Nang Wang

Optical emission spectroscopic studies of microwave enhanced diamond CVD using CH 4 /CO 2 plasmas

Diamond films have been grown using CH 4 /CO 2 gas mixtures in a microwave plasma deposition reactor. Optimum growth of well-faceted polycrystalline diamond occurs at approximately 50% CO 2 /50% CH 4 with a growth rate of approximately 1 m mh ’1 for a process pressure of 45 Torr and 1 kW applied power. Optical emission spectroscopic studies of CH 4 /CO 2 microwave plasmas have been performed for a range of diVerent process conditions, and the results correlated with diamond film quality and growth rate. Emission spectra (300‐800 nm) are presented for gas compositions ranging from 100% CO 2 to 100% CH 4 . Spectral peaks from electronically excited C 2 ,C 3 , CH, CO, O and H have been observed, and maxima in the ratios of the CH:C 2 , CH:C 3 and H:C 2 emission intensities are all found to correlate with process conditions for good quality diamond. By assuming that the rising spectral background at long wavelength can be attributed to blackbody radiation from macroscopic carbonaceous particles, we estimate the average gas temperature of the plasma to be 1925 K. For 100% CO 2 plasmas, emission peaks from atomic Mo are observed, indicating etching of the Mo substrate holder.

Accelerated formation of 110 K high Tc phase in the Ca-and Cu-rich Bi-Pb-Sr-Ca-Cu-O system

The crystal structure and superconducting properties of the Bi‐Pb‐Sr‐Ca‐Cu‐O system with Ca‐ and Cu‐rich nominal composition were investigated. A nearly single‐phased 110 K high Tc superconductor can be obtained with 852 °C/20 h sintering from the starting composition of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy. X‐ray diffraction patterns, resistivity measurement, diamagnetic susceptibility results, and scanning electron micrographs all indicate that the Ca‐ and Cu‐rich nominal composition would result in better superconducting properties than those of Ca:Sr=1:1 Bi‐Pb‐Sr‐Ca‐Cu‐O compounds in a much shorter sintering time.

Light Emitting Diode with Charge Asymmetric Resonance Tunneling

We suggest a system of two wells connected with Charge Asymmetric Resonance Tunneling (CART) as a basic element of light emitting diode (LED) structure for semiconductors with different masses of electrons and holes. The system consists of an emitter of electrons, an emitter of holes and an active layer. The hole emitter is coupled with the active in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is coupled to the active layer via a barrier. The barrier design uses the charge asymmetric resonance tunneling phenomenon which allows to make the barrier transparent for electrons and blocking for holes. Advantages of this design are: the increased capture efficiency of the electrons into the active layer due to direct resonance tunneling of the electrons from the electron emitter on bound electron level in the active quantum well, the suppression of electron leakage into the hole emitter, the elimination of the parasitic light generated outside the active layer, and the electron emitter acts also as a good current spreading layer. First results of experimental investigation and theoretical modeling of the CART LED devices are reported.

In situ monitoring of GaN epitaxial lateral overgrowth by spectroscopic reflectometry

The application of spectroscopic reflectometry to the monitoring of epitaxial lateral overgrowth of GaN in low pressure metalorganic vapor phase epitaxy has been investigated. Real-time vertical and lateral growth rates and hence thickness and wing width of the growing GaN are extracted. A vertical growth enhancement was clearly observed at an early stage, followed by vertical growth suppression until full coalescence was achieved. The lateral to vertical growth ratio was obtained showing clear time dependent characteristics. The observations were explained by considering the mass transport between the growing (0001) facets and the {112¯0} sidewall facets.

Resistive, magnetic, and structural studies of Tl0.5Pb0.5(Ca1−xMx)Sr2Cu2Oy compounds with M equal to the natural mixture of rare‐earth elements

We report the resistivity, structural, and magnetic susceptibility measurements for Tl0.5Pb0.5(Ca1−xMx)Sr2Cu2Oy superconductors with M equal to the natural mixture of rare‐earth elements and x=0, 0.05, 0.1, 0.2, and 0.3, respectively. The powder x‐ray diffraction profile indicates a single‐phase, P4/mmm tetragonal crystal structure with a=0.380±0.002 nm and c=1.205±0.002 nm for a Tl0.5Pb0.5(Ca0.9M0.1)Sr2Cu2Oy superconductor. Tc (zero)’s were found to be 79, 91, 101, 97, and 87 K for x=0, 0.05, 0.1, 0.2, and 0.3, respectively. Hc1 of Tl0.5Pb0.5(Ca0.9M0.1)Sr2Cu2Oy was estimated to be around 1.8 kG at 5 K, and Jc about 1.35×103 A/cm2 with a 20 kG applied field at 5 K. The Meissner signal of this sample indicates the Tc onset ∼100 K, and the mass diamagnetic susceptibility −χg at 5 K is 2.52×10−3 cm3/g (field cooled at 100 G).

Misfit dislocations and radiative efficiency of InxGa1-xN/GaN quantum wells

Abstract We report results of calculations of radiative efficiency of In x Ga 1− x N quantum wells embedded in wurtzite GaN epilayer. It was found that misfit dislocations with density up to ∼10 5–6 cm −1 could improve the quantum efficiency of the In x Ga 1− x N wells by more than 10 times because they reduce the quantum well built-in electric field. At higher densities, the misfit dislocations suppress the quantum efficiency of the wells since they produce an additional channel of nonradiative recombination.