[en] GaN thin films on Si(001) substrates are studied by infrared reflectance (IRR) spectroscopy at room temperature (RT). Variations in the IRR spectral line shape with the microstructure of GaN/Si(011) film are quantitatively explained in terms of a three-component effective medium model. In this model, the nominally undoped GaN film is considered to consist of three elementary components, i.e., single crystalline GaN grains, pores (voids), and inter-granulated materials (amorphous GaN clusters). Such a polycrystalline nature of the GaN/Si(001) films was confirmed by scanning electron microscopy measurements. It was demonstrated that based on the proposed three-component effective medium model, excellent overall simulation of the RT-IRR spectra can be achieved, and the fine structures of the GaN reststrahlen band in the measured RT-IRR spectra can also be interpreted very well. Furthermore, the volume fraction for each component in the GaN/Si(001) film was accurately determined by fitting the experimental RT-IRR spectra with the theoretical simulation. These results indicate that IRR spectroscopy can offer a sensitive and convenient tool to probe the microstructure of GaN films grown on silicon. [copyright] 2001 American Institute of Physics

[en] An innovative multiple-step rapid thermal annealing (RTA) technique was developed for obtaining high hole concentration in the nonpolar a-plane Mg-delta-doped AlGaN films grown on semi-polar r-plane sapphire substrates with metal organic chemical vapor deposition. The structural, optical, and electrical properties of the nonpolar a-plane p-AlGaN epi-layers were characterized with high-resolution x-ray diffraction, photoluminescence spectra, and Hall effect measurement. The characterization results demonstrated that the dissociation efficiency of the Mg–H bonds in the nonpolar a-plane p-AlGaN films could be improved significantly with the multiple-step RTA process. In fact, a hole concentration as high as 1.9  ×  10¹⁸ cm⁻³ was achieved with the multiple-step RTA technique, which was 35%, 9%, and 23% higher than those obtained with conventional one-step, two-step, and the recently renovated two-step RTA methods, respectively. (paper)

[en] Structural properties of In_xGa_1−xN/GaN multi-quantum wells (MQWs) grown on sapphire by metal organic chemical vapor deposition are investigated by synchrotron radiation x-ray diffraction (SRXRD), Rutherford backscattering/channelling (RBS/C) and high-resolution transmission electron microscopy. The sample consists of eight periods of In_xGa_1−xN/GaN wells of 2.1 nm thickness and 8.5 nm thickness of GaN barrier, and the results are very close, which verifies the accuracy of the three methods. The indium content in In_xGa_1−xN/GaN MQWs by SRXRD and RBS/C is estimated, and results are in general the same. By RBS/C random spectra, the indium atomic lattice substitution rate is 94.0%, indicating that almost all indium atoms in In_xGa_1−xN/GaN MQWs are at substitution, that the indium distribution of each layer in In_xGa_1−xN/GaN MQWs is very homogeneous and that the In_xGa_1−xN/GaN MQWs have a very good crystalline quality. It is not accurate to estimate indium content in In_xGa_1−xN/GaN MQWs by photoluminescence (PL) spectra, because the result from the PL experimental method is very different from the results by the SRXRD and RBS/C experimental methods. (cross-disciplinary physics and related areas of science and technology)

[en] InSb thin films were grown on GaAs substrates by metal organic chemical vapor deposition (MOCVD) and investigated by temperature-dependent spectroscopic ellipsometry (TD-SE). The refractive index, extinction coefficient, and dielectric function of the InSb films were extracted. The variation of critical point energies (E₁, E₁+Δ₁, E₂, $E_1^{\text{'}}$) related to the excited state transitions of InSb and the second energy derivatives of the dielectric function at different temperatures showed that the InSb thin film had high electrical and optical stability at the evaluated temperatures. TD-SE analysis revealed a temperature range suitable for the use of InSb/GaAs-based devices. Beyond 250°C, InSb was heavily oxidized to form a thin In-O layer, causing a pronounced change in the optical constants. The results indicated that optimized InSb thin films grown on GaAs by MOCVD possess good optical and structural properties.

[en] Ultrafast third-order nonlinear optical response of bulk 6H-SiC undoped and doped with different nitrogen concentrations are investigated utilizing femtosecond Z-scan and optical Kerr effect (OKE) techniques at the wavelength of 800nm. The Z-scan measurement shows that the third-order nonlinear optical susceptibilities of the doped samples are improved in comparison to the intrinsic sample. The OKE results additionally reveal that the instantaneous nonlinear optical response of the samples can be ascribed to the distortion of the electron cloud. The ultrafast transient spectroscopic measurements with the one-color and two-color pump-probe techniques demonstrate that the ultrafast recovery process in subpicosecond domain is induced by two-photon absorption process, while the slow relaxation component reflects the carrier dynamics of the excited electrons. (fundamental areas of phenomenology(including applications))

[en] We have reported room temperature infrared reflectance and transmission measurements at near normal incidence in the frequency range of 200–6500 cm⁻¹ to characterize 3C-SiC epitaxial layers grown on (1 0 0) Si by CVD in a vertical reactor configuration by using different Si/C ratios and growth time ranging from 2 min to 4 h. The effects of surface roughness and ‘conducting interfacial’ transition layer are meticulously included in the effective medium theory for simulating the infrared spectra to explicate observed damping of interference fringe contrasts with increasing frequencies. In a few epitaxially grown samples we have adopted Bruggeman's two-component model to elucidate atypical divots seen within the reststrahlen band by assuming the coexistence of crystalline and intergranular grains forming heterogeneous 3C-SiC. The estimated values of surface roughness and conducting transition layer are correlated very well with the existing data from optical interference and scanning probe microscopy. (paper)

[en] The non-polar a-plane $(11\stackrel{¯<!--\; ??\; -->}{2}0)$ AlGaN thin films with various Al compositions were grown successfully on the r-plane$\left(2\stackrel{¯<!--\; ??\; -->}{2}04\right)$ semi-polar sapphire substrates using the continuous growth, two-way pulsed-flow, and three-way pulsed-flow growth methods, respectively with metal-organic chemical vapor deposition. The optical properties and thermo-optic effect of these films were studied extensively for the first time with angle- and temperature-dependent spectroscopic ellipsometry (SE) under both isotropic and anisotropic fitting modes. The SE fitting results for the energy band-gap, the layer thickness, and the surface roughness of the non-polar GaN, AlGaN, and AlN thin films were found to be comparable or even consistent quite well with the characterization results of high-resolution x-ray diffraction, ultraviolet-visible absorption spectroscopy, and atomic force microscopy. It was revealed that the non-polar AlGaN has higher refractive index than its polar counterpart with the same Al composition, and the non-polar AlGaN with the lowest surface roughness could be achieved with the three-way pulsed-flow growth method. Moreover, it was demonstrated that the anisotropy for the non-polar AlGaN thin film increased with increasing the Al composition. These characterization results should be useful for the fabrication of the non-polar AlGaN-based high-temperature power and ultraviolet-polarized optoelectronic devices utilizing thermo-optical effect and optical anisotropy. (paper)

[en] Understanding the recombination nature in a polar InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) or similar device is critical for their further performance enhancements. This paper reports a new theoretical model for investigating the recombination dynamics in MQW LEDs, which more comprehensively takes both localized exciton recombination (LER) and free carrier recombination (FCR) into account. The obtained rates for LER, FCR and nonradiative recombination show a clear picture of recombination paths in a commercial blue MQW LED wafer. They can be also used to calculate the internal quantum efficiency without involving any extra measurements or prerequisites. This model may provide a universal solution to express the complicated recombination dynamics in various kinds of MQW LEDs. (paper)

[en] In this paper, we report a 2.6-fold deep ultraviolet emission enhancement of integrated photoluminescence (PL) intensity in AlGaN-based multi-quantum wells (MQWs) by introducing the coupling of local surface plasmons from Al nanoparticles (NPs) on a SiO₂ dielectric interlayer with excitons and photons in MQWs at room temperature. In comparison to bare AlGaN MQWs, a significant 2.3-fold enhancement of the internal quantum efficiency, from 16% to 37%, as well as a 13% enhancement of photon extraction efficiency have been observed in the MQWs decorated with Al NPs on SiO₂ dielectric interlayer. Polarization-dependent PL measurement showed that both the transverse electric and transverse magnetic mode were stronger than the original intensity in bare AlGaN MQWs, indicating a strong LSPs coupling process and vigorous scattering ability of the Al/SiO₂ composite structure. These results were confirmed by the activation energy of non-radiative recombination from temperature-dependent PL measurement and the theoretical three dimensional finite difference time domain calculations. (paper)

[en] Photoluminescence (PL) spectrum provides the most conventional measurement for emission properties of GaN-based light-emitting diodes (LEDs), in which Fabry–Perot oscillations are often observed modulating the emission peaks. A fitting model for PL intensity accounting the microcavity between air/GaN and GaN/sapphire heterostructure was proposed to treat the PL spectra of different GaN-based LEDs, extracting key factors that implied the interfacial optical properties. This approach was successfully verified by measurements of spectroscopic ellipsometry, then was applied to quantitatively analyse the interfacial-defect-related distortion of dielectric properties. The extracted oscillation coefficient is sensitive to the change of interface qualities and reveals the optical properties of internal interfaces. The new method may also be applied to the other heterojunction LEDs. (paper)