A freestanding GaN substrate over 2 inches in size was successfully prepared for the first time by hydride vapor phase epitaxy (HVPE) using GaAs as a starting substrate. In the experiment, a GaAs (111)A substrate with a SiO 2 mask pattern on its surface was used. A thick GaN layer was grown on the GaAs substrate at 1030 • C through the openings in the SiO 2 mask. By dissolving the GaAs substrate in aqua regia, a freestanding GaN substrate about 500 µm thick was obtained. The fullwidth at half maximum (FWHM) in the ω-mode X-ray diffraction (XRD) profile of GaN (0002) plane was 106 arcsec. The dislocation density of the GaN substrate obtained was determined to be as low as 2 × 10 5 cm −2 by plan-view transmission electron microscopy (TEM). Hall measurements revealed the n-type conductivity of the GaN substrate with typical carrier concentration and carrier mobility of 5 × 10 18 cm −3 and 170 cm 2 ·V −1 ·s −1 , respectively.

No abstract

No abstract

We present a reflection-type terahertz time-domain spectroscopic ellipsometry (THz-TDSE) technique for measuring the complex dielectric constants of thin-film materials without replacement of the sample. THz-TDSE provides complex dielectric constants from the ratio of the complex amplitude reflection coefficients between p- and s-polarized THz waves. The measured dielectric constants of doped GaAs thin films show good agreement with predictions of the Drude model, even though the film thickness is of the order of a tenth of the penetration depth of the THz waves. In addition, we demonstrate the measurements of soft-phonon dispersion in SrTiO(3) thin films deposited on a Pt layer. The obtained dielectric constants agree well with the predictions of a generalized harmonic oscillator model.

We have measured the soft-mode dispersion of SrTiO 3 bulk single crystals by terahertz time-domain spectroscopic ellipsometry (THz-TDSE). The real and imaginary parts of the dielectric function of the sample are independently derived from the complex amplitude reflectivities of p-and s-polarized terahertz pulses. The TO 1 soft-mode resonance is clearly observed at 2.62 THz (87 cm À1 ). The spectra of the complex dielectric function agree well with those predicted using the general harmonic oscillator model with parameters obtained from conventional Fourier-transform far-infrared (FT-FIR) reflection measurements. THz-TDSE developed in this study can be employed to investigate low-frequency dielectric dispersions.