Tzuen Rong Yang

Infrared reflectance analysis of GaN epitaxial layers grown on sapphire and silicon substrates

Abstract Infrared reflectance (IR) of GaN grown on sapphire and silicon substrates has been studied both theoretically and experimentally. The theoretical calculation of the IR spectra is based on the transfer matrix method. The IR spectral characteristics influenced by several factors, such as film thickness, incident angle, free carriers, are systematically examined. Combined with experimental results, surface scattering and interface layer effects are also studied. For GaN epilayers grown on sapphire, carrier concentrations and mobility are determined by fitting to the IR reststrahlen band and compared with the Hall measurement. The interface effect is demonstrated to cause a damping behaviour of the interference fringes away from the reststrahlen band. For GaN grown on Si, the IR spectra predicted the large surface roughness of the epilayers. A variation of IR reststrahlen band is correlated to the microstructures of the films, i.e. their polycrystalline nature of the GaN films grown on Si. A three-component effective medium model is proposed to calculate the IR spectra for polycrystalline GaN, and a qualitative correlation between the IR spectra and structure of the film is established. All results show that IR, as a non-destructive method, is efficient for characterising GaN epilayers in semiconductor processing.

Optical and structural properties of GaN materials and structures grown on Si by metalorganic chemical vapor deposition

Abstract GaN thin films, undoped, Si- and Mg-doped, and InGaN–GaN multiple quantum well (MQW) structures have been grown on Si (001) substrates with specially designed composite intermediate layers consisting of an ultra-thin amorphous Si layer and a GaN/AlGaN multilayered buffer by low pressure metalorganic chemical vapor deposition. Their optical and structural properties were studied by a variety of characterization techniques, including X-ray diffraction (XRD), Raman scattering, Fourier transform infrared reflectance, photoluminescence, scanning and transmission electron microscopy. Wurtzite GaN structure with the single crystalline grain size up to ∼2 μm is confirmed, and material properties are improved by adjusting the growth conditions and buffer layer structural design.

Critical currents of Bi:2212 doped by Fe and Ni

Abstract Measurements of the irreversible magnetization, ac susceptibility and electrical resistivity of Bi 2 Sr 2 Ca 1− x Er x (Cu 1− y M y ) 2 O 8+ d bulk are reported. Low concentrations of the doping elements increase the pinning force density and shift the magnetic irreversibility line towards higher fields. The intergranular critical current density was determined from ac susceptibility data by varying the field amplitude H ac in the range from 0.4 to 1000 A/m and from the irreversible part of magnetization using Bean’s model. The results were discussed in terms of SIS- and SNS-type models for granular superconductors.

An exact solution of the Lippmann–Schwinger equation in one dimension

We consider a one-dimensional scattering problem and establish a system of two integral equations for the local (self-consistent) electric field. The equations are derived using the standard Green’s function method. Their solution yields the local fields at any point of the system. To illustrate the efficacy of this approach, we apply it to a film and obtain analytical solutions that are identical to the Fabry–Perot solution. Some potential applications of the approach are also discussed.

Phase separation in Zn-doped InGaN grown by metalorganic chemical vapor deposition

Abstract Zn-doped InGaN thin films were deposited on GaN/sapphire by metalorganic chemical vapor deposition, and studied by a combination of high-resolution X-ray diffraction (HR-XRD), micro-photoluminescence (PL) and secondary ion mass spectrometry (SIMS). Indium phase separation is studied comparatively. HR-XRD exhibits a GaN band and a single band from InGaN for samples without phase separation, but two InGaN bands corresponding to different x(In) for samples with phase separation. PL excitation power dependence measurements reveal 2 sets of InGaN PL emissions for samples with phase separation, but only 1 set for samples without phase separation. SIMS data showed that phase separated InGaN:Zn films possess a high Zn concentration near the InGaN–GaN interface and non-uniform distributions of In and Zn contents, which are in contrast with data from InGaN:Zn films with no In-phase separation.

Effects of rare earth ion substitution for Ca in (Bi,Pb):2223 superconductors

Abstract The effect of partial replacement of Ca by Y, Er and Lu on the structural transport properties of (Bi 1.6 Pb 0.4 )(Sr 1.8 Ba 0.2 )(Ca 1− x R x ) 2 Cu 3 O y superconductors ceramics (0≤ x ≤0.03) have been investigated. The substitution of Y, Lu and Er at the Ca site induced the decrease of the volume fraction for the 2223 phase and the increase of the 2212 and 2201 phases volume fractions. In the normal state, all the samples are characterized by a linear temperature dependence of electrical resistivity and Hall concentrations. The rate of depression for the critical temperature in x =0.02 samples found to be nearly the same for all of these substitutions. The nature of T C suppression may be attributed to the increase of oxygen content in Bi 2 O 2 double layers with increasing Y, Lu and Er concentrations. The sample shows a linear dependence of intergranular temperature as a function of AC field amplitude, which is in agreement with the Muller critical state model.

Transport properties and ac susceptibility of (Bi1.6Pb0.4)Sr2Ca2Cu1−xCox)3Oy superconductors

Abstract We have investigated the effect of the partial substitution of Cu by Co in the stabilized 108 K phase Bi:2223, prepared by conventional solid state reaction method, by using electrical resistivity, Hall coefficient and ac susceptibility measurements. The critical temperatures and Hall concentration depend strongly on the Co content of the samples. Up to Co concentration of x =0.025 the bending of the ρ ( T ) curves, above T c , can be described in the terms of the Lawrence–Doniach theory. The intergrain and intragrain critical transition temperatures decrease by Δ T CG =8.5 K and Δ T j =11.2 K, respectively, as a result of the substitution of Co for Cu in Cu–O pyramids.

Hall effect, thermopower and electrical resistivity of doped RuSr 2GdCu2O8 system

Polycrystalline samples of Ru1-xSbxSr2GdCuO8 doped with Sb, in which magnetic order and superconductivity coexist with Tmagnetic≫Tc, were prepared by a solid state reaction technique. Lattice parameters, electrical resistivity, Hall and thermopower coefficients measurements on the same sintered ceramic samples are presented. Both Hall effect and thermopower show anomalous decreases below Tmagnetic which might be explained with a simple two-band model in the RuO2 layer at Tmagnetic. It was also observed that the Sb doping reduce the conductivity of the system and the transition temperature decreases with increasing Sb content. This may be due to a distortion of RuO6 octahedra which is responsible for the increase of hole localization.

Far-IR investigation of thin InGaN layers

AbstractWehavemeasuredthereflectionspectraofultra-thinIn x Ga 1 x N/GaNlayersonsapphiresubstrateswiththickGaNbufferlayerattemperaturesof300,200and90K.Wecouldnotobserveanysignalassociatedwiththeselayers.Comparingthemeasuredspectrawiththecalculatedonesforwell-mixedIn x Ga 1 x Nlayers,weconcludedthatoursampleshadnomixedstate.WealsoconcludedthattheseparatedInandNatomsdidnotformInNlayers.ThereasonforthefactthatwedidnotobservearesponsefromtheselayersistheinhomogeneousbroadeningoftheIn x Ga 1 x Nreflectionline.Wehavecalculatedtheabsorptionspectraofsmallparallelepiped-likeInNparticlesintheGaNmatrix.Ithasshownthattheabsorptionlinewidthforsuchsmallparallelepiped-likeparticlesisbiggerthanthatforthebulkInNmaterial.WeconcludedthatoursamplescontainedseparatedInanddemonstratedformationofInNclusters.r 2002ElsevierScienceB.V.Allrightsreserved. PACS: 78.30.Fs;78.66.Fd;78.67.Bf;42.25.BsKeywords: IRspectroscopy;InGaN;Mixedcrystals;Absorptionofnanoparticles 1. IntroductionAt present the wide-gap GaN-based nitridesemiconductorsattractagreatdealofattentionin research and development, because of theirusefultechnologicalapplicationsinlight-emittingdiodesinthenear-UV-blue-greenspectralregion[1],andtheirpotentialitiesforuseinthehigh-temperatureelectronics[2].In

Infrared Spectroscopy of Hg1-xZnxTe Alloys

Infrared spectra, including transmission and reflection, have been used to characterize Hg1-x Znx Te crystals with various compositions x from cryostat temperature to room temperature. The energy band gap of the crystals were determined from the transmission spectra and their temperature dependence was observed. The reflection spectra of Hg1-x Znx Te crystals have been measured in the wave-number region of 60 to 450 cm-1. The frequencies of optical mode lattice vibrations are determined and their temperature dependence is discussed. A correlation between the HgTe lattice vibration TO mode and the electronic band structure of Hg1-x Znx Te alloys was found and the temperature dependence of electronic states for Hg1-x Znx Te can be explained well by a clustering effect in crystals.